Study On Preparation And Photoelectrochemical Properties Of Ferroelectric Composites

Energy shortage and environmental pollution issues are two major challenges limiting the sustainable development of human society in the 21^st century.Heterogeneous photochemical reactions on semiconductor surfaces can be used to produce hydrogen by splitting water or to degrade organic contaminants.To achieve efficient and durable solar energy conversion,high photocatalytic activity photocatalysts require the following characteristics:effective and broad-band light absorption,efficient carrier separation,and superior stability.When a ferroelectric material is involved in a heterojunction,its spontaneous polarization field would induce considerable free carrier redistribution in the adjacent semiconductor and achieve excellent photocatalytic performance.This work just focuses on fabricating ferroelectrics-based semiconductor composites to exploit novel high activity photocatalysts and trying to find out the nature of polarization field promoting photocatalytic activity of ferroelectric heterojunctions.In the thesis’s work,titanium dioxide（TiO₂）nanotubes were prepared by two-step anodic oxidation and TiO₂/BaTiO₃core-shell nanotubes were prepared by hydrothermal reaction.Ferroelectric Bi₅Ti₃FeO₁₅films were prepared on FTO substrates by sol-gel method.Bi₅Ti₃FeO₁₅/BiOCl heterojunction thin films were synthesized by reaction of Bi₅Ti₃FeO₁₅ and HCl using ion replacement method.The microstructure and the photoelectrochemical properties of those ferroelectric composite samples were systematically characterized.The main results are as follows:1.Titanium dioxide nanotubes were prepared by two-step anodization.The effects of the second electrolysis voltage on the morphology and photoelectrochemical properties of nanotubes were investigated.TiO₂ nanotubes prepared by using 60V in both steps have the best light absorption and highest photocurrent.The photocurrent density is 1.04 m A/cm² at1.23V（vs the reversible hydrogen electrode（RHE））,and the light conversion efficiency is0.58%,which is 3.4 times of the value of one-step nanotube.The photocurrent density of two-step nanotubes decays by only 5%after 2 hour photoelectrochemical measurement under the 1.5 G illumination,showing the superior stability of the sample.2.TiO₂/BaTiO₃ core-shell nanotubes were synthesized using two-step nanotubes as base materials by hydrothermal method.The effects of reactant concentration and reaction time on the microstructure of the as-prepared samples were systematically investigated.The optimized synthesis process was determined.After hydrothermal treatment for 30 min,the obtained TiO₂/BaTiO₃ core-shell nanotube has a photocurrent density of 2.23 m A/cm²which is twice that of pure TiO₂ nanotubes,and the photoconversion efficiency reaches to1.6%which is 2.7 times of TiO₂ nanotubes.Electrochemical impedance spectroscopy shows that the hydrothermal treatment reduces the contact resistance between the titanium plate and titanium oxide as well as the resistance between the electrode and the solution,which favors the transport of photo-generated carriers and consequently improves photocatalytic performance.The results of open-circuit photovoltage decay（OCPVD）measurements show that the lifetime of photo-generated carriers in TiO₂/BaTiO₃ core-shell nanotubes is obviously increased by more than 5 times with respect to TiO₂ nanotubes,definitely indicating it is BaTiO₃ prolonging the lifetime of carriers,implying the internal electric field of ferroelectric barium titanate promotes the carrier separation.Using sodium sulfite as a hole-trapping agent,the photocurrents of TiO₂ nanotubes and TiO₂/BaTiO₃core-shell nanotubes were measured in sodium sulfite system and water system.The results indicate that the enhanced separation efficiency of photogenerated carriers is the main factor leading to the high photocatalytic activity of TiO₂/BaTiO₃ core-shell nanotubes with respect to TiO₂ nanotubes.We believe that the improvement of photo-generated carrier separation efficiency is mainly induced by internal electric field of ferroelectric barium titanate.3.Bi₅Ti₃FeO₁₅ films were prepared by sol-gel method on a FTO conductive glass substrate.Bi₅Ti₃FeO₁₅ films with different polarization orientations were obtained by applying an electric field in the crystallization process.The photocurrent measurements indicate that the photocurrent density of the positive polarized Bi₅Ti₃FeO₁₅ has the largest value among our the samples,which is 3.8 times that of the unpolarized sample,while photocurrent density of the negative polarized Bi₅Ti₃FeO₁₅ was 2.3 times that of the unpolarized sample.Bi₅Ti₃FeO₁₅/BiOCl heterojunction films with different polarization directions were successfully prepared by reacting in 0.5M hydrochloric acid.The results of photocurrent test show that the unpolarized heterojunction films exhibit a 1.6 times value of the unpolarized films.The photocurrent density of the Bi₅Ti₃FeO₁₅/BiOCl heterojunction films in positive and negative polarizations are 3.5 and 3.14 times that of unpolarized heterojunction films,respectively.The energy level structure of Bi₅Ti₃FeO₁₅ and BiOCl in aqueous system was obtained by MS curve test.It is found that the energy level structure is not conducive to the carrier separation.It is confirmed that the internal electric field of ferroelectrics improve the photocatalytic activity of Bi₅Ti₃FeO₁₅/BiOCl heterojunction films.