Photocurrent switching effect on platelet-like BiOI electrodes: influence of redox system, light wavelength and thermal treatment

The electrochemically deposited BiOI films with a thickness of several hundreds nanometers and composed of thin (about 30 nm) platelet-like crystallites demonstrate the photocurrent switching (sign inversion) in solutions containing various redox systems (I3−/I, quinone/hydroquinone, [Fe(CN)6]3−/[Fe(CN)6]4−). The potential of photocurrent switching (Esw) correlates with the equilibrium potential of the redox system and is determined by the exchange currents of anodic and cathodic processes (both dark and photoinduced). The possibility of both photocurrents observations is related to a small thickness of platelets that leads to insignificance of potential drop across them. As a result, separation of photocharges is determined by their capture at the BiOI-electrolyte interface rather than by a built-in space charge layer. High rate of photoelectrons and photoholes capture by electrolyte particles causes the diffusion-limited nature of photocurrent which largely depends on surface concentrations of oxidized and reduced forms of the redox system. Incident photon-to-current conversion efficiency increases monotonically with the wavelength decrease reaching sufficiently high values (∼30% at λ = 370 nm). Photocurrent sign inversion with the change of the incident light wavelength indicates that in BiOI layers deposited on ITO, the electric field originating from ITO/electrolyte contact is localized only near the BiOI/ITO interface, whereas most of the BiOI platelet network remains free of this field. Photocurrent spectra are highly influenced by heat treatment demonstrating a decrease in recombination losses in the case of annealing in an oxidizing (O2) atmosphere.