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FeP Nanoparticles Grown on Graphene Sheets as Highly Active NonPrecious-Metal Electrocatalyst for Hydrogen Evolution Reaction
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A synthetic route to FeP-GS hybrid sheets that show good stability and high electrocatalytic activity for hydrogen evolution reaction is reported. The materials are prepared via thermal phosphidation of pre-synthesized Fe3O4-GS hybrid sheets. Hydrogen, as a clean renewable energy source fuel without the emission of carbon dioxide, has been attracting a lot of attention as one of the most promising energy carrier of the future replacing traditional fossil fuels.1 On the other hand, conversion and storage solar energy and wind energy in the form of hydrogen by water splitting reaction is an attractive scheme to solve the intermittent nature of solar energy and wind energy. The water splitting reaction consists of half reactions: the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).2 To conduct electrochemical water splitting, overpotential (η) (required above the standard reaction potential) must be applied. Hence, efficient electrocatalyst are demanded to reduce the overpotential and increase the efficiency of OER and HER. Typically, precious metals such as platinum and its alloys are the most efficient electrocatalyst towards the HER for the production of molecular hydrogen from water. However, the high cost and low abundance prevent the large scale application of these precious metals for hydrogen production.3-4 Therefore, extensive efforts have been devoted to design and develop high-efficient and earth-abundant elements as HER electrocatalyst. To the present, a variety of non-noble (Fe, Co, Ni, and Mo) and metal-free materials (carbon-based) have been selected as effective candidates for catalyzing the electrochemical HER.5-8 Among these non-noble HER electrocatalyst, molybdenum-based compounds have drawn much attention. For example, MoS2,9 defect-rich MoS2,10 amorphous MoSX,11 Ni-Mo,12 NiMoNX,13 MoB,14 Mo2C,15 Cu2MoS4,16 Co0.6Mo1.4N2,17 and MoSe2,18 have been intensively studied. Very recently, metal-free HER electrocatalyst, comprised of carbon framework doped with different hybrid elements, have been reported to show high HER activity.19-22 Zheng et al. demonstrated that graphitic-carbon nitride with nitrogen-doped graphene to produce a metal-free hybrid catalyst, afford current densities of 10 mA/cm2 at η = 240 mV and a Tafel slope of 51.5 mV/dce.22 Furthermore, transitionmetal phosphides (TMPs), by the alloying of metals and phosphorus, are a known class of catalyst for hydrodesulfurization (HDS). Because both HER and HDS depend on the catalyst to reversibly bind hydrogen, it is expected that This journal is © The Royal Society of Chemistry [year]
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TMPs are a new class of HER catalyst. In recent years, several kinds of TMPs have emerged as an attractive HER electrocatalyst such as CoP,23-25 FeP,26 Ni2P,27-28 and MoP.29-30 On the other hand, the electrical conductivity of electrocatalyst is one of the most key factors that influence electrocatalytic efficiency. Taking this into account, carbon materials are ideal supports to improve the electrocatalytic activity because of their high electrical conductivity. Graphene, which is an atomic layers of carbon atoms arranged in a honeycomb network, has received considerable attention due to its unique two-dimensional structure and excellent physical and chemical properties.31-32 Until now, the combination of graphene sheets (GS) and various nanoparticles (NPs) as a new kind of hybrid materials has aroused extensive interest.33-42 For example, Dai et al. developed a selective solvothermal synthesis of MoS2 NPs on reduced graphene oxide sheets, which exhibited superior electrocatalytic activity in the HER relative to other MoS2 catalysts.43
Scheme 1. Illustration of the procedure for preparing FeP-GS.
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In this paper, we report the first synthesis of FeP NPs on GS and demonstrate their high HER electrocatalytic activity with small Tafel slope and low overpotential. The whole preparation strategy for constructing the FeP-GS is shown in Scheme 1 (for detailed experimental steps, please see Experimental Section in the ESI). Firstly, graphene oxide (GO) was prepared by oxidizing natural graphite via the modified Hummers method. Then, the oxidized graphite was exfoliated in N,N-Dimethylformamide
[journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
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Zhe Zhanga, Baoping Lua,b, Jinhui Haoa,b, Wenshu Yanga,b, and Jilin Tang*a
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(Figure S4). These observations indicate considerable deoxygenation of GO through the successively solvothermal and View Article Online thermal treatment, which is very important for improving FeP-GS DOI: 10.1039/C4CC05285D conductivity and enhancing HER. Figure 1c and 1d show the XPS profile of Fe 2p and P 2p, respectively. The Fe 2p spectrum shows two peaks with binding energy (BE) values at 710.9 and 724.0 eV, assigned to the Fe 2p3/2 and Fe 2p1/2 peaks, respectively. The P 2p spectrum shows two peaks at 129.3 and 130.2 eV corresponding to the BE of P 2p1/2 and P 2p3/2, respectively. Furthermore, the spectrum of P 2p also exhibits a peak with high BE of 133.6 eV, which could be attributed to PO43- or P2O5 caused by oxidation due to air contact.46
Figure 1. (a) XRD patterns of Fe3O4-GS and FeP-GS. XPS spectra in the C 1s (b), Fe 2p (c), and P 2p (d) regions for FeP-GS.
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First of all, X-ray diffraction (XRD) was used to characterize the structural information of Fe3O4-GS. Figure 1a shows the XRD pattern of the as-synthesis Fe3O4-GS (red line) by decomposition of Fe(acac)3 in autoclave. Several broad diffraction peaks in the representative XRD pattern of Fe3O4-GS corresponding to the (220), (311), (400), (422), (511), (440), and (533) reflections were observed. All of the diffraction peaks can be indexed as a pure cubic spine crystal structure of Fe3O4 with cell constant a = 8.39 Å (JCPDS card No. 19-0629).44 The broadening diffraction peaks indicated that the Fe3O4 NPs were small in size, which was also confirmed by transmission electron microscopy (TEM) images (see Figure 2). Furthermore, the formation of the Fe3O4GS hybrid composites can also be verified by XRD pattern, as the peak corresponding to graphite has not been observed. After phosphidation by thermal decomposition NaH2PO2, only the diffraction peaks of the FeP phase corresponding to the (002), (011), (200), (111), (202), (211), (103), (301), and (020) reflections are observed. All peaks in the XRD pattern (Figure 1a, black line) can be indexed to the orthorhombic cell of the FeP with space group Pnma (lattice constant a = 5.191 Å, b = 3.099 Å, and c = 5.792 Å, JCPDS card No. 89-2746).45 These observations suggest that the successful conversion of Fe3O4 into FeP. In addition to, the XRD of Fe3O4 NPs and FeP NPs are shown in Figure S1 in the ESI. Further evidence for the formation of the FeP-GS hybrid composites was obtained from X-ray photoelectron spectroscopy (XPS) analysis. The XPS survey of FeP-GS (Figure S3) revealed the presence of the elements C, O, P, and Fe, which indicated the formation of FeP NPs on GS. The element O comes from residual oxygen-containing functionalities in the GS and surface oxidation of FeP NPs due to air contact. Figure 1b shows the C 1s peaks of FeP-GS, the peak intensities of oxygen-containing functionalities in the FeP-GS are much smaller than that in graphite oxide 2 | Journal Name, [year], [vol], 00–00
Figure 2. TEM image (a) and HRTEM image (b) of Fe3O4-GS. TEM 60
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image (c) and HRTEM image (d) of FeP-GS.
The morphology of the Fe3O4-GS and FeP-GS were further characterized by TEM. TEM image (Figure 2a) shows that the Fe3O4-GS consist of single-layer two-dimensional GS decorated with Fe3O4 NPs and the individual Fe3O4 NPs are well separated from each other with high density and well distributed on the GS. Despite the high density of Fe3O4 NPs on GS, the sheets remained flat, and no crumpled sheets are observed (Figure S5). It can be seen that all the Fe3O4 NPs are coated on the GS and no aggregated or free NPs are detected. As shown from the Fe3O4 NPs in the high-resolution TEM (HRTEM) (Figure 2b), it is observed that small Fe3O4 NPs spread out on the surface of GS have a size of about 3-5 nm, which is in the superparamagnetic size range. The distance between two adjacent crystal planes of the NPs is determined to be 0.25 nm (Figure 2b), corresponding to the lattice spacing of the (311) planes of Fe3O4. After the phosphidation process, FeP NPs homogeneously attach to the surface of GS (Figure 2c) and FeP-GS display crumpled mono- or few-layer sheets (Figure S6). The spacing between two adjacent lattice planes is about 0.19 nm, corresponding to the (211) plane of FeP (Figure 2d). In order to evaluate electrocatalytic activity of FeP-GS, electrochemical measurements of the FeP-GS were conducted in This journal is © The Royal Society of Chemistry [year]
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(DMF) with the treatment of ultrasonication. Secondly, GS decorated with monodisperse Fe3O4 NPs were obtained by reacting between GO and iron (III) acetylacetonate (Fe(acac)3) in a DMF and tetraethylene glycol (TEG) mixed solvent at 180 °C for 2 h. Finally, the obtained intermediate Fe3O4-GS were converted to FeP-GS by phosphidation at 350 oC for 2 h with nitrogen as the carrier gas.
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0.5 M H2SO4 solution using a standard three-electrode system. FeP-GS were deposited onto glassy carbon electrode (GCE, 0.28 mg/cm2) as the working electrode. The HER activity of FeP NPs were also measured for comparison. Figure 3a shows the linear sweep voltammetry (LSV) curves for GCE modified with GS, FeP NPs, FeP-GS, and bare GCE, respectively. Both bare GCE and GS with no convincing HER activity were observed. In sharp contrast to FeP NPs, the FeP-GS have a small onset potential of 30 mV versus HER (black line). As shown in Figure 3b, FeP-GS achieve current densities of 10 mA/cm2 at overpotential 123 mV, whereas FeP NPs have a current density of 1 mA/cm2 under same overpotential. In order to investigate the hydrogen evolution mechanism at the FeP-GS, the Tafel plot was also conducted. Tafel plots are shown in Figure 3c in the low current density region. According to the Tafel equation (η=b log j + a, where j is the current density and b is the Tafel slope), it exhibits Tafel slopes of 67, 50, and 30 mV/dec for FeP NPs, FeP-GS, and Pt wire, respectively. At the same time, FeP-GS have a higher exchange current density (1.2 × 10-1 mA/cm2) compared with FeP NPs (1.7 × 10-2 mA/cm2). The Tafel slope for FeP-GS suggesting that the HER proceeds through a Volmer-Heyrovsky mechanism and electrochemical desorption process is the ratelimiting step. As another important aspect for catalysts, stability of FeP-GS was further probed in 0.5 M H2SO4. After cyclic voltammetry (CV) scanning for 1000 cycles, the LSV curves showed insignificant change compared with the first cycle (Figure 3d), suggesting good stability of FeP-GS in the HER process. The current-time (I-t) curve under η = 240 mV (Figure 3d, inset) suggests FeP-GS retained its electrocatalytic activity for at least 19 hours.
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subsequent thermal phosphidation to form FeP NPs on the surface of GS. It is shown that the obtained FeP-GS hybrid sheets View Article Online have high HER catalytic activity and good stability with a small DOI: 10.1039/C4CC05285D onset overpotential of 30 mV, a Tafel slope of 50 mV/dec, an exchange current density of 1.2 × 10-1 mA/cm2, and a 10 mA/cm2 current density at overpotential 123 mV. We attribute the high HER performance of the FeP-GS the following major factors: i) Intimate interactions and electronic coupling between the GS and FeP NPs should contribute to the high HER activity of the FeP-GS hybrid sheets. ii) GS offer the good conductivity and facilitate rapid electron transfer from the FeP NPs to the electrodes. iii) Better dispersion of FeP NPs on the GS with abundance of exposed catalytic sites play important roles to enhance the HER activity. Acknowledgement This work was supported by the National Basic Research Program of China (973 Program, No. 2011CB935800).
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State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China. E-mail:[email protected]. Tel/Fax: (+86) 431-85262734 b University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/ 1. 2.
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(b) Enlargement of the shadow region in (a). (c) Tafel plots of FeP NPs, 35
FeP-GS, and Pt wire. (d) LSV curves for FeP-GS in 0.5 M H2SO4 with a
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mV/s between +0.003 and -0.303 V (vs. RHE). Inset: A current-time (I-t) curve obtained for a hydrogen evolution reaction with FeP-GS at
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In summary, we present a facile two-step method to prepare FeP-GS hybrid sheets using Fe3O4-GS as an intermediate and This journal is © The Royal Society of Chemistry [year]
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N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 15729-15735. T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets and D. G. Nocera, Chem. Rev., 2010, 110, 6474-6502. R. Subbaraman, D. Tripkovic, D. Strmcnik, K. C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic and N. M. Markovic, Science, 2011, 334, 1256-1260. I. J. Hsu, Y. C. Kimmel, X. G. Jiang, B. G. Willis and J. G. Chen, Chem. Commun., 2012, 48, 1063-1065. F. Jiao and H. Frei, Energy Environ. Sci., 2010, 3, 1018-1027. D. Merki and X. L. Hu, Energy Environ. Sci., 2011, 4, 3878-3888. A. B. Laursen, S. Kegnaes, S. Dahl and I. Chorkendorff, Energy Environ. Sci., 2012, 5, 5577-5591. C. G. Morales-Guio, L.-A. Stern and X. Hu, Chem. Soc. Rev., 2014, DOI: 10.1039/c3cs60468c. T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 2007, 317, 100-102. J. F. Xie, H. Zhang, S. Li, R. X. Wang, X. Sun, M. Zhou, J. F. Zhou, X. W. Lou and Y. Xie, Adv. Mater., 2013, 25, 5807-5813. J. D. Benck, Z. B. Chen, L. Y. Kuritzky, A. J. Forman and T. F. Jaramillo, Acs Catalysis, 2012, 2, 1916-1923. J. R. McKone, B. F. Sadtler, C. A. Werlang, N. S. Lewis and H. B. Gray, Acs Catalysis, 2013, 3, 166-169. W. F. Chen, K. Sasaki, C. Ma, A. I. Frenkel, N. Marinkovic, J. T. Muckerman, Y. M. Zhu and R. R. Adzic, Angew. Chem. Int. Ed., 2012, 51, 6131-6135. H. Vrubel and X. L. Hu, Angew. Chem. Int. Ed., 2012, 51, 1270312706. W. F. Chen, C. H. Wang, K. Sasaki, N. Marinkovic, W. Xu, J. T. Muckerman, Y. Zhu and R. R. Adzic, Energy Environ. Sci., 2013, 6, 943-951. P. D. Tran, M. Nguyen, S. S. Pramana, A. Bhattacharjee, S. Y. Chiam, J. Fize, M. J. Field, V. Artero, L. H. Wong, J. Loo and J. Barber, Energy Environ. Sci., 2012, 5, 8912-8916.
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17. B. F. Cao, G. M. Veith, J. C. Neuefeind, R. R. Adzic and P. G. Khalifah, J. Am. Chem. Soc., 2013, 135, 19186-19192. 18. H. T. Wang, D. S. Kong, P. Johanes, J. J. Cha, G. Y. Zheng, K. Yan, N. A. Liu and Y. Cui, Nano Lett., 2013, 13, 3426-3433. 19. Y. Zheng, Y. Jiao, L. H. Li, T. Xing, Y. Chen, M. Jaroniec and S. Z. Qiao, Acs Nano, 2014, 8, 5290-5296. 20. M. Shalom, S. Gimenez, F. Schipper, I. Herraiz-Cardona, J. Bisquert and M. Antonietti, Angew. Chem. Int. Ed., 2014, 53, 3654-3658. 21. X. Wang, J. Wang, D. L. Wang, S. O. Dou, Z. L. Ma, J. H. Wu, L. Tao, A. L. Shen, C. B. Ouyang, Q. H. Liu and S. Y. Wang, Chem. Commun., 2014, 50, 4839-4842. 22. Y. Zheng, Y. Jiao, Y. H. Zhu, L. H. Li, Y. Han, Y. Chen, A. J. Du, M. Jaroniec and S. Z. Qiao, Nat. Commun., 2014, DOI: 10.1038/ncomms4783. 23. E. J. Popczun, C. G. Read, C. W. Roske, N. S. Lewis and R. E. Schaak, Angew. Chem. Int. Ed., 2014, 53, 5427-5430. 24. J. Q. Tian, Q. Liu, A. M. Asiri and X. P. Sun, J. Am. Chem. Soc., 2014, 136, 7587-7590. 25. Q. Liu, J. Tian, W. Cui, P. Jiang, N. Cheng, A. M. Asiri and X. Sun, Angew. Chem. Int. Ed., 2014, 53, 6710-6714. 26. Y. Xu, R. Wu, J. F. Zhang, Y. M. Shi and B. Zhang, Chem. Commun., 2013, 49, 6656-6658. 27. E. J. Popczun, J. R. McKone, C. G. Read, A. J. Biacchi, A. M. Wiltrout, N. S. Lewis and R. E. Schaak, J. Am. Chem. Soc., 2013, 135, 9267-9270. 28. L. G. Feng, H. Vrubel, M. Bensimon and X. L. Hu, Phys. Chem. Chem. Phys., 2014, 16, 5917-5921. 29. P. Xiao, M. A. Sk, L. Thia, X. Ge, R. J. Lim, J.-Y. Wang, K. H. Lim and X. Wang, Energy Environ. Sci., 2014, DOI: 10.1039/c4ee00957f. 30. Z. Xing, Q. Liu, A. M. Asiri and X. Sun, Adv. Mater., 2014, DOI: 10.1002/adma.201401692. 31. A. K. Geim, Science, 2009, 324, 1530-1534. 32. M. J. Allen, V. C. Tung and R. B. Kaner, Chem. Rev., 2010, 110, 132-145. 33. Z. Zhang, F. Xu, W. Yang, M. Guo, X. Wang, B. Zhanga and J. Tang, Chem. Commun., 2011, 47, 6440-6442. 34. S. Guo and S. Sun, J. Am. Chem. Soc., 2012, 134, 2492-2495. 35. X. Chen, G. Wu, J. Chen, X. Chen, Z. Xie and X. Wang, J. Am. Chem. Soc., 2011, 133, 3693-3695. 36. Z. Zhang, H. Chen, C. Xing, M. Guo, F. Xu, X. Wang, H. J. Gruber, B. Zhang and J. Tang, Nano Res., 2011, 4, 599-611. 37. S. Guo, S. Zhang, L. Wu and S. Sun, Angew. Chem. Int. Ed., 2012, 51, 11770-11773. 38. J. Duan, Y. Zheng, S. Chen, Y. Tang, M. Jaroniec and S. Qiao, Chem. Commun., 2013, 49, 7705-7707. 39. H. Wang, J. T. Robinson, G. Diankov and H. Dai, J. Am. Chem. Soc., 2010, 132, 3270-3271. 40. S.-M. Paek, E. Yoo and I. Honma, Nano Lett., 2009, 9, 72-75. 41. H. Wang, Y. Liang, Y. Li and H. Dai, Angew. Chem. Int. Ed., 2011, 50, 10969-10972. 42. J. Yang, D. Voiry, S. J. Ahn, D. Kang, A. Y. Kim, M. Chhowalla and H. S. Shin, Angew. Chem. Int. Ed., 2013, 52, 13751-13754. 43. Y. G. Li, H. L. Wang, L. M. Xie, Y. Y. Liang, G. S. Hong and H. J. Dai, J. Am. Chem. Soc., 2011, 133, 7296-7299. 44. H. P. Cong, J. J. He, Y. Lu and S. H. Yu, Small, 2010, 6, 169-173. 45. C. Qian, F. Kim, L. Ma, F. Tsui, P. D. Yang and J. Liu, J. Am. Chem. Soc., 2004, 126, 1195-1198. 46. J. Bai, X. Li, A. J. Wang, R. Prins and Y. Wang, J. Catal., 2012, 287, 161-169.
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This journal is © The Royal Society of Chemistry [year]
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Z. Zhang, B. Lu, J. Hao, W. Yang and J. Tang, Chem. Commun., 2014, DOI: 10.1039/C4CC05285D.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Cite this: DOI: 10.1039/c0xx00000x
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www.rsc.org/xxxxxx
FeP Nanoparticles Grown on Graphene Sheets as Highly Active NonPrecious-Metal Electrocatalyst for Hydrogen Evolution Reaction
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A synthetic route to FeP-GS hybrid sheets that show good stability and high electrocatalytic activity for hydrogen evolution reaction is reported. The materials are prepared via thermal phosphidation of pre-synthesized Fe3O4-GS hybrid sheets. Hydrogen, as a clean renewable energy source fuel without the emission of carbon dioxide, has been attracting a lot of attention as one of the most promising energy carrier of the future replacing traditional fossil fuels.1 On the other hand, conversion and storage solar energy and wind energy in the form of hydrogen by water splitting reaction is an attractive scheme to solve the intermittent nature of solar energy and wind energy. The water splitting reaction consists of half reactions: the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).2 To conduct electrochemical water splitting, overpotential (η) (required above the standard reaction potential) must be applied. Hence, efficient electrocatalyst are demanded to reduce the overpotential and increase the efficiency of OER and HER. Typically, precious metals such as platinum and its alloys are the most efficient electrocatalyst towards the HER for the production of molecular hydrogen from water. However, the high cost and low abundance prevent the large scale application of these precious metals for hydrogen production.3-4 Therefore, extensive efforts have been devoted to design and develop high-efficient and earth-abundant elements as HER electrocatalyst. To the present, a variety of non-noble (Fe, Co, Ni, and Mo) and metal-free materials (carbon-based) have been selected as effective candidates for catalyzing the electrochemical HER.5-8 Among these non-noble HER electrocatalyst, molybdenum-based compounds have drawn much attention. For example, MoS2,9 defect-rich MoS2,10 amorphous MoSX,11 Ni-Mo,12 NiMoNX,13 MoB,14 Mo2C,15 Cu2MoS4,16 Co0.6Mo1.4N2,17 and MoSe2,18 have been intensively studied. Very recently, metal-free HER electrocatalyst, comprised of carbon framework doped with different hybrid elements, have been reported to show high HER activity.19-22 Zheng et al. demonstrated that graphitic-carbon nitride with nitrogen-doped graphene to produce a metal-free hybrid catalyst, afford current densities of 10 mA/cm2 at η = 240 mV and a Tafel slope of 51.5 mV/dce.22 Furthermore, transitionmetal phosphides (TMPs), by the alloying of metals and phosphorus, are a known class of catalyst for hydrodesulfurization (HDS). Because both HER and HDS depend on the catalyst to reversibly bind hydrogen, it is expected that This journal is © The Royal Society of Chemistry [year]
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TMPs are a new class of HER catalyst. In recent years, several kinds of TMPs have emerged as an attractive HER electrocatalyst such as CoP,23-25 FeP,26 Ni2P,27-28 and MoP.29-30 On the other hand, the electrical conductivity of electrocatalyst is one of the most key factors that influence electrocatalytic efficiency. Taking this into account, carbon materials are ideal supports to improve the electrocatalytic activity because of their high electrical conductivity. Graphene, which is an atomic layers of carbon atoms arranged in a honeycomb network, has received considerable attention due to its unique two-dimensional structure and excellent physical and chemical properties.31-32 Until now, the combination of graphene sheets (GS) and various nanoparticles (NPs) as a new kind of hybrid materials has aroused extensive interest.33-42 For example, Dai et al. developed a selective solvothermal synthesis of MoS2 NPs on reduced graphene oxide sheets, which exhibited superior electrocatalytic activity in the HER relative to other MoS2 catalysts.43
Scheme 1. Illustration of the procedure for preparing FeP-GS.
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In this paper, we report the first synthesis of FeP NPs on GS and demonstrate their high HER electrocatalytic activity with small Tafel slope and low overpotential. The whole preparation strategy for constructing the FeP-GS is shown in Scheme 1 (for detailed experimental steps, please see Experimental Section in the ESI). Firstly, graphene oxide (GO) was prepared by oxidizing natural graphite via the modified Hummers method. Then, the oxidized graphite was exfoliated in N,N-Dimethylformamide
[journal], [year], [vol], 00–00 | 1
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Published on 06 August 2014. Downloaded by Pennsylvania State University on 12/08/2014 04:43:08.
Zhe Zhanga, Baoping Lua,b, Jinhui Haoa,b, Wenshu Yanga,b, and Jilin Tang*a
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(Figure S4). These observations indicate considerable deoxygenation of GO through the successively solvothermal and View Article Online thermal treatment, which is very important for improving FeP-GS DOI: 10.1039/C4CC05285D conductivity and enhancing HER. Figure 1c and 1d show the XPS profile of Fe 2p and P 2p, respectively. The Fe 2p spectrum shows two peaks with binding energy (BE) values at 710.9 and 724.0 eV, assigned to the Fe 2p3/2 and Fe 2p1/2 peaks, respectively. The P 2p spectrum shows two peaks at 129.3 and 130.2 eV corresponding to the BE of P 2p1/2 and P 2p3/2, respectively. Furthermore, the spectrum of P 2p also exhibits a peak with high BE of 133.6 eV, which could be attributed to PO43- or P2O5 caused by oxidation due to air contact.46
Figure 1. (a) XRD patterns of Fe3O4-GS and FeP-GS. XPS spectra in the C 1s (b), Fe 2p (c), and P 2p (d) regions for FeP-GS.
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First of all, X-ray diffraction (XRD) was used to characterize the structural information of Fe3O4-GS. Figure 1a shows the XRD pattern of the as-synthesis Fe3O4-GS (red line) by decomposition of Fe(acac)3 in autoclave. Several broad diffraction peaks in the representative XRD pattern of Fe3O4-GS corresponding to the (220), (311), (400), (422), (511), (440), and (533) reflections were observed. All of the diffraction peaks can be indexed as a pure cubic spine crystal structure of Fe3O4 with cell constant a = 8.39 Å (JCPDS card No. 19-0629).44 The broadening diffraction peaks indicated that the Fe3O4 NPs were small in size, which was also confirmed by transmission electron microscopy (TEM) images (see Figure 2). Furthermore, the formation of the Fe3O4GS hybrid composites can also be verified by XRD pattern, as the peak corresponding to graphite has not been observed. After phosphidation by thermal decomposition NaH2PO2, only the diffraction peaks of the FeP phase corresponding to the (002), (011), (200), (111), (202), (211), (103), (301), and (020) reflections are observed. All peaks in the XRD pattern (Figure 1a, black line) can be indexed to the orthorhombic cell of the FeP with space group Pnma (lattice constant a = 5.191 Å, b = 3.099 Å, and c = 5.792 Å, JCPDS card No. 89-2746).45 These observations suggest that the successful conversion of Fe3O4 into FeP. In addition to, the XRD of Fe3O4 NPs and FeP NPs are shown in Figure S1 in the ESI. Further evidence for the formation of the FeP-GS hybrid composites was obtained from X-ray photoelectron spectroscopy (XPS) analysis. The XPS survey of FeP-GS (Figure S3) revealed the presence of the elements C, O, P, and Fe, which indicated the formation of FeP NPs on GS. The element O comes from residual oxygen-containing functionalities in the GS and surface oxidation of FeP NPs due to air contact. Figure 1b shows the C 1s peaks of FeP-GS, the peak intensities of oxygen-containing functionalities in the FeP-GS are much smaller than that in graphite oxide 2 | Journal Name, [year], [vol], 00–00
Figure 2. TEM image (a) and HRTEM image (b) of Fe3O4-GS. TEM 60
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image (c) and HRTEM image (d) of FeP-GS.
The morphology of the Fe3O4-GS and FeP-GS were further characterized by TEM. TEM image (Figure 2a) shows that the Fe3O4-GS consist of single-layer two-dimensional GS decorated with Fe3O4 NPs and the individual Fe3O4 NPs are well separated from each other with high density and well distributed on the GS. Despite the high density of Fe3O4 NPs on GS, the sheets remained flat, and no crumpled sheets are observed (Figure S5). It can be seen that all the Fe3O4 NPs are coated on the GS and no aggregated or free NPs are detected. As shown from the Fe3O4 NPs in the high-resolution TEM (HRTEM) (Figure 2b), it is observed that small Fe3O4 NPs spread out on the surface of GS have a size of about 3-5 nm, which is in the superparamagnetic size range. The distance between two adjacent crystal planes of the NPs is determined to be 0.25 nm (Figure 2b), corresponding to the lattice spacing of the (311) planes of Fe3O4. After the phosphidation process, FeP NPs homogeneously attach to the surface of GS (Figure 2c) and FeP-GS display crumpled mono- or few-layer sheets (Figure S6). The spacing between two adjacent lattice planes is about 0.19 nm, corresponding to the (211) plane of FeP (Figure 2d). In order to evaluate electrocatalytic activity of FeP-GS, electrochemical measurements of the FeP-GS were conducted in This journal is © The Royal Society of Chemistry [year]
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(DMF) with the treatment of ultrasonication. Secondly, GS decorated with monodisperse Fe3O4 NPs were obtained by reacting between GO and iron (III) acetylacetonate (Fe(acac)3) in a DMF and tetraethylene glycol (TEG) mixed solvent at 180 °C for 2 h. Finally, the obtained intermediate Fe3O4-GS were converted to FeP-GS by phosphidation at 350 oC for 2 h with nitrogen as the carrier gas.
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0.5 M H2SO4 solution using a standard three-electrode system. FeP-GS were deposited onto glassy carbon electrode (GCE, 0.28 mg/cm2) as the working electrode. The HER activity of FeP NPs were also measured for comparison. Figure 3a shows the linear sweep voltammetry (LSV) curves for GCE modified with GS, FeP NPs, FeP-GS, and bare GCE, respectively. Both bare GCE and GS with no convincing HER activity were observed. In sharp contrast to FeP NPs, the FeP-GS have a small onset potential of 30 mV versus HER (black line). As shown in Figure 3b, FeP-GS achieve current densities of 10 mA/cm2 at overpotential 123 mV, whereas FeP NPs have a current density of 1 mA/cm2 under same overpotential. In order to investigate the hydrogen evolution mechanism at the FeP-GS, the Tafel plot was also conducted. Tafel plots are shown in Figure 3c in the low current density region. According to the Tafel equation (η=b log j + a, where j is the current density and b is the Tafel slope), it exhibits Tafel slopes of 67, 50, and 30 mV/dec for FeP NPs, FeP-GS, and Pt wire, respectively. At the same time, FeP-GS have a higher exchange current density (1.2 × 10-1 mA/cm2) compared with FeP NPs (1.7 × 10-2 mA/cm2). The Tafel slope for FeP-GS suggesting that the HER proceeds through a Volmer-Heyrovsky mechanism and electrochemical desorption process is the ratelimiting step. As another important aspect for catalysts, stability of FeP-GS was further probed in 0.5 M H2SO4. After cyclic voltammetry (CV) scanning for 1000 cycles, the LSV curves showed insignificant change compared with the first cycle (Figure 3d), suggesting good stability of FeP-GS in the HER process. The current-time (I-t) curve under η = 240 mV (Figure 3d, inset) suggests FeP-GS retained its electrocatalytic activity for at least 19 hours.
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subsequent thermal phosphidation to form FeP NPs on the surface of GS. It is shown that the obtained FeP-GS hybrid sheets View Article Online have high HER catalytic activity and good stability with a small DOI: 10.1039/C4CC05285D onset overpotential of 30 mV, a Tafel slope of 50 mV/dec, an exchange current density of 1.2 × 10-1 mA/cm2, and a 10 mA/cm2 current density at overpotential 123 mV. We attribute the high HER performance of the FeP-GS the following major factors: i) Intimate interactions and electronic coupling between the GS and FeP NPs should contribute to the high HER activity of the FeP-GS hybrid sheets. ii) GS offer the good conductivity and facilitate rapid electron transfer from the FeP NPs to the electrodes. iii) Better dispersion of FeP NPs on the GS with abundance of exposed catalytic sites play important roles to enhance the HER activity. Acknowledgement This work was supported by the National Basic Research Program of China (973 Program, No. 2011CB935800).
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State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China. E-mail:[email protected]. Tel/Fax: (+86) 431-85262734 b University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/ 1. 2.
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(b) Enlargement of the shadow region in (a). (c) Tafel plots of FeP NPs, 35
FeP-GS, and Pt wire. (d) LSV curves for FeP-GS in 0.5 M H2SO4 with a
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mV/s between +0.003 and -0.303 V (vs. RHE). Inset: A current-time (I-t) curve obtained for a hydrogen evolution reaction with FeP-GS at
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In summary, we present a facile two-step method to prepare FeP-GS hybrid sheets using Fe3O4-GS as an intermediate and This journal is © The Royal Society of Chemistry [year]
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