In situ real time monitoring of Pt-VO2 nanoparticle-nanowire assembly by GISAXS

C. T. Yavuz*, S. Lee, B. Lee, M. H. Kim, J. M. Baik, C. Larson, S. Seifert, S. Vajda, R. E. Winans, M. Moskovits, G. D. Stucky, A. M. Wodtke
Proc. SPIE, Vol. 7679, 76792D (2010). [DOI] [pdf]

Methanol is a hydrogen carrier for fuel cells and its chemical transformations are of great current interest. Methanol oxidation by vanadium oxides is well studied, hence, serves as a good measure for catalytic activity. Arrays of VO₂ nanowires grown on r-cut sapphire prove to be unique for the in situ catalytic activity tests. Here, we present size and morphology dependent activity of Platinum coated single crystalline VO₂ nanowires in methanol oxidation reactions using Grazing Incidence Small Angle X-ray Scattering (GISAXS). Our findings show an unexpected sintering behavior of Pt at temperatures as low as 200 °C.

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Pd-sensitized single vanadium oxide nanowires: highly-responsive hydrogen sensing based on the Mott transition

J. M. Baik, M. H. Kim, C. Larson, C. T. Yavuz, G. D. Stucky, A. M. Wodtke, M. Moskovits*
Nano Lett., 9 (12), 3980–3984 (2009). [DOI] [pdf]

Exceptionally sensitive hydrogen sensors were produced using Pd-nanoparticle-decorated, single vanadium dioxide nanowires. The high-sensitivity arises from the large downward shift in the insulator to metal transition temperature following the adsorption on and incorporation of atomic hydrogen, produced by dissociative chemisorption on Pd, in the VO₂, producing 1000-fold current increases. During a rapid initial process, the insulator to metal transition temperature is decreased by >10 °C even when exposed to trace amounts of hydrogen gas. Subsequently, hydrogen continues to diffuse into the VO₂ for several hours before saturation is achieved with only a modest change in the insulator to metal transition temperature but with a significant increase in the conductivity. The two time scales over which H-related processes occur in VO₂ likely signal the involvement of two distinct mechanisms influencing the electronic structure of the material one of which involves electron−phonon coupling pursuant to the modification of the vibrational normal modes of the solid by the introduction of H as an impurity.

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Growth of metal oxide nanowires from supercooled liquid nanodroplets

M. H. Kim, B. Lee, S. Lee, C. Larson, J. M. Baik, C. T. Yavuz, S. Seifert, S. Vajda, R. E. Winans, M. Moskovits, G. D. Stucky, A. M. Wodtke*
Nano Lett. 9 (12), 4138–4146 (2009). [DOI] [pdf]

Nanometer-sized liquid droplets formed at temperatures below the bulk melting point become supercooled as they grow through Ostwald ripening or coalescence and can be exploited to grow nanowires without any catalyst. We used this simple approach to synthesize a number of highly crystalline metal oxide nanowires in a chemical or physical vapor deposition apparatus. Examples of nanowires made in this way include VO₂, V₂O₅, RuO₂, MoO₂, MoO₃, and Fe₃O₄, some of which have not been previously reported. Direct evidence of this new mechanism of nanowire growth is found from in situ 2-dimensional GISAXS (grazing incidence small angle X-ray scattering) measurements of VO₂ nanowire growth, which provides quantitative information on the shapes and sizes of growing nanowires as well as direct evidence of the presence of supercooled liquid droplets. We observe dramatic changes in nanowire growth by varying the choice of substrate, reflecting the influence of wetting forces on the supercooled nanodroplet shape and mobility as well as substrate−nanowire lattice matching on the definition of nanowire orientation. Surfaces with defects can also be used to pattern the growth of the nanowires. The simplicity of this synthesis concept suggests it may be rather general in its application.

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Markedly improved CO2 capture efficiency and stability of gallium substituted hydrotalcites at elevated temperatures

C. T. Yavuz*, B. D. Shinall, A. V. Iretskii*, M. G. White, T. Golden, M. Atilhan, P. C. Ford, G. D. Stucky
Chem. Mater., 21 (15), 3473-3475 (2009). [DOI] [pdf]

Hydrotalcites (HTs) belong to a family of layered, double hydroxide solids that have been proven to be stable adsorbents under harsh conditions. The stoichiometry of these solids iswhere the divalent ion is typically Mg²⁺, the trivalent ion is typically Al³⁺, and the anion A is Cl⁻, NO₃⁻, or CO₃²⁻. Although important members of this family containing Ga³⁺ substituted for some fraction of the Al³⁺ have been synthesized, there are no existing reports of CO₂capture by these substituted hydrotalcites (SHTs). Here we report stable and reversible CO₂adsorbents with very high capture capacity that feature Ga substituted, potassium carbonate promoted HTs:

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