The xylene sensing performance of WO₃ decorated anatase TiO₂ nanoparticles as a sensing material for a gas sensor at a low operating temperature

Abstract

Here, pristine and WO₃ decorated TiO₂ nanoparticles were synthesized by a one-step hydrothermal method without the use of a surfactant or template and used to fabricate gas sensors. Various techniques were employed for the characterization of the structure and morphology of the as-prepared products. The gas-sensing characteristics of the fabricated sensors were investigated for various concentrations of xylene at different temperatures. At a low operation temperature of 160 °C, the sensors possess an excellent gas response, selectivity, linear dependence, low detection limitation, and repeatability as well as long-term stability. In particular, for the high gas response of the 10.0 mol% WO₃ decorated TiO₂ nanoparticles based sensor, its response reaches 92.53 for 10 ppm xylene, which is much higher than that of the pristine TiO₂ based sensor. And the detection limit is 1 ppm. Those values demonstrate the potential of using WO₃ decorated TiO₂ nanoparticles for xylene gas detection, particularly with low concentration xylene. Apart from this, the mechanism related to the advanced properties was also investigated and presented.

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1. Introduction

In recent years, indoor decoration pollution has been increasing day by day accompanied with the rapid development of modern industry. VOCs (volatile organic compounds) contained in adhesives, wallpaper, solid wood furniture, paints and antiseptic materials are thought to be the main reason of the increase of sick-house syndrome.¹ Thus VOC sensors have attracted attention to monitor the level of VOCs inside buildings and houses. Among those materials, formaldehyde, methanol, toluene and xylene are the major pollution. Xylene, a colorless liquid, is generally available as a mixture of o-xylene (CAS 95-47-6), m-xylene (CAS 108-38-3), and p-xylene (CAS 106-42-3). Those compounds are aromatic hydrocarbons that are a threat to human body.² Xylene is one such dangerous VOCs gas which may induce symptoms in human such as the irritation of eyes, nose, skin and so on.³ When human exposed themselves to a high concentration xylene, the obvious irritation of upper respiratory tract, pharyngeal hyperemia, dizziness, nausea, vomit, chest tightness, limb weakness, confusion and unsteady gait appeared in a short time. In view of the dangerousness of xylene, for instance, the American Conference of Governmental Industrial Hygienists has adopted a permissible exposure limit of 100 ppm in air as an 8 h hour time-weighted average, with a ceiling limit of 150 ppm.⁴ Moreover, the researchers have found that human who frequently under a low concentration xylene surrounding can cause some physical ill symptom than normal person.^5–11 For example, C. Manuela et al.⁶ had found that female urban workers who exposed to BTXs appeared to be higher than those able to determine the harmful effects on pregnancy; E. H. Lee et al.⁷ found that the probability of acquired color vision impairment was increased in workers exposed to mixed organic solvents, such as xylene, or mixed organic compounds, relative to non-exposed controls or to the general population; N. Kanjanasiranont et al.⁹ reported that the carbonyl compounds and BTEX derived from incomplete combustion were the pollutants of the greatest concern, where the outdoor workers had a higher cancer health effect risk from their inhalation levels than the currently acceptable limit. Therefore, the monitoring of xylene is essential for human health and the corresponding sensors with high performance are in great need.

So far, xylene sensors have not been extensively studied, and xylene gas sensors based on TiO₂ have been seldom reported. As an important, wide-energy-gap semiconductor, titanium dioxide (TiO₂) has been intensively studied as a key material for fundamental research and technological applications in the fields of semiconductors, lithium-ion batteries,¹² photocatalytic decomposition¹³ and solar cell,¹⁴ because of its good chemical stability, non-toxicity, abundance and low cost,¹⁵ etc. It is worth trying to fabricate xylene gas sensors with high performance using TiO₂ as the sensing materials. However, TiO₂ is a high resistance n-type semiconductor with conductivity that is too poor to be considered for gas sensing oxidative gases, and its low electrical conductivity inhibits its practical implementation as a gas sensor.¹⁶ For this reason, the TiO₂-based gas sensor has been extensively studied in order to improve its sensing performance. One strategy is to load noble mental or add foreign atoms to improve the sensing performance.^17–25 For instance, D. Z. Wang et al.¹⁷ reported that the ethanol gas response of palladium/TiO₂ nanobelt surface heterostructures is about 10 and 3 times than that of TiO₂ nanobelts and surface-coarsened TiO₂ nanobelts at the concentration of 500 ppm, respectively. L. H. Zhu et al.¹⁸ proved that the values of xylene gas response for 2 mol%, 4 mol%, 6 mol% Ni doped TiO₂ based xylene sensor are larger than the pure one, and among the three Ni doped samples, the 2 mol% doped TiO₂ displayed the highest gas response (4.4), which is 2.4 times larger than that of pure TiO₂ at 302 °C (about 1.8). R. J. Lv et al.¹⁹ reported that the gas response of Al₂O₃/meso-TiO₂ nanotubes based sensor is around 88.04% at a concentration of NO_x gas of 97 ppm, which is significantly greater than the 27.16% observed for meso-TiO₂ nanotube films at the same NO_x concentration. S. Kabcum et al.²¹ showed that the H₂ response of WO₃ sensors is strongly dependent on the operating temperature and Pd loading level. In particular, the 1.0 wt% Pd-loaded WO₃ nanorods exhibited an ultra-high response of 3.14 × 10⁶ with a short response time of 1.8 s to 3.0 vol% of H₂ at 150 °C. X. Liu et al.²⁵ observed that doping tungsten can significantly enhance the sensitivity of TiO₂ toward butane, and gas response toward 3000 ppm butane is increased from 6 to 17.8 through the doping of 5% tungsten. These results reveal that adding foreign atoms is an effective measure to enhance the gas sensing properties of TiO₂ nanomaterials. However, to the best of our knowledge, there are few reports on the xylene sensing properties of WO₃ decorated TiO₂ nanoparticles, which were prepared by hydrothermal method.

In this paper, the enhanced response, selective and repeatability xylene sensors fabricated by WO₃ decorated anatase TiO₂ nanoparticles were prepared by a simple hydrothermal method. To demonstrate the applications, the WO₃ decorated anatase TiO₂ nanoparticles based gas sensing properties were investigated. A comparative gas sensing study between the pure and WO₃ decorated anatase TiO₂ nanoparticles were performed. The results indicate that the sensors fabricated by WO₃ decorated anatase TiO₂ nanoparticles, in particular the W/Ti molar ratio is 10 : 100, shows an outstanding sensing properties towards xylene with enhanced response, selectivity and repeatability characteristics at a low temperature of 160 °C. At last, the effect of W decorating was investigated and a possible mechanism was proposed.

2. Experimental

2.1. Preparation of WO₃ decorated TiO₂

All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification.

WO₃ decorated TiO₂ powders were prepared by a simple low temperature hydrothermal method. The reactions can be described as follows:^26,27

TiOSO₄ + 2H₂O → TiO(OH)₂ + H₂SO₄ (1)

TiOSO₄ and (NH₄)₆H₂W₁₂O₄₀·xH₂O were used as titanium and tungsten sources, respectively. In typical synthesized experiments, 4.899 g of titanyl sulfate and appropriate amounts of (NH₄)₆H₂W₁₂O₄₀·xH₂O (the molar ratio of W/Ti were 0.0%, 1.0%, 2.5%, 5.0%, 7.5%, 10.0%, 20.0% and 30.0%, respectively) were dissolved in 50 mL of deionized water under vigorous stirring for 2 h to form a homogenous solution. Then, the solution was transferred into a Teflon-lined stainless steel autoclave with a capacity of 80 mL and reacted under hydrothermal conditions at a temperature of 180 °C for 4 h. The autoclaves were cooled down to room temperature in a standard atmosphere. Finally, the undecorated and WO₃ decorated TiO₂ products were centrifuged, the similar white precipitates were thoroughly washed with deionized water and dried at 60 °C for 12 h and the final products were obtained.

2.2. Characterization of as-synthesized WO₃ decorated TiO₂

X-Ray diffraction (XRD, Rigaku D/MAX-3B powder diffractometer) with a copper target and Kα1 radiation (λ = 1.54056 Å) was used for the phase identification, where the diffracted X-ray intensities were recorded as a function of 2θ. The sample was scanned from 20° to 65° (2θ) in steps of 0.02°. Transmission electron microscopy (TEM) measurement was performed on a Zeiss EM 912 Ω instrument at an acceleration voltage of 120 kV, while high-resolution transmission electron microscopy (HRTEM) characterization was done using JEOL JEM-2100 Electron Microscope (with an acceleration voltage of 200 kV). The samples for TEM were prepared by dispersing the final dry samples in ethanol, and this dispersing was then dropped on carbon–copper grids covered by an amorphous carbon film. The selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDX) spots pattern scanning analysis was performed by the TEM attachment. The nitrogen adsorption isotherm was measured at 77.3 K with a Micromeritics ASAP 2010 automated sorption analyzer. Prior to the measurement, the sample was degassed at 300 °C for 3 h under a vacuum. X-ray photoelectron spectroscopy (XPS) was carried out at room temperature in an ESCALAB 250 system. During XPS analysis, an Al Kα X-ray beam was adopted as the excitation source and the vacuum pressure of the instrument chamber was 1 × 10⁻⁷ Pa as read on the panel. Measured spectra were decomposed into Gaussian components by a least-square fitting method. Bonding energy was calibrated with reference to the C 1s peak (284.6 eV).

2.3. Fabrication and measurement of gas sensor

The fabrication of indirect-heating structure sensor was described in the literature.^28,29 WO₃ decorated TiO₂ nanoparticles were mixed with deionized water to form pastes, and then coated onto the outside of an alumina tube (4 mm in length, 1.2 mm in external diameter, and 0.8 mm in internal diameter) with a pair of Au electrodes and platinum wires installed at each end. A Ni–Cr alloy wire crossing the alumina tube was used as a resistor to ensure both substrate heating and temperature control by adjusting the heating voltage (V_h). Before measuring the gas sensing properties, the gas sensors were aged at a voltage of 5 V to improve their stability and repeatability. Gas sensing properties were measured by a WS-30A system (Weisheng Instruments Co. Zhengzhou, China) covered with a chamber (18 L in volume). For the purpose of clarity, sketch of the structure of the gas sensor, a photography of the measured system as well as the basic working principle of the gas sensor test is depicted in Fig. S1 (ref. 30) in the ESI.† The circuit voltage (V_c) was set at 5 V, and the output voltage (V_out) was set as the terminal voltage of the load resistor (R_L).³¹ During the test, the desired amounts of test gas were injected into a test chamber using a rheodyne after the base line of the sensor was stable. The desired concentrations of the testing gas are obtained by the volume of the analyte solution. An evaporator and two fans are installed to make the gas homogeneous immediately in the chamber. Note that the clean dry air was used as a reference gas and diluting gas for the different concentrations of target gas. It is well known that the gas response (β) was defined as the ratio of the electrical resistance in air (R_a) to that in target gas (R_g), namely β = R_a/R_g^30,32 for n-type gas sensors. Meanwhile, the operating temperature of the sensors was varied in the range from 100 to 340 °C.

3. Results and discussion

Fig. 1 shows the XRD patterns of the undecorated and WO₃ decorated TiO₂ products synthesized by the chemical reaction of TiOSO₄ and (NH₄)₆H₂W₁₂O₄₀·xH₂O, respectively.^26,27 It can be seen that all of the experimental diffraction peaks can be indexed to TiO₂ (anatase, JCPDS no. 21-1272, space group: I4₁/amd (141)) and tungsten oxide WO₃ (hexagonal, JCPDS no. 33-1387, space group: P6/mmm (191)). No obvious peaks from impurity are observed, indicating the high purity of the obtained products. The broader diffraction peaks, for instance, the overlapping of (103), (004) and (112), suggest the small crystallite size of the products. Besides, the diffraction peaks of WO₃ appeared with increasing the W decorating amount. The appearance of WO₃ diffraction peaks indicated that WO₃ could be incorporated on the surface of TiO₂ instead of incorporated into lattice of TiO₂ successfully.

Fig. 1 The XRD patterns for as-synthesized pure and WO₃ decorated TiO₂ nanoparticles. “*” is on behalf of the phase of WO₃.

The morphologies of as-synthesized WO₃ decorated TiO₂ were performed by TEM. Fig. 2 shows the structural features of 10.0% WO₃ decorated TiO₂. Fig. 2(a) clearly shows several WO₃ decorated TiO₂ with the size in micrometer. One can observe that the as-synthesized products have a rather uniform analogous shape as well as size. In order to ulteriorly see the surface topography of as-synthesized products, Fig. 2(b) shows the partial enlargement of Fig. 2(a), one can distinctly discovery that the WO₃ nanometer particles are uniformly distributed on the surface of TiO₂. Fig. 2(c) exhibits a high-resolution TEM (HRTEM) of several WO₃ decorated TiO₂, the sets of lattice fringes with an interplanar distance of about 0.352 nm can be for the (101) lattice planes of anatase TiO₂, the inset is the selected area electron diffraction pattern. It is worthwhile to note that no lattice fringes of WO₃ phases are found by elaborating examination of many HRTEM micrographs of 10 mol% WO₃ decorated TiO₂ nanoparticles. Again, the probable reason of this phenomenon can be put down to the rather smaller size of WO₃ nanoparticles and their lattice fringes were covered by the lattice fringes of TiO₂. To demonstrate the existence of WO₃, composition analysis was examined using energy-dispersive X-ray spectrometry (EDX) as for 10 mol% WO₃ decorated TiO₂ nanoparticles, as indicated in Fig. 2(d). The peaks of O, Ti and W (Cu peaks are attributed to the copper grids) can be clearly seen, suggesting the high purity of the products.

Fig. 2 TEM image and magnified TEM image of as-synthesized products, (a) TEM image of 10.0 mol% WO₃ decorated TiO₂ nanoparticles; (b) magnified TEM image of 10.0 mol% WO₃ decorated TiO₂ nanoparticles; (c) the corresponding HRTEM image with obvious TiO₂ lattice fringes, the inset is the selected area electron diffraction pattern; and (d) the corresponding spectrum of EDX.

In this work, nitrogen adsorption–desorption isotherms measurements were performed to detect the surface adsorption properties of sample. The N₂ adsorption–desorption isotherms are shown in Fig. 3. The isotherm can be categorized as type IV with small hysteresis loops observed at a relative pressure of 0.45–0.95. The specific surface area estimated from the BET method was 108.76 m² g⁻¹, which is comparable to the reported values of 108.9 m² g⁻¹ by the same method³³ and a little bigger than 7.5 mol% (98.77 m² g⁻¹) but smaller than that of 20.0 mol% (131.36 m² g⁻¹) (Fig. S2 and S3†). Inset of Fig. 3 illustrates its corresponding pore size distributions obtained from desorption branches. It can be concluded from the pore size distributions that as-synthesized 10.0 mol% WO₃ decorated TiO₂ nanoparticles have pores with diameter of about 3.562 nm, which are similar to 7.5 mol% (3.581 nm) (Fig. S2†) and 20.0 mol% (3.555 nm) (Fig. S3†) WO₃ decorated TiO₂ samples. However, the images of TEM do not show the porous structure, which indirectly illustrates that the pores are piled up by particles.

Fig. 3 Nitrogen adsorption–desorption isotherms of as-synthesized 10.0 mol% WO₃ decorated TiO₂ nanoparticles, and the inset is the corresponding pore size distributions.

Xylene is an important chemical and widely used raw materials, which can do harm to human and environments. Thus, it is significant to effectively detect and monitor it using suitable gas sensor. However, the xylene gas sensors response value reported are poor.^34,35 For instance, C. Yang et al.³⁵ reported that the response value of 3D flower-like CuO nanostructures based sensor is 3.6 to 1000 ppm at the temperature of 260 °C. Therefore, it is important to develop and search for a high-powered sensing material for xylene. Accordingly, the gas-sensing properties of the as-synthesized WO₃ decorated TiO₂ were investigated in this study. The operating temperature is one of the most important factors for the gas sensor, which highly determined the nature of the sensing materials and the gas-sensing process between the gas and the surface of materials. Normally, the first approach is to select the optimum temperature of gas sensor. The gas response of the sensors fabricated with WO₃ decorated TiO₂ materials was analyzed at working temperatures ranging from 100 °C to 340 °C at xylene ambient with a concentration of 200 ppm. As shown in Fig. 4(a). The sensors' response value first increased and then decreased with the temperature increasing. Obviously, at a low operating temperature, the low gas response (β = R_a/R_g) can be obtained for the target gas xylene do not have enough thermal energy to react with the surface electron of WO₃ decorated TiO₂ nanoparticles, which leads to a low response. With the operating temperature increasing, the thermal energy obtained is high enough to overcome the activation energy of the surface reaction.^36,37 Moreover, the reduction in gas response after the maximum is due to the low gas adsorption ability of the gas molecule at high temperature causes the low utilization rate of the sensing material, which is the reason for the reduction in gas response.^38–40 Furthermore, the gas response value of sensors reaches the highest at the temperature of 160 °C. Hence, 160 °C was chosen as the optimum temperature, and monitored different gases. From Fig. 4(b) we can clearly see that the response increased with the molar ratio of W when the W/Ti molar ratio was not too high and the response decreased when the W/Ti molar ratio became even larger (such as 20.0 mol%). The excessive surface reaction activity caused by over decorated WO₃ decreased the utility factors of the sensing body.²³ Therefore, the 10.0 mol% WO₃ decorated TiO₂ was chosen to further investigate the sensing properties.

Fig. 4 (a) Gas responses of the sensors based on pure and different molar ratios of WO₃ decorated TiO₂ nanoparticles at different operating temperature toward 200 ppm xylene, (b) the xylene response of different molar ratio of WO₃ decorated TiO₂ nanoparticles fabricated sensors.

In addition, selectivity is a remarkable parameter of the gas sensor to guarantee an exact recognition, the responses of the as-fabricated gas sensor to various types of gases (such as xylene, acetone, ammonia, ethanol, isopropanol, methylbenzene and n-butanol) were investigated at the optimum temperature of 160 °C. As shown in Fig. 5. One can see that the sensor shows higher response and better selectivity to xylene, and the response is about 1200 towards 200 ppm xylene, while to acetone, ammonia, ethanol, isopropanol, methylbenzene and n-butanol, the response are 116.86, 11.91, 26.87, 40.72, 202.51 and 168.43 at the same gas concentration, respectively. The response of xylene is about 6 times and 10 times higher than that of methylbenzene and acetone, respectively. The selectivity of the sensor is significantly influenced by several factors such as the lowest unoccupied molecule orbital (LUMO) energy of gas molecules and the amount of both oxygen and gas adsorption on the sensing material at different working temperatures.^41–43 The response could be enhanced for the low value of LUMO energy because of the relatively lower reactive energy. Furthermore, the lower LUMO energy also strengthens the ability to capture electrons from the surface of the sensing material of gas molecules, thus the gas-sensing performance can be improved.⁴¹

Fig. 5 Selectivity of xylene in relation to various gases for a gas sensor based on 10.0 mol% WO₃ decorated TiO₂ nanoparticles at a fixed concentration of 200 ppm at an operating temperature of 160 °C.

However, the xylene concentration of 200 ppm is higher than the threshold value of 100 ppm, which American Conference of Governmental Industrial Hygienists (ACGIH) had stipulated.⁴ And the researchers found that workers under a lower concentration of xylene surrounding day-to-day can cause palpable ocular region and air tube irritation than normal person.^5,6 Those researches show that the low concentrations of xylene can do a certain degree of harm to the workers' health.

Accordingly, the responses of the gas sensor based on 10.0 mol% WO₃ decorated TiO₂ as a function of low xylene concentrations from 1 to 10 ppm were investigated at optimum temperature. As shown in Fig. 6(a), the response value magnitude increases along with the increased concentration of xylene. The straight lines are calibration curves and experimental data which were fitted as:

β = 9.67758C_gas + 0.48933 (4)

where β is the gas response and C_gas is the gas concentration. The standard error of intercept and slope are 1.36241 and 0.21957, respectively. The correlative coefficient is 0.99539, indicating the sensor has a good linear dependence in the region from 1 to 10 ppm xylene. From the straight line obtained, we extrapolated a lower detection limit of 53 ppb to xylene. Fig. 6(b) shows the dynamic response–recovery time of the sensor at the xylene concentration of 10 ppm at 160 °C. The sensor shows excellent response value of 92.53 at a low temperature. Nevertheless, the shortage of as-fabricated sensor is its long response/recovery time, which may have some impacts on using in practical application. One approach to solve the deficiency is to heat the sensor at a high temperature, to accelerate the xylene degasification on the surface of the sensor. However, not all occasions can have the condition to heat the sensors. Hence, our following job is to ameliorate the response/recovery time of the xylene sensor by other synthetic technique or adding precious metal.

Fig. 6 The gas sensing properties of the sensor based on 10.0 mol% WO₃ decorated TiO₂ nanoparticles at an operating temperature of 160 °C. (a) Linear dependence relation between response sensitivity and gas concentration in a range of 1–10 ppm, (b) response and recovery characteristic of the gas sensor under xylene concentration of 10 ppm at the operating temperature.

In addition, repeatability is also one important parameter which can be used to evaluate the reliability of a fabricated sensor. To verify the repeatability of the sensor, the gas response evolutions in several cycles were tested toward 10 ppm xylene at 160 °C. Fig. 7 illustrates the reproducibility of the sensor based on the as-prepared WO₃ decorated TiO₂, revealing that the sensor maintains its initial response amplitude without a clear decrease upon 8 cycles successive sensing tests towards 10 ppm of xylene at 160 °C.

Fig. 7 Repeatability of the 10.0 mol% WO₃ decorated TiO₂ nanoparticles gas sensor to 10 ppm xylene at the operating temperature of 160 °C. Response sensitivity changes with time in continuous eight test cycles indicates the repeatability.

In practical applications, the long-term stability of a gas sensor has attained much attention for which the reliability of gas sensors and the service length were determined. To verify the stability of the sensor, the gas responses toward 10 ppm xylene over 28 days were tested at its optimal temperature. As shown in Fig. 8, the gas response evolution shows that the responses only have a small fluctuation, which is below 4.55% of its initial value. This illustrates a good stability of the gas sensor.

Fig. 8 Long-term stability of the 10.0 mol% WO₃ decorated TiO₂ nanoparticles gas sensor to 10 ppm xylene at the operating temperature of 160 °C.

A brief summary of the sensing performances of various xylene sensor materials based gas sensor toward xylene are shown in Table 1.^1,3,44–53 As it has been introduced, the responses of those xylene sensors toward xylene are poor. Moreover, most of the reported sensors show a bad response value property. For example, D. Acharyya and P. Bhattacharyya⁴⁴ reported the response value of ZnO nanoflowers based sensor towards 200 ppm xylene is 7.94. The response value for WO₃ thick film towards 1 ppm xylene is about 3.03.⁴⁶ Y. Vijayakumar et al.⁵² reported the response value for V₂O₅ thin films towards 100 ppm xylene is 27.9. The low response value naturally cannot meet the need of practical application. In this work, the response value of the sensor fabricated by as-synthesized 10.0 mol% WO₃ decorated TiO₂ can reach 92.54 with 10 ppm xylene and detection limits as low as 1 ppm.

Table 1 Comparison of varied material nanostructures in xylene sensing performances^a

Materials	C (ppm)	OT (°C)	Response	Ref.
a Note: OT: operating temperature, C: xylene concentration.
WO3 decorated TiO2	1	160	12.37	This work
	5		50.55
	10		92.54
Ni-deposited porous alumina	10	340	5.4	1
Cr-doped NiO	5	400	11.6	3
ZnO nanoflower	200	200	7.94	44
ZnO thick film + 10 wt% TiO2	100	370	50	45
WO3 thick film	1	400	3.03	46
Co-doped branched ZnO nanowires	5	375	20.25	47
Cr-loaded NiO core-in-hollow-shell structured micro/nanospheres	5	220	20.9	48
C-doped WO3	50	320	199	49
Ni-doped branched ZnO nanowire	5	400	42.44	50
SnO2 + MWCNTs	3.6	220	0.15	51
V2O5 thin films	100	300	27.9	52
α-Fe2O3	100	340	6.45	53

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In brief, gas sensor based on 10.0 mol% WO₃ decorated TiO₂ nanoparticles exhibits low operating temperature (160 °C), excellent response to a low concentration of xylene (1 to 10 ppm), good selectivity, and superior reproducibility and long-term stability. These observations indicated the as-synthesized WO₃ decorated TiO₂ nanoparticles are a promising gas-sensing material for xylene.

According to the results of the xylene-sensing tests, the tungsten oxide decorated TiO₂ leads to an unexpected remarkable increase in gas response towards xylene. The possible reason may be that WO₃ is moderately active in enhancing the chemical activity of xylene gas and thus increases the amount of adsorption on the surface of the sensing film. During adsorption and desorption on the surface of WO₃, some of xylene molecules are possibly converted to radicals or the catalytic promotion of WO₃ to methyl groups in xylene which have unpaired electrons and thus are chemically more active.^1,48 Our sensor materials are n-type semiconductors, which gas sensing performances depend on the change of surface occupation. According to Wolkenstein's model for semiconductors,⁵⁴ we propose an analogous model for the WO₃ decorated TiO₂, as schematically shown in Fig. 9, which is similar as our reported work.⁵⁵ In air atmosphere, oxygen adsorbs onto the surface of WO₃ decorated TiO₂, and electron transfers from conduction band to the oxygen molecules to form various kinds of oxygen ions with different valence states (O_2ads⁻, O_ads⁻, O_ads²⁻), which leads to the formation of a thick space-charge layer and a consequent high resistance of the sensor. This process can be described by the following equations:^55–57

O_2gas ↔ O_2ads (5)

O_2ads + e⁻ ↔ O_2ads⁻ (6)

O_2ads⁻ + e⁻ ↔ 2O_ads⁻ (7)

O_ads⁻ + e⁻ ↔ O_ads²⁻ (8)

Fig. 9 A schematic diagram of the proposed reaction mechanism of the 10.0 mol% WO₃ decorated TiO₂ nanoparticles based sensor in air and in xylene, respectively.

When the sensor is exposed to xylene, the reductive gas reacts with the oxygen adsorbed on the sensor surface. Then the electrons are released back to the conduction band of semiconductor, leading to a thinner space-charge layer and a lower potential barrier.⁵⁸ This process results in a decrease in the resistance and can be expressed by the following equations:^1,53

C₆H₄(CH₃)₂(gas) ↔ C₆H₄(CH₃)₂(ads) (9)

C₆H₄(CH₃)₂(gas) + 21O⁻ → 8CO₂(gas) + 5H₂O(gas) + 21e⁻ (10)

C₆H₄(CH₃)₂(ads) + 21O²⁻ → 8CO₂(gas) + 5H₂O(gas) + 42e⁻ (11)

In order to further explain the gas sensing mechanism, the surface/near chemical states of 10.0 mol% WO₃ decorated TiO₂ were measured by XPS measurements. As shown in Fig. 10. Fig. 10(a) shows the survey. One can find that only the peaks related to O, Ti and W are observed apart from C 1s peak at 284.6 eV, indicating a good purity of the as-synthesized samples. The high-resolution XPS spectrum of O 1s shows in Fig. 10(b) reveals two kinds of oxygen in the surface: the peak centers at 530.26 eV indexes lattice oxygen (O_lattice) while the peak at 532.23 eV presents adsorbed oxygen (O_x⁻).⁵⁸ O_x⁻ is attributed to the oxygen ions in the crystal lattice which is thought to be pretty stable and has no contribution to the gas response; meanwhile, O_x⁻ is attributed to the absorbed oxygen ions, which has a very important role in the gas sensing property.⁵⁹ Through calculating the area of O_x⁻ and O_lattice emission lines, the concentration of O_x⁻ to O 1s is estimated to be 32.94%, which is higher than the concentration of O_x⁻ of 7.5 mol% WO₃ decorated TiO₂ (26.21%) (Fig. S4†) and 20.0 mol% WO₃ decorated TiO₂ (31.51%) (Fig. S5†). That the xylene response value of 10.0% WO₃ decorated TiO₂ is higher than 7.5 mol% and 20.0 mol% WO₃ decorated TiO₂ can ascribe to this point. In Fig. 10(c), Ti 2p spectrum revealing the existence of two peaks of Ti 2p_1/2 and Ti 2p_3/2 at the position of 464.7 eV and 458.9 eV demonstrated that Ti ions in the sample present in a single of Ti⁴⁺ ions.⁶⁰ High-resolution W 4f spectra shown in Fig. 10(d) reveal two peaks of W 4f_5/2 and W 4f_7/2 at 38.1 eV and 36.0 eV, with a splitting of 2.1 eV, indicating that the tungsten element in the WO₃/TiO₂ exists in the form of W⁶⁺.^61,62 In addition, the presence of tungsten in XPS and the observable tungsten-related in XRD pattern further illustrate that tungsten is surely accreting on the surface of the TiO₂.

Fig. 10 (a) XPS survey spectrum of the 10.0 mol% WO₃ decorated TiO₂ nanoparticles, (b) high-resolution XPS spectrum of O 1s, (c) Ti 2p, and (d) W 4f for 10.0 mol% WO₃ decorated TiO₂ nanoparticles.

The significant enhancement of the gas response toward xylene is attributed to the synergistic effect of both the matrix and the enhancer. WO₃ and TiO₂ are n-type semiconductors, the band gap and work function of the two oxides are 2.7 eV and 4.41 eV,⁶³ 3.2 eV and 5.1 eV,⁶⁴ respectively. As WO₃ is coupled with TiO₂, the n–n type hetero-junctions are formed. As shown in Fig. 11(a). In such junction electron transfer can occur from semiconductor with low work function (WO₃) to the other with high work function (TiO₂), and the Fermi energy (E_F) of TiO₂ is different than that of WO₃, the electrons at higher energies side will flow across the interface to the unoccupied lower energy states, until the Fermi levels equalize,^65,66 as shown in Fig. 11(b) and (c), thus this process increases the resistance of WO₃. Moreover appropriate amount of heterojunction could promote higher oxygen adsorption on the sensor surface to a greater extent which might provide higher reaction sites. The sensor thus presents a higher resistance in air and the resistance change upon gas exposure is increased to a greater extent resulting in higher sensor response.⁶⁷ However, when the content of WO₃ reaches 20.0 mol%, the responses tended to become lower, which may be attribute to the fact that the WO₃ nanoparticles covered up too much active sites for xylene detection.

Fig. 11 Schematic diagram of (a) the mechanism for the enhancement caused by the metal-semiconductor and (b) n–n junction about to form between TiO₂ and WO₃; (c) after formation the n–n junction.

4. Conclusions

In conclusion, WO₃ decorated anatase TiO₂ nanoparticles were successfully synthesized by a simple and convenient facile hydrothermal method. Characterizations by XRD, TEM, BET and XPS showed that WO₃ decorated TiO₂ nanostructures comprised 1–3 nm spherical or oval WO₃ dispersed over the surface of polycrystalline TiO₂ nanoparticles. The sensors fabricated from WO₃ decorated anatase TiO₂ nanoparticles were used to detect xylene. The results of characterizations indicate that 10.0 mol% WO₃ decorated TiO₂ nanoparticles demonstrated significantly enhanced response to xylene compared with other counterpart. Also, 10.0 mol% WO₃ decorated TiO₂ based sensor exhibits high response towards xylene of a low concentration from 1 to 10 ppm at 160 °C, as well as good selectivity, excellent reproducibility and stability, illustrating the potential of using 10.0 mol% WO₃ decorated TiO₂ nanoparticles for xylene.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 51262029) and Program for Excellent Young Talents, Yunnan University.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1: the structure of the gas sensor and schematic structure along with the testing principle of the gas sensor; Fig. S2: nitrogen adsorption–desorption isotherms of as-synthesized 7.5 mol% WO₃ decorated TiO₂ nanoparticles; Fig. S3: nitrogen adsorption–desorption isotherms of as-synthesized 20.0 mol% WO₃ decorated TiO₂ nanoparticles; Fig. S4: (a) XPS survey spectrum of the 7.5 mol% WO₃ decorated TiO₂ nanoparticles, (b) high-resolution XPS spectrum of O 1s, (c) Ti 2p, and (d) W 4f for 7.5 mol% WO₃ decorated TiO₂ nanoparticles; Fig. S5: (a) XPS survey spectrum of the 20.0 mol% WO₃ decorated TiO₂ nanoparticles, (b) high-resolution XPS spectrum of O 1s, (c) Ti 2p, and (d) W 4f for 20.0 mol% WO₃ decorated TiO₂ nanoparticles. See DOI: 10.1039/c6ra09195d

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