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Mesocrystals of Rutile TiO2: Mesoscale Transformation, Crystallization, and Growth by a Biologic Molecules-Assisted Hydrothermal Process Shu-Juan Liu, Jun-Yan Gong, Bo Hu, and Shu-Hong Yu*

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 203–209

DiVision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, The School of Chemistry & Materials, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed February 29, 2008; ReVised Manuscript ReceiVed September 16, 2008

ABSTRACT: Using N,N′-dicyclohexylcarbodiimide (DCC) and L-serine as biologic additives, rutile TiO2 hollow spheres assembled by nanorods can be synthesized by a simple hydrothermal reaction route. A distinctive crystallization and transformation route of the rutile TiO2 hollow spheres has been proposed, which includes emergence of polycrystalline, mesoscale transformation to mesocrystals with morphologies of sectors, transformation of mesocrystals to bundles of rods based on oriented attachment, the simultaneous assembled process of sectors to solid spheres, and a cavitating process of solid spheres through the Ostwald ripening mechanism. Two ways of oriented attachment, side-by-side and end-to-end, were observed during the assembly process. It has been found that the presence of DCC and L-serine and their synergistic effects are essential for the formation of rutile TiO2 hollow spheres. The stability of the rutile TiO2 mesocrystals has been studied. The results have demonstrated that the mesocrystals could be maintained longer at lower temperature, while proper choice of organic additives can also enhance the stability of mesocrystals at higher temperature under solution conditions.

1. Introduction Recently, controllable synthesis of highly ordered superstructures of inorganic materials with distinctive shapes and sizes via self-assembly method has aroused considerable attention. Understanding the crystallization process plays a significant role in the preparation of various inorganic materials. With regard to the classical law of crystallization, the growth of crystals is typically considered to occur via atom-by-atom additions to an existing nucleus.1 While in contrast to the classical ones, there also exist nonclassical pathways that proceed through particlebased reaction systems.2 Of the particle-mediated crystallization pathways, mesoscopic transformation of self-assembled, metastable, or amorphous precursor particles into nanoparticulate superstructures is involved, which is generally considered as nonclassical crystallization. As one way of nonclassical crystallizations, mesocrystals are obtained via the mesoscopic transformation of precursor nanoparticles, which are interspaced by organic additives or by a second phase such as amorphous matter.3 Generally speaking, mesocrystals are oriented superstructures or colloidal crystals composed of individual nanocrystals that align in a common crystallographic fashion.4 Oriented attachment, another mechanism of nonclassical crystallizations, proceeds via the adjacent nanoparticles that are assembled by sharing a common crystallographic orientation and docking of these particles at a planar interface.5 From a thermodynamic viewpoint, via the elimination of pairs of highenergy surfaces, the surface free energy could be reduced substantially, which could drive the oriented attachment spontaneously.3 A representative scheme of classical and nonclassical crystallization pathways, which was brought forward by Co¨lfen et al. in recent years, illustrated three possible coarsening routes clearly existing in the nature.3,6 The first one is the classical crystallization model, which means that growth starts from * To whom correspondence should be addressed. Fax: + 86 551 3603040. E-mail: [email protected].

primary building units such as atoms, ions, or molecules to form clusters, and then grows further via ion-by-ion attachment until the formation of a single crystal. The second reaction route is that first via classical crystallization, primary nanoparticles were obtained; then, through mechanism of oriented attachment, oriented aggregation to form an iso-oriented crystal occurred; and finally, fusion took place in the inner part of the iso-oriented crystal to shape a single crystal. Different from the two pathways above, the third one undergoes nucleation clusters, primary nanoparticles, nanoparticles temporarily stabilized by organic additives or amorphous matters, and then mesocrystals via the process of mesoscale assembly and fusion of mesocrystals to iso-oriented crystals, and finally, via further fusion, a single crystal obtained. So far, abundant examples have been confirmed to adopt one or several crystallization stages in these routes. To the best of our knowledge, more and more mesocrystals built from threedimensional (3D) and well-aligned crystals while exhibiting scatting properties similar to a single crystal have been observed, such as CeO2,7 CdS,8 CoC2O4 · 2H2O,9 CaCO3,10 BaCrO4,11 CoPt3,12 and even Au13 and Ag.14 Moreover, considerable materials have been observed to adopt oriented attachment as their crystal-coarsening mechanism, from MnO multipods,15 PbWO4 dendrites,16 Au nanowires,17 and NiSe2 nanostars18 to heterostructured ZnWO4@MWO4 (M ) Mn, Fe) nanorods19 and so on. Generally, mesocrystals are considered as an intermediate state of a single crystal in the crystallization process of particle-based oriented aggregation. However, only several reports have investigated the transformation process from mesocrystals to a single crystal until now. Schwahn et al. studied the formation of D,L-alanine mesocrystals and their transformation into a single crystal by time-resolved experimental evidence of small angel neutron scattering (SANS) and dynamic light scattering (DLS) analysis for the first time.20 Recently, a continuous structural transition from polycrystalline to mesocrystal to single crystal has been observed in the crystallization of calcite with variation in the calcium concentration using

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poly(ethylene oxide)-b-poly(sodium 4-styrenesulfonate) diblock copolymer as additives by Co¨lfen et al.21 In addition, He et al. reported that the growth of complex single crystalline hollow icositetrahedral zeolite analcime architectures follows a reverse crystal growth process, which was through oriented aggregation of nanocrystals initially and then the process of surface recrystallization.22 Because of the low cost, high chemical stability, and excellent optical, electronic, and catalytic properties,23 TiO2 has recently received considerable attention. Several crystallization routes that constitute parts of the crystallization scheme reported by Co¨lfen et al.3,6 have been explored in the case of TiO2. Niederberger et al. introduced polydentate and charged ligands to study the oriented attachment of the anatase titania nanoparticles along the [001] direction.24 Cheon et al. adopted surfactant-assisted elimination of a high-energy facet as a means to control the shapes of anatase TiO2 nanocrystals with the mechanism of oriented attachment.25 Simultaneously, rodlike rutile TiO2 has been reported by Ribeiro et al.26 to undertake the nanocrystal growth process of oriented attachment, involving spontaneous self-organization of adjacent nanocrystals and coalescence. Zeng et al. prepared hollow anatase TiO2 nanospheres via the Ostwald ripening under hydrothermal conditions.27 Until now, although the syntheses of rutile TiO2 hollow spheres have been referred to now and then,28 no reports on the synthesis of such rutile TiO2 hollow spheres, whose crystallization pathway includes the distinctive process of mesoscale transformation of mesocrystals into a single crystal via the oriented attachment and Ostwald ripening, have been found. Herein, novel TiO2 hollow spheres with such distinctive crystallization processes have been synthesized under mild conditions in the presence of L-serine and N,N′-dicyclohexylcarbodiimide (DCC). The detailed transformation, the structural restructuring, and the shape evolution process will be discussed in particular. Moreover, the effects of biologic additives at different reaction stages and the stability of mesocrystals have been investigated.

2. Experimental Section All reagents were of analytical grade and used without further purification. 2.1. Synthesis of TiO2 Hollow Spheres. The synthesis was carried out in a Teflon-lined stainless steel autoclave. In a typical procedure, a transparent TiCl4 solution was prepared first via the dripping of 20 mL of TiCl4 into 50 mL of deionized water, whose Ti4+ concentration was 3.04 mol/L. Then, 0.5 g of anhydrous Na2CO3 and 0.06 g of L-serine were dissolved into 16 mL of deionized water to form a clear solution. After that, the mixture above was dropped into 9 mL of TiCl4 solution (3.04 mol/L) under vigorous magnetic stirring to form a homogeneous solution at room temperature. Then, 0.08 g of DCC was added into the solution above and stirred for about 5 min. Afterward, the above mixture was transferred into a Teflon-lined autoclave with a volume of 35 mL. The autoclave was sealed and maintained at 130 °C for different reaction periods and then cooled to room temperature naturally. After reaction, the white precipitate was separated from the solution by centrifugation, washed with deionized water and absolute ethanol several times, and dried in a vacuum at 60 °C for 4 h. 2.2. Characterization. The final products were examined by X-ray power diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive spectrum (EDS), and thermogravimetric analysis (TGA), respectively. XRD analyses were carried out on a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54056 Å), and the operation voltage and current were maintained at 40 kV and 40 mA, respectively. FESEM images were taken on a JEOL JSM-6700F field emission scanning electron

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Figure 1. XRD patterns of the products obtained in the presence of both DCC and L-serine at 130 °C for different reaction times: (A) 37 min, (B) 38 min, (C) 40 min, (D) 5 h, (E) 1 day, (F) 3 days, and (G) 9 days. microscope at 10 kV. TEM photographs were performed on a Hitachi (Tokyo, Japan) H-800 transmission electron microscope at an accelerating voltage of 200 kV. HRTEM and selected area electron diffraction (SAED) patterns were operated on a JEOL-2011 microscope at an acceleration voltage of 200 kV. EDS analysis was obtained with an EDS detector installed on the same HRTEM. TGA was carried out on a TGA-50H thermal analyzer (Shimadzu Corp.) with a heating rate of 10 °C min-1 in flowing air.

3. Results and Discussion 3.1. Influence of Reaction Time on the Formation of Rutile TiO2 Hollow Spheres. Figure 1 shows the XRD patterns of the as-prepared products obtained at 130 °C in the presence of L-serine and DCC after different reaction periods. All of the diffraction peaks can be indexed as pure rutile phase of TiO2 with cell parameters of a ) 4.58 and c ) 2.95, which are in good agreement with the literature values (JCPDS Card Number 78-2485). As the reaction time extended from 37 min to 9 days, it could be observed that the crystallinity increases and the broadening effect of diffraction peaks disappears gradually, indicating that the sizes of the building units of superstructures are increasing, accompanied with prolonged reaction times. The broadening effect of the diffraction peaks may result from the small sizes of the building units, the lattice imperfections, and stacking faults in the contacting areas between neighboring particles.29 Simultaneously, we could observe that the relative intensity of the peaks corresponding to the (110)/(101) and (110)/(211) planes varied significantly from the literature values, especially in the sample shown in Figure 1G, which indicates the possible orientation of the crystals and the possible anisotropic growth. A series of FESEM images for the samples obtained after different reaction periods in Figure 2 illustrate that many spheres, which are composed of superthin rods, can be obtained finally by a self-assembly process of sectors. After 3 days, the size of the spheres can go to 1.5-2 µm. The rods in the spheres were not tightly associated (Figure 2D). Small holes on the spheres were found. A half-broken one in the inset image of Figure 2D further proves that the TiO2 spheres grow with hollow interiors and the size of that takes a little more than one-quarter of the whole sphere. While further prolonging the reaction time to 9 days, a mixture of hollow spheres and incompact bundles of rods, whose diameters and lengths are much longer than those

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Figure 2. FESEM images of the samples obtained in the presence of both DCC and L-serine at 130 °C for different reaction times: (A) 40 min, (B) 5 h, (C) 1 day, and (D) 3 days.

Figure 3. HRTEM images of the sample prepared in the presence of both DCC and L-serine at 130 °C for 3 days.

before, is finally obtained (see the Supporting Information, Figure S1). It can be concluded that the formation process of superstructures underwent sectors, self-assembly of sectors, integrated solid spheres, hollow spheres with small holes, and a mixture of hollow spheres and incompact bundles of rods. The typical SAED pattern and HRTEM images taken from the selected area of the bundle of rods, which dropped from the integrated spheres, are shown in Figure 3. The appearance of periodic diffraction spots (insert image in Figure 3A) indicates that all of the rods in the selected area have self-assembled into highly oriented aggregates and diffract as a single crystal. The SAED pattern shows that some diffraction spots have been elongated slightly, implying that small misorientations deviating from perfect alignment between nanocrystallites are present in the bundles of rods in the spheres. The existence of that was further proved by the part in the black frame of Figure 3B, from which we could clearly observe the angles between different crystal lattices. Analyzed from the data, it can be concluded that the HRTEM images are viewed from the zone axis of [001], the lattice spacings of planes (11j0) and (110) are about 3.01

and 3.31 Å, respectively, and the rods grow along the direction of (11j0). In spite of the presence of slight oriented distortions, the rods show almost the same orientation (Figure 3B), and the bundle of rods has grown into one imperfect single crystal gradually via oriented attachment. The framed ones in Figure 3B indicated that although with imperfect attachment both ways of oriented attachment, end-to-end and side-by-side, are adopted in the process of reaction, which have not been detected in other TiO2 systems. It seems that every bundle of rods in the spheres shares the same orientation. It can be concluded that oriented attachment is one dominant growth way of the building units in the formation of the TiO2 spheres in the afterward crystallization stage, which could rationally explain the larger sizes of the rods obtained after reaction for 9 days. As the sizes of building units grow bigger, it is not surprising at all that the spheres become incompact and even a lot of bundles of rods have fallen off (see the Supporting Information, Figure S1). To obtain better understanding on the formation mechanism, the samples prepared after different growth stages are collected for a series of analysis. It is well-known that the hydrolyzing

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Figure 4. Series of HRTEM images of the samples prepared at 130 °C with DCC and L-serine addition for different reaction times: (A, B) 37 min, (C, D) 38 min, (E, F) 40 min, and (G, H) 5 h.

Figure 5. Energy-dispersive spectrum (EDS) taken on the sample prepared at 130 °C in the presence of both DCC and L-serine after reaction for 40 min.

rate of TiCl4 is so fast that the products that we took out when it just starts to hydrolyze have been crystallized. The corresponding ED and HRTEM in Figure 4A,B clearly illustrate its process of polycrystalline nature, and it seems that some areas have shown the orientations slightly. As the time prolonged to 38 min, the particles that constitute the small sectors have exhibited the same orientation, although still slight misorientation exists. Also, the elongated diffraction spots in the insert image in Figure 4C suggest the existence of misorientation deviating from perfect alignment between nanocrystallites in the mesocrystals. From the HRTEM image in Figure 4D, we could clearly see the boundaries between different nanoparticles, which are illustrated by the white arrows. The EDS analysis (Figure 5) further proves that these boundaries are constituted by organic additives, in which C and N elements could be detected. The peaks of Cu come from the Cu grid used in the experiment. It is no doubt that the TiO2 mesocrystals with morphology of sectors have been obtained. When the reaction

time reached 40 min, we saw that the orientation of nanoparticles was further perfected and that TiO2 mesocrystals with higher quality were produced. Simultaneously, in the reaction system assemblies of small mesocrystals of sectors to larger sectors, eventually the formation of a series of intact spheres is being carried out in concomitance with the orientation adjustment of nanoparticles. The TEM image (see the Supporting Information, Figure S2) and FESEM images in Figure 2 show the transformation process in detail. A lot of small sectors could be observed in the sample obtained for 37 min, and some of them have begun the assembly stage (see the Supporting Information, Figure S2). While from the image in Figure 2A, we could even obviously see the assembly trace between different sectors, whose subunits are assemblies of mesocrystals of small sectors. The insert image in Figure 2A illustrates the self-assembly process of spheres by larger sectors. As the reaction time extends to 5 h, we could obtain intact spheres with solid interiors (Figure 2B). Further HRTEM analysis (Figure 4H and Figure S3A of the Supporting Information), which were taken from the outward fringe (white frame part) and central part (black frame part) of one sector (Figure 4G), respectively, illustrates that for sectors, which are the building units of a sphere, the outward parts are composed of arrays of rods with nearly the same orientation while for the central parts both the extent of crystallization and the tropism are not very good. The corresponding ED pattern (see the Supporting Information, Figure S3B), which was taken from central parts of the sector, further prove this conclusion. In addition, we should mention that the outward parts of the spheres have grown into unattached rods with relatively larger sizes. Because of the above reasons, an Ostwald ripening mechanism may play a significant role in the whole process, which has been observed in many other systems.30 Consequently, nanoparticles in the central parts begin to dissolve, recrystallize, and grow on the outward fringe of the spheres. To some extent, this process has some similarity with the cavitating one as our group has reported before,31 in which both of them have suffered the nanocrystals stabilized by organic additives; then, some nanocrystals dissolve and the organic additives surrounding them will

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Figure 6. FESEM images of the products obtained at 130 °C for 3 days by addition of DCC and different kinds of amino acids: (A) L-histidine and (B) L-lysine.

Figure 7. FESEM images of the products obtained at 130 °C for 3 days with the addition of different organic additives: (A) without any additives, (B) with DCC only, and (C) with L-serine only.

go back into the solution. Along with the cavitating process of the spheres, the size of the spheres has increased from approximately 1.2 to 1.6 µm, which could provide other evidence for such a mechanism. At the same time, the adjacent nanoparticles inside the spheres undergo the process of oriented attachment and restructuring, which means the boundary area among nanoparticles in the mesocrystals begins to dissolve and pairs of high-energy surfaces are finally eliminated. The rectangular parts of HRTEM image in Figure 3B show the existence of this phenomenon. The corresponding HRTEM image suggested that both the end-to-end and the side-by-side oriented attachment are introduced in the formation of larger rods, which has rarely been reported in TiO2 system up till now. By further prolonging the reaction time to 9 days, perfect and independent single crystal of rods with a size of about 100 nm in length could finally be obtained (see the Supporting Information, Figure S4). 3.2. Effects of Biologic Additives. To investigate the actual role of biologic additives of L-serine and DCC in the process of crystallization, a series of contrast experiments were carried out systematically. Keeping the amount of DCC constant, the results show that the addition of different kinds of amino acids will result in the formation of TiO2 superstructures with various morphologies (Figure 6). It is found that in the products with L-histidine as the additive of amino acids, there exist two kinds of morphologies: One is spheres with nearly the same morphology as the ones with L-serine, and the other is the assembly of mesocrystals, which has been proved further with HRTEM (data not shown). To some extent, this result indicates that the stability of mesocrystals could reach a higher degree by proper choice of organic additives even at higher temperature. The one added with L-lysine produces a mixture composed of loose rods and broken spheres (Figure 6B). Moreover, the default experiments with using DCC and/or amino acids were also performed to examine the actual roles of the organic additives. Figure 7A indicates that a large number of TiO2 hollow spheres cannot be produced in the absence of any additives, and most of the particles are bundles of loose

Figure 8. TG curves of the series of products obtained at 130 °C with additions of DCC and L-serine at different reaction times: (A) 40 min, (B) 5 h, (C) 1 day, (D) 3 days, and (E) 3 days without any organic additives.

rods and only a small quantity of spheres were observed. This result demonstrated that DCC and amino acids were actually taking synergetic effects in the self-assembly process. Only when both of them are present in the solution can the synergetic effect be achieved effectively for the formation of the TiO2 spheres. To further prove the effects of amino acids and DCC in the whole crystallization process, we have made a series of TG analysis of the products obtained at different reaction stages. It is found that at the stage of assemblies of mesocrystals (Figure 8A), in which considerable nanoparticles are stabilized and surrounded by organic additives, the weight loss of products vs temperature could reach as high as 16 wt %. It is interesting that the prolonging of reaction time made the weight loss vs temperature lower from 16 to 5.3 wt %, the value of which approximates the one of 4.7 wt % without addition of any organic molecules very much. Also, we should note that most

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Figure 9. FESEM images of the products obtained at 60 °C for 3 days in the presence of both DCC and L-serine: Panel B is an enlarged image of panel A.

Figure 10. HRTEM image and SAED pattern taken from the sample obtained at 60 °C for 3 days with additions of both DCC and L-serine.

Scheme 1. Illustration of the Formation Mechanism of the TiO2 Rutile Hollow Spheresa

a “In” means that the reactions took place in the primary structure of the mesocrystals; “among” means that the assembly effect occurred among different primary mesocrystals.

fall comes from the value between Figure 8A and 8B, which may reflect the key change of the function of biologic molecules in the nanostructures. 3.3. Stability of the Assemblies of Mesocrystals. In comparison with the one at 130 °C, the products at a reaction temperature of 60 °C are also collected and analyzed in detail. From the FESEM images in Figure 9, we could see clearly that the products grow with the morphology of large sectors, and it seems that they are made up of superthin filaments. The HRTEM image and SAED pattern in Figure 10 discloses the nature of the large sectors. Actually, the large sectors are the assemblies of mesocrystals, which means the subunits of sectors are nanoparticles enclosed by organic molecules. The corresponding SAED pattern illustrates that the nature of small sectors is quasi-

single-crystal, albeit with small disorientation. Further investigations on the stability of the ones at 60 °C show that the structures of assemblies of mesocrystals at this temperature could be maintained for 3 months or more. The stability of the mesocrystals obtained at 130 °C can be kept for only several hours, which contrast obviously with the one at 60 °C. It is interesting to note that in the sample which is obtained with the addition of L-histidine and DCC at 130 °C, there also exist some assemblies of mesocrystals. Considering that, it could be concluded that mesocrystals should be maintained at low temperatures for longer time, but the phenomenon could also exist at higher temperatures for shorter time. By proper choice of organic molecules, we could also obtain mesocrystals maintained for longer time at higher temperatures.

Mesocrystals of Rutile TiO2

3.4. Formation Mechanism of Rutile TiO2 Hollow Spheres. The formation mechanism of the TiO2 hollow spheres is illustrated in Scheme 1. From that, we can see that the product obtained in the initial crystallization process is polycrystalline virtually. As the reaction proceeds further, the nanocrystallines that constitute the sectors rotate and orient in situ until nearly the same orientation is achieved. Then, TiO2 mesocrystals with the morphology of sectors are obtained. Simultaneously, the assembly process takes place accompanying the formation of mesocrystals. As illustrated above, assemblies of mesocrystals underwent and evolved from smaller sectors, then larger sectors, incompact spheres, compact solid spheres, and finally to hollow spheres. Nearly all of the crystallization fashions are adopted in the whole process. At the initial stage, the formation of mesocrystals takes a dominant role in the growth process. Then, the assembly procedure starts and oriented attachment begins to take effect in the inner parts of the hierarchical structures simultaneously. Both the ways of oriented attachment, that is, end-to-end and side-by-side, are adopted in the reaction system. At the same time, it is surprising to find that the cores begin to dissolve and recrystallize on the outer part of the spheres. Relatively independent rods are obtained in the peripheral parts of the sphere, and sizes of the spheres increase from 1.2 to 1.6 µm, which indicates that the Ostwald ripening also operates in the stage. Without doubt, both of the crystallization pathways, including the oriented attachment process and Ostwald ripening process, take place in concomitance with different assembly stages simultaneously. In conclusion, nearly all of the crystallization fashions in the existing crystallization systems nowadays are observed in the formation process of rutile TiO2 hollow spheres, from nonclassical crystallization pathways, mesocrystals and oriented attachment, to classical crystallization route, the Ostwald ripening.

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(NSFC,nos.50732006,20621061,and20671085),2005CB623601, the Partner-Group of the Chinese Academy of SciencessThe Max Planck Society, Anhui Development Fund for Talent PersonnelandAnhuiEducationCommittee(2006Z027,ZD20070041), and the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry. Supporting Information Available: FESEM and HRTRM images. This material is available free of charge via the Internet at http:// pubs.acs.org.

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(11) (12) (13) (14) (15) (16)

4. Conclusion In summary, an organic molecule-assisted hydrothermal route has been developed for producing unique rutile TiO2 hollow spheres assembled by nanorods in the presence of DCC and L-serine. The results demonstrate that the whole crystallization process of rutile TiO2 hollow spheres included several stages, that is, emergence of polycrystalline, mesoscale transformation to mesocrystals with morphologies of sectors, transformation of mesocrystals to bundles of rods based on oriented attachment, the simultaneous process of assembled sectors to solid spheres, and cavitating process of solid spheres through Ostwald ripening mechanism. In addition, two ways of oriented attachment process, that is, side-by-side and end-to-end, have been identified in the present system. Moreover, it has been found that the presence of DCC and L-serine and their synergistic effects is essential for the formation of hollow spheres. The amount of biologic additives included in the resultant products decreased gradually accompanied with prolonged reaction time. Furthermore, the relationship between stability of mesocrystals and reaction temperature has also been investigated. It has been demonstrated that generally low temperature is beneficial for the stability of rutile TiO2 mesocrystals, while proper choice of organic additives could also enhance the stability of the mesocrystals. This study could provide an example for better understanding the formation mechanism of other inorganic mesocrystals in the presence of organic additives. Acknowledgment. S.H.Y. acknowledges the special funding support from the National Natural Science Foundation of China

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