Photoinduced reduction of surface states in Fe:ZnO

TL;DR

This study investigates how photoinduced reduction of surface Fe states in nano-crystalline Fe:ZnO enhances its photocatalytic activity under visible light, revealing electronic structure changes and doping limitations.

Contribution

It provides new insights into the electronic states of Fe in ZnO and how surface reduction influences photocatalytic performance, highlighting doping concentration effects.

Findings

Fe reduces from 3+ to 2+ under illumination

Surface Fe states are deep in the bandgap and affect photocatalysis

Homogeneous Fe distribution is limited to low concentrations

Abstract

Transition metal doping is known to increase the photosensitivity to visible light for photocatalytically active ZnO. We report on the electronic structure of nano-crystalline Fe:ZnO, which has recently been shown to be an efficient photocatalyst. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the surface region of the nano-crystalline material. Electronic states corresponding to low-spin Fe 2+ are observed and attributed to crystal field modification at the surface. These states can be important for the photocatalytic sensitivity to visible light due to their deep location in the ZnO bandgap. X-ray absorption and x-ray photoemission spectroscopy suggest that Fe is only homogeneously distributed for concentrations up to 3%. Increased concentrations does not result in a higher concentration of Fe ions in the surface region. This is a crucial factor limiting the photocatalytic functionality of ZnO, where the most efficient doping concentration have been shown to be 2-4% for Fe doping. Using resonant photoemission spectroscopy we determine the location of Fe 3d states with sensitivity to the charge states of the Fe ion even for multi-valent and multi-coordinated Fe.