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Fabrication of large surface area nitrogen vacancy modified graphitic carbon nitride with improved visible-light photocatalytic performance
Diamond & Related Materials 91 (2019) 230–236
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Fabrication of large surface area nitrogen vacancy modified graphitic carbon nitride with improved visible-light photocatalytic performance
T
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Lei Lianga, Lei Shia, , Fangxiao Wangb a b
College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun 113001, China College of Chemistry, Chemical Engineering and Material Science, Shandong Normal University, Jinan 250014, China
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 Nitrogen vacancy Large surface area Photocatalytic performance Photocatalysis
In this work, nitrogen vacancy modified graphitic carbon nitride (g-C3N4) with large surface area was synthesized and analyzed by a series of instruments, including XRD, FTIR, XPS, EPR SEM, TEM, DRS and PL, etc. and the photocatalytic H2-evolution activity was investigated. The results indicated that the as-synthesized g-C3N4 with nitrogen vacancy exhibited stronger visible light response capability, enlarged specific surface area and notably separated rate of photoinduced charge carriers, which caused the as-synthesized photocatalyst possessing the higher hydrogen evolution rate (5250 μmol h−1 g−1) and excellent recycle stability. Evidently, this work could provide a new insight for preparing highly efficient photocatalyst.
1. Introduction In the past a few years, graphitic carbon nitride (g-C3N4) with some merits, including desirable visible-light responded activity, high stability and low-cost, has exhibited excellent performance in the fields of solar energy conversion, such as H2 and O2 production by splitting water [1–3], various kinds contaminant elimination [4–7], CO2 reduction to fuel [8], H2O2 generation [9], etc. Nevertheless, its intrinsic deficiency, for example inadequacy visible-light absorption, small specific surface area and poor photo-charge separated rate, restricted its visible-light photocatalytic performance to some extent. Therefore, a lot of effective methods were developed, for instance nonmetal elements doping [10,11], the introduction of porous structure [12–14], loading other semiconductors [15–17], specific morphology construction [18,19] and melamine based supramolecular thermopolymerization [20], the enhancement of photocatalytic performance was obtained. Although numerous accomplishment have been gained, it is still required to produce photocatalyst with high photocatalytic activity through enhancing light absorption capacity, enlarging surface area and accelerating photo-charge separated rate. In previous reports, for bulk g-C3N4, the introduction of nitrogen deficiency can effectively promote its photocatalytic property. Niu et al. have successfully prepared nitrogen-deficient modified g-C3N4 through high temperature (600 °C) thermal polymerization of dicyandiamide. Although this method was simple and as-prepared nitrogen-deficient modified g-C3N4 exhibited improved photocatalytic activity for
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generating %OH, photodegrading RhB and producing H2 by water splitting, its surface area was still low (17 m2 g−1) [21]. Hong et al. synthesized nitrogen-deficient g-C3N4 by oxidizing as-prepared g-C3N4 using weak oxidant ((NH4)2S2O3) in hydrothermal process [22]. Compared with pristine g-C3N4, the nitrogen-deficient g-C3N4 presented a broadened visible-light absorption spectrum and an improved charge separation efficiency, however, this method was time-consuming and surface area of resultant nitrogen-deficient g-C3N4 was about 10.4 m2 g−1. Low surface area made above two methods dissatisfactory. Therefore, it is necessary to develop a facile strategy to prepare nitrogen vacancy modified g-C3N4 with large specific surface area. Herein, large surface area nitrogen vacancy modified g-C3N4 was prepared through a simple route of two times heat treatment. It could be found that, after two times heat treatment, the surface area of the assynthesized g-C3N4 reach to 84 m2 g−1, and the remarkably improved visible light harvesting capability was obtained due to the introduction of nitrogen vacancy. In addition, the separation rate of photoinduced charge carriers was also promoted. These useful factors facilitated assynthesized g-C3N4 exhibiting greatly improved visible-light photocatalytic activity for H2 production and degradation of various dyes. 2. Experimental section 2.1. Sample preparation The prepared process of g-C3N4 was described as follows: 10 g urea
Corresponding author. E-mail address: [email protected] (L. Shi).
https://doi.org/10.1016/j.diamond.2018.11.025 Received 3 October 2018; Received in revised form 12 November 2018; Accepted 27 November 2018 Available online 28 November 2018 0925-9635/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. The XRD patterns of (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
Fig. 2. The FTIR spectra of as-made (a) g-C3N4 (550) and (b) g-C3N4 (550–650). Fig. 3. XPS spectra of (A) C 1s and (B) N 1s of (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
Table 1 Element content and BET surface area of g-C3N4 (550) and g-C3N4 (550–650). Samples
C/wt%
N/wt%
g-C3N4 (550) g-C3N4 (550–650)
34.12 34.53
58.99 57.65
C/N atomic ratio 0.67 0.70
BET surface area (m2 g−1) 50 84
was placed in a ceramic crucible with a cover, and heated to 550 °C for 2 h. The resultant yellow g-C3N4 sample was grinded, designated to gC3N4 (550). The prepared process of nitrogen vacancy modified g-C3N4 was described in detail: as-prepared g-C3N4 (550) was stayed in a ceramic ark with a cover and heat treated at 650 °C for 2 h under N2 atmosphere, as-obtained sample was collected and named g-C3N4 (550–650).
2.2. Characterizations The X-ray diffraction (XRD) measurement of g-C3N4 (550) and gC3N4 (550–650) were carried out on X-ray powder diffractometer (Bruker D8 Advance). Fourier transform infrared spectroscopy (FTIR) was collected on an Agilent Cary 5000 spectrum 100. Elemental analyses (EA) for the carbon and nitrogen contents were performed on a PerkinElmer series II CHNS/O analyzer 2400. The X-ray photoelectron spectroscopy (XPS) was detected on Thermo Fisher Scientific Escalab
Fig. 4. The EPR spectra of (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
250. Electron paramagnetic resonance (EPR) was measured on a Bruker EMX-10/12 EPR spectrometer. The morphology and microtopography of the products (g-C3N4 (550) and g-C3N4 (550–650)) were examined by scanning electron microscope (SEM, HITACHI SU8010) and 231
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B
A
5 m C
200 nm
D
5 m
200 nm
Fig. 5. The (A) SEM images and (B) TEM images of g-C3N4 (550), the (C) SEM images and (D) TEM images of g-C3N4 (550–650).
D -1
Potential (eV)
-0.53 0
-0.42 -0.07
CB
2.07
VB
H+/H2
1 2
2.15 Eg=2.68 eV
a
Eg=2.49 eV
b
Fig. 6. (A) UV–vis diffuse reflectance spectra, (B) a plot of (αhν)1/2 vs. photon energy (hν), (C) The VB XPS spectra and (D) electronic band structures of corresponding samples ((a) g-C3N4 (550) and (b) g-C3N4 (550–650)).
UV–vis spectrometer at room temperature. The photoluminescence spectra (PL) of g-C3N4 (550) and g-C3N4 (550–650) were measured by an Agilent Cary Eclipse spectrometer with an excitation wavelength of 325 nm. Transient photocurrent properties were detected using a Chenhua
transmission electron microscopy (TEM, JEM-2100F), respectively. BET surface area were collected at 77 K using a Quantachrome AutosorbIQ2-MP analyzer, samples were outgassed at 150 °C for 12 h prior to measurements. The UV–vis diffuse reflectance spectra (DRS) of g-C3N4 (550) and g-C3N4 (550–650) were recorded by an Agilent Cary 5000 232
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Fig. 8. (A) Photocatalytic H2 evolution rate of (a) g-C3N4 (550) and (b) g-C3N4 (550–650) (inset is HER) and (B) recycle of H2 produced of g-C3N4 (550–650).
Fig. 7. (A) The photoluminescence spectra and (B) transient photocurrent property of (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
27.5° to 27.7°, meaning that the interlayer stacking distance become narrow. This phenomenon manifested that the interlayer stacking order of as-prepared g-C3N4 (550–650) was strengthened, and related crystal structure could be more stable [23]. Whereafter, Fig. 2 demonstrates the FTIR spectra of as-made g-C3N4 (550) and g-C3N4 (550–650). The similar FTIR spectrogram could be observed for two simples, which revealed that the chemical structure of two samples was the same. The stretching vibration modes of triazine was represented at 809 cm−1, and some peaks in the range of 1100–1700 cm−1, including 1240, 1318, 1406, 1462, 1572 and 1639 cm−1, attributed to typical stretching modes of g-C3N4 heterocycles [24,25]. To further measure the elemental composition and chemical structure of g-C3N4 (550) and g-C3N4 (550–650), elemental analysis (EA) and X-ray photoelectron spectroscopy (XPS) were performed, respectively. Firstly, as shown in Table 1, the C/N atomic ratio for g-C3N4 (550) was 0.67, while the C/N atomic ratio for g-C3N4 (550–650) increased to 0.70, which suggested that nitrogen vacancy might be introduced in the g-C3N4 (550–650) through two time heat treatment. Moreover, the results of XPS were shown in Fig. 3. It could be seen that there were two peaks in the C 1s spectrum (Fig. 3A). The one at 284.6 eV was assigned to C atoms in the sp2 aromatic bonds, the other one at 288.0 eV was attributed to sp2-hybridized NeC]N [26]. The N 1s spectrum was shown in Fig. 3B, the peak could be deconvoluted into 398.4, 399.0, 400.5 and 404.4 eV, which could originate from the sp2 bonded N (N2eC), bridging nitrogen atoms in Ne(C)3, nitrogen hydrogen bond (CeNH or CeNH2) and the charging effects, respectively [27,28]. In order to further confirm nitrogen vacancy formed in g-C3N4, the
CHI760 electrochemical station (Shanghai, China) in a three-electrode quartz cells. Pt electrode, the saturated calomel electrode and the sample films (g-C3N4 (550) and g-C3N4 (550–650)) coated on ITO glasses were the counter electrode, the reference electrode and the working electrode, respectively. 0.1 M Na2SO4 was used as the electrolyte. The 300 W Xe lamp with 400 nm filters supplied light source. 2.3. Photocatalytic test The visible-light-induced photocatalytic evolution H2 from splitting water by g-C3N4 (550) or g-C3N4 (550–650) were carried out in a Pyrex top-irradiation reaction vessel connected to a closed quartz glass gascirculation system. 0.030 g sample was added in 50 mL aqueous solution containing triethanolamine (5 mL) and distill water (45 mL). Pt (3 wt%) was deposited on sample surface by the in-situ photodeposition method using H2PtCl6 as the precursor. The reactant solution was evacuated by vacuum pump for 0.5 h to remove air before illumination. In this system, 300 W Xe lamp with a 400 nm cutoff filter supplied visible light source. The product was analyzed by gas chromatography (GC7920, CEAULight, China), Ar was the carrier gas. 3. Results and discussion Fig. 1 presents the XRD patterns of as-made g-C3N4 (550) and gC3N4 (550–650). There were two peaks at 13.0° and 27.5°, corresponding to (100) and (002) planes of g-C3N4. Besides, the (002) diffraction peak width of g-C3N4 (550–650) become narrower than that of g-C3N4 (550), and the related peak position also slightly shifted from 233
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D
C
5 nm
100 nm
Fig. 9. The (A) XRD patterns, (B) FTIR spectra and (C–D) TEM images of used g-C3N4 (550–650) after three time reactions for splitting water to produce H2.
Fig. 10. The photodegradation curves of various dyes over (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
electron paramagnetic resonance (EPR) spectroscopy was measured, which could supply fingerprint evidences for probing the surface vacancies in semiconductor photocatalysts. As shown in Fig. 4, two samples displayed a single Lorentzian line in the magnetic field from 3490 to 3530 G, which was associated with the unpaired electrons of sp2-carbon atoms within π-conjugated aromatic rings. Compared to gC3N4 (550) with low EPR signal, g-C3N4 (550–650) showed the higher intensity of EPR signal, revealing nitrogen vacancy generated in g-C3N4 [29]. Therefore, combined with above results, the generation of nitrogen vacancy in g-C3N4 has been confirmed. The morphologies and microtopographies of resultant g-C3N4 (550) and g-C3N4 (550–650) were observed by SEM and TEM. As shown in Fig. 5A, g-C3N4 (550) was composed of irregularly curved layers, and the TEM image (Fig. 5B) further confirmed this result. For g-C3N4
(550–650), it was also formed by curved layers, but the average size of the layers was clearly declined due to in the introduction of a great number of cracks, and its TEM image (Fig. 5D) indicated that the thickness of the nanosheets decreased and some pores could be observed. Above results indicated that the large layer structure of g-C3N4 (550) has changed to smaller nanosheets of g-C3N4 (550–650) with porous structure after two times heat treatment. As a photocatalyst, specific surface area played a crucial role in its performance, the larger surface area could provide the more active sites, so that the improved photocatalytic activity might be expected [30]. Therefore, the BET surface area of g-C3N4 (550) and g-C3N4 (550–650) were detected and demonstrated in Table 1. Evidently, gC3N4 (550–650) exhibited larger specific surface area of 84 m2 g−1 than that of g-C3N4 (550) (50 m2 g−1). This result revealed that two times 234
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Finally, the photocatalytic activity of g-C3N4 (550) and g-C3N4 (550–650) was examined to the degradation of various organic dyes: Rhodamine B (RhB), Methylene Blue (MB), Methyl Orange (MO), with a dye concentration of 5 ppm and 30 mg of catalyst. The experimental process was similar with previous reports [33–35]. Fig. 10 describes the photodegradation curves of these dyes. Under the same reaction conditions, compared with g-C3N4 (550), g-C3N4 (550–650) could exhibited better photocatalytic performance, which further confirmed that g-C3N4 (550–650) was an effective photocatalyst.
heat treatment could enlarge resultant g-C3N4 specific surface area, conducing to offer more reaction active sites to boost the photocatalytic activity. Furthermore, the light-harvesting capability also affected photocatalytic performance of photocatalyst. Hence, the DRS of g-C3N4 (550) and g-C3N4 (550–650) were measured to investigate their optical property. As illustrated in Fig. 6A, compared with g-C3N4 (550), asmade g-C3N4 (550–650) exhibited clearly enhanced light responded property ranging from 450 to 800 nm, which might ascribe to the introduction of nitrogen vacancy [21]. And through the spectrum in Fig. 6B, their band gap energies were calculated using the Kubelka–Munk method. In detail, the band gap energy of g-C3N4 (550) was 2.68 eV, and g-C3N4 (550–650) exhibited two energy levels (2.49 and 2.14 eV), the former was intrinsic band gap energy, the latter might attribute to nitrogen vacancy, the similar phenomenon was reported in previous literature [31]. As we have known, the enhanced light-harvesting activity could benefit for improving the photocatalytic activity. Whereafter, the valence band (VB) XPS spectra of g-C3N4 (550) and g-C3N4 (550–650) were detected (Fig. 6C), their positions of the VB edge maxima were respectively estimated to be 2.15 and 2.07 eV. Combined with the results of VB XPS and band gap energy, the conduction band (CB) positions of g-C3N4 (550) and g-C3N4 (550–650) were calculated and shown in Fig. 6D. Obviously, for g-C3N4 (550–650), due to introduction of nitrogen vacancy, its potential of CB was negative shift to form the new conduction band, which would be as medium energy levels to accept the photogenerated electrons from the VB of g-C3N4 (550–650), resulting in more photons to be absorbed [32]. Finally, the separation, transfer and recombination rate of photoinduced charge carries also influence on the performance of photocatalyst. Therefore, photoluminescence spectrum (PL) of g-C3N4 (550) and g-C3N4 (550–650) were tested. As shown in Fig. 7A, g-C3N4 (550–650) displayed distinctly lower the PL intensity than g-C3N4 (550), meaning that the energy-wasteful charge recombination was mainly confined. The enhanced photocatalytic activity could be anticipated. Moreover, the emission peak of g-C3N4 (550–650) was redshift in comparison with g-C3N4 (550), agreeing with DRS results. To further compare the photoinduced charge separation rate of g-C3N4 (550) and g-C3N4 (550–650), photocurrent measurements were tested. As can be shown in Fig. 7B, compared with g-C3N4 (550), g-C3N4 (550–650) could exhibit the larger photocurrent density, implying higher separation rate of photoinduced electron-hole pairs. The photocatalytic performance for producing H2 of g-C3N4 (550) and g-C3N4 (550–650) was evaluated under visible light. Fig. 8A displays that H2 was persistently produced with irradiation time prolonging. Comparing with g-C3N4 (550), g-C3N4 (550–650) exhibited higher H2 evolved rate (HER). And inset indicates that the HER of gC3N4 (550) was 1925 μmol h−1 g−1, while that of g-C3N4 (550–650) was 5250 μmol h−1 g−1. Clearly, the rate of producing H2 over g-C3N4 (550–650) was 2.72 times than that of g-C3N4 (550). This result meant that, g-C3N4 (550–650) due to the introduction of N vacancy, larger surface area, stronger visible-light absorption and high efficient separated rate of photogenerated electron-hole pairs, could exhibit better visible-light photocatalytic H2 evolution. Moreover, the stability of photocatalyst is also a significant target to estimate its photocatalytic property. Hence, the recycling experiment for producing H2 over g-C3N4 (550–650) was measured under the same conditions. As demonstrated in Fig. 8B, the g-C3N4 (550–650) showed steady H2 produce rate for 12 h (three runs in total), implying outstanding photocatalytic stability. Meanwhile, for supplying more evidences to prove the stability of sample, the chemical structure and morphology of the used sample were further analyzed by XRD, FTIR and TEM. Data indicated that there was no clear difference in the XRD pattern (Fig. 9A), FTIR spectra (Fig. 9B), and morphology (Fig. 9C–D) of the sample compared with fresh sample, further revealing that the catalyst was very stable in process of photocatalytic H2 evolution.
4. Conclusions In summary, nitrogen vacancy modified large surface area g-C3N4 has been successfully prepared. The specific surface area of as-prepared photocatalyst reach to 84 m2 g−1, visible light absorption could extend to 800 nm, and photoinduced charge separation efficiency was also improved, these positive factors resulted in as-prepared photocatalyst exhibiting the significantly enhanced photocatalytic H2 evolution performance (5250 μmol h−1 g−1) and degraded activity for various dyes. In addition, as-prepared catalyst showed well stability of catalytic performance and chemical structure. We believe that this work will open a new window for exploiting markedly efficient metal-free photocatalysts in diverse applications. Acknowledgements We sincerely acknowledge the financial supported by talent scientific research fund of LSHU (No. 2016XJJ-080) and basic research projects of Liaoning Provincial Education Department (L2017LQN004). References [1] W.J. Ong, L.L. Tan, Y.H. Ng, S.T. Yong, S.P. Chai, Graphitic carbon nitride (g-C3N4)based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev. 116 (2016) 7159–7329. [2] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domenet, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80. [3] Y. Zhou, L. Zhang, W. Huang, Q. Kong, X. Fan, M. Wang, J. Shi, N-doped graphitic carbon-incorporated g-C3N4 for remarkably enhanced photocatalytic H2 evolution under visible light, Carbon 99 (2016) 111–117. [4] Y. Zhou, L. Zhang, J. Liu, X. Fan, B. Wang, M. Wang, W. Ren, J. Wang, M. Li, J. Shi, Brand new P-doped g-C3N4: enhanced photocatalytic activity for H2 evolution and Rhodamine B degradation under visible light, J. Mater. Chem. A 3 (2015) 3862–3867. [5] M. Ji, J. Di, Y. Ge, J. Xia, H. Li, 2D-2D stacking of graphene-like g-C3N4/ultrathin Bi4O5Br2 with matched energy band structure towards antibiotic removal, Appl. Surf. Sci. 413 (2017) 372–380. [6] Z. Wang, W. Guan, Y. Sun, F. Dong, Y. Zhou, W.K. Ho, Water-assisted production of honeycomb-like g-C3N4 with ultralong carrier lifetime and outstanding photocatalytic activity, Nanoscale 7 (2015) 2471–2479. [7] D.S. Wang, H. Sun, Q. Luo, X. Yang, R. Yin, An efficient visible-light photocatalyst prepared from g-C3N4 and polyvinyl chloride, Appl. Catal. B Environ. 156–157 (2014) 323–330. [8] J. Lin, Z. Pan, X. Wang, Photochemical reduction of CO2 by graphitic carbon nitride polymers, ACS Sustain. Chem. Eng. 2 (2014) 353–358. [9] Z. Zhu, H. Pan, M. Murugananthan, J. Gong, Y. Zhang, Visible light-driven photocatalytically active g-C3N4 material for enhanced generation of H2O2, Appl. Catal. B Environ. 232 (2018) 19–25. [10] S. Guo, Y. Tang, Y. Xie, C. Tian, Q. Feng, W. Zhou, B. Jiang, P-doped tubular g-C3N4 with surface carbon defects: universal synthesis and enhanced visible-light photocatalytic hydrogen production, Appl. Catal. B Environ. 218 (2017) 664–671. [11] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation of rhodamine B and methyl orange over boron-doped g-C3N4 under visible light irradiation, Langmuir 26 (2010) 3894–3901. [12] L. Shi, L. Liang, F. Wang, M. Liu, K. Chen, K. Sun, N. Zhang, J. Sun, Higher yield urea-derived polymeric graphitic carbon nitride with mesoporous structure and superior visible-light-responsive activity, ACS Sustain. Chem. Eng. 3 (2015) 3412–3419. [13] H. Yan, Soft-templating synthesis of mesoporous graphitic carbon nitride with enhanced photocatalytic H2 evolution under visible light, Chem. Commun. 48 (2012) 3430–3432. [14] S.S. Park, S.W. Chu, C.F. Xue, D.Y. Zhao, C.S. Ha, Facile synthesis of mesoporous carbon nitrides using the incipient wetness method and the application as hydrogen adsorbent, J. Mater. Chem. 21 (2011) 10801–10807. [15] L. Shi, L. Liang, J. Ma, F.X. Wang, J.M. Sun, Enhanced photocatalytic activity over
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Fabrication of large surface area nitrogen vacancy modified graphitic carbon nitride with improved visible-light photocatalytic performance
T
⁎
Lei Lianga, Lei Shia, , Fangxiao Wangb a b
College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun 113001, China College of Chemistry, Chemical Engineering and Material Science, Shandong Normal University, Jinan 250014, China
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 Nitrogen vacancy Large surface area Photocatalytic performance Photocatalysis
In this work, nitrogen vacancy modified graphitic carbon nitride (g-C3N4) with large surface area was synthesized and analyzed by a series of instruments, including XRD, FTIR, XPS, EPR SEM, TEM, DRS and PL, etc. and the photocatalytic H2-evolution activity was investigated. The results indicated that the as-synthesized g-C3N4 with nitrogen vacancy exhibited stronger visible light response capability, enlarged specific surface area and notably separated rate of photoinduced charge carriers, which caused the as-synthesized photocatalyst possessing the higher hydrogen evolution rate (5250 μmol h−1 g−1) and excellent recycle stability. Evidently, this work could provide a new insight for preparing highly efficient photocatalyst.
1. Introduction In the past a few years, graphitic carbon nitride (g-C3N4) with some merits, including desirable visible-light responded activity, high stability and low-cost, has exhibited excellent performance in the fields of solar energy conversion, such as H2 and O2 production by splitting water [1–3], various kinds contaminant elimination [4–7], CO2 reduction to fuel [8], H2O2 generation [9], etc. Nevertheless, its intrinsic deficiency, for example inadequacy visible-light absorption, small specific surface area and poor photo-charge separated rate, restricted its visible-light photocatalytic performance to some extent. Therefore, a lot of effective methods were developed, for instance nonmetal elements doping [10,11], the introduction of porous structure [12–14], loading other semiconductors [15–17], specific morphology construction [18,19] and melamine based supramolecular thermopolymerization [20], the enhancement of photocatalytic performance was obtained. Although numerous accomplishment have been gained, it is still required to produce photocatalyst with high photocatalytic activity through enhancing light absorption capacity, enlarging surface area and accelerating photo-charge separated rate. In previous reports, for bulk g-C3N4, the introduction of nitrogen deficiency can effectively promote its photocatalytic property. Niu et al. have successfully prepared nitrogen-deficient modified g-C3N4 through high temperature (600 °C) thermal polymerization of dicyandiamide. Although this method was simple and as-prepared nitrogen-deficient modified g-C3N4 exhibited improved photocatalytic activity for
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generating %OH, photodegrading RhB and producing H2 by water splitting, its surface area was still low (17 m2 g−1) [21]. Hong et al. synthesized nitrogen-deficient g-C3N4 by oxidizing as-prepared g-C3N4 using weak oxidant ((NH4)2S2O3) in hydrothermal process [22]. Compared with pristine g-C3N4, the nitrogen-deficient g-C3N4 presented a broadened visible-light absorption spectrum and an improved charge separation efficiency, however, this method was time-consuming and surface area of resultant nitrogen-deficient g-C3N4 was about 10.4 m2 g−1. Low surface area made above two methods dissatisfactory. Therefore, it is necessary to develop a facile strategy to prepare nitrogen vacancy modified g-C3N4 with large specific surface area. Herein, large surface area nitrogen vacancy modified g-C3N4 was prepared through a simple route of two times heat treatment. It could be found that, after two times heat treatment, the surface area of the assynthesized g-C3N4 reach to 84 m2 g−1, and the remarkably improved visible light harvesting capability was obtained due to the introduction of nitrogen vacancy. In addition, the separation rate of photoinduced charge carriers was also promoted. These useful factors facilitated assynthesized g-C3N4 exhibiting greatly improved visible-light photocatalytic activity for H2 production and degradation of various dyes. 2. Experimental section 2.1. Sample preparation The prepared process of g-C3N4 was described as follows: 10 g urea
Corresponding author. E-mail address: [email protected] (L. Shi).
https://doi.org/10.1016/j.diamond.2018.11.025 Received 3 October 2018; Received in revised form 12 November 2018; Accepted 27 November 2018 Available online 28 November 2018 0925-9635/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. The XRD patterns of (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
Fig. 2. The FTIR spectra of as-made (a) g-C3N4 (550) and (b) g-C3N4 (550–650). Fig. 3. XPS spectra of (A) C 1s and (B) N 1s of (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
Table 1 Element content and BET surface area of g-C3N4 (550) and g-C3N4 (550–650). Samples
C/wt%
N/wt%
g-C3N4 (550) g-C3N4 (550–650)
34.12 34.53
58.99 57.65
C/N atomic ratio 0.67 0.70
BET surface area (m2 g−1) 50 84
was placed in a ceramic crucible with a cover, and heated to 550 °C for 2 h. The resultant yellow g-C3N4 sample was grinded, designated to gC3N4 (550). The prepared process of nitrogen vacancy modified g-C3N4 was described in detail: as-prepared g-C3N4 (550) was stayed in a ceramic ark with a cover and heat treated at 650 °C for 2 h under N2 atmosphere, as-obtained sample was collected and named g-C3N4 (550–650).
2.2. Characterizations The X-ray diffraction (XRD) measurement of g-C3N4 (550) and gC3N4 (550–650) were carried out on X-ray powder diffractometer (Bruker D8 Advance). Fourier transform infrared spectroscopy (FTIR) was collected on an Agilent Cary 5000 spectrum 100. Elemental analyses (EA) for the carbon and nitrogen contents were performed on a PerkinElmer series II CHNS/O analyzer 2400. The X-ray photoelectron spectroscopy (XPS) was detected on Thermo Fisher Scientific Escalab
Fig. 4. The EPR spectra of (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
250. Electron paramagnetic resonance (EPR) was measured on a Bruker EMX-10/12 EPR spectrometer. The morphology and microtopography of the products (g-C3N4 (550) and g-C3N4 (550–650)) were examined by scanning electron microscope (SEM, HITACHI SU8010) and 231
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B
A
5 m C
200 nm
D
5 m
200 nm
Fig. 5. The (A) SEM images and (B) TEM images of g-C3N4 (550), the (C) SEM images and (D) TEM images of g-C3N4 (550–650).
D -1
Potential (eV)
-0.53 0
-0.42 -0.07
CB
2.07
VB
H+/H2
1 2
2.15 Eg=2.68 eV
a
Eg=2.49 eV
b
Fig. 6. (A) UV–vis diffuse reflectance spectra, (B) a plot of (αhν)1/2 vs. photon energy (hν), (C) The VB XPS spectra and (D) electronic band structures of corresponding samples ((a) g-C3N4 (550) and (b) g-C3N4 (550–650)).
UV–vis spectrometer at room temperature. The photoluminescence spectra (PL) of g-C3N4 (550) and g-C3N4 (550–650) were measured by an Agilent Cary Eclipse spectrometer with an excitation wavelength of 325 nm. Transient photocurrent properties were detected using a Chenhua
transmission electron microscopy (TEM, JEM-2100F), respectively. BET surface area were collected at 77 K using a Quantachrome AutosorbIQ2-MP analyzer, samples were outgassed at 150 °C for 12 h prior to measurements. The UV–vis diffuse reflectance spectra (DRS) of g-C3N4 (550) and g-C3N4 (550–650) were recorded by an Agilent Cary 5000 232
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Fig. 8. (A) Photocatalytic H2 evolution rate of (a) g-C3N4 (550) and (b) g-C3N4 (550–650) (inset is HER) and (B) recycle of H2 produced of g-C3N4 (550–650).
Fig. 7. (A) The photoluminescence spectra and (B) transient photocurrent property of (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
27.5° to 27.7°, meaning that the interlayer stacking distance become narrow. This phenomenon manifested that the interlayer stacking order of as-prepared g-C3N4 (550–650) was strengthened, and related crystal structure could be more stable [23]. Whereafter, Fig. 2 demonstrates the FTIR spectra of as-made g-C3N4 (550) and g-C3N4 (550–650). The similar FTIR spectrogram could be observed for two simples, which revealed that the chemical structure of two samples was the same. The stretching vibration modes of triazine was represented at 809 cm−1, and some peaks in the range of 1100–1700 cm−1, including 1240, 1318, 1406, 1462, 1572 and 1639 cm−1, attributed to typical stretching modes of g-C3N4 heterocycles [24,25]. To further measure the elemental composition and chemical structure of g-C3N4 (550) and g-C3N4 (550–650), elemental analysis (EA) and X-ray photoelectron spectroscopy (XPS) were performed, respectively. Firstly, as shown in Table 1, the C/N atomic ratio for g-C3N4 (550) was 0.67, while the C/N atomic ratio for g-C3N4 (550–650) increased to 0.70, which suggested that nitrogen vacancy might be introduced in the g-C3N4 (550–650) through two time heat treatment. Moreover, the results of XPS were shown in Fig. 3. It could be seen that there were two peaks in the C 1s spectrum (Fig. 3A). The one at 284.6 eV was assigned to C atoms in the sp2 aromatic bonds, the other one at 288.0 eV was attributed to sp2-hybridized NeC]N [26]. The N 1s spectrum was shown in Fig. 3B, the peak could be deconvoluted into 398.4, 399.0, 400.5 and 404.4 eV, which could originate from the sp2 bonded N (N2eC), bridging nitrogen atoms in Ne(C)3, nitrogen hydrogen bond (CeNH or CeNH2) and the charging effects, respectively [27,28]. In order to further confirm nitrogen vacancy formed in g-C3N4, the
CHI760 electrochemical station (Shanghai, China) in a three-electrode quartz cells. Pt electrode, the saturated calomel electrode and the sample films (g-C3N4 (550) and g-C3N4 (550–650)) coated on ITO glasses were the counter electrode, the reference electrode and the working electrode, respectively. 0.1 M Na2SO4 was used as the electrolyte. The 300 W Xe lamp with 400 nm filters supplied light source. 2.3. Photocatalytic test The visible-light-induced photocatalytic evolution H2 from splitting water by g-C3N4 (550) or g-C3N4 (550–650) were carried out in a Pyrex top-irradiation reaction vessel connected to a closed quartz glass gascirculation system. 0.030 g sample was added in 50 mL aqueous solution containing triethanolamine (5 mL) and distill water (45 mL). Pt (3 wt%) was deposited on sample surface by the in-situ photodeposition method using H2PtCl6 as the precursor. The reactant solution was evacuated by vacuum pump for 0.5 h to remove air before illumination. In this system, 300 W Xe lamp with a 400 nm cutoff filter supplied visible light source. The product was analyzed by gas chromatography (GC7920, CEAULight, China), Ar was the carrier gas. 3. Results and discussion Fig. 1 presents the XRD patterns of as-made g-C3N4 (550) and gC3N4 (550–650). There were two peaks at 13.0° and 27.5°, corresponding to (100) and (002) planes of g-C3N4. Besides, the (002) diffraction peak width of g-C3N4 (550–650) become narrower than that of g-C3N4 (550), and the related peak position also slightly shifted from 233
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D
C
5 nm
100 nm
Fig. 9. The (A) XRD patterns, (B) FTIR spectra and (C–D) TEM images of used g-C3N4 (550–650) after three time reactions for splitting water to produce H2.
Fig. 10. The photodegradation curves of various dyes over (a) g-C3N4 (550) and (b) g-C3N4 (550–650).
electron paramagnetic resonance (EPR) spectroscopy was measured, which could supply fingerprint evidences for probing the surface vacancies in semiconductor photocatalysts. As shown in Fig. 4, two samples displayed a single Lorentzian line in the magnetic field from 3490 to 3530 G, which was associated with the unpaired electrons of sp2-carbon atoms within π-conjugated aromatic rings. Compared to gC3N4 (550) with low EPR signal, g-C3N4 (550–650) showed the higher intensity of EPR signal, revealing nitrogen vacancy generated in g-C3N4 [29]. Therefore, combined with above results, the generation of nitrogen vacancy in g-C3N4 has been confirmed. The morphologies and microtopographies of resultant g-C3N4 (550) and g-C3N4 (550–650) were observed by SEM and TEM. As shown in Fig. 5A, g-C3N4 (550) was composed of irregularly curved layers, and the TEM image (Fig. 5B) further confirmed this result. For g-C3N4
(550–650), it was also formed by curved layers, but the average size of the layers was clearly declined due to in the introduction of a great number of cracks, and its TEM image (Fig. 5D) indicated that the thickness of the nanosheets decreased and some pores could be observed. Above results indicated that the large layer structure of g-C3N4 (550) has changed to smaller nanosheets of g-C3N4 (550–650) with porous structure after two times heat treatment. As a photocatalyst, specific surface area played a crucial role in its performance, the larger surface area could provide the more active sites, so that the improved photocatalytic activity might be expected [30]. Therefore, the BET surface area of g-C3N4 (550) and g-C3N4 (550–650) were detected and demonstrated in Table 1. Evidently, gC3N4 (550–650) exhibited larger specific surface area of 84 m2 g−1 than that of g-C3N4 (550) (50 m2 g−1). This result revealed that two times 234
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Finally, the photocatalytic activity of g-C3N4 (550) and g-C3N4 (550–650) was examined to the degradation of various organic dyes: Rhodamine B (RhB), Methylene Blue (MB), Methyl Orange (MO), with a dye concentration of 5 ppm and 30 mg of catalyst. The experimental process was similar with previous reports [33–35]. Fig. 10 describes the photodegradation curves of these dyes. Under the same reaction conditions, compared with g-C3N4 (550), g-C3N4 (550–650) could exhibited better photocatalytic performance, which further confirmed that g-C3N4 (550–650) was an effective photocatalyst.
heat treatment could enlarge resultant g-C3N4 specific surface area, conducing to offer more reaction active sites to boost the photocatalytic activity. Furthermore, the light-harvesting capability also affected photocatalytic performance of photocatalyst. Hence, the DRS of g-C3N4 (550) and g-C3N4 (550–650) were measured to investigate their optical property. As illustrated in Fig. 6A, compared with g-C3N4 (550), asmade g-C3N4 (550–650) exhibited clearly enhanced light responded property ranging from 450 to 800 nm, which might ascribe to the introduction of nitrogen vacancy [21]. And through the spectrum in Fig. 6B, their band gap energies were calculated using the Kubelka–Munk method. In detail, the band gap energy of g-C3N4 (550) was 2.68 eV, and g-C3N4 (550–650) exhibited two energy levels (2.49 and 2.14 eV), the former was intrinsic band gap energy, the latter might attribute to nitrogen vacancy, the similar phenomenon was reported in previous literature [31]. As we have known, the enhanced light-harvesting activity could benefit for improving the photocatalytic activity. Whereafter, the valence band (VB) XPS spectra of g-C3N4 (550) and g-C3N4 (550–650) were detected (Fig. 6C), their positions of the VB edge maxima were respectively estimated to be 2.15 and 2.07 eV. Combined with the results of VB XPS and band gap energy, the conduction band (CB) positions of g-C3N4 (550) and g-C3N4 (550–650) were calculated and shown in Fig. 6D. Obviously, for g-C3N4 (550–650), due to introduction of nitrogen vacancy, its potential of CB was negative shift to form the new conduction band, which would be as medium energy levels to accept the photogenerated electrons from the VB of g-C3N4 (550–650), resulting in more photons to be absorbed [32]. Finally, the separation, transfer and recombination rate of photoinduced charge carries also influence on the performance of photocatalyst. Therefore, photoluminescence spectrum (PL) of g-C3N4 (550) and g-C3N4 (550–650) were tested. As shown in Fig. 7A, g-C3N4 (550–650) displayed distinctly lower the PL intensity than g-C3N4 (550), meaning that the energy-wasteful charge recombination was mainly confined. The enhanced photocatalytic activity could be anticipated. Moreover, the emission peak of g-C3N4 (550–650) was redshift in comparison with g-C3N4 (550), agreeing with DRS results. To further compare the photoinduced charge separation rate of g-C3N4 (550) and g-C3N4 (550–650), photocurrent measurements were tested. As can be shown in Fig. 7B, compared with g-C3N4 (550), g-C3N4 (550–650) could exhibit the larger photocurrent density, implying higher separation rate of photoinduced electron-hole pairs. The photocatalytic performance for producing H2 of g-C3N4 (550) and g-C3N4 (550–650) was evaluated under visible light. Fig. 8A displays that H2 was persistently produced with irradiation time prolonging. Comparing with g-C3N4 (550), g-C3N4 (550–650) exhibited higher H2 evolved rate (HER). And inset indicates that the HER of gC3N4 (550) was 1925 μmol h−1 g−1, while that of g-C3N4 (550–650) was 5250 μmol h−1 g−1. Clearly, the rate of producing H2 over g-C3N4 (550–650) was 2.72 times than that of g-C3N4 (550). This result meant that, g-C3N4 (550–650) due to the introduction of N vacancy, larger surface area, stronger visible-light absorption and high efficient separated rate of photogenerated electron-hole pairs, could exhibit better visible-light photocatalytic H2 evolution. Moreover, the stability of photocatalyst is also a significant target to estimate its photocatalytic property. Hence, the recycling experiment for producing H2 over g-C3N4 (550–650) was measured under the same conditions. As demonstrated in Fig. 8B, the g-C3N4 (550–650) showed steady H2 produce rate for 12 h (three runs in total), implying outstanding photocatalytic stability. Meanwhile, for supplying more evidences to prove the stability of sample, the chemical structure and morphology of the used sample were further analyzed by XRD, FTIR and TEM. Data indicated that there was no clear difference in the XRD pattern (Fig. 9A), FTIR spectra (Fig. 9B), and morphology (Fig. 9C–D) of the sample compared with fresh sample, further revealing that the catalyst was very stable in process of photocatalytic H2 evolution.
4. Conclusions In summary, nitrogen vacancy modified large surface area g-C3N4 has been successfully prepared. The specific surface area of as-prepared photocatalyst reach to 84 m2 g−1, visible light absorption could extend to 800 nm, and photoinduced charge separation efficiency was also improved, these positive factors resulted in as-prepared photocatalyst exhibiting the significantly enhanced photocatalytic H2 evolution performance (5250 μmol h−1 g−1) and degraded activity for various dyes. In addition, as-prepared catalyst showed well stability of catalytic performance and chemical structure. We believe that this work will open a new window for exploiting markedly efficient metal-free photocatalysts in diverse applications. Acknowledgements We sincerely acknowledge the financial supported by talent scientific research fund of LSHU (No. 2016XJJ-080) and basic research projects of Liaoning Provincial Education Department (L2017LQN004). References [1] W.J. Ong, L.L. Tan, Y.H. Ng, S.T. Yong, S.P. Chai, Graphitic carbon nitride (g-C3N4)based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev. 116 (2016) 7159–7329. [2] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domenet, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80. [3] Y. Zhou, L. Zhang, W. Huang, Q. Kong, X. Fan, M. Wang, J. Shi, N-doped graphitic carbon-incorporated g-C3N4 for remarkably enhanced photocatalytic H2 evolution under visible light, Carbon 99 (2016) 111–117. [4] Y. Zhou, L. Zhang, J. Liu, X. Fan, B. Wang, M. Wang, W. Ren, J. Wang, M. Li, J. Shi, Brand new P-doped g-C3N4: enhanced photocatalytic activity for H2 evolution and Rhodamine B degradation under visible light, J. Mater. Chem. A 3 (2015) 3862–3867. [5] M. Ji, J. Di, Y. Ge, J. Xia, H. Li, 2D-2D stacking of graphene-like g-C3N4/ultrathin Bi4O5Br2 with matched energy band structure towards antibiotic removal, Appl. Surf. Sci. 413 (2017) 372–380. [6] Z. Wang, W. Guan, Y. Sun, F. Dong, Y. Zhou, W.K. Ho, Water-assisted production of honeycomb-like g-C3N4 with ultralong carrier lifetime and outstanding photocatalytic activity, Nanoscale 7 (2015) 2471–2479. [7] D.S. Wang, H. Sun, Q. Luo, X. Yang, R. Yin, An efficient visible-light photocatalyst prepared from g-C3N4 and polyvinyl chloride, Appl. Catal. B Environ. 156–157 (2014) 323–330. [8] J. Lin, Z. Pan, X. Wang, Photochemical reduction of CO2 by graphitic carbon nitride polymers, ACS Sustain. Chem. Eng. 2 (2014) 353–358. [9] Z. Zhu, H. Pan, M. Murugananthan, J. Gong, Y. Zhang, Visible light-driven photocatalytically active g-C3N4 material for enhanced generation of H2O2, Appl. Catal. B Environ. 232 (2018) 19–25. [10] S. Guo, Y. Tang, Y. Xie, C. Tian, Q. Feng, W. Zhou, B. Jiang, P-doped tubular g-C3N4 with surface carbon defects: universal synthesis and enhanced visible-light photocatalytic hydrogen production, Appl. Catal. B Environ. 218 (2017) 664–671. [11] S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation of rhodamine B and methyl orange over boron-doped g-C3N4 under visible light irradiation, Langmuir 26 (2010) 3894–3901. [12] L. Shi, L. Liang, F. Wang, M. Liu, K. Chen, K. Sun, N. Zhang, J. Sun, Higher yield urea-derived polymeric graphitic carbon nitride with mesoporous structure and superior visible-light-responsive activity, ACS Sustain. Chem. Eng. 3 (2015) 3412–3419. [13] H. Yan, Soft-templating synthesis of mesoporous graphitic carbon nitride with enhanced photocatalytic H2 evolution under visible light, Chem. Commun. 48 (2012) 3430–3432. [14] S.S. Park, S.W. Chu, C.F. Xue, D.Y. Zhao, C.S. Ha, Facile synthesis of mesoporous carbon nitrides using the incipient wetness method and the application as hydrogen adsorbent, J. Mater. Chem. 21 (2011) 10801–10807. [15] L. Shi, L. Liang, J. Ma, F.X. Wang, J.M. Sun, Enhanced photocatalytic activity over
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