1

Organic-Inorganic Hybrid Nanostructures

Prashant V. KamatRadiation Laboratory and

Dept Of Chemical & Biomolecular EngineeringUniversity of NotreDame

Notre Dame, Indiana 46556-0579Researchers/CollaboratorsAmy Dawson, Dr. T. Hirakawa Ella Jakob Dr. Girish KumarRavi Subramanian, Roxana Nicolaescu Dr. K. George Thomas (RRL, India)Istvan Robel Prof. Fukuzumi (Osaka U.)Said Barazzouk Prof. Imahori (Kyoto)T. Hasobe

Support: US DOE

Nanostructure Architectures for Energy Conversion

Photochemical Solar Cells− Assembly of molecular clusters

− Photoinduced charge separation

− Conversion of light energy into electricity

SS

S

S

S

S

S

S

SS

SS

SS

SS

SS

S

S

S

S

S

S

SS

SS

SS

SS

SS

S

S

S

S

S

S

SS

SS

SS

SS

SS

S

S

S

S

S

S

SS

SS

SS

SS

SS

S

S

S

S

S

S

SS

SS

SS

SS

e-

Pt

e-

OTEI-/ I3-

OTE: Optically Transparent Electrode

e-

hν

Porphyrin C60 Gold Nanoparticle

e-

Catalysis with Semiconductor and Metal Nanostructures− Design and characterization of new

materials− Photocatalytic production of hydrogen reactant products

hνe e

e

h hh

Ag

TiO2

reactant products

hνe e

e

h hh

reactant products

hνee ee

ee

hh hhhh

Ag

TiO2

Fuel Cells− Carbon nanotubes as a novel support

− Assembly of fuel cells and improving cell performance

CH3OH + H2O O2/air

H2OCO2

H+CH3OH + H2O O2/air

H2OCO2

H+O2/air

H2OCO2

H+

CO2

H+

Catalysis with Semiconductor and Metal Nanostructures

Synthesis Semiconductor Nanoparticles

Molecular Beam Epitaxy (MBE)

Metal Organic Chemical Vapor Deposition (MOCVD)

Colloidal Growth initiated by Chemical Reactions

Examples: Hydrolysis of titanium tetrachloride to produce TiO2

TiCl4 + 2H2O TiO2 + HCl

Reaction between cadmium and selenide compoundsCd2+ + Se2- CdSe (CdSe)n

A typical set up used in the synthesis of colloidal CdSe capped with TOPO. By controlling the temperature and arresting the colloidal growth one can control the size of the CdSe particles

2

Size control using reverse micellar system

Q-CdSeQ-CdSeQ-CdSe

Organic surfactants such as Aerosol-OT form reverse micelles in nonpolar solvents. Controlled addition of water yields desired size of particles

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

Wavelength/nm

a

b c

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

Wavelength/nm

a

b c

a

b c

Semiconductor Heterostructures

Cd2+ S2-TiO2

Cd2+

Cd2+

Cd2+

Cd2+

Cd2+

Cd2+

TiO2TiO2

CdS

Titanium

isopropoxide

HCl/H2O

Suspension ofSnO2 Colloids

Precipitation of SnO2@TiO2

Suspensionof SnO2@TiO2

TiO2@CdS

SnO2@TiO2

Organic capping or charge stabilization is important in achieving stable colloidal suspension

SnO2

TiOTiO22

CdSCdSSnOSnO22@TiO@TiO22@CdS@CdS

SnO2

TiOTiO22

SnO2

Confinement along 1, 2 and 3 dimensions. Analogous to a quantum well, quantum wire and quantum dot.

Quantum wells, wires and dots are often described using the analogy to a particle in a 1D box, a 2D box and a 3D box. This is because when the actual physical length scale of the system is smaller than the exciton Bohr radius or corresponding deBroglie wavelength, either or both the electron and hole experience confinment. In turn, theenergies of the carrier along that dimension of the material are no longer continuous as in the case where there is no confinement. The appearance of discrete states is one of the fundamental signatures of nanomaterials. Since solving the Schrodingerequation of a carrier to find its eigenvalues and eigenfunctions involves using boundary conditions one can also immediately predict that the actual shape of a quantum well, wire or dot will also play a role in dictating the ordering and spacing of states. A nanowire will have a similar but different progression of states than a quantum dot (or nanocrystal). The same applies to quantum wells as well as more exotic shapes of nanostructures.

For CdSe me = 0.13m0 , mh = 0.45m0 , ε = 9.4

Exciton Bohr Radius,

3

Schematic diagram of the molecular orbital model forband structure, adapted from ref 3.

Brus, Acc. Chem. Res., Vol. 23, 11990

The theoretical size dependence of the lowest excited state of four different semiconductors is shown in adjucentFigure. These curves can be approximated by

where me and mh are the effective masses and ε is the bulk optical dielectric coefficient. For every material, there is a diameter where the negative Coulombic attraction is balanced by the positive kinetic energy. In CdS this occurs at about a 70-A diameter, where each term is about 0.1 eV.

260 3403000

1.0

Abs.

Wavelength, nm

Bawendi, Acc. Chem. Res., 1999, 32, 389-396Kamat J. Phys. Chem., 1992, 96, 6829-34.

Size dependent absorption and emission spectra of colloidal CdSe quantum dots.

Examples of Size Quantization Effects

Emission from ZnO-quantum dots

CB

VB

S*

TiOTiO22

S+

O2

O2

Products

hν

hν

A

et ht

VB

–

+

+

–A−

B

B−

CB

Direct bandgap excitation of the semiconductor

Excitation of the molecules adsorbed on semiconductor substrate

Photocatalytic Activity of Semiconductor Nanoparticles

Emission

Charge Trapping

Interfacial Electron Transfer

4

Factors Controlling the Photocatalytic Activity

Bandgap energy determines minimum energy required to achieve charge separation

Energy levels of conduction and valence bands determine the energetics of electron transfer. (Quantized particles are more reductive/oxidative than the bulk semiconductors.)

Surface vacancies dictate the charge trapping and charge recombination processes

Interaction of molecules with surface and pH of the medium influence the charge transfer processes

H+/H2

-2.0

-1.0

0.0

+1.0

+2.0

+3.0

V vs. NHE

3.0 1.42.4 1.7

3.0

1.2

2.22.7 2.8 2.1 eV

VB

CBSIC

GaAsCdSe

CdSTiO2 MoS2Fe2O2

In2O3 WO3CdO

Eg

H+/H2

-2.0

-1.0

0.0

+1.0

+2.0

+3.0

V vs. NHE

3.0 1.42.4 1.7

3.0

1.2

2.22.7 2.8 2.1 eV

VB

CBSIC

GaAsCdSe

CdSTiO2 MoS2Fe2O2

In2O3 WO3CdO

Eg

400 500 600 700 800 9000.0

0.1

0.2

0.3

0.4

a

b

c

d

Abs

orba

nce

Wavelength, nm

0 100 200 300 400 5000.0

0.1

0.2

∆Abs

orba

nce

Time, s

Electron Accumulation/Trapping in TiO2 Colloids

Upon UV- irradiation TiO2 colloids prepared in ethanol turns blue as photogenerated electrons are trapped at the Ti4+ sites. These trapped electron survive charge recombination and exhibit long lifetimeSuch colorations are referred as photochromic effects –coloration induced by light

UV/N2 air

TiO2 TiO2(h + e) TiO2 (h) + ethanol TiO2 + productsTiO2 (e) TiO2 (etrap)TiO2 (etrap) + O2 TiO2 + O2

-

∆∆AA

0.015

0.051

0.033

Wavelength, nmWavelength, nm450 650550

50 ps

500 ps

2 ns

20 ns

Electron Accumulation/Trapping in ZnO Colloids

250 300 350 4000.0

0.2

0.4

0.6

0.8

1.0

Abso

rban

ce

Wavelength,nm

a,eb

cd

O2

O2

VB

CB

Vs2+/+

Electron accumulation in N2 + UV

–

+

VB hν

knrkr

Vs2+/+

CB– –– ––

+

Upon exposure to aira-d UV exposure of ZnO colloids in N2 for 30 min.e –Upon exposure to air

Electron accumulation causes band edge to shift to higher energies.

Photoinduced charge transfer between TiO2 semiconductor colloid and C60

hν

e

TiO2

C60CB

VB

hν

e

TiO2

C60CB

VB

0.00

0.05

0.10

0.15

0.20

400 500 600 700Wavelength (nm)

× 3

∆τ = 1 µs∆τ = 100 µs

0 50 100 150Time (µs)

420 nmC60 + TiO2

.01

∆A

Difference absorption spectra recorded following 308 nm laser pulse excitation of TiO2 colloidal suspension containing C60

5

hν

et ht

VB

–

+

+

–CB

2H2O+ O2

4OH−

H2

2H+2H+

H2

Pt

Hydrogen production is observed only in presence of a precious metal catalyst

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

What about gold and other noble metals?

Hydrogen Evolution rates for various Photocatalysts (µl/hr)

Pt/TiO2 7.7Pd/TiO2 6.7Rh/TiO2 2.8Ru/TiO2 0.2Sn/TiO2 0.2Ni/TiO2 0.1TiO2 <0.1

Toshima, J. Phys. Chem. 1985, 89, 1902

Photocatalytic water splitting reactions

TiO2

Pt

h

e

2H+

H2

4OH−

2H2O + O2hν

Role of nanometal 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. DumesicScience, 2004, 308, 1280-1283

+N-Br

Phase Transfer Catalyst in toluene+/-thiol

NaBH4

Hydrogen tetrachloro

aurate in water

NaBH4

toluene

Synthesis of Gold Colloids in Organic Solvents

A biphasic reduction method using alkanethiol and TOAB as stabilizers …….Brust et al, J. Chem. Soc., Chem. Commun. 1995, 1655-1656; Chemistry Materials, 1998, 10, 922.

N +

-Br

N +

N +N +

-Br

N+

-Br

N +-Br

-Br

-Br

25 nm

Au

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

Abs

orba

nce

Wavelength,nm

Thiol:Aua 4:1*(8nm)b 2:1(5nm)c 4:1(<3nm)*After reduction

a

b

c

400 500 600 7000.00

0.25

0.50

0.75

1.00I,2-dichlorobenzeneCyclohexane

Abs

orba

nce

Wavelength, nm

The peak position, λs, for metal nanoclusters can be expressed as

λs2 ≈ λp

2 /(ε∝+2 εm) where εm is the dielectric constant, λp is the bulk plasmon wavelength and can be expressed in the form

λp = (Ne2/ε0meff)-½

N is the conduction band electron density, meff is the effective mass

Surface Plasmon Absorption of Gold Nanoparticles

Solvent R.I., n λmax(nm)cyclohexane 1.426 526toluene 1.496 530o-xylene 1.501 532chlorobenzene 1.524 533o-dichloro- 1.551 535benzene

N+

-Br

N+

N+ N+-Br

N+-Br

N+-Br

-Br

-Br

SS

SSS

S S

S S

S

518

Alkanethiolate Gold Cluster Molecules with Core Diameters from 1.5 to 5.2 nm: Core and Monolayer Properties as a Function of Core Size, Hostetler et al Langmuir, 1998, 14,17-30Surface binding properties of tetraoctylammonium bromidecapped gold nanoparticles, George Thomas and Kamat, Langmuir, 2002, 18, 3722ε(ω) = n2(ω)

6

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

Abs

orba

nce

Wavelength,nm

Thiol:Aua 4:1*(8nm)b 2:1(5nm)c 4:1(<3nm)*After reduction

Controlling the sizeDifferent size Au particles can be prepared by adjusting thiol concentrations

SS

SSS

S S

S S

S

10 nm

(b)

10 nm10 nm

(a)

a

b

c5 nm5 nm

(c)

5 nm5 nm

(a)

N+

-Br

N+

N+ N+-Br

N+-Br

N+-Br

-Br

-Br

NBr

NBr

NBr

NBr

NBr

NBr

NBr

NBr

N Br

NBrN

Br

NBr

NBr

NBr

NBr

NBr

NBr

NBr

NBr

N Br

NBrN

Br

Au-ROD

NBr

NBr

NBr

NBr

NBr

NBr

NBr

NBr

N Br

NBrN

Br

NBr

NBr

NBr

NBr

NBr

NBr

NBr

NBr

N Br

NBrN

Br

Au-ROD

125 nm

125 nm400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0a- Au-R0b- Au-R50c- Au-R100d- Au-R150

dcba

Abso

rban

ce

Wavelength, nm

AR1

AR2

AR3AR 5

Controlling the shape

Aspect ratio =l/wl

25 nm

Kim, et al, Photochemical Synthesis of Gold NanorodsJ. Am. Chem. Soc., 124, 14317 (2002) George Thomas, et al., Unidirectional Plasmon Coupling through Longitudinal Self-assembly of Gold Nanorods. J. Phys. Chem. B, 108, 13066 (2004)

w

400 600 800 1000 12000.0

0.1

0.2

0.3

0.4jd

cb

a

Abs

orba

nce

Wavelength, nm

Self Assembly of Gold Nanorods

CS O

O H

C SO

OHCS O

O H

C SO

OHAu Au

Longitudinal interplasmon coupling via hydrogen bonding

50 nm

50 nm

50 nm

50 nm

Linear Assembly HS-(CH2)n-CO2Hn = 2, 10

George Thomas, K., Barazzouk, S., Ipe, B. I., Shibu Joseph, S. T. and Kamat, P. V., Unidirectional Plasmon Coupling through Longitudinal Self-assembly of Gold Nanorods. J. Phys. Chem. B, 2004, 108, 13066-13068.

Catalytic Nanomotors

Paxton et al J. Am. Chem. Soc. 2004, 126, 13424-13431

7

HS

Spacer Photoresponsive molecule

S SS

SSSSS

George Thomas & Kamat Accounts of Chemical Research 2003, 36, 888-898

Controlling the surface property

NCH3

O

S

NH3C

O

S

N CH3

O

S

NH3C O

S

NCH3

O

S

NH3C

O

S

S

SS

SS

S Au

O

S

O

S

O

S

OS

O

S

O

S

S

SS

SS

S Au

S H

SA uS

S

S

S

S

S

S

SS

SS

S

S

S

S

N

N H N

H NN H C O ( C H

2)n

S H

( n = 5 , 1 1 )

Nano Lett., 2002, 2, 29-35

Adv. Mater., 2004, 16, 975-978J. Am. Chem. Soc, 2005, 127, 1216-1228.

J. Phys. Chem. B, 2002, 106, 18-21

ee e e eee

h

hν

TiO2

Au

C2H5OH

EF EF

eeee ee ee eeeeee

hh

hν

TiO2

Au

C2H5OH

EF EF

COO−

S-

S-S

S

−OOCS-

S

-SS

COO−

S-

S-S

S

TiO2

Au

Au

Au

COO−

S-

S-S

S

COO−

S-

S-S

S

−OOCS-

S

-SS

−OOCS-

S

-SS

COO−

S-

S-S

S

COO−

S-

S-S

S

TiO2

Au

Au

Au

Jakob, M.; Levanon, H.; Kamat, P.V., Charge Distribution between UV-Irradiated TiO2 and Gold Nanoparticles. Determination of Shift in Fermi Level,. Nano Lett. 2003, 3, 353-358.

Subramanian, V., Wolf, E. E. and Kamat, P. V., Catalysis with TiO2/Au Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration. J. Am. Chem. Soc., 2004, 126, 4943-4950.

Charge Distribution in TiO2-Metal System Photoinduced charging and discharging in TiO2 nanoparticles

ee e e eee

h

hν

TiO2

Au

C2H5OH

EF EF

eeee ee ee eeeeee

hh

hν

TiO2

Au

C2H5OH

EF EF

Decreased blue coloration confirms electron transfer from charged TiO2 to gold nanoparticles

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

Abso

rban

ce

Wavelength,nm

TiO2a UV=0minb 30 minsc 60 minsd 120mins d

a

bc

TiO2 TiO2(h + e) C2H5OH TiO2(e)

TiO2(e) + Au TiO2 + Au(e)

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

Abso

rban

ce

Wavelength,nm

[Au]8nm

a 0µMb 43µMc 85µMd 126µMe 166µMf 206µM

d

ba

c

ef

-2 0 2 4 6 8

0.00

0.01

0.02

0.03

0.04

0.05

∆A

Time (µs)

a TiO2b Au(8nm)c Au(5nm)

a

b

c

-2 0 2 4 6 8

0.00

0.01

0.02

0.03

0.04

0.05

∆A

Time (µs)

a TiO2b Au(8nm)c Au(5nm)

a

b

c

675 nm

8

C60

TiO2/Au(e) + C60 TiO2/Au+ C60− ..(c-f)

TiO2(e) + Au TiO2/Au(e) (b)

TiO2 TiO2(e) ….(a)400 600 800 1000 1200-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Abso

rban

ce

Wavelength,nm

a TiO2,UV120b Au8,0.2mMc [C60]-5.9µMd [C60]-11.7µMe [C60]-17.5µMf [C60]-23.2µM

C60−

a

b

f

c

Au

Sequential Electron Transfer

e

ee

e

hν

e

TiO2

hC2H5OH

1 2 3 40

5

10

15

Red

uctio

n Ef

ficie

ncy

(%)

No Au8 nm Au

5 nm Au

3 nm Au

Effect of Gold Particle Size on the Catalytic Reduction Efficiency of TiO2 particles

10 nm

(b)

10 nm10 nm

(a)

a

b5 nm5 nm

(c)

5 nm5 nm

(c)

Vaidyanathan, Wolf, Kamat J. Am. Chem. Soc., 2004, 126, 4943-4950

E*f (TiO2)= Efb = -0.25 + 0.059 log ([C60]eq / [C60−])

[C60]eq = [C60]0 – [C60•⎯]

Determining the Fermi Level of the Composite Particles

MetalVB

CB

ee e

Redox couple

EfFEfhν

eE*fF

h

e e

e

E*f –Apparent Fermi Level

Efb –Flat band potential

-2706694TiO2 -Au(5nm)

-2907794TiO2 -Au(3nm)

-2505194TiO2 -Au(8nm)

-2303694TiO2

Ef*(mV)

[C60⎯](µM)

[C60]0(µM)

System

Subramanian, V., et al., Catalysis with TiO2/Au Nanocomposites. J. Am. Chem. Soc., 2004, 126, 4943-4950.

Particle Size Effect on the Shift in Flat Band Potential

Higher photocurrent observed with TiO2/Au composite confirms improved catalytic performance

A shift in the flat band potential is seen in TiO2 films deposited with metal nanoparticles

Larger shift observed with smaller particles parallel the Fermi level shift observed with particle systems

Cur

rent

(µA)

Voltage (V)

a TiO2b TiO2-Au(8nm)c TiO2-Au(5nm)

0 -1.2

ab

c

200

400

600

Vs. SCE

0.01 M NaOH

e

e

e

e

OTE TiO2 Au

h

h

h

RedOx

hν

9

Establishing the property of electron storage in metal Particles

reactant products

hνe e

e

h hh

Ag

TiO2

Ag+ NOOO

Ti O

DMFreflux

AgN

OOO

Ti ONOOO

Ti O

reflux

Ag

TiO2

NOOO

Ti ONOOO

Ti O

① Form TTEAIP-capped Ag metal

H2O② Form TiO2 shell

Ag+ NOOO

Ti O

DMFreflux

AgN

OOO

Ti ONOOO

Ti O

reflux

Ag

TiO2

NOOO

Ti ONOOO

Ti O

① Form TTEAIP-capped Ag metal

H2O② Form TiO2 shell

Hirakawa, Kamat Langmuir, 2004, 20, 5645-5647

e –

A−

N electrons N-1 electronsred shift

N+1 electronsblue shift

Metal – cluster

e –

B+

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

Charge transfer between cluster atoms and the surrounding species alters the electron density

The plasmon frequency of metal clusters can be expressed as,

ωp = (Ne2/ε0meff)½

N is the conduction band electron density, meff is the effective mass

Wavelength , nm

Abs

orba

nce

0

0.4

0.8

1.2

1.6

2

300 400 500 600 700

0s

10s30s

60s

air

Wavelength , nm

Abs

orba

nce

0

0.4

0.8

1.2

1.6

2

300 400 500 600 700

0s

10s30s

60s

air

ethanol products

hνe e

e

h hh

Ag

TiO2

λp= 470 nm

e

e

eAg

TiO2

λp= 430 nm

ethanol products

hνe e

e

h hh

Ag

TiO2

λp= 470 nm

ethanol products

hνe e

e

h hh

ethanol products

hνee ee

ee

hh hhhh

Ag

TiO2

λp= 470 nm

e

e

eAg

TiO2

λp= 430 nm

ee

ee

eeAg

TiO2

λp= 430 nm

Photoinduced charge separation and charging of metal core in Ag@TiO2

25 nm

Wavelength, nm

Abs

.

Ag @SiO2

0

0.04

0.08

0.12

0.16

0.2

300 400 500 600 700 800

(B)

50nm

Wavelength, nm

Abs

.

Ag @SiO2

0

0.04

0.08

0.12

0.16

0.2

300 400 500 600 700 800

(B)

Wavelength, nm

Abs

.

Ag @SiO2

0

0.04

0.08

0.12

0.16

0.2

300 400 500 600 700 800

(B)

50nm

(A)

hν

AgSiO2

Hirakawa and Kamat, J. Am. Chem. Soc. 127, ASAP (2005)

ethanol products

hνe e

e

h hh

Ag

TiO2

λp= 470 nm

e

e

eAg

TiO2

λp= 430 nm

ethanol products

hνe e

e

h hh

Ag

TiO2

λp= 470 nm

ethanol products

hνe e

e

h hh

ethanol products

hνee ee

ee

hh hhhh

Ag

TiO2

λp= 470 nm

e

e

eAg

TiO2

λp= 430 nm

ee

ee

eeAg

TiO2

λp= 430 nm

Charging and Discharging of Electrons in the Metal Core

Hirakawa, T. and Kamat, P. V., Electron Storage and Surface Plasmon Modulation in Ag@TiO2 Clusters.Langmuir, 2004, 20, 5645-5647

Time, min

Peak

pos

ition

of p

lasm

on, n

m

OnIrradiation Off

420

440

460

480

0 10 20 30 40380

400

420

440

Ag @TiO2

Ag @SiO2

(A)

Peak

pos

ition

of p

lasm

on, n

m

Time, min

Peak

pos

ition

of p

lasm

on, n

m

OnIrradiation Off

420

440

460

480

0 10 20 30 40380

400

420

440

Ag @TiO2

Ag @SiO2

(A)

Peak

pos

ition

of p

lasm

on, n

m

0

1

2

3

300 400 500 600 700 800

Wavelength, nm

Abs

.

420

430

440

450

0 1 2 3 4 5Concentration of TH, µM

Peak

pos

ition

, nm

After irradiation.

After addition of TH

0

1

2

3

300 400 500 600 700 800

Wavelength, nm

Abs

.

420

430

440

450

0 1 2 3 4 5Concentration of TH, µM

Peak

pos

ition

, nm

After irradiation.

After addition of TH

10

420

440

460

480

0 0.2 0.4 0.6 0.8 10

10

20

30

40

50

Irradiation time , min

Plas

mon

pea

k , n

m

No.

of s

tore

d el

ectr

ons

per p

artic

leThe plasmon frequency of silver clusters can be expressed as,

ωp = (Ne2/ε0meff)½

N is the conduction band electron density, meff is the effective mass

Electron Storage in Silver Nanoclusters

Potential applications in Microcapacitors, Photocatalysts & Optoelectronics

Wavelength, nm

1µs

Ag @TiO2

∆Ab

s, 1

0-3

420 nm

540 nm

-8.0

-6.0

-4.0

-2.0

0

2.0

4.0

6.0

8.0

360 460 560 660 760

TiO2

4

8

12

16

0 0.5 1.0 1.5

∆Ab

s., 1

0-3

Time, 10-4 s

Ag @TiO2

420 nm

-12.0

-8.0

-4.0

0.00E+000 0.5 1.0 1.5

Time, 10-4 s

∆Ab

s., 1

0-3

Ag @TiO2

540 nm

Probing the charge equilibration process with laser pulse excitation

AuAu FluorophoreFluorophore−OOC− Electron DonorTiO2

Conductingelectrode

e

e

hνe

Nanostructure architectures for charge rectification

Au

OOC S--

OOC S--

OOC S--

OS

OS

OS

Hybrid Assembly on an Electrode Surface

AuAuOS

HOOCS--

30 nm30 nm

+AuAu OS

HOOCS--

TiO2

OTE

11

a1 τ1(ns)

a2 τ2(ns)

<τ> (ns)

Air 0.55 2 1.28 34 33.2

Tol 1.26 3.9 0.78 25.9 21.6

ACN 0.63 1.22 0.59 9.19 8.2

• Efficient quenching of singlet excited state on TiO2 surface.

• No significant quenching on silica

0 10 20 300

500

1000

1500

2000

d

c

b

a

Cou

nts

Lifetime, ns

a- Airb- Tolc- ACNd- Scatterer

OTE/TiO2/Au-S-Pyrene Electrode

350 400 450 500 5500.0

0.2

0.4

0.6

0.8

1.0

c

b

aN

orm

aliz

ed E

mis

sion

Wavelength, nm

a- Air

b- Tol

c- ACN

Au

OOC S--

OOC S--

OOC S--

OS

OS

OS

TiO2

OTE

hν

AuAuOS

hν

AuAuOS

e

No emission

e

Emission

hν

Modulation of Fluorophore Emission Using Electrochemical Bias

Electrolyte, TBAP in ACN (0.1 M) RE: SCE,

CE: Pt Scan Rate, 1mV/sec

350 400 450 5000.0

0.2

0.4

0.6

0.8

1.0 -1.2 V

- 1.0V

- 0.75 V

- 0.50 V- 0.25 V

0 V

Nor

mal

ized

Em

issi

on

Wavelength, nm

OTETiOTiO22

-(CH2)n-S- -SCOO–

-(CH2)n-S- -SCOO–

-(CH2)n-S- -SCOO–

0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.20.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Em

issi

on

Potential, Volt

Em.@ 395 nm

e

P. V. Kamat; S. Barazzouk; S. Hotchandani, Electrochemical Modulation of Fluorophore Emission at a Nanostructured Gold Film. Angew. Chem. (Int. Ed.) 2002, 41, 2764-2767

600 650 700 750 8000.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300

1

2

3

4

I 0/I

[Au], µM

e

f

h

g

d

cb

a

Nor

mal

ized

Em

issi

on

Wavelength, nm

[Au}0

0.35 mM

Chla/Chla+•

Au

e

EF

e ee

e ee

1Chla*/Chla+•3Chla*/Chla+•

isce

Chla/Chla+•

Au

e

EF

e ee

e ee

Chla*/Chla+•

Chla*/Chla+•isce

Chla/Chla+•

Au

hν

e

EF

e ee

e ee

Chla*/Chla+•

Chla*/Chla+•isce

-1

0

1

E vs. NHE

-1

0

1

E vs. NHE

-1

0

1

E vs. NHE

Electron transfer between Excited Chlorophyll a and Au Colloids

Barazzouk, S., Kamat, P. V. and Hotchandani, S., Photoinduced Electron Transferbetween Chlorophyll a and Gold Nanoparticles. J. Phys. Chem. B, 2005, 109, 716-723.

12

650 700 750 800 850 9000.0

0.2

0.4

0.6

0.8

1.0Potential, mV vs. SCE

0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0-50-50 0-1000

λem= 750 nm

Nor

mal

ized

Em

issi

on

Time, sec

g

fe

d

c

b

a

Nor

mal

ized

Em

issi

on

Wavelength, nm

400 500 600 700 800 900 1000 1100-0.04

-0.02

0.00

0.02(A)b

a

∆A

Wavelength, nm

Excitation 532 nma. Chlab. Chla + Au

Electron Transfer in the Hybrid Assembly

Chla* + Au Chla+. + Au(e)

400 450 500 550 600 650 700 7500

4

8

12

16

20

Wavelength, nm

IPC

E, %

0.0

0.2

0.4

0.6

0.8

1.0

1.2

OTE/TiO2/Chla

Abs. Spec. Chl a

OTE/TiO2/Au/Chla

Abso

rban

ce

AuAu

h ν

e

AuAu

AuAuChl a

Chl a

Chl aAuAu

h ν

e

AuAu

AuAuChl a

Chl a

Chl ae

e

e

Photocurrent Action Spectrum of Chlorophyll Modified Electrodes

Semiconductor-metal-molecular assemblies can be engineered to tailor the properties of light harvesting assemblies

Gold nanoparticles improve the photocatalytic performance of semiconductor nanoparticles

Electrons can be stored in small nanoparticles using photolytic or electrochemical methods.

Charging of metal nanoparticles is convenient to modulate the catalytic properties of ordered assemblies

Hybrid Nanoassemblies for Energy Conversion