Remarkable improvement of W18O49/TiO2 heteronanowires in ambient temperature-responsive NO2-sensing abilities and its unexpected n-p transition phenomenon

For gas sensor applications, a unique heterostructure array of W18O49/TiO2 core-shell nanowires with high alignment and uniform shell layer were fabricated by thermal oxidation of W film followed by sputter deposition and annealing of TiO2. The ordering of the rough aligned W18O49-core nanowires formed from thermal oxidation of metallic W film is found to be modulated considerably by the sputter deposition of shell layer. Thicker shell results in better alignment. The gas-sensing characteristics of the as-prepared W18O49/TiO2 nanowire sensor are evaluated at room temperature to 150 °C by measuring the dynamic response over NO2 concentration ranging from 0.5 to 5 ppm. The W18O49/TiO2 core-shell nanowires show temperature-dependent p-n response characteristic reversal. At room temperature, it behaves as an abnormal p-type semiconductor and exhibits good NO2-sensing performances including high sensitivity, good selectivity and excellent dynamic response-recovery characteristics. It is found that, with improved alignment, the heteronanowires array responses to sub-ppm level of NO2 with ultrafast response and recovery rate at room temperature. The measured response times are shorter than 5 s and the sensor can be recovered completely within 50s. The underlying gas-sensing mechanism correlated to the p-type response inversion at room temperature is analyzed in detail.