Octahedral-shaped perovskite CaCu3Ti4O12 with dual defects and coexposed {(001), (111)} facets for visible-light photocatalysis

The finite light absorption and rapid charge carrier recombination are two major bottlenecks in semiconductor photolysis. Introducing defects and exposed facets at photocatalysts can significantly improve photoefficiency due to the synergetic effects of extending light response and effective usage of photoinduced charge carriers. However, the prime influential factors in surface defects and predominantly exposed facets that determine photocatalytic performance are still unclear. Herein, to combine the advantages of defects and exposed facets, we, for the first time, report a surfactant-free synthesis and boosted photolysis of a single-crystal CaCu3Ti4O12 octahedron with dual defects (oxygen and metal deficiencies of Cu+ and Ti3+) and coexposed ({001}, {111}) facets by a facile molten salt approach for visible light antibiotic decomposition. The utilized molten salt KCl acts as a structure directing agent, decreasing surface energy to form the octahedron shapes and coexposed facets. Asobtained sample displays a superior efficiency enhancement for antibiotic degradation, approximately 69 times faster than that of its counterpart, defect free CaCu3Ti4O12. Density functional theory (DFT) calculations and photodecomposition results show that existing of coexposed facets contributes to spatial separation of carriers, while dual defects improve light absorption, affect charge density and thus promote carrier transfer. The vacancy formation energies of oxygen and titanium defects on exposed facets were calculated. Moreover, dual defects, especially the Ti3+ species are greatly affected by defect dipoles of CaCu3Ti4O12 with distorted units of CuO4 and TiO6, in which TiO6 clusters make greater contributions on dipole-induced internal field changes. Finally, two concepts of surface heterojunction and defect dipole are revealed to unveil the synergetic roles of surface chemistry and defect engineering in tuning a photoactivity of crystal based inorganic semiconductors. With dual defects, predominantly coexposed facets and unique crystal shapes, this work reports a strategy and direction for rational synthesis of highly efficient visible light driven catalysts and provides new insights for unearthing the surface chemistry and defect engineering of inorganic photocatalysts.