Spin-State Transition Enhanced Oxygen Evolving Activity in Misfit

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Spin-state Transition Enhanced Oxygen Evolving Activity in Misfit-Layered Cobalt Oxide Nanosheets Xianbing Miao, Shiming Zhou, Liang Wu, Jiyin Zhao, and Lei Shi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02804 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Sustainable Chemistry & Engineering

Spin-state Transition Enhanced Oxygen Evolving Activity in Misfit-Layered Cobalt Oxide Nanosheets Xianbing Miao, Shiming Zhou,* Liang Wu, Jiyin Zhao and Lei Shi* Hefei National Laboratory for Physics Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. * E-mail: [email protected]; [email protected].

Abstract: Exploring efficient and economical electrocatalysts for oxygen evolution reaction (OER) is of great importance for large-scale water splitting. Transition-metal oxides and their derivatives have been explored as OER catalyst candidates owing to their earth-abundant reserves and environment-friendly features. However, their fewer electroactive sites and poor intrinsic activity of active sites greatly limit their catalytic efficiencies. Here, we successfully prepare misfit-layered-structured Bi2Sr2Co2O8+δ (BSCO) nanosheets via liquid exfoliation strategy. We find that the as-exfoliated nanosheets can serve as an efficient electrocatalyst for water oxidation. The single-unit-cell thick BSCO sheets exhibit promoted OER activity with the lower overpotential and smaller Tafel slope than the bulk counterpart. The improved performance is ascribed to the increased electrochemical surface areas and the enhanced activity of active sites due to the spin-state transition of cobalt ions at edge sites.

Keywords: Cobalt Oxide; Nanosheets; Spin-state transition; Oxygen evolution 1 Environment ACS Paragon Plus

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Introduction Ultrathin nanosheets of layered materials including graphene, (oxy)hydroxides, and transition metal dichalcogenides, have sparked worldwide attention because they exhibit fascinating physical and chemical properties owing to the extremely high surface area percentage and dimensional confinement.1-7 For ultrathin nanosheets, the unique structure can expose an high fraction of surface atoms with lowered coordinate number, which is usually accompanied by the formation of defects with structure disorders.8 Those disordered structures can engineer the band gap states and increase the density of states near Fermi level, which could serve as highly active sites for catalytic reactions.9,10 In particular, the ultrathin sheets fabricated by the top-down or bottom-up approaches have been reported to exhibit the superior oxygen evolving activity.11-21 Oxygen evolution reaction (OER), as the oxidative half reaction of water splitting, suffers from intrinsically sluggish kinetics owing to the involvement of a complex four electron oxidation process, hence electrocatalysts are required to facilitate the sluggish kinetics.22-24 Compared with the bulk, the enhanced kinetics in these nanosheets catalysts stem from the increased electrochemical surface areas and faster interfacial charge transfer, which help to decrease the catalytic reaction barriers and improve the catalytic efficiencies.18,19,25 The layered structures also exist in the complex transition metal oxides (TMOs), such as Ruddlesden-Popper-type layered perovskite26-28 and misfit-layered chalcogenide29,30. These TMOs have been widely tested as potential OER electrolysts. However, they usually suffer from higher overpotential and lower catalytic activities, which is mainly attributed to low numbers of exposed surface active sites in their bulks.27-29 Thus, increasing the electrochemical surface areas, especially exfoliating into ultrathin nanosheets, is eagerly anticipated to improve water oxidation performance. However, for

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most of them, it is extremely challenging to obtain their ultrathin nanosheets through liquid exfoliation due to their much stronger electrostatic interactions between layers.31,32 Fortunately, among misfit-layered oxide, a kind of material, namely Bi2Sr2Co2O8+δ (BSCO) that consists of an alternate stack of the CdI2-type CoO2 layer and the rock-salttype Bi2Sr2O4 (SrO-BiO-BiO-SrO) layer along c axis (see Figure 1), presents a rather weak van der Waals coupling between the BiO layers. Consequently, it is highly desirable to exfoliate BSCO into few-layer nanosheets to allow for exploring their OER performance.

Figure 1. Schematic illustration for the exfoliation of BSCO single crystals into ultrathin nanosheets. In this work, we successfully exfoliate BSCO single crystals into the single-unit-cell thick BSCO sheets through liquid exfoliation. The as-exfoliated BSCO nanosheets exhibit large electrochemical surface areas. More interestingly, the nanosheets present the spinstate transition from low spin to higher spin state for cobalt ions at edge sites, increasing the intrinsic activity of active sites by enhancing the charge transfer ability between electroactive cations and adsorbed intermediates. Benefiting from a greater number of active sites and higher intrinsic activity of surface active sites, the single-unit-cell thick BSCO sheets can effectively catalyze water oxidation with large mass activities of 51.09 A g-1 as well as the lower overpotential (0.44 V) at a current density of 10 mA cm-2 in alkaline electrolyte, superior to the bulk sample. 3 Environment ACS Paragon Plus


Results and discussion Ultrathin BSCO nanosheets were prepared through liquid exfoliation strategy. 200 ml Nmethyl-2-pyrrolidone (NMP) containing 100 mg BSCO single crystals was sonicated in ice water for 24 h. Afterward, BSCO nanosheets were obtained by centrifugation. Here, the single crystals were grown using a K2CO3-KCl flux method33, where BSCO polycrystalline powders, labelled as BSCO bulk, were prepared by the conventional solidstate reaction34 (see the Supporting Information for details). Transmission electron microscopy (TEM) image (Figure 2a) clearly indicates that the as-exfoliated nanosheets have a freestanding and sheet-like morphology with the lateral size of about 100 nm. The near transparency of the sheet supports their ultrathin nature. The high resolution TEM (HRTEM) image (Figure 2b) displays the periodic lattice fringes, revealing the singlecrystal structure of the obtained nanosheets. The interplane spacing of 0.25 nm was assigned to (020) plane of BSCO nanosheets. The molar ratio of Bi/Sr/Co in BSCO nanosheets is determined as 1:1.05:1.03 by the energy dispersive X-ray (EDS) spectrum analysis (Figure S2), suggesting a good stoichiometry of our samples. The thickness of BSCO nanosheets evaluated by atomic force microscopy (Figures 2c and 2d) is approximately 3.00 nm, which agrees well with the thickness of single-unit-cell in BSCO along c axial direction.30 The as-obtained product was fabricated into a film by a layer-by-layer assembly strategy.35 The X-ray diffraction (XRD) pattern of the film in Figure 2e clearly demonstrates the (00n) preferential orientation for ultrathin BSCO nanosheets, which is well consistent with the HRTEM image. The Raman spectra of BSCO nanosheets and bulk (Figure S3) show two characteristic peaks located at 430 and 600 cm-1, which can be


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Figure 2. Microstructural characterization of BSCO bulk and ultrathin nanosheets. (a) TEM. (b) HRTEM. (c,d) AFM image and corresponding height images. (e) XRD pattern. (f) XPS spectra of Co 2p. assigned to the in-plane and out-of-plane vibration modes E1g and A1g, respectively.36 The similar Raman spectra indicates the structural retention of BSCO during the exfoliated process. In addition, compared with the bulk, the peaks for the nanosheets show slightly



shift toward high wavenumbers, ascribing to the phonon confinement and the ultrathin thickness of the nanosheets.37 Figure 2f shows the Co 2p X-ray photoelectron spectra (XPS) of the nanosheets and bulk, which can be fitted by two spin-orbit doublets peaks of Co3+ and Co4+ ions and their shake-up satellites. No visible differences for the Co 2p XPS between two samples could be found, suggesting that the oxidation state of the Co ions remain unchanged during the exfoliated process.

Figure 3. OER activities of BSCO bulk and nanosheets. (a) Polarization curves. (b) Mass activity at η = 0.45 V. (c) Tafel slopes. (d) The current densities at 1.115 V vs RHE as a function of scan rates.


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The OER activity of BSCO nanosheets were evaluated in O2-saturated 0.1 M KOH solution using a standard three electrode system. As a reference, similar measurements were also conducted for BSCO bulk. Figure 3a shows the iR-corrected polarization curves for both the catalysts with current densities normalized to the geometric area of the electrodes. The onset overpotential (ηonset) of BSCO bulk is measured to be 0.47 V, indicating its inferior OER activity. Intriguingly, the single-unit-cell thick BSCO sheets exhibit a markedly small ηonset of 0.38 V for water oxidation. Furthermore, to achieve the current density of 10 mA cm-2, BSCO nanosheets only require a small overpotential (η) of 0.44 V. This value is significantly lower than that of bulk (far above 0.52 V), which can be comparable to that of currently reported highly efficient OER catalysts such as the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ (0.50 V)38, oxygen-deficient CYMO-350 (0.45 V)39, and the delithiated LiCoO2 (0.41 V)40. To directly assess the electrocatalytic performance of the catalysts, mass activities at η = 0.45 V are summarized in Figure 3b. The mass activities of BSCO nanosheets is 51.09 A g-1, which is about 18 times higher than that of bulk (2.81 A g-1). Meanwhile, the corresponding Tafel slopes (Figure 3c) also suggest that BSCO nanosheets possessed a small Tafel slope (59 mV dec-1), much lower than that of the bulk (113 mV dec-1). This remarkable reduction in Tafel slope suggests a possible shift in the rate-determining step of water oxidation from -OH adsorption to -OOH formation.41,42 In addition, chronopotentiometric measurements were performed to assess the stability of the catalysts for OER in alkaline electrolyte. As can be seen in Figure S4, the time dependence of the potential at the current density of 10 mA cm-2 shows that BSCO nanosheets exhibit a better stability than the bulk. All above results clearly



demonstrate that the single-unit-cell thick BSCO sheets is an efficient water oxidation electrocatalyst under the alkaline media. The great improvement in water oxidation performances has been widely reported in the various two dimensional ultrathin nanosheets, which are largely credited to the increased numbers of the electroactive sites.16,19,34 To evaluate the electrochemical surfaces areas (ECSAs) of the catalysts, the electrochemcial double layer capacitances (Cdl) was measured by a simple cyclic voltammetry method.43 As shown in Figures S5 and 3d, the BSCO nanosheets exhibit the larger Cdl of 48.3 mF cm-2, about 5 times higher than that of the bulk (9.1 mF cm-2). More importantly, relative to the bulk counterpart, BSCO nanosheets is still 3.6 times more active in the specific activity at η = 0.45 V normalized by ECSAs (Figure S6). This result clearly demonstrates that the significant enhancement of the OER activity is intrinsic. Notably, for cobalt oxides, recent studies have revealed that the spin states of cobalt ions play a vital role in their intrinsic electrocatalytic activity.44-48 For instance, Hsu et al.46 reported that the spin state of surface Co3+ ions on spinel Co3O4 was manipulated through surface treatment method. The high spin state Co3+ was found to significantly promote OER activity. Duan et al.47 showed that the enhanced water oxidation performance in Fe-doped LaCoO3 originated from the increased Co 3d-O 2p covalency due to the Co3+ spin state transition from low spin to high spin state. Our group48 investigated the nanosize effect of single pervoskite cobaltite on their electronic structure and found the existence of surface spin state transitions. As a result, the 80 nm LaCoO3 with the eg electron filling of ~ 1.2 exhibits the optimal intrinsic OER activity. The enhanced OER activity of these materials is mainly ascribed to the σ-bonding eg orbital having stronger overlap with the oxygen-related adsorbate than the π-bonding t2g orbital, which


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facilitates faster electron transfer between the catalytically active sites and adsorbed -OOH species.44

Figure 4. (a) Temperature dependent magnetizaition under H = 1 T. (b) Temperature dependent inverse susceptibilities under H = 1 T. The dotted lines are the fitting results by the Curie-Weiss law. (c) Nyquist plots. (d) FTIR spectra. To get insight into the spin structures of Co ions for the BSCO catalysts, the temperature dependent magnetizations as shown in Figure 4a were measured with a magnetic field of H = 1 T under field-cooling procedures. Above 200 K, the susceptibilities obtained from the magnetizations (χ = M/H) well obey the Curie-Weiss law: χ = C/(T-Θ), where C is Curie constant, and Θ is Curie-Weiss temperature. From the



fitting results (Figure 4b), the calculated effective magnetic moment per Co through µeff =√8C µB is about 0.70 and 1.23 µB for BSCO bulk and nanosheets, respectively. For the bulk, previous studies49,50 have revealed the coexistence of Co3+ and Co4+ ions both in low spin configuration (LS), i.e., t2g6eg0 and t2g5eg0. Using the obtained µeff, the molar ratio in Co3+: Co4+ ions for our powder sample is estimated to be 84%: 16%, which is close to the reported values.50,51 After exfoliating, an obvious increase in µeff is found, meaning a change in Co oxidation states or spin states of Co ions. Since the Co 2p XPS revealed no visible change in the Co oxidation states as discussed above, this increase would result from the spin-state transition of partial Co ions from LS to intermediate-spin state (IS: t2g5eg1 and t2g4eg1) or high spin state (HS: t2g4eg2 and t2g3eg2) owing to the competition between crystal-field splitting and intra-atomic exchange interaction in the cobalt oxides52-54. If the spin-state transition occurs in Co4+ ions, the increased volume fraction is estimated to be 8.5% for IS Co4+ or 3.2% for HS Co4+. Similarly, for the case of Co3+ ions, the volume fraction is about 12.8% IS Co3+ or 4.3% HS Co3+. For nanosized cobalt oxides, previous studies have revealed that the low spin state of Co3+ ions at the surface sites are favorable to turn into higher spin states owing to surface effects.45,48,55 For the single-unit-cell thick BSCO sheets, the CoO2 layers along c axis are sandwiched by the Bi2Sr2O4 layers (see Figure 1), and hence only the edge layers can be exposed. Thus, the spin-state transition of Co ions could occur in the exposed edge layers. Taking an edge thickness of 4∼5 unit cell (about 1 nm), we can establish a simple coreshell model to estimate the volume fraction of edge layers (see the Supporting Information for details). The estimated volume fraction of edge layers is about 6.5%. Assuming that Co ions within edge layers are transited to be in higher spin state, the


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volume fraction is 1.0% and 5.5% for Co4+ and Co3+ ions in higher spin state, respectively, which is much lower than that of IS Co4+(8.5%), HS Co4+(3.2%), and IS Co3+ (12.8%) derived from the increased magnetic moment. Hence, it seems that the spin-state transition from LS to HS state for Co3+ ions at edge layers is responsible for the increase in µeff. Practically, the HS state of Co3+ ions in cobalt compounds was recently reported to significantly enhance the charge transfer ability between electroactive Co ions and adsorbed intermediates, and thereby promote water oxidation activity.46-48 For our nanosheets, this feature is supported by the electrochemical impedance spectroscopy (Figure 4c) measured under OER conditions, where BSCO nanosheets have much smaller charge transfer resistance (20 Ω) than the bulk (120 Ω). On the other hand, since the half occupied eg orbital strongly overlapped with oxygen orbitals, the HS state of Co3+ ions can also facilitate oxygen-related intermediates adsorption.45,47 To assess the capability of the catalysts to adsorb intermediates in alkaline solution, Fourier transform infrared (FTIR) spectroscopy (Figure 4d) was carried out. For the nanosheets, an obvious higher intensity is found for the IR band around 3432 cm-1 corresponding to H-bonded stretching vibrations, indicating that BSCO nanosheets are more favorable for OH- adsorption to form -OH species. This leads to a change of the rate-determining step in OER from the -OH adsorption to the -OOH formation, which is well consistent with the significant reduced Tafel slope. Conclusions In summary, we reported ultrathin BSCO nanosheets as an efficient OER electrocatalyst. The single-unit-cell thick BSCO sheets obtained by liquid exfoliation exhibit the excellent OER activity with large mass activity of 51.09 A g-1, low overpotential of 0.44 V and small Tafel



slope of 59 mV dec-1, which are superior to the bulk counterpart. The insights gained form XPS analysis and magnetic measurements reveal the presence of the spin-state transition of cobalt ions at edge sites. The higher spin state of cobalt ions significant enhance the charge transfer ability between electrochemical active sites and adsorbed intermediates and hence improve water oxidation performance. Our work opens up the way to design the highly efficient OER catalysts. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental section, extra XRD and EDS results, Raman results, CV curves and polarization curves. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Science Foundation of China (Grant Nos. U1732149 and U1432134).


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The spin-state transition of Co3+ ions significantly enhances water oxidation performance for BSCO nanosheets.


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Spin-State Transition Enhanced Oxygen Evolving Activity in Misfit

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