Colloidal Semiconductor Particles Research Articles

STUDY OF CHITOSAN PROTECTIVE PROPETIES IN COLLOIDAL SOLUTIONS OF CADMIUM SELENIDE QUANTUM DOTS

In this paper, by a low-temperature colloidal aqueous method, without the use of toxic media, the quantum dots of cadmium selenide in the shell of chitosan were synthesized. Chitosan was used as a matrix for the growth of colloidal semiconductor particles, and it also stabilized the obtained particles.The purpose of this work was studying the protective properties of chitosan in colloidal solutions of cadmium selenide quantum dots. The objective of the study was to analyze the protective effect of chitosan in colloidal systems by analyzing the optical and rheological properties of the obtained solutions.The dependence of the light transmission of colloidal solutions of cadmium selenide quantum dots in the shell of chitosan on the concentration of polysaccharide was investigated.It has been established that the light transmission of colloidal solutions decreases with time, the lower the content of chitosan in solution, which indicates that the protective action of colloidal solutions of quantum dots of cadmium selenide increases with increasing concentration of the polysaccharide.A study was made of the kinematic viscosity of the obtained solutions with different concentrations of chitosan in time. It was established that when the quantum dots are formed in the chitosan matrix, the viscosity of the colloidal system decreases. The protective effect of chitosan increases with increasing concentration in the solution.

Organic synthesis using photoredox catalysis.

Natural photosynthesis is a remarkable chemical machinery that enables our life on earth and delivers a constant stream of oxygen and organic biomass. We should acknowledge this fact with humbleness, especially because we have not been able yet to mimic this process in a reliable way even after decades of intense research. The basic mechanistic principle behind photosynthesis is photoredox catalysis or light-driven charge separation, which leads to an energy harvesting process by taking advantage of the reduction products and filling the holes by a sacrificial electron donor, water. Fortunately, we can use the waste product from this process, oxygen, for breathing. For applications in organic synthesis, the principles of photoredox chemistry serve as guidelines, i.e., photoinduced electron transfer (PET) kinetics and thermodynamics as expressed in the Rehm–Weller and Marcus equations. For catalytic versions, the photoinduced redox processes require efficient and robust photocatalysts, and in many cases appropriate sacrificial components. In recent years, three major groups of light-absorbing molecules/materials have been (re)investigated, which facilitate a wide range of redox activation from their excited states: transition metal complexes (e.g., the thoroughly investigated Ru(bipy)3 and other Ru or Ir complexes) with strong MLCT transitions, organic dyes such as xanthene, porphyrine or phthalocyanine dyes (e.g., eosin Y), and colloidal semiconductor particles (e.g., TiO2) [1–9]. In addition, combinations of light-absorbing materials have been studied such as dye-coated semiconductor nanoparticles. On the substrate side, the focus is on redox-active donor/acceptor molecules, which range from all kind of aromatic, olefinic and carbonyl-type electron acceptor compounds to heteroatom-linked electron donors. The relevance of carbon–carbon bond formation for organic synthesis is also depicted in these processes, and in recent years, enantioselective versions of these processes as well as unusual activation and coupling modes have been developed. In contrast to the “traditional” catalysis areas such as metal-, organo- and biocatalysis, photoredox catalysis (and photocatalysis in general) is a young research field with regard to synthetic applications. The collection of papers in this Thematic Series on organic synthesis using photoredox catalysis shows this convincingly. It was a great pleasure to act as the editor of this Thematic Series on photochemical reactions, and I would like to thank all authors for their excellent contributions and the staff of the Beilstein-Institut for their professional support. Axel G. Griesbeck Cologne, April 2014

HETEROSTRUCTURED HYBRID COLLOIDAL SEMICONDUCTOR NANOCRYSTALS

Significant efforts in the field of colloidal semiconductor particles have been dedicated to the fabrication and study of hybrid metal–semiconductor nanoheterostructures, where the incorporation of the metal moiety may potentially enhance and/or expand existing applications of semiconductor nanoparticles. Many of these metal–semiconductor nanostructured constructs exhibit physical properties not found in either of their metal or semiconductor components, providing many opportunities for further investigation into interface and coupling effects between the two materials. We review some of the key research endeavors in this area, focusing mainly on the synthesis of the materials and the characterization of the various metal–semiconductor constructs, and highlighting some of the unique applications that have emerged from these efforts.

Photodriven reduction and oxidation reactions on colloidal semiconductor particles: Implications for prebiotic synthesis

Photoelectrochemical reactions on mineral surfaces may have contributed to the synthesis of prebiotic molecules on early Earth. Laboratory experiments were conducted to develop this idea. Colloidal particles of ZnS were selected as a prototypical semiconducting mineral material. We investigated (1) the capability of the ZnS colloid to reduce aqueous CO 2 under ultraviolet irradiation to yield formate in the presence of a sulfur hole scavenger and (2) to oxidize formate to yield CO 2 in the absence of a hole scavenger. In the presence of the hole scavenger, the initial quantum efficiency is 10% for formate production from CO 2 at pH 6.3. The reaction rate is sensitive to the pH of the solution, increasing with acidity for the pH range of 5–9. The initial formate production rate is proportional to the concentration of aqueous inorganic carbon from 13 to 65 mM, suggesting an absence of surface saturation. Photoproducts other than formate also form, including acetate and propionate, a finding which demonstrates the formation of carbon–carbon coupling products. In the absence of hole scavenger but in the presence of the ZnS colloid, formate is photooxidized with a quantum efficiency of 1.1%. The likely presence of semiconducting colloidal particles in the oceans of the prebiotic Earth suggests that photochemical reactions on their surfaces could have played a significant role in the synthesis of organic molecules.

Luminescence-based assessment of thermodynamic constants for electrostatic binding of non-luminescent dyes and atomic ions to colloidal semiconductor surfaces

Steady-state and time-resolved luminescence experiments were employed to study the electrostatic interaction of cationic photosensitizers and negatively charged colloidal semiconductor particles. Electrostatic adsorption of efficient photosensitizers on semiconductor surfaces is known to quench the photosensitizers’ luminescence. By monitoring the quenching, either directly or competitively, thermodynamic constants for binding of both luminescent and non-luminescent ruthenium and osmium complexes to negatively charged SnO2 particle have been determined. The function of atomic cations, such as Li+, Na+ and K+, in controlling the binding of cationic dyes to colloidal SnO2 has also been studied via competitive luminescence. From the study, semiconductor surface binding constants for these species have also been determined.

Kinetics of Nitroxyl Radical Oxidation by Ru(bpy)33+ following Photosensitization of Antimony-Doped Tin Dioxide Colloidal Particles

Uncharged, water-soluble nitroxyl free radicals are shown by laser flash photolysis to be effective reductants of the Ru(bpy)33+ (bpy = 2,2‘-bipyridine) that is produced via photosensitization at the surfaces of colloidal semiconductor particles composed of antimony-doped tin dioxide. The bimolecular rate constant for the reduction of Ru(bpy)33+ by the 3-carbamoyl-PROXYL radical in the colloidal suspension, (4.9 ± 0.2) × 108 M-1 s-1, is about one-third of the rate constant for the corresponding reaction in homogeneous aqueous solution, (1.4 ± 0.1) × 109 M-1 s-1. The high quantum efficiency for production of Ru(bpy)33+, 0.93 ± 0.01, together with the fact that the oxidized nitroxyl radicals are widely used as oxidants in organic syntheses, makes such systems of interest for effecting photocatalytic oxidation reactions.

H-aggregation and correlated absorption and emission of a merocyanine dye in solution, at the surface and in the solid state. A link between crystal structure and photophysical properties

Abstract The formation of H-aggregates as a function of solution, substrate and ambient variables is considered for the merocyanine dye 3-acetyl-5-12-(3-ethyl-2-benzothiazolydene) rhodanine. Colloidal semiconductor particles are shown to be a powerful tool to control the size of the aggregates. In water the blue shifted absorption band has been assigned to a dimer. Its spectrum has been isolated and the thermodynamical variables derived for the dissociation reaction are: ΔrG0=21.16 kJ/mol, ΔrH0=32.36 kJ/mol and ΔrS0=37.24 J/mol K. Exciton band absorption maxima for aggregates in solution and at the water-TiO2 and water-Al2O3 interface, respectively, have been correlated to the aggregation geometry using the extended dipole model in conjunction with crystallographic data. Microcrystals showing a hypsochromical shift in the absorption band have been produced within the pores of nanocrystalline semiconductor films. The calculation shows that these aggregates are needle shaped and are composed of about 2250 monomer units. A broad emission band appears when the organic molecules assemble in a head to tail stacking geometry which could be attributed to excimer fluorescence. It is not quenched by charge injection into TiO2 and indicates the existence of dislocations within the merocyanine stacks.

Effect of the Amount of Reactant on the Photon-Fluence Dependence in Laser-Induced Transient Formation at the Surface of Colloidal Semiconductor Particles

A kinetics model was described for the surface electron transfer at illuminated colloidal semiconductor particles to discuss the effect of the acceptor concentration. The principle is a modification of the recombination model which was previously proposed by the authors to understand the dependence of the electron-transfer quantum yield on the photon fluence. The modified recombination model was successfully applied to the experimental results, where the quantum yields for the electron transfer from colloidal CdS to methylviologen were obtained at various fluences as a function of acceptor concentration. The new model reveals that the Langmuir-type relationship between the reaction yield and the reactant concentration is not applicable at higher photon fluences

Light-Induced Redox Reactions in Nanocrystalline Systems

A review with 156 refs. on interfacial electron transfer reactions in colloidal semiconductor solns. and thin films and their application for solar light energy conversion and photocatalytic water purifn. Some of the topics discussed include; optical and electronic properties of colloidal semiconductor particles, quantum size effects in the photoluminescence of colloidal semiconductors, light-induced charge sepn., dynamics of interfacial charge transfer processes, properties and prepn. of nanocryst. semiconductor electrodes, energetics and operations of the nanoporous solar cell.

Analysis of Colloids. VI. Semiconductor Colloids of High Monodispersity by Preparative Size Exclusion Chromatography

Abstract The recently described method of size exclusion chromatography (SEC) of very small colloidal semiconductor particles was transferred from the analytical scale to the preparative one. The particular difficulties dealing just with SEC and the special situation when working with inorganic colloids are described. The influence of flow rate, sample volume, sample concentration and fraction volume were studied using CdS-sols with particle diameters ranging from 2nm to 8nm. Up to 200mg samples were separated in less than one hour. The fractions were generally characterized by analytical SEC and in two instances by transmission electron microscopy showing excellent consistency and standard deviations down to 11%.

The photopolymerisation of methacrylic acid by colloidal semiconductors

Methacrylic acid was photopolymerised using CdS and composite, CdS/HgS and CdS/TiO2, colloidal semiconductor particles as initiators. The previously proposed photoinitiation mechanism involving the photogenerated positive hole in the valence band of the CdS colloid was confirmed by the electron scavenging action of TiO2. The effect of pH on the course of the polymerisation was investigated and is discussed.

The electrophoresis of semiconductor particles

The role of colloidal semiconductor particles as photocatalysts in some environmentally important reactions, and the importance of the nature of the particle surface during such reactions is discussed. A summary of semiconductor particle-related electrochemical concepts is presented and the potential of electrophoresis for insitu non-perturbative interrogation of particle surface behaviour is described. The new technique of photoelectrophoresis and its utility in photocolloidal systems is also described. Finally, some recent advances in the field of semiconductor colloid chemistry are presented, with emphasis on those systems for which particle surface charge-related information would prove most useful.

Synthesis and characterisation of colloidal metal and semiconductor particles prepared in microemulsions

The preparation of colloidal particles of platinum ‘in situ’ in water-in-oil microemulsions formed by the aerosol-OT (AOT)–water–heptane system and the hexadecyltrimethylammonium bromide (CTAB)–water–heptane–chloroform system is described. In both systems, particle growth is controlled and particle sizes of the order of a nm or so are obtained. In the AOT system, there is only a weak correlation between the particle size and droplet size. To obtain further insight into the mechanism of particle growth, the kinetics of the process were investigated. Nucleation particles (Pt°) are produced in an initial step by the reduction of PtCl2–6(PtIV) using hydrazine as reducing agent. The kinetics of this stage were found to be erratic and this was attributed to a reaction of AOT with hydrazine. This also had the effect of flocculating the microemulsion system (forming networks) which drastically perturbed the size measurements made using the sedimentation velocity (ultracentrifuge) method. Fractal growth was also indicated by light-scattering measurements.

Size Control and Surface Modification of Colloidal Semiconductor Particles

AbstractSeveral methods for the control of semiconductor particle sizes, in particular in the quantum size regime, are surveyed. Rational choice of solvent polarity, reaction temperature, concentration of reactants, solution pH, and counterions will determine the size and size distribution of the particles. Sizes of colloidal particles formed in microheterogeneous environments will often be predetermined by the dimensions of the micropores and cavities within the network of the medium. This is the situation in zeolite and clay cavities, and in polymer networks. Self‐aggregation of amphiphillic molecules often generates microdomains which may be utilized to direct the particle precursors or the particles themselves into these domains and thus limit their growth. Finally, complexation at the surface of the particles may limit and control the growth processes of the particles. The latter approach also allows functional modification of the particles' surface.

Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles

Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles

Optical and photoredox properties of small colloidal semiconductor particles in the presence of trivalent lanthanide ions

Size quantization effects upon the optical properties of small size semi-conductor particles ( d < 50 A ̊ ) have been documented in several systems. As the particle size decreases the band gap energy increases accompanied by a blue-shifted emission, and consequently the photoredox activity should increase. We are studying the correlation between the recombination luminescence of CdS particles and the redox potential of the conduction band probed in terms of oxidative luminescence quenching via electron transfer to Ln 3+ Ln 2+ and Ln 3+ Ln redox couples associated with different redox potentials. The electron transfer process from the conduction band of small CdS particles to electron acceptors ( ΔG ° el. tr < 0) is size dependent.

Oxidation of In 2S 3 and In 2Se 3 colloids as studied by pulse radiolysis

Radiolytic oxidation of colloidal semiconductor particles of In 2S 3 ( D p ∼ 15 Å) and In 2Se 3 ( D p ∼ 25 Å) was studied in reactions with OH ḣ, Br h ̇ 2, I h ̇ 2 and (SCN) h ̇ 2 radicals using a pulse-radiolysis technique. The one-electron oxidation of colloids led to bleaching of semiconductor absorption as a result of their decomposition. The rate constants for the reaction of X h ̇ 2 anion radicals with colloidal particles were less than those predicted for the diffusion-controlled reactions and depended on the standard redox potential of the oxidants.

Size quantization of colloidal semiconductor particles in silicate glasses

A method for incorporating quantized particles of colloidal semiconductors in transparent silicate glasses has been developed. The absorption spectra of the particles in the glasses are identical to those of the starting colloidal aqueous solutions. The luminescence intensity of colloidal semiconductors in silicate glasses is much higher than that for colloids in liquid solutions. This is attributed to passivation of surface recombination sites by the silicate polymer.

Chemical effects on the optical properties of semiconductor particles

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChemical effects on the optical properties of semiconductor particlesY. Wang and N. HerronCite this: J. Phys. Chem. 1987, 91, 19, 5005–5008Publication Date (Print):September 10, 1987Publication History Published online1 May 2002Published inissue 10 September 1987https://doi.org/10.1021/j100303a023RIGHTS & PERMISSIONSArticle Views204Altmetric-Citations56LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (480 KB) Get e-Alerts

Small CdS particles in inverted micelles

Colloidal semiconductor particles of CdS, CuS, and PbS of extremely small dimensions have been obtained in reversed micelles, water-in-oil CTAB, and SDS mieroemulsions and lecithin vesicles. The effect of the relation between reactant and water-pool concentration and the effect of the water pool size have been studied. The spectroscopic characteristics of the particles have been examined and necessary comparisons have been made.