Electrochemical Impedance Study of the Hematite/Water Interface

Reactions taking place on hematite (α-Fe2O3) surfaces in contact with aqueous solutions are of paramount importance to environmental and technological processes. The electrochemical properties of the hematite/water interface are central to these processes and can be probed by open circuit potentials and cyclic voltammetric measurements of semiconducting electrodes. In this study, electrochemical impedance spectroscopy (EIS) was used to extract resistive and capacitive attributes of this interface on millimeter-sized single-body hematite electrodes. This was carried out by developing equivalent circuit models for impedance data collected on a semiconducting hematite specimen equilibrated in solutions of 0.1 M NaCl and NH4Cl at various pH values. These efforts produced distinct sets of capacitance values for the diffuse and compact layers of the interface. Diffuse layer capacitances shift in the pH 3–11 range from 2.32 to 2.50 μF·cm–2 in NaCl and from 1.43 to 1.99 μF·cm–2 in NH4Cl. Furthermore, these values reach a minimum capacitance at pH 9, near a probable point of zero charge for an undefined hematite surface exposing a variety of (hydr)oxo functional groups. Compact layer capacitances pertain to the transfer of ions (charge carriers) from the diffuse layer to surface hydroxyls and are independent of pH in NaCl, with values of 32.57 ± 0.49 μF·cm–2·s–φ. However, they decrease with pH in NH4Cl from 33.77 at pH 3.5 to 21.02 μF·cm–2·s–φ at pH 10.6 because of the interactions of ammonium species with surface (hydr)oxo groups. Values of φ (0.71–0.73 in NaCl and 0.56–0.67 in NH4Cl) denote the nonideal behavior of this capacitor, which is treated here as a constant phase element. Because electrode-based techniques are generally not applicable to the commonly insulating metal (oxyhydr)oxides found in the environment, this study presents opportunities for exploring mineral/water interface chemistry by EIS studies of single-body hematite specimens.