Semiconductors colloids and

The photoelectrochemical properties of 283 colloids prepared by chemical solution growth [193] have been demonstrated by carrying out oxidation and reduction processes under visible light irradiation. Charged stabilizers such as Nation were found to provide an effective microenvironment for controlling charge transfer between the semiconductor colloid and the redox relay. 

Optical absorption spectroscopy is often carried out on CD films to verify that the films have a bandgap expected from the deposited semiconductor. Additionally, since CD films are often nanocrystaUine and the most apparent effect of very small crystal size is the increasing bandgap due to size quantization (the effect is visible to the eye if the bandgap is in the visible region of the spectrum), absorption (or transmission) optical spectroscopy is clearly a fast and simple pointer to crystal size, since bandgap-size correlations have been made for a number of semiconductor colloids and films. 

Hotchandani, S. Kamat, P. V. Modification of electrode surface with semiconductor colloids and its sensitization with chlorophyll a, Chem. Phys. Lett. 1992, 191, 320. 

Figure 12. Control of electron transfer reactions at semiconductor solution interfaces (a) electron transfer at a semiconductor solution interface and degradative recombination processes (b) application of charged colloid as a supporting agent of semiconductor particles (c) application of a redox functionalized polymer as a supporting matrix for stabilization of semiconductor colloids and effective trapping of conduction band electrons.

The photcwurrent measurements establish that injection of charges occurs from the excited state of the dye to the conduction band of the semiconductor, but does not say anything about the identity of the dye excited state involved. Luminescence and time-resolved (transient) absorption studies on dye-semiconductor colloids and dye-coated films on conducting glass electrodes have been useful to establish the identity of the excited state and energetics of the sensitization process. 

Interfacial electron transfer in colloidal metal and semiconductor dispersions and photodecomposition of water. K. Kalyanasundaram, M. Gratzel and E. Pelizzelti. Coord. Chem. Rev., 1986, 69, 57 (338). 

Gaunt, J. A. Knight, A. E. Windsor, S. A. and Chechik, V. (2005). Stability and quantum yield effects of small molecules additives on solutions of semiconductor nanoparticles. /. Colloid and Interface Science, 290 (2), 437-443. 

Fig. 3.16 Reaction schemes of different CBD mechanisms for compound semiconductors (a) atom-by-atom process (b) aggregation of colloids and (c) mixed process. (Reprinted from [247], Copyright 2009, with permission from Elsevier)...

Thin film coatings of nanocrystalline semiconductors, as collections of quantum dots (QD or Q-dot) attached to a solid surface, resemble in many ways semiconductor colloids dispersed in a liquid or solid phase and can be considered as a subsection of the latter category. The first 3D quantum size effect, on small Agl and CdS colloids, was observed and correctly explained, back in 1967 [109]. However, systematic studies in this field only began in the 1980s. 

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

Spanhel L, Weller H, Henglein A (1987) Photochemistry of semiconductor colloids. 22. Electron injection from illuminated CdS into attached TiOa and ZnO particles. J Am Chem Soc109 6632-6635... 

A precursor of the studies on electron transfer reactions between short-lived radicals and colloidal particles was the development of a fast pulse radiolysis method to measure. the polarograms of radicals in the 10 s range . After considerable information had been acquired about the electron transfer reactions of a few dozen radicals at the mercury electrode, this compact electrode was replaced by metal colloids somewhat later, by semiconductor colloids These studies led to the detection of the electron-storing properties of certain colloids and of reactions of the stored electrons. 

Photoreactions on ZnO powder in aqueous suspension and in contact with gases have often been studied during the last few decades, and only a few aspects of this work are reviewed here. For example, nitrous oxide and methyl iodide were found to decompose when brought into contact at 20 °C with the illuminated surface of ZnO and nitrate, indigo carmine and p-nitrosodimethylaniline were found to be reduced in aqueous suspensions ZnO is of special interest as it is one of the standard electrode materials in conventional semiconductor electrochemistry and photo-electrochemistry Colloidal ZnO has not been available until recently. It... 

Differences in the absorption spectra of colloidal and macrocrystalline semiconductors were first recognized for CdS and AgBr The absorption of 3 nm particles of CdS in aqueous solution begins close to 515 nm, the wavelength at which bulk CdS starts to absorb however, the increase in absorption at shorter wavelengths is much less steep than for the macrocrystalline material (Fig. 6). The effect was first explained by a possible amorphous structure of the colloidal particles However, after it was shown by Brus and co-workers that the particles had an ordered struc-... 

X., Tohji, K., Jeyadevan, B., Shinoda,K., Ogawa, T., Arai, T., Hihara,T., and Sumiyama, K. (2002) Size- and shape-controls and electronic functions of nanometer-scale semiconductors and oxides. Colloids and Surfaces A Physicochemical and Engineering Aspects, 202 (2-3), 291-296. 

Whether there is currently a nanotechnology is a question of definition. If one asks whether there are (or are soon likely to be) commercial electronic fluidic, photonic, or mechanical devices with critical lateral dimensions less than 20 nm, the answer is no, although there may be in 10 to 20 years. There is, however, a range of important technologies—especially involving colloids, emulsions, polymers, ceramic and semiconductor particles, and metallic alloys—that currently exist. But there is no question that the field of nanoscience already exists. 

The approaches used for preparation of inorganic nanomaterials can be divided into two broad categories solution-phase colloidal synthesis and gas-phase synthesis. Metal and semiconductor nanoparticles are usually synthesized via solution-phase colloidal techniques,4,913 whereas high-temperature gas-phase processes like chemical vapor deposition (CVD), pulsed laser deposition (PLD), and vapor transfer are widely used for synthesis of high-quality semiconductor nanowires and carbon nanotubes.6,7 Such division reflects only the current research bias, as promising routes to metallic nanoparticles are also available based on vapor condensation14 and colloidal syntheses of high-quality semiconductor nanowires.15... 

I. Bedjat, P.V. Kamat, Capped semiconductor colloids. Synthesis and photo-electrochemical behavior of Ti02-capped SnC>2 nanocrystallites, J. Phys. Chem. 99 (1995) 9182-9188. 

Koch U, Fojtik A, Weller H, Henglein A (1985) Photochemistry of semiconductor colloids preparation of extremely small ZnO particles, fluorescence phenomena and size quantization effects. Chem Phys Lett 122 507-510... 

Bedja I, Kamat PV (1999) Capped semiconductor colloids synthesis and photoelectrochemical behavior of Xi02 capped Sn02 electrolyte. J Phys Chem 99 9182-9188... 

Several synthetic methods for the preparation of semiconductor nanoparticles have been reported. Colloidal and organometallic routes have probably been identified as the two major methods in use [11-16], although nano dimensional particles have been also synthesized in confined matrices such as zeolites [17], layered solids [18], molecular sieves [19,20], vesicles/micelles [21,22], gels [23,24], and polymers [25]. An ideal synthetic route should produce nanoparticles which are pure, crystalline, reasonably monodisperse and have a surface which is independently derivatized. 

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