Preparation Of High Efficient Photocatalytic Nano-TiO₂by Peroxotitanium Complex System

Nano-TiO₂materials have great potential advantages in the applications ofdye-sensitized solar cells, photocatalytic splitting of water, self-cleaning, sensor andother areas. Especially, the application in environmental pollution control is greatlyfocused due to its non-toxic, green process and no secondary pollution. However, largescale use of TiO₂is limited due to low quantum efficiency and insufficient use of visiblelight. Improvements in quantum yielding and response in the visible light regionbecome crucial to TiO₂photocatalytic application. Research in these two aspects wereconducted in this thesis through control synthesis of high energy facet exposed anataseTiO₂and design of nano-TiO₂with good pollutant adsorption capability by usingperoxotianium complex system.{010}-facet dominant anatase TiO₂nanocuboids,amorphous and crystalline nano-TiO₂with high photodegradation efficient in visiblelight were prepared to refer to designing new types of photocatalysts in the future. Themain results of this work are as follows:①Anatase TiO₂nanocuboids with up to75%exposed {010} facets were depositedon the Ti substrate via hydrothermal method. Reaction parameters such as reaction time,temperature and the amount of urea had profound effects on the synthesis ofnanocuboids; no nanocuboids could be deposited on the substrate with short reactiontime, low reaction temperature or without urea. H₂O₂was an essential condition for thereaction. The result of stability of {010} facets showed that the exposure percentage of{010} facets was unchanged during the ripening process while the phenomena ofetching and dissolving of {010} facets acompanied with morphology evolution fromnanocuboids to nanorods occured with long reaction time. Hydrogen titanate producedafter the pretreatment of Ti foil was beneficial to the nucleation and growth of anataseTiO₂. The gradual change of solution pH from acid to basic was not only beneficial tothe production of nanocuboids but also promoted the deposition of nanocuboids on thesubstrate.②Anatase TiO₂nanocuboids powder with {010} facets up to86%weresynthesized in pereoxotitanium complex-HCl-urea reaction system at180℃for4hwith1g urea. The reaction products were approximately hydrogen titanate with shortreaction time or at low reaction temperature. The percentage of nanocuboids increasedwith long reaction time or at high reaction temperater with diminishing of titanate. The composition of products was greatly influenced by urea; TiO₂with anatase and brookitecomposite phase was produced in the absence of urea and mixture of anatasenanocuboids and titanate produced with over dose of urea. The mechanism of evolutionof nanocuboids in solution could be divided into two stage; cyanate （NCO-） and peroxygroup stabilized {101} and {010} faces respectively in the first stage and OH-withperoxy group for {010} facets in the second stage. Hydrogen titanate was produced asthe result of the etching of TiO₂by OH-and then transformed to nanocuboids. Thephotoactivity under UV light showed that sample with86%{010} facet exhibited bestperformance and the photoreactivity of samples declined with duration of synthesis timedue to the etching of {010} facets.③Hierarchical amorphous nanostructures （HAN） modified with peroxy group withhigh surface areas and microporous structure were fabricated by direct liquidprecipitation method at low temperature. FT-IR and XPS results showed no sulfategroups on the surface. The dye adsorption percentage of HAN indicated that acidcondition was preferred for adsorption of Rhodamine B （RhB） and methyl orange （MO）.HAN fabricated with32mL H₂O₂showed highest adsorption performance in the test.The adsorption kinetics was in accordance with pseudo-second-order model. The resultof intraparticle diffusion model indicated that the rate determining step for adsorptionwas not intraparticle diffusion, but surface adsorption. Surface adsorption of HANfabricated with32mL H₂O₂accorded with Langumir adsorption isotherm model and itssaturated adsorption amounts of RhB and MO were85.824mg.g^-1and59.326mg.g^-1,respectively. Photodegradation of RhB and MO in the visible light showed that RhBcould be completely degraded by HAN samples in60min, but degradation of MOrevealed the HAN had different photoreactivity. The photodegradation mechanism wasdye self-sensitized photodegradation and complexes in the surface of TiO₂by reactionwith H₂O₂could promote the formation of hydroxyl radicals.④Peroxy group modified crystalline nano-TiO₂was fabricated by reflux method.Phase compositions of the samples were changed from anatase/amorphous TiO₂toanatase/rutile structure with increased reflux time. FT-IR spectra showed that sulfatespecies gradually coordinated on the TiO₂surface and UV-vis spectra results showed theresponse in the visible region deduced due to diminished peroxy group. The dyeadsorption in different pH solution showed acidic condition was beneficial to improvethe adsorption capacity. Adsorption isotherm for sample with3h refluxing timeaccorded with Langmiur adsorption model. Photodegradation tests revealed that samples with1.5h and3h refluxing time had quite similar photocatalytic properties.Degradation rate for samples refluxed for4.5h was faster for RhB than that for MOunder visible light and the sample refluxed for6h showed worst photoactivity. Cycleddegradation of RhB by the sample refluxed for3h were conducted and the sampleremained relative high degradation percentage up to80%in the third degradation testbenefiting for reuse. Powder suspension sedimentation experiment indicated that thesample refluxed for3h was completely precipitated within2h and favorable to berecycled in industry. Compared with HAN sample, the peroxy group modifiedcrystalline TiO₂photocatalyst acquired cooperative effect of UV excited and dyeself-sensitized photodegradation to achieve extraordinary efficient degradation of dyesin the sunlight.