Production of hydrogen and oxygen from water splitting reaction under visible light is a simple method for conversion of solar-to-hydrogen energy and it is a hopeful clean and renewable method for H-2 fuel generation. However, there is still a lack of potential materials with significant activity under visible light. Because of safety, chemical inertness, low cost, stability and other characteristics, transition metal oxide semiconductors have been widely applied as photocatalysts for hydrogen generation. Albeit, wide usage of semiconductor photocatalysts were prevented by its inability to exploit solar energy of visible region. Here we show synthesis of a nano-sized mixed metal oxide (MMO) Ca3MnO6 through wet-chemistry methods such as co-precipitation, ultrasonic, microwave, reflux, and hydrothermal methods. The nano-sized Ca3MnO6 has initially selected based on morphology and respective particle diameters. The selected sample shows a well-defined single crystal, free from any impurities, complete structural formation, and a band gap energy (E-g) of around 5.3 eV. The best product synthesized in ultrasonic method which shows the best morphology, purity and the highest efficiency for splitting of water to hydrogen and oxygen. Irrespective of preparation methods and morphologies, all samples split water into hydrogen and oxygen, as confirmed from their respective photocatalytic analysis. When the selected sample combined with (NH4)(2)Ce(NO3)(6), the single-crystal Ca3MnO6 nanoparticles split water into hydrogen and oxygen more efficiently under visible light. Our findings demonstrate the importance of nanostructured Ca3MnO6 single crystal photocatalysts in solar water splitting. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.