DOI: 10.1021/cg901559y
Formation and Self-Assembly of Cadmium Hydroxide Nanoplates in Molten Composite-Hydroxide Solution
2010, Vol. 10 4285–4291
Jing Zhang,†,# Yonghao Wang,‡,# Zhang Lin,*,† and Feng Huang*,‡ †
State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China, and ‡Key Laboratory of Optoelectronic Materials Chemistry and Physics, State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. # These two authors contributed equally. Received December 11, 2009; Revised Manuscript Received July 24, 2010
ABSTRACT: In this article, we report a novel molten hydroxide strategy to prepare bulk-sized complexes of three-dimensional (3D) nanostructures at a large scale. Taking cadmium hydroxide as an example, we revealed that the bulk-sized architectures were composed by cross-linked nanoplates, which could be fabricated by a one-step hydrothermal route in the presence of molten composite-hydroxide (NaOH þ KOH) at 200 °C. Powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analysis show that the growth of the well-defined nanostructures actually undergoes two stages. Initially, with the mixture of cadmium chloride and a large excess of composite-hydroxide, a prestructure of aggregating nanoparticles is formed. Afterward, the prestructure recrystallizes into nanoplates of 150 nm thickness and with the (001) plane as exposed surface. Meanwhile, these nanoplates are interweaved and organized into a globe-like structure and 3D aggregates. We speculate that the molten hydroxide plays a critical role not only in providing a strong surface effect to allow recrystallization and anisotropic growth but also in realizing the self-assembly of nanoparticles in inorganic solution. The molten compositehydroxide solution strategy was further extended into a calcium hydroxide system and the same assembling nanoplate architectures were successfully achieved.
*To whom correspondence should be addressed. E-mail:
[email protected] (Z.L.);
[email protected] (F.H.).
relies on the control of surface energy.14,15 So, understanding how the surface free energy varies with the material dimension is essential for controlling the reactivity, evolution, and stability of nanomaterials. For this purpose, specific organic surfactants or capping reagents are introduced to control the free energies of specific crystallographic surfaces.16-19 Such control could be very spatially specific, causing some planes to grow much faster than others and resulting in anisotropic morphologies and complex geometric forms. It has been well reported that surface-stabilized nanoparticles, acting as building blocks, can spontaneously self-assemble into diverse ordered nanostructures, driving by the interactions between surfaceabsorbed ligands instead of between the particles themselves.20,21 As another important alternative route, inorganic ions from a concentrated base or even molten salts have also been proven to provide strong surface adsorption ability that controls the growth mechanism, decreases their surface energies, and alters the polymorph of nanocrystals.22,23 In particular, concentrated hydroxide ions show their super strong effects adhering to the crystal surface. It has been demonstrated that under the strong interaction of highly concentrated hydroxide ions, a thermodynamic stable nanoscale ZnS, instead of bulk counterpart, can be formed.24,25 Furthermore, inorganic absorption effect shows its superior thermal stability, which will offer stable and consistent acting on the nucleation and growth of nanocrystals.26,27 Especially for the molten salts systems, their ion concentration will reach the maximum state to provide super surface effects. To avoid the higher temperature of molten salts, a binary salt system at the ratio of eutectic point is often selected to obtain liquid media at a lower temperature.22 Cd(OH)2 is a transparent, wide band gap semiconductor with a brucite-type layered structure. It has a number of
r 2010 American Chemical Society
Published on Web 09/15/2010
1. Introduction The superior physical properties of nanomaterials have stimulated the fabrication of complex and functional architectures, such as controlling the lateral placement of nanometersized objects into specific arrangements, for use in optical, optoelectronic, and magnetic storage devices.1-5 As an intriguing branch of nanomaterials, larger hierarchical assemblies by nanostructural building blocks are of great scientific and technological importance. Because of their advantages to interact with gases and liquids not only on the surface but also throughout the bulk, they are opening up new opportunities in the field of catalysis and separation technology, such as catalysts, fillers, microreactors, controlled release capsules, and chemical sensors and adsorbents.6-8 Large complicated nanostructures are often achieved by self-assembly strategies, including two major approaches; top-down and bottom-up assemblies. As for the top-down process such as photolithography, electron beam lithography, X-ray lithography, and contact molding processes, it requires the creation of nanoscale patterns over large lateral length scales at the precise location. Therefore, further miniaturization with the top-down approach is limited by the spatial resolution of lithography,9,10 whereas for the bottom-up strategy, where the process is often accomplished by a so-called “self-assembly” strategy, the main challenge lies in precisely immobilizing nanoscale building blocks, especially with the anisotropic units of one and two dimension.11-13 To this end, various strategies have been employed to tailor the formation of primary nanostructural units and the development of their assembling behaviors. But in most cases, the principle basically
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Figure 1. SEM images of cadmium hydroxide precipitates in molten composite-hydroxide at 200 °C for 24 h (a), 40 h (b), 88 h (c), and 5 days (d), respectively. Inset of (b) shows the surface of particle aggregates. Inset of (c) is broken ball-like aggregates of plates. Inset of (d) and image (e) are enlarged images for plate assemblies. (f) EDX spectrum of sample at 88 h and inset table of approximate element ratio. Au signal originating from spray treatment for SEM.
possible applications including the use in solar cells and as the precursor of its oxide and sulfide. Previously, various anisotropic nanostructures of Cd(OH)2 were prepared, such as nanowires, nanorings, and even hollow microspheres.28-31 However, well-organized self-assembly of Cd(OH)2 nanoplates is seldom reported. In this work, we selected binary molten alkali as the strong surface adsorption agent to fabricate nanostructural Cd(OH)2. Cd(OH)2 architectures with nanoscale plate units throughout the entire body were achieved by a molten alkali process. Under the super strong surface absorption effect of hydroxide ion, the surface energy of diverse crystal planes will be greatly varied, whereby anisotropic growth and self-assembly of nanoparticles are supposed to be realized by blocking the growth on the polar surfaces. This method provides a one-step, convenient, low-cost, and large-scale production route for the synthesis of nanostructures with well-controlled internal structures. Furthermore, comparing the strategy of organic surfactants, this inorganic molten salts approach is more advanced in thermal stability, facile adjustment of ions concentration, and additional charge effects, to control the synthesis of nanoparticles. This strategy has also been extended to synthesize calcium hydroxide systems exhibiting a similar polymorph and assembling structure. We speculate that this novel molten hydroxide strategy has a good potential to prepare bulksized complex of three-dimensional (3D) nanostructures at a large scale.
2. Experimental Section The synthesis of Cd(OH)2 is performed as follows. In a typical procedure, 10 g of mixed hydroxides (the mole ratio of NaOH/KOH is equal to 0.515/0.485, which is the ratio of its eutectic point about 170 °C) was placed in a 23 mL Teflon-lined autoclave and heated at 200 °C for 2 h to ensure a molten state alkali. Then, 0.3 g of anhydrous CdCl2 was added into the hydroxide vessel under stirring. The vessel was sealed and again put in a furnace at 200 °C. At the appropriate time interval, autoclave containers were taken out and quenched to room temperature. The solid products were collected and washed with ethanol and water until the pH was ∼7.0. The same procedures were performed for preparing samples at 180 and 220 °C. Fifteen millilters of 2 and 10 M (NaOH þ KOH) (nNaOH/nKOH = 0.515/0.485) were used to replace molten hydroxide for the reaction, respectively. For the synthesis of Ca(OH)2, all of the synthesis procedures are also the same as those stated above. Scanning electron microscopy (SEM) analyses were performed using a JSM-6700F equipped with an Oxford-INCA energy dispersive X-ray (EDX) spectroscopy, to characterize the morphology, size, and chemical composition of synthesized samples. Transmission electron microscopy (TEM) was used to confirm crystal structure and to determine the particle morphology. Samples were prepared for TEM study by dispersing the powder samples onto 200-mesh carbon-coated copper grids. TEM analyses were performed using a JEOL JEM2010 HRTEM at 200 kV. X-ray diffraction (XRD) was used to identify the crystal structure and average particle size. Diffraction data were recorded using a PANalytical X’ Pert PRO diffractometer with Cu KR radiation (45 kV, 40 mA) in the continuous scanning mode. The 2θ scanning range was from 15° to 85° in steps of 0.03° with a collection time of 20 s per step.
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3. Results After a very small amount of CdCl2 (∼0.0016 mol) was mixed with excess (NaOH þ KOH) solvents (∼0.2 mol) and coarsened at 200 °C, the morphology evolution of forming Cd(OH)2 was observed. As shown in the SEM images of Figure 1, after the chemical reaction of Cd2þ and OH-, Cd(OH)2 particles were produced. At the beginning of coarsening in molten hydroxide (until 24 h), particles with irregular shape at a size of around a hundred nanometers were obtained, as seen in Figure 1a. At 40 h, the aggregation of particles became concrete. The morphology of products reveals a cauliflower-like structure with a size of about 4 μm (Figure 1b). From the enlarged image (inset of Figure 1b), we can clearly see the uneven surface of the larger individual Cd(OH)2 particle. With the coarsening time prolonged to 88 h, the prestructure of Cd(OH)2 aggregates was recrystallized and further transformed to nanoplates, and meanwhile nanoplates were organized into a globe-like geometrical structure. As seen in Figure 1c, the architecture of ellipsoid was produced, which was constructed by intercrossed nanoscale plates (∼100 nm of thickness). From the broken ball (inset of Figure 1c), we can clearly see that the inner structure of the ellipsoid is composed of nanoplates. The enlarged SEM image for the split of ellipsoid (Figure 1e) also shows the building blocks of nanoplates for the larger complex architecture. An EDX spectrum in Figure 1f reveals the chemical composition of particles including cadmium and oxygen. To further check the thermal stability of organized architecture, we extended the coarsening time to 5 days and found that the globular complex was gradually decomposed and transformed to 3D aggregates by nanoplates (Figure 1d). The thickness of nanoplates is about 150 nm. From 88 h to 5 days, the phase structure and size of the plates tend to be stable, though the architectures of nanoplate aggregates kinetically change with the increased time. Powder XRD data of Cd(OH)2 particles at different growth stages are shown in Figure 2. Cd(OH)2 nanocrystals are mainly hexagonal phase, and the observed peaks in the XRD pattern can be indexed onto the hexagonal unit of Cd(OH)2 with the unit cell parameter a = 3.494 A˚ and c = 4.710 A˚ (space group P3m1 164, JCPDS No. 31-0228). From the XRD pattern, we also found some very weak impurity peaks of CdCO3 (marked as triangles), which was probably produced from the treatment process of washing and drying powder samples. With the coarsening time increasing from 24 h to 5 days, there is little change of the structure and size of particles. To investigate the exposure facets of the intercrossed nanoplates at different reaction stages, the morphology and crystal structure of products were further determined by TEM and selected area electron diffraction (SAED). As shown in Figure 3a,c, we can clearly see the lying and standing plates cross-linking together, which is a typical product after the reaction at 88 h and 5 days. The insets of Figure 3a,c show the SAED pattern of the lying plate. The SAED patterns can be both indexed as the hexagonal Cd(OH)2 with the electron beam directing on the (001) plane. As the SAED patterns are shooting along the [001] zone axis, we can readily get the diffraction from the planes (100) and (110). The corresponding HRTEM images of a Cd(OH)2 single plate are shown in Figure 3b,d. From the image of the (001) plate, we can clearly see the crystallographic axis directions of the hexagonal crystal lattice and the cross angle of 120°, which was indexed
Figure 2. Powder XRD patterns of cadmium hydroxide precipitates in molten composite-hydroxide at 200 °C for 24 h, 40 h, 88 h, and 5 days, respectively. The small diffraction peaks marked with triangles are probably from the impurity of cadmium carbonate.
in Figure 3d. Some small particles can also be seen attached to the big plate. Cd(OH)2 particles coarsening at other temperatures over the eutectic point of NaOH/KOH (∼170 °C) were also performed to check the influence of temperature on the morphology of cadmium hydroxide. As seen in Figure 4a-c, the products at 180, 200, and 220 °C for 88 h present a similar morphology, ball-like architecture assembled by nanoplates, though at high temperature, the size and aspect ratio of nanoplates becomes larger. Furthermore, the effect of hydroxide concentration on the structure of Cd(OH)2 was checked by preparing particles in 2 and 10 M (NaOH þ KOH) hydroxide solution (nNaOH/nKOH = 0.515/0.485). As we can see in Figure 4d-f, at low hydroxide concentration, only particles with micrometer size can be obtained. With the concentration of solution increasing to 10 M, the particles start to transform to lamellar structure, which can be clearly seen on the lateral part of the particles. While in the molten hydroxide system with the supreme “concentration” of hydroxide ions, nanoplates and their assembling complex are finally achieved. 4. Discussion The whole process from the prestructure of nanoparticle aggregates to the nanoplate architecture of Cd(OH)2 is proceeding in excess molten alkaline solution. During the reaction and coarsening process, the composite hydroxide of (NaOH þ KOH) not only acts as the reactant to produce Cd(OH)2 primary particles but also serves as the surrounding solution medium for the further evolution of nanocrystals. Molten hydroxide ions, as the super strong effect to absorb on the different crystal surfaces, play a critical role in producing nanoplate complex. As a comparison, when coarsening in hydroxide solution, the particles did not show the nanoplate assembling architectures. However, with hydroxide from low to high concentration, the products followed the evolution forming plates, which implied the potential of concentrated hydroxide in producing plates. Because the viscosity of molten hydroxide without any water is very large, this is probably the reason that the formation of Cd(OH)2 nanoparticle prestructure and reconstruction of nanoplate architecture are slow. On the basis of the above observation, the whole process of forming the Cd(OH)2 hierarchical structure could be illustrated in Figure 5. From the morphology evolution of Cd(OH)2
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Figure 3. (a, c) TEM images of cadmium hydroxide nanoplate complex in molten composite-hydroxide at 200 °C for 88 h and 5 days. Insets are the selected area electron diffraction (SAED) patterns acquired from a single plate with the zone axis [001]. (b, d) HRTEM of cadmium hydroxide single plate. In (d), crystallographic axis directions and angle are indicated. Some small particles can also be seen on the big plate.
Figure 4. (a-c) SEM images of cadmium hydroxide synthesized in molten composite-hydroxide at 180, 200, and 220 °C for 88 h. (d-f) SEM images of cadmium hydroxide made in 2 and 10 M (NaOH þ KOH) solution, and molten composite-hydroxide at 200 °C for 88 h.
particles, we can see that the formation of final stable products does not follow a straightforward pathway but involves the birth and deterioration of metastable prestructure. First, CdCl2 reacts with NaOH/KOH, which nucleates to form Cd(OH)2 nanoparticles. As the amount of CdCl2 is only about 1% of (NaOH þ KOH) solvents, the product of Cd(OH)2 particles
are continuously coarsened in the much more excess hydroxide solution, which has almost the same amount as that before reaction. So, the growth, aggregation, recrystallization, and self-assembly processes are always under a strong absorption effect of viscous molten hydroxide solution. After undergoing a long time of growth, the prestructure of nanoparticle aggregates
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Figure 5. Schematic representation of the formation and evolution of cadmium hydroxide nanoplate architectures. It involves the initial formation of prestructure by nanoparticle aggregates, then the recrystallization under the effect of molten hydroxide ions and self-assembly into the globular structure by cross-linked nanoplates, and further transformation to 3D nanoplate architecture.
was manufactured. In the view of the growth speed, this process might be controlled by an Ostwald ripening mechanism which is limited by the diffusion of solute ions,32 for the large viscosity of molten hydroxide without any water suspending the diffusion rate of ions. As mentioned before, the prestructure of nanoparticle aggregates shows uneven surfaces exposed to the molten hydroxide, which implies an imperfect and unstable state. Under the strong surface effect of molten hydroxide, the unstable prestructure will be easily transformed to a thermodynamically favored structure. Thermodynamically, the outcome of unstable prestructure originates from its lower activation barrier relative to the final products (as illustrated in Figure 6).14 With the further growth of particles, the surface free energy will play a critical role and determine the products of the stable polymorph with the lower free energy. Being in the molten hydroxide solution, the final formation of thermodynamically favored nanoplates is not only related to the crystal structure of Cd(OH)2 itself but also is dependent on the nanocrystal surface state and/or surface energy under the strong absorption of hydroxide. In the hexagonal structure of Cd(OH)2, OH- is arranged as a hexagonal close-packed matrix and Cd2þ occupies the hole of octahedron in the every other layer. Every Cd2þ is surrounded by six OH- and attaches each other to one layer, which builds up the lamellar structure of the Cd(OH)2 crystal and OH bonds with a relative weak force connecting the neighboring layers. This lamellar structure will facilitate the forming of plates. Structurally, for the hexagonal crystal system, the crystal face with smaller indices, such as {001} faces, has a higher atom density. In theory, the bonding coordination number of atoms on these surfaces is closer to that in the crystal body, so that they have a lower surface energy and the growth rate in the direction of is smaller. Moreover, when in the molten hydroxide solution, there is a varied amount of hydroxide groups strongly adsorbed on different surface planes of Cd(OH)2
Figure 6. Schematic illustration of energetics of two different polymorphs as a function of particle radius. The metastable polymorph with lower critical nucleation activation energy might occur first and then transform to the stable phase with a lower free energy.
Figure 7. Crystallographic illustration of Cd(OH)2 lamellar structure and its (001) surface state under the absorption of molten OH- ions.
nanoparticles, which depends on the corresponding surface density of atoms. Therefore, as shown in Figure 7, more oppositely charged hydroxide ions are attached on the (001) surface, which can be ascribed as hydrogen bonding between OH- ions. Combining these two points together, the (001)
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Figure 8. SEM images (a and b), XRD pattern (c), and TEM image with inset SAED pattern (d) showing the morphology and crystal structure of calcium hydroxide prepared at 200 °C for 4 days. The small diffraction peak marked with a triangle is from the impurity of calcium carbonate.
surface has a lower surface energy in molten hydroxide solution and the growth along the [001] zone axis will be suspended, which produces the nanoplates with the (001) flat surface and close to the hexagonal shape. In brief, under the strong surface effect of molten hydroxide ions, the prestructure of Cd(OH)2 nanoparticles recrystallized and formed the nanoplates. Besides, super concentrated hydroxide ions provide the glue effect to make an assembly of the nanoplates. To verify the self-assembly behavior of nanocrystals in molten hydroxide, we selected another system of Ca(OH)2, with the same hexagonal lattice and space group as those of Cd(OH)2, to make its hierarchical architecture by nanocrystals. By following the same precipitating procedure in the molten alkaline system, the globular architecture of Ca(OH)2 nanoplatelets can be also manufactured. As shown in Figure 8a, after coarsening at 200 °C for 4 days, the plate aggregates with the shape of a ball were produced under the strong effect of hydroxide ions. From Figure 8b and the inset of Figure 8a, we can see that the whole ball is a loose texture with intercross hexagonal discs and the thickness of plates is about 100 nm. Powder XRD in Figure 8c shows that the products are basically hexagonal phased Ca(OH)2 with the unit cell parameter a = 3.589 A˚ and c = 4.916 A˚ (space group P3m1 164, JCPDS no. 44-1481). Similarly, a very small amount of CaCO3 was detected during the process of post treatment. The SAED pattern (Figure 8d) from a single plate shows that the zone axis perpendicular to the plate is [001] direction and the electron diffraction from the planes parallel to the [001] zone axis can be obtained, such as (100) and (110), which is consistent with the preferred orientation of nanoplates in the Ca(OH)2 system. 5. Conclusion We have established a general applicable route for the fabrication of cadmium hydroxide architecture with intercrossed nanoplates. It is a simple, one-step, and self-assembly synthesis approach, in which molten hydroxide plays a critical role in determining the subsequent hierarchical assembly of
nanoplates with specific crystal faces. This one-step facile fabrication of nanoplate architecture has also been extended to the calcium hydroxide system with the same assembling structure. This work may provide a rational approach and has a good potential for large-scale fabrication of 3D nanostructure assembly with important scientific and technological applications. Acknowledgment. Financial support for this study was provided by the National Basic Research Program of China (2010CB933501), the National Natural Science Foundation of China (20971123), (20803082) and (21007070), the Outstanding Youth Fund (50625205), Natural Science Foundation of Fujian province (2010J0101), and Special Starting Foundation for Excellent Doctors in Chinese Academy of Sciences.
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