By using bifunctional surface modifiers (SH−R−COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO2 films. Upon visible light excitation, CdSe QDs inject electrons into TiO2 nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO2 nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 × 109 and 1.95 × 1011 s-1. The injected charge carriers in a CdSe-modified TiO2 film can be collected at a conducting electrode to generate a photocurrent. The TiO2−CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO2/CdSe interfaces and internal TiO2 grain boundaries.

Quantum dot solar cells. harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films.

The mechanisms of electrophoretic deposition (EPD) are discussed and their shortcomings identified. The kinetics of the processes involved are analyzed for constant-current and constant-voltage conditions. A method of determining the Hamaker constant of suspended particles is developed by modeling the relationship between the particle inter-action energy and the suspension stability. A three-probe dc technique is used to map the voltage profile around the depositing electrode, and the results are used to explain discrepancies between the calculated and experimentally observed voltage drops during deposition. A mechanism of deposition is proposed based on DLVO theory and particle double-layer distortion/thinning on application of a dc field to the suspension. Kinetic equations are developed for constant-current and constant-voltage EPD using mass balance conditions; these are verified by experiments. After the phenomenon is introduced and discussed, a critique of the application of EPD to the synthesis of ceramic shapes and coatings is given.

Electrophoretic Deposition (EPD): Mechanisms, Kinetics, and Application to Ceramics

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminesce...

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

Complex electrochemical reactions such as Oxygen Reduction Reaction (ORR) involving multi-electron transfer is an electrocatalytic inner-sphere electron transfer process that exhibit strong dependence on the nature of the electrode surface. This criterion (along with required stability in acidic electrolytes) has largely limited ORR catalysts to the platinum-based surfaces. New evidence in alkaline media, discussed here, throws light on the involvement of surface-independent outer-sphere electron transfer component in the overall electrocatalytic process. This surface non-specificity gives rise to the possibility of using a wide-range of non-noble metal surfaces as electrode materials for ORR in alkaline media. However, this outer-sphere process predominantly leads only to peroxide intermediate as the final product. The importance of promoting the electrocatalytic inner-sphere electron transfer by facilitation of direct adsorption of molecular oxygen on the active site is emphasized by using pyrolyzed metal porphyrins as electrocatalysts. A comparison of ORR reaction mechanisms between acidic and alkaline conditions is elucidated here. The primary advantage of performing ORR in alkaline media is found to be the enhanced activation of the peroxide intermediate on the active site that enables the complete four-electron transfer. ORR reaction schemes involving both outer- and inner-sphere electron transfer mechanisms are proposed.

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Fundamental mechanistic understanding of electrocatalysis of oxygen reduction on Pt and non-Pt surfaces: Acid versus alkaline media

The solvothermal reaction of sulfur, selenium, or tellurium with cadmium metal powder in different organic solvents in the temperature range 120−200 °C was investigated systematically to prepare ca...

Solvothermal Elemental Direct Reaction to CdE (E = S, Se, Te) Semiconductor Nanorod

Electrodeposition of CuInSe2 films from a sulphate bath

The electrophoretic deposition of CdS, CdSe, and their mixtures has been studied for the preparation of thin film anodes for photoelectrochemical cells. These films were obtained in a homogeneous state on SnO/sub 2//sup -/ coated glass substrates from the suspensions using polar organic dispersion media. Prior to photoelectrochemical experiments, the thin film anodes were thermally treated for 3 hr at 400/sup 0/-600/sup 0/C. An energy conversion efficiency of 2% was observed for the photoelectrochemical cells using these film anodes in S/sup 2 -//S /SUB lx/ /sup 2 -/ electrolytes.

Electrophoretically Deposited CdS and CdSe Anodes for Photoelectrochemical Cells

Electrochemical studies of the CdS film deposited onto a transparent conductive SnO2-coated glass slide by chemical spray deposition were performed in aqueous polysulphide solutions and the output characteristics of the electrochemical photocell (sprayed CdS film/polysulphide electrolyte/Pt) were measured. It was proved to be possible to construct a regenerative photocell free from the light losses through the polysulphide solution path by illuminating the CdS film anode through the SnO2-coated glass. Quantum yield and the maximum energy conversion efficiency of this photocell for monochromatic excitation of 480 nm were about 8% and 0.5%, respectively. It is concluded that the poorer quantum yield of this photocell employing CdS film than that of a photocell employing single crystal CdS can be attributed to the existence of lattice imperfections in the film.

Y. Ueno

Papers

Electrodeposition of CuInSe2 films from a sulphate bath

Electrophoretically Deposited CdS and CdSe Anodes for Photoelectrochemical Cells

Electrochemical solar cell based on a sprayed CdS film photoanode

Crystal growth of cadmium selenide by fused salt electrolysis and its photoelectrochemical properties

Photochargeable multilayer membrane device composed of CdS film and prussian blue battery