Elsevier

Materials Research Bulletin

Volume 47, Issue 11, November 2012, Pages 3855-3859
Materials Research Bulletin

One-pot synthesis of co-substituted manganese oxide nanosheets and physical properties of lamellar aggregates

https://doi.org/10.1016/j.materresbull.2012.08.008Get rights and content

Abstract

Co-substituted manganese oxide nanosheets, (Mn1−xCox)O2 have been synthesized in the form of a colloidal suspension via a simple one-pot method. Substitution effects on the structural, optical absorption, and magnetic properties are investigated for the nanosheets and their lamellar aggregates. The composition of the (Mn1−xCox)O2 nanosheets can be controlled continuously by adjusting the molar ratio of the starting materials. The solubility limit is x  0.20 based on the cell volume. In the 0.00  x  0.20 range, the band gap energy, magnetic moment, and Weiss temperature change systematically with x. The charge density of the (Mn,Co)O2 layer is independent of x (i.e., [(Mn,Co)O2]0.2−) and the cobalt ions are trivalent in low-spin state.

Highlights

Solid solution nanosheets, (Mn1−xCox)O2, synthesized via facile one-pot process. ► The structural characterization of nanosheets revealing a single (Mn,Co)O2 layer and the solubility limit as x  0.20. ► The invariant charge density of the layer upon Co substitution. ► Systematic dependence of magnetic and optical properties of the lamellar aggregates.

Introduction

Layered transition metal (TM) oxides such as RbCa2Nb3O10 [1], [2], KxMnO2 [3], [4], and CsxTi2−x/4x/4O4 [5], [6] have exchangeable alkali cations between chemically inert metal oxide layers (i.e., [Ca2Nb3O10], [MnO2]x, [Ti2−x/4x/4O4]x). Reactions with bulky organic molecules in aqueous solution at low temperatures allow these TM oxides to exfoliate into ultra-thin objects, termed often as ‘nanosheets’ [7]. The utmost feature of the TM oxide nanosheets is their exceptionally high specific surface area, rendering them promising candidates for a variety of applications, especially photocatalysis [8], [9], electrodes of electrochemical devices [10], [11], [12], spin electronics [13], and sensing [14]. Another point of interest is the turbostratic structure when the nanosheets are aggregated; when used as a cathode, LiMnO2 obtained by the aggregation of MnO2 nanosheets, suppresses the undesired phase transformation to a cubic spinel phase during Li charge/discharge process [10].

However, the synthesis of TM oxide nanosheets usually requires multistep processing, involving a high temperature solid state reaction (to yield a thermodynamically stable phase, e.g. RbCa2Nb3O10), protonation of interlayer alkali metal ions (e.g. to yield HCa2Nb3O10), and an acid–base reaction with quaternary ammonium cations (to finally yield negatively charged nanosheets, e.g. [Ca2Nb3O10], in the form of colloidal suspension). Mechanical exfoliation of metal oxides such as WO3 and MoO3 has also been utilized [15], [16]. Such multistep processing would be cumbersome considering future applications.

Recently, two reports were directed toward simplifying the synthetic procedure; concerning titanium and manganese oxide nanosheets [17], [18]. Tae et al. reported a rapid single step synthesis of TiO2 nanosheets through hydrolysis of titanium (IV) tetraisopropoxide in a tetramethyl ammonium hydroxide (TMA·OH) solution [17]. We also showed a rapid single step synthesis of MnO2 nanosheets by oxidizing Mn(II) ions in a TMA·OH solution at room temperature [18]. Since the latter method uses Mn(II)Cl2 (aq) as a manganese source, the addition of other soluble TM chlorides may offer a possibility to change and control the chemical composition of the nanosheets. In particular, the (Mn,Co)O2 system would be interesting, as a partial Co-for-Mn substitution in LiMnO2 is known to reduce the structural instability and improve the cycling performance as a cathode with a stable capacity of about 200 mAh/g [19]. In this study, a one-pot synthesis of the solid solution (Mn1−xCox)O2 nanosheets is presented. The substitution effects on structural, optical absorption, and magnetic characteristics were also investigated.

Section snippets

Synthesis

Typically, for the synthesis of (Mn1−xCox)O2 (x = 0.00, 0.05, 0.11, 0.15, 0.20, 0.26, and 0.29) nanosheets, 20 mL of a mixture of 1.2 M H2O2 and 1.2 M TMA·OH was added into 10 mL of a 0.6 M MCl2 aqueous solution consisting of MnCl2 and CoCl2 in a mole ratio x0, where x0 = Co/(Co + Mn). The resulting suspensions were stirred vigorously overnight in open air at 40 °C to let the reaction proceed further.

Lamellar aggregates of Ky(Mn1−xCox)O2·zH2O were obtained by mixing the colloidal suspensions of as-prepared

Characterization

A Seiko SPA400 atomic force microscopy (AFM) instrument was employed to visualize the surface topography of the nanosheets. Measurements were carried out in tapping mode with a silicon cantilever having a force constant of 20 N m−1. Samples were deposited onto fluorinated mica (Fluoro-Phologopite, Topy Industry Ltd.) precoated with poly(ethyleneimine) by spin coating (1000 rpm for 15 s and 2000 rpm for 30 s) of a 0.1 g L−1 colloidal suspension. UV–vis absorption spectra were taken in a quartz cell with

Synthesis of (Mn1−xCox) O2 nanosheets

After mixing MCl2 and TMA·OH/H2O2 solutions, purple-red solutions readily became dark brown, and then, with further stirring for 6 h, changed into semitransparent colloidal suspensions. All the obtained suspensions showed Tyndall light scattering, strongly suggesting the formation of dispersed particles. Additionally, the color change from yellowish brown to dark yellowish brown with increasing x0 also indicates a probable substitution of Mn by Co, as seen in Fig. 1. Sheet-like structures were

Conclusions

We have synthesized the (Mn1−xCox)O2 nanosheets through a one-pot method using a mixed solution consisting of MnCl2 and CoCl2. The cell volumes of these lamellar aggregates showed linear shrinkage upon the Co substitution in the region of 0.00  x  0.20. The composition x can be controlled continuously by adjusting the molar ratio x0 of the starting solution. The charge density of the inorganic layer is inert upon Co-for-Mn substitution. The substitution effects were observed in UV–vis spectra and

Acknowledgments

We thank Jun Kawamata for access to the atomic force microscopy experiment. This work was in part supported by Grant-in-Aid for Scientific Research (no. 17684018) from MEXT, Japan and by Core Research for Evolutional Science and Technology (CREST) from the Japan Science and Technology Corporation (JST). KK acknowledges the support by the Global COE program “Integrated Materials Science” (#B-024), and Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for young

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