A Review of Microwave Synthesis of Zinc Oxide Nanomaterials: Reactants, Process Parameters and Morphologies

Abstract

Zinc oxide (ZnO) is a multifunctional material due to its exceptional physicochemical properties and broad usefulness. The special properties resulting from the reduction of the material size from the macro scale to the nano scale has made the application of ZnO nanomaterials (ZnO NMs) more popular in numerous consumer products. In recent years, particular attention has been drawn to the development of various methods of ZnO NMs synthesis, which above all meet the requirements of the green chemistry approach. The application of the microwave heating technology when obtaining ZnO NMs enables the development of new methods of syntheses, which are characterised by, among others, the possibility to control the properties, repeatability, reproducibility, short synthesis duration, low price, purity, and fulfilment of the eco-friendly approach criterion. The dynamic development of materials engineering is the reason why it is necessary to obtain ZnO NMs with strictly defined properties. The present review aims to discuss the state of the art regarding the microwave synthesis of undoped and doped ZnO NMs. The first part of the review presents the properties of ZnO and new applications of ZnO NMs. Subsequently, the properties of microwave heating are discussed and compared with conventional heating and areas of application are presented. The final part of the paper presents reactants, parameters of processes, and the morphology of products, with a division of the microwave synthesis of ZnO NMs into three primary groups, namely hydrothermal, solvothermal, and hybrid methods.

1. Introduction

1.1. Nanotechnology

Nanotechnology is one of the most rapidly developing disciplines of science and technology. It was the reason for the global industrial revolution of the 21st century [1,2,3,4,5,6,7,8,9,10]. At present, this is a leading technology within various research areas, such as: physics, biology, chemistry, biochemistry, biotechnology, medicine, materials and biomedical engineering, electronics, optoelectronics, mechatronics, spintronics, energy generation, food and agriculture, environmental protection, and interdisciplinary fields [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. This technology enables testing, controlling, regulating, modifying, processing, producing, and using structures in which at least one of the dimensions does not exceed 100 nanometres (1 nm = 10⁻⁹ m) [39]. Materials in the nano-scale are characterised by new specific properties and phenomena, which sometimes considerably deviate from the characteristic properties of the same materials appearing in the micro-scale [40,41,42,43,44,45,46,47,48,49,50]. The observed changes to the physicochemical properties are caused primarily by two factors, namely the quantum confinement of electrons and increased share of surface atoms/ions in relation to atoms/ions present inside the particle. The big development of the specific surface area of the nanomaterial leads to an increase in the number of unsaturated coordination centres, defects and stresses in the crystal lattice, and in the chemical reactivity of particles [51,52].

Nanotechnology enables the application of nanomaterials to create innovative products, devices, and complex systems that make use of material properties in the nano-scale [11]. Products developed and created thanks to the use of nanotechnology are present in all aspects related to the human activity on Earth and in the outer space [2,3,4,5,6,7,8,9,10,11,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67]. The current research concerning the applications of nanotechnology in the broadly understood catalysis, optoelectronics, microelectronics, biomedicine and pharmacy concentrate on the control of the physicochemical properties of nanostructures (NSs) [68,69], including of the nano zinc oxide (ZnO) [70]. Numerous tests related to the use of nano ZnO for application purposes are in progress and attempts are made to produce them on an industrial scale. The dynamic development of methods of repeatable production of nano ZnO, characterised by high purity, controlled properties, high efficiency, and shortest synthesis duration possible, is desirable [71].

1.2. Bulk ZnO: Properties and Application

Owing to its exceptional physical (Table 1) and chemical properties, ZnO counts as a multifunctional material [72,73,74,75]. Its properties include high electrochemical coupling coefficient, broad range of radiation absorption, high photostability, low toxicity, biocompatibility, and biodegradability [76]. It occurs naturally in Earth’s crust in the form of a mineral called zincite (Zn_1−xM_xO). Zincite rarely occurs in a pure form; most often it includes dopants of other bivalent metals (among others: Mn(II) and Fe(II)), which is manifested in various colours of the mineral, among others, red, red and yellow, orange, and brown [77]. ZnO is insoluble in water and its powdered form is white. Due to its amphoteric properties it reacts both with acids and bases. ZnO can crystallise in three primary crystal systems: hexagonal wurtzite, cubic zinc blende, and cubic rocksalt (Figure 1). In normal conditions, the thermodynamically stable crystal structure of ZnO is wurtzite (Figure 2) [78]. Hexagonal ZnO is an II–VI semiconductor characterised by a wide band gap. A wide band gap (3.37 eV) and a high bond energy (60 meV) make ZnO a subject of enormous interest as regards applications in optoelectronics and electronics (among others, voltage dependent resistors) [79,80]. Owing to its antibacterial (disinfecting) properties, ZnO is widely applied as an ingredient of medications for various purposes [81,82]. The earliest information about the use of ZnO as an ingredient of therapeutic ointments for treating skin boils and ulcers comes already from 2000 BC [83,84,85,86]. It is common knowledge that ZnO was widely produced and used in the 13th century in Persia, where it was applied in treatment of eye inflammation and production of brass [86]. ZnO accelerates wound healing, which is why it is used as an ingredient of pastes (e.g., zinc paste) for treating skin inflammations (among others, acme, skin eruptions) and against itching. It is also commonly used in dentistry, e.g., as temporary tooth dressing [87] and root canal filling materials for deciduous teeth [88]. Moreover, ZnO is used as an additive in feed for pigs, where it helps protect piglets from diarrhoea by inhibiting the growth of pathogenic flora [89,90]. In the past, ZnO was also an ingredient of a medication for treating diarrhoea in humans, but at present, it is used in the oral form, most often as an ingredient of food products and dietary supplements, where it acts as a source of zinc (Zn²⁺). Zinc is the second most important trace element in the human body after iron, as it fulfils the catalytic, structural and regulatory function. The average zinc (Zn²⁺) content in the organism of an adult is 2–3 g [91], while the recommended daily average dose for a healthy adult is 8–11 mg [92]. ZnO is used as an ultraviolet (UV) radiation filter in sunscreen cosmetics (creams, balms, lotions). Nowadays ZnO is commonly used as an additive in such products as pigments (zinc white, zinc green), flame retardants, cements, ceramics, porcelain, glass, plastics, sealants, adhesives, lubricating oils and paints [70,76,89,90,93,94]. The presence of ZnO in paint, apart from changing the colour, contributes also to providing the paint coat with anti-corrosion and antifouling properties. The most important area of ZnO application is the rubber industry, where it is widely used as an activator of the conventional process of sulphur vulcanisation of natural rubber [95]. At present, as much as ca. 50% of the produced ZnO is used for the vulcanisation of rubber, which is a material for manufacturing tyres, sports equipment, garden and industrial hoses, shoe soles, belts, and other rubber products [96]. The presence of ZnO in rubber enhances its strength, hardness, dynamic stability, capacity to absorb heat, and provides important pigmenting properties.

1.3. Nano ZnO: Properties and Application

Due to its unique properties ZnO enjoys unfading popularity among research units worldwide (Figure 3). Increasingly often, researchers focus on ZnO in the nano-form, which is proven by the search results in the ScienceDirect scientific paper search engine (Figure 3). The number of hits was as follows: “zinc oxide”—264,763 hits, and “nano zinc oxide”—38,999 hits. The overall share of publications concerning “nano zinc oxide” among all the hitherto published papers about ZnO was ≈14.7%. The number of publications concerning nano ZnO have been growing dynamically year by year (Figure 4), while the annual share of publications concerning nano ZnO in 2020 was as high as 32.6% (Figure 4). Over the past 10 years, the number of scientific publications related to nano ZnO increased by ca. 400% (Figure 3). The growing popularity of nano ZnO results among others from the technological development of production of ZnO nanostructures, which are characterised by novel physical properties allowing hitherto unknown applications and possibilities [71,108,109,110].

The main limitation related to the synthesis and application of various types of nanopowders, obviously including nano ZnO, is the problem of repeatability of parameters of nanopowders. Most “wet methods” [73,76,86,111] used for producing metal oxide nanoparticles lead to obtaining powders with a broad particle size distribution, insufficient crystallinity degree, variable morphology and insufficient purity. This causes a range of problems with respect to their application in the broadly understood industry, where properties of raw materials are the crucial and decisive parameter for their choice and guarantee the performance characteristics of the product being manufactured. Research on ZnO nanomaterials (ZnO NMs) can be intensified in line with the development of new methods of production and control. The evolution of nanoparticle characterisation techniques [112] and the standardisation of nanotechnology [113] currently permit determining whether changes to the ZnO NMs properties are, e.g., caused by unrepeatability of methods of obtaining, actual changes to NMs features (among others, material ageing, etc.) or unrepeatability of research procedures. An important role in research on ZnO NMs is played by a network of Accredited Research Laboratories, where increasingly more accurate and appropriately validated measurement techniques are introduced. The large number of published scientific papers devoted to the issues of nano ZnO prove that the topic of synthesis, properties, and application of such powders is researched intensively. This indicates an enormous development potential of nano ZnO, which was noticed and appreciated by numerous research groups worldwide.

Nano ZnO is regarded as a safe material [114], which excellently fulfils the function of an ultraviolet (UV) radiation filter, because it permits creation of protective layers which are invisible to the human eye. Other advantages of the use of ZnO nanoparticles (NPs) as a UV filter in the area of personal hygiene and sun protection are, among others, long-term protection and broadband protection (UV-A (315–380 nm) and UV-B (280–315 nm)) [115]. The impact of contact between ZnO NPs and human skin is still being monitored and tested [116,117,118,119,120,121]. ZnO NPs are also applied in deodorants, medical and sanitary materials, glass, ceramic, as well as self-cleaning materials [115,122]. In optoelectronics, various ZnO nanostructures are used for producing e.g., refractive index sensors [123], surface-enhanced Raman scattering (SERS) sensors [124], lasers, UV detectors, and UV diodes [74,125,126,127,128]. ZnO nanostructures may serve for producing sensors of gases [129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152] such as steam (humidity, H₂O) [133], ammonia (NH₃) [129,130,131,143], nitrogen (N₂) [131], nitrogen monoxide (NO) [129,130,139], nitrogen dioxide (NO₂) [129,131,134,135,136,151,152]_, hydrogen (H₂) [129,130,131,132,149]_, ozone (O₃) [131,137]_, hydrogen sulfide (H₂S) [129,130,131,138,139], carbon monoxide (CO) [129,130,131,135,152], carbon dioxide (CO₂) [140], methane (CH₄) [141], acetylene (C₂H₂) [131], ethylene (C₂H₄) [147], ethane (C₂H₆) [129], 1,2-dichloroethane (C₂H₄Cl₂) [129], p-xylene (C₈H₁₀) [129], phenol (C₆H₅OH) [130], chlorobenzene (C₆H₅Cl) [130], methanol (CH₃OH) [131], ethanol (C₂H₅OH) [129,130,131,148,150], formaldehyde (HCHO) [129,130,131,151], acetaldehyde (CH₃CHO) [131], acetone ((CH₃)₂CO) [129,130,131,144,145,146], mixture of propane (C₃H₈) and butane (C₄H₁₀) [130,142], and triethylamine (C₆H₁₅N) [129]. The application of ZnO nanostructures in the commercial development of new gas sensors is quite strongly limited for the time being, which is a consequence above all of the ageing effect [153]. Thin films of nano ZnO are most often used as inorganic conductors in flexible and transparent devices [154,155], e.g., transparent electrodes, transparent windows, flat panel displays, and components of devices with the surface acoustic wave [156,157,158,159,160,161].

The antibacterial and antifungal properties of ZnO NPs have become the focus of interest of pharmacy, biomedicine and dentistry due to their low toxicity to the human organism [162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181]. It has been proved that ZnO NPs applied in textiles effectively fight bacteria, e.g., Staphylococcus aureus or Klebsiella pneumoniae [182]. In medicine, research is in progress on the application of ZnO NMs as a potential contrast agent (imaging), drug carrier, iron delivery, gene carrier, biosensor, a potential anti-cancer agent, in a photodynamic therapy, for prophylactic and therapeutic vaccines, support for antifungal treatments, in photocatalytic antibiotics, inhibition of influenza virus infection, diagnostic-therapeutic functions, wound dressing and tissue engineering [70,166,168,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213]. However, before implementing ZnO NMs in biomedical applications on a commercial scale, the toxicity of ZnO NMs must be carefully learnt and their toxicity mechanisms must be explained [121,162,171,174,178,179,214,215,216,217,218,219]. Once doped, they display new properties, e.g., electrical conductivity, magnetic, magneto-optical, photocatalytic, antibacterial and optical [74,161,220,221,222,223,224,225,226,227,228,229,230,231,232]. At present, various ZnO nanostructures are being used in attempts to produce a new generation of light-emitting diodes [233,234], lasers [235], field emission devices [236,237], memory carriers [238,239], solar cells [240,241,242,243,244,245,246], liquid crystals [247], polymer nanocomposites [248,249,250,251], food packaging materials [252,253,254,255,256,257,258], transparent ultraviolet light absorbers in unplasticised polymers [259], catalysts [260], photoluminescent NPs [261], photocatalysts [262,263,264,265,266,267,268,269,270,271,272,273], hybrid materials [272,273,274], chemical sensors [275,276], optical biosensors [277], nanofibrous materials [278,279], piezoelectric nanogenerators [280,281], nano-piezotronics [282], supercapacitors [155], flexible and transparent thin-film transistors [154,155], batteries [283,284], photoelectrochemical applications [285], water treatment [286,287,288,289,290,291], water filters [292], solar water splitting [266,293,294,295], functional coatings [296,297,298,299,300,301,302,303,304,305], and in crop cultivation [306,307]. ZnO nanostructures are investigated intensively for the purpose of their application production of chemical and biological sensors [190,308,309].

In agriculture, research is in progress on the application of the antifungal properties of ZnO NPs to protect crops [307,310]. ZnO NPs are used in a broad range of commercial applications, thus it is important to understand their fate and behaviour in soil, water, their manner of absorption and spreading/accumulation in plants, animals and microorganisms, but also interactions with impurities [311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334]. It must be borne in mind that the dynamically increasing the commercial application of ZnO NPs without appropriate supervision and tests may contribute to their adverse impact on land and water plants and animals [335,336].

The properties of nanostructures depend strongly on their size and shape [40,41,42,43,44,45,46,47,48,49,50]. The size-dependent effect of ZnO NPs has been observed on the: photocatalytic activity [337,338,339,340,341,342], catalyst activity [343], dielectric properties [344], piezoelectric property [345,346], breakdown voltage varistors [347], visible emission property of quantum dots displays [348], equilibrium constant of chemical reactions [349], gas sensing properties [350,351,352], thermal diffusivity of water nanofluid [353], photoluminescence [354], UV absorption [355,356], biomedical potential [357], toxicity [312,328,357,358,359,360,361,362], bioavailability [363], and interactions with biomatrices [364]. A nano ZnO material may be characterised by piezoelectric parameters of even a few times higher values than a bulk ZnO material [365]. The photoluminescence, band gap width, conductivity, or magnetic properties, in turn, may be controlled by doping ZnO NPs with transition metal ions (dopant quantity and type): e.g., Co, Mn, Cr, Ni, Fe, V) [74,223,224,225,226,227,228,229,230,231,232]. It was found that the UV sensing performance of doped ZnO improved in term of sensitivity and photoresponse properties compared to non-doped ZnO [366]. One of the goals of research concerning doped ZnO (diluted magnetic semiconductor—DMS) is to enable a combination of communication, memory, and data processing in a single device. They will serve for creating a new generation of smaller, faster and less energy-consuming devices than the present ones, acting based on a combination of conventional micro-electronics and spin-dependent effects [367,368].

In order that ZnO could be more willingly used in products on the industrial scale, the development of ZnO NPs synthesis methods is necessary to enable simultaneous precise control of the average particle size, obtaining narrow particle size distributions and their doping with transition ions. For the purpose of achieving the required performance characteristics, e.g., in pharmaceutical applications, it is also necessary that the material is homogeneous, fully crystalline, and characterised by high purity.

1.4. ZnO Market

It is estimated that the production of NPs of metal oxides in 2020 will be ca. 660 thousand tons, with the estimated production of ZnO NPs alone being between 45 and 56 thousand tons, i.e., ca. 7–8% of the market [115,122]. At present, ZnO NPs are used mainly for producing the following (Figure 5):

-

pharmaceuticals,

-

cosmetics,

-

paints,

-

various coatings,

-

antibacterial products,

-

electronics,

-

and in scientific research.

1.5. Obtaining ZnO Nanomaterials

The synthesis of ZnO NMs with repeatable properties is a quite difficult topic because some of the ZnO nanopowders available on the market are characterised by an insufficient degree of crystallinity, variable morphology, wide particle size distribution, presence of impurities and unrepeatability of properties. In addition, available ZnO nanopowders are mostly composed of a mixture of NPs agglomerates and aggregates, which can be confirmed by a test of average particle size for a dry powder using the laser diffraction method (ISO 13320:2020), while for suspension samples - using the dynamic light scattering method (ISO 22412:2017) [369].

The literature provides numerous chemical, physical and biological methods of producing ZnO nanostructures [73,76,86,154,169,170,171,172,173,174,270,307,370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390]. The following are most frequently enumerated methods of obtaining ZnO NMs [76]: co-precipitation with subsequent calcination, sol-gel method with subsequent calcination, mechanical synthesis combined with high-energy milling, hydrothermal synthesis and solvothermal synthesis. The unrepeatability of ZnO NMs properties arises above all from the limitations of the synthesis methods, unrepeatability of the reactants used and complexity of chemical reactions. The main obstacle in obtaining NMs with intended physicochemical properties is the lack of technologies that ensure the simultaneous control of, among others, average particle size, particle size distribution, shape, phase purity, dopant content, and particle agglomeration and aggregation. The synthesis of nanomaterials is currently at a new stage of development [391,392], at which the main criterion of the approach to obtaining ZnO NMs is understanding the synthesis mechanisms [393,394,395,396,397,398,399,400,401,402]. The familiarity with mechanisms of ZnO NMs synthesis allows obtaining repeatable and reproducible ZnO nanostructures.

Nowadays, when the humans are fully aware of their harmful environmental impact [403,404] and of the climate change on the planet [405], particular attention is drawn to technologies of obtaining such nanomaterials that are environment friendly. One of the most interesting dynamically developing “green technologies” of ZnO nanostructure syntheses is the “microwave assisted synthesis”, which is proved, e.g., by the constant trend of the growing number of publications concerning microwave synthesis of ZnO (Figure 6). At present, the number of scientific papers describing the microwave synthesis of ZnO accounts for ca. 10.9% of the number of all hitherto published papers describing the synthesis of ZnO, while the annual percentage share in 2020 was as high as ca. 16.0%. It must be noted that microwave synthesis is also commonly and willingly employed for obtaining different nanomaterials, e.g., gold (Au) [406,407], silver (Ag) [406,407,408], rhodium (Rh) [406,407], copper (Cu) [406,407], hydroxyapatite (HAp) [409], zirconium dioxide (ZrO₂) [410,411,412], titanium dioxide (TiO₂) [413], silicon oxide (SiO₂) [414], cerium dioxide (CeO₂) [415], tin oxides (SnO and SnO₂) [416], and nanocomposites (i.a. ZrO₂-AlO(OH) [417,418], MoS₂- polydopamine-Ag [419]).

2. Microwave Heating

Microwaves constitute the part of the electromagnetic spectrum with the wavelength (λ) between 1 mm and 1 m (Figure 7), which corresponds to the frequency range between 300 MHz (λ = 1 m) and 300 GHz (λ = 1 mm) [420,421]. Generally, microwaves are commonly used for commercial and military purposes, namely in cellular, Internet, and satellite communications, in radar technologies (radiolocation and radio navigation) and in the food industry (heating and drying). The International Telecommunication Union (ITU) [422] introduced the official division of the microwave range by frequency for the first time in 1953 [423,424]. ITU periodically organises World Radiocommunication Conferences (WRC) [425], during which legal radio regulations, i.e., international agreements controlling the use of the radio frequency spectra, are reviewed and corrected as necessary. In accordance with international agreements, some frequencies were assigned for industrial, scientific and medical purposes to avoid interferences in telecommunication. In each country, a dedicated governmental department deals with the assignment of frequencies and the frequency range as well as the terms and conditions of their use; in Poland, this role is fulfilled by the Ministry of Infrastructure [426]. The division of frequencies for industrial and scientific purposes is presented, e.g., by Torgovnikov [427]. Given the fact of the frequent use of microwave radiation in telecommunication, its individual frequencies were assigned to specific fields, e.g., microwave ovens and equipment employed at a laboratory were assigned the frequency of 2.45 GHz, which corresponds to the wavelength of ca. 12.25 cm.

The interaction of the microwave radiation with a substance may be divided into three types (Figure 8): absorption, transmission and reflection. Polar substances (e.g., solvents: H₂O, ethylene glycol) absorb microwave radiation, as a result of which they are heated. Non-polar substances may display low interactions with microwave radiation, for which they are a microwave transparent medium. The third type includes substances (electrical conductors, e.g., metals) which reflect the microwave radiation from their surface.

The heating of substances by microwave radiation has already been thoroughly described in the literature by numerous scholars [428,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457]. Microwave heating (MH) is based on the transformation of electromagnetic field energy stemming from microwave radiation into kinetic energy (heat) by interacting with the polar particles of the material. MH arising from the electric component of electromagnetic radiation may occur in four ways (Figure 9 and Figure 10): by rotation of dipoles (dipolar polarisation), ionic conduction (ionic polarisation), electronic polarisation (atomic polarisation), and interfacial polarisation (surface polarisation).

The first mechanism of MH consists of the rotation of dipoles having a dipole moment, which try to position themselves in line with the direction and sense of the variable electromagnetic field, which imposes their motion. During the rotation, the microwave radiation energy is converted into kinetic energy, which is transferred between the particles that collide and rub against each other. The result is a uniform spreading of heat in the heated material and a rapid increase in temperature. The second mechanism of MH based on ionic conduction concerns systems (solutions, suspensions) containing ions. Ions present in the microwave field move in line with the direction of the variable electric field. The collision of migrating ions with those moving in the opposite direction causes a heating effect, which is the stronger the higher the concentration and mobility of ions are. The third mechanism of MH constitutes the induction of a dipole moment by shifting the centre of the electron charge in relation to the nucleus. The fourth mechanism of MH is based on the polarisation of a material in the microwave field by accumulation of charges on the interface surface (particle boundaries, phase boundaries), i.e., partial charging of the surface (induced surface charges) through the action of the microwave radiation.

The capability of a substance (solvent) in the conditions of being radiated with microwaves at a given frequency and temperature of converting the microwave radiation energy is determined by the so-called “loss factor” or “loss angle” or “loss tangent”, tanδ. Loss factor is expressed as $\tan \mathsf{\delta }=\frac{{\epsilon }^{″}}{{\epsilon }^{\prime }}$, where ${\epsilon }^{″}$ is the dielectric loss (F·m⁻¹), which is indicative of the efficiency with which electromagnetic radiation is converted into heat and $\epsilon \prime$ is the dielectric constant (F·m⁻¹), which is indicative of the polarisability of molecules in the electric field [428]. The higher the tanδ value, the more efficient the heating of a given substance by microwave radiation will be. The tanδ and $\epsilon$ values depend on the electromagnetic radiation frequency and temperature. The tanδ values of popular pure solvents are presented in Table 2. The heating capability (tanδ) of solvents also depends on the substances it contains. The following classification of solvents was adopted depending on tanδ value:

-

low microwave absorbing, where tanδ value <0.1

-

medium microwave absorbing, where tanδ value ranges from 0.1 to 0.5

-

high microwave absorbing, where tanδ value is higher than 0.5.

Generally, microwaves are capable of heating only such solvents with particles being dipoles, e.g., water (H₂O), methanol (CH₃OH), ethanol (C₂H₅OH), ethylene glycol (C₂H₄(OH)₂), diethylene glycol ((C₂H₄OH)₂O), dimethylformamide ((CH₃)₂NCHO), dimethylsulfoxide ((CH₃)₂SO), ethyl acetate (CH₃COOC₂H₅), chloroform (CHCl₃), methyl chloride (CH₂Cl₂), acetic acid (CH₃COOH), acetonitrile (CH₃CN), and acetone ((CH₃)₂CO). Liquid (and solid) substances which do not have particles with the dipole moment and electric charge carriers do not absorb microwave radiation and do not heat. Therefore, such solvents as hexane (C₆H₁₄), benzene (C₆H₆), toluene (C₆H₅CH₃), diethyl ether (C₂H₅)₂O, tetrachloromethane (CCl₄), tetrahydrofuran (C₄H₈O), or polytetrafluoroethylene (PTFE) oil virtually do not heat under the influence of microwave radiation or heat insufficiently. Our experience shows that even a small quantity of an impurity that has a dipole moment and is present in a non-polar solvent may contribute to a considerable improvement of the solvent heating under the influence of microwave radiation.

It must be remembered that during the interaction of microwave radiation with unearthed metals, electric charge carriers (electrons) move under the influence of the electric component of radiation, which leads to metal polarisation. Therefore, unearthed metal subject to microwave radiation will heat intensively and the charges generated on its surface will cause electric discharges and sparking.

2.1. Comparison of Conventional Heating with Microwave Heating

Conventional methods of obtaining nanomaterials employ direct resistance heating and indirect resistance heating. In direct resistance heating, current flows directly through the feedstock itself, causing its heating. This type of heating is not popular because it mostly requires a high current to flow through the feedstock, which flows through the contacts connecting the power source with the feedstock but this causes a range of complications. Indirect resistance heating uses a heat source, i.e., a heating element (e.g., an electric heater). There are two possibilities of applying the heating element: at the direct contact with the feedstock (e.g., electric kettle) or at the indirect contact, where the heating element is in contact with the vessel walls and with the feedstock (e.g., heating mantle or heating jacket). The application of a heating element directly in the feedstock is not popular, which results mainly from heavy contamination caused by corrosion of heater walls and from accumulation of solid synthesis products on the heater surface. Heating jackets are used in the majority of conventional chemical reactors. It must be borne in mind that during conventional heating the heating jacket first heats the walls of a reaction chamber/vessel and subsequently the reaction feedstock, which is shown in Figure 11.

Conventional heating of the reactor feedstock involves the following disadvantages:

(a)

long heating time, which depends on the thermal conduction of the material of which the reaction chamber walls are made;

(b)

temperature maximums occur on the reaction vessel/chamber wall surface, which is one of the direct causes of the heterogeneity of the obtained products (so-called wall effect);

(c)

limited reaction control caused by a high thermal inertia of the system, which results from the heating of the heating jacket and the reaction chamber walls;

(d)

difficulties involved in the speed of the feedstock cooling process;

(e)

high heat losses.

MH is characterised by shorter heating times in relation to the conventional method. With optimum parameters (MH power, resonance structure), feedstock may be heated rapidly and uniformly due to the direct molecular heating by the energy of microwaves. MH is defined as endogenous or volumetric heating [461], which means that heat may be generated within the whole precursor mass (sample inside) rather than transferred from an external heat source (Figure 11). However, it must be borne in mind that microwave radiation does not always have to heat the whole sample volume. When electromagnetic radiation is cast on the surface of a given substance, a part of the radiation may reflect from the surface while the remaining part may penetrate inside the (volume of the) substance. Electromagnetic radiation which penetrated inside interacts with substance molecules and ions. Depending on the properties of a given substance, electromagnetic radiation may penetrate a given substance at various depths, which is illustrated in Figure 12. Penetration depth is the key parameter of each substance in the process of microwave radiation heating. The penetration depth of a field is defined as the distance from the surface to a certain internal point where the magnitude of field strength decreases to 1/e (=36.8%) of the original magnitude at the surface [462]. This parameter is described by the following formula [437]:

$$d_p=\frac{{\lambda }_0}{2\pi }{\left(\frac{1}{2\mu {\epsilon }_0{\epsilon }^{\prime }}\right)}^{1/2}[{(1+{(\tan \mathsf{\delta })}^2{)}^{\frac{1}{2}}-1]}^{-1/2}$$

where $d_p$—penetration depth (cm), ${\lambda }_0$—wavelength at vacuum conditions (cm), ${\mu }_0$—magnetic permeability of free space (H·m⁻¹), ${\mu }^{\prime }$—magnetic relative permeability (H·m⁻¹), ${\epsilon }_0$—permittivity in free space, (F·m⁻¹), ${\epsilon }^{\prime }$—relative permeability (F·m⁻¹), and $\tan \mathsf{\delta }$—loss angle. If considering, e.g., water, the penetration depth of the electromagnetic radiation with the frequency of 2.45 GHz at the temperature of 25 °C for deionised water is 2.88 cm, while for water with an addition of NaCl (0.5 M)—merely 0.45 cm [456] (Table 3). When the size of the heated substance is greater than the microwave penetration depth, only a part of the volume of this substance is heated, which is illustrated in Figure 12.

Reaction vessels of microwave reactors are mostly made of polytetrafluoroethylene (PTFE, one of the trade names is Teflon®), which is characterised by a low thermal conductivity (0.25 W/(m·K)) and acts as a thermal insulator, thanks to which a low temperature gradient is maintained in the reaction chamber [461,466,467]. Compared with conventional heating, MH has the following advantages:

(a)

No direct contact of heat source with heated material (contactless method).

(b)

Minimisation of the “wall effect” because the wall of the vessel (reaction chamber) is not heated directly.

(c)

Volumetric heating of the feedstock.

(d)

Instantaneous and precise electronic control. Quick heating switching on and off, e.g., heating process can be controlled with the accuracy of 1 s, namely after switching off the magnetron power unit, the heat source supply is interrupted immediately.

(e)

Rapid heating with preservation of low thermal gradients (rapid energy transfer) [468].

(f)

Heating uniformity (which is translated into the quality of obtained products, e.g., homogeneity and narrow size distribution of obtained nanomaterials [437,438,439,440,441,469,470]).

(g)

Shorter duration of syntheses/processes. Microwave synthesis, depending on type of organic compound obtaining, may be 5–2500 times quicker than in the case of the same syntheses carried out by conventional thermal methods [429,433,471].

(h)

Fewer side reactions [472,473].

(i)

Selectivity and purity of generated products [432,474].

(j)

Easy to conduct under solvent-free conditions [451].

(k)

Very high power densities developed in the processing zone [452].

(l)

Superior moisture levelling [452].

(m)

Energy saving [467].

(n)

Higher production efficiency (faster throughputs) [452].

(o)

Lower apparatus size (compact equipment) [452].

(p)

Shorter time of apparatus start-up.

(q)

A green chemistry approach [475,476,477,478,479,480].

2.2. Application of Microwave Heating, Chemical Microwave Apparatus

Microwave heating is used for various applications, among others:

(a)

food processing (np. home cooking, pasteurisation, drying) [445,446,447];

(b)

industrial application (np. drying, wood curing, rubber curing and vulcanisation, disinfection, coal pre-treatment and processing, ceramic processing, polymer processing, polymeric composites, ceramic composites, melting of glasses, melting of metallic materials, roasting of tea/coffee beans, plant extraction processes) [442,447,448,449,450,451,481,482,483],

(c)

waste treatment (np. medical waste, garbage, sludge) [447];

(d)

medical applications (np. sterilisation, drying, diagnosis, novel methods for treating inoperable tumours) [447,484];

(e)

analytical chemistry (laboratory sample processing, ashing, digestion, extraction, moisture analysis) [432,485];

(f)

chemical reactions: organic and inorganic synthesis (microwave assisted synthesis of e.g., medications, polymers and nanomaterials) [406,407,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,447,455,475,476,477,478,479,480,486,487,488,489,490,491,492,493,494,495].

Recent years have seen a noticeable constant trend of growing popularity of microwave technologies, which is proved e.g., by the increase in the number of publications that were published in the period 2011–2020 (Figure 13). According to the scientific search engine ScienceDirect, 304,138 scientific publications have been released so far which contain a reference to the keyword “microwave” (Figure 13).

The main reason for the increased interest in microwave technologies, which is visible particularly in chemical laboratories, is the dynamic development of microwave apparatus, in particular chemical reactors [428,461,496,497,498,499,500,501], and the decrease in apparatus prices. The use of microwave reactors involves a range of advantages, which are already discussed in the previous Section 2.1 describing the advantages of microwave heating. Control and measurement technologies have reached a very high development level over the past 25 years. Very difficult to control at the beginning of the 21st century, microwave devices are currently equipped with advanced control systems and their operation parameter repeatability matches older and well developed technologies. Microwave chemical reactors of a new generation are characterised by a high repeatability of processes thanks to, among others, the optimisation of reactor structure (batch or flow type) in terms of applied materials, shape and size of the reaction chamber/vessel, recess structure (single synthesis or parallel synthesis), classes of microwave applicators (monomode or multimode) and automation. The capability of monitoring of the real course of reaction (temperature (T), pressure (P)) in microwave chemical reactors is crucial for meeting the requirements of chemical product generation, e.g., in the pharmaceutical industry. New reactor designs enable the rapid cooling of products, e.g., through adiabatic opening of the reactor chamber, which enables freezing the course of reaction over just a few seconds after the finish of the heating process [461,502]. Microwave reactors can always experience a failure, just like other ones [503]. It must be emphasised that microwave ovens, which are often used for conducting chemical reactions, are not professional equipment. Processes performed in microwave ovens are characterised by unrepeatability, which is often related to:

-

random setting of the reaction vessel,

-

random geometry of the reaction vessel (shape and size),

-

impossibility to monitor the course of the process (temperature (T), pressure (P)).

One of the main reasons why microwave chemical reactors are popular is their heating speed. Figure 14 shows the kinetics of distilled water heating in an experimental reactor, MSS3, which was designed and constructed at the Laboratory of Nanostructures, Institute of High Pressure Physics, Polish Academy of Sciences (IHPP PAS). The MSS3 is a reactor intended exclusively for research applications with a reaction vessel made of quartz, which is cooled continuously. The selection of the appropriate microwave power (Figure 14) enables the precise control of kinetics of feedstock heating, which, as was proven later, permits the application of unique features to the synthesised nanomaterials (ZnO NPs) [502]. Differences between the dynamics of heating and cooling are perfectly visible in the chart (Figure 14). It is easier to heat a material because energy is supplied by the power of the microwave field, while cooling takes place through thermal conduction and this process is limited by the thermal conduction of the applied materials. Table 4 presents times after which the water sample reached the temperature of 100 °C and 140 °C depending on the applied microwave power.

3. Microwave Hydrothermal Synthesis of ZnO

The hydrothermal synthesis was defined in the literature [504] as a process in a closed system, in which chemical reactions take place exclusively in a water solvent at increased temperatures, at a pressure that is higher than atmospheric pressure (P > 101,325 Pa). This definition is often modified by scholars, e.g., Byrappa and Yoshimura [505] proposed a definition of the “hydrothermal reaction” as any heterogeneous chemical reaction in the presence of a solvent (whether aqueous or non-aqueous) above the room temperature and at a pressure greater than 1 atm in a closed system. For the purposes of this review, we assume that the “hydrothermal synthesis” is a process occurring in an aqueous environment (where mH₂O > 50%), with the pressure equal to or higher than atmospheric pressure. In hydrothermal processes, reaction products are mainly oxides and salts due to the properties of water as a solvent. The results of the literature review concerning microwave hydrothermal synthesis of ZnO are divided into the following subgroups:

(1)

Microwave hydrothermal synthesis of ZnO nanostructures without any additional heat treatment, where the literature review results [506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639] are summarised in Table 5.

(2)

Microwave hydrothermal synthesis of ZnO nanostructures with additional heat treatment, where the literature review results [585,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673] are summarised in Table 6.

(3)

Microwave hydrothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures without any additional heat treatment, where the literature review results [674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723] are summarised in Table 7.

(4)

Microwave hydrothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures with additional heat treatment, where the literature review results [724,725,726,727,728,729,730,731,732,733,734,735,736,737,738] are summarised in Table 8.

3.1. Reactants

The most popular reactants of the “Zn²⁺” zinc cation precursor used for ZnO synthesis according to the data derived from the literature review [506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738] (Table 5, Table 6, Table 7 and Table 8) are Zn(NO₃)₂·6H₂O, Zn(CH₃COO)₂·2H₂O, ZnCl₂, ZnSO₄·7H₂O and ZnO (Figure 15), respectively. The popularity of these reactants results from their low price and easy availability. Reactants being a source of “Zn²⁺” ions are commonly produced at a large scale mostly by several producers in each developed country. It should be emphasised that ZnCl₂ is one of the problematic reactants, which arises from the possibility that during the ZnO synthesis a stable by-product (impurity), called simonkolleite (Zn₅(OH)₈Cl₂·H₂O), is formed. If Zn(NO₃)₂·6H₂O is used as the reactant and if the temperature of the ZnO synthesis is too low (below 100 °C), one of the synthesis by-products (impurity) may be the salt Zn₅(OH)₈(NO₃)₂·2H₂O [515]. It must be borne in mind that ZnO obtained by the hydrothermal method, depending on the parameters employed (T, P, pH), may contain impurities being hydroxides, oxide hydroxides or basic salts. Depending on the quantity and size of crystallites of “impurities”, they are often invisible in the results of the X-ray powder diffraction analysis (XRD) because they are below the method’s detection limit. When discussing the results, it must be remembered that the XRD analysis is capable of indicating only crystalline impurities of ZnO.

The most popular reactants being hydroxide anion precursor chemicals “OH⁻” used for the ZnO synthesis according to the data derived from the literature review [506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738] (Table 5, Table 6, Table 7 and Table 8) (Figure 16) are sodium hydroxide (NaOH), ammonia water (NH₃·H₂O, NH₄(OH)), hexamethylenetetramine (urotropin, HMTA, C₆H₁₂N₄), potassium hydroxide (KOH), urea (CH₄N₂O, CO(NH₂)₂), hydrazine (N₂H₄·H₂O) as well as other amides and amines, e.g., 1-n-butyl-3-methyl imidazolium tetrafluoroborate (C₈H₁₅BF₄N₂), N,N dimethylformamide (C₃H₇NO), pyridine (C₅H₅N), aniline (C₆H₅NH₂), arginine (C₆H14N₄O₂), tris (hydroxymethyl) aminomethane (C₄H₁₁NO₃), bis(triaminomethyl) carbonate (C₃H₁₂N₆O₃), butyl-3-ethyl imidazolium tetrafluoroborate (C₈H₁₅BF₄N₂), dodecylamine (C₁₂H₂₇N), and 1-hexyl-2-ethyl-3-methylimidazoliumtetrafluoroborate (C₆H₁₁BF₄N₂).

The chemical properties of amines are similar to those of ammonia. These are compounds with alkaline properties, which form salts with acids and produce an alkaline reaction in aqueous solutions. The alkalinity of amines depends on substituents by the nitrogen atom. Aliphatic amines are, as a rule, more alkaline than ammonia while the alkaline properties of aromatic amines are weaker than those of ammonia. Amides undergo a slow hydrolysis, which leads to the formation of OH⁻ and the formation of an intermediate, Zn(OH)₂.

3.2. Surfactants

Surfactants are commonly used in nanotechnology for passivation of surface, control of shape, and giving of photophysical properties [740]. Similar impact on properties of nanomaterials, apart from various types of surfactants (anionic, nonionic, cationic and amphoteric), is exerted also by surface active polymers, amines, amino acids, proteins, and carbohydrates. One of the primary purposes of the use of surfactants in obtaining nanoparticles is their strong contribution to obtaining dispersoids (emulsions, suspensions, colloids), in which they counteract the agglomeration of NPs [741,742,743]. Surfactants enable achieving a dispersion of NPs in non-polar solvents, e.g., in lubricating oils [744]. Surfactants are also used for controlling the shape of nanomaterials [740,745]. The shape control mechanism is related above all to the adsorption of the surfactant particles on various planes of crystalline nucleation centres of crystallites; these particles selectively promote the growth of these centres or to the contrary: inhibit their growth. The following surfactants were used in microwave hydrothermal syntheses of ZnO NMs for the purpose of controlling the morphology:

-

Ethylenediamine (EDA, C₂H₈N₂) for obtaining nanoneedles [525].

-

Hexamethylenetetramine (HMT, (C₆H₁₂N₄)) for obtaining nanorods [525].

-

Triethyl citrate (C₁₂H₂₀O₇) for obtaining hexagonal disks [525].

-

Triethanolamine (TEA, C₆H₁₅NO₃) for obtaining nanosheets [521], pompon-like spheres [554], peach nut-like spheres [554], misshapen spheres [554], rugby-like nanostructures [565], raspberry-like nanostructures [566], hollow nanospheres [566], dumbbell-like [626], football-like shape [626], and spherical nanoparticles [565,566,625,645].

-

1,2,4,5-benzenetetracarboxylic acid (BTCA, C₆H₂(CO₂H)₄) for obtaining nanosheet [554] and elongated triangles [554].

-

Cetyltrimethylammonium bromide (CTAB, (C₁₉H₄₂BrN)) for obtaining heterogeneous shapes [531], wires [547], nanorods [576], wire-like architecture [597], flower-like microstructures composed of nanorods [597], and spike-like nanostructures [638].

-

Sodium di-2-ethylhexyl-sulfosuccinate (C₂₀H₃₆Na₂O₇S) for obtaining hexagonal rods [581], hexagonal prismatic [581], bihexagonal rod-like structures [581], wire-like architecture [597], and flower-like microstructures composed of nanorods [597] and rods [597].

-

Pluronic F127 (polyoxypropylene polyoxyethylene block copolymer) for obtaining heterogeneous shapes [531].

-

Polyethylene glycol 400 (PEG400, C_2nH_4n+2O_n+1) for obtaining nanorods [533], flowers [533], rod-like nanostructures [574], star-like nanostructures [574], particles with an irregular shape (plate and rod-like particles) [596], quasi-spherical shapes [620], flower-like structures [620], flower-like hierarchical structures [655], rod-like structures [673], and needle-like structures [673].

-

Acetyl acetate (ACAC, (CH₃CO)₂O) for obtaining rod-like structures [644].

-

Guanidinium carbonate ((CH₅N₃)₂·H₂CO₃) for obtaining spherical nanoparticles [644] and flower-like ZnO [644].

-

Polyvinyl alcohol 2000 (PVA2000, (C₂H₄O)_n) for obtaining spherical nanoparticles [578].

-

Polyvinyl Pyrrolidine (PVP, (C₆H₉NO)_n) for obtaining star-like nanostructures [526,558].

-

Triethyl citrate (C₁₂H₂₀O₇) for obtaining disk- and nut-like structures [525].

-

Tripotassium citrate for obtaining UFOs and balls-like structures [525].

-

Arginine (C₆H₁₄N₄O₂) for obtaining rods and flowers [543].

-

Albumen for obtaining whisker-like and rod-like nanostructures [585].

-

Triton X-100 (C₁₄H₂₂O(C₂H₄O)_n (n = 9–10)) for obtaining rods (400–800 nm) and flower structures [594].

3.3. Morphology

The morphologies of ZnO NMs reported in the literature are presented in Table 5, Table 6, Table 7 and Table 8 [506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738] along with a description of their properties. ZnO NMs synthesised with the use of microwave hydrothermal synthesis were characterised by the following shapes: belts-like [671], bundle-like [655], brush-like [572], calthrop-like framework [561], candles [525], dandelion-like [518,612], disks [525], dumbbell-like [626,641], football-like structure [626], hexagonal column [521,629], hexagonal nanoring [521], hexagonal bi-pyramidal [626], hexagonal prisms [513,694] hexagonally shaped prismatic [528], hexagonal tubular [664], hollow structures [521], javelins [540], lamellar-like [692,699], misshapen spheres [554], peach nut-like spheres [554], petals [644], nuts-like [525], pompon-like spheres [554], raspberry-like nanostructures [566], rhombic [663], rugby-like nanostructures [565], spindles [518], sponge-like [580], spike-like [638], thruster vanes [518], tetrapod-like [631,670], tubes [514,515,571], whiskers [585], wires [518,523,534,539,547,646,711], flakes [520,586,587,592,622,648,663,664], plates [511,532,572,579,596,629,651,658,694,736], star-like [506,526,535,536,558,574,631,679,703], heterogeneous structures [507,524,528,531,630,637,665,666,668,672,720,730,731], flower-like [508,510,512,513,519,520,527,528,529,530,532,533,535,538,541,543,545,550,553,555,556,557,558,559,560,561,562,563,564,565,566,571,572,579,582,593,594,597,598,599,620,621,622,631,639,649,654,655,663,670,704,734,737,738], needle-like [510,521,525,551,556,588,588,629,655,663,673,716,729], sheets [521,544,551,552,575,577,583,585,588,627,650,686,707,714], spherical particles [511,535,536,551,560,565,566,578,585,599,614,615,623,625,628,634,635,636,638,639,645,648,649,674,680,681,682,683,684,685,702,706,712,718,721,727,735], and rods [473,509,511,517,518,519,525,527,529,533,538,541,542,543,548,549,553,556,557,559,562,563,567,568,570,573,574,576,581,583,584,585,589,590,591,592,593,594,595,596,597,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,616,617,618,619,624,629,631,632,633,640,642,643,644,649,653,655,658,659,661,663,664,670,673,678,693,696,697,700,701,705,709,710,723,725,726,732].

Examples of various morphologies of ZnO are presented in Figure 17, Figure 18, Figure 19 and Figure 20. The modification of morphology of ZnO NMs with a 3D architecture is extremely important e.g., for producing sensors with high sensing capabilities [739]. The 3D architecture of ZnO NMs, characterised by a small particle size, a high specific surface area and porosity, enables the improvement of the sensing response of gas sensors [152].

3.4. Microwave Hydrothermal Synthesis of ZnO without Any Additional Heat Treatment

The microwave hydrothermal synthesis of ZnO NMs owes its widespread use and popularity to water (H₂O). This solvent is cheap, non-toxic and easy to dispose of. The main advantage of this solvent is that it can be easily separated from the product by e.g., centrifuging, drying with the use of a filter, in a vacuum dryer, or freeze-drying. Another advantage of water is that the majority of Zn²⁺ salts are soluble in it (among others: Zn(NO₃)₂·6H₂O, Zn(CH₃COO)₂·xH₂O, ZnCl₂·xH₂O, ZnSO₄·7H₂O).

Considering dispersoids of the reaction mixture in the microwave hydrothermal synthesis of ZnO NMs, it is possible to perform the synthesis in a solution or in a suspension. It must be underlined that currently the synthesis of ZnO NMs has reached a new stage of development, where the common requirement is to control of properties of the obtained ZnO NMs, their repeatability and reproducibility. One of examples of papers which report, e.g., control of the ZnO shape is the publication by Wang et al. [510]. They demonstrate an example of obtaining ZnO with different morphologies by using an imidazolium salt 1-n-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM]BF₄) (an ionic liquid). The aqueous mixture of Zn(NO₃)₂·6H₂O, NaOH and [BMIM]BF₄ was used as the precursor solution. They obtained a strongly alkaline pH of the solution, where Zn²⁺ existed mainly as Zn(OH)₄²⁻. The mechanism of the ZnO synthesis reaction proposed by Wang et al. [510] is as follows in Equations (1)–(4):

$$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{NO}}_3{}^{-}$$

(1)

$$\mathrm{NaOH}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}$$

(2)

$${\mathrm{Zn}}^{2+}+4{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }\mathrm{Zn}{\left(\mathrm{OH}\right)}_4{}^{2-}$$

(3)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_4{}^{2-}\stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}+2{\mathrm{OH}}^{-}$$

(4)

Wang et al. [510] controlled the change in the morphology from flower-like, to flower-like + needle-like and to needle-like by changing such parameters as concentration of reactants, temperature and synthesis duration.

Pulit-Prociak et al. [507] report the effect of process parameters on the size and shape of ZnO in their paper. They used Zn(NO₃)₂·6H₂O and NaOH for the synthesis. The mechanism of the ZnO synthesis reaction proposed by Pulit-Prociak et al. [507] was as follows in Equations (5)–(8):

$$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{NO}}_3{}^{-}$$

(5)

$$\mathrm{NaOH}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}$$

(6)

$${\mathrm{Zn}}^{2+}+2{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\to }\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\downarrow$$

(7)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}$$

(8)

Pulit-Prociak et al. [507] contributed to a change in the shape and size of aggregates of ZnO particles by changing the microwave power, synthesis duration and pressure.

Hu et al. [509] report obtaining ZnO rods using a water suspension composed of Zn(NO₃)₂·6H₂O and (CH₂)₆N₄ (hexamethylenetetramine, HMT) in their paper. The mechanism of the ZnO synthesis reaction proposed by Hu et al. [509] was as follows in Equations (9)–(12):

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{N}}_4+6{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }4{\mathrm{NH}}_3+4\mathrm{HCHO}$$

(9)

$${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{NH}}_4{}^{+}+{\mathrm{OH}}^{-}$$

(10)

$${\mathrm{Zn}}^{2+}+2{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\to }\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\downarrow$$

(11)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\downarrow \stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}$$

(12)

Hu et al. [509] changed the morphology from flower-like, to flower-like + needle-like, to needle-like by changing such parameters as concentration of reactants, temperature and synthesis duration.

Phuruangrat et al. [513] report the control of morphologies and a growth mechanism of hexagonal prisms with planar and pyramid tips of ZnO microflowers. They used Zn(NO₃)₂·6H₂O, NaOH, and hexamethylenetetramine (HMT) for the synthesis. Figure 21 presents the as-synthesised products which were pure hexagonal wurtzite ZnO microstructured flowers of hexagonal prisms with planar tips at pH 9 and of hexagonal prisms with hexagonal pyramid tips at pH 13.

Phuruangrat et al. [513] explained the formation of ZnO microflowers composed of petals in the shapes of hexagonal prisms with planar and pyramid as follows: the size and morphology of particles is controlled by the chemical state of Zn²⁺ ions in the solution. Thermodynamic and kinetic factors affect the process of precipitation of compounds containing Zn²⁺ in various ways. The chemical state of Zn²⁺ ions (dissociated form, dissociated complex form, sediment) strongly depends on the pH of the medium and may be strongly controlled by the pH of the solution and anionic type. By adding the NaOH solution gradually to the Zn(II) salt solution with the pH of 13, the Zn(OH)₄²⁻ complex is formed, which can be denoted by in Equations (13)–(16):

$$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{NO}}_3{}^{-}$$

(13)

$$\mathrm{NaOH}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}$$

(14)

$${\mathrm{Zn}}^{2+}+4{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }\mathrm{Zn}{\left(\mathrm{OH}\right)}_4{}^{2-}$$

(15)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_4{}^{2-}\stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}+2{\mathrm{OH}}^{-}$$

(16)

Due to a large number of growth units, [Zn(OH)₄]²⁻ complexes are able to easily adsorb on different sites of ZnO nanorods. The positively charged Zn-(0001) surfaces are the most reactive. Thus OH⁻ ions may stabilise the positive charge of the Zn-(0001) surfaces to some extent, allowing rapid growth along the [1] direction zone axis of the hexagonal phase, leading to the formation of ZnO hexagonal prisms with rod-like crystal. ZnO microflowers of hexagonal pyramid tips were obtained at a higher concentration of NaOH, where the pH of the solution was 13. It must be emphasised that higher concentrations of OH⁻ result in an increase in the solubility of Zn(OH)₂ and ZnO at the room temperature, which is related to the formation of an ionic complex, above all Zn(OH)₄²⁻. The solubility of Zn²⁺ complexes decreases in line with the increase in temperature during the microwave synthesis, which results in the precipitation of solid ZnO. The growth mechanism of ZnO microflowers with the petals of hexagonal pyramids may be explained as follows: when the pH value of the solution is high, a strong electrostatic interaction emerges between ions and polar surfaces, which leads to an increase in surface energies of polar surfaces (0001) and (1011) in comparison with the energy of the other surfaces of the hexagonal crystal. The growth rate of these polar surfaces decelerates and the polar surfaces appear as external surfaces with sharp edges. When the pH value is high, a strong electrostatic interaction emerges between ions and polar surfaces, which leads to an increase in surface energies of polar surfaces (0001) and (1011) in comparison with the energy of the other crystalline surfaces. The growth rate of these polar surfaces decelerates and the polar surfaces appear as external surfaces with sharp tips of the hexagonal prisms with pyramid tips.

Klofac et al. [574] report obtaining ZnO rods using a water suspension composed of Zn(CH₃COO)₂·2H₂O, aqueous ammonia, polyethylene glycol (PEG; Mr = 400), and cetyltrimethylammonium bromide (CTAB, C₁₉H₄₂BrN) in their paper. The mechanism of the ZnO synthesis reaction proposed by Klofac et al. [574] was as follows in Equations (17)–(21):

$${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{NH}}_4{}^{+}+{\mathrm{OH}}^{-}$$

(17)

$${\mathrm{Zn}}^{2+}+4{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{[\mathrm{Zn}{\left(\mathrm{OH}\right)}_4]}^{2-}$$

(18)

$${\mathrm{Zn}}^{2+}+4{\mathrm{NH}}_3\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4]}^{2+}$$

(19)

$${[\mathrm{Zn}{\left(\mathrm{OH}\right)}_4]}^{2-} \stackrel{{\mathrm{H}}_2\mathrm{O} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}+2{\mathrm{OH}}^{-}$$

(20)

$${[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4]}^{2+}+2{\mathrm{OH}}^{-}\stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +4{\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}$$

(21)

Klofac et al. [574] obtained star- or flower-like ZnO particles aggregated from rod-like components united at a common centre depending on the surfactant used.

Based on these papers, a general conclusion may be drawn that depending on pH and NH₃ derivatives the key role in reactions is played by stages related to the formation of complex compounds of zinc ions (Zn²⁺).

Details of other research papers concerning the microwave hydrothermal synthesis of ZnO without any additional heat treatment are presented in Table 5.

3.5. Microwave Hydrothermal Synthesis of ZnO Nanostructures with Additional Heat Treatment

The additional heat treatment process at the temperature from 150 °C to 900 °C (Table 6) of the microwave hydrothermal synthesis products is applied in order to remove organic impurities and to cause thermal decomposition of unreacted substrates and intermediates, e.g., Zn(OH)₂, Zn₅(OH)₈(NO₃)₂·2H₂O, Zn₅(OH)₈Cl₂·H₂O. The additional heat treatment process is applied mainly when reaction products are obtained in an open vessel in a microwave oven, which results in reaching a low reaction temperature (max. ca. 100 °C) (Table 6). The main disadvantage of the additional heat treatment in terms of structure of the obtained ZnO NMs is the sintering of particles and the increase in their size (recrystallisation process), which results in formation of large agglomerates/aggregates/conglomerates. Thus obtained ZnO NMs, despite the use of efficient homogenisation methods (e.g., ultrasounds), are characterised by a large average particle/agglomerate/aggregate size and high polydispersity in water suspensions.

When discussing aspects of application of ZnO NMs in a dry form, the additional heat treatment of samples improves the stability of nanostructures and gives unique properties by soaking ZnO in various gaseous atmospheres, which may result in the formation of oxygen vacancies in their structure. The controlled introduction of oxygen vacancies into ZnO NMs can manipulate their optical, electronic and surface properties [746]. An example that confirms the advantages of heat treatment is the paper by Gu et al. [654], where the authors used the obtained ZnO NMs for detecting gases. They used the aqueous solution of Zn(NO₃)₂ with varied urea contents for the synthesis. At the first stage, precursors were heated by microwaves for 40 min at the temperature of 90 °C, while at the second stage, the samples, after rinsing and drying, were soaked in various gaseous atmospheres (O₂, N₂, H₂ and air) for 3 h at the temperature of 400, 500, and 600 °C. Gu et al. [654] obtained ZnO in the form of 3D nanostructures. Figure 22 shows the impact of the change in the urea content on the shape of the obtained 3D structures. With the lowest urea content (0.3 g) urchin-like structures were obtained (Figure 22a), while as a result of adding a higher quantity of urea (3.0 g and 6.0 g) flower-like structures were obtained (Figure 22b,c). The change in the soaking temperature from 400 °C to 500 °C caused a change in the shape of the flower structure and a change in the morphology of the nanoflakes themselves (Figure 23a,b). Structures soaked at 600 °C were characterised by a ragged and porous structure of the nanoflakes (Figure 23c).

The paper by Li et al. [652] is another example of application of 3D structures of ZnO obtained by the microwave hydrothermal synthesis with an additional soaking process in applications related to gas detection. For the synthesis of ZnO hollow microspheres, Li et al. [652] used a solution created by mixing zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), trisodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O) with aqueous ammonia (NH₃·H₂O). At the first stage, precursors were heated by microwaves for 40 min at the temperature of 90 °C, while at the second stage the samples, after rinsing and drying, were soaked in an air environment for 4 h at the temperature of 400 °C. The mechanism of the ZnO synthesis reaction proposed by Li et al. [652] was identical with the mechanism proposed by Klofac et al. [574] per Equations (12)–(16). Figure 24 reveals how the authors controlled the morphology of the change to the microsphere filling by changing the synthesis duration. Figure 25 presents changes to the morphology caused by the change in the amounts of trisodium citrate dihydrate in the precursor solution.

Salah et al. [660] report developing a method of controlling the ZnO nanostructure size. They used an aqueous mixture of zinc nitrate dehydrate (Zn(NO₃)₂·6H₂O) with hexamethylenetetramine (C₆H₁₂N₄) as the reaction precursor. The first stage of microwave heating at the temperature of 120 °C lasted 10 min, and subsequently after rinsing and drying the synthesis products were soaked at 400 °C for 1 h. By applying various proportions of zinc nitrate dehydrate and hexamethylenetetramine in the precursor mixture, the authors [660] controlled the size of the structures within a range from several dozen nm to several µm (Figure 26). The change in the size of the ZnO structures was accompanied by the change in their morphology from spherical nanoparticles (Figure 26a) in micro sized hexagonal nanorods.

Details of other research papers concerning the microwave hydrothermal synthesis of ZnO with additional heat treatment are presented in Table 6.

3.6. Types of ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Hydrothermal Synthesis

The microwave hydrothermal synthesis enables obtaining ZnO doped with the following ions: Cd²⁺ [698], Ce⁴⁺ [686,687,693], Co²⁺ [688,689,690,691,728,729], Cu²⁺ [695], Cr³⁺ [727,728], Eu³⁺ [694,731], Fe²⁺ [678,679,680,681,682,683,684,685], Ga³⁺ [696,697], K⁺ [692], Mn²⁺ [674,675], Sn⁴⁺ [733], and Sr²⁺ [699].

The microwave hydrothermal synthesis enables obtaining the following composite and hybrid materials: ZnO/ZnMn₂O [676,677], ZnO/ZnFe₂O₄ [680,681,682,683,684,685], ZnO/ZnS [700], Au-decorated ZnO [701,702,738], Ag-decorated ZnO [638,703,704,705,706,735], reduced graphene oxide [707,708,709,710,711,712,713], ZrO₂ coated ZnO [717], Fe₃O₄/ZnO/AgBr [719], MoS₂/ZnO [720], ZnO doped CeO₂ [721], mesoporous Si@ZnO [722], ZnO/zinc aluminium hydroxide [723], ZnO/TiO₂ [724,725], ZnO/CuO [726], CdO-ZnO [730], and In₂O₃-ZnO [732].

3.7. ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Hydrothermal Synthesis without Any Additional Heat Treatment

The potential of the microwave hydrothermal synthesis in obtaining ZnO hybrid nanostructures is proved by the results achieved by Cho et al. [723]. The paper by Cho et al. [723] is one of the more interesting publications reporting a synthesis of hierarchical hexagonal zinc oxide/zinc aluminium hydroxide heterostructures through epitaxial growth using microwave irradiation. For obtaining a substrate being zinc aluminium layered double hydroxide heterostructures (ZnAl:LDH), the authors used the solution of zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), aluminium chloride (AlCl₃) and ammonia water (NH₃·H₂O). The microwave hydrothermal synthesis of ZnAl:LDH lasted 20 min at the temperature of 95 °C. The product, ZnAl:LDH substrate, was thoroughly rinsed and dried after the synthesis. Depending on the selected synthesis parameters and modifications of the precursor composition, the authors obtained various structures that grew on the surface of the ZnAl:LDH substrate, namely: hexagonal ZnO nanorods (Figure 27 and Figure 28), sunflower-like ZnO nanorods (Figure 29), ZnO nanotubes (Figure 30) and ZnO film (Figure 31). The process of ZnO nanorod growth on the substrate surface was as follows: the substrate (ZnAl:LDH) was introduced to the solution of the mixture of Zn(CH₃COO)₂·2H₂O and NH₄OH and was subjected to microwave heating (20 min, 95 °C), and subsequently the product was rinsed with deionised water and dried. On the surface of some of the substrates, the growing ZnO nanorods formed repeatable patterns, which can be seen in Figure 28. The formation mechanism of these regular patterns was not tested by the authors [723], but the occurring regular patterns indicate that the nucleation of ZnO on the surface of the ZnAl:LDH strongly depends on the atomic arrangements of the ZnAl:LDH surface. The process of sunflower-like ZnO nanorod growth on the substrate surface (Figure 29) was virtually identical to the growth of ZnO nanorods apart from the applied Zn(CH₃COO)₂·2H₂O concentration, which was increased twice relative to the original solution used for the growth of ZnO nanorods. The growth of ZnO nanotubes (Figure 30) on the ZnAl:LDH surface, in turn, consisted in introducing the substrate to the solution of the mixture of Zn(CH₃COO)₂·2H₂O with NH₄OH and subjected to microwave heating (20 min, 95 °C), and subsequently the suspension was aged at the room temperature for 10 h, after which the product was rinsed with deionised water and dried. For the synthesis of a 2D ZnO film on the surface of ZnAl:LDH heterostructures, the composition of the precursor solution was modified by adding tripotassium citrate monohydrate (HOC(COOK)(CH₂COOK)₂·H₂O). As a result of microwave heating, sandwich-like heterostructures were obtained (Figure 31). In our opinion, the results of that paper indicate that the microwaves could have potentially acted as a stimulator of a stereochemical reaction (i.e., spatial orientation of the structure). The reaction temperature was not high, merely 95 °C, where an identical temperature could have been reached by pre-soaking of substrates on a heating plate, but the clear spatial organisation of the structure (Figure 27, Figure 28, Figure 29, Figure 30 and Figure 31) suggests the participation of additional mechanisms, e.g., an electromagnetic field.

Details of other research papers concerning the microwave hydrothermal synthesis of ZnO with additional heat treatment are presented in Table 7.

3.8. ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Hydrothermal Synthesis with Additional Heat Treatment

As already mentioned, the additional soaking process of the microwave hydrothermal synthesis product aims, above all, to complete the conversion of the substrates or intermediates. In the case of a synthesis of doped ZnO, additional heat treatment mostly leads to the precipitation of foreign phases, e.g., Cr₂O₃ [727,728], Eu₂O₃ [731], and SnO₂ [733]. Depending on the type of gaseous atmosphere, dopants may also undergo a partial oxidation or reduction process. Synthesis products subjected to an additional soaking process may be also characterised by a compact structure, where the particles are sintered or strongly agglomerated, which is indicated, e.g., by the results of the research by Bhattia et al. [728] in Figure 32.

One of the purposes of applying an additional process of synthesis product soaking is to give unique features both to the doped ZnO materials and composite ZnO materials, which results in the possibility to apply them e.g., as photocatalysts [724,732,734], gas sensors [725], dilute magnetic semiconductors (DMS) for spintronics applications [728]. A good example of obtaining a hybrid material with the use of an additional soaking process is the paper by Cao et al. [736], which describes obtaining Ag/Ag₂SO₄/ZnO nanostructures. The authors [736] used the aqueous mixture of Zn(CH₃COO)₂·2H₂O, CO(NH₂), CS(NH₂)₂ and AgNO₃ as the precursor. The composition of the mixture was modified by changing the molar ratio of thiourea and Ag⁺, which was kept at 1:2, 1:1, 2:1, respectively. The samples were heated by microwaves for 30 min at the temperature of 170 °C, subsequently rinsed (water, ethanol) and were subjected to calcination for 4 h at the temperature of 500 °C in the air atmosphere. Figure 33 contains XRD results of an example as-synthesised sample (only after the microwave synthesis) and the XRD results of samples after an additional soaking process.

The as-synthesised sample reveals a considerable quantity of foreign inclusions, above all Zn₅(CO₃)₂(OH)₆ product (intermediate, by-product). The authors [736] proposed a mechanism of the formation of (Zn₅(CO₃)₂(OH)₆) by-product and main products (ZnO, Ag₂S), which is described by the following Equations (22)–(30):

$$\mathrm{CO}{({\mathrm{NH}}_3)}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{T} > 60 °\mathrm{C} }{\to }2{\mathrm{NH}}_3{}^{+}+{\mathrm{CO}}_2$$

(22)

$$\mathrm{CS}{({\mathrm{NH}}_3)}_2+2{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{T} > 60 °\mathrm{C} }{\to }2{\mathrm{NH}}_3{}^{+}+{\mathrm{H}}_2\mathrm{S}+{\mathrm{CO}}_2$$

(23)

$${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{NH}}_4{}^{+}+{\mathrm{OH}}^{-}$$

(24)

$${\mathrm{CO}}_2+2{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{CO}}_3{}^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(25)

$$5{\mathrm{Zn}}^{2+}+2{\mathrm{CO}}_3{}^{2-}+6{\mathrm{OH}}^{-}\stackrel{\mathrm{MH} }{\to }{\mathrm{Zn}}_5{\left({\mathrm{CO}}_3\right)}_2{\left(\mathrm{OH}\right)}_6\downarrow$$

(26)

$${\mathrm{Zn}}^{2+}+2{\mathrm{OH}}^{-}\stackrel{}{\to }\mathrm{Zn}{(\mathrm{OH})}_2\downarrow$$

(27)

$$\mathrm{Zn}{(\mathrm{OH})}_2\stackrel{ \mathrm{MH}}{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}$$

(28)

$${\mathrm{Zn}}_5{\left({\mathrm{CO}}_3\right)}_2{\left(\mathrm{OH}\right)}_6 \downarrow \stackrel{\mathrm{MH} }{\to }5\mathrm{ZnO}\downarrow +2{\mathrm{CO}}_2+3{\mathrm{H}}_2\mathrm{O}$$

(29)

$${\mathrm{H}}_2\mathrm{S}+2{\mathrm{Ag}}^{+}\stackrel{ \mathrm{MH}}{\to }{\mathrm{Ag}}_2\mathrm{S}\downarrow +2{\mathrm{H}}^{+}$$

(30)

It must be underlined that ZnO, Ag₂S and Zn₅(CO₃)₂(OH)₆ compounds were present in samples obtained only as a result of the microwave synthesis. However, after the calcination (Figure 34), the presence of such compounds as Ag₂SO₄ and metallic Ag was observed in the samples, which was related to the soaking of samples in an oxidising atmosphere (air). During the soaking, Ag₂S and Zn₅(CO₃)₂(OH)₆ decomposed. In the sample with the highest addition of thiourea also ZnS formed. The course of the oxidation process was explained by the following Equations (31)–(32):

$${\mathrm{Ag}}_2\mathrm{S}+{\mathrm{O}}_2\stackrel{500 °\mathrm{C},4 \mathrm{h}, \mathrm{air} }{\to }2\mathrm{Ag}\downarrow +{\mathrm{SO}}_2$$

(31)

$${\mathrm{Ag}}_2\mathrm{S}+2{\mathrm{O}}_2\stackrel{500 °\mathrm{C},4 \mathrm{h}, \mathrm{air} }{\to }2{\mathrm{AgSO}}_4\downarrow$$

(32)

An example morphology of the Ag/Ag₂SO₄/ZnO composite sample is shown in Figure 34. The obtained plates structure was composed of multiple nanoparticles and had rough and irregularly porous surfaces. Obtaining a plates structure was explained by Cao et al. [736] as follows: due to the hydrolysis reactions of urea and thiourea, Ag₂S/Zn₅(CO₃)₂(OH)₆/ZnO nanoparticles are obtained at the first stage of microwave heating. At the second stage, the existing nanoparticles rapidly formed into plates aggregates for the purpose of decreasing their surface energy. The plates shown in Figure 34 had the diameter of ca. 1.5–2 µm and the thickness of ca. 100–200 nm. The TEM image in Figure 34b indicates monodispersity of Ag/Ag₂SO₄ NPs with the diameters of ca. 4–7 nm, which were uniformly anchored on the surface of ZnO plates. Figure 34c presents two different kinds of the lattice fringes with 0.24 nm and 0.26 nm, corresponding to the distance of Ag (1 1 1) and ZnO (0 0 2) planes, respectively. Figure 34d confirms the qualitative composition of the Ag₂S/Ag₂SO₄/ZnO composite obtained by Cao et al. [736].

Details of other research papers concerning the microwave synthesis of ZnO nanocomposites or ZnO hybrid nanostructures with additional heat treatment are presented in Table 8.

4. Microwave Solvothermal Synthesis of ZnO

According to the literature [747], a solvothermal synthesis is a process in a closed system, in which chemical reactions take place exclusively in non-aqueous solvents at increased temperatures, at a pressure that is higher than atmospheric pressure (P > 101,325 Pa). The literature [748,749] provides a different definition of the solvothermal synthesis, e.g., it is a chemical reaction based on non-aqueous solvents or their mixtures. The application of water mixtures and organic solvents in syntheses of metal oxides made the definitions of the hydrothermal synthesis and the solvothermal synthesis often used interchangeably. However, this is not reasonable and there is no need to use these definitions interchangeably. The main reason for developing the solvothermal synthesis was attempts to obtain non-oxide materials (in particular metal particles) [747,748,749,750,751,752]. In syntheses with the use of organic solvents, the critical parameter is the temperature of the onset of initial decomposition, because after exceeding this temperature solvents begin undergoing the thermal decomposition, polymerisation or oxidation process. Solvothermal synthesis is much more expensive than hydrothermal synthesis, which results from the costs of application of organic solvents, their disposal and the need to adapt the laboratory (ventilation, laboratory fume hood, etc.). However, the costs related to the maintenance and safe use of reactors operating in milder conditions of solvothermal synthesis are considerably lower than in the case of high temperatures and pressures dedicated to hydrothermal syntheses. As a result, the solvothermal technology is becoming cheaper, safer, and more popular year by year.

For the purposes of this review, we assume that the “solvothermal synthesis” is a process occurring in an inorganic solvent environment (where m_s > 50%), with the pressure equal to or higher than atmospheric pressure. The results of the literature review concerning microwave solvothermal synthesis of ZnO are divided into the following subgroups:

(1)

Microwave solvothermal synthesis of ZnO nanostructures without any additional heat treatment, where the literature review results [402,573,758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797] are summarised in Table 10.

(2)

Microwave solvothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures without any additional heat treatment, where the literature review results [798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822] are summarised in Table 11.

(3)

Microwave solvothermal synthesis of undoped ZnO, ZnO nanocomposites or ZnO hybrid nanostructures with additional heat treatment, where the literature review results [823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838] are summarised in Table 12.

4.1. Reactants

The most popular organic solvents used for the microwave solvothermal synthesis of ZnO according to the data derived from the literature review [758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838] (Tables 10–12) are ethylene glycol (EG, C₂H₄(OH)₂), ethanol (C₂H₅OH), diethylene glycol (DEG, (HOCH₂CH₂)₂O), propanol (C₃H₇OH), methanol (CH₃OH), benzyl alcohol (C₆H₅CH₂OH) and other solvents (Figure 35). The following were used sporadically as a solvent in the microwave solvothermal synthesis: acetone (C₃H₆O) [778], acetonitrile (C₂H₃N, [789]), alkoxyethanol [772], butanediol (C₄H₈(OH)₂) [769], butoxyethanol (CH₃C₃H₆OC₂H₄OH) [772], ethoxyethanol (C₂H₅OC₂H₄OH) [772], glycerol ((CH₂OH)₂CHOH) [822], hexanol (C₆H₁₃OH) [766], methoxyethanol (CH₃OC₂H₄OH) [772], N,N-dimethylacetamide (CH₃CON(CH₃)₂) [792], mixture of oleic acid (C₈H₁₇CH=CH(CH₂)₇COOH) with oleylamine [807], propanediol (C₃H₆(OH)₂) [776], tetraethylene glycol (TTEG, C₈H₁₈O₅) [402], triethylene glycol (TEG, C₆H₁₄O₄) [402], tetrahydrofuran (C₄H₈O, THF) [795], and polyethylene glycol (PEG400) [768].

The popularity of solvents in microwave solvothermal syntheses is affected above all by such factors as availability, price, toxicity and boiling point (Table 9). Ethylene glycol and diethylene glycol are relatively cheap solvents, which are produced on an industrial scale and can be applied above all as components of coolants and as components for production of polyester resins. Ethanol and methanol are two most popular alcohols, which are applied on a global scale as primary solvents in organic syntheses. The application of organic solvents enables a considerable decrease in the pressure, temperature, and duration of the process in relation to a synthesis of the same product in water alone [839]. For example, the reaction of ZnO synthesis at the temperature of 230 °C in water is characterised by the equilibrium pressure of ca. ~28 bar [839], while in ethylene glycol for the same temperature the pressure is merely ~4 bar [502]. In a chemical reaction in the liquid state, only temperature is important, while pressure is only a derivative of the process conditions.

The most popular reactants of the “Zn²⁺” zinc cation precursor used for ZnO synthesis according to the data derived from the literature review [758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838] (Table 10, Table 11 and Table 12) are zinc acetate (Zn(CH₃COO)₂·xH₂O), zinc nitrate (Zn(NO₃)₂·6H₂O), zinc acetylacetonate (Zn(C₅H₇O₂)₂), zinc chloride (ZnCl₂), and other (Figure 36). The following were used sporadically as the “Zn²⁺” zinc cation precursor in the microwave solvothermal synthesis: ZnO [775,816,819,822], bis(acetylacetonato)zinc [789], bis(methylacetato)zinc [789], bis(dimethylmalonato)zinc [789].

The popularity of zinc acetate (68.7%) results mainly from its availability and solubility in most organic solvents.

The most popular reactants being hydroxide anion precursor chemicals “OH⁻” used for the ZnO synthesis according to the data collected from the literature review [758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838] (Table 10, Table 11 and Table 12) (Figure 37) are sodium hydroxide (NaOH), ammonia water (NH₃·H₂O, NH₄(OH)), as well as amides and amines, e.g., C₂H₈N₂ [775], hexamethylene tetramine (C₆H₁₂N₄) [786], triethanolamine (TEA, C₆H₁₅NO₃) [786], diethanolamine ((CH₂CH₂OH)₂NH) [791], thioacetamide (C₂H₅NS) [819]. In over a half of publications, no hydroxide anion precursor chemical “OH⁻” was used, because the “OH⁻” anion was formed during the synthesis as a result of a reaction of a Zn²⁺ salt with a solvent, forming basic salts [402].

4.2. Surfactants

As mentioned before, surfactants are commonly used in nanotechnology for passivation of surface, control of shape, and giving of photophysical properties [740]. The following surfactants were used in microwave solvothermal syntheses of ZnO NMs for the purpose of controlling the morphology:

-

Polyvinyl alcohol (PVA) for obtaining nanoflakes [779].

-

Polyethylene glycol 400 (PEG-400) for obtaining nanorods [779].

-

Polyvinylpyrrolidone (PVP) for obtaining flowers with rod-like petals [779] and nanorods [825,827,828,829,830].

-

Cetyltrimethylammonium-bromide (CTAB) for obtaining flowers with rod-like petals [779], nanorods [827,830] and chrysanthemum-like prismatic nanorods [838].

-

Polyethylene glycol for obtaining NPs [232].

-

Methylimidazole for obtaining uniform particles with approximately rhombic dodecahedron facets [832].

-

Triton X100 for obtaining nanorods [827,830].

4.3. Morphology

Morphologies of ZnO reported in the literature are presented in the description of properties in Table 10, Table 11 and Table 12 [758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838]. ZnO nanomaterials and micromaterials synthesised with the use of the microwave solvothermal synthesis had the following shapes: butterfly-like [765], cables [819], core-shell structures [816], hexagonal prisms [765], hierarchical architectures constructed by nanoparticles [806], mulberry-like structures [786], needle-like [785], peanut-like structures [765], pyramids [807], rice-like particles [777,787], spheroidal nanostructures [784], sword-like wires [765], tubes [770], wires [783,792], sheet-like structures [785], spheres [765,791], flower-like structures [573,775,779,785,786,787,794,838], rod-like shape structures [402,705,764,767,770,771,778,780,785,788,797,805,813,822,825,826,827,828,829,830], and spherical-like shape structures [181,402,758,759,760,761,762,763,764,766,769,772,773,774,776,778,780,789,790,793,795,796,798,799,800,801,802,803,804,808,809,810,811,812,814,818,823,824,825,826,827,828,829,830,831,833,834,835,836,837,864].

Examples of various morphologies of ZnO obtained by the microwave solvothermal method are presented in Figure 38, Figure 39, Figure 40, Figure 41 and Figure 42. Changes in the colours of suspensions of doped ZnO NMs caused by the type and quantity of dopant are reflected in Figure 43, Figure 44, Figure 45 and Figure 46, while Figure 47 shows a change in the colour of ZnO NPs colloid caused by the use of various sizes of NPs.

4.4. Microwave Solvothermal Synthesis of ZnO without Any Additional Heat Treatment

One of the main advantages of the solvothermal technology is the possibility to use mixtures of precursors in the form of solutions rather than sedimenting suspensions of hydroxides, as is the case in the majority of hydrothermal syntheses of ZnO in water. Thanks to that, numerous problems are avoided, above all with heterogeneity and unrepeatability of the precursor (suspension sedimentation) and the lack of stirring of the precursor suspension in the reactor, which critically affects the quality and homogeneity of the obtained products. The application of non-aqueous solutions or those with a limited water content eliminates the presence of competitive mechanisms during the ZnO NPs synthesis, which lead to formation of heterogeneous products, with varying morphology and chemical composition. Among various synthesis routes, the solvothermal method is noted as attractive for its simplicity and the possibility to have a better control of the reaction mechanism and the better rate of particle growth [402,753,754,755,756,757].

The microwave solvothermal synthesis enables also a ZnO NMs synthesis from a suspension, which consists in obtaining a suspension of a Zn²⁺ salt in an organic solvent with an addition of a substance precipitating the hydroxide anion precursor chemical “OH⁻”, e.g., NaOH.

4.5. Microwave Solvothermal Synthesis of ZnO from a Solution

During our research on the microwave solvothermal synthesis we noticed that by changing the water content in the solution obtained by dissolving zinc acetate in ethylene glycol it is possible to control the size of ZnO NPs precisely within the range from ca. 15 nm to 120 nm [758]. Figure 48 presents an example size distribution of ZnO NPs crystallites obtained from precursor solutions with varying H₂O contents. All microwave solvothermal syntheses were carried out with the same parameters (25 min, 230 °C). Due to the lack of access to research enabling us to analyse by-products, we proposed a general equation of the reaction (33) of ZnO NPs in ethylene glycol:

$$\mathrm{Zn}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·2{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2,{ \mathrm{H}}_2\mathrm{O}, \mathrm{MH} \left(\mathrm{T}, \mathrm{P}\right) }{\to }\mathrm{ZnO}\downarrow +\mathrm{other} \mathrm{products} \left(\mathrm{liquid} \mathrm{or} \mathrm{gas}\right)$$

(33)

We proved that by determining an experimental correlation between the average size of ZnO NPs and the water content in the precursor, a calibration curve of the ZnO NPs size is obtained, based on which particle size can be controlled. Of course, each calibration curve of ZnO NPs size refers to a specific lot of reactants for which it was determined. Each time, when changing the lot of reactants, the calibration curve of ZnO NPs size must be determined again to preserve the precision of particle size control.

Thorough research on the microwave solvothermal synthesis of ZnO NPs obtained from a precursor solution, zinc acetate dissolved in ethylene glycol with an addition of heavy water, allowed us to explain and verify the synthesis mechanism [402]. It was crucial for our experiment to track and learn the fate of water at various stages of the synthesis. Owing to the results of XRD tests of ZnO NPs synthesis products for different durations (Figure 49) and the elemental analysis, we managed to determine that the solid intermediate of the solvothermal synthesis of ZnO was lamellar hydroxy-zinc acetate (LHZA, Zn₅(OH)₈(CH₃COO)₂·2H₂O) with the lamellar structure (Figure 50a). Based on the results of the gas chromatography with mass spectrometry and Fourier transform infrared spectroscopy we proved that the liquid by-products were esters, mainly ethanediol monoacetate (HOC₂H₄OOCH₃), and small quantities of ethanediol diacetate (C₂H₄(OOCH₃)₂). Thanks to the water content analysis by the Karl Fischer method, we determined that also water was one of the liquid by-products.

Learning all the primary products of the solvothermal synthesis of ZnO allowed us to write a general equation of the reaction (34):

$$\mathrm{Zn}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2,{ \mathrm{H}}_2\mathrm{O}, \mathrm{MH} \left(\mathrm{T}, \mathrm{P}\right) }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}+\left(2-\mathrm{m}\right){\mathrm{HOC}}_2{\mathrm{H}}_4{\mathrm{OOCH}}_3+{\mathrm{mC}}_2{\mathrm{H}}_4{\left({\mathrm{OOCH}}_3\right)}_2$$

(34)

The above equation may be expressed in a more general way (35):

$$\mathrm{Zn}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2+\mathrm{alcohol}\stackrel{ \left(\mathrm{T}, \mathrm{P}\right) }{\to }\mathrm{ZnO}\downarrow +\mathrm{esters}+{\mathrm{H}}_2\mathrm{O}$$

(35)

An identical general equation of the reaction (35) was proposed by Du te al. [394], Tonto et al. [396], Šarić et al. [398], Zhao et al. [780], Yiamsawas et al. [840], and Yuan et al. [841], while the results achieved in the papers by Bilecka et al. [762], Bhatte et al. [769,776], and Demir [842] permit deriving a general equation independently (35). The results in the paper by Gotic et al. [843], which describes the solvothermal synthesis of Fe₃O₄ (hematite), and the results in the paper by Bilecka et al. [763], which describes the solvothermal synthesis of CoO, ZnO, Fe₃O₄, MnO and Mn₃O₄, permit deriving a general equation of the reaction (36) of synthesis of metal oxide (M_xO_z) NPs from a salt of a metal acetate (M(CH₃COO)_x) dissolved in an alcohol.

$$\mathrm{M}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_x+\mathrm{alcohol}\stackrel{ \left(\mathrm{T}, \mathrm{P}\right) }{\to }{\mathrm{M}}_y{\mathrm{O}}_z+\mathrm{esters}+{\mathrm{H}}_2\mathrm{O}$$

(36)

Learning the fate of heavy water allowed us to explain the mechanism of ZnO NPs size control in the microwave solvothermal synthesis, which we described in detail in our paper [402]. The mechanism of size control of ZnO NPs obtained by the microwave solvothermal synthesis may be divided into 4 stages (Republished with permission of ©IOP Publishing from [402], Copyright (2018), permission conveyed through Copyright Clearance Center, INC. All rights reserved.):

(1)

Dissolution of zinc acetate in ethylene glycol (37,38), preparation of the precursor with a specified H₂O content (39)–(41):

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{solid}\right)}\stackrel{70°\mathrm{C}, \mathrm{EG} }{\to }{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{dissolved}\right)}$$

(37)

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{solid}\right)}+ 2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{70°\mathrm{C}, \mathrm{EG} }{\to }{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2{}_{\left(\mathrm{dissolved}\right)}+2{\mathrm{H}}_2\mathrm{O}$$

(38)

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}\stackrel{{ \mathrm{H}}_2\mathrm{O}, \mathrm{EG}}{\leftrightarrow }\left({\mathrm{CH}}_3\mathrm{COO}\right){ \mathrm{Zn}}^{+}+{ \mathrm{CH}}_3{\mathrm{COO}}^{-}$$

(39)

$${\mathrm{CH}}_3{\mathrm{COO}}^{-}+{ \mathrm{H}}_2\mathrm{O}\stackrel{{ \mathrm{H}}_2\mathrm{O}, \mathrm{EG}}{\leftrightarrow }{\mathrm{CH}}_3\mathrm{COOH} +{ \mathrm{OH}}^{-}$$

(40)

$$\left({\mathrm{CH}}_3\mathrm{COO}\right){\mathrm{Zn}}^{+}+{\mathrm{OH}}^{-}\stackrel{{ \mathrm{H}}_2\mathrm{O}, \mathrm{EG}}{\leftrightarrow }\left({\mathrm{CH}}_3\mathrm{COO}\right)\left(\mathrm{OH}\right)\mathrm{Zn}$$

(41)

(2)

Formation (42)–(45) and growth of the intermediate (46):

$$5{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+8{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+x{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{T}, \mathrm{P} }{\to }{\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{precipitation}\right)}+8{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(42)

or possibly e.g.,

$$5{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+x{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{T}, \mathrm{P} }{\to }{\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{precipitation}\right)}+8{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2$$

(43)

$$5\left({\mathrm{CH}}_3\mathrm{COO}\right)\left(\mathrm{OH}\right)\mathrm{Zn}+3{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+x{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{T}, \mathrm{P} }{\to }{\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{precipitation}\right)}+3{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(44)

$$2\left({\mathrm{CH}}_3\mathrm{COO}\right)\left(\mathrm{OH}\right)\mathrm{Zn} + 3{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn} + x{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{T}, \mathrm{P} }{\to }{\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{precipitation}\right)}+6{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(45)

$${\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2\mathrm{O}+ 5{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn} + 8{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+x{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{T}, \mathrm{P} }{\to }\left(1+1\right){\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left(\mathrm{CH}3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{growth}\right)}+8{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(46)

nH₂O comes from the simultaneous esterification reaction (47) or (48)

$${\mathrm{C}}_2{\mathrm{H}}_4\left(\mathrm{OH}\right){ }_2+{\mathrm{CH}}_3\mathrm{COOH}\leftrightarrow {\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}$$

(47)

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+{\mathrm{CH}}_3\mathrm{COOH}\leftrightarrow {\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2·{\mathrm{H}}_2\mathrm{O}+{ \mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(48)

(3)

Achievement of equilibrium constant of the ester hydrolysis reaction for Equation (49) and at the same time of equilibrium constant of the esterification reaction (47) and decomposition of the intermediate caused by temperature (50):

$${\mathrm{HOC}}_2{\mathrm{H}}_4{\mathrm{OOCH}}_3+{ \mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+{\mathrm{CH}}_3\mathrm{COOH}$$

(49)

$${\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2\mathrm{O}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2 \stackrel{\mathrm{T}, \mathrm{P} }{\to } 5\mathrm{ZnO}+2{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}+5{\mathrm{H}}_2\mathrm{O}+x{\mathrm{H}}_2\mathrm{O}$$

(50)

(4)

Growth of existing ZnO NPs (51,52), which is confirmed by the results in Figure 51:

$$\mathrm{ZnO}+{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{\mathrm{T}, \mathrm{P} }{\to }\left(1+1\right){\mathrm{ZnO}}_{\left(\mathrm{particle} \mathrm{growth}\right)}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(51)

$$\mathrm{ZnO}+\left({\mathrm{CH}}_3\mathrm{COO}\right)\left(\mathrm{OH}\right)\mathrm{Zn}+{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{\mathrm{T}, \mathrm{P} }{\to }\left(1+1\right){\mathrm{ZnO}}_{\left(\mathrm{particle} \mathrm{growth}\right)}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(52)

The general equation of the microwave solvothermal synthesis reaction of ZnO NPs in ethylene glycol that takes into account obtaining only HOC₂H₄OOCH₃ ester is as follows:

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2,{ \mathrm{H}}_2\mathrm{O}, \mathrm{T}, \mathrm{P} }{\to }\mathrm{ZnO}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(53)

The detailed description of the above mechanism of the ZnO NPs synthesis, which is also presented in Figure 52, is included in the paper [402]. For the purposes of this review, we present a general description below:

• As a result of hydrolysis of zinc acetate, water leads to the formation of acetic acid, which participates in an esterification reaction with ethylene glycol during the microwave solvothermal synthesis.
• The products of the esterification reaction are esters and water. However, the course of the reaction of obtaining and growth of the intermediate, Zn₅(OH)₈(CH₃COO)₂·xH₂O, is possible only through the co-existence of the esterification reaction. Only water forming in the esterification reaction participates in reactions of obtaining/growth of the intermediate, Zn₅(OH)₈(CH₃COO)₂·xH₂O. Once the equilibrium constant of the esterification reaction is reached, the intermediate rapidly decomposes into ZnO NPs, H₂O and esters.
• The control of particle size arising from a change in the water content in the precursor is a consequence of the change in the quantity of formed crystalline nuclei of ZnO (NPs) relative to the remaining unconverted quantity of substrate (zinc acetate). After the decomposition of the intermediate into homogeneous nuclei of ZnO (NPs), no subsequent nuclei of ZnO (NPs) are formed as a result of further reactions. The only process that might occur is the growth of the existing nuclei of ZnO (NPs) until the still unreacted substrates are used up.
• Water fulfils the function of a catalyst in the described ZnO NPs solvothermal synthesis reaction. Water participates in the reaction with substrates and forms an unstable intermediate, Zn₅(OH)₈(CH₃COO)₂·xH₂O, which at the same time is a catalyst of the esterification reaction.

We believe that generally each ZnO solvothermal synthesis based on a precursor obtained as a result of dissolution of zinc acetate in an alcohol or a polyol will proceed according to the mechanism described by us above [402]. In this solvothermal synthesis, the alcohol/polyol fulfils a double function: the solvent of zinc acetate, and the reactant. Particular attention should be drawn to the fact that the lack of control over the H₂O content in organic solvents will be manifested in the lack of repeatability of properties of the obtained ZnO NMs. It must be borne in mind that precursors obtained based on organic solvents must be always closed tightly because they absorb moisture from air/environment.

Other authors considered a mechanism of ZnO synthesis from zinc acetate dissolved in an alcohol but without analysing the solid intermediate, e.g., Bhatte et al. [769,776] suggest that Zn(OH)₂ is the intermediate of the discussed solvothermal reaction of ZnO synthesis.

4.6. Microwave Solvothermal Synthesis of ZnO from a Suspension

The microwave solvothermal synthesis of ZnO NMs has one major advantage: once the reactants (Zn²⁺) have undergone a complete reaction, the further growth of ZnO NMs crystals through the Ostwald ripening process is considerably limited by the presence of the organic solvent. ZnO solubility in organic solvents is so low that it prevents recrystallisation. Of course, theoretically, the Ostwald ripening process may not take place where:

-

content of, ${\mathrm{m}}_{{\mathrm{H}}_2\mathrm{O}}=0 \mathrm{wt}\%$,

-

water being formed is collected physically or bound chemically,

-

other substances which may digest/dissolve ZnO are not formed.

A limited Ostwald ripening process in the solvothermal synthesis enables obtaining ZnO NPs with the size below 10 nm (quantum dots), which is extremely rare in the case of hydrothermal syntheses. ZnO quantum dots are NPs with the size ranging from 2 to 10 nm.

The paper by Kuo et al. [774] describes obtaining ZnO NPs from a precursor formed by precipitation of Zn(CH₃COO)₂ dissolved in isopropanol, in an aqueous solution of NaOH. The authors [774] proposed the following general equation of the reaction (54):

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+\mathrm{NaOH}\stackrel{}{\to }\mathrm{ZnO}+{\mathrm{H}}_2\mathrm{O}+2\mathrm{Na}\left({\mathrm{CH}}_3\mathrm{COO}\right)$$

(54)

The product of their synthesis were spherical ZnO NPs with the average size of 3.5 nm.

The paper by Saloga et al. [795] describes obtaining ZnO NPs through a multi-stage synthesis. First, their prepared zinc oleate by mixing ZnCl₂ with sodium oleate, which they rinsed with water and dried. Next, dry zinc oleate was dissolved in tetrahydrofuran and then tetrabutylammonium hydroxide was added to the solution. Thus obtained suspension was subjected to a microwave synthesis lasting 5 min at the temperatures of 125 °C, 150 °C, 175 °C and 200 °C. Saloga et al. [795] describe obtaining ZnO NPs with the average size of 2.6 nm (125 °C), 2.7 nm (150 °C), 3.1 nm (175 °C), and 3.8 nm (200 °C).

Saoud et al. [705] obtained nanorods from a suspension of a precursor obtained by dissolving Zn(NO₃)₂ in ethanol, subsequently adding such a quantity of the NaOH solution until the pH of the obtained solution reached 10. The suspension was exposed to microwave irradiation for approximately 10–15 min and was removed upon onset of boiling. The synthesis product was nanorods with diameters ranging from 10 to 20 nm (Figure 53).

Details of other research papers concerning the microwave solvothermal synthesis of ZnO without any additional heat treatment are presented in Table 10.

4.7. Types of ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Solvothermal Synthesis

The microwave solvothermal synthesis enables obtaining:

-

ZnO doped with the following ions: Al³⁺ [811,814,815], Co²⁺ [232,798,799,800,801,808,809,810,812,833,834,835], Cr³⁺ [232,809], Fe²⁺ [232,810,836], Ga³⁺ [815], In³⁺ [811], Mg²⁺ [817], Mn²⁺ [232,802,804,809,810,812,837], Ni²⁺ [232,809,810,813] and V⁵⁺ [810].

-

ZnO co-doped with the following ions: Co²⁺–Mn²⁺ [803] and Al³⁺–Ga³⁺ [815].

-

ZnO composites or ZnO hybrid structures: Ag–ZnO [181,805,806,838], Au–ZnO [807], Fe₃O₄@SiO₂/ZnO [818], coaxial ZnO/C/CdS nanocables [819], ZnO/reduced graphene oxide [819,821], Ag/ZnO/reduced graphene oxide [821], and carbon-coated ZnO nanorods [822].

4.8. ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Solvothermal Synthesis without Any Additional Heat Treatment

One of the main advantages of the solvothermal synthesis of doped ZnO is the multifunctionality of the organic solvent, which may fulfil the function of a reactant, a dopant stabilising agent, limit the uncontrolled growth of particles, and enables a precise control of the particle size.

Our research [798,799,801] proves that the solvothermal synthesis, through selection of appropriate reactants (Zn(CH₃COO)₂·2H₂O, Co(CH₃COO)₂·4H₂O, C₂H₄(OH)₂), enables obtaining Co doped ZnO with the nominal content of Co²⁺ ions being at least 15 mol% in the form of a single-phase material (without foreign phase inclusions). We used ethylene glycol, which has slightly reducing properties, which enabled stabilisation of the oxidation state of the Co²⁺ dopant during the synthesis. Co doped ZnO obtained by us displayed paramagnetic properties with a contribution of antiferromagnetic coupling of Co–Co pairs [798]. We also confirmed that the method, which we discovered, of ZnO particle size control in the microwave solvothermal synthesis [402,758] permitted also control of Co doped ZnO particle size within the size range of 20–53 nm.

Bhattacharyya et al. [805] and Wang et al. [806] described obtaining an Ag–ZnO composite with the use of a mixture containing identical components: (Zn(CH₃COO)₂·2H₂O, AgNO₃, C₂H₄(OH)₂ (solvent), H₂O and C₂H₄(OH)₂ (solvent)). Bhattacharyya et al. [805] carried out 15-min syntheses in a domestic microwave oven modified with a refluxing system, while Wang et al. [806] carried out 30-min syntheses at the temperature of 170 °C in a microwave reactor. Bhattacharyya et al. [805] obtained an Ag–ZnO composite depending on the Ag content (Ag:ZnO molar ratio of 0.02, 0.05, 0.15, and 0.22) with the nanodisk and nanorod shape (Figure 54), where Ag NPs sized 3.5 nm were inserted into the pores of ZnO. Wang et al. [806], in turn, obtained an Ag–ZnO composite composed of microspheres (Figure 55), where the increase in the content of Ag NPs with the diameter below 5 nm from 0.5 mol% to 3 mol% resulted in a decrease in the size of irregular semi-circular microspheres.

One of the most interesting papers is the one by Liu et al. [820], which describes obtaining reduced graphene oxide sheets covered with ZnO NPs. As the synthesis precursor, zinc acetate dissolved in diethylene glycol with the addition of reduced graphene oxide sheets was used. Synthesis reactions lasting 10 min were carried out in a microwave refluxing system. Products prepared with different amounts of zinc acetate 0.0023, 0.0046, 0.0069, 0.0092, and 0.0115 M. By changing the zinc acetate concentration, the authors controlled the degree of coverage of the reduced graphene oxide sheets surface, which is presented in Figure 56 and Figure 57. The authors [820] successfully obtained the ZnO NPs (10–14 nm) content in ZnO/reduced graphene oxide samples from 11.55 wt% to 26.89 wt%.

Details of other research papers concerning the ZnO nanocomposites or ZnO hybrid nanostructures obtained by the solvothermal synthesis without any additional heat treatment are presented in Table 11.

4.9. ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Solvothermal Synthesis with Additional Heat Treatment

One of the main purposes of additional heat treatment of microwave solvothermal synthesis products, among others, undoped ZnO NMs in the solvothermal synthesis according to the publication [823,824,825,826,827,828,829,830,831], was to remove unconverted reactants. A perfect example here is the cycle of papers by Brahma et al. [825,826,827,828,829,830], which describe the growth of ZnO nanorods on an amorphous or disordered surface (silica, glass and polymer substrates). Brahma et al. [825] used a mixture obtained by mixing zinc acetylacetonate dissolved in ethanol with polyvinylpyrrolidone dissolved in water for the synthesis. The obtained mixture together with the silica substrate immersed therein was introduced for a duration between 20 s and 5 min (800 W) in a domestic-type microwave oven equipped with a water-cooled condenser (reflux system) placed outside the microwave oven. After cooling the suspension, the substrates were removed, rinsed and dried. For the purposes of removing the surfactant residue, the covered substrates were soaked (2–5 min) at 500 °C in the air atmosphere. An example of the obtained morphology of the ZnO film grown on Si(100) is presented in Figure 58.

In the case of a microwave solvothermal synthesis of doped ZnO, additional heat treatment of samples may lead to an increase in the ZnO particle size (recrystallisation process), oxidation or reduction of dopant ions and formation of foreign inclusions. The results of our research [798] show the effect of soaking ZnO doped with Co²⁺ ions. We obtained Co doped ZnO from a solution obtained by dissolving a mixture of zinc acetate with cobalt acetate (II) in ethylene glycol. We performed syntheses lasting 25 min at the temperature of 220 °C in a microwave reactor. Figure 59 shows a homogeneous product of Zn_1−xCo_xO NPs synthesis (x = 0, 0.01, 0.05, 0.10, 0.15) in the form of spherical particles with the size of 30–40 nm without foreign phase inclusions. The obtained samples were subjected to additional soaking for 0.5 h at 800 °C in an oxidising (synthetic air, Figure 60) and a reducing (nitrogen, Figure 61) atmosphere. XRD tests proved the presence of a Co₃O₄ phase in the Zn_0.85Co_0.15O sample soaked in air, while the presence of metallic Co in the Zn_0.95Co_0.05O, Zn_0.90Co_0.10O and Zn_0.85Co_0.15O samples soaked in nitrogen. Co²⁺ ions underwent oxidation to Co³⁺ ions during the soaking in synthetic air, which is presented by the equation of the reaction (55).

$$4{\mathrm{Co}}^{2+}+{\mathrm{O}}_2\to 4{\mathrm{Co}}^{3+}+2{\mathrm{O}}^{2-}$$

(55)

Co²⁺ ions underwent reduction to metallic Co⁰ in nitrogen, which is presented by the equation of the reaction (56).

$$2{\mathrm{Co}}^{2+}+{\mathrm{N}}_2\to 2{\mathrm{Co}}^0+2{\mathrm{N}}^{2+}$$

(56)

The soaking of the Zn_1−xCo_xO NPs samples resulted in:

-

a change in their specific surface area from 37–39 m²/g to merely 3 m²/g,

-

an increase in the particle size from the range of 30–40 nm to the range of 50–2000 nm depending on the amount of Co,

-

a change in the morphology from homogeneous spherical NPs (Figure 59) to heterogeneous hexagonal or cubic NPs (Figure 60 and Figure 61).

Details of other research papers concerning the ZnO nanocomposites or ZnO hybrid nanostructures obtained by the solvothermal synthesis with additional heat treatment are presented in Table 12.

5. Microwave Hybrid Synthesis of ZnO

Microwave hybrid syntheses of ZnO NMs are divided into two following groups:

(1)

Microwave hybrid synthesis method of pure ZnO nano and microstructures, where the literature review results [844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886] are summarised in Table 13.

(2)

Microwave hybrid synthesis method of ZnO composites or ZnO hybrid structures, where the literature review results [887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927] are summarised in Table 14.

The following were used for obtaining ZnO nano- and microstructures in the literature:

(1)

Ultrasonic microwave synthesis, which consists in the use of a new generation of microwave reactors, which permit the presence of an ultrasonic homogeniser’s sonotrode in the precursor mixture during the microwave heating. The ultrasonic homogeniser during its operation converts electrical energy into mechanical energy by moving the tip of the titanium sonotrode immersed in the fluid with a high frequency (19.5–40 kHz). Due to its inertia, the fluid no longer catches up with the rapid motion of the sonotrode, which results in cavitation, i.e., formation of gas bubbles that rapidly collapse, which is accompanied by sudden pressure changes, and as a consequence creates an impact wave.

(2)

Microwave assisted combustion synthesis, which consists in an exothermic reaction of combustion of one of the reactants of the reaction mixture in an oxygen atmosphere. Generally, a mixture composed among others of a Zn²⁺ salt and an organic component (fuel) is thoroughly mixed. There are several possibilities of the final state of the reaction mixture, among others, powder, pressed pastilles, gel, emulsion. The ready reaction mixture is introduced to a microwave reactor or oven, subjected to microwave radiation, which leads to a rapid increase in the sample temperature and ignition of the fuel, resulting in the formation of a ZnO powder.

(3)

Microwave assisted annealing, which consists in decomposition of the reaction mixture to ZnO only under the influence of its heating as a result of microwave radiation.

(4)

Microwave assisted sintering, which consists in microwave soaking of the earlier obtained ZnO.

(5)

Microwave vapour deposition, which consists in ZnO deposition from a gaseous phase, mostly at the atmospheric pressure, on the wafer (substrate) surface. For example, powdered ZnO, Zn or a Zn²⁺ salt is introduced to a ceramic crucible made of Al₂O₃, which is closed with a cover to which the substrate is attached on its inside part. Under the influence of microwave heating, a plasma arc appears in the crucible, enabling the evaporation of the Zn²⁺ substrate, which is deposited at the same time in the form of thin films on the whole surface of the ceramic container in the form of ZnO. Of course, there are professional microwave based plasma deposition units, which enable the application of inert carrier gases (e.g., argon, helium) or such gases (e.g., O₂) that can participate in chemical reactions leading to the formation of ZnO layers.

5.1. Reactants

The most popular reactants of the “Zn²⁺” zinc cation precursor used for the hybrid ZnO synthesis according to the data derived from the literature review [844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927] (Table 13 and Table 14) are Zn(NO₃)₂·6H₂O, Zn(CH₃COO)₂·2H₂O, ZnO, metallic Zn, and others (Figure 62). Other reactants are ZnCl₂, ZnSO₄, and Zn(OH)₂.

5.2. Morphology

Morphologies of ZnO NMs reported in the literature are presented in Table 13 and Table 14 [844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927] along with a description of their properties. ZnO NMs synthesised with the use of the microwave hybrid method were characterised by the following shapes: block-shaped structures [859], cables [899], corn-like microstructures [862], cubes [925], fibres [871], fluff-like structures [923], foamy and porous structures [850], half-backed grenade-like structures [844], hourglass-like structures [845], nails [878], wrinkle structures [923], spindle-like structures [844,888], spindle-like to double-prism-like structures [844], tetrapods [883], platelets [849], needles [886,925], plates [902,903], sheet-like structures [855,877,889], flakes [853,902,903,925], flower-like structures [847,859,881,887], tubes [868,869,870,899,916], wires [867,874,880,883,899], layers (films) [856,857,858,865,873,891,900,912,913], rods [852,861,881,883,889,907,919,920], and spherical particles [846,848,851,861,863,864,884,885,889,893,905,908,910,921].

Examples of various morphologies of ZnO obtained by the microwave hybrid synthesis are presented in Figure 63, Figure 64, Figure 65, Figure 66, Figure 67, Figure 68 and Figure 69.

5.3. Synthesis of Pure ZnO by the Microwave Hybrid Method

An example of an interesting publication reporting results of the microwave and ultrasonic wave combined synthesis of ZnO micro-/nanostructures is the paper by Li et al. [844]. As the synthesis precursor, a solution obtained by mixing solutions of Zn²⁺ salts (zinc acetate dehydrate, zinc acetylacetonate hydrate, zinc chloride) with 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) was used. Later, the mixture was continuously sonicated for 5 min with the power of 1000 W. Subsequently, the mixed solutions were heated to 110 °C within 3 min and kept at this temperature for 17 min under microwave heating combined with discontinuous ultrasonic irradiation (1 s of sonication and 2 s of interruption) with the power of 500 W. After finishing the synthesis the products were centrifuged, thoroughly rinsed (water, ethanol) and subsequently dried. Li et al. [844] obtained various morphologies of ZnO products, including grenade-like, column-like, spindle-like, rod-like, shuttle-like and flower-like micro-/nanostructures. The size and shape of the ZnO nanostructures was controlled by the authors by changing the Zn/HEPES molar ratio, pH value, and Zn precursor. Figure 70 demonstrates the control of the ZnO morphology by changing the Zn/HEPES molar ratio. When the Zn/HEPES molar ratio was adjusted to 3:8, half-backed grenade-like ZnO microstructures were formed (Figure 70a,b). When the Zn/HEPES molar ratio was adjusted to 1:4, spindle-like ZnO microstructures were formed (Figure 70c,d). When the Zn/HEPES molar ratio was adjusted to 1:8, spindle-like to double-prism-like ZnO structures were formed (Figure 70e,f). When the Zn/HEPES molar ratio was adjusted to 1:20, short spindle-like ZnO nanostructures were formed (Figure 70g,h). When the Zn/HEPES molar ratio was adjusted to 1:40, irregular ZnO agglomerates were formed (Figure 70g,h). Figure 71 presents a change in the ZnO morphology caused by changes in the pH (7.4, 8.4, and 9.4) in a precursor with the constant Zn/HEPES molar ratio of 1:5. Figure 72, in turn, shows a change in the ZnO morphology caused by a change in the pH (7.4, 8.4, and 9.4) in a precursor with the constant Zn/HEPES molar ratio of 1:2.

An example of a publication showing results of obtaining ZnO with the use of the microwave induced combustion process is the paper by Cao et al. [847]. In this paper, an aqueous solution of zinc nitrate with an addition of urea was used as a reaction precursor. The samples were placed in an ordinary microwave oven and subjected to microwaves. After boiling, evaporating, and concentrating, the precursor rapidly foamed up, deflagrated, and released heat and gases. Synthesis products were heterogeneous ZnO microstructures, the morphology of which changed depending on the urea/Zn²⁺ molar ratio (1:1, 5:3, and 3:1), which is presented in Figure 73. The effect of the high temperature of the microwave induced combustion process was that the vast majority of the obtained synthesis products were ZnO microstructures.

Hu et al. [864] report obtaining ZnO NPs by sol-gel—microwave assisted annealing. As the reaction precursor, a suspension obtained by mixing a solution of zinc acetate in anhydrous ethanol with a solution of LiOH·H₂O in anhydrous ethanol was used. Hu et al. [864] performed a range of syntheses to examine the impact of temperature and duration on a change in ZnO NPs properties, namely:

-

for the same duration (20 min) at various reaction temperatures (30, 40, 50 and 60 °C),

-

for the same reaction temperature (50 °C) with various durations (10, 20, 30 and 40 min).

The product in the form of a transparent ZnO sol was removed from the microwave reactor, and subsequently introduced to hexanol and cooled to 4 °C. Thus prepared suspensions were centrifuged and the obtained white powders were dried in the air at the temperature of 80 °C. According to the Scherrer’s formula, the particle size of ZnO prepared at 30 °C for 20 min, 40 °C for 20 min, 50 °C for 10 min, 50 °C for 20 min, 50 °C for 30 min, 60 °C for 20 min, and 50 °C for 40 min are 3.86, 4.09, 4.33, 4.78, 4.96, 5.14, and 6.68 nm, respectively [864]. Zhou et al. [868] report the results of obtaining ZnO hexagonal tubes by the microwave sintering method. For the synthesis, a mixture of nanometric and micrometric ZnO was used, which was obtained by triturating for 1 h in an agate mortar and then pressed into pills with a hollow core, which itself acted as a form of a growth chamber. Thus prepared pastilles were placed in a microwave chamber on a quartz plate surface. The pastilles were subjected to microwave heating at the temperature of 1100–1350 °C for 40 min. The product was tubes (Figure 74) with the diameter exceeding 10 μm, length exceeding 100 μm, while wall thickness ranging from 0.5 to 1 μm. The hexagonal tubes growth process was explained by the authors [868] by proposing a vapour-solid mechanism.

Takahashi [871] reports obtaining ZnO nano-fibres by means of a domestic microwave oven. As the reaction precursor, he used a mixture of Zn powder and steel-wool, which was introduced to an Al₂O₃ crucible. The crucible was covered by a glass substrate, on which the crucible cover was placed (Figure 75a). The sample was subjected to microwave heating with the power of 1000 W for 30 s. As a result of microwave heating, ZnO nano-fibres (Figure 75b) with the diameter of ca. 50 nm and the length of 0.5–1 μm (Figure 75c) grew on the glass substrate.

Details of other research papers concerning the pure ZnO obtained by the microwave hybrid method are presented in Table 13.

5.4. Types of ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Hybrid Synthesis Method

The microwave hybrid synthesis enables obtaining:

-

ZnO doped with the following ions: Al³⁺ [890], Ba²⁺ [893], Co²⁺ [895,896], Cu²⁺ [904,914], Cr³⁺ [897], Eu³⁺ [907,908], Ga³⁺ [910,911], Mg²⁺ [915] and Sm³⁺ [919];

-

ZnO co-doped with the following ions: Ce²⁺-Cu²⁺ [894] and Mn²⁺-Co²⁺ [917], where M = Mn²⁺, Ni²⁺, Fe³⁺ and Cu²⁺ [914];

-

composite and hybrid materials: Ag-ZnO [887,888,889], Au-ZnO [887], Ag–ZnO–graphene [889], Al³⁺ doped ZnO/Sn doped In₂O₃ [891], Au/Fe₂O₃–ZnO [892], ZnO/BiOBr [898], Zn–ZnO [899,900], ZnO–ZrO₂ [901], ZnAl₂O₄/ZnO [902,903], Cu–ZnO–Al₂O₃ [905], Cu–ZnO [906], Fe₂O₃/ZnO [909], In₂O₃–Ga₂O₃–ZnO [912,913], MgO–ZnO [916], Sb₂O₃–MnO–CoO–Cr₂O₃–ZnO [918], Ti_xO_y–ZnO [920], ZnO/ZnFe₂O₄ [921], ZnO/multi-walled carbon nanotube [922], ZnO—exfoliated graphene [923], ZnO–expandable graphite [927], and ZnO–reduced graphene oxide [924,925].

5.5. Synthesis of ZnO Nanocomposites or ZnO Hybrid Nanostructures by the Microwave Hybrid Method

Dou et al. [889] report obtaining ZnO NPs, Ag/ZnO nanocomposites and Ag/ZnO/graphene nanocomposites by the microwave and ultrasonic wave combined method. In order to obtain products, the aqueous mixture of:

-

zinc nitrate with hexamethylenetetramine was used to obtain ZnO NPs;

-

zinc nitrate, silver nitrate with hexamethylenetetramine was used to obtain Ag/ZnO nanocomposites;

-

zinc nitrate, silver nitrate with hexamethylenetetramine and an addition of graphene was used to obtain Ag/ZnO/graphene nanocomposites.

Identical synthesis parameters were applied for each sample, namely after stirring the suspensions for 10 min, the reaction vessel with the sample was put into an ultrasonic microwave reaction system and kept at 90 °C for 2 h and sonicated for 30 min. Subsequently, the samples were centrifuged, rinsed with water and then dried in a vacuum dryer. Dou et al. [889] provide only the results of the morphology of Ag/ZnO/graphene nanocomposites (Figure 76).

Mary et al. [894] report obtaining ZnO co-doped with Ce²⁺ and Cu²⁺ ions by the microwave induced combustion process. Reaction precursors were obtained by dissolving zinc nitrate, cerium nitrate, copper nitrate in deionised water with an addition of urea as a fuel. The crucible with a precursor was placed in a microwave oven, where the precursor solution underwent various reactions for 8 min of heating. Namely, the solution first boiled, then underwent dehydration, and subsequently decomposition with emission of large quantities of vapours in the form of smoke. When the solution reached the spontaneous ignition point, the combustion heat released and transformed the sample into a solid powder. ZnO powders were prepared with the addition of copper (Cu) and cerium (Ce) of different molar ratios (Zn_1−2xCe_xCu_x)O with x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05 to ZnO. The general equation of the synthesis reaction is described by Equation (57).

$$\mathrm{Zn}{({\mathrm{NO}}_3)}_2+\mathrm{Cu}{({\mathrm{NO}}_3)}_2+\mathrm{Ce}{({\mathrm{NO}}_3)}_2\stackrel{\mathrm{urea}, \mathrm{water} \mathrm{MH} \left(\mathrm{T}\right)}{\to }{\mathrm{Zn}}_{1-2x}{\mathrm{Ce}}_x{\mathrm{Cu}}_x\mathrm{O}+\mathrm{gaseous} \mathrm{products}$$

(57)

The size of Zn_1−2xCe_xCu_xO crystallites was 35 nm, 38 nm, 40 nm, 44 nm, 46 nm and 46 nm depending on the dopant content of 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05, respectively. The SEM results (Figure 77) indicated that the size of Zn_1−2xCe_xCu_xO particles decreased in line with the increase in the dopant content. The structure of the obtained pure ZnO was polycrystalline, where the ZnO particle size reached several hundred nm, while for the Zn_0.90e_0.05Cu_0.05O sample the largest particle size reached up to several dozen nm.

Konou et al. [893] described Ba doped ZnO by the microwave assisted annealing process. Figure 78 shows a scheme according to which samples were obtained by Konou et al. [893]. First, NaOH (a molar ratio of OH⁻ and Zn²⁺ kept to 3) was dropped into solutions of zinc nitrate and barium chloride, and subsequently the obtained suspension was filtered, rinsed and dried. The obtained powders were soaked in a microwave furnace at the temperature of 500 °C for 30 min. The barium doping concentration of ZnO was 0, 1, and 2 at%. Figure 79 presents the morphology of ZnO, Ba (1 at%) doped ZnO and Ba (2 at%) doped ZnO. The obtained products were characterised by a compact (sintered) structure, which resulted from the application of a high soaking temperature (500 °C). All three samples were characterised by a heterogeneous shape and by a large range of particle/aggregate sizes although the crystallite size was the same in all samples (20–22 nm).

Kim et al. [927] described a microwave sintering synthesis of ZnO NPs/graphene nanocomposites, where individual steps of obtaining ZnO NPs/graphene nanocomposites are presented in Figure 80. Commercially available ZnO NPs and expandable graphite were used for the synthesis. The expandable graphite sample was introduced to a crucible (Al₂O₃) and subjected to microwave heating (1 kW, 1 min), after which the product was dispersed in ethanol and sonicated for 10 min to exfoliate the graphite. Subsequently, ZnO NPs together with exfoliated graphite (0.01 wt%) were mixed in ethanol through sonication. The mixture was filtered off on a filter paper and subsequently the powders were placed in a crucible (Al₂O₃) and heated in a microwave oven (1 kW, 5 min). Thus obtained ZnO-graphene nanocomposite powders were dispersed in ethanol and were spray-coated on a heated SiO₂ substrate using a spray gun (140–160 °C). Figure 81a presents the morphology of commercial ZnO NPs, while Figure 81b,c show SEM images of ZnO/graphene nanocomposites without the microwave sintering process and after the microwave sintering process, respectively. The main difference between the morphology of samples in Figure 81b,c was the emergence of secondary ZnO NPs with smaller sizes in the sample after the microwave sintering process, which is perfectly visible in TEM images (Figure 82). The results achieved by Kim et al. [927] are very interesting, because generally the sintering process of NPs leads to their size growth, but in this particular case, a secondary ZnO NPs phase with smaller sizes was obtained.

Zhang et al. report [899] obtaining Zn-ZnO nanocables and ZnO nanotubes by microwave vapour deposition with the use of a microwave plasma system. The scheme of their experiment is shown in Figure 83. Zinc powder present in the quartz crucible was placed inside a horizontal quartz tube. Pure hydrogen (H₂) was used as the protective gas and the carrier gas, which was introduced to the reaction chamber with the flow rate of 50–90 sccm. The pressure in the reaction chamber was maintained at ca. 4 Torr. Microwaves (400 W) were introduced with the use of a wave guide in the central part of the quartz tube for generating stable plasma. The temperature of the microwave plasma was estimated at about 1000 °C. As an additional heat source, a movable tubular furnace was used, which generated the temperature of 700 °C at a stretch of 20 cm, while the estimated temperature in the quartz tube along the same stretch was ca. 500 °C. After switching the heating off for 30 min, the H₂ flow was stopped and a black product covering the internal surface of the quartz tube was observed. For the purpose of oxidising the formed product, O₂/Ar gas (volume ratio 1:50) was introduced to the microwave plasma system for 15–30 min. Then, the tubular furnace was moved back-and-forth along the quartz tube until only white powder was left. The morphology of the obtained products could be controlled through the oxidation duration (15–30 min) and the H₂ flow rate (50–90 sccm). Figure 65f,g shows the morphology of the final products. The nanotubes had a uniform inner and outer diameter of about 10 and 40 nm, respectively, while the average length of the nanotubes was ca. 1 µm. Figure 65f,g reveal that the outer diameter of ZnO nanotubes is virtually identical to the outer diameter of Zn–ZnO nanocables.

Details of other research papers concerning the of ZnO nanocomposites or ZnO hybrid nanostructures obtained by the microwave hybrid method are presented in Table 14.

6. Conclusions

ZnO is a multifunctional material, among others thanks to its semi-conductor, optical, biological, antibacterial, piezoelectric properties. Probably, to the surprise of many, the popularity of ZnO contyinues to grow, which is mainly caused by the development of new methods of obtaining ZnO NMs. These methods permit a synthesis of various ZnO NMs, e.g., owing to the possibility to control the shape and size, and to modify the chemical composition (dopants, composites etc.), which defines their properties and enables their use in new applications. ZnO NMs are also promising unique components for producing various innovative devices. However, for the potential of all ZnO NMs to be fully tapped in consumer products, numerous comprehensive tests are still needed. Nowadays, the microwave heating technology is already a classic method for fabrication of distinct chemical compounds and materials, including ZnO NMs. This has become possible thanks to the availability of new professional microwave apparatus on the market, which is continually improved by the manufacturers to meet the users’ requirements. A microwave apparatus can be encountered increasingly often in laboratories, the industry, and virtually all households. Microwave heating is environment-friendly and has been classified as a green chemistry approach. The main advantages of microwave heating include speed, homogeneity, and purity (contactless method).

This comprehensive review describes the microwave synthesis of ZnO NMs (pure, doped, composites). We concentrated in particular on reactants, process parameters and morphologies of the obtained products. The microwave synthesis of ZnO NMs was divided into three primary groups: hydrothermal, solvothermal and hybrid methods. Hybrid methods include such methods as: ultrasonic microwave synthesis, microwave assisted combustion synthesis, microwave assisted annealing, microwave assisted sintering, and microwave vapour deposition. Statistics of the use of reactants were presented. The overall results point that the microwave synthesis of ZnO NMs has an enormous potential that enables obtaining a wide range of product morphologies, beginning with quantum dots, to core-shell structures and hierarchical structures, and ending with films on substrates (surface modification). The discovery and explanation of some of the mechanisms of microwave syntheses have made it possible to obtain ZnO NMs with controlled properties (among others size, shape) and has eliminated the unrepeatability of syntheses. The present review indicates that the microwave synthesis of ZnO NMs is an extremely vast research topic, with many phenomena yet to be explained. Moreover, new issues (effects, mechanisms) related to the microwave synthesis of ZnO NMs certainly remain to be discovered.

Unfortunately, the cited literature does not provide examples of the achieved daily capacity of the microwave synthesis of ZnO NMs. Nevertheless, based on our many years of experience with the microwave synthesis of nanomaterials, we have become convinced in practice that, thanks to microwave solvothermal synthesis, ca. 100 g of ZnO NPs can be achieved daily.