Showing papers by "Prashant V. Kamat published in 1996"

Environmental Photochemistry on Semiconductor Surfaces: Photosensitized Degradation of a Textile Azo Dye, Acid Orange 7, on TiO2 Particles Using Visible Light

Abstract: Photosensitized degradation of a textile azo dye, Acid Orange 7, has been carried out on TiO2 particles using visible light Mechanistic details of the dye degradation have been elucidated using diffuse reflectance absorption and FTIR techniques Degradation does not occur on Al2O3 surface or in the absence of oxygen The dependence of the dye degradation rate on the surface coverage shows the participation of excited dye and TiO2 semiconductor in the surface photochemical process Diffuse reflectance laser flash photolysis confirms the charge injection from the excited dye molecule into the conduction band of the semiconductor as the primary mechanism for producing oxidized dye radical The surface-adsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the oxidized dye radical and the photoinjected electrons Diffuse reflectance FTIR was used to make a tentative identification of reaction intermediates and end products of dye degradation

Nanostructured semiconductor films for photocatalysis. Photoelectrochemical behavior of SnO2/TiO2 composite systems and its role in photocatalytic degradation of a textile azo dye

Abstract: Nanostructured semiconductor films of SnO2, TiO2 and SnO2/TiO2 have been employed for electrochemically assisted photocatalytic degradation of a textile azo dye naphthol blue black (NBB). The degradation rate is significantly higher for SnO2/TiO2 composite films than SnO2 and TiO2 films alone. An effort has been made to correlate the photoelectrochemical behavior of these films to the rate of photocatalytic degradation of NBB. The enhanced degradation rate of NBB using composite semiconductor films is attributed to increased charge separation in these systems. Photoelectrochemical and photocatalytic degradation experiments carried out in both nitrogen- and oxygen-saturated solutions with an externally applied electrochemical bias provide useful information in optimizing semiconductor concentrations in a composite film.

Environmental Photochemistry on Semiconductor Surfaces. Visible Light Induced Degradation of a Textile Diazo Dye, Naphthol Blue Black, on TiO2 Nanoparticles

Abstract: Visible light induced degradation of the textile diazo dye Naphthol Blue Black (NBB) has been carried out on TiO2 semiconductor nanoparticles. Diffuse reflectance transient absorption and FTIR techniques have been used to elucidate the mechanistic details of the dye degradation. The failure of the dye to degrade on insulator surfaces such as Al2O3 or in the absence of oxygen further highlights the importance of semiconducting properties of support material in controlling the surface photochemical processes. The primary event following visible light excitation is the charge injection from the excited dye molecule into the conduction band of the semiconductor TiO2, producing the dye cation radical. This was confirmed by diffuse reflectance laser flash photolysis. The surface-adsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the dye cation radical and photoinjected electrons. Diffuse reflectance FTIR study facilitated identification of r...

Photosensitization of Nanocrystalline Semiconductor Films. Modulation of Electron Transfer between Excited Ruthenium Complex and SnO2 Nanocrystallites with an Externally Applied Bias

Abstract: In situ spectroelectrochemical measurements have been carried out to probe the charge injection from excited Ru(bpy)2(dcbpy)2+, Ru(II), into the SnO2 nanocrystallites. The dependence of luminescence yield and lifetime at various applied potentials suggests that the heterogeneous electron transfer from excited sensitizer into the semiconductor can be controlled by the externally applied electrochemical bias. The maximum quenching is seen at positive potentials while an increase in the luminescence yield and lifetime is seen at negative potentials. Laser flash photolysis of Ru(II)-modified SnO2 nanocrystalline film has been carried out to record the transient absorption spectra at different applied potentials. The yield of electron transfer product, Ru(III), decreases as the applied bias is switched to negative potentials. At an applied bias of −0.7 V the only observable transient is the excited Ru(II) complex (Ru(II)*). The maximum apparent electron transfer rate constant, ket (∼4 × 108 s-1), observed at p...

Role of reduction in the photocatalytic degradation of TNT

Abstract: The reduction of TNT by electrochemical and photoelectrochemical techniques has been investigated to obtain insight into the role of reductive transformations in a photocatalytic degradation process. TNT was observed to be labile to reductive transformation by a platinum electrode at electrochemical potentials commensurate with the flat band potential of TiO2, and aminodinitrotoluene species were detected as early reduction products. Oxygen did not influence the rate of reductive TNT transformation, but byproducts were more stable in aerated than deaerated conditions. Photocatalytic reduction of TNT was analyzed using CdS as a chromophore coupled to TiO2 and visible light excitation so that direct photolysis of TNT was precluded. Under deaerated conditions, reduction of TNT occurred through both direct and sensitized mechanisms, predominantly forming aminodinitrotoluene compounds as relatively stable byproducts. Photocatalytic transformation and mineralization of TNT was also examined with particulate fil...

Dye-Capped Semiconductor Nanoclusters. Excited State and Photosensitization Aspects of Rhodamine 6G H-Aggregates Bound to SiO2 and SnO2 Colloids

Abstract: SiO2 and SnO2 colloids are capped with a cationic dye, rhodamine 6G, by electrostatic interaction. The close packing of these dye molecules on the negatively charged SiO2 or SnO2 colloid results in the formation of H-aggregates. These aggregates are nonfluorescent but can inject electrons from the excited state into SnO2 colloids. The photophysical and photochemical properties of rhodamine-6G-aggregate on SiO2 and SnO2 colloids have been investigated using picosecond laser flash photolysis. Charge injection from the excited dye aggregate into SnO2 nanocrystallites occurs with a rate constant of 5.5 × 109 s-1. The application of these dye aggregates in extending the photoresponse of nanocrystalline SnO2 film has been demonstrated by constructing a photoelectrochemical cell. A maximum incident photon-to-photocurrent efficiency of ∼1% was observed for the photosensitized current generation. Fast reverse electron transfer between the injected electron and the cation radical of the dye aggregate is a limiting ...

Combine electrochemistry with photocatalysis

Abstract: Because a substantial percentage of colorant is lost during the dyeing process, remediation efforts have largely been focused on removing these dyes from the wastewater effluents of textile mills and other industrial colorant users. Incomplete decolorization of the effluent before discharge shifts the burden of treatment downstream. In publicly owned water treatment facilities, these dyes often end up as sludges that are dewatered and eventually deposited in landfills. There is a substantial economic impetus to develop a flow reactor to be used onstream by mills to treat colorant effluent and recycle the water. The authors have developed a photocatalytic approach using semiconductors for degrading several azo dyes. They recently found that deposition of semiconductor nanoclusters on a conducting glass surface provides a convenient way to manipulate the photocatalytic reaction by electrochemical methods. The thin semiconductor particulate film can be used as a photosensitive electrode in an electrochemical cell. The paper describes electrode preparation, the photoelectrochemical properties of TiO{sub 2} and SnO{sub 2}, reaction mechanism, and composite semiconductor films.

Fluorescence and photoelectrochemical behavior of chlorophyll a adsorbed on a nanocrystalline SnO2 film

Abstract: Fluorescence and photoelectrochemical studies of chlorophyll a (Chl a) adsorbed on nanocrystalline SnO2 film were carried out. The results of fluorescence and incident photon to current conversion efficiency (IPCE) as a function of applied bias suggest that the fluorescence quenching and the photocarrier generation are interrelated. Fluorescence quenching has thus been utilized to determine the photogeneration efficiency, η(e), of charges in a SnO2/Chl a based photoelectrochemical cell. A value of 0.75 was obtained for η(e) for unbiased cells. With an IPCE of 13%, η(e) of 75%, and a light harvesting efficiency of 70%, the charge collection efficiency of ∼23% was evaluated. These results suggest that the losses due to the charge recombination are a major factor that limit the efficiency of the cells.

Photochemistry of squaraine dyes: Excited triplet state and redox properties of crown ether squaraines

Abstract: The excited state and redox properties of three crown-ether derivatives of squaraines have been studied. Direct laser excitation of these dyes leads to the formation of excited triplets, dye radical cations and anions, via a biphotonic photoionization mechanism. All these transient species absorb strongly in the visible region. The triplet states of these dyes generated via triplet−triplet energy transfer from 3(1-pyrenecarboxaldehyde)* and 3C60* photosensitizers also produce dye radical cations and anions via a self-quenching process. The bimolecular rate constants for the self-quenching process are in the range of (0.5−1.9) × 109 M-1 s-1. The redox properties of these dyes have been investigated by electrochemical, spectroelectrochemical, and radiolytic methods. They exhibit two reversible oxidations in the potential range of 0.7−1.2 V vs SCE. The spectral features of electrochemically generated one electron oxidation product (λmax = 680 nm) match those of dye radical cations produced by photochemical a...

Picosecond dynamics of cresyl violet H‐aggregates adsorbed on SiO2 and SnO2 nanocrystallites

Abstract: The H‐aggregates of cresyl violet dye which are formed on the negatively charged SiO2 and SnO2 nanocrystallites, exhibit relatively short‐lived excitonic singlet state with a lifetime of about 35 ps. The difference absorption maximum (λmax=470 nm) is blue shifted compared to the corresponding singlet excited state of monomer (λmax=515 nm). Time‐resolved transient absorption measurements show that these dye aggregates are capable of injecting electrons from the triplet excited state into SnO2 nanocrystallites. The rate constant for heterogeneous electron transfer as measured from the formation of cation radical and electron trapping in SnO2 nanocrystallites was 2.0×108 s−1.

Dye-Capped Semiconductor Nanoclusters. One-Electron Reduction and Oxidation of Thionine and Cresyl Violet H-Aggregates Electrostatically Bound to SnO2 Colloids

Abstract: SnO2 colloids are capped with the cationic dyes thionine and cresyl violet by electrostatic interaction. The close packing of these dyes on the negatively charged SnO2 colloids resulted in the formation of H-aggregates. One-electron reduction and oxidation of these H-aggregates bound to SnO2 colloids has been investigated by pulse radiolysis. The spectral features of semireduced, •OH-adduct, and semioxidized forms of dye monomers and aggregates are compared by recording time-resolved transient absorption spectra. The bimolecular rate constants for the reaction of dye-capped SnO2 colloids with aqueous electrons (5−6 × 109 M-1 s-1) and •OH radicals (3.5 × 108 M-1 s-1) are nearly an order of magnitude smaller than those for the corresponding reactions of dye monomers. The slower diffusion of colloidal particles is likely to influence the diffusion kinetics of reactions involving dye-capped SnO2 colloids.

Sonochromic Effect in WO3 Colloidal Suspensions

Abstract: In recent years there has been a burst of activities in investigating sonolytic reactions. The usefulness of this technique in synthesizing colloidal semiconductors and metals and dissolution of MnO{sup 2} colloids has also been demonstrated. We have now employed semiconductor colloids to investigate the radical reactions in sonolytic processes. In this study we present our preliminary results from the reaction of WO{sup 3} colloids with sonolytically generated H atoms. Sodium tungstate, oxalic acid, and Acid Orange 7 were obtained from Aldrich. Acid Orange 7 was purified by column chromatography. All other chemicals were analytical reagents of highest available purity. The analysis experiments were carried out with a 640 kHz sonolysis setup of Ultrasonic Energy Systems (Panama City, FL). 24 refs., 4 figs.

Photochemistry of squaraine dyes. Part 10.—Excited-state properties and photosensitization behaviour of an IR sensitive cationic squaraine dye

Abstract: The singlet and triplet excited-state behaviour of a cationic squaraine dye, (Sq) in acetonitrile is investigated using pico- and nano-second laser flash photolysis. The dye in its protonated form has absorption and emission maxima at 780 and 812 nm, respectively. The singlet excited state exhibits a difference absorption maximum at 640 nm and a lifetime of 1.1 ± 0.1 ns. The triplet excited state, generated by T–T sensitization method exhibited a difference absorption maximum at 620 nm and a lifetime of 45 ± 5 µs. At higher laser excitation intensities, the dye underwent photoionization to yield a cation radical (abs. max. 630 nm). Photosensitization properties of the IR sensitive squaraine dye are demonstrated by modifying a nanocrystalline SnO2 electrode and exciting it with visible–IR light.

Photoinduced Charge Transfer Processes in Semiconductor Heterostructures. Capped vs. Coupled Systems

Abstract: By coupling or capping a metal oxide nanocrystallite with one or more semiconductors it is possible to design semiconductor heterostructures that are potentially useful in microelectronics or molecular devices. Photoinduced charge transfer processes in multicomponent semiconductor particulate systems can provide valuable information for improving the efficiency of charge separation. We have now synthesized a variety of semiconductor heterostructures with coupled and capped geometries. The charge transfer processes in metal oxide nanostructures of two different geometries have been investigated with nanosecond and picosecond laser flash photolysis. The geometry of these heterostructures has been found to control the interfacial charge transfer or charge collection efficiencies. The photoelectrochemical properties of capped and coupled semiconductor systems have also been made by preparing nanocrystalline thin films of these materials.