We prepared macro-porous silicon（MPS） by electrochemical corrosion in a double-tank cell on the surface of single-crystalline P-type silicon.Then,nano-WO_3 films were deposited on MPS layers by DC facing target reactive magnetron sputtering.The morphologies of the MPS and WO_3/MPS samples were investigated by using a field emission scanning electron microscope.The crystallization of WO_3 and the valence of the W in the WO_3/MPS sample were characterized by X-ray diffraction and X-ray photoelectron spectroscopy,respectively. The gas sensing properties of MPS and WO_3/MPS gas sensors were thoroughly measured at room temperature. It can be concluded that：the WO_3/MPS gas sensor shows the gas sensing properties of a P-type semiconductor gas sensor.The WO_3/MPS gas sensor exhibits good recovery characteristics and repeatability to 1 ppm NO_2.The addition of WO_3 can enhance the sensitivity of MPS to NO_2.The long-term stability of a WO_3/MPS gas sensor is better than that of an MPS gas sensor.The sensitivity of the WO_3/MPS gas sensor to NO_2 is higher than that to NH_3 and C_2H_5OH.The selectivity of the MPS to NO_2 is modified by deposited nano-WO_3 film.
Vanadium dioxide thin films were fabricated through annealing vanadium oxide thin films deposited by dual ion beam sputtering. X-ray diffraction （XRD）, atom force microscopy （AFM）, and Fourier transform infrared spectrum （FTIR） were employed to measure the crystalline structure, surface morphology, and infrared optical transmittance. The phase transition properties were characterized by transmittance. The results show that the annealed vanadium oxide thin film is composed of monoclinic VO2, with preferred orientation of （011）. The maximum of transmittance change is beyond 65% as the temperature increases from 20 to 80 C. The reversible changes in optical transmittance against temperature were observed. The change rate of transmittance at short wavelength is higher than that at long wavelength at the same temperature across semiconductor-metal phase transition. This phenomenon was discussed using diffraction effect.
We report on the fabrication and performance of a room-temperature NO2 gas sensor based on a WO3 nanowires/porous silicon hybrid structure. The W18O49 nanowires 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, WO3 nanowires 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 WO3 nanowires exhibits a much higher sensitivity than that based on the PS and pure WO3 nanowires in detecting NO2 gas at room temperature. The mechanism of the WO3 nanowires/PS hybrid structure in the NO2 sensing is explained in detail.
