Effect of dissolved oxygen in immobilized Ag-TiO2 and solvent impact in photodegradation of Reactive Red 4 dye

Visible light active silver (Ag) modified titanium dioxide (TiO2) photocatalysts has been widely studied. However, the presence of Ag2O during this photocatalyst modification reduces the photocatalytic performance. This study was conducted to determine the optimum percentage of Ag loading and elucidate the effect of dissolved oxygen (DO) in forming metallic Ag (Ag0) and Ag2O (Ag+) in the Ag-TiO2. AgNO3 and Degussa P25 TiO2 were used as an Ag precursor and TiO2 source respectively. Various purging times of nitrogen (N2) and solvent were conducted to compare the effect of DO and solvent impact on the photocatalyst. The photocatalytic activity was determined by photodegradation of 30 mg L-1 RR4 dye under a 55W fluorescent lamp. The optimum photocatalytic activity of immobilized Ag-TiO2 was obtained at 3 wt. % of Ag to TiO2 under 30 min of N2 purging (denoted as 3AT-DO30) with a 1st order rate constant value of 0.112 min-1. The photodegradation rate of immobilized Ag-TiO2 under different concentrations of dissolved oxygen has shown different degradation rates where it was ca. 0.112 > 0.085 > 0.059 > 0.048 > 0.041 min-1 for 3AT-DO30, 3AT-DO10, 3AT-DO0, 3AT-DO60 and TiO2 respectively. For the solvent impact study, the highest performance was achieved with the water-based prepared sample with a degradation rate of 0.096 min-1 surpassing other solvents-prepared samples. The acquired results from XRD (32.3°, 39.6°) and FTIR (509,1384 cm-1) indicated the presence of Ag+ and Ag0 onto TiO2 photocatalysts. The FESEM-EDX confirmed the composition of Ag in the photocatalysts with the obtained loading are 0.2, 0.6 and 0.9 wt. % of Ag corresponding to 1AT, 3AT and 5AT. The UV-Vis/DRS for Ag-decorated samples presented a strong visible light absorption and bandgap energy reduction due to Ag's surface plasmonic resonance (SPR) effect. From photoluminescence analysis, it was observed that the intensity of the signal for Ag-TiO2 is lower than the unmodified TiO2 suggesting the deposition of Ag lowered the e-/h+ recombination. The XPS analysis indicated the composition, oxidation states, and the existence of Ag+ and Ag0. The combination of a specific ratio of Ag+ and Ag0 in the Ag-TiO2 can reduced the e-/h+ recombination making the pairs able to produce hydroxyl radicals and enhance the photocatalytic performance. Therefore, the level of dissolved oxygen during the preparation of Ag-TiO2 is significant in designing a good formation of Ag+ and Ag0 which can influence the photocatalytic degradation of RR4 dye.