Taken together, the above data reflect that annealing in He atmosphere does not significantly change crystal structure, morphology, or porosity of BiOCl nanosheets. while BOC-O-300 shows no changes. This extended light absorption at longer wavelengths has been reported to originate from oxygen vacancies . EPR spectra were measured to verify the presence of OVs (Figure ). A significant peak appears positioned at g = 2.004, which is the typical signature for trapped electrons in OVs . With an increasing annealing temperature in helium atmosphere, the respective signal gradually becomes larger, indicating an increasing amount of OVs. Consistently, BOC-O-300 shows no response in the EPR spectra which relates to the absence of OVs after annealing in oxygen. XPS measurements have been employed to characterize OVs at the surface of semiconductors . The relative content of oxygen vacancies can be extracted from the O 1s and the corresponding metal high resolution core level XP spectra. For instance, it is possible to indirectly obtain the content of OVs through calculating the ratio of Ce 3+ /Ce 4+ in the Ce 3d spectrum or the relative area of O-Ce 3+ in O 1s spectrum . However, XPS is very surface sensitive with an information depth of few nanometers, depending on photoelectron kinetic energy. In our case, high resolution XP spectra of Bi 4f, O1s and Cl 2p regions remain almost unchanged with the different treatments indicating the absence of surface OVs (Fig. ). The XPS and EPR results taken together with the reported high reactivity of surface OVs in BiOCl , let us presume that surface OVs formed by annealing in He have been quickly replenished by subsequent exposure to ambient air during sample handling and preparation and that the OVs seen by EPR are located mainly in the bulk of the materials. Photoluminescence (PL) is commonly used to determine the energy positions of defect states in the band gap and evaluate separation efficiency of photogenerated charge carriers . The range of PL spectral features associated with OVs-created defect states is relative wide. The PL spectra of the BiOCl samples taken at room temperature show a broad distribution of PL intensity in the range of 340-500 nm (Fig. ). The distinct peak at 423 nm is associated with a defect state 2.93 eV above valence band maximum (denoted as S1 in Fig. ) and corresponds to the main recombination process of charge carriers. The required excitation energy of electrons trapped at S1 in BOC-H-250 sample is only 0.070 eV (= ECBM -ES1), which is close to 3kT (0.078 eV). This means that the trapped electrons can be excited thermally at 298 K. Therefore, S1 states are not regarded as effective recombination centers for BOC-H-250 and BOC-H-300 samples. However, this excitation energy reaches up to 0.23 eV in BOC-H-200, which makes it less likely that trapped electrons get thermally excited at 298 K. In this sample, additionally, a new deep defect state (denoted as S2) at 2.56 eV (485 nm) above VBM emerged. These two recombination centers related to defect states contribute to the high photoluminescence intensity for BOC-H-200 sample (Fig. ). The amount of S2 will decrease or even disappear with increasing annealing temperature. To investigate the mechanism of thermally induced OV formation, in-situ DRIFTS experiments on BOC-S0 were conducted at various temperatures and a pressure <1 mbar. As shown in Fig. , a peak centered at 2335 cm -1 appeared at 200 °C, which can be ascribed to physisorbed CO2 after thermal removal of water. The amount of physisorbed CO2 obviously increases as the amount of adsorbed water decreases. When the temperature increased to 300 °C, CO2 physisorption became stronger and a noticeable red shift is observed from 2335 cm -1 to 2332 cm -1 . This change originates from influence of an increased amount of bulk OVs. Meanwhile, a small peak at 2267 cm -1 emerged, which can be ascribed to CO2 chemisorption on Bi 3+ at the surface of defective