Synthesis And Characterization Of NaYF₄:Yb,tm/cds/TiO₂Visible/near-infrared Complex Photocatalysts

The world industrial development acceleration inevitably brought a series of environmentand energy related issues. To make full use of clean energy and develop environmentalpurification method are always world subjects. A series of studies showed that titanium oxide isefficient for solar photocatalytic decomposition of organic pollutants. Except that, it has theadvantages of simple synthetic method, stable physical and chemical properties, non-toxic andlow cost. Therefore, it is widely used in the photocatalytic degradation of pollutants andphotovoltaic solar cell electrode preparation. Titanium oxide is expected to solve the problems ofenvironmental pollution and the energy depletion. However, the3.2eV band gap of titaniumoxide results in that only the ultra violet part of sun light can be absorbed by titanium oxide togenerate photocatalysis. And the ultraviolet energy is only about5%of the whole solar spectrumenergy. Most of the spectral energy is concentrated in the visible （⁴8%） and near infrared（⁴4%） region respectively. The low utilization rate of solar energy limits the photocatalyticapplications of titanium dioxide. Many researches have shown that: composite photocatalystwith titanium dioxide as a host and cadmium sulfide whose bandgap is narrow as a sensitizer canextended the spectral response of titanium dioxide to visible region, thereby enhance thephotocatalytic efficiency for solar energy utilization. In this paper, we synthesized CdS/TiO₂composite photocatalyst through a multi-step hydrothermal method. Firstly, CdS nanoparticleswere synthesized through hydrothermal method. Then, TiO₂nanoparticles were deposited on thesurface of CdS by hydrolysis of butyl titanate and hydrothermal method. Scanning, transmissionelectron microscope and X-ray diffraction test results show that: the resulting materials are nanospheres with a size of about150nm. However, the spheres are polycrystalline which iscomposed of a large number of CdS and TiO₂nanoparticles of10nm. Compared with that ofpure TiO₂, the absorption spectral range composite photocatalyst have greatly expanded. Itcovers the spectral range of pure TiO₂and a part of visible region. In the photocatalytic activitytest, we found more than70%of methyl orange molecules in solution decomposed after9hoursirradiation under a xenon lamp with a420nm cut-off filters. In contrast, under the same testingcondition, the decomposition rates of methyl orange for pure CdS and TiO₂are about10%. The composite material showed a high catalytic activity under visible light. Benefiting from the porestructure of the CdS/TiO₂nano sphere, the material have a large specific surface area, this helpsto improve the photocatalytic efficiency. Except that, the CdS sensitizers are embedded in TiO₂nano clusters, which could effectively prevent the falling off of the sensitizers. Therefore the uselife of the catalyst is improved.In this paper, we also prepared and studied NaYF₄:Yb,Tm/CdS/TiO₂near infraredcomposite photocatalysis material.Up conversion materials can convert low energy photons to high energy photons. Underinfrared excitation, Yb/Tm co-doped NaYF₄upconversion material can emit visible light andultraviolet. The visible light can excite cadmium sulfide and the ultraviolet light can excitetitanium dioxide. Based on this principle, we prepared NaYF₄:Yb,Tm/CdS/TiO₂compositephotocatalyst, in which titanium oxide and cadmium sulfide are adhered to the surface of theNaYF₄:Yb,Tm microrod. The novel photocatalytic material can decompose organic dye underirradiation of980nm near infrared. SEM, TEM, XRD, absorption and emission spectrum resultsshow that: CdS and TiO₂are evenly connected to the surface of NaYF₄:Yb,Tm. Under980nmNIR excitation, except the645nm emission can not be absorbed by the CdS/TiO₂at theNaYF₄:Yb,Tm surface, each component of the emissions has attenuation of light intensity. Theresult of980nm near infrared driven photocatalytic activity measurement shows that: thesynthesized composite photocatalyst has good catalytic activity. After50hours irradiation, morethan90%of the methyl blue in solution decomposed. The photocatalytic efficiency is improvedsignificantly compared with that of NaYF₄:Yb, Tm/CdS or NaYF₄:Yb,Tm/TiO₂. The reason isthat this material is not only able to make full use of each component of upconversion emissions,but also benefits from the super strong photocatalytic activity of TiO₂. The luminescence lifetimemeasurements indicate that: The lifetime of Tm3+level in the composite material is shorter thanthat in pure NaYF₄:Yb,Tm, which proved that there is non-radiative energy transfer processbetween NaYF₄:Yb,Tm and CdS/TiO₂.