## 1 Introduction  
Due to global environmental difficulties and the depletion of fossil resources, sustainable energy generation is one of the most important tasks of this century [1]. Hydrogen is a viable alternative energy carrier because of its stability, abundance on Earth's surface, and lack of greenhouse gas emissions [2; 3; 4; 5; 6; 7]. Due to its renewability, the use of solar energy to make hydrogen is an appropriate technique for meeting the energy demand [4; 8; 9; 10; 11; 12; 13]. Photo reforming of organics in the aqueous phase [14; 15; 16] is an emerging technique able to combine wastewater treatment and energy production. The creation of visible--light active and stable photocatalytic/photo electrocatalytic materials is now the greatest obstacle for many researchers, despite a large number of photocatalysts suggested for hydrogen production [17]. Despite TiO\({}_{2}\) still representing the most used material, due to its advantages (chemical stability, photo corrosion resistance, non-toxicity, widespread availability, etc. ), it presents various problems such as a high electron-hole recombination rate (3.0 \(\times\) 10\({}^{10}\) M\({}^{-1}\) s\({}^{-1}\)[18]) and poor absorption in the visible light range, due to its wide band gap (about 3.2 eV) [19]. To overcome these disadvantages, many researchers have directed their efforts to improve the efficiency of bare TiO\({}_{2}\) through the combination of the semiconductor with other materials capable to extend the light absorption to the visible range and/or to reduce the recombination rate [8; 20]; on the other hand, many studies present in the literature are focused on the utilization of different semiconductors able to replace TiO\({}_{2}\)[21; 22; 23]. With reference to the first approach, noble metals (Ag, Pt, Au, Pd) have been found to be excellent co-catalysts for hydrogen production [24; 25], despite the fact that they have high costs; impregnation, co-precipitation or photodeposition methods are only some examples of the preparation methods proposed for metal-doped photocatalyst by several authors [26; 27]. Generally, noble metals have gained great attention due to their physicochemical characteristics, which effectively enhance the photocatalytic activity; in particular, the deposition of metals with a large work function, such as Ag, Au as well as Pt and Pd can reduce the recombination rate because of the formation of Schottky barriers at the metal/TiO\({}_{2}\) interface: being the work function of the metal higher than that of the titanium dioxide there is the formation of an electrons flow from TiO\({}_{2}\) to the metal to align the Fermi energy levels (EF), with a shift of the Fermi levels of the metal to the conduction band of TiO\({}_{2}\)[28]. The metal, with more negative energy levels, leads to a thermodynamically favourable proton ion reduction and favours the proton reduction also from a kinetic point of view. Hence, metal nanoparticles (a) act as antennas, producing a higher light absorption and (b) promote the transfer of free excited electrons into the conduction band of the semiconductor [29]. Despite the improvement in the photocatalytic activity of the metal-doped photocatalysts, these materials do not demonstrate a significant activation under visible light radiation. Hence, to extend the light adsorption to a wavelength higher than 400 nm, a different approach was proposed by some authors: among the techniques to improve the photocatalytic activity of the photocatalysts, many researchers have proposed the combination of semiconductors, such as TiO\({}_{2}\), with other materials capable of extending the visible light absorption to the visible range [8; 20; 30; 31]. The association of two (or more) different semiconductors (semiconductor-semiconductor heterojunction) was discussed in several articles [32; 33; 34; 35; 36; 37]. Generally, two different categories of semiconductor-semiconductor photocatalytic photocatalytic photocatalytic materials are reported: p-n semiconductor heterojunction and non-p-n heterojunction systems.