Heteroepitaxial MOF-on-MOF Photocatalyst for Solar-Driven Water Splitting

Assembly of different metal–organic frameworks (MOFs) into hybrid MOF-on-MOF heterostructures has been established as a promising approach to develop synergistic performances for a variety of applications. Here, we explore the performance of a MOF-on-MOF heterostructure by epitaxial growth of MIL-88B(Fe) onto UiO-66(Zr)–NH2 nanoparticles. The face-selective design and appropriate energy band structure alignment of the selected MOF constituents have permitted its application as an active heterogeneous photocatalyst for solar-driven water splitting. The composite achieves apparent quantum yields for photocatalytic overall water splitting at 400 and 450 nm of about 0.9%, values that compare much favorably with previous analogous reports. Understanding of this high activity has been gained by spectroscopic and electrochemical characterization together with scanning transmission and transmission electron microscopy (STEM, TEM) measurements. This study exemplifies the possibility of developing a MOF-on-MOF heterostructure that operates under a Z-scheme mechanism and exhibits outstanding activity toward photocatalytic water splitting under solar light.


■ Supplementary Figures and tables
Hence the number of BDC-NH2 ligands per unit cell is 6*0.79=4.76 ligands (6 ligands theoretically per unit cell).
Where I BDC and I BDC-NH 2 are the integration values corresponding to the peaks of BDC and BDC-NH2 ligands.Furthermore, the number of protons for each ligand peak are expressed as n H + BDC and n H + BDC-NH 2 (BDC has 1 peak counting for 4 protons whereas BDC-NH2 3 peaks counting each for 1 proton).
Finally, we can calculate the UiO-66-NH2 mass ratio: Where M w MIL-88B and M w UiO-66-NH 2 are the corresponding molecular weight of each MOF.

Calculations of MOF mass ratios from ICP-MS:
Starting with the mass concentration of Fe and Zr in the sample we can deduce their molar ratio: ular weights of Fe and Zr are expressed as M w Fe and M w Zr.
Hence, we can obtain the UiO-66(Zr)-NH2 molar ratio within the heterostructure: With n Fe and n Zr respectively the number of metals in one unit cell of MIL-88B(Fe) and UiO-66(Zr)-NH2 (i.e.UiO-66(Zr)-NH2 and MIL-88B(Fe) have respectively 6 Zr and 3 Fe in their unit cell).To calculate UiO-66(Zr)-NH2 mass ratio, we use the same formula as for the NMR measurements.
IR spectrum of the hybridized MOF exhibits the combination of the two main COO stretching bands from UiO-66(Zr)-NH2 and MIL-88B(Fe) in the 1600-1400 cm -1 range.The characteristic band at 1500 cm -1 corresponding to the C=C aromatic stretching in UiO-66(Zr)-NH2 or MIL-88B(Fe) MOF is also present.The metallic bands of UiO-66(Zr)-NH2 at 750 cm -1 (stretching Zr-O) and MIL-88B(Fe) at 450 cm -1 (Fe-O stretching) can be observed.Finally, the presence of a band at 1250 cm -1 which can be attributed to the C-N stretching of the amino group was observed in UiO-66(Zr)-NH2@MIL-88B(Fe).A set of capillaries containing MIL-88B(Fe) and UiO-66(Zr)-NH2@MIL-88B(Fe) materials thermally activated to remove physiosorbed solvent from the pores, were soaked in a series of solvents.Upon soaking, the peaks of MIL-88B(Fe) corresponding to (100), ( 101) and (002) planes shifted as observed on the XRPD pattern, showing a favoured expansion of the material in solvents such as pyridine, ethanol and DMF (open-form) whereas the component tends to shrink in presence of isopropanol or acetonitrile (close-form).A similar solvent-responsive phenomenon was also observed in the hybrid material.and UiO-66(Zr)-NH2@MIL-88B(Fe) (green lines).Table S3.Metal leaching as determined by ICP-OES analysis of the remaining solutions after exposing UiO-66(Zr)-NH2@MIL-88B(Fe) to photocatalytic OWS and HER conditions.

Figure S31 .
Figure S31.Study of sacrificial agents' effect over photocatalytic activity.

Figure
Figure S1.a) TEM image and b) distribution size counts of UiO-66-NH2 NPs (based on 3

Figure
Figure S1.a) TEM image and b) distribution size counts of UiO-66-NH2 NPs (based on 3 different batches of 60 nanoparticles).

Figure
Figure S3.a) Pore size distribution measurements (NLDFT model for cylindrical pores) of UiO-

Figure S12 .
Figure S12.High resolution XPS peaks and the best deconvolution for the C 1s (a), O 1s (b), N

Figure S13 .
Figure S13.High resolution XPS peaks overlay for the C 1s (a), O 1s (b), N 1s(c), Fe 2p (d) and spectra shows the presence of sp 2 aromatic carbons (284.4 eV) and carboxylate groups (288.0 eV) of the organic ligand.The hybrid heterostructure and the UiO-66(Zr)-NH2 material that have amino groups also exhibit the corresponding bands in C 1s and N 1s XPS regions at 286.0 and 399.0 eV, respectively.The broad O 1s XPS spectra is associated to the presence of oxygen atoms present in M-O (M: Zr or Fe) and COO-groups at about 529 and 531 eV, respectively.The Zr 3d XPS spectra of the hybrid heterostructure and UiO-66(Zr)-NH2 materials confirm the presence of Zr(IV) as deduced from the two bands at 182.0 and 184.4 eV corresponding to Zr 3d5/2 and Zr 3d3/2, respectively.XPS Fe 2p of MIL-88B(Fe) and the hybrid heterostructure reveal characteristic features of Fe (III) ions with two bands at 712 and 715 eV associated to Fe 2p3/2 and Fe 2p1/2, respectively.

Figure S16 .
Figure S16.Influence of the irradiation conditions on the photocatalytic activity of UiO-66(Zr)-

Figure S17 .
Figure S17.Mass spectrum obtained after the overall splitting reaction using labelled H 2 18 O
Figure S19.TGA profiles for former UiO-66(Zr)-NH2 and the same NPs after exposure to epitaxial conditions (dispersed in DMF and heated to 100 °C for 12h).

Table S1 .
Ratios MOF/Residue of UiO-66(Zr)-NH2 theoretical versus experimental with the decomposition profile normalized to 100 % of the residue.Between 200 and 350 ºC, UiO- Calculation of missing linkers defects from TGA: