Investigation of photocatalytic activity of titanium dioxide coating deposited on metallic substrates by plasma technique

摘要

Titanium dioxide (TiO_2) in the anatase crystalline structure corresponds to one of the most powerful photocatalytic materials available today. Photons with the energy equal (UV region) to or higher than its band gap (~3.2 e.V) are able to initiate a photo activation process in TiO_2, which creates hole/electrons pairs in the material. The hole/electron pair has high oxidizing and reducing power respectively which can split water into hydroxyl radicals and converting oxygen into superoxide. The hydroxyl and superoxide radicals can decompose various organic materials in contact with the TiO_2 surface, and therefore can be used a method for introducing self-cleaning or anti-bacterial surface. TiO_2 in the powder form and as coating has significant technological interest for use in many industrial and everyday-life application products. Metallic material such as stainless steel and lightweight aluminium alloys are widely used in several applications from household appliances, transportation, and hospitals to industrial environments. Therefore its combination with a functionalized self-cleaning and anti-microbial surface is extremely attractive for technological applications. Since the indented use of titanium dioxide coating is on various substrates, it is important to have a detailed investigation on the effect of metallic substrate on the photo-catalytic properties. For this investigation, TiO_2 coatings were made on aluminium alloy (AA 1050) and stainless steel (S316) substrates using pulsed DC reactive magnetron sputtering. The photo catalytic activity was measured using three experimental methods namely: (i) electrochemical methods such as Open Circuit Potential (OCP), Linear sweep voltammetry, and electrochemical Impedance Measurement, (ii) Kelvin probe microscopy, and (iii) Optical measurements using reflection. Microstructural investigation was carried out by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), and Atomic force microscope (AFM). The photocatalytic properties of TiO_2 film on different substrate varies significantly even though the microstructure of the film is not significantly varied. X-ray diffraction analysis showed that the film consists of mainly anatase phase and cross sectional SEM images show the both coating have thickness of 1.3 urn. The TiO_2 coating on stainless steel has significantly higher photocatalytic properties than on aluminium. Results from the electrochemical measurements and Kelvin probe together with TEM suggest that the presence of nano-scale oxide layer on the substrate surface interfaced with the TiO_2 coating has great impact on the photocatalytic activity. The reason for different photocatalytic activity is suspected to be the electrical properties of the oxide layer on steel and aluminium. Keywords: TiO_2 coatings, magnetron sputtering, photocatalytic effect.