*3.2. Characterization*

X-ray diffraction (XRD) patterns were recorded on a Malvern PANalytical Empyrean system (Malvern PANalytical, Hoeilaart, Belgium). Scanning electron microscopy (SEM) images and the corresponding energy dispersive X-ray spectroscopy (EDS) data were taken with a FEI-Q FEG250 system (Thermo Fisher Scientific, Zaventem, Belgium). X-ray photoelectron spectroscopy (XPS) data were recorded on an ESCALAB 250 spectrometer (Thermo Fisher Scientific, Beijing, China) with a monochromatized Al Ka X-ray as the excitation source, and the binding energies were calibrated by the C1s peak at 284.8 eV. Gas sorption measurements were performed on a 3Flex Surface Analyzer (Micromeritics, Unterschleissheim, Germany) where nitrogen (N2) adsorption-desorption isotherms were acquired at −196 ◦C and CO2 adsorption isotherm were acquired at 0 ◦C. Before the measurement, all the samples were degassed at 150 ◦C for 8 h under flowing N2. Thermogravimetric analysis (TGA) was performed from room temperature to 800 ◦C at a heating rate of 10 ◦C min−<sup>1</sup> in O2 on a TA-TGA Q500 (TA Instruments, Antwerp, Belgium). Fourier transformed infrared (FTIR) spectra were collected using a Bruker FTIR spectrophotometer (Bruker, Kontich, Belgium). UV-Vis di ffuse reflectance spectra were recorded with a Lambda-950 UV-Vis spectrometer (PerkinElmer, Mechelen, Belgium). The steady-state photoluminescence (PL) spectra were acquired on an Edinburgh FLS980 instrument (Edinburgh Instruments Ltd, Livingston, UK). The samples were prepared by filling a thin quartz cuvette (Macro cell 110-QS, 1 mm light path, Hellma, Kruibeke, Belgium) with the same volume of powders. Time-resolved PL spectra were acquired by a home-built confocal FLIM microscope, equipped with a single-photon counting device (Picoquant, Berlin, Belgium) Powders of CsPbBr3 and p-90Fe were dispersed in toluene and sonicated for 15 min. The suspension was then dropped onto a 20 × 20 mm cover slide and dried in the vacuum oven at 80 ◦C overnight. The excitation source was a 5 MHz pulsed 485 nm laser diode, and the emission was filtered by a 530 ± 25 nm band pass filter. Confocal fluorescence images were acquired on a Fluoview FV1000 confocal microscope (Olympus, Tokyo, Japan). An Olympus 20 × 0.75 NA air immersion objective lens was used. A 375 nm laser was used as the excitation source, and the detectors for fluorescence emission were set at 430–470 and 505–540 nm. The image size was 1024 × 768 pixels with a pixel dwell time of 4 μs.

#### *3.3. Photocatalytic CO2 Reduction Measurement*

The photocatalytic reduction of CO2 was performed in a homemade Pyrex reactor (volume: 150 mL). Visible light was generated by a 300 W Xe lamp with a 420 cut-o ff filter (Newport, Darmstadt, Germany) and positioned 5 cm away from the photocatalytic reactor. In a typical sample preparation, 20 mg photocatalyst was uniformly dispersed on a flat glass plate with an area of 4 cm2. The as-prepared sample plate was left in the vacuum oven at 80 ◦C overnight to remove the residual solvent. Before the reaction, helium flowed through the reactor for about 20 min to eliminate the air inside. Then, a mixture of CO2 and water vapor, generated by passing high purity CO2 (99.99%) gas through a water bubbler, flowed through the reactor for another 40 min in the dark. Afterwards, the reactor was closed o ff and light irradiation was started. The gas sample was evaluated every 1 h by gas chromatography (GC-2014, Shimadzu, Tokyo, Japan) equipped with a ShinCarbon packed column with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The carrier gas used in the GC-2014 was high purity helium. After the 16 h stability test, the sample was collected and treated at 80 ◦C in the vacuum oven overnight.
