Synthesis of TiO2-based nanocomposites by anodizing and hydrogen annealing for efficient photoelectrochemical water oxidation

Black TiO2 based nanomaterials have shown considerable potential towards photoelectrochemical water oxidation due to their low cost, raw material availability and operational stability. In this work, thermal annealing of anatase TiO2 nanotube arrays under hydrogen-argon atmosphere at high temperatures ( > 600 degrees C) is employed to induce significant oxygen deficiency and phase transitions to substoichiometric structures (Magneli phases), with the aim to limit interfacial recombination processes and increase the density of charge carriers. Partidenanowire porous TiO2 sub-stoichiometries are formed at 800 degrees C, which is quantitatively investigated via X-ray diffraction and Rietveld refinement. The photocurrent of this Ti-O compounds is increased by 2.4-fold at 1.23V(RHE) and the photoconversion efficiency is enhanced by 2.8-fold at a reduced bias under simulated solar illumination in comparison with pristine TiO2 nanotube arrays. The improved performance is owed to slower interfacial recombination, resulting in the increase of water oxidation selectivity from 65% to 92%, and by the increase in charge carrier density up to two orders of magnitude, as evaluated via electrochemical impedance spectroscopy. Finally, this method is compared to electrochemical hydrogen doping, with the latter exhibiting a significantly lower energy conversion efficiency.