Hematite/TiO2 Heterostructure Photoanodes for Efficient Solar-to-Fuel Conversion

Abstract

Photoelectrochemical (PEC) water splitting is a promising route for the conversion of solar energy into energy rich hydrogen fuel using water as a reactant. Synthesis of highly efficient semiconductor photoelectrode is the key challenge in this field. The photoelectrode should absorb a major part of visible light from the sunlight, the valence band and conducting band edge of semiconductor should be straddled with the oxygen evolution and hydrogen evolution potential, good stability in electrolyte medium and efficient charge transport properties in the semiconductor. So far, no single electrode satisfies all such criteria. In the quest of suitable semiconductor photoelectrodes, hematite (α-Fe2O3) is the earth abundant material, which has an optimum band gap of around 2.1 eV, high theoretical solar-to-hydrogen (STH) efficiency of 16.8%, a suitable band edge position for water splitting reaction and the good stability in broad pH electrolyte medium. However, the hematite suffers from the short hole-diffusion length, poor electronic conductivity, high electron-hole recombination rate, slow charge-transfer kinetics and sluggish oxygen evolution reaction (OER) kinetics still limits its practical STH efficiency. To mitigate these issues, we have developed hematite/TiO2 bi-layered photoanodes for enhanced charge separation and reduced recombination of electron-holes. The direct growth of the heterostructures with different thicknesses is achieved by the facile electron beam evaporation route via formation of metallic thin films followed by high temperature calcination in air atmosphere. The physico-chemical properties of as synthesized photoelectrodes are characterized using XRD, SEM and TEM. The photoelectrochemical properties are measured using linear sweep voltammetry, chronoamperometry and impedance spectroscopy under 1-sun illumination using three-electrode setup with platinum wire as a counter electrode and RHE as a reference electrode in 1 M KOH electrolyte. For comparison purpose, the dark current is also measured using the same experimental setup in the absence of light.