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.
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.
We report on the fabrication and performance of a room-temperature NO2gas sensor based on a WO3nanowires/porous silicon hybrid structure. The W18O49nanowires are synthesized directly from a sputtered tungsten film on a porous silicon(PS) layer under heating in an argon atmosphere. After a carefully controlled annealing treatment, WO3nanowires are obtained on the PS layer without losing the morphology. The morphology, phase structure, and crystallinity of the nanowires are investigated by using field emission scanning electron microscopy(FESEM), X-ray diffractometer(XRD), and high-resolution transmission electron microscopy(HRTEM). Comparative gas sensing results indicate that the sensor based on the WO3nanowires exhibits a much higher sensitivity than that based on the PS and pure WO3nanowires in detecting NO2gas at room temperature. The mechanism of the WO3nanowires/PS hybrid structure in the NO2sensing is explained in detail.
In this paper, porous silicon/V_2O_5 nanorod composites are prepared by a heating process of as-sputtered V film on porous silicon(PS) at 600?C for different times(15, 30, and 45 min) in air. The morphologies and crystal structures of the samples are investigated by field emission scanning electron microscope(FESEM), x-ray diffractometer(XRD), x-ray photoelectron spectroscopy(XPS), and Raman spectrum(RS). An improved understanding of the growth process of V_2O_5 nanorods on PS is presented. The gas sensing properties of samples are measured for NO_2 gas of 0.25 ppm^3 ppm at 25?C We investigate the effects of the annealing time on the NO_2-sensing performances of the samples. The sample obtained at600?C for 30 min exhibits a very strong response and fast response-recovery rate to ppm level NO_2, indicating a p-type semiconducting behavior. The XPS analysis reveals that the heating process for 30 min produces the biggest number of oxygen vacancies in the nanorods, which is highly beneficial to gas sensing. The significant NO_2 sensing performance of the sample obtained at 600?C for 30 min probably is due to the strong amplification effect of the heterojunction between PS and V_2O_5 and a large number of oxygen vacancies in the nanorods.
The effects of the surface and orientation of a WO3 nanowire on the electronic structure are investigated by using first principles calculation based on density functional theory（DFT）.The surface of the WO3 nanowire was terminated by a bare or hydrogenated oxygen monolayer or bare WO2 plane,and the[010]- and[001]-oriented nanowires with different sizes were introduced into the theoretical calculation to further study the dependence of electronic band structure on the wire size and orientation.The calculated results reveal that the surface structure, wire size and orientation have significant effects on the electronic band structure,bandgap,and density of states （DOS） of the WO3 nanowire.The optimized WO3 nanowire with different surface structures showed a markedly dissimilar band structure due to the different electronic states near the Fermi level,and the O-terminated[001] WO3 nanowire with hydrogenation can exhibit a reasonable indirect bandgap of 2.340 eV due to the quantum confinement effect,which is 0.257 eV wider than bulk WO3.Besides,the bandgap change is also related to the orientation-resulted surface reconstructed structure as well as wire size.
