Room temperature NO2-sensing properties of Ti-added nonstoichiometric tungsten oxide nanowires

Solvothermally synthesized W18O49 nanowires were modified by addition of titanium. The effect of the concentration and the introduction procedure of Ti additive on the microstructure and NO2 sensing properties of W18O49 nanowires were investigated. In order to study the influence of the additive introduction procedure, Ti addition was carried out by physical impregnation process and chemical synthesis process, in which the additive was introduced after and during nanowire synthesis, respectively. Introduction of Ti additive by chemical synthesis process suppressed the growth of one-dimensional nanowires along their axis direction and resulted in the secondary assembly of nanowires bundles forming unfavorable structure for gas sensing. By contrast, Ti addition preformed to as-synthesized nanowires with a proper additive concentration (2 at% in the present study) could create a favorable spotty configuration of tiny additive particles on the nanowire surface. The Ti-added W18O49 nanowires showed temperature-dependent response characteristic reversal, which behave as a p-type semiconductor at operating temperature below 100 °C. Ti addition not only lowered the optimal operating temperature of W18O49 nanowires sensors from 150 °C to room temperature but also increased the maximum value of sensor response. At room temperature, the Ti-added nanowires exhibited rapid response characteristic to NO2 gas.