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Structure evolution of graphene oxide during thermally driven phase transformation: is the oxygen content really preserved?

Sun P, Wang Y, Liu H, Wang K, Wu D, Xu Z, Zhu H - PLoS ONE (2014)

Bottom Line: In this work, the structure evolution of GO with mild annealing is closely investigated.These results are further supported by the density functional theory based calculations.The results also show that the amount of chemically bonded oxygen atoms on graphene decreases gradually and we propose that the strongly physisorbed oxygen species constrained in the holes and vacancies on GO lattice might be responsible for the preserved oxygen content during the mild annealing procedure.

View Article: PubMed Central - PubMed

Affiliation: School of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Key Laboratory of Materials Processing Technology of MOE, Tsinghua University, Beijing, P. R. China.

ABSTRACT
A mild annealing procedure was recently proposed for the scalable enhancement of graphene oxide (GO) properties with the oxygen content preserved, which was demonstrated to be attributed to the thermally driven phase separation. In this work, the structure evolution of GO with mild annealing is closely investigated. It reveals that in addition to phase separation, the transformation of oxygen functionalities also occurs, which leads to the slight reduction of GO membranes and furthers the enhancement of GO properties. These results are further supported by the density functional theory based calculations. The results also show that the amount of chemically bonded oxygen atoms on graphene decreases gradually and we propose that the strongly physisorbed oxygen species constrained in the holes and vacancies on GO lattice might be responsible for the preserved oxygen content during the mild annealing procedure. The present experimental results and calculations indicate that both the diffusion and transformation of oxygen functional groups might play important roles in the scalable enhancement of GO properties.

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Formation energies and reaction barriers for proposed evolution paths (a–c) of oxygen containing functional groups on GO, calculated by DFT based calculations.
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pone-0111908-g004: Formation energies and reaction barriers for proposed evolution paths (a–c) of oxygen containing functional groups on GO, calculated by DFT based calculations.

Mentions: To explore the possible migration and transition paths of oxygen-rich functional in graphene oxide, we performed DFT based first-principles calculations. Elementary binary reactions among hydroxyl, epoxide and carbonyl species chemisorbed on graphene were considered, following a recent work reported by Zhou et al.[29] The GO structure is modeled as a 5×6 super-cell of graphene, functionalized by specific oxygen-rich groups. The atomic structures, their energy differences and reaction barriers were obtained here by DFT based calculations, as summarized in Fig. 4. The plane-wave code Quantum-Espresso was used here with an energy cutoff of 70 Ry. Norm-conserving pseudopotentials [30] was used for the core-valence electron interaction and the Perdew-Burke-Ernzerhof (PBE) parametrization of generalized gradient approximation (GGA) was implemented for the exchange-correlation functional [31]. These settings have been verified to achieve a total energy convergence for the systems under exploration below 1 meV/atom. The energy difference of a reaction ΔE = Eprod−Ereact was calculated from relaxed structures of the reactant and product, respectively. The activation reaction barrier Eb was then calculated using the nudged elastic band (NEB) technique. A 10×10×1 Monkhorst-Pack mesh grid was set for k-point sampling, while only Г-point sampling was used for the reaction path searching, to save the computational demand. The vacuum region in our super-cell approach was set to 1.2 nm in the direction perpendicular to the basal plane of graphene. The energy diagrams in Fig. 5 show that the reaction from the carbonyl and hydrogen pairs to hydroxyl pairs is exothermic with ΔE = −2.45 eV and Eb = 2.33 eV, while the reaction from a carbonyl pair to the epoxides is endothermic with ΔE = 1.07 eV and Eb = 1.90 eV. Notably, the formation of a carboxyl group from carbonyl and hydroxyl groups is exothermic with negligible ΔE = −0.013 eV and Eb = 0.31 eV.


Structure evolution of graphene oxide during thermally driven phase transformation: is the oxygen content really preserved?

Sun P, Wang Y, Liu H, Wang K, Wu D, Xu Z, Zhu H - PLoS ONE (2014)

Formation energies and reaction barriers for proposed evolution paths (a–c) of oxygen containing functional groups on GO, calculated by DFT based calculations.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4221183&req=5

pone-0111908-g004: Formation energies and reaction barriers for proposed evolution paths (a–c) of oxygen containing functional groups on GO, calculated by DFT based calculations.
Mentions: To explore the possible migration and transition paths of oxygen-rich functional in graphene oxide, we performed DFT based first-principles calculations. Elementary binary reactions among hydroxyl, epoxide and carbonyl species chemisorbed on graphene were considered, following a recent work reported by Zhou et al.[29] The GO structure is modeled as a 5×6 super-cell of graphene, functionalized by specific oxygen-rich groups. The atomic structures, their energy differences and reaction barriers were obtained here by DFT based calculations, as summarized in Fig. 4. The plane-wave code Quantum-Espresso was used here with an energy cutoff of 70 Ry. Norm-conserving pseudopotentials [30] was used for the core-valence electron interaction and the Perdew-Burke-Ernzerhof (PBE) parametrization of generalized gradient approximation (GGA) was implemented for the exchange-correlation functional [31]. These settings have been verified to achieve a total energy convergence for the systems under exploration below 1 meV/atom. The energy difference of a reaction ΔE = Eprod−Ereact was calculated from relaxed structures of the reactant and product, respectively. The activation reaction barrier Eb was then calculated using the nudged elastic band (NEB) technique. A 10×10×1 Monkhorst-Pack mesh grid was set for k-point sampling, while only Г-point sampling was used for the reaction path searching, to save the computational demand. The vacuum region in our super-cell approach was set to 1.2 nm in the direction perpendicular to the basal plane of graphene. The energy diagrams in Fig. 5 show that the reaction from the carbonyl and hydrogen pairs to hydroxyl pairs is exothermic with ΔE = −2.45 eV and Eb = 2.33 eV, while the reaction from a carbonyl pair to the epoxides is endothermic with ΔE = 1.07 eV and Eb = 1.90 eV. Notably, the formation of a carboxyl group from carbonyl and hydroxyl groups is exothermic with negligible ΔE = −0.013 eV and Eb = 0.31 eV.

Bottom Line: In this work, the structure evolution of GO with mild annealing is closely investigated.These results are further supported by the density functional theory based calculations.The results also show that the amount of chemically bonded oxygen atoms on graphene decreases gradually and we propose that the strongly physisorbed oxygen species constrained in the holes and vacancies on GO lattice might be responsible for the preserved oxygen content during the mild annealing procedure.

View Article: PubMed Central - PubMed

Affiliation: School of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Key Laboratory of Materials Processing Technology of MOE, Tsinghua University, Beijing, P. R. China.

ABSTRACT
A mild annealing procedure was recently proposed for the scalable enhancement of graphene oxide (GO) properties with the oxygen content preserved, which was demonstrated to be attributed to the thermally driven phase separation. In this work, the structure evolution of GO with mild annealing is closely investigated. It reveals that in addition to phase separation, the transformation of oxygen functionalities also occurs, which leads to the slight reduction of GO membranes and furthers the enhancement of GO properties. These results are further supported by the density functional theory based calculations. The results also show that the amount of chemically bonded oxygen atoms on graphene decreases gradually and we propose that the strongly physisorbed oxygen species constrained in the holes and vacancies on GO lattice might be responsible for the preserved oxygen content during the mild annealing procedure. The present experimental results and calculations indicate that both the diffusion and transformation of oxygen functional groups might play important roles in the scalable enhancement of GO properties.

Show MeSH
Related in: MedlinePlus