TiO2 Nanobers Decorated with Monodispersed WO3 Heterostruture Sensors for High Gas Sensing Performance Towards H2 Gas

A simple spin coating method was used to prepare the WO 3 -TiO 2 heterostructural nanobers (HNFs). Various kinds of techniques, including XRD, SEM, TEM, EDS and XPS, have described the structures, chemical constitutions and morphologies of the samples. Following the decoration of the WO 3 nanocubes on the surface of TiO 2 nanobers, XPS ndings conrmed the presence of W 5+ and the excess proportion of both chemisorbed reactive oxygen and oxygen vacancies. WO 3 incorporated TiO 2 thin lm sensor showed high sensing response (78%), rapid response (20 s) and recovery time (23 s) with respect to other gas molecules (NH 3 , NO 2 , LPG and SO 2 ). The nding demonstrates that the WO 3 -TiO 2 sensor showed good selective response towards H 2 gas. A unique path was created by this work to build hetero-highly ordered mesoporous metal oxides junctions for applications in H 2 gas sensor based devices.


Introduction
With the need to identify the amount of toxic gases (NO, NO 2 , CO, SO 2 , H 2 S, etc.) in the air in real time, the development of high-performance gas sensors in the manufacturing, ecological and health sectors is essential. Due to its bene ts such as simple design, long lifetime, compact size and n, the semiconductor gas sensor is one of the most popular consistently applied sensors [1,2]. The core part of the semiconductor gas sensor as a sensitive coating. A multitude of compounds, such as ZnO [3,4], SnO 2 [5,6] and Fe 2 O 3 [7,8], were used to manufacture gas-sensitive coatings. However these frameworks have some drawbacks, such as low response, high energy consumption, and some humidity and temperature volatility. WO 3 has exceptionally high sensitivity for NO 2 detection compared to the conventional materials above [9,10], and is a suitable candidate for gas sensor coating materials. And it is also stated that the doping into WO 3 of another kind of oxides leads to the formation of semiconductor heterojunction, thereby increasing overall the coatings' gas sensitivity. TiO 2 is well established to be a kind of semiconductor material with excellent electrical properties.
Improvements have been documented in the gas sensitivity of semiconductor WO 3 induced by TiO 2 doping. However, the latest TiO 2 doping methods, which are not suitable for commercial development, are highly complex. In fact, the gas control problem of the TiO 2 -doped coatings has still not been clearly established. The WO 3 -based composite coatings doped with TiO 2 were prepared by liquid-phase plasma spraying in this paper as per that concern, and the gas-sensing process of TiO 2 -WO 3 composite coatings was extensively investigated. As already mentioned, it is also proved to be a potential method is tested loaded on a support material due to its delicate conductivity, strong catalytic properties, and remarkable chemical inertness. Nano ber WO 3 -TiO 2 may form n-n heterojunction, which can potentially present highly explosive localized areas and thus obtain unpredictable features for speci c applications. In this analysis, we describe a simple and successful spin coating technique has been used to fabricated the quasi-1D WO 3 nanoparticles-decorated TiO 2 heterostructural nano bers. High response, discern speci city, quick response time and recovery time for H 2 gas were demonstrated by the as-prepared WO 3 -TiO 2 material, making it a successful candidate for application in H 2 sensors. A sensor adsorption and reaction model has also been suggested. The increase in the e ciency of gas sensing can be due to the creation of heterojunctions seen between two material types.

Preparation of WO 3 /TiO 2 thin lms
The process of making WO 3 /TiO 2 lms involves two step synthesis processes. Hydrothermal and spin coating technique has been used to fabricate the bare WO 3 and TiO 2 thin lms, respectively. Firstly, WCl 6 was distributed in 30 mL deionized water through magnetic stirring for 20 min. Later, 5 mL of HCl aqueous solution was added and the reaction mixture was fully dissolved by thoroughly mixing for another 10 min. the reaction mixture was adapted to hydrothermal reaction (180 o C/12 h) and nally dried (80 o C/12 h) for further use. Spin coating method was used to fabricate the TiO 2 nanotubes. FTO substrate was used to deposit the thin lms. The raw materials of TiO 2 precursors (5 mL TTIP with 1:1 ratio of ethanol and DI water) was deposited on FTO glass substrate at speed of 2000 rpm for 1 min. The lms after drying at 120 o C for 5 min are subsequently heat-treated at 450 o C for 30 min in nitrogen ow.
In the process of WO 3 /TiO 2 composite. The prepared WO 3 nanopowder (0.5 g) was dispersed in the 0.5 g of TiO 2 nano bers and same experimental process of spin coating method has been repeated for each concentrations. The lms with WO 3 nanoparticles, TiO 2 nano bers and WO 3 /TiO 2 heterostrutures were labeled as WO 3 NPs, TiO 2 NFs and WO 3 /TiO 2 Hs, respectively.

Gas sensor set up with sensor region
The gas sensor of the resistive form was constructed and the schematic view is being shown in Fig. 1.
The detailed description of the sensor was mentioned already our previous reported work [11]. The mass ow controller (MFC) was used to modify gas concentrations at various ppm levels (0-1000 ppm). The gases were connected to the mass ow controller with a mixer from different cyclinder. The diluted gases were then evenly located in the testing reactor. The gas sensing response (S) was stated as (R G -R A )/R A x 100% [12], where the air resistance value and the corresponding gas were R G and R A , respectively.

Results And Discussion
3.1. X-ray diffraction (XRD) analysis Figure 2 shows the XRD pattern of WO 3 NPs, TiO 2 NFs and WO 3 /TiO 2 Hs lms respectively. The pattern clearly expose the bare WO 3 and TiO 2 are monoclinic (JCPDS card No. 43-1035) and anatase phase tetragonal rutile type structure (JCPDS card No. 21-1272). Sharp intense peaks without any contaminant suggest that fabricated lms are high order crystalline nature. The diffraction peaks of the WO 3 -TiO 2 can be indexed to the mixed WO 3 and TiO 2 with different phases and no apparent peak change relative to the pure materials, which shows that the end product contains of it rather than alloy WO 3 and TiO 2 nanocomposites.

Morphological studies
The morphological detection of the sensors was examined by SEM and TEM. Figure 2

Surface and elemental composition studies
Brunauer-Emmett-Teller (BET) method was used to describe the porous structure and clear surface areas of sensors through their N 2 adsorption-desorption analysis as well as pore size distribution curve ( Fig. 4a & b). All the samples display category IV nitrogen isotherm with a hysteresis loop, suggesting the features of mesopores [13][14][15][16][17]. Due to the heterostruture combination of WO 3 nanoparticles and TiO 2 nano bers can deliver the high surface area (104.7 m 2 /g) and pore size (17.4 nm)) than that of bare WO 3 (54.3 m 2 /g and 37.4 nm) and TiO 2 (77 m 2 /g and 30.2 nm). The chemical state and composition of elemental con guration was analyzed by XPS. The survey XPS of WO 3 /TiO 2 Hs shows the chief elements of W, Ti and O (Fig. 5a). Figure 5 (b-d) displays the high resolution spectrum of the W 4f, Ti 2p and O 1s spectra. The divided peaks based at the binding energies of 34.9 and 36.8 eV correspond to the standard binding energies of W 5+ [18,19] in the W 4f XPS spectrum. With binding energies at 465.2 eV and 459.6 eV, the Ti 2p XPS spectrum can be deconvoluted into two major peaks, corresponding to Ti 2p 1/2 and Ti

Gas sensing test
The e ciency of the gas sensing performance of the H 2 gas was tested by using the WO 3 , TiO 2 and WO 3 /TiO 2 sensor materials. Before gas sensing test the sensor samples were exposed to air atmosphere to identify the resistivity performance of the samples and the relevent graph illustrate that good rsisitive nature for all the sensor samples (Fig. 6a). The sensing response is shown in Fig. 6 (b). The dynamic response is drqastically enhanced with the increase of H 2 gas concentration from 0 to 1000 ppm (Fig. 6c). The maximum sensitiviy is achived by WO 3 /TiO 2 Hs (78%) than compared with bare WO 3 (27%) and TiO 2 (52%) sensor lms. In addition, the impact of the degree of doping on the ordered quality of the porous channel and relative humidity (RH) on sensing characteristics was also investigated and the relevent plot is shown in Fig. 7 (a). The analysis indicate that the sensor based on WO 3 /TiO 2 Hs offered the highest response, regardless of the test conditions. The high performance of the WO 3 /TiO 2 Hs based sensor ought to be bene cial for the unique and powerful structure effect and doping effect because the mesoporous structure could provide both high surface area and prosperous for hydrogen gas adsorption and diffusion (structure effect) while doping means improving defects and active site. For industrial cases, the reaction and recovery time of gas sensors is quite important. The H 2 gas concnetrtion I exposed to 1000 ppm at RT towards the all the sensor lms and the nding reveals that WO 3 /TiO 2 Hs sensors gained rapid response (20s) and recovery time (23 s) than other sensors ( Fig. 7b-d). The H 2 gas parameters of all the sensors are estimated and the values are displayed in Table 1. Finally, we carried out a response comparison of the sensors 1000 ppm of different target gases to a rm the progress in selectivity. Figure 8 (a-c) selectivity charecteristics graph of all the sensors, which is exposed to various target gases like, NH 3 , NO 2 , LPG and SO 2 . The sensor stability is often continually monitored. As shown in Fig. 8d, the curve showed a remarkably stable tendency toward 1000 ppm H 2 gas throughout a 60-day long-term stability calculation. In addition, no noticeable decline in response pattern is found for detecting 1000 ppm H 2 gas after 50 successive tests (Fig. 8d). These ndings show that the HNFs-based WO 3 /TiO 2 sensor has strong reproducibility and long-term reliability. The sensing mechanism with graphical sketch of the proposed sensor is shown in Fig. 9. The improved sensing performance of the HNFs-based WO 3 /TiO 2 sensor is due to the following reasons: On the one side, the Fermi level of WO 3 is lower for WO 3 /TiO 2 HNFs than for TiO 2 , which contributes to the transport of energy from TiO 2 to WO 3 till the level of Fermi energy is equivalent. As a consequence, on the side of WO 3 , the electron diffusion surface will develop, that will make it much easier to accumulate oxygen or target gasses on the WO 3 . Consequently, WO 3 nanospherical serve as a responsive active site on the surface of TiO 2 nanotubes and have a bene cial effect on H 2 sensor output. It is clearly suggest that the gas sensing e ciency of this sensor is obviously advantageous over that of other sensors.

Conclusions
In this report, the WO 3 nanoparticles incorporated TiO 2 heterostruuctures with mesoporous nature lms sensors were fabricated and tested the gas sensing response towards €h2 gas with at RT. The heterostruture which facilitated the fabrication of a sensitive and porous shaped sensing lm. WO 3 /TiO 2 heterostructure thin lm sensor showed high sensing response (78%), rapid response (20 s) and recovery time (23 s) with respect to other gas molecules (NH 3 , NO 2 , LPG and SO 2 ). In addition that the fabricated sensors also exhibits long term stability due no apparent loss in sensitivity after multiple cycle experiments. The nding demonstrates that the WO 3 -TiO 2 sensor showed good selective response towards H 2 gas. The WO 3 -TiO 2 HNFs sensor's excellent performance could be related to the existence of n-n junctions as well as the redox of W 6+ and W 5+ states. The results con rmed that the signi cant insight WO 3 -TiO 2 HNFs was a good approach for a high-performance H 2 sensor.