Unraveling the Ni-Co synergy in bifunctional hydroxide cocatalysts for better cooperation of CO2 reduction and H2O oxidation in 2D S-scheme photosynthetic systems

Abstract

ABSTRACT

Layered transition metal hydroxides show distinct advantages in separately co-catalyzing CO₂ reduction and H₂O oxidation at the electron-accumulating and hole-accumulating sites of wrapped heterojunction photocatalysts, while concurrently preventing side reactions and photocorrosion on the semiconductor surface. Herein, Ni-Co bimetallic hydroxides with varying Ni/Co molar ratios (Ni_xCo_1–x(OH)₂, x = 1, 0.75, 0.5, 0.25, and 0) were grown in situ on a model 2D/2D S-scheme heterojunction composed of Cu₂O nanosheets and Fe₂O₃ nanoplates to form a series of Cu₂O/Fe₂O₃@Ni_xCo_1–x(OH)₂ (CF@NiCo) photocatalysts. The combined experimental and theoretical investigation demonstrates that incorporating an appropriate amount of Co into Ni(OH)₂ not only modulates the energy band structure of Ni_xCo_1–x(OH)₂, balances the electron- and hole-trapping abilities of the bifunctional cocatalyst and maximizes the charge separation efficiency of the heterojunction, but also regulates the d-band center of Ni_xCo_1–x(OH)₂, reinforcing the adsorption and activation of CO₂ and H₂O on the cocatalyst surface and lowering the rate-limiting barriers in the CO₂-to-CO and H₂O-to-O₂ conversion. Benefiting from the Ni-Co synergy, the redox reactions proceed stoichiometrically. The optimized CF@Ni_0.75Co_0.25 achieves CO and O₂ yields of 552.7 and 313.0 μmol g_cat^–1 h^–1, respectively, 11.3/9.9, 1.6/1.7, and 4.5/5.9-fold higher than those of CF, CF@Ni, and CF@Co. This study offers valuable insights into the design of bifunctional noble-metal-free cocatalysts for high-performance artificial photosynthesis.