Chemically and Electrochemically Synthesized Graphene-Based Inks for 3D Printing of Advanced Electronics and Electromagnetic Shields
Creators: name: Erfanian, Elnaz
Publisher: name: Graduate Studies
Year: 2024
Subjects: subject: Graphene
subject: nanomaterial synthesis
subject: Direct ink writing
subject: 3D printing
subject: Electromagnetic-interference (EMI) shielding
subject: Engineering--Chemical

description: This thesis explores new approaches for the development of graphene-based inks for 3D printing of macro-scale patterned scaffolds for advanced applications, e.g., electronics and electromagnetic (EM) shields. These approaches include multiscale design of the inks, from nano-scale chemistry to micro-scale assembly. To achieve this, the chemical features of the nanosheets were fine-tuned by synthesizing different graphenes via both the electrochemical and chemical synthesis methods. The electrochemical method is considered safer, faster, and more environmentally friendly than chemical methods, primarily yielding pristine graphene with high electrical conductivity. However, due to the lack of surface functional groups, electrochemically synthesized graphene is challenging to disperse in water. In contrast, the chemical method produces highly oxidized graphene nanosheets, GO, which are easily dispersible in water due to their oxygen-containing functional groups but feature low electrical conductivity. In the first part of this thesis, graphene oxide (GO) is synthesized using modified Hummer method, selected for its water dispersibility, the most environmentally friendly medium. The resulting nanosheets exhibit both hydrophobic and hydrophilic characteristics; the hydrophobicity stems from the carbon basal plane, while the hydrophilicity originates from their functional groups. This duality makes it ideal for stabilizing Pickering emulsions featuring yield stress and proper rheological features, required for printing. Hence, utilizing emulsion templating, a non-printable graphene oxide dispersion was transformed into a printable yield-stress fluid, with the printing quality adjustable by modifying operational conditions. Although printable inks were developed by emulsion templating, the main constituent of these inks is GO, which is not conductive, limiting the application potential of the printed structure. GO can be transformed to reduced GO (rGO) via harsh chemical or thermal reduction post-processing methods. Thus, the second part of the thesis addresses the challenges of enhancing electrical conductivity in the conventional GO-based systems. Herein, an electrochemical synthesis approach was adopted, eliminating the need for harmful chemicals. However, this method introduced problems related to the graphene's limited water dispersibility. In this regard, electrochemically synthesized graphene (EGO) posed challenges in 3D printing, particularly its tendency to form droplets rather than a continuous filament. Thus, bio-based materials, i.e., TEMPO-oxidized cellulose nanofibrils (TOCNF), were employed as dispersants and rheological modifiers to improve the water compatibility of the highly conductive, nonpolar, electrochemically synthesized graphene nanosheets. A meticulous balance and optimization of TOCNF and EGO concentrations were required to achieve a printable ink that maintains the post-printed structure. This formulation allowed the identification of a 'window of printability,' creating a roadmap for fabricating 3D printed EGO-TOCNF ink. The high-fidelity printed structures maintained their form during drying and post-processing steps, enabling the fabrication of aerogels with tunable nano- and micro-scale designs. Hence, the multi-scale materials design of this work provided a unique opportunity to control the mechanical and electrical properties of the printed structures. Aerogels with a compression modulus ranging from 250-1096 kPa were obtained, and for the optimized ink, an EMI shielding effectiveness as high as 55.6 dB was achieved, eliminating the need for post-printing reduction processes. Although TOCNF bestows printability to graphene-based suspensions, it deteriorates the overall electrical conductivity due to its insulating nature. Thus, the third part of the thesis addresses the drawbacks of multicomponent inks, including the compromise in electrical conductivity when incorporating additives into graphene. A highly conductive all-graphene 3D printable aqueous ink was developed using a novel two-step electrochemical method with a specially designed intercalation step to control the surface functionality of the synthesized graphene nanosheets. Comprehensive characterization revealed the significant impact of graphene nanosheets’ physicochemical properties on the homogeneity, rheology, electrical conductivity, and EMI shielding effectiveness of their resultant aqueous-based inks. Phosphoric acid treatment was found to be the most effective in enhancing both printability and conductivity, achieving an electrical conductivity of 158 S/cm and an EMI shielding effectiveness of 50 dB at a 50 µm thickness, without the need for any post-processing reduction. Systematic experimentation with varying durations of phosphoric acid intercalation established that 10-minute intercalation produces inks with superior 3D printing fidelity, which can be converted to 3D patterned aerogels. This innovative approach enables rapid, continuous, and scalable manufacturing of lightweight, porous materials, avoiding environmentally harmful reductant chemistries or high-temperature processing. The elimination of a reduction step in the fabrication process aligns with industrial demands for energy-efficient production processes and high output rates, marking a significant advancement in the field of materials science. Thus, this work offers promising prospects for the application of graphene-based inks in advanced manufacturing technologies.