ZnS particles were grown over Cu2O cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu2O cubes stay photocatalytically inactive. ZnS-decorated Cu2O octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photoexcitation. Surprisingly, Cu2O rhombic dodecahedra give greatly suppressed photocatalytic activity after ZnS deposition. Electron paramagnetic resonance spectra agree with these experimental observations. Time-resolved photoluminescence profiles provide charge-transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu2O­(110)/ZnS­(200) plane interface. A modified Cu2O–ZnS band diagram is presented. Density functional theory calculations generating plane-specific band energy diagrams of Cu2O and ZnS match well with the experimental results, showing that charge transfer across the Cu2O­(110)/ZnS­(200) plane interface would not happen. This example further illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.

Semiconductor photocatalysis under natural sunlight is an emergent area in contemporary materials research, which has attracted considerable attention toward the development of catalysts for environmental remediation using solar energy. A series of five-layer Aurivillius-phase perovskites, Bi 5 ATi 4 FeO 18 (A = Ca, Sr, and Pb), are synthesized for the first time. Rietveld refinements of the powder X-ray diffraction data indicated orthorhombic structure for the Aurivillius phases with Fe largely occupying the central octahedral layer, whereas the divalent cations (Ca, Sr, and Pb) are statistically distributed over the cubo-octahedral A-sites of the perovskite. The compounds with visible-light-absorbing ability ( E g ranging from ∼2.0 to 2.2 eV) not only exhibit excellent collective photocatalytic degradation of rhodamine B–methylene blue (MB) and rhodamine B–rhodamine 6G mixture at pH 2 but also show almost 100% photocatalytic selective degradation of MB from the rhodamine B–MB mixture at pH 11 under natural solar irradiation. The selectivity in the alkaline medium is believed to originate from the combined effect of the photocatalytic degradation of MB by the Aurivillius-phase perovskites and the photolysis of MB. Although a substantial decrease in MB adsorption from the mixed dye solution (MB + RhB) together with slower MB photolysis at the neutral pH makes the selective MB degradation sluggish, the compounds showed excellent photocatalytic degradation activity and chemical oxygen demand removal efficacy toward individual RhB (at pH 2) and MB (at pH 11) under sunlight irradiation. The catalysts are exceptionally stable and retain good crystallinity even after five successive cyclic runs without any noticeable loss of activity in both the acidic and alkaline media. The present work provides an important insight into the development of layered perovskite photocatalysts for collective degradation of multiple pollutants and selective removal of one or multiple pollutants from a mixture. The later idea may open up new possibilities for recovery/purification of useful chemical substances from the contaminated medium through selective photocatalysis.

Topotactic ion exchange in open-framework solids and oxides with layered and tunnel structures has resulted in the formation of a variety of metastable functional materials that are inaccessible otherwise. These ion exchanges are primarily limited to the above structure types because of the presence of labile ions as loosely held charge-compensating cations/anions as in the framework or tunnel structures or the lability of the ions/charged motifs in interlayer galleries of layered oxides. While such topotactic exchanges are common in the above structure types, they are rare in the three-dimensional (3D) close-packed structures based solely on corner- and/or edge-connected polyhedral networks. Herein, we demonstrate divalent iron exchange in a close-packed all-octahedral-coordinated trirutile oxide. This has enabled the transformation of a near-ultraviolet-absorbing diamagnetic insulating oxide into a visible-light-active paramagnetic semiconductor. An ion exchange of this kind may open up avenues for the development of metastable functional oxides with a variety of other 3D structures and diverse properties.

Ag3PO4 nanoparticles were lightly deposited on Cu2O cubes, octahedra, and rhombic dodecahedra for facet-dependent photocatalytic degradation of methyl orange. Cu2O cubes remain inactive after loading with Ag3PO4. Drastic photocatalytic activity suppression occurs for Ag3PO4-decorated Cu2O octahedra. Surprisingly, total deactivation results for Cu2O rhombic dodecahedra after Ag3PO4 deposition. Electron paramagnetic resonance spectra and electrochemical impendence measurements support the experimental observations. Interfacial Ag3PO4 lattice planes have been identified. A modified band diagram is constructed to understand photocatalytic activity decay or deactivation because of large interfacial band bending to create an unfavorable situation for charge carrier transport across the heterojunctions. This study further illustrates that photocatalytic suppression can be often observed in many semiconductor heterojunctions and multiple interfacial plane combinations mean that photocatalytic outcomes are unpredictable.

A tri-α-PbO 2 -type oxide, Li 0.08 Fe 0.46 SbWO 6 , is synthesized for the first time by topotactic ion exchange of LiSbWO 6 in an aqueous Fe(II) solution at 60 °C for 4 days under an argon atmosphere. The structure of the resulting compound is ascertained by Rietveld refinement of PXRD data, and the topotactic nature of exchange is established. The Fe−Sb tungstate showed a considerable reduction in the band gap to 2.06 from 3.05 eV of the parent LiSbWO 6 . The compound is metastable in nature; it undergoes exothermic structural transitions or decompositions at around 790 and 950 °C and cannot be stabilized by conventional solid-state reactions. The compound shows an antiferromagnetic transition at ∼20 K, which is ascribed to the presence of Fe(II) dimers in the edge-shared chains of Li/FeO 6 octahedra running parallel to the cdirection in the tri-α-PbO 2 structure. The applicability of the Fe-exchanged compound for visible-light-driven semiconductor photocatalysis is investigated by degradation of RhB dye. The topotactic ion exchange in the tri-α-PbO 2 structure is unique and rare owing to its three-dimensional quasi-ordered structure with corner-and edge-connected moieties; however, plenty of them are reported in other structure types, including layered, tunnel, and framework structures. The strategy will be useful in constructing ordered or pseudo-ordered transitionmetal arrays in three-dimensional structures with intriguing electronic properties.