**1. Introduction**

Environmental problems related to water and air contamination, due to increasing world population and the resulting tremendous growth of industry and fuel combustion, have become a major concern of advanced science. In order to deal with this important problem, photocatalytic processes with employment of semiconductors are the most conventional approaches for water and air purification, along with alternative energy storage (e.g., H2) [1–4].

To date, different semiconductor nanoparticles such as TiO2, ZnO, Fe2O3, niobates, tantalates, and metal sulfides, and their underlying working mechanisms, have been investigated with the aim

of increasing their photocatalytic activity. It is well known that, besides the ability to decontaminate polluted air and water, a photocatalyst should meet certain requirements such as cost efficiency, stability, non-toxicity, and broad range response towards incident light. TiO2 is reported as the most durable photocatalyst, responding to all the above-mentioned requirements apart from broad range response to incident solar light due to its wide bandgap energy, (3.2 eV for anatase, 3.0 eV for rutile) which accounts for no more than 5% of the entire solar spectrum [1]. This lack of photocatalytic activity under visible light illumination allows the use of TiO2 as a UV blocker in sunscreens [5]. The tremendous interest in modification of titanium dioxide with different metals and oxides, to enable absorption of lower energy states and increase stability, has been rising over the last 20 years. Nonetheless, the range of visible-light photocatalysts is still restricted. Thus, it is essential to discover new and efficient photocatalytic materials that are sensitive to visible light.

Ag2O nanoparticles have been broadly utilized in various manufacturing areas as stabilizers, cleaning agents, electrode supplies, dyes, antioxidants, and catalysts for alkane activation and olefin [6,7]. Several papers have been published reporting the photocatalytic activity of Ag2O, Ag/Ag2O, Ag2O/semiconductors, and Ag/Ag2O/semiconductor composites, and some reviews are available [8–33]. Ag2O is reported to be a visible light active photocatalyst. However, due to its photosensitive and labile properties under incident light illumination, Ag2O is infrequently employed alone as a main photocatalyst rather than as a co-catalyst [8].

Wang et al. investigated the photocatalytic performance of Ag2O on the photocatalytic decolorization of methyl orange, rhodamine B, and phenol solution under fluorescent light irradiation, and concluded that the stability and high photocatalytic activity of Ag2O is maintained by the partial formation of metallic Ag on its surface during the photodecomposition of organic compounds [9]. Jiang et al. also reported the decomposition of methyl orange under visible light, ultraviolet light, near-infrared (NIR) light, and sunlight irradiation, using silver oxide nanoparticle aggregation. The superb photo-oxidation performance of Ag2O is kept almost constant after repeated exposure to light due to its narrow band gap, high surface area, and numerous crystal boundaries supplied by Ag2O quantum dots [13]. Several authors have claimed that an Ag/Ag2O structure exhibits 'self-stability' [9,10] during a photocatalytic run, due to rapid electron transfer from the excited Ag2O to Ag(0) [12,20].

Visible light active nanocomposites of Ag/Ag2O/TiO2 have been synthesized using different methods, such as a microwave-assisted method [28], a low-temperature hydrothermal method [32], a one-step solution reduction process in the presence of potassium borohydride [22], a simple pH-mediated precipitation [23], and a sol-gel method [27]. Moreover, Su et al. developed a novel multilayer photocatalytic membrane, consisting of an Ag2O/TiO2 layer stacked on a chitosan sub-layer immobilized onto a polypropylene [31]. Light-induced hydrogen production via photoreforming of aqueous glycerol has been scrutinized, employing Ag2O/TiO2 catalysts prepared by a sol-gel method with varying content of Ag2O (0.72–6.75 wt %) [30]. Hao et al. have reported that TiO2/Ag2O nanowire arrays forming a p-n heterojunction are applicable for enhanced photo-electrochemical water splitting [33]. Hu et al. reported the photocatalytic degradation of tetracycline under UV, visible, NIR, and simulated solar light irradiation with the Z-scheme between visible/NIR light activated Ag2O and UV light activated TiO2, using reduced graphene oxide as the electron mediator. They also investigated the stability of Ag2O, Ag2O/TiO2, and Ag2O/TiO2 in combination with reduced graphene oxide as an electron mediator. A large amount of Ag(0) was formed into Ag2O and Ag2O/TiO2 after four cycles of tetracycline photodegradation under UV, visible, and NIR illumination [23]. Ren et al. also observed the light-induced reduction of Ag2O during dye degradation in Ag2O/TiO2 suspensions. The authors suggested that the formation of Ag(0) contributed to the high stability of their photocatalyst [29]. The stabilization of Ag2O/TiO2 photocatalysts by Ag(0) formed at an initial stage of an experimental run has already been proposed earlier [11]. The photocatalytic stability of Ag-bridged Ag2O nanowire networks/TiO2 nanotubes, which were fabricated by a simple electrochemical method, revealed only an insignificant loss in performance, with respect to photocatalytic degradation of the dye acid

orange 7, under simulated solar light [15]. On the other hand, Kaur et al. reported a decrease of the degradation efficiency from 81% to 54%, after the third experimental run employing AgO2/TiO2 as the photocatalyst and the drug levofloxacin as the probe compound [24]. Very recently, Mandari et al. synthesized plasmonic Ag2O/TiO2 photocatalysts, which could absorb visible light by the resonant oscillation of the conduction band electrons under visible light illumination. With this method, they were able to improve the efficiency of TiO2 as a photocatalyst for hydrogen production by H2O splitting under natural solar light. The authors observed the formation of Ag(0) by light-induced reduction of Ag2O [26]. Light-induced reduction of Ag(I) to Ag(0) has also been reported for an Ag(0)/Ag(I) co-doped TiO2 photocatalyst [34].

The preceding discussion of published experimental results provoked doubt on the stability of Ag2O-containing photocatalysts under UV/vis illumination. Therefore, visible light harvesting Ag/Ag2O ⁄⁄ TiO2 photocatalysts for water treatment and photocatalytic hydrogen generation were synthesized. To the best of our knowledge, physical Ag/Ag2O ⁄⁄ TiO2 mixtures synthesized by the sonication of a suspension containing TiO2 (P25) and a self-prepared Ag/Ag2O were investigated for the first time. Ag/Ag2O ⁄⁄ TiO2 composites, prepared in situ by a simple precipitation method employing TiO2 and AgNO3, were also prepared, in order to evaluate the effect of the synthesis method on the photocatalytic activity. Additionally, the effect of the mass ratio of Ag/Ag2O was studied. The as-prepared mixtures and composites showed improved visible light activity for methylene blue (MB) bleaching, compared to blank TiO2, and high photocatalytic H2 production from a methanol-water mixture under artificial solar light illumination.

#### **2. Results**
