You can use the metal-seminductor or semconductor-semiconductor composites as photocatalysts. you can prepare the SiO2-TiO2 composite or you can dope the Si into the lottice of TiO2 but is not composite.

The photocatalyst you produce will be activated by a certain energy (given in eV) by photons. TiO2 e.g. has a band gap of 3.2eV and requires UV light to fully activate. You may dope TiO2 to decrease the band gap demand down to visible light. This is one main parameter. Moreover, the nanoproduct depends also of the application, therefore immobilized catalyst requires sol-gel procedures for coating and suspended catalyst requires production of non agglomerated powders, in the optimal range of 30nm. Crystalline phase, BET and "sol habit/phase" are important parameters, too. Mainly choice depends on application, the production process should then be tailored on the specific needs. You need to be more precise in defining the final application.