*3.3. The Photocatalytic Activity of Ag/Ag2O ⁄⁄ TiO2 Composites*

#### 3.3.1. Bleaching of Methylene Blue

When irradiated with light at wavelengths ≥ 410 nm, methylene blue was found to be bleached in the presence of the three TC composites (Figure 4d and Table 1). All TC composites exhibited a higher activity than the corresponding TM mixtures. As in the case of the TM materials, the rate of MB bleaching decreased with increasing amounts of TiO2. The increased reaction rates for MB bleaching in the presence of Ag2O containing solids, compared to the rate of photolysis under visible light illumination, were explained in Section 3.2.1 with an interfacial electron transfer from (excited) MB to Ag2O (cf. Figure 7). However, the experimental result is surprising when it is considered that the surfaces of the composites were smaller than the surfaces of the corresponding TM mixtures. A possible explanation may be due to the preparation method. For the TC materials, the Ag/Ag2O

was prepared in a TiO2 suspension. Therefore, the Ag/Ag2O was attached on the surface of the TiO2 particles. In contrast, in the TM mixtures large Ag/Ag2O particles were covered by TiO2, hindering the electron transfer from excited MB to the Ag2O, as discussed in Section 3.2.2.

The rate of MB bleaching in the presence of TC composite was significantly higher under UV/vis than under visible light illumination. As observed for the TM materials, the bleaching rates were lower in suspensions containing the composites than in suspensions containing only Ag/Ag2O or TiO2 (Figure 4b and Table 1).

XRD and XPS data indicate that Ag(I) was reduced, yielding Ag(0), during the light-induced bleaching reaction of MB in the presence of the composite TC 41. A stabilization of Ag2O by metallic silver, as claimed by several authors [9–12,20,29], was not observed. No XRD peaks that can be attributed Ag2O, were observed after two experimental runs of the composite. However, the ratios of the peak intensities due to metallic Ag and TiO2 obviously increased (Figure 8b). No Ag 3d5/2 and Ag 3d3/2 peaks, which can be attributed to Ag(I), were present either in the deconvoluted XPS spectra obtained after two experimental runs (Figure 9c and Figure S3). The XPS peak, which was attributed to the presence of Ag-O, also disappeared during the light-induced reaction (Figure 9d and Figure S3).

These observations support the statement made above that Ag/Ag2O cannot be called a photocatalyst. The XRD pattern shown in Figure 8b as well as the XPS data presented in the Figure 9c,d clearly evince that the Ag:Ag2O ratio changed during the light-induced bleaching of MB. Thus, the condition for a catalyst to exit a chemical reaction unchanged is not satisfied.

### 3.3.2. Light-Induced Hydrogen Evolution

The three TC composites were found to be able to promote light-induced H2 evolution from aqueous methanol. The calculated reaction rates were significantly larger than those of the corresponding TM mixtures. The highest H2 evolution rate was observed in the presence of TC 11 (Figure 5b and Table 1), which was also characterized by a high MB bleaching rate under UV/vis illumination. A possible mechanistic explanation for the high activity of the TC 11 composite is based on the assumption of synergistic effects, due to the presence of both Ag(0) and Ag2O at the TiO2 surface (Figure 10). TiO2 is excited by UV photons. The photogenerated conduction band electrons migrated to the Ag(0) attached to the TiO2 surface. In a subsequent step, interfacial electron transfer from Ag(0) to protons present in the surrounding electrolyte occurred, thus yielding molecular hydrogen. The valence band hole inside the TiO2 particle was filled by an electron from an attached Ag2O particle. Methanol was oxidized by this hole in the valence band of the Ag2O. According to this mechanism, Ag(0) acts as an electron sink, thus decreasing the electron-hole recombination, and as electrocatalyst for the hydrogen evolution reaction, while Ag2O is an electrocatalyst for the oxidation reaction of methanol yielding methanal. The supposition made here, that the methanol oxidation occurs at the Ag2O surface via electron transfer to the valence band of the excited TiO2, has already been proclaimed earlier [16,19,23,26]. It should be emphasized again that the energy of an electron in the conduction band of the Ag2O employed in this study is insufficient to reduce a proton (Figure 6). Consequently, excitation of TiO2 is a prerequisite for photocatalytic reforming of methanol. TiO2 is known to be a relatively inactive material for the photocatalytic reduction of protons. High evolution rates of molecular hydrogen are observed only in the presence of a co-catalyst. Ag2O was found here to be an unsuitable co-catalyst for the hydrogen evolution reaction, since electron transfer from the excited TiO2 can only occur into the conduction band of this material. The photocatalytic activities of the composites and mixtures discussed here are thus determined to a considerable extent by the competition between interfacial electron transfer to protons in the surrounding electrolyte, and to silver ions in Ag2O. The mechanism of the photocatalytic hydrogen evolution by reforming of organic compounds in the presence of the mixtures and composites employed in this study does not contradict the mechanism discussed for Ag/Ag2O ⁄⁄ TiO2 samples, which contain Ag2O with a significantly more negative conduction band energy than TiO2 [17,24,26,33].

**Figure 10.** Mechanism of hydrogen evolution from aqueous methanol under UV/vis illumination.

Changes in the respective mass fractions of TiO2, Ag, and Ag2O at constant total mass of the solid in suspension may have several impacts on the rate of hydrogen evolution. Increasing mass fractions of UV absorbing and scattering Ag and Ag2O reduces the number of photons to be absorbed by the TiO2, thus reducing the H2 evolution rate. A reduction of the mass fraction of metallic Ag may possibly slow down the interfacial electron transfer to the proton, while a reduction of the mass fraction of Ag2O might negatively affect the oxidation reaction. It should also be noted that Ag2O can act as a sink for a TiO2 conduction band electron (cf. Figure 6). These partially opposing effects may be responsible for the observed differences in the H2 evolution rates in the presence of the various TC composites (and TM mixtures).

### **4. Experimental Section**
