2, LCC-CNRS, Toulouse, , France
3, Rutgers, The State University of New Jersey, Piscataway, New Jersey, United States
4, Texas A&M University, College Station, Texas, United States
5, Wake Forest University, Winston-Salem, North Carolina, United States
The past five years have witnessed an increased interest in using metal organic frameworks (MOFs) as scaffolds to nucleate and stabilize metal nanoparticles (NPs) with a narrow size dispersion. Gold is particularly interesting because it exhibits a number of interesting properties in the form of nanoparticles for plasmonic, catalysis, optics, and even biology.
We pesent a new photoreduction approach (UV-vis irradiation of HAuCl4) that produces Au NPs inside MOFs without additional stabilizing or reducing agents, thus limiting the contaminations sources. Among several MOF structures with different pore sizes, we find that a relatively new MOF (MOF-808) – yields the smallest and most uniform Au NPs when functionalized with SH, compared to the well-studied UiO-66, or HKUST-1, MIL-101 (Cr), or even MOF-808 and MOF-808-NH2. Combining spectroscopy (IR absorption, Raman, XPS, UV-visible and LEIS) with imaging using high resolution transmission electron microscopy, we find that the diameter of Au-NPs synthesized in MOF-808-SH material (1.0 ± 0.5 nm with narrow distribution) is consistent with the pores size (1.07 ± 0.05 nm) principally due to a combination of a confinement effect relative to the pores sizes and a strong stabilization of the Au NPs surface by the -SH groups.
Care is taken to prove that the Au NPs are located inside the MOF structure because this is difficult to establish with TEM unless time consuming tomographic studies are performed, which can only be done in small selected parts of the sample. In contrast, spectroscopies such as LEIS, IR and Raman probe the whole volumes of MOF samples, thus providing information on the majority species in the sample. The most detailed information is obtained from LEIS and clearly shows that Au is located below the surface of the MOF microcrystals. IR and Raman provide additional information about the interactions within the pores, pointing to the chemical groups directly affected by the presence of the Au (through modification of molecular vibrations and formation of new bonds). These studies thus provide 1) a general view on the inclusion of Au NPs that is more representative of the whole material, and 2) a precise localization of the nanoparticles inside the channels of the porous materials.
Noting that the photocatalytic activities of Au NPs is strongly dependent on the size of the NPs within the 1-5 nm range, we doped MOF-808-SH with titanium by transmetalation prior to the synthesis of Au NPs and tested its photocatalytic activity, specifically the photoreduction of water. An activation period is observed in hydrogen production, which can be related to the growth of Au NPs from 1.0 ± 0.5 nm to 2.0 ± 0.9 nm, as localized surface plasmons are generated, necessary for photocatalytic activity.
Acknowledgement: This work was fully supported by the Department of Energy, Basic Energy Sciences, Grant No. DE-FG02-08ER46491.
 A. Dhakshinamoorthy et al., ACS Catalysis (2017), 7, 2896-2919.