In view of the hard economic situation, researchers resort to clean and renewable energy resources [1]. In the last few years, hydrogen (H₂) generation by water splitting under sunlight illumination is regarded as an ideal fuel for the future energetic area to solve hazardous environmental pollutants and energy crises [2].

Lately, in an attempt to widen the response range towards sunlight for semiconductors, the spinel ferrites have attracted great interest owing to their narrow bandgap, high photochemical stability, magnetic attitude, and easy recyclability [3,4]. Among several spinel ferrites, spinel cobalt ferrite (CoFe₂O₄), a magnetic p-type semiconductor, has acquired ever growing attention and has been widely examined in various areas including batteries, biomedicine, environmental reclamation, etc. [[5], [6], [7], [8]]. Particularly in the area of photocatalysis, researchers have exerted more extensive investigation on CoFe₂O₄ since its resource abundance, low cost, as well as environmental friendliness [9,10]. However, the aggregation of the particles themselves due to their high surface energy has restrained its photocatalytic (PC) performance. Therefore, diverse strategies have been proffered to boost the PC activity of CoFe₂O₄, especially the construction of the p-n junction. Bellamkonda and co-workers investigated for the first time an Ag@CoFe₂O₄/g-C₃N₄ plasmonic p-n heterojunction structure. They reported that this photocatalyst reached the rate of 335 μmol h⁻¹ for PC H₂ production. The high photoabsorption capacity of the Ag@CoFe₂O₄/g-C₃N₄ was enhanced by the combination of the internal electric field at the (CoFe₂O₄/g-C₃N₄) p-n heterojunction and the plasmonic effect of Ag [11]. Also, He et al. prepared p-n CoFe₂O₄/g-C₃N₄ heterojunctions by a coprecipitation method. An extensive specific surface area, a small band gap, and more photogenerated electrons demonstrate the great PC activity of p-n heterojunction. This nanocomposite displays a great PC hydrogen generation rate of 18.9 mmol g⁻¹ h⁻¹ [12]. Shao’s group revealed CoFe₂O₄/Cd₀.₉Zn_0.1S p-n junction photocatalyst obtained via a simple heat treatment procedure. The constitution of p-n junctions showed the greatest influential separation of photocarrier, which boosted the PC H₂ generation performance [13].

Synthesizing photocatalysts with high efficiency and low cost is the process of selecting proper and specific materials that can be utilized in the photocatalytic hydrogen production system to boost the efficiency of photocatalytic hydrogen production. It is mainly concerned with increasing the light absorption of the photocatalytic system, facilitating the rapid separation and transfer of photoelectrons, improving the redox capacity of the system to achieve excellent, efficient hydrogen production. For this reason, we perused the metal sulfide photocatalyst systems and found that their improvement was restricted by basic issues [14]. Mn_xCd_1-xS solid solution has countless advantages, which made the focus of attention in recent years as an example of superb visible light absorption capability, regulatable energy band positions, and lofty efficiency for PC H₂ generation. Meanwhile, elevated electron-hole pairs’ recombination speed and photocorrosion have impeded its PC application [15,16]. In this respect, several possible viewpoints have been investigated to improve its defects, like controlling morphology [17], doping with metal ions [18,19], and creating heterojunction [[20], [21], [22], [23]]. Creating a heterojunction, which is one of these perspectives, facilitates photocarrier separation. Especially, it can be build a p-n heterojunction by coupling Mn_xCd_1-xS, an n-type semiconductor, with a p-type semiconductor. The potent electric field positioned at the interface of the p-n heterojunction can potently direct the photocarriers to move in the reverse direction, resulting in productive charge separation and boosted PC efficiency [21,24]. Wang et al. constructed a low-cost Cu₂O/Mn_0.05Cd_0.95S p-n photocatalyst, which has a hydrogen production efficiency 2.8 times higher than pure MCS nanoparticles [25]. Gong et al. put forward that Mn_0.2Cd_0.8S nanorods were joined with CoWO₄, creating a p-n heterojunction that significantly evolved the hydrogen generation capability of Mn_0.2Cd_0.8S [26]. Han and coworkers also prepared the NiS/Mn_0.3Cd_0.7S photocatalyst as a p-n heterojunction by facile solvothermal method [27]. The results showed the enhancement of the PC activity of Mn_0.2Cd_0.8S by constructing heterojunction improves the performance of water splitting for hydrogen production [27].

Carbonaceous materials have also been commonly utilized in heterojunction structures to promote their PC efficiency. In particular, graphene is considered a promising matrix due to its wide surface area and lofty electron mobility [28]. However, getting pure graphene is a costly process. On the other hand, reduced graphene oxide (rGO), which resembles the structure of graphene and is easily produced, has been used in several applications, especially photocatalysis. Using reduced graphene in various heterojunctions is a way to improve PC activity. Pal et al. developed a CdS/rGO photocatalyst synthesized by the impregnation procedure followed by the H₂S gas reaction at high temperatures [29]. This heterojunction showed a powerful chemical interaction at the interface, so retard the recombination of charges. Hafeez et al. prepared a ternary InVO₄-g-C₃N₄/rGO nanocomposite by impregnation method. Introducing InVO₄ and rGO into g-C₃N₄ boosted the hydrogen production rate, and the ternary activity was 8 times higher than binary [30].