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.

This thesis presents a comprehensive study on stimuli-responsive materials derived from cellulose nanofibrils (CNFs), focusing on their synthesis, characterization, and performance evaluation in various applications. Renowned for their biodegradability, renewability, and robust mechanical properties, CNFs are explored in three primary contexts: moisture-responsive actuators, voltage-responsive actuators, and CO₂-responsive sensors.

The unique properties of CNFs, such as high tensile strength and surface area, are leveraged to achieve effective motion in response to moisture exposure. Specifically, CNFs are utilized to create bilayer, torsional, and tensile actuators. These actuators exhibit controllable and dynamic responses, making them suitable for applications in soft robotics and wearable technology.

In the realm of voltage-responsive actuators, this study investigates the impact of various electrolytes and counteranions on positively charged CNFs. It uncovers the critical role of electrolyte choice, ion migration and the plasticization effect within the CNFs matrix, resulting in volumetric expansion, which is pivotal to the actuation mechanism. These insights pave the way for CNFs applications requiring precise control of motion and flexibility in shape, such as in soft robotics.

The third area of application involves the development of a capacitive CO₂ sensor using CNFs-based foams functionalized with primary amines to enhance CO₂ capture through chemisorption. This functionalization turns the CNFs-based foam into an efficient dielectric layer (DE) for sensor applications. The addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to the DE further expands the scope of sensor's capacitance change in response to CO₂ exposure, underscoring its potential in environmental monitoring and CO₂ detection.

Overall, this thesis emphasizes the versatility and adaptability of CNFs as a sustainable biomaterial for developing stimuli-responsive devices. The insights gained from studying CNFs in these varied applications contribute significantly to materials science and open new avenues for research in sustainable, bio-based materials.