Preparation Of TiO₂ Nano-heterostructures And Optimization Of Their Properties

The development and utilization of resources promote the rapid development of human life,but at the same time it has great impacts on the climate system and the ecological environment.Nevertheless,these effects are not always good.The rapid development of social economy and technology results in many environmental problems,such as greenhouse gas emission,energy depletion,water pollution,aerosol emission,and formaldehyde residue.All of these problems pose a threat to human living environment.In order to relieve this series of problems,people have i:mproved in many aspects,such as the development of new energy?currently only dedicated to the development of solar energy conversion efficiency and solar hydrogen production?,degradation of pollutants?greenhouse gases,formaldehyde,water pollution,Etc.?,adsorption of dust particles?aerosols?and so on.Titanium dioxide?TiO₂?plays a role in photoelectric conversion,photocdegradation,photocatalytic for hydrogen production and adsorption due to its band structure and various properties.Meanwhile,due to its low price,non-toxic,high chemical stability,strong oxidizing,bio-compatible characteristics,titanium dioxide has potential application value in tackling pollutants and solving energy crisis.The process of synthesizing TiO₂ is very simple.TiO₂ is suitable for mass production,and it has a good foundation for practical application.Although the band gap width?3.2 eV?of TiO₂ is large,the proper structural design for TiO₂ can not only improve the photoelectrochemical performance but also improve its photocatalytic effect.With a suitable structural design,the transfer path of photogenerated electrons-hole pairs has been changed,while simultaneously,the final accumulation of electrons and holes,the spatial separation of electron-hole pairs,and the ability of redox reaction?redox potential?have also been charged.To some extent.the above factors can determine the photoelectric conversion efficiency and photocatalytic performance of the samples.In this paper,heterostructured TiO₂ nanotree arrays?NTAs?and TiO₂ nanosheets were synthesized by hydrothermal method.TiO₂ NTAs were further constructed for surface-modified metal quantum dots by magnetron sputtering and TiO₂ nanosheets were annealed with hydrogen annealing.The structures and other characterizations of counter sample were characterized.The correlation between photoelectrochemistry and photocatalytic performance was discussed.The main contents of the research work are as follows:The mixed phase TiO₂ NTAs were prepared on conductive glass?FTO?by secondary hydrothermal method.Ag quantum dots?QDs?were modified by magnetron sputtering method.Mixed phase TiO₂ NTAs are composed of the rutile and anatase phase.The anatase phase nanosheets are adhered to the nanorods?NRs?of the rutile phase,and the tops of the TiO₂ NTAs show the tetragonal morphology.When the other conditions such as the sputtering power are the same,the amount and size of the Ag QDs loading are controlled by the sputtering time.By scanning electron microscope?SEM?,transmission electron microscope?TEM?,Raman,X-ray diffraction?XRD?.X-ray photoelectron spectroscopy?XPS?,electrochemical workstation,the morphology,crystal structure,composition,optical properties,photoelectrochemical performance?PEC?of the prepared samples were tested.Ag/TiO₂ NTAs have larger light absorption range and absorption intensity than pure TiO₂ NTAs,and the cut-off wavelength has an obvious red shift.At the same time,surface plasmon resonance?SPR?occurs due to Ag QDs,and the plasmon resonance peak follows the condition of Ag QDs.When the size of Ag QDs is larger/increase,the peak intensity is higher,and the red-shift is more ob-vious.After five cycles,the photocurrent decays very slowly,essentially the same as the initial value Ag?15s?/TiO₂ NTAs have the highest photocurrent density reaching 0.52 mA/cm2,which is 104 times as that of pure TiO₂ NTAs.Mainly,because of Ag QDs forming a ohmic contact with TiO₂ and the anatase/rutile heterojunction,the transport mode of carriers in Ag/TiO₂ NTAs is different from that in pure TiO₂ NRs.The range and intensity of light absorption are enhanced by local plasmon resonance effect?LSPR?in Ag/TiO₂ NTAs.At the same time,the rutile and the anatase phase TiO₂ constitute a traditional type ? heterojunction,which effectively separates the electron-hole pairs in space and enhances the photoelectrochemical performance.The TiO₂ NTAs were prepared by secondary hydrothermal method,and annealed under a hydrogen atmosphere?the gas used was nitrogen-hydrogen gas,the ratio of H2/N2 was 5/95?,and the hydrogen was used to reduce the Ti⁴⁺ ions in the TiO₂ sample to make Ti⁴⁺being converted into Ti³⁺ state ions for self-doping.TiO₂ NTAs consist of nanorods of rutile phase and nanosheets of anatase phase,forming a tree-like traditional type ? heterostructure.Under this basis,after the atmosphere annealing,part of Ti⁴⁺ is reduced to Ti³⁺ ions,and certain defects are formed,so that the internal structure changes,and the photogenerated electron and hole transition processes change.In the case of annealing for the same time,the amount of internal defects of the sample and the amount of Ti³⁺ ions are changed by controlling the annealing temperature.At a bias voltage of 0 V,the hydrogen annealed titanium dioxide sample?H:TiO₂ NTAs?had the highest photocurrent density at annealing at 400 ?,and the current density was 0.2272 mA/cm2,which was 41 times than that of the non-hydrogen annealed sample.Due to the defects caused by hydrogen annealing,the presence of Ti³⁺ is to form sub-levels,which are beneficial to electron transitions and increase the absorption range and intensity of light.The anatase phase nanosheets were prepared by hydrothermal method in an acidic environment which caused by HF.And the powders were filtered,centrifuged,and washed to obtain the clean TiO₂ powder.The residual F-ion has a certain influence on the crystal plane of the main exposed surface in the TiO₂ powder.The photocatalysis capabilities of different crystal faces are also different.This will be covered in a subsequent article detailly.The sample was annealed in an atmosphere of H2/N2?5/95,ratio?.Subsequently,the photocatalysis of the two samples under sunlight and ultraviolet light was compared.Under ultraviolet?UV?light,the photocatalytic degradation rate of methyl orange by H:TiO₂ is 25.8%higher than that of unannealed TiO₂.We can draw a conclusion that:under the simulated natural light source AM 1.5 G,the photocatalytic degradation efficiency of H:TiO₂ powder is 10.5%higher than that of unannealed TiO₂ powder.Nanosheets have highly exposed{001}crystal faces,defects and Ti³⁺state ions.The heterojunction is made up of {001}crystal face and {101} crystal face.Since the Ti³⁺ state ions and the defects form sub-levels,the photogenerated electrons of the H:TiO₂ powder require less energy than the pristine TiO₂ powder during the transition,and the photocatalytic performance is better.The synergy between the heterojunction and the Ti³⁺ state ions or defects enhances the photocatalytic efficiency.