# Fileset [Manuscript.pdf](https://mdr.nims.go.jp/filesets/a9486f0e-a84a-4b3c-835e-25571de0f1e9/download) ## Creator [Nobuyuki Sakai](https://orcid.org/0000-0002-9395-6751), Katsutoshi Fukuda, [Renzhi Ma](https://orcid.org/0000-0001-7126-2006), Takayoshi Sasaki ## Rights This document is the Accepted Manuscript version of a Published Work that appeared in final form in Synthesis and Substitution Chemistry of Redox-Active Manganese/Cobalt Oxide Nanosheets, copyright © 2018 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acs.chemmater.7b04068[In Copyright](http://rightsstatements.org/vocab/InC/1.0/) ## Other metadata [Synthesis and Substitution Chemistry of Redox-Active Manganese/Cobalt Oxide Nanosheets](https://mdr.nims.go.jp/datasets/8de4a50c-6ffb-4ecb-8a66-b838d89e8c64) ## Fulltext Synthesis and Substitution Chemistry of Redox-Active Manga-nese/Cobalt Oxide Nanosheets Nobuyuki Sakai,*,† Katsutoshi Fukuda,‡ Renzhi Ma,† and Takayoshi Sasaki*,† †International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan and ‡Office of Society-Academia Collaboration for Innovation, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan. ABSTRACT: We report the synthesis and electrochemical properties of Co-substituted manganese oxide nanosheets (Mn1−xCoxO2). Polycrystalline samples of layered Na0.6Mn1−xCoxO2 (x = 0.2−0.5) were synthesized as starting materials. A linear decrease in the lattice constant a with increasing Co content supported the successful substitution of Co3+ ions for Mn3+ ions in the host layers. Acid-exchange treatment of the Na0.6Mn1−xCoxO2 powders resulted in the formation of H-Mn1−xCoxO2 while preserving the Mn/Co ratio and layered structure. Exfoliation of H-Mn1−xCoxO2 was achieved by reaction with tetra−n−butylammonium ions, yielding unilamellar Mn1−xCoxO2 (x = 0.2−0.5) nanosheets with a thickness of 0.8 nm. The optical absorption peak of the obtained Mn1−xCoxO2 nanosheets was continuously blueshifted as the Co content in-creased. The Mn1−xCoxO2 nanosheets exhibited well-defined redox peaks, which were shifted to a negative potential with increasing Co content. These results suggest that the 3d orbitals of Mn and Co are mixed owing to their statistical distri-bution in the nanosheets. The Mn1−xCoxO2 nanosheet electrodes showed a capacitance of 700−1000 F g−1 and improved cycle performance compared to MnO2 nanosheets. INTRODUCTION Metal oxide nanosheets have received increasing atten-tion because of their versatility for various applications owing to their properties based on their widely tunable composition and structure.1−6 The oxide nanosheets can be obtained by exfoliating their mother layered crystals into each elementary layer. The resultant nanosheets have a thickness of ~1 nm, while the lateral size ranges from submicrometers to several tens of micrometers. Due to such unusually high two-dimensional (2D) morphologies, the nanosheets are expected to show unique electronic and physicochemical properties different from the corre-sponding bulk materials. For example, titania nanosheets exhibit a shift in the band edge that increases the band gap energy in comparison with their bulk counterparts due to quantum size effects.7,8 Nanosheets with redox capabilities are promising for applications such as elec-trochemical capacitors and lithium-ion batteries because of their extremely large surface area. Such applications can be anticipated for nanosheets composed of RuO2,9,10 MnO2,11−16 CoO2,17−19 Mn1/3Co1/3Ni1/3O2,20 etc. Ruthenic acid nanosheets have been reported to exhibit a capacitance as large as 658 F g−1.9 Reassembled nanosheets of MnO221,22 or CoO219 have been used as cathodes for lithium-ion batter-ies. Modulation of the properties can be achieved by the el-emental substitution or doping of the redox-active nanosheets, as has been reported for bulk materials. For example, the influence of cobalt doping on the electro-chemical properties of layered manganese oxide has been studied to improve the cycle performance of lithium-ion batteries.23−25 The doping of different elements into redox-active nanosheets and its effect on the crystal structure, optical properties, and redox properties have been scarce-ly examined, although such properties would be largely modified. Examples of redox-active nanosheets doped with different elements include MnO2 nanosheets partly substituted with Co,26 Co/Ni,20 or Ru27. However, the re-dox properties have been examined only for Ru-substituted MnO2 nanosheets. The doping of 10% ruthe-nium into MnO2 nanosheets was reported to enhance the electrochemical capacitance of the reassembled material by ~40%.27 Hence, doping is one method to improve the redox properties of nanosheets to make them favorable for applications such as energy-storage devices. Because ruthenium is a noble metal and expensive, the replace-ment of ruthenium with 3d transition metal elements as dopants is expected to lead to possibilities for the devel-opment of various energy-storage materials. Hence, it is of interest to examine the effect of cobalt substitution, as cobalt is a 3d transition metal, on the crystal and elec-tronic structure of MnO2 nanosheets. In the present study, we synthesize Co-substituted MnO2 nanosheets (Mn1−xCoxO2) through the synthesis, protonation, and exfoliation of layered manganese oxide doped with cobalt. The starting material was synthesized by following the reported procedure.23−25 Although proto-nation of the starting material has been studied, the in-fluence of the doping amount on the crystal structure and the valence of constituent elements has not been report- ed.28 Although the synthesis of MnO2 nanosheets doped with cobalt has been examined via a bottom-up process,26 there is no direct evidence for the doping of Co into the lattice of MnO2 nanosheets. In addition, the structural aspects and electrochemical properties have not been studied. In this work, we conduct a systematic investiga-tion on the partial substitution of Co for Mn in layered Na-Mn1−xCoxO2 and H-Mn1−xCoxO2 structures, as well as Mn1−xCoxO2 nanosheets, via various techniques, including X-ray diffraction (XRD), elemental analysis, X-ray absorp-tion near edge structure (XANES), and X-ray photoelec-tron spectroscopy (XPS). The nanosheets obtained via exfoliation of the layered compound have a larger lateral size and higher crystallinity than those synthesized via the bottom-up process, which is beneficial for applica-tions such as electrochemical devices. We also examine the lattice constants, optical properties, and electrochem-ical properties of the Mn1−xCoxO2 nanosheets as a function of cobalt substitution, which may give insight into the crystal structure and electronic structure of the nanosheets. EXPERIMENTAL SECTION Reagents. All chemicals were of analytical grade and used as purchased. Milli-Q filtered water was used throughout the experiments. Synthesis of Na0.6Mn1−xCoxO2. Layered compounds, Na0.6Mn1−xCoxO2 (x = 0.2, 0.3, 0.4, 0.5), as starting materi-als were synthesized via a solution route according to the literature.23 Mn(CH3COO)2×4H2O, Co(CH3COO)2×4H2O, and NaNO3 in a stoichiometric ratio (1−x:x:0.6) were dis-solved in pure water and dried on a hot plate at 130 °C overnight. The resulting precipitates were pre-annealed in an oven at 300 °C for 20 h in air to decompose the organic components. Then, the materials were heated at 700 °C (x = 0.2), 730 °C (x = 0.3), 755 °C (x = 0.4), or 800 °C (x = 0.5) for 20 h in air and then quenched. Protonation. The resulting black powder (2 g) of Na0.6Mn1−xCoxO2 was dispersed and shaken in 0.1 mol dm−3 HCl solution (400 cm3) at room temperature. This proce-dure was repeated three times by renewing the acid solu-tion every 24 h. The resulting acid-exchanged product was filtered, washed with pure water, and dried in air. Exfoliation. A weighed amount (0.4 g) of the proto-nated form was dispersed in 100 cm3 of aqueous tet-ra−n−butylammonium hydroxide (TBAOH) solution (12−20 mmol dm−3). The resulting suspension was shaken moderately (120 rpm) for 2 weeks at ambient temperature and then centrifuged at 3000 rpm (1680 G) for 30 min to sediment/remove any unexfoliated material. The upper solution was collected to obtain a colloidal suspension of the nanosheets, which was dark green in color. Film Fabrication. Si wafer substrates were cleaned by immersion in a bath of methanol/HCl (1:1 in volume) and then concentrated H2SO4 for 30 min each. Indium tin oxide (ITO)-coated quartz glass substrates with a flat sur-face (Kuramoto Co., Ltd., sheet resistance = 10 W/sq.) were cleaned by sonication in acetone, ethanol, and ul-trapure water for 15 min each. Monolayer films were fab-ricated via a sequential adsorption procedure. First, the substrate was immersed in a polyethylenimine (PEI) solu-tion (1.25 g dm−3, pH 9) to make the surface positively charged. Then, the PEI-coated substrate was immersed in the colloidal suspension of nanosheets. The substrate was immersed in each solution for 20 min and then rinsed thoroughly with ultrapure water. Characterization. The contents of Na, Mn, and Co in Na0.6Mn1−xCoxO2 and its acid-exchanged phases were de-termined by inductively coupled plasma (ICP) optical emission spectrometry (Hitachi, SPS3520UV-DD) after dissolving the sample in a mixed solution of HCl and H2O2. Thermogravimetric-differential thermal analysis (TG−DTA, Rigaku, TG-8120) was performed in the tem-perature range of 25−1000 °C at a heating rate of 10 °C min−1. The mean valence of the oxidation state of Mn and Co in the samples was determined by redox titration.15,29 The sample was dissolved in sulfuric acid containing an excess amount of sodium oxalate as a reducing agent. The residual sodium oxalate was titrated against the standard aqueous solution of potassium permanganate to deduce the mean valence of the oxidation state of the Mn and Co ions. XRD data were collected using a powder diffractom-eter (Rigaku, Ultima IV) with graphite monochromatized Cu Ka radiation (l = 0.15405 nm). Scanning electron mi-croscopy (SEM) observations were carried out using a microscope (JEOL, JSM-6010LA). Atomic force microsco-py (AFM) images were collected in tapping mode using a Hitachi NanoNavi II Station and a SI-DF20 cantilever to examine the topography of the nanosheets deposited on the Si wafer substrate. Transmission electron microscopy (TEM) characterizations were performed on a JEOL JEM-3100F energy-filtering (Omega type) microscope. XPS measurements were performed on a PHI Quantera SXM (ULVAC-PHI) with an Al Ka source. In-plane XRD and XANES measurements were performed using synchrotron X-ray radiation at BL-6C, Photon Factory, High Energy Accelerator Research Organization. A spectrophotometer (Hitachi, U-4100) was employed to record the UV−vis absorption spectra. A potentiostat (Solartron, SI1287) was used for electrochemical measurements in a conventional three-electrode, single-compartment glass cell. An ITO substrate coated with Mn1−xCoxO2 nanosheets served as the working electrode. The counter and reference elec-trodes were platinum wire and Ag|Ag+, respectively. Pro-pylene carbonate containing 0.1 mol dm−3 LiClO4 was em-ployed as the supporting electrolyte. RESULTS AND DISCUSSION Synthesis of Na0.6Mn1−xCoxO2. Figure 1a shows the XRD data of the obtained black powder of Na0.6Mn1−xCoxO2 (x = 0.2). All the diffraction peaks can be indexed to the rhombohedral structure [space group: Rm (No. 166)] with refined unit-cell parameters of a = 0.28642(1) nm and c = 1.6953(1) nm (Table S1). All samples including other compositions (x = 0.3−0.5) were obtained in a single phase based on the rhombohedral structure. The lattice constant a linearly decreased as the Co con-tent (Co/(Mn+Co)) increased (Figure 2a), following Ve-gard's law. Hence, the crystal structure of the obtained 3 materials is the same as that of birnessite-type layered manganese oxide (K0.45MnO2) reported previously,11,30 and Co substitutes at Mn sites in a statistical distribution on the MnO2 planes of the crystals. Figure 1. XRD patterns of (a) Co-doped layered manganese oxides, Na-Mn1−xCoxO2 (x = 0.2, 0.3, 0.4, 0.5), and (b) their protonated materials, H-Mn1−xCoxO2 (x = 0.2, 0.3, 0.4, 0.5). The XRD intensity is shown on a log scale. Figure 2. (a) Lattice constant a and (b) interlayer spacing c/3 of Na- (filled circle) and H- (open circle) Mn1−xCoxO2 pow-ders as a function of cobalt content (x). The data for x = 0.0 (K0.45MnO2 and H0.13MnO2) were taken from ref 11 and 32, respectively. The chemical composition of the sample (x = 0.2) was determined to be Na0.56Mn0.81Co0.19O2 (Anal. Calcd: Na, 12.8%; Mn, 44.2%; Co, 11.1%. Found: Na, 13.0%; Mn, 44.7%; Co, 11.0%.) based on the results of elemental analysis by ICP and TG measurement. This formula agrees well with the nominal ratio of Na, Mn, and Co ions (0.6:0.8:0.2) during preparation. The composition of the other samples (x = 0.3−0.5) was also in good agreement with the prepa-ration ratio of Na:Mn:Co = 0.6:1−x:x (Table 1). This formu-la also yields a negative charge of 0.56 per Mn0.81Co0.19O2 layer. The redox titration with sodium oxalate suggested that the mean valence of Mn and Co, ZMn+Co, was 3.50, which is in good agreement with that estimated from the formula shown above (3.44). The slight difference in the mean valence (0.06, Figure S1) may be due to oxidation of the materials when dissolved in sulfuric acid during titra-tion. Similar values of ZMn+Co were obtained among all compositions of Na-Mn1−xCoxO2 synthesized (Table 1). This is compatible with the fact that the four samples have the same interlayer spacing of 0.57 nm (Figure 2b). Although the obtained ZMn+Co values are close to the ZMn value of K0.45MnO2, the interlayer spacing of K0.45MnO2 is larger by ~0.7 Å than that of Na-Mn1−xCoxO2. This is rea-sonable because the ionic radius of K+ (1.38 Å) in the in-terlayer space is larger by 0.36 Å than that of Na+ (1.02 Å).31 Table 1. Composition of the Obtained Layered Crys-tals of Na-Mn1−xCoxO2 and Their Protonated Materials and Mean Valence of Metal Ions Estimated by Chem-ical Titration To obtain quantitative information on the individual mean valence of Mn (ZMn) and Co (ZCo), XANES spectra in Mn K-edge and Co K-edge regions were measured for the samples and the reference compounds (Figure 3). The profiles of the Co K-edge spectra of Na-Mn1−xCoxO2 (x = 0.2−0.5) were similar to that of LiCoO2 and overlapped with each other, suggesting that the ZCo value is ~3 for all samples. In contrast, the profiles of Mn K-edge spectra were close to that of MnO2 and slightly shifted to higher energy as the cobalt content increased. This result sug-gests that the ZMn value is close to 4 and increases with increasing cobalt content. Based on the composition rati-os of Mn/Co and the ZMn+Co values deduced from the titra-tion, assuming that ZCo = 3, the ZMn values can be estimat-ed to be 3.62, 3.72, 3.85, and 3.92 for x = 0.2, 0.3, 0.4, and 0.5, respectively (Table S2). The change in the ZMn value with composition is qualitatively consistent with the shift in the Mn K-edge spectra. The increase in the value of ZMn can be considered to be a result of the substitution of Co3+ for Mn3+. Figure 3. (a) Mn K-edge and (b) Co K-edge XANES spectra for Na-Mn1−xCoxO2 and H-Mn1−xCoxO2 (x = 0.2 (red lines), 0.3 (blue lines), 0.4 (green lines), and 0.5 (purple lines)) and the reference compounds. The substitution of Co3+ for Mn3+ may induce the con-traction of the average bond length of (Mn/Co)−O and hence the lattice constant a, because the ionic radius of low-spin Co3+ (0.545 Å) is smaller than that of high-spin Mn3+ (0.645 Å).31 Although there are other combinations of high/low-spin Co3+/Mn3+, the estimated lattice constant a for low-spin Co3+ and high-spin Mn3+ is consistent with the experimental data (Figure S2). Based on the Mn/Co ratio, ZMn, and ionic radii of Co3+, Mn3+, Mn4+ (0.53 Å), and O2− (1.36 Å), the average bond lengths can be estimated to be 1.929, 1.917, 1.906, and 1.902 Å for x = 0.2, 0.3, 0.4, and 0.5, respectively (Table S2). The contraction ratios of the average bond lengths for x = 0.2, 0.3, 0.4, and 0.5 against that for K0.45MnO2 are 99.3%, 98.7%, 98.1%, and 97.9%, respectively, which are in good agreement with the con-traction trend of lattice constant a (99.3%, 98.9%, 98.3%, and 98.0% for x = 0.2, 0.3, 0.4, and 0.5) (Table S2). There-fore, we conclude that the Mn3+ ions are substituted by Co3+ ions in the MnO2 planes and the crystal structure of birnessite-type layered manganese oxide is preserved. Protonation of Na0.6Mn1−xCoxO2. The protonation of Na-Mn1−xCoxO2 (x = 0.2) was carried out by an acid leach-ing method. Based on the results of elemental analysis by ICP, TG measurement, and redox titration of the resulting material, H-Mn1−xCoxO2 (x = 0.2), the composition was formulated to be H0.22Na0.03Mn0.82Co0.18O2×0.7H2O (Anal. Calcd: Na, 0.7%; Mn, 44.6%; Co, 10.5%; H2O, 12.5%. Found: Na, 0.6%; Mn, 45.0%; Co, 10.7%; H2O, 12.6%.) The proton content was deduced by considering the total charge neutrality. The protonated material contains ~0.7 mol H2O in the interlayer space. The compositions of the other materials (x = 0.3−0.5) were also formulated in the same way (Table 1). The acid treatment significantly re-duced the content of Na in the material, while the ratio of Mn to Co in the host layers was preserved upon treatment. The mean valence of Mn and Co, ZMn+Co, revealed by the redox titration ranged from 3.75 to 3.50 for x = 0.2 to 0.5 (Table 1). Each value is larger than that before the acid treatment (Figure S1), suggesting that acid leaching in-duces not only proton exchange but also the oxidative deintercalation of Na+ ions, involving the disproportiona-tion of Mn3+ to Mn4+ and Mn2+, as has been demonstrated for various types of manganese oxides.30 The obtained H-Mn1−xCoxO2 (x = 0.2) powder showed an XRD pattern that can be indexed to the rhombohedral structure [space group: R m (No. 166)] (Figure 1b). The refined unit-cell parameters (a = 0.2834(1) nm, c = 2.165(3) nm, Table S3) are in good agreement with birnessite-type layered manganese oxide (H0.13MnO2×0.7H2O),11,15,32 sug-gesting that the structure of the Co-substituted MnO2 planes of the crystal remained intact upon protonation. The other compositions of H-Mn1−xCoxO2 (x = 0.3−0.5) also have the same structural features (Figure 1b). The unit-cell parameter a was nearly constant for H-Mn1−xCoxO2 powders with different cobalt contents (x), which is different from the results of Na-Mn1−xCoxO2 (Fig-ure 2a). Because the lattice constant a is correlated with the average bond length of (Mn/Co)−O, the average bond length can be estimated based on the composition ratio of Mn/Co and the ZMn+Co deduced from the titration, as-sumimg that ZCo = 3. The ZMn values were estimated to be 3.91, 3.92, 3.89, and 3.94 for x = 0.2, 0.3, 0.4, and 0.5, re-spectively, which are similar to those for H0.13MnO2×0.7H2O (Table S4).11,32 The result that the ZMn values are almost invariable against different cobalt con-tents is compatible with the XANES study showing over-lapping Mn K-edge spectra (Figure 3a). The average bond lengths can be estimated to be 1.901, 1.901, 1.904, and 1.901 Å for x = 0.2, 0.3, 0.4, and 0.5, respectively, which are in good agreement with the result that the lattice constant a is almost constant for the H-Mn1−xCoxO2 powders with different cobalt contents (x) (Table S4). The interlayer distance was expanded by 0.15 nm from 0.57 nm to 0.72 nm for the sample with x = 0.2 after pro-tonation, as indicated by the shift of the basal diffraction peak to a lower angle (Figure 1). This change can be ex-plained by the introduction of water molecules into the interlayer space since the water content increased upon protonation. Such increase in the interlayer distance was observed for the samples with other compositions (x = 0.3−0.5), although the expanded interlayer distance was dependent on the composition (Figure 2b). Because the amount of water content was almost the same, the differ-ence in the interlayer distance is attributable to the dif-ference in the mean valence of the metal ions. As the mean valence decreases with increasing cobalt content (x), the charge density of the host layers and the number of guest cations becomes large, resulting in a smaller inter-layer spacing. The samples were composed of plate-like crystals with lateral dimensions of 0.5−1 µm and 0.1−0.5 µm for the sample with x = 0.2 before and after the protonation, re-spectively (Figure S3). The decrease in the lateral size of the crystals is probably due to the dissolution of Mn2+ species produced by the disproportionation30 of Mn3+ dur-ing acid leaching. Because the molar ratio of Mn to Co did not change after the protonation process (Table 1), Co2+ species are also dissolved during the protonation process, and the dissolution rates of Mn2+ and Co2+ are similar. The dissolution of Co2+ ions was quantitatively confirmed by a color change of the acid solution used for protonation (Figure S4). The dissolution of Co2+ is caused by the dis-proportionation of Co3+, which should accompany the formation of Co4+.33−35 However, Co4+ can take one elec-tron from Mn3+, resulting in the formation of Co3+ and Mn4+ in the crystal, which is not contradictory to the as-sumption that all cobalt species are Co3+ on the basis of the XANES results. Moreover, the valence estimated from the dissolved amount of Co2+ and the process described above, ZMn', was consistent with that estimated from the titration and XANES studies (Table S5). It is worth mentioning the different mechanisms of charge compensation against cobalt doping in Na-Mn1−xCoxO2 and H-Mn1−xCoxO2. In the former case, since the number of guest cations and the charge of the host layers are fixed against a change in the cobalt content, the change of the mean valence of Mn is required to maintain the charge neutrality of the crystal. In the latter case, be-cause the number of guest cations is variable, the charge neutrality of the crystal can be maintained even without changing the mean valence of Mn. Exfoliation of protonated H-Mn1−xCoxO2. After shak-ing of the protonated samples dispersed in TBAOH solu-tion (TBA+/H+ = ~1) and subsequent centrifugation of the suspension, the top solution was collected. The top solu-tion was a colloidal suspension with a dark-green color. The suspension showed two broad absorption features in the UV−vis absorption spectrum (Figure 4a). One of the 3 features is an absorption peak at 368 nm for x = 0.2, which is similar to the spectrum of the MnO2 nanosheet suspen-sion. The molar extinction coefficient at the peak wave-length was estimated to be as large as ~104 mol−1 dm3 cm−1, which is equivalent to that for the MnO2 nanosheets.11,15 Such a large value is expected only for direct, allowed transitions. Hence, the absorption observed for the Mn1−xCoxO2 nanosheets is ascribable to an electronic tran-sition from the occupied band composed mostly of the oxygen 2p orbital to the unoccupied band composed mostly of the metal 3d orbital. The peak wavelength blueshifted as the Co content (x) increased (Figure 4b). Because the substitution of Mn for Co shifts the Fermi level of the metal band to more negative potential due to the increase in the number of electrons in the metal 3d orbitals, the energy required for the excitation should increase, resulting in the blueshifting of the absorption peak. The other feature is the appearance of an absorp-tion shoulder at 600−700 nm. This absorption should be attributable to Co3+ species in the nanosheets because the undoped MnO2 nanosheets do not have such absorption. These results support the homogeneous distribution of the Co ions into the MnO2 lattice. Figure 4. (a) UV−vis absorption spectra of colloidal suspen-sions of Mn1−xCoxO2 (solid lines, x = 0.2, 0.3, 0.4, 0.5) and MnO2 (dotted line) nanosheets. (b) Energy at the peak wave-length plotted as a function of Co content (x). To examine the morphology, composition, and crystal-linity, the exfoliated materials adsorbed on Si substrates pre-coated with polycations were investigated by AFM, XPS, and in-plane XRD. As shown in Figure 5a, AFM ob-servation detected many 2D materials with uniform thickness of ~0.8 nm, which is comparable to that of un-doped MnO2 nanosheets.11 The lateral size range from 100 to 500 nm is also compatible with that observed by TEM, as shown in Figure 5b. The ratios of Mn to Co in the Mn1−xCoxO2 nanosheets revealed by XPS studies (Figure S5, Table S6) agreed well with those of the H-Mn1−xCoxO2 powders, suggesting that the exfoliation process does not involve the change of the chemical composition of the host layers of H-Mn1−xCoxO2. On the other hand, the in-plane XRD measurements (Figure 6) showed diffraction peaks, which are derived from the 2D hexagonal structure. The refined lattice constant a of the Mn1−xCoxO2 (x = 0.2) nanosheets was 0.28384(4) nm, which is close to that be-fore exfoliation (a = 0.2834(1) nm). Similar to the H-Mn1−xCoxO2 powders, the lattice constant a was almost constant for the Mn1−xCoxO2 nanosheets with different ratios (x) of Co/(Mn+Co), indicating that the 2D architec-ture remained substantially unchanged upon exfoliation. The smaller lattice constant a of the Mn1−xCoxO2 nanosheets compared with that (a = 0.28593(5) nm) of MnO2 nanosheets32 is ascribable to the substitution of smaller Co3+ ions (0.545 Å) for larger Mn3+ ions (0.645 Å) in the MnO2 nanosheets. Based on the results shown above, we conclude that Mn1−xCoxO2 (x = 0.2−0.5) nanosheets with a plane composed of stoichiometric rati-os of Mn and Co atoms sandwiched by two planes of hex-agonally packed O atoms were obtained. Figure 5. (a) Typical AFM image and height profile of exfoli-ated Mn1−xCoxO2 (x = 0.2) nanosheets deposited on a Si sub-strate. (b) TEM image of exfoliated Mn1−xCoxO2 (x = 0.2) nanosheets. Figure 6. (a) In-plane XRD patterns of monolayer films of Mn1−xCoxO2 (x = 0.2, 0.3, 0.4, 0.5) and MnO2 nanosheets. The synchrotron X-ray wavelength was 0.12001 nm. (b) Refined lattice constants (a) of the Mn1−xCoxO2 nanosheets plotted as a function of cobalt content (x). Electrochemical properties of Mn1−xCoxO2 nanosheets. Electrochemical studies of the Mn1−xCoxO2 (x = 0.2−0.5) nanosheets were performed on monolayer films of the nanosheets fabricated on ITO substrates. Formation of the monolayer films was confirmed by AFM observation (Figure S6). Cyclic voltammograms (CV) in propylene carbonate containing 0.1 mol dm−3 LiClO4 gradually changed with increasing number of potential sweeps, and CV curves under steady-state conditions were obtained after 100 cycles (Figure 7a, Figure S7). The relative capacitance of the Mn1−xCoxO2 nanosheets gradu-ally decreased with the number of potential sweeps and reached ~85% after 200 cycles, which is greater than that of the MnO2 nanosheets, indicating that the cycle per-formance is improved by Co doping (Figure 7b). The in-plane XRD peaks were observed even after 200 cycles of potential sweeps (Figure S8), indicating that the 2D nanosheet structure was preserved. The CV curves of the Mn1−xCoxO2 (x = 0.2−0.5) nanosheet electrodes showed well-defined reduction and oxidation peaks (Figure 7c), which are similar to those reported previously for MnO2 nanosheet electrodes.36 These peaks can be ascribed to the reduction/oxidation of the Mn1−xCoxO2 nanosheets accompanied by the insertion/extraction of Li+ ions. The fact that only one pair of redox peaks is observed implies the formation of mixed orbitals of Mn and Co. The redox potential for the Mn1−xCoxO2 (x = 0.2) nanosheets was −0.42 V, which is more negative than that (−0.35 V) for MnO2 nanosheets, and the redox potential linearly shifted in the negative direction as the cobalt content in the Mn1−xCoxO2 nanosheets increased (Figure 7d). This result clearly indicates that the Fermi level of the Mn1−xCoxO2 nanosheets shifts toward a more negative potential with increasing x, which is qualitatively consistent with the energy shift of the absorption peak upon changing the cobalt content, as shown in Figure 4. These results sug-gest that the 3d orbitals of Mn and Co are mixed owing to their statistical distribution in the nanosheets. The mag-nitude of the redox potential shift, however, was almost half of that of the absorption peak energy. The redox po-tential shift is reflected only by the Fermi level shift, while the absorption energy shift is based on the shifts of both the excited state and the ground state. These results sug-gest that the substitution of Co3+ for Mn3+ shifts the ener-gy level of the highest edge of the occupied band, which is the origin of the electronic excitation, toward a more pos-itive potential. In addition to the faradaic peaks, the Mn1−xCoxO2 nanosheet electrodes showed capacitive cur-rents, similar to the MnO2 nanosheet electrodes. The ca-pacitance was estimated to be 700−1000 F g−1 on the as-sumption that the electrode is covered with a perfect monolayer film of nanosheets. This value is greater than that reported for Mn1−xRuxO2 nanosheets (360 F g−1),27 alt-hough the electrode system is different. The previous study examined restacked composites of the nanosheets, whereas the present study examined monolayer films of nanosheets. It is also important to investigate the electro-chemical capacitive properties of a single nanosheet. The present study clarified that Mn1−xCoxO2 nanosheets are promising candidates for electrochemical capacitors with high energy density and high cycle performance. Figure 7. (a) Cyclic voltammograms of monolayer films of Mn1−xCoxO2 (x = 0.2) nanosheet electrodes in a propylene carbonate solution containing 0.1 mol dm−3 LiClO4 at a sweep rate of 50 mV s−1. (b) Capacitance changes with potential sweeps. (c) Cyclic voltammograms of monolayer films of Mn1−xCoxO2 (x = 0.2, 0.3, 0.4, 0.5) and MnO2 (dotted line) nanosheet electrodes after 200 cycles. (d) Redox potential of Mn1−xCoxO2 (x = 0.2, 0.3, 0.4, 0.5) and MnO2 nanosheets plot-ted as a function of cobalt content (x). The potential values are given with respect to Ag|Ag+. CONCLUSIONS In the present work, we successfully synthesized Co-substituted MnO2 nanosheets with a composition of Mn1−xCoxO2 (x = 0.2−0.5) by exfoliating the protonated form of Na0.6Mn1−xCoxO2 and studied the electrochemical properties of the exfoliated Mn1−xCoxO2 nanosheets. Pro-tonation can be achieved along with the preservation of the chemical composition of the host layers of the starting material. We clarified different charge compensation mechanisms against cobalt doping in the two different systems, Na-Mn1−xCoxO2 and H-Mn1−xCoxO2. The ratio of Mn to Co did not change upon exfoliation, yielding Mn1−xCoxO2 (x = 0.2−0.5) nanosheets with a plane com-posed of stoichiometric ratios of Mn and Co atoms sand-wiched by two planes of hexagonally aligned O atoms. The obtained Mn1−xCoxO2 nanosheets showed improved cycle performance compared to MnO2 nanosheets and higher capacitance than the Mn1−xRuxO2 nanosheets, mak-ing them favorable for applications such as energy-storage devices. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Refinement data for XRD patterns of Na-Mn1−xCoxO2 and H-Mn1−xCoxO2. Estimated interatomic distance of (Mn/Co)−O based on valence and ionic radii for Na-Mn1−xCoxO2 and H-Mn1−xCoxO2. Valence of Mn for H-Mn1−xCoxO2 estimated from the amount of dissolved Co2+ ions. Mean valence of metal ions deduced by chemical titration or Na content. Lattice constants estimated on the basis of bond length. SEM images of the samples. XPS of Mn1−xCoxO2 nanosheets on Si. AFM images of a monolayer film of Mn1−xCoxO2 nanosheets on ITO. Changes of CV curves. In-plane XRD of Mn1−xCoxO2 nanosheets after CV. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: (N.S.) sakai.nobuyuki@nims.go.jp *E-mail: (T.S.) sasaki.takayoshi@nims.go.jp ACKNOWLEDGMENT This work was partly supported by the World Premier Inter-national Research Center Initiative on Materials Nanoarchi-tectonics (WPI-MANA), MEXT, Japan. N.S. acknowledges support from JSPS KAKENHI Grant Number JP16K14428. A part of this work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2014G571). We would like to thank Prof. Izumi Nakai and Ms. Chizuru Ohishi (Tokyo Univ. of Science) for their prelimi-nary work relating to this study. 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