Ionic liquid based dopant-free band edge shift in BiVO4 particles for photocatalysis under simulated sunlight irradiation

Abstract

Foreign elemental doping is a widely utilized strategy to modify the electronic structure of semiconductors. Herein, we present a dopant-free novel synthesis approach to control the electronic structure of a semiconductor. Utilizing butyl methyl imidazolium ([BMIM]Cl) and methoxyethyl methyl imidazolium ([M(MOE)Im][Cl]) chloride ILs, we prepared four different Bi and V based ILs: 3-butyl-1-methyl-1H-imidazol-3-ium vanadate [BMIm][VO₃], 3-(2-methoxyethyl)-1-methyl-1H-imidazol-3-ium vanadate [M(MOE)Im][VO₃], 3-butyl-1-methyl-1H-imidazol-3-ium tetrachlorobismate [BMIm][BiCl₄] and 3-(2-methoxyethyl)-1-methyl-1H-imidazol-3-ium tetrachlorobismate [M(MOE)Im][BiCl₄]. Owing to the bimetallic oxide nature of BiVO₄, these gels were mixed either with each other or with Bi/V commercial salts and simply heat-treated to obtain monoclinic BiVO₄. Depending on the IL, the bandgap energy of pure BiVO₄ will be redshifted (2.44 to 2.25 eV). The IL based synthesis induced oxygen vacancies and uplifted the BiVO₄ valence band edge as observed in the X-ray photoelectron spectroscopy (XPS). These effects were profound for IL anchored Bi; however, the side effects of this synthesis were chemisorption of a higher oxygen content and low reactivity of Bi with V to form an additional V₂O₅ phase. ILs acted as templates to form smooth spherical particles with improved crystallinity. [M(MOE)Im] based synthesis resulted in lower-order crystallinity and a large V–O bonding length of BiVO₄ compared to [BMIm] which may be ascribed to its lower-order cationic–anionic electrostatic attraction associated with the presence of oxygen in the ether-group for [M(MOE)Im]. [BMIm] cation-based synthesis suppressed photogenerated charge-recombination and resulted in a five-fold O₂ evolution of ∼30 μmol for 3 h (AM 1.5G illumination) compared to pure BiVO₄ which was better compared to the sample prepared by the conventional hydrothermal process. It also improved the photocurrent, and the MS plots have shown that the conduction band was not much affected; however, the defect density was larger for IL based synthesis.