Modulation of Photocatalytic CO 2 Reduction by n - p Codoping Engineering of Single-Atom Catalysts.

Transition metal (TM) single-atom catalysts (SACs) have been widely applied in photocatalytic CO ₂ reduction. In this work, n - p codoping engineering is introduced to account for the modulation of photocatalytic CO ₂ 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 _Cl ) in the dopant-defect pairs. Based on the formation energy of charged defects, neutral dopant-defect pairs for the Fe, Co, and Ni SACs ( P _TM ⁰ ) and the -1 e charge state of the Cu SAC-based pair ( P _Cu ^-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 _Cl 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 _Cu ^-1 also accumulates additional electrons surrounding Cu SACs and forms a half-occupied d _x ² _{- y} ² state, which further upshifts the d -band center and improves photocatalytic CO ₂ reduction. The metastability of Cl multivacancies limits the concentration of the n - p pairs with Cl multivacancies ( P _TM@nCl (n > 1)). Positively charged centers around the P _TM@nCl (n > 1) hinders the CO ₂ reduction by shielding the charge transfer to the CO ₂ molecule.