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Neutron reflectivity (NR) was used to study the sorption of Eu(III) by graphene oxide (GO) films exposed to ethanol solution of EuCl3. Most of the earlier sorption studies have been performed using GO dispersed in solution. In contrast, layered structure of GO films imposes limitations for penetration of ions between individual sheets. The analysis of NR data recorded before and after sorption under vacuum demonstrates an increase of GO film thickness due to sorption by 35–40%. The characterization of chemical state of Eu(III) sorbed by GO films by X-ray absorption near-edge structure (XANES) in high-energy resolution fluorescence detection (HERFD) method at the Eu L3 edge reveals that it remains the same as in anhydrous EuCl3. Analysis of all collected data including reference experiments with bulk GO samples allows to conclude that EuCl3 penetrates into GO interlayers with ethanol solution and remains trapped in interlayers after evaporation of ethanol. Sorption of EuCl3 results in nearly complete amorphization of film and likely formation of voids, thus making NR models based on specific volume of unit cell not valid for quantitative evaluation of Eu sorption. Limitations of NR method must be taken into account in future studies of sorption by thin films.

Graphite oxide (GO) is a hydrophilic, layered material prepared by oxidation of graphite. In the first part of this thesis, we studied materials produced from GO by intercalation and functionalization. The second part of the thesis was focused on supercapacitor applications of high surface area carbons prepared from GO using chemical activation. A detailed study of acetylated GO (AcGO) was performed to verify structure and properties of this material. Reports from 1960’s suggested that AcGO has “pillared” structure. Our analysis showed that the AcGO demonstrates expanded structure due to acetylation but exhibits negligible specific surface area and should not be considered as a pillared material. Pillared reduced GO (prGO) was prepared by applying mild annealing to GO material pillared with tetrapod-shaped amine molecules. PrGO showed relatively high surface area due to remaining pillaring molecules in the structure. The prGO is hydrophobic and exhibits 100x improved conductivity compared to precursor. PrGO is one of few true pillared structures reported in literature so far, and the first ever prepared starting from pillared GO. We also investigated the sorption of common dyes, methylene blue (MB), rose bengal (RB) and crystal violet (CV), by multilayered graphene oxide materials. We found that MB dissolved in ethanol intercalates the GO structure, as evidenced by significant expansion of inter-layer distance, and increase in weight due to sorption. In contrast to MB, GO is not easily intercalated by CV and RB dyes. We believe that the flat MB molecule shape allows easier insertion between GO layers compared to twisted and non-flat CV and RB molecules. Our results suggest that penetration into GO inter-layers depends not only on the size of molecules, but also on the shape. Temperature dependent study of structures formed by Brodie GO (BGO) in liquid alkyl alcohols was performed for a set starting from undecyl alcohol (no. of C=11) and up to behenyl alcohol (no.

Swelling is a property of hydrophilic layered materials, which enables the penetration of polar solvents into an interlayer space with expansion of the lattice. Here we report an irreversible swelling transition, which occurs in MXenes immersed in excess dimethyl sulfoxide (DMSO) upon heating at 362-370 K with an increase in the interlayer distance by 4.2 Å. The temperature dependence of MXene Ti3C2Tx swelling in several polar solvents was studied using synchrotron radiation X-ray diffraction. MXenes immersed in excess DMSO showed a step-like increase in the interlayer distance from 17.73 Å at 280 K to 22.34 Å above ∼362 K. The phase transformation corresponds to a transition from the MXene structure with one intercalated DMSO layer into a two-layer solvate phase. The transformation is irreversible and the expanded phase remains after cooling back to room temperature. A similar phase transformation was observed also for MXene immersed in a 2 : 1 H2O : DMSO solvent ratio but at a lower temperature. The structure of MXene in the mixed solvent below 328 K was affected by the interstratification of differently hydrated (H2O)/solvated (DMSO) layers. Above the temperature of the transformation, the water was expelled from MXene interlayers and the formation of a pure two-layer DMSO-MXene phase was found. No changes in the swelling state were observed for MXenes immersed in DMSO or methanol at temperatures below ambient down to 173 K. Notably, MXenes do not swell in 1-alcohols larger than ethanol at ambient temperature. Changing the interlayer distance of MXenes by simple temperature cycling can be useful in membrane applications, e.g. when a larger interlayer distance is required for the penetration of ions and molecules into membranes. Swelling is also very important in electrode materials since it allows penetration of the electrolyte ions into the interlayers of the MXene structure.

Here preparation of high surface area activated reduced graphene oxide (arGO) oxidized into a 3D analogue of defect-rich GO (dGO) is reported. Surface oxidation of arGO results in carbon to oxygen ratio C/O = 3.3, similar to the oxidation state of graphene oxide while preserving high BET surface area of about 880 m2 g−1. Analysis of surface oxidized arGO shows high abundance of oxygen functional groups which converts hydrophobic precursor into hydrophilic material. High surface area carbons provide the whole surface for oxidation without the need of intercalation and lattice expansion. Therefore, surface oxidation methods are sufficient to convert the materials into 3D architectures with chemical properties similar to graphene oxide. The "3D graphene oxide" shows high sorption capacity for U(VI) removal in an extraordinary broad interval of pH. Notably, the surface oxidized carbon material has a rigid 3D structure with micropores accessible for penetration of radionuclide ions. Therefore, the bulk "3D GO" can be used as a sorbent directly without dispersing, the step required for GO to make its surface area accessible for pollutants., This article also appears in: "Hot Topic: Carbon, Graphite, and Graphene"

High surface area carbons are so far the best materials for industrial manufacturing of supercapacitor electrodes. Here we demonstrate that pine cones, an abundant bio-precursor currently considered as a waste in the wood industry, can be used to prepare activated carbons with a BET surface area exceeding 3000 m2 g−1. It is found that the same KOH activation procedure applied to reduced graphene oxide (rGO) and pine cone derived biochars results in carbon materials with a similar surface area, pore size distribution and performance in supercapacitor (SC) electrodes. It can be argued that “activated graphene” and activated carbon are essentially the same kind of material with a porous 3D structure. It is demonstrated that the pine cone derived activated carbon (PC-AC) can be used as a main part of aqueous dispersions stabilized by graphene oxide for spray deposition of electrodes. The PC-AC based electrodes prepared using a semi-industrial spray gun machine and laboratory scale blade deposition of these dispersions were compared to pellet electrodes.

Half of today's global energy consumption is in the form of heating and cooling. Solar collectors are the most promising sustainable alternative to fossil fuels in this sector. The most important component in a solar collector is the receiver, which by use of a selective surface absorbs and converts solar irradiance to thermal energy. Herein, a novel selective surface for low-to mid-temperature solar collectors is developed, studied and presented. The surface is produced by electroplating a cobalt-chromium coating on a stainless steel substrate using an electrolyte based on a deep eutectic solvent. Our method makes use of trivalent instead of traditionally used hexavalent chromium, which significantly reduces health-related issues and makes it more environmentally benign. We obtain a coating of chromium doped cobalt where the surface exhibits an absorptance and emittance of 0.96 and 0.14, respectively, giving it a solar-to-thermal efficiency of 0.95. An observed loss in optical efficiency, is shown to correlate to an oxidation of the metallic cobalt to Co3O4 at elevated temperatures. We further show that this oxidation can be mitigated by dip-coating a protective silica top coating, which concurrently improves the optical selectivity of the surface. The present selective surface is efficient, cheap, scalable, and easy to produce sustainably, making it competitive to industry standards. We foresee that our method will have impact on the advancement of improved low-to mid-temperature solar collectors, assisting a faster transition towards a sustainable society.

Swelling of graphene oxide (GO) membranes and bulk graphite oxide under confinement conditions is found to produce pressures up to ≈220 bar. Swelling pressure is important to take into account in many applications of GO membranes, but it has not been previously reported. Swelling pressures are typically measured only for bulk materials. However, it is demonstrated that even µm thick GO membranes develop pressures 3–25 bar due to the volume expansion caused by swelling in water. A rather strong difference in kinetics of pressure increase is found for both graphite oxide and GO membranes in water and ethanol despite similar lattice expansion due to swelling. This effect is attributed to slower penetration of ethanol into GO interlayers. Significantly faster saturation of swelling pressure is found for GO membranes (few hours) as compared to bulk graphite oxides (weeks) due to a higher degree of compaction. Swelling pressure is an important factor in applications, which require confinement, encapsulation of GO membranes or using external pressure to limit the lattice expansion. Finally, the swelling pressure can be used as an estimation for the suction pressure developed in pervaporation or vapor permeation applications, which is suggested as a driving force in rapid water permeation across GO membranes.

A spray gun machine was used to deposit high‐surface‐area supercapacitor electrodes using green non‐toxic aqueous dispersions based on different kinds of high specific surface area nanostructured carbon materials: activated graphene (a‐rGO) and activated carbon (AC). Tuning the spray conditions and dispersion formulation allowed us to achieve good adhesion to stainless‐steel current collectors in combination with high surface area and a satisfactory mechanical stability of the electrodes. The specific surface area of approximately 2000 m2/g was measured directly on a‐rGO and AC electrodes showing only around a 20 % decrease compared to the precursor powder materials. The performance of the electrodes deposited on stainless‐steel and aluminum current collectors was tested in supercapacitor devices using three electrolytes. The electrodes were tested in an “as‐deposited” state and after post‐deposition annealing at 200 °C. The spray deposition method and post‐deposition annealing are completely compatible with roll‐to‐roll industrial production methods. The a‐rGO demonstrated superior performance compared to AC in supercapacitor electrodes with gravimetric capacitance, energy, and power density parameters, which exceed commercially available analogues. The formulation of the dispersions used in this study is environmentally friendly, as it is based on only on water as a solvent and commercially available non‐toxic additives (graphene oxide, fumed silica, and carbon nanotubes).

Intercalation of dyes into thin multilayered graphene oxide (GO) films was studied by neutron reflectivity and X-ray diffraction. Methylene blue (MB) penetrates the interlayer space of GO in ethanol solution and remains intercalated after the solvent evaporation, as revealed by the expansion of the interlayer lattice and change in chemical composition. The sorption of MB by thin GO films is found to be significantly stronger compared to the sorption of Crystal violet (CV) and Rose bengal (RB). This effect is attributed to the difference in the geometrical shape of planar MB and essentially nonflat CV and RB molecules. Graphite oxides and restacked GO films are found to exhibit different methylene blue (MB) sorptions. MB sorption by precursor graphite oxide and thin spin-coated films of GO is significantly stronger compared to freestanding micrometer-thick membranes prepared by vacuum filtration. Nevertheless, the sorption capacity of GO membranes is sufficient to remove a significant part of the MB from diluted solutions tested for permeation in several earlier studies. High sorption capacity results in strong modification of the GO structure, which is likely to affect permeation properties of GO membranes. Therefore, MB is not suitable for testing size exclusion effects in the permeation of GO membranes. It is not only hydration or solvation diameter but also the exact geometrical shape of molecules that needs to be taken into account considering size effects for penetration of molecules between GO layers in membrane applications.

Ball-milling under either air or argon is explored as a facile way to adjust the textural properties, surface chemistry and morphology of activated reduced graphene oxide to the requirements for optimum electrode materials in electrical double layer capacitors operating in aqueous (KOH), organic (tetraethylammonium tetrafluoroborate in acetonitrile) and ionic liquid (1-Ethyl-3-methylimidazolium bis(trifluoromethyls​ulfonyl)​imide) electrolytes. Ball-milling is evidenced to specifically remove excessively large pores through the collapse of mesoporosity with no negative effect on in-pore resistance in any electrolyte, a concomitant improvement in volume-based storage and no loss of gravimetric performance. Ball-milling under air results in high oxygen content in the materials, which brings about performance deterioration in tetraethylammonium tetrafluoroborate in acetonitrile, but not significantly in aqueous and ionic liquid electrolytes. The best performance is achieved using activated graphene ball-milling under argon, with a volumetric capacitance of 90, 60, 70 F.cm−3 and a characteristic cell response time of 0.28, 1.3 and 8 s for aqueous, organic and ionic liquid electrolyte. While the highest energy density of 25 Wh.L−1 is reached in ionic liquid electrolyte at a cell potential of 3 V, the highest practical power density of 15 kW.L−1 is measured in tetraethylammonium tetrafluoroborate in acetonitrile at the energy density of 10 Wh.L−1. Our study underscores that simple ball-milling can provide the best trade-off among multiple performance parameters, resulting in sufficiently high volumetric capacitance with no detriment in high-rate response and cycle stability.

Unlike many methods of chemical modification of Graphite Oxide (GO) reported during 1930-1960 and re-studied in much detail over the last decade, acetylation somehow escaped attention and remained almost completely unexplored. Acetylated Graphite Oxide (AcGO) was prepared using a reaction with acetic anhydride. Successful acetylation is evidenced by an increase in the average interlayer distance fromd(001) = 7.8 angstrom in the precursor GO to 10 angstrom in AcGO. The amount of oxygen in AcGO significantly decreased compared to the precursor GO (C/O = 2.2), reflecting partial reduction of GO in the process of acetylation and resulting in a scarcely functionalized material with C/O = 6.2. A theoretical model of the complete acetylation of GO results in a non-porous close packed molecular structure with an interlayer distance of similar to 10 angstrom, in good agreement with experiment. Remarkably, AcGO shows significant swelling despite the oxidation degree being comparable to that of reduced GO, which does not swell in polar solvents. Moreover, AcGO shows swelling in acetonitrile similar to that of the precursor GO but not in water, thus providing an example of selectivity in the sorption of common polar solvents. The low oxidation degree combined with selective swelling properties makes AcGO a promising material for membrane applications.

High-surface area activated graphene has a three-dimensional porous structure that makes it difficult to prepare dispersions. Here we report a general approach that allows the preparation of stable water-based dispersions/inks at concentrations of ≲20 mg/mL based on activated graphene using environmentally friendly formulations. Simple drying of the dispersion on the substrate allows the preparation of electrodes that maintain the high specific surface area of the precursor material (∼1700 m2/g). The electrodes are flexible because of the structure that consists of micrometer-sized activated graphene grains interconnected by carbon nanotubes (CNTs). The electrodes prepared using activated graphene demonstrate performance superior to that of reduced graphene oxide in supercapacitors with KOH and TEA BF4/acetonitrile electrolytes providing specific capacitance values of 180 and 137 F/g, respectively, at a specific current of 1 A/g. The high surface area of activated graphene in combination with the good conductivity of CNTs allows an energy density of 35.6 Wh/kg and a power density of 42.2 kW/kg to be achieved. The activated graphene dispersions were prepared in liter amounts and are compatible with most industrial deposition methods.

Graphene oxide (GO) pillared with tetrakis(4-aminophenyl)methane (TKAM) molecules shows a narrow distribution of pore size, relatively high specific surface area, but it is hydrophilic and electrically not conductive. Analysis of XRD, N2 sorption, XPS, TGA and FTIR data proved that the pillared structure and relatively high surface area (∼350 m2 g−1) are preserved even after thermal reduction of GO pillared with TKAM molecules. Unlike many other organic pillaring molecules, TKAM is stable at temperatures above the point of GO thermal reduction, as demonstrated by TGA. Therefore, gentle annealing results in the formation of reduced graphene oxide (rGO) pillared with TKAM molecules. The TKAM pillared reduced graphene oxide (PrGO/TKAM) is less hydrophilic as found using dynamic vapor sorption (DVS) and more electrically conductive compared to pillared GO, but preserves an increased interlayer-distance of about 12 Å (compared to ∼7.5 Å in pristine GO). Thus we provide one of the first examples of porous rGO pillared with organic molecules and well-defined size of hydrophobic slit pores. Analysis of pore size distribution using nitrogen sorption isotherms demonstrates a single peak for pore size of ∼7 Å, which makes PrGO/TKAM rather promising for membrane and molecular sieve applications.

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.

A spectrally selective surface is produced by electrodeposition of a Co-Cr coating on a stainless steel substrate. The plating bath consisted of CrCl3 center dot 6H(2)O and CoCl2 center dot 6H(2)O dissolved in a deep eutectic solvent (DES) of choline chloride and ethylene glycol. This DES enables the use of trivalent (Cr(III)) instead of hexavalent chromium (Cr(XI)) which significantly reduces health risks associated with chromium electroplating. The selective surface exhibits an absorptance (alpha) of 0.96 and an emittance (epsilon) of 0.13 at 100 degrees C making it well adapted for mid- to lowtemperature concentrating solar collectors. The 1.8 mu m thick coating exhibits a porous structure on the surface as well as throughout the entire coating. The surface of the coating comprises of Co oxides and hydroxides, while the bulk consists of metallic and oxidized Co and only small fractions of Cr compounds. Initial tests of the thermal stability indicate that the coating does not maintain optical properties at an acceptable level over a lifetime of 25 years. At approximately 438 degrees C significant parts of the coating is oxidized in an oxygen rich environment.