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2009, 113, 500–503 Published on Web 12/18/2008
Sonochemically Assisted Thermal Decomposition of Alane N,N-Dimethylethylamine with Titanium (IV) Isopropoxide in the Presence of Oleic Acid to Yield Air-Stable and Size-Selective Aluminum Core-Shell Nanoparticles K. A. Shiral Fernando,† Marcus J. Smith,† Barbara A. Harruff,† William K. Lewis,† Elena A. Guliants,† and Christopher E. Bunker*,‡ Sensors Technology Office, UniVersity of Dayton Research Institute, Dayton, Ohio 45469, Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson Air Force Base, Ohio 45433-7103 ReceiVed: October 21, 2008; ReVised Manuscript ReceiVed: December 08, 2008
Using sonochemistry to provide the thermal energy and mixing, we demonstrate the ability to synthesize air-stable aluminum nanoparticles of two different size distributions from the titanium-catalyzed thermal decomposition of alane. Characterization data indicate the presence of spherical face-centered-cubic aluminum nanoparticles with average sizes of either 5 or 30 nm that are capped with an organic shell. The average size of the nanoparticles correlates with the concentration of the passivation agent oleic acid, where a higher concentration results in smaller particles. Thermal analysis data demonstrates that at elevated temperatures (>550 °C), these particles react via a typical aluminum oxidation mechanism, whereas at low temperatures ( 650 °C) it is reported that the amorphous aluminum oxide grown through region I converts to γ-Al2O3, forming platelets that expose additional aluminum metal.2 The rate of oxidation is observed to increase and produce a significant increase in mass until the γ-Al2O3 platelets fully cover the remaining aluminum metal. Our results appear to correlate well with this description of the aluminum oxidation processswe observe a rapid increase in mass just after 550 °C that plateaus just before 600 °C, suggesting exposure of aluminum metal followed by oxide passivation. Additionally, our XRD analysis of a sample taken to 700 °C confirmed the formation of γ-Al2O3 while still showing unreacted fcc aluminum (Figure 2, bottom). Particle sizes estimated from the XRD peak widths using the Scherrer formula indicate a change from ∼15 nm at room temperature to ∼11 nm at 700 °C for the fcc aluminum and ∼3 nm for the γ-Al2O3 formed at 700 °C. A difference in our data, however, is that the rapid increase in mass just past 550 °C was preceded by a somewhat slower increase in mass that started at ∼400 °C. The rate is greater and the mass increase more significant than typical region I growth, indicating a somewhat different process. These observations appear to be corroborated by the DSC data. The two oxide growth steps we observe correlate with two exothermic peaks (B and C in Figure 4, bottom). The sharp peak C, which aligns with the region II mass increase, can be attributed to the exothermic reaction of oxide-coated aluminum nanoparticles and agrees well with literature reports for this process.5 The broad peak B, which yields a maximum near 500 °C, is both too broad and too early to be comfortably explained the same way. It would also suggest that the sample exists as a bimodal distribution of particles sizes, which is something not supported by the TEM data. Instead, we attribute the heat flow exhibited in peak B and the slow mass increase that precedes region II to oxide growth that occurs after the organic passivation layer is displaced at high temperatures. We should note that although these particles are passivated by oleic acid, oleic acid does provide oxygen from which an initial aluminum oxide layer can be formed. The presence of such a layer is supported by the ICP-MS data, which indicates 40% of the sample mass is active aluminum,20 and TGA in argon data, which confirms 35% of the mass is organic, leaving 25% of the sample mass to be an organic-provided aluminum oxide layer. The nature of this layer is currently unknown; however, the observation of the somewhat different region I oxidation behavior suggests it is unlike that of a naturally occurring aluminum oxide passivation layer. Although not yet achieving an oxide-free passivation of these aluminum nanoparticles, the results presented here do suggest the use of organic passivation compounds alters the reactivity such that energy is released at lower temperatures. Further investigations will focus on the effect of particle size on the oxidation properties and the role the organic passivation compound plays in forming the stabilizing shell (i.e., the nature of the initial oxide layer). Conclusion We have demonstrated the ability to produce stable aluminum nanoparticles of two different sizes using a sonochemically assisted technique. The size control is achieved by the organic passivation agent as it caps the growing nanoparticle surface to stop particle growth. We believe that an organic-provided oxide
Letters layer is formed in this process, and it is that layer that provides the observed particle stability. Both the FTIR and the thermal analysis confirm the presence of the organic passivation layer, and the thermal analysis combined with the ICP-MS data suggests the oxide layer. Further, the thermal analysis indicates aluminum oxidation occurring at relatively low temperatures once the organic has been removed and that the oxidation behavior differs from that of naturally formed oxide-coated aluminum particles. With the ability to tune the size of the aluminum nanoparticles and to manipulate the organic passivation layer, these organic passivated aluminum nanoparticles provide an interesting and valuable variation with which to study the oxidation process. Acknowledgment. We thank Drs. J. E. Spowart, C. A. Crouse, and D. K. Phelps for helpful discussions. We acknowledge the financial support of the Defense Threat Reduction Agency (DTRA, Grant no. HDTRA-07-1-0026), the Air Force Office of Scientific Research (AFOSR) through the continued support of Dr. Julian Tishkoff, the Air Force Research Laboratory (AFRL) through the support of nanoenergetics, and the Dayton Area Graduate Studies Institute (DAGSI) for support for M.J.S. Supporting Information Available: TGA/DSC analysis was performed following the same procedure reported in this manuscript for three commercially available Al nanopowders. These nanopowders are the Alpha Aeasar, Sigmal Aldrich, and MTI samples. The energy and mass dependence on temperature of these samples was recorded using a TA Instruments STD Q600 dual DSC/TGA instrument with open pan alumina sample cups. Samples were analyzed from room temperature to 700 °C using a 10 °C/min temperature profile under a constant flow of air. This material is available free of charge via the Internet at http://pubs.acs.org
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