Biomorphic Mesoporous Ce 1-x Zr x O 2 Nanostructures Fabricated from Paper Templates
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1094-DD07-11
Biomorphic Mesoporous Ce1-xZrxO2 Nanostructures Fabricated from Paper Templates Dickon H. L. Ng, and Jia Li Department of Physics, The Chinese University of Hong Kong, Shatin, N. T., Hong Kong ABSTRACT A simple and versatile procedure was developed for synthesizing biomorphic mesoporous Ce1-xZrxO2. Aqueous cerium nitrate and zirconium nitrate solutions, and paper were used as the starting materials. Under the structure-directed effect offered by the active hydroxyl groups of the cellulose in the paper templates, porous and fibrous structures of paper were replicated by the self-assembly of Ce1-xZrxO2 nanocrystallites after the paper underwent chemical infiltration and calcinations. The product, composing of interwoven network of fibers with diameter ranging from 10 to 20 µm, was a replica of the original paper structure only that each fiber was assembled by Ce1-xZrxO2 nanocrystallites with grain size of 3-10 nm. The templating function of cellulose and the mechanism in the formation of nanocrystallites were proposed. INTRODUCTION Natural materials possess unique hierarchically built anatomies in micro- and macroscales. Many of them could be utilized as templates for the synthesis of porous biomorphic materials. The products could have effective diffusion network, high availability of surface area, and well-defined orientation [1-3]. Various techniques have been developed for the conversions of biological materials such as wood [4,5], sea wool sponges [6], egg shell membrane [7] or cellulose-fiber materials [8,9], into different types of biomorphic materials. For examples, organic lignocellulosic structures were converted into SiC and TiC products by the gas-phase infiltration [10], liquid-phase infiltration [11], or sol-gel methods [12]. Some oxide-based products such as SiO2 [13] ZrO2 [14], Al2O3 [15], ZnO, Co3O4 and PdO [7] were also produced. The processing for these biomorphic oxides was based on the infiltration or coating of metal precursors onto the cell walls of the templates by using solutions which contained metallic nitrate [7], chlorides [15], or alkoxides [16] together with an oxide gel. Subsequent calcinations of the infiltrated samples in air resulted in the formation of the ceramic oxide on the wall of the templates and at the same time, the removal of the carbon content to produce the final biomorphic products [17]. The CeO2-based oxide is well known for its reversible reduction-oxidation (redox) behaviors, and is commonly used in three-way catalystic converters [18]. The catalytic efficiency could be much enhanced by CeO2 with small particle size and large surface area. Thus many types of porous and nanometer-sized CeO2 products were developed and reported, e.g. flowerlike mesoporous CeO2 by hydrothermal method [19], and large surface area CeO2 by using cetyltrimethyl ammonium bromide (CTAB) as templates [20]. However, in high temperature, CeO2 particles tended to coarsen resulting in smaller surface area and lower catalytic efficiency. To tackle this drawback, the binary oxide system
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