碱金属及碱土金属掺杂石墨相-C3N4光催化材料研究进展

Research Progress of Graphitic-C3N4 Photocatalytic Materials Doped with Alkali Metals and Alkaline-Earth Metals

石墨相碳化氮(g-C3N4)作为一种成本低廉、化学性质稳定、带隙窄的光催化剂,一直是材料科学领域的研究重点。虽然g-C3N4存在光生载流子复合率高、可见光利用率低、比表面积较小等缺点,但由于其聚合物的本身特性适合制备g-C3N4基复合材料,从而可以通过引入其他化学元素或异质结对g-C3N4进行改进,提高其光催化活性。与非金属共价掺杂不同,碱金属、碱土金属改性g-C3N4具有金属掺杂的非局域化特性,其表面活性位点增多,载流子分离率降低并且能使能带位置发生改变,从而具有较好的光催化性能,因此成为一个新的研究热点。综合考虑经济性和实用性,目前用来改善g-C3N4性能的碱/碱土金属元素多为锂(Li)、钠(Na)、钾(K)、钡(Ba)、镁(Mg)、钙(Ca)。现有的大部分数据表明,Li、Ca两种元素对g-C3N4的改性效果较好,尤其是Ca元素。同时结合不同制备工艺,如选择不同的前体,采用介孔材料作为催化剂载体,改变制备过程中的加热方式(控制升温速率、煅烧温度和时长),可以使g-C3N4的光催化活性进一步提高。虽然碱金属、碱土金属改性g-C3N4的理论依据是金属离子的引入会对能带结构和载流子迁移率产生影响,但金属离子与周围原子的相互作用和对能带的调控机理还未明确,实现碱金属/碱土金属可控改性g-C3N4也尚待研究。对碱金属、碱土金属改性g-C3N4的系统研究仍需继续进行大量的实验作为分析验证的基础。本文对国内外碱金属、碱土金属掺杂改性g-C3N4技术的发展现状进行了总结,归纳了改性g-C3N4的制备方法及应用范围,将改性g-C3N4在实际应用领域(氮氧化合物降解、光解水析氢、有机污染物降解)的光催化活性进行对比,按照掺杂元素种类和数量将其分为单掺杂和复合掺杂,并对其增强机理进行归纳整理,提出了当前碱金属、碱土金属改性g-C3N4发展所面临的问题,并对未来将要进行的工作及发展趋势进行了展望。

As a cost-effective,chemical stability,narrow energy band gap photocatalyst,graphitic carbon nitride(g-C3N4)has been noticed in the materials science.Pure g-C3N4 suffers from the fast recombination of photoinduced electron-hole pairs,insufficient sunlight absorption,low surface area.Typically,element doping and heterojunction based on g-C3N4 is known to be an efficient method to improve its photocatalytic performance,because g-C3N4 is polymer which is suitable for preparing composites based on its matrix.Different from the covalent bonding of non-metal doping,alkali metal-doped g-C3N4 and alkali-earth metal-doped g-C3N4 has been a hotspot due to the delocalization of metal doping,which endows the doped system with unique photocatalytic properties by providing more active sites,lowering the recombination of photoinduced electron-hole pairs and changing the band position.Considering economy and practicality,g-C3N4 modified with alkali metal and alkali-earth metal has been mostly prepared via lithium(Li),So-dium(Na),potassium(K),barium(Ba),magnesium(Mg)and calcium(Ca).The researches show that Li and Ca have better effect on the photocatalytic performance of g-C3N4,especially Ca.Moreover,the photocatalytic activity of g-C3N4 is expected to be further improved by diffe-rent preparation technology,such as different precursors selection,mesoporous material as catalyst carrier,the heating mode selection(controlling the heating rate,temperature and duration).The theoretical basis of alkali metal-doped or alkali-earth metal-doped g-C3N4 is the introduction of metal ions will have an impact on the band structure and carrier mobility.But the interaction of metal ions with surrounding atoms and the mechanism of modifying bandgap have not been determined,the realization of the controllable modification of alkali metal-doped or alkali-earth metal doped g-C3N4 is still to be studied.The syste-matic research of alkali metal-doped and alkali-earth metal doped g-C3N4 needs lots of experiments to form the basis of analysis and verification.This review concludes the development status of alkali metal-doped g-C3N4 and alkali-earth metal-doped g-C3N4,and provides elaborate descriptions about the preparation and application of modified g-C3N4.The photocatalytic activities of different modified g-C3N4 in practical application fields(nitrogen oxides removal,photocatalytic hydrogen evolution,degradation of organic pollutants)are compared.According to the types and quantities of doped elements,the catalyst is divided into single-doping and co-doping,and the enhancement mechanism is reorganized.This article presented the crucial issues of alkali metal-doping and alkali-earth metal-doping that need to be addressed in future research.