Zhi-Gang Gu

The surface barrier phenomenon at the loading of metal-organic frameworks

Loading with guest molecules is a crucial step for most applications of porous materials. For metal-organic frameworks, which are one of the most intensely investigated classes of porous materials, the experimentally determined rate of mass transfer into the material may vary by several orders of magnitude for different samples of the same material. This phenomenon is commonly attributed to the presence of so-called surface barriers, which appear to be omnipresent but poorly understood. Here we quantitatively study this phenomenon with a quartz crystal microbalance, using well-defined, highly crystalline, epitaxially grown thin films of metal-organic frameworks as a model system. Our results clearly demonstrate that surface barriers are not an intrinsic feature of metal-organic frameworks, as pristine films do not exhibit these limitations. However, by destroying the structure at the outer surface, for instance by exposure to air or water vapour, surface barriers are created and the molecular uptake rate is reduced.

Fullerene-like Polyoxotitanium Cage with High Solution Stability

We present the formation of the largest titanium-oxo cluster, [Ti42(μ3-O)60(OiPr)42(OH)12)](6-), with the first fullerene-like Ti-O shell structure. The {Ti42O60} core of this compound exemplifies the same icosahedral (Ih) symmetry as C60, the highest possible symmetry for molecules. According to the coordination environments, the Ti centers in this cluster can be arranged into a Platonic {Ti12} icosahedron and an Archimedean {Ti30} icosidodecahedron. The solution stability of this cluster was confirmed by electrospray ionization mass spectrometry. The spherical body of the {Ti42O60} core has an inside diameter of 1.05 nm and an outside diameter of 1.53 nm, which could be directly visualized by high-resolution transmission electron microscopy. Our results demonstrate that titanium oxide can also form fullerene-like shell structures.

Nanoporous designer solids with huge lattice constant gradients: multiheteroepitaxy of metal-organic frameworks.

We demonstrate the realization of hierarchically organized MOF (metal-organic framework) multilayer systems with pronounced differences in the size of the nanoscale pores. Unusually large values for the lattice constant mismatch at the MOF-MOF heterojunctions are made possible by a particular liquid-phase epitaxy process. The multiheteroepitaxy is demonstrated for the isoreticular SURMOF-2 series [ Liu et al. Sci. Rep. 2012 , 2 , 921 ] by fabricating trilayer systems with lattice constants of 1.12, 1.34, and 1.55 nm. Despite these large (20%) lattice mismatches, highly crystalline, oriented multilayers were obtained. A thorough theoretical analysis of the MOF-on-MOF heterojunction structure and energetics allows us to identify the two main reasons for this unexpected tolerance of large lattice mismatch: the healing of vacancies with acetate groups and the low elastic constant of MOF materials.

MOF‐Templated Synthesis of Ultrasmall Photoluminescent Carbon‐Nanodot Arrays for Optical Applications

Arrays of ultrasmall and uniform carbon nanodots (CDs) are of pronounced interest for applications in optical devices. Herein, we describe a low-temperature calcination approach with rather inexpensive reactants. After glucose molecules had been loaded into the pores of metal-organic frameworks (MOFs), well-defined CD arrays were produced by heating to 200 °C. The size and spacing of the CDs could be controlled by the choice of templating MOF: HKUST-1, ZIF-8, or MIL-101. The sizes of the obtained CDs were approximately 1.5, 2.0, and 3.2 nm, which are close to the corresponding MOF pores sizes. The CD arrays exhibited interesting photophysical properties, including photoluminescence with tunable emission and pronounced nonlinear optical (NLO) effects. The NLO properties of the obtained CD arrays were significantly different from those of a CD suspension, thus indicating the existence of collective phenomena.

Experimental and theoretical investigations of the electronic band structure of metal-organic frameworks of HKUST-1 type

The electronic properties of metal-organic frameworks (MOFs) are increasingly attracting the attention due to potential applications in sensor techniques and (micro-) electronic engineering, for instance, as low-k-dielectric in semiconductor technology. Here, the band gap and the band structure of MOFs of type HKUST-1 are studied in detail by means of spectroscopic ellipsometry applied to thin surface-mounted MOF films and by means of quantum chemical calculations. The analysis of the density of states, the band structure, and the excitation spectrum reveal the importance of the empty Cu-3d orbitals for the electronic properties of HKUST-1. This study shows that, in contrast to common belief, even in the case of this fairly “simple” MOF, the excitation spectra cannot be explained by a superposition of “intra-unit” excitations within the individual building blocks. Instead, “inter-unit” excitations also have to be considered.

Liquid-Phase Epitaxy Effective Encapsulation of Lanthanide Coordination Compounds into MOF Film with Homogeneous and Tunable White-Light Emission

As a new family of hybrid inorganic-organic materials with large porosity, metal-organic frameworks (MOFs) have received attractive attention recently on encapsulating functional guest species. Although the encapsulation of luminescent guest into bulk MOFs can tune the luminescent property, the powder composite materials are limited to the application in optical sensors and devices. In the present work, we use a modified liquid-phase epitaxial (LPE) pump method for the fabrication of lanthanide coordination compounds (LCCs)-encapsulated MOF thin film on substrate with high encapsulation efficiency. The resultant composite film reveals an oriented and homogeneous composite film, in which a white light emission by tuning the LCCs of red, blue and green emission can be obtained. This strategy may open new perspectives for developing high-encapsulation-efficiency, oriented, and homogeneous solid-state lighting composite films in the application of optical sensors and devices.

A Confined Fabrication of Perovskite Quantum Dots in Oriented MOF Thin Film

Organic-inorganic hybrid lead organohalide perovskites are inexpensive materials for high-efficiency photovoltaic solar cells, optical properties, and superior electrical conductivity. However, the fabrication of their quantum dots (QDs) with uniform ultrasmall particles is still a challenge. Here we use oriented microporous metal-organic framework (MOF) thin film prepared by liquid phase epitaxy approach as a template for CH3NH3PbI2X (X = Cl, Br, and I) perovskite QDs fabrication. By introducing the PbI2 and CH3NH3X (MAX) precursors into MOF HKUST-1 (Cu3(BTC)2, BTC = 1,3,5-benzene tricarboxylate) thin film in a stepwise approach, the resulting perovskite MAPbI2X (X = Cl, Br, and I) QDs with uniform diameters of 1.5-2 nm match the pore size of HKUST-1. Furthermore, the photoluminescent properties and stability in the moist air of the perovskite QDs loaded HKUST-1 thin film were studied. This confined fabrication strategy demonstrates that the perovskite QDs loaded MOF thin film will be insensitive to air exposure and offers a novel means of confining the uniform size of the similar perovskite QDs according to the oriented porous MOF materials.

Synthesis of homochiral zeolitic metal–organic frameworks with amino acid and tetrazolates for chiral recognition

By mixing amino acids and tetrazolate ligands, a series of homochiral zeolitic metal–organic frameworks (ZMOFs) with ABW topology have been synthesized, and these materials show permanent microporosity and potential enantioselective recognition ability.

Epitaxial Growth of Oriented Metalloporphyrin Network Thin Film for Improved Selectivity of Volatile Organic Compounds

This study reports an oriented and homogenous cobalt-metalloporphyrin network (PIZA-1) thin film prepared by liquid phase epitaxial (LPE) method. The thickness of the obtained thin films can be well controlled, and their photocurrent properties can also be tuned by LPE cycles or the introduction of conductive guest molecules (tetracyanoquinodimethane and C60 ) into the PIZA-1 pores. The study of quartz crystal microbalance adsorption confirms that the PIZA-1 thin film with [110]-orientation presents much higher selectivity of benzene over toluene and p-xylene than that of the PIZA-1 powder with mixed orientations. These results reveal that the selective adsorption of volatile organic compounds highly depends on the growth orientations of porphyrin-based metal-organic framework thin films. Furthermore, the work will provide a new perspective for developing important semiconductive sensing materials with improved selectivity of guest compounds.

Facile Synthesis of Metal-Loaded Porous Carbon Thin Films via Carbonization of Surface-Mounted Metal–Organic Frameworks

We report a facile approach to prepare metal-nanocatalyst-incorporated carbon thin films with uniform size distribution via carbonization of surface-mounted metal-organic frameworks (SURMOFs) and metal oxo-clusters loaded SURMOF. The calcinated thin films have high performance of methylene blue degradation and the reduction of nitrobenzene. This study describes a general strategy for preparing various nanoparticle-impregnated porous carbon thin films for applications in catalysis.