Abstract:

Rare earth upconversion materials NaYF₄:Yb,Tm with UV and blue light emission were synthesized by a one-step hydrothermal method. The composite material NaYF₄:Yb,Tm@TiO₂(P)@Bi₂WO₆ was prepared by sequentially laminating TiO₂ and Bi₂WO₆ onto NaYF₄:Yb,Tm using solvent/hydrothermal method. A series of characterization analyses were carried out on the materials, and the results showed that the composites had a tight cladding structure between them, and the specific surface area of the composite increased, but the pore size became smaller. XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy) confirmed the existence of oxygen vacancies in the materials. PL (photoluminescence spectroscopy) and UV-Vis DRS (UV-Vis diffuse reflectance) results indicated that TiO₂/Bi₂WO₆ could significantly absorb the emitting bule-ultraviolet light from NaYF₄:Yb,Tm. The effects of different catalyst dosage, 2,4-D concentration and external factors on the degradation reaction were investigated. The results showed that (a) the composite photocatalyst had excellent degradation performance for 2, 4-D, and the optimal degradation rate reached more than 96% within 3h under simulated sunlight irradiation, (b) the optimal catalyst dosage was at 1.0 g/L, and (c) the common anions in water had a large effect on the degradation and the cations had no effect; the strong acid condition promoted the reaction, the alkaline condition inhibited the reaction, and humic acid had a partially promotional effect on the reaction at a low concentration but inhibited the reaction obviously at a high concentration. The EPR (electron paramagnetic resonance) test and free radical trapping experiments found that h⁺ (photogenerated holes) was the active substance that played a major role in the removal of 2,4-D. The degradation intermediates were determined by GC-MS (gas chromatography mass spectrometry) and HPLC (high performance liquid chromatography), and two degradation pathways were hypothesized. In addition, a detailed toxicity evaluation of the degradation intermediates was carried out using T.E.S.T Toxicity Evaluation Software, and the results showed that the combined toxicity of the degradation products was significantly reduced.

Key words: 2, 4-dichlorophenoxyacetic acid, rare earth upconversion, mechanism, toxicity evaluation, degradation, catalyst, radical

摘要：

采用一步水热法合成了具有紫外光和蓝光发射的稀土上转换材料NaYF₄:Yb,Tm。采用溶剂/水热法依次将TiO₂和Bi₂WO₆复合在NaYF₄:Yb,Tm上，制备出复合材料NaYF₄:Yb,Tm@TiO₂(P)@Bi₂WO₆。对材料进行一系列表征分析，结果表明复合材料之间具有紧密的包覆结构，复合后的比表面积有所增大，但孔径变小，XRD（X射线衍射）、XPS（X射线光电子能谱仪）等证实了材料中氧空位的存在，PL（光致发光光谱）和UV-Vis DRS（紫外可见漫反射）结果显示TiO₂/Bi₂WO₆对NaYF₄:Yb,Tm上转换发射的蓝紫光有明显吸收。探究了不同催化剂投加量、2,4-D浓度和外界因素对降解反应的影响，结果表明：①复合光催化剂对2,4-D有着优异的降解性能，在模拟太阳光照射下3h内最优降解率达96%以上；②最佳催化剂投加量在1.0g/L；③水中常见阴离子对降解影响较大，阳离子基本没有影响；强酸条件下会促进反应，碱性条件下会明显抑制反应；腐殖酸低浓度下对反应有部分促进作用，但高浓度下抑制作用明显。通过EPR（电子顺磁共振）测试和自由基捕获实验，表明h⁺（光生空穴）是去除污染物2,4-D过程中起主要作用的活性物质。通过GC-MS（气质联谱）、HPLC（高效液相色谱）等手段测定降解中间产物，并推测出两条降解路径。此外，利用T.E.S.T毒性评估软件对降解中间产物进行了详细的毒性评价，结果显示降解后产物的综合毒性显著降低。

关键词: 2,4-二氯苯氧乙酸, 稀土上转换, 机理, 毒性评价, 降解, 催化剂, 自由基