| 1. |
- Chumakova, Natalia A., et al.
(author)
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Properties of Graphite Oxide Powders and Membranes as Revealed by Electron Paramagnetic Resonance Spectroscopy
- 2018
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:39, s. 22750-22759
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Journal article (peer-reviewed)abstract
- The spin probe technique was used to study graphite oxide (GO) powders swelled in polar liquids (CH3CN, CH3OH, and 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 interplane space of GO from the 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 mobilities. The small fraction of radicals located in the interplane 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 interplane space of GO. Such mobile media may be responsible for the unusual permeation properties of GOM, which is 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 the direction of magnetic field in the EPR spectrometer, was used for quantitative characterization of orientation alignment of GO planes within the membranes. Such an approach may serve as an elegant method to estimate the relative quality of membranes and other GO-layered structures.
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| 2. |
- Klechikov, Alexey, et al.
(author)
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Multilayered intercalation of 1-octanol into Brodie graphite oxide
- 2017
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In: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 9:20, s. 6929-6936
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Journal article (peer-reviewed)abstract
- Multilayered intercalation of 1-octanol into the structure of Brodie graphite oxide (B-GO) was studied as a function of temperature and pressure. Reversible phase transition with the addition/removal of one layer of 1-octanol was found at 265 K by means of X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC). The same transition was observed at ambient temperature upon a pressure increase above 0.6 GPa. This transition was interpreted as an incongruent melting of the low temperature/high pressure B-GO intercalated structure with five layers of 1-octanol parallel to GO sheets (L-solvate), resulting in the formation of a four-layered structure that is stable under ambient conditions (A-solvate). Vacuum heating allows the removal of 1-octanol from the A-solvate layer by layer, while distinct sets of (00 l) reflections are observed for three-, two-, and one-layered solvate phases. Step by step removal of the 1-octanol layers results in changes of distance between graphene oxide planes by similar to 4.5 angstrom. This experiment proved that both L- and A-solvates are structures with layers of 1-octanol parallel to GO planes. Unusual intercalation with up to five distinct layers of 1-octanol is remarkably different from the behaviour of small alcohol molecules (methanol and ethanol), which intercalate B-GO structure with only one layer under ambient conditions and a maximum of two layers at lower temperatures or higher pressures. The data presented in this study make it possible to rule out a change in the orientation of alcohol molecules from parallel to perpendicular to the GO planes, as suggested in the 1960s to explain larger expansion of the GO lattice due to swelling with larger alcohols.
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| 3. |
- Korobov, Mikhail V., et al.
(author)
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Sorption of polar organic solvents and water by graphite oxide : thermodynamic approach
- 2016
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In: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 102, s. 297-303
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Journal article (peer-reviewed)abstract
- Sorption of polar organic solvents CH3OH, C4H8O (THF), CH3CN, C3H7NO (DMF), C2H6OS (DMSO), C5H9NO (NMP) and water was quantitatively evaluated for Hummers (H-GO) and Brodie (B-GO) graphite oxides at T = 298K and at melting temperature (Tm) of the solvents. H-GO showed stronger sorption compared to B-GO for all studied solvents and the increase of sorption upon lowering temperature was observed for both H-GO and B-GO. Thermodynamic equations allowed to explain earlier reported "maximums" of swelling/sorption in the binary systems H-GO – solvent at Tm. The specific relation between the values of enthalpies of sorption and melting leads to the change of sign in enthalpies of sorption at Tm and causes maximal swelling/sorption. The same thermodynamic explanation was given for the "maximum" on the swelling vs. pressure dependence in B-GO and H-GO – H2O systems earlier reported at pressure of phase transition "liquid water-ice VI". Notably higher sorption of H2O was observed for H-GO compared to H-GO membrane (H-GOm) at high relative humidity (RH), RH > 0.75. Experimental sorption isotherm of H-GOm was used to simulate permeation rates of water through H-GOm and to estimate effective diffusion coefficient of water through the membrane.
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| 4. |
- Feicht, Patrick, et al.
(author)
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Systematic evaluation of different types of graphene oxide in respect to variations in their in-plane modulus
- 2017
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In: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 114, s. 700-705
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Journal article (peer-reviewed)abstract
- Graphene oxide samples prepared in various laboratories following a diversity of synthesis protocols based on Brodie's (BGO) and Hummers/Offeman's (HGO) methods were compared in respect of their in plane moduli. A simple wrinkling method allowed for a spatial resolution <1.5 pm by converting the wrinkling frequency. Quite surprisingly, a drastic variation of the in-plane moduli was found spanning the range from 600 GPa for the best BGO types, which is in the region of chemically derived graphene, all the way down to less than 200 GPa for HGO types. This would suggest that there are no two equal GO samples and GO should not be regarded a compound but rather a class of materials with very variable physical properties. While large differences between Brodie's and Hummers/Offeman's types might have been expected, even within the group of Hummers/Offeman's types pronounced differences are observed that, based on C-13 solid-state NMR, were related to over-functionalization versus over-oxidation.
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| 5. |
- Iakunkov, Artem, et al.
(author)
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Activated graphene as a material for supercapacitor electrodes : effects of surface area, pore size distribution and hydrophilicity
- 2019
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In: Physical Chemistry, Chemical Physics - PCCP. - : RSC Publishing. - 1463-9076 .- 1463-9084. ; 21:32, s. 17901-17912
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Journal article (peer-reviewed)abstract
- Activated reduced graphene oxide (a-rGO) is a material with a rigid 3D porous structure and high specific surface area (SSA). Using variation of activation parameters and post-synthesis mechanical treatment we prepared two sets of materials with a broad range of BET (N2) SSA ∼1000–3000 m2 g−1, and significant differences in pore size distribution and oxygen content. The performance of activated graphene as an electrode in a supercapacitor with KOH electrolyte was correlated with the structural parameters of the materials and water sorption properties. a-rGO is a hydrophobic material as evidenced by the negligibly small BET (H2O) SSA determined using analysis of water vapor sorption isotherms. However, the total pore volume determined using water vapor sorption and sorption of liquid water is almost the same as the one found by analysis of nitrogen sorption isotherms. Ball milling is found to provide an improved bulk density of activated graphene and collapse of all pores except the smallest ones (<2 nm). A decrease in the activation temperature from 850 °C to 550 °C is found to result in materials with a narrow micropore size distribution and increased oxygen content. Elimination of mesopores using ball milling or a lower activation temperature provided materials with better specific capacitance despite a significant decrease (by ∼30%) of the BET (N2) SSA. The best gravimetric and volumetric capacitances in KOH electrolyte were achieved not for samples with the highest value of the BET (N2) SSA but for materials with 80–90% of the total pore volume in micropores and an increased BET (H2O) SSA. Comparing the performance of electrodes prepared using rGO and a-rGO shows that a more hydrophilic surface is favorable for charge storage in supercapacitors with KOH electrolyte.
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| 6. |
- Klechikov, Alexey, et al.
(author)
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Hydrogen storage in high surface area graphene scaffolds
- 2015
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In: Chemical Communications. - : Royal Society of Chemistry. - 1359-7345 .- 1364-548X. ; 51:83, s. 15280-15283
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Journal article (peer-reviewed)abstract
- Using an optimized KOH activation procedure we prepared highly porous graphene scaffold materials with SSA values up to 3400 m2 g−1 and a pore volume up to 2.2 cm3 g−1, which are among the highest for carbon materials. Hydrogen uptake of activated graphene samples was evaluated in a broad temperature interval (77–296 K). After additional activation by hydrogen annealing the maximal excess H2 uptake of 7.5 wt% was obtained at 77 K. A hydrogen storage value as high as 4 wt% was observed already at 193 K (120 bar H2), a temperature of solid CO2, which can be easily maintained using common industrial refrigeration methods.
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| 7. |
- Mercier, Guillaume, et al.
(author)
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Porous Graphene Oxide/Diboronic Acid Materials : Structure and Hydrogen Sorption
- 2015
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:49, s. 27179-27191
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Journal article (peer-reviewed)abstract
- Solvothermal reaction of graphite oxide (GO) with benzene-1,4-diboronic acid (DBA) was reported previously to result in formation of graphene oxide framework (GOP) materials. The theoretical structure of GOFs consists of graphene layers separated by benzene-diboronic "pillars" with similar to 1 nm slit pores thus providing the opportunity to use it as a model material to verify the effect of a small pore size on hydrogen adsorption. A set of samples with specific surface area (SSA) in the range of similar to 50-1000 m(2)/g were prepared using variations of synthesis conditions and GO/DBA proportions. Hydrogen storage properties of GOF samples evaluated at 293 and 77 K were found to be similar to other nanocarbon trends in relation to SSA values. Structural characterization of GO/DBA samples showed all typical features reported as evidence for formation of a framework structure such as expanded interlayer distance, increased temperature of thermal exfoliation, typical features in FTIR spectra, etc. However, the samples also exhibited reversible swelling in polar solvents which is not compatible with the idealized GOF structure linked by benzenediboronic molecular pillars. Therefore, possible alternative nonframework models of structures with pillars parallel and perpendicular to GO planes are considered.
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| 8. |
- Sun, Jinhua, et al.
(author)
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Porous graphite oxide pillared with tetrapod-shaped molecules
- 2017
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In: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 120, s. 145-156
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Journal article (peer-reviewed)abstract
- Porous pillared graphene oxide (GO) materials were prepared using solvothermal reaction of Hummers GO with solution of Tetrakis(4-aminophenyl)methane (TKAm) in methanol. The intercalation of TKAm molecules between individual GO sheets, performed under swelling condition, results in expansion of inter-layer distance of GO from ∼7.5 Å to 13-14 Å. Pillaring GO with bulky, rigid 3D shaped TKAm molecules could be an advantage for the preparation of stable pillared structures compared to e.g. aliphatic or aromatic diamines. Insertion of TKAm molecules into inter-layer space of GO results in formation of interconnected network of sub-nanometer slit pores. The expanded GO structure prepared with optimized GO/TKAm composition shows Specific Surface Area (SSA) up to 660 m2/g which is among the highest reported for GO materials pillared using organic spacers. Modelling of GO structures pillared with TKAm molecules shows that maximal SSA of about 2300 m2/g is theoretically possible for realistic concentration of pillaring molecules in GO interlayers. Hydrogen sorption by pillared GO/TKAm is found to follow standard correlation with SSA both at ambient and liquid nitrogen temperatures with highest uptakes of 1.66 wt% achieved at 77 K and 0.25 wt% at 295 K. Our theoretical simulations show that pillared GO structures do not provide improvement of hydrogen storage beyond well-established physisorption trends even for idealized materials with subnanometer pores and SSA of 2300–3700 m2/g.
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| 9. |
- Tumanov, Nikolay A., et al.
(author)
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High-Pressure Study of Mn(BH4)(2) Reveals a Stable Polymorph with High Hydrogen Density
- 2016
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In: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 28:1, s. 274-283
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Journal article (peer-reviewed)abstract
- High-pressure behavior of alpha-Mn(BH4)(2) was studied up to 29.4 GPa in diamond anvil cells using powder Xray diffraction combined with DFT calculations and Raman spectroscopy, and two new polymorphs were discovered. The first polymorph, delta-Mn(BH4)(2), forms near 1 GPa and is isostructural to the magnesium analogue delta-Mg(BH4)(2). This polymorph is stable upon decompression to ambient conditions and can also be obtained by compression of alpha-Mn(BH4)(2) in a large-volume steel press as well as by high-energy ball milling. It shows a high volumetric density of hydrogen of 125 g H-2/L at ambient conditions. delta-Mn(BH4)(2) was refined in the space group I4(1)/acd with the cell parameters a = 7.85245(6), c = 12.1456(2) angstrom, and V = 748.91(1) angstrom(3) at ambient conditions; it can also be described in a stable P-4n2 superstructure. Its thermal stability was studied by in situ X-ray powder diffraction and thermal analysis coupled with mass-spectroscopy. delta-Mn(BH4)(2) transforms back to alpha-Mn(BH4)(2) upon heating in the temperature range of 67-109 degrees C in Ar (1 bar) or H-2 (100 bar) atmosphere, and a decomposition is initiated at 130 degrees C with the release of hydrogen and some diborane. Mn(BH4)(2) undergoes a second phase transition to delta'-Mn(BH4)(2) in the pressure range of 8.6-11.8 GPa. delta'-phase is not isostructural to the second high-pressure phase of Mg(BH4)(2), and its structure was determined in the root 2a X c supercell compared to the delta-phase and refined in the space group Fddd with a = 9.205(17), b = 9.321(10), c = 12.638(15) angstrom, and V = 1084(3) angstrom(3) at 11.8 GPa. Equations of state were determined for alpha- and delta-Mn(BH4)(2).
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