**5. Conclusions**

Ag/Ag2O was found to enhance the rate of light-induced bleaching of aqueous methylene blue under both UV/vis and vis illumination, in comparison to the bleaching in homogeneous solution. Even in suspensions containing mixtures and composites of Ag/Ag2O with TiO2 (P25), with varying mass ratios of Ag/Ag2O (20%, 50%, and 80%), the reaction rate was slightly increased under these illumination conditions. However, the bleaching rate of methylene blue was lower in the presence of the composites and mixtures than the rate measured for bare Ag/Ag2O. It is therefore suggested that the bleaching of methylene blue is initiated by an interfacial electron transfer from the excited organic probe compound to Ag2O. TiO2 layers covering the Ag2O seem to inhibit this electron transfer. Since Ag2O can transfer an electron neither to dissolved molecular oxygen nor to a proton for thermodynamic reasons, it is assumed that Ag<sup>+</sup> is reduced to Ag(0) in the processes investigated here. Results of XRD and XPS measurements support this assumption, and indicate that Ag/Ag2O is not stable under the experimental conditions employed in this study. A stabilization of Ag2O by metallic silver, as occasionally claimed, was not observed. Therefore, to address Ag/Ag2O as a (photo)catalytically active material does not seem appropriate.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/8/12/647/s1, Figure S1. EDX diagrams of all prepared photocatalysts. Figure S2. XRD patterns of TM 11 photocatalyst after one cycle of MB bleaching employing UV/vis light. Figure S3. XPS spectra of Ag 3d, O 1s and Ti 2p in the Ag/Ag2O, TM and TC materials. Figure S4. Emission spectrum of the used Xenon lamp. Figure S5. UV/vis spectra of aqueous methylene blue solutions obtained during illumination with visible light in the presence of Ag/Ag2O. Figure S6. Calibration curve for H2 experiment.

**Author Contributions:** Experiments and analysis: S.A.; XPS measurements: J.K.; Redaction of manuscript: S.A., R.D. & R.B.; Review: R.D.; R.B. & N.O.B.; Scientific support: R.D.; Supervision: D.W.B. & M.E.A.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors wish to thank Luis Granone for XRD measurements and Stephanie Melchers for SEM/EDAX images.

**Conflicts of Interest:** The authors declare no conflict of interest.
