Showing papers by "Prashant V. Kamat published in 2006"
TL;DR: By using bifunctional surface modifiers (SH-R-COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO(2) films and exhibits a photon-to-charge carrier generation efficiency of 12%.
Abstract: By using bifunctional surface modifiers (SH−R−COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO2 films. Upon visible light excitation, CdSe QDs inject electrons into TiO2 nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO2 nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 × 109 and 1.95 × 1011 s-1. The injected charge carriers in a CdSe-modified TiO2 film can be collected at a conducting electrode to generate a photocurrent. The TiO2−CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO2/CdSe interfaces and internal TiO2 grain boundaries.
1,738 citations
TL;DR: Improvement in the electrocatalytic activity of Pt particles toward oxygen reduction reaction (ORR) has been achieved by depositing them on a single wall carbon nanotubes (SWCNT) support, which opens up ways to utilize SWCNT/Pt electrocatalyst as a cathode in the proton-exchange-membrane-based hydrogen and methanol fuel cells.
Abstract: Significant enhancement in the electrocatalytic activity of Pt particles toward oxygen reduction reaction (ORR) has been achieved by depositing them on a single wall carbon nanotubes (SWCNT) support. Compared to a commercial Pt/carbon black catalyst, Pt/SWCNT films cast on a rotating disk electrode exhibit a lower onset potential and a higher electron-transfer rate constant for oxygen reduction. Improved stability of the SWCNT support is also confirmed from the minimal change in the oxygen reduction current during repeated cycling over a period of 36 h. These studies open up ways to utilize SWCNT/Pt electrocatalyst as a cathode in the proton-exchange-membrane-based hydrogen and methanol fuel cells.
293 citations
TL;DR: Femtosecond pump-probe spectroscopy experiments confirm the decay of the excited porphyrin in the SWCNT-H4P2+ assembly as it injects electrons intoSWCNT.
Abstract: Photochemical solar cells have been constructed from organized assemblies of single-walled carbon nanotubes (SWCNT) and protonated porphyrin on nanostructured SnO2 electrodes. The protonated form of porphyrin (H4P2+) and SWCNT composites form 0.5−3.0 μm-sized rodlike structures and they can be assembled onto nanostructured SnO2 films [optically transparent electrode OTE/SnO2] by an electrophoretic deposition method. These organized assemblies are photoactive and absorb strongly in the entire visible region. The incident photon to photocurrent efficiency (IPCE) of OTE/SnO2/SWCNT-H4P2+ is ∼13% at an applied potential of 0.2 V versus saturated calomel electrode. Femtosecond pump−probe spectroscopy experiments confirm the decay of the excited porphyrin in the SWCNT−H4P2+ assembly as it injects electrons into SWCNT. The dual role of SWCNT in promoting photoinduced charge separation and facilitating charge transport is presented.
179 citations
TL;DR: Well-dispersed Pt catalysts with very high utilization efficiencies for fuel cell reactions have been prepared by ethylene glycol reduction on polymer-wrapped single-walled carbon nanotubes (SWCNTs) to achieve better dispersion.
Abstract: Well-dispersed Pt catalysts with very high utilization efficiencies for fuel cell reactions have been prepared by ethylene glycol reduction on polymer-wrapped single-walled carbon nanotubes (SWCNTs). By wrapping the SWCNTs in a polymer such as polystyrene sulfonate, we are able to break up the nanotube bundles to achieve better dispersion. These polymer-wrapped SWCNTs with platinum nanoparticles deposited on them show very high electrochemically active surface areas. The increase in utilization efficiencies for platinum catalysts on these SWCNT supports can be attributed to the increased surface areas and the well-dispersed nature of the carbon support and catalyst. The catalyst dispersion facilitates diffusion of reactant species which in turn results in higher methanol oxidation currents and more positive potentials for oxygen reduction.
169 citations
TL;DR: The evaluation of carbon supports in a passive air breathing DMFC indicates that the observed power density depends on the nature and source of carbon nanostructures.
Abstract: Single-wall and multiwall carbon nanotubes are employed as carbon supports in direct methanol fuel cells (DMFC). The morphology and electrochemical activity of single-wall and multiwall carbon nanotubes obtained from different sources have been examined to probe the influence of carbon support on the overall performance of DMFC. The improved activity of the Pt-Ru catalyst dispersed on carbon nanotubes toward methanol oxidation is reflected as a shift in the onset potential and a lower charge transfer resistance at the electrode/electrolyte interface. The evaluation of carbon supports in a passive air breathing DMFC indicates that the observed power density depends on the nature and source of carbon nanostructures. The intrinsic property of the nanotubes, dispersion of the electrocatalyst and the electrochemically active surface area collectively influence the performance of the membrane electrode assembly (MEA). As compared to the commercial carbon black support, single wall carbon nanotubes when employed as the support for anchoring the electrocatalyst particles in the anode and cathode sides of MEA exhibited a approximately 30% enhancement in the power density of a single stack DMFC operating at 70 degrees C.
151 citations
TL;DR: Ultrafast relaxation dynamics of charge carriers in CdSe quantum wires with diameters between 6 and 8 nm are studied as a function of carrier density to define an optimal intensity for achieving optical gain in solution-grown nanowires given the different carrier-density-dependent scaling of relaxation rates.
Abstract: Ultrafast relaxation dynamics of charge carriers in CdSe quantum wires with diameters between 6 and 8 nm are studied as a function of carrier density. At high electron−hole pair densities above 1019 cm-3 the dominant process for carrier cooling is the “bimolecular” Auger recombination of one-dimensional (1D) excitons. However, below this excitation level an unexpected transition from a bimolecular (exciton−exciton) to a three-carrier Auger relaxation mechanism occurs. Thus, depending on excitation intensity, electron−hole pair relaxation dynamics in the nanowires exhibit either 1D or 0D (quantum dot) character. This dual nature of the recovery kinetics defines an optimal intensity for achieving optical gain in solution-grown nanowires given the different carrier-density-dependent scaling of relaxation rates in either regime.
124 citations
119 citations
114 citations
TL;DR: The mechanistic insights obtained in the present study should aid in designing semiconductor nanostructures that will maximize the charge separation efficiency and extend the response of the large band gap semiconductor TiO(2) into visible light regions.
Abstract: TiO(2) thin films prepared by a radio frequency magnetron sputtering (RF-MS) deposition method were found to show an enhanced photoelectrochemical response in the visible light region. By controlling the temperature and the gaseous medium during the deposition step, it was possible to control the properties of these films. The photoelectrochemical behavior of the sputtered TiO(2) thin films was compared with that of a commercial TiO(2) sample, and the sputtered films showed higher incident photon to the charge carrier generation efficiency (IPCE of 12.6% at 350 nm) as well as power conversion efficiency (0.33% at 1.84 mW/cm(2)) than the commercial TiO(2) sample. Femtosecond transient absorption spectroscopy experiments have revealed that a major fraction of photogenerated electrons and holes recombine within a few picoseconds, thus limiting photocurrent generation efficiency. The mechanistic insights obtained in the present study should aid in designing semiconductor nanostructures that will maximize the charge separation efficiency and extend the response of the large band gap semiconductor TiO(2) into visible light regions.
95 citations
TL;DR: In this paper, carbon nanotubes have emerged as new architectures for designing light-harvesting assemblies and they have been used as photoresponsive electrode materials and their role in the conversion of light energy into electricity is discussed.
78 citations
TL;DR: In this paper, the porphyrin-modified TiO2 composite electrodes have efficient light absorbing properties in the visible region, exhibiting the photoactive response under visible light excitation using I 3 − / I − redox couple.
TL;DR: Pulse radiolysis experiments confirm the spectroscopic fingerprint of the cation radical (or "trapped hole") with absorption bands in the 500-600 nm region and the electron transfer from the excited O PE-1 and OPE-2 to benzoquinone as characterized from the transient absorption spectroscopy illustrates the ability of these molecules to shuttle the electrons to acceptor moieties.
Abstract: The rigid rodlike character of phenyleneethynylenes and their ability to communicate charge/excitation energy over long distances have made them useful as molecular linkers in the light energy harvesting assemblies and molecular electronics devices. These linker molecules themselves possess rich photochemistry as evident from the relatively large yields of the excited singlet (0.5−0.66) and triplet (0.4−0.5) states of two model oligomers, 1,4-bis(phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-1) and 1,4-bis((4-phenylethynyl)phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-2). In particular, the long-lived triplet excited state is capable of undergoing deactivation by self-quenching processes such as ground-state quenching and triplet−triplet (T−T) annihilation. The T−T annihilation occurs with a nearly diffusion-controlled rate (∼2 × 109 M-1 s-1), and ground-state quenching occurs with a rate constant of ∼6 × 107 M-1 s-1. The electron transfer from the excited OPE-1 and OPE-2 to benzoquinone as characterized fr...
TL;DR: The close proximity of the chromophores on periphery of the gold core may lead to an electron transfer reaction and the products sustained for several nanoseconds before undergoing recombination, probably due to the stabilizing effect of the polar ethylene glycol moieties embedded between the Chromophore groups.
Abstract: Design of nanohybrid systems possessing several ruthenium trisbipyridine (Ru(bpy)32+) chromophores on the surface of gold nanoparticles, by adopting a place exchange reaction, was reported and their photophysical properties were tuned by varying the density of chromophores. The charge shift between the excited and ground-state Ru(bpy)32+ chromophores was reported for the first time, leading to the formation of Ru(bpy)3+ and Ru(bpy)33+. Electron-transfer products were not observed on decreasing the concentration of Ru(bpy)32+ functionalized on Au nanoparticles or in a saturated solution of unbound chromophores. The close proximity of the chromophores on periphery of the gold core may lead to an electron transfer reaction and the products sustained for several nanoseconds before undergoing recombination, probably due to the stabilizing effect of the polar ethylene glycol moieties embedded between the chromophore groups.
TL;DR: The absorption of 4,5-benzoindotricarbocyanine dye (IR125) in the infrared can be tuned by controlling the type of aggregation in different media and implications of such dye cluster films for harvesting infrared photons in a photoelectrochemical cell is discussed.
Abstract: The absorption of 4,5-benzoindotricarbocyanine dye (IR125) in the infrared can be tuned by controlling the type of aggregation in different media. Molecular clusters of this dye formed in a mixed solvent show broad absorption in the 550−950 nm region as compared to the absorption bands of J- and H-type aggregates. The molecular clusters of the carbocyanine dye are electrophoretically deposited as thin film on optically transparent electrodes using a dc electric field. These tricarbocyanine dye cluster films are photoactive in the infrared region and produce cathodic current when employed as photocathode in a photoelectrochemical cell. Transient absorption spectroscopy of the molecular clusters show short-lived singlet state in the picosecond time scale (lifetime 6 ps) and a charge separated state in the nanosecond time scale. Implication of such dye cluster films for harvesting infrared photons in a photoelectrochemical cell is discussed.
TL;DR: The structural transformations observed with Pt and Ir nanoparticles anchored on TiO2 is different than the clustering of gold atoms observed in theTiO2/Au system, and implications of such structural transformations on the photocatalytic activity of semiconductors are discussed.
Abstract: Structural transformations at the TiO2Pt and TiO2Ir interfaces during UV-irradiation have been probed by X-ray absorption spectroscopy. Oxidation by the photogenerated holes results in the intercalation of Pt and Ir into the Titania matrix. The structural transformations observed with Pt and Ir nanoparticles anchored on TiO2 is different than the clustering of gold atoms observed in the TiO2/Au system. Implications of such structural transformations on the photocatalytic activity of semiconductor photocatalyts are discussed.
TL;DR: In this article, the structure and activity of Pt nanoparticles anchored on two nanostructured carbon supports, C60 and carbon nanotubes (CNTs), were compared in direct methanol fuel cells.
Abstract: Comparison of the structure and activity of Pt nanoparticles anchored on two nanostructured carbon supports, C60 and carbon nanotubes (CNTs) provides insight into their electrocatalytic activity in direct methanol fuel cells. The local structure of platinum atoms during the initial stages of the catalytic oxidation of methanol was probed using x-ray absorption spectroscopy. A large fraction of the Pt atoms in the Pt–C60 nanocomposite continuously undergoes structural changes during the initial stages of methanol oxidation. The Pt-CNT system, however, proves to be more robust in maintaining its initial morphology and higher electrocatalytic activity. These observations reflect the importance of the carbon support in controlling the catalyst morphology and activity during methanol oxidation.
TL;DR: Carbon nanotubes, fullerenes, and mesoporous carbon structures constitute a new class of carbon nanomaterials with properties that differ signifi cantly from other forms of carbon such as graphite and diamond as discussed by the authors.
Abstract: Carbon nanotubes, fullerenes, and mesoporous carbon structures constitute a new class of carbon nanomaterials with properties that differ signifi cantly from other forms of carbon such as graphite and diamond. The ability to custom synthesize nanotubes with attached functional groups or to assemble fullerene (C60 and analogues) clusters into three-dimensional (3D) arrays has opened up new avenues to design high surface area catalyst supports and materials with high photochemical and electrochemical activity. Unlike the conventional graphite phase, carbon nanostructures possess metallic or semiconductor properties that can induce catalysis by participating directly in the charge transfer process. Further, the electrochemical properties of these materials facilitate modulation of their charge transfer properties and aid in the design of catalysts for hydrogenation, sensors, and fuel cells.
TL;DR: In this article, a phase-segregated network is proposed for efficient photoinduced electron transfer at the donor-acceptor interface, and intimate mixing of donor and acceptor is therefore beneficial for efficient charge separation.
Abstract: T he rapid consumption of fossil fuels has created unacceptable environmental problems such as greenhouse effects, which may lead to disastrous climatic consequences. Thus, renewable and clean energy such as that obtained by using solar cells is required to maintain the quality of human life as well as the environment. Progress is being made in the development of heterojunction organic solar cells, which possess an active layer of a conjugated donor polymer and an acceptor fullerene.1 In these polymer blends, efficient photoinduced electron transfer occurs at the donor-acceptor interface, and intimate mixing of donor and acceptor is therefore beneficial for efficient charge separation.1,2 For efficient transport of the positive charge carriers through the donor phase and of electrons via the acceptor phase to the electrodes, a phase-segregated network is required.
TL;DR: In this article, the porphyrin-modified TiO2 composite electrodes have efficient light absorbing properties in the visible region, exhibiting the photoactive response under visible light excitation using I 3 − / I − redox couple.
Abstract: TiO2 nanoparticles were modified with porphyrin derivatives, 5-[4-benzoic acid]-10,15,20-tris[3,5-di-tert-butylphenyl]-21H,23H-porphyrin ( Ar–H2P–COOH ), 5-[4-benzoic acid]-10,20-tris[3,5-di-tert-butylphenyl]-21H,23H-porphyrin ( H–H2P–COOH ), and 5,10,15,20-tetra[4-benzoic acid]-21H,23H-porphyrin ( H2P–4COOH ). The porphyrin-modified TiO2 nanoparticles were deposited on nanostructured OTE/SnO2 electrode together with nanoclusters of fullerene (C60) in acetonitrile–toluene (3/1, v/v) using an electrophoretic deposition technique to afford the porphyrin-modified TiO2 composite electrode denoted as OTE/SnO2/(porphyrin-modified TiO2 nanoparticle+C60)n. The porphyrin-modified TiO2 composite electrodes have efficient light absorbing properties in the visible region, exhibiting the photoactive response under visible light excitation using I 3 − / I − redox couple. The incident photon-to-photocurrent efficiency (IPCE) values of supramolecular nanostructured electrodes of porphyrin-modified TiO2 nanoparticles with fullerene [OTE/SnO2/( Ar–H2P–COO–TiO2 +C60)n, OTE/SnO2/( H–H2P–COO–TiO2 +C60)n, and OTE/SnO2/( H2P–4COO–TiO2 +C60)n] are much larger than those of the reference systems of porphyrin-modified TiO2 nanoparticles without C60 [OTE/SnO2/( Ar–H2P–COO–TiO2 )n, OTE/SnO2/( H–H2P–COO–TiO2 )n, and OTE/SnO2/( H2P–4COO–TiO2 )n]. In particular, the maximum IPCE value (41%) is obtained for OTE/SnO2/( H–H2P–COO–TiO2 +C60)n under the bias potential of 0.2 V versus SCE. This indicates that the formation of supramolecular complexes between porphyrins and fullerene on TiO2 nanoparticles plays an important role in improvement of the light energy conversion properties.
17 Feb 2006
TL;DR: In this paper, the influence of charging effect on the excited state behavior of chromophores on metal nanoparticles was investigated and used to improve the photoinduced charge separation efficiency of donor-acceptor type light harvesting assembly.
Abstract: Nanocluster-molecular assemblies provide innovative strategies for designing next generation nanodevices. Of particular interest are photoresponsive organic-inorganic hybrids which when assembled as twoor threedimensional architectures provide novel materials with tailored optical and photochemical properties [1-2]. Photoinduced electron transfer between gold nanoparticles and the chromophores has been investigated by femtosecond transient absorption spectroscopy as well as photoelectrochemical measurements. The electron transfer between the excited chromophore and the gold nanoparticle can be modulated by controlling externally applied potential. As the metal particles are charged, a decrease in the rate of electron transfer is seen. The influence of charging effect on the excited state behavior of chromophores on metal nanoparticles will be presented. Use of metal nanoparticles in improving the photoinduced charge separation efficiency of donoracceptor type light harvesting assembly will also be discussed.