A facile green antisolvent approach to Cu2+-doped ZnO nanocrystals with visible-light-responsive photoactivities

Abstract

An environmentally benign antisolvent method has been developed to prepare Cu²⁺-doped ZnO nanocrystals with controllable dopant concentrations. A room temperature ionic liquid, known as a deep eutectic solvent (DES), was used as the solvent to dissolve ZnO powders. Upon the introduction of the ZnO-containing DES into a bad solvent which shows no solvation to ZnO, ZnO was precipitated and grown due to the dramatic decrease of solubility. By adding Cu²⁺ ions to the bad solvent, the growth of ZnO from the antisolvent process was accompanied by Cu²⁺ introduction, resulting in the formation of Cu²⁺-doped ZnO nanocrystals. The as-prepared Cu²⁺-doped ZnO showed an additional absorption band in the visible range (400–800 nm), which conduced to an improvement in the overall photon harvesting efficiency. Time-resolved photoluminescence spectra, together with the photovoltage information, suggested that the doped Cu²⁺ may otherwise trap photoexcited electrons during the charge transfer process, inevitably depressing the photoconversion efficiency. The photoactivity of Cu²⁺-doped ZnO nanocrystals for photoelectrochemical water oxidation was effectively enhanced in the visible region, which achieved the highest at 2.0 at% of Cu²⁺. A further increase in the Cu²⁺ concentration however led to a decrease in the photocatalytic performance, which was ascribed to the significant carrier trapping caused by the increased states given by excessive Cu²⁺. The photocurrent action spectra illustrated that the enhanced photoactivity of the Cu²⁺-doped ZnO nanocrystals was mainly due to the improved visible photon harvesting achieved by Cu²⁺ doping. These results may facilitate the use of transition metal ion-doped ZnO in other photoconversion applications, such as ZnO based dye-sensitized solar cells and magnetism-assisted photocatalytic systems.