X-ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in

Colloidal-sized hematite spheroids exposed to aqueous NaCl solutions were investigated by X-ray photoelectron spectroscopy using the fast-frozen techn...
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J. Phys. Chem. C 2010, 114, 2613–2616

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X-ray Photoelectron Spectroscopy of Fast-Frozen Hematite Colloids in Aqueous Solutions. 2. Tracing the Relationship between Surface Charge and Electrolyte Adsorption Jean-Franc¸ois Boily* and Andrei Shchukarev Department of Chemistry, Umeå UniVersity, SE 901-87 Umeå, Sweden ReceiVed: August 25, 2009; ReVised Manuscript ReceiVed: January 7, 2010

Colloidal-sized hematite spheroids exposed to aqueous NaCl solutions were investigated by X-ray photoelectron spectroscopy using the fast-frozen technique. The O1s region provided evidence for (de)protonation reactions of surface (hydr)oxo groups of OH-enriched/O-depleted hematite surfaces. These results were also correlated to changes in sodium (Na 1s) and chloride (Cl 2p) contents with pH. Electrolyte ion surface loadings were successfully predicted using a classic thermodynamic adsorption model normalized for surface site density. These efforts pointed to ion-specific inner-Helmholtz plane capacitances. 1. Introduction Electrolyte ions are key players in the stabilization of charge at the metal (hydr)oxide/water interface.1 Classical analytical determinations of adsorbed electrolyte ions have, however, been largely prohibitive due to low adsorption energies, hence the paucity of reliable data in the literature. Predictive thermodynamic adsorption models2–7 have therefore mostly been constrained by the ionic strength dependence of potentialdetermining ions (H+, OH-) without any sound knowledge of the extent and mechanisms of electrolyte ion adsorption. X-ray (reflectivity, standing wave)8–10 and molecular dynamics (MD)11–13 techniques have started to provide insights into the distribution of electrolyte ions at solid/solution interfaces. These methods have notably been providing clues to the number and identity of surface groups required in stabilizing electrolyte ions. X-ray photoelectron spectroscopy (XPS) can also be used with such systems by freezing wet solid pastes to near liquid nitrogen temperatures.14–20 This technique provides surface concentrations of electrolyte ions in relation to metal and oxygen (O, OH, H2O) atoms at solid surfaces. It has notably been used in several recent studies from our group to follow sodium and chloride adsorption on iron, silica, aluminum, and manganese (oxy)(hydr)oxides.14–20 In a previous study20 this approach was used to study Na+ and Cl- interactions with oblate hematite (R-Fe2O3) particles exhibiting important proportions of the neutrally charged (001) basal plane. These particles, in contrast to any previously studied particles,14–20 induced the formation of a hydrohalite-like (NaCl · 2H2O) phase, which may have arisen from a threedimensional distribution of hydrated sodium and chloride ions at the solid/solution interface. In this second part of our series on XPS studies of frozen hematite pastes we extend our previous efforts to an important class of hematite spheroids (Figure 1) which, in contrast to their oblate counterpart, exhibit strongly amphoteric properties and do not promote hydrohalite-like phases. Of the many studies devoted to this material,21–25 the one of Schudel et al.26 contains one of the most comprehensive thermodynamic adsorption models, describing charge uptake, ζ potentials and colloidal stabilities over a range of pH and ionic strengths. For this study, XPS studies of frozen hematite pastes, concentrations of Na+ and Cl- ions, and surface OH * Corresponding author. Ph: [email protected].

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Figure 1. SEM image of the hematite used for this study (Hemcontrol).

and O groups provide further constraints to the model of Schudel et al.26 This study moreover adds insight into the stability of the hydroxylated surface of hematite27–31 through the use of particles that have been in contact with water for a period exceeding 10 years. 2. Methods 2.1. Mineral Synthesis and Characterization. Hematite (Hem-control) was synthesized in our laboratories in 1998 using the method described in part 1 of this study.20 The particles were then stored for 10 years at 298.2 ( 1.0 K as an aqueous suspension in a sealed polyethylene bottle. X-ray diffraction of the aged particles confirmed that hematite was the only crystalline phase present.20 Fourier transform infrared (FTIR) spectra were collected in vacuo in attenuated total reflectance mode (Golden Gate, Specac). These measurements were carried out using a Vertex 70v (Bruker) spectrometer equipped with a deuterated triglycine sulfate detector. 2.2. XPS on Fast-Frozen Wet Pastes. Aqueous suspensions of hematite were prepared at ionic strengths of 10 and 100 mM NaCl (Aldrich, dried at 220 °C). Aliquots of these suspensions were then equilibrated at pH values between 2 and 12 by addition of 10 mM HCl or 10 mM NaOH. These titrants for the 100 mM NaCl system also contained 90 mM NaCl to ensure a total ionic strength of 100 mM. The suspension pH values were adjusted by acidimetric and alkalimetric titrations from the pH of the suspension (∼5) to minimize the reprecipitation of dissolved iron (e.g., as an amorphous ferric oxy(hydr)oxide phase). The resulting suspensions were equilibrated for 1 h under constant stirring in a N2(g) atmosphere at 298.2 ( 1.0 K, after which they were centrifuged, decanted and analyzed by XPS (Figure 2).

10.1021/jp908197f  2010 American Chemical Society Published on Web 01/27/2010

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Boily and Shchukarev

Figure 2. Wide spectra of fast-frozen hematite samples at pH 3.4 and 12.0 prepared from pastes equilibrated in 10 mM NaCl at 298.2 K.

Figure 3. Fe 2p spectra of dry and fast-frozen hematite (pH 2-12) prepared from pastes equilibrated in 10 mM NaCl at 298.2 K. The 13 spectra of the fast-frozen samples are both overlapped (2-12) and staggered to show that changes well within 0.5 eV occurred with variations in pH. The spectrum of a dry goethite sample is also shown (bottom).

The procedures for the cryogenic XPS measurement and spectral manipulations are described in full detail in paper 1.20 Briefly, centrifuged wet pastes were frozen on a sample holder cooled at -170 °C under N2(g). They were then transferred to the analysis chamber under a pressure of (4-5) × 10-5 Pa. As this process results in the sublimation of loosely bound water molecules, the resulting samples tend to be enriched in water molecules that are strongly associated to the hematite surface and electrolyte ions.20 2.3. Thermodynamic Modeling. Thermodynamic adsorption models for H+/OH-, Na+, and Cl- adsorption were carried out with the basic Stern model using the equations and parameters of Schudel et al.26 All variations in ionic strength, which are mostly important at pH below 3 and above 11 in 10 mM NaCl, were explicitly taken into account in terms of variations in activity coefficients (Davies equation) and diffuse layer properties (Gouy-Chapman equation). All calculations were carried out by solving a set of nonlinear equations representing aqueous and surface mass action, mass balance, charge balance, and electric double layer equations using a Levenberg-Marquardt numerical iteration technique with Matlab (The Mathworks, Inc.). 3. Results and Discussion 3.1. Iron and Oxygen. The Fe 2p region (Figure 3) displays features characteristic of those of hematite with peaks at 724.5,

Figure 4. (a) Binding energies of O, OH, and H2O components of the O1s region of hematite control in three ionic strengths and of two hematite platelets (Hem-1 and Hem-8, both used in Shchukarev et al.20) in different ionic strengths and media. The average peak separation and associated standard deviation (3σ) are also shown. (b) HxO/Fe atomic ratios for Hem-control in 10 and 100 mM NaCl. The line at the bottom is a linear regression to the 10 mM OH/Fe data while the one at the top lies at O/Fe ) 1.5, the expected ratio for hematite.

719.5, 711.2, and 709.5 eV. The 709.5 eV peak, a distinguishing feature of hematite from other ferric (oxy)hydroxides, is predominant over the entire pH range. Its intensities, however, drop slightly, relative to the predominant 711.2 eV peak, in the samples prepared at pH 8.00-9.60. Comparable variations were also noted for the basal plane of hematite when analyzed in vacuo in grazing mode.32 These spectral variations, however, arose from undercoordinated surface (terminal) iron atoms, which are not likely to be present in aqueous conditions. A more probable explanation for our material involves changes in the coordination of iron by exchange of O2- for OH- and/or H2O, one that could have arisen through the formation of the hydroxylated surface layers discussed by Jang et al.30 This possibility is supported by FTIR spectra revealing O-H stretching (3400 cm-1) and water bending (1638 cm-1) bands, which have also been noted in proto- and hydrohematite.33 None of these bands were, however, indicative of any other iron (oxy)hydroxide phases (e.g., goethite, ferrihydrite). The O1s region also contains information for the coexistence of O, OH, and H2O in the frozen samples. The binding energies of these components, referenced to the hematite O value of 530.0 eV,34 were largely independent of pH (Figure 4a). Comparisons with other hematite particles prepared at different ionic strengths and media revealed variations no greater than 0.43 eV (3σ) between different O1s peaks. Changes in surface charge, electrolyte content, and hydration had consequently little effect

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Figure 5. Cl and Na data (atomic %) normalized for the estimated total surface OH concentration (see text) in 10 mM NaCl. The total ionic strength in the acidic and alkaline ranges were also affected by added HCl and NaOH and correspond to values up to 22 mM. The ionic composition of every solution was fully accounted in the thermodynamic calculations. The Na+/OHTot data were offset by -0.01 unit to provide an average of 0 in the 2-9 pH range. The solid curves with label “1” correspond to the model of Schudel et al.26 where the predictions for Cl- and Na+ are normalized for the total crystallographic site density. (Model parameters for curves 1: [tFeOH0.5-]Tot ) 8 sites/ nm2, pKproton,ads ) 9.2, pKelectrolyte,ads ) -0.3, CStern,Cl- ) CStern,Na+ ) 1.3 F/m2. Curves 2 were generated with CStern,Na+ ) 3.8 F/m2 with eq 2).

on these values. Similarly, the O content was relatively constant (Figure 4b) with O/Fe ratios of ∼1.1, namely ∼0.4 unit lower than the expected 1.5 value for stoichiometric hematite. This latter value is retrieved only by accounting for hydroxyl groups, present at OH/Fe ratios of ∼0.3-0.5. The region analyzed by XPS should consequently consist of Fe(III) atoms bound to bulk O, an important fraction of which has been exchanged for OH groups. Interestingly, the OH/Fe ratios exhibit a slope of ∼0.018 OH/ Fe per pH unit from pH 2 to 12. If this slope arises only from surface (de)protonation reactions it could correspond to a change of up to 0.54 sites/nm2 per pH unit.35,36 Although this estimate is strongly influenced by assumptions35,36 regarding the surface site stoichoimetry and by possible contributions from surface precipitates and/or adatoms, especially near the solubility minimum, the trend in the broader 2-12 pH range does remain consistent with the concept of surface (de)protonation reactions. Of the many surface species present on the hematite surface, singly coordinated oxygens (tFesO or sO) are expected to play a central role given previously estimated pK of 7.737 for the reaction:

-OH20.5+ T -OH0.5- + H+

(1)

Unfortunately, the expected correlated pH dependence of surface-bound H2O data cannot be verified due to the considerably greater scatter in these data. Contributions from triply coordinated oxygens (µ3-O) may also be responsible for the larger O/Fe values above pH 10 (Figure 4b). The relatively large scatter in these data, however, prevents any definitive conclusions in this regard. Nonetheless, the O1s spectra reveal changes in OH content with pH, an important portion of which arises from (de)protonation reactions involving an OH-enriched/Odepleted hematite surface. These groups are, in turn, responsible for the contents of charged electrolyte ions in the frozen pastes, which will be the object of the following section. 3.2. Na+ and Cl- Adsorption. The hematite pastes exhibit systematic variations in surface concentrations Na+ and Cl- ions with pH (Figure 5). These variations are best seen in pastes prepared in 10 mM NaCl as contributions from the residual

supernatant are small (0.01 Na/Fe) compared to surface-bound ions. Those in 100 mM NaCl were considerably more erratic, due the stronger salt concentrations and therefore not considered further. The Cl/Fe and Na/Fe values in the 10 mM NaCl system can be adequately predicted if the experimental and theoretical concentrations are normalized for the surface OH density of 15 sites/nm2.35 This was carried out by normalizing the atomic % values of Cl and Na to those of Fe, assuming an average O:Fe stoichiometry of 1:2.36 Theoretical surface Cl and Na concentrations were generated with the 8 sites/nm2 value of the Schudel et al.26 model and then normalized for surface OH density of 15 sites/nm2. This simple procedure connected the experimental to the modeling data by removing any model-derived surface site-electrolyte ion stoichiometry and site density. Although this normalization provides a good fit to the Cl/Fe data, it underestimated the Na/Fe data. This deviation can be partly understood by limitations of the potentiometric technique, from which the model (labeled “1”) was originally derived, in probing proton adsorption in alkaline conditions. Several alternative and/or complementary modeling strategies were consequently explored to account for this enhanced uptake of sodium, all of which adopt symmetric electrolyte ion adsorption constants to simulate the ionic strength independent isoelectric point of hematite.20 A first attempt involved distributing the charge of sodium between the inner- and outer-Helmholtz planes, according to the charge distribution model.37 This approach did not, however, improve the fit to the data. Another approach involved modeling sodium adsorption onto µ3-O0.5sites (pK ) 12). Attempts at including such a complex were made by testing various site densities, electrolyte binding constants, as well as considering a full multisite complexation model.37 These attempts, however, failed at generating sufficient concentrations of µ3-O0.5- · · · Na+ at pH 10-12 with physically reasonable modeling parameters, and were therefore abandoned. The most successful approach consists of expressing the capacitance of the inner-Helmholtz plane (Stern) as electrolyte ion-specific values, adjusted for surface loading.38 This possibility was used to obtain model “2” of Figure 5 with capacitance (CStern) expressed as

CStern ) CStem,ClfCl,ads + CStem,NafNa,ads

(2)

where f is the fraction of adsorbed electrolyte ion such that 1 ) fCl,ads + fNa,ads. This possibility alone provides a good fit to the data with CStern,Na ) 3.8 F/m2 and the original CStern,Cl ) 1.3 F/m2 value of Schudel et al. 26. The larger CStern,Na value can be explained by the smaller ionic radius of Na+ (CStern,Na ) ε · δNa-1; ε being permittivity and δNa ionic radius), which enables a smaller distance of approach to the surface, and hence a thinner Stern layer. Phenomena along these lines have already been addressed in terms of structure-making-breaking electrolytes ions39,40 and certainly warrant further investigation along these lines. Additional efforts in this series devoted to XPS studies of fast-frozen hematite wet pastes will address these issues further with electrolyte ions of varied sizes. 4. Conclusions This study uncovered key aspects of hematite surface reactivity in contact with water and electrolyte ions. Decadelong exposure of hematite spheroids to water (pH ∼ 5) did not result in the formation of a secondary phase but did result in surfaces containing up to 0.5 OH groups per Fe, consistent with

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previous claims30 for a hydroxyl-rich surface layers on this mineral. The concentrations of these groups are also affected by (de)protonation reactions and can be related to the sodium and chloride content of the hematite pastes. The Na 1s and Cl 2p data are best related to the thermodynamic model of Schudel et al.20 by normalization for the surface Fe content. Finally, these efforts point to the use of ion- and surface loading-dependent capacitance values in modeling adsorption reactions at the mineral/water interface. Acknowledgment. We thank J. Lu¨tzenkirchen for producing hematite. This project was supported by the Wallenberg and Kempe Foundations (Sweden). References and Notes (1) Bockris, J. O. M.; Khan, S. U. M. Surface Electrochemistry. A Molecular LeVel Approach; Plenum Press: New York, 1993. (2) Lu¨tzenkirchen, J.; Boily, J.-F.; Gunneriusson, L.; Lo¨vgren, L.; Sjo¨berg, S. J. Colloid Interface Sci. 2008, 317, 155. (3) Hiemstra, T.; van Riemsdijk, W. H. Geochim. Cosmochim. Acta 2009, 73, 4423. (4) Gaboriaud, F.; Ehrhardt, J. Geochim. Cosmochim. Acta 2003, 67, 967. (5) Rahnemaie, R.; Hiemstra, T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 2006, 293, 312. (6) Boily, J.-F.; Lu¨tzenkirchen, J.; Balme`s, O.; Beattie, J.; Sjo¨berg, S. Colloid Surf. A. 2001, 179, 11. (7) Villalobos, M.; Cheney, M. A.; Alcaraz-Cienfuegos, J. J. Colloid Interface Sci. 2009, 336, 412. (8) Fenter, P.; Sturchio, N. C. Prog. Surf. Sci. 2004, 77, 171. (9) Zhang, Z.; Fenter, P.; Cheng, L.; Sturchio, N. C.; Bedzyk, M. J.; Machesky, M. J.; Wesolowski, D. J. Surf. Sci. 2004, L95. (10) Fenter, P.; Cheng, L.; Rihs, S.; Machesky, M.; Bedzyk, M. J.; Stuchio, N. C. J. Colloid Interface Sci. 2000, 225, 154. (11) Predota, M.; Bandura, A.; Kubicki, J. D.; Lyov, S. N.; Cummings, P. T.; Chialvo, A. A.; Ridley, M. K.; Benezeth, P.; Anovitz, L.; Palmer, D. A.; Machesky, M. L.; Wesolowski, D. J. Langmuir 2004, 20, 4954. (12) Kerisit, S.; Cooke, D. J.; Marmier, A.; Parker, S. C. Chem. Commun. 2005, 3027. (13) Kerisit, S.; Ilton, E. S.; Parker, S. C. J. Phys. Chem. B 2006, 20491.

Boily and Shchukarev (14) Shchukarev, A.; Boily, J. F. Surf. Interface Anal. 2008, 40, 349. (15) Shchukarev, A. Surf. Interface Anal. 2006, 38, 682. (16) Shchukarev, A. AdV. Colloid Interface Sci. 2006, 122, 149. (17) Shchukarev, A.; Sjo¨berg, S. Surf. Sci. 2005, 584, 106. (18) Ramstedt, M.; Andersson, B. M.; Shchukarev, A.; Sjo¨berg, S. Langmuir 2004, 20, 8224. (19) Shchukarev, A.; Rosenqvist, J.; Sjo¨berg, S. J. Electron Spectrosc. Real. Phenom. 2004, 137, 171. (20) Shchukarev, A.; Boily, J.-F.; Felmy, A. R. J. Phys. Chem. 2007, 111, 18307. (21) Matijevic, E.; Kuo, R. J.; Kolny, H. J. Colloid Interface Sci. 1989, 133, 91. (22) Colic, M.; Fuerstenau, D. W.; Kallay, N.; Matijevic, E. Colloids Surf. 1991, 59, 169. (23) Penners, N. H. G.; Koopal, L. K. Colloids Surf. 1987, 28, 67. (24) Liang, L.; Morgan, J. J. Aquatic Sci. 1990, 52, 32. (25) Tiller, C. L.; O’Melia, C. R. Colloids Surf. A 1993, 73, 89. (26) Schudel, M.; Behrens, S. H.; Holthoff, H.; Kretzschmar, R.; Borkovec, M. J. Colloid Interface Sci. 1997, 196, 241. (27) Trainor, T. P.; Chaka, A. M.; Eng, P. J.; Newville, M.; Waychunas, G. A.; Catalano, J. G.; Brown, G. E., Jr. Surf. Sci. 2004, 573, 204. (28) Tanwar, K. S.; Lo, C. S.; Eng, P. J.; Catalano, J. G.; Walko, D. A.; Brown, G. E., Jr.; Waychunas, G. A.; Chaka, A. M.; Trainor, T. P. Surf. Sci. 2007, 601, 460. (29) Eggleston, C. M.; Stack, A. G.; Rosso, K. M.; Bice, A. M. Geochem. Trans. 2004, 5, 33. (30) Jang, J. H.; Dempsey, B. A.; Burgos, W. EnViron. Sci. Technol. 2007, 41, 7303. (31) Liu, P.; Kendelwicz, T.; Brown, G. E.; Nelson, E. J.; Chambers, S. A. Surf. Sci. 19998, 417, 53–65. (32) Droubay, T.; Chambers, S. A. Phys. ReV. B 2001, 64, 205414. (33) Stanjek, H.; Schwertmann, U. Clays Clay Mineral. 1992, 40, 347. (34) Junta-Rosso, J. L.; Hochella, M. F., Jr. Geochim. Cosmochim. Acta 1996, 60, 305. (35) Barro´n, V.; Torrent, J. J. Colloid Interface Sci. 1996, 177, 407. (36) Assuming a crystallographic density of 15 sites/nm2 and an average surface OH:Fe ratio of 1:2.20,36 (37) Venema, P.; Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1998, 198, 282. (38) Boily, J. F. J. Phys. Chem. C. 2007, 111, 1299. (39) Yang, Z.; Li, Q. F.; Chou, K. C. J. Phys. Chem. C. 2009, 113, 8201. (40) Lyklema, J. Chem. Phys. Lett. 2009, 467, 217–222.

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