Influence of lattice integrity and phase composition on the photocatalytic hydrogen production efficiency of ZnS nanomaterials

Natural marmatite photocatalyst for treatment of mineral processing wastewater to help zero wastewater discharge

Mineral processing wastewater (MPW) with large discharge and high toxicity affects environmental safety, and the realizing zero discharge of MPW is of great significance for reducing environmental pollution, saving water resources, and promoting the sustainable development of the mining industry. In this study, we reported natural marmatite (NM) as a low-cost and efficient photocatalyst for the treatment of MPW to help zero wastewater discharge. The photocatalytic activity of NM was evaluated by the removal of total organic carbon (TOC) from MPW under visible-light illumination, and the optimal degradation conditions were discussed. Results showed that superoxide free radicals (·O2-) were the dominant active species responsible for organic pollutants degradation, and 74.25% TOC removal was obtained after 120 min reaction under the optimum treatment conditions. Meanwhile, the wastewater treated by NM photocatalysis can be reused in the flotation system without adverse impact on the product index. Based on these findings, a model of zero wastewater discharge for flotation with the help of photocatalytic treatment was established, it indicated that the water of the whole system can be balanced without affecting the ore dressing index, which showed that visible light-driven photocatalyst has a promising application prospect in the treatment and recycling of industrial wastewater.

Crystal-Phase Engineering in Heterogeneous Catalysis

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

Recent Advances in Phase-Engineered Photocatalysts: Classification and Diversified Applications

Phase engineering is an emerging strategy for tuning the electronic states and catalytic functions of nanomaterials. Great interest has recently been captured by phase-engineered photocatalysts, including the unconventional phase, amorphous phase, and heterophase. Phase engineering of photocatalytic materials (including semiconductors and cocatalysts) can effectively affect the light absorption range, charge separation efficiency, or surface redox reactivity, resulting in different catalytic behavior. The applications for phase-engineered photocatalysts are widely reported, for example, hydrogen evolution, oxygen evolution, CO₂ reduction, and organic pollutant removal. This review will firstly provide a critical insight into the classification of phase engineering for photocatalysis. Then, the state-of-the-art development of phase engineering toward photocatalytic reactions will be presented, focusing on the synthesis and characterization methodologies for unique phase structure and the correlation between phase structure and photocatalytic performance. Finally, personal understanding of the current opportunities and challenges of phase engineering for photocatalysis will also be provided.

Intriguing physicochemical properties and impact of co-dopants on N-doped graphene oxide based ZnS nanowires for photocatalytic application

Superparamagnetic N-doped graphene oxide (GO)- with ZnS nanowires was synthesized by a one-step hydrothermal method by doping dilute amounts of Ga, Cr, In, and Al ions for water treatment and biomedical applications. In these experiments, to enhance their properties, 2% of Ga³⁺, In³⁺, and or Al³⁺ were codoped along with 2% Cr ions in these ZnS nanowires. The nanocomposite with the composition, In_0.02Cr_0.02Zn_0.96S, has better photocatalytic efficiency than other co-doped nanocomposites. The In (metalloids) and Cr (transition metal ion) are the best combinations to increase the magnetic properties which are beneficial for photocatalytic activity. Synthesized nanocomposite materials were characterized by several techniques such as X-ray diffraction, Field emission-scanning electron microscope (FESEM) with EDAX, vibrating sample magnetometer (VSM), UV-Vis, X-ray photoelectron spectroscopy (XPS), and fluorescence spectroscopy. The correlation of intriguing magnetic properties with their photocatalytic properties is also discussed. XPS was employed for the detection of surface defects, phase transformation, and the nature of chemical components present in the nanocomposites. The Frankel and substitutional defects have a direct impact on photocatalytic activity that was determined from the fluorescence (FL) spectroscopy. FL and XPS reveal that the Cr and In codoped composite has a higher percentage of defects hence its photocatalytic efficiency reaches 94.21%.

Coordination Complex Transformation-Assisted Fabrication for Hollow Chestnut-Like Hierarchical ZnS with Enhanced Photocatalytic Hydrogen Evolution

Hierarchical nanostructures (HNs) are possibly endowed with novel properties due to their complex three-dimensional (3D) structures. Here, we provide a novel stepwise growth strategy of Coordination Complex Transformation-Assisted Growth for fabricating HNs. By using this, we prepare a new wurtzite ZnS HNs-hollow chestnut-like hierarchical microspheres (HCHMs), which are mesoporous hollow microspheres with single crystalline nanorods arrayed densely and radially from the centre. The HCHMs formation depends on the stepwise decomposition of the two Zn2+ complexes ([Zn(en)m(H₂O)2(3-m)]2+ and [Zn(en)m(NH₃)2(3-m)]2+, natural number m < 3). As the reaction proceeds, [Zn2+] has been distinctly reduced due to the transformation from [Zn(en)m(H₂O)2(3-m)]2+ to [Zn(en)m(NH₃)2(3-m)]2+ with a high stability constant, leading to a low crystal growth rate to obtain single crystalline nanorods. Additionally, the generated bubbles (CO₂, NH₃) acting as a template can induce the generation of hollow structure. The as-prepared ZnS HCHMs show an enhanced photocatalytic hydrogen evolution activity due to the single crystalline wurtzite phase and the high surface area contributed by the hollow hierarchical structures, as well as the mesoporosity. The versatility of the coordination complex transformation-assisted growth strategy will open up new possibilities for fabricating HNs, especially for those transition metal ions with excellent complex capabilities.

Graphitic carbon nitride/CoTPP type-II heterostructures with significantly enhanced photocatalytic hydrogen evolution

CoTPP inhibits the recombination of electron-hole pairs through extracting holes from g-C₃N₄ thus dramatically enhancing photocatalytic hydrogen production under visible light irradiation.

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.

Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting

Over the past few decades, solar water splitting has evolved into one of the most promising techniques for harvesting hydrogen using solar energy. Despite the high potential of this process for hydrogen production, many research groups have encountered significant challenges in the quest to achieve a high solar-to-hydrogen conversion efficiency. Recently, ferroelectric materials have attracted much attention as promising candidate materials for water splitting. These materials are among the best candidates for achieving water oxidation using solar energy. Moreover, their characteristics are changeable by atom substitute doping or the fabrication of a new complex structure. In this review, we describe solar water splitting technology via the solar-to-hydrogen conversion process. We will examine the challenges associated with this technology whereby ferroelectric materials are exploited to achieve a high solar-to-hydrogen conversion efficiency.

The Study of Near-Band-Edge Property in Oxygen-Incorporated ZnS for Acting as an Efficient Crystal Photocatalyst

A wide gap semiconductor material has attracted attention as a heterophotocatalyst because of its light harvesting nature to be used in alternative energy production for the next generation. We, herein, grow and synthesize ZnS_(1-x)O _x series compounds using the chemical vapor transport (CVT) method with I₂ serving as the transport agent. Different crystals, such as undoped ZnS and oxygen-doped ZnS_0.94O_0.06 and ZnS_0.88O_0.12, revealed different bright palette emissions that were presented in photoluminescence spectra in our previous report. To study the electron-hole pair interaction of this sample series, the near-band-edge transitions of the sample series were characterized in detail by photoconductivity (PC) experiments. Additional results from surface photovoltage (SPV) spectra also detected the surface and defect-edge transitions from the higher oxygen-doped ZnS crystals. PC measurement results showed a red-shift in the bandgap with increasing incorporation of oxygen on ZnS. Consequently, the samples were subjected to photoirradiation by xenon lamp for the degradation of methylene blue (MNB) by acting as heterophotocatalysts. Undoped ZnS emerged as the best photocatalyst candidate with the fastest rate constant value of 0.0277 min^-1. In cubic {111} ZnS [{111} c-ZnS], the polarized Zn⁺ → S^- ions may play a vital role as a photocatalyst because of their strong electron-hole polarization, which leads to the mechanism for degradation of the MNB solution.

Combining ZnS with WS2 nanosheets to fabricate a broad-spectrum composite photocatalyst for hydrogen evolution

Compared with pure ZnS photocatalyst, the ZnS/WS₂ nanocomposite shows obviously enhanced hydrogen evolution activity and photostability.

Photocatalyzed Reduction of Bicarbonate to Formate: Effect of ZnS Crystal Structure and Positive Hole Scavenger

Zinc sulfide is a promising catalyst due to its abundance, low cost, low toxicity and conduction band position that enables the photoreduction of CO2 to formic acid. This study is the first to examine experimentally the photocatalytic differences between wurtzite and sphalerite under the parameters of size (micrometer and nanoscale), crystal lattice, surface area, and band gap on productivity in the photoreduction of HCO3(-). These photochemical experiments were conducted under air mass coefficient zero (AM 0) and AM 1.5 solar simulation conditions. We observed little to no formate production under AM 1.5, but found linear formate production as a function of time using AM 0 conditions. Compared to earlier reports involving bubbled CO2 in the presence of bicarbonate, our results point to bicarbonate as the species undergoing reduction. Also investigated are the effects of three hydroxylic positive hole scavengers, ethylene glycol, propan-2-ol (isopropyl alcohol, IPA) and glycerol on the reduction of HCO3(-). Glycerol, a green solvent derived from vegetable oil, greatly improved the apparent quantum efficiency of the photocatalytic reduction.

Visible light photocatalytic H2-production activity of wide band gap ZnS nanoparticles based on the photosensitization of grapheme

Visible light photocatalytic H(2) production from water splitting is considered an attractive way to solve the increasing global energy crisis in modern life. In this study, a series of zinc sulfide nanoparticles and graphene (GR) sheet composites were synthesized by a two-step hydrothermal method, which used zinc chloride, sodium sulfide, and graphite oxide (GO) as the starting materials. The as-prepared ZnS-GR showed highly efficient visible light photocatalytic activity in hydrogen generation. The morphology and structure of the composites obtained by transmission electron microscope and x-ray diffraction exhibited a small crystallite size and a good interfacial contact between the ZnS nanoparticles and the two-dimensional (2D) GR sheet,which were beneficial for the photocatalysis. When the content of the GR in the catalyst was 0.1%, the ZG0.1 sample exhibited the highest H(2)-production rate of 7.42 μmol h(−1) g(−1), eight times more than the pure ZnS sample. This high visible-light photocatalytic H(2) production activity is attributed to the photosensitization of GR. Irradiated by visible light, the electrons photogenerated from GR transfer to the conduction band of ZnS to participate in the photocatalytic process. This study presents the visible-light photocatalytic activity of wide bandgap ZnS and its application in H(2) evolution.

Defect Engineering and Phase Junction Architecture of Wide-Bandgap ZnS for Conflicting Visible Light Activity in Photocatalytic H₂ Evolution

ZnS is among the superior photocatalysts for H2 evolution, whereas the wide bandgap restricts its performance to only UV region. Herein, defect engineering and phase junction architecture from a controllable phase transformation enable ZnS to achieve the conflicting visible-light-driven activities for H2 evolution. On the basis of first-principle density functional theory calculations, electron spin resonance and photoluminescence results, etc., it is initially proposed that the regulated sulfur vacancies in wurtzite phase of ZnS play the key role of photosensitization units for charge generation in visible light and active sites for effective electron utilization. The symbiotic sphalerite-wurtzite phase junctions that dominate the charge-transfer kinetics for photoexciton separation are the indispensable configuration in the present systems. Neither ZnS samples without phase junction nor those without enough sulfur vacancies conduct visible-light photocatalytic H2 evolution, while the one with optimized phase junctions and maximum sulfur vacancies shows considerable photocatalytic activity. This work will not only contribute to the realization of visible light photocatalysis for wide-bandgap semiconductors but also broaden the vision on the design of highly efficient transition metal sulfide photocatalysts.

Insights into the structure-photoreactivity relationships in well-defined perovskite ferroelectric KNbO3 nanowires

1D perovskite-type orthorhombic KNbO₃ nanowires display RhB photodegradation about two-fold as large as their monoclinic counterparts and a synergy between ferroelectric polarization and electronic structure in photoreactivity enhancement is uncovered.

Structure–function correlations are a central theme in heterogeneous (photo)catalysis. In this study, the geometric and electronic structure of perovskite ferroelectric KNbO₃ nanowires with respective orthorhombic and monoclinic polymorphs have been systematically addressed. By virtue of aberration-corrected scanning transmission electron microscopy, we directly visualize surface photocatalytic active sites, measure local atomic displacements at an accuracy of several picometers, and quantify ferroelectric polarization combined with first-principles calculations. The photoreactivity of the as-prepared KNbO₃ nanowires is assessed toward aqueous rhodamine B degradation under UV light. A synergy between the ferroelectric polarization and electronic structure in photoreactivity enhancement is uncovered, which accounts for the prominent reactivity order: orthorhombic > monoclinic. Additionally, by identifying new photocatalytic products, rhodamine B degradation pathways involving N-deethylation and conjugated structure cleavage are proposed. Our findings not only provide new insights into the structure–photoreactivity relationships in perovskite ferroelectric photocatalysts, but also have broad implications in perovskite-based water splitting and photovoltaics, among others.

Effects of crystalline phase and morphology on the visible light photocatalytic H2-production activity of CdS nanocrystals

Optimized multi-armed CdS nanorods with a high percentage of wurtzite, prepared by a simple solvothermal route, exhibited high visible-light photocatalytic H₂-production performance.

Efficient visible-light photocatalytic hydrogen evolution and enhanced photostability of core/shell CdS/g-C3N4 nanowires

CdS/g-C3N4 core/shell nanowires with different g-C3N4 contents were fabricated by a combined solvothermal and chemisorption method and characterized by X-ray powder diffraction, scanning electronic microscopy, transmission electron microscopy, and UV-vis diffuse reflection spectroscopy. The photocatalytic hydrogen-production activities of these samples were evaluated using Na2S and Na2SO3 as sacrificial reagents in water under visible-light illumination (λ≥420 nm). The results show that after a spontaneous adsorption process g-C3N4 is successfully coated on CdS nanowires with intimate contact and can significantly improve the photocatalytic hydrogen-production rate of CdS nanowires, which reaches an optimal value of up to 4152 μmol h(-1) g(-1) at the g-C3N4 content of 2 wt %. More importantly, g-C3N4 coating can substantially reinforce the photostability of CdS nanowires even in a nonsacrificial system. The synergic effect between g-C3N4 and CdS, which can effectively accelerate the charge separation and transfer corrosive holes from CdS to robust C3N4, was proposed to be responsible for the enhancement of the photocatalytic activity and photostability. The possible conditions necessary for the synergic effect to work in a CdS/g-C3N4 core/shell configuration is also discussed.

Preparation of reduced graphene oxide by infrared irradiation induced photothermal reduction

We present a green and scalable route toward the formation of reduced graphene oxide (r-GO) by photothermal reduction induced by infrared (IR) irradiation, utilizing a bathroom IR lamp as the source of IR light. Thermogravimetric analysis, Raman, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirm the reduction of r-GO by IR light. Ultraviolet-visible-infrared spectra indicate that adsorption of IR light by original GO films is less than that of UV and visible light; but when GO is exposed to IR light, its adsorption of IR light increases very rapidly with time. The influence of the power density of the IR light on the structure and properties of r-GO was investigated. At high IR power density, the reduction reaction was so fierce that r-GO became highly porous due to the rapid degassing and exfoliation of GO sheets. The r-GO powder revealed good performance as the anode material for lithium ion batteries. At relatively low IR power density, the reduction process was found to be mild but relatively slow. Crack-free and uniform conductive r-GO thin films with a volume conductivity of 1670 S m(-1) were then prepared by two-step IR irradiation, i.e. first at low IR power density and then at high IR power density. Moreover, the r-GO films were also observed to exhibit obvious and reversible IR light-sensing behavior.

Enhanced photocatalytic hydrogen production activities of Au-loaded ZnS flowers

Au-nanoparticle-decorated ZnS nanoarchitectures were fabricated by a simple hydrothermal approach combined with a deposition-precipitation method. After the deposition-precipitation process, 5-nm Au nanoparticles were homogeneously dispersed on the ZnS surface. In addition, the band gap of ZnS was also narrowed by the incorporation of a small amount of Au(I) ions. The photocatalytic hydrogen production activities of all the samples were evaluated by using Na(2)S and Na(2)SO(3) as sacrificial reagents in water under a 350 W xenon arc lamp. The results show that the photocatalytic hydrogen production rate of ZnS nanoarchitectures can be significantly improved by loading Au cocatalysts and reaches an optimal value (3306 μmol h(-1) g(-1)) at the Au content of 4% wt. Although strong surface plasmon resonance (SPR) absorption of the Au nanoparticles was found in the Au-loaded samples, all of these samples exhibit no activities in the visible light region (λ > 420 nm). On the basis of this Au/ZnS system, the possible roles of Au deposition in improving the photocatalytic hydrogen production activity, especially the necessary condition for SPR effect of metal nanostructures to function in the visible-light photocatalysis, are critically discussed.

Synthesis and applications of graphene-based TiO(2) photocatalysts

Graphene is one of the most promising materials in the field of nanotechnology and has attracted a tremendous amount of research interest in recent years. Due to its large specific surface area, high thermal conductivity, and superior electron mobility, graphene is regarded as an extremely attractive component for the preparation of composite materials. At the same time, the use of photocatalysts, particularly TiO(2), has also been widely studied for their potential in addressing various energy and environmental-related issues. However, bare TiO(2) suffers from low efficiency and a narrow light-response range. Therefore, the combination of graphene and TiO(2) is currently one of the most active interdisciplinary research areas and demonstrations of photocatalytic enhancement are abundant. This Review presents and discusses the current development of graphene-based TiO(2) photocatalysts. The theoretical framework of the composite, the synthetic strategies for the preparation and modification of graphene-based TiO(2) photocatalysts, and applications of the composite are reviewed, with particular attention on the photodegradation of pollutants and photocatalytic water splitting for hydrogen generation.