*2.1. Deposition of TiO2 Anatase Thin Films*

Anatase TiO2 thin films were deposited by reactive DC magnetron sputtering on microscope slides glass substrates (Thermo Fischer Scientific, Waltham, MA, USA) using a Balzers UTT400 sputter system [27]. Two 5 cm diameter Ti targets (99.99% purity, Plasmaterials, Livermore, CA, USA) were used for deposition, and positioned 13 cm from the center of the sample holder, which was rotated at approximately 3 rpm, as described elsewhere [28]. Ar and O2 gases (both with 99.997% purity) were supplied using mass flow controllers. The Ar flow rate was set to 60 mL·min<sup>í</sup><sup>1</sup> and the total pressure in the chamber was adjusted to 20 mTorr, and kept constant in the experiment, yielding approximately the same kinetic energy of the ions impinging on the substrate and thus facilitate inter-comparisons between fabrication conditions. The O2 flow rate was varied in the experiments and two flow rates were employed, 2 mL·min<sup>í</sup><sup>1</sup> and 4 mL·min<sup>í</sup><sup>1</sup> , respectively, corresponding to an O2 partial pressure of *P*O2 = 0.65 mTorr and *P*O2 = 1.3 mTorr in the sputtering chamber. One extra sample was deposited at intermediate O2 pressure *P*O2 = 0.95 mTorr (3 mL·min<sup>í</sup><sup>1</sup> ) to confirm the dependence of physical properties on *P*O2 (see Table 1), but this film was not studied further here. The plasma was created by a DC power supply set to constant current mode at 0.75 A yielding 212 W direct power at *P*O2 = 0.65 mTorr O2, and 245 W at *P*O2 =1.3 mTorr O2.

At each O2 partial pressures a total of 5 samples were sputtered. One sample with a size of 25 mm × 25 mm for X-ray diffraction measurements, and two batches consisting of two samples each deposited on larger substrates (50 mm × 25 mm, covered with a mask, thus limiting the coated area to 45 mm × 20 mm, or 900 mm2 ) for the photo-catalytic experiments. Both sets of films were deposited for 35 min resulting in film thicknesses of 574 and 664 nm for the films sputtered at *P*O2 = 0.65 and 1.3 mTorr O2, respectively, as determined by surface profilometry (Bruker DektrakXT, Karlsruhe, Germany). The slight increase in sputtering rate from 16.4 to 19 nm·min<sup>í</sup><sup>1</sup> is likely to be due to the higher sputtering power obtained at 1.3 mTorr. The as-deposited samples were amorphous. To transform them into polycrystalline anatase they were calcined for 1 h at 500 °C. The temperature was ramped at 5 °C·min<sup>í</sup><sup>1</sup> and the samples were left to cool overnight.
