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Langmuir 1996, 12, 538-546
Modified Calcite Deposition Due to Ultrathin Organic Films on Silicon Substrates D. D. Archibald,*,† S. B. Qadri,‡ and B. P. Gaber† Laboratory for Molecular Interfacial Interactions, Code 6930, Center for Bio/Molecular Science and Engineering, and Division of Condensed Matter and Radiation Sciences, Code 6683, Naval Research Laboratory, Washington, D.C. 20375 Received April 25, 1995. In Final Form: August 3, 1995X In order to study the effect of organic surface chemistry on calcite nucleation, attachment, and growth, calcium carbonate was precipitated in the presence of various ultrathin-film organosilane-modified silicon wafers. The chemistry of the aminosilane surfaces was systematically changed by the coupling of various acidic molecules, without creating a geometric lattice of acidic functional groups. Optical microscopy, scanning electron microscopy with image analysis, and X-ray scattering were employed to characterize crystallite density and orientation normal to the surface. Calcite grown on amino-modified surfaces was produced with the equilibrium rhombohedral habit and had the 〈104〉 orientation. Surfaces of the silicon oxide, carboxylate, iminodiacetate, or phosphoramidate tended to favor the orientation of surface crystals along 〈001〉 or near the 〈001〉 axes of the crystal. Primarily this is a result of the affinity of the surface for cations, but functional-group-mediated ion ordering and/or stereochemical matching is also suggested by the much greater amount of crystal nucleation on the long-chain carboxylates when compared to shortchain carboxylates. Coupling of nitrilotriacetic acid (NTA) favored appearance of 〈110〉, 〈113〉, and 〈116〉 oriented crystals when compared to the other acid surfaces. Growth of calcite with relatively larger {110} faces was observed when the microcrystals were synthesized in the presence of freely soluble NTA. Appearance of these faces is a result of a relatively suppressed growth rate due to face-specific adsorption on the growing crystallites. Similarly, the enhancement of specific crystal surface binding by the substrate bound NTA is probably the mechanism influencing orientation of surface microcrystals. Two common structural features of the {110}, {113}, and {116} faces are the tilt of the carbonate plane at large angles from the face and the same angle of rotation of the carbonates about their 3-fold symmetry axes. That angle may enhance the ability of two NTA carboxylates to simultaneously occupy carbonate sites of these calcite faces. The fact that crystallite density and orientation are influenced by submonolayers of functional groups attests to the importance of electrostatic and stereochemical recognition of certain crystal faces even without matching of the geometric lattice.
Introduction Crystal nucleation and growth at organic interfaces often have an important role in biological mineralization and may also be involved in some geological cementation processes. Recently, crystal growth at synthetic organic interfaces has become an active area of research because of the relevance to understanding biomineralization1-7 as well as the more practical objectives of novel processing of thin films8-13 and substance purification.1,7 Model * Author to whom correspondence should be addressed. Internet:
[email protected]. † Laboratory for Molecular Interfacial Interactions. ‡ Division of Condensed Matter and Radiation Sciences. X Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Carter, P. W.; Ward, M. D. J. Am. Chem. Soc. 1994, 116, 769. (2) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. A. J. Phys. D: Appl. Phys. 1991, 24, 154. (3) Rajam, S.; Heywood, B. R.; Walker, J. B. A.; Mann, S.; Davey, R. J.; Birchall, D. B. J. Chem. Soc., Faraday Trans. 1991, 87 (5), 727. (4) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87 (5), 735. (5) Walker, J. B. A.; Heywood, B. R.; Mann, S. J. Mater. Chem. 1991, 1 (5), 889. (6) Rieke, P. C.; Bentjen, S. B.; Tarasevich, B. J.; Autrey, T. S.; Nelson, D. A. Mater. Res. Soc. Symp. Proc. 1990, 174, 69. (7) Frostman, L. M.; Bader, M. M.; Ward, M. D. Langmuir 1994, 10 (2), 576. (8) Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Rieke, P. C.; Tarasevich, B. J. Scanning Microsc. 1993, 7 (1), 423. (9) Feng, S.; Bein, T. Nature 1994, 368, 834. (10) Rieke, P. C.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer, D. R.; Fryxell, G. E. Langmuir 1993, 10 (3), 619-622. (11) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994, 264, 48. (12) Rieke, P. C.; Tarasevich, B. J.; Bentjen, S. B.; Fryxell, G. E.; Campbell, A. A. ACS Symp. Ser. 1992, No. 499, 61-75.
0743-7463/96/2412-0538$12.00/0
studies of crystallization at interfaces such as Langmuir monolayers suggest that control of crystallization by organic interfaces is affected by both the ability of the organic template to mimic the lattice of a two-dimensional face as well as the stereochemistry and orientation of the functional groups at the interface.2-5,14,15 It is unclear which of these factors will be more important in a particular surface-crystal growth system. In this study we examined patterns of crystal growth at well-defined interfaces in which there is no attempt to mimic a crystal lattice, but for which it is possible to systematically vary the functional groups that are present. Toward this goal of studying crystallization at chemically modified interfaces, we have used silicon substrates modified with aminosilane ultrathin films (UTFs) as a foundation for further surface modification with acidic functionalities (Figure 1). Silicon wafers provide a near ideal substrate because wafer flatness allows precise studies of crystal orientation and reduces the complicating factor of the microscopic roughness which might affect surface nucleation and thereby confound any effect of surface chemistry. Furthermore, the thin native oxide of silicon can be readily modified with organosilane chemistry which is highly versatile and well-studied.16 UTFs can be extensively evaluated with a number of techniques such as ellipsometry, contact angle, infrared spectroscopy, X-ray (13) Heuer, A. H.; Fink, D. J.; Laraia, V. J.; Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Rieke, P. C.; Thompson, D. H.; Wheeler, A. P.; Veis, A.; Caplan, A. I. Science 1992, 255, 1098. (14) Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492. (15) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4861. (16) Ulman, A. Ultrathin Organic Films; Academic Press: New York, 1991.
© 1996 American Chemical Society
Calcite Deposition
Figure 1. Linkage chemistries used in coupling of acidic functional groups to aminosilane ultrathin-film substrates. For linkage of the di- and trifunctional acids, DCDA and NTA, only a small mole fraction of activating agent (PyBOP plus HOBT) was used in order to favor formation of only one surface amide bond per molecule.
photoelectron spectroscopy, low-angle X-ray diffraction,17 and atomic force microscopy (AFM).18 With regard to influencing CaCO3 nucleation or adhesion, simple chemical functionalities were chosen with the aim of differential interaction with the various stereochemically distinct potential faces of the nascent calcite crystallite. In selection of surface moieties we looked to the principles developed through extensive studies of CaCO3 crystallization in the presence of surface-binding additives19-24 and under Langmuir monolayers.2-5 In the system studied here stereochemical matching2 between acidic functionalities and the nascent crystal surface is expected to be the main factor to differentiate the various faces of calcite. Experimental Section Materials. Polished silicon wafers (10 cm (001)) (from various manufacturers) were purchased from Wafernet (San Jose, CA). Trialkoxyaminosilanes were obtained from United Chemical Technologies (Bristol, PA) and included (3-aminopropyl)triethoxysilane (APS), (4-aminobutyl)triethoxysilane (ABS), [(N-(6aminohexyl)amino)propyl]trimethoxysilane (AHAP), [3-(N-(2(17) Peek, B. M.; Geer, R. E.; Ondris-Crawford, R. J.; Dulcey, C. S.; Ratna, B.; Calvert, J. M.; Shashidhar, R. Structure and Reactive Site Accessibility of Chemisorbed Aminosilane Layers Determined by X-Ray Reflectivity, XPS, and UV-Vis Spectroscopy (preprint). (18) Durfor, C. N.; Turner, D. C.; Georger, J. H.; Peek, B. M.; Stenger, D. A. Langmuir 1994, 10, 148. (19) Didymus, J. M.; Oliver, P.; Mann, S.; DeVries, A. L.; Hauschka, P. V.; Westbroek, P. J. Chem. Soc., Faraday Trans. 1993, 89 (15), 2891. (20) Mann, S.; Didymus, J. M.; Sanderson, N. P.; Heywood, B. R.; Samper, E. J. A. J. Chem. Soc., Faraday Trans. 1990, 86 (10), 1873. (21) Berman, A.; Addadi, L.; Weiner, S. Nature 1988, 331, 546. (22) Addadi, L.; Moradian, J.; Shay, E.; Maroudas, N. G.; Weiner, S. Proc. Natl. Acad. Sci. 1987, 84, 2732. (23) Addadi, L.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 4110. (24) Aizenberg, J.; Albeck, S.; Weiner, S.; Addadi, L. J. Cryst. Growth 1994, 142, 156.
Langmuir, Vol. 12, No. 2, 1996 539 aminoethyl)amino)propyl]trimethoxysilane (EDA), trimethoxysilylpropyldiethylenetriamine (DETA), (((((aminoethyl)amino)methyl)phenyl)ethyl)trimethoxysilane (PEDA). Pyrollidine(benzotriazolyloxy)tris(dimethylamino)phosphonium hexafluorophosphate (PyBOP) was supplied by Advanced Chemtech (Louisville, KY). Sigma Chemical Company (St. Louis, MO) supplied 1-hydroxybenzotriazole (HOBT). Most of the other materials were obtained from Aldrich (MIlwaukee, WI). We use the abbreviation d.H2O for tap water that was either deionized and doubly distilled or reverse osmosis filtered and passed through organic/colloid filters and a mixed bed deionizer. Preparation of Organosilane UTFs. Deposition of Alkoxyaminosilane Silicon Wafers. Silicon wafer substrates were cut to 1 in. widths and cleaned by the following procedure: (1) blown free of dust and one rinse with d.H2O; (2) soaked 30 min in 1:1 HCl(concn)/methanol; (3) rinsed four times with d.H2O; (4) soaked 30 min in H2SO4(concn); (5) rinsed four times with d.H2O; (6) soaked 30 min in 45-55 °C d.H2O; (7) rinsed once with methanol. Aminosilane films were prepared according to the procedures used in refs 17 and 18. One part silane was mixed with 95 parts methanol containing 1 mM acetic acid. Four parts d.H2O were added to the silane solution which was mixed briefly and then poured over the freshly cleaned wafers. The surface reaction was stopped after 15-20 min by rinsing the wafers with methanol (four times). The surfaces were then blown free of the solvent from the final rinse and warmed for 30 seconds on a hot plate set to low. Modification of Aminosilane Thin Films (Figure 1). (1) Nitrilotriacetic Acid (NTA) Coupling. Typically 0.95 mmol of NTA (1 equiv, MW ) 191.1 g/mol) and 4.2 equiv of diisopropylethylamine (DIEA) (700 µL, MW ) 129.3, F ) 0.742 g/mL) were added to 82 mL of dimethylformamide (DMF) (nom. 0.008% H2O) and stirred for 1 h to dissolve. We then added 0.1 equiv of HOBT (MW ) 135) and 0.1 equiv PyBOP (MW ) 520) and the carboxylate activation reaction was allowed to proceed for 30 min prior to addition of the aminosilanized wafers. After soaking overnight in the coupling solution, the wafers were rinsed with the following series of solvents: one rinse with DMF, one rinse with 1:1 DMF/ methanol, and three rinses with methanol. The last rinse solvent was rapidly blown off the surface with compressed nitrogen. (2) Docosanedioc Acid (DCDA) Coupling. We added 0.27 mmol of DCDA (1 equiv, MW ) 370.6) and 2.8 equiv of DIEA (132 µL, MW ) 129.3, F ) 0.742 g/mL) to 104 mL of 2:1 chloroform (ethanol free)/DMF (nom. 0.008% H2O) and stirred for 3 h to dissolve. The coupling procedure was then identical to that for NTA except the solvent rinsing series was different: two rinses with 1:1 chloroform/DMF, two rinses with 1:1 chloroform/methanol, and one rinse with methanol. (3) Amidophosphonation (adapted from ref 25). The reaction was carried out in a purged drybox. We added 1.44 mL of 2,4,6collidine and 1.0 mL of phosphorus oxychloride (POCl3) to 54 mL of acetonitrile (nom. 0.02% H2O) and the mixture poured over APS wafers. The wafers were reacted for 1 h, followed by the following rinses: two rinses with acetonitrile, one rinse with d.H2O, and three rinses with methanol. (4) Succinamidation. Adapting the method on p 3062 of ref 26, 3.5 g of succinic anhydride (SA) and 0.35 g of 4-(dimethylamino)pyridine were added to 35 mL of DMF (nom. 0.008% H2O) and stirred for 10 min to dissolve. Aminosilanized wafers were placed in the solution for 18 h, during which time the solution changed from yellow to reddish-orange. The wafers were rinsed one time with DMF, four times with H2O, and two times with methanol. Preparation of CaCO3 Surface Crystals. One wafer of a particular surface type was held in each glass reaction jar and the polished surface of the wafer was tilted downward about 15° in order to prevent collection of any crystals showering down from above. CaCO3 was formed by simultaneously pouring two freshly prepared reaction solutions over each wafer: (1) 35 mL of 5.0 mM CaCl2 in CO2-free d.H2O and (2) 35 mL of 5.0 mM Na2CO3 in CO2-free d.H2O (room temperature). More consistent crystallization results were obtained when the CO2 was removed by 1 h of boiling rather than by purging with nitrogen gas. The (25) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (26) Cuatrecasas, P. J. Biol. Chem. 1970, 245 (12), 3059.
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initial supersaturation was about 1400 (pH ) 10.3), resulting in faceted surface crystals within 7 min. Surface attachment of CaCO3 is thought to occur within a few minutes after addition of the solutions. Mineralized surfaces were removed from solution after varying times ranging from 1 to 24 h. Cleaning was accomplished with two brief rinses in d.H2O, and finally a gentle N2 blow to remove clinging droplets. Characterization Techniques. Ellipsometric values are from multiple measurements (typically five per specimen) made on a Gaertner Model 115C (Chicago, IL) using a clean section of the same wafer as the reference substrate and utilizing a 70° angle of incidence for the 632.8 nm He-Ne laser. A value of 1.45 was assumed to be a reasonable refractive index of the thin films. Advancing and receding contact angle values were measured for drops of d.H2O using an NRL Zisman type goniometer and a micrometer drive microsyringe. Optical microscopy (OM) was performed with a Nikon reflectance microscope equipped with BD PLAN lenses (5×/0.1, 10×/ 0.25, 20×/0.4, 40×/0.65) where the lens polarizer at the base of the lens was out during imaging. Images were recorded on 35mm Kodak TMX-100 film. The negatives were directly digitized using a Nikon LS-10E scanner. The resulting TIFF files were imported into the program NIH Image 1.54 (written by Wayne Rasband at the U.S. National Institutes of Health and available from the Internet by anonymous ftp from zippy.nimh.nih.gov) running on a Macintosh IIx. Particle areas, perimeters, and the lengths of minor and major ellipses were measured using the “analyze particles” tool after manually applying a threshold to the grayscale. Scanning electron microscope (SEM) images were recorded on an Electroscan ESEM 2020 (Wilmington, MA) after sputter coating samples with gold (Hummer, Technics, Inc.). The angles between crystal edges in digital SEM images were determined by using the measuring tools in SigmaScan/Image version 1.20.09 for Windows (Jandel Scientific, San Rafael, CA). Computer-generated models of oriented calcite {104} rhombohedra were produced using Shape for Windows version 5.0 (Shape Software, Kingsport, TN). The rhombohedrally-centered hexagonal unit cell is used to describe the calcite structure, faces, and directional axes (space group R3c (No. 167); 6 CaCO3; R ) β ) 90°, γ ) 120°; a1 ) a2 ) a3 ) 4.99 Å, c ) 17.06 Å). The four-symbol Miller-Bravais indices (h k i l) are abbreviated (h kl), where i ) -(h + k). X-ray diffraction (XRD) of the mineralized wafers was measured with a Rigaku (Japan) automated powder diffractometer coupled to a rotating anode X-ray target (50 kV, 200 mA); approximately 1 cm2 of the sample was illuminated with Cu KR radiation (λKR1 ) 1.540 50 Å, λKR2 ) 1.543 34 Å) and the scattered radiation was collected with a solid angle detector employing a graphite monochromator to remove the Cu Kβ. Intensities were corrected to compensate for the instrument response function. Scans were typically made in 0.02° steps and at a scan speed of 1 deg/min. The standard powder diffractogram data for calcite (Cu KR) was obtained from the JCPDS database.27 Crystallite orientations were estimated by both XRD and analyses of SEM data. The XRD technique is initially a θ-2θ scan of the crystals held in orientation with respect to the substrate. In this experiment only diffracting planes parallel to the substrate can produce significant diffraction intensity at the correct θ for a particular d-spacing. The SEM method assumes that flat calcite faces are the stable {104} faces. These faces are the most stable of calcite and frequently comprise all the large flat faces of cleaved samples of geological Iceland spar or synthetic microcrystals. Energy minimization surface modeling studies have indicated that there is essentially no surface reconstruction relative to the bulk structure.28 To facilitate determination of orientation from SEM data, Figure 2 and Table 1 were produced from mathematical models. Figure 2 contains images of regular rhombohedral calcite {104} displayed in 18 different orientations and showing one possible way each oriented rhombohedron could be truncated at the substrate. The orientations are the full set of X-ray diffracting planes and contains many, but not all, of the possible microcrystal orientations. Table 1 is derived from Figure (27) Joint Comittee on Powder Diffraction Standards - International Center for Diffraction Data, Swarthmore, U.K., 1986; File No. 24-27 (Calcite). (28) Titiloye, J. O.; Parker, S. J.; Osguthorpe, D. J.; Mann, S. J. Chem. Soc., Chem. Commun. 1991, 1494.
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Figure 2. Computed images of idealized regular {104} calcite rhombohedra in various orientations to a substrate and viewed directly down the surface normal without computed perspective. The orientation of the a1, a2, a3, and c crystallographic axes are indicated in the center of the crystal unless moved off-center for clarity. Dashed lines inside the crystal indicate either the hidden edges of the {104} form or one possible way in which the crystal could be truncated at the substrate. The angles listed in Table 1 were measured between the three lines meeting at the crystal apices in these images. 2 and for each orientation gives a set of characteristic angles that can be measured from SEM micrographs of real crystals. The following three-step procedure was used for indexing calcite microcrystals: I. SEM images were collected looking directly down the normal to the substrate. Using the topmost corner of the crystal as the central point, the three angles between edges were measured from the micrograph. These angles will be independent of face size. II. Using Table 1, the close matches ((5°) to the set of angles were determined and if there was more than one close match to the set of angles measured, the micrograph was examined and compared to the appropriate idealized oriented surface crystals in Figure 2. The number and
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Table 1. Indexing the Orientation of {104} Rhombohedral Calcite Attached to Surfaces and Imaged with SEMa orientation
angle 1 (deg)
angle 2 (deg)
angle 3 (deg)
〈012〉 〈104〉 〈001〉 〈110〉 〈113〉 〈101〉
152 129 120 180 144 125
104 129 120 110 82 125
104 102 120 70 134 110
〈018〉 〈116〉 〈211〉 〈122〉 〈1 0 10〉 〈214〉
150 178 170 144 126.5 152
105 85 112 128 126.5 132
105 97 78 88 107 76
〈119〉 〈100〉 〈015〉 〈2 1 10〉 〈134〉 〈318〉
158 120 169 164 136 161
102 120 95.5 96 94 77
100 120 95.5 100 130 122
a Listed angles are the three angles meeting at the apex in images of idealized regular {104} calcite rhombohedra viewed directly down the surface normal (as displayed in Figure 2).
Table 2. Characteristics of Aminosilane Ultrathin Films on Silicon Wafers extended organic measured H2O contact angle (deg) aminosilane molecular length (Å) ellipsometric UTF (theoretical) thickness (Å) (20 µL sessile) PEDA ABS AHAP APS EDA DETA oxide
12.5 6.7 13.5 5.5 9.1 12.3 ref
6-7 6-7 3-4 4-5 4-6 5-8 ref
61-64 51-53 50-54 48-52 39-44 36-46 {100} > {110} > {001}.28 Under normal synthetic conditions the high surface energy faces are not usually expressed.33 In circumstances where the substrate surface concentrated the calcium ions, greater amounts of 〈001〉 oriented nucleation have been seen and this is explained as an ionotropic effect22,23 causing higher local super(31) Kitano, Y. Bull. Chem. Soc. Jpn. 1962, 12, 1973. (32) No. 167 in International Tables for Crystallography, Vol. A, 3rd ed.; Hahn, T., Ed.; Kluwer Academic Publishers: Boston, MA, 1992. (33) Rajam, S.; Mann, S. J. Chem. Soc., Chem. Commun. 1990, 1789.
Figure 6. Illustration of the calcite unit cell structure indicating the definitions of δ and φ for quantifying orientations of carbonate ions with respect to an interface plane. These definitions are used in the explanation of the orientational preferences highlighted in Table 6. Note that for the purposes of indicating carbonate orientation, φ ) 60° is equivalent to φ ) 0°, due to 3-fold inversion and screw axes. Because of symmetry, the maximum value of φ is 30°. For φ ) 30° rotation of the carbonate trigonal plane about the c-axis, one carbonyl bond is pointing out of the idealized interfacial calcite surface while the other two are embedded equivalent distances into the surface, or vice versa. With φ ) 0°, one carbonyl bond is parallel to the interface. Calcium ions are displayed as large black spheres, while the carbonate ions have white atoms and bonds. The projection of the hexagonal cell unit cell is indicated by coarse dashed line as viewed down the a1-axis in (A) and down the c-axis in (B).
saturation and stabilization of bipolar faces. This phenomenon explains our results of calcite mineralization of silicon oxide or APS with submonolayers of coupled succinic acid (APS-SA) or phosphoramidate (APSPOCl3). With these specimens the nucleation densities were not significantly increased, but the 〈001〉 and 〈012〉 orientations were enhanced. The higher density mineralization of APS-DCDA and APS-NTA may be a result of a higher affinity for calcium ions or the ability to organize cations that are bound. Differences in total ion binding do not explain the difference between mineralization of analogous surfaces of the coupled long-chain (APS-DCDA) and short-chain (APS-SA) dicarboxylates, the latter of which probably contains more carboxylates but is a poorer nucleator. Improved nucleation and/or adhesion on APSDCDA could be a result of binding site flexibility due to the possibility of alkyl chain reorientation in either isolated molecules or domains of bound DCDA. With APS-NTA, the multidentate nature of the bound ligand may be enhancing cation binding affinity. In Table 6, along with the summary of microcrystal surface orientation results, we list two angles, δ and φ, which characterize the orientation of the carbonate ions with respect to the substrate. We define these angles as illustrated in Figure 6. δ is the degree that the carbonate plane tilts off the c-axis and φ is a measure of the rotational position of the carbonates about their 3-fold axes. Most of the highly favored orientations that appear predominant in the APS-DCDA XRD measurements and APS-Acids SEM measurements are a small δ of tilt off the c-axis (〈001〉, 〈018〉, and 〈1 0 10〉), with the exceptions being 〈110〉 and 〈101〉. We speculate that primarily the ionotropic effect tends to stabilize c and near-c faces on the developing surface microcrystallite. However, fractionally greater amounts of 〈110〉 on APS-DCDA contradict the argument that the ionotropic effect is the sole factor influencing crystal orientation on this surface. We think that the
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data indicate that the APS-NTA surface also has strong enhancement of the 〈110〉 orientation as measured by both XRD and SEM whereas the absolute enhancement of 〈110〉 was not measured on APS-Acids by the SEM indexing method. Measurement of a large fraction of 〈101〉 microcrystals by SEM of APS-Acids and a relative enhancement on APS-DCDA also does not support the purely ionotropic model for the microcrystal orientations on non-NTA acids. There is the possibility of specific molecular recognition by bound organic acids, but an alternative explanation for the SEM data is that some of the 〈1 0 10〉 crystals may have been misindexed as 〈101〉 even though the structures of their calcite-substrate interfaces would be quite different. In contrast, the observation of increased relative numbers of 〈119〉 microcrystals on APS-Acids is better supported by the morphological indexing method because the 〈119〉 is distinct from other orientations except 〈2 1 10〉, which would have a similar structure at the interface. In our system the NTA has an ionotropic influence and a measurable effect due to a specific molecular recognition interaction. In line with the ionotropic influence, on APSNTA, absolute increases in measured amounts of 〈119〉 and 〈001〉 were seen by XRD, and increases in 〈018〉 were measured by SEM. One of our most significant results is seen by direct comparison of APS-NTA orientational data to that from APS-DCDA or APS-Acids. Relatively preferred crystallite orientations on APS-NTA tend to have a large δ of carbonate ion planar tilt (〈110〉, 〈113〉, 〈116〉, 〈012〉, and 〈211〉), the smallest angle of this set being 48.6° for 〈116〉. Among the set of orientations preferred by APS-NTA, all except one rotation angle φ is at or near 30° indicating that two of the carbonyl oxygens are directed at equivalent angles and distances into the surface. We speculate that this angle produces pairs of carboxylate sites that are oriented for facile binding of symmetric dicarboxylates such as in the free iminodiacetate end of
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the bound NTA. The enhancement of 〈012〉 orientation is the exception to the φ rule and this deviation might be due to a combination of specific ligand interactions and the ionotropic effect, because this axis is bipolar like 〈001〉, but less unstable. The ultrathin films provide a good system for studying inorganic crystallization at organic interfaces. It was also shown that the SEM crystallite indexing method can be easily used to measure a large range of calcite orientations. This should support XRD studies and enhance future research in this area. In most of the specimens in this study the strongest influence on nucleation density and orientation appears to be ionotropic or electrostatic. However, the effect of ligand stereochemistry did have a measurable influence in the case of the immobilized NTA with its iminodiacetate ligand. It appears, though, that the orientational effect was less strong than that which can be achieved in the Langmuir monolayer studies where the ligands are presented in a two-dimensional lattice.2-5,14,15 However, direct comparison is not fully valid at this point because we have not quantified the density of the immobilized NTA. Toward the further study of the influence of functional group stereochemistry it is clear that the next step would be to both increase the number of ligand interactions per surface bound unit (i.e., immobilize face-specific oligomers or polymers) as well as increase the density of units, perhaps by employing selfassembled films.10,12 Acknowledgment. We thank Dr. Brian Peek for his advice in the area of surface modification and Dr. David Turner for his assistance with AFM. D.D.A. is supported by a National Research Council Research Associateship. This work was supported by the NRL Core Program in Crystal Engineering. LA950330A
