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Three-dimensional porous aerogel constructed by g-C3N4 and graphene oxide nanosheets with excellent visible-light photocatalytic performance Zhenwei Tong, Dong Yang, Jiafu Shi, Yanhu Nan, Yuanyuan Sun, and Zhongyi Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09503 • Publication Date (Web): 06 Nov 2015 Downloaded from http://pubs.acs.org on November 13, 2015

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ACS Applied Materials & Interfaces

Three-dimensional porous aerogel constructed by g-C3N4 and graphene oxide nanosheets with excellent visible-light photocatalytic performance Zhenwei Tong,ad Dong Yang,bc Jiafu Shi,c Yanhu Nan,bd Yuanyuan Sun,bd Zhongyi Jiang*ad a

Key Laboratory for Green Technology, School of Chemical Engineering and Technology, Tianjin

University, Tianjin 300072, China. b

Key Laboratory of Systems Bioengineering of Ministry of Education, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 30072, China c

School of Environmental Science and Engineering, Tianjin University, 300072 Tianjin, China

d

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,

China

Abstract: It is curial to develop the high-efficient, low-cost visible-light responsive photocatalyst for the application in solar energy conversion and environment remediation. Here, a three-dimensional (3D) porous g-C3N4/graphene oxide aerogel (CNGA) has been prepared by the hydrothermal co-assembly of two-dimensional g-C3N4 and graphene oxide (GO) nanosheets, in which g-C3N4 act as efficient photocatalyst, and GO support the 3D framework and promote the electron transfer simultaneously. In CNGA, the highly interconnected porous network renders numerous pathways for rapid mass transport, strong adsorption and multi-reflection of incident light; meanwhile the large planar interface between g-C3N4 and GO 1

ACS Paragon Plus Environment


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nanosheets increases the active site and electron transfer rate. Consequently, the methyl orange removal ratio over CNGA photocatalyst reaches up to 92% within 4 h, which is much higher than that of pure g-C3N4 (12%), 2D hybrid counterpart (30%) and most of representative g-C3N4-based photocatalysts. In addition, the dye is mostly decomposed into CO2 under natural sunlight irradiation, and the catalyst can also be easily recycled from solution. Significantly, when utilized for CO2 photoreduction, the optimized CNGA sample could reduce CO2 into CO with a high yield of 23 mmol g-1 (within 6 h), exhibiting about 2.3-fold increment compared to pure g-C3N4. The photocatalyst exploited in this study may become an attractive material in many environmental and energy related applications. Keywords: g-C3N4; graphene oxide; three-dimensional aerogel; visible-light; photocatalyst

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Introduction Visible-light photocatalysis has attracted tremendous interest due to their potential utilization for solar energy in the environmental remediation and energy conversion.1-2 Developing active and stable visible–light responsive photocatalyst is crucial for successful implementation of these applications. Generally, the performance of a photocatalyst depends heavily on its chemical and physical structure, since it can influence three major steps in photocatalysis: photon absorption, charge carrier transfer, and catalytic surface reactions.3-7 Till now, photocatalysts with diverse structures have been prepared and utilized, such as tubes, sheets and spheres, in which 3D porous structure has been proved to be an appealing and effective strategy.6, 8-9 Such structure can afford high-performance photocatalysis due to the large accessible surface area for facilitated adsorption and photoreaction. Besides, it has more efficient light harvesting ability due to the multi-reflection within interconnected open-framework.4, 6, 10 Moreover, 3D porous structure can substantially inhibit the aggregation or stacking of subunits, thus exposing more active sites for catalytic surface reactions.6, 11 Therefore, it is desirable to obtain visible-light photocatalysts with 3D porous structure for efficient photocatalysis. Construction of high-efficient 3D porous structure relies on the advanced materials and appropriate methods. In this regard, graphene aerogels (GA) with a typical 3D porous framework, large surface area and high adsorption capacity should be a preferred choice.12 Graphene aerogels have been successfully synthesized via the

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self-assembly of GO nanosheets with the π-π stacking and hydrogen bonds between nanosheets, and used for energy storage.13 However, up to now, GA have not be employed as photocatalyst due to its intrinsic physical property. Meanwhile, the GA-based composites have been actively explored in elevating photoelectrocatalytic activity and energy storage efficiency, owing to the distinctive attributes of GA,14-16 such as high interconnected open-framework, excellent electrical conductivity and without removal of template.17-18 In a recent work, Chen et al. prepared functional GA-based composites decorated with various metal/metal oxide particles via a co-assembly

method,

achieving

high

performance

as

electrodes

of

photoelectrochemical and supercapacitor devices.19 Zhang et al. prepared the TiO2/GA hybrid architecture with good conductivity, exhibiting a high specific capacity in lithium-ion batteries.20 However, to our knowledge, the application of the prepared GA-based aerogels have been limited to electrocatalysis and electrochemical supercapacitor; while GA-based composites as metal-free photocatalysts have not been exploited, although they may have advantages over metal/metal oxides such as low cost and high chemical stability for practical benefits. Graphic C3N4 (g-C3N4) is a fascinating photocatalyst for water splitting and pollutant degradation, since it can absorb the visible light with a medium band gap (2.4-2.8 eV).1,

21-22

Besides, the exceptional chemical stability, low cost, green

precursors and in particular the metal-free feature endow g-C3N4 with great promise in photocatalysis. However, the catalytic performance of g-C3N4 is dramatically

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restricted by its high electron–hole recombination and low specific surface area with inefficient light absorption. Hence, a g-C3N4/GO composite aerogel that manipulates the synergistic effect between g-C3N4 and GA, may offer a novel and feasible solution to high-performance visible-light photocatalyst.12-13 Concretely, the electron-hole recombination of g-C3N4 can be inhibited by good electrical conductivity of GO, the visible-light utilization may be intensified via the light multi-reflection across the connected open-framework, and the catalytic surface reaction can be intensified through the high adsorption capability.23-24 Moreover, the design and utilization of 3D g-C3N4/GO aerogel on the photodegradation and CO2 photoreduction applications are scarcely reported. In this study, the 3D porous g-C3N4/GO aerogels (CNGA) were synthesized via a facile hydrothermal co-assembly of two-dimensional GO and g-C3N4 nanosheets. The morphology, structure and property of GO, g-C3N4 nanosheets and CNGA were characterized in detail. The photocatalytic performance of CNGA was evaluated by the methyl orange (MO) degradation and CO2 reduction under visible light irradiation. CNGA may possess a high photocatalytic performance based on the coupling of g-C3N4 photocatalyst and GA. The effect of the aerogel structure on the photophysical and catalytic properties of CNGA was also elucidated. Due to the unique structure and property, 3D metal-free CNGA may become a novel and competitive photocatalyst for broad applications. Experimental

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sonication

hydrothermal

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freeze drying

co-assembly

g-C3N4/GO mixed aqueous solution

g-C3N4/GO hydrogel

GO

g-C3N4/GO aerogel

g-C3N4

Scheme 1. A schematic illustration for the synthetic procedure of CNGA.

Materials Melamine (C3H6N6), hydrogen peroxide aqueous solution (H2O2, 30 wt%) and sodium nitrate (NaNO3) were purchased from Tianjin Guangfu Technology Development Co. Ltd. Natural graphite flake (2500 mesh) was purchased from Qingdao Tianhe Graphite Co. Ltd. Concentrated sulfuric acid (H2SO4, 98 wt%), hydrochloric acid (HCl) and potassium permanganate (KMnO4) were bought from Tianjin Kewei Ltd. All chemical regents were of analytical grade, and used without further purification. The deionized water was used throughout this study. Synthesis of CNGA composites GO was prepared from graphite powder according to Hummer’s method.25 The g-C3N4 nanosheet was prepared via a simple thermal oxidation etching process as our previous report.26 In detail, bulk g-C3N4 was first synthesized by heating melamine at 550 oC for 4 h in static air with a rate of 2.5 oC min-1; the yellow product was collected and milled into powder for further utilization. The g-C3N4 nanosheet was fabricated by the thermal oxidation of bulk g-C3N4 at 500 oC for 2 h with a rate of 5

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o

C min-1. Then, the CNGA photocatalyst was synthesized via a hydrothermal

co-assembly of g-C3N4 and GO nanosheets and following freeze-drying as shown in Scheme 1. Typically, 10 mL suspension of g-C3N4 nanosheets (16 mg mL–1) was mixed with 10 mL suspension of GO nanosheets (4 mg mL–1), and the mixture was treated under ultrasonication for 30 min. Here, amphiphilic GO nanosheets can serve as the surfactant to disperse the g-C3N4 nanosheet, leading to the formation of g-C3N4/GO hybrid nanosheets. Subsequently, the resultant caesious stable suspension was put into a 50 mL Teflon vessel, and then sealed in an autoclave and heated at 180 o

C for 6 h. During the heating process, a 3D hydrogel forms by the co-assembly of

g-C3N4/GO hybrid nanosheets, which is similar to the formation of GO hydrogels via the self-assembly of GO nanosheets.13, 19 It is deduced that the π–stacking interactions and the hydrogen–bonding interactions between GO and g-C3N4 nanosheets are responsible for the formation of 3D framework. After washing with deionized water and freeze–drying overnight, a cylindrical CNGA was obtained finally. A series of CNGA photocatalysts were prepared by changing the mass ratio of g-C3N4 to GO (mg-C3N4: mGO= 4.0, 3.0, 2.0, 1.5 and 0.8), which are denoted as CNGA–4, CNGA–3, CNGA–2, CNGA–1.5 and CNGA–0.8, respectively. For comparison, a 2D g-C3N4/GO hybrid (mg-C3N4: mGO= 2.0) was also prepared by heating the mixed suspension of g-C3N4 and GO nanosheets at 80 oC for ~30 min in a thermostatic water-bath, which was denoted as 2D hybrid.27 Characterization

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The morphology and microstructure of as-prepared CNGA samples were observed by the scanning electron microscope (SEM, FEI Nova XL430 NanoSEM) and the transmission electron microscope (TEM, FEI Tecnai G2 F20) with an acceleration voltage of 200 kV. Energy dispersive X-ray analysis spectroscopy (EDS) was conducted at 90 K by using an FEI Nova XL430 NanoSEM equipped with a Bruker X-flash silicon drift detector (SDD) at 200 kV. Fourier transfer infrared spectroscopy (FTIR) was performed by using a Nicolet-560 FTIR spectroscope. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin-Elmer PHI 1600 ESCA with monochromatized Mg Kα X-ray radiation (1253.6 eV) and hemispherical electron energy analyzer. X-ray powder diffraction (XRD) patterns were recorded using a Rigaku D/max 2500V/PC X-ray diffractometer (Cu Kα, λ = 0.154 nm, 40 kV, 200 mA). The diffuse-reflectance spectrum (DRS) was recorded by a UV-vis spectrophotometer (U-3010, Hitachi) equipped with an integrating sphere, using BaSO4 as the reference. The specific surface area was measured by a MicromeriticsASAP2020 Surface Area and Porosity Analyzer and calculated by using the Brunauer−Emmett−Teller (BET) equation. Photocatalytic activity Photocatalytic activity of as-prepared CNGA samples was evaluated by the degradation of MO (20 mg L–1) in an aqueous solution under 500 W Xe lamp (Beijing AuLight Technology Co.) with a cutoff filter (420 nm). The light intensity was 0.15 mW cm–2 as measured using a UV-A radiometer (Photoelectric Instrument Factory,

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Beijing Normal University). Thirty milligram photocatalysts were dispersed in 30 mL MO solution in a 50 mL quartz tube. Prior to irradiation, the suspension was magnetically stirred for 1 h in the dark to ensure that the photocatalyst surface was saturated with MO molecules. Every irradiation interval, 2.0 mL suspension was sampled and analyzed with a UV-vis spectrophotometer at the maximal absorption wavelength of MO (465 nm). The MO degradation under natural light irradiation and the CO2 reduction under visible light irradiation reactions over CNGA were also tested and displayed in the supporting information. Results and discussion Characterization of CNGA composites

Figure 1. AFM images of (a) g-C3N4 and (b) GO nanosheets; SEM images of (c-d) CNGA-2 and (e) 2D hybrid; (f) HRTEM images of CNGA-2 (the inset is a magnified image of the area boxed by the dashed line).

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50

50

147.6 ± 25.2 nm Frequency/%

Frequency/%

40 30 20

155 ± 19.9 nm

40 30 20 10

10

0 100

0 100

150

Pore size/nm

200

150

200

Pore size/nm

50

50

191 ± 21.3 nm

232 ± 30 nm

40

40

Frequency/%

Frequency/%

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30

20

10

30

20

10

0

0 150

200

250

300

350

Pore size/nm

150

200

250

300

350

400

Pore size/nm

Figure 2. SEM images of (a) CNGA-4, (b) CNGA-3, (c) CNGA-1.5 and (d) CNGA-0.8.

The morphology and microstructure of CNGA were depicted by AFM, SEM and HRTEM. The AFM images in Figure 1a and b exhibit that the as-prepared g-C3N4 and GO nanosheets have similar laminar structure with the thickness of 1.762 and 0.676 nm, respectively. The SEM image in Figure 1c demonstrates that the CNGA composite possesses a porous, wrinkled, and fluffy microstructure stacked by nanosheets. From its enlarged image in Figure 1d, it is observed that the pore is highly interconnected, the mean pore size is about 170 nm, and the pore wall is composed of randomly oriented nanosheets; while the 2D hybrid counterpart only shows a planar microstructure with nonporous wrinkles about several micrometers long (Figure 1e). The EDX spectrum of CNGA confirms the presence of the C, O and N elements and

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the uniform dispersion of N element in the sample (Figure S1), suggesting that the g-C3N4 nanosheets are dispersed homogeneously in the CNGA composites. Meanwhile, the HRTEM image (Figure 1f) further exhibits that the nanosheets constituting the 3D framework are transparent flakes about several micrometers in size, and appear some corrugation and scrolling. Moreover, it can be seen from the inset in Figure 1f that the g-C3N4 nanosheet is tightly attached on the GO nanosheet, forming a sheet-on-sheet structure. It is noted that a distinct and coherent interface exists between GO and g-C3N4, suggesting the heterostructure formation. This heterostructure can accelerate the electron transfer from g−C3N4 to GO, thus benefiting for the photocatalytic activity. Besides macropores, CNGA−2 also contains the

mesopores

about

4

nm

in

diameter

determined

by

the

nitrogen

adsorption−desorption isotherm (Figure S2), which can be ascribed to the defect hole in the g−C3N4 nanosheet.28 The specific surface area of CNGA−2 is calculated to be 289.2 m2 g−1 based on BET method, which is slightly lower than that of GO aerogel (312.7 m2 g−1) prepared by the same method. This phenomenon can be assigned to the attachment of g−C3N4 nanosheets on GO, which enhances the thickness of framework. Additionally, the integral structure evolution of as-prepared CNGA samples with different mass ratios of g-C3N4 to GO is also characterized. As shown in Figure 2, the macropore size of CNGA can be regulated easily from 147±25 to 232±32 nm through changing the mass ratios of g−C3N4 to GO from 4.0 to 0.8.

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 g-C



 GO



3

b

N4

g-C3N4 CNGA-4 CNGA-3 CNGA-2 CNGA-1.5



GO

Transmittance /%

a

Intensity /a.u.

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CNGA-2

1724

1629

1675

g-C3N4

1072 1543 806

GO

10

20

30

40

50

1640

3500

60

2Theta /degree

3000

2500

2000

1240

1500

-1

1000

500

Wavelength /cm

Figure 3. (a) XRD patterns and (b) FTIR spectra of pure g−C3N4, GO, and as−prepared CNGA samples.

XRD patterns were performed in order to characterize the crystal structure of as−prepared CNGA samples, as well as pure g−C3N4 and GO. As exhibited in Figure 3a, the XRD spectrum of pure g-C3N4 exhibits two distinct peaks at 13.1o and 27.3o, which are indexed as the (100) and (002) plane of graphitic materials, respectively. The strong peak at 27.3o is a characteristic stacking reflection of conjugated aromatic systems, revealing a graphitic structure with an interlayer distance of 0.326 nm.29-30 For pure GO, only a sharp diffraction peak appears at around 10.8o, corresponding to the (002) reflection of GO.19 In the spectra of as-prepared CNGA samples, the characteristic peak at 27.3o is observed clearly, indicating the preservation of well−crystalline g−C3N4 structure in the composites. Meanwhile, a new peak at 26.2o, corresponding to the interlayer spacing of 0.344 nm appears and gradually increases with the increase of g-C3N4 content, and the characteristic peak of GO at 10.8o is significantly weakened in the case of CNGA samples. These results indicate that GO is partially converted into reduced GO (rGO) during the hydrothermal process, further

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indicative of the evolved graphitic structures.12 Figure 3b shows the FTIR spectra of pure g-C3N4, GO and CNGA-2. The FTIR spectrum of pure g-C3N4 presents three characteristic absorption bands at >3000, 1200–1650, and 420 nm)

1.0

MO, 13.5

36

Au/g-C3N4

2.5 h, 93%

500 W (>400 nm)

3.0

MO, 10

37

g- C3N4/rGO

1.25 h, 100%

1000W (>400 nm)

1.6

RhB, 5

33

CNT/g-C3N4

3 h, 100%

300 W (>400 nm)

1.0

MB, 10

38

Ag@C3N4

5 h, 100%

500 W (>420 nm)

0.5

MB, 3.74

29

Porous g-C3N4

4 h, 40%

500 W (>420 nm)

0.5

MB, 10

30

g-C3N4/Ag2O

30 min, 80%

300 W (>400 nm)

0.4

MO, 20

39

g-C3N4/SnS2

45 min, 98%

300 W (>400 nm)

0.1

MO, 10

40

CNGA

4 h, 92%

500 W (>420 nm)

1.0

MO, 20

this work

core-shell

In order to study the photocatalytic activity of CNGA in practice, the MO 19



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degradation under natural sunlight irradiation in Tianjin City was also conducted using CNGA-2 and CNGA-4 as the representatives. As shown in Figure S3, the color of MO solution over CNGA catalysts apparently fades away with the prolongation of reaction time. About 86% COD is removed by CNGA-2 after reaction for 15 h, which is higher about 26% than that by CNGA-4 (Figure S4). The COD removal confirms that MO is decomposed into CO2 under natural sunlight irradiation, but not a decoloration process merely. The catalytic activities of the CNGA samples were also evaluated by the photocatalytic reduction of CO2 using water vapor as a scavenger. Figure S5 exhibits that pure g-C3N4 and CNGA samples (without noble-metal addition) evolve CO as the main product (CO2 + 2H+ + 2 e– → CO + H2O, -0.53 eV vs. NHE). Similarly, the CNGA-2 also demonstrates a higher CO yield (23 mmol g-1) within 6 h, which is about 1.4 and 2.3 times higher than that of CNGA-4 and pure g-C3N4, respectively. The CO yield in this study is also superior to other reported photoreduction systems with CO as the main product using catalysts, such as Au-SrTiO3 (0.35 mmol h-1 g-1) and porous g-C3N4 (1.87 mmol h-1 g-1).34, 41 The excellent photocatalytic activity of CNGA can be ascribed to three factors as below. First, the photogenerated electrons of g-C3N4 nanosheets can transfer freely to GO surface via their large coherent interface, significantly inhibiting the radiative recombination of electron-hole pairs as confirmed by PL analysis. Second, the GO incorporation and 3D porous aerogel structure improve the visible light absorption,

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leading to more photogenerated electron-hole pairs for catalysis. At last, CNGA samples possess rather high adsorption capacity for MO solution (32 mg/mg), which is kinetically favorable for the surface chemical reaction. It has been widely reported that the adsorption of reactants is a prerequisite for the photocatalytic reaction, i.e. as a general phenomenon, the greater the adsorption is, the higher the degradation rate is.42 In this study, nearly 50% MO is adsorbed by CNGA-2; while only 3% MO is absorbed by pure g-C3N4. The 3D porous CNGA can capture plenty of MO molecules, and thus promoting the MO degradation, which is also suitable for the CO2 photoreduction. The continuous decreased absorbance of MO solution and increased CO production further confirm the rapid and synchronous adsorption-photocatalysis oxidation process under light irradiation. Since g-C3N4 and GO act different roles in the photocatalytic process, the balance between their contents can affect the activity of CNGA. When the layered g-C3N4 content is low, the excess GO nanosheets can rapidly transfer the photogenerated electrons, resulting in the increase of photocatalytic activity. On the contrast, when the g-C3N4 content is high, the GO nanosheets are not enough to disperse g-C3N4 or inhibit the recombination of electron-hole pairs, resulting in the decrease of photocatalytic activity. The CNGA composites would become a promising material for photocatalytic applications, considering its high dye decomposition and CO2 reduction capacity under visible light, together with the material availability (abundant elements on earth), facile preparation and no noble-metal addition.

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Photocatalytic mechanism No scavenger Methanol EDTA Benzoquinone

1.0 Dark

Light on

0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6

0.4

0.2

0.0 -1

0

1

2

3

4

Time/h

Figure 7. Effect of different scavengers on the MO degradation in the presence of CNGA-2.

To investigate the photocatalytic mechanism of CNGA, a radical trapping experiment was performed to explore the reactive radical species involved in the MO degradation over CNGA-2. Three radical scavengers including methanol (1: 15/V: V), benzoquinone (1 mmol L-1) and disodium ethylenediaminetetraacetate (EDTA, 10 mmol L-1) were added into the reaction system for trapping the specific reactive specie •OH, h+ and •O2-, respectively. As shown in Figure 7, a significant decrease in the photodegradation efficiency is observed by the addition of EDTA and methanol, while benzoquinone influences the photodegradation process only slightly. This result indicates that h+ and •OH are the major active species in the degradation process, while •O2− performs a minor effect.

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Scheme 2. Schematic diagram for illustrating the photodegradation (І) and photoreduction (П) processes over CNGA under visible light irradiation.

Based on the above results, a tentative photocatalytic mechanism is illustrated in Scheme 2. When the visible light irradiates on the CNGA catalyst, the incident light can be directly absorbed by CNGA and also scattered by its 3D porous framework. With the transmission of scattered light, it can be absorbed by the inner framework, leading to an enhanced visible light utilization. Then the g−C3N4 component is excited to produce electrons in the CB and keep the holes in the VB. Subsequently, the photo-excited electrons transfer to the GO surface for O2 reduction, due to the high conductivity and lower Fermi level of GO (-0.08 eV vs. NHE). The holes remained in the VB of g−C3N4 can directly degrade the MO molecules or react with the surface adsorbed OH– to generate •OH reactive radical for the MO oxidation. Generally, CO2 reduction reaction usually undergoes two processes: oxidizing water to generate hydrogen ions via the half-reaction (2H2O + 4h+→O2 + 4H+) and reducing CO2 to CH4 via acquiring 8-electrons process (CO2 + 8H+ + 8e–→CH4 + 2H2O).43 The edges of VB and CB of g-C3N4 are determined to be 1.4 and -1.3 eV, 23



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respectively. EVB of g-C3N4 is more positive than that of Eo (H2O/H+, 0.82 eV vs. NHE), and ECB is more negative than that of Eo (CO2/CH4, -0.24 eV vs. NHE). Under visible light irradiation, the photogenerated electrons transfer to the conducting network of GO, and then react with CO2 molecules adsorbed on the GO surface to produce CO. The holes remained in the VB of g-C3N4 can react with the surface adsorbed H2O. The separated electron-hole pairs fix the oxidative and reductive reactions primarily on g-C3N4 and GO surfaces, leading to highly efficient photocatalytic process. Conclusions In summary, a 3D porous g-C3N4/GO aerogel was prepared for efficient visible-light photocatalysis by using two-dimensional g-C3N4 and GO nanosheets as building blocks. The CNGA possesses highly interconnected framework with numerous macropores, and the pore wall is constructed by the robust sheet-on-sheet structures. The 3D porous framework facilitates the light absorption and the reactant adsorption; while the large planar interface between GO and g-C3N4 nanosheets accelerates the electron transfer rate and the separation of charge carriers. Consequently, the CNGA displays excellent photocatalytic activities for MO degradation, as well as for the CO2 reduction under visible light irradiation. Among all the CNGA samples, CNGA-2 exhibits the highest photocatalytic performance for MO degradation and CO2 reduction, which are 7.6 and 2.3 times higher than that of pure g-C3N4, respectively. The optimal combination of 3D porous structure with advanced materials in this kind

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of CNGA composites is believed as a promising strategy to prepare diverse kinds of photocatalysts with high catalytic performance.

ASSOCIATED CONTENT Supporting Information Additional experimental data including the EDX and Barrett−Joyner−Halenda (BJH) pore size distribution plot of CNGA-2 sample, MO degradation under natural light irradiation and CO2 photoreduction under visible light irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E−mail: [email protected]; Fax: +86−22−27406646; Tel: +86−22−27406646 (Zhongyi Jiang) ACKNOWLEDGMENTS The authors thank the financial support from the National Science Fund for Distinguished Young Scholars (21125627), National Natural Science Funds of China (21406163), Tianjin Research Program of Application Foundation and Advanced Technology (15JCQNJC10000), Program of Introducing Talents of Discipline to Universities (B06006). We are grateful for prof. Defa Wang for his advice and support on CO2 reduction experiment.

Reference (1) Liu, J.; Liu, Y.; Liu, N. Y.; Han, Y. Z.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. H. Metal-free Efficient Photocatalyst for Stable Visible Water Splitting via a Two-electron Pathway. Science 2015, 347, 970-974. 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(2) Cao, S. W.; Low, J. X.; Yu, J. G.; Jaroniec, M. Polymeric Photocatalysts Based on Graphitic Carbon Nitride. Adv. Mater. 2015, 27, 2150-2176. (3) Sun, Z. Q.; Kim, J. H.; Zhao, Y.; Bijarbooneh, F.; Malgras, V.; Lee, Y.; Kang, Y. M.; Dou, S. X. Rational Design of 3D Dendritic TiO2 Nanostructures with Favorable Architectures. J Am. Chem. Soc. 2011, 133, 19314-19317. (4) Li, H. X.; Bian, Z. F.; Zhu, J.; Zhang, D. Q.; Li, G. S.; Huo, Y. N.; Li, H.; Lu, Y. F. Mesoporous Titania Spheres with Tunable Chamber Stucture and Enhanced Photocatalytic Activity. J. Am. Chem. Soc. 2007, 129, 8406-8407. (5) Cao, S. W.; Yu, J. G. g-C3N4-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2014, 5, 2101-2107. (6) Li, G. L.; Chen, Q. W.; Lan, J. Facile Synthesis, Metastable Phase Induced Morphological Evolution and Crystal Ripening, and Structure-Dependent Photocatalytic Properties of 3D Hierarchical Anatase Superstructures. ACS Appl. Mater. Interfaces 2014, 6, 22561-22568. (7) Sun, J. H.; Zhang, J. S.; Zhang, M. W.; Antonietti, M.; Fu, X. Z.; Wang, X. C. Bioinspired Hollow Semiconductor Nanospheres as Photosynthetic Nanoparticles. Nat. commun. 2012, 1139-1145. (8) Qie, L.; Chen, W. M.; Xu, H. H.; Xiong, X. Q.; Jiang, Y.; Zou, F.; Hu, X. L.; Xin, Y.; Zhang, Z. L.; Huang, Y. H. Synthesis of Functionalized 3D Hierarchical Porous Carbon for High-performance Supercapacitors. Energ. Environ. Sci. 2013, 6, 2497-2504. (9) Fattakhova-Rohlfing, D.; Zaleska, A.; Bein, T. Three-dimensional Titanium Dioxide Nanomaterials. Chem. Rev. 2014, 114, 9487-9558. (10) Hwang, S. H.; Yun, J.; Jang, J. Multi-Shell Porous TiO2 Hollow Nanoparticles for Enhanced Light Harvesting in Dye-sensitized Solar Cells. Adv. Funct. Mater. 2014, 24, 7619-7626. (11) Zhang, J. S.; Zhang, M. W.; Yang, C.; Wang, X. C. Nanospherical Carbon Nitride Frameworks with Sharp Edges Accelerating Charge Collection and Separation at a Soft Photocatalytic Interface. Adv. Mater. 2014, 21, 4121-4126. (12) Hu, H.; Zhao, Z. B.; Wan, W. B.; Gogotsi, Y.; Qiu, J. Ultralight and Highly Compressible Graphene Aerogels. Adv. Mater. 2013, 25, 2219-2223. (13) Cong, H. P.; Ren, X. C.; Wang, P.; Yu, S. H. Macroscopic Multifunctional Graphene-Based Hydrogels and Aerogels by a Metal Ion Induced Self-Assembly Process. ACS Nano 2012, 6, 2693-2703. (14) Chen, Q.; Zhao, Y.; Huang, X. K.; Chen, N.; Qu, L. T. Three-dimensional graphitic carbon nitride functionalized graphene-based high-performance supercapacitors. J. Mater. Chem. A 2015, 3, 6761-6766. (15) Yang, Z.; Zhang, Y. J.; Schnepp, Z. Soft and Hard Templating of Graphitic Carbon Nitride. J. Mater. Chem. A. 2015, 3, 14081-14092. (16) Liu, W. J.; Cai, J. Y.; Li, Z. H. Self-Assembly of semiconductor nanoparticles/reduced graphene oxide composite aerogels for enhanced photocatalytic performance and facile recycling in aqueous photocatalysis. ACS Sustain. Chem. Eng. 2015, 3, 277-282. 26


Page 26 of 29

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Luo, W. B.; Chou, S. L.; Wang, J. Z.; Zhai, Y. C.; Liu, H. K. A Metal-Free, Free-Standing, Macroporous Graphene@g-C3N4 Composite Air Electrode for High-Energy Lithium Oxygen Batteries. Small 2015, 11, 2817-2824. (18) Mao, S.; Lu, G. H.; Chen, J. H. Three-dimensional Graphene-based Composites for Energy Applications. Nanoscale. 2015, 7, 6924-6943. (19) Niu, Z. Q.; Liu, L. L.; Zhang, L.; Shao, Q.; Zhou, W. Y.; Chen, X. D.; Xie, S. S. A Universal Strategy to Prepare Functional Porous Graphene Hybrid Architectures. Adv. Mater. 2014, 26, 3681-3687. (20) Qiu, B. C.; Xing, M. Y.; Zhang, J. L. Mesoporous TiO2 Nanocrystals Grown in Situ on Graphene Aerogels for High Photocatalysis and Lithium-Ion Batteries. J Am Chem Soc. 2014, 136, 5852-5855. (21) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen K.; Antonietti, M. A Metal-free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2008, 8, 76-80. (22)Wang, H. L.; Zhang, L. S.; Chen, Z. G.; Hu, J. Q.; Li, S. J.; Wang, Z. H.; Liu, J. S.; Wang, X. C. Semiconductor Heterojunction Photocatalysts: Design, Construction, and Photocatalytic Performances. Chem Soc Rev. 2014, 43, 5234-5244. (23) Ong, W. J.; Tan, L. L.; Chai, S. P.; Yong, S. T. Graphene Oxide as a Structure-directing Agent for the Two-dimensional Interface Engineering of Sandwich-like Graphene/gC3N4 Hybrid Nanostructures with Enhanced Visible-light Photoreduction of CO2 to Methane. Chem. Commun. 2014. (24) Gao, H. C.; Sun, Y. M.; Zhou, J. J.; Xu, R.; Duan, H. W. Mussel-inspired Synthesis of Polydopamine-functionalized Graphene Hydrogel as Reusable Adsorbents for Water Purification. ACS Appl. Mater. Interfaces 2013, 5, 425-432. (25) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339-1339. (26) Tong, Z. W.; Yang, D.; Xiao, T. X.; Tian, Y.; Jiang, Z. Y. Biomimetic Fabrication of g-C3N4/TiO2 Nanosheets with Enhanced Photocatalytic Activity toward Organic Pollutant Degradation. Chem. Eng. J. 2015, 260, 117-125. (27) Chen, C.; Yang, Q. H.; Yang, Y.; Lv, W.; Wen, Y.; Hou, P. X.; Wang, M.; Cheng, H. M. Self-Assembled Free-Standing Graphite Oxide Membrane. Adv. Mater. 2009, 21, 3007-3011. (28) Sano, T.; Tsutsui, S.; Koike, K.; Hirakawa, T.; Teramoto, Y.; Negishi, N.; Takeuchi, K. Activation of Graphitic Carbon Nitride (g-C3N4) by Alkaline Hydrothermal Treatment for Photocatalytic NO Oxidation in Gas Phase. J. Mater. Chem. A 2013, 1, 6489-6496. (29) Bai, X. J.; Zong, R. L.; Li, C. X.; Liu, D.; Liu, Y. F.; Zhu, Y. F. Enhancement of Visible Photocatalytic Activity via Ag@C3N4 Core–shell Plasmonic Composite. Appl. Catal. B-Environ. 2014, 147, 82-91. (30) Xu, J.; Wang, Y. J.; Zhu, Y. F. Nanoporous Graphitic Carbon Nitride with Enhanced Photocatalytic Performance. Langmuir. 2013, 29, 10566-10572. (31) Fu, J.; Chang, B. B.; Tian, Y. L.; Xi, F. N.; Dong, X. P. Novel C3N4–CdS Composite Photocatalysts with Organic–inorganic Heterojunctions: in situ Synthesis, 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Exceptional Activity, High Stability and Photocatalytic Mechanism. J. Mater. Chem. A. 2013, 1, 3083-3090. (32) Zhang, Z. Y.; Xiao, F.; Guo, Y. L.; Wang, S.; Liu, Y. Q. One-pot Self-assembled Three-dimensional TiO2-graphene Hydrogel with Improved Adsorption Capacities and Photocatalytic and Electrochemical Activities. ACS Appl. Mater. Interfaces 2013, 5, 2227-2233. (33) Li, Y. B.; Zhang, H. M.; Liu, P. R.; Wang, D.; Li, Y.; Zhao, H. J. Cross-linked g-C3N4/rGO Nanocomposites with Tunable Band Structure and Enhanced Visible Light Photocatalytic Activity. Small 2013, 9, 3336-3344. (34) Dong, G. H.; Zhang, L. Z. Porous Structure Dependent Photoreactivity of Graphitic Carbon Nitride under Visible Light. J. Mater. Chem. 2012, 22, 1160-1166. (35) Zhang, Q. F.; Myers, D.; Lan, J.; Jenekhe, S. A.; Cao, G. Applications of Light Scattering in Dye-sensitized Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 14982-14998. (36) Chen, Y. F.; Huang, W. X.; He, D. L.; Situ, Y.; Huang, H. Construction of Heterostructured g-C3N4/Ag/TiO2 Microspheres with Enhanced Photocatalysis Performance under Visible-Light Irradiation. ACS Appl. Mater. Interfaces 2014, 6, 14405-14414. (37) Cheng, N. Y.; Tian, J. Q.; Liu, Q.; Ge, C. J.; Qusti, A. H.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. P. Au-Nanoparticle-Loaded Graphitic Carbon Nitride Nanosheets: Green Photocatalytic Synthesis and Application toward the Degradation of Organic Pollutants. ACS Appl. Mater. Interfaces 2013, 5, 6815-6819. (38) Xu, Y. G.; Xu, H.; Wang, L.; Yan, J.; Li, H. M.; Song, Y. H.; Huang, L. Y.; Cai, G. B. The CNT Modified white C3N4 Composite Photocatalyst with Enhanced Visible-light Response Photoactivity. Dalton Trans. 2013, 42, 7604-7613. (39) Xu, M.; Han, L.; Dong, S. J. Facile Fabrication of Highly Efficient g-C3N4/Ag2O Heterostructured Photocatalysts with Enhanced Visible-light Photocatalytic Activity. ACS Appl. Mater. Interfaces 2013, 5, 12533-12540. (40) Zhang Z. Y.; Huang J. D.; Zhang M. Y.; Yuan Q.; Dong B. Ultrathin Hexagonal SnS2 Coupled with g-C3N4 Nanosheets as 2D/2D Heterojunction Photocatalyst toward High Photocatalytic Activity. Appl. Catal. B-Environ. 2015, 163, 298–305. (41) Zhou, H.; Guo, J. J.; Li, P.; Fan, T. X.; Zhang, D.; Ye, J. H. Leaf-architectured 3D Hierarchical Artificial Photosynthetic System of Perovskite Titanates Towards CO2 Photoreduction into Hydrocarbon Fuels. Sci. Rep. 2013, 3, 1667-1675. (42) Zhao J. C.; Wu T. X.; Wu K. Q.; Oikawa K.; Hidaka H.; Serpone N. Photoassisted Degradation of Dye Pollutants. 3. Degradation of the Cationic Dye Rhodamine B in Aqueous Anionic Surfactant/TiO2 Dispersions under Visible Light Irradiation: Evidence for the Need of Substrate Adsorption on TiO2 Particles. Environ. Sci. Technol. 1998, 32, 2394-2400. (43) Zhou, H.; Li, P.; Guo, J. J.; Yan, R. Y.; Fan, T. X.; Zhang, D.; Ye, J. H. Artificial Photosynthesis on Tree trunk Derived Alkaline Tantalates with Hierarchical Anatomy: towards CO2 Photo-fixation into CO and CH4. Nanoscale 2015, 7, 113-120. 28


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BRIEFS 3D porous CNGA photocatalyst was constructed by the co-assembly of two-dimensional g-C3N4 and GO nanosheets, which exhibits high catalytic efficiency in dye degradation and CO2 reduction. SYNOPSIS

a

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1.0 0.8

C/C0

0.6 GA g-C3N4

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2D hybrid CNGA-4 CNGA-3 CNGA-2 CNGA-1.5 CNGA-0.8

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hydrothermal co-assembly

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CO Evolution mmol g

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Three-Dimensional Porous Aerogel Constructed ... - ACS Publications

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