Two-Dimensional Cobalt-based Materials: Preparation And Their Photo-/Electro-Catalysis Applications

Developing renewable electrochemical energy conversion and storage technologies and the CO₂ converted again into value-added hydrocarbon compounds offer promising solutions to address the global energy scarcity and the associated environmental issues.Today,Oxygen evolution reaction(OER)as the key part of hydrogen production by electrolysis of water technology and photocatalytic CO₂reduction become the key research directions.Moreover,Co-based catalysts(eg,Co₃O₄,etc.),because of their excellent OER or CO₂ reduction performance,are the focus of current research.Therefore,in order to deepen the understanding of Co-based catalysts,the OER and photocatalytic CO₂ reduction were seclected as probe-reactions in this paper,and the effects from valence,geometry and size of Co were systematically studyed on the probe-reactions.The main conclusions are as follows:Firstly,in order to indentify the real catalytic sites of Co²⁺and Co³⁺on the surface of Co₃O₄ nanomaterials for the OER,we have selected the(111)plane exposed with only Co²⁺and the(110)-B plane exposed with only Co³⁺as the real structure models and designed size-controllable Co₃O₄ hexagonal platelets with(111)exposed surfaces only containing Co²⁺as model catalysts.The theoretical and experimental results further reveal that the high catalytic activity is based on the existence of Co³⁺in Co₃O₄,meaning that the Co³⁺is responsible for the OER reaction.Secondly,to further investigate the effect of Co ions with different geometric structures on OER reactions,we selected the Co₃O₄-7.5,Zn Co₂O₄-7.5,Co Al₂O₄-7.5and Co O-7.5 as the catalysts,where both Co³⁺in the center of octahedron(O_h)and Co²⁺in the center of tetrahedron(T_d),only Co³⁺in the center of O_h,Co²⁺in the center of T_d,and Co²⁺in the center of O_h,respectively.The geometrical configurations of different metal ions in the prepared materials and their catalytic activities in OER reactions were systematically studied.The electrochemical results demonstrate that Co³⁺(O_h)sites in Co₃O₄are the dominant active sites and the best geometrical configuration for OER and Co²⁺(O_h)sites exhibit better activity than Co²⁺(T_d).Finally,we successfully prepared ultrathin Co₁-Ti_1-xO₂ two-dimensional nanosheet catalysts to study the effect of catalytic activity by single-atom Co doping,and applied them in photocatalytic CO₂ reduction reaction.The experimental results demonstrate that single-atom Co-based catalysts Co₁-Ti_1-xO₂ exhibit the best catalytic activity and stability.Moreover,the reason why the doping of Co in the three target catalysts can significantly improve the catalytic activity and the reaction mechanism were studied by the DFT.