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Dual path mechanism in the thermal reduction of graphene oxide

A cost-effective and up-scalable route for the production of monolayer or few-layer graphene is the exfoliation of oxidized graphite to obtain graphene oxide (GO) patches, which can be then thermally reduced.Progressive elimination of oxygen from GO, which is an insulating material,  results in a gradual increase of the electronic mobility. However the transport properties of the thermally reduced GO are far from being comparable with those of the ideal dopant-free graphene as the sp2 conjugation is only partially retrieved because of the generation of topological defects and C vacancies in the graphene  lattice. Understanding the mechanisms of the thermal reactions is essential for defining  alternative routes able to limit the density of defects generated by carbon evolution.
The thermal reduction of GO has been the subject of a detailed study based on the experiments carried out at the SuperESCA beamline of Elettra by the research groups of CNR-ISC (Italy), University of Trieste (Italy) and Sincrotrone Trieste and on the theoretical modelling developed by the CNR-IOM Theory@Elettra group (Italy). A graphene monolayer grown on Ir(111) has been oxidized by oxygen atoms and high resolution fast XPS and thermal programmed desorption (TPD) together with  numerical modelling based on Density Functional Theory (DFT) were used to probe the species residing on the surface and those released in the gas phase during heating and to identify reaction pathways and rate-limiting steps.
Oxygen atoms preferably adsorb on intact, defect free graphene in bridge position over the C-C bonds forming epoxy structures without breaking the hexagonal network. At low oxygen coverage the  O1s spectrum (Fig.1a) shows only the contribution of the epoxy O atoms bonded to C atoms belonging to the graphene regions quasi free standing (O1 ) and weakly interacting (O2) with the Ir substrate. The TPD spectra measured during desorption (Fig1b) show that the only active reduction pathway is the recombination of two oxygen atoms into an O2 molecule which desorbs around 400-550 K, in agreement with the calculated activation energy of 1.13 eV, whereas CO and CO2 remain below the TPD detection limit. This proves that O atoms can be reversibly desorbed from the basal plane of perfect graphene layers without forming vacancies  in the C network. (Fig.1c).
The O1s spectrum measured on strongly oxidized graphene (Fig.2a) shows that epoxy groups (O1 and O2) are  accompanied by quinones (O3) and ethers (O4), the latter component becoming evident after the desorption of the O1 peak. In this case  during thermal evolution 10-15% of C lattice atoms are lost and indeed the TPD curves show that  O2 as well as CO/ CO2 mixtures are released in the gas phase between 350 and 530 K (Fig2b). The low temperature erosion of the graphene network, which for  C-based materials oxidized by molecular oxygen occurs around  600-670 K, indicates that the anticipated loss of C atoms is catalyzed by the reservoir of the epoxy O atoms diffusing on the basal plane. Calculations show that the relevant precursor for GO gasification are ether-lactone pairs (see Fig.2c) that form upon the interaction of diffusing epoxides with ether-epoxy pairs and dissociate releasing CO/CO2 mixtures with an activation barrier of only 1.1 eV. These results  identify a dual path mechanism in the thermal reduction of graphene oxide: at low surface density the O atoms adsorbed as epoxy groups evolve as O2 leaving the C network unmodified. At higher coverage the formation of other O-containing species opens competing reaction channels which consume the C backbone. These latter reactions are favoured in the presence of defects and become thus dominating for chemically synthesized GO.  The new scenario disclosed by this study points out that the key factor controlling the onset of lattice damage in GO is the surface density of epoxy species. To move forward, novel chemical strategies need to be found to scavenge the epoxy oxygens before they start to diffuse on the surface and trigger the extensive disruption of the graphene network.

Figure 1 (a) top: O1s core level spectrum measured on graphene/Ir(111) with an oxygen coverage of 0.03 ML;  bottom: 2D plot of the O1s spectral intensity vs. temperature. (b) TPD curves measured during annealing of the sample at a rate of 2 K/s. (c) Scheme of the cycloaddition reaction with O2 desorption

 

Figure 2 (a)  O1s  core level spectrum measured on graphene/Ir(111) with an oxygen coverage q=0.25 ML. In the O1s spectrum the weak component O3 indicates C=O double bonds, whereas the component O4 due to ethers cannot be disentangled from the intense O1 peak and becomes visible only above 350 K after epoxy desorption; bottom: 2D plot of the O1s spectral intensity vs. temperature. (b) TPD curves measured during the thermal annealing at a rate of 2 K/s. (c) Formation of the lactone-ether pairs which decompose releasing CO and CO2.

This research was conducted by the following team:

  • Rosanna  Larciprete, CNR-Istituto dei Sistemi Complessi, Roma, Italy
  • Stefano Fabris, Tao Sun, CNR-IOM DEMOCRITOS  and SISSA, Trieste, Italy
  • Paolo Lacovig, Silvano Lizzit, Sincrotrone Trieste S.C.p.A., Trieste, Italy.
  • Alessandro Baraldi, Physics Department, University of Trieste, Trieste, Italy

Reference

Rosanna  Larciprete, Stefano Fabris, Tao Sun, Paolo Lacovig, Alessandro Baraldi and Silvano Lizzit “Dual path mechanism in the thermal reduction of graphene oxide”,  J. Am. Chem. Soc. 133, 17315 (2011), doi: 10.1021/ja205168x
Last Updated on Monday, 16 April 2012 13:59