Surfactant-assisted water-based graphene conductive inks for flexible electronic applications
Graphical abstract
Introduction
In recent years, there has been an increasing scientific interest in flexible and wearable printed electronics for the fabrication of wireless sensors for Internet of Things (IoT) technologies [1]. Printing techniques are employed to transfer an ink onto the substrate. The printing technology helps in the generation of tailored patterns of electrical properties on different substrates, such as polymers, silicon, textiles, and paper. Particularly, the use of polymer substrate facilitates the development of wearable electronic devices that are flexible and stretchable [2], [3], [4], [5]. In this regard, inkjet printing has attracted considerable attention owing to its applicability in the fabrication of flexible electronic devices [6,10].
Studies on different types of conductive inks and fillers for the fabrication of printed conductive patterns have been conducted. However, most of these studies heavily rely on carbon-based materials, such as graphene and carbon nanotubes [7,8]. Particularly, there are several reports on the successful development of graphene conductive inks and their application in printed electronics. In the fabrication process of graphene conductive inks, solvents are important to ensure suitable viscosity for ink printing. However, the choice of solvent for ink production is mainly dependent on the type of device used and the nature of the printing film. To date, various types of solvents, such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), ethanol, acetone, terpineol, and xylene, have been used to prepare graphene conductive inks [9]. However, some of these solvents, such as NMP and DMF, are harmful, have a high boiling point, and are not environmentally friendly. Conversely, an undesirable surface tension is often generated when graphene is dispersed in solvents with low boiling points, such as ethanol, acetone, and isopropanol.
Based on the importance of environmentally friendly conductive ink for humans and the environment, research trend is moving toward the minimization of the use of organic solvents. Therefore, the possibility of replacing these solvents with eco-friendly alternatives in the formulation of conductive inks is currently being widely investigated. In fact, the choice of suitable environmentally sustainable solvents has currently become an important point of focus. The most preferred solvent is water owing to its non-toxic nature and suitable boiling point. However, it has a high surface tension of 72.8 mJm−2 [17]. Moreover, the dispersion of graphene in water is difficult due to the hydrophobic character of graphite carbon. Therefore, surfactants are commonly utilized to improve graphene dispersion in graphene inks through van der Waals force, hydrogen bonding, electrostatic activity, and π–π interactions. Several surfactants have been incorporated in graphene suspensions, including ionic, non-ionic, and polymeric types [11]. Table 1 presents some of the previous studies on graphene conductive inks using various printing techniques. In the past, numerous researchers have studied graphene conductive inks using different kinds of solvent and water. In the table, some of the solvents (NMP and DMF) are toxic and not suitable for future printed flexible/wearable electronic applications. Moreover, the sheet resistance of a water-based ink is considered to be too high. Hence, a high (100°C–400°C) annealing temperature is often required to reduce the sheet resistance. Unfortunately, such a high annealing temperature is damaging for flexible polymer substrates. Because of this, environmentally friendly solvents that produce lower sheet resistance and require lower temperature for annealing conductive tracks are more desirable. An extensive review of several literatures revealed that only limited studies have been conducted to evaluate the performance of water-based graphene conductive inks.
Therefore, in this contribution, a comparison analysis of the properties of water-based and surfactant-assisted water-based graphene conductive inks is conducted. Different types of surfactants (sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP), and Gum Arabic (GA)) were utilized to facilitate the dispersion of graphene. Moreover, the conductive inks were printed through inkjet printing on polyethylene terephthalate (PET) substrate. The dispersion of graphene in the PVP surfactant conductive inks, the morphology, and the mechanical performance of the graphene conductive pattern were also investigated.
Section snippets
Materials
The important chemicals utilized in this study are SDS, PVP, and GA, which were supplied by Sigma-Aldrich. A DuPont Mylar A PET with a 125 μm thickness was used as the flexible substrate. The substrate has an opaque white appearance and used for inkjet printing.
Methods
An electrochemical exfoliation technique was employed to synthesize the graphene used in this study as previously reported [19]. The water-based graphene conductive inks were prepared by adding 0.5 wt% of graphene and 0.1 wt% of the
Results and discussion
The chemical structure and possible changes in the functional groups of the water-based graphene conductive inks with or without surfactants were investigated via FTIR analysis. The FTIR spectra of the water-based and surfactant-assisted water-based graphene conductive inks are presented in Fig. 2. The notable peak in the spectra of the water-based inks as presented in the figure is the broad and intense stretching vibration peak of the O–H bond at 3414 cm−1, which is due to the retained
Conclusion
In this research, graphene conductive inks were fabricated using deionized (DI) water with different types of surfactants (SDS, PVP, and GA). The stability, wettability, and electrical conductivity of DI/PVP-based graphene conductive inks were significantly higher than those of SDS- and GA-based graphene conductive inks. Specifically, the stability of DI/PVP-based graphene conductive inks resulted in high conductivity, which was sustained even up to about 1 month following the initial
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors appreciate Universiti Sains Malaysia for providing the USM fellowship to the first author. We are also thankful to the Malaysia Ministry of Higher Education for the financial assistance through Research University Grant (grant no. 8014044). We would also like to thank the School of Materials and Mineral Resources Engineering, USM for the facilities.
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