Strategic design and mechanistic unveiling of Ni-based S-scheme heterojunctions for efficient CO2 photoreduction to C1 products: Coordination chemistry, spectroscopic evidence, and future directions

Tackling the rising anthropogenic CO₂ level aligns directly with the United Nations Sustainable Development Goals (SDGs), exclusively SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action). The CO₂ photoreduction into C₁ products, i.e., CO and CH₄, provides a favorable route to reduce greenhouse gas emissions. Among various modification strategies, the S-scheme heterojunction construction has a bright shine in the material chemistry field owing to its enhanced charge carrier separation efficiency, minimized charge carrier recombination, strong redox ability, and enhanced photocatalytic performance. The review significantly explores the recent advancements in Ni-based S-scheme junctions for CO₂ reduction, highlighting their interfacial charge transfer kinetics to achieve high selectivity. Firstly, the general CO₂ photoreduction mechanism was introduced to provide essential background knowledge. The article then probes the development of S-scheme mechanism and various classifications of S-scheme heterojunctions based on Ni-LDH, NiO, Ni(OH)₂, Ni-MOF, NiS, Ni₂P, etc., integrated with suitable semiconductors. Ni-based photocatalysts are a potential component within S-scheme heterojunctions because of their strong stability, suitable band potentials, cost-effectiveness, and earth-abundant nature. The advanced spectroscopic techniques, including DFT, KPFM, and ISI-XPS, are highlighted to validate the S-scheme charge transfer route. Moreover, the CO₂ reduction pathways were discussed with the primary focus on CO₂ adsorption, activation, intermediate stability, and C₁ product selectivity. Finally, the article outlines the respective challenges and future prospects in the field with the hope that this work offers strategic insight for developing photocatalytic CO₂ reduction technologies, supporting the pursuit of global carbon neutrality goals.