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Aug 21, 2015 - Here we report that excellent HER activity has been achieved with three-dimensional core/shell Co/Co3O4 nanosheets composed of a metall...
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Three-dimensional crystalline/amorphous Co/Co3O4 core/shell nanosheets as efficient electrocatalysts for hydrogen evolution reaction Xiaodong Yan, Lihong Tian, Min He, and Xiaobo Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02205 • Publication Date (Web): 21 Aug 2015 Downloaded from http://pubs.acs.org on August 23, 2015

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Three-dimensional crystalline/amorphous Co/Co3O4 core/shell nanosheets as efficient electrocatalysts for hydrogen evolution reaction Xiaodong Yan,† Lihong Tian,†,‡ Min He,†,‖ and Xiaobo Chen*,† † Department of Chemistry, University of Missouri – Kansas City, Kansas City, Missouri 64110, United States ‡ Hubei Collaborative Innovation Center for Advanced Organochemical Materials, Hubei University, Wuhan, 430062, China. ‖ Wuhan University of Science and Technology, Wuhan, Hubei 430081, China

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ABSTRACT: Earth-abundant, low-cost electrocatalysts with outstanding catalytic activity in electrochemical hydrogen evolution reaction are critical in realizing the hydrogen economy to lift our future welfare and civilization. Here, we report that excellent hydrogen evolution reaction activity has been achieved with three-dimensional core/shell Co/Co3O4 nanosheets composed of a metallic cobalt core and an amorphous cobalt oxide shell. A benchmark HER current density of 10 mA cm−2 is achieved at an overpotential of ~ 90 mV in 1 M KOH. The excellent activity is enabled with the unique metallic/oxide core/shell structure that allows a high electrical conductivity in the core and a high catalytic activity on the shell. This finding may open a door to the design and fabrication of earth-abundant, low-cost metal oxide electrocatalysts with a satisfactory hydrogen evolution reaction activity.

KEYWORDS: Cobalt/cobalt oxide, crystalline/amorphous core/shell, three-dimensional nanosheets, hydrogen evolution reaction, electrocatalysis

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Cost-effective, large-scale production of hydrogen is one of the keys to realizing the hydrogen economy in the decades-long dream towards a clean and renewable energy and environment future.[1−4] Hydrogen evolution from water by electrolysis is suggested as one of the viable ways to produce hydrogen on a large scale.[4] Noble metals, especially platinum, are still regarded as the best electrocatalysts for efficient hydrogen production.[5-8] However, their limited abundance and high cost have inevitably prevented the practical realization of the dreamed hydrogen economy. Thus, developing earth-abundant, low-cost electrocatalysts with a high catalytic activity in electrochemical hydrogen evolution reaction (HER) is critical in realizing the hydrogen economy to lift our future welfare and civilization. Over the past years, tremendous efforts have been devoted to the exploration of noble metalfree HER catalysts, and great progresses have been achieved with discoveries of highly active earth-abundant transition-metal based materials, such as phosphides,[9-15] dichalcogenides,[16-20] and Ni-Mo alloys.[21-23] Most of these catalysts are stable and/or have high activities in acidic conditions, and only very a few of them possess high and stable activities in alkaline media. However, hydrogen produced by electrolysis in alkaline conditions is widely employed in industries and at the cost of a moderate energy.[24] Therefore, the importance of developing highly efficient HER catalysts in alkaline electrolytes has always been emphasized. Although earth-abundant low-cost transition metal oxides, such as nickel oxide and cobalt oxide, have been studied as HER catalysts in alkaline media for many years, these oxides showed poor HER activities. Recently, metal-metal oxide/carbon hybrids (NiO/Ni/carbon nanotube and Co-CoOx/N-doped carbon) created by thermal decomposition showed excellent HER activities in alkaline electrolytes due to the synergetic effect of metal, metal oxide and functional carbons,[24,25] reviving these metal oxides as promising HER catalysts and triggering new research interest in

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these compounds. Despite of these great achievements, new strategies still needs to be developed to advance these metal oxides in practical applications. Inspired by the amorphous molybdenum sulphide that showed enhanced HER activity over the crystalline counterpart,[26-28] we anticipate that amorphous metal oxides covered on metallic core may be of high promise in achieving a high HER activity in alkaline conditions. In this study, we report the realization of such a novel catalytic system, that is, threedimensional (3D) crystalline/amorphous Co/Co3O4 core/shell nanosheets, with an excellent HER performance. The 3D Co3O4 nanosheets are grown on Ni foam using highly scalable solution growth method, and then the 3D Co/Co3O4 core/shell nanosheets are achieved by a lowtemperature hydrogen reduction. The hydrogen reduction leads to the formation of a 3D metallic mainframe for a higher electrical conductivity, and the surface amorphous cobalt oxide thin layer enriched with hydroxyl groups induces a high HER activity. Excellent HER activity in alkaline media is obtained by adjusting the composition and structure of Co/Co3O4 nanohybrids with the reduction condition. A benchmark HER current density of 10 mA cm−2 has been achieved at an overpotential of ~ 90 mV in 1 M KOH. More importantly, the Co/Co3O4 nanohybrids exhibit a very small onset potential (~ 30 mV). The excellent HER activity is enabled with the unique structure of the 3D crystalline/amorphous Co/Co3O4 core/shell nanosheets, which allow a high electrical conductivity in the core and a high HER activity on the surface. Thus, this study may open a new avenue to develop earth-abundant, low-cost electrocatalysts with high HER activities. The 3D Co3O4 nanosheets were first grown on nickel foams by reacting cobalt nitrate with hexamethylenetetramine at 90 °C in a water/ethanol solution with immersed nickel foams, followed by calcination at 300 °C in air. Their morphologies were checked with scanning electron microscopy (SEM). The surface of the nickel foam was largely covered with nanosheets (Figure

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1A) and the nanosheets self-assembled into flower-like structures (Figure 1B). On the other hand, the bare Ni foam (Supporting Information Figure S2) and Ni foam treated in a water/ethanol solution containing sodium nitrate and hexamethylenetetramine (Supporting Information Figure S3) displayed smooth surfaces. These indicated that the nanosheets were not grown from the Ni foam

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hexamethylenetetramine. The powders precipitated from the solution were annealed under the same condition for transmission electron microscopy (TEM) studies. The TEM images showed the nanosheets were composed of many small (5  8 nm in diameter) nanoparticles (Figure 1C, a corner of one Co3O4 nanosheet; see more images in Supporting Information Figure S4). These nanoparticles were well-crystalized with clearly resolved lattice fringes of Co3O4 (Figures 1D and S3). The adjacent plane distance of 4.65 Å corresponded well to the (111) plane of Co3O4 (JCPDS 43-1003) (Figures 1D). The 3D Co/Co3O4 core-shell nanosheets were obtained by chemical reduction of the above Co3O4 nanosheets at 200 °C for 3 h in a hydrogen atmosphere. After hydrogen reduction, the Ni foam was still well covered (Figure 1E), and the nanosheet morphology and self-assembled flowerlike structures were maintained (Figure 1F). The nanosheets were, however, apparently composed of some round and large (20  50 nm) nanoparticles which were covered with a thin layer of amorphous materials, displaying core/shell structures (Figures 1G; Supporting Information Figure S5). The high-resolution TEM (HRTEM) image further showed the primary nanoparticles had a crystalline core and an amorphous shell (Figure 1H). The core had clearly seen lattice fringes with adjacent plane distance of 2.17 Å, matched well with the (100) plane of Co crystal (JCPDS 050727). Thus, the core was Co metal. The Co nanoparticles likely had crystalline domains around 5 – 7 nm in diameter (Figure S5C), although the size of aggregated crystalline particle could extend

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to 20 – 30 nm in diameter (Figure 1H). The shell had a thickness of about 2 – 5 nm and had an amorphous characteristic, and was likely Co3O4 and/or CoO as indicated from the results in X-ray photoelectron spectroscopy (XPS). Thus, the Co/Co3O4 nanosheets were likely made of small Co/Co3O4 nanoparticles with a crystalline Co core and an amorphous Co3O4 and/or CoO shell. The X-ray diffraction (XRD) pattern (curve a in Figure 2A) of the Co3O4 nanosheets matched well with the standard XRD pattern (JCPDS 43-1003) (Supporting Information Figure S6A). The apparent diffraction peaks indicated well crystallized Co3O4 nanosheets. The grain size was calculated using Scherrer equation: τ = (kλ)/(βcosθ), where τ, k, λ, β, and θ are the mean size of the ordered (crystalline) domains, shape factor with a typical value of 0.9, X-ray wavelength, line broadening full width at half maximum (FWHM) peak height in radians, and Bragg angle, respectively.[29] The calculated grain size was 6.6 nm, consistent with the TEM observation. For the Co/Co3O4 nanosheets (curve b in Figure 2A), only one peak corresponding to (311) plane of Co3O4 was observed, and other diffraction peaks were attributed to Co (JCPDS 05-0727). The decreased (311) peak of Co3O4 and disappearance of other peaks suggested that the crystallinity of Co3O4 was largely reduced while some crystalline Co3O4 particles might still exist due to the partial reduction under this condition. The calculated grain size of the crystalline Co was 4.8 nm. Combined the information from the TEM observation, it was likely that one Co/Co3O4 particle contained several smaller crystalline Co nanograins/nanoparticles (Figure 1F). The XPS survey of both Co3O4 and Co/Co3O4 nanosheets were shown in Figure 2B. The spectra were similar: signals from Co and O elements were observed with C deposition from the atmosphere. All the spectra were calibrated with the C 1s peak to 284.6 eV. Figure 2C displayed the Co 2p core-level XPS spectra. Both Co3O4 and Co/Co3O4 nanosheets had two typical primary peaks from Co in Co3O4: Co 2p1/2 (795.0 eV),[30] and Co p3/2 (779.8 eV).[31] However, the

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corresponding satellites at 803.7 and 789.2 eV,[32] respectively, for Co3O4 nanosheets were shifted to 802.1 and 785.6 eV for Co/Co3O4 nanosheets. These shifts were likely from the influence of the Co metal on Co3O4 surface layer which showed some CoO feature.[33] The existence of metallic Co core in the Co/Co3O4 nanosheets was seen from the weak shoulder peaks around Co 2p1/2 at 793.5 eV and Co 2p3/2 at 778.5 eV.[30,34] Given that XPS can only probe the chemical information within a few atomic layers near the surface, it thus confirmed that the Co/Co3O4 nanosheets had a crystalline Co core and an amorphous Co3O4 shell. The O 1s XPS spectra were shown in Figure 2D. The Co3O4 nanosheets had two peaks near 529.2 and 530.2 eV, attributed to the lattice O2- in Co3O4,[35,36] and one small shoulder near 531.1 eV from the hydroxyl groups on the surface.[37−39] For the Co/Co3O4 nanosheets, the O2- peak shifted to 529.6 eV. This was explained by that the O2ions faced two different environments as bonded to Co2+ and Co3+ in Co3O4 (Co3O4 = CoO + Co2O3); these two bonding environments were merged into one in the amorphous Co3O4 shell in the Co/Co3O4 nanosheets, likely due to the apparent lattice structural alteration from the crystalline phase into the amorphous phase. Meanwhile, the intensity of the hydroxyl groups near 531.1 eV increased dramatically. This indicated that much more hydroxyl groups and likely more oxygen vacancies were created on the amorphous Co3O4 surface layers, consistent with the observations for hydrogenated metal oxides according in previous studies.[37−39] The catalytic properties of the Co3O4 and Co/Co3O4 nanosheets grown on Ni foams were investigated in a 1.0 M KOH solution using a typical three-electrode system. Bare Ni foam and commercial Pt/C were also tested for comparison. Potentials were reported versus the reversible hydrogen electrode (RHE). The polarization curves were shown in Figure 3A. All the polarization data were iR-corrected. Co/Co3O4 nanosheets exhibited a remarkably high activity with an onset potential of ~ 30 mV. To achieve an HER current density of 20 mA cm-2, an overpotantial of as

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small as 129 mV was needed. In comparison, Co3O4 nanosheets had an overpotential of 302 mV, and bare Ni required 390 mV at 20 mA cm−2. The catalytic activity of Co/Co3O4 nanosheets was also much better than the cobalt-cobalt oxide/N-doped carbon hybrids (CoOx@CN) that were the best cobalt oxide-based catalysts reported so far.[24] Although its activity was still slightly lower than Pt/C, the Co/Co3O4 nanosheets reported here were among the best alkali-based Pt-free catalysts studied so far, and even comparable to the best Pt-free acid-based catalysts, as shown in Table 1. The linear regions of the Tafel plots (Figure 2B) were fitted into the Tafel equation (ƞ = blog(j) + a, where b is the Tafel slop). The Tafel slope values of Pt/C, Co/Co3O4 nanosheets, Co3O4 nanosheets, and Ni foam were 28, 44, 49, and 83 mV/decade, respectively. This confirmed that Co/Co3O4 nanosheets had a high HER activity. Generally, there were two mechanisms involving the HER process in alkaline media, that is, Volmer (electrochemical hydrogen adsorption (Hads): H2O + e−  Hads + OH−, and Heyrovsky (chemical desorption: Had + H2O + e−  H2 +OH−) or Tafel (chemical desorption: Hads + Hads  H2) process.[40,41] Tafel slope of 120, 40, or 30 mV/decade was expected if the Volmer, Heyrovsky, or Tafel step was the rate-determining step, respectively.[40−42] Thus, the electrochemical hydrogen desorption (Heyrovsky process) was the rate-determining step for the HER on both Co/Co3O4 and Co3O4 nanosheets. The stability of Co/Co3O4 nanosheets was evaluated using a constant voltage technique. The result was shown in Figure 3C. At a constant overpotential of 120 mV, the current showed slight degradation during a long period of 6000 s. We performed XRD measurement on the Co/Co3O4 nanosheets after stability test to reveal the possible structural changes that caused the slight degradation in the polarization curves before and after stability test. As suggested from the XRD patterns (Supporting Information Figure S7), the content of the metallic cobalt in Co/Co3O4 nanosheets slightly

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increased after stability test, however, the diffraction peaks corresponding to Co3O4 did not show apparent change after stability test. As the crystalline Co3O4 likely located in the bulk based on previous analysis, this indicated that the increased metallic cobalt was more likely derived from the electrochemical reduction of the amorphous domain near the surface. Thus the slight reduction of the amorphous Co3O4 shell to Co metal in the shell likely led to the gradual degradation in performance. A slight difference was observed in the polarization curves of the Co/Co3O4 electrode before and after the stability test (Figure 3D). After stability test, the HER current slightly decreased at overpotentials < 200 mV but increased at overpotentials > 200 mV. This was likely due to the above gradual composition change due to the emergence of more Co from amorphous Co3O4 layer. On the other hand, these results suggested that the metallic cobalt contributed to a higher current at higher overpotentials, while the amorphous Co3O4 lowered the overpotential for the HER process. In another word, the metallic Co could act as a current reservoir while the amorphous Co3O4 shell could reduce the energy barrier for HER. The reduction temperature in hydrogen on Co3O4 nanosheets had a significant influence on the HER activity of the formed Co/Co3O4 nanosheets. All the Co3O4 nanosheets reduced in hydrogen had better HER performances than pristine Co3O4 nanosheets. The sample treated at 200 °C for 3 h (Co/Co3O4-200) displayed a better HER performance than those reduced at 150 (Co/Co3O4-150) and 300 (Co/Co3O4-300) °C (Figure 4A). No obvious metallic cobalt was detected in Co/Co3O4150, while only cobalt XRD peaks were observed in Co/Co3O4-300 (Supporting Information Figure S6A). And a thin layer of amorphous phase was also observed for Co/Co3O4-300 with HRTEM (Supporting Information Figure S6B). Apparently, increasing hydrogen treatment temperature would increase the percentage of crystalline Co3O4 being reduced. The remaining crystalline Co3O4 likely located in the bulk, as only a thin layer of amorphous phase (HRTEM)

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with Co3O4 chemical feature (XPS, surface chemical information) was observed. This indicated a suitable balance between Co and Co3O4 in the catalyst was crucial to a high catalytic activity likely from a synergistic effect of Co and amorphous Co3O4. The remarkably enhanced HER activity of the nanocomposites was explained with the synergistic effect between the metal core and the amorphous metal oxide shell, where the Co core could lower the internal resistance and act as electron reservoir for increased the HER current density, and the amorphous Co3O4 shell could lower the gas desorption energy barrier for a higher HER activity (based on Tafel analysis). The sample reduced at 200 °C for 3 h might have the optimal core/shell configuration to take full advantage of this synergistic effect. It was worth noting that a reduction peak was clearly observed prior to hydrogen evolution for all Co/Co3O4 nanosheets (Supporting Information Figure S8). This reduction peak was dependent on the reduction temperature, that is, Co/Co3O4-150 had a negligible reduction peak, and Co/Co3O4-200 had a very obvious reduction peak. And the reduction peak became smaller as the temperature increased. That reduction peak could be from the reduction of Co2+ to Co in the amorphous Co3O4 shell as the standard Co2+/Co redox potential is -0.28 V vs NHE, but might also be caused by the hydroxyl groups and/or oxygen vacancies as suggested previously, which showed a pseudo-capacitive behavior generated by the hydrogen reduction.[43,44] To reveal the HER kinetics on the surface of the catalysts, electrochemical impedance spectroscopy (EIS) analyses were performed. At an overpotential of 20 mV, two semicircles were observed in the Nyquist plots (Figure 4B), indicative of a two-time-constant behavior. The lowfrequency semicircle was correlated with hydrogen adsorption onto the electrodes forming hydride-type species, while the high-frequency semicircle was associated with the charge-transfer resistance.[41,45] Both adsorption impedance and charge-transfer resistance (Rct), especially adsorption impedance (Rad), experienced a tremendous decrease for Co/Co3O4 when compared to

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Co3O4. The onset potential displayed a substantial correlation with Rad, that is, the smaller the Rad, the more positive the onset potential was (Supporting Information Figure S9). This explained that the highly positive-shifted onset potential of Co/Co3O4 was the lower of the H2 adsorption energy barrier, consistent with previous Tafel analysis results. At a high overpotential of 220 mV, Bode plots suggested a one-time-constant process for all the electrodes (Supporting Information Figure S10), and Nyquist plots showed only one semicircle (Figure 4C). This suggested that the kinetic impedance played a critical role in determining the HER kinetics. To clearly reveal the relationship between HER current density (j) and Rct, the j-Rct plot was given in Figure 4D. The smaller the Rct, the larger the HER current density was. Therefore, the excellent HER performance of Co/Co3O4-200 was likely due to the low Rad and the fast charge-transfer kinetics on the surface of the electrode caused by the hydroxyl-enriched amorphous cobalt oxide. The diminished Rad was favorable for the Volmer process, while the decreased Rct facilitated both Volmer and Heyrovsky processes. It is expected that the Volmer-Heyrovsky process was greatly accelerated at the Co/Co3O4 interface. On the other hand, it was reported that hydroxide clusters facilitated the dissociation of water by weakening the O−H bond of the absorbed water,[46,47] and that more Lewis acidic groups promoted the activation of Lewis basic H2O through Lewis acid-base interaction, and thus led to improved HER activity as the HER process involved the adsorption of water onto the surface of the catalyst and the break of the O−H bond of the absorbed water.[42] We also noticed that the possible oxygen vacancy alone was unlikely to facilitate the HER process, because the Co3O4 nanosheets when vacuum-treated at 200 °C, which likely had more oxygen vacancy defects based on previous studies,[48,49] had a lower catalytic activity than Co3O4 nanosheets (Supporting Information Figure S11). However, the oxygen vacancies may also play a role in enhancing the adsorption of water through O atom as oxygen-vacancy-bearing oxide enabled more positive

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potential on the surface.[48,50] Thus, it can be concluded that the highly enhanced charge-transfer kinetics originated from the synergetic effect of the metallic cobalt and the amorphous cobalt oxide in the unique 3D core/shell nanosheet structure. The metallic core enhanced the electrical conductivity and acted as an electron reservoir to ensure a high current density, and the hydroxylenriched amorphous surface layer (along with some possible oxygen vacancies) favored the adsorption of water molecules and lowered the HER energy barrier. In summary, we demonstrate here a high-performance HER electrocatalyst in alkaline medium with 3D Co/Co3O4 metal/metal oxide core/shell nanosheets. Co3O4 nanosheets are first grown directly on Ni foam. Low-temperature treatment in hydrogen effectively induces the structural evolution from Co3O4 to Co nanosheets, leading to well-tuned cobalt−amorphous cobalt oxide hybrids. Best catalytic performance are achieved with a near-zero onset potential and an HER current density of 20 mA cm−2 at an overpotential of 129 mV. The high HER activity is attributed to the synergetic effect of metallic core and amorphous oxide shell in providing both good bulk conductivity and surface activity. Compared to Pt, cobalt and cobalt oxide possess the advantages of large abundance and low cost, thus these 3D crystalline/amorphous Co/Co3O4 core/shell nanosheets hold a promising future as practical catalyst candidates for hydrogen evolution from water electrolysis. Therefore, this study may provide us a new strategy to design new earthabundant, low-cost catalysts towards the realization of hydrogen economy. ASSOCIATED CONTENT Supporting Information Materials and Methods, Figures S1 to S10 are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT X. C. acknowledges the support from College of Arts and Sciences, University of Missouri − Kansas City, the University of Missouri Research Board (UMRB) and the University of Missouri Interdisciplinary Intercampus (IDIC) Programs. M. H. appreciates the financial supports from China Scholarship Council for oversee research. L. T. thanks the National Natural Science Foundation of China (No. 51302072) and China Scholarship Council for their financial supports. REFERENCES (1) Wang, H.; Zhang, Q.; Yao, H.; Liang, Z.; Lee, H.-W.; Hsu, P.-C.; Zheng, G.; Cui, Y. Nano Lett. 2014, 14, 7138–7144. (2) Luo, J.; Im, J. H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N. G.; Tilley, S. D.; Fan H. J.; Grätzel, M. Science 2014, 345, 1593−1596. (3) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. J. Am. Chem. Soc. 2014, 136, 4897−4900. (4) Wang, M.; Chen L.; Sun, L. Energy Environ. Sci. 2012, 5, 6763–6778. (5) Hsu, I. J.; Kimmel, Y. C.; Jiang, X.; Willis B. G.; Chen, J. G. Chem. Commun. 2012, 48, 1063– 1065. (6) Huang, X.; Zeng, Z.; Bao, S.; Wang, M.; Qi, X.; Fan Z.; Zhang, H. Nat. Commun. 2013, 4, 1444/1-1444/8.

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(21) McKone, J. R.; Sadtler, B. F.; Werlang, C. A.; Lewis, N. S.; Gray, H. B. ACS Catal. 2013, 3, 166−169. (22) Tang, X.; Xiao, L.; Yang, C.; Lu, J.; Zhang, L. Int. J. Hydrogen Energ. 2014, 39, 3055−3060. (23) Wang, X.; Su, R.; Aslan, H.; Kibsgaard, J.; Wendt, S.; Meng, L.; Dong, M.; Huang, Y.; Besenbacher, F. Nano Energy, 2015, 12, 9−18. (24) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. J. Am. Chem. Soc. 2015, 137, 2688−2694. (25) Gong, M.; Zhou, W.; Tsai, M.-C.; Zhou, J.; Guan, M.; Lin, M.-C.; Zhang, B.; Hu, Y.; Wang, D.-Y.; Yang, J.; Pennycook, S. J.; Hwang, B.-J.; Dai, H. Nat. Commun. 2014, 5, 4695/1-4695/6. (26) Merki, D.; Fierro, S.; Vrubel. H.; Hu, X. Chem. Sci. 2011, 2, 1262–1267. (27) Merki, D.; Vrubel, H.; Rovelli, L.; Fierro, S.; Hu, X. Chem. Sci. 2012, 3, 2515–2525. (28) Vrubel, H.; Merki, D.; Hu, X. Energy Environ. Sci. 2012, 5, 6136–6144. (29) Xia, T.; Zhang, W.; Wang, Z.; Zhang, Y.; Song, X.; Murowchick, J.; Battaglia, V.; Liu, G.; Chen, X. Nano Energy 2014, 6, 109–118. (30) Bonnelle, J. P.; Grimblot, J.; D’Huysser, A. J. Electron. Spectrosc. 1975, 7, 151−162. (31) McIntyre, N. S.; Johnston, D. D.; Coatsworth, L. L.; Davidson, R. D. Surf. Interface Anal. 1990, 15, 265–272. (32) Petitto, S. C.; Langell, M. A. J. Vac. Sci. Technol. A 2004, 22, 1690−1696. (33) Petitto, S. C.; Marsh, E. M.; Carson, G. A.; Langell, M. A. J. Molecular Catal. A: Chemical 2008, 281, 49−58. (34) Mandale, A. B.; Badrinarayan, S.; Date, S. K.; Sinha, A. P. B. J. Electron. Spectrosc. 1984, 33, 61−72. (35) Wagner, C. D.; Zatko, D. A.; Raymond, R.H. Anal. Chem. 1980, 52, 1445–1451.

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(36) Tyuliev, G.; Angelow, S. Appl. Surf. Sci. 1988, 32, 381−391. (37) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011, 331, 746−750. (38) Xia, T.; Zhang, C.; Oyler, N. A.; Chen, X. Adv. Mater. 2013, 25, 6905–6910. (39) Xia, T.; Wallenmeyer, P.; Anderson, A.; Murowchick, J.; Liu, L.; Chen, X. RSC Adv. 2014, 4, 41654–41658. (40) Conway, B. E.; Tilak, B. V. Electroch. Acta 2002, 47, 3571−3594. (41) Domínguez-Crespo, M. A.; Torres-Huerta, A. M.; Brachetti-Sibaja B.; Flores-Vela, A. Int. J. Hydrogen Energ. 2011, 36, 135−151. (42) Xu, Y.-F.; Gao, M.-R.; Zheng, Y.-R.; Jiang, J.; Yu, S.-H. Angew. Chem. Int. Ed. 2013, 52, 8546–8550. (43) Zhai, T.; Xie, S.; Yu, M.; Fang, P.; Liang, C.; Lu, X.; Tong, Y. Nano Energy 2014, 8, 255−263. (44) Lu, X.; Wang, G.; Zhai, T.; Yu, M.; Gan, J.; Tong, Y.; Li, Y. Nano Lett. 2012, 12, 1690– 1696. (45) Kucernak, A. R. J.; Sundaram, V. N. N. J. Mater. Chem. A 2014, 2, 17435–17445. (46) Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K.-C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M. Science 2011, 334, 1256−1260. (47) Danilovic, N.; Subbaraman, R.; Strmcnik, D.; Chang, K.-C.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Angew. Chem. 2012, 124, 12663–12666. (48) Xia, T.; Zhang, W.; Murowchick, J.; Liu, G.; Chen, X. Nano Lett. 2013, 13, 5289–5296. (49) Xia, T.; Zhang, Y.; Murowchick, J.; Chen, X. Catal. Today 2014, 225, 2−9. (50) Cheng, F.; Zhang, T.; Zhang, Y.; Du, J.; Han, X.; Chen, J. Angew. Chem. Int. Ed. 2013, 52, 2474 –2477.

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Figure Captions Figure 1. (A, B) SEM and (C, D) TEM images of Co3O4 nanosheets. (E, F) SEM and (G, H) TEM images of Co/Co3O4 nanosheets formed after heating Co3O4 nanosheets in hydrogen at 200 C for 3 h. Figure 2. (A) XRD patterns: (o) Co3O4 (JCPDS 43-1003), (#) Co (JCPDS 05-0727); (B) XPS survey; (C) Co 2p and (D) O 1s XPS spectra of (a) Co3O4 and (b) Co/Co3O4 nanosheets. Figure 3. Electrochemical characterization of Co/Co3O4 nanosheets obtained at 200 °C in hydrogen toward HER. (A) Polarization curves of bare Ni foam, Co3O4 nanosheets, Co/Co3O4 nanosheets, and Pt wire. (B) Tafel plots derived from (A). (C) Current-time characteristics of Co/Co3O4 nanosheets at an overpotential of 120 mV. (D) The polarization curves of Co/Co3O4 nanosheets before and after the current stability test. Figure 4. Electrochemical characterization of various Co/Co3O4 nanosheets obtained by annealing Co3O4 nanosheets in hydrogen at 150 °C (Co/Co3O4-150), 200 °C (Co/Co3O4-200), and 300 °C (Co/Co3O4-300), respectively. (A) Polarization curves. Nyquist plots at an overpotential of (B) 20 mV and (C) 220 mV. (D) Relationships between the HER current denisty and the charge-transfer resistance (Rct) at an overpotential of 220 mV.

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Figure 1. (A, B) SEM and (C, D) TEM images of Co3O4 nanosheets. (E, F) SEM and (G, H) TEM images of Co/Co3O4 nanosheets formed after heating Co3O4 nanosheets in hydrogen at 200 C for 3 h.

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o o

o

o (a) o

30

40

50

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(b)

(a)

1000

779.8

785.6

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795

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775

D

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(b) 530.2 529.2

(a)

805

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529.6

795.0

789.2

778.5

Co 803.7 802.1

600

O1s XPS

Intensity / a.u.

C

Co 2p XPS

(a)

800

Binding Energy / eV

2 Theta ()

(b)

Co 3s Co 3p

(b)

C 1s

#

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o

B

Co 2p

# # #

Intensity / a.u.

Intensity (a.u.)

A

Intensity / a.u.

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536

534

532

530

528

526

Binding Energy / eV

Figure 2. (A) XRD patterns: (o) Co3O4 (JCPDS 43-1003), (#) Co (JCPDS 05-0727); (B) XPS survey; (C) Co 2p and (D) O 1s XPS spectra of (a) Co3O4 and (b) Co/Co3O4 nanosheets.

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0

0.4

-20

0.3

Overpotential / V

j / mA cm

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-40 Ni Co3O4 Co/Co3O4 Pt/C

-60

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A -0.3

-0.2

-0.1

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49 Ni Co3O4 Co/Co3O4 Pt/C

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44 28

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j / mA cm

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2000

3000

4000

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E (V) vs. RHE

j / mA cm

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6000

-100 -0.4

D -0.3

t/s

-0.2

-0.1

0.0

0.1

0.2

E (V) vs. RHE

Figure 3. Electrochemical characterization of Co/Co3O4 nanosheets obtained at 200 °C in hydrogen toward HER. (A) Polarization curves of bare Ni foam, Co3O4 nanosheets, Co/Co3O4 nanosheets, and Pt wire. (B) Tafel plots derived from (A). (C) Current-time characteristics of Co/Co3O4 nanosheets at an overpotential of 120 mV. (D) The polarization curves of Co/Co3O4 nanosheets before and after the current stability test.

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0

A

-150

-40

-60

Co3O4 Co/Co3O4-150 Co/Co3O4-200 Co/Co3O4-300

-80

-100 -0.4

Z ()

j / mA cm

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-0.3

-0.2

-0.1

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Co3O4 Co/Co3O4-150 Co/Co3O4-200 Co/Co3O4-300

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0 0

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Co3O4 Co/Co3O4-150 Co/Co3O4-200 Co/Co3O4-300

D

80

j / mA cm

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-8

Z ()

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-4

Co/Co3O4-200

60 Co/Co3O4-300

40

20 Co/Co3O4-150

0

0

5

10

15

20

25

0

0

5

Co3O4

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Z ()

Figure 4. Electrochemical characterization of various Co/Co3O4 nanosheets obtained by annealing Co3O4 nanosheets in hydrogen at 150 °C (Co/Co3O4-150), 200 °C (Co/Co3O4-200), and 300 °C (Co/Co3O4-300), respectively. (A) Polarization curves. Nyquist plots at an overpotential of (B) 20 mV and (C) 220 mV. (D) Relationships between the HER current denisty and the charge-transfer resistance (Rct) at an overpotential of 220 mV.

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Table 1. Comparisons of the HER performance of the electrodes reported recently.

Catalyst

Mass loading (mg cm−2)

Electrolyte

20[a] (mV)

Ref.

NiFe LDH FeP NiP CoP CoOx/CN[b] NiO/Ni-CNT[c] Co/Co3O4

− − 1 0.92 0.42 0.28 0.85

1 M NaOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 1 M KOH 1 M KOH

~250 ~320 130 100 ~270 ~120 129

[2] [10] [11] [12] [24] [25] This work

[a] 20: overpotential at an HER current density of 20 mA cm−2, [b] CN: N-doped carbon, [c] CNT: carbon nanotube.

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TOC

Excellent activity in hydrogen evolution reaction in alkaline media has been achieved with 3-D crystalline/amorphous Co/Co3O4 core/shell nanosheets as electrocatalysts.

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