Controllable Cu⁺/Cu²⁺ ratio for the gas-sensing property of (Na, Cu) co-doped ZnO investigated by EPR and SPV

Herein, (Na, Cu) co-doped ZnO nanocrystals were prepared using the sol-gel method. By carefully balancing the Cu⁺/Cu²⁺ ratio through Na doping in Zn_0.95Cu_0.05O-based sensors, the gas-sensing activity can be significantly enhanced. Interestingly, we found that increasing Na doping results in the transformation of Cu²⁺ to Cu⁺ in the copper ion valence state. Furthermore, the XPS results indicate that Na⁺ ions increase the oxygen vacancies of the samples, which is in agreement with the electron paramagnetic resonance (EPR) results. The surface photovoltage (SPV) spectra indicate that (Na, Cu) co-doped ZnO nanocrystals with a high Cu⁺/Cu²⁺ ratio exhibit a high positive SPV response, demonstrating the excellent separation efficiency of photogenerated charges. Oxygen vacancies and the transformation of Cu²⁺ to Cu⁺ are presumed to be the driving factors responsible for UV light-activated NO₂ gas-sensing improvement in this study. This work demonstrates a strategy for improving the gas-sensing efficiency of ZnO-based sensors through leveraging the variable valence states of incorporated metal species.