New approach of surface modification consisting on mixing SnO₂ and TiO₂ colloids and packing them as a composite film (SnO₂-TiO₂) on a conducting glass surface has led to the increased sensitized photocurrent quantum yields. The photoinjected electrons from the sensitizer into TiO₂ are quickly transferred into lower lying conduction band of SnO₂ carriers thus promoting the charge separation. High internal photocurrent quantum yield for CdS film as sensitizer, approaching unity (f = 95%), has been obtained. In addition, composite SnO₂-TiO₂ nanocrystalline films improve the charge separation in adsorbed bis(2,2’-bipyridine)(2,2’-bipyridine-4,4’-dicarboxylic acid) ruthenium complex (Ru(II)) too. Higher internal photocurrent quantum yield (f = 92%) has been obtained for Ru(II) by replacing SnO₂ carriers against composite SnO₂-TiO₂ particles in the film. On the other hand, the later composition does not show any increase in the photovoltage

Spectral sensitization of nanocrystalline ZnO films has been carried out with ruthenium complex, bis(2,2'­bipyridine)(2,2'­bipyridine­4,4'­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 (_c) 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 _c. 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.

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

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, (e), of charges in a SnO₂/Chl a based photoelectrochemical cell. A value of 0.75 was obtained for (e) for unbiased cells. With an IPCE of 13%, (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.

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.

Transient absorption spectra have been recorded following 532 nm laser pulse excitation of a ruthenium complex, Ru(2,2’-bipyridine)2(2,2’-bipyridine-4,4’-dicarboxylic acid) (Ru(II)) modified nanocrystalline SnO2 film. The transients formed under different applied potentials indicate formation of oxidized sensitizer (Ru(III)) under positive bias and the excited sensitizer (Ru(II)*) under negative bias. These results show the possibility of controlling heterogeneous electron transfer at the semiconductor interface with an externally applied bias.

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.

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 x 108 s-1), observed at positive bias agrees with the previously determined electron transfer rate constants from emission lifetime and microwave conductivity experiments. The apparent rate constant for heterogeneous electron transfer is dependent on the applied bias, and it decreases as the difference between the pseudo-Fermi level of SnO2 and oxidation potential of Ru(II)* decreases. These results suggest that the decreased rate of charge injection is responsible for lower IPCE (incident photon-to-photocurrent efficiency) observed in photoelectrochemical cells under negative bias. No significant change in the rate of reverse electron transfer was observed at potentials greater that 0.4 V.

In our continuing efforts to surface-modify semiconductor colloids, we have suc-

ceeded in preparing TiO₂-capped SnO₂ (SnO₂@TiO₂) and TiO₂-capped SiO₂(SiO₂-

@TiO₂) colloids. The SnO₂@TiO₂ colloids are 80-100 Å in diameter and exhibit

improved photochromic and photocatalytic efficiencies compared to the native  col-

loids. The improved charge separation in this system was confirmed from the en-

hanced efficiency of hole trapping monitored from the absorption peak at 360 nm.

The photocatalytic properties of SiO₂@TiO₂ colloids are similar to that of native TiO₂

colloids. The capped semiconductor systems are useful for the oxidation of I^- and

SCN^-. For example, the quantum efficiency for I^- oxidation can be improved by a

factor of 2-3 upon capping the SnO₂ colloids with TiO₂. The distinction between the

capped and coupled semiconductor systems has been made by preparing nanocrys-

talline thin films in two different geometries and studying their photoelectrochemical

behavior

Thin, transparent films of nanocrystalline SnO2 semiconductor have been prepared from 30 Å diameter colloids.  The electron trapping process in SnO2 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 increased with increasing film thickness, but attained a limiting value at thickness greater than 0.75 mm. The migration of charge across the grain boundaries was a limiting factor for the photocurrent generation in thicker films. These SnO2 films are highly porous and exhibit strong affinity for adsorption of sensitizer molecules such as Ru(II)(2,2'-bipyridine-4,4'-dicarboxylic acid)perchlorate (Ru(bpy)2(dcbpy)2+).  SnO2 films modified with a Ru(bpy)2(dcbpy)2+, exhibit excellent photoelectrochemical response in the visible with a power conversion efficiency of ~1% at 470 nm. The rate constant for the charge injection process as measured from the analysis of luminescence decay of   Ru(bpy)2(dcbpy)2+* on SnO2 surface, was in the range of 7.6 x 107  s-1 - 0.82 x107 s-1.

Nanocrystalline thin SnO2 semiconductor  films (thickness £1 μm) have been modified with chlorophyll b (chl-b) by electrodeposition and absorption methods for use as novel photosensitive electrodes in photoelectrochemical cell.  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.

UV excitation of TiO2 colloids in ethanol leads to the trapping of holes and electrons.  Photoelectrochromic effects are seen as these trapped charge carriers exhibit strong absorption in the red and UV regions.  Electron or hole scavengers such O2 or triethanolamine can selectively scavenge these stored charge carriers.  A simple photocatalytic method has been presented to carry out one-electron reduction of  C60 in 50/50 (v/v) benzene/ethanol.  The charge transfer between the excited TiO2 semiconductro colloid and C60 occurs with a quantum efficiency of 24%.  The dependence of observed quantum yield of C60-formation on the concentration of C60 shows the existence of an association equilibrium between the two clusters (K_app =   57 500M^-1).

Electron storage effects in quantized WO3 colloids have been investigated by spectroelectrochemical and photochemical methods. Electrons trapped within the colloidal particles exhibit blue coloration with absorption in the red-IR region.  From picosecond flash photolysis experiments, we estimate the rate constant for electron trapping to be 10¹⁰ s^-1.  The trapped electrons are stable in an inert atmosphere and can be utilized to reduce substrates such as thiazine and oxazine dye which have reduction potentials less negative than the conduction band of WO3.  The rate constants for the heterogeneous electron transfer at the semiconductor/eletcolyte interface are in the range (0.7 – 2.4)x10⁹ m^-1S^-1.