In-situ temperature-controlled diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to assess surface oxygen vacancy formation. Steady state IR spectra were recorded in the region of 400 to 3950 cm -1 at a resolution of 2 cm -1 on a Bruker Vertex 70v spectrometer with a room temperature DTGS detector. Pristine BOC-S0 was used as starting sample and heated at 200 °C, 250 °C, and 300 °C in vacuum, respectively. For the oxidation process, dry O2 (≥99.95%) was introduced into the cell at a flow rate of 2 cm 3 min -1 at 400 °C for 6 h to oxidize defective BiOCl while the static pressure of the cell was 10 mbar. After cooling down to 298 K, DRIFTS spectra were collected in static vacuum (1 mbar). Dry KBr powder was used as the reference.