2D materials such as graphene, graphene oxide (GO), reduced graphene oxide (rGO) and MXene, possess unique properties, e.g., high carrier mobilities, mechanical flexibility, good thermal conductivity, and high optical and UV adsorption. They are potentially applicable in the fields of electronics, optoelectronics, catalysts, energy storage facilities, sensors, solar cells, lithium batteries, and so on. Normally, weak interactions and irregular packing or stacking of 2D layers may adversely offset or weaken to some extent their 2D effects such as mechanical and electrical properties at a macroscale. In this regard, it is required to spatially organize 2D materials into macroscopic forms of a well-defined shape (e.g. fibers, films, or 3D structures) in a way that can simultaneously preserve favorable 2D properties and functions shown at the nanoscale, and facilitate their compatibility with the state-of-the-art industrial processes. In my thesis, different types of 2D materials, here GO, rGO and MXene together with polymers were rationally assembled into functional composite materials. The synergistic molecular crosslinking strategy was utilized and controlled in such composite materials for the sake of better performance. My thesis mainly involves four parts:

(1) Tough and strong GO composite films via a polycationitrile approach. The interface between GO nanosheets was reinforced via an intermolecular covalent crosslinking approach called “polycationitrile chemistry”. As a result, the mechanical performance of the as-prepared GO-based composite films was enhanced and maintained even at an extremely high relative humidity of 98%.

(2) rGO-poly(ionic liquid) (PIL) composite films with high mechanical performance. The rGO/PIL composite films were designed and fabricated, where the synergistic supramolecular interactions between PIL and rGO layer enable high electrical conductivity and favorable mechanical properties.

(3) Regenerated cellulose (RC)/MXene composite nanofibers for personal heating management. I harnessed a biodegradable RC-based fibrous matrix to bond with inorganic MXene nanoflakes via electrospinning method. Via hybridization, the as-formed RC/MXene nanofibers present a promotion of mechanical performance and photothermal conversion capability. As a personal heating cloth, it realizes energy-saving outdoor thermoregulatory.

(4) RC/MXene solar absorber for solar-driven interfacial water evaporation. The RC/MXene composite nanofibers integrate considerable merits of excellent mechanical performance, wettability, and fast steam generation rate. The RC/MXene solar absorber offers significant values for the practical application of solar-driven steam generation.

The development of sustainable materials from biobased resources is essential due to environmental concerns posed by fossil-based materials. Lignin is a chemically complex biopolymer that exists in woody tissues of vascular plants. Lignin has many useful properties such as antioxidant activity, thermal stability, UV-absorbance, rigidity and so on. However, an inherent challenge of lignin relates to its complex molecular structures and poor solubility in water and common solvents. One strategy to utilize lignin is to fabricate lignin nanoparticles (LNP) that produce colloidally stable dispersions in water. This thesis aims to develop LNP-based materials which can be used in photonic crystals and photothermal films towards energy-efficient functional materials.

The first part of the thesis focused on elucidation of the phenomena occurring during centrifugation-assisted assembly of LNP-photonic crystal (L-PC). L-PC with rainbow coloration or separate colors were produced by controlling the polydispersity index (PDI), particle size (150 to 240 nm), and assembly of LNPs. In a follow-up work, an improved method was developed to increase the yield of L-PCs. The effects of factors such as initial lignin concentration, and dilution time on the particle size and PDI of formed LNPs were studied. Empirical models were established to predict the size of LNPs and successfully used to control the resulting color of L-PCs. Moreover, the nanostructure of L-PCs was investigated. 

To harness lignin’s ability to absorb solar energy (light wavelength: 250–2500 nm), LNP-based composite films and coatings with photothermal performance were developed in the second part of the thesis. LNP-chitosan films and coatings were prepared and applied to indoor heat management. The LNPs content was adjusted from 10 to 40 wt%. By incorporating LNPs, the mechanical strength and photothermal properties of the films were improved compared to the pure chitosan film. Moreover, LNP-silver-chitosan (CC-Ag@LNP) films were prepared by using LNPs as a reducing agent. Silver ions were reduced on the surface of LNPs with UV-light assistance, and the hybrid nanoparticles were used to prepare films by casting. The CC-Ag@LNP films exhibited improved wet-strength and exhibited antibacterial performance against Escherichia coli (sterilization effect > 99.9%).

Overall, this thesis contributes to both the fundamental insight in lignin aggregation to colloidal particles and showcases ways to control their assembly and incorporation into macroscopic materials with added functionality.