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Ca2+-Keggin Anion Colloidal Particles as Templates for the Growth of Star-Shaped Calcite Crystal Assemblies Debabrata Rautaray, Sudhakar R. Sainkar, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune - 411 008, India Received January 24, 2003. In Final Form: May 29, 2003 The formation of star-shaped CaCO3 crystals by reaction of an aqueous solution of phosphotungstic acid and Ca2+ ions with CO2 is described. The phosphotungstate ions [(PW12O40)3- Keggin ions] complex with the Ca2+ ions in solution leading to the formation of highly uniform nanometer-sized colloidal particles of the salt, calcium phosphotungstate. Reaction of these nearly spherical colloidal particles with CO2 results in the growth of calcite with very regular, star-shaped morphology. Energy dispersive analysis of X-rays of the star-shaped calcite particles indicated the presence of the Keggin ions in the crystals suggesting that they play the role of nanoprecursors in the growth of calcite crystals with such unique morphology.
Introduction Control over the crystallography and morphology of minerals is an important goal in the area of crystal engineering. Synthesis of advanced inorganic materials with control over crystallographic structure, size, and morphology is often driven by commercial requirements in areas as diverse as electronics, pigments, cosmetics, ceramics, and medical industries.1,2 Insofar as ceramic engineering is concerned, much of the research has centered on the use of biomimetic templates such as Langmuir monolayers at the air-water3a-e and liquidliquid interface,3f,g self-assembled monolayers (SAMs),4 lipid bilayer stacks,5 and functionalized polymer surfaces6 to achieve such control. This is not surprising given the exquisite range of morphologies exhibited by biominerals in comparison with their synthetic counterparts. An important example is that of CaCO3 in the skeletal plate of sea urchins where a porous, spongelike fenestrated structure of calcite crystals is observed which bears little resemblance to the regular rhombohedral morphology of synthetic calcite.7,8 Biominerals based on calcium carbonate are complicated by formation of the three stable polymorphs, viz., calcite, * To whom correspondence should be addressed. Ph: +91 20 5893044. Fax: +91 20 5893952. E-mail:
[email protected]. (1) Mann, S.; Ozin, G. A. Nature 1996, 382, 313. (2) Matijevic, E. Curr. Opin. Colloid Interface Sci. 1996, 1, 176. (3) (a) Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492. (b) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681. (c) Litvin, A. L.; Valiyaveettil, S.; Kaplan, D. L.; Mann, S. Adv. Mater. 1997, 9, 124. (d) Buijnsters, P. J. J. A.; Donners, J. J. J. M.; Hill, S. J.; Heywood, B. R.; Nolte, R. J. M.; Zwanenburg, B.; Sommerdijk, N. A. J. M. Langmuir 2001, 17, 3623. (e) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9 and references therein. (f) Rautaray, D.; Kumar, A.; Reddy, S.; Sainkar, S. R.; Pawaskar, N. R.; Sastry, M. CrystEngComm 2001, 45. (g) Reddy, S.; Rautaray, D.; Sainkar, S. R.; Sastry, M. Bull. Mater. Sci. 2003, 26, 283. (4) (a) Kuther, J.; Nelles, G.; Seshadri, R.; Schaub, M.; Butt, H. J.; Tremel, W. Chem. Eur. J. 1998, 4, 1834. (b) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (5) (a) Sastry, M.; Kumar, A.; Damle, C.; Sainkar, S. R.; Bhagwat, M.; Ramaswamy, V. CrystEngComm 2001, 21. (b) Rautaray, D.; Kumar, A.; Reddy, S.; Sainkar, S. R.; Sastry, M. Cryst. Growth Des. 2002, 2, 197. (c) Damle, C.; Kumar, A.; Bhagwat, M.; Sainkar, S. R.; Sastry, M. Langmuir 2002, 18, 6075. (6) (a) Feng, S.; Bein, T. Science 1994, 265, 1839. (b) Falini, G.; Gazzano, M.; Ripamonti, A. Adv. Mater. 1994, 6, 46. (c) Donners, J. J. J. M.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. J. Am. Chem. Soc. 2002, 124, 9700. (7) Donnay, G.; Pawson, D. L. Science 1969, 166, 1147. (8) Park, R. J.; Meldrum, F. C. Adv. Mater. 2002, 14, 1167.
aragonite, and vaterite.9 In addition to the biomimetic templates mentioned above, an interesting possibility is the use of functionalized nanoparticles as nucleating sites for the growth of calcium carbonate crystals.10,11 Tremel and co-workers have shown that the transition from SAMs of mercaptophenols on gold thin films (two-dimensional Euclidean space) to mercaptophenol-derivatized gold colloidal particles (curved two-dimensional space) as templates for the crystallization of CaCO3 and SrCO3 resulted in interesting differences in morphology of the minerals.10c Such an approach based on functionalized nanoparticles as nucleation centers enables the synthesis of much larger quantities of the crystals by “heterogeneous nucleation in homogeneous solutions” and leads to frustrated growth of the crystals consequent to the nanoscale curvature of the template.10c Keggin ions form a subset of polyoxometalates and have the general formula (XM12O40)(8-n)-, where M stands for W or Mo and X stands for heteroatoms such as P, Si, or Ge with n being the valency of X. Keggin ions undergo stepwise multielectron redox processes without undergoing a structural change.12 Recently, Troupis, Hiskia, and Papaconstantinou have shown that photochemically reduced Keggin ions [(SiW12O40)4-] when exposed to aqueous metal ions such as Ag+, AuCl4- , Pd2+, and PtCl62- resulted in the formation of the corresponding metal nanoparticles of reasonable monodispersity.13 Matijevic and co-workers have shown that salts of Keggin ions with cesium,14a thorium, and zirconium14b cations can form uniform micron-sized colloidal particles in an aqueous medium. In this paper, we develop the approach of Matijevic et al. and investigate the possibility of using nanometer-size colloidal particles of Keggin anions complexed with Ca2+ cations as a new (9) McGrath, K. M. Adv. Mater. 2001, 13, 989. (10) (a) Nagtegaal, M.; Seshadri, R.; Tremel, W. Chem. Commun. 1998, 2139. (b) Kuther, J.; Seshadri, R.; Nelles, G.; Butt, H.-J.; Knoll, W.; Tremel, W. Adv. Mater. 1998, 10, 401. (c) Kuther, J.; Seshadri, R.; Nelles, G.; Assenmacher, W.; Butt, H.-J.; Mader, W.; Tremel, W. Chem. Mater. 1999, 11, 1317. (11) Lee, I.; Han, S. W.; Choi, H. J.; Kim, K. Adv. Mater. 2001, 13, 1617. (12) (a) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (b) Kogan, V.; Izenshtat, Z.; Neumann, R. Angew. Chem., Int. Ed. 1999, 38, 3331. (13) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Angew. Chem., Int. Ed. 2002, 41, 1911. (14) (a) Perez-Maqueda, L. A.; Matijevic, E. Chem. Mater. 1998, 10, 1430. (b) Koliadima, A.; Perez-Maqueda, L. A.; Matijevic, E. Langmuir 1997, 13, 3733.
10.1021/la034128g CCC: $25.00 © 2003 American Chemical Society Published on Web 06/28/2003
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Figure 1. SEM images recorded from (A) calcium phosphotungstate colloidal particles formed by reacting an aqueous solution of phosphotungstic acid with Ca2+ ions for 1 h (see text for details) and (B-D) star-shaped CaCO3 crystals under different stages of magnification obtained by reacting calcium phosphotungstate colloidal particles (shown in (A)) with CO2 for 2 days. The inset of (A) shows the growth of CaCO3 crystals obtained by reacting CaCl2 with CO2 in solution.
class of inorganic templates for the growth of CaCO3 crystals. More specifically, we show below that reaction of an aqueous solution of phosphotungstic acid [PTA, H3(PW12O40)] with Ca2+ ions leads to the formation of highly uniform, stable colloidal particles of calcium phosphotungstate. The calcium phosphotungstate colloidal particles may then be reacted with CO2 to yield large quantities of star-shaped CaCO3 crystals of uniform size in solution. X-ray diffraction analysis of the star-shaped structures indicated that they were composed of calcite crystallites. Presented below are details of the investigation.
of 30 mA, while SEM measurements were carried out on a Leica Stereoscan-440 instrument equipped with a Phoenix energy dispersive analysis of X-rays (EDAX) attachment. The Ca-PTA colloidal particles and CaCO3 crystals collected on Si(111) substrates were analyzed by FTIR spectroscopy in the diffuse reflectance mode at a resolution of 4 cm-1 on a Perkin-Elmer FTIR Spectrum-1 spectrometer. To understand the role of the Ca-PTA colloidal particles in modulating the morphology of the CaCO3 crystals, control experiments were performed wherein crystallization of CaCO3 was accomplished directly in solution by bubbling of CO2 in an aqueous solution of CaCl2 under an electrolyte concentration identical to that mentioned above. The CaCO3 crystals in the control experiment were collected and subjected to SEM analysis.
Experimental Details Phosphotungstic acid (H3PW12O40) and calcium chloride were obtained from Aldrich Chemicals and used as received. An aqueous mixture of CaCl2 (50 mL, 1 × 10-3 M) and phosphotungstic acid (50 mL, 1 × 10-3 M) solutions was taken in a beaker and allowed to react for 1 h. The aqueous mixture of CaCl2 and PTA solutions was clear even after 1 h of mixing. CO2 was then bubbled slowly at a volumetric flow rate of 480 mL/h through this solution, and aliquots were taken from the reaction medium after 12, 24, 36, and 48 h of reaction for further analysis. After about 10 h of CO2 bubbling, a faint turbidity was observed in the solution that increased progressively with time. The aliquots at different times of reaction were allowed to stand, thus enabling the CaCO3 crystals to settle at the bottom of the container. The CaCO3 crystals were then loaded onto glass substrates for X-ray diffraction (XRD) studies and Si(111) substrates for scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy measurements. Films of calcium phosphotungstate (Ca-PTA) colloidal particles prior to reaction with CO2 were also analyzed by XRD, SEM, and FTIR spectroscopy. XRD measurements of the different films were carried out in the transmission mode using Cu KR radiation on a Philips PW 1830 instrument operating at 40 kV and a current
Results and Discussion Figure 1A shows a representative SEM image recorded from a drop-cast film of the solution obtained by reaction of aqueous CaCl2 and H3PW12O40 for 1 h on a Si(111) substrate. The image shows highly uniform colloidal particles of predominantly spherical morphology. An analysis of 100 such particles from several SEM micrographs resulted in determination of an average particle size of 57 ( 8 nm. In earlier studies on the reaction of Keggin ions with metal cations, the formation of similar highly uniform spherical colloidal particles has been observed.14 Matijevic et al. have reported the formation of uniform crystalline colloidal particles of cesium phosphotungstate and amorphous thorium phosphotungstate particles of size ∼ 1-2 µm prepared by aging solutions containing the metal ions and phosphotungstic acid in stoichiometric ratios at 90 °C for 1 h.14a It was also observed that the presence of an anionic surfactant AVANEL S-150 in the reaction medium modulated the particle shape of Cs-phosphotungstate from spherical to an octahedral
Star-Shaped Calcite Crystal Assemblies
Figure 2. XRD patterns recorded from films of calcium phosphotungstate colloidal particles (curve 1) and CaCO3 crystals obtained by reacting the calcium phosphotungstate particles with CO2 (curve 2, see text for details), deposited on glass. The Bragg reflections marked *, +, and O correspond to crystalline calcium phosphotungstate, calcite, and vaterite phases, respectively.
Figure 3. (A) FTIR spectra of calcium phosphotungstate colloidal particles (curve 1) and calcium carbonate crystals obtained by reacting calcium phosphotungstate particles with CO2 (curve 2). (B) EDAX spot profile analysis of one of the star-shaped CaCO3 crystals shown in Figure 1D.
morphology.14a EDAX analysis of one of the colloidal particles shown in Figure 1A confirmed the presence of W, P, O, and Ca (data not shown). No carbon signal was observed. A quantitative analysis of the various elements yielded a Ca/P/W/O ratio of 3:1.7:22.5:65 in good agreement with the expected ratio of 3:2:24:80 from a stoichiometric complex of the form Ca3(PW12O40)2]. The XRD pattern recorded from the Ca-PTA colloidal particle film of Figure 1A is shown in Figure 2, curve 1. A number of Bragg reflections are identified (marked with *) and have been indexed with reference to the XRD pattern obtained by Koliadima et al. for thorium and zirconium tungstosilicic particles.14b From the line broadening of the Bragg reflections in the XRD pattern of the Ca-PTA colloidal particles and the Debye-Scherrer equation,15 the average particle size was calculated to be 55 nm which is consistent with the particle size of 57 ( 8 nm obtained from the SEM analysis. The Debye-Scherrer analysis was based on the use of a Lorentzian fitting function and is, therefore, not particularly sophisticated. Figure 3A shows the FTIR spectrum recorded from the Ca-PTA colloidal particle sample on a Si(111) substrate (curve 1). Prominent (15) Jeffrey, J. W. Methods in crystallography; Academic Press: New York, 1971.
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absorption bands are seen at 983 and 1080 cm-1. The band at 983 cm-1 is assigned to the W-O stretching vibration, while the 1080 cm-1 band arises from the P-O stretching vibration of the primary Keggin anion structure.14a From the EDAX, XRD, and FTIR results, we conclude that the reaction of calcium ions with phosphotungstic acid results in the formation of highly uniform crystalline calcium phosphotungstate colloidal particles of 57 ( 8 nm average size. Figure 1B-D shows SEM images at different magnifications recorded after the formation of CaCO3 crystals by bubbling CO2 into the calcium phosphotungstate colloidal particle solution for 2 days. The low-magnification SEM image (Figure 1B) shows the presence of densely populated, very regular star-shaped CaCO3 crystals of uniform size (average size ∼ 1-8 µm). That the reaction of almost completely spherical calcium phosphotungstate colloidal particles with CO2 results in the evolution of such symmetrical particles is a salient feature of this work. The EDAX spectrum recorded from one of the star-shaped particles is shown in Figure 3B. An analysis of the EDAX spectra recorded from eight different star-shaped particles yielded an average atomic percentage contribution of 2.87% Ca, 10.1% C, 64.38% O, 20.97% W, and the remaining 1.75% P to the particles, thus clearly showing the presence of the Keggin ions in the symmetrical structures. The Ca/P and Ca/W ratios in the star-shaped calcite structures of 1.63:1 and 1:7.31 are close to those observed in the precursor Ca-PTA colloidal particles, indicating that Ca-PTA particles are indeed the source of calcite crystallites by reaction with CO2. An increase in the carbon signal is observed in the star-shaped CaCO3 assemblies which we recollect was not present in the precursor PTA particles. This clearly indicates reaction of calcium ions in the Ca-PTA colloidal particles and the formation of CaCO3 in the star-shaped crystal assemblies. The XRD pattern recorded from the CaCO3 crystals shown in Figure 1B is displayed in Figure 2, curve 2. A number of Bragg reflections are identified and have been indexed. The reflections identified by a + sign arise from the CaCO3 phase and have been indexed with reference to the unit cell of the calcite crystallographic structure (a ) b ) 4.989 Å, c ) 17.062 Å, space group D3D6 - R3*c).4b,16 There is also evidence of formation of a small percentage of the polymorph vaterite in the XRD pattern (circles, Figure 2, curve 2). In addition to these Bragg reflections, reflections observed in the precursor calcium phosphotungstate colloidal particles persist in the star-shaped calcite particles as well (reflections labeled as stars) indicating that not all of the calcium phosphotungstate phase has reacted with CO2. While we could not estimate the percentage contribution of the unreacted Ca-PTA phase in the sample, analysis of the calcite and vaterite phases yielded an 86 and 14% contribution, respectively, of these two phases. The XRD data thus clearly indicate that the star-shaped CaCO3 crystals seen in the SEM image (Figure 1B) belong to the calcite polymorph. Higher magnification SEM images of the individual CO2-reacted Ca-PTA particles (Figure 1C,D) show the morphology of the particles in much greater detail. The CaCO3 particles appear to be composed of flat calcite crystallites self-assembled into the characteristic starshaped superstructure (Figure 1D). The morphology of the calcite crystallites bears no resemblance to the wellknown rhombohedral structure normally observed in (16) The XRD patterns were indexed with reference to the unit cell of the calcite structure (a ) b ) 4.989 Å, c ) 17.062 Å, space group D3D6 - R3c, ASTM chart card no. 5-0586).
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Figure 4. (A-D) Representative SEM images of CaCO3 crystals obtained by reacting calcium phosphotungstate colloidal particles with CO2 at different time intervals of reaction.
calcite crystals. All the CaCO3 particles possess a 3-fold rotation symmetry about an axis perpendicular to the plane of the substrate. Furthermore, the star-shaped structures appear to have additional symmetry perpendicular to the plane of the substrate. The SEM images indicate that the structures are made of two symmetrical tetrahedral units joined at one vertex but rotated by 60° relative to each other. A Debye-Scherrer analysis of the line-broadening of the Bragg peaks corresponding to the calcite phase resulted in an average calcite crystallite size of ca. 64 nm. The size of the calcite crystallites is consistent with the size of the precursor Ca-PTA colloidal particles (Figure 1A) and suggests a mechanism for the formation of star-shaped calcite assemblies by a possible method of self-assembly. This will be discussed below. In addition to the star-shaped calcite crystals observed in Figure 1C, a big spherical crystal is observed at the top right-hand corner of the image (Figure 1C). These spherical structures occurred with a much smaller frequency than the starshaped structures seen in Figure 1D. EDAX analysis of the large spherical structure yielded only Ca, C, and O signals at a ratio close to that expected for CaCO3. No evidence of either a W or P signal from the Keggin ions was observed in the EDAX analysis of the spherical particles. We believe these large structures could correspond to CaCO3 crystals of either the calcite or vaterite polymorph grown in solution without mediation of Keggin ions. As mentioned briefly in the Introduction, the ability to grow calcite crystals with such complex morphology is an important challenge to materials chemists and crystal engineers.7,8 Further evidence for the growth of the calcite polymorph with the novel Ca-PTA inorganic precursor was provided by FTIR measurements of the star-shaped CaCO3 crystals (Figure 3A, curve 2). Prominent absorption bands are observed at 889 cm-1 (W-O-W stretching vibration) and 712 cm-1 in the CaCO3 crystals. The latter
vibrational mode is characteristic of calcite17 and thus agrees with the XRD evidence provided above. The W-O (983 cm-1) and P-O (1080 cm-1) stretch vibrations observed in the calcium phosphotungstate film (Figure 3A, curve 1) have disappeared after the formation of calcite, indicating some disruption to the native Keggin structure consequent to calcite growth. The SEM, XRD, and FTIR results presented above show that colloidal particles of Ca-PTA of ca. 57 nm sizes are transformed to star-shaped CaCO3 particles of dimensions 1-8 µm after reaction with CO2 and that post-CO2 reaction, the particles obtained consist of a mixture of unreacted Ca-PTA, calcite, and vaterite. To understand better the formation of large, star-shaped calcite assemblies, the kinetics of crystallization was monitored as a function of time of reaction of calcium phosphotungstate colloidal particles with CO2. Figure 4A-D shows representative SEM micrographs recorded after 12, 24, 36, and 48 h of bubbling CO2 in the Ca-PTA colloidal particle solution, respectively. After 12 h of reaction (Figure 4A), the colloidal particles have already grown to ca. 0.5-0.8 µm size but with little evidence of the final star-shaped structure. The particles show some variation in morphology from the predominantly spherical shape observed in the precursor Ca-PTA colloidal particles (Figure 1A). After 24 h of reaction of Ca-PTA with CO2, a very large percentage of the particles have grown in size and now exhibit a distinct and highly irregular morphology. From this image, it appears that growth of the CaCO3 crystallites proceeds from within the precursor Ca-PTA colloidal particles and leads to their assembly on the surface of the precursor particles. After 36 h of reaction (Figure 4C), the colloidal particles have evolved considerably in size and shape and (17) (a) Naka, K.; Tanaka, Y.; Chujo, Y. Langmuir 2002, 18, 3655. (b) Nassrallah-Aboukais, N.; Boughriet, A.; Laureyns, J.; Aboukais, A.; Fischer, J. C.; Langelin, H. R.; Wartel, M. Chem. Mater. 1998, 10, 238.
Star-Shaped Calcite Crystal Assemblies
the underlying Ca-PTA core that was quite evident in the particles after 24 h of reaction (Figure 4B) is no longer seen. The overall morphology of the particles bears a clear resemblance to the final star-shaped morphology obtained after 48 h of reaction (Figure 4D). From the timedependent SEM studies discussed above, we believe the following mechanism is responsible for formation of the star-shaped calcite crystallite assembly. During the initial stages of CO2 bubbling (up to ∼12 h), the Ca-PTA colloidal particles continue to age and grow thereby increasing in size from 57 nm to ca. 0.5 µm. During further CO2 reaction, CaCO3 growth takes place from within the Ca-PTA precursor particles. The percentage of the CaCO3 phase (seen as flat calcite particles evolving from the underlying Ca-PTA colloidal particle template) increases with time of reaction, finally leading to the characteristic star-shaped structures after 48 h of reaction. It is not unreasonable to expect that periodic arrangement of Ca2+ ions within the crystalline calcium phosphotungstate colloidal superstructure would lead to CaCO3 growth along welldefined directions in the colloidal particle. We speculate that these well-defined calcite growth directions in the calcium phosphotungstate crystals could be along ion channels in the crystal through which CO2 can diffuse rather easily. Such a channeling effect is frequently observed in Rutherford backscattering studies of crystals wherein the crystal becomes “transparent” to high-energy He ions along specific directions.18 Therefore, the symmetry of the star-shaped CaCO3 crystals observed should mirror the symmetry of ion channels in the calcium phosphotungstate colloidal particles. Further experiments are in progress using different Keggin ions and other polyoxometalates to understand this line of thought better. The large increase in size of the assemblies between 36 and 48 h of reaction is interesting and may be due to growth in size of the individual precursor particles further to formation of additional CaCO3 or to self-assembly of some of the individual structures (seen in Figure 4C). The latter process appears unlikely given that the much (18) Johnson, B. C.; McCallum, J. C. Nucl. Instrum. Methods B 2002, 190, 602.
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smaller calcite crystallite assemblies after 36 h of reaction (Figure 4C) already possess a rudimentary starlike superstructure. The correspondence in size estimated from XRD analysis between the calcite crystallites and the underlying Ca-PTA precursor appears to be fortuitous. It is clear that the Ca-PTA colloidal particles play an important role in formation of star-shaped calcite crystal assemblies. This is convincingly shown by a control experiment wherein CaCO3 crystals were grown in solution in the absence of PTA under conditions similar to those used for the above studies. The inset of Figure 1A shows a representative SEM image of CaCO3 crystals grown in solution in the control experiment. It is clear that the CaCO3 particles possess a morphology quite different from that observed for crystals grown in the presence of calcium phosphotungstate colloidal particles. A number of rhombohedral crystals are observed in the control experiment, and XRD measurements of films of the crystals confirmed that they were indeed calcite crystals (data not shown). In conclusion, the formation of star-shaped CaCO3 crystals by reaction of an aqueous solution of phosphotungstic acid and Ca2+ ions with CO2 is described. The phosphotungstate ions [(PW12O40)3- Keggin ions] complex with the Ca2+ ions in solution leading to the formation of highly uniform nanometer-sized colloidal particles of the salt, calcium phosphotungstate. Reaction of these colloidal particles with CO2 results in the growth of calcite with very regular star-shaped morphology from the calcium phosphotungstate precursor phase. The metal cationKeggin ion colloidal particles thus represent a new class of inorganic precursor material with exciting possibilities for the growth of different minerals with potentially different morphologies and will form the basis of future studies. Acknowledgment. D.R. thanks the Department of Science and Technology (DST), Government of India, for financial assistance. LA034128G
