Water Emulsions Stabilized by Partially Reduced Graphene

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CO2/Water Emulsions Stabilized by Partially Reduced Graphene Oxide Chengcheng Liu, Jianling Zhang, Xinxin Sang, Xinchen Kang, Bingxing Zhang, Tian Luo, Xiuniang Tan, Buxing Han, Li-Rong Zheng, and Jing Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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ACS Applied Materials & Interfaces

CO2/Water Emulsions Stabilized by Partially Reduced Graphene Oxide ‡

‡

‡

‡

Chengcheng Liu,†, Jianling Zhang,*, † , Xinxin Sang,† , Xinchen Kang,†, Bingxing Zhang,†, ‡

§

Tian Luo,† Xiuniang Tan,† Buxing Han,†, Lirong Zheng, and Jing Zhang †

‡

§

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and

Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R.China. ‡

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,

Beijing 100049, P.R.China. §

Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese

Academy of Sciences, Beijing 100049, P.R.China. KEYWORDS: graphene oxide, CO2, water, emulsion, catalysis

ABSTRACT: Using functional materials to stabilize emulsions of carbon dioxide (CO2) and water is a promising way to expand the utility of CO2 and functional materials. Here we demonstrate for the first time that the partially reduced graphene oxide (rGO) can well stabilize the emulsion of CO2 and water without the aid of any additional emulsifier or surface modification for rGO. More interestingly, such a novel kind of emulsion provides a facile and versatile route for constructing highly porous three-dimensional rGO materials, including rGO,

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metal/rGO and metal oxide/rGO networks. The as-synthesized Au/rGO composite is highly active in catalyzing 4-nitrophenol reduction and styrene epoxidation.

1. INTRODUCTION Water and carbon dioxide (CO2) are the two most abundant solvents on earth. The simultaneous utilization of CO2 and water is very promising owing to the environmentally benign properties of these two solvents and unique features of CO2 such as readily available, inexpensive, nontoxic, nonflammable, adjustable and easily released.1 However, water and liquid CO2 are immiscible with each other because the physicochemical properties of CO2 are much different from those of water, e.g. CO2 has no dipole moment, very low dielectric constant and polarizability per volume.2 To form emulsions of CO2 and water with the aid of stabilizers is an efficient way to combine these two solvents. Generally, the amphiphilic molecules (e.g. surfactant)3-6 are adopted to stabilize emulsions of CO2 and water. As an alternative of amphiphilic molecule, the solid can act as emulsifier for the two immiscible solvents with some advantages: 1) the particle assemblies at interface are highly attractive for surfactant-free material synthesis7 and in-situ catalytic reactions;8 2) the solid can be easily separated and recycled by centrifugation or filtration after use. Nevertheless, the studies on emulsifying CO2 and water by solid are largely restricted,9,10 mainly because of the difficulty in overcoming the strong Hamaker attraction between particles caused by the weak Van der Waals forces of CO2, which often leads to droplet floculation and coalescence. To develop solid emulsifiers with desirable amphiphilicity to CO2 and water is of great significance, but still remains a severe challenge. Graphene oxide (GO) is a well-known two-dimensional (2D) carbon material, which has found wide applications in different fields.11-13 It is known that GO is amphiphilic because the carboxylic acid at the edges of nanosheet gives GO hydrophilicity while the basal plane of


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hydrophobic polyaromatic endows GO hydrophobicity.14 Moreover, the surface wettability of GO can be easily modulated by the reduction degree of GO. Here we propose for the first time the utilization of the partially reduced GO (rGO) for emulsifying CO2 and water. It was found that rGO can well stabilize the emulsion of CO2 and water by assembling at interface, without the aid of any additional emulsifier or surface modification for rGO. Moreover, the formation and breakage of the emulsion are reversible, which can be switched by pressurization and depressurization. Such CO2 and water emulsions stabilized by rGO provide promising routes for constructing highly porous three-dimensional (3D) rGO materials. 2. RESULTS AND DISCUSSION 2.1. Emulsion formation. GO was first synthesized by a modified Hummers method15 and its emulsification for liquid CO2 and water was investigated. It shows that GO cannot emulsify CO2 and water at 303.2 K and 7.30 MPa, which may be due to the strong hydrophilicity of GO. To get a desirable amphiphilicity to CO2 and water, the partially reduced GO flakes were synthesized16 (Figure S1 of the Supporting Information, SI). The wettability of the rGO was measured through the aqueous phase by static contact angle measurement (Figure S2). The rGO exhibits a contact angle of around 63.9°, larger than that of the GO (25.6°). It indicates a less hydrophilic surface of rGO as compared with that of GO. The rGO was further characterized by X-ray diffraction (XRD), Raman and Fourier transformed infrared (FT-IR) spectra, which prove that GO was partially reduced to rGO (Figure S3-5). The partial reduction endows rGO suitable amphiphilicity for emulsifying CO2 and water. The rGO was dispersed in water with a concentration of 5 mg mL-1 (Figure 1a) and then CO2 was charged into the aqueous rGO dispersion at 302.2 K. As the pressure was higher than the vapor pressure of CO2 at 302.2 K (7.05 MPa), an upper liquid CO2 phase occurred (Figure 1b). The CO2/H2O/rGO mixture was


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stirred for 10 min at 7.30 MPa, resulting in the formation of emulsion with a turbid appearance (Figure 1c). The emulsion can keep stable for at least 12 h against coalescence. Interestingly, the formation and breakage of the emulsion could be tuned reversibly by pressurization and depressurization. After depressurizing to be lower than 7.05 MPa, the emulsion shown in Figure 1c changed into the mixture of lower aqueous rGO dispersion and upper CO2 gas. The emulsion could be recovered by pressurizing the gas again to be higher than 7.05 MPa.

Figure 1. Photographs of (a) rGO dispersed aqueous solution, (b) rGO aqueous solution with upper liquid CO2 phase at 7.30 MPa and (c) emulsion stabilized by rGO at 7.30 MPa. (d) Electrical conductivities of CO2/H2O/rGO mixture at different pressures. The inset in d illustrates the reversible transition from rGO dispersion to emulsion by pressurization and depressurization. The concentration of rGO in water is 5 mg mL-1 and temperature is 302.2 K. To identify the microstructure of the emulsion shown in Figure 1c, the electrical conductivities of the CO2/H2O/rGO mixture were determined at different pressures and 302.2 K (Figure 1d). The aqueous dispersion of rGO is conductive (~400 µs cm-1) because of the partially ionization of rGO in water. With the addition of CO2, the electrical conductivity first increases obviously, which is caused by the carbonation of dissolved CO2 in water, and then keeps nearly unchanged


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(~700 µs cm-1) even as CO2 turns into liquid at pressures higher than 7.05 MPa. It proves that the emulsion shown in Figure 1c corresponds to a CO2-in-H2O emulsion, i.e. CO2 droplets are dispersed in water continuous phase. As illustrated in the inset of Figure 1d, the rGO nanosheets assemble at CO2-water interface to create a rigid protective barrier around the dispersed CO2 droplet, thus stabilizing CO2-in-H2O emulsion. The emulsifications of rGO materials for CO2 and water were investigated at different conditions. First, the phase behavior of CO2/H2O/rGO mixture in presence of HAuCl4 was investigated at 7.30 MPa and 302.2 K. The results prove that the rGO can well emulsify CO2 and HAuCl4 aqueous solution. Second, the possibility of rGO hybrid (e.g. TiO2/rGO) for emulsifying CO2 and water was detected. The pre-formed rGO immobilized with TiO2 nanoparticles (see synthesis details for TiO2/rGO in supporting information) was used as emulsifier for CO2 and water at 7.30 MPa and 302.2 K. The phase behavior observation reveals that the emulsion of CO2 and water can form with the aid of TiO2/rGO solid. 2.2. 3D rGO materials derived from the emulsions. The CO2 and water emulsions provide novel routes for constructing rGO and rGO hybrid networks. As illustrated in Scheme 1a, the emulsion of CO2 and water stabilized by rGO is frozen in liquid nitrogen, and the liquids (i.e. CO2 and water) were subsequently removed by depressurization and freeze drying. Therefore, the rGO solid with a macroporous structure, which is templated by CO2 droplets, can be obtained. For the synthesis of Au/rGO network, HAuCl4 aqueous solution was used as water phase for emulsion formation, while other experimental conditions were the same with those above (Scheme 1b). No additional reducing reagent was needed because rGO can act as reductant to directly synthesize Au nanoparticles.16 For the synthesis of TiO2/rGO network, the pre-formed TiO2-loaded rGO was used as emulsifier and the reconstructed TiO2/rGO composite


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was derived by a similar freeze depressurization and drying procedure (Scheme 1c). The CO2 and water emulsion templating route for constructing 3D rGO materials has many advantages: 1) rGO itself can stabilize the emulsion and no additional emulsifier is needed, thus the complex demulsifying and washing procedures suffered from the conventional emulsions are avoided; 2) high-pressure CO2 helps the exfoliation and dispersion of rGO nanosheets by penetrating into the interlayers of rGO;17 3) high-pressure CO2 favors the in-situ formation of small nanoparticles and their dispersion on rGO nanosheets owing to low-viscosity effect;18,19 4) CO2 simplifies the post-processing procedure because it can be easily released by depressurization.

Scheme 1. Schematic illustration for (i) forming emulsions in CO2 and water with the aid of rGO or rGO hybrid and (ii) constructing rGO network (a), Au/rGO network (b) and TiO2/rGO network (c) by freeze depressurization and drying for emulsions. The reconstructed rGO synthesized by Scheme 1a presents a 3D network with macropores in range of 0.5-10 µm, as can be seen from the scanning electron microscope (SEM) image shown


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in Figure 2a. The N2 adsorption-desorption isotherms exhibit a typical type-IV mode with a distinct hysteresis loop (Figure 2f). It proves that the macroporous rGO network is also mesoporous, which may be caused by the aggregation of rGO nanosheets as evidenced by SEM image. The mesopore size distribution curve, calculated from the Barret-Joyner-Halenda method, shows a pore size distribution centered at 3.9 nm. The BET surface area (SBET) and total pore volume (Vt) are 241 m2 g-1 and 0.39 cm3 g-1, respectively. For FT-IR spectra, the stretching vibration of hydroxyl group of the rGO network shows a prominent red shift (3395 cm-1) as compared with that of pristine rGO (3414 cm-1) (Figure S5). It implies the formation of hydrogen bondings between the residual oxygenated functional groups on rGO network, which may be caused by the assembly of rGO at CO2-water interface and result in the crosslinking of nanosheets to 3D network. No obvious changes were observed for the XRD and Raman spectra of the pristine rGO and rGO network (Figure S3 and S4). Furthermore, the 3D macroporous rGO network was obtained from the emulsion stabilized by more reduced GO (see characterizations in Figure S6). The macropore size of the rGO network distributes in a range of 0.2-3.0 µm, which is smaller than that shown in Figure 2a. It can be ascribed to the smaller droplets in the emulsion stabilized by more reduced GO because of the template effect of droplets to macropores.


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Figure 2. SEM and TEM images of (a) rGO, (b-d) Au/rGO and (e) TiO2/rGO derived from CO2in-H2O emulsions. The insets in d and e are the high-resolution TEM images of Au and TiO2 nanoparticles, respectively. Scale bars, 10 µm, 500 µm, 10 µm, 30 nm and 50 µm in a-e, respectively. (f) N2 adsorption-desorption isotherms of rGO (black), Au/rGO (red) and TiO2/rGO (green). The Au/rGO synthesized by Scheme 1b shows a macroporous network structure (Figure 2b and c), similar to the pure rGO network shown in Figure 2a. The transmission electron microscope (TEM) image reveals that the ultra-small Au nanoparticles (~2 nm) are immobilized uniformly on rGO network, with a narrow size distribution of 1-3 nm (Figure 2d). From the highresolution TEM image, the interplanar spacings for the lattice fringe of 0.23 nm were observed (inset in Figure 2d), corresponding to the (111) lattice plane of face-centered cubic metallic gold. The effective reduction from AuIII to Au0 was further proved by X-ray photoelectron spectroscopy (XPS, Figure S7). The Au/rGO network is mesoporous (~2 nm), as determined by N2 adsorption-desorption isotherms (Figure 2f). The SBET and Vt of Au/rGO are 355 m2 g-1 and


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0.57 cm3 g-1, respectively. For the TiO2/rGO synthesized by Scheme 1c, the anatase TiO2 nanoparticles about 3.5 nm are dispersed uniformly on rGO support (Figure 2e and Figure S810). The N2 adsorption-desorption isotherms (Figure 2f) reveal that the TiO2/rGO network is also mesoporous with SBET, Vt and mesopore size of 311 m2 g-1, 0.23 cm3 g-1 and 3.4 nm, respectively. These results indicate that the CO2-in-H2O emulsion is versatile in producing different kinds of highly porous 3D rGO materials. 2.3. Catalytic activity of the Au/rGO. The as-synthesized rGO composites combine the advantages of ultra-small nanoparticles and hierarchically macro- and mesoporous rGO support, endowing them promising candidates as catalysts. The catalytic activity of the Au/rGO for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), which is very useful and important in practical applications,20 was tested. The conversion of 4-NP to 4-AP with reaction time was determined by UV-vis spectra (Figure S11). 4-AP cannot be formed without catalyst in 12 h. Remarkably, 4-NP is reduced to 4-AP completely within 1 min with the addition of a small amount of Au/rGO catalyst (Au: 30.85 µmol L-1) (Entry 1 in Table 1). Even as gold concentration is as low as 7.71 µmol L-1, 4-NP can be completely converted to 4-AP in 1 min (Entry 3). As gold concentration is lower than 7.71 µmol L-1, the time for the complete conversion of 4-NP increases slightly with the less amount of catalyst (Entries 4-6). The recovered catalyst after running 3 cycles exhibits a similar catalytic performance without visible reduction in the conversion for the same reaction time (Entries 7 and 8, Figure S12). For comparison, Au/rGO was synthesized by a conventional method, i.e. the reduction of HAuCl4 in rGO aqueous solution under ultrasonication, and its catalytic activity for 4-NP reduction was tested. As gold concentration is 7.71 µmol L-1, a reaction time as long as 50 min is needed for the complete conversion of 4-NP (Entry 9 and Figure S13). This low catalytic activity can be


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attributed to the larger Au nanoparticles supported on rGO sheets synthesized in aqueous dispersion (~35 nm, Figure S14). Further, as listed in Entries 10 and 11, the Au/rGO catalyst synthesized by CO2 and water emulsion route is much more active than the reported Au/rGO, which needs 12 min for the complete 4-NP conversion even with a Au concentration as high as 40.61 µmol L-1.21 The catalytic activity of the as-synthesized Au/rGO network for 4-NP reduction is also higher than that of Au/SiO2/GO ternary catalyst (Entry 12).22 Table 1. Catalytic performance of Au/rGO for 4-NP reduction. Entry

Catalyst

CAu

C4-NP

CNaBH4

Timea

(µmol L-1)

(mmol L-1)

(mol L-1)

(min)

1b

Au/rGO

30.85

8.75

0.62

1

2b

Au/rGO

12.34

8.75

0.62

1

3b

Au/rGO

7.71

8.75

0.62

1

4b

Au/rGO

5.61

8.75

0.62

2

5b

Au/rGO

4.84

8.75

0.62

3

6b

Au/rGO

3.98

8.75

0.62

6

7c

Au/rGO

7.71

8.75

0.62

1

8c

Au/rGO

7.71

8.75

0.62

1

9d

Au/rGO

7.71

8.75

0.62

50

10b

Au/rGO

7.71

0.093

0.0067

1

11e

Au/rGO

40.61

0.093

0.0067

12

12f

Au/SiO2/GO

7.50

0.24

0.095

3

a

Time denotes the minimum reaction time for complete reduction of 4-NP. bAu/rGO synthesized by CO2 and water emulsion route. cAu/rGO was reused for further two runs. dAu/rGO synthesized by reduction of HAuCl4 in aqueous rGO dispersion. eCatalyst reported in ref. 20. f Catalyst reported in ref. 22 The styrene epoxidation is important for producing styrene oxide, which is an industrially important intermediate for the synthesis of fine chemicals and pharmaceuticals.23,24 Numerous


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studies have been focused on the use of Au nanoparticles as catalysts for styrene epoxidation;25-31 however, the catalytic activities are largely restricted by low styrene conversion and epoxide selectivity. Here the styrene epoxidation over the as-synthesized Au/rGO network was tested. Tert-butyl hydroperoxide was used as oxidant and the reaction was carried out at 353.2 K, with catalyst (10 mg, Au: 1.43 wt%) and styrene (0.4 mL) in 5 mL acetonitrile. As listed in Table 2, the styrene conversion and selectivity to styrene oxide increase with prolonged reaction time and can reach 82.5 % and 60.0 % at 24 h, respectively (Entries 1-3). The turnover frequencies (TOF), defined as the mole of substrate consumed per mole of gold per hour, could reach values of 940 h-1 at 2 h, 420 h-1 at 5 h and 170 h-1 at 24 h, respectively. For the Au/rGO catalyst synthesized by the conventional aqueous reduction method, the TOFs are 340 h-1 at 2 h, 280 h-1 at 5 h, 110 h-1 at 24 h, respectively (Entries 4-6), much lower than the catalytic activities of the Au/rGO network synthesized by CO2 and water emulsion route (Entries 1-3). Also, the styrene epoxidation catalyzed by Au/rGO network presents higher conversion and selectivity to styrene oxide than the reported Au nanoparticles supported on GO sheets (with TOF of 291 h-1 at 5 h and 105 h-1 at 24 h in Entries 7 and 8).22 As far as we know, the TOF of the Au/rGO network synthesized by CO2 and water emulsion route is higher than those of the supported Au nanoparticles (e.g. Au/CaO,27 Au/Yb2O3,28 Au/hydroxyapatite,29 Au/barium titanate nanotube,30 Au/carbon nanocomposite31) for styrene epoxidation by t-butyl hydroperoxide.


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Table 2. Catalytic performance of Au/rGO for styrene epoxidation. Entry

Catalyst

Time

Conversion

Selectivity

TOF

(h)

(%)

(%)

(h-1)

1a

Au/rGO

2

38.8

24.9

940

2a

Au/rGO

5

49.7

38.8

420

3a

Au/rGO

24

82.5

60.0

170

4b

Au/rGO

2

14.4

35.2

340

5b

Au/rGO

5

29.9

42.5

280

6b

Au/rGO

24

56.8

52.3

110

7c

Au/GO

5

29.1

13.8

291

8c

Au/GO

24

50.2

15.7

105

a

Au/rGO synthesized in CO2 and water emulsion. bAu/rGO synthesized in aqueous rGO dispersion. cCatalyst reported in ref. 22. The high catalytic activity of the Au/rGO network can be explained from the following three aspects. First, the Au nanoparticles are ultra-small, which increases the density of catalytic active sites.32 It is worth noting that the GO-supported Au nanoparticles reported are subject to limitation of large particle size (>5 nm) and a secondary support (e.g. SiO2, TiO2) is requisite to produce smaller particles.22,33 Here the formation of ultra-small Au nanoparticles on rGO can be attributed to the CO2-water interfacial directing rGO assembly and Au3+ reduction, through which the particle growth and aggregation can be well prevented. Second, the hierarchically macro- and mesoporous structure of rGO makes Au nanoparticles highly exposed and enhances the diffusion of substrates and products, which is favorable for accelerating reactions.34 Third, the interaction between Au nanoparticles and rGO is weak, as evidenced from XPS, FT-IR, Raman and synchrotron X-ray absorption spectroscopy (Figure S7, S15-17). Such “naked” Au


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nanoparticles are highly active because the low-coordinated gold atoms on bare gold clusters provide major active sites for catalytic reactions.35 3. CONCLUSIONS In summary, we demonstrate for the first time the formation of CO2 and water emulsions stabilized by rGO or rGO hybrid, from which the hierarchically macro- and mesoporous rGO, metal/rGO and metal oxide/rGO networks have been fabricated. The as-synthesized Au/rGO composite has shown high catalytic performance for 4-nitrophenol reduction and styrene epoxidation. The emulsions composed of rGO, CO2 and water have many advantages than conventional emulsions and are highly attractive towards applications. It is anticipated that such novel emulsions would find a wide utilization in different fields, such as emulsion polymerization, interfacial catalysis and fabrication of rGO composite superstructures. 4. EXPERIMENTAL SECTION 4.1. Materials. CO2 (>99.995% purity) was provided by Beijing Analytical Instrument Factory. Graphite flake (Natural, 325 mesh, 99.8%), styrene (99%) and NaBH4 (98%) were supplied by Alfa Aesar. Analytical grade KMnO4 was purchased from Shanghai Chemical Reagents Company. Hydrazine solution (85 wt% in water) and ammonia solution (25 wt% in water) were purchased from Sinopharm Chemical reagent Beijing Co., Ltd. Analytical grade NaNO3, dodecane, t-butyl hydroperoxide (70 wt% in water) and tetrabutyl titanate (TBOT) (>98% purity) were purchased from J&K Scientific Co., Ltd. 98% H2SO4, 30% H2O2 aqueous solution, 4-nitrophenol (4-NP, A. R. grade), acetonitrile, ethanol and deionized water were provided by Beijing Chemical Reagent Company. HAuCl4·4H2O was produced by Shenyang Jinke Reagent Factory. All these materials were used directly without further purification.


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4.2. Synthesis of rGO. 80 mL GO aqueous solution (2.5 mg mL-1) was added to a 100 mL stainless autoclave, then 136 µL of hydrazine solution (85 wt% in water) and 280 µL of ammonia solution (25 wt% in water) were added to the solution.16 After the mixture was vigorously shaken for a few minutes, the solution was incubated in an oil bath (368.2 K) for 1 h with vigorous stirring by magnon. After the reaction, the obtained rGO in the solution was processed by rotary evaporation and then vacuum dried to remove the residual N2H4. 4.3. Phase behavior observation of CO2/water emulsions. For the rGO-stabilized emulsion formation, CO2 was charged into the autoclave containing rGO aqueous dispersion (5 mg mL-1) under stirring at 302.2 K. The phase behavior was observed at different pressures.9 The apparatus consisted of a high-pressure view cell of 50 mL with a quartz view window, a magnetic stirrer, a high-pressure syringe pump, a pressure gauge, a constant temperature water bath and a gas cylinder. The water bath temperature was controlled by a HAAKE D8 digital controller and the accuracy of the temperature measurement was ±0.05 K. The pressure gauge was composed of a pressure transducer (FOXBORO/ICT, Model 93) and an indicator, whose accuracy was up to ±0.025 MPa in the pressure range of 0-20 MPa. For the formation of rGO-stabilized emulsion in presence of HAuCl4, 25 mM HAuCl4 aqueous solution was added to rGO solution (5 mg mL-1). Then CO2 was charged into the solution under stirring until the suitable pressure (7.30 MPa) was reached and the phase behavior was observed. For the emulsion stabilized by TiO2/rGO, CO2 was charged into the autoclave containing TiO2/rGO aqueous dispersion (5 mg mL-1) under stirring at 302.2 K until the suitable pressure (7.30 MPa) was reached and the phase behavior was observed. 4.4. Determination of electrical conductivities of CO2/H2O/rGO mixture. The apparatus for the high-pressure conductivity measurement was similar to that reported previously.36 The


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apparatus consisted mainly of a high-pressure stainless steel vessel, a conductivity cell, a high pressure syringe pump, a pressure gauge, a magnetic stirrer, and a gas cylinder. Both working electrode and counter electrode were made of Pt foil. The conductivity was determined by a conductivity meter with a precision of ±0.5%, which was produced by Shanghai Precision Scientific Instrument Co., Ltd. (Model DDS-307). The cell constant was calibrated with standard KCl aqueous solutions. In a typical experiment, a suitable amount of rGO aqueous dispersion was added into the high-pressure vessel at 302.2 K. Then CO2 was charged into the solution under stirring until the suitable pressure was reached. The electrical conductivities of the CO2/H2O/rGO mixture were recorded at different pressures. 4.5. Synthesis of rGO network from emulsion. 1.6 mL of rGO aqueous dispersion (5 mg mL-1) was added into a high-pressure cell (4.6 mL). Then CO2 was charged into the cell until a desired pressure was reached. After stirring the mixture for 10 min at 302.2 K, the high-pressure cell was put into liquid nitrogen and frozen for 10 min. Then CO2 was released slowly in refrigerator for 20 h, followed by freeze drying to remove water. The rGO solid was obtained without further processing. 4.6. Characterizations. The morphologies of the as-synthesized rGO materials were characterized by SEM (HITACHI S-4800), TEM (JEM-1011, JEOL, Japan) and high-resolution TEM (FEI Tecnai G2 F20U-TWIN). The porosity properties were gained from N2 adsorptiondesorption isotherms using a Micromeritics ASAP 2020M system. The gold concentration in the Au/rGO composite was determined by induced coupled plasma-atomic emission spectroscopy (VISTA-MPX). FT-IR spectra were obtained using a Bruker Tensor 27 spectrometer, and the samples were prepared by KBr pellet method. XRD was performed on a Rigaku D/max-2500 diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 200 mA. Raman spectrum was


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tested by LabRAM ARAMIS. UV-vis spectrum was determined using UV-vis spectroscopy (Shimadzu UV-2550). XPS was determined by VG Scientific ESCALab220i-XL spectrometer. The XAS experiment was carried out at Beamline 1W2B at Beijing Synchrotron Radiation Facility (BSRF). The raw data were energy-calibrated, background corrected and normalized using Ifeffit software. The contact angles were measured at 25 oC through aqueous phase by static contact angle instrument (Harke-SPCAX1). GO or rGO was first squashed in a tablet machine at pressure of 10 KPa for 20 seconds. Then the photograph of the contact angle was taken after placing a water droplet (volume=3 µL) on the surface of the sample film in air. The start time for taking the photo is 3.7 s. 4.7. Catalytic test of 4-NP reduction. The reduction of 4-NP was carried out in a quartz cuvette and monitored using a UV-vis spectroscopy at 298.2 K.22 For comparison, the yellow aqueous 4-NP solution was prepared and measured prior to monitoring the changes of absorption. In a typical experiment, Au/rGO (1.7 mg, Au: 1.43 wt%) was added a total of 2.8 mL of aqueous 4-NP solution (0.1 mM) and stirred for 1 min. Then the mixture was mixed with 0.2 mL of fresh NaBH4 solution (0.1 M). After reaction for 1 min, the solution was quickly subjected to UV-vis measurement. To study the reusability of the catalysts, the recycled catalyst was washed with deionized water 4 times. The procedure of 4-NP reduction was repeated 3 times. 4.8. Catalytic test of styrene epoxidation. The styrene oxidation was carried out in a 25 mL flask. The flask was charged with catalyst (10 mg, Au: 1.43 wt%), styrene (0.4 mL, 0.3588 g, 3.445 mmol) and acetonitrile (5 mL). With N2 flowing through the flask under atmospheric pressure, the mixture was stirred for 15 min at room temperature. Then 1.667 g of t-butyl hydroperoxide (70 wt% in water) was added to the flask and the mixture was heated to 353.2 K under stirring. After reaction for a certain time, the catalyst was separated and the product was


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mixed with dodecane as internal standard. The mixture was analyzed by gas chromatograph (Agilent 6820). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. More details of experimental procedures, results and discussion (PDF) AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (21525316, 21673254) and Chinese Academy of Sciences (QYZDY-SSW-SLH013).


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Photographs of (a) rGO dispersed aqueous solution, (b) rGO aqueous solution with upper liquid CO2 phase at 7.30 MPa and (c) emulsion stabilized by rGO at 7.30 MPa. (d) Electrical conductivities of CO2/H2O/rGO mixture at different pressures. The inset in d illustrates the reversible transition from rGO dispersion to emulsion by pressurization and depressurization. The concentration of rGO in water is 5 mg mL-1 and temperature is 302.2 K. 155x61mm (150 x 150 DPI)


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Schematic illustration for (i) forming emulsions in CO2 and water with the aid of rGO or rGO hybrid and (ii) constructing rGO network (a), Au/rGO network (b) and TiO2/rGO network (c) by freeze depressurization and drying for emulsions. 152x89mm (118 x 118 DPI)



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SEM and TEM images of (a) rGO, (b-d) Au/rGO and (e) TiO2/rGO derived from CO2-in-H2O emulsions. The insets in d and e are the high-resolution TEM images of Au and TiO2 nanoparticles, respectively. Scale bars, 10 µm, 500 µm, 10 µm, 30 nm and 50 µm in a-e, respectively. (f) N2 adsorption-desorption isotherms of rGO (black), Au/rGO (red) and TiO2/rGO (green). 161x81mm (116 x 116 DPI)


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Water Emulsions Stabilized by Partially Reduced Graphene

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