*3.2. The Photocatalytic Activity of Physical Ag/Ag2O ⁄⁄ TiO2 Mixtures*

#### 3.2.1. Bleaching of Methylene Blue

When irradiated with light at wavelengths ≥ 410 nm, methylene blue was found to be bleached in the presence of Ag/Ag2O, and mixtures of this material with TiO2. The rate of MB bleaching decreased with increasing amounts of TiO2. Of course, TiO2 itself was found to be photocatalytically inactive, since it was not excited under this illumination condition (Figure 4c and Table 1). The electron transfer reaction resulting in the observed bleaching of the MB solution occurred at the surface of the Ag2O, as discussed in Section 3.1. According to the SEM images (Figure 2a–d), the surface of the Ag2O was increasingly covered by TiO2 as the content of this oxide in the mixture increased. The interfacial electron transfer was inhibited by this TiO2 layer (Figure 7). The reaction rates suggest that this inhibition increased with increasing amounts of TiO2 on the Ag/Ag2O surface. Consequently, the TiO2-rich mixtures TM 11 and TM 14 exhibited rates of bleaching almost the same as the rate of photolysis in homogeneous solution (Table 1). Interfacial electron transfer from excited MB to TiO2 (which is thermodynamically possible; cf. Figure 6) obviously did not contribute significantly, since no MB bleaching was observed under visible light illumination of suspensions containing only this photocatalyst.

**Figure 7.** Possible mechanism of MB bleaching by Ag2O and Ag2O-containing mixtures and composites under visible light illumination.

The situation was different when the TM mixtures were illuminated with UV/vis light. The rate of MB bleaching in the presence of the Ag/Ag2O ⁄⁄ TiO2 mixtures was found to increase with increasing TiO2 content. However, the rates were always lower than the rates determined for suspensions containing only Ag/Ag2O or bare TiO2 (Figure 4a and Table 1). These rates cannot be explained solely by the optical properties of the suspensions. Of course, as the Ag/Ag2O content increases, more UV photons are absorbed by Ag2O. They are thus no longer available for the excitation of the TiO2 that results in decreasing amounts of charge carriers in the TiO2 and, consequently, decreasing rates of MB degradation. However, the MB bleaching rate calculated for the TiO2-rich TM 14 mixture suggests that not all photogenerated charge carriers were used in the desired MB bleaching reaction, but some were lost by reactions between excited TiO2 and Ag/Ag2O, resulting in the reduction of Ag+.

XRD measurements revealed the reduction of Ag+ during the light-induced bleaching of MB under UV/vis illumination. The ratios of the peak intensities corresponding to Ag2O and TiO2 of the mixture TM 41 and the composite TC 41 were significantly lower after two experimental runs than before illumination (Figure 8). On the other hand, the ratios of the peak intensities attributed to metallic Ag and TiO2 obviously increased. In the case of the Ag/Ag2O ⁄⁄ TiO2 mixture TM 11, apart from the TiO2 peaks, the only visible XRD peaks could be assigned to AgCl and Ag(0) after illumination of a suspension containing MB (Figure S2). The new peaks in the diffractogram, which are indexed to AgCl, were possibly formed by a reaction between Ag+ and Cl<sup>−</sup> known to be present

at the surface of TiO2 P25 [39]. This reaction certainly explains the decrease of the Ag2O peaks in the diffractogram. However, this explanation does not exclude that Ag2O is also transformed by a light-induced reduction reaction, yielding Ag(0).

**Figure 8.** XRD patterns of (**a**) TM 41 and (**b**) TC 41 after two cycles of MB bleaching employing UV/vis light.

The conclusion from the XRD data, that Ag(I) was reduced yielding Ag(0) during the light-induced bleaching of MB in the presence of the mixture TM 41, is supported by the results of the analysis of XPS data taken before and after two experimental runs (Figure 9a,b and Figure S3). It becomes obvious from Figure 9a that the Ag 3d5/2 and Ag 3d3/2 peaks of Ag2O in the mixture TM 41 decreased in intensity and broadened, while the Ag(0) 3d5/2 and Ag(0) 3d3/2 peaks increased in intensity after two photocatalytic reactions. Furthermore, the deconvolution of the O 1s peaks denotes that the peak corresponding to the Ag-O bond had a lower intensity compared to the same peak observed before the reaction, indicating significant changes occurred during the light-induced MB bleaching reaction (Figure 9b). These changes were mainly due to the light-induced reduction of Ag+ yielding Ag(0). Again, the condition of stability of a catalyst was not satisfied.

**Figure 9.** *Cont*.

**Figure 9.** High-resolution XPS spectra of the Ag 3d and O 1s signals of TM 41 (**a**,**b**) and TC 41 (**c**,**d**) before and after two experimental runs.

#### 3.2.2. Light-Induced Hydrogen Evolution

From a thermodynamic point of view, excited TiO2 is able to transfer a conduction band electron to a proton present at the photocatalyst surface (Figure 6). This electron transfer is, however, known to be a kinetically inhibited process. Therefore, it is necessary to deposit an electrocatalyst at the TiO2 surface, which accelerates the interfacial electron transfer. Ag(0) is known to be a suitable, though relatively inactive, electrocatalyst [50,51]. In this work as well, pure TiO2 showed only a very low photocatalytic activity with regard to H2 evolution from aqueous methanol. When using the TM materials, a significant increase in the amount of H2 evolved (consequently corresponding with an increase in the reaction rate) during six hours of illumination of the mixture was observed with increasing TiO2 content (Figure 5a and Table 1). On the one hand, this can be explained by the fact that a significant portion of the UV photons was absorbed by Ag2O being inactive under this illumination condition, and thus was not available for the desired H2 evolution reaction. However, this portion decreased with increasing TiO2 amount of the mixture. On the other hand, some of the TiO2 conduction band electrons were transferred to the Ag2O, where they were consumed to reduce Ag<sup>+</sup> to Ag(0). These electrons were therefore also not available for the desired reaction. Obviously, these undesired electron losses are lower the higher the mass fraction of TiO2 in the physical mixture, resulting in increasing H2 evolution rates with increasing mass fraction of TiO2.
