Synthesis and characterization of novel three-dimensional graphene structures: From status quo to impending states and applications

Student thesis: Doctoral Thesis

Abstract

GO-substrate interactions & properties There is no denial to the interminable importance of 2D crystals such as graphene. Since the successful isolation of graphene it was desired to arrange these nano-sheets in a three-dimensional (3D) design so as to exploit excellent electrical, thermal and piezo-resistive properties. In this thesis work, we have come up with an idea of creating 3D structures of chemically modified graphene (CMG), also known as reduced graphene oxide (rGO), via a method which is scalable to industrial grade and produces good quality rGO foams/sponges with excellent electrical conductivity, controllable density, shapes and sizes. The method of composed of two-steps, 1) coating of graphene oxide (GO) on a polymeric foam/sponge template and 2) removing the template from the core while simultaneously reducing GO to rGO in order to restore the graphene like properties of GO. Since it's a template based method, therefore, we have studied the interaction of GO and rGO with different intricate polymeric and other surfaces. GO was coated on fibers and fabrics of polymers such as polyester, nylon, cotton, wool, Kevlar as well as on glass fibers. It was found that the GO, hydrophilic, in nature coats well on intricate shapes of polymers when the coating is done on a highly hydrophobic substrate such as Teflon® at slightly elevated temperatures (i.e. 80 C). GO was then reduced to rGO in order to restore its graphene like properties. Aforementioned commercially available fibers and fabrics were coated with reduced graphene (rGO) in a layer by layer fashion such that the individual fibers of the fabric get wrapped by rGO and the fabric looks like dyed with grey color as evident from close-up photographic images of the fabrics before and after coating process. Detailed morphological characterization of the rGO coated fabric, done via scanning electron microscopy (SEM), showed that the rGO is well adhered to the surface of the fibers in such a way that the rGO looks like an integral part of each individual fiber. Cross-sectional SEM images of individual fibers revealed the rGO coating and polymeric core. The SEM images of the twisted vii individual rGO coated fibers show that the coating remains undamaged until a twist angle of about 1800°. This is the first time that the interaction of GO and rGO with such intricate surfaces such fibers of a fabric was studied. Formation of graphene based 3D structures from the knowledge of GO-substrate interaction Since GO was able to coat on intricate surfaces and was able to take the shape of the substrate, therefore, it was desired to coat GO on intricate 3D structures of biodegradable polymer templates which can then be removed from the core either chemically or thermally. Polyurethanes (PU) are such polymers which are readily available at low prices and are removable completely as volatiles at temperatures of 400-800 °C. GO was then successfully coated on commercially available PU foams/sponges, available in different shapes, sizes, microstructures and densities. The GO-coated PU was then heated at about 1000 C in order to reduce GO to rGO and simultaneously release the PU completely as volatiles. This facile and scalable method was used to successfully create free-standing large monoliths (15cm long) of graphene foams (GF) with tunable shapes, sizes, microstructures, densities and electrical conductivities. SEM micrographs of the GFs show that it mimics the microstructure of its respective template. GF were also tested for their mechanical strength under compressive loads using an Instron microtester. Raman spectroscopy, thermogravimetric analysis and energy dispersive spectroscopy (EDS) showed that the formed graphene is pure and all the PU has been removed successfully from the core of the GF. The behavior of GF under compressive load is similar to that of porous ceramics (mullite) structures. However, unlike porous ceramics the GF doesn't completely collapse into powder under compression. Rather, high compressive loads results in its compaction. viii Sensing behavior of GF & graphene coated fabrics Graphene is semi-metallic to semiconducting in nature with finite resistance. This property of graphene makes it piezoresistive, therefore, any external perturbation such as force/pressure or deformation causes graphene to change its resistance. Stretchable and flexible conductors of GF were made by infusing polydimethylsiloxane (PDMS) inside GF via vacuum infusion process. The infusion of PDMS inside GF didn't decrease its electrical conductivity. The flexible and stretchable GF-PDMS conductor was tested for their mechanical properties in compression and it showed no degradation in mechanical strength when compressed up to 50% strain for at least 1000 times. The instron® microtester was coupled with an electrochemical workstation for in situ measurement of resistance when a force or strain is applied to the GF-PDMS conductor. When the composite is compressed 20% and 30% of its original thickness, resistance changes of 270 and 850% respectively, are recorded. The change in resistance increases linearly with the applied stress until a compressive stress of about 300 kPa and later increases exponentially on higher stresses. It reaches a change in resistance value of about 3500% on the application of compressive stress of 500 kPa. GF was also fabricated via coating of GO on metallic foam and subsequently reducing it to rGO and removing the metal foam via chemical etching with fuming hydrochloric acid (HCl). The GF created from these methods were at least 20 times more pressure sensitivie than previously reported graphene based stretchable and flexible conductors. Such high pressure sensitivity was exploited in making a wrist band which was able to measure blood pressure and the pulse rate when attached to the wrist of a human being. In addition to GF-PDMS flexible conductors, the rGO coated textiles were also tested for their piezo-resistive property. An in situ applied pressure in compression, on the fabric, of about 2500 KPa changed its resistance 8 krelative to its original resistance with a durability of up to 6000 cycles. Single rGO coated fibers quarantined and tested for their strain sensitivity showed that an in situ bending radius of 1mm changes its resistance to 326 ±21 ix krelative to its original resistance. With the help of a read-out circuit it was also showed that quarantined rGO coated fibers arranged in a 2x2 grid format sense the position of the applied force. Finally, the conductive rGO coated Nylon fabric was successfully demonstrated to be used as an electrode for electrocardiography (ECG) analysis replacing the expensive and irritating to skin commercial wet Ag/AgCl electrodes. The dry textile electrode of rGO coated Nylon showed 97% accuracy when compared to the conventional electrode.
Date of AwardMar 2016
Original languageAmerican English
SupervisorKin Liao (Supervisor)

Keywords

  • Graphene; Graphene oxide; E-textile; Graphene foam; Pressure sensor.

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