*2.1. Characterization of the Prepared Materials*

The powder X-ray diffraction (XRD) patterns of Ag/Ag2O, physical mixtures of Ag/Ag2O ⁄⁄ TiO2 with increasing amounts of TiO2 (20 mass% (TM 41), 50% (TM 11), and 80% (TM 14)), and in situ prepared Ag/Ag2O ⁄⁄ TiO2 composites (20 mass% TiO2 (TC 41), 50% (TC 11), and 80% (TC 14)) are shown in Figure 1. The XRD peaks for Ag/Ag2O at 26.7◦, 32.8◦, 38.1◦, 54.9◦, 65.4◦, and 68.8◦ perfectly correlate to the (110), (111), (200), (220), (311), and (222) crystal planes of cubic Ag2O (JCPDS 41–1104). The three peaks at 44.3◦, 64.7◦, and 77.5◦ are indexed to the (200), (200), and (311) crystal planes of cubic Ag(0), respectively (JCPDS 04-0783) [35,36].

The TiO2 containing mixtures (TM) and composites (TC) exhibit diffraction peaks at 25◦, 38◦, 48◦, 54◦, 55◦, 63◦, 69◦, 71◦, 75◦, and 83◦, which are attributed to the tetragonal phase of anatase TiO2, whereas one peak at 27.8◦ corresponds to the tetragonal phase of rutile TiO2. Figure 1a presents the patterns of the TM mixtures, where two phases of titania were present. The two strongest peaks of Ag2O become more prominent, with the Ag2O mass ratio increasing from TM 14 to TM 41. The small diffraction peaks situated at 44.4◦, 64.2◦, and 77.5◦ are indexed to the (200), (200), and (311) plane of metallic Ag(0) (JCPDS 04-0783) [20]. The strongest peak of Ag(111) might likely be masked by the TiO2 peak at 2θ = 38◦. The diffraction peaks in the TM mixture patterns correspond to the cubic structure of Ag2O and the cubic structure of Ag [35,36]. Figure 1b illustrates the XRD patterns of the TC composites. As the figure shows, no significant difference between the two preparations methods was observed, except that in TiO2-rich composites TC 11 and TC 14 no Ag2O diffraction peaks were observed, suggesting a complete reduction of Ag2O to metallic silver Ag(0) during the preparation of these composites. The XRD pattern of TiO2 is presented for comparison. The diffractogram clearly indicates the presence of two TiO2 phases with predominance of the anatase phase (JCPDS 21–1272).

**Figure 1.** X-ray diffraction (XRD) patterns of (**a**) TiO2 containing mixtures (TM), and (**b**) TiO2 containing composites (TC). The diffractograms of Ag/Ag2O and TiO2 are included in both figures.

In order to investigate the oxidation states of the silver species present on the materials, X-ray photoelectron spectroscopy (XPS) was performed. The results of the XPS analysis for all samples are shown in Figure S3. The deconvolution of the high-resolution spectra for Ag 3d reveals that silver was present in more than one oxidation state in all samples. The binding energies of Ag 3d at 367.5 and 373.5 eV are assigned to the Ag 3d5/2 and Ag 3d3/2 photoelectrons respectively, indicating the presence of silver in the +1 oxidation state. The other two peaks of Ag 3d5/2 and Ag 3d3/2, at 368.3 and 374.3 eV respectively, confirm the existence of silver in the Ag(0) state. These binding energies are in good agreement with the values reported for Ag(I) in Ag2O and Ag(0) [16,37,38]. The peaks for O 1s, located in the ranges of 528.9–530.1 eV and 530.5–531.2 eV, are ascribed to O2<sup>−</sup> in Ag2O and TiO2 respectively (Figure S3). From the Ti 2p core-level spectrum, two peaks at about 464.3 and 458.7 eV can be assigned to the Ti 2p1/2 and Ti 2p3/2 spin–orbital components respectively, which correspond to the characteristic peaks of Ti4+.

The SEM images of blank TiO2, Ag/Ag2O, TM mixtures, and TC composites are presented in Figure 2. Ag/Ag2O showed well-defined particles with particle sizes ranging from 100 nm to 500 nm (Figure 2a). The small particles that contrast as white spots correspond to the metallic silver Ag(0) distributed on the surface of silver oxide, which is in agreement with the XRD results. The EDX reveals that the sample contained Ag and O without any other impurities (Figure S1).

Figure 2b–d shows SEM images of the physical mixtures of Ag/Ag2O with TiO2. It becomes obvious from these images that Ag/Ag2O changed its shape during preparation of the mixtures by sonification of aqueous suspensions of the oxides. The increasing loading of the Ag2O platelets with TiO2 is also clearly recognizable in these figures. In the Ag/Ag2O ⁄⁄ TiO2 mixture with the highest mass fraction of TiO2 (TM 14), the appearance was apparently determined by the titanium dioxide distributed over the underlying surface of the Ag2O platelets (Figure 2d). This was also reflected in the specific surface area (SSA) of the materials. The TiO2 (P25) used in this work is known to have an average diameter and specific surface area of 21 nm and about 50 m<sup>2</sup> g<sup>−</sup>1, respectively [39]. The specific surface area of the Ag/Ag2O synthesized in this work was determined to be 2.7 m2 g<sup>−</sup>1. As expected, the specific surface area of the Ag/Ag2O ⁄⁄ TiO2 mixtures was found to increase with increasing TiO2 content (Table 1), resulting in a SSA of 38.5 m2 g−<sup>1</sup> for TM 14.

**Figure 2.** SEM pictures of (**a**) Ag/Ag2O, (**b**) TM 41, (**c**) TM 11, (**d**) TM 14, (**e**) TiO2 (P25), (**f**) TC 41, (**g**) TC 11, and (**h**) TC 14.

SEM images of the TC composites are presented in Figure 2f–h. The image of the TiO2-poor composite TC 41 clearly shows the large Ag/Ag2O particles covered with TiO2 (Figure 2f). The specific surface area of this composite was determined to be 8.4 m2 g<sup>−</sup>1, thus being equal within the limits of the experimental error to the surface area of the corresponding physical mixture TC 41 (SSA = 9.7 m2 g<sup>−</sup>1). The images of the composites richer in TiO2 (TC 11 and TC 14) seemed to be dominated by aggregates or agglomerates of small TiO2 particles.

The optical properties of TiO2 and the as-prepared Ag-containing mixtures and composites were investigated by UV/vis diffuse reflectance spectroscopy (Figure 3). Ag/Ag2O, as well as the TM, and TC materials, had a dark brown to black color. They displayed strong absorption over the whole UV and visible range (200 nm–800 nm). TiO2 showed only the absorption band below 405 nm, which matches the band gap energy of 3.06 eV calculated from the formula λ = 1239.8/Ebg due to the charge transfer from O (valence band) to Ti (conduction band).

**Figure 3.** UV/vis diffuse reflectance spectra of (**a**) TiO2, Ag/Ag2O, TM mixtures, and (**b**) TC composites.

Ag/Ag2O exhibited a band gap energy < 1.5 eV, which is in agreement with the reported value of 1.3 ± 0.3 eV [40]. The scattering of the reported values might be due to different particle diameters, as shown for TiO2 [41]. Electrochemical measurements in suspensions yielded flat band potentials of −0.4 V and +0.3 V vs. NHE for TiO2 and Ag2O, respectively. The value measured here for the flat band potential of Ag2O is also in reasonably good agreement with published values [42,43].

### *2.2. Photocatalytic Performance of the Materials*

The photocatalytic activity of all materials described above was investigated, employing methylene blue (MB) as the probe compound. The materials in aqueous suspensions were excited by the full output of a xenon lamp (UV/vis illumination), and by Xe light after passing a UV cut-off filter (≥410 nm, vis illumination). Figure 4 illustrates the bleaching of an aqueous solution of MB and the MB-containing suspensions. Photolysis of MB (initiated by the direct excitation of the probe compound) was observed under both UV/vis and visible light illumination. The bleaching of MB was significantly accelerated by the presence of Ag/Ag2O. Under UV/vis illumination, Ag/Ag2O was found to be nearly as active as TiO2 (P25), which is well known to be a very efficient photocatalyst suitable to degrade MB [44] (Figure 4a). In the presence of Ag/Ag2O, MB was bleached very rapidly even when exposed to visible light. As expected, TiO2, having a bandgap energy of 3.1 eV, was found to be inactive under vis illumination (Figure 4c).

In the presence of mixtures of Ag/Ag2O with TiO2, MB was bleached under UV/vis illumination only in the presence of the TiO2-rich TM 14, with a significantly increased rate compared to the rate of MB photolysis. In suspensions containing TM 41 and TM 11, the rate of bleaching was almost the same

as the rate of photolysis (Figure 4a). Exposure to visible light in the presence of the Ag/Ag2O-rich TM 41 resulted in bleaching of MB with a slightly increased rate. In contrast, the TiO2-rich mixtures TM 11 and TM 14 were virtually inactive under this illumination condition (Figure 4c).

**Figure 4.** Bleaching of MB in the presence of Ag/Ag2O, TiO2, the TM mixtures and the TC composites under UV/vis (**a**,**b**) and under vis light only (**c**,**d**).

(**c**) (**d**)

In the presence of the composites TC, MB was bleached with significantly faster reaction rates than the rate of photolysis when exposed to UV/vis and visible light. The rates were, however, lower than the rate of bleaching in the presence of the bare TiO2 (Figure 4b,d). Interestingly, while increasing the amount of TiO2 in the TC composites, the visible light activity of the materials seemed to decrease, thus confirming the essential influence of Ag/Ag2O on MB bleaching under illumination with wavelengths ≥ 410 nm.

As a second test reaction for the activity of the materials, the UV/vis light-induced evolution of molecular hydrogen by reforming of aqueous methanol was used. Figure 5 shows the amount of H2 vs. illumination time in the presence of TiO2, Ag/Ag2O, and the prepared mixtures and composites. No H2 evolution was observed in the presence of Ag/Ag2O and the Ag/Ag2O-rich TM 41. In the presence of all other materials, the evolution of H2 was detected. However, large amounts of H2 were only evolved with the materials TM 14 (104 μmol/6 h) and TC 11 (174 μmol/6 h).

**Figure 5.** The amount of H2 evolved from aqueous methanol under UV/vis illumination of Ag/Ag2O, TiO2, (**a**) TM mixtures and (**b**) TC composites vs. illumination time.

Many authors have reported that the kinetic behavior of photocatalytic reactions can be described by a Langmuir–Hinshelwood rate law, with the two limiting cases of zero-order and first-order kinetics [45,46]. To calculate the initial rates *r*<sup>0</sup> of the bleaching of methylene blue, first-order kinetics have been assumed (*r*<sup>0</sup> = k*C*0). To determine the rate constant k, the data given in Figure 4 have therefore been fitted with *C* = *C*<sup>0</sup> exp(−k*t*). The initial rates are given in Table 1.


**Table 1.** Brunauer-Emmet-Teller (BET) surface area, initial rates of methylene blue (MB) bleaching and H2 generation in the presence of Ag/Ag2O, TiO2, the TM mixtures and the TC composites.

#### **3. Discussion**
