Shape Evolution of One-Dimensional Single-Crystalline ZnO Nanostructures in a Microemulsion System Jun Zhang, Ling-Dong Sun,* Xiao-Cheng Jiang, Chun-Sheng Liao, and Chun-Hua Yan*
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 2 309-313
State Key Laboratory of Rare Earth Materials Chemistry and Applications & PKU-HKU Joint Lab in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received July 25, 2003;
Revised Manuscript Received October 3, 2003
ABSTRACT: Shape evolution of one-dimensional (1D) single-crystalline ZnO nanostructures, which were produced via a facile microemulsion-based approach, was clarified by SEM observations by stepwise prolonging the reaction time. It suggested that at the initial stage the nucleation process dominated and the shape of ZnO nanostructures was preferably confined by the microemulsion droplets and took spherical forms. With the extension of the reaction time, the growth gradually governed the process and the shape of ZnO nanostructures evolved to 1D by experiencing a nanoparticles and nanorods coexistance period. The evidenced evolution process from nanoparticles to high aspect ratio and single crystalline 1D nanostructures suggested that the formation process might involve a directed aggregation growth process mediated by microemulsion droplets, in which microemulsion droplets play an important role in modulating crystal size and shape through controlling nucleation rate and nuclei size at the initial stage. On the basis of this postulation, we can control the diameter and aspect ratio of 1D ZnO nanostructures by altering the chain of the cosurfactant or the molar ratio of quaternary microemulsion components. The proposed formation mechanism of 1D ZnO nanostructures is useful for understanding shape evolution and provides guidance for the morphology control of other nanostructures in a microemulsion-based system. Introduction Currently, the fabrication of nanomaterials with a controllable size and shape is of great scientific and technological interest, mainly due to the significant influences of size and shape on their properties.1-4 For example, the optical and catalytic properties of certain types of nanocrystals are expected to be variable with their shape, which may afford a desirable possibility of tuning the performances of nanomaterials to open up new applications.5,6 Studies of size and shape control of nanocrystals may greatly contribute to the understanding of quantum size phenomena and give deep insights into the crystallization mechanism of materials in nanosized scale. However, fine control of size and shape simultaneously is fairly difficult because of the lack of understanding of the crystallization behavior of nanocrystals.7 This would hinder the progress on research of size and shape control of nanocrysals. In comparison with other semiconductor nanocrystals, the crystallization of colloid nanocrystals, especially CdSe and CdS, has been extensively investigated, and it is suggested that in some cases their nucleation and growth process is far from following the classic model.7 As a result, CdSe and CdS nanocrystals with wellcontrolled size, size distribution, and shape, and even architecture have been richly presented by simply modulating the reaction conditions.8 From this point of view, systematic investigations of the crystallization mechanism of nanocrystals in various synthetic systems are, thus, of obvious importance for developing novel strategies to obtain size- and shape-controllable nano* Chun-Hua Yan. College of Chemistry, Peking University, Beijing 100871, China; fax & tel: 86-10-62754179; e-mail: chyan@ chem.pku.edu.cn.
crystals. Recently, there are numerous results afforded to deeply explore shape-controllable synthesis of semiconductors, especially ZnO, due to its promising wide range of applications.9-14 Well-defined nanostructures of ZnO with abundant shapes, such as nanobridges and nanonails,15 nanobelts,16 hierarchical nanostructures,17 nanocables and nanotubes,18 nanoneedles,19 and dendritic nanowires,20 are achieved. Nanorods with a regular shape21 and a diameter as small as 50 nm are also presented.22 However, fundamental comprehension of the formation mechanism of so many diverse shapes of ZnO is far from complete. It is believed that the discrepancies of the formation mechanism under different growing environments is vitally responsible for shape variations of ZnO, and thus research for understanding the crystallization process of such nanocrystals is attracting more attention than ever before and is becoming urgently important for obtaining nanomaterials with a desired size, shape, and architecture. On the other hand, it is also particularly expected for new strategies to be explored to obtain size- and shapecontrollable ZnO nanocrystals, because ZnO is of considerable importance in nanotechnology due to its potential applications in nanolasers,6 solar cells,23 or advanced optoelectronic devices,24-26 and precise control of size and shape is necessary for further integration and construction of nanobuilding blocks into nanodevices. Up to now, various strategies, for instance, the high-temperature physical evaporation,27 template induced growth,28-30 precursor decomposition method, etc.,31-34 have been developed to meet such demands. Among them, morphology-controllable synthesis by means of a solution route and microemulsion-assisted approach is becoming more and more attractive due to its mildness, simplicity, and lack of expensive equip-
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ment. However, a fundamental understanding of the formation mechanism of ZnO with diverse shapes by these techniques is still inadequate, although a few works have been reported so far.21,22,31-37 Coupled with these considerations, herein, a facile, reproducible, and effective technique by hydrothermally decomposing Zn(OH)42- in a microemulsion system was presented to obtain shape-controllable nanostructures of ZnO, and more importantly, to achieve insight into the formation mechanism of one dimensional (1D) ZnO nanostructures. The crystallization process of ZnO was monitored by microscopes using a few steps during the reaction, which suggested that the formation mechanism for 1D single-crystalline ZnO nanostructures might involve a directed aggregation growth process mediated by microemulsion droplets. Following this growth mechanism, the diameter of 1D ZnO nanostructures can be controlled by adjusting the reaction conditions. Experimental Section All chemicals (analytical grade reagents) were purchased from Beijing Chemicals Co. Ltd., and used as received without further purification. Water was distilled twice. Zn(OH)42precursor solution was prepared by mixing 0.5 mol/L ZnAc2 and 5 mol/L NaOH solutions (volume ratio, v/v ) 1:1, pH ≈ 14). The procedure is described as follows. Typically, Zn(OH)42precursor solution, surfactant cetyltrimethylammonium bromide (CTAB), cosurfactant n-hexanol, and solvent n-heptane were mixed with various molar ratio to form a microemulsionbased system in a vessel under constantly stirring. The mixture was then transferred into a 25-mL Teflon-lined autoclave and heated to a given temperature for a certain time. After the reaction finished, the autoclave containing the samples was cooled to room temperature naturally. The white precipitate depositing on the bottom of the autoclave was collected and washed with absolute ethanol and distilled water for several times. Finally, ZnO samples were obtained by centrifugation and dehydration of the precipitate in a vacuum at 60-70 °C. In our studies, a typical microemulsion system with the composition of 1 g of CTAB, 1.2 mL of Zn(OH)42- solution, 2.0 mL of n-hexanol, and 11.2 mL of n-heptane was selected as the reaction media based on our previous investigations. The decomposition reaction of Zn(OH)42- precursor was conducted at a stepwise prolonged time to capture the shape evolution of ZnO at each reaction stage. The crystallization processes were monitored by microscopes. The size and morphology of ZnO were characterized by scanning electron microscopes (SEM, Amary, FE-1910 and JEOL, JSM-6700F) and transmission electron microscope (TEM, Hitachi, H-9000NAR). A small drop of the sample redispersed by ethanol was deposited on silicon substrate or a carbon film coated copper grid, and then dried in the air for SEM and TEM characterizations, respectively. Dry powder samples were used for the structural measurements with an X-ray powder diffractometer (XRD, Rigaku, D/max-2000, Cu KR radiation).
Results and Discussion Shape Control. There are some reports on the preparation of 1D ZnO nanostructures. Most of them focused on synthetic processes unrelated to wet chemical routes, such as vapor transport,6 anodic alumina membranes templates,30 and physical vapor deposition approaches.16 Parts of these approaches are beneficial to obtain well-crystallized ZnO nanocrystals and easy for realizing size and shape control; however, more or less,
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Figure 1. XRD patterns of 1D ZnO prepared by hydrothermal treatment of microemulsion with the composition of (a) 1 g of CTAB, 1.2 mL of Zn(OH)42-, 2.0 mL of n-hexanol, and 11.2 mL of n-heptane; and (b) 1 g of CTAB, 1.2 mL of Zn(OH)42-, 5.0 mL of n-hexanol, and 8.2 mL of n-heptane, under 180 °C for 13 h. The bottom is a standard XRD pattern of ZnO (JCPDS No. 36-1451).
they may involve complex procedures, expensive equipment, and rigid experimental conditions. Moreover, most of the previous reports on hydrothermal preparation of ZnO are based on the decomposition of hydroxide or products of hydrolyzed zinc salt in pure water or a simple solvent system. Recently, some results of ZnO synthesis via a surfactant-assistant method show the well-controlled shape of ZnO, which indicates the important function of surfactant modulation during the formation of ZnO crystals.21,33 Our approach in a microemulsion system, presented here, was carried out not only to produce nanostructured ZnO, but also to pursue the growth mechanism of 1D ZnO structure. Typically, the sample obtained by hydrothermal treatment of a Zn(OH)42- precursor in a CTAB quaternary microemulsion under 180 °C for a certain time preferably adopted a rodlike shape with different diameters corresponding to different reaction conditions. XRD patterns indicated that all samples exhibited a wurtzite structure (hexagonal phase, space group P63mc), as shown in Figure 1. All the diffraction peaks are well assigned to hexagonal-phased ZnO (JCPDS card, no. 361451). Compared with the standard diffraction pattern, as shown in Figure 1 (bottom), the strong diffraction intensities along the [101] direction as well as the weak diffraction intensities along the [002] direction are similar to our previous results33 and a recent report,21 implying the preferred growth orientation of ZnO. The representative TEM image of ZnO obtained in a reaction media composed of 1 g of CTAB, 1.2 mL of a Zn(OH)42solution, 2.0 mL of n-hexanol, and 11.2 mL of n-heptane after a 13-h heat treatment is shown in Figure 2. It can be seen clearly that ZnO rodlike structures with good crystallinity are successfully produced via this simple microemulsion-based process. The rodlike shape of samples is relatively uniform with an average diameter around 350 nm and length of up to tens of microns. The ED pattern in the inset of Figure 2 indicates the singlecrystal nature of ZnO rods with hexagonal structure, which is consistent with the XRD characterizations (Figure 1a). Shape Evolution. To pursue the growth mechanism of ZnO presented in Figure 2, the time-dependent experiments were carried out to monitor the evolving process by recording the shape of the sample after a suitable reaction time. The series SEM images in Figure
Shape-Evolution of ZnO Nanostructures
Figure 2. TEM image of 1D ZnO corresponding to the sample in Figure 1a. The inset is ED pattern, showing the singlecrystal nature of 1D ZnO.
3 show the morphology in each reaction stage corresponding to the reaction time span from 10 min to 8 h. From the image in Figure 3a, it can be found that when reacting the sample for 10 min, the obtained ZnO exhibits the forms of uniform nanoparticles with an average diameter as small as 30 nm and weak aggregation. Normally, CTAB quaternary microemulsion droplets are apt to form uniform spheres of nanoscale at
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room temperature.38-40 The precursor, Zn(OH)42-, can be dispersed homogeneously inside the droplets. With this aspect, the microemulsion droplets can act as microreactors, and play a role in controlling nucleation rate and nuclei size during this stage, which would eventually affect the shape formation of the final products. Thus, the shape of ZnO stays in the form of nanoparticles, and basically no 1D structure can be found at this stage. However, collision, conglutination, and amalgamation of different microemulsion droplets simultaneously occurred during the reaction, especially under the hydrothermal condition. As a result, aggregation of ZnO nanoparticles is observed. When prolonging the reaction time to 30 min, it is observed from the SEM image shown in Figure 3b that besides aggregated nanoparticles, ZnO nanorods also appeared. The length and diameter of the nanorods are not uniform, and most of them do not exceed 100 nm in diameter. The reason for above observation is that with the extension of the heating time, the nucleation of ZnO gradually finished and the directed aggregation growth mediated by microemulsion droplets might occur. The interaction between the droplets resulted in a linear aggregation, and provided a favorable environment for
Figure 3. Series of SEM images of shape evolution of 1D ZnO presented in Figure 2 with stepwise prolonging of the reaction time (a) 10 min, (b) 30 min, (c) 1 h, (d) 2 h, (e) 4 h, and (f) 8 h, exhibiting the evolution of ZnO nanoparticles to 1D nanorods.
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the recrystallization into a single-crystalline 1D structure along a preferred orientation.37 However, under higher temperature, the shape of microemulsion droplets themselves would be changed slightly,41 and the confinement function of the droplets would be eventually destroyed if the heating time was long enough. From our point of view, microemulsion droplets definitely played a critical role in the formation of 1D ZnO nanostructures at the initial formation stage because it has been proved in our previous works that without the microemulsion-based system, just using H2O or n-heptane as reaction media, almost no 1D ZnO nanostructure formed, but flower- and snowflake-like aggregates were observed.32 The time-dependent experiments were taken subsequently by prolonging the heating time to 1, 2, 4, and 8 h. The representative SEM images of obtained ZnO samples are correspondingly shown in Figure 3c-f. Figure 3c indicates that when the reaction is performed for 1 h, the obtained ZnO almost exhibits uniform rod shape with an average diameter of about 200 nm. Figure 3d∼f shows that the rodlike ZnO grows up gradually and becomes more perfect with the increase of heating time. Interestingly, during this growth process, the ends of the rods evolve from pyramid- to prism-like shapes, which can be seen clearly from Figures 3e and 2, respectively. This is also ascribed to the crystal growth habit of ZnOsits growth velocities along different crystal planes greatly differ.42,43 In addition, it is found from Figure 3b-f that some nanorods are liable to form a homocentrical bundle, which is more evident in Figure 3e. This phenomenon has also been observed in other shapes of ZnO.32 Generally, for the growth mechanism of the asprepared 1D ZnO nanostructures, the roles of microemulsion should be taken into account. Normally, without the assistance of additive, the crystallization of ZnO also shows a tendency toward 1D nanostructures under hydrothermal conditions,44 but can only result in irregular ellipsoidal shapes. But in our case, when the microemulsion-based system is utilized, the formation of homogeneous ZnO nanorods might be induced and achieved via the directed aggregation growth process mediated by the microemulsion droplets, as suggested in previous works.45,46 Thus, it is much easier to get well-controlled 1D ZnO nanostructures and it would be possible to tune the diameter by modulating the nucleation rate and nuclei size by means of altering the microemulsion drop size. It is also suggested that surfactant CTAB is beneficial to the transport and orderly stacking of the crystal growth units.47 The mechanism in our case is analogous to nanorods formation by “oriented attachment” of nanoparticles, which was suggested by C. Pacholski et al.,37 but with the assistance of microemulsion droplets here. The droplets in microemulsion-based synthesis may play an important role in confining the size and shape of the crystal nucleus at the nucleation stage, which will have effects on the size and shape of the final products. At the growth stage, it is believed that the confined growth of ZnO crystallites first occurs inside the droplets, and then the directed aggregation-based crystal growth is caused by the interaction of microemulsion droplets. According to this postulation, with the assistance of
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Figure 4. TEM (a) and HRTEM (b) images (the inset is the ED pattern) of 1D ZnO nanorods shown in Figure 1b. The lattice spacing of 2.59 Å corresponds to (002) plane, implying the preferred growth orientation of 1D ZnO along (002) plane.
microemulsion droplet interactions, initial ZnO nanoparticles inside the droplets ultimately form into linear aggregations, and then recrystallize into a 1D structure along the preferred growth orientation. Influences of the Reaction Conditions. As far as a quaternary CTAB microemulsion is concerned, it is found that interface rigidity and curve diameter of microemulsion droplets, which determine the characteristic of the droplets, can be easily adjusted by changing the ratio of cosurfactant to solvent, in our case, the ratio of n-hexanol to n-heptane.38 The droplet size can be adjusted to afford the possibility to control the nuclei size of ZnO formed inside the droplets, which would affect the diameter of the nanorods during the following formation process. By increasing the molar ratio of n-hexanol to n-heptane (5.0-8.2) while maintaining other parameters, the diameter of the nanorods decreased obviously, as shown in the representative TEM image in Figure 4, in which most of the nanorods are less than 100 nm in diameter. Compared with the sample presented in Figure 2 (molar ratio of n-hexanol to n-heptane is 2.0-11.2), the diameter of nanorods achieved at this reaction conditions decreased evidently. From Figure 4a, it can be seen that the high-resolution TEM image (Figure 4b) shows the lattice spacing of 2.59 Å, corresponding to a lattice constant of the (002) plane of wurtzite structured ZnO, and implies the preferred growth orientation of ZnO nanorods along the (002) plane.22 ED pattern in the inset of Figure 4b confirms the growth orientation as well as reveals the singlecrystal nature of ZnO nanorods. When adjusting the molar ratio of n-hexanol/n-heptane to 3.0/10.2 and lowering the reaction temperature to 140 °C, ZnO nanowires with a diameter ranging from 30 to 150 nm and length up to several micrometers is obtained, which has been previously reported by us.33 For the quaternary microemulsion system, the length of hydrocarbon chain of cosurfactant is also important to modulate the droplets. We replaced n-hexanol by butanol to investigate the influence of cosurfactant species on the nanorods. It is indicated that by utilizing butanol instead of n-hexanol, maintaining the other parameters as the sample presented in Figure 2, the diameter of the obtained ZnO rods increased, as shown in Figure 5. The sample shows uniform rodlike structures with a diameter of 1 µm and length of up to tens of micrometers. This also provides evidence for the modulation effect of the microemulsion on the dimensions of the final products.
Shape-Evolution of ZnO Nanostructures
Figure 5. TEM image of 1D ZnO prepared under the same conditions of the sample in Figure 2, except for replacing n-hexanol with n-butanol.
Conclusion In this paper, we present a facile, reproducible, and effective route to obtain 1D single-crystalline ZnO nanostructures by decomposing a simple Zn(OH)42precursor in a microemulsion system, and we further investigate the formation mechanism by monitoring shape evolution by stepwise prolonging the reaction time. It is found that microemulsion droplets play a critical role in modulating the shape and size of 1D ZnO nanostructures. The formation of 1D ZnO nanostructures is attributed to a directed aggregation growth process mediated by microemulsion droplets. The results may reinforce the understanding of the formation mechanism of materials prepared in surfactant-contained systems and may provide guidance for controlling size and shape of other nanomaterials. Under this guidance, by optimizing the microemulsion components and altering other reaction conditions, it is promising to obtain 1D ZnO nanostructures with diameters less than 10 nm, which would be of significant importance in fundamental studies of the quantum effect of nanostructures and potential applications. Acknowledgment. This work was supported by NSFC (Nos. 20001002, 20221101, and 20023005), MOST (G19980613), MOE (the Foundation for University Key Teacher), and the Founder Foundation of PKU. References (1) Heath, J. M. In Nanoscale Materials, special issue of Acc. Chem. Res. 1999, 32. (2) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802-804. (3) Holms, J.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471-1473. (4) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. Adv. Mater. 2003, 15, 353-389. (5) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; EISayed, M. A. Science 1996, 272, 1924-1926. (6) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897-1899. (7) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 13891395. (8) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 33433353. (9) Zhang, W. X.; Wen, X. G.; Yang, S. H.; Berta, Y.; Wang, Z. L. Adv. Mater. 2003, 15, 822-825. (10) Dai, Y.; Zhang, Y.; Wang, Z. L. Chem. Phys. Lett. 2003, 375, 96-101.
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