DOI: 10.1021/cg900927j
Synthesis of a Nanofibrous Manganese Oxide Octahedral Molecular Sieve with Co(NH3)63þ Complex Ions as a Template via a Reflux Method
2010, Vol. 10 3355–3362
Haojie Cui,†,‡ Xionghan Feng,† Wenfeng Tan,† Wei Zhao,† Ming Kuang Wang,§ Tou Ming Tsao,§ and Fan Liu*,† †
Key Laboratory of Subtropical Agricultural Resources and Environment, Ministry of Agriculture, Huazhong Agricultural University, Wuhan 430070, China, ‡Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China, and §Department of Agricultural Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan Received August 5, 2009; Revised Manuscript Received June 7, 2010
ABSTRACT: Todorokite-type manganese oxide octahedral molecular sieves (OMS-1) have been extensively studied due to their potential applications as materials. In this study, a nanofibrous todorokite-type, tunnel structure, manganese oxide molecular sieve material was successfully synthesized from a layered precursor with Co(NH3)63þ complex ions as a template director by a refluxing method (namely, Co(N)-todorokite). X-ray photoelectron spectroscopy (XPS) indicates that complex ions are located in the tunnel of the Co(N)-todorokite in the form of [Co(NH3)x]3þ (4 < x < 6) ions, while keeping the trivalence of the cobalt ions. Scanning electron microscopic (SEM) and high resolution transmission electron microscopic (HRTEM) images reveal that this material has a fibrous morphology with a thickness of 20-40 nm, and a lattice fringe spacing of 0.96 nm, corresponding to the (100) plane of the todorokite structure. The [010] HRTEM fringe images show that the tunnels contained a constant triple-chain width (3 3) along the c and a axes. The Co(N)-todorokite consists of a chemical composition of Co0.11N0.48H1.39MnO1.99 3 H2Ox. Thermogravity analysis (TGA) indicates that these nanofibers are thermally stable up to 400 °C. The Brunauer-Emmett-Teller (BET) surface area for Co(N)-todorokite is 98.2 m2/g, which is higher than that of bulk todorokite materials. The Horvath-Kawazoe (HK) plot shows a major pore size distribution peak centered at 0.71 nm for Co(N)-todorokite.
*Corresponding author. Tel: þ86-27-87280271; e-mail:
[email protected]. edu.cn.
current interest.1,22 To obtain higher performance from OMS1 materials as catalysts, one of the common techniques is to increase the surface area by decreasing the particle size.10 More recently, increasing interest has been focused on onedimensional (1D) manganese oxide nanorods for microwave absorbers.23 However, a mixture of platelike and needlelike or fibrous morphology commonly appears in synthetic OMS-1 materials prepared by different methods, including hydrothermal and reflux methods.2,7,8,15,20,24 A neat fibrous morphology of Mn-oxides, containing Mg2þ, can be obtained when a colloidal precursor and a high crystallinity precursor are used to synthesize OMS-1 materials under hydrothermal conditions.10,25 Todorokite-type octahedral molecular sieves, products of microwave-assisted hydrothermal treatment, with cubelike morphology can be observed.8 The morphological differences can be accounted for by the initial layered precursors and their treatment methods during the formation process of OMS-1 materials. Most OMS-1 materials are structure-directed by a variety of metal cation templates, and it has been observed that the crystal morphologies and sizes of synthetic OMS-1 materials depend on these foreign metals.7,24 OMS-1 materials are predominantly prepared using Mg2þ as the template cation in the tunnels,2,3,5-10,12-15,24 although other divalent cations such as Cu2þ,2,3,5 Co2þ,3,7,24 Ni2þ,2,3,5,7,13,24 and Zn2þ are also used under hydrothermal conditions.3,5,24 As in most syntheses, size, charge, and polarizability of various template cations must be considered.1 Many works have carried out synthetic research on OMS-1 materials, but a limited number of studies have discussed the effect of complex ions as template directors on the formation process and particle morphology of synthesized OMS-1 materials.
r 2010 American Chemical Society
Published on Web 06/29/2010
Introduction Manganese oxide octahedral molecular sieves (OMS) have been extensively studied due to their potential applications as ion sieves, molecular sieves, catalysts, and rechargeable battery materials.1 Todorokite-type manganese oxides are one group of the octahedral molecular sieve (OMS-1) family, containing 6.9 6.9 A˚ tunnels (3 3 framework structure).2-4 A variety of methods, such as hydrothermal treatment,2,3,5-7 microwave heating,8,9 sol-gel,10 and annealing methods,11 have been developed to synthesize todorokite-like manganese oxide OMS materials with different properties. These synthetic phases have served to be effective cathode materials,12 highly active oxidative catalysts,13 and selective ion-exchange agents for heavy metals from water solutions.14 Reflux treatment, a convenient method due to its operation in a relative low pressure and temperature, has been developed to prepare OMS materials15,16 and other nanostructure metal oxides.17-19 However, todorokite formation is influenced by the interaction between interlayer cations and MnO6 layers of buserite under reflux conditions.20 No todorokite is obtained after the refluxing of Cu-buserite and Co-buserite prepared by direct ion exchanging with Na-buserite using CuCl2 and CoCl2 solutions.21 The number of metal ions used as templates for todorokite formation is limited. Therefore, further studies and extensive applications are underway to improve the formation of todorokite using different template ions under reflux conditions. The effect of the crystal morphology of manganese oxide materials on their physicochemical properties is a subject of
pubs.acs.org/crystal
3356
Crystal Growth & Design, Vol. 10, No. 8, 2010
Cui et al.
Complex ions can exhibit different effects in controlling the morphology and properties of produced OMS-1 materials in comparison with ordinary template cations due to their larger size and different hydrate forms. In this study, a todorokite-type manganese oxide OMS, containing cobalt, was synthesized from a layered precursor with the Co(NH3)63þ complex ion as a template by a reflux method. The synthetic todorokite shows a nanofibrous morphology, which is different from that of todorokite-type materials containing magnesium prepared under the same reflux conditions, and also different from that of todorokitetype materials, containing cobalt, synthesized by hydrothermal methods. This synthesis method may be extended to explore synthesis size- and morphology-tunnel OMS materials using other complex ions under reflux conditions. Experimental Section Preparation of Na-buserite. Na-buserite was prepared as follows: 250 mL of 5.5 mol/L NaOH solution (
