# Conductivity Enhancement of Graphene and Graphene Derivatives by Silver Nanoparticles
## 1 Introduction  
The development of compatible materials and inks and their use in appropriate printing methods represents an emerging methodology to produce novel, large area, highly flexible printed electronic devices [1]. To date, printed electronics demand new classes of materials and multiple types of inks that are highly attractive for potential applications, including photovoltaics, batteries, antennas, sensors, biosensors, etc. [2; 3; 4; 5; 6; 7; 8]. The production of conductive inks suitable for flexography, gravure, screen, or inkjet printing on several substrates is of high importance for the industry. Thus far, Ag nano-ink is the most popular conductive ink used in functional printing due to its excellent electrical conductivity and strong antioxidant characteristics [9]. The main drawbacks to the use of nanosilver are the high cost of the raw material and the electromigration that deteriorates its potential uses. Copper is also often used for conductive inks due to its high conductivity; however, copper suffers from oxidation [10]. Recently, carbon-based nanocomposites have attracted much interest in printing technology for the development of novel, low-cost, stable, and effective conductive inks. In this regard, various carbon nanoallotropes such as carbon nanotubes, graphene, and their derivatives have been examined, affording a plethora of potential products [11; 12; 13; 10]. Graphene is a single-atom thick sheet of carbon atoms arrayed in a honeycomb pattern and the world's thinnest, strongest, and stiffest material [14]. Because of its remarkable inherent carrier mobility and unique carrier transport properties, graphene is expected to be an excellent conductive pigment [15; 16; 17]. On the other hand, graphene is nowadays produced at a low cost from an abundant and natural raw material such as graphite witheffective and scalable procedures. The most widely used graphene precursors are reduced graphene oxide (rGO) and the few layered graphene nanosheets (FLG) that are produced by the liquid-phase exfoliation of graphite in several liquids or water solutions [18]. The latter has much higher electrical conductivity, but it is produced with a much lower yield compared to rGO, which is produced from the treatment of graphite with strong oxidants and the reduction of the as-formed graphene oxide (GO). Furthermore, FLGs are only rarely dispersed in specific solvents, and thus their incorporation into industrial inks has a lot of difficulties, such as low yield and inhomogeneity. rGO has been used as an alternative conductive pigment. The dispersibility of rGO is also very low, similar to FLG. In several cases, reduction of GO is followed by the functionalization of rGO with hydrophilic groups, leading to finally water-dispersible graphene derivatives. Although functionalized rGO (\(f\)-rGO) has lower conductivity compared to FLG, it outcompetes pristine graphene due to the great advantage of its high dispersibility. Nonetheless, the conductivity of pure graphene ink is low for most applications and not comparable with that of metals, restricting its extensive use. Several efforts have been devoted to increase the electrical conductivity of graphene inks. Among other things, a graphene/carbon nanotube hybrid has been developed that can be easily mixed with hydrophilic polymers and resins to produce highly conductive water-based nanocomposites suitable for gravure printing [11]. The development of a gravure ink based on a water dispersible and highly conductive \(f\)-rGO starting from GO and using 2,4-diamino benzene sulfonic acid (2,4-DBSA) as a reducing agent and functionalization agent has recently been presented by [PERSON] et al. [19]. Another interesting approach to increasing the electrical conductivity of graphene inks is to decorate graphene nanosheets with various metal nanostructures.