Nanoparticle Core/Shell Architectures within MOF Crystals Synthesized by Reaction Diffusion

Abstract

Metal–organic frameworks (MOFs) have attracted considerable attention on account of their applications in molecular separations, gas storage, catalysis, and chemical sensing. Recently, there has been a growing interest in using these highly ordered microporous architectures as host matrices or templates with incorporated metal or metal oxide nanoclusters or nanoparticles (NPs). Such hybrid NPMOF structures are promising materials for gas storage and catalysis. The strategies available for the incorporation of metal or metal oxide NPs into MOFs include solvent-free gasphase loading, solution impregnation, incipient wetness impregnation, solid grinding, and microwave irradiation. Most of these techniques, however, involve relatively cumbersome processes such as pretreating the MOF (e.g., by solvent exchange or MOF activation) or particle loading (by introduction of reducing agents, heating, irradiation, etc.), and none allows any spatial control over the NP loading within the MOF crystals. Here, we describe a procedure in which reaction-diffusion processes inside the MOF crystals mediate the deposition of NPs either in a uniform or in a location-specific fashion, with the latter leading to the formation of core/shell architectures (which are of interest in the context of multistep catalysis). In our method, cyclodextrin-based MOF (CD-MOF) crystals are immersed in a metal (here, Ag or Au) salt solution, and the OH counterions—which are homogenously distributed in the CD-MOF at concentrations of about 1.33m—reduce this salt to the respective metal NPs. By coupling the diffusion of salt precursors with their reduction inside the CD-MOFs it is possible to deposit the NPs only at the core of the MOF crystal and such that the thickness of the NP-free shell depends on the concentration of the HAuCl4 used. Subsequent deposition of another type of NPs gives rise to core/ shell architectures. NPs of all types can be readily liberated by dissolving the CD-MOFs in water—for the core/shell NP/CDMOFs, the release of the two different types of NPs is then sequential. Two types of millimeter-sized CD-MOFs were used. The first type was synthesized from g-cyclodextrin (g-CD) and RbOH following the reported procedures (see Experimental Section for details). As illustrated in Figure 1a–c, these Rb-CD-MOF single crystals (for powder X-ray diffraction (PXRD) spectra, see Section 1 in the Supporting Information) were rectangular prisms up to about 2 2 1 mm in size with nanosized cavities (ca. 1.7 nm across) and 1D channels connecting them (channel cross-section: ca. 8 8 ). The second type of MOFwas also made from g-CD, but CsOH was used as the alkali metal source. These Cs-CDMOF crystals also comprised cavities of approximately 1.7 nm in diameter connected by channels with cross-sections of about 8 8 2 (Figure 1a,b). Following the procedure described in the Experimental Section, Cs-CD-MOF single crystals were grown that had an overall truncated-octahedron shape and the diameters of these crystals were up to 5 mm (Figure 1d, see also the PXRD spectra in Section 2 of the Supporting Information). In the context of the present work, the key feature of the CD-MOFs is that they contain hydroxide counterions (one per metal center) which can work either alone or cooperatively with the cyclodextin units to reduce metal salt Figure 1. a) A unit cell of Rbor Cs-CD-MOF crystals synthesized from g-CD and RbOH or CsOH, respectively (red: oxygen; gray: carbon; purple: Rb or Cs). b) The 1D channels in Rbor Cs-CD-MOF crystals. c) and d) The optical images of the millimeter-sized Rb-CD-MOF and Cs-CD-MOF crystals, respectively.