Efficient COD Removal Coinciding with Dye Decoloration by Five-Layer Aurivillius Perovskites under Sunlight-Irradiation

An absorption edge in the visible region and a slow recombination of photogenerated electron–hole pairs in semiconductors are desirable for efficient sunlight-driven photocatalysis. One of the strategies to harvest visible-light instead of ultraviolet is to identify or develop new semiconductors with a band gap below 3 eV. The five-layer Aurivillius phase perovskites, Bi_6–<i>x</i>La_<i>x</i>Ti₃Fe₂O₁₈ (<i>x</i> = 0, 1), where the band gap ranges from ∼2.02–2.57 eV, have been identified as potential members. The compounds, Bi_6–<i>x</i>La_<i>x</i>Ti₃Fe₂O₁₈ (<i>x</i> = 0, 1), are synthesized by solid state reaction and characterized by PXD, FE-SEM, EDS, UV–vis DRS, and PL spectroscopy. La-substitution into Bi₆Ti₃Fe₂O₁₈ results in a sluggish recombination of photogenerated electron–hole pairs in Bi₅LaTi₃Fe₂O₁₈ as compared to the parent. The compounds showed remarkable photocatalytic performance toward Rhodamine B (RhB) degradation (more than 96%) within 30 min of sunlight-irradiation under mild acidic medium. Dye degradation is found to be coincident with COD removal. Moreover, the photocatalytic cycle test demonstrated the catalysts to be highly stable and reusable after five catalytic cycles without any noticeable decrease in the activity. Because of the fact that reactive oxygen species (ROS) are generated by sunlight-irradiation over the layered perovskite catalysts and that up to 95% COD removal takes place within 30 min, the catalysts may find practical application in dye/organic contaminant degradation or waste treatment that is sustainable without incurring any additional energy cost

Excellent Sun-Light-Driven Photocatalytic Activity by Aurivillius Layered Perovskites, Bi_5–<i>x</i>La_<i>x</i>Ti₃FeO₁₅ (<i>x</i> = 1, 2)

Aurivillius phase layered perovskites, Bi_5–<i>x</i>La_<i>x</i>Ti₃FeO₁₅ (<i>x</i> = 1, 2) are synthesized by solid-state reaction. The compounds are characterized by powder X-ray diffraction (PXD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), UV–vis diffuse reflectance (UV–vis DRS), and photoluminescence (PL) spectroscopy. UV–vis DRS data revealed that the compounds are visible light absorbing semiconductors with band gaps ranging from ∼2.0–2.7 eV. Photocatalytic activity studies by Rhodamine B (RhB) degradation under sun-light irradiation showed that these layered oxides are very efficient photocatalysts in mild acidic medium. Scavenger test studies demonstrated that the photogenerated holes and superoxide radicals (O₂^•–) are the active species responsible for RhB degradation over the Aurivillius layered perovskites. Comparison of PL intensity, dye adsorption and ζ-potential suggested that a slow e^––h⁺ recombination and effective dye adsorption are crucial for the degradation process over these photocatalysts. Moreover, relative positioning of the valence and conduction band edges of the semiconductors, O₂/O₂^•–, ^•OH/H₂O potential and HOMO–LUMO levels of RhB appears to be responsible for making the degradation hole-specific. Photocatalytic cycle tests indicated high stability of the catalysts in the reaction medium without any observable loss of activity. This work shows great potential in developing novel photocatalysts with layered structures for sun-light-driven oxidation and degradation processes largely driven by holes and without any intervention of hydroxyl radicals, which is one of the most common reactive oxygen species (ROS) in many advanced oxidation processes

Photocatalytic Activity Suppression of CdS Nanoparticle-Decorated Cu₂O Octahedra and Rhombic Dodecahedra

Wurtzite CdS nanoparticles have been lightly deposited on Cu₂O cubes, octahedra, and rhombic dodecahedra to examine facet effects on the interfacial charge transfer in a photocatalytic reaction. Instead of an expected photocatalytic activity enhancement on the basis of a favorable band alignment at the heterojunction, CdS-decorated Cu₂O octahedra and rhombic dodecahedra show drastically reduced photocatalytic activities. Further increasing the CdS deposition amount leads to complete suppression of photocatalytic activity. Cu₂O cubes remain inactive even after CdS deposition. Transmission electron microscopy analysis reveals epitaxial growth of the (101) planes of CdS on the (110) planes of a Cu₂O rhombic dodecahedron, whereas the (110) planes of CdS align parallel to the (111) planes of a Cu₂O octahedron. Because facet-dependent photocatalytic activity can be understood from different degrees of band bending at the crystal surfaces, significantly upward bending for the CdS-contacting planes can explain the observed photocatalytic inactivity. This work demonstrates that strong facet effects tuning the band energies of both semiconductors at the heterojunctions make the predictions of an enhanced photocatalytic activity, simply through bulk band energy alignment analysis, highly unreliable