Poisoning prevention of TiO2 photocatalyst coatings sputtered on soda-lime glass by intercalation of SiNx diffusion barriers

Amorphous titanium dioxide coatings were deposited at high pressure (about 5 Pa) on cold soda-lime glass by reactive magnetron sputtering. In order to obtain anatase form known for its photocatalytic properties, an annealing at 450 °C was required. The morphology of the photocatalyst coating, examined by scanning and transmission electron microscopy, exhibits a columnar structure with closed intercolumnar porosity. The concentration profile obtained by means of sputtered neutral mass spectrometry revealed the presence of sodium into the TiO2 coating due to the Na diffusion from the soda-lime glass during the annealing step. This level of contamination reaches 5.5 at.% in the whole film thickness. Since the presence of Na in TiO2 coatings is harmful to the photocatalytic properties, a silicon nitride (SiNx) coating, acting as a Na diffusion barrier, was intercalated between the glass substrate and the TiO2 coating. The concentration profiles showed that the TiO2 coating deposited on SiNx/glass substrate is sodium-free. Furthermore, photocatalytic tests revealed that TiO2/SiNx/glass system is about threefold more efficient than TiO2/glass, showing both the poisoning effect of Na and the superior efficiency of SiNx as a diffusion barrier. To illustrate this, diffusion barriers made of silicon dioxide have also been grown and compared to SiNx.