Water splitting to hydrogen and oxygen molecules suing semiconductors as photocatalysts to store solar energy, is one of the “Holy Grails” in solar energy conversion¹. The performance of a photocatalyst is largely limited by four requirements: 1) long term stability with respect to photo corrosion, 2) a narrow band gap materials to absorb a large fraction of solar light, 3) suitable band edges that match the redox potential of water, and 4) slow charge carrier recombination systems. In particular, with respect to point 4, many artificial Z-scheme based photocatalytic systems have been widely investigated². Recently, Rao et al.,³ reported an all-solid state Z-scheme anisotropic ZnO/Pt/Cd_0.8Zn_0.2S heterojunction system which exhibited a high photocatalytic activity owing to an efficient electron-hole transfer between the ZnO conduction band and the CdS valence band through Pt nanoparticles (using benzyl alcohol and acetic acid as sacrificial reagents). However, this system is not stable over the long term mainly because ZnO is stable in a very narrow pH range (6-8)⁴, whereas the ideal pH for hydrogen production, either by sacrificial or direct water splitting, is often outside this range. As a consequence, the original Z-scheme structure is gradually destroyed following the dissolution of the ZnO which subsequently causes the decomposition of Cd_0.8Zn_0.2S.

In this work, we have developed a novel TiO₂/Ag-Pd/Cd(Zn)S based Z-scheme photocatalytic system. This system was found to be stable in the presence of aqueous solution containing benzyl alcohol/acetic acid (the role of both has been investigated). Catalysts were prepared and characterized by XRD, XPS, TEM and UV-Vis and the catalytic tests were conducted over the complete series of the Z-Scheme system and its individual components such as Ag-Pd/TiO₂ and Cd_0.8Zn_0.2S. The hydrogen generation rate of the best system and that of its components were 3.3 mmol/h.g_catal. (TiO₂/Ag-Pd/Cd_0.8Zn_0.2S), 1.1 mmol/h.g_catal. (Ag-Pd/TiO₂) and 0.5 mmol/h.g _catal. (Cd_0.8Zn_0.2S) respectively under identical conditions as described below.