Influence of oxygen vacancy defects and cobalt doping on optical, electronic and photocatalytic properties of ultrafine SnO2-δ nanocrystals

Ultrafine pure and cobalt doped SnO2-δ nanocrystals (Sn1-xCoxO2-δ, 0 ≤ x ≤ 0.05) were synthesized by microwave-assisted hydrothermal method. The as-prepared nanocrystals have single phase tetragonal rutile structure. With increase of Co content (x > 0.01), Co entered into SnO2 lattice in mixed Co2+/Co3+ state. Pronounced blue shift of the band gap with cobalt doping originated from the combined effect of quantum confinement and Burnstain-Moss shift. Raman and photoluminescence study revealed oxygen deficient structure of SnO2-δ for which the prevalent defects are in the form of in-plane oxygen vacancies. Co-doping induced decrease of in-plane oxygen vacancy concentration and luminescence quenching. SnO2-δ exhibited significantly better photocatalytic activity under UV light irradiation, than Co-doped samples due to better UV light absorption and increased concentration of in-plane oxygen vacancies which, as shallow donors, enable better electron-hole separation and faster charge transport.