Enhanced electrical, optical and chemical properties of graphene oxide through a novel phase transformation
Graphene oxide (GO) is a versatile,
solution-processable candidate material for next-generation,
large-area, ultrathin electronics, optoelectronics, energy
conversion and storage technologies. GO is an atom-thick sheet of
carbon functionalized with several oxygen-containing groups
dominated by the epoxy and hydroxyl functional groups on the basal
plane, with carboxyls and lactols at the sheet edges. It is well
known that reduction of GO at temperatures > 150°C leads to the
removal of oxygen atoms from the carbon plane, leading to the
formation of reduced GO (rGO) structures. Although GO has been
utilized for multiple applications in the last decade, our
understanding of the structure-property relationships at the
atomic-level has still been lacking owing to the amorphous nature
and chemical inhomogeneity of GO, which has in turn limited our
ability to design and tailor GO nanostructures for high-performance
applications. In particular, the material's structure and its
structural evolution at mild annealing temperatures (< 1000°C)
has been largely unexplored. In this thesis, we use a combination
of first-principles computations, classical molecular dynamics
simulations based on reactive force fields and experiments to model
realistic GO structures and develop a detailed understanding of the
relationship between the carbon-oxygen framework and the sheet
properties, at the atomic level. Based on our understanding, we
demonstrate a new phase transformation in GO sheets at mild
annealing temperatures (50-80°C), where the oxygen content is
preserved and as-synthesized GO structures undergo a phase
separation into prominent oxidized and graphitic domains
facilitated by oxygen diffusion. Consequently, as-synthesized GO
that absorbs mainly in the ultraviolet region becomes strongly
absorbing in the visible region, photoluminescence is blue shifted
and electronic conductivity increases by up to four orders of
magnitude. We then use this novel phase transformation to improve
two sets of applications. 1) We demonstrate that cell capture
devices making use of phase transformed-GO substrates have higher
capture efficiencies compared to devices making use of
as-synthesized GO substrates. 2) We show that the reduction of
phase transformed-GO leads to better electrical properties of rGO
thin films. Our results fill an important gap and establish a
complete theory for structural evolution of GO over the entire
range of temperatures, i.e. from room temperature to ~1000°C. Taken
together, this structural transition in GO enables us to predict
and control the sheet properties in new ways, as opposed to
reduction, which is till date the only handle to control the
structure of GO. This could potentially open the door for
completely new applications or for enhancing the performance of
existing applications based on GO.