Study the Active Sites in Porous Nickel Oxide Nanosheets by

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Study the Active Sites in Porous Nickel Oxide Nanosheets by Manganese Modulation for Enhanced Oxygen Evolution Catalysis Tian Tian, Hong Gao, Xichen Zhou, Lirong Zheng, Jianfeng Wu, Kai Li, and Yong Ding ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01206 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Energy Letters

Study the Active Sites in Porous Nickel Oxide Nanosheets by Manganese Modulation for Enhanced Oxygen Evolution Catalysis Tian Tian,⊥,† Hong Gao,⊥,† Xichen Zhou,† Lirong Zheng,ǁ Jianfeng Wu,† Kai Li, § and Yong Ding †, ‡,§

†

*

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metals

Chemistry and Resources Utilization of Gansu Province, and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ‡

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical

Physics, Chinese Academy of Sciences, Lanzhou 730000, China ǁ

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, Beijing 100049, China §

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, China

ACS Paragon Plus Environment

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ABSTRACT. Identifying active sites of oxygen evolution reaction catalysts is essential for studying water oxidation mechanisms. Herein, the active site of nickel oxide nanosheets by manganese modulation is investigated in electrocatalytic oxygen evolution system. The electronic structure could be realized by Mn modulation. The intrinsic catalytic activity Ni3+ (t2g6eg1) and Jahn-Teller active Mn3+ (t2g3eg1) species, which act synergistically to promote the elctrocatalytic oxygen evolution reaction. X-ray absorption near edge structure analysis indicates that the Ni3+ and Mn3+ in Ni0.75Mn0.25 nanosheets should result from nickel vacancies and oxygen vacancies, respectively. The resulting Ni0.75Mn0.25 nanosheets show much higher oxygen evolution activity than those of NiO and Mn2O3. Density functional theory calculations indicate the Ni and Mn act synergistically to promote the forming O-O bond. Our work provides a comprehensive understanding the active site of porous nickel oxide nanosheets by manganese modulation and rational design of electrocatalysts with precisely engineered structures and electrical properties.

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ACS Energy Letters

Increasing energy consumption and environmental problem require new energy resources to substitute for fossil fuels.1 Hydrogen fuel generated by electrochemical water splitting becomes one of the most efficient chemical methods for renewable energy storage. However, the oxygen evolution reaction (OER) remains the bottleneck of water splitting because of the intrinsically very sluggish kinetics associated with multistep proton-coupled electron transfer.2-5 Therefore, developing highly efficient water oxidation catalysts is crucial. The iridium and ruthenium oxides represent excellent electrocatalysts for water oxidation reaction, but the scarcity and highcost of precious metals limit their practical application on a large scale.6 It is highly desirable and urgent to develop new electrocatlysts with both excellent performance and low cost. Transition metal oxide electrocatalysts based on nickel compounds have been discovered as ideal candidates for OER due to their earth abundant nature, environmental benignity, low cost, unique 3d electron number and special eg orbitals.7-9 For the 3d transition elements, Ni has good interaction strength with OHad, satisfying the Sabatier principle for catalyst design.10 To further understand the Ni-based catalysts water oxidation mechanisms, it is imperative to identify the active sites during the OER reaction. One of the ways to improve water oxidation is the modulation of the electronic structure.11 For the electronic modulation, the eg orbital percentage is considered as the most important factor for OER catalysts due to its easy react with oxygenrelated adsorbents.12 According to Yang’s pioneering work,4 the eg orbital occupancy is very important for electrocatalysts based 3d transition metal. For Ni-based species, Ni3+ (t2g6eg1) is an ideal candidates for OER catalysts, which is very close to the optimal eg1.2 electronic configuration.13 Similarly, a high spin Mn3+ (t2g3eg1) is also an important active site for water oxidation catalysis.14 However, the Ni3+ (t2g6eg1) ion is the most unstable Ni oxidation states, it is of crucial importance to stabilize this species by electronic regulation. manganese oxide involve


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reversible cycling among Mn2+, Mn3+, and Mn4+ oxidation.15 On the other hand, numerous studies show that structural defects have a significant effect on the activity of electrocatalysts.16 It is well known that electrocatalysts with oxygen vacancies can result in higher catalytic activity.14, 17-19 The generation of oxygen vacancies could improve the electronic conductivity.20 Moreover, metal vacancies can modulation the electronic structure of the catalysts surface and improve the catalytic activity due to their multifarious electron and orbital distributions.13, 21-23 Therefore, introducing vacancy into electrocatalysts is a promising way to modulation the electronic structure. So far, many studies reported the manganese doping improves the electrochemical oxygen evolution reaction.24, 25 However, the active sites of manganese doping electrocatalysts are rarely studied at current stage. Inspired by the above mentioned, we developed a simple and facile strategy to fabricate manganese modulation nickel oxide nanosheets with systematically engineered structure for dramatically accelerated OER process. The corresponding merits of the porous nickel manganese oxide nanosheets are summarized as follows: 1) Porous nanomaterials are generally attractive because they can expose more active sites for contact with electrolyte and facilitate the transfer of ionic species. 2) The electronic modulation of the manganese enhances the intrinsic activity. 3) The formation nickel manganese oxide nanosheets stabilize the intrinsic Ni3+ (t2g6eg1) and Mn3+ (t2g3eg1) species. Furthermore, we used X-ray absorption spectroscopy (XAS) to elucidate the effects of Mn in Ni-based oxides. The high catalytic activity of Ni0.75Mn0.25 is due to unsaturated surface Ni3+ and Mn3+ active sites. Density functional theory (DFT) calculations indicate that the Ni and Mn act synergistically to promote the forming O-O bond. To the best of our knowledge, this is the first time identify the active sites in porous nickel oxide nanosheets by manganese modulation in electrochemical oxygen evolution.


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ACS Energy Letters

Figure 1. Schematic illustration of the synthesis of porous NiMn oxide nanosheets. Precursor morphology-directed method has been proven to be effective for preparation of porous oxides with specific morphology. In this paper, precursor nanosheets were synthesized at room temperature by a simple coprecipitation method. Porous NiMn oxide nanosheets were prepared by direct calcinations of the NiMn LDH precursor nanosheets (Figure 1). The X-ray diffraction (XRD) pattern (Figure S1a) indicates the LDH structure of the compound (JCPDS 38-0715). After calcinations at 350°C, the different ratios of NiMn-LDH were transformed into porous NiMn oxides that denoted as NixMn1-x. Particularly, we focus on the sample of Ni0.75Mn0.25 oxide because it shows the best catalytic activity. The XRD peaks of the Ni0.75Mn0.25 (Figure S1b) are indexed to Ni6MnO8 (JCPDS 42-0749). Thermal gravimetric analysis result exhibits the decomposition of surface adsorbed species and hydroxide layers, as well as the formation of oxides (Figure S2). As shown in Figure S3 and S4, other ratios of NiMn oxides were also successfully synthesized. The energy-dispersive X-ray (EDX) result confirms that Mn, Ni and O are the only elements composing of samples (Cu and C derived from Cu grid, Figure S5). The atomic ratios of Ni/Mn (Table S1) in the as-synthesized samples were determined by EDX data and inductively coupled plasma atomic emission spectroscopy (ICP-AES).


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Transmission electron microscopy (TEM) image of NiO is shown in Figure S6a, the pure NiO reveals the morphology of flake with the mean particle size of 5 nm. The pure Mn2O3 shows the nanosheet shape (Figure S6b). After doping the manganese into the nickel oxide, nickel manganese oxides are porous nanosheets display irregular 2D sheet-like shape (Figure 2a-c). High-resolution transmission electron microscopy (HRTEM) image of the Ni0.75Mn0.25 (Figure 2e) shows lattice fringe spacing of 0.186 and 0.208 nm, corresponding to the (420) and (400) facets of cubic Ni6MnO8. Moreover, the numerous small pits shown in Figure 2d-f illustrate the porous structure of nickel manganese nanosheets. Figure 2g shows the nitrogen adsorption– desorption isotherms and the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curves of the Ni0.75Mn0.25. The average pore diameter is approximately 7.9 nm for Ni0.75Mn0.25, which has been illustrated by Barrett–Joyner–Halenda (BJH) pore-size distribution curve (inset of Figure 2g). This porous structure has very high Brunauer–Emmett–Teller (BET) surface area (152 m2 g-1) for Ni0.75Mn0.25 (Table S2). The BET analysis of other samples is shown in in Figure S7. Atomic force microscopy (AFM) was applied to evaluate the thickness of Ni0.75Mn0.25 nanosheets as-synthesized. As shown in Figure 2h, the average scanning height of the Ni0.75Mn0.25 nanosheets is 4.4 nm (Figure 2i), which indicates that the ultrathin 2D nanosheets have been successfully prepared. It is known that the composition of the catalysts can affect the OER performance significantly. Therefore, to investigate the modulation effect after the Mn doping, a series of electrochemical measurements were conducted using catalysts with different Mn contents to optimize the composition. As shown in Figure S8, the Mn content plays an important role for the catalytic activities of NiMn oxides. When the molar ratio of Ni/Mn is 3:1, that is, Ni0.75Mn0.25, the best water oxidation properties are obtained with the highest catalytic current density(Figure S8 a-c),


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Figure 2. TEM and HRTEM images of Ni0.67Mn0.33 (a, d), Ni0.75Mn0.25 (b, e) and Ni0.83Mn0.17 (c, f); Nitrogen adsorption–desorption isotherm curve for Ni0.75Mn0.25 (g); AFM image of Ni0.75Mn0.25 nanosheets (h, i). the lowest Tafel slope (overpotential (η) vs log(j), Figure S8d), the smallest charge transfer resistance (Figure S8e) and the highest TOF (Figure S8f). Such excellent performance of Ni0.75Mn0.25 is attributed to the modulation effect between Ni and Mn. From the Figure S8a, the redox peak shift with the Ni/Mn ratio, which further confirms the modulation effect. Similar tests were also carried out using samples NiO and Mn2O3 for comparison. In Figure 3a, the oxidation peak located in 1.37 V vs RHE is assigned to Ni2+ to Ni3+, which has been reported in most of Ni-based electrocatalysts.8, 26, 27 Interestingly, the Ni2+ to Ni3+ oxidation peak for Ni0.75Mn0.25 is


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Figure 3. (a) LSV curves of Ni0.75Mn0.25-LDH, Ni0.75Mn0.25, NiO, Mn2O3 and glassy carbon (inset shows the η10 of Ni0.75Mn0.25-LDH, Ni0.75Mn0.25 and NiO), (b) Tafel slopes and (c) Nyquist plots of Ni0.75Mn0.25, NiO and Mn2O3 at an applied potential of 1.65 V, (d) Chronoamperometric durability test for the Ni0.75Mn0.25 at a constant current density of ~ 10 mA cm-2 (inset shows the corresponding polarization curves before and after the stability test) , (e) Charging current density differences (∆J = Ja − Jc) plotted against scan rates, (f) Amount of experimental and theoretical O2 and H2 evolution by Ni0.75Mn0.25 at a constant oxidative current of 1 mA.


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more intense than that of NiO, implying more active sites formation on porous Ni0.75Mn0.25 nanosheets. As shown in Figure 3a, bare glassy carbon shows negligible activity, while the Mn2O3 catalyst shows very poor electrocatalytic activity. The Ni0.75Mn0.25-LDH is able to produce oxygen, but relatively low OER activity is reflected in the measured potential window. Obviously, at η of 400 mV, Ni0.75Mn0.25 can achieve a current density of 84.0 mA cm-2, which is much higher than those of NiO (14.0 mA cm-2) and Mn2O3 (1.4 mA cm-2). The BET normalized LSV curves were conducted to evaluate the intrinsic activity (Figure S9). Compared with NiO, the BET surface area increases with the decrease of Mn content among NixMn1-x (Table S3). Ni0.83Mn0.17 has the largest BET surface area among the catalysts of NixMn1-x, but it has the lowest catalytic performance. The BET surface area does not determine the catalytic performance. It can be clearly seen that after normalization with BET, the Ni0.75Mn0.25 shows the best intrinsic activity. To gain insight into the catalytic OER kinetics, the Tafel plots were analyzed based on the polarization curves. The Tafel slope of Ni0.75Mn0.25 is 91 mV/dec (Figure 3b), which is lower than those of NiO (123 mV/dec) and Mn2O3 (247 mV/dec). The lower Tafel slope of Ni0.75Mn0.25 indicates the facile electron transfer for water oxidation on the surface of nanosheets.28 To further confirm the excellent OER electrocatalytic efficiency of Ni0.75Mn0.25, electrochemical impedance spectroscopy (EIS) was performed. The diameter of the semicircle in Nyquist plot of Ni0.75Mn0.25 is very small in comparison with those of NiO and Mn2O3 (Figure 3c), suggesting Ni0.75Mn0.25 catalysts have lower charge transfer resistance (Rct), and improved conductivity. The EIS data can be fitted by an equivalent voigt circuit. The values of electrolyte resistivity (Rs) and electron transfer resistivity (Rct) are listed in Table S4, the Ni0.75Mn0.25 has smaller Rct than those of the other samples. The intrinsic activities of catalysts were further assessed by turnover


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frequency (TOF) assuming all the metal atoms to be catalytically active (Table S5). It is found that Ni0.75Mn0.25 exhibits the highest TOF of ~4.85×10-2 s-1 at η = 400 mV, which is ~ 6.3 and ~ 60 times higher than those of NiO and Mn2O3, respectively. Meanwhile, the η required to achieve the current density of 10 mA cm-2, which is a metric relevant to solar fuel synthesis.29 The η of ~297 mV for Ni0.75Mn0.25 is much smaller than those of NiO (380 mV) and Mn2O3. The OER catalytic performance of Ni0.75Mn0.25 is also compared with those of Ni-based and Mnbased electrocatalysts (Table S6). Sample Ni0.75Mn0.25 shows the best electrocatalytic activity among all the electrocatalysts listed. We investigated the OER catalytic durability of the Ni0.75Mn0.25 via a chronoamperometric method (I-t). The catalytic current density of Ni0.75Mn0.25 stabilizes at least 20 h, revealing the robust activity of Ni0.75Mn0.25 during water oxidation process (Figure 3d). The polarization curve (the inset picture) of the Ni0.75Mn0.25 catalyst after 20 h electrolysis affords a similar curve to the initial cycle. All the results disclose that Ni0.75Mn0.25 can serve as an efficient and stable catalyst to drive water oxidation. The structural stability of Ni0.75Mn0.25 is further characterized by TEM and FTIR spectra. The TEM images (Figure S10a-c) of the catalyst after the OER stability test confirm that the porous nanosheet structure is maintained. As shown in Figure S10d, an amorphous layer of metal oxyhydroxide can be observed on the surface, which is consistent with recent publications.30-32 Our TEM results reveal structural transformation occurred on the surface of oxides and metal oxhydroxide produced. As shown in Figure S11, a strong and sharp absorption peak at around 425 cm-1 is attributed to eg electron in the anti-bonding orbital.33 The FTIR band at 1384 cm-1 can be assigned to the nitrate vibrations.34 The broad band at 3438 cm-1 and weak band at 1633 cm-1 are attributed to surface absorption -OH stretching vibration and bending vibration, respectively. The FTIR bands with four absorption bands in the region 900-1250 cm-1 are assigned to the


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characteristic absorption peak of Nafion. The FTIR spectra demonstrate the structure of Ni0.75Mn0.25 is stable before and after reaction. The electrochemical active surface areas (ECSAs) of NiMn oxides were obtained from cyclic voltammetry (CV) curves with different scan rates (Figure S12 a-c). By plotting the △J against the scan rate, the double layer capacitance (Cdl, Figure S13) are achieved, and they are employed to calculate the corresponding ECSAs (see calculation details in ESI).35, 36 The ECSAs of NiMn oxides with different Mn content are listed in Table S7. Obviously, the largest ECSA is achieved when the Mn content is 0.25 in NiMn oxides. This is a clue that incorporating Mn into Ni oxides is a feasible way to modulate the ECSAs of NiMn samples. As a comparison, we also investigated ECSAs of NiO and Mn2O3 in Figure S14 and Figure 3e. From Table S5, the ECSA of Ni0.75Mn0.25 indeed is higher than that of NiO, suggesting that the doping Mn to the Ni-based oxides can lead to the increased active surface area. To further investigate the water oxidation activity and surface area. The current density averaged by ECSA was obtained (Figure S15). The specific capacitance for oxides normally taken to be 60 uF cm-2.37 In our study, the specific capacitance (Cdl) is far less than 60 uF cm-2. It should be noted that Cdl is a reliable indicator of ECSA when the catalyst has excellent electronic conductivity.38 In order to compare the electrochemical active surface area normalized activity, we divided all the samples into two groups: the first group is samples of NixMn1-x (x= 0.67, 0.75, and 0.83) and the second group is samples of NiO and Mn2O3. The ECSA-normalized current density for NixMn1-x should be more reliable. As shown in Figure S15, the Ni0.75Mn0.25 has the largest current density after normalized by ECSA. At last, the current density averaged by Ni amount (integrated from the redox peaks of nickel) was investigated. The sample of Ni0.75Mn0.25 has the largest current


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density among the samples (Figure S16). From the above results, we found evidence that the improved activity is not only due to the surface area. Faradaic efficiency test was performed to further prove that the observed current densities originate from water oxidation rather than side reactions. The generated O2 by Ni0.75Mn0.25 at a constant oxidative current of 1 mA was quantitatively measured by gas chromatography (the H2 generated from counter electrode in three-electrode system). The good agreement of the experimentally produced and theoretically calculated amount of O2 (Figure 3f) reveals the Faradaic efficiency of ~ 100%, confirming O2 is the only product of the electrode reaction. In order to understand the electronic structure of Mn doped modulation catalysts, XAS technique was explored. The energy position of the absorption edge suggests for the mean oxidation state of the probed element.39 Ni K-edge spectra of NiO and Ni0.75Mn0.25 samples are similar. Obviously, the XAFS adsorption edge of Ni0.75Mn0.25 was shifted to higher energies (Figure 4a), indicating the increased average valence state of Ni0.75Mn0.25.40 From the R space plot for Ni0.75Mn0.25 (Figure 4b), the first Ni-O shell has a distance of ~ 2.06 Å and coordination number of 6.1 (Table 1), which is the typical structure of Ni-O octahedral. However, in the NiNi shell, the coordination number of NiO is 12 and Ni0.75Mn0.25 is 9.5 (Table 1), indicating the existence of Ni vacancies (VNi) in Ni0.75Mn0.25 nanosheets.13 The average Ni-Ni distance for Ni0.75Mn0.25 ~ 2.97Å is slightly larger than that of NiO (~ 2.95 Å) (Table 1). The larger DebyeWaller parameters (σ2) are found in Ni0.75Mn0.25 structure, indicating that the Ni centers in the Ni0.75Mn0.25 appear structural distortion. Likewise, we studied the surface atomic structure of Mn in the porous Ni0.75Mn0.25 nanosheets by XANES (Figure 4c). It is obvious that the Mn’s binding energy is less than that of MnO2, indicating that the average manganese oxidation state in Ni0.75Mn0.25 nanosheets is lower than +4.


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Figure 4. (a) Ni K-edge XANES spectra (inset shows the magnification in panel for easy viewing); (b) Ni K-edge Fourier-transformed EXAFS spectra corresponds to a specific structural motif that is schematically depicted; (c) Mn K-edge XANES spectra; (d) Mn K-edge Fouriertransformed EXAFS spectra; (e) XPS spectra of Ni 2p; (f) XPS spectra of O 1s.


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Table 1. Parameters obtained by simulation of EXAFS spectra. Catalyst

Shell

N

R(Å)

σ2(Å)2

Ni-O

6.1

2.06

0.008

Ni-Ni

9.5

2.97

0.008

Ni-O

6.0

2.08

0.006

Ni-Ni

12.0

2.95

0.006

Mn-O

4.3

1.91

0.005

Mn-Mn

5.0

2.98

0.006

Mn-O

6.0

1.90

0.004

Mn-Mn

4.0

2.84

0.008

Ni0.75Mn0.25

NiO

Ni0.75Mn0.25

MnO2

N = coordination number; R = absorber-backscatter distance; σ = Debye–Waller parameter.

From the Fourier transform (FT) of the EXAFS spectrum (Figure 4d), it is determined that the first Mn-O shell has a prevalence distance of ∼1.91 Å (Table 1), which is assigned to the MnO6. In the Mn-O shell, the coordination number for Ni0.75Mn0.25 is only 4.3 compared to 6.0 for MnO2, suggesting the presence of oxygen vacancies (Vo).41 The second prominent FT peak (Figure 4d) corresponds to the Mn-Mn distance (di-µ-oxo bridged). The average Mn-Mn distance of 2.98 Å for Ni0.75Mn0.25 is longer than that of the MnO2 (2.84 Å), implying that Mn centers in Ni0.75Mn0.25 emerge more structural distortion. The longer Mn-Mn distance in the sample indicates the Jahn-Teller effect characteristic of high-spin Mn(Ⅲ) ion.42 The XANES and EXAFS spectra of NiMn oxides with different Mn content were further examined in details, as shown in Figure S17 and Table S8. The oxygen and nickel vacancies contribute to structural flexibility that is important for catalytic process in water oxidation on the surface.


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The presence of Ni vacancies and oxygen vacancies in Ni0.75Mn0.25 are further confirmed by X-ray photoelectron spectroscopy (XPS). For Ni0.75Mn0.25 nanosheets, the intense peaks for Ni 2p3/2 are located at 854.4 and 856.1 eV (Figure 4e), suggesting the existence of Ni2+ and Ni3+.43 The Ni 2p1/2 region shows two peaks at 871.9 and 873.8 eV, corresponding to the binding energies of Ni2+ and Ni3+, respectively.13 The high resolution XPS spectrum of O 1s is displayed in Figure 4f. The peaks at 531.0 eV is assigned to the Ni-O,44 whereas the peak at 532.3 eV is attributed to oxygen vacancies,45 and the peak at 533.2 eV is associated with surface adsorbed water molecules.46 Figure S18 shows Mn 2p XPS spectra, a major peak at binding energy of 642.0 eV is indexed to the Mn3+,47-49 which matches with the XANES results. The peak at 645.5 eV and a satellite peak around 653.8 eV are assigned to the Mn4+ species.50 Additionally, a shoulder peak at 638 eV is in line with previous report.51 The Ni3+ and Mn3+ in Ni0.75Mn0.25 nanosheets should be resulted from nickel vacancies and oxygen vacancies, respectively. The XPS spectra of the Ni0.75Mn0.25 after the OER stability test confirm that all the original valence of Ni, Mn and O are kept (Figure S19). However, the percentage of Ni3+ (Figure S19a) after reaction (41%) is larger than that of before reaction (36%), revealing that some Ni2+ converts to Ni3+. The percentage of Mn3+ after reaction is 39.5% (Figure S19b), which is higher than that of before reaction (36.6%). The above results combined TEM images of Ni0.75Mn0.25 after OER test indicate that the formation of an amorphous layer of metal oxyhydroxide on the surface. For the O 1s (Figure S19c), the peak at 535.9 eV after reaction is assigned to the O=C-OH,52 which may be resulted from Nafion that was used to fixed the Ni0.75Mn0.25 sample on the surface of the electrode. Electron paramagnetic resonance (EPR) was also used to probe the valence state of Ni in Ni0.75Mn0.25. The EPR signals with g=2.172 is attributed to Ni3+ species (Figure S20a).40


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Unfortunately, we did not find the Mn3+ signals from Figure S20a. The Mn3+ contains even number of unpaired electrons, which makes its EPR signal very difficult to be detected.53, 54 Here, we used VersaLab to detect the presence of Mn+3. Figure S20b shows the inverse susceptibility (1/χ) plot of Ni0.75Mn0.25. From 250 to 300 K, inverse susceptibility is fitted by Curie-Weiss law. The effective magnetic moments (µeff) reflect the numbers of unpaired electrons in sample Ni0.75Mn0.25. From the fitted curves, the C is 1.834 and the µeff = 3.83µ‫ܤ‬. We also used SLS, SHS and VHS to calculate the µeff (see calculation details in ESI). We calculated µeff is 2.57µ‫ܤ‬. The values of µeff are calculated according to two different methods and the difference of 1.26µ‫ܤ‬, which reveals that the electrons located at eg is larger than 1. From a known relationship that E0 (maximum inflection point of the absorption edge) scales with oxidation state (Figure S21), we conclude that the Ni electronic configuration is t2g6eg1.6 and Mn is t2g3eg0.5 for Ni0.75Mn0.25, respectively. As previously mentioned, the occupancy of eg orbital in 3d transition-metal-based electrocatalysts is crucial for water oxidation.4 Electrocatalysts with too low or too high eg occupation (0 or 2) are not desirable, which will result in poor water oxidation performance. When the electronic configuration of eg is 1.2, the catalyst can effectively adsorb the reactant molecules, and afford the best performance of electrocatalytic oxygen evolution.4 Compared with a t2g6eg2 of Ni2+ in NiO, Ni3+ active sites in Ni0.75Mn0.25 possess a near-unity occupancy of the eg orbital t2g6eg1, which is superior for efficient OER performance.55 Moreover, Mn3+ (t2g3eg1) is also an important active site feature in water oxidation catalysis.14 We suppose that MnIII−O bonds in edge sharing octahedral at the surface are more catalytically effective due to these weaker, more flexible bonds. In Dau’s work,56 they concluded that a higher fraction of longer


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Mn−Mn bridge is indicative of a more disordered structure, which longer bond is assumed as necessary condition for water oxidation.57, 58 To further understand the advantages of Ni0.75Mn0.25 nanosheets, we applied density functional theory (DFT) to calculate the free energy based on the following mechanism for water oxidation: H2O + *→*OH + H+ + e-

(1)

*OH→*O + H+ + e-

(2)

H2O + *O → *OOH + H+ + e-

(3)

*OOH →O2 + * + H+ + e-

(4)

Figure 5. DFT calculation. (a) NiO-OH, (b) NiO-O, (c) NiO-OOH, (d) Ni0.75Mn0.25-OH, (e) Ni0.75Mn0.25O-O, (f) Ni0.75Mn0.25O-OH, and (g) DFT-calculated adsorption energies of the intermediates, H2O (l), *OH, *O and *OOH on the surfaces of NiO and Ni0.75Mn0.25.


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In the equations, the symbol “*” present the adsorption site. The adsorbed free energy for OER intermediates (including *OH, *O, and *OOH) are calculated. The overpotential η defined in equation is as follows: η = G (*OOH)- 1.23 V The OH- prefers to be adsorbed on the Ni-Mn bridge sites for Ni0.75Mn0.25, which acts synergistically to promote the forming O-O band. Similarly, the OH- absorbs on the Ni-O bridge sites for NiO species. As shown in the free-energy diagram in Figure 5, the rate-limiting step for NiO and Ni0.75Mn0.25 is the formation of *OOH, which requires overpotential of 0.86 and 0.42 eV, respectively. Clearly, the catalytic activity of OER is enhanced since the Mn is doped into NiO. In summary, we demonstrate an ingenious design of Mn-modulated nickel oxide nanosheets for highly efficient water oxidation. The significantly improved catalytic performance of Ni0.75Mn0.25 is mainly benefit from tuning the surface electronic states and forming defective sites. In addition, the formation nickel manganese oxide nanosheets stabilize the intrinsic Ni3+ (t2g6eg1) and Mn3+ (t2g3eg1) species. The high intrinsic catalytic activity of Ni0.75Mn0.25 nanosheets is mainly due to the intrinsic Ni3+ and Mn3+ sites, which act synergistically to promote the forming O-O bond and facilitate charge-transfer. DFT calculations further confirm that the Ni and Mn in catalyst Ni0.75Mn0.25 act synergistically to promote the forming O-O bond. This work opens an avenue to understand the active site of porous nickel oxide nanosheets by manganese modulation and rational design of highly efficient electrocatalysts with precisely engineered structures and electrical properties. ASSOCIATED CONTENT Supporting Information.


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Experimental details, materials characterization, and supporting data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Yong Ding: 0000-0002-5329-8088. Author Contributions ⊥

T.T. and H.G. contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grants Nos. 21773096 and 21707059), Fundamental Research Funds for the Central Universities (lzujbky-2018-k08) and the Natural Science Foundation of Gansu (17JR5RA186) and the Open Funds of the State Key Laboratory of Rare Earth Resource Utilization (RERU2017001). A portion of this work was performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS. We thank Anhui Key Laboratory of Condensed Matter Physics at Extreme Conditions, High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031, China for EPR measurements of Ni0.75Mn0.25. We also thank Professor Chunxi Zhang for the analysis of EPR and Professor Daqiang Gao for the analysis of Versalab.


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REFERENCES (1) Chow, J.; Kopp, R. J.; Portney, P. R. Energy Resources and Global Development. Science 2003, 302, 1528-1531. (2) Zheng, M.; Ding, Y.; Yu, L.; Du, X.; Zhao, Y. In Situ Grown Pristine Cobalt Sulfide as Bifunctional Photocatalyst for Hydrogen and Oxygen Evolution. Adv. Funct. Mater. 2017, 27, 1605846-1605857. (3) Huang, J.; Zhang, Y.; Ding, Y. Rationally Designed/Constructed CoOx/WO3 Anode for Efficient Photoelectrochemical Water Oxidation. ACS Catal. 2017, 7, 1841-1845. (4) Jin, S.; Kevin, J. M.; Hubert, A. G.; John, B. G.; Yang, S.-H. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383-1385. (5) Tahir, M.; Pan, L.; Zhang, R.; Wang, Y.-C.; Shen, G.; Aslam, I.; Qadeer, M. A.; Mahmood, N.; Xu, W.; Wang, L. et al. High-Valence-State NiO/Co3O4 Nanoparticles on Nitrogen-Doped Carbon for Oxygen Evolution at Low Overpotential. ACS Energy Lett. 2017, 2, 2177-2182. (6) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399-404. (7) Faber, M. S.; Jin, S. Earth-abundant Inorganic Electrocatalysts and Their Nanostructures for Energy Conversion Applications. Energy Environ. Sci. 2014, 7, 3519-3542. (8) Xu, K.; Chen, P.; Li, X.; Tong, Y.; Ding, H.; Wu, X.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y. Metallic Nickel Nitride Nanosheets Realizing enhanced Electrochemical Water Oxidation. J. Am. Chem. Soc. 2015, 137, 4119-4125. (9) Long, X.; Lin, H.; Zhou, D.; An, Y.; Yang, S. Enhancing Full Water-Splitting Performance of Transition Metal Bifunctional Electrocatalysts in Alkaline Solutions by Tailoring CeO2–Transition Metal Oxides–Ni Nanointerfaces. ACS Energy Lett. 2018, 3, 290-296. (10) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M(Ni,Co,Fe,Mn) Hydr(oxy)oxide Catalysts. Nature Mater. 2012, 11, 550-557. (11) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y. L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nat. Commun. 2013, 4, 2439. (12) Maiyalagan, T.; Jarvis, K. A.; Therese, S.; Ferreira, P. J.; Manthiram, A. Spinel-type Lithium Cobalt Oxide as a Bifunctional Electrocatalyst for the Oxygen Evolution and Oxygen Reduction Reactions. Nat. Commun. 2014, 5, 3949-3947. (13) Zhao, Y.; Jia, X.; Chen, G.; Shang, L.; Waterhouse, G. I.; Wu, L. Z.; Tung, C. H.; O'Hare, D.; Zhang, T. R. Ultrafine NiO Nanosheets Stabilized by TiO2 from Monolayer NiTiLDH Precursors: An Active Water Oxidation Electrocatalyst. J. Am. Chem. Soc. 2016, 138, 6517-6524. (14) Kim, J.; Yin, X.; Tsao, K. C.; Fang, S.; Yang, H. Ca2Mn2O5 as Oxygen-deficient Perovskite Electrocatalyst for Oxygen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 1464614649.


20

Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(15) Takashima, T.; Hashimoto, K.; Nakamura, R. Inhibition of Charge Disproportionation of MnO2 Electrocatalysts for Efficient Water Oxidation Under Neutral Conditions. J. Am. Chem. Soc. 2012, 134, 18153-18156. (16) Jia, Y.; Zhang, L.; Gao, G.; Chen, H.; Wang, B.; Zhou, J.; Soo, M. T.; Hong, M.; Yan, X.; Qian, G.; et al. A Heterostructure Coupling of Exfoliated Ni-Fe Hydroxide Nanosheet and Defective Graphene as a Bifunctional Electrocatalyst for Overall Water Splitting. Adv. Mater. 2017, 29, 1700017-1700025. (17) Guo, Y.; Tong, Y.; Chen, P.; Xu, K.; Zhao, J.; Lin, Y.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y. Engineering the Electronic State of a Perovskite Electrocatalyst for Synergistically Enhanced Oxygen Evolution Reaction. Adv. Mater. 2015, 27, 5989-5994. (18) Ling, T.; Yan, D. Y.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X.; Mao, J.; Du, X. W.; Hu, Z.; Jaroniec, M.; et al. Engineering Surface Atomic Structure of Single-crystal Cobalt (II) Oxide Nanorods for Superior Electrocatalysis. Nat. Commun. 2016, 7, 12876-12884. (19) Sun, Y.; Gao, S.; Lei, F.; Xie, Y. Atomically-thin Two-dimensional Sheets for Understanding Active Sites in Catalysis. Chem. Soc. Rev. 2015, 44, 623-636. (20) Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L. Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2016, 55, 5277-5281. (21) Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J. Rapid Room-temperature Synthesis of Nanocrystalline Spinels as Oxygen Reduction and Evolution Electrocatalysts. Nat Chem. 2011, 3, 79-84. (22) Wang, Y.; Zhang, Y.; Liu, Z.; Xie, C.; Feng, S.; Liu, D.; Shao, M.; Wang, S. Layered Double Hydroxide Nanosheets with Multiple Vacancies Obtained by Dry Exfoliation as Highly Efficient Oxygen Evolution Electrocatalysts. Angew. Chem. Int. Ed. 2017, 56, 5867-5871. (23) Liu, Y.; Cheng, H.; Lyu, M.; Fan, S.; Liu, Q.; Zhang, W.; Zhi, Y.; Wang, C.; Xiao, C.; Wei, S.; et al. Low Overpotential in Vacancy-rich Ultrathin CoSe2 Nanosheets for Water Oxidation. J. Am. Chem. Soc. 2014, 136, 15670-15675. (24) Zhao, A.; Masa, J.; Xia, W.; Maljusch, A.; Willinger, M. G.; Clavel, G.; Xie, K.; Schlogl, R.; Schuhmann, W.; Muhler, M. Spinel Mn-Co Oxide in N-doped Carbon Nanotubes as a Bifunctional Electrocatalyst Synthesized by Oxidative Cutting. J. Am. Chem. Soc. 2014, 136, 7551-7554. (25) Tang, T.; Jiang, W. J.; Niu, S.; Liu, N.; Luo, H.; Chen, Y. Y.; Jin, S. F.; Gao, F.; Wan, L. J.; Hu, J. S. Electronic and Morphological Dual Modulation of Cobalt Carbonate Hydroxides by Mn Doping toward Highly Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. J. Am. Chem. Soc. 2017, 139, 8320-8328. (26) Smith, R. D.; Prevot, M. S.; Fagan, R. D.; Trudel, S.; Berlinguette, C. P. Water Oxidation Catalysis: Electrocatalytic Response to Metal Stoichiometry in Amorphous Metal Oxide Films Containing Iron, Cobalt, and Nickel. J. Am. Chem. Soc. 2013, 135, 11580-11586. (27) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel-iron Oxyhydroxide Oxygen-evolution Electrocatalysts: the Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136, 6744-6753. (28) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis. ChemCatChem. 2010, 2, 724-761. (29) Gorlin, Y.; Jaramillo, T. F. A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132, 13612-13614.


21


Page 22 of 23

(30) Hung, S.-F.; Hsu, Y.-Y.; Chang, C.-J.; Hsu, C.-S.; Suen, N.-T.; Chan, T.-S.; Chen, H. Unraveling Geometrical Site Confinement in Highly Efficient Iron-Doped Electrocatalysts toward Oxygen Evolution Reaction. Adv. Energy Mater. 2018, 8, 1701686. (31) Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Araújo, J. F.; Reier, T.; Dau, H.; Strasser, P. Reversible Amorphization and the Catalytically Active State of Crystalline Co3O4 During Oxygen Evolution. Nat. Commun. 2015, 6, 8625. (32) Suen, N. T.; Hung, S. F.; Quan, Q.; Zhang, N.; Xu, Y. J.; Chen, H. M. Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (33) Shetkar, R. G.; Salker, A. V. Solid State and Catalytic CO Oxidation Studies on Zn1−xNixMnO3 System. Mater. Chem. Phys. 2008, 108, 435-439. (34) Hadj-Sadok Ouaguenouni, M.; Benadda, A.; Kiennemann, A.; Barama, A. Preparation and Catalytic Activity of Nickel–manganese Oxide Catalysts in the Reaction of Partial Oxidation of Methane. Comptes Rendus Chimie 2009, 12, 740-747. (35) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977-16987. (36) Fan, K.; Chen, H.; Ji, Y.; Huang, H.; Claesson, P. M.; Daniel, Q.; Philippe, B.; Rensmo, H.; Li, F.; Luo, Y.; et al. Nickel-vanadium Monolayer Double Hydroxide for Efficient Electrochemical Water Oxidation. Nat. Commun. 2016, 7, 11981. (37) Trasatti, S.; Petrii, O. Real Surface Area Measurements in Electrochemistry. Pure Appl. Chem. 1991, 63, 711-734. (38) Batchellor, A. S.; Boettcher, S. W. Pulse-Electrodeposited Ni–Fe (Oxy)hydroxide Oxygen Evolution Electrocatalysts with High Geometric and Intrinsic Activities at Large Mass Loadings. ACS Catal. 2015, 5, 6680-6689. (39) Wiechen, M.; Zaharieva, I.; Dau, H.; Kurz, P. Layered Manganese Oxides for WaterOxidation: Alkaline Earth Cations Influence Catalytic Activity in a Photosystem II-like Fashion. Chem. Sci. 2012, 3, 2330-2339. (40) He, Q.; Wan, Y.; Jiang, H.; Pan, Z.; Wu, C.; Wang, M.; Wu, X.; Ye, B.; Ajayan, P. M.; Song, L. Nickel Vacancies Boost Reconstruction in Nickel Hydroxide Electrocatalyst. ACS Energy Lett. 2018, 3, 1373-1380. (41) Zaharieva, I.; Chernev, P.; Risch, M.; Klingan, K.; Kohlhoff, M.; Fischer, A.; Dau, H. Electrosynthesis, Functional, and Structural Characterization of a Water-oxidizing Manganese Oxide. Energy Environ. Sci. 2012, 5, 7081-7089. (42) Mantel, C.; Hassan, A. K.; caut, J. P.; Deronzier, A.; Collomb, M.-N. l.; Duboc-Toia, C. A High-Frequency and High-Field EPR Study of New Azide and Fluoride Mononuclear Mn(III) Complexes. J. Am. Chem. Soc. 2003, 125, 12337-12344. (43) Ding, Y.; Wang, Y.; Su, L.; Zhang, H.; Lei, Y. Preparation and Characterization of NiO– Ag Nanofibers, NiO Nanofibers, and Porous Ag: Towards the Development of a Highly Sensitive and Selective Non-enzymatic Glucose Sensor. J. Mater. Chem. A 2010, 20, 9918-9926. (44) Vasilyeva, I. G.; Asanov, I. P.; Kulikov, L. M. Experiments and Consideration about Surface Nonstoichiometry of Few-Layer MoS2 Prepared by Chemical Vapor Deposition. J. Phys. Chem. C 2015, 119, 23259-23267. (45) Xu, W.; Lyu, F.; Bai, Y.; Gao, A.; Feng, J.; Cai, Z.; Yin, Y. Porous Cobalt Oxide Nanoplates Enriched with Oxygen Vacancies for Oxygen Evolution Reaction. Nano Energy 2018, 43, 110-116.


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(46) Wang, H.; Qing, C.; Guo, J.; Aref, A. A.; Sun, D.; Wang, B.; Tang, Y. Highly Conductive Carbon–CoO Hybrid Nanostructure Arrays with Enhanced Electrochemical Performance for Asymmetric Supercapacitors. J. Mater. Chem. A 2014, 2, 11776-11783. (47) Zhang, L.; Shi, L.; Huang, L.; Zhang, J.; Gao, R.; Zhang, D. Rational Design of HighPerformance DeNOx Catalysts Based on MnxCo3–xO4 Nanocages Derived from Metal–Organic Frameworks. ACS Catal. 2014, 4, 1753-1763. (48) Chen, Z.; Yang, Q.; Li, H.; Li, X.; Wang, L.; Tsang, Chi. S. Cr–MnOx Mixed-oxide Catalysts for Selective Catalytic Reduction of NOx with NH3 at Low Temperature. J. Catal. 2010, 276, 56-65. (49) Zhang, D.; Zhang, L.; Shi, L.; Fang, C.; Li, H.; Gao, R.; Huang, L.; Zhang, J. In Situ Supported MnO(x)-CeO(x) on Carbon Nanotubes for the Low-temperature Selective Catalytic Reduction of NO with NH3. Nanoscale 2013, 5, 1127-1136. (50) Yu, Q.; Luo, R.; Bai, X.; Yang, W.; Lu, Y.; Hou, X.; Peng, T.; Liu, X.; Kim, J.-K.; Luo, Y. Rational Design of Double-confined Mn2O3/S@Al2O3 Nanocube Cathodes for Lithium-sulfur Batteries. J. Solid State Electrochem. 2018, 22, 849-858. (51) Zhao, J.; Chen, J.; Xu, S.; Shao, M.; Zhang, Q.; Wei, F.; Ma, J.; Wei, M.; Evans, D. G.; Duan, X. Hierarchical NiMn Layered Double Hydroxide/Carbon Nanotubes Architecture with Superb Energy Density for Flexible Supercapacitors. Adv. Funct. Mater. 2014, 24, 2938-2946. (52) Xu, Q.; Pu, P.; Zhao, J.; Dong, C.; Gao, C.; Chen, Y.; Chen, J.; Liu, Y.; Zhou, H. Preparation of Highly Photoluminescent Sulfur-doped Carbon Dots for Fe(III) Detection J. Mater. Chem. A 2015, 3, 542-546. (53) Campbell, K. A.; Lashley, M. R.; Wyatt, J. K.; Nantz, M. H.; Britt, R. D. Dual-Mode EPR Study of Mn(III) Salen and the Mn(III) Salen-Catalyzed Epoxidation of cis-βMethylstyrene. J. Am. Chem. Soc. 2001, 123, 5710-5719. (54) Huang, D.; Zhang, X.; Ma, C.; Chen, H.; Chen, C.; Liu, Q.; Zhang, C.; Liao, D.; Li, L. Aggregation of Tetranuclear Mn Building Blocks with Alkali Ion: Syntheses, Crystal Structures and Chemical Behaviors of Monomer, Dimer and Polymer Containing [Mn4(U3-O)2]8+ Units. Dalton Trans. 2007, 0, 680-688. (55) Liu, Y.; Xiao, C.; Lyu, M.; Lin, Y.; Cai, W.; Huang, P.; Tong, W.; Zou, Y.; Xie, Y. Ultrathin Co3S4 Nanosheets that Synergistically Engineer Spin States and Exposed Polyhedra that Promote Water Oxidation under Neutral Conditions. Angew. Chem. Int. Ed. 2015, 54, 11383 -11387. (56) Zaharieva, I.; Gonzalez-Flores, D.; Asfari, B.; Pasquini, C.; Mohammadi, M. R.; Klingan, K.; Zizak, I.; Loos, S.; Chernev, P.; Dau, H. Water Oxidation Catalysis-role of Redox and Structural Dynamics in Biological Photosynthesis and Inorganic Manganese Oxides. Energy Environ. Sci. 2016, 9, 2433-2443. (57) Robinson, D. M.; Go, Y. B.; Mui, M.; Gardner, G.; Zhang, Z.; Mastrogiovanni, D.; Garfunkel, E.; Li, J.; Greenblatt, M.; Dismukes, G. C. Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis. J. Am. Chem. Soc. 2013, 135, 3494-3501. (58) Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F. In Situ X-ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525-8534.


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Study the Active Sites in Porous Nickel Oxide Nanosheets by

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