J. Am. Chem. Soc. 1997, 119, 5455-5456
Morphological Control of Nanocrystalline Aluminum Nitride: Aluminum Chloride-Assisted Nanowhisker Growth Joel A. Haber,† Patrick C. Gibbons,‡ and William E. Buhro*,† Departments of Chemistry and Physics Washington UniVersity St. Louis, Missouri, 63130-4899 ReceiVed September 25, 1996 Herein we report a morphologically selective synthesis of nanocrystalline aluminum nitride (nano-AlN) by low-temperature nitridation of nanocrystalline aluminum (nano-Al).1 Particle morphologies are varied from predominately equiaxed to predominately whisker-like, apparently by the presence of vaportransport species during nitridation. Whisker formation appears to be due to an increased volatility of aluminum induced by the large surface area of nano-Al and the action of volatile aluminum chlorides. The altered electronic, magnetic, and mechanical properties of nanocrystalline materials have received much emphasis;2 our results illustrate their altered reactivities.3 To our knowledge, procedures allowing selective synthesis of various nanoparticle morphologies by purposeful variations in reaction conditions are rare.4 Air-sensitive nano-Al was produced by the catalytic decomposition of H3Al(NMe2Et)5 with Ti(O-i-Pr)4 (0.05-1.2 mol %, eq 1). The nano-Al powders were 99 wt % Al by elemental 1,3,5-Me3C6H3 Ti(O-i-Pr)4 cat.
H3Al(NMe2Et) 9 8 nano-Al + NMe2Et + 3/2H2 (1) ∼164 °C analysis,6 and mean crystallite dimensions of 40-180 nm were determined by Scherrer analysis of XRD line broadening.7 † ‡
Department of Chemistry. Department of Physics. (1) For other syntheses of nano-AlN, see: (a) Vissokov, G. P.; Stefanov, B. I.; Gerasimov, N. T.; Oliver, D. H.; Enikov, R. Z.; Vrantchev, A. I.; Balabanova, E. G.; Pirgov, P. S. J. Mater. Sci. 1988, 23, 2415-2418. (b) Chow, G. M.; Xiao, T. D.; Chen, X.; Gonsalves, K. E. J. Mater. Res. 1994, 9, 168-174. (c) Wade, T.; Park, J.; Garza, E. G.; Ross, C. B.; Smith, D. M.; Crooks, R. M. J. Am. Chem. Soc. 1992, 114, 9457-9464. (d) Bolt, J. D.; Tebbe, F. N. In AdVances in Ceramics; American Ceramic Society: Westerville, OH, 1989; Vol 26, pp 69-76. (e) Ramesh, P. D.; Rao, K. J. AdV. Mater. 1995, 7, 177-17. (f) Adjaottor, A. A.; Griffin, G. L. J. Am. Ceram. Soc. 1992, 75, 3209-3214. (g) Jung, W.-S.; Ahn, S.-K. J. Mater. Chem. 1994, 4, 949-953. (h) Johnston, G. P.; Muenchausen, R. E.; Smith, D. M.; Foltyn, S. R. J. Am. Ceram. Soc. 1992, 75, 3465-3468. (i) Sood, R. R.; Southam, F. W.; Raghavan, N. S. Eur. Pat. 88308209.1, 1989, document number 0 308 116 A1, Chem. Abstr. an 111:9752 ca. (j) Pratsinis, S. E.; Wang, G.; Panda, S.; Guiton, T. Weimer, A. W. J. Mater. Res. 1995, 10, 512-520. (2) (a) Andres, R. P.; Averback, R. S.; Brown, W. L.; Brus, L. E.; Goddard, W. A., III; Kaldor, A.; Louie, S. G.; Moscovits, M.; Peercy, P. S.; Riley, S. J.; Siegel, R. W.; Spaepen, F.; Wang, Y. J. Mater. Res. 1989, 4, 704-736. (b) Gleiter, H. Prog. Mater. Sci. 1989, 33, 223-315. (3) Rieke, R. D. Science 1989, 246, 1260-1264. (4) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A. Science 1996, 272, 1924-1926. (b) Tanori, J.; Pileni, M. P. AdV. Mater. 1995, 7, 862-864. (5) Frigo, D. M.; van Eijden, G. J. M.; Reuvers, P. J.; Smit, C. J. Chem. Mater. 1994, 6, 190-195. A powerful explosion occurred during the synthesis of H3Al(NMe2Et), which extensively deformed a steel fume-hood enclosure. Details are in the Supporting Information. (6) nano-Al (28 nm) produced using 0.5 mol % Ti(O-i-Pr)4 was consolidated by hot pressing at 100 °C for 1 h at 350 MPa in an Ar-filled glove box. Elemental analysis of a portion of this pellet was performed by Glow Discharge Mass Spectrometry analyzing for 25 elements. The primary impurities were (wt %): C, 0.23; O, 0.25; N, 0.055; Cl, 0.14; Ti, 0.32; Al, 99.0 (by difference). Minor impurities were (ppm by wt): Li, 1.3; B, 0.15; F, 50°C/min was used branched and comblike crystals were common, indicating a significant change in growth kinetics that made additional growth directions accessible. Among the known whisker-growth mechanisms, the vaporliquid-solid (VLS) and vapor-solid (VS) mechanisms are the most likely to function under the present conditions.12 In the VLS mechanism, whiskers grow from liquid flux droplets attached to whisker tips. We observed no such flux droplets, and addition of various potential flux materials did not promote whisker formation.12 Additionally, the variety of observed crystallite morphologies and whisker-growth directions is more (8) In a typical run, the MgO tube was loaded with nano-Al, 2 µm Al, 20 µm Al, or an Al/AlCl3 mixture and inserted into a fused-silica tube sealed on one end and fitted with an Ultratorr valve assembly on the other end. The assembly was removed from the glovebox, and the MgO tube was placed into a tube furnace. After appropriate purging of the connecting lines, the valve was opened to N2 (1 atm). The sample was heated to 900-1100 °C for 15 min to 10 h to produce nano-AlN. (9) nano-AlN was prepared by heating nano-Al under N2 at 1100 °C for 10 h. Elemental analysis performed by Galbraith laboratories with airless handling found (wt %): Al, 63.5; N, 32.2; Ti, 0.32; C,
