## 2 Materials and Methods  
TiO\({}_{2}\) commercial powder (\(\geq\)99.0%), Polyethyleneglycol 2000 (PEG 2000), Polyethyleneglycol 4000 (PEG 4000), C\({}_{2}\)H\({}_{5}\)OH (\(\geq\)99.0%), AgNO\({}_{3}\), and HNO\({}_{3}\) (\(\geq\)69.0%) were obtained from Fluka (Buchs, Switzerland). Glass substates (76 \(\times\) 26 \(\times\) 1 mm) were supplied by ISO-LAB (Schweitenkirchen, Germany). Silver standard solution for FAAS 1000 mg/L (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany) and double-deionized water (Millipore purification system Synergy, France) was used for the preparation of FAAS calibration standard solutions. For the photocatalytic tests, commercially available Malachite green oxalate (MG, \(\lambda_{\text{max}}\) = 615 nm, Croma-Gesellschaft mbH & Co.) and Methylene Blue (MB, \(\lambda_{\text{max}}\) = 660 nm, Fluka) were selected as model contaminants. Both distilled and tap (drinking) water were utilized in the photocatalytic tests to simulate pollutant degradation as it occurs in natural water systems. Tap water was obtained from Sofia municipal water supply and was of naturally low hardness (Na\({}^{+}\) < 5.01 mg/L, Ca\({}^{2+}\) < 10.74 mg/L, Cl\({}^{-}\) < 5 mg/L, NO\({}_{3}\)\({}^{-}\) < 0.94 mg/L, etc.) The powder catalysts were prepared as follows: PEG (M\({}_{\text{W}}\) 2000 or 4000) was dissolved in 50 mL ethanol (to obtain 14 wt%) under magnetic stirring at 70 \({}^{\circ}\)C, until a clear solution was obtained. The starting commercial TiO\({}_{2}\) powder was also suspended in 60 mL for 15 min at 23 \(\pm\) 2 \({}^{\circ}\)C (to obtain 7 wt%) for 15 min at 23 \(\pm\) 2 \({}^{\circ}\)C. The resulting dispersion was mixed with the PEG-ethanol solution, sonicated (15 kHz) for 30 min, and magnetically stirred for an additional 15 min to obtain the suspension used for TiO\({}_{2}\) depositions. The glass slide substrates were coated employing a dip-coating technique performed at room temperature at a withdrawal rate of 0.9 cm/min. A total of five coating cycles were applied. After each coating cycle, the samples were dried at 100 \({}^{\circ}\)C for 2 min and finally annealed for one hour at 500 \({}^{\circ}\)C to burn off the organic material. The resulting TiO\({}_{2}\) films had a loading of 38.5 mg, covering an area of about 35 cm\({}^{2}\) (\(\approx\)1.1 mg/cm\({}^{2}\)). The films obtained from two stabilizers, PEG 2000, and PEG 4000, are denoted as TP2 and TP4, respectively. The coating procedure is illustrated in Figure 1.  
To produce silver co-catalytically modified TiO\({}_{2}\) films, photo-deposition was employed. The Ag/TiO\({}_{2}\) films were photo-fixed by immersion in 10\({}^{-2}\) M aqueous AgNO\({}_{3}\) for 20 min, followed by UV light exposure and rinsing with water (Figure 1). Subsequently, the Ag-modified films were dried for ten minutes at 100 \({}^{\circ}\)C to remove nitrate ions. These films were denoted as Ag, 10\({}^{-2}\)/TP2 and Ag, 10\({}^{-2}\)/TP4 for the films obtained from the PEG 2000 stabilizer and PEG 4000 stabilizer, respectively. To study the effects of the Ag-photodeposition solution concentration, additional films were prepared via the same procedure, but in 10\({}^{-3}\) and 10\({}^{-4}\) M AgNO\({}_{3}\) solutions, denoted as Ag, 10\({}^{-3}\)/TP2 and Ag, 10\({}^{-4}\)/TP2 for the PEG 2000-derived samples, and Ag, 10\({}^{-3}\)/TP4, and Ag, 10\({}^{-4}\)/TP4 for the PEG 4000-derived films, respectively. The morphology of the samples was examined by SEM (Hitachi TM4000, 15 kV, Krefeld, Germany). The specific surface areas were measured using at low-temperature nitrogen adsorption (\(-\)196 \({}^{\circ}\)C) with a Quantachrome NOVA 1200e instrument. Energy-dispersive X-ray spectroscopy with a Bruker AXS detector (Microanalysis GmbH, Berlin, Germany) was used to determine the elemental composition. X-ray diffractometer (Siemens D500 with CuK\(\alpha\) radiation, Karlsruhe, Germany) was employed for XRD analysis of pure, Ag/TP2, and Ag/TP4 powder films to investigate the crystalline structure.