## 1 Introduction  
The development of alternative energy sources is attracting considerable attention due to the growing demand for energy carriers, as well as the negative impact of greenhouse gases on the climate [1, 2]. Hydrogen is one of the most promising energy carriers owing to its high calorific value, wide distribution, and absence of toxic products during combustion [3]. However, traditional methods for producing hydrogen are based on the use of fossil fuels, in particular natural gas [4]. The development of alternative methods of hydrogen production, using only renewable energy sources, will allow the transition to egreen\({}^{\star}\) hydrogen [5]. One of the most promising methods is the photocatalytic hydrogen evolution from aqueous solutions on semiconductor materials, since this technology allows one to convert the energy of sunlight into the energy of chemical bonds. Under the action of light irradiation with a certain wavelength, photogenerated charge carriers are formed in the semiconductor--an electron and a hole--and then participate in redox reactions on the surface of the semiconductor, which in this case is a photocatalyst [6, 7, 8, 9]. The first semiconductor material used as a photocatalyst was titanium dioxide, and photocatalysts based on it are still widely used today, despite the wide band gap of TiO\({}_{2}\) (3.0-3.2 eV). Significant progress has been made in the field of materials for photocatalytic hydrogen production. Among others, photocatalysts with high activity under the action of visible radiation have been developed: metal organic frameworks [10; 11; 12], metal sulfides [13; 14; 15; 16; 17; 18], metal-doped perovskite [19; 20], and (oxy)nitrides of Ti and Ta [21; 22; 23]. However, sulfides and nitrides tend to undergo photoconrosion and decrease in activity during long-term photocatalytic tests. At the same time, the synthesis of MOFs or the doping of perovskites is not always attractive for practical applications due to the complexity and cost of their synthesis. A promising semiconductor material of interest to researchers is graphitic carbon nitride g-C\({}_{3}\)N\({}_{4}\) due to its band structure, stability, and lack of toxicity [24; 25]. The narrow band gap (2.7 eV) provides g-C\({}_{3}\)N\({}_{4}\) with activity under visible light irradiation, which makes up a significant part of the solar spectrum [26; 27; 28]. Another advantage of g-C\({}_{3}\)N\({}_{4}\) is the strongly negative position of the conduction band (-1.3 V vs. NHE), which provides the significant reduction potential of photogenerated electrons [29; 30]. Thus, the band structure of g-C\({}_{3}\)N\({}_{4}\) is favorable for the effective occurrence of reduction reactions under visible light. In addition, g-C\({}_{3}\)N\({}_{4}\) can be produced in one step from available precursors such as melamine, urea, and other nitrogen-containing organic compounds. However, pristine g-C\({}_{3}\)N\({}_{4}\) has low activity in photocatalytic reactions due to the high rate of recombination of photogenerated electrons and holes, as well as poor adsorption of reagents on the photocatalyst surface [31]. The g-C\({}_{3}\)N\({}_{4}\) performance in the photocatalytic H\({}_{2}\) evolution reaction can be improved in several ways. The change of morphology, leading to the breakage of interlayer hydrogen bonds, allows the lifetime of photogenerated charge carriers [32] to increase. The increased surface area allows more reactant to adsorb, which also increases the reaction rate [33]. The coating of the semiconductor surface with g-C\({}_{3}\)N\({}_{4}\) quantum dots allows the shortening of the distance for electrons and holes to migrate to the surface, reducing the rate of charge recombination [34]. A widely used method for modifying g-C\({}_{3}\)N\({}_{4}\) is the deposition of particles of metals or their compounds on the surface of the photocatalyst.