Inner-Helmholtz potential development at the hematite (α-Fe2O3) (001) surface

Electric potentials of the (0 0 1) surface of hematite were measured as a function of pH and ionic strength in solutions of sodium nitrate and oxalic acid using the single-crystal electrode approach. The surface is predominantly charge-neutral in the pH 4-14 range, and develops a positive surface potential below pH 4 due to protonation of mu-OH0 sites (pK(1,1,0,int) = -1.32). This site is resilient to deprotonation up to at least pH 14 (-pK (1,1,0, int) >> 19). The associated Stern layer capacitance of 0.31-0.73 F/m(2) is smaller than typical values of powders, and possibly arises from a lower degree of surface solvation. Acid-promoted dissolution under elevated concentrations of HNO3 etches the (0 0 1) surface, yielding a convoluted surface populated by -OH20.5+ sites. The resulting surface potential was therefore larger under these conditions than in the absence of dissolution. Oxalate ions also promoted (0 0 1) dissolution. Associated electric potentials were strongly negative, with values as large as -0.5 V, possibly from metal-bonded interactions with oxalate. The hematite surface can also acquire negative potentials in the pH 7-11 range due to surface complexation and/or precipitation of iron species (0.0038 Fe/nm(2)) produced from acidic conditions. Oxalate-bearing systems also result in negative potentials in the same pH range, and may include ferric-oxalate surface complexes and/or surface precipitates. All measurements can be modeled by a thermodynamic model that can be used to predict inner-Helmholtz potentials of hematite surfaces. (C) 2011 Elsevier Ltd. All rights reserved.