Defective Indium Tin Oxide Forms an Ohmic Back Contact to an n-Type CuO Photoanode to Accelerate Charge-Transfer Kinetics for Enhanced Low-Bias Photoelectrochemical Water Splitting.

In significant contrast to the tremendous research efforts mostly geared to addressing the severe hole accumulation at the back contact of a p-type CuO photocathode with a fluorine-doped tin oxide (FTO) substrate, sluggish electron transfer from an n-type CuO photoanode to a tin-doped indium oxide (ITO) substrate has been largely overlooked. To tackle this issue that has been reported to largely limit the photoelectrochemical performance of n-type CuO photoanodes at a low bias, the present contribution puts forward a strategy to introduce oxygen vacancies into the ITO substrate via an unprecedented yet facile electrochemical approach. Such defect engineering turns out to decrease the work function of the ITO substrate, which in turn approaches the conduction band extremum of n-CuO to highly efficiently extract the photoexcited electrons therein. Moreover, the dendritic growth of n-CuO is, in the meantime, interfered by the oxygen vacancy manifested as pinholes distributed over the ITO substrate, which is thereby crystallized into several small grains with augmented surface roughness that is in favor of the injection of the photoexcited hole into the electrolyte. Such facile interfacial charge-transfer kinetics leads to a significant cathodic shift amounting to 200 mV of the onset potential to 0 V, whereat the n-CuO photoanode deposited on the defective ITO substrate delivers the maximum photocurrent density reaching 2 mA cm and, more significantly, its applied bias photon-to-current efficiency (ABPE) reaches 1.1%, which is among the highest performance reported to date for a variety of state-of-the-art metal oxide-based photoanodes in the literature.