Surface potentials of (001), (012), (113) hematite (α-Fe2O3) crystal faces in aqueous solution

Abstract

Hematite (α-Fe2O3) is an important candidate electrode for energy system technologies such as photoelectrochemical water splitting. Conversion efficiency issues with this material are presently being addressed through nanostructuring, doping, and surface modification. However, key electrochemical properties of hematite/electrolyte interfaces remain poorly understood at a fundamental level, in particular those of crystallographically well-defined hematite faces likely present as interfacial components at the grain scale. We report a combined measurement and theory study that isolates and evaluates the equilibrium surface potentials of three nearly defect-free single crystal faces of hematite, titrated from pH 3 to 11.25. We link measured surface potentials with atomic-scale surface topology, namely the ratio and distributions of surface protonation-deprotonation site types expected from the bulk structure. The data reveal face-specific points of zero potential (PZP) relatable to points of zero net charge (PZC) that lie within a small pH window (8.35-8.85). Over the entire pH range the surface potentials show strong non-Nernstian charging at pH extremes separated by a wide central plateau in agreement with surface complexation modeling predictions, but with important face-specific distinctions. We introduce a new surface complexation model based on fitting the entire data set that depends primarily only on the proton affinities of two site types and the two associated electrical double layer capacitances. The data and model show that magnitudes of surface potential biases at the pH extremes are on the order of 100 mV, similar to the activation energy for electron hopping mobility. An energy band diagram for hematite crystallites with specific face expression and pH effects is proposed that could provide a baseline for understanding water splitting performance enhancement effects from nanostructuring, and guide morphology targets and pH for systematic improvements in efficiency.

A Density Functional Theory Study of the Adsorption of Benzene on Hematite (α-Fe2O3) Surfaces

“…The work function, calculated as the difference between the electrostatic potential in the vacuum region and the Fermi energy of the (0001) surface, is 4.4 eV, similar to the results of Wang et al [62] and Jin et al [155]. The other surface structure studied in this work is the {011  2} surface, known to be one of the dominant growth faces exposed on natural α-Fe 2 O 3 which has been studied in the past by both experimental [156][157][158] and theoretical groups [63,69,70]. It can have a number of non-dipolar terminations, all of which we have considered, but we have used the most stable termination (see Figure 8) to investigate the adsorption properties of benzene.…”

Hematite(001)-liquid water interface from hybrid density functional-based molecular dynamics

“…The hematite layer was initially set up to have bulk unit cell geometry followed by adding the surface oxygen layer necessary to make the slab symmetric [9] and protonating the surface according to the point of zero charge [29]. This way, for both terminations there was no net dipole for the slabs along the surface normal [30].…”

The Effects on the Response of Metal Oxide and Fluorite Single Crystal Electrodes and the Equilibration Process in the Interfacial Region

“…The value of pHstirr should not be confused with pHpzp of crystal itself which is defined by the thermodynamic equilibrium constants of interfacial reactions (Equation 1). For most investigated metal oxide crystal planes [37,38,40,41] the broad electroneutrality region were obtained in which case the accurate value of pHpzp was hard to obtained, but rather pHpzp zone. The value of pHiep for the same metal oxides crystal planes obtained from streaming potential results is found to be in the narrow pH region between pH = 2.0 and pH = 4.5 (Table 3).…”

“…[18][19][20][21][22][23][24][25] The ISFET electrode was already used as a sensitive electronic device for determining the concentration of H + ions in the solutions. Twelve years ago, the first metal oxide single crystal electrode (SCrE) was constructed ( Figure 2c) [26] and since then have been developed (Figure 2d) [9,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] A special case of the single crystal electrode is the ice electrode, [43,44] i.e. a metal wire covered by a thin layer of ice (Figure 2b).…”