S-scheme heterojunctions have become a hot topic in photocatalysis. Copper (Cu) compounds are a versatile family of photocatalytic materials, including oxides (CuO, Cu₂O), binary oxides (CuBi₂O₄, CuFe₂O₄), sulfides (Cu_xS, (1 ≤ x ≤ 2)), selenides (CuSe), phosphides (Cu₃P), metal organic frameworks (MOFs), etc. These materials are characterized by narrow bandgaps, large absorption coefficients, and suitable band positions. To further increase the efficiency of photoinduced charge separation, Cu-based photocatalytic materials are widely integrated into S-scheme heterojunctions and exploited for the hydrogen evolution reaction (HER), CO₂ reduction, H₂O₂ generation, N₂ fixation, and pollutant degradation. This review comprehensively discusses recent progress in Cu-based S-scheme heterojunctions, and highlights their considerable potential for targeted applications in sustainable energy conversion, environmental remediation, and beyond. The fundamentals of S-scheme charge transfer, the design principles and verification tools are summarized. Then, the review describes the Cu-based photocatalytic materials, categorized according to their chemical composition, and their integration in S-scheme heterojunctions for photocatalytic applications. In particular, the implications of the S-scheme charge transfer mechanism on promoting the catalytic activity of selected systems are analyzed. Finally, current limitations and outlooks are provided to motivate future studies on developing novel and advanced Cu-based S-scheme photocatalysts with high performance and studying the underlying photocatalytic mechanisms.

Solar conversion of CO₂ into energy-rich products is one of the sustainable solutions to lessen the global energy shortage and environmental crisis. Pitifully, it is still challenging to attain reliable and affordable CO₂ conversion. Herein, we demonstrate a facile one-pot approach to design core-triple shell Mn, C-codoped ZnO hollow spheres as efficient photocatalysts for CO₂ reduction. The Mn ions, with switchable valence states, function as “ionized cocatalyst” to promote the CO₂ adsorption and light harvesting of the system. Besides, they can capture photogenerated electrons from the conduction band of ZnO and provide the electrons for CO₂ reduction. This process is continuous due to the switchable valence states of Mn ions. Benefiting from such unique features, the prepared photocatalysts demonstrated fairly good CO₂ conversion performance. This work is endeavoured to shed light on the role of ionized cocatalyst towards sustainable energy production.

Electrocatalytic CO₂ reduction reaction (CO₂RR) to multi‐carbon products (C₂₊) in acidic electrolyte is one of the most advanced routes for tackling our current climate and energy crisis. However, the competing hydrogen evolution reaction (HER) and the poor selectivity towards the valuable C₂₊ products are the major obstacles for the upscaling of these technologies. High local potassium ions (K⁺) concentration at the cathode's surface can inhibit proton‐diffusion and accelerate the desirable carbon‐carbon (C−C) coupling process. However, the solubility limit of potassium salts in bulk solution constrains the maximum achievable K⁺ concentration at the reaction sites and thus the overall acidic CO₂RR performance of most electrocatalysts. In this work, we demonstrate that Cu nanoneedles induce ultrahigh local K⁺ concentrations (4.22 M) – thus breaking the K⁺ solubility limit (3.5 M) – which enables a highly efficient CO₂RR in 3 M KCl at pH=1. As a result, a Faradaic efficiency of 90.69±2.15 % for C₂₊ (FE_C2+) can be achieved at 1400 mA.cm⁻², simultaneous with a single pass carbon efficiency (SPCE) of 25.49±0.82 % at a CO₂ flow rate of 7 sccm.