Showing papers by "Prashant V. Kamat published in 1995"

Enhanced Rates of Photocatalytic Degradation of an Azo Dye Using SnO2/TiO2 Coupled Semiconductor Thin Films.

Abstract: We present here for the first time the results from electrochemically assisted photocatalytic experiments using coupled TiO{sub 2}/SnO{sub 2} semiconductor thin films in the degradation of textile dye effluent. We show that by using such a system the oxidative efficiency of photocatalytic semiconductor systems in degrading a commercial azo dye such as Acid Orange 7 (AO7) can be improved. The results presented here on the decolorization of the azo dye AO7 represents a major step forward in the development of new advanced oxidation processes for the treatment of such industrial waste. The improved charge separation as a result of coupling two semiconductor systems with different energy levels and the applied anodic bias is responsible for the enhancement in the rate of photocatalytic degradation. 20 refs., 6 figs.

Capped Semiconductor Colloids. Synthesis and Photoelectrochemical Behavior of TiO2 Capped SnO2 Nanocrystallites

Abstract: In our continuing efforts to surface-modify semiconductor colloids, we have succeeded in preparing TiO{sub 2}-capped SnO{sub 2} (SnO{sub 2} @ TiO{sub 2}) and TiO{sub 2}-capped SiO{sub 2} (SiO{sub 2} @ TiO{sub 2}) colloids. The SnO{sub 2} @ TiO{sub 2} colloids are 80-100 A in diameter and exhibit improved photochromic and photocatalytic efficiencies compared to the native colloids. The improved charge separation in this system was confirmed from the enhanced efficiency of hole trapping monitored from the absorption peak at 360 nm. The photocatlytic properties of SiO{sub 2} @ TiO{sub 2} colloids are similar to that of native TiO{sub 2} colloids. The capped semiconductor systems are useful for the oxidation of I{sup -} and SCN{sup -}. For example, the quantum efficiency for I{sup -} oxidation can be improved by a factor of 2-3 upon capping the SnO{sub 2} colloids with TiO{sub 2}. The distinction between the capped and coupled semiconductor systems has been made by preparing nanocrystalline thin films in two different geometries and studying their photoelectrochemical behavior. 54 refs., 9 figs.

Rate Constants for Charge Injection from Excited Sensitizer into SnO2, ZnO, and TiO2 Semiconductor Nanocrystallites

Abstract: Independent microwave absorption and luminescence measurements have been carried out to monitor the charge injection from excited bis(2,2-bipyridine) (2,2{prime}-bipyridine-4,4{prime}-dicarboxylic acid)ruthenium(II) cation, Ru(bpy){sub 2}(dcbpy){sup 2+}, into SnO{sub 2}, ZnO, and TiO{sub 2} nanocrystallites. The luminescence decay showed at least two components, and the faster of the two decay processes gave rate constants of (1-3) x 10{sup 8} s{sup -1}. The growth of microwave absorption was delayed from the laser pulse by a process showing a rate constant similar to that of the fast decay portion of the luminescence. With ZnO, a second, slower growth was seen in the microwave absorption, and its rate corresponded with that of the slower luminescence decay. The appearance of microwave conductivity at rates corresponding with that of the luminescence decay directly confirms heterogeneous electron transfer from excited dye to the semiconductor particle. The existence of two rates suggests differing adsorption and/or injection sites. In contrast to the behavior of excited Ru(bpy){sub 2}(dcbpy){sup 2+} charge injection from the excited singlet states of chlorophyll a and b on SnO{sub 2} film was very fast and could not be resolved (k > 5 x 10{sup 8} s{sup -1}) in the microwave experiment. 49 refs., 5 figs., 1 tab.

Photochemistry of Ru(bpy)2(dcbpy)2+ on Al2O3 and TiO2 Surfaces. An Insight into the Mechanism of Photosensitization

Abstract: Diffuse reflectance studies have been undertaken to investigate the photochemical behavior of a ruthenium complex, bis(2,2{prime}-bipyridine)(4,4{prime}-dicarboxy-2,2{prime}-bipyridine)ruthenium(II) [Ru(bpy)2(dcbpy){sup 2+}], on the surface of Al{sub 2}O{sub 3} and TiO{sub 2} particles. Decreased emission yield and lifetime of excited Ru(bpy){sub 2}(debpy){sup 2+} on TiO{sub 2} surface indicated dominance of the charge injection process in deactivating excited sensitizer. The rate constant for the heterogeneous electron transfer rate constant as measured from the analysis of luminescence decay was in the range (1.0-5.5) x 10{sup 8} s{sup -1}. Upon irradiation with visible light, the degassed sample of Ru(bpy)2{sup +} (dcbpy){sup 2+}-coated TiO{sub 2} particles turned blue as photogenerated electrons got trapped at the TiO{sub 2} surface. The blue color disappeared instantly upon exposure to air. Irreversible degradation of the sensitizer was noticeable when air-equilibrated samples were photolyzed with visible light for an extended period. Diffuse reflectance FTIR studies confirmed the semiconductor surface assisted photodegradation of Ru(bpy){sub 2}(dcbpy){sup 2+}. 36 refs., 9 figs.

Electrochemically Active Nanocrystalline SnO2 Films: Surface Modification with Thiazine and Oxazine Dye Aggregates

Abstract: Thin films of SnO{sub 2}, nanocrystallites have been surface-modified with thionine, methylene blue, and oxazine 170 by adsorption from the corresponding dye solutions. The strong electrostatic interaction between the cationic dye and the negatively charged semiconductor nanocrystallites results in close packing of the dye on the semiconductor surface. These closely packed H-aggregates of the adsorbed dye are active both electrochemically and photoelectrochemically. Electron transfer from semiconductor nanocrystallites into the adsorbed dye aggregates leads to bleaching of the colored film. The extent of dye bleaching which is readily controlled by the applied potential, has been probed by spectroelectrochemical measurements. The photocurrent action spectra of these dye-modified SnO{sub 2} films indicate charge injection from excited dye aggregate into the semiconductor nanocrystallites with an incident photon-to-photocurrent efficiency of < 1 %.

Photochemistry on Surfaces. Intermolecular Energy and Electron Transfer Processes between Excited Ru(bpy)32+ and H-Aggregates of Cresyl Violet on SiO2 and SnO2 Colloids

Abstract: Cresyl violet, a cationic dye (CV{sup +}), forms H-aggregates on the negatively charged SiO{sub 2} and SnO{sub 2} colloids. These aggregates exhibit broad absorbance around 520 nm. By coadsorbing a sensitizer, Ru(bpy){sub 3}{sup 2+}, we are able to characterize the triplet excited state and reduced form of dye-aggregates on the colloidal SiO{sub 2} and SnO{sub 2} suspensions. On SiO{sub 2} surfaces, the excited state quenching of Ru(bpy){sub 3}{sup 2+} by dye-aggregates proceeds via an energy transfer mechanism. Picosecond laser flash photolysis experiments indicate that such a surface-promoted energy transfer is completed within 20 ps. On the other hand dye-aggregates adsorbed onto SnO{sub 2} colloids undergo photosensitized reduction since the excited sensitizer, Ru(bpy){sub 3} {sup 2+}, is efficiently quenched by the semiconductor support. The role of support material in promoting energy and electron transfer processes is described. 87 refs., 11 figs.

Singlet and Triplet Excited State Behaviors of C60 in Nonreactive and Reactive Polymer Films

Abstract: The excited state behavior of C[sub 60] has been investigated in polystyrene (PS) and poly(9-vinylcarbazole) (PVCz) polymer films. Picosecond and nanosecond laser flash photolysis techniques are employed to probe the transient absorption properties of singlet and triplet excited states in nonreactive (PS) and reactive (PVCz) microenvironments. In PS film, the absorption properties and lifetimes of these excited states are very similar to those observed in solutions. Encapsulation of C[sub 60] with PS results in the suppression of excited state self-quenching processes. Excited state charge transfer complex formation dominates in PVCz film. The absence of long-lived electron transfer products indicates that the charge separation in PVCz film is short-lived. 40 refs., 6 figs.

Electrochemical and photoelectrochemical properties of monoaza-15-crown ether linked cyanine dyes : photosensitization of nanocrystalline SnO2 films

Abstract: Photophysical, electrochemical, and photosensitization properties of newly synthesized crown other derivatives of cyanine dyes have been investigated. These dyes exhibit strong absorption in the visible (450-560 nm) range with a solvent-dependent fluorescence quantum yield ranging from 0.006 to 0.11 and adsorb strongly on SnO{sub 2} nanocrystallites. The singlet excited state is readily quenched when adsorbed on SnO{sub 2} nanocrystallites as a result of a charge injection process. The photosensitization properties of these cyanine dyes in extending the photoresponse of SnO{sub 2} semiconductor have been investigated by probing the dependence of photocurrent and photovoltage on the wavelength and intensity of excitation. A maximum photon to photocurrent conversion efficiency of approximately 1% has been observed. 44 refs., 10 figs., 2 tabs.

Tailoring nanostructured thin films

Abstract: Chemical vapor deposition or molecular beam epitaxy has been the preferred technique for depositing thin semiconductor films. Compared with these ultrahigh vacuum deposition techniques, chemical and electrochemical approaches to casting thin films are relatively simple and inexpensive. These films have important technological applications in chemical sensors, microelectronics, electrooptics, imaging science, photovoltaics, and photocatalysis. In recent years the author has been investigating the photophysical and photocatalytic properties of quantized semiconductor colloids. Most of the experiments described here have been tested only in the laboratory. The commercial-scale production of photoelectrochemical cells based on the Ru complex-modified TiO{sub 2} system, and the photocatalytic reactor is already being considered in both the United States and Europe. Prototype photocatalytic reactors for purifying air and water are already in the market. This technology has ample scope for industries to tailor semiconductor nanostructures to their needs. The development of multicomponent semiconductor nanostructures with capped and coupled geometry is another important area where future efforts need to be directed.

Excited Triplet and Reduced Forms of C84

Abstract: The triplet excited state behavior of C[sub 84] has been probed by the triplet - triplet energy transfer method using pulse radiolytically generated biphenyl triplets in benzene. The triplet excited state has a weak absorption in the UV (difference absorption maxima at 310 and 345 nm) but no significant absorption in the visible. The rate constants for energy transfer from [sup 3]BP* and [sup 3]C[sub 60]* are 4 [times] 10[sup 9] and 4.7 [times] 10[sup 9] M[sup [minus]1] s[sup [minus]1], respectively. Triplet excited state properties of [sup 3]C[sub 84]* are compared with other fullerenes. A photocatalytic method has also been successfully employed to reduce C[sub 84] in a UV-irradiated colloidal TiO[sub 2] suspension. 36 refs., 4 figs., 1 tab.

Photophysical and Photoelectrochemical Behavior of Poly[styrene-co-3-(acrylamido)-6-aminoacridine]

Abstract: Photochemically and photoelectrochemically active copolymer poly[styrene-co-3-(acrylamido)-6-aminoacridine] (PS-AA) has been synthesized. The absorption properties of this copolymer (PS-AA) are dependent on the acidity of the medium as it can exist in neutral and singly and doubly protonated forms in solution. The excited singlet and triplet state properties of PS-AA are very similar at different acidities, indicating thereby the singly protonated species is the only dominant species that is responsible for excited-state emission. The fluorescence and triplet quantum yields are 0.16 ± 0.03 and 0.22 ± 0.06, respectively. The feasibility of employing these polymer films as photosensitive electrode materials has been demonstrated. A photon-to-photocurrent conversion efficiency (IPCE) of <1% has been observed with a PS-AA-coated nanocrystalline SnO 2 film.

Crown ether derivatives of squaraine: new near-infrared-absorbing, redox-active fluoroionophores for alkali metal recognition

Abstract: Three monoaza crown ether bisphenylsquaraine dyes, 1–3, with excellent solubilities in both polar and non-polar solvents, have been synthesised. The fluorescence yields of these red-sensitive dyes are dependent on the solvent polarity. They are capable of binding quantitatively with alkali metal ions, Li–, Na+, K+, as is seen from their fluorescence quenching and shift in the oxidation potentials. The fluorimetric and electrochemical experiments indicate the potential application of these dyes for cation recognition.

Electrochemically Active Nanocrystalline SnO2 Films: Surface Modification with Thiazine and Oxazine Dye Aggregates.

Abstract: Thin films of SnO{sub 2}, nanocrystallites have been surface-modified with thionine, methylene blue, and oxazine 170 by adsorption from the corresponding dye solutions. The strong electrostatic interaction between the cationic dye and the negatively charged semiconductor nanocrystallites results in close packing of the dye on the semiconductor surface. These closely packed H-aggregates of the adsorbed dye are active both electrochemically and photoelectrochemically. Electron transfer from semiconductor nanocrystallites into the adsorbed dye aggregates leads to bleaching of the colored film. The extent of dye bleaching which is readily controlled by the applied potential, has been probed by spectroelectrochemical measurements. The photocurrent action spectra of these dye-modified SnO{sub 2} films indicate charge injection from excited dye aggregate into the semiconductor nanocrystallites with an incident photon-to-photocurrent efficiency of < 1 %.

CdSe-SnO2 coupled semiconductor thin films: Electrochemical and photoelectrochemical studies

Abstract: The coupled OTE/SnO2/CdSe electrodes have been prepared by sequential deposition of SnO2 and CdSe colloidal films onto an optically transparent electrode (OTE), and their electrochemical and photoelectrochemical properties have been studied. It is observed that coupling SnO2 film with CdSe particles has extended its photoresponse into the visible. The incident photon-to-photocurrent conversion efficiency (IPCE) for CdSe film which is doubled when coupled with SnO2 film, shows a better charge separation in the coupled film. The cyclic voltammetric results of [Fe(CN)6]3−/4− with OTE, OTE/CdSe and OTE/SnO2/CdSe electrodes suggest that the coupling of CdSe with SnO2 creates an energy barrier that hinders electron flow from SnO2 to CdSe and into the electrolyte. Photocurrent stability of the cell employing OTE/SnO2/CdSe electrode has been improved by increasing the thickness of SnO2 particulate film. The photoelectrochemical performance of the cell has been evaluated and a net conversion efficiency of 2.25% has been obtained

Excited Triplet and Reduced Forms of CM

Abstract: visible absorption spectra of c8429 and the reduction potentials of c8427-31 have already been reported. At least six reversible reductions have been characterized using cyclic v o l t a m r r ~ e t r y . ~ ~ ~ ~ Although distinctively different electronic properties of these two isomers were observed in electrochemical and ESR studies, the separation of the two isomers of c 8 4 has not yet been achieved. Consequently, a mixture of the two isomers has been used in this investigation. We report here, for the first time, experimental results which characterize triplet excited state properties of c84. We also report the photoelectrochemical reduction of c 8 4 in colloidal Ti02 suspension. Experimental Section Materials. Mass-pure C84 was obtained via selective solvent extraction followed by HPLC chromatography with a semipreparative Buckyclutcher column (Regis) run in recycling mode. Details of the selective solvent enrichment techniques will be published later.26a The purity of the sample was checked by surface analysis via laser ionization (SALI) mass spectrometry (MS), a technique that has been used extensively at SRI Intemational for analysis of fullerene samples.27b SALI-MS showed that the fullerene purity exceeded 99.5%, with the only impurity peaks belonging to c 8 2 and c86; in particular, no c60 or c 7 0 was present in the SALI-MS spectrum obtained. The