**1. Introduction**

Photocatalytic and superhydrophillic thin films of anatase TiO2 have attracted large research interest since the early 1970s and find application as self-cleaning, anti-fogging and anti-bacterial coatings [1–5]. Due to their intrinsic beneficial physical properties, TiO2 films are suitable for architectural coatings, and commercially available self-cleaning glasses, tiles, cement, porcelain based on TiO2 already exist [6–8].

There are a number of ways to deposit TiO2 onto a glass substrate, including chemical methods such as dip-coating using sol-gel techniques [9], spray-pyrolysis [10], and atomic layer deposition [11]. Physical deposition techniques such as magnetron sputtering are industrial up-scalable technologies which allow for fast and controlled deposition of films with good optical and mechanical properties [12]. An added possibility for DC magnetron sputtering deposition of anatase TiO2 is that the film textured can be controlled, and preferred orientated film growth in the <004> direction has been reported with improved photocatalytic properties [13].

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Typically, the surfaces of TiO2 nanocrystals are dominated by the stable {101} facets, with only a small amount of {001} facets, leading to a truncated bipyramidal shape [14,15]. The {001} facets have two times higher surface energy than {101} facets, but is expected to exhibit higher reactivity [16]. Ab initio calculations show that water dissociates on the (001) surface, while it only adsorbs molecularly on the (101) [17]. Altering the preparation conditions it was found that TiO2 nanoparticles made by solvothermal synthesis can be prepared with altered <001>/<101> facet ratio [18–20]. In several studies it was reported that this led to an improved photocatalytic activity for the degradation of a number of liquid and gas-phase contaminants [21–23]. There are much fewer studies on sputter deposited TiO2 films. It has been reported that sputtered TiO2 films on biased substrates alter the texturing and preferred orientation depending on bias voltage, leading to an improved photodegradation rate of acetaldehyde as a function of increasing orientation [24]. Other reports show that changing total pressure [25], and O2 partial pressure [26] may also affect film growth and preferential orientation of anatase films.

In the present study, we present results on the effect of preferred crystal grain orientation on the photocatalytic reactivity of anatase TiO2 films prepared by DC magnetron sputtering. The preferred grain orientation is shown to be related to the partial pressure of O2 in the reaction chamber. The effect of orientation on their photocatalytic properties was investigated. Both the apparent rate constant and intensity dependence of the photo-oxidation of methylene blue dye in liquid phase was studied. Our results show that even though the apparent reaction rate of the oriented films is higher, the increase in orientation does not affect the intensity dependence of the photo-degradation rate.
