Synthesis of AlN Hollow Nanospheres via an in Situ Generated

210009, China. J. Phys. Chem. C , 2008, 112 (30), pp 11331–11335. DOI: 10.1021/jp8016979. Publication Date (Web): July 8, 2008. Copyright © 200...
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J. Phys. Chem. C 2008, 112, 11331–11335

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Synthesis of AlN Hollow Nanospheres via an in Situ Generated Template Method Fan Zhang, Qiang Wu,* Pei Xiao, Yanwen Ma, Yemin Hu, Xizhang Wang, Chunyan Wang, and Zheng Hu* Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu ProVincial Laboratory for Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China

Yinong Lu¨ College of Materials Science and Engineering, Nanjing UniVersity of Technology, Nanjing 210009, China ReceiVed: February 23, 2008; ReVised Manuscript ReceiVed: May 20, 2008

An in situ generated template method has been developed via the simple reaction of AlCl3 with NH3 for the synthesis of AlN hollow nanospheres with diameters of 80-400 nm and shell thickness of ∼15 nm. On the basis of the detailed characterization on the intermediate compounds, the preparation process has been elucidated which includes the first formation of the AlCl3 · xNH3@AlN core-shell nanostructures followed by the decomposition of the inner compound around 1100 °C. This reaction route successfully extended the traditional CVD reaction of AlCl3 with NH3 from producing solid AlN powder to preparing AlN hollow nanospheres. 1. Introduction Hollow nanospheres show some unique advantages such as low effective density, high specific surface area, and distinct optical, electrical and magnetic properties;1 hence, they have great potential applications in many fields, e.g., in catalysis, drug delivery, chromatography, separation and photonic devices.2 To date, many methods such as sol-gel process,3 spray drying4 and self-assembly5 have been employed for the synthesis of hollow nanospheres of various materials.6 The general strategy is to coat the spherical templates, which could be liquid or emulsion droplets,7 gas bubbles,1c metal particles8 and colloid spheres9 with thin layers of the desired materials, followed by the removal of the template through calcination or wet chemical etching. As an important member of group III nitrides, which is one of the most significant material systems for bandgap engineering,10 AlN has attracted increasing interest in recent years due to its high intrinsic thermal conductivity, low thermal expansion coefficient, small even negative electron affinity, excellent piezoelectricity and dielectric properties. AlN nanoscale quantum confinement geometries are expected to have wide potential applications in future nanodevices such as ultraviolet light diodes,11 flexible pulse-wave sensors12 and nanoscale mechanical resonators.13 In recent years, various one-dimensional AlN nanostructures including nanotubes,14 nanowires,15 nanobelts16 and nanocones17 have been successfully synthesized and some novel properties such as field emission18 and light-emitting19 have been observed as expected. In contrast, little progress has been achieved in zero-dimensional AlN hollow nanospheres and even the synthesis of these nanostructures is still a challenging topic. It is noticed that AlN is usually synthesized with metal Al powder or anhydrous AlCl3 as typical Al precursors.14–17 Very recently, with Al nanoparticles precursor, a self-templated synthesis route has been developed with which polycrystalline AlN hollow nanospheres ranging from 20 to 200 nm in diameter have been obtained based on Kirkendall effect.20 * Corresponding authors. Tel: 0086-25-83686015. Fax: 0086-2583686251. E-mail: Z.H., [email protected]; Q.W., [email protected].

As known, the chemical vapor deposition (CVD) reaction between AlCl3 and NH3 has been intensively studied in the past decades.21 With this reaction, high-purity fine AlN powder could be produced, which is very useful for the fabrication of ceramic bodies for advanced electronics applications; however, only solid AlN powders have been reported to date.21 In this paper, by optimizing the experimental setup and reaction conditions, this traditional CVD route has been successfully extended to the synthesis of AlN hollow nanospheres. Briefly, the gaseous reaction of AlCl3 with NH3 first leads to the formation of AlCl3 · xNH3@AlN core-shell nanostructures and a subsequent calcination around 1100 °C could effectively decompose the inner compound to form the AlN hollow nanospheres. Similar route might be applied to explore the hollow nanospheres of other nitrides with corresponding metal chloride precursors. 2. Experimental Section The synthesis of AlN hollow nanospheres was conducted in a horizontal tubular furnace with two temperature zones (Figure 1). Typically, 0.5 g anhydrous AlCl3 was placed in the low temperature zone, and then the furnace was evacuated and flushed with Ar for several times to remove oxygen and moisture. After the high temperature zone was heated to 1000 °C at a rate of 10 °C/min under the protection of Ar flow, the low temperature zone was heated to 170 °C at a rate of 20 °C/ min. At first, the sublimated AlCl3 vapor was carried by Ar gas (260 cm3/min) to the high temperature zone and then mixed with NH3-N2 gas (NH3, 4 vol %) of 340 cm3/min. After the reaction lasted for about 3 h, the product was collected from the tube wall near the outlet where the temperature is below 250 °C. Then, this product was calcined at 1100 °C under the mixture gas of NH3 (250 cm3/min) and Ar (300 cm3/min) for another 3 h, and finally a white product was obtained. X-ray diffraction (XRD) experiments were carried out on a Philips X’pert Pro X-ray diffractometer with Cu KR radiation of 1.5418 Å. The morphology and structure of the product were analyzed by transmission electron microscopy (TEM, JEM1005), scanning electron microscopy (SEM, LEO-1530VP) and

10.1021/jp8016979 CCC: $40.75  2008 American Chemical Society Published on Web 07/08/2008

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Figure 1. (a) Two-temperature-zone tubular furnace along with an indication about the synthesis process for samples 1-3 in Figure 2. (b) Corresponding temperature profile when the central temperature of the high-temperature zone is 1000 °C. The fan-cooling and water-cooling could effectively decrease the interference between the low and high temperature zones.

high-resolution transmission electron microscopy (HRTEM, JEM2010) equipped with an energy dispersive X-ray analyzer (EDX, Thermo NORAN). Raman spectrum of the product was characterized by Raman microscope (Raman, Renishaw Raman microscope 514 nm Ar+ laser) and the photoluminescence spectrum was measured at room temperature with a He-Cd laser excitation at 325 nm (Amino Bowman Series-2 spectrometer). 3. Results and Discussion XRD profile of the product as-obtained at the first stage can be mainly assigned to ammonium chloride (NH4Cl) (#1, Figure 2a). As known, the following reaction, i.e., AlCl3(g) + NH3(g) f AlN(s) + 3HCl(g), could easily occur at temperature above 700 °C.21d Hence, NH4Cl formed through the subsequent reaction between the pregenerated HCl and the reactant NH3 in the downstream below 250 °C, i.e., HCl(g) + NH3(g) f NH4Cl(s).21a–c Because the NH4Cl is much more than the AlN species, which was produced in the high-temperature region and carried downstream, the diffraction peaks from AlN species could not be distinguished from the strong ones from NH4Cl. The corresponding SEM image of the as-prepared sample reveals that the mixture of NH4Cl and AlN mainly exists as irregular nanoparticles (Figure 2b). After dispersing the product into distilled water and ultrasonic treatment for several minutes, spherical nanoparticles with diameter about 80-400 nm could be obtained as shown in TEM image (Figure 2c). The corresponding XRD profile (#2, Figure 2a) is in agreement with that of AlN, as expected, because NH4Cl was easily dissolved in water. The core-shell structure for this AlN-containing species could be clearly observed with the shell thickness of about 15 nm. Interestingly, by 3 h calcination of the product (Figure 2b) in the mixture gas of NH3/Ar at 1100 °C, the contained NH4Cl

was driven out and polycrystalline hollow nanospheres of hexagonal AlN were obtained as confirmed by different characterization results. The XRD profile for the well-crystallized h-AlN is obtained (#3, Figure 2a), SEM and TEM images (Figure 2d,e) clearly show the spherical hollow feature, and the selected area electron diffraction (SAED) pattern (Figure 2f) indicates the polycrystalline nature of the product. As learned from Figure 2c, the AlN-containing core-shell structure is composed of a dense outer shell and a loose core consisting of tiny powder. We have further made a HRTEM characterization on this sample (Figure 3a) and found that the dense shell is composed of crystalline nanoparticles with dimension about 8 nm. The distance between two neighboring lattice fringes is about 0.249 nm, in accordance with the d002 value of hexagonal AlN (Figure 3a). The EDX spectrum of the dense shell, which was obtained by grazing-incidence of electron beam to the shell, shows signals of Al, N and O (Figure 3d). The Al and N signals should come from the AlN shell, and the O signal from the oxygen contamination, which is inevitable for AlN products.14a,15c,20a By vertical-incidence of the electron beam to the core-shell structure, the EDX spectrum could tell us the elemental information from both the outer shell and the inner core, as shown in Figure 3e. In comparison with Figure 3d for the outer shell, a new small Cl signal appears in addition to the expected Al, N and O signals. This indicates that the inside tiny powder is composed of chloride-containing substances. These substances decomposed and condensed during the 3 h calcination in NH3/Ar at 1100 °C; consequently, AlN hollow nanospheres were formed as revealed in Figure 2 and further supported by HRTEM characterization (Figure 3b,c). The corresponding EDX spectra for both grazing- and verticalincidence cases only showed the Al, N and O signals and the Cl signal disappeared (Figure 3f,g), indicating the release of Cl species due to the decomposition of the inside chloride-

Synthesis of AlN Hollow Nanospheres

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Figure 2. (a) XRD patterns of samples 1-3: 1, the as-prepared product at the first reaction stage; 2, after dissolving NH4Cl in sample 1 by distilled water; 3, after 3 h calcination of sample 1 in the mixture gas of NH3/Ar at 1100 °C. (b) SEM image of sample 1. (c) TEM image of sample 2. (d) SEM image of sample 3. (e), (f) TEM image and corresponding SAED pattern of sample 3.

Figure 3. (a) Typical HRTEM image of the core-shell structure in Figure 2c. (b) HRTEM image of the AlN hollow nanospheres. (c) Typical HRTEM image of the shell part of the AlN hollow nanospheres. (d) EDX spectrum collected by grazing-incidence of electron beam to the core-shell structure in (a). (e) EDX spectrum collected by vertical-incidence of the electron beam to the core-shell structure in (a). (f), (g) EDX spectra of the AlN hollow nanospheres for grazing- and vertical-incidence cases, respectively. Note: The dotted line in (a) represents the boundary between the inner core and the outer shell for the core-shell nanostructure. The lattice fringes of 0.249 nm in (a) and (c) is in accordance with d002 value of h-AlN.

containing substance. From above results, it is concluded that AlN hollow nanospheres have been synthesized from the reaction of anhydrous AlCl3 and NH3, followed by calcination treatment in NH3/Ar. Raman spectrum of the AlN hollow nanospheres exhibits the vibration modes at 610.3, 657.2, 669.0 and 911.9 cm-1 (Figure 4a), which could be well assigned to A1(TO), E2(high), E1(TO),

and the mixture of E1(LO) and A1(LO) mode of h-AlN, respectively.16,22 In comparison with the Raman spectrum of the AlN hollow nanospheres made from the Al nanopowder precursor, which only showed an intensive E2 (high) mode but very weak other modes due to the abundant defects arising from the impurities (e.g., δ-Al2O3) and surface or interfacial atoms,20a the full appearance of the Raman modes indicates the quite good

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Figure 4. (a) Raman and (b) PL spectra of the AlN hollow nanospheres.

Figure 6. Typical TEM images of the AlN products obtained by annealing the core-shell nanostructures at (a) 700 °C, (b) 900 °C, (c) 1100 °C and (d) 1200 °C, respectively.

Figure 5. (a) XRD spectrum of the AlCl3 · xNH3 intermediate adduct compound that could be indexed to AlCl3 · 3NH3 to high degree (JCPDS card No. 44-0557). (b) XRD spectrum of the product (AlN particles) by annealing the AlCl3 · xNH3 powder around 1000 °C in NH3/N2 (NH3, 4 vol %) of 340 cm3 min-1 for 3 h. (c) TEM image of the product in (b).

purity and crystallization of the AlN hollow nanospheres in the present study. This indicates that the new preparation method with anhydrous AlCl3 precursor is superior to the previous reports with Al nanopowder precursor in the synthesis of AlN hollow nanospheres.20 Photoluminescence (PL) spectrum of the AlN hollow nanospheres is shown in Figure 4b. It is quite similar to the PL spectrum of the AlN hollow nanospheres made from Al nanopowder precursor.20a In addition to a broadband centered at 464 nm (2.67 eV) related to oxygen impurities17a,23 or nitrogen vacancies24,25 and a emission around 411 nm (3.02 eV) from native defects,25 the emission around 531nm (2.33 eV) might be the characteristics of AlN hollow nanostructures.20a,26 On the basis of the above characterization results, the formation mechanism of the AlN hollow nanospheres could be deduced. As seen from Figure 1, after sublimation of anhydrous AlCl3 at low temperature zone (∼170 °C), the gaseous AlCl3

species was carried by Ar and mixed with NH3/N2, then transported to the high temperature zone. During this process, AlCl3 and NH3 first formed AlCl3 · xNH3 powder, which is a kind of Lewis acid-Lewis base adduct compound, via AlCl3 + xNH3 fAlCl3 · xNH3 in the temperature range of 400-700 °C (roughly corresponding to the region of -13 to -11 cm in Figure 1a).21a,d,e This is confirmed by the XRD spectrum of the powder collected from the tube wall of the -13 to -11 cm region, which could be indexed to AlCl3 · 3NH3 to a high degree, as shown in Figure 5a. During the floating process, to pass through the high-temperature region of about 700-1000 °C (ca. -11 to +10 cm in Figure 1a), the AlCl3 · xNH3 powder agglomerated to spherical aggregates21d with the surface layer decomposed to AlN shell via AlCl3 · NH3 f AlN + 3HCl21a,d,e due to the short time of only about 5 s (see Supporting Information). The surface AlN shell then inhibited the further decomposition of the inside AlCl3 · xNH3; hence the core-shell nanostructures formed with the in situ generated AlCl3 · xNH3 as template, as revealed in Figures 2c and 3a,d,e. This is supported by the long-time annealing of the AlCl3 · xNH3 powder in NH3/N2 around 1000 °C, which resulted in the formation of AlN particles rather than the core-shell structures (Figure 5b,c). Actually the omen of the core-shell structures also existed in the similar reaction process between AlCl3 and NH3 in literature.21d When the core-shell nanostructures were further calcined at 1100 °C in NH3/Ar for 3 h, the inside AlCl3 · xNH3 species decomposed and condensed; hence AlN hollow nanospheres formed (Figure 2e). In fact, the decomposition of the inside AlCl3 · xNH3 species started at about 700 °C.21a,d,e The higher the annealing temperature, the more completely the inside AlCl3 · xNH3 decomposed, and the more compact the formed AlN species were, and hence, the more cavities there were inside the shell. This could be learned from the morphological evolution of the products with annealing temperature in the range of 700-1100 °C (Figure 6a-c). Further increasing the annealing temperature to 1200 °C resulted in the collapse of the hollow structure and AlN particles were then obtained (Figure 6d). This indicates the quite good high temperature endurance up to 1100 °C for the AlN hollow nanospheres prepared by this in situ generated template method.

Synthesis of AlN Hollow Nanospheres 4. Conclusions In summary, by optimizing the experimental setup and reaction conditions, an in situ generated template method has been developed via the reaction of AlCl3 with NH3 for the synthesis of AlN hollow nanospheres. AlCl3 and NH3 first formed the AlCl3 · xNH3 intermediate adduct compound in the upriver of the reactor around 400-700 °C, which agglomerated to spherical aggregates with the surface layer decomposed to AlN shell during passing quickly through the high-temperature region of about 700-1000 °C. Consequently the AlCl3 · xNH3@ AlN core-shell nanostructures were formed with the in situ generated AlCl3 · xNH3 as template, and a subsequent calcination around 1100 °C in NH3/Ar effectively decomposed the inner compound. The AlN hollow nanospheres thus formed have the diameter of 80-400 nm and shell thickness of about 15 nm. This reaction route successfully extended the traditional CVD reaction of AlCl3 with NH3 from producing solid AlN powder to preparing AlN hollow nanospheres. A similar route might be applied to explore the hollow nanospheres of other nitrides with corresponding metal chloride precursors. Acknowledgment. This work was financially supported by NSFC(20525312and20601013),“973”program(2007CB936302), MOE (NCET-04-0449) and the Foundation of Jiangsu Province (BK2005416). Supporting Information Available: Characterization of the AlCl3 · xNH3 intermediate adduct compound and its decomposition product by long-time annealing. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Murphy, C. J. Science 2002, 298, 2139–2141. (b) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642–7643. (c) Peng, Q.; Dong, Y. J.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 3027– 3030. (d) Sun, Y. G.; Mayers, B.; Xia, Y. N. AdV. Mater. 2003, 15, 641– 646. (e) Bao, J. C.; Liang, Y. Y.; Xu, Z.; Si, L. AdV. Mater. 2003, 15, 1832–1835. (f) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714. (g) Liu, Q.; Liu, H. J.; Han, M.; Zhu, J. M.; Liang, Y. Y.; Xu, Z.; Song, Y. AdV. Mater. 2005, 17, 1995–1999. (h) Pan, Y.; Hu, K. F.; Hu, Y. M.; Fu, J. J.; Lu¨, Y. N.; Dai, Z. D.; Hu, Z.; Chen, Y. Small 2005, 1, 1199–1203. (i) Fan, H. J.; Go¨sele, U.; Zacharias, M. Small 2007, 3, 1660–1671. (2) (a) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647– 1650. (b) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111– 1114. (c) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321–323. (d) Gallis, K. W.; Araujo, J. T.; Duff, K. J.; Moore, J. G.; Landry, C. C. AdV. Mater. 1999, 11, 1452–1455. (e) Lin, H. P.; Mou, C. Y. Science 1996, 273, 765–768. (3) (a) Zhong, Z. Y.; Yin, Y. D.; Gates, B.; Xia, Y. N. AdV. Mater. 2000, 12, 206–209. (b) Shiho, H.; Kawahashi, N. Colloid Polym. Sci. 2000, 278, 270–274. (4) Iida, M.; Sasaki, T.; Watanabe, M. Chem. Mater. 1998, 10, 3780– 3782. (5) (a) Discher, B. M.; Won, Y. Y.; Ege, D. S.; Lee, J. C. M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143–1146. (b) Caruso, F.; Shi, X. Y.; Caruso, R. A.; Susha, A. AdV. Mater. 2001, 13, 740–744.

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