Dependence of the gas sensing properties of ZnO nanowires on their microstructure

Two different types of ZnO nanowires were synthesized by the thermal evaporation of different material powders: ZnS powders for ZnO nanowires A; and a mixture of ZnO and graphite powders for ZnO nanowires B. ZnO nanowires A were found to have a bamboo structure consisting of many hexagonal plates each of which is a single crystal, whereas ZnO nanowires B were single crystals with no grain boundary at all. ZnO nanowires B had a far higher degree of preferred orientation than ZnO nanowires A. The sensing properties of ZnO nanowires A were superior to those of ZnO nanowires B in terms of the sensing response and sensing speed. ZnO nanowires A showed an approximately 440 % response at 5 ppm NO2 at 300 °C, whereas ZnO nanowires B showed an approximately 150 % response under the same condition. ZnO nanowires A showed considerably shorter response and recovery times than ZnO nanowires B under the same condition. The superior sensing properties of ZnO nanowires A compared to ZnO nanowires B can be explained mainly by carrier modulation of the depletion layer formed across the grain boundaries of nanowires A.