Preparation And Properties Of Z-Scheme Metal-Doped G-C₃N₄ Photocatalysts

Conversion of solar energy into available energy by utilizing photocatalytic semiconductor technology is one of the important scientific researches concerning energy crisis and environmental contamination.Recently,graphitic carbon nitride（g-C₃N₄）,a new-type polymeric semiconductor benefiting from its suitable bandgap structure and chemical stability,has attracted tremendous attention in the field of photocatalysis research.In this dissertation,a small amount IA-IIIA metal ions was doped into the matrix of g-C₃N₄,resulting in a significant enhancement of photocatalytic performance toward water splitting.Characterizations of XRD,XPS,TEM,PL,UV-vis DRS,N₂ adsorption-desorption isotherms and photoelectric chemical tests were carried out to investigate the relationship between the compositions and the properties of the as-prepared photocatalysts,and to explore the possible mechanism for the enhanced photocatalytic activity.The main contents and results are shown as follows:1.Alkali ion-incorporated g-C₃N₄ photocatalysts were fabricated through a two-step mechanicallapping and thermopolymerization process by using metal chloride and melamine as the precursors.After the modification,the as-prepared catalysts exhibit high photocatalytic activities for H₂ evolution.The optimal photocatalytic H₂-evolution rates over Na ion-doped g-C₃N₄ and K ion-doped g-C₃N₄are 182.3μmol?h^-1 and 225.0μmol?h,respectively.The H₂-evolution rate on the K ion-doped catalyst is 21.4 times as high as that on the pristine one.2.The results indicate that the enhanced activity of the catalysts could be ascribed as follows:i)carbon-deficient interband levels are introduced under the the conduction band minimum of g-C₃N₄,resulting in the enhanced optical absorption by a two-photon photocatalytic system（N 2p→C 2p and N 2p→Metal excitation）;ii)a Z-scheme Ohm-contact heterojunction is formed viaπ-conjugated covalent bonds between carbon and nitrogen atoms of g-C₃N₄,contributing to the improved separation of photoinduced carriers due to the efficient electron transfer between the host and the guest;the doping of the metal ions is effective to inhibit the competitive H₂O₂ generation with O₂ production,beneficial to the H₂ evolution on the modified catalysts.3.The contents of terminal amino and sp³ N of g-C₃N₄ were decreased by the substitution of the terminal H sites of melamine with the metal ions in the thermopolymerization process.It shows that the substitution has a significant effect on the photocatalytic performance of the as-prepared catalysts.With the increasing outer electrons of the doped atoms and the increasing coordination number with melamine,the sp³ N content and the degree of polymerization of the modified g-C₃N₄reduce,resulting in the decreasing photocatalytic activity in the order of Na⁺-doped g-C₃N₄>Mg²⁺-doped g-C₃N₄>Al³⁺-doped g-C₃N₄.Compared to the Na⁺-doped one,the H₂-evolution rates on the Mg²⁺-doped catalyst and the Al³⁺-doped catalyst were decreased to 67.1μmol?h^-1and 21.7μmol?h^-1,respectively.4.With the increasing metallicity among Li,Na and K atoms,the electron density of their covalent nitrogen atoms boost,contributing to a similar increase in the electron density of their nearest carbon atoms through theπ-conjugated network.This will give rise to the decreasing levels of the carbon-deficient interbands,which leads to the improved optical absorption and the increased photocatalytic activity in the order of Li⁺-doped g-C₃N₄<Na⁺-doped g-C₃N₄<K⁺-doped g-C₃N₄.