Controllable Preparation And Microstructure Manipulation Of Two-dimensional Layered Tungsten Disulfide And Its Gas Sensing Properties For Ammonia At Room Temperature

Nowadays,toxic and explosive gas leakage has become one of the major problems facing industrial production,therefore,the harmful gas detection has been paid more and more attention.The resistance-type semiconductor gas sensor as an important tool for the detection of harmful gases,since their simple preparation method,low cost and easy to carry make them more suitable for application.The current resistive semiconductor gas sensors are primarily based on metal oxides as sensitive materials,which typically require that the sensor's minimum operating temperature should be typically above 200 °C.However,for flammable and explosive gases such as ammonia,high temperature detection will bring serious safety hazards;maintaining gas sensors at high temperature for a long time will also lead to higher power consumption.Hence,the development of a novel resistive semiconductor gas sensor capable of operating at room temperature with low power consumption,which can efficiently and safely detect flammable and explosive gases is also one of the future directions of gas sensors.In recent years,two-dimensional layered transition metal sulfides have become the major attention in the field of room temperature gas sensing materials due to their narrow band gap,high specific surface area and easy structure control.However,there is a widespread problem that the base current of layered transition metal sulfides after interaction with NH₃,which is difficult to recovery at room temperature.It seriously restricts the application of such materials in the field of gas sensing.Therefore,it is very necessary to explore the intrinsic mechanism of layered metal sulfides interaction with NH₃,point out the reason why it is difficult to recover at room temperature and propose a solution.In this paper,tungsten disulfide?WS₂?was studied as a typical layered transition metal sulfide.Firstly,the surface adsorption process of tungsten disulfide and NH₃ gas in the air background was discussed.We report a novel CVD method to prepare the monolayer WS₂ nanosheet,which improves the rapid recovery performance at room temperature.Combined with theoretical calculations and experimental results,the reason why bulk WS₂ is the difficulty recovery after interaction with ammonia gas at room temperature has been disscussed.Finally,a novel room temperature gas sensing material with high response,rapid recovery and strong stability to ammonia at room temperature was obtained by introducing defects and quantum dot modification.In addition,the intrinsic relationship between layer thickness,defects and surface modification and gas-aerobic gas sensitivity at room temperature is established,which provides theoretical support and method for future research on two-dimensional materials in the field of gas sensing.Firstly,the morphology and structure of the bulk WS₂ material and its surface gas adsorption characteristics were studied.Characterization shows that the bulk layer WS₂ is a high-purity,high-quality hexagonal structure with the size of 5 ?m and the thickness of above 200 nm.Comparing the adsorption response of bulk WS₂ with ammonia in different background atmospheres,it can be seen that NH₃ and O2 molecules can directly adsorb on the surface of WS₂ for direct electronic interaction.The analysis results show that NH₃ and O2 molecule can promote each other to be adsorped on the surface of WS₂.Secondly,a novel liquid precursor has been developed by CVD method successfully to form the monolayer WS₂ nanosheets?size:100 nm,thickness: 1 nm?.The gas-sensing results of NH₃ gas at room temperature showed that the room temperature recovery of monolayer WS₂ was greatly improved?the recovery time was 598.2 ± 61.3 s with 250 ppm NH₃?.The adsorption of NH₃ molecules at different binding sites of WS₂ was simulated by theoretical calculations.It was found that bulk WS₂ mainly adsorbs ammonia gas in interlayers,which makes the strong adsorptiona and difficult recovery.While NH₃ gas was only adsorbed on the surface of monolayer WS₂,leading to the weak adsorption and the rapid recovery.All the results demonstrated that reducing the number of WS₂ layers can greatly improve the recovery of WS₂ materials and ammonia gas at room temperature.In order to solve the low yield of two-dimensional WS₂ material prepared by CVD method and the cumbersome device preparation,this paper further simplifies the device preparation process by large-scale preparation of different layers WS₂ based on lithium ion intercalation method.The gas-sensing results of different layer-thickness WS₂ samples determined that there was a linear relationship between the number of layers and the recovery time after WS₂ interacting with NH₃ gas.In addition,we found that the 2D WS₂ flakes prepared by lithium ion intercalation method are rich in vacancy defects and thus have stronger adsorption and dissociation effect on NH₃ gas.Compared with the 2D WS₂ sample prepared by CVD method,the response is enhanced after interaction for the same concentration of ammonia gas.Moreover,the recovery time was also significantly reduced by half?recovery time was 271.9 ± 27.2 s at 250 ppm NH₃?.In order to further improve the room temperature NH₃ gas sensitivity,TiO₂ quantum dots with 3-4 nm size were successfully loaded on the surface of the defect-rich 2D WS₂ flakes.By optimizing the loading molar ratio,the optimal TiO₂ quantum dot loaded WS₂ were obtained?molar ratio is 0.44?.Finally,this new gas-sensing material has high sensitivity to ammonia at room temperature?250 ppm response intensity is 43.72 ± 1.63%,17 times larger than the pure 2D WS₂ sample?,high selectivity,fast recovery?average recovery time is only 174.43 ± 13.75 s at 250 ppm NH₃ gas?and strong stability.Furthermore,the adsorption mechanism of TiO₂ QDs/WS₂ composites with NH₃ molecules was also discussed.The interface depletion layer area was identified as a key factor to improve gas-sensing response and stability.