CoO nanocrystal is a well-known photocatalyst for overall water splitting; however it suffers from a very short lifetime of only 1 h. The poor stability is derived from carrier recombination-induced thermal oxidation. Amorphous materials are generally omitted from photocatalysts due to the tailing effect. Here, we report that efficient solar overall water splitting into H-2 and O-2 with an expected 2:1 stoichiometry is achieved by two-dimensional amorphous CoO nanoflakes without any cocatalysts or scavengers, and no deactivation is detected even after working for 120 h. We also establish that the mediation of mid-gap states resulting from the amorphization is responsible for the carrier separation and consequent high stability. Further, graphene oxide is grafted to construct a bionic Z-scheme structure, promoting the gases evolution rates by a factor of 13. Overall, these findings reveal the potential of CoO for practical applications in the future hydrogen economy, and shed new light on the design of novel amorphous photocatalysts. (C) 2019 Elsevier Inc. All rights reserved.