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Prashant V Kamat

Publications and source records attributed to Prashant V Kamat.

34 records · Page 2Linked to original sources
PubMed2005

Charge separation and catalytic activity of Ag@TiO2 core-shell composite clusters under UV-irradiation.

Photocatalytic properties of Ag@TiO2 composite clusters have been investigated using steady state and laser pulse excitations. Photoexcitation of TiO2 shell results in accumulation of the electrons in the Ag core as evidenced from the shift in the surface plasmon band from 460 to 420 nm. The stored electrons are discharged when an electron acceptor such as O2, thionine, or C60 is introduced into the system. Charge equilibration with redox couple such as C60/C60*- shows the ability of these core shell structures to carry out photocatalytic reduction reactions. The charge separation, charge storage, and interfacial charge-transfer steps that follow excitation of the TiO2 shell are discussed.

Journal Article↗
PubMed2005

Photovoltaic cells using composite nanoclusters of porphyrins and fullerenes with gold nanoparticles.

Novel organic solar cells have been prepared using quaternary self-organization of porphyrin (donor) and fullerene (acceptor) units by clusterization with gold nanoparticles on nanostructured SnO2 electrodes. First, porphyrin-alkanethiolate monolayer-protected gold nanoparticles (H2PCnMPC: n is the number of methylene groups in the spacer) are prepared (secondary organization) starting from the primary component (porphyrin-alkanethiol). These porphyrin-modified gold nanoparticles form complexes with fullerene molecules (tertiary organization), and they are clusterized in acetonitrile/toluene mixed solvent (quaternary organization). The highly colored composite clusters can then be assembled as three-dimensional arrays onto nanostructured SnO2 films to afford the OTE/SnO2/(H2PCnMPC+C60)m electrode using an electrophoretic deposition method. The film of the composite clusters with gold nanoparticle exhibits an incident photon-to-photocurrent efficiency (IPCE) as high as 54% and broad photocurrent action spectra (up to 1000 nm). The power conversion efficiency of the OTE/SnO2/(H2PC15MPC+C60)m composite electrode reaches as high as 1.5%, which is 45 times higher than that of the reference system consisting of the both single components of porphyrin and fullerene.

Journal Article↗
PubMed2005

Photoinduced electron transfer between chlorophyll a and gold nanoparticles.

Excited-state interactions between chlorophyll a (Chla) and gold nanoparticles have been studied. The emission intensity of Chla is quenched by gold nanoparticles. The dominant process for this quenching has been attributed to the process of photoinduced electron transfer from excited Chla to gold nanoparticles, although because of a small overlap between fluorescence of Chla and absorption of gold nanoparticles, the energy-transfer process cannot be ruled out. Photoinduced electron-transfer mechanism is supported by the electrochemical modulation of fluorescence of Chla. In absence of an applied bias, Chla cast on gold film, as a result of electron transfer, exhibits a very weak fluorescence. However, upon negatively charging the gold nanocore by external bias, an increase in fluorescence intensity is observed. The negatively charged gold nanoparticles create a barrier and suppress the electron-transfer process from excited Chla to gold nanoparticles, resulting in an increase in radiative process. Nanosecond laser flash experiments of Chla in the presence of gold nanoparticles and fullerene (C60) have demonstrated that Au nanoparticles, besides accepting electrons, can also mediate or shuttle electrons to another acceptor. Taking advantage of these properties of gold nanoparticles, a photoelectrochemical cell based on Chla and gold nanoparticles is constructed. A superior performance of this cell compared to that without the gold film is due to the beneficial role of gold nanoparticles in accepting and shuttling the photogenerated electrons in Chla to the collecting electrode, leading to an enhancement in charge separation efficiency.

Chlorophyll↗
PubMed2005

Enhancement of light-energy conversion efficiency by multi-porphyrin arrays of porphyrin-peptide oligomers with fullerene clusters.

Organic photovoltaic cells using supramolecular complexes of porphyrin-peptide oligomers (porphyrin-functionalized alpha-polypeptides) with fullerene demonstrate remarkable enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible and near-infrared regions by increasing the number of porphyrin units in alpha-polypeptide structures. A high power conversion efficiency (eta) of 1.3% and a maximum incident photon-to-photocurrent efficiency (IPCE) of 42% were attained using composite clusters of porphyrin-peptide octamer and fullerene. These results clearly show that the formation of a molecular assembly between fullerene and multi-porphyrin arrays with a polypeptide backbone controls the electron transfer efficiency in the supramolecular complex, which is essential for the light-energy conversion.

Electrochemistry↗
PubMed2004

Photoelectrochemical properties of supramolecular composite of fullerene nanoclusters and 9-mesityl-10-carboxymethylacridinium ion on SnO2.

[structure: see text] The photovoltaic cell composed of both fullerene nanoclusters and 9-mesityl-10-carboxymethylacridinium ion exhibits significant enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible and near-infrared regions as compared with the reference system containing only each component.

Journal Article↗
PubMed2004

Self-assembled linear bundles of single wall carbon nanotubes and their alignment and deposition as a film in a dc field.

A one-step process of solubilization of single wall carbon nanotubes (SWCNT) in an organic solvent has enabled us to polarize them asymmetrically in a dc electric field. Quaternary ammonium ion-capped SWCNTs readily suspend in organic solvents; under the influence of a dc electric field, they assemble as stretched bundles anchored on the positive electrode. At low dc applied field (approximately 40 V), all of the SWCNTs from the suspension are deposited on the electrode, thus providing a simple methodology to design robust SWCNT films. At higher applied voltages (>100 V), the SWCNT bundles stretch out into the solution and orient themselves perpendicular to the electrode surface. The alignment of these bundles is responsive to the ON-OFF cycles of the applied electric field. The possibility of modulating the alignment of SWCNT in an electric field opens new ways to achieve electrical contacts in nano- to micro-devices.

Journal Article↗
PubMed2004

Photoinduced electron storage and surface plasmon modulation in Ag@TiO2 clusters.

The reversible charging and discharging effects associated with photoexcitation of a TiO2 shell in a Ag@TiO2 composite are described. The photoinduced charge separation in the TiO2 shell is followed by electron injection into the silver core. Interestingly, the charging of the silver core is associated with the shift in the surface plasmon band from 460 to 430 nm. The stored electrons are discharged upon exposure of the charged Ag@/TiO2 composite to an electron acceptor. As the electrons from the silver core are discharged, the original surface plasmon absorption of the Ag core is restored.

Absorption↗
PubMed2004

Catalysis with TiO2/gold nanocomposites. Effect of metal particle size on the Fermi level equilibration.

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 TiO(2) nanoparticles under steady-state and laser pulse excitation. Equilibration with the C(60)/C(60)(-) redox couple provides a means to determine the apparent Fermi level of the TiO(2)-Au composite system. The size-dependent shift in the apparent Fermi level of the TiO(2)-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 TiO(2) --> Au --> C(60) has provided mechanistic and kinetic information on the role of metal in semiconductor-assisted photocatalysis and size-dependent catalytic activity of metal-semiconductor nanocomposites.

Journal Article↗
PubMed2004

Selective electrochemical detection of ionic and neutral species using films of Suwannee River humic acid.

An electroanalytical method has been developed to investigate the uptake of redox-active species by the 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 disk electrode. Neutral molecules such as 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 such as 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 identification of 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.

Electrochemistry↗
PubMed2003

Dynamics of photoinduced electron-transfer processes in fullerene-based dyads: effects of varying the donor strength.

Two classes of fullerene-based donor-bridge-acceptor (D-B-A) systems containing donors of varying oxidation potentials have been synthesized. These systems include fullerenes linked to heteroaromatic donor groups (phenothiazine/phenoxazine) as well as substituted anilines (p-anisidine/p-toluidine). In contrast to the model compound, an efficient intramolecular electron transfer is observed from the fullerene singlet excited state in polar solvents. An increase in the rate constant and quantum yield of charge separation (kcs and phi 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 (lambda) of C60 coupled with large negative free energy changes (-delta G degree) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products.

Journal Article↗
PubMed2003

Mechanism of hydroxyl radical-induced breakdown of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-d).

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 (2,4-dichlorophenoxyacetic acid). The experimental data on the reactivity of the hydroxyl radical in the degradation of the herbicide 2,4-D were obtained from gamma-radiolysis experiments with both (18)O-labeled and unlabeled water. These were complemented by computational studies of the (.)OH attack on 2,4-D and 2,4-DCP (2,4-dichlorophenol) in the gas phase and in solution. These studies firmly established the kinetically controlled attack ipso to the ether functionality as the main reaction pathway of (.)OH and 2,4-D, followed by homolytic elimination of the ether side chain. 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/elimination on the aromatic ring, the oxidative breakdown of 2,4-DCP occurs through (.)OH addition followed by either elimination of chlorine or formation of the ensuing dichlorophenoxyl radical.

2,4-Dichlorophenoxyacetic Acid↗
PubMed2003

Synergy of combining sonolysis and photocatalysis in the degradation and mineralization of chlorinated aromatic compounds.

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 toward the degradation of polar compounds but causes the build up of undesirable chemical intermediates. In contrast to sonolytic degradation, photocatalysis is very effective toward 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.

Environmental Pollution↗
PubMed2003

Electrophoretic assembly of naturally occurring humic substances as thin films.

Polydisperse humic acid thin films on optically transparent electrodes (OTEs) 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 assembly of SHA aggregates on the electrode surface. The ability of these thin films to incorporate redox-active species such as ferrocene from solution is 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.

Electrochemistry↗
PubMed2003

Chromophore-functionalized gold nanoparticles.

We report in this Account the design of chromophore-functionalized metal nanoparticles and the detailed investigation of the ground- and excited-state interactions between the metal nanoparticles and the photoactive molecules. The methodologies adopted for organizing chromophore-functionalized gold nanoparticles on two-dimensional surfaces and the mechanistic details of various deactivation channels of the photoexcited chromophores, such as energy and electron transfer to the metal nanoparticle, are presented. The possible applications of such chromophore-functionalized gold nanoparticles in photovoltaics, light-mediated binding and release of biologically important molecules such as amino acid derivatives, and fluorescent display devices are described.

Binding Sites↗