Abstract
A novel method for fabricating Magnéli phase (MP) TinO2n-1 (4<n<10), carbothermal reduction sieving, in air atmosphere was introduced. The influence of the reduction temperature and reduction time on the phase structure and resistivity of reduction product was investigated. The results show that increasing the reduction temperature and prolonging the reduction time are beneficial for the reduction of TiO2 to MP TinO2n-1. MP TinO2n-1 (n=4, 5) powder was obtained after reduction at 1350 °C for 20 min, and its particle size is 0.5~8 μm according to results of scanning electron microscopy analysis. Resistivity of the reduction product is decreased significantly with prolonging the reduction time at 1350 °C. The minimum resistivity of 79.3 Ω‧cm is achieved for the product after reduction at 1350 °C for 50 min, and the phase composition is mainly Ti3O5.
Science Press
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The anatase TiO2 powder was prepared into particles of 3~5 mm in size with deionized water, and the particles were dried at 120 °C for 2 h. Then the TiO2 particles were placed into a graphite crucible, and the periphery of the TiO2 particles was covered with graphite powder. Subsequently, the TiO2 particles were reduced in a box-type furnace under different reduction conditions. The mixture of graphite powder and reduced particles was then sieved using a vibrating screen. Finally, the reduced particles were washed by alcohol and then the products were crushed by milling.
Fig.1 Schematic diagram of preparation process of TinO2n-1
The reduction products were identified using X-ray diffraction (XRD) with Cu Kα radiation (D8 ADVANCE, Bruker, Germany) with 2θ=10°~90°. The microstructure was studied using scanning electron microscopy (SEM, Quanta-400, FEI Corporation, Netherlands) and X-ray photoelectron spectroscopy (XPS, 250XI, Thermo Electron, USA). The resistivity and density of the specimens were tested using four-probe measurements (ST2722-SZ, JG, China).
Fig.2a shows XRD patterns of the raw material and five specimens prepared at 950~1350 °C for 20 min. The main phase of the raw material and specimen reduced at 950 °C is clearly the anatase TiO2 phase, whereas the rutile TiO2 phase appears in the specimen reduced at 1050 °C. This phenom-enon indicates that the phase-transformation temperature of TiO2 is 950~1050 °C. The phase of the specimens reduced at 1050~1250 °C still contains rutile phase TiO2, but the peak shape is gradually broadened with increasing the reduction temperature. When the reduction temperature reaches 1350 °C, the component is transformed into a mixture of Ti5O9, Ti4O7, and TiO2 (rutile). These results show that the reduction extent of titanium dioxide is increased with increasing the tempe-rature from 950 °C to 1350 °C. Therefore, the proper redu-ction temperature for synthesizing TinO2n-1 is about 1350 °C when reduction time is set as 20 min.
Fig.2b shows the XRD patterns of specimens prepared by reducing anatase TiO2 in air at 1350 °C for 5~50 min. The phase in specimens reduced for 5~10 min is mainly rutile TiO2, and the phase-transformation time of TiO2 at 1350 °C is less than 5 min. With further reducing the specimens, Ti9O17 appears after reduction for 15 min, and the phases become Ti5O9 and Ti4O7 when the reduction time increases to 20 min. After reduction for 30 min, the Ti3O5 phase appears while the peak intensities of TiO2, Ti4O7, and Ti5O9 phases decrease. After reduction for 50 min, the specimens are reduced almost exclusively to Ti3O5, and Ti4O7 and Ti5O9 phases disappear. These results indicate that when carbothermal reduction in air at 1350 °C proceeds from 0 min to 50 min, the TiO2 (anatase), TiO2 (rutile), Ti9O17, Ti5O9, Ti4O7, and Ti3O5 phases appear orderly.
To investigate the change mechanism of surface components during carbothermal reduction, Ti 2p peaks of the raw material and specimens reduced at 1350 °C for 20 min were obtained by XPS analysis (Fig.3). For the raw material anatase TiO2, the binding energy of Ti 2p exhibits a sharp peak without shoulder peak. The peaks around 458.47 and 464.17 eV can be regarded as the T
Fig.4 shows SEM morphologies of anatase TiO2 material reduced at 1350 °C for 20 min. Fig.4a shows that the particles are irregularly shaped and distributed homogeneously, whereas Fig.4b shows clearly that there are a small number of large particles with many small pores on the surfaces and large particle gap. In addition, a loose structure is formed by CO gas escaping easily to the atmosphere, and the particle sizes are 0.5~8 μm.
The powder was compacted by a uniaxial press under a pressure of 18 MPa into a cylinder for resistivity and density measurement. The resistivity and density of different specimens are listed in
Fig.5 shows transmission electron microscope (TEM) images of MP TinO2n-1 reduced at 1350 °C for 50 min. The crystallite with an interplanar spacing of about 0.354 nm is consistent with the distance of (110) crystalline plane of the Ti3O5, and the interplanar crystallite appears to be surrounded by amorphous layers. To investigate the light absorption of the prepared MP TinO2n-1, the absorption spectrum of the specimens was obtained by ultraviolet visible (UV-vis) diffuse reflectance spectrum from 200 nm to 900 nm at room temperature, as shown in Fig.6. The light absorption performance is enhanced with increasing the reduction time.
1) Powders of Magnéli phase (MP) TinO2n-1 (4<n<10) were prepared by carbothermal reduction sieving method in air atmosphere. With prolonging the reduction time at 1350 °C, the phase of TiO2 (anatase), TiO2 (rutile), Ti9O17, Ti5O9, Ti4O7, and Ti3O5 appears orderly.
2) The MP TinO2n-1 powder exhibits low conductivity at room temperature because of the existence of a certain amount of TiO2 (rutile) in the powders. The minimum resistivity of 79.3 Ω‧cm is achieved after the MP TinO2n-1 powder is reduced at 1350 °C for 50 min.
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