Univ. of Warwick. |
Graphene oxide has had many of researchers fall upon it as
it retains much of the properties of pure graphene, but it is much easier and
cheaper to make in bulk quantities; easier to process; and its significant
oxygen content appears to make it soluble in water. However new research led by
Univ. of Warwick Chemist Dr Jonathan P. Rourke
and Physicist Univ. of Warwick Physicist Dr Neil Wilson, has found that that last
assumption is incorrect. Graphene oxide’s solubility literally comes out in the
wash.
Drs Rourke and Wilson’s
team made their discovery when treating the graphene oxide with sodium
hydroxide (NaOH) in an attempt to increase the usefulness of the oxygen
containing functional groups believed to be bound to the graphene.
Unfortunately it seemed to make things worse rather than better. At high enough
concentrations of NaOH Dr Rourke was left with a black suspension.
The Warwick
led researchers recalled that it had been shown that oxidation debris adheres
to carbon nanotubes but the weak nature of the connection of this oxidation
debris to the carbon nanotubes meant that a wash with a base can remove the
oxidative debris. Experiments showed that in that particular case oxidative
debris was found to make up almost a quarter of the mass of the “oxidized
carbon nanotubes”. The researchers felt a similar process maybe happening in
the graphene oxide they were studying.
The results may also help explain the inordinately high
levels of oxygen people were claiming to find in graphene oxide. Chemists were
already struggling to identify enough plausible carbon to oxygen bonds to
accommodate the amounts of oxygen believed to form part of graphene oxide.
Univ. of Warwick. |
On centrifuging the black liquid the Warwick team was left with a pile of black
powder that turned out to be graphene oxide that may once have been soluble
before the application of the base but which refused to show any sign of being
soluble again in its current state. The black material was found to be similar
to graphene itself; in particular it was shown to consist of very large sheets
of electrically conducting carbon atoms, unlike the insulating graphene oxide.
The remaining liquid was also dried to give a white powder
that the Warwick
researchers showed contained the oxidative debris (OD). The OD was shown to be made
up of small, low molecular weight compounds (i.e. less than 100 atoms).
The graphene oxide recovered from washing process formed
about 64% of the mass of the graphene oxide at the start of the process. The
recovered OD or oxidative debris formed at least 30% of the weight of the mass
of the original graphene oxide.
Drs Rourke and Wilson’s team believe this shows that much of
the oxygen that was believed to be closely bonded to the carbon in the graphene
oxide was actually not bonded at all but simply lying on top of the graphene
sheets, loosely connected to them as oxidative debris. This oxidative debris
contained a large quantity of oxygen that simply came out in the wash when the
graphene oxide was treated with sodium hydroxide.
This creates a problem for researchers
depending on an easily soluble form of graphene oxide as the level of
solubility found so far was directly dependent on the high quantities of oxygen
believed to be bound to the carbon in the graphene oxide. If much of that
oxygen so easily falls away, so will the levels of solubility.
Drs Rourke and Wilson say “Our
results suggest that models for the structure of graphene oxide need
revisiting. These results have important implications for the synthesis and
application of chemically modified graphene particularly where direct covalent functionalization
of the graphene lattice is required.”
