Construction and Application of Semiconductor Quantum Dots and Carbon Dots Based 0D/2D Heterojunction Photocatalysts Enhanced Publication

半导体量子点和碳点基零维/二维异质结光催化剂构筑及应用 Construction and Application of Semiconductor Quantum Dots and Carbon Dots Based 0D/2D Heterojunction Photocatalysts 1 first-author 张栋琪(1996-)，男，江苏常熟人，硕士，2021年于江苏大学获得硕士学位，主要从事零维/二维半导体量子点基光催化剂的设计与合成及在光催化制氢中应用的研究。Email: 736111980@qq.com http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208866&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208872&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208877&type= 张栋琪 (1996-)，男，江苏常熟人，硕士，2021年于江苏大学获得硕士学位，主要从事零维/二维半导体量子点基光催化剂的设计与合成及在光催化制氢中应用的研究。Email: 736111980@qq.com 736111980@qq.com 1 1 1 corresp E-mail: liuyh@ujs.edu.cn E-mail: liuyh@ujs.edu.cn 刘艳红(1981-)，女，湖南邵东人，博士，助理研究员，2009年于中国科学院长春光学精密机械与物理研究所获得博士学位，主要从事基于量子点和二维材料的复合纳米结构的设计、制备及催化应用的研究。Email: liuyh@ujs.edu.cn http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208883&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208888&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208893&type= 刘艳红 (1981-)，女，湖南邵东人，博士，助理研究员，2009年于中国科学院长春光学精密机械与物理研究所获得博士学位，主要从事基于量子点和二维材料的复合纳米结构的设计、制备及催化应用的研究。Email: liuyh@ujs.edu.cn liuyh@ujs.edu.cn 2 corresp qingpipa@ujs.edu.cn qingpipa@ujs.edu.cn 李丽霞(1978-)，女，河北邯郸人，博士，副教授，2013年于南京理工大学获得博士学位，主要从事基于纳米金属和水凝胶等高性能复合催化剂的构筑及环境催化应用的研究。Email: qingpipa@ujs.edu.cn http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208900&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208905&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208911&type= 李丽霞 (1978-)，女，河北邯郸人，博士，副教授，2013年于南京理工大学获得博士学位，主要从事基于纳米金属和水凝胶等高性能复合催化剂的构筑及环境催化应用的研究。Email: qingpipa@ujs.edu.cn qingpipa@ujs.edu.cn 1 corresp maobd@ujs.edu.cn maobd@ujs.edu.cn 毛宝东(1982-)，男，山东荷泽人，博士，研究员，博士研究生导师，2012年于美国凯斯西储大学获得博士学位，主要从事环境友好新型量子点的光电性质调控、超快光谱动力学及光催化和电催化应用的研究。E-mail: maobd@ujs.edu.cn http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208917&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208930&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208933&type= 毛宝东 (1982-)，男，山东荷泽人，博士，研究员，博士研究生导师，2012年于美国凯斯西储大学获得博士学位，主要从事环境友好新型量子点的光电性质调控、超快光谱动力学及光催化和电催化应用的研究。E-mail: maobd@ujs.edu.cn maobd@ujs.edu.cn 江苏大学　化学与化工学院，江苏 　 镇江 　 212013 江苏大学　化学与化工学院，江苏 　 镇江 　 212013 School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China 江苏大学　环境与安全工程学院，江苏 　 镇江 　 212013 江苏大学　环境与安全工程学院，江苏 　 镇江 　 212013 School of Environmental and Safety Engineering, Jiangsu University, Zhenjiang 212013, China School of Environmental and Safety Engineering, Jiangsu University, Zhenjiang 212013, China 半导体量子点因其具有精准的尺寸调控、独特的光电特性、丰富的表面活性位点等优势，在光催化剂设计和机理研究中获得了广泛关注。与传统半导体量子点主要作为光吸收单元不同，新兴的碳点更是在增加光吸收、促进电荷分离和增加表面反应位点等光催化不同环节均展现出优异的应用潜力。然而，量子点光催化剂由于小尺寸带来电荷复合严重、易团聚、稳定性差等问题而限制了其光催化性能。解决这些问题的主要途径之一是将零维(0D)量子点负载到超薄的二维(2D)纳米片上，形成0D/2D纳米复合材料，使量子点更加分散和稳定，且2D纳米材料促进的加速电荷转移能够抑制光生电荷的复合，从而可以有效地改善量子点基光催化剂的催化活性和稳定性。本文系统阐述了半导体量子点和碳点基0D/2D异质结光催化剂的构筑及应用，着重讨论了不同类型0D/2D异质结的光催化作用机理及面临的挑战，最后对其未来发展进行了分析和展望。 Semiconductor quantum dots(QDs) have attracted much attention in the design and mechanism of photocatalysts due to their precise size control, unique photoelectric properties and abundant surface active sites. Unlike traditional semiconductor QDs that are mainly used as light absorption units, the emerging carbon dots show excellent application potential in different aspects of photocatalysis, such as enhancing light absorption, promoting charge separation, and increasing surface reaction sites. However, the small size of QDs photocatalysts also leads to serious charge recombination, easy agglomeration and poor stability, which severely limits their photocatalytic performance. One of the main pathways to solve these problems is to load zero dimensional(0D) QDs on ultra-thin two-dimensional(2D) nanosheets to form 0D/2D nanocomposites, which can make the QDs better dispersed and more stable. The accelerated charge transfer promoted by 2D nanosheets can inhibit the recombination of photogenerated charges carriers, which can subsequently improve the activity and stability of QDs-based photocatalysts. In this review, the construction and application of semiconductor QDs and carbon dots based 0D/2D heterojunction photocatalysts are systematically introduced, with special attention to the discussion of the mechanism and challenges of different types of 0D/2D heterojunction photocatalysts. Finally, future development is analyzed and prospected for the 0D/2D heterojunction photocatalysts. 光催化 制氢 0D/2D异质结 半导体量子点 碳点 photocatalysis hydrogen production 0D/2D heterojunction semiconductor quantum dots carbon dots 1　引言 1 随着整个世界工业的迅猛发展以及人类社会的日益进步，传统化石燃料的使用造成可利用资源持续减少，环境受到严重污染并逐渐恶化，海平面持续上升等一系列问题 [ 1 1 - 2 2 1-2 ] 。氢能作为公认的清洁能源，受到了越来越多研究者的关注。氢能除了具有资源丰富、燃烧性能好和热值高等特点以外，还具有环保性能好、经济效益高和用途广泛等特点 [ 3 3 ] 。当前制氢途径主要分为化石燃料重整、氯碱工业副产氢、电解水制氢和生物质新型制氢等，但是都受限于过高的成本和较低的效率 [ 4 4 ] 。近年来，光解水产氢技术可以通过太阳光驱动光催化剂产生光生电子并进行反应生成氢气，而太阳光作为取之不尽的自然能源，在一定程度上可以改善能源危机和环境问题。 为了更好地利用太阳光谱中43%的可见光，可见光响应的窄带隙材料的研究受到了研究人员的广泛关注。其中，量子点具有量子限域效应、比表面积大和可见光区消光系数高等独特特性而被广泛应用于光催化领域。然而，传统量子点基光催化剂由于容易发生团聚、稳定性差而限制了其光催化性能。解决这些问题的主要途径之一是将量子点负载到超薄的二维纳米片上，形成零维/二维(0D/2D)纳米复合材料，两部分之间的相互作用可以使量子点更加均匀分散和稳定，而2D纳米材料促进的电荷转移可以有效改善电荷分离和迁移，从而抑制光生载流子的复合。因此，0D/2D复合材料可显著提高光电性能，在光催化领域受到了广泛关注。 目前，针对0D/2D光催化剂构筑的研究已经被广泛报道，但是仍然面临着制备方法复杂、动力学研究不足等困难。同时，与传统半导体量子点主要作为光吸收单元不同，碳点更是在增加光吸收、促进电荷分离和增加表面反应位点等光催化的不同环节均展现出优异的应用潜力。本文系统介绍了传统半导体量子点和碳点基0D/2D复合异质结光催化剂，并根据引入的二维材料类型以及碳点的作用机理对0D/2D异质结进行了系统归纳总结，并对其发展进行了初步展望。 2　光催化析氢基本原理 1 光催化产氢可以分为三步:(1)半导体的本征光吸收；(2)光生电荷的分离和迁移；(3)表面发生氧化还原反应。主要步骤如 图1 图1 所示。首先，当光催化剂受到外界能量( hν )大于其禁带宽度( E g )的光照时，被激发产生的电子( e - )从价带(VB)跃迁到导带(CB)上，并在价带上留下带正电的空穴( h + )；然后，电子和空穴分离并扩散到光催化剂表面；最后，扩散到表面的电子将水中氢离子还原并生成氢气，空穴把水氧化成氧气 [ 5 5 - 6 6 5-6 ] 。然而，实际上在光生载流子移动的同时会发生不可避免的复合，导致光催化效率低。 由此可见，半导体材料要想实现光催化分解水达到产氢的目的需要满足以下4个要求:(1)合适的带隙，光催化剂的带隙( E g )应大于1.23 V，但应该小于3.0 V，最好覆盖大部分或整个可见光区域；(2)合适的导带(CB)和价带(VB)位置，导带应比H 2 析出的能级(0 V vs. NHE,pH=0)更负，而价带应比O 2 /H 2 O氧化电位(1.23 V vs . NHE,pH=0)更正，才能进行水分解的氧化还原反应；(3)有效的电荷分离；(4)良好的化学稳定性和低成本。量子点中尺寸和组分可调的能带结构为光解水产氢提供了合适的平台。 光催化剂异质结结构工程是在光催化剂内部促进电子空穴分离的最实用和有效的方法之一 [ 7 7 ] ，包括传统的Ⅱ型异质结 [ 8 8 ] 、p-n型异质结 [ 9 9 ] 和Z型异质结 [ 10 10 ] 。AgIn 5 S 8 /Ti http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21208991&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209006&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209000&type= 等异质结的光生载流子被有效分离，延长了载流子的寿命，以改善H 2 的析出和稳定性。与此同时，研究人员发现，异质结的构筑对其他纳米材料改善其自身结构和性能也有帮助。CaIn 2 S 4 /Ti http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209012&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209022&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209017&type= 、ZnIn 2 S 4 /W http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209085&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209096&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209088&type= 和AgIn 5 S 8 /Bi 2 W http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209102&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209114&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209106&type= 均由于高效的电荷分离而展现出优于单体的光解水性能。 3　0D/2D异质结光催化剂简介 1 3.1　量子点简介 2 量子点是把激子在3个空间方向上约束住的半导体纳米结构，一般尺寸在2~10 nm范围内的近球形半导体，其粒径小于或接近其相应半导体材料的激子波尔半径 [ 15 15 - 16 16 15-16 ] 。量子点主要包括Ⅱ-Ⅵ族、Ⅳ-Ⅵ族、Ⅲ-Ⅴ族和Ⅳ族等几类。 胶体量子点在不同领域均有广泛的应用。如量子点因其高光吸收效率、合成简单及量子限域效应使其在敏化太阳能电池及薄膜太阳能电池中有较好的应用 [ 17 17 - 18 18 17-18 ] ；而量子点中尺寸可调的电子结构、表面化学和表面陷阱态工程在光电探针器件中有着独特的优势 [ 19 19 ] ；电致发光器件中量子点通过其窄光谱发射和尺寸可调性允许跨越宽色域而备受关注 [ 20 20 ] ；量子点又因其可调的尺寸、形状和组成特性( 图2 图2 )在光催化中成为最有希望的材料之一 [ 21 21 ] ；量子点的单一激发、多元发射、荧光强、抗光漂白和可调的荧光发射波长等特性使得其在生物成像及量子点传感器有着很强的应用 [ 22 22 ] 。在光催化应用中，与体相半导体光催化剂相比，量子点材料因其有更多的反应活性位点、更大的比表面积、更短的载流子迁移距离和独特的量子限域效应等光学和物理性质，使得量子点光催化剂材料具有突出的优势。 近些年，Ⅰ-Ⅲ-Ⅵ和Ⅲ-Ⅴ族等不含镉或铅等重金属元素、对环境友好的量子点吸引了越来越多的关注。Ⅰ-Ⅲ-Ⅵ族硫化物主要由(Ⅰ=Ag,Cu;Ⅲ=In,Ga;Ⅵ=S,Se)组成，如CuInS 2 、AgInS 2 、CuGaS 2 和AgGaS 2 等。由于Ⅰ-Ⅲ-Ⅵ族量子点光催化剂有着比CdS、CdSe和PbSe等更宽的响应范围和更高的消光系数 [ 23 23 ] ，经过对Ⅰ-Ⅲ-Ⅵ量子点的研究，已经清楚了其具体性质。但是对于光催化制氢的实际商业化应用依然有较大的差距。因此，研究者们通过各种策略在追求着更高的性能。 3.2　量子点光催化应用 2 量子点由于其组分可调，可以调节其带隙，并且由于其优异的光电性能，代表了最有前途的用于制氢的可见光响应光催化剂。早期研究主要集中于Ⅱ-Ⅵ族量子点，如CdS和CdSe及其复合结构基光催化剂。实际上，体相Ⅰ-Ⅲ-Ⅵ硫化物是首次报道的用于光催化分解水的带隙可调的光催化剂 [ 25 25 ] 。Ⅰ-Ⅲ-Ⅵ族硫化物光催化剂的早期研究主要集中在组分调整对光催化性能的影响，如(AgIn) x Zn 2(1- x ) S 2 、(CuIn) x Zn 2(1- x ) S 2 等固溶体材料，在很宽的范围内提供了连续可调的带隙。然而，由于其成分和结构的复杂性，制备过程通常需要高温退火以提高结晶度和减少缺陷，这也导致其尺寸增大和比表面积降低。为了进一步改进，纳米结构可以缩短光生载流子向催化剂表面的扩散距离，有效地提高表面析氢反应的效率。近年来，研究人员发展了简单的水热法和溶剂热法来制备高比表面积的Ⅰ-Ⅲ-Ⅵ固溶体纳米结构，如多孔微球和层状纳米结构，它们可以提供丰富的表面活性中心。总之，高结晶度和小尺寸的纳米颗粒一直是光催化应用中追求的理想结构 [ 26 26 - 27 27 26-27 ] 。除了传统的组分和结构调控外，量子点还具有尺寸和表面配体精细调控的独特优势，从而为助催化剂负载和异质结构构筑提供了新的机遇 [ 28 28 ] 。 表面配体不仅决定了量子点的水/有机分散性，还可以作为表面催化中心。通常，在高温高沸点有机溶剂条件下合成的量子点表面通常覆盖一层具有长烃尾的有机配体(如油胺、油酸和氧化三辛基膦) [ 29 29 ] ，以获得高结晶度和高分散性。它们很容易分散在非极性溶剂或聚合物基体中，但很难分散在水相中。原始的长链配体通常与短链双极性配体(如3-巯基丙酸)交换，在光催化过程中提供了更高的水胶体稳定性和方便的电荷传输通道，有利于光催化活性的提高。用短链配体直接水溶液法制备了大量的Ⅰ-Ⅲ-Ⅵ量子点光催化剂。这些短链配体包覆的胶体量子点具有更好的电荷分离、高导电性和增强的光催化活性 [ 30 30 ] 。此外，Mao等发现混合配体修饰的Ag-In-Zn-S量子点对光催化性能表现出优异的协同效应，其中，与L-半胱氨酸相比，3-巯基丙酸的引入导致PL寿命延长和非辐射复合减少，这可归因于3-巯基丙酸更强的盖层效应 [ 31 31 ] 。 碳点(CDs)作为一类新兴的碳基材料，一般指一种准球形的离散的碳纳米颗粒，其尺寸通常 < 10 nm [ 32 32 - 34 34 32-34 ] 。碳点在光催化应用中，除了类似于传统量子点的光吸收作用，还是一种优良的电子介质，具有较高的电子电导率和较大的电子存储容量。这归因于其sp2石墨碳成分，可以促进异质结复合材料之间的电子转移，抑制了光电催化中电子-空穴的复合，提高了催化性能。除此之外，碳点还通过有效地拓展太阳光的响应范围吸收更多的可见光，发挥了光敏剂的作用。这使得碳点成为目前用作光敏剂的镉基化合物的理想替代品。碳点还具有上转换光致发光特性，能够同时吸收多个波长较长的光子，发射波长较短的光子。碳点作为新兴的碳纳米材料，凭借其独特的物理化学、光学和电子特性，在生物传感、光电子学、光伏、光催化有机合成和光催化氨基酸缩合等众多领域显示出巨大的发展势头 [ 35 35 - 37 37 35-37 ] 。特别是碳点丰富的光学和电子特性，包括高效光收集、可调节光致发光(PL)、上转换光致发光和光诱导电子转移，在光催化应用方面引起了人们的极大兴趣 [ 38 38 - 44 44 38-44 ] 。 例如，Wei等利用碳点的强吸收和电子缓冲特性，以及NiS和CdS之间形成内部p-n异质结的优点，制备了NiS/CdS/CDs复合材料，较二元杂化催化剂(NiS/CDs和CdS/CDs)相比，具有显著提高的析氢性能。在双助催化剂体系中，碳点可以保留其激发态光电子，并促进其光吸收，说明碳点在其中具有作为电子受体及电子给体的双重功能的作用 [ 45 45 ] 。碳点作为反应活性位点可以极大地促进催化性能，这也是光催化中碳点可以提升活性的一个重要原因 [ 46 46 ] 。如Wu等通过酚醛缩合法，将碳点、有机染料分子原花青素和4-甲氧基苯甲醛组成一种无金属的光催化剂，在海水中进行光合作用产生H 2 O 2 。其中，原花青素作为光敏剂将所产生的电子转移至满是反应活性位点的碳点上进行还原反应。结果表明，复合物在海水中以1 776 μmol·g -1 ·h -1 的速率光生H 2 O 2 ，是单体的4.8倍 [ 47 47 ] 。 3.3　二维材料 2 2004年，石墨烯的发现引领了二维材料的快速发展，开辟了一个新的领域 [ 48 48 ] 。之后科学家们又探索了大量其他二维层状材料，如过渡金属二硫属元素化合物(TMD)、过渡金属氧化物、石墨相氮化碳(g-C 3 N 4 )和六方氮化硼(h-BN)等 [ 49 49 - 50 50 49-50 ] 。二维材料通常由层内强共价键和层间弱范德华力组成，前者导致平面内的稳定性，后者维持了层状结构。由于其具有新颖的特性和潜在的多种用途，是研究最为广泛的材料之一。目前已应用于光伏器件、晶体管、光电探测器、光催化、电催化和锂离子电池等领域 [ 51 51 - 52 52 51-52 ] 。 2D结构也展现出优异的催化性能，可归因于2D材料的独特性质。即:(1)二维层状材料比表面积比其体相结构更大，能够为催化反应提供更多的反应活性位点；(2)其二维特性通过最小化电子到达固体/水界面之前必须迁移的距离来分离光生电子和空穴。因此，在光解水产氢中使用二维材料可以提高催化性能 [ 53 53 - 54 54 53-54 ] 。2D纳米材料作为一种新兴的纳米材料，在与其他材料结合时可以作为一种优秀的基底材料，有利于控制所设计的光催化剂的形貌，提高光催化性能。近年来，人们采取了多种策略将不同的材料负载到2D材料表面以提高其活性 [ 55 55 - 57 57 55-57 ] 。 2D材料可以与0D量子点材料复合，作为一种新的构筑单元，以制备界面异质结构。将量子点半导体与2D纳米材料相结合可以很好地弥补其缺陷，在光催化剂的制备方面显示出更广阔的发展趋势。这些材料已经成功地作为电子介质、共催化剂和保护层与其他半导体材料结合使用。下面我们围绕传统量子点基和碳点基0D/2D异质结分别予以介绍。 3.4　传统半导体量子点基0D/2D异质结 2 随着近几十年来对光催化材料结构的深入研究，光催化剂的制备已不再仅仅是通过不同材料的直接结合来完成。越来越多的人致力于对材料的微观形貌控制，根据不同维度纳米结构和性质的精细调控，通过构筑各种跨维度异质结(如0D/1D,0D/2D,0D/3D,1D/2D和2D/2D等)以完善其物理和化学性质，实现增强光活性的完美结合，代表了一个重要的发展趋势 [ 58 58 - 63 63 58-63 ] 。 其中0D量子点与2D纳米片具有良好的匹配性，可以形成0D/2D异质结来促进光生载流子的分离，并协同二者在光吸收和活性位点的优势 [ 64 64 ] 。在这里，量子点因其可调的带隙和优越的光吸收能力等优势而被认为是理想的光催化制氢材料之一。0D量子点具有比表面积大、电荷迁移距离短和尺寸易于控制等优点。与较大的粒子相比，它们具有更强的氧化还原能力、更大的比表面积和量子限域效应，这大大促进了它们的光催化应用 [ 65 65 - 66 66 65-66 ] 。但由于易光腐蚀、自聚集和高光致发光等特点，限制了其应用 [ 67 67 ] 。2D纳米片作为优秀的基底，能够有效避免0D量子点易于团聚的缺点，有助于量子点良好地分散在纳米片上；2D形貌能够保持原有的稳定性，并赋予其较大的比表面积，大大提高了体系的光催化效率；超薄的2D纳米片特性可以加速0D量子点的光生电荷分离和迁移 [ 68 68 ] 。 近年来报道了大量0D/2D结构光催化剂，在光催化领域表现出了优异的性能。根据引入2D材料的作用和类型，目前主要的0D/2D复合光催化剂分为以下三类:(1)促进电荷分离型0D/2D复合光催化剂；(2)助催化剂基复合光催化剂；(3)半导体异质结型0D/2D复合光催化剂。 3.4.1　促进电荷分离型0D/2D复合光催化剂 3 0D/2D复合光催化剂已经得到了快速的发展，其中量子点和石墨烯纳米片的复合是最成功的例子之一。石墨烯具有优越的电子导电性、独特的二维结构、大的表面积比和结构稳定性等特点，在和量子点构筑纳米复合物后具有出色的电荷转移和分离能力，从而大大提高了光催化活性和稳定性。 Fang等 [ 69 69 ] 和Liu等 [ 70 70 ] 分别将CdS量子点负载到石墨烯纳米片上，使量子点均匀分布在石墨烯纳米片上，有效解决了量子点易于团聚而导致的稳定性差的问题，从而增强了光催化性能和稳定性。Liu等 [ 71 71 ] 合成了0D/2D石墨烯/CdS量子点纳米复合材料，并通过时间分辨荧光光谱观察到光生电子从CdS到石墨烯片皮秒级的超快转移。光谱分析表明，石墨烯作为电子受体和传输介质可以显著降低CdS光催化剂中光生电子和空穴的复合速率。Zhang等 [ 72 72 ] 以水热法制备了TiO 2 纳米片/石墨烯片复合物为载体，CdSe量子点溶液为前驱体，通过沉淀法合成了0D/2D CdSe量子点/石墨烯/TiO 2 三元复合物。石墨烯的加入拓宽了CdSe量子点的可见光吸收范围，并利用TiO 2 纳米片作为传输电子介质实现了有效的电荷分离。结果显示，CdSe量子点/石墨烯/TiO 2 复合材料在水中对亚甲基蓝的降解表现出很高的可见光催化活性和稳定性( 图3 图3 )。 此外，Ⅰ-Ⅲ-Ⅵ基量子点与石墨烯的工作之前已有很多报道。例如，Sun等 [ 73 73 ] 将Ag掺杂的ZnIn 2 S 4 原位合成在还原氧化石墨烯纳米片上，这种掺杂方式极大地增加了载流子密度和电荷传输效率。同时，Ag∶ZnIn 2 S 4 量子点和RGO之间存在良好接触的界面，这使RGO成为电子受体和传输介质，从而有效地延长了光生电荷载流子的寿命。优化过的纳米复合材料展现出优异的产氢性能并在可见光照射下表现出良好的稳定性。 3.4.2　助催化剂基0D/2D复合光催化剂 3 二维层状材料还包括过渡金属化合物，如MoS 2 、WS 2 和NiS等，在光催化复合体系中通常作为助催化剂出现。MoS 2 由于其在酸性和碱性介质中的近零氢吸附能，被认为是一种很有前途的电解水和光解水析氢的助催化剂。理论研究表明，电子密度集中在Mo位，有利于吸附质子(H + )生成理想的Mo—H键。MoS 2 暴露边缘上的活性S原子提高了其光/电催化活性 [ 74 74 - 75 75 74-75 ] ,Yan等 [ 76 76 ] 证明MoS 2 中的不饱和活性S原子可以改善MoS 2 的电催化性能。 图4(a) 图4(a) 显示了MoS 2 的微观结构及其产氢的助催化机制示意图。由于配位不同，MoS 2 分子中S原子的活性不同，单配位红色标记的S原子活性最高，高于双配位橙色标记的S原子活性。 Zhang等 [ 78 78 ] 报道了4~14 nm的CdS和单层 M S 2 复合纳米晶( M =W或Mo)，光生电子扩散到 M S 2 侧边活性位点的距离非常小，且 M S 2 单层的富硫面选择性生长于CdS的富Cd(0001)面形成异质结，极大地提高了光生电子空穴在界面处的的分离效率，从而提高了光催化活性( 图4(b) 图4(b) )。Lee等 [ 79 79 ] 在MoS 2 表面引入CdS量子点形成0D/2D异质结构，提高了电化学分解水的析氢反应活性。 Liu等 [ 80 80 ] 将约4.2 nm的黑磷量子点(BP QDs)锚定在厚度约3 nm的MoS 2 纳米片上。BP的导带比MoS 2 更负，并且由于MoS 2 价带为正，因此半导体被光激发之后，BP导带产生的光生电子能够快速迁移到MoS 2 的导带上，同时光生空穴能够马上迁移到BP的价带上，有效地加快电荷分离，减少了光生载流子复合。因此，所制备的BP QDs/MoS 2 纳米化合物分别在可见光和近红外光照下显示出优异的光降解亚甲基橙能力，分别比单体BP QDs和MoS 2 高13倍和27倍。 Ning等 [ 81 81 ] 将0D AgInZnS量子点和2D MoS 2 纳米片结合起来构建0D/2D异质结，并系统地研究了Ag掺杂量、牺牲剂和MoS 2 助催化剂负载量对光催化活性的影响。结果表明，MoS 2 纳米片的引入使复合物的AQE达到41%(400 nm处)，并且复合物具有40.1 mmol·g -1 ·h -1 的产氢速率。Zhang等 [ 82 82 ] 使用一种简单的量子点介导的沉淀方法把Zn-AgIn 5 S 8 QD锚定在超薄NiS纳米片上构筑0D/2D纳米复合材料( 图5 图5 )。由于NiS纳米片中丰富的活性位点和有效的电荷传输而具有明显的光催化制氢能力，展现出优异的产氢能力和稳定性。 3.4.3　半导体异质结型0D/2D复合光催化剂 3 通过将两种不同能级位置的半导体材料复合并发生电荷转移的结构称为异质结，可以加快光生载流子的转移，根据电荷传输路径可以分为Ⅱ型、Z型、S型等。已有量子点与各种2D半导体纳米结构复合，如AgVO 3 QDs/g-C 3 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209266&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209279&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209273&type= 、CdS/ZnO [ 85 85 ] 、AgInS 2 /Ti 3 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209288&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209300&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209296&type= 、Zn-AgIn 5 S 8 /g-C 3 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209306&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209318&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209313&type= 和Zn-AgIn 5 S 8 /α-Fe 2 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209326&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209337&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209330&type= 等。0D/2D的结构可以将量子点中的光生电子和空穴更好地分离，转移至2D半导体从而达到高效的电子空穴分离效率。 Niu等 [ 85 85 ] 设计了非晶态CdS纳米颗粒和超薄ZnO纳米片构成的新型0D/2D CdS/ZnO异质结。分散的CdS纳米颗粒能提供与ZnO纳米薄片最大的界面接触并且展露出更多的反应活性位点，缩短它们之间的电荷迁移距离，降低载流子复合率，两者之间存在协同偶联作用。与纯CdS和ZnO相比，CdS/ZnO复合材料拥有更优异的光催化性能。 g-C 3 N 4 是研究最为广泛的二维半导体催化剂，与其他材料结合时，可以作为一种优良的基底，有利于控制所设计的光催化剂的形貌，提高光催化性能。因此，近年来人们采取了许多策略将不同的材料装载到g-C 3 N 4 表面以增强活性 [ 55 55 - 56 56 55-56 ] 。其中，0D纳米材料由于具有高比表面积和短电荷转移路径等优点，与g-C 3 N 4 结合引起了极大关注。例如，在AgVO 3 QDs/g-C 3 N 4 光催化剂中，AgVO 3 QDs在g-C 3 N 4 纳米片表面具有更好的分散性和更小的尺寸，并且它们之间形成了紧密的0D/2D Ⅱ型异质结构，这有助于它们在可见光下具有优异的降解性能 [ 83 83 ] 和沙门氏菌消毒性能 [ 84 84 ] 。Yang等 [ 87 87 ] 通过在g-C 3 N 4 纳米片上原位生长Zn-AgIn 5 S 8 量子点制备一系列Zn-AgIn 5 S 8 /g-C 3 N 4 0D/2D纳米复合材料，以改善电荷分离。在该系统中，g-C 3 N 4 纳米片充当的是电荷转移介质而非光敏单元。经过优化，当g-C 3 N 4 的负载比为10%时，复合物最大析氢速率达到0.35 mmol·g -1 ·h -1 ，是纯Zn-AgIn 5 S 8 QDs的1.39倍( 图6 图6 )。 针对Z型异质结，Zhang等 [ 88 88 ] 通过静电吸附在正负Zeta电位下设计并合成了0D Zn-AgIn 5 S 8 量子点/2D α-Fe 2 O 3 纳米片复合体系( 图7 图7 )。经过优化，复合物的最大析氢速率达到1.7 mmol·g -1 ·h -1 ，是纯量子点的3.5倍。采用电子自旋共振测试观察到，在光照条件下ZAIS/α-Fe 2 O 3 检测到属于·O 2- 和·OH的自由基信号，证明了复合材料能够将O 2 还原为·O 2- 并且能把—OH氧化为·OH，说明ZAIS/α-Fe 2 O 3 Z型异质结的成功构筑。Qin等 [ 86 86 ] 在Ti 3 C 2 纳米片的表面上原位生长具有不同负载比例的0D AgInS 2 量子点。该类具有紧密接触的0D/2D复合材料可以形成Z型异质结，从而可以加速Ti 3 C 2 表面电子的积累。紫外-可见漫反射光谱证明AgInS 2 量子点的引入拓宽了Ti 3 C 2 的吸收边，使得复合物在450~800 nm范围内展现出高的光吸收强度，从而增加载流子的活性。 3.5　碳点基0D/2D异质结 2 碳点由于具有激发/发射可调的光致发光性质，以及上转换发光、化学发光和电化学发光等独特光学性能，在光催化领域显示出巨大的潜力。包括:(1)连续、宽吸收光谱、强荧光活性、良好的光稳定性、可调的光致发光和优异的光电性能；(2)由于多种原材料和合成方法使碳点具有丰富的表面官能团从而拥有优异的吸电子/给电子能力；(3)价格低廉，生物相容性强，对环境友好。 与传统半导体量子点主要作为光吸收单元不同，碳点在增加光吸收、促进电荷分离和增加表面反应位点等光催化的不同环节均展现出优异的应用潜力，获得了广泛关注 [ 34 34 , 89 89 - 90 90 89-90 ] 。依据碳点的作用，碳点基0D/2D光催化剂可以分为增加光吸收型、促进电荷分离型、表面反应位点型和电荷集储型四类，下面我们分别予以简单介绍。 3.5.1　增加光吸收型 3 碳点在紫外区域表现出显著的光学吸收，并且能够延伸到可见光范围 [ 91 91 ] 。芳香族C http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209376&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209387&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209382&type= C双键中存在π-π * 跃迁，而C http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209395&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209429&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=21209418&type= O键或其他表面基团中存在n-π * 跃迁是吸收光谱中的肩峰 [ 92 92 - 93 93 92-93 ] 。据报道，碳点在与宽禁带半导体(如TiO 2 )结合期间用作光敏剂，将光活性区域从紫外区域延伸到可见光区域 [ 94 94 - 95 95 94-95 ] 。另外，碳点除了在紫外到可见光范围内表现出很强的光吸收外，还具有明显的上转换发光特性。 图8(a) 图8(a) 显示了长波长光(500~1 000 nm)激发的碳点的PL光谱，上转换发射位于325~425 nm范围。碳点的这种上转换PL性质归因于多光子活化过程 [ 96 96 ] 。 当TiO 2 /CDs或SiO 2 /CDs复合光催化剂经过光照射之后，由于碳点的上转换性质，能够发出较短波长的光(325~425 nm)，进而激发TiO 2 或SiO 2 形成电子/空穴对 [ 98 98 - 100 100 98-100 ] 。电子/空穴对与吸附的氧化剂/还原剂(O 2 /OH - )反应，产生活性氧自由基(·O 2- /·OH)，然后进行染料降解。当碳点附着在TiO 2 或SiO 2 的表面上时，碳点带边的相对位置允许电子从TiO 2 或SiO 2 的表面迁移，电子可以沿着碳点的导电网络自由穿梭 [ 101 101 ] ，促进电荷分离，减少光生载流子的复合( 图8(b) 图8(b) ) [ 101 101 ] 。TiO 2 或SiO 2 上寿命更长的空穴证明了这类复合光催化剂的优异活性 [ 97 97 ] 。 在一些实验中，已经报道了碳点在各种2D纳米片表面的成功沉积 [ 102 102 ] 。结果表明，复合材料的光催化性能得到了提高，这归因于有效地拓宽了光吸收范围。Dong等 [ 103 103 ] 用柠檬酸为碳点前驱体，采用共沉淀法合成了Fe 3 O 4 /WO 3 /CDs三元纳米复合材料。紫外-可见吸收光谱表明，Fe 3 O 4 /WO 3 /CDs复合材料的吸收峰强度高于Fe 3 O 4 /WO 3 。除此之外，Fe 3 O 4 /WO 3 /CDs在紫外区具有明显的吸收峰，归因于碳点中的π-π * 跃迁。与Fe 3 O 4 /WO 3 相比，碳点的比表面积增加了90.21%，为反应提供了更多的吸附位点，对提高光催化性能至关重要。 另外，由于碳点前驱体的多样性，所合成的碳点表现出可调谐的光学特性，特别是碳点的上转换荧光。材料发出的光波长比激发源短，可以把宽带隙半导体的光吸收范围延长到可见光区域，甚至近红外区域。Luo等 [ 104 104 ] 通过水热法在SnNb 2 O 6 超薄纳米片上制备了具有上转换特性的氮化碳量子点(CNQDs)。制备的CNQDs/SnNb 2 O 6 0D/2D纳米复合材料在可见光照射下具有良好的光催化产氢性能( λ > 420 nm)。CNQDs的上转换特性使CNQDs/SnNb 2 O 6 纳米复合材料具有在波长大于600 nm处光催化产氢的能力。此外，引入的CNQDs可以作为有效的电子捕收剂，促进光生载流子的转移，瞬态光电流响应和电化学阻抗谱证实了这一点。最后提出了CNQDs/SnNb 2 O 6 纳米复合材料光催化分解水制氢的机理( 图9 图9 )。 3.5.2　促进电荷分离型 3 二维纳米结构具有独特的优点，包括高效的电荷分离、光捕获能力以及可调谐性。与2D半导体纳米结构相结合，碳点作为优良的电子受体，其独特的电子转移性和良好的稳定性使其成为一种良好的光催化助催化剂。g-C 3 N 4 作为目前研究最为深入的二维半导体催化剂之一，与碳点结合时，可以作为一种优良的基底，有利于控制所设计的光催化剂的形貌，提高光催化性能。因此，近年来人们采取了许多策略将碳点装载到g-C 3 N 4 表面以增强活性。其中，碳点由于具有光捕获能力以及可调谐性和短电荷转移路径等优点，与g-C 3 N 4 结合引起了极大关注 [ 105 105 - 107 107 105-107 ] 。 Kang等 [ 108 108 ] 以CdS为光催化剂、碳点为助催化剂，制备了一系列0D/2D 碳点/硫化镉(CDs/CdS)复合材料来作为水/海水中高效制氢的光催化剂。在这种情况下，优化过的CDs/CdS在水和海水中的产氢速率分别为6.70 mmol·h -1 ·g -1 和4.64 mmol·h -1 ·g -1 ，在420 nm处的AQE分别为19.3%和11.8%。其中，碳点具有良好的活性和稳定性，不仅大大提高了电荷分离效率，还能抵御海水中各种离子组分的干扰。 Chen等 [ 109 109 ] 通过在Bi 2 WO 6 纳米片上自组装碳点构筑了全光谱响应的0D/2D CDs/Bi 2 WO 6 异质结构，复合物具有较高的光生载流子分离效率和全光谱吸收。同时，超薄Bi 2 WO 6 纳米片特性使得复合物具有优异的光降解能力。密度泛函理论计算表明，碳点与Bi 2 WO 6 之间的互补导电和价带边杂化能明显提高复合材料电子-空穴对分离效率。Liu等 [ 110 110 ] 制备的三元三维CDs/Bi/TiO 2 光催化剂，通过精心设计的电荷传输功能，增强了光催化性能。碳点充当有效的电子提取器，以加速电子-空穴对的分离，并且Bi/TiO 2 三维骨架的结构极大地促进了整个太阳光谱范围内的光响应。与TiO 2 相比，在全光谱照射下，CDs/Bi/TiO 2 催化剂对苯酚和罗丹明B的光催化降解性能大大提高。 Pan等 [ 111 111 ] 通过简便的水热法合成碳点修饰的g-C 3 N 4 /TiO 2 纳米异质结。通过热缩合制备g-C 3 N 4 纳米片，水热法合成TiO 2 纳米片，随后将碳点依次引入到g-C 3 N 4 和TiO 2 的纳米片表面。CDs/g-C 3 N 4 /TiO 2 纳米复合材料的产氢量显着提高，分别比g-C 3 N 4 /TiO 2 、CDs/TiO 2 和CDs/g-C 3 N 4 高12,6,1.5倍。光催化制氢的提高归因于碳点的众多优越性能，包括它们的大电子存储容量和有效促进光生载流子分离能力，这导致了过氧化氢分解的高催化活性，如 图10(c) 图10(c) 所示。 3.5.3　表面反应位点型 3 二维纳米片的相对较大的横向尺寸为表面反应提供了高密度的活性中心。同时碳点表面含有大量的羟基、羧基和氨基等基团，这些基团具有优异的水溶性和化学反应性。这是碳点优越于传统半导体量子点的特性，为碳点基0D/2D复合光催化剂提供了更为丰富的结构和功能可调性。 Yu等 [ 114 114 ] 首次用碳点和Pt纳米粒子作为光催化制氢的双催化剂来修饰ZnIn 2 S 4 纳米片。负载碳点和Pt纳米颗粒的ZnIn 2 S 4 纳米片的产氢速率为1 032.2 μmol·h -1 ·g -1 ，复合物在420 nm处的表观量子效率为2.2%。其光催化速率远远高于纯ZnIn 2 S 4 、负载Pt或者负载碳点的ZnIn 2 S 4 。催化性能的增强是由于碳点具有良好的结晶性、较强的光吸收、较高的导电性以及碳点作为催化活性位点可以有效地捕获光生电子从而发生还原反应( 图11 图11 )。 Bian等 [ 115 115 ] 在TiO 2 纳米片上负载碳点，利用碳点的表面官能团作为吸附中心来吸附Cr(Ⅵ)。此外，碳点可以捕获光诱导电子，抑制它们与空穴的复合。在Cr(Ⅵ)的光还原过程中，CDs/TiO 2 的活性是纯TiO 2 的5.4倍。CDs/TiO 2 表面的正电荷有利于Cr(Ⅵ)的选择性吸附和Cr(Ⅲ)的快速脱附，对Cr(Ⅵ)的光催化还原和稳定性有明显的促进作用。另外，Kang等 [ 116 116 ] 构筑的CDs/g-C 3 N 4 复合材料在 λ =(420 ± 20) nm和 λ =(580±15) nm时的表观量子效率分别为16%和6.3%。与传统的一步四电子反应不同，CDs/g-C 3 N 4 光催化剂在可见光照射下通过两步两电子反应生成氢气和氧气。g-C 3 N 4 负责第一步(光催化生成H 2 和H 2 O 2 )，碳点负责第二步(化学催化H 2 O 2 分解)。碳点作为反应活性位点使第二阶段的H 2 O 2 分解和氧气的生成变得更加容易，从而复合物也达到了优异的催化性能( 图12 图12 )。 3.5.4　电荷集储型 3 碳点是一种优良的电子介质，不仅具有较高的电子电导率而且还有较大的电子存储容量，这归因于sp2石墨碳成分。这促进了异质结复合材料之间的电子转移，抑制了光电催化中电子-空穴的复合，提高了性能。其他类型光催化剂的光生电子可以穿梭在碳点的导电网络中，从而延缓光生载流子在结界面的复合。 例如，Wei等 [ 45 45 ] 利用碳点的强吸收和电子缓冲特性，将碳点均匀分布在CdS/NiS纳米片表面，制备了CdS/NiS/CDs复合材料，与单一碳点光催化剂和二元杂化复合物(NiS/CDs和CdS/CDs)相比，具有显著的促析氢性能。可以发现，碳点既是优良的电子给体，又是良好的电子受体和电子储存单元，能有效地分离电子和空穴。Kang等 [ 117 117 ] 以碳点为固体电子介质进行电荷存储，构建了BiVO 4 /CDs/CdS Z型光催化剂，在可见光下表现出良好的光催化活性和稳定性。BCC50复合物(BiVO 4 /CDs/CdS,BiVO 4 /CDs质量比为50%)的光催化活性最好，析氢和析氧速率分别为1.24 μmol·h -1 和0.61 μmol·h -1 。Z型光催化剂的光催化活性的提高可以归因于碳点有效的电荷存储从而导致光生载流子的高效分离和传输，以及电子和空穴寿命的延长( 图13 图13 )。 4　总结与展望 1 本文阐述了半导体量子点和碳点基0D/2D异质结光催化剂的最新研究进展，重点介绍了其构筑、应用及作用机制。总体而言，0D/2D异质结光催化剂因其广泛和多学科的应用而得到高度评价，有望作为解决各种环境和能源相关问题的有力手段，其构筑及性能研究已成为光催化领域一个突出的研究课题，并仍在迅速发展。 这类0D/2D复合物综合量子点和纳米片的优势，有效减少了量子点自聚集并且展露出更多的反应活性位点，缩短了它们之间的电荷迁移距离，降低了载流子复合率。量子点有规整的尺寸组分和表面形貌，对于机理的研究和催化设计有重要的指导作用。而2D材料具有快速的电荷传输和丰富的活性位点的优势。二者的结合为光催化机理的阐释和高性能催化剂的开发提供了一种丰富的创新平台。其中，传统半导体量子点主要作为光吸收单元，和各个类型的2D材料进行复合，发展相对成熟，机理研究也比较深入。根据引入的二维材料及其作用机理，已经开发出电荷分离型0D/2D复合光催化剂、助催化剂基0D/2D复合光催化剂和半导体异质结型0D/2D复合光催化剂。而碳点除光吸收外，还有促进电荷分离、增加表面活性位点和电荷集储等功能，据此碳点基0D/2D复合光催化剂可分为增加光吸收型、促进电荷分离型、表面反应位点型和电荷集储型。 目前量子点和碳点基0D/2D异质结光催化剂相关的报道很多，但是深入的催化剂设计和界面机理研究仍需要更多的探索。在这些0D/2D异质结光催化剂能够用于商业应用之前，仍然存在一些需要解决的问题和难点。 (1)量子点的形貌和尺寸在决定其性能和选择性应用方面起着关键作用，开发简便、低成本和绿色的量子点合成方法，实现更多量子点结构和尺寸精确控制至关重要。 (2)二维纳米材料领域发展迅猛，新型超薄二维纳米材料不断涌现，探索兼容新型二维纳米材料的整合以及0D/2D复合体系界面结构调控仍是一个挑战性的课题。 (3)载流子动力学研究对于高效0D/2D异质结光催化剂设计至关重要，但现在仍然局限于瞬态吸收和时间分辨荧光等粒子内的光物理复合过程，面向实际催化过程的原位或实时载流子动力学研究仍比较缺乏。 本文专家审稿意见及作者回复内容的下载地址:http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20210204. 参考文献: WANG X C , MAEDA K , THOMAS A , et al . 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Carbon dots as solid-state electron mediator for BiVO4/CDs/CdS Z-scheme photocatalyst working under visible light 10.37188/CJL.20210204 O482.31 A 1000-7032(2021)08-1278-19 国家自然科学基金（21908081, 21501072）；江苏特聘教授项目资助 Supported by National Natural Science Foundation of China (21908081, 21501072); Jiangsu Specially-appointed Professors Program 2021-06-08 2021-06-20 2021-08-01 2021-10-19 2021 发光学报 08 42