Titanium dioxide sensitization

The main electroceramic apphcations of titanium dioxide derive from its high dielectric constant (see Table 6). Rutile itself can be used as a dielectric iu multilayer capacitors, but it is much more common to use Ti02 for the manufacture of alkaline-earth titanates, eg, by the cocalciuation of barium carbonate and anatase. The electrical properties of these dielectrics are extremely sensitive to the presence of small (<20 ppm) quantities of impurities, and high performance titanates require consistently pure (eg, >99.9%) Ti02- Typical products are made by the hydrolysis of high purity titanium tetrachloride. 

The reduction of gun erosion by the use of additives to the gun powder has been studied at the US Naval Ordnance Station (NOS), Indian Head, Md. During the investigation of one of the additives, namely titanium dioxide (Ti02) or Swedish additive, it was necessary to determine small amts of Ti in gun powder residues taken from the gun after firing. As only very small amts of Ti were expected in the samples, the very sensitive technique of NAA was used for the analysis (Ref 2)... 

Pigment-Sensitized Dye Photooxidation. Dye degradation sensitized by titanium dioxide is a recognized phenomenon (16). 

Polycrystalline GaN UV detectors have been realized with 15% quantum efficiency [4], This is about 1 /4 of the quantum efficiency obtained by crystalline devices. Available at a fixed price, however, their increased detection range may well compensate their lack in sensitivity. Furthermore, new semiconductor materials with a matching band gap appear as promising candidates for UV detection if the presumption of the crystallinity is given up. Titanium dioxide, zinc sulfide and zinc oxide have to be mentioned. The opto-electronic properties and also low-cost production processes for these compound semiconductors have already been investigated to some extent for solar cell applications [5]. 

In the near future, UV photodiodes made from polycrystalline wide band-gap semiconductors may fill the gap in the market. Although they have a lower sensitivity (photocurrent per area) they promise to have a better merit-rating in terms of photocurrent per sensor costs. The other major drawback of polycrystalline photodiodes, the risetime of micro- to milliseconds, is not relevant for household applications. Fuji Xerox Laboratories in Japan are developing visible-blind UV photodiodes made from polycrystalline GaN [12], while twlux AG in Berlin, Germany is developing visible-blind UV photodiodes made from polycrystalline titanium dioxide [13]. A prototype is shown in Fig. 5.45. 

Vlachopoulos, N., P. Liska, A. J. McEvoy, and M. Gratzel (1987), "Efficient Spectral Sensitization of Polycrystalline Titanium Dioxide Photoelectrodes", Surface-Science 189/190, 823-831. 

The antenna effect as it is found in natural photosynthetic systems is an attractive tool for increasing light absorption of solar cells. Some of the work done on dye sensitization of polycrystalline titanium dioxide shows aspects of antenna behavior [76,83-87]. Most of the problems in the systems where an electron is injected into the semiconductor arise in the regeneration process of... 

Kay A (1996) Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder. Solar Energy Mater Solar Cells 44 99-117 Wang X, Zhi L, Muellen K (2008) Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Lett 8 323-327... 

Applying ion beams, surface-sensitive analysis and modification in atomic and electronic structures of inorganic materials have been developed. Ion beam modification of titanium dioxide (Ti02), carbon-based materials, and the analysis of Nb/Cu multilayers and VO2 using ion beam are described as follows. 

Much effort has gone into development of catalysts for photochemical reactions, initially with the objective of converting solar energy into storable fuels (typically H2 from the photolysis of water) but, more recently, mainly for the destruction of noxious pollutants such as chlorocarbons. There are two ways in which a catalyst may be involved in a photochemical reaction it may simply provide a surface on which the reactants can be adsorbed, so that, when a molecule of one reactant is activated by absorption of light, a molecule of the other is held in close proximity to facilitate reaction (a catalyzed photoreaction)-, or it may itself be excited by the absorption of light and then activate the adsorbed molecules (a sensitized photoreaction). The latter mode is the more relevant to the theme of this chapter, and is exemplified by the photocatalytic properties of titanium dioxide, Ti02-14 15... 

The attention devoted to supramolecular sensitizers containing multifold chromophoric and electroactive centers arises from the construction of molecular devices based on nanometric and well-defined molecular architectures [4]. The use of these species for sensitization of titanium dioxide has provided fundamental insights into interfacial electron-transfer processes. 

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. 

Titanium dioxide is a light-sensitive semiconductor, and absorbs electromagnetic radiation in the near UV region. The energy difference between the valence and the conductivity bands in the solid state is 3.05 eV for rutile and 3.29 eV for anatase, corresponding to an absorption band at <415 nm for rutile and <385 nm for anatase. 

The crystal structure of the pigments is determined by X-ray analysis which is sensitive enough to determine 0.3-0.5% anatase in the presence of 99.7-99.5% rutile. For standards, see Table 1 (Titanium dioxide pigments Methods of analysis and Specification ). 

Titanium dioxide (Ti02) has been attracting much attention for its important role in water photo-oxidation and photocatalyst, as well as a base material for dye-sensitized solar cells. A number of studies have been conducted on the mechanisms of interfacial photo-anodic reactions but the reported mechanisms still remain sketchy, and the detailed molecular mechanism has not yet been clarified. The main reason for confusion may arise from the possibility that the reaction mechanism depends on detailed chemical structures of the electrode surface. This implies that studies with well-defined surfaces are of key importance. 

The actions of photoexcited semiconductor particles on organic compounds under oxygen is of significant importance from both practical and basic aspects. Semiconductors like titanium dioxide and cadmium sulfide were shown to induce oxidation of olefins and aromatic hydrocarbons under oxygen, and also to sensitize isomerization of unsaturated systems. The mechanisms of these reactions are discussed. 

The nanocrystalline solids are metal oxides, especially titanium dioxide [54-58], Various dyes are used. Transition metal complexes such as (65) and (66) have broad absorption bands and allow the harvesting of a large fraction of sunlight [54,58], Fluorescent dyes are also used, such as Eosin-Y (67) [57], Dye-sensitized nanocrystalline solar cells are now giving efficiencies in excess of 10% [54,58], compared to just 1 % ten years ago [3],... 

Turner, G.M., M.C. Beard, and C.A. Schmuttenmaer. 2002. Carrier localization and cooling in dye-sensitized nanocrystalline titanium dioxide. J. Phys. Chem. B 106 11716-11719. 

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