Metal-based cocatalysts have been widely applied in the photocatalytic CO₂ reduction reaction (CO₂RR) and the integration of cocatalysts/semiconductors is expected to enhance the selectivity of the catalytic reaction.

It is highly important to develop efficient and cheap photocatalysts for hydrogen production. Herein, a series of p–n heterojunction Mn_0.5Cd_0.5S/CuCo₂S₄ has been successfully synthesized for the first time by the hydrothermal impregnation method. Mn_0.5Cd_0.5S/CuCo₂S₄ loading with 12 wt % CuCo₂S₄ shows the highest H₂ evolution rate of 15.74 mmol h^–1 g^–1 under visible light (λ ≥ 420 nm) irradiation, which is about 3.15 and 15.28 times higher than that of bare Mn_0.5Cd_0.5S (4.99 mmol h^–1 g^–1) and CuCo₂S₄ (1.03 mmol h^–1 g^–1), respectively. In addition, it shows a relatively good stability during the five recycle tests, with about 20% loss of reaction rate compared to that of the first cycle. The superior photocatalytic performance is attributed to the effective separation and transfer of photogenerated charge carriers because of the formation of the p–n junction. The samples are systematically characterized by X-ray diffraction, ultraviolet–visible (UV–vis), diffuse reflectance spectroscopy, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray photoelectron spectroscopy, photoluminescence, EIS, and so on. UV–vis and EIS show that CuCo₂S₄ can effectively improve the visible light response of Mn_0.5Cd_0.5S/CuCo₂S₄ and promote the electron transfer from CuCo₂S₄ to the conduction band of Mn_0.5Cd_0.5S, so as to improve the photocatalytic efficiency. This study reveals that the p–n heterojunction Mn_0.5Cd_0.5S/CuCo₂S₄ is a promising photocatalyst to explore the photocatalysts without noble metals.

Hydroxyl groups facilitate the oxidation of SO₂ and NO by epoxy groups on carbon materials.