Optimisation of HiPIMS photocatalytic titania coatings for low temperature deposition

Abstract

Titanium dioxide in its anatase form is widely used in photocatalytic applications due to its high photocatalytic activity, stability and low cost. Titania coatings directly deposited by conventional magnetron sputtering tend to have an amorphous microstructure. For the anatase structure to develop, substrate heating or post-deposition thermal treatment is usually required, with the anatase crystal phase generally forming at temperatures in excess of 400°C. This precludes the choice of thermally sensitive substrate materials for the photoactive coating.Depending on the nature of the driving voltage waveform, high power impulse magnetron sputtering (HiPIMS) has been shown to deliver a relatively low thermal flux to the substrate, whilst still allowing the direct deposition of crystalline titania coatings. Consequently, this technique offers the potential to deposit photocatalytically active titania coatings directly onto polymeric substrates and, therefore, opens up a range of new applications. In the present work a range of titanium dioxide thin films were deposited by HiPIMS onto glass substrates in order to study the influence of various process parameters, such as pressure, pulse frequency and pulse duration on coating structure and photocatalytic properties. The photocatalytic properties of the coatings were assessed by their ability to degrade the organic dye methylene blue under UV and fluorescent light irradiation. The degradation rate of methylene blue was calculated by measuring its absorption peak height at 665nm in continuous mode under UV/fluorescent light source. The hydrophilic properties of the coatings were also investigated by measuring the contact angle of water droplets on the coating surfaces. Experimentally, the optimum conditions to maximise the photocatalytic performance of the coatings were found. The influence of various deposition parameters on the photocatalytic properties and crystal structure of the coatings is discussed.Optimised coatings then were deposited onto polymeric substrates, such as polyethylene terephthalate (PET) and polycarbonate, to assess the suitability of using this method for high-energy, low-temperature deposition of photoactive titania coatings and the relevance of the optimised condition was tested for other types of substrates other than glass. It was found that titania coatings deposited by HiPIMS directly onto polymeric substrates showed relatively high levels of activity in their as-deposited state.The ability to deposit crystalline titania with photocatalytic functionality at temperatures low enough to enable the use of polymer substrates is a significant advancement in the field. It could potentially allow the production of high volumes of photocatalytic material on substrates, such as polymer web, which is not possible with current deposition techniques.