TiO₂（B） Nanosheet-started Phase Controlled Synthesis Of TiO₂Photocatalyst And Their Photocatalytic Properties

Up until now,TiO2crystallizes in four common polymorphs: anatase, rutile andbrookite and TiO2(B). Therein, anatase and rutile phases are common phases. But brookiteand TiO2(B) phases draw less attention in comparison with anatase and rutile phases. AnataseTiO2has proven to show higher photocatalytic activities than rutile. Further study indicatesthat antase/rutile heterojunction can significantly boost the activity of TiO2. However, therestill be a controversy for the activity of brookite and brookite based heterojunction, mainlydue to the difficulty of developing a general route to phase selective synthesis of TiO2. In thisthesis, we develop a general soft chemical route using TiO2(B) nanosheet as precursor tosynthesize common TiO2phases especially brookite. Benfting from the structural similarityand ultrathin thickness feature, TiO2(B) nanosheet precursor can readily transform to purephase of anatase, rutile and brookite as well as mixed phase of brookite/anatase.Photocatalytic activity tests reveal the activity difference between different TiO2samples. Thecontents of the paper are listed as following:1. Pure brookite was synthesized by hydrothermal treatment of TiO2(B) nanosheets in thepresence of NaCl in aqueous ammonia solution. The role of Na+ion in the formation of thebrookite from TiO2(B) nanosheets is also studied and the result indicates that Na+play animportant role to formation of brookite, We also investigate the influence of some anion ionsor surfactant（including F-、SO42-、CTAB、PEG、SDS）on the morphology or the phasecomposition of the product. The result shows F-、SDS and CTAB can influence morphology orthe phase composition of the product, F-can promote the formation of anatase and change themorphology of the product. When CTAB was employed as surfactant, anatase nanorod wereobtained and nanostructured brookite was not observed. When CTAB was employed assurfactant, phase transformation from TiO2（B）to other phase was inhibited. Some other ionsand surfactants were also selected; they have no effect on the morphology of product and thephase transition process.2. Pure anatase, rutile, as well as anatase/brookite mixture were also prepared usingTiO2(B) nanosheets as a precursor. The ratio of anatase to brookite phase can be easilycontrolled by tuning NaCl concentration in aqueous ammonia, and pure rutile was obtained byhydrothermal treatment of TiO2(B) nanosheets in weak acid solutions. The formationmechanism of TiO2with controlled phases was proposed by investigating the phasecomposition and morphology of the intermediates of pure TiO2(B) after reaction for6h，12hand18h.3. We also systematically studied the phase-dependent photocatalytic activity towardgaseous acetaldehyde degradation. The results shows that the sequence of photocatalyticactivity of TiO2photocatalysts with various phase structures and similar BET value wasbrookite/antase>brookite>anatase>rutile, the brookite/antase mixture with71wt%brookite shows the highest photocatalytic activity. Based on the above result, we propose the surfacephase junction between anatase and brooktie can enhance the charge separation, thusimproving the photocatalytic activity of anatase/brookite mixture. The study above alsoindicates that brookite is a promising photocatalyst.