Published on Web 03/24/2005
A General Nonaqueous Route to Binary Metal Oxide Nanocrystals Involving a C-C Bond Cleavage Nicola Pinna, Georg Garnweitner, Markus Antonietti, and Markus Niederberger* Contribution from the Max Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany Received December 21, 2004; E-mail:
[email protected] Abstract: A widely applicable solvothermal route to nanocrystalline iron, indium, gallium, and zinc oxide based on the reaction between the corresponding metal acetylacetonate as metal oxide precursor and benzylamine as solvent and reactant is presented. Detailed XRD, TEM, and Raman studies prove that, with the exception of the iron oxide system, where a mixture of the two phases magnetite and maghemite is formed, only phase pure materials are obtained, γ-Ga2O3, zincite ZnO, and cubic In2O3. The particle sizes lie in the range of 15-20 nm for the iron, 10-15 nm for the indium, 2.5-3.5 nm for gallium, and around 20 nm for zinc oxide. GC-MS analysis of the final reaction solution after removal of the nanoparticles showed that the composition is rather complex consisting of more than eight different organic compounds. Based on the fact that N-isopropylidenebenzylamine, 4-benzylamino-3-penten-2-one, and N-benzylacetamide were the main species found, we propose a detailed formation mechanism encompassing solvolysis of the acetylacetonate ligand, involving C-C bond cleavage, as well as ketimine and aldol-like condensation steps.
Introduction
The nonaqueous synthesis of oxidic compounds started decades ago, when investigations of Gerrard et al. on the interaction of alcohols with silicon tetrachloride gave evidence that this reaction resulted in the formation of hydrated silica and alkyl chlorides.1 Many years later, related processes were applied to the preparation of monolithic silica2 and metal oxide gels.3 The extension of these nonhydrolytic reactions to the synthesis of titania nanocrystals4 was a first step in opening up new pathways to a large variety of metal oxide nanoparticles such as iron oxides,5 ZrO2,6 HfO2/HfxZr1-xO2,7 ZnO,8 or ferrites.9 Most of these procedures still rely on the use of surfactants such as trioctylphosphine oxide (TOPO) to control the crystal growth and to provide solubility. However, TOPO (1) Gerrard, W.; Woodhead, A. H. J. Chem. Soc. 1951, 519. (2) Corriu, R. J. P.; Leclercq, D.; Lefevre, P.; Mutin, P. H.; Vioux, A. J. NonCryst. Solids 1992, 146, 301. (3) Corriu, R. J. P.; Leclercq, D.; Lefevre, P.; Mutin, P. H.; Vioux, A. J. Mater. Chem. 1992, 2, 673. (4) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. (5) (a) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (b) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (c) Redl, F. X.; Black, C. T.; Papaefthymiou, G. C.; Sandstrom, R. L.; Yin, M.; Zeng, H.; Murray, C. B.; O’Brien, S. P. J. Am. Chem. Soc. 2004, 126, 14583. (6) Joo, J.; Yu, T.; Kim, Y. W.; Park, H. M.; Wu, F. X.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 6553. (7) Tang, J.; Fabbri, J.; Robinson, R. D.; Zhu, Y. M.; Herman, I. P.; Steigerwald, M. L.; Brus, L. E. Chem. Mater. 2004, 16, 1336. (8) (a) Shim, M.; Guyot-Sionnest, P. J. Am. Chem. Soc. 2001, 123, 11651. (b) Cozzoli, P. D.; Curri, M. L.; Agostiano, A.; Leo, G.; Lomascolo, M. J. Phys. Chem. B 2003, 107, 4756. (9) (a) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (b) Zeng, H.; Rice, P. M.; Wang, S. X.; Sun, S. J. Am. Chem. Soc. 2004, 126, 11458. 5608
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is toxic, leads to impurities in the final product, and hampers the application of these nanomaterials in electronic and sensing devices. More advanced processes are based on the use of solvents, which act as reactant as well as control agent for particle growth, and thus allow the synthesis of high-purity nanomaterials. Another important point on the way to industrial scale-up is the generalization of a specific synthesis methodology, so that the same reaction setup can be used for as many different materials as possible. Regarding these requirements, the “benzyl alcohol route” is particularly versatile for the synthesis of diverse binary metal oxides,10-15 perovskites,16,17 and nanohybrid materials.18 However, this approach is rather restricted, when metal alkoxides are either commercially not available or only at very high price, or when metal halide precursors are undesirable because of halide impurities in the final oxidic material. To circumvent some of these drawbacks, we present in this work an alternative route to nanocrystalline iron, indium, (10) Niederberger, M.; Bartl, M. H.; Stucky, G. D. J. Am. Chem. Soc. 2002, 124, 13642. (11) Niederberger, M.; Bartl, M. H.; Stucky, G. D. Chem. Mater. 2002, 14, 4364. (12) Niederberger, M.; Garnweitner, G.; Krumeich, F.; Nesper, R.; Co¨lfen, H.; Antonietti, M. Chem. Mater. 2004, 16, 1202. (13) Pinna, N.; Antonietti, M.; Niederberger, M. Colloids Surf., A 2004, 250, 211. (14) Pinna, N.; Garnweitner, G.; Antonietti, M.; Niederberger, M. AdV. Mater. 2004, 16, 2196. (15) Pinna, N.; Neri, G.; Antonietti, M.; Niederberger, M. Angew. Chem., Int. Ed. 2004, 43, 4345. (16) Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. Angew. Chem., Int. Ed. 2004, 43, 2270. (17) Niederberger, M.; Garnweitner, G.; Pinna, N.; Antonietti, M. J. Am. Chem. Soc. 2004, 126, 9120. (18) Pinna, N.; Garnweitner, G.; Beato, P.; Niederberger, M.; Antonietti, M. Small 2005, 1, 112. 10.1021/ja042323r CCC: $30.25 © 2005 American Chemical Society
Nonaqueous Route to Binary Metal Oxide Nanoparticles
ARTICLES
gallium, and zinc oxide, based on the reaction between the corresponding metal acetylacetonate and benzylamine. In the case of the presented metal oxides, it is particularly advantageous for economic reasons to use acetylacetonates instead of metal alkoxides. The reaction between acetylacetonates and oleylamine has already been reported for the synthesis of indium and manganese oxide nanoparticles by Park et al.19,20 The authors proposed that the particle formation involved the thermal decomposition of the acetylacetonate, but they did not provide any further details. While studying the mechanism, we found that nanoparticle formation proceeds via a novel pathway involving a C-C bond cleavage in the acetylacetone molecule. In principle, this observation is antagonistic to the formation mechanism found in the BaTiO3 system, where a C-C bond formation occurred between benzyl alcohol and the 2-propanolate ligand.17 Experimental Details Synthesis. All synthesis procedures were carried out in a glovebox (O2 and H2O
