Effect of surface chemistry and textural properties on carbon dioxide uptake in hydrothermally reduced graphene oxide

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Available at www.sciencedirect.com

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Effect of surface chemistry and textural properties on carbon dioxide uptake in hydrothermally reduced graphene oxide Zhu-Yin Sui, Bao-Hang Han

*

National Center for Nanoscience and Technology, Beijing 100190, China

A R T I C L E I N F O

A B S T R A C T

Article history:

We developed a facile method to obtain bulk quantities of three-dimensional porous

Received 9 July 2014

materials through hydrothermal treatment of aqueous graphene oxide (GO) dispersion at

Accepted 6 November 2014

different temperatures. The morphology and textural properties of hydrothermally reduced

Available online 15 November 2014

GO (HRGO) were characterized by scanning electron microscopy, X-ray diffraction, and nitrogen adsorption–desorption measurements. X-ray photoelectron spectroscopy, Raman spectroscopy, and infrared spectroscopy were used to analyze their chemical properties. The as-prepared HRGO not only exhibited three-dimensional porous network structure, but also possessed high specific surface area and large pore volume. Controllable surface functionalities on graphene sheets and textural properties enabled the HRGO to show an excellent carbon dioxide capture performance. The HRGO prepared at 100 °C exhibited higher carbon dioxide adsorption capacity (2.4 mmol g1 at 1.0 bar and 273 K) than those of the other two porous materials prepared at 80 and 120 °C. It was found that in addition to textural properties, the excellent adsorption performance can also be ascribed to various surface interactions between carbon dioxide and HRGO, including acid–base interaction, polar interaction, and hydrogen bonding. This study can be helpful to the development of porous materials for carbon dioxide uptake and separation. Ó 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Global carbon dioxide emissions largely resulting from the combustion of fossil fuels have increased steadily in the past few decades and have partly led to global warming and some environmental issues. It is reported that the concentration of carbon dioxide in the atmosphere has increased from 270 ppm before the industrial revolution to more than 390 ppm today [1]. Therefore, to deal with the problem of global warming, the storage and separation of carbon dioxide gas from the atmosphere is currently a serious challenge. To date,

* Corresponding author: Fax: +86 10 8254 5576. E-mail address: [email protected] (B.-H. Han). http://dx.doi.org/10.1016/j.carbon.2014.11.014 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved.

many types of sorbents have been proposed as potential sorbents for carbon dioxide, including metal–organic frameworks [2–5], zeolitic imidazolate frameworks [6,7], microporous organic polymers [8,9], mesoporous silicas [10,11], and porous carbons [12,13]. These materials generally possess high specific surface area and permanent porosity, which make them highly competitive in carbon dioxide uptake. Microporous carbons are considered to be ideal solid adsorbents for carbon dioxide owing to their intriguing properties, such as high porosity, low cost, and easy regeneration. It has been shown that micropores less than 1 nm play an

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important role in carbon dioxide uptake, especially at ambient conditions. KOH activation is a well-known method to produce microporous carbons with high porosity and their surface area can be up to 3000 m2 g1 [14,15]. Many research groups have reported that microporous carbons possess excellent adsorption performance for carbon dioxide, and their adsorption capacities can reach 5.8 mmol g1 [16], 6.2 mmol g1 [17], 7.0 mmol g1 [14], and 8.9 mmol g1 [15] at 1 bar and 273 K. Graphene, a class of two-dimensional carbon nanomaterials, has attracted extensive concern for its potential use in various applications, such as hydrogen storage [18–21], carbon dioxide capture [22–24], and electrode material in electrochemical energy devices [25–27]. Chemical reduction of exfoliated graphite oxide is considered to be a common method to prepare graphene in a large scale [28]. Graphene oxide (GO) is an intermediate during the synthesis of graphene starting from graphite by a solution chemistry method and possesses various polar functional groups, such as hydroxyl, epoxy, carbonyl, and carboxyl [29]. Recently, there have been extensive researches focusing on exploring the preparation of graphene-based porous materials via chemical activation in order to expand their practical applications into carbon dioxide uptake and separation. Srinivas et al. reported the synthesis of a range of highly porous carbons from GO precursor with KOH chemical activation. They showed that the as-prepared porous materials exhibited excellent adsorption ability (16.4 mmol g1) for carbon dioxide at 300 K and 20 bar [13]. Chandra et al. reported the preparation of a nitrogen-doped porous carbon via chemical activation of a polypyrrole–graphene composite and the resulting nitrogendoped porous carbon possessed high carbon dioxide adsorption capacity (4.3 mmol g1, 1.0 bar) and selectivity (16) of carbon dioxide over nitrogen at 298 K [12]. A nitrogen-doped microporous carbon, prepared by Saleh et al. via chemical activation of polyindole-modified GO sheets, showed high carbon dioxide adsorption capacity of 3.0 mmol g1 at 298 K and 1.0 bar [30]. Graphene-based microporous carbon materials are highly promising in the field of carbon dioxide uptake due to their large surface area, easy surface modification, and being low cost. In the recent years, theoretical studies reported by Wood et al. using density functional theory (DFT) have shown that polar groups may be promising candidates for enhancing carbon dioxide adsorption capacity by strengthening adsorption and activating exposed edges and terraces to introduce additional binding sites [31]. It is well-known that there are many polar groups existing in the basal plane and edge of GO sheets [29]. Thus, GO sheet with a large number of oxygen-containing groups can be considered as a useful building block to construct porous materials with excellent carbon dioxide capture capacity. The work on carbon dioxide capture of GO-based porous materials is still limited [32–34]. This is likely due to the difficulties in preventing the restacking of GO sheets and increasing the specific surface area of GO-based porous materials. Recently, pillaring molecular structures have been applied to keep GO layers apart to obtain porous materials with high accessible surface area [35]. Burress et al. reported the synthesis of porous GO frameworks through a well-known reaction between boronic acids and hydroxyl groups of GO sheets [32]. In a similar way, Srinivas

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et al. reported the detailed synthesis of porous GO frameworks by reacting GO sheets with linear boronic acid pillaring units in a solvothermal reaction. They demonstrated that the resulting porous GO frameworks exhibited high isosteric heat of adsorption (Qst) and adsorption capacity for hydrogen [33]. In our work, we prepared three-dimensional (3D) porous materials with controllable C/O ratios through a facile hydrothermal treatment of aqueous GO dispersion at different temperatures. The as-prepared porous materials exhibited high surface area and large pore volume. The advantage of functionalized graphene materials was demonstrated by their enhanced performance as carbon dioxide adsorbents. We compared the uptake capacity, Qst, and selectivity of carbon dioxide over nitrogen for the as-prepared porous materials and further discussed the effect of surface chemistry and textual properties of HRGO on carbon dioxide uptake, which may be useful for the preparation of porous materials with enhanced carbon dioxide storage and separation capacity.

2.

Experimental section

2.1.

Materials

Natural flake graphite (average particle diameter of 20 lm, 99 wt% purity) was obtained from Yingshida graphite Co. Ltd., Qingdao, China. Sulfuric acid (98 wt%), hydrogen peroxide (30 wt%), potassium permanganate, sodium nitrate, hydrochloric acid, tert-butanol, and ethanol are of analytical grade and were purchased from Beijing Chemical Reagents Company. All these chemical reagents were used directly without further purification. Ultra-pure water (18 MX cm) used in all experiments was obtained from a Millipore-ELIX water purification system.

2.2.

Preparation

GO was prepared by a modified Hummers’ method using natural flake graphite as starting material [36–39]. Aqueous GO dispersion was sonicated for 60 min before using. A total of 20 mL of aqueous GO dispersion (10 mg mL1) was sealed in a Teflon-lined stainless-steel autoclave and heated at different temperatures (80, 100, and 120 °C) for 16 h. The autoclave was then cooled to room temperature. The hydrothermally treated GO dispersion was found to form a hydrogel. The as-made hydrogels were washed with tert-butanol before freeze-drying to replace water within the network of hydrogels. This is because tert-butanol is easier to be sublimated than water. Finally, the as-prepared hydrogels were freezedried under vacuum (less than 20 Pa) for 24 h to completely remove the solvent. The obtained porous materials were denoted as HRGO-x, wherein x represented the hydrothermal temperature.

2.3.

Instrumental characterization

Scanning electron microscopy (SEM) images were observed by using a Hitachi S-4800 microscope (Hitachi Ltd., Japan) equipped with a Horiba energy dispersive X-ray (EDX) spectrometer at an accelerating voltage of 4–8 kV. X-ray diffraction

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(XRD) patterns of these porous materials were measured from 4° to 60° by using a Philips X’Pert PRO X-ray diffraction instrument. X-ray photoelectron spectroscopy (XPS) data were collected with an ESCALab220i-XL electron spectrometer (VG Scientific Ltd., U.K.) using Al Ka radiation at 300 W. Thermal gravimetric analysis (TGA) was conducted by using a Pyris Diamond thermogravimetric/differential thermal analyzer through heating the samples to 800 °C at 10 °C min1 in the atmosphere of nitrogen. The ultraviolet–visible (UV–Vis) spectra were recorded with a Perkin–Elmer Lamda 950 UV–Vis–NIR spectrophotometer. Infrared (IR) spectra were recorded in KBr pellets by using a Spectrum One Fourier transform infrared (FTIR) spectrometer (Perkin–Elmer Instruments Co. Ltd., U.S.A.). Raman spectra were obtained by using a Renishaw inVia Raman spectrometer (Renishaw plc, U.K.) with a 514 nm laser as an excitation source. Nitrogen and carbon dioxide adsorption isotherms were recorded by a Micromeritics TriStar II 3020 surface area and porosity analyzer (Micromeritics Instrument Corporation, U.S.A.). The carbon dioxide and nitrogen adsorption isotherms of the samples were obtained at 273 and 287 K. Each sample was measured at least twice, and the average value was used as the final result. Before measurement, the as-prepared porous materials (HRGO-80, HRGO-100, and HRGO-120) were degassed under vacuum at 100 °C for 12 h. The specific surface area and pore size distribution (PSD) of HRGO-80, HRGO-100, and HRGO-120 were obtained from nitrogen physisorption at 77 K using ASAP2020 volumetric adsorption analyzer (Micromeritics Instrument Corporation, U.S.A.). The PSD was calculated from the adsorption branch

with the non-local density functional theory (NLDFT) approach. Micropore volume was calculated by using the t-plot method. In addition, HRGO-80 was regenerated under vacuum condition at 80 °C for 6 h to conduct the cycling adsorption–desorption test. The adsorption selectivity (S) of carbon dioxide over nitrogen in HRGO was obtained according to a simplified ideal adsorbed solution theory (IAST) [40]. IAST has been applied to carbon dioxide uptake in different adsorbents, such as metal– organic frameworks and microporous organic polymers [41,42]. The adsorption selectivity of carbon dioxide over nitrogen was calculated according to the following equation: S¼

V1 =V2 P1 =P2

where V1 and V2 are the adsorbed amount of carbon dioxide at 0.15 bar and nitrogen at 0.85 bar, respectively; P1 and P2 are the equilibrium partial pressure of carbon dioxide (0.15 bar) and nitrogen (0.85 bar) in the bulk gas phase, respectively.

3.

Results and discussion

The preparation process of HRGO-120 is illustrated in Fig. 1a. It is well-known that the hydrothermal method is often applied to assemble GO sheets into a three-dimensional network [43,44]. The typical hydrogel can be easily prepared by heating 10 mg mL1 of aqueous GO dispersion sealed in a Teflon-lined autoclave at different temperatures for 16 h. Interestingly, these hydrogels prepared at different hydrothermal

Fig. 1 – Illustration (a) of the preparation process of HRGO-120, digital pictures (b) of the as-made hydrogels at different hydrothermal temperatures, and SEM images (c) and (d) of HRGO-100.

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temperatures exhibit different volume shrinkage (Fig. 1b). The hydrothermal process was monitored by UV–vis absorption spectra (Fig. S1). The absorption peak (227 nm) of GO corresponding to p ! p* electron transitions of aromatic C@C red-shifts to 230, 254, and 263 nm after hydrothermal treatment at 80, 100, and 120 °C, respectively, indicating the partial restoration of the electronic conjugation structure and deoxygenation of GO with the increase in hydrothermal temperatures. In this work, freeze-drying was used to dry the asprepared wet gels and prepare porous materials with high porosity. After lyophilization, the solvent is removed from the hydrogel to avoid the destruction of the 3D structure and produce highly porous materials. The formation of hydrogels, driven by hydrophobic interaction, hydrogen bonding, and p–p stacking interactions between GO and graphene sheets, was also previously observed by others [43,45,46]. Seen from Fig. 1b, the apparent sizes of the as-prepared hydrogels obviously decrease with the increase in hydrothermal temperatures and more water is expelled from the as-made hydrogels, so that more compact and stronger hydrogels are obtained. After freeze-drying, the as-prepared HRGO porous materials possess low density (14–81 mg cm3) owing to their open rich structures. The HRGO-100 exhibits a foam-like structure with interconnected pores as shown in Fig. 1c and d. The cell walls are made up of assembled sheets produced during the freezing process where individual sheets are pushed together. Macropores (not less than 50 nm) of HRGO-100 can be obviously observed and their pore size is in the range of submicrometer to several micrometers. The formation of cross-linking sites existing in the framework can be attributed to partial overlapping of flexible sheets. The as-made HRGO-80 and HRGO-120 also exhibit similar network structures with large pores as shown in Fig. S2. XPS and EDX analysis were applied to characterize chemical properties of the as-prepared porous materials. From the XPS spectra shown in Fig. 2a, all of these porous materials show C1s and O1s and the calculated C/O atom ratios

increase from 2.03 (GO) to 2.10 (HRGO-10) to 2.79 (HRGO-100) to 4.59 (HRGO-120). EDX analysis reveals that the oxygen contents of GO, HRGO-80, HRGO-100, and HRGO-120 are 43.1, 41.3, 36.5, and 21.3 wt%, respectively. These results indicate that with the increase in hydrothermal temperatures, the oxygen content has an obvious decreasing tendency due to the loss of oxygen-containing groups, which is consistent with previous reports [43,47]. The high-resolution C1s spectra shown in Fig. 2b show the presence of sp2 carbon (284.8 eV), C–O (epoxyl and hydroxyl, 286.8 eV), carbonyl (C@O, 287.9 eV), and carboxyl (C(O)–OH, 289.1 eV), and the comparison of the C1s XPS spectra of different samples reveals an obvious decrease in the signals for C–O, C@O, and C(O)–OH, indicating that the oxygen content can be tuned in a controllable way by changing hydrothermal temperatures (Table S1). The reduction of oxygenated functional groups is also confirmed by the FTIR spectra as shown in Fig. S3a. The absorption band at 1720 cm1 can be attributed to C@O stretching vibrations from carbonyl and carboxyl and its intensity gradually decreases with the increase in hydrothermal temperatures. TGA curves (Fig. 3a) of the yielded porous materials also confirm that the oxygen content is highly controllable. The slow and continuous weight loss can be attributed to the decomposition of oxygenated functional groups [48,49], and

a

100

Mass Remaining / %

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HRGO-120

80

HRGO-100

60

HRGO-80 GO

40

20 100

200

300

400

500

600

700

800

o

Temperature / C

a

b

C 1s

b

O 1s

HRGO-120 C/O = 4.59

HRGO-120

C/O = 2.79

HRGO-80

HRGO-100

C/O = 2.10

HRGO-80

Intensity / a.u.

Intensity / a.u.

Intensity / a.u.

GO HRGO-100

HRGO-80

HRGO-100

GO

200

300

400

HRGO-120

GO

C/O = 2.03

500

Binding Energy / eV

600

282

284

286

288

290

292

Binding Energy / eV

Fig. 2 – XPS full spectra (a) and C1s spectra (b) of GO, HRGO80, HRGO-100, and HRGO-120. (A colour version of this figure can be viewed online.)

10

20

30

40

50

60

2 theta / degree Fig. 3 – TGA curves (a) and XRD patterns (b) of GO, HRGO-80, HRGO-100, and HRGO-120. (A colour version of this figure can be viewed online.)

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the residue weight increases gradually as the hydrothermal temperature increases. All these data indicate that oxygenated groups can be gradually decreased in a controllable way through the hydrothermal treatment of GO dispersion at different temperatures. More structural information of the as-made porous materials was revealed by Raman spectra (Fig. S3b). There are two common characteristic peaks in Raman spectra, D-band (1350 cm1) and G-band (1590 cm1), which can be assigned to sp3 carbon atoms of the defect structure (disordered graphitic lattice) and sp2-hybridized carbon atoms, respectively [20]. Compared with GO, the intensity ratio of the D- to G-band (ID/IG) of HRGO-80, HRGO-100, and HRGO-120 slightly increases from 0.80 to 0.85 to 0.87 to 0.93, indicating that a few more defect sites are introduced into the 3D network during the hydrothermal process [20]. Fig. 3b shows XRD patterns of the as-made porous materials. The GO has a sharp peak at 11.6°, corresponding to an interlayer spacing of 0.76 nm. This peak still exists in HRGO-80, indicating that the interlayer spacing of HRGO-80 does not

obviously change. However, this peak completely disappears and a broad peak centered at about 24° (d002 of ca. 0.37 nm) is observed for HRGO-100 and HRGO-120, thus confirming the reduction of GO and partial recovery of graphitic structure [50]. The porous properties of the as-prepared porous materials were characterized through nitrogen adsorption–desorption measurement. As shown in Fig. 4a, the nitrogen adsorption– desorption isotherms of HRGO-80, HRGO-100, and HRGO-120 are quite similar and exhibit type IV isotherms with type H3 hysteresis, which is indicative of the presence of mesoporosity and slit-shaped pores. In addition, the adsorbed volume increases sharply at high relative pressure, implying that there are macropores existing in HRGO, which is consistent with the result of SEM. The nitrogen adsorption–desorption isotherm of HRGO-120 reveals a Brunauer–Emmett–Teller (BET) specific surface area of 870 m2 g1, which is higher than those of nitrogen-doped graphene framework (280 m2 g1) [51] and monolithic graphene aerogels (510 m2 g1) [50],

60

1800

b

-1

1400

GO-100

50

3

Adsorbed Volume / cm (STP) g

1600

3

Adsorbed Volume / cm (STP) g

-1

a

1200 HRGO-120

1000 800 600 400

HRGO-100

200

HRGO-80

0 0.0

0.2

0.4

0.6

0.8

GO-80

40

GO-120

30 20 10 0 0.0

1.0

0.2

Relative Pressure (P/P0 )

-1

1.0

d

-1 3

HRGO-100

Qst (kJ mol )

0.8

70

C

50 40

HRGO-80

30

HRGO-120 20 10 0 0.0

0.6

Pressure (bar)

Adsorbed Volume / cm (STP) g

60

0.4

0.2

0.4

0.6

0.8

1.0

1.2 -1

CO2 uptake (mmol g )

1.4

60

HRGO-80

50 40 30 20 10 0

0

2

4

6

8

10

Cycle

Fig. 4 – (a) Nitrogen adsorption–desorption isotherms at 77 K and (b) carbon dioxide adsorption–desorption isotherms at 273 K of HRGO-80, HRGO-100, and HRGO-120 (solid symbols for adsorption and empty symbols for desorption), Qst for HRGO80, HRGO-100, and HRGO-120 as a function of the amount of carbon dioxide adsorbed (c), and the cyclability of HRGO-80 for carbon dioxide uptake (d). (A colour version of this figure can be viewed online.)

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Table 1 – Gas uptake and selectivity of carbon dioxide over nitrogen for HRGO-80, HRGO-100, and HRGO-120 at 273 K. N2 uptake Selectivity CO2 uptake at 0.15 bar/cm3 at 0.85 bar/cm3 (V1/V2)/(p1/p2) (STP) g1 (STP) g1

Sample

HRGO-80 20.0 HRGO-100 21.8 HRGO-120 12.2

1.1 1.4 2.8

102 91 25

-1

groups, and carbon dioxide adsorption capacity of HRGO-80, HRGO-100, and HRGO-120, we can come to the conclusion that surface chemistry and textural properties in HRGO are two important factors for the enhancement of carbon dioxide uptake, which is consistent with previous report [55]. Furthermore, the carbon dioxide uptake isotherms of HRGO-100 and HRGO-120 are almost reversible seen from Fig. 4b. However, there is a slight hysteresis in the carbon dioxide adsorption–desorption isotherms of HRGO-80, implying that the uptake of carbon dioxide is essentially via the physisorption or weak chemisorption, which results from the surface interactions between carbon dioxide and the oxygenated functional groups. The isosteric heat of adsorption is calculated based on adsorption isotherms of carbon dioxide at different temperatures through the Clausius–Clapeyron equation [56] (Fig. S5a–f). The plots of Qst as a function of carbon dioxide uptake for HRGO-80, HRGO-100, and HRGO-120 are presented in Fig. 4c. The initial Qst varies with the oxygen content from 46 (HRGO-120, 17.9 at.% O) to 51 (HRGO-100, 26.4 at.% O) to 56 kJ mol1 (HRGO-80, 32.2 at.% O) at initial pressure. The Qst value of HRGO-80 is lower than that of the metal–organic framework material of CuBTTri [57], but comparable to polyamine-tethered PPNs (40–63 kJ mol1) [41]. Especially, it is the highest among reported values for organic porous materials [58–61]. High initial Qst value further indicates the strong adsorbent–adsorbate interaction between

60 50

HRGO-100

CO2 N2

3

suggesting the prominent porous structure of the as-made porous materials. Table S2 summarizes the porous properties of the as-made porous materials obtained from the nitrogen adsorption–desorption measurements. Micropore volumes of HRGO-80, HRGO-100, and HRGO-120 were calculated by using the t-plot method to be 0.06, 0.11 and 0.16 cm3 g1, respectively. To further investigate the microporosity, the PSD was analyzed through the NLDFT. As shown in Fig. S4, the pore sizes of HRGO-80 and HRGO-100 mainly range from 0.52 to 0.66 nm, respectively. However, HRGO-120 shows an increased pore size at about 0.66 and 0.95 nm, respectively. This is likely due to the restacking of graphene sheets at higher hydrothermal temperature. The carbon dioxide adsorption–desorption isotherms of the as-made porous materials are shown in Fig. 4b. The HRGO-100, which has an intermediate oxygen content (26.4 at.%) and specific surface area, exhibits the highest carbon dioxide uptake capacity (53.4 cm3 g1, 2.4 mmol g1) at 273 K and 1.0 bar, which is lower than those of microporous carbons [14–17], comparable to those of other graphene-based porous materials [23,34]. Under similar conditions, the HRGO120, which has the highest specific surface area (870 m2 g1) but lowest oxygen content, displays the lowest carbon dioxide adsorption capacity (41.3 cm3 g1, 1.8 mmol g1). Therefore, in addition to specific surface area, some other factors play important roles in determining the carbon dioxide adsorption performance. Firstly, pore size is considered to be a key factor to determine the carbon dioxide uptake. According to the microporous analysis, both HRGO-80 and HRGO-100 possess a smaller micropore (pore size below 0.6 nm) compared with HRGO-120, which may be more favorable to the carbon dioxide uptake [52]. In addition, based on the chemical analysis of the as-prepared pore materials, we conclude that oxygencontaining functional groups may enhance carbon dioxide adsorption on the HRGO. As we all know, oxygen in carbonyl and epoxy is slightly basic, so there is acid–base interaction between carbon dioxide and HRGO, which is beneficial to gas uptake. The effects of oxygenated functional groups on carbon dioxide adsorption were analyzed by Liu and coworkers [53]. Their calculations demonstrated that oxygencontaining functional groups can enhance carbon dioxide adsorption. Wood et al. also indicated that the presence of hydroxyl and carboxyl can largely enhancing gas binding [31]. The enhancement can be ascribed to strong polar interaction of carbon dioxide with these oxygen-containing functional groups. Xing et al. showed that the introduction of nitrogen species into the carbon surface can facilitate the hydrogen bonding between carbon dioxide and carbon surface [54]. There are many hydroxyl and carboxyl in HRGO, so hydrogen bonding between carbon dioxide and hydroxyl and carboxyl is a possible factor to improve the carbon dioxide adsorption capacity of HRGO. Therefore, these various surface interactions, which belong to surface chemistry, including acid–base interaction, polar interaction, and hydrogen bonding can largely enhance the carbon dioxide uptake capacity of HRGO-80 and HRGO-100 owing to the presence of a great amount of oxygenated functional groups. The high hydrothermal temperatures can lead to the reduction of surface groups, thus weakening the surface interactions. Based on specific surface area, the amount of oxygenated functional

Adsorbed Volume / cm (STP) g

CARBON

40 30 20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

Pressure (bar) Fig. 5 – Carbon dioxide and nitrogen adsorption capacity for HRGO-100 at 273 K. (A colour version of this figure can be viewed online.)

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the GO-based porous material and carbon dioxide molecules. However, this value is lower than the energy of covalent bonds, so we further investigated the regeneration of HRGO80 as shown in Fig. 4d. It can be seen that the cycling was performed for ten times with no noticeable changes being observed in carbon dioxide uptake. Thus, the HRGO-80 with a large number of oxygen-containing groups reported here can be totally regenerated over multiple cycles without obvious loss of adsorption performance for carbon dioxide. Based on the excellent carbon dioxide uptake capacity, moderate specific surface area, and high Qst for the as-made porous materials in this study, we also studied the selectivity of carbon dioxide over nitrogen of HRGO. Flue gas emitting from coal-fired power plants contains about 15% carbon dioxide at total pressures of around 1.0 bar, so carbon dioxide adsorption capacity at about 0.15 bar (partial pressure of carbon dioxide in flue gas) is more relevant to realistic postcombustion applications [41]. We estimated the selectivity of carbon dioxide over nitrogen by using experimental single-component isotherms and IAST [40,42] model at an equilibrium partial pressure of 0.85 bar (nitrogen) and 0.15 bar (carbon dioxide) in the bulk phase. These 3D porous materials exhibit high carbon dioxide over nitrogen selectivities as shown in Table 1 and Fig. 5. The gas selectivities of carbon dioxide over nitrogen were found to obviously increase from 25 for HRGO-120 to 102 for HRGO-80.

4.

Conclusions

Aqueous GO dispersion has been used to generate a range of 3D porous materials through a hydrothermal method. This is the first systematic report on the preparation of HRGO at different hydrothermal temperatures. The as-prepared porous materials possess high porosity and 3D network structure with interconnected pores. Even though high hydrothermal temperature significantly increases the specific surface area and pore volume, there is no marked increase in the carbon dioxide adsorption capacity. Further analysis indicates that in addition to textural properties, carbon dioxide adsorption performance can also be linked to surface interactions, including acid–base interaction, polar interaction, and hydrogen bonding between carbon dioxide molecules and oxygencontaining functional groups. Our results are consistent with theoretical predictions that oxygen-containing groups are beneficial to carbon dioxide uptake and separation, which can be helpful to the development of porous materials. In addition, it is well-known that specific surface area and oxygenated functional groups are two very important factors in the field of supercapacitor, so these GO-based porous materials prepared in this work can be excellent electrode materials.

Acknowledgements The financial support of the National Science Foundation of China (Grant no. 21374024 and 61261130092) and the Ministry of Science and Technology of China (Grant 2014CB932200) is acknowledged. We thank Mr. Dong-Xing Ma for his assistance in the XPS measurement.

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.carbon. 2014.11.014.

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