Research Progress of Gas Sensing Performance of 2D Hexagonal WO3

Metal oxide semiconductor gas sensing materials have attracted great research interest in the gas sensor field due to their outstanding physical and chemical properties, low cost, and easy preparation. Among them, two-dimensional hexagonal tungsten trioxide (2D h-WO3) is especially interesting because of its high sensitivity and selectivity to some gases. We firstly introduce the characteristics of 2D h-WO3 gas sensing materials and discuss the effects of microstructure, oxygen vacancy, and doping modification on the gas sensing properties of 2D h-WO3 mainly. Finally, we explore the application of 2D h-WO3 gas sensing materials and propose some research directions.


INTRODUCTION
As a critical component of the intelligent detection system, the gas sensor (Lee et al., 2018) has been widely used in environmental monitoring (Ji et al., 2019a), respiratory analysis, explosive gases, and automobile exhaust detection. Based on different working mechanisms, the developed gas sensors include semiconductor gas sensors (Morrison, 1987a;Zhang et al., 2021), polymer gas sensors (Zee and Judy, 2001), and electrochemical gas sensors (Tierney and Kim, 1993). Among them, the semiconductor gas sensors can also be divided into resistive and non-resistive types, while the resistive semiconductor gas sensors have advantages of high sensitivity and easy preparation (Seiyama et al., 1962). Meanwhile, compared with carbon and other organic gas sensing materials, the resistive metal oxide gas sensors (Nazemi et al., 2019) have become the research hotspot due to their high responsivity (Demarne and Grisel, 1988) and excellent selectivity (Morrison, 1987b). As a highly sensitive metal oxide gas sensing material, tungsten trioxide (WO 3 ) has attracted extensive attention because of its unique physical and chemical properties (Salje and Viswanathan, 1975), and its applications in photocatalysis (Dong et al., 2017) and electrochromic (Adhikari and Sarkar, 2014).
WO 3 is a typical metal oxide semiconductor with various phase transition structures, while different phases can induce different gas sensitivity. The stable structures at room temperature are m-WO 3 and h-WO 3 . In recent years, as the most stable structure, m-WO 3 has attracted much attention (Hübner et al., 2010;Oison et al., 2011), but bulk m-WO 3 gas sensors are not sensitive to some gases at 25°C-500°C, such as CO (Ahsan et al., 2012) and H 2 S (Szilágyi et al., 2010). Therefore, it is urgent to improve the gas sensitivity of WO 3 at room temperature effectively. Xu et al. (2008) found that the sensitivity of h-WO 3 almost linearly increases with CO concentration at room temperature. Szilágyi et al. (2010) found that h-WO 3 becomes more sensitive than m-WO 3 compared to m-WO 3 when the concentration of H 2 S is 10 ppm. Meanwhile, the large hexagonal and trigonal tunnel structures of h-WO 3 result in it having a high specific surface area (as shown in Figure 1) (Balaji et al., 2009), indicating that h-WO 3 is an excellent candidate material for gas sensors.
To effectively improve the gas sensitivity of the material, various preparation methods have been used to prepare WO 3 nanomaterials on various dimensions (0D, 1D, 2D, and 3D) (Qin et al., 2010;Zhang et al., 2010;D'Arienzo et al., 2014). Among them, 2D nanomaterials are widely used because of their high specific surface area and abundant oxygen vacancies (Yang et al., 2016;Liu et al., 2017;Yang et al., 2017). The unique characteristics of 2D WO 3 nanostructure compared with the bulk material include (1) higher specific surface area, which will provide more interaction area between tested gases and gas sensor surface molecules; (2) quantum confinement effects, due to the inherently small size of nanostructure materials, that can significantly affect charge transport, electronic band structure, and optical properties (Zheng et al., 2011). Based on this, we mainly review the effects of microstructure, oxygen vacancy, and doping modification on the gas sensing performance of 2D h-WO 3 and explore the application prospect of the 2D h-WO 3 gas sensor.

CHARACTERISTICS OF 2D h-WO 3 GAS SENSING MATERIAL
As a kind of metal oxide semiconductor, 2D h-WO 3 gas sensing material has been an excellent candidate material for gas sensors due to its advantages of easy preparation, stable crystal structure, high specific surface area, and abundant oxygen vacancies. Table 1 shows some typical preparation methods of 2D h-WO 3 . Among them, the hydrothermal method is the most widely used. According to this method (Kitagawa et al., 2009;Szilágyi et al., 2010;Ji et al., 2019b), (NH 4 ) 10 W 12 O 41 ·5H 2 O is firstly put into a high-pressure cauldron as the raw material. Then, under high temperature and high pressure, (NH 4 ) 10 W 12 O 41 ·5H 2 O recrystallizes to obtain precipitates (h-WO 3 crystals). Finally, the precipitates are removed and washed several times with deionized water to obtain the final product. Compared with vapor/liquid phase deposition methods, the hydrothermal method is simple and economical, and can prepare nanomaterials with high purity, good chemical uniformity and high dispersion. 2D h-WO 3 is classified as the surfacecontrolled gas sensor by a gas sensing mechanism.

Stable Crystal Structure
The phases of WO 3 can transform when it is annealed under different conditions. However, it does not simply form new nanostructures, but the original WO 6 octahedron distorts and twists to a certain extent and thus can form different crystal phases. The phase transition with temperature of WO 3 is shown in Figure 2 (Salje et al., 1997;Vogt et al., 1999;Roussel et al., 2000), which is monoclinic II (ε- Gerand et al. (1979) found that stable hexagonal WO 3 (h-WO 3 ) can be prepared by dehydration method at 200°C-400°C. Tian et al. (2020) has calculated the gas (O 2 ) sensing on hexagonal WO 3 (001) surface by using the pseudopotentials method based on the density functional theory (DFT). The formation energy (E form ) of the h-WO 3 (001) monolayer is  −5.87 eV, indicating that the h-WO 3 (001) monolayer is stable. The carrier mobility μ calculated from the energy band is 886 cm 2 V −1 s −1 (as shown in Table 2) at T 300 K. The value is higher than that of 2D GeP 3 (Gerand et al., 1979) and MoS 2 (Cai et al., 2014) and is about 88 times higher than that of bulk WO 3 (Yamazoe et al., 2003), which implies that 2D h-WO 3 may have excellent gas sensing performance.
High Specific Surface Area Sun et al. (2015) investigated the high surface area tunnels in 3D h-WO 3 by low-pressure CO 2 adsorption isotherms with nonlocal density functional theory fitting (NLDET), transmission electron microscopy (TEM), and thermal gravimetric analysis. They found that h-WO 3 has a large hexagonal tunnel structure (the diameter is 3.67 Å) and high specific surface area (45.585 m 2 /g). Meanwhile, the large lateral size and ultrathin thickness of 2D materials provide it with ultrahigh specific surface areas and high ratios of exposed surface atoms (Zhang, 2015), significantly improving the gas sensing performance of 2D h-WO 3 .

Abundant Oxygen Vacancies
The conduction band of 2D WO 3 mainly consists of W-5d electrons, and the valence band mainly consists of O-2p electrons (Niklasson et al., 2004). Chatten et al. (2005) found that abundant oxygen vacancies are related to the energy gap between O-2p and W-5d orbitals in non-stoichiometric tungsten oxide. Makarov and Trontelj (1996) pointed out that the oxygen vacancies in 2D WO 3 can affect the conductivity and carrier concentration, and further affect the gas sensing performance of WO 3 . For example, Tian et al. (2020) found that oxygen vacancies provide electrons to O 2 gas molecules on the WO-terminated h-WO 3 (001) surface, thus effectively improving the gas sensing performance of h-WO 3 (001) surface to O 2 .

INFLUENCING FACTORS OF 2D h-WO 3 ON GAS SENSING PERFORMANCE
When the gas sensors are exposed to the air, O 2 molecules are physically or chemically adsorbed on the surface of 2D h-WO 3 .
The oxygen will be dissociated and capture the electrons from the conduction bands of 2D h-WO 3 , generating ionized oxygen species (mainly O − ). This leads to a decrease in the number of electrons on the surface and forming an electron depletion region (EDR), which causes the first change in resistance. When the sensors are exposed to the target gas, the gas molecules are adsorbed on the surface of 2D h-WO 3 . Then, the gas molecules react with pre-absorbed oxygen and change the number of the electrons of ionized oxygen species, increasing the density of carriers in the 2D h-WO 3 . It results in the second change in resistance (Deng et al., 2015;Li et al., 2015;Liu et al., 2016).

Effect of Microstructure on Gas Sensing
Performance of 2D h-WO 3 Figure 3 shows different microstructures of h-WO 3 . It can be seen that h-WO 3 nanosheets and films can provide more gas molecular absorption sites because of their obvious orientation, small particle size, large specific surface area, and no agglomeration. However, h-WO 3 nanoparticles, nanowires, and nanospheres have a negative effect on gas transportation and reaction due to serious agglomeration or large particle size. Moreover, we also find from Table 3 that h-WO 3 nanosheets and films have the highest responsiveness (R) and wider detection scope (S) to H 2 , NH 3 , H 2 S, and NO 2 , compared with nanowires, nanorods, nanospheres, and nanoparticles. Different h-WO 3 nanomaterials have exhibited different gas sensing performance due to their different microstructures. Among them, 2D h-WO 3 nanomaterials show important application prospects in the gas sensing field due to their excellent gas sensing performance.

Effect of Oxygen Vacancy on Gas Sensing Performance of 2D h-WO 3
In 1964, Kevane (1964) found that oxygen vacancies are easy to form in the preparation of metal oxide semiconductors. Makarov and Trontelj (1996) found that the oxygen vacancies would affect the conductivity, thus further affecting the gas sensing performance of WO 3 . However, the expression of oxygen vacancy on metal oxide semiconductor surfaces is not in agreement (Gillet et al., 2003). Until 2018, Tian et al. (2018) established a theory based on surface oxygen density (do) and unitedly expressed the oxygen vacancies on the WO 3 surface (   Yamazoe et al.(2003).
Frontiers in Chemistry | www.frontiersin.org December 2021 | Volume 9 | Article 786607 3 0.68 eV and 0.002e, respectively, compared with the O-terminated h-WO 3 (001) surface. For defective WO-terminated, the values of decrease are 0.4 eV and 0.011e, respectively. The result shows that the adsorption and sensing ability of CO on the defective O-and WO-terminated h-WO 3 (001) surface decreases. The oxygen vacancy inhibits the oxidation reaction of reducing gas CO on the 2D h-WO 3 (001) surface, which reduces the gas sensing performance of the 2D h-WO 3 .
Oxygen vacancy also inhibits the gas sensing performance of other reducing gases (H 2 S, CH 4 , H 2 ) on the 2D h-WO 3 (001) surface (Szilágyi et al., 2010;Tian et al., 2017;Wu et al., 2019) ( Table 5). However, the inhibitory effect of oxygen vacancy on H 2 S and CH 4 is unapparent. Although the gas sensing performance of H 2 S is inhibited by oxygen vacancy, the value (1.85 eV) is still large enough for effective adsorption of H 2 S on the surface. The adsorption sensing ability of CH 4 on the 2D h-WO 3 (001) surface is weak and the inhibition of oxygen vacancy makes it difficult to spontaneously adsorb on defective WO-terminated h-WO 3 (001) surface. Moreover, oxygen vacancy has the strongest inhibitory effect on the gas sensing performance of H 2 on the 2D h-WO 3 (001) surface. The adsorption energy decreases from 2.62 to 0.16 eV and the charge transfer decreases from 0.635e to 0.065e. The gas adsorption ability of H 2 on the 2D h-WO 3 (001) surface greatly reduces with the decrease of surface oxygen density.
TABLE 3 | Relationship between microstructure, particle size, and gas sensitivity of H 2 , NH 3 , H 2 S, and NO 2 in h-WO 3 (S is the detection scope, R is the responsiveness, and C is concentration).
Frontiers in Chemistry | www.frontiersin.org December 2021 | Volume 9 | Article 786607 4 shows that the adsorption and sensing ability of O 2 are improved on the defective O-and WO-terminated h-WO 3 (001) surface. The oxygen vacancy activates the O-O bond of O 2 and promotes the reduction reaction of oxidizing gas O 2 on the 2D h-WO 3 (001) surface, which improves the gas sensing performance of the 2D h-WO 3 .
These results indicate that the effect of oxygen vacancy on gases with different redox properties is different. For reducing gases, the oxygen vacancy inhibits their oxidation reactions on the 2D h-WO 3 (001) surface and then reduces the gas sensing performance of the reducing gases. On the contrary, for oxidizing gases, the oxygen vacancy promotes the reduction reaction and then improves the gas sensing performance.

Effect of Doping Modification on Gas Sensing Performance of 2D h-WO 3
Various methods have been performed to improve the gas sensing performance, to overcome the defects of pure metal oxides such as low sensitivity, low selectivity, and long response time for some gases . Among them, noble metal doping is one of the most common and effective methods. Due to the high electronic activity of noble metal elements, the activation energy of the reaction can be reduced during the contact reaction between the gas sensing material and the target gas, thus improving the gas sensing performance of the materials (Xu et al., 1990) when they react with target gases. Based on this, noble metals such as Au, Ag, Pd, and Pt are usually doped on WO 3 films to improve their sensitivity and selectivity to NO x (Penza et al., 1998;Chen and Tsang, 2003), H 2 S (Stankova et al., 2004;Hurtado-Aular et al., 2021), CH 3 COCH 3 (Feng et al., 2021), etc.
Recently, the gas sensing performance of CO adsorption on the 2D h-WO 3 (001) surface doped with noble metals Cu, Ag, and Au were investigated by using DFT (as shown in Table 6) (Hurtado-Aular et al., 2021). They found that the incorporation of Au and Cu atoms improves the surface activity of the material and the absorptivity of CO on the 2D h-WO 3 (001) surface. Meanwhile, the doped Au and Cu atoms provide a large number of electrons. The charge transfer increases, which effectively improves the sensing performance of CO on the 2D h-WO 3 (001) surface.
Theoretically, noble metal doping promotes the adsorption and sensing ability of the target gas on 2D h-WO 3 surface, and then improves the gas sensing performance of 2D h-WO 3 . However, the experimental study on the mechanism of improving the gas sensing performance of noble metal doped h-WO 3 films is still insufficient.

SUMMARY AND PROSPECT
The research progress of the gas sensing performance of 2D h-WO 3 has been reviewed. Firstly, we briefly summarize the   Frontiers in Chemistry | www.frontiersin.org December 2021 | Volume 9 | Article 786607 5 characteristics of 2D h-WO 3 gas sensing materials. Then, the effects of microstructure, oxygen vacancy, and doped metal on the performance of 2D h-WO 3 gas sensors are mainly discussed. We find that the 2D h-WO 3 gas sensor has better gas sensing performance than other WO 3 nanomaterials due to their small particle size and large specific surface area. Moreover, the effect of oxygen vacancy on the gas sensitivity of different oxidationreducing gases on 2D h-WO 3 is different. Besides, we also note that noble metal doping can improve the gas sensing performance of 2D h-WO 3 due to the high electronic activity of noble metals and the reduction of reaction activation energy.
As we all know, 2D h-WO 3 is an excellent candidate material for metal oxide semiconductor gas sensors, which has vital research significance and wide application prospects in gas sensors. However, there are still some unsolved problems in 2D h-WO 3 that need to be completely solved, such as the low sensitivity and low selectivity to some gases. To solve the above problems, the possible solutions include the following: (1) Photoactivation method (i.e., activation of reactants by light), which can improve the sensitivity and selectivity effectively. Deng et al. (2012) activated mesoporous WO 3 sensing material and improved the sensitivity of WO 3 to HCHO by using visible light irradiation at room temperature. Moreover, Trawka et al. (2016) enhanced the sensitivity and selectivity of WO 3 -based gas sensors greatly by ultraviolet irradiation. (2) Noble metal doping method improves sensitivity and selectivity. Adding precious metal catalysts has become an important method to improve the gas sensing performance of metal oxide semiconductors, because the catalyst has a great influence on the resistance and sensitivity of semiconductor gas sensing materials (Krebs and Grisel, 1993).

AUTHOR CONTRIBUTIONS
All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.