Charge Transport at Ti-Doped Hematite (001)/Aqueous Interfaces
Solid-state transport and electrochemical properties of Ti-doped hematite (α-(Ti_xFe_(1-x))_2O_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 Fe^(2+) concentration and Fe^(3+) 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.
© 2015 American Chemical Society. Received: December 3, 2014. Revised: February 2, 2015. Publication Date (Web): February 3, 2015. This work was supported by the Geosciences Research Program in the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Portions of this research were performed using the Environmental Molecular Science Laboratory (EMSL) and the Advanced Light Source (ALS), both national scientific user facilities sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and Office of Basic Energy Sciences, respectively. EMSL is located at Pacific Northwest National Laboratory (PNNL), a multiprogram national laboratory operated for DOE by Battelle. We gratefully acknowledge the analytical assistance of Tim Droubay (MBE synthesis), Mark Engelhard (XPS), and Vaithiyalingam Shutthanandan (PIXE) at PNNL.