Atomic-Level Co3O4 Layer Stabilized by Metallic Cobalt

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Atomic-level Co3O4 layer stabilized by metallic Cobalt nanoparticles: A highly active and stable electrocatalyst for oxygen reduction Min Liu, Jingjun Liu, Zhilin Li, and Feng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16549 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

Atomic-level Co3O4 layer stabilized by metallic Cobalt nanoparticles: A highly active and stable electrocatalyst for oxygen reduction Min Liu, Jingjun Liu*, Zhilin Li, Feng Wang*

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P R China E-mail: [email protected] (J. Liu), [email protected] (F. Wang)

Tel/Fax: +86-10-64411301

Keywords: atomic-level Co3O4 layer, vacancies defects, band gap, work function, oxygen reduction reaction

ABSTRACT

Developing atomic-level transition oxides may be one of the most promising ways for providing ultrahigh electro-catalytic performance for oxygen reduction reaction (ORR), compared with their bulk counterparts. In this paper, we developed a set of atomically thick Co3O4 layers covered on Co nanoparticles, through partially reducing Co3O4 nanoparticles using melamine as a reductive additive at an elevated 1

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temperature. Compared with the original Co3O4 nanoparticles, the synthesized Co3O4 with a thickness of 1.1 nm exhibits remarkably enhanced ORR activity and durability, which are even higher than those obtained by a commercial Pt/C in an alkaline environment. The superior activity can be attributed to the unique physical and chemical structures of the atomic-level oxide featuring the narrowed band gap and decreased work function, caused by the escaped lattice oxygen and the enriched coordination-unsaturated Co2+ in this atomic layer. Besides, the outstanding durability of the catalyst can result from the chemically epitaxial deposition of the Co3O4 on the cobalt surface. Therefore, the proposed synthetic strategy may offer a smart way to develop other atomic-level transition metals with high electro-catalytic activity and stability for energy conversion and storage devices.

1. INTRODUCTION Oxygen reduction reaction (ORR) has been considered as one of the most critical electrochemical processes in many industrial fields, such as fuel cells,1 metal-air batteries,2 and energy-efficient chlor-alkali electrolysis.3 However, one of the major challenges in these above fields is the sluggish kinetics of the ORR in acidic or alkaline environments. To solve the knotty issue, low-cost and highly active electro-catalysts are required to promote the ORR process. By now, the developed ORR catalysts include platinum-based alloys,4-6 metal-free heteroatom-doped carbons,7,8 transition metal-nitrogen-carbon materials,9-11 and spinel oxides.12-14 Among these above catalysts, the spinel oxides have been recognized as one of the 2


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most promising ORR catalysts with reasonable activity and considerable stability, concerning their earth-abundant source and simple preparation procedure with respect to other catalysts.15 However, the large-scale practical application of the transition metal oxides as catalysts for the ORR is limited by some drawbacks to some extent. One lies in the relatively low chemical adsorption of active oxygen (O2) on surface of the oxides like Co3O4, Mn3O4, caused by low concentration of catalytically active sites for the ORR.16-21 The densities of the active sites strongly depend on their vacancies and defects existed in these oxides. Another challenging issue arises from the high electrochemical activation barrier of the adsorbed O2, which makes the adsorbed O2 difficult to trap the first electron and break the strong O=O bond, that is, a rate-limiting step of the ORR.22,23 Finally, since the ORR displays a multi-electron reaction pathway, the production of hydrogen peroxide (H2O2) as intermediate is always high for these oxides, compared to that of Pt-based catalysts.23 To address these above issues, it is required to synthesize a new class of transition oxides with unique nanostructures for achieving high adsorption energy and low activation barrier for the active oxygen.24,25 Based on this understanding, fabricating atomically thin transition oxides may give a feasible technical way to improve their catalytic activity. As reported by Xie et al.,16 the atomically thin SnO2 sheets can remarkably increase the activity for CO oxidation, compared with the bulk counterpart. Moreover, Tierui Zhang et al.26 fabricated monolayer δ-MnO2 nanosheets 3



on a nickel foam, and suggested that the ultrathin manganese dioxide should be responsible for the remarkably improved activities for the oxygen evolution and hydrogen evolution reactions. Recently, other investigators27 also confirmed that the atomic-level Co3O4 layer can efficiently lower activation barrier of carbon dioxide electro-reduction through promoting chemical adsorption of CO2 on the oxide surface. Inspired by the above results, the atomic-level thin Co3O4 may substantially improve ORR activity due to its unique physical and chemical structures. Compared with the bulk counterpart, the atomic-level Co3O4 possesses the increased oxygen vacancies and decreased metal cation oxidation state for charge neutrality, which can efficiently lead to a significantly narrowed band gap.28,29 Recently, Liu et al.23 confirmed that the narrowed band gap of the oxide can reduce activation barrier of active oxygen, which results in the improved ORR activity. As stated by them, the narrowed band gap contributes to the excitation of the d-band electrons from the valence band to the conduction band in the oxide, facilitating the ORR kinetics. Besides, the oxygen vacancy defects in Co3O4 can also act as n-type donors, which can increase electrical conductivity of the oxide to facilitate the ORR kinetics.28,30 Therefore, developing atomic-level thin oxides may open a door to significantly improve the electro-catalytic activity of the oxide. However, the fabrication of the atomic-level oxides is very difficult, even by using various intricate methods including mechanical exfoliation, liquid exfoliation, and chemical vapor deposition

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(CVD).31-35 In addition, the chemical or electrochemical stability of these ultrathin oxides is also poor relative to their bulk counterparts. Herein, we proposed a facile method to fabricate atomically thin Co3O4 layer through annealing the carbon-supported Co3O4 nanoparticles mixed with melamine as a reductive additive at 700 ℃ in an argon (Ar) atmosphere. The thickness of the thin oxide layer grown on metallic cobalt nanoparticles, can be tuned simply by controlling the added amount of melamine. The synthesized atomically thin oxide with a thickness of 1.1 nm exhibits surprisingly enhanced activity and durability towards the ORR, which is even superior to a commercial Pt/C catalyst (E-TEK). The exact origin of the improved activity may be attributed to a narrowed band gap and an upshifted Fermi level, caused by abundant coordination-unsaturated Co atoms and lots of oxygen vacancies in this atomic-level Co3O4. Therefore, this proposed synthetic strategy offers a smart way to fabricate atomically thin oxides with high activity and stability. 2. EXPERIMENTAL SECTION 2.1 Synthesis of atomically thin Co3O4 grown on metallic cobalt Prior to the synthesis of the atomic-level Co3O4 layer stabilized by metallic cobalt nanoparticles supported on a carbon black (denoted as atomic-level Co3O4@Co/C), we firstly fabricated the carbon-supported Co3O4 nanoparticles (denoted as Co3O4/C) by a simple pyrolysis under 300 ℃. The detailed fabrication procedures of the Co3O4/C hybrid can be described as follows. First, nitrate hexahydrate 5



(Co(NO3)2·6H2O) (1.52 g) was dissolved into 50 ml ultrapure water, followed by vigorous stirring for 20 min. Then, 1.4 g carbon black (Vulcan XC-72), which was treated by a concentrated nitric acid (HNO3) at 120 ℃ for 10 h and then washed with ultrapure water until the pH is 7. Afterword, the treated carbon was added to the above solution containing nitrate hexahydrate and sonicated for almost 30 min. After drying at 80 ℃, the mixture was transferred to a porcelain crucible and treated in muffle furnace under 300 ℃ for 30 min to obtain Co3O4/C product (the mass ratio of Co3O4 to carbon is about 50 wt%). After that, the atomic-level Co3O4@Co/C composite was synthesized by using the above Co3O4/C hybrid as a precursor, through annealing the precursor mixed with a certain amount of melamine as a reductive additive at 700 ℃ in an Ar atmosphere. The typical fabrication procedures are shown as follows. The above synthesized Co3O4/C hybrid (0.1 g), melamine (0.2 g) and 20 mL ultrapure water were blended and transferred to a rotary evaporation system flask with a water temperature of 60 ℃ and a rotation rates of 80 rpm for almost 30 min under a vacuum to obtain a uniform grey-dark mixture. The grey-dark mixture was further thermal annealed at 700 ℃ in an Ar atmosphere for 30 min with a heating rate of 5 ℃ min-1. After that, the furnace was cooled down to the room temperature, yielding the final atomic-level Co3O4@Co/C composite. 2.2 Characterization The morphology of the atomic-level Co3O4@Co/C composite was characterized by using transmission electron microscope (TEM) (JEOL JMS-2010 microscope) and 6


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an aberration-corrected scanning transmission electron microscope (STEM, JEOL ARM200F) with energy dispersive X-ray spectroscope (EDX). The structure of the composite was analyzed by Raman spectroscopy (LabRam HR800) with a visible laser (λ=532 nm) and X-ray diffraction (XRD) (Rigaku RINT 2200 V/PC) with Cu Kα radiation (l = 1.5406 Å) at an angle ranging from 10° to 90° with a scan rate of 1 °min-1. The electronic structure of the sample was analyzed by X-ray photoelectron spectroscopy (XPS) using ESCALAB 250 (Thermo Fisher Scientific, USA) with monochromatic Al Kα X-radiation (beam size 500 µm). In order to determine the exact thickness of the atomic-level Co3O4 layer on metallic Co, the as-synthesized composite was etched by a 1 KV Ar+ ion sputtering XPS, corresponding to a sputter rate of 1.0 nm min-1. The optical absorption spectra were recorded on a Shimadu UV-2450 spectrophotometer in a wave length range of 190-900 nm. Moreover, The Co K-edge EXAFS spectrum for this sample was recorded at the 1W1B beamline of Beijing Synchrotron Radiation Laboratory, China. Besides, ultraviolet photoelectron spectrum (UPS) was performed on an AXIS ULTRA DLD spectrometer with a monochromatic He lamp (hν=21.22 eV) as excitation source at an applied bias voltage of 8 eV. 2.3 Electrochemical measurements All the electrochemical measurements were carried out in a conventional three-electrode cell by using a ring-disk electrode rotator (Pine, Corp.). A commercial glassy carbon (GC, 0.247 cm2) was served as the working electrode. A saturated 7



calomel electrode (SCE) and a graphitic rod were used as the reference and the counter electrode, respectively. For the working electrode, the loading of the catalyst is 0.6 mg/cm-2, while the loading of a commercial Pt/C (20 wt% Pt, E-TEK) is 0.1 mg/cm-2. The ORR activities of these catalysts were tested in O2-saturated 1 M NaOH solution by linear sweep voltammetry (LSV) with a scan rate of 5 mV s-1 at 1600 rpm at the room temperature (298 K). Cyclic voltammograms (CVs) were recorded with a scan rate of 50 mV s-1 in nitrogen or oxygen-saturated 1 M NaOH solution. 3. RESULTS AND DISCUSSION 3.1. Atomic-level Co3O4 layer stabilized by metallic Co Figure 1A shows the typical fabrication procedure of atomic-level Co3O4 layer stabilized by metallic Co particles supported on a carbon black, through a facial two-step solid-phase method. In the first step, the presence of some oxygenated functional groups (carboxylic or hydroxyl groups) on the HNO3-treated carbon can serve as the active sites for chemical adsorption of Co ions through electrostatic interaction. It leads to uniform nucleation and growth of small spinel Co3O4 nanoparticles on carbon (Co3O4/C), as evidenced in Figure S1. In the second step, utilizing the synthesized Co3O4/C as precursor, we have prepared the atomically thin Co3O4 layer through the partial reduction of the Co3O4/C mixed a certain amount of melamine as a reductive additive at 700 ℃ in an Ar atmosphere. During this reduction stage, the initially reduced metallic cobalt atoms by the melamine, localized at the oxide surface, will take place an inward diffusion from the surface to the 8


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interior of the oxide, accompanied by an outward diffusion of the lattice O ions in the inside to the oxide surface. The mutual diffusion process can be triggered by the high energy of the reduced metallic cobalt phase with high-density lattice defects, caused by the remove of the coordinated O atoms, according to the well-known nanoscale Kirkendall effect or defect-induced Kirkendall diffusion processes.36 The diffusion process must happen because we can achieve the complete reduction of the oxide to cobalt in a short period of time (> 2 h), as confirmed by the control experiments shown in Figure S2. Moreover, it has been recognized that the Co atom diffusion is faster than that of O atom in the Co3O4 phase.37 The fast inward diffusion of the Co atoms may lead to keep the solid-type particles instead of hollow-type ones during the reduction of the Co3O4 nanoparticles.

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Figure 1. (A) Schematic illustration for the synthesis process of the ultrathin Co3O4@Co/C composite. (B-C) TEM and STEM elemental line scan of the Co3O4@Co/C composite. (D-G) STEM-EDX elemental mapping of this composite: the green is Co while the red is O. (H-I) high magnification STEM dark and bright field images of the Co3O4@Co/C composite. (J) A schematic illustration of the unique interfacial structure of these formed Co3O4@Co composite. According to the above reduction process shown in Figure 1A, it is possible to simply tune the thickness of the oxide on the metallic cobalt surface by controlling the added amount of melamine. As the mass fraction of the added melamine to the oxide is fixed at 2:1, the typical morphology of the obtained product is shown in Figure 1B. The obtained nanoparticles are uniformly dispersed on the surface of the carbon black 10


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matrix with no observable aggregation. As depicted in Figure 1C, the average size of these nanoparticles is approximately 10.5 nm, as confirmed by the size histogram (Figure 1B inset). For the nanoparticles, the presence of Co3O4 phase can be verified by the Raman spectroscopy in Figure S3. But, no any Co3O4 diffraction peaks were observed in Figure S4, which reveals that the amount of the oxide is very small relative to the major metallic cobalt phase. To further investigate the structure of these nanoparticles,

high-angle annular dark-field

scanning

transmission

electron

microscopy-energy dispersive X-ray spectroscopy (HAADF-STEM-EDS) line scan analysis was performed across the typical nanoparticles, as shown in Figure 1C. As observed, the recorded signal intensity of Co line-scan profile is weak at both edges of the particles but is strong at the core region. It implies that the cobalt oxide is probably located at the surface of the metallic cobalt particles, forming an oxide-encapsulated-metal nanostructure. In addition, the distribution of the oxide phase with respect to the cobalt phase is shown in Figure 1D-G. It further confirm that Co3O4 thin layer covers the Co particles. The STEM image shows that the obtained Co3O4 layer is ultrathin and its thickness is about 1.1 nm, as shown in Figure 1H. Moreover, for the core region of the particles shown in Figure 1H, the inter-planar distance is about 0.20 nm, corresponding to the (111) plane of face-centered cubic Co (PDF #15-0806). At the outside oxide region, the recorded the lattice spacing is about 0.16 nm, which is slightly larger than 0.155 nm of the (511) lattice plane of cubic spinel Co3O4 (PDF#43-1003). The slight lattice expansion for the oxide may be 11



attributed to the lattice mismatch between the Co3O4 and Co phases. The lattice distortion also illustrates that there exists an epitaxial growth of the Co3O4 along (111) lattice plane of metallic Co. The epitaxial growth can firmly stabilize the atomic-level Co3O4 phase through the coherent Co atom at the interface between the Co3O4 and Co phases. Moreover, the Co3O4 phase has high-density lattice defects, caused by the missing of some coordinated O atoms, as marked as a red dashed circle shown in Figure 1I. Figure 1J shows the schematic illustration of the oxygen vacancy defects confined in the atomic layer. The presence of the abundant oxygen vacancy in the oxide lattice can decrease the coordination number of the neighboring metal sites, which makes these unsaturated Co atoms own more reactive sites for the ORR.38 3.2. The integrity of the atomically thin Co3O4 layer

Figure 2. (A-B) The XPS spectra of Co2p3/2 and O1s for the Co3O4@Co/C composite at different ion bombardment time (t=0, 20, 40, 60, 80 s). (C) Schematic illustrating the structure of the Co3O4@Co/C composite and the thickness of the Co3O4 layer.

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For the purpose of elucidating the integrity or perfection of the atomic-level Co3O4 covered on Co nanoparticles (Co3O4@Co/C), we performed Ar+ ion bombardment for the sample as a functional of times, t=0, 10, 20, 40, 60, 80 s. As evidenced from Figure 2A, the recorded three peaks observed at 780.3 eV, 783 eV, and a peak at 788 eV can be respectively assigned to oxidization sate Co3+, Co2+, and the satellite peak of the oxidation state of Co3O4.39 The intensity of these components related to Co3O4 phase is obviously decreased with increasing ion bombardment time. Furthermore, after Ar+ ion bombardment 80 s, these above characteristic peaks of Co3O4 phase are disappeared and only metallic Co 2p3/2 at 778.58 eV is clearly observed.8 Similarly, as manifested by the O 1s spectra in Figure 2B, the peak at 529.89 eV is associated with oxygen atoms (O2-) in Co3O4 lattice.40 The other two peaks around 530 eV and 532.3 eV are assigned to the oxygen atoms connected to carbon in C-O and O-C=O bonds, respectively.14 Moreover, with increasing bombardment time, the intensity of the lattice O2- related to Co3O4 is also decreased and then disappeared. It also reveals that the Co3O4 layer completely wraps the metallic Co up, even though it is very ultrathin. Moreover, based on the Ar+ ion bombardment rate, the calculated thickness of the Co3O4 layer is approximately 1.1 nm, which is in good agreement with the STEM results (Figure 1H). 3.3. The significantly enhanced ORR activity The ORR activity of the ultrathin Co3O4@Co/C catalyst was firstly evaluated by cyclic voltammetry (CV) measurements in O2-saturared 1 M NaOH solution. For 13



comparison, the activities of the original Co3O4/C and a commercial Pt/C (20 wt% Pt, E-TEK) were also measured under the same condition. As shown in Figure 3A, during the ORR, the recorded peak potential and current for the Co3O4@Co/C catalyst are much more positive and higher than those of the original Co3O4/C, suggesting the outstanding ORR performance of the catalyst. This finding can be verified by the results of the linear sweep voltammetry (LSV) curves for these catalysts shown in Figure 3B. In this figure, we can see that the catalyst exhibits the superior improved ORR relative to the original Co3O4/C. Moreover, the half-wave potential of the catalyst is 0.89 V (vs RHE), which is more positive 80 mV than that of Co3O4/C (0.81 V), as shown in Figure 3C. In addition, the half-wave potential of the catalyst is 10 mV higher than that of the commercial Pt/C. At 0.9 V versus RHE, the recorded kinetic current density (Jk) of each catalyst is given in Figure 3D. The Jk is determined by the Koutecky-Levich equation below: ଵ ௃

ଵ

ଵ

ೖ

ಽ

=௃ +௃

(1)

Where, J is the measured current density, Jk is the kinetic current density and JL is the diffusion-limited current density. For the Co3O4@Co/C catalyst, the recorded Jk is 5.51 mA cm−2 at 0.9 V, which is much higher than that obtained by the Co3O4/C (0.12 mA cm−2), as shown in Figure 3D. These above results confirm that the formation of the atomic-level Co3O4 layer on the metallic Co nanoparticles should be responsible for the remarkably enhanced ORR activity, which is superior to the commercial Pt/C catalyst that has been currently regarded as the best catalyst for the ORR. 14


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Figure 3. (A) Cyclic voltammograms for the ultrathin Co3O4@Co/C, Co3O4/C, and a commercial Pt/C (20 wt % of Pt relative to carbon) in O2-saturated 1 M NaOH. (B) RDE LSV curves of these above samples and the commercial Pt/C at a rotation rate of 1600 rpm in O2-saturated 1 M NaOH solution. (C) The corresponding half-wave potentials. (D) The kinetic-limiting current density (Jk) at 0.89 V. (E) H2O2 yields of these samples and Pt/C, based on the results shown in Figure S8. (F) The Tafel plots at the low overpotential region. To give insight into the ORR pathway, some electrochemical parameters including the electron transfer number (n) and hydrogen peroxide yields were determined for the Co3O4@Co/C catalyst. First, as shown in Figure S5, the calculated electron transfer number is 3.88 at a potential region ranging from 0.60 to 0.75 V, similar to that obtained by the commercial Pt/C (3.92), as shown in Figure S6. It suggests that a nearly 4-electron process occurred on the Co3O4@Co/C catalyst for the ORR. In contrast, the Co3O4/C shows a much lower electron transfer number of 3.70, indicating poor electro-catalysis selectivity (Figure S7). This conclusion can be 15



further verified by the results shown in Figure 3E. As observed, the determined hydrogen peroxide yield on the catalyst is significantly lower than that obtained by the Co3O4. As shown in Figure 3F, the similar Tafel slope has been observed for the as-synthesized catalyst and Pt/C suggesting the similar ORR kinetics over two of them.41 Moreover, the atomically thin Co3O4 catalyst also displays the good tolerance to methanol crossover poisoning effect during the ORR, which is superior to the commercial Pt/C, as verified by Figure 4A. Also, the catalyst exhibits an outstanding stability during the ORR, which surpasses that of the commercial Pt/C in 1 M NaOH solution, as shown in Figure 4B. The excellent durability can be associated with the epitaxial growth between the atomic-level Co3O4 phase and the metallic Co phase at the interface between them, as verified by Figure1I. Therefore, the Co3O4 layer grown on the metallic Co may be an ideal cathode catalyst for the ORR in alkaline solution. The remarkably enhanced activity may be attributed to the narrowed band gap and the decreased work function for the thin layer of cobalt oxide.

Figure 4. (A) LSVs of the Co3O4@Co/C and Pt/C catalysts in O2-saturated 1 M NaOH with (dash line) and without (solid line) 1 M methanol at a scan rate of 5 mV 16


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s-1. (B) Chronoamperometric responses of the Co3O4@Co/C and commercial Pt/C in saturated 1 M NaOH solution at 0.89 V versus RHE and 900 rpm. To verify the effect of the atomically thin Co3O4 layer on the ORR activity, we have fabricated a set of Co3O4 layers with different thicknesses, by changing the added amount of melamine relative to the Co3O4/C precursor. The morphologies of the obtained composites are shown in Figure S9 and their thicknesses of the oxides in the above composites were determined by XPS equipped with Ar+ ion bombardment (Figure S10). Correspondingly, their ORR activities have been determined by the linear sweep voltammetry (LSV) measurements, as illustrated in Figure S11. As expected, there exists an almost linear tendency that the ORR activity increases along with decreasing the thickness of the Co3O4 layers for these composites. 3.4. Origin of the ORR activity The origin of the remarkably enhanced ORR activity of the Co3O4@Co/C composite can be attributed to the following two aspects: (1) structural defects caused by the lattice oxygen vacancies existed in the atomically thin Co3O4 phase and (2) unique electronic structure of the oxide phase. It is believed that the thinner oxide will inevitably lead to the more vacancy defects, caused by escaping of the lattice oxygen atoms from the oxide surface.24 In our case, the atomic-level thin Co3O4 should contain many vacancy defects. To further verify this hypothesis, we performed the magnitudes of k2-weighted Fourier transforms (FT) of the cobalt K-edge EXAFS spectra, as shown in Figure 5A. For the ultrathin Co3O4 layer, there exists a very weak 17



characteristic peak at ~1.34 Å, which is assigned to single scattering paths of the cobalt ion to the closest neighbouring crystal oxygen (Co-O), compared with the strong corresponding peak for the bulk Co3O4.42 Moreover, the recorded position of this Co-Co (Td) characteristic peak is shifted by 0.2 Å to the low R direction, relative to that of the Co3O4 counterpart, as shown in Figure 5A. It has been reported that for the oxide layers, the reduced distances for M-O and M-M bonds imply an obvious distortion surrounding the vacancies of ultrathin oxide layers.25 Such surface distortion of ultrathin Co3O4 layer has been observed in Figure 1I. These above results clearly reveal the formation of many oxygen vacancies in the Co3O4 layer shown in Figure 5B. This finding is in agreement with the results shown in Figure 1I-J. First, the presence of the oxygen vacancy defects can lower the electrochemical activation energy of the adsorbed O2 and facilitate the adsorbed O2 dissociation into highly reactive oxygen atoms,16,24,19 which favors the ORR kinetics. Second, the escaped lattice oxygen atoms can decrease the coordination number of the neighbouring Co atoms, leading to more Co2+ species than Co3+ ions in the defective oxide through the charge balance between the O and Co ions, as verified by the results in Figure S12. As a result, the increase in the content of the unsaturated Co2+ species should contribute to the improved ORR activity, since they have been commonly regarded as the mainly active sites for the ORR.43 Others further confirmed that Co2+ ions can efficiently strengthen the adsorption and activation of active O2, leading to a fast ORR kinetic rate.23 18


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Figure 5. (A) The Fourier transformed (FT) spectra of the synthesized Co3O4@Co/C and the original Co3O4/C composite. (B) Schematic illustration of the ultrathin Co3O4 layer with vacancies for adsorbing O2. (C) The energy level diagram for the ultrathin Co3O4 layer with different thickness (2nm, 1.4 nm and 1.1 nm) and the Co3O4/C. The potential of O2/OH- is 5.201 eV, which corresponds to a potential of 4.801 V (vs NHE) relative to the Vacuum level. More importantly, the oxygen vacancy defects in the ultrathin Co3O4 layer also have a direct effect on the electronic structure of the oxide, caused by an energy level splitting to form an additional donor level (defect level) below the conduction band, which results in a narrowed band gap (Eg).44 To give a direct proof for the close correlation between the band gap and thickness of the oxide, we have performed 19



UV-vis spectra to determine their electronic band gaps for the different Co3O4 samples with different thickness (2 nm, 1.4 nm and 1.1 nm), as shown in Figure S13A. Besides, their valence band (VB) and conduction band (CB) positions to the Fermi level were further determined by the leading edges linear extrapolation of XPS valence band spectra to the base line, as depicted in Figure S13B. Taken together these above results, the obtained energy level diagrams for these oxides are shown in Figure 5C. There exists a narrowed trend of the band gap as the oxide thickness decreases, as observed from the diagrams. Definitely, the Eg value for each of all the oxides with different thickness (2 nm, 1.4 nm and 1.1 nm) is 3.11, 3.09 and 3.06 eV respectively, which are much smaller than that of the original Co3O4 nanoparticles (3.18 eV). Clearly, the thinnest oxide (1.1 nm) displays the narrowest electronic band gap (Eg) (3.06), among all the tested oxides. The narrowed Eg may play a key role in governing the ORR activity of these oxides, as reported by other investigators.45,23 On the one hand, the narrowed band gap of the Co3O4 makes the d-band electrons in Co ions more easily excited from valence band to the conduction band, which can lead to lowered activation energy of active O2 in electro-catalytic process.23 As a result, the adsorbed O2 is easier to trap the first electron from the oxides and break the strong O=O bond to facilitate the ORR kinetics, that is, a rate-limiting step of the ORR.22 This process can facilitate the ORR kinetics, as verified by the optimal activity of the oxide (1.1nm) shown in Figure 3. Therefore, based on the smallest bandgap and ideal

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energy level structure of the ultrathin Co3O4, the oxide exhibits a superior ORR activity, which is in agreement with the results shown in Figure 3 and Figure S11.

Figure 6. (A) The UPS spectra in the secondary cut-off region of the Co3O4@Co/C and the Co3O4/C. (B) The correlation between the potential at -1 mA cm-2 shown in Figure 5(B) and the work function value obtained from (A). (C) Schematic diagram for the electron transfer from catalyst to oxygen in the ORR process. In addition, the reduction potential of O2/OH- (5.201 eV, corresponding to potentials of 0.401 V, vs NHE) is between the valence and conduction band levels for all the oxides, as shown in Figure 5C. Compared with the other oxides, the thinnest oxide (1.1 nm) displays more negative VB and CB levels and the redox potential of O2/OH- is more close to its VB level. This unique energy level of the oxide can 21



facilitate the excited electrons transferring into the O2/OH- level to produce OHduring the ORR process.45-47 To confirm that the electrons transfer easily from the atomic-level oxide to active oxygen, the work function (φ) of the atomic-level oxide was determined by UPS measurements. The work function, defined as the minimum energy required to add or remove an electron from solid material,28 is an important parameter for charge-exchange between the oxide and the adsorbed O2 molecule. As shown in Figure 6A, the recorded secondary electron cutoff spectra show that the work function of the ultrathin Co3O4 layer (5.6 eV) decreases by about 0.6 eV, compared with that of the original Co3O4 (6.2 eV). It is noted that the electro-catalytic activity increases along with the decrease of work function, as shown in Figure 6B. For the atomic-level thin Co3O4, the reduced work function is attributed to the presence of a defect level, as illustrated in Figure 6C. Therefore, the decrease in work function can substantially reduce the barrier height of charge exchange during the ORR over the oxide. 4. CONCLUSIONS In summary, we propose a facile method to fabricate atomically thin Co3O4 through simply annealing the carbon-supported Co3O4 nanoparticles mixed with melamine as a reductive additive at 700 ℃ in an Ar atmosphere, without adding any other additives. The obtained atomic-level Co3O4 phase is completely covered and stabilized on the surface of metallic cobalt nanoparticles through an epitaxial deposition of Co3O4 along the (111) lattice plane of the metal Co. The thickness of the 22


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oxide layers can be tuned by controlling the amount of melamine. This work establishes a clear quantitative correlation between the thicknesses of these synthesized atomically thick Co3O4 and their electro-catalytic activities in an alkaline solution. Among them, the thinnest oxide (1.1nm) exhibits the best ORR activity and the lowest H2O2 production, superior to the Co3O4 nanoparticle. The improved ORR activity can be attributed to the narrowed band gap and the decreased work function of atomic-level oxide, caused by the abundant structural oxygen vacancy defects in this atomic-level oxide. This study may provide a promising route to synthesize other atomic-level transition metal oxides such as iron and manganese oxides with high property and stability for energy conversion and storage devices.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the Internet at http://pubs.acs.org

Additional characterization results including TEM, XRD, Raman, XPS, UV-vis spectra and additional details about electro-chemical measurements (PDF)

AUTHOR INFORMATION Corresponding Author *(J. Liu) Tel: +86-10-64411301. E-mail: [email protected]

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* (F. Wang) Tel: +86-10-64451996. E-mail: [email protected]

ACKNOWLEDGMENTS

This work was supported by National Natural Science Funds of China (Grant Nos. 51572013). The authors thank beam line 1W1B of Beijing Synchrotron Radiation Facility (BSRF) for providing the beam time.

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Energy level diagram of atomic-level Co3O4 layer with vacancy defects for electron transfer and adsorbing O2 in the ORR electro-catalytic process

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Atomic-Level Co3O4 Layer Stabilized by Metallic Cobalt

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