Ultrathin LiCoO2 Nanosheets: An Efficient Water-Oxidation Catalyst

Jan 27, 2017 - Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Ac...
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Ultrathin LiCoO2 Nanosheets: An Efficient Water-Oxidation Catalyst Jianghao Wang,† Liping Li,‡ Haiquan Tian,† Yuelan Zhang,‡ Xiangli Che,† and Guangshe Li*,†,‡ †

Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China ‡ State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *

ABSTRACT: Ultrathin cation-exchanged layered metal oxides are promising for many applications, while such substances are barely successfully synthesized to show several atomic layer thickness, owing to the strong electrostatic force between the adjacent layers. Herein, we took LiCoO2, a prototype cation-exchanged layered metal oxide, as an example to study. By developing a simple synthetic route, we synthesized LiCoO2 nanosheets with 5−6 cobalt oxide layers, which are the thinnest ever reported. Ultrathin nanosheets thus prepared showed a surprising coexistence of increased oxidation state of cobalt ions and oxygen vacancy, as demonstrated by magnetic susceptibility, X-ray photoelectron, electron paramagnetic resonance, and X-ray absorption fine spectra. This unique feature enables a higher electronic conduction and electrophilicity to the adsorbed oxygen than the bulk. Consequently ultrathin LiCoO2 nanosheets provided a current density of 10 mA cm−2 at a small overpotential of a mere 0.41 V and a small Tafel slope of ∼88 mV/decade, which is strikingly followed by an excellent cycle life. The findings reported in this work suggest that ultrathin cation-exchanged layered metal oxides could be a next generation of advanced catalysts for oxygen evolution reaction. KEYWORDS: LiCoO2, nanosheets, mixed state, electrocatalysis, oxygen evolution reaction Fe doped Co3O4, NiFe-LDHs, and so on.11−18 Unfortunately, the electroactivity of noble metal free catalysts for OER is still poor and needs to be markedly improved. In this regard, cation-exchanged layered metal oxides could be the alternatives due to the nature of unique layered structure. LiCoO2 is a prototype cation-exchanged layered metal oxide, which has been taken as a famous cathode material for Li-ion batteries, and also a typical nonprecious-metal-based catalyst for OER.19 If cation-exchanged layered metal oxides, like LiCoO2, could be prepared to show a thickness of several atomic layers, one may expect to have a significantly promoted OER performance, since (i) Co−O octahedral sites in the layered LiCoO2 are highly active for OER, and exposing octahedral sites (like the {001} plane of LiCoO2) as much as possible provides an effective measure to improve the catalytic activity; and (ii) electronic states (including valence state and spin state) and oxygen vacancy are also the key for OER, and modulation of electronic states and oxygen vacancy in the uniquely exposed active plane of cation-exchanged layered

1. INTRODUCTION Low dimensional materials with several atomic layers are receiving more and more attention owing to their fascinating physical and chemical properties that are essential for many applications (e.g., solar water splitting, regenerative fuel cells, and rechargeable metal−air batteries).1−4 Taking water oxidation in alkaline solutions as an example, oxygen evolution reaction (OER), 4OH− → O2 + 2H2O + 4e−, occurs really difficultly because of a high activation barrier that couples with the need for transfer of four electrons and four protons.5−7 To promote this OER process, many catalysts have been synthesized. At present, typical catalysts used in water splitting reactions are primarily based on noble metals (e.g., Pt, Ru, Ir) and their alloys/compounds.5,8−10 However, the scarcity and high cost of these precious metals limit their large-scale industrial applications. Consequently, great efforts have been made to discover highly active catalysts without or with less precious metal. In this quest, first-row transition metal oxide derived electrocatalytic systems are promising, because of their good catalytic activity for OER and abundant availability. Till now, there have been many noble metal free electrocatalysts for OER, which include Mn3O4/CoSe2, hollow Co3O4 microspheres, ZnCo-LDHs, CoMoO4 nanosheets, LT-LiCoO2, © 2017 American Chemical Society

Received: November 20, 2016 Accepted: January 27, 2017 Published: January 27, 2017 7100

DOI: 10.1021/acsami.6b14896 ACS Appl. Mater. Interfaces 2017, 9, 7100−7107

Research Article

ACS Applied Materials & Interfaces

water, washed, and filtered to remove the residual fluxes. Finally, the products were dried at 130 °C for 24 h. LiCoO2 nanosheets thus obtained were marked as LiCoO2-B. Partial Delithiation of Bulk LiCoO2 Sample Materials. In a typical procedure, LiCoO2-B was dispersed in a stoichiometric amount of 0.15 M aqueous solution of K2S2O8. This suspension was kept at 55 °C while being stirred. After stirring for 36 h, the liquid phase was removed by filtration and the remaining solid was extensively washed with deionized water at 55 °C. Finally, the products were dried at 80 °C for 24 h, just like those previously reported.27 The products were marked as LiCoO2-BD. 2.2. Sample Characterizations. Phase structures of the samples were characterized by powder X-ray diffraction (XRD) (Cu Kα, λ = 1.5418 Å) on a Rigaku Miniflex apparatus. The morphologies of the samples were observed by field-emission scanning electron microscopy (SEM) (JEOL, model JEM-2010) and transmission electron microscopy (TEM) (JEOL, model JEM-2010). Atomic force microscopy (AFM) study in the present work was measured on a Dimension ICON. The nanosheets were adsorbed onto a mica substrate. The chemical compositions of the samples were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). The valence states of cobalt, lithium, and oxygen were examined by X-ray photoelectron spectroscopy (XPS) on an ESCA-LAB MKII apparatus performed with a monochromatic Al Kα X-ray source. The charging shift was calibrated using the C 1s photoemission line at a binding energy of 284.6 eV. Raman spectra were recorded in the backward geometry on Labram HR800 Evolution at room temperature. Excitation was carried out on the powder samples using the 632.8 nm wavelength line from a He−Ne laser. Magnetic susceptibility measurements were performed in a temperature range 1.8−300 K using a vibrating sample magnetometer (VSM) of Quantum Design MPMS SQUID-VSM system. Electron paramagnetic resonance (EPR) spectra were recorded using a JEOL JES-FA200 ESR spectrometer at room temperature with frequency of 9.062 GHz. The X-ray absorption fine structure (XAFS) data were collected at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). 2.3. Electrochemical Characterization. Homogeneous catalyst inks were prepared by mixing 16 mg of the sample and 73.2 μL of 5% Nafion solution in 4 mL of deionized water with 4 mL of ethanol under sonication for 30 min. The catalyst ink was then coated onto the rotating-disk electrode (RDE) at a loading of 0.1 mgoxide cm−2 disk and dried at room temperature. Before the coating of the catalyst, the glassy carbon electrode (5 mm diameter) was polished with Al2O3 paste (0.05 mm) and washed ultrasonically with double-distilled water. All the electrochemical measurements were performed with Hg/HgO reference electrode and graphite counter electrode at a scan rate of 10 mV s−1 in 0.1 M KOH. The rotation speed of RDE was set at 1,600 rpm for all measurements. In this work, the potentials are displayed versus RHE by the RHE calibration: E(RHE) = E(Hg/HgO) + 0.098 + 0.0591 × pH, where pH = 13 in 0.1 M KOH solution. The mass activity was normalized to the loading of the oxide catalyst. Cyclic voltammetry (CV) measurements with different scan rates (1, 5, 10, 20, 30, 40 mV/s) were used to determine the electrochemical double layer capacitances (EDLC, Cdl). Chronoamperometry was tested at an applied potential of 1.7 V at pH = 13 to study the durability of the electrocatalysts.

metal oxides allows one to find noble metal free catalysts with highly promoted electroactivity for OER. Even so, ultrathin cation-exchanged layered metal oxides are really difficult to prepare to show several atomic layers thickness, owing to the strong electrostatic force between the adjacent layers. For example, conventional methods of preparing such ultrathin metal oxides have to undergo complicated steps from ion exchange to osmotic swelling and exfoliation.20 Although ultrathin nanosheets could be obtained by this method, low yield and time consumption become two additional concerns. Among all preparation methods, modified hydrothermal reaction has shown merits in high yield and control over the components and morphologies of nanomaterials. So far, various nanostructures of LiCoO2, including nanoparticles, nanoplates, nanospheres, and nanowires, have been prepared and reported.21−24 Despite this progress, ultrathin LiCoO2 nanosheets are barely prepared to show a thickness of several layers. As a result, it is still not clear if ultrathin LiCoO2 nanosheets could show improved catalytic activities for OER. Herein, we developed a new synthetic strategy that combines the advantage of chemical exfoliation and hydrothermal method, leading to ultrathin and high-quality LiCoO2 nanosheets. Such a unique feature of nanosheets enables a coexistence of increased oxidation state of cobalt ions and oxygen vacancy, surprisingly different from the bulk. As a consequence, ultrathin LiCoO2 nanosheets have shown a highly promoted catalytic performance toward oxygen evolution reaction, when compared to the bulk.

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. All chemical regents used were of analytical grade without further treatment. Synthesis of α-Co(OH)2 Nanosheets. In a typical procedure, 1.9034 g of CoCl2·6H2O, 0.4676 g of NaCl, and 6.3 g of hexamethylenetetramine (HMT) were dissolved in a mixture of 200 mL of deionized water and 40 mL of ethanol, just following the procedure reported in the literature.25 The mixed reactants were heated at 90 °C for 1 h with magnetic stirring. The system was then allowed to cool to room temperature. The resulting green powder was collected by centrifugation and washed several times with deionized water and ethanol, and subsequently dried at 60 °C. For convenience, the product was marked as Co(OH)2-B. 40 mg of α-Co(OH)2 and 40 mL of distilled water were added into a weighing bottle with a capacity of 100 mL. The weighing bottle was then sealed with a rubber stopper and sonicated in a sonic bath (DS7510DTH) for 6 h. During this procedure, retaining a constant temperature is very important, which could prevent the transformation of α-Co(OH)2 to Co3O4. Herein, the ice bath was used to keep the system at about 0 °C. Then, the resultant dispersions were centrifuged at a rate of 5000 rpm for 5 min to remove the incompletely exfoliated particles. The concentration of Co(OH)2 dispersions was about 0.2 mg/mL. The products were marked as Co(OH)2-NS. Synthesis of LiCoO2 Nanosheets. 2 g of LiOH and 10 g of KOH were added into 40 mL of α-Co(OH)2 dispersions. The dark solution was then transferred to a 100 mL Teflon-lined stainless autoclave, and treated in an oven at 200 °C for 12 h. The resulting powder was collected by centrifugation and washed by deionized water thoroughly. Then, the isolated dark brown powder was dried under vacuum. Finally, about 7 mg of powder was obtained. The obtained LiCoO2 nanosheets were marked as LiCoO2-NS. Synthesis of Bulk LiCoO2 Materials. Bulk LiCoO2 was synthesized via a molten salt method previously reported.26 CoO, LiOH·H2O, and KCl were mixed in terms of molar ratios of 1:1:2 and pestled. The mixtures were heated at 850 °C for 8 h in air and then cooled to ambient temperature. The products were immersed in deionized

3. RESULTS AND DISCUSSION In this work, ultrathin nanosheets LiCoO2 were initiated through developing a combined soft chemical strategy. A detailed procedure is illustrated in Figure 1a. Bulk Co(OH)2 sheets (named as Co(OH)2-B) were first obtained by a conventional hydrothermal method, just following a procedure reported elsewhere.25 Then, the resultant green powders were sonicated for 6 h to get a colloidal solution that contains Co(OH)2 nanosheets. Co(OH)2 nanosheets (named as Co(OH)2-NS) were collected by centrifuging the resultant dispersions at 5000 rpm for 5 min. XRD, Tyndall effect, SEM, and AFM data are used to 7101

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Figure 1. Schematic illustration for the formation process of LiCoO2-NS.

characterize the samples so as to provide direct evidence for the formation of Co(OH)2-B and Co(OH)2-NS, as shown in Figures S1−3. Eventually, LiCoO2-NS were obtained by a hydrothermal method under a strong alkaline condition. The general reaction could be described as follows: 4Co(OH)2 (s) + 4Li+ + O2 + 4OH− = 4LiCoO2 (s) + 6H 2O

Shown in Figure 2 are the tapping-mode AFM image and corresponding height profile of the sample, LiCoO2-NS.

Figure 3. (a) Layered crystal structure of LiCoO2; (b) XRD pattern of LiCoO2 nanosheet-based film, where the data in blue and black colors are the bulk LiCoO2 and corresponding standard pattern for LiCoO2 (JCPDS, No. 50-0653); and (c, d) TEM and HRTEM images of the obtained LiCoO2-NS.

Figure 2. (a, b) Tapping mode AFM image with a scale bar of 500 nm and corresponding height profile of the as-obtained nanosheets deposited on a mica substrate. The different height profiles correspond to the height of the nanosheets shown in panel b.

which completely differs from LiCoO2-bulk (Figure 3b). For the latter case, the XRD pattern depicted in the blue line matches the standard diffraction data for LiCoO2 (JCPDS, No. 50-0653) without any apparent orientation. The formation of LiCoO2-NS with a layered structure is also confirmed by Raman spectra. As shown in Figure S6, two Raman bands observed at 593.5 and 482.7 cm−1 are assigned to the A1g mode of Co−O stretching vibration and the Eg mode of O−Co−O bending vibration, respectively.29 Notably, a red shift is seen for both bands when comparing to the LiCoO2-B sample, which could be explained in terms of dual facts, (i) phonon confinement effect due to its ultrathin feature and (ii) cationic nonstoichiometry (like Li deficiency in this case) as reported in other metal oxide nanomaterials.30 To clarify the second possibility, the chemical compositions for the as-prepared samples were comparatively studied by ICP and XPS: For LiCoO2-NS, as determined by ICP, the molar ratio of Li to Co is 0.58, far below unity as expected, which largely differs from the molar ratio of 0.99 for LiCoO2-B. The chemical composition of LiCoO2-NS was further investigated by XPS. The obtained survey data confirm that the sample consists of elements Li and Co only (Figure S7). The binding energy of Li 1s is at about 54 eV, closer to that previously reported for LiCoO2.31 Two peaks can be clearly identified in O 1s core level spectra: One at 529.4 eV is ascribed to the oxygen atoms bound to metals, while another at 531.0 eV is attributed to the surface oxygen species such as hydroxyls that might be adsorbed at oxygen vacancy site.32 The presence of oxygen vacancies in LiCoO2-NS was demonstrated by EPR,

One can see large sheet-like morphology with a height of 2−3 nm, which provides direct and solid evidence for ultrathin LiCoO2 nanosheets. The thickness of LiCoO2-NS is varied slightly from about five to six layers of Co−O layers (Figure 3a). The thickness distribution of LiCoO2 nanosheet is shown in Figure S4. Such a tiny difference in thickness could be closely related to the sonicated exfoliation that produces precursor, Co(OH)2 of different dimension (Figure S3). It should be noted that there exist lots of pits in nanosheets (Figure 2a). These pits on the surface of nanosheets may enrich oxygen vacancies that provide more active sites for catalytic reaction, as suggested elsewhere.28 From the TEM image in Figure 3c, it is clear that the as-obtained particles possessed sheet-like morphology with a size of about several hundred nanometers. Moreover, all these sheets are nearly transparent, confirming the ultrathin nature. Even so, these ultrathin nanosheets were highly crystalline and showed distinct exposed facets. Figure 3d has given the clear crystalline lattice with an interplanar spacing of 2.40 Å, which matches well the lattice distance of planes (110) and (010) of bulk LiCoO2. The Fourier transform pattern in the inset of Figure 3d is consistent with a typical feature of the reciprocal lattice projected along the [001] zone axis. Therefore, the as-obtained ultrathin nanosheets grew along the ab plane by exposing the facets at top and bottom surfaces. Such an exposure of {001} planes for the nanosheets with most Co−O octahedral sites is also demonstrated by the highly preferred (003) orientation in layer-by-layer assembled film, 7102

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Figure 4. (a) Temperature-dependent magnetic susceptibilities for LiCoO2-NS and LiCoO2-B, and (b) electron paramagnetic resonance spectra (EPR) of LiCoO2-NS, partially delithiated bulk LiCoO2 sample (LiCoO2-BD) and LiCoO2-B. Inset of panel (a) shows the variation of 1/(χ − χ0) with temperature for LiCoO2-NS.

OH•o and O•2 radicals.38 Large amounts of oxygen vacancies in LiCoO2-NS might be caused by a few of the oxygen species dissolved in precursor solution and lithium deficiencies. From these results, one can reach a conclusion that ultrathin LiCoO2 nanosheets with unique electronic states and {001} planes exposed were successfully synthesized by the current combined solution chemistries. To deeply reveal the structure characteristic of LiCoO2 nanosheets, extended X-ray absorption fine structure spectroscopy (EXAFS) was performed. As shown in Figure 5a, Co K-edge EXAFS k3χ(k) oscillation curve for the ultrathin nanosheet displays a noticeable difference in 2−10 Å−1 in comparison with the bulk material, implying the different local atomic arrangements of cobalt atoms. Moreover, Fourier transform (FT) curves of bulk and nanosheet are mainly characterized by two main peaks at 1.50 and 2.43 Å (Figure 5b), corresponding to the nearest Co−O and next nearest Co−Co coordination, respectively. Strikingly, the intensities of these peaks for LiCoO2 nanosheets decreased obviously. The decrease of Co−O coordination number from 6.0 to 5.8 for nanosheets (Table S1) also confirms the existence of oxygen vacancy, consistent with the observation of magnetic and EPR analysis above. Ultrathin LiCoO2 nanosheets have shown {001} plane exposure with special electronic state of cobalt and large amount of oxygen vacancy, which is divined as an effective catalyst based on mechanism of OER. The possible reaction pathway of OER could be proposed as follows:39

a complement technique as mentioned later, because of the limit of XPS analysis in distinguishing the existence of oxygen vacancy. Further, two spin splitting peaks of 2p3/2 and 2p1/2 were observed in the Co 2p core level region. The presence of two satellites apart from the main peak at 790 and 805 eV indicates the existence of Co3+. A careful data analysis shows that Co 2p3/2 peak is not symmetric with a shoulder peak located at 781 eV, and full width at half-maximum (fwhm) of Co 2p3/2 peak in LiCoO2-NS is 2.25 eV, which is broader than that of 1.95 eV for bulk LiCoO2.33 Therefore, Co ions in LiCoO2-NS should be in a mixed valence state. Due to the multiple electronic effect of Co ions, it is hard to deconvolute Co 2p3/2 peaks accurately. Lithium content in LiCoO2 compound directly determines the average valence state of Co ions and moreover oxygen vacancies. To get insight into the mixed valence state of Co ions, magnetic susceptibility of LiCoO2-NS was measured (Figure 4a). The relevant data for LiCoO2-B is also displayed for comparison. It is seen that LiCoO2-NS possessed a magnetism stronger than LiCoO2-B. For the latter one, the magnetic susceptibility is near to zero, consistent with the low spin state of Co3+ in stoichiometric LiCoO2-B. Temperature-dependent magnetic susceptibility for LiCoO2-NS well obeys the Curie−Weiss law in two temperature regions (below 100 K and above 175 K). From their linear relationship between 1/(χ − χ0) and temperature (inset of Figure 4a, low temperature region), the calculated effective magnetic moment per Co is 0.67 μB. Based on the effective magnetic moment of low-spin Co4+ (1.73 μB),34 the content of Co4+ is calculated to be about 39%, closer to the expected value of 42% estimated from the charge neutrality of Li0.58CoO2. Such a slight difference might arise from the presence of oxygen vacancy. Fitting the high temperature data gave a magnetic moment of 1.05 μB. The increase of effective magnetic moment suggests a transition of spin state, as happened in many cobalt oxides.35 The presence of high spin state cobalt ions is really crucial for the excellent oxygen evolution. To prove the existence of oxygen vacancy in LiCoO2 nanosheets as for many other metal oxide nanomaterials,36,37 EPR spectra were recorded. Comparing to both counterpart samples (partially delithiated bulk LiCoO2 (LiCoO2-BD) and LiCoO2-B), LiCoO2-NS showed a relatively strong signal at about g = 2.0 (Figure 4b). This signal is attributed to the oxygen vacancies that trap electrons to become single electron vacancies V•o or to adsorb atmospheric H2O and O2 molecules to produce

OH− + * → OH* + e−

(1)

OH* + OH− → O* + H 2O + e−

(2)

O* + OH− → OOH* + e−

(3)

OOH* + OH− → O2 + H 2O + e−

(4)

where * signifies the active site in catalysts, i.e., tetravalent cobalt ions or oxygen vacancy in nanosheets. In order to further illuminate the advantages of LiCoO2-NS as the catalysts for OER, the scheme of OER mechanism is illustrated in Figure 6. As illustrated in Figure 6, LiCoO2-NS exposed with {001} plane could provide a large amount of catalytic sites for OER. Besides these, the presence of oxygen vacancy and Co4+ with high spin state could enhance the electrophilicity of the adsorbed O (Figure 6), and thus facilitate the reaction of an OH− anion with an adsorbed O atom on the catalytic active 7103

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Figure 5. (a) Co K-edge EXAFS k3χ(k) oscillation functions and (b) the corresponding Fourier transform curves for ultrathin LiCoO2 nanosheet and bulk LiCoO2.

sites to form the adsorbed −OOH species, which has been considered as a rate-limiting step for OER.40 Moreover, the coexistence of mixed valence and spin state for cobalt ions in LiCoO2-NS might also increase the number of free electrons, as evidenced by a higher magnetic susceptibility at room temperature when compared to LiCoO2-B. Higher number of free electrons may give rise to an improved charge transport during the OER process. To verify these predictions, oxygen evolution measurements were performed on LiCoO2-NS, LiCoO2-B, and a partially delithiated bulk LiCoO2 sample (LiCoO2-BD), respectively. Figure 7 compares the polarization curves of these samples (sample loading is set at 0.1 mg cm−2) in 0.1 M KOH with a scan rate of 10 mV s−1. All data were recorded in a standard three-electrode electrochemical cell. Due to the effect of ohmic

Figure 6. Electronic structure of LiCoO2-NS and a model proposed for water oxidation on nanosheet surface.

Figure 7. Comparison of OER performances for samples LiCoO2-NS, LiCoO2-BD, and LiCoO2-B: (a) polarization curves, (b) Tafel plots, (c) mass activities at different overpotentials, and (d) chronoamperometry of LiCoO2-NS, LiCoO2-BD, and LiCoO2-B recorded at 1.7 V and 1600 rpm. 7104

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Figure 8. Comparison of overpotential at onset and current density at 10 mA cm−2 for LiCoO2 nanosheets (LiCoO2-NS) with LiCoO2-BD, LiCoO2-B, and other Co-based catalysts reported previously. The symbol “***” represents our catalysts, whereas the data in oblique lines are redrawn from the references, and the detailed information is shown in Table S2.

Figure 9. (a, c, e) Electrochemical double layer capacitance curves on LiCoO2-NS, LiCoO2-BD, and LiCoO2-B with different scan rates. (b, d, f) Plots of current densities at 0.5 V versus scan rates of different samples. The electrochemical surface area of LiCoO2-NS is much larger than others. 7105

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conduction electrons and, thereby, improve the charge transport during the OER process.

resistance, the as-measured reaction currents cannot reflect the intrinsic behavior of electrocatalysts. In the present study, resistance tests were made using an iR correction for all initial data. During the measurement, an overpotential is required to achieve a current density of 10 mA cm−2, a metric relevant to solar fuel production.35,36 As expected, LiCoO2-NS gave an overpotential of η = 0.41 V, the smallest among all samples. Further, ultrathin LiCoO2-NS exhibited a large anodic current density of 19.9 mA cm−2 at 1.7 V, which is about 3 times larger than that of LiCoO2-BD and about 19 times larger than LiCoO2-B. Moreover, the corresponding Tafel plots in Figure 7b indicate that ultrathin LiCoO2 sample possesses a smallest Tafel slope at 88 mV decade−1. A small Tafel slope is really important for practical applications, because it could give a remarkable increase of OER rate. It should be noted that the mass activity of LiCoO2-NS is 95.6 A g−1 at an overpotential of 0.41 V, much higher than that of 14 A g−1 for LiCoO2-BD and that of 2.9 A g−1 for LiCoO2-B. The variation of current density with time was also investigated. Accordingly, the durability of the electrocatalysts for OER at an applied potential of 1.7 V at pH = 13 was evaluated by chronoamerometry. As shown in Figure 7d, the current density for LiCoO2-NS was still as high as 11.2 mA cm−2 after running for 1 h, which is nearly one order of magnitude higher than those of LiCoO2-BD and LiCoO2-B. A slight decrease of current density is also observed, which might be related to the partial oxidation of catalyst during the OER process as reported elsewhere,41 leading to a decrease of oxygen vacancy content. As mentioned above, oxygen vacancy is very important for enhancing the electrophilicity of the adsorbed O, and thus facilitates the reaction of an OH− anion with an adsorbed O atom on the catalytically active sites to form the adsorbed −OOH species. The decrease of oxygen vacancy content could be the primary reason for the reduction of the catalytic activity, as claimed elsewhere.42 To further demonstrate the excellent OER activity of LiCoO2-NS, we compared the overpotentials of onset and current density with other Co-based catalysts reported previously. Under similar test conditions, LiCoO2-B and LiCoO2-BD exhibited an OER activity lower than most reported catalysts, whereas the overpotential for LiCoO2-NS was lower than those previously reported for most other catalysts, as shown in Figure 8. The detailed data of catalytic activity and more comparison are given in Table S2. It is obvious that LiCoO2-NS has an excellent catalytic activity at a relatively low loading. All these results demonstrate that ultrathin LiCoO2 nanosheets featured by oxygen vacancy and special electron configuration of Co ions could act as the efficient catalyst for OER. To confirm the high exposure of effective active sites for LiCoO2-NS, electrochemical double layer capacitances (Cdl) are measured, which are expected to be linearly proportional to the effective surface area. The values of Cdl for LiCoO2-NS, LiCoO2-BD, and LiCoO2-B were 20410, 327, and 168 uF cm−2, respectively (Figure 9). Since Cdl is proportional to the active area of electrocatalysts, larger Cdl value may give rise to a higher active area. That is, LiCoO2-NS should possess the largest active area among all samples studied. The remarkably enhanced activity of the ultrathin nanosheets LiCoO2 could be attributed to the following aspects: (i) {001} facets of LiCoO2 nanosheets may have the most active sites; (ii) the coexistence of special electronic state of cobalt ions and oxygen vacancies could decrease the adsorption energy of oxygen species and, thus, increase the catalytic activity of OER; and most importantly, (iii) mixed valence and spin state of cobalt could increase the number of

4. CONCLUSIONS We report on a facile method to prepare ultrathin LiCoO2 nanosheets with {001} facets exposed. These ultrathin nanosheets have only 5−6 cobalt oxide layers, and are thus merited by a surprising coexistence of mixed valence/spin state for cobalt ions and oxygen vacancies. Such merits enable ultrathin LiCoO2 nanosheets to possess a high electron conduction as well as an electrophilicity to the adsorbed oxygen. Consequently, when tested as an OER catalyst, ultrathin LiCoO2 nanosheets showed an excellent activity. The mass activity of ultrathin LiCoO2 nanosheets was 95.6 A/g at an overpotential of 410 mV, much higher than that of bulk LiCoO2. Since LiCoO2 belongs to a prototype cation-exchanged layered metal oxide with many counterparts, the findings reported here would open up a new route to prepare ultrathin 2D layered metal oxides with cation-exchanged feature for OER and many other important chemical reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14896. XRD, SEM, AFM, Raman, XPS, and EXAFS results and photograph of a colloidal suspension (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guangshe Li: 0000-0002-3278-1804 Funding

This work was financially supported by the National Natural Science Foundation of China (No. 21025104, 21271171, 21671077, and 21571176). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Beijing Synchrotron Radiation Facility (BSRF) for their technical assistance. REFERENCES

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DOI: 10.1021/acsami.6b14896 ACS Appl. Mater. Interfaces 2017, 9, 7100−7107