A Comprehensive Review on Aero-Materials: Present and Future Perspectives

Abstract

Recently, a new class of materials with very high porosity and ultra-lightweight, namely, semiconductor aero-materials, has attracted the attention of many researchers. Semiconductor aero-materials, due to their special properties, can be used in the development of devices applied in biomedical, electronics, optoelectronic, energy conversion and storage, sensors, biosensors, catalysis, automotive, and aeronautic industries. Although aero-materials and aerogels are similar, different methods of obtaining them are used. Aerogels are synthesized from organic, inorganic, or hybrid precursors, the main characteristic being that they are gel-like solids with a high air content (99.9%) in the structure. Thus, three-dimensional (3D) interconnected porous network chains are formed, resulting in light solid-state structures with very high porosity due to the large number of air pores in the network. On the other hand, to obtain aero-materials with controlled properties such as morphology, shape, or the formation of 3D hollow structures, sacrificial templates are used. Thus, sacrificial structures (which can be easily removed) can be obtained depending on the morphology of the 3D structure to be obtained. Therefore, this review paper offers a comprehensive coverage of the synthesis methods of different types of semiconductor aero-materials that use ZnO tetrapod, ZnO(T), as a sacrificial template, related to the present and future perspectives. These ZnO(T) sacrificial substrates offer several advantages, including diverse synthesis processes and easy removal methods that occur simultaneously with the growth of the desired aero-materials.

1. Introduction

The last several decades have seen an increase in demand for materials with innovative and enhanced properties, and advances in materials science have resulted in the discovery of novel materials. Among these, 3D structures have attracted particular attention and were intensively studied due to the extraordinary potential in terms of morpho-structural characteristics in numerous applications, e.g., energy storage [1,2,3,4], water treatment [5], oil–water separation [6], gas sensors [7], supercapacitors [8], biosensors [9] catalysts [10], and dye-sensitized solar cells [11]. The 3D materials can have different architectures such as foams, networks, woven fabrics, aerogels, hydrogels, sponges, and frameworks [12] and can be synthesized by several methods, e.g., spray drying method [13], solution phase route [14], self-assemblies [12], freeze-drying [15], and chemical vapor deposition [12].

Aerogels are porous solid materials with interconnected 3D networks, remarkably lightweight due to their high air content, exhibiting a low density of approximately 0.003 g cm⁻³ and a correspondingly large surface area. Generally, aerogels are defined as gels in which the liquid has been replaced by air [16]. They exist in a variety of shapes, sizes, and forms—including monoliths, powders, and films—and can be classified as aerogels, xerogels, cryogels, or hydrogels depending on their synthesis method. Aerogels can be inorganic, organic, or composite, depending on their chemical structure. Thus, inorganic precursors such as alkoxides or metal salts (silica, alumina, and carbon) are used to obtain inorganic aerogels. Organic precursors such as phenolic formaldehyde resin are used in the synthesis of organic aerogels, and for composite aerogels, both inorganic and organic precursors are used [17]. Figure 1a shows several examples of aerogels’ application, as well as different types of aerogels based on the precursors used in the synthesis process. Among conventional methods, the wet chemical technique is well-known and widely studied for the production of various aerogels, and the sol–gel method represents one of the most applied techniques for aerogel synthesis [18]. In Figure 1b, the main steps used in the sol–gel synthesis process are shown, starting with precursors’ mixing, hydrolysis, polycondensation, gelation, aging, and drying. The main parameters of the process are solution pH, precursor concentration, and time [19].

Table 1. Some related aero-materials based on synthesis methods and proposed applications.

Type of Material	Synthesis Method	Application	References
Aerographite	Chemical vapor deposition (CVD)	Optoelectronic technologies, ultra-lightweight, electrically conductive, electrode material/supercapacitor	[20,21,22,23]
Aerographite-GaN	Hybrid vapor-phase epitaxy (HVPE)	Electronic, photonic, and sensor applications	[24]
Aero-β-Ga2O3/ZnGa2O4 nanocomposite	HVPE	Electrochemical application	[25]
Aero-Ga2O3	HVPE	Photocatalytic Water purification	[26]
Aero-GaN	HVPE	Ultra-lightweight pressure sensor/portable electrical equipment	[27,28]
Aero-TiO2	Atomic layer deposition (ALD)	Photocatalytic degradation of drugs (tetracycline) Photocatalytic applications	[29,30]
Hybrid Aero-SiO2/rGO	Wet-chemical coating	Enhanced and rapid volumetric photothermal response	[31]
Aero-ZnS	HVPE	Micro-fluidic applications	[32,33]

presents some related aero-materials based on synthesis methods and application areas.

Recently, a new class of porous materials, called aero-materials, has attracted the attention of the scientific community because of their specific qualities. Due to their extraordinary properties, aero-materials have been intensively studied and used in various applications. Aero-materials are similar to aerogels in terms of structural properties but are obtained through different technologies. The principle of obtaining the aero-materials consists in using sacrificial templates on which the materials of interest are grown; subsequently, the templates are removed, and thus, highly porous materials with special properties are obtained. It should be noted that after removing the template, the obtained aero-material almost completely takes on the shape of the tetrapod rods. Thus, extremely porous and ultra-lightweight aero-materials are obtained. Figure 2 shows a schematic illustration for obtaining aero-materials using ZnO sacrificial templates.

Within this review paper, the recent developments about semiconductor aero-materials, a new class of ultralight and high porosity materials, with different applications in biomedical, electronic, sensors, energy conversion storage, and optoelectronics fields, were presented. A state-of-the-art approach was related to the morphology and shape of these special aero-materials with controlled properties grown on ZnO(T) sacrificial templates, which can be easily removed depending on the morphology of the 3D structure. The main objective of this research is to give a comprehensive coverage of these novel semiconductor materials, presenting the synthesis technologies and applications of ZnO(T) template, as well as the main types of aero-materials. Finally, a perspective covering directions and challenges related to the development and application of aero-materials for multiple applications is provided.

2. Semiconductor Aero-Materials

2.1. ZnO Tetrapods Synthesis Methods

ZnO, a semiconductor oxide having the direct and wide bandgap of 3.37 eV, with a hexagonal-wurtzite crystal structure, and the various defects in the crystal, which are usually undesirable [34], nevertheless make ZnO nanostructures considered a multitasking material and promising candidate for different applications such as electronics [35], lasers [36], optoelectronic [37], biomedicine [38], pro-ecological systems [39], sensors [40], convertors [41], photocatalysts [42], and energy generators [43]. There are many studies in the literature that present the production and application of the nanoscale ZnO in powder form [44,45,46,47]. The application of ZnO nanostructures with various shapes in different fields has attracted considerable attention in recent years. Thus, various synthesis methods have been used to obtain different morphologies of ZnO, such as nanowires [48,49], nanorods [50,51], nanoparticles [52,53], and tetrapods [54,55,56,57]. Since the 19th century, when the tetrapod shape was unique in coastal engineering, the unique 3D morphology has offered easy accessibility at the nanoscale level due to the forces applied to one arm, which is transferred to the others, offering high stability. The theoretical studies [58] provide specific insights about the mechanism of growth of tetrapods, and maybe some experimental demonstrations can confirm them. Subsequently, it was found that 3D nanostructures obtained by assembling nanoscale structures into complex 3D structures [14] offer much improved properties compared to 1D structures. Over the past 25 years, tetrapod-shaped ZnO nanostructures have been well-known to the research community, and even the growth mechanism has been discussed and studied, but a clear understanding of the processes is lacking [59,60,61].

Nowadays, several synthesis methods have been utilized for growing different metal oxide nano-microstructures. Hydrothermal synthesis, direct current (DC) thermal plasma technique, chemical vapor deposition, microwave chemistry, combustion synthesis, and flame-based synthesis have all been used to obtain metal oxide nanostructures, especially ZnO nano-tetrapods. Therefore, a scalable, cost-effective, and simple synthesis method capable of producing large quantities of various metal oxide tetrapods remains highly investigated for diverse applications (Table 2). 3D ZnO(T) exhibits very promising technological potential in terms of applications; thus, efficient methods for the growth of various types of tetrapod-based networks are demanded.

2.1.1. Combustion Method (CM)

CM has been widely researched for manufacturing different oxides, including those of titanium, silicon, aluminum, or zirconium. Notably, it has been demonstrated that when zinc is combusted at high temperature in the presence of air, the resulting ZnO particles exhibit a distinctive tetrapod shape. Similarly, L. Chen et al. developed a simple, innovative, and cost-effective combustion method known as the melting–combustion method (MCM), which eliminates the use of volatile salt precursor in the fabrication process. The nano-ZnO(T) exhibits high purity, structural uniformity, and excellent dispersion. The use of a non-metal catalyst along with an appropriate ratio of a gas mixture of oxygen and acetylene guarantees the high purity of the final product [62]. S. Rackauskas et al. constructed a vertical flow reactor in an air atmosphere for the combustion process of ZnO(T) that was able to control the processes inside the reactor [63]. The operating temperature ranged from 750 to 850 °C, while the ZnO(T) size remained unaffected by reactor conditions, attributed to self-sustaining combustion. As a result, they obtained a final product with controlled geometry, featuring an average leg diameter of 15 ± 5 nm and a length of 200 ± 100 nm. To enhance optical properties, ZnO(T) was decorated with Au NPs, achieving strong attachment via DNA strands [61].

A cage-like nano-tetrapod ZnO, ZnO(nT), was successfully synthesized by Y.-N. Zhao et al. by a novel combustion oxidation method at a temperature of 850 °C. These materials, based on cage-like ZnO(nT) having an improved UV emission, have been used for the fabrication of nanodevices and nano-sensors [64]. Another simple melting combustion method was studied by H. Y. Zahran et al. to synthesize ZnO(nT), and the combustion process was carried out without any catalyst or additives under static conditions. The high surface area and good homogeneity of ZnO(nT) make them suitable for applications in different technology fields [65].

2.1.2. DC Thermal Plasma

The advantages of using the DC thermal plasma technique include the low cost and high yield rate (0.8–1.0 kg h⁻¹) in producing high ZnO(nT). H.-F. Lin highlighted that ZnO(nT) with a high ratio of tetrapod-like nanoparticles is a very promising candidate for the sponge-like photocatalytic filter application [66]. Chiu et al. developed a photoanode film consisting of tetrapod-like ZnO nanoparticles, synthesized using a DC plasma technique. The film with a thickness of 42.2 μm provided good electron transport and good photovoltaic performance. ZnO(T) exhibited a high energy conversion efficiency of 4.9% with a high short-circuit photocurrent density of 12.3 mA cm² [67]. Two different types of ZnO(nT), with short and long arms, were synthesized by C.-H. Lee et al. and have been successfully demonstrated the application in dye-sensitized solar cells (DSSCs) as photoelectrodes. Among these, the DSSCs incorporating short-arm ZnO exhibited higher energy conversion efficiency compared to those using long-arm ZnO. Furthermore, electrochemical impedance spectroscopy has revealed that short-arm ZnO had superior electron transport properties over the long-arm ZnO [68]. H.-F. Lin et al. synthesized ZnO nanoparticles using a novel DC plasma reactor that operated at 70 kW and atmospheric pressure, as presented in Figure 3a. The DC plasma working conditions, such as plasma flow rates, carrier, and quenching gases, were in the range of 200, 10, and 3000 slm [69].

2.1.3. Microwave

A. S. Afify et al. synthesized ZnO(T) for humidity sensors utilizing an ambient microwave evaporation technique in the air atmosphere without the need for chemical solvents or precursors. The humidity-sensing properties of the sensors based on ZnO(T) exhibited a significant sensitivity response of 30% towards relative humidity [70]. In another study, P. Chamoli et al. presented a rapid microwave synthesis (300 W, 180 s) of ZnO anisotropic tetrapods over graphene and exhibited promising results in terms of photocatalytic performance in degradation of three types of organic dyes (methylene blue (MB)—71% removal, methylene orange (MO)—45% removal, and Rhodamine B (RhB)—91.6% removal) under UV and visible light irradiations [71].

E. Seok et al. reported a simple way to synthesize ZnO(T) by an atmospheric pressure microwave plasma jet system, proposing a facile methodology to fabricate some new flexible photodetectors using ZnO(T). High-quality ZnO(T) was synthesized using applied plasma power of 1200 W and mixing gases of O₂ and N₂ [72]. A new “environmentally green” microwave evaporation method of Zn powder in air with a very stable, reliable, and long-lasting photosensing ability was employed in the study by A. Amir et al. Furthermore, to obtain some new ZnO(T), no organic solvents or precursors were required [73]. As it is illustrated in Figure 3b, E. Seok et al. has synthesized ZnO(T) in an APMP jet system ignited using a mixture of O₂ and N₂ (5 N purity, 50:50 vol.%) with a flow rate of 10 L min⁻¹, a frequency of 2.45 GHz, and the applied plasma power being of 1200 W [72].

Figure 3. A schematic illustration of techniques for ZnO growth using (a) DC plasma thermal reactor, reprinted with permission from [69] Copyright 2005, Elsevier B.V.; (b) atmospheric pressure microwave plasma jet system [72]; (c) thermal evaporation method [74]; (d) vapor phase growth reactor, reprinted with permission from [75] Copyright 2010, WILEY-VCH Verlag GmbH & Company KG.

Figure 3. A schematic illustration of techniques for ZnO growth using (a) DC plasma thermal reactor, reprinted with permission from [69] Copyright 2005, Elsevier B.V.; (b) atmospheric pressure microwave plasma jet system [72]; (c) thermal evaporation method [74]; (d) vapor phase growth reactor, reprinted with permission from [75] Copyright 2010, WILEY-VCH Verlag GmbH & Company KG.

2.1.4. Thermal Oxidation (TO)

The TO technique, which utilizes metal zinc powder combined with oxidizing agents such as hydrogen peroxide (H₂O₂) and ammonium persulfate ((NH₄)₂S₂O₈), presents a simple approach for synthesizing ZnO(T). The TO technique, which utilizes metal zinc powder combined with oxidizing agents such as hydrogen peroxide (H₂O₂) and ammonium persulfate ((NH₄)₂S₂O₈), presents a simple approach for synthesizing ZnO(T). Y. Daejeong et al. investigated the production of ZnO(T) in a furnace with the heating temperature gradually raised to 1000 °C in an atmospheric condition for 3 min. From morpho-structural characterization results, the average diameter and length of ZnO(T) leg were 45.3 nm and 1.57 μm, and the lattice spacing was 0.252 nm, respectively [76]. Another thermal oxidation method, utilizing mixed solutions, including methanol (CH₃OH), ethanol (C₂H₅OH), and hydrogen peroxide (H₂O₂), was applied to synthesize ZnO(T) tetrapods. B. Chawalit and colleagues heated this mixture to 1000 °C under normal atmospheric conditions, yielding ZnO(T) structures with a leg length of approximately 8.17 μm, a leg diameter of 1.17 μm, and a tip diameter of 47.8 nm [77]. In another study, B. Aziz et al. [78] synthesized ZnO(nT) using a simple thermal oxidation method. The samples were then irradiated with phosphorus ions at varying doses to highlight their photoluminescence effect. The photoluminescence (PL) spectra indicated that the near band emission and deep-level emission peaks increased with higher dose, and these emission peaks are related to defects. Specifically, the PL spectra revealed that emissions at 3.31 eV and 3.26 eV are attributed to a conduction band transition involving phosphorus-related acceptors and a donor-to-acceptor pair transition, respectively. Moreover, N. Hongsith et al. employed an oxidation reaction technique, heating a mixture of zinc powder and hydrogen peroxide solution at 1000 °C under normal atmospheric pressure. This process yielded tetrapod ZnO nanostructures with leg dimensions ranging from 200 to 1000 nm and lengths of 1 to 5 mm. They concluded that this growth model could be generalized to explain the formation of hexagonal structures in other materials, such as GaN [79]. In another study, F. H. Alsultany et al. synthesized 3D ZnO(nT) on a catalyst-free glass substrate using the thermal evaporation method within a horizontal tube furnace system, and the experimental installation shown in Figure 3c features a two-zone tube furnace. Zn powder, serving as the source material, was positioned in the first zone and gradually heated to 650 °C at a rate of 10 °C min⁻¹, while the substrate zone (second zone) was maintained at 425 °C [74].

2.1.5. Flame Transport Synthesis (FTS)

Y.K. Mishra et al. have demonstrated a versatile and single-step synthesis of ZnO(T) with various arm morphologies using a simple FTS. These ZnO(T) structures have been utilized for the photocatalytic degradation of methylene blue solution under UV light at ambient temperature. Tetrapods with varying arm morphologies exhibit over 95% photocatalytic activity against methylene blue under UV irradiation. Notably, tetrapods with sharp needle-like arms achieve near-complete dye degradation in approximately 10 min [80].

T. E. Antoine et al. [81] synthesized some large quantities of ZnO(T) with a high degree of reproducibility using a highly cost-effective and efficient approach, the flame transport method. Due to the self-supported characteristics, these tetrapods have a wide range of applications, from antiviral activities to serving as templates for designing 3D networks and growing multifunctional composite structures [21,82].

2.1.6. Vapor Phase Growth (VPG)

D. Calestani et al. optimized a vapor-phase synthesis process to achieve reproducible and selective tetrapod growth. The resulting nanostructures were utilized to develop low-cost, tetrapod-based sensor prototypes for testing responses to various gases [83]. L. Zanotti et al. outlined several advantages of vapor phase growth in their research, including the efficient production of ZnO(T), the ability to isolate ZnO(T) from undesirable metal particles or other ZnO nanostructures, and the operation of this method at low temperatures through precipitation from a liquid suspension. The researchers have used a reactor as illustrated in Figure 3d for the vapor growth of ZnO(T), the temperature used was between 650 and 700 °C, and the pressure of the Zn vapors was in the range 37–82 mbar. To obtain a large amount of solid ZnO nuclei, they used argon (100 sccm) and oxygen (about 5 sccm) gases in the “nucleation” region [75].

2.1.7. Chemical Vapor Deposition (CVD)

Besides various growth methods, the CVD technique stands out for its ability to produce high-quality films while also being well-suited for large-scale production. For understanding the growth model of tetrapod-like ZnO, the intense study of ZnO(T) is essential [84,85]. B.-B. Wang et al. reported the production of ZnO nanomaterials by CVD method, and with the help of SEM analysis, the structure of the tetrapods junction was proposed. The results support further research into joint interface development and mechanical strength [86]. In another study, O. A. Lyapina and co-workers described the synthesis of ZnO(T) using CVD technology on silicon substrates, and they also examined the influence of synthesis conditions on the shape and dimension of the ZnO(T). Thus, they highlighted that at low zinc concentrations in the reaction system, elongated rods are predominant, and in contrast, at high zinc supersaturations, tetrapods with hexagonal bulbs are formed [87].

Table 2. Characteristics and synthesis methods of ZnO(T) depending on the application domain.

Synthesis Methods	Advantages	Material Purity/Precursor	Investigated Properties	Application/Proposed Domain	Ref.
MCM	No volatile salts, which makes the process cheap	99.995% bulk industrial, metal Zn	Morpho-structural, optical, luminescence, photoluminescence, electrical conductivity	Growth mechanism, shock-resistance, optical, technological, nanodevices, and nanosensors	[62,63,64]
DC	Synthesis of various oxide nanoparticles at low cost and high yield	95%–99% metal zinc	Morpho-structural, photostability, optical, current–voltage, electrochemical, and photocurrent–voltage	Photocatalytic, photovoltaic process, and photoelectrode	[66,67,68,69]
Microwave	Synthesis quickly and low energy consumption	99.9% Zn powder	Morpho-structural, humidity sensing, photosensitivity, optical, photocatalytic, and recyclability	Industrial activity, degradation of organic dyes and flexible photosensors	[70,71]
TO	Simple and low-cost method, depending on the substrate	99.99% Zn powder	Morpho-structural, optical, photoluminescence, and kinetics	Gas sensor and UV photodetection	[74,76,78,79]
FTS	Facile and cost-efficient	Zn microparticles	Morpho-structural, optical, photocatalytic, fluorescently	Nanosensing devices, gas sensor, sacrificial template	[80,81,88]
VPG	Easy for different ZnO nanostructures, but the morphologies are mixed together	99.999% powder and Zn foil	Morpho-structural, conductance, relative humidity, sensor response	Gas sensors, optoelectronic	[75,83]
CVD	Low-cost method compared with the PVD method	99.999% Zn powder	Morpho-structural, optical, photoluminescence, growth mechanisms, luminescence	Nanooptics, nanoelectronics, nanodevices	[85,86,87]

Table 2. Characteristics and synthesis methods of ZnO(T) depending on the application domain.

Synthesis Methods	Advantages	Material Purity/Precursor	Investigated Properties	Application/Proposed Domain	Ref.
MCM	No volatile salts, which makes the process cheap	99.995% bulk industrial, metal Zn	Morpho-structural, optical, luminescence, photoluminescence, electrical conductivity	Growth mechanism, shock-resistance, optical, technological, nanodevices, and nanosensors	[62,63,64]
DC	Synthesis of various oxide nanoparticles at low cost and high yield	95%–99% metal zinc	Morpho-structural, photostability, optical, current–voltage, electrochemical, and photocurrent–voltage	Photocatalytic, photovoltaic process, and photoelectrode	[66,67,68,69]
Microwave	Synthesis quickly and low energy consumption	99.9% Zn powder	Morpho-structural, humidity sensing, photosensitivity, optical, photocatalytic, and recyclability	Industrial activity, degradation of organic dyes and flexible photosensors	[70,71]
TO	Simple and low-cost method, depending on the substrate	99.99% Zn powder	Morpho-structural, optical, photoluminescence, and kinetics	Gas sensor and UV photodetection	[74,76,78,79]
FTS	Facile and cost-efficient	Zn microparticles	Morpho-structural, optical, photocatalytic, fluorescently	Nanosensing devices, gas sensor, sacrificial template	[80,81,88]
VPG	Easy for different ZnO nanostructures, but the morphologies are mixed together	99.999% powder and Zn foil	Morpho-structural, conductance, relative humidity, sensor response	Gas sensors, optoelectronic	[75,83]
CVD	Low-cost method compared with the PVD method	99.999% Zn powder	Morpho-structural, optical, photoluminescence, growth mechanisms, luminescence	Nanooptics, nanoelectronics, nanodevices	[85,86,87]

2.2. Type of Aero-Materials

2.2.1. Aerographite

Aerographite materials were discovered about 10 years ago. Besides the quality of the “lightest material” in those times, those materials have other special properties that differ from other lightweight materials, such as aerogels. Most aerographite materials are derived from highly porous ZnO networks consisting of interconnected micrometer-thick rods with a 3D architecture, often resembling tetrapods. ZnO is one of the most extensively studied semiconductor compounds within the metal oxide group, with research on this material continuously expanding, especially in the field of low-dimensional structures such as nanodots, nanotubes, nanotetrapods, and nanomultipods [20]. Template materials based on ZnO possess several advantages; besides a wide variety of morphologies, ZnO can be easily dissolved in acids or bases, and removal is simple. Through the deposition process, the template is removed from aerographite, while a carbon layer covers the ZnO lattice. Aerographite is lightweight yet strong enough to withstand significant deformations. For aerographite applications, achieving a high-quality, optimized structure is crucial, necessitating an in-depth understanding of its growth principles through the CVD technique used in carbon foam fabrication. Consequently, the CVD process stands as one of the most promising methods for the commercial production of aerographite, wherein a carbon source is introduced into a flowing gas stream [22]. Figure 4 presents a small overview of the variety within the aerographite family [21] characterized by a seamless composition, and the interconnected network under consideration consists of closed-shell microtubes. The values of the density of these microtubes are approximately 2 mg cm⁻³, which is much lower than that of the lightest aerogels reported in the literature. Despite their internal graphitic structure providing a significantly greater specific surface area, these materials suffer from reduced flexibility and damage tolerance, comparable to that of conventional silica or carbon aerogels.

The formation of hierarchical 3D aerographites with micro- and nanocrystalline ZnO(T) was developed by Tiginyanu et al., using a simple magnetron sputtering process, as very promising candidates for optoelectronic devices (Figure 5A) [20]. Aerographite microtubes within the network were uniformly coated with a continuous ZnO nanocrystalline film. The deposited ZnO film, approximately 150 nm thick, exhibited a nano-granular structure with an average nanocrystal size of about 100 nm, while the tetrapod arms measured approximately 100 nm or less in diameter.

From the applicability aspect, aerographite is a promising candidate for use as scaffolds in the deposition of various solid-state nanoparticles, facilitating the development of hybrid nanocomposite materials with flexible 3D architectures. Consequently, certain 3D structures have been developed using ZnO micro- and nanocrystals grown on aerographite materials, enabling their application in micro- and nano-optoelectronics, particularly for constructing flexible broadband photodetectors spanning the ultraviolet to infrared range. Recently, great progress has been made in the development of 3D interconnected systems based on different semiconductors, and gallium nitride remains an interesting subject. A. Schuchardt et al. reported a novel method for obtaining 3D hybrid networks based on GaN microstructures and nanostructures using highly porous microtubular aerographite templates, as is presented in Figure 5B [24].

The exceptional mechanical flexibility, distinctive surface morphology, and direct growth capability of these highly porous aerographite networks make them ideal backbones for non-agglomerated growth of GaN active nanostructures, leading to the development of multifunctional 3D composites. Graphene, carbon nanotubes, and carbon nanorings have attracted significant scientific interest due to their unique characteristics and potential applications [89,90]. Using these carbon porous 3D networks as templates to obtain composite materials can lead to the growth of non-agglomerated nanostructures, aspects very important for the application field [91]. Various structures, such as graphene and graphene oxide, have been used for the growth of nano- and microstructures, GaN, and the limitations are related to the templates. Baek and collaborators [92] have presented techniques for growing high-quality GaN microdisks on amorphous silicon oxide substrates created on silicon. Utilizing micropatterned graphene films as a starting point for growth, they discovered that the epitaxial lateral overgrowth of GaN onto graphene layers presents promising opportunities for the development of advanced devices. Graphene films, transferred onto SiO₂/Si substrates and shaped into microdots, serve as a crucial foundation for the development of ZnO nanostructures. These ZnO nanostructures, in turn, act as an important intermediary layer for the subsequent growth of GaN. Furthermore, the use of graphene dots with intermediary ZnO nanowalls enables the epitaxial lateral overgrowth of highly crystalline GaN microdisks, characterized by their hexagonal facets. R. Yang et al. [93] presented a general approach for synthesizing large-scale GaN nanostructures using the GO-assisted CVD method, demonstrating that GaN surface morphology can be controlled by GO concentrations. Their findings open new possibilities for GaN nanomaterial-based biomedical devices. Han et al. [94] reported the synthesis of GaN nanorods coated with graphitic carbon layers through carbon layer deposition on pre-produced GaN nanorods using a conventional CVD method. They observed that methane as a hydrocarbon source enables precise control over the number of carbon layers, which serve as chemically inert protective coatings for GaN nanorods. Additionally, they highlighted that adding more carbon layers further extends the lifetime of the nanomaterials. Another study describes the synthesis of 3D aerographite networks formed from interconnected tubular tetrapods of multilayer graphene, providing valuable insights into their mechanical properties and the design of similar carbon-based aerogels. In Figure 5A, the authors exhibit the morphology of the aerographite/ZnO hybrid nanomaterial, aerographite being presented as an ultra-lightweight, highly porous, and mechanically flexible graphite-based 3D network, and the tubes have micrometer-scale diameters and ultra-thin walls, approximately 15 nm thick [20]. Figure 5B illustrates the direct and rapid growth of GaN nano- and microstructures on aerographite tube surfaces, forming a flexible, interconnected 3D aerographite-GaN hybrid network via the HVPE technique. During the HVPE process, homogeneous GaN nano- and microstructure growth occurs on both the inner and outer surfaces of the graphitic tubes, with multiple growth directions indicating direct free growth. The resulting GaN nano- and microstructures adhere strongly to the thin graphite walls, preventing agglomeration [24]. Figure 5C illustrates the morphologies of aerographite tetrapods, where the authors investigate the mechanical behavior of single tetrapods and predict a base for other future networks with different densities and characteristics. The growth strategy for nano- and microtubular aerographite tetrapods network from sacrificial ceramic templates by the CVD method illustrates, by SEM morphologies, the direct conversion of white ZnO network into black aerographite network [23]. The ZnO(T) structures feature uniform hexagonal cylindrical arms that taper towards their tips, with arm lengths ranging from 15 to 30 mm. Figure 5D reveals that the aerographite tetrapods are hollow, featuring a nano-microtubular arm morphology, in contrast to the solid initial ZnO template. The SEM image confirms the complete transformation of the ZnO network into aerographite while preserving its tetrapodal structure. However, the AG tetrapods exhibit slight crumbling, primarily attributed to their hollow nano-microtubular morphology [95].

Figure 5. (A) Growing of 3D hybrid nanomaterials on aerographite: (a) schematic sputtering process for decorating the aerographite with ZnO tetrapods; (b,c) SEM morphologies of aerographites before and after decorating with tetrapods [20]. (B) 3D network aerographite–gallium nitride from aerographite template: (a) HVPE process for the growth of hybrid network on aerographite; (b) digital image of AG-GaN; (c–e) SEM images on different magnifications [24]. (C) Synthesis of aerographite tetrapods: (a) CVD representation of the synthesis of t-AG from ZnO tetrapods; (b,c) SEM micrographs of tetrapods-ZnO (b) and converted t-AG networks (c); (d) SEM morphology of a t-AG arm; (e) TEM image of an aerographite tube [23]. (D) Growth process for aerographite tetrapods from ceramic template: (A,B) schematic illustration of CVD conversion process from ZnO template to aerographite network; (C) schematic conversion process of ZnO arms into tubular carbon arms; (D–F) SEM images of aerographite networks at different conversion stages [95].

Figure 5. (A) Growing of 3D hybrid nanomaterials on aerographite: (a) schematic sputtering process for decorating the aerographite with ZnO tetrapods; (b,c) SEM morphologies of aerographites before and after decorating with tetrapods [20]. (B) 3D network aerographite–gallium nitride from aerographite template: (a) HVPE process for the growth of hybrid network on aerographite; (b) digital image of AG-GaN; (c–e) SEM images on different magnifications [24]. (C) Synthesis of aerographite tetrapods: (a) CVD representation of the synthesis of t-AG from ZnO tetrapods; (b,c) SEM micrographs of tetrapods-ZnO (b) and converted t-AG networks (c); (d) SEM morphology of a t-AG arm; (e) TEM image of an aerographite tube [23]. (D) Growth process for aerographite tetrapods from ceramic template: (A,B) schematic illustration of CVD conversion process from ZnO template to aerographite network; (C) schematic conversion process of ZnO arms into tubular carbon arms; (D–F) SEM images of aerographite networks at different conversion stages [95].

W. Gong et al. reported the production of ZnO nanorods used as a template and carbon nanotubes using dopamine as a carbon source [96]. ZnO can be obtained in various morphologies, including tetrapods, disks, rings, and nanorods, but among them, the one-dimensional ZnO nanorods are the most attractive for the fabrication of tubular nanostructures [97,98]. Moreover, different structures based on ZnO nanorods as a template for obtaining tubular nanostructures of platinum [97], titanium dioxide [98], and polypyrrole [99] were reported; the interactions between ZnO and carbon have not been exploited to obtain carbon replicas of ZnO nanorods [96]. Recently, the research papers highlighted the properties of graphene material like foam structure related to porosity, an excellent electrically conductive, and very mechanically stable material used for Li-ion car batteries or supercapacitors as electrode material. Therefore, aerographite was designed as a lightweight material but extremely robust to support strong deformation. To overcome this challenge, a single-step CVD technique based on ZnO networks was realized through the hierarchical design of aerographites [21] I. Plesco et al. synthesized 3D flexible networks based on aerographite, decorated with InP micro- and nanocrystallites, using the HVPE method. Initially, the 3D aerographite networks were fabricated via the CVD process, with ZnO(T) serving as a sacrificial template. After removing the template, a 3D graphite structure was obtained that almost completely preserved the template architecture, followed by the growth of the 3D aerographite–InP networks using the HVPE method. Thus, it was found that the InP nanocrystals grew on both surfaces (inner, outer) of the 3D graphite structure with a uniform distribution, no agglomeration, and high elasticity. Thus, the characteristics of the synthesized structures recommend them successfully for applications in tactile/strain sensors and other devices requiring flexible electronic components [100]. Figure 6 presents SEM images of AG decorated with InP microcrystallites, which have grown on both the outer surface and the interior of the AG structure. The InP micro-crystallites are relatively uniformly distributed across the external surface of the AG network, with sizes ranging from a few nanometers to micrometer scales and exhibiting well-defined shapes. The results indicate that the AG-InP network undergoes less than 1% plastic deformation after 10 cyclic compressions, underscoring the structural robustness of the AG-InP 3D hybrid material. Additionally, the primary changes in electrical resistivity occur within the compression range of up to 10%, corresponding to the Hookean elastic regime. This suggests that the resistivity decrease under compression is primarily due to the increased number of conductive pathways within the AG-InP network structure.

F. Rasch et al. [101] introduced a versatile method for wet-chemical assembly of 2D carbon nanomaterials into macroscopic 3D networks with different shapes and sizes. They used a flame transport synthesis, and for template fabrication with various geometries and dimensions (Figure 7b), an amount of ZnO powder is pressed into pellets using a rough mold, as can be seen in Figure 7a. SEM images show ZnO(T) network at different magnifications, and due to its arrangement of randomly positioned, interconnected microrods measuring a diameter of 1–3 µm and 20–30 µm in length, this structure exhibited an extended volume that can be readily accessed from the exterior. Owing to the simplicity, their approach allows for a combination of various nanomaterials to form freestanding, lightweight (<2 mg cm⁻³⁾ hybrid networks. Electrical characterization of the networks revealed an ultralow percolation threshold of the composites (0.05 vol%) and notably high conductivity (8 S m⁻¹ at 4 mg cm⁻³).

2.2.2. Aero-β-Ga₂O₃

N. Wolff et al. recently published a study to obtain hybrid materials based on aero-β-Ga₂O₃/ZnGa₂O₄ nanocomposite networks. Thus, initially, the precursor material aero-GaN(ZnO) was obtained by epitaxial growth of GaN on ZnO(T) by the HVPE method. During the process, the growth of aero-GaN on the tetrapods, but also the decomposition of the ZnO template, took place. In the next step, the aero-β-Ga₂O₃/ZnGa₂O₄ nanocomposite networks were obtained by applying a heat treatment, in air, of the precursor material aero-GaN(ZnO). During the process, the growth of aero-GaN on the tetrapods, but also the decomposition of the ZnO template, took place. In the next step, the aero-β-Ga₂O₃/ZnGa₂O₄ nanocomposite networks were obtained by applying a heat treatment, in air, of the precursor material aero-GaN(ZnO) [25]. In Figure 8A, the steps of the synthesis process of aerodynamic networks based on gallium oxide are schematically presented.

Figure 8B presents the surface morphologies of pristine ZnO(T), aero-GaN(ZnO) networks, and aero-Ga₂O₃/ZnGa₂O₄ network structures. The micrographs reveal structural modifications at different annealing temperatures (950–1100 °C), while the hexagonal shape and porosity remain preserved. Cross-sectional analysis of the tubes indicates that the wall thickness of the aero-Ga₂O₃/ZnGa₂O₄ sample annealed at 950 °C is greater than that of the sample annealed at 1100 °C, suggesting inhomogeneities during GaN growth or variations in synthesis parameters. I. Plesco et al. reported the obtaining of a new type of photocatalyst based on aero-β-Ga₂O₃ synthesized by annealing aero-GaN by epitaxial growth using a sacrificial ZnO microtetrapod template. To improve the photocatalytic performance, aero-β-Ga₂O₃ was functionalized with Au or Pt nanodots. The obtained aero hybrid materials were tested under both UV and visible light for the degradation of Methylene Blue dye. The results obtained in the photocatalytic degradation tests highlighted the superior performance of aero-Ga₂O₃ functionalized with Au and Pt, compared to aero-Ga₂O₃ [26]. Figure 9 shows a schematic representation of the steps for the preparation of aero-Ga₂O₃ and aero-Ga₂O₃ functionalized with Au.

2.2.3. Aero-GaN

In another study, T. Braniste et al. grew thin layers of aero-GaN on sacrificial templates of ZnO networks of microtetrapods by the HVPE method. The authors found that during the synthesis, two processes occur almost simultaneously, such as the growth of GaN and the partial decomposition of the ZnO template. This aspect was also observed in another study carried out in a previous work published by the same authors [27]. Thus, aero-GaN has the appearance of hollow micro-tetrapods, and inside there is a very thin film of zinc oxide. A very important aspect of the obtained materials is the dual hydrophobic/hydrophilic property, meaning that the outer surface of the tetrapod arms is superhydrophobic, and the free ends are superhydrophilic [102]. Figure 10 presents a schematic representation of hydrophobic and hydrophilic properties for the aero-GaN micro-tetrapod. Also, SEM images for aero-GaN exhibited a network of interpenetrated micro-tetrapods, and the image of a broken arm shows its tubular structure with a nanoscale thickness of the walls.

M. Dragoman et al. reported the fabrication of ultraporous GaN aero-materials composed of interconnected hollow micro-tetrapods. Aero-GaN was obtained by HVPE on ZnO(T). Notably, during the GaN layer growth process at 850 °C, the decomposition of the sacrificial ZnO template commenced. However, a very thin ZnO film remains undecomposed on the inner surface of the GaN micro-tetrapods. The ultraporous GaN aero-material was used to fabricate a pressure sensor, and sensing measurements showed that the electrical conductance exhibits a linear dependence on the applied pressure (the pressure used in the tests was up to 40 atm) [28]. Its nondimensional sensitivity varied from 16.2 10⁻³ at low pressure (5 atm) to 7.4  10⁻³ at high pressure (40 atm).In Figure 11, SEM images show the initial interconnected network of ZnO microtetrapod, ZnO(mT), template (a), as well as the resulting aero-GaN material following the removal of the sacrificial layer ZnO (b) and (c). Based on the SEM images, the authors observed that the aero-GaN was composed of tetrapod-shaped GaN microtubes that were arranged in a random manner, with a wall thickness of approximately 70 nm and a porosity level greater than 93%.

I. Tiginyanu et al. [27] demonstrated that the HVPE technique enables controlled deposition of homogeneous thin layers of GaN. They emphasized that the wall thickness of the hollow aero-tetrapod arm depends on the deposition time at 850 °C, which can be adjusted from approximately 15 nm to a few hundred nanometers (Figure 12f,i). Additionally, they observed that higher growth temperatures accelerate the decomposition rate of the ZnO template during the process.

2.2.4. Aero-TiO₂

V. Ciobanu et al. reported a study in which they obtained a new nanomaterial based on hollow TiO₂ microtetrapods grown on a sacrificial ZnO template by the atomic layer deposition (ALD) method. Thus, the obtained TiO₂ microtetrapod-based aero-materials featured a hollow microtetrapod design that offers a very interesting morphology. The tubular shape of the microtetrapods provided a large contact surface with pollutants, which has led to a superior degradation efficiency. Furthermore, the interconnected structure of the tetrapods presented a great advantage in dynamic processes due to improved mechanical stability. Figure 13 presents SEM images, which highlight the unique design of aero-TiO₂, a schematic presentation of obtaining aero-materials starting from ZnO(T) [29]. The arms of aero-TiO₂ microtetrapods range from 20 to 40 µm in length and 1 to 3 µm in diameter, with TiO₂ microtube walls measuring approximately 50 nm thick.

A comparative study on obtaining aero-TiO₂ with controlled morphology and crystal structure was carried out by V. Ciobanu et al. [30] SEM images from Figure 14 showed that after wet etching process to remove the sacrificial ZnO template, the walls of the TiO₂ microtubes are dense and solid, but the structures obtained after etching process in HVPE system consisted of perforated microtubes. From a structural point of view, it was found that the heat treatment applied at a temperature of 400 °C resulted in the formation of the anatase crystalline phase of TiO₂ microtubes, and in the case of annealing at 800 °C, a mixture of crystalline phases, anatase and rutile, was obtained. Etching within the HVPE system resulted in the formation of a mixture containing anatase and rutile phases. The authors concluded that the obtained materials could serve as promising candidates for photocatalytic applications as well as sensor development.

2.2.5. Other Aero-Materials

L. M. Saure et al. synthesized a photothermal SiO₂/rGO hybrid in a wet chemical coating process, using ZnO(T)as a sacrificial template. Thus, the functionalization with reduced graphene oxide was achieved by treating the ZnO templates with graphene oxide flakes. After that, the ZnO templates were treated with a solution to form an amorphous SiO₂ layer on the ZnO(T), and the sacrificial ZnO templates were removed by treatment in 1 M HCl acid solution. The authors demonstrated that the hybrid aero-material SiO₂/rGO exhibits a significantly enhanced photothermal response compared to pristine carbon nanomaterial [31]. Figure 15 schematically highlights the synthesis steps of the macroscopic nano- and micro-engineered transducer material. In Figure 15b, the authors present a photograph of cylindrical aero-rGO with varying volumetric loading of rGO. Figure 15c,d reveal SEM images showing the homogeneous distribution of rGO flakes on the surface of SiO₂ microtubes, while Figure 15e illustrates a SiO₂ microtube arm decorated with an rGO flake.

V. Ursaki et al. [32] synthesized aero-ZnS by HVPE from Sn₂S₃ crystal precursors, using ZnO tetrapods as sacrificial templates. The authors demonstrated that the used technology allows control of both the composition and the crystallographic phase content of the obtained aerogels. The technological approach used in obtaining aero ZnS is more advantageous than the HVPE method (frequently used in obtaining aero-materials). Moreover, during the technological procedure, two processes take place: the sublimation of Sn₂S₃ crystals with the formation of ZnS and the decomposition of ZnO(mT). As seen in Figure 16, SEM images present ZnS(mT) with arms of 10–30 µm obtained from ZnO template through HVPE method over a period of 4 h (Figure 16a), and when the time is increased to 8 h (Figure 16b), the arm surface became more granulated.

Plesco et al. [33] synthesized aero-ZnS with self-organized architectures by the HVPE method, using CdS as a precursor. To obtain the 3D structure of the hollow ZnS microtetrapod, ZnO(T) was used as a sacrificial template. During the synthesis of the aero-material, the sacrificial template was also removed. A very important property of the obtained aero-materials is their extremely low density (5.6 mg cm⁻³), as expected because it is a very important characteristic of aero-materials; but also they exhibit dual behavior, hydrophilic/hydrophobic, thus, the outer wall surface demonstrates hydrophobicity, while the inner wall surface is hydrophilic. These properties of aero-ZnS make it a promising candidate for microfluidic applications. SEM was used to highlight the structures of ZnS hollow tetrapods. Thus, Figure 17A(a) presents ZnO(T) structures used as sacrificial scaffolds, while Figure 17A(b) shows ZnO–CdS core–shell structures following CdS layer growth via the HVPE process at 650 °C. The results indicate that during ZnO scaffold decomposition (Figure 17A(c)), the process initiates from the shell and progresses toward the center.

A. Schuchardt et al. synthesized a flexible 3D aerographite–GaN hybrid network by rapidly growing GaN nano- and microstructures directly on the surface of an aerographite template. Initially, the 3D aerographite structures were obtained by CVD on ZnO(T) templates [24]. During the aerographite synthesis process, the sacrificial ZnO template was removed. In the second step, the aerographite networks were used as templates for the growth of GaN nano- and microstructures via the HVPE method. Figure 18 presents SEM images of the aerographite–GaN 3D hybrid network, showing varying degrees of GaN coverage (Figure 18a–c), from partial to full. Notably, even as the GaN content increased, the architecture of the 3D aerographite remained intact. Additionally, GaN growth occurred on both the outer and inner surfaces of the aerographite tubes, influenced by the morphology of these 3D templates (Figure 18h–i). The HVPE method enables precise control over the uniformity and morphology of GaN nanostructures on aerographite tetrapods by adjusting deposition time and growth temperature (Figure 18d–f). Figure 18h illustrates GaN development along the inner surface of the tube, likely resulting from HVPE reactant infiltration through openings or pores in the graphitic framework. Figure 18i presents an example of a hexagonal-prism-shaped GaN nano- and microstructure formed within the graphitic microtube.

According to the conclusions of the authors of this study, from the results of the cathodoluminescence measurements, the obtained composite materials may be successfully used in electronic, photonic, and sensor applications.

3. Conclusions and Future Perspectives

This paper provides a systematic review of semiconductor aero-materials, a novel category of ultra-lightweight, highly porous, three-dimensional materials featured with diverse functional applications, and can be considered a potential alternative for aerogels. Generally, the materials with 3D architectural structure are successfully applied in different fields, such as the production and storage of energy, environmental protection, sensors, electronic and optoelectronic devices, and biomedical. Unlike traditional aerogels, where the starting point is gels, the aero-materials are generally made from sacrificial templating approaches, among which ZnO(T) is especially interesting due to its morphology, ease of removal, and compatibility with various deposition processes. ZnO(T) represents excellent sacrificial templates for such structured aero-materials, with controlled porosity, geometry, and mechanical properties. Various synthesis methods have been investigated to control the synthesis of ZnO(T), and each method offers differential advantages associated with scalability, cost, purity, and morphological control. The use of these sacrificial components presents multiple advantages, such as some of the synthesis methods are simple, cheap, and versatile (e.g., FTS) and the development of methods for removing the sacrificial substrate that can be performed at the same time as the growth of semiconductor aero-materials. Aerographite manufacturing, a carbon-based aero-material synthesized through CVD on ZnO(T) template, is one of the most notable developments. Aerographite materials possess exceptional mechanical strength, electrical conductivity, and adaptability for hybridization. They serve as robust platforms for the growth of semiconductor nanostructures (e.g., ZnO, GaN, and InP), leading to advanced functionalities in sensors, photodetectors, and optoelectronic devices. Some aero-materials, such as aero-TiO₂, aero-Ga₂O_3, and aero-ZnS, have demonstrated superior performance in photocatalytic applications, sensor development, and microfluidic applications, due to their hollow tubular morphologies and large surface areas. Also, composite aero-materials (e.g., aero-β-Ga₂O₃/ZnGa₂O₄, AG-InP, SiO₂/rGO hybrid, flexible 3D aerographite–GaN) can be successfully used in electronic, photonic, photothermal, sensory, and photocatalytic degradation applications. The technologies for synthesizing aero-materials can be updated by multiple improvements, and countless challenges may arise. In this context, obtaining precise morphological control of semiconductor aero-materials, optimizing the removal of the sacrificial template, exploring alternative template materials, and integrating hybrid functionalities are demands of great interest that will need further research. In conclusion, aero-materials represent a challenge in materials science, combining structural ability with multifunctional performance, and adjustable architectures and properties make them highly suitable for further applications. Even if semiconductor aero-materials have exhibited great characteristics and excellent results in various domains, some important technological challenges must be overcome in order to allow widespread application and industrial use; the following main areas are significant research priorities:

• Control of structural and synthesis method—The synthesis methods that use ZnO(T) as sacrificial templates suffer from limited control over size distribution, pore connectivity, and wall thickness. Improved methodologies for predictable nano- and microscale structural control are required. Moreover, most present synthesis techniques (e.g., HVPE, CVD, and TO) demand high temperatures and possibly toxic reagents; thus, developing low-energy, sustainable, and solvent-free methods is crucial for environmental protection and energy-efficient production.
• Surface functionalization—Many applications require specific functional groups or surface changes (e.g., biosensing and catalysis); thus, more research on post-synthesis functionalization techniques that maintain structural integrity is needed.
• Application level and testing—For application in real-world devices, semiconductor aero-materials must be tested under severe temperatures, cyclic stress, humidity, and chemically severe environments. For applications on specific prototypes, more effort is required to progress from proof-of-concept research to practical prototypes, and more tests for biomedical applications.