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无机材料学报, 2023, 38(10): 1176-1182 DOI: 10.15541/jim20230096

研究论文

MoS2/g-C3N4 S型异质结的构建及光催化性能研究

马润东,1,2, 郭雄1,2, 施凯旋1,2, 安胜利1,2, 王瑞芬,1,2, 郭瑞华1,2

1.内蒙古科技大学 材料与冶金学院, 包头 014010

2.稀土资源绿色提取与高效利用教育部重点实验室, 包头 014010

S-type Heterojunction of MOS2/g-C3N4: Construction and Photocatalysis

MA Rundong,1,2, GUO Xiong1,2, SHI Kaixuan1,2, AN Shengli1,2, WANG Ruifen,1,2, GUO Ruihua1,2

1. School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China

2. Laboratory of Green Extraction & Efficient Utilization of Light Rare-Earth Resources, Ministry of Education, Baotou 014010, China

通讯作者: 王瑞芬, 副教授. E-mail:wrf2008@imust.edu.cn

收稿日期: 2023-02-25   修回日期: 2023-03-13   网络出版日期: 2023-04-11

基金资助: 内蒙古自治区科技计划(2021GG0042)
内蒙古自治区高等学校青年科技英才(NJYT22064)
内蒙古自治区自然科学基金(2022MS05018)

Corresponding authors: WANG Ruifen, associate professor. E-mail:wrf2008@imust.edu.cn

Received: 2023-02-25   Revised: 2023-03-13   Online: 2023-04-11

Fund supported: Inner Mongolia Autonomous Region Science and Technology Program(2021GG0042)
Inner Mongolia Autonomous Region Youth Science and Technology Excellence in Higher Education(NJYT22064)
Inner Mongolia Autonomous Region Natural Science Foundation Program(2022MS05018)

摘要

制备高效稳定的光催化剂对于光催化技术的发展至关重要。本研究采用超声辅助沉积加低温煅烧的方法制备了2H相MoS2/g-C3N4 S型异质结光催化剂(MGCD), 并综合考察了材料的相结构、微观形貌、光吸收性能、X射线光电子能谱、电化学交流阻抗和光电流等对光催化性能的影响。结果表明: 经过超声辅助沉积-煅烧处理, MoS2微米球发生破碎分散结合在g-C3N4纳米片层表面上并形成异质结。可见光下5%MGCD(添加5% MoS2)对罗丹明B(RhB)在20 min时的降解率达到了99%, 且样品重复使用5次后对RhB的降解率仍能达到95.2%, 表现出良好的光催化性能及稳定性。从内建电场形成的角度进一步分析表明, 异质结中MoS2与g-C3N4间耦合形成的内建电场引起的能带弯曲可以有效引导载流子的定向迁移, 并促进光生载流子的分离, 从而提高了光催化反应效率。异质结光催化剂的自由基捕获实验表明: O2-和·OH在催化降解RhB中是主要的活性物种, h+的贡献次之。

关键词： 石墨相氮化碳; MoS2; S型异质结; 稳定性; 光催化机理

Abstract

Preparation of highly efficient and stable photocatalysts is crucial for the development of photocatalysis technology. In this study, the method of ultrasonic-assisted deposition and low-temperature calcination was used to prepare MoS2/g-C3N4 S-type heterojunction photocatalyst (MGCD). Effects of the phase structure, micro-morphology, optical absorption performance, X-ray photoelectron spectroscopy, electrochemical AC impedance, and photocurrent of the materials on the photocatalytic activity were comprehensively investigated. The results show that, after ultrasonic-assisted deposition-calcination treatment, MoS2 microspheres were broken, dispersed and combined on the surface of g-C3N4 nanosheets, and formed a kind of heterojunction. Under visible light, the degradation rate of 5%MGCD (with 5% MoS2 addition) for Rhodamine B (RhB) reached 99% in 20 min, and still reach 95.2% when the sample was reused for 5 times, showing good photocatalytic performance and stability. Further analysis from the point of view of the formation of built-in electric field shows that the band bending caused by built-in electric field, coupled with MoS2 and g-C3N4 in heterojunction, can effectively guide the directional migration of carriers, which can efficiently promote the separation of photogenerated carriers, thus improving the efficiency of photocatalytic reaction. Free radical capture experiment of heterojunction photocatalyst reveals that O2- and ·OH are the main active species in the catalytic degradation of RhB, followed by H+.

Keywords： g-C3N4; MoS2; S-type heterojunction; stability; photocatalytic mechanism

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本文引用格式

马润东, 郭雄, 施凯旋, 安胜利, 王瑞芬, 郭瑞华. MoS2/g-C3N4 S型异质结的构建及光催化性能研究. 无机材料学报, 2023, 38(10): 1176-1182 DOI:10.15541/jim20230096

MA Rundong, GUO Xiong, SHI Kaixuan, AN Shengli, WANG Ruifen, GUO Ruihua. S-type Heterojunction of MOS2/g-C3N4: Construction and Photocatalysis. Journal of Inorganic Materials, 2023, 38(10): 1176-1182 DOI:10.15541/jim20230096

石墨相氮化碳(g-C3N4)作为一种二维类石墨层状光催化剂, 具有较稳定的化学性质和优异的可见光响应能力, 被广泛应用于光电催化领域[1], 但g-C3N4存在带隙较宽、载流子复合率高和可见光响应范围不足等问题[2]。对g-C3N4基光催化剂的改性研究, 扩大其对光的吸收范围需要较窄的带隙, 而获得或保留强氧化还原能力的电子和空穴则需要更正的价带和更负的导带电位, 因此构建合适的异质结构是改善g-C3N4光催化性能的理想路径[3-4]。

2H相MoS2具有独特的带隙结构和层间较弱的范德瓦尔斯力, 被广泛应用于润滑和光电催化领域。Wang等[5]通过水热一锅法将g-C3N4薄膜涂敷在MoS2纳米球的表面并成功构建了MoS2/g-C3N4 II型异质结, 应用于可见光下降解甲苯, 制备的异质结光催化剂的光催化活性比g-C3N4提升了1.3倍, 实验表明构建II型异质结可有效抑制g-C3N4表面光生载流子的复合, 其内部光生电子迁移至还原电位更低的MoS2表面并作为MoS2的电子补充剂参与光催化反应。II型异质结作为一种半导体改性方法, 可有效减少催化剂中光生载流子的复合, 但往往以牺牲氧化还原性更强的h+和光生电子为代价, 并不能促进光催化反应的高效进行[6-7]。Chu等[8-9]采用超声+溶剂热法制备了MoS2/g-C3N4的 2D/2D异质结, 光催化N还原反应中NH3的产率达到18.5 μg·h−1·mg-1, 比纯g-C3N4和MoS2表现出更优异的反应活性, 研究发现, MoS2/g-C3N4异质结交界处的Mo位点(0.53 eV)比MoS2边缘处的Mo位点 (0.87 eV)的N氢化稳定自由能()低得多, 说明MoS2与g-C3N4的耦合能够有效激活异质结交界的边缘Mo位点并降低N还原反应中氢化反应的能垒, 促进N2→*N2H的反应进行。

本工作以廉价的(NH4)6Mo7O24·4H2O和CS(NH2)2作为Mo源和S源, 采用水热法制备花簇状MoS2微球, 并采用超声沉积+煅烧法构建了MoS2/g-C3N4 S型异质结, 通过引入MoS2, 不仅可以调整禁带宽度, 扩大可见光的吸收范围, 同时可以利用异质结由能带交错而产生的电荷传输间接减少光生载流子的复合, 进而促进光催化反应, 并从内建电场形成的角度分析了异质结电荷的传输及光催化降解机理。

1 实验方法

1.1 光催化剂制备

MoS2和g-C3N4光催化剂的制备: 称取40 g尿素放入200 mL带盖刚玉坩埚中, 并用锡箔纸封住, 置于马弗炉中。以5 ℃/min的升温速率逐渐升温至550 ℃并保温240 min, 取出、研磨得到淡黄色粉末状g-C3N4。将2.471 g钼酸铵和5.32 g硫脲溶于80 mL去离子水中, 超声处理3 h后置于磁力搅拌器上再搅拌3 h后装入100 mL反应釜中, 在200 ℃反应24 h, 分离上层清液, 得到的黑色粉末用去离子水/乙醇反复洗涤5次, 将pH调至7制得黑色粉末状MoS2, 将制得的g-C3N4 和MoS2分别在氮气氛围下以5 ℃/min的速率升温至300 ℃并保温3 h, 并将煅烧后的g-C3N4记为GCD、煅烧后的MoS2记为MOS。

MoS2/g-C3N4复合光催化剂的制备: 按照MoS2质量分数为1%、3%、5%、7%称取MoS2和g-C3N4粉末, 将g-C3N4粉末置于100 mL烧杯中, 分别加入30 mL乙醇和40 mL去离子水, 超声3 h后再搅拌3 h, 经过滤置于80 ℃干燥箱烘干8 h, 得到MoS2/g-C3N4复合光催化剂。将复合光催化剂在氮气氛围下以5 ℃/min的速率升温至300 ℃并保温3 h, 制得MoS2/g-C3N4异质结结构光催化剂, 并标记为x%MGCD(其中, x%分别为1%、3%、5%和7%)。

1.2 光催化性能及自由基捕获

采用20 mg/L的RhB溶液为模拟降解物进行光催化实验、稳定性及自由基捕获实验, 催化剂投加量为20 mg/L。无光照条件下搅拌30 min达到吸脱附平衡后分别取4 mL左右反应液, 开启光源, 每隔10 min取样, 经离心后测量上层清液的吸光度。收集暗反应阶段吸出的悬浊液和光反应完成后反应瓶中剩余悬浊液, 经抽滤、洗涤后重新进行第二次光催化实验, 循环测试5次来评价样品的光催化稳定性, 并通过添加相应的猝灭剂BQ (O2-)、KI(h+)和TBA(·OH)参与载流子捕获实验来探究光催化机理。

1.3 表征测试

采用德国Bruker D8-ADVANCE型衍射仪进行XRD表征; 采用德国蔡司Sigma300型场发射扫描电镜和日本日立 HT7800 透射电镜观察表面微观形貌; 分别采用日本日立U-3900型紫外-可见漫反射光谱仪和U-4600荧光光谱仪测试DRS和PL荧光光谱。采用FT-IR1200型傅里叶变换红外光谱仪测试FT-IR光谱。采用北京泊菲莱PCX-50C型多通道光化学反应器进行光催化反应, 光照强度260 mW/cm2; 采用荷兰IVIUM电化学工作站测试样品的光电性能。

2 结果与讨论

2.1 物相、微观形貌

不同样品的XRD谱图由图1(a)所示, 其中2θ=13.1°和27.3°左右的衍射峰来自g-C3N4的(100)和(002)晶面衍射, 分别对应着三嗪环面内的平铺和氮化碳的层间堆叠[11]。随着MoS2含量增大, 2θ=27.3°处衍射峰逐渐发生右移, 说明随着MoS2的引入, g-C3N4分子的晶面间距有所缩小, 且该衍射峰的半峰宽逐渐变大, g-C3N4晶粒度逐渐变小, 这有助于增大光催化剂的比表面积并暴露更多的反应活性位点, 促进光催化反应的高效进行。煅烧前后MoS2样品的XRD图谱如图1(b)所示, 其中2θ=14.1°、33.7°、39.5°和58.8°处衍射峰分别对应MoS2的(002)、(101)、(103)、(110)晶面, 与标准卡片(PDF#75-1539)相符合[10], 可以看出经保护气氛下煅烧, MoS2的结晶度明显提高并具有(002)晶面择优取向, 这有助于提高MoS2的致密度和稳定性并巩固异质结结构, 煅烧后MoS2没有出现杂峰, 也体现出MoS2良好的热稳定性。

图1

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图1   不同样品的XRD谱图(a), 煅烧前后的MoS2的XRD谱图(b)和样品的红外光谱图(c)

Fig. 1   XRD patterns of different samples (a), XRD patterns of MoS2 samples before and after calcination (b), and FT-IR spectra of samples (c)


不同样品的红外光谱图如图1(c)所示, 指纹区478 cm-1处的吸收峰归属于MoS2中Mo-S键垂直于基面振动[12], 810 cm-1处的吸收峰归属于g-C3N4中庚三氮环的面外弯曲振动, 1360和1600 cm-1处出现的吸收峰归属于碳氮杂环的弯曲振动[13]。官能团区中2830 cm-1处的宽峰对应缔合态N-H键的伸缩振动模式, 3500 cm-1处的吸收峰对应缔合态O-H键的伸缩振动模式, 来自于超声时样品表面的吸附氧。通过对比可以看出, 随着MoS2含量的增加, 810 cm-1处的振动吸收峰强度增大, 表明MoS2的复合明显影响了g-C3N4分子所处的化学环境, 也充分说明二者成功构建了异质结, 这与XRD分析结果一致。

不同样品的微观形貌如图2所示。由图2(a)所示, GCD呈现出不规则的花片状, 这是由g-C3N4纳米片通过物理吸附的方式团聚成块并降低自身表面能达到的稳定状态。由图2(b)可见, 合成的MoS2外观呈花球状, 花球内部是为了降低表面能而自组装卷曲团聚而成的褶皱状纳米片[14]。图2(c, d)给出了5%MGCD的表面形貌照片, g-C3N4呈纳米片状, 未观察到MoS2, 这可能是MoS2含量少、分散所致。5%MGCD样品的HR-TEM照片如图2(e, f)所示, 从图2(e)中可以观察到g-C3N4呈纳米薄片状, MoS2微球破碎后的纳米片分布在g-C3N4表面。从图2(f)观察到MoS2的晶格条纹, 其晶格间距对应(002)晶面, 同时在其周边观测到致密的g-C3N4晶格条纹, 晶格间距为0.33 nm, 对应(002)晶面, 两者紧密结合, 表明二者成功构建了异质结结构。如元素分布(图2(g~k))所示, 复合样品中MoS2花球经超声-煅烧后发生破碎, 形成的纳米片层分散、点缀修饰在g-C3N4表面, 并无杂相, 也充分显示了MoS2的分散性和热稳定性良好。

图2

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图2   g-C3N4 (a), MoS2 (b)和5%MGCD(c, d)的SEM照片、HR-TEM照片(e, f)、元素分布总图(g)及其元素C(h), N(i), S(j), Mo(k)分布图

Fig. 2   FE-SEM images of g-C3N4 (a), MoS2 (b) and FE-SEM images (c, d), HR-TEM images (e, f),element mapping (g) and C (h), N (i), S (j), Mo (k) mapping of 5%MGCD


2.2 光电学性能

图3(a)是样品的紫外-可见吸收光谱图, 随着MoS2引入量增大, 样品的吸收带边逐渐发红移。图3(b)是结合Tauc-plot公式得到的样品的带隙图, 当MoS2引入量为5%和7%时, 样品带隙分别为2.45和2.38 eV, 说明MoS2与g-C3N4复合后, 样品的带隙更窄, 光生电子的跃迁也更加容易。借助Tauc法并结合能带公式${{E}_{\text{CB}}}=X-{{E}_{\text{c}}}-{{E}_{\text{g}}}/2$和${{E}_{\text{VB}}}={{E}_{\text{CB}}}+{{E}_{\text{g}}}$计算两种材料的能带结构[15], 式中, Ec为标准氢电极的偶极矩, 取4.5 eV, Eg为半导体的禁带宽度, ECB、EVB分别代表半导体的导带和价带值。X为半导体的绝对电负性, 数值上等于半导体中各个组分原子的几何平均值, 通过查阅文献, XGCD取经验值为4.73[16], 进一步计算得到其EVB和ECB分别为1.57和–1.11 eV; XMOS计算值为5.32, 结合能带公式进一步计算得到其EVB和ECB分别为1.77和–0.13 eV。

图3

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图3   样品的紫外-可见吸收光谱图(a), Tauc曲线(b), 荧光光谱图(c)、瞬态光电流响应图谱(d)及EIS图谱(e)

Fig. 3   UV-Vis (a) spectra, Tauc curves (b), PL spectra (c), instantaneous meter photocurrents (d), and EIS (e) patterns of samples

Colorful figures are available on website


为了进一步研究MoS2与g-C3N4接触界面的电荷转移及光生载流子的分离状况, 测试了样品的光电性能。图3(c)为不同样品在370 nm激发波长下的PL谱图, GCD和5%MGCD样品的发射峰都集中在450 nm附近, 5% MGCD的荧光强度远小于g-C3N4, 说明其载流子的复合程度显著下降, 进一步说明MoS2与g-C3N4成功耦合, 并且由其交错的能带结构和能级差异引导光生电子在不同的HOMO和LUMO能级中传导, 减少光生载流子的复合。样品的瞬态光电流响应如图3(d)显示, 相较于纯g-C3N4, 异质结催化剂的光电流强度有明显提高, 这也意味着复合材料中载流子的分离率更高, 寿命更长, 更有助于光催化反应的高效进行[17-18]。电化学交流阻抗(EIS)图谱常被用于研究半导体光催化材料内部载流子的分离与载流子传质的特性, 其中Nyquist圆弧半径大小代表半导体电极表面电荷的转移的阻力[19], 5%MGCD异质结样品的Nyquist圆弧半径比GCD样品的更小(图3(e)), 说明5%MGCD异质结催化剂电极/电解质液表面的电荷的传输阻抗更小, 更有助于载流子有效转移[20-21]。

2.3 光催化性能及光催化稳定性研究

图4(a)为不同样品对RhB降解率对比图, 可以看到复合光催化剂的光催化效率均优于纯相g-C3N4和MOS。且当MoS2引入量为5%时, 催化20 min样品对RhB的降解率可以达到99%, 较纯相g-C3N4明显提升。良好的稳定性是评价光催化剂综合性能的重要标准[22-23], 5%MGCD样品的降解率随时间变化图(图4(b)), 循环使用5次时, 样品对RhB的降解率仍能保持在95.2%, 说明样品具有良好的光催化稳定性。由样品使用前后的XRD图谱(图4(c))可知, 循环实验五次以后光催化剂g-C3N4的(100)和(002)晶面的衍射峰仍保持较明显的峰型, 说明该复合光催化剂具有良好的相结构稳定性[24-25]。

图4

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图4   (a) 不同光催化剂样品对RhB随时间变化的降解率对比图, (b) 5%MGCD的循环降解率随时间变化图, (c) 5%MGCD循环降解实验前后的XRD谱图, 以及(d) 自由基捕获实验图

Fig. 4   Degradation rate of different photocatalyst samples (a), cyclic degradation rate of 5%MGCD with time (b), XRD patterns of 5%MGCD before and after circulation experiment (c), and histogram of free radical trapping experiment (d)


为了探究光催化剂的光催化机理, 对光催化降解过程进行了自由基捕获实验, 结果如图4(d)所示, 降解过程中添加BQ、TBA和KI活性物种牺牲剂后, 5%MGCD异质结催化剂对RhB的光催化降解率分别下降到61.72%、54.88%和76.47%, 在降解过程中, BQ和TBA对相应活性物种的猝灭效应较为明显, 说明起降解作用的主要活性物种为·OH和O2-, h+对光催化降解的贡献较小。

2.4 光催化机理研究

结合自由基捕获实验, 以BQ为牺牲剂可以有效抑制O2-生成, 说明g-C3N4与MoS2形成的异质结能够有效保留g-C3N4中还原性更强的光生电子并促进O2发生还原反应，生成活性物种O2-, 其载流子传输途径不同于传统的Z型/II型异质结。如图5所示, 通常半导体的平带电位比N型半导体导带底部低0.1~0.3 eV[26]。图5(a)给出了g-C3N4和MoS2形成界面的费米能级及能带变换情况, 当受到一系列大于两者带隙的可见光照射后, 光生电子由稳态变成激发态并发生跃迁, 受费米能级差异的影响, 电荷会从g-C3N4流入MoS2(如图5(b)所示), g-C3N4区的费米能级逐渐降低, 同时MoS2区的费米能级逐渐升高, 并在交界处生成空间电荷区, 伴随产生的内建电场方向由g-C3N4区指向MoS2区(如图5(c)所示), 沿g-C3N4区的电荷电势至MoS2不断下降, 同时电子的电势能-qV沿相同的方向不断增大, 内建电场会在费米能级弯曲的同时拉高g-C3N4的能带, 拉低MoS2能带, 因此, g-C3N4区的光生电子需要克服电势能垒才能回落至MoS2区, 即抑制电荷的输出, 同时允许空穴输入。MoS2区的电荷为高势能态, 能带的下弯允许电子的输出, 同时抑制空穴输入, 氧化电位较低的g-C3N4-VB区h+和MoS2-CB区的光生电子会优先复合并保留具有更强氧化还原电位的h+和e-[27-28]。

图5

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图5   (a) 异质结能带图, (b) 样品异质结的构建及能费米能级的弯曲, (c) 内建电场引起的能带弯曲图

Fig. 5   Energy bands diagram of heterojunction (a), construction of heterojunction of sample and bending of Fermi energy level (b), and diagram of energy band bending caused by built-in electric field (c)


结合上述实验结果分析进一步提出光催化降解机理, 如图6所示, g-C3N4与2H相MoS2紧密结合形成S型异质结, 光照使半导体的光生电荷发生跃迁, 内建电场会使MoS2导带处的电子优先回落至g-C3N4的价带, 保留了MoS2价带处氧化性更强的光生空穴[29]。在g-C3N4导带处的电荷具有较强的还原性, 能够将吸附于催化剂表面的氧气还原成活性物种O2-并降解罗丹明B, 同时, O2-与溶液中的H+、e-结合生成双氧水也能促进·OH的生成, 并共同参与反应[30-31]。在可见光照射下, MoS2的电子在电场作用下迁移至g-C3N4并与光生空穴复合, 同时, 大量g-C3N4空穴在电场牵引下向MoS2更强氧化电位移动并更有效促进 RhB降解反应。两种常规N型半导体的载流子为光生e-。在载流子捕获实验中, h+的猝灭效应不显著, 可能是h+的数量少导致的。

图6

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图6   样品的光催化机理图

Fig. 6   Photocatalytic mechanism diagram of sample


3 结论

通过简便易操作的超声沉积+煅烧法将g-C3N4、MoS2两种半导体耦合并巩固, 制备了g-C3N4/MoS2 S型异质结。S型异质结的构筑有效提升了样品的光催化活性, 当MoS2含量为5%时, 光照20 min复合光催化剂对RhB的降解率达到99%, 比纯相氮化碳有明显提升。不同于II型和Z型异质结, 内建电场的加持下, S型异质结的电子-空穴传输途径有效抑制了g-C3N4光生载流子的复合, 并保留了强氧化性的h+和强还原性e-用于反应生成O2-、·OH等活性物种协助降解RhB。通过充分利用MoS2良好的分散性及热稳定性的特点, 对g-C3N4实现了少量低成本的复合改性工作。

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