Photoluminescence and hydrogen evolution properties of ZnS:Eu quantum dots

In the era of Photonics, design and development of novel rare earth ion-doped quantum dots (QDs) for optoelectronic applications has gained significant interest owing to their outstanding characteristics. Simultaneously, the creation of a new class of photocatalytic materials on the nanoscale is also imperative for environmental purification. Thus, we report on wet chemical synthesis, the structural, morphological, and optical characteristics, fluorescence, and hydrogen evolution of ZnS:Eu (0, 2, 4, and 6 at%) QDs for optoelectronic and photocatalytic applications. Comprehensive structural studies depicted that Eu³⁺ ions were efficiently substituted into the host matrix and altered the original structure of the ZnS compound. The emission spectra of the ZnS:Eu QDs exhibited distinctive red fluorescence owing to the transition of dopant ions in ⁵D₀ - ⁷F₁, ⁵D₀ - ⁷F₂, ⁵D₀ - ⁷F₃, and ⁵D₀ - ⁷F₄ energy levels of the 4f orbital of the Eu³⁺ ions. Moreover, the photocatalytic properties of ZnS:Eu (6 at%) QDs possess better catalytic efficiency toward hydrogen evolution through a water splitting mechanism under simulated sunlight irradiation. The observed photocatalytic phenomenon in the synthesized samples agreed well with the luminescence properties exhibited by the QDs.     

Synthesis of ZnS nanoparticles of required size by precipitation in aerosol microdroplets

Zinc sulphide (ZnS) nanoparticles were prepared by an aerosol method from zinc acetate and sodium sulphide (Na₂S) aqueous solutions. Aqueous solution of zinc acetate was dispersed into the form of microdroplets, which were introduced by airflow to vigorously stirred aqueous solution of Na₂S, which was in excess. Microdroplets served as microreactors, so the reaction took place only in limited volume. Particle size distribution was studied by transmission electron microscopy and by dynamic light scattering measurements. In this work, the equation that allows us to predict the final size distribution of ZnS nanoparticles using exact concentration of zinc acetate was derived and ZnS nanoparticles with predicted mean particle diameter around 50 and 70?nm were successfully synthetised.     

ZnO/ZnS heterostructures grown on Zn foil substrate by hydrothermal method for photoelectrochemical water splitting

ZnO nanorod arrays (NRAs) grown on the Zn foil substrate in the dual alkaline solution by hydrothermal method were first successfully utilized for photoelectrochemical (PEC) water splitting. ZnO NRAs were sufficiently covered on the Zn foil substrate via the reasonable match of the pH value, reaction temperature and reaction time through intensive experimental works. To enhance PEC performance, ZnS was coated on the surfaces of ZnO NRAs by ion exchange. The morphological, structural, optical and PEC characteristics of as-grown ZnO NRAs and ZnO/ZnS heterostructures were investigated. Compared with the pristine smooth and hexagonal ZnO NRAs, the surfaces of ZnO/ZnS heterostructures become rough and rounded in shape. Through the sulfidation treatment, some ZnO/ZnS heterostructures with a diameter of no more than 60 nm break and fade away, leading to a decrease in number density of ZnO/ZnS heterostructures. The texture coefficients of (002) diffraction peaks of ZnO decrease from 2.32 to 2.158. ZnO/ZnS heterostructures exhibit efficient separation rate of electron-hole pairs and enhanced visible light absorption, and thus have higher photocurrent density and photoconversion efficiency. In the polysulfide electrolyte solution, ZnO/ZnS heterostructures show good stability.     

Synthesis and photoluminescence properties of ZnS nanobowl arrays via colloidal monolayer template

Two-dimensional Zinc sulfide (ZnS) nanobowl arrays were synthesized via self-assembled monolayer polystyrene sphere template floating on precursor solution surface. A facile approach was proposed to investigate the morphology evolution of nanobowl arrays by post-annealing procedure. Photoluminescence (PL) measurement of as-grown nanoarrays shows that the spectrum mainly includes two parts: a purple emission peak at 382 nm and a broad blue emission band centering at 410 nm with a shoulder around 459 nm, and a blue emission band at 440 nm was obtained after the annealing procedure. ZnS nanoarrays with special morphologies and PL emission are benefits to their promising application in novel photoluminescence nanodevice.     

Impact of gypsum supersaturated process water on the interactions between silica and zinc sulphide minerals

Flotation recovery and selectivity problems have been reported in the flotation of fine sulphide minerals in gypsum supersaturated process water. In this study, the effect of gypsum supersaturated solution on the interactions between silica and sphalerite (ZnS) minerals was examined by observing deposition behaviour of silica nanoparticles on sphalerite surface using a quartz crystal microbalance with dissipation (QCM-D). Significant deposition of silica nanoparticles on ZnS coated sensor surface was observed in the gypsum supersaturated solution, indicating consequential slime coating of silica fines on sphalerite mineral surface. Substantial deposition of silica nanoparticles on SiO₂ coated surface was also observed suggesting strong homo-aggregation of silica fines in the gypsum supersaturated solution. The interaction behaviour between silica–sphalerite and silica–silica is mainly attributed to the high calcium concentration of the gypsum supersaturated solution. Similar deposition behaviour of silica nanoparticles onto ZnS or SiO₂ coated sensor surface was observed in 800 ppm calcium solution, which is similar to the calcium concentration of the gypsum supersaturated solution. Colloidal force measurement between a silica particle and a fractured sphalerite surface or a silica wafer surface by an atomic force microscopy (AFM) revealed attractive van der Waals force between the mineral particles in both gypsum supersaturated solution and 800 ppm calcium solution. The high calcium concentration of the gypsum supersaturated solution induced the hetero-aggregation between silica and sphalerite, accounting for the observed decrease in flotation selectivity.     

Facile preparation of ZnS/CdS core/shell nanotubes and their enhanced photocatalytic performance

In this paper, ZnS/CdS core/shell nanotubes were successfully synthesized by combining hydrothermal treatment and ion exchange conversion, and the significant influence of CdS content in the shell on photo absorption and photocatalytic activity was also investigated. The core/shell nanotubes structure of CdS deposition on both sides of ZnS nanotube was confirmed by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The room temperature PL spectra of ZnS/CdS core/shell nanotubes indicated that CdS on the shell can reduce the recombination of photon-generated electron and hole. The photocatalytic activity tests prove that ZnS/CdS nanotubes have much higher photocatalytic hydrogen production activity than ZnS nanotube and CdS nanotube. Under the irradiation of visible light, the highest photocatalytic hydrogen production rate of 110 μmol h⁻¹ g⁻¹ is observed over the ZnS/CdS core/shell nanotubes with CdS/ZnS molar ratio of 1:4, which is about 11.02 and 5.56 times more active than ZnS nanotube and CdS nanotube, respectively. The improved performance of ZnS/CdS samples can be due to the strong photo response in the visible light region and the efficient separation of electron–hole pairs.     

Effect of porosity and hexagonality on the infrared transmission of spark plasma sintered ZnS ceramics

Tailoring phase transition and microstructural evolution during sintering is crucial for the fabrication of ZnS ceramics transparent to infrared (IR) radiation. Herein, we have described the phase transition, microstructure, and related IR transmission of spark-plasma-sintered ZnS ceramics in terms of sintering temperature and pressure. The pore characteristics of spark-plasma-sintered ZnS ceramics were evaluated using Mie scattering theory. Changes in hexagonality and residual pore characteristics of the microstructure affected IR transmission of the sintered specimens. High temperature and pressure condition of SPS were found to increase excessive hexagonal phase (>20%), mainly contributing to a transmittance decay in the range 2–4 μm.     

Atomistic tight-binding computations of the electronic properties of ZnSe/ZnS core/shell nanocrystals under applied electric field

The effect of applied electric field on the electronic properties of spherical ZnSe/ZnS core/shell nanocrystals of experimentally relevant size is investigated by the atomistic tight-binding theory. Using this model, the calculations show that a range of electronic properties, including the single-particle spectra, atomistic characters, charge densities, excitonic energies, ground-state coulomb energies, overlaps of the electron and hole wave functions and oscillation strengths, all depend on the strengths of the applied electric field. The spatial distributions of the electron and hole wave functions are induced by the applied electric field. The analysis demonstrates a clear manipulation of the electronic properties of ZnSe/ZnS core/shell nanocrystals by introducing and varying the applied electric field strengths. According to the comprehensive investigations, I suppose that these atomistic computations will be of prospective help for experimental works concentrated on the new optoelectronic devices based on the applied electric field.     

An efficient ZnS-UV photocatalysts generated in situ from ZnS(en)0.5 hybrid during the H2 production in methanol–water solution

Highly active ZnS-UV was obtained in situ from ZnS(en)_0.5 hybrid during the hydrogen formation using a methanol–water solution under UV irradiation. X-ray diffraction patterns and UV spectroscopy for both ZnS-UV and ZnS-400 obtained from the calcination of the ZnS(en)_0.5 hybrid showed similar structural and photophysical properties; however, the efficiency of the ZnS-UV semiconductor was 7 times higher (4825 μmol h⁻¹ g⁻¹) compared to the ZnS-400. The highest H₂ production was obtained using a UV lamp of very low intensity (2.2 mW cm⁻¹) and it is attributed to a quantum size effect caused by the slow elimination of ethylenediamine (en) in the structural ZnS layer during the UV irradiation.     

Photocatalytic hydrogen production by flower-like graphene supported ZnS composite photocatalysts

Flower-like graphene (FG) prepared by a transformer coupled plasma enhanced chemical vapor deposition method was used as support for the preparation of composite photocatalysts. Small ZnS particles were formed on the surface of FG by a hydrothermal process with ZnCl₂ and Na₂S precursors. The surface morphology, surface area, surface chemistry, crystalline property, optical properties, photogenerated current and photocatalytic hydrogen production activity of the FG-ZnS photocatalysts were investigated by using the X-ray diffraction, scanning electron microscope, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflectance spectra, photocurrent response, photoluminescence spectra, electrochemical impedance spectra and photocatalytic hydrogen production tests. The maximum hydrogen production rate of FG-ZnS composite photocatalyst ZS-G0.02 was 11600 μmol g^?1h^?1 under UV light irradiation at a graphene/ZnCl₂ precursor weight ratio of 0.02. The flower-like structure of FG may help the light absorption, adsorption of sacrificing agents in the solution, and separation of photogenerated carriers. In comparison with pristine ZnS photocatalyst, the FG-ZnS nanocomposites exhibits enhanced photocatalytic hydrogen production activity.