Organic-Free and Selectively Oriented Recrystallization for Design of

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Organic-Free and Selectively Oriented Recrystallization for Design of Natisite Microstructures Stanislav Ferdov* Department of Physics, University of Minho, 4800-058 Guimaraes, Portugal

bS Supporting Information ABSTRACT: In organic-free system, microstructures of natisite [Na2(TiO)SiO4] via mechanisms of selective and oriented dissolution and recrystallization have been revealed. The growth of faceted natisite crystals is followed by selective dissolution that extends along the c-axis limited by the borders of the pinacoidal face. A similar effect is also observed in nonfaceted disk-shaped crystals where the total number of steps of selective dissolution and recrystallization were observed in one batch including the (1) formation of disk-shaped crystals and (2) dissolution and recrystallization of the top central part of the crystal (3) that extend to the particle periphery and along the c-axis resulting in a ring-shaped crystal. In less alkaline conditions, a polyhedral aggregation of natisite particles give rise to pseudo-octahedral aggregates that undergo selective and oriented Ostwald ripening, linearly extended from the corners to the center of the particles, which created uniform four-gate hollow microstructures. The present work enabled us to reveal two mechanisms for selective formation of titanosilicate microstructures that can be applied for the construction of a new type of hollow aggregates with controlled number and location of the apertures.

’ INTRODUCTION In the past decade, hollow microstructures have attracted intense research due to their wide variety of applications comprising delivery vesicles for drugs, dyes, inks, microcontainers for artificial cells, protection shield for proteins, enzymes, DNA, and different catalytic reactions.1 Presently, the general strategy for preparation of hollow microstructures is via layer-by-layer self-assembly of preformed nanoparticles onto a sacrificial core as a template.2 The dimension and architecture of the obtained hollow microstructures are mainly determined by the template used. A disadvantage of this method is that the shells are difficult to keep intact after the removal of the template at a high calcination temperature. Alternative routes employed to prepare hollow microstructures involve Ostwald ripening,3 the Kirkendall effect,4 self-assembly of building blocks through hydrophobic interactions,5 or chemically induced self-transformation.6 The majority of reported hollow microstructures have been prepared in shape of spheres,6,7 and new morphological types are still a challenge. In this respect, there are prepared hollow nanospindels of PbWO4,8 octahedral SnO2,9 hexagon-based drums of ZnO,10 octahedral Cu2O nanocages,11 Cu2O hollow cubes,12 Cu2O nanoframes,13 18-facet polyhedral Cu7S4 hollow nanocages,14 Fe2O3 hollow nanocubes,15 CaTiO3 hollow cubes16 and nanocubes,17 hollow zeolite analcime icositetrahedra,18 and r 2011 American Chemical Society

PbTe nanoboxes.19 In a greater part of the structures, the hollow part is sealed by the inorganic shell, and the postsynthesis access to the empty space is not possible. In this respect, the preparation of hollow microstructures having an external access to the voids is another challenge. Nowadays, a number of microstructures prepared via one-step template-free, surfactant-assisted methods including Ostwald ripening have attracted significant interest due the less complicated fabrication. However, the detailed mechanism of formation of these microstructures has been just recently elucidated. Zhou and co-workers17,18,20 showed the phenomenon of reversed crystal growth whose appearance and detailed study in various materials will be a progressively growing area. In the last two decades, a new class of stable microporous and dense silicates that involves novel mixed polyhedral framework oxides has been synthesized in laboratory conditions. The archetypal material is based on titanosilicates, although there is tremendous scope for introducing many other transition metals. These materials not only have potential novel applications in the fields normally associated with zeolites but also possible applications in the areas of optoelectronics, nonlinear optics, batteries, Received: June 9, 2011 Revised: August 19, 2011 Published: August 24, 2011 4498

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Figure 1. Relations between the composition of the initial batch and the morphology of the obtained natisite crystals.

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Figure 3. Powder XRD patterns with indexed (00l) and (h00) reflections of different morphological types of natisite produced at (a) TiO2/ SiO2 = 1.2, Na2O/TiO2 = 8.3; (b) TiO2/SiO2 = 0.5, Na2O/TiO2 = 200; (c) TiO2/SiO2 = 0.2 0.5, Na2O/TiO2 = 8 15; (d) TiO2/SiO2 = 0.1, Na2O/TiO2 = 50; (e) TiO2/SiO2 = 0.05, Na2O/TiO2 = 100.

Figure 2. Refined unit cell parameters of the as-synthesized natisite samples (1 5) as compared with the ones in ICDD (6) (PDF No. 0541128).

magnetic materials, hierarchical pore structures, and sensors,21 which makes them of particular interest. To date, just a few reports on the synthesis of natisite are available, and no hollow or microstructures have been described.22 This material is unique with its rare five-coordinated titanium included in a water-free framework. The structure is composed of layers built of chess mate-arranged SiO4 tetrahedra sharing all of their corners with TiO5 semioctahedra. In between the layers resides one crystallographically unique Na cation that compensates the charge.22 In this work, an example of organic-free, selectively shaped microstructures and aggregates has been revealed. By way of variation of the Na2O/TiO2 and TiO2/SiO2 ratios, selectively perforated truncated bipyramidal and disk-shaped crystals as well as geometrically unique hollow microstructures of natisite aggregates are synthesized. The mechanisms of microstructural formation are discussed.

’ EXPERIMENTAL SECTION Synthesis. Detailed synthesis studies were held in the system:

aNa2O bTiO2 10SiO2 675H2O, where 30 e a e 100; 0.5 e b e 12, at a temperature of 200 °C, crystallization time of 24 h, and autogenous pressure. No organics were used as reactants or templates. The initial gels were obtained by mixing NaOH, SiO2, TiCl4 (SigmaAldrich), and distilled water in appropriate ratios. In a typical procedure,

Figure 4. Change of the fwhm of (200) reflection of natisite crystals with different morphology. SiO2 was added to the alkaline aqueous solution and then mixed with the hydrolyzed TiCl4 brought to the boiling point. The mixture was homogenized using a mechanical stirrer (300 rpm) for 60 min at room temperature. The gels were subsequently transferred into 45 mL Teflonlined autoclaves. The hydrothermal treatment was terminated by quenching of the autoclaves in cold water. The run products were washed with distilled water and dried at 50 °C. Characterization. The powder X-ray diffraction (XRD) patterns of the synthesized material were collected in a step-scan regime (step 0.02° and time 1 s) on a Bruker D8 Discover diffractometer with Cu Kα radiation in the range 2θ, 5 70°. The scanning electron microscopy (SEM) images and chemical analysis (EDS) were realized with NanoSEM-FEI Nova 200 (FEG/SEM). The EDS probing on the synthesized samples indicated average values of Na:Si:Ti = 1:0.6:0.4, which is in fair agreement with the expected ones for natisite Na:Si:Ti = 1:0.5:0.5. The Le Bail fits were performed using TOPAS-3 software package.23

’ RESULTS AND DISCUSSION Figure 1 presents the relation between the obtained crystal morphology and the initial batch composition. It was found that TiO2/SiO2 and Na2O/TiO2 ratios control not only the morphology but also the process of particle aggregation. By varying these two ratios, five different run products were observed as 4499

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Figure 5. (a) SEM images showing truncated bipyramidal crystals of natisite; the inset shows that together with the well-faced crystals exist crystals with partly dissolved {00l} face; (b) top view of well-faced crystal and (c) its stereographic projection.

follows—truncated bipyramidal crystals, disk-shaped crystals, mixture of disk- and needle-shaped crystals, pillowlike aggregates, and pillowlike aggregates with four well-defined apertures on the corners. In the selected experimental conditions when the ratio TiO2/SiO2 (1.2) in the initial batch is close to the one in the natisite structure [Na2(TiO)SiO4; TiO2/SiO2 = 1], it appeared well-faced crystals truncated bipyramidal crystals. However, when in the initial batch the content of SiO2 is significantly more (TiO2/SiO2 = 0.05 0.1) than the expected one in the formula of natisite, then the run product change to aggregates of particles. This is an indication that the highly nonstoichiometric initial batch triggers a crystal nucleation that leads to particle aggregation. This finding is valid only for the ratios of the structure-forming cations (Ti4+ and Si4+), and the concentration of the charge compensation Na+ cations can be far more deviated from the expected in the crystal structure. Another interesting aspect is that the obtained aggregates can be prepared with or without well-defined openings on the corners. It was found that the increased Na2O/TiO2 ratio contributes for the formation of four well-defined holes on the particle corners. In the cases when in the initial gel the TiO2/SiO2 ratio varies between 0.2 and 0.5, the run product can be as disk-shaped crystals (high alkaline content) or mixture of disk- and needle-shaped crystals (low alkaline content). The synthesized samples were initially studied by powder XRD. All diffractograms can be indexed in tetragonal natisite structure with space group P4/nmm and starting from the

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Figure 6. SEM images of truncated bipyramidal crystals of natisite showing (a) well localized selective dissolution of {00l} face and undermined surface layer; (b) a crystal with {00l} face in its initial stages of dissolution (c) showing a few rectangular carved borders.

previously reported lattice parameters [a = b = 6.49(7); c = 5.08(4) Å].22 The Le Bail analyses showed small enlargement of the lattice dimension in the well-faceted truncated bipyramidal crystals (Figure 2 and Figure S1 in the Supporting Information). The powder XRD pattern of the same sample shows strong preferential orientation along (00l) and broaden (h00) reflections with diminished intensity (Figures 3 and 4). Apparently, this is an indication for structural defects along (h00) planes, which was supported by the estimated longer a = b parameter of the lattice (Figure 2). Considering the layered nature of the natisite structure, a possible reason for the observed broadening can be related with stack disorder of the layer sequences, but this can be proved by further polymorph modeling. Less intense but similar preferential orientation along (00l) was observed in the disk-shaped and in the mixture of disk- and needle-shaped crystals (Figure 3). Here, the peak broadening of (h00) reflections are smaller, and the refined unit cell parameters show similar values to standard ones (Figure 2a c). As it was mentioned, the pillowlike aggregates can be prepared with or without well-defined holes on the corners as the particles with less open holes show higher preferred orientation (Figure 3). It is visible that the process of face dissolution is not regular and affects some of the particles, but in all cases, the dissolution starts from the top central parts of the crystal surface, and it is limited by the borders of {00l} face (Figure 5). Another peculiar characteristic is the rectangular profile of some of excavated pinacoidal faces (Figure 6). In fact, the observed figures of dissolution are a strong indication of the 4500

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Figure 7. SEM image of (a) disk-shaped crystals with vulnerable top central part and mixture of (Na2O/TiO2 = 200) and (b) disk- and needle-shaped natisite crystals (Na2O/TiO2 = 8 15). The inset shows a particle with a rectangular carved top part.

real symmetry of the crystal, which is well-known phenomenon,24 however, to the best of our knowledge, this effect has not been observed in titanosilicates. In other particles, the dissolution is considerably progressed, and an undermined thin layer is visible (Figure 5), which indicates a well-defined selectivity of the observed process. SEM images of the as-synthesized truncated bipyramidal crystals are shown in Figure 5. It was noticed that some of the crystal faces suffered a selective dissolution that has drawn attention for a more detailed study. By rule, in the tetragonal syngony, there are three simple forms that cross the main vertical axis and the horizontal axes. Thus, three of the forms must be parallel to the main axis, and only one should cross it. Additionally, there are always three simple forms of pyramidal type, three prisms and one basal pinacoide. Hence, the indexed simple forms of the truncated bipyramidal crystals indicated three different crystal faces {001}, {h0l}, and {0kl}, which possess four 2-fold and one 4-fold axes (Figure 5). Considering the Curie Wulff25 rule and Bravis26 law, the observed pinacoidal crystal faces should have a higher surface energy than the pyramidal ones, and when a change of the physicochemical conditions occurs, these faces will be the first to suffer changes in an attempt to minimize the surface energy of the particle. In some crystals with integral {00l} faces (Figure 6a), curvatures and increased interfacial area can be seen, which is the first indication that the crystal tries to decrease its surface energy27 via Ostwald ripening.28 At this stage, two scenarios are possible—morphology change or intense process of coalescence of lattice vacancies that will create growing pinholes on the face with the highest surface energy. Apparently, the second sequence of events brought to a well localized hole on the pinacoidal face. The peculiar finding here is that at the applied organic-free conditions the only face to suffer dissolution is {00l}, and the pyramidal faces remain intact. A similar process but in its complete stage was observed in the disklike crystals. These crystals can be prepared alone or in mixture with needlelike ones (Figure 7). The high Na2O/TiO2 ratio leads to the formation of disklike crystals, while the low one brings a mixture of disklike and needlelike crystals of natisite. The main difference between the two disk shapes is the apparent vulnerability of the top central part of the crystals synthesized at a higher alkaline content. The coincident location of the most vulnerable crystal part is indirect evidence for similar orientation of the disk-shaped and truncated bipyramidal crystals. Thus, the rectangular-shaped forms

of dissolution on the top of natisite particles were also observed (Figure 7). Another similar aspect is the smeared truncated bipyramidal shape and outlined edges whose relicts can be seen in the rims of some particles (Figure 8a,a1). It is interesting to note that at the applied synthesis conditions the run product from one batch comprises different evolution steps of dissolution and recrystallization entirely localized along the c-axis of the crystals (Figure 8). In the beginning, small apertures reveal the growth of well-faceted crystals that consumed the tiny layer of the relatively flat surface. The beginning of process comprises the top central parts of the particle (Figure 8a,a1). Further development contours the undermined top thin layer that is extensively consumed and traced by circular borders comprising the central aperture (Figure 8b,b1). The excavation of the top central part continues localized along the c-axis creating a chiseled aperture (Figure 8c,c1). At this stage, there is a well-formed opening whose central part is almost empty and complete along the c-axis. As a result from the recrystallization, it is visible that a pile of bigger crystals is the last to be dissolved (Figure 8c,c1). In some particles, the aperture in the middle is completely empty, and it has a bigger diameter, indicating a further evolution of the process from the middle to the periphery (Figure 8d). Because of the relatively short time for synthesis, it was not possible to capture a stage where all particles have uniformity. This demonstrates that the process of nucleation, crystal growth, and recrystallization is not homogeneous. The advantage of this aspect is that at the same time can be seen different points in time of the crystal evolution. Summarizing, in a like manner, the truncated bipyramidal crystals and the disk-shaped ones suffer changes localized on the top central part of the particle. The SEM images indicate a combination of processes that attempt to minimize the surface energy of the crystal. In the beginning, sites with relatively high surface energies (typical examples include steps, point defects, and stacking faults),29 concentrated on the top central part of the crystal, provoke a recrystallization. At the same time, relicts of pyramidal crystal faces are observed. These parts of faces are not complete counterpart of the ones in the truncated bipyramidal crystals and together with the highly extended interfacial area demonstrate competition between the process of morphology reconstruction and development of crystal defects. Finally, the process of dissolution vanishes the crystal faces, and it crates a well-localized aperture in the middle of the particle. Another interesting phenomenon is the formation of a new type of hollow microstructure, four gate polyhedral aggregates of natisite (Figure 9). By varying the above-mentioned chemical 4501

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Figure 9. (a) SEM image of the as-synthesized four-gate hollow micropillows of natisite. (b and c) SEM images of the natisite particles showing details of the localized openings.

Figure 8. Disk-shaped natisite crystals from one and the same batch showing different stages of selectively localized recrystallization and dissolution: (a and a1) initial stage of recrystallization with well-defined faceted crystals under the consumed flat surface, (b and b1) traced and partly consumed undermined layer with well-defined hole in the middle, (c and c1) collapsed layer with well-defined hole in the middle of the particle, and (d) advanced stage of dissolution where a ring-shaped crystal with a big aperture is formed.

composition of the initial gel, these aggregates were prepared in uniform particles whose four corners can be with or without well-defined opening (Figures 9 and 10). A broken micropillow shows well-defined and localized directions of recrystallization (Figure 11). Apparently, the orientation of the process follows

Figure 10. SEM image of pillowlike natisite particles with semiopened corners.

lines between the four corners of the pseudo-octahedral aggregates. As a result, cross-like channels are formed. The top borders of these channels are marked by big, partly faced crystals. Considering that these crystals are not observed on the surface of the aggregates, one can conclude that the process of recrystallization starts from the corner and selectively continues to the internal parts of the aggregates. As a support of this state, in the center of the broken pillow, an aggregation of smaller particles 4502

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was observed, and no hollow space is visible. This indicates that the corners can be considered as a starting point for the Ostwald ripening. Within the internal part of the aggregate, the coexistence of small and big crystals is well visible, which indicates that during its formation existed an oriented particle size distribution that later triggered the oriented Ostwald ripening. Briefly, unlike the common case where the Ostwald ripening leads to the formation of relatively homogeneous hollow space, the presented example shows selective and oriented process that creates a specific cross-like hollow space. This is a phenomenon that to the best of our knowledge has not been described so far. A time-dependent study can reveal if the hollow part will comprise the whole particle or will stay localized. The experimental results obtained in the present work clearly demonstrated the formation mechanisms of the natisite hollow crystals as illustrated in Figure 12. The truncated bipyramidal and disk-shaped crystals undergo one and the same selective dissolution of the top central part, but in the latter, the process progressed until the formation of a ring-shaped crystal. The pseudo-octahedral natisite aggregates were subjected to a rare type of selective transformation. As a result, a cross-like hollow space accessible by four geometrically unique apertures was formed.

Figure 11. (a) SEM image of broken hollow micropillow and (b) indicating the directions of recrystallization.

’ CONCLUSION In conclusion, we have developed a simple organic-free procedure for the synthesis of different naisite microstructures, and the mechanisms for their formation are established and described. These are the first reported titanosilicate microstructures. The novelties of the work include (1) preparation of faceted and nonfaceted natisite crystals with selectively located holes on the top central part of the solid and (2) a new type of polyhedrally shaped hollow aggregates with external access to the

Figure 12. Schematic illustration showing two different Ostwald ripening mechanisms in (a) truncated bipyramidal, (b) disk-shaped, and (c) pillowlike crystals. 4503

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Crystal Growth & Design hollow part via four apertures situated on the corners of the particle. The selective induction of the holes formation is explained via Curie Wulff rule and Bravis law; however, the formation of the hollow pseudo-octahedral aggregates follows other principles. By careful analysis of the internal space relation between the particles comprising the hollow area, it is demonstrated that the shaping of the cross-like channels of pseudooctahedral aggregates extends outward via Ostwald ripening. In fact, it follows a route based on selectively oriented aggregation of particles with different sizes and distributions.

’ ASSOCIATED CONTENT

bS

Supporting Information. Extraction from the Le Bail fits. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +351 253 510 468. Fax: + 351 253 510 461. E-mail: sferdov@fisica.uminho.pt.

’ ACKNOWLEDGMENT This work was supported by FCT, project PTDC/CTM/ 108953/2008. ’ REFERENCES (1) (a) Tamber, H.; Johansen, P.; Merkle, H. P.; Gander, B. Adv. Drug Delivery Rev. 2005, 57, 357–376. (b) Trehan, A.; Sinha, V. R. J. Controlled Release 2003, 90, 261–280. (c) Tracy, M. A. Biotechnol. Prog. 1998, 14, 108–115. (d) Raymond, M.-C.; Neufeld, R. J.; Poncelet, D. Artif. Cell Blood Sub. 2004, 32, 275–291. (e) Hammer, D. A.; Discher, D. E. Annu. Rev. Mater. Sci. 2001, 31, 387–404. (f) Molvinger, K.; Quignard, F.; Brunel, D.; Boissiere, M.; Devoisselle, J.-M. Chem. Mater. 2004, 16, 3367–3372. (g) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908–7914. (2) (a) Zhong, Z. Y.; Yin, Y. D.; Gates, B.; Xia, Y. N. Adv. Mater. 2000, 12, 206–209. (b) Zhao, X. F.; Cheung, T. L. Y.; Zhang, X. T.; Ng, D. H. L.; Yu, J. G. J. Am. Ceram. Soc. 2006, 8, 2960–2963. (c) Caruso, F.; Lichtenfeld, H.; Giersig, M.; M€ohwald, H. J. Am. Chem. Soc. 1998, 120, 8523–8524. (d) Valtchev, V.; Mintova, S. Microporous Mesoporous Mater. 2001, 43, 41–49. (e) Valtchev, V. Chem. Mater. 2002, 14, 956– 958. (f) Valtchev, V. Chem. Mater. 2002, 14, 4371–4377. (3) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492–3495. (4) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714. (5) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348–351. (6) Yu, J.; Guo, H.; Davis, S. A.; Mann, S. Adv. Funct. Mater. 2006, 16, 2035–2041. (7) (a) Xiang, Q.; Yu, J.; Jaroniec, M. Chem. Commun. 2011, 47, 4532–4534. (b) Yu, J.; Zhang, J. Dalton Trans. 2010, 39, 5860–5867. (c) Liu, S.; Yu, J.; Jaroniec, M. J. Am. Chem. Soc. 2010, 132, 11914– 11916. (d) Yu, H.; Yu, J.; Liu, S.; Mann, S. Chem. Mater. 2007, 19, 4327– 4334. (e) Cai, W.; Yu, J.; Mann, S. Microporous Mesoporous Mater. 2009, 122, 42–47. (f) Cai, W.; Yu, J.; Gu, S.; Jaroniec, M. Cryst. Growth Des. 2010, 10, 3977–3982. (8) Geng, J.; Zhu, J.-J.; Lu, D.-J.; Chen, H.-Y. Inorg. Chem. 2006, 45 (20), 8403–8407. (9) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930–5933.

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