Hexagonal WO3nanorods are fabricated by a facile hydrothermal process at 180?C using sodium tungstate and sodium chloride as starting materials. The morphology, structure, and composition of the prepared nanorods are studied by scanning electron microscopy, X-ray diffraction spectroscopy, and energy dispersive spectroscopy. It is found that the agglomeration of the nanorods is strongly dependent on the PH value of the reaction solution. Uniform and isolated WO3nanorods with diameters ranging from 100 nm–150 nm and lengths up to several micrometers are obtained at PH = 2.5and the nanorods are identified as being hexagonal in phase structure. The sensing characteristics of the WO3nanorod sensor are obtained by measuring the dynamic response to NO2with concentrations in the range 0.5 ppm–5 ppm and at working temperatures in the range 25?C–250?C. The obtained WO3nanorods sensors are found to exhibit opposite sensing behaviors, depending on the working temperature. When being exposed to oxidizing NO2gas, the WO3nanorod sensor behaves as an n-type semiconductor as expected when the working temperature is higher than 50?C, whereas, it behaves as a p-type semiconductor below 50?C. The origin of the n- to p-type transition is correlated with the formation of an inversion layer at the surface of the WO3nanorod at room temperature. This finding is useful for making new room temperature NO2sensors based on hexagonal WO3nanorods.
WO 3 bulk and various surfaces are studied by an ab-initio density functional theory technique.The band structures and electronic density states of WO 3 bulk are investigated.The surface energies of different WO 3 surfaces are compared and then the (002) surface with minimum energy is computed for its NH 3 sensing mechanism which explains the results in the experiments.Three adsorption sites are considered.According to the comparisons of the energy and the charge change between before and after adsorption in the optimal adsorption site O 1c,the NH 3 sensing mechanism is obtained.
Hexagonal WO_3 nanorods were synthesized through a facile hydrothermal method. The nanorods properties were investigated by scanning electron microscope(SEM), transmission electron microscope(TEM), energy dispersive spectroscopy(EDS), and x-ray diffraction(XRD). The NO_2-sensing performances in terms of sensor response, response/recovery times and repeatability at room temperature were optimized by varying the heat treatment temperature of WO_3 nanorods. The optimized NO_2sensor(400-℃-annealed WO_3 nanorods) showed an ultra-high sensor response of 3.2 and short response time of 1 s to 5-ppm NO_2. In addition, the 400-℃-annealed sample exhibited more stable repeatability.Furthermore, dynamic responses measurements of annealed samples showed that all the annealed WO_3 nanorods sensors presented p-type behaviors. We suppose the p-type behavior of the WO_3 nanorods sensor to be that an inversion layer is formed in the space charge layer when the sensor is exposed to NO_2 at room temperature.Therefore, the 400-℃-annealed WO_3 nanorods sensor is one of the most energy conservation candidates to detect NO_2 at room temperature.
Density functional theory (DFT) calculations are employed to explore the NO 2 -sensing mechanisms of pure and Ti-doped WO 3 (002) surfaces. When Ti is doped into the WO 3 surface, two substitution models are considered: substitution of Ti for W 6c and substitution of Ti for W 5c . The results reveal that substitution of Ti for 5-fold W forms a stable doping structure, and doping induces some new electronic states in the band gap, which may lead to changes in the surface properties. Four top adsorption models of NO 2 on pure and Ti-doped WO 3 (002) surfaces are investigated: adsorptions on 5-fold W (Ti), on 6-fold W, on bridging oxygen, and on plane oxygen. The most stable and likely NO 2 adsorption structures are both N-end oriented to the surface bridge oxygen O 1c site. By comparing the adsorption energy and the electronic population, it is found that Ti doping can enhance the adsorption of NO 2 , which theoretically proves the experimental observation that Ti doping can greatly increase the WO 3 gas sensor sensitivity to NO 2 gas.
Density functional theory (DFT) calculations are conducted to explore the interaction of H_2 with pure and Tidoped WO_3 (002) surfaces.Four top adsorption models of H_2 on pure and Ti-doped WO_3 (002) surfaces are investigated respectively,they are adsorption on bridging oxygen O_(1c),absorption on plane oxygen O_(2c),absorption on 5-fold W_(5c) (Ti),and absorption on 6-fold W_(6c).The most stable and H_2 possible adsorption structure in the pure surface is H-end oriented to the surface plane oxygen O_(2c) site,while the favourable adsorption sites for H_2 in a Ti-doped surface is not only an O_(2c) site but also a W_(6c) site.The adsorption energy,the Fermi energy level E_F,and the electronic population are investigated and the H_2-sensing mechanism of a pure-doped WO_3 (002) surface is revealed theoretically:the theoretical results are in good accordance with our existing experimental results.By comparing the above three terms,it is found that Ti doping can obviously enhance the adsorption of H_2.It can be predicted that the method of Ti-doped into a WO_3 thin film is an effective way to improve WO_3 sensor sensitivity to H_2 gas.
The NO2 gas sensing behavior of porous silicon(PS) is studied at room temperature with and without ultraviolet(UV) light radiation.The PS layer is fabricated by electrochemical etching in an HF-based solution on a p +-type silicon substrate.Then,Pt electrodes are deposited on the surface of the PS to obtain the PS gas sensor.The NO2 sensing properties of the PS with different porosities are investigated under UV light radiation at room temperature.The measurement results show that the PS gas sensor has a much higher response sensitivity and faster response-recovery characteristics than NO2 under the illumination.The sensitivity of the PS sample with the largest porosity to 1 ppm NO2 is 9.9 with UV light radiation,while it is 2.4 without UV light radiation.We find that the ability to absorb UV light is enhanced with the increase in porosity.The PS sample with the highest porosity has a larger change than the other samples.Therefore,the effect of UV radiation on the NO2 sensing properties of PS is closely related to the porosity.
