Metal Oxide Nanowires Grown by a Vapor–Liquid–Solid Growth Mechanism for Resistive Gas-Sensing Applications: An Overview

Metal oxide nanowires (NWs) with a high surface area, ease of fabrication, and precise control over diameter and chemical composition are among the best candidates for the realization of resistive gas sensors. Among the different techniques used for the synthesis of materials with NW morphology, approaches based on the vapor–liquid–solid (VLS) mechanism are very popular due to the ease of synthesis, low price of starting materials, and possibility of branching. In this review article, we discuss the gas-sensing features of metal oxide NWs grown by the VLS mechanism, with emphasis on the growth conditions and sensing mechanism. The growth and sensing performance of SnO2, ZnO, In2O3, NiO, CuO, and WO3 materials with NW morphology are discussed. The effects of the catalyst type, growth temperature, and other variables on the morphology and gas-sensing performance of NWs are discussed.


Introduction
One-dimensional (1D) materials such as nanofibers (NFs), nanorods (NRs), nanotubes, and nanowires (NWs) have two dimensions in the range of 1-100 nm, but one dimension is much longer [1].In particular, NWs have lengths in the range of micrometers and high aspect ratios of 20 or higher.Their cross-sections are usually round [2][3][4].NWs generally have high crystalline quality, have tunable composition and properties, and, due to their morphology, they can also be incorporated in different devices with different designs [5,6].
In general, "top-down" and "bottom-up" synthesis methods are employed for the synthesis of nanostructures [7].The former methods such as lithography techniques use bulk materials; by sculpting or etching them, nanostructures can be produced [8,9].However, due to nanoscale dimensions, in some cases, the quality of the nanostructures cannot be adequately controlled [2].In bottom-up approaches, controlled crystallization from vapor or liquid sources is possible, which leads to the realization of high-quality nanostructures [2].Based on the above approaches, there are different methods available to synthesize metal oxide NWs including hydrothermal [10], sol-gel [11], sonochemical [12], lithography [13], confinement growth [14], pulse laser deposition (PLD) [15], direct thermal oxidation [16], and chemical vapor deposition (CVD) [17].
In 1964, Wagner [18] presented a vapor-liquid-solid (VLS) mechanism for the synthesis of Si whiskers on a Au-deposited Si substrate using SiCl 4 and H 2 source materials.By heating above 363 • C, which is above the eutectic point of Au-Si, a Au-Si liquid droplet can be formed.Subsequently, additional Si in the vapor phase is readily adsorbed on the droplet, increasing the Si amount in the droplet.After being supersaturated with Si atoms, the driving force for the crystallization of excess Si at the liquid-solid interface is provided, and Si whiskers grow.After becoming fully grown, the droplet remains on the tip of the whiskers through surface tension [19,20].Later, Holonyak and co-workers [21] reported the synthesis of GaP whiskers using a Zn catalyst.Subsequent research by Givargizov [22] and others further explained the details of the VLS mechanism.In general, any unidirectional growth of crystalline materials with the help of a liquid phase and sources provided from a vapor phase is related to the VLS growth mechanism [23].Different materials such as metal oxides [24] and metal sulfides [25] are currently synthesized by this mechanism via different techniques [26,27].
The advantages of this mechanism include (i) the synthesis of good quality crystalline 1D nanostructures such as NWs, (ii) the possibility of tuning chemical compositions, (iii) a simple setup, and (iv) low activation energy compared to other vapor deposition approaches.Nevertheless, sometimes a high growth temperature is a limiting factor for approaches which use this technique [28].Moreover, this approach cannot be used for the growth of metallic NWs; for some applications, the existence of a metal catalyst at the tip of nanostructures may degrade its properties [29].However, for gas-sensing purposes, the presence of a metal catalyst, which is usually a noble metal, enhances the sensing response by chemical sensitization and electronic sensitization.It should be noted that today, mostly metal oxides with a particulate morphology are used for the realization of gas sensors.They have a high surface area, and their contact points are considered good sources of resistance modulation.However, extensive agglomeration often occurs among them, leading to an extensive decrease in their surface area and closing of open pores among them.Hence, the sensing performance is decreased, especially when the sensor is working at a high temperature due to agglomeration among particles.An alternative way is to use the metal oxides with NW morphology.Metal oxide NWs have stable chemical properties as well as a large surface area.In particular, when the diameter of NWs approaches their Debye lengths, significant modulation of the resistance can occur in the presence of target gases.Moreover, NWs can have large mechanical deformations without cracking, being so small that they are incredibly robust and tough.Hence, peculiar assemblies can be achieved on flexible substrates to develop high-performance flexible/wearable gas sensors.Furthermore, it is reported that NPs have no efficient self-heating effects, which typically form agglomerations.In contrast, NWs with 1D morphology have good efficiency in self-heating mode [30][31][32][33][34]. Therefore, it seems that metal oxide NWs can be promising materials for the realization of highly sensitive, flexible, and low-power-consumption gas sensors.

VLS Growth Mechanism and Fundamentals
Often, the VLS mechanism has four steps (Figure 1): (i) the transport of vapor phase precursors, (ii) adsorption and dissolution of vapors at the droplet, resulting in catalyst alloy formation, (iii) diffusion of dissolved species in droplets, and (iv) precipitation and growth at the liquid-solid (L/S) interface.Catalyst alloy formation for two-and three-valence metal oxides such as ZnO and In2O3 can be described as follows [36].From MDPI: M(v) + Au(l) = AuM(l) for two-valence metal oxides (1) 2M(v) + 2Au(l) = 2AuM(l) for three-valence metal oxides The formation of NWs proceeds as follows [36]: AuM(l) + M(v) + ½ O2 = AuM(l) + MO(s) for two-valence metal oxides (3) 2AuM(l) + 4M(v) + 3O2 (g) = 2AuM(l)+ 2M2O3(s) for three-valence metal oxides (4) Figure 2 schematically shows a typical thermal CVD setup for the synthesis of NWs using a VLS mechanism [37].The process is usually performed in a system composed of a horizontal tube furnace, a pump system, an alumina or quartz tube, a gas supply, and a gas control unit.The carrier gas enters the inlet of the tube.The starting material is put on a crucible, and then it is put at the center of the tube, in which the maximum temperature is provided.The substrate at a lower temperature is placed downstream at a fixed distance from the alumina boat for collecting growth products.Upon evacuating the tube, thermal vaporization occurs, and growth starts on the substrate [38,39].Catalyst alloy formation for two-and three-valence metal oxides such as ZnO and In 2 O 3 can be described as follows [36].From MDPI: M(v) + Au(l) = AuM(l) for two-valence metal oxides (1) 2M(v) + 2Au(l) = 2AuM(l) for three-valence metal oxides The formation of NWs proceeds as follows [36]: O 2 = AuM(l) + MO(s) for two-valence metal oxides (3) 2AuM(l) + 4M(v) + 3O 2 (g) = 2AuM(l)+ 2M 2 O 3 (s) for three-valence metal oxides (4) Figure 2 schematically shows a typical thermal CVD setup for the synthesis of NWs using a VLS mechanism [37].The process is usually performed in a system composed of a horizontal tube furnace, a pump system, an alumina or quartz tube, a gas supply, and a gas control unit.The carrier gas enters the inlet of the tube.The starting material is put on a crucible, and then it is put at the center of the tube, in which the maximum temperature is provided.The substrate at a lower temperature is placed downstream at a fixed distance from the alumina boat for collecting growth products.Upon evacuating the tube, thermal vaporization occurs, and growth starts on the substrate [38,39].Catalyst alloy formation for two-and three-valence metal oxides such as ZnO and In2O3 can be described as follows [36].From MDPI: M(v) + Au(l) = AuM(l) for two-valence metal oxides (1) The formation of NWs proceeds as follows [36]: AuM(l) + M(v) + ½ O2 = AuM(l) + MO(s) for two-valence metal oxides (3) 2AuM(l) + 4M(v) + 3O2 (g) = 2AuM(l)+ 2M2O3(s) for three-valence metal oxides (4) Figure 2 schematically shows a typical thermal CVD setup for the synthesis of NWs using a VLS mechanism [37].The process is usually performed in a system composed of a horizontal tube furnace, a pump system, an alumina or quartz tube, a gas supply, and a gas control unit.The carrier gas enters the inlet of the tube.The starting material is put on a crucible, and then it is put at the center of the tube, in which the maximum temperature is provided.The substrate at a lower temperature is placed downstream at a fixed distance from the alumina boat for collecting growth products.Upon evacuating the tube, thermal vaporization occurs, and growth starts on the substrate [38,39].In more detail, usually, clean and macro-defect-free Si substrates are used for VLS growth.However, sometimes alumina or glass are also used.First, a layer of catalyst, usually Au, is coated over the substrate using different techniques such as sputtering.Subsequently, the substrate is put in a reaction tube, and it is heated where the catalyst is melted.Depending on the type of catalyst and substrate, the droplet starts to melt at a certain temperature.For Si substrates, by heating above the eutectic point of 363 • C in the Si-Au system, a droplet will appear.Then, the vapor-phase gas of the desired material to be grown flows through the reaction tube.The starting material can be in a solid form, and by using different heating techniques, it will evaporate.Vapor-phase atoms reach the droplet surface using diffusion or direct impingement.Indeed, due to the larger sticking coefficient of the liquid droplet relative to that of the solid substrate, the vapor-phase atoms prefer to be deposited onto the droplet and form an alloy.When the droplet is supersaturated with vapor-phase atoms, the driving force for the growth is provided, and crystal growth occurs at the L/S interface by precipitation where unidirectional growth starts, leading to the growth of NWs.During the growth of metal oxides, Ar or N 2 as the carrier gas and oxygen gas are used.O 2 gas can affect the stoichiometry and formation of metal oxide NWs.The NW diameter is mainly affected by droplets sizes.Moreover, the growth rate mostly depends on supersaturation, which is governed by the amount of the vapor-phase precursor and the substrate temperature.Often, the droplet is attached at the tip of the NW [40].Hence, 1D nanostructures with a high aspect ratio and vertically aligned NWs can be synthesized [41,42].
When there is no catalyst for the growth of nanostructures, it is known as vaporsolid (VS), and growth occurs in a random distribution on the substrate.Moreover, the motivation for growth is the decrease in surface free energy.Upon the first condensation, the as-formed seeds serve as nucleation sites for subsequent growth.Actually, they facilitate directional growth to lower the surface energy [33].However, the nanostructure produced by the VLS mechanism shows much higher aspect ratios and better crystalline quality relative to those grown by the VS mechanism [43].Different synthesis techniques including physical routes such as PLD, thermal evaporation, and chemical routes such as CVD are used to provide the vapor-phase atoms for the growth of NWs and to tune the crystallinity, diameter, and aspect ratio in the VLS growth mechanism [33,44].

Metal Catalyst
Since the growth, growth site, and diameter of NWs are mainly governed by catalysts, catalysts are essential during VLS growth [45].A liquid alloy is usually formed with the material within its growth conditions.A good metal catalyst should have these merits: (i) The precursor should be soluble in the catalyst to supersaturate it, and (ii) the catalyst should not have a low surface tension due to the lack of stability.Furthermore, it causes the formation of flatter droplets, which are not suitable for VLS growth [2].(iii) The vapor pressure of the catalyst should be small, and (iv) it must be chemically inert [46].Generally, Au is commonly used as a catalyst since it is inert and can be easily coated onto a substrate while it forms a eutectic with most substrates [47].However, sometimes other catalysts are also used.For example, ultrathin (15 nm) GeS NWs have been grown in the presence of Bi as a catalyst [48].

Effect of Deposition Temperature
The selection of the source temperature depends on the volatile nature of the source material.However, it is slightly lower than the melting point (T m ) of the starting material.The temperature can affect the droplet sizes, the amount of the generated vapors, and the length of NWs.Generally, higher deposition temperatures lead to smaller and denser droplets, resulting in the formation of thinner and denser NWs.At lower temperatures, the NWs become thicker and shorter.Indeed, at higher temperatures, more vapors and faster diffusion lead to the formation of denser, thinner, and longer NWs.However, very high temperatures lead to a high desorption rate, lessening the crystal growth [49].

Effect of the Deposition Pressure
As the pressure inside the chamber decreases, the generated vapors are transferred to be adsorbed on the Au droplets in the lower-temperature region, leading to crystal growth.The growth rate drops as the pressure decreases because less of the vapors can be moved to the lower temperature zone, leading to a slower growth rate [13].

Effect of the Oxygen Gas Flow
Metal vapors are oxidized more at a higher oxygen flow rate.Hence, the growth rate often increases with the oxygen flow rate.However, it should be noted that a very high oxygen flow rate induces the growth of NWs that are too long [13].In the remaining parts of this review paper, we discuss the gas-sensing properties of metal oxide NWs grown by the VLS growth mechanism.It should be noted that there are already some review papers discussing VLS growth of different materials for different applications.However, as far as we know, there is no review paper related to VLS-grown metal oxide NWs for gas-sensing applications.

SnO 2 NWs
The T m of Sn is only 231.9 • C. As a result, the SnO 2 NWs can be easily synthesized by the VLS growth mechanism [50,51].Other morphologies such as SnO 2 nanobelts have also been reported by using this mechanism [52].In addition, doping of other elements through starting materials or through the catalyst is possible [53].During VLS growth, doping can be performed, and Sb is a stable dopant which can be easily incorporated into the SnO 2 lattice.In this context, Sb-doped SnO 2 NWs were synthesized by the VLS growth mechanism using Sn and Sb powders.They were synthesized at 950 • C for 90 min on a Au-coated (20 nm) Si/SiO 2 substrate.Figure 3a,b show SEM micrographs of as-grown NWs and a single NW with a Au NP on its tip, respectively, demonstrating the VLS growth mechanism.Moreover, Figure 3c,d exhibit the EDS spectra captured at the Au NP and at the NW surface, respectively [54], in which the presence of Sb as the dopant can be confirmed.Even though these NWs probably had good potential for gas sensing, their sensing performance was not studied.As a result, this system needs to be explored more in regard to this aspect in the future.In other research, SnO2 NWs were grown on bare SnO2 (10 nm) and Au (10 nm)coated SiO2/Si (100) substrates at 900 °C for 1 h.The rectangular cross-section was obtained for the NWs deposited onto pristine and Au-coated substrates, while the NWs grown on a SnO2-coated substrate look like nanoribbons.Based on the H2 gas-sensing results, the sensor fabricated using SnO2 NWs grown on the Au-coated substrate had a higher response than other gas sensors.It displayed a response of 11.5 to 1000 ppm H2 gas at 100 In other research, SnO 2 NWs were grown on bare SnO 2 (10 nm) and Au (10 nm)-coated SiO 2 /Si (100) substrates at 900 • C for 1 h.The rectangular cross-section was obtained for the NWs deposited onto pristine and Au-coated substrates, while the NWs grown on a SnO 2 -coated substrate look like nanoribbons.Based on the H 2 gas-sensing results, the sensor fabricated using SnO 2 NWs grown on the Au-coated substrate had a higher response than other gas sensors.It displayed a response of 11.5 to 1000 ppm H 2 gas at 100 • C. The Au NPs on the tip of the NWs improved the conductivity of NWs and resulted in easy H 2 gas dissociation.In fact, Au NPs are not only helpful for the VLS growth of NWs but also enhance the response of the sensor.Furthermore, the high surface area allowed more H 2 gases to be adsorbed on NWs, contributing to the sensing signal [55].
Kim et al. [56] synthesized SnO 2 NWs and investigated the effect of temperature (700-900 • C) on their morphologies and diameters in the presence of Bi as a catalyst.Most of the NWs which were synthesized at 700 • C (Figure 4a) had a polygonal-shaped NP on their tips (inset in Figure 4a).The diameters of the NWs and NPs were 30-120 nm and 60-180 nm, respectively.Moreover, in the products grown at 800 • C, both particle-ended NWs and particle-free NWs (Figure 4b and inset) were observed.Figure 4c and its inset show that NWs grown at 900 • C had no particles on their tips.As shown in Figure 4d, when a mixed powder of Bi/Sn was evaporated, liquid droplets containing Sn, Bi, and O were adsorbed on the substrate.After supersaturation with these elements, short SnO 2 nuclei precipitated at the S/L interface.By constantly dissolving the mentioned elements onto the NPs, SnO 2 NWs grew along with crystalline Bi 2 Sn 2 O 7 NPs on their tips.The sample grown at 700 • C was used as an oxygen gas sensor; at 25 • C, it was able to detect oxygen gas with a stable signal.However, no selectivity study was reported.Sometimes, SnO powder can be used instead of Sn powder for the growth of Sn NWs [57].For example, Cakestani et al. [58] used commercial SnO powder rather th elemental Sn powder to better control the evaporation process.The dissociation of S only takes place at 800-1000 °C.2SnO (s) → Sn (l) + SnO2 (s) Furthermore, in spite of pure liquid Sn, the mixture of SnO2 (s) and Sn (l) did not fl inside the crucible.The dissociation starts in the presence of the carrier gas (Ar).Then vapors are moved to the substrates and are condensed into liquid droplets.Crystallizat into SnO2 NWs occurs in the presence of oxygen gas.However, sensing studies were carried out.In another study, SnO2 NWs were grown at 800 °C (using Sn powder) and Sometimes, SnO powder can be used instead of Sn powder for the growth of SnO 2 NWs [57].For example, Cakestani et al. [58] used commercial SnO powder rather than elemental Sn powder to better control the evaporation process.The dissociation of SnO only takes place at 800-1000 • C. 2SnO (s) → Sn (l) + SnO 2 (s) (5) Furthermore, in spite of pure liquid Sn, the mixture of SnO 2 (s) and Sn (l) did not flow inside the crucible.The dissociation starts in the presence of the carrier gas (Ar).Then, Sn vapors are moved to the substrates and are condensed into liquid droplets.Crystallization into SnO 2 NWs occurs in the presence of oxygen gas.However, sensing studies were not carried out.In another study, SnO 2 NWs were grown at 800 • C (using Sn powder) and 980 • C (using SnO powder) for 30 min on Si substrates.Furthermore, hierarchical SnO 2 nanostructures were synthesized by producing SnO 2 NWs first, followed by subsequent SnO 2 branching.FE-SEM images of different SnO 2 nanostructures are provided in Figure 5a-d.According to the sensing results, the responses of hierarchical SnO 2 nanostructures were higher than those of SnO 2 NWs for LPG and NH 3 gases (Figure 6a,b).The width of the EDL as well as the height of the barrier potentials change in the presence of the target gas.Improved sensing characteristics of SnO 2 hierarchical nanostructures are related to (i) the porous nature due to the hierarchical structure, (ii) the existence of much more active sites, and (iii) the presence of more homojunctions barriers between SnO 2 -SnO 2 NWs in hierarchical nanostructures (Figure 6c,d) [59].Au-coated Si substrates on which trenches with widths of 10 to 80 µm were produced by a photolithographic process were used for the growth of networked SnO 2 NWs at 900 • C for 15 min [60].In general, as the trench width decreased, the number of junctions increased (Figure 7a-f).The sensor with a 10 µm wide trench displayed a response of 3 to 10 ppm NO 2 at 300 • C. Indeed, both the densely packed NWs and small trench size limited the flow of Sn vapor during VLS growth, mainly resulting in the growth of NWs at the upper surface of the trench.This eventually led to a poor response to NO 2 gas due to a shortage of enough NWs for sensing of the gas.For the sensor with the 20 µm wide trench, the response increased to 25.However, further increasing the trench width resulted in decreased performance.This sensor had the largest overlapped area among NWs, leading to a lot of homojunctions between the NWs and a significant modulation of the resistance in the NO 2 gas medium.Hence, the trench width should be optimized to obtain maximum response for a gas sensor.Au-coated Si substrates on which trenches with widths of 10 to 80 µm were produced by a photolithographic process were used for the growth of networked SnO2 NWs at 900 °C for 15 min [60].In general, as the trench width decreased, the number of junctions increased (Figure 7a-f).The sensor with a 10 µm wide trench displayed a response of 3 to 10 ppm NO2 at 300 °C.Indeed, both the densely packed NWs and small trench size limited the flow of Sn vapor during VLS growth, mainly resulting in the growth of NWs at the upper surface of the trench.This eventually led to a poor response to NO2 gas due to a shortage of enough NWs for sensing of the gas.For the sensor with the 20 µm wide trench, the response increased to 25.However, further increasing the trench width resulted in decreased performance.This sensor had the largest overlapped area among NWs, leading to a lot of homojunctions between the NWs and a significant modulation of the resistance in the NO2 gas medium.Hence, the trench width should be optimized to obtain maximum response for a gas sensor.Branched NWs can offer a large surface area, as an advantage for sensing applications.In this regard, by initially using metallic Sn powders, SnO2 NWs were grown on a Au (10 nm)-Si substrate at 960 °C for 1 h.Then, In2O3 NWs branches were grown on the stem of SnO2 NWs at 860 °C for 1 h using a self-catalytic VLS process.At 300 °C, the branched NWs comprised of SnO2 and In2O3 revealed a response of 1.4 to 10 ppm CO gas.The sensing mechanism was related to active centers due to oxygen vacancy defects, Branched NWs can offer a large surface area, as an advantage for sensing applications.In this regard, by initially using metallic Sn powders, SnO 2 NWs were grown on a Au (10 nm)-Si substrate at 960 • C for 1 h.Then, In 2 O 3 NWs branches were grown on the stem of SnO 2 NWs at 860 • C for 1 h using a self-catalytic VLS process.At 300 • C, the branched NWs comprised of SnO 2 and In 2 O 3 revealed a response of 1.4 to 10 ppm CO gas.The sensing mechanism was related to active centers due to oxygen vacancy defects, which facilitated the adsorption of oxygen and enhanced the interaction with CO gas, and the formation of heterojunctions between In 2 O 3 and SnO 2 along with the large surface area of the branched NWs [61].
One of unique features of the VLS growth mechanism is that the growth can occur directly on the electrodes on the substrate.In a relevant study using Sn powders, SnO 2 NWs were directly fabricated on an electrode substrate comprised of Pt (5000 Å), Ti (500 Å), and Au (200 Å) layers.Growth at 700-800 • C for 20 to 30 min was carried out in the presence of different flow rates of O 2 (1 to 10 sccm).The diameters of the NWs synthesized under different conditions were 20-60 nm.The optimal sensor fabricated from the NWs with the smallest diameter displayed a response of 43 to 10 ppm NO 2 gas with a response time (t res ) of 38 s and a recovery time (t rec ) of 25 s at 200 • C. The presence of plenty of homojunctions between SnO 2 -SnO 2 NWs led to improved response performance [62].In another research work, SnO 2 NWs were directly grown on Au-coated Si having electrodes with a gap size of 5 µm and using Sn powders at 750 • C for 30 min.The sensor displayed a high response of 110 to 5 ppm NO 2 gas at 200 • C with fast dynamics.The formation of an electron depletion layer (EDL) around each NW as well as the formation of potential barriers at the SnO 2 -SnO 2 NW junctions contributed to the sensing signal [63].In another research work, single-crystalline SnO 2 NWs were directly synthesized on a SiO 2 /Si substrate having electrodes by the VLS growth mechanism at 900 • C. SnO 2 NWs with diameters of 30-200 nm and lengths of several micrometers were synthesized.At 150 • C, the sensor revealed a response of 7 to 1000 ppm H 2 [64].However, no other sensing tests such as selectivity were reported.
In another work, SnO 2 NWs were synthesized on an interdigitated electrode (Ti, Pt, and Au; 10, 100, and 3 nm, respectively) substrate patterned by the VLS mechanism at 900 • C for 10 min, where subsequent attachment of Co 3 O 4 was carried out via a sol-gel route.The response of Co 3 O 4 NP-attached SnO 2 NWs to 50 ppm acetone gas at 300 • C was ~7 times more than that of the bare sensor.In fact, in contact areas, due to the flow of electrons from SnO 2 to Co 3 O 4 , the initial conduction channel inside the SnO 2 NWs decreased relative to pure SnO 2 NWs and subsequent significant expansion of the conduction channel in an acetone atmosphere, resulting in a high response to acetone gas [65].In a similar study, SnO 2 NWs were initially grown on an electrode substrate with a very similar procedure described previously [65].Then, Fe 2 O 3 NPs were coated onto SnO 2 NWs by spin coating of a Fe 3 + solution and thermal annealing.As a result, desired NWs with good morphology were obtained (Figure 8a-d).At 300 • C, the Fe 2 O 3 NP-coated SnO 2 NW sensor showed an almost six times higher response relative to the pristine sensor (Figure 8e), along with good selectivity to ethanol gas (Figure 8f).The presence of Fe 2 O 3 -SnO 2 heterojunctions was attributed as the main reason for improved gas response, relative to the pristine sensor.Indeed, for the pristine sensor, the variations in EDL upon exposure to ethanol gas were not significant (Figure 9a).However, for the decorated sensor, the initial volume of the conduction channel was significantly decreased, owing to the presence of Fe 2 O 3 NPs in the air.By subsequent exposure to ethanol, a remarkable change in the conduction channel inside the SnO 2 NWs occurred (Figure 9b), resulting in a larger response than that from the pure sensor [66].CuO is a promising candidate for H 2 S gas sensing due to direct reaction with H 2 S gas [67].In this context, SnO 2 NWs initially were synthesized at 900 • C for 10 min on a Au/Si substrate.Then, by sputtering, a Cu layer was coated onto the SnO 2 NWs.Finally, branches of SnO 2 NWs were synthesized at 300-700 • C for 30 min.For the NWs grown at 700 • C, CuO NPs were formed at their tips.The presence of CuO significantly enhanced the response to H 2 S. The primary reason for the enhanced selectivity to H 2 S is related to the reaction between CuO and H 2 S gases, as follows.
Hence, CuO with semiconducting properties was transformed into CuS with high conductivity, which has much better conductivity than CuO.This significantly increased the resistance modulation [68].In another study related to H 2 S gas sensing, SnO 2 NWs were prepared at 900 • C for 1 h after the deposition of a Au layer on porous Si.In Figure 10a-f, the growth of SnO 2 NWs on porous Si is shown.The results of H 2 S gas sensing showed an improved response of SnO 2 NWs/porous Si relative to porous Si (Figure 10g-i).The heterojunctions between SnO 2 and Si caused a flow of electrons from SnO 2 to Si and vice versa.Hence, the hole conduction volume in the porous Si was considerably decreased.In H 2 S gas medium, due to the combination of electrons by holes, the hole resistance increased.Moreover, defects at the interfaces between the two materials were created due to lattice mismatch between them, leading to an enhanced response due to providing additional favorable adsorption sites [69].
CuO is a promising candidate for H2S gas sensing due to direct reaction with H2S ga [67].In this context, SnO2 NWs initially were synthesized at 900 °C for 10 min on a Au/S substrate.Then, by sputtering, a Cu layer was coated onto the SnO2 NWs.Finally, branche of SnO2 NWs were synthesized at 300-700 °C for 30 min.For the NWs grown at 700 °C CuO NPs were formed at their tips.The presence of CuO significantly enhanced the re sponse to H2S.The primary reason for the enhanced selectivity to H2S is related to th reaction between CuO and H2S gases, as follows.
CuO + H2S = CuS + H2O ( 6Hence, CuO with semiconducting properties was transformed into CuS with hig conductivity, which has much better conductivity than CuO.This significantly increase the resistance modulation [68].In another study related to H2S gas sensing, SnO2 NW were prepared at 900 °C for 1 h after the deposition of a Au layer on porous Si.In Figur 10a-f, the growth of SnO2 NWs on porous Si is shown.The results of H2S gas sensin showed an improved response of SnO2 NWs/porous Si relative to porous Si (Figure 10g i).The heterojunctions between SnO2 and Si caused a flow of electrons from SnO2 to Si an vice versa.Hence, the hole conduction volume in the porous Si was considerably de creased.In H2S gas medium, due to the combination of electrons by holes, the hole re sistance increased.Moreover, defects at the interfaces between the two materials were cre ated due to lattice mismatch between them, leading to an enhanced response due t providing additional favorable adsorption sites [69].Noble metal decoration via electronic and chemical sensitizations can boost the gas sensing performance.In this regard, SnO2 NWs were synthesized by a similar method a described previously [62].Then, SnO2 NWs were uniformly decorated with Pd NPs b radiolysis using 60 Co γ-rays.Pd NPs in the range of 10-35 nm were uniformly deposite over the surface of SnO2 NWs.The Pd-decorated gas sensor showed a higher response t NO2 gas relative to the pristine sensor.The response was 19 compared to only 0.1 ppm NO2 gas and 66 to 1 ppm at 200 °C [70].The good features of the Pd-decorated sensor ar Noble metal decoration via electronic and chemical sensitizations can boost the gassensing performance.In this regard, SnO 2 NWs were synthesized by a similar method as described previously [62].Then, SnO 2 NWs were uniformly decorated with Pd NPs by radiolysis using 60 Co γ-rays.Pd NPs in the range of 10-35 nm were uniformly deposited over the surface of SnO 2 NWs.The Pd-decorated gas sensor showed a higher response to NO 2 gas relative to the pristine sensor.The response was 19 compared to only 0.1 ppm NO 2 gas and 66 to 1 ppm at 200 • C [70].The good features of the Pd-decorated sensor are related to the catalytic activity of Pd NPs towards oxygen and NO 2 gases, as well as the creation of Schottky contacts between Pd and SnO 2 NWs.Modulation of the Schottky barriers in the NO 2 atmosphere led to remarkable variations in the resistance and appearance of the sensing signal.Similarly, SnO 2 NWs were grown on an electrode substrate.Then, Pd decoration was performed using UV-light irradiation (λ = 360 nm) of a PdCl 2 solution in the presence of SnO 2 NWs.By subsequent annealing at 500 • C, Pd NPs were dispersed on SnO 2 NWs as isolated islands.Figure 11a-f show that SnO 2 NWs with an approximate diameter of 50 nm and desired phases, along with Pd decoration, were successfully produced.The response to 10 ppm H 2 gas at 300 • C was boosted by 141.4% after Pd decoration in addition to better selectivity to H 2 gas.Pd NPs facilitated the dissociation of O 2 and H 2 gases.H 2 decomposes to atomic H by a spillover effect, and it moves to the neighboring SnO 2 surface by reaction with oxygen species, resulting in a significant decrease in sensor resistance (Figure 11g-h).Moreover, formation of Pd and SnO 2 Schottky barriers led to the modulation of the sensor resistance in the H 2 gas atmosphere due to a change in the height of barriers in the H 2 gas atmosphere [71].In another study, SnO2 NWs were prepared using a VLS mechanism on a Au-deposited Al2O3 substrate at 900 °C for 1 h.Pd embedding on SnO2 was used by applying a flame for 5 s at 1300 °C on PdCl2 containing SnO2 NWs.The sensor showed a higher response than the SnO2 NW sensor to NO2 gas, along with faster dynamics.The presence of oxygen vacancies, which are regarded as favorable sites for gas adsorption (Figure 12a), the formation of Schottky junctions between Pd and SnO2-x (Figure 12b,c), and the spillover effect of Pd NPs (Figure 12d) all contributed to the response [72].In another study, SnO 2 NWs were prepared using a VLS mechanism on a Au-deposited Al 2 O 3 substrate at 900 • C for 1 h.Pd embedding on SnO 2 was used by applying a flame for 5 s at 1300 • C on PdCl 2 containing SnO 2 NWs.The sensor showed a higher response than the SnO 2 NW sensor to NO 2 gas, along with faster dynamics.The presence of oxygen vacancies, which are regarded as favorable sites for gas adsorption (Figure 12a), the formation of Schottky junctions between Pd and SnO 2-x (Figure 12b,c), and the spillover effect of Pd NPs (Figure 12d) all contributed to the response [72].

In2O3 NWs
In2O3 is an n-type metal oxide which is highly used for the sensing of differen Pristine and doped In2O3 NWs have also been widely synthesized by VLS growth By control of the temperature of precursors and source materials, different 1 nanostructures can be synthesized [76,77].Figure 13 schematically shows the depe of the In2O3 morphology on the growth temperature [78,79].Indeed, by careful over the synthesis temperature, it is possible to obtain the desired morphology.E alyst-free growth of In2O3 NWs has been reported [80].There have been studies in the synthesis of In2O3 NWs by VLS growth for gas-sensing applications, which viewed below.

In 2 O 3 NWs
In 2 O 3 is an n-type metal oxide which is highly used for the sensing of different gases.Pristine and doped In 2 O 3 NWs have also been widely synthesized by VLS growth [73][74][75].By control of the temperature of precursors and source materials, different 1D In 2 O 3 nanostructures can be synthesized [76,77].Figure 13 schematically shows the dependency of the In 2 O 3 morphology on the growth temperature [78,79].Indeed, by careful control over the synthesis temperature, it is possible to obtain the desired morphology.Even catalyst-free growth of In 2 O 3 NWs has been reported [80].There have been studies involving the synthesis of In 2 O 3 NWs by VLS growth for gas-sensing applications, which are reviewed below.In2O3 powder was used for the growth of In2O3 NWs with the help of a Pd c 1500 °C for 1.5 h.The thinnest NWs were as narrow as 20 nm.The response of th fabricated from these NWs was about 7 to 25 ppm of acetone at 400 °C [81].Howe the growth temperature and sensing temperature were high.Moreover, no s study was performed.In another study, In2O3 NWs were produced by the VL nism.They had a length of 5 µm and diameters in the range of 10-100 nm.Tw were prepared from the single NW or NWs.The single NW sensor showed a res 2.6 to 0.5 ppm at 200 °C, while the In2O3 multi-NW sensor revealed a response of ppm NO2 gas [82].However, it should be noted that making a sensing device wit NW is challenging.To explore the effect of the electrode gap (10, 30, and 40 µ NWs were grown from metallic In powder on the sensor with different electrode (Figure 14a-d).The growth was carried out at 700 °C for 0.5 h.The sensors were in a self-heating condition using a low voltage of 1.5 V.Among different sensors, NW sensor with an electrode gap of 40 µm exhibited the best response of 1.37 to 1 along with response and recovery times of less than 12 and 35 s, respectively [83 ver, the consumed power increased with a decrease in the electrode gap size.Ind a small gap (10 and 30 µm), the density of the NWs between the gap was high leading to high power consumption.Despite this, in the large gap (40 µm) elect density of the NWs was low, resulting in low power consumption.In 2 O 3 powder was used for the growth of In 2 O 3 NWs with the help of a Pd catalyst at 1500 • C for 1.5 h.The thinnest NWs were as narrow as 20 nm.The response of the sensor fabricated from these NWs was about 7 to 25 ppm of acetone at 400 • C [81].However, both the growth temperature and sensing temperature were high.Moreover, no selectivity study was performed.In another study, In 2 O 3 NWs were produced by the VLS mechanism.They had a length of 5 µm and diameters in the range of 10-100 nm.Two sensors were prepared from the single NW or NWs.The single NW sensor showed a response of 2.6 to 0.5 ppm at 200 • C, while the In 2 O 3 multi-NW sensor revealed a response of 2.1 to 0.5 ppm NO 2 gas [82].However, it should be noted that making a sensing device with a single NW is challenging.To explore the effect of the electrode gap (10, 30, and 40 µm), In 2 O 3 NWs were grown from metallic In powder on the sensor with different electrode gap sizes (Figure 14a-d).The growth was carried out at 700 • C for 0.5 h.The sensors were operated in a self-heating condition using a low voltage of 1.5 V.Among different sensors, the In 2 O 3 NW sensor with an electrode gap of 40 µm exhibited the best response of 1.37 to 1000 ppm along with response and recovery times of less than 12 and 35 s, respectively [83].Moreover, the consumed power increased with a decrease in the electrode gap size.Indeed, with a small gap (10 and 30 µm), the density of the NWs between the gap was highly dense, leading to high power consumption.Despite this, in the large gap (40 µm) electrode, the density of the NWs was low, resulting in low power consumption.In another study, In2O3 NWs were synthesized at 800 °C for 1 h, and the effect of the flow rate of the carrier gas on the morphology of NWs was explored.A low flow rate (50 mL min −1 ) of the carrier gas resulted in an insufficient supply of gas to the substrate; hence, In2O3 NPs were formed.By increasing the flow rate to 100 mL min −1 , the supply of atoms was increased to form the 1D In2O3 NWs.Hence, due to the higher reactive stick possibility of gas on the catalyst droplet and precipitation at the catalyst-NW interface, high-aspectratio NWs with lengths of 1-5 µm and diameters of 100 nm were synthesized.However, the increase in the flow rate to 100 mL min −1 led to the formation of microrods with a low aspect ratio.Using In2O3 NWs, a gas sensor was fabricated which showed a low response of 1.2 to 1000 ppm H2 gas at 250 °C along with a sensor tres of 48 s and a trec of 176 s [84].However, the response was very low for practical applications.In another interesting study, Au-catalyst-loaded nickel foams were initially synthesized by Au sputtering for 5, 10, 20, and 30 s.Then, using In and Sn powders (mass ratio of 10:1), Sn-doped indium In2O3 NWs were fabricated at 835℃ for 0.5 h.At 125 °C, the sensor prepared with 10 nmthick Au catalyst showed the highest sensing response of 160 to 100 ppm ethylene glycol along with fast response and recovery times of 8.7 s and 19.3 s, respectively.For a Au sputtering time of 5 s, the density of NWs on NiO was small.Moreover, for sputtering times of 20 and 30 s, NWs completely covered the NiO surface and significantly decreased the number of p-NiO/n-Sn-doped In2O3 heterojunctions, causing a decrease in the sensing response.Under the optimal sputtering time (10 s), sufficient amounts of p-n junctions were formed, leading to improved gas response [85].Furthermore, the Ni 3+ ions had a catalytic effect, and the highest amount of adsorbed oxygen and oxygen vacancies in the optimal sensor contributed to the sensing signal.To synthesize Zn-doped In2O3, ZnO and In2O3 powders with graphite powder were heated at 900 °C for 30 min to grow Zn-doped in2O3 NWs on a Au-coated (9 nm) alumina substrate.The optimized molar fraction of Zn was 3% since higher amounts of Zn resulted in the formation of a ZnO phase.Due to the different sizes of Au catalysts, NWs with different diameters (70-150 nm) were obtained.Interestingly, at high temperatures (300 °C and above), the sensor showed an unusual nto-p transition.Zn-doping can significantly increase the oxygen vacancies in In2O3 and surface defects.Higher defects led to more adsorption of oxygen species from ambient air.At high temperatures, adsorbed oxygen ions diffused into the bulk regions and filled the oxygen vacancies, leading to a decrease in electrons.Furthermore, capturing of electrons by NO2 gas led to a further decrease in the electrons.Hence, a down shift of the Fermi In another study, In 2 O 3 NWs were synthesized at 800 • C for 1 h, and the effect of the flow rate of the carrier gas on the morphology of NWs was explored.A low flow rate (50 mL min −1 ) of the carrier gas resulted in an insufficient supply of gas to the substrate; hence, In 2 O 3 NPs were formed.By increasing the flow rate to 100 mL min −1 , the supply of atoms was increased to form the 1D In 2 O 3 NWs.Hence, due to the higher reactive stick possibility of gas on the catalyst droplet and precipitation at the catalyst-NW interface, high-aspect-ratio NWs with lengths of 1-5 µm and diameters of 100 nm were synthesized.However, the increase in the flow rate to 100 mL min −1 led to the formation of microrods with a low aspect ratio.Using In 2 O 3 NWs, a gas sensor was fabricated which showed a low response of 1.2 to 1000 ppm H 2 gas at 250 • C along with a sensor t res of 48 s and a t rec of 176 s [84].However, the response was very low for practical applications.In another interesting study, Au-catalyst-loaded nickel foams were initially synthesized by Au sputtering for 5, 10, 20, and 30 s.Then, using In and Sn powders (mass ratio of 10:1), Sn-doped indium In 2 O 3 NWs were fabricated at 835 • C for 0.5 h.At 125 • C, the sensor prepared with 10 nm-thick Au catalyst showed the highest sensing response of 160 to 100 ppm ethylene glycol along with fast response and recovery times of 8.7 s and 19.3 s, respectively.For a Au sputtering time of 5 s, the density of NWs on NiO was small.Moreover, for sputtering times of 20 and 30 s, NWs completely covered the NiO surface and significantly decreased the number of p-NiO/n-Sn-doped In 2 O 3 heterojunctions, causing a decrease in the sensing response.Under the optimal sputtering time (10 s), sufficient amounts of p-n junctions were formed, leading to improved gas response [85].Furthermore, the Ni 3+ ions had a catalytic effect, and the highest amount of adsorbed oxygen and oxygen vacancies in the optimal sensor contributed to the sensing signal.To synthesize Zn-doped In 2 O 3 , ZnO and In 2 O 3 powders with graphite powder were heated at 900 • C for 30 min to grow Zn-doped in 2 O 3 NWs on a Au-coated (9 nm) alumina substrate.The optimized molar fraction of Zn was 3% since higher amounts of Zn resulted in the formation of a ZnO phase.Due to the different sizes of Au catalysts, NWs with different diameters (70-150 nm) were obtained.Interestingly, at high temperatures (300 • C and above), the sensor showed an unusual n-to-p transition.Zn-doping can significantly increase the oxygen vacancies in In 2 O 3 and surface defects.Higher defects led to more adsorption of oxygen species from ambient air.At high temperatures, adsorbed oxygen ions diffused into the bulk regions and filled the oxygen vacancies, leading to a decrease in electrons.Furthermore, capturing of electrons by NO 2 gas led to a further decrease in the electrons.Hence, a down shift of the Fermi level (E F ) in NWs occurred.In addition, by Zn-doping, Zn 2+ replaced In 3+ and contributed to the downward shift of the E F .The down shift of the E F caused the transfer of electrons in the opposite path, resulting in an opposite sensor response at high temperatures.Zn-doped In 2 O 3 NWs showed a 30 times higher response to ethanol relative to the pristine sensor, due to the high oxygen vacancies and higher defects in Zn-doped In 2 O 3 NWs [86].
Using In 2 O 3 powder, In 2 O 3 NWs were grown at 1500 • C for 1.5 h.The growth was performed on bare alumina and alumina seeded by metallic "In" NPs.For the former case, micrometer-sized nanostructures were formed; in the second case, connected NWs were formed.The NWs gas sensor had higher responses to acetone and NO 2 gases relative to a thick wire gas sensor at 400 • C owing to the higher surface area of NWs, which provided a lot of adsorption sites for the incoming gas [87].Materials 2023, 16, x FOR PEER REVIEW level (EF) in NWs occurred.In addition, by Zn-doping, Zn 2+ replaced In 3+ and contr to the downward shift of the EF.The down shift of the EF caused the transfer of ele in the opposite path, resulting in an opposite sensor response at high temperatur doped In2O3 NWs showed a 30 times higher response to ethanol relative to the p sensor, due to the high oxygen vacancies and higher defects in Zn-doped In2O3 NW Using In2O3 powder, In2O3 NWs were grown at 1500 °C for 1.5 h.The grow performed on bare alumina and alumina seeded by metallic "In" NPs.For the forme micrometer-sized nanostructures were formed; in the second case, connected NW formed.The NWs gas sensor had higher responses to acetone and NO2 gases relati thick wire gas sensor at 400 °C owing to the higher surface area of NWs, which pr a lot of adsorption sites for the incoming gas [87].Growth of In2O3 nanostructures In based on the VLS mechanism on a Au-coated SiO2 substrate was carried out at 7 °C for 1 h.At 700 °C, uniform In2O3 nanostructures were grown.At 800 °C, the di and length of the NWs were 70-80 nm and several micrometers, respectively.The p displayed a high density of NWs.At 900 °C, In2O3 nanoneedles with diameters of 1 nm and lengths about 4-5 µm were formed.H2 gas-sensing results showed that th NWs had a higher response relative to In2O3 nanoneedles due to the high surface In2O3 NWs relative to nanoneedles.However, the sensor temperature was high at [88].In2O3 NWs were prepared at 800 °C for 1 h on a Au-coated (3 nm) Si substrate Pt was sputtered onto In2O3 NWs, and by subsequent annealing, Pt islands were f on In2O3 NWs.At 50 °C, decorated NWs displayed a response of 4 to 400 ppm O2 gas the pristine sensor showed no response to this gas.Indeed, oxygen species were ad and disassociated on Pt NPs and then migrated onto the In2O3 NWs, where the adsorbed, leading to a high response to oxygen gas [89].Using a mixture of In2 In2Se3 powders (10:1 volume ratio), In2O3 NWs were synthesized by the VLS mech on a Au-coated (3 nm) Si wafer at 850 °C for 1 h.Second, a solution of Ti 4+ ions wa coated onto an In2O3 NW template, and after being prepared at 500 °C for 2 h, TiO (10-30 nm) were dispersed on the surface of NWs.As shown in Figure 15a-d, the s sized NWs had high aspect ratios (diameters = 50 to 100 nm).  Figure 15e displays the acetone gas-sensing performance of both gas sensors at 250 • C. Clearly, the TiO 2 -decorated In 2 O 3 NW gas sensor displayed a higher response to acetone relative to the bare gas sensor.Moreover, both gas sensors showed a high stability over a period of 30 days (Figure 15f).SnO 2 -SnO 2 homojunctions in both gas sensors contributed to the sensing signal.However, in the decorated gas sensor, the formation of TiO 2 -In 2 O 3 heterojunctions provided additional sources of the resistance modulation, causing a higher sensing performance relative to the pure gas sensor.Furthermore, the surface areas of pristine and TiO 2 -decorated NWs were 4.25 and 6.72 m 2 /g, respectively, leading to more adsorption sites on the decorated gas sensor, contributing to the sensor signal [90].
In and Mg mixed powders (weight ratio of 1:1) were taken for the growth of In 2 O 3 NWs, in which Mg lowered the melting point of the In-Mg alloy.The NWs were prepared at 800 • C for 1 h.Then, Pd was sputtered on In 2 O 3 NWs, and subsequent heat treatment was carried out at 700 • C, resulting in the dispersion of Pd NPs on the surface of NWs.The decorated sensor showed a higher response to NO 2 gas relative to the bare sensor.Enhanced sensing features were related to the catalytic effect of Pd on O 2 and NO 2 gases as well as the generation of Schottky junctions between Pd and In 2 O 3 in intimate contact [91].In another study, In 2 O 3 NWs were grown at 800-1000 • C on a Au (3 nm)-coated SiO 2 /Si substrate using metallic In powders.At 800 and 850 • C, NWs were not formed.As the growth temperature enhanced, the length increased while the diameter of the NWs decreased.Indeed, NWs with a higher aspect ratio were prepared at high temperatures due to the larger indium vapor pressure.At 300 • C, the responses to 100 ppm ethanol gas for the samples grown at 900, 950, and 1000 • C were 1.84, 3.78, and 13.97, respectively.The good response of the optimal gas sensor was related to the narrower diameter of NWs grown at 1000 • C, leading to a larger surface area.Moreover, for narrower NWs, most of the NWs were depleted of electrons, and by exposure to ethanol gas, significant variation in the width of the EDL leads to a higher response.In addition, the optimal sensor showed good selectivity to ethanol in the presence of methanol and acetone gases [92].

ZnO NWs
ZnO is a highly popular semiconducting metal oxide for gas-sensing usages, thanks to its high mobility of carriers, high stability, ease of synthesis, and low price.Using a mixed Zn and Sn powder, ZnO NWs were grown on glass substrates with interdigitated ITO electrodes where Sn was used as the catalyst for the growth of ZnO NWs at 900 • C.After the growth of ZnO NWs, Fe 2 O 3 NPs were attached to them using a chemical process.A α-Fe 2 O 3 /ZnO NW gas sensor showed a higher response to CO gas relative to both pristine Fe 2 O 3 and ZnO gas sensors.At 300 • C, it revealed a response of 18.8 to 100 ppm CO gas.The formation of heterojunctions between α-Fe 2 O 3 and ZnO and the high surface area of ZnO NWs caused an enhanced gas response relative to other gas sensors [93].Branching NWs offer a high surface area along with many heterojunctions, which are highly demanded for sensing studies.In a relevant study, core SnO 2 NWs were synthesized at 1050 • C for 2 h on a Si substrate.Then, different thicknesses (1, 3, 10, and 30 nm) of Au were deposited over the SnO 2 NWs, and ZnO branches on SnO 2 were grown at 500 • C for 1 h [94].Based on the SEM images presented in Figure 16a-d, when using a thick Au layer (10 and 30 nm), no branched structures were formed.Moreover, the diameters of the branches grown using Au with thicknesses of 1 and 3 nm were 30 and 34.5 nm, respectively.The thin (1, 3 nm) Au layers resulted in small and disconnected particles upon heating, leading to the growth of thin ZnO branches (Figure 16e).However, thick (10 and 30 nm) Au layers resulted in the formation of a connected and rough Ag layer upon heating.Hence, nuclei interfere during the initial growth, forming thick cluster-like ZnO (Figure 16f).A sensor was fabricated from the branched NWs, and at 250 • C it showed a response of 6 to 100 ppm NO 2 gas.The formation of ZnO-ZnO homojunctions along with the formation of SnO 2 -ZnO heterojunctions was attributed to the sensing mechanism.The change in the barrier height of junctions in the NO 2 gas atmosphere significantly modulated the sensor resistance [94].In another study, the growth of ZnO NWs using Zn powders was performed in the presence of Au-coated Si substrates at 900 to 1050 °C for 1 h to explore the impact of temperature.Figure 17a,c,g,e show SEM micrographs of ZnO NWs synthesized at 900, 950, 1000, and 1050 °C, respectively.According to Figure 17b,d,f,h  In another study, the growth of ZnO NWs using Zn powders was performed in the presence of Au-coated Si substrates at 900 to 1050 • C for 1 h to explore the impact of temperature.Figure 17a,c,g,e show SEM micrographs of ZnO NWs synthesized at 900, 950, 1000, and 1050 • C, respectively.According to Figure 17b,d,f,h, the diameters of NWs grown at the aforementioned temperatures were 200-550, 90-190, 50-150, and 30-90 nm, respectively.
Accordingly, with increasing growth temperature, NWs with smaller diameters were synthesized thanks to the faster diffusion of atoms along the NWs, instead of remaining on the stems of the NWs, resulting in the creation of thinner NWs. Figure 18a-c shows a typical TEM and TEM-EDS analysis of a single NW, demonstrating that the growth was performed using VLS analysis, where Au NPs at the tip of a ZnO NW were detected.The sensor fabricated from the sample with the smallest diameter was used for oxygen-sensing studies at 25 • C. It showed a sensitivity of 100% to 100 ppm oxygen gas.However, no selectivity study was performed [95].
In another study, ZnO NWs were directly grown at 950 • C for 1 h using the VLS method on an electrode substrate.Tri-layered Ti/Pt and Au (5 nm) electrodes were applied on a SiO 2 /Si substrate.For Pt decoration using sputtering, the thickness of the Pt (5 and 10 nm) and heat-treating temperature (500-750 • C) were changed, whereas for Pd decoration, the UV irradiation time (1, 5, 10, and 20 s) was varied.Figure 19a,b demonstrate the different steps for the synthesis and sensing fabrication of Pd or Pt-decorated gas sensors.The particle size of Pt on ZnO NWs increased with increasing annealing temperature.For samples calcined at 600 and 650 • C, the sizes were 18 and 28 nm, respectively, for the 10 nm Pt layer and 20 and 30 nm, respectively, for the 5 nm Pt layer.Moreover, for Pd NPs, under 5 s of irradiation, the largest particle size of 24 nm was resulted.At 25 • C, under the self-heating mode with 20 V of applied voltage, the highest response of Pt-decorated (initial thickness = 5 nm, annealed at 600 • C) ZnO NWs to 50 ppm toluene was 2.86.In addition, the highest response of Pd-decorated (UV irradiation time, 5 s) ZnO NWs to 50 ppm benzene was 2.20.Therefore, it was possible to realize selective benzene and toluene gas sensors using the decoration of Pd and Pt on ZnO NWs.Chemical sensitizations of Pd and Pt NPs as well as the formation of Schottky junctions between Pd and ZnO and Pt and ZnO resulted in enhanced gas performance [96].Accordingly, with increasing growth temperature, NWs with smaller diameters were synthesized thanks to the faster diffusion of atoms along the NWs, instead of remaining on the stems of the NWs, resulting in the creation of thinner NWs. Figure 18a-c shows a typical TEM and TEM-EDS analysis of a single NW, demonstrating that the growth was performed using VLS analysis, where Au NPs at the tip of a ZnO NW were detected.The In another study, ZnO NWs were directly grown at 950 °C for 1 h u method on an electrode substrate.Tri-layered Ti/Pt and Au (5 nm) electrodes on a SiO2/Si substrate.For Pt decoration using sputtering, the thickness of 10 nm) and heat-treating temperature (500-750 °C) were changed, whereas f tion, the UV irradiation time (1, 5, 10, and 20 s) was varied.Figure 19a,b dem different steps for the synthesis and sensing fabrication of Pd or Pt-decorated The particle size of Pt on ZnO NWs increased with increasing annealing tem samples calcined at 600 and 650 °C, the sizes were 18 and 28 nm, respect 10 nm Pt layer and 20 and 30 nm, respectively, for the 5 nm Pt layer.Moreove under 5 s of irradiation, the largest particle size of 24 nm was resulted.At 25 self-heating mode with 20 V of applied voltage, the highest response of Pt-d tial thickness = 5 nm, annealed at 600 °C) ZnO NWs to 50 ppm toluene was tion, the highest response of Pd-decorated (UV irradiation time, 5 s) ZnO NW benzene was 2.20.Therefore, it was possible to realize selective benzene and sensors using the decoration of Pd and Pt on ZnO NWs.Chemical sensitizatio Pt NPs as well as the formation of Schottky junctions between Pd and ZnO ZnO resulted in enhanced gas performance [96].Using the VLS growth mechanism, ZnO NWs were fabricated directly on alumina with Au electrodes at 900 °C for 20 min (Figure 20a).Through the cation exchange and by evaporation of NiCl2 at 700 °C, ZnO NWs were transformed into NiO NWs (Figure 20b).Then, ZnO NWs were grown on NiO NWs using Zn powder at 500 °C for 10 min.Finally, Ni-doped ZnO-branched NiO NWs were formed (Figure 20c).Figure 21a-d   Using the VLS growth mechanism, ZnO NWs were fabricated directly on alumina with Au electrodes at 900 • C for 20 min (Figure 20a).Through the cation exchange and by evaporation of NiCl 2 at 700 • C, ZnO NWs were transformed into NiO NWs (Figure 20b).Then, ZnO NWs were grown on NiO NWs using Zn powder at 500 • C for 10 min.Finally, Ni-doped ZnO-branched NiO NWs were formed (Figure 20c).Figure 21a-d show different characterizations of the synthesized materials, demonstrating the formation of materials with favorable branched morphology and composition.
Using the VLS growth mechanism, ZnO NWs were fabricated directly on alumina with Au electrodes at 900 °C for 20 min (Figure 20a).Through the cation exchange and by evaporation of NiCl2 at 700 °C, ZnO NWs were transformed into NiO NWs (Figure 20b).Then, ZnO NWs were grown on NiO NWs using Zn powder at 500 °C for 10 min.Finally, Ni-doped ZnO-branched NiO NWs were formed (Figure 20c).Figure 21a-d    The fabricated sensor displayed a response of 42.44 to 5 ppm p-xylene at 400 °C, which is better than pristine ZnO and NiO NWs along with good selectivity (Figure 22).The improved sensitivity of Ni-doped branched ZnO NWs was attributed to the presence of a lot of Schottky barriers formed between the branches, and the selectivity to xylene was related to the catalytic performance of the Ni dopant [97].Similarly, Co-doped ZnObranched ZnO NWs were prepared using the same procedure described previously [97] using CoCl2 powders instead of NiCl2 powders.The response to 5 ppm p-xylene at 400 °C was ~19.60.Again, the improved sensitivity to p-xylene was attributed to the catalytic effect of Co and the formation of plenty of Schottky barriers between branches and stems [98].The fabricated sensor displayed a response of 42.44 to 5 ppm p-xylene at 400 • C, which is better than pristine ZnO and NiO NWs along with good selectivity (Figure 22).The improved sensitivity of Ni-doped branched ZnO NWs was attributed to the presence of a lot of Schottky barriers formed between the branches, and the selectivity to xylene was related to the catalytic performance of the Ni dopant [97].Similarly, Co-doped ZnO-branched ZnO NWs were prepared using the same procedure described previously [97] using CoCl 2 powders instead of NiCl 2 powders.The response to 5 ppm p-xylene at 400 • C was ~19.60.Again, the improved sensitivity to p-xylene was attributed to the catalytic effect of Co and the formation of plenty of Schottky barriers between branches and stems [98].In another research work, ZnO NWs were directly prepared at 950 °C for 1 h on a SiO2/Si substrate with electrodes consisting of Ti/Pt and Au (3 nm) (Figure 23a,b).Then, γ-ray radiolysis using 60 Co was employed for Pd decoration.A dose rate of 10 kGy h −1 for 3 h and subsequent annealing at 500 °C for 1 h in air resulted in the formation of Pd with an average diameter of 20 nm on ZnO NWs (Figure 23c,d).In another research work, ZnO NWs were directly prepared at 950 • C for 1 h on a SiO 2 /Si substrate with electrodes consisting of Ti/Pt and Au (3 nm) (Figure 23a,b).Then, γ-ray radiolysis using 60 Co was employed for Pd decoration.A dose rate of 10 kGy h −1 for 3 h and subsequent annealing at 500 • C for 1 h in air resulted in the formation of Pd with an average diameter of 20 nm on ZnO NWs (Figure 23c,d).In another research work, ZnO NWs were directly prepared at 950 °C for 1 h on a SiO2/Si substrate with electrodes consisting of Ti/Pt and Au (3 nm) (Figure 23a,b).Then, γ-ray radiolysis using 60 Co was employed for Pd decoration.A dose rate of 10 kGy h −1 for 3 h and subsequent annealing at 500 °C for 1 h in air resulted in the formation of Pd with an average diameter of 20 nm on ZnO NWs (Figure 23c,d).At 24 • C, it displayed a response of 1.02 to 100 ppb CO gas, while the pristine sensor did not show any response.Generally, Pd is oxidized to PdO in air, and in the presence of CO gas it is reduced to Pd.Hence, the electron flow between the Pd/PdO phase and the ZnO NWs affected the sensor response.The catalytic activity of Pd to Co also affected the sensing performance [99].
ZnO NWs were synthesized on Ti-Pt electrode SiO 2 /Si substrates via the VLS growth mechanism.Subsequently, Au layers (3,5,10,20, and 30 nm) were sputter-deposited onto the ZnO NWs followed by annealing.The size of the Au NPs increased with an increase in the initial Au thickness from 3 to 30 nm.The sensor with a 10 nm-thick Au displayed the largest response to CO gas at 7 V in the self-heating mode.The catalytic effect of Au towards CO gas and the formation of Schottky barriers between Au and ZnO led to an enhanced gas sensor [100].Networked ZnO NW sensors were grown at 950 • C for 1 h via growth by the VLS mechanism.They were directly grown on SiO 2 -/Si substrates having tri-layered electrodes consisting of Au (3 nm)/Pt (100 nm)/Ni (50 nm) with different electrode gaps of 10, 20, 50, and 60 µm.In this way, it was possible to control the number of junctions between the as-grown NWs.For the thinnest spacing of 10 µm, networked NW junctions formed more densely, and by increasing the electrode gap, less dense junctions were formed.Moreover, for the electrodes with gaps of 60 µm, no junctions were formed.Therefore, the narrower the gap between electrodes, the denser the junctions between ZnO NWs.Based on the sensing results, at 350 • C, the sensor with a smaller gap exhibited a higher response to NO 2 gas thanks to the creation of more ZnO-ZnO homojunctions and, consequently, more modulation of the sensing resistance [101].
Because of their novel structure that includes the head group, backbone, and end group, self-assembled monolayers (SAMs) have different functionalities, leading to the possibility of their use for the detection of different gases [102].In this regard, ZnO NWs with diameters ranging from 10 to 20 nm were grown using a Au catalyst VLS mechanism with ZnO powders at 1200 • C for 12 min and then functionalized with 3-aminopropyl trimethoxysilane (APTMS) and 3-glycidoxypropyltrimethoxysilane (GLYMO) SAMs.The 10 × 10 −3 M APTMS-functionalized ZnO NWs sensor displayed an improved response to acetone at 300 • C along with acceptable selectivity.The sensing enhancement was related to the significant modulation of the EDL width and the reaction between the amino groups (-NH 2 ) of APTMS SAMs and acetone molecules.Indeed, the reaction between the acetone electrophilic "C" atom and nucleophilic (-NH 2 ) − takes place, resulting in the formation of imine ((H 3 C) 2 -C=N) and H 2 O molecules, eventually modulating the resistance [103].

Other Metal Oxide NWs
In addition to SnO 2 , In 2 O 3 , and ZnO NWs, some other semiconducting metal oxides have also been synthesized by VLS growth for gas-sensing usages.We will review and discuss them.Moumen et al. [104] prepared crystalline Bi 2 O 3 NWs via the VLS mechanism using three catalysts.Pt, Cu, and Au thin layers with thicknesses of less than 3 nm were sputter-deposited onto Al 2 O 3 substrates.Using Bi 2 O 3 powder, the growth of nanostructures was performed at 1000 • C, and the substrate temperature was set to 550 • C. Figure 24a-c show SEM images of Bi 2 O 3 nanostructures grown using different catalysts, namely, Au, Pt, and Cu, respectively.NWs with ~100 nm diameters and 6 µm lengths were grown using a Au catalyst (Figure 24d,e), while micropillars with irregular shapes were grown using a Pt catalyst.The use of a Cu catalyst led to the formation of few microrods.Gas-sensing measurements using Bi2O3 NWs demonstrated that the sensor was mor sensitive to ethanol gas (Figure 24f).Homojunctions between Bi2O3-Bi2O3 NWs acted as resistance modulation source during exposure to ethanol gas.Furthermore, the presenc of Au NPs at the top of NWs enhanced the sensing performances due to their catalyt effect towards the dissociation of oxygen and ethanol gases as well as the formation o potential barriers with Bi2O3 NWs.Moreover, WO3 NWs are a good candidate to be grow by the VLS mechanism.In this regard, using WO3 powder, WO3 NWs were grown at 110 °C over Pt or Au catalysts with the same thicknesses (3 nm) on alumina substrates [105 Moreover, the temperature of substrate was varied between 500 and 700 °C, and the dep osition times were set to 15 and 20 min.In the case of the Pt catalyst, no growth of NW occurred at 525 and 580 °C, while at 700 °C NRs were grown.In the case of the Au catalys NWs were grown at 525 °C as the optimal substrate temperature, and they had small d Gas-sensing measurements using Bi 2 O 3 NWs demonstrated that the sensor was more sensitive to ethanol gas (Figure 24f).Homojunctions between Bi 2 O 3 -Bi 2 O 3 NWs acted as a resistance modulation source during exposure to ethanol gas.Furthermore, the presence of Au NPs at the top of NWs enhanced the sensing performances due to their catalytic effect towards the dissociation of oxygen and ethanol gases as well as the formation of potential barriers with Bi 2 O 3 NWs.Moreover, WO 3 NWs are a good candidate to be grown by the VLS mechanism.In this regard, using WO 3 powder, WO 3 NWs were grown at 1100 • C over Pt or Au catalysts with the same thicknesses (3 nm) on alumina substrates [105].Moreover, the temperature of substrate was varied between 500 and 700 • C, and the deposition times were set to 15 and 20 min.In the case of the Pt catalyst, no growth of NWs occurred at 525 and 580 • C, while at 700 • C NRs were grown.In the case of the Au catalyst, NWs were grown at 525 • C as the optimal substrate temperature, and they had small diameters (10-30 nm).Furthermore, the WO 3 NWs prepared over a longer deposition time were denser, with their length in the micrometer range.However, NWs prepared for 15 min revealed lengths and diameters in the nanometer range.The WO 3 NW sensor was prepared using optimized WO 3 grown using a Au catalyst for 15 min.It displayed a response of 103 to 10 ppm of H 2 S at 400 • C and a high response of 170 to 300 ppb ozone at 200 • C. In particular, the high response to ozone was related to the dipole moment of ozone and its central atom, which has a positive charge, and the other side atoms, which have a negative charge.Upon interaction of the central atom with the WO 3 surface, it accepted electrons from WO 3 , and the other two atoms formed O 2 gas.Thanks to the high electron affinity of ozone, the amount of adsorbed oxygen species on the surface of the sensor increased, causing an increase in the EDL thickness and resistance in the presence of ozone.Kaur et al. [106,107] synthesized NiO NWs on an alumina substrate by the VLS mechanism using Au, Pt, and Pd catalysts for gas-sensing studies.The growth was performed at 1400 and 1450 • C, and the temperature of the substrate was varied between 900 and 1000 • C. For both Pt and Pd catalysts, NWs along with some other nanostructures were formed, while the use of a Au catalyst resulted in the formation of only NiO NWs.Under optimal conditions with Au catalysts, i.e., growth at 1400 • C and a substrate temperature of 930 • C, NiO NWs with small diameters (16-50 nm) were fabricated.The NiO sensor showed its best performance to acetone and ethanol gases at 500 • C and H 2 and CO gases at 300 • C. Hence, the working temperature can tune the selectivity to different gases.However, the working temperatures were relatively high, especially for acetone and ethanol detection, resulting in high power consumption in real applications.In another work, NiO NWs were synthesized using NiO powder at 1400 • C for 15 min on Au/alumina substrates.Then, ZnO NWs (with a size of 9 nm) were grown on NiO NWs without any catalyst at 1200 • C. At 400 • C, the branched sensor showed almost twice the response for both acetone and ethanol gases, relative to pristine NiO NWs [108].β-Ga 2 O 3 nanostructures have also been prepared by the VLS process [109].In a study, single-crystalline β-Ga 2 O 3 NWs on silicon (100) substrates were fabricated using Ga 2 O 3 powder at 975, 1000, 1025, and 1050 • C for 30 min.Then, Au decoration was performed on the synthesized NWs.The diameter of the NWs was increased from 122 to 133 nm with growth temperature.A gas sensor was realized from the sample grown at 1025 • C. The CO gas-sensing features of Au decorated on multiple β-Ga 2 O 3 NWs with Au-decorated single β-Ga 2 O 3 NW and single pristine β-Ga 2 O 3 NW were compared at RT.It was found that the sensor fabricated from a single Au-decorated β-Ga 2 O 3 NW showed an enhanced response to CO gas with faster dynamics [110].
The VLS growth mechanism using evaporation of Fe sources has been less studied.In fact, due to the high melting points of metallic Fe and its oxides, Fe 2 O 3 NWs, which are thermodynamically the most stable type of iron oxide, are often produced by thermal oxidation routes using oxidation of Fe foil [111][112][113][114].In a rare report about the VLS growth mechanism of iron oxide NWs in 2006, PLD-assisted VLS growth resulted in iron oxides with different morphologies such as NRs, NWs, and nanobelts.Fe 3 O 4 in pellet form was used as the source material.Then, it was evaporated by laser beams.By PLD, it is possible to maintain the stoichiometry of the source material due to the high heating rate.Furthermore, it needs a lower substrate temperature relative to other evaporation techniques [115].In this way, Fe 2 O 3 NWs were formed.However, the synthesized NWs were not used for sensing studies.Hence, it seems that VLS-grown Fe 2 O 3 NWs for gas sensing require further studies.
Table 1 provides the gas-sensing performance of different gas-sensing materials synthesized by the VLS growth mechanism.It can be seen that metal oxides synthesized by this method can be successfully used for the detection of various gases at different temperatures.Note: LOD = limit of detection; t res /t rec : response time/recovery time; selectivity ratio: the highest response to target gas divide to the highest response to an interfering gas.(-): no data available.
Metal oxide NW gas sensors used as resistive gas sensors have advantages such as high response, high stability, fast response and recovery times, low price, and ease of fabrication and use, relative to other types of gas sensors [33].Moreover, in comparison to other 1D morphologies such as NF gas sensors [116], NWs generally have better crystallinity, and they can be easily branched so that their surface area can be further increased [117].However, the gas sensors with NW morphology are still in a non-commercial state, mainly due to non-uniform growth and poor control over density and the distribution of assynthesized NWs [118].

Conclusions
In this review paper, we discussed the VLS growth mechanism of semiconducting metal oxides for gas-sensing applications.The synthesis of SnO 2 , In 2 O 3 , and ZnO NWs via the VLS growth mechanism for gas-sensing applications is the most reported relative to other gas sensors such as Bi 2 O 3 , Ga 2 O 3 , and WO 3 .This mechanism is mostly suitable for metals or metal oxides with melting points lower than ~1400 • C. Hence, no studies have been performed on the synthesis of Fe 2 O 3 NWs grown by the VLS mechanism for gas-sensing studies yet.By controlling the growth temperature, catalyst type, and growth time, the formation of NWs with desired diameters and aspect ratios is possible.Different metal catalysts are employed for the growth of NWs.However, Au is the most commonly used catalyst due to its inertness and ease of coating on substrates as well as the formation of a low-temperature eutectic Si substrate, which is the most common type of substrate.VLS-grown metal oxides can be successfully used for the detection of various gases, depending on the synthesized materials, sensing temperature, and nature of the target gas.Generally, branched NWs as well as hierarchical NWs result in the realization of gas sensors with higher performance relative to NWs due to having a larger surface area along with the presence of more homojunctions.In particular, for branched NW when the core and branch materials are synthesized from different materials, with the formation of heterojunction barriers, the core and branch NWs can significantly affect the gas-sensing properties.Moreover, decoration of noble metals on the surface of NWs can improve the sensing properties due to the catalytic effect of noble metal NPs as well as the formation of Schottky barriers in the interfaces between them.Doping is another strategy which can improve the sensing properties of metal oxide NWs.Generally, it leads to the formation of structural defects such as oxygen vacancies, which are considered good sites for gas adsorption.In future studies, the effect of high energy irradiation, bimetallic decoration, and hybrid formation with conducting polymers and graphene materials can be explored for gas-sensing applications.

Figure 2 .
Figure 2. Schematic of the synthesis setup for the growth of metal oxide NWs [37] (Used with permission from Elsevier ® ).

Figure 2 .
Figure 2. Schematic of the synthesis setup for the growth of metal oxide NWs [37] (Used with permission from Elsevier ® ).

Figure 2 .
Figure 2. Schematic of the synthesis setup for the growth of metal oxide NWs [37] (Used with permission from Elsevier ® ).

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Figure 4 .
Figure 4. SEM images of SnO2 NW-Bi2Sn2O7 NP composites grown at (a) 700, (b) 800, and (c) °C.The insets show SEM images of the tips of NWs.(d) Schematic of the growth mechanism (Used with permission from Elsevier ® ).

Figure 4 .
Figure 4. SEM images of SnO 2 NW-Bi 2 Sn 2 O 7 NP composites grown at (a) 700, (b) 800, and (c) 900 • C. The insets show SEM images of the tips of NWs.(d) Schematic of the growth mechanism [56] (Used with permission from Elsevier ® ).

Figure 8 .
Figure 8. SEM micrographs of (a) pure SnO 2 NWs and (b) Fe 2 O 3 NP-coated SnO 2 NWs along with (c) TEM and (d) HR-TEM images [66].The insets are larger magnification images.(e) Response to 200 ppm ethanol gas versus temperature and (f) selectivity patterns of gas sensors [67] (Used with permission from Elsevier ® ).

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16,  x FOR PEER REVIEW 13 of 33 related to the catalytic activity of Pd NPs towards oxygen and NO2 gases, as well as the creation of Schottky contacts between Pd and SnO2 NWs.Modulation of the Schottky barriers in the NO2 atmosphere led to remarkable variations in the resistance and appearance of the sensing signal.Similarly, SnO2 NWs were grown on an electrode substrate.Then, Pd decoration was performed using UV-light irradiation (λ = 360 nm) of a PdCl2 solution in the presence of SnO2 NWs.By subsequent annealing at 500 °C, Pd NPs were dispersed on SnO2 NWs as isolated islands.Figure11a-fshow that SnO2 NWs with an approximate diameter of 50 nm and desired phases, along with Pd decoration, were successfully produced.The response to 10 ppm H2 gas at 300 °C was boosted by 141.4% after Pd decoration in addition to better selectivity to H2 gas.Pd NPs facilitated the dissociation of O2 and H2 gases.H2 decomposes to atomic H by a spillover effect, and it moves to the neighboring SnO2 surface by reaction with oxygen species, resulting in a significant decrease in sensor resistance (Figure11g-h).Moreover, formation of Pd and SnO2 Schottky barriers led to the modulation of the sensor resistance in the H2 gas atmosphere due to a change in the height of barriers in the H2 gas atmosphere[71].

Materials 2023 ,Figure 13 .
Figure 13.One-dimensional In2O3 nanostructures synthesized by the VLS growth method with permission from Elsevier ® ).
Growth of In 2 O 3 nanostructures using In based on the VLS mechanism on a Au-coated SiO 2 substrate was carried out at 700-900 • C for 1 h.At 700 • C, uniform In 2 O 3 nanostructures were grown.At 800 • C, the diameter and length of the NWs were 70-80 nm and several micrometers, respectively.The product displayed a high density of NWs.At 900 • C, In 2 O 3 nanoneedles with diameters of 150-200 nm and lengths about 4-5 µm were formed.H 2 gas-sensing results showed that the In 2 O 3 NWs had a higher response relative to In 2 O 3 nanoneedles due to the high surface area of In 2 O 3 NWs relative to nanoneedles.However, the sensor temperature was high at 400 • C [88].In 2 O 3 NWs were prepared at 800 • C for 1 h on a Au-coated (3 nm) Si substrate.Then, Pt was sputtered onto In 2 O 3 NWs, and by subsequent annealing, Pt islands were formed on In 2 O 3 NWs.At 50 • C, decorated NWs displayed a response of 4 to 400 ppm O 2 gas, while the pristine sensor showed no response to this gas.Indeed, oxygen species were adsorbed and disassociated on Pt NPs and then migrated onto the In 2 O 3 NWs, where they were adsorbed, leading to a high response to oxygen gas [89].Using a mixture of In 2 S 3 and In 2 Se 3 powders (10:1 volume ratio), In 2 O 3 NWs were synthesized by the VLS mechanism on a Au-coated (3 nm) Si wafer at 850 • C for 1 h.Second, a solution of Ti 4+ ions was spin-coated onto an In 2 O 3 NW template, and after being prepared at 500 • C for 2 h, TiO 2 NPs (10-30 nm) were dispersed on the surface of NWs.As shown in Figure 15a-d, the synthesized NWs had high aspect ratios (diameters = 50 to 100 nm).

Figure 18 . 33 Figure 19 .
Figure 18.(a) TEM image of a single ZnO NW grown at 950 • C. (b,c) TEM-EDS spectra taken from different areas in (a) [95] (Used with permission from Elsevier ® ).Materials 2023, 16, x FOR PEER REVIEW 22 of 33 show different characterizations of the synthesized materials, demonstrating the formation of materials with favorable branched morphology and composition.

Figure 19 .
Figure 19.(a) Synthesis procedure of Pt and Pd-decorated ZnO NWs and (b) fabrication of a sensor and the self-heating mode of operation [96] (Used with permission from Elsevier ® ).
show different characterizations of the synthesized materials, demonstrating the formation of materials with favorable branched morphology and composition.

Table 1 .
Gas-sensing properties of VLS-grown sensing materials.