Advanced Interface Engineering in Gradient Core/Shell Quantum Dots Enables Efficient Photoelectrochemical Hydrogen Evolution

Semiconductor core/shell quantum dots (QDs) are considered promising building blocks to fabricate photoelectrochemical (PEC) cells for the direct conversion of solar energy into hydrogen (H2). However, the lattice mismatch between core and shell in such QDs results in undesirable defects and severe carrier recombination, limiting photo-induced carrier separation/transfer and solar-to-fuel conversion efficiency. Here, an interface engineering approach is explored to minimize the core-shell lattice mismatch in CdS/CdSexS1-x (x = 0.09-1) core/shell QDs (g-CSG). As a proof-of-concept, PEC cells based on g-CSG QDs yield a remarkable photocurrent density of 13.1 mA cm-2 under AM 1.5 G one-sun illumination (100 mW cm-2), which is ≈54.1% and ≈33.7% higher compared to that in CdS/CdSe0.5S0.5 (g-CSA) and CdS/CdSe QDs (g-CS), respectively. Theoretical calculations and carrier dynamics confirm more efficient carrier separation and charge transfer rate in g-CSG QDs with respect to g-CSA and g-CS QDs. These results are attributed to the minimization of the core-shell lattice mismatch by the cascade gradient shell in g-CSG QDs, which modifies carrier confinement potential and reduces interfacial defects. This work provides fundamental insights into the interface engineering of core/shell QDs and may open up new avenues to boost the performance of PEC cells for H2 evolution and other QDs-based optoelectronic devices.

Using rice husk ash as a SiO2 source in the preparation of SiO2/Nb2O5 and SiO2/ZnS heterostructures for photocatalytic application

This work presents the synthesis of SiO₂/Nb₂O₅ and SiO₂/ZnS heterostructures using the microwave-assisted hydrothermal (MAH) method, which is fast and has low temperature. The silica used in the synthesis was obtained by burning the rice husk without any pre- or post-treatments. The obtained samples were characterized using various techniques such as X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), Fourier transform infrared (FTIR), and UV-visible. The obtained silica was found to be amorphous, and the materials used for modification showed characteristic of the type of synthesis used. SEM images showed that Nb₂O₅ and ZnS interacted with the SiO₂ surface, filling the voids. In the photocatalytic process, the heterostructures showed enhanced decolorization efficiency for dyes such as rhodamine B (RhB) and methylene blue (MB) compared to SiO₂. For RhB, the silica decolorized approximately 24%, and for MB, it discolored approximately 27%; SiO₂/Nb₂O₅ showed 91.24% decolorization efficiency for RhB and 72.77% MB, while SiO₂/ZnS showed approximately 96% for RhB and 100% for MB. All samples were tested under the same conditions. This demonstrates that the use of rice husk residue not only improves the photocatalytic activity of heterostructures but also promotes the utilization of improperly discarded residues.

Facile synthesis of ZnO/ZnS hollow nanorods via Kirkendall effect with enhanced photocatalytic degradation of methylene blue

Because of the growing concerns about environmental issues, the search of proficient semiconductor catalysts for pollutants degradation from contaminated water is one of the interesting areas of research. Due to the larger surface area, hollow nanomaterials with hollow interior and outer thickness illustrate a class of significant nanostructured materials. The enhanced surface area provides remarkable applications of the hollow nanomaterials in catalysis. In Kirkendall effect, pores are formed owing to the diverse diffusion rates of two nanomaterials in a diffusion couple. Here, we have introduced the facile hydrothermal synthesis of hollow nanorods of ZnO/ZnS via Kirkendall effect using ZnO nanorods (NRs). The morphologies, optical properties, compositions, and crystal structures of the as synthesized materials are systematically studied using UV-vis, PXRD, FESEM, TEM, EDS, XPS, etc. The process of synthesis and growth mechanism of hollow NRs is suggested based on the Kirkendall effect. A hollow nanomaterial, envisaged being highly efficient for molecule adsorption on its surface, the as synthesized materials were used for the photocatalytic degradation of methylene blue (MB) dye. MB degradation efficiency of 96% within 60 min was performed over ZnO/ZnS hollow NRs, which was 2.6-fold greater than that of ZnO. The rate constant of ZnO/ZnS heterostructure was 0.045 min^-1, which was 5.5 times larger than that of bare ZnO. We have concluded our work in the directions towards the synthesis of various semiconductor hollow nanostructures for the varied catalytic reactions.

ZnO/ZnS-Polyvinyl Alcohol Hydrogel for Photocatalytic H2-Generation

The separation of nanoparticles from a solution-based photocatalytic reaction is a significant problem in practical applications. To address the issue, we developed a new photocatalyst composite based on ZnO-ZnS heterojunction (ZnOS) embedded in polyvinyl alcohol (PVA) hydrogel, which showed satisfactory results for photocatalyst recycling. PVA-ZnOS composite hydrogel was fabricated by freezing-induced gelation, which enabled the encapsulation of ZnOS nanoparticles into polymeric matrices. PVA hydrogel served as a promising candidate in photocatalytic applications due to its excellent properties such as high transparency, porosity, hydrophilicity, and stability under ultraviolet (UV) light. PVA-ZnOS hydrogel showed worthy activity in H2 generation from Na2S/Na2SO3 aqueous solution under UV radiation with a production rate of 18.8 µmol.h−1. PVA-ZnOS composite hydrogel is a separation-free photocatalyst, which is prospective in a solution-based photocatalytic reactor.

Transition metal sulfides meet electrospinning: versatile synthesis, distinct properties and prospective applications

One-dimensional (1D) electrospun nanomaterials have attracted significant attention due to their unique structures and outstanding chemical and physical properties such as large specific surface area, distinct electronic and mass transport, and mechanical flexibility. Over the past years, the integration of metal sulfides with electrospun nanomaterials has emerged as an exciting research topic owing to the synergistic effects between the two components, leading to novel and interesting properties in energy, optics and catalysis research fields for example. In this review, we focus on the recent development of the preparation of electrospun nanomaterials integrated with functional metal sulfides with distinct nanostructures. These functional materials have been prepared via two efficient strategies, namely direct electrospinning and post-synthesis modification of electrospun nanomaterials. In this review, we systematically present the chemical and physical properties of the electrospun nanomaterials integrated with metal sulfides and their application in electronic and optoelectronic devices, sensing, catalysis, energy conversion and storage, thermal shielding, adsorption and separation, and biomedical technology. Additionally, challenges and further research opportunities in the preparation and application of these novel functional materials are also discussed.

Heterojunction interface of zinc oxide and zinc sulfide promoting reactive molecules activation and carrier separation toward efficient photocatalysis

Heterojunction photocatalysts, which can alleviate the low carrier separation efficiency and insufficient light absorption capacity of a single catalyst, have received widespread attention. However, the specific interfacial structure of the heterojunction and its effect on the photocatalytic reaction is still unclear. Herein, a battery of zinc oxide/zinc sulfide (ZnO@ZnS) heterojunction microspheres with different degrees of sulfuration were successfully constructed via a facile hydrothermal method. The as-prepared photocatalysts shown decent aerobic nitric oxide (NO) oxidation performance under visible light irradiation, and the results of various characterization techniques illustrated that the superior photoactivity could be ascribed to the spatial separation of photoinduced electron-hole pairs due to the synergy of the internal electric field and the band offset. More importantly, density functional theory (DFT) calculations revealed that the heterojunction interface can significantly promote the generation of reactive oxygen species (ROS) and NO⁺ reaction intermediates and thus accelerate the photocatalytic reaction. Finally, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technology was used to time-dependently monitor the NO oxidation process, revealing the photocatalytic mechanism. This work investigated the role of the heterojunction interface in the gas-phase catalytic reaction, broadening the practical application of the ZnO@ZnS heterojunction.

A radially controlled ZnS interlayer on ultra-long ZnO-Gd2S3 core-shell nanorod arrays for promoting the visible photocatalytic degradation of antibiotics

Nanorod (NR) arrays offer commendable visible-light-driven photocatalytic performances. Herein, we describe the construction of a ternary ZnO-ZnS-Gd₂S₃ nanostructural array in which a sulfidation process is used to decorate a Gd₂S₃ shell layer with a ZnS interface over vapor-phase-grown vertically-aligned ZnO. With control over the shell-wall thickness, the shell layer of ∼25 nm wall thickness on the ultra-long ZnO NR arrays exhibited a higher catalytic efficiency close to 3.3, 2.0, 1.2, and 1.8 times those of the bare ZnO, the ZnO-ZnS, the Gd₂S₃-decorated (∼10 nm) and Gd₂S₃ shell-layered (∼40 nm) ZnO-ZnS core-shell structures, respectively. The core-shell geometry and the shell-wall thickness with maximized contact interface afforded increased light absorption in the visible region and effectively retarded the recombination rate of the photoinduced charge carriers by confining electrons and holes separately, thus providing advantages in terms of the degradation of the pharmaceutical residue tetracycline and the industrial pollutant 4-nitrophenol in wastewater.

ZnO@ZnS nanorod-array coated titanium: Good to fibroblasts but bad to bacteria

Cell-selective toxic titanium is highly desired in clinical dental practice. Herein, based on the in situ conversion of ZnO to ZnO@ZnS, nanorod-array structured coatings with a controllable release features of zinc (Zn), has been successfully fabricated by a two-step hydrothermal method to endow titanium surface with cell-selectivity, i.e. boosting the functions (attachment and migration) of human gingival fibroblasts (HGnFs) while acting against the invasion of pathogenic bacteria. The improved functions of HGnFs over the ZnO@ZnS nanorod-array were attributed to the material's optimized zinc release, which was decreased from an order of 3.5 mg L^-1 to about 0.3 mg L^-1 (within the first week). But more importantly, this concentration still had a high antibacterial efficacy up to 100% (against both the S. aureus and E. coli, 10⁷ CFU mL^-1). This study demonstrated that a ZnO@ZnS nanorod-array coating could be a promising strategy to endow titanium dental implants with improved soft tissue sealing and effectively reduce peri-implantitis.

Ultrasound-assisted synthesis of rGO/Sb4O5Cl2/Sb2S3 for a high photo-catalytic rate

The ternary chalcogenide photo-catalyst rGO–Sb₄O₅Cl₂–Sb₂S₃ presents excellent photo-catalysis performance for MO degradation and its photo-erosion is drastically inhibited.

Controllable synthesis of a 3D ZnS@MoO3 heterojunction via a hydrothermal method towards efficient NO purification under visible light

Heterojunctions can deliver superior photocatalytic efficiency via modulating the surface-interface reaction, enhancing light absorption and hindering charge carrier recombination.

An investigation of the photovoltaic parameters of ZnS grown on ZnO(101̄1)

The selective growth of ZnS on ZnO (zinc nitrate versus acetate precursors) affects the photovoltaic parameters when the material is used as a photoanode in solar cells.

Sputtering control of Ag2O decoration configurations on ZnO nanorods and their surface arrangement effects on photodegradation ability toward methyl orange

In this study, a combinational strategy for synthesizing ZnO nanorod arrays interlaced with Ag₂O particles was proposed. Hydrothermally derived ZnO nanorod templates were sputter coated with Ag₂O particles. The sputtered Ag₂O particles can be decorated on the surfaces of the ZnO nanorod arrays with a randomly dispersive or continuous coverage characteristic by controlling the sputtering duration. Structural analysis revealed the formation of satisfactory crystalline ZnO-Ag₂O composite nanorods through the hydrothermal and sputtering methods. The ZnO-Ag₂O composite nanorods exhibited a significantly enhanced photoactivity compared with that of pristine ZnO nanorods under light irradiation. Moreover, the Ag₂O content and the coverage feature of the ZnO-Ag₂O composite nanorods influence the photodegradation of methyl orange solution by the composite nanorods under light irradiation. The photodegradation efficiency of the ZnO nanorods was substantially enhanced when the Ag₂O particles were decorated on the surfaces in a dispersive manner. This can be attributed to the optimal content of Ag₂O particles and their randomly dispersive characteristic on the surface of the composite nanorods, which resulted in the efficient transfer of photocarriers and markedly suppressed the electron-hole recombination rate.

Highly selective surface adsorption-induced efficient photodegradation of cationic dyes on hierarchical ZnO nanorod-decorated hydrolyzed PIM-1 nanofibrous webs

Selectivity of catalysts toward harmful cationic pollutants in industrial wastewater remains challenging but is of crucial importance in environmental remediation processes. Here, we present a complex network of a hydrolyzed polymer of intrinsic microporosity (HPIM)-based electrospun nanofibrous web with surface functional decoration of ZnO nanorods (NRs) as a hierarchical platform for selective and rapid degradation of cationic dyes. Over a single species or binary mixtures, cationic dyes were selectively adsorbed by the HPIM surface, which then rapidly degraded under simultaneous photoirradiation through the ZnO NRs. Both HPIM and ZnO exhibited high electronegative surfaces, which induced the selectivity towards the cationic dyes and rapidly degraded the pollutants with the production of reactive oxygen species under photoirradiation. Further, as a free-standing web, the catalytic network could be easily separated and reused without any significant loss of catalytic activity after multiple cycles of use. The hierarchical platform of ZnO/HPIM-based heterostructures could be a promising catalytic template for selective degradation of synthetic dyes in mixed wastewater samples.

Two Hybrid Au-ZnO Heterostructures with Different Hierarchical Structures: Towards Highly Efficient Photocatalysts

A new paradigm for photocatalysts based on two different hierarchically structured honeycomb and porous cylindrical Au-ZnO heterostructures was successfully developed via a straightforward and cost-effective hydrothermal method under different preparation conditions, which can be promising for industrial applications. The photocatalytic performance of all as-prepared samples under the illumination of sunlight was evaluated by the photocatalytic degradation of rhodamine B (RhB) and malachite green (MG) aqueous solutions. The results show that the photocatalytic degradation efficiency of RhB and MG was 55.3% and 40.7% for ZnO, 95.3% and 93.4% for the porous cylindrical Au-ZnO heterostructure, and 98.6% and 99.5% for the honeycomb Au-ZnO heterostructure, respectively. Compared with those from the ZnO, the results herein demonstrate an excellent reduction in the photoluminescence and improvement in the photocatalysis for the Au-ZnO hybrids with different morphologies. These results were attributed not only to the greatly improved sunlight utilization efficiency due to the surface plasmon resonance (SPR) absorption of Au nanoparticles in the visible region coupled with the UV light utilization by the ZnO nanostructures and multi-reflections of the incident light in the pore structures of their interior cavities but also to the high charge separation efficiency and low Schottky barrier generated by the combination of Au nanoparticles and ZnO micromaterials. Moreover, the honeycomb Au-ZnO heterostructure had a high Au content, surface area and surface oxygen vacancy (O_V), which enabled photocatalytic properties that were higher than those of the porous cylindrical Au-ZnO heterostructures. In addition, two different formation mechanisms for the morphology and possible photocatalytic mechanisms are proposed in this paper.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

This review describes an in-depth overview and knowledge on the variety of synthetic strategies for forming metal sulfides and their potential use to achieve effective hydrogen generation and beyond.

Engineering Charge Transfer Characteristics in Hierarchical Cu₂S QDs @ ZnO Nanoneedles with p⁻n Heterojunctions: Towards Highly Efficient and Recyclable Photocatalysts

Equipped with staggered gap p-n heterojunctions, a new paradigm of photocatalysts based on hierarchically structured nano-match-shaped heterojunctions (NMSHs) Cu₂S quantum dots (QDs)@ZnO nanoneedles (NNs) are successfully developed via engineering the successive ionic layer adsorption and reaction (SILAR). Under UV and visible light illumination, the photocatalytic characteristics of Cu₂S@ZnO heterojunctions with different loading amounts of Cu₂S QDs are evaluated by the corresponding photocatalytic degradation of rhodamine B (RhB) aqueous solution. The results elaborate that the optimized samples (S3 serial specimens with six cycles of SILAR reaction) by means of tailored the band diagram exhibit appreciable improvement of photocatalytic activities among all synthesized samples, attributing to the sensitization of a proper amount of Cu₂S QDs. Such developed architecture not only could form p⁻n junctions with ZnO nanoneedles to facilitate the separation of photo-generated carries but also interact with the surface defects of ZnO NNs to reduce the electron and hole recombination probability. Moreover, the existence of Cu₂S QDs could also extend the light absorption to improve the utilization rate of sunlight. Importantly, under UV light S3 samples demonstrate the remarkably enhanced RhB degradation efficiency, which is clearly testified upon the charge transfer mechanism discussions and evaluations in the present work. Further supplementary investigations illustrate that the developed nanoscale Cu₂S@ZnO heterostructures also possess an excellent photo-stability during our extensive recycling photocatalytic experiments, promising for a wide range of highly efficient and sustainably recyclable photocatalysts applications.

Efficient Charge Separation in Plasmonic ZnS@Sn:ZnO Nanoheterostructure: Nanoscale Kirkendall Effect and Enhanced Photophysical Properties

Tetravalent Sn doped ZnO nanocrystals show excellent plasmonic absorbance in the visible region. Plasmonic ZnS@Sn:ZnO core-shell heterostructures have been synthesized by the anion exchange process where the O^2- is exchanged by S^2- anion. An increase of sulfur concentration induces interior hollow structures arising from the different diffusion rates of O^2- and S^2- ions. Gradual transformation of wurtztie ZnO nanocrystals in the anion exchange process stabilizes the wurtzite crystalline phase of ZnS. Carrier concentration and various types of intrinsic defect states in both ZnO and ZnS result in ultraviolet, blue, and green emissions. The coexistence of exciton-plasmon coupling in the same nanoparticle and efficient electron-hole separation in type II heterostructure increases the photocatalytic activity and photo current gain.

Synthesis of TiO2–ZnS nanocomposites via sacrificial template sulfidation and their ethanol gas-sensing performance

TiO₂–ZnS core–shell composite nanorods were synthesized by using ZnO as a sacrificial shell layer in a hydrothermal reaction.

ZnO–TiO2 composites and ternary ZnTiO3 electrospun nanofibers: the influence of annealing on the photocatalytic response and reusable functionality

By using understanding from the construction of composites to ternary-phased 1D NFs, we design a layout for ZnO–TiO₂ composite and ZnTiO₃ electrospun NFs with different band structures as a function of the annealing temperature with the possibility of defect states.

Citric acid-assisted hydrothermal synthesis of a self-modified Bi2SiO5/Bi12SiO20 heterojunction for efficient photocatalytic degradation of aqueous pollutants

The phase composition and weight ratio of Bi₁₂SiO₂₀ and Bi₂SiO₅ in the heterojunction can be tuned by simply adjusting the CA dosage.

Pd nanocube decoration onto flexible nanofibrous mats of core–shell polymer–ZnO nanofibers for visible light photocatalysis

Plasmonic enhancement for electron–hole separation efficiency and visible light photocatalysis was achieved by Pd nanocube decoration on a ZnO nanolayer coated onto electrospun polymeric (polyacrylonitrile (PAN)) nanofibers.