Photocatalytic Performance Improvement Of Nc-TiO₂ Through The Synergistic Effect Of Photonic Crystals And Upconversion Nanoparticles RE³⁺:NaYF₄

As a photocatalyst that has been widely studied,TiO₂ has a very promising application prospect.However,the biggest disadvantage of TiO₂is that it can only absorb ultraviolet light with a wavelength less than 387 nm which accounts for 4%of sunlight.This led to restrictions on its application.Although the photoresponse range of TiO₂ currently can be expanded to visible light regionby surface noble metal deposition,ion doping and quantum dot sensitization,etc.,the utilization of sunlight is still limited.When the upconversion nanoparticles?UCNPs?continuouslyabaorbsuccessive near infrared radiation,itcould emit ultraviolet or visible fluorescence,whichcan be absorbed by sensitized TiO₂ absorption of.Itscould further broadthe sunlight harvestof TiO₂.However,the fluorescence efficiency of UCNPs is usually too low.The photonic crystals?PCs?can be used tocouplewith UCNPs,which could enhance its upconversion fluorescence intensity.And this synergistic effect from the PCs and UCNPs could be used to improve the photocatalytic performance of TiO₂.There are several main aspects to the research content of this paper:?1?Lanthanide rare earth ions Yb³⁺/Tm³⁺doped UCNPs NaYF4:Yb/Tm and lanthanide rare earth ions Yb³⁺/Er³⁺doped UCNPs NaYF₄:Yb/Er were prepared by high temperature thermal decomposition method.The effects of variables on the preparation of hexagonal phase UCNPs were investigated,such as reaction temperature,the amount of oleic acid and F^-ions.UCNPs withgood dispersibility,uniform particle size and high fluorescence intensity can be obtained,when the reaction temperature at 300?,the ratio of oleic acid and 1-octadecene at 3:7 and the molar ratio of F^-to the total amount of doped rare earth ions at 3.3:1.We also obtained the hydrophilic UCNPs NaYF₄:Yb/Tm and NaYF₄:Yb/Er by ligand exchange method by replacing the oleic acid ligand with citrate ligand.Using fluorescence spectroscopy,there are four characteristic fluorescence emission peaks of NaYF₄:Yb/Tm at 360nm,448nm,478nm and 800nm,and three characteristic fluorescence emission peaks of NaYF₄:Yb/Er at 528nm,544nm and 654nm.Meanwhile,the fluorescence mechanism of the corresponding characteristic emission peak was explained.?2?A series of monodisperse SiO₂ microspheres with different diameters were prepared by seed growth method.The SiO₂ opal PC films were successfully assembled by the room temperature floating method.And the photonic bandgap?PBG?of PC films can be adjusted by controlling the diameter of SiO₂ microspheres.Various of PC/UCNPs composite films were prepared through spin-coating UCNPs on the surface of SiO₂ opal PCs with different PBG.The effect of overlapping extent between PBG and UCNPs'emission on upconversion emission enhancement was studied.The result shows that when the SiO₂ Opal PCs with the PBG at 448 nm was composted with NaYF₄:Yb/Tm,the upconversion fluorescence at 448 nm wasenhanced mostly,which is 34.71-fold.Similarly the highest enhancement,23.13-fold,was observed for fluorescence peak of NaYF₄:Yb/Er at 544nm when composited with SiO₂ opal PCs having PBG at 544 nm.According to the analysis,the highest enhancement can be obtained when the PBG of PC's is exactly overlapping with the wavelength of upconversion fluorescencethrough coupling with PCs by utilizing the PBG and light localization effect of PCs.?3?The nanocrystalline TiO₂ was prepared by hydrothermal method and then composited with PCs/UCNPs films.The TiO₂/PCs/UCNPs composite film was then sensitized by Cd S quantum dots?Cd SQDs?.The photodegradation of rhodamine B was used as a probe reaction to study the effects of PCs,UCNPs,QDs and the synergistic effect of them on the photocatalytic performance of TiO₂.The result shows that the absorption edge of TiO₂ can be extended to the visible light range of 500nm by Cd S QDs sensitization effect.And 1.63-fold enhancement effect of photocatalytic performance of Cd S QDs sensitized TiO₂ was observed.Meanwhile,little enhancement on the photocatalytic performance was discovered with the UCNPs alone,due to its low upconversion fluorescence intensity.However,through the synergistic effect of PCs with 448 nm PBG and NaYF₄:Yb/Tm,2.57-fold enhancement effect on the photocatalytic performance of Cd S QDs sensitized TiO₂was obtained.Moreover,although the upconversion fluorescence of NaYF₄:Yb/Er at544 nm could be enhanced by the PCs with 544 nm PBG.It bears out that fluorescence with this wavelength cannot be absorbed by Cd SQDs sensitized TiO₂.Therefore,the photocatalytic performance of TiO₂ did not be enhanced by the composite of PCs and NaYF₄:Yb/Er.It is verified that the synergistic effect of PC and UCNPs is an effective method to improve the photocatalytic performance of TiO₂.And the synergistically enhanced fluorescence emission peaks need to be within the photoresponse range of quantum dot-sensitized TiO₂ in order to maximize this synergistic effect.