Summary of novel Joule heating procedure and record performance of the fabricated RGO film.
Summary of novel Joule heating procedure and record performance of the fabricated RGO film.

Heating reduced graphene oxide (RGO) films to high temperatures boosts conductivity and mobility to a record high, report researchers from the University of Maryland [Wang et al., Materials Today (2017), doi: 10.1016/j.mattod.2017.10.008].

Graphene’s remarkable properties have prompted research and efforts to find application for the material in many areas. It can be produced in three distinct ways: mechanical exfoliation, chemical vapor deposition (CVD), and chemical functionalization. But since the yield of mechanical exfoliation is extremely low and the cost of CVD is high, chemical functionalization has become the most widely used method to produce graphene.

Typically, chemical functionalization produces a synthetic precursor of graphene known as graphene oxide (GO) – a single monolayer of graphite with various oxygen-containing functional groups. Reduced GO (RGO), which is similar to pristine graphene, can be produced by thermal, chemical, electrochemical or photochemical reduction of GO.

“RGO is similar to graphene in terms of structure and electronic properties [but its] previously reported conductivity and mobility values were small, which limits applications in many fields,” explains Liangbing Hu.

Now, however, Hu and his colleagues have found that a two-step thermal reduction process can transform the conductivity and mobility of RGO opening up new potential applications.

“We obtained a reduced graphene oxide (RGO) film with record high conductivity of 6300 S cm-1 and mobility of 320 cm2 V-1 s-1,” he told Materials Today.

The boost to RGO’s properties is the result of an improved Joule heating procedure in which a freestanding GO film is first gradually heated to 1000 K and held at that temperature for an hour in an argon-filled furnace. The 1000 K-reduced RGO film is then suspended in a vacuum chamber above a substrate and heated a second time to 3000 K.

“The two-step reduction procedure with a curved film is novel, which is the core for achieving a stable 3000 K Joule heating reduction and leading to the record-performance RGO film,” says Hu.

The researchers believe that the high-temperature thermal reduction process serves to drive off oxygen-containing functional groups and other impurities, which tend to trap or scatter charge carriers traveling through graphitic networks. The 3000 K-reduced RGO films also show larger sp2 domains. The combination of these changes to the physical nature of RGO films appears to give rise to the greatly improved conductivity and mobility values observed by the team.

“RGO films with such high conductivity and mobility can be used in many fields,” adds Hu, “and our approach for fabricating RGO films is very practical and reliable.”