Showing papers by "Prashant V. Kamat published in 2004"
TL;DR: The size-dependent shift in the apparent Fermi level of the TiO(2)-Au composite shows the ability of Au nanoparticles to influence the energetics by improving the photoinduced charge separation.
Abstract: 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 siz...
1,896 citations
TL;DR: In this paper, the difference absorption spectra of SiO2−Ag colloids in the presence of citrate ions show an increase in the absorption at 410 nm with increase in concentration ofcitrate.
Abstract: Citrate ion, a commonly used reductant in metal colloid synthesis, undergoes strong surface interaction with silver nanocrystallites. The slow crystal growth observed as a result of the interaction between the silver surface and the citrate ion makes this reduction process unique compared to other chemical and radiolytic synthetic methods. Addition of citrate ions to preformed silver colloids (Ag-capped SiO2) results in the complexation of citrate with silver colloids. The difference absorption spectra of SiO2−Ag colloids in the presence of citrate ions show an increase in the absorption at 410 nm with increase in concentration of citrate. The apparent association constant as determined from these absorption changes is 220 M-1. Pulse-radiolysis studies show that citrate ions complex with the silver seeds and influence the particle growth. For example, one of the primary intermediates, Ag2+ produced in the radiolytic reduction of silver ions, readily interacts with citrate to form a complex absorbing in th...
719 citations
TL;DR: In this paper, a new paradigm for tuning the optical properties of gold nanorods by organizing them longitudinally, using thioalkylcarboxylic acid based bifunctional molecules is reported.
Abstract: We report a new paradigm for tuning the optical properties of gold nanorods by organizing them longitudinally, using thioalkylcarboxylic acid based bifunctional molecules. The rationale behind the selection of the bifunctional molecule is based on the fact that the thiol group binds to the ends of the nanorods, which further assembles in a longitudinal fashion through cooperative hydrogen bonding between the carboxylic groups. A generalized procedure for uniaxial plasmon coupling through longitudinal self-assembly of Au nanorods, initially to dimers and further to linear assemblies, is presented. Uniaxial modulation of interplasmon coupling through stepwise self-assembly of Au nanorods will have application in nanoelectronics and plasmonics.
427 citations
TL;DR: In this paper, a reproducible way to obtain films of varying amounts of single-walled carbon nanotubes (SWCNTs) on electrode surfaces using electrophoretic deposition is presented.
Abstract: We show here, for the first time, a reproducible way to obtain films of varying amounts of single-walled carbon nanotubes (SWCNTs) on electrode surfaces using electrophoretic deposition. We deposit these nanotubes in a facile manner on an optically transparent electrode (OTE) and investigate its performance as an electrode material in the presence of platinum for methanol oxidation and oxygen reduction. Our focus here is on the deposition of the SWCNT on the electrodes, the characterization of the nanotubes on the electrode surface, and the cyclic voltammetry of methanol oxidation and oxygen reduction using these nanostructured carbon electrodes with platinum electrodeposited on them. The nanotubes retain their structure on the electrode surface, and we can obtain electrodes with relatively thick films of the CNTs. The high surface area and porosity of these films enable us to use relatively small amounts of platinum and yet obtain excellent currents. We see a remarkable enhancement in methanol oxidation ...
333 citations
TL;DR: The reversible charging and discharging effects associated with photoexcitation of a TiO2 shell in a Ag@TiO2 composite are described and the charging of the silver core is associated with the shift in the surface plasmon band from 460 to 430 nm.
Abstract: 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.
288 citations
TL;DR: The possibility of modulating the alignment of SWCNT in an electric field opens new ways to achieve electrical contacts in nano- to micro-devices.
Abstract: 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-cap...
255 citations
TL;DR: In this paper, a low photon-to-current conversion efficiency of 1.5% suggests that most of the photogenerated charge carriers are lost in the recombination process, and transient absorption experiments confirm the charge separation following the laser-pulse excitation of SWCNTs.
Abstract: Electrophoretically deposited single-wall carbon nanotube (SWCNT) films on optically transparent electrodes are photoelctrochemically active and generate photocurrent upon visible excitation A low photon-to-current conversion efficiency of 015% suggests that most of the photogenerated charge carriers are lost in the recombination process Time-resolved transient absorption experiments confirm the charge separation following the laser-pulse excitation of SWCNTs Relaxation of photogenerated charge carriers in the SWCNT to the lowest energy gap occurs in ∼1 ps
151 citations
TL;DR: In this article, a redox-active molecular assembly was employed as an electron relay that greatly minimizes the interaction with the excited dye while maintaining the effectiveness of the regeneration cycle.
Abstract: The interaction between the excited sensitizer and the redox couple in a photochemical solar cell is an important factor that can decrease the photon-conversion efficiency. We have now employed C60 clusters to separate the Ru(bpy)2(dcbpy)2+ (Ru(II) complex) and I3-/I- couple to minimize the sensitizer−redox couple interactions. The C60-modified electrodes (viz., OTE/SnO2/Ru(II)/C60 and OTE/TiO2/Ru(II)/C60) delivered photocurrent with greater efficiency than did the SnO2 and TiO2 films modified with only a Ru(II) dye (viz., OTE/SnO2/Ru(II) and OTE/TiO2/Ru(II)). The luminescence quenching of Ru(II)* by I3-, which occurs with a rate constant of 1.9 × 1010 M-1 s-1, is suppressed following the deposition of a layer of C60 clusters. This paper presents a novel concept of employing a redox-active molecular assembly as an electron relay that greatly minimizes the interaction with the excited dye while maintaining the effectiveness of the regeneration cycle.
148 citations
145 citations
TL;DR: In this article, a composite molecular nanoclusters of dendritic porphyrin and fullerene were used for organic photovoltaic cells, which absorb light over entire spectrum of visible light.
Abstract: Novel organic solar cells have been prepared using molecular clusters of porphyrin dendrimer (donor) and fullerene (acceptor) dye units assembled on SnO2 electrodes. The molecular clusters of porphyrin with dendritic structure and fullerene exhibit controlled size and shape in contrast with the reference systems (a porphyrin dimer and a porphyrin−fullerene dyad) without dendritic structure in TEM images, which show rather irregular and smaller clusters. The composite molecular nanoclusters of dendritic porphyrin and fullerene prepared in acetonitrile/toluene mixed solvent absorb light over entire spectrum of visible light. The comparison of photoelectrochemical properties of composite molecular cluster of porphyrin and fullerene with that of molecular cluster of porphyrin−C60 dyad with covalent linkage shows the importance of interpenetrating structure in each network to transport hole and electron efficiently. Furthermore, organic photovoltaic cells using clusters of supramolecular complexes of V-shaped ...
144 citations
TL;DR: In this article, the role of the hydroxyl radical in the breakdown of the common herbicide 2,4-D (2,4dichlorophenoxyacetic acid), its breakdown product 2, 4-DCP (2.4-dichlormyl-phospholophenol), and related compounds is elucidated through the determination of degradation rates, analyses of the transformation intermediates, and studies using computational chemistry methods.
Abstract: The massive use of herbicides in the agricultural industry has led to measurable levels in natural bodies of water, many of which supply the human population with fresh drinking water. Advanced oxidation processes such as photocatalysis, sonolysis, and radiolysis show promise in eliminating the dangers of exposure to herbicides and the products of their natural breakdown. A basic understanding of the mechanistic details involved in the oxidative transformations remains the key for improving the effectiveness of the advanced oxidation processes. The role of the hydroxyl radical in the breakdown of the common herbicide 2,4-D (2,4-dichlorophenoxyacetic acid), its breakdown product 2,4-DCP (2,4-dichlorophenol), and related compounds is elucidated through the determination of degradation rates, analyses of the transformation intermediates, and studies using computational chemistry methods.
TL;DR: In this article, a fullerene nanoclusters were electrophoretically deposited on optically transparent electrode surfaces to provide an electrochemical window to carry out methanol oxidation.
Abstract: Films of C60 clusters were electrophoretically deposited on optically transparent electrode surfaces. These C60 films constitute a new class of carbon electrodes with properties that differ from graphite and diamond electrodes. The electrophoretically deposited C60 cluster film is highly porous and is quite stable to oxidative potentials. Hence C60 film provides an electrochemical window to carry out oxidation processes. Upon electrodeposition of platinum particles, these nanostructured carbon films show remarkable activity toward 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.
TL;DR: In this paper, the design of chromophore-functionalized metal nanoparticles and the detailed investigation of the ground and excited-state interactions between the metal particles and the photoactive molecules are reported.
Abstract: 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.
TL;DR: 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.
TL;DR: In this paper, a number of porphyrin molecules self-organized on three-dimensional gold nanoclusters to form monolayer-protected gold nanocusters (MPCs) that act as an efficient photocatalyst for the uphill reduction of HV2+ by BNAH to produce 1-benzylnicotinamidinium ion (BNA+) and hexyl viologen radical cation (HV·+).
Abstract: π-Complex formation between porphyrins and their radical cations plays an important role in self-exchange electron transfer between neutral porphyrins and the radical cations, leading to negative activation enthalpies when the stabilization energy of the π-complex is larger than the activation energy for the intracomplex electron transfer in the π-complex. A number of porphyrin molecules are self-organized on three-dimensional gold nanoclusters to form monolayer-protected gold nanoclusters (MPCs) that act as an efficient photocatalyst for the uphill reduction of HV2+ by BNAH to produce 1-benzylnicotinamidinium ion (BNA+) and hexyl viologen radical cation (HV·+). Such three-dimensional architectures of porphyrin MPCs with large surface area allow supramolecular π-complexation between MPCs and HV2+, resulting in fast electron transfer from the singlet excited state of porphyrin to HV2+ on MPCs. The π-π interaction is exploited to develop efficient photovoltaic devices consisting of porphyrin and fullerene assemblies which have an enhanced light-harvesting efficiency throughout the solar spectrum together with a highly efficient conversion of the harvested light into electrical energy.
TL;DR: An electroanalytical method has been developed to investigate the uptake of redox-active species by the humic acid substances and opens up new avenues to examine the interactions and transport of charged species in a humic Acid environment.
Abstract: 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 interaction...
01 Jan 2004
TL;DR: In this article, the authors proposed a method to fabricate almost perfect and uniformly shaped nanotubular crystals, which order spontaneously by self-assembly of derivatized fullerenes.
Abstract: Design of molecular systems, which can spontaneously self-assemble and generate three dimensionally extended structures such as clusters is of interest in “chemical nanotechnology”. Of particular interest is the utilization of fullerene materials for developing next generation nanodevices. Recent studies on molecular systems based on particularly the dyads and triads, have renewed the interest towards building photovoltaic systems and mimicking photosynthesis. Organizing molecules as nanoclusters or binding them to inorganic nanoparticles serve as important building blocks in developing new generation nanodevices. Assembly of such nanostructures in an orderly fashion would assist in designing optoelectronic nanodevices that can perform specific functions such as light induced energy and electron transfer processes. Thus, designing strategies to obtain robust films of carbon nanostructures on desired electrode surfaces with a well-controlled morphology remains a major challenge. Fullerenes and their derivatives readily form self-assembled molecular clusters in mixed solvents. These molecular clusters are optically transparent and thermodynamically stable. Recent studies have demonstrated interesting photochemical and photoelectrochemical properties of fullerene-based cluster systems. Furthermore, one can dope these spherical fullerene assemblies with electron donors such as ferrocene, amines, phenothiazine. The local concentration of electron donors is much higher in these clusters. Formation of long lived electron transfer products, following the photoexcitation of the fullerene cluster and various electron donors were confirmed through flash photolysis studies. The size and shape of these clusters can be altered with structural modifications. These clusters can be readily assembled as 3dimensional arrays on a conducting glass electrode. An efficient method has also been reported to fabricate almost perfect and uniformly shaped nanotubular crystals, which order spontaneously by self-assembly of derivatized fullerenes. These interesting fullerene nanostructures show unusual electrochemical sensing and light energy