Photoelectrochemical And Photocatalytic Properties Of Quantum Dot-sensitized TiO₂ Film

Self-organized TiO₂ film,prepared by anodic oxidation process,had been applied in many fields including:solar cells,photocatalysis,lithium-ion batteries and supercapacitors due to its stability,nontoxicity and low cost proterties.However,pristine TiO₂ suffers from poor photoabsorption over the entire visible light range due to its wide band gaps?3.2 eV for anatase?,and the high recombination efficiency of electron-hole simultaneously limited further photoelectrochemical applications of TiO₂.In this article,the optimization of TiO₂ nanotube array?NTs?prepared by the anodic oxidation method had been studied in detail,then TiO₂ nanowires and nanotubes?NWs/NTs?had been prepared,quantum dot?QDs?to be a sensitizer to enhance its absorption ability,and the following paragraphes shows the innovation in detail1.Previous works showed that depositing QDs on TiO₂ NTs through traditional methods can cause to obvious clogging on the surface of NTs.In our work,ultrasound-assisted hydrothermal method was to synthesize CdSe QDs,which ensured that QDs enter into NTs without any obvious clogging,and this method can easily control the partical size of QDs.The results showed that the sample prepared by ultrasound-assisted hydrothermal method hold well-dispersed morphology,and QDs had entered into NTs without any obvious clogging.Under the radiation of visible light,the as-prepared products displayed stable photoelectrochemical properties.Tthe photo current density of CdSe sensitized TiO₂ NTs?CdSe/TiO₂ NTs?can reach to 0.30 mA cm^-2,which is about five-fold of bare TiO₂ NTs.When been doped by Cobalt element,the photo current density improved with the increased of the doping concentration,and the maximum density was captured at the doping concentration of at_10%(0.47 mA cm^-2).In this experiment,the QDs-sensitization can expand the absorption range of TiO₂ NTs,at the same time,the arrangement of QDs in the TiO₂ NTs improve the refractive index and absorption rate of light in the tube,thus CdSe/TiO₂ NTs can achieve better photoelectrochemical.The doping of Cobalt can change the band structure of CdSe QDs,and the band gap had been lowered with the results of improved photon absorption ability,therefore Co-CdSe/TiO₂ NTs can lead to higher photo current conversion efficiency.2.When been applied in photocatalytic fields,the size of TiO₂ nanoparticles is small,and separating TiO₂ nanoparticles from the pollutants is a big problem at the end of the photocatalysis,which deeply limited the practical photocatalytic application of TiO₂.In this work,the NTs can be furthered oxidated into nanowires and nanotubes?NWs/NTs?through changing the electrolyte,oxidating time,oxidating voltage.Compared to the bare NTs,the NWs/NTs structure can hold border specific surface area.Thus,NWs/NTs can absorb more photons,and the unique composite structure can lode more QDs and then formed a well stable heterojunction.In this work,the TiO₂ NWs/NTs was chose as the depositing substrate,CdS QDs with faster carrier transmission rate was chose as the sensitizer,and then CdS QDs were deposited on TiO₂ NWs/NTs through successive ion layer adsorption and reaction?SILAR?method.The result showed that TiO₂ NWs/NTs was prepared successfully with border specific surface area,and NWs was located on the surface of NTs.The as-prepared QDs was deposited on NTs and NWs simultaneously and then formed a well stable heterojunction.This heterojunction material expand the light absorption range to 500nm,and causing to a lower recombination efficiency of electron-hole,contributing to the improved photoelectrochemical and photocatalytic activities.Under the radiation of visible light,the sample prepared by SILAR 3 times?CdS?3 cycles?/TiO₂ NWs/NTs?exhibited the best photoelectrochemical performance(1.04 mA cm^-2).Moreover,when been applied in photocatalytic methylene blue solution,TiO₂ NWs/NTs displayed better photocatalytic efficiency than the bare TiO₂ NTs?achieving to 96.50%?.The unique morphological nanostructure of TiO₂,which guaranteeing the photocatalytic stability and repeatability of the prepared heterogeneous materials,thus photocatalytic efficiency of CdS/TiO₂ NWs/NTs can still up to 95.80%after 5 identical photocatalytic tests.3.Moreover,the photoelectrochemical and photocatalytic properties of CdSe and CdS QDs co-sensitized TiO₂ NWs/NTs was deeply studied in this work.The as-prepared product CdS/TiO₂ NWs/NTs was as the substrate to deposit CdSe QDs through electrochemical depositing method with the formation of CdSe/CdS/TiO₂NWs/NTs.The experiment result showed that two kind of QDs were deposited on NWs and NTs simultaneously,and the partical size of the two QDs were less than 10nm.The co-sensitized sample CdSe/CdS/TiO₂ NWs/NTs move the light absorption range to 600 nm.Under the radiation of visible light,the highest photo current density is up to 1.58 mA cm^-2,which is about 1.5-fold compared to CdS/TiO₂ NWs/NTs.When been applied in photocatalytic methylene blue solution,the photocatalytic efficiency of CdSe/CdS/TiO₂ NWs/NTs can achieve to 99.77%,and this data can still up to 97.84%after 5 identical photocatalytic tests.