Enhanced Photocatalytic Activity of Ultrathin Ba₅Nb₄O₁₅ Two-Dimensional Nanosheets

Anisotropic two-dimensional (2D) nanosheets of the layered perovskite, Ba₅Nb₄O₁₅, with thicknesses of 5–10 nm and lateral sizes of 300–1200 nm, were synthesized by a hydrothermal route. The influences of the 2D morphology of the material on the crystal and electronic structures, light absorption properties, and photocatalytic activity were investigated. The ultrathin nanosheets showed much-enhanced photocatalytic activity compared to both thick nanosheets (∼30 nm) and micrometer-sized particles for the evolution of H₂ from water splitting under UV light illumination. This enhanced activity is predominantly attributed to the larger surface area, higher optical absorption, and charge separation ability of the 2D nanosheet, which results from the variation of the local crystal structure arising from the ultrathin morphology of the Ba₅Nb₄O₁₅

BiVO₄/WO₃/SnO₂ Double-Heterojunction Photoanode with Enhanced Charge Separation and Visible-Transparency for Bias-Free Solar Water-Splitting with a Perovskite Solar Cell

Coupling dissimilar oxides in heterostructures allows the engineering of interfacial, optical, charge separation/transport and transfer properties of photoanodes for photoelectrochemical (PEC) water splitting. Here, we demonstrate a double-heterojunction concept based on a BiVO₄/WO₃/SnO₂ triple-layer planar heterojunction (TPH) photoanode, which shows simultaneous improvements in the charge transport (∼93% at 1.23 V vs RHE) and transmittance at longer wavelengths (>500 nm). The TPH photoanode was prepared by a facile solution method: a porous SnO₂ film was first deposited on a fluorine-doped tin oxide (FTO)/glass substrate followed by WO₃ deposition, leading to the formation of a double layer of dense WO₃ and a WO₃/SnO₂ mixture at the bottom. Subsequently, a BiVO₄ nanoparticle film was deposited by spin coating. Importantly, the WO₃/(WO₃+SnO₂) composite bottom layer forms a disordered heterojunction, enabling intimate contact, lower interfacial resistance, and efficient charge transport/transfer. In addition, the top BiVO₄/WO₃ heterojunction layer improves light absorption and charge separation. The resultant TPH photoanode shows greatly improved internal quantum efficiency (∼80%) and PEC water oxidation performance (∼3.1 mA/cm² at 1.23 V vs RHE) compared to the previously reported BiVO₄/WO₃ photoanodes. The PEC performance was further improved by a reactive-ion etching treatment and CoO_<i>x</i> electrocatalyst deposition. Finally, we demonstrated a bias-free and stable solar water-splitting by constructing a tandem PEC device with a perovskite solar cell (STH ∼3.5%)

Anomalous Behaviors of Visible Luminescence from Graphene Quantum Dots: Interplay between Size and Shape

For the application of graphene quantum dots (GQDs) to optoelectronic nanodevices, it is of critical importance to understand the mechanisms which result in novel phenomena of their light absorption/emission. Here, we present size-dependent shape/edge-state variations of GQDs and visible photoluminescence (PL) showing anomalous size dependences. With varying the average size (<i>d</i>_a) of GQDs from 5 to 35 nm, the peak energy of the absorption spectra monotonically decreases, while that of the visible PL spectra unusually shows nonmonotonic behaviors having a minimum at <i>d</i>_a = ∼17 nm. The PL behaviors can be attributed to the novel feature of GQDs, that is, the circular-to-polygonal-shape and corresponding edge-state variations of GQDs at <i>d</i>_a = ∼17 nm as the GQD size increases, as demonstrated by high-resolution transmission electron microscopy