Three-Dimensional Undoped Crystalline SnO₂ Nanodendrite Arrays Enable Efficient Charge Separation in BiVO₄/SnO₂ Heterojunction Photoanodes for Photoelectrochemical Water Splitting

Instead of using the doped SnO2 nanostructured scaffolds as transparent conducting electrodes, in this work, a three-dimensional (3D) undoped crystalline SnO2 nanodendrite (ND) array is developed on fluorine-doped tin oxide substrate to be the scaffold of the visible-light-driven photoanodes for photoelectrochemical (PEC) water splitting. The performances of the undoped SnO2 nanorod (NR) and ND arrays are investigated by the formation of staggered (type-II) heterojunction photoanodes using BiVO4 as a model photocatalyst. The hole-scavenger-assisted PEC measurements indicate that the charge separation efficiencies of 88% and 55% are respectively obtained in the 3D BiVO4/SnO2 ND array and one-dimensional BiVO4/SnO2 NR array photoanodes at 1.2 V vs the reversible hydrogen electrode under front illumination (through electrolyte to photoanode). We suggest that the presence of SnO2 branches in the BiVO4/SnO2 ND array increases the volumes of the depletion regions in both BiVO4/SnO2 heterojunction and BiVO4/electrolyte heterojunction compared to the BiVO4/SnO2 NR array, resulting in the enhanced charge separation efficiency and photocurrent density in the BiVO4/SnO2 ND array photoanode. The results demonstrate that the 3D undoped crystalline SnO2 ND array is a promising semiconductor core scaffold to couple with the visible-light-driven photocatalyst shell for the formation of the type-II heterojunction photoanode with superior charge separation efficiency.