Titanium dioxide surface chemistry

Herrmann, M. and Boehm, H.-P. (1969). On the chemistry of the titanium dioxide surface — II. Acidic hydroxyl groups on the surface (in German). Z. Anorg. Allg. Chem., 368, 73-86. 

Titanium dioxide differs from silica mainly in two respects (1) the Ti + ions are octahedrally coordinated in all three modifications of TiOji (2) the Ti—0 bond is more pronouncedly ionic than the Si—O bond. Using Pauling s electronegativity values (297), one calculates a 63% ionic character for the Ti—0 single bond versus 50% for Si—O. In SiOj, there is certainly some double bond character involving 3d orbitals of the Si atom, causing lowered ionic character. Therefore, characteristic differences should be expected regarding the surface chemistry. 

Titanium dioxide occurs in three crystalline modifications anatase, rutile, and brookite. In all three forms, each Ti + ion is surrounded by six 0 ions and each ion has three Ti + neighbors. Both anatase and rutile are important white pigments which are produced on a large scale. Even though their surface chemistry is very important for their technological application, astonishingly little has been published in the chemical literature on this subject. However, it is very likely that many investigations have been undertaken in industrial laboratories. 

Bilmes, S.A., P. Mandelbaum, F. Alvarez and N.M. Victoria (2000). Surface and electronic structure of titanium dioxide photocatalysts. Journal of Physical Chemistry B, 104, 9851-9858. 

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). 

Fig. 6 Influence of surface chemistry of pigments on theophylline release from pellets coated with Eudragit RS 30 D containing dibutyl phthalate as a plasticizer. (V) Titanium dioxide as fine platelets ( ) mica (x) talc (O) red iron oxide fine platelets. (From Ref. f)...

Both the surface chemistry of titanium dioxide and the kinetics of photoreactions occurring at Ti02 are markedly affected by its interaction with water. 

Replacing adsorbed solvent molecules and ions by chelating agents, a method known as surface modification, changes the energetic situation of such surface states and may considerably change the chemistry taking place at the surface of titanium dioxide [362],... 

Gustafsson R. I, A. Orlov, P. T. Griffiths, R. A. Cox and R. M. Lambert (2006) Reduction of NO2 to nitrous acid on illuminated titanium dioxide aerosol surfaces implications for photocatalysis and atmospheric chemistry. Chemical Communication (Cambridge) 37, 3936-3938... 

Ti02 and metal-doped Ti02 are prone to a variety of applieations such as catalytic gas-phase conversion of moleeules or photo-oxidation of organic molecules in aqueous solutions [117]. For the first aspect, titanium dioxide exhibits a very rich surface organie chemistry, due to the ability of titanium cations to exist in different oxidation states and coordination envirorunents [118]. On the other hand, the interfaces between highly dispersed metal phases on the Ti02 substrate may deeply influenee the macroscopic behaviour of these materials, in view of the modifieations of the eleetronie structure, morphology and thermal stability of the metal [119]. 

The properties of nanomaterials depend on a wider range of parameters than macroscopic materials across many length scales. Relationships between the properties of nanomaterials and their size, composition, impurities (both internally and superficially), the surface chemistry and degree of agglomeration are not well established. As an example one can present the relationship between size of particle and phase stability of titanium dioxide. Macroscopically the rutile phase is stable and the anatase phase metastable, but when the particle size is under 20 nm this sim-ation is reversed. There is an intrinsic relationship between the two dependencies, size and phase [337]. 

There can be reinforcements added to polymeric matrices such as cellulosic fibers, and fillers such as titanium dioxide incorporated into roll thermoplastics, to improve performance properties or reduce cost bases. These host matrices can include polypropylene, polyethylene, polystyrene, polyamide, and PVC to name a few. In these circumstances, the addition of dispersants and/or coupling agents is needed to attract the more highly polar fibers and fillers to the less polar polymer matrix. These added constituents will form a level of presence (depending upon their inclusion ratio relative to the bulk) at the composite surface which must be considered relative to atmospheric plasma chemistry prescription. 

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