Polyvinylpyrrolidone (PVP) is used in the synthesis of Ag nanoparticles (NPs) with controlled shape, most commonly producing cubes. The mechanism for shape control is unclear but believed by many to be caused by preferential binding of PVP to Ag(100) facets compared to Ag(111) facets and assumed by most to be the result of thermodynamic control, whereby facets with lower interfacial free energy predominate. To investigate this mechanism, we measured adsorption isotherms of PVP on different-shaped Ag NPs, to determine the thermodynamics of PVP adsorption to Ag(100) and Ag(111) facets. The equilibrium adsorption constant is independent of PVP molecular weight and depends only weakly on NP shape (and thus Ag facet). The equilibrium adsorption constant for PVP on Ag(111) (2.8 M −1 ) is about half that on Ag(100) (5 M −1 ). From a Wulff construction, this difference is not nearly enough to produce cubes via thermodynamic control. This result indicates the importance of kinetic control of the Ag nanoparticle shape by PVP, as has recently been proposed.
We report a robust, low-cost method to attach transition metal ions directly to the surface of anatase TiO2 rod-shaped nanocrystals with preservation of the host nanocrystal morphology and phase. The procedure has been optimized to achieve quantitative control of metal ion loading on the surface of the nanorods. The metal ion can be attached to the nanocrystal surface up to fullmonolayer coverage, after which the surface becomes saturated and there is no further addition.
We report the formation of high surface area hollow Mn 3 O 4 nanoparticles that form as a result of the galvanic reaction of Cu 2+ with MnO nanocrystals concomitant with a nanoscale Kirkendall effect. The MnO nanocrystals were prepared according to the ultralarge scale synthesis reported by Hyeon, which allowed the preparation of hollow Mn 3 O 4 in multigram quantities. Ex-situ analyses with transmission electron microscopy and powder X-ray diffraction show the morphology and phase stability of the hollow particles correlate with DSC-TGA data and show collapse of the hollow particles at temperatures greater than 200 °C. Electrodes fabricated from hollow Mn 3 O 4 exhibited excellent initial Li ion storage capability (initial discharge capacity = 1324 mAh/g) but poor cycling performance (97% loss of discharge capacity after 10th cycle), whereas Mn 3 O 4 -MWCNT electrodes exhibited good reversibility and discharge capacity of 760 mAh/g after 100 cycles.
X-ray photoelectron spectra were obtained for a series of M-TiO 2 samples in which transition metal ions are directly attached to the surface of anatase TiO 2 nanocrystals. The samples were prepared using CrCl 3 .nH 2 O, MnCl 2 .nH 2 O, FeCl 2 .nH 2 O, CoCl 2 .nH 2 O, NiCl 2 .nH 2 O, and CuCl 2 .nH 2 O as metal sources. We observed spontaneous air oxidation of the metal for the Mn-TiO 2 and Fe-TiO 2 samples as indicated by rapid color changes. X-ray photoelectron spectroscopy (XPS) data confirms the oxidation states of the metals Cr, Co, and Ni are unchanged from the precursor, Mn and Fe are oxidized, and Cu is in a more reduced state. The reduction of Cu likely arises during the XPS experimenta phenomenon well-documented in the literature; whereas UV-visible data of the Cu-TiO 2 dispersions are consistent with Cu 2+ .
We
report a thermolytic reduction of silver precursors in the presence
of anatase TiO2 nanorods to form Ag–TiO2 hybrid nanocrystals (HNCs). Upon changing the reaction conditions,
the size and number density of Ag on the HNCs could be adjusted. The
size and number density of Ag on the HNCs were found to have an inverse
relation. We assess the hydrogen evolution of TiO2 nanorods,
P25 TiO2, and Ag–TiO2 HNCs in methanol/water
under xenon lamp irradiation. The turnover frequency for hydrogen
evolution on silica-supported Ag–TiO2 was 1.4 ×
10–4 s–1, greater than that of
the anatase TiO2 nanorods (9.8 × 10–6 s–1) or the coupled anatase/rutile TiO2 (P25 catalyst; 5.2 × 10–5 s–1).
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