Controlled Synthesis of GaN@SiO2 Particles in Preventing the

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Controlled Synthesis of GaN@SiO2 Particles in Preventing the Hydrolysis of GaN Keyan Bao,* Ge Guo, Lianfeng Zhang, Ruoyu Liu, Hongxian Sun, and Zhiguo Zhong College of Chemistry and Pharmacy Engineering, Nanyang Normal University, Henan, 473061 P. R. China

bS Supporting Information ABSTRACT: GaN small particles are sensitive to water molecules and easily hydrolyzed into GaOOH when exposed to atmosphere. One of the perfect solutions is to coat them with SiO2, which serves as a protective layer against adsorption of water molecules on the GaN surface and effectively prevents the hydrolysis of GaN. Herein, the synthesis of GaN@SiO2 with morphologies of microrhombuses and microrods is demonstrated. The products of GaN@SiO2 were characterized by XRD, indicating that there was almost no hydrolysis of GaN@ SiO2 after one year exposition in the air. The photoluminescence (PL) of GaN microrhombuses and microrods exhibited emission peaks in the blue region, as did the GaN@SiO2 products. The luminescence intensity increased after SiO2 coating, while the PL position was not shifted. The silica shell effectively prevented the hydrolysis of GaN and successfully preserved the luminescence properties of GaN microrhombuses and microrods.

1. INTRODUCTION Extensive research on refractory group III nitride semiconductors continues at a dizzying pace, motivated to a large extent by the development of new blue- and green-light-emitting diodes and lasers in practical optoelectronic devices.1 The recent development of commercial blue-light emitters based on GaN has propelled these materials into the mainstream of interest. GaN, an important IIIV semiconductor with a direct band gap of 3.39 eV at room temperature, has wide use in optical devices operating at blue and ultraviolet wavelengths and in high-temperature electronic devices.2,3 GaN materials have been prepared by many methods, such as chemical vapor deposition,4 metalorganic chemical vapor deposition,5 molecular beam epitaxy,6 halide vapor-phase epitaxy,7 arc discharge,8 magnetron sputtering,9 chemical thermal-evaporation process,10 etching method,11 and ammonolysis route.1215 Up to now, many structures of GaN including nanoparticles,1621 nanowires,2231 microrods,3234 nanobelts,35 hollow spheres (spindles),36 and tubes3741 have been successfully synthesized. Nanomaterials often display high chemical reactivity due to their low dimensionality and a high surface-to-volume ratio. The reactivity may lead to oxidation and contamination and dramatic changes in morphologies and properties of nanostructures. Thus, it is extremely important to have a protective sheath made of thermally and chemically stable materials to enhance their performances. Graphite coatings could act as chemically inert protecting layers for GaN nanomaterials, and several groups have coated GaN nanomaterials with graphite layer by various methods, such as arc discharge in nitrogen atmosphere,42 two-step catalytic reaction,43 microwave plasma-enhanced chemical vapor deposition,44 annealing experiment based on surface decoration with small metal clusters,45 and a carbon-assisted chemical vapor deposition.46 Furthermore, the synthesis of GaN/PMMA r 2011 American Chemical Society

composite was also reported.47 Chevtchenko and co-workers reported that the surface of GaN films could be passivated by SiNx and SiO2.48 In 2008, Dyuzheva et al.49 reported that GaOOH single crystals with a size of about 0.125 mm 0.125 mm 0.650 mm could be obtained by the reaction of a hydrolysis and an oxidation of GaN at high pressure: GaN þ 2H2O f GaOOH þ NH3. As to the materials, the embodiment of their capabilities is related to both properties and stability, because the latter directly determines the application conditions. In this article, we did systematic investigations on the stability of the GaN micromaterials. Our experimental data showed that GaN small particles gradually hydrolyzed into GaOOH when exposed to air, and all of the products transferred into GaOOH after one year. Experimental results indicate that one of the perfect solutions to this problem is the fabrication of silica-coated GaN materials, where the SiO2 layer serves as a protective layer against adsorption of water molecules3 on the GaN surface and effectively prevents the hydrolysis of GaN. In this paper, we developed a facile synthetic method to prepare GaN@SiO2 with microrhombuses and microrods morphologies. The products were characterized by powder X-ray diffraction (XRD), indicating that there is almost no hydrolysis of GaN@SiO2 microrhombuses and microrods after one year. The photoluminescence (PL) emission spectrum of the GaN@SiO2 microrhombuses indicates that the luminescence intensity increased after SiO2 coating but the PL position did not shift. Therefore, the characteristics of GaN@SiO2 products have the potential to extend the application of GaN materials. Received: March 20, 2011 Revised: May 16, 2011 Published: May 26, 2011 13200

dx.doi.org/10.1021/jp202624a | J. Phys. Chem. C 2011, 115, 13200–13206

The Journal of Physical Chemistry C

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2. EXPERIMENTAL METHODS 2.1. Synthesis of GaOOH Microrhombuses. In a typical synthesis, under continual stirring, 0.88 g of GaCl3 (5 mmol) was dissolved into a mixture of 20 mL of deionized water and 25 mL of N,Ndimethylformamide (DMF). Then 0.8 g of poly(vinylpyrrolidone) K-30 (PVP) was introduced to the above solution. Then the solution was transferred to a 50 mL Teflon-lined autoclave. The autoclave was sealed, maintained at 180 °C for 15 h, and then allowed to cool to room temperature naturally. The product was retrieved by filtration, washed several times with distilled water and absolute ethanol, and dried under vacuum at 60 °C for 6 h. 2.2. Synthesis of GaOOH Microrods. In total, 0.35 g of (5 mmol) metal Ga (99.99%) was dissolved into 5 mL of hydrochloric acid. After the solution was vaporized nearly dry, 20 mL of deionized water was added to form a clear solution. Under continual stirring , some diluted ammoniawater was introduced to the solution until the pH reached 6. Then 25 mL of DMF was mixed with the above solution. At last 1.0 g of PVP (K-30) was added into the solution. Then the mixture was transferred into a 50 mL Teflon-lined autoclave. The autoclave was sealed, maintained at 190 °C for 24 h, and then allowed to cool to room temperature naturally. The resulting white precipitate was retrieved by filtration, washed several times with distilled water and absolute ethanol, and dried under vacuum at 60 °C for 6 h. 2.3. Preparation of GaN Microrhombuses and Microrods. A 0.5 g amount of the as-prepared GaOOH microrhombuses or microrods was loaded into a quartz crucible which was then horizontally placed in the quartz tube of a tubular furnace. The quartz tube was evacuated for 20 min. Then a pure NH3 flow was set within the quartz tube at a constant rate of 20 sccm (standard cubic centimeters per minute). The system was heated to 850 °C with heating rates of 8 °C min1 under flowing NH3 and then maintained at this temperature for 100 min. The sample was then allowed to cool to room temperature with flowing NH3 and was subsequently sealed in a plastic tube for further characterization. 2.4. Silica Coating on GaN Microrhombuses and Microrods. Silica-coated GaN microrhombuses and microrods were prepared via the modified St€ober method.50 Typically, 40 mg of the GaN products were ultrasonically dispersed in a solution containing 80 mL of isopropyl alcohol and 8 mL of distilled water for 2050 min to form a suspension. Under vigorous stirring, ammonia (3.5 mL, 25 wt %) was added to the mixture solution, followed by the addition of TEOS (0.067 mL) in isopropyl alcohol (10 mL) 3 times within 6 h. After being stirred for 6 h, the reaction mixture was centrifuged and washed several times with isopropyl alcohol, absolute ethanol, and distilled water and dried under vacuum at 60 °C for 8 h. 2.5. Characterization. Powder X-ray diffraction (XRD) measurement was carried out with a Philips X’Pert diffractometer (CuKR λ = 1.541 874 Å; nickel filter; 40 kV, 40 mA). Field emission scanning electron microscope (FESEM) images were taken on a JEOL JSM-6300F SEM. Transmission electron microscopy (TEM) images, high-resolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SAED) were performed on a JEOL JEM-2010 microscope operating at 200 kV. TGA-DTA measurements were carried out on a Diamond TG/differential thermal analysis (DTA) thermal analyzer (Perkin-Elmer) with a heat rate of 10 K/min in air or N2. Photoluminescence (PL) measurements were carried out on a Perkin-Elmer LS-55 luminescence spectrometer using a pulsed Xe lamp. The Raman spectrum was obtained on the JY LABRAM-HR laser micro-Raman spectrometer with 514.5 nm emission lines.

Figure 1. (a, b) FESEM images, (c, d) TEM images, (e) HRTEM image, and (f) corresponding SAED pattern of GaOOH microrhombuses.

3. RESULTS AND DISCUSSION In our experiment, the XRD patterns show that GaN gradually hydrolyzed into GaOOH when exposed to air, and all of the products transferred into GaOOH after one year. It is reported that S. A. Chevtchenko investigated the effects of SiNx and SiO2 surface passivation on the optical and electrical properties of GaN layers with different carrier concentrations.48 It is found that the main effect of such passivation on the PL spectra leads to an increase of the near-band-edge emission intensity and a reduced density of charged surface states at reverse bias, and thereby a reduction in the device leakage current. These above effects are attributed to the removal of oxygen from the surface of GaN and the subsequent formation of a protective layer during passivation. Hence, we have strong inclination to generalize this idea to prevent the hydrolysis of GaN. Theoretically the SiO2 coating on the surface of GaN can reduce the contact of water molecules with GaN, which can be preventing the hydrolysis of GaN. Silicacoated GaN microrhombuses and microrods were prepared via the modified St€ober method. The experiment results indicate that the SiO2 shell serves as a protective layer against adsorption of water molecule on the GaN surface and effectively prevents the hydrolysis of GaN (as shown below in Figure 11). The GaN@SiO2 products were prepared in main three steps: first, orthorhombic GaOOH microrhombuses and microrods were solvothermally prepared as precursors. Second, wurtzite GaN microrhombuses and microrods were synthesized via ammoniating GaOOH precursors. Last, GaN@SiO2 microrhombuses and microrods were fabricated by the modified St€ober method. GaOOH microrhombuses and microrods were synthesized through the hydrothermal method. The products were characterized by X-ray diffraction (XRD) pattern and were found to be the orthorhombic phase of GaOOH (JCPDS PDF no. 06-0180, a = 4.58 Å, b = 9.80 Å, c = 2.97 Å) (Figure S1 of the Supporting Information). The morphologies and the structures of the GaOOH products were checked by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), 13201

dx.doi.org/10.1021/jp202624a |J. Phys. Chem. C 2011, 115, 13200–13206


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Figure 3. XRD patterns of the as-prepared GaN products: (a) GaN microrhombuses and (b) GaN microrods.

Figure 2. (a, b) FESEM images, (c, d) TEM images, (e) HRTEM image, and (f) corresponding SAED pattern of GaOOH microrods.

high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Figure 1a shows the FESEM image of the as-prepared GaOOH, exhibiting that the product consists of uniform microrhombuses with lengths in the range of 35 μm. The high-magnification image (Figure 1b) indicates that the GaOOH microrhombuses have smooth surfaces. Figure 1c and d are representative TEM images of GaOOH microrhombuses, indicating that the microrhombuses are solid structure, about 4 μm in length. Figure 1e exhibits the HRTEM image of a single GaOOH microrhombus, revealing that the crystal planes have lattice spacing of about 0.410 nm corresponding to the (110) plane of the orthorhombic phase of GaOOH. The corresponding SAED pattern (Figure 1f) can be indexed as (021), (131), and (110) planes of orthorhombic GaOOH, indicating that the asprepared GaOOH microrhombus is single crystal. FESEM and TEM images in Figure 2 reveal the morphology and structure of GaOOH microrods. As shown in Figure 2a, the product is composed of large numbers of GaOOH microrods with smooth surfaces. The GaOOH microrods are usually 24 μm in length and 500800 nm in cross section (Figure 2b). The TEM images in Figure 2c and d exhibit a typical solid structure of microrods. Figure 2e is a HRTEM image of the GaOOH microrods, which indicates an interplanar spacing of about 0.49 nm corresponding to the (020) plane of orthorhombic GaOOH crystal. The corresponding SAED pattern is presented in Figure 2f, which can be indexed to the typical (001), (021), and (020) planes of orthorhombic GaOOH. The analysis of HRTEM in combination with SAED jointly confirms that the as-prepared GaOOH rod is single crystal. The wurtzite GaN microrhombuses and microrods can be synthesized by ammoniating of orthorhombic GaOOH microrhombuses and microrods in a quartz tube at 850 °C under ammonia flow atmosphere for 100 min. All the reflection peaks can be indexed as wurtzite GaN, which is in good agreement with the standard data (JCPDS PDF no. 74-0243, a = 3.195 Å, c = 5.182 Å). No peaks of impurities such as Ga2O3 and Ga were detected, revealing that the orthorhombic GaOOH microrhombuses and microrods were

Figure 4. (a, b) FESEM images, (c, d) TEM images, (e) HRTEM image, and (f) corresponding SAED pattern of GaN microrhombuses.

completely converted into wurtzite GaN microrhombuses and microrods (Figure 3). Typical SEM images of the as-prepared GaN microrhombuses are shown in Figure 4a and b. On the basis of these data, it can be observed that the sample consists of microrhombuses with lengths in the range of 45 μm. Careful observation from the highmagnification image (Figure 4b) indicates that the rough surfaces of GaN microrhombuses are made of densely packed GaN nanoparticles about 50150 nm in size. Figures 4c and d are the TEM images of the GaN product, confirming that the product is composed of solid structure of GaN microrhombuses about 4.5 μm in length. As shown in Figure 4e, the lattice fringes of the (010) plane of wurtzite GaN with a d spacing of 0.275 nm can be clearly seen. Detailed structural analysis of GaN nanoparticles was carried out with SAED. The corresponding SAED spots (Figure 4f) can be indexed as wurtzite GaN (102), (112), and (010) planes. Figures 5a and b show low-magnification and middle-magnification FESEM images of wurtzite GaN microrods, which have rough surfaces made of small nanoparticles, quadrate cross sections of about 500900 nm in diameter, and the lengths of 13202



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Figure 5. (a, b) FESEM images, (c, d) TEM images, (e) HRTEM image, and (f) corresponding SAED pattern of GaN microrods.

Figure 7. (a) TGA, (b) DTA of GaN microrhombuses; (c) TGA, (d) DTA of GaN microrods.

Figure 6. (a, parts i, ii, iii) XRD patterns of the products shown in (b), (c), (d), respectively. FESEM images of the products obtained by calcinations of orthorhombic GaOOH microrhombuses at 850 °C (b) for 10, (c) 30, and (d) 100 min.

about 24 μm. Figure 5c is a representative TEM image of a single GaN microrod, indicating that the microrod is solid structure and about 4 μm in length. Figure 5d is a high-magnification TEM image of the single GaN microrod, revealing that the GaN microrods are composed of small nanoparticles with diameters of about 50150 nm. Detailed structural analysis of a single GaN nanoparticle was carried out with HRTEM and SAED. Figure 5e is a HRTEM image that shows that the crystal planes have lattice spacing of about 0.275 nm corresponding to the (100) plane of wurtzite GaN. The corresponding SAED pattern (Figure 5f) can be indexed as wurtzite GaN (100), (010), and (110) planes. Several experiments were carried out under different reaction conditions making the understanding of the processes of orthorhombic GaOOH microrhombuses and microrods transformation

into wurtzite GaN microrhombuses and microrods possible. The growth process of wurtzite GaN microrhombuses is taken as an example. Figure 6a exhibits the XRD patterns of the three products obtained by calcinations of GaOOH microrhombuses at 850 °C for 10, 30, and 100 min, respectively, indicating that the three products are rhombohedral structure of Ga2O3 (JCPDS PDF no. 74-1610, a = 4.98 Å, c = 13.43 Å), rhombohedral Ga2O3 with minor amount of wurtzite GaN, and wurtzite GaN, respectively. The morphologies of the samples were further investigated by FESEM. As shown in Figure 6bd, the three products are all composed of microrhombuses, and the lengths are in the range of 45 μm. The observations suggest that the initial GaOOH structural motifs were unaffected by the subsequent high-temperature chemical transformations processing, with the surfaces becoming rough only compared with that of the GaOOH precursors. It is clear that the formation of GaN microrhombuses involves two steps: (1) First step: the orthorhombic GaOOH microrhombuses decomposed into rhombohedral Ga2O3 microrhombuses without destroying the GaOOH framework. As shown in Figure 6b, the rhombohedral Ga2O3 product is composed of microrhombuses with lengths in the range 45 μm. (2) Second step: rhombohedral Ga2O3 microrhombuses transformed into wurtzite GaN microrhombuses via a high-temperature reaction. Pure wurtzite GaN could be obtained at 850 °C for 100 min. Figures 6bd display the FESEM images of the products obtained at 850 °C for 10, 30, and 100 min respectively, revealing that the initial structural motifs were unaffected. 13203



Figure 8. Raman spectra of (a) GaN microrhombuses and (b) GaN microrods.

Based on the above discussion, the possible formation process could be suggested as follows: 2GaOOH f Ga2 O3 þ H2 O Ga2 O3 þ 2NH3 f 2GaN þ 3H2 O The thermal stabilities of the as-prepared GaN microrhombuses and microrods were examined by TGA-DTA under flowing air. The TGA-DTA curves of GaN microrhombuses oxidation tests (air flow) are shown in Figure 7a and b. From the TGA curve (Figure 7a), it is found that the weight of the product does not change significantly below 684 °C. From 684 to 970 °C, the weight of the powder increases gradually by about 9.9%. As shown in the DTA curve (Figure 7b), there is only one big exothermic peak which starts at 684 °C, ends at 977 °C, and centers at 950 °C. Combining the results of the two curves, we can reach the following conclusions: The sample has basically not been oxidized from room temperature to 684 °C. From 684 to 977 °C, the sample suffered gradual oxidation. The oxidation process became intensified as the temperature rised to 950 °C. The TGA-DTA curves of GaN microrods oxidation tests (air flow) are shown in Figure 7c and d, which are similar to the TGADTA curves of GaN microrhombuses. These reveal that the asprepared GaN microrhombuses and microrods have excellent thermal stability and antioxidation property. It is known that Raman scattering is a valuable tool for probing phonon excitation in semiconductors. It can give useful information about impurity, grain size, porosity, and crystal chemistry. Figure 8a and b shows the Raman spectra of the as-prepared GaN microrhombuses and microrods, respectively. The two spectra clearly indicate that Raman peaks appear at about 251, 312, 421, 568, and 729 cm1. The first-order modes at about 568 and 729 cm1 exhibit the feature of red-shifts as compared to values of 570 and 735 cm1 for bulk GaN, which is attributed to the nanometer size effect.3 The second-order Raman modes at about 251, 312, and 421 cm1 are assigned to the zone-boundary phonon, activated by crystal imperfections and finite size effects, and the acoustic overtone of wurtzite GaN .26,32 A study of the optical properties of GaN microrhombuses and microrods is desirable, because their optical properties are directly linked to their potential optoelectronic applications. Figure 9 shows the room-temperature photoluminescence (PL) emission and excitation spectra of the as-prepared wurtzite GaN microrhombuses and microrods. Figures 9a and b are the room-

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Figure 9. Curves ac: room-temperature photoluminescence emission spectra of the GaN microrhombuses and microrods and the GaN microrhombuses exposed to the air for one year. Curves i, ii, iii: roomtemperature photoluminescence excitation spectra of the GaN microrhombuses and microrods and the GaN microrhombuses exposed to the air for one year.

temperature PL emission spectra of the as-prepared GaN microrhombuses and microrods with excitation wavelength of 370 nm. The PL emission spectrum of the wurtzite GaN microrhombuses exhibits an emission peak located at 450 nm, and the wurtzite GaN microrods reveal an emission peak centered at 448 nm. Figures 9i and ii are the room-temperature PL excitation spectra of the as-prepared GaN microrhombuses and microrods with emission wavelength of about 450 and 448 nm. The positions of the two PL emissions spectra of GaN microrhombuses and GaN microrods are possibly attributable to the existence of defect.51 The above study showed that the as-prepared GaN microrhombuses and GaN microrods have excellent thermal stability and optical properties; they can be expected to become good candidates for research in optical and optoelectronic device. However, the theoretical calculation shows that GaN material is sensitive to water molecules and it easily hydrolyzes into GaOOH when exposed to atmosphere. GaNðsÞ þ 2H2 OðgÞ f GaOOHðsÞ þ NH3 ðgÞ ΔH ϕf , 298 ðJ 3 mol-1 Þ 109621 241814 705300 45940 ΔSϕf , 298 ðJ 3 K -1 Þ 29:706

188:724

51:56

192:669

ðΔr GϕT ¼ Δr H ϕf , T TΔr SϕT ÞΔr Gϕ298 ¼ 109:5 KJ 3 mol-1 Our experimental data showed that GaN gradually hydrolyzed into GaOOH when exposed to air, and all of the products transferred into GaOOH after one year. In order to extend the application of GaN materials, surface coating and surface modification can be applied. Silica-coated GaN microrhombuses and microrods were prepared via the modified St€ober method. The reflection peaks of the products after being treated with the TEOS solution can be indexed as wurtzite GaN, which are in good agreement with the standard data (JCPDS PDF no. 740243, a = 3.195 Å, c = 5.182 Å). No other peaks were detected, revealing the formation of amorphous SiO2 (Figure S2 of the Supporting Information). The morphologies of the products were investigated using TEM and SEM. Figure 10a and b present low-magnification and high-magnification TEM images of GaN 13204



Figure 10. (a) Low-magnification and (b) high-magnification TEM images of GaN microrhombuses coated with silica showing a core/shell structure. (c) Low-magnification and (d) high-magnification TEM images of GaN microrods coated with silica showing a core/shell structure.

Figure 11. (a) XRD pattern of GaN microrhombuses and (b) XRD pattern of GaN@SiO2 microrhombuses exposed to air after one year.

microrhombuses coated with a silica layer. It is can be clearly observed that the GaN microrhombuses have a core/shell structure and the silica shell is clearly visible. The thickness of the silica shell is uniform at about 3050 nm. Figures 10c and d show low-magnification and high-magnification TEM images of the core/shell structure of GaN@SiO2 microrods. The thickness of the silica shell is uniform (about 40 nm), and the shell surface appears to be smooth. Note that the silica shell was homogeneous on each individual GaN particle. As a result, the shape of each GaN particle was essentially retained during silica coating (Figure S3 of the Supporting Information). PL measurement was used to test the optical properties of GaN@SiO2 materials. Take microrhombuses as an example. The room-temperature PL emission spectrum of the GaN@SiO2 microrhombuses shown in Figure 9c indicates that the luminescence intensity increased after coating but the PL position did not shift. Amorphous SiO2 is almost insulating (Eg > 5.4 eV); the SiO2-coated GaN microrhombuses and microrods reported here are structures of semiconductorinsulator in the radial direction. The silica shell effectively prevented the hydrolysis of GaN and successfully preserved the luminescence properties. The colors of the GaN microrhombuses and microrods changed from yellow to white when exposed to the air for one year, while the GaN@SiO2 microrhombuses and microrods did

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not. XRD was used to verify the crystal structure and the phase of the materials. Take microrhombuses as an example. Figures 11a and b are the XRD patterns of GaN microrhombuses and GaN@SiO2 microrhombuses exposed to the air for one year, respectively. All the reflection peaks in Figure 11a can be indexed as orthorhombic phase of GaOOH, which reveals that the products of GaN microrhombuses were completely hydrolyze into orthorhombic GaOOH. Figure 11b is the XRD pattern of the product of GaN@SiO2 microrhombuses exposed to air for one year. The main reflection peaks can be indexed as wurtzite GaN. The two small diffraction peaks marked with stars at 22.84° and 27.38° can be indexed as (020) and (110) diffractions of orthorhombic GaOOH. It is obvious that the SiO2-coated GaN microrhombuses were hardly hydrolyzed. In this case, the SiO2 layer serves as a protective layer against adsorption of water molecule on the GaN surface, while it did not obviously change the optical property. The effect of passivation on GaN surface was delayed the hydrolysis compared with the unpassivation samples; thus, it extends the application field of GaN materials.

4. CONCLUSIONS We developed a facile synthetic method to prepare GaN@ SiO2 with microrhombuses and microrods morphologies. The GaN@SiO2 products were prepared in main three steps: first, orthorhombic GaOOH microrhombuses and microrods were solvothermally prepared as precursors. Second, wurtzite GaN microrhombuses and microrods were synthesized via ammoniating GaOOH precursors. Last, GaN@SiO2 microrhombuses and microrods were fabricated by the modified St€ober method. The products of GaN@SiO2 particles were stable in atmosphere, while GaN particles were sensitive to water molecules and easily hydrolyzed into GaOOH. SiO2 shell serves as a protective layer against adsorption of water molecules on the GaN surface and effectively prevents the hydrolysis of GaN. The PL of GaN microrhombuses and microrods exhibited emission peaks in the blue region, as did the GaN@SiO2. The luminescence intensity increased after SiO2 coating, while the PL position was not shifted. The silica shell effectively prevented the hydrolysis of GaN and successfully preserved the luminescence properties, which is of special interest for a variety of applications including optical devices and high-temperature electronic devices. ’ ASSOCIATED CONTENT

bS

Supporting Information. XRD patterns of the as-prepared GaOOH precursors and GaN@SiO2 products; SEM images of GaN@SiO2 products. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The support of this work by the Key scientific and technological project of Henan Province (092102210021), (092102210194) and Special Foundation of Nanyang Normal University (No. ZX2011002), (No. Nytc2004k02) is gratefully acknowledged. 13205



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