Heat treated anodised titanium surfaces showing enhanced photocatalytic inhibition of microbial fouling J. Gopal* 1 , R. P. George 1 , P. Muraleedharan 1 , S. Kalavathi 2 , G. Mangamma 2 and H. S. Khatak 1 Titanium, an otherwise perfect condenser tube material in sea water applications, is challenged by the problem of severe biofouling. Anatase, one of the two commercially important crystalline forms of titanium dioxide, possesses excellent photocatalytic activity (PCA). It has been shown in the earlier studies by the authors that anodisation of titanium produces anatase type of TiO 2 capable of photocatalytic inhibition of microbial adhesion under near UV light illumination. The present study investigates the influence of anodising voltage and anodising time on the photocatalytic inhibition of Pseudomonas sp., a frequent coloniser of natural biofilms formed on titanium surfaces. The effect of heat treatment of anodised surfaces on PCA was also studied. The results showed that heat treatment resulted in a significant enhancement of PCA. The surface oxide was characterised using glancing incidence X-ray diffraction and atomic force microscopy and the results indicate a marked increase in the cystallinity of the anatase film on the heat treated anodised surfaces. Attempts have also been made to understand the mechanism underlying the photocatalytic inactivation of the bacterial cells on TiO 2 surfaces by studying their growth characteristics. Keywords: Photocatalytic activity, Heat treatment, Bactericidal effect, Adhesion, Rutile Introduction Semiconductor photocatalysis offers convenient routes to the purification of air and water and provides self- maintaining clean surfaces. 1,2 TiO 2 has attracted a great deal of attention as a photocatalyst due to its excellent photochemical properties, non-toxicity and low cost and is considered a multifunctional material. It is used as a pigment, photocatalyst, filler, coating, photoconductor, UV filter, etc. TiO 2 appears in three crystalline poly- morphic phases, rutile, anatase and brookite. Anatase phase of titania is preferred in dye sensitised solar cells 3 and catalysis whereas rutile is mostly used in the area of dielectrics 4 and high temperature oxygen gas sensors. 5 Matsunaga et al. reported for the first time in 1985 the antibacterial effect of TiO 2 photocatalytic reactions. 6 Today, photocatalytic antibacterial tiles, antifogging glass and air cleaners are among the commercial appli- cations of the photocatalytic activity (PCA) of TiO 2 based on its self-cleaning ability. 7 Anodic oxidation (anodising) is a commonly used surface treatment, especially on aluminium alloys for structural application to improve the corrosion or wear resistance. 8 The application of anodic oxidation to the surfaces of titanium and its alloys is more recent. Anodisation of titanium at room temperature forms titanium dioxide on the surface, which is predominantly anatase. Since the microbial fouling of titanium surfaces is the major problem with respect to the use of titanium in the sea water cooled condensers of power plants, the self-sterilising ability of anatase type of TiO 2 thin films yielded the idea of growing a thin film of the anatase on the biofouling prone titanium surface to reduce the attachment of these organic living cells, using the above mentioned industrially feasible process of anodisation. Photocatalytic activity is the ability of a material to catalyse oxidation/reduction reactions on illumination by light of suitable wavelength. Most of the semicon- ducting oxides exhibit this property. When such materials are illuminated with light of appropriate wavelength, electron hole pairs are produced in the oxide by the transfer of a valence band electron to conduction band. Photocatalytic activity strongly depends on the surface redox potential, the band gap and the lifetime of photo generated electron hole pairs. Anatase, which has a larger band gap, tends to increase the surface redox potentials and prolong the carrier lifetime in comparison 1 Corrosion Science and Technology Division, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India 2 Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India *Corresponding author, email judy@igcar.gov.in ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 30 September 2006; accepted 11 October 2006 194 Surface Engineering 2007 VOL 23 NO 3 DOI 10.1179/174329407X174425