Free-Standing Atomically Thin ZnO Layers via Oxidation of Zinc Chalcogenide Nanosheets

Monolayer ZnO represents a class of new two-dimensional (2D) materials that are expected to exhibit unique optoelectronic properties and applications. Here we report a novel strategy to synthesize free-standing atomically thin ZnO layers via the oxidation of hydrothermally grown ultrathin zinc chalcogenide nanosheets. With micrometer-scaled lateral size, the obtained ultrathin ZnO layer has a thickness of ∼2 nm, and the layered structure still maintained well after high temperature oxidation. The thermal treatment strongly improves the crystal quality as well without inducing cracks or pinholes in the ultrathin layers. The atomically thin ZnO layers are highly luminescent with dominant green emission. High quality white light is obtained from the mixed phosphors containing the ZnO layers, exhibiting their potential as compelling ultraviolet-excited phosphors

Bioinspired Formation of 3D Hierarchical CoFe₂O₄ Porous Microspheres for Magnetic-Controlled Drug Release

Bioinspired by the morphology of dandelion pollen grains, we successfully prepared a template-free solution-based method for the large-scale preparation of three-dimensional (3D) hierarchical CoFe₂O₄ porous microspheres. Besides, on the basis of the effect of the reaction time on the morphology evolution of the precursor, we proposed an in situ dissolution–recrystallization growth mechanism with morphology and phase change to understand the formation of dandelion pollenlike microspheres. Doxorubicin hydrochloride, an anticancer drug, is efficiently loaded into the CoFe₂O₄ microspheres. The magnetic nanoparticles as field-controlled drug carriers offer a unique power of magnetic guidance and field-triggered drug-release behavior. Therefore, 3D hierarchical CoFe₂O₄ porous microspheres demonstrate the great potential for drug encapsulation and controlled drug-release applications

Effective Formation of Oxygen Vacancies in Black TiO₂ Nanostructures with Efficient Solar-Driven Water Splitting

Black TiO₂ nanomaterials have attracted considerable attention since they usually exhibit excellent photocatalytic activities. Herein, we report the facile preparation of black TiO₂ nanostructures with ultrathin hollow sphere morphology, high crystalline quality, small grain size (∼8 nm), and ultrahigh surface area (168.8 m² g^–1) through Al reduction. Electron paramagnetic resonance (EPR) spectra demonstrate the existence of oxygen vacancies in black TiO₂ nanostructures, which could increase the donor density and effectively promote the separation and transportation of photogenerated electron–hole pairs. The black TiO₂ nanostructures exhibit a high solar-driven hydrogen generation rate (56.7 mmol h^–1 g^–1) under the full spectrum of solar light, which is nearly 2.5 times than that of pristine TiO₂ nanostructures and superior to those kinds of black TiO₂ photocatalytic materials reported previously

Three-Dimensional Porous Nickel Frameworks Anchored with Cross-Linked Ni(OH)₂ Nanosheets as a Highly Sensitive Nonenzymatic Glucose Sensor

A facile and scalable in situ microelectrolysis nanofabrication technique is developed for preparing cross-linked Ni­(OH)₂ nanosheets on a novel three-dimensional porous nickel template (Ni­(OH)₂@3DPN). For the constructed template, the porogen of NaCl particles not only induces a self-limiting surficial hot corrosion to claim the “start engine stop” mechanism but also serves as the primary battery electrolyte to greatly accelerate the growth of Ni­(OH)₂. As far as we know, the microelectrolysis nanofabrication is superior to the other reported Ni­(OH)₂ synthesis methods due to the mild condition (60 °C, 6 h, NaCl solution, ambient environment) and without any post-treatment. The integrated Ni­(OH)₂@3DPN electrode with a highly suitable microstructure and a porous architecture implies a potential application in electrochemistry. As a proof-of-concept demonstration, the electrode was employed for nonenzymatic glucose sensing, which exhibits an outstanding sensitivity of 2761.6 μA mM^–1 cm^–2 ranging from 0.46 to 2100 μM, a fast response, and a low detection limit. The microelectrolysis nanofabrication is a one-step, binder-free, entirely green, and therefore it has a distinct advantage to improve clean production and reduce energy consumption

Porous CoO Nanostructure Arrays Converted from Rhombic Co(OH)F and Needle-like Co(CO₃)_0.5(OH)·0.11H₂O and Their Electrochemical Properties

Novel CoO nanostructure arrays on nickel foam with needle-like and rhombic morphologies have been prepared by using urea and hexamethylenetetramine as hydrolysis agents through fluoride-assisted hydrothermal method, respectively. The possible formation mechanism and effect factors of the novel arrays were systematically investigated by X-ray diffraction, scanning and transmission electron microscopies, N₂ sorption, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The precursor Co­(CO₃)_0.5(OH)·0.11H₂O or Co­(OH)F plays a crucial role in the formation of needle-like or rhombic arrays. As-prepared precursor arrays can be further converted to corresponding CoO arrays by annealing at 450 °C for 2 h without signification alteration of one-dimensional (1D) morphology. When tested as anodes for lithium ion batteries (LIBs) without the addition of other ancillary materials (carbon black and binder), the synthesized CoO arrays with needle-like and rhombic morphologies deliver ultrahigh initial discharge capacities of 1973.3 and 1447.9 mAh g^–1, respectively. In addition, they also maintain high reversible capacities of 710 and 719 mAh g^–1 at 0.2 C after 50 cycles, respectively

Effects of Organic Cation Length on Exciton Recombination in Two-Dimensional Layered Lead Iodide Hybrid Perovskite Crystals

In recent years, 2D layered organic–inorganic lead halide perovskites have attracted considerable attention due to the distinctive quantum confinement effects as well as prominent excitonic luminescence. Herein, we show that the recombination dynamics and photoluminescence (PL) of the 2D layered perovskites can be tuned by the organic cation length. 2D lead iodide perovskite crystals with increased length of the organic chains reveal blue-shifted PL as well as enhanced relative internal quantum efficiency. Furthermore, we provide experimental evidence that the formation of face-sharing [PbI₆]^4– octahedron in perovskites with long alkyls induces additional confinement for the excitons, leading to 1D-like recombination. As a result, the PL spectra show enhanced inhomogeneous broadening at low temperature. Our work provides physical understanding of the role of organic cation in the optical properties of 2D layered perovskites, and would benefit the improvement of luminescence efficiency of such materials

A H₂O₂ Oxidation Approach to Ti₃C₂/TiO₂ for Efficient Photocatalytic Removal of Distinct Organic Pollutants in Water

To develop versatile photocatalysts for efficient degradation of distinct organic pollutants in water is a continuous pursuit in environment remediation. Herein, we directly oxidize Ti3C2 MXene with hydrogen peroxide to produce C-doped anatase TiO2 nanowires with aggregates maintaining a layered architecture of the MXene. The Ti3C2 MXene provides a titanium source for TiO2, a carbon source for in situ C-doping, and templates for nanowire aggregates. Under UV light illumination, the optimized Ti3C2/TiO2 exhibits a reaction rate constant 1.5 times that of the benchmark P25 TiO2 nanoparticles, toward photocatalytic degradations of trace phenol in water. The mechanism study suggests that photogenerated holes play key roles on the phenol degradation, either directly oxidizing phenol molecules or in an indirect way through oxidizing first the surface hydroxyl groups. The unreacted Ti3C2 MXene, although with trace amounts, is supposed to facilitate electron transfer, which inhibits charge recombination. The unique nanostructure of layered aggregates of nanowires, abundant surface oxygen vacancies arising from the carbon doping, and probably the Ti3C2/TiO2 heterojunction guarantee the high photocatalytic efficiency toward removals of organic pollutants in water. The photocatalyst also exhibits an activity superior to, or at least comparable to, the benchmark P25 TiO2 toward photodegradations for typical persistent organic pollutants of phenol, dye molecule of rhodamine B, antibiotic of tetracycline, pharmaceutical wastewater of ofloxacin, and pesticide of N,N-dimethylformamide, when evaluated in total organic carbon removal

Simple Approach to Improving the Amplified Spontaneous Emission Properties of Perovskite Films

Organo-lead halide perovskite has emerged as a promising optical gain media. However, continuous efforts are needed to improve the amplified spontaneous emission (ASE) even lasing properties to evade the poor photostability and thermal instability of the perovskites. Herein, we report that simply through the coating of polymer layer, the CH₃NH₃PbBr₃ polycrystalline films prepared by a modified sequential deposition process show remarkably enhanced photoluminescence and prolonged decay lifetime. As a result, under nanosecond pulse pumping, the ASE threshold of the perovskite films is significantly reduced from 303 to 140 μJ/cm². Furthermore, the light exposure stability is improved greatly after the polymer coating. We confirmed that the polymer layer plays the roles of both surface passivation and symmetric waveguides. Our results may shed light upon the stable and sustained output of laser from perovskite materials

Violet Emission in ZnO Nanorods Treated with High-Energy Hydrogen Plasma

Violet photoluminescence was observed in high-energy hydrogen-plasma-treated ZnO nanorods at 13 K. The photoluminescence spectrum is dominated by a strong violet emission and a shoulder attributed to excitonic emission. The violet emission shows normal thermal behavior with an average lifetime of about 1 μs at 13 K. According to the time-resolved and excitation density-dependent photoluminescence, it was found that the violet emission is determined by at least two emitting channels, which was confirmed by annealing experiments. Evidence was also given that the violet emission is related to hydrogen. We suggested that the hydrogen-related complex defects formed under high-energy hydrogen plasma treatment are responsible for this violet emission

2D Behaviors of Excitons in Cesium Lead Halide Perovskite Nanoplatelets

Fundamental to understanding and predicting the optoelectronic properties of semiconductors is the basic parameters of excitons such as oscillator strength and exciton binding energy. However, such knowledge of CsPbBr₃ perovskite, a promising optoelectronic material, is still unexplored. Here we demonstrate that quasi-two-dimensional (quasi-2D) CsPbBr₃ nanoplatelets (NPLs) with 2D exciton behaviors serve as an ideal system for the determination of these parameters. It is found that the oscillator strength of CsPbBr₃ NPLs is up to 1.18 × 10⁴, higher than that of colloidal II–VI NPLs and epitaxial quantum wells. Furthermore, the exciton binding energy is determined to be of ∼120 meV from either the optical absorption or the photoluminescence analysis, comparable to that reported in colloidal II–VI quantum wells. Our work provides physical understanding of the observed excellent optical properties of CsPbBr₃ nanocrystals and would benefit the prediction of high-performance excitonic devices based on such materials