One-Step Synthesis of NiMn-Layered Double Hydroxide Nanosheets Efficient for Water Oxidation

Transcription

1 Communication Layered Double Hydroxides One-Step Synthesis of NiMn-Layered Double Hydroxide Nanosheets Efficient for Water Oxidation Ruiqi Li, Yuqian Liu, Haobo Li, Man Zhang, Yiran Lu, Lin Zhang, Jianping Xiao, Frank Boehm, and Kai Yan* Layered double hydroxides (LDHs) are noble metal free 2D materials promising for water oxidation. One-step synthesis of ultrathin NiMn- LDHs nanosheets is successfully achieved at room temperature avoiding the multiple tedious steps (e.g., hydrothermal treatment, exfoliation). The as-prepared NiMn-LDHs (1.3 nm thickness) exhibit the twofold enhancement of the activity and a reduction of overpotential by 80 mv at 10 ma cm 2 in comparison with the traditional NiMn-LDHs in 0.1 m NaOH, which is superior to the previously reported LDH-derived electrocatalysts. The combination of theoretical and experimental results manifest that the largely enhanced electrocatalytic water oxidation activity of NiMn-LDHs nanosheets is associated with the highly exposed active sites with a nearly optimal intermediates (*OH and *O) adsorption energy. In view of the ever-growing energy crisis as well as the worsening environmental issue, there has been an increasing thrust on exploring alternative renewable and sustainable energy solutions to fossil fuels. [1 3] Electrochemical water splitting provides a promising strategy to capacitate the efficient energy conversion and storage. [1,4 9] Unfortunately, because of the multistep with four-electron-transfer, water oxidation is severely hindered by the sluggish kinetics and thermodynamics, leading to a large overpotential, great energy loss, and low performance of R. Li, Y. Liu, Dr. M. Zhang, Prof. K. Yan Guangdong Provincial Key Laboratory of Environmental Pollution and Remediation Technology School of Environmental Science and Engineering Sun Yat-sen University 135 Xingang Xi Road, Guangzhou , China yank9@mail.sysu.edu.cn Dr. H. Li, Prof. J. Xiao Institute of Natural Sciences Westlake Institute for Advanced Study Westlake University Hangzhou , China Dr. Y. Lu, Dr. L. Zhang School of Engineering Brown University Providence, RI RI02906, USA Dr. F. Boehm Department of Chemistry Lakehead University Thunderbay, ON P7B5E1, Canada The ORCID identification number(s) for the author(s) of this article can be found under DOI: /smtd electrocatalysts. [2,10 13] Generally, the development of highly efficient water oxidation catalysts combining low kinetic barriers with long-term electrocatalytic stability is being pursued actively. To date, numerous 3d transition metal-derived materials have been acted as potential electrocatalysts for water oxidation process, principally due to their earth-abundance, the chemical versatility, diversified nanostructures, ecofriendly character and theoretically high catalytic activity. [14 19] The large interlayer distance of LDHs structure can afford great electrochemically accessible surface area to the electrolyte. In addition, the flexible and adjustable composition of LDHs, as well as the edge-sharing octahedral MO 6 layers consisted in LDHs (e.g., NiCo-, NiFe-LDHs) have exhibited excellent catalytic activity in electrochemical catalysis. [20 24] However, the layered space of bulk LDHs often stack together, leading to poor electronic conductivity, electron transportation and blocked active sites. To overcome these issues, various strategies (e.g., the association of LDHs with conductive nanocrabon, doping or plasma treatment and exfoliation) have been frequently utilized to obtain the high performance. [11,17,24 27] Among different methodologies, exfoliation could increase accessible active sites, tune the electronic effect, and tailor the surface properties of electrocatalysts. For example, Hu et al. reported the exfoliation of bulk NiFe- and NiCo-LDHs into nanosheets in the formamide solution, the resulted nanosheets exhibited a 4.5-fold enhancement compared with the bulk LDHs. [25] Subsequently, various morphologies of LDHs (e.g., NiFe-, NiCo-, CoMn-LDHs) have been exfoliated. [13,18,19] However, the exfoliation of LDHs is often performed in high boiling point solvents (e.g., formamide, N-methylpyrrolidone), the strategy often involves the high cost, tedious procedure, restacking in the aqueous environment and blocking the catalytic active sites because of the strong adsorption of organic solvent on the surface of LDHs nanosheets. Herein, we reported one-step facile synthesis of ultrathin NiMn-LDHs nanosheets were successfully and controllably achieved at room temperature under ultrasonic irradiation, resulting in a more homogeneous and easy control process. This new route could combine the advantageous characters of the solution-based and solid-state reactions, involving direct interactions of metal precursors, atoms and ions. The as-obtained nanosheets exhibited the twofold enhancement of (1 of 5)

2 the performance toward the NiMn-LDHs synthesized by the traditional co-precipitation method, whereas a reduction of overpotential by 80 mv was achieved. The much enhanced activity was competitive with other LDHderived electrocatalytic materials reported to date. Besides, the combination of theoretical and experimental results documented that the much enhanced electrocatalytic activity was associated with the highly exposed surface structure with a nearly optimal intermediates (*OH and *O) adsorption energy. Transmission electron microscopy (TEM) was firstly utilized to compare the textural difference between the bulk NiMn-LDHs and as-obtained nanosheets. The bulk NiMn-LDHs exhibited nanoplate s morphology with clear aggregation (Figure 1a,b), where the large nanoplates were around dozens of nanometers and the crystalline nature was mainly amorphous. In comparison, TEM images (Figure 1c,d) of NiMn-LDHs nanosheets display multiple thin single layers without the detectable stacking, which confirmed that the ultrasonic assistance could efficiently prevent the stacking or aggregation. Figure 2d indicates the ultrathin nanosheets morphology with a lateral size less than several nanometers (Inset in Figure 1d). The energy dispersive X-ray (EDX) spectrum of the NiMn-LDHs nanosheets (Figure S1, Supporting Information) shows the peaks of Ni and Mn where they should be and an Mn/Ni atomic ratio of 0.46, which is close to the original ratio. As a direct tool, atomic force microscopy (AFM) Figure 1. TEM images of the bulk NiMn-LDHs a,b) synthesized by the traditional co-precipitation and hydrothermal method, and NiMn-LDHs nanosheets c,d) prepared in one-step without the hydrothermal treatment at room temperature. was used to investigate variations in thickness between bulk materials and nanosheets. Bulk NiMn-LDHs exhibited a layered plate morphology with a thickness of 50 nm (Figure 2a,c). In contrast, as-made ultrathin NiMn-LDHs nanosheets had a thickness of 1.3 nm (Figure 2b,d). An X-ray diffraction (XRD) pattern was then used to study the crystal phase structure of the as-obtained NiMn-LDHs materials. Figure 2. AFM images of bulk NiMn-LDHs a,c) synthesized via a traditional co-precipitation and hydrothermal method, and NiMn-LDHs nanosheets b,d), which were prepared through a one-step method without hydrothermal treatment at room temperature (2 of 5)

3 The diffraction pattern (Figure S2, Supporting Information) of NiMn-LDHs synthesized by the traditional co-precipitation and hydrothermal method indicated the characteristic reflections (003), (006), and (009), suggesting the successful synthesis of NiMn-LDHs with high crystallinity. [17,28] Several peaks at 20.5, 31.2, 40.5 from Ni(OH) 2 clearly presented in the diffraction pattern, which was possible because of the Jahn Teller effect of Mn 2+ that distorted the layered structure during the synthesis. Based on the Scherrer equation, the mean size estimated from (003) facet was of 50 nm. For the NiMn-LDHs nanosheets, it only exhibited a diffraction peak at 27, indicating ultrathin nanosheets were produced. X-ray photoelectron spectroscopy (XPS) was subsequently used to determine the chemical and electronic states of NiMn-LDHs nanosheets. The XPS survey (Figure S3, Supporting Information) clearly exhibits the peaks of Ni, Mn, C, and O where they should be. The XPS spectra (Figure S4a, Supporting Information) of Ni shows two deconvoluted peaks at and ev. The strong Ni 2p 3/2 satellite appeared at ev indicated the octahedral Ni 2+ in the lattice of LDHs. The two peaks (Figure S4b, Supporting Information) of Mn 2p 3/2 and Mn 2p 1/2 at and ev were believed to be the prominent Mn 2+ in the exfoliated nanosheets. The N 2 adsorption desorption was utilized to estimate the surface area and pore volume of the fabricated NiMn-LDHs nanosheets. A type IV isotherm (Figure S2b, Supporting Information) with H 3 hysteresis loop at P/P 0 = was obtained, confirming the mesoporous structure. According to the adsorption formula of P/[V(P 0 P)] = 1/[V m C] + [(C 1)P]/[V m CP 0 ] (more details are described in the Supporting Information), the surface area of 85.6 m 2 g 1 was computed from the adsorption curve. The total volume (V p ) of 0.26 cm 3 g 1 was calculated from the adsorbed amount at a relative pressure P/P 0 of While the bulk NiMn-LDHs synthesized by the traditional method display very limited surface area of 39.2 m 2 g 1 and smaller volume of 0.18 cm 3 g 1. Barrett Joyner Halenda method was used to count the pore size distribution of the as-obtained NiMn-LDHs. The size distribution (Inset of Figure S2b, Supporting Information) of the NiMn-LDHs nanosheets exhibited a narrow size distribution with the average pore size of 2.8 nm (Figure S2b, Supporting Information), where the bulk NiMn-LDHs display a relatively broad size dispersion with an average size of 4.3 nm. These porous structures were mainly from the space of different layers. To investigate their oxygen evolution reaction (OER) catalytic properties, a three-electrode system connected with rotating disk electrode was utilized using the as-made LDHs as a working electrode in 0.1 m NaOH. Figure 3a shows that overpotential of NiMn-LDHs nanosheets was significantly shifted to lower value (e.g., a shift of 80 mv at 10 ma cm 2 ) in comparison with the bulk NiMn-LDHs material, suggesting ultrathin nanosheets had better electron transportation and more exposed active sites. The redox peaks of NiMn-LDHs nanosheets at 0.19 V overpotential were ascribed to the Ni 2+ /Ni 3+ redox process (Figure 3a). Ultrathin NiMn-LDHs nanosheets exhibited a strong anodic current wall, displaying the lowest onset potential and the potential applied to reach a current density of 20 ma cm 2 Figure 3. a) LSV curves for OER on bulk NiMn-LDHs and ultrathin NiMn-LDHs nanosheets. b) The corresponding Tafel plots. c) Cycles stability of the exfoliated NiMn-LDHs nanosheets. d) DFT calculated free energy diagrams for the OER process from H 2 O to O 2 on the NiMn-LDHs structures. The optimized configurations of the adsorption intermediates of *OH, *O and *OOH are shown inside. Dark blue: Mn; green: Ni; red: O; white: H (3 of 5)

4 was at 1.62 V, while the bulk NiMn-LDHs was at 1.71 V. These further documented ultrathin NiMn-LDHs nanosheets with better electronic conductivity had more active sites and larger active surface areas. [28] The overpotential versus log (current density) are plotted based on the Tafel equation (η = b log j + a) in Figure 3b. The Tafel slope of the as-obtained NiMn-LDHs nanosheets and bulk NiMn-LDHs material was of 47 and 93 mv dec 1, respectively, confirming high performances of NiMn-LDHs nanosheets in OER and the Volmer reaction as the rate-determining step. Multiple cycling tests were performed to check the durability of the as-obtained NiMn-LDHs nanosheets in OER, as shown in Figure 3c. Superior cycles performances were reached over 5000 cycles without the detectable change. We choose the overpotential required for 10 ma cm 2 closely matches the spectrum for a 10% efficient solar-to-fuel device [11,29,30] and the Tafel slope b to benchmark our materials with other LDHs-derived catalysts (Table S1, Supporting Information). It was clear to see the as-obtained NiMn-LDHs nanosheets exhibited a much lower overpotential and were even better than the currently active candidates for oxygen evolution reaction. The electrochemical active surface area (ECSA) was performed and determined from the double-layer capacitances (C dl ) via cyclic voltammograms. As shown in Figure S5 in the Supporting Information, the linear slope of NiMn-LDHs nanosheets was 8.1 mf cm 2 that was twice higher than that of bulk NiMn- LDHs (3.9 mf cm 2 ). Besides, Figure S6 in the Supporting Information displays the electrochemical impedance spectra (EIS) of bulk and nanosheets. Both EIS curves consist of two clear semicircles. Fundamentally, the high-frequency semicircle is primarily associated with charge transfer resistance, whereas the lowfrequency semicircle is related to the mass-diffusion process. [18,21] Clearly, both charge transfer and mass-diffusion resistance of the NiMn-LDHs nanosheets were largely reduced when compared with the bulk NiMn-LDHs. The high ECSA and decreased resistance significantly accelerated the reaction kinetics of the NiMn- LDHs nanosheets. A comparison between quantum chemical calculations and experimental observations has been performed to better understand the surface structure of NiMn-LDHs nanosheets and the OER mechanism at the atomic level. A model of NiMn-LDHs structures with the atomic ratio Ni:Mn = 2:1 has been built to simulate the OER process on the catalyst. Since the entire process is a four-electron-transfer process, there are four intermediate steps. During the first step, a water molecule is oxidized on one active site of the oxide surface and one proton and one electron are released in order to form a surface adsorbed HO* intermediate: H 2 O(l) + * HO* + H + + e. That HO* intermediate is further oxidized to O* specie: HO* O* + H + + e. A second water molecule is splitted on the top of the previously formed O* species to form a surface adsorbed superoxide intermediate: H 2 O(l) + O* HOO* + H + + e. This intermediate is oxidized in order to release the oxygen molecule: HOO* * + O 2 (g) + H + + e. The reaction free energy of each step can be denoted as G 1, G 2, G 3, and G 4. According to the previous theoretical study, the adsorption free energy difference between the two important intermediates *O and *OH, that is G 2, can be used as a descriptor for the reaction activity. For a variety of metal oxides, the optimal activity is achieved at G 2 around 1.61 ev. [31,32] As shown in Figure 3d, G 2 was calculated to be 1.54 ev on NiMn-LDHs nanosheets structure, which is quite close to 1.61 ev. Therefore, the catalyst based on such active site structures showed good potential for OER. Such theoretical results corresponded well with the high OER performance observed in electrochemical test experiments (Figure 3a c). In summary, one-step synthesis of ultrathin NiMn-LDHs nanosheets has been rationally designed and successfully achieved at room temperature avoiding the multiple tedious steps of hydrothermal treatment and exfoliation. NiMn-LDHs nanosheets (1.3 nm thickness) display twofold enhancement of the activity toward the traditional NiMn-LDHs in 0.1 m NaOH solution. Besides, NiMn-LDHs nanosheets led to a reduction of overpotential by 80 mv at 10 ma cm 2. This was a striking observation and in agreement with computational predictions. The combination of theoretical and experimental results manifested that the enhanced electrocatalytic activity and durability of NiMn-LDHs nanosheets were closely associated with the highly exposed active sites with a nearly optimal intermediates (*OH and *O) adsorption energy. This work offers a novel strategy to design low-cost and high-performance electrocatalysts that are promising for water oxidation. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements R.L. and Y.Liu contributed equally to this work. This work was supported by National Key R&D Program of China (2018YFD ), National Natural Science Foundation of China ( ), Science and Technology Planning Project of Guangdong Province (2014A ), Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (2018K02), and Hundred Talent Plan (201602) from Sun Yat-sen University. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conflict of Interest The authors declare no conflict of interest. Keywords durability, enhancement, nanosheets, NiMn-LDHs, water oxidation Received: September 4, 2018 Revised: September 16, 2018 Published online: November 2, 2018 [1] J. S. Luo, J. H. Im, M. T. Mayer, M. Schreier, M. K. Nazeeruddin, N. G. Park, S. D. Tilley, H. J. Fan, M. Grätzel, Science 2014, 345, [2] J. F. Yu, Q. Wang, D. O Hare, L. Sun, Chem. Soc. Rev. 2017, 46, [3] M. W. Kanan, D. G. Nocera, Science 2008, 321, [4] K. Yan, T. A. Maark, A. Khorshidi, V. A. Sethuraman, A. A. Peterson, P. R. Guduru, Angew. Chem., Int. Ed. 2016, 55, (4 of 5)

5 [5] J. L. Fillol, Z. Codolà, I. Garcia-Bosch, L. Gómez, J. J. Pla, M. Costas, Nat. Chem. 2011, 3, 807. [6] K. Yan, Y. R. Lu, Small 2016, 12, [7] O. Diaz-Morales, I. Ledezma-Yanez, M. T. Koper, F. Calle-Vallejo, ACS Catal. 2015, 5, [8] H. H. Shi, H. F. Liang, F. W. Ming, Z. C. Wang, Angew. Chem., Int. Ed. 2017, 56, 573. [9] F. W. Ming, H. F. Liang, H. H. Shi, X. Xu, G. Mei, Z. C. Wang, J. Mater. Chem. A 2016, 4, [10] M. S. Burke, L. J. Enman, A. S. Batchellor, S. H. Zou, S. W. Boettcher, Chem. Mater. 2015, 27, [11] M. S. Burke, M. G. Kast, L. Trotochaud, A. M. Smith, S. W. Boettcher, J. Am. Chem. Soc. 2015, 137, [12] N. B. Halck, V. Petrykin, P. Krtil, J. Rossmeisl, Phys. Chem. Chem. Phys. 2014, 16, [13] R. Liu, Y. Y. Wang, D. D. Liu, Y. Q. Zou, S. Y. Wang, Adv. Mater. 2017, 29, [14] A. J. Tkalych, H. L. Zhuang, E. A. Carter, ACS Catal. 2017, 7, [15] S. M. Yin, W. G. Tu, Y. Sheng, Y. H. Du, M. Kraft, A. Borgna, R. Xu, Adv. Mater. 2018, 30, [16] W. J. Zhou, X. J. Wu, X. H. Cao, X. Huang, C. L. Tan, J. Tian, H. Liu, J. Y. Wang, H. Zhang, Energy Environ. Sci. 2013, 6, [17] H. F. Liang, F. Meng, M. Cabán-Acevedo, L. S. Li, A. Forticaux, L. C. Xiu, Z. C. Wang, S. Jin, Nano Lett. 2015, 15, [18] Y. Wang, Y. Zhang, Z. Liu, C. Xie, S. Feng, D. Liu, M. Shao, S. Wang, Angew. Chem. 2017, 129, [19] F. Song, X. L. Hu, J. Am. Chem. Soc. 2014, 136, [20] X. Long, J. K. Li, S. Xiao, K. Y. Yan, Z. L. Wang, H. N. Chen, S. H. Yang, Angew. Chem., Int. Ed. 2014, 53, [21] J. Bao, X. D. Zhang, B. Fan, J. J. Zhang, M. Zhou, W. L. Yang, X. Hu, H. Wang, B. Pan, Y. Xie, Angew. Chem., Int. Ed. 2015, 54, [22] M. Gong, Y. G. Li, H. L. Wang, Y. Y. Liang, J. Z. Wu, J. G. Zhou, J. Wang, T. Regier, F. Wei, H. J. Dai, J. Am. Chem. Soc. 2013, 135, [23] K. Yan, T. Lafleur, J. J. Chai, C. Jarvis, Electrochem. Commun. 2016, 62, 24. [24] W. T. Hong, M. Risch, K. A. Stoerzinger, A. Grimaud, J. Suntivich, Y. Shao-Horn, Energy Environ. Sci. 2015, 8, [25] F. Song, X. L. Hu, Nat. Commun. 2014, 5, [26] Y. Y. Wang, C. Xie, Z. Y. Zhang, D. D. Liu, R. Chen, S. Y. Wang, Adv. Funct. Mater. 2018, 28, [27] D. W. Chen, M. Qiao, Y. R. Lu, L. Hao, D. D. Liu, C. L. Dong, Y. F. Li, S. Y. Wang, Angew. Chem., Int. Ed. 2018, 57, 8691 [28] M. W. Louie, A. T. Bell, J. Am. Chem. Soc. 2013, 135, [29] C. C. L. McCrory, S. Jung, J. C. Peters, T. F. Jaramillo, J. Am. Chem. Soc. 2013, 135, [30] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. X. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, [31] I. C. Man, H. Y. Su, F. Calle-Vallejo, H. A. Hansen, J. I. Martínez, N. G. Inoglu, J. Kitchin, T. F. Jaramillo, J. K. Nørskov, J. Rossmeisl, ChemCatChem 2011, 3, [32] J. Rossmeisl, Z. W. Qu, H. Zhu, G. J. Kroes, J. K. Nørskov, J. Electroanal. Chem. 2007, 607, (5 of 5)