Dopant-induced strain moulation regulates the water oxidation performance of hematite photoanodes

Abstract

Hematite (α-Fe₂O₃) is one of the most promising photoanode materials in solar water splitting owing to its high theoretical performance, low cost and earth abundance. However, it has low practical efficiency due to its mismatching opto-electronic properties. Doping and annealing have been utilized extensively to improve bulk conductivity and surface reactivity. However, the underlying mechanisms for these improvements are still not fully understood. In this study, we investigated the interplay of doping and annealing conditions on crystal size-strain and electrochemical processes. Doping with Ti⁴⁺ greatly enhanced bulk conductivity by not only increasing the n-type character of the material, but also shortening electron hopping distance due to lattice compression. However, this is countered by increased lattice strain and smaller crystallite size. A model was developed to explain the interplay of size-strain effect and electrical conductivity at various dopant levels. Co-doping with F⁻ further improved performance without causing additional lattice strain. Although nitrogen annealing is necessary to reduce bulk charge transport resistance, it was found that air annealed hematite has more favorable surface charge transfer properties. A multistep-annealing protocol was then conducted, first in nitrogen, and then at a lower temperature in air, to balance the bulk and surface electrochemical processes. Our work highlights how high performant hematite can be produced by balancing these synthesis parameters.