Metal–organic frameworks (MOFs)1,2,3 are known for their specific interactions with gas molecules4,5; this, combined with their rich and ordered porosity, makes them promising candidates for the photocatalytic conversion of gas molecules to useful products6. However, attempts to use MOFs or MOF-based composites for CO2 photoreduction6,7,8,9,10,11,12,13 usually result in far lower CO2 conversion efficiency than that obtained from state-of-the-art solid-state or molecular catalysts14,15,16,17,18, even when facilitated by sacrificial reagents. Here we create ‘molecular compartments’ inside MOF crystals by growing TiO2 inside different pores of a chromium terephthalate-based MOF (MIL-101) and its derivatives. This allows for synergy between the light-absorbing/electron-generating TiO2 units and the catalytic metal clusters in the backbones of MOFs, and therefore facilitates photocatalytic CO2 reduction, concurrent with production of O2. An apparent quantum efficiency for CO2 photoreduction of 11.3 per cent at a wavelength of 350 nanometres is observed in a composite that consists of 42 per cent TiO2 in a MIL-101 derivative, namely, 42%-TiO2-in-MIL-101-Cr-NO2. TiO2 units in one type of compartment in this composite are estimated to be 44 times more active than those in the other type, underlining the role of precise positioning of TiO2 in this system.
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The following are available in Supplementary Information: additional crystallographic information and PXRD data; data from SEM, TEM, selected-area electron diffraction, STEM; results of gas-sorption measurements; diffuse reflectance UV–visible spectra; elemental analyses; CO2 reduction and O2 production data; results from TAS, X-ray absorption fine structure measurements, and XPS; and DFT simulations.
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We thank staff at beamlines BL14B1, BL14W1, BL15U1 and BL08U1-A (Shanghai Synchrotron Radiation Facility), and at beamline BL04B (National Synchrotron Radiation Laboratory, Hefei, China), for providing the beam time and assistance with synchrotron experiments. We also thank staff of the High Magnetic Field Laboratory of the Chinese Academy of Sciences (CHMFL) for collecting EPR data. We thank S. Osami, T. Masahiko, Y. Katsuya and K. J. Ohara for collecting the absorption coefficient and PDF data at BL04B2 and BL15XU in the National Institute for Materials Science in Japan; Y. Yang and J. Huang at the Dalian Institute of Chemical Physics for collecting nanosecond TAS data; and A. Carlsson and S. Lazar from Thermo Fisher, I. Onishi from JEOL Ltd, and P. Liu from Shanghai Jiaotong University for their invaluable assistance with TEM. Other characterizations were provided by the test centre and Core Research Facilities of Wuhan University. We thank W. Zhang, J. Cao, X. Cai, Z. Dong, H. Jiang, Q. Liu, B. Bie, F. Ke and H. Cong from Wuhan University and Z. Pan and W. Hong from Xiamen University for their help. Financial support was provided by the Natural Science Foundation of China (21471118, 91545205, 91622103, 21971199) and the National Key Basic Research Program of China (2014CB239203, 2018YFA0704000). L.Z. thanks the National Natural Science Foundation of Jiangsu Province for support (BK20151248). T.P. thanks the Natural Science Foundation of China for support (21573166). Y.M. thanks the Commission for Science and Technology of Shanghai Municipality (17ZR1418600), the Shanghai Pujiang Program (17PJ1406400) and the Young Elite Scientist Sponsorship Program by CAST (2017QNRC001) for support. The Recruitment Program for Foreign Experts, China (O.T.) and the grant EM02161943 from CħEM, SPST, ShanghaiTech are acknowledged by Y.M., H.S.C., P.O. and O.T. We also thank the scientists at Berkeley Global Science Institute, UC-Berkeley, for discussions.
The authors declare no competing interests.
Peer review information Nature thanks Matthew Cliffe, Fernando Uribe Romo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, b, 2D projection of the reconstructed reciprocal space of the MOF (MIL-101-Cr; a) and the TiO2-in-MOF (23%-TiO2-in-MIL-101-Cr; b) samples along the  direction. The inset in b is the differential electrostatic potential obtained from the 3D diffraction data of 23%-TiO2-in-MIL-101-Cr subtracted from that of MIL-101-Cr. The yellow circles at top right in a and b indicate the diffraction points with highest resolution. Scale bars, 1 nm−1.
a, b, Elemental maps of single-crystal 23%-TiO2-in-MIL-101-Cr (a) and 42%-TiO2-in-MIL-101-Cr (b); the elements are given at the top right of each coloured map. The uniform TiO2 distribution in both samples is clear. Also shown (bottom right in a and b) is the atomic ratio Ti/Cr (in %) for the five sampling points indicated at top left of a and b. The green dashes in these plots indicate the Ti/Cr ratio of the bulk materials. The Ti/Cr atomic ratios at the five distinct positions in one single crystal are almost identical. Scale bars: a, 100 nm; b, 50 nm.
a, HAADF and b, iDPC images. The sample thicknesses are indicated on the left in yellow. Red and blue outlines (bottom right of each panel) are overlaid on the images to highlight the position of molecular compartments, with the unit cell in orange. Scale bars, 20 nm.
As Extended Data Fig. 3 but for 23%-TiO2-in-MIL-101-Cr.
As Extended Data Fig. 3 but for 42%-TiO2-in-MIL-101-Cr.
Extended Data Fig. 6 Fourier diffractograms of the images of MOF and TiO2-in-MOF samples in Extended Data Figs. 3–5.
a–c, Fourier diffractograms of the iDPC images in Extended Data Figs. 3–5, respectively; d–f, Fourier diffractograms of the HAADF images in Extended Data Figs. 3–5, respectively. Insets, contrast plots for the spots used to derive the resolution (boxed, labelled with indices). The dashed yellow circles indicate the resolution of each sample.
Extended Data Fig. 7 Line profiles for pure MOF and TiO2-in-MOF samples at different sample thickness in the HAADF images.
a, Illustration of the mesopore distribution in MOF single crystal in different directions, from which the sample thickness is derived. b, Line profiles across mesopores of different types in HAADF images at identical sample thickness. The line profiles of MIL-101-Cr, 23%-TiO2-in-MIL-101-Cr and 42%-TiO2-in-MIL-101-Cr are marked in green, red and blue, respectively. The black lines represent those obtained from the symmetry averaged image of pure MIL-101-Cr as references.
a, The energy level diagram of TiO2, reaction intermediates and products. h+, holes; e−, electrons. b, Electron transfer pathway of the photocatalytic reaction illustrated in TiO2-in-MOF compartments composed of Cr clusters.
This file contains Supplementary Methods, Supplementary Discussions, Supplementary Figures 1 to 102, Supplementary Tables 1 to 16, and Supplementary References, and provides information about materials synthesis, identification of TiO2 within the pores of MOF, discussion of catalytic performance and mechanism.
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Jiang, Z., Xu, X., Ma, Y. et al. Filling metal–organic framework mesopores with TiO2 for CO2 photoreduction. Nature 586, 549–554 (2020). https://doi.org/10.1038/s41586-020-2738-2