**4. Conclusions**

In summary, we have reported the in situ synthesis of CsPbBr3/MIL-100(Fe) for CO2 photoreduction under visible light irradiation. Compared to anti-solvent deposition and physical mixing methods, the composites obtained by this new route show a significantly improved CO2 photoreduction'activity. H2O is necessary for the in situ growth of the MOF structure, but excessive H2O initiates the transformation of CsPbBr3 to CsPb2Br5. Through optimization of the composite material composition, the highest CO production rate is 20.4 μmol g<sup>−</sup><sup>1</sup> <sup>h</sup>−1, which is about 4.5 times that of the parent materials. The composite showed good stability in a 16 h photocatalytic reaction, where H2O is involved as the reactant. The introduction of MIL-100(Fe) endowed the composites with a largely increased surface area and enhanced light-harvesting capability in the visible light region. The perfect band matching between CsPbBr3 and MIL-100(Fe) attributes to better electron-hole separation and transfer. The findings here could serve as a steppingstone for further developing MHP photocatalysts, involving MOF-based heterojunctions.

*Catalysts* **2020**, *10*, 1352

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/11/1352/s1, Figure S1: XPS spectra of p-90Fe, Figure S2: Photoluminescence spectra of the as-prepared samples at an excitation wavelength of 380 nm, Figure S3: Typical SEM images of CsPbBr3, MIL-100(Fe) and p-90Fe, and EDS pattern of the selected region in p-90Fe, Figure S4: XRD pattern of p-90Fe before and after a 16 h photocatalytic reaction, Figure S5: N2 adsorption-desorption isotherms and CO2 adsorption isotherms of the photocatalysts, Figure S6: N2 adsorption-desorption isotherms and pore size distribution curves of the p-90Fe before and after reaction, Figure S7: Valence band XPS spectra of MIL-100(Fe), CsPbBr3 and p-90Fe, Table S1: Summary of the reported photocatalytic CO2 reduction performance of perovskite-based and traditional photocatalysts under various illumination conditions.

**Author Contributions:**Conceptualization, R.C. and J.H.; methodology, R.C., E.D.,M.B.J.R. and J.H.; formal analysis,R.C.; writing—original draft preparation, R.C.; writing—review and editing, M.B.J.R., E.D. and J.H.; supervision, J.H. and M.B.J.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the European Union (Horizon 2020) Marie Sklodowska-Curie innovation program (Grant No. 722591, in the form of a Ph.D. fellowship to R.C.), the Research Foundation—Flanders (FWO) through a postdoctoral fellowship to E.D. (FWO Grant No. 12O3719N), and research projects to J.H. and M.B.J.R (FWO Grant Nos. G098319N and ZW15\_09-GOH6316), the KU Leuven Research Fund (C14/15/053 and C14/19/079), and the Flemish governmen<sup>t</sup> through long term structural funding Methusalem (CASAS2, Meth/15/04).

**Conflicts of Interest:** The authors declare no conflict of interest.
