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Solution-Liquid-Solid-Induced Tip-Growth of Filled-GaN Nanotubes on MCM-48 Microspheres Ligang Gai,†,‡,* Haihui Jiang,†,‡ Wanyong Ma,† Deliang Cui,‡ Ning Lun,§ and Qilong Wang‡ School of Chemical Engineering, Shandong Institute of Light Industry, Jinan 250100, People’s Republic of China, State Key Lab of Crystal Materials, Shandong UniVersity, Jinan 250100, People’s Republic of China, and School of Materials Science & Engineering, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: August 31, 2006; In Final Form: NoVember 3, 2006
Using MCM-48 microspheres (MMS) as the support, GaN nanotubes, with branched tubules and encapsulated crystalline GaN nanoparticles, have been prepared by the reaction of GaCl3 with excess NaN3 in dry benzene solution. The key steps in our approach are as follows: (1) by stirring at low-temperature of 40 °C for 12 h, the starting materials are adsorbed on the MMS surface with the occurrence of in-situ gallium azide precursor; (2) at higher temperature, under the reduction of active sodium atoms (Na*) derived from the decomposition of surplus NaN3, liquid Ga droplets occur acting as catalytic centers for the growth of GaN nanotubes; (3) at the same time, small GaN nanoparticles, accompanying tube growth, are captured into the tube due to the capillarity-induced filling. Based on the solution-liquid-solid (SLS) model and hodograph concept, a SLSinduced tip-growth mechanism for the formation of filled-GaN nanotubes on MMS is proposed. The new nanostructures presented here enrich the nanoscale community with structural complexity and enable greater potential applications.
1. Introduction One-dimensional (1D) GaN nanostructures have attracted much attention because of their great potential for new visible and UV optoelectronic applications.1 To date, many approaches have been developed for the fabrication of GaN nanorods or nanowires, such as laser ablation,2 template-induced growth,3 metal4- or oxide-assisted synthesis,5 and other methods.6 These methods involve either a high-temperature chemical vapor deposition (CVD), vapor-phase epitaxy (VPE), or molecular beam epitaxy (MBE) process. Furthermore, the vapor-liquidsolid (VLS)7 mechanism dominates the growth process of GaN nanorods or nanowires. On the basis of the analogy to the VLS process, Buhro et al. have pioneered a solution-liquid-solid (SLS) model for the growth of InP, InAs, and GaAs nanowires in organic solution.8 Xie et al.9 have proposed a lamella rollingup mechanism for the synthesis of GaP and InP nanowires in aqueous solution containing cetyltrimethylammonium cations (CTA+). Compared with nanowires or nanorods, nanotubular structures are of special scientific interest due to the possible confinement effect. In the case of GaN nanotubes, several high-temperature synthesis strategies have been successfully designed, such as the templating method,10,11 metal-assisted growth,12,13 and the CVD process.14 Although the solvothermal route is an effective way to prepare GaN nanoparticles at low temperature,15-17 to the best of our knowledge, no synthesis of GaN nanotubes by the solvothermal route has been reported. We have developed a hydroxyl-promoting approach to prepare GaN nanorods on SBA-15 microparticles, of which the * To whom correspondence should be addressed. E-mail: liganggai@ yahoo.com. † Shandong Institute of Light Industry. ‡ State Key Lab of Crystal Materials, Shandong University. § School of Materials Science & Engineering, Shandong University.
grooved morphology and exterior surface hydroxyls are highlighted.18 Herein we report the solvothermal synthesis of GaN nanotubes with unique morphologies at low temperatures of 280-300 °C, using MCM-48 microspheres (MMS) as the support. The formation mechanisms related to the GaN nanotubes on MMS are discussed. 2. Experimental Section 2.1. Synthesis of GaN Nanotubes. Our work is inspired by the ‘step-edge decoration’ idea,19 wherein oxygen-containing functionalities (e.g., carbonyls, ethers, hydroxyls, etc.), which are formed on the graphite surface by electrochemical oxidation, facilitate the nucleation of a new phase on the decorated graphite surface. In addition, it is known that there are plenty of hydroxyls naturally distributed on the surface of mesoporous silica.20 Therefore, it is reasonable to employ MMS as the support in our experiments. As a kind of mesoporous silica, MMS was prepared by the sol-gel method according to the procedure similar to that described by Romero et al.21 In a typical synthesis, 15 mmol GaCl3 (A.R.), 60 mmol NaN3 (A.R.), 50 mL benzene (A.R., dried by sodium flakes), and 0.8 g freshly calcined MMS were introduced into a stainless steel container in a glove box at N2 (99.999%) atmosphere, with a filling ratio up to 30%. The container was then fixed to an autoclave. After being purged with high purity N2 (99.999%), the autoclave was sealed, a vacuume made, and the mixture stirred at 40 °C for 12 h at a rate of 250 r/min. Subsequently, the autoclave temperature was raised to 280-300 °C for 1224 h, at a rate of 1.1 °C/min. After that, the autoclave was cooled naturally to room temperature. The resulting dark-gray product was filtered and rinsed with acetone, absolute ethanol, 10% HF aqueous solution, and deionized water, sequentially. After being dried in a vacuum at 70 °C for 6 h, the product was obtained.
10.1021/jp065650p CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007
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Figure 2. A typical Far-FTIR spectrum of GaN nanotubes. Figure 1. XRD patterns of GaN samples: (a) synthesized at 300 °C for 12 h in the presence of MMS, (b) synthesized at 300 °C for 24 h in the absence of MMS. Black dot (b) in Figure 1a denotes cubic GaN.
2.2. Characterization. The GaN samples were characterized by X-ray powder diffraction (XRD), far Fourier transformation infrared spectroscopy (far-FTIR), transmission electron microscopy (TEM), and room-temperature photoluminescence (PL). XRD examinations were carried out on a Rigaku D/Max-γA X-ray diffractometer with filtered Cu KR radiation. Far-FTIR spectra were taken on a Nicolet NEXUS-670 infrared spectrometer using paraffin oil as the dispersant. For TEM analyses, samples were dripped onto coated Cu grids after sonication in ethanol. TEM images were taken on a Philips Tecnai Twin20U and a Japan JEM-2100 high-resolution transmission electron microscope with an energy dispersive spectrometer (EDS), operating at 200 kV. Room-temperature PL spectra were recorded by an Edinburgh FLS-920 fluorescent spectrometer with a 450 W xenon lamp excited at 290 nm, using a filter through wavelengths over 315 nm. 3. Results and Discussion 3.1. Composition, Morphologies, and Structures. Figure 1ashows a typical XRD pattern of the GaN samples obtained in the presence of MMS. The peak positions in Figure 1a are in good agreement with the reference values (JCPDS card, No. 76-0703). In contrast to that of Figure 1b, the diffraction intensity of peaks increases prominently, implying that the crystallinity of the samples can be improved greatly by using MMS as the promoter. Furthermore, compared with (100) and (101) peaks, the unusually strong (002) peak indicates a preferential orientation along the c-axis of wurtzite GaN.10 The weak peak (marked with dot in Figure 1a) can be indexed to the (111) peak of cubic GaN in the rocksalt structure, which usually occurs under high pressure.15 Far-FTIR measurement is usually used to verify an inorganic compound, the fingerprint-zone of which occurs in the farinfrared region. Figure 2 shows a typical far-FTIR transmittance spectrum of the GaN nanotubes. A strong peak centered at 552 cm-1 and a shoulder peak around 540 cm-1 can be attributed to E1 and A1 transverse optical (TO) model of wurtzite GaN, respectively.22 A broad peak centered at 180 cm-1 can be ascribed to the Ga-N vibrational band.23 A broad weak peak around 480 cm-1 may be related to surface oxidation of the GaN nanostructures during sample processing due to the activity in nanoscale. The peak at 72 cm-1 can be assigned to the band edge effect. Figure 3a shows a typical TEM image of the as-synthesized GaN nanotubes, having outer diameters of 50-150 nm, tube wall thickness of ca. 10 nm, and lengths up to several micrometers. It can be seen clearly that a dense particle is
encapsulated at the tip of each tube. In some cases, nanotubes packed in array (Figure 3b), tubules branched from the trunk (Figure 3b), and branched nanotube with ‘T’ morphology (Figure 3c) can be observed. Examination of a filled-nanotube reveals an open-end and wave-like surface (Figure 3d), while a tube without confined nanoparticles shows an open-end and straight surface (Figure 3c). The high-resolution TEM image of a single unfilled-tube shows an amorphous tube wall (Figure 3e). The corresponding EDS spectrum (Figure 3f) of the tube gives the overall molar ratio of ca. 1:0.93 for Ga to N, indicating the tube to be GaN. The oxygen element in Figure 3f might come from surface oxidation during sample processing. The high-resolution TEM image of a single filled-nanotube (Figure 4a)displays crystallized GaN nanoparticles confined in the tube (Figure 4b), which is confirmed by the corresponding selectedarea electron diffraction (SAED) pattern (Figure 4d). The diffraction rings in Figure 4d from inner to outer is (100), (002), and (101) of wurtzite GaN (JCPDS card, No. 76-0703), respectively. Lattice spacing of ca. 0.26 nm (Figure 4c) of a nanocrystal in Figure 4b corresponds to the d002 of wurtzite GaN (JCPDS card, No. 76-0703), which is consistent with the SAED and XRD results. Although a GaN layer covers the dense particle at the tip of each tube, the corresponding EDS spectrum shows an unusual molar ratio of ca. 3:1 for Ga to N, implying that the core of the dense particle is metal gallium. 3.2. Open End and Wave-like Surface of the GaN Nanotubes. When the molar ratio of GaCl3/NaN3 is 1:4, only a small quantity of active gallium atoms (Ga*) can be generated from GaCl3 or Ga(N3)317 by the reduction of active sodium atoms (Na*) derived from the thermal decomposition of surplus NaN3. At the same time, the formed active nitrogen atoms (N*) are dispersed as gas in the whole reaction system. According to the surface diffusion growth theory,24 the smaller the surface concentration, the more difficult the surface nucleation. As a result, amorphous GaN nanotube walls occur. In addition, when the flux of atoms diffusing from the periphery and feeding tube growth is larger than that of atoms impinging into the inner diameter, open-end nanotubes occur (Figure 3, c and d).24 As for the wave-like surface of the tubes, Menon et al.25 had pointed out that the surface buckling of III-V semiconductors was caused by surface relaxation, originating from orbital rehybridization of the surface atoms. In our GaN nanotubes, the stress on the concave sides of the tubes, which is caused by the confined particles, might contribute to the surface undulation of the tubes. The straight morphology of a tube without confined particles (Figure 3c) gives counterevidence of the above conjecture. 3.3. Proposed Formation Mechanism of the Filled-GaN Nanotubes on MMS. In view of the excess NaN3 used according to stoichiometry, at the low-temperature stage (40
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Figure 3. (a-d) TEM images of GaN nanotubes prepared at 300 °C for 12 h in the presence of MMS. (e) High-resolution TEM image of an open-end GaN nanotube wall marked with an arrow in Figure 3c. (f) EDS spectrum corresponding to the tube marked with an arrow in Figure 3c. Be and Cu elements are attributed to the detecting window material and Cu grids, respectively.
°C for 12 h), [Ga(N3)3]n17 and NaN3 coexist on the surface of MMS (Figure 5, A and B). At the higher temperature, active gallium atoms (Ga*) are formed as mentioned above (Figure 5C). GaN nanoparticles occur due to the decomposition of [Ga(N3)3]n or via the reaction of fresh Ga* with N*. At the same time, segregation of Ga atoms takes place, following a decrease in activity, leading to an increase in the size of catalytic center, i.e., liquid Ga droplet (Figure 5D). Buhro et al. have proposed a SLS mechanism for the synthesis of GaAs nanofibers.8 Stach et al. have reported the growth of GaN nanowires via a selfcatalytic VLS mechanism.26 Both of the mechanisms involve the liquid Ga droplets as catalytic centers. Herein, the growth process of GaN nanotubes resembles but is not identical with the SLS mechanism, while metal droplets acting as catalysts at the tips can be generated in organic solution. With respect to the growth mechanisms for the synthesis of nanotubes, two principal models have been proposed, i.e., ‘tipgrowth’27,28 and ‘base-growth’.27,28 Until Amelinckx et al. introduced the concept of a spatial-velocity hodograph to quantitatively describe the extrusion of a carbon tubule from a catalytic particle, the distinction between the two mechanisms did not appear to be essential.29 On the basis of the state of the nanotube tips and the surface morphology of MMS (see Supporting Information), which reminds us of the hodograph concept,29 we suggest that in our experiments, GaN nanotubes grow on MMS via the ‘tip-growth’ mechanism involving a SLS incipience, perhaps according to the scenario as shown in Figure 5 (Figure 5, D to G). At the first stage, the reaction species dissolve into the liquid Ga droplets in contact with MMS substrate, and the SLS tube growth is initiated by supersaturating the liquid Ga. Then the metal Ga particle is lifted off by the growing tube, with N*
deposited at the annular contact with the particle and at the bottom contact with the support. Subsequently, the nanotube falls away from the support due to loose contact between the tube and the support. Here the stirring during the experimental process may play an important role in the formation of freestanding tubes. At the same time, newly formed Ga droplets and small GaN nanoparticles, accompanying the tube growth, can be captured into the tube due to the capillarity-induced filling.30 As a result, the tube filled with particles occurs. Further growth may presumably be continued by surface diffusion24,29,31 and feedstock supplied to the tip particle.29,32 Once a layer covers the tip particle, the surface diffusion will dominate the tube growth.29 Following the increase of the encapsulated GaN particles in a tube, crystalline GaN nanofibers will occur (see Supporting Information). Certainly, we cannot exclude the possibility for the formation of GaN nanofibers via the SLS model. However, our experimental results provide an insight into the formation process of one-dimensional structures. 3.4. Nanotubes in Bundle and Branched Tubules. Li et al.33 reported a large-scale synthesis of aligned carbon nanotubes by the chemical vapor deposition (CVD) method, using iron nanoparticles embedded in mesoporous silica as catalysts. Kong et al. investigated the effect of catalyst supports on the growth of carbon nanotubes32,34 by the same method. They believed that carbon nanotubes in bundles occurred, because active catalyst particles were uniformly and closely distributed on the support, which had an amorphous structure and isotropic surface. In our experiments, quasi-amorphous mesoporous silica MMS with an isotropic surface is used as the support. Therefore, GaN nanotubes in bundles (Figure 3b) occur, whereby maximizing the van der Waals interactions between the walls of the tubes.32,35 Once the newly formed Ga droplets are captured into
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Figure 4. (a) A TEM image of filled-GaN nanotubes. (b) High-resolution TEM image of the area marked with a square in Figure 4a. (c) Highmagnification image corresponding to the framed nanocrystal in Figure 4b. (d) SAED pattern corresponding to the area marked with a square in Figure 4a. (e) EDS spectrum taken from the dense particle at the tip of a tube marked with a circle in Figure 4a.
Figure 5. Schematic illustration of the growth mechanism of a filled-GaN nanotube on MMS: (A and B) Reactants are adsorbed and [Ga(N3)3]n occurs on the surface of MMS, promoted by the surface hydroxyls (blue dots) via a coordination-catalytic step. (C) Active Ga* (cyan dots) occur by the reduction of active Na* generated from the decomposition of surplus NaN3, accompanying with the occurrence of GaN nanoparticles (gray dots). (D) Incipient solution-liquid-solid (SLS) growth of a GaN nanotube catalyzed by the Ga liquid droplet (dark dot). (E) The Ga droplet is ‘lifted’ away from the support by the growing tube. (F) The growing tube falls away from the support and GaN nanoparticle are ‘sucked’ into the tube by capillary filling. (G) Further growth of the tube mainly controlled by surface diffusion, accompanying with the filling of GaN nanoparticles and Ga droplets. The ‘sucked’ Ga droplets are responsible for the formation of branched tubules (Figure 3, b and c).
a tube and contact the tube wall, catalytic growth of branched GaN tubules will take place (Figure 3b). 3.5. PL of the GaN Nanotubes. Figure 6 shows a typical room-temperature photoluminescence (PL) emission spectrum of the GaN samples excited at 290 nm, using a filter through wavelengths over 315 nm. A broad emission band ranging from 300 to 570 nm is observed. Peak deconvolution by Gaussian distribution shows that the broad band is composed of five bands centered at 324, 376, 420, 443, and 560 nm. The blue
luminescence (BL) band with the highest intensity at 376 nm (3.30 eV), near the band-edge emission of bulk polycrystalline GaN,36 has also been observed in GaN nanoparticles,15,16 nanowires,37 and nanorods.38 The BL band with higher intensity around 443 nm (2.80 eV), which is ascribed to surface states or defects in GaN nanowires by Zhang et al.,39 has been observed in GaN nanotubes12 and nanorods18 as well. The peak at 324 nm (3.83 eV) might be assigned to discrete states situated on a conduction band and a valence band due to the quantum
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Figure 6. A room-temperature PL spectrum of GaN nanotubes excited at 290 nm, using a filter through wavelengths over 315 nm. PL spectrum (black line) is deconvoluted into Gaussian distributions (green lines).
confinement effect in GaN nanoparticles,40 which features the filled-GaN nanotubes. The weak band at 420 nm (2.96 eV) can be attributed to the transition from ON (O substituting for N) to NGa (N antisite).38 The small peak around 560 nm (2.22 eV) might be ascribed to the yellow luminescence (YL) band.41 Jian et al. have pointed out that the YL band is seen in high conductivity GaN (N deficient) and disappears in semiinsulating GaN (N rich),38 which is consistent with the EDS analysis of the obtained GaN nanotubes. However, further work is needed to clarify the underlying mechanism for the PL of GaN nanotubes. 4. Conclusions In conclusion, a simple but novel solvothermal method using mesoporous silica MMS as the support has been developed to synthesize GaN nanotubes with unique morphologies at low temperature (280-300 °C). It is believed that unique morphologies may bring about unique properties of nanostructures. A SLS-induced tip-growth mechanism for the growth of nanotubes in organic solution is suggested. This mechanism makes it possible to prepare other III-V or II-VI semiconductor nanotubes. Supporting Information Available: Field-emission SEM image of MCM-48 microspheres (Figure S1). TEM image (Figure S2a) and SAED pattern (Figure S2b) of the nanofibers. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Ponce, F. A.; Bour, D. P. Nature 1997, 386, 351. (2) Duan, X. F.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. (3) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Hu, Y. D. Science 1997, 277, 1287.
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