Highly concentrated quantum size ZnO sols have been synthesized and used for sol-gel dip processi... more Highly concentrated quantum size ZnO sols have been synthesized and used for sol-gel dip processing to fabricate optically transparent ZnO films on commercial glass substrates. Controlled sintering of the films produces c-axis oriented ZnO crystallites, without altering the optical transparency. The resulting coatings have shown to be nanoporous, e.g., they have shown the ability to serve as a host for quantum size CdS clusters. In these CdS-ZnO composites, a rapid electron transfer onto ZnO takes place after photoinduced electron-hole pair generation in CdS. The stored electrons cause a blue shift of the absorption threshold of ZnO that is attributed to exciton polarization within the created excess electric field.
ABSTRACTFilms and monoliths, containing clusters (sizes < 5 nm) of the binary semiconductor Cd... more ABSTRACTFilms and monoliths, containing clusters (sizes < 5 nm) of the binary semiconductor CdS and sandwiched CdS-PbS, were prepared via multifunctional inorganic-organic sol-gel processing. As a sulfur source, hexamethyldisilylthiane was employed. In precursor sols, the metal sulfide clusters are carrying functionalized silanes acting as stabilizing centers as well as inorganic and organic network formers. Hydrolysis and condensation produces an inorganic network yielding viscous liquids useful to prepare optically transparent films or monoliths. The final organic cross-linking at T < 100°C results in materials of variable spectral response, thickness and optical density. In preliminar degenerate four-wave mixing experiments, self-diffraction from laser-induced gratings was observed on unsupported 200 μm thick CdS-PbS doped monoliths. The maximum first order grating efficiency, measured at different wavelengths between 490 and 520 nm, was 0.5 · 10−3 and the corresponding cal...
ABSTRACT Alloyed ZnxTiyOz nanostructures were prepared by a thermal condensation process employin... more ABSTRACT Alloyed ZnxTiyOz nanostructures were prepared by a thermal condensation process employing ethanolic mixtures of zinc acetate dihydrate and titanium alkoxide, without additional use of additives. It has been found that the nanocrystal growth takes place at temperatures above 350 °C yielding cubic spinel Zn2TiO4, cubic ZnTiO3, or hexagonal ilmenite ZnTiO3. Their preferential appearance and size tuning between 5 and 80 nm was possible by varying the starting sol condensation conditions, Zn/Ti-ratio and sintering temperature. The electronic transitions in these ternary oxide semiconductor nanocrystals were observed at photon energies between 3.6 and 3.8 eV that reflects the presence of a wider optical gap than that known for the corresponding binary oxides ZnO and TiO2. We also noticed a strong influence of Eu3+ doping on the resulting structural and optical properties. Despite its large ionic size, this lanthanide efficiently intervenes in the evolution process by blocking the nanocrystal growth and the cubic-to-hexagonal transformation in ZnTiO3. Its preferential incorporation into spinel phase is manifested by the activation of a strong red intrashell fluorescence and a significant Stark splitting of the 5FJ states. The comparison of the spectral response and asymmetry ratio values deduced from the experimental Eu3+ luminescence data collected on structurally similar Zn2TiO4 and ZnTiO3 nanophases suggests the hypothesis that cubic ZnTiO3 has a structure of an inverse defect spinel. Our study has not confirmed the existence of the previously reported and often recalled defect spinel Zn2Ti3O8.
Nanokristalline Partikel enthaltendes Material, wobei die nanokristallinen Partikel aus II/VI-Hal... more Nanokristalline Partikel enthaltendes Material, wobei die nanokristallinen Partikel aus II/VI-Halbleitern aus zwei oder mehreren der Komponenten Cd, Zn, Hg und S, Se, Te ausgewahlt sind und die aus einem stabilisierende Liganden enthaltenden Sol abgeschiedenen Partikel als matrixfreier Film angeordnet sind, wobei der matrixfreie Film durch Vernetzung der nanokristallinen Partikel untereinander uber bifunktionelle Liganden gebildet ist.
Aggregation, Gelation, and Crystal Growth in Concentrated ... Contribution from the Water Chemist... more Aggregation, Gelation, and Crystal Growth in Concentrated ... Contribution from the Water Chemistry Program, University of Wisconsin, Madison, Wisconsin 53706. Receiced May 31, 1990. Revised Manuscript Received October 19, I990 ... Abstract: A new synthesis of ZnO wurtzite ...
... The curing procedure took usually 1-2 minutes. Optical absorption spectra from the colloidal ... more ... The curing procedure took usually 1-2 minutes. Optical absorption spectra from the colloidal solutions and cured sol-gel films on glass substrates were taken with an OMEGA 30 spectrophotometer (Bruins Instruments). Cluster ...
ABSTRACT The role of orange based pectin in the nucleation and growth of silver and gold nanopart... more ABSTRACT The role of orange based pectin in the nucleation and growth of silver and gold nanoparticles is addressed. Pectin is a complex polysaccharide found in fruits such as oranges, lemons, passion fruits or apples. It displays smooth and hairy chain regions containing hydroxyl-, ester-, carboxylate- and eventually amine groups that can act as surface ligands interacting under various pH conditions more or less efficiently with growing nanometals. Here, a high methoxy pectin (&gt;50% esterified) was used as a stabilizer/reducing agent in the preparation of gold, silver and silver–gold nanoparticles. Commercial pectin (CP) and pectin extracted from orange bagasse (OP) were used. Optionally, trisodium citrate or oxalic acid we used to reduce AgNO3 and HAuCl4 in aqueous environment. Characterization methods included UV–vis absorption spectroscopy, transmission electron microscopy, electron diffraction and energy-dispersive X-ray spectroscopy. The results show that under different pH conditions, pectin and reducing agents allow producing various nanostructures shapes (triangles, spheres, rods, octahedrons and decahedrons) often with high polydispersity and sizes ranging between 5 nm and 30 nm. In addition, depending on Ag/Au-ratio and pH, the surface plasmon bands can be continuously shifted between 410 nm and 600 nm. Finally, pectin seems to be a highly efficient stabilizer of the colloidal systems that show a remarkable stability and unchanged optical spectral response even after five years.
25 years of research devoted to semiconductor photo-catalysis brought about several new materials... more 25 years of research devoted to semiconductor photo-catalysis brought about several new materials concepts currently explored in environmental sciences, solar cell technologies and preparative "green" chemistry. A real breakthrough is however yet to come because photo-stable non-toxic visible-light harvesting nano-architectures are still not available. At least two strategies of "doping" exist that shifts the optical spectra of white pigments TiO 2 and ZnO from the UV into the visible spectral range. The first route is based on embedding transition metal cations (Fe 3+ , Pt 4+ , V 5+ etc.) or their complexes into the nanocrystals. Of particular interest to us is the second recently reported sputtering of metal oxide targets in Ar-N 2 gas mixtures offering colored powders and layers of zinc [1] -and titanium [2] oxynitrides. In our work, we developed two new wet chemical routes to colored Zn x Ti y O w-z N z powders and nanocrystalline waveguides on glass. The fir...
Optically transparent CdS membranes have been synthesized. Colloidal Q-CdS particles (particle di... more Optically transparent CdS membranes have been synthesized. Colloidal Q-CdS particles (particle diameter d{sub p} < 40 â«) carrying short phosphate chains and excess Cd{sup 2+} ions have been directly converted into a microporous membrane form. By controlling ionic strength and particle concentrations, one can link self-organized fusion-free aggregates and avoid formation of powder-like flocculation products. The resultant unsupported membranes exhibit different mechanical properties (e.g., rigidity, delayed elasticity, and solubility) in the presence of water depending upon the preparation method employed. The membrane form of CdS has distinctively different photophysical properties than the precursor colloidal form. Conversion of weakly red luminescent colloids (broad band at 700 nm) into membranes activated an intense room temperature band edge luminescence (BEL) (narrow bands between 450 and 500 nm) attributed to the recombination of excitons and/or shallowly trapped electron/...
The classical theories for overall crystallization kinetics are first reviewed, then discussed an... more The classical theories for overall crystallization kinetics are first reviewed, then discussed and extended. To model the crystallization phenomena during polymer processing, we have chosen an extension of Ozawa’s theory which allows an easy determination of the kinetic parameters from anisothermal DSC experiments. The kinetic law is then coupled with the energy equation. Two applications are presented: cast-film extrusion and injection molding.
Berichte der Bunsengesellschaft für physikalische Chemie, 1987
CV curves only show one peak. Although the spectral data do not prove a two-step charge-transfer ... more CV curves only show one peak. Although the spectral data do not prove a two-step charge-transfer mechanism in these pH regions for the smooth Ag electrode, the fact that the SERS and CV curves establish the existence of the intermediate strongly suggests that a two-step mechanism ...
ABSTRACT Electrons and positive holes have been produced in colloidal TiO2 particles upon illumin... more ABSTRACT Electrons and positive holes have been produced in colloidal TiO2 particles upon illumination with near-u.v. light. In the absence of adsorbed reactants, the charge carriers recombine. In the presence of an adsorbed reactant for holes [such as SCN- or poly(vinyl alcohol)], an excess of long-lived electrons remains on the particles and finally reacts with a reactant in the bulk solution. In the presence of an adsorbed reactant for electrons (such as a platinum deposit or methyl viologen), an excess of long-lived holes remains and finally reacts with an oxidizable compound. The processes observed include the reduction of water to form H2, the reduction of tetranitromethane and halothane and the oxidation of Br- and various organic compounds. Methyl viologen, MV2+, was found to be reduced to the radical MV.+ and also to be oxidized, to a lesser extent, to form a fluorescing compound. The absorption spectra of the long-lived excess of electrons and positive holes trapped at the interface were measured using flash photolysis.
Berichte der Bunsengesellschaft für physikalische Chemie, 1987
ABSTRACT The luminescence of colloidal CdS in solutions containing sodium polyphosphate as stabil... more ABSTRACT The luminescence of colloidal CdS in solutions containing sodium polyphosphate as stabilizer is activated by the addition of either Ag(ClO4) or colloidal Ag2S. The degree of activation depends on how strongly the CdS and Ag2S particles are attached to each other, and this attachment is controlled by the concentration of polyphosphate and added divalent metal ions which bind to polyphosphate. The CdS fluorescence in these solutions can be excited by light of wavelengths longer than the wavelength of the absorption edge of CdS. The results are explained in terms of an interfacial region in which anion vacancies and incorporated silver centers, which deliver electrons into the conduction band of CdS upon illumination with photon energies less than the bandgap energy of CdS, are present. The preparation of Q-CdS sols which fluoresce with quantum yields close to 100% is also described. Strong fluorescence occurs when defect sites, at which radiationless recombination takes place, are blocked.
Highly concentrated quantum size ZnO sols have been synthesized and used for sol-gel dip processi... more Highly concentrated quantum size ZnO sols have been synthesized and used for sol-gel dip processing to fabricate optically transparent ZnO films on commercial glass substrates. Controlled sintering of the films produces c-axis oriented ZnO crystallites, without altering the optical transparency. The resulting coatings have shown to be nanoporous, e.g., they have shown the ability to serve as a host for quantum size CdS clusters. In these CdS-ZnO composites, a rapid electron transfer onto ZnO takes place after photoinduced electron-hole pair generation in CdS. The stored electrons cause a blue shift of the absorption threshold of ZnO that is attributed to exciton polarization within the created excess electric field.
ABSTRACTFilms and monoliths, containing clusters (sizes < 5 nm) of the binary semiconductor Cd... more ABSTRACTFilms and monoliths, containing clusters (sizes < 5 nm) of the binary semiconductor CdS and sandwiched CdS-PbS, were prepared via multifunctional inorganic-organic sol-gel processing. As a sulfur source, hexamethyldisilylthiane was employed. In precursor sols, the metal sulfide clusters are carrying functionalized silanes acting as stabilizing centers as well as inorganic and organic network formers. Hydrolysis and condensation produces an inorganic network yielding viscous liquids useful to prepare optically transparent films or monoliths. The final organic cross-linking at T < 100°C results in materials of variable spectral response, thickness and optical density. In preliminar degenerate four-wave mixing experiments, self-diffraction from laser-induced gratings was observed on unsupported 200 μm thick CdS-PbS doped monoliths. The maximum first order grating efficiency, measured at different wavelengths between 490 and 520 nm, was 0.5 · 10−3 and the corresponding cal...
ABSTRACT Alloyed ZnxTiyOz nanostructures were prepared by a thermal condensation process employin... more ABSTRACT Alloyed ZnxTiyOz nanostructures were prepared by a thermal condensation process employing ethanolic mixtures of zinc acetate dihydrate and titanium alkoxide, without additional use of additives. It has been found that the nanocrystal growth takes place at temperatures above 350 °C yielding cubic spinel Zn2TiO4, cubic ZnTiO3, or hexagonal ilmenite ZnTiO3. Their preferential appearance and size tuning between 5 and 80 nm was possible by varying the starting sol condensation conditions, Zn/Ti-ratio and sintering temperature. The electronic transitions in these ternary oxide semiconductor nanocrystals were observed at photon energies between 3.6 and 3.8 eV that reflects the presence of a wider optical gap than that known for the corresponding binary oxides ZnO and TiO2. We also noticed a strong influence of Eu3+ doping on the resulting structural and optical properties. Despite its large ionic size, this lanthanide efficiently intervenes in the evolution process by blocking the nanocrystal growth and the cubic-to-hexagonal transformation in ZnTiO3. Its preferential incorporation into spinel phase is manifested by the activation of a strong red intrashell fluorescence and a significant Stark splitting of the 5FJ states. The comparison of the spectral response and asymmetry ratio values deduced from the experimental Eu3+ luminescence data collected on structurally similar Zn2TiO4 and ZnTiO3 nanophases suggests the hypothesis that cubic ZnTiO3 has a structure of an inverse defect spinel. Our study has not confirmed the existence of the previously reported and often recalled defect spinel Zn2Ti3O8.
Nanokristalline Partikel enthaltendes Material, wobei die nanokristallinen Partikel aus II/VI-Hal... more Nanokristalline Partikel enthaltendes Material, wobei die nanokristallinen Partikel aus II/VI-Halbleitern aus zwei oder mehreren der Komponenten Cd, Zn, Hg und S, Se, Te ausgewahlt sind und die aus einem stabilisierende Liganden enthaltenden Sol abgeschiedenen Partikel als matrixfreier Film angeordnet sind, wobei der matrixfreie Film durch Vernetzung der nanokristallinen Partikel untereinander uber bifunktionelle Liganden gebildet ist.
Aggregation, Gelation, and Crystal Growth in Concentrated ... Contribution from the Water Chemist... more Aggregation, Gelation, and Crystal Growth in Concentrated ... Contribution from the Water Chemistry Program, University of Wisconsin, Madison, Wisconsin 53706. Receiced May 31, 1990. Revised Manuscript Received October 19, I990 ... Abstract: A new synthesis of ZnO wurtzite ...
... The curing procedure took usually 1-2 minutes. Optical absorption spectra from the colloidal ... more ... The curing procedure took usually 1-2 minutes. Optical absorption spectra from the colloidal solutions and cured sol-gel films on glass substrates were taken with an OMEGA 30 spectrophotometer (Bruins Instruments). Cluster ...
ABSTRACT The role of orange based pectin in the nucleation and growth of silver and gold nanopart... more ABSTRACT The role of orange based pectin in the nucleation and growth of silver and gold nanoparticles is addressed. Pectin is a complex polysaccharide found in fruits such as oranges, lemons, passion fruits or apples. It displays smooth and hairy chain regions containing hydroxyl-, ester-, carboxylate- and eventually amine groups that can act as surface ligands interacting under various pH conditions more or less efficiently with growing nanometals. Here, a high methoxy pectin (&gt;50% esterified) was used as a stabilizer/reducing agent in the preparation of gold, silver and silver–gold nanoparticles. Commercial pectin (CP) and pectin extracted from orange bagasse (OP) were used. Optionally, trisodium citrate or oxalic acid we used to reduce AgNO3 and HAuCl4 in aqueous environment. Characterization methods included UV–vis absorption spectroscopy, transmission electron microscopy, electron diffraction and energy-dispersive X-ray spectroscopy. The results show that under different pH conditions, pectin and reducing agents allow producing various nanostructures shapes (triangles, spheres, rods, octahedrons and decahedrons) often with high polydispersity and sizes ranging between 5 nm and 30 nm. In addition, depending on Ag/Au-ratio and pH, the surface plasmon bands can be continuously shifted between 410 nm and 600 nm. Finally, pectin seems to be a highly efficient stabilizer of the colloidal systems that show a remarkable stability and unchanged optical spectral response even after five years.
25 years of research devoted to semiconductor photo-catalysis brought about several new materials... more 25 years of research devoted to semiconductor photo-catalysis brought about several new materials concepts currently explored in environmental sciences, solar cell technologies and preparative "green" chemistry. A real breakthrough is however yet to come because photo-stable non-toxic visible-light harvesting nano-architectures are still not available. At least two strategies of "doping" exist that shifts the optical spectra of white pigments TiO 2 and ZnO from the UV into the visible spectral range. The first route is based on embedding transition metal cations (Fe 3+ , Pt 4+ , V 5+ etc.) or their complexes into the nanocrystals. Of particular interest to us is the second recently reported sputtering of metal oxide targets in Ar-N 2 gas mixtures offering colored powders and layers of zinc [1] -and titanium [2] oxynitrides. In our work, we developed two new wet chemical routes to colored Zn x Ti y O w-z N z powders and nanocrystalline waveguides on glass. The fir...
Optically transparent CdS membranes have been synthesized. Colloidal Q-CdS particles (particle di... more Optically transparent CdS membranes have been synthesized. Colloidal Q-CdS particles (particle diameter d{sub p} < 40 â«) carrying short phosphate chains and excess Cd{sup 2+} ions have been directly converted into a microporous membrane form. By controlling ionic strength and particle concentrations, one can link self-organized fusion-free aggregates and avoid formation of powder-like flocculation products. The resultant unsupported membranes exhibit different mechanical properties (e.g., rigidity, delayed elasticity, and solubility) in the presence of water depending upon the preparation method employed. The membrane form of CdS has distinctively different photophysical properties than the precursor colloidal form. Conversion of weakly red luminescent colloids (broad band at 700 nm) into membranes activated an intense room temperature band edge luminescence (BEL) (narrow bands between 450 and 500 nm) attributed to the recombination of excitons and/or shallowly trapped electron/...
The classical theories for overall crystallization kinetics are first reviewed, then discussed an... more The classical theories for overall crystallization kinetics are first reviewed, then discussed and extended. To model the crystallization phenomena during polymer processing, we have chosen an extension of Ozawa’s theory which allows an easy determination of the kinetic parameters from anisothermal DSC experiments. The kinetic law is then coupled with the energy equation. Two applications are presented: cast-film extrusion and injection molding.
Berichte der Bunsengesellschaft für physikalische Chemie, 1987
CV curves only show one peak. Although the spectral data do not prove a two-step charge-transfer ... more CV curves only show one peak. Although the spectral data do not prove a two-step charge-transfer mechanism in these pH regions for the smooth Ag electrode, the fact that the SERS and CV curves establish the existence of the intermediate strongly suggests that a two-step mechanism ...
ABSTRACT Electrons and positive holes have been produced in colloidal TiO2 particles upon illumin... more ABSTRACT Electrons and positive holes have been produced in colloidal TiO2 particles upon illumination with near-u.v. light. In the absence of adsorbed reactants, the charge carriers recombine. In the presence of an adsorbed reactant for holes [such as SCN- or poly(vinyl alcohol)], an excess of long-lived electrons remains on the particles and finally reacts with a reactant in the bulk solution. In the presence of an adsorbed reactant for electrons (such as a platinum deposit or methyl viologen), an excess of long-lived holes remains and finally reacts with an oxidizable compound. The processes observed include the reduction of water to form H2, the reduction of tetranitromethane and halothane and the oxidation of Br- and various organic compounds. Methyl viologen, MV2+, was found to be reduced to the radical MV.+ and also to be oxidized, to a lesser extent, to form a fluorescing compound. The absorption spectra of the long-lived excess of electrons and positive holes trapped at the interface were measured using flash photolysis.
Berichte der Bunsengesellschaft für physikalische Chemie, 1987
ABSTRACT The luminescence of colloidal CdS in solutions containing sodium polyphosphate as stabil... more ABSTRACT The luminescence of colloidal CdS in solutions containing sodium polyphosphate as stabilizer is activated by the addition of either Ag(ClO4) or colloidal Ag2S. The degree of activation depends on how strongly the CdS and Ag2S particles are attached to each other, and this attachment is controlled by the concentration of polyphosphate and added divalent metal ions which bind to polyphosphate. The CdS fluorescence in these solutions can be excited by light of wavelengths longer than the wavelength of the absorption edge of CdS. The results are explained in terms of an interfacial region in which anion vacancies and incorporated silver centers, which deliver electrons into the conduction band of CdS upon illumination with photon energies less than the bandgap energy of CdS, are present. The preparation of Q-CdS sols which fluoresce with quantum yields close to 100% is also described. Strong fluorescence occurs when defect sites, at which radiationless recombination takes place, are blocked.
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