DOI: 10.11817/j.ysxb.1004.0609.2021-36436
新型Bi2O3/ZnFe2O4光阳极的制备及其光电催化降解有机物性能
王子衿,刘芳洋,贾 明,蒋良兴,赖延清,李 劼
(中南大学 冶金与环境学院,长沙 410083)
摘 要:采用化学水浴法制备Bi2O3薄膜,并通过电沉积结合后退火工艺在其表面负载ZnFe2O4进行改性,通过XRD、SEM、Raman、XPS、UV-vis等对Bi2O3样品及Bi2O3/ZnFe2O4复合薄膜进行表征。以制备的薄膜作为光电极,研究其光电化学性能,并首次报道其在光电催化降解有机污染物中的应用。结果表明:ZnFe2O4的复合拓宽Bi2O3的吸光范围,提高光吸收系数,抑制光电化学反应过程中光生载流子的复合,从而使Bi2O3/ZnFe2O4复合薄膜在Na2SO4溶液中的光响应电流密度是纯相材料的4倍左右,AM1.5光照下的光电降解亚甲基蓝性能相比于Bi2O3有明显提高。
关键词:异质结;Bi2O3/ZnFe2O4;光电化学;降解
文章编号:1004-0609(2021)-05-1320-10 中图分类号:TF80 文献标志码:A
引文格式:王子衿, 刘芳洋, 贾 明, 等. 新型Bi2O3/ZnFe2O4光阳极的制备及其光电催化降解有机物性能[J]. 中国有色金属学报, 2021, 31(5): 1320-1329. DOI: 10.11817/j.ysxb.1004.0609.2021-36436
WANG Zi-jin, LIU Fang-yang, JIA Ming, et al. Preparation and photoelectrochemical property of novel Bi2O3/ZnFe2O4 photoanode[J]. The Chinese Journal of Nonferrous Metals, 2021, 31(5): 1320-1329. DOI: 10.11817/j.ysxb.1004.0609.2021-36436
随着社会的发展,能源危机和环境污染问题越来越严重,染料、杀菌剂、油污、有机溶剂等工业废水中的有机污染物普遍具有毒性和致癌性,对人类产生极大的危害[1-3]。如何高效、环保、节能地去除这些有机物成为关注的焦点。在各种处理方法中,半导体光电催化技术因具有“环境友好”以及高效温和的特点而被广泛研究,其利用太阳能分解水产生氢气并将有机污染物矿化成对环境温和的二氧化碳和水[4-6]。在已开发的众多半导体材料中,铋系氧化物由于Bi 6s和O 2p轨道的杂化效应使其相比于TiO2,ZnO等传统半导体材料对可见光响应更好[7]。然而由于光生电子空穴对的严重复合,纯相氧化铋的光电催化效率较低。为了解决这一问题,研究者们通过负载稀有金属作为助催化剂的方式提升材料的光电催化性能[8-9]。但是大部分助催化剂(Pt,Au)非常昂贵并且利用率低,因此寻找一种具有高可见光响应、理想能带结构的半导体材料与Bi2O3形成异质结构就变得尤为关键。铁基氧化物半导体材料由于其禁带宽度小,对光的利用范围可拓宽至可见光区域,并且成本低廉,稳定性高,对环境友好,自身的磁性使其便于回收等特点,在光电催化领域展现出了良好的应用前景[10-15]。然而,目前为止有关Bi2O3/ZnFe2O4薄膜光电极的研究还未见报道。
本文通过化学水浴法合成Bi2O3基底,采用电沉积结合后退火工艺制备复合Bi2O3/ZnFe2O4光电极。通过对比Bi2O3样品与Bi2O3/ZnFe2O4复合电极的结构,形态及光电化学性能发现,ZnFe2O4的复合有效提升了薄膜的吸光能力与可见光响应。并作为敏化剂与Bi2O3形成了异质结构,减少了载流子在材料体相和表面的复合,提高了光生电子空穴对的分离效率,实现了材料光电化学性能的提升。
1 实验
1.1 试剂
实验所用试剂包括:Bi(NO3)2·5H2O,浓HNO3(国药集团药业股份有限公司生产),Zn(CH3COO)2·2H2O,FeSO4·7H2O,三乙醇胺(TEOA),NaOH(上海阿拉丁生化科技股份有限公司生产),以上试剂均为分析纯。实验用水为去离子水。
1.1.1 Bi2O3电极的制备
首先对FTO基底进行清洗:丙酮超声清洗15 min,氨水超声清洗15 min,无水乙醇超声清洗15 min,去离子水(18.2 MΩ/cm)超声清洗15 min,最后氮气吹干备用。称取一定量的Bi(NO3)2·5H2O溶解于去离子水中,获得Bi(NO3)2溶液,加入适量的TEOA,逐滴添加NaOH饱和溶液调节反应体系的pH至13左右;将干燥洁净的FTO玻璃放入溶液中,恒温60 ℃下水浴反应120 min。样品用去离子水冲洗干净后在空气中350 ℃下退火1 h,加热速率为5 ℃/min,然后自然冷却至室温。
1.1.2 Bi2O3/ZnFe2O4复合电极的制备
称取一定量的FeSO4溶解于去离子水中,获得FeSO4溶液,以预先制备的Bi2O3为阳极,石墨电极为阴极,饱和甘汞电极为参比电极在FeSO4溶液中持续电沉积1 h。沉积过程在常温无搅拌下进行,沉积电压为1 V。配制20 mL 1 mol/L的乙酸锌溶液,取0.5 mL乙酸锌溶液滴到沉积后的薄膜上,400 ℃下空烧1 h,加热速率为5 ℃/min,自然冷却至室温。薄膜退火后浸入1 mol/L的NaOH溶液中1 h,除去薄膜表面多余的ZnO。最后将薄膜冲洗干净,吹干备用。
1.2 材料的表征及检测技术
通过X射线衍射仪(XRD, Rigaku-TTRIII, Cu Kα (1.54056 ), 250 mA, 40 kV)表征样品的晶体结构。通过场发射扫描电子显微镜(SEM, Zeiss, MERLIN compact, 10 kV)表征样品的形貌和结构,通过紫外-可见分光光度计(UV-vis, Shimadzu)表征样品的吸光性能并通过台阶仪(Veeco, Dektak150 Surface Profiler)测试薄膜的厚度并计算样品的光吸收系数及禁带宽度。通过X射线光电子能谱仪(XPS,Thermo fisher Scientific,K-Alpha+)表征样品的元素组成及结合方式。沉积过程及(光)电化学性能测试均在PARSTAT4000电化学工作站中进行。
1.3 Bi2O3与Bi2O3/ZnFe2O4电极光电催化降解亚甲基蓝
称取一定量的亚甲基蓝溶解于去离子水中,获得亚甲基蓝溶液,称取一定量的Na2SO4溶解于溶液中作为电解质。亚甲基蓝溶液预先在暗态下搅拌30 min达到吸脱附平衡。以Bi2O3电极或Bi2O3/ZnFe2O4电极为阳极,石墨电极为阴极,饱和甘汞电极为参比电极,在AM1.5光照下进行亚甲基蓝的光电降解实验,外加偏压为1 V。反应每1 h取一次样,在紫外可见分光光度计上测定627 nm下的吸光值,降解效果用以下公式表示:
(1)
式中:η为亚甲基蓝去除率;A0为亚甲基蓝溶液的初始吸光度;At为反应时间为t时亚甲基蓝溶液的吸光度。
2 结果与讨论
2.1 样品表征
2.1.1 XRD分析
图1所示为Bi2O3薄膜与Bi2O3/ZnFe2O4复合薄膜的XRD谱。根据XRD谱可知,Bi2O3的主要衍射峰与PDF标准卡片(JCPDS No. 65-1209)一致。其中在2θ为27.945°、31.754°、32.687°、46.215°、46.900°的衍射峰分别对应Bi2O3晶体的(201)、(002)、(220)、(222)、(400)晶面,表明实验成功合成了纯相四方型β-Bi2O3[16-17]。复合改性后材料在2θ为29.919°、35.264°、42.844°、65.339°出现新的衍射峰,和标准卡片(JCPDS No. 22-1012)相匹配,分别对应ZnFe2O4晶体的(220)、(311)、(400)、(531)晶面[18-19]。证明电沉积结合后退火工艺成功合成了Bi2O3/ZnFe2O4复合材料,并且复合后Bi2O3的衍射峰强度增大,峰型更加尖锐,说明后退火工艺使Bi2O3基底的结晶性增加。
图1 Bi2O3薄膜与Bi2O3/ZnFe2O4复合薄膜的XRD谱
Fig. 1 XRD patterns of Bi2O3 film and Bi2O3/ZnFe2O4 composite film
2.1.2 XPS分析
图2所示为Bi2O3/ZnFe2O4复合薄膜的XPS全谱图及Bi、Zn、Fe、O的分峰拟合结果。从XPS全谱(见图2(a))可知样品表面主要由Bi、O、Fe、Zn这四种元素(C为仪器校正元素)组成。图2(b)所示为Bi 4f的高分辨XPS图谱,电子结合能为164.1eV 和159.1eV 的峰对应Bi 4f5/2与Bi 4f7/2的电子,说明Bi主要以+3价的形式存在[8, 16, 20-21]。图2(c)所示为Zn 2p的高分辨XPS图谱,电子结合能为1044.1 eV和1021.5 eV的峰对应Zn 2p1/2和Zn 2p3/2的电子,说明Zn主要以+2价的形式存在[11]。图2(d)所示为Fe 2p 的高分辨XPS图谱,通过拟合可知,电子结合能在724.4 eV和710.6 eV处的峰与文献中ZnFe2O4的Fe 2p1/2和Fe 2p3/2一致,另外,732.5 eV和717.7 eV的卫星峰确认了Fe在复合薄膜中为+3价[22-24]。图2(e)为O 1s的高分辨XPS谱,Bi2O3/ZnFe2O4表面O 1s的峰较复杂,经分峰拟合得最大峰值分别为531.5 eV和529.6 eV。其中531.5 eV的峰与材料表面的缺陷、污染物或其他组分有关,例如氢氧根或吸附氧等[25-26]。529.6 eV的峰和Bi2O3与ZnFe2O4的O2-的峰对应[27-29]。其中,Bi 4f5/2与Bi 4f7/2的电子结合能与NIST数据库中Bi2O3 (CAS Registry No: 1304-76-3)的Bi峰相比均出现负偏移,可能是由于Bi2O3与ZnFe2O4之间化学键合使界面电子重新排列,使Bi的化学态发生了变化,这对光生载流子在界面的传输是十分有利的[12]。
图2 Bi2O3/ZnFe2O4的XPS谱
Fig. 2 XPS spectra of Bi2O3/ZnFe2O4
2.1.3 SEM分析
图3(a)和(b)所示为Bi2O3与Bi2O3/ZnFe2O4复合薄膜的表面形貌图。Bi2O3呈片状,长度约2 μm,表面比较光滑,Bi2O3/ZnFe2O4薄膜比较粗糙并在表面发现大量颗粒状物质,图3(c)~(e)所示为Bi2O3- ZnFe2O4薄膜Bi、Fe、Zn的面扫描图谱,薄膜上检测到了Fe和Zn的分布,结合之前XRD和XPS的测试结果可以确认ZnFe2O4颗粒均匀负载在Bi2O3片上,ZnFe2O4的复合使Bi2O3表面变粗糙,从而可增大薄膜的比表面积。复合材料的大比表面积为光电催化反应提供了更多的活性位点,缩短光生电子空穴的迁移距离,从而改善材料的光电催化性 能[30-31]。
2.2 复合薄膜的光电催化活性
2.2.1 吸光性能
图3 Bi2O3薄膜与Bi2O3/ZnFe2O4复合薄膜的表面形貌以及Bi,Fe,Z的面扫描元素分布
Fig. 3 Surface morphologies of Bi2O3 film(a) and Bi2O3/ZnFe2O4 composite film(b), and Bi(c), Fe(d), Zn(e) surface scan elements distribution of Fig. 3(b)
通过测量紫外-可见分光光谱表征Bi2O3/ ZnFe2O4复合薄膜吸光性能,如图4(a)所示,ZnFe2O4的复合使薄膜的光透过率明显降低,提升了材料的光吸收能力,提高了薄膜的光吸收系数,促进了材料对可见光的捕获。根据Kubelka–Munk公式计算Bi2O3和ZnFe2O4的禁带宽度:
(2)
式中:h为普朗克常数;v为光子的频率,Hz;Eg为半导体的禁带宽度,eV;n为常数。如图4(d)所示,Bi2O3和ZnFe2O4均为直接带隙半导体,Bi2O3的禁带宽度为2.29 eV,ZnFe2O4的禁带宽度为1.92 eV,与文献[33-34]报道的一致。
2.2.2 光电化学性能
在三电极体系中测试材料的光电化学性能,电解质为1 mol/L的Na2SO4溶液。测试过程中,入射光从薄膜一侧入射。图5(a)所示为Bi2O3与Bi2O3/ ZnFe2O4在斩光情况下的I-V曲线。在AM1.5光源的照射下,Bi2O3与Bi2O3/ZnFe2O4均有光电流信号。其中Bi2O3/ZnFe2O4复合薄膜光电流达约4 μA/cm2 (1.2V (vs RHE)),是Bi2O3的4倍左右。与Bi2O3相比,Bi2O3/ZnFe2O4复合薄膜的“尖峰”现象有所降低,“尖峰”现象是由于光生电子-空穴对复合或光电催化过程中组分的副反应引起的[32],这说明ZnFe2O4的负载减少了光生载流子的复合。通过测量Bi2O3与Bi2O3/ZnFe2O4的电化学阻抗谱(见图5(b))发现,Bi2O3/ZnFe2O4复合薄膜具有更小的Nyquist圆弧半径,说明光生载流子在传输的过程中阻抗减小[2, 19, 35],从侧面证实了Bi2O3与ZnFe2O4之间异质结的形成。复合后,薄膜的暗电流有所减小,说明薄膜的稳定性有所提高,从XRD谱可知,这可能与复合薄膜中Bi2O3结晶性更好有关。通过测量Bi2O3和ZnFe2O4的莫特-肖特基曲线(见 图5(c)和(d))可知,Bi2O3与ZnFe2O4均为N型半导体[36-37],通过将直线部分外延到X轴上取截距得Bi2O3的平带电位为0.58 V (vs RHE),高于ZnFe2O4 (0.25 V (vs RHE)),根据文献调研,大多数N型半导体的导带电位约在平带电位以上0.2 V[38-40]。Bi2O3与ZnFe2O4的导带电位约为0.38 V和0.05 V (vs RHE),结合UV-vis计算的禁带宽度,Bi2O3与ZnFe2O4的价带电位估计为2.67 V和1.97 V (vs RHE),Bi2O3的导带和价带位置均低于ZnFe2O4的导带和价带位置,满足type-Ⅱ型异质结构的形成条件[41-43]。光照下,电子-空穴对被激发并在自建电场的作用下向相反方向移动,成功抑制了光生载流子的复合,从而使复合薄膜的光电流明显提高。
图4 Bi2O3与Bi2O3/ZnFe2O4的UV-vis透过光谱以及Bi2O3与ZnFe2O4的带隙能曲线
Fig. 4 UV-vis transmittance spectra of Bi2O3 and Bi2O3/ZnFe2O4(a), band gap energy curves of Bi2O3(b) and ZnFe2O4 (c)
图5 Bi2O3与Bi2O3/ZnFe2O4的光电响应曲线、Bi2O3与Bi2O3/ZnFe2O4的交流阻抗图以及Bi2O3和ZnFe2O4的莫特-肖特基曲线
Fig. 5 Photoelectric response curves of Bi2O3 and Bi2O3/ZnFe2O4(a), AC impedance diagram of Bi2O3 and Bi2O3/ZnFe2O4(b) and Mott-schottky curve of Bi2O3(c) and ZnFe2O4(d)
2.2.3 光电催化降解亚甲基蓝
实验以亚甲基蓝为目标降解物考察Bi2O3/ ZnFe2O4电极的光电催化氧化性能,图6(a)和(b)所示分别为Bi2O3(a)与Bi2O3/ZnFe2O4(b)电极光电降解过程中亚甲基蓝的紫外-可见吸收光谱,627 nm处吸收峰为亚甲基蓝的特征峰,595 nm处的肩峰为亚甲基蓝的二聚物[44]。根据Bi2O3与Bi2O3/ ZnFe2O4电极光电降解亚甲基蓝的去除率随时间变化曲线(见图6(d))可知,Bi2O3电极光照4 h后亚甲基蓝去除率为30.4%,溶液中亚甲基蓝溶度趋于平稳,材料对亚甲基蓝的降解效果达到饱和。Bi2O3/ ZnFe2O4电极在4 h内亚甲基蓝去除率达到42.9%,较Bi2O3电极有显著提升,同时亚甲基蓝的浓度曲线仍保持下降趋势,因此,优化光电化学池的结构,充分发挥光电化学池的性能是后续研究需要解决的问题。由光电降解过程中Bi2O3与Bi2O3/ZnFe2O4电极的电流-时间曲线(见图6(c))可知,在整个降解过程中,Bi2O3/ZnFe2O4电极相比Bi2O3具有更 大的光电催化降解电流,与其光吸收性能和光电化学性能一致,意味着其具有更强的光电降解能力。光电降解实验结果再次证明了ZnFe2O4的复合改善了Bi2O3的光电转换效率,提升了材料的光电催化性能。
图6 亚甲基蓝紫外-可见吸收光谱在Bi2O3与Bi2O3/ZnFe2O4电极光电降解下随时间的变化曲线、光电降解过程中Bi2O3与Bi2O3/ZnFe2O4电极的电流-时间曲线以及Bi2O3与Bi2O3/ZnFe2O4电极光电降解亚甲基蓝的去除率随时间变化曲线
Fig. 6 UV-visible absorption spectra of methylene blue during photodegradation of Bi2O3(a) and Bi2O3/ZnFe2O4(b) electrodes, current-time curve of Bi2O3 and Bi2O3/ZnFe2O4 electrodes during photodegradation(c) and removal rate of methylene blue photodegradation by Bi2O3 and Bi2O3/ZnFe2O4 electrodes varies with time(d)
3 结论
1) 采用化学水浴法合成了Bi2O3基底,然后通过电沉积结合后退火工艺制备Bi2O3/ZnFe2O4复合薄膜,通过UV-vis、EIS、M-S等表征手段证明ZnFe2O4的复合成功改善了Bi2O3薄膜的光电催化性能。Bi2O3/ZnFe2O4电极在AM1.5光源照射下4 h内亚甲基蓝去除率达到42.9%,光电降解性能较Bi2O3电极有显著提升并且亚甲基蓝浓度保持下降趋势。
2) 窄带隙ZnFe2O4的负载促进了Bi2O3对可见光的捕获,使材料的吸收带边红移,提高了薄膜的光吸收效率。
3) Bi2O3与ZnFe2O4的复合形成了type-Ⅱ型异质结构,抑制了光生电子空穴对的复合,实现了高密度光子吸收与电荷分离。这些对于光电催化降解有机污染物效率的提高具有重要意义。
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Preparation and photoelectrochemical property of novel Bi2O3/ZnFe2O4 photoanode
WANG Zi-jin, LIU Fang-yang, JIA Ming, JIANG Liang-xing, LAI Yan-qing, LI Jie
(School of Metallurgy and Environment, Central South University, Changsha 410083, China)
Abstract: Bi2O3 thin film was prepared by chemical bath deposition, and ZnFe2O4 was prepared by electrodeposition and post-annealing. The Bi2O3 and Bi2O3/ZnFe2O4 composite films were characterized by XRD, SEM, Raman, XPS and UV-vis. The photoelectrochemical performance of the prepared film as a photoelectrode and its application in photoelectrocatalytic degradation of organic pollutants was studied for the first time. The results show that the load of ZnFe2O4 enhances the absorption range of Bi2O3, increases the light absorption coefficient and inhibits the recombination of carriers at the interface between semiconductor and solution. The photocurrent density of Bi2O3/ZnFe2O4 composite film is 4 times of pure phase material in the Na2SO4 solution. The performance of methylene blue photoelectric degradation under AM1.5 illumination is significantly improved compared with pure Bi2O3.
Key words: heterojunction; Bi2O3/ZnFe2O4; photoelectrochemical; degradation
Foundation item: Project(51674298) supported by the National Natural Science Foundation of China; Project (2017JJ3384) supported by the Natural Science Foundation of Hunan Province, China; Project(2018M630910) supported by the 63rd Batch of China Postdoctoral Science Foundation
Received date: 2020-06-25; Accepted date: 2021-03-31
Corresponding author: JIANG Liang-xing; Tel: +86-731-88830649; E-mail: lxjiang@csu.edu.cn
(编辑 王 超)
基金项目:国家自然科学基金资助项目(51674298);湖南省自然科学基金资助项目(2017JJ3384);第63批中国博士后科学基金资助项目(2018M630910)
收稿日期:2020-96-25;修订日期:2021-03-31
通信作者:蒋良兴,教授,博士;电话:0731-88830649;E-mail:lxjiang@csu.edu.cn