Facile Fabrication of Large-Aspect-Ratio g-C3N4 Nanosheets for

Feb 1, 2017 - (1) Since the seminal discovery of graphene, a host of two-dimensional (2D) nanomaterials have attracted much research interest over the...
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Facile Fabrication of Large-Aspect-Ratio g‑C3N4 Nanosheets for Enhanced Photocatalytic Hydrogen Evolution Li Jun Fang, Yu Hang Li, Peng Fei Liu, Dan Ping Wang, Hui Dan Zeng,* Xue Lu Wang,* and Hua Gui Yang Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China S Supporting Information *

ABSTRACT: Exfoliation of bulk graphitic carbon nitride (BCN) into two-dimensional (2D) nanosheets is one of the effective strategies to improve its photocatalytic performance. Compared with BCN, the 2D g-C3N4 nanosheets (CNNS) have larger specific surface areas and more reaction sites. With the etching assistance of anhydrous ethylenediamine, BCN can be successfully peeled off into 2D CNNS with a large lateral size of more than 15 μm which is much larger than that of other works. After appropriate etch by anhydrous ethylenediamine, the specific surface area of g-C3N4 expands from 4.7 to 31.1 m2 g−1 and the photocatalytic hydrogen evolution rate increases 7.4 times, from 4.8 to 35.3 μmol h−1. In contrast to other reported methods, the strategy to fabricate 2D CNNS in this work is convenient and it is the first time to report the fabrication of 2D CNNS with the assistance of alkaline reagent. KEYWORDS: Photocatalysis, Hydrogen evolution, Graphitic carbon nitride nanosheets, Liquid etching, Anhydrous ethylenediamine

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INTRODUCTION In addition to the composition and arrangement of ingredients, dimensionality is a crucial role in determining the fundamental properties of materials.1 Since the seminal discovery of graphene, a host of two-dimensional (2D) nanomaterials have attracted much research interest over the past decade, such as transition metal dichalcogenides, layered metal oxides, transition metal carbides and layered-double hydroxides.2,3 Compared with their bulk counterparts, 2D nanosheets possess some unique properties, for instance larger specific surface area, strong quantum confinement of electrons which endow them with numerous potential applications, such as optoelectronic/ electronic devices, catalysis, energy storage, conversion, sensor and biomedicine.4 As a promising organic semiconductor with 2D layer structure, graphitic carbon nitride (g-C 3 N 4) has been investigated in various fields such as heterogeneous catalysis, CO2 reduction, fuel cells, biomedical applications and sewage detoxification due to its proper band edges, nontoxic and earthabundant properties.5−8 But the performance of bulk g-C3N4 (BCN) is far from satisfying due to its intrinsic drawbacks, for instance low specific surface area, fast recombination of photogenerated electrons and holes.9−11 To address these issues, nanocrystallization design is one of the valid categories that can strikingly expand the specific surface area, improve electron−phonon interaction and prolong the lifetime of charge carriers.12 Meanwhile, recent theoretical investigations reveal that nanocrystallization design might lead to different electronic and optical properties for g-C3N4.13 Hence, it seems plausible to anticipate better photocatalytic performance of g-C3N4 © 2017 American Chemical Society

through the strategy of nanocrystallization. BCN with 2D characteristic structure is constituted with C−N layers with a layer distance of about 0.33 nm that are connected with each other through hydrogen bonds and weak van der Waals force.6,14 Analogous to graphene, 2D g-C3N4 nanosheets (CNNS) can be fabricated by peeling off the bulk counterpart. Until now, plenty of efforts have been carried out to prepare CNNS including thermal oxidation etching, liquid ultrasonic exfoliation or chemical methods with the assistance of strong acids and/or oxidants.12 However, immature operations and immanent shortcomings of these exfoliation technologies restrict their large-scale application. For instance, some of these processes are cumbersome and need the assistant liquid with proper surface energy that should match well with that of g-C3N4. Besides, long-time sonication or strong oxidation treatment would result in the damage of nanosheets and the small lateral sizes inevitably.12 Furthermore, some prepared 2D CNNS are instable and tend to aggregation again during applications.15,16 Therefore, it is still worthy and desirable to explore new strategies to fabricate 2D CNNS. In this work, we develop a novel exfoliation strategy to produce large-aspect-ratio 2D CNNS through utilizing anhydrous ethylenediamine (AED) as the assistant reagent which can greatly alleviate the above problems. By optimizing the etching treatment time, the thickness of the obtained 2D CNNS can reach about 3−4 nm and the lateral size is more Received: November 10, 2016 Revised: January 13, 2017 Published: February 1, 2017 2039

DOI: 10.1021/acssuschemeng.6b02721 ACS Sustainable Chem. Eng. 2017, 5, 2039−2043

Letter

ACS Sustainable Chemistry & Engineering than 15 μm. In comparison with that of BCN, the specific surface area of 2D CNNS can be improved about 7 times, from 4.7 to 31.1 m2 g−1. As expected, under the irradiation of visible light, the photocatalytic hydrogen (H2) evolution rate of gC3N4 increases from 4.8 to 35.3 μmol h−1. Notably, the preparation processing is simple and does not need long-time ultrasonic or especial gas assistance. To the best of our knowledge, it is the first time to prepare 2D CNNS with the assistance of alkaline reagent and we hope that this study may enrich the methods to prepare 2D CNNS and paves a new way to fabricate other 2D materials.

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RESULTS AND DISCUSSION As clearly illustrated in Figure 1a, 2D CNNS are prepared by drastically stirring BCN in AED in a screw-neck bottle (20 mL)

Figure 2. (a−c) TEM and HR-TEM images of CNNS. (d) Typical AFM image showing the lateral dimensions and thickness of CNNS.

may be caused by the gradual degraded of polymeric melon units (Figure S3d−f). As demonstrated with AFM (Figure 2d), the thickness of the prepared CNNS is around 3−4 nm. In view of the layer distance, there would be around 13 layers in each nanosheet. In addition, the lateral size of the sample is more than 15 μm which is larger than that of other work. X-ray diffraction (XRD) patterns were used to study the crystal structures of the photocatalysts. As shown in Figure 3a, 2D Figure 1. (a) Diagrammatic sketch for the exfoliation of CNNS. (b) Photographs of BCN and CNNS powders. (c) Status of 5 mg photocatalysts in 10 vol % TEOA aqueous solution after natural sedimentation for different days. Distinct tyndally was observed after 40 d. The left is BCN and the right is CNNS.

at room temperature for 24 h. Because of the same amine groups (−NH2) in both AED and g-C3N4, the assistant reagents can penetrate into the internal part of g-C3N4 easily. Different than the protonation of acidic reagent in previous work, AED with strong corrosivity cuts off the hydrogen bonds existing between the CN layers and then unties BCN into nanosheets, which is similar to the method of thermal oxidation etching.16−18 Because of the strong corrosivity, AED collapses the skeleton of g-C3N4 as the covalent bonds of C−N can also be snipped so the key point of this work is to control the stir time. As shown in Figure S1, the color of g-C3N4 becomes lighter gradually when extending the etching time. After stirring for 24 h, the color of g-C3N4 turns from yellow to off-white (Figure 1b). It is worth noting that the obtained 2D CNNS turn more stable in 10 vol % triethanolamine (TEOA)/water solution than BCN and distinct tyndally can still be observed in the solution after natural sedimentation for 40 days that may be due to the larger specific surface areas (Figure 1c and Figure S2). The morphologies of BCN and 2D CNNS were characterized through transmission electron microscopy (TEM) and atomic force microscopy (AFM). Compared with Figure S3, 2D CNNS have clear laminar structure like silk veil and are more transparent in Figure 2. In contrast to the smooth surface of BCN, the surface of 2D CNNS are curled and rough in order to minimize the surface energy.19 When the etching time is increased, the surface is covered with more plications which

Figure 3. (a) XRD patterns of BCN and CNNS. (b) FTIR spectra of BCN and CNNS. XPS spectra for (c) C 1s, (d) N 1s in BCN and CNNS.

CNNS have two clear peaks consistent with BCN implying that the primary crystal structure of g-C3N4 is largely retained after etching by AED. The two typical peaks at around 13.1° (001) and 27.3° (002) derive from the in-plane ordering of tri-striazine motifs and the periodic stacking of layers of conjugated aromatic segments, respectively.20,21 Compared CNNS with BCN, the reflection peak (002) shifts from 27.3° to 27.6° which mainly attributes to the decrease stacking distance between CNNS layers.22,23 Meanwhile, when extending etching time, the peak at 13.1° becomes less significant owing to the 2040

DOI: 10.1021/acssuschemeng.6b02721 ACS Sustainable Chem. Eng. 2017, 5, 2039−2043

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ACS Sustainable Chemistry & Engineering decreased planar size of the layers.24 Notably, CN-60 has a different XRD pattern indicating that the original crystal structure of g-C3N4 is decomposed after long hours etching procedure that is in agreement with the results of TEM (Figure S4). Fourier transform infrared (FTIR) spectroscopy reveals the chemical structure of BCN and 2D CNNS. As can be seen from Figure 3b, the two samples have the similar FTIR spectra which are consistent with the results of XRD. The broad peak in the region from 3000 to 3600 cm−1 is contributed by the NH stretching of amine groups or water. The set of peaks between around 1900 and 900 cm−1 attributes to the stretching modes of s-triazine derivatives. The sharp peak at around 810 cm−1 stems from the typical breathing mode heptazine ring system. Clearly, some peaks of 2D CNNS are sharper than that of BCN, which is due to the more ordered packing of tri-s-triazine motifs in the nanosheets.19 At 2180 cm−1, the vibration peak becomes more distinct which might descend from more H2O molecules or amino groups bonding on the surface of g-C3N4, implying that 2D CNNS have enlarged open-up surface.25 The chemical compositions and valence states of backbone elements in BCN and 2D CNNS were investigated by elemental analysis (EA) and X-ray photoelectron spectroscopy (XPS). As seen in Figure S5a, the three peaks assigned to C 1s, N 1s and O 1s signals are observed clearly. The generated O 1s signal might be due to the absorbed H2O or CO2 on the surface of the samples.19 As displayed in Figure 3c,d, 2D CNNS have the similar C 1s and N 1s spectra with BCN revealing that AED etching treatment has little influence on the CN aromatic framework within 24 h. However, the CN-60 has distinctly different C 1s and N 1s signals from BCN illustrating that the structure of g-C3N4 had be destroyed after much longer time etching treatment which is consistent with the results of XRD patterns. Three peaks centered at binding energies of 284.6, 287.13 and 287.83 eV are obtained after deconvolution of the C 1s spectrum. The three peaks are attributed to the graphitic carbon (sp2 CC bonds) and the sp2-hybridized carbon covalent binding with N species ((C)3N and NCN), respectively.26 For the N 1s spectrum, the small peak sited at 403.54 eV derives from the positive charge localization in heterocycles. The big N 1s spectrum from 396.4 to 402.7 eV can be resolved into three small peaks, located at binding energy of 398.4, 400.0 and 400.9 eV attributing to sp2hybridized nitrogen in triazine ring (CNC), tertiary nitrogen N(C)3 groups and amino functions caring hydrogen (CNH), respectively.27 As presented in Table S1, the ratio of C to N sustains at around 0.64 according to the results of EA meaning that the backbones of g-C3N4 are maintained well after nanocrystallization.28 While, the percentages of H increased from 2.19% to 2.77% which may be due to the adsorbed H2O on the enlarged surface area that is in agreement with the results of FTIR. The optical properties and the band gap energy of BCN and 2D CNNS were studied by optical absorption spectra and fluorescence emission spectra. With respect to the intrinsic absorption edge, the evident hypsochromic shift from 450 to 445 nm is observed in Figure 4a. In addition, other samples also show obvious blue shift phenomenon (Figure S6). Meanwhile, the derived bandgaps determined from the (ahυ)1/2 versus photon-energy plots are 2.7 and 2.76 eV for BCN and 2D CNNS, respectively (inset in Figure 4a). The bandgap is larger by 0.06 eV, which may be caused by the widely acknowledged quantum confinement effect by changing the conduction and

Figure 4. (a) UV−vis diffuse reflectance spectra of BCN and CNNS. (b) PL spectra under 400 nm excitation of BCN and CNNS. (c) Mott−Schottky plots collected at various frequencies in dark of BCN and CNNS. (d) Bandgap structures of BCN and CNNS.

valence band edges in opposite directions.15 The photoluminescence (PL) spectrum is an effective testing method to study the separation and recombination rate of photocarriers of photocatalysts. As we can see in Figure 4b, under the irradiation of 400 nm, the emission peak shifts from 450 to 437.5 nm. Besides, the emission peaks of other samples also turn shorter than that of BCN (Figure S7). The result of PL is consistent with the results of UV−vis owing to the same reason.29 Mott−Schottky plots were obtained to investigate the valence and conduction band edge of the BCN and 2D CNNS (Figure 4c).30,31 Apparently, the Mott−Schottky plots of the two catalysts have the positive slope of the linear plots under various frequencies implying that both of the two samples are typical n-type semiconductors.32 Combined with the results of UV−vis, the calculated conduction band for 2D CNNS is about −1.22 eV versus Ag/AgCl which thermodynamically enables photocatalytic water reduction (H+/H2: −0.59 V vs Ag/AgCl) although which is more positive by 0.26 eV than that for BCN (Figure 4d). The electrochemical impedance spectroscopies (EIS) were recorded in a 0.2 M Na2SO4 aqueous solution with 10 vol % TEOA under an applied bias of −1.0 V vs Ag/AgCl in light. The equivalent Randle circuit is presented in the inset of Figure 5a. Rs and CPE represent for the electrolyte resistance and the constant phase element, respectively. Rct stands for the charge transfer resistance at the solid/electrolyte interface. It is obvious that the diameter of the semicircular Nyquist curve of 2D CNNS is smaller than that of BCN implying that the resistance at the solid/electrolyte interface is significantly decreased and the charge from the conduction band of 2D CNNS could transfer to the protons more smoothly.33 The photocatalytic performance of the prepared samples were analyzed through measuring the amount of H2 generated from water reduction. 30 mg samples with 3.0 wt % platinum (Pt) as cocatalyst were dispersed in the system of 10 vol % TEOA/water mixtures under the irradiation of visible light (λ > 420 nm). The photocatalytic results of the control experiments are displayed in Figure S8. It is apparent that 24 h is the optimal time perspective to refine the performance of g-C3N4. The photocatalytic H2 evolution rate can be improved from 4.8 to 19.9 μmol h−1. Compared with the pristine g-C3N4 benchmark, the photocatalytic H2 evolution rate is improved by 7.4 times 2041

DOI: 10.1021/acssuschemeng.6b02721 ACS Sustainable Chem. Eng. 2017, 5, 2039−2043

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ACS Sustainable Chemistry & Engineering

CNNS with larger specific surface areas show higher photocatalytic H2 evolution performance than BCN under visible light irradiation. The superior performance can be attributed to the unique 2D structure. The prepared 2D CNNS with large lateral size of more than 15 μm can be applied in other fields, such as sewage treatment, photoelectrochemistry, etc. In addition, the processing procedure is simple and coarse. For the first time, alkaline reagent is applied as the assistant reagent to produce 2D CNNS and we hope that this method can be extended to fabricate other 2D materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02721. Details of experiments, more photographs, TEM images, XPS, UV−vis, PL spectra and photocatalytic H 2 evolution performance of photocatalysts and the results of EA (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*X. L. Wang. E-mail: [email protected]. Tel/Fax: +8621-64252127. *H. D. Zeng. E-mail: [email protected]. Tel/Fax: +86-2164252127.

Figure 5. (a) EIS spectra of BCN and CNNS. Inset: An equivalent circuit for BCN and CNNS. (b) Comparison the visible-light photocatalytic H2 evolution of BCN and CNNS. (c) Wavelengthdependent H2 evolution of CNNS. The inset is photocatalytic durability test for CNNS under visible light irradiation. All the experiments were conducted in the presence of 30 mg photocatalysts, 3 wt % Pt, 10 vol % TEOA aqueous solution at room temperature under visible light irradiation (λ > 420 nm) (inset of panel c).

ORCID

Xue Lu Wang: 0000-0002-4149-0187 Author Contributions

All authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest.

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after nanocrystallization, from 4.8 to 35.3 μmol h−1 as shown in Figure 5b. In addition, the photocatalytic reduction of water was also tested under the irradiation of different wavelength. As depicted in Figure 5c, the H2 evolution rates have the similar trend with the optical absorption spectrum of 2D CNNS indicating that the photoinduced electrons in g-C3N4 are responsible for the H2 evolution. Obviously, 2D CNNS can even response to wavelength as long as 550 nm. At 420 nm, the calculated apparent quantum efficiency of 2D CNNS is about 2.77%. No clear decay in the H2 evolution rate is observed during photocatalytic test for 35 h indicating that 2D CNNS are stable, which is consistent with the favorable dispersibility of the sample displayed in Figure 1c. The enhancement of the photocatalytic activity of 2D CNNS is attributed to the synergistic effects of larger specific areas, modulation of the conductive band and decrease of resistance at the solid/electrolyte interface. 2D CNNS with larger special surface areas and more active edges can offer more catalytic sites for H2 evolution reaction. In addition, the unique 2D structure has advantages of diffusion of mass and the transfer of photogenerated carriers. Also, the dispersibility of 2D CNNS in the reaction reagent is improved which is conducive to photocatalyic reaction. Furthermore, charge carriers can react with proton at lower resistance.

ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (21573068 and 21603073), Program of Shanghai Subject Chief Scientist (15XD1501300), Fundamental Research Funds for the Central Universities (WD1313009 and WD1514303) and China Postdoctoral Science Foundation Funded Project (2016M591615).

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CONCLUSIONS In summary, BCN can be successfully exfoliated into 2D nanosheets in the etching reagent of AED. The obtained 2D 2042

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DOI: 10.1021/acssuschemeng.6b02721 ACS Sustainable Chem. Eng. 2017, 5, 2039−2043