J. Phys. Chem. C 2009, 113, 925–929
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Formation of Hollow Gallium Nitride Spheres via Silica Sphere Templates Chun-Neng Lin and Michael H. Huang* Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan ReceiVed: October 19, 2008; ReVised Manuscript ReceiVed: NoVember 19, 2008
We report the formation of hollow GaN spheres using silica sphere templates. First, silica spheres with an average diameter of ∼130 nm were synthesized. A mixture of GaCl3, silica spheres, water, and urea in 2-propanol was prepared and heated to 100 °C for 24 h to generate silica spheres with γ-Ga2O3 nanoparticle shells. Decomposition of urea and its reaction with water slowly increased the solution pH to ∼8; this controlled reaction is the key to forming uniform γ-Ga2O3 shells with a thickness of about 13 nm. The amounts of urea and water have been varied to find the optimal conditions for the preparation of the oxide shells. Transfer of the colloidal particle solution to silicon substrates and ammonolysis at 850 °C for 6 h produced the SiO2-GaN core-shell nanostructures. Immersion of the silicon substrates in an HF solution removed the silica cores, and hollow GaN spheres with a shell thickness of around 8 nm were formed. The morphologies and crystal structures of the oxide and nitride shells have been carefully examined. The GaN shell materials show an absorption band at 350-360 nm and a broad defect-related emission band centered at around 570 nm. Introduction Gallium nitride (GaN) is an important III-V semiconductor with a wide direct band gap of ∼3.4 eV. Because of its potential for optoelectronic applications, fabrication of GaN nanostructures such as nanowires and nanoparticles has been an area of particular research interest in recent years.1-3 Various synthetic strategies including metathesis reactions,4,5 pyrolysis of singlesource precursors,6-9 and reactions between separate gallium and nitrogen reagent sources have been employed to directly form GaN nanocrystals and quantum dots.3,10 Although direct synthesis of individual GaN nanoparticles has been successfully demonstrated using the above methods, assembly of these particles for the fabrication of novel GaN nanostructures such as uniform hollow spheres remains challenging. To make GaN hollow spheres with uniform thickness, identification of reaction conditions favoring an even distribution of a gallium source over the entire surfaces of the spherical cores is critical to its success. Hollow GaN spheres have been prepared by the conversion of hollow Ga2O3 spheres, formed using carbon spheres from glucose or sucrose as templates, in an ammonia atmosphere at 700-900 °C.11 In another study, vapor-phase transport of gallium vapor from GaCl3 powder heated to 1000-1200 °C generated nanosized Ga droplets on substrates.12 A liquid gallium-ammonia source gas interface reaction led to the formation of hollow GaN spheres with rough surfaces. More recently, Eu2+-doped GaN-SiO2 nanocomposites were synthesized by mixing silica spheres with Ga(NO3)3 and Eu(NO3)3 at 85 °C, followed by ammonolysis at 900 °C.13 Dispersed Eu2+-doped GaN particles, rather than a continuous coverage, were obtained. Clearly, available methods for the preparation of hollow GaN spheres with a uniform and continuous shell are very limited; the development of new synthetic approaches is highly desirable. In this paper, we present a new approach for the preparation of hollow GaN spheres using silica sphere templates. In the first step, a urea-assisted reaction generated γ-Ga2O3 shells on * To whom correspondence should be addressed. E-mail: hyhuang@ mx.nthu.edu.tw.
the surfaces of the silica spheres. Transfer of the colloidal particle solution to silicon substrates and heat treatment under ammonia flow produced the SiO2-GaN core-shell nanostructures. Hollow GaN spheres were synthesized after the removal of the silica cores with HF solution. The crystal structures of the shells have been carefully characterized. Optimal reaction conditions necessary for the formation of uniform γ-Ga2O3 and GaN shells have been identified. Optical properties of the products were also examined. Experimental Section Preparation of Monodispersed Silica Spheres. The silica colloidal spheres with an average size of 130 nm were synthesized on the basis of a literature procedure with slight modification.14 In a typical experiment, 5 mL of tetraethyl orthosilicate (99.0%, Fluka), 5 mL of deionized water, and 50 mL of 99.5% ethanol were mixed in a 100 mL polyethylene container. While the mixture was stirred vigorously, 1 mL of ammonium hydroxide (28-30 wt %, Showa) was added to the solution. The resulting mixture was rapidly stirred for 12 h at room temperature. The suspended white colloidal particles were collected by centrifugation at 8000 rpm for 10 min and redispersed in ethanol or water several times. Finally, the asprepared silica colloids were transferred into 2-propanol (99.8%, Aldrich), which had previously been dried using calcium hydride granules (95%, Aldrich) to remove water and diluted to a concentration of 30 mg/mL as a stock solution. Deposition of γ-Ga2O3 Shells on the Silica Sphere Cores. A volume of 8.5 mL of dry 2-propanol containing 6 mg of the silica spheres and 20 µL of deionized water was first prepared in a 22.5 mL glass vial with a Teflon-lined polypropylene cap. Then 0.18 g of urea pellets (reagent grade, J. T. Baker) was added and dissolved. A 0.2 M gallium(III) chloride stock solution was prepared by dissolving 0.2 g of GaCl3 in dry 2-propanol. GaCl3 was stored in a glovebox. Subsequently, 0.5 mL of the gallium chloride stock solution was injected into the silica and urea mixture. The molar ratio of urea to gallium ion is 30:1. The solution was stirred and heated at a rate of 1 °C/ min to 100 °C in an oil bath for 24 h. The products were
10.1021/jp8092429 CCC: $40.75 2009 American Chemical Society Published on Web 12/30/2008
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Figure 1. (a, b) SEM and TEM images of the as-synthesized bare silica spheres. (c, d) SEM and TEM images of the silica spheres after the deposition of γ-Ga2O3 shells.
Figure 2. (a) XRD patterns of the silica spheres with a layer of γ-Ga2O3 shell. (b) XRD pattern of the SiO2-GaN core-shell nanostructures after the SiO2-γ-Ga2O3 core-shell spheres were heated to 850 °C for 6 h under ammonia flow.
collected by centrifugation at 6000 rpm for 7 min and washed with two rounds of deionized water and one round of 95% ethanol. Finally, the white powder was dispersed and stored in ethanol. Formation of GaN Shells. The γ-Ga2O3-coated silica spheres dispersed in ethanol were dropped onto a 1.5 cm × 1.5 cm Si(100) substrate and dried at 60 °C in an oven for about 3 h. The substrate was loaded into the center of a quartz tube with a 1 in. diameter, which was then placed in a tube furnace (Lindberg/Blue). One side of the quartz tube was connected to a steel gas cylinder of anhydrous ammonia (Air Products San
Fu), and the other side was linked to an oil bubbler followed by a flask containing 500 mL of distilled water to trap the remaining ammonia gas. The quartz tube was first purged with ammonia flow at 50 sccm for 1 h to remove moisture and oxygen. Under the same ammonia flow rate, the furnace temperature was raised to 850 °C at a rate of 5 °C/min and held for 6 h. After that, the furnace cooled naturally. Part of the obtained pale yellow powder was scraped off the substrate and dispersed in 2-propanol for characterization. Acidic Etching of Silica Cores. The inner silica cores can be removed by using hydrofluoric acid (HF; 40%, RiedeldeHae¨n). Because HF is an extremely corrosive chemical, the whole process was carefully conducted in a hood. First, 500 mL of saturated calcium D-gluconate monohydrate (98%, Alfa Aesar) aqueous solution was prepared in a polyethylene (PE) container. The initial concentrated acid was diluted to make 10 mL of 8% HF aqueous solution in a 20 mL PE vial with a cap. The previously used Si(100) substrates and new carbon filmcoated copper grids loaded with the silica-GaN core-shell nanostructures were immersed in this etching solution for 10 s and immediately rinsed in 10 mL of water and then ethanol to remove the remaining acid and produced silicate. The samples were dried naturally in a hood. Last, the HF solution and rinsed solution were added to the calcium D-gluconate solution that sequesters the fluoride ions. Instrumentation. Field-emission scanning electron microscopy (FE-SEM) images were obtained by using Hitachi S4700 and JEOL JSM-7000F scanning electron microscopes operated at 5 kV. An energy-dispersive spectroscopy (EDS) detector was attached to the JEOL electron microscope for elemental analysis. Transmission electron microscopy (TEM) images were collected by using JEOL JEM-2000 FX II and JEM-3000F transmission electron microscopes operated at 160 and 300 kV, respectively. X-ray diffraction (XRD) patterns were taken on a Shimadzu XRD-6000 X-ray diffractometer with Cu ΚR radiation. A JASCO V-570 spectrophotometer was used for acquiring UV-vis absorption spectra of the samples. Diffuse reflectance spectra were taken by using a Hitachi U-3310 spectrophotometer equipped with integrating spheres. A He-Cd laser with an
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Figure 3. (a, b) SEM and TEM images of the SiO2-GaN core-shell nanostructures. (c, d) SEM and TEM images of the hollow GaN spheres.
Figure 4. (a) High-resolution TEM image of a portion of a spherical GaN shell after the removal of the silica core. (b, c) Respective circled regions in panel a showing the lattice fringes of GaN. (d) SAED pattern of the hollow GaN spheres.
excitation wavelength of 325 nm was employed for measuring the photoluminescence spectra. Results and Discussion For this study, the key to forming GaN shells is to search for a way to uniformly deposit the gallium source on the surfaces of the silica spheres. Previously, we have demonstrated the formation of arrays of GaN and InN nanorods within the nanoscale channels of mesoporous silica SBA-15 powder via the infiltration of metal sources (i.e., GaCl3 and In(NO3)3) and a subsequent ammonolysis process.15 The lack of strong interactions between GaCl3 and the surfaces of silica spheres means that a simple addition of this gallium source to a silica sphere solution is unlikely to lead to a uniform and dense deposition for the subsequent heat treatment. We considered the formation of Ga(OH)3 or GaO(OH) shells,16 followed by a conversion to Ga2O3 and then GaN as a possible approach to fabricate hollow GaN spheres. CNO- species from the decomposition of urea can react with H+ and H2O (CNO- + 2H+ +
2H2O f NH4+ + H2CO3); this reaction has been shown to increase the solution pH in a moderate manner.16,17 In this work, we adopted this slower and more controllable reaction route by mixing and heating a solution of silica spheres and GaCl3 in the presence of urea to uniformly coat gallium species onto the silica spheres. Before the reaction, the solution pH as measured by a pH test paper was very acidic because of the deprotonation of possible Ga(H2O)63+ species. After the reaction at 100 °C for 24 h, the solution pH increased to about 8. Tin oxide hollow colloids and R-Fe2O3 particles have also been synthesized by adding urea to the reaction mixture.18,19 Figure 1 shows the SEM and TEM images of the assynthesized silica spheres and after the deposition of γ-Ga2O3 shells. (Confirmation of the composition of the shell layer will be given later.) Smooth-surfaced silica spheres with an average diameter of ∼130 nm were obtained. These silica spheres were used as templates for the formation of gallium-containing shells. After the deposition of gallium species, the surfaces of the silica spheres became rough, and uniform shells were formed. Fine
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Figure 5. Solution UV-vis absorption spectra of bare silica spheres and the SiO2-GaN core-shell nanostructures.
Figure 6. Diffuse reflectance spectrum of the SiO2-GaN core-shell nanostructures. The inset shows a plot of (Rhν)2 vs hν for the determination of the direct band gap of the GaN nanostructures.
particles just a few nanometers in diameter were evenly distributed over the surfaces of the silica spheres. No separate aggregated particles from the silica spheres were found. The average diameter of the coated spheres increased to ∼150 nm. The shell thickness was about 13 nm as estimated from the TEM images. EDS analysis of the coated spheres reveals strong signals for gallium, silicon, and oxygen (see the Supporting Information). The composition of the shell material was determined from its XRD pattern (see Figure 2a). The diffraction pattern matches well with that of cubic γ-Ga2O3 (JCPDS file no. 20-0426), which is a rarely observed phase for gallium oxide. Broad and weak peaks of γ-Ga2O3 were recorded, reflecting the nanocrystalline nature of the shell particles. The broad peak in the region of 15-30° 2θ is from the amorphous silica spheres. The absence of GaOOH peaks suggests that any initially formed Ga(OH)3 or GaOOH precipitate can be further dehydrated to form γ-Ga2O3 nanoparticles under the present solvothermal synthesis conditions. γ-Ga2O3 nanoparticles and nanotubes have also been synthesized by hydrothermally treating a mixture of Ga(NO3)3, NaOH, and HNO3 at 100 °C for 2 days.20 To form reasonably uniform gallium oxide shells, the amounts of urea and water added to the reaction mixture were varied. If the amount of urea added was reduced to 0.06 g, or one-third of the amount used in this study, thinner shells were produced (see the Supporting Information). The solution pH also remained acidic. The thinner shells may not withstand further treatment to form hollow GaN spheres. Due to the low solubility of urea in 2-propanol, addition of a larger amount of urea than that used in this study is not desirable. Water is considered a reactant
Lin and Huang from the equation of urea decomposition. We found that the use of 20-50 µL of water gave the best products with a uniform coating of γ-Ga2O3 nanoparticles and separated silica spheres. Addition of 200 µL of water accelerated the hydrolysis process; the more rapid formation of precipitate led to an uneven coating of nanoparticles on the silica spheres. We also found that lowering the reaction temperature to 70-90 °C decreased the amount of nanoparticle coating. After the heat treatment at 850 °C with ammonia flow, the γ-Ga2O3 shells were transformed into GaN shells. Parts a and b of Figure 3 display the SEM and TEM images of SiO2-GaN core-shell nanostructures. The silica spheres remained separated. The surfaces of the spheres became rougher due to the formation of GaN nanocrystals. In some spheres, the GaN shells did not completely cover the silica cores. This is understandable considering the original thin oxide shells and the extent of atom migration during the high-temperature nitridation process. This porous shell structure has also been reported for GaN tubes with porous nanowalls after nitridation of β-Ga2O3 tubes.21 Figure 2b is the XRD pattern of the SiO2-GaN core-shell nanostructures. Only diffraction peaks of hexagonal GaN were recorded, indicating a complete conversion from oxide to nitride with the ammonolysis process. The broad peak from the amorphous silica spheres can still be observed. Parts c and d of Figure 3 offer the SEM and TEM images of the hollow GaN spheres after treatment of the SiO2-GaN core-shell nanostructures with HF solution to remove the templates. The spherical shape of the GaN shells with porous walls was preserved. The hollow spheres also look more transparent. Higher magnification TEM images of these hollow spheres show that the shell thickness is around 8 nm, which is thinner than the γ-Ga2O3 shells. The shrinkage of the shell thickness and the formation of porous walls not only result from the reaction of gallium oxide and ammonia, forming GaN (γ-Ga2O3 + 2NH3 f 2GaN + 3H2O), but also are related to the decomposition of ammonia (2NH3 f N2 + 3H2). The evolved hydrogen gas can react with gallium oxide to generate Ga2O gas (γ-Ga2O3 + 2H2 f Ga2O + 2H2O), and this will deplete some of the gallium source needed to form GaN. We also found that the use of a nitridation temperature of 850 °C gave the best hollow spheres with good maintenance of the shells. At lower nitridation temperatures of 700-800 °C, XRD peaks of GaN are barely visible. If the nitridation temperatures are raised to 875-900 °C, the spherical GaN shell structure cannot be maintained as more Ga2O gas is produced (see the Supporting Information). The detailed analysis of single hollow GaN spheres was carried out. Figure 4a presents a high-resolution TEM image of a portion of a spherical GaN shell after the removal of the silica core. It reveals that the shell is composed of connected tiny domains of nanocrystalline GaN. Enlarged lattice fringe images are given in Figure 4b,c. Lattice fringes with d spacings of 2.60 and 2.74 Å were measured, which should correspond respectively to the (002) and (100) lattice planes of hexagonal GaN. The selected-area electron diffraction (SAED) pattern of the hollow spheres shows a ring pattern that can be matched to that of GaN, further verifying the polycrystalline nature of the shell materials (see Figure 4d). The optical properties of the GaN shell materials were examined. Figure 5 gives the solution UV-vis absorption spectra of bare silica spheres and the SiO2-GaN core-shell nanostructures. A single absorption band located at 350-360 nm was measured for the GaN shell materials. This band position is close to the band gap value of bulk GaN. To further determine the band gap value of the nanocrystalline GaN shells,
Formation of Hollow GaN Spheres a diffuse reflectance spectrum of the SiO2-GaN core-shell nanostructures was taken (see Figure 6). The absorption band maximum is at around 340 nm. From the plot of (Rhν)2 vs hν, a band gap value of 3.13 eV was determined. This number is smaller than the band gap value for bulk GaN. Previously, a smaller band gap value of 3.22 eV was also obtained from the diffuse reflectance spectra of ultralong ZnO nanowires using the same instrument, and the same band gap value was found for bulk ZnO.22 Photoluminescence spectra of the SiO2-GaN core-shell nanostructures were also taken (see the Supporting Information). No near-band-edge emission peak was measured, possibly due to the presence of a large amount of crystal defects from the aggregated nanocrystalline GaN particles. The GaN shells exhibit a broad emission band centered at around 570 nm. This band is believed to be the defect-related emission band. Conclusion We have demonstrated the fabrication of hollow GaN spheres using silica sphere templates. To achieve a relatively uniform coating of GaN shells on the silica spheres, a urea-assisted reaction in 2-propanol was adopted to first form thin γ-Ga2O3 shells. The amounts of urea and water added to the reaction mixture were varied to find the optimal conditions favorable for the preparation of separated silica spheres with thin gallium oxide shells. In the next step, the coated silica spheres were subject to an ammonolysis process to convert the oxide shells to the nitride shells. After the removal of the silica sphere templates with HF solution, hollow GaN spheres with a shell thickness of around 8 nm were synthesized. The morphology and crystal structures of the oxide and nitride shells have been carefully characterized. Absorption and emission properties of the GaN shell materials have also been studied. This synthetic approach to make hollow GaN shells should be extendable to grow thin GaN and perhaps other metal nitride nanostructures of various other shapes, provided silica templates of certain structures are available. Acknowledgment. We thank the National Science Council of Taiwan for the financial support of this research (Grant NSC95-2113-M-007-031-MY3). The assistance of Shih-Cheng Chang and Prof. Shangjr Gwo in the measurements of photoluminescence spectra is also acknowledged.
J. Phys. Chem. C, Vol. 113, No. 3, 2009 929 Supporting Information Available: EDS spectrum of the silica spheres with deposition of γ-Ga2O3 shells, additional SEM and TEM images of the silica spheres with γ-Ga2O3 shells and GaN shells, and emission spectrum of the SiO2-GaN core-shell nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Johnson, J. C.; Choi, H.-J.; Knutsen, K. P.; Schaller, R. D.; Yang, P.; Saykally, R. J. Nat. Mater. 2002, 1, 106. (b) Gradecˇak, S.; Qian, F.; Li, Y.; Park, H.-G.; Lieber, C. M. Appl. Phys. Lett. 2005, 87, 173111. (2) Kim, H.-M.; Kang, T. W.; Chung, K. S. AdV. Mater. 2003, 15, 567. (3) Sardar, K.; Rao, C. N. R. AdV. Mater. 2004, 16, 425. (4) (a) Wang, J.; Grocholl, L.; Gillan, E. G. Nano Lett. 2002, 2, 899. (b) Grocholl, L.; Wang, J.; Gillan, E. G. Chem. Mater. 2001, 13, 4290. (5) Pan, G.; Kordesch, M. E.; Van Patten, P. G. Chem. Mater. 2006, 18, 5392. (6) Sardar, K.; Dan, M.; Schwenzer, B.; Rao, C. N. R. J. Mater. Chem. 2005, 15, 2175. (7) Pan, G.; Kordesch, M. E.; Van Patten, P. G. Chem. Mater. 2006, 18, 3915. (8) Mic´ic´, O. I.; Ahrenkiel, S. P.; Bertram, D.; Nozik, A. J. Appl. Phys. Lett. 1999, 75, 478. (9) Manz, A.; Birkner, A.; Kolbe, M.; Fischer, R. A. AdV. Mater. 2000, 12, 569. (10) Biswas, K.; Sardar, K.; Rao, C. N. R. Appl. Phys. Lett. 2006, 89, 132503. (11) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 3827. (12) Yin, L.-W.; Bando, Y.; Li, M.-S.; Golberg, D. Small 2005, 1, 1094. (13) Mahalingam, V.; Tan, M.; Munusamy, P.; Gilroy, J. B.; van Veggel, F. C. J. M. AdV. Funct. Mater. 2007, 17, 3462. (14) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (15) (a) Yang, C.-T.; Huang, M. H. J. Phys. Chem. B 2005, 109, 17842. (b) Chang, S.-C.; Huang, M. H. Inorg. Chem. 2008, 47, 3135. (16) Tas¸, A. C.; Majewski, P. J.; Aldinger, F. J. Am. Ceram. Soc. 2002, 85, 1421. (17) Shaw, W. H. R.; Bordeaux, J. J. J. Am. Chem. Soc. 1955, 77, 4729. (18) Lou, X. W.; Yuan, C.; Archer, L. A. Small 2007, 3, 261. (19) Ocan˜a, M.; morales, M. P.; Serna, C. J. J. Colloid Interface Sci. 1999, 212, 317. (20) Zhao, Y.; Frost, R. L.; Martens, W. N. J. Phys. Chem. C 2007, 111, 16290. (21) Ding, S.; Lu, P.; Zheng, J.-G.; Yang, X.; Zhao, F.; Chen, J.; Wu, H.; Wu, M. AdV. Funct. Mater. 2007, 17, 1879. (22) Kuo, T.-J.; Lin, C.-N.; Kuo, C.-L.; Huang, M. H. Chem. Mater. 2007, 19, 5143.
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