Hematite coated, conductive Y doped ZnO nanorods for high efficiency solar water splitting

For the first time, hematite (α-Fe₂O₃) crystals were electrochemically deposited over vertically aligned conductive zinc oxide nanorods (NR) to form a specially designed 3D heterostructure with a unique triple layer structure. The structure formed with a thin layer of ZnFe₂O₄ sandwiched between the hematite and the ZnO, which forms a barrier to reduce the back migration of holes. Hence, the charge separation is significantly improved. The small unequal bandgaps of α-Fe₂O₃ and ZnFe₂O₄ help to enhance and broaden visible light absorption. The electron transportation was further improved by yttrium doping in the ZnO (YZnO) NRs, resulting in increased conductivity. This allowed the vertically aligned NRs to perform as electron highways, which also behave as effective optical waveguides for improved light trapping and absorption, since ZnO absorbs little visible light. All these benefits made the unique structures suitable for high performance photoelectrochemical (PEC) water splitting. Optimisation of α-Fe₂O₃ thickness led to a photocurrent density improvement from 0.66 to 0.95 mA cm^-2 at 1.23 V_RHE. This was further improved to 1.59 mA cm^-2 by annealing at 550 °C for 3 h, representing a record-breaking photocurrent for α-Fe₂O₃/ZnO systems. Finally IPCE confirmed the successful generation and transfer of photoelectrons under visible light excitation in the specifically designed heterostructure photoanode, with 5% efficiency for blue light, and 15% for violet light.