环境污染是当今社会面临的重大问题之一, 半导体光催化剂作为一种新型的绿色科技物质, 能够降解有机污染物(如甲基蓝、罗丹明B、苯酚等), 一直是人们关注和研究的重要课题之一.以锐钛矿型TiO
MOFs是一种通过多齿状有机基团连接金属离子或基团形成的一维、二维或三维的空间网格结构的金属有机复合物[3-4], 因其具有大的孔隙率、大比表面积、可变孔径和可变的功能基团而在气体存储、催化领域、光电领域等方面受到广泛关注.在MOFs材料中MIL-125(Ti)是一个典型代表, 它是以循环八聚物TiO
因此, 可以将MIL-125(Ti)与BiOI进行复合, 形成一种能带相匹配的具有异质结结构的复合物.这样不仅能保留MIL-125(Ti)比表面积大的优点, 还能够有效吸收可见光, 从而在可见光的照射下有效地降解有机污染物[5-7].本文通过一步共沉淀法制备异质结结构MIL-125(Ti)/BiOI, 探究该催化剂在可见光辐射下对罗丹明B的光催化降解效果, 并结合其表征, 从机理上进行分析.
1 实验部分 1.1 实验试剂与仪器试剂:五水硝酸铋、碘化钾、乙二醇、柠檬酸、对苯二甲酸(H
仪器: BL-GHX-V型光化学反应仪(Photochemical Reactions Instrument); U-3900型紫外可见分光光度计(UV-Vis); D/MAX2500PC型X射线衍射仪(XRD); Hitachi S4480型扫描电子显微镜(SEM); Fluoromax-4型荧光分光光度计(PL); N
量取216mL DMF, 甲醇24mL, 倒入500mL的烧杯中, 磁力搅拌30min; 称取12.0g(0.0722mol)H
称取0.3520g(0.7256mmol)Bi(NO
称取80mg MIL-125(Ti)/BiOI样品于石英管中, 加入转子, 称取80mg/L的RhB(罗丹明B)溶液10g, 加去离子水稀释到10mg/L.暗反应30min, 使其达到吸附-脱附平衡, 再将其超声15min, 防止因颗粒团聚对实验结果产生影响[9-10].在光化学反应仪中, 500W氙灯辐射下, 光催化反应90min(前50min, 每10min取1个样, 后面每20min取1个样, 共8个样).离心分离(8000r/min)5min, 取上清液, 用紫外可见分光光度计测试罗丹明B的吸收强度, 在最强吸收峰处计算罗丹明B的归一化浓度.比较不同掺杂比例(Ti:Bi)的降解效果.归一化浓度的计算公式为
$ \begin{align*} Y=\frac{C}{C_{0}}\times/100\%, \end{align*} $ |
其中,
由图 1可知, M-BiOI-
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图 1 MIL-125(Ti)、BiOI和M-BiOI- |
用260nm紫外光做激发光源, 分别测试DMF(空白), 5mmol/L MIL-125(Ti)的DMF溶液, 5mmol/L MIL-125(Ti)/Bi(NO
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图 2 DMF、MIL-125(Ti)的DMF溶液和MIL-125(Ti)/Bi(NO |
一般而言, 由于半导体材料对能量高于其吸收限的光子有很强的吸收, 能产生额外的光生电子-空穴对, 这些载流子一边向材料表面扩散, 一边通过各种复合机制复合, 而PL的强度与复合的概率成正比, PL强度越低, 光生电子-空穴复合概率越低[13].如图 2所示, 5mmol MIL-125(Ti)/Bi(NO
制备的M-BiOI-
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图 3 M-BiOI-1 (a)、M-BiOI-2 (b)、M-BiOI-3 (c)、M-BiOI-4 (d)、M-BiOI-5 (e)、M-BiOI-6 (f)的SEM照片 Fig.3 SEM images of M-BiOI-1 (a), M-BiOI-2 (b), M-BiOI-3 (c), M-BiOI-4 (d), M-BiOI-5 (e), M-BiOI-6 (f) |
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图 7 MIL-125(Ti)、BiOI和M-BiOI- |
图 4为M-BiOI-5(根据图 3的SEM照片, 可知M-BiOI-5中BiOI在MIL-125(Ti)表面分布最均匀, 所以这里选择M-BiOI-5做EDX图谱分析)的EDX图谱及各元素映射图像.
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图 4 4 M-BiOI-5的EDX(a)以及各元素的元素映射图像(b)-(f) Fig.4 (a) EDX spectrum, (b)-(f) corresponding elemental mapping images of M-BiOI-5 |
由图 4(a)可以很直观地看出M-BiOI-5中C、Ti、Bi、I、O等元素含量的对比情况, 同时可以看出没有其他的元素, 说明制备的样品纯度很高. 图 4(b)至图 4(f)分别为C、Ti、Bi、I、O的元素映射图像, 可以看出在M-BiOI-5中, BiOI均匀地分布在MIL-125(Ti)的表面, 这进一步印证了BiOI和MIL-125(Ti)形成了比较好的微观结构.
2.1.5 BET测试图 5为BiOI、MIL-125(Ti)、M-BiOI-
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图 5 BiOI、MIL-125 (Ti)和M-BiOI- |
表 1所示为BiOI、MIL-125(Ti)、M-BiOI-
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表 1 BiOI、MIL-125 (Ti)和M-BiOI- |
当掺杂BiOI后, M-BiOI-
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图 8 MIL-125(Ti)、BiOI和M-BiOI- |
图 6为MIL-125(Ti)、BiOI和M-BiOI-
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图 6 MIL-125(Ti)、BiOI和M-BiOI- |
对于晶体半导体, 光吸收和能带遵循光电效应, 公式[18]为
$ \begin{align*} \alpha hv =A(hv - E_{\rm g} )^{n/2}, \end{align*} $ |
其中,
由图 7可以看出, BiOI的能带大概为1.8eV, 与文献[21]中1.72eV基本相符; M-BiOI-
图 8为MIL-125(Ti)、BiOI和M-BiOI-
为更好地理解M-BiOI-
$ \begin{align*} {\rm ln}\frac{C_{0}}{C}=k_{\rm app}t, \end{align*} $ |
其中,
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图 9 MIL-125(Ti)、BiOI和M-BiOI- |
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表 2 MIL-125(Ti)、BiOI和M-BiOI- |
由表 2可知, 对应样品的动力学反应速率常数随着BiOI的掺杂比例的增加而增加(除了M-BiOI-6), M-BiOI-5的
光催化剂的重复性测试在工业生产中是非常重要的, 它可以降低操作费用, 提高经济效益[24].结合前面测试结果分析, 本文认为M-BiOI-5具有最优的催化性能, 因此对M-BiOI-5光催化测试前后进行XRD表征, 结果如图 10所示.由图 10可知, M-BiOI-5在降解罗丹明B前后的XRD图并无太大变化, 说明在降解过程中催化剂的结构并没有受到破坏, 反应过程中催化剂并没有参与化学反应. 图 11是M-BiOI-5在可见光条件下3次循环降解罗丹明B的降解曲线图.由图 11可以看出, 在3次循环降解后, 其光催化降解效率并无明显降低, 由此可以认为M-BiOI-5具有很好的循环性, 且有很好的工业应用价值.
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图 10 M-BiOI-5光催化性能测试前和测试后的XRD图 Fig.10 XRD patternsof M-BiOI-5 before and after photocatalytic performance testing |
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图 11 可见光条件下M-BiOI-53次循环降解罗丹明B的降解曲线图比较 Fig.11 Degradation curve for three cycles of degrading RhB for M-BiOI-5 under visible light |
采用Mulliken理论分别对MIL-125(Ti)和BiOI的能带进行计算[25-26], 公式为
$ \begin{align*} &E_{\rm VB}=X - E_{\rm e}+ 0.5 E_{\rm g}, \\ &E_{\rm CB}=E_{\rm VB}- E_{\rm g}, \end{align*} $ |
其中,
图 12所示, BiOI的导带能级的位置(0.58eV)比MIL-125(Ti)的导带能级(
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图 12 MIL-125(Ti)/BiOI异质结结构光催化剂机理图 Fig.12 Mechanism of photocatalyst of heterostructure for MIL-125(Ti)/BiOI |
利用一步共沉淀法合成了不同掺杂比例的MIL-125(Ti)/BiOI异质结光催化剂.根据XRD谱图可知, M-BiOI-
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