According to reference the modification of

The suppression of the photocatalytic activity was assigned to the low reducing power of the photogenerated electrons and the covering of the TiO2 nanoparticles with inactive silica shells. The formation of TiOSi bonding through condensation reactions between hydroxyl groups from TiO2 surface and silanol groups from the silane compounds results in creation of coating around the TiO2 nanoparticles which suppresses the photocatalytic activity of composite nanoparticles in comparison with bare TiO2[33].