Photocatalytic thin films prepared by plasma curing of non-reactive siloxane dispersions containing TiO2 particles

Photocatalytic TiO2 layers prepared by PVD or CVD processes are already produced industrially. The deposited TiO2 needs a sufficiently high concentration of the photocatalytically active anatase phase which is produced at higher substrate temperature. As a consequence, the PVD/CVD techniques are limited to heat-resistant substrates. A novel approach for plasma hybrid coating technology is presented here using the advantages of a cold low-pressure plasma process in combination with non-reactive siloxane (polydimethylsiloxane - PDMS) dispersions containing sub-micron TiO2 particles for surface functionalisation. As the result, a mechanically stable composite film containing TiO2 particles with a diameter of up to several times the average coating thickness is generated. The film consists of a top SiO2-like surface layer and, depending on the initial film thickness, an underlying gradually demethylated and crosslinked siloxane. Although the TiO2 particles are partially covered by a thin matrix layer of several nanometres, the thin coating shows photocatalytic activity as demonstrated by UV-degradation of methylene blue. The photocatalytic activity depends on the TiO2 particle concentration in the film and can be further enhanced by exposing the TiO2 surface by CF4 plasma etching of the composite film.