Control of Oxidation Level in Graphene Oxide by Annealing

PI: K. Andre Mkhoyan

Figure 1. (a) SEM and digital image (inset) of graphite and (b) AFM image of GO sheets [1], (c) the actual transistor, where channel consists of several GO sheets, (d) source-drain current as a function of gate voltage at different T [2].

Oxidized graphene, often known as graphene oxide (GO), is a nano-scale material that has all the essential ingredients for potentially revolutionizing our current technology. These GO sheets are only three atomic layers thick when they are fully oxidized and only one atomic layer thick when they are free of oxygen (i.e., graphene), making them the thinnest possible materials that can ever be made. In addition, a solubility of GO in water and other common solvents allows its uniform deposition onto different substrates (Si wafer, glass, plastic etc.) making GO a very attractive material for a variety of applications, from building new-generation circuit elements to elastic membranes (Fig. 1). From a purely scientific point of view, GO sheet is a unique system where a single atom of oxygen is bonded to carbon atoms in solid form which is ideal for the study of the very fundamentals of material formation: bonding of two individual yet different atom species.

The level of oxidation in GO films can be controlled in two ways: (i) at the initial stage of graphite oxidation, by adjusting the composition of chemicals in the modified Hummers method which results in GO flakes and (ii) by controlling the reduction of oxygen from highly oxidized GO by annealing or exposing the GO to hydrazine. Oxygen reduction by annealing is the most efficient and practical approach for obtaining GO with different oxidation levels. Our early electron microscopy study and ab initio DFT-based calculations of GO films showed that oxygen atoms during the oxidation process attach to graphene sites randomly and convert sp² carbon bonds in graphene to sp³ (Fig. 2a,b). The results show that the ratio of 1:5 O to C is sufficient to transform 40% of the original carbon bonds of graphene from sp² into sp³. As a direct consequence, the atomic structure of GO sheets is highly distorted. Now the two fundamental questions are - What does happen to these sp³ bonds when oxygen is removed? and, can we control the sp³ and sp² fractions of the carbon bonds by controlling oxygen detachment from carbon in GO?

Figure 2. (a) ADF-STEM image of GO sheets and (b) the atomic structure of GO determined by STEM-EELS, (c) a new-generation TEM heating holder for the atomic-resolution analysis (from Protochips, Inc.).

In this research I propose a detailed experimental exploration of the local atomic bonding between oxygen and carbon atoms in GO and the structural modification of these bonds (including bond breaking) as a function of annealing temperature. This will require measurements as fine as the direct imaging of a single oxygen atom and the structural changes in GO with temperature increase. EELS measurements from the O and C atoms will also be carried out. Electronic transitions in O and C atoms will provide a direct measurement of changes in electron energy levels and local electronic density-of-states for a single O and C atoms as a function of temperature. Finally, the results will be compared and correlated with electronic and mechanical structures of GO films calculated using ab initio DFT-based calculations. The results, when achieved, will be the first-ever direct experimental observation of the modifications of the bonds and bond breaking as a function of temperature at the single-atom-level, and it will be accomplished by direct imaging.

1. S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, and R.S. Ruoff, Graphene-based composite materials, Nature 442, 282 (2006).

2. G. Eda, G. Fanchini and M. Chhowalla, Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material, Nature Nanotech. 3, 270 (2008).