Novel ultrathin BiOBr nanosheets have been successfully synthesized without using any expensive chemicals. The obtained nanosheets with dominantly exposed {001} facets possess unique concave geometric shapes. The immiscible solvent system used is believed to be the key to the formation of the concave ultrathin nanosheets, whose growth mechanism is proposed based on the time-dependent experiment. The unique micromorphology and microstructure of nanosheets not only improve the light absorption but also facilitate the separation and transfer of charge carriers. Therefore, the photocatalytic activity toward the degradation of rhodamine B(RhB) are obviously improved compared with BiOBr microspheres. This work develops a simple, room temperature and cost-efficient strategy for fabricating concave ultrathin nanosheets, which can be extended to other Bi-based oxides. (c) 2017 Elsevier B.V. All rights reserved.