G-O is obtained by exfoliating graphite oxide. Reducing this material removes most of the oxygen-containing functional groups, producing a material that has a higher electrical conductivity than G-O itself. Thanks to its high flexibility and unique optical properties, rG-O thin films are promising for a variety of thin-film electronic and optoelectronic device applications.

Although several techniques exist for reducing G-O, most of them are not very environmentally friendly because they rely on toxic reducing agents. Others require high temperatures (greater than 1000 °C) and ultrahigh vacuum, incompatible with polymer substrates.

New and simple electrochemical process

Xiaohan Wang, Harry Chou and Iskandar Kholmanov of the University of Texas at Austin in the US and Rodney Ruoff of the IBS Center for Multidimensional Carbon Materials and the Ulsan National Institute of Science and Technology in Korea, have now developed a new and simple electrochemical process to simultaneously reduce and delaminate G-O films from metal substrates. In their technique, the researchers sandwich a thin layer of G-O sheets between a metal substrate and a polymer-supporting layer. The metal substrate plays a double role here as it also serves as the cathode in an electrolytic cell, explains Wang.

“During the electrochemical reaction, our G-O films begin to reduce from their edges,” he tells nanotechweb.org. “While this is going on, hydrogen bubbles produced by water electrolysis gently and continually delaminate the already reduced G-O films from the metal substrate, yielding rG-O films supported on the polymer layer.”

'Green' and straightforward

So how is this technique better than existing metal-based methods to reduce G-O? “For one, during the G-O electrochemical reduction and delamination processes, the metal substrate is preserved – either fully or nearly fully,” says Kholmanov. “This means that the substrate can be used over and over again to fabricate rG-O films – something that may significantly lower production costs compared with other methods. Secondly, our technique is ‘green’, straightforward and compatible with continuous production processes, such as roll-to-roll.”

The rG-O films thus produced were made into transparent conductive films, he adds, and the method we describe could be applied to make low-cost rG-O films for a wide range of practical applications – including coatings, protective layers, thin-film energy storage systems and membranes.

The researchers, reporting their work in ACS Nano DOI: 10.1021/acsnano.5b03814, say that their technique works well for making rG-O films less than 200 nm thick. For thicker films, however, inefficient charge transfer through the rG-O layer needs to be addressed, admits Kholmanov. “We could try solving this problem by creating channels through which charge would better flow. One possibility might be to use G-O/metal nanowire composites, for example, where the nanowires act as the charge transfer channels.”