ZnO is being used more and more in products such as acoustic wave filters, light-emitting diodes, photodetectors, waveguides, gas sensors, and solar cells. It combines being a semiconductor with a wide bandgap, with remarkable chemical and heat stability. Paper #5 describes a simple yet powerful method for creating ordered and well-defined arrays of pure ZnO of varying dimensions using nothing more exacting than aqueous solutions and moderate temperatures. The ZnO nanorods are formed from the reaction of zinc nitrate and the complex amine methenamine, the secret being to carefully control their concentrations. When such a solution is heated at 95 º C for several hours in the presence of a substrate, like tin oxide-fluoride glass or silicon/silicon oxide wafers, then on their surfaces well-formed nanorods of ZnO will grow. These are 100-200 nanometers (nm) wide and up to 10 microns (10,000 nm) long. Analysis by energy-dispersive spectroscopy coupled to field-emission scanning electron microscopy, showed no contamination of the nanorods by other elements. By reducing the concentration of the reactants, and controlling the experimental conditions, Vayssieres shows that crystalline nanowires of ZnO are produced, and these are narrower than the rods, typically 10-20 nm diameter, and several microns in length. (See also L. Vasyssieres, International Journal of Nanotechnology, 1[1/2]:1-41, 2004.)Vayssieres is currently working on highly oriented nanostructures of transition metal and lanthanide oxides. "The ultimate goal is to design and fabricate purpose-built nano materials at low cost and on a large scale and to have a better fundamental understanding of their structural/physical relationships," he says. "We need to demonstrate how size and morphology relates to the electronic structure and physical properties of one-dimensional metal oxides for the development of functional nanodevices." His most recent paper is in Angewandte Chemie International Edition, (L. Vayssieres, M. Graetzel, 43[28]: 3666-70, 2004). The other paper trumpeting the wonders of ZnO is #1, which comes from the stable of Omar Yaghi of the University of Michigan, whose work has previously featured in this column (17[1]: 7, January/February 2004; see also the interview in 15[6]: 3-4, November/December 2004). The older paper concerned the storage of methane in an open framework formed from ZnO and organic dicarboxylates; this paper reports on the ability of such a compound to store hydrogen gas within its open lattice structure. The commonly accepted name for such materials is MOFs (metal organic frameworks), and some of these have cube-like structures with oxygen atoms at the corners of the cubes to which are bonded four zinc ions tetrahedrally placed, and these in turn are interconnected through benzene-dicarboxylate linking molecules. The paper shows that there are two kinds of hydrogen-binding site within the cavities: the zinc oxide atoms and the benzene rings of the dicarboxylate linkers. The world appears to be moving to a hydrogen-based economy, and one day hydrogen-fed fuel cells may provide the energy for motorized vehicles. To make these economical will require some method of storing hydrogen gas, ideally absorbed into some kind of lightweight material, and Yaghi’s compounds appear to offer an ideal solution to the problem. The cavities are large and at liquid nitrogen temperatures (77 K) they are each capable of holding 17 molecules of hydrogen (H2). However, at room temperature this drops to significantly fewer molecules per cavity, and falls short of the target set by the U.S. Department of Energy for hydrogen storage for automotive use, which is 6.5% H2 by weight. More recently, progress has been made towards the target of 6.5%, and Yaghi has been able to increase the number of retained H2 molecules by varying the dicarboxylate linkers. In particular, the network with interconnecting hydropyrene dicarboxylate links, which have four fused rings, shows a great deal of promise. Fuller details are given in J.LC. Rowsell, et al., Journal of the American Chemical Society., 126(18): 5666-7, 2004. 
Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.
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wo
papers in the current Hot Ten involve zinc, and in ways that could soon
lead to commercial developments. The first is paper #1, which reports a
zinc compound that has the potential for storing hydrogen gas. The
second is paper #5, which reports how zinc oxide (ZnO) nanorods and
nanowires can be grown in a rather surprising way: from aqueous
solution. This latter paper is the work of Lionel Vayssieres, currently
at the National Institute for Materials Science, Tsukuba, Japan.
View the 
.
