Charge Transport at Ti-Doped Hematite (001)/Aqueous Interfaces

Solid-state transport and electrochemical properties of Ti-doped hematite (alpha-(TixFe1-x)(2)O-3 (001) epitaxial thin films (x = 0.15, 0.21, and 0.42) were probed to achieve a better understanding of doped hematite for photoelectrochemical (PEC) applications. Room temperature resistivity measurements predict a resistivity minimum near x = 0.25 Ti doping, which can be rationalized as maximizing charge compensating Fe2+ concentration and Fe3+ electron accepting percolation pathways simultaneously. Temperature dependent resistivity data are consistent with small polaron hopping, revealing an activation energy that is Ti concentration dependent and commensurate with previously reported values (approximate to 0.11 eV). In contact with inert electrolyte, linear MottSchottky data at various pH values indicate that there is predominantly a single donor for Ti-doped hematite at x = 0.15 and x = 0.42 Ti concentrations. Two slope MottSchottky data at pH extremes indicate the presence of a second donor or surface state in the x = 0.21 Ti-doped film, with an energy level approximate to 0.7 eV below the Fermi level. MottSchottky plots indicate pH and Ti concentration dependent flatband potentials of -0.2 to -0.9 V vs SHE, commensurate with previously reported data. Flatband potentials exhibited super-Nernstian pH dependence ranging from -69.1 to -101.0 mV/pH. Carrier concentration data indicate that the Fermi energy of the Ti-doped system is Ti concentration dependent, with a minimum of 0.15 eV near x = 0.25. These energy level data allow us to construct an energy band diagram for Ti-doped hematite electrode/electrolyte interfaces, and to determine a Ti-doping concentration that reduces bulk resistivity while also reducing the formation of surface states for these photoanodes.