Electronic Band Structure Engineering of Transition Metal Oxide-N,S-Doped Carbon Catalysts for Photoassisted Oxygen Reduction and Oxygen Evolution Catalysis

A novel "dual isolation of metal active sites" heterojunction engineering is developed in this work. The double isolation of the active sites is achieved through M-N-4 (M = Co, Fe) coordinating bonds in sulfonated-substituted metalloporphyrin (MTPPS) and electrostatic interactions of MTPPS absorbed with poly(3,4-ethylenedioxythiophene) networks. Subsequent "in situ copyrolysis" over the precursors ensures effective contact between p-type CoFe2O4 and n-type Fe2O3 species with n-type N,S-doping carbon support, respectively. This leads to p-n CoFe2O4-N,S-C and n-n Fe2O3-N,S-C heterojunctions. Meanwhile, CoFe2O4-N,S-C has a higher E-VB energy level (-6.22 eV) than Fe2O3-N,S-C (-6.31 eV), achieving lower energy barrier, and thus superior oxygen reduction reaction (ORR) performance with a higher half-wave potential (0.84 V) of CoFe2O4-N,S-C than Fe2O3-N,S-C (0.82 V). In addition, CoFe2O4-N,S-C also exhibits higher oxygen evolution reaction (OER) performance with 100 mV lower overpotential (0.41 V) than Fe2O3-N,S-C. The lower overpotential is a result of larger energy gap between E-CB energy level and Fermi level of CoFe2O4-N,S-C. This is the first demonstration of a novel p-n heterojunction with a special band structure that plays a key role in highly active ORR and OER bifunctional catalysis. Moreover, CoFe2O4-N,S-C displays significant photoelectrochemical enhancement upon light irradiation.