Colloidal titanium dioxide

Surface polarity can also be independently evaluated by physical means. deMayo and coworkers have assigned surface polarity of silica gel particles by observing shifts in the absorption spectra of absorbed spiropyrans which are sensitive to solvent polarity . Darwent and coworkers have shown that kinetic salt effects follow surface charge on colloidal titanium dioxide and, with zeta potential measurements, that surface area and charge could be separately evaluated... 

The photoelectrochemical reduction of the N = N double bond of the diaryl azo dye methyl orange can be similarly sensitized by colloidal titanium dioxide les, isoj The reaction was sensitive to pH and the identity of the organic redox reaction could be shifted by conducting the photoreaction in the presence of surfactants. Cationic surfactants increased the efficiency of oxidative cleavage by inhibiting charge recombination. Polyvinyl alcohol instead favored reduction. The ambident photoactivity of methyl orange thus makes it an attractive probe for activity of irradiated semiconductor suspensions. 

As has been shown by time-resolved flash photolysis measurements in colloidal titanium dioxide suspensions trapping is a very fast process. Rothenberger et al. performed picosecond and nanosecond transient absorption experiments on titanium dioxide and observed that the electron trapping time was faster than 30 ps, the time resolution of their laser system [4e]. The trapping time for holes was estimated to be < 250 ns. In a recent picosecond study by Serpone et al. on titanium dioxide colloids solutions of varying diameters it was observed that the spectra of trapped electrons as well as of trapped holes are fully developed after a laser... 

Serpone et al. have examined colloidal titanium dioxide sols (prepared by hydrolysis of TiCl4) with mean particle diameters of 2.1, 13.3, and 26.7 nm by picosecond transient absorption and emission spectroscopy [5]. Absorption decay for the 2.1 nm sols was found to be a simple first-order process, and electron/hole recombination was 100% complete by 10 ns. For the 13.3 and 26.7 nm sols absorption decay follows distinct second-order biphasic kinetics the decay times of the fast components decrease with increase in particle size. 10 ns after the excitation pulse, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (x > 20 ns) reflect recombination of deep-trapped electrons and holes. 

Umapathy, S. Cartner, A. M. Parker, A. W. Hester, R. E. Time-resolved resonance Raman spectroscopic studies of the photosensitization of colloidal titanium dioxide, J. Phys. Chem. 1990, 94, 8880. 

Bahnemann, D.W., J. Monig and R. Chapman (1987b). Efficient photocatalysis of the irreversible one-electron and two-electron reduction of halothane on platinized colloidal titanium dioxide in aqueous suspension. Journal of Physical Chemistry, 91(14), 3872-3788. 

Furthermore, the transient formation of cation radicals can be observed when a colloidal titanium dioxide suspension is flashed in the presence of an olefin [56]. The flash photolysis experiments also show that the surface influences the subsequent chemistry of the photogenerated intermediate. In fact, oxygenation and isomerization dominated the chemistry observed for tran -stilbene, with the same product distribution obtained upon starting with either the cis or the trans isomer (Eq. 3). 

Physical techniques for evaluating surface polarity led deMayo and coworkers to assign relative rates of reaction on silica gel particles from shifts in the absorption spectra of absorbed spiropyrans [76, 77]. Similarly, Darwent and coworkers demonstrated that kinetic salt effects correlate with surface charge and with zeta potential measurements on colloidal titanium dioxide [80]. 

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. 

Serpone N., Sharma D. K., Moser J. and Gratzel M. (1987), Reduction of acceptor relay species by conduction-band electrons of colloidal titanium dioxide. Light-induced charge separation in the picosecond time domain , Chem. Phys. Lett. 136, 47-51. 

Chadwick, M.D. et al.. Surface charge properties of colloidal titanium dioxide in ethylene glycol and water. Colloids Surf. A, 203, 229, 2002. 

Yezek, L. et al.. Adsorption of sodium dodecyl sulfate to colloidal titanium dioxide An electrophoretic fingerprinting investigation, J. Colloid Interf. Sci., T15. 227, 2000. 

Yezek, L., et al.. Changes in the zeta potential of colloidal titanium dioxide after exposure to a ratio frequency electric field using a circulating sample. Colloids Surf., 141, 67, 1998. 

B. O Regan, M. Graetzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal titanium dioxide films. Nature, 1991, 353,131-1AO. 

In the most commonly studied configuration of the DSSC, the electron conductor is a wide band gap, nanocrystalhne, metal oxide film. Colloidal titanium dioxide is most often used although other wide band gap metal oxides are candidates. The sensitizer is a transition metal-based (usually ruthenium), organic dye, with excited state free energy sufficient to reduce the semiconductor, and containing ligands, such as carboxylates or phosphonates, which facilitate bonding to the semiconductor... 

G. Rothenberger, D. Fitzmaurice, M. Gratzel, Optical electrochemistry spectroscopy of conduction-band electrons in transparent metal-oxide semiconductor-films - optical determination of the flat-band potential of colloidal titanium-dioxide Aims, J. Phys. Chem. 1992, 96(14), 5983-5986. 

Interaction of particles is a very important topic in particle technology concerning both natural and industrial fields, and this work is part of a more comprehensive investigation in to solid-liquid separations. The behaviour of single particles and non-interacting concentrated suspensions is well understood thanks to Stokes and other fundamental theories [1]. Nevertheless, there is a problem when the particles are able to interact with each other in order to form different dynamic entities [2]. The main aim of this paper is to study how colloidal titanium dioxide interacts, starting from an... 

Attia YA. (Ed.). Sol-gel processing and applications. New York Plenum Press, 1994 Bahnemann D., Henglein J.L., Spanhel L. Flash photolysis observation of the absorption spectra of trapped positive holes and electrons in colloidal titanium dioxide. J. Phys. Chem. 1984 88 709-711... 

Adsorption of ionic surfactants on to polar substrates will be affected by pH. The relationship between the amount of NaDS adsorbed on to colloidal alumina and the stability ratio, W, of alumina dispersions is shown in Fig. 9.5 at two different pH values, 6.9 and 1,2, The zero point of charge of alumina is pH 9.1 thus the higher surface charge at pH 6.9 ensures that hydrocarbon chain interactions occur at a lower surfactant concentration than at pH 7.2. As the pH is lowered further this effect is accentuated [12]. The effect of pH on adsorption of lauryl sulphate ions and tetradecylpyridinium ion is shown clearly in Rupprecht s [6] results using colloidal titanium dioxide as the adsorbate (Fig. 9.6). 

Dispersant - Tamol 960 Rohm Haas Nonionic Surfactant - Triton N-101 Rohm Haas 2-Amino-2-methyl-l-propanol - AMP-95 Angus " Antifoam - Colloid 640 Colloids Titanium Dioxide - TiPure R-900 DuPont Zinc Oxide - Pasco 311 Pacific Smelting Miea - Mica 325WG KMG Talc - rr-325, Vanderbilt... 

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