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Highly efficient photocatalytic hydrogen evolution by nickel phosphide nanoparticles from aqueous solution†
Received 1st July 2014, Accepted 18th July 2014
Shuang Cao,a Yong Chen,*a Chuan-Jun Wang,a Ping Hea and Wen-Fu Fu*ab
DOI: 10.1039/c4cc05026f www.rsc.org/chemcomm
Monodispersed nickel phosphide (Ni2P) nanoparticles were for the first time applied to photocatalytic hydrogen evolution from lactic acid aqueous solution under visible light LED irradiation using CdS nanorods as a photosensitizer. The system exhibited high photocatalytic hydrogen-generating activity and excellent stability in aqueous acidic media.
Hydrogen, which is considered as an ideal candidate for the replacement of fossil fuels, has attracted much attention and will become a significant development strategy in the future.1–3 Since Honda and Fujishima discovered photocatalytic splitting of water on TiO2 electrodes in 1972, various semiconductors have been intensively investigated as photocatalysts for hydrogen production.4,5 CdS, due to its desired valence and conduction band positions, is considered as one of the potential promising candidates for visiblelight driven photocatalytic water splitting.6,7 However, the photocatalytic efficiency of CdS is severely restricted by the fast recombination of photoexcited charge carriers.8,9 In most work reported so far, co-catalysts loaded on the surface of semiconductor particulates could promote the separation of photoexcited electrons and holes.10,11 By introducing noble metal co-catalysts, such as Pt,12,13 Rh,14 and Ru15 into CdS, high photocatalytic hydrogen production efficiency can be reached. Considering the wide range of applications, it is necessary to develop more efficient and robust catalysts, especially those based on non-precious and earth-abundant elements, for photocatalytic systems. In this context, molybdenum- and nickel-based catalysts, such as MoS2,16–18 NiS,19–21 and Ni(OH)2,22 have been reported as efficient co-catalysts for CdS. However, these systems were mostly heterogeneous and the metal catalysts usually suffered from serious agglomeration especially under prolonged light irradiation, which a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials and HKU-CAS Joint Laboratory on New Materials, Technical Institute of Physics and Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: [email protected], [email protected] b College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc05026f
This journal is © The Royal Society of Chemistry 2014
was an obstruction to achieve a high-efficiency solar energy harvesting system. Nickel phosphide comprised of inexpensive and earth-abundant elements was a highly active hydrodesulfurization (HDS) catalyst23–25 and was predicted to be a hydrogen evolution reaction (HER) catalyst by density functional theory (DFT) calculations in 2005.26 According to the DFT calculations, the Ni hollow sites and the Ni–P bridge site have an ensemble effect, which plays an essential role in the high hydrogen-generating activity. The use of Ni2P as an electrocatalyst for the hydrogen evolution reaction has been further investigated since 2013, which showed excellent activity and stability in acidic solution.27,28 However, to the best of our knowledge, there has been no report on photocatalytic hydrogen evolution with nickel phosphide nanoparticles (Ni2P NPs) as a catalyst. Herein, we report for the first time the evaluation of a novel and low-cost photocatalytic system using CdS nanorods (CdS NRs) as a photosensitizer, Ni2P NPs as a catalyst and lactic acid as a sacrificial electron donor in aqueous solution under visible light irradiation. The present system not only exhibited efficient light-driven hydrogen evolution activity but also showed excellent stability in aqueous acidic media. A turnover number (TON) of 26 300 for the first 20 h of irradiation with an initial turnover frequency (TOF) of 2110 h 1 was achieved at a Ni2P NP concentration of 1.0 10 6 M. And the catalyst maintained high photocatalytic activity even after more than 100 h of irradiation under intense LED light. CdS NRs with polyethyleneimine (PEI) as a ligand were prepared according to literature methods.29–31 The as-prepared CdS NRs can be readily dissolved in water, and were characterized by UV-vis (Fig. S1, ESI†) and fluorescence (Fig. S2, ESI†) spectroscopy, transmission electron microscopy (TEM, Fig. S3, ESI†), and X-ray diffraction (XRD, Fig. S4, ESI†). The UV-vis absorption spectrum shows a shoulder absorption peak at B430 nm, which indicates that the obtained CdS NRs have sufficient absorption under visible light irradiation. The fluorescence spectrum of CdS NRs shows a narrow peak at 450 nm, which is attributed to the direct electron–hole recombination. There is another broad and weak emission at B600 nm due to the trap-related electron–hole recombination and the surface state emission. The TEM images show highly
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Fig. 1 (a) TEM and (b) HRTEM images, (c) powder XRD pattern and (d) EDX spectrum of Ni2P NPs.
monodispersed nanorods of 2.5–5 nm diameter and 20–30 nm length, with a lattice spacing of ca. 0.33 nm, which can be ascribed to the (002) plane of wurtzite CdS. In addition, the XRD pattern also demonstrates the peaks at 2y = 26.5, 43.6, 47.7 and 51.81, which are in good agreement with the expected CdS phase (JCPDS Card No. 41-1049). The water-soluble Ni2P NPs capped with poly(vinylpyrrolidone) (PVP) were obtained by a one-pot synthesis route.32 The experimental details were described in the ESI.† The TEM images clearly reveal the formation of spherical nanoparticles with a relatively narrow size distribution (Fig. 1a). The particle sizes ranging from 5 to 8 nm are consistent with the observations from dynamic light scattering (DLS) (Fig. S5, ESI†). Lattice fringes with a spacing of 2.21 Å are evident in the high-resolution TEM image (Fig. 1b) and can be indexed to the high intense (111) reflection of Ni2P.33 The XRD pattern of as-prepared Ni2P NPs (Fig. 1c) shows four sharp diffraction peaks with 2y at 40.7, 44.6, 47.4 and 54.21, which can be indexed to the (111), (201), (210) and (002) reflections, respectively, and match well with the expected phase of Ni2P (JCPDS card No. 03-0953). In addition, energy dispersive X-ray (EDX) data reveal the ratio of Ni and P in the sample to be close to 2 : 1 (Fig. 1d), further confirming the successful formation of the desired phase. The signals of Cu, C and O come from the carbon-coated copper grid substrate. The photogenerating hydrogen activity of the Ni2P NPs was evaluated in the system using CdS NRs as a photosensitizer and lactic acid as a sacrificial agent. Fig. 2a shows the pH effect on the H2 evolution in the system containing CdS NRs, Ni2P NPs and LA. A maximal rate of H2 evolution was achieved at pH 3.0, while less amounts of H2 were obtained at either lower or higher pH values. The likely reason was that the hydrolyzation equilibrium of lactic acid (HA 2 H+ + A , pKa = 3.86) is very sensitive to the pH value. At a lower pH value, the equilibrium of HA to A and H+ is suppressed and lactic acid cannot function as an effective electron donor.34,35 At a higher pH value, protonation on Ni2P NP surfaces may become more
10428 | Chem. Commun., 2014, 50, 10427--10429
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Fig. 2 Visible light-driven hydrogen production in aqueous solution (10 mL) containing 0.5 mL lactic acid (5% v/v). (a) H2 evolution at various pH values, containing 1.7 10 4 M CdS NRs and 5.0 10 5 M Ni2P NPs. (b) H2 evolution as a function of Ni2P NP concentrations containing 1.7 10 4 M CdS NRs at pH 3.0. (c) The TON of the system containing 1.0 10 6 M Ni2P NPs and different concentrations of CdS NRs at pH 3.0. (d) Comparison of photogenerating hydrogen with different components after 10 h of irradiation at pH 3.0. The concentrations of CdS and Ni2P are 1.7 10 4 M and 5.0 10 5 M, respectively. LED light: l Z 420 nm, 30 3 W.
thermodynamically unfavourable according to the reported DFT calculations.26 The concentrations of Ni2P NPs also play a vital role in the efficiency of H2-evolution. At a fixed amount of CdS NRs, increasing the concentration of Ni2P NPs from 1.0 10 6 M to 1.0 10 4 M (the concentrations of CdS NRs and Ni2P NPs were measured by ICP-AES) resulted in a substantial increase of the amount of H2 evolved (Fig. 2b). There is no doubt that the more Ni2P NPs existed the more active catalytic sites can be provided for hydrogen evolution reaction. Fig. 2c shows the H2 evolution rates with different CdS NR concentrations. Progressive addition of CdS NRs to the reaction solution from 3.5 10 5 M to 7.0 10 4 M significantly improved the rate of H2 production. To our surprise, the H2 production reached a TON of 26 300 in 20 h of irradiation based on Ni2P NPs and an initial TOF of 2110 h 1 in the first 2 h for the system containing 7.0 10 4 M CdS NRs and 1.0 10 6 M Ni2P NPs. So it is obvious that Ni2P NPs are a robust catalyst for photocatalytic hydrogen production from aqueous solution under visible light irradiation. Control experiments proved that the components CdS NRs, Ni2P NPs, lactic acid, and light were all essential for efficient H2 evolution. The absence of any of the components or light led to no appreciable amounts of H2 evolution (Fig. 2d). Furthermore, photocurrent tests were performed to study the excitation and transfer of photogenerated charge carriers in photocatalysts under visible light irradiation. Among the three samples of CdS NRs, Ni2P NPs and CdS NRs–Ni2P NPs, the CdS NRs–Ni2P NPs system shows much higher photocurrent response than CdS NRs, whereas there is almost no response for Ni2P NPs. Thus, we proposed that the CdS NRs–Ni2P NPs system may achieve more effective charge separation than CdS
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and 2013CB632403) and the Ministry of Science and Technology (2012DFH40090). We thank the Natural Science Foundation of China (21273257, 21371175 and U1137606), Beijing Natural Science Foundation (2142033), and CAS-Croucher Funding Scheme for Joint Laboratories. Y.C. acknowledges Chinese Academy of Sciences (100 Talents Program) for funding support.
Notes and references
Fig. 3 Time course of photocatalytic H2 production for the system containing 7.0 10 4 M CdS NRs, 1.0 10 6 M Ni2P NPs and 0.5 mL lactic acid (5% v/v) in 10 mL aqueous solution (pH 3.0) with visible light irradiation. Addition of CdS NRs (7.0 10 6 mol) or lactic acid (0.2 mL) was marked at the indicated time (pH was adjusted by 1.0 M NaOH).
NRs, which is advantageous for the high photocatalytic efficiency according to the reported literature (Fig. S6, ESI†).6,8 It has been reported that Ni2P NPs exhibited outstanding stability under acid electrocatalytic conditions and less than 0.25% of Ni2P NPs was removed from the electrode in 1.0 M H2SO4.28 We further examined the durability of Ni2P NPs under the optimal photocatalytic conditions (Fig. 3). The results reveal that the system containing 7.0 10 4 M CdS NRs, 1.0 10 6 M Ni2P NPs and 0.5 mL lactic acid in 10 mL aqueous solution (pH 3.0) displays high hydrogen evolution activity and can sustain for more than 20 h, and then slow down slightly. However, when additional aliquots of CdS NRs or a mixture of CdS NRs and LA was added to the solution at the time of 26 h and 70 h, respectively, the hydrogen production efficiency restored promptly to the initial state. These observations indicate that Ni2P NPs are very robust under the reaction conditions and still retain high catalytic activity after more than 100 h of irradiation. The reduction of hydrogen evolution activity can be attributed to the photocorrosion of the photosensitizer CdS NRs and the consumption of the sacrificial electron donor lactic acid.19 In conclusion, Ni2P NPs were found to be a highly active and stable catalyst for photocatalytic hydrogen evolution from acid aqueous solution. Under optimal conditions, the TON of 26 300 for the first 20 h of irradiation and an initial TOF of 2110 h 1 were achieved based on Ni2P NPs. After more than 100 h of irradiation, the Ni2P NPs still retained their exceptional catalytic activity. The work shows that Ni2P NPs, which were comprised of inexpensive and earth-abundant elements, could serve as an important candidate material for photocatalytic H2-evolution in water. We believe that this work will stimulate further research efforts in the development of inexpensive and earth-abundant first row transition metal phosphide nanoparticles for photogenerating hydrogen reaction. This work was financially supported by the National Key Basic Research Program of China (973 Program 2013CB834804
This journal is © The Royal Society of Chemistry 2014
1 A. J. Esswein and D. G. Nocera, Chem. Rev., 2007, 107, 4022. 2 D. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295. ´nchez-Sa ´nchez, M. C. Alvarez-Galvan, F. Valle 3 R. M. Navarro, M. C. Sa and L. L. G. Fierro, Energy Environ. Sci., 2009, 2, 35. 4 A. Fujishima and M. Honda, Nature, 1972, 238, 37. 5 X. Chen, S. Shen, L. Guo and S. Mao, Chem. Rev., 2010, 110, 6503. 6 X. J. Lv, W. F. Fu, H. X. Chang, H. Zhang, J. S. Cheng, G. J. Zhang, Y. Song, C. Y. Hu and J. H. Li, J. Mater. Chem., 2012, 22, 1539. 7 Z. Fang, Y. Wang, J. Song, Y. Sun, J. Zhou, R. Xu and H. Duan, Nanoscale, 2013, 5, 9830. 8 J. Zhang, Y. Wang, J. Jin, J. Zhang, Z. Lin, F. Huang and J. Yu, ACS Appl. Mater. Interfaces, 2013, 5, 10317. 9 M. Matsumura, S. Furukawa, Y. Saho and H. Tsubomura, J. Phys. Chem., 1985, 89, 1329. 10 X. Zong, J. Han, G. Ma, H. Yan, G. Wu and C. Li, J. Phys. Chem. C, 2011, 115, 12202. 11 P. Tongying, F. Vietmeyer, D. Aleksiuk, G. J. Ferraudi, G. Krylova and M. Kuno, Nanoscale, 2014, 6, 4117. 12 Q. Li, B. Guo, J. Yu, J. Ran, B. Zhang, H. Yan and J. R. Gong, J. Am. Chem. Soc., 2011, 133, 10878. ´rdoba and T. Lian, 13 K. Wu, H. Zhu, Z. Liu, W. Rodrı´guez-Co J. Am. Chem. Soc., 2012, 134, 10337. 14 D. E. Aspnes and A. Heller, J. Phys. Chem., 1983, 87, 4919. 15 F. E. Osterloh, Chem. Mater., 2008, 20, 35. 16 X. Zong, H. Yan, G. Wu, G. Ma, F. Wen, L. Wang and C. Li, J. Am. Chem. Soc., 2008, 130, 7176. 17 T. Jia, A. Kolpin, C. Ma, R. C. T. Chan, W. M. Kwok and S. C. E. Tsang, Chem. Commun., 2014, 50, 1185. 18 G. Chen, D. Li, F. Li, Y. Fan, H. Zhao and Y. Luo, Appl. Catal., A, 2012, 443, 138. 19 W. Zhang, Y. Wang, Z. Wang, Z. Zhong and R. Xu, Chem. Commun., 2010, 46, 7631. 20 J. Meng, F. Li, Y. Hu, L. Xu, Z. Sun and J. Liu, Mater. Res. Bull., 2013, 48, 2111. 21 J. Zhang, S. Z. Qiao, L. Qi and J. Yu, Phys. Chem. Chem. Phys., 2013, 15, 12088. 22 J. Ran, J. Yu and M. Jaroniec, Green Chem., 2011, 13, 2708. 23 H. Song, J. Wang, Z. Wang, H. Song, F. Li and Z. Jin, J. Catal., 2014, 311, 257. 24 S. T. Oyama, T. Gott, H. Zhao and Y. K. Lee, Catal. Today, 2009, 143, 94. 25 S. J. Sawhill, D. C. Phillips and M. E. Bussell, J. Catal., 2003, 215, 208. 26 P. Liu and J. A. Rodriguez, J. Am. Chem. Soc., 2005, 127, 14871. 27 E. J. Popczum, 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. 28 L. Feng, H. Vrubel, M. Bensimon and X. Hu, Phys. Chem. Chem. Phys., 2014, 16, 5917. 29 S. Li, L. Zhang, T. Jiang, L. Chen, Y. Lin, D. Wang and T. Xie, Chem. – Eur. J., 2014, 20, 311. 30 C. B. Li, Z. J. Li, S. Yu, G. X. Wang, F. Wang, Q. Y. Meng, B. Chen, K. Feng, C. H. Tung and L. Z. Wu, Energy Environ. Sci., 2013, 6, 2597. 31 Z. Zhuang, X. Lu, Q. Peng and Y. Li, J. Am. Chem. Soc., 2010, 132, 1819. 32 J. Wang, A. C. J. Peck and J. B. Tracy, Chem. Mater., 2009, 21, 4462. 33 A. W. Burns, M. E. Bussell and S. L. Brock, Adv. Funct. Mater., 2007, 17, 3933. 34 F. Wang, W. G. Wang, X. J. Wang, H. Y. Wang, C. H. Tung and L. Z. Wu, Angew. Chem., Int. Ed., 2011, 50, 3193. 35 Z. J. Li, X. B. Li, J. J. Wang, S. Yu, C. B. Li, C. H. Tung and L. Z. Wu, Energy Environ. Sci., 2013, 6, 465.
Chem. Commun., 2014, 50, 10427--10429 | 10429
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Highly efficient photocatalytic hydrogen evolution by nickel phosphide nanoparticles from aqueous solution†
Received 1st July 2014, Accepted 18th July 2014
Shuang Cao,a Yong Chen,*a Chuan-Jun Wang,a Ping Hea and Wen-Fu Fu*ab
DOI: 10.1039/c4cc05026f www.rsc.org/chemcomm
Monodispersed nickel phosphide (Ni2P) nanoparticles were for the first time applied to photocatalytic hydrogen evolution from lactic acid aqueous solution under visible light LED irradiation using CdS nanorods as a photosensitizer. The system exhibited high photocatalytic hydrogen-generating activity and excellent stability in aqueous acidic media.
Hydrogen, which is considered as an ideal candidate for the replacement of fossil fuels, has attracted much attention and will become a significant development strategy in the future.1–3 Since Honda and Fujishima discovered photocatalytic splitting of water on TiO2 electrodes in 1972, various semiconductors have been intensively investigated as photocatalysts for hydrogen production.4,5 CdS, due to its desired valence and conduction band positions, is considered as one of the potential promising candidates for visiblelight driven photocatalytic water splitting.6,7 However, the photocatalytic efficiency of CdS is severely restricted by the fast recombination of photoexcited charge carriers.8,9 In most work reported so far, co-catalysts loaded on the surface of semiconductor particulates could promote the separation of photoexcited electrons and holes.10,11 By introducing noble metal co-catalysts, such as Pt,12,13 Rh,14 and Ru15 into CdS, high photocatalytic hydrogen production efficiency can be reached. Considering the wide range of applications, it is necessary to develop more efficient and robust catalysts, especially those based on non-precious and earth-abundant elements, for photocatalytic systems. In this context, molybdenum- and nickel-based catalysts, such as MoS2,16–18 NiS,19–21 and Ni(OH)2,22 have been reported as efficient co-catalysts for CdS. However, these systems were mostly heterogeneous and the metal catalysts usually suffered from serious agglomeration especially under prolonged light irradiation, which a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials and HKU-CAS Joint Laboratory on New Materials, Technical Institute of Physics and Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: [email protected], [email protected] b College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc05026f
This journal is © The Royal Society of Chemistry 2014
was an obstruction to achieve a high-efficiency solar energy harvesting system. Nickel phosphide comprised of inexpensive and earth-abundant elements was a highly active hydrodesulfurization (HDS) catalyst23–25 and was predicted to be a hydrogen evolution reaction (HER) catalyst by density functional theory (DFT) calculations in 2005.26 According to the DFT calculations, the Ni hollow sites and the Ni–P bridge site have an ensemble effect, which plays an essential role in the high hydrogen-generating activity. The use of Ni2P as an electrocatalyst for the hydrogen evolution reaction has been further investigated since 2013, which showed excellent activity and stability in acidic solution.27,28 However, to the best of our knowledge, there has been no report on photocatalytic hydrogen evolution with nickel phosphide nanoparticles (Ni2P NPs) as a catalyst. Herein, we report for the first time the evaluation of a novel and low-cost photocatalytic system using CdS nanorods (CdS NRs) as a photosensitizer, Ni2P NPs as a catalyst and lactic acid as a sacrificial electron donor in aqueous solution under visible light irradiation. The present system not only exhibited efficient light-driven hydrogen evolution activity but also showed excellent stability in aqueous acidic media. A turnover number (TON) of 26 300 for the first 20 h of irradiation with an initial turnover frequency (TOF) of 2110 h 1 was achieved at a Ni2P NP concentration of 1.0 10 6 M. And the catalyst maintained high photocatalytic activity even after more than 100 h of irradiation under intense LED light. CdS NRs with polyethyleneimine (PEI) as a ligand were prepared according to literature methods.29–31 The as-prepared CdS NRs can be readily dissolved in water, and were characterized by UV-vis (Fig. S1, ESI†) and fluorescence (Fig. S2, ESI†) spectroscopy, transmission electron microscopy (TEM, Fig. S3, ESI†), and X-ray diffraction (XRD, Fig. S4, ESI†). The UV-vis absorption spectrum shows a shoulder absorption peak at B430 nm, which indicates that the obtained CdS NRs have sufficient absorption under visible light irradiation. The fluorescence spectrum of CdS NRs shows a narrow peak at 450 nm, which is attributed to the direct electron–hole recombination. There is another broad and weak emission at B600 nm due to the trap-related electron–hole recombination and the surface state emission. The TEM images show highly
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Fig. 1 (a) TEM and (b) HRTEM images, (c) powder XRD pattern and (d) EDX spectrum of Ni2P NPs.
monodispersed nanorods of 2.5–5 nm diameter and 20–30 nm length, with a lattice spacing of ca. 0.33 nm, which can be ascribed to the (002) plane of wurtzite CdS. In addition, the XRD pattern also demonstrates the peaks at 2y = 26.5, 43.6, 47.7 and 51.81, which are in good agreement with the expected CdS phase (JCPDS Card No. 41-1049). The water-soluble Ni2P NPs capped with poly(vinylpyrrolidone) (PVP) were obtained by a one-pot synthesis route.32 The experimental details were described in the ESI.† The TEM images clearly reveal the formation of spherical nanoparticles with a relatively narrow size distribution (Fig. 1a). The particle sizes ranging from 5 to 8 nm are consistent with the observations from dynamic light scattering (DLS) (Fig. S5, ESI†). Lattice fringes with a spacing of 2.21 Å are evident in the high-resolution TEM image (Fig. 1b) and can be indexed to the high intense (111) reflection of Ni2P.33 The XRD pattern of as-prepared Ni2P NPs (Fig. 1c) shows four sharp diffraction peaks with 2y at 40.7, 44.6, 47.4 and 54.21, which can be indexed to the (111), (201), (210) and (002) reflections, respectively, and match well with the expected phase of Ni2P (JCPDS card No. 03-0953). In addition, energy dispersive X-ray (EDX) data reveal the ratio of Ni and P in the sample to be close to 2 : 1 (Fig. 1d), further confirming the successful formation of the desired phase. The signals of Cu, C and O come from the carbon-coated copper grid substrate. The photogenerating hydrogen activity of the Ni2P NPs was evaluated in the system using CdS NRs as a photosensitizer and lactic acid as a sacrificial agent. Fig. 2a shows the pH effect on the H2 evolution in the system containing CdS NRs, Ni2P NPs and LA. A maximal rate of H2 evolution was achieved at pH 3.0, while less amounts of H2 were obtained at either lower or higher pH values. The likely reason was that the hydrolyzation equilibrium of lactic acid (HA 2 H+ + A , pKa = 3.86) is very sensitive to the pH value. At a lower pH value, the equilibrium of HA to A and H+ is suppressed and lactic acid cannot function as an effective electron donor.34,35 At a higher pH value, protonation on Ni2P NP surfaces may become more
10428 | Chem. Commun., 2014, 50, 10427--10429
ChemComm
Fig. 2 Visible light-driven hydrogen production in aqueous solution (10 mL) containing 0.5 mL lactic acid (5% v/v). (a) H2 evolution at various pH values, containing 1.7 10 4 M CdS NRs and 5.0 10 5 M Ni2P NPs. (b) H2 evolution as a function of Ni2P NP concentrations containing 1.7 10 4 M CdS NRs at pH 3.0. (c) The TON of the system containing 1.0 10 6 M Ni2P NPs and different concentrations of CdS NRs at pH 3.0. (d) Comparison of photogenerating hydrogen with different components after 10 h of irradiation at pH 3.0. The concentrations of CdS and Ni2P are 1.7 10 4 M and 5.0 10 5 M, respectively. LED light: l Z 420 nm, 30 3 W.
thermodynamically unfavourable according to the reported DFT calculations.26 The concentrations of Ni2P NPs also play a vital role in the efficiency of H2-evolution. At a fixed amount of CdS NRs, increasing the concentration of Ni2P NPs from 1.0 10 6 M to 1.0 10 4 M (the concentrations of CdS NRs and Ni2P NPs were measured by ICP-AES) resulted in a substantial increase of the amount of H2 evolved (Fig. 2b). There is no doubt that the more Ni2P NPs existed the more active catalytic sites can be provided for hydrogen evolution reaction. Fig. 2c shows the H2 evolution rates with different CdS NR concentrations. Progressive addition of CdS NRs to the reaction solution from 3.5 10 5 M to 7.0 10 4 M significantly improved the rate of H2 production. To our surprise, the H2 production reached a TON of 26 300 in 20 h of irradiation based on Ni2P NPs and an initial TOF of 2110 h 1 in the first 2 h for the system containing 7.0 10 4 M CdS NRs and 1.0 10 6 M Ni2P NPs. So it is obvious that Ni2P NPs are a robust catalyst for photocatalytic hydrogen production from aqueous solution under visible light irradiation. Control experiments proved that the components CdS NRs, Ni2P NPs, lactic acid, and light were all essential for efficient H2 evolution. The absence of any of the components or light led to no appreciable amounts of H2 evolution (Fig. 2d). Furthermore, photocurrent tests were performed to study the excitation and transfer of photogenerated charge carriers in photocatalysts under visible light irradiation. Among the three samples of CdS NRs, Ni2P NPs and CdS NRs–Ni2P NPs, the CdS NRs–Ni2P NPs system shows much higher photocurrent response than CdS NRs, whereas there is almost no response for Ni2P NPs. Thus, we proposed that the CdS NRs–Ni2P NPs system may achieve more effective charge separation than CdS
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and 2013CB632403) and the Ministry of Science and Technology (2012DFH40090). We thank the Natural Science Foundation of China (21273257, 21371175 and U1137606), Beijing Natural Science Foundation (2142033), and CAS-Croucher Funding Scheme for Joint Laboratories. Y.C. acknowledges Chinese Academy of Sciences (100 Talents Program) for funding support.
Notes and references
Fig. 3 Time course of photocatalytic H2 production for the system containing 7.0 10 4 M CdS NRs, 1.0 10 6 M Ni2P NPs and 0.5 mL lactic acid (5% v/v) in 10 mL aqueous solution (pH 3.0) with visible light irradiation. Addition of CdS NRs (7.0 10 6 mol) or lactic acid (0.2 mL) was marked at the indicated time (pH was adjusted by 1.0 M NaOH).
NRs, which is advantageous for the high photocatalytic efficiency according to the reported literature (Fig. S6, ESI†).6,8 It has been reported that Ni2P NPs exhibited outstanding stability under acid electrocatalytic conditions and less than 0.25% of Ni2P NPs was removed from the electrode in 1.0 M H2SO4.28 We further examined the durability of Ni2P NPs under the optimal photocatalytic conditions (Fig. 3). The results reveal that the system containing 7.0 10 4 M CdS NRs, 1.0 10 6 M Ni2P NPs and 0.5 mL lactic acid in 10 mL aqueous solution (pH 3.0) displays high hydrogen evolution activity and can sustain for more than 20 h, and then slow down slightly. However, when additional aliquots of CdS NRs or a mixture of CdS NRs and LA was added to the solution at the time of 26 h and 70 h, respectively, the hydrogen production efficiency restored promptly to the initial state. These observations indicate that Ni2P NPs are very robust under the reaction conditions and still retain high catalytic activity after more than 100 h of irradiation. The reduction of hydrogen evolution activity can be attributed to the photocorrosion of the photosensitizer CdS NRs and the consumption of the sacrificial electron donor lactic acid.19 In conclusion, Ni2P NPs were found to be a highly active and stable catalyst for photocatalytic hydrogen evolution from acid aqueous solution. Under optimal conditions, the TON of 26 300 for the first 20 h of irradiation and an initial TOF of 2110 h 1 were achieved based on Ni2P NPs. After more than 100 h of irradiation, the Ni2P NPs still retained their exceptional catalytic activity. The work shows that Ni2P NPs, which were comprised of inexpensive and earth-abundant elements, could serve as an important candidate material for photocatalytic H2-evolution in water. We believe that this work will stimulate further research efforts in the development of inexpensive and earth-abundant first row transition metal phosphide nanoparticles for photogenerating hydrogen reaction. This work was financially supported by the National Key Basic Research Program of China (973 Program 2013CB834804
This journal is © The Royal Society of Chemistry 2014
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