BiOCl. Interestingly, these two peaks shift back to their original positions after oxygen treatment at 300 °C for 16 h. This is possibly because most of bulk OVs are healed and lead to reduction of the binding strength of CO2 molecules at the surface. The CO2 adsorption process can be understood as depicted in Fig. . We anticipate residues of the D-mannitol from synthesis as well as other adventitious carbon species reacting with lattice oxygen of BiOCl as the source for CO2 in the DRIFTS experiments. This viewpoint is confirmed by the decrease of hydrocarbon related absorption features between 3000-2800 cm -1 (C-H stretching region) upon increasing temperature. Moreover, D-mannitol starts to decompose when the temperature increases above 300 °C according to literature . The formation and recovery process of bulk OVs can be described in Kröger-Vink notation as follows. Firstly, bulk OVs are formed in BiOCl samples and O2 is released at low temperature (eqn. 1). Secondly, D-mannitol or adventitious carbon decomposition to produce CO2 and water and physisorption occurs at the surface of BiOCl (eqn.