Disorder Transformation,Band Engineering And Photoelectric Performances Of Nano Titania Under Wet Chemistry

Nano TiO₂ is a multifunctional metal oxide semiconductor with a wide band gap,and it has always been a major issue to improve its solar energy utilization by band engineering.The black disordered titania that synthesized through hydrogenated annealing of TiO₂ nanocrystals（NCs）was an efficient and stable photocatalyst for water-splitting and organic pollutants degradation.This approach to engineer the electronic and band structure of TiO₂ by disturbing the regular lattice and creating disorder on NCs was an important breakthrough in band engineering of metal oxide semiconductors in recent years.This paper is carried out based on a new thought of disorder transformation of TiO₂ NCs to obtain optimized solar energy utilization and photoelectric performances under wet chemistry,which reveals its chemical mechanism,as well as the relationship and physical essence between degree of disorder and the corresponding band structure and photoelectric performances,confirming that disorder engineering under wet chemistry is able to achieve highly efficient solar energy utilization of TiO₂.We have developed innovative technologies of heat treatment and sonochemistry to conduct the disorder engineering of TiO₂ under wet chemistry,obtaining a series of structurally and functionally graded nanocomposites of disordered titania as photocatalysts and photovoltaic materials,and this new method could be applied to the property optimization of other metal oxides with semiconductor characters,which establishes a low-cost,conveniently-operate,easily-control,universal and original technology for band engineering,and promotes the spread of materials for solar photoelectric conversion and environmental cleaning.The main research contents and results are as follows:（1）The amorphous titania that contains lots of hydroxyls was prepared as precursor,and structurally graded nanocomposites of disordered titania were obtained through simple heat treatment,which achieved the optimization of band structure and photocatalysis.During this process,hydroxyls in materials decreased by dehydration along with the total disorder structure turned to regular lattice of anatase gradually.The material structures could be continuously controlled by heating temperature,which obtained a series of structurally graded nanocomposites of disordered titania.The results indicated the critical role of hydroxyl amount in the degree of disorder under wet chemistry.The modifications of disorder engineering included band gap narrowing,optical absorption enhancement,specific surface area increase and photocatalytic activity promotion.（2）Sonochemistry was creatively used to control the hydroxylation of amorphous titania,obtaining highly disordered amorphous titania,which achieved the optimization of band structure and photocatalysis.Amorphous titania precursor was prepared through one-step aqueous synthesis with titaniunm sulfate in alkaline solution.High-intensity ultrasonic irradiation induced hydroxyl injection into precursor,and the degree of disorder in products was increased with the extension of ultrasonic time.On one hand,the highly disordered amorphous titania contained longer band tails,and the valence band maximum blue-shifted from 2.75eV to2.15eV under Fermi level,making narrower actual band gap and higher absorption of visible-light,which improved solar energy utilization and turned the color in appearance from white to black gradually.On the other hand,the specific surface area and the porosity was nearly doubled and improved for about 1.3 times with the degree of disorder increasing,producing more locations for photocatalytic reaction.And high intensity of disorder defects effectively restrain the recombination of photon-excited electrons and holes,promoting the photo-quantum efficiency.Combined with these factors,solar-driven and visible-light-driven photocatalytic activities in organic pollutants degradation of amorphous titania were enhanced for about 4.5 times and about 5 times respectively.（3）The surface disorder transformation and photoelectric performances optimization of TiO₂ NCs were achieved through sonochemical hydroxylation at the first time.The degree of disorder could be easily controlled by sonochemistry reaction time,acidity or alkalinity in reaction and the average size of TiO₂ NCs precursor,which obtained a series of structurally and functionally graded nanocomposites with TiO₂ NC@amorphous titania core-shell structure.The sonochemical hydroxylation was based on acoustic cavitation.High-intensity ultrasonic irradiation under’ wet chemistry created lots of energy-concentrated hot-spots,which produced hydrogen radicals and hydroxyl radicals by water activation.These radicals hit the surface of TiO₂ NCs with high-speed water jets,which gave rise to the recomposition of chemical bonds by mass transfer and energy exchange.The hydroxyl injection into TiO₂ NCs induced the distortion of regular lattice,i.e.the disorder transformation in surface layers.Based on this principle,extending sonochemistry reaction time,providing alkaline environment and shrinking the average diameter of Ti02 NCs precursor were all benefited to increase the degree of disorder of composite titania,which monotonously improve the solar energy utilization,making the photocatalytic activity and the photoelectric conversion efficiency of assembled DSSC reach 2.33 times and 2.42 times of the usually used P25 NCs respectively.The degree of disorder was found out non-monotonously influencing the photo-quantum efficiency,because the disorder transformation diffused from surface layers to bulk in TiO₂ NCs,and when the degree of disorder was excessive,the surface defects turned to bulk defects,which provided the annihilating centers of photon-excited free charges,leading to the decrease of photo-quantum efficiency and further being bad for both the photocatalytic activity and the photoelectric conversion efficiency of assembled DSSC.This poses a challenge for disorder engineering of TiO₂ NCs under wet chemistry,and only if the degree of disorder was rationally controlled to conduct both the solar energy utilization and the photo-quantum efficiency to reach ideal conditions,could perfect photoelectric performances be obtained.（4）Property optimization through the new approach of sonochemical hydroxylation was successfully applied to other metal oxides that have similar semiconductor characters,such as ZnO、ZrO₂、Fe₂O₃ and SnO₂ NCs.Sonochemistry reaction also gave rise to the disorder transformation in surface layers and band engineering of these metal oxide semiconductor NCs,which improved their solar energy utilization and photo-quantum efficiency,and their photocatalytic activities were enhanced for about 27%（ZnO）、43%（ZrO₂）、46%（Fe₂O₃）and 74%（SnO₂）respectively.The disorder engineering and property optimization of metal oxide semiconductor NCs by sonochemistry under wet chemistry has been proven to be universally effective,which exhibits broad prospects because it is green,economical and easily-control.