Synthesis and characterization of graphene oxide with enhanced mechanical properties

Abstract

With the ever-increasing multiple threats and conflicts, and with the insufficiency of the actual protection systems made of polymeric fibers such as Kevlar, Dyneema, and Zylon against improvised explosive devices (IEDs) and lethal ammunition, the development of personal protection systems with heightened mechanical proprieties has received great interest in this decade, and this interest will continue to increase as the market of personal protective armor systems is predicted to rise to $5.3 billion by 2024. As it can offer solutions and evolutions for today's world, nanotechnology holds -undoubtedly- the opportunity to provide the breakthroughs that defense technology so desperately needs. The emergence of nanomaterials with exotic proprieties makes them an excellent choice for ballistic armor materials, among the most ideal ones is graphene: graphene is already known as the world's strongest material with a theoretical modulus of more than TPa. Moreover, graphene has a low density, which is a very interesting propriety for body armor application since it provides better mobility for the soldier due to its lightweight attribute and fatigue reduction. According to recent studies, graphene has also an intrinsic ability to absorb sudden impacts and dissipate their high energy. what is essential at this point is to effectively exfoliate the raw material: graphite into large quantities of high-quality graphene. Graphene oxide (GO), the oxidative derivative of pristine graphene can be produced via a solution-based chemical exfoliation method which is a top-down process susceptible to economical large-scale production. The oxygen groups of GO can increase also its interaction with different functional groups of polymeric fibers such as Kevlar fibers, or with the polymer constituting the "brick" part of the nacre-like protective system. GO can be further converted to graphene through chemical or thermal reduction, which makes the potential of fabricating graphene-based body armors very high soon. However, GO quality remains the determinant factor and the big challenge to overcome in order to integrate GO into body armor systems. The main objective of this thesis is to enhance GO quality and to make its chemical synthesis more suitable for large-scale production. By using expanded graphite as a starting material instead of natural graphite flakes, we promoted the synthesis of GO with large sheets (average of ~ 37 μm) and low defects degree thanks to the effective oxidant diffusion into graphene galleries after enlarging the interlayer spacing. The expanded graphite was obtained easily by treating graphite with cooled piranha solution without any washing or drying steps, and without involving any heat treatment nor requiring advanced equipment, unlike the traditional methods which require harsh conditions and result in a high cost and severe environmental pollution. An expansion volume of 430 ml/g was achieved under room temperature with mass ratios of +100 mesh graphite to sulfuric acid of 1:100 and hydrogen peroxide to sulfuric acid of 1:10. Thanks to this expansion, the oxidation temperature could be reduced from 50 °C to 35 °C and the oxidation time could be reduced to half. XRD, XPS, and NMR have shown that GO synthesized via this route that we called the "enhanced method" —reported to the best of our knowledge for the first time— has a high oxidation degree, while UV, XPS, and Raman have manifested the retain of more aromatic rings i.e. low defects compared to Tour group's method. Furthermore, after using the industrially suitable doctor-blade technique and hydroiodic acid (HI) reduction, rGO film obtained through this method has achieved a tensile strength of 190 MPa, a toughness of 5.7 MJ m-3 which is promising for the mass production of expanded graphite (EG) and GO due to the method simplicity, cost-effectiveness, and low environmental impact. The enhanced synthesis method was further used but with four different graphite sizes to study their effect on the volume expansion and GO properties in the scope of producing large graphene oxide from initially large graphite flakes. Other enhancements were done like reducing the acid quantity to reduce the total cost and make the synthesis more environmentally friendly, operating it at room temperature (20 °C), and minimizing the oxidant quantity to restrict any over-oxidation which can lead to more defects hardly removed through reduction. The strength and toughness were found to increase with increasing the starting graphite material size, except GO+100 mesh which was unexpectedly inferior to GO200 mesh. In this study, GO50 mesh exhibited the highest failure strength and toughness at 232 MPa and 11.3 MJ m-3 respectively, but despite that its starting material has a much larger size than that of GO200 mesh, the difference between their tensile curves was not that pronounced. This research work concludes that the starting graphite size can play an important role, but larger graphite flakes' size does not always lead to better GO despite that this trend is ordinarily correct. XPS shows that impurities such as organosulfate, and carboxylic groups located on the sheets' edges can reduce the properties of the final GO even if it is obtained from large graphite flakes. Raman and morphology studies reveal that as larger flakes need higher oxidant quantity, harsher oxidation may exist to overcome the diffusion-controlled oxidation pathways until achieving the flakes centers, which cuts off the sheets into smaller ones and creates more cracks and defects. Thus, there exists a balance between the large building blocks needed and the defects induced. In this study, evidence of how using +50 mesh graphite -with the enhancements made to the already enhanced method- can improve the resulting GO mechanical properties. However, to confirm the graphite size effect on GO properties, it would be better if a graphite size larger than +50 mesh can be used and the final GO size as well as its mechanical properties before and after reduction can be determined. It is important to make other characterizations for the graphite flakes and the resulting GO such as elemental analysis, XRD, and AFM. More than the tensile tests, advanced characterization can be made to GO and rGO free-standing films such as nanoindentation test, split Hopkinson bar, and gas gun measuring system for testing high strain rate behavior, and for determining failure stress and absorbed specific energy under dynamic conditions. For the immediate use of GO in the current body armor systems, Kevlar fibers can be coated with GO synthesized and enhanced through this thesis, then GO can be reduced to further ameliorate its mechanical properties. Our proposed enhanced method susceptible to cost-effective mass-scale production of GO makes the fabrication of such a body armor attainable in the near future.