Epitaxial, Energetic, and Morphological Synergy on Photocharge Collection of the Fe2TiO5/ZnO Nanodendrite Heterojunction Array Photoelectrode for Photoelectrochemical Water Oxidation

Conformal orthorhombic Fe2TiO5 layers are epitaxially grown on the m-planes of the ZnO nanodendrite (ND) scaffold with the lattice matching relations of (100)Fe2TiO5∥(110)ZnO and (010)Fe2TiO5∥(001)ZnO using metal–organic decomposition. The photoelectrochemical water splitting performance of the epitaxial Fe2TiO5/ZnO ND heterojunction array photoanode with an ND length of 2 μm is substantially enhanced compared to the planar Fe2TiO5 photoanode. The photocurrent density increases from 0.033 to 1.04 mA cm–2 at 1.23 V versus reversible hydrogen electrode (RHE) under AM 1.5G (100 mW cm–2). It is mainly ascribed to the improved charge separation and injection efficiencies of the photoelectrode with a vertical ND structure in coalition with the epitaxial Fe2TiO5/ZnO heterojunction of type-II band alignment. At 1.23 V versus RHE, a Faradaic efficiency of ∼90% for solar water oxidation is acquired from the epitaxial Fe2TiO5/ZnO ND heterojunction array photoanode. By elongating the length of the ND heterojunction array from 2 to 3 μm, 1.5-fold enhancement in the photocurrent density is obtained because of the decoupled light harvesting and hole transport paths. With further deposition of the cocatalyst cobalt phosphate (Co-Pi), the Co-Pi/Fe2TiO5/ZnO ND heterojunction array photoelectrode exhibits a further elevated photocurrent density of 2.14 mA cm–2 at 1.23 V versus RHE.