A DFT Study on S-Scheme Heterojunction Consisting of Pt Single Atom Loaded G-C3N4 and BiOCl for Photocatalytic CO2 Reduction

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.

Automated summary

In this study, the photocatalytic properties of a Pt-C3N4/BiOCl heterojunction photocatalyst were investigated using density functional theory calculations. The results showed that the heterojunction photocatalyst, composed of single-atom Pt-loaded g-C3N4 and BiOCl, exhibited enhanced photocatalytic activity for CO2 reduction. The introduction of Pt single-atom cocatalysts improved the CO2 reduction performance of the g-C3N4/BiOCl S-scheme photocatalysts by strengthening the interfacial electric field and reducing the energy barrier.