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Facile synthesis of carbon self-doped g-C3N4 for enhanced photocatalytic hydrogen evolution
Journal Pre-proof Facile synthesis of carbon self-doped g-C3N4 for enhanced photocatalytic hydrogen evolution Jingsheng Cao, Huiqing Fan, Chao Wang, Jiangwei Ma, Guangzhi Dong, Mingchang Zhang PII:
S0272-8842(19)33485-6
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.008
Reference:
CERI 23661
To appear in:
Ceramics International
Received Date: 7 November 2019 Revised Date:
29 November 2019
Accepted Date: 2 December 2019
Please cite this article as: J. Cao, H. Fan, C. Wang, J. Ma, G. Dong, M. Zhang, Facile synthesis of carbon self-doped g-C3N4 for enhanced photocatalytic hydrogen evolution, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.008. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Facile synthesis of carbon self-doped g-C3N4 for enhanced photocatalytic hydrogen evolution
Jingsheng Cao, Huiqing Fan*, Chao Wang, Jiangwei Ma, Guangzhi Dong, Mingchang Zhang
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
*Corresponding author, E-mail: [email protected]
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Abstract: Graphite carbon nitride (g-C3N4) is an appealing metal-free photocatalyst for hydrogen evolution, but the potential has been limited by its poor visible-light absorption and unsatisfactory separation of photo-induced carriers. Herein, a facile one-pot strategy to fabricate carbon self-doped g-C3N4 composite through the calcination of dicyanamide and trace amounts of dimethylformamide is presented. The as-obtained carbon self-doped catalyst is investigated by X-ray photoelectron spectroscopy (XPS), confirming the substitution of carbon atoms in original sites of bridging nitrogen. We demonstrate that the as-prepared materials display remarkably improved visible-light absorption and optimized electronic structure under the premise of principally maintaining the tri-s-triazine based crystal framework and surface properties. Furthermore, the carbon doped g-C3N4 composite simultaneously weakens the transportation barrier of charge carriers, suppresses charge recombination and raises the separated efficiency of photoinduced holes and electrons on account of the extension of pi conjugated system. As a result, carbon self-doped g-C3N4 exhibits 4.3 times greater photocurrent density and 5.2 times higher hydrogen evolution rate compared with its bulk counterpart under visible light irradiation.
Keywords: Graphite carbon nitride, Carbon doping, Hydrogen evolution, Photocatalysis, Photocurrent.
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1. Introduction Nowadays, semiconductor based photocatalysis has attracted widespread attention because of its high performance in utilizing the clean and exhaustible solar energy, which is considered as a green and sustainable route for addressing the crisis of environment and energy in future [1]. Among various photocatalysts, graphitic carbon nitride has aroused great enthusiasm in academic communities, which was first reported for photocatalytic water splitting by Wang [2]. As a conjugated polymeric semiconductor, g-C3N4 owns promising prospects of photocatalysis due to its metal free nature, easy synthesis, appealing optical and electronic properties, and superior stability of thermal and chemistry. Unfortunately, the photocatalysis efficiency of pristine g-C3N4 is still restricted on account of its limited specific surface area, weak absorption of visible-light and high recombination rate of photoexcited carriers. To address these drawbacks, many approaches have been developed, such as increasing the surface area and active sites of g-C3N4 by pre-polymerizing melamine [3], in situ intercalating lithium chloride ions [4], or construction of nanostructure in morphologies [5-11], and improving carriers utilization by doped with foreign elements [12-18] or formation of g-C3N4 based heterostructures [19-28]. Carbonaceous materials have been widely applied in functional nanomaterials fields because of their good compatibility [29-30]. Recently, researchers have proposed that the hybrids between g-C3N4 and carbonaceous materials can improve the absorption of visible-light and promote the efficiency of photoinduced charge separation. Che et al. designed hetero-structured carbon ring (Cring)-C3N4 with extended in-plane pi 3
conjugated electric field, the lifetime and diffusion length of photocarrier were significantly increased up to 10 times [31]. Yu et al. incorporated all-carbon aromatic rings with g-C3N4 to optimize electronic property and photocatalytic activity [32]. He et al. constructed composites of g-PAN and g-C3N4 for improvement of photocatalytic performance and the nanosheets of g-PAN acted as efficient transport channels to facilitate photogenerated carrier separation [33]. Wang et al. implanted carbon quantum dot into g-C3N4 nanotubes through the copolymerization of carbon quantum dots (CQDs) precursor and urea, which exhibited an obvious improved quantum yield of 10.94% at 420 nm [34]. Obviously, constructing composition between carbon source and g-C3N4 is an effective method to optimize intrinsic electronic structure and enhance catalytic performance. As a widely acknowledged way of improving catalysts’ performance, self-doping has an obvious advantage of altering the electronic structure without the introduction of any heteroatoms [35-36], which might act as centers of charge carriers’ recombination [37-38]. Dong et al. first employ density functional theory (DFT) to calculate that carbon self-doping in g-C3N4 resulted in the formation of big delocalized pi bonds for the reason that bridging nitrogen atoms can be replaced by carbon atoms, which is beneficial for photocatalysis on account of enhanced charge carrier mobility and conductivity [39]. This is further confirmed by several reports about successfully synthesis of carbon self-doped g-C3N4 [39-41]. Depended on the pervious report, we note that it worth studying the mechanism attribution for carbon doped g-C3N4 in photocatalysis performance. 4
Herein, we develop a facile one-pot approach of synthesizing carbon self-doped g-C3N4 through calcining the hybrid between dicyanamide and small amount of dimethylformamide, in which dimethylformamide acts as the cost-effective carbon source. The crystal structures, morphologies, electronic structures and optical properties are detailedly investigated and the exact doping position of carbon is determined. The photocatalytic properties of hydrogen evolution in carbon doped g-C3N4 are evaluated under visible-light irradiation. The resultant photocatalyst shows remarkably improved photocatalysis activity and the mechanism therein is intensive investigated.
2. Experimental 2.1. Materials preparation In a typical synthetic procedure, 4 g dicyanamide and a certain volume of dimethylformamide (0mL, 0.5 mL) were full mixed and calcined in an alumina crucible at 550 ºC for 2 h in air and the ramping rate was 3 ºC per minute. After cooling down products to room temperature and grounding them into fine powders, the samples were finally obtained and donated as CN and CD-CN, respectively (CN is the pristine g-C3N4 and CD-CN is C-doped g-C3N4). Chemicals involved in this experiment are used at analytical grade as received. 2.2. Characterization A powder X-ray diffractometer (X'pert, Philips, Eindhoven, The Netherlands) was employed to record X-Ray diffraction (XRD) patterns ranging from 5 º-90 º using Cu Kα radiation. Fourier transform infrared spectra was examined on a Fourier transform infrared spectroscope (TENSOR27, Bruker, Billerica, MA, USA). The scanning 5
electron microscopy (JSM-6701F, JEOL, Tokyo, Japan) was used for obtaining the surface morphologies (SEM) images. Nitrogen adsorption-desorption measurements were conducted for the BET specific surface area (TriStar ΙΙ 3020, Micromeritics, GA, USA) at 77 K and the Barrett-Joyner-Halenda (BJH) method was used for calculation of distribution of pore size. X-ray photoelectron spectroscopy spectra (XPS) was measured for chemical state analysis (Axis Supra, Kratos Analytical, Manchester, UK). The element analyzer (Vario EL cub, Elementar, Langenselbold, Germany) was used for the measurement of elemental proportion of C, N, O, H. Diffuse reflection spectroscopy (DRS) was tested using BaSO4 as reference by a spectrophotometer (UV3150; Shimadzu Corporation, Kyoto, Japan). The spectrophotometer (FLS980, Edinburgh Instruments, Edinburgh, UK) was employed in measuring photoluminescence (PL) spectra and time-resolved decay fluorescence spectra at room temperature and the excitation wavelength was 340 nm. 2.3. Photoelectrochemical measurements Electrochemical test was carried out with an electrochemical workstation (CHI660E, Chenhua, Shanghai, China) and the measurement was conducted in a standard three-electrode system where a platinum plate, a saturated Ag/AgCl electrode and an indium-tin oxide (ITO) conductive film glass electrode deposited with g-C3N4 based samples are employed as the counter electrode, the reference electrode and the working electrode, respectively. For preparation of the working electrode, an ITO glass (0.8 × 0.8 cm2) was coated by CN and CD-CN based viscous slurry. The slurry was prepared by fully mixing ethyl cellulose (3 mg), g-C3N4 (5 mg), terpineol (40 mg) and 6
lauric acid (2 mg) [3]. Subsequently the ITO glass with coated slurry was heated at 350 ºC for 0.5 h for enhanced adhesion. 0.5 M Na2SO4 aqueous solution was employed as the electrolyte. The plots of Mott-Schottky (M-S) were recorded ranging from -0.2 V to 0.7 V at 200 Hz. In particular, flat-band potential was calculated depended on the below M-S relation [42-44]:
=
−
−
(1)
where Csc is the space charge region capacitance, Nd is the charge carriers density, ε0 is the free space permittivity, E is the tested potential, ε is the material’s dielectric constant and EFB is the flat band potential which is the approximate value of conduction band potential (CB). Electrochemistry impedance spectroscopy (EIS) was determined at 0.5 V amplitude and frequency range was from 1000 kHz to 0.01 Hz. The photocurrent spectra were measured without bias voltage and the interval time of light on and off was 50 s. 2.4. Evaluation of the photocatalytic activity The photocatalysis activity of catalysts was evaluated by hydrogen generation under the irradiation of visible-light (λ > 400 nm). The irradiation source was obtained by a 300 W Xe lamp (MAX-302, USA) with a cut off filter of 400 nm. 50 mg photocatalyst was fully dispersed in 100 mL aqueous solution containing 10 vol% triethanolamine (TEOA) and 1 wt% Pt using H2PtCl6 as the precursor, which was on the surface of catalyst because of in situ photo-deposited. Cooling water circulation system was utilized to keep the temperature of reaction solution at 10 ºC. Before irradiation, the reaction system was bubbled several times to entirely remove air. The generated 7
hydrogen was analyzed by a gas chromatography device which was equipped with thermal conductive detector (TCD) and nitrogen acted as the carrier gas.
3. Results and discussion Figure 1A displays the XRD patterns of as-synthesized samples. Despite the introduction of dimethylformamide in the synthesis of CD-CN, XRD patterns show identical peaks of CN and CD-CN at 13.0º and 27.4º, which confirms that the intrinsic layer graphitic structure is preserved. The diffraction peaks at 13.0º and 27.4º refer to (100) and (002) plane which corresponds to the in-plane repeat units of tri-s-triazine and the stacking interlayers of conjugated aromatic structures, respectively. As shown in the insert, the (002) plane full width at half maximum (FWHM) in CD-CN decrease in small quantity compared with CN, indicating the crystallinity of CD-CN are slightly improved. No obvious shift of peaks is observed, revealing the interlamellar spacing of two samples are same. The above results indicate that low dosage of dimethylformamide introduced could hardly disturb the molecular network of carbon nitride. FT-IR spectroscopy is further carried out to characterize functional groups in CN and CD-CN. As shown in Figure 1B, the intense peak in 806 cm-1 originates from heptazine units’ out-of-plane bending and the typical skeletal stretching vibrations of tri-s-triazine heterocyclic (C6N7) rings appear over 1200-1650 cm-1.The broad band centered at 2800 to 3400 cm-1 is ascribed to the N-H (N-H2) stretching vibrations. Not only the XRD patterns but also FT-IR spectra in CD-CN display all the characteristic peaks of pristine g-C3N4, manifesting the original framework of carbon nitride remains unchanged after the introduction of dimethylformamide during calcination. 8
The contents of C, N, O, H are further determined by elemental analysis in table 1. The results reveal a C/N mass ratio of 0.61 for CD-CN which is higher than that for CN (0.59), confirming the success in carbon incorporation. Trace amount of O is detected while the quantity of O element is nearly the same in two samples, and the origin is comprehensive studied in the XPS analysis. The observed residual content of hydrogen emanates from the partial condensation of tri-s-triazine units, which is consistent with the broad bands in FT-IR spectra. To study element’s chemical states and the exact position of doped carbon, XPS was conducted. In Figure S2, the XPS survey spectrum displays two intense peaks referring to C 1s and N 1s, indicating that CN and CD-CN are mainly composed of carbon and nitrogen. The weak peaks (ca. 530 eV) corresponding to O 1s are nearly same in two patterns and no obvious difference has been found between them in the high resolution spectra in the insert. This is probably for the reason that the oxygen in dimethylformamide has been released during the calcining, and the signal in O 1s belongs to the absorbed H2O molecules on the surface of samples [45-46]. The high resolution C 1s spectra is shown in Figure 2A, there are two conspicuous peaks which are located at ca. 284.6 and 288.0 eV in CN and CD-CN, representing adventitious graphitic carbon (C=C) used as a calibration reference and sp2-hybridized carbon (N=C-N) in the network of graphitic carbon nitride, respectively. A slight increase of 0.1 eV in the peak attributed to sp2-hybridized carbon in CD-CN indicates that the interaction between carbon source and g-C3N4 has induced change in chemical states after the incorporation of doped carbon [40,47]. The deconvolution of N 1s has four peaks centered at 398.5, 399.7, 401.0 and 404.1 eV. The hybridized sp2 9
aromatic N (C-N=C, 398.5 eV) in the triazine units displays the dominant signal peak in both samples. The peaks at binding energy of 399.7 eV and 401.0 eV are ascribed to bridging nitrogen atoms (N-(C)3) and amino groups carrying hydrogen (NH bonds or NH2 bonds) respectively. The weakest peaks (404.1 eV) in CN and CD-CN are associated with the charging effects or positive charge localization of cyano-groups and heterocycles. Figure 2C shows the percentage of different N atoms above calculated by peaks integrated. Compared with CN, the content of bridging N in CD-CN decrease from 17.28% to 16.95%, implying that the bridging N is replaced by carbon atoms. The reduce of NH groups in CD-CN result from a higher degree of crystallinity, which has been confirmed in XRD patterns. In addition, accompanied with the replacement of carbon in original sites of bridging N, the percentage composition of C-N=C groups have increased from 70.75% to 71.17%, which benefits the construction of pi conjugated system. From the analysis above, we confirm that carbon doped g-C3N4 has been successfully manufactured and propose the doping path of carbon atoms in Figure 2D, where the bridging N atoms are substituted by C atoms. In consideration of homogeneous substitution in lattice nitrogen by carbon will alter the hybridization type, the delocalized big pi bonds have been formed among the carbon substituted tri-s-triazine units [39], which contributes to prompting electrical conductivity of CD-CN. However, it is difficult to distinguish C-doped carbon nitride from the g-C3N4 matrix based on XRD patterns and FT-IR spectra as a result of the ultra-small amount of dimethylformamide and the similar properties of CN and CD-CN. 10
The morphology of CN and CD-CN are displayed in Figure 3A and 3B. The surface morphologic structure of two samples have no obvious difference between each other and they are composed of dense thick layers and bulk agglomerates, which is consistent with the typical graphitic carbon nitride. Figure S2A shows the nitrogen adsorption-desorption isotherms, Figure S2B demonstrates the BJH pore-size distribution curves of CN and CD-CN, and the porous characteristics calculated through above measurement are displayed in Table S1. Figure S2A illustrates that all samples are of type IV isotherms, implying the mesopores’ existence. This is further confirmed by the pore size distribution of both samples (Figure S2B), which exhibits presence the of mesopores (2~3nm) in CN and CD-CN. As displayed in Table S1, the BET surface area as well as total pore volume is slightly increased in CD-CN when compared with CN, while the average pore size in CD-CN are less than CN. This is beneficial to photocatalytic reaction for the reason that the large area in surface and small diameter in pores size would provide more abundant reaction sites [3]. In consideration of the tiny enhancement of porous characteristics in CD-CN, we regard this part as the secondary factor accounting for its enhanced photocatalysis performance.
The interaction between doped carbon atoms and g-C3N4 networks will undoubtedly give rise to some differences in its optical properties and electronic band structure. As shown in the UV-vis diffuse reflection spectra (Fig. 4A), both samples display a typical semiconductor intrinsic absorption in the blue region, while the absorption intensity of CD-CN in the region of visible-light are remarkably enhanced compared to CN. This could be reflected in their digital images, in which CD-CN 11
exhibits darker color. In addition, the intrinsic absorption edge of CD-CN shows a noticeable red shift with respect to CN. As shown in Fig. 4B, the bandgaps of CN and CD-CN calculated by corresponding Kubelka-Munk plots are 2.72 eV and 2.66 eV respectively, indicating the narrowed bandgap (0.06 eV) of CD-CN. This phenomenon is because of the quantum confinement effects and the increase of the conjugation length [48], which is consistent with the outcome of carbon substitution. The extended pi-conjugation and narrow band gap would benefit electron hole pairs separation and photoelectron transferring [34]. The electrochemical M-S plots in Figure 4C for two samples display typical n-type characteristic and the determined conduction bands (CB) of CN and CD-CN are -1.21 V and -0.94 V (vs. Ag/AgCl hydrogen electrode) respectively. The valence band (VB) in CD-CN (1.92 eV) is determined a shift of 0.21 eV than CN (1.71 eV) in Figure S3, demonstrating the variation of the electronic structure. Hence the schematic band structure of CN and CD-CN is shown in Fig. 4D. It is concluded that the thermodynamic photocatalytic condition about hydrogen evolution produced by water splitting can be satisfied by both samples, and the CB of CD-CN is more positive than CN, which reduces the thermionic emission induced by high potential barrier height of the photocatalyst-Pt contact [49]. As mentioned above, optimizing electronic structure leads to great alteration in charge carrier transfer and separation. As exhibited in Fig. 4E, PL spectrum originates from the recombination of holes at VB and electrons at CB. And CD-CN has a more intense intensity of PL emission with respect to CN, manifesting that the introduction of doped carbon in networks of g-C3N4 can act as an electron buffer which effectively 12
expedite transport of excited electrons and delocalization of pi-electron in the conjugated system, accordingly raising the separation efficiency of photo-excited holes and electrons [32]. This is supported by the results calculated in time-resolved spectroscopy (Fig. 4F). Both samples decay exponentially in fluorescent intensity, while CD-CN displays slow decay kinetics. To further analyze the decay curves, the average lifetimes of photo-induced carriers in CN and CD-CN are calculated by bi-exponential function fitting, and the results are listed in Table 2. In detail, the shortest and longest lifetime of CN and CD-CN are 1.39 ns (67.31%), 5.61ns (32.69%) and 1.35 ns (62.82%), 5.49 ns (37.18%) respectively, demonstrating that the content of longest lifetime in CD-CN has a distinct increase compared to CN. The average fluorescent lifetime (τ) was deduced by the following equation [50]:
=
(2)
We can find that compared to the average fluorescent lifetime of CN (4.18 ns), CD-CN possess a prolonged lifetime of 4.27 ns. The improved lifetime is related to the enhanced electron transport resulted from the decreased electronic transmission resistance. What’s more, a longer lifetime means that the photogenerated holes and electrons have a more probability to be hunt by reactive catalyst and utilized in the photo-redox reaction. To further explore the separation and transport performance of photoinduced charge, electrochemical impedance spectroscopy (EIS) and transient photocurrents are measured. EIS Nyquist plots of CN and CD-CN electrodes in same illumination environment are shown in Figure 5A. CD-CN displays a smaller arc radius when 13
compared to CN, revealing that photoinduced hole and electron pairs in CD-CN encounter a decreased resistance in process of transfer and can be separated effectively. In Figure 5B, we can see that the current density with no light irradiation in CD-CN electrode display a higher baseline than CN electrode, indicating that not only the amount but also transport ability of intrinsic charge carriers has been improved after the formation of delocalized big pi bonds in CD-CN. Moreover, all electrodes show rapid and repeatable transient photocurrent responses, while CD-CN has a higher photocurrent density of 6.4 µAcm-2 which is 4.3 times that of CN (1.5 µAcm-2). The enhancement of photocurrent in CD-CN further confirms that the great electronic transportation competence of carbon doped g-C3N4. Moreover, the increase absorption in visible-light of CD-CN also account for the enhanced photocurrent, in line with UV-vis diffuse reflectance spectra. The photocatalysis ability of prepared samples was evaluated by production of H2 in the irradiation of visible light. In figure 5C, along with the irradiation time increasing, hydrogen evolution of the as-synthesized photocatalyst is in linear growth process. The 8 hours activity of hydrogen production in CN is only 6.78 µmol because of its high recombination rate of photoexcited electrons and holes and limited visible-light responsive competence. After incorporated with doped carbons, the carbon nitride has a dramatically increase on the hydrogen evolution to 35.5 µmol in 8 hours. The inset histogram shows that the hydrogen evolution rate in CD-CN (8.88 µmol/h) is 5.2 times that of CN (1.70 µmol/h), which is ascribed to accelerated separation of photoexcited carrier and high use efficiency of visible light. Furthermore, the durability of CD-CN in 14
hydrogen evolution is measured and Figure 5D displays the result. The hydrogen evolution activity of CD-CN is maintained without evident deactivation after four experimental cycles for 12 h, manifesting the stability of catalysts. Depended on the analysis above, we propose the enhanced photocatalytic mechanism of CD-CN in Figure 6. Firstly, compared with CN, the C-doped carbon nitride has a narrower bandgap and stronger absorbing ability of visible light, providing more separated photoinduced electrons at conduction band (CB) and holes at valence band (VB) under visible-light. Secondly, the interaction between substituted carbon atoms and graphite carbon nitride network result in the creation of delocalized big pi bonds, which can optimize the electronic structure of CD-CN, reducing the barrier of photocarrier transfer and enhancing the electron transportation, as verified by PL spectra and EIS analysis. By contrast, pristine graphite carbon nitride possesses plenty of bridging nitrogen atoms which are unable to form conjugation with ambient carbon atoms, bringing about inconsecutive and small area of pi conjugated system in CN. Thus, there will be more and faster transfer electrons in CD-CN participating in the reaction of hydrogen evolution than CN. Consequently, the carbon doped g-C3N4 has a drastically improved photocatalytic activity.
4. Conclusion To sum up, we have successfully synthesized carbon doped g-C3N4 composite through a facile, green and low-cost approach, which introduces slight amount of dimethylformamide into the matrix of g-C3N4 and primarily maintains the tri-s-triazine based crystal structure. It has been demonstrated in experimental terms that the 15
self-doped carbon could induce the variation of intrinsic electronic band structure via the substitution of bridging N and the consequential extension of pi conjugated system, leading to a narrowed bandgap and significantly enhanced visible light absorption. Furthermore, the photo-excited charge carriers of modified sample are greatly improved in separation and transfer, thus dramatically enhancing the photocatalytic efficiency for hydrogen evolution. This work may provide a new promising method of developing carbon doped g-C3N4-based photocatalysts in the field of harnessing solar energy.
Acknowledgement This work is supported by the National Defense Science Foundation (32102060303), the National Nature Science Foundation (51672220), the SKLSP Project (2019-TZ-04) of China, and the Fundamental Research Funds for the Central Universities of NPU (3102019GHXM002). Here we also express appreciation for the Analytical & Testing Center of Northwestern Polytechnical University.
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g-C3N4-modified CdS heterostructure with enhanced photocatalytic activity, Applied Surface Science, 358 (2015) 385-392. [28] M.W. Kadi, R.M. Mohamed, Increasing visible light water splitting efficiency through synthesis route and charge separation in measoporous g-C3N4 decorated with WO3 nanoparticles, Ceram Int, 45 (2019) 3886-3893. [29] L. Song, C. Guo, T. Li, S. Zhang, C60/graphene/g-C3N4 composite photocatalyst and mutually- reinforcing synergy to improve hydrogen production in splitting water under visible light radiation, Ceram Int, 43 (2017) 7901-7907. [30] Wang S, Yang H, Yi Z, et al. Enhanced photocatalytic performance by hybridization of Bi2WO6 nanoparticles with honeycomb-like porous carbon skeleton[J]. Journal of environmental management, 2019, 248: 109341. [31] W. Che, W.R. Cheng, T. Yao, F.M. Tang, W. Liu, H. Su, Y.Y. Huang, Q.H. Liu, J.K. Liu, F.C. Hu, Z.Y. Pan, Z.H. Sun, S.Q. Wei, Fast Photoelectron Transfer in (C-ring)-C3N4 Plane Heterostructural Nanosheets for Overall Water Splitting, Journal of the American Chemical Society, 139 (2017) 3021-3026. [32] Y. Yu, W. Yan, W. Gao, P. Li, X. Wang, S. Wu, W. Song, K. Ding, Aromatic ring substituted g-C3N4 for enhanced photocatalytic hydrogen evolution, Journal of Materials Chemistry A, 5 (2017) 17199-17203. [33] F. He, G. Chen, Y. Yu, S. Hao, Y. Zhou, Y. Zheng, Facile approach to synthesize g-PAN/g-C3N4 composites with enhanced photocatalytic H2 evolution activity, ACS Applied Materials & Interfaces, 6 (2014) 7171-7179. [34] Y. Wang, X. Liu, J. Liu, B. Han, X. Hu, F. Yang, Z. Xu, Y. Li, S. Jia, Z. Li, Y. Zhao, 21
Carbon Quantum Dot Implanted Graphite Carbon Nitride Nanotubes: Excellent Charge Separation and Enhanced Photocatalytic Hydrogen Evolution, Angewandte Chemie. International Ed. in English, 57 (2018) 5765-5771. [35] J. Fang, H. Fan, M. Li, C. Long, Nitrogen self-doped graphitic carbon nitride as efficient visible light photocatalyst for hydrogen evolution, Journal of Materials Chemistry A, 3 (2015) 13819-13826. [36] Wang S, Yang H, Wang X, et al. Surface disorder engineering of flake-like Bi2WO6 crystals for enhanced photocatalytic activity[J]. Journal of Electronic Materials, 2019, 48(4): 2067-2076. [37] F. Zuo, L. Wang, T. Wu, Z.Y. Zhang, D. Borchardt, P.Y. Feng, Self-Doped Ti3+ Enhanced Photocatalyst for Hydrogen Production under Visible Light, Journal of the American Chemical Society, 132 (2010) 11856-11857. [38] J.S. Zhang, J.H. Sun, K. Maeda, K. Domen, P. Liu, M. Antonietti, X.Z. Fu, X.C. Wang, Sulfur-mediated synthesis of carbon nitride: Band-gap engineering and improved functions for photocatalysis, Energy & Environmental Science, 4 (2011) 675-678. [39] G. Dong, K. Zhao, L. Zhang, Carbon self-doping induced high electronic conductivity and photoreactivity of g-C3N4, Chemical Communications, 48 (2012). [40] N. Bao, X. Hu, Q. Zhang, X. Miao, X. Jie, S. Zhou, Synthesis of porous carbon-doped g-C3N4 nanosheets with enhanced visible-light photocatalytic activity, Applied Surface Science, 403 (2017) 682-690. [41] Z. Zhao, Y. Sun, F. Dong, Y. Zhang, H. Zhao, Template synthesis of carbon self-doped g-C3N4 with enhanced visible to near-infrared absorption and photocatalytic 22
performance, Royal Society of Chemistry, 5 (2015) 39549-39556. [42] Fattah-Alhosseini A. Passivity of AISI 321 stainless steel in 0.5 M H2SO4 solution studied by Mott–Schottky analysis in conjunction with the point defect model[J]. Arabian Journal of Chemistry, 2016, 9: S1342-S1348. [43] Cardon F, Gomes W P. On the determination of the flat-band potential of a semiconductor in contact with a metal or an electrolyte from the Mott-Schottky plot[J]. Journal of Physics D: Applied Physics, 1978, 11(4): L63. [44] Morrison S R. Electrochemistry at semiconductor and oxidized metal electrodes[J]. 1980. [45] K. Li, F.-Y. Su, W.-D. Zhang, Modification of g-C3N4 nanosheets by carbon quantum dots for highly efficient photocatalytic generation of hydrogen, Applied Surface Science, 375 (2016) 110-117. [46] X. Fan, L. Zhang, M. Wang, W. Huang, Y. Zhou, M. Li, R. Cheng, J. Shi, Constructing carbon-nitride-based copolymers via Schiff base chemistry for visible-light photocatalytic hydrogen evolution, Applied Catalysis B: Environmental, 182 (2016) 68-73. [47] Y. Li, S. Wu, L. Huang, J. Wang, H. Xu, H. Li, Synthesis of carbon-doped g-C3N4 composites with enhanced visible-light photocatalytic activity, Materials Letters, 137 (2014) 281-284. [48] L. Ma, H. Fan, M. Li, H. Tian, J. Fang, G. Dong, A simple melamine-assisted exfoliation of polymeric graphitic carbon nitrides for highly efficient hydrogen production from water under visible light, Journal of Materials Chemistry A, 3 (2015) 23
22404-22412. [49] J.Z. Chen, L.P. Zhang, Z.H. Lam, H.B. Tao, Z.P. Zeng, H.B. Yang, J.Q. Luo, L. Ma, B. Li, J.F. Zheng, S.P. Jia, Z.J. Wang, Z.P. Zhu, B. Liu, Tunneling Interlayer for Efficient Transport of Charges in Metal Oxide Electrodes, Journal of the American Chemical Society, 138 (2016) 3183-3189. [50] Lewis C, Ware W R, Doemeny L J, et al. The Measurement of Short‐Lived Fluorescence Decay Using the Single Photon Counting Method[J]. Review of Scientific Instruments, 1973, 44(2): 107-114.
24
Figure and table captions: Figure 1. (A) XRD patterns (inserted FWHM of (002) peaks) and (B) FTIR spectra of CN and CD-CN. Figure 2. High resolution XPS spectra (A) C 1s, (B) N 1s and (C) relative contents for different types of nitrogen in N 1s deconvolution of CN and CD-CN; (D) schematic of the structure transformation for tri-s-triazine monomer of graphitic carbon nitride before and after carbon doped; blue: carbon atom, gray: nitrogen atom. Figure 3. Typical SEM images of (A) CN and (B) CD-CN. Figure 4. (A) UV-vis DRS spectra (inserted photograph), (B) Kubelka-Munk plots, (C) Mott– Schottky plots, (D) band structure of alignments, (E) PL spectra and (F) time-resolved fluorescence decay spectra of CN and CD-CN. Figure 5. (A) EIS Nyquist plots, (B) transient photocurrent response, (C) hydrogen evolution curves (insert shows the hydrogen evolution rate) and (D) cycling testing of CN and CD-CN. Figure 6. Proposed mechanism for Pt-deposited CN and CD-CN of visible-light photocatalytic performance, TEOA is triethanolamine and D is oxidation products. Table 1. Elemental composition of CN and CD-CN. Table 2. Fluorescence lifetimes and relative percentages of CN and CD-CN.
25
Table 1. Elemental composition of CN and CD-CN.
Sample
C (wt%)
N (wt%)
O (wt%)
H (wt%)
C/N
CN
36.35
61.12
0.25
2.28
0.59
CD-CN
36.88
60.56
0.22
2.34
0.61
Table 2. Fluorescence lifetimes and relative percentages of CN and CD-CN.
Sample CN
CD-CN
Component τ1 τ2 τ τ1 τ2 τ
Lifetime (ns) 1.39 5.61 4.18 1.35 5.49 4.27
Intensity (%) 67.31 32.69 62.82 37.18
Figure 1. (A) XRD patterns (inserted FWHM of (002) peaks) and (B) FTIR spectra of CN and CD-CN.
Figure 2. High resolution XPS spectra (A) C 1s, (B) N 1s and (C) relative contents for different types of nitrogen in N 1s deconvolution of CN and CD-CN; (D) schematic of the structure transformation for tri-s-triazine monomer of graphitic carbon nitride before and after carbon doped; blue: carbon atom, gray: nitrogen atom.
Figure 3. Typical SEM images of (A) CN and (B) CD-CN.
Figure 4. (A) UV-vis DRS spectra (inserted photograph), (B) Kubelka-Munk plots, (C) Mott–Schottky plots, (D) band structure of alignments, (E) PL spectra and (F) time-resolved
fluorescence decay spectra of CN and CD-CN.
Figure 5. (A) EIS Nyquist plots, (B) transient photocurrent response, (C) hydrogen evolution curves (insert shows the hydrogen evolution rate) and (D) cycling testing of CN and CD-CN.
Figure 6. Proposed mechanism for Pt-deposited CN and CD-CN of visible-light photocatalytic performance, TEOA is triethanolamine and D is oxidation products.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
N/A
S0272-8842(19)33485-6
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.008
Reference:
CERI 23661
To appear in:
Ceramics International
Received Date: 7 November 2019 Revised Date:
29 November 2019
Accepted Date: 2 December 2019
Please cite this article as: J. Cao, H. Fan, C. Wang, J. Ma, G. Dong, M. Zhang, Facile synthesis of carbon self-doped g-C3N4 for enhanced photocatalytic hydrogen evolution, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.008. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Facile synthesis of carbon self-doped g-C3N4 for enhanced photocatalytic hydrogen evolution
Jingsheng Cao, Huiqing Fan*, Chao Wang, Jiangwei Ma, Guangzhi Dong, Mingchang Zhang
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
*Corresponding author, E-mail: [email protected]
1
Abstract: Graphite carbon nitride (g-C3N4) is an appealing metal-free photocatalyst for hydrogen evolution, but the potential has been limited by its poor visible-light absorption and unsatisfactory separation of photo-induced carriers. Herein, a facile one-pot strategy to fabricate carbon self-doped g-C3N4 composite through the calcination of dicyanamide and trace amounts of dimethylformamide is presented. The as-obtained carbon self-doped catalyst is investigated by X-ray photoelectron spectroscopy (XPS), confirming the substitution of carbon atoms in original sites of bridging nitrogen. We demonstrate that the as-prepared materials display remarkably improved visible-light absorption and optimized electronic structure under the premise of principally maintaining the tri-s-triazine based crystal framework and surface properties. Furthermore, the carbon doped g-C3N4 composite simultaneously weakens the transportation barrier of charge carriers, suppresses charge recombination and raises the separated efficiency of photoinduced holes and electrons on account of the extension of pi conjugated system. As a result, carbon self-doped g-C3N4 exhibits 4.3 times greater photocurrent density and 5.2 times higher hydrogen evolution rate compared with its bulk counterpart under visible light irradiation.
Keywords: Graphite carbon nitride, Carbon doping, Hydrogen evolution, Photocatalysis, Photocurrent.
2
1. Introduction Nowadays, semiconductor based photocatalysis has attracted widespread attention because of its high performance in utilizing the clean and exhaustible solar energy, which is considered as a green and sustainable route for addressing the crisis of environment and energy in future [1]. Among various photocatalysts, graphitic carbon nitride has aroused great enthusiasm in academic communities, which was first reported for photocatalytic water splitting by Wang [2]. As a conjugated polymeric semiconductor, g-C3N4 owns promising prospects of photocatalysis due to its metal free nature, easy synthesis, appealing optical and electronic properties, and superior stability of thermal and chemistry. Unfortunately, the photocatalysis efficiency of pristine g-C3N4 is still restricted on account of its limited specific surface area, weak absorption of visible-light and high recombination rate of photoexcited carriers. To address these drawbacks, many approaches have been developed, such as increasing the surface area and active sites of g-C3N4 by pre-polymerizing melamine [3], in situ intercalating lithium chloride ions [4], or construction of nanostructure in morphologies [5-11], and improving carriers utilization by doped with foreign elements [12-18] or formation of g-C3N4 based heterostructures [19-28]. Carbonaceous materials have been widely applied in functional nanomaterials fields because of their good compatibility [29-30]. Recently, researchers have proposed that the hybrids between g-C3N4 and carbonaceous materials can improve the absorption of visible-light and promote the efficiency of photoinduced charge separation. Che et al. designed hetero-structured carbon ring (Cring)-C3N4 with extended in-plane pi 3
conjugated electric field, the lifetime and diffusion length of photocarrier were significantly increased up to 10 times [31]. Yu et al. incorporated all-carbon aromatic rings with g-C3N4 to optimize electronic property and photocatalytic activity [32]. He et al. constructed composites of g-PAN and g-C3N4 for improvement of photocatalytic performance and the nanosheets of g-PAN acted as efficient transport channels to facilitate photogenerated carrier separation [33]. Wang et al. implanted carbon quantum dot into g-C3N4 nanotubes through the copolymerization of carbon quantum dots (CQDs) precursor and urea, which exhibited an obvious improved quantum yield of 10.94% at 420 nm [34]. Obviously, constructing composition between carbon source and g-C3N4 is an effective method to optimize intrinsic electronic structure and enhance catalytic performance. As a widely acknowledged way of improving catalysts’ performance, self-doping has an obvious advantage of altering the electronic structure without the introduction of any heteroatoms [35-36], which might act as centers of charge carriers’ recombination [37-38]. Dong et al. first employ density functional theory (DFT) to calculate that carbon self-doping in g-C3N4 resulted in the formation of big delocalized pi bonds for the reason that bridging nitrogen atoms can be replaced by carbon atoms, which is beneficial for photocatalysis on account of enhanced charge carrier mobility and conductivity [39]. This is further confirmed by several reports about successfully synthesis of carbon self-doped g-C3N4 [39-41]. Depended on the pervious report, we note that it worth studying the mechanism attribution for carbon doped g-C3N4 in photocatalysis performance. 4
Herein, we develop a facile one-pot approach of synthesizing carbon self-doped g-C3N4 through calcining the hybrid between dicyanamide and small amount of dimethylformamide, in which dimethylformamide acts as the cost-effective carbon source. The crystal structures, morphologies, electronic structures and optical properties are detailedly investigated and the exact doping position of carbon is determined. The photocatalytic properties of hydrogen evolution in carbon doped g-C3N4 are evaluated under visible-light irradiation. The resultant photocatalyst shows remarkably improved photocatalysis activity and the mechanism therein is intensive investigated.
2. Experimental 2.1. Materials preparation In a typical synthetic procedure, 4 g dicyanamide and a certain volume of dimethylformamide (0mL, 0.5 mL) were full mixed and calcined in an alumina crucible at 550 ºC for 2 h in air and the ramping rate was 3 ºC per minute. After cooling down products to room temperature and grounding them into fine powders, the samples were finally obtained and donated as CN and CD-CN, respectively (CN is the pristine g-C3N4 and CD-CN is C-doped g-C3N4). Chemicals involved in this experiment are used at analytical grade as received. 2.2. Characterization A powder X-ray diffractometer (X'pert, Philips, Eindhoven, The Netherlands) was employed to record X-Ray diffraction (XRD) patterns ranging from 5 º-90 º using Cu Kα radiation. Fourier transform infrared spectra was examined on a Fourier transform infrared spectroscope (TENSOR27, Bruker, Billerica, MA, USA). The scanning 5
electron microscopy (JSM-6701F, JEOL, Tokyo, Japan) was used for obtaining the surface morphologies (SEM) images. Nitrogen adsorption-desorption measurements were conducted for the BET specific surface area (TriStar ΙΙ 3020, Micromeritics, GA, USA) at 77 K and the Barrett-Joyner-Halenda (BJH) method was used for calculation of distribution of pore size. X-ray photoelectron spectroscopy spectra (XPS) was measured for chemical state analysis (Axis Supra, Kratos Analytical, Manchester, UK). The element analyzer (Vario EL cub, Elementar, Langenselbold, Germany) was used for the measurement of elemental proportion of C, N, O, H. Diffuse reflection spectroscopy (DRS) was tested using BaSO4 as reference by a spectrophotometer (UV3150; Shimadzu Corporation, Kyoto, Japan). The spectrophotometer (FLS980, Edinburgh Instruments, Edinburgh, UK) was employed in measuring photoluminescence (PL) spectra and time-resolved decay fluorescence spectra at room temperature and the excitation wavelength was 340 nm. 2.3. Photoelectrochemical measurements Electrochemical test was carried out with an electrochemical workstation (CHI660E, Chenhua, Shanghai, China) and the measurement was conducted in a standard three-electrode system where a platinum plate, a saturated Ag/AgCl electrode and an indium-tin oxide (ITO) conductive film glass electrode deposited with g-C3N4 based samples are employed as the counter electrode, the reference electrode and the working electrode, respectively. For preparation of the working electrode, an ITO glass (0.8 × 0.8 cm2) was coated by CN and CD-CN based viscous slurry. The slurry was prepared by fully mixing ethyl cellulose (3 mg), g-C3N4 (5 mg), terpineol (40 mg) and 6
lauric acid (2 mg) [3]. Subsequently the ITO glass with coated slurry was heated at 350 ºC for 0.5 h for enhanced adhesion. 0.5 M Na2SO4 aqueous solution was employed as the electrolyte. The plots of Mott-Schottky (M-S) were recorded ranging from -0.2 V to 0.7 V at 200 Hz. In particular, flat-band potential was calculated depended on the below M-S relation [42-44]:
=
−
−
(1)
where Csc is the space charge region capacitance, Nd is the charge carriers density, ε0 is the free space permittivity, E is the tested potential, ε is the material’s dielectric constant and EFB is the flat band potential which is the approximate value of conduction band potential (CB). Electrochemistry impedance spectroscopy (EIS) was determined at 0.5 V amplitude and frequency range was from 1000 kHz to 0.01 Hz. The photocurrent spectra were measured without bias voltage and the interval time of light on and off was 50 s. 2.4. Evaluation of the photocatalytic activity The photocatalysis activity of catalysts was evaluated by hydrogen generation under the irradiation of visible-light (λ > 400 nm). The irradiation source was obtained by a 300 W Xe lamp (MAX-302, USA) with a cut off filter of 400 nm. 50 mg photocatalyst was fully dispersed in 100 mL aqueous solution containing 10 vol% triethanolamine (TEOA) and 1 wt% Pt using H2PtCl6 as the precursor, which was on the surface of catalyst because of in situ photo-deposited. Cooling water circulation system was utilized to keep the temperature of reaction solution at 10 ºC. Before irradiation, the reaction system was bubbled several times to entirely remove air. The generated 7
hydrogen was analyzed by a gas chromatography device which was equipped with thermal conductive detector (TCD) and nitrogen acted as the carrier gas.
3. Results and discussion Figure 1A displays the XRD patterns of as-synthesized samples. Despite the introduction of dimethylformamide in the synthesis of CD-CN, XRD patterns show identical peaks of CN and CD-CN at 13.0º and 27.4º, which confirms that the intrinsic layer graphitic structure is preserved. The diffraction peaks at 13.0º and 27.4º refer to (100) and (002) plane which corresponds to the in-plane repeat units of tri-s-triazine and the stacking interlayers of conjugated aromatic structures, respectively. As shown in the insert, the (002) plane full width at half maximum (FWHM) in CD-CN decrease in small quantity compared with CN, indicating the crystallinity of CD-CN are slightly improved. No obvious shift of peaks is observed, revealing the interlamellar spacing of two samples are same. The above results indicate that low dosage of dimethylformamide introduced could hardly disturb the molecular network of carbon nitride. FT-IR spectroscopy is further carried out to characterize functional groups in CN and CD-CN. As shown in Figure 1B, the intense peak in 806 cm-1 originates from heptazine units’ out-of-plane bending and the typical skeletal stretching vibrations of tri-s-triazine heterocyclic (C6N7) rings appear over 1200-1650 cm-1.The broad band centered at 2800 to 3400 cm-1 is ascribed to the N-H (N-H2) stretching vibrations. Not only the XRD patterns but also FT-IR spectra in CD-CN display all the characteristic peaks of pristine g-C3N4, manifesting the original framework of carbon nitride remains unchanged after the introduction of dimethylformamide during calcination. 8
The contents of C, N, O, H are further determined by elemental analysis in table 1. The results reveal a C/N mass ratio of 0.61 for CD-CN which is higher than that for CN (0.59), confirming the success in carbon incorporation. Trace amount of O is detected while the quantity of O element is nearly the same in two samples, and the origin is comprehensive studied in the XPS analysis. The observed residual content of hydrogen emanates from the partial condensation of tri-s-triazine units, which is consistent with the broad bands in FT-IR spectra. To study element’s chemical states and the exact position of doped carbon, XPS was conducted. In Figure S2, the XPS survey spectrum displays two intense peaks referring to C 1s and N 1s, indicating that CN and CD-CN are mainly composed of carbon and nitrogen. The weak peaks (ca. 530 eV) corresponding to O 1s are nearly same in two patterns and no obvious difference has been found between them in the high resolution spectra in the insert. This is probably for the reason that the oxygen in dimethylformamide has been released during the calcining, and the signal in O 1s belongs to the absorbed H2O molecules on the surface of samples [45-46]. The high resolution C 1s spectra is shown in Figure 2A, there are two conspicuous peaks which are located at ca. 284.6 and 288.0 eV in CN and CD-CN, representing adventitious graphitic carbon (C=C) used as a calibration reference and sp2-hybridized carbon (N=C-N) in the network of graphitic carbon nitride, respectively. A slight increase of 0.1 eV in the peak attributed to sp2-hybridized carbon in CD-CN indicates that the interaction between carbon source and g-C3N4 has induced change in chemical states after the incorporation of doped carbon [40,47]. The deconvolution of N 1s has four peaks centered at 398.5, 399.7, 401.0 and 404.1 eV. The hybridized sp2 9
aromatic N (C-N=C, 398.5 eV) in the triazine units displays the dominant signal peak in both samples. The peaks at binding energy of 399.7 eV and 401.0 eV are ascribed to bridging nitrogen atoms (N-(C)3) and amino groups carrying hydrogen (NH bonds or NH2 bonds) respectively. The weakest peaks (404.1 eV) in CN and CD-CN are associated with the charging effects or positive charge localization of cyano-groups and heterocycles. Figure 2C shows the percentage of different N atoms above calculated by peaks integrated. Compared with CN, the content of bridging N in CD-CN decrease from 17.28% to 16.95%, implying that the bridging N is replaced by carbon atoms. The reduce of NH groups in CD-CN result from a higher degree of crystallinity, which has been confirmed in XRD patterns. In addition, accompanied with the replacement of carbon in original sites of bridging N, the percentage composition of C-N=C groups have increased from 70.75% to 71.17%, which benefits the construction of pi conjugated system. From the analysis above, we confirm that carbon doped g-C3N4 has been successfully manufactured and propose the doping path of carbon atoms in Figure 2D, where the bridging N atoms are substituted by C atoms. In consideration of homogeneous substitution in lattice nitrogen by carbon will alter the hybridization type, the delocalized big pi bonds have been formed among the carbon substituted tri-s-triazine units [39], which contributes to prompting electrical conductivity of CD-CN. However, it is difficult to distinguish C-doped carbon nitride from the g-C3N4 matrix based on XRD patterns and FT-IR spectra as a result of the ultra-small amount of dimethylformamide and the similar properties of CN and CD-CN. 10
The morphology of CN and CD-CN are displayed in Figure 3A and 3B. The surface morphologic structure of two samples have no obvious difference between each other and they are composed of dense thick layers and bulk agglomerates, which is consistent with the typical graphitic carbon nitride. Figure S2A shows the nitrogen adsorption-desorption isotherms, Figure S2B demonstrates the BJH pore-size distribution curves of CN and CD-CN, and the porous characteristics calculated through above measurement are displayed in Table S1. Figure S2A illustrates that all samples are of type IV isotherms, implying the mesopores’ existence. This is further confirmed by the pore size distribution of both samples (Figure S2B), which exhibits presence the of mesopores (2~3nm) in CN and CD-CN. As displayed in Table S1, the BET surface area as well as total pore volume is slightly increased in CD-CN when compared with CN, while the average pore size in CD-CN are less than CN. This is beneficial to photocatalytic reaction for the reason that the large area in surface and small diameter in pores size would provide more abundant reaction sites [3]. In consideration of the tiny enhancement of porous characteristics in CD-CN, we regard this part as the secondary factor accounting for its enhanced photocatalysis performance.
The interaction between doped carbon atoms and g-C3N4 networks will undoubtedly give rise to some differences in its optical properties and electronic band structure. As shown in the UV-vis diffuse reflection spectra (Fig. 4A), both samples display a typical semiconductor intrinsic absorption in the blue region, while the absorption intensity of CD-CN in the region of visible-light are remarkably enhanced compared to CN. This could be reflected in their digital images, in which CD-CN 11
exhibits darker color. In addition, the intrinsic absorption edge of CD-CN shows a noticeable red shift with respect to CN. As shown in Fig. 4B, the bandgaps of CN and CD-CN calculated by corresponding Kubelka-Munk plots are 2.72 eV and 2.66 eV respectively, indicating the narrowed bandgap (0.06 eV) of CD-CN. This phenomenon is because of the quantum confinement effects and the increase of the conjugation length [48], which is consistent with the outcome of carbon substitution. The extended pi-conjugation and narrow band gap would benefit electron hole pairs separation and photoelectron transferring [34]. The electrochemical M-S plots in Figure 4C for two samples display typical n-type characteristic and the determined conduction bands (CB) of CN and CD-CN are -1.21 V and -0.94 V (vs. Ag/AgCl hydrogen electrode) respectively. The valence band (VB) in CD-CN (1.92 eV) is determined a shift of 0.21 eV than CN (1.71 eV) in Figure S3, demonstrating the variation of the electronic structure. Hence the schematic band structure of CN and CD-CN is shown in Fig. 4D. It is concluded that the thermodynamic photocatalytic condition about hydrogen evolution produced by water splitting can be satisfied by both samples, and the CB of CD-CN is more positive than CN, which reduces the thermionic emission induced by high potential barrier height of the photocatalyst-Pt contact [49]. As mentioned above, optimizing electronic structure leads to great alteration in charge carrier transfer and separation. As exhibited in Fig. 4E, PL spectrum originates from the recombination of holes at VB and electrons at CB. And CD-CN has a more intense intensity of PL emission with respect to CN, manifesting that the introduction of doped carbon in networks of g-C3N4 can act as an electron buffer which effectively 12
expedite transport of excited electrons and delocalization of pi-electron in the conjugated system, accordingly raising the separation efficiency of photo-excited holes and electrons [32]. This is supported by the results calculated in time-resolved spectroscopy (Fig. 4F). Both samples decay exponentially in fluorescent intensity, while CD-CN displays slow decay kinetics. To further analyze the decay curves, the average lifetimes of photo-induced carriers in CN and CD-CN are calculated by bi-exponential function fitting, and the results are listed in Table 2. In detail, the shortest and longest lifetime of CN and CD-CN are 1.39 ns (67.31%), 5.61ns (32.69%) and 1.35 ns (62.82%), 5.49 ns (37.18%) respectively, demonstrating that the content of longest lifetime in CD-CN has a distinct increase compared to CN. The average fluorescent lifetime (τ) was deduced by the following equation [50]:
=
(2)
We can find that compared to the average fluorescent lifetime of CN (4.18 ns), CD-CN possess a prolonged lifetime of 4.27 ns. The improved lifetime is related to the enhanced electron transport resulted from the decreased electronic transmission resistance. What’s more, a longer lifetime means that the photogenerated holes and electrons have a more probability to be hunt by reactive catalyst and utilized in the photo-redox reaction. To further explore the separation and transport performance of photoinduced charge, electrochemical impedance spectroscopy (EIS) and transient photocurrents are measured. EIS Nyquist plots of CN and CD-CN electrodes in same illumination environment are shown in Figure 5A. CD-CN displays a smaller arc radius when 13
compared to CN, revealing that photoinduced hole and electron pairs in CD-CN encounter a decreased resistance in process of transfer and can be separated effectively. In Figure 5B, we can see that the current density with no light irradiation in CD-CN electrode display a higher baseline than CN electrode, indicating that not only the amount but also transport ability of intrinsic charge carriers has been improved after the formation of delocalized big pi bonds in CD-CN. Moreover, all electrodes show rapid and repeatable transient photocurrent responses, while CD-CN has a higher photocurrent density of 6.4 µAcm-2 which is 4.3 times that of CN (1.5 µAcm-2). The enhancement of photocurrent in CD-CN further confirms that the great electronic transportation competence of carbon doped g-C3N4. Moreover, the increase absorption in visible-light of CD-CN also account for the enhanced photocurrent, in line with UV-vis diffuse reflectance spectra. The photocatalysis ability of prepared samples was evaluated by production of H2 in the irradiation of visible light. In figure 5C, along with the irradiation time increasing, hydrogen evolution of the as-synthesized photocatalyst is in linear growth process. The 8 hours activity of hydrogen production in CN is only 6.78 µmol because of its high recombination rate of photoexcited electrons and holes and limited visible-light responsive competence. After incorporated with doped carbons, the carbon nitride has a dramatically increase on the hydrogen evolution to 35.5 µmol in 8 hours. The inset histogram shows that the hydrogen evolution rate in CD-CN (8.88 µmol/h) is 5.2 times that of CN (1.70 µmol/h), which is ascribed to accelerated separation of photoexcited carrier and high use efficiency of visible light. Furthermore, the durability of CD-CN in 14
hydrogen evolution is measured and Figure 5D displays the result. The hydrogen evolution activity of CD-CN is maintained without evident deactivation after four experimental cycles for 12 h, manifesting the stability of catalysts. Depended on the analysis above, we propose the enhanced photocatalytic mechanism of CD-CN in Figure 6. Firstly, compared with CN, the C-doped carbon nitride has a narrower bandgap and stronger absorbing ability of visible light, providing more separated photoinduced electrons at conduction band (CB) and holes at valence band (VB) under visible-light. Secondly, the interaction between substituted carbon atoms and graphite carbon nitride network result in the creation of delocalized big pi bonds, which can optimize the electronic structure of CD-CN, reducing the barrier of photocarrier transfer and enhancing the electron transportation, as verified by PL spectra and EIS analysis. By contrast, pristine graphite carbon nitride possesses plenty of bridging nitrogen atoms which are unable to form conjugation with ambient carbon atoms, bringing about inconsecutive and small area of pi conjugated system in CN. Thus, there will be more and faster transfer electrons in CD-CN participating in the reaction of hydrogen evolution than CN. Consequently, the carbon doped g-C3N4 has a drastically improved photocatalytic activity.
4. Conclusion To sum up, we have successfully synthesized carbon doped g-C3N4 composite through a facile, green and low-cost approach, which introduces slight amount of dimethylformamide into the matrix of g-C3N4 and primarily maintains the tri-s-triazine based crystal structure. It has been demonstrated in experimental terms that the 15
self-doped carbon could induce the variation of intrinsic electronic band structure via the substitution of bridging N and the consequential extension of pi conjugated system, leading to a narrowed bandgap and significantly enhanced visible light absorption. Furthermore, the photo-excited charge carriers of modified sample are greatly improved in separation and transfer, thus dramatically enhancing the photocatalytic efficiency for hydrogen evolution. This work may provide a new promising method of developing carbon doped g-C3N4-based photocatalysts in the field of harnessing solar energy.
Acknowledgement This work is supported by the National Defense Science Foundation (32102060303), the National Nature Science Foundation (51672220), the SKLSP Project (2019-TZ-04) of China, and the Fundamental Research Funds for the Central Universities of NPU (3102019GHXM002). Here we also express appreciation for the Analytical & Testing Center of Northwestern Polytechnical University.
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Figure and table captions: Figure 1. (A) XRD patterns (inserted FWHM of (002) peaks) and (B) FTIR spectra of CN and CD-CN. Figure 2. High resolution XPS spectra (A) C 1s, (B) N 1s and (C) relative contents for different types of nitrogen in N 1s deconvolution of CN and CD-CN; (D) schematic of the structure transformation for tri-s-triazine monomer of graphitic carbon nitride before and after carbon doped; blue: carbon atom, gray: nitrogen atom. Figure 3. Typical SEM images of (A) CN and (B) CD-CN. Figure 4. (A) UV-vis DRS spectra (inserted photograph), (B) Kubelka-Munk plots, (C) Mott– Schottky plots, (D) band structure of alignments, (E) PL spectra and (F) time-resolved fluorescence decay spectra of CN and CD-CN. Figure 5. (A) EIS Nyquist plots, (B) transient photocurrent response, (C) hydrogen evolution curves (insert shows the hydrogen evolution rate) and (D) cycling testing of CN and CD-CN. Figure 6. Proposed mechanism for Pt-deposited CN and CD-CN of visible-light photocatalytic performance, TEOA is triethanolamine and D is oxidation products. Table 1. Elemental composition of CN and CD-CN. Table 2. Fluorescence lifetimes and relative percentages of CN and CD-CN.
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Table 1. Elemental composition of CN and CD-CN.
Sample
C (wt%)
N (wt%)
O (wt%)
H (wt%)
C/N
CN
36.35
61.12
0.25
2.28
0.59
CD-CN
36.88
60.56
0.22
2.34
0.61
Table 2. Fluorescence lifetimes and relative percentages of CN and CD-CN.
Sample CN
CD-CN
Component τ1 τ2 τ τ1 τ2 τ
Lifetime (ns) 1.39 5.61 4.18 1.35 5.49 4.27
Intensity (%) 67.31 32.69 62.82 37.18
Figure 1. (A) XRD patterns (inserted FWHM of (002) peaks) and (B) FTIR spectra of CN and CD-CN.
Figure 2. High resolution XPS spectra (A) C 1s, (B) N 1s and (C) relative contents for different types of nitrogen in N 1s deconvolution of CN and CD-CN; (D) schematic of the structure transformation for tri-s-triazine monomer of graphitic carbon nitride before and after carbon doped; blue: carbon atom, gray: nitrogen atom.
Figure 3. Typical SEM images of (A) CN and (B) CD-CN.
Figure 4. (A) UV-vis DRS spectra (inserted photograph), (B) Kubelka-Munk plots, (C) Mott–Schottky plots, (D) band structure of alignments, (E) PL spectra and (F) time-resolved
fluorescence decay spectra of CN and CD-CN.
Figure 5. (A) EIS Nyquist plots, (B) transient photocurrent response, (C) hydrogen evolution curves (insert shows the hydrogen evolution rate) and (D) cycling testing of CN and CD-CN.
Figure 6. Proposed mechanism for Pt-deposited CN and CD-CN of visible-light photocatalytic performance, TEOA is triethanolamine and D is oxidation products.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
N/A