(103b) Graphene Metal Oxide Nanoassemblies for Dye-Sensitized Solar Cells

Tremendous interest exists towards synthesizing
nanoassemblies for dye-sensitized solar cells (DSSCs) using earth abundant and
friendly materials with green synthetic approaches. Recently, two-dimensional graphene has initiated a
rapid exploration of its potential in a broad range of fields including energy
conversion and catalysis [1]. Catalyst particles
can be incorporated directly onto an individual graphene sheet, providing
enhanced properties for DSSCs [2]. Graphene is a good electron acceptor and can  serve as
charge trapping sites and reduce the electron-hole recombination rate to
enhance the photoefficiency of titania[3].

In this
research, uniform TiO₂ nanowires were grown on the
surface of graphene sheets using a sol-gel method in supercritical carbon
dioxide (scCO₂) to help exfoliate graphene while preventing agglomeration and
controlling the particle size. Fe doped TiO₂
on the graphene sheets were also prepared in both ethanol and scCO₂
to extend its band gap to the visible region.[4] TiO₂ nanoparticles
less than 5 nm (when   ethanol was used as a solvent) and TiO₂
nanowires less than 40 nm (when scCO₂ was used as a solvent) were uniformly
decorated on the graphene sheets. Both doped TiO₂ nanoparticles and
nanowires showed smaller crystal size, and higher visible absorption, surface
area, and higher efficiency solar cells compared to similar materials without
graphene.[5] Theoretical band structures have been studied using the Vienna
ab-initio Simulation Package (VASP)[6] based on the Density Functional Theory
(DFT). Band gap and adsorption energy values of structural TiO₂ were calculated respectively in the bulk
and on graphene by physisorption and chemisorption. Functionalized graphene
sheets decrease significantly the band gap but increase the binding energy
values, due to carboxylate adsorption sites, of
structural TiO₂ compounds which facilitate
DSSC performance.

[1]        I.V. Lightcap, T.H.
Kosel, P.V. Kamat, Nano Letters 10 (2010) 577-583.

[2]        H. Zhang, X. Lv, Y. Li,
Y. Wang, J. Li, ACS Nano 4 (2009) 380-386.

[3]        X.Y.L.H.-P.C. Zhang,
X-L;  Linb, Y., Journal of Materials Chemistry 20 (2010) 2801-2806.

[4]   Nasrin Farhangi, Yaocihuatl Medina-Gonzalez , Rajib Roy Chowdhury,
and Paul A. Charpentier, Nanotechnology (2012), In Press.

[5] Nasrin Farhangi, Rajib Roy Chowdhury, Yaocihuatl Medina-Gonzalez,
Madhumita B. Ray and Paul A. Charpentier, Applied Catalysis B: Environmental
110 (2011) 25? 32.

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