CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402180
Synthesis of Potassium-Modified Graphitic Carbon Nitride with High Photocatalytic Activity for Hydrogen Evolution Ming Wu, Jun-Min Yan,* Xian-nian Tang, Ming Zhao, and Qing Jiang*[a] Potassium-modified graphitic carbon nitride (K-g-C3N4) nanosheets are synthesized by a facile KCl-template method that holds the advantage of easy removal of residual template. A combination of XRD, X-ray photoelectron spectroscopy, and inductively coupled plasma analyses are utilized to characterize the obtained resultant K-g-C3N4 architectures, which are composed of nanosheets of variable thickness (< 10 nm). Photocatalytic hydrogen evolution experiments under visible light irra-
diation showed that K-g-C3N4 nanosheets have high photocatalytic activities (up to about thirteen times higher than that of pure g-C3N4) as well as good stability (no reduction in activity within 16 h); both features emanate from their unique structural characteristics. These results illustrate the viability of this methodology for the facile synthesis of efficient heterogeneous photocatalysts for potential commercial applications.
Introduction Semiconductor photocatalysts have attracted great attention for their application in solar energy conversion.[1, 2] In recent years, TiO2 has been regarded as a good photocatalyst for its high activity, great stability, nontoxicity, and low cost.[3, 4] However, the practical application of TiO2 is seriously limited by its wide band gap of 3.2 eV, which makes TiO2 mainly absorb UV light (l < 400 nm) that accounts for only 4 % of solar energy.[5, 6] Therefore, it is highly desirable to develop new photocatalysts with high photocatalytic activities and stabilities under visible light irradiation. Graphitic carbon nitride (g-C3N4), as an interesting metal-free polymeric organic semiconductor, has gained more and more worldwide attention.[7–9] As a promising metal-free photocatalyst for solar energy conversion, g-C3N4 possesses appropriate band position (band gap of 2.7 eV), excellent thermal and chemical stabilities, and good optical features.[10–13] However, the photocatalytic efficiency of pure g-C3N4 is still far from the level required for practical applications because of its high recombination rate of photogenerated electron–hole pairs for efficient use of solar energy.[14–16] To solve this problem many strategies have been proposed, such as doping with foreign ions (B, P, Ag, Fe, etc.), increasing surface area, and combining with other semiconductors.[17–24] To this end, g-C3N4 with different structures has been synthesized using various hard silica templates, such as Santa Barbara amorphous (SBA) silica and silica particles.[25–27] However, these techniques require the use of hazardous chemicals (e.g., NH4HF2) to remove the hard silica templates. [a] M. Wu, Dr. J.-M. Yan, X.-n. Tang, Dr. M. Zhao, Dr. Q. Jiang Key Laboratory of Automobile Materials Ministry of Education School of Materials Science and Engineering Jilin University, Changchun 130022 (P.R. China) E-mail: [email protected] [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Herein, we first synthesize potassium-modified g-C3N4 (K-gC3N4) nanosheets using KCl as a soft template. Residual KCl can be subsequently removed easily by washing with water. The obtained unique structure of K-g-C3N4 can effectively reduce the recombination rate of photogenerated electron–hole pairs; therefore, the photocatalytic activity and stability of K-g-C3N4 under visible light irradiation is enhanced greatly relative to pure g-C3N4.
Results and Discussion A series of K-g-C3N4 nanosheets were produced using KCl/gC3N4 weight ratios of 1:1, 5:1, 10:1, 15:1, and 17.5:1; the resulting architectures are denoted herein as K1-g-C3N4, K5-g-C3N4, K10-g-C3N4, K15-g-C3N4, and K17.5-g-C3N4, respectively. Pure gC3N4 has the bulk structure, whereas K10-g-C3N4 is composed of curved nanosheets with different thicknesses (Figure 1 a, b, and d).[28] From high-resolution TEM (HRTEM) images of K10-gC3N4 (Figure 1 c, red arrows) it is clear that the thicknesses of these nanosheets range from 1.5 to 6.3 nm. This structure is beneficial for photocatalytic reactions (vide infra). From XRD patterns of pure g-C3N4 and K10-g-C3N4 (Figure 2 a), it can be found that K10-g-C3N4 has a signal that corresponds to the (002) plane, similarly to pure g-C3N4 ; however, it is slightly shifted relative to g-C3N4 (from 27.48 to 28.08). In fact, all (002) signals of specimens with K (Figure 2 b) are located at higher angles (27.78 for K1-g-C3N4 ; 28.08 for K1-g-C3N4, K10-g-C3N4, and K17.5-g-C3N4), which means that the layer distance of the (002) plane of g-C3N4 is decreased after K modifying.[29, 30] Moreover, after K modifying, the (001) signal at 138 disappeared (Figure 2 a and b), indicating that the in-plane structure is destroyed or changed.[31, 32] The compositions and chemical states of K-g-C3N4 and gC3N4 were further tested by X-ray photoelectron spectroscopy (XPS) for comparison. C, N, and O were detected in survey ChemSusChem 2014, 7, 2654 – 2658
2654
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
spectra of both structures (Figure 3 a); O originated from hightemperature treatment in air during synthesis.[29, 33] For K10-gC3N4, K (but no Cl) was detected (Figure 3 a and d), indicating the existence of K in this specimen through the preparation method.[34] That was further supported by inductively coupled plasma (ICP) analyses, which showed that K content in K1-gC3N4, K10-g-C3N4, and K17.5-g-C3N4 was 12.5, 13.9, and 14.1 wt %, respectively. This means that the increase in the amount of KCl added initially (from 1 to 17.5 g) resulted in an increase of K content in as-prepared specimens. Because gC3N4 has N H2 and NH species[10] (Figure 3 b and c),K + from KCl may react with the amino species from g-C3N4 to form N–K bonds on g-C3N4 and H + , and thus K10-g-C3N4 is formed. The peak of K 2p at 292.6 eV (Figure 3 d, red trace) is the same as that in potassium azide (KN3).[35] This may prove the existence of N K bonds in K10-g-C3N4. Also, no clear binding-energy shift can be found on N 1s and C 1s between K10-g-C3N4 and g-C3N4 (Figure 3 a and b), and all the peaks imply the typical Figure 1. Middle-resolution TEM images of (a) pure g-C3N4 and (b) K10-gchemical states of N and C in g-C3N4.[10, 29, 36] Therefore, K modiC3N4 ; (c) HRTEM image of K10-g-C3N4 ; (d) low-resolution TEM image of K10g-C3N4. fying did not change the main structure of g-C3N4. Figure 4 shows the Raman spectra of K10-g-C3N4 and g-C3N4. The peaks at 725 and 995 cm 1 are assigned to the breathing modes of triazine rings,[37] and their densities decrease after K modifying. The decrease in density indicates a charge transfer occurring between g-C3N4 aromatic rings and K atoms.[38] In addition, the peak at 773 cm 1 of g-C3N4 corresponds to the out-of-plane bending mode of graphitic domains of g-C3N4 ;[37] this peak shifts to 788 cm 1 after K modifying, which may also stem from charge transfer between g-C3N4 layers and K atoms.[39] To investigate the electronic band structure of K-gFigure 2. (a) XRD patterns of pure g-C3N4 and K10-g-C3N4 ; (b) XRD patterns of all K-gC3N4 samples. C3N4 nanosheets, UV/Vis diffuse reflectance spectra and photoluminescence (PL) spectra of K10-g-C3N4 and pure g-C3N4 were obtained. The UV/ Vis adsorption of K10-g-C3N4 has a red shift relative to that of pure g-C3N4 (Figure 5 a) and the band gap of K10-g-C3N4 is determined to be 2.66 eV (Figure 5 a, inset), which is lower than that of pure g-C3N4 (2.71 eV). Therefore, K10-g-C3N4 has a higher efficiency with regard to visible light absorption and thus may have higher photocatalytic activity than g-C3N4. Figure 5 b shows the PL spectra of K10-g-C3N4 and g-C3N4 excited at 400 nm at room temperature. Pure g-C3N4 has a PL intensity that is much higher than that of K10-g-C3N4, with an emission peak at 457 nm. Because a lower peak intensity represents a lower reFigure 3. (a) XPS spectra; (b) N 1s high-resolution XPS spectra; and (c) high-resolution XPS spectra of C 1s for K10combination rate of photogenerg-C3N4 and pure g-C3N4 ; (d) high-resolution XPS spectra of K 2p and Cl 2p for K10-g-C3N4. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2014, 7, 2654 – 2658
2655
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
tivity of K10-g-C3N4 is stable and the evolution of hydrogen does not change during a 16 h period. Based on the above analyses, the photocatalytic mechanism of K-g-C3N4 can be explained as follows: g-C3N4 sheets with different thickness have different band gaps and the thinner the sheets the wider the band gap.[41, 42] Moreover, photogenerated electrons can be transferred freely between nanosheets with different band gaps.[43] K-g-C3N4 is composed of nanosheets of variable thickness (Figure 1 b and c). We chose two hypothetical nanosheets with different thicknesses (nanosheets A and B) for discussion purposes and hypothesized that nanosheet B was thicker than nanosheet A. Thus, nanosheet A has a wider Figure 4. Raman spectra (325 nm excitation) of g-C3N4 and K10-g-C3N4. band gap. Because the valence-band (VB) and conductionband (CB) positions of different nanosheets are not certain, there are three types of heterojunctions between the two nanosheets (Figure 8). However, irrespective of the type of heterojunction (Figure 8 a and b), photogenerated electrons can be transferred from a nanosheet with a higher CB to a nanosheet with a lower CB.[44, 45] As the electrons are transferred, the recomFigure 5. (a) UV/Vis diffuse reflectance spectra of K10-g-C3N4/pure g-C3N4 ; (b) PL spectra of K10-g-C3N4/pure gbination rate of photogenerated C3N4 (400 nm excitation). electron–hole pairs can be inhibited. Heterojunctions of the third type can also reduce the recombination rate of photogeneratated electron–hole pairs,[10, 29, 40] K10-g-C3N4 can efficiently ed electron–hole pairs (see the Z-scheme in Figure 8 c).[46, 47] hinder the recombination rate of photogenerated electronhole pairs relative to pure g-C3N4. Therefore, photogenerated electrons can be transferred between all nanosheets of variable thickness and the recombinaPhotocatalytic hydrogen evolution of as-prepared specimens tion rate of photogenerated electron–hole pairs can be deloaded with Pt (0.5 wt %) as a co-catalyst are evaluated under creased greatly, which agrees well with PL results (Figure 5 b). visible light irradiation (l > 420 nm), using triethanolamine Based on the above experimental and theoretical discussion, (10 vol %) as the sacrificial reagent. From Figure 6 a, it can be the high photocatalytic activity of K10-g-C3N4 for hydrogen seen that all K-g-C3N4 specimens have photocatalytic activities that are much higher than those of pure g-C3N4 (7.3 mmol h 1). evolution from water under visible light irradiation emanates Especially, K10-g-C3N4 has the highest photocatalytic activity from its unique architecture. (about thirteen times higher than that of g-C3N4) due to which the average hydrogen evolution rate can reach 102.8 mmol h 1. Conclusions In addition, the curved nanosheets of K10-g-C3N4 are maintained well after reaction (Figure 7). Figure 6 b shows the staWe successfully synthesized potassium-modified graphitic bility of K10-g-C3N4 within 16 h. Clearly, the photocatalytic accarbon nitride (K-g-C3N4) nanosheets by a KCl templating method using no hazardous chemicals. The synthesized K-g-C3N4 possesses a special architecture, which is characterized by nanosheets of variable thickness. Due to this architecture, K-g-C3N4 has high photocatalytic activity and stability for hydrogen evolution from water under visible light irradiation relative to pure g-C3N4. The present study may provide a new and facile method for the design and synthesis of heterogeneous photocatalysts.
Experimental Section Figure 6. (a) Hydrogen evolution rates of K-g-C3N4 (variable K content) and pure g-C3N4 ; (b) stability test of hydrogen evolution for K10-g-C3N4 under visible light irradiation (l > 420 nm).
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chemicals: Potassium chloride (KCl, Beijing Chemicals Works, 99.5 %), melamine (C3H6N6, Sinopharm Chemical ChemSusChem 2014, 7, 2654 – 2658
2656
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org amount of added KCl to 20 g led to a significant decrease in yields of K-modified specimens.
Figure 7. TEM image of K10-g-C3N4 after the hydrogen evolution reaction.
Characterization: XRD analyses were performed on a Rigaku D/ max-2500pc X-ray diffractometer with CuKa irradiation (l = 1.5406 ) at a scan rate of 28 min 1. TEM analyses were performed using a JEOL JEM-2010 microscope (accelerating voltage = 200 kV). XPS analyses were performed using an ESCALABMk II (Vacuum Generators) spectrometer using AlKa radiation (300 W). All binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. UV/Vis diffuse reflectance spectra were recorded on a UV-3600 spectrophotometer. PL spectra were recorded on a RF-5301 fluorescence spectrophotometer (SHIMADZU). Catalysts were all analyzed before their photocatalytic applications. The contents of K in specimens were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Jarrell Ash). Raman spectra were collected on a LabRAM ARAMIS instrument (laser wavelength 325 nm). Photocatalytic tests: Reactions were carried out in a Pyrex-top irradiation reaction vessel connected to a glass-closed gas system. H2 production was achieved by dispersing catalyst (100 mg) in an aqueous solution (100 mL) containing triethanolamine (10 vol %) as a sacrificial electron donor. Pt (0.5 wt %) was loaded on the surface of the as-prepared catalyst through the in situ photodeposition method[11] by using K2PtCl6 as the Pt source. The reaction solution was evacuated to remove air completely prior to irradiation under a xenon lamp (CEL-HXF 300, 320 < l < 2500 nm, 300 W) with a cutoff filter (AULIGHT, 420 nm). The temperature of the reaction solution was maintained below 10 8C by a flow of cooling water during the reaction. The evolved gas was analyzed by GC, on an insturment (GC7900) equipped with a thermal conductive detector (TCD), using N2 as the carrier gas. Stability experiments: The hydrogen evolution reaction catalyzed by K10-g-C3N4 (0.5 wt % Pt) was firstly tested for 4 h by using the method described above. Then, the reaction solution was evacuated to remove the generated H2 gas completely. The photocatalytic reaction was continued for another 4 h. That reaction was performed four times (16 h in total). For pure g-C3N4, we only tested its stability for 4 h.
Figure 8. Three possible electron-transfer mechanisms between different nanosheets of K-g-C3N4.
Reagent Co., Ltd, 98 %), potassium hexachioroplatinate (K2PtCl6, Sinopharm Chemical Reagent Co., Ltd, 40.0–40.3 % Pt basis), triethanolamine [(HOCH2CH2)3N, Sinopharm Chemical Reagent Co., Ltd, 78 %] were used without any purification. Ultrapure water with the specific resistance of 18.2 MW cm was obtained by reverse osmosis followed by ion-exchange and filtration. Preparation of K-g-C3N4 : Typically, KCl (10 g) and C3H6N6 (1 g) were mixed with grinding for 30 min. Then, the mixture was heated at 550 8C in a furnace for 4 h in static air at a ramp rate of 2.5 8C min 1. After natural cooling to ambient temperature, the sample was added to water (100 mL) with magnetic stirring for 12 h at room temperature. At last, the solid sample obtained by centrifugation was washed with water ( 8) to remove most of KCl. Samples with different KCl:g-C3N4 weight ratios (1:1, 5:1, 10:1, 15:1, 17.5:1) were prepared by the method mentioned above, and were denoted herein as K1-g-C3N4, K5-g-C3N4, K10-g-C3N4, K15-g-C3N4, and K17.5-g-C3N4, respectively. Notably, further increases in the
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Acknowledgements This work is supported in part by National Natural Science Foundation of China (51101070); National Key Basic Research, Development Program (2010CB631001); Program for New Century Excellent Talents in University of the Ministry of Education of China (NCET-09-0431); Jilin Province Science and Technology Development Program (201101061); Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (3C1137282416); and Jilin University Fundamental Research Funds. Keywords: electron microscopy · hydrogen · nanostructures · photochemistry · template synthesis [1] [2] [3] [4] [5] [6]
A. Fujishima, K. Honda, Nature 1972, 238, 37 – 38. A. Kudo, Y. Miseki, Chem. Soc. Rev. 2009, 38, 253 – 278. X. B. Chen, S. S. Mao, Chem. Rev. 2007, 107, 2891 – 2959. J. G. Yu, J. R. Ran, Energy Environ. Sci. 2011, 4, 1364 – 1371. M. Wu, J. M. Yan, M. Zhao, Q. Jiang, ChemPlusChem 2012, 77, 931 – 935. X. B. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331, 746 – 750.
ChemSusChem 2014, 7, 2654 – 2658
2657
CHEMSUSCHEM FULL PAPERS [7] B. Chai, T. Peng, P. Zeng, J. Mao, J. Mater. Chem. 2011, 21, 14587 – 14593. [8] Y. Wang, X. C. Wang, M. Antonietti, Angew. Chem. Int. Ed. 2012, 51, 68 – 89; Angew. Chem. 2012, 124, 70 – 92. [9] G. Liu, P. Niu, C. H. Sun, S. C. Smith, Z. G. Chen, G. Q. Lu, H. M. Chen, J. Am. Chem. Soc. 2010, 132, 11642 – 11648. [10] J. H. Li, B. Shen, Z. H. Hong, B. H. Lin, B. F. Gao, Y. L. Chen, Chem. Commun. 2012, 48, 12017 – 12019. [11] X. C. Wang, K. Maeda, A. Thomas, K. Takanable, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8, 76 – 80. [12] S. Chu, Y. Wang, Y. Guo, J. Y. Feng, C. C. Wang, W. J. Luo, X. X. Fan, Z. G. Zou, ACS Catal. 2013, 3, 912 – 919. [13] Q. Xiang, J. G. Yu, M. Jaroniec, J. Phys. Chem. C 2011, 115, 7355 – 7363. [14] J. S. Zhang, M. W. Zhang, R. Q. Sun, X. C. Wang, Angew. Chem. Int. Ed. 2012, 51, 10145 – 10149; Angew. Chem. 2012, 124, 10292 – 10296. [15] Y. D. Hou, A. B. Laursen, J. S. Zhang, G. G. Zhang, Y. S. Zhu, X. C. Wang, S. Dahl, I. Chorkendorff, Angew. Chem. Int. Ed. 2013, 52, 3621 – 3625; Angew. Chem. 2013, 125, 3709 – 3713. [16] H. Xu, J. Yan, Y. G. Xu, Y. H. Song, H. M. Li, J. X. Xia, C. J. Huang, H. L. Wan, Appl. Catal. B 2013, 129, 182 – 193. [17] Z. Z. Lin, X. C. Wang, Angew. Chem. Int. Ed. 2013, 52, 1735 – 1738; Angew. Chem. 2013, 125, 1779 – 1782. [18] Y. P. Yuan, S. W. Cao, Y. S. Liao, L. S. Yin, C. Xue, Appl. Catal. B 2013, 140, 164 – 168. [19] L. Ge, C. C. Han, J. Liu, Y. F. Li, Appl. Catal. A 2011, 409, 215 – 222. [20] X. F. Chen, J. S. Zhang, X. Z. Fu, M. Antonietti, X. C. Wang, J. Am. Chem. Soc. 2009, 131, 11658 – 11659. [21] J. D. Hong, X. Y. Xiao, Y. S. Wang, R. Xu, J. Mater. Chem. 2012, 22, 15006 – 15012. [22] Y. W. Zhang, J. H. Liu, G. Wu, W. Chen, Nanoscale 2012, 4, 5300 – 5303. [23] A. Paracchino, V. Laporte, K. Sivula, M. Grtzel, E. Thimsen, Nat. Mater. 2011, 10, 456 – 461. [24] B. Chai, T. Y. Peng, J. Mao, K. Li, L. Zan, Phys. Chem. Chem. Phys. 2012, 14, 16745 – 16752. [25] X. H. Li, X. C. Wang, M. Antonietti, Chem. Sci. 2012, 3, 2170 – 2174. [26] J. S. Zhang, F. S. Guo, X. C. Wang, Adv. Funct. Mater. 2013, 23, 3008 – 3014. [27] X. H. Li, X. C. Wang, M. Antonietti, ACS Catal. 2012, 2, 2082 – 2086. [28] W. W. Lei, D. Portehault, D. Liu, S. Qin, Y. Chen, Nat. Commun. 2013, 4, 1777. [29] J. S. Zhang, M. W. Zhang, G. G. Zhang, X. C. Wang, ACS Catal. 2012, 2, 940 – 948.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org [30] M. J. Bojdys, J. O. Mller, M. Antonietti, A. Thomas, Chem. Eur. J. 2008, 14, 8177 – 8182. [31] J. H. Zhang, X. F. Chen, K. Takanabe, K. Maeda, K. Domen, J. D. Epping, X. Z. Fu, M. Antonietti, X. C. Wang, Angew. Chem. Int. Ed. 2010, 49, 441 – 444; Angew. Chem. 2010, 122, 451 – 454. [32] J. H. Zhang, G. G. Zhang, X. F. Chen, S. Lin, L. Mçhlmann, G. Dolega, G. Lipner, M. Antonietti, S. Blechert, X. C. Wang, Angew. Chem. Int. Ed. 2012, 51, 3183 – 3187; Angew. Chem. 2012, 124, 3237 – 3241. [33] J. H. Liu, T. K. Zhang, Z. C. Wang, G. Dawson, W. Chen, J. Mater. Chem. 2011, 21, 14398 – 14401. [34] L. Ge, C. C. Han, Appl. Catal. B 2012, 117, 268 – 274. [35] http://srdata.nist.gov/xps/Default.aspx. [36] Y. W. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach, R. S. Ruoff, Science 2011, 332, 1537 – 1541. [37] Y. Shiraishi, S. Kanazawa, Y. Sugano, D. Tsukamoto, H. Sakamoto, S. Ichikawa, T, Hirai, ACS Catal. 2014, 4, 774 – 780. [38] X. J. Bai, R. L. Zong, C. X. Li, D. Liu, Y. F. Liu, Y. F. Zhu, Appl. Catal. B 2014, 147, 82 – 91. [39] Y. L. Chen, J. H. Li, Z. H. Hong, B. S, B. Z. Lin, B. F. Gao, Phys. Chem. Chem. Phys. 2014, 16, 8106 – 8113. [40] S. Martha, A. Nashim, K. M. Parida, J. Mater. Chem. A 2013, 1, 7816 – 7824. [41] S. B. Yang, Y. J. Gong, J. S. Zhang, L. Zhan, L. L. Ma, Z. Y. Fang, R. Vajtai, X. C. Wang, P. M. Ajayan, Adv. Mater. 2013, 25, 2452 – 2456. [42] P. Niu, L. L. Zhang, G. Liu, H. M. Cheng, Adv. Funct. Mater. 2012, 22, 4763 – 4770. [43] F. Dong, Z. W. Zhao, T. Xiong, Z. L. Ni, W. D. Zhang, Y. J. Sun, W. K. Ho, ACS Appl. Mater. Interfaces 2013, 5, 11392 – 11401. [44] S. Ye, L. G. Qiu, Y. P. Yuan, Y. J. Zhu, J. Xia, J. F. Zhu, J. Mater. Chem. A 2013, 1, 3008 – 3015. [45] Y. Sui, J. H. Liu, Y. W. Zhang, X. K. Tian, W. Chen, Nanoscale 2013, 5, 9150 – 9155. [46] X. W. Wang, G. Liu, Z. G. Chen, F. Li, L. Z. Wang, G. Q. Lu, H. M. Chen, Chem. Commun. 2009, 3452 – 3454. [47] H. Tada, T. Mitsui, T. Kiyonaga, T. Akita, K. Tanaka, Nat. Mater. 2006, 5, 782 – 786.
Received: March 14, 2014 Revised: May 6, 2014 Published online on July 14, 2014
ChemSusChem 2014, 7, 2654 – 2658
2658
Synthesis of Potassium-Modified Graphitic Carbon Nitride with High Photocatalytic Activity for Hydrogen Evolution Ming Wu, Jun-Min Yan,* Xian-nian Tang, Ming Zhao, and Qing Jiang*[a] Potassium-modified graphitic carbon nitride (K-g-C3N4) nanosheets are synthesized by a facile KCl-template method that holds the advantage of easy removal of residual template. A combination of XRD, X-ray photoelectron spectroscopy, and inductively coupled plasma analyses are utilized to characterize the obtained resultant K-g-C3N4 architectures, which are composed of nanosheets of variable thickness (< 10 nm). Photocatalytic hydrogen evolution experiments under visible light irra-
diation showed that K-g-C3N4 nanosheets have high photocatalytic activities (up to about thirteen times higher than that of pure g-C3N4) as well as good stability (no reduction in activity within 16 h); both features emanate from their unique structural characteristics. These results illustrate the viability of this methodology for the facile synthesis of efficient heterogeneous photocatalysts for potential commercial applications.
Introduction Semiconductor photocatalysts have attracted great attention for their application in solar energy conversion.[1, 2] In recent years, TiO2 has been regarded as a good photocatalyst for its high activity, great stability, nontoxicity, and low cost.[3, 4] However, the practical application of TiO2 is seriously limited by its wide band gap of 3.2 eV, which makes TiO2 mainly absorb UV light (l < 400 nm) that accounts for only 4 % of solar energy.[5, 6] Therefore, it is highly desirable to develop new photocatalysts with high photocatalytic activities and stabilities under visible light irradiation. Graphitic carbon nitride (g-C3N4), as an interesting metal-free polymeric organic semiconductor, has gained more and more worldwide attention.[7–9] As a promising metal-free photocatalyst for solar energy conversion, g-C3N4 possesses appropriate band position (band gap of 2.7 eV), excellent thermal and chemical stabilities, and good optical features.[10–13] However, the photocatalytic efficiency of pure g-C3N4 is still far from the level required for practical applications because of its high recombination rate of photogenerated electron–hole pairs for efficient use of solar energy.[14–16] To solve this problem many strategies have been proposed, such as doping with foreign ions (B, P, Ag, Fe, etc.), increasing surface area, and combining with other semiconductors.[17–24] To this end, g-C3N4 with different structures has been synthesized using various hard silica templates, such as Santa Barbara amorphous (SBA) silica and silica particles.[25–27] However, these techniques require the use of hazardous chemicals (e.g., NH4HF2) to remove the hard silica templates. [a] M. Wu, Dr. J.-M. Yan, X.-n. Tang, Dr. M. Zhao, Dr. Q. Jiang Key Laboratory of Automobile Materials Ministry of Education School of Materials Science and Engineering Jilin University, Changchun 130022 (P.R. China) E-mail: [email protected] [email protected]
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Herein, we first synthesize potassium-modified g-C3N4 (K-gC3N4) nanosheets using KCl as a soft template. Residual KCl can be subsequently removed easily by washing with water. The obtained unique structure of K-g-C3N4 can effectively reduce the recombination rate of photogenerated electron–hole pairs; therefore, the photocatalytic activity and stability of K-g-C3N4 under visible light irradiation is enhanced greatly relative to pure g-C3N4.
Results and Discussion A series of K-g-C3N4 nanosheets were produced using KCl/gC3N4 weight ratios of 1:1, 5:1, 10:1, 15:1, and 17.5:1; the resulting architectures are denoted herein as K1-g-C3N4, K5-g-C3N4, K10-g-C3N4, K15-g-C3N4, and K17.5-g-C3N4, respectively. Pure gC3N4 has the bulk structure, whereas K10-g-C3N4 is composed of curved nanosheets with different thicknesses (Figure 1 a, b, and d).[28] From high-resolution TEM (HRTEM) images of K10-gC3N4 (Figure 1 c, red arrows) it is clear that the thicknesses of these nanosheets range from 1.5 to 6.3 nm. This structure is beneficial for photocatalytic reactions (vide infra). From XRD patterns of pure g-C3N4 and K10-g-C3N4 (Figure 2 a), it can be found that K10-g-C3N4 has a signal that corresponds to the (002) plane, similarly to pure g-C3N4 ; however, it is slightly shifted relative to g-C3N4 (from 27.48 to 28.08). In fact, all (002) signals of specimens with K (Figure 2 b) are located at higher angles (27.78 for K1-g-C3N4 ; 28.08 for K1-g-C3N4, K10-g-C3N4, and K17.5-g-C3N4), which means that the layer distance of the (002) plane of g-C3N4 is decreased after K modifying.[29, 30] Moreover, after K modifying, the (001) signal at 138 disappeared (Figure 2 a and b), indicating that the in-plane structure is destroyed or changed.[31, 32] The compositions and chemical states of K-g-C3N4 and gC3N4 were further tested by X-ray photoelectron spectroscopy (XPS) for comparison. C, N, and O were detected in survey ChemSusChem 2014, 7, 2654 – 2658
2654
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
spectra of both structures (Figure 3 a); O originated from hightemperature treatment in air during synthesis.[29, 33] For K10-gC3N4, K (but no Cl) was detected (Figure 3 a and d), indicating the existence of K in this specimen through the preparation method.[34] That was further supported by inductively coupled plasma (ICP) analyses, which showed that K content in K1-gC3N4, K10-g-C3N4, and K17.5-g-C3N4 was 12.5, 13.9, and 14.1 wt %, respectively. This means that the increase in the amount of KCl added initially (from 1 to 17.5 g) resulted in an increase of K content in as-prepared specimens. Because gC3N4 has N H2 and NH species[10] (Figure 3 b and c),K + from KCl may react with the amino species from g-C3N4 to form N–K bonds on g-C3N4 and H + , and thus K10-g-C3N4 is formed. The peak of K 2p at 292.6 eV (Figure 3 d, red trace) is the same as that in potassium azide (KN3).[35] This may prove the existence of N K bonds in K10-g-C3N4. Also, no clear binding-energy shift can be found on N 1s and C 1s between K10-g-C3N4 and g-C3N4 (Figure 3 a and b), and all the peaks imply the typical Figure 1. Middle-resolution TEM images of (a) pure g-C3N4 and (b) K10-gchemical states of N and C in g-C3N4.[10, 29, 36] Therefore, K modiC3N4 ; (c) HRTEM image of K10-g-C3N4 ; (d) low-resolution TEM image of K10g-C3N4. fying did not change the main structure of g-C3N4. Figure 4 shows the Raman spectra of K10-g-C3N4 and g-C3N4. The peaks at 725 and 995 cm 1 are assigned to the breathing modes of triazine rings,[37] and their densities decrease after K modifying. The decrease in density indicates a charge transfer occurring between g-C3N4 aromatic rings and K atoms.[38] In addition, the peak at 773 cm 1 of g-C3N4 corresponds to the out-of-plane bending mode of graphitic domains of g-C3N4 ;[37] this peak shifts to 788 cm 1 after K modifying, which may also stem from charge transfer between g-C3N4 layers and K atoms.[39] To investigate the electronic band structure of K-gFigure 2. (a) XRD patterns of pure g-C3N4 and K10-g-C3N4 ; (b) XRD patterns of all K-gC3N4 samples. C3N4 nanosheets, UV/Vis diffuse reflectance spectra and photoluminescence (PL) spectra of K10-g-C3N4 and pure g-C3N4 were obtained. The UV/ Vis adsorption of K10-g-C3N4 has a red shift relative to that of pure g-C3N4 (Figure 5 a) and the band gap of K10-g-C3N4 is determined to be 2.66 eV (Figure 5 a, inset), which is lower than that of pure g-C3N4 (2.71 eV). Therefore, K10-g-C3N4 has a higher efficiency with regard to visible light absorption and thus may have higher photocatalytic activity than g-C3N4. Figure 5 b shows the PL spectra of K10-g-C3N4 and g-C3N4 excited at 400 nm at room temperature. Pure g-C3N4 has a PL intensity that is much higher than that of K10-g-C3N4, with an emission peak at 457 nm. Because a lower peak intensity represents a lower reFigure 3. (a) XPS spectra; (b) N 1s high-resolution XPS spectra; and (c) high-resolution XPS spectra of C 1s for K10combination rate of photogenerg-C3N4 and pure g-C3N4 ; (d) high-resolution XPS spectra of K 2p and Cl 2p for K10-g-C3N4. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2014, 7, 2654 – 2658
2655
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org
tivity of K10-g-C3N4 is stable and the evolution of hydrogen does not change during a 16 h period. Based on the above analyses, the photocatalytic mechanism of K-g-C3N4 can be explained as follows: g-C3N4 sheets with different thickness have different band gaps and the thinner the sheets the wider the band gap.[41, 42] Moreover, photogenerated electrons can be transferred freely between nanosheets with different band gaps.[43] K-g-C3N4 is composed of nanosheets of variable thickness (Figure 1 b and c). We chose two hypothetical nanosheets with different thicknesses (nanosheets A and B) for discussion purposes and hypothesized that nanosheet B was thicker than nanosheet A. Thus, nanosheet A has a wider Figure 4. Raman spectra (325 nm excitation) of g-C3N4 and K10-g-C3N4. band gap. Because the valence-band (VB) and conductionband (CB) positions of different nanosheets are not certain, there are three types of heterojunctions between the two nanosheets (Figure 8). However, irrespective of the type of heterojunction (Figure 8 a and b), photogenerated electrons can be transferred from a nanosheet with a higher CB to a nanosheet with a lower CB.[44, 45] As the electrons are transferred, the recomFigure 5. (a) UV/Vis diffuse reflectance spectra of K10-g-C3N4/pure g-C3N4 ; (b) PL spectra of K10-g-C3N4/pure gbination rate of photogenerated C3N4 (400 nm excitation). electron–hole pairs can be inhibited. Heterojunctions of the third type can also reduce the recombination rate of photogeneratated electron–hole pairs,[10, 29, 40] K10-g-C3N4 can efficiently ed electron–hole pairs (see the Z-scheme in Figure 8 c).[46, 47] hinder the recombination rate of photogenerated electronhole pairs relative to pure g-C3N4. Therefore, photogenerated electrons can be transferred between all nanosheets of variable thickness and the recombinaPhotocatalytic hydrogen evolution of as-prepared specimens tion rate of photogenerated electron–hole pairs can be deloaded with Pt (0.5 wt %) as a co-catalyst are evaluated under creased greatly, which agrees well with PL results (Figure 5 b). visible light irradiation (l > 420 nm), using triethanolamine Based on the above experimental and theoretical discussion, (10 vol %) as the sacrificial reagent. From Figure 6 a, it can be the high photocatalytic activity of K10-g-C3N4 for hydrogen seen that all K-g-C3N4 specimens have photocatalytic activities that are much higher than those of pure g-C3N4 (7.3 mmol h 1). evolution from water under visible light irradiation emanates Especially, K10-g-C3N4 has the highest photocatalytic activity from its unique architecture. (about thirteen times higher than that of g-C3N4) due to which the average hydrogen evolution rate can reach 102.8 mmol h 1. Conclusions In addition, the curved nanosheets of K10-g-C3N4 are maintained well after reaction (Figure 7). Figure 6 b shows the staWe successfully synthesized potassium-modified graphitic bility of K10-g-C3N4 within 16 h. Clearly, the photocatalytic accarbon nitride (K-g-C3N4) nanosheets by a KCl templating method using no hazardous chemicals. The synthesized K-g-C3N4 possesses a special architecture, which is characterized by nanosheets of variable thickness. Due to this architecture, K-g-C3N4 has high photocatalytic activity and stability for hydrogen evolution from water under visible light irradiation relative to pure g-C3N4. The present study may provide a new and facile method for the design and synthesis of heterogeneous photocatalysts.
Experimental Section Figure 6. (a) Hydrogen evolution rates of K-g-C3N4 (variable K content) and pure g-C3N4 ; (b) stability test of hydrogen evolution for K10-g-C3N4 under visible light irradiation (l > 420 nm).
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chemicals: Potassium chloride (KCl, Beijing Chemicals Works, 99.5 %), melamine (C3H6N6, Sinopharm Chemical ChemSusChem 2014, 7, 2654 – 2658
2656
CHEMSUSCHEM FULL PAPERS
www.chemsuschem.org amount of added KCl to 20 g led to a significant decrease in yields of K-modified specimens.
Figure 7. TEM image of K10-g-C3N4 after the hydrogen evolution reaction.
Characterization: XRD analyses were performed on a Rigaku D/ max-2500pc X-ray diffractometer with CuKa irradiation (l = 1.5406 ) at a scan rate of 28 min 1. TEM analyses were performed using a JEOL JEM-2010 microscope (accelerating voltage = 200 kV). XPS analyses were performed using an ESCALABMk II (Vacuum Generators) spectrometer using AlKa radiation (300 W). All binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. UV/Vis diffuse reflectance spectra were recorded on a UV-3600 spectrophotometer. PL spectra were recorded on a RF-5301 fluorescence spectrophotometer (SHIMADZU). Catalysts were all analyzed before their photocatalytic applications. The contents of K in specimens were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Jarrell Ash). Raman spectra were collected on a LabRAM ARAMIS instrument (laser wavelength 325 nm). Photocatalytic tests: Reactions were carried out in a Pyrex-top irradiation reaction vessel connected to a glass-closed gas system. H2 production was achieved by dispersing catalyst (100 mg) in an aqueous solution (100 mL) containing triethanolamine (10 vol %) as a sacrificial electron donor. Pt (0.5 wt %) was loaded on the surface of the as-prepared catalyst through the in situ photodeposition method[11] by using K2PtCl6 as the Pt source. The reaction solution was evacuated to remove air completely prior to irradiation under a xenon lamp (CEL-HXF 300, 320 < l < 2500 nm, 300 W) with a cutoff filter (AULIGHT, 420 nm). The temperature of the reaction solution was maintained below 10 8C by a flow of cooling water during the reaction. The evolved gas was analyzed by GC, on an insturment (GC7900) equipped with a thermal conductive detector (TCD), using N2 as the carrier gas. Stability experiments: The hydrogen evolution reaction catalyzed by K10-g-C3N4 (0.5 wt % Pt) was firstly tested for 4 h by using the method described above. Then, the reaction solution was evacuated to remove the generated H2 gas completely. The photocatalytic reaction was continued for another 4 h. That reaction was performed four times (16 h in total). For pure g-C3N4, we only tested its stability for 4 h.
Figure 8. Three possible electron-transfer mechanisms between different nanosheets of K-g-C3N4.
Reagent Co., Ltd, 98 %), potassium hexachioroplatinate (K2PtCl6, Sinopharm Chemical Reagent Co., Ltd, 40.0–40.3 % Pt basis), triethanolamine [(HOCH2CH2)3N, Sinopharm Chemical Reagent Co., Ltd, 78 %] were used without any purification. Ultrapure water with the specific resistance of 18.2 MW cm was obtained by reverse osmosis followed by ion-exchange and filtration. Preparation of K-g-C3N4 : Typically, KCl (10 g) and C3H6N6 (1 g) were mixed with grinding for 30 min. Then, the mixture was heated at 550 8C in a furnace for 4 h in static air at a ramp rate of 2.5 8C min 1. After natural cooling to ambient temperature, the sample was added to water (100 mL) with magnetic stirring for 12 h at room temperature. At last, the solid sample obtained by centrifugation was washed with water ( 8) to remove most of KCl. Samples with different KCl:g-C3N4 weight ratios (1:1, 5:1, 10:1, 15:1, 17.5:1) were prepared by the method mentioned above, and were denoted herein as K1-g-C3N4, K5-g-C3N4, K10-g-C3N4, K15-g-C3N4, and K17.5-g-C3N4, respectively. Notably, further increases in the
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Acknowledgements This work is supported in part by National Natural Science Foundation of China (51101070); National Key Basic Research, Development Program (2010CB631001); Program for New Century Excellent Talents in University of the Ministry of Education of China (NCET-09-0431); Jilin Province Science and Technology Development Program (201101061); Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (3C1137282416); and Jilin University Fundamental Research Funds. Keywords: electron microscopy · hydrogen · nanostructures · photochemistry · template synthesis [1] [2] [3] [4] [5] [6]
A. Fujishima, K. Honda, Nature 1972, 238, 37 – 38. A. Kudo, Y. Miseki, Chem. Soc. Rev. 2009, 38, 253 – 278. X. B. Chen, S. S. Mao, Chem. Rev. 2007, 107, 2891 – 2959. J. G. Yu, J. R. Ran, Energy Environ. Sci. 2011, 4, 1364 – 1371. M. Wu, J. M. Yan, M. Zhao, Q. Jiang, ChemPlusChem 2012, 77, 931 – 935. X. B. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331, 746 – 750.
ChemSusChem 2014, 7, 2654 – 2658
2657
CHEMSUSCHEM FULL PAPERS [7] B. Chai, T. Peng, P. Zeng, J. Mao, J. Mater. Chem. 2011, 21, 14587 – 14593. [8] Y. Wang, X. C. Wang, M. Antonietti, Angew. Chem. Int. Ed. 2012, 51, 68 – 89; Angew. Chem. 2012, 124, 70 – 92. [9] G. Liu, P. Niu, C. H. Sun, S. C. Smith, Z. G. Chen, G. Q. Lu, H. M. Chen, J. Am. Chem. Soc. 2010, 132, 11642 – 11648. [10] J. H. Li, B. Shen, Z. H. Hong, B. H. Lin, B. F. Gao, Y. L. Chen, Chem. Commun. 2012, 48, 12017 – 12019. [11] X. C. Wang, K. Maeda, A. Thomas, K. Takanable, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8, 76 – 80. [12] S. Chu, Y. Wang, Y. Guo, J. Y. Feng, C. C. Wang, W. J. Luo, X. X. Fan, Z. G. Zou, ACS Catal. 2013, 3, 912 – 919. [13] Q. Xiang, J. G. Yu, M. Jaroniec, J. Phys. Chem. C 2011, 115, 7355 – 7363. [14] J. S. Zhang, M. W. Zhang, R. Q. Sun, X. C. Wang, Angew. Chem. Int. Ed. 2012, 51, 10145 – 10149; Angew. Chem. 2012, 124, 10292 – 10296. [15] Y. D. Hou, A. B. Laursen, J. S. Zhang, G. G. Zhang, Y. S. Zhu, X. C. Wang, S. Dahl, I. Chorkendorff, Angew. Chem. Int. Ed. 2013, 52, 3621 – 3625; Angew. Chem. 2013, 125, 3709 – 3713. [16] H. Xu, J. Yan, Y. G. Xu, Y. H. Song, H. M. Li, J. X. Xia, C. J. Huang, H. L. Wan, Appl. Catal. B 2013, 129, 182 – 193. [17] Z. Z. Lin, X. C. Wang, Angew. Chem. Int. Ed. 2013, 52, 1735 – 1738; Angew. Chem. 2013, 125, 1779 – 1782. [18] Y. P. Yuan, S. W. Cao, Y. S. Liao, L. S. Yin, C. Xue, Appl. Catal. B 2013, 140, 164 – 168. [19] L. Ge, C. C. Han, J. Liu, Y. F. Li, Appl. Catal. A 2011, 409, 215 – 222. [20] X. F. Chen, J. S. Zhang, X. Z. Fu, M. Antonietti, X. C. Wang, J. Am. Chem. Soc. 2009, 131, 11658 – 11659. [21] J. D. Hong, X. Y. Xiao, Y. S. Wang, R. Xu, J. Mater. Chem. 2012, 22, 15006 – 15012. [22] Y. W. Zhang, J. H. Liu, G. Wu, W. Chen, Nanoscale 2012, 4, 5300 – 5303. [23] A. Paracchino, V. Laporte, K. Sivula, M. Grtzel, E. Thimsen, Nat. Mater. 2011, 10, 456 – 461. [24] B. Chai, T. Y. Peng, J. Mao, K. Li, L. Zan, Phys. Chem. Chem. Phys. 2012, 14, 16745 – 16752. [25] X. H. Li, X. C. Wang, M. Antonietti, Chem. Sci. 2012, 3, 2170 – 2174. [26] J. S. Zhang, F. S. Guo, X. C. Wang, Adv. Funct. Mater. 2013, 23, 3008 – 3014. [27] X. H. Li, X. C. Wang, M. Antonietti, ACS Catal. 2012, 2, 2082 – 2086. [28] W. W. Lei, D. Portehault, D. Liu, S. Qin, Y. Chen, Nat. Commun. 2013, 4, 1777. [29] J. S. Zhang, M. W. Zhang, G. G. Zhang, X. C. Wang, ACS Catal. 2012, 2, 940 – 948.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org [30] M. J. Bojdys, J. O. Mller, M. Antonietti, A. Thomas, Chem. Eur. J. 2008, 14, 8177 – 8182. [31] J. H. Zhang, X. F. Chen, K. Takanabe, K. Maeda, K. Domen, J. D. Epping, X. Z. Fu, M. Antonietti, X. C. Wang, Angew. Chem. Int. Ed. 2010, 49, 441 – 444; Angew. Chem. 2010, 122, 451 – 454. [32] J. H. Zhang, G. G. Zhang, X. F. Chen, S. Lin, L. Mçhlmann, G. Dolega, G. Lipner, M. Antonietti, S. Blechert, X. C. Wang, Angew. Chem. Int. Ed. 2012, 51, 3183 – 3187; Angew. Chem. 2012, 124, 3237 – 3241. [33] J. H. Liu, T. K. Zhang, Z. C. Wang, G. Dawson, W. Chen, J. Mater. Chem. 2011, 21, 14398 – 14401. [34] L. Ge, C. C. Han, Appl. Catal. B 2012, 117, 268 – 274. [35] http://srdata.nist.gov/xps/Default.aspx. [36] Y. W. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach, R. S. Ruoff, Science 2011, 332, 1537 – 1541. [37] Y. Shiraishi, S. Kanazawa, Y. Sugano, D. Tsukamoto, H. Sakamoto, S. Ichikawa, T, Hirai, ACS Catal. 2014, 4, 774 – 780. [38] X. J. Bai, R. L. Zong, C. X. Li, D. Liu, Y. F. Liu, Y. F. Zhu, Appl. Catal. B 2014, 147, 82 – 91. [39] Y. L. Chen, J. H. Li, Z. H. Hong, B. S, B. Z. Lin, B. F. Gao, Phys. Chem. Chem. Phys. 2014, 16, 8106 – 8113. [40] S. Martha, A. Nashim, K. M. Parida, J. Mater. Chem. A 2013, 1, 7816 – 7824. [41] S. B. Yang, Y. J. Gong, J. S. Zhang, L. Zhan, L. L. Ma, Z. Y. Fang, R. Vajtai, X. C. Wang, P. M. Ajayan, Adv. Mater. 2013, 25, 2452 – 2456. [42] P. Niu, L. L. Zhang, G. Liu, H. M. Cheng, Adv. Funct. Mater. 2012, 22, 4763 – 4770. [43] F. Dong, Z. W. Zhao, T. Xiong, Z. L. Ni, W. D. Zhang, Y. J. Sun, W. K. Ho, ACS Appl. Mater. Interfaces 2013, 5, 11392 – 11401. [44] S. Ye, L. G. Qiu, Y. P. Yuan, Y. J. Zhu, J. Xia, J. F. Zhu, J. Mater. Chem. A 2013, 1, 3008 – 3015. [45] Y. Sui, J. H. Liu, Y. W. Zhang, X. K. Tian, W. Chen, Nanoscale 2013, 5, 9150 – 9155. [46] X. W. Wang, G. Liu, Z. G. Chen, F. Li, L. Z. Wang, G. Q. Lu, H. M. Chen, Chem. Commun. 2009, 3452 – 3454. [47] H. Tada, T. Mitsui, T. Kiyonaga, T. Akita, K. Tanaka, Nat. Mater. 2006, 5, 782 – 786.
Received: March 14, 2014 Revised: May 6, 2014 Published online on July 14, 2014
ChemSusChem 2014, 7, 2654 – 2658
2658
