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Iodine Modified Carbon Nitride Semiconductors as Visible Light Photocatalysts for Hydrogen Evolution Guigang Zhang, Mingwen Zhang, Xinxin Ye, Xiaoqing Qiu, Sen Lin, and Xinchen Wang* Conjugated polymers have been widely used in a variety of fields including industrial production, daily life, aviation, as well as biomedicine.[1] This is mainly benefited from the excellent intrinsic thermoplasticity and thermosetting properties of organic backbones. However, the inherently weak electrical conductivity of polymer materials greatly restricted their practical applications. Since the pioneering work of Heeger et al.[2] much attention has been paid to conductive polymers due to their unique electronic and optical properties that allow their promising applications in plastic electronic and photovoltaic devices. In principle, redox doping strategies have been widely used to increase the electrical conductivity of conjugated polymers, such as polyacetylene (PA), polythiophene (PT), polyaniline (PANI), polypyrrole (PPV) and poly(3,4-ethylenedioxythiophene) (PEDOT), typically using sodium and iodine to create n-type and p-type conductive polymers, respectively.[3] In virtue of the unique conducting properties, a variety of semiconductor devices have been fabricated. For example, the discovery of polymer light-emitting diodes (PLED) in 1990 extremely expands the applications of LED, which is mainly referred to its distinct features, including flexibility, simple process and low cost.[4] It is noted that most current conjugated polymers feature one dimensional (1D) linear-backbone structures, being suffered from high exciton binding energy due to quantum effect of charge carrier.[5] Very recently, great attentions have been paid to a new class of polymeric semiconductor, graphitic carbon nitride (g-CN), which is a prototypical two-dimensional (2D) conjugated polymer featuring an extended delocalized pi electron system. The application of 2D g-CN polymers are now being explored in the field of catalysis, photocatalysis, bioimaging and so on,[6–8] because they offer several features that distinguish them from linear macromolecules. For example, the binding energy of Frenkel-type excitons in 2D planar conjugated polymers is lower than those in linear-backbone polymer.[9] In addition, the performances of 2D conjugated polymers are strongly depend on the exciton splitting to generate energized electron and hole in π-delocalized electronic systems.[10] Thus, 2D conjugated polymers are recommended as host materials for solar energy
G. Zhang, M. Zhang, X. Ye, Prof. X. Qiu, Dr. S. Lin, Prof. X. Wang Research Institute of Photocatalysis Fujian Provincial Key Laboratory of Photocatalysis-State Key Laboratory Breeding Base and College of Chemistry and Chemical Engineering Fuzhou University Fuzhou, 350002, People’s Republic of China E-mail: [email protected]
DOI: 10.1002/adma.201303611
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applications. Furthermore, being a race case of prototypical 2D polymer, g-CN was recently introduced as solid state conjugated semiconductors for photocatalytic hydrogen evolution, water photo-oxidation and CO2 reduction via sunlight, extending the photocatalytic materials from inorganic semiconductors to organic/polymeric counterparts.[7,11] Although the pristine g-CN shows moderate photocatalytic performance, there is still much room for improvement. The organic nature of the 2D carbon nitride polymer offers ample choice of chemical protocols to engineer the molecular structure, electronic structure and texture to boost the performance. Indeed, inorganic and organic chemistry protocols have been applied to modify carbon nitride photocatalysts, such as fabrication of nanostructure by hard-templating approach, dye sensitization, copolymerization, hybridization and doping with metal atoms (e. g., Fe, Zn, Cu) or non-metal atoms (e. g., S, B, P).[12–16] Even though, doping with iodine to modify the optical and electrical properties of the inorganic materials (e.g.,. I-doped TiO2) and traditional 1D conjugated polymers have been widely adopted,[17] to the best of our knowledge, the modification of g-CN with iodine for visible light photocatalysis has not yet been reported. In this paper, we have developed a facile in-situ modification of g-CN by using dicyandiamide (DCDA) and iodine ion as the carbon nitride precursor and dopant, respectively. For a systematic study on the doping effect, other halide ions (F−, Cl−, and Br−) have also been applied to functionalize g-CN. The thus obtained samples were subjected to various characterizations in chemical and structural alternations after the modification. The photocatalytic activities of the modified g-CN solids have been evaluated in an assay of photocatalytic H2 evolution under visible light irradiation, by using Pt and triethanolamine as co-catalyst and electron donor, respectively. Further integration of the current modification scheme by coupling with the texture engineering of 2D carbon nitride materials could generate a series of functional carbon nitride nanoarchitectures for diverse applications in (photo) catalysis, organocatalysis and photosynthesis. In Figure S1, Supporting Information all the modified g-CN samples show enhanced optical absorption at the light spectrum region below 420 nm, indicating that the doping of halide ions can indeed optimize the band structures of the polymeric carbon nitride. Among them, only iodine doped g-CN (CN-I) exhibits an obvious red-shift of the absorption band edge, with a wide shoulder tail extending the optical absorption region from 420 nm to 600 nm. This is mainly ascribed to the effective extension of aromatic carbon nitride heterocycle by I ions.[16] Besides, the introduce of iodine will generate impurity energy levels above the valence band edge. In this case, the band gap of the carbon nitride is reduced. This can be further proved by the DFT calculation results and the XPS valance spectra (Figure S2 and S3). This enhanced optical property is expected
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to find important application in photocatalysis, since more vis621.5 eV ible photons might be harvested to run photocatalytic reactions. Therefore, the photocatalytic activities of pure g-CN and halide 624.4 eV 619.4 eV ions doped g-CN (CN-X, wherein X represents halide ions) were 1 evaluated in an assay of hydrogen evolution with visible light (λ > 420 nm), by taking triethanolamine and Pt as holes scavenger and co-catalyst, respectively. As shown in Figure S4, all I 3d 5/2 I 3d 3/2 modified CN-X samples show accelerated hydrogen evolution rate (HER) in comparison with the pristine g-CN. Moreover, as the reductive potential of the halide ion increased, the HER 2 of the modified CN-X speeded up. The CN-I shows the fastest HER (about 38 μ mol h−1), 2 times faster than that of pure g-CN (14 μ mol h−1). Based on the optical characterization and hydrogen evolution activity, iodine modification is considered as 640 635 630 625 620 615 a good candidate to facilitate the photocatalytic performance. Binding Energy / eV To reveal the exact origin of the enhanced photocatalytic Figure 1. High-resolution XPS spectra of the CN-I sample before (1) and properties, we examined the structural features of the pristine after (2) the photocatalytic reaction. g-CN and modified samples. The chemical valence state of the elements in CN-I was checked by X-ray photoelectron spectroscopy (XPS). As displayed in Figure S5a, the survey XPS spectra of CN-I contain only C, N, O and I elements. The binding The intimate interaction between iodine and carbon nitride energies for C 1s, N 1s, O 1s and I 3d are 284.6 e V, 398.7 extended the π-conjugated system might be beneficial for the e V, 532.0 e V and 628.4 e V, respectively. This result confirms mobility of photo-generated carriers, which will be further conthe existence of iodine in the CN-I sample. According to the firmed later. XPS analysis, the I content was determined to be 1.6, which To obtain the comprehensive understanding of the effect of is roughly equal to the initial amount used in the preparation. iodine species, the doping density of iodine was investigated. Furthermore, high resolution XPS spectra of I 3d exhibits two A series of samples were synthesized (denoted as CN-Ix, × multiple peaks centered at 621.5 and 633.3 eV, corresponding = 0.1, 0.3, 0.5, 1.0 and 2.0 respectively, where x refers to the to the I 3d5/2 and I 3d3/2, respectively (Figure S 5b). Clearly, I initial mass amount of the iodine). The apparent color of the 3d5/2 can be deconvoluted with three peaks locating at 619.4, CN-Ix samples changed from pale yellow to deep brown with 621.5 and 624.4 eV, which are associated with the I−, I+ and increasing content of iodine. Figure 2a shows the optical I7+, respectively. Small amount of I− mainly root from the preproperties of the pure g-CN and CN-Ix solids, as measured cursor, while I+ and I7+ are oxidation products of I− during the by UV-vis Diffuse Reflection Spectra. It is clear that all samreaction process, by which the electrons in I− were donated into ples show typical semiconductor absorption at blue region, the carbon nitride matrix. Iodine is viewed as electron donor to which is associated with the typical semiconductor absorption provide electrons, and the resultant I+ and I7+ are stabilized by of graphitic carbon nitride.[7] This indicates that the intrinsic electron-rich carbon nitride networks. This is a typical n-type backbone structures of g-CN were not changed by the modidoping modification of the semiconductors that facilitates the fication. Powder-XRD, FT-IR and solid state C13 NMR charac[ 19 ] hydrogen evolution. To reveal the exact position of iodine, terizations for g-CN and CN-Ix samples further certify that no theoretical calculations were adopted. As shown in Figure S6 evidence structure destroy occurred after iodine modification and S7, the sp2-bonded N is prone to be substituted by I atom. (Figure S9). This finding is very important, since the mainteAccording to the Bader Charge analysis,[20] iodine atom has nance of 2D conjugated backbone structures is a prerequisite positive charges +1.01 |e|, clearly indicating there is significant electron transfers from (a) (b) the iodine atom to the CN substrate. Our calculation result is in good agreement with CN the experimental observation that the valence state of iodine atom is detected to be positive (as shown in Figure 1). This is further proved by the UV Raman spectra (Figure S8), a weak CN-I2.0 CN-I2.0 peak which is attributed to the iodine species is observed. Furthermore, chemical state of iodine in the samples before and after photocatalytic reaction was also tested. As shown CN in Figure 1, negligible difference can be 400 500 600 700 800 450 500 550 600 650 700 observed for the high-resolution XPS spectra Wavelength / nm Wavelength / nm of CN-I2.0 before and after the reaction. This indicates that the iodine was stabilized Figure 2. UV-vis Diffuse Reflection Spectra (a) and Photoluminescence spectra under 400 nm by electron-rich carbon nitride networks. excitation at 298 K (b) for CN and CN-Ix samples.
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4 for the π-delocalized electronic systems. 0.6 (a) (b) Besides, as the doping density increased, an CN off on CN-I1.0 evident red-shift of the absorption band edge CN-I1.0 emerged. This designates a band-gap nar0.4 rowing after iodine modifications. The gap decreased from 2.75 e V for pristine g-CN 2 to 2.69 eV for CN-I2.0, with a total narrowed 0.2 value about 0.06 e V. The band gaps of the CN pure g-CN and CN-Ix samples are summarized in Table S1. As the doping contents increased, a noticeable improved optical 0 0.0 0 20 40 0 2 4 6 absorption between 420 nm and 650 nm is 5 Re(z) / ohm × 10 Time / s created. This can be seen more clearly by the enhanced optical absorption between CN-Ix Figure 4. Electrochemical Impedance Spectroscopy (a) and transient photocurrents (λ > and g-CN (Figure S12), demonstrating that 420 nm) (b) for CN and CN-I samples. 1.0 the optical absorption properties can be well controlled by tuning the content of iodine. increased. This indicates the increased density state of conducThe charge-carriers separation/recombination rates of the tion band after the electron donation from the iodine species.[16] photo-excited carriers were next investigated by room temperaIn the next set of experiment, the pure g-CN and CN-I samture photoluminescence spectra (PL) under excitation waveples were evaluated by the photo-electrochemical experiments. length of 400 nm, because they are key parameters in deterFirstly, electrochemical impedance spectroscopy (EIS) in the mining the photocatalytic performance. As shown in Figure 2b, dark was carried out. In Figure 4a, a marked decrease Nyquist all CN-Ix samples exhibit a broad emission peak centering at plots diameter for CN-I1.0 is observed, which suggests that the about 460 nm, which can be attributed to the band-band PL electronic resistance of CN-I1.0 is smaller than that of pristine phenomenon with the energy of light which is approximately g-CN.[14a] On the other hand, an enhanced photocurrent for equals to the band-gap energy (2.7 eV) of g-CN.[18] Obviously, CN-I1.0 is generated, which is nearly 3 times higher than that of the PL intensity decreased as the doping contents of the iodine the pure g-CN (Figure 4b), strongly illustrating that the mobility increased. The PL quenching in principle indicates a suppressed of the photo-excited charge carriers is promoted.[18a] Although recombination rate of the photo-induced charge carriers.[18c] the increased electrical conductivity of the iodine doped g-CN This is mainly benefited from the expended conjugated system, is still moderate, which is mainly due to the relatively rough which is conduced to hasten the mobility of the free charge preparation strategy of the electrodes, it can be viewed as a feacarriers.[14] The lower recombination rate of charge carriers is sible modification strategy to increase the charge transfer rate proposed to facilitate the heterogeneous photocatalysis. The of polymeric carbon nitride. It should be noted that the elecextension of the covalent system can be further revealed by the trochemical characterizations illustrate an encouraging result room-temperature electron paramagnetic resonance (EPR). In favoring for separation and migration of the photo-induced Figure 3, only one single Lorentzian line centered at a g value charge carriers. of 2.0034 is detected for both pristine g-CN and CN-I samples It is also found that the textural properties of g-CN were in the magnetic field from 3465 to 3650 G. The g value equivamodified by in-situ non-metal doping strategy as illustrated lent to 2.0034 is assigned to lone pair electrons in sp2-carbon in in Figure S10. The SEM image of bulk g-CN shows a typical a typical heptazine graphitic carbon nitride.[14] In addition, the 2-D graphitic accumulation with large pieces of thick layers. EPR intensity greatly strengthened when the doping density When the g-CN was modified with iodine, many micro-pores on the surface of the graphitic layers emerged. This is mainly CN-I2.0 due to the release of ammonia gas during the pyrolysis process. The thermal-induced pores contribute to the enlarged surface area, which was further confirmed by the low temperature N2 BET analysis. As shown in Table S1, as the doping density increased, the surface area is enlarged from 12 (for pure CN g-CN) to 23 m2 g−1 (for CN-I2.0), which is nearly one time larger than that of pristine g-CN. The larger surface area is expected to provide more active sites to adsorb reaction species. This is extremely important for the heterogeneous photocatalytic reactions, as confirmed by the following photocatalytic hydrogen evolution experiments. From what has been discussed above, one can safely conclude that the iodine modification can evidently improve 3480 3510 3540 the optical, emission and electronic properties of the g-CN. B /G The pure g-CN and CN-I samples were therefore systemically evaluated in an assay of photocatalytic hydrogen evolution by Figure 3. Room temperature EPR spectra for CN and CN-Ix samples.
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loading 3 wt.% Pt as co-catalyst and using triethanolamine as hole scavenger. Figure 5a shows that when the doping content increased (from 0.1 to 1.0), the hydrogen evolution rate (HER) gradually speeded up. The activity decreased when further increasing the iodine content to 2.0 (30 μ mol h−1), but it is still higher than that of the pure g-CN (14 μ mol h−1). When the doping content is 1.0, an optimum HER (ca. 38 μ mol h−1 with an apparent quantum of 2.4% at 420 nm) is achieved, which is nearly 2 times faster than that of pristine g-CN (14 μ mol h−1). The HER of the solids sample was also evaluated under different wavelength irradiation. Interestingly, the photocatalytic activities of CN-I1.0 is well coincidence with its optical 808
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absorption (Figure 5b), suggesting that the main driving force of the photocatalytic reaction is the harvested visible photons. In addition, CN-I still shows photocatalytic activity (HER = 1 μ mol h−1) even when the irradiation wavelength is extended to as long as 600 nm. However, the pure g-CN is already inert (HER < 0.1 μ mol h−1) when the illuminate wavelength is just 500 nm. This result strongly illustrates that CN-I solid possesses advanced optical property to drive photochemical reactions. Moreover, the overall amount of produced H2 increased as a function of time when the irradiation wavelength is 550 nm (Figure 5b inset). After 8 hours reaction, the overall amount of H2 gas ran up to 13.5 μ mol. In order to check the stability of the catalyst, we carried out hydrogen production catalyzed by CN-I1.0 under a prolonged visible light irradiation (λ > 420 nm) of 20 h. As shown in Figure 5c, it is clear that no obvious decay of HER was observed after 4 consecutive cycle’s reactions. This suggests the well stability of CN-I catalyst against photocorrosion. After the reaction, the catalyst was recovered and then was characterized in terms of optical properties and intrinsic structure by DRS, XRD and FT-IR respectively (Figure S11). Indeed, there is virtually no noticeable alternation in the structure of the catalyst before and after the reaction, again reflecting the robust nature of carbon nitride based photocatalysts. In summary, a facial in-situ modification strategy has been developed using DCDA and ammonium iodine as starting materials to obtain CN-I solids. The modification endows carbon nitride materials with enhanced optical absorption, enlarged surface area, and accelerated charge carriers transfer rate as well as the increased hydrogen evolution rate. Concluded from the photocatalytic results, iodine has been regarded as the optimum dopant, and 1.0 has been considered to be the best doping density. Interestingly, CN-I still shows photocatalytic activity even the wavelength is extended to 600 nm while pristine g-CN is inactive at just 500 nm. This well demonstrates the advantage of non-metal doping to optimize the band structure and texture of the polymeric catalyst. This paper highlights the importance of iodine doping to increase the optical and electrical properties, as well as photocatalytic activities of this newly-developed conjugated carbon nitride semiconductor. It is still anticipated that the innovative doping strategy presented herein could facially couple to the already known modification tools of 2D carbon nitride,[12–16] further developing a new family of light-harvesting 2D platforms for the efficient and sustained utilization of solar radiation for a variety of advanced applications, including CO2 photofixation, organic photosynthesis and pollutant controls.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work is financially supported by the National Basic Research Program of China (2013CB632405), the National Natural Science
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Received: August 3, 2013 Revised: September 18, 2013 Published online: October 30, 2013
[1] a) J. Fréchet, Science 1994, 263, 1710; b) G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, A. J. Heeger, Nature 1992, 357, 477; c) H. Sirringhaus, N. Tessler, R. H. Friend, Science 1998, 280, 1741. [2] a) C. Chiang, C. Fincher, Y. Park, A. Heeger, H. Shirakawa, E. Louis, S. Gau, A. MacDiarmid, Phys. Rev. Lett. 1977, 39, 1098; b) C. Chiang, Y. Park, A. Heeger, H. Shirakawa, E. Louis, A. MacDiarmid, J. Chem. Phys. 1978, 69, 5098; c) Y. Park, M. Druy, C. Chiang, A. MacDiarmid, A. Heeger, H. Shirakawa, S. Ikeda, J. Polym. Sci., Polym. Lett. Ed. 1979, 17, 195; d) A. Patil, A. Heeger, F. Wudl, Chem. Rev. 1988, 88, 183. [3] a) H. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, Nature 2009, 3, 649; b) K. Shimamura, Makromol. Chem., Rapid Commun. 1982, 3, 269; c) E. Ehrenfreund, Z. Vardeny, O. Brafman, Phys. Rev. Lett. 1986, 57, 2081; d) Z. Wang, E. Scherr, A. MacDiarmid, A. Epstein, Phys. Rev. B 1992, 45, 4190; e) J. Joo, Z. Oblakowski, G. Du, J. Pouget, E. Oh, J. Wiesinger, Y. Min, A. MacDiarmid, A. Epstein, Phys. Rev. B 1994, 49, 2977; f) X. Zeng, T. Ko, J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 1993. [4] J. Burroughes, D. Bradley, A. Brown, R. Marks, K. Mackay, R. Friend, P. Burns, A. Holmes, Nature 1990, 347, 539. [5] G. Yu, J. Gao, J. Hummelen, F. Wudl, A. Heeger, Science 1995, 270, 1789. [6] F. Goettmann, A. Thomas M. Antonietti, Angew. Chem., Int. Ed. 2007, 46, 2717. [7] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8, 76. [8] X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan, Y. Xie, J. Am. Chem. Soc. 2013, 135, 18. [9] a) T. Hasegawa, Y. Iwasa, T. Koda, H. Kishida, Y. Tokura, S. Wada, H. Tashiro, H. Tachibana, M. Matsumoto, Phys. Rev. B 1996, 54, 11365; b) M. Vissenberg M. J. Jong, Phys. Rev. Lett. 1996, 77, 4820. [10] a) R. Österbacka, C. An, X. Jiang, Z. Vardeny, Science 2000, 287, 839; b) J. Hou, Z. Tan, Y. Yan, Y. He, C. Yang, Y. Li, J. Am. Chem. Soc. 2006, 128, 4911; c) H. Sirringhaus, P. Brown, R. Friend, M. Nielsen, K. Bechgaard, B. Langeveld-Voss, A. Spiering, R. Janssen, E. Meijer, P. Herwig, D. Leeuw, Nature 1999, 401, 685; d) M. McGehee, A. Heeger, Adv. Mater. 2000, 12, 1655.
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Foundation of China (21033003, 21173043 and 21203026) and the Department of Education of Fujian Province in China.
[11] a) K. Maeda, X. Wang, Y. Nishihara, D. Lu, M. Antonietti, K. Domen, J. Phys. Chem. C 2009, 113, 4940; b) X. Wang, K. Maeda, X. Chen, K. Takanabe, K. Domen, Y. Hou, X. Fu, M. Antonietti, J. Am. Chem. Soc. 2009, 131, 1681; c) G. Dong, L. Zhang, J. Mater. Chem. 2012, 22, 1160. [12] a) S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P. M. Ajayan, Adv. Mater. 2013, 25, 2452; b) J. Sun, J. Zhang, M. Zhang, M. Antonietti, X. Fu, X. Wang, Nature Commun. 2012, 3, 1139; c) X. Li, J. Zhang, X. Chen, A. Fischer, A. Thomas, M. Antonietti, X. Wang, Chem. Mater. 2011, 23, 4344; d) J. Zhang, F. Guo, X. Wang, Adv. Funct. Mater. 2013, 23, 3008. [13] a) K. Takanabe, K. Kamata, X. Wang, M. Antonietti, J. Kubota, K. Domen, Phys. Chem. Chem. Phys. 2010, 12, 13020; b) Y. Wang, J. Hong, W. Zhang, R. Xu, Catal. Sci. Technol. 2013, 3, 1703. [14] a) J. Zhang, X. Chen, K. Takanabe, K. Maeda, K. Domen, J. Epping, X. Fu, M. Antonietti, X. Wang, Angew. Chem. Int. Ed. 2010, 49, 441; b) J. Zhang, G. Zhang, X. Chen, S. Lin, L. Möhlmann, G. Dołe¸ga, G. Lipner, M. Antonietti, S. Blechert, X. Wang, Angew.Chem. Int. Ed. 2012, 124, 3237. [15] a) Y. Hou, A. B. Laursen, J. Zhang, G. Zhang, Y. Zhu, X. Wang, S. Dahl, Ib Chorkendorff, Angew. Chem. Int. Ed. 2013, 52, 3621; b) J. Zhang, M. Grzelczak, Y. Hou, K. Maeda, K. Domen, X. Fu, M. Antonietti, X. Wang, Chem. Sci. 2012, 3, 443; c) Q. Xiang, J. Yu, M. Jaroniec, J. Phys. Chem. C 2011, 115, 7355. [16] a) X. Chen, J. Zhang, X. Fu, M. Antonietti, X. Wang, J. Am. Chem. Soc. 2009, 131, 11658; b) G. Liu, P. Niu, C. Sun, S. Smith, Z. Chen, G. Lu, H. Cheng, J. Am. Chem. Soc. 2010, 13, 11642; c) J. Hong, X. Xia, Y. Wang, R. Xu, J. Mater. Chem. 2012, 22, 15006; d) X. Wang, X. Chen, A. Thomas, X. Fu, M. Antonietti, Adv. Mater. 2009, 21, 1609; e) Z. Ding, X. Chen, M. Antonietti, X. Wang, ChemSusChem. 2011, 4, 274; f) Z. Lin, X. Wang, Angew. Chem. Int. Ed. 2013, 52, 1735. [17] a) G. Liu, Z. Chen, C. Dong, Y. Zhao, F. Li, G. Lu, H. Cheng, J. Phys. Chem. B 2006, 110, 20823; b) S. Tojo, T. Tachikawa, M. Fujitsuka, T. Majima, J. Phys. Chem. C 2008, 112, 14948; c) X. Hong, Z. Wang, W. Cai, F. Lu, J. Zhang, Y. Yang, N. Ma, Y. Liu, Chem. Mater. 2005, 17, 1548; d) W. Su, Y. Zhang, Li, L., Wu, X. Wang, J. Li, X. Fu, Langmuir 2008, 24, 3422; e) Z. Yao, H. Nie, Z. Yang, X. Zhou, Z. Liu, S. Huang, Chem. Commun. 2012, 48, 1027. [18] a) G. Zhang, J. Zhang, M. Zhang, X. Wang, J. Mater. Chem. 2012, 22, 8083; b) Y. Cui, Z. Ding, X. Fu, X. Wang, Angew. Chem. Int. Ed. 2012, 124, 11984; c) J. Zhang, M. Zhang, R. Sun, X. Wang, Angew. Chem. Int. Ed. 2012, 51, 10145. [19] G. Liu, L. Wang, H. Yang, H. Cheng, G. Lu, J. Mater. Chem. 2010, 20, 831. [20] G. Henkelman, A. Arnaldsson, H. Jonsson, Comput. Mater. Sci. 2006, 36, 254.
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Iodine Modified Carbon Nitride Semiconductors as Visible Light Photocatalysts for Hydrogen Evolution Guigang Zhang, Mingwen Zhang, Xinxin Ye, Xiaoqing Qiu, Sen Lin, and Xinchen Wang* Conjugated polymers have been widely used in a variety of fields including industrial production, daily life, aviation, as well as biomedicine.[1] This is mainly benefited from the excellent intrinsic thermoplasticity and thermosetting properties of organic backbones. However, the inherently weak electrical conductivity of polymer materials greatly restricted their practical applications. Since the pioneering work of Heeger et al.[2] much attention has been paid to conductive polymers due to their unique electronic and optical properties that allow their promising applications in plastic electronic and photovoltaic devices. In principle, redox doping strategies have been widely used to increase the electrical conductivity of conjugated polymers, such as polyacetylene (PA), polythiophene (PT), polyaniline (PANI), polypyrrole (PPV) and poly(3,4-ethylenedioxythiophene) (PEDOT), typically using sodium and iodine to create n-type and p-type conductive polymers, respectively.[3] In virtue of the unique conducting properties, a variety of semiconductor devices have been fabricated. For example, the discovery of polymer light-emitting diodes (PLED) in 1990 extremely expands the applications of LED, which is mainly referred to its distinct features, including flexibility, simple process and low cost.[4] It is noted that most current conjugated polymers feature one dimensional (1D) linear-backbone structures, being suffered from high exciton binding energy due to quantum effect of charge carrier.[5] Very recently, great attentions have been paid to a new class of polymeric semiconductor, graphitic carbon nitride (g-CN), which is a prototypical two-dimensional (2D) conjugated polymer featuring an extended delocalized pi electron system. The application of 2D g-CN polymers are now being explored in the field of catalysis, photocatalysis, bioimaging and so on,[6–8] because they offer several features that distinguish them from linear macromolecules. For example, the binding energy of Frenkel-type excitons in 2D planar conjugated polymers is lower than those in linear-backbone polymer.[9] In addition, the performances of 2D conjugated polymers are strongly depend on the exciton splitting to generate energized electron and hole in π-delocalized electronic systems.[10] Thus, 2D conjugated polymers are recommended as host materials for solar energy
G. Zhang, M. Zhang, X. Ye, Prof. X. Qiu, Dr. S. Lin, Prof. X. Wang Research Institute of Photocatalysis Fujian Provincial Key Laboratory of Photocatalysis-State Key Laboratory Breeding Base and College of Chemistry and Chemical Engineering Fuzhou University Fuzhou, 350002, People’s Republic of China E-mail: [email protected]
DOI: 10.1002/adma.201303611
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applications. Furthermore, being a race case of prototypical 2D polymer, g-CN was recently introduced as solid state conjugated semiconductors for photocatalytic hydrogen evolution, water photo-oxidation and CO2 reduction via sunlight, extending the photocatalytic materials from inorganic semiconductors to organic/polymeric counterparts.[7,11] Although the pristine g-CN shows moderate photocatalytic performance, there is still much room for improvement. The organic nature of the 2D carbon nitride polymer offers ample choice of chemical protocols to engineer the molecular structure, electronic structure and texture to boost the performance. Indeed, inorganic and organic chemistry protocols have been applied to modify carbon nitride photocatalysts, such as fabrication of nanostructure by hard-templating approach, dye sensitization, copolymerization, hybridization and doping with metal atoms (e. g., Fe, Zn, Cu) or non-metal atoms (e. g., S, B, P).[12–16] Even though, doping with iodine to modify the optical and electrical properties of the inorganic materials (e.g.,. I-doped TiO2) and traditional 1D conjugated polymers have been widely adopted,[17] to the best of our knowledge, the modification of g-CN with iodine for visible light photocatalysis has not yet been reported. In this paper, we have developed a facile in-situ modification of g-CN by using dicyandiamide (DCDA) and iodine ion as the carbon nitride precursor and dopant, respectively. For a systematic study on the doping effect, other halide ions (F−, Cl−, and Br−) have also been applied to functionalize g-CN. The thus obtained samples were subjected to various characterizations in chemical and structural alternations after the modification. The photocatalytic activities of the modified g-CN solids have been evaluated in an assay of photocatalytic H2 evolution under visible light irradiation, by using Pt and triethanolamine as co-catalyst and electron donor, respectively. Further integration of the current modification scheme by coupling with the texture engineering of 2D carbon nitride materials could generate a series of functional carbon nitride nanoarchitectures for diverse applications in (photo) catalysis, organocatalysis and photosynthesis. In Figure S1, Supporting Information all the modified g-CN samples show enhanced optical absorption at the light spectrum region below 420 nm, indicating that the doping of halide ions can indeed optimize the band structures of the polymeric carbon nitride. Among them, only iodine doped g-CN (CN-I) exhibits an obvious red-shift of the absorption band edge, with a wide shoulder tail extending the optical absorption region from 420 nm to 600 nm. This is mainly ascribed to the effective extension of aromatic carbon nitride heterocycle by I ions.[16] Besides, the introduce of iodine will generate impurity energy levels above the valence band edge. In this case, the band gap of the carbon nitride is reduced. This can be further proved by the DFT calculation results and the XPS valance spectra (Figure S2 and S3). This enhanced optical property is expected
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to find important application in photocatalysis, since more vis621.5 eV ible photons might be harvested to run photocatalytic reactions. Therefore, the photocatalytic activities of pure g-CN and halide 624.4 eV 619.4 eV ions doped g-CN (CN-X, wherein X represents halide ions) were 1 evaluated in an assay of hydrogen evolution with visible light (λ > 420 nm), by taking triethanolamine and Pt as holes scavenger and co-catalyst, respectively. As shown in Figure S4, all I 3d 5/2 I 3d 3/2 modified CN-X samples show accelerated hydrogen evolution rate (HER) in comparison with the pristine g-CN. Moreover, as the reductive potential of the halide ion increased, the HER 2 of the modified CN-X speeded up. The CN-I shows the fastest HER (about 38 μ mol h−1), 2 times faster than that of pure g-CN (14 μ mol h−1). Based on the optical characterization and hydrogen evolution activity, iodine modification is considered as 640 635 630 625 620 615 a good candidate to facilitate the photocatalytic performance. Binding Energy / eV To reveal the exact origin of the enhanced photocatalytic Figure 1. High-resolution XPS spectra of the CN-I sample before (1) and properties, we examined the structural features of the pristine after (2) the photocatalytic reaction. g-CN and modified samples. The chemical valence state of the elements in CN-I was checked by X-ray photoelectron spectroscopy (XPS). As displayed in Figure S5a, the survey XPS spectra of CN-I contain only C, N, O and I elements. The binding The intimate interaction between iodine and carbon nitride energies for C 1s, N 1s, O 1s and I 3d are 284.6 e V, 398.7 extended the π-conjugated system might be beneficial for the e V, 532.0 e V and 628.4 e V, respectively. This result confirms mobility of photo-generated carriers, which will be further conthe existence of iodine in the CN-I sample. According to the firmed later. XPS analysis, the I content was determined to be 1.6, which To obtain the comprehensive understanding of the effect of is roughly equal to the initial amount used in the preparation. iodine species, the doping density of iodine was investigated. Furthermore, high resolution XPS spectra of I 3d exhibits two A series of samples were synthesized (denoted as CN-Ix, × multiple peaks centered at 621.5 and 633.3 eV, corresponding = 0.1, 0.3, 0.5, 1.0 and 2.0 respectively, where x refers to the to the I 3d5/2 and I 3d3/2, respectively (Figure S 5b). Clearly, I initial mass amount of the iodine). The apparent color of the 3d5/2 can be deconvoluted with three peaks locating at 619.4, CN-Ix samples changed from pale yellow to deep brown with 621.5 and 624.4 eV, which are associated with the I−, I+ and increasing content of iodine. Figure 2a shows the optical I7+, respectively. Small amount of I− mainly root from the preproperties of the pure g-CN and CN-Ix solids, as measured cursor, while I+ and I7+ are oxidation products of I− during the by UV-vis Diffuse Reflection Spectra. It is clear that all samreaction process, by which the electrons in I− were donated into ples show typical semiconductor absorption at blue region, the carbon nitride matrix. Iodine is viewed as electron donor to which is associated with the typical semiconductor absorption provide electrons, and the resultant I+ and I7+ are stabilized by of graphitic carbon nitride.[7] This indicates that the intrinsic electron-rich carbon nitride networks. This is a typical n-type backbone structures of g-CN were not changed by the modidoping modification of the semiconductors that facilitates the fication. Powder-XRD, FT-IR and solid state C13 NMR charac[ 19 ] hydrogen evolution. To reveal the exact position of iodine, terizations for g-CN and CN-Ix samples further certify that no theoretical calculations were adopted. As shown in Figure S6 evidence structure destroy occurred after iodine modification and S7, the sp2-bonded N is prone to be substituted by I atom. (Figure S9). This finding is very important, since the mainteAccording to the Bader Charge analysis,[20] iodine atom has nance of 2D conjugated backbone structures is a prerequisite positive charges +1.01 |e|, clearly indicating there is significant electron transfers from (a) (b) the iodine atom to the CN substrate. Our calculation result is in good agreement with CN the experimental observation that the valence state of iodine atom is detected to be positive (as shown in Figure 1). This is further proved by the UV Raman spectra (Figure S8), a weak CN-I2.0 CN-I2.0 peak which is attributed to the iodine species is observed. Furthermore, chemical state of iodine in the samples before and after photocatalytic reaction was also tested. As shown CN in Figure 1, negligible difference can be 400 500 600 700 800 450 500 550 600 650 700 observed for the high-resolution XPS spectra Wavelength / nm Wavelength / nm of CN-I2.0 before and after the reaction. This indicates that the iodine was stabilized Figure 2. UV-vis Diffuse Reflection Spectra (a) and Photoluminescence spectra under 400 nm by electron-rich carbon nitride networks. excitation at 298 K (b) for CN and CN-Ix samples.
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4 for the π-delocalized electronic systems. 0.6 (a) (b) Besides, as the doping density increased, an CN off on CN-I1.0 evident red-shift of the absorption band edge CN-I1.0 emerged. This designates a band-gap nar0.4 rowing after iodine modifications. The gap decreased from 2.75 e V for pristine g-CN 2 to 2.69 eV for CN-I2.0, with a total narrowed 0.2 value about 0.06 e V. The band gaps of the CN pure g-CN and CN-Ix samples are summarized in Table S1. As the doping contents increased, a noticeable improved optical 0 0.0 0 20 40 0 2 4 6 absorption between 420 nm and 650 nm is 5 Re(z) / ohm × 10 Time / s created. This can be seen more clearly by the enhanced optical absorption between CN-Ix Figure 4. Electrochemical Impedance Spectroscopy (a) and transient photocurrents (λ > and g-CN (Figure S12), demonstrating that 420 nm) (b) for CN and CN-I samples. 1.0 the optical absorption properties can be well controlled by tuning the content of iodine. increased. This indicates the increased density state of conducThe charge-carriers separation/recombination rates of the tion band after the electron donation from the iodine species.[16] photo-excited carriers were next investigated by room temperaIn the next set of experiment, the pure g-CN and CN-I samture photoluminescence spectra (PL) under excitation waveples were evaluated by the photo-electrochemical experiments. length of 400 nm, because they are key parameters in deterFirstly, electrochemical impedance spectroscopy (EIS) in the mining the photocatalytic performance. As shown in Figure 2b, dark was carried out. In Figure 4a, a marked decrease Nyquist all CN-Ix samples exhibit a broad emission peak centering at plots diameter for CN-I1.0 is observed, which suggests that the about 460 nm, which can be attributed to the band-band PL electronic resistance of CN-I1.0 is smaller than that of pristine phenomenon with the energy of light which is approximately g-CN.[14a] On the other hand, an enhanced photocurrent for equals to the band-gap energy (2.7 eV) of g-CN.[18] Obviously, CN-I1.0 is generated, which is nearly 3 times higher than that of the PL intensity decreased as the doping contents of the iodine the pure g-CN (Figure 4b), strongly illustrating that the mobility increased. The PL quenching in principle indicates a suppressed of the photo-excited charge carriers is promoted.[18a] Although recombination rate of the photo-induced charge carriers.[18c] the increased electrical conductivity of the iodine doped g-CN This is mainly benefited from the expended conjugated system, is still moderate, which is mainly due to the relatively rough which is conduced to hasten the mobility of the free charge preparation strategy of the electrodes, it can be viewed as a feacarriers.[14] The lower recombination rate of charge carriers is sible modification strategy to increase the charge transfer rate proposed to facilitate the heterogeneous photocatalysis. The of polymeric carbon nitride. It should be noted that the elecextension of the covalent system can be further revealed by the trochemical characterizations illustrate an encouraging result room-temperature electron paramagnetic resonance (EPR). In favoring for separation and migration of the photo-induced Figure 3, only one single Lorentzian line centered at a g value charge carriers. of 2.0034 is detected for both pristine g-CN and CN-I samples It is also found that the textural properties of g-CN were in the magnetic field from 3465 to 3650 G. The g value equivamodified by in-situ non-metal doping strategy as illustrated lent to 2.0034 is assigned to lone pair electrons in sp2-carbon in in Figure S10. The SEM image of bulk g-CN shows a typical a typical heptazine graphitic carbon nitride.[14] In addition, the 2-D graphitic accumulation with large pieces of thick layers. EPR intensity greatly strengthened when the doping density When the g-CN was modified with iodine, many micro-pores on the surface of the graphitic layers emerged. This is mainly CN-I2.0 due to the release of ammonia gas during the pyrolysis process. The thermal-induced pores contribute to the enlarged surface area, which was further confirmed by the low temperature N2 BET analysis. As shown in Table S1, as the doping density increased, the surface area is enlarged from 12 (for pure CN g-CN) to 23 m2 g−1 (for CN-I2.0), which is nearly one time larger than that of pristine g-CN. The larger surface area is expected to provide more active sites to adsorb reaction species. This is extremely important for the heterogeneous photocatalytic reactions, as confirmed by the following photocatalytic hydrogen evolution experiments. From what has been discussed above, one can safely conclude that the iodine modification can evidently improve 3480 3510 3540 the optical, emission and electronic properties of the g-CN. B /G The pure g-CN and CN-I samples were therefore systemically evaluated in an assay of photocatalytic hydrogen evolution by Figure 3. Room temperature EPR spectra for CN and CN-Ix samples.
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loading 3 wt.% Pt as co-catalyst and using triethanolamine as hole scavenger. Figure 5a shows that when the doping content increased (from 0.1 to 1.0), the hydrogen evolution rate (HER) gradually speeded up. The activity decreased when further increasing the iodine content to 2.0 (30 μ mol h−1), but it is still higher than that of the pure g-CN (14 μ mol h−1). When the doping content is 1.0, an optimum HER (ca. 38 μ mol h−1 with an apparent quantum of 2.4% at 420 nm) is achieved, which is nearly 2 times faster than that of pristine g-CN (14 μ mol h−1). The HER of the solids sample was also evaluated under different wavelength irradiation. Interestingly, the photocatalytic activities of CN-I1.0 is well coincidence with its optical 808
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absorption (Figure 5b), suggesting that the main driving force of the photocatalytic reaction is the harvested visible photons. In addition, CN-I still shows photocatalytic activity (HER = 1 μ mol h−1) even when the irradiation wavelength is extended to as long as 600 nm. However, the pure g-CN is already inert (HER < 0.1 μ mol h−1) when the illuminate wavelength is just 500 nm. This result strongly illustrates that CN-I solid possesses advanced optical property to drive photochemical reactions. Moreover, the overall amount of produced H2 increased as a function of time when the irradiation wavelength is 550 nm (Figure 5b inset). After 8 hours reaction, the overall amount of H2 gas ran up to 13.5 μ mol. In order to check the stability of the catalyst, we carried out hydrogen production catalyzed by CN-I1.0 under a prolonged visible light irradiation (λ > 420 nm) of 20 h. As shown in Figure 5c, it is clear that no obvious decay of HER was observed after 4 consecutive cycle’s reactions. This suggests the well stability of CN-I catalyst against photocorrosion. After the reaction, the catalyst was recovered and then was characterized in terms of optical properties and intrinsic structure by DRS, XRD and FT-IR respectively (Figure S11). Indeed, there is virtually no noticeable alternation in the structure of the catalyst before and after the reaction, again reflecting the robust nature of carbon nitride based photocatalysts. In summary, a facial in-situ modification strategy has been developed using DCDA and ammonium iodine as starting materials to obtain CN-I solids. The modification endows carbon nitride materials with enhanced optical absorption, enlarged surface area, and accelerated charge carriers transfer rate as well as the increased hydrogen evolution rate. Concluded from the photocatalytic results, iodine has been regarded as the optimum dopant, and 1.0 has been considered to be the best doping density. Interestingly, CN-I still shows photocatalytic activity even the wavelength is extended to 600 nm while pristine g-CN is inactive at just 500 nm. This well demonstrates the advantage of non-metal doping to optimize the band structure and texture of the polymeric catalyst. This paper highlights the importance of iodine doping to increase the optical and electrical properties, as well as photocatalytic activities of this newly-developed conjugated carbon nitride semiconductor. It is still anticipated that the innovative doping strategy presented herein could facially couple to the already known modification tools of 2D carbon nitride,[12–16] further developing a new family of light-harvesting 2D platforms for the efficient and sustained utilization of solar radiation for a variety of advanced applications, including CO2 photofixation, organic photosynthesis and pollutant controls.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work is financially supported by the National Basic Research Program of China (2013CB632405), the National Natural Science
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Received: August 3, 2013 Revised: September 18, 2013 Published online: October 30, 2013
[1] a) J. Fréchet, Science 1994, 263, 1710; b) G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, A. J. Heeger, Nature 1992, 357, 477; c) H. Sirringhaus, N. Tessler, R. H. Friend, Science 1998, 280, 1741. [2] a) C. Chiang, C. Fincher, Y. Park, A. Heeger, H. Shirakawa, E. Louis, S. Gau, A. MacDiarmid, Phys. Rev. Lett. 1977, 39, 1098; b) C. Chiang, Y. Park, A. Heeger, H. Shirakawa, E. Louis, A. MacDiarmid, J. Chem. Phys. 1978, 69, 5098; c) Y. Park, M. Druy, C. Chiang, A. MacDiarmid, A. Heeger, H. Shirakawa, S. Ikeda, J. Polym. Sci., Polym. Lett. Ed. 1979, 17, 195; d) A. Patil, A. Heeger, F. Wudl, Chem. Rev. 1988, 88, 183. [3] a) H. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, Nature 2009, 3, 649; b) K. Shimamura, Makromol. Chem., Rapid Commun. 1982, 3, 269; c) E. Ehrenfreund, Z. Vardeny, O. Brafman, Phys. Rev. Lett. 1986, 57, 2081; d) Z. Wang, E. Scherr, A. MacDiarmid, A. Epstein, Phys. Rev. B 1992, 45, 4190; e) J. Joo, Z. Oblakowski, G. Du, J. Pouget, E. Oh, J. Wiesinger, Y. Min, A. MacDiarmid, A. Epstein, Phys. Rev. B 1994, 49, 2977; f) X. Zeng, T. Ko, J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 1993. [4] J. Burroughes, D. Bradley, A. Brown, R. Marks, K. Mackay, R. Friend, P. Burns, A. Holmes, Nature 1990, 347, 539. [5] G. Yu, J. Gao, J. Hummelen, F. Wudl, A. Heeger, Science 1995, 270, 1789. [6] F. Goettmann, A. Thomas M. Antonietti, Angew. Chem., Int. Ed. 2007, 46, 2717. [7] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8, 76. [8] X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan, Y. Xie, J. Am. Chem. Soc. 2013, 135, 18. [9] a) T. Hasegawa, Y. Iwasa, T. Koda, H. Kishida, Y. Tokura, S. Wada, H. Tashiro, H. Tachibana, M. Matsumoto, Phys. Rev. B 1996, 54, 11365; b) M. Vissenberg M. J. Jong, Phys. Rev. Lett. 1996, 77, 4820. [10] a) R. Österbacka, C. An, X. Jiang, Z. Vardeny, Science 2000, 287, 839; b) J. Hou, Z. Tan, Y. Yan, Y. He, C. Yang, Y. Li, J. Am. Chem. Soc. 2006, 128, 4911; c) H. Sirringhaus, P. Brown, R. Friend, M. Nielsen, K. Bechgaard, B. Langeveld-Voss, A. Spiering, R. Janssen, E. Meijer, P. Herwig, D. Leeuw, Nature 1999, 401, 685; d) M. McGehee, A. Heeger, Adv. Mater. 2000, 12, 1655.
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Foundation of China (21033003, 21173043 and 21203026) and the Department of Education of Fujian Province in China.
[11] a) K. Maeda, X. Wang, Y. Nishihara, D. Lu, M. Antonietti, K. Domen, J. Phys. Chem. C 2009, 113, 4940; b) X. Wang, K. Maeda, X. Chen, K. Takanabe, K. Domen, Y. Hou, X. Fu, M. Antonietti, J. Am. Chem. Soc. 2009, 131, 1681; c) G. Dong, L. Zhang, J. Mater. Chem. 2012, 22, 1160. [12] a) S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P. M. Ajayan, Adv. Mater. 2013, 25, 2452; b) J. Sun, J. Zhang, M. Zhang, M. Antonietti, X. Fu, X. Wang, Nature Commun. 2012, 3, 1139; c) X. Li, J. Zhang, X. Chen, A. Fischer, A. Thomas, M. Antonietti, X. Wang, Chem. Mater. 2011, 23, 4344; d) J. Zhang, F. Guo, X. Wang, Adv. Funct. Mater. 2013, 23, 3008. [13] a) K. Takanabe, K. Kamata, X. Wang, M. Antonietti, J. Kubota, K. Domen, Phys. Chem. Chem. Phys. 2010, 12, 13020; b) Y. Wang, J. Hong, W. Zhang, R. Xu, Catal. Sci. Technol. 2013, 3, 1703. [14] a) J. Zhang, X. Chen, K. Takanabe, K. Maeda, K. Domen, J. Epping, X. Fu, M. Antonietti, X. Wang, Angew. Chem. Int. Ed. 2010, 49, 441; b) J. Zhang, G. Zhang, X. Chen, S. Lin, L. Möhlmann, G. Dołe¸ga, G. Lipner, M. Antonietti, S. Blechert, X. Wang, Angew.Chem. Int. Ed. 2012, 124, 3237. [15] a) Y. Hou, A. B. Laursen, J. Zhang, G. Zhang, Y. Zhu, X. Wang, S. Dahl, Ib Chorkendorff, Angew. Chem. Int. Ed. 2013, 52, 3621; b) J. Zhang, M. Grzelczak, Y. Hou, K. Maeda, K. Domen, X. Fu, M. Antonietti, X. Wang, Chem. Sci. 2012, 3, 443; c) Q. Xiang, J. Yu, M. Jaroniec, J. Phys. Chem. C 2011, 115, 7355. [16] a) X. Chen, J. Zhang, X. Fu, M. Antonietti, X. Wang, J. Am. Chem. Soc. 2009, 131, 11658; b) G. Liu, P. Niu, C. Sun, S. Smith, Z. Chen, G. Lu, H. Cheng, J. Am. Chem. Soc. 2010, 13, 11642; c) J. Hong, X. Xia, Y. Wang, R. Xu, J. Mater. Chem. 2012, 22, 15006; d) X. Wang, X. Chen, A. Thomas, X. Fu, M. Antonietti, Adv. Mater. 2009, 21, 1609; e) Z. Ding, X. Chen, M. Antonietti, X. Wang, ChemSusChem. 2011, 4, 274; f) Z. Lin, X. Wang, Angew. Chem. Int. Ed. 2013, 52, 1735. [17] a) G. Liu, Z. Chen, C. Dong, Y. Zhao, F. Li, G. Lu, H. Cheng, J. Phys. Chem. B 2006, 110, 20823; b) S. Tojo, T. Tachikawa, M. Fujitsuka, T. Majima, J. Phys. Chem. C 2008, 112, 14948; c) X. Hong, Z. Wang, W. Cai, F. Lu, J. Zhang, Y. Yang, N. Ma, Y. Liu, Chem. Mater. 2005, 17, 1548; d) W. Su, Y. Zhang, Li, L., Wu, X. Wang, J. Li, X. Fu, Langmuir 2008, 24, 3422; e) Z. Yao, H. Nie, Z. Yang, X. Zhou, Z. Liu, S. Huang, Chem. Commun. 2012, 48, 1027. [18] a) G. Zhang, J. Zhang, M. Zhang, X. Wang, J. Mater. Chem. 2012, 22, 8083; b) Y. Cui, Z. Ding, X. Fu, X. Wang, Angew. Chem. Int. Ed. 2012, 124, 11984; c) J. Zhang, M. Zhang, R. Sun, X. Wang, Angew. Chem. Int. Ed. 2012, 51, 10145. [19] G. Liu, L. Wang, H. Yang, H. Cheng, G. Lu, J. Mater. Chem. 2010, 20, 831. [20] G. Henkelman, A. Arnaldsson, H. Jonsson, Comput. Mater. Sci. 2006, 36, 254.
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