*3.1. The Photocatalytic Activity of Ag/Ag2O*

It is well known that methylene blue is photocatalytically oxidized in the presence of TiO2 under illumination with photons having an energy equal to or larger than the bandgap energy of the semiconductor. The photocatalytic degradation of methylene blue in the presence of molecular oxygen is reported to follow Equation (1) [44].

$$\text{C}\_{16}\text{H}\_{18}\text{N}\_{3}\text{SCl} + 25.5\text{O}\_{2} \xrightarrow{\text{TiO}\_{2} + h\nu \ge 3.2\text{ eV}} \text{HCl} + \text{H}\_{2}\text{SO}\_{4} + 3\text{HNO}\_{3} + 16\text{CO}\_{2} + 6\text{H}\_{2}\text{O} \quad \text{(1)}$$

The energetic positions of the valence and conduction bands of TiO2 and Ag2O, and the reduction potentials of some species (possibly) present in the surrounding electrolyte are shown in Figure 6. As becomes obvious from this Figure, the conduction band electrons generated by UV illumination of TiO2 are able to reduce O2 adsorbed at the semiconductor surface. From a thermodynamic point of view, valence band holes at the TiO2 surface have an energy suitable to oxidize H2O/OH−, yielding OH radicals. These OH radicals are generally assumed to be the oxidizing species in photocatalytic MB degradation.

**Figure 6.** The electrochemical potentials (vs. NHE) of the valence and conduction bands of TiO2 and Ag2O, and the reduction potentials of some species (possibly) present in the surrounding electrolyte. MB, MB•−, MB•+, MBT, and MBS denote the MB ground state, the semi-reduced MB, the oxidized MB, the excited triplet state, and the excited singlet state of MB, respectively. The one electron reduction potentials have been calculated with data given in References [44,47,48].

With the assumption that the flat band potential of Ag2O, which has been determined to be + 0.3 V vs. NHE at pH 7, was equal to the conduction band edge of this semiconductor, and a bandgap energy Eg = 1.5 eV, the valence band position was calculated to be +2.0 V vs. NHE. Xu and Schoonen reported a value of +0.2 V vs. NHE for the energy of the Ag2O conduction band [49]. As becomes obvious from Figure 6, excited Ag2O was neither able to reduce O2 nor to oxidize H2O/OH−. Consequently, the mechanism of MB bleaching observed in the presence of Ag/Ag2O (Figure 4 and Table 1) was different from the MB degradation mechanism in the presence of TiO2. A possible explanation for the decolorization of MB in the presence of Ag/Ag2O is that MB is excited by light of suitable wavelength (Equation (2), MB\* = MB<sup>S</sup> and or MBT), which is subsequently followed by electron injection into the conduction band of Ag2O (Equation (3)).

$$\begin{array}{c} \text{MB} \stackrel{\text{h\nu}}{\rightarrow} \text{MB} \text{\*} \end{array} \tag{2}$$

$$\rm{MB} \* + \rm{Ag\_2O} \to \rm{MB^{+\bullet}} + \rm{Ag\_2O} \{e^-\} \tag{3}$$

As an alternative to these reactions, the direct oxidation of MB by valence band holes according to

$$\text{Ag}\_2\text{O} \rightarrow \text{Ag}\_2\text{O} \{\text{h}^+ + \text{e}^-\} \tag{4}$$

$$\rm Ag\_2O \{ h^+ + e^- \} \, + \, MB \, \rightarrow Ag\_2O \{ e^- \} \, + \, MB^{+\bullet} \tag{5}$$

has to be considered. Both mechanisms require an electron transfer between Ag2O and MB. Despite the low surface area available for this reaction, the electron transfer between the solid and the probe compound appears to be very efficient.

It is well known that Ag2O is sensitive to light and decomposes under illumination. However, it has been suggested that Ag(0) being present in Ag/Ag2O acts as an electron sink and accepts the conduction band electron of Ag2O, thus inhibiting the reduction of Ag+ and stabilizing the Ag2O [9,10,12,20]. However, the possibility cannot be excluded that Ag+ is reduced during the processes given in the Equations (2)−(5), yielding Ag(0), since no other suitable electron acceptor is available. Regardless of whether the electrons reduce Ag+ or become stored in Ag(0), Ag/Ag2O is not acting as a photocatalyst, because the material changes irreversibly during the reaction.

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The potential of the Ag2O conduction band electron is more positive than the reduction potential of the H+/H2 couple (Figure 6). Consequently, light-induced proton reduction yielding H2 is thermodynamically impossible in suspensions containing only Ag/Ag2O. This is in accordance with the experimental results reported in Section 2.2.
