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（英文）
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摘要
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.
Molecular doping has been proven to be an effective approach to adjusting the electronic structure of polymeric carbon nitride(PCN) and thus improving its optical properties and photocatalytic activity. Herein, theobromine, a compound composed of an imidazole ring and a pyrimidine ring, was first copolymerized with urea to prepared doped PCN. Experimental investigations and theoretical calculations indicate that, a narrowing in band gap and a positive shift in valence band positon happened to the theobromine doped PCN, owing to the synergistic effect between the pyrimidine ring and the imidazole ring in the theobromine molecule. Moreover, it is shown that the doping with theobromine at a suitable mass fraction makes the obtained sample exhibit decreased photoluminescent emission, enhanced photocurrent density, and reduced charge-transport resistance. Consequently, an enhancement in the photocatalytic activity for water oxidation is found for the sample, which oxygen evolution rate is 4.43 times higher than that of the undoped PCN. This work sheds light on the choice of the molecular dopants for PCN to improve its photocatalytic performance.
Molecular doping has been proven to be an effective approach to adjusting the electronic structure of polymeric carbon nitride(PCN) and thus improving its optical properties and photocatalytic activity. Herein, theobromine, a compound composed of an imidazole ring and a pyrimidine ring, was first copolymerized with urea to prepared doped PCN. Experimental investigations and theoretical calculations indicate that, a narrowing in band gap and a positive shift in valence band positon happened to the theobromine doped PCN, owing to the synergistic effect between the pyrimidine ring and the imidazole ring in the theobromine molecule. Moreover, it is shown that the doping with theobromine at a suitable mass fraction makes the obtained sample exhibit decreased photoluminescent emission, enhanced photocurrent density, and reduced charge-transport resistance. Consequently, an enhancement in the photocatalytic activity for water oxidation is found for the sample, which oxygen evolution rate is 4.43 times higher than that of the undoped PCN. This work sheds light on the choice of the molecular dopants for PCN to improve its photocatalytic performance.
引文
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