This research fabricated and tested various graphene-related 1D, 2D and 3D materials. We described how using specifically designed graphene foam (GF) as templates can transform its unique structures and excellent properties to new materials. Graphene, GF, Polypyrrole (PPY), Polyaniline (PANI), PPY-GF, PANI-GF, SiC foam, SiC nanowires and SiC nanoflakes will be described in this thesis. The chemical vapour deposition method was used to produce graphene and GFs. PPY-GF, PPY, PANI and PANI-GF were prepared by both chemical and electrochemical (Chronopotentiometry) methods. SiC foams were produced by a low-cost carbothermal reduction of SiO with GF, and then the SiC nanoflakes were separated from SiC nanowires and purified via a multistep sonication process. The synthesised materials were characterised by a variety of techniques such as SEM, EDX, XRD, TEM, Raman, AFM and TGA. The electrochemical properties of the materials were measured in a three electrode cell using cyclic voltammetry (CV), galvanostatic charge-discharge and A.C impedance spectroscopy techniques. The mechanical properties of the GF and SiC foams were investigated by conducting compression tests under in-situ SEM imaging. The as-produced graphene in this research was few layer graphene with layer number varies from 2 to 15. The GFs was found to be extremely light weight with an average density value of 5 mg cm-3. Using GF as electrode materials for supercapacitors, we obtained 100% capacity retention after 10,000 of charge-discharge cycles. The PPY-GF composite electrode exhibited an outstanding specific capacitance of 660 Fg-1, which is superior to the performance of most of the existing PPY-CNT, PPY-graphite and PPY-Graphene electrodes reported to date. In contrast to the PPY which shows a big structure degradation and a 30% capacity loss after only hundreds of CV cycles, the PPY-GF composite showed nearly 100% capacity retention after 6,000 cycles of charge-discharge. Our post-test characterisations showed no structural loss for the GF and PPY-GF. The excellent pseudocapacitive performance of the electrodes was found to be related to three key parameters: the open porosity feature of the GF which provides short pathways for ion diffusion and charge transportation, the dual charge storage mode in the composite, and the excellent mechanical properties of the GF. Due to its high flexibility and void spaces, the GF played successfully the role as a holder and stabilizer for the electroactive materials in protecting them from any structural degradation during the repeated ion intercalation-de-intercalation processes. In the SiC project, we have successfully created extremely light-weighted SiC foams with a density range of 9-20 mg cm-3, with various shapes, by using the GF as templates. These foams are the lightest reported SiC structures, and they consist of hollow trusses made from 2D SiC and long 1D SiC nanowires growing from the trusses, edges and defect sites. The 1D SiC nanowires, being confirmed as 3C-structure, appeared in a variety of shapes and sizes and are highly flexible; the 2D SiC is hexagonal, and upon breakup the resulting 2D nanoflakes have an average size of 2 µm and a thickness value of 2-3 nm which is 5-9 layers of SiC. They, to the best of our knowledge, are probably the thinnest and largest reported SiC flakes. Ultimately, in this research we have successfully produced two extremely lightweight and simultaneously strong foams: the GF and SiC foam. We have explored the GFs by efficiently addressing a key issue in the cycle life of energy storage devices, by creating an ideal architecture of such 3D GF-based electrodes. We have developed a completely novel 3D SiC structure made from continuously linked 2D layered SiC reinforced with 1D SiC nanowires. In-situ compression studies have revealed that both the GF and SiC foams can recover significantly, up to 85% in the case of GF, after compression strain exceeding 70%. The SiC foam did not experience any dramatic failure under the compression loads, as do in conventional ceramics. Compared with most existing lightweight foams of similar density, the present 3D SiC exhibited superior compression strengths and an significantly enhanced strength-to-weight ratio.

The mounting concern for renewable energies from ecologically conscious alternatives is growing in parallel with the demand for portable energy storage devices, fuelling research in the fields of electrochemical energy storage technologies. The supercapacitor, also known as electrochemical capacitor, is an energy storage device possessing a near infinite life-cycle and high power density recognized to store energy in an electrostatic double-layer, or through a pseudocapacitance mechanism as a result of an applied potential. The power density of supercapacitors far exceeds that of batteries with an ability to charge and discharge stored energy within seconds. Supercapacitors compliment this characteristic very well with a cycle life in excess of 106 cycles of deep discharge within a wide operational temperature range, and generally require no further maintenance upon integration. Conscientious of environmental standards, these devices are also recyclable. Electrochemical capacitors are currently a promising candidate to assist in addressing energy storage concerns, particularly in hybridized energy storage systems where batteries and supercapacitors compliment each other’s strengths; however specific challenges must be addressed to realize their potential. In order to further build upon the range of supercapacitors for future market applications, advancements made in nanomaterial research and design are expected to continue the materials development trend with a goal to improve the energy density through the development of a cost-efficient and correspondingly plentiful material. However, it is important to note that the characteristic power performance and exceptional life-cycle should be preserved alongside these efforts to maintain their niche as a power device, and not simply develop an alternative to the average battery. It is with this clear objective that this thesis presents research on an emerging carbon material derived from an abundant precursor, where the investigations focus on its potential to achieve high energy and power density, stability and integration with other electroactive materials. Activated carbons have been the dominant carbon material used in electric double-layer capacitors since their inception in the early 1970s. Despite a wide range of carbon precursors and activation methods available for the generation of high surface area carbons, difficulties remain in controlling the pore size distribution, pore shape and an interconnected pore structure to achieve a high energy density. These factors have restricted the market growth for supercapacitors in terms of the price per unit of energy storage. Activation procedures and subsequent processes for these materials can also be energy intensive (i.e. high temperatures) or environmentally unfriendly, thus the challenge remains in fabricating an inexpensive high surface-area electroactive material with favourable physical properties from a source available in abundance. Double-layer capacitive materials researched to replace active carbons generally require properties that include: high, accessible surface-area; good electrical conductivity; a pore size distribution that includes mesopore and micropore; structural stability; and possibly functional groups that lend to energy storage through pseudocapacitive mechanisms. Templated, fibrous and aerogel carbons offer an alternative to activated carbons; however the drawbacks to these materials can include difficult preparation procedures or deficient physical properties with respect to those listed above. In recent years nanostructured carbon materials possessing favourable properties have also contributed to the field. Graphene nanoplatelet (GNP) and carbon nanotube (CNT) are nanostructured materials that are being progressively explored for suitable development as supercapacitor electrodes. As carbon lattice structured materials either in the form of a 2-dimensional sheet or rolled into a cylinder both of these materials possess unique properties desirable in for electrode development. In the proceeding report, GNPs are investigated as a primary material for the synthesis of electrodes in both a pure and composite form. Three projects are presented herein that emphasize the suitability of GNP as a singular carbon electrode material as well as a structural substrate for additional electroactive materials. Investigation in these projects focuses on the electrochemical activity of the materials for supercapacitor devices, and elucidation of the physical factors which contribute towards the observed capacitance. An initial study of the GNPs investigates their distinct capacitive ability as an electric double-layer material for thin-film applications. The high electrically conductivity and sheet-like structure of GNPs supported the fabrication of flexible and transparent films with a thickness ranging from 25 to 100 nm. The thinnest film fabricated (25 nm) yielded a high specific capacitance from preliminary evaluation with a notable high energy and power density. Furthermore, fast charging capabilities were observed from the GNP thin film electrodes. The second study examines the use of CNT entanglements dispersed between GNP to increase the active surface area and reduce contact resistances with thin-film electrodes. Through the use MWNT/GNP and SWNT/GNP composites it was determined that tube aspect ratio influences the resulting capacitive performance, with the formation of micropores in SWNT/GNP yielding favourable results as a composite EDLC. The third study utilizes electrically conducting polypyrrole (PPy) deposited onto a GNP film through pulse electrodeposition for use as a supercapacitor electrode. Total pulse deposition times were evaluated in terms of their corresponding improvements to the specific capacitance, where an optimal deposition time was discovered. A significant increase to the total specific capacitance was observed through the integration PPy, with the majority charge storage being developed via psuedocapacitive redox mechanisms. A summary of the studies presented here centers on the development of GNP electrodes for application in high power supercapacitor devices. The potential use for GNP in both pure and composite electrode films is explored for electrochemical activity and capacitive capabilities, with corresponding physical characterization techniques performed to examine influential factors which contribute to the final results. The work emphasizes the suitability of GNP material for future investigations into their application as carbon or carbon composite electrodes in supercapacitor devices.

The mounting concern for renewable energies from ecologically conscious alternatives is growing in parallel with the demand for portable energy storage devices, fuelling research in the fields of electrochemical energy storage technologies. The supercapacitor, also known as electrochemical capacitor, is an energy storage device possessing a near infinite life-cycle and high power density recognized to store energy in an electrostatic double-layer, or through a pseudocapacitance mechanism as a result of an applied potential. The power density of supercapacitors far exceeds that of batteries with an ability to charge and discharge stored energy within seconds. Supercapacitors compliment this characteristic very well with a cycle life in excess of 106 cycles of deep discharge within a wide operational temperature range, and generally require no further maintenance upon integration. Conscientious of environmental standards, these devices are also recyclable. Electrochemical capacitors are currently a promising candidate to assist in addressing energy storage concerns, particularly in hybridized energy storage systems where batteries and supercapacitors compliment each other’s strengths; however specific challenges must be addressed to realize their potential. In order to further build upon the range of supercapacitors for future market applications, advancements made in nanomaterial research and design are expected to continue the materials development trend with a goal to improve the energy density through the development of a cost-efficient and correspondingly plentiful material. However, it is important to note that the characteristic power performance and exceptional life-cycle should be preserved alongside these efforts to maintain their niche as a power device, and not simply develop an alternative to the average battery. It is with this clear objective that this thesis presents research on an emerging carbon material derived from an abundant precursor, where the investigations focus on its potential to achieve high energy and power density, stability and integration with other electroactive materials. Activated carbons have been the dominant carbon material used in electric double-layer capacitors since their inception in the early 1970s. Despite a wide range of carbon precursors and activation methods available for the generation of high surface area carbons, difficulties remain in controlling the pore size distribution, pore shape and an interconnected pore structure to achieve a high energy density. These factors have restricted the market growth for supercapacitors in terms of the price per unit of energy storage. Activation procedures and subsequent processes for these materials can also be energy intensive (i.e. high temperatures) or environmentally unfriendly, thus the challenge remains in fabricating an inexpensive high surface-area electroactive material with favourable physical properties from a source available in abundance. Double-layer capacitive materials researched to replace active carbons generally require properties that include: high, accessible surface-area; good electrical conductivity; a pore size distribution that includes mesopore and micropore; structural stability; and possibly functional groups that lend to energy storage through pseudocapacitive mechanisms. Templated, fibrous and aerogel carbons offer an alternative to activated carbons; however the drawbacks to these materials can include difficult preparation procedures or deficient physical properties with respect to those listed above. In recent years nanostructured carbon materials possessing favourable properties have also contributed to the field. Graphene nanoplatelet (GNP) and carbon nanotube (CNT) are nanostructured materials that are being progressively explored for suitable development as supercapacitor electrodes. As carbon lattice structured materials either in the form of a 2-dimensional sheet or rolled into a cylinder both of these materials possess unique properties desirable in for electrode development. In the proceeding report, GNPs are investigated as a primary material for the synthesis of electrodes in both a pure and composite form. Three projects are presented herein that emphasize the suitability of GNP as a singular carbon electrode material as well as a structural substrate for additional electroactive materials. Investigation in these projects focuses on the electrochemical activity of the materials for supercapacitor devices, and elucidation of the physical factors which contribute towards the observed capacitance. An initial study of the GNPs investigates their distinct capacitive ability as an electric double-layer material for thin-film applications. The high electrically conductivity and sheet-like structure of GNPs supported the fabrication of flexible and transparent films with a thickness ranging from 25 to 100 nm. The thinnest film fabricated (25 nm) yielded a high specific capacitance from preliminary evaluation with a notable high energy and power density. Furthermore, fast charging capabilities were observed from the GNP thin film electrodes. The second study examines the use of CNT entanglements dispersed between GNP to increase the active surface area and reduce contact resistances with thin-film electrodes. Through the use MWNT/GNP and SWNT/GNP composites it was determined that tube aspect ratio influences the resulting capacitive performance, with the formation of micropores in SWNT/GNP yielding favourable results as a composite EDLC. The third study utilizes electrically conducting polypyrrole (PPy) deposited onto a GNP film through pulse electrodeposition for use as a supercapacitor electrode. Total pulse deposition times were evaluated in terms of their corresponding improvements to the specific capacitance, where an optimal deposition time was discovered. A significant increase to the total specific capacitance was observed through the integration PPy, with the majority charge storage being developed via psuedocapacitive redox mechanisms. A summary of the studies presented here centers on the development of GNP electrodes for application in high power supercapacitor devices. The potential use for GNP in both pure and composite electrode films is explored for electrochemical activity and capacitive capabilities, with corresponding physical characterization techniques performed to examine influential factors which contribute to the final results. The work emphasizes the suitability of GNP material for future investigations into their application as carbon or carbon composite electrodes in supercapacitor devices.

Oxide ceramics have been increasingly researched for high temperature thermoelectric (TE) applications. SrTiO3 based materials are promising candidates due to its chemical and thermal stability. In this study, oxide ceramics of composition (1-x)SrTiO3-(x)La1/3NbO3 (0 smaller or equal to x smaller or equal to 0.3) were prepared by single-step solid state sintering in Ar/5%H2 at 1700 K. The density of all the samples prepared was above 90%. All the samples were predominantly single-phase compositions crystallised with a cubic structure in Pm ̅3m space group. The impact of oxygen deficiency, A-site vacancies and mixed oxidation states of Ti3+/Nb4+ on electrical and thermal transport properties was assessed. Optimum TE properties were obtained for x=0.2 (Sr0.8La0.067Ti0.8Nb0.2O(3-delta) = L2), which has 13.4% A-site vacancies. The ZT values improved from 0.2 to 0.27 at 1000 K, with an increase in sintering time from 8 hours to 48 hours, due to increased carrier concentration. Complex interplay of oxygen vacancies and excess donor substitution on A/B-sites of L2 (substituting 5-10% Sr/Ti with La/Nb) exhibited 35% improvement in ZT values, whilst maintaining the A-site vacancies and core-shell structures within grains, which reduced the thermal conductivity by ~50% compared to undoped SrTiO3 samples, due to strong phonon scattering. A facile method to incorporate metallic inclusions (2.5 wt% Fe/Cu) at grain boundaries in L2 ceramics is demonstrated. The modified compositions displayed a maximum ZT of ~0.37 at 1000 K for L2 samples containing metallic inclusions due to increased carrier concentration (5.5 x 10^21 carriers/cm^3) and carrier mobility (2.4 cm^2/(V.s).The addition of graphene/Graphene Oxide (GO) flakes in L2 ceramics has been investigated to improve the electrical conductivity of L2 composites without significantly increasing the thermal conductivity. Spark plasma sintering (SPS) of the composite powders at 1473 K and 50 MPa produced dense samples (>95% relative density) with a homogeneous dispersion of graphene/GO flakes, for loadings smaller or equal to 1.0 wt%. The effect of interaction and distribution of graphene/GO flakes within the ceramics on TE properties is investigated. The composite samples demonstrate anisotropic ZT values, with 20% improvement in the direction normal to the orientation of graphene flakes. A novel sintering method has been proposed which has strong industrial potential. The L2 based composites were sintered in Air at 1700 K (ramp rate: ±300 K/min), whilst samples were covered uniformly. Strong reducing conditions and evolution of secondary phases in the microstructure helped achieve, the very low electrical resistivity of ~3.0 x 10^(-6) ohm.m at room temperature. Secondary phases, sub-micron voids in the grains and A-site vacancies reduced the lattice thermal conductivity (~2.0 W/m.K), comparable to the lowest lattice thermal conductivity achievable (~1.5 W/m.K) at 1000 K and obtain a maximum ZT of 0.4 at 1000 K for L210G-Air/C composites.

No description available.

No description available.

Cette thèse est consacrée à la production de matériaux 2D en phase liquide, en utilisant des approches pouvant permettre la production en masse de graphène et de matériaux apparentés. Notre objectif est de surmonter certains problèmes critiques pour le traitement et l'utilisation pratique des encres à base de matériaux 2D et de fournir une compréhension approfondie de la relation structure-propriétés dans ces matériaux, constituant des étapes obligatoires pour leurs applications futures. Cette thèse porte principalement sur l'UILPE et l'exfoliation électrochimique du graphène et du disulfure de molybdène (MoS2), qui ont été choisis comme matériaux prototypes à 2 dimensions. Les approches synthétiques sont combinées à une caractérisation physico-chimique des matériaux produits, à l'aide de techniques telles que l'AFM, la microscopie électronique, la spectroscopie XPS et Raman, ainsi qu'à une caractérisation électrique. Des applications dans le domaine de la détection et de l'électronique ont été explorées et ont permis de démontrer que des approches d'exfoliation en phase liquide pouvaient être utilisées pour obtenir un contrôle précis des propriétés des matériaux 2D ouvrant la voie à leur intégration en tant que matériaux actifs dans de nouveaux dispositifs multifonctionnels. / This thesis is devoted to the production in liquid-phase of two-dimensional materials, by using approaches that may enable mass production of graphene and related materials. We aim to overcome some issues that are critical for the processing and practical use of 2D materials-inks and to provide a deep understanding of the structure-properties relationship in such materials being mandatory steps toward their future applications. This thesis mainly focuses on ultrasound-induced liquid-phase exfoliation and electrochemical exfoliation of graphene and molybdenum disulfide, which have been chosen as prototypical 2D materials. The synthetic approaches have been combined with a multiscale physico-chemical and electrical characterization of the produced materials, by employing techniques such as AFM, XPS and Raman spectroscopy. Applications in the field of sensing and electronics have been explored and allowed to demonstrate that liquid-phase exfoliation approaches can be conveniently employed to achieve a fine control on the properties of 2D materials paving the way to their integration as active materials in novel multifunctional devices.

The growing incidences of orthopedic problems globally have created a huge demand for strong bioactive materials for bone tissue engineering. Over the years, studies have shown chemical, physical, and mechanical properties of biomaterials influence the cellular interactions at the material-tissue interface, which subsequently controls biological response to materials. Strong biomaterials with surface properties that actively direct cellular response hold the key for engineering the next generation orthopedic implants. With its unique properties graphene can be used to reinforce poly (ε-caprolactone) (PCL) to prepare strong and bioactive polymer nanocomposites for bone tissue regeneration. The thesis entitled ―Engineering bioactive and multifunctional graphene polymer composites for bone tissue regeneration” systematically studies the effect of different chemically functionalized and metal-graphene hybrid nanoparticles in PCL composites for bone tissue engineering. The thesis comprises of seven chapters. Chapter 1 is an outline review on the impact of graphene and graphene derived particles to prepare supporting substrates for tissue regeneration and the associated cell response to multifunctional graphene substrate. This chapter discusses how cells interact with different graphene based particles and the interplay between cells performance and multifunctional properties of graphene based substrates. Chapter 2 describes the role, if any, of the functionalization of graphene on mechanical properties, stem cell response and bacterial biofilm formation. PCL composites of graphene oxide (GO), reduced GO (RGO) and amine-functionalized GO (AGO) were prepared at different filler contents (1%, 3% and 5%). Although the addition of the nanoparticles to PCL markedly increased the storage modulus, this increase was higher for GO and AGO than with RGO. In vitro cell studies revealed that the AGO and GO particles significantly increased human mesenchymal stem cell (hMSC) proliferation. AGO was most effective in augmenting stem cell osteogenesis leading to mineralization. Bacterial studies revealed that interaction with functionalized GO induced bacterial cell death due to membrane damage which was further accentuated by amine groups in AGO. The synergistic effect of oxygen containing functional groups and amine groups on AGO-reinforced composites renders the optimal combination of improved modulus, favorable stem cell response and biofilm inhibition desired for orthopaedic applications. In Chapter 3, toward preparing strong multi-biofunctional materials, poly(ethylenimine) (PEI) conjugated graphene oxide (GO_PEI) was synthesized using poly(acrylic acid) (PAA) as spacer and incorporated in PCL at different fractions. GO_PEI significantly promoted proliferation and formation of focal adhesions in hMSCs on PCL. GO_PEI was highly potent in inducing stem cell osteogenesis leading to 90% increase in alkaline phosphatase activity and mineralization over neat PCL with 5% filler content and was 50% better than GO. Remarkably, 5% GO_PEI was as potent as soluble osteo-inductive factors. Increased adsorption of osteogenic factors due to the amine and oxygen containing functional groups on GO_PEI augment stem cell differentiation. GO_PEI was also highly efficient in imparting bactericidal activity with 85% reduction in counts of E. coli colonies compared to neat PCL at 5% filler content and was more than twice as efficient as GO. This may be attributed to the synergistic effect of the sharp edges of the particles along with the presence of the different chemical moieties. Thus, in contrast to using labile biomolecules, GO_PEI based polymer composites can be utilized to prepare bioactive resorbable biomaterials for fabricating orthopedic devices for fracture fixation and tissue engineering. Chapter 4 describes the preparation of hybrid nanoparticles of graphene sheets decorated with strontium metallic nanoparticles and its advantages in bone tissue engineering. Strontium-decorated reduced graphene oxide (RGO_Sr) nanoparticles were synthesized by facile reduction of graphene oxide and strontium nitrate. X-ray diffraction, transmission electron microscopy, and atomic force microscopy revealed that the hybrid particles were composed of RGO sheets decorated with 200 – 300 nm metallic strontium particles. Thermal gravimetric analysis further confirmed the composition of the hybrid particles as 22 wt% of strontium. Macroporous tissue scaffolds were prepared incorporating RGO_Sr particles in PCL. The PCL/RGO_Sr scaffolds were found to elute strontium ions in aqueous medium. Osteoblast proliferation and differentiation was significantly higher in the PCL scaffolds containing the RGO_Sr particles in contrast to neat PCL and PCL/RGO scaffolds. The increased biological activity can be attributed to the release of strontium ions from the hybrid nanoparticles. This study demonstrates that composites prepared using hybrid nanoparticles that elute strontium ions can be used to prepare scaffolds with osteoinductive property. These findings have important implications for designing the next generation of biomaterials for use in tissue regeneration. Chapter 5 discusses the use of hybrid graphene-silver particles (RGO_Ag) to reinforce PCL and compared with PCL/RGO and PCL/Ag composites containing RGO and silver nanoparticles (AgNPs), respectively. RGO_Ag hybrid particles were well dispersed in the PCL matrix unlike the RGO and AgNPs due to enhanced exfoliation. RGO_Ag led to 77 % increase in the modulus of PCL and provided a conductive network for electron transfer. Electrical conductivity increased four orders of magnitude from 10-11 S/cm to 10-7 S/cm at 5 wt % filler that greatly exceeded the improvements with the use of RGO and AgNP in PCL. RGO_Ag particles reinforced in PCL showed sustained release of silver ions from the PCL matrix unlike the burst release from PCL/Ag. PCL/RGO_Ag and PCL/RGO composites were non-toxic to hMSCs and supported osteogenic differentiation unlike the PCL/Ag composites which were highly toxic at ≥3% filler content. The PCL/RGO_Ag composites exhibited good antibacterial effect due to a combination of silver ion release from the AgNPs and the mechanical rupture induced by the RGO in the hybrid nanoparticles. Thus, the synergistic effect of Ag and RGO in the PCL matrix uniquely yielded a multifunctional material for use in implantable biomedical devices and tissue engineering. Chapter 6 presents investigation of potential differences in the biological response to graphene in polymer composites in the form of 2D substrates and 3D scaffolds. Results showed that osteoblast response to graphene in polymer nanocomposites is markedly altered between 2D substrates and 3D scaffold due to the roughness induced by the sharp edges of graphene at the surface in 3D but not in 2D. Osteoblast organized into aggregates in 3D scaffolds in contrast to more well spread and randomly distributed cells on 2D discs due to the macro-porous architecture of the scaffolds. Increased cell-cell contact and altered cellular morphology led to significantly higher mineralization in 3D scaffolds compared to 2D. This study demonstrates that the cellular response to nanoparticles in composites can change markedly by varying the processing route. Chapter 7 summarizes the important results and future directions of the work. This chapter provides general conclusions arising from this study, and makes suggestions for future work designed to provide a greater understanding of the in vivo response in terms of bio-distribution of the released functionalized graphene from the scaffold or substrate must be assessed with special attention on their accumulation or excretion.