Photoelectrochemistry and

Nanoparticle Research

Research Activity January 2003 - December 2003

Prashant V. Kamat Radiation Laboratory

and Chemical and Biomolecular Engineering

University of Notre Dame Notre Dame, Indiana 46556-0579

Tel. (574 631-5411 Fax (574) 631-8068 E-mail: PKAMAT@nd.edu

Website: http://www.nd.edu/~pkamat

Research Funding Office of Basic Energy Sciences, U.S. Department of Energy

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Research Program Photoelectrochemical Processes for Light Energy Conversion

• Charge transfer processes in semiconductor and

metal nanostructures • Semiconductor-metal nanocomposites • Photochemical and radiation chemical processes on

surfaces • Excited state & redox processes of novel molecular

assemblies • Environmental remediation

External Collaborators K. George Thomas (RRL, Trivandrum) S. Hotchandani (U. of Quebec) K. Vinodgopal (Indiana U., N. W.) S. Padmaja (Unilever Research) S. Fukuzumi (Osaka University) H. Imahori (Kyoto University) M. Anpo (Osaka Prefecture University)

Collaborators at Notre Dame Joan Brennecke, Bruce Bunker, Olaf Wiest, and Eduardo Wolf

Visiting Scientists K. Vinodgopal (Indiana U., N. W.) K. George Thomas (RRL, India) S. Hotchandani (U. of Quebec)

Postdoctoral Research Associate Zeena S. Pillai (August 2001-Sept 2003) Shailesh Sharma (April 2002-March 2003) Tsutomo Hirakawa (April 2003-present) Girish Kumar (Aug. 2003-present) Mariko Hirakawa (Oct. 2003- present)

Graduate Students Roxana Nicolaescu (Chemistry, U. of Notre Dame) Julie Peller (Chemistry, U. of Notre Dame) Ravi Vaidyanathan (Chem. Eng. U. of Notre Dame) Said Barazzouk (Chemistry, U. of Quebec) Taku Hasobe (Chemistry, Osaka U) Hisashi Kikuchi (Osaka Prefecture Univ.)

Undergraduate Students M. Haria (Co-Op student, McMaster U,) Jan-Aug 2003

Ph. D. Dissertations Julie Peller (Chemistry, UND, May 2003), “Hydroxyl radical mediated transformations of Herbicide, 2,4-Dichlorophenoxyacetic acid and related compounds”.

Professional Activities

• Senior Editor/Editorial Board Member, Journal of Physical Chemistry

• Editor/ Editorial Board Member, Research on Chemical Intermediates

• Editorial Board Member, Langmuir • Editorial Board Member, Fullerene Science &

Technology • Editorial Board Member, International Journal of

Photoenergy • Editorial Board Member, Interface • Chairman, Fullerenes, Nanotubes and Carbon

Nanostructures Division (The Electrochemical Society)

• Co-Editor, Book on Fullerenes Volume 13, 2003 (The Electrochemical Society)

• Co-organizer, Fullerene Symposium at The Electrochemical Society annual meeting, Paris, May 2003.

• Co-organizer, Symposium on Nanotechnology, at The Electrochemical Society annual meeting, Paris, May 2003.

• Panel member of NSF-SBIR Review Committee • Panel member of EPA Grand Challenge Workshop

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Significant Accomplishments Fermi Level Equilibration in Semiconductor-Metal Nanocomposites. Oxide semiconductor

colloids (TiO2, and ZnO) are capable of undergoing charge storage under UV-irradiation. By making use of this electron accumulation property and the ability to titrate the stored electrons with a redox couple (C60/ C60

•−), we succeeded in determining the Fermi level of TiO2 and TiO2-Au nanoparticluate systems. The charge distribution between the semiconductor and metal nanoparticles was found to shift Fermi level to more negative potentials. A Fermi level shift of –22 mV observed for the Au-TiO2 nanocomposite is indicative of improved charge separation in semiconductor-metal systems and demonstrates its usefulness for improving the efficiency of photocatalytic reactions.

Surface Interactions of CdSe Quantum Dots : The excited state interaction between CdSe nanocrystals and a hole acceptor, p-phenylenediamine (PPD), was elucidated using emission and transient absorption spectroscopies. The changes in the photophysical properties of CdSe nanocrystals arising from the interaction with PPD were different than those obtained with an aliphatic amine, n-butylamine (n-BA). At low concentrations (<0.5 mM) n-butylamine enhances the emission of CdSe quantum dots whereas PPD effectively quenches the emission at similar concentrations. The low oxidation potential of PPD (E = 0.26 V vs NHE) enables it to act as an effective scavenger for photogenerated holes. A surface bound complexation equilibrium model was proposed to explain the quenching phenomenon. The transient absorption measurements confirmed the formation of PPD cation radical and subsequent formation of coupling product. Formation of such charged species at the surface extends the bleaching recovery over several microseconds

Assembly of Molecular Clusters on Electrode Surfaces for Photocurrent Generation. Composite molecular nanoclusters of fullerene and porphyrin were prepared in acetonitrile/toluene mixed solvent to absorb light over entire spectrum of visible light. Both tubular shaped or square shaped microcrystallites were obtained by varying composition and evaporation conditions. The highly colored composite clusters were further assembled as 3 dimensional array onto nanostructured SnO2 films using electrophoretic deposition approach. The composite cluster films exhibit an incident photon-to-photocurrent efficiency (IPCE) as high as 17 % at an applied potential of 0.2 V vs. SCE which is significantly higher than the additive effect observed from either porphyrin (IPCE = 1.6 %) or fullerene clusters (IPCE = 5.0 %) under similar photoelectrochemical conditions. The high IPCE observed with porphyrin and C60 clusters demonstrate the synergy of these systems towards yielding efficient photoinduced charge separation within these composite nanoclusters.

Environmental Remdiation: By simultaneously carrying out high frequency sonolysis and photocatalysis we have succeeded in achieving faster and complete mineralization with no build up of toxic intermediates even at very low catalyst loadings. Whereas sonolysis is quite effective in the initial degradation of chlorinated aromatic molecules, complete mineralization is difficult to achieve. Photocatalysis is selective towards the degradation of polar aromatics and causes the build up of undesirable chemical intermediates. The synergy of combining the two advanced oxidation processes was established using 2,4-dichlorophnoxy acetic acid as a model ccompound. The primary events of the oxidative transformation of N-heterocycles, e.g. Quinoline, by different radicals, was elucidated using radiolysis and sonolysis. The hydroxyl radical reacts by adding to both rings of quinoline to form the OH adducts. On the other hand, SO4

•−

oxidizes the quinoline to its radical cation, which further undergoes hydrolysis to yield also the OH adducts.

Nanostructured humic acid membranes on optically transparent electrodes (OTE) were prepared by electrophoretic deposition from a solution of Suwanee River Humic acid (SHA) in ethanol/acetonitrile. The thickness of the film and the rate of deposition of SHA are dependent on the applied voltage and the concentration of the solution. Tapping mode Atomic Force Microscopy (TM-AFM) confirmed the presence of a three-dimensional, porous, assembly of SHA clusters on the electrode surface. The ability of these membranes to incorporate redox-active species such as ferrocene from solution opens new ways to probe binding properties of various chemical species present in the environment.

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Awards and Recognition Served as Editor of the scientific journal, Research on Chemical Intermediates (2001-Sep 2003); Serving as Senior Editor of the Journal of Phyical Chemistry starting October 2003.

Member of Board of Directors (2002-2004) and Finance committee (2003-2008) of the Electrochemical Society

Serving the term as a Chairman of the Fulerenes, Nanotubes and Carbon Nanostructures Division of the Electrochemical Society (2000-2004) Japan Society for Promotion of Science (JSPS) Fellowship, July-August 2003

Delivered Plannary/invited lectures at the meetings and chaired sessions at the Internatinal Photochemical Society Conference, Nara, Japan; 11th International Conference on Colloid and Surface Science, Brazil; MPND Conference Orleans, France; The American Chemical Society Meeting, New Orleans, and The Electrochemical Society meeting, Paris, France.

Panel member, Grand Challenge Workshop organized by EPA

Served as a member of the Editorial Advisory Board of the ACS journals, Langmuir starting January 2000 and Journal of Physical Chemistry starting Jan 2001, Fullerene Science and Technology, and International Journal of Photoenergy starting September 2001. Assumed the position as a Senior Editor of the Journal of Physical Chemistry starting October 2003.

Invited Lectures

Photoinduced charge separation in organic-inorganic hybrid nanostructures. Bowling Green State University, Bowling Green, OH. April 16, 2003.

Luminescent semiconductor nanostructures for monitoring low level contaminants. Grand Challenge Workshop sponsored by Environmental Protection Agency Washington, DC Invited May 8-9, 2003

Photochemical activity of indo dye in homogeneous and heterogeneous media. Unilever Research. Edgewater, NJ. June 19,20, 2003.

Kamat, P. V. (2003). Nanostructures in solar photochemistry and beyond. Kyoto University. Kyoto, Japan. July 21, 2003.

Photoinduced charge separation in organic - inorganic hybrid nanostructures. Nagoya University. Nagoya, Japan. July 23, 2003.

Inorganic hybrid nanostructures for light energy conversion. Saga University. Saga, Japan. July 24, 2003.

Charge distribution and Fermi-level equilibration in semiconductor-metal nanocomposites. Osaka Prefecture University. Osaka, Japan. August 1, 2003.

Kamat, P. V. (2003). Photoinduced charge separation in organic - inorganic hybrid nanostructures. Osaka University. Osaka, Japan. August 2, 2003.

Photoresponsive Hybrid Nanostructures , Univ. of Notre Dame (Chem. Eng. Sept.2 2003)

Photoinduced charge separation in organic - inorganic hybrid nanostructures, Marquette University (Chemistry, Sept. 5, 2003)

Photoresponsive Hybrid Nanostructures, Auburn University (Chemistry Dept., Oct. 2, 2003)

Inorganic-organic hybrid nanostructures for light energy conversion, Univ. of Missouri at St. Louis (Chemistry Dept., Oct. 20, 2003)

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Meeting Presentations Kamat, P. V., Jakob, E. and Levanon, H. 2003 C60/C60

-

Redox couple as a probe in the determination of Fermi level of semiconductor nanoparticles. Annual Meeting ot eh Electrochemical Society. Paris, France Invited April 27-May 2, 2003 Subramanian, V., Wolf, E. E. and Kamat, P. V. 2003 Fermi-level equilibration in semiconductor-metal nanocomposites. Annual Meeting of the Electrochemical Society Paris, France Invited April 27-May 2, 2003 Kamat, P. V., Subramanian, V. and Sharma, S. 2003 Semiconductor-metal and semiconductor-semiconductor nanoassemblies on electrode surfaces. Annual Meeting of the Electrochemical Society Paris, France Invited April 27-May 2, 2003 Subramanian, V., Wolf, E. E. and Kamat, P. V. 2003 Charge storage and charge equilibration in semiconductor-metal nanoparticles. International Symposium on Materials Processing for Nanostructured Devices - MPND 2003 Sologne, France Invited May 4-6, 2003 Kamat, P. V. 2003 Photoinduced charging events in semiconductor-metal and metal-fluorophore hybrid nanoassemblies. XXI Internationsl Photochemistry Conference Nara, Japan Invited July 16-20, 2003 Kamat, P. V. 2003 Charge distribution and Fermi-level equilibration in semiconductor-metal nanocomposites. DOE Solar Photochemistry Meeting Granlibakken, CA Invited June 8-12, 2003 Peller, J., Wiest, O. and Kamat, P. V. 2003 Combining light and sound to modulate photochemical processes at TiO2 surface. Annual Meeting of the American Chemical Society New Orleans, LA Invited March 23-27, 2003 Vinodgopal, K., Subramanian, N. and Kamat, P. V. 2003 Incorporation of electroactive metal species in humic acid membranes. Annual Meeting of the American Chemical Society New Orleans, LA March 23-27, 2003 Kamat, P. V., Huehn, R. and Nicolaescu, R. 2003 Semiconductor nanostructures for detection and degradation of low level organic contaminants from water. Annual Meeting of the American Chemical Society New Orleans, LA, March 23-27, 2003

Kamat, P. V., Subramanian, V. and Wolf E. E. 2003 Photosensitization of nanostructured TiO2 films with CdSe Quantum Dots, Annual Meeting of the Electrochemical Society Orlando, Invited Oct 13-16, 2003 K. Vinodgopal Mehul Haria, Dan Meisel and Kamat, P. V., Fullerene Based Nanostructured Carbon Electrodes for Methanol Oxidation Annual Meeting of the Electrochemical Society Orlando, Oct 13-16, 2003

Graduate Research Presentations Annual Meeting of the American Chemical Society New Orleans, LA March 23-27, 2003

Nicolaescu, R. Radiolytic transformations of Quinoline

Notre Dame Environmental Education Research (NDEER) symposium ednesday, November 12, 2003.

Peller, J. OH: A strong oxidant for the remediation of chlorinated organic contaminants in water. Nicolaescu, R. Radiolytic transformations of Quinoline Subramanian, V. Analysis of a Semiconductor - Metal Photocatalyst Using a Continuous Batch Flow Photocatalytic Reactor

AIChE annual meeting, San Franscisco, November 2003

Subramanian, V. Photoinduced Transformations at the Semiconductor -Metal Interface in Films and Solutions Subramanian, V. Photophysical, Photocatalytic and Electrochemical Aspects of Semiconductor -Metal Nanocomposite Systems Subramanian, V. Fermi Level Equilibration in TiO2/Au Nanocomposites. Effect of Particle Size

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Publications

Published Articles 1. George Thomas, K. and P.V. Kamat,

Chromophore Functionalized Gold Nanoparticles. Acc. Chem. Res., 2003, 36, 888-898

2. Subramanian, V., E.E. Wolf, and P.V. Kamat, Influence of Metal/Metal --Ion Concentration on the Photocatalytic Activity of TiO2-Au Composite Nanoparticles. Langmuir, 2003, 19, 469-474.

3. Adams, D., L. Brus, C.E.D. Chidsey, S. Creager, C. Cruetz, C.R. Kagan, P.V. Kamat, M. Lieberman, S. Lindsay, R.A. Marcus, R.M. Metzger, M.E. Michel-Beyerle, J.R. Miller, M.D. Newton, D.R. Rolison, O. Sankey, K.S. Schanze, J. Yardley, and X. Zhu, Charge Transfer on the Nanoscale. J. Phys. Chem. B, 2003, 107, 6668-6697.

4. Vinodgopal, K., V. Subramanian, S. Carrasquillo, and P.V. Kamat, Electrophoretic Deposition of Naturally Occurring Humic Substances as Thin Films. Environ. Sci. Technol., 2003, 37, 761-765.

5. Lahiri, D., V. Subramanian, T.Shibata, E.E. Wolf, B.A. Bunker, and P.V. Kamat, Photoinduced Transformations at Semiconductor/Metal Interfaces: XAFS Investigation of UV-irradiated TiO2 /Au Films. J. Appl. Phys., 2003, 93, 2575-2582.

6. Kamat, P.V. and K. George Thomas, Molecular Assembly of Fullerenes as Nanoclusters and Nanostructured Films, in Nanoscale Materials, L. Liz-Marzan and P.V. Kamat, Editors. 2003, Kluwer Academic/Plenum Boston. p. 475-494.

7. Peller, J., O. Wiest, and P.V. Kamat, Synergy of combining sonolysis and photocatalysis in the degradation and mineralization of chlorinated aromatic compounds. Environ. Sci. Technol., 2003, 37, 1926-1932.

8. Nicolaescu, R., O. Wiest, and P.V. Kamat, Radical induced oxidative transformations of Quinoline. J. Phys. Chem. A, 2003, 107, 427-433.

9. Subramanian, V., E.E. Wolf, and P.V. Kamat, Green Emission to Probe Photoinduced Charging Events in ZnO-Au Nanoparticles. Charge Distribution and Fermi-Level Equilibration. J. Phys. Chem. B, 2003, 107, 7479-7485.

10. Sharma, S., Z.S. Pillai, and P.V. Kamat, Photoinduced charge transfer between CdSe nanocrystals and p-phenelenediamine. J. Phys. Chem. B, 2003, 107, 10088-10093.

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

12. Liz-Marzan, L. and P.V. Kamat, Nanoscale Materials. Boston Kluwer Academic/Plenum Publishers. 2003, 499pp (Edited Book).

13. Subramanian, V., P.V. Kamat, and E.E. Wolf, Mass-Transfer and Kinetic Studies during the Photocatalytic Degradation of an Azo Dye on Optically Transparent Electrode Thin Film. Ind. Eng. Chem. Res., 2003, 42, 2131-2138.

14. Hasobe, T., H. Imahori, S. Fukuzumi, and P.V. Kamat, Nanostructured assembly of porphyrin clusters for light energy conversion. J. Mater. Chem., 2003, 13, 2515 - 2520.

15. Peller, J., O. Wiest, and P.V. Kamat, Mechanism of Hydroxyl Radical-Induced Breakdown of the Herbicide 2,4-Dichlorophenoxyacetic Acid. Chemistry, European J., 2003, 9, 5379-5387.

16. Hasobe, T., H. Imahori, S. Fukuzumi, and P.V. Kamat, Light Energy Harvesting Using Mixed Molecular Nanoclusters. Porphyrin and C60 Cluster Films for Efficient Photocurrent Generation. J. Phys. Chem. B, 2003, 107, 17205-17212

17. Kamat, P.V. and D. Meisel, Nanoscience Opportunities in Environmental Remediation. Comptes Rendus Chimie, 2003, 6, 999-1007

18. George Thomas, K., V. Biju, D.M. Guldi, P.V. Kamat, and M.V. George, Photoinduced Electron Transfer Processes in Fullerene Based Dyads with Heteroaromatic Donors. Chem. Phys. Chem., 2003, 4, 1299-1307.

19. Jakob, M., H. Levanon, and P.V. Kamat, C60/C60

- Redox couple as a probe in the Determination of Fermi Level of Semiconductor Nanoparticles, in Fullerenes-2003, P.V. Kamat, D.M. Guldi, and F. D'Souza, Editors. 2003, ECS, Pennigton, NJ.

20. Hasobe, T., H. Imahori, S. Fukuzumi, and P.V. Kamat, Quaternary Self-Organization of Porphyrin and Fullerene Units by Clusterization with Gold Nanoparticles on SnO2 Electrodes for Organic Solar Cells. J. Am. Chem. Soc., 2003,125 14962-14963.

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Submitted/Acepted for Publication

1. Vinodgopal, K., V. Subramanian, and P.V. Kamat, The Selective Electrochemical Detection of Model Pollutant Species Using Films of Naturally Occurring Humic Acid K. Environ. Sci. & Technol.., 2003, in press.

2. George Thomas, K., S. Barazzouk, B.I. Ipe, S.T. Shibu Joseph, and P.V. Kamat, Unidirectional Plasmon Coupling through Longitudinal Self-assembly of Gold Nanorods. J. Am. Chem. Soc., 2003, submitted.

3. Subramanian, V., E.E. Wolf, and P.V. Kamat, Catalysis with TiO2/Au Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration. J. Am. Chem. Soc., 2003, 125, submitted.

4. Kamat, P.V., R. Huhen, and R. Nicolaescu, Semiconductor Nanostructures For Detection and Degradation Of Low Level Organic Contaminants From Water, in Nanotechnology and the Environment: Applications and Implications, P.Alivisatos, et al., Editors. 2003, The American Chemical Society: Washington, D.C.

5. Kamat, P.V., Photochemistry and Electrochemistry of Nanoassemblies, in Chemistry of Nanomaterials, C. N. R. Rao, A. Muller, and A.K. Cheetham, Editors. 2003, Jon Wiley Interscience: New York.

6. Vinodgopal, K., M. Haria, D. Meisel, and P. Kamat, Fullerene Based Carbon Nanostructures for Methanol Oxidation. Nano Lett., 2004, 4, submitted.

7. Hasobe, T., Y. Kashiwagi, M.A. Absalom, Kohei Hosomizu, M.J. Crossley, H. Imahori, P.V. Kamat, and S. Fukuzumi, Supramolecular Photovoltaic Cells of Porphyrin Dendrimers and Fullerene. Adv. Mater. (Int. Ed.), 2004, submitted.

8. Pillai, Z.S. and P.V. Kamat, What Factors Control the Size and Shape of Silver Nanoparticles in the Citrate Ion Reduction Method. J Phys. Chem. B, 2003, 107, in press.

Monographs/Special Volumes

1. Nanoscopic Materials, Edited by L. Liz-Marzan and P. V. Kamat, Kluwer Academic Publishers, 2003.

2. Fullerenes and NanotubesThe Building Blocks of Next Generation Nanodevices, Fullerenes, Volume 13, Proceeding Volume of the Electrochemical Society, P. V. Kamat, D. M. Guldi, and F. D’Souza, Electrochemical Society, Pennigton, NJ

Congratulations Dr. Julie Peller! (2003 Commencement Ceremony)

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Abstracts of Some Recent Submissions

Acc. Chem. Res., 2003, 36, 888-898. Chromophore Functionalized Gold Nanoparticles. George Thomas, K. and P.V. Kamat,

We report in this account the design of chromophore functionalized metal nanoparticles and the detailed investigation of the ground as well as excited state interactions between the metal nanoparticles and the photoactive molecules. Mechanistic details of various deactivation channels of the photoexcited chromphores, such as energy and electron transfer, to the metal nanoparticle are presented. The methodologies adopted for organizing chromophore functionalized gold nanoparticles on to two-dimensional surfaces and their possible applications in photovoltaics and fluorescent display devices are described.

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J. Am. Chem. Soc., 2003, 125, submitted Catalysis with TiO2/Au Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration.

Subramanian, V., E.E. Wolf, and P.V. Kamat

Photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles. Such a charge distribution has direct influence in dictating the energetics of the composite by shifting the Fermi level to more negative potentials. The transfer of electrons to Au nanoparticles has now been probed by exciting TiO2 nanoparticles under steady state and laser pulse excitation. Equilibration with the C60/C60

− redox couple provides a means to determine the apparent Fermi level of the TiO2-Au composite system. The size-dependent shift in the apparent Fermi level of the TiO2-Au composite (20 mV for 8 nm diameter and 40 mV for 5 nm and 60 mV

for 3 nm gold nanoparticles) shows the ability of Au nanoparticles to influence the energetics by improving the photoinduced charge separation. Isolation of individual charge transfer steps from

UV-excited TiO2 → Au → C60 has provided mechanistic and kinetic information on the role of metal in semiconductor-assisted photocatalysis and size-dependent catalytic activity of metal-semiconductor nanocomposites.

J. Mater. Chem., 2003,13, 2515 - 2520. Nanostructured assembly of porphyrin clusters for light energy conversion. Hasobe, T., H. Imahori, S. Fukuzumi, and P.V. Kamat

Free base porphyrin molecules form well-defined and ordered nanoclusters in mixed solvents with absorption characteristics that significantly differ from those of the monomer form. These self-assembled crystallites in acetonitrile/toluene (9:1, v/v) can be deposited as thin films on nanostructured TiO2 electrode using an electrophoretic technique. These porphyrin cluster assembly is highly photoactive and capable of undergoing charge separation under visible light excitation. Photoexcitation of the porphyrin film electrode assembly in a photoelectrochemical cell with visible light produces relatively high photocurrent generation. A maximum photocurrent of 0.15 mA cm-2 and a photovoltage of 250 mV were attained using I3

−/I− redox couple. The electron flow to TiO2 electrode can be facilitated by application of a positive potential. An incident photon-to-photocurrent generation efficiency (IPCE) of 2.0 % has been achieved at an applied bias potential of 0.2 V vs. SCE. The broad photoresponse of these crystallites throughout the visible range as well as the ease of assembling them on the electrode surface opens up new avenues for harvesting a wide wavelength range of solar light.

Environ. Sci. Technol., 2004, in press. The Selective Electrochemical Detection of Model Pollutant Species Using Films of Naturally Occurring Humic Acid K. Vinodgopal, K., V. Subramanian, and P.V. Kamat

An electroanalytical method has been developed to investigate the uptake of redox active species by the naturally occurring humic acid substances. The Suwannee river humic acid (SHA) films were first cast on a glassy carbon electrode using an electrophoretic approach. The binding of a series of redox active species to these SHA films was then probed using cyclic voltammetry at a rotating disc electrode. Neutral molecules like hydroquinone and cationic species such as methyl viologen are able to bind with the humic membrane and exhibit high redox activity within the film. On the other hand anionic species like ferrocyanide are unable to attach themselves to the SHA films and thus exhibit negligible electrochemical activity. Cyclic voltammetric study of SHA films has also facilitated the determination of the partitioning constants and identifying the effect of coadsorbed ions (Ca2+) on the binding of redox species. The electroanalytical method described in this study opens up new avenues to examine the interactions and transport of charged species in a humic acid environment.

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J. Phys. Chem. B, 2003, 107, 12105 - 12112. Light Energy Harvesting Using Mixed Molecular Nanoclusters. Porphyrin and C60 Cluster Films for Efficient Photocurrent Generation. Hasobe, T., H. Imahori, S. Fukuzumi, and P.V. Kamat

Composite molecular nanoclusters of fullerene and porphyrin prepared in acetonitrile/toluene mixed solvent absorb light over entire spectrum of visible light. Upon slow evaporation of the solvent on the copper grid, these mixed nanoclusters undergo close packed stacking to produce either tubular shaped or square shaped microcrystallites and differ from those obtained from single component clusters. The highly colored composite clusters can be assembled as 3 dimensional array onto nanostructured SnO2 films using electrophoretic deposition approach. The composite cluster films exhibit an incident photon-to-photocurrent efficiency (IPCE) as high as 17 % at

an applied potential of 0.2 V vs. SCE which is significantly higher than the additive effect observed from either porphyrin (IPCE = 1.6 %) or fullerene clusters (IPCE = 5.0 %) under similar photoelectrochemical conditions. The high IPCE observed with porphyrin and C60 clusters demonstrate

the synergy of these systems towards yielding efficient photoinduced charge separation within these composite nanoclusters.

Adv. Mater. 2003, submitted. Supramolecular Photovoltaic Cells of Porphyrin Dendrimers and Fullerene.

Hasobe, T., Y. Kashiwagi, M.A. Absalom, K. Hosomizu, M.J. Crossley, H. Imahori, P.V. Kamat, and S. Fukuzumi

Organic photovoltaic cells using supramolecular complexes of porphyrin dendrimers with fullerene demonstrate remarkable enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible and infrared regions relative to the reference system. This demonstrates that molecular assemblies of C60 and porphyrins with dendric structures are effective in light energy conversion systems.

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J. Phys. Chem. B, 2004, 108, in press What Factors Control the Size and Shape of Silver Nanoparticles in the Citrate Ion Reduction Method? Zeena Pilai and P.V. Kamat We have investigated the early reduction steps using pulse radiolysis to understand the role of sodium citrate towards the growth of particles. Steady state and pulse radiolysis experiments provide evidence for the multiple roles of citrate ions as a reductant, complexant and stabilizer that collectively dictate the size and shape of silver nanocrystallites.

Nano Lett., 2004, 4, submitted. Fullerene Based Carbon Nanostructures for Methanol Oxidation

Vinodgopal, K., M. Haria, D. Meisel, and P. Kamat

Films of C60 clusters were electrodeposited on optically transparent electrode surfaces. These C60 films constitute a new class of carbon electrodes with properties that differ from graphite and diamond electrodes. These films are quite stable to oxidative potentials and hence provide an electrochemical window to carry out oxidation processes. The electrophoretically deposited C60 cluster film is highly porous and consists of C60 clusters that are assembled in orderly fashion. Upon electrodeposition of

platinum particles, these nanostructured carbon films show remarkable activity towards methanol oxidation. The

dependence of methanol oxidation on the amount of platinum and C60 in a half cell reaction demonstrates the role of fullerene nanoclusters as new form of carbon support.

Comptes Rendus Chimie., 2003 6, 999-1007 Nanoscience Opportunities in Environmental Remediation P. V. Kamat and D. Meisel

Semiconductor nanostructures play an important role in developing smart materials that can simultaneously sense and destroy harmful chemical contaminants from our environment. This review article highlights some recent advances of nanoscience in the area of environmental remediation. The potential for improving the effectiveness of photocatalytic processes by means of semiconductor-metal nanocomposites and merits of combining two advanced oxidation processes are discussed.

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10

J. Am. Chem. Soc., 2003, 125, 14962-14963 Quaternary Self-Organization of Porphyrin and Fullerene Units by Clusterization with Gold Nanoparticles on SnO2 Electrodes for Organic Solar Cells. Hasobe, T., H. Imahori, S. Fukuzumi, and P.V. Kamat

Herein we report novel organic solar cells prepared using quaternary self-organization of porphyrin (donor) and fullerene (acceptor) dye units by clusterization with gold nanoparticles on SnO2 electrodes. Remarkable enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible and infrared relative to the reference systems demonstrates that the quaternary organization approach provides novel perspective for the development of efficient organic solar cells.

Chemistry, Eur. J, 2003, 9, 5379-5387. Mechanism of Hydroxyl Radical-Induced Breakdown of the Herbicide 2,4-Dichlorophenoxyacetic Acid J. Peller, O. Wiest and P. V. Kamat

Oxidative transformations by the hydroxyl radical are significant in advanced oxidation processes for the breakdown of organic pollutants, yet mechanistic details of the reactions are lacking. A combination of experimental and computational methods has been employed in this study to elucidate the reactivity of the hydroxyl radical with the widely used herbicide 2,4-D. The experimental data on the reactivity of the hydroxyl radical in the degradation of the herbicide 2,4-D was attained from γ-radiolysis experiments using both 18O-labeled water and unlabeled water. This was complemented by computational studies of the •OH attack on 2,4-D and 2,4-DCP (2,4-dichlorophenol) in gas phase and in solution. These studies established the main reaction pathway of •OH on 2,4-D. In addition, the majority of the early intermediates in the reaction between the hydroxyl radical and 2,4-DCP, the major intermediate, were identified experimentally. While the hydroxyl radical attacks 2,4-D by •OH-addition to the aromatic ring, the oxidative breakdown of 2,4-DCP occurs through both •OH-addition and the formation of the dichlorophenoxyl radical.

Langmuir, 2003. 19, 469-474. Influence of Metal/Metal --Ion Concentration on the Photocatalytic Activity of TiO2-Au Composite Nanoparticles V. Subramanian, E. E. Wolf, P. V. Kamat

Gold capped TiO2 nanocomposite particles have been

synthesized by reducing [AuCl4] on the surface of preformed TiO2 colloids in acetonitrile. Such semiconductor-metal nanocomposites are considered to be beneficial towards improving the efficiency of photocatalytic oxidation process. Whereas photogenerated electrons of TiO2 colloids readily reduce Au(III) ions in deaerated solution, the photogenerated holes are capable of oxidizing the preexisting gold layer on TiO2 particles in aerated solutions. The experimental conditions under which photocatalysis experiments are carried out dictate the net concentration of metal and metal ion species at the semiconductor/metal interface and hence the overall catalytic activity. Using photocatalytic oxidation of SCN as a probe we have investigated the influence of Au0 and Au(III) ions on interfacial hole transfer. The ratio of metal/metal ion concentration (viz., [Au0]/[Au(III)]) at the TiO2 interface was found to dictate the overall photocatalytic process in the semiconductor/metal nanocomposite.

J. Phys. Chem. B 2003, 107, 6668-6697

Charge Transfer on the Nanoscale D. Adams, L. Brus, C. E. D. Chidsey, S. Creager, C. Cruetz, C. R. Kagan, P. V. Kamat, M. Lieberman, S. Lindsay, R. A. Marcus, R. M. Metzger, M. E. Michel-Beyerle, J. R. Miller, M. D. Newton, D. R. Rolison, O. Sankey, K. S. Schanze, J. Yardley, X. Zhu

This is the report of a DOE sponsored workshop organized to discuss the status of our understanding of charge transfer processes on the nanoscale and to identify research and other needs for progress in nanoscience and nanotechnology. The current status of basic electron-transfer research, both theory and experiment, is addressed, with emphasis on the distance dependent measurements, and we have attempted to integrate terminology and notation of solution electron transfer kinetics with that of conductance analysis. The interface between molecules or nanoparticles and bulk metals is examined and new research tools that advance description and understanding of the interface are presented. The state of the art efforts in the area of molecular electronics is summarized along with furture research needs. Finally, novel strategies of nanoscale architectures based on photochemistry, catalysis and electrocatalysis are presented for enhancing the efficiencies of energy conversion.

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Environ. Sci. Technol. 2003, 37, 761-765 Electrophoretic Assembly of Naturally Occurring Humic Substances as Membranes K. Vinodgopal, V. Subramanian, S. Carrasquillo, P. V. Kamat

Nanostructured humic acid membranes on optically transparent electrodes (OTE) have been prepared by electrophoretic deposition from a solution of Suwanee River Humic acid (SHA) in ethanol/acetonitrile. The thickness of the film and the rate of deposition of SHA are dependent on the applied voltage and the concentration of the solution. Tapping mode Atomic Force Microscopy (TM-AFM) confirms the presence of a three-dimensional, porous, assembly of SHA clusters on the electrode surface. The ability of these membranes to incorporate redox-active species such as ferrocene from solution has been demonstrated by cyclic voltammetry experiments. A linear dependence of the peak current for the oxidation of ferrocene as a function of scan rate indicates that the ferrocene is incorporated into the humic membrane.

J. Appl. Phys. 2003, 93, 2575-2582. Photoinduced Transformations at Semiconductor/Metal Interface: EXAFS Investigation of UV-irradiated Au-TiO2 Films

D. L. Dey V. Subramanian, T.Shibata, E. E. Wolf, B. A. Bunker, and P. V. Kamat

Nanostructured TiO2 films modified with gold nanoparticles are photoelectrochemically active under UV-excitation and deliver photocurrent when used as a photoanode in a photoelectrochemical cell. The photogenerated holes are capable of oxidizing gold at the TiO2 interface and cause the overall photoelectrochemical performance of the composite film to decrease. As gold ions generated during photooxidation interact with the TiO2 nanoparticles, the band edge shifts into the visible. X-ray absorption fine structure (XAFS) of pre- and post- UV-irradiated TiO2/Au samples have provided quantitative local structural information for these samples. We have observed a remarkable change of the near neighbor species between these samples. The Au ions which are coordinated mostly with Ti and oxygen atoms prier to UV illumination exhibits clustered gold atom features following UV-illumination.

Environ. Sci. Technol. 2003, 37, 1926-1932. Synergy of Combining Sonolysis and Photocatalysis in the Degradation and Mineralization of Chlorinated Aromatic Compounds J. Peller, O. Wiest, and P. V. Kamat

Merits of using advanced oxidation processes such as sonolysis and photocatalysis as well as a combination of the two have been explored using model herbicides such as 2,4-dichlorophenoxy acetic acid and 2,4-dichlorophenoxypropionic acid and the chlorinated phenols 2,4-dichlorophenol and 2,4,6-trichlorophenol. Whereas

sonolysis is quite effective in the initial degradation of chlorinated aromatic molecules, complete mineralization is difficult to achieve. Photocatalysis is selective towards the degradation of polar aromatics and causes the build up of undesirable chemical intermediates. In contrast to sonolytic degradation, photocatalysis is very effective towards achieving complete mineralization. By simultaneously carrying out high frequency sonolysis and photocatalysis we have succeeded in achieving faster and complete mineralization with no build up of toxic intermediates even at very low catalyst loadings. The synergy of combining the two advanced oxidation processes is discussed.

J. Phys. Chem. A 2003, 107, 427-433. Radical Induced Oxidative Transformation of Quinoline R. Nicolaescu, O. Wiest, P. V. Kamat

The primary events of the oxidative transformation of Quinoline, an environmental pollutant, by different radicals, have been investigated using pulse radiolysis. The hydroxyl radical reacts by adding to both rings of quinoline to form the OH adducts. On the other hand, SO4

•− oxidizes the quinoline to its radical cation, which further undergoes hydrolysis to yield also the OH adducts. The possibility of the quinoline radical cation to undergo hydrolysis was independently verified by generating the radical cation under laser induced photoionization conditions. The quinoline radical cation produced in acetonitrile shows high reactivity towards water. The rate for OH reaction is higher (k = 1 x 1010 M-1s-1) than that found for SO4

•− reaction (k = 3.5 x 109 M-1s-1). Quinoline is much less reactive towards •N3 and •O− radicals. Mechanistic understanding of the primary oxidation pathway of quinoline could aid in designing strategies for the abatement of pollutants containing nitrogen heterocycles.

Nanoscale Materials (Eds.: L. Liz-Marzan, P. V. Kamat), Kluwer Publishers, Boston, 2003, pp 475-494. Assembling Fullerenes as Nanoclusters and Films

P. V. Kamat and K. George Thomas Fullerenes and their derivatives readily form self-

assembled molecular clusters in mixed solvents. These molecular clusters are optically transparent and thermodynamically stable. Furthermore, one can dope these spherical fullerene assemblies with electron donors such as Ferrocene, amines, phenothiazine. The size and shape of these clusters can be altered with structural modifications. These clusters can be readily assembled as 3-dimensional array on a conducting glass electrode. These interesting fullerene nanostructures show unusual electrochemical sensing and light energy conversion properties. Salient features of these nanostructures are described here.

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J. Phys. Chem. B, 2003, 107, 10088-10093

Photoinduced charge transfer between CdSe nanocrystals and p-phenelenediamine. S. Sharma, , Z.S. Pillai, and P.V. Kamat

The excited state interaction between CdSe nanocrystals and a hole acceptor, p-phenylenediamine (PPD), is probed using emission and transient absorption spectroscopies. The changes in the photophysical properties of CdSe nanocrystals arising from the interaction with PPD are compared with an aliphatic amine, n-butylamine (n-BA). At low concentrations (<0.5 mM) n-butylamine enhances the emission of CdSe quantum dots whereas PPD effectively quenches the emission at similar concentrations. The low oxidation potential of PPD (E = 0.26 V vs NHE) enables it to act as an effective scavenger for photogenerated holes. A surface bound complexation equilibrium model has been proposed to explain the quenching phenomenon. The transient absorption measurements confirm the formation of PPD cation radical and subsequent formation of coupling product.

J. Phys. Chem. B, 2003, 107, 10088-10093

Green Emission to Probe Photoinduced Charging Events in ZnO-Au Nanoparticles. Charge Distribution and Fermi-Level Equilibration. Subramanian, V., E.E. Wolf, and P.V. Kamat,

Photoinduced electron accumulation in ZnO nanoparticles results in the bleaching of the exciton band as well as quenching of green emission. In the absence of an electron scavenger, photogenerated electrons are stored near the conduction band edge and promote charge recombination via a nonradiative process. When gold nanoparticles are added to pre-UV-irradiated ZnO colloids, only partial recovery of the emission is seen. Pt nanoparticles on the other hand caused almost complete

recovery of the quenched emission as the electrons are discharged into the solution. The charge distribution between UV-irradiated ZnO and gold nanoparticles results

in equilibration of the Fermi level. Basic understanding of the interaction between the semiconductor and metal layers leading to Fermi-level equilibration is important for evaluating the role of noble metals in photocatalytic reactions.

Ind. Eng. Chem. Res. 2003, 42, 2131-2138. Mass-Transfer and Kinetic Studies during the Photocatalytic Degradation of an Azo Dye on Optically Transparent Electrode Thin Film Subramanian, V., P. V. Kamat and E. E. Wolf,

We have studied the various parameters that affect the kinetics of the degradation process for an azo dye in an aquatic flow system using a novel batch continuous-loop flow reactor with TiO2 colloids immobilized on a flat substrate. A detailed study of physical parameters including light intensity, flow rate, stirring, catalyst loading, dye concentration, and temperature has been performed to obtain the reaction kinetics independent of mass-transport limitations. We have studied the effect of metal nanoparticles on the photocatalytic activity of TiO2 under the optimized conditions of a flow reactor to determine the role of metal nanoparticles in the photocatalytic degradation of AO7

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

TiO2 nanoparticles when subjected to UV irradiation exhibit blue coloration as electrons are stored within the particles. Upon contact with gold nanoparticles, a partial disappearance of the blue color is seen as the stored electrons are transferred from TiO2 to Au nanoparticles. The charge distribution between the semiconductor and metal nanoparticles causes the Fermi level to shift to more negative potentials. By employing C60/C60

- as a probe-redox couple, we were able to estimate the apparent Fermi levels of TiO2 and TiO2/Au nanoparticles. A Fermi level shift of -22 mV observed for the Au-TiO2 nanocomposite is indicative of improved charge separation in semiconductor-metal systems and demonstrates its usefulness for improving the efficiency of photocatalytic reactions.

Chem. Phys. Chem., 2003, 4, 1299-1307 Photoinduced Electron Transfer Processes in Fullerene Based Dyads with Heteroaromatic Donors. George Thomas, K., V. Biju, D.M. Guldi, P.V. Kamat, and M.V. George

Two classes of fullerene-based donor-bridge-acceptor (D-B-A) systems containing donors of varying oxidation potentials have been synthesized. An increase in the rate constant and quantum yield of charge separation (kcs and cs) has been observed for both classes of dyads, with decrease in the oxidation potentials of the donor groups. This observation indicates that the rates of the forward electron transfer fall in the normal region of the Marcus curve. The long-lived charge separation enabled the characterization of electron transfer products, namely, the radical cation of the donor and radical anion of the pyrrolidinofullerene, by using nanosecond transient absorption spectroscopy. The small reorganization energy ( ) of C60 coupled with large negative free energy changes (- G°) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products.

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E-mail: PKAMAT@nd.edu

Website: http://www.nd.edu/~pkamat