Synthesis of a Hexagonal Phase ZnS Photocatalyst for High CO Selectivity in CO2 Reduction Reactions

ZnS materials exhibit very negative potential of the conduction band, which is promising in photocatalytic reduction reactions. Unfortunately, previously reported ZnS materials for photocatalysis are mainly in the cubic phase, which produce high activity for H₂ evolutions and low activity toward CO₂ reductions. Herein, a hexagonal phase ZnS photocatalyst is fabricated for highly efficient CO₂ reduction reactions. The hexagonal ZnS nanoplates with the pure phase and well crystallization are synthesized via three-step solvothermal methods. In photocatalytic CO₂ reduction reactions under an aqueous solution environment, the hexagonal ZnS produces a CO selectivity of 21%, which is distinctly higher than that of 0.2% for commonly used cubic ZnS. The energy band study suggests that hexagonal ZnS possesses a slightly more negative conduction band and wider bandgap than cubic ZnS. Theoretical calculations reveal that the hexagonal ZnS possesses increased electron density around Zn atoms as that of cubic ZnS. Furthermore, hexagonal ZnS exhibits relatively reduced absorption energy of CO₂ reduction intermediates and increased absorption energy of H* as cubic ZnS, which result in better selectivity toward CO₂ reduction reactions. This study offers deep insights into the synthesis and electronic structure of hexagonal ZnS for CO₂ reduction reactions, which inspire the design of highly active photocatalysts for artificial photosynthesis.

Sulfidogenic Bioreactor-Mediated Formation of ZnS Nanoparticles with Antimicrobial and Photocatalytic Activity

The use of sulfidogenic bioreactors is a biotechnology trend to recover valuable metals such as copper and zinc as sulfide biominerals from mine-impacted waters. In the present work, ZnS nanoparticles were produced using "green" H₂S gas generated by a sulfidogenic bioreactor. ZnS nanoparticles were physico-chemically characterized by UV-vis and fluorescence spectroscopy, TEM, XRD and XPS. The experimental results showed spherical-like shape nanoparticles with principal zinc-blende crystalline structure, a semiconductor character with an optical band gap around 3.73 eV, and fluorescence emission in the UV-visible range. In addition, the photocatalytic activity on the degradation of organic dyes in water, as well as bactericidal properties against several bacterial strains, were studied. ZnS nanoparticles were able to degrade methylene blue and rhodamine in water under UV radiation, and also showed high antibacterial activity against different bacterial strains including Escherichia coli and Staphylococcus aureus. The results open the way to obtain valorous ZnS nanoparticles from the use of dissimilatory reduction of sulfate using a sulfidogenic bioreactor.

Prospects of ZnS and ZnO as smart semiconductor materials in light-activated antimicrobial coatings for mitigation of severe acute respiratory syndrome coronavirus-2 infection

We carried out theoretical and experimental analyses of ZnO and ZnS nanoparticles as smart semiconductor materials in light-activated antimicrobial coating for application in masks. We used low-cost hydrothermally processable precursors to direct the growth of the coatings on cotton fabric. Both ZnO and ZnS coatings had high reactivities as disinfection agents in photocatalysis reactions for the degradation of a methylene blue dye solution. Also, these coatings showed excellent UV protection properties. For understanding at the molecular level, the broad-spectrum biological activities of the ZnO and ZnS coatings against Fusarium Oxysporum fungi, Escherichia coli bacteria, and severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) virus and their variants, were investigated computationally. Hexagonal Zn6O6 and Zn6S6 clusters were used as models for the simulations through excited- and ground-state calculations. The theoretical findings show that changes in the local chemical environment in these excited systems have a profound impact on their physical and chemical properties and thus, can provide a better understanding to engineer new functional materials in light-activated antimicrobial coatings for the mitigation of SARS-CoV-2 infection.

Zinc-Based Nanoparticles Reduce Bacterial Biofilm Formation

Biofilm formation is important for microbial survival in hostile environments and a phenotype that provides microorganisms with antimicrobial resistance. Zinc oxide (ZnO) and Zinc sulfide (ZnS) nanoparticles (NPs) present potential antimicrobial properties for biomedical and food industry applications. Here, we aimed to analyze, for the first time, the bactericidal and antibiofilm activity of ZnS NPs against Staphylococcus aureus, Klebsiella oxytoca, and Pseudomonas aeruginosa, all medically important bacteria in developed countries. We compared ZnS NPs antimicrobial activity to ZnO NPs, which have been extensively studied. Using the colorimetric XTT reduction assay to observe the metabolic activity of bacterial cells and the crystal violet assay to measure biofilm mass, we demonstrated that ZnS and ZnO had similar efficacy in killing planktonic bacterial cells and reducing biofilm formation, with S. aureus being more susceptible to both therapeutics than K. oxytoca and P. aeruginosa. Crystal violet staining and confocal microscopy validated that Zn NPs inhibit biofilm formation and cause architectural damage. Our findings provide proof of principle that ZnS NPs have antibiofilm activity, and can be potentially used in medical and food industry applications, such as treatment of wound infections or package coating for food preservation. IMPORTANCE Zinc (Zn)-based nanoparticles (NPs) can be potentially used in medical and food preservation applications. As proof of principle, we investigated the bactericidal and antibiofilm activity of zinc oxide (ZnO) and zinc sulfide (ZnS) NPs against medically important bacteria. Zn-based NPs were similarly effective in killing planktonic and biofilm-associated Staphylococcus aureus, Klebsiella oxytoca, and Pseudomonas aeruginosa cells. However, S. aureus was more susceptible to these investigational therapeutics. Although further studies are warranted, our findings suggest the possibility of future use of Zn-based NPs in the treatment of skin infections or preservation of food.

A novel zinc sulfide impregnated carbon composite derived from zeolitic imidazolate framework-8 for sodium-ion hybrid solid-state flexible capacitors

The pyrolysis of metal-organic frameworks (MOFs) is an easy approach to prepare metal oxides as well as nanoporous carbon with high specific surface area. In the present work, for the first time, ZIF-8 (zeolitic imidazolate framework-8) has been pyrolyzed under different conditions to derive two products, i.e., highly porous carbon (C) and zinc sulfide (ZnS) infused carbon (ZnS@C). These two materials, i.e., nanoporous C and ZnS@C, have been investigated as a negative and a positive electrode, respectively, for potential application in a hybrid asymmetrical solid-state supercapacitor device (HASD). The controlled pyrolysis approach for the preparation of ZnS@C has yielded uniformly distributed ZnS nanoparticles inside the carbon structure. A 1.8 V HASD has been assembled, which delivered an excellent energy density of 38.3 W h kg^-1 (power density of 0.92 kW kg^-1) along with the greatly desirable feature of cycling stability. The proposed selection of materials as electrodes is promising to develop futuristic hybrid capacitors.

The Influence of CBD Parameters on the Energy Gap of ZnS Narcissus-Like Nanostructured Thin Films

Recently, the efficient preparation techniques of zinc sulfide (ZnS) nanostructured films have drawn great attention due to their potential applications in optoelectronics. In this study, the low-cost and high-yield chemical bath deposition (CBD) technique was used to deposit ZnS nanostructured thin films. The effect of various deposition parameters such as time, pH, precursor concentration, and temperature on the morphology and energy bandgap (Eg) of the prepared thin films were investigated. The characterization of the prepared thin films revealed the formation of polycrystalline ZnS with Narcissus-like nanostructures. Moreover, the optical characterization showed inverse proportionality between both the transmission and Eg of the nanostructured thin films and the variation of the deposition parameters. A range of different Eg values between 3.92 eV with 20% transmission and 4.06 eV with 80% transmission was obtained. Tuning the Eg values and transmission of the prepared nanostructured films by manipulating the deposition parameters of such an efficient technique could lead to applications in optoelectronics such as solar cells and detectors.

The Effect of the Deposition Method on the Structural and Optical Properties of ZnS Thin Films

ZnS is a wide band gap material which was proposed as a possible candidate to replace CdS as a buffer layer in solar cells. However, the structural and optical properties are influenced by the deposition method. ZnS thin films were prepared using magnetron sputtering (MS), pulsed laser deposition (PLD), and a combined deposition technique that uses the same bulk target for sputtering and PLD at the same time, named MSPLD. The compositional, structural, and optical properties of the as-deposited and annealed films were inferred from Rutherford backscattering spectrometry, X-ray diffraction, X-ray reflectometry, Raman spectroscopy, and spectroscopic ellipsometry. PLD leads to the best stoichiometric transfer from target to substrate, MS makes fully amorphous films, whereas MSPLD facilitates obtaining the densest films. The study reveals that the band gap is only slightly influenced by the deposition method, or by annealing, which is encouraging for photovoltaic applications. However, sulphur vacancies contribute to lowering the bandgap and therefore should be controlled. Moreover, the results add valuable information towards the understanding of ZnS polymorphism. The combined MSPLD method offers several advantages such as an increased deposition rate and the possibility to tune the optical properties of the obtained thin films.

Effect of electron-phonon interaction and valence band edge shift for carrier-type reversal in layered ZnS/rGO nanocomposites

The artificial stacking of nanohybrid films helps to enhance their properties and thus intrigues researchers to explore this possibility in emerging technologies. The layer-by-layer approach was used to fabricate samples of zinc sulfide/reduced graphene oxide (ZnS/rGO) by using spin coating technique. The structure and optoelectronic properties has been extensively studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV-VIS-NIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Hall measurements. Raman spectrum elucidates the phonon contribution of ZnS and breathing mode of κ-point phonons and sp² bonds of carbon atoms of rGO. The electron-phonon interactions reveal reduction in electron mobility and enhancement in holes contribution with rGO content leading to surface charge transfer doping (SCTD). XPS results explain the valence band edge and conduction band edge to form type-I band alignment to reconfirm carrier-type reversal. A change in the dispersion of refractive indices along with a small rise in the value of absorption coefficient in terahertz (THz) region for ZnS/rGO nanocomposite films has been observed. These results will open up new opportunities to furthering the science of this technologically important class of materials for future electronics.

On Tailoring Co-Precipitation Synthesis to Maximize Production Yield of Nanocrystalline Wurtzite ZnS

Pyroelectric materials can harvest energy from naturally occurring ambient temperature changes, as well as from artificial temperature changes, notably from industrial activity. Wurtzite- based materials have the advantage of being cheap, non-toxic, and offering excellent opto-electrical properties. Due to their non-centrosymmetric nature, all wurtzite crystals have both piezoelectric and pyroelectric properties. Nanocrystalline wurtzite ZnS, being a room temperature stable material, by contrast to its bulk counterpart, is interesting due to its still not well-explored potential in piezoelectric and pyroelectric energy harvesting. An easy synthesis method-a co-precipitation technique-was selected and successfully tailored for nanocrystalline wurtzite ZnS production. ZnS nanopowder with nanoparticles of 3 to 5 nm in size was synthesized in ethyl glycol under medium temperature conditions using ZnCl₂ and thiourea as the sources of Zn and S, respectively. The purified and dried ZnS nanopowder was characterized by conventional methods (XRD, SEM, TEM, TG and FTIR). Finally, a constructed in-house pilot plant that is able to produce substantial amounts of wurtzite ZnS nanopowder in an environmentally friendly and cost-effective way is introduced and described.

Phase-dependent structural, optical, phonon and UV sensing properties of ZnS nanoparticles

In order to investigate the influence of crystal structure on the photoresponse properties of ZnS nanoparticles under UV light exposure, electrical measurements on polyvinylpyrrolidone capepd cubic and hexagonal ZnS nanoparticles with average particle size 30-40 nm have been successfully carried out. Low-temperature solvothermal synthesis yields pure and highly crystalline w-ZnS nanoparticles with good stability. Different structures of prepared ZnS nanoparticles have been identified and confirmed with powder x-ray diffraction, high-resolution field-emission transmission electron microscopy and non-resonant Raman spectroscopic techniques, respectively. The energy band gap and emission properties of cubic and hexagonal ZnS have been studied using UV-vis absorption and photoluminescence spectroscopy. The interaction of phonons with free carriers and transport properties has a vital influence on the electronic and optical properties of nanomaterials. Hence, the resonant and non-resonant Raman spectral studies have been carried out for the prepared samples. Studies on the I-V characteristics and the mechanism behind the enhancement of photocurrent for the cubic and hexagonal ZnS nanoparticle-based photodetectors have shown an enhanced photocurrent for hexagonal ZnS nanoparticles rather than the cubic.

A Raman spectral probe on polar w-ZnS nanostructures and surface optical phonon modes in nanowires

In the present study, different morphologies of ZnS nanostructures have been synthesized through a hydrothermal method and their Raman spectral modes are investigated. Raman scattering from surface optical (SO) modes has been seen and identified as a strange shoulder band of LO at 340 cm-1 in nanowires (NWs) with a hexagonal wurtzite structure of ZnS in air medium. X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques have been used to confirm the hexagonal phase and the modulation in the surface during the growth process, which causes the translational symmetry breaking to activate the SO mode. The appearance of a strong SO mode in NWs has been obviously confirmed by the frequency downshift of the SO mode in different dielectric media with dielectric constants ranging from 1 to 2.56. The SO phonon mode shift due to the roughness in the NW faces has been estimated from the wave-vector that activates the SO mode and an approximate dielectric continuum (DC) model has been used to understand the SO modes in NWs. The surface perturbation responsible for the activation of the SO mode has been estimated and is compared with the surface modulation along the growth axis of the NW from the TEM images.

Organic-to-water dispersible Mn:ZnS–ZnS doped core–shell quantum dots: synthesis, characterization and their application towards optical bioimaging and a turn-off fluorosensor

Dot-in-dot core/shell Mn:ZnS/ZnS QDs as a good fluorescent agent for bioimaging and a turn-off fluorescent probe for detection of heavy metal ions.

Facile Microwave-assisted Synthesis Manganese Doped Zinc Sulfide Nanoparticles

Undoped and manganese doped zinc sulfide nanoparticles were produced by a fast, one-step and two-component microwave-assisted synthesis method. The solid phase retains around 78% of the initial Mn concentration, as revealed by Particle Induced X-ray Emission analysis. X-ray diffraction patterns confirmed zinc blende structure and in the transmission electron microscopy images, nanoparticles with triangular prism and cube shapes were observed, respectively with an average particle size around 7 nm and 13 nm. Dried powders of zinc sulfide nanoparticles, doped with 0.1 mol% and 0.7 mol% of Mn ions, show highest brilliance of luminescence under UV light. Increasing dopant levels resulted in a diminishing emission that vanishes above 4% of dopant concentration. The synthesis of ZnS was monitored and two main events were detected, one at 145 °C corresponding to the sol-gel phase formation and another after ~3 min at 300 °C where the precipitation of the zinc sulfide nanoparticles occurs.

Physico-chemical investigation of ZnS thin-film deposited from ligand-free nanocrystals synthesized by non-hydrolytic thio-sol-gel

Ultra-small and monodispersed zinc sulfide nanocrystals (NCs) (d ≤ 3 nm) have been prepared without the use of any surfactants by a synthetic route using benzyl mercaptan as a source of sulfur. The prepared NCs are dispersible in highly polar solvents and display the capability to closely pack-up in a bulky film. The NCs were characterized by TEM, XRD and UV-vis optical absorption as well as by steady-state and time-resolved photoluminescence (PL) spectroscopies. Uniform films of ZnS were spin-coated on glass and ITO-glass substrates using a NCs dispersion in N,N-dimethylformamide. The NCs and the resulting films were characterized by morphological and optoelectronic probing techniques such as AFM, SEM, diffuse reflectance, PL and photoelectron spectroscopy in air. These physical investigations confirmed that the chalcogenide NCs grown by this method have the potential to be utilized directly as photocatalysts and are potentially useful building-blocks/starting materials for the fabrication of semiconductor thin films for optoelectronic applications such as LED, luminescent screens, field effect transistor and solar cells. Insights on the chemistry involved in the NCs growth have been provided revealing that their formation proceeds through a mechanism involving a thioether elimination reaction.

Manipulating the Structural and Electronic Properties of Epitaxial SrCoO2.5 Thin Films by Tuning the Epitaxial Strain

Structure determines material's functionality, and strain tunes the structure. Tuning the coherent epitaxial strain by varying the thickness of the films is a precise route to manipulate the functional properties in the low-dimensional oxide materials. Here, to explore the effects of the coherent epitaxial strain on the properties of SrCoO_2.5 thin films, thickness-dependent evolutions of the structural properties and electronic structures were investigated by X-ray diffraction, Raman spectra, optical absorption spectra, scanning transmission electron microscopy (STEM), and first-principles calculations. By increasing the thickness of the SrCoO_2.5 films, the c-axis lattice constant decreases, indicating the relaxation of the coherent epitaxial strain. The energy band gap increases and the Raman spectra undergo a substantial softening with the relaxation of the coherent epitaxial strain. From the STEM results, it can be concluded that the strain causes the variation of the oxygen content in the BM-SCO2.5 films, which results in the variation of band gaps with varying the strain. First-principles calculations show that strain-induced changes in bond lengths and angles of the octahedral CoO₆ and tetrahedral CoO₄ cannot explain the variation band gap. Our findings offer an alternative strategy to manipulate structural and electronic properties by tuning the coherent epitaxial strain in transition-metal oxide thin films.

The Critical Role of Thioacetamide Concentration in the Formation of ZnO/ZnS Heterostructures by Sol-Gel Process

ZnO/ZnS heterostructures have emerged as an attractive approach for tailoring the properties of particles comprising these semiconductors. They can be synthesized using low temperature sol-gel routes. The present work yields insight into the mechanisms involved in the formation of ZnO/ZnS nanostructures. ZnO colloidal suspensions, prepared by hydrolysis and condensation of a Zn acetate precursor solution, were allowed to react with an ethanolic thioacetamide solution (TAA) as sulfur source. The reactions were monitored in situ by Small Angle X-ray Scattering (SAXS) and UV-vis spectroscopy, and the final colloidal suspensions were characterized by High Resolution Transmission Electron Microscopy (HRTEM). The powders extracted at the end of the reactions were analyzed by X-ray Absorption spectroscopy (XAS) and X-ray diffraction (XRD). Depending on TAA concentration, different nanostructures were revealed. ZnO and ZnS phases were mainly obtained at low and high TAA concentrations, respectively. At intermediate TAA concentrations, we evidenced the formation of ZnO/ZnS heterostructures. ZnS formation could take place via direct crystal growth involving Zn ions remaining in solution and S ions provided by TAA and/or chemical conversion of ZnO to ZnS. The combination of all the characterization techniques was crucial to elucidate the reaction steps and the nature of the final products.

Geometry strategy for engineering the recombination possibility of excitons in nanowires

We proposed a geometry strategy to engineer the radiative recombination possibility and thus the lifetime of excitons in nanowires of some photovoltaic semiconductors by using theoretical analysis and first-principles calculations. We demonstrated that the shape can engineer the symmetry of the wave-functions of band-edge states and influence the radiative recombination possibility. The nanowires need to satisfy the following requirements to forbid the radiative recombination possibility of band-edge excitons: (i) wurtzite structure; (ii) pxy-characterized wave-function of VBM state and (iii) C3v-symmetry shape. The geometrical symmetry results in the pxy-characterized C3v-symmetry wave-function of VBM state and leads to forbidden radiative recombination of band-edge excitons. The geometry strategy offers a flexible proposal to prolong the exciton lifetime, leaving optical absorption impregnable.

Identifying and rationalizing the morphological, structural, and optical properties of [Formula: see text]-Ag2MoO4 microcrystals, and the formation process of Ag nanoparticles on their surfaces: combining experimental data and first-principles calculations

We present a combined theoretical and experimental study on the morphological, structural, and optical properties of β-Ag2MoO4 microcrystals. β-Ag2MoO4 samples were prepared by a co-precipitation method. The nucleation and formation of Ag nanoparticles on β-Ag2MoO4 during electron beam irradiation were also analyzed as a function of electron beam dose. These events were directly monitored in real-time using in situ field emission scanning electron microscopy (FE-SEM). The thermodynamic equilibrium shape of the β-Ag2MoO4 crystals was built with low-index surfaces (001), (011), and (111) through a Wulff construction. This shape suggests that the (011) face is the dominating surface in the ideal morphology. A significant increase in the values of the surface energy for the (011) face versus those of the other surfaces was observed, which allowed us to find agreement between the experimental and theoretical morphologies. Our investigation of the different morphologies and structures of the β-Ag2MoO4 crystals provided insight into how the crystal morphology can be controlled so that the surface chemistry of β-Ag2MoO4 can be tuned for specific applications. The presence of structural disorder in the tetrahedral [MoO4] and octahedral [AgO6] clusters, the building blocks of β-Ag2MoO4, was used to explain the experimentally measured optical properties.

Structural, optical, and magnetic properties of NiMoO4 nanorods prepared by microwave sintering

We report on the structural, optical, and magnetic properties of α,β-NiMoO₄ nanorods synthesized by annealing the NiMoO₄:nH₂O precursor at 600°C for 10 minutes in a domestic microwave. The crystalline structure properties of α,β-NiMoO₄ were investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR), and Raman (FT-Raman) spectroscopies. The particle morphologies and size distributions were identified by field emission microscopy (FE-SEM). Experimental data were obtained by magnetization measurements for different applied magnetic fields. Optical properties were analyzed by ultraviolet-visible (UV-vis) and photoluminescence (PL) measurements. Our results revealed that the oxygen atoms occupy different positions and are very disturbed in the lattice and exhibit a particular characteristic related to differences in the length of the chemical bonds (Ni-O and Mo-O) of the cluster structure or defect densities in the crystalline α,β-NiMoO₄ nanorods, which are the key to a deeper understanding of the exploitable physical and chemical properties in this study.

Photochemical synthesis of size-tailored hexagonal ZnS quantum dots

ZnS quantum dots were synthetized at room temperature using a simple photochemical process involving ketyl radicals. Through the simple adjustment of reagent concentration, the method allows the control of nanoparticle size. Transmission electron microscopy confirmed that the nanomaterial adopts the hexagonal structure.