###### Abstract  
Electrical and optical properties of graphene/silver nanoparticles hybrid suspensions intended for use in inkjet printing technologies were studied. Few-layered graphene particles were manufactured via a direct ultrasonic-assisted liquid-phase exfoliation route in water/surfactant system, whereas silver nanoparticles were synthetized using a polyol process. Hybrid suspensions for graphene/silver nanoparticles mixtures showed significant reduction in mean particle size while electrical conductivity remained almost intact even after thorough centrifugation. Structuring effects in mixed colloids were very pronounced as both electrical conductivity and optical transmission showed maxima at 65 wt.% graphene. Suspensions with conductivities above 300 \(\mu\)Sm/cm, much higher than previously reported, were obtained, and resulted in the manufacturing of films with less than 10% optical absorption throughout the visible region. These samples did not demonstrate absorption peaks attributed to silver nanoparticles' surface plasmon resonance, which is suitable for transparent electrode applications. Suspension properties at optimal composition (65 wt.% graphene) are very promising for printed electronics as well as transparent conductive coating applications. In the paper, we establish that the optimal suspension composition matches that of the film; therefore, more attention should be paid to carefully studying electrically conductive suspensions. graphene; silver nanoparticles; inkjet printing; transparent electrodes; electrical conductivity; optical absorption; centrifugation +
Footnote †: journal: Applied Sciences  
However, it is almost impossible to achieve a sufficiently high degree of reduction to minimize defects and oxygen-containing groups and provide high electrical conductivity of the particles. Direct liquid-phase exfoliation is a route that, at the cost of decreased yield of few-layered particles, does not lead to generation of defects in the graphene plane. Therefore, choosing highly crystalline graphite, such as natural graphite or highly oriented pyrolytic graphite (HOPG), enables one to obtain low-defect few-layered graphene particles directly in the form of stabilized suspensions. Numerous studies report N-methyl-2-pyrrolidone (NMP) [6; 7] or dimethylformamide (DMF) [8] as preferable media for exfoliation. The choice of an eco-friendly exfoliation medium should, however, be also taken into account; therefore, water- or alcohol-based systems are much more preferable. Several studies report water-surfactant systems as being suitable for high-yield exfoliation [9; 10]; the choice of surfactants is thoroughly treated in several studies as well [11; 12]. On the other hand, silver nanoparticles (AgNPs) have extremely low electrical resistance and sintering temperatures. Moreover, being metals, AgNPs do not suffer from high local contact resistance. Their main drawbacks are deterioration in air and sulfur-containing gases, as well as a relatively high price. Several techniques have been described to achieve monodisperse spherical AgNPs, including cryosynthesis [13] and plasma-induced synthesis [14]. Many applications require AgNPs of different morphologies, which is challenging to achieve when the process is thermodynamically-controlled; therefore, chemical routes that provide kinetic control of particle growth should also be considered. Soft chemical methods, especially polyol synthesis, liquid-phase reduction of silver salts in polyhydroxyl alcohols, are more promising and widely used due to the opportunity to achieve fine control of the synthesis parameters so that spherical NPs [15], polyhedral AgNPs [16], or silver nanowires (AgNWs) [17] can be obtained at high selectivity and high yield.