Abstract:

Photocatalytic CO₂ reduction to renewable hydrocarbon fuels provides a feasible protocol for alleviating the greenhouse effect and addressing energy shortage. However, the CO₂ reduction activity of a single-component photocatalyst is very low because of two problems. One is the fast recombination of photogenerated charge carriers, which leads to low photon efficiency, while the other is the large energy barrier to CO₂ activation. There have been considerable research efforts to develop photocatalysts with improved CO₂ reduction performance. For example, step-scheme (S-scheme) heterojunctions have been developed to improve charge carrier separation and enhance the redox abilities of photocatalysts. Single-atom metals have also been applied cocatalysts to optimize the reaction thermodynamics. Thus, the synergy between S-scheme heterojunctions and single-atom metal cocatalysts is anticipated to promote both charge carrier transfer and CO₂ reduction reaction processes. In this study, a Pt-C₃N₄/BiOCl heterojunction photocatalyst is modeled, composed of single-atom Pt-loaded g-C₃N₄ and BiOCl, and its photocatalytic properties are studied using density functional theory calculations. Its structure and electronic property are explored, and the process of CO₂ conversion is also simulated. The charge density difference results show that electrons in g-C₃N₄ are transferred to BiOCl owing to the higher Fermi level of g-C₃N₄ than that of BiOCl. Therefore, an interfacial electric field from g-C₃N₄ to BiOCl is established at the g-C₃N₄/BiOCl interface. Under light irradiation, charge carrier transfer in the g-C₃N₄/BiOCl composite is consistent with the S-scheme mechanism. Specifically, the photogenerated electrons in the CB of BiOCl recombine with the photogenerated holes in the VB of g-C₃N₄, while the photogenerated electrons in the CB of g-C₃N₄ and the photogenerated holes in the VB of BiOCl are retained. After the loading of Pt atom at each sixfold cavity of g-C₃N₄, the work function of g-C₃N₄ decreases, thereby enlarging the difference between the Fermi levels of the two semiconductors. Consequently, more electrons are transferred from Pt-C₃N₄ to BiOCl, and the strength of the interfacial electric field is increased. This enhanced electric field is beneficial to the S-scheme charge transfer in Pt-C₃N₄/BiOCl heterojunctions. Besides, based on the calculated variation in reaction energy, the rate-limiting step involved in CO₂ reduction on g-C₃N₄/BiOCl heterojunction is the hydrogenation of CO₂ to COOH, which has an energy barrier of 1.13 eV. After Pt loading, the hydrogenation of CO to HCO is the rate-limiting step and the corresponding energy increase is 0.71 eV. These results manifest that the introduction of Pt single-atom cocatalysts improves the CO₂ reduction performance of g-C₃N₄/BiOCl S-scheme photocatalysts by strengthening the interfacial electric field and reducing the energy barrier. This study provides guidance for constructing metal-atom-incorporated S-scheme heterojunction photocatalysts to realize efficient CO₂ reduction.

Key words: Step-scheme heterojunction, Density functional theory, Photocatalytic CO₂ reduction, Single-atom Pt, Carbon nitride, Bismuth oxychloride, Internal electric field