## 1 Introduction  
Hydrogen is regarded as one of the promising alternatives to fossil fuels that may help satisfy the environmental and social requirements for clean and efficient energy. Photocatalytic water splitting under visible-light irradiation is a promising method of producing clean fuel because it generates hydrogen using sunlight [1, 2, 3]. Much effort has been directed toward developing highly efficient and stable photocatalysts for water splitting using building blocks and synthesis methods as required. Carbon nitride (C3N4), an emerging metal-free visible-light-activated photocatalyst, has been widely studied owing to its excellent optical, thermal, and electrical characteristics and low cost [4, 5, 6, 7]. Vacancy defects have been widely used to modify the performance of photocatalytic materials [8]. The fast electron-hole recombination of C3N4 greatly limits the improvements of itsphotocatalytic performance [9]. For these reasons, vacancies are generally introduced to promote electron transfer and the band arrangement is modified to inhibit carrier recombination. C atoms have a larger atomic radius and surface area than N atoms, can accept more energy, and have a smaller molecular weight, which makes it easier to sputter C atoms from the surface. Carbon vacancies can inhibit carrier recombination by capturing photogenerated electrons and holes. Due to the formation of carbon vacancies, g-C\({}_{3}\)N\({}_{4}\) photocatalysts tend to release more photogenerated electrons to narrow the gap [10]. The introduction of porous structures is often accompanied by the formation of vacancies. [PERSON] et al. synthesized porous g-C\({}_{3}\)N\({}_{4}\) nanosheets (HGCNs) with carbon vacancies. In this structure, the holes in the plane produce more active exposed edges and reduce aggregation. Additionally, the carbon vacancies expand the light absorption range to the near-infrared region and reduce the recombination of photogenerated carriers. Consequently, the photocatalytic activity of HGCNs for producing H is approximately 20 times higher than that of the bulk structure under visible-light irradiation [11]. [PERSON] et al. prepared g-C\({}_{3}\)N\({}_{4}\) with carbon vacancy (Cv-g-C\({}_{3}\)N\({}_{4}\)) by a simple two-step calcination process. The photocatalytic efficiency of Cv-g-C\({}_{3}\)N\({}_{4}\) is 12.3% higher than that of pristine g-C\({}_{3}\)N\({}_{4}\). When the amino and carbon vacancies generated by charge compensation appear simultaneously, the carbon vacancy enables Cv-g-C\({}_{3}\)N\({}_{4}\) to adsorb O\({}_{2}\) more strongly via Lewis acid-base interactions between the N atom of the amino group and O\({}_{2}\), and it is easier for Cv-g-C\({}_{3}\)N\({}_{4}\) to transfer sufficient photogenerated electrons to O\({}_{2}\) to produce O\({}_{2}\)\({}^{-}\)[10]. The existence of carbon vacancies can promote the effective separation of photogenerated electron-hole pairs. Following characterization using experimental methods and DFT calculations, tuning the formation of terminal amine functional groups around the carbon vacancy can generate preferred docking sites for the co-catalyst, enhancing its proximity [12]. Compared with pristine g-C\({}_{3}\)N\({}_{4}\), the distribution of electrons and holes in the carbon-deficient g-C\({}_{3}\)N\({}_{4}\) is clearly separated, with the majority of the carriers being trapped in the triazine ring fragments where the defects are located. Therefore, the charge transfer between the fragments is centered on the carbon-deficient triazine ring, and it is no longer limited to intra-fragment charge transfer and multi-center charge transfer between fragments [13]. C\({}_{3}\)N\({}_{4}\) can be obtained by calcination of melamine or urea as the precursor. In order to further improve the performance of C\({}_{3}\)N\({}_{4}\), some researchers use molten salt method and double calcination to obtain C\({}_{3}\)N\({}_{4}\) with high activity [14; 15].