Rational Design of S-Scheme CeO2/Bi2MoO6 Microsphere Heterojunction for Efficient Photocatalytic CO2 Reduction

Abstract

In the pursuit of efficient photocatalytic carbon dioxide (CO₂) conversion, the use of artificial semiconductors powered by solar energy offers great potential for simulating natural carbon cycling. However, the efficiency of photocatalytic CO₂ conversion remains suboptimal, primarily due to inadequate separation of photogenerated charges, which hinders the performance of semiconductor-based CO₂ reduction. Consequently, recent research efforts have focused on identifying ideal materials for CO₂ photocatalytic conversion. Among the candidate materials, the structure of Bi₂MoO₆ consists of alternating layers of (Bi₂O₂)²⁺ and perovskite-like (MoO₄)²⁻ layers with shared oxygen atoms between them. This inherent charge distribution within Bi₂MoO₆ creates an inhomogeneous electric field, facilitating the efficient separation of photogenerated charge carriers. The morphology and structure of a catalyst significantly influence the rate of recombination of photogenerated charge carriers. Research has shown that ultrathin Bi₂MoO₆ nanosheets, compared to other 2D and 3D structures of Bi₂MoO₆ materials, possess longer fluorescence lifetimes, providing more opportunities for the separation of photogenerated charge carriers. However, Bi₂MoO₆ still exhibits relatively low catalytic efficiency due to its insufficiently negative conduction band position (ranging between −0.2 and −0.4 V). To address this limitation, a viable strategy is to load a semiconductor with a more negatively positioned conduction band onto Bi₂MoO₆, creating an S-scheme heterojunction. In this study, Bi₂MoO₆ nanosheets were synthesized through a hydrothermal method, and simultaneously, CeO₂ nanoparticles were grown on their surfaces, forming an S-scheme heterojunction modified with Ce³⁺/Ce⁴⁺ ion bridges. Time-resolved photoluminescence (TRPL) and photoelectrochemical tests demonstrated the enhanced charge separation effect of this heterojunction. In situ X-ray photoelectron spectroscopy (In situ XPS) analysis and theoretical calculations further confirmed that photogenerated electrons follow an S-scheme mechanism, transferring from Bi₂MoO₆ to CeO₂. Experimental results revealed that the photocatalytic CO₂ reduction efficiencies of CeO₂/Bi₂MoO₆, Bi₂MoO₆, and CeO₂ were 65.3, 14.8, and 1.2 μmol∙g⁻¹∙h⁻¹, respectively. Compared to pure Bi₂MoO₆, the catalytic efficiency of the CeO₂/Bi₂MoO₆ composite catalyst for CO₂ photocatalytic reduction to CO improved by a factor of 3.12. This enhancement in photocatalytic CO₂ conversion performance can be attributed to the synergistic interaction between the S-scheme heterojunction and Ce³⁺/Ce⁴⁺ ion bridging, resulting in enhanced light absorption, efficient charge separation, and redox capabilities of the composite catalyst. This study offers valuable insights into the rational design and construction of novel S-scheme heterojunction photocatalysts.

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