现代化工

现代化工 ›› 2025, Vol. 45 ›› Issue (7) : 218-228. DOI: 10.16606/j.cnki.issn0253-4320.2025.07.035

科研与开发

S型异质结光催化剂MIL-88B(Fe)/CdS可见光高效降解2-氯苯酚

王智娟 ¹^,^* ,
缪应纯 ¹ ,
王旭东 ¹ ,
胡柠檬 ²

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Enhanced photocatalytic degradation of 2-chlorophenol over S-type heterojunction photocatalyst MIL-88B(Fe)/CdS under visible light

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Received		Published
2025-02-21		2025-07-20
Issue Date	Revised Date
2025-07-15	2025-02-21

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摘要

光催化处理含酚废水是消除其对水体环境污染的有效手段。研究制备了一种新型的S型异质结光催化剂MIL-88B(Fe)/CdS,并将其用于2-氯苯酚的光催化降解。在优化条件下,可见光辐照2 h后,2-氯苯酚的脱除效率达到88.4%。2-氯苯酚较高的去除效率一方面源于多孔MIL-88B(Fe)材料对2-氯苯酚的吸附作用,另一方面是CdS对MIL-88B(Fe)的表面修饰提升了材料的光催化活性。这是因为二者紧密接触且具有相匹配的费米能级,从而使CdS和MIL-88B(Fe)产生能带弯曲且界面间形成了内建电场,这使得CdS和MIL-88B(Fe)界面间形成光生电子S型转移路径,从而加快了光生载流子的迁移,并促进了光生电子-空穴对的分离。MIL-88B(Fe)/CdS在较宽的pH(3~9)值范围内,均可对2-氯苯酚起到光催化降解作用。通过一系列光学和光电化学表征对MIL-88B(Fe)/CdS的光催化机制进行了分析,h⁺和·OH对2-氯苯酚的降解起主要作用。

Abstract

Photocatalytic treatment of phenol-containing wastewater is an effective means to eliminate its environmental impact on water bodies.In this study,a novel iron-based S-type heterojunction photocatalyst,MIL-88B(Fe)/CdS,is prepared and applied to photocatalytic degradation of 2-chlorophenol.Under the optimized conditions,the removal efficiency of 2-chlorophenol reaches 88.4% over MIL-88B(Fe)/CdS after 2 h of visible light irradiation.Such a satisfactory removal efficiency of 2-chlorophenol originates from the adsorption of porous MIL-88B(Fe) for 2-chlorophenol,and also results from the improved photocatalysis activity caused by the surface decoration of MIL-88B(Fe) with CdS.Thanks to the matched Fermi energy level and intimate contact between MIL-88B(Fe) and CdS,their energy band bends and an internal-built electric field is formed between the interfaces,thus leading to the formation of an S-type transfer route for photogenerated electrons between MIL-88B(Fe) and CdS interfaces.Thus,the migration of photogenerated carriers is accelerated,which facilitates the separation of photogenerated electron-hole pairs.Besides,MIL-88B(Fe)/CdS is able to play a photocatalytic role in degradation of 2-chlorophenol over a wide pH range (3-9).The photocatalytic mechanism of MIL-88B(Fe)/CdS is analyzed via a series of optical and photoelectrochemical characterization,verifying that h⁺ and OH play a major role in the degradation of 2-chlorophenol.

Graphical abstract

关键词

光催化 / MIL-88B(Fe) / S-型异质结 / 2-氯苯酚降解

Key words

photocatalysis / MIL-88B(Fe) / S-type heterojunction / 2-chlorophenol degradation

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College of Resources, Environment and Chemistry, Chuxiong Normal University, Chuxiong 675000, China), AuthorCompanyExt(id=1241416189227791202, tenantId=1226480274814496768, journalId=1226484258170171394, articleId=1240720080524243766, companyId=1241416189215208288, language=CN, country=null, province=null, city=null, postcode=null, companyName=null, departmentName=null, remark=2.楚雄师范学院资源环境与化学学院,云南楚雄 675000)])])] 王智娟,缪应纯,王旭东,胡柠檬. S型异质结光催化剂MIL-88B(Fe)/CdS可见光高效降解2-氯苯酚[J]. 现代化工, 2025, 45(7): 218-228 DOI:10.16606/j.cnki.issn0253-4320.2025.07.035

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氯酚(CP),作为一种有机污染物,具有生物降解率低、自然环境中可持续时间长、致畸、致癌、致突变风险高的特点,1976年被美国环境保护局(USEPA)列为优先控制的危险物质^[1-2],在造纸、合成染料、塑料、制药和纺织等行业的排放废水中经常被检测到^[2-4]。近半个世纪以来,含氯酚废水的处理一直受到科研人员的不断关注,吸附^[5]、生物处理^[6]、高级氧化^[7]、电化学方法^[8]等已经被广泛用于含氯酚废水的处理,但吸附法仅发生污染物相转移,会造成二次污染,生物方法处理周期长,高级氧化和电化学方法需持续输入氧化剂或电能,因此进一步探索高效经济的氯酚去除方法迫在眉睫。

光催化因绿色环保、太阳能取之不竭、易于操作等优点已成为降解甚至矿化有机污染物的常用方法^[4,9]。目前,开发可见光响应性强、光生电子-空穴对复合率低、氧化电位高的光催化剂仍然是高效清除氯酚的关键,另外光催化反应有典型的表面反应特性,故良好的光催化剂还需对污染物具有适当的吸附能力。金属有机框架材料(MOFs)自诞生以来,因其有序、可调的孔结构和比表面积大的特点,在吸附中得到了广泛应用^[10-11]。自2007年Alvaro Mercedes验证了它们的半导体特性以来^[12],MOFs在光催化领域的研究也在持续深入。因此,MOFs已成为构建高效光催化剂的优秀候选者。其中,常见的Fe-MOFs之一MIL-88B(Fe)带隙小(2.39~2.87 eV)^[13-14]、化学稳定性高^[15]、具有典型的呼吸及溶胀效应^[16]、地壳含量丰富、制备过程简单,从而可用于光催化领域。但MIL-88B(Fe)的光生电子-空穴对极易复合,抑制了其光催化活性^[17]。目前,已有通过形貌调控^[17-18]、缺陷设计^[19]、MOF结合MOF^[20]、稀土元素掺杂^[15]、与其他半导体构建异质结^[14]等措施来提升MIL-88B(Fe)光催化效率的相关报道。其中,基于氧化型(OP)和还原型(RP)光催化剂间匹配的费米能级、内建电场(IEF)和能带弯曲原理构建的S型异质结不仅可加速e^-和h⁺的分离,还能保留复合材料的高氧化还原性,从而获得令人满意的光催化效果^[21]。CdS可见光响应性好,还原能力高,因此常被选为RP与其他半导体构建异质结。据报道,含CdS的S型异质结复合材料,如间苯二酚-甲醛球/CdS^[22]、CdS/BiOCl^[23]、BiOIO₃/CdS^[24]、WOx/CdS^[25]已成功应用于光催化H₂O₂生成、制备H₂、污染物降解、苯甲醇氧化、CO₂还原等。

受上述内容启发,本文通过在MIL-88B(Fe)颗粒表面原位生长CdS,制备了一种CdS修饰的MIL-88B(Fe)S型异质结复合材料,并将其用于降解氯酚代表污染物之一—2-氯苯酚。MIL-88B(Fe)/CdS对2-氯苯酚表现出较高的去除率一方面源于其在MIL-88B(Fe)/CdS上的吸附效果,另一方面源自其在光催化剂表面被活性氧化物种(ROS)氧化降解。此外,论文还对MIL-88B(Fe)/CdS进行了XPS、EPR等表征和能带结构测试,深入分析了 MIL-88B(Fe)/CdS电荷转移机制和2-氯苯酚的降解机理。

1 实验

1.1 化学试剂

所用试剂有六水氯化铁(FeCl₃·6H₂O,≥99.0%,Greagent)、对苯二甲酸(H₂BDC,≥99.0%,Adamas)、N,N-二甲基甲酰胺(DMF,≥99.8%,Adamas)、二水乙酸镉(≥99.0%,Greagent)、硫脲(≥99.0%,Greagent)、甲醇(≥99.9%,Greagent)、乙醇(99.9%,Adamas)、2-氯苯酚(2-CP,99.9%,Adamas)、乙二胺四乙酸二钠(EDTA-2Na,≥99.0%,天津众联)、对苯醌(p-BQ,99.9%,Adamas)和异丙醇(IPA,≥99.7%,Greagent)。

1.2 MIL-88B(Fe)/CdS的制备

首先,将4.6 mmol (1.244 g)FeCl₃·6H₂O和4.6 mmol (0.764 g)对苯二甲酸溶解在50 mLDMF溶剂中,充分搅拌混合后将其转移至100 mL水热反应釜中,于150℃下反应12 h。反应结束后,离心分离出固体产物MIL-88B(Fe)并分别用DMF和无水乙醇各洗涤两次,最后置于60℃烘箱进行干燥。准确称取100 mg MIL-88B(Fe)置于50 mL DMF中超声30 min,然后分别加入0.5 mL(或1.0 mL,1.5 mL)乙酸镉DMF溶液(15.9 mg/mL)和硫脲DMF溶液(5.25 mg/mL)后持续搅拌1 h。之后,将悬浊液转移至100 mL水热反应釜并于200℃下反应2 h。反应结束后,离心分离出固体产物MIL-88B(Fe)/CdS-x并分别用DMF和甲醇各洗涤两次,最后置于60℃烘箱干燥。所得复合材料中CdS的质量比分别为5%,10%和15%,分别标记为MIL-88B(Fe)/CdS-5,MIL-88B(Fe)/CdS-10和MIL-88B(Fe)/CdS-15。CdS的制备过程与MIL-88B(Fe)/CdS-x类似,但不添加MIL-88B(Fe)。

1.3 光催化剂表征

通过X射线衍射仪(XRD,D8 ADVANCE,Bruker,德国)对光催化剂的结晶性和物相进行分析(使用Cu Kα辐照,λ=1.541 8 Å,扫描速率是5°/min)。利用X射线光电子能谱(XPS,ESCALab 250Xi,Thermo Fisher,美国)对材料的表面元素含量及价态进行分析。通过配有能量色散X射线光谱仪(EDS)的扫描电镜SEM(S4800,Hitachi,日本)对MIL-88B(Fe)/CdS-x形貌、表面元素组成及映射图进行分析。使用表面积孔分析仪(tristar Ⅱ3020,美国)测试材料的比表面积和孔径分布。在紫外/可见/近红外分光光度计(Hitachi U4100,日本)上测试材料的紫外-可见漫反射光谱(DRS)。利用荧光分光光度计(日立F-7000,日本)在325 nm的激发波长下测试材料的稳态光致发光(PL)光谱。利用电子顺磁共振波谱仪(EPR,Bruker EMXplus,德国)对5,5-二甲基-1-吡咯啉-N-氧化物(DMPO)试剂自旋俘获信号进行测试。通过傅里叶变换红外光谱仪(FT-IR,Thermo Scientific,美国)对MIL-88B(Fe)/CdS-x的官能团进行测定。利用电化学工作站(上海辰华,CH1760D,中国)对材料的光电化学性能进行测试。

1.4 活性测试及分析

在100 mL的石英杯中对2-氯苯酚的降解活性进行测试,光源采用配有420 nm滤波片的500 W氙灯。每次降解实验中,将一定量MIL-88B(Fe)/CdS-x分散在50 mL浓度为15 mg/L的2-CP溶液中,然后在可见光照射下将混合物磁力搅拌120 min。每隔一定时间,取出1 mL悬浮液并用0.22 μm聚四氟乙烯膜进行过滤除去固相以便滤液进行液相色谱分析。将2-CP溶液用0.1 mol/L的NaOH或0.1 mol/L的HCl调节pH至3,5,7,9来研究液相pH对2-CP降解效果的影响。实验中分别加入IPA、p-BQ和EDTA-2Na作为·OH,·

O 2 -

和h⁺的淬灭剂进行淬灭实验。2-CP溶液的浓度由高效液相色谱(HPLC,Thermo Fisher,USA)进行分析。

2 结果与讨论

2.1 光催化剂表征

利用XRD对MIL-88B(Fe)及其含CdS复合物的物相进行分析。如图1(a)所示,原始MIL-88B(Fe)在2θ为9.3°(002),10.6°(101),12.5°(102),21.9°(202)和26.7°(211)出现了一系列X射线衍射峰,这与文献[26-27]的结果一致,证实了MIL-88B(Fe)被成功合成。对于MIL-88B(Fe)/CdS-x样品,随着CdS含量比增加,MIL-88B(Fe)的典型衍射峰逐渐变弱,而CdS在24.9°(100),26.6°(002),28.3°(101),43.9°(110),48.1°(103)和52.1°(112)处的典型衍射峰(JCPDS No.80-0006)逐渐增强^[28],这说明不同比例含量的CdS成功原位生长在MIL-88B(Fe)表面。

随后利用傅里叶变换红外光谱(FT-IR)对样品的官能团进行了分析,结果如图1(b)所示。位于552 cm^-1的吸收峰可归因于H₂BDC的—COOH和Fe³⁺之间形成的铁氧簇中Fe—O键的伸缩振动^[18]。1 658 cm^-1和1 562 cm^-1处的吸收峰为连接到芳香环的C=O键的不对称伸缩振动,C=O键的对称伸缩振动出现在1 388 cm^-1^[14,29]。在749 cm^-1处观察到对苯二甲酸连接体苯环中C—H键的弯曲振动^[14,18]。上述结果与文献[29-30]结果高度一致,再次证实了MIL-88B(Fe)的成功制备。然而,MIL-88B(Fe)/CdS-x的FT-IR光谱中并未观察到CdS在618 cm^-1处的典型吸收峰^[28,31],这可能是其含量较低的缘故。

图1(c)、(d)和表1显示了MIL-88B(Fe)和MIL-88B(Fe)/CdS-x的氮气吸附-脱附等温线、孔径分布曲线和孔结构参数。原始MIL-88B(Fe)为典型的Ⅳ型吸附等温线且在P/P⁰为0.8时呈现出H4型介孔回滞环^[18,29],这说明MIL-88B(Fe)表现出介孔结构特征。如图1(c)所示,MIL-88B(Fe)的孔径宽度主要分布在4.2 nm和23.5 nm处。当引入CdS后,MIL-88B(Fe)/CdS-x呈现出典型的Ⅲ型吸附等温线且其H3型回滞环呈明显缩小状,这说明MIL-88B(Fe)的部分介孔被CdS占据,介孔体积下降。如图1(d)所示,MIL-88B(Fe)/CdS-x的孔径宽度主要分布在约4.1 nm和7.9 nm处,这说明原始MIL-88B(Fe) 23.5 nm左右大小的介孔被CdS逐渐占据,且其比表面积和孔体积分别从原始MIL-88B(Fe)的568.88 m²/g和0.17 cm³/g降低至46.62 m²/g和0.14 cm³/g(MIL-88B(Fe)/CdS-5),38.36 m²/g和0.083 cm³/g(MIL-88B(Fe)/CdS-10),38.04 m²/g和0.086 cm³/g(MIL-88B(Fe)/CdS-15)。

所制备光催化剂的形貌如图2(a)~(d)所示。原始MIL-88B(Fe)显示出典型的纺锤状形貌[图2(a)]并暴露出{101}和{002}晶面。在图2(b)~(d)圆圈标记的位置可以明显观察到CdS纳米粒子随机黏附在MIL-88B(Fe)表面,且随着乙酸镉和硫脲添加量的增加,CdS纳米粒子的数量变得更多,颗粒直径逐渐变大。如图2(e)所示,从MIL-88B(Fe)/CdS-5到MIL-88B(Fe)/CdS-10和MIL-88B(Fe)/CdS-15,Cd和S元素峰强度的逐渐增加也再次证实了复合样品MIL-88B(Fe)/CdS-x中CdS含量的增大。用CdS对MIL-88B(Fe)进行表面修饰且二者紧密地界面接触可以促进光生载流子的转移和分离,从而提高2-CP的光催化降解效率。

利用XPS对所制备光催化剂的元素组成和化学价态进行了进一步分析。如图3(a)所示,MIL-88B(Fe)/CdS-10的XPS全谱中Fe、O、C、S和Cd元素的共存再次说明成功制备了MIL-88B(Fe)和CdS的二元复合物。图3(b)中,对于纯CdS,位于161.6 eV和162.9 eV的峰可归属于S^2-的S 2p_3/2和S 2p_1/2轨道。而对于二元复合物MIL-88B(Fe)/CdS-10,S^2-的S 2p_3/2和S 2p_1/2反卷积峰移动到更高结合能位置,即161.9 eV和163.2 eV处,此外在168.8 eV处观察到一个可归属于S—O键的峰^[32],这说明MIL-88B(Fe)和CdS间存在紧密的原子间键合。如图3(c)所示,纯CdS样品,结合能位于405.3 eV和410.2 eV处的峰分别为Cd 3d_5/2和Cd 3d_3/2,而在MIL-88B(Fe)/CdS-10二元复合物中,Cd 3d_5/2和Cd 3d_3/2峰明显地移向了高结合能方向(405.6 eV和412.3 eV)。如图3(d)所示,纯MIL-88B(Fe)样品,位于530.2、531.9 eV和533.6 eV处的3个O 1s反卷积峰可分别归属于铁氧簇的Fe—O键(O1)^[29]、有机配体的晶格氧(O2)^[27]和催化剂表面吸附水的H—O键(O3)^[31,33]。值得注意的是,引入CdS后,MIL-88B(Fe)/CdS-10的O1反卷积峰结合能不变,而其O2和O3反卷积峰的结合能均下降了0.2 eV。图3(e)显示了样品的C 1s谱图,MIL-88B(Fe)在284.8、286.5 eV和288.8 eV处的C 1s拟合峰可分别归属于C—C^[34]、C—O^[35]和O=C—O^[27]。和初始MIL-88B(Fe)相比,MIL-88B(Fe)/CdS-10的C—O和O=C—O键的结合能分别降低了0.1 eV和0.2 eV。Fe 2p的XPS谱图见图3(f),MIL-88B(Fe)表面的Fe 2p3/2和Fe 2p1/2峰的结合能分别位于711.6 eV和724.6 eV,同时伴有718.7 eV和731.2 eV处的卫星峰。另外,MIL-88B(Fe)的Fe 2p3/2峰可拟合为结合能是711.4 eV和714.9 eV处的两个峰,分别归属于八面体 Fe(Ⅲ)和四面体Fe(Ⅲ)物种。和初始MIL-88B(Fe)相比,MIL-88B(Fe)/CdS-10复合物的Fe 2p_3/2峰的结合能从711.6 eV负移至711.4 eV。根据以上分析,不难发现与纯MIL-88B(Fe)和CdS相比,MIL-88B(Fe)/CdS-10异质结光催化剂的Fe、O、C和Cd、S元素的结合能均发生了一定程度移动。S 2p和Cd 3d轨道的结合能呈现出明显的正移,说明CdS的电子密度降低;反之,O 1s、C 1s和Fe 2p峰略微向结合能降低的方向移动,这说明MIL-88B(Fe)的电子密度增加。元素结合能的变化证实由于MIL-88B(Fe)和CdS间费米能级的差异^[36],当二者发生紧密的界面接触时,电子会从CdS转移到MIL-88B(Fe),在MIL-88B(Fe)和CdS的界面处产生内建电场(IEF),从而促使二者间形成S-型异质结,大大促进了光生载流子的分离。

2.2 光催化性能

为了具体评估MIL-88B(Fe)/CdS-x的光催化性能,在可见光照射下进行了一系列2-CP的光催化降解实验,结果如图4(a)所示。含2-CP模拟废水在可见光辐照下,若不添加任何光催化剂,经120 min后,2-CP的C/C₀值降低至0.73,表明2-CP具有一定程度的光自降解性,但不显著。当体系加入12 mg MIL-88B(Fe)时,2-CP的C/C₀值急剧降至0.25,这证实了MIL-88B(Fe)有较好的光催化能力。当MIL-88B(Fe)表面被不同量CdS修饰后,MIL-88B(Fe)/CdS-5、MIL-88B(Fe)/CdS-10和MIL-88B(Fe)/CdS-15对2-CP的移除率分别达到72.1%、88.4%和81.3%,且其相应的降解速率常数(k_obs)从初始MIL-88B(Fe)的0.010 min^-1分别提高至0.011、0.018 min^-1和0.014 min^-1[图4(b)]。考虑到原始MIL-88B(Fe)和MIL-88B(Fe)/CdS-x对2-CP潜在的吸附作用,还进行了2-CP的暗吸附实验作为对照。如图4(c)所示,经暗吸附120 min后,42.3%的2-CP可被MIL-88B(Fe)吸附,当CdS的负载量为5%、10%和15%时,2-CP的吸附率略有下降,分别降至21.2%、36.7%和32.9%。显然,多孔MIL-88B(Fe)对2-CP的吸附作用对2-CP的去除有一定贡献,但更重要的是表面吸附的2-CP能被催化剂表面的活性氧物种(ROS)进行原位攻击和氧化,从而加速整个光催化氧化过程,因为ROS在液体中扩散和迁移时易被淬灭。此外,MIL-88B(Fe)/CdS-x对2-CP暗吸附效果下降可能是因为原位生长的CdS颗粒遮蔽或阻塞了部分MIL-88B(Fe)孔道,使得MIL-88B(Fe)/CdS-x比表面积和孔体积下降所致。

从图4(c)可以看出,CdS表面修饰的MIL-88B(Fe)对2-CP的光催化降解贡献由MIL-88B(Fe)的32.7%(75.0%-42.3%=32.7%)分别扩大至50.9%(MIL-88B(Fe)/CdS-5,72.1%-21.2%=50.9%)、51.7%(MIL-88B(Fe)/CdS-10,88.4%-36.7%=51.7%)和48.4%(MIL-88B(Fe)/CdS-15,81.3%-32.9%=48.4%),这证实MIL-88B(Fe)/CdS-x对2-CP降解的光催化性能优于初始MIL-88B(Fe),但CdS量对2-CP降解效果的影响并不明显。此外,当同时添加11 mg MIL-88B(Fe)和1 mg CdS时(与复合物MIL-88B(Fe)/CdS-10中MIL-88B(Fe)和CdS的量相当),2-CP的脱除率仅为51.0%,远低于MIL-88B(Fe)/CdS-10的效果,这证实MIL-88B(Fe)/CdS-10光催化性能的提升源于复合材料异质结的构建,而非额外添加了CdS。

此外,对影响2-CP光催化降解过程的关键过程参数(初始溶液pH和催化剂用量)也进行了分析。因MIL-88B(Fe)/CdS-10显示出最高的光催化活性,故选MIL-88B(Fe)/CdS-10进行后续实验。如图5(a)、(b)所示,当MIL-88B(Fe)/CdS-10用量从 4 mg增加到16 mg时,2-CP的降解效率呈火山状趋势,即先增加后减少。MIL-88B(Fe)/CdS-10的最佳用量是12 mg,其k_obs达到0.018 min^-1。这是因为较大的催化剂用量能提高光生载流子浓度,从而使催化剂表面生成更多的ROS来参与2-CP的氧化降解过程,但若MIL-88B(Fe)/CdS-10过量,由于光的屏蔽和散射效应^[37],MIL-88B(Fe)/CdS-10的可见光吸收性会大大降低,从而降低了其光催化活性。如图5(c)、(d)所示,随着液体初始pH从3增加到9,2-CP的光降解效率变化不大,这意味着MIL-88B(Fe)/CdS-10对2-CP的光催化降解能在较宽的pH范围内进行。

2.3 光催化活性的增强机制

如前文所述,初始MIL-88B(Fe)上复合CdS可大大提高材料对2-CP的光催化降解性能,测试了初始MIL-88B(Fe)和典型MIL-88B(Fe)/CdS-10的一系列光学和光电化学性质,结果如图6所示。图6(a)显示MIL-88B(Fe)对250~380 nm范围的紫外光有强吸收,对可见光的吸收稍弱,其光吸收边缘为515 nm,这是MIL-88B(Fe)在光照下存在O^Ⅱ→Fe^Ⅲ的配体至金属电荷转移(LMCT)现象所致^[14]。而CdS对400~490 nm范围的可见光有强吸收作用,其光吸收边缘为556 nm。复合CdS后,MIL-88B(Fe)/CdS-10在可见光区域的吸收性能有明显上升,且其光吸收边缘得到极大扩展,扩展至616 nm,这说明MIL-88B(Fe)/CdS-10能更好地吸收可见光,从而提升催化剂的光生载流子浓度。图6(b)为MIL-88B(Fe)和MIL-88B(Fe)/CdS-10的稳态荧光光谱,结果显示MIL-88B(Fe)/CdS-10比MIL-88B(Fe)表现出更低的荧光发射强度,这说明MIL-88B(Fe)/CdS-10的光生电子和空穴对的复合受到明显抑制,从而促进更多的光生载流子能参与到表面反应中。如图6(c)所示,MIL-88B(Fe)/CdS-10的瞬时光电流密度明显高于初始MIL-88B(Fe)的瞬时光电流密度,这也再次印证了MIL-88B(Fe)表面原位生长CdS后,材料的光生载流子得到有效分离,光生载流子的复合受到抑制。如图6(d)所示,与MIL-88B(Fe)相比,MIL-88B(Fe)/CdS-10的阻抗弧半径更小,这说明MIL-88B(Fe)/CdS-10具有更低的电荷转移电阻和更好的电荷转移能力。通常Tafel斜率越小,电流密度随过电位增加而提升得越快,反应动力学越快,催化活性越高。图6(e)显示MIL-88B(Fe)/CdS-10的Tafel斜率为100.6 mV/dec,远低于初始MIL-88B(Fe)的Tafel斜率(376.4 mV/dec),这再次证明MIL-88B(Fe)/CdS-10有更快的电子转移能力,可获得更高的反应速率。相比于初始MIL-88B(Fe),MIL-88B(Fe)/CdS-10的可见光吸收能力明显提高,且有更快的载流子迁移能力和低的电荷转移阻力,从而使得MIL-88B(Fe)/CdS-10的光生电子和空穴的分离性能大大提升,两方面共同作用使得更多的光生载流子参与到2-CP的光降解中,从而提高了复合材料的光催化性能。

2.4 光生载流子转移机制

为揭示MIL-88B(Fe)/CdS-10光生载流子的转移机制,首先对MIL-88B(Fe)/CdS-10的能带结构进行了分析,如图6(f)~(h)所示。根据公式分别计算光催化材料的带隙。式中,α,h和ν分别为吸光系数、普朗克常数和光的频率;n与半导体类型有关,直接带隙半导体n值取1/2,间接带隙半导体n为2;A为常数^[38]。根据文献,CdS和MIL-88B(Fe)的n值均取均为1/2^[13,38],则(αhν)² vs.hν的Tauc图见图6(f),结果显示初始MIL-88B(Fe)和CdS的带隙分别为2.69 eV和2.34 eV,MIL-88B(Fe)/CdS-10复合物的带隙因窄带隙CdS的引入降低至2.55 eV。

(α h ν) 1 / n = A (h ν - E g)

如图6(g)、(h)所示,MIL-88B(Fe)和CdS的莫特-肖特基图(MS)显示二者均具备n-型半导体属性,因为MS图的斜率为正。从MS图得到的MIL-88B(Fe)和CdS的平带电位(E_fb)分别为 -0.09 V和-1.07 V(vs.SCE)。对于n型半导体,其导带电位(E_CB)一般比其平带电势低约0.10 V^[33,39]。故MIL-88B(Fe)和CdS的导带电位是-0.19 V和 -1.17 V(vs.SCE),即-0.05 V和-0.93 V(vs.NHE)。带隙为价带电位(E_VB)和导带电位之差,则MIL-88B(Fe)和CdS的价带电位分别为2.64 eV和1.41 eV(vs.NHE)。MIL-88B(Fe)和CdS接触前的能带结构见图7(a),两者间匹配的能带边缘为之形成Ⅱ型或S型异质结提供了可能。

根据文献[40-41],MIL-88B(Fe)和CdS的功函数分别为5.71 eV和4.29 eV,意味着和MIL-88B(Fe)相比,CdS具有更高的费米能级。因此,当CdS和MIL-88B(Fe)密切接触时,电子将自发地从CdS转移到MIL-88B(Fe)直到二者的费米能级趋于一致,XPS结果也恰好证实了这一电子转移过程。由于电子转移,在CdS和MIL-88B(Fe)的两侧分别形成了电子耗尽区和电子积累区,从而形成了由CdS指向MIL-88B(Fe)的内建电场(IEF)。此外,由于CdS和MIL-88B(Fe)两侧的电子损失和积累使得二者的能带分别向上和向下弯曲。在可见光照射下,IEF驱使MIL-88B(Fe)导带的光生电子与CdS价带的光生空穴结合,从而构成S-型电子转移过程,这不仅加速了光生载流子的分离还拓宽了光催化剂的氧化还原能力。

为进一步证实MIL-88B(Fe)/CdS-10 S-型异质结的界面电荷转移机制,通过电子自旋共振(ESR)对超氧阴离子(·

O 2 -

)和羟基(·OH)自由基进行了测定。如图7(b)、(c)所示,若二者间形成Ⅱ型异质结,则CdS导带的光生电子将移至MIL-88B(Fe)导带,而MIL-88B(Fe)价带空穴将转移到CdS价带。因为MIL-88B(Fe)的导带电位(-0.05 V vs.NHE)高于O₂/·

O 2 -

(-0.33 V vs.NHE),且CdS价带电位低于·OH/OH^-(2.38 V vs.NHE)^[42],故无法生成·

O 2 -

和·OH。只有MIL-88B(Fe)和CdS间形成S-型异质结才能解释DMPO-·OH和DMPO-·

O 2 -

强信号的出现,即更多的光生电子和空穴分别留在CdS导带和MIL-88B(Fe)价带上,高的CdS导带电位(-0.93 V vs.NHE)和MIL-88B(Fe)价带电位(2.64 V vs.NHE)促使溶解氧或H₂O/OH^-被还原或氧化为 ·

O 2 -

和·OH。

2.5 光催化机理

为进一步明确MIL-88B(Fe)/CdS-10对2-CP的光催化降解机制,在最佳反应条件下进行了淬灭实验。如图8所示,当体系分别加入过量EDTA(h⁺的淬灭剂)和IPA(·OH的淬灭剂)时,2-CP的移除率分别从88.4%降至72.0%和78.8%,说明h⁺和 ·OH均对2-CP的光催化降解有一定贡献。通常,在光催化反应体系中,p-BQ能与·

O 2 -

以(0.9-1.0)×10⁹ M^-1·s^-1^[43]的速率进行反应,从而抑制·

O 2 -

对有机污染物的氧化降解能力。但从图8可见,在MIL-88B(Fe)/CdS-10光催化降解体系中加入p-BQ,2-CP的移除率并未下降,反而大大提高至99.8%。这很可能是因为醌类化合物可以很容易地被光还原为·OH和半醌自由基^[44]。此外,p-BQ对光生电子的捕获阻碍了e^-和h⁺的复合,从而使更多的h⁺可以将H₂O/OH^-氧化为·OH^[44],h⁺和·OH的增加促进了2-CP的降解。

因此,2-CP的光催化降解机制主要是:当MIL-88B(Fe)/CdS-10在可见光照射下,MIL-88B(Fe)和CdS均产生光生空穴和电子,且MIL-88B(Fe)和CdS间形成的内建电场和它们的能带弯曲促使形成S型电子转移过程,从而加速了载流子分离。CdS导带电子与溶解氧反应生成·

O 2 -

,MIL-88B(Fe)价带空穴与H₂O/OH^-反应生成·OH,h⁺和·OH主导了2-CP的降解。

3 结论

本文通过在MIL-88B(Fe)表面原位生长CdS纳米粒子,制备了一种新型铁基S型异质结光催化剂并将其用于可见光下消除典型的氯酚污染物2-CP。在50 mL浓度为15 mg/L的2-CP溶液中,当加入12 mg MIL-88B(Fe)/CdS-10时,光照2 h后 2-CP的去除率达到88.4%。和原始MIL-88B(Fe)相比,MIL-88B(Fe)/CdS-10因其增强的可见光吸收、更快的载流子转移和S型异质结弱化的电子-空穴对复合使其对2-CP的光催化贡献由32.7%提高到51.7%。此外,MIL-88B(Fe)/CdS-10对2-CP的光催化降解可在pH为3~9的较宽范围内操作,光催化活性几乎没有差异。总之,MIL-88B(Fe)/CdS-10新型光催化剂的构造为解决氯酚污染问题提供了一种可行的方法。

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