TY  - GEN
ID  - heidok14023
AV  - public
TI  - Investigation of dye-functionalized
TiO2 nanoparticles using vibrational
sum-frequency-generation
spectroscopy
Y1  - 2012/11/16/
N2  - The steadily increasing demand for energy worldwide has resulted in the depletion of the existing
fossil energy resources and the pollution of the atmosphere by greenhouse gases such as
CO2, which are responsible for global warming. To curb these problems, research activities aiming
at CO2 conversion into value-added products, e.g. fuels like methanol, using sunlight have
intensified over the last few years. In this work, TiO2 nanoparticles functionalized with perylene-based
dyes containing either a carboxylic acid or anhydride group as molecular anchor to the
TiO2 surface were studied as potential photocatalyst for solar light-driven CO2 reduction. The
dye binding geometry is of particular importance since it influences the electron transfer from the
dye to TiO2 and hence the photocurrent output of any given dye/TiO2 system. Two dyes, ID1157
and ID1152, structurally identical apart from their anchor group, were selected to allow direct
comparison. In addition, a simple model substance bearing an anhydride group was investigated
to facilitate the interpretation of the lesser known anhydride binding mode. Surprisingly, despite
their structural similarity, the observed photocurrent amplitude of the ID1157/TiO2 system
containing a carboxylic acid anchor was much higher than for the ID1152/TiO2 system containing
an anhydride anchor. With the help of ultraviolet/visible (UV/Vis) absorption, infrared (IR),
Raman and vibrational sum-frequency generation (VSFG) spectroscopy it was sought to determine
whether the photocurrent signals were correlated to the dissimilar anchor groups and their
binding modes. From the UV/Vis spectra it was found that the anhydride group opened upon
binding. As for the IR and Raman studies it could be concluded that the carboxylic acid anchor
of ID1157 and both carboxylate groups of the opened anhydride of ID1152 bound via a bidentate
bridging pattern. Furthermore, it was shown from the background-suppressed VSFG spectra of
the bound dyes in air and water that these adsorbed in an orderly fashion, ID1157 more so than
ID1152, but with both only slightly disturbed in the presence of water. In addition, it was found
that all molecules in the ID1157 dye layer were adsorbed on the TiO2 surface via chemisorption.
By contrast, the ID1152 dye layer was composed of chemisorbed as well as physisorbed dyes, the
latter being coordinated via a closed anhydride group. Also, ID1157 displayed a higher surface
density of adsorbed molecules compared to ID1152. From a preliminary polarization analysis, it
was suggested that the dyes with carboxylic acid anchor adopted a tilted binding geometry. In
view of the results obtained on the binding geometry of the dyes ID1157 and ID1152 it was possible
to identify some criteria important for the generation of a high photocurrent: 1) binding of
the dye via chemisorption with 2) a high surface density and possibly through 3) a tilted geometry.
These findings have important implications for the understanding of the mechanism of dye
functionalization. Finally, methanol and CO2 adsorption on TiO2 films was investigated by VSFG
spectroscopy. Only molecularly adsorbed methanol was observed which was easily displaced
by water or methanol/water mixtures. However, CO2 adsorption could not be detected in the
spectral range which was investigated.
A1  - Keese, Anna
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/14023/
ER  -