Excellent Toluene Sensing Properties Based on Monodispersed WO3 Nanoparticles
Authors: Sun Yanfeng , Wang Xi , Lu Jingyuan , Lu Geyu 
Published: May 2020

Introduction

Tungsten trioxide (WO3) has been demonstrated to possess high sensitivity and selectivity for gas sensing [
1,2
], and used to detect NOx, NH3, H2S and O3 gases. However, studies on toluene are comparatively few. It was demonstrated that smaller particles with controlled pore sizes contribute to better performance for gas detection compared with their corresponding bulk forms [3,4]. Nevertheless, the particle agglomeration during operation at elevated temperatures affects the diffusion of gases into the sensing bodies and leads to a decrease in their sensitivity
We here demonstrate the synthesis of dispersed WO3 nanoparticles with porous nanostructure through a facile solvothermal process. Gas sensing investigation indicates the sensor based on the synthesized WO3 samples with suitable hexamethylenetetramine amount and sintering at 500 °C shows remarkably superior response, outstanding selectivity and fast response/recovery time towards toluene.

Experimental

WO3 nanoparticles were prepared through a facile solvothermal method. Hexamethylenetetramine (HMT) with different HMT/WCl6 molar ratios (0, 0.5, 1 and 2) and WCl6 were dissolved in n-butanol solution, and the corresponding obtained samples were labelled as H0-W, H0.5-W, H1-W and H2-W, respectively. Typically, 0.79 g WCl6 was dissolved completely into a mixed solution containing 0.28 g HMT and 30 mL n-butanol under vigorous and continuous stirring using a magnetic stirrer for 20 min at room temperature accompanied by the precipitate emergence. Afterwards, the uniform solution was placed into a 45 mL Teflonlined stainless-steel autoclave, followed by tightly sealed and stored at 160 °C for 24 h in an electric oven. Then, the precipitate was gathered and washed with distilled water and absolute ethanol for six times by centrifugal separation before drying at 80°C overnight. Finally, the obtained products were calcined under various air atmospheres at 400°C, 500°C and 600°C for 2 h using a muffle furnace (2°C min−
1) and were named as 400°C-WO3, 500°C-WO3 and 600°C-WO3.

Results and Conclusions

Twelve main characteristic diffraction peaks of XRD patterns can be assigned to the monoclinic structure of WO3 according to JCPDS card no.72-677 (Fig. 1a). The intensity becomes stronger with the increase of sintering temperature from 400°C to 500°C due to the crystal growth, then decreases at 600°C caused by crystal distortion. XPS analyses (Fig. 1b-c) indicate that regardless of the samples synthesized at various HMT proportions or sintered at different temperatures, the H1-W nanoparticles sintered at 500°C own the least contents of surface-chemisorbed oxygen (OC). With the increase of HMT amount, the morphologies of the samples will transform from flower-like nanostructure to coagulated nanoparticles (Fig. 3) and dispersed WO3 nanoparticles can be synthesis with suitable HMT amount (Fig. 3c). The inlet in Fig. 3c clearly demonstrates that H1-W has a narrow size distribution. The morphologies of the H1-W will transform from flocculent clusters to coagulated nanoparticles after annealing at high temperature (Fig. 4). Schematic diagrams of H1-W preparation and the morphology-controlled process are shown in Fig.5. Gas investigation indicates that the sensor based on H1-W nanoparticles sintered at 500 °C possesses superior sensing properties towards toluene at the working temperature of 225 °C, including high response (132 to 100 ppm toluene) and fast response/recovery times (2 s/6 s) (Fig. 6).
Generally, high gas response is accompanied with more oxygen defects, which will absorb more chemisorbed oxygen species to react with the target gas. However, this rule does not account for the explanation of the highest response of the sensor based on H1-W nanoparticles sintered at 500 °C. The sensor based on H1-W nanoparticles sintered at 500 °C possesses the highest resistance (over 300 MΩ, which is almost 20 times higher than those of other gas sensors) in air among all sensors. Thus, the resistance of the sensor will change considerably when only a spot of toluene gas reacts with the surface of the sensor. The high resistance of the sensor based on H1-W nanoparticles sintered at 500 °C can be attributed to the high crystallization as shown in the XRD results (Fig. 1a) and the grain boundary among the nanoparticles. Uniform size distribution is also very important when nanoparticles are used as the sensing materials. When the sensing body is composed by nanoparticles with a wide size distribution, the accumulation pores formed by large particles will be filled up by tiny particles. Then the sensing body will become compact which makes it difficult for the tested gas to diffuse deeply into the sensing material. The results imply that monodispersed WO3 nanoparticles with high resistance and porous nanostructure are a promising candidate for ultrafast toluene detection.
References
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Figure 1