Electrochemical Impedance Study of the Hematite/Water Interface Kenichi Shimizu, † Andrzej Lasia, ‡ and Jean-Franc ̧ ois Boily* ,† † Department of Chemistry, Umeå University, Umeå SE-901 87, Sweden ‡ Department of Chemistry, University of Sherbrooke, Sherbrooke QC J1K 2R1, Canada ABSTRACT: Reactions taking place on hematite (α-Fe 2 O 3 ) 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 semi- conducting electrodes. In this study, electrochemical impe- dance 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 NH 4 Cl 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 NH 4 Cl. 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 NH 4 Cl 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 NH 4 Cl) 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. 1. INTRODUCTION Processes occurring at hematite/water interfaces play critical roles in a variety of natural and technological processes. Hematite is an abundant mineral in the earth’s crust and contributes to several biogeochemical cycles. 1 It also has various industrial applications, given its high chemical robustness, mechanical strength, facile synthesis, attractive band gap (2.1 eV), and low cost. For instance, it is considered for photoassisted water-splitting reactions and in the fabrication of gas-sensing electrodes. 2 A fundamental knowledge of the properties controlling the hematite/water interface reactivity is thereby important to predicting the behavior of this important mineral in these different settings. Fundamental experimental studies on hematite/water inter- face chemistry have focused on various forms of colloidal particles 3-7 as well as single or cut crystals. 8-18 Recent notable efforts along these fronts led to interfacial water structures, 9-13 ion adsorption, 14-16 and electric potentials 19-24 of this important system. Although interfacial electrical potentials, other than shear plane potentials, are generally not measurable in insulating metal (oxy)(hydr)oxides of environmental interest, some hematite specimens can be found or made with sufficient n-type semiconductivity to be used as electrode materials. Hematite electrode surfaces have consequently been used to follow interfacial reactions, including potential following, ion adsorption, 19,21 ligand-promoted etching, 21 surface-to-bulk electron transfer, 20 and photoassisted water splitting. 6,7 Open circuit potentials and cyclic voltammetry are common measurement approaches in these cases. Electro- chemical impedance spectroscopy (EIS) has received, in contrast, much less attention in the study of hematite, 25-28 although it is a very sensitive technique for studying interfacial phenomena, including fundamental reaction kinetics and mechanisms. 29-31 EIS measures the opposition of flow of an alternating current of various frequencies applied to a cell, such as a hematite body in contact with an aqueous solution (Figure 1). The ratio of Fourier transforms of voltage and current can then be modeled using an equivalent circuit consisting of a combination of resistors and capacitors representing various frequency-resolved interfacial processes (Figure 1). These measurements can notably be used to distinguish between slow chemical reactions and fast electrostatic reactions involving the transfer of ions through compact and diffuse layers. The few EIS studies of Received: February 27, 2012 Revised: April 19, 2012 Published: April 27, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 7914 dx.doi.org/10.1021/la300829c | Langmuir 2012, 28, 7914-7920