SRecently, semiconductor photocatalytic water decomposition has been improved by integrating appropriate co-catalysts. Due to the sufficient catalytic site and easy preparation, cobalt sulfide-based cocatalysts have been widely applied as co-catalysts for various semiconductors toward photocatalytic hydrogen evolution ^[1][2][3][4][50,51,52,53]. Fu et al. have illustrated that combining a hollow cobalt sulfide (CoS_x) polyhedral cocatalyst with g-C₃N₄ can effectively accelerate the separation of photoinduced charges in g-C₃N₄ and provide an abundant active site to promote redox reactions ^[5] (Figure 6a) [54]. In addition, the hollow structure of the CoS_x polyhedron can also allow multiple reflections of light to enhance the light collection of g-C₃N₄. Thus, the photocatalytic performance of the 2% CoS_x/g-C₃N₄ hybrids was significantly better than that of the blank g-C₃N₄. Obviously, the incorporation of cobalt sulfide could act as a cocatalyst to accelerate the separation and transfer of photo-generated electron-hole pairs and reduce the overpotential of the hydrogen production reaction. Qiu et al. reported that CdS nanorods loaded with CoS₂ nanoparticles exhibited excellent photocatalytic hydrogen production activity, which was 13 times higher than that of pristine CdS NRs samples, and the optimized CoS₂/CdS NRs photocatalyst had high stability and recyclability ^[6][55].

In addition to the cobalt sulfide single component cocatalyst, multicomponent cocatalysts exhibit superior co-catalytic activity than single component cocatalysts. For example, Li et al. have reported an excellent composite photocatalyst by combining CoS with Co(OH)₂ on g-C₃N₄ to construct a dual cocatalyst ^[7][56]. The photocatalytic hydrogen production rate of the CoS/Co (OH)₂/g-C₃N₄ composite photocatalyst is 311 times higher than that of pure g-C₃N₄, which is due to the synergistic effect of the dual cocatalysts. In the dual cocatalyst system, CoS cocatalyst acts as an electron acceptor to facilitate the separation of photogenerated carriers, and Co(OH)₂ can also act as a conductor to diffuse photon-generated electrons. Moreover, in addition to acting as a co-catalyst, cobalt sulfide has also been reported as a semiconductor for H₂ production. For example, Zhang et al. used a simple hydrothermal synthesis method to in situ grow two-dimensional ZnIn₂S₄ on one-dimensional hollow Co₉S₈ nanotubes to form a Co₉S₈/ZnIn₂S₄ heterostructure ^[8][57]. Type-I heterostructures are constructed when the Co₉S₈ nanotubes are covered with ZnIn₂S₄ nanosheets. When the Co₉S₈/ZnIn₂S₄ composites are excited to generate electron-hole pairs, the photogenerated electrons can migrate rapidly from the CB of ZnIn₂S₄ to that of Co₉S₈. Consequently, the Co₉S₈/ZnIn₂S₄ heterostructure achieves a higher photocatalytic activity than pure ZnIn₂S₄. 

In addition to being used as a cocatalyst for photocatalytic H₂ production, cobalt sulfide can also be used as efficient photocatalytic for CO₂ reduction ^[9][10][64,65]. For example, Zhang et al. have composited the hollow Co₉S₈ nanocages with ZnIn₂S₄ nanosheets and CdS quantum dots to construct a ternary composite photocatalyst ^[11][29]. The hollow structure of Co₉S₈ nanocages promotes multiple reflections of sunlight in the cavity, which enhanced the light absorption of ZnIn₂S₄ nanosheets and CdS quantum dots. In addition,, the ternary composite photocatalyst form a double Z-type heterojunction, which facilitates the separation and migration of photogenic electron hole pairs. Therefore, the photocatalytic performance of the Co₉S₈@ZnIn₂S₄/CdS hybrid is obviously better than that of blank CdS and ZnIn₂S₄.

Moreover, photocatalytic reduction of CO₂ to methanol is another ideal approach for solar energy conversion. Ma et al. have prepared carbon nitride (CN) loaded with cobalt sulfide (CS) as a cocatalyst. The optimized CS/CN photocatalyst was 2.3 times more selective for CH₃OH than CN ^[12][66]. It was confirmed that the introduction of cobalt sulfide can improve the selectivity of CH₃OH. The cobalt sulfide not only provides the H₂O oxidation center but also can significantly weaken the overpotential of the H₂O oxidation half reaction, thus effectively avoiding the formation of strongly oxidized radicals.

Furthermore, Wang et al. have reported hierarchical FeCoS₂-CoS₂ double-shelled nanotubes as a composite photocatalyst for CO₂ reduction ^[13][67]. FeCoS₂-CoS₂ composites can be obtained after ion-exchange reactions and sulfidation reactions with MIL-88A as precursors. FeCoS₂-CoS₂ composites present a uniform hierarchical nanosheet structure. When the Ru(bpy)₃²⁺ is used as the photosensitizer, the optimal FeCoS₂-CoS₂ composite shows excellent photocatalytic activity with a CO generation rate of 28.1 µmol h⁻¹, which is better than the individuals of FeCoS₂ and CoS₂ and their physical mixtures sample. The unique hierarchical nanosheet structure reduces diffusion length and enhances scattering in the cavity, which inhibits electron-hole recombination and exposes active sites for redox reactions, thus improving the photocatalytic activity of the FeCoS₂-CoS₂ composite.

Neither humans nor the earth’s ecosystem can survive without the ability to synthesize ammonia ^[14][68]. The production of this foundation sustaining life on earth is based on both industrial and biological fixation levels of 200 × 10⁶ tons per year ^[15][69]. At present, nitrogen fixation is principally carried out in three ways: (i) biological nitrogen fixation. Some micro-organisms, such as nitrogen-fixing bacteria, use their own nitrogenase to fix N₂ molecules for biological nitrogen fixation; (ii) high-energy nitrogen fixation in geochemical processes, such as lightning; (iii) the energy-intensive Haber–Bosch method for industrial nitrogen fixation. However, biological and geochemical nitrogen fixation solely account for a tiny fraction of the fixed nitrogen supply. The Haber–Bosch process, which uses N₂ and H₂ as sources and iron-based compounds as the main material, is currently the main route for the synthesis of industrial ammonia. Nevertheless, this process requires a great deal of energy input while generating large emissions of by-products (such as carbon dioxide), which may cause environmental hazards. Hence, developing high-selectivity photocatalysts for nitrogen-reducing ammonia is challenging and interesting research ^[15][69]. Recently, Yuan et al. have demonstrated that loading Ru/CoS_x to g-C₃N₄ nanosheets can effectively activate N₂ molecules and facilitate the separation of light-induced electron-hole pairs in g-C₃N₄ ^[16][70]. In comparison with pure CN, Ru-Vs-CoS/CN shows obviously enhanced photocatalytic activity, reaching 1.28% apparent quantum efficiency at 400 nm and 0.042% solar-to-ammonia efficiency. The excellent nitrogen reduction reaction performance is attributed to the fact that the sulfur vacancies in CoS_x can effectively promote the selective chemisorption of N₂ molecules. In addition, an N₂ molecule is bridged against the side-on Ru-Co center by the undercoordination of Ru and Co atoms at the Ru/CoS_x interface. Furthermore, the plasmonic Ru/CoS_x interface enhances light absorption to generate energetic charge-carriers, accelerates charge separation and transfer, and therefore kinetically facilitates the fixation of N₂. This confirms that the presence of vacancies on the surface of cobalt sulfide-based nanomaterials exhibits excellent photocatalytic NRR performance, as it can modify the electronic structure, decrease the coordination number of surface atoms, facilitate the formation of dangling bonds, and greatly promote the formation of N₂ chemisorption and activation. The N₂-fixation mechanism indicates the hydrogen evolution reaction (HER) on Ru occurs easily due to its good free energy of hydrogen production (−0.07 eV). Meanwhile, the active hydrogen adsorption on Co and desorption on S limit the hydrogen evolution reaction (HER) on Ru.

Recent research shows that cobalt sulfide-based materials, such as CoS, CoS₂, and Co₃S₄, are important candidate catalysts for photocatalytic organic pollutants degradation ^{[17][18][19][20]}[71,72,73,74]. For instance, Co_2.67S₄ shows excellent photocatalytic degradation efficiency of methylene blue (MB) under UV, visible, and near-infrared irradiation ^[21][75]. The valence state change of cobalt ions effectively separates electrons from holes and accelerates electron transfer, thus enhancing the activity of photocatalytic degradation. In addition to single cobalt-based sulfide materials, cobalt sulfide, as a co-catalyst, can be combined with host semiconductors for photocatalytic degradation. For example, Tang et al. have designed a two-dimensional CoS/BiOBr heterojunction, which shows a 5.3-fold higher degradation rate as compared to pure BiOBr ^[22][76]. When the BiOBr and the CoS combine to construct the CoS/BiOBr heterojunction photocatalyst, the electrons on the CB of the CoS can be easily transferred to the CB of the BiOBr. In addition, the VB of BiOBr can oxidize glyphosate directly, producing small molecules or ions (PO₄³⁻, etc.). Simultaneously, some holes also migrate from BiOBr to CoS, leading to effective photogenerated charge carrier separation and thereby boosting the photocatalytic performance of the CoS/BiOBr composite.

Moreover, Zhang et al. have covered uniformly MoS₂ nanosheets on CoS₂ nanoparticles to construct CoS₂/MoS₂-nitrogen-doped graphene aerogels for photocatalytic organic pollutants degradation ^[23][77]. When MoS₂ is combined with CoS₂, the band gap of MoS₂ can be narrowed and the optical response range can be expanded. At the same time, CoS₂ can effectively accelerate the charge separation and increase the surface-active sites. Taking advantage of these advantages, the optimized three-dimensional CoS₂/MoS₂-nitrogen-doped graphene aerogel photocatalyst can degrade pollutants up to 97.1% within 60 minimums and still maintain 95.1% after three cycles.