Modulation of Photocatalytic CO₂ Reduction by n-p Codoping Engineering of Single-Atom Catalysts

Transition metal (TM) single-atom catalysts (SACs) have been widely applied in photocatalytic CO₂ reduction. In this work, n-p codoping engineering is introduced to account for the modulation of photocatalytic CO₂ reduction on a two-dimensional (2D) bismuth-oxyhalide-based cathode by using first-principles calculation. n-p codoping is established via the Coulomb interactions between the negatively charged TM SACs and the positively charged Cl vacancy (V) in the dopant-defect pairs. Based on the formation energy of charged defects, neutral dopant-defect pairs for the Fe, Co, and Ni SACs (P⁰) and the -1e charge state of the Cu SAC-based pair (P^-1) are stable. The electrostatic attraction of the n-p codoping strengthens the stability and solubility of TM SACs by neutralizing the oppositely charged V defect and TM dopant. The n-p codoping stabilizes the electron accumulation around the TM SACs. Accumulated electrons modify the d-orbital alignment and shift the d-band center toward the Fermi level, enhancing the reducing capacity of TM SACs based on the d-band theory. Besides the electrostatic attraction of the n-p codoping, the P^-1 also accumulates additional electrons surrounding Cu SACs and forms a half-occupied d²_-² state, which further upshifts the d-band center and improves photocatalytic CO₂ reduction. The metastability of Cl multivacancies limits the concentration of the n-p pairs with Cl multivacancies (P (n > 1)). Positively charged centers around the P (n > 1) hinders the CO₂ reduction by shielding the charge transfer to the CO₂ molecule.