Nanostructured Architectures

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Nanostructured Architectures for Energy
Conversion Prashant V. Kamat Radiation
Laboratory and Dept Of Chemical Biomolecular
Engineering University of NotreDame Notre Dame,
Indiana 46556-0579
Support US DOE
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Unique Aspects of nanostructures

• Organization of molecular-particle composites
• 2- and 3-D assemblies
• Control of electronic and surface properties

• Applications
• Optoelectronics, photonics, displays
• Chemical and biosensors
• Catalysis photovoltaics and fuel cells,

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The U.S. tax code offers a 2,000 consumer credit
for hybrid car owners and a deduction of up to
100,000 for people who buy the largest SUVs for
business use!
..It's inevitable. But just how soon will the
vital fuel become so scarce and expensive that
we're forced to make hard choices about how we
live? Some experts, in fact, think the world
production peak is already here. The timing rests
largely on the actions of Middle East producers
and on moves to conserve and to develop
unconventional sources.
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Need for alternative energy sources Solar Energy
Overcome the environmental issues Greenhouse
effect Decrease the cost per watt by improving
the efficiency solar paint, flexible cells
Efficiency of Photovoltaic Devices
Margolis, Science 285, 690, 1999)
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Semiconductor (bulk)/metal (or redox couple)
Issues Charge separation and charge transport
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Simple assemblies of elementary nanoobjects
Wire connected to Metal or SC Nanoparticle
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Areas where nanotechnolgy can contribute
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Semiconductor nanoparticles as photocatalysts

• Direct bandgap excitation of the semiconductor
  results in electron-hole separation
• Photogenerated holes as well as hydroxyl radicals
  oxidize the organic contaminant at the TiO2
  surface
• Electrons are scavenged by oxygen

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Semiconductor (e.g., TiO2) nanoparticles for
hydrogen production

CB

e
t
h
t

hn

4OH-
VB
2H2O O2
Hydrogen production is observed only in presence
of a precious metal catalyst. It is important
that we look into other metals as catalysts
What about gold and other noble metals?
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Role of nanogold in catalysis
Size and support dependency of catalytic activity
of gold clusters on titania, Haruta, et al
Catal. Today, 1997, 36, 153 Structural and
electronic properties of Au on TiO2(110), Yang,
Z.X., R.Q. Wu, and D.W. Goodman, Phys. Rev. B,
2000, 6, 14066-14071. High-performance
nanocatalysts for single-step hydrogenations.
Thomas, J. M., Johnson, B. F. G., Raja, R.,
Sankar, G. and Midgley, P. A., Acc. Chem. Res.,
2003, 36, 20-30. (Bimetallic nanoparticles
(Ru6Pd6, Ru6Sn, Ru10Pt2, and Ru12Ag4) anchored
within silica nanopores exhibit high
activities) Catalysis with TiO2/gold
nanocomposites. Effect of metal particle size on
the Fermi level equilibration, Subramanian, V.,
E. Wolf, and P.V. Kamat, J. Am. Chem. Soc.
2004, 126, 4943-4950 The Structure of
Catalytically Active Au on Titania. Chen, M. S.
and Goodman, D. W., Science, 2004, Published
online Powering Fuel Cells with CO via Aqueous
Polyoxometalates and Gold Catalysts Won Bae Kim,
T. Voitl, G. J. Rodriguez-Rivera, J. A. Dumesic
Science, 2004, 308, 1280-1283
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How does the metal influence the catalytic
activity in a semiconductor/metal composite? How
does the metal shuttle electrons across the
interface in nanostructures? Can control of size
be an important parameter?
Goodman, Science, 2004, Online
Dumesic, Science, 2004, 308, 1280-1283
Polyoxometalate-gold catalyst oxidizes CO in a
WGS reaction and stores charge
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Quantized double layer charging effects Murray
et al. Science, 1998, 280, 2098 and Anal. Chem.
1999, 71, 3703
Quantized conductance through individual rows of
suspended gold atoms H. Ohnishi, Y. Kondo K.
Takayanagi Nature, 395, 780 (1998)
DV e/CCLU
where e is the electronic charge and CCLU is
capacitance (aF)
...metal core potentials change by gt0.1V
increments for single electron transfers at the
electrode electrolyte interface
unit conductance G0 2e2/h.
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Charge Distribution in TiO2-Metal System and its
role in improving the energetics of composite
catalyst
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Effect of Gold Particle Size on the Catalytic
Reduction Efficiency of TiO2 particles
Vaidyanathan, Wolf, Kamat J. Am. Chem. Soc.,
2004, 126, 4943-4950
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Effect Gold Perticle Size on the Fermi Level of
the TiO2-Au Composite Catalyst
Ef (TiO2) Efb -0.25 0.059 log (C60eq /
C60-) C60eq C600 C60?
CB
e
e
e
hn
Redox couple
VB
Metal
Ef Apparent Fermi Level Efb Flat band potential
Subramanian, V., et al., Catalysis with TiO2/Au
Nanocomposites. J. Am. Chem. Soc., 2004, 126,
4943-4950.
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Establishing the property of electron storage in
metal Particles
Hirakawa, Kamat Langmuir, 2004, 20, 5645-5647
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Effect of excess charge on the electronic
properties
Kreibig et al Mie-plasmon spectroscopy A tool
of surface science. in Fine Particles Science and
Technology,Ed. E. Pelizzetti,NATO, p499
e
e
Metal cluster
A-
B
N electrons
N -1 electrons red shift
N1 electrons blue shift
Charge transfer between cluster atoms and the
surrounding species alters the electron density
The plasmon frequency of metal clusters can be
expressed as, ?p (Ne2/e0meff)½ N is the
conduction band electron density, meff is the
effective mass
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Photoinduced charge separation and charging of
metal core in Ag_at_TiO2
(A)
25 nm
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Charging and Discharging of Electrons in the
Metal Core
Hirakawa, T. and Kamat, P. V., Electron Storage
and Surface Plasmon Modulation in Ag_at_TiO2
Clusters. Langmuir, 2004, 20, 5645-5647
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Semiconductor-Metal Composite Catalysts for
Energy Conversion
Gold nanoparticles (2-8 nm diameter) improve the
photocatalytic performance of semiconductor
nanoparticles These composites are potentially
useful in the hydrogen production Cactivity of
other oxide-metal and bimetallic composites need
to be explored
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Organic Solar Cells
Selecting Chromophores that can harvest visible
light
(Mimicking photosynthesis using porphyrins and
chlorophyll)
Molecular clusters formed in mixed solvents
exhibit broader absorption bands
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Molecular Clusters with Donor Acceptor Moeities
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Designing Molecular Clusters with Donor-Acceptor
Moities
Assembling van der Waals complexes of
donor-acceptor chromophores
The structure of H2TPP-C60 reveals a zigzag chain
of alternating porphyrin/C60 interactions. C60 is
centered over the porphyrin with electron-rich
66 ring-juncture C-C bonds in close approach to
the plane of the porphyrin core.
Selective Supramolecular Porphyrin/Fullerene
Interactions. Boyd, P. D. W., et al ., J. Am.
Chem. Soc, 1999, 121, 10487-10495 and Sun et al
J. Am. Chem. Soc. 2002, 124, 6604-6612
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Unique properties of C60 Clusters

• Assist in building supramolecular assemblies
• Increased absorption in the visible
• Improved Charge separation
• Incorporation of redox active species in cluster
  assemblies
• Capable of shuttling electrons in nanostructured
  semiconductor films

Spherical Bilayer Vesicles of Fullerene based
surfactantsZhou et al Science, 2001, 291, 1944
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Self Assembled Molecular Clusters
Hasobe, Fukuzumi, KamatJ. Mater. Chem., 2003,
13, 2515 2520 and J. Phys. Chem. B, 2003,
107, 12105 12112
Nanoclusters
(H2PC60)n
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Charge Seperation and Charge Propagation in
Cluster Films
hn
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Assembling H2P-C60 clusters on electrode surfaces
Electrophoretic deposition
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Hasobe, T., Imahori, H., Kamat, P. V. and
Fukuzumi, S., Photovoltaic Cells using composite
nanoclusters of porphyrins and fullerenes with
gold nanoparticles. J. Am. Chem. Soc, 2004, 126,
in press.
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Porphyrin-Gold-C60 as Light Harvesting Assembly

C60 0.3 mM 0
H2Pc-Au-C60
H2Pc-C60
Power Conversion Efficiency 2
Hasobe et al J. Am. Chem. Soc. 2003, 125,
14962-14963.
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Dependence of alkane thiol chain length
H2P-(CH2)n-SH n 5, 11, 15 (a)-(c)
C60H2P0.31 mM (d) 0.38 mM

• Gold nanocore provides the foundation for
  anchoring H2P and C60
• C-15 chain length of the bridge provides a better
  geometrical configuration for H2P and C60
  interaction
• The precise role of god nanoparticle in promoting
  gold nanocore in promoting electron transfer and
  transport will be explored in future studies

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Supramolecular Photovoltaics
OTE/SnO2/(H2P)2C60m
C60 a. 0 mM b. 0.06 c. 0.13 d. 0.31
OTE/SnO2/(H2P)nC60m
a. n4 b. n8 c. n16 d. n1
Hasobe, T., Kashiwagi, Y., Absalom, M. A., Kohei
Hosomizu, Crossley, M. J., Imahori, H., Kamat, P.
V. and Fukuzumi, S., Supramolecular Photovoltaic
Cells of Porphyrin Dendrimers and Fullerene. Adv.
Mater., 2004, 16, 975-978
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CONCLUSIONS

• Opportunities exist for Nanotechnology to promote
  revolutionary and important breakthroughs in
  energy technology.
• Gold nanoparticles show unique property of
  quantized storage of electrons and exhibit size
  dependent catalytic properties.
• Higher photoconversion efficiency observed using
  molecular cluster-metal hybrid assemblies
  demonstrate new opportunities of developing
  organic solar cells.