The superiority of a hierarchical photocatalyst for water treatment applications is mostly rationalized in terms of two features: light harvestability and adsorption capability. Not only a conclusive evidence to support these claims is missing, knowledge on the 'key material property' governing photocatalyst performance is also unclear. Herein, a hierarchical BiOBr photocatalyst was studied in comparison with its plate-like counterpart. Found from the photocatalytic water treatment experiments, the hierarchical BiOBr exhibited three times faster reaction kinetics compared to the plate-like BiOBr. While light harvestability and adsorption capability of the two structures was not significantly different, a ca. 36% higher photocurrent and a ca. 16% longer charge carrier lifetime observed in hierarchical BiOBr demonstrated its superior charge carrier separability. Compared to other material properties, crystal disorders were found to predominantly influence the photocatalytic activity, which was verified through Raman spectroscopy, high resolution transmission electron microscopy, and X-ray diffraction analyses. The findings provide an insight into the role of crystallographic disorders in hierarchical photocatalysts which is a useful advancement towards the pursuit of rational photocatalyst design particularly for interfacial photocatalytic water treatment applications. (C) 2020 Elsevier Inc. All rights reserved.