Transferable Graphene Oxide Films with Tunable Microstructures

Abstract

This report describes methods to produce large-area films of graphene oxide from aqueous suspensions using electrophoretic deposition. By selecting the appropriate suspension pH and deposition voltage, films of the negatively charged graphene oxide sheets can be produced with either a smooth "rug" microstructure on the anode or a porous "brick" microstructure on the cathode. Cathodic deposition occurs in the low pH suspension with the application of a relatively high voltage, which facilitates a gradual change in the colloids' charge from negative to positive as they adsorb protons released by the electrolysis of water. The shift in the colloids' charge also gives rise to the brick microstructure, as the concurrent decrease in electrostatic repulsion between graphene oxide sheets results in the formation of multilayered aggregates (the "bricks"). Measurements of water contact angle revealed the brick films (79°) to be more hydrophobic than the rug films (41°), a difference we attribute primarily to the distinct microstructures. Finally, we describe a sacrificial layer technique to make these graphene oxide films free-standing, which would enable them to be placed on arbitrary substrates.

Electrophoretic deposition of graphene-related materials: A review of the fundamentals

“…A reduction process in principle occurs on the cathode [58]. However, as seen in the example above, the 'reduction' of GO has also been observed to occur on the anode surface during anodic EPD processes, leading to production of reduced GO films [9,40,49,79,136,137]. The extent of reduction does not depend on deposition time [49], but is mainly controlled by voltage.…”

“…For both acidic and basic conditions, GO exhibits net negative surface charge when suspended in water. In order to obtain a stable suspension, a net zeta potential higher than about -30 mV is usually required which can be obtained across a broad range of pH for GO [15,16,79]. Recently, ultrasonicated ozonemodified and electrochemically-exfoliated graphene oxide has been proposed as a route for preparing stable GRM suspensions suitable for EPD [80].…”

“…For example, GO dispersions are generally stabilised electrostatically by the charge remaining on the conjugate base of ionised acidic surface oxides; during EPD, water electrolysis at the anode can lead to local acidification, (re)protonation of the charged groups and a loss of stabilising repulsion [49]. Depending on the initial pH of the GO suspension (net initial charge of the GO sheets), deposit formation can be induced on either anode or the cathode, with both options attributed to charge neutralization mechanism [79]. In addition, other effects may also contribute specifically for GRMs, including the removal of oxide groups by GO 'reduction' processes that may happen at either anode or cathode (see Section 5).…”

“…Several studies have reported that EPD of GO at high voltages causes a simultaneous 'reduction' (more accurately, de-oxygenation) of GO during deposition [8,9,79]. This phenomenon may appear beneficial for the production of rGO films, since it potentially avoids the need for a post-EPD GO reduction process, and may prevent the passivation of the growing film due to the insulating nature of the GO.…”

“…In a study by Hasan et al, the microstructure of GO films was modulated by using acidic or basic conditions [79]. As illustrated in Figure 18, when basic conditions (pH=11.5) were used, GO sheets were deposited on the anode with a parallel alignment to the electrode's surface.…”