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

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

Kelvin–Helmholtz Instability Augmented by von Kármán Vortex Shedding during an Oil Droplet Impact on a Water Pool

The impact of an oil droplet on a water surface has been explored with the aid of computational fluid dynamics simulations. The study reveals the details of the spatiotemporal evolution of such a ternary system with a triplet of interfaces, e.g., air–water, oil–water, and oil–air, when the impact velocity of the oil droplet with the water surface is high. The oil droplet is found to flatten, spread, stretch, and eventually dewet on the water surface of the deep crater to show a host of interesting post-impact flow morphologies. Furthermore, at higher impact velocities, the formation of a biphasic oil–water crown is observed followed by the ejection of secondary water droplets from the crown tip due to capillary instability. The rapidly spreading oil film on the “crater” of the water surface is found to undergo Kelvin–Helmholtz instability before dewetting the same due to cohesion failure. Subsequently, the formation of an array of secondary oil droplets is observed during the process of dewetting. The dominant wavelength evaluated from the linear stability analysis of a representative flow system could faithfully predict the simulated spacing of dewetted oil droplets floating on the crater. Importantly, the variations in Laplace pressure around the curvatures of the undulatory interfaces along with sharp viscosity gradients across the three-phase contact line is found to engender interesting recirculation patterns, which eventually shed to form a coherent wake region in air near the crater. We also uncover the conditions under which the counter-rotating vortices shed along the oil–water interface resembling a von Kármán vortex street

Kelvin–Helmholtz Instability Augmented by von Kármán Vortex Shedding during an Oil Droplet Impact on a Water Pool

The impact of an oil droplet on a water surface has been explored with the aid of computational fluid dynamics simulations. The study reveals the details of the spatiotemporal evolution of such a ternary system with a triplet of interfaces, e.g., air–water, oil–water, and oil–air, when the impact velocity of the oil droplet with the water surface is high. The oil droplet is found to flatten, spread, stretch, and eventually dewet on the water surface of the deep crater to show a host of interesting post-impact flow morphologies. Furthermore, at higher impact velocities, the formation of a biphasic oil–water crown is observed followed by the ejection of secondary water droplets from the crown tip due to capillary instability. The rapidly spreading oil film on the “crater” of the water surface is found to undergo Kelvin–Helmholtz instability before dewetting the same due to cohesion failure. Subsequently, the formation of an array of secondary oil droplets is observed during the process of dewetting. The dominant wavelength evaluated from the linear stability analysis of a representative flow system could faithfully predict the simulated spacing of dewetted oil droplets floating on the crater. Importantly, the variations in Laplace pressure around the curvatures of the undulatory interfaces along with sharp viscosity gradients across the three-phase contact line is found to engender interesting recirculation patterns, which eventually shed to form a coherent wake region in air near the crater. We also uncover the conditions under which the counter-rotating vortices shed along the oil–water interface resembling a von Kármán vortex street

Kelvin–Helmholtz Instability Augmented by von Kármán Vortex Shedding during an Oil Droplet Impact on a Water Pool

The impact of an oil droplet on a water surface has been explored with the aid of computational fluid dynamics simulations. The study reveals the details of the spatiotemporal evolution of such a ternary system with a triplet of interfaces, e.g., air–water, oil–water, and oil–air, when the impact velocity of the oil droplet with the water surface is high. The oil droplet is found to flatten, spread, stretch, and eventually dewet on the water surface of the deep crater to show a host of interesting post-impact flow morphologies. Furthermore, at higher impact velocities, the formation of a biphasic oil–water crown is observed followed by the ejection of secondary water droplets from the crown tip due to capillary instability. The rapidly spreading oil film on the “crater” of the water surface is found to undergo Kelvin–Helmholtz instability before dewetting the same due to cohesion failure. Subsequently, the formation of an array of secondary oil droplets is observed during the process of dewetting. The dominant wavelength evaluated from the linear stability analysis of a representative flow system could faithfully predict the simulated spacing of dewetted oil droplets floating on the crater. Importantly, the variations in Laplace pressure around the curvatures of the undulatory interfaces along with sharp viscosity gradients across the three-phase contact line is found to engender interesting recirculation patterns, which eventually shed to form a coherent wake region in air near the crater. We also uncover the conditions under which the counter-rotating vortices shed along the oil–water interface resembling a von Kármán vortex street

Kelvin–Helmholtz Instability Augmented by von Kármán Vortex Shedding during an Oil Droplet Impact on a Water Pool

The impact of an oil droplet on a water surface has been explored with the aid of computational fluid dynamics simulations. The study reveals the details of the spatiotemporal evolution of such a ternary system with a triplet of interfaces, e.g., air–water, oil–water, and oil–air, when the impact velocity of the oil droplet with the water surface is high. The oil droplet is found to flatten, spread, stretch, and eventually dewet on the water surface of the deep crater to show a host of interesting post-impact flow morphologies. Furthermore, at higher impact velocities, the formation of a biphasic oil–water crown is observed followed by the ejection of secondary water droplets from the crown tip due to capillary instability. The rapidly spreading oil film on the “crater” of the water surface is found to undergo Kelvin–Helmholtz instability before dewetting the same due to cohesion failure. Subsequently, the formation of an array of secondary oil droplets is observed during the process of dewetting. The dominant wavelength evaluated from the linear stability analysis of a representative flow system could faithfully predict the simulated spacing of dewetted oil droplets floating on the crater. Importantly, the variations in Laplace pressure around the curvatures of the undulatory interfaces along with sharp viscosity gradients across the three-phase contact line is found to engender interesting recirculation patterns, which eventually shed to form a coherent wake region in air near the crater. We also uncover the conditions under which the counter-rotating vortices shed along the oil–water interface resembling a von Kármán vortex street

Kelvin–Helmholtz Instability Augmented by von Kármán Vortex Shedding during an Oil Droplet Impact on a Water Pool

The impact of an oil droplet on a water surface has been explored with the aid of computational fluid dynamics simulations. The study reveals the details of the spatiotemporal evolution of such a ternary system with a triplet of interfaces, e.g., air–water, oil–water, and oil–air, when the impact velocity of the oil droplet with the water surface is high. The oil droplet is found to flatten, spread, stretch, and eventually dewet on the water surface of the deep crater to show a host of interesting post-impact flow morphologies. Furthermore, at higher impact velocities, the formation of a biphasic oil–water crown is observed followed by the ejection of secondary water droplets from the crown tip due to capillary instability. The rapidly spreading oil film on the “crater” of the water surface is found to undergo Kelvin–Helmholtz instability before dewetting the same due to cohesion failure. Subsequently, the formation of an array of secondary oil droplets is observed during the process of dewetting. The dominant wavelength evaluated from the linear stability analysis of a representative flow system could faithfully predict the simulated spacing of dewetted oil droplets floating on the crater. Importantly, the variations in Laplace pressure around the curvatures of the undulatory interfaces along with sharp viscosity gradients across the three-phase contact line is found to engender interesting recirculation patterns, which eventually shed to form a coherent wake region in air near the crater. We also uncover the conditions under which the counter-rotating vortices shed along the oil–water interface resembling a von Kármán vortex street

Unleashing the Potential of Coupled Substituted 3D Niobate Perovskite Oxides in Cocatalyst-free Photocatalytic Hydrogen Evolution

Many 3D titanate and niobate perovskites are useful photocatalysts for hydrogen evolution but are active only under UV-light. Various strategies, including the use of noble metal-based catalysts/cocatalysts and heterojunction formation, are adopted to induce visible-light absorption and enhance photocatalytic activity. Here, a simple coupled-substitution approach in 3D niobate perovskite oxides, Na0.5Ca0.5M0.25Nb0.75O3 (M = Cr, Mn, Fe, and Co), is demonstrated to show enhanced hydrogen evolution without using heterojunction systems or noble metal catalysts/cocatalysts. The approach is innovative since the incorporation of a non-Jahn–Teller transition metal ion induces local octahedral distortion in the structure, while the Jahn–Teller active ions show negligible to no distortion in the presence of Nb5+, a d0 system, that is known to undergo second-order Jahn–Teller distortions. The effects of octahedral distortion on the structure, optical absorption, and in cocatalyst-free hydrogen evolution are discussed. The Cr compound shows a hydrogen evolution of ∼200 μmol g–1 h–1, the highest rate among all the compounds in the series, due to its fast charge transfer dynamics. The study offers important insights into the understanding of underlying factors influencing the photocatalytic activity of the perovskite-based materials for the development of practical photocatalytic systems