Unusual properties and reactivity at the nanoscale

Ready transformation and reactivity is observed at the nanoscale. Silica/silicon (SiOx) nanospheres of diameter %30 nm generated from equimolar Si/SiO2 mixtures are found to display enhanced catalytic and reactive properties relative to commonly employed silica support surfaces including fumed silica. A unique oxygen deficient synthesis has been used to generate SnO2Kx nanowires, nanoribbons, and nanotubes displaying the phase coalescence of both ground state rutile and orthohombic structures at pressures well below 0.5 bar. This is in contrast to the bulk where 150 kbar of pressure must be used to generate the orthohombic structure. This unique reactivity and ready transformation is found to accompany the facile (seconds) room temperature nitridation of TiO2 nanocolloids, efficiently producing photocatalytically active TiO2Kx Nx nanoparticles absorbing light well into the visible region. In distinct contrast the direct nitridation of TiO2 submicron particles does not produce the conversion to the oxynitride at room temperature. q 2004 Elsevier Ltd. All rights reserved. An exciting aspect of research at the nanoscale results as nanostructures hold the potential to display an enhanced and unexpected reactivity relative to that at the micron scale and bulk phase. Further, their formation and interaction may be accompanied by phase transformations not commonly observed in bulk systems. Here, we outline two simple approaches to generate select nanostructures, which are characterized by these distinguishing attributes. We have used a modified-flow tube furnace configuration carefully calibrated for temperature, temperature gradients, entrainment gas flow rate, and total pressure, and variable Si/SiO2 mixtures to generate silica (SiOx) nanospheres [1,2] which are found to display enhanced catalytic [3] activity and unexpected reactivity [4]. These structures can be agglomerated to wire-like configurations subsequently providing a means to grow silica nanotubes [2]. Also within this configuration, we have used layered Sn/SnO mixtures to generate SnOx nanostructures which, at pressures of a few hundred Torr, readily display a phase coexistence between 0022-3697/$ see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.047 * Corresponding author. Address: Schools of Physics, Material Science and Engineering, and Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0430, USA. Tel.: C1 404 894 4029; fax: C1 404 894 9958. E-mail address: [email protected] (J.L. Gole). rutile and orthohombic crystal structures [5] normally observed at pressures in excess of 150 kbar in the bulk. The efficacy of these processes at the nanoscale has suggested the application of a highly efficient nitriding process [6] to produce TiO2KxNx nanoparticles which, in distinction from TiO2, present highly photocatalytically active quantum dots absorbing light well into the visible region. Fig. 1 corresponds to an exemplary TEM micrograph of dispersed silica (SiOx) nanospheres of diameter—30 nm generated in gram quantities on a cold plate placed in the gas flow field of our high temperature synthesis source. Nearly monodisperse particle size distributions for a given experimental run can center on a size between 45 and 8 nm. TEM [1,2], X-ray diffraction [1,2] and ESR [7] measurements demonstrate that the silica nanospheres are amorphous and absent of dangling bonds and defect sites. More recent studies suggest, however, that it will be possible to modify the stoichiometry of these structures from SiOx(xO1) to SixO(xR2) with a judicious choice of experimental conditions. This may, of course, influence defect structure, unpaired electron density, and crystallinity. The silica nanospheres (Fig. 1) display some surface properties similar to fumed silica produced by the flame hydrolysis of SiCl4 (Cab–O–Sil) albeit in the absence of Journal of Physics and Chemistry of Solids 66 (2005) 546–550 www.elsevier.com/locate/jpcs