Towards a highly efficient simulated sunlight driven photocatalyst: a case of heterostructured ZnO/ZnS hybrid structure

Synthesis, Characterization, and Electronic Properties of ZnO/ZnS Core/Shell Nanostructures Investigated Using a Multidisciplinary Approach

ZnO/ZnS core/shell nanostructures, which are studied for diverse possible applications, ranging from semiconductors, photovoltaics, and light-emitting diodes (LED), to solar cells, infrared detectors, and thermoelectrics, were synthesized and characterized by XRD, HR-(S)TEM, and analytical TEM (EDX and EELS). Moreover, band-gap measurements of the ZnO/ZnS core/shell nanostructures have been performed using UV/Vis DRS. The experimental results were combined with theoretical modeling of ZnO/ZnS (hetero)structures and band structure calculations for ZnO/ZnS systems, yielding more insights into the properties of the nanoparticles. The ab initio calculations were performed using hybrid PBE0 and HSE06 functionals. The synthesized and characterized ZnO/ZnS core/shell materials show a unique three-phase composition, where the ZnO phase is dominant in the core region and, interestingly, the auxiliary ZnS compound occurs in two phases as wurtzite and sphalerite in the shell region. Moreover, theoretical ab initio calculations show advanced semiconducting properties and possible band-gap tuning in such ZnO/ZnS structures.

Band Gap Engineering of Newly Discovered ZnO/ZnS Polytypic Nanomaterials

We report on a new class of ZnO/ZnS nanomaterials based on the wurtzite/sphalerite architecture with improved electronic properties. Semiconducting properties of pristine ZnO and ZnS compounds and mixed ZnO1−xSx nanomaterials have been investigated using ab initio methods. In particular, we present the results of our theoretical investigation on the electronic structure of the ZnO1−xSx (x = 0.20, 0.25, 0.33, 0.50, 0.60, 0.66, and 0.75) nanocrystalline polytypes (2H, 3C, 4H, 5H, 6H, 8H, 9R, 12R, and 15R) calculated using hybrid PBE0 and HSE06 functionals. The main observations are the possibility of alternative polytypic nanomaterials, the effects of structural features of such polytypic nanostructures on semiconducting properties of ZnO/ZnS nanomaterials, the ability to tune the band gap as a function of sulfur content, as well as the influence of the location of sulfur layers in the structure that can dramatically affect electronic properties. Our study opens new fields of ZnO/ZnS band gap engineering on a multi-scale level with possible applications in photovoltaics, light-emitting diodes, laser diodes, heterojunction solar cells, infrared detectors, thermoelectrics, or/and nanostructured ceramics.

Enhanced Photocatalytic and Antibacterial Activities of K2Ti6O13 Nanowires Induced by Copper Doping

Cu-doped K2Ti6O13 (Cu–KTO) nanowires were prepared using a combination of sol–gel and hydrothermal methods to improve the photocatalytic and antibacterial performance of K2Ti6O13 (KTO) nanowires. The Cu–KTO nanowires maintained the monoclinic structure of KTO. The Cu2+ ions could enter into the lattice of KTO by substituting for certain Ti4+ ions and cause the formation of defects and oxygen vacancies. The UV–Visible absorption spectra showed that after Cu doping, the absorption edge of KTO moved to the visible region, indicating that the band gap decreased and the ability to absorb visible light was acquired. The photocatalytic properties of the Cu–KTO nanowires with different doping amounts were assessed by simulating the photodegradation of rhodamine B (RhB) under simulated sunlight irradiation. The 1.0 mol% Cu–KTO nanowires showed the best photocatalytic performance, and 91% of RhB was decomposed by these nanowires (the catalyst dose was only 0.3 g/L) within 5 h. The performance of the Cu–KTO nanowires was much better than that of the KTO nanowires. The Cu–KTO nanowires also showed high antibacterial activity for Escherichia coli (ATCC 25922) of up to 99.9%, which was higher than that of the pure KTO samples. Results proved that Cu doping is an effective means to develop multifunctional KTO nanomaterials. It can be used to degrade organic pollutants and remove harmful bacteria simultaneously in water environments.

Facile and scalable production of heterostructured ZnS-ZnO/Graphene nano-photocatalysts for environmental remediation

A facile and eco-friendly strategy is described for the synthesis of ZnS-ZnO/graphene heterostructured nano-photocatalysts for the first time. This solvent-free and technologically scalable method involves solid-state mixing of graphite oxide (GO), Zn salt and surfeit yet non-toxic elemental sulfur using ball-milling followed by thermal annealing. The as-formed hybrids are composed of uniformly distributed in-situ formed ZnS-ZnO nanoparticles simultaneously within the thermally reduced GO (graphene) matrix. A series of hybrid compositions with varying content of ZnS/ZnO and graphene were prepared and thoroughly characterized. Further, the effect of heterostructure composition on the photocatalytic properties was investigated under visible-light illumination. The synergistic ZnS-ZnO/graphene hybridization promoted the band-gap narrowing compared to the pristine ZnS nanoparticles. The ZnS:ZnO composition was controlled by graphite oxide under thermal treatment and observed to be a crucial factor in enhancement of photocatalytic activity. As a proof of concept, the phase optimized and surface enhanced ZnS-ZnO/graphene nano-photocatalysts was tested towards visible light driven photocatalytic degradation of environmentally harmful organic dyes and toxic phenol molecules from aqueous media. The presented cost-effective strategy provides high potential in large-scale production of heterostructured nano-photocatalysts for environmental remediation and photocatalytic greener production of hydrogen.

Surface crystal feature-dependent photoactivity of ZnO–ZnS composite rods via hydrothermal sulfidation

ZnO–ZnS core–shell composite rods were synthesized using a two-step facile hydrothermal methodology wherein different sulfidation durations were employed. The effects of sulfidation duration on the morphology and crystalline quality of ZnS shell layers on the surfaces of ZnO rods were investigated. A ZnS shell layer with visible granular features was obtained in the adequately controlled 3 h sulfidation process. A structural analysis demonstrated that the ZnS shell layers of ZnO–ZnS composite rods synthesized after 3 h sulfidation were in a well-defined crystalline cubic zinc blend phase. Moreover, optical properties revealed that these composite rods had a higher light-harvesting ability than those obtained after 1 and 2 h sulfidation. The density of surface crystal defects and the photoexcited charge separation efficiency of the composite rods were associated with changes in the microstructure of the synthesized ZnS shell layers. The optimal sulfidation duration of 3 h for the ZnO–ZnS composite rods resulted in the highest photocatalytic activity for the given photodegradation test conditions. The improved light harvesting and charge transport at the ZnO–ZnS heterointerface accounted for the enhanced photocatalytic activity of the ZnO–ZnS composite rods synthesized after 3 h sulfidation.

ZnO–ZnS core–shell composite rods were synthesized using a two-step facile hydrothermal methodology wherein different sulfidation durations were employed.

Enhanced photocatalytic hydrogen production from aqueous sulfide/sulfite solution by ZnO0.6S0.4 with simultaneous dye degradation under visible-light irradiation

Photocatalytic hydrogen (H₂) production was performed by visible-light-driven (VLD) ternary photocatalyst, zinc oxysulfide (ZnO_0.6S_0.4) in the presence of sulfide/sulfite (S₂^2-/SO₃^2-) sacrificing system, with simultaneous azo-dye Reactive Violet 5 (RV5) degradation. Enhancement in both RV5 degradation and H₂ production was achieved, with the promotion of H₂ production after decolorization of RV5. The effect of initial concentration of RV5 was found to be influential on the enhancement of H₂ during the simultaneous processes, with a maximum of 110% increase of H₂ produced. The mechanism of the simultaneous system was investigated by scavenger study and intermediate analysis, including Fourier transform-infrared (FTIR) spectroscopy and total organic carbon (TOC) analysis. It was confirmed that the partial degradation of RV5 and presence of dynamic organic intermediates contributed to the enhancement in H₂ production. The present study revealed the feasibility of developing VLD photocatalysis as a sustainable and environmentally friendly technology for concurrent organic pollutant degradation with energy generation.

Thiourea assisted hydrothermal synthesis of ZnS/CdS/Ag2S nanocatalysts for photocatalytic degradation of Congo red under direct sunlight illumination

Schematic illustration for the photodecomposition of Congo red dye using ZnS/CdS/Ag₂S nanocomposites under solar light.

Review on ZnO hybrid photocatalyst: impact on photocatalytic activities of water pollutant degradation

Zinc oxide (ZnO) is one of the most widely used benchmark standard photocatalysts in the field of environmental applications. However, the large band gap of ZnO and the massive recombination of photogenerated charge carriers, especially in its nanosize, limit the overall photocatalytic efficiency. This can be further overcome by modifying the electronic band structure of ZnO by hybridization with a narrow band gap material, including metal, metal oxide, carbon based, and polymeric based. Indeed, ZnO hybridization with the respective materials contributed to its sensitizer by shifting the absorption wavelength to the visible region of the spectrum. This review encompasses several advancements made in the mentioned aspects, and also some of the new physical insights related to the charge transfer events, such as charge carrier generation, trapping, detrapping, and their transfer to surface, are discussed for each strategy of the hybrid ZnO. The synergistic effects in the mixed polymorphs of ZnO and also the theories proposed for their enhanced activity are reported. The review also highlights the potential application of ZnO hybrid for different kinds of pollutants from different wastewater sources.

ZnO/ZnS heterostructures for hydrogen production by photoelectrochemical water splitting

ZnO/ZnS heterostructures anodized under stirring conditions and in glycerol/water electrolyte succeeded as being a photococatalyst for photoelectrochemical water splitting.

Band Gap Reduction in ZnO and ZnS by Creating Layered ZnO/ZnS Heterostructures

Wurtzite-type zinc oxide (ZnO) and zinc sulfide (ZnS) have electronic band gaps that are too large for light-harvesting applications. Using screened hybrid density-functional methods, we show that the band gaps of ZnO and ZnS can be dramatically reduced by creating layered ZnO/ZnS bulk heterostructures in which m contiguous monolayers of ZnO alternate with n contiguous monolayers of ZnS. In particular, the band gap decreases by roughly 40% upon substitution of every tenth monolayer of ZnS with a monolayer of ZnO (and vice versa) and becomes as low as 1.5 eV for heterostructures with m = 3 to m = 9 contiguous monolayers of ZnO alternating with n = 10 - m monolayers of ZnS. The predicted band gaps of layered ZnO/ZnS heterostructures span the entire visible spectrum, which makes these materials suitable for photovoltaic device engineering.

Reactive oxygen species (ROS) mediated enhanced anti-candidal activity of ZnS–ZnO nanocomposites with low inhibitory concentrations

Enhanced antifungal activity against the yeast species Candida albicans, Candida tropicalis and Saccharomyces cerevisiae was displayed by ZnS–ZnO nanocomposites prepared by a simple precipitation technique.

Novel ZnO–ZnS nanowire arrays with heterostructures and enhanced photocatalytic properties

The graphical abstract exhibits the SEM images and schematic representation of ZnO–ZnS heterostructure NW arrays, the schematic energy band diagram and the rates of the photocatalytic H₂ at different temperatures.

Fabrication of TiO2/porous carbon nanofibers with superior visible photocatalytic activity

TiO₂/PCNFs heterostructures were successfully fabricated by combining an electrospinning technique with a hydrothermal method, exhibiting enhanced visible photocatalytic activity.

A template-free facile approach for the synthesis of CuS–rGO nanocomposites towards enhanced photocatalytic reduction of organic contaminants and textile effluents

Copper sulfide–reduced graphene oxide nanocomposites were synthesized hydrothermally from copper nitrate and thiourea as precursor materials.

Hybrid ZnO/ZnS nanoforests as the electrode materials for high performance supercapacitor application

Heterostructured ZnO/ZnS nanoforests are prepared through a simple two-step thermal evaporation method at 650 °C and 1300 °C in a tube furnace under the flow of argon gas, respectively.

Compositionally controlled band gap and photoluminescence of ZnSSe nanofibers by electrospinning

A simple, non-toxic, low-priced, and reproducible manipulation, which meets the standards of green chemistry, is introduced for the synthesis of ZnS_xSe_1−x nanofibers. ZnS_xSe_1−x nanofibers have been prepared in the entire composition range from ZnSe to ZnS by using a low-cost wet-chemical method.