Preparation Of Upconversion Photocatalysts Based On Rare Earth Doped CaF₂ And Their Applications In Degradation Of Organic Contaminants

Photocatalysis is a technology o f environmental pollution treatment and energy production through the reaction driven by sunlight. However, only the ultraviolet（UV） light accounted for 5% in solar spectrum and part of visible light can be utilized in the conventional photocatalytic process. In order to improve the utilization efficiency of sunlight, in recent years, a kind of composite photocatalyst possessing both upconversion and photoactivity has become research hotspot. As the starting point, the rare earth ions doped CaF₂ is used to combine with TiO₂ semiconductor in this work, and the NIR light will be upconverted to UV and short-wavelength visible light to excite TiO₂ to improve the photocatalytic activities. Three parts of active-core/active-shell structure preparation, heterostructure formation, and upconversion emission intensity enhancement are investigated. Due to the small absorption edge of TiO₂, the Bi VO₄ semiconductor with larger absorption edge is chosen to incorporate into upconversion photocatalyst, and the utilization efficiency of the upconverted light can be improved. Finally, the mixed ferrites fabricated from electroplating wastewater are used as the magnetic agents to combine with the best photocatalyst of Zn O prepared from electroplating wastewater, and the magnetite upconversion photocatalysts with the advantages of full-spectrum absorption and heterostructure properties are obtained to meet the upconversion and recycling purposes. The important conclusions are summarized below:（1） Upconversion photocatalyst of Er³⁺/Yb³⁺-（CaF₂/TiO₂） possesses the active-core/active-shell structure, and the active centers of Er³⁺/Yb³⁺ present both in the CaF₂ cores and TiO₂ shells. The absorption edge of Er³⁺/Yb³⁺-（CaF₂/TiO₂） is 425 nm, and the red light（657 nm）, green light（540 a nd 523 nm）, violet light（408 nm）, and UV light（379 nm） can be emitted under 980 nm NIR light excitation, which will be helpful for the photocatalytic reactions. The degradation of the methyl orange（MO） simulated wastewater show that the NIR（720 < λ < 1100 nm） driven degradation rate over Er³⁺/Yb³⁺-（CaF₂/TiO₂） is 32.1% after 12 h. Under the full-spectrum light（xenon lamp） irradiation for 4 h, the degradation rate of MO over Er³⁺/Yb³⁺-（CaF₂/TiO₂） is 63.3%, higher than those of pure TiO₂（52.6%）, CaF₂/TiO₂（45.9%）, and Er³⁺/Yb³⁺-TiO₂（48.6%）.（2） To improve the photocatalytic activities of the CaF₂/TiO₂ upconversion photocatalysts, the heterostructure is introduced to synthesize Er³⁺/Tm³⁺/Yb³⁺-（CaTi O₃/CaF₂/TiO₂）. The upconversion photocatalyst keeps the banded morphology of the cotton templates, and the surfaces of its（Ca0.8Yb0.2）F₂.2 cores are loaded with many anatase nanoparticles and rutile nanorods. The heterostructures can be formed between anatase and rutile, as well as among anatase, rutile, and the remaining CaTi O₃, which will provide efficient electron-hole pair separation for Er³⁺/Tm³⁺/Yb³⁺-（CaTi O₃/CaF₂/TiO₂）. The absorption edge of Er³⁺/Tm³⁺/Yb³⁺-（CaTi O₃/CaF₂/TiO₂） is 409 nm, and the red light（652 nm）, green light（541 and 522 nm）, blue light（485 and 455 nm）, violet light（409 nm）, and UV light（381, 364, 351, and 292 nm） can be emitted under 980 nm NIR light excitation, which will be helpful for the photocatalytic reactions. The degradations of the MO simulated wastewater show that the NIR（λ ≥ 780 nm） driven degradation rate over Er³⁺/Tm³⁺/Yb³⁺-（CaTi O₃/CaF₂/TiO₂） is 52.3% after 3 h, which are 1.6 and 2.5 times as high as those of Er³⁺/Tm³⁺/Yb³⁺-（CaTi O₃/TiO₂） and Er³⁺/Tm³⁺/Yb³⁺-（CaTi O₃/CaF₂）, respectively. Under UV- vis-NIR light irradiation for 10 min, the degradation rate of MO over Er³⁺/Tm³⁺/Yb³⁺-（CaTi O₃/CaF₂/TiO₂） is 90.3%, apparently higher than those of Er³⁺/Tm³⁺/Yb³⁺-（CaTOi 3/TiO₂） and Er³⁺/Tm³⁺/Yb³⁺-（CaTOi 3/CaF₂）.（3） The photocatalytic activities of upconversion photocatalysts are clo sely related to their upconversion emissions. In order to enhance the upconversion emission light, the upconversion photocatalyst of Er³⁺/Tm³⁺/Yb³⁺-（CaWO₄/TiO₂/CaF₂） is synthesized. The obtained upconversion photocatalyst consists of the remaining CaWO₄ and the TiO₂/（Ca0.8Yb0.2）F₂.2 coatings.（Ca0.8Yb0.2）F₂.2 is found to connect with TiO₂ nanocrystals, instead of being coated by TiO₂. In comparison to Er³⁺/Tm³⁺/Yb³⁺-（CaWO₄/CaF₂）, TiO₂ coatings improve the upconversion emission intensity of Er³⁺/Tm³⁺/Yb³⁺-（CaWO₄/TiO₂/CaF₂）. The absorption edge of Er³⁺/Tm³⁺/Yb³⁺-（CaWO₄/TiO₂/CaF₂） is 425 nm, and the red light（659 nm）, green light（544 and 525 nm）, blue light（477 nm）, violet light（410 nm）, and UV light（382 and 365 nm） can be emitted under 980 nm NIR light excitation, which will be helpful for the photocatalytic reactions. Under 980 nm and NIR light（λ ≥ 780 nm） irradiations for 3 h, the degradation rates of MO over Er³⁺/Tm³⁺/Yb³⁺-（CaWO₄/TiO₂/CaF₂） are 20.3% and 46.6%, respectively, higher than those（12.7% and 38.2%） of Er³⁺/Tm³⁺/Yb³⁺-（CaWO₄/TiO₂）. Under UV-vis-NIR light irradiation for 30 min, the degradation rate of MO over Er³⁺/Tm³⁺/Yb³⁺-（CaWO₄/TiO₂/CaF₂） is 91.7%, higher than that（84.8%） of Er³⁺/Tm³⁺/Yb³⁺-（CaWO₄/TiO₂）.（4） The three TiO₂-based upconversion photocatalysts mentioned above possess small absorption edges, and thus the upconversion photocatalyst of Er³⁺/Tm³⁺/Yb³⁺-CaF₂/Bi VO₄ with larger absorption edge is fabricated. The obtained upconversion photocatalyst consists of the regular dendritic morphology of Bi VO₄, and the（Ca0.8Yb0.2）F₂.2 particles were deposited homogeneously on the surface of Bi VO₄. In comparison to pure Er³⁺/Tm³⁺/Yb³⁺-CaF₂, the upconversion emission intensity of Er³⁺/Tm³⁺/Yb³⁺-CaF₂/Bi VO₄ can be improved by more than 8 times, since the crystal field symmetry has been broken by the substituted Bi³⁺ ions in Er³⁺/Tm³⁺/Yb³⁺-CaF₂. The absorption edge of Er³⁺/Tm³⁺/Yb³⁺-CaF₂/Bi VO₄ is 506 nm, and the red light（658 nm）, green light（543 and 524 nm）, blue light（485 nm）, violet light（408 nm）, and UV light（379 and 361 nm） can be emitted under 980 nm NIR light excitation, which will be helpful for the photocatalytic reactions. The degradations of the MO simulated wastewater show that the NIR（980nm） driven degradation rate over Er³⁺/Tm³⁺/Yb³⁺-CaF₂/Bi VO₄ is 10.1% after 6 h. Under UV-vis-NIR light irradiation for 3 h, the degradation rate of MO over Er³⁺/Tm³⁺/Yb³⁺-CaF₂/Bi VO₄ is 39.0%, higher than that of pure Bi VO₄, while the photocatalytic activity of sewage sludge templated Er³⁺/Tm³⁺/Yb³⁺-CaF₂/Bi VO₄-TiO₂ has been improved, with the value of 60.1%.（5） In the perspective of using all the upconversion emission light and the recovery of the upconversion photocatalysts by magnetic field, both of the mixed ferrites and the Zn O semiconductor prepared from electroplating wastewater are combined with the rare earth ions and Ca2+ ions to form the magnetite upconversion photocatalyst of Er³⁺/Tm³⁺/Yb³⁺-（CaF₂/Zn Fe₂O₄/Zn O）. Er³⁺/Tm³⁺/Yb³⁺-（CaF₂/Zn Fe₂O₄/Zn O） includes the（Ca0.8Yb0.2）F₂.2 cores, and the surfaces of（Ca0.8Yb0.2）F₂.2 are loaded with many Zn O and Zn Fe₂O₄ nanocrystals. The saturated magnetization of Er³⁺/Tm³⁺/Yb³⁺-（CaF₂/Zn Fe₂O₄/Zn O） is 1.08 emu/g, which can be separated by the magnetic field. The red light（656 nm）, green light（541 and 523 nm）, blue light（484 nm）, violet light（410 nm）, and UV light（381 and 362 nm） can be emitted from Er³⁺/Tm³⁺/Yb³⁺-（CaF₂/Zn Fe₂O₄/Zn O） under 980 nm NIR light excitation. Er³⁺/Tm³⁺/Yb³⁺-（CaF₂/Zn Fe₂O₄/Zn O） possesses the advantage of full- spectrum utilization, and all the upconversion emission light can be absorbed directly for photocatalysis. The degradations of the MO simulated wastewater show that the NIR（λ ≥ 780nm） driven degradation rate over Er³⁺/Tm³⁺/Yb³⁺-（CaF₂/Zn Fe₂O₄/Zn O） is 13.2% after 3 h. Under vis-NIR light irradiation for 3 h, the degradation rates of MO over Er³⁺/Tm³⁺/Yb³⁺-（CaF₂/Zn Fe₂O₄/Zn O） is 43.7%, apparently higher than that of Ca-Zn magnetite precursors.