282378 Growing Titania Nanowires Using Sol-Gel Chemistry in Supercritical CO2
Growing Metal Oxide Nanowires using Sol-Gel Chemistry Using
P. A. Charpentier*, N. Farhangi, Qasem Alsharari, S. Ayissi
Department of Chemical and Biochemical Engineering, Faculty of Engineering,
University of Western Ontario, London, Ontario Canada, N6A5B9.
Supercritical carbon dioxide (scCO2) provides a new approach for the synthesis of nanomaterials and nanodevices due to its low viscosity, high diffusivity, and “zero” surface tension . This paper will review our methodology and new synthetic and density functional theory (DFT) modelling data for understanding the directed self-assembly of titania and other metal alkoxides in scCO2 using sol-gel chemistry. This methodology has allowed the formation of well defined nanostructures of TiO2, and modified titania with ZrO2, Fe, N and other dopants by this template-free scalable method. Well defined nanospheres, wires, tubes and other unique morphologies have been formed, with diameters ranging from 5 to 50 nm. Other unique properties including high surface areas, tunable morphologies, and 3-dimensional monolithic structures have been enabled. Nanowires have advantages over other nanostructures in terms of direct electron transport (to100 X) to electrodes compared to other morphologies and reduced recombination.  Tri and multi- metallic nanowires were successfully prepared using this methodology with high metal oxide dispersion, which is very challenging to be achieved using other methods, such CVD. Metal dispersion is a critical parameter for providing not only high surface area but also more active catalytic sites for reactions to take place.
The formation of these structures has been monitored by in situ FTIR along with offline mass-spectrometry, Raman, NMR, XRD, XPS and other techniques in which the titania alkoxides were found to form hexamer crystal structures that react and self-assemble by sol-gel and supramolecular chemistry through the acetate ligands. Also, uniform TiO2 nanowires and TiO2/ZrO2 nanotubes were grown from the surface of graphene sheets, which acted as a template for sol-gel growth. Doped TiO2 nanowires showed smaller crystal size, higher visible absorption, surface area, thermal stability, photocatalytic activity and higher efficiency solar cells compared to similar materials without graphene. The directed self-assembly and interactions between graphene and TiO2 were comprehensively studied using the Vienna ab-initio Simulation Package (VASP) based on Density Functional Theory (DFT). Molecular, Rutile and Anatase TiO2 were decorated on the graphene sheets in 3 different positions corresponding to physical adsorption phenomena (physisorption): the top, the bridge and the hollow sites. An additional position corresponding to a chemical adsorption phenomenon (chemisorption) was created by the organic functionalization process of graphene: the carboxylate site. In all cases Rutile (Ti2O4) and Anatase (Ti4O8) showed higher stability than a molecular TiO2 while adsorbed on graphene. In the specific case of functionalized graphene, the carboxylate site showed a much higher binding energy value confirming the significant difference between physisorption (~2 eV) and chemisorptions (~5 eV).
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