Gas sensing mechanism of ZnO: Na nanocrystals at room temperature using surface photovoltage spectroscopy

ZnO nanocrystals with different Na doping concentrations were prepared on PET flexible substrates sputtered with Au/Ti interdigital electrodes by a simple sol gel method. The relationship between the photoassisted room temperature NO2 gas sensing mechanism and surface photovoltage was explored by characterizing the microstructure and optical properties of the sample. The X-ray diffraction (XRD) results show that all samples have the hexagonal wurtzite structure, and Na doping does not exhibit diffraction peaks of Na and its oxides. The room temperature gas sensitivity test results show that Na doped ZnO nanocrystals have excellent room temperature gas sensitivity performance, and 0.94 mg/m3 NO2 is detected, which significantly improves the gas sensitivity response compared to pure ZnO nanocrystals. The experimental results of surface photovoltage spectroscopy (SPV) and ultraviolet visible spectrophotometer (UV-vis) indicate that the room temperature gas sensitivity of doped ZnO samples may be related to their surface defect content and defect energy levels. Na doping can significantly enhance the separation of photo generated charges, while also introducing more oxygen defects (Vo) and active sites to promote the reaction between NO2 gas and surface adsorbed ionized oxygen defects. In addition, the blue shift of the optical bandgap and the newly generated defect energy levels further enhance the sensitivity of NO2 gas.