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Atomic Force Microscopy Investigation of the Mechanism of Calcite Microcrystal Growth under Kitano Conditions Phillip S. Dobson,† Lucy A. Bindley, Julie V. Macpherson,* and Patrick R. Unwin* Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K. Received July 29, 2004 A combined atomic force microscopy (AFM)-inverted optical microscopy technique has been used to image the surface of calcite single microcrystals, with dimensions of 10-20 µm, at high resolution. The microcrystals were grown on a glass substrate using the Kitano method, a process that involves the outgassing of carbon dioxide from a saturated solution of calcium carbonate. The resulting increase in the supersaturation of the solution, with respect to calcium carbonate, induces crystallization. It is demonstrated, for the first time, that calcite microcrystals formed in this way exhibit a single spiral growth hillock on the (101 h 4) surface, as evidenced by a spiral step pattern, indicating that growth occurs at steps arising from an individual screw dislocation. The subsequent reactivity of these crystals under Kitano conditions has been followed in situ using AFM imaging.
Introduction Since its invention in 1986, atomic force microscopy (AFM)1 has proved invaluable for high-resolution topographical investigations of the growth and dissolution of inorganic crystal surfaces.2-4 Initial work involved the ex situ investigation of crystal surfaces after a defined period of exposure to an appropriate growth or dissolution solution.5 In this way, the nature and rate of reaction was inferred from observations of the crystal surface, such as the interatomic step spacing.6 In situ imaging of crystal surfaces is possible by operating the atomic force microscope in a fluid cell environment.7-10 For such measurements, the atomic force microscope tip is often employed to topographically monitor the dynamic process of interest, while the crystal is exposed to an appropriate solution.11-13 Rates of step advancement or retreat across the crystal surface, or step movement at dislocation sites, as the surface grows or * To whom correspondence should be addressed. E-mail:
[email protected] (J.V.M.);
[email protected] (P.R.U.). † Present address: Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland. (1) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930-933. (2) Liang, Y.; Baer, D. R.; McCoy, J. M.; Lafemina, J. P. J. Vac. Sci. Technol., A 1996, 14, 1368-1375. (3) Shiraki, R.; Rock, P. A.; Casey, W. H. Aquat. Geochem. 2000, 6, 87-108. (4) Onuma, K.; Ito, A.; Tateishi, T.; Kameyama, T. J. Cryst. Growth 1995, 154, 118-125. (5) Ester, G. R.; Halfpenny, P. J. J. Cryst. Growth 1998, 187, 111118. (6) Price, R.; Ester, G. R.; Halfpenny, P. J. Proc. R. Soc. London, Ser. A 1999, 455, 4117-4130. (7) Onuma, K.; Ito, A.; Tateishi, T. J. Cryst. Growth 1996, 167, 773776. (8) Hillner, P. E.; Gratz, A. J.; Manne, S.; Hansma, P. K. Geology 1992, 20, 359-362. (9) Hillner, P. E.; Manne, S.; Gratz, A. J.; Hansma, P. K. Ultramicroscopy 1992, 42-44, 1387-1393. (10) Liang, Y.; Baer, D. R.; Lea, A. S. Mater. Res. Soc. Symp. Proc. 1995, 355, 409-414. (11) Ester, G. R.; Price, R.; Halfpenny, P. J. J. Cryst. Growth 1997, 182, 95-102. (12) Campbell, P. A.; Ester, G. R.; Halfpenny, P. J. J. Vac. Sci. Technol., B 1996, 14, 1373-1377. (13) Jones, C. E.; Unwin, P. R.; Macpherson, J. V. ChemPhysChem 2003, 4, 139-146.
dissolves, can be used to quantify surface reactivity.14,15 The majority of these studies have involved relatively large single crystal faces, several millimeters across, with a high density of defect and dislocation sites.16 Although the atomic force microscope images only a small area of interest, with a field of view at the micron level, the whole crystal surface responds to the imposed supersaturated or undersaturated solution conditions. Thus, processes occurring at neighboring sites on the crystal surface, outside the scan area, may interfere with growth or dissolution in the area probed by the atomic force microscope tip. Moreover, it may be difficult to adequately control mass transport to macroscale crystal surfaces, which is a key factor to consider when investigating interfacial reactivity.17-19 To overcome these potential problems, some recent studies have reported imaging of single microcrystals, but many of these investigations have largely been ex situ.20-23 The growth and dissolution kinetics of the geologically important mineral calcite24 has been studied extensively using many different techniques, with the crystal subject to a variety of conditions.25-29 AFM has proved particularly (14) Jones, C. E.; Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2351-2359. (15) Tang, R. K.; Orme, C. A.; Nancollas. G. H. J. Phys. Chem. B 2003, 107, 10653-10657. (16) Dove, P. M.; Platt, F. M. Chem. Geol. 1996, 127, 331-338. (17) Unwin, P. R.; Macpherson, J. V. Chem. Soc. Rev. 1995, 24, 109120. (18) Macpherson, J. V.; Unwin, P. R. Prog. React. Kinet. 1995, 20, 185-244. (19) Unwin, P. R. J. Chem. Soc., Faraday Trans. 1998, 94, 31833196. (20) Plomp, M.; Buijnsters, J. G.; Bo¨gels, G.; van Enckevort, W. J. P.; Bollen, D. J. Cryst. Growth 2000, 209, 911-923. (21) Kim, H.; Garavito, R. M.; Lal, R. Colloids Surf., B 2000, 19, 347-355. (22) Baker, A. A.; Helbert, W.; Sugiyama, J.; Miles, M. J. Appl. Phys. A 1998, A66, S559-S563. (23) Archibald, D. D.; Gaber, B. P.; Hopwood, J. D.; Mann, S.; Boland, T. J. Cryst. Growth 1997, 172, 231-248. (24) Reddy, M. M. In Mineral Scale Formation and Inhibition; Amjad, Z., Ed.; Plenum Press: New York, 1995. (25) Gutjar, A.; Dabringhaus, H.; Lacmann, R. J. Cryst. Growth 1996, 158, 296-309. (26) Swinney, L. D.; Stevens, J. D.; Peters, R. W. Ind. Eng. Chem. Fundam. 1982, 21, 31-36. (27) Compton, R. G.; Unwin, P. R. Philos. Trans. R. Soc. London, Ser. A 1990, 330, 1-45.
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powerful for investigating calcite surfaces, enabling atomistic theories of growth and dissolution to be validated with experimental surface topographical data.30-32 Previous AFM studies of calcite crystal growth have focused largely on natural (“Iceland Spar”) macroscopic crystals (dimensions of several millimeters to centimeters) exposed to a solution with a fixed, low supersaturation. These studies have provided information on the mechanisms of growth31,33 as well as the effect on growth of a variety of inhibitors, including macromolecules34-36 and metal ions.37,38 In the 1960s, Kitano developed a method for producing supersaturated CaCO3 solutions,39 which involved saturating a solution of CaCO3 with CO2 (see eq 1) and then evolving CO2 into the atmosphere to supersaturate the solution with respect to CaCO3.
CaCO3(s) + CO2(g) + H2O(l) T Ca2+(aq) + 2HCO3-(aq) (1) While this methodology was initially developed as a model for CO2-rich waters expelled from the Earth’s crust, the method has more recently been used to prepare supersaturated solutions in order to grow calcite microcrystals.40 Such investigations have been extended to include the study of CaCO3 crystal growth in the presence of additives41,42 and on foreign substrates, including synthetic polymers,43-45 Langmuir monolayers of stearic acid molecules,46 and functionalized alkanethiols on gold47 and silver.48 These studies have demonstrated that monolayers may be selected to control calcium carbonate growth and encourage the predominance of particular polymorphs, particularly calcite. Moreover, the surface template also has the ability to induce the oriented nucleation of complementary crystal faces.49 However, (28) Brown, C. A.; Compton, R. G.; Narramore, C. A. J. Colloid Interface Sci. 1993, 160, 372-379. (29) Compton, R. G.; Pritchard, K. L.; Unwin, P. R. J. Chem. Soc., Chem. Commun. 1989, 249-251. (30) Reddy, M.; Plummer, L. N.; Busenberg, E. Geochim. Cosmochim. Acta 1981, 45, 1281-1289. (31) Teng, H. H.; Dove, P. M.; DeYoreo, J. J. Geochim. Cosmochim. Acta 2000, 64, 2255-2266. (32) Coles, B. A.; Compton, R. G.; Suarez, M.; Booth, J.; Hog, Q.; Sanders, G. H. W. Langmuir 1998, 14, 218-225. (33) Teng, H. H.; Dove, P. M.; DeYoreo, J. J. Geochim. Cosmochim. Acta 1999, 63, 2507-2512. (34) Walters, D. A.; Smith, B. L.; Belcher, A. M.; Paloczi, G. T.; Stucky, G. D.; Morse, D. E.; Hansma, P. K. Biophys. J. 1997, 72, 1425-1433. (35) Smith, B. L.; Paloczi, G. T.; Hansma, P. K.; Levine, R. P. J. Cryst. Growth 2000, 211, 116-121. (36) Teng, H. H.; Dove, P. M. Am. Mineral. 1997, 82, 878-887. (37) Davis, K. J.; Dove, P. M.; De Yoreo, J. J. Science 2000, 290, 1134-1137. (38) Astilleros, J. M.; Pina, C. M.; Fernandez-Diaz, L.; Putnis, A. Geochim. Cosmochim. Acta 2000, 64, 2965-2972. (39) Kitano, Y. Bull. Chem. Soc. Jpn. 1962, 35, 1980-1985. (40) Sato, K.; Kumagai, Y.; Watari, K.; Tanaka, J. Langmuir 2004, 20, 2979-2981. (41) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Proc. R. Soc. London, Ser. A 1989, 423, 457-471. (42) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 344, 692-695. (43) Donners, J. J. J. M.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. J. Am. Chem. Soc. 2002, 124, 9700-9701. (44) Pai, R. K.; Hild, S.; Ziegler, A.; Marti, O. Langmuir 2004, 20, 3123-3128. (45) Agarwal, P.; Berglund, K. A. Cryst. Growth Des. 2004, 4, 479483. (46) Duffy, D. M.; Harding, J. H. J. Mater. Chem. 2002, 12, 34193425. (47) Ku¨ther, J.; Seshadri, R.; Knoll, W.; Tremel, W. J. Mater. Chem. 1998, 8, 641-650. (48) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500-4509. (49) Han, Y.-J.; Aizenberg, J. Angew. Chem., Int. Ed. 2003, 42, 36683670.
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despite these studies, the mechanism of calcite crystal growth on a surface from a Kitano solution is not known. In this paper, we examine the mechanism of calcite microcrystal formation under Kitano conditions. First, microcrystals grown on glass surfaces using the Kitano technique have been imaged ex situ in order to identify the surface morphology and hence the mechanism of crystal growth. Second, we investigate the reactivity of these microcrystals in situ when they are subjected again to Kitano growth conditions. These studies not only reveal detailed information on calcite crystal growth but also serve to demonstrate the advantages of investigating microcrystals to understanding crystal growth processes. Developing a methodology for the AFM study of microscale crystals is important in that it opens up the possibility of studying crystals with few defects and allows the investigation of materials that are not available on the macroscale. Moreover, there is considerable interest in understanding size effects in crystal reactivity, particularly for nanoscale crystals and features.50 Experimental Section All solutions were prepared from Milli-Q reagent water (Millipore Corp). CaCO3 growth solutions were prepared as described by Kitano.39 In brief, an excess of CaCO3 powder (ACS reagent, Sigma) was placed in a vessel containing Milli-Q water. The solution was stirred continuously, while CO2 (produced from dry ice) was bubbled through the solution for 90 min. The solution was then filtered rapidly using a Buchner flask, and CO2 was again bubbled through the resulting liquor for 60 min. Finally, this solution was filtered using a 0.2 µm syringe filter (Minisart high flow, Sartorius, Germany). Microcrystal samples for AFM imaging were prepared by placing clean borosilicate glass disks (10 mm diameter, 1 mm thick, UQG Ltd, Cambridge, U.K.) into a Petri dish filled with the Kitano solution. The dish was left open to the air in a clean environment to allow dissolved CO2 to evolve from the solution. This resulted in an increase in the solution pH, reducing the solubility of CaCO3 and thereby inducing the formation of CaCO3 microcrystals at the glass/solution interface. After 18 h, the disks were removed from the solution and rapidly blown dry under a stream of nitrogen (BOC, white spot). After inspection using an optical microscope (Olympus BH2, Japan), these samples were investigated immediately using an atomic force microscope. A Molecular Imaging (MI) atomic force microscope (U.S.A.) and fluid cell were employed for all measurements. The atomic force microscope was equipped with a piezoelectric scanner that facilitated a maximum scan range of 30 µm × 30 µm. Si3N4 triangular beam cantilevers with integrated pyramidal tips (nominal spring constant of 0.12 N m-1) were employed for all imaging experiments. For simultaneous optical and AFM imaging, the atomic force microscope was transferred to an inverted microscope (Willovert, Hund, Germany) and secured using a stage that had been modified in house. The microcrystals deposited from the growth solution were typically 5-20 µm in size. For the experiments described, it was necessary to locate the atomic force microscope tip on the top surface of a microcrystal, a process achievable by direct observation with the optical microscope. As shown in Figure 1, the resolution attainable with the setup allowed individual microcrystals to be addressed with the atomic force microscope tip. Moreover, by observing the morphology of the crystal with optical microscopy, it was possible to determine the crystallographic orientation of the surface under investigation. Once the tip was engaged on the crystal surface, an initial scan was obtained over an area much smaller than the microcrystal surface. After an initial preview, the scan area was increased and recentered until observation, with the inverted microscope, indicated that the region of the AFM image contained the maximum area without any risk of the tip moving from the top surface of the crystal. (50) Tang, R. K.; Orme, C. A.; Nancollas, G. H. ChemPhysChem 2004, 5, 688-696.
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Figure 1. Optical micrograph of the atomic force microscope cantilever and crystals observed through an inverted microscope. Microcrystals were imaged both in air, to determine the surface morphology of crystals at the end of the Kitano process, and subsequently under solution. In the latter case, the tip was withdrawn a short distance (
