Formation of Microcrystals, Micropuddles, and Other Spatial

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Formation of Microcrystals, Micropuddles, and Other Spatial Inhomogenieties in Surface Reactions under Ambient Conditions: An Atomic Force Microscopy Study of Water and Nitric Acid Adsorption on MgO(100) and CaCO3(101 h 4) Brenda J. Krueger,† Jean L. Ross,‡ and Vicki H. Grassian*,† Department of Chemistry and the Central Microscopy Research Facility, The University of Iowa, Iowa City, Iowa 52242 Received May 24, 2005. In Final Form: July 7, 2005 In this study, atomic force microscopy (AFM) is used to image freshly cleaved MgO(100) and CaCO3(101h 4) as these surfaces undergo reaction with water and nitric acid under ambient conditions of temperature, pressure, and relative humidity. The reaction of water and nitric acid results in the formation of hydroxylated and nitrated surfaces, respectively. It is clear from the AFM images that there are spatial inhomogenieties and surface features that form on micrometer and nanometer length scales as these reactions proceed. These features, which include hillocks, patches, microcrystallites, and micropuddles, are due to surface and phase segregation as a result of facile ion mobility in the presence of adsorbed water. In addition, instabilities and oscillations in the AFM images provide an indication of liquid formation and the deliquescence (i.e., a solid to liquid-phase transition) of nitrate salts as a function of relative humidity.

Introduction Scanning probe microscopy (SPM), which includes scanning tunneling and atomic force microscopy (STM and AFM), is becoming an increasingly important tool in modern chemical research.1-10 The interaction of the probe tip with a sample can yield important information on the nature of the sample surface including the spatial arrangement of atoms and molecules, magnetic field heterogeneities, and the local electronic configuration on micrometer, nanometer, and atomic length scales. SPM often provides a more detailed molecular understanding of the chemical problem being investigated. For example, if there are two chemically distinct species on a surface it can be difficult to tell from spectroscopy alone whether the two species are homogeneously mixed on the surface or if there are distinct boundaries or phase separation of these two species. Spectroscopic probes give a signature of an ensemble average of the entire sample but do not easily yield any information as to the heterogeneity of the ensemble, whereas because of the inherent spatial resolution of the technique SPM can resolve this issue. In addition, SPM techniques have opened up a new dimension to imaging studies because these techniques can be used under a variety of conditions including ambient environ* To whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry. ‡ Central Microscopy Research Facility. (1) Poirier, G. Chem. Rev. 1997, 97, 1117-1128. (2) Takano, H.; Kenseth, J. R.; Wong, S.-S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845. (3) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (4) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (5) Guo, X.-C.; Madix, R. J. Acc. Chem. Res. 2003, 36, 471. (6) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520. (7) Gracias, D. H.; Chen, Z.; Shen, Y. R.; Somorjai, G. A. Acc. Chem. Res. 1999, 32, 930. (8) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491. (9) Kubby J. A.; Boland, J. J. Surf. Sci. Rep. 1996, 26, 61. (10) Ikai, A. Surf. Sci. Rep. 1996, 26, 261.

ments as well as in solution. There have been several studies on the use of scanning probe techniques to study the nature of surfaces under ambient conditions of temperature and relative humidity.11-16 However, little has been done to utilize scanning probe techniques to investigate the reaction chemistry of surfaces under these conditions. As shown here, AFM can provide new details of the reaction chemistry of oxide and carbonate surfaces under ambient conditions. Because oxides are important heterogeneous catalysts,17,18 biomedical materials,19 and environmental interfaces20-22 and carbonates are ubiquitous minerals in biological and geochemical systems that play a major role in global CO2 exchange23 and represent a reactive component of the mineral dust aerosol present in the troposphere,25,26 understanding the reaction chemistry of oxide and carbonate surfaces under ambient conditions is important for many reasons. (11) Fenter, P.; McBride, M. T.; Srager, G.; Sturchio, N. C.; Bosbach, D. J. Phys. Chem. B 2001, 105, 8112. (12) Bokern, D. G.; Ducker, W. A. C.; Hunter, K. A.; McGrath, K. M. J. Cryst. Growth 2002, 246, 139. (13) Schlegel, M. L.; Nagy, K. L.; Fenter, P.; Sturchio, N. C. Geochim. Cosmochim. Acta 2002, 66, 3037. (14) Stipp, S. L. S.; Gutmannsbauer, W.; Lehmann, T. Am. Mineral. 1996, 81, 1. (15) Dokou, E.; Zhang, L.; Barteau, M. A. J. Vac. Sci. Technol., B 2002, 20, 2183. (16) Xu, L.; Bluhm, H.; Salmeron, M. Surf. Sci. 1998, 407, 251. (17) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994. (18) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides; Wiley: New York, 190. (19) Jones, F. H. Surf. Sci. Rep. 2001, 42, 75. (20) Stumm, W. Chemistry of the Solid-Water Interface; John Wiley and Sons: New York, 1992. (21) Brown, G. E., Jr.; Henrich, V. E.; Case, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gratzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Chem. Rev. 1999, 99, 77. (22) Al-Abadleh, H. A.; Grassian, V. H. Surf. Sci. Rep. 2003, 52, 63. (23) Robbins, L. L.; Fabry. V. J. In Carbon Dioxide Chemistry: Environmental Issues; Paul, J., Pradier, C., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1994; p 301.

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The particular surfaces of interest here include freshly cleaved MgO(100) and CaCO3(101 h 4). The chemistry of H2O and HNO3 on these two surfaces is investigated. We are interested in understanding the molecular-level details of these reactions, and it is our intent here to show how AFM can be used to learn more about the spatial inhomogenieties of these reactions. Spatial inhomogenieties in surface reactions involving NaCl surfaces are well documented. Allen et al. used electron microscopy to show that NaNO3 microcrystals form following the reaction of HNO3 with NaCl particles.27,28 Elegant AFM studies by Zangmeister and Pemberton further demonstrate important spatial features of reactions involving HNO3 and H2SO4 on NaCl surfaces29,30 that could not be easily discerned from spectroscopic studies alone. In particular, it was shown that mobile strings of NaNO3 form from the reaction of nitric acid and NaCl.29 These strings eventually result in the formation of NaNO3 microcrystallites. Mobile structures of NaHSO4 were also seen for the reaction of H2SO4 on NaCl. In addition, phase separation and phase transitions between two NaHSO4 phases, R and β, were observed.30 Several previous AFM studies have shown that MgO(100) and CaCO3(101 h 4) surfaces exposed to the ambient environment undergo surface roughening with the formation of nanometer-sized spatial features that result from the reaction of water with these surfaces.31-36 The current study expands upon these previously published AFM results and uses AFM to further probe the surface chemistry of water and nitric acid with these two surfaces. As discussed here, hillocks, patches, microcrystallites, micropuddles, and other spatial inhomogenieties form in surface reactions of water and nitric acid on h 4) under ambient conditions of MgO(100) and CaCO3(101 temperature and relative humidity.

were adjusted to obtain the best image quality. These scan rates correspond to image acquisition times of ∼ 8-10 min. Because of the use of a tube scanner, which causes a flat surface to have a bowl shape, all images were flattened using an AFM shareware program WSxM (http://www.nanotec.es). This program is similar to the software included with the AFM instrument. A fluid cell attachment was used to control the vapor above the sample. The fluid cell has a volume of 0.08 mL.29 A silicon O-ring along with self-sealing tubing connected to both the inlet and outlet port was used to isolate the cell. To control reactant concentrations, the outlet port was sealed; however, when a dry N2 purge was needed to flush out the cell, the outlet tubing was opened. The reactants were contained in a 4 mL vial with a rubber septum cap. A syringe of known volume was inserted into the vial, and a known amount of vapor from the headspace was taken. The vapor was then injected into the tubing connected to the inlet port for reaction studies. The ambient temperature was 296 K in these studies. Single crystals of calcium carbonate (101h 4) and magnesium oxide (100) were cleaved using a sharp edge. Single crystals of MgO were purchased from M. T. I. Corporation and were freshly cleaved prior to reaction. Single crystals of Icelandic spar calcite, CaCO3, were purchased from Atomergic Chemicals Corporation and were also freshly cleaved prior to reaction. The cleaved samples were ∼1 cm on each side. A 3:1 solution of sulfuric acid/nitric acid (ACS PLUS Certified, Fisher Scientific) was used to ensure high-purity, dry nitric acid vapor. The nitric acid solution was transferred in 1 mL aliquots into the vials in an N2 environment. Prior to transfer of the reactive gas, the fluid cell was purged with nitrogen so that the relative humidity could be determined by the amount of water vapor added (Optima water, Fisher Scientific). Depending on the desired vapor pressure, a 1 mL, 10 µL, or 100 µL syringe was used to introduce the vapor into the fluid cell. The vapor pressures of nitric acid and water are 7.5 and 21.1 Torr, respectively, at the temperatures used in this study.

Experimental Section

h 4) ExFreshly Cleaved MgO(100) and CaCO3(101 posed to the Ambient Environment at ca. 15% RHsSurface Roughness and Hillock Formation Due to Surface Hydroxylation. It is well known that MgO and CaCO3 surfaces undergo hydroxylation under ambient conditions through reaction with atmospheric water.31-36 There have been a few studies that have shown that AFM can be used to investigate the spatial heterogeneity of the hydroxylation process. In a prelude to our studies of the surface chemistry of HNO3 with freshly cleaved h 4) under ambient conditions of MgO(100) and CaCO3(101 temperature and relative humidity, we have monitored the hydroxylation process under ambient conditions. Figure 1a shows three panels of AFM data taken within the first few hours of cleaving a MgO single crystal and exposing the (100) surface plane to the ambient environment. The room temperature was constant at 296 K, and the relative humidity was 15 ( 2% RH. The left and middle panels show height images at two different magnifications. The left panel shows a 3 µm × 3 µm image, and the middle panel shows a 1 µm × 1 µm image. The line profile plot in the right panel indicates that the surface soon after cleaving is relatively smooth with steps showing minimal roughening and surface features on the order of a monatomic step. To quantify the surface roughness, the root-mean-square roughness, R, is determined. To calculate R, height variations from an average height are used. It has been previously shown that factors such as the AFM mode of operation as well as image size can effect the values obtained for RMS roughness.38 Simpson et al.

Noncontact tapping-mode AFM was used to collect all of the images shown here because tip effects were evident in contactmode AFM measurements.37 A Nanoscope IIIa MultiMode AFM instrument equipped with an extender electronics module for phase-imaging measurements was used in these experiments (Veeco). The contrast in the phase image was used as a qualitative tool to detect spatial variations of the chemical properties of the surface. Scan rates for all images ranged from 0.75 to 1.5 Hz and (24) Sundquist, E. T. In The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present; Sundquist, E. T., Broecker, W. S., Eds.; Geophysical Monograph 32; American Geophysical Union: Washington, DC, 1985; pp 5-59. (25) Usher, C. R.; Michel, A. E.; Grassian, V. H. Chem. Rev. 2003, 103, 4883. (26) Song, C. H.; Carmichael, G. R. Atmos. Chem. 2001, 40, 1. (27) Allen, H. C.; Laux, J. M.; Vogt, R.; FinlaysonPitts, B. J.; Hemminger, J. C. J. Phys. Chem. 1996, 100, 6371. (28) Allen, H. C.; Mecartney, M. L,; Hemminger, J. C. Microsc. Microanal. 1998, 4, 23. (29) Zangmeister, C. D.; Pemberton, J. E. J. Phys. Chem. B 1998, 102, 8950. (30) Zangmeister, C. D.; Pemberton, J. E. J. Am. Chem. Soc. 2000, 122, 12289. (31) Stipp, S. L. S.; Eggleston, C. M.; Nielsen, B. S. Geochim. Cosmochim. Acta 1994, 58, 3023. (32) Stipp, S. L. S.; Gutmannsbauer, W.; Lehmann, T. Am. Mineral. 1996, 81, 1. (33) Stipp, S. L. S. Geochim. Cosmochim. Acta 1999, 63, 3121. (34) Liu, P.; Kendelewicz, T.; Brown, G. E., Jr.; Parks, G. A. Surf. Sci. 1998, 412/413, 287. (35) Aswal, D. K.; Muthe, K. P.; Tawde, S.; Chodhury, S.; Bagkar, N.; Singh, A.; Gupta, S. K.; Yakhmi, J. V. J. Cryst. Growth 2002, 236, 661. (36) Liu, P.; Kendelewicz, T.; Nelson, E.; Parks, G. A.; Brown, G. E., Jr. SSRL Activity Report - Experimental Progress Reports 1997, 7, 287. (37) Krueger, B. Ph.D. Dissertation, University of Iowa, Iowa City, IA, 2005.

Results

(38) Dokou, E.; Zhang, L.; Barteau, M. A. J. Vac. Sci. Technol., B 2002, 20, 2183.

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Figure 1. MgO(100) surface cleaved and imaged with tapping-mode AFM under ambient conditions: left panel, 3 µm × 3 µm AFM images (600 nm scale bar); center panel, 1 µm × 1 µm images (210 nm scale bar); and right panel, line profiles. Boxes in the images shown in the left panel are the 1 µm × 1 µm images shown in the center panel. Lines drawn in the images shown in the center panel correspond to the profile plots shown in the right panel. Images are shown after cleaving the MgO crystal to expose the (100) surface to the ambient environment for different amounts of time: (a) 2 h (0 days), (b) 2 days, (c) 6 days, (d) 13 days, and (e) 21 days.

have done a study on the surface roughness calculations in contact-mode and tapping-mode atomic force microscopy and concluded that tapping-mode AFM provides a better measure of surface roughness.39 Using the 1 µm × 1 µm images shown in the middle panel of Figure 1a, R is calculated from the following equation

where N is the number of pixels in the image, zi is the height of the ith pixel, and zm is the mean image height.38 The calculated value of R is 0.42 nm and is on the order of one monatomic step height and one unit cell.34,35,40

The remaining images shown in Figure 1b-e were obtained over a 3-week time period. Because of the relatively long time period, the samples were moved out of the AFM head between measurements. When the surfaces were imaged again, attempts were made to put the sample back in the AFM in such a way that images of the same or nearby regions could be obtained. The remaining AFM data shown in Figure 1b-e clearly show that the surface changes as it is exposed to the ambient environment. After 2 days at ca. 15% RH, there is evidence of “white spots” in the image. As seen in both the 3 µm × 3 µm and 1 µm × 1 µm images, the number and size of these spots grow over the time. It is evident that the spots are nucleating at the steps and the number of spots grows as a function of time. Rows of white spots decorating

(39) Simpson, G. J.; Sedin, D. L.; Rowlen, K. L. Langmuir 1999, 15, 1429.

(40) Sangwal, K.; Gorostiza, P.; Sanz, F.; Borc, J. Cryst. Res. Technol. 2000, 35, 959.

(zi|zm)2 |1/2 i)1 N|1 N

R)|

∑

(1)

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Table 1. Root-Mean-Square Surface Roughness Measurements Following Exposure of MgO(100) and CaCO3(101 h 4) to the Ambient Environment at ca. 15% RH MgO

2h

2 days

6 days

13 days

21 days

RMS roughness (nm)

0.42

0.34

1.10

1.68

2.82

CaCO3

5.5 h

3 days

7 days

13 days

21 days

RMS roughness (nm)

0.38

0.33

0.47

0.45

0.51

the steps are clearly seen in the images collected 6 and 13 days after exposing the cleaved MgO(100) surface to the ambient environment. The white spots on the surface grow in height and width, indicating that the surface is becoming quite rough. The line profiles shown in the panels on the right in Figure 1b-e suggest that as the surface is exposed to the ambient environment the white spots are now best described as hillocks, in agreement with the results of Liu et al.34 To quantify the surface roughness, R is calculated for the 1 µm × 1 µm image shown in Figure 1b-e. These values of R for the different images are tabulated in Table 1. It is clear from the data in Table 1 that the surface roughens considerably and is a factor of 6 times rougher after 21 days compared to what it was initially a few hours after cleaving the surface. The white spots that grow into hillocks are due in large part to the dissociative adsorption of water to form Mg(OH2) on the surface according to reaction 2

MgO + H2O f Mg(OH)2

(2)

Similar to what is observed here, a recent Fourier transform infrared (FTIR) spectroscopy study of freshly cleaved MgO(100) also indicates that the surface hydroxylation process occurred over a time period of several days. Following initial cycles of water adsorption/desorption at room temperature, there were no absorption bands evident in the FTIR spectrum indicating the formation of hydroxyl groups on the surface. However, after several days of repeated adsorption/desorption cycles, a sharp feature at 3700 cm-1, due to the formation of surface hydroxyl groups, is observed.41 In a recent AFM study done in conjunction with X-ray photoelectron spectroscopy (XPS), Aswal et al. showed that a MgO(100) surface exposed to the ambient environment at higher relative humidity of ∼70% for 2 days consisted of Mg(OH)2 and MgCO3, indicating that the magnesium oxide surface is reactive not only toward water vapor in the environment but also toward carbon dioxide.35 Magnesium carbonate is formed from the reaction of the MgO surface with CO2 present under ambient conditions according to reaction 3

MgO + CO2 f MgCO3

(3)

The magnesium hydroxide on the surface has also been shown to react with carbon dioxide in the following reaction to form magnesium carbonate35

Mg(OH)2 + CO2 f MgCO3 + H2O

(4)

The relative importance of the direct reaction of CO2 with MgO(100) versus the reaction of CO2 with Mg(OH2) on the surface is not well documented. In addition, it is also (41) Foster, M.; Furse, M.; Passno, D. Surf. Sci. 2002, 502-503, 102.

unclear whether the kinetics and extent of formation of surface carbonate are relative-humidity-dependent. In contrast to the increase in surface roughness and the formation of hillocks on the magnesium oxide surface, the time-dependent behavior of freshly cleaved calcium carbonate is very different. Figure 2 shows AFM data for single-crystal calcium carbonate after cleaving to expose the (101 h 4) surface plane to the ambient environment. The left and middle panels of Figure 2a show height images for CaCO3(101 h 4) 5.5 h after cleaving the crystal and exposing the surface to the ambient environment. Even in these early images, the formation of surface inhomogenieties is seen. At first these inhomogenieties appear along step edges as indicated by the arrows in height images shown in Figure 2a. A few nucleation sites within the terraces are also observed. As the surface is exposed to the ambient environment for longer periods of time, more and more of the surface begins to change as seen in the height images shown in Figure 2b-e. The line profiles also shown in Figure 2 are informative and provide an indication of the differences in the change in surface h 4) is exposed to the structure that results as CaCO3(101 ambient environment compared to MgO(100). The line profiles indicate that that the initial surface is fairly flat and then as the surface changes it is not roughening to the same degree found for the magnesium oxide surface. The line profiles shown in Figure 2 are on the same scale as in Figure 1 so that these can be directly compared. The calculated values of the surface roughness for each of the 1 µm × 1 µm images shown in Figure 2 are tabulated h 4) surface in Table 1. The data show that the CaCO3(101 roughens over the time scale that these measurements were made but much less than what was observed for the MgO(100). The spatial inhomogenieties observed in these images appear to be more like patches than hillocks. Previous studies have determined that freshly cleaved h 4) surfaces exposed to the ambient environment CaCO3(101 undergo calcium carbonate hydroxylation to yield surface species identified as S‚CO3H and S‚CaOH (where S‚ represents the calcite surface) in XPS studies.42 The hydroxylation occurs according to reaction

CaCO3 + H2O f Ca(OH)(CO3H)

(5)

The hydroxlyation of calcium carbonate surfaces according to reaction 5 has also been confirmed in FT-IR studies.43 As proposed in the Discussion section, one possible reason that the surface roughens less in the carbonate reaction may be due to differences in the surface mobility of the bicarbonate ion compared to that of the hydroxide ion. If there is less surface mobility, then the tendency to form larger surface structures will be suppressed. Further evidence for the importance of ionic mobility in the growth of surface features is presented in the next section. Freshly Cleaved MgO(100) Exposed to Nitric Acid at 25% RHsThe Formation of Mg(NO3)2 Microcrystalline‚Hydrates and Phase Transformations of Mg(NO3)2 as an f(RH). As discussed in the Introduction, surface reaction of nitric acid with NaCl forms well-defined microcrystallites of NaNO3.27-29 AFM data including 3D and height images and line profiles show that similar microstructures are seen upon reaction of MgO(100) with nitric acid. The AFM data shown in Figure 3a and b are for a 1 µm × 1 µm area of a freshly cleaved MgO(100) before and (42) Stipp, S. L.; Hochella, M. F., Jr. Geochim. Cosmochim. Acta 1991, 55, 1723. (43) Neagle, W.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1990, 86, 181.

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Figure 2. CaCO3(101 h 4) surface cleaved and imaged with tapping-mode AFM under ambient conditions: left panel, 3 µm × 3 µm AFM images (600 nm scale bar); center panel, 1 µm × 1 µm images (210 nm scale bar); and right panel, line profiles. Boxes in the images shown in the left panel are the 1 µm × 1 µm images shown in the center panel. Lines drawn in the images shown in the center panel correspond to the profile plots shown in the right panel. Images are shown after cleaving the CaCO3 crystal to expose the (101 h 4) surface to the ambient environment for different amounts of time: (a) 5.5 h (0 days), (b) 3 days, (c) 7 days, (d) 13 days, and (e) 21 days.

after reaction with nitric acid vapor at 15% RH, respectively. The image taken after reaction was recorded at low relative humidity under a dry nitrogen purge over the sample. There are clearly new structures present on the surface after reaction with nitric acid vapor. On the basis of previous infrared studies,44 these surface features are concluded to be microcrystals of magnesium nitrate that appear to nucleate on steps that are still evident on the surface. The microcrystals show evidence for polygon-type symmetry and are between 0.2 and 1 µm in width and ∼45 nm in height. The outline of a hexagon has been superimposed on one of the microcrystallites to see this symmetry better. The other AFM images, shown in Figure 3c-e, have been recorded of the nitric acid MgO(100) surface but at higher relative humidity. The line profiles shown next to the AFM images in c and d indicate that as the relative humidity increases the height of the microcrystals in-

creases by approximately 20%. This increase in height is consistent with infrared studies that have shown that as the relative humidity increases, crystalline magnesium nitrate hydrate can incorporate water into its structure.44 The infrared data suggest that the crystalline magnesium nitrate hydrate goes from a tetra- to a penta- to a hexahydrate salt. The AFM data indicate that as water is incorporated into the structure it swells along the surface normal. At the highest relative humidity (Figure 3e), the AFM image shows an increase in the number of smaller features on the surface. Besides an increase in the number of small surface features, there is a nearly a doubling in the height of the larger surface structure as seen in the line profiles. The surface begins to become difficult to image, and there (44) Al-Abadleh, H. A.; Grassian, V. H. J. Phys. Chem. B 2003, 107, 10829.

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Figure 3. Tapping-mode AFM images (3 µm × 3 µm) viewed as 3D (left panel) and height (center panel, 600 nm scale bar) and line profiles (right panel) for a MgO(100) surface: (a) ∼2 h after cleaving under ambient conditions (25 ( 5% RH) and (b) after the surface was exposed to 1 mL of HNO3 vapor, allowed to sit overnight, and then purged with dry N2. The surface was then exposed to increasing aliquots of water vapor corresponding to (c) 12.5, (d) 25, and (e) 75% RH. The outline of a hexagon has been placed around one of the structures, proposed to be magnesium nitrate, that forms on the surface; see text for further details.

is an instability in the image, especially on the tops of the larger structures as indicated in what appears to be oscillations and larger variations in the line profiles. These larger variations in the line profiles are reproducible. The changes in height and the instability and oscillations in the images of the surface are all consistent with the deliquescence of the salt structures at the higher relative humidity as determined with FTIR spectroscopy. h 14) Exposed to Nitric Freshly Cleaved CaCO3(101 Acid at 25% RHsThe Formation of Ca(NO3)2 Micropuddles. AFM data including phase and height h 4) before and images along with line profiles of CaCO3(101 after reaction with nitric acid are presented in Figure 4. h 4) 1 µm × 1 µm image of The freshly cleaved CaCO3(101

the surface shown in Figure 4a shows features ∼1-2 nm in height due to hydroxyl groups adsorbed to the surface under ambient conditions of 24 ( 5% RH. The surface of this particle shows extensive hydroxyl formation in a short amount of time due to the higher ambient relative humidity. After the surface is exposed to 100 µL of HNO3 vapor at 24 ( 5% RH overnight and purged with dry N2, Figure 4b shows what appears to be droplets on the surface. The round shape of these particles would indicate that they are droplets and not crystals. These droplets are proposed to be composed of calcium nitrate and are on the order of 200 nm and average 50-60 nm in height. An outline of an oval is placed around one of the droplet features in the image. Although difficult to see in these

H2O, HNO3 Adsorption on MgO(100) and CaCO3(101 h 4)

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Figure 4. Tapping-mode AFM images (1 µm × 1 µm) viewed as phase (left panel, 200 nm scale bar), height (center panel, 200 nm scale bar), and line profiles (right panel) for a CaCO3(101 h 4) surface (a) ∼1 h after cleaving under ambient conditions (24 ( 5% RH) and (b) after the surface was exposed to 100 µL of HNO3 vapor, allowed to sit overnight, and then purged with dry N2. The surface was then exposed to increasing aliquots of water vapor totaling (c) 12.5, (d) 25, (e) 54% and (f) 75% RH. The outline of an oval has been placed around one of the structures, proposed to be calcium nitrate, that forms on the surface; see text for further details.

images, there is also some oscillation noise associated with these particles, which is indicative of their liquid nature. As increasing amounts of water vapor are added, Figure 4c-e, the large puddles appear to stay the same; however, there is the appearance of smaller puddles as the relative humidity increases. At the highest relative humidity, ∼75% RH (Figure 4f), the water puddles increase, the images appear noisy, and it is difficult to image the surface. Discussion Oxide and carbonate surfaces are important environmental interfaces because they play a role in several environmental processes including environmental catalysis and remediation, heterogeneous atmospheric chemistry, and aqueous geochemistry.22,45 As such, it is

important to understand the molecular-level details of the reactivity of these interfaces with atmospheric gases such as water and nitric acid. Here we used AFM to investigate the nature of two representative oxide and carbonate surfaces, MgO and CaCO3, respectively, under ambient conditions and following reaction with nitric acid, a trace atmospheric gas. Magnesium oxide is an ideal model for ionic metal oxides because it forms the rock salt structure and has a relatively simple electronic structure that can be modeled using ab initio calculations. For rock salt crystals, the most stable surface face is the (100) plane.17 In the case of calcium carbonate, there are two stable polymorphs at ambient pressure and temperature, (45) Al-Abadleh, H. A.; Al-Hosney, H. A.; Grassian, V. H. J. Mol. Catal. A 2005, 22.

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calcite and aragonite.46 The bulk crystal structure of calcite is rhombohedral, whereas for aragonite it is orthorhombic. For calcite, the (101 h 4) surface plane is the lowest-energy surface plane.47 The AFM studies shown here indicate that upon exposure to the ambient environment these two surfaces, h 4), react, and as already disMgO(100) and CaCO3(101 cussed, it has been previously shown by photoelectron34 and FTIR41 spectroscopy that the dominant reaction is with adsorbed water to form hydroxylated surfaces. In the case of MgO(100), there is an increase in hydroxylation due to long exposures to the ambient environment, which in these studies is near 15% RH. As the MgO(100) surface becomes hydroxylated, the surface roughens considerably. h 4) surface undergoes hydroxyIn contrast, the CaCO3(101 lation at this relative humidity but with little change in the surface roughness. The cause of this dissimilarity may be due to differences in the mobility of the surface ions and the thermodynamic stability of the crystallites that form on the surface. In a recent study using scanning polarization force microscopy,48 Kendall and Martin have shown that for CaCO3(101 h 4) the surface ions become mobile only at relative humidity above 55% RH. The fact that at ca. 15% RH the AFM images show patches but no hillocks agrees with their results in that there is minimal formation of crystallites on the surface. Although no comparable studies have been done on MgO(100), the fact that there is the formation of hillocks suggests that the surface ions must be more mobile at low relative humidity. These ions are seen to nucleate as Mg(OH)2 crystals at step edges on the surface. Following the reaction of nitric acid with MgO and CaCO3 at low relative humidity, there are also spatial features observed on both surfaces suggesting once again the importance of ionic mobility in these reactions to form aggregates of some type, whether crystallites or puddles. These surface features are proposed to be associated with the nitrate salts that form from these reactions. In the case of the reaction of nitric acid with MgO(100), previous FTIR studies have shown that crystalline hydrates form near 15% RH. In particular, Al-Abadleh and Grassian found the formation of crystalline magnesium nitrate hydrate, Mg(NO3)2‚nH2O, 4 < n e 6.44 The extent of hydration was found to increase as a function of relative humidity until a crystalline hexahydrate, Mg(NO3)2‚6H2O, formed immediately before deliquescence which occurred near 55% RH. In the AFM images, it is seen that the height of the nitrate crystals increases with relative humidity, suggesting that water is easily intercalated into the salt layers. For pure Mg(NO3)2‚6H2O, the crystal structure is monoclinic, with the water groups forming a distorted octahedral around the magnesium ion.49 Studies by Chang and Irish have shown that the tetrahydrate, Mg(NO3)2‚ 4H2O, maintains a coordination number of 6; however two of the waters in the octahedral are replaced by nitrates.50 The changes observed in the z axis upon increasing relative humidity (Figure 3) indicate that the microcrystallites on the surface may initially form the tetrahydrate that as the RH increases converts to hexahy(46) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Interscience Publishers: New York, 1963; Vol. 2. (47) de Leeuw, N. H.; Parker, S. C. J. Phys. Chem. B 1998, 102, 2914. (48) Kendall, T.; Martin, S. T. Geochim. Cosmochim. Acta 2005, 69, 3257. (49) Braibanti, A.; Triripicchio, A.; Lanfredi, A. M. M.; Bigoli, F. Acta Crystallogr. 1969, B25, 354. (50) Chang, T.-C. G.; Irish, D. E. Can. J. Chem. 1972, 51, 118-125.

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drate with an orientation of the crystal such that the water replaces the nitrate ions in the octahedral positions and causes a swelling in the crystallite along the surface normal. In the case of the reaction of nitric acid with CaCO3(101 h 4), previous FTIR studies have indicated that the calcium nitrate salt undergoes deliquescence at low relative humidity from a metastable phase and that the crystalline tetrahydrate, the stable hydrate for calcium nitrate, does not form in the reaction.51 In the AFM experiment, submicrometer dropletlike features or micropuddles are seen in the AFM phase and height images. This is consistent with the spectroscopic study. As the relative humidity increases, it can most easily be observed in the phase images that smaller droplets form and that at the highest relative humidity the sample becomes difficult to image as a continuous liquid coats the surface. The results presented here, along with previous studies on NaCl, indicate that reactions to produce nitrate salts on the surface (in this case, from reaction with HNO3) lead to crystallite formation and other spatial inhomogenieties on environmental interfaces.27-31 A recent study by Dubowski et al. have shown that for borosilicate glass surfaces, nitric acid can leach out certain elements such as Zn from the glass to form crystals of Zn(NO3)2 on the surface.52 In this case, both the bulk and surface mobility of ions are needed to form these surface structures. Because little is known about the mobility of ions on oxide and carbonate surfaces under conditions of relative humidity, temperature, and pressure found in the environment, additional studies in this area may be particularly helpful in understanding the reactivity of these surfaces in the natural environment and the formation of the surface structures shown here. The studies of Kendall and Martin on carbonate surfaces are a useful starting point. Similarly, earlier studies by Luna et al. using surface polarization force microscopy on NaCl surfaces have provided important information about ion hydration and mobility under ambient conditions.53 Thus, future studies using surface polarization force microscopy combined with reaction chemistry on these surfaces may provide additional information on these reactions that has hitherto never been obtained. Conclusions AFM has been used to image MgO(100) and CaCO3(101 h 4) surfaces as these surfaces undergo reaction with water and nitric acid under ambient conditions of temperature, pressure, and relative humidity. The AFM data show that as these surfaces react, surface structures are seen to form. These features, which include the formation of hillocks, patches, microcrystallites, and micropuddles, are due to surface and phase segregation as a result of facile ion mobility in the presence of adsorbed water. In addition, instabilities and oscillations in the AFM images provide an indication of liquid formation and deliquescence (i.e., a solid-to-liquid phase transition) of nitrate salts as a function of relative humidity. Acknowledgment. We are grateful for the support of the National Science Foundation through a creativity extension of grants CHE99-88434 and CHE05-03854. In (51) Al-Abadleh, H. A.; Krueger, B. J.; Ross, J. L.; Grassian, V. H. Chem. Commun. 2003, 2796. (52) Dubowski, Y.; Sumner, A. L.; Menke, E. J.; Gaspar, D. J.; Newberg, J. T.; Hoffman, R. C.; Penner, R. M.; Hemminger, J. C.; Finlayson-Pitts, B. J. Phys. Chem. Chem. Phys. 2004, 6, 3879. (53) Luna, M.; Rieutord, F.; Melman, N. A.; Dai, Q.; Salmeron, M. J. Phys. Chem. A 1998, 102, 6793.

H2O, HNO3 Adsorption on MgO(100) and CaCO3(101 h 4)

addition, acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. We also thank Dr. Kenneth Moore, the Director of the University of Iowa’s Central Microscopy Research Facility, for his support of these

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studies, Professor Michelle Foster for her help with sample preparation and Professor Scot Martin and Dr. Treavor Kendall for a copy of their paper prior to publication. LA051378J