盐酸处理制备高结晶氮化碳及其增强光催化产氢活性（英文）

由于氢气燃烧具有高能量和零污染的优点,氢能一直被认为是解决环境污染和全球能源危机问题的新能源.而光催化剂可以将太阳能转化为氢能,是目前制氢最理想的方式.近年来,研究者们的目光已经转向非金属光催化剂,其中氮化碳光催化剂因其化学稳定性好、成本低和无毒性而备受关注.但是传统的利用含氮前驱体通过热聚合得到的氮化碳呈无定形或半结晶结构,导致其光催化活性很差.而熔盐法制备的结晶氮化碳(CCN)则具有优异的光催化产氢性能.但是,熔盐法得到的CCN依然没达到理想的结晶度.在本文中,我们用盐酸(HCl)洗涤处理熔盐法制备的产物,进一步提高了CCN的结晶度.结果表明,随着盐酸水溶液浓度的增加,制备样品的结晶度增大,在盐酸浓度为0.1 mol/L时,样品结晶度达到最大值.这是因为盐酸水溶液可以去除CCN末端氨基中的一些钾离子,导致聚合位点被释放,所以进一步提高了样品的结晶度.而当盐酸浓度进一步提高到0.2 mol/L时,氮化碳结构因为过高的盐酸浓度被破坏,导致结晶度反而下降.以0.1 mol/L盐酸水溶液处理得到的0.1HCCN样品具有良好的光催化产氢性能,在以三乙醇胺为牺牲剂时,其光催化产氢速率达到683.54μmol h^-1 g^-1,在420 nm处的量子效率为6.6%,光催化产氢速率分别是CCN和块状氮化碳的2倍和10倍.光催化活性的提高主要有两个原因:样品结晶度的提高和钾离子嵌入xHCCN样品的中间层.其中,样品结晶度的提高可以减少样品中的表面缺陷以及破坏结构中的氢键,从而增加了光生载流子的迁移,减少了电子空穴对的复合位点,这都非常有利于光催化反应的进行.而插入到xHCCN中间层的钾也促进了光生电子的转移.这是因为桥连的氮原子(N1)并不会被激发产生光生电子,因此抑制了光生电子在七嗪单元之间的迁移,而插入到xHCCN中间层的K可以增加电子的离域性,延长π共轭体系,从而促进光生电子的转移,进一步提高光催化产氢活性.本研究为熔盐法的进一步发展提供了新的思路.

Enhanced photocatalytic hydrogen production activity of highly crystalline carbon nitride synthesized by hydrochloric acid treatment

Crystalline carbon nitride(CCN)prepared by a molten-salt method is attracting increased attention because of its promising properties and excellent photocatalytic activity.In this work,we further improve the crystallinity of CCN through synthesis by the molten-salt method under the action of aqueous hydrochloric acid(HCl)solution.Our results showed that the crystallinity of the as-prepared samples increased with increasing HCl concentration and reached the maximum value at 0.1 mol L^-1.This can be attributed to the removal of some potassium ions(K+)from the terminal amino groups of CCN by the aqueous HCl solution,which results in a release of the polymerization sites.As a result,the crystallinity of the as-prepared samples further increased.Moreover,the obtained 0.1 highly crystalline carbon nitride(0.1HCCN;treated with 0.1 mol L^-1 aqueous HCl solution)exhibited an excellent photocatalytic hydrogen evolution of 683.54μmol h^-1 g^-1 and a quantum efficiency of 6.6%at 420 nm with triethanolamine as the sacrificial agent.This photocatalytic hydrogen evolution was 2 and 10 times higher than those of CCN and bulk carbon nitride,respectively.The enhanced photocatalytic activity was attributed to the improved crystallinity and intercalation of K+into the xHCCN interlayer.The improved crystallinity can decrease the number of surface defects and hydrogen bonds in the as-prepared sample,thereby increasing the mobility of the photoinduced carriers and reducing the recombination sites of the electron-hole pairs.The K+intercalated into the xHCCN interlayer also promoted the transfer of the photoinduced electrons because these ions can increase the electronic delocalization and extend theπ-conjugated systems.This study may provide new insights into the further development of the molten-salt method.