【作者】 张代生；

【导师】 秦伟平；

【作者基本信息】 吉林大学 ， 物理电子学， 2011， 博士

【摘要】 光催化是通过催化剂利用光能进行物质转化的一种方式。由于光催化在治理环境污染方面具有极大的应用前景,受到了很多研究者的关注。TiO2以其催化活性高、稳定性好、无毒和成本低等有点,被广泛应用于光催化降解有机和无机污染物的研究。但是由于TiO2的禁带宽度达到了3.2eV,只有波长小于387nm的紫外光才能将其激发产生光催化作用。紫外光在整个太阳光谱中所占的比例只有～5%,而可见光和近红外光的比例则分别为～48%和～44%。对太阳能的利用率低限制了TiO2光催化的实际应用。人们在合成具有可见光响应特性的TiO2催化剂方面做了大量研究,但是,与可见光能量相当的近红外光仍然没有被用于光催化。上转换材料可以吸收低能量光子并发射高能量光子,Yb/Tm共掺杂的氟化物在980nm近红外光的激发下可以发出紫外光,而紫外光可以激发TiO2。我们提出可以用这种红外—紫外上转换材料敏化TiO2,得到一种可以用红外光激发的新型光催化材料。如果将这两种材料结合,那么将得到的复合材料制成以TiO2包覆上转换材料的核/壳结构是一种理想的选择。因为这种结构既不会减少TiO2与被降解物质的接触,又能最大限度的保护氟化物上转换材料,避免外界环境对上转换材料的破坏而造成对其发光性能的不利影响。基于上述原因,我们开展了下面的一些工作。(1)根据实验的需要,以及在实际测试过程中的不断探索,设计一系列新型的红外光催化反应器,这些反应器都可以利用近红外光完成光催化的测试。最初我们设计了可以利用980nm二极管激光、太阳光中的近红外光作为激发光源的反应器,并使用它完成了红外光催化反应的测试。随着我们研究工作的深入,上述两种反应器已经不能满足测试的需要,因此我们又设计了以980nm光纤激光以及用小功率氙灯为激发光源的反应器。本文中还对这些光催化反应器的优点和缺点做了详细的分析和讨论。(2)通过微乳液法合成了YF3：Yb3+,Tm3+微米晶,然后以聚乙烯吡咯烷酮(PVP)为表面活性剂和空间稳定剂,通过钛酸丁酯(TBOT)水解,首次合成了TiO2包覆稀土离子掺杂氟化物上转换材料的核/壳结构粒子。通过实验发现,TBOT的浓度以及使用适合的表面活性剂,对于最终得到的样品的形貌有很大影响。TEM测试结果表明,当使用PVP为表面活性剂、钛酸丁酯的浓度为0.003mmol/mL时,得到的TiO2包覆层最为均匀,厚度约为10-20nm。测试了980nm红外光激发下YF3：Yb3+,Tm3+微米晶的上转换发光光谱,观察到了290nm、346nm和362nm的紫外发光和451nm、477nm和643nm的可见光发射峰,并对上转换发光机理进行了分析。通过对TiO2包覆YF3：Yb3+,Tm3+微米晶上转换光谱的测量,表明均匀的包覆层对上转换发光强度影响较小。通过对这一结果的分析,我们认为形貌均匀的包覆层可以减少对激发光的散射,从而使激发光的强度降低的较少,因此更有利于上转换发光。(3)我们提供了一种将红外—紫外上转换材料与TiO2相结合用于光催化的新方法。首先用水热法合成YF3：Yb3+,Tm3+纳米晶,再通过上面所述的钛酸丁酯水解制备了核/壳结构的YF3：Yb3+，Tm3+/TiO2纳米粒子。透射电镜照片表明在YF3：Yb3+,Tm3+上转换纳米晶表面形成了均匀的Ti02包覆层。通过YF3：Yb3+,Tm3+与核/壳结构纳米粒子在980nm二极管激光激发下的上转换发光光谱的比较和分析,证明TiO2包覆层可以有效的吸收上转换材料发射的紫外光。核/壳结构纳米粒子吸收光谱的测试结果说明这中光催化材料可以吸收980nm左右的近红外光。使用这种核/壳结构的纳米粒子作为催化剂,在980nm激光和太阳光中近红外光(λ>700 nm)的激发下,分别进行了分解亚甲基蓝的实验。在980nm激光照射30 h后,亚甲基蓝的分解量为61%,在太阳光中的近红外光照射9h后,亚甲基蓝的分解量达到了51%。这表明YF3：Yb3+,Tm3+/TiO2纳米粒子在近红外光的激发下具有光催化活性。以紫外光为激发光源分解甲基橙,核/壳结构纳米粒子和TiO2的催化活性基本相同,说明由于上转换发光材料包覆在TiO2内部,因此对TiO2在紫外光下的催化活性基本没有不利影响。此外,这种核/壳结构的组合方式更便于进一步改进TiO2,以获得一种对于紫外光、可见光和近红外光辐照均有响应的新型光催化材料。本章的研究首次报道了一种将近红外光能量用于光催化的新方法,相同的方法在提高基于TiO2的光化学和光电材料对太阳能的利用效率方面有广阔的应用前景。(4)使用PVP为螯合剂和表面活性剂,通过溶剂热法制备了形貌均匀的NaYF4:Yb,Tm纳米晶,纳米晶的尺寸为50nm左右。利用保留在NaYF4:Yb,Tm纳米晶表面的PVP作为表面活性剂,通过钛酸乙酯(TEOT)水解法可以直接在纳米晶表面均匀包覆一层TiO2,这种方法使TiO2包覆上转换材料的制备过程过程大大简化,因此对合成TiO2包覆上转换材料的红外光催化剂具有重要的意义。当TEOT的水解反应时间为15min、45min和180min时,TiO2包覆层的厚度分别为10nm、14nm和16.5nm。这一结果表明这种方法可以通过控制钛酸乙酯的水解反应时间,可以很容易的调节TiO2包覆层的厚度,从而可以调节TiO2在核/壳结构红外光催化材料中的质量分数。这就为进一步探讨TiO2层厚对这种红外光催化材料的催化性能的影响提供了一种简单有效的途径。更多还原

【Abstract】 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.更多还原