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材料研究学报, 2023, 37(11): 871-880 DOI: 10.11901/1005.3093.2022.669

研究论文

Ag3PO4/MIL-125(Ti) Z型异质结的构建及其光催化还原Cr(VI)的性能

孙玉伟1,2, 陈畴1, 祁昕1, 任楚奇1, 汤茜1,2, 滕洪辉,1,2, 任百祥1,2

1.吉林师范大学工程学院 四平 136000

2.吉林省高校环境材料与污染控制重点实验室 四平 136000

Synthesis of Z-scheme Ag3PO4/MIL-125(Ti) Heterojunction and Its Performance in Photocatalytic Reduction of Cr(VI)

SUN Yuwei1,2, CHEN Chou1, QI Xin1, REN Chuqi1, TANG Qian1,2, TENG Honghui,1,2, REN Baixiang1,2

1.College of Engineering, Jilin Normal University, Siping 136000, China

2.Key Laboratory of Environmental Materials and Pollution Control, Education Department of Jilin Province, Siping 136000, China

通讯作者: 滕洪辉，教授，hhteng2022@163.com，研究方向为工业废水处理

责任编辑: 吴岩

收稿日期: 2022-12-19   修回日期: 2023-03-03  

基金资助: 吉林省科技厅项目(YDZJ202201ZYTS368)
吉林省科技厅项目(20210101391JC)
吉林省教育厅项目(JJKH20220451KJ)
吉林省教育厅项目(JJKH20230517KJ)

Corresponding authors: TENG Honghui, Tel: 13944400855, E-mail:hhteng2022@163.com

Received: 2022-12-19   Revised: 2023-03-03  

Fund supported: Department of Science and Technology of Jilin Province(YDZJ202201ZYTS368)
Department of Science and Technology of Jilin Province(20210101391JC)
Department of Education of Jilin Province(JJKH20220451KJ)
Department of Education of Jilin Province(JJKH20230517KJ)

作者简介 About authors

孙玉伟，女，1986年生，副教授，博士

摘要

将Ag3PO4纳米颗粒原位沉积在圆饼状MIL-125(Ti)的表面制备出Ag3PO4/MIL125(Ti) Z型异质结光催化剂，分别用XRD、SEM、EDS、UV-vis、FTIR、EIS和PL等手段表征其晶相结构、形貌特征、光吸收性能、价带结构和电荷分离效率，研究了在模拟太阳光照射下Ag3PO4沉积量不同的Ag3PO4/MIL125(Ti)光催化剂还原Cr(Ⅵ)的性能，以及在光催化过程中初始溶液的pH值和催化剂投加量等的影响。结果表明，Ag3PO4的沉积有效提高了MIL-125(Ti)光催化还原性能。Cr(VI)溶液初始浓度为10 mg/L、pH为2时，Ag3PO4/MIL-125(Ti)-2对Cr(VI)的还原率可以达到96.9%。带隙结构计算和自由基捕获实验的结果表明， Ag3PO4/MIL-125(Ti)中的光生载流子符合Z型转移机制。

关键词： 无机非金属材料; MIL-125(Ti); Ag3PO4; Z型异质结; 光催化; 还原Cr(VI)

Abstract

A composite of Z-scheme Ag3PO4/MIL-125(Ti) heterojunction was synthesized by loading Ag3PO4 nano particles on the surface of round-shaped MIL-125(Ti). The Ag3PO4/MIL-125(Ti) composite can effectively improve the utilization of light and charge separation efficiency. The crystal-structure, morphology, optical absorption, valence band structure and charge separation efficiency of the prepared Ag3PO4/MIL-125(Ti) composite were characterized by XRD、SEM、EDS、UV-vis、FTIR、EIS and PL testing methods. Under a simulated solar irradiation, the performance of Cr(VI) reduction by Ag3PO4/MIL125(Ti) composite with different deposition amounts of Ag3PO4 was studied. Furthermore, the effect of solution pH and catalyst dosage in the photocatalytic reduction process was also discussed. The photocatalytic test results indicated that the deposition of Ag3PO4 effectively improved the photocatalytic reduction performance of MIL-125(Ti). When the concentration of Cr(VI) solution was 10mg/L and pH was 2, the reduction rate of Cr(VI) by Ag3PO4/MIL-125(Ti)-2 could reach to 96.9%. The results of bandgap structure calculation and free radical trapping experiments show that photon-generated carriers in Ag3PO4/MIL-125(Ti) are conformed to Z-scheme mechanism.

Keywords： inorganic non-metallic materials; MIL-125 (Ti); Ag3PO4; Z-scheme heterojunction; photocatalytic; reduction of Cr(VI)

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本文引用格式

孙玉伟, 陈畴, 祁昕, 任楚奇, 汤茜, 滕洪辉, 任百祥. Ag3PO4/MIL-125(Ti) Z型异质结的构建及其光催化还原Cr(VI)的性能[J]. 材料研究学报, 2023, 37(11): 871-880 DOI:10.11901/1005.3093.2022.669

SUN Yuwei, CHEN Chou, QI Xin, REN Chuqi, TANG Qian, TENG Honghui, REN Baixiang. Synthesis of Z-scheme Ag3PO4/MIL-125(Ti) Heterojunction and Its Performance in Photocatalytic Reduction of Cr(VI)[J]. Chinese Journal of Materials Research, 2023, 37(11): 871-880 DOI:10.11901/1005.3093.2022.669

在电镀、纺织、制药和皮革等行业，Cr(VI)浓度过高的废水危害人体健康、动植物生长和污染环境[1,2]。化学沉淀、离子交换和吸附等传统的废水处理方法易产生二次污染、循环性差且能耗过高[3~5]。与传统的方法相比，光催化技术的能耗低、效率高且不易产生二次污染[6]。

金属有机骨架结构(Metal organic frameworks, MOFs)材料，由金属簇或无机金属离子通过配位键连接而成。MOFs材料具有较大的比表面积、多种金属中心和易于调整的形貌[7,8]，可用于光催化领域[9~11]。MIL-125(Ti)是一种制备成本低、易于获取的经典MOFs材料，但是较宽的带隙能限制其在光催化领域的应用[12~14]。制备复合材料和染料敏化等方式，可提高其光能利用率。Li等[15]用溶剂热法制备了一种以TiO2为壳、以MIL-125(Ti)为芯的核壳结构复合材料，具有良好的光催化还原性能。Hu等[16]制备了MIL-125(Ti)/ZnIn2S4复合材料，Ti4+与Ti3+之间的界面电荷转移和协同作用可高效去除RhB和Cr(VI)。Han等[17]通过RhB敏化MIL-125(Ti)，光照60 min对甲基橙的降解效率达到90%。Ag3PO4是一种窄带隙(2.45 eV)半导体材料，在可见光区具有光催化活性。Ag3PO4的价带能级较低和导带中有π*轨道，这种特殊的能带结构使其氧化能力和电子迁移速率提高，具有高效降解水体中污染物的性能[18]。但是，在光催化过程中Ag3PO4易被光腐蚀，使其光催化效率和循环利用率降低[19,20]。将MOFs与Ag3PO4结合制备的复合材料，如Ag/Ag3PO4/HKUST-1[21]、Ag3PO4/MIL-101/NiFe2O4[22]、Ag3PO4/超薄MOF纳米片[23]等，具有优异的光催化性能和稳定性。鉴于此，本文用原位沉积将Ag3PO4纳米颗粒沉积在MIL-125(Ti)上制备复合光催化剂Ag3PO4/MIL-125(Ti)，研究其光催化还原Cr(VI)的性能。

1 实验方法

1.1 样品的制备

实验用材料有：对苯二甲酸(H2BDC)、N-N-二甲基甲酰胺(DMF)、无水甲醇(MeOH)，钛酸四丁酯(TBT)，硝酸银(AgNO3)、十二水合磷酸氢二钠(Na2HPO4·12H2O)、异丙醇(IPA)、草酸铵(AO)、对苯醌(BQ)、过硫酸钾(K2S2O8)、硫酸(H2SO4)、氢氧化钠(NaOH)以及去离子水。

用溶剂热法制备圆饼状MIL-125(Ti)。将1.375 g 的H2BDC溶解在22.5 mL的DMF和2.5 mL的MeOH的混合溶液中，在常温下搅拌10 min后将1.5 mL TBT快速加入到混合溶液中，继续搅拌至溶液澄清。将澄清的溶液转移至容积为50 mL、温度为150℃的聚四氟乙烯反应釜中加热24 h，冷却后去除上清液得到白色沉淀。将白色沉淀分别用DMF和MeOH洗涤三次后转移至温度为60℃的真空干燥箱内烘干12 h，制备出MIL125(Ti)样品。

用原位沉积法制备Ag3PO4/MIL125(Ti)复合光催化剂。在常温避光条件下，将适量的MIL125(Ti)超声分散在50 mL去离子水中，分别加入一定量AgNO3溶液(0.043 mol/L)和Na2HPO4·12H2O溶液(0.014 mol/L)后持续搅拌12 h。离心分离所得产物并用去离子水充分洗涤，然后将其移至温度为60℃的真空干燥箱烘干8 h，制备出Ag3PO4/MIL-125(Ti)样品。改变上述反应过程中MIL125(Ti)、AgNO3和Na2HPO4·12H2O的质量，可制备出Ag3PO4的质量分数分别为10%、20%和30%的Ag3PO4/MIL125(Ti)复合光催化剂，及其依次命名为Ag3PO4/MIL125(Ti)-1、Ag3PO4/MIL125(Ti)-2和Ag3PO4/MIL125(Ti)-3。

1.2 性能表征

用D-max/3C型X射线衍射仪(XRD)测试样品的晶相结构；用S-570型扫描电镜(SEM)和能谱(EDS)测定样品的形貌特征和元素组成；用UV3600型紫外可见漫反射仪(UV-vis)测定样品的光吸收性能；用CARY 630型红外光谱(FTIR)测试样品的价键结构；用F4500型荧光光谱(PL)谱和瑞士万通PGSTAT 302型电化学工作站测定样品的电荷分离效率。

在光催化还原实验中，用北京中教金源CELHXF300型氙灯模拟太阳光，考察不同样品对Cr(VI)的还原性能。先取20 mg光催化剂分散在体积为100 mL浓度为10 mg/L的Cr(VI)溶液中，调节溶液的pH值为3。暗反应30 min至吸附解析平衡后，开光进行90 min的光催化还原实验。每10 min取样测定Cr(VI)浓度，其中用紫外可见分光光度法(GB-T 7467-1987)测定Cr(VI)的浓度，其最大吸收波长为544 nm。

2 实验结果和讨论

2.1 样品的结构和形貌

图1给出了不同样品的XRD谱。可以看出，在MIL125(Ti)的谱中衍射峰分别出现在6.79°、9.83°、11.69°、15.06°、15.45°、16.63°、17.92°、19.07°和19.64°处，与文献中MIL125(Ti)衍射峰的位置一致[24]。在Ag3PO4的谱中位于20.9°、29.7°、33.3°、36.6°、47.8°、52.7°、55.0°和57.3°的特征衍射峰分别对应(110)，(200)，(210)，(211)，(310)，(222)，(320)和(321)晶面[25]。在Ag3PO4/MIL125(Ti)-2的谱中可观察到MIL125(Ti)和Ag3PO4的两组特征衍射峰，表明用原位沉积法可制备Ag3PO4/MIL125(Ti)复合光催化剂。

图1

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图1   不同样品的XRD谱

Fig.1   XRD patterns of different samples


不同样品的形貌如图2a所示。可以看出，MIL125(Ti)呈形状规整的圆饼状，表面光滑，直径约为2.3 μm。图2b给出了Ag3PO4的SEM形貌，可见Ag3PO4呈颗粒状。MIL125(Ti)与Ag3PO4复合后的Ag3PO4/MIL125(Ti)-2仍然呈圆饼状，Ag3PO4颗粒均匀分布在MIL125(Ti)表面，使其表面变得粗糙(图2c)。图2d~g给出了Ag3PO4/MIL125(Ti)-2的EDS能谱，可见其中有元素Ti、Ag、O、P，进一步证明已经将Ag3PO4成功地沉积在MIL125(Ti)的表面。

图2

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图2   不同样品的SEM形貌和Ag3PO4/MIL125(Ti)-2的EDS能谱

Fig.2   SEM of MIL125(Ti) (a), Ag3PO4 (b), Ag3PO4/MIL125(Ti)-2 (c) and EDS spectrum of Ag3PO4/MIL125(Ti)-2 (d~g)


2.2 光吸收性能和价键结构

从图3a可见，MIL125(Ti)的吸收光谱集中在紫外光区，吸收边界为354 nm。Ag3PO4在紫外光谱和可见光区都有较强的吸收能力，其吸收边界为550 nm。与MIL-125(Ti)相比，Ag3PO4/MIL125(Ti)-2的光吸收范围扩展到可见光区，吸收边界为442 nm。这表明，MIL125(Ti)与Ag3PO4复合能有效提高光吸收能力，使Ag3PO4/MIL125(Ti)-2能被可见光激发进行光催化反应[26]。图3b表明，MIL125(Ti)和Ag3PO4带隙能分别为3.44 eV和2.13 eV。图3c表明，由价带谱测得的MIL-125(Ti)的HUMO能级和Ag3PO4的价带(VB)电位分别为3.17 eV和1.77 eV。根据公式[27]

ECB=EVB-Eg

计算出MIL125(Ti)的LUMO电位为-0.27 eV，Ag3PO4的导带(CB)电位为-0.36 eV。其中ECB为导带位置，EVB为价带位置，Eg为带隙能。基于带隙能的测试和计算，得出的MIL125(Ti)和Ag3PO4的能带结构在图3d中给出。

图3

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图3   不同样品的紫外-可见漫反射光谱、(αhv)1/2与hv的曲线、MIL-125(Ti)和Ag3PO4的VB谱以及MIL125(Ti)和Ag3PO4的能带结构

Fig.3   UV-vis spectrum (a), (αhv)1/2 vs. hv curves of different samples (b), VB profiles of MIL-125(Ti) and Ag3PO4 (c) and the band structure diagrams of MIL125(Ti) and Ag3PO4 (d)


图4给出了用FTIR测定的不同样品的价键结构。所有样品在3430 cm-1处的宽吸收峰归因于吸附在样品表面的残余水分子H—O—H的弯曲振动[28]。在MIL125(Ti)的红外光谱中，位于400~800 cm-1范围内的吸收峰是Ti—O—Ti振动引起[29]，位于1410、1597和1700 cm-1处的吸收峰则分别对应O—C—O的对称伸缩振动[29]、苯环的C=C振动[30]和芳香羧酸[31]。在Ag3PO4/MIL125(Ti)-2的红外光谱中，除属于MIL125(Ti)的吸收峰以外，位于537和989 cm-1处的两个吸收峰与磷酸盐中P—O的伸缩振动有关[32]。对红外光谱分析进一步证明，已经成功地制备出Ag3PO4和MIL125(Ti)复合光催化剂。

图4

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图4   不同样品的红外光谱

Fig.4   Infrared spectrum of different samples


2.3 电荷分离效率

图5a给出了样品的EIS图。与MIL-125(Ti)和Ag3PO4相比，Ag3PO4/MIL-125(Ti)-2的EIS半径更小，表明光生载流子迁移时受到的阻力更小，有利于光生载流子的转移和分离。图5b给出了样品的PL光谱，与MIL-125(Ti)相比，Ag3PO4/MIL-125(Ti)复合光催化剂的发射峰强度更弱，表明Ag3PO4的引入阻止了光生载流子的复合。在所有样品中，Ag3PO4/MIL-125(Ti)-2的发射峰最小，表明其光生电子和空穴的分离效率最高，与EIS图相符。

图5

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图5   不同样品的EIS谱和PL光谱

Fig.5   EIS (a) and PL spectrum (b) of different samples


2.4 光催化性能

2.4.1 光催化还原Cr(VI)的性能

以容积为100 mL、浓度为10 mg/L的Cr(VI)溶液作为底物，考察了样品在模拟太阳光下的还原性能。图6a表明，暗反应30 min后光催化体系达到吸附-解吸平衡。不加光催化剂光照90 min后Cr(VI)还原率为17.8%，使用MIL125(Ti)和Ag3PO4光催化剂对Cr(VI)的还原率分别为41.6%和34.2%。这表明，二者光催化还原Cr(VI)的性能有限。MIL125(Ti)和Ag3PO4二者复合对Cr(VI)的光催化还原率明显提高，Ag3PO4/MIL125(Ti)-2的光催化还原性能最佳。光照90 min对Cr(VI)的还原率为77.6%，还原速率k为0.0132 min-1，分别是MIL125(Ti)和Ag3PO4的2.58倍和3.53倍。其原因是，Ag3PO4的引入拓展了复合光催化剂的吸光范围，并促进了光生载流子的分离[33]。值得注意的是，复合光催化剂中Ag3PO4的质量比升至30%后Ag3PO4/MIL125(Ti)-3对Cr(VI)的还原率反而降低至41.6%，可能是过量的Ag3PO4纳米颗粒团聚使光生载流子重新复合所致[34]。

图6

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图6   不同样品光催化还原性能和光催化还原Cr(VI)的一级动力学曲线

Fig.6   Photocatalytic reduction performance of different samples (a) and the first-order kinetic curves (b) of the photoca-talytic reduction of Cr(VI)


2.4.2 光催化还原Cr(VI)的影响因素

溶液的pH值对Cr(VI)的还原效率有较大的影响 [35]，因此以Ag3PO4/MIL125(Ti)-2为光催化剂，用H2SO4和NaOH调节Cr(VI)溶液的初始pH值，考察了Ag3PO4/MIL125(Ti)-2对Cr(VI)的光催化还原性能。从图7a、b可见，在酸性条件下Ag3PO4/MIL-125(Ti)-2对Cr(VI)的光催化还原速率明显高于碱性和中性条件。溶液的初始pH值为2时Ag3PO4/MIL-125(Ti)-2的光催化还原性能最佳。模拟太阳光光照90 min后Cr(VI)的还原率为96.9%，其反应速率常数k为0.0276 min-1。其原因是，酸性条件有助于Cr2O7-和HCrO4-与H+快速反应生成Cr3+(式(2)和(3))[36]。同时，pH值不同产物Cr3+的存在形式也不同(式(2)~(4))。在中性和碱性条件下生成的Cr(OH)3容易沉积在MIL125(Ti)孔径中，使活性位点减少和反应速率降低[37]。

Cr2O72-+14H++6e-→2Cr3++7H2O
HCrO4-+7H++3e-→Cr3++4H2O
CrO72-+4H2O+3e-→CrOH3+5OH-

图7

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图7   不同初始pH值对光催剂还原性能的影响、不同投加量对光催剂还原性能的影响以及不同有机酸对光催化剂还原性能的影响

Fig.7   Effects of different initial pH values on the photocatalytic reduction performance (a, b), the effects of different dosage on the photocatalytic reduction performance (c, d), the effects of different organic acids on the photocatalytic reduction performance (e, f)


图7c、d给出了光催化剂投加量对其还原Cr(VI)性能的影响。可以看出，随着Ag3PO4/MIL-125(Ti)-2投加量的增大Cr(VI)的光催化还原率呈先提高后降低的趋势。Ag3PO4/MIL-125(Ti)-2的投加量为30 mg时Cr(VI)的光催化还原率最高，达到94.9%。其原因是，随着催化剂质量的增加光生e-/h+对的数量随之增加[38]。当Ag3PO4/MIL-125(Ti)-2的投加量升至50 mg时，对Cr(VI)的光催化还原率反而下降至84.7%。其原因是，过量的光催化剂分散在溶液中，颗粒的散射导致对光的利用率降低[34]。

以柠檬树、酒石酸和草酸为小分子有机酸，对Cr(VI)光催化还原效率的影响如图7e、f所示。加入小分子有机酸不同程度地促进了Ag3PO4/MIL-125(Ti)-2光催化还原Cr(VI)的效率。加入酒石酸后光催化还原Cr(VI)反应速率常数k达到0.0696 min-1，是不加入小分子有机酸时(k=0.0135 min-1)的5.16倍。其原因是，小分子有机酸与光生h+发生反应促进了光生e-和h+的分离，使参与还原Cr(VI)的光生e-增多[39]。

本文制备的Ag3PO4/MIL-125(Ti)-2与文献中MOF基光催化材料对Cr(VI)还原性能的比较，列于表1。可以看出，Ag3PO4/MIL-125(Ti)-2 对Cr(VI)的光催化还原性能优于表1中列出的光催化剂，表明Ag3PO4/MIL-125(Ti)-2具有良好的光催化还原Cr(VI)的性能。

表1   不同MOFs基光催化剂去除Cr(VI)活性的比较

Table1  Comparison of photocatalytic Cr(VI) removal activities of various MOFs-based photocatalysts

Catalyst and dosage /mg·L-1

Pollutants

/ mg·L-1

IrradiationpHEfficiency /%Ref.
Pt/MIL-125-NH2/1000Cr(VI)/15300 W Xe lamp675.0% (120 min)[40]
Bi2S3@NH2-MIL-125(Ti)/100Cr(VI)/10300 W Xe lamp777.0% (120 min)[41]
CdS/MIL-125(Ti)/500Cr(VI)/48300 W Xe lamp(cut~420 nm)635.0% (70 min)[42]
Pd@MIL-101/1000Cr(VI)/10125 W light pressure mercury lamp646.4% (240 min)[43]
MIL-125-derived TiO2@C/300Cr(VI)/5–361.8% (90 min)[44]
Ag3PO4/MIL-125(Ti)-2Cr(VI)/10300 W Xe lamp677.6% (90 min)This work
Ag3PO4/MIL-125(Ti)-2Cr(VI)/10300 W Xe lamp296.9% (90 min)This work

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2.4.3 光催化剂的稳定性

进行6次光催化循环实验，考察Ag3PO4/MIL-125(Ti)-2的稳定性。图8a表明，4次光催化循环实验后Ag3PO4/MIL-125(Ti)-2的回收率为90.7%，对Cr(VI)的光催化还原率仍保持较高的水平(69.3%)。循环次数增至6次后Ag3PO4/MIL-125(Ti)-2对Cr(VI)的光催化还原率由77.6%降低至61.7%，其原因可能是在光催化过程中催化剂损耗。图8b给出了Ag3PO4/MIL-125(Ti)-2在6次光催化反应前后XRD谱的对比。可以看出，经过光催化反应后Ag3PO4/MIL-125(Ti)-2中对应的Ag3PO4和MIL-125(Ti)特征峰位置没有变化，说明二者的晶型没有改变，表明催化剂具有良好的稳定性。

图8

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图8   光催化循环性能和光催化反应前后Ag3PO4/MIL-125(Ti)-2的XRD谱

Fig.8   4 cycles of photocatalytic reduction experiment (a) and XRD patterns (b) of Ag3PO4/MIL-125(Ti)-2 before and after 6 cycles


2.4.4 光催化还原机制

分别用AO、BQ、IPA和K2S2O8作为h+、·O2-、·OH和e-的捕获剂，测定了光催化反应过程中的活性物种[45]。从图9可见，K2S2O8的加入大大降低了Ag3PO4/MIL-125(Ti)-2对Cr(VI)的光催化还原率，说明光生e-在Cr(VI)的还原过程中有决定性作用。加入BQ时Cr(VI)的还原率有所下降，其原因可能是O2/·O2-介导的还原反应中电子传递降低[4]。与之相反的是，加入AO和IPA后Cr(VI)的还原率显著提高，因为光生h+被捕获后与光生e-的分离效率提高，参与还原Cr(VI)的光生e-更多，而·OH被捕获后限制了Cr(III)再次被氧化为Cr(VI)。

图9

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图9   活性物种的捕获性能

Fig.9   Experimental results of active species capture


根据上述实验结果和分析并结合MIL125(Ti) 和Ag3PO4的能带结构(图3)，分析Ag3PO4/MIL-125光催化还原Cr(VI)的机制。图10a表明，在模拟太阳光照射下Ag3PO4和MIL-125均被激发，光生e-分别由Ag3PO4的VB跃迁至CB，由MIL-125(Ti)的HUMO能级跃迁至LUMO能级，而后位于Ag3PO4中CB上的e-转移至MIL-125(Ti)的LUMO能级参与Cr(VI)的还原和·O2-的形成。位于MIL-125(Ti) HUMO能级上的h+转移至Ag3PO4的VB，进而与H2O/-OH反应生成·OH。但是，由于MIL-125(Ti)的LUMO电位为-0.27 eV，位于其上的e-不能与O2反应生成·O2- (-0.33 V vs. NHE)[46]，这与活性物种捕获实验结果不符。这表明，Ag3PO4/MIL-125(Ti)中的光生e-和h+更可能遵循Z型转移机制。如图10b所示，在模拟太阳光照射下Ag3PO4和MIL-125被激发，位于MIL-125(Ti)的LUMO能级上的光生e-转移至Ag3PO4的VB与其上的h+复合(如 式(5)和(6))，留下位于Ag3PO4的CB上的e-将Cr(VI)还原为Cr(III)。同时，e-促进O2转化为·O2-，·O2-有助于Cr(VI)的还原(式(7)~(9))[47]。同时，·O2-还可能按照式(10)~(12)的反应参与Cr(III)氧化产生·OH[48,49]。而位于MIL-125(Ti)的HUMO能级上的h+与OH-/H2O反应生成·OH，这些·pOH与h+将低价态Cr重新氧化为Cr(VI) (式(13))[50]。Ag3PO4和MIL-125(Ti)形成的Z型异质结使光生e-和h+的分离效率提高，加快光了催化还原Cr(VI)的进程。

图10

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图10   Ag3PO4/MIL-125(Ti)的光催化还原Cr(VI)机制

Fig.10   Mechanism of photocatalytic reduction of Cr(VI) by Ag3PO4/MIL-125(Ti)


Ag3PO4/MIL-125(Ti)+hv→Ag3PO4(h++e-)
MIL-125(Ti)(h++e-)
Ag3PO4(h+)+MIL-125(Ti)(e-)→Heat or hv
Ag3PO4(e-)+Cr(VI)→Cr(V)→Cr(IV)→Cr(III)
O2+e-→·O2-
·O2-+Cr(VI)→Cr(V)+O2
2·O2-+2H+→H2O2+O2
   H2O2+Cr(III)+H+→Cr(VI)+H2O+·OH
MIL-125(Ti)(h+)+H2O/OH-→·OH
Cr(V)+h+/·OH→Cr(VI)+OH-

3 结论

用原位沉积法制备Ag3PO4/MIL-125(Ti) Z型异质结光催化剂，改变Ag3PO4的质量分数可调控其光催化还原Cr(VI)的性能。Ag3PO4/MIL-125(Ti)具有比MIL-125(Ti)更优异的光催化还原性能。Ag3PO4质量分数为20%的Ag3PO4/MIL-125(Ti)-2其光催化还原性能最佳。Ag3PO4的加入可有效提高复合光催化剂的光吸收性能，光生e-和h+在Ag3PO4和MIL-125(Ti)遵循Z型转移机制，能促进二者的分离，使其光催化还原Cr(VI)的效率提高。Ag3PO4/MIL-125(Ti)光催化剂具有良好的稳定性。

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