- Email: [email protected]
Enhanced charge separation ability and visible light photocatalytic performance of graphitic carbon nitride by binary S, B co-doping
Materials Research Bulletin 107 (2018) 477–483
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Enhanced charge separation ability and visible light photocatalytic performance of graphitic carbon nitride by binary S, B co-doping ⁎
Xiaoxue Han, Chengkai Yao, Aili Yuan, Fengna Xi, Xiaoping Dong , Jiyang Liu
T
⁎
Department of Chemistry, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 S, B co-doping Visible light photocatalysis Dye photo-degradation
As a metal-free visible light photocatalyst, the photocatalytic performance of graphitic carbon nitride (g-C3N4) is limited. To improve the activity of g-C3N4, we herein report a binary S, B co-doped g-C3N4 (SBCN) by thermally treating the precursor mixture of thiourea, melamine and boron oxide. Characterization results show the obtained SBCN has ultrathin sheet-like morphology and these doped heteroatoms are uniform distributed in the sample. Compared to the pristine g-C3N4, the SBCN has a small band gap with additional absorption in visible light region, decreased photoluminescence emission and electric resistance and enhanced photocurrent response. These advantages endue SBCN with excellent photocatalytic activity for degradation of organic dyes, and its reaction rate is 6.5 times higher than that of the g-C3N4. Moreover, the successive cycles demonstrate that this SBCN possesses a superior stability and reusability. Finally, we propose a possible photocatalytic mechanism based on the band structure and mainly active species.
1. Introduction Nowadays, the problem of energy shortage and environmental deterioration become more and more serious. It is a prospective way to use solar sources as a clean energy. Semiconductor photocatalysis provides promising approaches in water splitting and pollutant degradation utilizing solar energy directly and has attracted considerable attention from research experts in materials science and chemistry [1–3]. Graphitic carbon nitride (g-C3N4) is composed of stacked 2D layers which are constructed from tri-s-triazine units connecting through planar amino groups. The g-C3N4 possesses high thermal and chemical stability, as well as an appropriate band structure and a narrow band gap of 2.7 eV allowing it to be driven under visible light when acting as a photocatalyst for solar resource conversion [4–7]. Therefore, ever since its emergence, g-C3N4 has attracted extensive attention in the field of visible light photocatalysis for its unique atomic and electronic structures [8–13]. However, g-C3N4 has some shortcomings, such as low visible light utilization efficiency, small specific surface area and rapid recombination of the photo-generated electron–hole pairs. Experimental and theoretical investigations have demonstrated that doping with nonmetal elements can enhance the visible light photocatalytic performance of g-C3N4. Up to date, various nonmetal elements have been introduced to obtain heteroatom doped g-C3N4 [14–37], such as N-
⁎
doped g-C3N4 [15], B-doped g-C3N4 [16–18], P-doped g-C3N4 [19–23], S-doped g-C3N4 [24–32] and O-doped g-C3N4 [33–35]. The heteroatom dopants can significantly affect the electronic structure of g-C3N4, and therefore promote visible light harvest, suppress photo-excited charge recombination and enhance photocatalytic activity. For instance, the introduction of sulfur dopant is indeed effective in extending the optical response of g-C3N4 in visible light region, and meanwhile promotes the separation of photo-generated carriers [27,31]. The narrowed band gap and the enhanced photocurrent response are also revealed in B doped gC3N4, which bestow an increased photocatalytic performance upon Bdoped g-C3N4 in comparison with the pristine g-C3N4 [16]. Very recently, researchers have focused on the co-doping with binary nonmetal elements. Compared to single element doping, the co-doped g-C3N4 materials recombine the advantage of single elements and present much higher activities than those of single element doped g-C3N4. For instance, You et al introduced S and O atoms into the g-C3N4 lattice that provided a facile way to adjust the intrinsic electronic and band structure of g-C3N4 and improved the photocatalytic performance [36]. Hu et al synthesized oxygen functionalized S–P co-doped g-C3N4 nanorods using a hydrothermal post-treatment in the absence of H2O2 and exhibited outstanding visible light activity under anoxic conditions [37]. All these binary elements co-doped g-C3N4 materials depict much more efficient photocatalytic behavior than their corresponding single element doped g-C3N4 and the pristine g-C3N4 as well.
Corresponding authors. E-mail addresses: [email protected] (X. Dong), [email protected] (J. Liu).
https://doi.org/10.1016/j.materresbull.2018.08.021 Received 23 November 2017; Received in revised form 3 August 2018; Accepted 14 August 2018 Available online 18 August 2018 0025-5408/ © 2018 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
Fig. 1. SEM images of (a) the pristine g-C3N4 and (b) the SBCN samples; element mapping of (c) C, (d) N, (e) S and (f) B in SBCN; (g) TEM image of SBCN.
emission scanning electron microscope (SEM) equipped with an energydispersive X-ray spectrometer (EDS). The microstructures of materials were measured by transmission electron microscopy (TEM) observation performed on a JEOL JEM-2100 electron microscope. Fourier transform infrared spectra (FT-IR) were obtained on a Nicolet Avatar 370 spectrophotometer using the standard KBr disk method. UV–vis diffuse reflectance spectroscopy (DRS) was performed on a Shimadzu UV-2450 spectrophotometer using BaSO4 as the reference. Photoluminescence (PL) spectra of materials were measured using a fluorescence spectrophotometer with an excitation wavelength at 340 nm by Edinburgh FL/ FS900 spectrophotometer. X-ray photoelectron spectra (XPS) was analyzed on a Kratos AXIS Ultra DLD instrument with an Al Ka monochromatic source. The electrochemical tests were performed on a CHI660E electrochemical workstation (Chenhua, Shanghai, China), respectively using platinum plate, Ag/AgCl electrode and Na2SO4 aqueous solution (0.2 M) as counter electrode, reference electrode and electrolyte solution. Working electrodes were prepared by the following process: Firstly, the substrate of ITO glass was repeatedly rinsed with ethanol and deionized water. Secondly, 25 mg photocatalysts and 1 mL binder (5% polyvinylidene fluoride, PVDF, in N-methyl pyrrolidone) were mixed and stirred for 1 h. The slurry was coated on ITO glass by spin-coating, and then dried at 70 °C overnight to obtain the working electrodes. Mott–Schottky plots were recorded at an AC voltage magnitude of 5 mV with the frequency of 1000 Hz, and electrochemical impedance spectra were measured with the frequency from 100 kHz to 0.01 Hz. The photocurrent measurement was achieved under xenon lamp illumination.
In this paper, we report a pathway to improve the photocatalytic performance by introducing S and B atoms into the g-C3N4 lattice. Compared to the pristine, S doped and B doped g-C3N4 materials, the co-doped g-C3N4 exhibits much smaller band gap with additional absorption in visible light region that could greatly improve the solar light utilization. Furthermore, the heteroatom dopants change the electronic structure and therefore promote the photo-induced charge separation. We evaluated the photocatalytic activity of the obtained S,B co-doped g-C3N4 by decomposing organic dyes under visible light irradiation. And we also investigated its stability, the main active species in the photocatalytic process and finally proposed a possible photocatalytic mechanism. 2. Experimental 2.1. Material preparation The pristine g-C3N4 (CN) was synthesized using melamine as precursors, according to our published literatures [38,39]. The heteroatom doped CN samples were obtained by the similar process with the addition of dopant precursors. In a typical experiment for preparing S,B co-doped CN (SBCN), boron oxide (0.02 g) and melamine (2 g) were dispersed into 10 mL ethanol. After evaporating ethanol at 80 °C, the white resultant was mixed with 10 g thiourea in an agate mortar. The obtained mixture was transferred into an alumina crucible with a cover and then heated at 520 °C in a muffle furnace for 4 h. Finally, the yellow product was collected and milled into powder for further use. For comparison, the S-doped CN (SCN) and B-doped CN (BCN) photocatalysts were prepared following the same procedure mentioned above in the absence of boron oxide or thiourea, respectively.
2.3. Photocatalytic tests The photocatalytic performance of materials was appraised by the photocatalytic degradation of Rhodamine B (RhB) in aqueous solution under visible light irradiation using a 300 W xenon lamp (HSX-F300, Beijing NBet) as the light source. In a typical photocatalytic experiment, 0.1 g of catalyst was dispersed into 100 mL of 10 mg L−1 RhB solution. The suspension was stirred in the dark for 60 min prior to the illumination to ensure the adsorption–desorption equilibrium. During the
2.2. Characterizations The crystal structure of samples was characterized by X-ray diffraction (XRD) on a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd.) using Cu Ka radiation. The morphology and composition of materials were analyzed by Quanta 250FEG field 478
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
stretching vibrations of C-N heterocycle in the wavenumber range of 1200∼1600 cm−1 and the characteristic breathing mode of s-triazine at 810 cm−1 are still presented in the SCN, BCN and SBCN samples, suggesting the structural preservation of conjugated heterocycles after heteroatom doping. The optical absorption of samples was investigated by using UV–vis DRS. As shown in Fig. 2c, the CN holds a sharp absorption edge at 460 nm. These heteroatom doped samples, SCN, BCN and SBCN have similar spectral aspect to the pristine CN, demonstrating that all these samples have similar electronic structure. We could find the absorption in the long wavelength region was enhanced in SCN, BCN and SBCN samples, compared to CN. It should be related to the doped levels of heteroatoms, which can sufficiently absorb solar light and then improve the photocatalytic performance. The band gap (Eg) values of the pristine CN and the heteroatom doped samples were estimated by plotting (αhν)1/2 as a function of the photon energy (hν). As illustrated in Fig. 2d, the Eg values of CN, SCN, BCN and SBCN are respectively 2.70, 2.51, 2.58 and 2.48 eV. XPS technology was used to investigate the chemical environment of elements in SBCN sample. From the survey spectrum in Fig. 3a, four elements, C, N, S and B, are all checked in the SBCN sample, and the atomic ratios of S and B are 0.61% of S and 0.18% of B, respectively. Fig. 3b shows C 1 s XPS spectrum of SBCN. The peak located at 284.6 eV (C1) is attributed to adventitious carbon contamination (CeC and C]C), and the 288.1 eV signal (C2) is from the conjugated CN heterocycle [31]. Moreover, an additional peak at 286.3 eV (C3) is related to the CeS bond [32]. The N 1 s spectrum (Fig. 3c) can be divided into three peaks located at 398.5 eV (N1), 399.5 eV (N2) and 401.eV (N3), which are assigned to the sp2-hybridized aromatic N atoms bonded to carbon atoms (CeN]C), sp3-hybridized N atoms of N(eC)3 and terminal amino functions (CeNH2), respectively [17,34]. The O 1 s spectrum (Fig. 3d) centered at 532.0 eV is mainly from the adsorbed water on surface [37]. Fig. 3e depicts the S 2p spectrum that is fitted into two peaks. The peak at 165.0 eV could be reasonably attributed to CeS bonds formed in the g-C3N4 lattice via substituting N, and another peak at 168.3 eV is ascribed to S]O in the intermediate product. The B 1 s signal at 192.1 eV in Fig. 3f is consistent with the reported CeNB group
photoreaction process, 3.5 mL of the suspension was sampled at 20 min intervals for 2 h, and then the slurry sample was centrifuged to separate the photocatalyst particles. The concentration was determined using a Shimadzu UV-2450 spectrophotometer at 554 nm for RhB. 3. Results and discussion 3.1. Physicochemical properties of S, B co-doped g-C3N4 The morphology and microstructure of SBCN were observed by electronic microscopy technologies (Fig. 1). In comparison with the dense bulky morphology of pristine CN (Fig. 1a), a number of irregular thin and flexible sheets are located on the SBCN sample (Fig. 1b). It should be related to the gas release from the additional precursors in the thermal treatment, which is similar to the chemical-blowing synthesis of CN or carbon nanosheets [39,40]. The presence and distribution of elements in SBCN were demonstrated by EDS. As shown in Fig. 1c-f, four elements, C, N, S and B, are all checked and their distributions are well uniform in SBCN sample. This result proves these heteroatoms of S and B elements were successfully doped into the SBCN. The TEM image of SBCN (Fig. 1g) presents a 2D lamellar morphology, and the low contrast indicates its thin thickness. Some dark zones in the image should be assigned to the edge corrugation of these ultrathin sheets in SBCN. Fig. 2a shows XRD patterns of the as-prepared SBCN, as well as CN, SCN and BCN samples. Two distinct diffraction peaks at ∼13.0° and ∼27.4° are indexed as (100) and (002) planes, respectively, corresponding to the in-plane structural packing motif and interlayer stacking of aromatic segments [38–41]. After heteroatoms were introduced, the intensity of these peaks apparently decreases in the SCN and BCN samples. And, these diffractions of SBCN further weakens, which is in accordance with the thin thickness of SBCN observed in SEM and TEM images. However, from the enlarged XRD pattern of SBCN (inset of Fig. 2a) we can still find the characteristic diffractions of graphitic carbon nitride, indicating that the heteroatom dopants do not influence the crystal structure of CN. Fig. 2b depicts the FT-IR spectra of pristine CN and all the heteroatom doped CN samples. Some typical
Fig. 2. (a) XRD patterns, (b) FTIR spectra, (c) UV–vis absorption spectra and (d) plots of (αhv)1/2 vs. hv of CN, SCN, BCN and SBCN. 479
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
Fig. 3. XPS spectra of SBCN: (a) survey; (b) C 1 s, (c) N 1 s, (d) O 1 s, (e) S 2p and (f) B 1 s.
hole pairs.
[16]. PL emission spectroscopy is an effective route to investigate the separation efficiency of photo-generated charges. Before diffusing to the surface, most photo-generated charges of CN recombine, resulting in an intensive PL emission (Fig. 4a). It is apparent that these heteroatom doped samples have PL spectra alike to CN, but the intensity drastically decreases under the 340 nm excitation. This result suggests that these SCN, BCN and SBCN samples have similar electronic structure to CN, whereas the separation of photo-excited electron-hole pairs has been significantly promoted. Among these heteroatom doped samples, the SBCN has the lowest intensity of PL emission. It implies that the SBCN sample possesses the most sufficient separation efficiency of photo-induced carriers. The migration ability of charges in semiconductor photocatalysts can be well described by their electrochemical impedance spectra (EIS). Fig. 4b displays Nyquist plots of EIS for CN, SCN, BCN and SBCN samples. The arc radius represents the electric resistance of charge transfer. Apparently, the SBCN presents the smallest radius in all four samples, indicative of its lowest resistance for photo-generated carrier transport. The transient photocurrent responses of samples were measured during repeated ON/OFF illumination cycles (Fig. 4c). As the lamp each was initiated, all the four samples exhibited prompt and reproducible photocurrent responses. The photocurrent rapidly dropped to almost zero as turning off the lamp and reverted once the lamp was turn on again. As shown in Fig. 4c, the SBCN has the largest photocurrent response among these four photocatalysts, confirming it has the most sufficient separation yield of photo-generated electron-
3.2. Visible light photocatalytic performance of S, B co-doped g-C3N4 The photocatalytic activity of SBCN was estimated by degradation of RhB under visible light irradiation. Fig. 5a shows the change for absorption spectra of RhB aqueous solution under visible light irradiation with the presence of SBCN sample. The characteristic absorption of RhB at 554 nm does not decrease from the adsorption time of 20 min to 60 min, suggesting the adsorption-desorption equilibrium of RhB has reached. After turning on the lamp, this absorption decreases greatly with the increase of irradiation time and almost disappears after 80 min. Besides this characteristic absorption, some absorption bands in UV region also exhibit a declining trend. It can be ascribed to the decomposition of aromatic rings to small molecules. The photocatalytic performance of pristine CN and various heteroatom doped CN samples is compared in the Fig. 5b. RhB is very stable, and almost no RhB is decomposed without the presence of any photocatalysts after 120 min irradiation. The pristine CN gives a decomposition ratio of ∼50%, and the heteroatom doped SCN and BCN depict a similar photocatalytic degradation ratio of ∼ 80% after 120 min visible light illumination. In contrast, the SBCN sample illustrates an efficient degradation behavior that can decompose almost 100% RhB within 80 min. To better understand the photocatalytic behavior, the photocatalytic kinetic study was estimated with the following equation: -ln(C/C0) = kt (Fig. 5c). The calculated rate constant (k) of SBCN is 0.0495 min−1 that is 6.5 times higher than that of the pristine CN.
Fig. 4. (a) PL spectra, (b) Nyquist plots of EIS and (c) photocurrent responses of CN, SCN, BCN and SBCN. 480
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
Fig. 5. (a) UV–vis absorption spectral changes during the photocatalytic degradation of RhB in SBCN; (b) Comparison of visible light photocatalytic performances of CN, SCN, BCN and SBCN for degrading RhB and (c) their pseudo-first-order kinetics fittings; (d) the cycle behaviors of SBCN.
Fig. 6. Mott-Schottky curves of (a) the pristine CN and (b) the SBCN; (c) schematic illustration for band structure of CN and SBCN; (d) the photocatalytic activity of SBCN for degrading RhB with the presence of various scavengers.
photocatalytic performance of SBCN with the pristine CN for removal of various organic dyes, including methyl blue (MB), Congo red and reactive violet under visible light irradiation (Fig. S1), where the SBCN presents better photocatalytic behavior than the pristine CN.
The stability and reusability of photocatalysts is vital for their practical application. We evaluated the recycling property of SBCN material by directly adding concentrated dye solution to the mixture after the last photocatalytic test. As shown in Fig. 5d, the successive five cycle experiments exhibit similar photocatalytic performance, indicating its superior stability and reusability. We also compared the 481
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
Fig. 7. The possible photocatalytic mechanism of SBCN under visible light irradiation.
However, they can directly oxidize RhB because of the relatively small oxidizing potential of RhB (0.97 V vs. NHE) [42]. On the other hand, the photo-excited electrons on CB can reduce the adsorbed O2 to form % O2− radicals that can further react with water to generate ∙OH radicals. These %O2− and %OH radicals have strong oxidation ability to decompose organic dyes to small molecules.
3.3. Possible photocatalytic mechanism of S, B co-doped g-C3N4 For understanding the photocatalytic mechanism of SBCN, we employed the Mott-Schottky (MS) electrochemical technology to determine the semiconductive behavior and the flat band potential (Efb) of these samples. Therefore, according to the band gap values based on the result from UV–vis spectra, we can illustrate their band alignment. As revealed in Fig. 6a, the pristine CN shows a MS curve with a positive slope, implying its n-type semiconductive behavior. It is in accordance with CN materials in published literatures [40,41]. After doping with heteroatoms, the n-type behavior is remained in SBCN sample, as seen in Fig. 6b. The intercept at the x axis by drawing the tangent of MS curve is the Efb value that is regarded as the Fermi level (Ef) of semiconductors. In general, the Ef closes to the conduction band (CB) in ntype semiconductor and is near to the valence band (VB) of p-type semiconductors. Therefore, from their MS curves we can estimate the edge potential of CB in CN and SBCN to be -1.28 and -0.77 vs. NHE, respectively. Furthermore, the VB edge potential is calculated following the below formula: Eg = EVB – ECB. And thus the band structures are illustrated in Fig. 6c. To determine the main active species in this photocatalytic process, we employed various scavengers to quench different active species. Herein, tertiary butanol (TBA) is a hydroxyl radical (%OH) quencher, benzoquinone (BQ) is a superoxide radical (%O2−) scavenger and disodium ethylenediaminetetraacetate (EDTA-2Na) is a hole (h+) scavenger. As shown in Fig. 6d, when TBA and BQ are added, the photodegradation efficiency for RhB is significantly suppressed. However, the degradation efficiency of RhB is not suppressed but somewhat improved by adding EDTA-2Na. That means %O2− and %OH should be the main active species in photocatalytic process of SBCN for degradation of organic dyes under visible light irradiation. The enhancement of photocatalytic activity by quenching h+ with EDTA-2Na may be ascribed to that the recombination of photo-generated charges is significantly inhibited. Based on the above results, we propose the possible photocatalytic mechanism for the degradation of organic dyes with SBCN, as depicted in Fig. 7. Under visible light illumination, the electrons on VB are excited and cross the band gap to the CB of SBCN, meanwhile leaving equivalent holes on VB. Because of the thinner sheet-like morphology, much more electrons and holes can transfer to the surface of SBCN before recombination in comparison to the bulky CN. Considering the redox potential of %OH/OH− (2.7 V vs. NHE), the photo-induced holes on VB (1.71 V vs. NHE) cannot oxidize water to produce %OH radicals.
4. Conclusion In summary, we have successfully synthesis an S, B co-doped g-C3N4 material during a facile and similar method to prepare the pristine gC3N4. The co-doped heteroatoms effectively change the intrinsic electronic and band structure of g-C3N4, which results in the sufficient light absorption, the low internal resistance and the high separation of photo-generated charges. Consequently, this SBCN shows much higher activity than the pristine g-C3N4, SCN and BCN samples for removal of organic pollutants. So far, several nonmetal elements co-doping systems have been developed, and all of them present enhanced photocatalytic performance than single element doped materials. Based on these results, the modification of g-C3N4 by multi-nonmetal element co-doping is an efficient strategy to promote the photocatalytic activity of g-C3N4. In the further investigation, we will continue to study the controllable synthesis of other co-doping systems and explore their photocatalytic behavior for other photocatalytic applications. Acknowledgements This work was financially supported from the financial support from the Zhejiang Provincial Natural Science Foundation of China (No. LY17B010004, LY17B050007) and the 521 talent project of ZSTU. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
482
A. Kubacka, M. Fernandez-García, G. Colon, Chem. Rev. 112 (2012) 1555–1614. M. Chong, B. Jin, C.W.K. Chow, C. Saint, Water Res. 44 (2010) 2997–3027. P. Zhou, J. Yu, M. Jaroniec, Adv. Mater. 26 (2014) 4920–4935. J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S. Lee, J. Zhong, Z. Kang, Science 347 (2015) 970–974. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. A. Jorge, D. Martin, M.T.S. Dhanoa, A. Rahman, N. Makwana, J. Tang, A. Sella, F. Cora, S. Firth, J. Darrand, P. McMillan, J. Phys. Chem. C 117 (2013) 7178–7185. J. Zhang, F. Guo, X. Wang, Adv. Funct. Mater. 23 (2013) 3008–3014. Y. Wang, X. Wang, M. Antonietti, Angew. Chem. Int. Ed. 51 (2012) 68–89. W. Ong, L. Tan, Y. Ng, S. Yong, S. Chai, Chem. Rev. 116 (2016) 7159–7329. J. Xu, Y. Li, S. Peng, G. Lu, S. Li, Phys. Chem. Chem. Phys. 15 (2013) 7657–7665.
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
[28] [29] [30] [31] [32]
S. Yan, Z. Li, Z. Zou, Langmuir 25 (2009) 10397–10401. D. Chen, J. Yang, H. Din, Appl. Surf. Sci. 391 (2017) 384–391. J. Wen, J. Xie, X. Chen, X. Li, Appl. Surf. Sci. 391 (2017) 72–123. G. Dong, K. Zhao, L. Zhang, Chem. Commun. 48 (2012) 6178–6180. J. Fang, H. Fan, M. Li, C. Long, J. Mater. Chem. A 3 (2015) 13819–13826. S. Yan, Z. Li, Z. Zou, Langmuir 26 (2010) 3894–3901. D. Gao, Y. Liu, P. Liu, M. Si, D. Xue, Sci. Rep. 6 (2016) 35768–35774. N. Sagara, S. Kamimura, T. Tsubota, T. Ohno, Appl. Catal. B 192 (2016) 193–198. Y. Zhang, T. Mori, J. Ye, M. Antonietti, J. Am. Chem. Soc. 132 (2010) 6294–6295. T. Ma, J. Ran, S. Dai, M. Jaroniec, S. Qiao, Angew. Chem. Int. Ed. 54 (2015) 4646–4650. S. Hu, L. Ma, J. You, F. Li, Z. Fan, F. Wang, D. Liu, J. Gui, RSC Adv. 4 (2014) 21657–21663. Y. Zhou, L. Zhang, J. Liu, X. Fan, B. Wang, M. Wang, W. Ren, J. Wang, M. Li, J. Shi, J. Mater. Chem. A 3 (2015) 3862–3867. L. Zhang, X. Chen, J. Guan, Y. Jiang, T. Hou, X. Mu, Mater. Res. Bull. 48 (2013) 3485–3491. J. Hong, X. Xia, Y. Wang, R. Xu, J. Mater. Chem. 22 (2012) 15006–15012. L. Ye, S. Chen, Appl. Surf. Sci. 389 (2016) 1076–1083. G. Liu, P. Niu, C. Sun, S. Smith, Z. Chen, G. Lu, H. Cheng, J. Am. Chem. Soc. 132 (2010) 11642–11648. L. Cao, R. Wang, D. Wang, Mater. Lett. 149 (2015) 50–53.
[33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
483
J. Chen, Z. Hong, Y. Chen, B. Lin, B. Gao, Mater. Lett. 145 (2015) 129–132. M. Seredych, T. Bandosz, Carbon 107 (2016) 895–906. S. Stolbov, S. Zuluaga, J. Phys.: Condens. Matter 25 (2013) 0855077pp. L. Ge, C. Han, X. Xiao, L. Guo, Y. Li, Mater. Res. Bull. 48 (2013) 3919–3925. L. Li, W. Fang, P. Zhang, J. Bi, Y. He, J. Wang, W. Su, J. Mater. Chem. A 4 (2016) 12402–12406. Z. Huang, J. Song, L. Pan, Z. Wang, X. Zhang, J. Zou, W. Mi, X. Zhang, L. Wang, Nano Energy 12 (2015) 646–656. J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, Chem. Commun. 48 (2012) 12017–12019. H. Ma, Y. Li, S. Li, N. Liu, Appl. Surf. Sci. 357 (2015) 131–138. R. You, H. Dou, L. Chen, S. Zheng, Y. Zhang, RSC Adv. 7 (2017) 15842–15850. S. Hu, L. Ma, Y. Xie, F. Li, Z. Fan, F. Wang, Q. Wang, Y. Wang, X. Kang, G. Wu, Dalton Trans. 44 (2015) 20889–20897. F. Cheng, J. Yan, C. Zhou, B. Chen, P. Li, Z. Chen, X. Dong, J. Colloid Interf. Sci. 468 (2016) 103–109. J. Yan, C. Zhou, P. Li, B. Chen, S. Zhang, F. Xi, X. Dong, J. Liu, Colloids Surf. A 508 (2016) 257–264. J. Yan, X. Han, X. Zheng, J. Qian, J. Liu, X. Dong, F. Xi, Mater. Res. Bull. 94 (2017) 423–427. J. Yan, X. Han, J. Qian, J. Liu, X. Dong, F. Xi, J. Mater. Sci. 52 (2017) 13091–13102. T. Takizawa, T. Watanabe, K. Honda, J. Phys. Chem. 82 (1978) 1391–1396.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Enhanced charge separation ability and visible light photocatalytic performance of graphitic carbon nitride by binary S, B co-doping ⁎
Xiaoxue Han, Chengkai Yao, Aili Yuan, Fengna Xi, Xiaoping Dong , Jiyang Liu
T
⁎
Department of Chemistry, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 S, B co-doping Visible light photocatalysis Dye photo-degradation
As a metal-free visible light photocatalyst, the photocatalytic performance of graphitic carbon nitride (g-C3N4) is limited. To improve the activity of g-C3N4, we herein report a binary S, B co-doped g-C3N4 (SBCN) by thermally treating the precursor mixture of thiourea, melamine and boron oxide. Characterization results show the obtained SBCN has ultrathin sheet-like morphology and these doped heteroatoms are uniform distributed in the sample. Compared to the pristine g-C3N4, the SBCN has a small band gap with additional absorption in visible light region, decreased photoluminescence emission and electric resistance and enhanced photocurrent response. These advantages endue SBCN with excellent photocatalytic activity for degradation of organic dyes, and its reaction rate is 6.5 times higher than that of the g-C3N4. Moreover, the successive cycles demonstrate that this SBCN possesses a superior stability and reusability. Finally, we propose a possible photocatalytic mechanism based on the band structure and mainly active species.
1. Introduction Nowadays, the problem of energy shortage and environmental deterioration become more and more serious. It is a prospective way to use solar sources as a clean energy. Semiconductor photocatalysis provides promising approaches in water splitting and pollutant degradation utilizing solar energy directly and has attracted considerable attention from research experts in materials science and chemistry [1–3]. Graphitic carbon nitride (g-C3N4) is composed of stacked 2D layers which are constructed from tri-s-triazine units connecting through planar amino groups. The g-C3N4 possesses high thermal and chemical stability, as well as an appropriate band structure and a narrow band gap of 2.7 eV allowing it to be driven under visible light when acting as a photocatalyst for solar resource conversion [4–7]. Therefore, ever since its emergence, g-C3N4 has attracted extensive attention in the field of visible light photocatalysis for its unique atomic and electronic structures [8–13]. However, g-C3N4 has some shortcomings, such as low visible light utilization efficiency, small specific surface area and rapid recombination of the photo-generated electron–hole pairs. Experimental and theoretical investigations have demonstrated that doping with nonmetal elements can enhance the visible light photocatalytic performance of g-C3N4. Up to date, various nonmetal elements have been introduced to obtain heteroatom doped g-C3N4 [14–37], such as N-
⁎
doped g-C3N4 [15], B-doped g-C3N4 [16–18], P-doped g-C3N4 [19–23], S-doped g-C3N4 [24–32] and O-doped g-C3N4 [33–35]. The heteroatom dopants can significantly affect the electronic structure of g-C3N4, and therefore promote visible light harvest, suppress photo-excited charge recombination and enhance photocatalytic activity. For instance, the introduction of sulfur dopant is indeed effective in extending the optical response of g-C3N4 in visible light region, and meanwhile promotes the separation of photo-generated carriers [27,31]. The narrowed band gap and the enhanced photocurrent response are also revealed in B doped gC3N4, which bestow an increased photocatalytic performance upon Bdoped g-C3N4 in comparison with the pristine g-C3N4 [16]. Very recently, researchers have focused on the co-doping with binary nonmetal elements. Compared to single element doping, the co-doped g-C3N4 materials recombine the advantage of single elements and present much higher activities than those of single element doped g-C3N4. For instance, You et al introduced S and O atoms into the g-C3N4 lattice that provided a facile way to adjust the intrinsic electronic and band structure of g-C3N4 and improved the photocatalytic performance [36]. Hu et al synthesized oxygen functionalized S–P co-doped g-C3N4 nanorods using a hydrothermal post-treatment in the absence of H2O2 and exhibited outstanding visible light activity under anoxic conditions [37]. All these binary elements co-doped g-C3N4 materials depict much more efficient photocatalytic behavior than their corresponding single element doped g-C3N4 and the pristine g-C3N4 as well.
Corresponding authors. E-mail addresses: [email protected] (X. Dong), [email protected] (J. Liu).
https://doi.org/10.1016/j.materresbull.2018.08.021 Received 23 November 2017; Received in revised form 3 August 2018; Accepted 14 August 2018 Available online 18 August 2018 0025-5408/ © 2018 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
Fig. 1. SEM images of (a) the pristine g-C3N4 and (b) the SBCN samples; element mapping of (c) C, (d) N, (e) S and (f) B in SBCN; (g) TEM image of SBCN.
emission scanning electron microscope (SEM) equipped with an energydispersive X-ray spectrometer (EDS). The microstructures of materials were measured by transmission electron microscopy (TEM) observation performed on a JEOL JEM-2100 electron microscope. Fourier transform infrared spectra (FT-IR) were obtained on a Nicolet Avatar 370 spectrophotometer using the standard KBr disk method. UV–vis diffuse reflectance spectroscopy (DRS) was performed on a Shimadzu UV-2450 spectrophotometer using BaSO4 as the reference. Photoluminescence (PL) spectra of materials were measured using a fluorescence spectrophotometer with an excitation wavelength at 340 nm by Edinburgh FL/ FS900 spectrophotometer. X-ray photoelectron spectra (XPS) was analyzed on a Kratos AXIS Ultra DLD instrument with an Al Ka monochromatic source. The electrochemical tests were performed on a CHI660E electrochemical workstation (Chenhua, Shanghai, China), respectively using platinum plate, Ag/AgCl electrode and Na2SO4 aqueous solution (0.2 M) as counter electrode, reference electrode and electrolyte solution. Working electrodes were prepared by the following process: Firstly, the substrate of ITO glass was repeatedly rinsed with ethanol and deionized water. Secondly, 25 mg photocatalysts and 1 mL binder (5% polyvinylidene fluoride, PVDF, in N-methyl pyrrolidone) were mixed and stirred for 1 h. The slurry was coated on ITO glass by spin-coating, and then dried at 70 °C overnight to obtain the working electrodes. Mott–Schottky plots were recorded at an AC voltage magnitude of 5 mV with the frequency of 1000 Hz, and electrochemical impedance spectra were measured with the frequency from 100 kHz to 0.01 Hz. The photocurrent measurement was achieved under xenon lamp illumination.
In this paper, we report a pathway to improve the photocatalytic performance by introducing S and B atoms into the g-C3N4 lattice. Compared to the pristine, S doped and B doped g-C3N4 materials, the co-doped g-C3N4 exhibits much smaller band gap with additional absorption in visible light region that could greatly improve the solar light utilization. Furthermore, the heteroatom dopants change the electronic structure and therefore promote the photo-induced charge separation. We evaluated the photocatalytic activity of the obtained S,B co-doped g-C3N4 by decomposing organic dyes under visible light irradiation. And we also investigated its stability, the main active species in the photocatalytic process and finally proposed a possible photocatalytic mechanism. 2. Experimental 2.1. Material preparation The pristine g-C3N4 (CN) was synthesized using melamine as precursors, according to our published literatures [38,39]. The heteroatom doped CN samples were obtained by the similar process with the addition of dopant precursors. In a typical experiment for preparing S,B co-doped CN (SBCN), boron oxide (0.02 g) and melamine (2 g) were dispersed into 10 mL ethanol. After evaporating ethanol at 80 °C, the white resultant was mixed with 10 g thiourea in an agate mortar. The obtained mixture was transferred into an alumina crucible with a cover and then heated at 520 °C in a muffle furnace for 4 h. Finally, the yellow product was collected and milled into powder for further use. For comparison, the S-doped CN (SCN) and B-doped CN (BCN) photocatalysts were prepared following the same procedure mentioned above in the absence of boron oxide or thiourea, respectively.
2.3. Photocatalytic tests The photocatalytic performance of materials was appraised by the photocatalytic degradation of Rhodamine B (RhB) in aqueous solution under visible light irradiation using a 300 W xenon lamp (HSX-F300, Beijing NBet) as the light source. In a typical photocatalytic experiment, 0.1 g of catalyst was dispersed into 100 mL of 10 mg L−1 RhB solution. The suspension was stirred in the dark for 60 min prior to the illumination to ensure the adsorption–desorption equilibrium. During the
2.2. Characterizations The crystal structure of samples was characterized by X-ray diffraction (XRD) on a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd.) using Cu Ka radiation. The morphology and composition of materials were analyzed by Quanta 250FEG field 478
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
stretching vibrations of C-N heterocycle in the wavenumber range of 1200∼1600 cm−1 and the characteristic breathing mode of s-triazine at 810 cm−1 are still presented in the SCN, BCN and SBCN samples, suggesting the structural preservation of conjugated heterocycles after heteroatom doping. The optical absorption of samples was investigated by using UV–vis DRS. As shown in Fig. 2c, the CN holds a sharp absorption edge at 460 nm. These heteroatom doped samples, SCN, BCN and SBCN have similar spectral aspect to the pristine CN, demonstrating that all these samples have similar electronic structure. We could find the absorption in the long wavelength region was enhanced in SCN, BCN and SBCN samples, compared to CN. It should be related to the doped levels of heteroatoms, which can sufficiently absorb solar light and then improve the photocatalytic performance. The band gap (Eg) values of the pristine CN and the heteroatom doped samples were estimated by plotting (αhν)1/2 as a function of the photon energy (hν). As illustrated in Fig. 2d, the Eg values of CN, SCN, BCN and SBCN are respectively 2.70, 2.51, 2.58 and 2.48 eV. XPS technology was used to investigate the chemical environment of elements in SBCN sample. From the survey spectrum in Fig. 3a, four elements, C, N, S and B, are all checked in the SBCN sample, and the atomic ratios of S and B are 0.61% of S and 0.18% of B, respectively. Fig. 3b shows C 1 s XPS spectrum of SBCN. The peak located at 284.6 eV (C1) is attributed to adventitious carbon contamination (CeC and C]C), and the 288.1 eV signal (C2) is from the conjugated CN heterocycle [31]. Moreover, an additional peak at 286.3 eV (C3) is related to the CeS bond [32]. The N 1 s spectrum (Fig. 3c) can be divided into three peaks located at 398.5 eV (N1), 399.5 eV (N2) and 401.eV (N3), which are assigned to the sp2-hybridized aromatic N atoms bonded to carbon atoms (CeN]C), sp3-hybridized N atoms of N(eC)3 and terminal amino functions (CeNH2), respectively [17,34]. The O 1 s spectrum (Fig. 3d) centered at 532.0 eV is mainly from the adsorbed water on surface [37]. Fig. 3e depicts the S 2p spectrum that is fitted into two peaks. The peak at 165.0 eV could be reasonably attributed to CeS bonds formed in the g-C3N4 lattice via substituting N, and another peak at 168.3 eV is ascribed to S]O in the intermediate product. The B 1 s signal at 192.1 eV in Fig. 3f is consistent with the reported CeNB group
photoreaction process, 3.5 mL of the suspension was sampled at 20 min intervals for 2 h, and then the slurry sample was centrifuged to separate the photocatalyst particles. The concentration was determined using a Shimadzu UV-2450 spectrophotometer at 554 nm for RhB. 3. Results and discussion 3.1. Physicochemical properties of S, B co-doped g-C3N4 The morphology and microstructure of SBCN were observed by electronic microscopy technologies (Fig. 1). In comparison with the dense bulky morphology of pristine CN (Fig. 1a), a number of irregular thin and flexible sheets are located on the SBCN sample (Fig. 1b). It should be related to the gas release from the additional precursors in the thermal treatment, which is similar to the chemical-blowing synthesis of CN or carbon nanosheets [39,40]. The presence and distribution of elements in SBCN were demonstrated by EDS. As shown in Fig. 1c-f, four elements, C, N, S and B, are all checked and their distributions are well uniform in SBCN sample. This result proves these heteroatoms of S and B elements were successfully doped into the SBCN. The TEM image of SBCN (Fig. 1g) presents a 2D lamellar morphology, and the low contrast indicates its thin thickness. Some dark zones in the image should be assigned to the edge corrugation of these ultrathin sheets in SBCN. Fig. 2a shows XRD patterns of the as-prepared SBCN, as well as CN, SCN and BCN samples. Two distinct diffraction peaks at ∼13.0° and ∼27.4° are indexed as (100) and (002) planes, respectively, corresponding to the in-plane structural packing motif and interlayer stacking of aromatic segments [38–41]. After heteroatoms were introduced, the intensity of these peaks apparently decreases in the SCN and BCN samples. And, these diffractions of SBCN further weakens, which is in accordance with the thin thickness of SBCN observed in SEM and TEM images. However, from the enlarged XRD pattern of SBCN (inset of Fig. 2a) we can still find the characteristic diffractions of graphitic carbon nitride, indicating that the heteroatom dopants do not influence the crystal structure of CN. Fig. 2b depicts the FT-IR spectra of pristine CN and all the heteroatom doped CN samples. Some typical
Fig. 2. (a) XRD patterns, (b) FTIR spectra, (c) UV–vis absorption spectra and (d) plots of (αhv)1/2 vs. hv of CN, SCN, BCN and SBCN. 479
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
Fig. 3. XPS spectra of SBCN: (a) survey; (b) C 1 s, (c) N 1 s, (d) O 1 s, (e) S 2p and (f) B 1 s.
hole pairs.
[16]. PL emission spectroscopy is an effective route to investigate the separation efficiency of photo-generated charges. Before diffusing to the surface, most photo-generated charges of CN recombine, resulting in an intensive PL emission (Fig. 4a). It is apparent that these heteroatom doped samples have PL spectra alike to CN, but the intensity drastically decreases under the 340 nm excitation. This result suggests that these SCN, BCN and SBCN samples have similar electronic structure to CN, whereas the separation of photo-excited electron-hole pairs has been significantly promoted. Among these heteroatom doped samples, the SBCN has the lowest intensity of PL emission. It implies that the SBCN sample possesses the most sufficient separation efficiency of photo-induced carriers. The migration ability of charges in semiconductor photocatalysts can be well described by their electrochemical impedance spectra (EIS). Fig. 4b displays Nyquist plots of EIS for CN, SCN, BCN and SBCN samples. The arc radius represents the electric resistance of charge transfer. Apparently, the SBCN presents the smallest radius in all four samples, indicative of its lowest resistance for photo-generated carrier transport. The transient photocurrent responses of samples were measured during repeated ON/OFF illumination cycles (Fig. 4c). As the lamp each was initiated, all the four samples exhibited prompt and reproducible photocurrent responses. The photocurrent rapidly dropped to almost zero as turning off the lamp and reverted once the lamp was turn on again. As shown in Fig. 4c, the SBCN has the largest photocurrent response among these four photocatalysts, confirming it has the most sufficient separation yield of photo-generated electron-
3.2. Visible light photocatalytic performance of S, B co-doped g-C3N4 The photocatalytic activity of SBCN was estimated by degradation of RhB under visible light irradiation. Fig. 5a shows the change for absorption spectra of RhB aqueous solution under visible light irradiation with the presence of SBCN sample. The characteristic absorption of RhB at 554 nm does not decrease from the adsorption time of 20 min to 60 min, suggesting the adsorption-desorption equilibrium of RhB has reached. After turning on the lamp, this absorption decreases greatly with the increase of irradiation time and almost disappears after 80 min. Besides this characteristic absorption, some absorption bands in UV region also exhibit a declining trend. It can be ascribed to the decomposition of aromatic rings to small molecules. The photocatalytic performance of pristine CN and various heteroatom doped CN samples is compared in the Fig. 5b. RhB is very stable, and almost no RhB is decomposed without the presence of any photocatalysts after 120 min irradiation. The pristine CN gives a decomposition ratio of ∼50%, and the heteroatom doped SCN and BCN depict a similar photocatalytic degradation ratio of ∼ 80% after 120 min visible light illumination. In contrast, the SBCN sample illustrates an efficient degradation behavior that can decompose almost 100% RhB within 80 min. To better understand the photocatalytic behavior, the photocatalytic kinetic study was estimated with the following equation: -ln(C/C0) = kt (Fig. 5c). The calculated rate constant (k) of SBCN is 0.0495 min−1 that is 6.5 times higher than that of the pristine CN.
Fig. 4. (a) PL spectra, (b) Nyquist plots of EIS and (c) photocurrent responses of CN, SCN, BCN and SBCN. 480
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
Fig. 5. (a) UV–vis absorption spectral changes during the photocatalytic degradation of RhB in SBCN; (b) Comparison of visible light photocatalytic performances of CN, SCN, BCN and SBCN for degrading RhB and (c) their pseudo-first-order kinetics fittings; (d) the cycle behaviors of SBCN.
Fig. 6. Mott-Schottky curves of (a) the pristine CN and (b) the SBCN; (c) schematic illustration for band structure of CN and SBCN; (d) the photocatalytic activity of SBCN for degrading RhB with the presence of various scavengers.
photocatalytic performance of SBCN with the pristine CN for removal of various organic dyes, including methyl blue (MB), Congo red and reactive violet under visible light irradiation (Fig. S1), where the SBCN presents better photocatalytic behavior than the pristine CN.
The stability and reusability of photocatalysts is vital for their practical application. We evaluated the recycling property of SBCN material by directly adding concentrated dye solution to the mixture after the last photocatalytic test. As shown in Fig. 5d, the successive five cycle experiments exhibit similar photocatalytic performance, indicating its superior stability and reusability. We also compared the 481
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
Fig. 7. The possible photocatalytic mechanism of SBCN under visible light irradiation.
However, they can directly oxidize RhB because of the relatively small oxidizing potential of RhB (0.97 V vs. NHE) [42]. On the other hand, the photo-excited electrons on CB can reduce the adsorbed O2 to form % O2− radicals that can further react with water to generate ∙OH radicals. These %O2− and %OH radicals have strong oxidation ability to decompose organic dyes to small molecules.
3.3. Possible photocatalytic mechanism of S, B co-doped g-C3N4 For understanding the photocatalytic mechanism of SBCN, we employed the Mott-Schottky (MS) electrochemical technology to determine the semiconductive behavior and the flat band potential (Efb) of these samples. Therefore, according to the band gap values based on the result from UV–vis spectra, we can illustrate their band alignment. As revealed in Fig. 6a, the pristine CN shows a MS curve with a positive slope, implying its n-type semiconductive behavior. It is in accordance with CN materials in published literatures [40,41]. After doping with heteroatoms, the n-type behavior is remained in SBCN sample, as seen in Fig. 6b. The intercept at the x axis by drawing the tangent of MS curve is the Efb value that is regarded as the Fermi level (Ef) of semiconductors. In general, the Ef closes to the conduction band (CB) in ntype semiconductor and is near to the valence band (VB) of p-type semiconductors. Therefore, from their MS curves we can estimate the edge potential of CB in CN and SBCN to be -1.28 and -0.77 vs. NHE, respectively. Furthermore, the VB edge potential is calculated following the below formula: Eg = EVB – ECB. And thus the band structures are illustrated in Fig. 6c. To determine the main active species in this photocatalytic process, we employed various scavengers to quench different active species. Herein, tertiary butanol (TBA) is a hydroxyl radical (%OH) quencher, benzoquinone (BQ) is a superoxide radical (%O2−) scavenger and disodium ethylenediaminetetraacetate (EDTA-2Na) is a hole (h+) scavenger. As shown in Fig. 6d, when TBA and BQ are added, the photodegradation efficiency for RhB is significantly suppressed. However, the degradation efficiency of RhB is not suppressed but somewhat improved by adding EDTA-2Na. That means %O2− and %OH should be the main active species in photocatalytic process of SBCN for degradation of organic dyes under visible light irradiation. The enhancement of photocatalytic activity by quenching h+ with EDTA-2Na may be ascribed to that the recombination of photo-generated charges is significantly inhibited. Based on the above results, we propose the possible photocatalytic mechanism for the degradation of organic dyes with SBCN, as depicted in Fig. 7. Under visible light illumination, the electrons on VB are excited and cross the band gap to the CB of SBCN, meanwhile leaving equivalent holes on VB. Because of the thinner sheet-like morphology, much more electrons and holes can transfer to the surface of SBCN before recombination in comparison to the bulky CN. Considering the redox potential of %OH/OH− (2.7 V vs. NHE), the photo-induced holes on VB (1.71 V vs. NHE) cannot oxidize water to produce %OH radicals.
4. Conclusion In summary, we have successfully synthesis an S, B co-doped g-C3N4 material during a facile and similar method to prepare the pristine gC3N4. The co-doped heteroatoms effectively change the intrinsic electronic and band structure of g-C3N4, which results in the sufficient light absorption, the low internal resistance and the high separation of photo-generated charges. Consequently, this SBCN shows much higher activity than the pristine g-C3N4, SCN and BCN samples for removal of organic pollutants. So far, several nonmetal elements co-doping systems have been developed, and all of them present enhanced photocatalytic performance than single element doped materials. Based on these results, the modification of g-C3N4 by multi-nonmetal element co-doping is an efficient strategy to promote the photocatalytic activity of g-C3N4. In the further investigation, we will continue to study the controllable synthesis of other co-doping systems and explore their photocatalytic behavior for other photocatalytic applications. Acknowledgements This work was financially supported from the financial support from the Zhejiang Provincial Natural Science Foundation of China (No. LY17B010004, LY17B050007) and the 521 talent project of ZSTU. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
482
A. Kubacka, M. Fernandez-García, G. Colon, Chem. Rev. 112 (2012) 1555–1614. M. Chong, B. Jin, C.W.K. Chow, C. Saint, Water Res. 44 (2010) 2997–3027. P. Zhou, J. Yu, M. Jaroniec, Adv. Mater. 26 (2014) 4920–4935. J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S. Lee, J. Zhong, Z. Kang, Science 347 (2015) 970–974. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. A. Jorge, D. Martin, M.T.S. Dhanoa, A. Rahman, N. Makwana, J. Tang, A. Sella, F. Cora, S. Firth, J. Darrand, P. McMillan, J. Phys. Chem. C 117 (2013) 7178–7185. J. Zhang, F. Guo, X. Wang, Adv. Funct. Mater. 23 (2013) 3008–3014. Y. Wang, X. Wang, M. Antonietti, Angew. Chem. Int. Ed. 51 (2012) 68–89. W. Ong, L. Tan, Y. Ng, S. Yong, S. Chai, Chem. Rev. 116 (2016) 7159–7329. J. Xu, Y. Li, S. Peng, G. Lu, S. Li, Phys. Chem. Chem. Phys. 15 (2013) 7657–7665.
Materials Research Bulletin 107 (2018) 477–483
X. Han et al.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
[28] [29] [30] [31] [32]
S. Yan, Z. Li, Z. Zou, Langmuir 25 (2009) 10397–10401. D. Chen, J. Yang, H. Din, Appl. Surf. Sci. 391 (2017) 384–391. J. Wen, J. Xie, X. Chen, X. Li, Appl. Surf. Sci. 391 (2017) 72–123. G. Dong, K. Zhao, L. Zhang, Chem. Commun. 48 (2012) 6178–6180. J. Fang, H. Fan, M. Li, C. Long, J. Mater. Chem. A 3 (2015) 13819–13826. S. Yan, Z. Li, Z. Zou, Langmuir 26 (2010) 3894–3901. D. Gao, Y. Liu, P. Liu, M. Si, D. Xue, Sci. Rep. 6 (2016) 35768–35774. N. Sagara, S. Kamimura, T. Tsubota, T. Ohno, Appl. Catal. B 192 (2016) 193–198. Y. Zhang, T. Mori, J. Ye, M. Antonietti, J. Am. Chem. Soc. 132 (2010) 6294–6295. T. Ma, J. Ran, S. Dai, M. Jaroniec, S. Qiao, Angew. Chem. Int. Ed. 54 (2015) 4646–4650. S. Hu, L. Ma, J. You, F. Li, Z. Fan, F. Wang, D. Liu, J. Gui, RSC Adv. 4 (2014) 21657–21663. Y. Zhou, L. Zhang, J. Liu, X. Fan, B. Wang, M. Wang, W. Ren, J. Wang, M. Li, J. Shi, J. Mater. Chem. A 3 (2015) 3862–3867. L. Zhang, X. Chen, J. Guan, Y. Jiang, T. Hou, X. Mu, Mater. Res. Bull. 48 (2013) 3485–3491. J. Hong, X. Xia, Y. Wang, R. Xu, J. Mater. Chem. 22 (2012) 15006–15012. L. Ye, S. Chen, Appl. Surf. Sci. 389 (2016) 1076–1083. G. Liu, P. Niu, C. Sun, S. Smith, Z. Chen, G. Lu, H. Cheng, J. Am. Chem. Soc. 132 (2010) 11642–11648. L. Cao, R. Wang, D. Wang, Mater. Lett. 149 (2015) 50–53.
[33] [34] [35] [36] [37] [38] [39] [40] [41] [42]
483
J. Chen, Z. Hong, Y. Chen, B. Lin, B. Gao, Mater. Lett. 145 (2015) 129–132. M. Seredych, T. Bandosz, Carbon 107 (2016) 895–906. S. Stolbov, S. Zuluaga, J. Phys.: Condens. Matter 25 (2013) 0855077pp. L. Ge, C. Han, X. Xiao, L. Guo, Y. Li, Mater. Res. Bull. 48 (2013) 3919–3925. L. Li, W. Fang, P. Zhang, J. Bi, Y. He, J. Wang, W. Su, J. Mater. Chem. A 4 (2016) 12402–12406. Z. Huang, J. Song, L. Pan, Z. Wang, X. Zhang, J. Zou, W. Mi, X. Zhang, L. Wang, Nano Energy 12 (2015) 646–656. J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, Chem. Commun. 48 (2012) 12017–12019. H. Ma, Y. Li, S. Li, N. Liu, Appl. Surf. Sci. 357 (2015) 131–138. R. You, H. Dou, L. Chen, S. Zheng, Y. Zhang, RSC Adv. 7 (2017) 15842–15850. S. Hu, L. Ma, Y. Xie, F. Li, Z. Fan, F. Wang, Q. Wang, Y. Wang, X. Kang, G. Wu, Dalton Trans. 44 (2015) 20889–20897. F. Cheng, J. Yan, C. Zhou, B. Chen, P. Li, Z. Chen, X. Dong, J. Colloid Interf. Sci. 468 (2016) 103–109. J. Yan, C. Zhou, P. Li, B. Chen, S. Zhang, F. Xi, X. Dong, J. Liu, Colloids Surf. A 508 (2016) 257–264. J. Yan, X. Han, X. Zheng, J. Qian, J. Liu, X. Dong, F. Xi, Mater. Res. Bull. 94 (2017) 423–427. J. Yan, X. Han, J. Qian, J. Liu, X. Dong, F. Xi, J. Mater. Sci. 52 (2017) 13091–13102. T. Takizawa, T. Watanabe, K. Honda, J. Phys. Chem. 82 (1978) 1391–1396.