Current Research Projects:
Nanomaterials for Energy Storage. Low-dimensional materials, such as graphene and carbon nanotubes (CNT) exhibit exceptional high carrier mobility, high electrical and thermal conductivity and large specific surface area. By controlling synthesis and assembly, those exotic properties are able to be extended to macro-scale, which is important for applications like solar cells, supercapacitors and lithium ion batteries. In our lab, we have developed the method to form covalently bonded graphene and carbon nanotubes, which has been proven an efficient way to build high performance supercapacitor electrodes. Currently we are exploring the techniques to synthesis and assembly well-organized 3D bi-continuous low-dimensional materials. Those materials are kinetically ideal electrodes/supports for electrochemical energy storage devices, which is important for electrode’s reaction mechanism and performance studies. The on-going research includes novel lithium-ion battery and lithium-air battery electrode materials, battery electrode reaction mechanism study and hybrid materials based asymmetric supercapacitors.
Figure 1. Left: Scheme for the synthesis of CNT directly from graphene. Middle: Theoretical models suggested the atomic structure of the CNT/graphene junction. Right: High resolution STEM image of junction areas with an overlayed structural sketch.
Conjugated polymers with hydrogen-bonding on the main chain. Hydrogen-bonding is a strong and directional intermolecular interaction. It has potential to be used to “lock in” molecular packing and improve the long-range order of organic molecules. Our group is dedicated to develop the conjugated polymers with hydrogen-bonding on the main chain, which is targeted to form desired molecular packing between conjugated polymer molecules. Furthermore, the dynamic formation of hydrogen-bonding can provide additional functions such as patentability to the polymers.
One example here is the conjugated polymers with latent hydrogen-bonding on the main chain that were synthesized using Suzuki coupling reaction. The resulting polymers with latent hydrogen-bonding can be converted to the actual hydrogen-bonded polymers by thermal annealing or UV irradiation. By removing the protection group and forming hydrogen-bonding, the polymers exhibit bathochromic shift over those with latent hydrogen-bonding, indicating a hydrogen-bonding mediated enhancement of π-π stacking. In addition, the fused hydrogen-bond sites and π-conjugated units lead to closely packed polymer chains, resulting in insoluble pigment-like polymers. This drastic solubility change from polymers with latent hydrogen-bonding to hydrogen-bonded polymers can be used to pattern conjugated polymers directly. The photolithography of the conjugated polymer with latent hydrogen-bonding was demonstrated in our lab and the patterned electrochromic devices were fabricated as in the following figure.
Figure 2. Conjugated polymers synthesized by the group.
Nano-manufacturing. The scalable manufacturing of nanomaterials is pivot important for many future applications where the conventional materials can’t provide required performances. We are developing scalable methods to synthesis and control the nanomaterials for the devices with unconventional performance. Current projects include transparent electrodes based on patterned metal nanowire and substrates for surface enhanced Raman spectroscopy.
Figure 3. The transparent electrode fabricated in the lab (read the full story)
Polymer composites with 2D fillers. Two-dimensional (2D) nanomaterials, such as graphene, have ultrahigh Young’s modulus and ultimate strength. They are ideal candidates to improve and extend properties of polymers. In addition, 2D sheets are expected to achieve percolation in polymer composites at ultralow concentrations. Currently we are exploring two composite systems: graphene rubber composites and graphene polyurethane composites.
Graphene catalyst support. The organized two dimensional materials (such as graphene) can provide large surface area, avoid catalyst aggregation and serve as an ideal substrate for small catalyst particles. We are interested in synthesizing catalyst on the porous graphene templates for selective oxidation reactions.