Design, Synthesis And NIR-Responsive Photocatalytic Properties Of TiO₂@NaYF₄：Yb,Tm Core-shell Composite

In the question of realizing the sustainable development, environmental pollutionand the exhaustion ofresource are becoming the new challenges which the humanmust face in21stcentury. Solar energy is the most important natural resources, whichdirectly or indirectly provide the energy for maintaining normal human life activities.How to make the best of the solar energy for the the survival and development ofhuman beings is a big problem to be solved. Semiconductor photocatalysis technologyis a process that converts light energy into chemical energy, which directly use theexcitation light to drive the oxidation-reduction reactions. From the point of resourcesutility, it is of great significance for the development photocatalytic technology.Since Fujishima and Honda discovered photocatalytic water-splitting oncrystalline TiO2electrodes for hydrogen prodution in1972, research interest in TiO2photocatalysis has grown significantly owing to its strong oxidizing power underultraviolet light, extraordinary chemical stability, and environmental friendly andbiocompatible features. Heterogeneous photocatalytic technology refers that thephotochemical reactions occurred between the catalyst material and its surfaceadsorption. Therefore, light and a catalyst are the two essential factors. However, TiO2requires UV light to be activated as a result of its large bandgap. The ultravioletregion of the solar radiation is only of the total of about~5%. Therefore, more than90%of overall solar spectrum cannot be utilized to activate this photocatalyst forphotocatalysis.To resolve this problem, much effort has been devoted to extending theabsorption of TiO2to the visible light region for better use of solar energy. To reachthis goal, several strategies have been proposed by adjusting the bandgap towardvisible light energies through the introduction of noble metals, cationic substitutionsand anionic doping. It is a promising way to use the upconversion fluorescentmaterials for the photocatalysis. In2010, the previous work by our group first demonstrated NIR-driven photocatalysis of broadband semiconductor TiO2that wascombined with YF3:Yb,Tm to form a core-shell structure, where YF3:Yb,Tm acts as amedium for converting NIR to UV and visible light via multiphoton upconversionprocesses. On the basis of the above work in our group, we further optimized thepreparation process of the upconversion/semiconductor composite. NaYF4upconversion micro/nanocrystals were succcessfully coated with titania via throughcontrolling the hydrolysis and condensation rate of TBOT. Furthermore, a NIRphotocatalytic mechanism, including the energy transfer process betweenupconversion luminescence particles and TiO2and the actual origin of the organicpollutant degradation were investigated. The following conclusions were obtained.(1) NaYF4:Yb,Tm nanophosphors were synthesized by a hydrothermal orsolvothermal process. TiO2@NaYF4:Yb,Tm core-shell particles were synthesized viaa method similar to a St ber process. The structure and electron microscopy analysisindicate that the anatase titania nanocrystals were attached around cubicNaYF4:Yb,Tm particles, forming TiO2@NaYF4:Yb,Tm core-shell nanostructures. Theshell thickness can be adjusted by changing the relative concentration of the precursorof upconverting particles and TBOT. This method for TiO2@NaYF4:Yb,Tmcomposite can be generalized to the preparation of other inorganic/semiconductorcomposite systems.(2) The synthetic parameters for TiO2@NaYF4:Yb,Tm composites were furtheroptimized based on the previous work by our group. For example, TBOT was chosenas the precursor of TiO2to improve the coating quality due to its small hydrolysisratio; the hydrothermal method was used as a thermal treatment way to control theparticle growth and improve the crystallinity; the NaYF4rather than YF3was chosenas the core component to enhance the upconverting luminescence efficiency.(3) The upconverting luminescence and absorption data indicate that the UVphoton energy generated via upconversion process of NaYF4:Yb,Tm can be absorbedby the anatase TiO2around NaYF4:Yb,Tm particles. The fluorescence dynamicanalysis of Tm3+indicates that the energy migration between NaYF4:Yb,Tm and TiO2is a fluorescence resonance energy transfer (FRET) process for TiO2@NaYF4:Yb,Tm core–shell composites while it is a radiation-reabsorption process forNaYF4:Yb,Tm-TiO2mixtures. The comparison of photocatalytic efficiency betweenTiO2@NaYF4:Yb,Tm composites and the mixtures that FRET is an importantmechanism in the NIR photocatalytic activity.(4) The nature of NIR-responsive photocatalysis of TiO2@NaYF4:Yb,Tmcomposites was demonstrated via the parallel experiments.(i) MB solution wasirradiated with NIR light in the absence of NaYF4:Yb,Tm and TiO2. In this case,almost no degradation of MB occurred.(ii) MB solution was irradiated with NIR lightin the presence of NaYF4:Yb,Tm, and only about8%of MB was decomposed. Thedetection of OH indicates that almost no OH was generated in the case of withoutcatalyst or with NaYF4:Yb,Tm while OH was generated for TiO2@NaYF4:Yb,Tmcomposites. In addition, it should be noted that the relationship between the OHgeneration rate and the photocatalytic activity implies that the NIR photocatalyticactivity arises from OH. These results demonstrate that the degradation of organicchemicals induced by upconverting/semiconductor composite catalysts occurspredominantly by photocatalysis rather than photolysis or thermolysis.(5) The factors on the photocatalytic efficiency of the composite was investigated.It should be noted that the pH value is important to the photocatalytic activity of theTiO2@NaYF4:Yb,Tm composite in aqueous solutions. For example, an acidic solutionis beneficial to improving the photocatalytic activity of the composite for methylorange (MO) dye system under NIR irradiation.