In an embodiment, the present disclosure pertains to photocatalysts with high solar-to-hydrogen overall water splitting efficiency. In an embodiment, the photocatalyst is a nanocrystalline cobalt (II) oxide (CoO) nanoparticle. In some embodiments, the present disclosure pertains to methods of synthesizing the photocatalysts disclosed herein. Such a method may comprise using femtosecond laser ablation of cobalt oxide micropowders. In some embodiments, such a method comprises mechanical ball milling of cobalt oxide micropowders. In an embodiment, the photocatalyst disclosed herein decomposes water under visible light without the aid of any co-catalysts or sacrificial reagents. In some embodiments, the present disclosure pertains to methods of splitting water to produce hydrogen.