Preparation Of TiO₂ Nanocrystallines With High Photocatalytic Activity By Sol-gel Method At Low Temperature

TiO₂ is the most promising photocatalyst currently. It has been widely used in the field ofenvironmental protection, construction, agriculture due to its chemical stability, acid and alkaliresisting, non-toxic, and strong oxidation and reduction property, etc.. In recent years, thepreparation of the TiO₂ nanocrystallines by modified sol-gel method at temperatures below150℃has become the research focus. The prepared TiO₂ particles are very small, which ownlarge surface area and strong photocatalytic activity. In this study, the TiO₂ nanocrystallineswere prepared at room temperature. Moreover, The formation mechanism of the sol-gel-derivedTiO₂ nanocrystallines at low temperature was studied. In order to enlarge the photoresponserange and improve its photocatalytic activity, the doping of the TiO₂ Nanocrystals was asloresearched.The photocatalysis mechanism of the semiconductor and the properties of thenano-materials were introduced firstly, then the absolute advantage of the TiO₂ nanocrystallinesin the photocatalysis filed was highlighted. On the basis, the preparation of the TiO₂nanocrystallines by the sol-gel method was studied. TiO₂, nitrogen doped TiO₂, vanadium andsulfur co-doped TiO₂ were prepared by controlling the pH value, water amount W value, anddoping amount in the system. X-ray diffraction (XRD), UV-Vis absorbance spectra (UV-Visabs), transmission electron microscopy (TEM), High resolution transmission electronmicroscopy (HRTEM), Energy Dispersive X-ray Detector (EDX), X-ray photoelectronspectroscopy (XPS), Infrared spectra (IR), and Brunauer Emmett Teller (BET) were used tounderstand the optical property, crystallization property, microstructure, electronic bandstructure and photocatalytic property of the prepared TiO₂ nanocrystallines. The main researchresults are as follows:Tetrabutyl titanate and ethanol were choosed as precursor and solvent, respectively. TiO₂sol was obtained by the hydrolysis and polycondensation of tetrabutyl titanate in large amountof water. After aging for several hours, the TiO₂ powders were prepared by drying the aging solunder infrared lamp. By characterization the TiO₂ sol and powders with several measurements,the TiO₂ nanocrystalline was obtained at room temperature. The results showed that a pure anatase phase was formed at pH 1, in contrast to the formation of particles with a mixture ofanatase and brookite at pH 2-7. The crystalline size of the particles was about 4 nm, and the sizewas well-distributed. To study the mechanism of sol-gel-derived TiO₂ nanocrystalline at lowtemperature, the pH value, water amount and preparation process has influenced the formationof the TiO₂ nanocrystalline significantly.When the water content is small, the hydrolysis oftetrabutyl titanate proceeds insufficiently. Therefore, the Ti groups contain Ti-O-C, which actasstructural impurities and inhibit crystallization. However, in thecase of a large amount ofwater, and the process is adding the tetrabutyl titanate to the water, so the tetrabutylhydrolyzescompletely. If the hydrolysis goes to completion before polycondensation proceedssignificantly, the Ti groups will exist in the form of an inorganic substance containing Ti-OHgroups, which is beneficial for the formation of the unit-[TiO₆] octahedron. For the TiO₂ crystalstructure, rutile and anatase belong to the tetragonal crystal system, and brookite belongs to theorthorhombic crystal system. These crystal structures can be obtained through the rearrangementof the [TiO₆] octahedron units. On the other hand, the different pH value leads to the differentH⁺ concertration in the system. When pH is small, the concentration of H⁺ is high, so theconcentration of Ti-OH₂⁺ is high. There are more H⁺ and O^2- for the arrangement, connectionand the removing of the H₂O molecular, which is beneficial to the mass loss of the watermolecules when polycondensation proceeds. Therefore, the formation of an anatase structure isfavored when the pH is 1. On the contrary, Higher pH value favors less loss of water molecules.Therefore, part of bthe [TiO₆] octahedra combined by sharing corners, resulting in the formationof a brookite structure at pH 2-7. With increasing aging time, after the rearrangement of the[TiO₆] octahedra, the TiO₂ nucleates and grows, resulting in clusters of TiO₂ crystalstructure.And the crystal lattice structure completes gradually, forming the regular crystalstructure with the aging time. The photocatalytic activity of the prepared pure TiO₂nanocrystalline was aslo investigated. The results suggested that the TiO₂ nanocrystallineprepared at pH 1 all showed better activity than P25. However, the TiO₂ prepared at 2-7 showslower activity than P25. Moreover, the sample TiO₂-pH1-W400 prepared at pH1, W100 ownedthe highest photocataltic activity.Triethylamine was choosed as the nitrogen source to nitriding the prepared pure TiO₂, andthe nitrogen doped anatase TiO₂ was prepared at low temperature successfully. The crystalline size was about 4 nm, which was lower than the pure TiO₂ from the whole size distribution, thenitrogen existed in the TiO₂ crystalline lattice in the form of interval. As the nitrogen dopingconcentration increases, the crystal size decreases, the specific surface area increased first andthen decreased gradually, the absorption edge shift to the long wave. In addition, the UV andvisible light photocatalytic performance of studies show that when the nitrogen-doped amountis from 0 ml to 20 ml, the photocatalytic activity increased. The property was best when thedoping amount was 20ml. The further doping amount increasing resulted in the weaking of thephotocatalytic activity. Such diffenences under UV ligh can largely be attributed torecombination rate of the electron-hole pairs affected by the local state generated by nitrogendoping, the grain size, specific surface area. Such diffenences under visible ligh can mainly beattributed to the doped impurity energy level and band gap changes, grain size, specific surfacearea.Vanadyl Sulfate was choosed as the vanadium and sulfur source.The vanadium and sulfurco-doped TiO₂ was prepared at 50℃successfully. The crystalline size was about 4 nm, withuniform grain size distribution. XPS spectra showed that vanadium and sulfur replace Ti⁴⁺respectively by V⁵⁺, S⁶⁺. With the doping of vanadyl sulfate increased from 0% to 20%, thecrystal size decreases, the specific surface area increased first and then decreased gradually, theabsorption edge shift to the long wave. In addition, the UV and visible light photocatalyticperformance of studies show that when the doped amount is from 0 to 10%, the photocatalyticactivity increased. The property was best when the doping amount was 10%. The further dopingamount increasing resulted in the weaking of the photocatalytic activity. Such diffenences underUV ligh can largely be attributed to recombination rate of the electron-hole pairs affected by thelocal state generated by nitrogen doping, the grain size, specific surface area. Such diffenencesunder visible ligh can mainly be attributed to the doped impurity energy level and band gapchanges, grain size, specific surface area.