SURFACE MODIFICATION AND CHEMISTRY OF HEMATITE-BASED CATALYSTS FOR WATER OXIDATION: MODEL SURFACES, NANOMATERIALS, AND THIN FILMS

Abstract

Hematite-based electrocatalysts are widely used for water oxidation, but these catalysts suffer from its low reaction kinetics. To help elucidate detailed reaction mechanisms associated with water oxidation, water chemisorption and reaction as well as structural changes induced by Ni incorporation into the α-Fe2O3(0001) surface was studied. Incorporation of Ni into the near-surface region of hematite changes the structure of the (0001) surface by the formation of FeO-like domains on the topmost layer. Electrochemical measurements demonstrated that Ni incorporation leads to higher current density and lower onset potential than the unmodified α-Fe2O3 surface.

To extend the surface science study to real catalysts, hematite nanocrystals were synthesized with continuous tuning of the aspect-ratio and fine control of the surface area ratio (from 98% to 30%) of the (0001) facet with respect to other surfaces. Ni doping forms a uniformly doped NixFe2-xO3 surface overlayer that improves the electrocatalytic activity of water oxidation. The enhancement of water oxidation activity by Ni-doping increased as the surface area ratio of the (0001) facet of hematite nanocrystals increased, consistent with the theoretical predictions and surface science studies.

Then, a composite oxide film photoelectrode comprised of α-Fe2O3 and WO3 were prepared, and exhibited a water oxidation photocurrent onset potential as low as 0.43 V vs. RHE. This result represents one of the lowest onset potentials measured for hematite-based PEC water oxidation systems. The composition of the films differs between the surfaces and bulk, with tungsten found to be concentrated in the surface region. Post-reaction Raman spectroscopy characterization demonstrates that water interacts with surface WO3 crystals, an event that is associated with the formation of a hydrated form of the oxide.

Lastly, the surface chemistry of H2O on hematite nanoplates is investigated by studying water adsorption and desorption behavior via ATR-FTIR, then correlating it with the surface chemistry study on hematite model surfaces. By adapting the specular reflectance geometry, hematite nanoplates were incorporated into a spectroelectrochemistry setup to simultaneously evaluate electrode performance and monitor the evolution of surface-bound species as a function of applied potential, which are consistent with the surface science studies mentioned above.