Showing papers by "Prashant V. Kamat published in 1994"

Preparation and Photoelectrochemical Characterization of Thin SnO2 Nanocrystalline Semiconductor Films and Their Sensitization with Bis(2,2'-bipyridine)(2,2'-bipyridine-4,4'-dicarboxylic acid)ruthenium(II) Complex

Abstract: Thin, transparent films of SnO 2 semiconductor have been prepared from 30-A-diameter colloids. Scanning electron microscopy and X-ray powder diffraction studies indicate a three-dimensional network of SnO 2 nanocrystallites of particle diameter ≤50 A. These thin nanocrystalline films exhibit reversible electrochromic effects. The electron trapping process in SnO 2 particles has been investigated by both spectroelectrochemical and laser flash photolysis techniques. These electrodes are photoelectrochemically active in the UV region with incident photon-to-photocurrent conversion efficiency of 20% at 280 nm. The photocurrent increases with increasing film thickness but attains a limiting value at thickness greater than 0.75 μm

Electrochemically Assisted Photocatalysis. 2. The Role of Oxygen and Reaction Intermediates in the Degradation of 4-Chlorophenol on Immobilized TiO2 Particulate Films

Abstract: The electrochemically-assisted photocatalytic degradation of 4-chlorophenol (4-CP) using immobilized TiO[sub 2] particulate films has been investigated by analyzing reaction intermediates under a variety of experimental conditions. The degradations were carried out in both nitrogen- and oxygen-saturated solutions to explore the role of reduced oxygen species and molecular oxygen in the formation of reaction intermediates and in the mineralization of 4-CP. The degradation rate can be greatly improved even in a nitrogen-saturated atmosphere by applying an anodic bias to the TiO[sub 2] film electrodes. 4-Chlorocatechol (4-CC) is the predominant intermediate observed in oxygen-saturated solutions, whereas hydroquinone (HQ) is the primary intermediate in nitrogen-saturated solutions. Molecular oxygen plays an important role in the enhancement of the electrochemically assisted photocatalytic decay rate of 4-CP and the subsequent degradation of reaction intermediates, viz., 4-CC and HQ. 37 refs., 7 figs., 1 tab.

A photocatalytic approach for the reductive decolorization of textile azo dyes in colloidal semiconductor suspensions

Abstract: Two representative commercially used textile azo dyes, Acid Orange 7 and Direct Blue 1, have been decolorized using colloidal TiO[sub 2] and WO[sub 3] photocatalytic systems. Under UV irradiation, these dyes undergo rapid decolorization as they are reduced at the semiconductor surface by the trapped electrons. The trapping of electrons in irradiated semiconductor colloids and their participation in the dye reduction process have been probed by steady-state and laser flash photolysis techniques. The quantum efficiency for such a reductive process has been determined to be 4.7%. The rate constant for the electron transfer between the excited semiconductor colloid and the dye is of the order of 10[sup 8] M[sup [minus]1] s[sup [minus]1]. This photocatalytic decolorization approach has potential applications in the treatment of textile dye wastes. 28 refs., 8 figs.

Photochemistry of textile azo dyes. spectral characterization of excited state, reduced and oxidized forms of acid orange 7

Abstract: The spectral characterization of the singlet and triplet excited states of an azo dye (Acid Orange 7 (AO)) was performed using picosecond and nanosecond laser flash photolysis. The excited singlet state lifetime, estimated from the bleaching recovery, is around 135 ps. The triplet excited state generated by triplet—triplet (TT) energy transfer shows a difference absorption maximum at 650 nm with a lifetime of μs. The oxidized and reduced forms of the dye were generated by reacting AO with pulse radiolytically generated azide (N3) and eaq radicals. These radicals were also detected as the primary intermediates in visible-laser-induced photoionization. The photoelectrochemical reduction of the dye in UV-irradiated TiO2 colloidal suspension results in the irreversible decolorization of the dye.

Photochemistry of Squaraine Dyes. 8. Photophysical Properties of Crown Ether Squaraine Fluoroionophores and Their Metal Ion Complexes

Abstract: The photophysical properties of a series of red-sensitive, highly fluorescent bis[4-(monoaza-crown ether)-phenyl]squaraine fluoroionophores are reported. These dyes are soluble in both polar and nonpolar solvents with fluorescence quantum yield varying from 0.01 to 0.85. These dyes readily form aggregates in aqueous solutions. These aggregates dissociate in [beta]-cyclodextrin solutions as the dye molecules form 1:1 complex with [beta]-cyclodextrin. This complexation process is accompanied by a significant enhancement in the fluorescence yields of these dyes. Complexation of the fluoroionophores with alkali-metal ions brings about drastic reductions in their fluorescence quantum yield as well as significant changes in their redox properties. 35 refs., 9 figs., 3 tabs.

Electrochromic and photoelectrochromic behavior of thin WO[sub 3] films prepared from quantum size colloidal particles

Abstract: Thin films of WO[sub 3] are cast from quantum size colloids onto an optically transparent electrode. These thin particulate films are found to exhibit reversible electrochromic and photoelectrochromic effects. A blue coloration of the WO[sub 3] particulate film is seen when electrons are injected into the conduction band of WO[sub 3] by electrochemical or UV-excitation methods. The onset potential of the electrochromic effect is dependent on the pH and corresponds to the flat band potential of WO[sub 3]. Spectroelectrochemical and microwave absorption experiments suggest that trapped electrons are the major species responsible for the blue coloration of the WO[sub 3] particulate film. 34 refs., 9 figs.

Chlorophyll b-modified nanocrystalline sno2 semiconductor thin film as a photosensitive electrode

Abstract: Nanocrystalline thin SnO2 semiconductor films (thickness≤1 μm) have been modified with chlorophyll b (Chl‐b) by electrodeposition and adsorption methods for use as novel photosensitive electrodes in photoelectrochemical cells. Excitation of Chl‐b with monochromatic light produced photocurrents with an incident photon‐to‐photocurrent efficiency of around 8.5%. The charge injection from excited Chl‐b into the conduction band of the semiconductor SnO2 crystallites has been probed by time‐resolved microwave absorption.

Interaction of semiconductor colloids with J-aggregates of a squaraine dye and its role in sensitizing nanocrystalline semiconductor films

Abstract: The role of bis(2,4,6-trihydroxyphenyl)squaraine, Sq, in sensitizing large bandgap semiconductors has been investigated in the present study. The dye in its aggregate form readily interacts with the TiO2 colloids giving rise to a new charge transfer band in the red region. The apparent association constant for the dye aggregate and TiO2 colloid as determined from a Benesi-Hildebrand plot is 1600 M-1. Nanocrystalline semiconductor films prepared from TiO2, ZnO, and SnO2 colloids have been modified with Sq to probe the photosensitization effects. Both dye monomers and aggregates were found to participate in the charge injection process. An incident photon-to-photocurrent efficiency up to 0.7% has been observed.

Photoelectrochemical Behavior of Thin CdSe and Coupled TiO2/CdSe Semiconductor Films.

Abstract: Photoelectrochemical effects at CdSe thin film electrodes (thickness 30-200 [angstrom]) have been investigated by monitoring open-circuit voltage and short-circuit current at varying film thicknesses and incident light intensities. Unlike the situation in conventional photoelectrochemical cells that employ single-crystal or polycrystalline semiconductors, the charge separation in thin CdSe films is not controlled by the space charge layer at the semiconductor-electrolyte interface, but it is controlled by the differing rates of electron and hole transfer into the solution. Quick decay of the photocurrent, even in the presence of a redox couple such as [Fe(CN)[sub 6]][sup 3[minus]/4[minus]], suggests high degree of charge recombination within the thin CdSe film. It is possible to improve the photocurrent stability of a thin CdSe film by coupling it with a TiO[sub 2] particulate film. The improved charge separation in the coupled semiconductor system has a beneficial effect in improving the photocurrent stability of thin semiconductor films. 22 refs., 8 figs., 1 tab.

The Sensitized Photocatalysis of a Mixed Reactant System of 4-Chlorophenol and 4-Nitrophenol

Abstract: Sensitized photocatalysis has been demonstrated with colored pollutants such as nitrophenols. With visible light excitation of an adsorbed nitrophenolic compound, charge injection into the conduction band of the semiconductor occurs and subsequent transformation of nitrophenols to a variety of products is possible. These phenomena have been observed in powder systems of TiO 2 , but have not been clearly detailed in aqueous systems. This work reports the results of aqueous TiO 2 slurry experiments in which the degradation of 4-nitrophenyl and 4-chlorophenyl is studied in single reactant and mixed reactant systems. The primary objectives of this work are to establish the reaction pathway occurring in sensitized photocatalysis and to determine if sensitization can induce/drive the degradation of non-colored pollutants. The influence of organic concentration, oxygen and light energy are discussed and byproduct data are presented.