Synthesis of sodium cobalt oxide submicron tubules using citric acid-based sol-gel route assisted by porous aluminum oxi
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Synthesis of sodium cobalt oxide submicron tubules using citric acid-based sol-gel route assisted by porous aluminum oxide membrane
Chia-Jyi Liu, Long-Jiann Shih, and Hsueh-Jung Huang Department of Physics, National Changhua University of Education, Changhua 50007, Taiwan. ABSTRACT We report synthesis of the sodium cobalt oxide γ-NaxCoO2 submicron tubules using the citric acid-based sol-gel route with the assistance of commercial porous anodized aluminium oxide (AAO) membrane with a nominal pore size of 200nm as supporting templates. The γNaxCoO2 submicron tubules can be obtained at 500ºC using a rapid-heat-up procedure and held for 30 min. The submicron tubules have a diameter of 200-250 nm. The products are investigated using various techniques including powder x-ray diffraction, field emission scanning electron microscope and transmission electron microscope. The atomic ratio of Na/Co of theγ-NaxCoO2 submicron tubules is determined using energy dispersive x-ray spectra. The atomic ratio of Na/Co tends to be less than 0.5 due to the tendency of Na+ de-intercalating from its parent structure during the tubules collecting process. INTRODUCTION Thermoelectric materials can be considered as a kind of promising energy-conversion material in harmony with our environmental need. Thermoelectric oxides have the advantage of high thermal stability, wide temperature range of application, low cost of starting materials, and low toxicity. Good thermoelectric materials requires high figure of merit. The figure of merit, Z, is expressed as Z = S2σ/κ, where S, σ, and κ are the thermopower, electrical conductivity and thermal conductivity, respectively. The layered sodium cobalt oxides have gained much interest in recent years due to the discovery of large thermoelectric power in combination with low electrical resistivity and low thermal conductivity in NaxCoO2 [1,2]. The γ phase of NaxCoO2 is isomorphous to α-NaMnO2 and has a hexagonal lattice with the space group of P63/mmc (No. 194). There are four crystallographically distinct positions: Co at 2a (0,0,0), Na(1) at 2b (0,0,0.25), Na(2) at 2d (0.333,0.667,0.75), and O at 4f (0.333,0.667,z) [3,4] in γ-NaxCoO2, which comprises two CoO2 layers with the Na ions located in two different crystallographic sites between them. The γ-NaxCoO2 has the lattice constants of a-axis of ca. 2.8 Å and c-axis of ca. 10.9 Å. Therefore, it has an interlayer spacing of ca. 5.45 Å between the two CoO2 layers. Interestingly, γ-Na0.7CoO2 can be topotactically transformed to superconducting oxyhydrates Nax(H2O)yCoO2 with Tc = 4.6 K upon oxidative intercalation of water molecules [5-7] . One-dimensional conductors is expected to enhance thermoelectric figure of merit Z = S2/ρk through enhancing thermopower by increasing ∂g(E)/∂E near the Fermi energy, where g(E) is the
Figure 1. Porous AAO membrane obtained from Grace Davison Discovery Sciences (Whatman Anodisc 13) having a nominal pore size of 200 nm. It can be readily seen that some of the pores are connected due to the broken bridges
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