Showing papers by "Prashant V. Kamat published in 1997"

Dye capped semiconductor nanoclusters. role of back electron transfer in the photosensitization of sno2 nanocrystallites with cresyl violet aggregates

Abstract: Adsorption of a cationic dye, cresyl violet, on SnO2 and SiO2 nanoclusters and nanocrystalline thin films results in the formation of H-aggregates. These dyes are photochemically and electrochemically active and extend the photoresponse of large bandgap semiconductors such as SnO2. Photocurrent generation in dye capped nanocrystalline films of SnO2 has been demonstrated with visible light excitation. A photon-to-photocurrent generation efficiency around 1% has been observed at 510 nm. Back electron transfer between the photoinjected electron and the oxidized sensitizer plays an important role in controlling the efficiency of net electron transfer. Transient absorption and microwave absorption measurements of the dye aggregate capped SnO2 films suggest that the back electron transfer is multiexponential and most is completed within a few hundred nanoseconds. The activation energy of the back electron transfer process is very low (∼1.7 kJ/mol).

Semiconductor nanoclusters-physical, chemical, and catalytic aspects

Abstract: Introduction (P.V. Kamat, D. Meisel). Preparation and characterization of semiconductor nanoparticles (H. Weller, A. Eychmuller). The mechanics of nanoscale suspensions (K.T. Miller, C.F. Zukoski). Sonochemistry in colloidal systems (F. Grieser). Charge transfer in nanoparticles (D. Meisel). Spectroscopy of metal colloids -- some comparisons with semiconductor colloids (P. Mulvaney). Surface characterization of nanostructured systems (M. Tomkiewicz). Quantum well and superlattice electrodes (A.J. Nozik). Pseudopotential theory of nanometer silicon quantum dots (L.--W. Wang, A. Zunger). Luminescent porous silicon: synthesis, chemistry, and applications (M.J. Sailor, J.L. Heinrich, J.M. Lauerhaas). Composite semiconductor nanoclusters (P.V. Kamat). Photoelectron transfer in nanocomposite films, layer by layer self--assembled from polycations and anionic semiconductors (J.J. Fendler). Semiconductor nanocrystals in photoconductive polymers: charge generation and charge transport (Y. Wang). Nanostructure and size quantization in chemical solution deposited semiconductor films (S. Gorer, G. Hodes). Electrodeposition and characterization of nanocrystalline semiconductor films (K. Rajeshwar, N.R. de Tacconi). Nanocrystalline electronic junctions (M. Gratzel). Nanostructures in analytical chemistry (E. Pelizzetti, C. Minero). Semiconductor--mediated photocatalysis for organic synthesis (Y. Li, L. Wang). Application of nanoparticles in the photocatalytic degradation of water pollutants (N. Serpone, R.F. Khairutdinov). Applications in photocatalytic purification of air (X. Fu, W.A. Zeltner, M.A. Anderson). Author index. Subject index.

Photoelectrochemistry of Composite Semiconductor Thin Films. Photosensitization of SnO2/CdS Coupled Nanocrystallites with a Ruthenium Polypyridyl Complex

Abstract: The charge injection from an excited Ru(II) complex (viz., (Ru(bpy)2(L))2+, where L = 4-(2,2‘-bipyrid-4-yl)phenyl diphosphonate) into SnO2 and SnO2/CdS films is investigated to elucidate the beneficial effect of composite semiconductor systems. The electrons injected from excited Ru(II) into CdS are transferred quickly into SnO2 nanoclusters, thus promoting the charge separation. Photoelectrochemical measurements suggest improved photon to photocurrent charge carrier generation efficiency in the SnO2/CdS composite system. The formation of the oxidation product, Ru(III), and the recovery of the sensitizer are investigated using transient absorption spectroscopy. The rate constant for the back electron transfer in the SnO2/CdS/Ru(II) composite system is slower by a factor of 2−3 than the one observed in the SnO2/Ru(II) system. The beneficial aspects of composite semiconductor systems in photochemical solar cells are presented.

Photosensitization of Nanocrystalline ZnO Films by Bis(2,2‘-bipyridine)(2,2‘-bipyridine-4,4‘-dicarboxylic acid)ruthenium(II)

Abstract: Spectral sensitization of nanocrystalline ZnO films has been carried out with ruthenium complex, bis(2,2‘-bipyridine)(2,2‘-bipyridine-4,4‘-dicarboxylic acid) ruthenium(II) or (Ru(II)). An incident photon to current conversion efficiency (IPCE) of 14% has been obtained for unbiased ZnO/Ru(II) photoelectrochemical cells. This low IPCE has been attributed to the poor light harvesting efficiency (LHE) and charge collection (ηc) efficiency. The weak interaction between Ru(II) and ZnO surface which results in poor uptake of Ru(II) on ZnO is responsible for a lower LHE while various charge recombination processes are the cause of poor ηc. To better understand the process of photosensitization of ZnO by Ru(II), the dependence of IPCE and fluorescence on applied bias, and other photoelectrochemical measurements have also been carried out.

Ultrafast study of interfacial electron transfer between 9-anthracene-carboxylate and tio2 semiconductor particles

Abstract: The excited state dynamics of 9-anthracene-carboxylic acid adsorbed onto the surface of TiO2 semiconductor particles were examined with ca. 250 fs time resolution. A combination of transient absorption and time-resolved anisotropy measurements show that approximately 76% of the photo-excited dye molecules transfer an electron to the TiO2 particles. The time scale for the forward electron transfer reaction was determined to be ⩽1 ps. The 9-anthracene-carboxylate radical cations produced by this reaction undergo back electron transfer on a 54 ps time scale. A more accurate estimate of the forward electron transfer reaction time is not possible, due to the contribution to the transient absorption signal from adsorbed dye molecules that do not transfer electrons to TiO2. These nonreactive species are deactivated by either nonradiative decay or fluorescence emission. The fluorescence spectrum from the dye molecules bound to the TiO2 particles is very different to that of the free dye in solution. The free dye ...

Ultrafast Photophysical Investigation of Cresyl Violet Aggregates Adsorbed onto Nanometer-Sized Particles of SnO2 and SiO2

Abstract: The relaxation dynamics of cresyl violet H-aggregate dimers adsorbed onto SnO2 or SiO2 colloidal particles has been examined with ca. 200 fs time resolution. These experiments were performed by monitoring both the ground state recovery of the excited dye molecules and the transient absorption signal in the region of the dye radical cation. For cresyl violet−SiO2, the ground state recovery is a single exponential with a 2.9 ± 0.2 ps time constant. Transient absorption measurements that monitored the excited electronic state of the dye show a similar 2.5 ± 0.4 ps decay. The observed dynamics for cresyl violet−SiO2 is assigned to internal conversion followed by vibrational relaxation of the adsorbed cresyl violet dimers. The similarity of the transient absorption and bleach recovery time constants shows that vibrational relaxation is extremely rapid, i.e., internal conversion is the rate-limiting step. For cresyl violet−SnO2, the ground state recovery is biexponential with time constants of 2.4 ± 0.4 ps (∼80...

Photocatalytic reduction of azo dyes Naphthol Blue Black and Disperse Blue 79

Abstract: Photocatalytic reduction of two textile azo dyes, Naphthol Blue Black (NBB) and Disperse Blue 79 (DB79) has been carried out in colloidal WO3 and TiO2 suspensions. Under bandgap excitation of the semiconductor colloids these dyes undergo irreversible reduction as they react with the trapped electrons. The quantum efficiency for the photocatalytic reduction of these dyes were 5.4% and 4.8% for NBB and DB79 respectively. The kinetics and mechanism of the interfacial charge transfer in these colloidal suspension has been elucidated with transient absorption spectroscopy. The reaction between the dye and trapped electrons is diffusion limited and occurs with rate constants of 1.1×108 M−1s−1 and 4.0×107 M−1s−1 for NBB and DB79 respectively.

Photostabilization of Organic Dyes on Poly(styrenesulfonate)-Capped TiO2 Nanoparticles

Abstract: Photostabilization of phenosafranin dye on poly(styrenesulfonate)-capped TiO2 nanoparticles has been investigated using diffuse reflectance spectroscopy. The dye degrades oxidiatively on a native TiO2 surface under visible light excitation as the excited dye participates in the charge injection process. The interaction of the dye with the semiconductor surface can be significantly reduced by capping the TiO2 particles first with poly(styrenesulfonate). The increase in fluorescence emission and decrease in dye degradation rate with increasing polymer capping suggest the protective role of poly(styrenesulfonate) capping. Diffuse reflectance laser flash photolysis studies further elucidate surface photochemical processes of phenosafranin on native and polymer-capped TiO2 nanoparticles.

Composite semiconductor nanoclusters

Abstract: Publisher Summary This chapter discusses some important features of chemically synthesized semiconductor heterostructures. By controlling the methods of preparation it is possible to tailor the photocatalytic property of these composite semiconductor nanoclusters. Their ability to carry out redox processes, with greater efficiency and selectivity than the single component systems has made them potential candidates for the conversion and storage of solar energy, and in the mineralization of chemical pollutants. These systems have potential applications in designing novel microheterogeneous assemblies, microelectronics, and chemical sensors. The feasibility of synthesizing composite semiconductor nanoclusters, by chemical precipitation or electrochemical deposition, opens up a wide array of possibilities, in utilizing these materials for chemical sensors, electrooptics, microelectronics, imaging technology, and photovoltaics. The chapter classifies composite semiconductor nanoclusters into two categories— namely, capped and coupled type heterostructures. The capped nanoclusters essentially have a coreshell geometry, while in a coupled system two semiconductor nanoclusters are in contact with each other. Photo-induced charge transfer processes in single and multicomponent semiconductor nanostructures are discussed in the chapter.

Photocatalytic degradation of 4-chlorophenol : a mechanistically-based model

Abstract: The photocatalytic degradation of 4-CP was mathematically modelled using the mechanistic insights and data presented in an earlier study [1]. The solution and surface concentrations of reacting species were calculated by solving a system of differential equations that account for oxidation reactions of dissolved and adsorbed species, adsorption and desorption, reduction of oxygen, and hole-electron recombination. The differential equations were integrated over discrete time-periods and annular regions of the photoreactor. The resulting model predicts the trends observed in studies in other laboratories using different experimental apparati. Using the model it is possible to predict effects of reactor geometry, TiO2 loading, light intensity, and mixing on the course of TiO2 photocatalytic oxidation. The model verifies the importance of surface reactions, and reveals the need to better understand the fate and role of oxygen in TiO2 photocatalytic systems.

Picosecond dynamics of an IR sensitive squaraine dye. Role of singlet and triplet excited states in the photosensitization of TiO2 nanoclusters

Abstract: The photosensitization properties of a novel squaraine dye is probed using picosecond laser flash photolysis. The dye which exists in a protonation equilibrium with a pKa of 4.9 has a strong absorption in the IR region. The protonated form of the dye is highly fluorescent and is readily quenched in colloidal TiO2 suspensions. The association constant as determined from the quenching studies is 1.25×103 M−1. The singlet excited state of the protonated form of the dye participates in the charge injection process with a heterogeneous electron transfer rate constant of 8.3×109 s−1. The unprotonated form of the dye injects electrons from both the singlet and triplet excited state with electron transfer rate constants of 1×1010 s−1 and 1.25×109 s−1, respectively.

Excited States and Reduced and Oxidized Forms of C76(D2) and C78(C2v

Abstract: The singlet and triplet excited state properties of C76(D2) and C78(C2v‘) have been investigated using picosecond laser flash photolysis and pulse radiolysis. Both C76 and C78 are weakly fluorescent as they undergo internal nonradiative conversion and intersystem crossing to generate a triplet excited state. Unlike their predecessor, C60, the excited larger fullerenes have structured spectral bands in the visible/NIR region. The triplet excited states of C76 and C78 were also generated independently by energy transfer from triplet excited biphenyl with a diffusion-controlled rate (ket ≈ 1 × 1010 s-1). These larger fullerenes readily undergo one-electron reduction and oxidation by reacting with radiolytically generated (CH3)2•C(OH) and (CH2Cl2)•+ radicals. The absorption features of excited states and chemical reactivity of C76 and C78 are compared with those of C60, C70, and C84. The characterization of radical anions of these larger fullerenes is also made with complementary reduction experiments with UV...

Photosensitization of composite metal oxide semiconductor films

Abstract: New approach of surface modification consisting on mixing SnO 2 and TiO 2 colloids and packing them as a composite film (SnO 2 -TiO 2 ) on a conducting glass surface has led to the increased sensitized photocurrent quantum yields. The photoinjected electrons from the sensitizer into TiO 2 are quickly transferred into lower lying conduction band of SnO 2 carriers thus promoting the charge separation. High internal photocurrent quantum yield for CdS film as sensitizer, approaching unity (Φ = 95%), has been obtained. In addition, composite SnO 2 -TiO 2 nanocrystalline films improve the charge separation in adsorbed bis(2,2'-bipyridine)(2,2'-bipyridine-4,4'-dicarboxylic acid) ruthenium complex (Ru(lI)) too. Higher internal photocurrent quantum yield (Φ = 92%) has been obtained for Ru(II) by replacing SnO 2 carriers against composite SnO 2 -TiO 2 particles in the film. On the other hand, the later composition does not show any increase in the photovoltage.

Radical reactions of C84

Abstract: Pulse radiolysis and steady-state radiolysis experiments describing the radical and electron transfer reactions of C84 are reported here for the first time. C84 reacts readily with radiolytically generated chloromethyl (•CCl3) and trichloromethylperoxyl (CCl3OO•) radicals in CCl4. The formation of the radical adduct has been confirmed from its characteristic absorption in the UV (320 nm) and visible (480 nm). Radical-induced oxidation in 1,2-dichloroethane (1,2-DCE) resulted in a short lived transient absorbing at 920 nm. Reduction of C84 in toluene/2-propanol/acetone could be conveniently followed by formation of an absorption band with an absorption maximum at 960 nm.

Capped semiconductor colloids: Synthesis and photochemistry of CdS capped SnO2 nanocrystallites

Abstract: Cds-capped SnO2 (CdS@SnO2) and SiO2 (CdS@SiO2) colloids of 50–80 A in diameter have been prepared in aqueous medium. Significant quenching of CdS emission is observed in the CdS@SnO2 system as the electrons are injected from the excited CdS shell into the SnO2 core. Photoinduced charge separation in this system also results in transient bleaching in the 450 nm region. Picosecond laser flash photolysis studies of composite semiconductor nanoclusters that highlight the effect of the metal oxide core on the photophysical properties of the outer CdS shell are described.

Photoelectrochemical Behavior of Coupled SnO2/CdSe Nanocrystalline Semiconductor Films.

Abstract: A photoelectrochemical cell with a coupled SnO2|CdSe nanocrystalline semiconductor electrode has been prepared by sequential deposition of SnO2 and CdSe films onto an optically transparent electrode (OTE), and its photoelectrochemical behavior has been studied. The results show that the coupling of CdSe with SnO2 leads to an improvement in the performance of OTE|SnO2|CdSe over OTE|CdSe cells in terms of increased incident photon-to-current conversion efficiency, increased stability and smaller reversal of current. The favorable positioning of the energy bands of SnO2 and CdSe is responsible for the above observations. Various photoelectrochemical parameters of the OTE|SnO2|CdSe cell obtained for an incident light power of 0.31 mW cm−2 at 470nm, are as follows: Isc ≈ 25–30 μA cm−2, Voc ≈ 0.5–0.6 V, ƒƒ = 0.47 and a power conversion efficiency of about 2.25%.