Design and synthesis of effective, eco- friendly, low-price semiconductor-based photocatalyst/solar-energy materials and their application for photocatalytic transformation of plentiful/perilous hazardous chemicals (e.g. H2S) into hydrogen green fuel and other valuable elements is a promising/sustainable strategy, which is highly in demand from environmental, energy and fuel as well as chemical and technological standpoints. To this end, herein, a set of new quaternary, Ni-doped n-type solid-solution (alloy) semiconductor compounds, viz. (NixFe0.2Zn0.7-xII)-Fe-II-Zn-III .S (x = 0, 0.02, 0.05 and 0.1) were synthesized through a facile one-pot hydrothermal route and employed for the photocatalytic conversion of alkaline H2S medium (pH = 11) to hydrogen fuel and elemental sulfur. The investigations revealed that by adding Ni into the ternary photocatalyst-base (Fe0.2Zn0.7S), with decreasing the size of constituting nanoparticles and enlarging the photocatalyst surface area, the absorption intensity was strengthened. Among the materials under consideration, the lowest charge recombination (photoluminescence emission), highest photocurrent, and greatest displacement in open circuit and flat-band potentials were witnessed for x = 0.05 , the photocatalyst with maximum performance to produce H-2 and S. Finally, by using photoelectrochemical data and depicting the energy-diagram of system, a detailed discussion was provided and the phenomenon was justified from physicochemical viewpoint. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.