Template-Free Synthesis of Highly Uniform α-GaOOH Spindles and

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Template-Free Synthesis of Highly Uniform r-GaOOH Spindles and Conversion to r-Ga2O3 and β-Ga2O3 Hai-Sheng Qian,† Poernomo Gunawan,† Yun-Xia Zhang,† Guo-Feng Lin,† Jian-Wei Zheng,‡ and Rong Xu*,†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1282–1287

School of Chemical & Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore 637459, and Institute of High Performance Computing, 1 Science Park Road, #01-01, The Capricorn, Singapore Science Park II, Singapore 117528 ReceiVed October 13, 2007; ReVised Manuscript ReceiVed December 1, 2007

ABSTRACT: Monodispersed single crystalline R-GaOOH spindles have been prepared via a simple wet-chemical route at 60 °C, in which GaCl3 was used as the gallium source and ammonia as the alkali. The R-GaOOH spindles obtained at pH values of 10.0 and 11.0 have a hierarchical layered nanostructure and are comprised of many nanoplatelets. The investigation of the products formed at early growth stages indicates that the spindles are formed by a self-assembly process via oriented attachment. During this process, the pH value and the ammonia molecule have great influence on the morphology of the final products. Smooth prism-like R-GaOOH crystals were obtained at a lower pH value of 8.0 when ammonia was used or when ammonia was replaced by ethylenediamine at pH values of 8.0 and 10.0. The single-phase R-Ga2O3 and β-Ga2O3 spindles can be obtained by thermal treatment of the R-GaOOH spindles at 600 and 900 °C, respectively. The morphological characteristics of the pristine R-GaOOH spindles are well maintained in the oxide products in terms of good dispersion, size uniformity and layered nanostructure. The β-Ga2O3 spindle product exhibits a strong blue luminescence emission under the excitation wavelength of 250 nm.

1. Introduction Inorganic materials with novel architectures consisting of nanobuilding units, such as nanoparticles, nanorods, nanobelts, etc., often possess unique and peculiar optical/electronic/ magnetic/mechanical properties that are useful for many potential applications.1 Recently, there has been increasing interest in pursuing effective synthesis strategies for the fabrication of advanced materials with complex shape and hierarchical organization. Various methodologies have been developed such as biomimetic mineralization,2 polymer assisted self-assembly,3 Kirkendall type diffusion,1a hard- and soft-templating synthesis,4 etc. A great variety of hierarchically organized materials have been synthesized. For example, Kotov and co-workers developed a self-assembly process to obtain a layered clay-polyelectrolyte structures.5 Polymer directed formation of unusual CaCO36 and ZnO7 pancake nanostructures were achieved. CuO dandelion constructed from oriented attachment of one-dimensional nanoribbons was reported by Liu and Zeng.8 Among all the processes, simple and scalable routes with rationally designed experimental parameters for mimicking complex and oriented nanostructures are always very attractive. As one of the most important types of semiconductor, Ga2O3 has been extensively studied because of its wide applications in optoelectronic devices including flat panel display, sensor, solar energy conversion devices, etc.9 Among various known polymorphs, β-Ga2O3 is a thermally stable form. Other polymorphs of gallium oxide including R-, χ-, δ-, and ε-G2O3 can all be converted to β-Ga2O3 at T > 870 °C.10 For the largescale synthesis of crystalline Ga2O3, the most convenient and frequently applied route is via thermal treatment of the precursor, gallium oxyhydroxide (R-GaOOH), which can be readily synthesized by wet chemical methods.10a The structure of R-GaOOH is analogous to that of diaspore (R-AlOOH), which * Corresponding author. E-mail: [email protected]. Tel: +65 67906713. Fax: +65 67947553. † Nanyang Technological University. ‡ Institute of High Performance Computing.

has a orthorhombic crystal symmetry.11,12 To generate Ga2O3 of hierarchical structures for advanced applications, it is important to first develop a method to fabricate R-GaOOH particles with organized structures. Second, it is also critical that such organization shall be well preserved even under the harsh conditions of high temperature treatment. Finally, the occurrence of aggregation due to thermal sintering among particles has to be kept minimal. Synthesis of R-GaOOH and Ga2O3 nano- and microcrystals of different morphologies have been reported by several groups using a protein filament templated process, sonochemical reaction, forced hydrolysis, hydrothermal treatment, sol–gel method, etc.13–17 For examples, a low temperature enzyme catalytic process was developed using the protein filaments as both the template and the catalyst for the hydrolysis and polycondensation of gallium nitrate to yield either R-GaOOH or gallium oxides attached to the filament.13 The sonochemical reaction of an aqueous solution of GaCl3 was used to form R-GaOOH cylinders rolled up in a scroll-like layered structure with a metallic gallium core.14 Formation of rod-like R-GaOOH was reported using forced hydrolysis or hydrothermal treatment of aqueous solutions containing gallium nitrate or β-Ga2O3 precursors.16 More interestingly, it was found that spindle-like R-GaOOH single crystalline particles that consist of oriented subnanoplates can be obtained by a homogeneous precipitation of gallium nitrate with urea15 or laser ablation of gallium metal in the presence of cationic surfactant cetyltrimethylammonium bromide.18 In both syntheses, the exact mechanisms of forming the hierarchically organized spindle-like particles were not yet clear, as the presence of various species generated by urea decomposition,19 and the surfactant cation in the solution led to complicated systems. Upon thermal transformation to β-Ga2O3 at 750 °C and above, their particles aggregated and the pristine layered structure in the spindle-like particle was not clearly observed. In the present study, highly uniform R-GaOOH spindles of layered nanostructures have been synthesized by a simple precipitation route using ammonia at a low temperature of 60

10.1021/cg701004w CCC: $40.75  2008 American Chemical Society Published on Web 02/28/2008

Synthesis of Highly Uniform R-GaOOH Spindles

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Figure 1. XRD patterns of samples obtained from precipitation of GaCl3 by ammonia solution at 60 °C with an aging of 18 h at different pH values of (a) 8.0 and (b) 10.0.

°C. It has been found that the morphology of R-GaOOH crystal can be tuned easily by adjusting the pH of the solution. The oxide products of R- and β-Ga2O3 obtained at 600 and 900 °C, respectively, do not show particle aggregation and the individual oxide spindles still exhibit the layered nanostructure.

Figure 2. FESEM images of samples obtained from precipitation of GaCl3 by ammonia solution at 60 °C with an aging of 18 h at different pH values of (a, b) 8.0 and (c, d) 10.0.

2. Experimental Section 2.1. Synthesis of r-GaOOH. All chemicals are analytical grade and were used without further purification. In a typical synthesis, 2 mmol of gallium chloride (GaCl3, 99.99%, ultra dry, Sigma-Alddrich) was dissolved in 20 mL of deionized water to form a clear solution in a bottle (40 mL in total volume) under magnetic stirring. To this solution, an aqueous ammonia solution (Kanto Chemical, ∼1.5 M) was added drop by drop until the pH of the mixture reached 8.0, 10.0, and 11.0, respectively. The bottle was then capped airtight and the temperature of the mixture was raised to 60 °C and aged for 18 h. The resulting precipitates in white color were collected, centrifuged, and washed with deionized water and ethanol for three times respectively, and finally dried at 60 °C for 5 h. In another synthesis, ammonia solution was replaced by EDA (ethylenediamine, 99%, Alfa Aesar), and the pH of the mixture was adjusted to 8.0 and 10.0, while other experimental conditions were kept the same. 2.2. Thermal Conversion of r-GaOOH to r-Ga2O3 and β-Ga2O3. The as-prepared R-GaOOH was calcined at 600 and 900 °C, respectively, in static air with a heating rate of 1 °C min-1 and dwelling time of 3 h. 2.3. Characterization. The crystalline phase of samples were characterized by X-ray diffraction, which was operated on a Bruker D8 Advance 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 20 mA, respectively. The morphology and size of the samples were investigated by field-emission scanning electron microscopy (FESEM, JEOL-6700F) and transmission electron microscopy (TEM, JEOL 3010). Selected area electron diffraction (SAED) pattern was obtained on JEOL JEM 2010F. Photoluminescence (PL) spectra of calcined samples and the commercial bulk β-Ga2O3 (99.99%, Aldrich) were recorded at room temperature using a Shimadzu RF-5301PC spectrofluorophotometer with an excitation wavelength of 250 nm.

3. Results and Discussion 3.1. Synthesis of Highly Uniform Spindle-Like r-GaOOH. Figure 1 shows the X-ray diffraction patterns of the samples prepared at 60 °C for 18 h using ammonia as the precipitation agent. The XRD results of other precipitates are not shown since they all have similar diffraction patterns as those shown in Figure 1. All the peaks can be indexed to the orthorhombic R-GaOOH phase (JCPDS File no. 06-180) with cell constants a ) 4.58 Å, b ) 9.80 Å, c ) 2.97 Å. According

Figure 3. FESEM images of samples obtained from precipitation of GaCl3 by ammonia solution at 60 °C with an aging of 5 h at a pH of 11.0 (a) a general view and (b) a magnified view.

to previous investigation on the precipitation of gallium chloride in hydrochloric solutions by various alkalis, it is known that the fresh precipitates exist as an amorphous gallium hydroxide Ga(OH)3, which transforms to crystalline R-GaOOH during the aging.20 All the as-prepared samples in this work upon precipitation and aging at 60 °C using either ammonia or EDA exhibit a well crystallized R-GaOOH structure, indicating a complete transformation of the amorphous Ga(OH)3 precursor in the present synthesis conditions. The morphology and size of the products were investigated by field-emission scanning electron microscopy (FESEM). Figures 2 and 3 show the images of R-GaOOH synthesized at different pH values using ammonia. At a pH value of 8.0, the product appears as prism-like submicron particles of about 500–800 nm in lengths, which can be normally explained by the outside embodiment of the unit-cell replication and amplification of the orthorhombic phase. These particles have smooth surfaces and are well dispersed. It is interesting to observe that the products obtained at higher pH values have different morphologies. At a pH of 10.0, uniform hierarchical spindlelike R-GaOOH crystals are formed with the average length and width of ca. 1.2 µm and 700 nm, respectively (Figures 2c,d). The SEM images of the product synthesized at a higher pH value of 11.0 with a shorter aging time of 5 h are shown in Figure 3a. This product exhibits a similar spindle-like morphology. These spindles are composed of many well aligned thin nanoplatelets, exhibiting a layered nanostructure. The edges of the individual nanoplatelets can be easily distinguished on the surface of the spindles in the magnified images (Figures 2d and 3b). The aspect ratio of the spindles increases from about 1.5

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Figure 4. (a) TEM image and (b) the corresponding electron diffraction pattern of a single R-GaOOH spindle, prepared from precipitation of GaCl3 by ammonia solution at 60 °C with an aging of 18 h at a pH of 10.0. The inset in (a) illustrates the nanoplatelet crystal orientation.

at pH of 10.0 to about 4 with their average length and width changed to 1.5 µm and 400 nm respectively at pH of 11.0. It is noted that the current syntheses did not involve any specific additives or templates, apart from the gallium chloride precursor and ammonia solution. Our system is thus distinctively different from that reported by Tas et al. in which gallium oxyhydroxide spindles were obtained under the acidic condition of a pH about 2.0–2.5.15 Many other species, such as OH-, CO32-, CN-, NH4+, etc., were expected to be present in their system due to urea decomposition.19 Our observations indicate that the pH value of the solution associated with the concentration of ammonia added during the precipitation plays a key role in the formation the highly uniform spindles with tunable aspect ratios. The structure of the spindles was further investigated by TEM and SAED. Figure 4a displays the image of a representative R-GaOOH spindle obtained at the pH of 10.0. Consistent with the SEM results, it can be clearly observed that the spindlelike particle is comprised of face-to-face assembled nanoplatelets. The corresponding SAED pattern (Figure 4b) reveals a single-crystalline structure of the sample and the diffraction spots can be indexed to a pure orthorhombic phase of R-GaOOH. The crystal orientations of the nanoplatelets along the length, width and thickness are determined to be [001], [100], and [010], respectively, as illustrated in the inset of Figure 4a. In agreement with the previous results by Huang and Yeh,18 the preferential growth direction of R-GaOOH nanoplatelets is along the [001] direction (c axis). This is associated with the characteristics of R-GaOOH crystal structure. Similar to diaspore, the structure of R-GaOOH consists of double chains of edge-sharing octahedra and two-thirds of the octahedral interstices are occupied by gallium, while the oxygen-hydroxyl sheets form a hexagonal closed-packed array.11,12 In principle, a crystal face with more closely packed atoms has lower density of the unsaturated bonds and thus a lower specific surface free energy.21 Based on the gallium atom density difference, the crystal plane with the lowest and highest surface energy among {100}, {010}, and {001} is {010} and {001}, respectively. As a result, the crystal grows preferentially in the [001] direction and the largest facet of an equilibrium shape of a diaspore-like crystal belongs to {010} faces. Besides {010}, the equilibrium crystal is also bounded with {110}, {111}, and {021} facets.11 To gain a better understanding on the growth mechanism of these uniform spindles, the products formed at early stages were collected for TEM analysis. It is found that the fresh precipitates immediately recovered upon precipitation at room temperature are comprised of aggregates of amorphous particles (Figure 5a). It is interesting to observe that some spindle-like nanoparticles of 400–500 nm in length and 100–250 nm in width are formed after a short aging time of 15 min at 60 °C (Figure 5b). On the

Figure 5. TEM images of the products obtained at early stages of selfassembled spindle-like R-GaOOH with layered nanostructures synthesized at a pH of 10.0 (shown in Figure 2c,d). The corresponding reaction durations are (a) fresh precipitates at room temperature; (b) aged at 60 °C for 15 min; (c) aged at 60 °C for 30 min; and (d) aged at 60 °C for 70 min.

basis of our TEM observation, each of these nanoparticles is essentially a single nanoplatelet, which should be generated from dissolution-precipitation of the amorphous Ga(OH)3 precursor. As we have observed here, the transformation of Ga(OH)3 to R-GaOOH phase at pH 10.0 is very fast and the whole process may be described by the following reactions. NH3 + H2O S NH4 + OH-

(1)

Ga3+ + 3OH- S Ga(OH)3

(2)

Ga(OH)3 S GaOOH + H2O

(3)

The appearance of needle-like ends of the spindle-like nanoplatelets implies fast growth in the [001] direction. At longer aging times of 30 and 70 min, more spindles appear in the products at the expense of the amorphous particles (Figure

Synthesis of Highly Uniform R-GaOOH Spindles

5c,d). At the same time, the size of the spindles has grown larger in all directions and the size uniformity is greatly improved. The length of the spindles obtained at 70 min almost reaches that of the final product of ca. 1.2 µm (Figure 5d) with a very small size variation, while the width is slightly smaller at around 500 nm. According to these time-dependent studies, it is known that the primary nanoplatelets of the spindle-like R-GaOOH form very quickly after the precipitation of Ga3+ in the solution with a pH of 10.0 at 60 °C. The nanoplatelets aggregate with the same crystal faces, that is, side-to-side for {100} planes and face-to-face for {010} planes. The former type of aggregation is directly evidenced from the two attached thin nanoplatelets in Figure 5b (in the circle). It appears that such a self-assembly process proceeds readily once the nanoplatelet is formed based on the observation of very few free-standing nanoplatelets coexisting with the growing spindles (Figure 5c, 30 min aging). On the basis of the SEM observations and the size difference between the initial nanoplatelets and the final spindles, it can be seen that the spindles are obtained with a preferential [010] aggregation direction. Although the constituent nanoplatelets in a same spindle of the final products are observed with different lengths and widths (Figures 2 and 3b), single crystals of highly uniform sized spindles are resulted from the oriented attachment growth process.22 It has been reported that pH is an important factor in morphological and dimensional control particularly for low dimensional nanocrystals.23 Previous studies showed that the crystallinity and the morphology of R-GaOOH crystals are sensitive to pH and the type of alkali.15,20 For the first time we demonstrate in this work that R-GaOOH single crystals with distinctively different morphologies and aspect ratios can be obtained by adjusting the pH value as the sole parameter by the amount of aqueous ammonia solution. It was reported that at pH close to 7.0, gallium is almost completely hydrated as Ga(OH)3.24 Upon further increase in pH, dissolution of the amorphous Ga(OH)3 phase occurs, which is accompanied with the formation of negatively charged complexes, such as [Ga(OH)4]- or [Ga2(OH)8]2-.17 The concentration of OHdetermines that of the soluble hydroxy complexes which plays an important role in the growth of R-GaOOH crystals. At a high pH value of 10.0 or above, the concentration of the gallium hydroxy complexes is high. Therefore, the growth of R-GaOOH nanocrystal proceeds sufficiently fast to form a nanoplatelet which is elongated in the [001] direction and bounded with {010} planes as the largest faces due to the surface energy difference in different crystal faces as discussed earlier. Besides providing basic media for precipitation, NH3 · H2O could also coordinate with the trivalent gallium species to form Ga(NH3)x3+ complexes and facilitate the transportation of gallium to the growing sites of the nucleate seeds with OH- ligands attached.25 As a result, at a higher pH value of 11.0 and the corresponding higher concentration of NH3 · H2O, the primary nanoplatelets become larger in the [001] direction as observed from the SEM images. On the other hand, the individual nanoplatelets are not stable due to a high surface-to-volume ratio. Under such a circumstance, aggregation is energetically favored for the formation of larger crystals with reduced interfacial energy of the primary nanoplatelets.22 For anisotropic nanoplatelets, the surface reduction is the most effective when the nanoplatelets are self-assembled through orientated face-to-face aggregation. In this regard, the observed preferential attachment in the [010] direction in our spindles is most probably driven by the energy minimization, which is also the mechanism proposed for the syntheses of other self-assembled materials in the absence of

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Figure 6. FESEM images of samples obtained from precipitation of GaCl3 by EDA at 60 °C with an aging of 18 h at different pH values of (a) 8.0 and (b) 10.0.

surfactant and organic ligands.22a,b In particular, a similar template-free process was reported by Liu et al. for the fabrication of nanoribbon-based hierarchical CuO nanosheets.26 At a lower pH value of 8.0, no self-assembled spindles were obtained. The prism-like smooth crystals should be produced via the conventional Ostwald ripening during the aging process.27 It is interesting to note that, when ammonia is replaced by EDA, no spindle-like products were formed even though the pH value of the mixture was raised to 10.0. As shown in Figure 6, only irregularly sized prism-like R-GaOOH crystals were obtained at both pH values of 8.0 and 10.0. EDA as a bidentate ligand is commonly used as structural directing agents for the synthesis of nanostructured materials, such as Mg(OH)2, ZnO, β-Co(OH)2, etc.28 In this work, although the exact roles of EDA are not yet elucidated, the possible formation of gallium complexes with EDA may lead to different nucleation and crystallization rates of R-GaOOH phase. In addition, EDA adsorbed on the crystal faces as impurity molecules also affects the overall shape and morphology of the obtained crystals. Hence, besides the pH value, the type of alkali also influences the crystal morphology due to different molecular interactions between the alkali and gallium species. 3.2. Conversion to r-Ga2O3 and β-Ga2O3. Many efforts have been made to synthesize R-Ga2O3 and β-Ga2O3 by thermal transformation of some precursors including gallium oxyhydroxide since last century.10a,b,29 It was generally observed that R-Ga2O3 forms at lower calcination temperatures and is converted to β-Ga2O3 at higher temperatures. The temperature ranges for these transformations depend on the type and properties of the gallium precursors.10b,17,18 In this work, well dispersed and uniformly sized R-GaOOH spindles of hierarchical structures (shown in Figure 2c,d) were employed as the precursor to fabricate R-Ga2O3 and β-Ga2O3 crystals. Figure 7 shows the XRD results of the samples calcined at 600 and 900 °C for 3 h. The XRD pattern (Figure 7a) for the sample calcined at 600 °C can be readily identified as a hexagonal phase of R-Ga2O3 (JCPDS card no. 06-0503) with lattice constants a ) 4.979 Å and c ) 13.42 Å. When the calcination temperature was increased to 900 °C, the resulting sample exhibits a monoclinic β-Ga2O3 phase (JCPDS card no. 41-1103) with lattice constants a ) 12.22 Å, b ) 3.038 Å, c ) 5.807 Å, and β ) 103.82 (Figure 7b). No peaks of impurity phases can be

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Figure 7. XRD patterns of samples obtained after calcination of the sample shown in Figure 2c,d at different temperatures for 3 h, (a) 600 °C and (b) 900 °C.

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Figure 9. PL spectra of Ga2O3 products obtained after calcination and the commercial β-Ga2O3 with an excitation wavelength of 250 nm, (a) R-Ga2O3 obtained at 600 °C; (b) commercial β-Ga2O3; and (c) β-Ga2O3 obtained at 900 °C.

440 nm can be observed for both β-Ga2O3 spindles and the commercial β-Ga2O3 powder. The blue luminescence in Ga2O3 is attributed to the recombination of an electron on a donor formed by oxygen vacancies (VO) and a hole on an acceptor formed by gallium vacancies (VGa).31 The β-Ga2O3 spindles generated in this work probably have more surface defects and are less crystalline than the commercial β-Ga2O3 due to their hierarchical nanostructure. The observed stronger blue emission and a slight blue-shift in β-Ga2O3 spindles is consistent with the presence of the defects.32 In addition, all the samples exhibit a UV emission band at around 368 nm, which are assigned to the recombinations a self-trapped excitation.31a

4. Conclusion

Figure 8. FESEM images of samples obtained after calcination of the sample shown in Figure 2c,d at different temperatures for 3 h, (a, b) 600 °C and (c, d) 900 °C.

observed in our XRD results, indicating that both R-Ga2O3 and β-Ga2O3 obtained in this work are single-phase compounds. The morphologies of the obtained R-Ga2O3 and β-Ga2O3 have been observed and shown in Figure 8. Although previous reports have demonstrated that well-dispersed gallium oxide rods in nano- or micronsized scale can be obtained from thermal conversion of R-GaOOH rods,15,16a,b aggregation and fusion of spindle-like particles were always observed.15,18 In contrast to previous results, calcination at temperatures of 600 and 900 °C did not cause any significant changes in the spindle morphology in the present study. As shown in Figure 8, the oxide products well preserve the features of the pristine R-GaOOH spindles in terms of good dispersion, size uniformity and layered nanostructures. Thus, high quality R-Ga2O3 and β-Ga2O3 spindles with a hierarchical nanostructure can be fabricated in a large scale using the current method. The presence of nanosized pores on the surface of β-Ga2O3 is presumed to be due to the elimination of water from the constituent OH- groups in the precursor.15,30 Smaller pores can also be observed on the surface of R-Ga2O3 product, although they are not visible in Figure 8b due to a low magnification. Figure 9 displays the room temperature PL spectra of R-Ga2O3 and β-Ga2O3 spindles obtained in this work, as well as a commercial β-Ga2O3 powder under an excitation wavelength of 250 nm. Broad-band blue emissions centered at around

In summary, single crystalline R-GaOOH with different morphologies have been successfully prepared in high yield at different pH values at 60 °C, in which GaCl3 was used as the gallium source and ammonia or EDA used as the alkali. It is the first time for the report of synthesizing highly uniform spindle-like R-GaOOH single crystals with layered nanostructures at low temperature. Ammonia solution and the pH value of the reaction play important roles in the formation hierarchical structured R-GaOOH spindles. It has also been demonstrated that the aspect ratio of the spindles could be adjusted from 1.5 to 4 by changing the pH value of the solution from 10.0 to 11.0. An oriented attachment mechanism has been proposed for the formation of such unusual spindles with layered nanostructures. Furthermore, the oxide products of R-Ga2O3 and β-Ga2O3 obtained by thermal transformation of the R-GaOOH precursor maintain the morphological characteristics of the pristine spindles. The β-Ga2O3 spindle product exhibits a strong blue luminescence emission possibly due to the defects presented in the hierarchical nanostructure. The synthesis method used in this work is simple and does not involve costly surfactants or sophisticated equipments; it provides a convenient route to synthesize hierarchically nanostructured gallium oxyhydroxide and oxides for advanced applications. Acknowledgment. The authors greatly acknowledge the research funding support from Agency for Science, Technology and Research (A-Star), Singapore (PSF0521010016).

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