Enhancing Photoresponse of Nanoparticulate α-Fe₂O₃Electrodes by Surface Composition Tuning

A facile and general procedure to modify the surface composition of nanostructured α-Fe 2 O 3 electrodes is reported. The method involves uniformly covering nanostructured α-Fe 2 O 3 electrodes with a solution containing metal ions, which form a coating layer of Fe 2 O 3 −M x O y solid solutions on α-Fe 2 O 3 nanoparticles 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 M x O y oxide layers are removed by dissolution in 1 M NaOH solution where α-Fe 2 O 3 particles and their solid solution coating layers are stable. In this study, Al 3+ and Sn 4+ ions were incorporated using this method into the surface of a nanoparticulate α-Fe 2 O 3 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 Al 3+ and Sn 4+ ions on the surface of the α-Fe 2 O 3 particles. These surface-treated samples showed significantly enhanced photocurrent compared with the untreated samples, and samples containing both Al 3+ and Sn 4+ ions showed the best performance. However, the Al, Sn, and Al/Sn treatments did not increase the electrical conductivity or carrier density of the α-Fe 2 O 3 electrode. Instead, they commonly resulted in a significant reduction in transient photocurrent observed with the untreated α-Fe 2 O 3 electrode. These results indicate that the main effect of Al 3+ and Sn 4+ ions forming the surface coating layers is to reduce the surface states or back reactions caused by electrons leaking at the α-Fe 2 O 3 /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 α-Fe 2 O 3 in a controlled manner.