Facile Synthesis of Graphene Sponge from Graphene Oxide for

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Facile Synthesis of Graphene Sponge from Graphene Oxide for Efficient Dye-sensitized H2 Evolution Weiying Zhang, Yuexiang Li, and Shaoqin Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01805 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 1, 2016

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ACS Applied Materials & Interfaces

Facile Synthesis of Graphene Sponge from Graphene Oxide for Efficient Dye-sensitized H2 Evolution Weiying Zhang, Yuexiang. Li*, Shaoqin Peng Department of Chemistry, Nanchang University, Nanchang 330031, P.R.China

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ABSTRACT: Graphene is an advanced carbon energy material due to its excellent properties. Reduction of graphene oxide (GO) is the most promising mass production route of graphene/reduced graphene oxide (rGO). To maintain graphene’s properties and avoid restacking of rGO sheets in bulk, the preparation of 3-dimensional porous graphene sponge via 2-dimensional rGO sheets is considered as a good strategy. This article presents a facile route to synthesize graphene sponge by thermal treating GO powder at low temperature of 250 oC under N2 atmosphere. The sponge possesses macroporous structure (5-200 nm in size) with BET specific surface area 404 m2 g-1 and high conductivity. The photocatalytic H2 production activity of the rGO sponge with a sensitizer Eosin Y (EY) and co-catalyst Pt was investigated. The rGO sponge shows highly efficient dye-sensitized photocatalytic H2 evolution compared to that obtained via a chemical reduction method. The maximum apparent quantum yield (AQY) reaches up to 75.0 % at 420 nm. The possible mechanisms are discussed. The synthesis method can be expanded to prepare other graphene-based materials.

KEYWORDS: Graphene sponge, thermal treatment, phase separation, dye-sensitized, H2 evolution


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1. Introduction Graphene, a 2-dimensional single sheet of sp2-hybrized carbon atoms, possesses huge specific surface area, high electron mobility and chemical stability.1-3 Due to the outstanding performances, it has been widely employed as an advanced energy material in many fields such as supercapacitors, batteries, solar cells and photocatalysts.4-11 In particular, graphene, as an excellent support of semiconductors or dyes to improve their charge separation and transfer, is extensively used to prepare composite photocatalysts which exhibit high activity for watersplitting.10-15 Conversion of solar energy into H2 (a clean and green chemical fuel) via photocatalysis is the most promising strategy to resolve the global energy crisis and environment pollution.16-18 For the application, cheap and large-scale production technologies of graphene should be developed. Graphene has been prepared by several technologies such as mechanical exfoliation of graphite, chemical vapor deposition and reduction of graphene oxide (GO). The first approach is low productivity, the second one is high-cost, and thus the both are unsuitable for the scalable production and application. Reduction of GO which is obtained by chemical oxidation exfoliation of graphite, becomes the most promising mass production route of graphene. In this process, GO can be restored to the graphene structure (reduced graphene oxide, rGO) by chemical and thermal treatment reduction.8,19-25 However, for the chemical reduction approach, the toxicity of the reductants and the time-consuming production process limit its mass application. More seriously, the formed rGO sheets can restack into thick multilayer structure via van der waals force, decreasing significantly the material’s specific surface area and conductivity.26,27 To prevent the restacking and to maintain conductivity of individual graphene sheets in bulk, the construction of 3-dimensional porous graphene sponge via 2-dimensional rGO sheets is considered as a good strategy.28-30 rGO sponge can be fabricated from GO by using various templates, e.g. silica sphere.28 The method is complex and costly, unfitting for large scale 3 ACS Paragon Plus Environment


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production. rGO sponge can be also prepared from GO sponge/aerogel by chemical reduction using reducing vapor or thermal treatment at high temperature (600-1000 oC).29,30 However, the production process of GO sponge is complex and tedious. For example, transformation of GO hydrogel into its sponge/aerogel needs special drying technologies (freeze drying and critical point drying) which are time-consuming and high-cost in order to avoid pore structure collapsing.31 rGO sponge can be directly obtained by thermal expansion of compact GO solid/powder dried in air, which is a facile, cheap and green method without using any toxic reducing reagents. Although thermal exfoliation of the GO powder into rGO sponge has been achieved, harsh preparation conditions are needed such as ultra-high vacuum,22 reductant H2,23 high temperature,24 rapid heating rate and long treatment time.25 It is highly desirable to develop a thermal exfoliation method to prepare rGO sponge from GO powder under facile conditions. Dye-sensitized solar cell as an effective strategy to convert solar energy into electricity has been widely investigated.32 Recently, photocatalytic H2 production via dye-sensitization has received much attention.33-36 A dye-sensitized catalyst for hydrogen production is usually composed of a light-absorbing dye, sensitized matrix and cocatalyst for hydrogen evolution reaction (HER). EY, a xanthene dye, shows the high activity for dye-sensitized H2 evolution.3236

Pt is considered as the best HER cocatalyst owing to the very small overpotential. The

sensitized matrix as the support of a dye and cocatalyst plays an important role in the separation and transfer of photogenerated charge pairs. rGO is an efficient sensitized matrix for hydrogen evolution.3,11 However, studies on a graphene sponge as the sensitized matrix have not been reported. Herein, rGO sponge is successfully fabricated by thermal treatment of compact GO powder in N2 flow under normal pressure at the temperature of 250 oC for only 5 min. The sponge shows macroporous structure, high specific surface area and conductivity. The photocatalytic H2 production activity of the rGO sponge with a sensitizer EY and co-catalyst Pt was investigated. 4 ACS Paragon Plus Environment

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The rGO sponge displays high-efficient dye-sensitized photocatalytic H2 evolution under the visible irradiation. The maximum apparent quantum yield (AQY) reaches up to 75.0 % at 420 nm, much higher than the most reported data for dye-sensitized H2 evolution. The green, cheap and facile synthesis method can be expanded to prepare other graphene-based materials.

2. Experimental Section 2.1. Preparation of graphene sponge All the reagents except for EY (biological reagent) were analytical grade without further purification. GO was synthesized according to ref. 37 by the modified Hummers method. The prepared paper-like GO was cut into smaller fragments, heated to 250 oC with a heating rate of 10 oC min-1 in a tubular furnace with N2 flow rate of 100 mL min-1 and then maintained at the temperature for 5 min. Finally, the sample was cooled to 80 oC under N2 atmosphere. The obtained fluffy rGO sponge sample was labeled as rGOT. For a comparison, GO was reduced by sodium borohydride (NaBH4).20 0.20 g of GO powder was scattered in 200 mL of distilled water by sonication for 2 h. 1.0 g of NaBH4 was added, the mixture was heated to 80 oC under stirring and then kept for 10 h. Finally, the suspension was filtered, washed with distilled water several times, and then dried in an oven at 80 oC for 10 h. The obtained product was named as rGOC. 2.2. Characterization X-ray photoelectron spectra (XPS) were performed on an ESCALAB250xi XPS spectrometer fitted with an Al Kα X-ray source using C1s peak at 284.8 eV as the internal reference. X-ray diffraction (XRD) patterns were obtained on an XD-2/3 diffractometer employing nickelfiltered Cu Kα radiation. Scan electron microscopy (SEM) images were observed on a JEOL JSM-6701F, and high-resolution transmission electron microscopy (HRTEM) images on JEOL JEM-2100. Specific surface area and adsorption-desorption tests were measured on ASAP2000 5 ACS Paragon Plus Environment


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(USA) under N2 atmosphere at 77 K. TG-DTA test was performed on TG/DTA PYRIS DIAMOND (USA) under N2 flow from room temperature to 800 oC. Electrochemical impedance spectra (EIS) were taken in a 3-compartment cell using an IVIUMSTAT (Netherlands) electrochemical workstation. The working electrode was prepared by the similar method described in ref. 38. Firstly, a homogeneous ink was prepared via ultrasonic dispersion of GO, rGOT sponge or rGOC (2.0 mg) in a Nafion solution (50.0 µL in 4 mL of distilled water). Then, 10.0 µL of the ink was dropped on a glass carbon electrode (diameter 3 mm). Lastly, the electrode was obtained after drying at room temperature. The counter electrode, reference electrode, and the used electrolyte were the same as in ref. 39. The photoluminescence (PL) spectra of EY before and after adding rGOT and rGOC were recorded on a fluorospectrophotometer (Hitachi F-7000). 5.0 mg of rGOT or rGOC was put into 100 mL of aqueous EY (4.0×10-5 mol L-1) solution. After 4 h sonication and 4-fold dilution, the test sample was obtained. 2.3. Photocatalytic H2 evolution activity Photocatalytic H2 production reaction was conducted under visible light irradiation in a closed 190 mL Pyrex cell. Typically, 5.0 mg of rGOT or rGOC, EY, trimethylamine (TMA), and H2PtCl6 reaction solutions in total 100 mL were added in the cell. The detailed reaction conditions including the light resource and reaction atmosphere, and the gas chromatograph parameters for determining the product H2 were described in ref. 38. The intensity of the incident light (400-700 nm) was 239.2 µmol m-2 s-1, measured on a FGH-1 actinometer. AQY was calculated based on the following formula.

AQY [%] = 2 ×

mole of hydrogen evolved × 100 mole of incident photon

(1)

The monochromatic AQYs at various incident wavelengths were determined by using LED lamps (UVEC-4) as the irradiation resources. The other photocatalytic reaction conditions were the same as in ref. 38. 6 ACS Paragon Plus Environment

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3. Results Figure 1 displays the XPS spectra of C1s for GO, rGOT and rGOC. The XPS spectra were fitted to evaluate the reduction degree of rGOT. Compared with GO, the oxygen-containing functional group (HO-C=O, C=O, C-O-C, and C-OH) peaks of rGOT decrease sharply, while the sp2 C=C bond carbon peak (284.6 eV) increases significantly. The total C/O atomic ratio is calculated, and the ratio of rGOT increases to 5.7 from 2.5 of GO, clearly demonstrating effective reduction of GO through the facile thermal treatment. Note that there is still a certain amount of sp3 carbons in rGOT, which may make the rGOT sheets flexible (a single graphene sheet is rigid due to sp2 π-conjugated plane). The total C/O atomic ratio of rGOC is 4.9, suggesting the lower reduction degree of rGOC than that of rGOT. Interestingly, although the C/O ratio of rGOT is greater than that of rGOC, namely lower total oxygen content, the content of C-OH group of rGOT is significantly larger than that of rGOC. FTIR spectra of GO and rGOT were depicted as Figure S1 (Supporting Information). Compared to the GO spectrum, the intensities of oxygen-containing functional group peaks of rGOT decrease remarkably. For GO, the C=C peak occurs at 1587 cm-1, while for rGOT, a new peak at 1631 cm-1 can be assigned to δC=C skeletal vibrations of unoxidized graphitic domains,40 confirming further the recovery of sp2 π-conjugated net in rGOT. Figure S2 (Supporting Information) shows the Raman spectra of rGOT and rGOC. There are two prominent peaks, namely D band and G band. The D band intensity of rGOT at about 1325 cm1

is lower than that of rGOC, indicating that the defect concentration of rGOT is less than that of

rGOC.41 This result is consistent with that from the XPS analysis.



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Figure 1. High resolution XPS spectra of C 1s for GO, rGOT and rGOC. The morphology and microstructure of the rGOT powder were observed by SEM and HRTEM. As shown in Figure 2a, rGOT exhibits the 3 dimensional sponge-like nanostructure with the various irregular pores (pore size range: about 5-200 nm). Figure 2b indicates that the exfoliated sheets consist of both planar and bending domains which form the pore structure. HRTEM image (Figure 2c) confirms further the presence of the pore structure formed by about 3-10 atom-thick rGOT sheets, while the stacking rGOC sheets is very thick (Figure S3, Supporting Information), much greater than 10 atom layers (the inset of Figure S3). 8 ACS Paragon Plus Environment

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Figure 2. The SEM (a,b), HRTEM(c) image of rGOT (the yellow dash lines represent rGOT layers); the XRD patterns of rGOC and rGOT (d); Nitrogen adsorption and desorption isotherms of rGOT (e); the photo of rGOC and rGOT (f). XRD analysis (Figure 2d) displays that rGOT exhibits a very wide and weak diffraction peak centered at 25.0° which is assigned to graphite layer structure, indicating also the poor (irregular) stacking of rGOT sheets and the presence of many bending domains, while the peak 9 ACS Paragon Plus Environment


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for rGOC is narrow and strong, suggesting good stacking (regular) of graphene sheets to well form layer-stacking structure. The adsorption isotherm of rGOT (Figure 2e) shows a slight adsorption at the low relative pressure region and a sharp adsorption in the relative high pressure region (P/P0 > 0.9), that is, a type III adsorption with H3 hysteresis loop, indicating the macropore structure raised from the rGOT sheets. The BET surface area of precursor GO is 44 m2 g-1. However, the surface area of rGOT dramatically expands to 404 m2 g-1, much larger than that of rGOC (66 m2 g-1). Figure 2f shows that the volume (not compressed) of the leavened fluffy rGOT is more than 30 times higher than that of rGOC with the same mass (apparent densities are 9.5 mg cm-3 and 285.0 mg cm-3 respectively), confirming further formation of the sponge structure. The conductivity of GO, rGOT and rGOC is verified by their EIS (Figure 3a). The highfrequency semicircle corresponds to the charge-transfer resistance (RCT) at the interface of the electrode contacting with the electrolyte solution.42 The semicircle of GO is significantly greater than those of rGOT and rGOC, indicating much lower conductivity for GO, and higher conductivity for rGOT and rGOC.43,44 The Nyquist diagrams of the rGOT and rGOC electrodes were fitted by employing the equivalent circuit as shown in the inset, where Re, CPE, RCT and Wo represent the electrolyte impedance between the working and reference electrode, the constant phase element simulating the nonideal capacitor, the charge transfer resistance, and the Warburg element, respectively.42 The RCT values for rGOT and rGOC are 89.9 and 135 Ω⋅cm2, respectively, which may be due to higher reduction degree of rGOT than that of rGOC. Figure 3b displays the TG and DTA curves of GO which were performed in N2 atmosphere from room temperature to 800 oC. The mass loss of about 20% from room temperature to 150 o

C can be ascribed to the desorption of the physical-adsorbed water on the surface of GO and

in the inner (interlayer). In this temperature range, we cannot observe the leavening of GO, suggesting that the slow release of interlaminar water cannot make stacked GO sheets be exfoliated to form the sponge. From 150 to 220 oC, mass loss of GO reaches about 50%, and at 10 ACS Paragon Plus Environment

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the same time the DTA curve shows a strong exothermic peak at 216 oC. In this process, two reactions take place: (1) the dehydration between the neighboring oxygen-containing groups, namely loss of abundant ‘chemical’ H2O,22,45 (2) the decarbonation by release of CO and CO2.22,27,46 In the temperature range, violent release of the gases exfoliates effectively the stacked GO sheets to form pore structure. Thus, the volume explosion can be observed (Figure 2f).



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Figure 3. (a) Nyquist diagrams of GO, rGOC and rGOT. (b) TG and DTA curves of the pristine GO sample. (c) The PL spectra of aqueous EY solution before and after adding rGOC and rGOT. Conditions: 1.0×10-5 mol L-1 EY; 12.5 µg mL-1 rGO; excited wavelength 480 nm. Figure 3c gives the PL spectra of aqueous EY solution before and after adding rGOC and rGOT. The EY solution shows a strong emission peak at about 542 nm. When rGOC or rGOT is introduced into the EY solution, the peak intensity sharply declines. The quenching effect of rGOT is much greater than that of rGOC, indicating that the electron transfer from the photoexcited EY molecules to rGOT is more effective than to rGOC. EY can be adsorbed onto rGO via hydrogen bond and π-π stacking interaction.47 Because the hydrogen bond energy between EY and rGO is about 107.9 kJ mol-1 (a moderate hydrogen bond), the interaction via hydrogen bond should be much stronger than that via π-π stacking. Thus, the adsorption via hydrogen bond is main. The adsorptions of EY onto rGOT and rGOC do not depend on the EY concentration in the range from 1.0×10-5 to 4.0 ×10-5 mol L-1 (Table S1, Supporting Information), indicating that the adsorption is a strong interaction, that is, via hydrogen bond. The adsorption of EY onto rGOT is about 2 times as high as onto rGOC, in agreement with their hydroxyl content (Figure 1). Figure 4 demonstrates the photocatalytic H2 production activity of EY-sensitized GO and rGO with Pt as a co-catalyst via in situ photodeposition. GO exhibits very low EY-sensitized photocatalytic H2 evolution activity (only 12.7 µmol for 3 h irradiation) because of the poor conductivity and low specific surface area. rGOT displays the extremely high photocatalytic activity (361.3 µmol), about 28.4 times as high as GO. rGOC shows the lower activity (165.1 µmol). The activity of EY-rGOT/Pt system is 2.2 times as high as that of EY-rGOC/Pt. Compared with rGOC, rGOT sponge as support for the sensitizer EY cannot ensure the excitation of all dyes, leading to decreased light utilization. However, rGOT sponge as the sensitized matrix shows much higher activity than rGOC, which can be attributed to the larger hydroxyl group content for the dye EY adsorption onto rGOT (Figure 1), more effective 12 ACS Paragon Plus Environment

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photogenerated electron injection from EY to rGOT (Figure 3c), and the higher conductivity for the electron transfer along rGOT sponge sheets to loaded Pt (Figure 3a).

Figure 4. Photocatalytic H2 production over EY-sensitized GO, rGOT and rGOC with Pt as a co-catalyst via in situ photodeposition. Conditions: 50 µg mL-1 GO or rGO; 2.0×10-4 mol L-1 EY; 7.7×10-6 mol L-1 H2PtCl6; 7.7×10-2 mol L-1 TMA, pH 10.0; 3 h irradiation. The reaction conditions of EY-rGOT/Pt system including H2PtCl6 concentration, EY concentration and pH were optimized. As shown in Figure S4a (Supporting Information), the amount of H2 evolution increases with H2PtCl6 concentration to 1.93×10-5 mol L-1, and then maintains almost unchanged. No hydrogen produces when there is no EY, indicating that the visible-light activity originates from the sensitization of EY (Figure S4b, Supporting Information). The activity rises with increase of the EY concentration, achieves a maximum at 4.0×10-4 mol L-1, and then reduces. The pH value remarkably affects the H2 production activity. With increasing pH value, the activity increases to a maximum at 10.0, and then declines quickly (Figure S4c, Supporting Information). Thus, the optimal initial conditions are 1.93×105

mol L-1 H2PtCl6 (7.5 wt% vs. rGOT), 4.0×10-4 mol L-1 EY, and pH 10.0. The highest H2

evolution amount is 207.0 µmol in an hour irradiation under the above optimal conditions, and the corresponding AQY reaches up to 28.0 % in visible range.



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Higher Pt loading on rGOT is needed in the above optimal reaction system. Because Pt is rare and expensive, it is desirable to load resource-abound and inexpensive HER cocatalysts on the sensitized matrix to substitute Pt. The potential candidates can include transition metal phosphides48-54 and the composite of graphene and Ni38 etc. Table 1. Performance of recent dye-sensitized photocatalysts for H2 production AQY (%) or H2

No. Photocatalyst

Reaction conditions

1

EY-rGO sponge/Pt

λ≥420 nm, TMA, pH 10.0

75.0 % at λ = 420 nm

this work

2

EY-rGO/Pt

λ≥420 nm, triethanolamine (TEOA), pH 7.0

9.3 % at λ = 520 nm

20

3

EY-rGO/Pt

λ≥420 nm, TEOA, pH 7.0

3.75 mmol g-1 h-1

47

4

Ru(dcbpy)32+-rGO/Pt


4.89 %

55

5

TPPH-rGO/Pt

Full spectrum, CTAB, TEOA

3.6 %

3

6

EY-rGO/Pt

λ≥420 nm, TMA, pH 10.0

23.4 % at λ = 520 nm

39

7

EY-rGO/NiSx


32.5 % at λ = 430 nm

11

8

EY-rGO/Pt-Sn


87.2 % at λ = 430 nm

56

9

EY-Fe3O4@rGO/Pt

λ≥420 nm, TEOA, pH 11.0

14.6 % at λ = 430 nm

57

10

EY-rGO/Ni

λ≥420 nm, TMA, pH 10.0

30.3 % at λ = 470 nm

38

11

EY-AlSiW11/Pt

λ≥420 nm, TEOA, pH 10.0

28.0 % at λ = 520 nm

36

12

EY-C3N4/Pt


18.8 %

58

13

ErB-TiO2/Pt

λ = 400 and 550 nm, TEOA

15.9 µmol in 2 h

59

14

PHIV/CoP

λ＞420 nm, TEOA, pH 1.0

160.7 µmol in 40 min

54

evolution activity

Ref

We also investigated the effect of incident wavelengths using monochromatic light sources on photocatalytic hydrogen production in EY-rGOT/Pt system under the optimal conditions. Figure 5a displays the AQYs for EY-rGOT/Pt at various incident wavelengths. The highest AQY achieves 75.0 % at 420 nm, much higher than the most reported data as shown in Table1. In the long-wavelength region, the catalytic activity decreases quickly because the absorption of EY declines fast with increasing light wavelength. To improve the activity in this region, dye co-sensitization strategy (using dyes with complementary absorption spectra) can be 14 ACS Paragon Plus Environment

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adopted.33,60 Also, new efficient dyes should be developed to expand their absorption spectra into the long-wavelength visible and even panchromatic light.33 Figure 5b displays the time courses of photocatalytic hydrogen evolution in EY-rGOT/Pt and EY-rGOC/Pt reaction systems. After four reaction cycles, the activity over EY-rGOT/Pt declines only 22 % compared to that for the first cycle, while that of EY-rGOC/Pt decreases 60 %. This indicates rGOT is relatively stable in the reaction system. A small decrease in the photocatalytic H2 production activity after each reaction cycle in the two reaction systems could be ascribed to the losses of rGOT and rGOC in the filtrating processes. Interestingly, the activity of rGOC decreases significantly in the third cycle, while that of rGOT decreases slowly. The reason is that both the recovered rGOT and rGOC were kept in renewed reaction solution for one night after the second cycle, which would lead to serious aggregation of rGOC but not for rGOT. To confirm the above assumption, we have tested the size change of rGOT and rGOC before and after resting for one night. As shown in Table S2 (Supporting Information), the average size of rGOT hardly changes after the resting, while rGOC increases by 18 %. Further investigation is needed to understand the mechanism.



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Figure 5. (a) Effect of incident light wavelength on AQY of the EY-rGOT/Pt reaction system. Conditions: 50 µg mL-1 rGOT; 4.0×10-4 mol L-1 EY; 1.93×10-5 mol L-1 H2PtCl6; 7.7×10-2 mol L-1 TMA, pH 10.0; irradiation 1 h. (b) Time courses of photocatalytic H2 production in EYrGOT/Pt and EY-rGOC/Pt reaction systems. Conditions: 100 µg mL-1 rGO; 4.0×10-4 mol L-1 EY; 3.86×10-5 mol L-1 H2PtCl6; 7.7×10-2 mol L-1 TMA, pH 10.0; After each 3 h reaction cycle, the recovery of the used catalyst and next cycle were conducted as in ref. 38.

4. Discussion GO is an oxygen-functionalized graphene sheet with some epoxy and hydroxyl groups on the basal plane and carboxyl and lactol at the sheet edges.61 It consists of mixed sp2-sp3 hybridized phase, that is, bare sp2 graphitic domains (about 3 nm) are well divided by small oxidized sp3 16 ACS Paragon Plus Environment

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fragments largely containing epoxy and hydroxyl groups with a small quantity of carbonyl.40 In this case, GO is poor conductive.

Figure 6. (a) Schematic illustration of phase-separation and leavening in the thermal treatment process of GO. (b) The schematic illustration of the adsorption of EY and EY•- onto rGO via hydrogen bond. (c) The SEM image of Pt loading on rGOT sheets via in situ photocatalytic H2 evolution reaction. (d) The suggested mechanism of EY-sensitized photocatalytic H2 evolution over rGOT with Pt as co-catalyst under visible light irradiation. On annealing GO, the both epoxy and hydroxyl groups can easily diffuse along the GO basal plane, because their activation barrier are low (0.83 and 0.30 eV, respectively).40 Thus, even when heating at 80 oC for several days, the isolated sp3 fragments can effectively interact and merge into the larger sp3 domains, leading to separation of the sp2 and sp3 domains (phase separation), formation of larger sp2 graphitic/planar domains and enhancement of the 17 ACS Paragon Plus Environment


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conductivity.40 When GO is heated to higher temperature such as 150 oC, the phase separation should take place very fast. Moreover, because our heating rate is slow (10 oC min-1), there is enough time for epoxy and hydroxyl groups to diffuse into merged domains before the dehydration and decarbonation reactions occur (>150

o

C), leading to high oxygen

accumulation degree in the merged sp3 domains. As a result, with progressive heating from 150 to 250 oC, the two reactions take place violently at the oxygen-concentrated domains and release a large amount of H2O, CO and CO2 gases to exfoliate effectively stacked GO sheets, resulting in formation of the graphene sponge. Figure 6a shows the proposed phase-separation and leavening process in thermal treatment of GO. During the annealing, “self-healing” or “regraphitization” of the accumulation domains also happens to enhance the conductivity. Thus, the formed rGOT sponge exhibits high conductivity. At the same time, besides the presence of oxygen-containing groups, various defects including topological defects and vacancies are formed during the “self-healing”,62,63 which makes the domains themselves transform into the bending parts of the pore structure (Figure S5, Supporting Information). Hydrogen bond interaction is critical to achieving long-range donor–acceptor electronic coupling in biological systems.64 Since there is hydrogen bond between EY and rGOT, the electron-transfer between photoexcited EY and rGOT could be realized by the hydrogen bond. Based on the adsorption experiment (Table S1, Supporting Information), at higher EY concentration, only a small part of EY molecules are adsorbed onto rGOT sponge, whereas most of EY molecules exist in the solution. The adsorbed EY molecules can directly inject electrons into rGOT when the molecules are excited by visible light. How can the molecules in the solution transfer the electrons to rGOT? When a dye EY in the solution is irradiated by visible light, the excited EY* can form by the electron transition from HOMO to LUMO orbital. Because the concentration of TMA (7.7×102

mol L-1) is very high compared with that of EY (4.0×10-4 mol L−1) in the reaction system,

TMA could effectively quench EY* to produce reductive radical EY•-.65 18 ACS Paragon Plus Environment

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hυ EY → EY*

(2)

EY* + TMA → EY•- + oxidized TMA

(3)

Since the formed EY•- radical is very stable (the lifetime in the millisecond range),66 it should diffuse from bulk solution to the surface of rGOT via “Brownian motion”. As shown in Figure 6b, both EY and EY•- can form a hydrogen bond between the phenoxyl and hydroxyl of rGOT. Because EY•- has one more electron than EY, the phenoxyl oxygen of EY•- as the hydrogen acceptor bears a higher negative charge than that of EY, leading to enhancement of the hydrogen bond strength. As a result, when EY•- diffuses to the surface of rGOT, it can replace the adsorbed EY, inject the electron into rGOT and then transform into adsorbed EY again. Thus, although the most EY molecules exist in bulk solution, after photoexcitation, the formed EY•- radicals can still inject the electrons into rGOT to achieve high efficiency for photocatalytic hydrogen evolution. EY•- + rGOT → rGOT- + EY

(4)

The injected electrons can be effectively transferred along the planes to the bending domains of rGOT and be trapped by the defects in the areas. The defects act as active sites for in-situ photodeposition of Pt and for photocatalytic hydrogen evolution.67 PtCl62- + 4 rGOT- → Pt + rGOT + 6 Cl-

(5)

The SEM characterization (Figure 6c) shows that Pt indeed preferentially deposits at the bending domains besides edges, confirming that the above assumption is correct. After the Pt deposition, the EY•- radicals can continuously inject the electrons into rGOT by reaction 4 and then transfer to Pt, which reduce water into H2. 2 rGOT- + 2 H2O → 2 rGOT + 2 OH- + H2 (on Pt)

(6)



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Based on above results, the mechanism for EY-sensitized photocatalytic H2 evolution over rGOT with sensitizer EY and co-catalyst Pt under visible light irradiation is proposed as Figure 6d.

5. Conclusions In summary, rGOT sponge was successfully prepared by facile thermal treatment of compact graphene oxide powder at low temperature of 250 oC under N2 atmosphere. The sponge exhibits macroporous structure with BET specific surface 404 m2 g-1 and high conductivity. The pore structure is consisted of both planar and bending domains. The former is mainly graphitic domains with sp2 carbons, while the latter contains various defects including oxygencontaining groups, vacancies and topological defects. The obtained graphene shows highly efficient dye-sensitized photocatalytic H2 evolution activity. The highest AQY is up to 75.0 % at 420 nm. This work develops a facile, green and cheap method to synthesize rGOT sponge from GO. The sponge is an excellent matrix for dye-sensitized hydrogen evolution, much better than the sheet-like rGOC and most reported rGO matrices.

ASSOCIATED CONTENT Supporting Information The characterization results including FTIR spectra of GO and rGOT; Raman spectra of rGOC and rGOT; HRTEM images of rGOC and rGOT; the optimal reaction conditions of EY-rGOT/Pt for photocatalytic hydrogen production activity; the adsorption amount of EY onto rGO sheets, and average sizes of rGOC and rGOT before and after resting. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail: Yuexiang [email protected]. 20 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No.21563019, 21163012) and the Nature Science Foundation of the Jiangxi Province (No.20151BAB203).

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Table of contents

Graphene sponge synthesized by a facile thermal treatment of graphene oxide exhibits highly efficient dye-sensitized photocatalytic H2 evolution activity.


Facile Synthesis of Graphene Sponge from Graphene Oxide for

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