Enhancing Photoresponse of Nanoparticulate α-Fe2O3 Electrodes by Surface Composition Tuning

A facile and general procedure to modify the surface corn position of nanostructured alpha-Fe2O3 electrodes is reported. The method involves uniformly covering nanostructured alpha-Fe2O3 electrodes with a solution containing metal ions, which form a coating layer of Fe2O3 - MxOy solid solutions on alpha-Fe2O3 nanopartides upon heating in air. Because of the high surface area and surface reactivity of nanoparticles, a solid-state reaction can be induced at the surface of nanostructured electrodes using a mild heating condition that does not cause an aggregation of nanostructures. Any unreacted residual MA oxide layers are removed by dissolution in 1 M NaOH solution where alpha-Fe2O3 particles and their solid solution coating layers are stable. In this study, Al3+ and Sn4+ ions were incorporated using this method into the surface of a nanoparticulate alpha-Fe2O3 electrode prepared by electrodeposition. Although scanning electron microscopy and X-ray diffraction studies did not show any detectable morphological or structural changes, energy-dispersive spectroscopy and X-ray photoelectron spectroscopy studies confirmed the presence of Al3+ and Sn4+ ions on the surface of the alpha-Fe2O3 particles. These surface-treated samples showed significantly enhanced photocurrent compared with the untreated samples, and samples containing both Al3+ and Sn4+ ions showed the best performance. However, the Al, Sn, and Al/Sn treatments did not increase the electrical conductivity or carrier density of the alpha-Fe2O3 electrode. Instead, they commonly resulted in a significant reduction in transient photocurrent observed with the untreated alpha-Fe2O3 electrode. These results indicate that the main effect of Al3+ and Sn4+ ions forming the surface coating layers is to reduce the surface states or back reactions caused by electrons leaking at the alpha-Fe2O3/electrolyte interface. The composition tuning method described in this study, which preserves the native nanostructured electrode morphology, will make it possible to investigate the role of various metal ions on the photoresponse of alpha-Fe2O3 in a controlled manner.