Multi-layer graphene oxide alone and in a composite with nanosilica: Preparation and interactions with polar and nonpolar adsorbates

Applied Surface Science 387 (2016) 736–749

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Multi-layer graphene oxide alone and in a composite with nanosilica: Preparation and interactions with polar and nonpolar adsorbates V.M. Gun’ko a,∗ , V.V. Turov a , V.I. Zarko a , O.V. Goncharuk a , A.K. Matkovsky a , G.P. Prykhod’ko a , Yu.M. Nychiporuk a , E.M. Pakhlov a , T.V. Krupska a , D.Yu. Balakin b , ˛ c , A.I. Marynin d , A.I. Ukrainets d , B. Charmas c , L.S. Andriyko a , J. Skubiszewska-Zieba M.T. Kartel a a

Chuiko Institute of Surface Chemistry, 17 General Naumov Street, 03164 Kyiv, Ukraine Institute of Physics, 46 Prospect Nauki, 03028 Kyiv, Ukraine c Faculty of Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland d National University of Food Technology, 68 Volodymyrska Street, 01033 Kyiv, Ukraine b

a r t i c l e

i n f o

Article history: Received 24 March 2016 Received in revised form 30 June 2016 Accepted 30 June 2016 Available online 1 July 2016 Keywords: MLGO Nanosilica Polar adsorbates Nonpolar adsorbates Interfacial phenomena Freezing-melting hysteresis effects

a b s t r a c t Freeze-dried multi-layer graphene oxide (MLGO), produced from natural flake graphite using ionic hydration method, demonstrates strong interactions of functionalized carbon sheets with polar or nonpolar adsorbates or co-adsorbates depending on the characteristics of dispersion media. Interactions of MLGO with a mixture of water and n-decane in chloroform media provide specific surface area (Su ) in contact with unfrozen liquids greater than 1000 m2 /g corresponding to stacks with 3–5 carbon layers. Electrostatic interactions between functionalized carbon sheets in dried MLGO are very strong. Therefore, nonpolar molecules (benzene, decane, nitrogen) cannot penetrate between the sheets. Water molecules can effectively penetrate between the sheets, especially if MLGO is located in weakly polar CDCl3 medium. In this case, n-decane molecules (co-adsorbate) can also penetrate into the sheet stacks and locate around nonpolar fragments of the sheets. The Su value of MLGO being in contact with unfrozen water can reach 360 m2 /g, but upon co-adsorption of water with decane Su = 930 m2 /g, i.e., hydrophobic interactions of the mentioned fragments with decane are stronger that with co-adsorbed water. Water alone (0.25 or 0.5 g/g) bound to MLGO in a mixture with fumed silica A-300 in air or CDCl3 media can provide Su = 30–50 m2 /g. Pores in wetted MLGO or MLGO/A-300 mainly correspond to mesopores. Nanosilica does not provide significant opening of the MLGO sheet stacks during their mechanical mixing. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Various carbons are important materials for industrial, medicinal and other applications [1]. Among them graphenes such as single (SLG) and multi (MLG) layer graphenes, single (SLGO) and multi (MLGO) layer graphene oxides are the most promising materials for applications in industry and medicine [1–10]. However, these materials remain relatively expensive (especially SLG and SLGO) despite significant reduction of their cost during last years. Therefore, searching of less expensive pathways of preparation of materials of graphene family is of great practical importance. One of these ways could be based on natural cheap raw and relatively pure

∗ Corresponding author. Fax: +380 44 4243567. E-mail address: vlad [email protected] (V.M. Gun’ko). http://dx.doi.org/10.1016/j.apsusc.2016.06.196 0169-4332/© 2016 Elsevier B.V. All rights reserved.

materials such as natural flake graphite treated, e.g., using relatively cheap modified methods of preparation of graphite oxide [11,12]. An additional way to reduce the final cost can be based on preparation of composites with graphene materials and other cheaper materials, e.g., fumed silica, natural disperse minerals, polymers, etc. Note that previous investigations of carbon materials in combination with fumed silica showed that carbon layers can practically completely cove the silica surface [13]. Note that graphene materials are effectively used in composites with various metal oxides [14–18]. In composites based on the graphene materials, their fundamental properties, caused by a specific 2D structure, relatively large sizes of carbon sheets, and significant amounts of O-containing functionalities in SLGO and MLGO, play an important role in adsorption of polar or nonpolar, low- or high-molecular weight compounds, reactions in the gas and liquid phases, electro-physical

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properties (e.g. anisotropic electro-conductivity), etc. [19–34]. Surface O-containing functionalities in SLGO or MLGO determine the behavior of these carbons in both gaseous and liquid media or composites [35–50]. Therefore, the efficiency of applications of the graphene materials depends strongly on control of the interfacial phenomena at their surfaces because these materials are very flexible and soft and can be collapsed and agglomerated or disagglomerated during heating, evacuation, changes in pH value of the solutions, presence of bound oxide or metallic nanoparticles, etc. [1–5,19,20,51–58]. Composites with graphene oxide and various metal oxides can be used as effective catalysts, adsorbents, polymer fillers, etc. [59–73], and interfacial phenomena play an important role in these composites per se and, therefore, in their applications. Preparation of a monolayer (or close) carbon coverage of the matrices (carriers) is of interest from both theoretical and practical points of view because the matrices are cheaper than SLGO or MLGO. Interactions of the graphene materials with water and various aqueous solutions and suspensions are of importance because these materials are frequently used in the aqueous media. Features of these interactions depend on many factors such as types of dispersion media (humid air, aqueous or more complex media, pH, and salinity), content and structure of solutes or adsorbates and co-adsorbates, temperature, mechanical loading, etc. Therefore, investigations of the interfacial phenomena are of importance to control the properties of the graphene materials, including a thin layer of MLGO located at a surface of nanooxides, in different media, and these investigations were aimed in the present work for a deeper insight into the mentioned above problems. MLGO produced from flake graphite as a precursor was studied alone and in a mechanical mixture with nanosilica A-300 located in different media (air, aqueous, CDCl3 pure and with addition of CD3 CN, trifluoroacetic acid F3 CCOOD (TFAA), and nondeuterated n-decane), as well as adsorption of water, benzene, toluene, dimethylsulfoxide (DMSO), n-hexane, and n-decane and evaporation of water, n-hexane, and n-decane bound to MLGO. A variety of conditions, adsorbates and methods used allows us to accumulated information for better understanding the properties of MLGO studied.

2. Materials and methods 2.1. Materials Multi-layer graphene oxide (MLGO) was prepared using natural flake graphite (Zaval’evsk coal field, Ukraine; flake sizes <0.2 mm) as a precursor using a modified method of ionic hydration described in detail elsewhere [11,12]. Briefly, a solution of concentrated H2 SO4 (0.65 L) was added to 20 g of flake graphite, heated at 45 ◦ C to appearance of blue color of graphite bisulphate and cooled to 10–15 ◦ C. KMnO4 (72 g) was added by small portions during constant stirring at 20 ◦ C. The mixture was heated to 400 ◦ C. It was stirred to a slurred state and then aged for 20 h at room temperature. Water (120 mL) was added with constant stirring at 45 ◦ C. The mixture was maintained at 45 ◦ C for 1 h, cooled to 10–15 ◦ C, and 1 L of water was added, and then 70 mL of 28% H2 O2 was added by small portions with constant stirring. The obtained mixture of light yellow color was centrifuged. The residue was suspended in 3% aqueous solution of HCl (2 L) and again centrifuged. The residue was suspended in bidistilled water and centrifuged, and this procedure was repeated four-five times to neutral pH of the suspension. The dried residue after final centrifugation was freeze-dried at −24 ◦ C to 25 ◦ C at 10−3 Torr. The MLGO samples were also dried and degassed by evacuation up to 10−3 Torr (this sample was studied only by DSC method). The final dry MLGO has very low bulk density and light brown color similar to that of commercial MLGO or SLGO (Cheap

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Tubes, Inc.) used in comparative investigation with MLGO produced from natural flake graphite. Fumed silica A-300 (Pilot plant of Chuiko Institute of Surface Chemistry, Kalush, Ukraine, SBET = 285 m2 /g, bulk density 0.045 g/cm3 ) was used to prepare a mechanical mixture with MLGO (4:1 w/w) carefully stirred in a porcelain mortar. This A-300/MLGO ratio was selected because it provides the carbon amount enough to prepare a surface layer close to a monolayer coverage of nanosilica [13,53,54]. 2.2.

1H

NMR

The 1 H NMR spectra were recorded at 200–285 K using a Varian 400 Mercury spectrometer of high resolution with the 60◦ probe pulses with the duration of 1 ␮s and a bandwidth of 20 kHz. The temperature of samples was controlled by means of a Bruker VT1000 device with relative mean errors of ±1 K. Relative mean errors smaller than ±10% for 1 H NMR signal intensity were determined for samples in different dispersion media. To prevent supercooling of the systems, the temperature dependences of concentration of unfrozen water and decane were determined on heating of samples pre-cooled to 200–210 K. The signals of water molecules from ice or frozen decane did not contribute the 1 H NMR spectra recorded here because of features of the measurement technique using static samples and narrow bandwidth [54]. Some details of the lowtemperature 1 H NMR spectroscopy used are described in electronic Supplementary Information (SI) file. Deuterated organic compounds (solvents CDCl3 and CD3 CN) and trifluoroacetic acid F3 CCOOD (TFAA) and non-deuterated n-decane were used in low-temperature 1 H NMR spectroscopy measurements with static samples. Deuterated compounds were used to prevent their contribution into 1 H NMR signals of water and decane bound to MLGO or MLGO/A-300. 2.3. DSC Differential scanning calorimetry (DSC) investigations of interactions of MLGO and A-300/MLGO with nonpolar (benzene, toluene, and n-decane) and polar (water and DMSO) or a mixture of water and decane adsorbates were carried out using a PYRIS Diamond (Perkin Elmer Instruments, USA) differential scanning calorimeter calibrated at different heating rates using standard samples such as distilled water (melting temperature Tm = 0 ◦ C) and indium (Tm = 156.6 ◦ C) supplied by the producer and using the recommended standard calibration procedure (see also the SI file). On the basis of the methods sensitive to transition of phase (such as 1 H NMR and DSC), different versions of cryoporometry, relaxometry and thermoporometry were developed to study the structural and textural (specific surface area, S, pore volume, V, pore size distributions, PSD) characteristics of a variety of solid and soft materials and bioobjects in non-dried states [54,74–86]. 2.4. Adsorption and desorption To analyze the adsorption characteristics of MLGO, the adsorption of water and benzene was studied using an adsorption apparatus with a McBain–Bark quartz scale at 293 K. Samples were evacuated at 10−3 Torr and 473 K for several hours to a constant weight, then cooled to 293 ± 0.2 K, and the adsorption of water or benzene was studied at varied relative pressures p/ps . The measurement accuracy was 1 ± 10−3 mg with a relative mean error of ±5%. Low-temperature (77.4 K) adsorption-desorption of nitrogen was measured using a Micromeritics ASAP 2420 adsorption analyzer.

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Fig. 1. HRTEM images of MLGO prepared using natural flake graphite at different magnifications of (a and b) pure carbon sheets and (c and d) multi-layer structures with ash particles.

Evaporation of water (1 g per 0.05 g of MLGO), hexane (1 g per 0.05 g or 66.3 mg per 20.1 mg of MLGO), and n-decane (1.05 g per 0.05 g of MLGO) was studied at a constant temperature (20 ± 1 ◦ C) for samples studied in comparison with evaporation of pure liquids without an adsorbent using an ABT 220-5 DM (Kern, Germany) analytical balances. To reduce the errors caused by changes in environmental conditions, the evaporation of liquids from open glass vials (volume 10 cm3 ) was carried out simultaneously for all samples including a control sample without MLGO [87]. To analyze the interaction of water with MLGO and the presence of O-containing functionalities, desorption of water, CO, CO2 and O was studied at a heating rate ␤ = 1.93 K/s using one-pass (OP) TPD time-of-flight (ToF) MS method (pressure in a chamber 8 × 10−5 Pa, sample weight 0.5 mg, with a short distance (∼0.5 cm) between a sample and a MS detector) with a MSC-3 (“Electron”, Sumy, Ukraine) ToF mass-spectrometer [88]. Thermogravimetric (TG and differential TG, DTG) measurements with differential thermal analysis (DTA) were carried out in an inert (nitrogen) atmosphere at 20 to 1200 ◦ C using a Derivatograph C (Paulik, Paulik & Erdey, MOM, Budapest) apparatus. Samples of 25–28 mg were placed in a ceramic crucible heated at a heating rate of 10 ◦ C/min. Desorption of water and others was studied using the TG/DTA method.

2.5. Microscopy High resolution transmission electron microscopy, HRTEM images were recorded for MLGO using JEM-2100F (Japan). A powder sample was added to acetone (for chromatography) and sonicated. Then a drop of the suspension was deposited onto a copper grid with a thin polymeric film. After acetone evaporation, sample particles remaining on the film were studied with HRTEM. The MLGO and MLGO/A-300 morphology was studied using scanning electron microscopy (SEM) with Quanta 3D FEG (FEI) (voltage 5 kV). 2.6. Infrared spectroscopy Infrared (IR) spectra of thin films of freeze-dried MLGO mixed with KBr (1:100, treated in a microbreaker as a grinding device for 3 min) or after drying (in air) of aqueous suspension (6 wt.%) of MLGO were recorded using a Specord M80 (Carl Zeiss) spectrophotometer. 2.7. Particle sizing Particle size distributions were studied using a Zetasizer Nano ZS (Malvern Instruments) apparatus. Distilled water with the amounts of MLGO of 0.05 or 0.5 wt.% was utilized to prepare the suspensions sonicated for 2–10 min using an ultrasonic disperser (Sonicator

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Fig. 2. SEM images of (a) MLGO alone (ash particles are visible), (b) in a mixture with nanosilica A-300, and (c and d) A-300 alone.

Misonix, power 500 W and frequency 22 kHz). The pH value was constant of 2.26 (CMLGO = 0.5 wt.%) or 2.9 (CMLGO = 0.05 wt.%). These relatively low values of pH are due to the presence of the COOH groups in the MLGO structure, as well as residual amounts of acids used on the material preparation. 2.8. Theoretical modelling A structure with two GO layers was computed by the MM+ method with the VEGA ZZ 3.1.0 program suit [89]. The model geometry was drawn using the UCSF Chimera package (version 1.11) developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco (supported by NIGMS P41-GM103311, http://www.cgl.ucsf.edu/chimera) [90]. For visualization of the fields around functionalized carbon sheets, the Torch program (version 10.4.2, Rev. 24876, 2016) was used [91,92]. The geometry of two-sheet model of graphene alone and interacting with a silica nanoparticles was optimized using the PM7 method (MOPAC 2016) [93]. 3. Results and discussion 3.1. Structural features MLGO prepared from natural flake graphite as a precursor demonstrates relatively large (micrometer scaled) carbon sheets (Figs. 1 and 2 a ) with a short-range order in nano-scaled structures (Fig. 1a, insert and Fig. S1 in the SI file). However, HRTEM image (Fig. 1a) shows that the carbon structure in MLGO is rather

amorphous. Additionally, there are ash particles (Figs. 1 c,d and 2 a), which composed mainly of carbon with a certain content of silica and other minor admixtures (see Figs. S2–S4 in SI). Similar ash particles can be found in commercial MLGO (Cheap Tubes Inc.) (see Fig. S5c,d in SI). Note that the commercial MLGO is more amorphous (Fig. S5) than MLSO produced from natural flake graphite (see insert in Figs. 1 a and S1 in SI). A mixture of MLGO with nanosilica A-300 (ratio 1:4 w/w) has the morphology (Fig. 2b) rather similar to that of fumed silica alone (Fig. 2c and d) because of a minor amount of MLGO (20 wt.%) in the composite. This means that secondary particles of MLGO (Fig. 2a) can be decomposed to smaller structures (Fig. 2b) during mechanical mixing with nanosilica. The surface properties of the composite are rather similar to those of MLGO due to the formation of a thin carbon layer at a surface of silica particles (vide infra). A strong influence of MLGO on the properties of the composite with A-300 is observed upon comparison of the TG, DTG, and DTA curves of MLGO alone (Fig. 3a), A-300 alone and their mixture MLGO/A-300 (Fig. 3b). The curves for the composite are much more similar to those for MLGO than that for nanosilica. The main processes during heating of the samples in the inert atmosphere are (i) water desorption with two DTG peaks at 92 ◦ C and 217 ◦ C (∼4.5 wt.% for MLGO) and 80 ◦ C and 209 ◦ C (1.2 wt.% for MLGO/A-300); (ii) several associative desorption processes of removal of water and some other molecules formed from O-containing functionalities with the weight loss of ∼4 wt.% at 220–530 ◦ C, ∼3 wt.% at 530–750 ◦ C, and ∼0.5 wt.% at 750–1200 ◦ C (for MLGO). Note that both intact water desorption and associative desorption of O-containing molecules occur at lower temperatures for MLGO/A-300 than that for MLGO

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Fig. 3. TG/DTG/DTA thermograms of (a) MLGO alone and (b) in a mixture with nanosilica (1: 4 w/w) (curves 1–3) or silica A-300 alone (curves 4 (TG), 5 (DTG) and 6 (DTA)). Table 1 Assignment of the IR bands of MLGO. Band (cm−1 )

Assignment

3700–2500 2890–2800 1730 1630 1425, 1394 1225 1115, 1040

O H (surface hydroxyls, H2 O) C H C O COOH, C O, H2 O C C, C O H C O C C O, C C

alone. This result can be explained by partial decomposition of the stacks of soft large carbon sheets during mechanical mixing with nanosilica, i.e. opening of pores between the sheets in large stacks in the aggregates that were closed (collapsed) in the initial MLGO (vide infra). Similar decomposition was observed for other carbon materials (activated carbons, carbon blacks) during mixing of them with nanosilica [13,94]. Thus, water more strongly interacts with MLGO than with nanosilica because of a large amount of O-containing functionalities at a MLGO surface. This interaction is of an electrostatic character (see Figs. S7–S9 in SI). The electrostatic fields are predominant for both dry and wetted MLGO (comp. the field view images for dry and wetted two-layer GO in Figs. S8 and S9 in SI). According to the EDAX data for MLGO (see Fig. S3 in SI), it contains approximately 11 at.% of oxygen, which appears in different O-containing functionalities (Fig. 4, Table 1). This value is simi-

lar to that for commercial MLGO (∼10%), and it is in agreement with the weight loss (taking into account the desorption of intact water molecules) observed during heating of MLGO from 220 ◦ C to 1200 ◦ C (Fig. 3a). The main portion of the oxygen in MLGO (Figs. 3, 4 and S3 in SI) corresponds to different surface functionalities, which start to be intensively decomposed at approximately 500 ◦ C and higher (Fig. 3). The line of CO (which can correspond to decomposition of carboxylic, epoxy and other functionalities with one O atom per group) is more intensive than line CO2 (formed due to decomposition mainly of groups with two O atoms per group). Water is intensively desorbed at lower temperatures due to the presence of intact water molecules (molecular desorption occurs at lower temperatures than associative one) and OH groups (associative desorption of water occurs at higher temperatures). The position of the main peaks (Fig. S6) is similar to that of the second DTG peak at 200 ◦ C (Fig. 3). Peaks of CO2 and H2 O (Fig. S6) have close positions due to bimolecular reactions of COOH and COH groups with the elimination of H2 O and CO2 . The decomposition of the epoxy groups can strongly contribute the CO line. The IR spectra of MLGO (Fig. 4, Table 1) are in agreement with the spectra of graphene oxide analyzed in detail elsewhere [95]. The IR spectra show the presence of various O-containing functionalities, which play a predominant role in all interfacial phenomena studied here. Some bands demonstrate decreasing splitting after suspending and drying (Fig. 4, comp. curves 2 and 1). Additionally, the amount of strongly bound water (3200–2500 cm−1 ) increases for the second sample. These results show than suspending and simple drying in air (i.e., non-freeze-drying) lead to stronger aggregation of carbon sheets. The particle size distributions (PaSD) in the aqueous suspensions of MLGO demonstrate (Fig. 5) that an increase in time of sonication (tus ) of a more diluted suspension (0.05 wt.%) resulted in rather diminution of particle sizes (Fig. 5a–c). For a more concentrated suspension (0.5 wt.%), the particle sizes increased with increasing tus (Fig. 5d–f). However, at tus = 10 min the PaSD becomes bimodal (Fig. 5f) due to decomposition of a fraction of secondary structures, as well as for the more diluted suspension with time tus (Fig. 5b and c). As a whole, MLGO studied demonstrated typical sizes of carbon sheets observed previously for commercial MLGO samples [34,44,53]. This confirms that the proposed synthesis method is effective and appropriate for the MLGO preparation.

3.2. Interfacial phenomena The presence of a lot of O-containing functionalities at a surface of graphene sheets provides the hydrophilic properties of the MLGO surface that cause stronger adsorption of polar water molecules than nonpolar benzene molecules (Fig. 6). However, the adsorption of water is low at p/ps < 0.9 due to collapsed structure of MLGO strongly dried and degassed before the adsorption measurements. Water at a large amount can reduce sheet–sheet interactions in MLGO stacks that leads to a significant increase in the volume of pores (voids between the sheets) filled by water. The carbon sheets with a great number of O-containing functionalities (see IR spectra in Fig. 4 and Table 1) can strongly interact in dense stacks (see a model structure in Figs. S7–S9) with several layers. Therefore, water molecules, which can penetrate between functionalized carbon sheets, can be slowly desorbed in contrast to benzene molecules, which do not penetrate between densely packed carbon sheets strongly O-functionalized. This difference in the interactions of water and benzene with the MLGO stacks can cause appearance of open and close hysteresis loops in the adsorption-desorption isotherms of water (more strongly interacting with MLGO) and benzene (more weakly interacting with MLGO), respectively (Fig. 6).

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Fig. 4. Infrared spectra of freeze-dried MLGO mixed with dry KBr (1:100, treated in a microbreaker for 3 min) (curve 1) or suspended (6 wt.% aqueous suspension) and dried in air to form a very thin film (curve 2) (insert shows a weakly hydrated two-sheet model of MLGO optimized using the PM7 method).

Another consequence of the strong interactions between oxidized carbon sheets in the stacks of dried and degassed MLGO is very low adsorption of nitrogen (similar to benzene). Therefore, the SBET value of MLGO estimated from the nitrogen adsorption is low (SBET ≈ 2 m2 /g). This value rather corresponds to the outer surface area of MLGO particles. The benzene adsorption gives a slightly greater value SBET ≈ 15 m2 /g. This difference can be caused by stronger penetration of benzene molecules between carbon sheets in the stacks than nitrogen molecules can do. In the case of commercial SLGO (including particles with one-three carbon layers) degassed at 150 ◦ C, the value of SBET was also low (∼50 m2 /g). From theoretical estimations, the value of SBET for MLGO with five carbon layer stacks in average should be about 800–900 m2 /g. This large surface area can appear for MLGO studied under specific conditions in certain media [34] (see Table S2 in SI). However, not only the dispersion medium affects the MLGO stack structure, but also MLGO affects the structure of bound liquid layers both polar (e.g., water) and nonpolar (n-hexane or n-decane). This influence leads to faster evaporation of liquids in the presence of a relatively small amount of MLGO in the suspension (Fig. 7). This difference is minimal for n-decane (Fig. 7b, very slow evaporation) and maximal for n-hexane (Fig. 7a, fast evaporation). The effect increases with decreasing residual amount of a liquid. This is due to an increase in the surface area at the boundary with the gas phase and easier diffusion of the molecules toward evaporation centers at a solid surface. There the energy of intermolecular interactions is smaller than that in the volume or at the surface of bulk liquid with maximal surface tension. This result can be explained by diminution of a number of neighboring molecules and increasing their disorder at the interface of a non-uniform solid surface with mosaic hydrophilic (surface hydroxyls and other O-containing functionalities) and hydrophobic (pure carbon sheet fragments without oxygen) structures. A decrease in the surface tension is confirmed by a relatively small value of ␥S of bound water (Table 2), since it tends to decrease with decreasing amount of adsorbate [54]. Note that for all adsorbates studied on the initial stage, when the solid particles are completely immersed into the liquid, evaporation of free liquid occurs slightly faster than that for the suspension. However, as the amount of the liquid decreases and solid particles appear over the liquid surface, the evaporation is accelerated.

The polar functionalities, especially acidic COOH groups, attached to carbon sheets of MLGO affect the characteristics of bound water (as well as pH of the suspensions), which is characterized by broader 1 H NMR spectra than bulk free water (Fig. 8). Deprotonation of acidic groups with the formation of cations, such as H5 O2 + or H9 O4 + , results in the downfield shift of the 1 H NMR spectra of strongly associated water (SAW) at ␦H > 4 ppm. Other water structures characterized by the upfield shift correspond to weakly associated water (WAW) at ␦H = 1–2 ppm [54]. For deeper insight into the interfacial phenomena at a surface of MLGO and MLGO/nanosilica, these samples have been studied using DSC (Figs. 6 and 9, Table S1 and Figs. S10–S22 in SI) and lowtemperature 1 H NMR spectroscopy (Figs. 8, 10 and 11, Table 2, and Figs. S23–S29 and Table S2 in SI). An increase in relative amounts of polar (Fig. S10 in SI) or nonpolar (Fig. S11) liquids (i.e., an increase in the associativity of adsorbate molecules and a decrease in the effects caused by the interfaces [54]) leads to an increase in the exothermic effects during cooling and freezing of the adsorbates and endothermic effects during melting (Table S1). This result is in agreement with enhanced evaporation of disordered liquids in the presence of MLGO (Fig. 7). The effect of delay of freezing and melting DSC peaks (i.e., the hysteresis effect) in respect to the freezing-melting temperatures depends on a type of adsorbate, the amounts of adsorbate (co-adsorbates) and adsorbent and a cooling-heating rate. Several endotherms and exotherms can be observed due to the presence of various structures of adsorbate, which are larger or smaller, more or less ordered, and weaker or stronger interacting with the adsorbent surface and located in pores of different sizes. For water, five types of the structures such as bulk water (which does not sense the presence of an adsorbent), weakly (WBW) and strongly (SBW) bound waters (frozen close to 273 K or far from 273 K), and strongly (SAW) and weakly (WAW) associated waters (3D structures of larger sizes and 2D or branched 3D structures of smaller sizes, respectively) can be determined using both DSC and 1 H NMR spectroscopy methods [54]. For such long and linear molecules as n-decane, there are additional factors related to their conformerization (due to bending of the CC-chain and rotation of the groups around the C C bonds) and ordering-disordering of the molecules in the clusters and domains [87,96]. An increase in bending of the CC-chain and disordering of the molecular clusters lead to a decrease in intermolecular inter-

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Fig. 5. Particle size distributions of MLGO at content of (a–c) 0.05 wt.% or (d–f) 0.5 wt.% and pH (a–c) 2.9 or (d–f) 2.26 sonicated for (d) 2, (a) 4, (b and e) 5 and (c and f) 10 min (curves 1–3 correspond to the distributions in respect to the light scattering intensity, particle volume and particle number, respectively).

action energy. This leads to easier evaporation of them, as well as certain changes in the shifts of the exotherms or endotherms in the DSC thermograms. These effects at the interfaces depend on the amount and strength of adsorption surface sites affecting the structure of the adsorption layers. The presence of nanosilica in the MLGO/A-300 composite causes smaller effects on the behavior of a water/decane mixture than the amounts of the liquids, e.g., the effect of a strong increase in the content of water (Fig. S12 in SI). These effects can be explained by

coverage of silica particles by carbon structures and changes in the associativity of water with increasing its amount (similar to the effects observed in the TG/DTG/DTA curves, Fig. 3). Note that heating at temperatures higher that Tm (melting points) of both liquids (Fig. S12b) results in the appearance of several endotherms caused by changes in the organization of liquid clusters and domains and evaporation of water (60–120 ◦ C). Evaporation of n-decane (boiling point Tb ≈ 174 ◦ C) gives a small effect (at 170–200 ◦ C) due much

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Fig. 6. Adsorption isotherms of water and benzene for MLGO.

Fig. 8. 1 H NMR spectra of water bound to MLGO (curve 1) experimental (12 g of water per 1 g of MLGO at 270 K), theoretically calculated for water bound to MLGO (curve 2) or free nanodroplet with 2000H2 O (curve 3) using the PM7 method and a correlation function linking atomic charges qH (PM7) and the value of ␦H (calculated using GIAO/B3LYP/6-31G(d,p) for smaller systems [54]).

Fig. 7. Evaporation of (a) n-hexane (at smaller (curves 1 and 2) and larger (curves 3 and 4) amounts), (b) water or n-decane alone and bound to MLGO.

smaller content of decane (0.578 mg) than water (4.509 mg) in the studied sample. The pore size distributions, PSD (Fig. 9) calculated with the thermoporometry method [54] using the DSC melting thermograms of water, decane and water/decane at T < Tm = 0 ◦ C (water) or −29.3 ◦ C (n-decane) show the effects of co-adsorption of water and decane and mixing of MLGO with nanosilica. During co-adsorption of water and decane, water is located in larger pores than in the case of the adsorption of water alone. This is typical result [54] caused by diminution of the Gibbs free energy of the system with decreasing area of contacts between such immiscible liquids as water and decane. This effect leads to much smaller surface area in contacts with unfrozen water bound to MLGO placed in nonpolar solvents (Table 2, Su ). Mechanical mixing of MLGO with nanosilica results in location of water (Fig. 9a) or decane (Fig. 9b) in narrower pores than in the case of their co-adsorption onto MLGO alone. This effect can be due to partial decomposition of the carbon sheet stacks and

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Table 2 Characteristics of water unfrozen at T < 273 K or n-decane at T < 243 K and bound to MLGO and MLGO/silica estimated by NMR-cryoporometry. Medium

h (g/g)

cd ( g/g)

Su (m2 /g)

Su,nano (m2 /g)

Su,meso (m2 /g)

Su,macro (m2 /g)

Vu,nano (cm3 /g)

Vu,meso (cm3 /g)

Vu,macro (cm3 /g)

␥S (J/g)

−G (kJ/mol)

(K)

MLGO MLGOa MLGO MLGOa MLGO MLGOa MLGO/A-300 MLGO/A-300a MLGO/A-300 MLGO/A-300a MLGO/A-300 MLGO/A-300a MLGO/A-300 MLGO/A-300 MLGO/A-300

Air Air CDCl3 + TFAA CDCl3 + TFAA CDCl3 + TFAA + CD3 CN CDCl3 + TFAA + CD3 CN Air Air CDCl3 CDCl3 CDCl3 + TFAA CDCl3 + TFAA Air CDCl3 6CDCl3 + 1CD3 CN

2 2 2 2 2 2 0.1 0.1 0.1 0.1 0.1 0.1 0.24 0.47 0.47

1.5 1.5 1.5 1.5 1.5 1.5 0.07 0.07 0.07 0.07 0.07 0.07 0 0 0

104 50 126 934 358 665 30 11 13 30 71 43 36 43 48

6 0 36 0 191 0 20 0 5 0 64 0 19 22 10

98 50 87 934 166 665 10 11 8 30 8 43 17 18 36

0 0 4 0 0 0 0.1 0 0.1 0 0 0 0 3 2

0.003 0 0.017 0 0.074 0 0.009 0 0.003 0 0.025 0 0.007 0.010 0.004

0.783 0.094 1.224 1.406 1.591 1.172 0.086 0.020 0.091 0.040 0.069 0.050 0.164 0.241 0.378

0 0 0.068 0 0 0 0.001 0 0.002 0 0 0 0 0.036 0.020

11.7

2.63

35.0

2.51

46.5

3.40

4.1

2.88

3.3

2.65

7.4

3.50

5.3 6.1 7.6

3.54 2.79 2.92

265.4 238.1 259.4 226.0 259.7 234.6 252.3 232.8 256.6 197.3 234.4 179.5 258.8 262.0 263.5

Note. h is the hydration degree of samples (amounts of water in gram added per gram of dry material); cd is the content of decane, G is the changes in the Gibbs free energy of water layer closely located to a surface; ␥S is the modulus of the total changes in the Gibbs energy of bound water unfrozen at T < 273.15 K; Suw is the specific surface area in contact with unfrozen water or unfrozen decane, Su,nano and Vu,nano , Su,meso and Vu,meso , Su,macro and





Tm,0

Vu,macro are the specific surface area and pore volume of nanopores at R < 1 nm, mesopores at 1 nm < R < 25 nm, and macropores at R > 25 nm, respectively, filled by unfrozen water; and < Tm >= Tmin

average melting temperature, Tm,0 is the melting temperature of individual bulk ice or frozen decane, Tmin correspond to the temperature at which Cu = 0. a Calculations were carried out for unfrozen decane (others are for unfrozen water).

Tm,0 dCu (T ) TdT/ dT Tmin

dCu (T ) dT dT

is the

Fig. 9. Pore size distributions (PSD) calculated using DSC melting thermograms for (a) water and (b) decane bound to MLGO or MLGO/A-300 as water or decane alone (curves 1) or a mixture of water and decane (curves 2 and 3).

their aggregates upon interaction of MLGO with silica nanoparticles and their secondary structures (aggregates of primary particles and agglomerates of aggregates [54], Fig. 2). Additionally, location of adsorbed molecules in narrow voids between silica nanoparticles in their aggregates can contribute the PSD at R < 3.5 nm. Additional information on the properties of MLGO can be obtained using low-temperature 1 H NMR spectroscopy of static samples containing polar (water, acetonitrile, and TFAA) or nonpolar (n-decane) liquids bound to MLGO and MLGO/nanosilica being in different dispersion media (air, chloroform). For wetted MLGO placed in air, signals of water (␦H = 5.5–8.0 ppm) and decane (CH2 and CH3 groups around ␦H ≈ 1 ppm) decrease with decreasing temperature (Fig. S23a and b in SI). This decrease is strong at T < Tm,w of water (water signals) or at T < Tm,d decane (decane signals) due partial freezing-out of bound liquids [54]. Changes in the dispersion media from air to CDCl3 with addition of TFAA (Fig. S23c) or CD3 CN + TFAA (Fig. S23d) leads to stronger narrowing of signals of decane than that of water. This can be explained by the H-D exchange reactions for water and TFAA and the appearance of additional signal of ‘acidic’ protons, i.e., Zundel (H5 O2 + ) and

V.M. Gun’ko et al. / Applied Surface Science 387 (2016) 736–749

Sample

V.M. Gun’ko et al. / Applied Surface Science 387 (2016) 736–749

745

Fig. 10. Size distribution of (a and b) water and (c and d) decane clusters and domains in pores of MLGO at 2 g H2 O + 1.5 g C10 H22 per gram of MLGO in air (curves 1), CDCl3 (2 g/g) + F3 CCOOD (1.5 g/g) (curves 2), and CDCl3 (2 g/g) + CD3 CN (1 g/g) + F3 CCOOD (1.5 g/g) (curves 3); (a and c) differential and (b and d) incremental distributions.

Eigen (H9 O4 + ) cations. Changes in the temperature behavior of the amounts of unfrozen water and decane depend on the type of the dispersion media (Figs. S23, S25 and S27 in SI). The presence of TFAA results in a small downfield shift of water signals without their splitting. However, signals of decane (␦H = 0.7–2.0 ppm) split (Fig. S23c), but after addition of CD3 CN the splitting disappears (Fig. S23d). Water bound to mosaic hydrophobic-hydrophilic surfaces of MLGO (see models in Figs. 4, S7-S9 in SI) can form WAW (␦H = 1–2 ppm) and SAW (␦H > 3 ppm) structures that is affected by weakly polar (chloroform) dispersion medium. Signals of WAW are observed in the region of signals of decane. However, change in the dispersion medium type does not lead to a strong re-distribution of signal intensity of the main peaks (Fig. S23), ant it is not higher than 10%. Consequently, contribution of WAW can be small. According to the temperature dependences of the amount of unfrozen decane (Fig. S26a), melting of frozen decane bound to MLGO in air starts close to its Tm and stops at 288 K. Freezingmelting delay is observed for decane bound to MLGO studied using the DSC method (Fig. S11c). In chloroform and especially with addition of TFAA, decane melting occurs at lower temperatures, but delay of complete melting remains (Fig. S26a). This effect can be explained by textural changes of soft MLGO particles due to penetration of adsorbates between the carbon sheets in the stacks. Therefore, the specific surface area in contact with unfrozen decane and water strongly increases (Table 2). The PSD calculated using the

NMR cryoporometry method (Fig. 10) show enhanced contribution of nanopores and narrow mesopores in the CDCl3 media. However, the tendencies in changes in the location of unfrozen water and decane are opposite for a mixture with CDCl3 + CD3 CN + TFAA in comparison with CDCl3 + TFAA, since contribution of narrow pores filled by unfrozen water increases but that for decane decreases. In both cases, the values of S and V for nanopores and mesopores are much larger than that for MLGO/decane/water in air (Table 2). Note that contribution of macropores for all samples studied is small (Table 2, Figs. 10 and 11). Mechanical mixing of MLGO with nanosilica (1:4 w/w) affects the structural characteristics of the stacks of carbon sheets (Table 2, Figs. 11, S24 and S28). After addition of small amounts of water (0.1 g/g) and decane (0.075 g/g) water can penetrate into narrow pores of the MLGO/A-300 mixture placed in air. This effect increases in the chloroform or CDCl3 /TFAA dispersion media. Note that the ratio of signal intensity of water (5–6 ppm) and decane (1–2 ppm) corresponds to the content ratio of adsorbates at 285 K (Fig. S24a). The signal ratio decreases with decreasing temperature because the freezing-melting temperature of bulk decane is lower than that of water by 30 K. In the chloroform dispersion medium, the shape of signals changes, since two signals at 5.5–6 ppm and 44.5 ppm are observed (Fig. S24b). This effect can be explained by location of water in pores of different sizes (Fig. 11c and d) with various degrees of the associativity of adsorbed molecules. In this medium, location of decane in pores also changes since its pene-

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V.M. Gun’ko et al. / Applied Surface Science 387 (2016) 736–749

Fig. 11. Size distribution of (a–d) water and (e and f) decane clusters and domains in pores of MLGO/A-300 for (a and b) A-300 wetted by aqueous suspension (6 wt.%) of MLGO in amount of 0.25 g/g (curves 1) and 0.5 g/g placed in different media air (curves 1), CDCl3 (curves 2), and 6 CDCl3 + 1 CD3 CN (curves 3); (c–f) water (0.1 g/g) and decane (0.075 g/g) bound to MLGO/A-300 (1:4) in air (curves 1), CDCl3 (curves 2) and CDCl3 + F3 CCOOD (curves 3); (a, c and e) differential and (b, d and f) incremental distributions.

V.M. Gun’ko et al. / Applied Surface Science 387 (2016) 736–749

tration into nanopores strongly increases (Fig. 11e and f, curves 1 and 2). Note that the ratio of signals at 4–6 and 1–2 ppm at 285 K slightly decreases in the chloroform medium (from 1.8 in air to 1.6 in CDCl3 ). Therefore, one can assume that a portion of WAW can be formed in narrow pores. In the CDCl3 + TFAA medium, the downfield shift of signals of SAW and WAW is observed (Fig. S24c) due to the H-D exchange reaction between water and CF3 COOD resulting in appearance of solvated ‘acidic’ protons (i.e., Zundel and Eigen cations) with great values of ␦H . In this medium, localization of water and decane clusters and domains changes (Table 2, Fig. 11). At least, there are two types of bound water structures characterized by different activity as a solvent and dissolved different amounts of TFAA. A greater amount of dissolved TFAA corresponding to signals at greater values of ␦H (Fig. S24c) can be caused by acid molecules dissolved in larger water structures located in larger pores. In the case of greater amounts of water bound to MLGO and nanosilica upon wetting of A-300 by 6 wt.% aqueous suspension of MLGO (Figs. 11 a and b, S25 and S28), the temperature behavior of 1 H NMR signals, as well as the amounts of unfrozen water vs. temperature (Fig. S27), changes in comparison with the systems analyzed above. In air, signal of bound water (added 0.25 g/g of the MLGO suspension) is asymmetric (Fig. S25a) due to the presence of, at least, two types of water structures (at ␦H ≈ 5 and 6 ppm) located in narrower and broader pores and interacting with silica and MLGO surfaces. In the CDCl3 medium (Fig. S25b), both signals become narrower. Weaker signal at greater value of ␦H can be attributed to water bound in narrow pores of MLGO (Fig. 11a and b, Table 2). Stronger signal (Fig. S25b) is due to water bound in broader voids between nanoparticles of silica in their aggregates (Fig. 11a and b). In complex dispersion media (6 CDCl3 + 1 CD3 CN (Fig. S27c) and 6 CDCl3 + 1 CD3 CN + 1 F3 CCOOD (Fig. S25d)), there are signals of water with hydrogen bonds to acetonitrile molecules (at ␦H ≈ 2 ppm), WAW (␦H ≈ 1.5 ppm), and SAW (4–6 ppm). Weak signal at ␦H ≈ 7.2 ppm is due to CHCl3 presented as an admixture in CDCl3 . Addition of TFAA into the dispersion medium results in the downfield shift of water signals, which can be stronger and weaker dependent on the activity of water as a solvent that is varied vs. the sizes of water structures located in pores of different sizes with different structures of the pore walls with silica and MLGO. Note that in the case of commercial MLGO and SLGO (Table S2 and Fig. S29 in SI), the specific surface area in contact with unfrozen water depends strongly on the amounts of water due to disaggregation of the carbon sheets with increasing amount of water.

4. Conclusion MLGO produced from natural flake graphite as a precursor (flakes < 0.2 mm) using a modified method of ionic hydration and freeze-dried has low bulk density and typical light brown color. During interaction of MLGO with a mixture of water (2 g/g) and decane (1.5 g/g) the surface area in contact with unfrozen liquids at T < Tf can be greater than 1000 m2 /g that corresponds to the formation of stacks with 3–5 carbon sheets (layers). Interaction between the carbon sheets in dry MLGO is very strong and nonpolar molecules such as benzene, decane, and nitrogen practically cannot penetrate between the sheets, i.e., intercalation adsorption is small. Water molecules can effectively penetrate (this is rather intercalation adsorption) between the sheets if MLGO is located in weakly polar (CDCl3 ) dispersion medium. In this case, decane molecules can also penetrate into the carbon sheet stacks. The value of the specific surface area of MLGO in contact with unfrozen water can reach 360 m2 /g but the value of S for decane can be up to 930 m2 /g. Thus, despite the presence of a lot of O-containing functionalities

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(∼11 at.% of oxygen in MLGO), hydrophobic interactions of MLGO with decane can be stronger that with water. The main driving force of the textural changes is the requirement of a minimal surface area of contacts between immiscible liquids such as water and chloroform or decane. Note that water without decane at lower amounts (0.25 or 0.5 g/g) both in air or CDCl3 dispersion media can provide much smaller value of the specific surface area (30–50 m2 /g) of MLGO/A-300 (1:4 w/w) composite in contact with unfrozen water, which does not form continuous coverage of the solid surface. Pores in wetted MLGO or MLGO/A-300 mainly correspond to mesopores partially filed by unfrozen liquids at T < Tf . Thus, nanosilica does not provide significant opening of MLGO stacks and their aggregates during mechanical mixing of the materials. The MLGO structure remains similar to that of MLGO alone because nanosilica is rather ‘soft’ matter in respect to aggregates of nanoparticles and agglomerates of aggregates. Acknowledgments The authors are grateful to European Community, Seventh Framework Programme (FP7/2007–2013), Marie Curie International Research Staff Exchange Scheme (IRSES grant No. 612484) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.06. 196. References [1] J. Zhang, M. Terrones, C. Rae, R. Mukherjee, M. Monthioux, N. Koratkar, Y.S. Kim, R. Hurt, E. Frackowiak, T. Enoki, Y. Chen, Y. Chen, A. Bianco, Carbon science in 2016: Status, challenges and perspectives, Carbon 98 (2016) 708–732. [2] X. Zhuang, Y. Mai, D. Wu, F. Zhang, X. Feng, Two-dimensional soft nanomaterials: a fascinating world of materials, Adv. Mater. 27 (2015) 403–427. [3] B.Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906–3924. [4] C. Soldano, A. Mahmood, E. Dujardin, Production, properties and potential of graphene, Carbon 48 (2010) 2127–2150. [5] Y. Zhu, D.K. James, J.M. Tour, New routes to graphene, graphene oxide and their related applications, Adv. Mater. 24 (2012) 4924–4955. [6] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [7] K.S. Subrahmanyam, S.R.C. Vivekchand, A. Govindaraj, C.N.R. Rao, A study of graphenes prepared by different methods: characterization, properties and solubilisation, J. Mater. Chem. 18 (2008) 1517–1523. [8] Y.J. Hu, J.A. Jin, H. Zhang, P. Wu, C.X. Cai, Graphene: synthesis functionalization and applications in chemistry, Acta Phys.—Chim. Sin. 26 (2010) 2073–2086. [9] Y. Tang, H. Guo, L. Xiao, S. Yu, N. Gao, Y. Wang, Synthesis of reduced graphene oxide/magnetite composites and investigation of their adsorption performance of fluoroquinolone antibiotics, Colloids Surf. A: Physicochem. Eng. Aspects 424 (2013) 74–80. [10] X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H. Zhang, Graphene-based materials: synthesis, characterization, properties, and applications, Small 7 (2011) 1876–1902. [11] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [12] V.E. Muradyan, M.G. Ezernitskaya, V.I. Smirnova, N.M. Kabaeva, Yu.N. Novikov, Z.N. Parnes, M.E. Volpin, Transformation of graphite oxide in conditions of ionic hydration, J. Gen. Chem. 61 (1991) 2626–2629. [13] V.M. Gun’ko, Ya.V. Zaulychnyy, B.I. Ilkiv, V.I. Zarko, Yu.M. Nychiporuk, Yu.G. ˛ Textural and Ptushinskii, E.M. Pakhlov, R. Leboda, J. Skubiszewska-Zieba, electronic characteristics of mechanochemically activated composites with nanosilica and activated carbon, Appl. Surf. Sci. 258 (2011) 1115–1125. [14] I. Sheet, A. Kabbani, H. Holail, Removal of heavy metals using nanostructured graphite oxide, silica nanoparticles and silica/graphite oxide composite, Energy Procedia 50 (2014) 130–138. [15] A. Waheed, A. Majeed, N. Iqbal, W. Ullah, A. Shuaib, U. Ilyas, F. Bibi, H.M. Rafique, Specific capacitance and cyclic stability of graphene based

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