Pure and Sn-, Ga- and Mn-doped ZnO gas sensors working at different temperatures for formaldehyde, humidity, NH3, toluene and CO

ZnO and Sn-, Ga- and Mn-doped ZnO nanoparticles were prepared by a coprecipitation method, and characterized by scanning electron microscopy (SEM), energy dispersive spectra (EDS), X-ray diffraction (XRD) and Raman spectra. The gas sensing properties were studied using formaldehyde, relative humidity, NH3, toluene and CO as the probes. The results show that all particles have wurtzite ZnO phase, though Sn–ZnO has a relatively smaller particle (and crystallite) size than the other three samples. Gas sensing property tests reveal that the temperature where the gas sensing maximum is gained (TM) is changed by different dopants: Sn–ZnO and Mn–ZnO have relatively lower TM (∼100°C lower) compared with that of pure ZnO, while Ga–ZnO has the same TM as pure ZnO except in CO sensing. Thermoluminescence (TL) spectra were used to investigate the mechanism of TM change. The peak positions of Ga–ZnO and ZnO are the same at 300–350°C, while that of Sn–ZnO shifts to 250–300°C, which might contribute to the same TM of Ga–ZnO and pure ZnO and relatively lower TM of Sn–ZnO. In the case of Mn–ZnO, the luminescence emission is evidently limited by its black color.