We are trying to intermingle the compelling chemistry of MOFs with the uniqueness of nanomaterials to give new functions for energy storage and conversion applications. Multivariate MOFs with different structures and properties will be hybridized with various nanomaterials, such as nanoparticles, nanowires, and carbon-based nanomaterials, with a view of using their combined functions for photocatalysis, heterogeneous catalysis, electrochemical devices, and water harvesting from air. As a long-term plan, I will expand this idea and hybridize other classes of materials, such as enzymes, molecular catalysts, proteins, living cells, and drugs, with various porous crystalline materials [covalent-organic frameworks (COFs), zeolitic imidazolate frameworks (ZIFs) and so on] for energy storage and conversion applications.
Metal-Organic Frameworks (MOFs)
The easy of modification of the constituents’ geometry, size, and functionality in MOFs has led to more than 20,000 different MOFs being reported and studied within the past decade. Similarly, the synthesis and applications of nanomaterials have been extensively developed. As such, the design and synthesis of new structures for these materials are no longer formidable tasks. Upon consideration of the required properties in target applications, we can either choose existing MOFs and nanomaterials from known library or develop new structures if necessary. In any cases, an understanding of the structure and chemistry of each material is essential.
MOF coating on nanomaterials
Surface functionalization is key for the unique functions of nanomaterials. I believe MOF coatings on nanomaterials provides a facile way to control the chemistry occurring at the interface between the nanomaterial and an incoming substrate, as MOFs are well-defined units for which the spatial arrangement of functional organic and inorganic units, porosity, density, and thickness can be precisely controlled. Further, the chemistry occurring at the organic and inorganic parts of MOFs can be tuned by combining with nanomaterials. For example, surface plasmonic enhancement of the nanomaterials would facilitate the transfer of highly intensified energy to MOFs for higher photosynthetic activity.
Multi-layer combinations of MOFs and nanomaterials
Strategies for combining multivariate layers of different metals and MOFs are largely unexplored. By having layers of porous MOFs intercalating metal layers, the active surface of the metals are exposed to the exterior, and the access to which is controlled by the MOF structures. Diffusion of guest molecules can be also controlled by MOFs, thus the equilibrium of chemical reactions can be manipulated to enhance catalytic performance. More importantly, the combination of different metal layers will give tandem catalytic reactions in a MOF layer intercalating the metal layers. Moreover, MOF structures can also be designed to active for catalysis, so they can directly contribute to the catalytic reactions.
Embedding Nanomaterials in MOFs
Inorganic nanoparticles that occupy an intermediate size regime between small molecules and extended structures can be embedded in MOFs by growing the MOFs around nanoparticles. In this method, nanoparticles with controlled composition, size and morphology can be placed in a precisely manipulated environment of organic and inorganic units of MOFs. By designing the chemical and physical environment of MOFs, the reaction pathway can be fully controlled and tailored. Moreover, multi-step catalytic reactions would be possible by combining multivariate nanoparticles with catalytically active MOFs.
For electrochemical energy storage and conversion devices, such as fuel cells, supercapacitors, metal-air batteries, and redox flow batteries, the materials are better to be electrically and ionically conductive and combine porosity and high surface area. It is also advantageous for such materials to be crystalline in order to allow ions to flow in and out of pores without obstruction. In this context, COFs, METs, and CATs are ideal candidates as active electrode materials, or can be combined with existing electrode materials to help to overcome their challenging issues. The knowledge and experience from as-developed materials can be also applied to the design of porous materials and their combinations with the existing materials to enhance the electrochemical performance.
Photocatalysts for artificial photosynthesis
Direct conversion of solar energy to storable fuels offers a promising means of reducing the reliance on fossil fuels. One of the most critical issues that I have been mainly working on is the development of new materials for light harvesting, CO2 reduction and H2O oxidation. My approach is to synthesize the MOFs with photocatalytic links and inorganic joints to facilitate such reactions. In addition, photocatalytic MOFs can also be intermingled with inorganic nanoparticles known to be efficient in harvesting solar energy and transferring it around to enhance photocatalytic activities.
In heterogeneous catalysis, enhancement of catalytic activity and high product selectivity is very important. MOFs themselves can directly make catalytic reactions by designing the organic and inorganic parts catalytically-active, or being able to support catalytically-active species such as nanoparticles and control their activity and selectivity. The flexibility of MOF structures with which their physical and chemical properties are varied can be used to enhance the catalytic performance of practical reactions.
Gas storage and separation
Porous materials typically provide lots of spaces for gas storage and, once the affinity was tuned for specific gas molecules, separate target gas among the gases introduced to MOFs. Although most of porous materials targeted to increase gas storage property for multiple gases (such as CH4, CO2, H2…), there are still many challenges remained. I have worked on CO2 and CH4 storage with the MOF having heterogeneous pore system and suggested a simple way to enhance the storage capability without designing complex structures.