Nanometric Heterostructures for Photocatalytic Degradation: Mechanisms, Preparation, and Applications—A Review

Abstract

Nanomaterials are the subject of an increasingly growing number of investigations. Due to their unique characteristics, they have found wide applications in disciplines like electronics, catalysis, and medicine. In the field of catalysis, one can mention their application for the photocatalytic removal of various contaminants, both inorganic (e.g., toxic chromates) and organic (pesticides, dyes, pharmaceuticals, etc.). Such applications are important because the contamination of the natural environment is constantly growing. Distinct nanomaterials, such as quantum dots, have been widely utilized for photo-based wastewater treatment, but the current research activity is directed toward investigations of combined nanometric heterostructures which exhibit additional and useful properties in comparison with standalone components. However, the preparation of such heterostructures may be demanding and more complicated than that of standalone components, and the mechanism of improved photocatalytic performance may vary among different types of heterostructures. The aim of this review is to fill the existing gap in the literature and present mechanisms (type-II, Z-scheme, and S-scheme), methods of preparation (sol–gel, hydrothermal, solvothermal, sonochemical methods, and others), and applications of nanometric heterostructures in the photocatalytic degradation of organic pollutants.

1. Introduction

The recent few decades brought about the exponential development of studies at the nano scale in which extraordinary phenomena were observed. The increasingly growing applications of nanomaterials, especially, are the main driving force for the development of nanotechnology. The current definition of a nanoparticle, provided by the International Union of Pure and Applied Chemistry, says it is a particle of arbitrary shape but with a size (in at least one dimension) lying in the range of 10⁻⁹ to 10⁻⁷ m [1]. The mentioned phenomena on the nano scale lie somewhere between the quantum and macroscopic point of view; some of them are more similar to the behavior of single molecules, and, simultaneously, others are rather similar to that exhibited by bulk substances. This fact contributes to the observed extraordinary behavior of nanomaterials.

One typical example is the relatively large surface-to-volume ratio, which favors the adsorption of chemical molecules on their surface. This feature is especially demanded in catalysis, including photocatalysis, which is of special concern in this review [2]. Nanoparticles also exhibit a relatively high surface energy—due to many unsaturated chemical bonds on their surfaces—which contributes to the ability of nanostructures to conduct and enhance various chemical reactions [2]. For example, quantum dots—a special case of crystalline nanomaterials—found applications in the catalytic degradation of various organic compounds, especially. This is a highly important issue because many bio-active organic molecules originating from various branches of industry, as well as from households, such as dyes, pharmaceuticals, and preservatives, are present in the natural environment, posing a risk for humans and other living organisms. In this context, photocatalytic degradation is promising as an environment-friendly and efficient technology for wastewater treatment.

The attention of scientists dealing with nanomaterials has recently shifted to the construction of heterostructured composites, utilizing the most valuable properties of their components and, preferably, exhibiting some emergent properties due to a synergetic effect [3,4,5]. The biggest problems in photocatalysis, which may be at least partially solved by constructing heterostructures, are rapid charge–carrier recombination, inappropriate redox potentials, and insufficient light absorption (see Figure 1) [6,7,8,9].

Figure 1. Schematic representation of physical and chemical phenomena during the photocatalytic process. Reproduced from ref. [9] under the Creative Commons license without modifications.

The main types of heterostructures are type-II, Z-scheme, and S-scheme heterostructures [9,10]. In the type-II heterostructure (Figure 2I), electrons with higher energy from the first semiconductor recombine with holes of higher energy from the second semiconductor [9,10]. In the Z-scheme heterostructure (Figure 2II), the situation is the total opposite; the carriers with lower energy recombine, leaving carriers with higher energy [9,10]. In the S-scheme heterostructure (Figure 2III), due to the contact between two distinct semiconductors, the energies of the conduction and valence bands in both of them change, and an intrinsic electric field is generated which results in the movement of electrons in a manner similar to that exhibited by the Z-scheme heterostructure [9,10].

Figure 2. Schematic representation of action of distinct types of heterostructures: (I) type-II heterostructure, (II) Z-scheme heterostructure, and (III) S-scheme heterostructure. Reproduced from [10] under the Creative Commons license without modifications.

Currently, there exist review articles describing the synthesis of heterostructures or their applications, but they are focused on only one aspect between these two, and they deal with a specific compound or group of compounds such as TiO₂, metal halide perovskites, or chalcogenides [10,11,12,13,14,15,16,17,18,19,20,21]. Thus, the aim of this review is to present—in a comprehensive way—the recent literature on the preparation and applications of nanometric heterostructures in the photocatalytic degradation of organic compounds, with special emphasis on the underlying mechanisms of their improved performance compared with their standalone components.

2. Methods of Preparation of Nanometric Heterostructures

Heterostructures used for photocatalysis can be synthesized in many ways, some of which are as follows: sol–gel [22], ultrasound-assisted [23], solvothermal [24], and hydrothermal [25] methods. It is often the case that—because of the complexity of the aforementioned structures—these methods are used simultaneously in order to successfully obtain the desired products. An example of this can be found in the work of E.E. El-Katori et al. [26] in which they have combined a sol–gel approach (to obtain ZnO and NiO nanoparticles) with a sonochemical method to manufacture NiO/ZnO nanocomposites with varying NiO concentrations (0.5, 1, and 5 wt%). In another work, the authors [27] have combined the sol–gel method with the hydrothermal approach to synthesize ZnO rod-like structures on TiO₂ nanorods.

2.1. Sol–Gel Methods

The sol–gel method is a wet chemical process often used for the synthesis of oxide-based materials from hydrolyzable precursors. Such precursors are often halides, sulfates, nitrates, alkoxides, or carboxylates of the respective metals. The typical procedure for the synthesis of nanoparticles by the sol–gel method (as presented in Figure 3) is as follows: a portion of the appropriate precursor is dissolved in distilled water (or other solvent) to hydrolyze it. To this solution, a precipitating agent is then added (for example, ammonium hydroxide), and, after continuous stirring, a sol–colloidal solution of metal oxide nanoparticles is obtained. This solution then undergoes condensation, after which a wet gel is obtained. The condensation process happens spontaneously during the aging of the sol at room temperature, and, after gelation comes to an end, the gel is washed (for example, with distilled water) and dried. Dry gel is then often calcined to obtain the final product.

Figure 3. Schematic presentation of the sol–gel synthesis process. Reproduced under the Creative Commons license from ref. [28].

The simplicity of this process allows for its easy modification. For example, to synthesize thin NiO/ZnO nanocomposite films, the authors [26] used a solution of surfactant called “Triton X-100” as a structuring and templating agent, which, after the nanocrystals have grown to a certain size, inhibits them from assembling, thus improving the crystalline quality. Another modification that was used for the synthesis of NiO/ZnO heterostructures by T. Tangcharoen et al. [29] was the employment of an auto combustion process. In this approach, the authors added diethanolamine as a fuel to the solution of Zn(NO₃)₂ and Ni(NO₃)₂ in ethanol and then heated it to a temperature of 200 °C while stirring constantly. After some time, the solution gelified and self-ignited, and, after that, a dark brown powder was obtained and calcined at 1000 °C. The sol–gel method was also employed for the synthesis of pyrophyllite/TiO₂ heterostructures [30]. The natural clay was reacted with TiO₂ nanoparticles obtained from the hydrolysis of titanium tetrabutoxide, which gelified after adding a 15 mL/10 mL EtOH/distilled water solution dropwise. The gel obtained this way was then, similarly to the other previously described processes, aged, washed, dried, sieved, and calcined.

Other heterostructures that were synthesized with the help of the sol–gel method are as follows: Ag-modified TiO₂/ZnO nanotree arrays [27], Bi₂O₃-SiO₂ [31], and a ZnO nanorod/CuAlO₂ nanofiber heterojunction grown on nickel plates [32].

Bismuth silicate was synthesized in a typical way, but with the additional use of a surfactant. Structures for the modification with Ag QDs were prepared using a combined sol–gel and hydrothermal approach, as stated previously, and silver quantum dots were introduced by photodeposition. ZnO nanorod/CuAlO₂ nanofiber heterojunctions were obtained by first synthesizing CuAlO₂ structures on Ni plates using the typical sol–gel method with the addition of ethylene glycol and dimethylformamide to facilitate the better mixing of the reagents. After obtaining CuAlO₂ structures, ZnO nanorods were synthesized on top of them by first creating a ZnO seed film using the sol–gel method and then growing nanorods from it via the hydrothermal approach. The Ag-modified TiO₂/ZnO nanotree array synthesis process is presented in the diagram below (Figure 4), and their structure can be seen in the SEM images below (Figure 5).

Figure 4. Schematic representation of the synthesis of Ag-modified TiO₂/ZnO heterostructures (green color indicated substrate). Reproduced under the Creative Commons license from ref. [27] without modifications.

Figure 5. SEM images of (a) TiO₂ nanorod arrays, (b) Ag-modified TiO₂ nanorod arrays, (c) TiO₂ nanorod arrays/ZnO nanoparticles, (d) TiO₂/ZnO heterojunction nanotree arrays, (e) Ag-modified TiO₂/ZnO heterojunction nanotree arrays, and (f) an EDS spectrum of Ag-modified TiO₂/ZnO heterojunction nanotree arrays. Reproduced under the Creative Commons license from ref. [27] without modifications.

As previously stated, calcination is often the last step in the sol–gel synthesis method. However, in the recent literature, it is possible to find examples of a calcination-free approach. For example, it was employed by K. Yang et al. [33] by adding an organic semiconductor molecule, 1,4-naphthalene dicarboxylic acid (NA), to a solution of titanium precursor (tetrabutyl titanate).

The summary of all sol–gel methods can be seen in the table below (Table 1).

Table 1. Summary of the presented sol–gel synthesis methods.

Material	Method	Precursors	Key Steps	Calcination	Special Features
Pyrophyllite/ TiO2 [30]	Sol–gel synthesis	Pyrophyllite, Ti[OC(CH3)3]4	Exfoliation of clay, TiO2 incorporation	750 °C (optimized)	Anatase/rutile mix; high surface area
Bi2SiO5 (Bi2O3SiO2) [31]	Surfactant-mediated sol–gel	Bi(NO3)3·5H2O, Si(OC2H5)4	Surfactant-assisted gelation, Si doping into Bi2O3, calcination	600 °C	Enhanced visible-light activity; excellent dye degradation and reusability
Ag-Modified TiO2/ZnO Nanotree Arrays [27]	Hydrothermal + Sol–gel + Photo-deposition	Ti(O(CH2)3CH3)4 Zn(CH3COO)2·2H2O	TiO2 hydrothermal growth, ZnO sol–gel coating and hydrothermal growth, Ag quantum dots photodeposition	480 °C (for TiO2)	Nanotree 3D array; Ag plasmon enhancement; 8× photocurrent boost
ZnO Nanorod /CuAlO2 Nanofiber Network on nickel [32]	Sol–gel for CuAlO2 + Hydrothermal for ZnO	Cu(C2H3O)2·H2O Al(NO3)3·9H2O ZnNO3·6H2O	Sol–gel CuAlO2 fibers dip-coated on Ni plates, hydrothermal ZnO rod growth	600 °C (Ni plates); Two-step at 300 °C and 1150 °C (CuAlO2)	3D fibrous heterostructure; strong photocurrent; blue-shift tunable absorption
TiO2-NA Organic-Inorganic Hybrid [33]	Calcination-free sol–gel	Ti(OC4H9)4	Low-temp sol–gel with NA doping	None (calcination-free)	Very high surface area; narrow bandgap (2.1 eV); robust visible-light H2 production
Titania–Clay Composite [22]	Sol–gel (in situ clay exfoliation)	Organoclay Ti[OCH(CH3)2]4	Clay exfoliation with Ti precursor, gelation	550 °C	Optimized TiO2 loading on clay; enhanced solar-driven pollutant degradation
Mesoporous NiO/ZnO [26]	Sol–gel with surfactant templating	NiSO4·6H2O ZnSO4·7H2O	Sol–gel synthesis using Triton X-100, doping NiO onto ZnO	600 °C (ZnO) 400 °C (NiO)	Mesoporous structure; superior photocatalytic and anticorrosion properties
NiO/ZnO [29]	Sol–gel auto-combustion	Zn(NO3)2·6H2O Ni(NO3)2·6H2O	Sol–gel preparation, self-combustion of gel	1000 °C	Tunable solid solutions; strong UV absorption; versatile for photocatalysis and optics

2.2. Hydrothermal Methods

Hydrothermal synthesis is a simple method for obtaining nanoparticles, similar to the sol–gel approach, by dissolving a precursor in distilled water. Depending on the material that one wants to obtain, an addition of other reagents, such as NaOH, EDTA, or NH₄H₂PO₄, may be required [19,34]. The obtained solution is then placed in an autoclave and heated. After the reaction ends, the final product is washed and dried [35]. Such obtained products are often annealed or calcined [35].

This method, with some modifications, was used to synthesize the CuO@ZnO p-n heterojunction nanocomposite [36]. In this work, the authors prepared ZnO nanoparticles from the Zn(NO₃)·6H₂O and EDTA mixture dissolved in 30 mL of water, 10 mL for zinc precursor, and 20 mL for EDTA, and the EDTA solution was added dropwise to the precursor solution. After drying, the white solid was collected and calcined to obtain said nanoparticles. To acquire the nanocomposite, the authors dispersed the previously obtained ZnO nanoparticles in toluene and added copper (II) acetate monohydrate to this suspension. This mixture was then heated under reflux for 6 h in a nitrogen atmosphere. After this, the resulting material was filtered, washed with ethanol, and calcined. This method allowed for the synthesis of nanostructures with a high photocatalytic activity and long-term stability towards photochemical hydrogen production and a reduction in methylene blue [36].

In another work by Bessoussa et al. [34], the authors used a two-step hydrothermal method for the synthesis of Bi₂S₃/TiO₂ nano-heterostructures. TiO₂ nanorod arrays were synthesized by the hydrothermal reaction of HCl, water, and titanium isopropoxide. The reaction was carried out in a Teflon-covered autoclave, which was placed inside a furnace for 5 h at 180 °C, followed by the cleaning, drying, and calcination of the obtained samples. Bismuth nanoparticles were synthesized by reacting bismuth nitrate and thiourea in a similar process (10 h of heating instead of 5, and the annealing process was skipped). To obtain the aforementioned heterostructure, a solution of bismuth and sulfur precursors were transferred to an autoclave, and, subsequently, the as-prepared TiO₂ nanoparticles were also placed in the same autoclave. Further synthesis was carried out in the same way as in the case of bismuth nanoparticles. Similarly to CuO@ZnO nanostructures, the synthesized Bi₂S₃/TiO₂ structures are reported to have excellent photocatalytic performance and stability for the elimination of methylene blue.

Other nano-heterostructures that were synthesized by hydrothermal approaches include SnO₂/α-Fe₂O₃ [19], TiNT@AnNP (titanate nanotubes decorated with anatase nanoparticles) [37], and Fe₂O₃/Bi₂S₃ nano-heterostructured rods [38].

The summary of all hydrothermal methods can be seen in the table below (Table 2).

Table 2. Summary of the presented hydrothermal synthesis methods.

Material	Method	Precursors	Key Steps	Calcination	Special Features
CuO@ZnO [36]	Hydrothermal + Reflux	Zn(NO3)·6H2O, Cu(CO2CH3)2·H2O	ZnO by EDTA chelation; CuO via reflux in toluene	700 °C (ZnO), 400 °C (composite)	p–n junction, efficient H2 evolution
Bi2S3/TiO2 [34]	Two-step Hydrothermal	Ti[OCH(CH3)2]4, Bi(NO3)3·5H2O	TiO2 NRAs then Bi2S3 deposition	400 °C (TiO2)	Branched nanoflowers, solar light responsive
SnO2/ α-Fe2O3 [19]	Hydrothermal Epitaxial	SnCl4·5H2O, Fe(NO3)3·9H2O (nanocubes), FeCl3 (nanospindles)	Epitaxial SnO2 on α-Fe2O3 nanospindles/cubes	None specified	Lattice-guided growth, minimized mismatch
TiNT@AnNP [37]	Microwave-assisted hydrothermal	TiO2(anatase)	TiO2 suspension subjected to microwave irradiation	None specified	Fast one-pot synthesis, tunable by temperature/time
Fe2O3/Bi2S3 nanorods [38]	One-step hydrothermal	Bi(NO3)3·5H2O, Fe(NO3)3·9H2O, thiourea	Solution of precursors heated in an autoclave	None specified	Simple, scalable, Fe2O3-enhanced Bi2S3 rods

2.3. Solvothermal Methods

Solvothermal methods are similar to hydrothermal methods in almost every aspect except the solvent used. While the hydrothermal approach refers to the synthesis in aqueous solutions above the boiling point of water, the solvothermal synthesis is conducted in non-aqueous solutions at relatively high temperatures. This change in solvent can often be desirable, for example, in situations when the precursors or other necessary reagents used are not well-soluble in water. In addition to that, the difference in solvents used is reported to have a significant influence on the properties of synthesized nanoparticles [39,40].

An example of such a synthesis can be seen in the work of J. Feng et al. [41], in which the authors have synthesized two-dimensional layered graphitic carbon nitride (g-C₃N₄)/SnS₂ p-n heterojunction composites exhibiting excellent photocatalytic activity and reusability in the process of the photodegradation of Prussian blue. The bulk g-C₃N₄ was obtained through a thermal polymerization route in which 10 g of melamine was annealed at 550 °C with a heating speed of 5 °C per minute and then maintained for 4 h. After that, the obtained yellow product was ground into powder and used in the next step of the synthesis in which SnS₂ nanoparticles were synthesized in the presence of the aforementioned carbon nitride. Tin disulfide nanoparticles were synthesized by heating a solution of the Sn precursor, thioacetamide, and g-C₃N₄ for 12 h at 180 °C. The obtained product was then washed with deionized water and ethanol.

In a different work, Zhang et al. [42] have obtained MIL-53(Fe) hybrid magnetic composites (MHMCs), which are MOF-based heterostructures and which comprises iron terephthalate metal–organic frameworks (MOF) (MIL-53(Fe)) and Fe₃O₄ nanospheres. Such composites were synthesized by a simple solvothermal procedure in which a solution of Fe₃O₄ nanospheres, iron precursor, and terephthalic acid in DMF (dimethylformamide) was heated in a Teflon-lined autoclave for 12 h at 150 °C. After the heat treatment, the products were collected by centrifugation, cleaned with distilled water by suspending them in said water overnight, and then dried under vacuum at 60 °C for 24 h.

Other structures synthesized by the solvothermal approach include the following: nanosheet-assembled BiOCl/ZnO nanoflowers where ZnO nanoparticles were anchored on a BiOCl flower-like nanostructure [43]; β-Bi₂O₃/(BiO)₄(OH)₂CO₃ composites, which are reported to exhibit great photocatalytic activity for the degradation of Rhodamine B, Congo red, and tetracycline hydrochloride [39]; and g-C₃N₄/TiO₂ hybrid photocatalysts [44]. The said titanium dioxide hybrid catalyst, as a result of the addition of graphitic carbon nitride and the use of a solvothermal approach, exhibits higher photocatalytic activity under solar irradiation compared to the bare TiO₂.

The summary of all solvothermal methods can be seen in the table below (Table 3).

Table 3. Summary of the presented solvothermal synthesis methods.

Material	Method	Precursors	Key Steps	Special Features
β-Bi2O3/ (BiO)4(OH)2CO3 [39]	Solvothermal	NaBiO3·2H2O	Solvothermal treatment at 160 °C	Enhanced sunlight-driven photocatalysis by improved charge separation
g-C3N4/SnS2 [41]	Thermal polymerization + solvothermal	Melamine, SnCl4·5H2O, Thioacetamide	Annealing of melamine, then solvothermal treatment at 180 °C	Synergistic p-n junction effect for efficient charge separation
BiOCl/ZnO [43]	Facile solvothermal	Zn(Ac)2·2H2O, Bi(NO3)3·5H2O	Solvothermal treatment at 160 °C	Effective charge carrier separation, enhanced UV photocatalytic efficiency
MIL–53(Fe) hybrid magnetic composites [42]	One-pot solvothermal	FeCl3·6H2O, Terephthalic acid	Solvothermal treatment at 150 °C	Multifunctional MOF-based photocatalyst with steady visible-light performance
g-C3N4/TiO2 [44]	Solvothermal	Melamine, TiO2	Solvothermal treatment at 160 °C	Broadened activation spectrum, improved visible-light absorption

2.4. Ultrasound-Assisted Methods

Sonochemistry can be shortly described as the use of ultrasound for the conduction of or enhancement in chemical reactions (Figure 6). The main driving force behind this enhancement is the phenomenon called acoustic cavitation. Under ultrasound irradiation, pre-existing gas microbubbles present in the solvent used as a reaction medium begin to oscillate, which causes them to grow until they attain a certain critical size. After reaching said size, they begin to implode. In this process, an extreme environment is created locally within which the temperature and pressure can increase up to 5000 K and 150 MPa. To generate the ultrasounds, a variety of sources can be used, such as ultrasound cleaners or probes. Typical sonochemical setups are, in turn, often quite simple, both in deployment and execution, and may be easily combined with other synthetic methods.

Figure 6. Schematic presentation of the sonochemical synthesis process.

Such an approach can be found in the work of A. Moradi Golsheikh et al. [45], in which the authors have synthesized reduced graphene oxide (rGO) sheets decorated with ZnS nanospheres. The GO was obtained by a simplified Hummers’ method and then stirred for 16 h in the presence of an aqueous solution of zinc acetate dihydrate to ensure the adsorption of zinc ions to the surface of the GO sheets. After that, thioacetamide was added to said mixture, and this solution was then sonicated for 15 min using an ultrasonic horn. In this process, a ZnS/rGO nanocomposite was obtained after centrifugation and washed with distilled water and ethanol several times. The authors claim that such an obtained product prevents the agglomeration of ZnS nanoparticles present, which, in turn, allows for smaller nanoparticles to grow, increasing the active surface area compared to the bare ZnS.

Another photocatalyst that was prepared by a sonochemical approach is a composite of ZnO and Bi₂O₃ nanoparticles [46]. Here, the authors have mixed Bi₂O₃ with a 0.4 M mixture of ZnCl₂, and precipitated it with ammonia water until it reaches pH = 11. The collected and dried precipitate was then subjected to ultrasonic irradiation in absolute ethanol. After that, the obtained catalyst was calcined at 500 °C for 4 h.

Other examples of heterostructures with potential photocatalytic applications that were synthesized with the use of ultrasound include the following: the CdS/COF (covalent organic frameworks) composite [47] and Ag/ZnO composite [48]. The cadmium sulfide and COF composite was prepared by irradiating a mixture of cadmium acetate dihydrate, thioacetamide, and the previously synthesized COFs with ultrasound for 1 h. Such an obtained product was then dried in an oven for 12 h. This simple procedure enabled the synthesis of a catalyst with a high photodegradation efficiency towards bisphenol A (BPA) under UV–Vis irradiation. The aforementioned process also occurred faster (with a shorter reaction time) compared to other catalysts referenced by the authors [47].

A summary of all sonochemical methods is presented in the table below (Table 4).

Table 4. Summary of the presented sonochemical synthesis methods.

Material	Method	Precursor	Key Steps	Photo-Catalytic Application	Key Findings
ZnO/Bi2O3 [46]	Co-precipitation + sonication	Bi(NO3)3 ZnCl2	Precipitation with NH4OH, then ultrasonic treatment in ethanol and calcination	Diesel de-sulfurization (thiophene model)	Up to 95.7% sulfur removal; enhanced surface area and charge separation
Ag/ZnO [48]	Sonochemical deposition	Zn(NO3)2 · 6H2O AgNO3	ZnO by precipitation, then ultrasonic treatment in the presence of AgNO3	Degradation of methylene blue	Improved UV light photodegradation efficiency compared to bare ZnO
CdS/COF [47]	Ultrasound-assisted synthesis	(CH3COO)2Cd·2H2O Thioacetamide COF	COF mechano-chemically, then ultrasonic treatment in the presence of CdS precursor mixture	Degradation of bisphenol-A	0.5 wt% COF in composite showed ~85.7% BPA removal in 3 h; increased surface area (up to 600 m2/g) and active species generation
ZnS/rGO Nano-composite [45]	Hummers’ (rGO) Sonochemical (nano-composite)	Zn(CH3COO)2	GO dispersion, then sonochemical treatment to grow ZnS on GO	Degradation of methylene blue	Higher surface area, good photocatalytic performance

2.5. Other Methods of Synthesis

Besides the four methods presented earlier, there are others worth mentioning, such as co-precipitation, microwave-assisted, and seed-mediated growth. The first method, co-precipitation, was employed for the synthesis of cube-shaped cobalt-doped zinc oxide nanoparticles (Co@ZnO) [49], strontium oxide and zinc oxide nanocomposites (SrO/ZnO) [50], and flake-like copper manganite (CuMn₂O₄) [51] nanoparticles. This type of synthesis is typically conducted as follows (as exemplified by the synthesis of SrO/ZnO nanocomposites). A strontium precursor and a zinc precursor were mixed in 100 mL of ethanol under continuous stirring. Then, a 10 mL aqueous solution of sodium hydroxide was added dropwise to the previously obtained mixture and stirred for 2 h. In this way, a white precipitate was obtained and washed with deionized water and absolute ethanol. It was then dried at 60 °C for 1 h and annealed at 800 °C for 2 h. A schematic representation of this procedure is shown in Figure 7.

Figure 7. Schematic of wet chemical method followed to synthesize SrO/ZnO nanocomposites. Reproduced from ref. [50] under the Creative Commons license.

An interesting modification to this process can be found in the work of Asmaa I. Meky et al. [49], in which the authors have used red algae (Pterocladia Capillacea) extract as a reducing, stabilizing, and capping agent due to it containing many different biomolecules that can perform the role of such agents. SEM images of nanoparticles obtained in this way are shown in Figure 8.

Figure 8. SEM image of Co@ZnO NPs (nanoparticles) synthesized with the use of Pterocladia Capillacea. Reproduced from ref. [49] under the Creative Commons license.

The seed-mediated growth method allows for the controlled growth of new materials onto existing, smaller nanoparticles. It can be used for the synthesis of both simple and complex structures, such as gold-doped copper oxide Core–Shell Mesoporous Nanospheres (Au@Cu_xO) [52], heterostructured Cu_1.94S-MS (M = Zn, Cd, Mn) [53], and alloyed CuNS₂ (N = In, Ga) nanocrystals [53]. As the name suggests, this procedure involves the use of previously synthesized seed particles, such as gold nanoparticles (in the case of Au@Cu_xO synthesis) or Cu_1.94S nanoparticles (in the case of the synthesis of Cu-M-S and Cu-N-S₂ systems), which are mixed with an appropriate metal precursor solution (sometimes in the presence of additional reagents such as n-oleyl-1,3-propanediamine) and then heated to obtain the desired nanostructures. SEM images of Au@Cu_xO nanospheres obtained via the seed-mediated growth method are shown in Figure 9.

Figure 9. The growth process for Au@CuxO nanostructures. (a–e) The SEM images and (f) the corresponding XRD patterns of the products obtained after (a) 5 min, (b) 15 min, (c) 30 min, (d) 45 min, and (e) 60 min, respectively. Reproduced from ref. [52] under the Creative Commons license.

Microwaved-assisted processes are often used to accelerate conventional synthesis methods such as sol–gel or solvothermal synthesis. This is achieved by replacing the standard heating source (e.g., oil bath) with microwave irradiation. This results in a decrease in both the time and energy required to carry out these processes while maintaining the same physical–chemical properties of conventionally obtained nanostructures [54]. Using this method, various hetero-nanostructures were obtained, including rGO-ZnO/CuO nanocomposites [55], Fe₃O₄/SiO₂/TiO₂ core-2-layer-shell nanocomposites [56], and bismuth vanadate nanoflowers decorated with gold nanoparticles (Au-BiVO₄NF) [54]. SEM images of synthesized Fe₃O₄/SiO₂/TiO₂ core-2-layer-shell nanocomposites are shown in Figure 10.

Figure 10. SEM images of Fe₃O₄/SiO₂/TiO₂ nanocomposites synthesized by the microwave-assisted method at (g) 100 °C, (h) 150 °C, and (i) 200 °C. Reproduced from ref. [56] under the Creative Commons license.

The summary of all methods presented in this subsection is provided in Table 5.

Table 5. Summary of the presented synthesis methods.

Material	Method	Precursor	Key Steps	Special Features
Co@ZnO nano-particles [49]	Co-precipitation	Zn(CH3CO2)2·2H2O, Co(CH3CO2)2·4H2O	Co-precipitation using P. Capillacea algae extract; calcination for 3 h at 500 °C	100% degradation of ciprofloxacin (10 mg/L) under visible light within 15 min; good recyclability
SrO/ZnO nano-composites [50]	Co-precipitation	Sr(CH3COO)2·5H2O Zn(CH3COO)2·2H2O	Co-precipitation; calcination for 2 h at 800 °C	Efficient malachite green degradation under sunlight
CuMn2O4 nano-particles [51]	Co-precipitation	MnSO4·H2O, CuCl2·2H2O	Co-precipitation from copper and manganese salts; calcination for 2 h at 600 °C	Efficient (86% and 92%) degradation of methylene blue and malachite green, respectively
Au@CuxO nano-spheres [52]	Seed-mediated growth	HAuCl4·3H2O Cu(acac)2	Gold seed NPs synthesis, then addition of copper precursor	Efficient degradation of methyl orange under visible light (97.8% within 120 min); tunable size
Cu1.94S-MS (M = Zn, Cd, Mn) [53]	Seed-mediated growth	Cu(acac)2 Zn(acac)2 Cd(OAc)2 Mn(OAc)2	Cu1.94S seeds by thermolysis (with purification), then addition of corresponding metal precursor	Enhanced H2 evolution; all heterostructured NCs outperform bare Cu1.94S
CuNS2 (N = In, Ga) [53]	Seed-mediated growth	Cu(acac)2 Ga(acac)3 In(acac)3	Cu1.94S seeds by thermolysis (without purification), then addition of corresponding metal precursor	Enhanced H2 evolution; CuGaS2 nanocrystals outperform bare Cu1.94S
Au-BiVO4NF [54]	Microwave-assisted	HAuCl4·4H2O Bi(NO3)3·5H2O NH4VO3	10 min of microwave irradiation: 5 min for gold precursor solution, and then another 5 min after the addition of previously prepared BiVO4 solution	Reduced synthesis time compared to conventional heating sources (from 4 h to 10 min); efficient (95%) degradation of methylene blue under UV–Visible light irradiation
rGO-ZnO/CuO nano-composites [55]	Microwave-assisted	Graphite powder Zn(CH3COO)2·2H2OCu(NO3)2·3H2O	GO by improved Hammer’s method, reduction with ascorbic acid; reduced GO used with other precursors to obtain desired nanocomposites	Enhanced Methylene blue and 4-nitrophenol degradation
Fe3O4/SiO2/TiO2 nano-composites [56]	Microwave-assisted	FeCl3·6H2O FeSO4·7H2O Si(OC2H5)4 Ti[OCH(CH3)2]4	Magnetic Fe3O4 NPs synthesis, coating with SiO2 and TiO2 sols	Efficient (94% after 120 min) ciprofloxacin antibiotic removal; good reusability and stability

3. Photocatalytic Applications of Nanometric Heterostructures

3.1. Mechanisms of Action of Heterostructures

The main problems in photocatalysis are the recombination of photo-generated electrons and holes, the insufficient oxidizing and reducing potentials of these carriers, and the too-low absorption of light [57]. The recombination may take place in the bulk of the photocatalyst as well as on its surface. The oxidizing/reducing properties of the photocatalyst are related to the energy levels of the conduction and valence bands (CB and VB, respectively) (see Figure 11). The differences in energy of these bands increase with the increase in the energy band gap of the photocatalyst. However, at the same time, the ability to absorb more light is lowered.

Figure 11. Various physical and chemical phenomena occurring during the action of light on the standalone semiconducting photocatalyst: 1—excitation, 2—charge carriers’ separation and transfer, and 3—recombination of charge carriers. Reproduced from ref. [12] under the Creative Commons license without modifications.

To address these issues, a formation of heterostructures is recently considered [57]. Such heterostructures are formed between two semiconducting photocatalysts. There are many types of heterojunctions, among which the most popular are type-II, Z-scheme, and S-scheme [57]. A schematic representation of various types of heterostructures is presented in Figure 12. The formation of heterostructure may be evidenced by techniques such as transient photocurrent spectroscopy (by an increase in the photocurrent), photoluminescence emission spectroscopy (by a decrease in the luminescence peak), and electrochemical impedance spectroscopy (by an increase in the photocurrent), which detects the diminished charge carriers’ recombination.

Figure 12. Schematic representation of distinct types of heterostructures (CB—conduction band, VB—valence band, PC A—photocatalyst A, PC B—photocatalyst B, IEF—intrinsic electric field, OP—oxidative potential, and RP—reductive potential): (a) type-II heterostructure, (b) Z-scheme heterostructure, and (c) S-scheme heterostructure. Reproduced from ref. [12] under the Creative Commons license without modifications.

In the heterostructure of type-II (see Figure 12a), the photogenerated electrons from the CB of the first photocatalyst (blue color in Figure 12a) travel to the CB (with lower energy) of the second photocatalyst (green color in Figure 12a), while the photogenerated holes from the VB of the second photocatalyst travel to the VB (also with lower energy) of the first photocatalyst.

In effect, electrons gather on the CB of the second photocatalyst, while the holes gather on the VB of the first photocatalyst [58,59]. Although the recombination of carriers is lowered, their redox potentials are also lowered (similarly to the case of the type-I heterojunction), which may negatively influence the ability to conduct various reactions. The mechanisms underlying the type-II heterojunction are questionable from the dynamical point of view: the repulsive forces between electrons in the CB of the second photocatalyst and between holes in the VB of the first photocatalyst should block the transfer of carriers, together with the action of attractive forces between electrons and holes lying in the respective conduction and valence bands [57].

The Z-scheme heterojunction (see Figure 12b) is designed to separate the carriers and preserve the redox abilities at the same time [57,60,61]. It was inspired by natural photosynthesis [62]. Similarly to the type-II mechanism, the constituent semiconductors have staggered configurations of conduction and valence bands [57,60,61]. The mechanism is as follows: photogenerated holes from the VB of the first photocatalyst (blue color in Figure 12b) react with the donor of electrons (D) and form the acceptor of electrons (A), which then react with the photogenerated electrons from the CB of the second photocatalyst (green color in Figure 12b), recovering the donor [57,60,61]. Due to that, high-energy electrons in the CB of the first photocatalyst and high-energy holes in the VB of the second photocatalyst are preserved [57,60,61]. Such a result is equivalent to the separation of photogenerated carriers, while, at the same time, the redox ability is conserved. Examples of D/A pairs include the following redox systems: Fe³⁺/Fe²⁺, IO₃⁻/I⁻, and I₃⁻/I⁻ [57,60,61]. Thus, such heterostructures are coupled with a solution phase, which narrows the scope of their applications and constitutes one of several disadvantages. Another problem is that the electrons in the CB of the first photocatalyst have greater energy than the electrons in the CB of the second photocatalyst, and, consequently, they are more likely to react with the acceptor. Similar behavior is observed for the holes. These side and unwanted reactions dominate the desired mechanism [57,60,61]. The next disadvantage is linked with the ability of some ions—incorporated in the D/A pairs—to absorb light competitively to photocatalysts. Yet another disadvantage corresponds to the sensitivity of some D/A pairs to pH variations. For example, the Fe³⁺/Fe²⁺ pair does not work well under alkaline conditions due to the precipitation of hydroxides or oxides. On the other hand, the IO₃⁻/I⁻ pair does not work well under strong acidic conditions due to the comproportionation reaction.

The disadvantages of classical-Z-scheme-type heterostructures are resolved in the so-called all-solid-state (or indirect) Z-scheme heterojunctions, which were proposed quite recently, in 2006 by Tada and cooperators [57,63]. In this type of heterojunction, the intermediate redox pair is replaced by a solid conductor that links two solid photocatalysts [63]. It is postulated that the separation of photogenerated carriers is enhanced due to the shortened length of charge transfer [63]. The linking solid conductor allows for the transfer of electrons from the CB in the second photocatalyst to the VB of the first photocatalyst. However, the synthesis process of such a heterostructure may be problematic due to the imprecise control of metal deposition (i.e., metal particles may be randomly distributed on the surfaces of constituent photocatalysts). Moreover, the metal particles may compete with photocatalysts in the process of light absorption. Another problem is that, from the thermodynamical point of view, the flow of carriers should be opposite to the postulated one—that is, the electrons from the CB of the first photocatalyst should travel to the holes in the VB of the second photocatalyst [57]. Additionally, the linking metal forms a Schottky-type barrier with neighboring semiconductors, which blocks the flow of electrons [57].

There were also reports on the so-called direct Z-scheme heterostructures (Figure 12b), composed of only two photocatalysts (similarly to the type-II heterostructure), in which an internal electric field is postulated to exist, causing the desired charge transfer [57]. Examples of such heterostructures are ZnO/CdS and g-C₃N₄/TiO₂ heterojunctions [64,65]. Experiments showed the improved performance of these heterostructures; the mechanism of their structure is still disputable [57,64,65].

The next generation of heterostructures acts in accordance with the S-scheme (or Step-scheme) mechanism [57]. The S-scheme-type heterojunction (see Figure 12c) consists of two semiconductors, one of which is the oxidative type (green color in Figure 12c) and the second is the reductive type (blue color in Figure 12c). The oxidation photocatalyst has relatively lower energies for both CB and VB, while the reduction photocatalyst has relatively higher energies of these bands [57]. Thus, in oxidation photocatalyst, holes are relatively more reactive than electrons, which contributes to the oxidative properties. Similarly, in a reduction photocatalyst, electrons are relatively more reactive than holes, which contributes to the reductive properties. When both photocatalysts are combined in a heterojunction, the interface is polarized—on the part of the oxidation photocatalyst, there exists a negative charge due to the donation of electrons from the CB of the reduction photocatalyst, while, on the part of the reduction photocatalyst, there exists a positive charge [57]. As a result, an internal electric field is created, which causes the movement of relatively low-energy electrons from the CB of the oxidation photocatalyst to the relatively low-energy holes in the VB of the reduction photocatalyst, which allows for their elimination and the preservation of relatively high-reactive electrons in the reductive photocatalyst and holes in the oxidation photocatalyst [57].

3.2. Examples of Applications of Nanometric Heterostructures

By combining the noble metal Au, which presents plasmonic properties, with a porous metal–organic framework and BiOBr (BOB) photocatalyst, Li and cooperators achieved a ternary heterostructure which was then used for the photocatalytic degradation of Cr(VI) and antibiotic norfloxacin (NOR)—both separately and simultaneously [66]. The heterostructure was derived using a combination of the solvothermal method (the synthesis of metal–organic framework MIL-101(Fe) from terephthalic aldehyde and FeCl₃ in dimethylformamide and the subsequent solvothermal synthesis of BiOBr from Bi(NO₃)₃·5H₂O with the addition of the previously derived MIL-101(Fe)) and photoreduction (decoration of MIL-101(Fe)/BiOBr heterostructure with Au nanoparticles formed by the reduction of HAuCl₄ in methanol/H₂O solution under xenon lamp) [66]. In that way, the authors were able to produce heterostructures with a varied Au content (0.5, 1, and 3 wt%). The scheme of preparation, as well as the basic characterizations, is presented in Figure 13.

Figure 13. (a) Schematic presentation of synthesis of ternary plasmonic S-scheme Au/MIL-101(Fe)/BiOBr heterostructure; (b) SEM image of standalone BiOBr (BOB); (c–e) SEM images of Au/MIL-101(Fe)/BiOBr heterostructure; (f,g) TEM images of this heterostructure; (h) HR-TEM image of this heterostructure; and (i) EDX and elemental mapping images of this heterostructure. Reproduced under the Creative Commons license from ref. [66] without modifications.

The photocatalytic degradation experiments were performed by simply illuminating the suspension of the heterostructure photocatalyst in a medium containing degraded contaminants—norfloxacin and Cr(VI)—whose concentrations were monitored using UV–Vis spectrophotometry and the diphenylcarbazide method, respectively [66]. The incorporation of a porous MIL-101(Fe) metal–organic framework into the heterostructure caused the increased adsorption of Cr(VI) species; thus, the degree of Cr(VI) photoreduction was greater in the binary MIL-101(Fe)/BiOBr or ternary Au/MIL-101(Fe)/BiOBr heterostructures than in the case of the standalone BiOBr or MIL-101(Fe) [66]. Indeed, the reported values of the specific surface area were as follows: 362.17 m²/g for MIL-101(Fe), 10.2 m²/g for BiOBr, and 241.13 m²/g for the ternary heterostructure [66]. The binary heterostructure contains an S-scheme heterojunction, which suppresses the recombination of photo-generated carriers; thus, it performed better than standalone components. The photocatalytic performance was additionally improved by taking advantage of the localized surface plasmon resonance (LSPR) effect exhibited by gold nanoparticles. However, the Au content had to be optimized because—despite the beneficial LSPR effect—a too-large number of covering Au nanoparticles could lead to an excessive covering of the reactive sites of the photocatalyst. In this study, it was found that the optimal Au content was 1 wt% [66]. Similar photocatalytic experiments were conducted to destruct the molecules of antibiotic norfloxacin, and the results were also similar—the ternary heterostructure showed the best activity. In the case of the simultaneous removal of norfloxacin and Cr(VI), a synergy effect was observed because the first one acted as a quencher of photo-generated positive holes (h⁺), which contributed to the decreased probability of the recombination of carriers—so the photo-generated electrons could react with Cr(VI) species instead of recombining [66].

Li and cooperators derived a binary heterostructure hydrothermally with an S-scheme heterojunction consisting of Mn_0.5Cd_0.5S and Bi₂MoO₆ compounds [67]. The first compound, which was present in the form of nanoparticles, acted as a reduction center, while the latter, which, in turn, was present in the form of microparticles, acted as an oxidation center [67]. This Mn_0.5Cd_0.5S/Bi₂MoO₆ binary heterostructure was utilized as a photocatalyst in the degradation of tetracycline under simple irradiation with a xenon lamp during which the catalyst was suspended in a tetracycline solution [67]. The XPS (X-ray photoelectron spectroscopy) results evidenced the formation of the Mo-S bond in the heterostructure, which was indicated by shifts of the binding energies of Mn, Cd, and S elements toward a higher energy and of Bi and Mo toward lower energies [67]. This Mo-S bond improved the separation of photo-generated carriers—the removal of tetracycline after 60 min of irradiation was reported as 56.1% for standalone Mn_0.5Cd_0.5S, 44.2% for standalone Bi₂MoO₆, and 90.7% for the heterostructure with an optimized Mn_0.5Cd_0.5S content [67]. In this case, an optimal content of sulfide was also observed—after exceeding the optimal value, a decrease in photocatalytic performance was observed. It is noteworthy that the heterostructure outperformed its components despite a lower value of the specific surface area, which was 47.22 m²/g for the heterostructure, 53.39 m²/g for Bi₂MoO₆, and 21.98 m²/g for Mn_0.5Cd_0.5S [67]. Indeed, the heterostructure showed a much greater photocurrent density than the standalone components under the electrochemical impedance spectroscopy investigation [67]. The incorporation of Mn_0.5Cd_0.5S nanoparticles also improved the wettability of the heterostructure, which, additionally, contributed to the enhanced photocatalytic performance [67].

A hydrothermal route was also applied by Liang et al. to synthesize a Z-scheme heterostructure photocatalyst consisting of acidified graphitic carbon nitrogen (ag-C₃N₄), carbon quantum dots (CQDs), and CdIn₂S₄ microparticles [68]. Such a ternary ag-C₃N₄/CQDs/CdIn₂S₄ heterostructure was used in the photocatalytic degradation of ibuprofen. The experiments were conducted similarly to two previously described cases—under simulated visible light generated with a xenon lamp with a cutoff filter (>420 nm) [68]. The catalysts worked in a suspension. In this study, the improved photocatalytic activity was linked with the ability of carbon quantum dots to transfer the electrons from the conduction band of CdIn₂S₄ to holes in the valence band of ag-C₃N₄, allowing for their recombination [68]. The incorporated CQDs also improved the absorption of visible light.

Akbarzadeh and cooperators synthesized a tertiary heterostructure g-C₃N₄/Ag/AgCl/BiVO₄ using the hydrothermal method [69]. It was applied in the photocatalytic degradation of ibuprofen, reaching 94.7% of degradation after 1 h [69]. Despite the relatively low specific surface area (3.67 m²/g) of the g-C₃N₄/Ag/AgCl/BiVO₄ heterostructure, its activity is improved due to the increased absorption of visible light and the suppressed recombination of photo-generated carriers.

Yet another study on photocatalytic degradation of ibuprofen was performed by Liu and coauthors [70]. They applied a two-dimensional g-C₃N₄/Bi₂WO₆/rGO heterostructure, which allowed for the achievement of c.a. 93% and 98.6% degradation of the target compound under visible and sunlight conditions, respectively [70]. In this case, the improved performance of the heterostructure is attributed to the higher crystallization degree, relatively large specific surface area, and improved charge transfer [70]. This heterostructure exhibited a Z-scheme behavior—the photogenerated electrons were transported from the conduction band of Bi₂WO₆ via reduced graphene oxide (rGO) to the positive holes in the valence band of g-C₃N₄ [70].

A g-C₃N₄/Bi₂WO₆/NCQs heterostructure is another example of a 2D Z-scheme photocatalyst that was applied in the photocatalytic degradation of two distinct model pollutants: rhodamine B and tetracycline under visible light generated by an 800 W xenon lamp with a cutoff filter (>420 nm) [71]. In such a heterostructure, g-C₃N₄ and Bi₂WO₆ constituted two-dimensional nanostructures, and nitrogen-doped carbon quantum dots (NCQs) acted as a linkage between them, allowing the Z-scheme behavior [71].

Chen et al. applied a hierarchical g-C₃N₄/CdIn₂S₄ heterostructure for two distinct photocatalytic applications: H₂ photoproduction due to water splitting and 4-nitroaniline selective photoreduction to 4-phenylenediamine [72]. However, the photocatalytic setup was quite similar to the previously described studies—the photocatalyst worked in suspension, and a xenon lamp with a 400 nm cutoff filter was used as a source of visible light [72]. The heterostructure had hierarchical construction in which CdIn₂S₄ nanosheets were wrapped by mesoporous g-C₃N₄ nanosheets during the hydrothermal synthesis [72]. The heterostructure showed improved photocatalytic activity due to the formation of an interface between its components that improved the separation of photogenerated holes and electrons [72]. This was evidenced by the photoluminescence emission spectra in which the emission peak originates from the unwanted recombination of carriers. In the case of a constructed heterostructure, the authors observed a significant lowering of the emission peak at 438 nm corresponding to the band gap of pure g-C₃N₄, which is 2.83 eV [72]. The improvement in photocatalytic activity was observed despite the fact that the wrapping by g-C₃N₄ diminishes the absorption of light in comparison with pristine CdIn₂S₄ [72]. The increased separation of photogenerated carriers was further evidenced by the transient photocurrent responses and electrochemical impedance spectroscopy investigations [72]. Additionally, the wrapping of CdIn₂S₄ was further beneficial due to the increase in its photostability [72].

Rao et al. used another heterostructure based on CdIn₂S₄ for the examination of H₂ photogeneration, that of CoMoS₄/CdIn₂S₄ composition [73]. Similarly to the previously described studies, the construction of such a heterostructure allowed for the enhanced separation of photo-generated carriers, as evidenced by transient photocurrent spectroscopy, photoluminescence emission spectroscopy, and electrochemical impedance spectroscopy measurements [73]. Additionally, the heterostructure exhibited an improved absorption of light in comparison to pristine CdIn₂S₄ [73]. It is noteworthy that, for the heterostructure with an optimized CoMoS₄ content (5%), the H₂ photogeneration rate was 8.4-fold greater than in the case of pristine CdIn₂S₄ [73].

Dhandole and cooperators constructed a heterostructured photoanode consisting of CdIn₂S₄ flower-like nanostructures, TiO₂ nanorods, and fluorine-doped tin oxide (FTO) [74]. The heterostructure was derived in a two-step hydrothermal approach in which, firstly, a layer of TiO₂ nanorods was deposited, and, secondly, a layer of CdIn₂S₄ was added [74]. The constructed heterostructured photoanode showed an enlarged photocurrent density (three times higher than that of the TiO₂/FTO heterostructured anode) due to, again, the improved separation of the photogenerated carriers and improved absorption of light [74]. Interestingly, the formation of a heterostructured anode improved the photostability of the chalcogenide CdIn₂S₄ material [74].

Many other studies on the photocatalytic applications of various heterostructures were also performed. However, the typical mechanisms underlying the improved performance of the studied heterostructures were similar to those presented in the above examples. Among the more interesting features that deserve outlining, the utilization of hollow spheres allowing for multiple reflections of light may be mentioned [75]. Other applications of heterostructures include the following: the photocatalytic reduction of Cr(VI) by the ZnIn₂S₄/CdS nanocomposite [76], H₂ photogeneration and methyl orange degradation by a core-shell CdIn₂S₄/Ta₃N₅ heterostructure [77], the TiO₂/g-C₃N₄/Bi₂WO₆ ternary Z-scheme heterostructure for the degradation of rhodamine B, methylene blue, and phenol under visible light [78], the multi-hierarchical TiO₂/g-C₃N₄/Ag-AgBr dual Z-scheme hetereostructure for H₂ generation and methyl orange degradation [79], the Z-scheme photocatalyst g-C₃N₄/Pt/macroporous-(TiO₂@carbon) for the selective conversion of CO₂ with H₂O to CH₄ under visible light [80], and the Bi₂WO₆/Ni-MoS₂ heterogeneous photocatalyst for the photocatalytic degradation of rhodamine B [81], among others. The reader is referred to Table 6 for a short summary of the recent studies on the photocatalytic applications of nanometric heterostructures.

Table 6. A short summary of recent studies on photocatalytic applications of nanometric heterostructures.

Heterostructure	Type of Heterostructure	Photocatalytic Application	Degraded Compounds or Performed Reactions	Experimental Details	Key Findings	Reference
Au/MIL-101(Fe)/BiOBr	S-scheme	Photocatalytic degradation	Cr(VI) norfloxacin	Simultaneous removal under visible light (300 W xenon lamp)	53.3-times-greater Cr(VI) removal rate in comparison with standalone BiOBr 2-times-greater norfloxacin removal rate in comparison with standalone BiOBr Removal of Cr(VI) was improved in the presence of norfloxacin due to improved usage of photo-generated carriers	[66]
Mn0.5Cd0.5S/Bi2MoO6	S-scheme	Photocatalytic degradation	Tetracycline norfloxacin enrofloxacin oxytetracycline	300 W xenon lamp	Mo-S bond in the heterostructure improves separation of photo-induced carriers and inhibits photocorrosion of photocatalyst	[67]
ag-C3N4/CQDs/CdIn2S4	Z-scheme	Photocatalytic degradation	Ibuprofen	Visible light (300 W xenon lamp)	Carbon quantum dots connect acidified g-C3N4 with CdIn2S4 and improve adsorption of visible light Carbon quantum dots transport electrons from conduction band of CdIn2S4 to recombine with holes in valence band of ag-C3N4	[68]
g-C3N4/Ag/AgCl/BiVO4		Photocatalytic degradation	Ibuprofen	Visible light	Despite relatively low specific surface area (3.67 m2/g) of g-C3N4/Ag/AgCl/BiVO4 heterostructure, its activity is improved due to increased absorption of visible light and suppressed recombination of photo-generated carriers	[69]
g-C3N4/Bi2WO6/rGO	2D Z-scheme	Photocatalytic degradation	Ibuprofen	Visible light and sun light	Improved performance of heterostructure is attributed to the higher crystallization degree, relatively large specific surface area, and improved charge transfer	[70]
g-C3N4/Bi2WO6/NCQs	2D Z-scheme	Photocatalytic degradation	Rhodamine B tetracycline	Visible light (800 W xenon lamp)	High separation efficiency of photo-induced carriers stands for the improved photocatalytic efficiency of the heterostructure	[71]
g-C3N4/CdIn2S4	Hierarchical	Photocatalytic water splitting and photo-redox reaction	H2 photogeneration 4-nitroaniline photoreduction	Visible light (300 W xenon lamp with 400 nm cutoff filter)	Interfacial contact between heterostructure components improves the separation of photo-generated carriers Wrapping of CdIn2S4 by g-C3N4 improves the photostability	[72]
CoMoS4/CdIn2S4	Binary	Photocatalytic water splitting	H2 photogeneration	Simulated sunlight (300 W xenon lamp with 420 nm cutoff filter)	Combination of the components in heterostructure improved the absorption of light For heterostructure with optimized CoMoS4 content (5%), the H2 photogeneration rate was 8.4-fold greater than that for pristine CdIn2S4	[73]
CdIn2S4/TiO2/FTO	Ternary	Photoanode	Photocurrent generation	Three-electrode system: heterostructured photoanode as working electrode, Ag/AgCl as the reference electrode, Pt wire as the counter electrode; illumination by simulated sunlight	The constructed heterostructured photoanode (CdIn2S4/TiO2/FTO) exhibited photocurrent density three times higher than that of binary TiO2/FTO heterostructured anode due to improved separation of photogenerated carriers and improved absorption of light	[74]
ZnIn2S4/CdIn2S4/CdS	Ternary hollow	Photoelectrochemistry	Determination of carbohydrate antigen 19-9	Three-electrode system with ZnIn2S4/CdIn2S4/CdS/ITO (indium-tin oxide) anode as working electrode illuminated by a light source generating electromagnetic wave of 430 nm with 180 W/m2 intensity	Application of hollow spheres improved the utilization of light by multiple reflections (not observed in the not-hollow version) and accelerates the charge carriers’ transport	[75]

4. Conclusions

The studies of nanometric heterostructures and their applications, including photocatalysis, are constantly growing. Due to their varied and extraordinary properties, such heterostructures have found many applications in photocatalysis, including the following: the photocatalytic degradation of distinct organic pollutants, the photoreduction of inorganic and organic toxic compounds, CO₂ to CH₄ conversion, the construction of photoanodes for biosensing, H₂ evolution during water splitting, and others. The most important factors contributing to the improvement in photocatalytic activity include the following: enlarged specific surface areas, an enhanced light absorption ability, increased photogenerated carriers’ transport (i.e., inhibited recombination), and improved photostability. It is worth noting that the postulated mechanisms of action of various types of heterostructures (S-scheme, Z-scheme, etc.) are still disputable, and significant scientific effort should be performed to understand the principles of action of heterostructures. As well as expanding applications, the studies of the methods of the synthesis of the demanded heterostructures are also increasing. The way of preparation is very important and may affect the properties of the heterostructured photocatalyst. Typical ways of synthesis studied so far include the following: solvo- and hydrothermal routes, the sol–gel method, ultrasound-assisted and/or microwave-assisted synthesis, co-precipitation, and seed-mediated growth. However, there is still a need for investigations of novel methods allowing for a reduction in environmental and financial costs, as well as for the achievement of the desired properties of heterostructures. The scalability and reproducibility of syntheses are also very important issues and need to be addressed in future investigations.