聚合物氮化碳材料光激发过程研究进展

近年来,不含金属元素的聚合物氮化碳半导体以其独特的材料组成和电子结构特征吸引了研究人员的广泛兴趣,在诸如光催化、光致发光、光电化学等光激发相关领域具有潜在的应用前景。聚合物氮化碳光激发过程由能带结构、载流子行为、激子效应等因素所主导,对于这一过程的研究在材料性能的优化、应用领域的拓展以及应用机制的理解等方面具有重要意义。围绕这一主题,本文综述了近年来聚合物氮化碳材料光激发过程研究中取得的最新进展,简单介绍了聚合物半导体光激发过程的研究方法,分别讨论了光激发下材料中的载流子行为和激子过程,进而总结了常用改性策略对于材料光激发过程的调控机制,此外还对材料光响应特性功能化的研究进行了概括。     

高温氮化热聚合法制备氮化碳框架材料用于可见光光催化反应

石墨相氮化碳是一种聚合物半导体材料(带隙宽度约为2.7 eV),具有独特的和可调控的光学和电子性质,能够作为半导体光催化剂用于驱动一系列光催化反应,在能源和环境领域具有潜在应用前景.利用简单的热聚合法,在空气或氮气中高温焙烧(500？700 oC)富氮前驱体可以合成氮化碳聚合物.通常,这些富氮前驱物含有三嗪单元(如三聚氰胺和三聚硫氰酸原料)或在热聚合过程中会生成三嗪单元(如氰胺和二聚氰胺原料).由于热聚合反应过程受到反应动力学限制,氮化碳半导体材料的聚合度和结晶度不高,且比表面积较小,使其在光催化反应过程中存在传质作用差、激子结合能高和光生载流子复合严重等问题,不利于光催化反应进行.本课题组发展了氮化碳光催化剂的合成新方法(高温氮化),该方法抑制了热聚合过程中三嗪中间体的快速分解,促进了氮化碳的聚合.我们将所制备的催化剂用于光催化分解水产氢反应,发现高温氮热反应制备的氮化碳样品(CNC)的催化性能显著优于传统氮化碳.傅立叶红外光谱(FT-IR)、X射线光电子能谱(XPS)和13C固体核磁共振谱(13C NMR)的表征结果表明, CNC光催化剂具有与传统氮化碳类似的化学结构和组成(七嗪基本结构单元).然而,对于高温氮化热聚合方法制备得到的七嗪基氮化碳聚合物光催化性能增强的原因并不清楚.基于此,本文采用X射线粉末衍射(XRD)、场发射透射电镜(FE-TEM)、原子力显微镜(AFM)和比表面积(BET)测试等手段研究了不同制备方法得到的氮化碳基光催化剂在可见光光催化分解水产氢反应中催化性能差异的原因. XRD结果表明, CNC系列样品的XRD谱与体相氮化碳相似,具有石墨相氮化碳特征的层间堆积(002)衍射峰和面内重复单元(100)衍射峰.与传统石墨相氮化碳相比, CNC在27o附近的衍射峰发生明显偏移.其(002)晶面衍射峰从27.5o增大到27.8o,使(002)晶面间距从0.325 nm减小到0.322 nm.进一步观察发现, CNC系列样品与体相氮化碳相比,其衍射峰出现明显窄化,且衍射强度增加,表明由高温氮化热聚合法制得的产物具有更好的结晶度.通常,半导体晶体结构缺陷会阻碍光生载流子的快速迁移和分离,提高氮化碳聚合物的结晶度可有效改善其光催化氧化还原反应. TEM结果表明,传统石墨相氮化碳是由大块的(厚重的)片层堆积形成,而高温氮化合成的CNC-3则是由纳米薄片组成,这种形貌差异可能是因为活性前驱体(氨气和三聚氰氯)的使用改善了反应动力学过程.另外, CNC-3纳米片上有一些地方发生卷曲,这种卷曲能够有效减小纳米片表面张力,降低其表面能,使纳米片结构稳定存在,类似于石墨烯中的碳卷曲行为. CNC-3的AFM结果进一步证实形成了纳米片结构,其厚度均匀,约为3–6 nm.我们构筑的这种纳米薄片结构具有高度敞开的平面结构,有利于光生电子-空穴从体相迁移到表面,可有效提高半导体的光催化性能. BET结果表明, CNC系列样品的比表面积均比传统g-C3N4的比表面积大,且随焙烧温度升高而增大. CNC光催化剂增大的比表面积改善了多相光催化反应的传质扩散过程,增加了表面反应活性位,有利于提高氮化碳聚合物的光催化活性.     

高温氮化热聚合法制备氮化碳框架材料用于可见光光催化反应（英文）

石墨相氮化碳是一种聚合物半导体材料(带隙宽度约为2.7 e V),具有独特的和可调控的光学和电子性质,能够作为半导体光催化剂用于驱动一系列光催化反应,在能源和环境领域具有潜在应用前景.利用简单的热聚合法,在空气或氮气中高温焙烧(500 700 oC)富氮前驱体可以合成氮化碳聚合物.通常,这些富氮前驱物含有三嗪单元(如三聚氰胺和三聚硫氰酸原料)或在热聚合过程中会生成三嗪单元(如氰胺和二聚氰胺原料).由于热聚合反应过程受到反应动力学限制,氮化碳半导体材料的聚合度和结晶度不高,且比表面积较小,使其在光催化反应过程中存在传质作用差、激子结合能高和光生载流子复合严重等问题,不利于光催化反应进行.本课题组发展了氮化碳光催化剂的合成新方法(高温氮化),该方法抑制了热聚合过程中三嗪中间体的快速分解,促进了氮化碳的聚合.我们将所制备的催化剂用于光催化分解水产氢反应,发现高温氮热反应制备的氮化碳样品(CNC)的催化性能显著优于传统氮化碳.傅立叶红外光谱(FT-IR)、X射线光电子能谱(XPS)和13C固体核磁共振谱(13C NMR)的表征结果表明,CNC光催化剂具有与传统氮化碳类似的化学结构和组成(七嗪基本结构单元).然而,对于高温氮化热聚合方法制备得到的七嗪基氮化碳聚合物光催化性能增强的原因并不清楚.基于此,本文采用X射线粉末衍射(XRD)、场发射透射电镜(FE-TEM)、原子力显微镜(AFM)和比表面积(BET)测试等手段研究了不同制备方法得到的氮化碳基光催化剂在可见光光催化分解水产氢反应中催化性能差异的原因.XRD结果表明,CNC系列样品的XRD谱与体相氮化碳相似,具有石墨相氮化碳特征的层间堆积(002)衍射峰和面内重复单元(100)衍射峰.与传统石墨相氮化碳相比,CNC在27o附近的衍射峰发生明显偏移.其(002)晶面衍射峰从27.5o增大到27.8o,使(002)晶面间距从0.325 nm减小到0.322 nm.进一步观察发现,CNC系列样品与体相氮化碳相比,其衍射峰出现明显窄化,且衍射强度增加,表明由高温氮化热聚合法制得的产物具有更好的结晶度.通常,半导体晶体结构缺陷会阻碍光生载流子的快速迁移和分离,提高氮化碳聚合物的结晶度可有效改善其光催化氧化还原反应.TEM结果表明,传统石墨相氮化碳是由大块的(厚重的)片层堆积形成,而高温氮化合成的CNC-3则是由纳米薄片组成,这种形貌差异可能是因为活性前驱体(氨气和三聚氰氯)的使用改善了反应动力学过程.另外,CNC-3纳米片上有一些地方发生卷曲,这种卷曲能够有效减小纳米片表面张力,降低其表面能,使纳米片结构稳定存在,类似于石墨烯中的碳卷曲行为.CNC-3的AFM结果进一步证实形成了纳米片结构,其厚度均匀,约为3–6 nm.我们构筑的这种纳米薄片结构具有高度敞开的平面结构,有利于光生电子-空穴从体相迁移到表面,可有效提高半导体的光催化性能.BET结果表明,CNC系列样品的比表面积均比传统g-C3N4的比表面积大,且随焙烧温度升高而增大.CNC光催化剂增大的比表面积改善了多相光催化反应的传质扩散过程,增加了表面反应活性位,有利于提高氮化碳聚合物的光催化活性.     

结晶氮化碳的制备与改性策略

石墨相氮化碳(g-C_3N_4)是最具代表性的二维有机聚合物半导体材料,其具有可见光响应性能、稳定化学结构和优良的生物相容性等优点,在环境和能源领域有非常广阔的应用前景。但是,普通g-C_3N_4材料的热聚合不完全,其体相和表面的缺陷多,因此光生载流子易复合,光催化活性不高。近年来,高活性结晶氮化碳(CCN)的研究得到了国内外学者的广泛关注。本文总结了目前CCN制备及其改性方法:5种代表性制备方法,包括传统熔盐法、预热熔盐法、固态盐法、溶剂法和质子化法;4种代表性CCN的改性方法,包括缺陷引入、形貌控制、单原子修饰和材料复合。文章重点介绍了 CCN制备原理、结构特征与光催化性能。最后,对CCN的制备与改性方法进行了评价,并对其研究方向进行了展望。     

共轭聚合物薄膜中光生载流子的产生及输运机制

对共轭聚合物光生载流子的产生机制进行了初步探讨,分析了由最初产生的电子 空穴对经过晶格驰豫之后形成极化子 激子的热离化过程,认为同一链上的激子会迅速复合,只有链间激子对光电流作出贡献.研究了共轭聚合物中载流子的输运机制,导出了共聚物的电导率公式,其计算值与实验结果符合,我们认为是极化子的链间跃迁实现了整个共聚物的电导和光致发光,较好地解释了实验事实.     

石墨相氮化碳的液相合成及光催化性能研究进展

石墨相氮化碳是类石墨层状聚合物材料,因其特殊的能带和电子结构,近年来成为功能材料研究领域的热点.液相合成法具有温和多变的特性,是石墨相氮化碳合成的重要途径.本文作者就现阶段液相介质合成氮化碳的主要方法进行了介绍,主要包括液相电沉积、脉冲激光烧蚀、溶剂热合成法等.介绍了不同液相介质和合成参数对制备氮化碳材料晶型、形貌等的影响.同时就溶剂热合成氮化碳在光催化领域的研究进展进行了总结.在未来的研究中,液相合成法将极大的丰富氮化碳材料结构优化的途径,有助于推动多功能聚合物材料的深入研究.     

高性能双极性聚合物半导体材料与晶体管器件

有机聚合物半导体材料与晶体管器件是融合了化学、材料、半导体以及微电子等学科的前沿交叉研究方向.聚合物半导体材料分子是该领域研究的重要内容,其中双极性聚合物分子半导体材料,兼具了电子和空穴的双重载流子输运能力而受到学术界的广泛关注.本文总结了双极性聚合物半导体材料与器件的研究进展,重点介绍了我们在D-A型双极性聚合物分子半导体材料设计、加工技术与器件制备以及功能应用方面的研究工作,并论述了双极性聚合物分子半导体材料与器件研究过程中存在的科学问题及发展方向.     

纳米晶/聚合物太阳能电池

纳米晶/聚合物太阳能电池作为一种新型光伏器件成为近年来的研究热点。通过改变纳米晶的形貌及尺寸来调节材料本身的带隙从而改善光吸收特性,并且无机半导体材料本身具有高的电子迁移率和良好的热稳定性,以上特性使该类电池具有巨大的发展潜力。本文从纳米晶的种类、形状和尺寸、表面配体及纳米晶与聚合物界面性能等方面综述了纳米晶/聚合物太阳能电池的研究现状。纳米晶形貌、太阳光利用率和载流子传输效率是影响电池效率的主要因素。文中指出开展窄带隙纳米晶的合成、优化纳米晶/聚合物电池结构、解析纳米晶与聚合物界面激子传输机理等改善该类电池性能的途径,旨在为提高纳米晶/聚合物太阳能电池的效率提供借鉴经验。     

激子的增殖与第三代太阳能电池

通过激子增殖来提高太阳能电池的光-电转换效率是近年来颇受重视的一个研究方向.本文对激子增殖产生的途径与机制,以及在其形成过程中应注意的各种因素作了扼要的介绍和讨论,诸如:冲击离子化过程、声子瓶颈现象、热-电子的冷却过程、Auger重合过程,以及热-电子的电子转移过程等,同时也对检测这一过程的重要方法——瞬态(泵浦-探测)吸收光谱(TA)作了简要介绍,并通过某些实例对这一现象在作为第三代太阳能电池中光-电转换材料的半导体量子点内情况作了说明.     

含咪唑环与嘧啶环的可可碱掺杂的氮化碳聚合物及其增强的光催化性能

氮化碳聚合物(PCN)是一种有潜力的聚合物型半导体光催化剂,具有原料廉价、物理化学稳定性好以及合适的带边等优点,使其在光催化分解水产氢产氧、降解染料以及抑菌等方面具有很大的潜力.但是由于高电负性的N原子被低电负性的C原子均匀地取代,增加了PCN内部电子传输的难度,使得光生电子–空穴对的复合度增加,进而光催化活性降低.由于PCN的分子结构可调控,所以可以通过分子掺杂来改变氮化碳分子结构,提高光催化活性.常用的分子有机分子,比如吡啶类化合物、嘧啶类化合物以及噻吩类化合物.研究发现,强电负性元素的引入可以改变氮化碳的电子分布,所以含有两个N原子的咪唑类化合物理论上对氮化碳的光催化活性提升帮助更大.由于此类化合物还未见有报道.因此,本文将同时含有咪唑环和嘧啶环的可可碱与尿素反应,生成了咪唑环与嘧啶环共掺杂的氮化碳聚合物,并通过一系列的表征方法验证了咪唑环与嘧啶环成功引入到氮化碳聚合物结构中;然后利用紫外可见光谱(UV-vis),荧光发射光谱(PL),电子顺磁共振(EPR)等实验与DFT理论计算共同验证了咪唑环与嘧啶环共掺杂的氮化碳光学性能;最后通过光催化分解水产氧和降解罗丹明B(RhB)来评价改性后氮化碳的活性.UV-vis测试结果表明,改性后的PCN不仅本征吸收发生红移,而且在波长450到550 nm之间有一个明显的吸收峰,这是由于引入咪唑环和嘧啶环后本征n→π~*电子跃迁所致.并且改性后的PCN的禁带宽度相比于未改性有所降低,说明其可利用的可见光范围增加.PL和EPR结果表明,改性后的PCN不仅光生载流子的复合得到了极大地抑制,而且能够产生更多的孤对电子.通过XPS价带谱,莫特–肖特基曲线以及DFT理论计算推断出改性前后PCN的带边位置,发现改性后PCN的价带位置更正,说明其产生的空穴氧化能力更强.光催化分解水产氧和降解RhB发现,最优改性样品CN40的产氧和降解RhB活性分别是未改性氮化碳的4.43倍和5.1倍.这说明通过咪唑环和嘧啶环共掺杂改性后的氮化碳的光催化活性确实得到了大幅度提升.最后通过添加各种牺牲剂和ESR/DMPO表明·O_2~-和空穴是降解RhB的主要因素.综上所述,通过咪唑环和嘧啶环共掺杂改性氮化碳聚合物,不仅提高了其光吸收能力,抑制了光生载流子的复合,产生更多的孤对电子,而且使得价带位置正移,提高了价带空穴的氧化能力,光催化活性显著提高.     

Helical Graphitic Carbon Nitrides with Photocatalytic and Optical Activities

Graphitic carbon nitride can be imprinted with a twisted hexagonal rod‐like morphology by a nanocasting technique using chiral silicon dioxides as templates. The helical nanoarchitectures promote charge separation and mass transfer of carbon nitride semiconductors, enabling it to act as a more efficient photocatalyst for water splitting and CO₂ reduction than the pristine carbon nitride polymer. This is to our knowledge a unique example of chiral graphitic carbon nitride that features both left‐ and right‐handed helical nanostructures and exhibits unique optical activity to circularly polarized light at the semiconductor absorption edge as well as photoredox activity for solar‐to‐chemical conversion. Such helical nanostructured polymeric semiconductors are envisaged to hold great promise for a range of applications that rely on such semiconductor properties as well as chirality for photocatalysis, asymmetric catalysis, chiral recognition, nanotechnology, and chemical sensing.     

Synthesis of Layered Carbonitrides from Biotic Molecules for Photoredox Transformations

The construction of layered covalent carbon nitride polymers based on tri-s-triazine units has been achieved by using nucleobases (adenine, guanine, cytosine, thymine and uracil) and urea to establish a two-dimensional semiconducting structure that allows band-gap engineering applications. This biomolecule-derived binary carbon nitride polymer enables the generation of energized charge carrier with light-irradiation to induce photoredox reactions for stable hydrogen production and heterogeneous organosynthesis of C−O, C−C, C−N and N−N bonds, which may enrich discussion on chemical reactions in prebiotic conditions by taking account of the photoredox function of conjugated carbonitride semiconductors that have long been considered to be stable HCN-derived organic macromolecules in space.     

Fabrication of porous graphitic carbon nitride-titanium dioxide heterojunctions with enhanced photo-energy conversion activity

Porous graphite-phase polymeric carbon nitride (GPPCN)/TiO₂ donor-acceptor heterojunction was facilely fabricated through the combination of a template technique with a co-calcination process, which exhibited much higher photoelectric activity compared to pristine carbon nitride and TiO₂. The precursor of porous GPPCN (pGPPCN), porous melem, was prepared by using a green template, calcium carbonate, which could be easily removed by diluted hydrochloride. The pGPPCN/TiO₂ heterojunction was then obtained by the assembly and subsequent co-calcination of TiO₂ nanoparticles with porous melem. The formation of pGPPCN/TiO₂ donor-acceptor heterojunction prepared by this method showed improved surface area and light absorption. Moreover, the composite presented much higher photo-energy conversion activity than those of GPPCN, pGPPCN and TiO₂, which could be mainly ascribed to the high charge carrier separation efficiency. This study provides a new approach for the design and development of various photocatalysts with high efficiency for applications in energy fields.     

Directional Charge Transport in Layered Two‐Dimensional Triazine‐Based Graphitic Carbon Nitride

Triazine‐based graphitic carbon nitride (TGCN) is the most recent addition to the family of graphene‐type, two‐dimensional, and metal‐free materials. Although hailed as a promising low‐band‐gap semiconductor for electronic applications, so far, only its structure and optical properties have been known. Here, we combine direction‐dependent electrical measurements and time‐resolved optical spectroscopy to determine the macroscopic conductivity and microscopic charge‐carrier mobilities in this layered material “beyond graphene”. Electrical conductivity along the basal plane of TGCN is 65 times lower than through the stacked layers, as opposed to graphite. Furthermore, we develop a model for this charge‐transport behavior based on observed carrier dynamics and random‐walk simulations. Our combined methods provide a path towards intrinsic charge transport in a direction‐dependent layered semiconductor for applications in field‐effect transistors (FETs) and sensors.     

Nanofluidic Ion Transport and Energy Conversion through Ultrathin Free‐Standing Polymeric Carbon Nitride Membranes

Ions transport through confined space with characteristic dimensions comparable to the Debye length has many applications, for example, in water desalination, dialysis, and energy conversion. However, existing 2D/3D smart porous membranes for ions transport and further applications are fragile, thermolabile, and/or difficult to scale up, limiting their practical applicability. Now, polymeric carbon nitride alternatively allows the creation of an ultrathin free‐standing carbon nitride membrane (UFSCNM), which can be fabricated by simple CVD polymerization and exhibits excellent nanofluidic ion‐transport properties. The surface‐charge‐governed ion transport also endows such UFSCNMs with the function of converting salinity gradients into electric energy. With advantages of low cost, facile fabrication, and the ease of scale up while supporting high ionic currents, UFSCNM can be considered as an alternative for energy conversion systems and new ionic devices.     

Amorphous and Crystalline 2D Polymeric Carbon Nitride Nanosheets for Photocatalytic Hydrogen/Oxygen Evolution and Hydrogen Peroxide Production

Photocatalytic reactions, including hydrogen/oxygen generation, water splitting and hydrogen peroxide production, are regarded as a renewable and promising method to harvest and use solar energy. The key to achieving this goal is to explore efficient photocatalysts with high productivity. Recently, two‐dimensional (2D) polymeric carbon nitride nanosheets were reported as efficient photocatalysts toward various products because of their outstanding properties, such as high specific surface area, more reactive sites, the quantum effect in thickness and unique electronic properties. This minireview attempts to overview recent advances in the preparation, structure and properties of crystalline and amorphous carbon nitride nanosheets, and their applications in photocatalytic hydrogen/oxygen evolution, water splitting and hydrogen peroxide production. We also thoroughly discuss the effect of defects, dopants and composites on the photocatalytic efficiency of these carbon nitride nanosheets. Finally, we outlook the ongoing opportunities and future challenges for 2D carbon nitride nanosheets in the field of photocatalysis.     

Two-dimensional polymeric carbon nitride: structural engineering for optimizing photocatalysis

As a two-dimensional (2D) material, polymeric carbon nitride (g-C₃N₄) nanosheet holds great potentials in environmental purification and solar energy conversion. In this review, we summarized latest progress in the optimization of photocatalytic performance in 2D g-C₃N₄. Some of the latest structural engineering methods were summed up, where the relevant influences on the behaviors of photoinduced species were emphasized. Furthermore, the construction strategies for band structure modulation and charge separation promotion were then discussed in detail. A brief discussion on the opportunity and challenge of 2D g-C₃N₄-based photocatalysis are presented as the conclusion of this review.     

A Facile Steam Reforming Strategy to Delaminate Layered Carbon Nitride Semiconductors for Photoredox Catalysis

The delamination of layered crystals that produces single or few‐layered nanosheets while enabling exotic physical and chemical properties, particularly for semiconductor functions in optoelectronic applications, remains a challenge. Here, we report a facile and green approach to prepare few‐layered polymeric carbon nitride (PCN) semiconductors by a one‐step carbon/nitrogen steam reforming reaction. Bulky PCN, obtained from typical precursors including urea, melamine, dicyandiamide, and thiourea, are exfoliated into few‐layered nanosheets, while engineering its surface carbon vacancies. The unique sheet structures with strengthened surface properties endow PCNs with more active sites, and an increased charge separation efficiency with a prolonged charge lifetime, drastically promoting their photoredox performance. After an assay of a H₂ evolution reaction, an apparent quantum yield of 11.3 % at 405 nm was recorded for the PCN nanosheets, which is much higher than those of PCN nanosheets. This delamination method is expandable to other carbon‐based 2D materials for advanced applications.