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Vol. 8. Issue 4.
Pages 3580-3588 (July - August 2019)
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Vol. 8. Issue 4.
Pages 3580-3588 (July - August 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.05.016
Open Access
In situ construction and sensing mechanism of TiO2–WO3 composite coatings based on the semiconductor heterojunctions
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Yangguang Yaoa, Jianhui Yuana,
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yuanjianhui0002@163.com

Corresponding author.
, Xiaoxiao Chena, Liming Tana, Qingshan Gua, Wenjie Zhaob, Jieshi Chena
a School of Materials Engineering, Shanghai Collaborative Innovation Center of Laser Advanced Manufacturing Technolog, Shanghai University of Engineering Science, Shanghai 201620, China
b Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science, Ningbo, 315201, China
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Table 1. The spray parameters used in this research.
Table 2. Average grain sizes calculated by the Debye–Scherrer formula.
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Abstract

Tungsten oxide (WO3) is one of the most promising gas-sensitivity materials owing to its high decomposition temperature and stability of microstructure. The doping of oxides can significantly improve its gas-sensitivity properties. In this paper, WO3 based coatings doped with TiO2 were prepared via plasma spraying technology by using mixed feedstock suspensions containing a precursor as WCl6 and a certain ratio of nano-TiO2. The characterization of microstructures and properties of the TiO2–WO3 composite coatings were achieved and the gas sensitivity mechanism was researched. During the suspension plasma spraying, the solvent of the precursor droplets was evaporated and then the nucleation, crystallization and growth of WO3 occurred. The crystallinity and crystal grains of WO3 increased observably as the plasma spraying power improved. With the increase of TiO2 in coatings, the average grain size of the TiO2–WO3 composite coatings decreased at first and then increased. And the gas-sensitive properties of the composite coatings were significantly better than the pure WO3 coatings. The gas-sensitive properties improvement of TiO2–WO3 composite coatings doped with TiO2 was firstly caused by the decrease of grains size, which leaded to an increase of the gas adsorption sites. And the Fermi level effect of the composite metal oxide heterojunction formed by TiO2 and WO3 also markedly enhanced the gas sensitivity properties of the composite coatings.

Keywords:
Gas sensitivity
Heterojunction
Liquid-phase plasma spraying
WO3
Full Text
1Introduction

With the requirement of real-time detection of the concentration of harmful gases (NO, NO2, CO, SO2, H2S, etc.) in air, it is urgent to develop high performance gas sensors in the industrial, environmental and medical fields. The semiconductor gas sensor is one of the most successful commercially applied sensors because of its advantages such as simple structure, long service life, small size and no maintenance [1,2]. The gas sensitive coating is a core part of the semiconductor gas sensor. A variety of materials have been used to prepare the gas-sensitive coatings, such as zinc oxide (ZnO) [3,4], tin oxide (SnO2) [5,6] and iron oxide (Fe2O3) [7,8]. But there are some limitations in these materials such as the low resolution, high power consumption, and instability in certain temperature and humidity. Compared with the traditional materials above [9,10], WO3 has extremely high sensitivity for NO2 detection, and has been an ideal material for gas sensor coatings. And it is also reported that doping another kind of oxides into WO3 leads to the formation of semiconductor heterojunction, thus can effectively improve the gas sensitivity of the coatings.

At present, most of the commercial semiconductor sensors coatings were prepared by screen printing and magnetron sputtering processes. However, the screen-printed coatings displayed a low iterative feature of their structures. And a time-consuming process of high temperature post-treatment was also unavoidable in screen printing (in order to remove the organic binder and improve the bond strength of the coatings) [11–13]. The magnetron sputtering which needed a high vacuum chamber exhibited a slow sputtering rate, a high cost, and an uncontrollable of the porosity in film [14,15]. Therefore, it was necessarily to develop a high-efficiency and low-cost technique for the preparation of gas sensor coatings. Liquid-phase plasma spraying was an emerging technology for the preparation of coatings with controllable microstructures. The grains and defects sizes in the coatings prepared by liquid-phase plasma spraying were smaller and controllable due to the use of metal salts as precursors. And a larger specific surface area was obtained in these coatings. At the same time, the coatings prepared by liquid-phase plasma spraying showed a higher bonding strength. And there were no need of post-heat treatment and vacuum chamber in liquid-phase plasma spraying. Compare with the traditional technique above, liquid-phase plasma spraying displayed distinct advantages in economizing costs and raising efficiency [16–19]. It is more suitable for the preparation of gas sensor coatings.

It has been reported that doping of metal oxides can significantly improve the performance of WO3 based gas sensors. He [20] sputtered a layer of V2O5 catalyst film on the surface of the WO3 thin film and found that the V2O5 coating with thickness of 20nm showed a porous structure and an excellent sensitivity. A series of W/Cr oxides (W/Cr=1:6, 1:2, and 3:2) were prepared by Diao [21]. And one of the oxides with the W/Cr of 3:2 displayed the maximum response ability for NO2 gas in the concentration range of 20–300ppm. Wang [22] prepared a kind of SiO2–WO3 composite films with SiO2 content between 0 and 20wt%. It was found that the grain size of WO3 decreased with the increase of SiO2, and the sensitivity of the composite film with 5% SiO2 was much higher than that of pure WO3. It can be seen that improving the gas sensing properties of coatings by doping of metal oxides is a hotspot in recent research. However, the influence of the types and proportions of doped metal oxides on the gas sensing properties of coatings is not clear. And the gas sensing mechanism of the composite coatings needs to be further studied. It is well known that TiO2 is a kind of semiconductor material with excellent electrical properties. The improvement in the gas sensitivity of semiconductor WO3 caused by the doping of TiO2 has been reported. Yet the existing technologies for doping of TiO2 are extremely complicated, which are not appropriate for industrialized production. In addition, the gas sensing mechanism of the coatings doped by TiO2 has not been clearly recognized. According to above consideration, the WO3 based composite coatings doped with TiO2 were prepared by liquid-phase plasma spraying in this paper, and the gas-sensing mechanism of TiO2-WO3 composite coatings was studied systematically.

2Experimental materials and methods2.1Preparation of sprayed slurry

For slurry preparation, tungsten chloride (WCl6) as a precursor was dissolved in the solvent (mixture of distilled water and ethanol with the ratio of 1:1). Then an appropriate amount of nano-TiO2 (particle size 5–10nm) was added to the solution. The ammonia water was also added to the slurry in order to adjust the pH. The mechanical stirring and ultrasonic vibration methods were used to keep the slurry uniform and stable. During the coating deposition, the slurry was delivered into an atomizer by peristaltic pump. Compressed air of 0.3MPa was used as the atomizing gas. Thus the slurry was injected into the plasma flame in the form of tiny droplets, and the WO3 solid crystals were formed after heating and evaporating. The purity of all the chemical reagents used in this paper reached to AR grade. The WCl6, NH3·H2O and TiO2 were bought from Aladdin Reagent Co. Ltd. And the C2H6O, C3H6O, HO(C2H4O)nH and H2O were bought from Sinopharm Chemical Reagent Co., Ltd.

2.2Preparation of gas-sensitive coatings

The substrates used for preparing the gas-sensitive coatings were divided into three layers, where in the bottom layer was a Pt heater, the middle layer was an Al2O3 ceramic plate, and the top layer was Au interdigital electrode (as shown in Fig. 1a). The substrates were cleaned by ultrasonic and air dried. Then the substrates were firstly fixed on a stainless steel plate, and the lower portion of Au electrodes was covered by a mask plate. The WO3 based composite coating was then deposited on the upper portion of Au interdigital electrode via plasma spraying by using the slurry as a feedstock (Fig. 1b). The coating deposition was made by APS-2000K plasma spray system (AVIC Beijing Aeronautical Manufacturing Technology Research Institute, China). The spray parameters used in this research were listed in Table 1. For comparison purpose, the pure WO3 coating was prepared under the same conditions.

Fig. 1.

(a) The substrates used for preparing the gas-sensitive coatings, (b) the sample of gas sensor, (c) a model illustrating the change of the precursor (WCl6) during the plasma spraying.

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Table 1.

The spray parameters used in this research.

Number  Current (A)  Voltage (V)  Slurry transportation rate (ml/min)  Spraying distance (mm) 
400  50  15  170 
500  60  15  170 
550  60  15  170 
600  60  15  170 
2.3Characterization of microstructure and properties of coatings

The microstructure of the pure WO3 and TiO2–WO3 composite coatings was observed using a field emission scanning electron microscope (FESEM, Hitachi S-550N, Japan) equipped with a detector of secondary electrons and an energy dispersives pectrometer (EDS). The morphology, sizes and bonding state of the grains in coatings were characterized by transmission electron microscope (TEM, FEI Tecnai F20, Netherlands). X-ray diffraction (XRD, Bruker AXS, Netherlands) and Raman spectrometer (RS, Renishaw in Via Reflex, Britain) were employed to detect the crystal structure and valence of the material in the coatings. The performance of the prepared gas-sensitive coatings was measured. Before testing, the gas-sensing coating was aged at 200°C for at least three days in order to increase its stability. The sensitivity of the coating was calculated according to Eq. (1)[23]:

where Rg was the resistance of sensor texted in the measurement atmosphere and Ra was that of sensor texted in the air.

3Results and discussion3.1Effect of plasma power on coatings

A model illustrating the change of the precursor (WCl6) during plasma spraying was schematically depicted (Fig. 1c). It was clear that the formation of coating including several steps as follow: breakdown and refining of dropletsevaporationconcentrationnucleationgrowthformation of solid particlesmelted of solid particlesformation of coating. It was well known that the liquid-phase plasma spraying parameters played an important role in each of the steps. And the microstructure of the coatings would be significantly influenced by these parameters.

The effects of the plasma power on the crystallinity and morphology of the coatings were investigated. Fig. 2 showed the XRD and Raman spectra of WO3 coatings prepared under different plasma powers. It could be seen that there were no WO3 peaks in the XRD of the coating deposited at a plasma power of 20kW. When the plasma power reached 30kW, the WO3 peaks of the coating became obviously. With the increase of the plasma power (from 30kW to 36kW), the WO3 peaks of the coatings were more and more sharp, which indicated that the crystallinity of the coatings increased. The XRD also displayed that the monoclinic WO3 were predominant in all of the coatings prepared by liquid-phase plasma spraying (Fig. 2a). At the same time, the Raman result showed that there were four wavenumber peaks, which were located at 273cm−1, 328cm−1, 715cm−1 and 807cm−1, referring to the monoclinic WO3 in the prepared coatings (Fig. 2b). And it was kept in line with the result of XRD.

Fig. 2.

(a) XRD spectra and (b) Raman spectra of WO3 coatings prepared under different plasma powers. (c)–(e) Corresponding surface morphology of the WO3 coatings, respectively. (f) TEM of WO3 coatings, (g) HRTEM of WO3 coatings.

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The surface morphology of the WO3 coatings prepared under different plasma powers (30kW, 33kW, and 36kW) were also shown in Fig. 2. It can be seen that all of the WO3 coatings presented a loose and porous structure, which was beneficial to the gas sensitivity of coatings (Fig. 2c–e). And as the plasma power raise up to 36kW, the WO3 had grown larger due to the higher heat input during plasma spraying (Fig. 2e). It was well-known that the larger grains may lead to a lower gas sensitivity of the coatings. Therefore, the plasma power used for the following preparation of WO3 based coatings was fixed to 33kW in this paper. The morphology of the cross-section in all the coatings obtained in this research was almost the same as that of the surface. And the thickness of the pure WO3 coating was about 3μm. The TEM and HRTEM of the obtained coatings were shown in Fig. 2f and g, respectively. It was clear that the platy shaped WO3 particles illustrated a high crystalline and a growth direction of 002. The distances of crystal planes were 0.39nm (Fig. 2g) which were approximately of the same size of the interspaces of monoclinic WO3 (002) crystal faces.

3.2Microstructure of TiO2–WO3 composite coatings

A series of TiO2–WO3 composite coatings doped with different amount of TiO2 were prepared, and the pure WO3 coating was fabricated under the same plasma power (33kW) for comparison. The thickness of the composite coatings was about 5μm and the porosity was about 8%. The XRD patterns of these composite coatings and pure WO3 coating were shown in Fig. 3a. It could be seen that the peaks in TiO2–WO3 composite coatings were obviously wider than that in the pure WO3 coating, indicating that the particle sizes of the composite coatings were relatively small. There were only WO3 and TiO2 without any new phases in all the composite coatings. It could be concluded that no combination reactions between the WO3 and TiO2 occurred. When the mass ratio of MTiO2:MWCl6 reached 10:100 in the composite coating, the TiO2 peaks appeared in the XRD. In order to further characterize the presentation state of TiO2 and WO3 in the composite coatings, the Raman spectroscopy test was carried out (Fig. 3b). It could be seen that the peaks of both the monoclinic WO3 and anatase TiO2 began to appear with the mass ratio of MTiO2:MWCl6 grew up to 10:100 in the composite coating. When the amount of TiO2 doped in coatings was lower, the peaks of TiO2 disappeared. It was probably due to that the quantity of TiO2 was too little and they were uniformly dispersed. The result of Raman spectroscopy was also kept in line with the XRD (Fig. 3).

Fig. 3.

(a) XRD and (b) Raman spectra of the composite coatings and pure WO3 coating.

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The Debye–Scherrer formula was also used to calculate the average grain size of the particles in the composite coatings. The results were shown in Table 2. It could be seen that the average grain size of the composite coating decreased with the increase of TiO2. While the mass ratio of MTiO2:MWCl6 grew up to 20:100, the average grain size of the composite coating became larger. The smallest average grain size was 10.64nm, which was achieved at a mass ratio of MTiO2:MWCl6 was 10:100. It was reported that the smaller grain size may lead to a larger specific surface area, which was beneficial to the gas sensing performance of the coatings. Thus more attention would be paid to the TiO2–WO3 coating with the mass ratio of MTiO2:MWCl6 was 10:100.

Table 2.

Average grain sizes calculated by the Debye–Scherrer formula.

MTiO2:MWCl6  Average grain size(nm) 
0:100  17.77 
1:100  15.18 
5:100  14.85 
10:100  10.64 
20:100  18.94 

The surface morphology and element distribution of the composite coating (MTiO2:MWCl6=10:100) were tested by EDS (Fig. 4). It can be seen that the WO3 was evenly distributed around TiO2 in this coating, and there was a boundary layer between WO3 and TiO2 (Fig. 4a–d), which would suggest the generation of semiconductor heterojunctions.

Fig. 4.

EDS of the composite coating (MTiO2:MWCl6=10:100): (a) the surface morphology and the element distribution: (b) O, (c) Ti and (d) W. (e) TEM of pure WO3 coating, (f) TiO2–WO3 composite coating, (g) HRTEM image of TiO2–WO3 composite coating.

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In order to illustrate the microstructure of coatings more intuitively, the transmission electron microscopy of a pure WO3 coating and a TiO2–WO3 composite coating (MTiO2:MWCl6=10:100) were observed (Fig. 4). It could be seen that the average particle size of the pure WO3 coating was about 20–30nm (Fig. 4e), while that of the TiO2–WO3 composite coating was about 10–20nm (Fig. 4f), indicating that the doping of TiO2 could effectively inhibit the growth of WO3. From the high resolution of Fig. 4g, it could be found out that there were two kinds of crystals with different brightness in the composite coating. The distance of crystal face in the brighter one was 0.39nm, which corresponded to that of WO3 (002) crystal face. The distance of crystal face in the darker one was 0.24nm, which corresponded to that of TiO2 (004) crystal face. Furthermore, the WO3 and TiO2 crystals in the figure were closely combined. In conclusion, the high-resolution image provided a clear evidence for the presence of WO3 and TiO2 semiconductor heterojunctions in the composite coating.

3.3Gas sensing properties of TiO2–WO3 coatings

In this paper, two kinds of coatings prepared using a raw material of pure WCl6 and the mixture of TiO2 and WO3 (MTiO2:MWCl6=10:100), respectively, were the main study object. And the gas sensitivities of them were investigated deeply. In the gas sensitivity test, the gas sensors worked at a temperature of 100°C, 150°C and 200°C, respectively. The response resistance signals of the above mentioned coatings to 100ppm NO2 at different temperatures were shown in Fig. 5. It could be seen that the response and recovery time of TiO2–WO3 composite coating to 100ppm NO2 was shorter than that of pure WO3 coating at 100°C, 150°C and 200°C (Fig. 5a–c). The sensitivity (S) of the coatings at different temperatures was calculated by Eq. (1), as shown in Fig. 5d. It was observed that the sensitivity of both the TiO2–WO3 composite coating and pure WO3 coating decreased as the temperature raised. Therefore the coatings prepared in this paper might have better sensitivity at a lower temperatures (100°C). It might lead to a lower energy consumption and be more suitable for the application of industrial. At the same time, it was obviously that the sensitivity of the TiO2–WO3 composite coating was better than that of the pure WO3 coating. Especially at 100°C, the former was an order of magnitude higher than the latter.

Fig. 5.

The response resistance signals of the TiO2–WO3 composite coatings to 100ppm NO2 at (a) 100°C, (b) 150°C, (c) and 200°C. (d) The corresponding sensitivity (S) at different temperatures.

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3.4Analysis of the gas sensing mechanism

During the liquid-phase plasma spraying process, the heating and cooling rates of the precursor (WCl6) was extremely high. And WO3 was formed in this non-equilibrium reaction. Therefore, this kind of WO3 was generally a nonstoichiometric compound. There would be a variety of defects on its surface [24]. And a high capacity to attract gas was obtained by the WO3 prepared using liquid-phase plasma spraying technology [25]. At the same time, there were many bare crystalline surfaces in this type of WO3 crystal. When the bare crystalline surfaces were exposed in the air, the oxygen molecules would be captured on the surfaces and a lot of electrons would transfer from the surfaces to oxygen molecules [26]. Therefore, on the near surfaces of the WO3 semiconductor, many positively charged donors were left, which would lead to a formation of the space charge layer on the surfaces [27]. And there would be a contact barrier between the WO3 grains at last. It was obviously that the electron transfer between WO3 grains would be limited by the contact barrier (as shown in Fig. 6a). As the height of contact barrier was positively correlated with the oxygen concentration, the higher oxygen concentration, the larger contact barrier between the WO3 grains, which would lead to a higher resistance of the WO3 semiconductor. If the WO3 grains were exposed in a reducing gas, the reducing gas molecules would react with the oxygen attracted on the bare crystalline surfaces of WO3. And a part of the electrons trapped by the oxygen before might be returned to the WO3, the thicknesses of the space charge layers on the surface of WO3 grains were reduced. The height of the contact barrier between the WO3 grains might also decrease, and the quantity of electrons crossing the contact barrier might increased, which would result in a reduction in resistance of WO3 semiconductor (as shown in Fig. 6b). Furthermore, the oxidizing gas exhibited a high electron affinity. If the WO3 grains were exposed in an oxidizing gas, on the other hand, the oxidizing gas molecules would capture electrons from the oxygen attracted on the bare crystalline surfaces of WO3[28]. On the other hand, they might directly capture electrons from the WO3. As a result, the height of the contact barrier between the WO3 grains might increase, leading to an increase in resistance of WO3 semiconductor (as shown in Fig. 6c). The schematic of the fundamental cause of the gas-sensing properties obtained by the WO3 coating was shown in Fig. 6. At the same time, the coatings prepared by liquid-phase plasma spraying contained a quantity of tiny WO3 grains (Fig. 4) and presented a loose and porous structure (Fig. 2). The specific surface area of WO3 coating would be rather large, thus a large number of gas adsorption sites were provided in the coating, and the gas-sensing performance of the WO3 coating would be excellent.

Fig. 6.

The schematic of the fundamental cause of the gas-sensing properties obtained by the WO3 coating in different atmospheres: (a) in air, (b) in reducing gas and (c) in oxidizing gas.

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Based on the analysis above, it could be seen that the TiO2 and WO3 semiconductors were uniformly distributed in the TiO2–WO3 composite coatings (Fig. 4). Since TiO2 and WO3 were both n-type semiconductors, the close contact between them would lead to the formation of an n–n-type heterojunction at the interface (Fig. 7). The difference of Fermi energy between the TiO2 and WO3 semiconductors might induce a directional migration of the electrons in the n–n-type heterojunction (Fig. 7a) [29,30]. Then an electron depletion layer in TiO2 and an electron accumulation layer in WO3 were formed, and the thermal equilibrium was achieved (Fig. 7b). It was well known that the electron accumulation layer would promote the oxygen adsorption on the surfaces of gas sensitive materials. And the electron depletion layer might lead to a reduction in conductivity of the composite gas sensitive materials [31,32]. Therefore, the height of the contact barrier would be increased. And a larger initial resistance was obtained by the composite gas sensitive materials, which might indirectly improve the gas-sensing properties of composite materials. This was the reason why the TiO2–WO3 composite coating exhibited even more excellent gas-sensing properties than the pure WO3 coating under the same condition (Fig. 5d).

Fig. 7.

The formation of an n–n-type heterojunction at the interface between TiO2 and WO3 (a) the directional migration of electrons, (b) the formation of an electron depletion layer in TiO2 and an electron accumulation layer in WO3.

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4Conclusions

In the present study, a series of WO3 based coatings doped with TiO2 were prepared by liquid-phase plasma spraying. The microstructure and properties of TiO2–WO3 composite coatings has been investigated. The main conclusions are as follows:

  • (1)

    The most suitable plasma power for the preparation of gas sensitive coatings was 33kW. As the mass ratio of MTiO2:MWCl6 was 10:100, the TiO2–WO3 composite coating obtained the smallest average grain size (about 10.64nm). And this composite coating displayed an excellent sensitivity to NO2, especially at a lower temperatures (100°C), the sensitivity of the composite coating was about 10 times higher than that of the pure WO3 coating.

  • (2)

    The TiO2-doping led to a decrease of the average grain size and an increase of the specific surface area in the TiO2–WO3 composite coating. At the same time, an n–n-type heterojunction was formed at the interface of TiO2 and WO3.

  • (3)

    The electron depletion layer and electron accumulation layer formed by different Fermi energy between the TiO2 and WO3 semiconductors increased the resistance of the composite coating. It can be conclude that TiO2–WO3 composite coating exhibited rather high gas-sensing properties.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgement

The research was partially supported by the National Natural Science Foundation of China (51301192, 51805316), and the Foundation of Key Laboratory of Marine Materials and Related Technologies, Chinese Academy of Sciences (Grant No. 2017K06).

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Journal of Materials Research and Technology

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