*2.1. Preparation of Coated Surfaces*

The substrate material for all coatings was stainless steel AISI 304 2B (75 × 25 × 1.6 mm3 ). Coatings were produced using either closed field unbalanced magnetron sputtering (CFUBMS) [21] or by spray-coating with a TiO2 sol. Table 1 shows the coatings produced.


**Table 1.** Preparation method of coated surfaces.

Coatings T1–T3 were deposited using reactive magnetron sputtering in a Teer Coatings UDP 450 coating system. One titanium target (99.5% purity) was used for the deposition of TiO2. Argon (99.998% purity) was used as the working gas and oxygen (99.5% purity) as the reactive gas. The working pressure was 1 mbar. Ag (99.95% purity) was used as the dopant. Advanced Energy Pinnacle Plus pulsed DC power supplies were used to power the titanium magnetrons and bias the substrates. An Advanced Energy DC power supply was used to power the silver target. 10–30 substrates were ultrasonically cleaned in acetone prior to loading into the chamber in order to remove surface contaminants. The substrates were aligned on a flat plate parallel to the surface of the metal targets at a distance of 150 mm from the target plane. A high rotational speed of 10 rpm was applied to the substrates to ensure enhanced mixing of silver and titanium within the coatings rather than the preferential formation of multilayer coatings. The substrates were ion-cleaned for a period of 20 min prior to the coating deposition using a bias voltage of í400 V and a low current of 0.2–0.35 A on the targets. The coatings were deposited at a bias voltage of í40 V. A thin layer of Ti was initially deposited as the adhesion layer prior to the introduction of oxygen to the deposition chamber. The amount of oxygen was controlled using an optical emission monitor, using conditions known to produce stoichiometric TiO2 [26]. A pulsed-DC power of 2.5 kW was used on the Ti target at frequency 50 kHz and a duty of 97.75% (in synchronous mode). A continuous DC power of 70 W in the case of T2 and 150 W in the case of T3 was applied to the Ag target to vary the dopant content. The deposition rate was 17–22 nm/min depending on Ag content and coatings with thickness of 0.8–1 μm were produced. No additional heating was used during the coating process and the temperature did not exceed 200 °C during the process.

U1 and U2 coatings were deposited using reactive magnetron sputtering in a Teer Coatings UDP 450 coating system as described above. Two opposing magnetrons were fitted with titanium targets and one with the Mo dopant metal target (99.5% purity). The magnetrons with the titanium targets were in the closed field configuration and driven in pulsed DC sputtering mode using a dual channel Advanced Energy Pinnacle Plus supply at a frequency of 100 kHz and a duty of 50% (in synchronous mode). The Mo metal target was driven in a continuous DC mode (Advanced Energy MDX). The Ti targets were operated at a constant time-averaged power of 1 kW and the dopant target was operated at 180 W. Stainless steel samples were mounted on a substrate holder, which was rotated between the magnetrons at 4 rpm during deposition. The target to substrate separation was 8 cm. The titanium and Mo targets were cleaned by pre-sputtering in a pure argon atmosphere for 10 min. Deposition times were adapted to obtain a film thickness of 0.8–1 μm (deposition rate was 7.5 nm/min). The sputtered films were post deposition annealed at 600 °C for 30 min. in air.

Coating MC was prepared by spray-coating with a proprietary water-based TiO2 sol using the following method. This transparent, neutral sol contained 2% TiO2 (as anatase). Degreased stainless steel coupons were fixed to aluminium panels (approximately 150 × 100 mm2 ). The panels with attached coupons were accurately weighed. The TiO2 sol (0.2–0.3 g) was sprayed onto the aluminium panel with the attached coupons in a slow, steady motion, sweeping the panel in horizontal stripes from top to bottom, using a Badger Airbrush 200-3 model spray kit (Badger Air-Brush Co., Franklin Park, IL, USA). After air-drying for at least 15 min, the spraying procedure was repeated until 0.8–1.0 g/m2 of TiO2 sol was delivered to the surface. After air-drying overnight, the aluminium panel with the attached stainless steel coupons was re-weighed to give an accurate measurement of the weight per area of the coating.
