Shin-Yi Yang

Design and tailoring of a hierarchical graphene-carbon nanotube architecture for supercapacitors

Stacking of individual graphene sheets (GS) is effectively inhibited by introducing one-dimensional carbon nanotubes (CNTs) to form a 3-D hierarchical structure which significantly enhances the electrochemical capacitive performances of GS-based composites. From SEM images, inserting proper quantity of CNTs as nanospacers can effectively impede the stacking of GS and enlarge the space between GS sheets, leading to obtain a highly porous nanostructure. The specific capacitance of GS-CNTs-9-1 (∼326.5 F g−1 at 20 mV s−1) is much higher than that of GS material (∼83 F g−1). Furthermore, the energy and power densities of GS-CNTs-9-1 are respectively as high as 21.74 Wh kg−1 and 78.29 kW kg−1, revealing that the hierarchical graphene-CNT architecture provides remarkable effects on enhancing the capacitive performance of GS-based composites. Therefore, the GS-CNT composites are promising carbon materials for supercapacitors.

Preparation and properties of graphene oxide/polyimide composite films with low dielectric constant and ultrahigh strength via in situpolymerization

This study proposes an effective approach using in situpolymerization, to fabricate large-area graphene oxide (GO)/polyimide (PI) composite films with outstanding mechanical properties. The GO/PI composite films provide ultrahigh tensile strength (up to 844 MPa) and Youngs modulus (20.5 GPa). The NH2-functionalized GO (ODA-GO) is a versatile starting platform for polymer grafting, promoting excellent dispersion of GO within the polymer matrix, and forming strong links with the polymer to facilitate load transfer. The Youngs modulus of the integrated GO–PI composite films with 3.0 wt% ODA-GO loading is 15 times greater, and the tensile strength is 9 times greater than comparable properties of pure PI film. The dielectric constant decreases with increasing GO content and a dielectric constant (Dk) of 2.0 was achieved. This approach provides a strategy for developing ultrahigh performance GO–polymer composite materials.

Effect of Molecular Chain Length on the Mechanical and Thermal Properties of Amine-Functionalized Graphene Oxide/Polyimide Composite Films Prepared by In Situ Polymerization

This study fabricates amine (NH(2))-functionalized graphene oxide (GO)/polyimide(PI) composite films with high performance using in situ polymerization. Linear poly(oxyalkylene)amines with two different molecular weights 400 and 2000 (D400 and D2000) have been grafted onto the GO surfaces, forming two types of NH(2)-functionalized GO (D400-GO/D2000-GO). NH(2)-functionalized GO, especially D400-GO, demonstrated better reinforcing efficiency in mechanical and thermal properties. The observed property enhancement are due to large aspect ratio of GO sheets, the uniform dispersion of the GO within the PI matrix, and strong interfacial adhesion due to the chemical bonding between GO and the polymeric matrix. The Youngs modulus of the composite films with 0.3 wt % D400-GO loading is 7.4 times greater than that of neat PI, and tensile strength is 240% higher than that of neat PI. Compared to neat PI, 0.3 wt % D400-GO/PI film exhibits approximately 23.96 °C increase in glass transition temperature (T(g)). The coefficient of thermal expansion below T(g) is significantly decreased from 102.6 μm/°C (neat PI) to 53.81 μm/°C (decreasing 48%) for the D400-GO/PI composites with low D400-GO content (0.1 wt %). This work not only provides a method to develop the GO-based polyimide composites with superior performances but also conceptually provides a chance to modulate the interfacial interaction between GO and the polymer through designing the chain length of grafting molecules on NH(2)-functionalized GO.

Effects of Multiwalled Carbon Nanotubes Functionalization on the Morphology and Mechanical and Thermal Properties of Carbon Fiber/Vinyl Ester Composites

Multiwalled carbon nanotube (MWCNT)/carbon fiber (CF)/vinyl ester (VE) laminate composites have been fabricated in this study. Pristine MWCNTs were treated with acid solution, which formed numerous oxygen-containing functional groups onto their surface, resulting in COOH-MWCNTs. Thereafter, acrylic functional groups were grafted onto the COOH-MWCNTs to generate acryl-MWCNTs. Three types of MWCNTs (pristine MWCNTs, COOH-MWCNTs, and acryl-MWCNTs) were used to reinforce the CF/VE-based composites. The dispersion of MWCNTs in the VE matrix and the interfacial interaction between MWCNTs and the VE matrix were investigated. Thereafter, the individual reinforcement efficiencies of these MWCNTs are compared. The flexural strength of the MWCNT/CF/VE composite with 1.0 phr acryl-MWCNTs content is 29.8% greater than that of neat CF/VE composites, and the flexural modulus of the MWCNT/CF/VE composite is 9.9% higher than that of neat CF/VE composites. Compared with neat CF/VE composites, 1.0 phr acryl-MWCNT/CF/VE composites exhibit an approximately 19.9 °C increase in glass transition temperature (Tg). The coefficients of thermal expansion significantly decreased from 47.2 ppm/°C of the neat CF/VE composites to 35.6 ppm/°C of the acryl-MWCNTs/CF/VE composites with 1 phr acryl-MWCNT content. This study provides a method for developing acryl-MWCNT/CF/VE composites with good dispersion of MWCNTs in VE matrix and strong interfacial interaction between the MWCNTs and VE matrix for enhancing the stress transfer from VE matrix to CF reinforcement.

Effect of Octa(aminophenyl) Polyhedral Oligomeric Silsesquioxane Functionalized Graphene Oxide on the Mechanical and Dielectric Properties of Polyimide Composites

An effective method is proposed to prepare octa(aminophenyl) silsesquioxane (OAPS) functionalized graphene oxide (GO) reinforced polyimide (PI) composites with a low dielectric constant and ultrastrong mechanical properties. The amine-functionalized surface of OAPS-GO is a versatile starting platform for in situ polymerization, which promotes the uniform dispersion of OAPS-GO in the PI matrix. Compared with GO/PI composites, the strong interfacial interaction between OAPS-GO and the PI matrix through covalent bonds facilitates a load transfer from the PI matrix to the OAPS-GO. The OAPS-GO/PI composite film with 3.0 wt % OAPS-GO exhibited an 11.2-fold increase in tensile strength, and a 10.4-fold enhancement in tensile modulus compared with neat PI. The dielectric constant (D(k)) decreased with the increasing content of 2D porous OAPS-GO, and a D(k) value of 1.9 was achieved.

Effect of extended polymer chains on properties of transparent graphene nanosheets conductive film

This study examined the intercalation reaction of graphite oxide (GO) with poly(acryl amide)/poly(acrylic acid) (PMA) as a method to control the spacing between GOs. The interlayer spacing of GO was increased from 0.80 to 1.21 nm by grafting PMA on the GO surface. To fabricate transparent conductive films (TCFs), GOs must be reduced to graphene nanosheets (GNS) by a two-step chemical reduction with increased conductivity. The intercalated polymer chains of poly(acrylic acid) between GNS were extended as the carboxylic acid groups were deprotonated by the Na+ ions of NaBH4 on reduction, which efficiently inhibits GNS aggregation and restacking. The Na+ bonding on the polymer chains also facilitates electron transfer between the layers, yielding lower surface electrical resistance at the same GNS film thickness. The PMA grafted GNS (NE-PMA-GNS) composite films show the lowest sheet resistance of 2.11 × 102 Ω □−1, which is one order of magnitude less than that without grafting polymer (NE-GNS, 1.86 × 103 Ω □−1); moreover, instead of 0.22, the ratio of DC conductivity to optical conductivity (σDC/σOP) was 2.60. The higher σDC/σOP ratio indicates a higher TCFs performance.

Self-assembly of graphene onto electrospun polyamide 66 nanofibers as transparent conductive thin films

A simple method was developed to assemble graphite oxide (GO) densely onto electrospun (ES) polyamide 66 (PA66) nanofibrous membranes, used as a guide for the deposition of graphene nanosheet (GNS) conductive networks for preparing transparent conductive thin film (TCF). The main advantage of this technique by comparison with previous methods is that graphene does not form a uniform coating, but a percolated conductive network, when guided by PA66 nanofiber templates. A low surface coverage of the transparent substrate by GNS resulted in high transmittance. Polyvinylpyrrolidone-stabilized GO (PVP-GO) was prepared as a modifier for improving the adsorption to the nanofibers. The resulting PVP-GO material could adsorb well on PA66 nanofibers due to stronger hydrogen bonds. Hence, a lower sufficient concentration of PVP-GO (0.050 wt%) solution was required than that for GO solution (0.100 wt%) to fabricate a complete conductive path through a possible enriched adsorption process. For TCF applications, a reduction step is essential because as-deposited GO is non-conductive. In this work, we reduced GO to GNS by a combination of chemical reduction and thermal annealing. The TCF optical transmittance also could be improved after thermal annealing at 350 °C above the PA66 melting point. Light scattering by PA66 nanofibers was found as the main cause of reduced transmittance. A fused film, obtained after electrospinning PA66 solution for 120 s, and immersing in 0.050 wt% PVP-GO solution, exhibits a surface resistance of 8.6 × 10³ Ω/square, while maintaining 88% light transmittance.

Fabrication of a silver nanowire-reduced graphene oxide-based electrochemical biosensor and its enhanced sensitivity in the simultaneous determination of ascorbic acid, dopamine, and uric acid

Silver nanowire/reduced graphene oxide nanocomposites (AgNW/rGO) are synthesized using a two-step process: preparation of silver nanowire/graphene oxide (AgNW/GO) and the microwave-assisted hydrothermal (MAH) method. The nanocomposites are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) analyses. Ascorbic acid (AA), dopamine (DA), and uric acid (UA) are determined simultaneously on the AgNW/rGO-modified screen-printed carbon electrodes (SPCEs) by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The results reveal that AgNW/rGO-modified SPCEs exhibit well-resolved oxidation peaks with a negative shift in peak potential and enhanced peak currents in the simultaneous determination of AA, DA and UA in comparison with the pure rGO-modified SPCEs, demonstrating the superior catalytic activity of AgNW/rGO composites. AgNW/rGO-modified SPCEs show the linear response of AA, DA and UA in the concentration range of 45–1550, 40–450 and 35–300 μM with a detection limit of 0.81, 0.26 and 0.30 μM (S/N = 3), respectively. The covalent bonds between AgNWs and rGOs are expected to suppress the random attachment of AgNWs and facilitate the electron transfer and reactant transport by constructing a porous and continuous electrically conductive network. The excellent sensitivity of AgNW/rGO composites makes them become promising electrode materials in the field of electrochemical biosensors.

Preparation of transparent, conductive films by graphene nanosheet deposition on hydrophilic or hydrophobic surfaces through control of the pH value

This study used sodium borohydride to reduce graphene oxide to graphene nanosheets (GNS), which contain the carboxylic functional group that becomes carboxyl (–COOH) or carboxylate anion (–COO−)-type in the acid or alkaline environment, respectively. The GNS with didodecyldimethylammonium bromide (DDAB) particles becomes hydrophilic (A-GNS) or hydrophobic (B-GNS) in property, through control of the pH value, which can be dispersed efficiently in water or a water/THF medium to deposit on the hydrophilic (poly(acrylic acid-acryl amide)) or hydrophobic (polystyrene) substrate for preparing the transparent, conductive film (TCF) by spin coating. The DDAB particles can be removed by washing with nitric acid, and the optimal performances of the TCFs are then obtained (HA-GNS and HB-GNS). The surface electrical resistance of HA-GNS (1.5 × 103 Ω □−1 at transmittance of 82%) is similar to that of HB-GNS (2.1 × 103 Ω □−1 at transmittance of 81%), which demonstrated that, not only the hydrophilic, but also the hydrophobic surface can be chosen to prepare the TCF when the hydrophilic and hydrophobic GNS can be prepared.

Graphene nanosheets deposited on polyurethane films by self-assembly for preparing transparent, conductive films

This study prepared graphene nanosheet (GNS)-based transparent, conductive films (TCFs) by a self-assembly method. We used the water-borne polyurethane (WPU, with sulfonate functional groups) film as the substrate. Aggregation and restacking of the GNS were inhibited efficiently by attracting the octadecyl trimethyl ammonium chloride surfactant (cationic surfactant) to the surface of the GNS (GNS-O), which can in turn attract sulfonate groups to the WPU surface. The GNS-O was deposited on WPU to form TCFs. We obtained highly transparent and electrically conductive thin films after treatment with nitric acid (GNS-OA). The GNS-OA composite films showed a maximum sheet electrical resistance of 1.5 × 103 Ω □−1, with a light transmittance of up to 79% and a ratio of DC conductivity to optical conductivity of 0.88.