chemical and physical properties. In general, SiO2 is a SiO4 tetrahedra, where each silica

atom is bonded to four oxygen atoms and each oxygen atom is bound to two silica atoms.

 HO OH
 HO-Si-OH HO-Si-OH
 I I
 HO 0 OH
 I I 1



 HO-S--OH HO-Si-OH



Figure 2-5. Linkages of SiO2 tetrahedras (Hench and West, 1990).

 There has been much research on the different preparation methods for creating a

solid SiO2-TiO2 material with photocatalytic ability. The characteristics (e.g., pore size,

surface charge, mechanical strength, and adsorption sites) of the final product are

dependent on the synthesis conditions and the type of interaction between TiO2 and SiO2.

There are two forms of interaction: physical forces of attraction (such as Van der Waals

forces) and chemical bonding (creation of a Ti O Si bond). The physically supported

TiO2 on SiO2 preparation methods have been the least researched, while the chemically

bonded TiO2 on SiO2 methods (also called mixed oxides) have been thoroughly

investigated (Gao and Wachs, 1999).

 The most widely used methods of preparation (for details on these individual

methods, see Appendix A) for creating mixed oxides are sol-gel hydrolysis and

coprecipitation. Chemical vapor deposition, precipitation, and impregnation are the

methods commonly used for creating supported oxides, yet there is evidence of Ti-O-Si

bonds attaching the titania to the silica surface (Chun et al., 2001). A problem with

supported oxides is the inconsistency in the coating from one batch to the next and the