Showing papers by "Prashant V. Kamat published in 2002"

Photophysical, photochemical and photocatalytic aspects of metal nanoparticles

Abstract: Unique electronic and chemical properties of metal nanoparticles have drawn the attention of chemists, physicists, biologists, and engineers who wish to use them for the development of new generation nanodevices. Metal nanoparticles such as gold and silver show noticeable photoactivity under UV−visible irradiation as is evident from the photoinduced fusion and fragmentation processes. Binding a photoactive molecule (e.g., pyrene) to metal nanoparticle enhances the photochemical activity and renders the organic−inorganic hybrid nanoassemblies suitable for light-harvesting and optoelectronic applications. The nature of charge-transfer interaction of fluorophore with gold surface dictates the pathways with which the excited-state deactivates. Obtaining insight into energy and electron-transfer processes is important to improve the charge separation efficiencies in metal−fluorophore nanoassemblies and photocatalytic activity of metal−semiconductor composites.

A “Sense and Shoot” Approach for Photocatalytic Degradation of Organic Contaminants in Water

Abstract: A “sense and shoot” approach has been developed for the photocatalytic degradation of organic contaminants from aqueous solutions. The presence of aromatic compounds such as chlorinated phenols is readily detected from the quenching of visible emission of ZnO semiconductor film. The emission quenching is quantitative and can be analyzed on the basis of adsorption equilibrium between ZnO and organic molecules. High apparent association constant values (Ka = (1−5) × 104 M-1) make possible the detection of these organic molecules at very low concentration levels. For example, 1 ppm of 4-chlorocatechol in water results in a 15% decrease of ZnO emission. Interestingly, the same ZnO film is capable of degrading aromatic compounds present in water under UV irradiation. The concept of adopting a strategy of simultaneous sensing and degradation in photocatalysis is illustrated in this paper.

Interparticle electron transfer between size-quantized CdS and TiO2 semiconductor nanoclustersDedicated to Professor Frank Wilkinson on the occasion of his retirement.

Abstract: Photoinduced electron transfer in a size-quantized CdS/TiO2 composite system has been investigated using emission and transient absorption spectroscopy. Quantum-sized CdS and TiO2 particles were synthesized in reverse micelles using di-octyl sulfosuccinate (Aerosol-OT, AOT) as the surfactant stabilizer. The particle sizes of CdS and TiO2 were controlled by varying water-to-surfactant molar ratio, wo = [H2O]/[AOT], with values of 1, 2.5, 5 and 10. The blue-shift in the absorption onset confirmed size-quantization of these semiconductor particles. Electron transfer from photoexcited CdS to TiO2 was found to depend on the particle size of TiO2, where charge transfer was observed only when TiO2 particles were sufficiently large (>12 A). Interactions with smaller size TiO2 particles (⩽10 A) with CdS instead led to enhancements in emission with an increase in quantum yield from 2.3% to 8.8%. Picosecond laser flash photolysis experiments have been carried out to elucidate the interparticle electron transfer processes in the CdS/TiO2 reverse micellar system.

Photoinduced Charge Separation in a Fluorophore−Gold Nanoassembly

Abstract: We report the first spectroscopic demonstration of direct electron transfer between a gold nanoparticle and a surface-bound fluorophore induced by pulsed laser irradiation. Binding of pyrene thiol directly to the gold nanoparticle results in quenching of its singlet excited state. The suppression of S1−T1 intersystem crossing process as well as the formation of pyrene radical cation confirm the excited-state interaction between the metal nanoparticle and the surface-bound fluorophore. The charge separation is sustained for several microseconds before undergoing recombination.

Fullerene-Functionalized Gold Nanoparticles. A Self-Assembled Photoactive Antenna-Metal Nanocore Assembly

Abstract: A self-assembled photoactive antenna system containing a gold nanoparticle as the central nanocore and appended fullerene moieties as the photoreceptive hydrophobic shell is designed by functionalizing a gold nanoparticle with a thiol derivative of fullerene. Upon suspension of fullerene-functionalized gold nanoparticles (Au−S−C60) in toluene we observe formation of 5−30 nm diameter clusters. The ease of suspending these nanoassemblies in organic solvents allows us to probe the excited state interactions by spectroscopic methods. The quenching of fluorescence emission as well as decreased yields of triplet excited state suggest the participation of excited singlet in the energy transfer to the gold nanocore. Application of electrophoretically deposited Au−S−C60 nanoassemblies on optically transparent electrodes in the photoelectrochemical conversion of light energy has been demonstrated.

Surface Binding Properties of Tetraoctylammonium Bromide-Capped Gold Nanoparticles

Abstract: Surface interaction of gold nanoparticles with solvents and functionalized organic molecules is probed using the changes in the surface plasmon absorption band. A red shift in the surface plasmon band is observed with increase in solvent dielectric constant for solvents that do not complex with metal surface. A plot of the square of the observed position of the surface plasmon bands of Au nanoparticles in these solvents as a function of medium dielectric function shows a linear dependence. Interestingly, the surface plasmon band position remains unaffected in polar solvents (with nonbonding electrons) and is attributed to the direct interaction of these solvents with the gold surface. Binding of gold nanoparticles with organic molecules containing functional groups such as −SCN or −NH2 were found to dampen the surface plasmon bands. Binding of phenyl isothiocynate (PITC) and 15N-labeled benzylamine to gold nanoparticles was probed using NMR techniques, and in both cases, two sets of signals were observed,...

Nanoparticles in advanced oxidation processes

Abstract: Photocatalysis using semiconductor nanoparticles as an advanced oxidation technique (AOT) has been a focus of research by a number of groups during the last two decades. The photocatalytic approach has been adopted successfully to develop self-cleaning glasses and air purification systems. Enhancing the photoconversion efficiency, maximizing the rate of degradation, and extending the photoresponse of the semiconductor catalyst into the visible range still pose a major challenge. Recent developments that address these issues and new approaches that expand the scope of semiconductor and metal nanoparticles in AOTs and related applications are presented in this overview.

Photoinduced transformations in semiconductor-metal nanocomposite assemblies*

Abstract: Semiconductor-metal nanocomposites provide a simple and convenient way to tai- lor the properties of photocatalysts. Modification of semiconductor surface improves charge separation and promotes interfacial charge-transfer processes in nanocomposite systems. Charge accumulation in the metal layer results in Fermi-level equilibration raising the quasi- Fermi level of the composite close to the conduction band level of the oxide semiconductor. Phototransformations of such composites—including morphological changes, interfacial charge-transfer processes and photocurrent generation of TiO 2 -capped gold colloids—are presented in this review article.

Tuning the properties of CdSe nanoparticles in reverse micelles

Abstract: A new and simple approach of synthesizing size-quantized CdSe colloids in reverse micellar suspension is described. The room temperature reaction between Cd2+ and selenosulfate is carried out within the water pool of di-octyl sulphosuccinate (Aerosol-OT) reverse micelles. The size dependent absorption and emission properties of these small CdSe particles (3—5 nm) are described. The Q-sized CdSe nanoparticles exhibit an emission yield of 0.13. Up to a factor of two enhancement in the emission efficiency can be achieved following the surface functionalization of CdSe colloids with triethyl amine.

Photophysical, Photochemical and Photocatalytic Aspects of Metal Nanoparticles

Abstract: Unique electronic and chemical properties of metal nanoparticles have drawn the attention of chemists, physicists, biologists, and engineers who wish to use them for the development of new generation nanodevices. Metal nanoparticles such as gold and silver show noticeable photoactivity under UV−visible irradiation as is evident from the photoinduced fusion and fragmentation processes. Binding a photoactive molecule (e.g., pyrene) to metal nanoparticle enhances the photochemical activity and renders the organic−inorganic hybrid nanoassemblies suitable for light-harvesting and optoelectronic applications. The nature of charge-transfer interaction of fluorophore with gold surface dictates the pathways with which the excited-state deactivates. Obtaining insight into energy and electron-transfer processes is important to improve the charge separation efficiencies in metal−fluorophore nanoassemblies and photocatalytic activity of metal−semiconductor composites.

Clusters of Bis- and Tris-Fullerenes

Abstract: Bis- and tris-fullerene derivatives [bis-(C60) and tris-(C60)] form suspendable clusters in mixed solvents (80% acetonitrile and 20% toluene). These clusters are optically transparent and exhibit interesting nanostructures with sizes ranging from 100 nm to 1 μm and shapes varying from elongated wires to entangled spheres. Ground- and excited-state properties of the clusters of the mono-, bis-, and tris-fullerene derivatives are compared with the corresponding properties of their monomeric forms through steady-state and time-resolved transient absorption spectroscopy. The singlet excited-state quenching of bis- and tris-fullerene derivatives by N-methylphenothiazine (NMP) occurs with rate constants of 5.5 × 109 and 5.3 × 109 M-1 s-1, respectively. The charge separation in these clustered fullerene derivatives is probed by monitoring the formation and decay of the C60 radical anion and the radical cation of NMP. An increase in the number of fullerene moieties in the molecule helps in the formation of ordere...

Unusual electrocatalytic behavior of ferrocene bound fullerene cluster films

Abstract: Fullerene clusters containing a redox couple (ferrocene) have been assembled as nanostructured films on conducting glass and carbon electrodes using an electrophoretic approach. These films show irreversibility in the reduction waves when the electrochemical scan is limited to cathodic cycles (potential range of 0 to −0.8 V versus SCE). However, the reversibility in the cyclic voltammograms can be restored by extending the scans to the anodic region (+0.75 to −0.8 V versus SCE). The ferrocene couple reoxidizes the stabilized C60 anion–tetrabutylammonium complex in the cluster assembly during the oxidation cycle. We present here a simple approach of incorporating a redox active species for improving the electrochemical activity of fullerene films. The electrochemical measurements that illustrate the electrocatalytic property of C60 cluster films are described.