An overview of semiconductors/layered double hydroxides composites: Properties, synthesis, photocatalytic and photoelectrochemical applications

Layered Double Hydroxides as Systems for Capturing Small-Molecule Air Pollutants: A Density Functional Theory Study

Air pollutants are usually formed by easily spreading small molecules, representing a severe problem for human health, especially in urban centers. Despite the efforts to stem their diffusion, many diseases are still associated with exposure to these molecules. The present study focuses on modeling and designing two-dimensional systems called Layered Double Hydroxides (LDHs), which can potentially trap these molecules. For this purpose, a Density Functional Theory (DFT) approach has been used to study the role of the elemental composition of LDHs, the type of counterion, and the ability of these systems to intercalate NO2 and SO2 between the LDH layers. The results demonstrated how the counterion determines the different possible spacing between the layers, modulating the internalization capacity of pollutants and determining the stability degree of the system for a long-lasting effect. The variations in structural properties, the density of states (DOS), and the description of the charge transfer have been reported, thus allowing the investigation of aspects that are difficult to observe from an experimental point of view and, at the same time, providing essential details for the effective development of systems that can counteract the spread of air pollutants.

Recent Developments in Nickel-Based Layered Double Hydroxides for Photo(-/)electrocatalytic Water Oxidation

Layered double hydroxides (LDHs), especially those containing nickel (Ni), are increasingly recognized for their potential in photo(-/)electrocatalytic water oxidation due to the abundant availability of Ni, their corrosion resistance, and their minimal toxicity. This review provides a comprehensive examination of Ni-based LDHs in electrocatalytic (EC), photocatalytic (PC), and photoelectrocatalytic (PEC) water oxidation processes. The review delves into the operational principles, highlighting similarities and distinctions as well as the benefits and limitations associated with each method of water oxidation. It includes a detailed discussion on the synthesis of monolayer, ultrathin, and bulk Ni-based LDHs, focusing on the merits and drawbacks inherent to each synthesis approach. Regarding the EC oxygen evolution reaction (OER), strategies to improve catalytic performance and insights into the structural evolution of Ni-based LDHs during the electrocatalytic process are summarized. Furthermore, the review extensively covers the advancements in Ni-based LDHs for PEC OER, including an analysis of semiconductors paired with Ni-based LDHs to form photoanodes, with a focus on their enhanced activity, stability, and underlying mechanisms facilitated by LDHs. The review concludes by addressing the challenges and prospects in the development of innovative Ni-based LDH catalysts for practical applications. The comprehensive insights provided in this paper will not only stimulate further research but also engage the scientific community, thus driving the field of photo(-/)electrocatalytic water oxidation forward.

Rational Design of MIL-68(In) Derived Multiple Sulfides with Well Confined Quantum Dots and the Promoted Photocatalytic Hydrogen Generation

Quantum dots (QDs) of metal sulfides were proven to be excellent cocatalysts in visible-light-driven photocatalytic reactions. Metal organic frameworks (MOFs) possess a 3D porous channel that effectively confines small QDs and preserves their high catalytic activity by preventing their aggregation. In order to precisely construct the ternary metal sulfides of ZnS/ZnIn2S4/In2S3 with well-maintained Zn-AgInS2 (ZAIS) QDs, an in situ sulfurization combining a subsequent Zn(II)-exchange strategy was employed in this work. First, the ZAIS QDs were incorporated into MIL-68(In), which were then used as the precursors to precisely construct the ternary metal sulfides of ZnS/ZnIn2S4/In2S3 with well maintained ZAIS QDs through an in situ sulfurization combining subsequent Zn(II)-exchange strategy. When the optimized nanocomposites (QDs@M-t-Zn, where t is the sulfurization time) were applied in visible light-induced photocatalytic hydrogen generation, the resulting QDs@M-24h-Zn showed a significantly improved hydrogen evolution rate of 448.96 μmol g-1 h-1, which values are clearly higher than those of MIL-68(In), QDs@MIL-68(In), and M-24h-Zn without the presence of ZAIS QDs. To elucidate the increased photocatalytic mechanism, the optical patterns and the batch electrochemical investigations were combined. It has been discovered that the matching band potentials and the close contact heterojunction enhance interface charge transfer, which in turn encourages photocatalytic hydrogen production. This study demonstrates the well-thought-out design of the uniform confinement architecture inherited from MOF QD-assisted multinary metal sulfides photocatalysts.

Recent progress in ZnCr and NiCr layered double hydroxides and based photocatalysts for water treatment and clean energy production

In pursuit of advancing photocatalysts for superior performance in water treatment and clean energy generation, researchers are increasingly focusing on layered double hydroxides (LDHs) which have garnered significant attention due to their customizable properties, morphologies, distinctive 2D layered structure and flexible options for modifying anions and cations. No review has previously delved specifically into ZnCr and NiCr LDH-based photocatalysts and therefore, this review highlights the recent surge in ZnCr and NiCr-based LDHs as potential photocatalysts for their applications in water purification and renewable energy generation. The structural and fundamental characteristics of layered double hydroxides and especially ZnCr-LDHs and NiCr-LDHs are outlined. Further, the various synthesis techniques for the preparation of ZnCr-LDHs, NiCr-LDHs and their composite and heterostructure materials have been briefly discussed. The applicability of ZnCr-LDH and NiCr-LDH based photocatalysts in tackling significant issues in water treatment and sustainable energy generation is the main emphasis of this review. It focuses on photocatalytic degradation of organic pollutants in wastewater, elucidating the principles and advancements for enhancing the efficiency of these materials. It also explores their role in H2 production through water splitting, conversion of CO2 into valuable fuels and NH3 synthesis from N2, shedding light on their potential for clean energy solutions. The insights presented herein offer valuable guidance for researchers working towards sustainable solutions for environmental remediation and renewable energy generation.

Type-1 α-Fe2O3/TiO2 photocatalytic degradation of tetracycline from wastewater using CCD-based RSM optimization

Antibiotic pollution in water is a growing threat to public health and the environment, leading to the spread of antimicrobial-resistant bacteria. While photocatalysis has emerged as a promising technology for removing antibiotics from water, its limited efficiency in the visible light range remains a challenge. In this study, we present a novel method for the photocatalytic degradation of tetracycline, the second most commonly used antibiotic worldwide, using α-Fe₂O₃/TiO₂ nanocomposites synthesized via rapid sonochemical and wet impregnation methods. The nanocomposites were characterised and tested using a range of techniques, including BET, TEM, FTIR, XRD, FESEM, EDS, and UV-Vis. The RSM-CCD method was also used to optimize the degradation process by varying four key variables (initial concentration, photocatalyst quantity, irradiation time, and pH). The resulting optimized conditions achieved a remarkable degradation rate of 97.5%. We also investigated the mechanism of photodegradation and the reusability of the photocatalysts, as well as the effect of light source operating conditions. Overall, the results demonstrate the effectiveness of the proposed approach in degrading tetracycline in water and suggest that it may be a promising, eco-friendly technology for the treatment of water contaminated with antibiotics.

Dyes Confinement in the Nano Scale and Converting Poly Vinyl Alcohol to Be Optical-Active Polymeric Nanocomposites with High Thermal Stability

In the present research, groups of nanolayered structures and nanohybrids based on organic green dyes and inorganic species are designated to act as fillers for PVA to induce new optical sites and increase its thermal stability through producing polymeric nanocomposites. In this trend, different percentages of naphthol green B were intercalated as pillars inside the Zn-Al nanolayered structures to form green organic-inorganic nanohybrids. The two-dimensional green nanohybrids were identified by X-ray diffraction, TEM and SEM. According to the thermal analyses, the nanohybrid, which has the highest amount of green dyes, was used for modifying the PVA through two series. In the first series, three nanocomposites were prepared depending on the green nanohybrid as prepared. In the second series, the yellow nanohybrid, which was produced from the green nanohybrid by thermal treatment, was used to produce another three nanocomposites. The optical properties revealed that the polymeric nanocomposites depending on green nanohybrids became optical-active in UV and visible regions because the energy band gap decreased to 2.2 eV. In addition, the energy band gap of the nanocomposites which depended on yellow nanohybrids was 2.5 eV. The thermal analyses indicated that the polymeric nanocomposites are thermally more stable than that of the original PVA. Finally, the dual functionality of organic-inorganic nanohybrids that were produced from the confinement of organic dyes and the thermal stability of inorganic species converted the non-optical PVA to optical-active polymer in a wide range with high thermal stability.

Optimized fabrication of Cu-doped ZnO/calcined CoFe‒LDH composite for efficient degradation of bisphenol a through synergistic visible-light photocatalysis and persulfate activation: Performance and mechanisms

A novel magnetically separable Cu/ZnO/CoFe‒CLDH composite, whose synthesis was optimized using the Taguchi approach, was optimally synthesized by hydrothermally coupling Cu-doped ZnO and calcined CoFe-LDH. The synthesized Cu/ZnO/CoFe‒CLDH was applied to construct a synergistic process of integrating visible-light photocatalysis (VPC) with persulfate activation (PSA) and to degrade bisphenol A (BPA). Various characterizations proved that Cu/ZnO/CoFe‒CLDH possessed excellent physicochemical, optoelectronic and magnetic properties, thereby enhancing the catalytic performance. The Cu/ZnO/CoFe‒CLDH composite achieved highly efficient BPA degradation during the synergistic VPC‒PSA process, and its reaction rate constant (0.74 h^-1) was 6.17-, 4.11-, and 2.85-fold higher than that of Cu/ZnO, CoFe‒CLDH, and Cu/ZnO/CoFe‒CLDH (VPC only), respectively. Moreover, the effects of the catalyst dosage, initial pollutant concentration, solution pH, persulfate dosage and coexisting ions on BPA degradation were comprehensively investigated. Radical-trapping experiments revealed that the contributions of ·OH, SO₄·^‒, ·O₂^-, and ¹O₂ involved in BPA degradation. Based on the intermediates identified by LC/MS, the main BPA degradation pathways were determined, the overall trend of which reflects a decreasing ecotoxicity. This study verified the effectiveness of the synergistic VPC‒PSA process with Cu/ZnO/CoFe‒CLDH, which could be used as a new reference for removing organic micropollutants from water.

Recent advances in the hybridization of cellulose and semiconductors: Design, fabrication and emerging multidimensional applications: A review

Cellulose is a plentiful, biodegradable, renewable, and natural polymer in the world that can be widely utilized in the production of polymer nanocomposites. Cellulose is developed in nanomaterials owing to its remarkable inherent features of low density, non-toxicity, and affordability, as well as the amazing sample characteristics of strength and thermal stability. Recently, there has been a lot of interest in organic-inorganic composites because of their adaptable qualities. Cellulose and semiconductors have exciting properties, and new combinations of both materials may result in efficient functional hybrid composites with distinct properties. Lately, a huge study was reported on cellulose and semiconductor-based nanocomposites. In this review, we summarize the present research development in the preparation methods, structure, features, and possible applications of multifunctional cellulose and semiconductor-based nanocomposites. The cellulose/semiconductor based nanocomposites have massive potential applications in the areas of photodegradation of organic dyes, hydrogen production, metal removal, biomedical, and sensor applications. It is also assumed that this article will promote additional investigation and will establish innovative capabilities to enhance novel cellulose and semiconductor based nanocomposites with new and exciting applications.

Fe-Ce/Layered Double Hydroxide Heterostructures and Their Derived Oxides: Electrochemical Characterization and Light-Driven Catalysis for the Degradation of Phenol from Water

Fe-Ce/layered double hydroxides (LDHs) were synthesized via a facile route by exploiting the "structural memory" of the LDH when the calcined MgAlLDH and ZnAlLDH were reconstructed in the aqueous solutions of FeSO₄/Ce(SO₄)₂. XRD analysis shows the formation of heterostructured catalysts that entangle the structural characteristics of the LDHs with those of Fe₂O₃ and CeO₂. Furthermore, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, TG/DTG, SEM/EDX and TEM results reveal a complex morphology defined by the large nano/microplates of the reconstructed LDHs that are tightly covered with nanoparticles of Fe₂O₃ and CeO₂. Calcination at 850 °C promoted the formation of highly crystallized mixed oxides of Fe₂O₃/CeO₂/ZnO and spinels. The photo-electrochemical behavior of Fe-Ce/LDHs and their derived oxides was studied in a three-electrode photo-electrochemical cell, using linear sweep voltammetry (LSV), Mott-Schottky (M-S) analysis and photo-electrochemical impedance spectroscopy (PEIS) measurements, in dark or under illumination. When tested as novel catalysts for the degradation of phenol from aqueous solutions, the light-driven catalytic heterojunctions of Fe-Ce/LDH and their derived oxides reveal their capabilities to efficiently remove phenol from water, under both UV and solar irradiation.

Theoretical Study on the Swelling Mechanism and Structural Stability of Ni3Al-LDH Based on Molecular Dynamics

layered double hydroxide (LDH) as a kind of 2D layer material has a swelling phenomenon. Because swelling significantly affects the adsorption, catalysis, energy storage, and other application properties of LDHs, it is essential to study the interlayer spacing, structural stability, and ion diffusion after swelling. In this paper, a periodic computational model of Ni₃Al-LDH is constructed, and the supramolecular structure, swelling law, stability, and anion diffusion properties of Ni₃Al-LDH are investigated by molecular dynamics theory calculations. The results show that the interlayer water molecules of Ni₃Al-LDH present a regular layered arrangement, combining with the interlayer anions by hydrogen bonds. As the number of water molecules increases, the hydrogen bond between the anion and the basal layer gradually weakens and disappears when the number of water molecules exceeds 32. The hydrogen bond between the anion and the water molecule gradually increases, reaching an extreme value when the number of water molecules is 16. The interlayer spacing of Ni₃Al-LDH is not linear with the number of water molecules. The interlayer spacing increases slowly when the number of water molecules is more than 24. The maximum layer spacing is stable at around 19 Å. The interlayer spacing, binding energy, and hydration energy show an upper limit for swelling: the number of water molecules is 32. When the number of interlayer water molecules is 16, the water molecules' layer structure and LDH interlayer spacing are suitable for anions to obtain the maximum diffusion rate, 10.97 × 10^-8 cm²·s^-1.

Functional Regulation of ZnAl-LDHs and Mechanism of Photocatalytic Reduction of CO2: A DFT Study

Defect engineering and heteroatom doping can significantly enhance the activity of zinc-aluminum layered double hydroxides (ZnAl-LDHs) in photocatalytic CO2 reduction to fuel. However, the in-depth understanding of the associated intrinsic mechanisms is limited. Herein, we systematically investigated Zn vacancies (VZn), oxygen vacancies (VO), and Cu doping on the geometry and electronic structure of ZnAl-LDH using density functional theory (DFT). We also revealed the related reaction mechanism. The results reveal the concerted roles of VO, VZn, and doped-Cu facilitate the formation of the unsaturated metal complexes (Znδ+-VO and Cuδ+-VO). They can localize the charge density distribution, function as new active centers, and form the intermediate band. Simultaneously, the intermediate band of functionalized ZnAl-LDHs narrows the band gap and lowers the band edge location. Therefore, it can broaden the absorption range of light and improve the selectivity of CO. Additionally, the unsaturated metal complex lowers the Gibbs free energy barrier for effective CO2 activation by bringing the d-band center level closer to the Fermi level. The work provided guidance for developing LDH photocatalysts with high activity and selectivity.

Highly effectual photocatalytic degradation of tartrazine by using Ag nanoparticles decorated on Zn-Cu-Cr layered double hydroxide@ 2D graphitic carbon nitride (C3N5)

Pollution of water resources is one of the main concerns of many countries. This issue originates from the entry of diverse pollutants, including dye compounds, into water sources. In this work, ternary Zn-Cu-Cr layered double hydroxides (LDH) supported on graphitic carbon nitride (g-C₃N₅) decorated by silver nanoparticles (C₃N₅-LDH-Ag) was first prepared. Application of various characterization techniques such as SEM, XRD, and FT-IR revealed that the synthesized nanocomposite was composed of Zn-Cu-Cr LDH nanoparticles, g-C₃N₅ nanosheets, and Ag nanoparticles. The prepared nanomaterials were employed for the photodegradation of tartrazine in aqueous solutions. It was found that the C₃N₅-LDH-Ag catalyst outperformed their pure g-C₃N₅, Zn-Cu-Cr LDH, and C₃N₅-LDH composite in photocatalytic degradation of tartrazine under visible light irradiation. Tartrazine (20 mg/L) can be entirely removed by 0.25 g/L C₃N₅-LDH-Ag photocatalyst under 1 h visible light irradiation (200 W) at pH 6 with a rapid degradation rate constant (k) that is 4.4, 3.9, and 2.6 times higher than that of pure C₃N₅, Zn-Cu-Cr LDH, and C₃N₅-LDH component, respectively. The formation of hydroxyl radicals on the surface of C₃N₅-LDH-Ag as the main active species was approved by the capturing experiment. The finding results approved the stability and reusability of C₃N₅-LDH-Ag in four photocatalytic degradation cycles. In general, our findings revealed that the synthesized nanocomposite could be employed as an efficient photocatalyst in environmental remediation.

Advanced nanomaterials for highly efficient CO2 photoreduction and photocatalytic hydrogen evolution

At present, CO₂ photoreduction to value-added chemicals/fuels and photocatalytic hydrogen generation by water splitting are the most promising reactions to fix two main issues simultaneously, rising CO₂ levels and never-lasting energy demand. CO₂, a major contributor to greenhouse gases (GHGs) with about 65% of the total emission, is known to cause adverse effects like global temperature change, ocean acidification, greenhouse effects, etc. The idea of CO₂ capture and its conversion to hydrocarbons can control the further rise of CO₂ levels and help in producing alternative fuels that have several further applications. On the other hand, hydrogen being a zero-emission fuel is considered as a clean and sustainable form of energy that holds great promise for various industrial applications. The current review focuses on the discussion of the recent progress made in designing efficient photocatalytic materials for CO₂ photoreduction and hydrogen evolution reaction (HER). The scope of the current study is limited to the TiO₂ and non-TiO₂ based advanced nanomaterials (i.e. metal chalcogenides, MOFs, carbon nitrides, single-atom catalysts, and low-dimensional nanomaterials). In detail, the influence of important factors that affect the performance of these photocatalysts towards CO₂ photoreduction and HER is reviewed. Special attention is also given in this review to provide a brief account of CO₂ adsorption modes on the catalyst surface and its subsequent reduction pathways/product selectivity. Finally, the review is concluded with additional outlooks regarding upcoming research on promising nanomaterials and reactor design strategies for increasing the efficiency of the photoreactions.

Formation of Cr-based layered double hydroxide: effect of the amendments

Chromium is one of the eight most popular inorganic soil pollutants in China, and its bioavailability is determined by the chemical states. Amendments, which are able to change the chemical forms of chromium and decrease its bioavailability, have received considerable attention in recent years. In this work, the formation of Cr-based layered double hydroxides (LDHs) and the immobilization of Cr in solution and soil were systemically investigated. The formation of Cr-based LDHs is strongly depended on the layer charges, aging temperatures and reaction time, as identified by X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray absorption fine structure (XAFS) spectrum. According to the pot experiment results, the concentration of Cr in the overground part of Brassica Chinensis L. was significantly decreased to 1.50-2.03 µg kg^- 1 in the present of amendments. In total, the finding of LDHs formation on amendments and the thermodynamic stability of LDHs provides a new insight into the remediation of Cr-polluted soils.

Recent Breakthrough in Layered Double Hydroxides and Their Applications in Petroleum, Green Energy, and Environmental Remediation

The fast development of the world civilization is continuously based on huge energy consumption. The extra-consumption of fossil fuel (petroleum, coal, and gas) in past decades has caused several political and environmental crises. Accordingly, the world, and especially the scientific community, should discover alternative energy sources to safe-guard our future from severe climate changes. Hydrogen is the ideal energy carrier, where nanomaterials, like layered double hydroxides (LDHs), play a great role in hydrogen production from clean/renewable sources. Here, we review the applications of LDHs in petroleum for the first time, as well as the recent breakthrough in the synthesis of 1D-LDHs and their applications in water splitting to H2. By 1D-LDHs, it is possible to overcome the drawbacks of commercial TiO2, such as its wide bandgap energy (3.2 eV) and working only in the UV-region. Now, we can use TiO2-modified structures for infrared (IR)-induced water splitting to hydrogen. Extending the performance of TiO2 into the IR-region, which includes 53% of sunlight by 1D-LDHs, guarantees high hydrogen evolution rates during the day and night and in cloudy conditions. This is a breakthrough for global hydrogen production and environmental remediation.

Structural Aspects of “Memory Effect” for MgGa LDHs: New Data Obtained by Simulation of XRD Patterns for 1D Disordered Crystals

Simulation of diffraction patterns for 1D disordered crystals was used to investigate the structure of the initial CO32− containing MgGa LDHs with a different Mg2+/(Mg2+ + Ga3+) ratio equal to 0.67, 0.75, and 0.80; mixed oxides obtained by calcination of LDHs at a temperature of 550 °C; and the hydroxide obtained by hydration of MgGa oxide with the Mg2+ content of 0.80. The initial LDHs contain lamellar inclusions of manasseite structure (polytype 2H1) in the hydrotalcite structure (3R1). A loss of water at 200 °C leads to the formation of a metastable dehydrated phase where layers are packed, as in polytypes 3R2 and 1H, with turbostratic disorder. The structure of mixed oxides is also layered and consists of periclase-like octahedral layers and spinel-like octahedral-tetrahedral layers. Hydration of the oxides results in restoring the initial layered hydrotalcite structure (polytype 3R1) for Mg2+ mole fractions 0.67 and 0.75. For the Mg2+ content of 0.80, the phase composition is represented by the hydroxide with hydrotalcite structure and the layered mixed hydroxide with the alternation of hydrotalcite and brucite lamellar domains, which was also revealed by calculation of diffraction patterns using models of 1D disordered crystals.

Preparing a dual-function BiVO4/NiFe-LDH composite photoanode for enhanced photoelectrocatalytic wastewater treatment and simultaneous hydrogen evolution

The proposed PEC degradation over the BiVO4/NiFe-LDH photoelectrode under visible light irradiation and simultaneous hydrogen evolution at the cathode.

Hollow microspherical Bi2MoO6/Zn–Ti layered double hydroxide heterojunction for efficient visible-light photocatalytic degradation of organic contaminants

The construction of a Bi2MoO6/Zn–Ti layered double hydroxide heterojunction can effectively enhance the photocatalytic activity owing to efficient charge separation.

New Aptamer/MoS2/Ni-Fe LDH Photoelectric Sensor for Bisphenol A Determination

Here, a new type of PEC aptamer sensor for bisphenol A (BPA) detection was developed, in which visible-light active MoS₂/Ni-Fe LDH (layered double hydroxide) heterostructure and aptamer were used as photosensitive materials and biometric elements, respectively. The combination of an appropriate amount of MoS₂ and Ni-Fe LDH enhances the photocurrent response, thereby promoting the construction of the PEC sensor. Therefore, we used a simple in situ growth method to fabricate a MoS₂/Ni-Fe LDH sensor to detect the BPA content. The aptasensor based on aptamer/MoS₂/Ni-Fe LDH displayed a linear range toward a BPA of 0.05-10 to 50-40,000 ng L^-1, and it has excellent stability, selectivity and reproducibility. In addition, the proposed aptamer sensor is effective in evaluating real water samples, indicating that it has great potential for detecting BPA in real samples.

Mussel-inspired triple bionic adsorbent: Facile preparation of layered double hydroxide@polydopamine@metal-polyphenol networks and their selective adsorption of dyes in single and binary systems

To effectively address the serious human health challenges and ecological damage caused by organic dyes in wastewater, we developed a novel bionic adsorbent (LDH@PDA@MPNs) for the selective adsorption and removal of malachite green (MG) and crystalline violet (CV). The adsorbent was prepared using a facile two-step method based on mussel-inspired chemistry and metal complexation. The physicochemical structure, surface morphology, and composition of the LDH@PDA@MPNs were characterized by scanning electron microscopy, Fourier-transform infrared spectrometry, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Adsorption of MG and CV with the LDH@PDA@MPNs was evaluated. Under optimal conditions, the maximum adsorption of MG and CV by the adsorbent was 89.608 and 40.481 mg/g, respectively. The adsorption kinetics showed that the experimental data were in good agreement with the pseudo-second-order kinetic model, and the equilibrium adsorption isotherm data fitted well with the Freundlich model. The thermodynamic results indicated that the adsorption of the dyes on LDH@PDA@MPNs was a spontaneous endothermic process. Importantly, the bionic adsorbent not only shows high removal efficiency by easy regeneration with low-cost reagents but also exhibits high selectivity for dyes in both single and binary systems. Therefore, LDH@PDA@MPNs have the potential to adsorb and remove dyes from complex wastewater solutions.

An overview on non-spherical semiconductors for heterogeneous photocatalytic degradation of organic water contaminants

Because of their carcinogenicity and mutagenicity, the elimination of organic contaminants from surface and subsurface water is a subject of environmental significance. Conventional water decontamination approaches such as membrane separation, ultrafiltration, adsorption, reverse osmosis, coagulation, etc., have relatively higher operating costs and can generate highly toxic secondary contaminants. On the other hand, heterogeneous photocatalysis, an advanced oxidation process (AOP), is considered a clean and cost-effective process for organic pollutants degradation. Owing to their distinctive structure and physicochemical properties non-spherical semiconductors have gained considerable limelight in the photocatalytic degradation of organic contaminants. The current review briefly introduces a wide range of organic water contaminants. Recent advances in non-spherical semiconductor assembly and their photocatalytic degradation applications are highlighted. The underlying mechanism, fundamentals of photocatalytic reactions, and the factors affecting the degradation performance are also alluded including the current challenges and future research perspectives.

In situ stabilization of the adsorbed Co2+ and Ni2+ in rice straw biochar based on LDH and its reutilization in the activation of peroxymonosulfate

The stabilization of heavy metals adsorbed in the spent biochar remains a challenge, and the risk of secondary pollution under environmental changes will rise under inadequate post-treatment. Moreover, the safe and sufficient re-utilization of the spent biochar with heavy metals loaded have attracted extensive attention. In this research, the performance and mechanism of rice straw biochar (RSBC) pyrolyzed at different temperature for nickel and cobalt adsorption were investigated, and the stabilization of the adsorbed heavy metals was achieved via a simple two-step strategy: the adsorption of metal ions and the hydrothermal process to form the stable layered double hydroxides (LDH) on biochar, with a leaching rate below 0.005% evaluated by EPA toxicity characteristic leaching procedure (TCLP). Meanwhile, the stabilized RSBC-LDH can be reused as an excellent catalyst in the activation of peroxymonosulfate (PMS) to degrade organic pollutants efficiently. This work eliminated the risk of heavy metal desorption from the spent biochar, and enabled a new strategy for the optimized utilization of biochar in environmental remediation.

Layered double hydroxides and related hybrid materials for removal of pharmaceutical pollutants from water

Pharmaceuticals and their by-products are recalcitrant contaminants in water. Moreover, the high consumption of these drugs has many detrimental effects on body waters and ecosystems. In this timely review, the advances in molecular engineering of layered double hydroxides (LDH) that have been used for the removal of pharmaceutical pollutants are discussed. The approach starts from the strategies to obtain homogeneous synthesis of LDH that allow the doping and/or surface functionalization of different metals and oxides, producing heterojunction systems as well as composites with carbon and silica-based materials with high surface area. Adsorption is considered as a traditional removal of pharmaceutical pollutants, so the kinetic and mechanism of this phenomenon are analyzed based on pH, temperature, ionic strength, in order to obtain new insights for the formation of multifunctional LDH. Advanced oxidation methodologies, mainly heterogeneous photocatalysis and Fenton-like processes, stand out as the more efficient even to obtain the mineralization of the drugs. The LDH have the advantage of structural memory that favors regeneration processes. The reconstruction of calcined LDH can be used to improve drug removal, through a combination of adsorption capacity/catalytic activity. A meticulous analysis of the persistence, toxicity and bioaccumulation of the most common pharmaceuticals has allowed us to highlight the ability of the LDH to remove recalcitrant drugs at relatively low concentrations (ppm, ppb), in contrast to other mixed oxide nanostructures and homogeneous oxidation processes. In this sense, the mechanism of drug removal by LDH is discussed based on the importance of the use of composites, scavenger agents, Fenton and electro-Fenton processes, membranes, thin films and coatings, among others. In addition, the ecotoxicity of LDH is also reviewed to indicate that these layered structures can exhibit biocompatibility or high toxicity depending on the adsorbed drug and ions/metals that compose them. Undoubtedly, the LDH have a unique flexible structure with adsorption capacity and catalytic activity, facts that explain the important reasons for their extensive use in the environmental remediation of pharmaceutical pollutants from water.

Fast microwave-assisted preparation of nickel-copper-chromium-layered double hydroxide as an excellent electrocatalyst for water oxidation

Herein, we describe a simple microwave method for the doping of Cu2+ into NiCr-LDH and the preparation of ternary Ni2.25Cu0.75Cr-LDH as a superior electrocatalyst for water oxidation in a neutral solution. The obtained Ni2.25Cu0.75Cr-LDH was characterized by XRD, DRS, TEM and FE-SEM techniques. The results showed that Ni2.25Cu0.75Cr-LDH was formed with a size of 30 nm. In order to examine the water oxidation activity of Ni2.25Cu0.75Cr-LDH, the as-prepared samples were used as an electrocatalyst-modified carbon paste electrode in neutral solution. The electrochemical results revealed that the optimized Ni2.25Cu0.75Cr-LDH presented extraordinary water oxidation activity with a low onset potential of 1.40 V (vs. RHE) and an overpotential of 170 mV compared to other molar ratios (Ni2.5Cu0. 5Cr-LDH), (Ni2CuCr-LDH), and bimetallic (CuCr-LDH), and even outperformed NiCr-LDH. Also, a small Tafel slope of 31 mV and high durability of 14 h could be obtained for Ni2.25Cu0.75Cr-LDH. The excellent OER could be assigned to the decreased band gap energy and increased charge transfer at Ni2.25Cu0.75Cr-LDH. Therefore, Ni2.25Cu0.75Cr-LDH is a promising water oxidation catalyst owing to its improved charge transfer ability.

Photoactive Heterostructures: How They Are Made and Explored

In our review we consider the results on the development and exploration of heterostructured photoactive materials with major attention focused on what are the better ways to form this type of materials and how to explore them correctly. Regardless of what type of heterostructure, metal–semiconductor or semiconductor–semiconductor, is formed, its functionality strongly depends on the quality of heterojunction. In turn, it depends on the selection of the heterostructure components (their chemical and physical properties) and on the proper choice of the synthesis method. Several examples of the different approaches such as in situ and ex situ, bottom-up and top-down, are reviewed. At the same time, even if the synthesis of heterostructured photoactive materials seems to be successful, strong experimental physical evidence demonstrating true heterojunction formation are required. A possibility for obtaining such evidence using different physical techniques is discussed. Particularly, it is demonstrated that the ability of optical spectroscopy to study heterostructured materials is in fact very limited. At the same time, such experimental techniques as high-resolution transmission electron microscopy (HRTEM) and electrophysical methods (work function measurements and impedance spectroscopy) present a true signature of heterojunction formation. Therefore, whatever the purpose of heterostructure formation and studies is, the application of HRTEM and electrophysical methods is necessary to confirm that formation of the heterojunction was successful.

Efficient solar heterogeneous photocatalytic degradation of metronidazole using heterojunction semiconductors hybrid nanocomposite, layered double hydroxides

This study focuses on the synthesis of various nanocomposites with heterojunction structures, MgAl-LDH (LDH = layered double hydroxides) hybrid with semiconductor such as MoO₃ and CuO. These solids were synthesized by co-precipitation method at constant pH and have been characterized extensively using atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and transmission electron microscopy-energy dispersive X-ray (TEM-EDX) methods. The catalytic activity of nanocomposites was tested in the photocatalytic degradation under solar irradiation of emerging pollutants as the pharmaceutical metronidazole (MNZ). The experimental parameters, including initial MNZ concentration, the nature of oxide incorporate in the photocatalyst, catalyst loading were explored. All the synthesized samples showed high photocatalytic performances; the highest photocatalysis efficiency was achieved with the photocatalyst dose 1.5 g/L and initial MNZ concentration of 10 mg/L at neutral pH. The photocatalytic experimental results were fitted very well to the Langmuir-Hinshelwood model. From the obtained results the calcined LDH/semiconductors could be efficient for the photocatalytic process under solar irradiation of pharmaceuticals and may contribute in environmental remediation.

Layered Double Hydroxide (LDH) Based Photocatalysts: An Outstanding Strategy for Efficient Photocatalytic CO2 Conversion

CO2 conversion to solar fuels/chemicals is an alluring approach for narrowing critical issues of global warming, environmental pollution, and climate change, caused by excess atmospheric CO2 concentration. Amongst various CO2 conversion strategies, photocatalytic CO2 conversion (PCC) is considered as a promising approach, which utilizes inexpensive sunlight and water with a photocatalyst material. Hence, development of an efficient and a stable photocatalyst is an essential activity for the respective scientific community to upscale the PCC research domain. Until today, metal oxides, such as TiO2, ZnO, etc., are categorized as standard photocatalysts because of their relative stability, abundant availability and low cost. However, their performance is tethered by limited light absorption and somewhat physical properties. Recently, layered double hydroxides (LDHs) have offered an exciting and efficient way for PCC due to their superb CO2 adsorption and moderate photocatalytic properties. The LDH based photocatalysts show marvelous physiochemical and electrical properties like high surface area, stability, and excellent conductivity. In the present review article, a summarized survey is portrayed regarding latest development for LDH based photocatalysts with a focus on synthesis strategies employing various photocatalyst materials, influencing parameters and possible mechanism involved in PCC to useful fuels and chemicals like CO, CH4, CH3OH, and H2.

Recent innovations in functionalized layered double hydroxides: Fabrication, characterization, and industrial applications

Layered Double Hydroxides (LDHs) are a group of hydrotalcite-like nano-sized materials with cationic layers and exchangeable interlayer anions. The wide range of divalent and trivalent cationic metals and anionic compounds are employed in the synthesis of LDH materials, which have improved their importance among the researchers. Because of their high anion exchange property, memory effect, tunable behavior, bio-friendly, simple preparation, and their affordability, these nano-materials are essentially interested today. Modification of LDHs improves their behaviours to make them appropriate in industrial fields, including biological, adsorbent, mechanical, optical, thermal, electrical fields, etc. This review has critically discussed the structural features, main properties, and also clarified the most important methods of modification and intercalation of LDH nano-materials. Moreover, some novel reported researches related to the successful modification of LDH materials have been characterized and briefly the advantages, disadvantages, and applications are presented in the industrial fields.

Nanoparticles in Agroindustry: Applications, Toxicity, Challenges, and Trends

Nanotechnology is a tool that in the last decade has demonstrated multiple applications in several sectors, including agroindustry. There has been an advance in the development of nanoparticulated systems to be used as fertilizers, pesticides, herbicides, sensors, and quality stimulants, among other applications. The nanoencapsulation process not only protects the active ingredient but also can affect the diffusion, interaction, and activity. It is important to evaluate the negative aspects of the use of nanoparticles (NPs) in agriculture. Given the high impact of the nanoparticulated systems in the agro-industrial field, this review aims to address the effects of various nanomaterials on the morphology, metabolomics, and genetic modification of several crops.

Comparative Study of ZnO Thin Films Doped with Transition Metals (Cu and Co) for Methylene Blue Photodegradation under Visible Irradiation

We synthesized and characterized both Co-doped ZnO (ZnO:Co) and Cu-doped ZnO (ZnO:Cu) thin films. The catalysts’ synthesis was carried out by the sol–gel method while the doctor blade technique was used for thin film deposition. The physicochemical characterization of the catalysts was carried out by Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction, and diffuse reflectance measurements. The photocatalytic activity was studied under visible irradiation in aqueous solution, and kinetic parameters were determined by pseudo-first-order fitting. The Raman spectra results evinced the doping process and suggested the formation of heterojunctions for both dopants. The structural diffraction patterns indicated that the catalysts were polycrystalline and demonstrated the presence of a ZnO wurtzite crystalline phase. The SEM analysis showed that the morphological properties changed significantly, the micro-aggregates disappeared, and agglomeration was reduced after modification of ZnO. The ZnO optical bandgap (3.22 eV) reduced after the doping process, these being ZnO:Co (2.39 eV) and ZnO:Co (3.01 eV). Finally, the kinetic results of methylene blue photodegradation reached 62.6% for ZnO:Co thin films and 42.5% for ZnO:Cu thin films.

Photodegradation of volatile organic compounds catalyzed by MCr-LDHs and hybrid MO@MCr-LDHs (M = Co, Ni, Cu, Zn): the comparison of activity, kinetics and photocatalytic mechanism

The efficient removal of high-concentration volatile organic compounds (VOCs) has been a challenging task.

A new biocompatible ternary Layered Double Hydroxide Adsorbent for ultrafast removal of anionic organic dyes

It would be of great significance to introduce a new biocompatible Layered Double Hydroxide (LDH) for the efficient remediation of wastewater. Herein, we designed a facile, biocompatible and environmental friendly layered double hydroxide (LDH) of NiFeTi for the very first time by the hydrothermal route. The materialization of NiFeTi LDH was confirmed by FTIR, XRD and Raman studies. BET results revealed the high surface area (106 m²/g) and the morphological studies (FESEM and TEM) portrayed the sheets-like structure of NiFeTi nanoparticles. The material so obtained was employed as an efficient adsorbent for the removal of organic dyes from synthetic waste water. The dye removal study showed >96% efficiency for the removal of methyl orange, congo red, methyl blue and orange G, which revealed the superiority of material for decontamination of waste water. The maximum removal (90%) of dyes was attained within 2 min of initiation of the adsorption process which supported the ultrafast removal efficiency. This ultrafast removal efficiency was attributed to high surface area and large concentration of -OH and CO₃²⁻ groups present in NiFeTi LDH. In addition, the reusability was also performed up to three cycles with 96, 90 and 88% efficiency for methyl orange. Furthermore, the biocompatibility test on MHS cell lines were also carried which revealed the non-toxic nature of NiFeTi LDH at lower concentration (100% cell viability at 15.6 μg/ml). Overall, we offer a facile surfactant free method for the synthesis of NiFeTi LDH which is efficient for decontamination of anionic dyes from water and also non-toxic.