US EPA

2953759 

Journal Article 

Electrochemical Signatures of Crystallographic Orientation and Counterion Binding at the Hematite/Water Interface 

Shimizu, K; Boily, JF 

2015 

Yes 

Journal of Physical Chemistry C
ISSN: 1932-7447
EISSN: 1932-7455 

119 

11 

5988-5994 

10.1021/jp511371c 

WOS:000351557800025 

The interfacial electrochemistry of hematite (a-Fe2O3) is a key aspect for understanding the behavior of this important mineral phase in photocatalytic water-splitting devices as well as in terrestrial and atmospheric systems. Nano- to microsized particles are often multifaceted and exhibit terminations of varied crystallographic orientations and structures. As structure often controls reactivity, this study was devised to identify the impact of crystallographic orientation on the electrochemical response of hematite electrode surfaces contacted with technologically, geochemically, and environmentally important solutions of inorganic ions (NaCl, NaHCO3, and NH4Cl). Electrochemical impedance spectroscopy (EIS) measurements of single hematite crystals oriented along the (001) and (012) faces were used for this purpose. The EIS responses of the electrodes were described in terms of an equivalent electrical circuit that accounts for fast bulk and slower interfacial processes. Capacitance and resistance values for the bulk processes confirmed the anisotropic conductivity attributes of hematite and supported the use of the EIS data for interpreting the crystallographic orientation dependence of interfacial processes. These efforts extracted diffuse (C-dl) and compact (T-ad) layer capacitances and resistance (R-ad), as well as relaxation times pertaining to the re-equilibration of interfacial species during EIS. Capacitance values confirmed the greater charge-storing capability of the (012) face (C-dl = 110 mu F.cm(-2); T-ad = 335 mu F.cm(-2).s(-phi)) compared to the (001) face (C-dl = 0.20.6 mu F.cm(-2); T-ad = 0.20.6 mu F.cm(-2).s(-phi)). This was also confirmed through the resistance values pertaining to the transfer of charge carriers across the compact plane, which were lower (R-ad = 0.00.8 M Omega.cm(-2)) on the (012) face than on the (001) face (R-ad = 14 M Omega.cm(-2)). Binding of chloride and (bi)carbonate on the (012) face under acidic conditions was associated with an increase in capacitance values and relaxation times. The lowest capacitances and relaxation times occurred in the pH 8-9 region, which correspond to a likely point of zero charge. The capacitance values in NH4Cl were considerably lower than in NaCl and NaHCO3, owing to hydrogen bonding between the NH4+/NH3 species and surface (hydr)oxo groups. Such interactions can block protonation reactions and can be translated to negligible relaxation times for this system. Collectively, these findings underpin the interdependency of the hematite electrode surface orientation on its electrochemical signatures for important inorganic ions of direct relevance to technological and natural systems.