Thin Solid Films 596 (2015) 242–249

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Nanoporous spongy graphene: Potential applications for hydrogenadsorption and selective gas separation

Nikolaos Kostoglou a,b,⁎, Georgios Constantinides c, Georgia Charalambopoulou d, Theodore Steriotis d,Kyriaki Polychronopoulou e, Yuanqing Li f, Kin Liao f, Vladislav Ryzhkov g, ChristianMitterer b, Claus Rebholz a,⁎⁎a Department of Mechanical and Manufacturing Engineering, University of Cyprus, 1678 Nicosia, Cyprusb Department of Physical Metallurgy and Materials Testing, Montanuniversitӓt Leoben, 8700 Leoben, Austriac Research Unit for Nanostructured Materials Systems, Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology, 3036 Lemesos, Cyprusd National Center for Scientific Research Demokritos, Agia Paraskevi Attikis, 15310 Athens, Greecee Department of Mechanical Engineering, Khalifa University of Science, Technology and Research, Abu Dhabi, UAEf Department of Aerospace Engineering, Khalifa University of Science, Technology and Research, Abu Dhabi, UAEg Nanotube Production Department, Fibrtec Incorporation, TX, 75551 Atlanta, USA

⁎ Correspondence to: N. Kostoglou, Department of PhyTesting, Montanuniversitӓt Leoben, 8700 Leoben, Austria.⁎⁎ Corresponding author.

E-mail addresses: nikolaos.kostoglou@stud.unileoben.claus@ucy.ac.cy (C. Rebholz).

http://dx.doi.org/10.1016/j.tsf.2015.06.0600040-6090/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 May 2015Received in revised form 15 June 2015Accepted 16 June 2015Available online 21 July 2015

Keywords:GrapheneNanoporous spongeWet reductionFreeze dryingGas sorptionGas selectivity

In the presentwork, a nanoporous (porewidth ~ 0.7 nm) graphene-based sponge-likematerialwith large surfacearea (~350 m2/g) was synthesized by wet chemical reduction of graphene oxide in combination with freeze-drying. Surfacemorphology and elemental composition were studied by scanning and transmission electronmi-croscopy combinedwith energy dispersive X-ray spectroscopy. Surface chemistrywas qualitatively examined byFourier-transform infrared spectroscopy, while the respective structure was investigated by X-ray diffractionanalysis. Textural properties, including Brunauer–Emmet–Teller (BET) surface area, micropore volume and sur-face area as well as pore size distribution, were deduced from nitrogen gas adsorption/desorption data obtainedat 77 K and up to 1 bar. Potential use of the spongy graphene for gas storage and separation applicationswas pre-liminarily assessed by low-pressure (0–1 bar) H2, CO2 and CH4 sorptionmeasurements at different temperatures(77, 273 and 298K). The adsorption capacities for each gaswere evaluatedup to ~1 bar, the isosteric enthalpies ofadsorption for CO2 (28–33 kJ/mol) and CH4 (30–38 kJ/mol) were calculated using the Clausius–Clapeyron equa-tion, while the CO2/CH4 gas selectivity (up to 95:1) was estimated using the Ideal Adsorbed Solution Theory(IAST).

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Graphene is considered one of themost promising nanomaterials foradvanced applications in the fields of nanoelectronics and nanosensors[1,2], coatings and thin films technology [3,4], nanocomposite struc-tures [5,6], catalytic chemistry [7,8], environmental engineering[9–11], gas separation and storage [10–13], aswell as energy conversionand storage [14–17]. Visualized as a two-dimensionalmonolayer of sp2-bonded carbon atoms forming multiple hexagonal cells, this recentlyisolated [18,19] and extensively studied carbon allotrope combines aseries of unique properties, including high electrical and thermalconductivity, remarkable strength and stiffness and an impressivelylarge surface area relative to its mass [20–22]. Based on theoretical

sical Metallurgy and Materials

ac.at (N. Kostoglou),

calculations an individual graphene sheet can provide a specific surfacearea as high as 2630 m2/g (i.e. 1315 m2/g for each side) [23]. However,single planar graphene sheets are thermodynamically unstable and usu-ally form few- or multi-layer stacks, as well as curved nanostructures(e.g. nanotubes, fibers, fullerenes, etc.).

Production of graphene has beenwidely reported over thepast yearsmostly through micro-mechanical exfoliation/peeling of pyrolyticgraphite [18,24], epitaxial growth by chemical vapor deposition orhigh-temperature segregation [25] and reduction of graphite- orgraphene-oxide using thermal annealing, solvothermal/hydrothermal,electrochemical or microwave methods [26–30]. Especially the latteris considered the “bottleneck” in the development of graphene technol-ogy due to the need to optimize the reduction process and produce pris-tine graphene in high yield [31]. A plethora of porous graphene-basednanostructures, including nanosheets [32–34], sponges [35–40] andfoams [41–43], have been synthesized with potential to be used as ad-vanced nanoelectronic components for energy conversion and storagedevices [29,33,34,37–39,43,48] and as highly-efficient adsorbents forliquid substances and gases [32,35,36,40–42,44–47]. Specifically, they

243N. Kostoglou et al. / Thin Solid Films 596 (2015) 242–249

have been proposed as electrodes and substrates for super-capacitors[29,43], batteries [33,34,38], fuel cells [39] and solar cells [48], as wellas sorption materials for water purification from oils and organic sol-vents [35,36,41], sequestration of unwanted greenhouse emissionssuch as carbon dioxide (CO2) [45–47] and storage of highly-dense ener-gy carriers such as methane (CH4) and hydrogen (H2) [32,40,42,44,45].These nanoscale materials are considered attractive for the aforemen-tioned applications as they combine low densities, large surface areas,tunable pore sizes and volumes, excellent thermal and chemical stabil-ity, environmental friendliness, non-toxicity and low production cost[16,49,50].

Focusing onmaterials-based gas storage applications, an intense re-search effort has been devoted towards the development of few-layergraphene or graphene oxide (GO) sorbents with narrow microporesizes and large surface areas that can physically adsorb/store variousgases (e.g. H2, CO2, CH4) into their porous network [13]. Srinivas et al.[45] produced a series of nanoporous (1.2–2.6 nm) and high-surfacearea (916–1894 m2/g) GO-derived carbons by potassium hydroxide(KOH) activation that demonstrated a favorable behavior towards CO2

and CH4 adsorption. Lyth et al. [42] synthesized a high-surface area(1269 m2/g) graphene foam through the combustion of sodiumethoxide (C2H5ONa) that showed a significant H2 storage capacity of2.1 wt.% at 77 K and 10 bar. Recently, Sui et al. [47] prepared hydrother-mally reduced GO sampleswith large surface areas (420–870m2/g) andnarrow-size micropores (~0.6 nm) that exhibited high CO2 adsorptioncapacities up to 2.4 mmol/g at 273 K and 1 bar.

In the current study, we have prepared a nanoporous spongy deriv-ative of GO by combining wet chemical reduction (i.e. hydrothermaltreatment using a reducing agent) and freeze-drying methods. The as-prepared spongy material was extensively characterized with respectto its morphology, elemental composition, surface chemistry, texture/porosity and structure using scanning electron microscopy (SEM) andtransmission electronmicroscopy (TEM), energy dispersive X-ray spec-troscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), nitro-gen (N2) adsorption at 77 K and X-ray diffraction (XRD), respectively.Great emphasis was placed on investigating its behavior towards H2

cryo-adsorption at 77 K, as well as CO2 and CH4 adsorption at 273 Kand 298 K, on the basis of systematic low-pressure (0–1 bar) measure-ments. The interaction strength of both CO2 and CH4 with the graphenesurface as well as the CO2 over CH4 selectivity were two of the main gassorption properties evaluated and discussed in this study within thecontext of potential gas storage and separation applications.

2. Materials and experimental methods

2.1. Graphene sponge synthesis

The graphene sponge-like material (denoted hereafter as GS) wassynthesized by wet chemical reduction of GO using hydriodic acid(HI) followed by freeze drying. The original GOwas prepared by oxidiz-ing graphite powder (Sigma-Aldrich.; b20 μm) using a modifiedHummer's method, as already described in previous studies [37,51].For the preparation of GS, about 120 mg of GO were easily dispersedin 60 ml aqueous solution by ultra-sonication for 30 min using a con-ventional ultra-sonic bath. Next, 1 ml of HI (Sigma-Aldrich; 30%) wasadded and the mixture was placed inside a polytetrafluoroethylene(PTFE)-lined stainless steel autoclave reactor for 12 h at 180 °C. Agraphene hydrogel (GH) was subsequently collected from the reactorand was instantly submerged into de-ionized water for 24 h to removeany residual HI from its surface. Water was removed by 75 wt.% fromthe as-prepared GH through room temperature evaporation in orderto obtain a final product with an enhanced surface area [37,40]. In afinal step, the partially dried GH was freezed with liquid N2 and thenfreeze-dried in vacuum overnight to remove the excess water. By theend of this procedure, a black and fluffy powder was obtained.

2.2. Characterization techniques

Surface morphology was studied with a Quanta 200 (FEI) SEM usingan acceleration voltage of 20 kV. The specimenwas sputter-coated withsilver (SC7640, Quorum Technologies) in argon atmosphere to avoidany charging effects during imaging. The elemental composition of thesamplewas subsequently analyzed using an EDVACGenesis X-ray anal-ysis probe at various sites and spectral patternswere generated for eachsession of analysis. High resolution images were obtained by a PhilipsCM-20 TEM with a lanthanum hexaboride (LaB6) filament using an ac-celeration voltage of 200 kV.

FT-IR spectra were collected by a Thermo Scientific Nicolet 6700 FT-IR spectrometer in the mid-infrared region of 4000–400 cm−1. Theinstrument was equippedwith an attenuated total reflection (ATR) dia-mond crystal, a N2 purging system for CO2 and moisture and a wide-range mercury cadmium telluride (MCT) detector cooled by liquid N2.XRD patternswere collected by a Rigaku R-AXIS IV imaging plate detec-tormounted on a RigakuRU-H3R rotating copper (Cu) anodeX-ray gen-erator (λ = 1.54 Å). The Origin Lab-9 software was used for thedeconvolution of the diffractogram in the 2θ region 0–62° and the de-tailed analysis of the structural features.

N2 and H2 adsorption/desorption isotherms at 77 K were recordedup to 1 bar using an Autosorb 1-MP (Quantachrome) volumetric gassorption analyzer. Ultra-pure N2 (99.999%) and H2 (99.9999 %) gaseswere used. Prior tomeasurement, the samples (~40mg)were degassedunder high vacuum (10−6 mbar) at 250 °C for approximately 12 h. Po-rous properties were determined by analyzing the N2 adsorption/desorption data using Quantachrome's ASiQWin software. Total specificsurface area (SSA)was calculated by themulti-point Brunauer–Emmet–Teller (BET) method in the relative pressure range 0.05 b P/P0 b 0.20.Micropore volume andmicropore SSA were both calculated by the Car-bon Black statistical thickness equation (t-plot). Pore size distribution(PSD) was estimated by the Quenched Solid Density Functional Theory(QSDFT) using the nitrogen–carbon equilibrium transition kernel at77.4 K for slit-shaped pores.

CO2 and CH4 adsorption/desorption isothermal curves wererecorded both at 273 K and 298 K and up to 1 bar using a Micromeritics3Flex volumetric gas sorption analyzer along with a PolySciencecirculating bath (50:50 vol.% water:ethylene glycol mixture) for main-taining constant temperature during experiment. Same degassing con-ditions were applied to the samples (~50 mg) as previously described(10−6 mbar and 250 °C for 24 h), while both carbon dioxide and meth-ane gas of ultra-high purity (99.9999%) was used. The isosteric en-thalpies of adsorption (ΔΗ), towards both CO2 and CH4, at a constantsurface coverage (θ)were extracted using the Clausius–Clapeyron equa-tion [52] for the two operating temperatures (i.e. 273 and 298 K):

ΔΗ ¼ −R∂ ln Pð Þ∂ 1=Tð Þ

� �θ

ð1Þ

where, R is the universal gas constant (8.314 J mol−1 K−1), P is thepressure in mbar and T is the temperature in K. The adsorption selectiv-ity of CO2 over CH4 was calculated using the Ideal Adsorbed SolutionTheory (IAST) approach [53,60]. The single component adsorption iso-therms were described by fitting the data with the following virial-type equation:

P ¼ vK

exp C1vþ C2v2 þ C3v3 þ C4v4� � ð2Þ

where, P is the pressure inmbar, v is the amount adsorbed inmmol g−1,K is the Henry constant inmmol g−1 mbar−1 and Ci are the constants ofthe virial equation. The free energy of desorption at a given value of

20 µm

5 µm

1 µm

Fig. 1. SEM images of the as-prepared GS material at three different magnifications.

244 N. Kostoglou et al. / Thin Solid Films 596 (2015) 242–249

temperature and pressure G(T,P) of the gas is obtained from the analyt-ical integration of Eq. (2):

G T; Pð Þ ¼ RTZP

0

nPdP ¼ RT vþ 1

2C1v2 þ 2

3C2v3 þ 3

4C3v4 þ 4

5C4v5

� �ð3Þ

The free energy of desorption is a function of temperature and pres-sure G(T,P) and describes the minimum work (i.e. Gibbs Free Energy)required to completely degas the adsorbent surface. For a binary mix-ture of components i and j, Eq. (4) yields the individual pure loadingsvio and vj

o at the same free energy of desorption:

G0i v0i� � ¼ G0

j v0j�

: ð4Þ

The partial pressure of components i and j in an ideal adsorptionmixture can be given:

Pyi ¼ P0i v0i� �

xi ð5Þ

Pyj ¼ P0j v0j�

xj: ð6Þ

There, yi=(1− yj) and xi=(1− xj) are themolar fractions of com-ponent i in the gas phase and the adsorbed phase, respectively, and Pi

o

and Pjo are the pure components pressures of i and j, respectively. The

selectivity (Si,j) of the adsorbates i and j and the total pressure (P) ofthe gas mixture can be obtained from Eqs. (7) and (8), respectively:

Si; j ¼xi=yix j=yj

¼ P0j

P0i

ð7Þ

P ¼Xj

i

P0i xi: ð8Þ

3. Results & discussion

3.1. Morphological features

The SEM images, shown in Fig. 1, reveal the sponge-likemorphologyof the synthesized GS. Specifically, it exhibits a three-dimensional net-work of open cell/strut structures (size b 5 nm), thus potentiallyallowing gas and liquids to penetrate through, while at the same timeinteracting with the significantly increased surface area of the material.It appears that the employed synthesis process promotes the exfoliationof graphene stacks and their connection into a continuous network.Moreover, high-magnification TEM images (Fig. 2) support the SEM ob-servations by indicating a “wrinkled” structure with folded regionsformed due to the overlap of exfoliated graphene layers.

3.2. Elemental composition

In all of the recorded EDS patterns the highest peak always occurredfor the C Kα (carbon) peak at around 0.3 keV and the O Kα (oxygen)peak at around 0.5 keV. The occurrence of an Ag peak is related to thematerial used for sputter-coating the specimen for improved conductiv-ity. Peaks of Si, Ca, Cu and S are related to impurities incorporated duringthe synthesis process of GS. Their overall % atomic composition is lessthan 1 % (Table 1). As expected, carbon dominates the spectrum with~83 at.% or ~74 wt.%. The increased oxygen percentage (~15 at.% or~18 wt.%) of the graphene nano-sponge system is related to the wetchemical reduction of the original GO using HI (Section 2.1). It appearsthat the reduction process retains some oxygen within the graphenenetwork at a gravimetric ratio of 1 to 4 (specifically O/C ~ 0.24).

3.3. Surface chemistry

The FT-IR spectrum of the GS, shown in Fig. 3, exhibits three charac-teristic vibrational bands in the mid-infrared region 2000–400 cm−1

[54,55]. The strong band around 1556 cm−1 is related to the skeletal vi-brationmode of graphene sheets and indicates the aromatic C_C bond.The band observed at around 1722 cm−1 corresponds to the C_Ostretching vibration mode of carbonylic and carboxylic surface groups.The third band spotted at around 1028 cm−1 is probably attributed toC–O and/or C–OH stretching vibrations of various oxygen-based func-tionalities (i.e. carboxyls, epoxyls, alkoxyls and phenols). Hence, theFT-IR observations agree well with the EDS analysis (Section 3.2) andfurther confirm the presence of residual oxygen (and/or oxygen con-taining groups) in the GS material.

3.4. Textural/porosity properties

N2 adsorption/desorption isotherms curves at 77 K are presented inFig. 4. The synthesized GS demonstrated a type IV sorption isotherm

Fig. 2. High-magnification TEM image of the as-prepared GS material.

Fig. 3. FT-IR spectrum of the as-prepared GS material in the mid-infrared region.

245N. Kostoglou et al. / Thin Solid Films 596 (2015) 242–249

based on the International Union of Pure and Applied Chemistry(IUPAC) classification [56], typical of mesoporous materials (porewidth 2–50 nm); the H3-type hysteresis branch indicates pores withslit-shape. Moreover, the characteristic single-step observed next tothe point of loop closure (P/P0 ~ 0.5) strongly implies that desorptionis achieved through themechanism of “spontaneous” cavitation. Specif-ically, N2 is evaporated from the mesopores through the spontaneousnucleation and growth of gaseous bubbles inside the condensed phase[57]. At higher relative pressures (up to P/P0 ~ 0.99) the adsorptioncurve does not reach a saturation point, but instead it extends indefi-nitely in a vertical direction. This behavior represents an unrestrictedmulti-layer adsorption of N2 due to the presence of macropore surface(pore width N 50 nm). At lower relative pressures (P/P0 b 0.05) the GSmaterial demonstrates enhanced N2 physisorption, most probablydue to microporosity (pore width b 2 nm). Specifically, as shownin the inset of Fig. 4a, the adsorbed volume begins from ~52 cm3/g atP/P0 ~ 10−3, while it reaches up to ~78 cm3/g at P/P0 ~ 0.05.

Table 2 summarizes the porosity-related properties of the GS mate-rial based on the different methods applied on the nitrogen sorptiondata (i.e. multi-point BET, Carbon Black t-plot, QSDFT). The GS exhibitsa BET specific surface area of 348.2 m2/g, which is much larger thanthat of the non-exfoliated graphite oxide (i.e. 10–20 m2/g). The micro-pore surface area of 87.6 m2/g (0.04 cm3/g in terms of micropore vol-ume), determined by the Carbon Black t-plot method, corresponds tothe ~25% of the overall BET surface area. The SSA derived by theQSDFT method (348.7 m2/g) agrees well with the BET value, while thecumulative volume of pores with width less than 14 nm is estimatedat 0.42 cm3/g. The PSD curve (Fig. 4b) exhibits two distinct peaks; thesharper one, detected at ~0.7 nm, is related to the presence of ultra-micropores while the second broader one, between 2.5 and 4.5 nm(average value ~ 3.4 nm), could be considered an artifact attributed tothe cavitation mechanism.

Table 1Elemental composition expressed by atomic (at.%) and weight (wt.%) percent of the as-prepared GS material based on the EDS analysis.

Composition C O Ca Ag⁎ Si Cu S

at.% 83.29 15.15 0.68 0.61 0.13 0.08 0.06wt.% 74.29 17.99 2.01 4.90 0.27 0.40 0.14

⁎ Ag relates to the material used for sputter-coating.

3.5. Structural components

Fig. 5 represents the deconvoluted XRD pattern of the GS materialafter subtraction of a linear background using Origin Lab-9 software.

Fig. 4. (a) N2 adsorption and desorption isotherms at 77 K of the as-prepared GSmaterial;the inset shows the adsorption behavior at the lower relative pressures (P/P0 b 0.05) and(b) differential and cumulative pore size distribution (PSD) as determined by the QSDFTmethod.

Table 2Textural/porosity properties of the as-prepared GS material based on N2 adsorption/de-sorption isotherms at 77 K.

SBET[m2/g]

Smicro

[m2/g]Smicro(p)

[%]Vmicro

[cm3/g]SQSDFT[m2/g]

VQSDFT

[cm3/g]WQSDFT

[nm]

348.2 87.6 25.1 0.04 348.7 0.42 0.66

SBET: total specific surface area calculated by the multi-point Brunauer–Emmet–Teller(BET) method, Smicro: micropore specific surface area calculated by the t-plot method,Smicro(p): % percentage of micropore specific surface area to BET specific surface area((Smicro/SBET) ×100), Vmicro: micropore volume calculated by the Carbon Black statisticalthickness equation (t-plot), SQSDFT: total specific surface area calculated by the QuenchedSolid Density Functional Theory (QSDFT)method, VQSDFT: cumulative pore volume (poreswith width b 14 nm) calculated by the QSDFTmethod,WQSDFT: average pore width calcu-lated by the QSDFT method.

Table 3Structural components of the as-prepared GS material based on the Gaussiandeconvolution of the XRD pattern.

Reflection Line[color]

2θ[°]

D[Å]

FWHM[°]

Lc[Å]

La[Å]

Layers[no.]

RA[counts]

CF[%]

002 Red 24.80 3.59 6.54 12.2 – 4–5 87,195 22002 Green 24.80 3.59 3.23 24.7 – 8 28,806 7002 Blue 24.93 3.57 16.4 4.8 – 2–3 284,451 71100 Magenta 44.01 2.06 3.76 – 46 – 18,017 –100 Yellow 43.07 2.10 1.44 – 120 – 7593 –100 Dark

yellow43.07 2.10 14.32 – 12 – 220,481 –

2θ: diffraction angle, d: interlayer distance calculated by Bragg's law, FWHM: full-width athalf maximum, Lc: crystallite thickness estimated by Scherrer equation, La: crystallite lat-eral size estimated by Scherrer equation, RA: Reflection area, CF: % carbon fraction.

246 N. Kostoglou et al. / Thin Solid Films 596 (2015) 242–249

The broad hump within the range of 2θ ~ 12–40ο was fitted with wideand narrowGaussian peaks at 2θ ~ 26°, representing thinner and thickergraphene nanoparticles, respectively. The peak at 2θ ~ 25–26.6° corre-sponds to aromatic carbon atoms (i.e. graphitic carbon) [58,59], whilethere was no sign of amorphous carbon (expected at 2θ ~ 20°). It is as-sumed that a total area of the aforementioned peaks within the rangeof 2θ ~ 12–40ο, after subtraction of a possible background, representsall the carbon atoms in the sample.

Table 3 shows results of the best deconvolution of Gaussian peaksrepresenting three types of few-layered graphene-like crystallites. Theinterlayer distances (d) were calculated by Bragg's law, while crystallitethicknesses (Lc) and lateral sizes (La) were estimated from the 002 and100 reflections, respectively, using Scherrer's equations. Two- andthree-layered graphene-like crystallites, having lateral sizes of justabout 1.2 nm represent about 71wt.% of all the carbon in the GS sample.The lateral size roughly corresponds to the average pore size (~0.7 nm),as determined by the QSDFT method in Section 3.4. The second largefraction of carbon (22 wt.%) is made of 4–5 layered graphene-like crys-tallites with lateral size of about 4.6 nm. The largest crystallites makejust 7 wt.% of the carbon and are about 12 nm in lateral size and2.47 nm in thickness (about 8 layers). Finally, even though Ca wasfound at 0.6 at.% by EDS (Section 3.2), no reflections of calcite (CaCO3)are clearly seen in the diffractogram, indicating that the CaCO3 structurewas probably decomposed under the preparation of the GS.

3.6. Gas sorption, storage and selectivity

Low-pressure H2 cryo-adsorption/desorption isothermal curves re-corded at 77 K are shown in Fig. 6. As similarly described in our previous

Fig. 5.DeconvolutedXRD pattern of the as-prepared GSmaterial after subtraction of linearbackground (see Table 3 for color code).

work [40], the GS exhibits a fully reversible isotherm (type I based onIUPAC) and no hysteresis loop appears between adsorption and desorp-tion. The H2 storage capacity reaches up to 53.9 cm3/g or 0.48wt.% at at-mospheric pressure (~1 bar) and liquid nitrogen temperature (~77 K),which is a relatively moderate value compared to other porousgraphene-based carbons found in the literature for similar experimentalconditions [32,42,44]. However, considering its BET surface area value(~348 m2/g) and the low-pressure regime, the GS exhibits an adequateH2 uptake to SSA ratio of 1.37 × 10−3 wt.% m−2 g. This is in good agree-mentwith Xia et al. [46], who evaluated the H2 adsorption behavior of ahierarchical porous graphene (BET SSA ~ 239 m2/g and microporeSSA ~ 88 m2/g) derived by CO2 activation of graphite oxide at 750 °C,that demonstrated a capacity of 0.46 wt.% at 77 K and 1 bar. Moreover,an enhancedH2 physisorption behavior is observed at the lower operat-ing pressures (b10 mbar), as shown in the inset of Fig. 6, which is mostprobably attributed to microporosity (pore width b2 nm). Therefore, itseems that the recorded H2 storage capacity at 77 K is attributed to theavailable surface area of the GSmaterial, as well to the fraction ofmicro-pores acting as “strong” adsorption/binding sites for the H2 moleculesdue to the overlapping of the potential fields of the opposite pore walls.

Low-pressure CO2 and CH4 adsorption isothermal curves wererecorded both at 273 K and 298 K (Figs. 7a and 8a). The isothermsare completely reversible (i.e. without a hysteresis loop) forboth gases at both temperatures with desorption following exactlythe same path as adsorption (desorption is not shown for claritybetween the two temperatures). As expected, the uptake of bothgases is enhanced at 273 K rather than 298 K (i.e. lower tem-peratures favor physisorption processes) and specifically by 8.61 cm3/g(or 0.38 mmol/g) and 3.71 cm3/g (or 0.17 mmol/g) for CO2 and

Fig. 6. Low-pressure H2 adsorption and desorption isothermal curves at 77 K of the as-prepared GS material; the inset shows the adsorption behavior at the lower pressures(P b 10 mbar).

Fig. 7. (a) Low-pressure CO2 adsorption isothermal curves at 273 K (square symbols) and298 K (circle symbols) of the as-prepared GSmaterial and (b) isosteric heat of adsorptioncalculated by the Clausius–Clapeyron equation.

Fig. 8. (a) Low-pressure CH4 adsorption isothermal curves at 273 K (square symbols) and298 K (circle symbols) of the as-prepared GSmaterial and (b) isosteric heat of adsorptioncalculated by the Clausius–Clapeyron equation.

Table 4Gas storage capacities of the as-prepared GSmaterial towardsH2, CO2 and CH4 adsorption.

Gas Temperature[K]

Pressure[bar]

Cv

[cm3/g]Cn[mmol/g]

Cg

[wt%]Cg/SBET[wt%/ m2/g]

H2 77 1.01 53.95 2.40 0.48 1.37 × 10−3

CO2 273 1.01 19.39 0.86 3.64 10.45 × 10−3

CO2 298 1.01 10.78 0.48 2.06 5.91 × 10−3

CH4 273 0.96 5.72 0.25 0.39 1.12 × 10−3

CH4 298 0.96 2.01 0.08 0.12 0.34 × 10−3

Cv: volume of adsorbed gas at standard temperature–pressure (STP) conditions, Cn:mole of adsorbed gas, Cg: gravimetric capacity of adsorbed gas calculated by the ratio[mgas/(mgas + msample)], (Cg/SBET): ratio between gravimetric capacity and BET specificsurface area.

247N. Kostoglou et al. / Thin Solid Films 596 (2015) 242–249

CH4, respectively, at ~1 bar pressure. As shown in Fig. 7a, the CO2

storage capacity is 19.39 cm3/g (or 0.86 mmol/g) and 10.78 cm3/g (or0.48 mmol/g) at 273 K and 298 K, respectively, and ~1 bar. Theisosteric enthalpy of adsorption (Fig. 7b), calculated at these twotemperatures using the Clausius–Clapeyron equation (Eq. (1)), beginsfrom ~33 kJ/mol at low CO2 coverage (~2 cm3/g), then decreases to~28 kJ/mol and finally slightly increases to ~29 kJ/mol at higher loadings(~11 cm3/g). These values (i.e. 28–33 kJ/mol) are larger than the onespresented for KOH activated carbons [45], comparable to CO2 activatedporous graphene [46], and smaller than the hydrothermally reducedGO samples prepared by Sui et al. [47]. Based on Fig. 8a, the CH4 storagecapacity is 5.72 cm3/g (or 0.25mmol/g) and 2.01 cm3/g (or 0.08mmol/g)at 273 K and 298 K, respectively, and ~1 bar. The adsorbed CH4 amountsby the GSmaterial are considered low compared to other similar studies[45,61,65]. The equivalent isosteric heat of adsorption in this case(Fig. 8b) ranges between 30 and 38 kJ/mol, corresponding to a low CH4

coverage (0.2–2 cm3/g). Table 4 summarizes the recorded storage capac-ities (expressed in terms of adsorbed volume, mole or mass) of the GSmaterial towards H2, CO2 and CH4 gas sorption close to 1 bar under dif-ferent temperatures.

The CO2/CH4 selectivity at low-pressures was estimated by adoptingthe IAST model and using the experimental adsorption isotherms ofthese gases at 273 K and 298 K. As observed in Fig. 9, the GS materialpresents a loading dependent gas selectivitywhich increases by increas-ing the pressure. The IAST-based CO2/CH4 selectivity values are in the

range of 45–95 and 40–80 at 273 K and 298 K, respectively, which areactually much higher than those of porous chalcogels (up to 35) andcarbonized metal organic frameworks (up to 27) under similar condi-tions [60,61]. Hence, the GS material selectively adsorbs CO2 in amuch greater extent than CH4 (ratio 95:1) close to ambient conditions(i.e. 273 K and 0.66 bar), thus potentially allowing its use in a numberof processes where gas separation is a key factor, such as pre- andpost-combustion in CO2 sequestration/capture and natural gas purifica-tion/sweetening. The separation procedure leads to the upgrade of theraw gas, making it suitable for different applications, including fuelcells [62] and higher-efficiency power plants [63,64]. Besides the nature

Fig. 9. CO2/CH4 gas selectivity of the as-prepared GSmaterial based on the IASTmodel andcalculated by the low-pressure (0–1 bar) adsorption isotherms at 273 K and 298 K.

248 N. Kostoglou et al. / Thin Solid Films 596 (2015) 242–249

(i.e. polarizable carbon) and the textural properties (i.e. enhanced sur-face area and microporosity) of the GS material, the different electronicproperties among the two studied gases have a significant effect on itsgas selectivity. Specifically, CO2 is more polarizable than CH4 (2.9against 2.6 × 10−24 cm−3), while at the same time it exhibits a signifi-cant quadrupole moment (13.4 × 10−40 C m2) in comparison to thenon-polar CH4 [63]. Therefore, CO2 interacts through dispersion forcesmore significantly with the polarizable graphene surface (aromatic π-electron system) predominantly due to its large quadrupole moment[65]. Future work on the GS material will be focused on increasing itsavailable BET surface area by simultaneously enhancing its microporefeatureswith the view to improve further the gas sorption-related prop-erties presented in this study.

4. Conclusions

A facile and few-step synthetic approachwas adopted, involvingwetchemical reduction and freeze drying methods, in order to produce ananoporous, high-surface area and three-dimensional graphene-basedsponge-like material. Extensive characterization was carried out withthe view to investigate properties related to morphology, elementalcomposition, surface chemistry, structure and texture/porosity. SEMimages revealed a continuous network of exfoliated graphene stacks,while observations by TEM showed “wrinkles” formed by the overlapof exfoliated graphene layers. EDS micro-analysis indicated that carbondominates thematerial's atomic composition (~83 at.%), followed by re-sidual oxygen (~15 at.%), while FT-IR studies confirmed the presence ofsurface oxygen-based functionalities (carboxyls, carbonyls etc). XRDanalysis suggested that about 71 wt.% of all carbon is composed oftwo- and three-layered graphene crystallites (lateral size ~1.2 nm),while 22 wt.% is made of 4–5 layers (~4.6 nm). BET and microporesurface areaswere estimated to be ~350m2/g and ~88m2/g, respective-ly, while pore size distribution showed a maximum at ~0.7 nm (i.e.ultra-micropores), based on the N2 adsorption/desorption data at77 K. H2, CO2 and CH4 gas sorption properties were examined by low-pressure (0–1 bar) measurements at different temperatures. Fully re-versible sorption isotherms were recorded for all the studied gaseswith gravimetric capacities of ~0.5 wt.% H2 at 77 K and 1 bar, aswell as ~3.7 wt.% CO2 and ~0.4 wt.% CH4 at 273 K and ~1 bar.Isosteric enthalpies of adsorption were determined between 28–33and 30–38 kJ/mol towards CO2 and CH4, respectively, while the spongygraphene exhibited a high CO2 over CH4 selectivity of 95:1 at near-ambient conditions (273 K and ~0.7 bar).

Acknowledgments

This research was partially supported by EC FP7-INFRASTRUCTURESproject H2FC (GA No. 284522) and by Khalifa University Internal Re-search Fund (KUIRF L1 2013/210014 and KUIRF 2014/210047).

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