Electrochemical Signatures of Crystallographic Orientation and

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Electrochemical Signatures of Crystallographic Orientation and Counterion Binding at the Hematite/Water Interface Kenichi Shimizu, and Jean-François Boily J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511371c • Publication Date (Web): 27 Feb 2015 Downloaded from http://pubs.acs.org on March 4, 2015

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The Journal of Physical Chemistry

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

and

K. Shimizu and J.-F. Boily*

Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden *[email protected]

Abstract The interfacial electrochemistry of hematite (α-Fe2O3) is a key aspect for understanding the behaviour of this important mineral phase in photocatalytic water-splitting devices as well as in terrestrial and atmospheric systems. Nano- to micro-sized 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 (Cdl) and compact (Tad) layer capacitances and resistance (Rad), 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 (Cdl=1–10 µF·cm-2; Tad=3–35 µF·cm-2·s- ϕ) compared to the (001) face (Cdl=0.2–0.6 µF·cm-2; Tad=0.2–0.6 µF·cm-2·s- ϕ). This was also confirmed through the resistance values pertaining to the transfer of charge carriers across the compact plane, which were lower (Rad=0.0–0.8 MΩ·cm-2) on the (012) face than on the (001) face (Rad=1–4 MΩ·cm-2). Binding of chloride and (bi)carbonate on the (012) face under acidic conditions was associated to 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.

1. Introduction The hematite (α-Fe2O3)/water interface plays key roles in technological and natural processes.1-3 Reactions taking place with inorganic ions are of particular interest for these two areas, considering the widespread occurrence and reactivity of these compounds. They have already been the subject of a great number of studies involving slurries of nano- to micro-sized particles4-8 as well as single crystal particles.9-19 Studies involving crystal-rod truncation14-16, X-ray reflectivity12, 17, tomography20-22, atomic and scanning tunnelling microscopy18 as well as X-ray photoelectron spectroscopy (XPS)6, 23 have been particularly useful in describing the hematite surface structure, topography, composition and reactivity with solutions of inorganic ions. As naturally occurring forms of hematite occur as n-type semiconductors, with a band gap of 2.1 eV, surface reactions can also be of a (photo)electrochemical nature, which are far less understood. This form of reactivity is especially attractive in the pursuit of routes for hydrogen production by solar water splitting24-29 and in the use of doped specimens (e.g. Co, Ti, V, etc.) to overcome otherwise intrinsically fast electron-hole recombination rates in this material28, 30. Resolution of the electrochemical attributes of the hematite/water interface is, therefore, essential to push these efforts forward, as well as to predict biogeochemical31-32 and atmospheric33 processes where these forms of interactions also occur.

Electrochemical characterisation of hematite often involves direct open-circuit potential (Eoc) measurement34-37, cyclic voltammetry (CV)7, 38 as well as electrochemical impedance spectroscopy (EIS)38-39 of deposited nanoparticles or (epitaxially grown) supported thin-film electrode surfaces7, 40-43. Studies on single-crystal specimens have, on the other hand, been more instrumental in resolving the crystallographic orientation dependence on surface reactivity and have almost exclusively involved Eoc measurements44,45, whereas CV and EIS24, 38, 46-48 have, on the other hand, not been as widely used for such sample types, but there have been two recent studies from our group38, 49. In those studies, an equivalent electrical circuit, representing the bulk and interfacial electrochemical response of a hematite electrode in contact with aqueous solutions of NaCl and NH4Cl, was developed from EIS data and used to extract interfacial properties. EIS is particularly useful for distinguishing between (electro)chemical processes involving electron transport through the hematite bulk and electrostatic reactions involving the transfer of charge carriers (ions) through compact and diffuse layers. Diffuse layer (Cdl) and adsorption (Tad) capacitance values extracted from EIS measurements of hematite electrodes were notably related to the interfacial speciation of electrolyte ions, such as those resolved by cryogenic XPS6, 23, 50. Those efforts, which were achieved on a purposely roughened surface, are now taken further in this study by focusing on mechanically and chemically polished hematite surfaces along the (001) and (012) faces. The basal (001) plane is ideally a charge-

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neutral and amphoterically silent plane that is terminated by doubly coordinated hydroxo (µ-OH) groups. It is, however, more realistically terminated by configurations of the (OH)3-Fe-H3O3-R (oxygen-terminated) or (OH)3-Fe-Fe-R (iron adatom) type.51 In contrast, the (012) surface ideally contains a greater intrinsic number of proton active sites (-OH, µ-OH, µ3-OH) and is more corrugated than the (001) counterpart and, as a consequence, it has a greater charge-storage capacity. In this work, EIS resolves the electrochemical signatures of these two hematite faces, which are exposed to aqueous solutions of three commonly occurring inorganic salts in natural and technological systems: NaCl, NaHCO3 and NH4Cl. The study of these inorganic ions enables us to study the impact of (1) hydroxo (de)protonation reactions in a relatively indifferent NaCl background electrolyte, (2) (non-)specific binding of (bi)carbonate (e.g. –OH0.5+ HCO3–OCO21.5- + H2O)52 as a dominant species in circumneutral to alkaline solutions and (3) strong hydrogen bonding by ammonia (e.g. –OH···NH3)6 as a potentially important species that blocks (de)protonation reactions. All EIS data collected for this work were modelled using an equivalent circuit model (Fig. 1) with resistance (R) and capacitance (C) values of various interfacial and bulk hematite processes, generating estimations for relaxation times of charge-carrier transfer across the compact plane. These efforts shed further insight into the driving mechanisms that control hematite/water interface reactivity in terms of (1) crystallographic orientation and (2) the nature of counter ion/interface interactions.

Fig. 1 The hematite electrode/water interface (top) and the equivalent circuit model used for EIS modelling (bottom). The equivalent circuit model consists of two components: a solid phase and an aqueous phase. The solution side of the interface is represented by the solution resistance (Rs), a diffuse layer capacitance (Cdl), and a Constant Phase Element CPE as well as a resistance for charge-carrier transfer from the diffuse to the compact layer (Rad). The solid-phase terms include the capacitance of the spacecharging layer (Csc), the ohmic resistance (R1) as well as the charge transport (R2) and charge diffusion (Zw).

2. EXPERIMENTAL 2.1 Electrode preparation and characterisation Hematite electrodes were cut to a size of 5 × 5 × 3 mm3 from natural specimens of unknown origin. These electrodes were cut and physically and chemically polished along the or directions by SurfaceNet (Germany). The samples were then sonicated in our laboratory for 1 min periods, first in acetone and then in ethanol and finally in water. After thoroughly rinsing the crystal surface with ultrapure water, the samples were dried under a stream of dry N2 (g). Note that these surfaces were never annealed and are therefore deemed as more naturally equilibrated surfaces with aqueous solutions. The surface topographies were imaged using an atomic force microscope (AFM; PICO PLUS, Agilent, USA)

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under tapping mode, using a 100 µm scanner, a scanning resolution of 512 × 512 pixels and with acoustically driven cantilevers operating at a resonance frequency in the 320–370 kHz range. The resulting images revealed a generally flat surface with topographical variations of the order of < 10 Å.49 We stress that, although the surfaces cannot be a truly idea crystallographic representation of their intended termination, the bulk is highly oriented along this plane. Elemental compositions at the sample surfaces were also determined by XPS, using a Kratos Axis Ultra electron spectrometer equipped with a delay line detector. The spectrometer was equipped with a monochromated AlKα source operated at 150 W, a hybrid lens system with a magnetic lens, which enables analysis over an area of 0.3 × 0.7 mm2, and a charge neutraliser. All spectra exhibited the characteristic Fe 2p3/2 and O 1s lines of hematite, with no detectable impurities other than atmospheric carbon-based contaminants. Although the surfaces were low in contaminant levels, we should stress that the samples were of natural origin and were electrically conductive (10–16 kW). They therefore contained some level of dopants below the detection limit of the XPS instrument (0.02 at.% in the top ~10 nm of the sample surface). In fact, our estimated dopant concentration (N) from our conduction values, using the relationship 1/ρ=µNq (where ρ is the resistance in Ω·cm, µ=200 cm2 V-1 s-1 is the charge mobility of the hematite bulk and q=1.6022 × 10-19 C), is N≈2.0−3.1 × 1012 cm-3 (0.10−0.15 nmol/mol hematite). 2.2 Eoc and EIS measurements The Eoc and EIS measurements were carried out using a conventional three-electrode cell configuration, consisting of the working hematite single-crystal electrode, a Ag/AgCl reference electrode (3 M KCl, Mettler Toledo Inlab® Reference) and a platinum mesh auxiliary electrode. The underside of the working electrode was connected to a Cu wire using Ag epoxy and then mounted against a Teflon O-ring in a single Teflon body, exposing a 7.07 mm2 circular area to the aqueous solution. Although most traditional setups have all electrodes in close contact with one another, the flow-through setup used in this work has the working electrode connected by a narrow capillary tube to the reference and counter electrode. The experimental set-up, which was described in a previous study49, enables fresh aqueous solutions to be reacted with the electrode surface. Aqueous solutions of 100 mM NaCl, NaHCO3 or NH4Cl were pumped through the cell using a 3 × 3 mm2 capillary from an external titration vessel, where the mother solution was monitored for pH, and, with the exception of NaHCO3 solutions, it was continuously degassed from atmospheric CO2 (g) with N2 (g). In an effort to ensure proper mixing and to avoid accumulation of air bubbles at the electrode surface, the solution arrived from the 3 × 3 mm2 capillary at a 45° angle to the surface, and left at a right angle. The experiments began in thoroughly degassed solutions and were then titrated to alkaline conditions with NaOH or NH4OH, and subsequently to acidic conditions using HCl. All titrants were at the same ionic strength and ionic composition as the background electrolyte and were standardised prior all experiments. The Eoc and pH values were monitored for at least 40 min at each solution composition that was considered in this work. An EIS experiment was carried out thereafter by applying a bias potential corresponding to the experimentally determined Eoc value and by changing the frequency from 100 kHz to 0.1 Hz using a working amplitude of 25 mV. Measurements were conducted using a Princeton Applied Research 273A potentiostat/galvanostat, with a model 1250A frequency-response analyser (Solartron Analytical) and ZPlot® (v. 3.10, Scribner Associates Inc.) as the operating interface. The EIS


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data were fitted to the equivalent circuit model of Fig. 1, using the program ZView® (v. 3.1, Scribner Associates Inc.). A variety of models were tested during the course of these procedures and this circuit is based on our previous studies on hematite38; it was chosen based on its ability to reproduce the data, as evaluated by a statistical analysis used to objectively determine the optimal number of significant adjustable parameters. 3. RESULTS AND DISCUSSION 3.1 Potential development at the hematite/water interface Equilibration of the electrode surfaces was determined by the stability of solution pH and Eoc measurements. The latter were more sluggish than pH changes, yet reversible as long as the waiting period at each solution composition exceeded 90 min, an equilibrium period that is typically associated with drifts of less than 0.3 mV.min-1 (Fig. 2b). All of the Eoc data revealed smaller Eoc vs. pH slopes on the (012) face (6 mV/pH) than on the (001) face (43 mV/pH) in NaCl. These results, thus, fall in line with those of Chatman et al.45, 53 on freshly annealed electrode surfaces in circumneutral-to-acidic conditions, although they do not reproduce the super-Nerstian behaviour they reported in the alkaline region. Differences in electrode surface structure could be responsible for these results but it should however be noted that a more systematic of comparison of annealed and water-equilibrated hematite electrode surfaces would be required to address these discrepancies.

greater interfacial resistance of this surface. We also note that both NaCl- and NaHCO3-bearing systems are characterised by systematic decreases in the imaginary impedance from alkaline to mildly acidic conditions, whereas the opposite occurs in NH4Cl. These results thereby underscore important differences caused by the NH4+ ion, which, again, forms hydrogen bonds directly with the hematite surface hydroxo groups.6

Fig 3. Nyquist plots showing complex-plane plots (Re=real; Im=imaginary) for the hematite electrode oriented along the (001) face in NaCl (a,b), NaHCO3 (c,d) and NH4Cl (e,f). Full lines show fits obtained with the equivalent circuit model of Fig. 1 using Equation 1. Salient modelling parameters are reported in Figs. 5 and 6.

Fig 2. The Eoc of the hematite electrodes in contact with solutions of NaCl, NaHCO3 and NH4Cl (left), and examples of the drift in Eoc values in various experiments for the (012) face in contact with NaCl (right).

Our Eoc values for the (001) face are largely independent of the solution composition and are neutral at pH 11.5–12.0 for the (001), relative to the Ag/AgCl reference. Those of the (012) face are, on the other hand, more sensitive to the solution composition and become neutral at pH 9.5 in NH4Cl, 10.7 in NaHCO3 and 11.5 in NaCl. This order correlates with the greater affinity of (bi)carbonate species over chloride at the hematite surface. The lower crossing point of NH4Cl, on the other hand, is proposed to arise from the blocking of protonation sites from the direct hydrogen bonding of NH4+/NH3 to the surface hydroxo groups6, as will be discussed further in the next section.

3.2 EIS of the hematite/water interface The EIS response of the (001)- and (012)-terminated electrodes (Figs. 3 and 4) consistently exhibits two predominant frequency-resolved regions. The 10 kHz–200 Hz region (Figs. 3b,d,f and 4b,d,f) arises from phenomena of the hematite bulk, which is manifested in the 0–0.6 MΩ (Real Z) region of the Nyquist plots. The region below 200 Hz is related to interfacial and aqueoussolution phenomena, and is shown in the region above 0.6 MΩ (real Z) up to 3 MΩ at the (001) and 30 MΩ at the (012) face. The tenfold increase in the range of the (012) data, thus, already points to the

Fig 4. Nyquist plots showing complex-plane plots (Re=real; Im=imaginary) for the hematite electrode oriented along the (012) face in NaCl (a,b), NaHCO3 (c,d) and NH4Cl (e,f). Full lines show fits obtained with the equivalent circuit model of Fig. 1, using Equation 1. Salient modelling parameters are reported in Figs. 5 and 6. All EIS data could be modelled with an electric circuit model of the hematite/water interface developed in a previous study by our group38, 49. In this model, impedance responses corresponding to the high frequency 10 kHz–200 Hz region are modelled using a RC circuit (R1, R2, Csc, Zw), pertaining to hematite bulk processes, but which were modified to account for the response to ionic strength on the experimental data (Ccell, Rcell). This originally unforeseen response arose from the separation of the reference and counter electrodes in our new experimental setup, which may have induced lower relaxation times for the electrolyte ions. The region below 200 Hz is, in contrast to the region pertaining to hematite bulk processes, strongly affected by variations



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in pH value from interfacial and aqueous solution processes, and will be used to account for the effects of such (de)protonation of surface hydroxo groups, (non-)specific binding of (bi)carbonate and ammonium/ammonia species. Interfacial processes are, therefore, described in terms of double-layer charging (Cdl) from free electrolyte ions and the transport of a charge carrier across the compact layer (CPE and Rad). The latter can, thus, account for surface hydroxo groups (–OH), (de)protonation (e.g. –OH0.5- + H+ –OH20.5+), direct hydrogen bonding (e.g. –OH···NH3) or coordination (e.g. –OH0.5- + HCO3- –OCO21.5- + H2O). This equivalent circuit was used to model all EIS data collected for this study (Figs. 3 and 4) and generated circuit parameters, including their uncertainties, for hematite bulk (Fig. 5) and interfacial (Fig. 6) processes. The impact of crystallographic orientation and interfacial processes, involving (de)protonation, ammonia/ammonium as well as (bi)carbonate complexation reactions, will be presented first. Relaxation times for salient interfacial processes will then be discussed in the final part of this report. 3.2.1 Equivalent circuit parameters Circuit parameters pertaining to the hematite bulk are largely pH-independent and predominantly controlled by crystallographic orientation (Fig. 5). The space-charge capacitance (Csc) of the depletion layer is about 1.0–1.5 orders of magnitude larger in the electrode oriented along the (001) face than along the (012) face, whereas the opposite holds for the charge trap and diffusion (ZW). This result is strongly related to the anisotropic electron-hopping processes in hematite54-55, which are facilitated in the directions that are perpendicular to the (001) face. Its impact is also seen in the ohmic resistance (R1) component of the cell, which is 1.0–1.5 orders of magnitude larger at the (001) plane, as well as in the bulk charge-carrier transport resistance (R1), which was required for this plane, but not for the (012) face. We also note that the cell capacitance (Ccell) or resistance (Rcell) components, used to describe the dielectric relaxation of the electrolytes in the bulk solution, were unaffected by the crystallographic orientation of the electrodes. Some of the bulk parameters were affected by the nature of the background electrolyte. This may be caused by the remaining shortcomings of the equivalent circuit model although, the response of subsurface components cannot be overlooked. The space-charge region could still be responsive to interfacial chemistry, especially where direct bonding to the surface hydroxo groups impacts the ntype carrier density. Parameters for the interfacial processes (Fig. 6) are not only strongly affected by the crystallographic orientation of the electrode, but also by the pH value and the identity of the electrolyte solution, as for the hematite bulk. Both interfacial capacitances (Cdl and Tad) denote a larger charge-storage capability for the (012) orientation, which can be understood by the diversity of protonactive hydroxo groups and the relatively more corrugated interfacial structure, resulting in greater possibilities for solvent-assisted charge-carried stabilisation. Conversely, the adsorption resistance (Rad) is smaller for this plane, and reaches a minimum in the pH 8–9 range, which is close to the expected point of zero charge (pzc) for this face. A minimum of the Cdl value in this pH range was also seen in our previous study on a roughened hematite surface, but here it is manifested by a sharp drop in value from the acidic region in the NaCl-bearing system, with pH-independent values in the pH 8–12 range.

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Fig 5. Capacitance and resistance components of the hematite bulk and external cell components: (a) space-charge capacitance, (b) ohmic resistance, (c) Warburg impedance for the charge trap, (d) diffusion resistance, (e) cell capacitance and (f) cell resistance.

Fig 6. Capacitance and resistance components of the hematite/water interface: (a,b) diffuse-layer capacitance (Cdl), (c,d) CPE capacitance (Tad) and (e,f) compact-layer resistance (Rad). Note -2 - that the unit of Tad is µF⋅cm ⋅s with =1 in NaCl and NaHCO3 and =0.5 in NH4Cl. All results are corrected for the geometrical surface 2 area of the electrode (7.07 mm ). The strong pH dependence of Cdl and Tad at the (012) face, in contrast to the largely pH-independent values for the (001) face, are additional indications of the greater ion-binding and, thus, chargestorage capabilities of this face. Thus, increasing Cdl and Tad values of the (012) face contacted to NaCl and NaHCO3 solutions below the pzc correlate with surface charge development promoted by chloride and bicarbonate binding. The substantially lower Cdl values in NH4Cl result from direct hydrogen-bonding interactions with surface hydroxo groups (–O···[H···NH3]+), a mechanism that blocks protonation and therefore charge development. The pH independence of the Cdl and Tad values in the NH4Cl solutions for the (001) face may be an indication of the unchanged interfacial speciation of NH4+ species, especially at pH values below its pKa. Finally, we also note that the sharp break in the pH dependence of Cdl values at the pzc arises, in contrast, from the greater contributions of interfacially-bound sodium ions, as for instance previously noted by X-ray photoelectron spectroscopy23, to the overall capacitance. 3.2.2 Relaxation time of interfacial charge carriers The equivalent circuit parameters discussed in the previous section provide a means to estimate the relaxation times (τ) of specific interfacial processes perturbed by the externally applied alternating-current (AC) voltages during an EIS experiment. The most relevant to this study concerns the transfer of ions (charge carriers) from the diffuse layer to the compact plane. These


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relaxation times can be derived from our Tad and Rad values in order to first obtain the capacitance of the charge-carrier process56: 1⁄ φ

1-φφ

Cad =Tad ·Rad

(1)

where ϕ=0.67-0-72 for the (012) and ϕ=0.50 for the (001) plane pertain to the ideality of the capacitor. The relaxation time is then obtained through the following relationship: τ = Cad ⋅Rad

(2)

The resulting values, shown in Fig. 7, range from 0 to 4 s and are, consequently, comparable to the timescales extracted by pressurejump measurements of iron oxides.34

Fig 7. EIS-derived relaxation times of the hematite/water interface. Values of the NaCl- and NaHCO3-bearing systems for the (012) face are strongly pH dependent, as are their counterpart Tad and Rad values, and show a clear minimum in the pH 8–9 range, namely, the expected pzc. This result suggests that the system reequilibrates at a faster rate when the surface is neutrally charged. The pH independence and lower relaxation times of the NaClbearing system for the neutrally charged (001) face supports this concept further. We note that the NaHCO3-bearing system relaxes, within a comparable and generally pH-independent timeframe, to that of the (012) face. Larger relaxation times below the pzc suggest that the strongly charged mineral surface inhibits re-equilibration of the altered protonation states of the hydroxo groups to their original states under these conditions. The NH4Cl system represents, however, a contrasting case. Relaxation times are, in this case, almost negligible on the (012) face. This can be taken as an indication that surface hydroxo–ammonia interactions re-equilibrate much more rapidly than protonation and/or are predominantly unperturbed by the externally applied AC current. The rapidity with which the cation reaches equilibrium with the surface functional groups may be a key aspect in the reactivity of ammonia. The 1–2 s relaxation times at the (001) face may, on other hand, provide indirect evidence for yet unresolved interactions which ought to be resolved in future studies. 4. CONCLUSIONS EIS measurements of crystallographically oriented hematite electrode surfaces provide an insightful view into mineral/water interfacial chemistry. This study showed that the dependence of the equivalent circuit on the crystallographic orientation, pH and ion identity can be explained by the interdependent nature of the hematite/water interface structure, composition and ion-binding mechanisms. Crystallographic orientation effects are observed

through bulk electrical conduction processes, which are facilitated in directions perpendicular to the (001) face. Interfacial processes can be effectively captured through the capacitance (diffuse layer and adsorption) values, denoting the greater charge-storage capability of the (012) surface, as well as through lower electrical adsorption resistances. Relaxation times extracted by EIS-derived capacitance and resistance values are lowest at the expected pzc and are larger at greater surface potential and ion loadings. These findings also support recent efforts in our group for developing thermodynamic adsorption models that, in contrast with traditional approaches, invoke pH and ionic-loading dependence of the compact layer (e.g. Stern or inner-Helmholtz) capacitance values. This concept, which was recently implemented in the Variable Capacitance Model57, expresses the overall interfacial capacitances in terms of a combination of capacitances of ionspecific electric double layer structures, and therefore one that is pHdependent. An important leap that remains, however, is to subject EIS-derived capacitances and potentially even resistance values to such models. The apparent mismatch of capacitance values between EIS (on the order of 1–10 µF·cm-2) and thermodynamic adsorption modelling (on the order of 20–200 µF·cm-2) represents a key challenge in this area, and is thus calling for renewed efforts in resolving this issue. Such efforts are particularly important in our quest to resolve mineral/water interfacial processes, which are central to our understanding of the functioning of (photo)catalytic attributes of hematite particle surfaces in terrestrial and atmospheric settings, as well as in water-splitting devices tests for hydrogen-gas production.

Acknowledgements This work was supported by the Carl-Tryggers and ÅForsk Foundations, and by the Swedish Research Council (2012-2976). 1. Hochella, M. F., Jr.; Lower, S. K.; Maurice, P. A.; Penn, R. L.; Sahai, N.; Sparks, D. L.; Twining, B. S., Nanominerals, Mineral Nanoparticles, and Earth Systems. Science 2008, 319, 1631-5. 2. Casey, W. H.; Rustad, J. R.; Spiccia, L., Minerals as Molecules--Use of Aqueous Oxide and Hydroxide Clusters to Understand Geochemical Reactions. Chemistry 2009, 15, 4496-515. 3. Paracchino, A.; Laporte, V.; Sivula, K.; Gratzel, M.; Thimsen, E., Highly Active Oxide Photocathode for Photoelectrochemical Water Reduction. Nature Materials 2011, 10, 456-461. 4. Boily, J. F.; Shchukarev, A., X-Ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in Aqueous Solutions. 2. Tracing the Relationship between Surface Charge and Electrolyte Adsorption. Journal of Physical Chemistry C 2010, 114, 2613-2616. 5. Schudel, M.; Behrens, S. H.; Holthoff, H.; Kretzschmar, R.; Borkovec, M., Absolute Aggregation Rate Constants of Hematite Particles in Aqueous Suspensions: A Comparison of Two Different Surface Morphologies. Journal of Colloid and Interface Science 1997, 196, 241-253. 6. Shimizu, K.; Shchukarev, A.; Boily, J. F., X-Ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in Aqueous Solutions. 3. Stabilization of Ammonium Species by Surface (Hydr)Oxo Groups. Journal of Physical Chemistry C 2011, 115, 6796-6801. 7. Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W., Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with "Co-Pi"-Coated Hematite Electrodes. Journal of the American Chemical Society 2012, 134, 16693-16700.



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Electrochemical Signatures of Crystallographic Orientation and

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