80 -

 70-
 0 1.1
 S60 mW/cm^2
 2 A 0.75
 S50 mW/cm^2

 40 0.45
 W mW/cm^2
 30

 20

 10
 0 5 10 15 20 25 30 35 40
 Percent TiO2

Figure 4-6. Destruction ofRR (pH 7.6) after 2 hours UV exposure versus TiO2 loading as
 a function of flux.

 The plateau can be explained by the TiO2 is agglomerating in solution during gel

preparation, which was visually observed with TiO2 loadings greater than 8%, and

therefore is decreasing the effective surface area of titania available for excitation and/or

oxidation and reduction reactions on its surface. The 12% optimal loading (wt/vol

precursor) at the lowest energy is approximately equal to 30 wt%, which was found to be

an optimum for destruction of organic compounds with other methods of producing

silica-titania composites (Jung and Park, 2000; Chun et al., 2001; Anderson and Bard,

1995).

 The structure of RR includes sodium sulfonate groups that would make the

compound anionic when the sodium disassociates as the compound dissolves. Therefore,

the effect of pH on adsorption and destruction of RR was also investigated. It was

observed that at a pH range of 3 to 10, there was no adsorption of RR dye to the silica-