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Shell-Engineering of Hollow g-C3N4 Nanospheres by Copolymerization for Photocatalytic Hydrogen Evolution Dandan Zheng, Chenyang Pang, Yuxing Liu, and Xinchen Wang*
Published on 06 May 2015. Downloaded by University of North Dakota on 18/05/2015 02:48:19.
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Incorporation of aromatic motifs into the nanosized shell of hollow carbon nitride nanospheres has been reported to develop functional photosynthetic structures for solar energy application. This modification results in an extended πconjugation system, red-shift of the optical absorption, and an improved charge separation in the shell, while still keeping the unique hollow polymeric nanoarchitectures. This strategy enables the tuning of the semiconductor properties of the shell substance in the hollow carbon nitride nanostructures to generate redox species to enhance photocatalytic activity for hydrogen evolution with visible light. Melon-based graphitic carbon nitride polymer (typically termed as g-C3N4 for simplicity), which consists of two-dimensional (2D) conjugated planes packed together with tri-s-triazine repeating units through van der Waals interactions, has recently emerged as a metal-free photocatalyst for relevant reactions, including water splitting,1 CO2 reduction,2 phenol synthesis from benzene,3 and the selective oxidation of alcohols.4 Nanostructure engineering is an efficient protocol to tailor the morphology, texture, electronic structure, surface properties and the photochemical function of gC3N4. To date, various nanoarchitectural g-C3N4 materials have been developed, such as mesozeolites, nanorods, nanowires, nanosheets and helical nanorods, which practically allows its advanced applications in fields ranging from catalysis, photocatalysis, chemical sensors, to biological imaging.5 Recently, hollow g-C3N4 nanospheres (HCNS) has emerged as a new nanoarchitecture of polymeric g-C3N4 due to its unique three-dimensional hollow structural polymers with semiconductor properties.6 Crucial to the physical properties of these freestanding hollow nanospheres is their oustanding morphological, thermal and chemical stability against sintering in the air up to 500 oC. Meanwhile, this stable conjugated nanospheres with hollow structure can act as a host scaffold for designing multifunctional nanoarchitectures, with large surface areas for both their exterior and interior surfaces that increase the amount of light absorbed by the photonic effects and promote surfacedependent reactions by shuttling both the electronic and chemical species.7 However, due to the defective polymerization of organic networks, this aromatic π-conjugated system in its pristine form restricted by some problems, such as insufficient sunlight absorption (an hypsochromic/blue shift of absorption threshold This journal is © The Royal Society of Chemistry [year]
from ca. 470 nm of g-C3N4 to ca. 420 nm of HCNS), and fast charge recombination, limiting its photochemical functions.8 It is therefore desired to address such drawbacks to make full use of the material and structure functions of HCNS for photocatalysis. Various strategies such as post-annealing,9 combining inorganic quantum dots (i.e. CdS, CdSe, MoS2), and constructing artificial Z-Scheme system have been developed and the activity of the modified catalysts has been improved to some extent.10 Despite the efforts in this research field, the grand challenge to efficient utilization of sunlight is still remained before such a photocatalytic process becomes economically feasible. A cross-linked conjugated HCNS semiconductor is an organic material in nature and therefore can be easily modified through grafting organic groups in its structure by organic chemistry protocols using donor and acceptor motifs.11 Therefore, invitation a variety of new monomer as building blocks with the desired compositions and electronic structures to modify the conjugated polymeric network of HCNS by copolymerization, can basically extend the delocalization of π electrons to red-shift the optical absorption towards deeper visible light region, to promote exciton splitting to generate free charge and to strengthen surface properties.12 Considering the ample choice of co-monomers and organic chemistry protocols, it has been regarded as a versatile chemical pathway to realize the micro-adjustion of hollow polymeric carbon nitride nanostructure, while the stable hollow conjugated polymer architectures can be well remained by careful controls in the synthesis process. The functional HCNS thus developed provides a valuable platform for constructing highly organized photosynthetic systems for solar energy conversion.
Scheme 1. Synthetic strategy. An illustration of the CNST.
In this paper, three typical monomers, barbituric acid (BA), 2aminobenzonitrile (ABN) and 2-aminothiophene-3-carbonitrile (ATCN) were selected. We first focus on the heteromolecular of ATCN to carry out a detailed study, because ATCN was reported [journal], [year], [vol], 00–00 | 1
to have high electron mobility and has been extensively used as building blocks for the construction of opto-electronic polymers, acting as the chromophore center to harvest photons.13 Therefore, micro-adjustion the HCNS network via integrating ATCN in the π-conjugated system is a feasible manner to modify its bulk electronic features and surface/interface properties as well, which in principle can help to optimize the physical and chemical properties for photocatalysis.14 Detailed synthesis was provided in the Experimental Section of SI. Here, in brief, ATCN was added in the synthesis of HCNS using cyanamide as the precursor. In principle, ATCN with chemically activating side groups of amino and cyano to undergo nucleophilic reaction is a recommended monomer to graft thiophene motifs in the HCNS skeleton. For simplicity, the resulting samples are denoted as CNSTx, where x (0.005, 0.01, 0.05, 0.1) refers to the weighed in amount of ATCN. The morphology, structure and optical\electric properties of the samples were characterized using standard physical and chemical techniques. The as-synthesized materials were tested as photocatalysts for hydrogen generation from aqueous solutions under visible light illumination (λ > 455 nm), using photodeposited Pt nanoparticles as hydrogen-evolution catalysts.
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similar to HCNS, indicating that copolymerization do not change the overall morphology of HCNS. HCNS copolymerized with View Article Online other organics, such as barbituric acid (BA), DOI: 2-Aminobenzonitrile 10.1039/C5CC03143E (ABN), have also been prepared and manifested in the Supporting Information (Fig. S1). To further study such copolymerization behavior, elemental mapping was carried out to characterize the sample of CNST0.05. In Fig. 1c, it clearly shows that C, N and S are homogeneously distributed in hollow spheres. This result together with the XPS analysis proved the incorporation of ATCN in the π-conjugated system of HCNS. On the XPS spectra of CNST0.05, three elements (C, N and S) are observed. In Fig. 2a, the binding energies (BEs) collected for S2p3/2 and S2p1/2 are determined as 164.0 eV and 165.2 eV, respectively, similar to the typical peaks of thiophene-S reported by previous literatures (where S2p3/2 and S2p1/2 at 163.8 and 165.0 eV).15 Note that these BEs are quite different from those (163.9 and 168.5 eV) of sulfur doped HCNS. The successful combination of ATCN groups in CN network gradually increases the C/N molar ratio from 0.73 (pure HCNS) to 0.78 of CNST0.05 as well as resulting in the sulfur species (Table 1). This result is further certified by solid-state 13C NMR spectra. In Fig. 2b, a new broad peak centering at 105 ppm is clearly observed for the CNST sample, suggesting the incorporation of ATCN species in HCNS. The results described above indicate that ATCN groups have been grafted on the surfaces of HCNS via copolymerization, by which more delocalized electronic system would be expected due to the strong donor characteristic of ATCN.12 All samples feature 2wt% hydrogen, indicating the melon structure.
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Fig. 2. Analysis of CNST0.05 sample, with HCNS as reference. a) XPS spectra of S 2p; b) solid-state 13C NMR spectra.
Fig. 1 Surface morphology characterization. a), b) SEM images of HCNS and CNST0.05 samples; c), d) TEM images of HCNS and CNST0.05 samples, respectively; and (e) elemental mapping of CNST0.05 sample.
The morphology and texture of modified HCNS samples were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1a shows a typical SEM image of HCNS, while Fig. 1b depicts the SEM image of the modified CNST0.05 sample. These images reveal that the uniform hollow structures with a diameter of ~320 nm are retained after post-engineering, indicating the robust nature of the hollow polymer. The textural structure of CNSTx particles are directly presented by TEM images. As shown in Fig. 1d, the polymeric shell thickness of CNST0.05 approached to 56 nm, 2 | Journal Name, [year], [vol], 00–00
As shown in Fig. S3a, X-ray diffraction (XRD) patterns of CNSTx and HCNS show that the crystal-phase structures are highly related. The XRD peak at 27.6° is attributed to the (002) reflection of a graphitic-like structure with a d-value of 0.323 nm, whilst the peak at 13.1° is related to the in-plane repeating motifs of the heptazine network.16 However, the (002) peak becomes broader and gradually less intense with increasing amount of ATCN, indicating the slight disorder of the structure, potentially by the additional carbon functionality introduced by ATCN. This result is also confirmed by the Fourier transformed infrared (FTIR) spectra (Fig. S3b), where not any obvious change is detected for the CNSTx samples compared with pure HCNS.17 In the further set of experiments, the textural properties of the CNST0.05 sample were accessed from N2 absorption-desorption isotherm. In Fig. S4, a typical IV isotherm with pronounced Htype hysteresis loop is observed, suggesting the presence of welldeveloped mesopores in polymeric matrix.18 The corresponding textural parameters, such as specific surface area (SBET) and pore This journal is © The Royal Society of Chemistry [year]
ChemComm
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ChemComm Accepted Manuscript
This journal is © The Royal Society of Chemistry [year]
increased to 0.1, the photocatalytic activity dramatically decreased. The excessive dopant seems to turn the material in View Article Online part to a degenerate semiconductor by DOI: spoiling the electronic 10.1039/C5CC03143E 13 structure, which is detrimental to photocatalysis. The photocatalytic activities of CNST0.05 is well coincidence with its optical absorption (Fig. 4b), suggesting that the main driving force of the photocatalytic reaction is the harvested visible photons.19 Meanwhile, when extending the wavelength of incident light to 550 nm, the CNST0.05 sample still shows much enhanced photocatalytic activity (HER = 79 μmol·h-1), while the pure HCNS is practically inert (with HER less than 1 μmol·h-1). This result is regarded as promising as it is the first polymer photocatalyst that shows such a high HER at wavelength > 550 nm, which actually covers a sufficient section of solar spectrum.
F(R)
volume are almost constant, compared with HCNS (Table S1). It is clear that the chemical skeleton of HCNS is mostly unaltered after the copolymerization of a small amount of organic monomer that is enough to modify the semiconductive properties. With increasing ATCN content in HCNS matrix, the sample colors changes from pale yellow for pure HCNS to a much deeper orange, well addressing the poor absorption in the visible range caused by the chemical and nanostructure engineering. Accordingly, the UV-Vis (Fig. 3a) reflectance spectrum shows a remarkable red-shift of optical absorption from 420 to 700nm with increasing ATCN content compared to HCNS.18c The enhanced visible light harvesting indicates that the combination of electron-rich ATCN aromatic donors in the HCNS matrix promote electron delocalization in the π-conjugated system and thus narrow the semiconductive band-gap energy. In Fig. 3b, a strong photoluminescence (PL) emission peak is observed for the pristine HCNS, due to the radiative recombination of charge carriers. Obviously, the otherwise strong PL quenches quickly for the CNSTx samples, indicating that the energy-wasteful charge recombination happening in the semiconductor is suppressed. In Fig. 3c, the intensity of electron paramagnetic resonance (EPR) spectra, originating from unpaired electrons, increases progressively with integrating more and more ATCN in HCNS. ATCN doping extended the delocalization of π-conjugated system, and in principle, it can efficiently promote exciton dissociation to produce free electrons and holes for relevant photoredox reactions. As expected, this extended π-conjugated system sufficiently promote the generation of photochemical radical pairs and thus exhibiting an enhanced EPR signal when irradiated with visible light (Fig. S5). Electrochemical impedance spectroscopy (EIS) study of the charge transfer rate in the dark reveals the expected semicircular Nyquist plots for HCNS and CNST0.05, but with a much decreased diameter for the latter (Fig. 3d). This result demonstrates that ATCN incorporation in the HCNS skeleton can indeed improve the electronic conductivity of polymer matrix to promote the charge separation. The considerably enhanced photocurrent generated by CNST0.05 is another evidence of ATCN modification, which show an enhancement in the Iph of CNST0.05 over unmodified HCNS by a factor of five (Fig. 3d insert). Associated with the red-shifted optical absorption, improved charge separation and transport capabilities, HCNS after copolymerisation with organics is now expected to be favourable for photocatalysis.18 Fig. 4a shows the photocatalytic activity of modified HCNS samples toward hydrogen evolution from water/triethanolamine mixtures with visible light (λ > 455 nm) and Pt (3 wt %) as cocatalyst. As shown in Fig. 4a, an overall enhanced hydrogen evolution rate (HER) was found for all the modified samples, especially for the ATCN doped samples. The optimum HER of CNST reached to 278 μmol·h-1, which is about 3 times faster than that of the pure HCNS. This result indicated that copolymerisation can optimize the π-conjugated system to simultaneously promote mass transfer and charge separation for photochemical applications that rely on the generation and separation of light-stimulated charge carriers.12 As a result of ATCN doping, almost all the modified samples show a remarkable improvement photocatalytic activity over the pristine HCNS (Fig. S6a). In particular, with the ATCN content
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Fig. 3. (a) UV-Vis diffuse reflectance spectra, (b) photoluminescence spectra and (c) electron paramagnetic resonance spectra of CNSTx photocatalysts. (d) EIS Nyquist plots for CNST0.05 and HCNS; inset shows the Photocurrent generation performance of CNST0.05 and HCNS.
The stability of CNST0.05 samples was examined by performing experiments on the photocatalyst under similar conditions after four recycles (Fig. 4b insert). The H2 produced increases steadily with irradiation time, without noticeable deactivation. The samples before and after the photocatalytic operations were also investigated by TEM, XRD, FTIR and XPS analysis. The elemental image (Fig. 4c) of the used HCNS depicted an apparently unaltered hollow nanosphere and spatially homogeneous of Pt element. This result is confirmed by the XPS spectra of the used CNST, and the BE of Pt 4f is clearly observed (Fig. S8). The XPS spectra of C1s and N1s after the reaction remained unchanged. Furthermore, no distinct variations can be distinguished in XRD and FTIR spectrum between the CNST sample before and after the reaction (Fig. S9). These observations support the robust nature and high operation stability of the hollow functional polymers for photocatalytic hydrogen evolution.
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Higher Education (20133514110003).
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Notes and references
Fig. 4. a) Photocatalytic H2 evolution rate (λ > 455 nm, 20 mg cat.). b) Wavelength dependence of hydrogen evolution rate on CNST0.05 and stability test of Pt/CNST0.05 photocatalyst with prolonged visible light irradiation (λ > 455 nm). c) Elemental mapping of CNST0.05 after photocatalytic activity test.
Conclusions Taken together, we have shown that the optical and electronic properties of the shell of hollow carbon nitride nanostructure can be adjusted by standard organic protocols, here exemplified by molecular engineering of HCNS matrix using aromatic monomers. The incorporation of ATCN in CN skeleton can extend the π-conjugated system, adjusting its semiconductor properties, such as engineering the band structure with tuneable bandgap and promoting the charge migration and separation. Meanwhile, the morphology of the modified HCNS depicted an apparently unaltered by adjusting the ATCN/cyanamide molar ratio in a controllable fashion. The optimized CNSTx exhibit a remarkable enhanced photocatalytic activity under visible light irradiation. This interesting finding affords us a feasible chemical pathway to realize the molecular engineering of hollow carbon nitride capsules, providing a flexible semiconductor scaffold for constructing/assembling polymeric systems for the sustainable utilization of solar radiation and for the application in life science, such as drug delivery or bioimaging.
Acknowledgements This work was supported by the National Basic Research Program of China (2013CB632405), the NSF of China (21425309 and 21173043), the State Key Laboratory of NBC Protection for Civilian (SKLNBC2013-04
4 | Journal Name, [year], [vol], 00–00
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K), and the Specialized Research Fund for the Doctoral Program of View Article Online
DOI: 10.1039/C5CC03143E
State Key Laboratory of Photocatalysis on Energy and Environment College of Chemistry, Fuzhou University, Fuzhou 350002, People’s Republic of China E-mail: [email protected]. † Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c000000x/ 1 a) X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater., 2009, 8, 76; b) X. C. Wang, X. F. Chen, A. Thomas, X. Z. Fu, M. Antonietti, Adv. Mater., 2009, 21, 1609; c) Y. Zheng, L. H. Lin, B. Wang, X. C. Wang, Angew. Chem. Int. Ed., 2015, DOI: 10.1002/anie.201501788R1. 2 a) R. Kuriki, K. Sekizawa, O. Ishitani, K. Maeda, Angew. Chem. Int. Ed., 2015, 54, 2406; b) K. Maeda, D. Lu, K. Domen, Angew. Chem. Int. Ed., 2013, 52, 6488. 3 X. J. Ye, Y. J. Cui, X. C. Wang, ChemSusChem., 2014, 7, 738. 4 a) B. H. Long, Z. X. Ding, X. C. Wang, ChemSusChem., 2013, 6, 2024; b) Y. Chen, J. S. Zhang, M. W. Zhang, X. C. Wang, Chem. Sci., 2013, 4, 3244. 5 a) Y. Zheng, L. H. Lin, X. J. Ye, F. S. Guo, X. C. Wang, Angew. Chem. Int. Ed., 2014, 53, 11926; b) G. Siller, N. Severin, S. Y. Chong, T. Björkman, R. G. Palgrave, A. Laybourn, M. Antonietti, Y. Z. Khimyak, A. V. Krasheninnikov, J. P. Rabe, U. Kaiser, A. I. Cooper, A. Thomas, M. J. Bojdys, Angew. Chem. Int. Ed., 2014, 53, 7450; c) J. J. Duan, S. Chen, M. Jaroniec, S. Z. Qiao, ACS Nano, 2015, 9, 931; d) J. S. Zhang, M. W. Zhang, L. H. Lin, X. C. Wang, Angew. Chem. Int. Ed., 2015, DOI: 10.1002/anie.201501001. 6 a) J. H. Sun, J. S. Zhang, M. W. Zhang, M. Antonietti, X. Z. Fu, X. C. Wang, Nat. Commun., 2012, 3, 1139; b) J. S. Zhang, M. W. Zhang, C. Yang, X. C. Wang, Adv. Mater., 2014, 26, 4121. 7 a) X. W. Lou, L. A. Archer, Z. C. Yang, Adv. Mater., 2008, 20, 3987; b) H. J. Huang, S. B. Yang, R. Vajtai, X. Wang, P. M. Ajayan, Adv. Mater., 2014, 26, 5160. 8 J. S. Zhang, X. F. Chen, K. Takanabe, K. Maeda, K. Domen, J. Epping, X. Z. Fu, M. Antonietti, X. C. Wang, Angew. Chem. Int. Ed., 2010, 49, 441. D. D. Zheng, C. J. Huang, X. C. Wang, Nanoscale, 2015, 7, 465. 9 10 Y. S. Jun, W. H. Hong, M. Antonietti, A. Thomas, Adv. Mater., 2009, 21, 4270. 11 a) Y. S. Jun, J. Park, S. U. Lee, A. Thomas, W. H. Hong, G. D. Stucky, Angew. Chem. Int. Ed., 2013, 52, 11083; b) D. J. Martin, K. P. Qiu, S. A. Shevlin, A. D. Handoko, X. W. Chen, Z. X. Guo, J. W. Tang, Angew. Chem. Int. Ed., 2014, 53, 9240. 12 J. S. Zhang, G. G. Zhang, X. F. Chen, S. Lin, L. Möhlmann, G. Dołęga, G. Lipner, M. Antonietti, S. Blechert, X. Wang, Angew. Chem. Int. Ed., 2012, 51, 3183. 13 J. S. Zhang, M. W. Zhang, S. Lin, X. Z. Fu, X. C. Wang, J. Catal., 2014, 310, 14. 14 M. W. Zhang, X. C. Wang, Energy Environ. Sci., 2014, 7, 1902. 15 A. Sathyapalan, S. C. Ng, A. Lohani, T. T. Ong, H. Chen, S. Zhang, Y. M. Lam, S. G. Mhaisalkar, Thin Solid Films, 2008, 516, 5645. 16 K. Wang, Q. Li, B. S. Liu, B. Cheng, W. K. Ho, J. G. Yu, Appl. Catal. B: Environ., 2015, 176, 44. 17 a) P. Niu, L. L. Zhang, G. Liu, H. M. Cheng, Adv. Funct. Mater., 2012, 22, 4763; b) Z. Z. Lin, X. C. Wang, Angew. Chem. Int. Ed., 2013, 52, 1735. 18 a) R. G. Li, H. X. Han, F. X. Zhang, D. E. Wang, C. Li, Energy Environ. Sci., 2014, 7, 1369; b) A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg, Y. Bando, Adv. Mater., 2005, 17, 1648; c) S. N. Talapaneni, G. P. Mane, A. Mano, C. Anand, D. S. Dhawale, T. Mori, A. Vinu, ChemSusChem, 2012, 5, 700. 19 a) G. G. Zhang, M. W. Zhang, X. J. Ye, X. Q. Qiu, S. Lin, X. C. Wang, Adv. Mater., 2014, 26, 805; b) A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J. O. Müller, R. Schlögl, J. M. Carlsson, J. Mater. Chem., 2008, 18, 4893.
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Aromatic monomers have been grafted on photocatalytic hollow carbon nitride nanospheres by copolymerizaiton to strengthen optical and electronic properties.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: D. zheng, C. Pang, Y. Liu and X. Wang, Chem. Commun., 2015, DOI: 10.1039/C5CC03143E.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Shell-Engineering of Hollow g-C3N4 Nanospheres by Copolymerization for Photocatalytic Hydrogen Evolution Dandan Zheng, Chenyang Pang, Yuxing Liu, and Xinchen Wang*
Published on 06 May 2015. Downloaded by University of North Dakota on 18/05/2015 02:48:19.
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Incorporation of aromatic motifs into the nanosized shell of hollow carbon nitride nanospheres has been reported to develop functional photosynthetic structures for solar energy application. This modification results in an extended πconjugation system, red-shift of the optical absorption, and an improved charge separation in the shell, while still keeping the unique hollow polymeric nanoarchitectures. This strategy enables the tuning of the semiconductor properties of the shell substance in the hollow carbon nitride nanostructures to generate redox species to enhance photocatalytic activity for hydrogen evolution with visible light. Melon-based graphitic carbon nitride polymer (typically termed as g-C3N4 for simplicity), which consists of two-dimensional (2D) conjugated planes packed together with tri-s-triazine repeating units through van der Waals interactions, has recently emerged as a metal-free photocatalyst for relevant reactions, including water splitting,1 CO2 reduction,2 phenol synthesis from benzene,3 and the selective oxidation of alcohols.4 Nanostructure engineering is an efficient protocol to tailor the morphology, texture, electronic structure, surface properties and the photochemical function of gC3N4. To date, various nanoarchitectural g-C3N4 materials have been developed, such as mesozeolites, nanorods, nanowires, nanosheets and helical nanorods, which practically allows its advanced applications in fields ranging from catalysis, photocatalysis, chemical sensors, to biological imaging.5 Recently, hollow g-C3N4 nanospheres (HCNS) has emerged as a new nanoarchitecture of polymeric g-C3N4 due to its unique three-dimensional hollow structural polymers with semiconductor properties.6 Crucial to the physical properties of these freestanding hollow nanospheres is their oustanding morphological, thermal and chemical stability against sintering in the air up to 500 oC. Meanwhile, this stable conjugated nanospheres with hollow structure can act as a host scaffold for designing multifunctional nanoarchitectures, with large surface areas for both their exterior and interior surfaces that increase the amount of light absorbed by the photonic effects and promote surfacedependent reactions by shuttling both the electronic and chemical species.7 However, due to the defective polymerization of organic networks, this aromatic π-conjugated system in its pristine form restricted by some problems, such as insufficient sunlight absorption (an hypsochromic/blue shift of absorption threshold This journal is © The Royal Society of Chemistry [year]
from ca. 470 nm of g-C3N4 to ca. 420 nm of HCNS), and fast charge recombination, limiting its photochemical functions.8 It is therefore desired to address such drawbacks to make full use of the material and structure functions of HCNS for photocatalysis. Various strategies such as post-annealing,9 combining inorganic quantum dots (i.e. CdS, CdSe, MoS2), and constructing artificial Z-Scheme system have been developed and the activity of the modified catalysts has been improved to some extent.10 Despite the efforts in this research field, the grand challenge to efficient utilization of sunlight is still remained before such a photocatalytic process becomes economically feasible. A cross-linked conjugated HCNS semiconductor is an organic material in nature and therefore can be easily modified through grafting organic groups in its structure by organic chemistry protocols using donor and acceptor motifs.11 Therefore, invitation a variety of new monomer as building blocks with the desired compositions and electronic structures to modify the conjugated polymeric network of HCNS by copolymerization, can basically extend the delocalization of π electrons to red-shift the optical absorption towards deeper visible light region, to promote exciton splitting to generate free charge and to strengthen surface properties.12 Considering the ample choice of co-monomers and organic chemistry protocols, it has been regarded as a versatile chemical pathway to realize the micro-adjustion of hollow polymeric carbon nitride nanostructure, while the stable hollow conjugated polymer architectures can be well remained by careful controls in the synthesis process. The functional HCNS thus developed provides a valuable platform for constructing highly organized photosynthetic systems for solar energy conversion.
Scheme 1. Synthetic strategy. An illustration of the CNST.
In this paper, three typical monomers, barbituric acid (BA), 2aminobenzonitrile (ABN) and 2-aminothiophene-3-carbonitrile (ATCN) were selected. We first focus on the heteromolecular of ATCN to carry out a detailed study, because ATCN was reported [journal], [year], [vol], 00–00 | 1
to have high electron mobility and has been extensively used as building blocks for the construction of opto-electronic polymers, acting as the chromophore center to harvest photons.13 Therefore, micro-adjustion the HCNS network via integrating ATCN in the π-conjugated system is a feasible manner to modify its bulk electronic features and surface/interface properties as well, which in principle can help to optimize the physical and chemical properties for photocatalysis.14 Detailed synthesis was provided in the Experimental Section of SI. Here, in brief, ATCN was added in the synthesis of HCNS using cyanamide as the precursor. In principle, ATCN with chemically activating side groups of amino and cyano to undergo nucleophilic reaction is a recommended monomer to graft thiophene motifs in the HCNS skeleton. For simplicity, the resulting samples are denoted as CNSTx, where x (0.005, 0.01, 0.05, 0.1) refers to the weighed in amount of ATCN. The morphology, structure and optical\electric properties of the samples were characterized using standard physical and chemical techniques. The as-synthesized materials were tested as photocatalysts for hydrogen generation from aqueous solutions under visible light illumination (λ > 455 nm), using photodeposited Pt nanoparticles as hydrogen-evolution catalysts.
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similar to HCNS, indicating that copolymerization do not change the overall morphology of HCNS. HCNS copolymerized with View Article Online other organics, such as barbituric acid (BA), DOI: 2-Aminobenzonitrile 10.1039/C5CC03143E (ABN), have also been prepared and manifested in the Supporting Information (Fig. S1). To further study such copolymerization behavior, elemental mapping was carried out to characterize the sample of CNST0.05. In Fig. 1c, it clearly shows that C, N and S are homogeneously distributed in hollow spheres. This result together with the XPS analysis proved the incorporation of ATCN in the π-conjugated system of HCNS. On the XPS spectra of CNST0.05, three elements (C, N and S) are observed. In Fig. 2a, the binding energies (BEs) collected for S2p3/2 and S2p1/2 are determined as 164.0 eV and 165.2 eV, respectively, similar to the typical peaks of thiophene-S reported by previous literatures (where S2p3/2 and S2p1/2 at 163.8 and 165.0 eV).15 Note that these BEs are quite different from those (163.9 and 168.5 eV) of sulfur doped HCNS. The successful combination of ATCN groups in CN network gradually increases the C/N molar ratio from 0.73 (pure HCNS) to 0.78 of CNST0.05 as well as resulting in the sulfur species (Table 1). This result is further certified by solid-state 13C NMR spectra. In Fig. 2b, a new broad peak centering at 105 ppm is clearly observed for the CNST sample, suggesting the incorporation of ATCN species in HCNS. The results described above indicate that ATCN groups have been grafted on the surfaces of HCNS via copolymerization, by which more delocalized electronic system would be expected due to the strong donor characteristic of ATCN.12 All samples feature 2wt% hydrogen, indicating the melon structure.
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Fig. 2. Analysis of CNST0.05 sample, with HCNS as reference. a) XPS spectra of S 2p; b) solid-state 13C NMR spectra.
Fig. 1 Surface morphology characterization. a), b) SEM images of HCNS and CNST0.05 samples; c), d) TEM images of HCNS and CNST0.05 samples, respectively; and (e) elemental mapping of CNST0.05 sample.
The morphology and texture of modified HCNS samples were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1a shows a typical SEM image of HCNS, while Fig. 1b depicts the SEM image of the modified CNST0.05 sample. These images reveal that the uniform hollow structures with a diameter of ~320 nm are retained after post-engineering, indicating the robust nature of the hollow polymer. The textural structure of CNSTx particles are directly presented by TEM images. As shown in Fig. 1d, the polymeric shell thickness of CNST0.05 approached to 56 nm, 2 | Journal Name, [year], [vol], 00–00
As shown in Fig. S3a, X-ray diffraction (XRD) patterns of CNSTx and HCNS show that the crystal-phase structures are highly related. The XRD peak at 27.6° is attributed to the (002) reflection of a graphitic-like structure with a d-value of 0.323 nm, whilst the peak at 13.1° is related to the in-plane repeating motifs of the heptazine network.16 However, the (002) peak becomes broader and gradually less intense with increasing amount of ATCN, indicating the slight disorder of the structure, potentially by the additional carbon functionality introduced by ATCN. This result is also confirmed by the Fourier transformed infrared (FTIR) spectra (Fig. S3b), where not any obvious change is detected for the CNSTx samples compared with pure HCNS.17 In the further set of experiments, the textural properties of the CNST0.05 sample were accessed from N2 absorption-desorption isotherm. In Fig. S4, a typical IV isotherm with pronounced Htype hysteresis loop is observed, suggesting the presence of welldeveloped mesopores in polymeric matrix.18 The corresponding textural parameters, such as specific surface area (SBET) and pore This journal is © The Royal Society of Chemistry [year]
ChemComm
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ChemComm Accepted Manuscript
This journal is © The Royal Society of Chemistry [year]
increased to 0.1, the photocatalytic activity dramatically decreased. The excessive dopant seems to turn the material in View Article Online part to a degenerate semiconductor by DOI: spoiling the electronic 10.1039/C5CC03143E 13 structure, which is detrimental to photocatalysis. The photocatalytic activities of CNST0.05 is well coincidence with its optical absorption (Fig. 4b), suggesting that the main driving force of the photocatalytic reaction is the harvested visible photons.19 Meanwhile, when extending the wavelength of incident light to 550 nm, the CNST0.05 sample still shows much enhanced photocatalytic activity (HER = 79 μmol·h-1), while the pure HCNS is practically inert (with HER less than 1 μmol·h-1). This result is regarded as promising as it is the first polymer photocatalyst that shows such a high HER at wavelength > 550 nm, which actually covers a sufficient section of solar spectrum.
F(R)
volume are almost constant, compared with HCNS (Table S1). It is clear that the chemical skeleton of HCNS is mostly unaltered after the copolymerization of a small amount of organic monomer that is enough to modify the semiconductive properties. With increasing ATCN content in HCNS matrix, the sample colors changes from pale yellow for pure HCNS to a much deeper orange, well addressing the poor absorption in the visible range caused by the chemical and nanostructure engineering. Accordingly, the UV-Vis (Fig. 3a) reflectance spectrum shows a remarkable red-shift of optical absorption from 420 to 700nm with increasing ATCN content compared to HCNS.18c The enhanced visible light harvesting indicates that the combination of electron-rich ATCN aromatic donors in the HCNS matrix promote electron delocalization in the π-conjugated system and thus narrow the semiconductive band-gap energy. In Fig. 3b, a strong photoluminescence (PL) emission peak is observed for the pristine HCNS, due to the radiative recombination of charge carriers. Obviously, the otherwise strong PL quenches quickly for the CNSTx samples, indicating that the energy-wasteful charge recombination happening in the semiconductor is suppressed. In Fig. 3c, the intensity of electron paramagnetic resonance (EPR) spectra, originating from unpaired electrons, increases progressively with integrating more and more ATCN in HCNS. ATCN doping extended the delocalization of π-conjugated system, and in principle, it can efficiently promote exciton dissociation to produce free electrons and holes for relevant photoredox reactions. As expected, this extended π-conjugated system sufficiently promote the generation of photochemical radical pairs and thus exhibiting an enhanced EPR signal when irradiated with visible light (Fig. S5). Electrochemical impedance spectroscopy (EIS) study of the charge transfer rate in the dark reveals the expected semicircular Nyquist plots for HCNS and CNST0.05, but with a much decreased diameter for the latter (Fig. 3d). This result demonstrates that ATCN incorporation in the HCNS skeleton can indeed improve the electronic conductivity of polymer matrix to promote the charge separation. The considerably enhanced photocurrent generated by CNST0.05 is another evidence of ATCN modification, which show an enhancement in the Iph of CNST0.05 over unmodified HCNS by a factor of five (Fig. 3d insert). Associated with the red-shifted optical absorption, improved charge separation and transport capabilities, HCNS after copolymerisation with organics is now expected to be favourable for photocatalysis.18 Fig. 4a shows the photocatalytic activity of modified HCNS samples toward hydrogen evolution from water/triethanolamine mixtures with visible light (λ > 455 nm) and Pt (3 wt %) as cocatalyst. As shown in Fig. 4a, an overall enhanced hydrogen evolution rate (HER) was found for all the modified samples, especially for the ATCN doped samples. The optimum HER of CNST reached to 278 μmol·h-1, which is about 3 times faster than that of the pure HCNS. This result indicated that copolymerisation can optimize the π-conjugated system to simultaneously promote mass transfer and charge separation for photochemical applications that rely on the generation and separation of light-stimulated charge carriers.12 As a result of ATCN doping, almost all the modified samples show a remarkable improvement photocatalytic activity over the pristine HCNS (Fig. S6a). In particular, with the ATCN content
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Fig. 3. (a) UV-Vis diffuse reflectance spectra, (b) photoluminescence spectra and (c) electron paramagnetic resonance spectra of CNSTx photocatalysts. (d) EIS Nyquist plots for CNST0.05 and HCNS; inset shows the Photocurrent generation performance of CNST0.05 and HCNS.
The stability of CNST0.05 samples was examined by performing experiments on the photocatalyst under similar conditions after four recycles (Fig. 4b insert). The H2 produced increases steadily with irradiation time, without noticeable deactivation. The samples before and after the photocatalytic operations were also investigated by TEM, XRD, FTIR and XPS analysis. The elemental image (Fig. 4c) of the used HCNS depicted an apparently unaltered hollow nanosphere and spatially homogeneous of Pt element. This result is confirmed by the XPS spectra of the used CNST, and the BE of Pt 4f is clearly observed (Fig. S8). The XPS spectra of C1s and N1s after the reaction remained unchanged. Furthermore, no distinct variations can be distinguished in XRD and FTIR spectrum between the CNST sample before and after the reaction (Fig. S9). These observations support the robust nature and high operation stability of the hollow functional polymers for photocatalytic hydrogen evolution.
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Higher Education (20133514110003).
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Notes and references
Fig. 4. a) Photocatalytic H2 evolution rate (λ > 455 nm, 20 mg cat.). b) Wavelength dependence of hydrogen evolution rate on CNST0.05 and stability test of Pt/CNST0.05 photocatalyst with prolonged visible light irradiation (λ > 455 nm). c) Elemental mapping of CNST0.05 after photocatalytic activity test.
Conclusions Taken together, we have shown that the optical and electronic properties of the shell of hollow carbon nitride nanostructure can be adjusted by standard organic protocols, here exemplified by molecular engineering of HCNS matrix using aromatic monomers. The incorporation of ATCN in CN skeleton can extend the π-conjugated system, adjusting its semiconductor properties, such as engineering the band structure with tuneable bandgap and promoting the charge migration and separation. Meanwhile, the morphology of the modified HCNS depicted an apparently unaltered by adjusting the ATCN/cyanamide molar ratio in a controllable fashion. The optimized CNSTx exhibit a remarkable enhanced photocatalytic activity under visible light irradiation. This interesting finding affords us a feasible chemical pathway to realize the molecular engineering of hollow carbon nitride capsules, providing a flexible semiconductor scaffold for constructing/assembling polymeric systems for the sustainable utilization of solar radiation and for the application in life science, such as drug delivery or bioimaging.
Acknowledgements This work was supported by the National Basic Research Program of China (2013CB632405), the NSF of China (21425309 and 21173043), the State Key Laboratory of NBC Protection for Civilian (SKLNBC2013-04
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K), and the Specialized Research Fund for the Doctoral Program of View Article Online
DOI: 10.1039/C5CC03143E
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DOI: 10.1039/C5CC03143E
Aromatic monomers have been grafted on photocatalytic hollow carbon nitride nanospheres by copolymerizaiton to strengthen optical and electronic properties.
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