Synthesis And Charactorization Of Near-infrared Photocatalysis Based On Upconversion Micro/Nano Crystals@TiO₂ Core/Shell Particles

Photocatalysis is a process of transforming materials by catalyst with utilization of photon energy. Photocatalysis has gained significant interest as it has great potential in the environmental remediation. TiO2 has widely studied has been widely studied on the degradation of inorganic or organic pollutants for its high activity, stability, avirulence and cheapness. The bandgap of pure TiO2 is up to 3.2 eV; hence, the ultraviolet (UV) light (λ< 380 nm) is necessary to activate pure TiO2 for photocatalytic reactions. However, the percentage of UV light in the solar spectrum is only 5%, which is very low compared to the visible light (-48%) and the near-infrared (NIR) light (-44%). The low-usage of sunlight has been restraining the photocatalytic efficiency of pure TiO2 in the environmental remediation. Numerous methods have been adopted to modify TiO2 for the utilization of visible light. However, the NIR of large fraction in sunlight remains untapped for photocatalysis.The upconversion materials emit one higher-energy photon after absorbing two or more lower-energy photons. Yb/Tm co-doped fluoride has ultraviolet (UV) emissions under the excitation of 980 nm near-infrared (NIR) light. We propose that the UV-to-NIR upconversion materials could be used as sensitizers for TiO2 to obtain new photocatalysts which can be activated by NIR light. It is a good method to coat TiO2 on the surface of upconversion particles for the combination of them, because this will protect the fluoride and will not reduce the adsorption of pollutants on the surface of TiO2. Our studies are as follows.(1) New reactors for the photocatalysis under NIR excitation were fabricated according to the experimental conditions. The new reactors can be used in the measurement of photocatalysis reaction. In the beginning, we fabricated the reactors which are effective for the photocatalsis under 980 nm diode laser and the NIR in sunlight. The reactors equipped with 980 nm fiber laser and low-power xenon lamp were designed as the development of our study. The advantages and disadvantages of the new reactors were discussed in the paper.(2) YF3:Yb3+,Tm3+ microcrystals were prepared by a microemulsion method. For the first time, the fluoride microcrystals were successfully coated with TiO2 by hydrolysis of titanium n-butoxide (TBOT) with polyvinylpyrrolidone K-30 (PVP) as a stabilizer and surfactant. The surfactant and concentration of TBOT had great influence on the morphology of the core/shell samples. The uniform TiO2 coatings with thickness of 10-20 nm can be prepared when the PVP was used and the concentration of TBOT was 0.003 mmol/mL. The upconversion luminescence properties of YF3:Yb3+,Tm3+ microcrystals were studied under 980-nm excitation. The 290 nm,346 nm.362 nm UV emissions and 451 nm,477 nm,643 nm visible emissions were observed and the mechanism of upconversion was discussed. The upconversion spectrum of TiO2-coated YF3:Yb3+, Tm3+ illustrated that the uniform TiO2 coatings compromise the upconversion emissions minimally. This maybe caused by the weak scattering effect by the uniform coatings on the excitation light. The little reduction of the excitation is beneficial for the upconversion emissions.(3) We presented a novel method of combining TiO2 and UV-to-NIR upconversion agents for photocatalysis. YF3:Yb3+,Tm3+ nanocrystals were prepared by hydrothermal method, and then the YF3:Yb3+,Tm3+/TiO2 core/shell nanoparticls were synthesized by the TBOT hydrolysis method mentioned above. TEM images illustrated that the uniform TiO2 coatings were formed on the surface of YF3:Yb3+.Tm3+ nanocrystals. The upconversion spectrum of YF3:Yb3+,Tm3+and YF3:Yb3+,Tm3+/Ti02 indicated that the TiO2 coatings can effectively absorb the UV emissions of YF3:Yb3+,Tm3+. The absorbance spectra of YF3:Yb3+,Tm3+/TiO2 particles indicated that the 980 nm NIR can be absorbed by the particles. After 30 h irradiation by 980 nm NIR,61% of MB was decomposed by the YF3:Yb3+,Tm3+/TiO2 particles. After 9 h NIR irradiation of sunlight,58% of MB has been degraded by YF3:Yb3+,Tm3+/TiO2 NPs. The results indicate that the YF3:Yb3+,Tm3+/TiO2 NPs have photocatalytic activity under NIR irradiation. Additionally, the catalytic activities of the pure TiO2 and YF3:Yb3+,Tm3+/TiO2 NPs were compared by decomposing methyl orange (MO) under UV irradiation. The results indicate that the uniform TiO2 shell reduce greatly the optical filting effect of YF3:Yb3+,Tm3+ on the UV light. It is the first time to use NIR light as driving source for photocatalysis. The same strategy has great potential in improving the utility rate of solar energy for photochemical and photoelectrical applications based on TiO2 materials.(4) NaYF4:Yb,Tm nanocrystals were prepared by a solvothermal method with PVP as chelating agent and surfactant. The nanocrystals were uniform with size of about 50 nm. The nanocrystals could be coated with TiO2 directly with the aid of PVP retained on the surface of the nanocrystals. This simplified the process of preparing of TiO2 coatings. The TiO2 shell thickness of TiO2 shell was～10 nm when the hydrolysis time was 15 min. At 45 min and 180 min, the thickness of TiO2 shell reached～14 nm and-16.5 nm, respectively. The shell thickness was readily adjusted through controlling the reaction time of TEOT hydrolysis. Thereby it is easy to change the weight percentage of TiO2 in the core/shell nanoparticles, which has great influence on the efficiency of composite photocatalysts.