碱金属掺杂的石墨相氮化碳在可见光光催化制氢中的比较研究

摘要

利用半导体光催化技术将太阳能转化为清洁化学能源是解决能源危机和环境问题的最有潜力的途径之一.过去几十年,许多半导体包括氧化物、硫化物和氮化物均表现出光催化活性.然而,半导体光催化的实际应用仍然受制于其较低的太阳能转化效率.解决上述问题的方法之一是发展高效的可见光光催化制氢材料.近年来,石墨相氮化碳(g-C3N4)作为一种聚合物半导体材料,受到了光催化研究人员的广泛关注.g-C3N4具有可见光吸收能力、合适的导带价带位置、良好的热稳定性和化学稳定性,且制备方法简单和结构易调控,是一种极具潜力的光催化制氢材料.然而g-C3N4仍然仅能吸收波长450 nm以下的光,且其光生电子和空穴极易复合,因而光催化制氢效率较低.目前,研究人员采用了多种改性方法来增强g-C3N4的光催化性能,其中通过元素掺杂进行能带结构调控是一种非常有效的策略.而碱金属原子(Li,Na和K)被认为可有效进入g-C3N4的内部结构,通过引入缺陷来拓宽g-C3N4的光吸收范围和提高光生电荷的分离效率.不过到目前为止,尚未见系统的比较研究来深入理解不同碱金属元素掺杂的g-C3N4在可见光光催化制氢中的性能差异.本文采用X射线衍射(XRD)、氮气吸附-脱附测试、紫外可见漫反射光谱(UV-vis DRS)、时间分辨荧光光谱(TRPL)、X射线光电子能谱(XPS)、光电化学测试和光催化制氢测试等表征和测试手段比较研究了不同碱金属元素掺杂的g-C3N4在结构、光学性质、能带结构、电荷转移能力和光催化性能等方面的差异.XRD结果表明,碱金属掺杂可导致g-C3N4的层间距离增大,且碱金属原子半径越大,g-C3N4的层间距离越大.氮气吸附-脱附测试结果表明,碱金属掺杂可提高g-C3N4的比表面积,其中Na掺杂的最高.UV-vis DRS和XPS谱结果表明,依Li,Na,K的顺序,碱金属掺杂导致g-C3N4带隙逐渐变窄,使得可见光吸收能力逐渐增强,且其导带和价带位置逐渐下移.TRPL和光电化学测试结果显示,碱金属掺杂有效抑制了g-C3N4的光生载流子复合和促进了光生载流子的转移,其中Na掺杂的g-C3N4的光生载流子利用效率最高.可见光光催化制氢实验表明,碱金属掺杂显著提升了g-C3N4的光催化性能,其中以Na掺杂的g-C3N4性能最佳,其产氢速率(18.7μmol h–1)较纯的g-C3N4(5.0μmol h–1)可提高至3.7倍.由此可见,g-C3N4的掺杂改性是一个对其微结构和能带结构的优化调控过程,最终获得最优的光催化性能.%Photocatalytic hydrogen production based on semiconductor photocatalysts has been considered as one of the most promising strategies to resolve the global energy shortage. Graphitic carbon nitride (g-C3N4) has been a star visible-light photocatalyst in this field due to its various advantages. How-ever, pristine g-C3N4 usually exhibits limited activity. Herein, to enhance the performance of g-C3N4, alkali metal ion (Li+, Na+, or K+)-doped g-C3N4 are prepared via facile high-temperature treatment. The prepared samples are characterized and analyzed using the technique of XRD, ICP-AES, SEM, UV-vis DRS, BET, XPS, PL, TRPL, photoelectrochemical measurements, photocatalytic tests, etc. The resultant doped photocatalysts show enhanced visible-light photocatalytic activities for hydrogen production, benefiting from the increased specific surface areas (which provide more active sites), decreased band gaps for extended visible-light absorption, and improved electronic structures for efficient charge transfer. In particular, because of the optimal tuning of both microstructure and electronic structure, the Na-doped g-C3N4 shows the most effective utilization of photogenerated electrons during the water reduction process. As a result, the highest photocatalytic performance is achieved over the Na-doped g-C3N4 photocatalyst (18.7 μmol/h), 3.7 times that of pristine g-C3N4 (5.0 μmol/h). This work gives a systematic study for the understanding of doping effect of alkali metals in semiconductor photocatalysis.