Effects of post-deposition heat treatment on nanostructured TiO₂-C composite structure and antimicrobial properties

The growth of cases of nosocomial infections is an urgent issue in health care facilities. Reducing cross-contamination from high-frequency touch surfaces is a key area of interest. Robust TiO₂ coatings on touch surfaces could reduce pathogen viability if the material could be modified to increase photocatalytic activity (PCA) under visible light. Research has focused on increasing the bandgap by use of dopants and rutile-anatase heterojunctions, using nanoparticles to increase active surface area, and sensitizing TiO₂ with carbon. We have previously reported 11 μm thickness nanostructured anatase and rutile mixed phase TiO₂ and carbon composite coatings (NsARC) which are antimicrobial under visible light. Here we investigate the role of the co-deposited carbon in the antibacterial performance. Coatings of 4.0–6.0 μm thicknesses nanostructured anatase were deposited by pulsed-pressure metalorganic chemical vapor deposition (pp-MOCVD) on stainless steel and glass substrates, and one set was heat-treated to remove the co-deposited amorphous carbon. Morphology, phase and adhesion were not affected by the short heat treatment at 500 °C in air. The water contact angle was reduced under visible light to a much greater extent for the carbon-free sample. The antimicrobial performance of the coatings was marginally enhanced by removing the carbon. While the carbon may be enhancing the visible light PCA of the TiO₂, we conclude that it is also interfering with the transport of radical oxygen species to the bacteria and reducing the active surface area.