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

## TL;DR

This paper shows how adjusting the Cu+/Cu2+ ratio in ZnO nanocrystals improves gas-sensing performance for detecting NO2 at room temperature.

## Contribution

The study introduces a controllable Cu+/Cu2+ ratio via Na doping to enhance ZnO-based gas sensors.

## Key findings

- Increasing Na doping transforms Cu2+ to Cu+ and increases oxygen vacancies in ZnO.
- High Cu+/Cu2+ ratio in (Na, Cu) co-doped ZnO leads to improved photogenerated charge separation and gas-sensing performance.
- The sensors can detect NO2 at room temperature with a limit as low as 250 ppb.

## Abstract

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 Zn0.95Cu0.05O-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.

Increased Na doping promotes the transformation of Cu2+ to Cu+ and introduces more oxygen vacancies of ZnO-based sensors through (Na, Cu) co-doping for room-temperature NO2 detection with a limit as low as 250 ppb.