We successfully stabilize the delta phase of bismuth oxide (delta-Bi2O3) down to room temperature through a simple hydrothermal method. The prepared delta-Bi2O3, which is composed of ultrathin nanosheets, possesses excellent crystallinity. When delta-Bi2O3 is doped with Fe3+, the photocatalytic activity for the mineralization of a number of refractory organic compounds is improved. The improved photocatalytic activity is caused by an increased electron population upon the Fe3+ doping. Additional thermal treatment through a post-calcination at 500 degrees C under air atmosphere results in further increases of photocatalytic activity due to the improved crystallinity, extended light absorption, and better incorporation of Fe3+ into the host lattice. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.