Photoelectrochemical response and electronic structure analysis of mono-dispersed cuboid-shaped Bi2Fe4O9 crystals with near-infrared absorption

Abstract

An n-type mono-dispersed cuboid-shaped Bi₂Fe₄O₉ semiconductor is synthesized via a hydrothermal method in concentrated NaOH solution. It is demonstrated that Bi₂Fe₄O₉ phase is formed by the reaction of Bi₂₅FeO₄₀ crystal and amorphous Fe(OH)₃. A doctor-blade technique is employed to determine the orientation of Bi₂Fe₄O₉ cuboids. It is found that the cuboids are preferentially grown along [001] direction with side facets (110) and (10) parallel to it. The UV-visible-near infrared absorption spectrum shows that besides two broad absorption edges in visible spectrum region, remarkable near-infrared absorption is also observed, indicating Bi₂Fe₄O₉ is a promising semiconductor capable of utilizing all solar band energy. Hence, steady and distinct photocurrents are measured to be 0.35 μA cm⁻², 7 μA cm⁻² and 33 μA cm⁻² under near-infrared irradiation, visible-light and simulated sunlight, respectively. First principle calculation is used to reveal the electronic structure of Bi₂Fe₄O₉ and the derived band gap is 1.23 eV, which agrees well with our experimental value of 1.29 eV. The calculation results also show that Bi₂Fe₄O₉ is an indirect bandgap semiconductor which is contrary to previous results. Besides, the extra absorption peak at around 700 nm in the UV-visible-near infrared spectrum should be attributed to the intervalence charge transfer induced by unevenly distributed [FeO₆]⁹⁻ octahedrons and [FeO₄]⁵⁻ tetrahedrons rather than the previously reported d–d transition which is both spin and Laporte forbidden. Our work provides deep insights into the nature of the band structure of Bi₂Fe₄O₉, and will facilitate the design of composite photoanodes that can response to near-infrared light.