Black Phosphorus Modified Co3O4 through Tuning Electronic

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Black Phosphorus Modified Co3O4 through Tuning Electronic Structure for Enhanced Oxygen Evolution Reaction Fangbing Shi, Keke Huang, Ying Wang, Wei Zhang, Liping Li, Xiyang Wang, and Shouhua Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04078 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Applied Materials & Interfaces

Black Phosphorus Modified Co3O4 through Tuning Electronic Structure for Enhanced Oxygen Evolution Reaction Fangbing Shi,† Keke Huang,† Ying Wang,‡ Wei Zhang,§ Liping Li,† Xiyang Wang† and Shouhua Feng*, †

†State

Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, Changchun 130012, People’s Republic of China ‡State

Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of

Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China §Electron

Microscopy Center, and School of Materials Science & Engineering, Jilin

University, Changchun 130012, People’s Republic of China KEYWORDS: engineering electronic structure, hybrids, black phosphorus, Co3O4, oxygen evolution reaction

ABSTRACT: Spinel Co3O4, consisting of two mixed valence states: Co2+ and Co3+, has attracted enormous interests as a promising electrocatalyst for oxygen evolution reaction (OER). Proper control the relative proportion of Co2+/Co3+ in cobalt oxide can greatly tune the electronic structure and further optimize its catalytic performance. Herein, a hybrid coupling Co3O4 with black phosphorus

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(Co3O4@BP) is designed as an efficient catalyst for OER. Electrons migration from BP to Co3O4 is achieved in Co3O4@BP, owing to the higher Fermi level of BP than that of Co3O4. Efficient electron transfer can not only create massive active sites with abundant Co2+, but also remarkably suppress the deterioration of BP. Particularly, Co3O4@BP catalyst outperforms the pristine Co3O4 by over 4 times and is even 20 times higher than that of bare BP at a potential of 1.65 V vs RHE. Our finding provides insightful understanding for electronic engineering in Co3O4@BP by balancing advantages and utilizing drawbacks of Co3O4 and BP. 1. INTRODUCTION Oxygen evolution reaction (OER), as a half-cell reaction of water splitting, provides a clean and sustainable source for energy conversion and storage.1-3 However, OER is kinetically sluggish because this four electron-proton coupled process requires a high overpotential to overcome the kinetic barrier.4, 5 For this purpose, numerous efforts have been made to improve electrode kinetics and a variety of electrocatalysts have been designed to potentially replace those preciousmetal oxides (i.e. IrO2 and RuO2).6-10 Co3O4, a promising material for OER, has been widely explored as OER electrocatalyst due to its relatively low-cost and earth-abundant features.1,

8, 11, 12

Previous study has demonstrated that in Co3O4

system, Co2+ and Co3+ hold different responsibilities for OER performance. Co2+ serves as the essential active site dominating the OER activity; whereas Co3+ tends to bond with hydroxy groups, which restricts the catalytic activity of the whole Co3O4.13 Thus, proper control the relative proportion of Co2+/Co3+ ratio in spinel cobalt oxide has been identified as a promising strategy to engineer its electronic structure and thus optimize catalytic performance.8,

14

The relative ratio of

Co2+/Co3+ in cobalt oxide could be tuned by treating pristine Co3O4 with plasma-


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engraved,8 NaBH4 reduced,14 or a third element doped.15 However, oxygen vacancies (VO) will be formed through the above process, and the stability of VO formed remains an open question, especially under the high oxidation conditions during OER.16 Moreover, much attention has been paid to pursue the synergistic effect of Co3O4-based catalysts, by coupling Co3O4 with Au, Pt and N-doped graphene,11,

17, 18

whereas Co3O4-based hybrids with more Co2+ to modify the

electronic structure of catalysts is rarely reported. Because it is still highly challenging to select a suitable catalyst which can help tune the relative ratio of Co2+/Co3+ and subsequently enhance the electrocatalytic performance. Black phosphorus (BP), as an up-rising member in two-dimensional (2D) materials, has attracted tremendous interests since the first report in 2014.19-21 Owing to its tunable direct band gap, unique anisotropic properties, strong optical absorption and high carrier mobility, BP has been extensively studied for optical and electronic applications.22-26 Recent investigations demonstrate that layered BP is a promising candidate for water electrolysis to produce oxygen and hydrogen.2730

Nevertheless, further improving the catalytic performance of few-layered BP

nanosheets (BP NSs) for oxygen evolution reactions (OER), especially long-term catalysis, is severely restricted by its degradation due to the presence of active lone-pairs exposed at the surface.31-33 Recent years have witnessed a flourish of interest in effectively passivating the exposed lone pairs.34,

35

Conversely, less

emphasis has been placed on utilizing the active lone-pairs of BP to engineer its electronic structure. It is still a big challenge to elaborate hybrids electrocatalysts not only possessing electronic structure innovation and abundant active sites, but also satisfying complementary advantages of two catalysts. With these key concepts of electronic structure modulation, active sites and kinetic mechanism in mind, selecting the BP NSs and spinel cobalt oxide as a ACS Paragon Plus Environment

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model system will be useful for balancing advantages and utilizing drawbacks for OER. Herein, we present a facile method to synthesize a well-designed hybrid of Co3O4 nanoparticles (NPs) and BP NSs (denoted as Co3O4@BP), which is firstly used as a stable and efficient electrocatalyst to produce oxygen. Electronic structure tuning in Co3O4@BP can be achieved by meeting the complementary advantages of Co3O4 and BP. Due to the relatively higher Fermi level of BP than that of Co3O4, a steady and efficient flow of electron will be transferred from BP to Co3O4. Consequently, it can effectively suppress the deterioration of BP NSs without sacrificing the main performance of OER. On the other hand, partial Co3+ ions is reduced to Co2+ with abundant electrons transferred from BP to Co3O4, which gives birth to more active sites for OER, and accordingly enhances the electrochemical activity. 2. EXPERIMENTAL SECTION Synthesis of Co3O4@BP hybrids. Co3O4@BP hybrid were synthesized by adding 10.0 mg of Co3O4 into 60 mL of BP nanosheets N-methyl-2-pyrrolidone (NMP) solvent. Afterward, the mixture was treated by ultrasonic wave for 30 minutes and then kept stirring for 24 h. The products were washed with ethanol thoroughly, and collected by high-speed centrifugation, resulting in Co3O4@BP. For preparing Co3O4@BP with different ratio of BP/Co3O4, the exfoliated BP nanosheets (~10 mg) was re-dispersed in 60 ml NMP solvent to serve as processed material, and then 40, 20, and 10 mg of Co3O4 were added. Besides, the exfoliated BP nanosheets (~20 and 40 mg) was re-dispersed in 60 ml NMP solvent to serve as processed material, and then 10 mg of Co3O4 were added. Samples prepared as such are referred to as 4-1, 2-1 1-1, 1-2 and 1-4, and the different ratio of Co/P in Co3O4@BP was estimated by the Inductively Coupled Plasma-Atomic Emission


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Spectrometry (ICP-AES) analysis, and the Co3O4@BP presented in this work mainly suggests the sample (1-1) prepared with the ratio of Co/P by 1:0.67. Electrochemical Tests. A typical three-electrode setup was performed on 660E electrochemical

workstation

(CHI,

Inc.,

Shanghai)

to

characterize

the

electrochemical water oxidation properties. Saturated Hg (Ⅰ) | Hg2Cl2 and Pt electrode were respectively used as the reference and the counter electrode. The electrocatalytic activities towards OER were examined in 1.0 M KOH solution by obtaining polarization curves with a scan rate of 5 mV s-1, and all tests were performed at room temperature. The working electrode was prepared by adding 5 mg as-prepared samples and 80 uL Nafion solution (5 wt %) to 0.5 mL isopropanol and sonicated for 0.5 h. Afterward, the slurry (15 uL) was deposited onto a glassy carbon electrode (GCE, 5 mm in diameter). The stability tests of Co3O4@BP was performed by potential cycling at a sweep rate of 100 mV s-1 from 0.2 to 0.3 V vs RHE for 1000 cycles in 1.0 M KOH. All the potentials demonstrated in this work were transformed to reversible hydrogen electrode (RHE) and iR compensation was applied for the linear sweep voltammetry. Turnover Frequency (TOF). The TOF value is evaluated by the following equation:36 TOF= (j×A)/(4×F×m) where j (A cm−2) is on behalf of the current density at E= 1.65 V; A represents the geometric surface area of the electrode (cm−2); 4 indicates the passage of electrons of the OER via a four electron-transfer reaction of 4OH-→2H2O+O2+4e-; F stands for the Faradaic constant (96485 C mol−1); m symbolizes the moles of the active materials that are loaded onto the GCE. 3. RESULTS AND DISCUSSION


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Typically, Co3O4@BP was synthesized by sonicating the ultrathin BP NSs and Co3O4 in N-methyl-2-pyrrolidone (NMP), followed by stirring overnight. The crystal structures of the Co3O4 NPs, BP crystals and Co3O4@BP are characterized by using powder X-ray diffraction (XRD). Figure S1 shows that the diffraction peaks of Co3O4@BP are well indexed to cubic Co3O4 (JCPDS no. 65–3103) and orthorhombic BP (JCPDS no. 65–2491). In contrast with the XRD pattern acquired from the BP crystals, the sharp diffraction peak of BP at 34.6° indexed to (040) plane remains the strongest peak, which reveals that the layered nature of the BP crystals is well maintained in [email protected] Moreover, the successful inheritance of the typical lamellar morphology is also evidenced by scanning electron microscopy (SEM) images (Figure S2). The detailed morphological and structure of the BP NSs and Co3O4@BP were analyzed by using transmission electron microscopy (TEM), high-resolution TEM (HR-TEM) and selected area electron diffraction (SAED). As shown in Figure 1a, the exfoliated BP NSs exhibit a smooth lamellar morphology. From highresolution TEM (HR-TEM), the lattice fringes with d-spacing of 0.26 nm are observed (Figure 1d), which can be attributed to (040) plane of BP. The corresponding SAED pattern (inset in Figure 1d) consists of discrete spots. After the Co3O4 NPs were deposited on the surface of BP, plenty of tiny nanoparticles are uniformly dispersed on the surface of the BP NSs (Figure 1b, c). The HR-TEM image (Figure 1e) clearly displays the different lattice fringes with d-spacing of 0.29, 0.24 and 0.26 nm, in accordance with the (220) and (311) of Co3O4 and (040) of BP, respectively. SAED pattern (Figure 1f) indicates that the main spots match well with the structures of both cubic Co3O4 and orthorhombic BP. Furthermore, energy dispersive X-ray (EDX) elemental mapping (Figure 1g) further confirms


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the hybridization of BP and Co3O4, which is consistent with the aforementioned TEM, HR-TEM and SAED results.

Figure 1. TEM and HRTEM images of BP nanosheets (a, d) and Co3O4@BP (b, c and e). SAED patterns of BP nanosheets (inset in d) and Co3O4@BP (f). STEM-EDS elemental mapping images (g) of Co3O4@BP.


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To achieve fast interfacial electron transfer between BP and Co3O4, the formation of intimate interaction is crucial. However, considering that Co3O4 NPs are dispersed on BP NSs by facile ultrasonic treatment, it is reasonable to ask whether there is a strong interaction in Co3O4@BP. To address this confusing issue, a control experiment was conducted by an ultrasonic process of the pure Co3O4 in NMP as a comparative study. Figure 2a shows the Raman spectra of the ultrasonicated Co3O4, pure Co3O4, Co3O4@BP, and BP. Four peaks at 468, 510, 606, and 670 cm-1 are assigned to the Eg, F12g F22g, and A1g modes of Co3O4.38 The A1g mode of Co3O4 is attributed to the octahedral site (Co3+) because of the Co-O mode, and the Eg stretching is assigned to the characteristic vibration of tetrahedral site (Co2+) due to the O-Co-O bending.39 The relative vibration intensity of the CoO bending (Co3+)/the O-Co-O stretching (Co2+) is calculated for ultrasonicated Co3O4, pure Co3O4, Co3O4@BP. As can be seen in the Raman spectrum with coordinate values (Figure S3), the position of characteristic vibration peaks and the A1g/Eg ratio for the pure Co3O4 (2.78) and ultrasonicated Co3O4 (2.38) shows that there is negligible change for the pure Co3O4 before and after ultrasonic treatment. By contrast, the A1g/Eg ratio drops to 1.59 for Co3O4@BP. This dramatic decline suggests that a weak Co-O bending (Co3+) and a strong O-Co-O stretching (Co2+) are present in the Co3O4@BP. Besides, three peaks located at 360, 435, and 463 cm−1 are assigned to A1g, B2g and A2g modes of BP.40 For Co3O4@BP, the characteristic peaks shift to higher and lower wavenumber compared with that of pure Co3O4 and bare BP, respectively. In general, such shifts of characteristic peaks suggest an intimate interfacial interaction between different constituents of the hybrid.41 The blue and yellow iso-surfaces (Figure 2b) depict the interfacial interaction at the adjacent interface between Co3O4 and BP. Besides, the strong interaction between BP and Co3O4 in Co3O4@BP is also revealed by the O 1s XPS


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spectra (Figure 2c). The O 1s XPS spectrum of pure BP nanosheets shows the peak at 532.1 eV corresponds to P-O bond.42,

43

The peak for Co3O4 nanoparticles

located at 531.2 eV is assigned to low-coordinated oxygen defects, and that located at 529.8 eV originates from the Co–O species.8 In contrast to the O 1s spectrum of pure BP and Co3O4, a new peak at 533.4 eV is analyzed for Co3O4@BP, which could be attributed to the formation of P–O–Co coupling by the surface oxygenic functional groups of BP and Co3O4. The forming mechanism of P–O–Co coupling resembles that of C–O–Co and C–O–Ni.44,

45

This P–O–Co coupling further

indicates that the Co3O4 NPs and the BP NSs are strongly hybridized with each other.

Figure 2. (a) Raman spectra of BP crystals, Co3O4 nanoparticles and Co3O4@BP. (b) The side view at the adjacent interface of Co3O4 and BP in Co3O4@BP. (c) O 1s XPS


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spectra of BP nanosheets, Co3O4 nanoparticles and Co3O4@BP. (d) P 2p XPS spectra of BP crystal and Co3O4@BP. (e) Co 2p XPS spectra of Co3O4 nanoparticles and Co3O4@BP.

In particular, electron transfer is revealed by using XPS, which is powerful to probe electronic interactions and states of the surface elements. The surveyed spectrum (Figure S4) of Co3O4@BP indicates the presence of P, Co, O, and C elements. Three peaks at 129.81, 130.7, and 133.8 eV are presented in BP crystals (Figure 2d), respectively corresponding to P 2p3/2, P 2p1/2 as well as oxidized phosphorus (P-O).46,

47

Two main peaks of Co 2p3/2 and 2p1/2 along with two

satellites are detected for the synthesized Co3O4 (Figure 2e), which are ascribed to Co ions with different valence states (i.e. Co2+ and Co3+).8 Interestingly, when BP NPs are coupled with Co3O4, the typical peaks of P 2p exhibit a shift to higher binding energies (Figure 2d). Generally, in XPS analysis, the enhancive binding energy suggests a lessened screening effect as a result of the reduction of electron density, nevertheless the weakened binding energy indicates an enhanced electron density.48,

49

Therefore, in our case, the higher binding energy shifts of P 2p in

Co3O4@BP can be attributed to the weakened electron density of BP. This occurrence is supposed to arise from the electron transfer from BP to Co3O4 through the contact interface, thus resulting in the decrease of electron density surrounding BP. Besides, those peaks of Co 2p in Co3O4@BP also exhibit a shift to higher binding energies (Figure 2e), implying that the electron density surrounding Co3O4 does not increase. Both the binding energies of P 2p and Co 2p are positively shifted by ca. 0.15 and 0.54 eV, respectively, compared with the bare BP and pure Co3O4 (Figure 2d and e), which indicates the decrease of the whole


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electronic density in Co3O4@BP, i.e., the transferred electron that previously exposed at the BP surface does not surround the surface of Co3O4 but become delocalized around two Co3+ atoms and one Co2+ atom, as well as the adjacent O atoms. Obviously, a higher degree of electron delocalization is obtained in the BP modified Co3O4 than the pristine Co3O4. To gain further insight into the surface electronic properties, the relative atomic ratio of Co2+/Co3+ for Co3O4 is analyzed, which could be acquired by calculating the area covered by the fitted curve.8 As can be clearly observed in Figure S5, the atomic ratio of Co2+/Co3+ for the Co3O4@BP (0.80) is higher than that of pure Co3O4 (0.44) and ultrasonicated Co3O4 (0.42), suggesting that relatively abundant Co2+ ions are present in the Co3O4@BP. That is, Co3+ ions is partially reduced to Co2+ with massive electron transferred from BP. Since Raman and XPS analyses show negligible change of the pure Co3O4 before and after ultrasonic treatment, it is believed that the migration of electron from BP to Co3O4 does occur and more Co2+ than Co3+ are is indeed obtained in the Co3O4@BP hybrid. To further verify the local electronic states and electronic structures of Co3O4@BP, Fourier-transformed extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) were studied. Three dominant peaks in Figure 3a derive from scattering of different coordination shells. The peaks located at II and III regions are ascribed to the Co–Co bonds. The peak located at I region is resulted from two Co–O bonds that consist of a tetrahedral shell (Co2+–O) and an octahedral shell (Co3+–O).16 However, the distance of tetrahedral Co2+–O bond and octahedral Co3+–O bond is too close to be distinguished. It is not clear whether Co3+ ions is reduced to Co2+ when partial Co3+–O breaks based on the EXAFS results. By contrast, XANES is sensitive to the change of electronic states and structures. Generally, there are three main


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features in the Co K-edge XANES spectra for Co3O4 (Figure 3b), the pre-peak in (i) region is ascribed to 1s→3d electronic dipole-forbidden transition; (ii) region from pre-peak is used to study the oxidation state of Co atom; and the main peak in (iii) region is attribute to 1s→4p electronic dipole-allowed transition.16, 50 Hence, the change of oxidation state can be clearly observed from the inset (ii) in Figure 3b. An obvious negative shift as compared to that of pristine Co3O4 can be observed in Co3O4@BP, indicating that the decreased oxidation state of Co. This result further confirms the reduction of Co when Co3O4 NPs are coupled with BP NPs, which is in accordance with the change of Co valence state observed from XPS characterization.

Figure 3. (a) The R-space Fourier-transformed Co K-edge EXAFS and (b) Co K-edge XANES spectra of Co3O4 and Co3O4@BP. The inset shows the magnification of (ii) region for Co K-edge XANES.

Previous works have clearly revealed that in Co3O4 system the divalent Co2+ serves as active sites for water oxidation, and dominates the OER activity.13 Partial


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Co3+ is reduced to Co2+ due to effective migration of electron from BP to Co3O4, resulting in plentiful Co2+ ions in Co3O4@BP. Thus, it is expected to improve the catalytic performance for OER. Indeed, the integration of BP with Co3O4 does give rise to superior electrocatalytic activity. The OER performance for Co3O4@BP is evaluated by using three-electrode equipment. The LSV polarization curves with and without iR-compensation are illustrated in Figure S6. The LSV curves of Co3O4@BP in comparison with BP NSs, Co3O4 NPs, and bare glassy carbon electrode (GCE) are presented in Figure 4a. Both of the bare BP NSs and Co3O4 NPs alone enable to produce oxygen, but their activity is rather poor. In contrast, Co3O4@BP exhibits dramatically enhanced electrocatalytic activity, implying that engineering electronic structure of the hybrid is indispensable towards the superior OER activity. The OER specific activity for the pristine Co3O4, bare BP NSs and Co3O4@BP are respectively 5.3, 1.1 and 23.5 mA cm−2 at the potential of 1.65 V vs RHE. The Co3O4@BP catalyst outperforms the pristine Co3O4 by over 4 times and is even 20 times higher than that of bare BP NSs. It is well known that catalytic activity for OER is generally evaluated by calculating the overpotential at a current density of 10 mA cm−2, which is roughly the same current density of a 10% efficiency for solar water splitting.51 As shown in Figure S7, Co3O4@BP exhibits excellent OER activity at a low overpotential of 400 mV to deliver 10 mA cm−2, which is 45 mV larger than that of RuO2. Besides, an overpotential of 460 mV is needed for Co3O4@BP to achieve 50 mA cm−2, while RuO2 needs 530 mV to reach the same current density. Furthermore, the influence of different ratio of Co/P in Co3O4@BP toward OER is also investigated (Figure S8), and the corresponding overpotential at a current density of 10 mA cm−2 shows the optimal catalytic activity for Co3O4@BP when the ratio of Co/P is 1:0.67.


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Figure 4. The polarization curves (a), Tafel plots (b) and Nyquist plots (c) of the electrocatalysts, the electrical equivalent circuit and the magnifying Nyquist plots are inset in (c), RS stands for series resistance and RCT stands for charge transfer resistance. (d) The polarization curves of Co3O4@BP before and after 1000 CV cycles. The potentiostatic test (e) for 55 h, and the fluctuations caused by the bubble's release are showed in the red box by black arrows. (f) Stability of current densities at 1.65 V for pure BP NSs and Co3O4@BP.

In order to reveal the intrinsic OER kinetic behavior of the catalysts, Tafel slope values (Figure 4b) obtained from the polarization curves are analyzed. It can provide crucial information toward the rate-limiting step of the OER process. ACS Paragon Plus Environment

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Generally, in alkaline solution, the OER mechanism is initialized with electron transfer on the metal site (M) and then undergoes consecutive reaction steps for converting OH− to O2,52 which can be described as follows: M + OH− ⇌ M-OH + e−

(1)

2MOH ⇌ MO + H2O

(2)

MO + OH− ⇌ M−OOH

(3)

2M−OOH ⇌ MO + M + H2O + O2

(4)

Theoretically, the Tafel slope values of OER are calculated to be approximately 120, 60, and 40 mV dec-1, respectively.2, 53 As shown in Figure 4b, Co3O4 and BP exhibit Tafel slope values of 128 and 92 mV dec-1, suggesting that the OER process in the two catalysts Co3O4 and BP is rate controlled at the initial stage where the catalyst tends to bond with hydroxy groups and accompanied with electron transfer (reaction 1 with featured Tafel slope of 120 mV dec-1). Contrastingly, a value of 63 mV decade−1 is calculated for Co3O4@BP, revealing that the OER reaction of Co3O4@BP is controlled by the chemical reaction with O−O formation in an intermedium on the active sites, while the process of electron transfer is no longer the major step that restricts the OER performance for Co3O4@BP. Figure 4c shows the electrochemical impedance spectra (EIS) of the three catalysts. The Nyquist plots fitted with corresponding equivalent circuit and the fitting data from EIS (Table S1) shows that the charge transfer resistances (RCT) of Co3O4@BP is much smaller than that of pure Co3O4 NPs and bare BP NSs. Besides, the turnover frequency (TOF) is also calculated as an important index to value the intrinsic activity for OER. As summarized in Table S2, the TOF value of BP NSs and Co3O4 NPs are 0.7×10-2 and 2.42×10-2 s-1 at the overpotential of 0.32 V, whereas the values of the Co3O4@BP with different ratio of Co/P is 6.08×10-2 ACS Paragon Plus Environment

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1.29×10-2, 2.05×10-2, 2.44×10-2, and 3.59×10-2 s-1, respectively. All results of the electrochemical analysis demonstrate that the excellent OER property of Co3O4@BP can be ascribed to the process of electron migration from BP to Co3O4, which is favorable for the efficient use of active sites. In addition, due to the preferential transfer of the active lone pairs from the surface of BP to Co3O4, effective protection of BP NSs from degradation will be achieved in Co3O4@BP, thus manifest high OER activity especially for long-term stability. The excellent stability of Co3O4@BP is confirmed by the consecutive CV tests. From the polarization curves (Figure 4d), the catalytic activity of Co3O4@BP shows no obvious change in the overpotential after 1000 cycles. The durability of Co3O4@BP is further evidenced by applying chronopotentiometry test (Figure 4e and Figure S9). Co3O4@BP retains its activity even after 55 h electrocatalytic test. As shown in Figure 4f, the pristine BP NSs show a current density of 0.43 mA cm−2 at 20 s, while it is 11.6 mA cm−2 for Co3O4@BP at 20 s, i.e., >26 folds higher than that of the pristine BP NSs. After 4000 s, the current density of the pristine BP NSs remains at only 29.45% of its initial value, while it remains at 89.78% for the Co3O4@BP. The SEM, TEM, and HRTEM data (Figure S10, 11) for Co3O4@BP after OER test further confirms that Co3O4@BP is indeed structurally stable. The XPS spectra of P 2p, Co 2p and O 1s after stability test are showed in Figure S12. P 2p spectra shows three binding energies respectively assigned to P 2p3/2, P 2p1/2, and oxidized phosphorus (P-O).47 The surface of BP nanosheets would be slightly oxidized during and after OER test, contributing to the oxidized phosphorous in BP nanosheets. The atomic ratio of Co2+/Co3+ in Co 2p for the Co3O4@BP (0.56) after OER test is lower than the Co3O4@BP (0.80) before OER test, but higher than that of pure Co3O4 (0.44), suggesting that Co2+ is the actual active sites for OER. In Figure S12c, an extra peak located at 536.4 eV for Co3O4@BP after OER test


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could be ascribed to the adsorbed CO2,54 compared with the O 1s peaks of Co3O4@BP before OER test. And the P–O–Co coupling still remains in O 1s for the Co3O4@BP after OER test, further indicating intimate interaction of BP and Co3O4 in Co3O4@BP. Besides, the Raman of Co3O4@BP after OER test was measured. Previous work has demonstrated that the intensity ratio of the A1g/A2g sensitively depends on BP degradation because of oxidation, and when the intensity ratio shows A1g/A2g < 0.6, it confirms that the basal planes of BP are unoxidized.55 Therefore, we analyzed the A1g/A2g ratio for Co3O4@BP before and after OER test. As shown in Figure S13, the intensity ratio of Co3O4@BP after OER test shows A1g/A2g 0.56, which is slightly higher than that of Co3O4@BP before OER test (0.55), confirming the stability of Co3O4@BP. Furthermore, the electrochemical surface area (ECSA) is evaluated by the cyclic voltammograms (CV) technique at different scan rates (Figure S14). The double layer capacitance (Cdl) for Co3O4@BP (28.90 mF cm−2) is much larger than that of Co3O4 (0.12 mF cm−2) and BP (0.74 mF cm−2), implying that more active surface area is created in Co3O4@BP. To further understand the electron transfer in Co3O4@BP for excellent OER performance. The UV/Vis absorption spectra and ultraviolet photoelectron spectroscopy (UPS) were obtained to determine the band gaps and the positions of Fermi level (EF). As a p-type semiconductor, BP possesses an adjustable band gap from ~ 0.3 eV in bulk to ~ 2 eV in monolayer.49 Herein, BP NSs and Co3O4 NPs exhibit a band gap of 1.88 and 2.55 eV, respectively. (Figure S15). The value of work function can be calculated by subtracting the electron cut off energy (Ecut off) from the He I UPS spectra at excitation energy of 21.22 eV, which stands for the difference between the vacuum level (0 eV) and EF,56 are counted to be 3.93 and 4.71 eV for BP and Co3O4, respectively (Figure 5a, b). Figure 5c shows simple


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energy band diagram for Co3O4@BP heterojunction. In order to realize EF equilibrium, electron will migrate from the higher EF of BP NSs (-3.93 eV) to Co3O4 NPs (-4.71 eV). As a result, effective electron migration can not only protect BP from degradation without sacrificing much performance of OER, but also create massive active sites in Co3O4. Thus, compared to pure Co3O4 and bare BP electrocatalysts, Co3O4@BP exhibits much better stability and higher electrocatalytic activity.

Figure 5. UPS spectrum of BP NSs (a) and Co3O4 NPs (b). Energy band diagram (c) of Co3O4@BP.


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4. CONCLUSIONS In conclusion, we have synthesized an efficient electrocatalyst consisting of Co3O4 nanoparticles and black phosphorus nanosheets via a facile ultrasonic treatment process. In comparison to the single Co3O4 and BP, the Co3O4@BP electrocatalysts exhibited enhanced electrocatalytic activity and long-term stability for OER in alkaline solution. Co3O4@BP outperforms the pristine Co3O4 by over four-fold and is even twenty-fold higher than that of bare BP NPs. The dramatically enhanced electrocatalytic performance for Co3O4@BP could be attributed to the complementary advantages between Co3O4 and BP, in which efficient electron migration from BP to Co3O4 not only tune the relative ratio of Co2+/Co3+ but also effectively prevent the BP NSs from deteriorating. Besides, the restack of 2D BP NSs can be suppressed due to the deposition of Co3O4 NPs, and simultaneously the agglomeration of Co3O4 NPs can be prevented when they are supported on BP NSs. The detailed demonstrations of electronic structure modulation and kinetic mechanism towards OER reactions are expected to provide fundamental guidelines to design high-efficiency water oxidation electrocatalysts. ASSOCIATED CONTENT

Supporting Information Additional experimental results; detailed XRD, SEM, Raman and XPS characterizations of samples; other electrochemical test results; SEM, TEM, HR-TEM, Raman and XPS characterizations after the OER tests; and TOF comparisons of samples (PDF)


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

Funding Sources

This work was supported by National Natural Science Foundation of China (grants 21427802, 21671076 and 21621001). W.Z. thanks the Open Foundation from State Key Laboratory of Inorganic Synthesis and Preparative Chemistry at Jilin University (No. 2019-02).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank Beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.


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Black Phosphorus Modified Co3O4 through Tuning Electronic

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