Gas Phase Production of Metal Oxide Nanowires Using a Microwave Plasma Reactor

Nanowires have attracted great interest because of their promise in a variety of applications such as electronics, optoelectronics, nano electro-mechanical devices and catalysis. Hence, the synthesis of bulk quantities of nanowires with controlled composition and high crystallinity is important. However, bulk production of nanowires has been a challenge using the techniques known so far. Over the last several years, our lab has developed and demonstrated several direct reaction schemes for producing a variety of metal oxide, nitride and sulfide nanowires. Irrespective of the gas phase excitation (thermal or plasma), the reaction between metal and gas phase proceeded via two modes: In the first mode, a high density of nanowires nucleate and grow from one large metal droplet and in the other mode, one metal droplet led the growth of a single nanowire. However, till date, all the above reactions were carried out using metal supported on a substrate. So, it is not obvious if one can produce compound nanowires by reacting metal powders directly in the gas phase using appropriate gas precursors. In this work, we conducted experiments to investigate the feasibility of reacting metal powders directly in the gas phase for the production of metal oxide nanowires using a newly designed microwave plasma jet reactor. This new reactor is designed such that a high density microwave plasma discharge in a jet like manner can be sustained inside a two inch quartz tube, at powers ranging from 300 W – 3 kW. The plasma jet is shaped by controlling the gas flow rates. The desired metal powder (or metal-containing precursor) is introduced into the quartz tube via gravity feed and is conveyed into the plasma by gravity. The product is collected at the bottom in a cup. Initial results show a theoretical nanowire production capacity to the tune of kilograms per day. Figure 1 shows a few grams of zinc oxide nanowires synthesized in this reactor. The nanowire production efficiency is about 8090% and is being optimized by the reactor design as well as the process chemistry. Pure nanowires are obtained by a dispersion in 1-methoxy 2-proponal followed by a horn sonication and gravity sedimentation. References: 1. S. Sharma and M.K. Sunkara, “Direct Synthesis of Gallium Oxide Tubes, Nanowires, and Nanopaintbrushes”, J. Am. Chem. Soc. 124 (41), 12289 (2002). 2. M. Mozeti , U. Cvelbar, M. K. Sunkara, and S. Vaddiraju, “A Method for the Rapid Synthesis of Large Quantities of Metal Oxide Nanowires at Low Temperatures”, Advanced Materials, 17, 2138 (2005). 3. M.K. Sunkara and S. Sharma, “Direct synthesis of oxide nanostructures of low-melting metals”, US Patent No. 7182812 (2007). 4. S. Vaddiraju, A. Mohite, A. Chin, M. Meyyappan, G. Sumanasekera, B.W. Alphenaar, M.K. Sunkara, “Mechanisms of 1D Crystal Growth in Reactive Vapor Transport: Indium Nitride Nanowires” Nano Lett., 5, 1625-1631 (2005). 5. R. Rao, H. Chandrasekaran, S. Gubbala, M.K. Sunkara, C. Doraio, S. Jin and A.M. Rao, ”Synthesis of lowmelting metal oxide and sulfide nanowires and nanobelts”, J. of Electronic Materials, 35 (5): 941-946 (2006). Figure 1. Picture showing 2 grams of assynthesized zinc oxide nanowires. 213th ECS Meeting, Abstract #771, © The Electrochemical Society