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Notes Surface Oxidation States in Si/SiO2 Nanostructures Prepared from Si/SiO2 Mixtures James L. Gole,† Brian D. Shinall,‡ Alexei V. Iretskii,‡ Mark G. White,*,‡ and Ann S. Erickson§ School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, and School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100 Received June 17, 2003
Heterogeneous catalysts, adsorbents, and solid-state electronic sensors are derived from either single or multiple metal oxides sometimes combined with high surface area oxides such as silica or alumina. The surface oxidation state of these solids at least in part determines their performance. Current materials preparation techniques require postsynthesis treatment(s) to achieve the desired surface oxidation state(s). We have reported the preparation of nonporous nanoparticles,1 which appear spherical and nearly monodisperse (Figure 1), from the reaction/interaction of Si and SiO2 at 1300 °C under argon or nitrogen atmospheres.2,3 Here, employing these nanospheres, we show arresting data on preparing a solid with the desired surface oxidation state without a postsynthesis treatment. The surface oxidation states of this solid are adjusted by varying the amounts of the metalloid ions/ atoms in the starting materials. The inherent simplicity of this technique is general to many mixed metal oxides without the need for liquid solvents, and thus, the preparation is environmentally friendly. Finally, supported metal oxides can be prepared in two steps, thus making the new technique economically attractive as a replacement for the current preparation methods that often require up to five steps in the synthesis. This study also demonstrates a surprising explanation for the activity of fumed silica. The “silica” nanospheres, Figure 1, have been used as a support for a well-dispersed copper oxide catalyst and for the selective conversion of ethanol to acetaldehyde.4 With a variation of the experimental conditions, we have also produced silica nanowires with diameters in the range 60-80 nm. In this report, we describe the surprising surface chemistry of these silica nanostructures and compare them to fumed silica using results from X-ray photoelectron spectroscopy and the phenol hydroxylation reaction in excess hydrogen peroxide. This reaction is * To whom correspondence should be addressed. E-mail:
[email protected];
[email protected]. † School of Physics, Georgia Institute of Technology. ‡ School of Chemical Engineering, Georgia Institute of Technology. § Department of Chemistry and Department of Physics, Reed College, Portland, OR 97202. (1) Gole, J. L.; et al. Appl. Phys. Lett. 2000, 76, 2346. (2) Gao, R. P.; Wang, Z. L.; Gole, J. L.; Stout, J. D. Adv. Mater. 2000, 12, 1938. (3) Prokes, S. M.; Carlos, W. E.; Seals, L.; Lewis, S.; Gole, J. L. Mater. Lett. 2002, 54, 85. (4) Gole, J. L.; White, M. G. J. Catal. 2001, 204 (1), 249-252.
Figure 1. TEM of virtually “monodisperse” SiO2 nanospheres synthesized by gas-phase condensation from Si/SiO2.
particularly useful for identifying acid sites on solids5 and surface oxidation state(s).6 All reactions were completed at 298 K under vigorous stirring, so that mass transport did not influence the observed results. Solids (0.05-0.1 g) were dried at 373 K overnight prior to use. For a typical run, 0.021 mmol of phenol was added to 8.4 mmol of H2O2 and this mixture was stirred for 1/2 h prior to adding the solids. Powdered samples of Cab-O-Sil (Aldrich, 390 ( 40 m2/g) and silica nanospheres were examined separately by X-ray photoelectron spectroscopy, XPS, to determine their surface oxidation states. The powders were mixed with colloidal gold and then pressed into a lead foil (99.95% metals, Alfa Aesar). Binding energies of the samples were determined relative to the colloidal gold standard. The Si 2p, highresolution electron spectrum of Cab-O-Sil showed a single peak at a binding energy of 100.35 eV. The electron spectrum of the nanospheres, prepared by heating to 1300 °C an equimolar mixture of Si4+ and Si0, showed a single peak at 103.15 eV. Additional spectra were checked to insure that changing the flood gun potential did not change the peak shapes. This result is consistent with the assumption that the potential of the Au reference particles was the same as the potential of the insulating powders that we have examined. The hydroxylation of phenol in hydrogen peroxide at 298 K was completed for fumed silica (Cab-O-Sil), 98 mg; silica nanowires, 51 mg, nanospheres (batch 1), 53.1 mg; (5) Allian, M.; Germain, A.; Cseri, T.; Figueras, F. Heterogeneous Catalysis and Fine Chemicals III; Elsevier Science: Amsterdam, 1993. (6) Shuvalov, V. F.; Moravskii, A. P. trans. from Dokl. Akad. Nauk SSSR 1977, 234 (6), 1402-1405.
10.1021/la035072t CCC: $27.50 © 2004 American Chemical Society Published on Web 11/21/2003
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Figure 2. Conversion of phenol at 298 K over nanospheres, nanowires, and Cab-O-Sil. Results of phenol hydroxylation with hydrogen peroxide at 298 K are shown: (diamonds) blank run, no solids present; (squares) Cab-O-Sil, 0.098 g; (filled circles) nanospheres batch 1, 0.053 g; (open circles) nanospheres batch 2, 0.049 g; (filled triangles) nanowires, 0.054 g. The initial concentrations of the reactants were the same for each run: phenol (1.85 mM) and hydrogen peroxide (8.4 mM). Table 1. Fractional Yields and Conversion Ratesa solid resorcinol nanospheres Cab-O-Sil
phenol conversion +catechol & rate, 1/(g h) hydroquinone resorcinol )total 0.267 0.0264
0.027 0.0046
0.077 0.011
0.104 0.0156
a Fractional conversion rates/mass of silica and fractional yields to products.
and silica nanospheres (batch 2), 48.9 mg. A blank shows less than a 5% conversion of phenol in the first 67 h. The results are depicted in Figure 2. The initial reaction rates were 0.56 µmol/(h g) over fumed silica (Cab-O-Sil), 1.7 µmol/(g h) over the nanowires, and 4.87 µmol/(h g) over the silica nanospheres. The difference in the specific reaction rates cannot be explained by the difference in specific surface areas for the Cab-O-Sil and nanospheres (390 and
