Highly [210] Oriented Porous α-Fe2O3/Fe3O4/ Fe Mesocrystalline Photoanode for Efficient Water Splitting

The electrocatalytic activity of hematite (alpha-Fe2O3) in photoelectrochemical (PEC) water splitting is hindered mainly by its slow water oxidation kinetics and low absorption coefficient. In this report, we have demonstrated a facile, one-step growth of highly porous alpha-Fe2O3/Fe3O4/Fe heterojunction via controlled annealing of the Fe substrate in the presence of a moist atmosphere. This effectively improves the PEC water-splitting performance by enhancing bulk carrier density and charge transfer at the interface. The bulk transport of photogenerated electrons in this n-type catalyst is improved due to the conducting Fe3O4 underlayer through metallic Fe at the back contact. By comparing performances of nonporous mesocrystals (n-PMC) and porous mesocrystals (PMC), the latter being oxidized in water vapor, it is seen that the PMC photoanode exhibits higher PEC activity toward the PEC water splitting than n-PMC by means of a twofold improved photocurrent density, anodically shifted flat band potential from 0.16 VRHE to 0.37 VRHE, and better charge carrier transfer ability. While the presence of both the oxide phases as well as metallic Fe, their growth sequence, surface chemistry, and electronic structure were characterized using X-ray diffraction (XRD), grazing incidence XRD, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), the measurements do not show much changes in n-PMC and PMC; their morphology, investigated with SEM/TEM (scanning/tunneling electron microscopy), shows major differences. Interestingly, neither the core nor the valence band XPS of n-PMC and PMC samples show much differences, clearly hinting at sole morphological contributions toward improved catalytic performances in PMC. The oxide layer thicknesses are further determined to be similar to 6 mu m from interference fringes of Fourier transform infrared spectra. Thus, it is concluded that our cost-effective yet effortlessly grown PMCs of the alpha-Fe2O3/Fe3O4/Fe heterojunction are superior photoelectrocatalysts for water splitting and outperform the widely studied thin films of alpha-Fe2O3 due to their unique porous morphology. The present study thus offers new facile avenues to grow nanostructured oxide on the surface of metal substrates, which can easily be extended for other low-cost substrates, viz., zinc and copper, for realizing their potential as the electrode material for application in PEC water splitting.