Alkali Treatment for Enhanced Photoelectrochemical Water Oxidation on Hematite Photoanode

As one of the most popular photoanode materials for photoelectrochemical (PEC) water splitting, hematite (alpha-Fe2O3) suffers from low conductivity, severe electron hole recombination, and sluggish water oxidation kinetics unfortunately. Herein, we report an alkali-treatment method to effectively accelerate the water oxidation kinetics of alpha-Fe2O3 and titanium doped alpha-Fe2O3 (Ti:alpha-Fe2O3) nanorod array photoanodes. The purpose of Ti-doping is to increase the conductivity of alpha-Fe2O3. The photocurrent densities increased 3- and 2-times for alpha-Fe2O3 and Ti:alpha-Fe2O3 photoanodes after KOH treatment, respectively. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses demonstrated that a conformal thin layer grafted with hydroxyl (OH) groups was formed on the hematite surface. Linear sweep voltammetry (LSV) curves under light irradiation and in the dark indicated that the thin OH-grafted overlayer behaved like an electrocatalyst to accelerate the water oxidation kinetics on hematite photoanodes. Moreover, XPS valence band (XPS-VB) spectra, Mott-Schottky analysis, and electrochemical impedance spectroscopy (EIS) revealed that a type II heterojunction was in situ formed by the OH-grafted overlayer on the hematite nanorod surface, which substantially enhanced the surface charge separation efficiency. The improved PEC performance could be attributed to the accelerated water oxidation kinetics and enhanced surface charge transfer.