An electrochemical impedance study of the hematite/water interface
2012 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 28, no 20, 7914-7920 p.Article in journal (Refereed) Published
Reactions taking place on hematite (α-Fe2O3) surfaces contacted with aqueous solutions are of paramount importance for environmental and technological processes. 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 mm-sized single-body hematite electrodes. This was carried out by developing equivalent circuit models to impedance data collected on a semi-conducting 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 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. They however decrease with pH in NH4Cl from 33.77 at pH 3.5 to 21.02 μF·cm-2·s-ϕ at pH 10.6 due to 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 non-ideal behavior of this capacitor, here treated as a constant phase element. As electrode-based techniques are generally not applicable to the commonly insulating metal (oxyhydr)oxides found in the environment, this study opens opportunities for exploring mineral/water interface chemistry by EIS studies of single-body hematite specimens.
Place, publisher, year, edition, pages
2012. Vol. 28, no 20, 7914-7920 p.
IdentifiersURN: urn:nbn:se:umu:diva-55071DOI: 10.1021/la300829cPubMedID: 22540260OAI: oai:DiVA.org:umu-55071DiVA: diva2:525449