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C: Physical Processes in Nanomaterials and Nanostructures
Properties of Graphite Oxide Powders and Membranes as Revealed by Electron Paramagnetic Resonance Spectroscopy Natalia A Chumakova, Anastasya T. Rebrikova, Alexandr V. Talyzin, Nikita A. Paramonov, Andrey Kh Vorobiev, and Mikhail Valerievich Korobov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07221 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018
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The Journal of Physical Chemistry
Properties of Graphite Oxide Powders and Membranes as Revealed by Electron Paramagnetic Resonance Spectroscopy Natalia A. Chumakova1, Anastasya T. Rebrikova1, Alexandr V.Talyzin2, Nikita A. Paramonov1, Andrey Kh. Vorobiev1 and Mikhail V. Korobov1,* 1
Department of Chemistry, Moscow State University, Leninskie Gory 1-3, Moscow 119991,
Russia 2
Umeå University, Department of Physics, S-90187 Umeå, Sweden
* Corresponding author. E-mail:
[email protected] Abstract The spin probe technique was used to study graphite oxide (GO) powders swelled in polar liquids (CH3CN, CH3OH, H2O) and liquid-free GO membranes (GOM). The nitroxide radicals TEMPO
(2,2,6,6-tetramethylpiperidine-N-oxyl)
and
TEMPOL
(4-hydroxy-2,2,6,6-
tetramethylpiperidine-N-oxyl) readily penetrated into the inter-plane space of GO from solution. Electron Paramagnetic Resonance (EPR) spectra of these radical probes were sensitive to molecular mobility and orientation ordering within the internal space of GO. The radicals embedded in swelled GO were in two states with different rotational mobility. The small fraction of radicals located in the inter-plane space of GO and detected in the broad range of temperatures was in the state of fast rotation, similar to the same radicals dissolved in bulk liquids thus providing experimental evidence of formation of a liquid-like media within the inter-plane space of GO. Such mobile media may be responsible for the unusual permeation properties of GOM, reported in the literature. Second, less mobile fraction of radicals was found to be immobilized at the internal surface of GO and was sensitive to phase transformations in the swelled GO structures. The transformations were detected as anomalies at temperature dependences of rotational mobility of radicals. The detected dependence of EPR spectra of probe radicals on orientation of GOM relative to direction of magnetic field in EPR spectrometer was used for quantitative characterization of orientation alignment of GO planes within the membranes. Such approach may serve as an elegant method to estimate the relative quality of membranes and other graphene oxide layered structures.
Introduction
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Graphite oxide (GO) is a hydrophilic derivative of graphite* which received wide attention recently as a precursor for preparation of various graphene based materials2-4. Of particular interest are GO membranes (GOM) which were suggested for filtration and highly selective separation of gaseous and liquid mixtures5-7. GO is a layered structure formed by graphene parallel planes each of them randomly decorated with the oxygen containing groups (-OH, -O-, COOH- and etc.). GO powders easily swelled in polar liquids, i.e. sorption of the liquids into the inter-plane space is accompanied by the increase of the inter-plane distances from ~5-7 Å in the dry state up to ~20 Å in the saturated states8,9. The corresponding values of sorption of polar liquids into GO were reported in10,11. Swelling and sorption are known to depend on temperature, external pressure and on the synthesis procedure (Brodie vs Hummers methods, B-GO and HGO, respectively)8,9,12,13. For B-GO such temperature and pressure dependences take the form of phase transitions in the bulk 3D phases, similar to the incongruent melting transitions in the binary systems14,15. However these transformations yet were never examined at the molecular level. Much attention was attracted to the properties of polar liquids, sorbed into the confined inter-plane space of GO and GOM. The mobility of liquids (mostly of water) within the interplane space of GO was examined by several experimental methods e.g. quasi-elastic neutron scattering16 and neutron diffraction17, broadband dielectric spectroscopy18, impedance spectroscopy19, etc. It was demonstrated that sorbed liquids did not experience bulk or confined freezing/melting phase transition11,18. Based on this observation the state of liquids was characterized as “strongly bound to the host molecules”18. On the other hand, molecules of water inside GO were reported to possess some rotational freedom16,19. Also fast permeation of water through GOM20 pointed to certain translational freedom of the H2O molecules within the interplane space. This translational mobility was attributed to the «hydrophobic regions of pristine graphene capillaries», where travelling along the inter-plane space is not hindered by the interaction with oxygen-containing groups of GO20. Several authors have claimed that the role of hydrophobic unoxidized capillaries was overestimated and they were not large enough to account for the observed permeation of water21-23. More comprehensive model of the internal structure of hydrated GOM was recently proposed24. Experimental data20 provoked modeling of movable H2O structures in the inter-plane space25-28 and several different low-dimensional ordered forms of structured water were proposed including bi- and trilayers25, sliding hexagonal ice layers26 or layers consisting of different coexisting clusters of water27,28. Formation of *
We use the term “Graphite oxide” rather than “Graphene oxide” in accordance with the nomenclature recommended in ref. 1. 2 ACS Paragon Plus Environment
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“square ice” was observed by scanning tunneling microscopy29. However, the absence of additional reflections in the X-ray Diffraction spectrum (XRD) of swelled GO structures points out to complete disorder of molecules of the sorbed polar liquid in the inter-plane space9. One may conclude that experimental data on the properties of sorbed liquids inside the GO powders and GOM are incomplete and in part controversial. Of particular interest are new experimental measurements and methods capable to give a deeper insight into the state of these sorbed polar liquids. Such novel data are of crucial importance primarily for understanding of liquids permeation through GOM and for rational modeling of filtration and separation processes at GOM. Recent progress in practical applications of GOM calls for quantitative characterization of their properties and for standardization of the membranes30. The inter-plane distance in GOM is one of the key parameters which can be easily measured by XRD31-33. Another useful characteristic of GOM could be alignment of the GO layers within the membrane32. The alignment of layered structures e.g. of liquid crystals or membranes, is frequently characterized by order parameters. The order parameters are values of Legendre functions averaged over all possible orientations 34-36. Analyses of the EPR spectra of nitroxide radicals (spin probes) embedded in the studied medium is known as a powerful technique producing the detailed information on internal mobility and molecular orientation alignment37-44 in different systems. To the best of our knowledge, EPR spectroscopy was never used to examine GO swelled materials before, though nitroxide radicals were introduced into GO dry films45. Also intrinsic EPR signal of GO was used to characterize GO-based composite materials46,47. Here we apply the spin probe method to examine the B-GO and H-GO powders, saturated with different polar liquids, and the liquid-free H-GO membranes. The objectives of this study were investigating of the state and mobility of liquids in the inter-plane space of GO, detecting of the phase transаformations in the swelled GO structures and monitoring of the orientation alignment of the graphene oxide planes within the dry GOM.
Experimental Materials GO prepared by the Hummers’ method (H-GO) was purchased from ACS Material. Samples prepared by Brodie’s method (B-GO) were provided by Dr. T. Szabo and the synthetic procedure was described elsewhere48. The comparative description of both materials was given in ref.13. The samples were dried to a constant mass in the desiccators with P2O5 for 3-5 days11. The C/O ratio as measured by X-ray Photoelectron Spectroscopy (XPS) after drying was 2.47 3 ACS Paragon Plus Environment
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and 2.85, for H-GO and B-GO, respectively. The H-GO membranes were prepared according to the procedure described in23. Organic solvents exploited (CH3CN, CH3OH) were specially re-distilled before use. The resulting purity was >99,8%. Milli-Q water was used for measurements.
Introducing of spin probes The structures of spin probe molecules TEMPO (2,2,6,6-tetramethylpiperidin-N-oxyl), TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) and ChR (cholestane, 3beta-doxyl5alpha-cholestane) are presented in Fig. 1.
Fig. 1. Stable nitroxide radicals used in this study.
To introduce the spin probes into the inter-layer space of GO the radicals were dissolved in the corresponding polar liquid to form the solutions with the concentration ~ 1·mmol•l-1 and then 1-2 mg of B-GO/H-GO were mixed with the excess of such solutions. The resulting samples were equilibrium heterogeneous mixtures of the swelled GO and of the free solvent as confirmed by equilibrium sorption data11. Trace amounts of radicals were equilibrated in these two phases. After preparation some samples were gently dried to remove polar liquid partially or completely. The radicals therewith were left within GO. To make GOM sample for EPR measurements a free-standing membrane 2-5 µm thick (see Table 1) and with the diameter 40 mm was cut into pieces to fit to EPR tube with the diameter 4 mm. The probe radicals were introduced into the inter-plane space of GOMs in the same way as into the GO powders (see above). After preparation the GOM samples were gently dried by heating and pumping in the 4 ACS Paragon Plus Environment
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same manner as the GO powders and then put into the EPR tube. Mass of the GOM sample in the EPR tube was 1-2 mg. Some samples were prepared by equilibration of the initially dry radical-containing GO samples with the unsaturated vapors of the polar liquids. By this means we eliminated the possibility of condensation of bulk polar liquids in the samples and/or filling of the interlamellas’ voids in GO powders. The amount of radicals in the samples studied was ~1015 spin•mg-1. The molar ratio
nRadicals / nGO was below 2•10-5 and in the swelled samples
-5 nRadicals / ( nGO + nPolar liquid )
