Low Temperature Ambient Pressure Synthesis Of High Performance Titania Photocatalysts

Owing to its merits of environmental benignity,chemical stability,non-toxicity,high biocompatibility and low cost,titanium oxide?TiO₂?has been widely used in many fields such as photocatalysis environment remediation.Hydrothermal and solvothermal methods achieve TiO₂ with various morphologies in forms of both thin film and powder;yet high temperature and pressure environment are often demanded,which hinder the large scale production.In addition,most of the synthesis methods involve expensive organic titanium alkoxides that adsorb water easily and are not easy to operate.Therefore,it's highly necessary and urgent to develop a simple method to synthesize TiO₂,which should also be easy to scale up.Utilizing the interactions between inorganic titanium source of metal titanium,potassium titanyl oxalate and hydrogen peroxide?H₂O₂?,in this thesis,TiO₂ with various nanostructures were successfully synthesized under low temperature and ambient pressure.The decomposition of hydrogen titanate synthesized via different reactions and the low temperature crystallization of TiO₂ were studied in detail,together with the simultaneous doping in TiO₂.TiO₂ powders and thin films with high photocatalytic performance were obtained.Potassium titanyl oxalate?K₂TiO?C₂O₄?₂?was introduced into the low temperature reaction system of titanium plate and H₂O₂ at 80?,to achieve TiO₂thin films with controlled phases and morphologies.With a low potassium titanyl oxalate concentration,hydrogen titanate nanowires were obtained via the oxidation of titanium plate by H₂O₂ while flower-like rutile mesocrystals were directly gained by the reaction of potassium titanyl oxalate and H₂O₂.The subsequent calcination decomposed hydrogen titanate to anatase with the morphology well retained,thus achieving anatase nanowire and flower-like rutile mesocrystal coexisted film.With high potassium titanyl oxalate concentration,the oxidation of titanium plate by H₂O₂ to hydrogen titanate nanowire was inhibited,and thin film composed of flower-like rutile mesocrystals only was obtained.When utilized to assist the photodegradation of sulfosalicylic acid in water under UV light illumination,rutile mesocrystals exhibited similar performance to that of anatase,demonstrating that the mesocrystal structure can effectively improve the performance of photocatalysts.Melamine and tungstic acid were added into the reaction system of titanium plate and H₂O₂,and W,N co-doped TiO₂ nanobelt film was gained.Hydrogen titanate nanobelts were firstly obtained after the low temperature reaction at 80?,which decomposed to anatase by calcination,with morphology well retained.Tungstic acid interacted with H₂O₂ to form peroxocomplex,which participated in the reaction and achieved W doping.The hydrolysis of melamine led to hydroxyl groups,through which the hydrolyzates bonded to the titanate and achieved N doping in TiO₂.With the co-doping effect of 2.6 at%W and 3.1 at%N,the band gap of TiO₂nanobelt film was greatly reduced to 2.3 eV and visible light response was obtained.Thanks to both the belt structure and co-doping,the W,N co-doped TiO₂ nanobelt film exhibited superior photocatalytic activity,achieving a degradation efficiency of rhodamine B under visible or UV light illumination three times that of anatase TiO₂ nanobelt film synthesized by the conventional alkali-hydrothermal method.The hydrolysis-dissolution-precipitation procedure of potassium titanyl oxalate in H₂O₂ with the additive of nitric acid was utilized to fabricate TiO₂ with various nanostructures.With a low K+/H+ ratio,potassium titanyl oxalate firstly hydrolyzed to hydrogen titanate nanowire assembled microspheres,which further dissolved and precipitated under acid condition and rutile mesocrystal nanorod assembled hollow TiO₂ microspheres were finally achieved.With a K+/H+ ratio beyond 1,K+ cannot be completely replaced by H+ and there was no additional H+ to provide acidic condition,nanosheet-assembled yolk-shell microspheres were achieved.Titanates containing K+ were firstly gained,and nanosheet-assembled yolk-shell anatase TiO₂ microspheres were obtained after a subsequent proton exchange process and calcination.Assisted by the specific structure,the yolk-shell TiO₂ microspheres exhibited improved photocatalytic performance.Hydrogen titanate nanowire assembled microspheres were gained by direct oxidation of titanium powder with H₂O₂.The low temperature transformation and crystallization of TiO₂ were achieved by an acid treatment in aqueous HCl or H₂SO4 solution.Hydrogen titanate nanowires were converted to rutile nanorods through the HCl treatment.This process could be promoted by increasing the HCl concentration or the reaction temperature.Controllable transformation from hydrogen titanate nanowires to rutile nanorods could be achieved by fine controlling the HCl concentration and temperature.The product with the coexistence of hydrogen titanate nanowires and rutile nanorods exhibited the best dye adsorption ability.Hydrogen titanate nanowires were converted to single crystalline nanorod assembled rutile TiO₂ nanoflowers by H₂SO4 treatment.Ni doping was simultaneously achieved by adding NiSO4·6H₂O into the H₂SO4 solution,with the morphology and phase composition of the product well retained.When utilized to assist the photodegradation of rhodamine B under Xe-lamp,un-doped and Ni-doped rutile TiO₂ nanoflowers exhibited efficiencies 2 times and 4 times that of P25,respectively.Single crystalline hydrogen titanate nanowire arrays were synthesized through an alkali-hydrothermal method,which were subjected to a low temperature H₂SO4 treatment followed with calcination.Top opened nanowire with nanoprotrusions on the surface,which was hierarchical yet single crystalline as a whole,was obtained.Hydrogen titanates underwent dissolution-precipitation process in the H₂SO4 solution.Since the hydrogen titanate was composed of curved layer structure,the dissolution started from the top and further developed deep into the nanowire along the axial direction to form tubular structure.TiO₂ precipitated back to hydrogen titanate nanowire along[100]direction by epitaxial growth.In addition,unreacted hydrogen titanate skeleton underwent topotactic transformation during the subsequent calcination to form single crystal anatase grown along[100]direction,therefore,the whole hierarchical nanowire was single-crystalline.When utilized to photodegrade rhodamine B under the illumination of UV light,the hierarchical nanowire array films demonstrated efficiencies 3 times and 9 times that of anodized nanotube films and alkali-hydrothermally synthesized nanowire films.When the target changed to phenol,the hierarchical nanowire array films demonstrated efficiencies 1.4 times and 2.4 times that of anodized nanotube films and alkali-hydrothermally synthesized nanowire films,respectively.Spindle-like nanoparticles assembled hydrogen titanate aggregates were gained based on the reaction of potassium titanyl oxalate and H₂O₂ under room temperature.Their low temperature transformation and crystallization process was investigated during hot water treatment,and compared with thermal treatment.Calcination decomposed hydrogen titanate to anatase with morphology well retained while anatase nanoparticles and rutile nanorods co-assembled TiO₂ hierarchical aggregates were gained after hot water treatment.Hot water treatment can effectively avoid the grain growth and the decrease of specific surface area,while the surface hydroxyl groups can be well preserved.As a result,TiO₂ derived by hot water treatment showed an excellent photocatalytic performance.The efficiency towards photodegradation of rhodamine B under UV light illumination was 2 times that of P25.