In this study, we report the influence of carbon-based spacer-oxidized derivatives of fullerenes (fullerenols) C60(OH)26−32 and graphene oxide nanoribbons on the performance and pressure stability of ...graphene-oxide-based composite membranes. The impact of the intercalant shape and composition on the permeance of the selective layers for water vapors has been studied under pressure gradients. It is shown that the insertion of ball-shaped fullerenols between graphene oxide nanoflakes allows a suppression in irreversible permeance loss to 2−4.5% and reversible permeance loss to <25% (at 0.1 MPa), while retaining large H2O/N2 selectivities of up to ~30,000. The demonstrated approach opens avenues for the highly effective stabilization of GO membranes at elevated pressures for industrial-scale dehumidification.
A comparative study of transport characteristics of composite membranes based on graphene oxide prepared by Hummers' (H-GO) and Brodie (B-GO) methods is presented. By using Raman and XPS spectroscopy ...combined with gas and vapor measurements at non-zero pressure drop, it is shown that the difference in preparation methods results not only in different composition and microstructure of the membranes, but also in different water vapor permeability and resistance towards pressure drops during membrane performance. The H-GO samples are found to be more defective and stronger oxidized with C/O ratio of 1.8, whereas B-GO revealed a total C/O ratio of 2.6 with more perfect microstructure. The higher oxidation degree of H-GO membranes allows one to achieve higher water vapor permeability (up to ~170 Barrer at 100 % humidity) but dramatically lower stability towards pressure revealing the irreversible loss in permeability up to 46 % during the application of pressure drop of 1 bar. In contrast, B-GO membranes show slightly lower permeability (~140 Barrer at 100 % humidity) but enhanced pressure stability revealing the irreversible permeability loss of only 4 % at pressure drop of 1 bar which is about 10-fold smaller compared to H-GO stability. This could be explained by the difference in microstructural features of the H-GO and B-GO. Graphene oxide prepared by Hummer's method has more flexible and defective nanosheets, whereas Brodie's method gives rise to more rigid nanosheets with more perfect microstructure. The obtained results suggest that it is possible to prepare graphene oxide membranes with high resistance towards pressure using only the composition-microstructure interplay without additional modification with pressure-stabilizing agents.
Composite membranes are formed based on ultrathin 20 nm-thick selective layers of graphene oxide nanoflakes deposited on porous anodic alumina substrates. The long-term dynamics of permanent gases ...transport and water vapor permeability across the composite membranes is measured during 240 days (8 months). It is revealed that the permeability towards permanent gases remains nearly constant during a prolonged period of time. Contrary, water vapor flux decreases rapidly within the first 30 days from the membrane preparation moment and reaches about 80% of permeability loss during 8 months. The rapid decrease of membrane permeability during the first month could be attributed to a gradual packing of graphene oxide nanoflakes, particularly, locating in the surface sublayers, into more tight microstructure due to the evaporation of remaining solvent (membrane drying) under ambient conditions. Further decrease in permeability during more prolonged time could be caused additionally by deoxygenation of surface GO nanoflakes preventing water vapors diffusion into the GO film. This phenomenon, the so called "ageing" accompanies graphene oxide thin films similarly to some types of highly-permeable polymers. Holding the aged membrane under saturated water vapors, and even liquid water, didn't allow one to revitalize completely its permeability. The obtained results should be taken into account when designing membranes and other devices based on graphene oxide and its derivatives.
One of the possible ways to enhance photocatalytic activity is the use of photonic band gap (PBG) materials. Anodic titania photonic crystals (PhCs) are promising PBG structures, whose structure can ...be tuned easily by anodizing conditions. However, the effect of various doping approaches on the photocatalytic activity of anodic titania PhCs has never been studied before. Here anodic titania PhCs were synthesized by cycling anodizing of titanium with voltage versus charge density modulation followed by annealing at 450 °C in air. The PhCs were post-treated by electrochemical reduction in 0.1 M Na2SO4, annealing in H2/Ar mixture or NH3. The phase composition and structure of titania PhCs were studied by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy. Photocatalytic activity of PhCs in the degradation of the methylene blue dye decreases after electrochemical reduction and annealing in NH3, whereas the annealing in H2/Ar at 600 °C for 2 h leads to an increase in the degradation rate constant by 60%.
•Cyclic anodizing is applied for the preparation of 1D titania photonic crystals (PhCs).•Amount of oxygen vacancies in TiO2 increases after annealing in H2/Ar and NH3.•Photocatalytic activity of PhCs is demonstrated for methylene blue (MB) degradation.•Photocatalytic activity of PhCs decreases after electroreduction or annealing in NH3.•Annealing in H2/Ar leads to a 60% increase in the rate constant of MB degradation.
A series of K2NiF4-like complex oxides Nd2-xCaxNiO4 (x = 0–0.4) was obtained by the freeze drying synthesis method. Thermal decomposition of these precursors in H2 at 900 °C resulted in the ...formation of Ni/(Nd2O3,CaO) composites with different Nd2O3/CaO ratios. Their microstructure was similar to the products of the redox exsolution of perovskites: spherical 30 nm particles of Ni metal were allocated at the surface of dense agglomerates of Nd2O3 and CaO grains. The as-obtained composites demonstrated significant catalytic activity, selectivity, and stability in the reactions of dry reforming and partial oxidation of methane (DRM and POM, respectively) at T ≥ 800 °C.
A surface of the oxide grains, obtained by the reduction of Nd2-xCaxNiO4, is coated with Ni nanoparticles that demonstrate a significant catalytic activity. Display omitted
•Full thermal reduction of Nd2-xCaxNiO4 causes a formation of Ni metal, Nd2O3 and CaO.•A microstructure of as-obtained composites is similar to the products of exsolution.•A surface of (Nd2O3, CaO) agglomerates is coated with 30 nm spherical Ni particles.•High catalytic activity and selectivity in the reforming of methane was observed.
Surfactant-assisted ball milling of the Fe powder in paraffin has been used for fabrication of core–shell nanocrystalline particles. The local atomic structure of the bulk and surface layers of the ...mechanically milled particles has been studied using X-ray absorption spectroscopic techniques with synchrotron radiation from the DORIS storage ring at DESY, Hamburg. Regardless of milling environment composition, the as-prepared powders were shown to be characterized by a significant drop in the EXAFS signal intensity and coordination numbers of the Fe–Fe pairs due to the formation of nanocrystalline state in the particles. It has been shown that an addition of perfluorononanoic acid as a surfactant has a more prominent effect on the structure of the shell layers. The effect is revealed as an appearance of light element atoms (O, F, C) in the local atomic environment of the Fe atoms due to formation of oxide, carbide and adsorbed structures of different types in the particle shell.
•Local structure of Fe powder mechanically milled in paraffin environment has been studied.•Nanocrystalline state results in decrease in EXAFS signal and Fe–Fe coordination numbers.•Significant changes occur in the Fe particle surface (TEY mode) especially with surfactant additives.•Depending on milling environment, oxides/carbides/fluorides/carboxylates form on surface.•Different variants of possible adsorption interaction have been considered.
