May Day Graphene
20 August 2014
In view of the success of the Graphene-themed Christmas meeting at Manchester, it was felt that a follow-up in London would be generally welcomed. It was held on 1 May 2014 at the RSC's headquarters at Burlington House and had another good attendance, about 40 in all. One major difference from the Manchester meeting was a poster session that proved very popular with 13 on display during the tea and AGM break.
The first invited lecture was given by Prof Nicole Grobert of the University of Oxford, a past vice-Chairman of the BCG. She addressed the problem of growing relative large domains of graphene under controlled conditions. As she pointed out, there was considerable industrial interest in this, most methods concentrating on exfoliation or Controlled Vapour Deposition (CVD) - these methods were also featured in later presentations at the meeting. Chemically exfoliated graphene showed a range of sheets in a rather random 3D arrangement, while CVD gives better quality material and gives the possibility of easier scale-up. HRTEM (high-resolution transmission electron microscopy) shows a very uniform defect-free surface. Prof Grobert pointed out that one source of defects was a sort of ripple effect that does show up in HRTEM and arises from atomic irregularities on the surface of the substrate on which the graphene was formed during CVD process. The position of the substrate in the furnace, whether it is near the on-coming gas flow or in the middle, also affects the quality of the graphene product. Her group had experimented with Pt/SiO2 substrates. Si atoms become incorporated in the silica and smooth out the surface. They had made good quality graphene of up to 1mm size using this technique. Removing the graphene from the substrate still tended to show irregularities because of atomic asperities in the latter.
Chris Howard (UCL) talked about manipulating graphene by chemical doping, effectively creating hole and electron defects by adding atoms and molecules. He started from the other end of carbon scale, with graphite and specifically, the intercalation compound C8K. He looked at similarities between that and potassium intercalated between two or more graphene layers. Raman spectroscopy was a very useful technique for following changes consequent on K-doping. The potassium was added in vapour form in situ and the progress of intercalation followed by observing the decrease of G band intensity at ~1600cm-1 and the growth of 2D bands at ~2700cm-1 .The phenomena were closely similar up to 4 graphene layers.
He then talked about doping graphene in a liquid medium. Sonication of graphite was known to give graphene in low yields. The extension to using the technique to exfoliate doped graphite seemed worth exploring. C60 and SWNTs can be dissolved in a solution of K in ammonia, replacing it with THF (tetrahydrofuran), it was then sonicated to give a solution of doped graphene sheets. These could then be functionalised or electrodeposited.
After a tea break, Tom Trevethan (Surrey) showed some fascinating videos of simulated random walk migrations of vacancies in graphene, made by irradiation. There are obviously applications of this work to neutron irradiation of graphite, where apparently the defects are confined initially to the outmost graphene layers. In graphen, carbon atoms are removed by the radiation and the resulting bivacancy can then move to form larger vacancies with other defect sites. Activation energies for the migration process were calculated. In the larger defects, where adjacent 5- and 7- membered rings are formed, the neighbouring carbon atoms are in a strained state, some being under compression and others under tension.
Tom Proctor (Salford) then described some work he had been involved with where graphene was hydrogenated under pressure. Graphene can be directly hydrogenated with hydrogen atoms, disrupting the conductivity and forming local band gaps. Ultimately, the fully hydrogenated form is graphene, which can exist in boat and chair conformations. His work attempted to use molecular hydrogen for the hydrogenation process. Both high pressure, up to 200 GPa and high temperatures, up to 200°C were needed to do this and they were collecting enough data to be able to draw up a phase diagram of the area of stability. The progress of hydrogen was again followed by Raman spectroscopy using a special diamond anvil cell. The D bands at 1300cm-1 and the G bands at 1600cm-1 were particularly useful as their relative intensity depended upon the ratio of sp3/sp2 Carbon atoms in the partially hydrogenated graphene sheet.
The final paper, given by Helen Thomas (Warwick), was more chemical in nature, appropriate as Malcolm Heggie pointed out to the RSC HQ. The group at Warwick had previously shown that graphene oxide (GO), as commonly prepared from graphite, was actually a mixture and highly oxygenated part could be removed by washing with a base to leave a black flaky insoluble material on which they had been working. Helen described the properties of this material, which the nmr spectra show keeps a graphene-based structure. The oxygen that it contains is in an epoxy form, which can be opened with suitable sulphur-containing reactants to make a new compound that they labelled GOSH. The hydrogen atom in GOSH can be etherated to make for example a –GOSBu thioether. This has a special affinity for gold nanoparticles that can be used to characterise it in TEM.
Helen's paper was originally submitted as a poster and as that contains more information than the brief account here.
Vice-Chair, British Carbon Group