Showing papers by "Prashant V. Kamat published in 2000"

Improving the Photoelectrochemical Performance of Nanostructured TiO2 Films by Adsorption of Gold Nanoparticles

Abstract: TiO2 films cast on conducting glass plates were modified by adsorbing gold nanoparticles (5 nm diameter) from a toluene solution. Selective formation of Au islands and larger particles on the TiO2 surface could be seen from transmission electron micrographs and AFM images. Although the adsorbed Au particles are larger in diameter (50−70 nm), they retain surface plasmon characteristics, similar to those observed for smaller gold nanoparticles in solutions. A 3-fold enhancement in photocurrent generation has been achieved upon modification of TiO2 films with Au nanoparticles. This improved photoelectrochemical performance is explained on the basis of improved interfacial charge-transfer kinetics of the TiO2/Au composite film. Such semiconductor−metal composite films have potential applications in improving the performance of photoelectrochemical solar cells.

Combinative Sonolysis and Photocatalysis for Textile Dye Degradation

Abstract: The merits of combining two advanced oxidation processes, viz., sonolysis and photocatalysis, have been evaluated by investigating the degradation of an azo dye, naphthol blue black (NBB), using a high-frequency ultrasonic generator and UV−photolysis. An additive effect on the degradation rate of the parent compound is observed when the sonolysis and photocatalysis experi ments were carried out in a simultaneous or sequential manner. Sonolysis is effective for inducing faster degradation of the parent dye, while TiO2 photocatalysis is effective for promoting mineralization.

Electrodeposition of C60 Cluster Aggregates on Nanostructured SnO2 Films for Enhanced Photocurrent Generation

Abstract: Clusters of C60 formed in acetonitrile (ACN)/toluene solutions are deposited as thin films on nanostructured SnO2 electrodes under the influence of an electric field. These films exhibit remarkable photoelectrochemical activity under visible light excitation. The photocurrent observed with these novel electrodes (∼0.14 mA/cm2 obtained with visible light excitation, λ > 425 nm, 130 mW/cm2) is 2−3 orders of magnitude larger than previously reported values for a fullerene system. Direct electron transfer between the excited C60 clusters deposited on SnO2 nanocrystallites and the redox couple (I3-/I-) in solution is responsible for the enhanced photocurrent generation.

Dye-Capped Gold Nanoclusters: Photoinduced Morphological Changes in Gold/Rhodamine 6G Nanoassemblies

Abstract: Au nanoparticles (particle diameter ∼2 nm) prepared by thiocyanate reduction method are too small to exhibit the characteristic surface plasmon band. Addition of Rhodamine 6G (Rh 6G) to the colloidal gold solution brings about significant changes in the absorption spectrum. Two distinct peaks appear at 507 and 537 nm. Close packing of the cationic dye molecules on the gold surface induces intermolecular and intercluster interactions. Furthermore, the adsorption of the cationic dye on the gold surface results in surface charge neutralization causing the Au/dye assembly to aggregate. When the Au/Rh-6G solution was subjected to 532 nm laser pulse irradiation for long time intervals, we observed distinct changes in the absorption spectrum due to morphological changes. A growth in the particle size (5−20 nm) is observed as a result of melting and fusion of gold nanoparticles. The photoinduced morphological changes have been elucidated using picosecond laser flash photolysis. The multiphoton process leading to ...

Nanostructured thin films of C60-aniline dyad clusters: electrodeposition, charge separation, and photoelectrochemistry.

Abstract: Clusters of C60-aniline dyads are deposited as thin films on nanostructured SnO2 electrodes under the influence of an electric field. At low applied DC voltage ( 50 V) they are deposited on the electrode surface as thin films. The C60- aniline dyad cluster films when cast on nanostructured SnO2 films are photoelectrochemically active and generate photocurrent under visible light excitation. These nanostructured fullerene films are capable of delivering relatively large photocurrents (up to approximately 0.2 mA cm(-2), photoconversion efficiency of 3-4%) when employed as photoanodes in photoelectrochemical cells. Both luminescence and transient absorption studies confirm the formation of charge transfer product (C60 anion) following UV/Vis excitation of these films. Photo-induced charge separation in these dyad clusters is followed by the electron injection from C60-anion moiety into the SnO2 nanocrystallites. The oxidized counterpart is reduced by the redox couple present in the electrolyte, thus regenerating the dyad clusters. The feasibility of casting high surface area thin fullerene films on electrode surfaces has opened up new avenues to utilize dyad molecules of sensitizer bridge donor type in light energy conversion devices, such as solar cells.

Photosensitization Aspects of Pinacyanol H-Aggregates. Charge Injection from Singlet and Triplet Excited States into SnO2 Nanocrystallites

Abstract: The singlet and triplet excited-state behavior of a symmetric carbocyanine dye, 1,1‘-diethyl-2,2‘-carbocyanine (commonly referred as pinacyanol), adsorbed on SiO2 and SnO2 nanocrystallites has been investigated using transient absorption spectroscopy. The adsorption of the dye molecules on the negatively charged SiO2 or SnO2 colloids results in H-type aggregation. When excited with a 532 nm laser pulse we observe a short-lived (τ 5 × 1010 and 7 × 10...

Complexation of Gold Nanoparticles with Radiolytically Generated Thiocyanate Radicals ((SCN)2

Abstract: Pulse radiolytically generated (SCN)2•- radicals bind strongly to the gold nanoparticle surface, with an apparent equilibrium constant of 4.7 × 103 M-1. The resultant complex has a strong absorption band at 390 nm which quickly undergoes chemical changes to yield an oxidation product, [Au(SCN)2]-, with an absorption maximum at 320 nm. The spectral changes associated with the chemical interaction between SiO2/Al2O3-stabilized gold nanoparticles and (SCN)2•- are elucidated using time-resolved transient absorption spectroscopy.

Radiolytic Reduction of Hexachlorobenzene in Surfactant Solutions: A Steady-State and Pulse Radiolysis Study

Abstract: Steady-state and pulse radiolysis experiments have been performed to gain insight into the mechanism of hexachlorobenzene (HCB) degradation in nonionic surfactant (Plurafac RA-40) solutions. This understanding is important for the environmental application of radiolysis to remediate soils contaminated with chlorinated aromatic compounds or to treat surfactant solution wastes from soil washing processes. Steady-state experiments showed that, after an applied dose of 50 kGy, reductive dechlorination of HCB to trichlorobenzene occurs under reducing conditions. Under oxidizing conditions at the same dose, reductive dechlorination proceeds more slowly to yield tetrachlorobenzene. Radiolytic experiments on the surfactant alone showed that the reaction rate constant between hydroxyl radicals and RA-40 (1.09 × 109 M-1 s-1) was nearly 2 orders of magnitude higher than that between hydrated electrons and RA-40 (2.0 × 107 M-1 s-1). Reaction kinetics analysis indicates efficient hydroxyl radical scavenging by surfact...

Role of Hydroxypropylcellulose in Free Radical Induced Reactions of the Anthraquinone Based Textile Dye, Uniblue A:

Abstract: Radical reactions that involve one-electron reduction and oxidation of the anthraqui none based dye, Uniblue A, are studied by pulse radiolysis in the presence and absence of hydroxypropyl cellulose, a water soluble cellulose model. The dye is readily reduced by e aq- and 2-propanol radicals in aqueous solution. A comparison of the transient spectrum obtained in the reaction of the dye molecules and hydroxyl radicals with those from the reaction with one-electron oxidants, such as SO4*- and N3*, suggests that hydroxyl radicals react by means of both electron-transfer and adduct formation. In radiolytic experiments where the concentration of hydroxypropyl cellulose greatly exceeds that of Uniblue A, hydroxyl radicals react exclusively with the soluble cellulose to generate cellulose radicals. Subsequent reaction of the cellulose radicals with Uniblue A leads to the formation of the one-electron oxidized form of the dye.