Phase transformations in CdSe quantum dots induced by reaction time

This paper reports the synthesis of high-quality TOP/TOPO capped CdSe nanocrystals using a simple colloidal method. The effects of varying the reaction time on the size and crystallinity of the synthesized CdSe quantum dots were studied in detail. Powder X-ray diffraction (PXRD) analysis showed a dependence of the crystallite phases as a function of reaction time, from cubic zinc-blende to hexagonal wurtzite and back again to the cubic phase. PXRD data was found to be consistent with high resolution transmission electron microscopy (HRTEM) images of the particles, whilst optical spectroscopy demonstrated the quantum dot nature of the particles. The reaction time-dependent phase behavior is best accounted for by rapid growth along the {111} crystallite facets, after which the facial facets then 'catch-up', resulting once again in cubic symmetry.3 page(s

La₂O₃ Promoted Pd/rGO Electro-catalysts for Formic Acid Oxidation

High activity, a low rate of CO poisoning, and long-term stability of Pd electro-catalysts are necessary for practical use as an anode material in direct formic acid fuel cells. Achieving a high degree of Pd nanoparticle dispersion on a carbon support, without agglomeration, while maintaining a facile electron transfer through the catalyst surface are two challenging tasks to be overcome in fulfilling this aim. Herein, we report the effect of addition of La/La-oxides on the efficiency of Pd nanoparticles supported on reduced graphene oxide (rGO) for formic acid electro-oxidation reaction. A series of electro-catalysts with different Pd–La molar ratios were successfully synthesized and characterized using a range of techniques including PXRD, XPS, TEM, FTIR, and Raman spectroscopy and then tested as anode materials for direct formic acid fuel cells. We explore that the lanthanum species (La/La-oxide) significantly promote the activity and stability of Pd catalyst toward electrocatalytic oxidation of formic acid. The metallic ratio is found to be critical, and the activity order of various catalysts is observed as follows; Pd₃₀La₇₀/rGO > Pd₈₀La₂₀/rGO > Pd₇₀La₃₀ rGO. The obtained mass specific activity for Pd₃₀La₇₀/rGO (986.42 A/g) is 2.18 times higher than that for Pd/rGO (451 A/g) and 16 times higher than that for Pd/C (61.5 A/g) at given onset peak potentials. The high activity and stability of the electro-catalysts are attributed to the uniform dispersion of Pd nanoparticles over the rGO support, as evidenced from TEM images. It is believed that the role of La species in promoting the catalyst activity is to disperse the catalyst particles during synthesis and to facilitate the electron transfer via providing a suitable pathway during electrochemical testing

Synthesis and characterization of organic-inorganic hybrid materials prepared by sol-gel and containing ZnxCd1-xS nanoparticles prepared by a colloidal method

Nanocomposite materials based on a hybrid organic–inorganic ureasilicate matrix doped with ZnxCd1 xS nanoparticles were prepared. ZnxCd1 xS nanoparticles with different compositions (Zn/Cd mole ratio) were prepared through a colloidal method using reverse micelles. Previously to dispersion within the matrix precursors used to prepare the hybrid gel composite, the nanoparticles surface was modified in order to improve compatibility and stability with the matrix and to assure the preservation of the original optical properties of the nanoparticles. The matrix was obtained by the reaction between a silicon alkoxide modified by an isocyanate group and a di-amine functionalized oligopolyoxyethylene (Jeffamine ED-600), which by subsequent hydrolysis and condensation reactions formed a mechanically stable and highly transparent solid network containing the ZnxCd1 xS nanoparticles. The materials were characterized by absorption, steady-state and time-resolved photoluminescence spectroscopy and by HRTEM. The obtained nanocomposites show a high transparency in the visible range accounting for the good dispersion of the nanoparticles within the matrix. The results obtained confirmed the preservation of the original optical properties of the nanoparticles after their incorporation into the ureasilicate matrix, showing that the developed method is suitable for the production of materials with potential applications in which it is necessary to take advantage of the optical properties of the nanoparticles incorporated. The HRTEM analysis confirmed that the dispersed nanoparticles show a high level of crystallinization.This work has been partially supported by the Fundacao para a Ciencia e a Tecnologia (FCT). L.F. Goncalves thanks the FCT for the PhD grant SFRH/BD/38262/2007. The authors gratefully acknowledge the financial support by Centro de Quimica [project F-COMP-01-0124-FEDER-022716 (ref. FCT Pest-C/Qui/UI0686/2011)-FEDER-COMPETE] and Centro de Fisica (Universidade do Minho). The authors would like to thank the Electron Microscope Unit of the Mark Wainwright Analytical Centre at The University of New South Wales, for scientific and technical suppor

Synthesis and characterization of organic-inorganic hybrid materials prepared by sol-gel and containing CdS nanoparticles prepared by a colloidal method using poly(N-vinyl-2-pyrrolidone)

This paper describes the synthesis and characterization of CdS nanoparticles (NPs) stabilized with poly(N-vinyl-2-pyrrolidone) and their further immobilization on a hybrid organic–inorganic matrix produced by the sol–gel process. The production of the hybrid matrix doped with CdS NPs was carried out in two steps. In the first step a precursor, designated diureasil precursor, was synthesized from the reaction between the terminal amine groups of a,x-diamine-poly(oxyethylene-co-oxypropylene) and the isocyanate group of 3-isocyanatopropyltriethoxysilane. The next step involved the hydrolysis and condensation reactions of ethoxy groups attached to silicon, this step resulting in the formation of a crosslinked siliceous network linked through urea bonds to a poly(oxyethylene)/ poly(oxypropylene) chain. The NPs were added to the diureasil precursor before the gelation process to allow a homogeneous dispersion of the NPs within the matrix. The developed method allowed the transfer of colloidal NPs to a solid matrix without the need of exchange the capping agents or the solvent. The materials were characterized by absorption, steady-state photoluminescence spectroscopy and by TEM. The results obtained showed the presence of CdS NPs with quantum size effect dispersed within the diureasil matrix. The obtained nanocomposites show a high transparency in the visible range accounting for the good dispersion of the NPs within the matrix. The TEM analysis confirmed that the NPs are uniformly dispersed within the diureasil matrix.This work has been partially supported by the Fundação para a Ciência e a Tecnologia (FCT). L. F. Gonçalves thanks the FCT for the PhD grant SFRH/BD/38262/2007. The authors gratefully acknowledge the financial support by Centro de Química [project F-COMP-01-0124-FEDER-022716 (ref. FCT Pest-C/Qui/UI0686/2011)-FEDER-COMPETE] and Centro de Física (Universidade do Minho). The authors would like to thank the Electron Microscope Unit of the Mark Wainwright Analytical Centre at The University of New South Wales, for scientific and technical support