TiO2/ZnO heterostructure nanowire based NO2 sensor

A highly selective NO2 sensor based on TiO2/ZnO heterostructure nanowires (NWs) has been demonstrated successfully. The sensor film is realised by modifying the ZnO NWs surface with Ti (~30 nm) and subsequent annealing at 350 °C. XPS studies indicated that the Ti is present in the form of TiO2 on the NW surface. Low annealing temperature of 350 °C is attributed to the presence of a mixed phase containing both rutile and anatase phase, forming TiO2–ZnO heterostructure film. The heterostructure film exhibited a sensor response of ~7 towards 5 ppm of NO2 at an operating temperature of 200 °C. Moreover, NO2 detection was achieved with faster response kinetics wherein the response and recovery times of 65 and 98 s, respectively were obtained. Interestingly the response parameters were found to be independent of the NO2 concentration i.e., no significant variation in response/recovery times were observed for different gas concentrations. Additionally, a stable response for the measurement period of over 70 days was also obtained. A sensing mechanism has been established based on the outcomes of sensing characteristics, photoluminescence, X-ray photoelectron spectroscopy and Kelvin probe studies. Our results indicated that the TiO2/ZnO NW heterostructure film is highly sensitive and selective towards NO2.