Mapping Electrochemical Heterogeneity at Iron Oxide Surfaces: A

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Mapping Electrochemical Heterogeneity at Iron Oxide Surfaces: A Local Electrochemical Impedance Study Marie Lucas, and Jean-François Boily Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03849 • Publication Date (Web): 01 Dec 2015 Downloaded from http://pubs.acs.org on December 5, 2015

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Mapping Electrochemical Heterogeneity at Iron Oxide Surfaces: A Local Electrochemical Impedance Study Marie Lucas and Jean-François Boily* Department of Chemistry, Umeå University, SE-901 87, Sweden ABSTRACT: Alternating current scanning electrochemical microscopy (AC-SECM) was used for the first time to map key electrochemical attributes of oriented hematite (α-Fe2O3) single crystal surfaces at the micron-scale. Localized electrochemical impedance spectra (LEIS) of the (001) and (012) faces provided insight into the spatial variation local double layer capacitance (Cdl) and charge transfer resistance (Rad). These parameters were extracted by LEIS measurements in the 0.4-8000 Hz range to probe the impedance response generated by the redistribution of water molecules and charge carriers (ions) under an applied AC. These variations were attributed to local variations in the local conductivity of the sample surfaces. Comparison with global EIS measurements on the same sample surfaces uncovered highly comparable frequency-resolved processes, broken down as contributions from the bulk hematite, the interface as well as the microelectrode/tip assembly. This work paves the way for new studies aimed at mapping electrochemical processes at the meso-scale on this environmentally and technologically important material.

keywords: Hematite, scanning electrochemical microscopy, AC-SECM, local impedance, double layer capacitance. 1. INTRODUCTION The electric double layer (EDL) is an inherent feature of mineral/water interfaces, and its properties are key for predicting a score of natural and technological processes. Its structure and composition are, in particular, strongly linked to the intricate molecular-scale nature of mineral-water and mineral-ion interactions which, collectively, account for the electrochemical nature of the interface. Simple EDL models often invoked in thermodynamic adsorption models employ compact (e.g. Helmholtz-Perrin, Stern) and diffuse layer (e.g. GouyChapman) capacitances as means for conveying the chargestoring capability of interfaces. Although capacitance can be experimentally resolved in metal or metal halide electrodes, those of (semi)insulating mineral surfaces are, at best, indirectly constrained given inherent difficulties in simultaneously measuring surface charge and potential. This has consequently long posed limitations in adequately predicting electrochemical phenomena in mineral/water interfacial systems.

of (i) hematite bulk (Zhem), (ii) interfacial (Zint) and (iii) electrolyte solution (Zsol) phenomena, such that: = Z + + + (1) The additional Zcell term accounts for the electrical circuitry of the experimental set-up. The Zint term accounts for describing the interfacial electrochemical phenomena in terms of (i) a diffuse layer capacitance (Cdl) working in parallel with (ii) a capacitance (Cad) and resistance (Rad) of adsorption of a charge carrier (e.g. ion) across the compact layer. This model thus attempts to distinguish fast (ionic diffusion in the diffuse layer; Cdl) from slower (hydrogen bonded charge carriers in the compact layer; Cad and Rad) relaxation processes perturbed by applied AC fields. We also note that the electrochemical responses denoted in Eq. 1 may be related to one another. For example, the charge depletion region expressed in Zhem may possible be a response of the electric double layer expressed in Zint.. This latter term should, in turn, be pH dependent while others should not.

Fortunately, electrochemical measurements of semiconducting minerals have opened new possibilities for exploring charged mineral/water interfaces, and for extending newly resolved concepts to other insulating minerals. Hematite (αFe2O3) single crystals with n-type semi-conductivity has been particularly instrumental in this regard, both through the acquisition of open circuit potentials as surrogates to innerHelmholtz potentials, and for probing various interfacial electrochemical processes. 1,2,3,4,5,6,7,8 Our recent electrochemical impedance spectroscopy (EIS)9 work on hematite single crystals exposed to aqueous solutions has, in particular, enabled us to describe these systems in terms of the equivalent circuit shown in Figure 1. Typical EIS measurements probe rearrangement of water molecules and ionic (charge-carrier) under applied alternating currents (AC) (0.1 - 100 000 Hz). Our model describes the total impedance (Z) of the system in terms

Our previous EIS efforts4 revealed a strong response of electrolyte ion identity (NaCl vs. NH4Cl vs. NaHCO3), and therefore indirectly ionic interfacial distributions and coordination environments, on the magnitude of these parameters. Strong crystallographic orientation effects were also resolved through these measurements, and especially through weaker adsorption resistance (Rad) and larger capacitance (Zad, Cdl) at the (012) face than on the (001) face. These differences can be readily explained by the anisotropic nature of ion binding at different crystallographic orientations. Ideally-speaking this difference can be rationalized in terms of the greater density and diversity of proton active functional groups (-OH, µ-OH, µ3-OH) on the (012) faces, compared to the relatively more proton silent (001) faces where µ-OH groups predominate and retain their protonation states under circumneutral pH conditions. These differences can, however, be substantially attenuated by surface roughness. The anisotropic behaviors of the electrodes

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can also be understood in terms of the space charge capacitance of the depletion layer, which could be linked to the strongly anisotropic nature of electron conduction in the hematite bulk (facilitated perpendicular to the (001) face).

Figure 1. Equivalent circuit model used for global EIS measurements of single hematite crystal surfaces.10 The bulk electrolyte side is represented by a solution resistance (Zsol=Rsol) terms (Eq. 1). The hematite/water interface is represented by the diffuse layer capacitance (Cdl), a constant phase element (Zad) and a resistance (Rad) for the charge carrier transfer. The hematite bulk portion includes the capacitance of the spacecharging layer (Csc), ohmic resistance (R1), charge transport (R2) as well as charge diffusion (Zw). The latter pertains to the electron-hole recombination, namely charge trap and diffusion, and is treated as a constant phase element (CPE; Zw = −1 −0.5 T (jω) ). A RC term for the cell is also included, as discussed in our previous work21, but has no significance to the hematite-electrolyte system per se. EIS of single crystal electrodes thus provides key insight into the electrochemical responses of hematite electrodes, but only as the average of very large (>> 0.1 cm2) surface area. As a result, concurrent challenge in this field lies in grasping the impact of local variations in electrode surface structure and composition on electrochemical processes. While many thermodynamic adsorption models work on the basis of homogeneous distributions of reaction centers, structural and chemical heterogeneity, especially at molecular up to the micron (cf. meso) scale, could potentially play considerable roles in accounting for globally observed reactions. For instance, we could anticipate that reactive hotspots brought by large local dopant concentrations could alter mineral surface reactivity, and even call for the development of new (e.g. thermodynamic) models accounting for heterogeneous processes at the meso-scale. Scanning electrochemical microscopy offers possibilities for mapping such forms of heterogeneity. Furthermore, recent developments in the area of alternating current (AC) SECM opened new possibilities for probing local electrochemical phenomena without any electrochemical mediator11-12. The technique notably enables local EIS (LEIS) experiments at the micron scale, and, thus possibilities for mapping local key parameters (double layer capacitance, charge transfer resistance, solution resistance.) over a given electrode surface region. In this study we mapped for the first time these parameters of hematite surfaces oriented along the (001) and (012)

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faces, and draw comparisons with global EIS (GEIS) measurements. 2. MATERIALS AND METHODS 2.1. Chemicals and materials. All chemicals were used as received. Acetone was obtained from Fisher scientific (UK, Loughborough), ethanol from Solveco (Sweden, Roserberg) and methanol from VWR (Fontenay-sous-Bois, France). Sodium chloride (NaCl) was purchased from VWR (analar normapur, Leuven, Belgium). NaCl electrolyte solution was prepared using either Millipore water (Milli-Q plus 185, QPAK® purification pack) or an ELGA and Veolia water Purelab® chorus dispenser (model PC1ANRXM1, Purification pack-RO Feed LC232) in a total ionic strength of 1 mM. Hematite electrodes are from a natural hematite specimen from an unspecified location in Brazil. They were cut as well as chemically and mechanically polished along the (001) (3.13 × 3.14 × 1.99 mm) and (012) (4.95 × 5.00 ×4.94 mm) faces by SurfaceNet (Rheine, Germany). Prior each experiment, specimens were successively sonicated for 1 min periods in acetone, ethanol, methanol, and a mixture of 50% (vol.) methanol 50% (vol.) to remove organic impurities. The samples were finally rinsed with water and kept in water until use. Storage in water ensured that electrode surfaces were fully equilibrated with respect to the aqueous phase, and therefore their electrochemical responses were representative of this state 13. 2.2. Hematite characterization The orientations of the samples were first confirmed by X-ray diffraction (XRD), using a Bruker-AXS D8Advance X-ray diffractometer with a Våntec-1 detector and a line-focused Cu Kα radiation tube. Scans were performed as continuous detectors from 20° to 80° with a fixed X-ray source angle of 5° to ensure a high degree of surface interaction over the samples. Generator settings were set to 30 kV and 25 mA to obtain a small penetration depth. The diffractograms were then evaluated with DIFFRAC.EVA software using the PDF-2 reference database. These measurements confirmed that both samples were single crystals with the expected crystallographic orientations. Still, they could potentially exhibit mixtures of different terminations, such as those discussed in the recent literature14 15 16 17. Surface atomic compositions of both specimens were determined by X-ray photoelectron spectroscopy (XPS). All XPS data were recorded with a Kratos Axis Ultra electron spectrometer equipped with a delay line detector. A monochromated Al Kα source operated at 150 W, a hybrid lens system with magnetic lens providing an analysis area of 0.3 × 0.7 mm and a charge neutralizer were used for the measurements. All spectra were processed with Kratos software. Survey spectra of the crystals were collected from 1100 to 0 eV of binding energy at a pass energy of 160 eV. High resolution spectra for Fe 2p, O 1s and C 1s were then collected at pass energy of 20 eV with resolution of 0.1 eV. The binding energies were adjusted with respect to the C 1s line of aliphatic lines set at 285.0 eV. These measurements showed that no impurities


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could be detected other than carbon based aliphatic carbon, which is otherwise unavoidable. We note that because the hematite crystals were of natural origin and were electrically conductive, their dopant levels must be below the detection limit (0.02%) of the XPS technique. Previous estimates 1 based on the bulk conductivity of these samples in fact point to dopant levels or the order of nmol per mol of hematite. Finally, electrode surfaces were imaged by Atomic Force Microscopy (AFM; Fig. S1). Measurements were carried out under tapping mode (AFM; PICO PLUS, Agilent USA) using a 100 µm scanner, a scanning resolution of 512×512 pixels, and an acoustically driven cantilever operating at a resonance frequency between 350 and 450 kHz. Images showed that surface roughness did not exceed 15 Å at the (001) and 250 Å at the (012) faces. As these topographical reliefs were smaller than the resolution of the z-axis under AC-SECM, the electrochemical responses detected by the microelectrodes should predominantly arise from electrochemical parameters of the interface. 2.3 EIS All electrochemical measurements were carried out using a BioLogic AC-SECM workstation (M 470). The BioLogic Micro-Tricell was used with a custom made Teflon holder, fitting the bottom of the cell (Fig. S2). The hematite sample was inserted such that only the surface of interest was in contact with a 1 mM NaCl solution equilibrated with atmospheric CO2, with a pH of 5.9. The electrolyte solution that was constantly renewed with a peristaltic pump throughout the course of the experiments. A classical three-electrode setup was used for all measurements. The reference electrode was a pseudo reference (Ag wire) for all AC experiments, and an Ag/AgCl reference electrode (3 M KCl, BASi) for all DC experiments. A Pt plate was used as a counter electrode. Finally, we note that the hematite crystal is the working electrode in GEIS, while it is the microelectrode (Fig. S2) in LEIS. 2.3.1 GEIS GEIS experiments were carried out with the underside of the hematite crystal connected to an electrical Cu wire fixed using silver epoxy. The method for GEIS experiment was described by Shimizu et al.4 Briefly, OCP measurements were conducted before all GEIS experiment and until the potential was stable, after which GEIS full spectra (100 000-0.1 Hz) were recorded. The data were fitted using the equivalent circuit of Figure 1, first using the program ZView (SAI - ZView 3.4c) to ensure that the data and fitting procedures were consistent with our previous GEIS efforts (10 1 4). The data were then fitted using a code that we wrote in the environment of Matlab (The Mathworks, Inc). This code uses a MultiStart algorithm that finds global and physically-realistic combinations of electrical circuit parameters based upon a non-linear least square minimization of the deviation of the modeled total impedance to the experimental data. This optimization procedure makes use of a robust trust region reflective algorithm and gradient estimation obtained by finite-differencing.

LEIS experiments were carried out using a 25 µm working microelectrode (Cromsol, U-23/25, Sweden) using the configuration depicted in Figure S2. The microelectrode was handpolished with emery paper (grade 4000) and sonicated in ethanol and water to remove any residues. The tip of the working electrode was then approached to the hematite surface at a distance of 2.0 ± 0.5 µm using a low frequency (0.5 kHz) ACapproach protocol.18 This approach protocol was developed, and tested for reproducibility by repeating the procedure 50 times (Fig. S2). The accuracy of this procedure was important especially considering that the low frequency AC signal was topography-dependent. Once the tip was in place LEIS experiments were performed at constant distance from the substrate over a 7 × 7 grid with 10 µm steps spanning a 60 × 60 µm2 surface area. No specific area was targeted for mapping as we only sought to record representative LEIS data for the purpose of this study. Measurements were performed by applying an AC of 100 mV at 45 frequencies in the 0.4-8000 Hz range. Preliminary experiments showed that repeated LEIS results over a single coordinate every 15 min over the course of 21 h were highly reproducible (Fig. 3 left). All 4D AC-SECM12 dataset were recorded with the M470 software (BioLogic). The quality of random sets of spectra was evaluated by Kramers-Kronig transform tests. Fitting of the 49 spatially resolved LEIS spectra was carried out using a modification of the aforementioned Matlab code for GEIS. This program determines the bestfitting combination of electrochemical parameters for an equivalent circuit accounting for an RC model of the tip and of a circuit for the hematite/water interface, as will be discussed further in this work. The code also provides an automated means for fitting and mapping a large number of EIS data, as well as testing for parameter sensitivity. The search for the best-fitting equivalent circuit first involved an investigation for the dimensionality of the 49 LEIS spectra collected on each electrode surface. This analysis builds upon our recent efforts at employing chemometric/Linear Algebraic methods as objective means at evaluating EIS data 19. This was effectively carried out by separate singular value decomposition (SVD) analyzes of the frequency dependence of the real (Z’) and imaginary (Z’’) parts of the impedance data, whereby Z=USVT and where Zm××n a 2D matrix of m frequencies and n spectra. The left-singular unit vectors in U are eigenvectors of ZZT and contain linearly-independent vectors describing the variance of the data. The right-singular vectors in V are eigenvectors of ZTZ and contain information on the loadings of all vectors. Finally, the diagonal matrix S contains the square roots of the non-zero eigenvalues of ZZT and ZTZ and denotes the lengths of vectors in U. The dimensionality (d) of the data was determined by analysis of S, using the Factor Indicator Function (IND)20. The data can thereafter be represented by Zm××n=Um××mSm××dVn××nT + Em××n where the latter term is the unaccounted portion of the data most often associated with more negligible contributions as well as experimental noise and uncertainties. Also, in the case of circuit elements in series, where the total impedance is a linear combination of individual circuit components, the abstract vectors in U where rotated to a real electrochemical space using the Multivariate Curve Resolution (MCR) method21. This method resolved the fre-

2.3.2 LEIS


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quency-resolved profiles of individual circuit components (Zi) and their loadings (Li) such that Zi > 0, Li > 0 and Z=Σ(Zi·Li). 3. RESULTS AND DISCUSSION The GEIS data of both electrodes (Fig. 2) were highly consistent with those previously obtained on different hematite samples and experimental apparatus10,19,1,4. These notably reveal a high frequency component (< < 0.5 MΩ from hematite bulk contributions, and a low frequency component from the hematite/water interface1,10, 19. Our findings thus confirmed further the important anisotropy on the impedance response of the (001) and (012) faces. These responses were, moreover, effectively modeled with the equivalent circuit model of Figure 1 with a greater resistivity for charger carrier transport in the hematite bulk in the sample oriented along the (001) face (Table 1). The parameters for the interfacial processes also confirmed the notion for lower interfacial capacitances and for greater adsorption resistance at the basal (001) face. Again, the larger chargestorage capacity of the (012) face stems from greater density of proton active functional groups coupled with facilitated charge transport processes with the depletion layer 4.

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LEIS measurements over a 60 × 60 µm2 area of the same electrode surfaces revealed markedly more resistive processes and considerably less crystallographic orientation dependence than seen by GEIS (Figs. 3 & S3). Contributions from the hematite bulk could therefore be diminished in relation to GEIS yet, as modeling will suggest, they cannot be neglected to account for our LEIS data. We note that a test involving 86 repeated scans over a single analysis point over the course of 28 hours (Fig. 3) confirmed that the electrode response was not affected during LEIS and that the results are highly reproducible. Thus, variations in LEIS data seen over a surface, such as the ones shown along a single 7 × 1 scan line in Figure 4, should chiefly arise from the inhomogeneous electrochemical response of the sample surface.

Figure 3. Nyquist plot from 86 LEIS measurements at a single spot at the (012) face. Inset shows data in the 0-30 MΩ range for both –Im(Z) and Re(Z).

Figure 2. GEIS data (points) and model fit (red lines) for the (001) (left) and (012) (right) faces. Collected in the 0.6 Hz – 100 kHz range. Table 1. GEIS values of the interfacial parameters for the equivalent circuit model (Fig. 2). (001) (012) Interface 1.46 9.06 Cdl (µF⋅cm-2) 12.6 75.1 Zad (µF⋅cm-2⋅s-α) 0.58 0.68 α1 115 12.2 Rad (kΩ⋅cm2) Hematite bulk 10.1 0.361  (MΩ⋅cm2) 54.5 5.73 R2 (kΩ⋅cm2) TW(µF⋅cm-2⋅s2.37 14.0 0.5 2 ) -2 -0.5 7.97 CS (nF⋅cm ⋅s ) 11.2 1. The non-ideality factor for Zad.term of the the CPE of the interface. 2. Pertains to the electron-hole recombination, namely charge trap and diffusion, and is treated as a constant phase element −1 −0.5 (CPE; Zw = T (jω) ).

Figure 4. Examples of model fits to experimental data along one 7 x 1 scan line on the (001) face (a,b) and the (001) face (c,d). Left: entire dataset in the 0.4 – 8000 Hz range. Right: High frequency data of the data shown in left. A SVD-based dimensionality analysis of the 49 LEIS spectra collected on each electrode surface provided strong evidence for the use of three circuit components to account for the data


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(Fig. 5). These three components account for 97% of the variant in the LEIS data, with the remaining associated to smaller unresolvable processes including experimental noise and uncertainties. A MCR analysis of the three dominant orthonormal vectors retrieved by SVD uncovered their frequencyresolved dependence (Fig. 5). Two of these components are highly comparable to those that we previously derived for bulk hematite (high frequencies) and the interface (low frequencies) through GEIS measurements (Fig. 2). Variations in the responses of these components are thus the key evidence for the electrochemical heterogeneities of the surfaces under study. A third low frequency should arise from the contributions of the glass shield/tip assembly of the microelectrode (labeled as ‘tip’ in Fig.5). Maps of the relative importance these three MCR-based components, shown in Figure S3, underscore their important spatial variations over the 60 × 60 µm2 electrode area under study. Further insight into these variations can be made by modeling the impedance spectra using an equivalent circuit representing the hematite bulk, the interface, as well as the tip of the microelectrode.

Figure 5. Representative frequency-resolved impedance (real on left and imaginary on right) derived from GEIS (a) and LEIS (b,c) data at the (001) face. Lines in (a) and (c) are from equivalent circuit modeling (cf. Figure 1 for GEIS and Figure 6 for LEIS model). Red line is sum of contributions from all circuit components. Lines in (b) are MCR-derived impedance profiles of the three dominant vectors accounting for 97% of the variance of the data collected in 49 spectra.

Equivalent circuit modeling of the LEIS data was carried out with the following modification of Eq. 1: = Z + + (2) whereby the Zcell term of Eq. 1 was replaced by a new Ztip term accounting for the resistance (Rtip) and capacitance (Ctip) contributions of the microelectrode (tip) assembly to the system. Our model is a combination of our original GEIS model (Fig. 1) for the hematite bulk and the interface, and the one devel-

oped for LEIS (Fig. 6). The model thus subscribes to the notion that the contributions of the electrode area that are not covered by the glass shield of the tip are insignificant 18. The model was thereafter developed by investigating the impact of each of the terms of Eq. 1 in accounting for the experimental data. A statistical analysis of the χ2 of our objective function was used as an objective criterion for modifying the model. Firstly, simplification of the Zhem term used in GEIS for a RC component provided an equally good fit to the data, and was therefore preferred. This simplified representation was also motivated by the lack of crystallographic orientation dependence on the data, which partly suggests that LEIS probes a near interfacial region of the hematite electrode surface that could be less affected by the anisotropic nature of electron conduction. Secondly, possibilities for simplifying the Zint term by, for instance, removal of the Cdl term or use of an ideal capacitor in Zint worsened the fit to the data. The interfacial portion was consequently described as in our original model. Thirdly, attempts at describing the Ztip term as purely capacitive (or constant phase element) interface, as originally described in Bandarenka et al. 11 12 systematically gave worst fits to the data than assuming a pure RC circuit. Fourthly, inclusion of a solution resistance term (Zsol) in series did not improve the fit to the data, while adding it in parallel worsened the fit. This term was therefore not included in Eq. 2. Finally, any other attempts at fitting the data with any of the terms of Eq. 2 either individually, as combinations of terms, gave considerably poorer fits to the data. In summary, our equivalent modeling attempts strongly concur with our model-free MCR analysis of the data. Model predictions moreover give rise to highly comparable frequency-resolved contributions to the impedance spectra, as shown in Figure 5c.

Figure 6. Equivalent circuit model used for LEIS modeling and the schematic view of the setup components. The schematic representation of the system is taken after Bandarenka et al. 11 for SECM, and adapted for the model of Shimizu and Boily 10 for the hematite/water interface. Note the solution resistance is in series, as discussed in the text. Maps of electrochemical parameters derived from our proposed model are shown in Figure 7 for interfacial parameters, and in Figures S5 and S6 for the tip and bulk hematite, respectively. These maps reveal the considerable electrochemical heterogeneity of the hematite surfaces under study, yet none of the strong crystallographic orientation dependence extracted by GEIS (Table 1) can be seen in LEIS (Table 2). Attempts at area-normalizing these parameters were also made assuming the cross-sectional area of the microelectrode (491 µm2).


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Those efforts give, for instance for the (001) face, greater interfacial capacitances (e.g. Cdl = 33 µF/cm2, Zad = 77 µF/cm2) and lower resistances (e.g. 0.33 kΩ⋅cm2) than in GEIS (Table 1). One possibility explaining this finding is that the probed area is larger than the tip of the microelectrode. A complementary observation could also lie in the limited ability of the LEIS technique in fully probing charge carrier processes at the near hematite/water interface, and more specifically, those that are strongly coupled to the hematite bulk via the depletion layer. A distinction ought therefore to be made between the equivalent circuit models derived by LEIS compared to those by GEIS. In this sense, development of theoretical (e.g. molecular) models predicting the response of various interfacial charge carriers under an externally applied AC would be strongly useful for further studies.

Still, the variations captured by LEIS are telling of considerable electrochemical heterogeneity at the micron scale. Local variations surface structure (Fig. S1) can, on the one hand, be responsible for variations in proton-active (-OH) site densities, and therefore adsorption reactivity. Possible examples include dissolution or precipitation zones, multidomain surfaces, structural defects or perhaps even antiphase boundaries. In addition to this, compositional (e.g. dopant) inhomogeneities at the near interface could possibly affect the conductivity of the surface, and therefore electrochemical work of adsorption reactions. We however suspect that these levels of heterogeneity are not resolvable at the micron scale. As such, correlating structural and chemical maps of surfaces with LEIS-derived properties should strengthen the interpretative framework in future studies.

Table 2. LEIS equivalent circuit parameters

4. OUTLOOK The importance of hematite surface electrochemistry stems upon its natural abundance in Earth’s upper crust, its key role in biogeochemical and technological processes, including its potential role in the development of sustainable energy production via photocatalytic water splitting reactions. A score of techniques — crystal-rod truncation17,22, X-ray reflectivity23,24, tomography25,26, atomic and scanning tunneling microspopy27, X-ray photoelectron microscopy (XPS)28,29, molecular modelling30,31 and electrochemical measurements4,32,15 — have moreover provided much insight into hematite surface structure, topography composition, reactivity with inorganic ions and complexation agents, as well as electrochemical behavior. These studies paved the way towards a detailed understanding of hematite surface structure and composition on reactivity. Knowledge on its electrochemical reactivity is, however, far less constrained. The AC-SECM/LEIS efforts detailed in this study provided new insights into the electrochemical properties at a mesoscale (a few micrometers) of key hematite surfaces under environmentally relevant conditions. LEIS revealed variations in electrochemical properties associated to bulk hematite and interfacial processes. Strong possibilities accounting for this anisotropy over the sample surfaces considered in this study include heterogenous disributions of dopants in the near hematite surface, as well as structural variations. We also note that the range of parameters extracted by LEIS was not fully compatible with GEIS, despite our findings showing that both techniques retrieve comparable frequency-resolved processes. Our efforts thus represent the very first step required to explore the rich electrochemistry of hematite surfaces at the meso-scale. Salient ongoing developments in our laboratory include mapping the photoelectrochemical responses hematite surfaces, and ought to include in the future more coupled surface-to-bulk electron transport processes. These, amongst other concurrent efforts in the field, should contribute in advancing our fundamental knowledge of key mineral/water interfacial processes, which are very much relevant to natural phenomena, but also to opening possibilities for developing technologies involving hematite as a catalytic material.

(001)

(012)

Zad (nFs-α) α

0.01-0.31 (=0.16) 0.22-0.61 =0.38) 0.50-0.82 (=0.63)

0.01-0.26 (=0.18) 0.26-0.88 (=0.64) 0.50-0.77 (=0.55)

Rad (MΩ)

1-127 (=67)

1-397 (=67)

Rhem (MΩ) Chem(nF)

6.6-245 (=27) 4.4-247 (=4.8)

10.9-105 (=21)

Cdl (nF)

5.3-610 (=91)

Figure 7. Spatial distribution of the diffuse layer capacitance (Cdl) a resistance for charge-carrier transfer (Rad) from the diffuse and the compact layer, a constant phase element (Zad) contributing to the charge transfer and its associated non-ideal factor (α).


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Eggleston, C. M.; Stack, A. G.; Rosso, K. M.; Higgins, S. R.; Bice, A. M.; Boese, S. W.; Pribyl, R. D.; Nichols, J. J. The Structure of Hematite (??Fe2O3) (001) Surfaces in Aqueous Media: Scanning Tunneling Microscopy and Resonant Tunneling Calculations of Coexisting O and Fe Terminations. Geochim. Cosmochim. Acta 2003, 67, 985–1000.

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Lützenkirchen, J.; Preočanin, T.; Stipić, F.; Heberling, F.; Rosenqvist, J.; Kallay, N. Surface Potential at the Hematite (001) Crystal Plane in Aqueous Environments and the Effects of Prolonged Aging in Water. Geochim. Cosmochim. Acta 2013, 120, 479–486.

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Fenter,

Corresponding Author * E-mail: [email protected]. Tel: +46 73 833 2678

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Carl-Tryggers foundation, the ÅForsk foundation and the Swedish Research Council (#20122976). Nils Skoglund is thanked for assistance in the XRD analyses.

ABBREVIATIONS EIS, Electrochemical Impedance Spectroscopy; AC-SECM, Alternating Current Scanning Electrochemical Microscopy; XRD, X-Ray Diffraction; XPS, X-ray Photoelectron Spectroscopy;

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Mapping Electrochemical Heterogeneity at Iron Oxide Surfaces: A

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