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Macroscopic 3D Porous Graphitic Carbon Nitride Monolith for Enhanced Photocatalytic Hydrogen Evolution Qinghua Liang, Zhi Li, Xiaoliang Yu, Zheng-Hong Huang,* Feiyu Kang, and Quan-Hong Yang* As a promising and alternative way to alleviate the current worldwide energy and environmental issues, artificial photosynthesis that mimics the natural conversion and generation of chemical fuels has drawn tremendous interest in the past few decades.[1] Despite the fact that much progress has been made, photocatalysis technology is still restricted by the lack of highly efficient photocatalysts for practical applications. This has motivated considerable research on pursuing low-cost, stable, efficient, and eco-friendly photocatalysts.[2] Among many semiconductor-based photocatalysts, polymeric graphitic carbon nitride (g-C3N4 or g-CN) has attracted much attention owing to its many excellent properties including extraordinary chemical stability, easy preparation, 2D structure, metal-free composition, visible light response, and tunable electronic structure[1e,3] since the pioneering study in 2009.[1c] It is believed that g-CN will play an increasingly significant role in the fields of photocatalysis.[1e,3,4] Nevertheless, it still suffers from some drawbacks, such as small specific surface area (SSA), low visible light utilization efficiency, and rapid recombination of photogenerated carriers.[3,5] For these reasons, many impressive results have been obtained by modifying g-CN by coupling with other materials such as carbon dots,[1e] graphene,[6] hydrogenase,[7] semiconductors,[8] aromatic compounds,[9] and doping with heteroatoms such as P,[10] F,[11] O,[12] B,[11,13] I,[5a] S,[14] and Fe,[15] and introducing nitrogen vacancies[16] to overcome the aforementioned problems. Besides, a variety of forms of nanostructured g-CN including nanosheets,[17] porous Q. Liang, X. Yu, Prof. F. Kang, Prof. Q.-H. Yang Shenzhen Key Laboratory for Graphene-Based Materials and Engineering Laboratory for Functionalized Carbon Materials Graduate School at Shenzhen Tsinghua University Shenzhen 518055, P. R. China E-mail: [email protected] and [email protected] Q. Liang, X. Yu, Prof. Z.-H. Huang Key Laboratory of Advanced Materials (MOE) School of Materials Science and Engineering Tsinghua University Beijing 100084, P. R. China E-mail: [email protected] Q. Liang, Prof. Z. Li Technical Institute of Physics and Chemistry Chinese Academy of Sciences (CAS) Beijing 100190, P. R. China Prof. Q.-H. Yang School of Chemical Engineering and Technology Tianjin University Tianjin 300072, P. R. China
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structures,[18] nanospherical frameworks,[5b] nanorods,[19] etc. have also been developed in recent years. In particular, Antonietti and Wang et al. have performed many pioneering studies to promote the development of g-CN.[1c,3c,9–11,14a,15,20] However, almost all studies reported thus far have been focused on g-CN in the powder form that suffers from a complicated recovery step during the photocatalytic process, setting a hurdle for the real applications. Moreover, the random aggregation of g-CN nanosheets in a powder decreases their surface accessibility, which, unfortunately, is highly important for the catalytic performance. Therefore, an ordered and macrosized assembly of highly accessible g-CN nanosheets is of great significance for device-based performance and real application of g-CN. Though some recent advances have been made to produce some assembly of g-CN hybridized with graphene or carbon nanotubes,[21] the construction of a 3D macroform of g-CN is still a big challenge. Consequently, it is of great urgency to develop a synthesis method for producing high efficiency macroscopic 3D g-CN photocatalyst with better recovery properties for the industrial and practical applications.[22] Here, we report the first synthesis of a macroscopic 3D porous g-CN monolith (PCNM) by a well-designed template method, that is, one-step thermal polymerization of urea inside the framework of an intentionally selected yet very cheap template, melamine sponge (MS), normally found in kitchen, that is free of any other functionalizing or crosslinking agents. Combining extremely high loading of urea with lightweight and good water absorbability of MS makes the method practicable for the synthesis of PCNM. Also, note that both urea and melamine are precursors for preparing g-CN.[3a–c,5] The resultant PCNM possesses a 3D interconnected network composed of 2D porous g-CN nanosheets. With such a unique structure, PCNM is lightweight and freestanding, and has good structural stability and a large SSA. Remarkably, as a typical application, the 3D PCNM exhibits significantly improved performance compared to its powder counterpart toward photocatalytic water splitting for hydrogen evolution under visible light. The general synthesis route leading to the macroscopic 3D PCNM is illustrated in Figure 1. A commercially available MS is directly used as both the template and the supporting framework (Figure 1a). After dipping it in the saturated aqueous urea solution followed by freeze-drying (Figure 1b), the MS was completely and homogeneously filled with urea (Figure S1, Supporting Information). Heating the MS filled with urea at 550 °C for 4 h results in a macroscopic 3D PCNM that has a very low mass density of ≈35 mg cm−3 (Figure 1c), while directly heating MS in the same condition results in an N-enriched carbon foam (Figures S1–S3 and Table S1, Supporting Information). Note that upon heating the original white
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MS (40 × 20 × 20 mm3) transformed into a smaller piece of light black PCNM (28 × 14 × 14 mm3), as shown in Figure 1c. The isotropic 30% shrinkage is caused by two synergistic processes: heating-induced shrinking of the MS framework and volume decrease resulting from thermal polymerization of the pyrolysis intermediates of urea and melamine (Figures S4 and S5, Supporting Information). The resultant PCNM can be easily cut into different shapes with a blade (Figure 1d). Heating urea in the same way without MS yields g-CN powder as a reference. Direct evidence for the formation of g-CN is obtained using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetry (TG), and X-ray photoelectron spectroscopy (XPS) analysis. Similar to the powdered g-CN, the XRD pattern of PCNM in Figure 2a clearly has two characteristic diffraction peaks at 13.6° (100) and 27.2° (002) that are, respectively, attributed to the in-plane repeating unites of the continuous heptazine framework and the stacking of the conjugated aromatic structure with a spacing of ≈0.32 nm (inset in Figure 2a).[5b,14b,18a,19a] The broader and weaker diffraction peaks indicate the shorter correlation lengths of the interlayer periodicity of tri-s-triazine building blocks in PCNM.[5b,19a] FT-IR spectra shown in Figure 2b exhibit typical peaks at 3100–3500, 1200–1600, and 810 cm−1, which are ascribed to the vibrational absorption of N H and O H, aromatic C N heterocyclic unites and the triazine unit (inset in Figure 2b), respectively.[5b,17a,b,18a] In the XPS survey spectrum, only C, N, and O elements are detected. The weak O 1s peak may be due to the surface absorbed H2O, as confirmed by the FT-IR results (Figure 2b). The C/N molar ratio of PCNM is close to the theoretical value of ideal g-CN (≈0.73). The chemical groups and bonding characteristics of PCNM are further confirmed by the high-resolution C 1s and N 1s spectra (insets in Figure 2e). There are three kinds of carbon species, including 288.7 eV for N C N, 286.5 eV for C N, 285.1 eV for C C in the C 1s spectrum.[13,21a] Moreover, the N 1s spectrum can be well fitted to four N species, i.e., 398.7 eV for C N C, 399.5 eV for N (C)3, 401.0 eV for N H, and 405.0 eV for π excitation of the
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Figure 1. Illustration of the preparation of a macroscopic 3D PCNM.
C N conjugated structure.[5b,13,21a] Both C 1s and N 1s XPS spectra confirm the basic heptazine heterocyclic structure of g-CN in PCNM. Most typical bands assigned to the vibration modes of CN heterocycles can be identified in the Raman spectrum of PCNM (Figure S3, Supporting Information). In addition, no obvious D- and G-bands typically assigned to carbon materials were detected in the Raman spectrum of PCNM. The above results further suggest that the main composition of the obtained PCNM is g-CN. TG was further carried out to study the thermal stability of PCNM (Figure 2d). Compared to the g-CN powder, a larger weight loss from 30 to 100 °C in the TG curve is attributed to the evaporation of a larger amount of absorbed H2O, which is due to the larger SSA of the PCNM. The decomposition of both PCNM and g-CN powder occurs from 560 to 700 °C, which is consistent with previous reports.[18a,21c] The less-than-2 wt% residues should be nitrogen enriched amorphous carbon originated from MS (Table S1, Supporting Information) promoted by the catalytic charring effect of isolated g-CN nanosheets,[23] suggesting the possible formation of nitrogen enriched carbon/CN heterojunction in the PCNM. Little nitrogen enriched carbon is the main cause for the gray black color of PCNM, and the presence of heterojunction in PCNM is useful for charge separation of g-CN.[3b] The representative N2 adsorption isotherm of PCNM is of type IV with an H2-type hysteresis loop at a high relative pressure ranging from 0.75 to 1 (Figure 2c), suggesting the presence of mesopores and macropores. Accordingly, a sharp peak at 4 nm and a broad peak ranging from 30 to 170 nm are identified in the pore size distribution curve (inset in Figure 2c), which are, respectively, in corresponding to the diameter of the inner cavity of the g-CN nanosheets and the size of the slitlike pores formed between the g-CN nanosheets. The SSA and pore volume are calculated to be 78 m2 g−1 and 0.76 cm3 g−1, respectively, which are much higher than those of the g-CN powder (40 m2 g−1 and 0.28 cm3 g−1). The higher SSA and larger pore volume may be caused by the effective prevention of the stacking of g-CN nanosheets by the MS framework during the polymerization of urea. The mercury porosimetry results also confirm the presence of abundant macropores in PCNM (Figure S6, Supporting Information). The resultant PCNM has a good mechanical strength although it has a very low mass density. A piece of PCNM about 12 × 17 × 17 mm3 can support a 100 g weight with little deformation (Figure 2f and S7, Supporting Information). In addition, PCNM shows acceptable elasticity. The unloading curves obtained from compression and rebound tests are above the line y = 0 when returning to the original state (Figure S8, Supporting Information), indicating a partial volume recovery without serious deformation of PCNM. The 3D porous structure of PCNM is readily identified with scanning and transmission electron microscopy (SEM and TEM). As shown in Figure 3a,b, we can clearly observe the 3D interconnected network and hierarchical pores with diameter in the range 200–400 nm. A magnified SEM image reveals that the interconnected networks are composed of irregularly shaped 2D porous nanosheets with a thickness of approximately 30 nm. A consistent result was obtained from TEM images showing a typical porous interconnected structure formed by the g-CN nanosheets joining together (Figure 3c). It
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hydrogen in the presence of Pt as a cocatalyst and triethanolamine as a sacrificial electron donor under visible light (Supporting Information). As shown in Figure 4a, the photocatalytic activity, in terms of the hydrogen evolution rate (29.0 µmol h−1), is 2.84 times higher than that of the g-CN powder (10.2 µmol h−1). We believe the significant enhancement in photocatalytic hydrogen evolution of the 3D PCNM compared to g-CN powder can be attributed to the following reasons. On one hand, the larger SSA of PCNM (Figure 2e) leads to an increased surface reactivity for the photocatalytic reaction compared to the g-CN powder. On the other hand, PCNM makes use of more visible light compared to g-CN powder. The diffuse reflectance spectrum (DRS) shown in Figure 4b demonstrates that PCNM shows significantly improved light harvesting ability above 450 nm in the optical spectrum compared to the powdered g-CN, which may be caused by the multiple reflections of incident light within the interconnected network of porous g-CN nanosheets and the presence of N-enriched carbon/g-CN heterojunction.[18b] The result is also reflected by the smaller bandgap energy of PCNM (2.05 eV) than the g-CN powder (2.68 eV). Besides, PCNM has a more effective separation of photogenerated charge carriers under the excitation of visible light compared to the g-CN powder, as demonstrated by the photoluminescence (PL) spectra and photochemical Figure 2. (a) XRD patterns, (b) FT-IR spectra, (c) N2 sorption isotherms and (d) TG curves measurements. In Figure 4c, no obvious of PCNM (solid line) and g-CN powder (dotted line). (e) XPS survey spectrum with the corresponding C 1s and N 1s spectra (insets) of PCNM. (f) Photographs of PCNM showing that peak is found in the PL emission spectrum of PCNM, while the g-CN powder shows a it could support a 100 g weight. The insets in (a, b) are the ideal structures of g-CN and the strong intrinsic fluorescence emission peak inset in (c) is the corresponding pore size distribution. at 465 nm, suggesting a larger barrier to charge recombination in PCNM.[5,6] In accordance with the PL should be mentioned that the g-CN powder prepared without MS shows irregularly shaped particles (Figure S9, Supporting spectra, PCNM shows a much smaller arc radius in the electroInformation), demonstrating that the MS template plays a critchemical impedance spectrum (EIS, Figure 4d), indicating its ical role in the formation of PCNM. An enlarged TEM image faster interface charge transport.[5a,6a,17b] Moreover, obviously in Figure 3d obviously reveals the presence of abundant pores increased PL life time (Figure S11, Supporting Information) in the nanosheets with the diameters ranging 30–150 nm. The and photocurrent density are observed in the PCNM electrode selected area electron diffraction (SAED) pattern showing two compared to the g-CN powder case (inset in Figure 4d), sugdiffuse diffraction rings indicates the disordered structure of gesting its more efficient separation of photo-generated charge the porous g-CN nanosheets (inset in Figure 3d).[24] A highcarriers,[5,6][8d] which may be attributed to the unique structure resolution TEM image of the fine microstructure shows that because the 3D interconnected network of porous nanosheets these nanosheets have sharp edges and a turbostratic structure ensures sufficient mass transportation and also shortens the (Figure S10, Supporting Information). Such a 3D intercondiffusion length of charge migration and thus facilitates elecnected network of porous nanosheets containing many junctron relocalization on surface edge sites to hinder the charge tions and the open pores provide a good combination of infilrecombination.[5b] These results also agree well with previous [ 25 ] tration rate and surface area, and are also favorable for mass reports of g-CN obtained from thermal polycondensation of supramolecular organization of carbon nitride precursors. transfer and charge separation.[5b,26] [18b,26,27] Owing to the unique 3D interconnected network, a large SSA, and probably more catalytically active sites, the resultant PCNM The catalytic stability of PCNM was evaluated by was evaluated for photocatalytic water splitting to produce measuring photocatalytic hydrogen evolution under the same
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Figure 3. a,b) SEM and c,d) TEM images of the resultant PCNM. The inset in (d) is the corresponding SAED.
reaction conditions for three cycles. There is no obvious deactivation with time under continuous visible light illumination for 9 h (Figure S12, Supporting Information). Both the
3D interconnected network and the porous g-CN nanosheets are retained after such a 9 h experiment (Figure S13, Supporting Information). Pt nanoparticles with an average size of
Figure 4. (a) Photocatalytic activity for H2 production, (b) DRS spectrum, (c) PL emission spectra, and (d) EIS plot and photocurrent-time dependence of PCNM (solid line) and the g-CN powder (dotted line).
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2 nm are well distributed on the edges of the porous g-CN nanosheets (Figure S13, Supporting Information), demonstrating the possible utilization of PCNM as a stable matrix for supporting other catalysts. The application of PCNM as the electrocatalyst for fuel cells and exploration of the synthesis method for preparing other macroscopic 3D porous materials is under way. In conclusion, we have shown the first synthesis of a macroscopic 3D porous graphitic carbon nitride monolith (PCNM) by a one-step thermal polymerization of urea inside the framework of a melamine sponge. Because of its abundant porosity, high specific surface area, good visible light capture, as well as superior charge separation efficiency, PCNM exhibits excellent photocatalytic activity, which is 2.84 times higher than that of g-CN powder for hydrogen evolution under visible light. In view of its freestanding structure, good mechanical strength, acceptable elasticity, and superior photocatalytic activity, the macroscopic PCNM shows great potential for the applications in the fields of water splitting, pollutant degradation, CO2 reduction, and energy storage devices. It is believed that such a well-designed approach paves the way for preparing other 3D g-CN hybrid materials for real applications.
[4] [5]
[6]
[7] [8]
[9] [10] [11] [12]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
[13] [14]
Acknowledgements
[15]
Financial support from National Basic Research Program of China (2014CB932400), National Natural Science Foundation of China (Nos. 51302274, 51372167, and U1401243), Guangdong Province Innovation R&D Team Plan (No. 2009010025), and Shenzhen Basic Research Project (Nos. JCYJ20130402145002430 and ZDSYS20140509172959981) was gratefully acknowledged. The authors also thank Dr. Ruitao Lv for helpful discussions.
[16]
Received: April 29, 2015 Revised: May 28, 2015 Published online: July 2, 2015
[17]
[18]
[19] [1] a) L.-Z. Wu, B. Chen, Z.-J. Li, C.-H. Tung, Acc. Chem. Res. 2014, 47, 2177; b) K. Chang, Z. Mei, T. Wang, Q. Kang, S. Ouyang, J. Ye, ACS Nano 2014, 8, 7078; c) X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8, 76; d) W. Wang, J. Chen, C. Li, W. Tian, Nat. Commun. 2014, 5, 4647; e) J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S.-T. Lee, J. Zhong, Z. Kang, Science 2015, 347, 970. [2] a) Y. Qu, X. Duan, Chem. Soc. Rev. 2013, 42, 2568; b) J. Ran, J. Zhang, J. Yu, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev. 2014, 43, 7787; c) H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Chem. Soc. Rev. 2014, 43, 5234; d) Q. Xiang, J. Yu, M. Jaroniec, Chem. Soc. Rev. 2012, 41, 782. [3] a) Y. Zheng, J. Liu, J. Liang, M. Jaroniec, S. Z. Qiao, Energy Environ. Sci. 2012, 5, 6717; b) Z. Zhao, Y. Sun, F. Dong, Nanoscale 2015, 7, 15; c) Y. Wang, X. Wang, M. Antonietti, Angew. Chem. Int. Ed. 2012,
4638
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[20]
[21]
51, 68; d) S. Cao, J. Low, J. Yu, M. Jaroniec, Adv. Mater. 2015, 27, 2150; e) Y. Gong, M. Li, Y. Wang, ChemSusChem 2015, 8, 931. Y. Zheng, Y. Jiao, Y. Zhu, L. H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec, S. Z. Qiao, Nat Commun. 2014, 5, 3783. a) G. Zhang, M. Zhang, X. Ye, X. Qiu, S. Lin, X. Wang, Adv. Mater. 2014, 26, 805; b) J. Zhang, M. Zhang, C. Yang, X. Wang, Adv. Mater. 2014, 26, 4121. a) Y. Hou, Z. Wen, S. Cui, X. Guo, J. Chen, Adv. Mater. 2013, 25, 6291; b) Y. Zhang, T. Mori, L. Niu, J. Ye, Energy Environ. Sci. 2011, 4, 4517; c) A. Du, S. Sanvito, Z. Li, D. Wang, Y. Jiao, T. Liao, Q. Sun, Y. H. Ng, Z. Zhu, R. Amal, S. C. Smith, J. Am. Chem. Soc. 2012, 134, 4393. C. A. Caputo, M. A. Gross, V. W. Lau, C. Cavazza, B. V. Lotsch, E. Reisner, Angew. Chem. Int. Ed. 2014, 53, 11538. a) W. Ma, D. Han, M. Zhou, H. Sun, L. Wang, X. Dong, L. Niu, Chem. Sci. 2014, 5, 3946; b) J. Zhang, M. Grzelczak, Y. Hou, K. Maeda, K. Domen, X. Fu, M. Antonietti, X. Wang, Chem. Sci. 2012, 3, 443; c) Y. Wang, R. Shi, J. Lin, Y. Zhu, Energy Environ. Sci. 2011, 4, 2922; d) Y. Hou, F. Zuo, A. P. Dagg, J. Liu, P. Feng, Adv. Mater. 2014, 26, 5043; e) X. Chen, X. Huang, Z. Yi, Chem. Eur. J. 2014, 20, 17590. M. Zhang, X. Wang, Energy Environ. Sci. 2014, 6, 1902. Y. J. Zhang, T. Mori, J. H. Ye, M. Antonietti, J. Am. Chem. Soc. 2010, 132, 6294. Y. Wang, J. S. Zhang, X. C. Wang, M. Antonietti, H. R. Li, Angew. Chem. Int. Ed. 2010, 49, 3356. J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, Chem. Commun. 2012, 48, 12017. Z. Lin, X. Wang, Angew. Chem. Int. Ed. 2013, 52, 1735. a) J. S. Zhang, J. H. Sun, K. Maeda, K. Domen, P. Liu, M. Antonietti, X. Z. Fu, X. C. Wang, Energy Environ. Sci. 2011, 4, 675; b) G. Liu, P. Niu, C. H. Sun, S. C. Smith, Z. G. Chen, G. Q. Lu, H. M. Cheng, J. Am. Chem. Soc. 2010, 132, 11642. X. Chen, J. Zhang, X. Fu, M. Antonietti, X. Wang, J. Am. Chem. Soc. 2009, 131, 11658. P. Niu, L.-C. Yin, Y.-Q. Yang, G. Liu, H.-M. Cheng, Adv. Mater. 2014, 26, 8046. a) P. Niu, L. Zhang, G. Liu, H.-M. Cheng, Adv. Funct. Mater. 2012, 22, 4763; b) S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P. M. Ajayan, Adv. Mater. 2013, 25, 2452; c) K. Schwinghammer, M. B. Mesch, V. Duppel, C. Ziegler, J. Senker, B. V. Lotsch, J. Am. Chem. Soc. 2014, 135, 1730; d) X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan, Y. Xie, J. Am. Chem. Soc. 2012, 135, 18. a) J. Zhang, F. Guo, X. Wang, Adv. Funct. Mater. 2013, 23, 3008; b) Y.-S. Jun, E. Z. Lee, X. Wang, W. H. Hong, G. D. Stucky, A. Thomas, Adv. Funct. Mater. 2013, 23, 3661; c) A. Fischer, M. Antonietti, A. Thomas, Adv. Mater. 2007, 19, 264. a) Y. Zheng, L. Lin, X. Ye, F. Guo, X. Wang, Angew. Chem. Int. Ed. 2014, 53, 11926; b) X. H. Li, X. C. Wang, M. Antonietti, Chem. Sci. 2012, 3, 2170. a) X. Wang, S. Blechert, M. Antonietti, ACS Catalysis 2012, 2, 1596; b) M. Groenewolt, M. Antonietti, Adv. Mater. 2005, 17, 1789; c) 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; d) Y. Wang, H. Li, J. Yao, X. Wang, M. Antonietti, Chem. Sci. 2011, 2, 446; e) J. Zhang, M. Zhang, R.-Q. Sun, X. Wang, Angew. Chem. Int. Ed. 2012, 51, 10145; f) X.-H. Li, J.-S. Chen, X. Wang, J. Sun, M. Antonietti, J. Am. Chem. Soc. 2011, 133, 8074; g) Y. Zhang, A. Thomas, M. Antonietti, X. Wang, J. Am. Chem. Soc. 2008, 131, 50. a) T. Y. Ma, S. Dai, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 2014, 53, 7281; b) H. Huang, S. Yang, R. Vajtai, X. Wang, P. M. Ajayan, Adv. Mater. 2014, 26, 5160; c) Y. Zhao, F. Zhao, X. Wang, C. Xu, Z. Zhang, G. Shi, L. Qu, Angew. Chem. Int. Ed.
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[24] D. Y. Zhong, G. Y. Zhang, S. Liu, E. G. Wang, Q. Wang, H. Li, X. J. Huang, Appl. Phys. Lett. 2001, 79, 3500. [25] L. Z. Fan, Y. S. Hu, J. Maier, P. Adelhelm, B. Smarsly, M. Antonietti, Adv. Funct. Mater. 2007, 17, 3083. [26] Y. Ishida, L. Chabanne, M. Antonietti, M. Shalom, Langmuir 2014, 30, 447. [27] M. Shalom, S. Inal, C. Fettkenhauer, D. Neher, M. Antonietti, J. Am. Chem. Soc. 2013, 135, 7118.
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2014, 53, 13934; d) Q. Chen, Y. Zhao, X. Huang, N. Chen, L. Qu, J. Mater. Chem. A 2015, 3, 6761. [22] Y. Li, J. Chen, L. Huang, C. Li, J.-D. Hong, G. Shi, Adv. Mater. 2014, 26, 4789. [23] Y. Shi, S. Jiang, K. Zhou, C. Bao, B. Yu, X. Qian, B. Wang, N. Hong, P. Wen, Z. Gui, Y. Hu, R. K. K. Yuen, ACS Appl. Mat. Interfaces 2013, 6, 429.
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Macroscopic 3D Porous Graphitic Carbon Nitride Monolith for Enhanced Photocatalytic Hydrogen Evolution Qinghua Liang, Zhi Li, Xiaoliang Yu, Zheng-Hong Huang,* Feiyu Kang, and Quan-Hong Yang* As a promising and alternative way to alleviate the current worldwide energy and environmental issues, artificial photosynthesis that mimics the natural conversion and generation of chemical fuels has drawn tremendous interest in the past few decades.[1] Despite the fact that much progress has been made, photocatalysis technology is still restricted by the lack of highly efficient photocatalysts for practical applications. This has motivated considerable research on pursuing low-cost, stable, efficient, and eco-friendly photocatalysts.[2] Among many semiconductor-based photocatalysts, polymeric graphitic carbon nitride (g-C3N4 or g-CN) has attracted much attention owing to its many excellent properties including extraordinary chemical stability, easy preparation, 2D structure, metal-free composition, visible light response, and tunable electronic structure[1e,3] since the pioneering study in 2009.[1c] It is believed that g-CN will play an increasingly significant role in the fields of photocatalysis.[1e,3,4] Nevertheless, it still suffers from some drawbacks, such as small specific surface area (SSA), low visible light utilization efficiency, and rapid recombination of photogenerated carriers.[3,5] For these reasons, many impressive results have been obtained by modifying g-CN by coupling with other materials such as carbon dots,[1e] graphene,[6] hydrogenase,[7] semiconductors,[8] aromatic compounds,[9] and doping with heteroatoms such as P,[10] F,[11] O,[12] B,[11,13] I,[5a] S,[14] and Fe,[15] and introducing nitrogen vacancies[16] to overcome the aforementioned problems. Besides, a variety of forms of nanostructured g-CN including nanosheets,[17] porous Q. Liang, X. Yu, Prof. F. Kang, Prof. Q.-H. Yang Shenzhen Key Laboratory for Graphene-Based Materials and Engineering Laboratory for Functionalized Carbon Materials Graduate School at Shenzhen Tsinghua University Shenzhen 518055, P. R. China E-mail: [email protected] and [email protected] Q. Liang, X. Yu, Prof. Z.-H. Huang Key Laboratory of Advanced Materials (MOE) School of Materials Science and Engineering Tsinghua University Beijing 100084, P. R. China E-mail: [email protected] Q. Liang, Prof. Z. Li Technical Institute of Physics and Chemistry Chinese Academy of Sciences (CAS) Beijing 100190, P. R. China Prof. Q.-H. Yang School of Chemical Engineering and Technology Tianjin University Tianjin 300072, P. R. China
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structures,[18] nanospherical frameworks,[5b] nanorods,[19] etc. have also been developed in recent years. In particular, Antonietti and Wang et al. have performed many pioneering studies to promote the development of g-CN.[1c,3c,9–11,14a,15,20] However, almost all studies reported thus far have been focused on g-CN in the powder form that suffers from a complicated recovery step during the photocatalytic process, setting a hurdle for the real applications. Moreover, the random aggregation of g-CN nanosheets in a powder decreases their surface accessibility, which, unfortunately, is highly important for the catalytic performance. Therefore, an ordered and macrosized assembly of highly accessible g-CN nanosheets is of great significance for device-based performance and real application of g-CN. Though some recent advances have been made to produce some assembly of g-CN hybridized with graphene or carbon nanotubes,[21] the construction of a 3D macroform of g-CN is still a big challenge. Consequently, it is of great urgency to develop a synthesis method for producing high efficiency macroscopic 3D g-CN photocatalyst with better recovery properties for the industrial and practical applications.[22] Here, we report the first synthesis of a macroscopic 3D porous g-CN monolith (PCNM) by a well-designed template method, that is, one-step thermal polymerization of urea inside the framework of an intentionally selected yet very cheap template, melamine sponge (MS), normally found in kitchen, that is free of any other functionalizing or crosslinking agents. Combining extremely high loading of urea with lightweight and good water absorbability of MS makes the method practicable for the synthesis of PCNM. Also, note that both urea and melamine are precursors for preparing g-CN.[3a–c,5] The resultant PCNM possesses a 3D interconnected network composed of 2D porous g-CN nanosheets. With such a unique structure, PCNM is lightweight and freestanding, and has good structural stability and a large SSA. Remarkably, as a typical application, the 3D PCNM exhibits significantly improved performance compared to its powder counterpart toward photocatalytic water splitting for hydrogen evolution under visible light. The general synthesis route leading to the macroscopic 3D PCNM is illustrated in Figure 1. A commercially available MS is directly used as both the template and the supporting framework (Figure 1a). After dipping it in the saturated aqueous urea solution followed by freeze-drying (Figure 1b), the MS was completely and homogeneously filled with urea (Figure S1, Supporting Information). Heating the MS filled with urea at 550 °C for 4 h results in a macroscopic 3D PCNM that has a very low mass density of ≈35 mg cm−3 (Figure 1c), while directly heating MS in the same condition results in an N-enriched carbon foam (Figures S1–S3 and Table S1, Supporting Information). Note that upon heating the original white
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MS (40 × 20 × 20 mm3) transformed into a smaller piece of light black PCNM (28 × 14 × 14 mm3), as shown in Figure 1c. The isotropic 30% shrinkage is caused by two synergistic processes: heating-induced shrinking of the MS framework and volume decrease resulting from thermal polymerization of the pyrolysis intermediates of urea and melamine (Figures S4 and S5, Supporting Information). The resultant PCNM can be easily cut into different shapes with a blade (Figure 1d). Heating urea in the same way without MS yields g-CN powder as a reference. Direct evidence for the formation of g-CN is obtained using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetry (TG), and X-ray photoelectron spectroscopy (XPS) analysis. Similar to the powdered g-CN, the XRD pattern of PCNM in Figure 2a clearly has two characteristic diffraction peaks at 13.6° (100) and 27.2° (002) that are, respectively, attributed to the in-plane repeating unites of the continuous heptazine framework and the stacking of the conjugated aromatic structure with a spacing of ≈0.32 nm (inset in Figure 2a).[5b,14b,18a,19a] The broader and weaker diffraction peaks indicate the shorter correlation lengths of the interlayer periodicity of tri-s-triazine building blocks in PCNM.[5b,19a] FT-IR spectra shown in Figure 2b exhibit typical peaks at 3100–3500, 1200–1600, and 810 cm−1, which are ascribed to the vibrational absorption of N H and O H, aromatic C N heterocyclic unites and the triazine unit (inset in Figure 2b), respectively.[5b,17a,b,18a] In the XPS survey spectrum, only C, N, and O elements are detected. The weak O 1s peak may be due to the surface absorbed H2O, as confirmed by the FT-IR results (Figure 2b). The C/N molar ratio of PCNM is close to the theoretical value of ideal g-CN (≈0.73). The chemical groups and bonding characteristics of PCNM are further confirmed by the high-resolution C 1s and N 1s spectra (insets in Figure 2e). There are three kinds of carbon species, including 288.7 eV for N C N, 286.5 eV for C N, 285.1 eV for C C in the C 1s spectrum.[13,21a] Moreover, the N 1s spectrum can be well fitted to four N species, i.e., 398.7 eV for C N C, 399.5 eV for N (C)3, 401.0 eV for N H, and 405.0 eV for π excitation of the
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Figure 1. Illustration of the preparation of a macroscopic 3D PCNM.
C N conjugated structure.[5b,13,21a] Both C 1s and N 1s XPS spectra confirm the basic heptazine heterocyclic structure of g-CN in PCNM. Most typical bands assigned to the vibration modes of CN heterocycles can be identified in the Raman spectrum of PCNM (Figure S3, Supporting Information). In addition, no obvious D- and G-bands typically assigned to carbon materials were detected in the Raman spectrum of PCNM. The above results further suggest that the main composition of the obtained PCNM is g-CN. TG was further carried out to study the thermal stability of PCNM (Figure 2d). Compared to the g-CN powder, a larger weight loss from 30 to 100 °C in the TG curve is attributed to the evaporation of a larger amount of absorbed H2O, which is due to the larger SSA of the PCNM. The decomposition of both PCNM and g-CN powder occurs from 560 to 700 °C, which is consistent with previous reports.[18a,21c] The less-than-2 wt% residues should be nitrogen enriched amorphous carbon originated from MS (Table S1, Supporting Information) promoted by the catalytic charring effect of isolated g-CN nanosheets,[23] suggesting the possible formation of nitrogen enriched carbon/CN heterojunction in the PCNM. Little nitrogen enriched carbon is the main cause for the gray black color of PCNM, and the presence of heterojunction in PCNM is useful for charge separation of g-CN.[3b] The representative N2 adsorption isotherm of PCNM is of type IV with an H2-type hysteresis loop at a high relative pressure ranging from 0.75 to 1 (Figure 2c), suggesting the presence of mesopores and macropores. Accordingly, a sharp peak at 4 nm and a broad peak ranging from 30 to 170 nm are identified in the pore size distribution curve (inset in Figure 2c), which are, respectively, in corresponding to the diameter of the inner cavity of the g-CN nanosheets and the size of the slitlike pores formed between the g-CN nanosheets. The SSA and pore volume are calculated to be 78 m2 g−1 and 0.76 cm3 g−1, respectively, which are much higher than those of the g-CN powder (40 m2 g−1 and 0.28 cm3 g−1). The higher SSA and larger pore volume may be caused by the effective prevention of the stacking of g-CN nanosheets by the MS framework during the polymerization of urea. The mercury porosimetry results also confirm the presence of abundant macropores in PCNM (Figure S6, Supporting Information). The resultant PCNM has a good mechanical strength although it has a very low mass density. A piece of PCNM about 12 × 17 × 17 mm3 can support a 100 g weight with little deformation (Figure 2f and S7, Supporting Information). In addition, PCNM shows acceptable elasticity. The unloading curves obtained from compression and rebound tests are above the line y = 0 when returning to the original state (Figure S8, Supporting Information), indicating a partial volume recovery without serious deformation of PCNM. The 3D porous structure of PCNM is readily identified with scanning and transmission electron microscopy (SEM and TEM). As shown in Figure 3a,b, we can clearly observe the 3D interconnected network and hierarchical pores with diameter in the range 200–400 nm. A magnified SEM image reveals that the interconnected networks are composed of irregularly shaped 2D porous nanosheets with a thickness of approximately 30 nm. A consistent result was obtained from TEM images showing a typical porous interconnected structure formed by the g-CN nanosheets joining together (Figure 3c). It
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hydrogen in the presence of Pt as a cocatalyst and triethanolamine as a sacrificial electron donor under visible light (Supporting Information). As shown in Figure 4a, the photocatalytic activity, in terms of the hydrogen evolution rate (29.0 µmol h−1), is 2.84 times higher than that of the g-CN powder (10.2 µmol h−1). We believe the significant enhancement in photocatalytic hydrogen evolution of the 3D PCNM compared to g-CN powder can be attributed to the following reasons. On one hand, the larger SSA of PCNM (Figure 2e) leads to an increased surface reactivity for the photocatalytic reaction compared to the g-CN powder. On the other hand, PCNM makes use of more visible light compared to g-CN powder. The diffuse reflectance spectrum (DRS) shown in Figure 4b demonstrates that PCNM shows significantly improved light harvesting ability above 450 nm in the optical spectrum compared to the powdered g-CN, which may be caused by the multiple reflections of incident light within the interconnected network of porous g-CN nanosheets and the presence of N-enriched carbon/g-CN heterojunction.[18b] The result is also reflected by the smaller bandgap energy of PCNM (2.05 eV) than the g-CN powder (2.68 eV). Besides, PCNM has a more effective separation of photogenerated charge carriers under the excitation of visible light compared to the g-CN powder, as demonstrated by the photoluminescence (PL) spectra and photochemical Figure 2. (a) XRD patterns, (b) FT-IR spectra, (c) N2 sorption isotherms and (d) TG curves measurements. In Figure 4c, no obvious of PCNM (solid line) and g-CN powder (dotted line). (e) XPS survey spectrum with the corresponding C 1s and N 1s spectra (insets) of PCNM. (f) Photographs of PCNM showing that peak is found in the PL emission spectrum of PCNM, while the g-CN powder shows a it could support a 100 g weight. The insets in (a, b) are the ideal structures of g-CN and the strong intrinsic fluorescence emission peak inset in (c) is the corresponding pore size distribution. at 465 nm, suggesting a larger barrier to charge recombination in PCNM.[5,6] In accordance with the PL should be mentioned that the g-CN powder prepared without MS shows irregularly shaped particles (Figure S9, Supporting spectra, PCNM shows a much smaller arc radius in the electroInformation), demonstrating that the MS template plays a critchemical impedance spectrum (EIS, Figure 4d), indicating its ical role in the formation of PCNM. An enlarged TEM image faster interface charge transport.[5a,6a,17b] Moreover, obviously in Figure 3d obviously reveals the presence of abundant pores increased PL life time (Figure S11, Supporting Information) in the nanosheets with the diameters ranging 30–150 nm. The and photocurrent density are observed in the PCNM electrode selected area electron diffraction (SAED) pattern showing two compared to the g-CN powder case (inset in Figure 4d), sugdiffuse diffraction rings indicates the disordered structure of gesting its more efficient separation of photo-generated charge the porous g-CN nanosheets (inset in Figure 3d).[24] A highcarriers,[5,6][8d] which may be attributed to the unique structure resolution TEM image of the fine microstructure shows that because the 3D interconnected network of porous nanosheets these nanosheets have sharp edges and a turbostratic structure ensures sufficient mass transportation and also shortens the (Figure S10, Supporting Information). Such a 3D intercondiffusion length of charge migration and thus facilitates elecnected network of porous nanosheets containing many junctron relocalization on surface edge sites to hinder the charge tions and the open pores provide a good combination of infilrecombination.[5b] These results also agree well with previous [ 25 ] tration rate and surface area, and are also favorable for mass reports of g-CN obtained from thermal polycondensation of supramolecular organization of carbon nitride precursors. transfer and charge separation.[5b,26] [18b,26,27] Owing to the unique 3D interconnected network, a large SSA, and probably more catalytically active sites, the resultant PCNM The catalytic stability of PCNM was evaluated by was evaluated for photocatalytic water splitting to produce measuring photocatalytic hydrogen evolution under the same
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Figure 3. a,b) SEM and c,d) TEM images of the resultant PCNM. The inset in (d) is the corresponding SAED.
reaction conditions for three cycles. There is no obvious deactivation with time under continuous visible light illumination for 9 h (Figure S12, Supporting Information). Both the
3D interconnected network and the porous g-CN nanosheets are retained after such a 9 h experiment (Figure S13, Supporting Information). Pt nanoparticles with an average size of
Figure 4. (a) Photocatalytic activity for H2 production, (b) DRS spectrum, (c) PL emission spectra, and (d) EIS plot and photocurrent-time dependence of PCNM (solid line) and the g-CN powder (dotted line).
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2 nm are well distributed on the edges of the porous g-CN nanosheets (Figure S13, Supporting Information), demonstrating the possible utilization of PCNM as a stable matrix for supporting other catalysts. The application of PCNM as the electrocatalyst for fuel cells and exploration of the synthesis method for preparing other macroscopic 3D porous materials is under way. In conclusion, we have shown the first synthesis of a macroscopic 3D porous graphitic carbon nitride monolith (PCNM) by a one-step thermal polymerization of urea inside the framework of a melamine sponge. Because of its abundant porosity, high specific surface area, good visible light capture, as well as superior charge separation efficiency, PCNM exhibits excellent photocatalytic activity, which is 2.84 times higher than that of g-CN powder for hydrogen evolution under visible light. In view of its freestanding structure, good mechanical strength, acceptable elasticity, and superior photocatalytic activity, the macroscopic PCNM shows great potential for the applications in the fields of water splitting, pollutant degradation, CO2 reduction, and energy storage devices. It is believed that such a well-designed approach paves the way for preparing other 3D g-CN hybrid materials for real applications.
[4] [5]
[6]
[7] [8]
[9] [10] [11] [12]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
[13] [14]
Acknowledgements
[15]
Financial support from National Basic Research Program of China (2014CB932400), National Natural Science Foundation of China (Nos. 51302274, 51372167, and U1401243), Guangdong Province Innovation R&D Team Plan (No. 2009010025), and Shenzhen Basic Research Project (Nos. JCYJ20130402145002430 and ZDSYS20140509172959981) was gratefully acknowledged. The authors also thank Dr. Ruitao Lv for helpful discussions.
[16]
Received: April 29, 2015 Revised: May 28, 2015 Published online: July 2, 2015
[17]
[18]
[19] [1] a) L.-Z. Wu, B. Chen, Z.-J. Li, C.-H. Tung, Acc. Chem. Res. 2014, 47, 2177; b) K. Chang, Z. Mei, T. Wang, Q. Kang, S. Ouyang, J. Ye, ACS Nano 2014, 8, 7078; c) X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8, 76; d) W. Wang, J. Chen, C. Li, W. Tian, Nat. Commun. 2014, 5, 4647; e) J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S.-T. Lee, J. Zhong, Z. Kang, Science 2015, 347, 970. [2] a) Y. Qu, X. Duan, Chem. Soc. Rev. 2013, 42, 2568; b) J. Ran, J. Zhang, J. Yu, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev. 2014, 43, 7787; c) H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Chem. Soc. Rev. 2014, 43, 5234; d) Q. Xiang, J. Yu, M. Jaroniec, Chem. Soc. Rev. 2012, 41, 782. [3] a) Y. Zheng, J. Liu, J. Liang, M. Jaroniec, S. Z. Qiao, Energy Environ. Sci. 2012, 5, 6717; b) Z. Zhao, Y. Sun, F. Dong, Nanoscale 2015, 7, 15; c) Y. Wang, X. Wang, M. Antonietti, Angew. Chem. Int. Ed. 2012,
4638
wileyonlinelibrary.com
[20]
[21]
51, 68; d) S. Cao, J. Low, J. Yu, M. Jaroniec, Adv. Mater. 2015, 27, 2150; e) Y. Gong, M. Li, Y. Wang, ChemSusChem 2015, 8, 931. Y. Zheng, Y. Jiao, Y. Zhu, L. H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec, S. Z. Qiao, Nat Commun. 2014, 5, 3783. a) G. Zhang, M. Zhang, X. Ye, X. Qiu, S. Lin, X. Wang, Adv. Mater. 2014, 26, 805; b) J. Zhang, M. Zhang, C. Yang, X. Wang, Adv. Mater. 2014, 26, 4121. a) Y. Hou, Z. Wen, S. Cui, X. Guo, J. Chen, Adv. Mater. 2013, 25, 6291; b) Y. Zhang, T. Mori, L. Niu, J. Ye, Energy Environ. Sci. 2011, 4, 4517; c) A. Du, S. Sanvito, Z. Li, D. Wang, Y. Jiao, T. Liao, Q. Sun, Y. H. Ng, Z. Zhu, R. Amal, S. C. Smith, J. Am. Chem. Soc. 2012, 134, 4393. C. A. Caputo, M. A. Gross, V. W. Lau, C. Cavazza, B. V. Lotsch, E. Reisner, Angew. Chem. Int. Ed. 2014, 53, 11538. a) W. Ma, D. Han, M. Zhou, H. Sun, L. Wang, X. Dong, L. Niu, Chem. Sci. 2014, 5, 3946; b) J. Zhang, M. Grzelczak, Y. Hou, K. Maeda, K. Domen, X. Fu, M. Antonietti, X. Wang, Chem. Sci. 2012, 3, 443; c) Y. Wang, R. Shi, J. Lin, Y. Zhu, Energy Environ. Sci. 2011, 4, 2922; d) Y. Hou, F. Zuo, A. P. Dagg, J. Liu, P. Feng, Adv. Mater. 2014, 26, 5043; e) X. Chen, X. Huang, Z. Yi, Chem. Eur. J. 2014, 20, 17590. M. Zhang, X. Wang, Energy Environ. Sci. 2014, 6, 1902. Y. J. Zhang, T. Mori, J. H. Ye, M. Antonietti, J. Am. Chem. Soc. 2010, 132, 6294. Y. Wang, J. S. Zhang, X. C. Wang, M. Antonietti, H. R. Li, Angew. Chem. Int. Ed. 2010, 49, 3356. J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, Chem. Commun. 2012, 48, 12017. Z. Lin, X. Wang, Angew. Chem. Int. Ed. 2013, 52, 1735. a) J. S. Zhang, J. H. Sun, K. Maeda, K. Domen, P. Liu, M. Antonietti, X. Z. Fu, X. C. Wang, Energy Environ. Sci. 2011, 4, 675; b) G. Liu, P. Niu, C. H. Sun, S. C. Smith, Z. G. Chen, G. Q. Lu, H. M. Cheng, J. Am. Chem. Soc. 2010, 132, 11642. X. Chen, J. Zhang, X. Fu, M. Antonietti, X. Wang, J. Am. Chem. Soc. 2009, 131, 11658. P. Niu, L.-C. Yin, Y.-Q. Yang, G. Liu, H.-M. Cheng, Adv. Mater. 2014, 26, 8046. a) P. Niu, L. Zhang, G. Liu, H.-M. Cheng, Adv. Funct. Mater. 2012, 22, 4763; b) S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P. M. Ajayan, Adv. Mater. 2013, 25, 2452; c) K. Schwinghammer, M. B. Mesch, V. Duppel, C. Ziegler, J. Senker, B. V. Lotsch, J. Am. Chem. Soc. 2014, 135, 1730; d) X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan, Y. Xie, J. Am. Chem. Soc. 2012, 135, 18. a) J. Zhang, F. Guo, X. Wang, Adv. Funct. Mater. 2013, 23, 3008; b) Y.-S. Jun, E. Z. Lee, X. Wang, W. H. Hong, G. D. Stucky, A. Thomas, Adv. Funct. Mater. 2013, 23, 3661; c) A. Fischer, M. Antonietti, A. Thomas, Adv. Mater. 2007, 19, 264. a) Y. Zheng, L. Lin, X. Ye, F. Guo, X. Wang, Angew. Chem. Int. Ed. 2014, 53, 11926; b) X. H. Li, X. C. Wang, M. Antonietti, Chem. Sci. 2012, 3, 2170. a) X. Wang, S. Blechert, M. Antonietti, ACS Catalysis 2012, 2, 1596; b) M. Groenewolt, M. Antonietti, Adv. Mater. 2005, 17, 1789; c) 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; d) Y. Wang, H. Li, J. Yao, X. Wang, M. Antonietti, Chem. Sci. 2011, 2, 446; e) J. Zhang, M. Zhang, R.-Q. Sun, X. Wang, Angew. Chem. Int. Ed. 2012, 51, 10145; f) X.-H. Li, J.-S. Chen, X. Wang, J. Sun, M. Antonietti, J. Am. Chem. Soc. 2011, 133, 8074; g) Y. Zhang, A. Thomas, M. Antonietti, X. Wang, J. Am. Chem. Soc. 2008, 131, 50. a) T. Y. Ma, S. Dai, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 2014, 53, 7281; b) H. Huang, S. Yang, R. Vajtai, X. Wang, P. M. Ajayan, Adv. Mater. 2014, 26, 5160; c) Y. Zhao, F. Zhao, X. Wang, C. Xu, Z. Zhang, G. Shi, L. Qu, Angew. Chem. Int. Ed.
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[24] D. Y. Zhong, G. Y. Zhang, S. Liu, E. G. Wang, Q. Wang, H. Li, X. J. Huang, Appl. Phys. Lett. 2001, 79, 3500. [25] L. Z. Fan, Y. S. Hu, J. Maier, P. Adelhelm, B. Smarsly, M. Antonietti, Adv. Funct. Mater. 2007, 17, 3083. [26] Y. Ishida, L. Chabanne, M. Antonietti, M. Shalom, Langmuir 2014, 30, 447. [27] M. Shalom, S. Inal, C. Fettkenhauer, D. Neher, M. Antonietti, J. Am. Chem. Soc. 2013, 135, 7118.
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2014, 53, 13934; d) Q. Chen, Y. Zhao, X. Huang, N. Chen, L. Qu, J. Mater. Chem. A 2015, 3, 6761. [22] Y. Li, J. Chen, L. Huang, C. Li, J.-D. Hong, G. Shi, Adv. Mater. 2014, 26, 4789. [23] Y. Shi, S. Jiang, K. Zhou, C. Bao, B. Yu, X. Qian, B. Wang, N. Hong, P. Wen, Z. Gui, Y. Hu, R. K. K. Yuen, ACS Appl. Mat. Interfaces 2013, 6, 429.
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