(653c) Growing Titania Nanowires Using Sol-Gel Chemistry in Supercritical CO2

Growing Metal Oxide Nanowires using Sol-Gel Chemistry Using

 Supercritical CO₂

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 [1]. 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.[2] 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. [3] 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.[2] 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.[4]  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.[5] The directed self-assembly and interactions between graphene and TiO₂ were
comprehensively studied using the Vienna ab-initio Simulation Package (VASP)[6]
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).

1)     
R. Sui, Paul A. Charpentier*, Synthesis of Metal and Silicon
Oxides Nanostructures by Direct Sol-Gel Reactions in Supercritical Fluids, Chem.
Rev., (2012) In Press.

2)     
Ruohong Sui, John M. H. Lo, Nasrin Farhangi, 
and Paul A. Charpentier, Self?Assembly of 1D Nanostructures by Linear
Condensation of Metal?Acetate Complexes in Supercritical Carbon Dioxide,
Accounts of Chemical Research (2012), Submitted.

3)     
Oudel, P., & Qiao, Q. (2012).
One dimensional nanostructure/nanoparticle composites as photoanodes for
dye-sensitized solar cells. Nanoscale, 4(9), 2826-2838. 

4)     
Nasrin Farhangi, Yaocihuatl
Medina-Gonzalez , Rajib Roy Chowdhury, and Paul A. Charpentier, Using
Supercritical CO₂ for growing TiO₂/Graphene
Nanoassembles: Synthesis and Photocatalytic Effect, Nanotechnology
(2012), In Press.

5)     
Nasrin Farhangi, Rajib Roy
Chowdhury, Yaocihuatl Medina-Gonzalez, Madhumita B. Ray and Paul A. Charpentier
Visible light active Fe doped TiO2 nanowires grown on Graphene using
Supercritical CO₂, Applied
Catalysis B: Environmental 110 (2011) 25? 32.

6)     
G. Kresse, D. Joubert, Phys. Rev. B 59 (1999)
1758