E.F. Demarchi*; M.F. Oliveira; L.P.Gabriel; A.L.C. Pereira
School of Applied Sciences, University of Campinas (UNICAMP), Limeira, 13484-350, Brazil
Abstract
Recently, there has been a great interest in conductive graphene-based polymer composites, due to its demonstrated potential for several applications, like for advanced sensors, photovoltaic cells, energy storage, scaffolds for tissue engineering, etc. In this work, graphene oxide (GO) was synthesized by modified Hummers’ method, and then deposited by dip-coating over rotary jet spun polycaprolactone (PCL) fibers membranes. The thermal reduction of GO films and of the GO incorporated to the polymer membranes was obtained by annealing or in contact with a hot plate. Different time and temperature parameters for the GO reduction were compared and optimized. The effectiveness of the GO reduction was clearly verified by electrical conductivity measurements, using a four-point probe. The initially insulating produced PCL-GO membranes showed to become electrically conductive after the thermal treatment, with the resistance dropping up to 3 orders of magnitude.
Keywords: Graphene Oxide, Rotary jet spinning, Polycaprolactone, Conductive polymer composite, Thermal reduction.
1. Introduction
Graphene is a two-dimensional material, which has attracted enormous interest since its discovery in 2004 [1], due to its unique properties, including a large surface area and a great electronic mobility under ambient conditions [2].
New possible applications of graphene-based-materials in large scale have started through the chemical synthesis of graphene oxide (GO) and production of reduced graphene oxide (rGO) [3,4]. The graphene oxide is commonly produced via ‘top-down’ techniques, that uses graphite as the starting material, in which through an oxidation and ultrasonication processes, results in a dispersion of GO monolayers [3,5].
The GO is highly functionalized with oxygen-functional groups as hydroxyl, epoxy, carboxyl and carbonyl, and these groups provide ample stability to the GO sheets, allowing an efficient and uniform distribution within composites. However, the functional groups need to be removed to restore the electrical properties of graphene [6].
Polycaprolactone (PCL) and its composites with rGO have been studied, looking for optimized and scalable manufacturing routes, for many different applications, such as biosensors, supercapacitors, electronic inks for printing electronic flexible circuits and, biomedical engineering [7,8].
2. Materials and methods
2.1 Samples Preparation
Graphene oxide solution preparation: The GO solution was prepared by the modified Hummers’ method, a low cost chemical synthesis, which provides a high quality GO. In summary, the synthesis started with an oxidation reaction involving pyrolytic graphite, sulphuric acid, potassium permanganate and phosphoric acid. In sequence, hydrogen peroxide was added to the system to eliminate any remaining ions from the previous reaction. The resulting solid was then repeatedly washed and centrifuged to obtain a neutral pH (pH = 7) in
PCL membranes and GO incorporation: The PCL membranes were produced by rotary jet spinning technique, with a solution of PCL/Chloroform at a total concentration of 9% (wt/v), [10], and sequentially the GO was incorporated in the PCL membranes by dip-coating [4].
For the formation of thin films of GO, the GO solution was deposited onto a flexible and insulating substrate (transparency film sheet) by drop-casting [11].
2.2 Thermal reduction and electrical conductivity measurements
To restore the electrical conductivity of the GO, and obtain a conductive polymer composite, GO is thermally reduced, but under different techniques. The GO films were annealed using a vacuum oven, while the PCL-GO samples were reduced using a hot plate [3-6,11], in which the parameters used are shown in Table 1.
Table 1. Parameters used for the reduction of PCL-GO and GO samples Samples Temperature (°C) Time Reduction method
PCL-rGO1 225 5 min Hot Plate
PCL-rGO2 225 45 min Hot Plate
rGO1 180 96 h Oven
rGO2 195 96 h Oven
To evaluate the electrical conductivity of the reduced PCL-GO and GO samples, the four point probe method is used [5,11,12], to obtain the I-V and the resistance per time curves, in which a Keithley 6220 current source and a Keithley 2100 digital multimeter were used to gather the measured data.
3. Results and Discussion
The PCL-GO composites and the GO films were produced and its resistance have been measured, as shown in figure 1, in which it is seen that both materials have a poor electrical conductivity, behaving as insulators, presenting a very high resistance, of 1.10.1 MΩ for the GO, and 2.30.1 MΩ for the PCL-GO composite.
Figure 1: Resistance as a function of time measurements for PCL-GO and GO.
After thermally reducing the samples, the IV curves have been generated, as shown in figures 2 and 3, for PCL-GO and GO films, respectively.
Figure 2: IV curves for PCL-GO, before and after the thermal reduction. Electrical resistance of the sample before reduction is 2.3 M. After thermal treatment of the sample at 225°C in contact with a hot plate for 5 minutes and for 45 minutes, the resistance decreases to 123 k
and to 8.3 k, respectively.
For the PCL-GO composite, it can be observed that even for the reduction time of 5 minutes, it is possible to start reducing the GO, as observed by the clear decrease in the resistance, from 2.3 M before the thermal treatment to 123 k. Moreover, it is also observed a more significant reduction when increasing the time to 45 minutes and keeping the same temperature, 225°C, dropping the resistance to 8.3 kΩ.
Figure 2: IV curves for GO films, before and after the annealing and GO reduction.
Resistances obtained were 1.1 M before reduction, 10.7 k and 5.3 k after reduction at
It is already possible to observe in figure 3 the effect of annealing on the electrical conductivity for the GO films, in which the resistance dropped from 1.1 M to 10.7 k after annealing for 96 hours at 180°C. And keeping the same reduction time, 96h, but slightly increasing the temperature only 15 degrees, the resistance dropped by a half, demonstrating how effective the thermal reduction works, even for low temperatures and its small variation.
4. Conclusions
Graphene oxide was chemically synthesized by modified Hummers’ method, and PCL-GO composite was obtained, incorporating the GO in the PCL membranes by dip-coating. In parallel, GO films were produced by drop-casting onto a transparency film. The samples of PCL-GO and GO films have been thermally reduced using a hot plate and annealed on a vacuum oven, respectively. It was possible to observe a significant reduction of the samples, restoring the electrical conductivity, as demonstrated by dropping the resistance up to 3 orders of magnitude, measured through the four-point probe method. Small changes in temperatures demonstrated affecting strongly the GO reduction, while changing the reduction time is shown to affect the PCL-GO electrical behaviour, besides being seen that the reduction effectiveness starts at small times, demonstrating a high potential for several applications.
Acknowledgments
This project is being carried out with the support of CAPES (Coordination for the Improvement of Higher Education); Grant number: 88887.513359/2020-00
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