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Electrodeposition of platinum nanoparticles onto porous GaN as a binder-free electrode for hydrogen evolution reaction
Chemical Physics Letters 737 (2019) 136796
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Electrodeposition of platinum nanoparticles onto porous GaN as a binderfree electrode for hydrogen evolution reaction Hui Huanga,b, Chao Wanga,b, Shao-Hui Zhanga,b, Long Zhangb, Ge-Bo Pana,b, a b
T
⁎
School of Nano Technology and Nano Bionics, University of Science and Technology of China, 230026 Hefei, PR China Division of Interdisciplinary Research, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 215123 Suzhou, PR China
H I GH L IG H T S
can be applied to HER directly, which is very significant in cost saving. • Pt/PGaN electrode showed excellent catalytic performances for HER. • Pt/PGaN • Pt/PGaN is of potential application both in electrochemical and photoelectrochemical water splitting.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen evolution reaction Gallium nitride Electrodeposition Binder-free electrode
Electrochemical water splitting has been recognized as a sustainable and clean method to produce hydrogen. Herein, we report the electrodeposition of platinum nanoparticles (PtNPs) onto porous GaN (PGaN) electrode obtained by photoelectrochemical etching planar GaN as a binder-free electrode for hydrogen evolution reaction (HER). The synthesized Pt/PGaN electrode showed excellent catalytic performances for HER with an overpotential of 98 mV at 10 mA/cm2, a Tafel slope of 85 mV/dec and an outstanding stability. Furthermore, this work may provide a concept to prepare a dual function electrode for both electrochemical and photoelectrochemical water splitting.
1. Introduction As the cleanest fuel, hydrogen is considered as one of the most promising energy sources in the 21st century [1–3]. Currently, most of hydrogen is produced from the conversion of fossil fuels while producing large amounts of harmful gas, which is obviously contrary to the original intention of developing clean energy. Electrochemical water splitting has been recognized as a sustainable and clean method to produce hydrogen, which could become an attractive candidate in the future hydrogen economy [4,5]. Generally, Pt-based nanocatalysts are considered to be the best catalysts for hydrogen evolution reaction (HER) [6,7]. However, their scarcity and high cost limit their widespread commercialization [8]. Electrodeposition is a bottom-up method for nanomaterial preparation. The electrolyte containing Pt can be recycled many times and Pt nanocatalyst can be deposited onto electrode surface directly. Therefore, electrodeposition is suitable for fabricating noble metal nanocatalyst such as Pt nanoparticles (PtNPs) onto the surface of electrode [9,10]. Pt-decorated binder-free electrode can be applied to HER directly,
⁎
eliminating the process of conventional catalyst coating and binder adding, which is very significant in cost saving [11]. Considering that acid or alkaline solution is indispensable for HER, the electrode material must be excellent in chemical stability, thermal stability and electrical conductivity [12]. Gallium nitride (GaN), with good electrical and optical properties, is a direct bandgap (3.4 eV) semiconductor material [13]. Due to its outstanding chemical stability, thermal stability and electrical conductivity, and its strong interaction with metals, GaN is an ideal electrode material [14]. Compared to other Pt-decorated binder-free electrodes such as stainless steel mesh [2], carbon cloth [15] and nickel foam [16], Pt-decorated GaN can offer abundant Mott-Schottky heterojunctions [17]. It makes PtNPs more electron rich and reductive, which is conducive to improving the catalytic performance [18,19]. Inaddition, GaN has also received great attention in the field of photoelectrochemical water splitting due to its wide bandgap [20–22]. Therefor, GaN electrode is expected to prepare a dual function electrode for both electrochemical and photoelectrochemical water splitting.
Corresponding author at: School of Nano Technology and Nano Bionics, University of Science and Technology of China, 230026 Hefei, PR China. E-mail address: [email protected] (G.-B. Pan).
https://doi.org/10.1016/j.cplett.2019.136796 Received 21 July 2019; Received in revised form 20 September 2019; Accepted 25 September 2019 Available online 26 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 737 (2019) 136796
H. Huang, et al.
Fig. 1. Schematic illustration showing the fabrication of Pt/PGaN binder-free electrode.
impedance spectroscopy (EIS) was measured at a bias of −0.1 V (vs. RHE) with a test frequency range of 0.1 Hz to 105 Hz. Stability was evaluated by comparing polarization curves (LSV) before and after 1000 cycles of CV. The voltage range of CV was from 0.2 to −0.6 V (vs. RHE) and the scan speed was 50 mV/s.
Recently, our group discovered that highly porous GaN (PGaN) could be easily obtained by photoelectrochemical etching [23]. Herein, a method for two-step fabrication of Pt/PGaN as a binder-free electrode for HER is reported. Pt/PGaN shows excellent catalytic performances for HER and can be applied to HER directly, eliminating the process of conventional catalyst coating. In addition, Pt/PGaN may be designed as a dual function electrode for both electrochemical and photoelectrochemical water splitting.
3. Results and discussion Fig. 2a shows a typical SEM image of planar GaN surface. The surface of planar GaN is smooth and flat, and no streaks and wrinkles could be observed. Fig. 2b shows the porous structure is obtained after photoelectrochemical etching. In addition, the pore shape is hexagon, which is consistent with the wurtzite structure (hexagonal crystal system) of GaN (0001) [24]. Fig. 2c shows the pore diameter of PGaN is within the range of 40–80 nm. The average diameter of the holes is 56.5 nm. Fig. 2d shows a typical CV of PGaN in the mixed solution containing 0.5 M NaCl and 1 mM H2PtCl6. Peak A at −0.25 V (vs. Ag/ AgCl) can be attributed to the reduction of Pt (IV) to Pt (II). Peak B at −0.4 V (vs. Ag/AgCl) can be attributed to the generation of Pt nanoparticles. Peak C at −0.9 V (vs. Ag/AgCl) can be attributed to the oxidation of hydrogen. Peak D at −0.3 V (vs. Ag/AgCl) and peak E at −0.2 V (vs. Ag/AgCl) can be attributed to the oxidation of Pt(IV) and Pt (II), respectively [25]. Fig. 3a shows PtNPs are simply deposited on planar GaN electrode surface (Pt/GaN). The diameter of PtNPs is within the range of 10 to 50 nm. Fig. 3b shows PtNPs are deposited on PGaN (Pt/PGaN). The diameter of PtNPs on PGaN is between 5 and 30 nm, which is smaller than that on planar GaN. The smaller PtNPs are formed ascribed to the following reasons. Firstly, a strong peak of hydrogen evolution (Fig. 2d) arises when the potential is lower than −0.9 V (vs. Ag/AgCl). Hydrogen bubbles at the interface of PGaN surface and electrolyte can effectively inhibit the growth of crystal nuclei of PtNPs [26]. Secondly, PtNPs are likely to deposit on the inner wall of the pores, exhibit a three-dimensional structure. Lastly, similar to the steps of graphite surface [27], hexagonal porous structure has many edges, which provides abundant defective sites. The defective sites are conductive to the electrodeposition of PtNPs. By comparison, it can be concluded that PGaN is advantageous for electrodeposition of PtNPs. In order to research the influence of different cycles of CV on its morphology, PtNPs were prepared onto PGaN by CV with the cycles of 1, 3, 5, and 7, named Pt-1, Pt-3, Pt-5 and Pt-7, respectively. As the number of cycles increases, PtNPs begin to agglomerate and cover the porous structure of PGaN gradually (Fig. S1). In Pt-7, PtNPs almost fill the entire holes and agglomerate into a film. The EDS spectrum of Pt/PGaN is shown in Fig. 3c. The peaks at 1.1 keV and 9.2 keV can be attributed to Ga, while the peak at 2.1 keV can be attributed to Pt. In order to investigate the crystal structure of Pt nanoparticles, X-ray diffraction (XRD) tests were performed. As shown in Fig. 3d, a strong diffraction peak at the 2θ angle of 35.2° belongs to
2. Experimental 2.1. Fabrication of PGaN electrode Single-crystal n-type GaN (0 0 0 1) films were grown on sapphires by hydride vapor phase epitaxy. The Si-doped GaN layer is 5 μm thick with the carrier concentration of 4.8 × 1018 cm−3. The size of GaN chips is 1.3 cm × 0.3 cm. As shown in Fig. 1, photoelectrochemical etching was performed using the front-side illumination of 300 W Xenon lamp under a standard two-electrode cell system. GaN chips with front-side ohmic contact made by indium point and platinum wire were used as the anode and cathode, respectively. Ionic liquid 1-ethyl-3-methylimidazolium triflate was used as etchant. Etching voltage was 5 V and etching time was 20 min. 2.2. Fabrication of Pt/PGaN binder-free electrode Electrodeposition experiments were performed at room temperature using an electrochemical workstation by a standard three-electrode cell. PGaN, Pt wire, and Ag/AgCl electrodes (saturated with 3 M KCl) were used as working, counter and reference electrodes, respectively. The electrolyte was a mixed solution containing 1 mM H2PtCl6 and 0.5 M NaCl. Pt nanoparticles were deposited onto porous GaN by 3 cycles of cyclic voltammetry (CV). The voltage range was from open circuit voltage to −1.0 V (vs. Ag/AgCl) with a scan rate of 50 mV/s. After electrodeposition, the sample was rinsed with deionized water thoroughly and blown dry with nitrogen. 2.3. Electrochemical measurements All electrochemical tests were performed on an electrochemical workstation (AUT87464) at room temperature. The Pt/PGaN, a saturated Ag/AgCl and a graphite rod were used as working electrode, reference electrode and counter electrode, respectively. The electrolyte was 0.5 M H2SO4 solution. Before the test, high-purity nitrogen gas was continuously introduced into electrolytic bath for 30 min until electrolyte was saturated. The catalytic performances for HER of the samples were measured by linear sweep voltammetry (LSV) with a scan speed of 5 mV/s. Tafel slope is obtained by re-mapping on the linear sweep voltammetry curve by Tafel equation. The electrochemical 2
Chemical Physics Letters 737 (2019) 136796
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Fig. 2. (a) SEM image of planar GaN. (b) SEM image of PGaN. (c) Pore size distribution histogram of PGaN. (d) Cyclic voltammogram of PGaN in 0.5 M NaCl and 1 mM H2PtCl6.
the GaN (0 0 2) crystal plane. The diffraction peaks at the 2θ angles of 40.62 and 47.06° can be attributed to Pt (1 1 1) and Pt (2 0 0) crystal planes, respectively. These two diffraction peaks correspond well to Pt (JCPDS card No. 87-0640). As depicted in Fig. 4a, the pure GaN electrode is almost a straight line, which indicates the poor catalytic performance for HER. The overpotentials of Pt/GaN, Pt/PGaN and Pt/C at 10 mA/cm2 are 130, 98
and 38 mV, respectively. Fig. 4b shows the Tafel slopes of Pt/GaN, Pt/ PGaN and Pt/C are 105, 85 and 32 mV/dec, respectively. Obviously, the electrocatalytic performance of Pt/PGaN is better than that of Pt/GaN, which can be ascribed to the following reasons. On one hand, the porous structure increases the contact area between electrode and electrolyte, which facilitates the charge transfer between them. On the other hand, PGaN facilitates the growth of PtNPs. As is demonstrated in
Fig. 3. (a) SEM image of Pt/GaN. (b) SEM image of Pt/PGaN. (c) EDS spectrum of Pt/PGaN. (d) XRD pattern of Pt/PGaN. 3
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Fig. 4. (a) Polarization curves for pure GaN, Pt/GaN, Pt/PGaN and Pt/C in 0.5 M H2SO4. (b) Tafel plots obtained from the polarization curves. (c) Linear sweep voltammetry (LSV) of Pt/PGaN before and after 1000 CV cycles. (d) Nyquist plots of Pt/PGaN.
interests or personal relationships that could have appeared to influence the work reported in this paper.
the foregoing, PtNPs deposited on PGaN are smaller in diameter indicating higher catalytic performances for HER [28]. The overpotentials of Pt-1, Pt-3, Pt-5 and Pt-7 at 10 mA/cm2 are 110, 98, 100 and 90 mV, respectively (Fig. S2). As the number of CV increases, catalytic performances for HER increases slowly. Pt-3, Pt-5 and Pt-7 have little difference in catalytic performances. In order to improve the utilization of Pt catalysts and save costs, Pt-3 is the best choice in the current situation. After 1000 CV cycles, the polarization curve of Pt/PGaN is nearly the same as the initial one, indicating the excellent stability (Fig. 4c). This interesting stability can be related to the bind-free electrode via strong bondformed between PtNPs and PGaN [29]. Fig. 4d shows the electrochemical impedance spectroscopy (EIS) of Pt/GaN and Pt/PGaN. The charge-transfer resistances (Rct) of Pt/GaN and Pt/PGaN are 46.8 and 14.2 Ω, respectively. Obviously, Pt/PGaN possesses faster interfacial electron transfer kinetics than Pt/GaN during HER. It is worth mentioning that the external circuit resistance of Pt/PGaN is slightly higher than that of Pt/GaN. This may be because photoelectrochemical etching causes some damage to the doped layer on the surface and reduces the conductivity of GaN.
Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 61773372), the Key Research Program of Jiangsu Province (BE2015073), and the Chinese Academy of Science. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.136796. References [1] G. Zhao, K. Rui, S.X. Dou, W. Sun, Adv. Funct. Mater. (2018) 1803291. [2] D. Li, Y. Li, B. Zhang, Y.H. Lui, S. Mooni, R. Chen, S. Hu, H. Ni, Materials (Basel) 11 (2018). [3] J. Wang, F. Xu, H. Jin, Y. Chen, Y. Wang, Adv. Mater. 29 (2017). [4] H. Wang, X.B. Li, L. Gao, H.L. Wu, J. Yang, L. Cai, T.B. Ma, C.H. Tung, L.Z. Wu, G. Yu, Angew. Chem. Int. Ed. Engl. 57 (2018) 192. [5] X. Huang, M. Leng, W. Xiao, M. Li, J. Ding, T.L. Tan, W.S.V. Lee, J. Xue, Adv. Funct. Mater. 27 (2017) 1604943. [6] X. Zhong, Y. Qin, X. Chen, W. Xu, G. Zhuang, X. Li, J. Wang, Carbon 114 (2017) 740. [7] A. Oh, Y.J. Sa, H. Hwang, H. Baik, J. Kim, B. Kim, S.H. Joo, K. Lee, Nanoscale 8 (2016) 16379. [8] N. Cheng, S. Stambula, D. Wang, M.N. Banis, J. Liu, A. Riese, B. Xiao, R. Li, T.K. Sham, L.M. Liu, G.A. Botton, X. Sun, Nat. Commun. 7 (2016) 13638. [9] V. Shokhen, D. Zitoun, Electrochim. Acta 257 (2017) 49. [10] X. Chia, N.A.A. Sutrisnoh, M. Pumera, ACS Appl. Mater. Interfaces 10 (2018) 8702. [11] Y. Liang, Q. Liu, A.M. Asiri, X. Sun, Y. Luo, ACS Catal. 4 (2014) 4065. [12] Q.M. Jiang, M.R. Zhang, L.Q. Luo, G.B. Pan, Talanta 171 (2017) 250. [13] H.-I. Chen, K.-C. Chuang, C.-H. Chang, W.-C. Chen, I.P. Liu, W.-C. Liu, Sens. Actuators, B 246 (2017) 408. [14] Y. Zhao, S.-J. Qin, Y. Li, F.-X. Deng, Y.-Q. Liu, G.-B. Pan, Electrochim. Acta 145 (2014) 148. [15] X. Han, X. Wu, Y. Deng, J. Liu, J. Lu, C. Zhong, W. Hu, Adv. Energy Mater. 8 (2018) 1800935. [16] S. Huang, X. Zhu, B. Cheng, J. Yu, C. Jiang, Environ. Sci. Nano 4 (2017) 2215. [17] Z.-H. Xue, H. Su, Q.-Y. Yu, B. Zhang, H.-H. Wang, X.-H. Li, J.-S. Chen, Adv. Energy Mater. 7 (2017) 1602355.
4. Conclusions In summary, we have demonstrated an approach to prepare Pt/ PGaN by electrodeposition. The synthesized Pt/PGaN electrode showed excellent catalytic performances for HER with an overpotential of 98 mV at 10 mA/cm2, a Tafel slope of 85 mV/dec and an outstanding stability. The influences of different cycles of CV on its morphology and HER performance have been studied. Pt/PGaN can be applied to HER directly, eliminating the process of conventional catalyst coating, which is very significant in cost saving. In addition, Pt/PGaN may be designed as a dual function electrode for both electrochemical and photoelectrochemical water splitting. Declaration of Competing Interest The authors declare that they have no known competing financial 4
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Appl. Phys. 80 (1996) 3228. [25] A.M. Feltham, M. Spiro, Chem. Rev. 71 (1971) 177. [26] M.-R. Zhang, X.-Q. Chen, G.-B. Pan, Sens. Actuators, B 240 (2017) 142. [27] E.C. Walter, B.J. Murray, F. Favier, G. Kaltenpoth, M. Grunze, R.M. Penner, J. Phys. Chem. B 106 (2002) 11407. [28] E. Kemppainen, A. Bodin, B. Sebok, T. Pedersen, B. Seger, B. Mei, D. Bae, P.C.K. Vesborg, J. Halme, O. Hansen, P.D. Lund, I. Chorkendorff, Energy Environ. Sci. 8 (2015) 2991. [29] R. Karimi Shervedani, M. Torabi, F. Yaghoobi, Electrochim. Acta 244 (2017) 230.
[18] X.H. Li, M. Antonietti, Chem. Soc. Rev. 42 (2013) 6593. [19] J. Hou, Y. Sun, Y. Wu, S. Cao, L. Sun, Adv. Funct. Mater. 28 (2018) 1704447. [20] Y.-H. Yeh, T.-Y. Yu, M.-C. Liu, Y.-J. Cheng, Int. J. Hydrogen Energy 42 (2017) 27066. [21] M.A. Hassan, J.-H. Kang, M.A. Johar, J.-S. Ha, S.-W. Ryu, Acta Mater. 146 (2018) 171. [22] Z.A. Syed, Y. Hou, X. Yu, S. Shen, M. Athanasiou, J. Bai, T. Wang, ACS Photon. 6 (2019) 1302. [23] M.-R. Zhang, S.-J. Qin, H.-D. Peng, G.-B. Pan, Mater. Lett. 182 (2016) 363. [24] X.H. Wu, L.M. Brown, D. Kapolnek, S. Keller, B. Keller, S.P. DenBaars, J.S. Speck, J.
5
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Electrodeposition of platinum nanoparticles onto porous GaN as a binderfree electrode for hydrogen evolution reaction Hui Huanga,b, Chao Wanga,b, Shao-Hui Zhanga,b, Long Zhangb, Ge-Bo Pana,b, a b
T
⁎
School of Nano Technology and Nano Bionics, University of Science and Technology of China, 230026 Hefei, PR China Division of Interdisciplinary Research, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 215123 Suzhou, PR China
H I GH L IG H T S
can be applied to HER directly, which is very significant in cost saving. • Pt/PGaN electrode showed excellent catalytic performances for HER. • Pt/PGaN • Pt/PGaN is of potential application both in electrochemical and photoelectrochemical water splitting.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen evolution reaction Gallium nitride Electrodeposition Binder-free electrode
Electrochemical water splitting has been recognized as a sustainable and clean method to produce hydrogen. Herein, we report the electrodeposition of platinum nanoparticles (PtNPs) onto porous GaN (PGaN) electrode obtained by photoelectrochemical etching planar GaN as a binder-free electrode for hydrogen evolution reaction (HER). The synthesized Pt/PGaN electrode showed excellent catalytic performances for HER with an overpotential of 98 mV at 10 mA/cm2, a Tafel slope of 85 mV/dec and an outstanding stability. Furthermore, this work may provide a concept to prepare a dual function electrode for both electrochemical and photoelectrochemical water splitting.
1. Introduction As the cleanest fuel, hydrogen is considered as one of the most promising energy sources in the 21st century [1–3]. Currently, most of hydrogen is produced from the conversion of fossil fuels while producing large amounts of harmful gas, which is obviously contrary to the original intention of developing clean energy. Electrochemical water splitting has been recognized as a sustainable and clean method to produce hydrogen, which could become an attractive candidate in the future hydrogen economy [4,5]. Generally, Pt-based nanocatalysts are considered to be the best catalysts for hydrogen evolution reaction (HER) [6,7]. However, their scarcity and high cost limit their widespread commercialization [8]. Electrodeposition is a bottom-up method for nanomaterial preparation. The electrolyte containing Pt can be recycled many times and Pt nanocatalyst can be deposited onto electrode surface directly. Therefore, electrodeposition is suitable for fabricating noble metal nanocatalyst such as Pt nanoparticles (PtNPs) onto the surface of electrode [9,10]. Pt-decorated binder-free electrode can be applied to HER directly,
⁎
eliminating the process of conventional catalyst coating and binder adding, which is very significant in cost saving [11]. Considering that acid or alkaline solution is indispensable for HER, the electrode material must be excellent in chemical stability, thermal stability and electrical conductivity [12]. Gallium nitride (GaN), with good electrical and optical properties, is a direct bandgap (3.4 eV) semiconductor material [13]. Due to its outstanding chemical stability, thermal stability and electrical conductivity, and its strong interaction with metals, GaN is an ideal electrode material [14]. Compared to other Pt-decorated binder-free electrodes such as stainless steel mesh [2], carbon cloth [15] and nickel foam [16], Pt-decorated GaN can offer abundant Mott-Schottky heterojunctions [17]. It makes PtNPs more electron rich and reductive, which is conducive to improving the catalytic performance [18,19]. Inaddition, GaN has also received great attention in the field of photoelectrochemical water splitting due to its wide bandgap [20–22]. Therefor, GaN electrode is expected to prepare a dual function electrode for both electrochemical and photoelectrochemical water splitting.
Corresponding author at: School of Nano Technology and Nano Bionics, University of Science and Technology of China, 230026 Hefei, PR China. E-mail address: [email protected] (G.-B. Pan).
https://doi.org/10.1016/j.cplett.2019.136796 Received 21 July 2019; Received in revised form 20 September 2019; Accepted 25 September 2019 Available online 26 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 737 (2019) 136796
H. Huang, et al.
Fig. 1. Schematic illustration showing the fabrication of Pt/PGaN binder-free electrode.
impedance spectroscopy (EIS) was measured at a bias of −0.1 V (vs. RHE) with a test frequency range of 0.1 Hz to 105 Hz. Stability was evaluated by comparing polarization curves (LSV) before and after 1000 cycles of CV. The voltage range of CV was from 0.2 to −0.6 V (vs. RHE) and the scan speed was 50 mV/s.
Recently, our group discovered that highly porous GaN (PGaN) could be easily obtained by photoelectrochemical etching [23]. Herein, a method for two-step fabrication of Pt/PGaN as a binder-free electrode for HER is reported. Pt/PGaN shows excellent catalytic performances for HER and can be applied to HER directly, eliminating the process of conventional catalyst coating. In addition, Pt/PGaN may be designed as a dual function electrode for both electrochemical and photoelectrochemical water splitting.
3. Results and discussion Fig. 2a shows a typical SEM image of planar GaN surface. The surface of planar GaN is smooth and flat, and no streaks and wrinkles could be observed. Fig. 2b shows the porous structure is obtained after photoelectrochemical etching. In addition, the pore shape is hexagon, which is consistent with the wurtzite structure (hexagonal crystal system) of GaN (0001) [24]. Fig. 2c shows the pore diameter of PGaN is within the range of 40–80 nm. The average diameter of the holes is 56.5 nm. Fig. 2d shows a typical CV of PGaN in the mixed solution containing 0.5 M NaCl and 1 mM H2PtCl6. Peak A at −0.25 V (vs. Ag/ AgCl) can be attributed to the reduction of Pt (IV) to Pt (II). Peak B at −0.4 V (vs. Ag/AgCl) can be attributed to the generation of Pt nanoparticles. Peak C at −0.9 V (vs. Ag/AgCl) can be attributed to the oxidation of hydrogen. Peak D at −0.3 V (vs. Ag/AgCl) and peak E at −0.2 V (vs. Ag/AgCl) can be attributed to the oxidation of Pt(IV) and Pt (II), respectively [25]. Fig. 3a shows PtNPs are simply deposited on planar GaN electrode surface (Pt/GaN). The diameter of PtNPs is within the range of 10 to 50 nm. Fig. 3b shows PtNPs are deposited on PGaN (Pt/PGaN). The diameter of PtNPs on PGaN is between 5 and 30 nm, which is smaller than that on planar GaN. The smaller PtNPs are formed ascribed to the following reasons. Firstly, a strong peak of hydrogen evolution (Fig. 2d) arises when the potential is lower than −0.9 V (vs. Ag/AgCl). Hydrogen bubbles at the interface of PGaN surface and electrolyte can effectively inhibit the growth of crystal nuclei of PtNPs [26]. Secondly, PtNPs are likely to deposit on the inner wall of the pores, exhibit a three-dimensional structure. Lastly, similar to the steps of graphite surface [27], hexagonal porous structure has many edges, which provides abundant defective sites. The defective sites are conductive to the electrodeposition of PtNPs. By comparison, it can be concluded that PGaN is advantageous for electrodeposition of PtNPs. In order to research the influence of different cycles of CV on its morphology, PtNPs were prepared onto PGaN by CV with the cycles of 1, 3, 5, and 7, named Pt-1, Pt-3, Pt-5 and Pt-7, respectively. As the number of cycles increases, PtNPs begin to agglomerate and cover the porous structure of PGaN gradually (Fig. S1). In Pt-7, PtNPs almost fill the entire holes and agglomerate into a film. The EDS spectrum of Pt/PGaN is shown in Fig. 3c. The peaks at 1.1 keV and 9.2 keV can be attributed to Ga, while the peak at 2.1 keV can be attributed to Pt. In order to investigate the crystal structure of Pt nanoparticles, X-ray diffraction (XRD) tests were performed. As shown in Fig. 3d, a strong diffraction peak at the 2θ angle of 35.2° belongs to
2. Experimental 2.1. Fabrication of PGaN electrode Single-crystal n-type GaN (0 0 0 1) films were grown on sapphires by hydride vapor phase epitaxy. The Si-doped GaN layer is 5 μm thick with the carrier concentration of 4.8 × 1018 cm−3. The size of GaN chips is 1.3 cm × 0.3 cm. As shown in Fig. 1, photoelectrochemical etching was performed using the front-side illumination of 300 W Xenon lamp under a standard two-electrode cell system. GaN chips with front-side ohmic contact made by indium point and platinum wire were used as the anode and cathode, respectively. Ionic liquid 1-ethyl-3-methylimidazolium triflate was used as etchant. Etching voltage was 5 V and etching time was 20 min. 2.2. Fabrication of Pt/PGaN binder-free electrode Electrodeposition experiments were performed at room temperature using an electrochemical workstation by a standard three-electrode cell. PGaN, Pt wire, and Ag/AgCl electrodes (saturated with 3 M KCl) were used as working, counter and reference electrodes, respectively. The electrolyte was a mixed solution containing 1 mM H2PtCl6 and 0.5 M NaCl. Pt nanoparticles were deposited onto porous GaN by 3 cycles of cyclic voltammetry (CV). The voltage range was from open circuit voltage to −1.0 V (vs. Ag/AgCl) with a scan rate of 50 mV/s. After electrodeposition, the sample was rinsed with deionized water thoroughly and blown dry with nitrogen. 2.3. Electrochemical measurements All electrochemical tests were performed on an electrochemical workstation (AUT87464) at room temperature. The Pt/PGaN, a saturated Ag/AgCl and a graphite rod were used as working electrode, reference electrode and counter electrode, respectively. The electrolyte was 0.5 M H2SO4 solution. Before the test, high-purity nitrogen gas was continuously introduced into electrolytic bath for 30 min until electrolyte was saturated. The catalytic performances for HER of the samples were measured by linear sweep voltammetry (LSV) with a scan speed of 5 mV/s. Tafel slope is obtained by re-mapping on the linear sweep voltammetry curve by Tafel equation. The electrochemical 2
Chemical Physics Letters 737 (2019) 136796
H. Huang, et al.
Fig. 2. (a) SEM image of planar GaN. (b) SEM image of PGaN. (c) Pore size distribution histogram of PGaN. (d) Cyclic voltammogram of PGaN in 0.5 M NaCl and 1 mM H2PtCl6.
the GaN (0 0 2) crystal plane. The diffraction peaks at the 2θ angles of 40.62 and 47.06° can be attributed to Pt (1 1 1) and Pt (2 0 0) crystal planes, respectively. These two diffraction peaks correspond well to Pt (JCPDS card No. 87-0640). As depicted in Fig. 4a, the pure GaN electrode is almost a straight line, which indicates the poor catalytic performance for HER. The overpotentials of Pt/GaN, Pt/PGaN and Pt/C at 10 mA/cm2 are 130, 98
and 38 mV, respectively. Fig. 4b shows the Tafel slopes of Pt/GaN, Pt/ PGaN and Pt/C are 105, 85 and 32 mV/dec, respectively. Obviously, the electrocatalytic performance of Pt/PGaN is better than that of Pt/GaN, which can be ascribed to the following reasons. On one hand, the porous structure increases the contact area between electrode and electrolyte, which facilitates the charge transfer between them. On the other hand, PGaN facilitates the growth of PtNPs. As is demonstrated in
Fig. 3. (a) SEM image of Pt/GaN. (b) SEM image of Pt/PGaN. (c) EDS spectrum of Pt/PGaN. (d) XRD pattern of Pt/PGaN. 3
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Fig. 4. (a) Polarization curves for pure GaN, Pt/GaN, Pt/PGaN and Pt/C in 0.5 M H2SO4. (b) Tafel plots obtained from the polarization curves. (c) Linear sweep voltammetry (LSV) of Pt/PGaN before and after 1000 CV cycles. (d) Nyquist plots of Pt/PGaN.
interests or personal relationships that could have appeared to influence the work reported in this paper.
the foregoing, PtNPs deposited on PGaN are smaller in diameter indicating higher catalytic performances for HER [28]. The overpotentials of Pt-1, Pt-3, Pt-5 and Pt-7 at 10 mA/cm2 are 110, 98, 100 and 90 mV, respectively (Fig. S2). As the number of CV increases, catalytic performances for HER increases slowly. Pt-3, Pt-5 and Pt-7 have little difference in catalytic performances. In order to improve the utilization of Pt catalysts and save costs, Pt-3 is the best choice in the current situation. After 1000 CV cycles, the polarization curve of Pt/PGaN is nearly the same as the initial one, indicating the excellent stability (Fig. 4c). This interesting stability can be related to the bind-free electrode via strong bondformed between PtNPs and PGaN [29]. Fig. 4d shows the electrochemical impedance spectroscopy (EIS) of Pt/GaN and Pt/PGaN. The charge-transfer resistances (Rct) of Pt/GaN and Pt/PGaN are 46.8 and 14.2 Ω, respectively. Obviously, Pt/PGaN possesses faster interfacial electron transfer kinetics than Pt/GaN during HER. It is worth mentioning that the external circuit resistance of Pt/PGaN is slightly higher than that of Pt/GaN. This may be because photoelectrochemical etching causes some damage to the doped layer on the surface and reduces the conductivity of GaN.
Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 61773372), the Key Research Program of Jiangsu Province (BE2015073), and the Chinese Academy of Science. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cplett.2019.136796. References [1] G. Zhao, K. Rui, S.X. Dou, W. Sun, Adv. Funct. Mater. (2018) 1803291. [2] D. Li, Y. Li, B. Zhang, Y.H. Lui, S. Mooni, R. Chen, S. Hu, H. Ni, Materials (Basel) 11 (2018). [3] J. Wang, F. Xu, H. Jin, Y. Chen, Y. Wang, Adv. Mater. 29 (2017). [4] H. Wang, X.B. Li, L. Gao, H.L. Wu, J. Yang, L. Cai, T.B. Ma, C.H. Tung, L.Z. Wu, G. Yu, Angew. Chem. Int. Ed. Engl. 57 (2018) 192. [5] X. Huang, M. Leng, W. Xiao, M. Li, J. Ding, T.L. Tan, W.S.V. Lee, J. Xue, Adv. Funct. Mater. 27 (2017) 1604943. [6] X. Zhong, Y. Qin, X. Chen, W. Xu, G. Zhuang, X. Li, J. Wang, Carbon 114 (2017) 740. [7] A. Oh, Y.J. Sa, H. Hwang, H. Baik, J. Kim, B. Kim, S.H. Joo, K. Lee, Nanoscale 8 (2016) 16379. [8] N. Cheng, S. Stambula, D. Wang, M.N. Banis, J. Liu, A. Riese, B. Xiao, R. Li, T.K. Sham, L.M. Liu, G.A. Botton, X. Sun, Nat. Commun. 7 (2016) 13638. [9] V. Shokhen, D. Zitoun, Electrochim. Acta 257 (2017) 49. [10] X. Chia, N.A.A. Sutrisnoh, M. Pumera, ACS Appl. Mater. Interfaces 10 (2018) 8702. [11] Y. Liang, Q. Liu, A.M. Asiri, X. Sun, Y. Luo, ACS Catal. 4 (2014) 4065. [12] Q.M. Jiang, M.R. Zhang, L.Q. Luo, G.B. Pan, Talanta 171 (2017) 250. [13] H.-I. Chen, K.-C. Chuang, C.-H. Chang, W.-C. Chen, I.P. Liu, W.-C. Liu, Sens. Actuators, B 246 (2017) 408. [14] Y. Zhao, S.-J. Qin, Y. Li, F.-X. Deng, Y.-Q. Liu, G.-B. Pan, Electrochim. Acta 145 (2014) 148. [15] X. Han, X. Wu, Y. Deng, J. Liu, J. Lu, C. Zhong, W. Hu, Adv. Energy Mater. 8 (2018) 1800935. [16] S. Huang, X. Zhu, B. Cheng, J. Yu, C. Jiang, Environ. Sci. Nano 4 (2017) 2215. [17] Z.-H. Xue, H. Su, Q.-Y. Yu, B. Zhang, H.-H. Wang, X.-H. Li, J.-S. Chen, Adv. Energy Mater. 7 (2017) 1602355.
4. Conclusions In summary, we have demonstrated an approach to prepare Pt/ PGaN by electrodeposition. The synthesized Pt/PGaN electrode showed excellent catalytic performances for HER with an overpotential of 98 mV at 10 mA/cm2, a Tafel slope of 85 mV/dec and an outstanding stability. The influences of different cycles of CV on its morphology and HER performance have been studied. Pt/PGaN can be applied to HER directly, eliminating the process of conventional catalyst coating, which is very significant in cost saving. In addition, Pt/PGaN may be designed as a dual function electrode for both electrochemical and photoelectrochemical water splitting. Declaration of Competing Interest The authors declare that they have no known competing financial 4
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