SiO<i><sub>x</sub></i> Ultrathin Layer Coverage Effect on the (Photo)catalytic Activities of Rutile TiO<sub>2</sub>

A two-step method consisting of 1,3,5,7-tetramethycyclotetrasiloxane chemisorption and the postheating has formed a uniform and dense silica-like SiO<sub><i>x</i></sub> ultrathin layer (SiO<sub><i>x</i></sub>-UTL) on the surface of rutile TiO<sub>2</sub>. Diffuse reflectance infrared Fourier transform and solid-state <sup>29</sup>Si NMR spectroscopy showed that three-dimensional Si−O−Si networks grow to become rigid by repeating the treatment. The coverage with the SiO<sub><i>x</i></sub>-UTL eliminates the surface acid sites, leading to an almost complete loss of the photocatalytic activity at the thicknesses above 0.36 ± 0.03 nm. On the basis of a tunneling model, the significant restriction of the photocatalytic activity can be accounted for by the excellent barrier effect of the SiO<sub><i>x</i></sub>-UTL for the photogenerated charge carrier transfer from TiO<sub>2</sub> to acceptors in solution.