(80e) Effect of Precursor on Crystal Structure of Titania Synthesis in Supercritical Sol – Gel Reaction

Titania has been applied as a photocatalyst in water treatment technology. On the development of titania, there are the drawbacks, the calcination step for the crystallization could cause the reduction of the surface area. Recently, the carbon / titania composite fabrication have attracted attention on the enhancement of the formation of mixed crystal structure of anatase and rutile phase which suppresses the recombination of electron and hole (A. Chatterjee et al., J. Raman Spectrosc. 42, 1075 – 1080 (2011)). Typically, anatase crystal of the titania can be formed in the calculation at the lower temperature and then the rutile crystal phase at around600 ℃. Graphite or carbon nanotube were used for incorporation of carbon source into titania.These composite are expected to increasing of the photocatalytic activity. However, they are expensive and synthesized in complex reaction process.

In our research group, titania particles with various morphologies were synthesized by a sol-gel reaction in supercritical carbon dioxide( M. Kinoshita et al., J. of Supercritical Fluids 116 (2016)). Supercritical carbon dioxide has been applied as both synthesis and drying solvents for the fabrication of nanostructured titania particles because of the unique properties, such as low surface tension, high solubility and diffusivity. We found that the titanium precursor affects the remained amount of carbon in the titania particles. It was confirmed that the titania from the lower reactive precursor contained the larger amount of carbon and expected to reduction of the calcination temperature for the formation of the mixed crystal structure of the anatase and rutile phases.

In this work, the effect of the titanium precursor on the particle fabrication is investigated. The sol-gel reaction for synthesis of titania particle was conducted using the three types of titanium precursors acetic acid (HAc) in in supercritical carbon dioxide. Titanium isopropoxide (TIP), titanium ethylhexoxide (TEO) and titanium diisopropoxide bis acetylacetonate (TDB) used as their precursors. A high-pressure vessel with 34 ml was used for the synthesis of titania particle in supercritical carbon dioxide. The known amounts of the precursor and HAc were loaded into the high-pressure vessel before supplying supercritical carbon dioxide into the system. The molar ratio of HAc to the precursor was 10. Carbon dioxide from the cylinder was passed through the cell including the silica gel in order to remove the trace amount of water. The purified carbon dioxide was liquefied at a chiller and pressurized by a feed pump. The pressurized carbon dioxide was supplied into the system in the air thermostat and achieved to the supercritical fluid. The supercritical carbon dioxide was installed into the high-pressure vessel in which the titanium precursor and HAc were loaded. The sol-gel reactions in supercritical carbon dioxide were conducted in the by a batch type operation at 313 to 333 K and 20.0 MPa. The mixture inside the high-pressure vessel was stirred by a magnetic stirrer. After the sol-gel reaction for 24 h, the contents in the vessel were dried by supercritical carbon dioxide for 6 h. The system was depressurized and then the dried titania particle was removed from the high-pressure vessel. The titania particles from the sol-gel reaction and drying using supercritical carbon dioxide was calcined at 973 K in air for 2 h for the crystallization. The synthesized titania particle was also analyzed by scanning electron microscope (SEM), x-ray diffraction (XRD), thermogravimetric analysis (TGA) and second-ion mass spectroscopy (SIMS). The fibrous titania was fabricated using TIP precursor, whereas block and sphere ones were synthesized in case of TEO and TDB precursors. It is considered that the different intermediate during the sol-gel reaction was formed due to their difference of reaction rate. The amount of carbon in titania was measured by SIMS analysis. In case of TIP, the carbon was hardly observed inside the titania particles. On the other hand, the large amount of carbon was in the particles from TEO and TDB. It is thought that the sol-gel reaction did not sufficiently proceed and unreacted carbon group remained in titania due to their low reactivity of TEO or TDB. The calcinated titania from TDB or TEO has the low weight loss in the TGA analysis. The ratio of anatase and rutile was calculated from XRD analysis results. The ratio of rutile in the titania from TDB and TEO are larger than those from TIP precursor. From TGA abd XRD results, it was suggested that the large amount of the residual carbon in the titania could enhance the transition of the crystal structure from anatase to rutile phases.