Precursor-modified strategy to synthesize thin porous amino-rich graphitic carbon nitride with enhanced photocatalytic degradation of RhB and hydrogen evolution performances

前驱体改性法制备薄层多孔富氨基的石墨相氮化碳用于光催化降解RhB和光催化制氢

The photocatalytic activity of carbon nitride(CN)materials is mainly limited to small specific surface areas,limited solar absorption,and low separation and mobility of photoinduced carriers.In this study,we developed a precursor-modified strategy for the synthesis of graphitic CN with highly efficient photocatalytic performance.The precursor dicyandiamide reformed by different acids undergoes a basic structural change and transforms into diverse new precursors.The thin porous amino-rich HNO_(3)-CN(5H-CN)was calcined by dicyandiamidine nitrate,formed by concentrated nitric acid modified dicyandiamide,and presented the best photocatalytic degradation rate of Rh B,more than 34 times that of bulk graphitic CN.Moreover,the photocatalytic hydrogen evolution rate of 5H-CN significantly improved.The TG-DSC-FTIR analyses indicated that the distinguishing thermal polymerization process of 5H-CN led to its thin porous amino-rich structure,and the theoretical calculations revealed that the negative conduction band potential of 5H-CN was attributed to its amino-rich structure.It is anticipated that the thin porous structure and the negative conduction band position of 5H-CN play important roles in the improvement of the photocatalytic performance.This study demonstrates that precursor modification is a promising project to induce a new thermal polycondensation process for the synthesis of CN with enhanced photocatalytic performance.

半导体光催化是一种利用半导体将太阳能转换为高能化学能的绿色技术,在可再生清洁能源生产和污染物修复领域有着巨大的应用前景.石墨相氮化碳(g-C_(3)N_(4))作为一种环境友好的非金属半导体,因其制备工艺简单、来源丰富、热稳定性和化学稳定性好、可见光吸收范围及特殊的电子性能而受到广泛关注.但一般常用氮源前驱体如二氰二胺、三聚氰胺等所制备的块状石墨相氮化碳存在团聚、比表面积小和光生载流子分离效率低等问题,严重抑制了其光催化活性.本文采用前驱体改性法制得具有高效光催化活性的石墨相氮化碳.利用氰基在酸性条件下易水解这一特性,通过调节不同种类和浓度的酸(硝酸、盐酸、磷酸等)改性二氰二胺,制得一系列新前驱体,通过焙烧制备石墨相氮化碳.采用X射线粉末衍射、X射线光电子能谱、傅立叶变换红外光谱、透射电子显微镜和场发射扫描电子显微镜等表征手段对前驱体及氮化碳的结构及微观形貌进行研究.结果表明,通过浓硝酸改性二氰二胺成功制得脒基脲硝酸盐,其煅烧后所得的HNO_(3)-CN(5H-CN)催化剂具有较好的薄层多孔结构,且面内三均三嗪环末端具有丰富的氨基官能团.TG-FTIR结果表明,5H-CN通过不同于传统氮化碳的热缩合过程,导致了其多孔富氨基的结构.光催化性能测试表明,5H-CN对光催化降解罗丹明B(Rh B)具有最佳的催化活性,其准一级速率常数达0.05316 min^(-1),是普通块状石墨相氮化碳(CN)的34倍.此外,5H-CN的光催化制氢性能也远远高于CN.通过紫外-可见漫反射光谱、莫特-肖特基曲线和瞬态光电流测试等方法研究催化剂的形貌结构对光催化活性的影响.结果表明,5H-CN催化剂具有较高的光催化活性主要归因于其薄层多孔结构提供了更大的比表面积(148.76 m^(2)g^(-1)),表面有更多的活性位点,同时有助于光生载流子的有效分离;其面内三均三嗪环的末端边缘丰富的氨基结构使得其能带结构发生变化,更负的导带位置使其光生电子的还原能力更强,从而有利于光催化反应的进行.其光催化机理归纳如下:在5H-CN催化剂光催化降解Rh B过程中,O_(2)·^(-)作为最主要的活性物种可与空穴(h+)同时氧化催化剂表面的Rh B分子,从而达到光催化降解Rh B的作用;在5H-CN催化剂光催化制氢过程中,铂(3wt%Pt)作为助催化剂可以负导带上的光生电子(e-)快速迁移,迁移的e-可以直接还原水中的氢质子生成氢气.