Semiconductive colloids

A final interesting development is that of quantum-dot-loaded particles. Quantum dots [52] are themselves very small semiconducting colloids (1-lOnm) which fluoresce due to quantum confinement. They can be trapped within a polymer or silica [53] colloid. Since quantum dots photobleach much less than organic dyes, they can be used for experiments involving long-term observation. 

Chapter 4 (Modification and passivation of colloidal particles) deals with passivation of metal and semiconductive colloids in situ reactions, such as chemical reductions, photoreductions, polymerizations or thermal decompositions. The preparation of stable organics- or stabilizer-protected particles is very important to permit studies the novel... 

Let us add here that the fabrication of polycrystalline semiconductive films with enhanced photoresponse and increased resistance to electrochemical corrosion has been attempted by introducing semiconductor particles of colloidal dimensions to bulk deposited films, following the well-developed practice of producing composite metal and alloy deposits with improved thermal, mechanical, or anti-corrosion properties. Eor instance, it has been reported that colloidal cadmium sulfide [105] or mercuric sulfide [106] inclusions significanfly improve photoactivity and corrosion resistance of electrodeposited cadmium selenide. 

It is possible that colloidal photochemistry will provide a new approach to prebiotic syntheses. The work described previously on redox reactions at colloidal ZnS semiconductor particles has been carried on successfully by S. T. Martin and co-workers, who studied reduction of CO2 to formate under UV irradiation in the aqueous phase. ZnS acts as a photocatalyst in the presence of a sulphur hole scavenger oxidation of formate to CO2 occurs in the absence of a hole scavenger. The quantum efficiency for the formate synthesis is 10% at pH 6.3 acetate and propionate were also formed. The authors assume that the primeval ocean contained semiconducting particles, at the surface of which photochemical syntheses could take place (Zhang et al 2007). 

Other inorganic reactions shown to be photo-induced at colloidal semiconducting metal oxide surfaces include the synthesis of ammonia from water and nitrogen (19) and the oxidation of halide ions 1 ,... 

Leigh, D.S. (1996) Soil chronosequence of Brasstown Creek, Blued Ridge Mountains, USA. Catena 26 99-114 Leland, J.K. Bard, A.J. (1987) Photochemistry of colloidal semiconducting iron oxide polymorphs. J. Phys. Chem. 91 5076-5083 Lengweiler, H. Buser.W. Feitknecht, W. 

The colloidal or powder particle can be composed of either insulating, semiconductive, or conductive molecules. While only semiconductor particles are likely to be photoactive per se (by virtue of the energy gap between the filled valence band and the vacant conduction band), photoactivity of adsorbates can be mediated at the surface of other solids [18] which are often used themselves, or in conjunction with an irradiated semiconductor, as catalytic sites for alteration of kinetics of dark reactions initiated by photoexcitation. 

Many materials exist that have dimensions in the range of 1 rnn to several micrometers. Recall that colloidal particles (e.g., latex particles from emulsion polymerization, colloidal silica or alumina, etc.) fall in the range from about 10 nm to 1000 nm (1 jxm). A few examples of nanoparticles that are designed with more specific structures or geometries include carbon nanotubes, metal clusters, nanoscale magnetic crystals, and semiconducting ... 

Krishnaswamy, R. Kalyanikutty, K. P. Kanishka, B. Sood. A. K. Rao, C. N. R. Nonlinear Viscoelasticity of Ultrathin Nanocrystalline Semiconducting Films of CdS and CuS at Liquid-Liquid Interfaces. J. Colloid Interface Sd., (submitted for publication). 

Leland, J.K. and A.J. Bard (1987). Photochemistry of colloidal semiconducting iron oxide polymorphs. Journal of Physical Chemistry, 91, 5076-5083. 

Figure 21 UV-vis-NIR spectra of different SWNTs. (a) Spectrum of SWNTs ropes produced by laser ablation in comparison with the colloidal graphite, (b) Symmetric transitions (DOS) between lowest subbands in the semiconducting (A, B) and the metallic (C) tubes, (c) HiPco samples in suspended sodium dodecyl sulfate (SDS) micelles. Individually suspended tubes shows 50 meV blue shit with respect to the rope material. (Reprinted with permission from Ref. 133. 2003 American Chemical Society)...

When referring to Ti02-based photocatalytic systems it is important to note that, in most cases, the semiconducting oxide is associated there with a noble metal or/and a noble metal oxide catalyst. While the role played by these catalysts in (partial) cathodic reactions seems relatively well understood it remains less clear with regard to the photoanodic reactions. In particular, the exact function of the extensively used ruthenium dioxide catalyst has been questioned The role of Ru02 as a hole-transfer catalyst has, for example, been established through laser-photolysis kinetic studies in the case of photo-oxidation of halide (Br and CP) ions in colloidal titanium dioxide dispersions. In fact, the yields of Brf and ClJ radical anions, photogenerated in the course of these reactions. 

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