(6ha) Understanding of Nanoparticle Self-Assembly Mechanisms and Its Applications to Energy Storage and Bio-Medical Applications

The study of self-assemblies is a research topic in which I am very interested because it provides an efficient way to fabricate material structures (i.e., the formation of complex nanostructures, such as nanowires, branched structures, and nanoporous single crystals, as well as unique nanocluster structures, such as hetero-nanostructured materials and bottom-up nanofabrication methods, such as superlattice crystal structures) at large scales in a controlled process. One of the most interesting examples is the unique assembly of inorganic nanocrystals or colloid stability that involves control of superlattice crystal structure/cluster morphology, crystal growth via oriented attachment (OA) and spontaneous wetting-dewetting process of polymers or organic ligands and characterization of ionic structure at solid-liquid interfaces. The coupling between energetics (,e.g., van der Waals force, Electrostatic force, Steric hindrance force, and Brownian force ) and dynamics (i.e., hydrodynamics) plays a crucial role. Moreover, non-DLVO forces can determine the short separation distance at equilibrium state. Therefore, the coupling in self-assembly process is expected to be correlated closely with various molecular details (e.g., surface modifications via ligand or polymer coating and solvent structures at interfaces) due mainly to the length scales of crystals (i.e., O(1) nm). In addition, micelle shape, which is determined by chain length of hydrophilic chain and hydrophobic chain, can affect surface energy of particle and determine particle morphology during crystal growth in solution. In addition, the unique nature of particles would create additional complexity (e.g., the tensorial nature of the dielectric property due to the crystallinity).

Teaching Interests:

In addition to supporting extant courses in the Chemical Engineering Department, I endeavor to design graduate and undergraduate level courses in Colloid Interfacial Phenomena and Microfluidics and Thermodynamics. My intention is to guide students from fundamentals to related recent research topics. The following list provides for courses I am prepared to teach:

• Colloid Interfacial Phenomena and Microfluidics (Graduate level): Colloid interfacial science is the study of the behavior of particles. Scientific interest, along with experimental techniques and theoretical interpretations of interparticle forces, allows for the effective manufacture of synthetic dispersions for coating, enhanced oil recovery, the development of new fuels, environmental pollution, ceramics fabrication, corrosion phenomena, biotechnology, and separation processes. My aim is to impart a quantitative understanding grounded in basic theory and coupled to experiments on well-characterized model systems. I provide for the physical side of colloid science. Subjects range from individual forces, acting between submicron particles suspended in a liquid through the equilibrium, and dynamic properties of dispersion. Relevant forces include Brownian motion, electrostatic repulsion, attraction due to dispersion forces, attraction and repulsion caused by soluble polymers, and viscous forces arising from relative motion between particles and liquid.
• Thermodynamics (Undergraduate level): This course introduces the concept of thermodynamics form a chemical-engineering viewpoint. This class specifically targets an undergraduate level. It is relevant to students for studying the treatment of equilibrium thermodynamics in sufficient detail to solve a wide variety of problems. This class is designed to be conceptually-based, with examples of natural phenomenon, meant to provide students with a solid foundation of this subject. Conceptual development, numerous problems taking form as weekly assignments, and term-projects, promote deep learning and provide students the ability to apply thermodynamics to real-world engineering problems.

Research Abstract:

The study of self-assemblies have attracted increasing attention because it provide an efficient way to fabricate material structures(i.e., the formation of complex nanostructures, such as nanowires, branched structures, and nanoporous single crystals, as well as unique nanocluster structures, such as hetero-nanostructured materials and bottom-up nanofabrication methods, such as superlattice crystal structures) at large scales in a controlled process. For these reasons, interesting examples what I studied before are the self-assembly characterization of nanomaterials, simulation of nanoparticle coagulations and spontaneous wetting/dewetting process of polymers or organic ligands.

To understand kinetic details of the self-assembly process, the self-assembly of gold NPs are further analyzed by calculating the contributions of various forces involved [3-4]. Our results show that these forces, including Brownian force (F_Br), van der Waals force (F_vdW), electrostatic force (F_elec), and hydrodynamic force (F_D), and their interplay play a key role in NP self-assembly process. The superlattice structure can be controlled via solvents or ligands. For a given particle pair, the repulsive steric hindrance force (F_Sh) or hydration force (F_hyd) increases with decreasing h and balances with attractive F_vdW at an equilibrium separation, here measured as h_eq, = 2.8 nm. This prediction is validated by our experimental observations; the particles assembled into a superlattice with an average h of 2~3 nm. Meanwhile, I observed 0.7 nm of h as NPs dispersed in aqueous solution contained salts. It is demonstrated that 0.7 nm of h was attributed to the formation of strong water structure on the NP surface due to hydrated ions.

Based on these knowledge, I developed the hetero-structured metal oxide nanoparticles for lithium ion battery anode materials and gold-nanorods encapsulated by gas generating amphiphilic block copolymers under external stimulus. The synthesized Co₂MnO₄ hetero-structured nanoparticles from CoO/Mn₃O₄ core/shell nanoparticles demonstrated superior electrochemical properties with a large capacity and high stability during fast charging/discharging cycles for potential applications as advanced lithium-ion battery (LIB) electrode materials. The heterogeneous structures of Cu_3.8Ni/CoO and Cu_3.8Ni/MnO nanoparticles could enhance the lithium ion mobility and improve the life cycle, and these materials are therefore promising candidates as high-power-density and high-energy-density anode materials for lithium-ion batteries in diverse applications, such as electrical vehicles. Meanwhile, block copolymers coated gold nanorods showed a greater absorbance/heat generation rate under IR irradiation. This system produce ultrasound contrast enhancement effects under continuous exposure to IR light for >20 min.

These results provide insights into the self-assembly of nanoparticle and polymer, and its applications. The obtained knowledge of interactions between surface coating materials can be extended to core-shell/hetero-structured nanomaterials or polymer coated nanomaterials that can be applied to the field of lithium ion batteries and ultrasound contrast.

References:

[1] M Boles, M Engel, and D. V. Talapin. Chemical reviews 116, (2016), p11220-11289.

[2] J. M. Yuk, J. Park, P.Ercius, K. Kim, D. J. Hellebusch, M. F. Crommie, J. Y. Lee, A. Zettl,; A. P. Alivisatos, , Science 336, (2012), p61-64.

[3] W.B. Russel, W B. Russel, D. A Saville and W. R. Schowalter, in “Colloidal dispersions”, ed G.K. Batchelor (Cambridge university press, New York) p3-22

[4] J. N. Israelachvili, in “Intermolecular and surface forces”, Academic press, (Elsevier, Santa Barbara) p3-22