The XRD pattern of BiOCl samples are shown in Fig. . X-ray diffraction peaks of all BiOCl samples are in accordance with tetragonal phase BiOCl (JCPDS card no. 06-0249). When increasing temperature to 300 °C in helium or oxygen, all BiOCl samples retain their original tetragonal crystal structure, with no obvious diffraction peaks of Bi metal are observed. This indicates that annealing in either O2 or He atmospheres did not significantly change the crystal structure of BiOCl samples. In addition, the annealing process has little effect on the average crystallite sizes of BiOCl samples as estimated via the Scherrer formula (Table ). However, intensities of the diffraction peaks of BiOCl nanosheets obviously have changed. Using the intensity of the (001) crystal plane as reference, relative crystallinity of BiOCl noticeably increased from 1.00 to 1.68 after annealing in oxygen while it decreased from 1.00 to 0.58, 0.65, 0.55 at 200°C, 250°C, 300°C in helium, respectively (Table ). Intensity of the (020) crystal facet follows the same trend. These changes are possibly caused by the increasing number of defects during annealing in He. Additionally, N2 physisorption isotherms of the BiOCl samples were collected to calculate specific surface areas and pore volume (Fig. ). The BET surface area of BOC-S0 is 19.53 m 2 g -1 and its pore volume is 0.11 cm -2 g -1 . These values show a negligible change after treatment at various temperatures in helium and oxygen. The data of physical properties are summarized in Table .