Efficient photoelectrochemical water oxidation enabled by an amorphous metal oxide-catalyzed graphene/silicon heterojunction photoanode

Abstract

Silicon is a promising photoelectrochemical (PEC) material owing to its earth abundance, high carrier mobility and narrow bandgap. However, a bare Si photoelectrode is prone to photocorrosion and its photovoltage is affected by surface states on the electrode in PEC measurements. In the present work, a simple and inexpensive method for the construction of a Si/graphene heterojunction is presented. The high barrier at the solid/solid junction results in an open-circuit voltage of 490 mV, making it an excellent heterojunction light absorber to generate a substantial photovoltage for PEC water oxidation. A TiO₂ thin layer deposited on the Si/graphene structure is effective to protect the heterojunction from the electrolyte while favoring the interfacial charge transport from the buried junction. The introduction of a FeNiCoO_x co-catalyst thin film onto the TiO₂ protected heterojunction yields a high photocurrent density of ∼19 mA cm⁻² at an applied potential of 1.5 V vs. RHE in 1.0 M NaOH solution under one sun simulated solar illumination. In contrast to the unprotected Si/graphene electrode, the activity of the TiO₂ protected photoanode is sustained for several hours in an alkaline electrolyte. This work clearly reveals the important role of graphene in the performance improvement of the Si-based photoanodes and sheds light on the introduction of novel materials onto silicon-based electrodes to achieve higher photovoltages for efficient PEC reactions.