Research Projects
1. Heteronuclear Gold Clusters:
In recent years, transition metal clusters have attracted considerable attention owing to their interesting physicochemical properties. Gold clusters and nanoparticles, in particular, exhibit very important electronic, optical, chemical and catalytic properties, which have been extensively studied in several areas, such as quantum dots, catalysis, and sensing. The stabilities, dynamics and physicochemical properties of gold clusters highly depend on size, quantized electronic structures, and unique geometric structures. Efforts have been made to study the structure-property relationships of gold clusters with less than 100 atoms. However, only a few studies have actually focused on heteronuclear gold clusters. The introduction of different metal atoms into gold clusters will greatly influence the electronic and photophysical properties.
In this project, we will develop novel synthetic methods to introduce metals, such as Platinum group metals, into gold clusters. According to the polyhedral skeletal electron pair theory and the eighteen electron rule, structures of heteronuclear clusters can be predicted. For example, for clusters with the molecular formula MaAubLc, there are limited combinations of a, b, c that will result in scientifically possible clusters. In combination with computational calculations, heteronuclear gold clusters with superior properties can be designed and developed.
2. MOF Immobilized Catalysts for Photocatalytic Hydrogen Production:
Secondary metal centers can be easily introduced into MOF voids which have been extensively studied for potential applications as these added metal centers can serve as catalytic active sites to promote a wide range of reactions. There are generally three strategies to functionalize an existing MOF with secondary metal centers (a) starting from linkers with pre-designed secondary functional groups and incorporate metal centers via post-synthetic modifications, (b) starting from pre-formed linkers with coordinated metal centers, (c) starting from non-functionalized ligand and introduce metal complexes through post-synthetic linker exchange.
The gravimetric energy density of hydrogen (142 MJ/Kg) is much higher than that of natural gas (55.5 MJ/Kg) and gasoline (46.4 MJ/Kg). Hydrogen has been considered as a green energy source as water is the only combustion product. Industrially, hydrogen is produced through steam reforming of natural gas which normally require high temperature (700 – 1100 oC) and catalysts. However, the biological system, called hydrogenase, is able to split water at room temperature. Inorganic/organometallic synthetic groups have tried to synthesize Fe2S2 clusters to mimic the biological system to catalyze hydrogen evolution reaction (HER). Recently, photocatalytic HER attracted increasing attention because it uses solar energy from the sun. In such systems, photosensitizers usually present to act as antennas which will absorb photons and inject electrons to the catalyst, usually Fe2S2 clusters which are capable of reducing protons to generate molecular hydrogen as shown in (a). In this project, we will fully utilize the versatility of MOFs to incorporate both photosensitizers and catalytic active centers to the surface of MOF cavities to produce MOF-based photocatalytic HER system. As illustrated in (b), the photosensitizer and catalytic active center can be anchored on linkers with the identical size and incorporated into MOFs through either solvothermal reactions or post-synthetic-modifications.
3. Metal-Organic Framework Composites:
The development of materials chemistry, including that of nanomaterials and polymers, has boomed in recent decades due to increased industrial demands. In targeting advanced functional materials, researchers have sought to combine different materials in order to integrate the strength of each component for desired properties and applications. For one, surface modified nanoparticles have been extensively investigated in diverse applications, including photodynamic therapy, bioimaging, catalysis, and dye‐sensitized solar cells (DSSC). Recently, MOF nanocrystals have attracted increased attention for their tunable particle sizes and readily achievable surface modification.
(1) Core-Shell MOF Composites
As discussed above, only a handful of examples have been reported concerning MOF core‐shell structures that fully use their advantages. In response, this part of the project will seek to develop a general strategy for synthesizing nanoparticles that capsulated by MOFs. For MOF growth, the interaction of metal ions and organic linkers is normally greater than that of these two components with the nanoparticle surface. Consequently, it remains highly difficult to synthesize these core‐shell structures by using one‐pot solvothermal reactions, largely because they yield isolated MOF crystals. It is thus critical to increase the interaction of nanoparticles and MOF layers, for example, by covalently attaching the MOF to desired nanostructures. In general, MOF‐decorated nanostructures are superior to those decorated by small molecules. For example, in DSSC, the efficiency is often limited by photosensitizers since only one layer of molecular photosensitizers—usually porphyrins—can be covalently attached to the TiO2 surface. This obstacle could be overcome by replacing molecular photosensitizers with MOFs, which contain built‐in porphyrins as part of their structural backbones. It is thus expected that the efficiency will be greatly improved, since multiple layers of photosensitizers can be introduced via MOF growth and the porous nature of MOFs will allow substrates to diffuse in and out of the nanoparticle surface.
(2) Mixed-Matrix-Membranes
Membranes have been widely studied for applications in the separation of gas mixtures, particularly in fuel cells. Due to the low thermal stability of organic polymer membranes and the difficulty of controlling the pore size, researchers have attempted to mix nanoparticles with membranes as a mean to produce mixed‐matrix membranes (MMMs). Nevertheless, there is a lack of comprehensive theory, regarding how MMMs are formed, that can direct researchers in rationally altering MMMs for different applications. For the purposes of separation, since MOF pores and functionalities can be precisely controlled, the resulting MMMs can also be tuned for different gas mixtures. MOFs with exceptionally small pores can be used to construct MMMs in order to separate hydrogen gas from other gases such as methane, ethane, and carbon dioxide. Amine functionalized MOFs, for instance, are excellent candidates for separating carbon dioxide from hydrocarbons. It is also possible to govern the hydrophilicity of MOFs and the membrane material as a means to separate hydrocarbons from water. Another crucial factor is the MOF crystal size since smaller MOF crystals can promise a more uniform dispersion in order to produce MMMs with evenly distributed MOF crystals. The nano‐ and micro‐MOF crystals can be readily obtained and controlled by varying synthetic conditions.