(6fs) Learning from Nature: Rational Design of Multifunctional Hybrid Materials

One of nature’s most powerful tools to combine disparate material properties (e.g., mechanical strength, surface tension, permeability, color, catalytic properties) in a synergistic way is to integrate dissimilar components at the nanometer scale. For example, mother-of-pearl, or nacre—the iridescent material lining the inside of seashells—is made almost entirely of brittle calcium carbonate. Nevertheless, it exhibits excellent toughness and mechanical resilience because the calcium carbonate plates are arranged in a “brick-and-mortar” fashion and glued together by a tiny fraction of biopolymers. Such a meticulous integration leads to a material with properties superior to the sum of the individual properties. Recent advancements in nanotechnology are opening up new ways to approach nature-inspired organic–inorganic hybridization engineering and make novel, multi-functional nanocomposites.^[1] However, much remains unknown about how nature can miniaturize hybridization synergistically, without sacrificing properties of the hybrid components, and how we can leverage this information to design nature-inspired nanocomposite hybrids. To account for these unknowns, my research will aim to understand what interaction should be controlled to maximize desired properties by inspiring by nature creatures. I will utilize that idea to solve technical problems and develop practical applications for energy, electrochemistry, electrical, mechanical, chemical, and environment related challenges.

Research Experiences:

My graduate work provided me with an in-depth tool kit for engineering hybridization at the macro- and molecular level that will be invaluable towards designing practical applications. In particular, I developed multifunctional hybrid materials to adapt mechanical flexibility and electrochemical performance synergistically, without sacrificing either properties in a given volume. Flexible batteries with high energy storage and load-bearing mechanical properties are essential for emerging wearable devices and flexible electronics which provide portability, user comfort, and flexibility of device design. Unfortunately, conventional batteries are bulky and rigid, and their components are often brittle. I developed flexible electrode materials for lithium ion batteries by adapting the brick-and-mortar structure of nacre.^[2-3] Vanadium pentoxide (V₂O₅) is a promising redox-active material for flexible battery electrodes, but is limited by its mechanical rigidity, low ion and electron conductivity, and severe volumetric changes upon cycling. I addressed those problems by hybridizing V₂O₅ with an amphiphilic copolymer bearing electron- and ion-conducting blocks, poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO), to form a highly flexible battery electrode. The flexible electrode mimics the brick-and-mortar structure of tough seashells, with V₂O₅ layers arranged in parallel and glued together by the copolymer. This structure significantly enhances mechanical flexibility and toughness without sacrificing the electrochemical performance of the batteries. My research revealed how the diblock copolymer creates synergy and why homopolymers do not, suggesting that tailoring intra- and inter-molecular interactions and preventing unfavorable phase separations are critical to combine multiple properties in a synergistic way.^[4-6]

As another hybridization, I engineered surface-agonistic stretchable conductive coatings by mimicking the natural layer-by-layer structure of seashells.^[7] Stretchable, bendable, and foldable conductive coatings which provide stable functionality while undergoing mechanical deformation are also crucial for wearable electronics and biometric sensors. MXenes are a new family of ultrathin atomic nanosheet materials that are mostly composed of layers of metals like titanium interleaved by carbon and/or nitrogen. Due to their high electrical conductivity (e.g., greater than graphene) and the variety of elements they can be composed of (e.g., compared to graphene containing only carbon), the emerging MXenes are currently booming as novel inorganic nanosystems in applications ranging energy storage and catalysis to biomedical applications. However, it is still extremely difficult to form thin MXene coatings that can withstand extreme mechanical deformation. I replicated the layer-by-layer structure of resilient seashells with negatively charged MXene and a cationic polyelectrolyte to synergistically integrate high conductivity, mechanical robustness, and mechanical flexibility into a composite coating material, and thereby address the main challenges in current MXene composites: loss of functionality under mechanical stress. I revealed the underlying mechanism of electromechanical coupling by using numerical modeling and geometric analysis, which showed reversible microscale gap formation and a corresponding change in electron conduction pathways under bending and stretching. These MXene multilayer coatings can be deposited onto nearly any surface, including fabrics and fibers, regardless of chemistry, topography, or softness, providing the basis for further development and the implementation of MXene-based conductive conformal coatings in a growing number of flexible conductive materials. Also, I developed ultrafast humidity sensors by adapting wood expansion as its moisture content changes.^[8] For example, wood is constantly expanding and contracting but wood does not move equally in all directions due to the grain structure. I replicated MXene/polyelectrolyte multilayers vertically stacked to a substrate to achieve a rapid response to dynamic changes in humidity, such as for humidity respiration. I proposed the mechanism that as humidity changes, the reversible insertion of water molecules into the MXene/polyelectrolyte multilayer results in changing their interlayer distance and tunneling resistance between MXene sheets and proved by using numerical modeling and experimental measurements including in situ quartz crystal microbalance and in situ ellipsometry.

To complete the foundation for my own research program, I cultivated an electron microscopy tomography–quantitative morphometry skill set during my postdoctoral work that I will couple with my hybridization engineering tool kit. Specifically, I developed low-dose electron microscope tomography that enables investigations of beam-sensitive soft materials with heterogenous nanoscale morphologies.^[9] As a model soft material, I studied polyamide membranes which are used in water desalination, purification of chemicals and pharmaceuticals, and recycling of catalysts. Polyamide membranes has been used in separation industries for over 30 years, however, it is challenging to develop the predictive design of nanometer thin polyamide membranes for two major reasons: (i) it is difficult to investigate the nonperiodic heterogenous nanostructures that emerge from synthesis and (ii) characterization of the 3D soft material structure at the nanoscale and thus quantitative understanding of their synthesis–structure–functionality relationship has been limited. To address these issues, I developed a 3D electron microscopy imaging-quantitative morphometry platform. More specifically, I took over 60 projections at various tilt angles from −60° to +60° and reconstructed a full 3D view with nanometer resolution (i.e., one 3D pixel is 6.8 Å). Next, I quantified various structural parameters governing film transport properties, such as void volume, local curvature, thickness mapping in 3D, and surface-to-volume ratio from the 3D reconstructed image as single data set. I also demonstrated that reaction conditions (i.e., reactant concentrations, reaction time) can serve as a handle on structure parameters like pore structure, local curvature, thickness, and interconnectivity. To this end, I undertook low-dose electron tomography and quantitative morphometry efforts that provide a means to map out functionally relevant, 3D features of polyamide crumples and to extract a wide variety of transport-related properties. 3D imaging and quantitative characterization of soft matter will bridge the structural organization in space and the functionality of materials.

My future research focuses concern (i) the ageing mechanisms of rechargeable batteries and the development of strategies to prolong their life, (ii) how loss of electromechanical properties of stretchable conductors could be mitigated by mimicking human elbow skin structure, where micro winkles exist and maintain flexibility over 80 years of human life. To address these questions, my research program will merge my expertise in hybridization engineering, electrochemistry, polymer, and electron microscopy tomography-quantitative morphometry. In my future research, I will seek to develop fundamental understandings of the mechano-electrochemical and mechano-electrical response mechanisms and failure modes at both the macroscopic and atomic scale. My foundation in chemical engineering and materials science leaves me well-situated to undertake this research program in interdisciplinary engineering.

Teaching Interests:

In my academic career, I have had the great fortune of being a teaching assistant on multiple occasions, for courses covering chemical engineering thermodynamics (at Texas A&M University), separation processes (at Hanyang University). I also served as laboratory instructor in a course on numerical analysis for chemical engineers (at Texas A&M University). In the process, I was able to experience first-hand how an instructor’s passion could motivate and engage young, bright minds. I learned teaching know-how to provide additional support to students to tackle challenging subject matter and to improve lecture quality. For example, I will start my classes with a 5-minute overview of the previous and current lecture subjects to make sure students know exactly what they are learning and doing for the day and I will write my lecture notes by hand as class proceeds, to pace delivery of material, provide peer-teaching opportunities, and use small group collaboration. In addition to my teaching assistant experience, I developed my teaching skills through ENGR681: Preparing Future Faculty-Professional Development at Texas A&M University. In particular, I learned the importance of balance between micro- and macro-management to give students room to be autonomous and have an opportunity to improve themselves with their own knowledge.

Beyond the classroom, I have had the privilege of mentoring six undergraduates, two masters students and two PhD students in laboratories at Texas A&M University and Hanyang University. More than half of them produced peer-reviewed publications and patents under my mentorship that are rewarding experience while pursing career in academia. I am interested and qualified in teaching most of the major courses for Chemical Engineering including Thermodynamics, Separation Processes, Transport Processes, Reaction Engineering, Physical/Organic Chemistry, Polymer Chemistry, Electrochemistry, Numerical Methods, and Advanced Mathematics for chemical engineers at both undergraduate and graduate level. Additionally, I am interested in developing special interdisciplinary topic courses at the graduate level on new research topics (e.g., Quantitative Data Analysis, Polymer-Inorganic Hybrid Materials).

Selected Publications:

[1] M. Morris, H. An, J. L. Lutkenhaus, T. H. Epps III, “Harnessing the power of plastics: nanostructured polymer systems in lithium-ion batteries” ACS Energy Letters 2017, 2(8), 1919-1936.

[2] H. An, J. Mike, K. Smith, L. Swank, Y. Lin, S. Pesek, R. Verduzco, J. L. Lutkenhaus, “Highly flexible self-assembled V₂O₅ cathodes enabled by conducting diblock copolymers” Scientific Reports 2015, 5, 14166.

[3] H. An, X. Li, C. Chalker, M. Stracke, R. Verduzco, J. L. Lutkenhaus, “Conducting block copolymer binders for carbon-free hybrid vanadium pentoxide cathodes with enhanced performance” ACS Applied Materials & Interfaces 2016, 8 (42), 28585.

[4] H. An, X. Li, K. A. Smith, Y. Zhang, R. Verduzco, J. L. Lutkenhaus, “Regioregularity and molecular weight effects in redox active poly(3-hexylthiophene)-block-poly(ethylene oxide) electrode binders” ACS Applied Energy Materials 2018, 1(11), 5919.

[5] X. Li, H. An, J. Strzalka, J. L. Lutkenhaus, R. Verduzco, “Self-doped conjugated polymeric binders improve the capacity and mechanical properties of V₂O₅ cathodes” Polymers 2019, 11(4), 589.

[6] K. Sarang, A. Miranda, H. An, E. Oh, R. Verduzco, J. L. Lutkenhaus, “Poly(fluorene-alt-naphthalene diimide) as n-type polymer electrodes for energy storage” ACS Applied Polymer Materials 2019, 1(5), 1155.

[7] H. An, T. Habib, S. Sha, H. Gao, M. Radovic, M. J. Green, J. L. Lutkenhaus, “Surface-agonistic highly stretchable and bendable conductive MXene multilayers” Science Advances 2018, 4(3), eaaq0118.

[8] H. An, T. Habib, S. Sha, H. Gao, A. Patel, M. Radovic, M. J. Green, J. L. Lutkenhaus, “Water sorption in MXene-polyelectrolyte multilayers for ultrafast humidity sensing” ACS Applied Nano Materials 2019 2 (2), 948–955.

[9] X. Song, J. W. Smith, J. Kim, N. J. Zaluzec, W. Chen, H. An, J. M. Dennison, D. G. Cahill, M. A. Kulzick, Q. Chen, “Unraveling the morphology–function relationships of polyamide membranes using quantitative electron tomography” ACS Applied Materials & Interfaces 2019, 11 (8), 8517–8526.

[10] C. Chalker*, H. An*, J. Zavala, A. Parija, S. Banerjee, J. L. Lutkenhaus, J. Batteas, “Fabrication and electrochemical performance of structured mesoscale open shell V₂O₅ networks” Langmuir 2017, 33(24), 5975-5981. (*co-first)

[11] H. An*, D. Song*, J. Lee, E. Kang, J. Jaworski, J. Kim, Y. Kang, “Promotion of strongly anchored dyes on the surface of titania by tetraethyl orthosilicate treatment for enhanced solar cell performance” Journal of Materials Chemistry A 2014, 2, 2250. (*co-first)

[12] D. Song*, H. An*, J. Lee, J. Lee, H. Choi, I. Park, J. Kim, Y. Kang, “Densely packed siloxane barrier for blocking electron recombination in dye-sensitized solar cells” ACS Applied Materials & Interfaces 2014, 6(15), 12422. (*co-first)

[13] Lee, H. Chang, H. An, S. Ahn, J. Shim, J. Kim, “A protective layer approach to solvatochromic sensors” Nature Communications 2013, 4, 2461.