Comparative Study on NO2 and H2s Sensing Mechanisms of Gas Sensors Based on WS2 Nanosheets

Abstract

Introduction Metal sulfides have been actively explored in a wide range of potential applications in optoelectronic [1], energy storage [2], sensor [3], and photocatalytic devices [4] because of their excellent electrical properties and active defects sites . In particular, metal sulfides such as SnS2, MoS2 and WS2 are of interest to researchers because of their narrow band gap, large surface-to-volume ratio and excellent gas selectivity. However, the exploration of the sensing mechanism of metal sulfide-based gas sensors is still in its infancy and lacks experimental verification. Especially the research on the sensing mechanism of WS2 is very scarce. In this paper, we report a facile synthesis of WS2 nanosheets with an average cross-sectional size of about 10 nm by sulfurization of the WO3 nanomesh. The sensor based on WS2 nanosheets exhibits p-type sensing behavior and excellent selectivity to NO2 gas at a low temperature of 160 °C. Its excellent sensing performance can be attributed to the active sites of the thin nanosheets for the adsorption of NO2 and the strong physical affinity between WS2 nanosheets and NO2 gas molecules. In addition, we designed sensing experiments of WS2 nanosheets toward NO2 and H2S in different background atmospheres. Surprisingly, the effect of oxygen in the background gas on the sensing response of NO2 and H2S gas is different, revealing the different sensing mechanisms of WS2 nanosheets for NO2 and H2S gas. The study of the influence of oxygen content in the background atmosphere on the gas response of the sensor is of great significance for the application of the sensor in some anaerobic environments such as aerospace, mine and plateau. Synthesis of WS2 nanosheets The WO3 nanomesh was synthesized according to the procedures detailed in our previous report. In brief, WO3 nanomesh was synthesized by hydrothermal recrystallization of H2WO4 in oxidizing environment at 180 °C for 12 h under autogenic pressure with 0.2 M thiourea as the morphology-controlling agent. The WS2 nanosheets were obtained by sulfurization of the WO3 nanomesh. In a tipical process, 1g sulphur powder and 0.1 g WO3 nanomesh were placed at left end of heating zone (the upstream side) and the hot center of the tube furnace, respectively. The tube furnace was evacuated by using a mechanical pump and then filled up with N2. The evacuation and flushing steps were repeated three times to minimize O2 in the tube furnace. Then, the furnace was heated from room temperature to 500 °C using a heating rate of 5 oC/min, and the sulfurization was performed for a period of 1h at 500 oC in a flow of 80 sccm N2. After the sulfurization, the furnace was naturally cooled down to room temperature. Method The as-prepared WS2 was mixed with a few drops of deionized water and absolute ethanol to form a slurry by grounding in an agate mortar. The sensing slurry was deposited on the Au interdigital electrode by a hand-brushing method, and then the electrode was placed in an oven and dried at 70 °C overnight. The gas sensing measurements were performed by a computer-controlled gas mixing and data-acquisition system. The sensor response was defined as the ratio of resistance ([(Ra /Rg )-1]×100) (%) to oxidizing gas and ([(Rg /Ra )-1]×100) (%) to reducing gas), where where Ra and Rg were the sensor resistance in air and in target gas atmosphere, respectively. The response and recovery times of the sensors were defined as the time required for 90% of the total resistance change. Results and Conclusions In summary, we have reported the gas sensor based on WS2 nanosheets synthesized by the sulfurization of the WO3 nanomesh. The p-type WS2 nanosheets sensor exhibits excellent NO2 sensing characteristics in terms of high response, short response/recovery time, low detection limit and good selectivity. This is attributed to the interactions between the gas molecules and edge and in-plane sites of the thin units of WS2 nanosheets. Moreover, we tested the response of WS2 nanosheets sensor to NO2 and H2S under different background atmospheres. We found that WS2 nanosheets do not require oxygen for NO2 sensing, but oxygen is required for H2S sensing. This result indicates that WS2 nanosheets have different sensing mechanisms for different gases. Specifically, the NO2-sensing mechanism of WS2 nanosheets is based on NO2 physisorption model, while the H2S-sensing mechanism is based on classical oxidation reaction model. This study presents the basic mechanism of the WS2 nanosheets sensor that can be used as reference for other metal sulfide sensors.