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

Abstract

Solid-state transport and electrochemical properties of Ti-doped hematite (α-(TixFe1-x)2O3 (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 (≈ 0.11 eV). In contact with inert electrolyte, linear Mott–Schottky 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 Mott–Schottky 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 ≈0.7 eV below the Fermi level. Mott–Schottky 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.