Atomic Force Microscopy and X-ray Photoelectron Spectroscopy Study

Jul 30, 2012 - At 70% RH, CaCO3 (101̅4) surfaces undergo rapid formation of a metastable amorphous calcium carbonate layer, which in turn serves as a...
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Atomic Force Microscopy and X‑ray Photoelectron Spectroscopy Study of NO2 Reactions on CaCO3 (101̅4) Surfaces in Humid Environments Jonas Baltrusaitis*,† and Vicki H. Grassian* Departments of Chemistry and Chemical and Biochemical Engineering, University of Iowa, Iowa City Iowa 52242, United States S Supporting Information *

ABSTRACT: In this study, alternating current (AC) mode atomic force microscopy (AFM) combined with phase imaging and X-ray photoelectron spectroscopy (XPS) were used to investigate the effect of nitrogen dioxide (NO2) adsorption on calcium carbonate (CaCO3) (101̅4) surfaces at 296 K in the presence of relative humidity (RH). At 70% RH, CaCO3 (1014̅ ) surfaces undergo rapid formation of a metastable amorphous calcium carbonate layer, which in turn serves as a substrate for recrystallization of a nonhydrated calcite phase, presumably vaterite. The adsorption of nitrogen dioxide changes the surface properties of CaCO3 (1014̅ ) and the mechanism for formation of new phases. In particular, the first calcite nucleation layer serves as a source of material for further island growth; when it is depleted, there is no change in total volume of nitrocalcite, Ca(NO3)2, particles formed whereas the total number of particles decreases. This indicates that these particles are mobile and coalesce. Phase imaging combined with force curve measurements reveals areas of inhomogeneous energy dissipation during the process of water adsorption in relative humidity experiments, as well as during nitrocalcite particle formation. Potential origins of the different energy dissipation modes within the sample are discussed. Finally, XPS analysis confirms that NO2 adsorbs on CaCO3 (101̅4) in the form of nitrate (NO3−) regardless of environmental conditions or the pretreatment of the calcite surface at different relative humidity.

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INTRODUCTION Calcite, one of the most abundant calcium carbonate minerals, is a constituent of limestone, other sediments, such as calcsilicate materials diopside, tremolite and alkaline igneous rocks, such as carbonatites and some nepheline-syenites.1 Due to its presence in the atmosphere as a component of mineral dust aerosol, calcite reactivity has been investigated in heterogeneous atmospheric chemistry,2−5 as well as in dissolution studies.6−11 Additionally, calcium carbonate is a building material, as well as used in ceramics and glasses where interactions with trace atmospheric gases can result in enhanced deterioration.3,12 NO2 is a trace atmospheric gas emitted from both natural and anthropogenic sources, such as biomass or fossil fuel combustion. It is also related to ozone and hydroxyl radical atmospheric cycling.13,14 In the presence of atmospheric aerosols or their components, NO2 can react with a range of different uptake coefficients, adsorption mechanisms, and gaseous products, depending on temperature and relative humidity.15−17 Recently, it has been shown that CaCO3 undergoes heterogeneously reaction with NO2 at different RH,16 as well as facilitates synergistic NO2 and SO2 redox reactions.18 These heterogeneous reactions effectively transform calcite particles into particles with very different properties.19 © 2012 American Chemical Society

The surface chemistry of calcium carbonate under humid environment has been investigated in a number of studies.3,20,21 Enhanced ionic mobility on the calcite surface in the presence of water as relative humidity was observed and characterized by surface sensitive imaging techniques.4,21−23 Various scanning probe microscopy (SPM) techniques have been used to characterize calcite surface interactions with water to study the dissolution kinetics and crystal growth mechanisms.12,24−36 These studies utilize both constant and intermittent contact AFM4,22,23,37−40 and scanning polarization-force microscopy (SPFM).21,41,42 However, to our knowledge only a handful of studies have attempted to obtain both morphological and spectral information on interfacial phenomena on calcite surfaces in the presence of water vapor. In this study, we combined AC mode atomic force microscopy (AFM) combined with phase imaging at controlled environmental conditions and X-ray photoelectron spectroscopy (XPS) to investigate the effects of relative humidity on adsorptive properties of CaCO3 (101̅4) surfaces toward NO2. Freshly cleaved surface, as well as surfaces prereacted with NO2, were exposed to high (70%) RH, while the temperature was kept Received: May 25, 2012 Revised: July 18, 2012 Published: July 30, 2012 9001

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Figure 1. Experimental setup of (a) relative humidity and temperature controlled AFM apparatus40 and (b) custom-designed XPS system.5 The AFM consists of (i) relative humidity preparation system, (ii) closed environmental cell connected to the scanner head and a sample heater/cooler and, (iii) environmental monitor and control system. The XPS system consists of (i) surface analysis chamber equipped with monochromated X-ray source, (ii) transfer antechamber, and (iii) reaction chamber. 9002

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constant at 296 K. Phase imaging yielded complementary information on the formation of the inhomogeneities and different phases on the surface of CaCO3 (101̅4). Finally, chemically modified calcite surfaces, such as those reacted with NO2, present a different picture of surface spatial inhomogeneities when compared to unreacted surfaces.

gas was introduced into the reaction chamber, equilibrated for 30 min, evacuated and then either transferred into samples analysis chamber for the XPS analysis or removed from the instrument and transferred into AFM environmental cell. The time necessary for the transfer of the sample after the reaction to AFM environmental cell was within few minutes. Data processing was performed using commercially available CasaXPS software.43 All spectra were calibrated using the adventitious C 1s peak at 285.0 eV. A Shirley-type background was subtracted from each spectrum to account for inelastically scattered electrons that contribute to the broad background. The components of the peaks contain a Gaussian/Lorentzian product with 30% Lorentzian and 70% Gaussian character. Sources and Purity of Reagents. Commercially available Icelandic spar calcium carbonate single crystals, obtained from Ward’s Scientific, were cleaved with a razor edge to expose a fresh CaCO3 (1014̅ ) surfaces. For AFM analysis, the single crystal sample was mounted onto a stainless steel puck (1.2 × 0.8 × 0.5 mm, Asylum Research) using conductive silver paint (Ted Pella). Nitrocalcite (Ca(NO3)2·4H2O from Catalonia, Spain) and used as received. Nitrocalcite surface elemental composition was confirmed by XPS. For reactions done inside the XPS reaction chamber, crystal and particulate samples were mounted onto a copper stub (Kratos Inc.) with conductive silver paint or pressed into In foil, and was loaded into the sample entry chamber and immediately evacuated. Total exposure to the atmosphere after cleaving was less than one minute. For relative humidity studies, distilled water (Optima grade from Fisher Scientific) was used. Prior to use the distilled water was outgassed several times with consecutive freeze−pump− thaw cycles. Research-grade purity nitrogen dioxide was purchased from Matheson. Nitrogen gas (99.998%) was purchased from Airgas.

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EXPERIMENTAL METHODS AND REAGENTS Atomic Force Microscopy: Experimental Setup. The experimental setup used for these studies has been described before.40 Briefly, an Asylum Research MFP-3D AFM instrument was coupled with a custom build environmental cell was used to investigate CaCO3 (101̅4) surfaces at controlled relative humidity. A schematic of the apparatus is shown in Figure 1a. It consists of: (i) a gas handling system to control relative humidity; (ii) an environmental cell connected to the scanner head and coupled to a sample heater/cooler; and (iii) environmental monitor and control system. To prepare a gas flow stream of known relative humidity, dry nitrogen is passed through an Erlenmeyer flask filled with water. Polypropylene tubing is used to transport nitrogen and was not submerged into water, not to induce scanning artifacts due to the acoustic waves from gas bubbling system. Relative humidity and temperature inside of the closed cell is monitored using a model HIH3610 relative humidity sensor from Honeywell Inc. with 2% accuracy at 298 K, which is magnetically attached to the heater/cooler element. Low force constant Si cantilevers were used (CSC37/AlBS from MikroMash, cantilever A with a typical force constant of 1.2 N/m) to minimize any tip effects on the surface. Alternating current (AC) imaging mode was used in all experiments. Typically, 70% RH was used in these experiments. Images were acquired using 512 × 512 resolution were acquired. X-ray Photoelectron Spectroscopy. A detailed description of the XPS experimental setup is provided elsewhere.5 Briefly, custom-designed Kratos Axis Ultra X-ray photoelectron spectroscopy system, equipped with a reaction chamber, was used in NO2 reactions and analysis experiments and is shown in Figure 1b. The surface analysis chamber is equipped with monochromatic radiation at 1486.6 eV from an aluminum Kα source using a 500 mm Rowland circle silicon single crystal monochromator. The X-ray gun was operated using a 15 mA emission current at an accelerating voltage of 15 kV. Low energy electrons were used for charge compensation to neutralize the sample. High resolution spectra were acquired in the region of interest using the following experimental parameters: 20−40 eV energy window; pass energy of 20 eV; step size of 0.1 eV, dwell time of 1000 ms and an X-ray spot size of 700 × 300 μm. . The absolute energy scale was calibrated to the Cu 2p2/3 peak binding energy of 932.6 eV using an etched copper plate. From the surface analysis chamber, the sample was directly transferred to the auxiliary reaction chamber via the transfer antechamber by means of two sample transfer rods. The reaction chamber was fabricated from stainless steel and is approximately 3 L in volume and was retrofit with a leak valve, a pressure transducer (BOC Edwards WRG-S-NW35), a pumping system (Boc Edwards TIC) and two Pyrex glass windows. The pumping system consists of a rotary pump, a foreline trap (both BOC Edwards) and an EXT75DX turbomolecular pump with 60 L/s pumping capacity. The pumping system is separated from the reaction chamber by a hand valve (Granville-Philips Co.). For a typical reaction, NO2

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RESULTS AND DISCUSSION Atomic Force Microscopy of Unreacted CaCO3 (101̅4) Surfaces in the Presence of Relative Humidity at 296 K. The time sequence of AFM images of freshly cleaved CaCO3 (1014̅ ) surfaces exposed to 70% RH is shown in Figure 2, with that of freshly cleaved shown in Figure 2a. The surface exhibits steps of 0.32 ± 0.2 nm, characteristic of those of calcite cleaved in air.38,40 CaCO3 (101̅4) cleaved in air at low RH values (50 nN) loading forces, for a sample immersed into 60 μM CaCO3 solution, a contact force of >100 nN was required for step displacement to begin.48 Surfaces formed by spontaneous deposition at high supersaturations showed large defects,

dissipation properties similar to those observed in RH experiments earlier. Thus, we performed AFM force measurements to further elucidate the nature of different energy dissipation on these crystallites. Force Spectroscopy of CaCO3 (101̅4) Surfaces in the Presence of Relative Humidity at 296 K. Force spectroscopy was used to probe the force related properties of the particles formed on the CaCO3 (101̅4) surfaces in the presence of relative humidity and NO2. Force curves were acquired in contact mode on the CaCO3 (1014̅ ) surfaces prereacted with 10 mTorr NO2 for 30 min and then exposed to 70% RH for 30 min. Different spots selected to probe are shown in Figure 6. A total of five repetitions were performed in a single spot in four different spots representing particles formed and CaCO3 (101̅4) surfaces around (Figure 6a). A trigger point of 5 nN, force distance of 500 nm, and scan rate of 1 s with no dwell on the surface were used as typical settings. These results are shown in Figure 6b. There is more adhesion between the tip and the CaCO3 (101̅4) surfaces and/or first nucleation layer of the CaCO3 (101̅4) surface, than between the tip and the particles at 70% RH. This confirms the phase imaging data, where there was less energy dissipation on particles and more on the first nucleation layer. Consequentially, this energy dissipation phenomenon is at least partially due to the different adhesive forces between the tip and particles or first nucleation 9006

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including three-dimensional deposits (hillocks).27 Furthermore, deposition during scanning yielded atomically flat surfaces. Rates of enhanced growth were estimated up to 70 nm/min at 270 nN loading force. However, CaCO3 monolayer cleavage surface in ambient air (∼30% RH) were found to be totally resistant to tip induced wear.48 Rates of material movement in our experiments are significantly smaller (on the order of 5 nm/min), much less than those in pure water at ambient PCO2 (2.5−0.5 nm/s)33 and are not expected to originate from tip− sample induced artifacts. X-ray Photoelectron Spectroscopy of CaCO3 (101̅4) and Related Calcium-Containing Compounds. XPS spectra of the calcium and nitrogen containing reference compound nitrocalcite, as well as CaCO3 (101̅4) reacted under various environmental conditions, were acquired to better understand the surface chemistry of NO2 and H2O, two important atmospheric gases. The spectra of O1s and N1s regions are shown in Figure 7 and binding energy values

Table 1. Assignment of XPS Binding Energies in CaCO3 (1014̅ ) and Related Calcium-Containing Compounds binding energy, eV region

assigned species

this worka

O1s

CO32−

531.5 533.2

N1s

NO3− OH− NO3−

407.5

refb 531.9 (ref 20), 531.7 (ref 5), 531.2, 531.3, 531.4 (ref 69 and references therein) 533.2 (ref 70) 533.2 (ref 20) 407.4−407.6 9 (refs 75, 72, and 71)

a

Calibrated to the C1s peak at 285.0 eV. bCalibrated to the Au4f peak at 84.0 eV in ref 20, CO32− peak at 289.5 eV; Au4f peak at 84.0 eV in ref 69 and references therein; ref 75 calibrated to the C1s peak at 283.6 (values in Table 1 have been shifted by 1.4 eV from literature data); ref 72 referenced to the C1s peak in pump oil; ref 70 referenced to the C1s peak at 285 eV.

533.2 eV in nitrocalcite can be assigned to nitrate ion (NO3−) in nitrocalcite lattice. It correlates well with nitrate peak reported previously for NaNO3 at 533.2 eV.70 The shoulder at 533.2 eV in CaCO3 (101̅4) exposed to 10 mTorr NO2 at 70% RH spectrum unambiguously shows the presence of adsorbed nitrate ion (NO3−) on the surface of calcite. Additionally, the peak at 533.2 eV can also be contributed from crystalline water that can form in the process of recrystallization of the calcium carbonate on the surface and/or hydroxyl groups.20 This peak has increased twice in intensity from ∼7 to ∼15% of the total O1s area after the 10 mTorr NO2 reaction experiment thus showing that both adsorbed H2O/OH− and NO2 contribute to its intensity. In N1s region, the strong peak at 407.5 eV can be unambiguously assigned to nitrate ion (NO3−),71−73 either as structural (nitrocalcite) or when adsorbed on the surface (CaCO3 (101̅4) exposed to 10 mTorr NO2 at 70% RH). Thus, NO2 adsorbs strongly on calcite surfaces in the form of nitrate ion (NO3−). From XPS data presented above, several mechanistic aspects of these reactions became apparent. First, water molecules adsorb on calcite surface as indicated by the growth of a peak at 533.2 eV at ∼15 Å thickness, as reported recently67 H 2O(g) → H 2O(a)

(1)

Second, it can then dissociate to form surface hydroxyl/ bicarbonate groups via CaCO3(s) + H 2O(g) → Ca(OH)(HCO3)(s)

(2)

Third, when exposed to 10 mTorr NO2 for 30 min, the CaCO3 (101̅4) surface reacts to form adsorbed nitrate (NO3−) via reaction with surface oxygen or hydroxyl sites

Figure 7. XPS high resolution O1s and N1s binding energy regions following ex situ cleaved CaCO3 (1014̅ ), followed by reaction with water vapor at 70% RH for 180 min, followed by reaction with 10 mTorr NO2 for 30 min followed by further reaction with water vapor at 70% RH for 180 min, as well as a standard nitrocalcite surface. All reactions and measurement were performed at 296 K. Assignments of different photoelectron peaks are given in Table 1.

NO2 (g) + O−(s) → NO3−(s)

(3)

Fourth, at high relative humidity values, adsorbed water layer can react directly with the adsorbing NO2 to form HNO3 and HONO16

summarized in Table 1 together with literature and reference values. To assign the peaks arising from photoelectron emission from this mineral, we used binding energy values of known organic and inorganic compounds previously reported in the literature. The carbonate (CO32−) peak in the O1s region, originating from bulk calcium carbonate is located at 531.5 eV, close to the experimental values reported previously.5,20,69 The peak at

H 2O(a) + 2NO2 (a) → HNO3(a) + HONO(g)

(4)

The similarity between the XPS N1s region spectra of nitrocalcite and the CaCO3 (101̅4) after reaction with NO2 indicates that the final product is calcium nitrate, Ca(NO3)2, on the calcite surface. Additionally, the binding energy of the crystalline water in nitrocalcite overlaps with that of the nitrate (NO3−) peak at 533.2 eV, thus not allowing us to distinguish 9007

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between trapped H2O and NO3−. Ni and Ratner in their comprehensive XPS and TOF-SIMS investigation showed that pure calcium carbonate polymorphs have differences in their Ca2p and O1s spectra.74 From the AFM data presented here it can be seen that CaCO3 (1014̅ ) surfaces under environmental conditions can possess several phases simultaneously thus making their unambiguous identification difficult. Importantly, NO2 reacts with calcite surfaces regardless of the order in which gases are introduced. This shows that different adsorption sites may be involved in H2O and NO2 adsorption. While NO2 adsorbs on surface oxygen sites on nonhydroxylated CaCO3 (101̅4), it can also adsorb on reactive hydroxyl groups on the surface exposed to 70% RH or react with molecular water via reaction 4.

CONCLUSIONS AC mode atomic force microscopy, combined with phase imaging and XPS spectroscopy was employed to investigate the surface reactivity of CaCO3 (101̅4) toward NO2 in the presence of water as RH. Freshly cleaved surfaces, as well as surfaces reacted with NO2 were investigated at 296 K. Topography images provided information on the time evolution of the nucleation products on the surface, whereas phase imaging combined with force measurements provided information on the inhomogeneous energy dissipation areas present on the surface. A complex two-layer based mechanism takes place when CaCO3 (101̅4) is exposed to RH and NO2, depending on the order these atmospheric gases are introduced. XPS analysis shows that the NO2 adsorbed product was nitrate (NO3−) and that the first nucleation layer, possibly metastable hydrated form of calcite, was a source of material for both more stable vaterite and nitrocalcite phases. ASSOCIATED CONTENT

S Supporting Information *

Details on AFM phase imaging and force spectroscopy are provided. This information is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Deer, W. A.; Howie, R. A.; Zussman, J. An Introduction to the Rock-Forming Minerals; Prentice Hall Englewood Cliffs, NJ, 1966. (2) Laskin, A.; Iedema, M. J.; Ichkovich, A.; Graber, E. R.; Taraniuk, I.; Rudich, Y. Faraday Discuss. 2005, 130, 453−468. (3) Al-Hosney, H. A.; Grassian, V. H. Phys. Chem. Chem. Phys. 2005, 7, 1266−1276. (4) Usher, C. R.; Baltrusaitis, J.; Grassian, V. H. Langmuir 2007, 23, 7039−7045. (5) Baltrusaitis, J.; Usher, C. R.; Grassian, V. H. Phys. Chem. Chem. Phys. 2007, 9, 3011−3024. (6) Pokrovsky, O. S.; Mielczarski, J. A.; Barres, O.; Schott, J. Langmuir 2000, 16, 2677−2688. (7) Lea, A. S.; Amonette, J. E.; Baer, D. R.; Liang, Y.; Colton, N. G. Geochim. Cosmochim. Acta 2001, 65, 369−379. (8) Pokrovsky, O. S.; Schott, J. Environ. Sci. Technol. 2002, 36, 426− 432. (9) Jones, C. E.; Unwin, P. R.; Macpherson, J. V. ChemPhysChem 2003, 4, 139−146. (10) Zuddas, P.; Pachana, K.; Faivre, D. Chem. Geol. 2003, 201, 91− 101. (11) Dobson, P. S.; Bindley, L. A.; Macpherson, J. V.; Unwin, P. R. Langmuir 2005, 21, 1255−1260. (12) Morse, J. W.; Arvidson, R. S. Earth-Sci. Rev. 2002, 58, 51−84. (13) Felix, J. D.; Elliott, E. M.; Shaw, S. L. Environ. Sci. Technol. 2012, 46, 3528−3535. (14) Zhang, R.; Tie, X.; Bond, D. W. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 1505−1509. (15) Finlayson-Pitts, B. J.; Wingen, L. M.; Sumner, A. L.; Syomin, D.; Ramazan, K. A. Phys. Chem. Chem. Phys. 2003, 5, 223−242. (16) Li, H. J.; Zhu, T.; Zhao, D. F.; Zhang, Z. F.; Chen, Z. M. Atmos. Chem. Phys. 2010, 10, 463−474. (17) Goodman, A. L.; Miller, T. M.; Grassian, V. H. J. Vac. Sci. Technol., A 1998, 16, 2585−2590. (18) Liu, C.; Ma, Q.; Liu, Y.; Ma, J.; He, H. Phys. Chem. Chem. Phys. 2012, 14, 1668−1676. (19) Saliba, N. A.; Chamseddine, A. Atmos. Environ. 2012, 46, 256− 263. (20) Stipp, S. L.; Hochella, M. F., Jr. Geochim. Cosmochim. Acta 1991, 55, 1723−1736. (21) Kendall, T. A.; Martin, S. T. J. Phys. Chem. A 2007, 111, 505− 514. (22) Krueger, B. J.; Ross, J. L.; Grassian, V. H. Langmuir 2005, 21, 8793−8801. (23) Stipp, S. L. S.; Konnerup-Madsen, J.; Franzreb, K.; Kulik, A.; Mathieu, H. J. Nature (London) 1998, 396, 356−359. (24) Hochella, M. F., Jr. Mineral. Soc. Series 1995, 5, 17−60. (25) Komatsu, H.; Miyashita, S.; Nakada, T.; Sazaki, G.; Chernov, A. A. Advances in the Understanding of Crystal Growth Mechanisms; Elsevier: Amsterdam, 1997; pp 515−529. (26) Arvidson, R. S.; Ertan, I. E.; Amonette, J. E.; Luttge, A. Geochim. Cosmochim. Acta 2003, 67, 1623−1634. (27) McEvoy, A. L.; Stevens, F.; Langford, S. C.; Dickinson, J. T. Langmuir 2006, 22, 6931−6938. (28) Hillner, P. E.; Gratz, A. J.; Manne, S.; Hansma, P. K. Geology 1992, 20, 359−362. (29) Hillner, P. E.; Manne, S.; Gratz, A. J.; Hansma, P. K. Ultramicroscopy 1992, 42−44, 1387−1393. (30) Gratz, A. J.; Hillner, P. E.; Hansma, P. K. Geochim. Cosmochim. Acta 1993, 57, 491−495. (31) Liang, Y.; Baer, D. R.; McCoy, J. M.; Amonette, J. E.; LaFemina, J. P. Geochim. Cosmochim. Acta 1996, 60, 4883−4887. (32) Liang, Y.; Baer, D. R. Surf. Sci. 1997, 373, 275−287. (33) Jordan, G.; Pammensee, W. Geochim. Cosmochim. Acta 1998, 62, 941−947. (34) Teng, H. H.; Dove, P. M.; Orme, C. A.; De Yoreo, J. J. Science 1998, 282, 724−727. (35) Shiraki, R.; Rock, P. A.; Casey, W. H. Aquatic Geochem. 2000, 6, 87−108.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: J.B., [email protected]; V.H.G., [email protected]. Present Address †

Current address: PhotoCatalytic Synthesis Group, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, Meander 225, P.O. Box 217, 7500 AE Enschede, The Netherlands.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on upon work partially supported by the National Science foundation under Grant CHE-0952605. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We thank Keith Jones, Jason Li, Jeff Honeyman, and Deron Walters from Asylum Research for countless discussions regarding AFM AC mode combined with phase imaging. We also acknowledge the Central Microscopy Research Facility (CMRF) for instrumental resources. 9008

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(36) Teng, H. H.; Dove, P. M.; De Yoreo, J. J. Geochim. Cosmochim. Acta 2000, 64, 2255−2266. (37) Hausner, D. B.; Reeder, R. J.; Strongin, D. R. J. Colloid Interface Sci. 2006, 305, 101−110. (38) Stipp, S. L. S.; Gutmannsbauer, W.; Lehmann, T. Am. Mineral. 1996, 81, 1−8. (39) Stipp, S. L. S. Geochim. Cosmochim. Acta 1999, 63, 3121−3131. (40) Baltrusaitis, J.; Grassian, V. H. Surf. Sci. 2009, 603, L99−L104. (41) Na, C.; Kendall, T. A.; Martin, S. T. Environ. Sci. Technol. 2007, 41, 6491−6497. (42) Sun, J.-L.; Hu, J.; Zhang, Y.; Chen, S.-F.; Ouvang, Z.-Q.; Li, M.Q. Nucl. Sci. Tech. 1998, 9, 90−91. (43) Fairley, N. CasaXPS. In CasaXPS; 2.3.14dev39 ed.; http://www. casaxps.com/, 1999−2012. (44) Britt, D. W.; Hlady, V. Langmuir 1997, 13, 1873−1876. (45) Gratz, A. J.; Hillner, P. E. J. Cryst. Growth 1993, 129, 789−793. (46) Liang, Y.; Baer, D. R.; McCoy, J. M.; LeFemina, J. P. J. Vac. Sci. Technol., A 1996, 14, 1368−1375. (47) De Giudici, G. Am. Mineral. 2002, 87, 1279−1285. (48) Park, N.-S.; Kim, M.-W.; Langford, S. C.; Dickinson, J. T. Langmuir 1996, 12, 4599−4604. (49) Garcia, R.; Tamayo, J.; Calleja, M.; Garcia, F. Appl. Phys. A: Mater. Sci. Process. 1998, 66, S309−S312. (50) Magonov, S. N.; Elings, V.; Whangbo, M. H. Surf. Sci. 1997, 375, L385−L391. (51) Bar, G.; Brandsch, R.; Whangbo, M.-H. Langmuir 1998, 14, 7343−7347. (52) Kopp-Marsaudon, S.; Leclere, P.; Dubourg, F.; Lazzaroni, R.; Aime, J. P. Langmuir 2000, 16, 8432−8437. (53) James, P. J.; Antognozzi, M.; Tamayo, J.; McMaster, T. J.; Newton, J. M.; Miles, M. J. Langmuir 2001, 17, 349−360. (54) Aime, J. P.; Boisgard, R.; Nony, L.; Couturier, G. J. Chem. Phys. 2001, 114, 4945−4954. (55) Nony, L.; Cohen-Bouhacina, T.; Aime, J. P. Surf. Sci. 2002, 499, 152−160. (56) Paredes, J. I.; Gracia, M.; Martinez-Alonso, A.; Tascon, J. M. D. J. Colloid Interface Sci. 2005, 288, 190−199. (57) Tan, S.; Sherman, R. L., Jr.; Qin, D.; Ford, W. T. Langmuir 2005, 21, 43−49. (58) Clarkson, J. R.; Price, T. J.; Adams, C. J. J. Chem. Soc., Faraday Trans. 1992, 88, 243−249. (59) Brooks, R.; Clark, L. M.; Thurston, E. F. Trans. R. Soc. London 1950, A243, 145−167. (60) Bischoff, J. L.; Fitzpatrick, J. A.; Rosenbauer, R. J. J. Geol. 1993, 101, 21−33. (61) Suess, E.; Balzer, W.; Hesse, K. F.; Mueller, P. J.; Ungerer, C. A.; Wefer, G. Science 1982, 216, 1128−1131. (62) Jansen, J. H. F.; Woensdregt, C. F.; Kooistra, M. J.; Van der Gaast, S. J. Geology 1987, 15, 245−248. (63) Tsuno, H.; Kagi, H.; Akagi, T. Chem. Lett. 2002, 960−961. (64) Ito, T.; Matsubara, S.; Miyawaki, R. J. Mineral. Petrol. Econ. Geol. 1999, 94, 176−182. (65) Tlili, M. M.; Ben Amor, M.; Gabrielli, C.; Joiret, S.; Maurin, G.; Rousseau, P. J. Raman Spectrosc. 2002, 33, 10−16. (66) Brecevic, L.; Nielsen, A. E. J. Cryst. Growth 1989, 98, 504−510. (67) Bohr, J.; Wogelius, R. A.; Morris, P. M.; Stipp, S. L. S. Geochim. Cosmochim. Acta 2010, 74, 5985−5999. (68) Park, N.-S.; Kim, M.-W.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 1996, 80, 2680−2686. (69) Blanchard, D. L., Jr.; Baer, D. R. Surf. Sci. 1992, 276, 27−39. (70) Aduru, S.; Contarini, S.; Rabalais, J. W. J. Phys. Chem. 1986, 90, 1683−1688. (71) Overbury, S. H.; Mullins, D. R.; Huntley, D. R.; Kundakovic, L. J. Catal. 1999, 186, 296−309. (72) Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Inorg. Chem. 1969, 8, 2642−2647. (73) Wagner, A. J.; Wolfe, G. M.; Fairbrother, D. H. Appl. Surf. Sci. 2003, 219, 317−328. (74) Ni, M.; Ratner, B. D. Surf. Interface Anal. 2008, 40, 1356−1361.

(75) Burger, K.; Tschismarov, F.; Ebel, H. J. Electron Spectrosc. Relat. Phenom. 1977, 10, 461−465.

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