Surface engineering of layered double hydroxide (LDH) nanoparticles for polymer flame retardancy

Support based metal incorporated layered nanomaterials for photocatalytic degradation of organic pollutants

An effective approach to producing sophisticated miniaturized and nanoscale materials involves arranging nanomaterials into layered hierarchical frameworks. Nanostructured layered materials are constructed to possess isolated propagation assets, massive surface areas, and envisioned amenities, making them suitable for a variety of established and novel applications. The utilization of various techniques to create nanostructures adorned with metal nanoparticles provides a secure alternative or reinforcement for the existing physicochemical methods. Supported metal nanoparticles are preferred due to their ease of recovery and usage. Researchers have extensively studied the catalytic properties of noble metal nanoparticles using various selective oxidation and hydrogenation procedures. Despite the numerous advantages of metal-based nanoparticles (NPs), their catalytic potential remains incompletely explored. This article examines metal-based nanomaterials that are supported by layers, and provides an analysis of their manufacturing, procedures, and synthesis. This study incorporates both 2D and 3D layered nanomaterials because of their distinctive layered architectures. This review focuses on the most common metal-supported nanocomposites and methodologies used for photocatalytic degradation of organic dyes employing layered nanomaterials. The comprehensive examination of biological and ecological cleaning and treatment techniques discussed in this article has paved the way for the exploration of cutting-edge technologies that can contribute to the establishment of a sustainable future.

Facile synthesis of silver doped manganese oxide nanocomposite with superior photocatalytic and antimicrobial activity under visible spectrum

Water pollution and antimicrobial resistance (AMR) have become two global threats; 80% of diseases and 50% of child deaths are due to poor water quality. In this study, hydrothermal processing was employed to manufacture manganese oxide nanorods. Silver dopant was deposited on the surface of manganese oxide. XRD diffractogram confirmed the facile synthesis of Ag/Mn2O3 nanocomposite. XPS survey analysis demonstrated silver content of 9.43 atom %. Photocatalytic measurements demonstrated the outstanding efficiency of the Ag-Mn2O3 compared to virgin oxide particles under visible radiation. Degradation efficiencies Mn2O3 and Ag/Mn2O3 on methyl orange (MO) dye was found to be 53% and 85% under visible spectrum. Silver dopant was found to decrease the binding energy of valence electrons; this action could support electron-hole pair generation under visible spectrum and could promote catalytic performance. Ag/Mn2O3 NPs demonstrated most effective performance (95% removal efficiency) at pH 3; this could be ascribed to the electrostatic attraction between positively charged catalyst and the negatively charged MO. Ag/Mn2O3 demonstrated enhanced antibacterial activity against Gram-positive Staphylococcus aureus (S. aureus) (19 mm ZOI), and Gram-negative Escherichia coli (E. coli) (22 mm ZOI) respectively; the developed nanocomposite demonstrated advanced anti-film activity with inhibition percentage of 95.5% against E. coli followed by 89.5% against S. aureus.

Unveiling a robust and high-temperature-stable two-dimensional ZnAl layered double hydroxide nanosheet based flexible triboelectric nanogenerator

Triboelectric nanogenerators have the ability to harvest low- and mid-frequency vibrational energy from the environment; however, achieving stable performance of the nanogenerator device in high-temperature conditions remains challenging. In this work, a flexible and temperature-stable polyvinyl alcohol (PVA)/layered double hydroxides (LDH) nanocomposite-based triboelectric nanogenerator was developed to harvest unexploited vibrational energy for the first time. Crystalline ZnAl LDH nanosheets grown by a hydrothermal route are used to fabricate the high-performance flexible nanogenerator. The ZnAl LDH exhibits fire-retardancy and high-temperature stability (∼500 °C). A triboelectric nanogenerator based on the ZnAl LDH-PVA nanocomposite generated a very high output voltage of 60 V even under a low vertical pressure of 1 kgf. Surprisingly, the developed device shows ultra-stable output performance even up to a temperature of 200 °C. In addition, a ZnAl LDH-nanosheet-reinforced PVA nanocomposite film shows very high dielectric constant of about 5 × 105 at the low-frequency side. The tremendous increase in the output voltage and stable performance are discussed in terms of the high dielectric constant and synergistic effect of the LDH nanosheets and PVA. Furthermore, the device was also used to monitor human body movements such as finger and wrist bending to develop self-powered sensors.

Investigation of the physico-chemical properties of Ni-Mg-Al-La catalysts from ultrasound-assisted synthesis

This work reports on the ultrasonic synthesis of layered double hydroxides (LDH), also known as hydrotalcite-type materials. We have studied the influence of ultrasonic irradiation parameters (power, time, temperature) on the physicochemical properties of Ni2Mg4Al1.8La0.2 hydrotalcite-type precursors and related mixed oxides (MO). The low-frequency acoustic cavitation (22 kHz) was applied during the precipitation and aging steps of co-precipitation synthesis and the results were compared to the classical preparation route. The materials were characterized by ATR-FTIR, XRD, N2 adsorption-desorption, SEM-EDX, S/TEM-HAADF, and XPS. Using the combination of acoustic cavitation-assisted precipitation and aging steps, XRD experiments show a higher purity hydrotalcite phase and a better incorporation of lanthanum ions into the LDH structure. As expected, morphological characterization shows a reduction in average crystallite size and an increase in surface area and pore volume, combined with a drastic reduction in synthesis time (45 min at room temperature versus 19 h at 60 °C in conventional synthesis). The insertion of a larger quantity of La is observed by S/TEM-EDSX mapping which also shows a better distribution of lanthanum atoms within the LDH and mixed oxide structures.

Electroplating wastewater treatment by in-situ formation of organic anions and inorganic anions intercalated layered double hydroxides

The electroplating wastewater containing various metal ions was treated by adding sodium dodecyl benzene sulfonate (SDBS) and regulating pH value, and the resulting precipitates were characterized by X-ray diffraction (XRD). The results showed that organic anions intercalated layered double hydroxides (OLDHs) and inorganic anions intercalated layered double hydroxides (ILDHs) were in-situ formed to remove heavy metals during the treatment process. In order to reveal the formation mechanism of the precipitates, SDB- intercalated Ni-Fe OLDHs, NO3- intercalated Ni-Fe ILDHs and Fe3+-DBS complexes were synthsized by co-precipitation at various pH values for comparison. These samples were characterized by XRD, Fourier Transform infrared (FTIR), element analysis as well as the aqueous residual concentrations of Ni2+ and Fe3+ were detected. The results showed that OLDHs with good crystal structures can be formed as pH≤7, while ILDHs began to form at pH = 8. When pH < 7, complexes of Fe3+ and organic anions with the ordered layered structure were formed firstly, and then with increase in pH value, Ni2+ inserted into the solid complex and the OLDHs began to form. However, Ni-Fe ILDHs were not formed when pH ≤ 7. The Ksp (Solubility Product Constant) of OLDHs was calculated to be 3.24 × 10-19 and that of ILDHs was 2.98 × 10-18 at pH = 8, which suggested that OLDHs might be easier to form than ILDHs. The formation process of ILDHs and OLDHs were also simulated through MINTEQ software, and the simulation output verified that OLDHs could be easier to form than ILDHs at pH ≤ 7. Information from this study provides a theoretical basis for effective in-situ formation of OLDHs in wastewater treatment.

Multifaceted potential applicability of hydrotalcite-type anionic clays from green chemistry to environmental sustainability

Hydrotalcite-like anionic clays (HTs) also known as Layered double hydroxides (LDHs) have been developed as multifunctional materials in numerous applications related to catalysis, adsorption, and ion-exchange processes. These materials constitute an important class of ionic lamellar solid clays of Brucite-like structure which comprise of consecutive layers of divalent and trivalent metal cations with charge balancing anions and water molecules in interlayer space. These materials have received increasing attention in research due to their interesting properties namely layered structure, ease of preparation, flexible tunability, ability to intercalate different types of anions, electronic properties, high thermal stability, high biocompatibility, and easy biodegradation. Moreover, HTs/LDHs have unique tailorable and tuneable characteristics such as both acidic and basic sites, anion exchange capability, surface area, basal spacing, memory effect, and also exhibit high exchange capacities, which makes them versatile materials for a wide range of applications and extended their horizons to diverse areas of science and technology. This study enlightens the various rational researches related to the synthetic methods and features focusing on synthesis and/or fabrication with other hybrids and their applications. The diverse applications (namely catalyst, adsorbent to toxic chemicals, agrochemicals management, non-toxic flame retardants, and recycling of plastics) of these multifunctional materials related to a clean and sustainable environment were also summarized.

Cooperative Effect of ZIF-67-Derived Hollow NiCo-LDH and MoS2 on Enhancing the Flame Retardancy of Thermoplastic Polyurethane

In this work, a novel three-dimensional (3D) hollow nickel-cobalt layered double hydroxide (NiCo-LDH) was synthesized using zeolitic imidazole framework-67 (ZIF-67) as a template, and then utilized to functionalize molybdenum disulfide (NiCo-LDH/MoS2) via electrostatic force. Flame retardant thermoplastic polyurethane (TPU) composites were prepared by the melt blending method. Compared to pure TPU, NiCo-LDH/MoS2 filled TPU composite was endowed with a decrease of 30.9% and 55.7% of the peak heat release rate (PHRR) and the peak smoke production rate (PSPR), respectively. Furthermore, the addition of NiCo-LDH/MoS2 can significantly improve the thermal stability and char yield of the TPU composite. The catalytic carbonization effect and dilution effect of NiCo-LDH, and the barrier effect of MoS2 nanosheets enable TPU composites with excellent flame retardancy and toxic gas suppression ability.

Synthesis of 3D Hollow Layered Double Hydroxide-Molybdenum Disulfide Hybrid Materials and Their Application in Flame Retardant Thermoplastic Polyurethane

The development of high-performance thermoplastic polyurethane (TPU) with high flame retardancy and low toxicity has always been the focus of its research. In this paper, the novel 3D hollow layered double hydroxide/molybdenum disulfide (LDH/MoS₂) hybrid materials were synthesized by hydrothermal method using the MIL-88A as in situ sacrificial template and MoS₂ as synergistic flame retardant. Among all TPU composites, the peak heat release rate, total heat release rate, and total smoke release rate of TPU/NiFeTb-LDH/MoS₂ were reduced by 50.9%, 18.2%, and 35.8% compared with pure TPU, respectively. The results of the thermogravimetric infrared analysis demonstrated that the contents of combustible volatiles (hydrocarbons) and toxic volatiles (CO and HCN) emitted from TPU/LDH/MoS₂ were significantly reduced, indicating that LDH/MoS₂ hybrid materials can dramatically enhance the fire safety of TPU composites. Combined with the analysis of carbon residues and thermal stability of TPU composites, the enhanced flame retardancy and smoke suppression performances are primarily attributed to the catalytic carbonization of LDH and the physical barrier effect of MoS₂.

Precise Control of a Yolk-Double Shell Metal-Organic Framework-Based Nanostructure Provides Enhanced Fire Safety for Epoxy Nanocomposites

Nanomaterials derived from metal-organic frameworks (MOFs) are highly promising as future flame retardants for polymeric materials. The precise control of the interface for polymer nanocomposites is taking scientific research by storm, whereas such investigations for MOF-based nanofillers are rare. Herein, a novel yolk-double shell nanostructure (ZIF-67@layered double hydroxides@polyphophazenes, ZIF@LDH@PZS) was subtly designed and introduced into epoxy resin (EP) as a flame retardant to fill the vacancy of yolk/shell construction in the field. Meanwhile, the interface of the polymer nanocomposites can be further accurately tailored by the outermost layer of the nanofillers from PZS to Ni(OH)₂ (NH), by which hollow nanocages with treble shells (LDH@PZS@NH) were obtained. It is remarkably interesting that LDH@PZS@NH endows the EP with the lowest peak of heat release rate in the cone calorimeter test, but the total heat and smoke releases (THR and TSP) of the nanocomposites are even higher than those of the neat polymer. In contrast, EP blended with ZIF@LDH@PZS shows outstanding comprehensive performance: with 2 wt.%, the limiting oxygen index is increased to 29.5%, and the peak heat release rate is reduced by 26.0%. The impact and flexural strengths are slightly lowered, while the storage modulus is enhanced remarkably compared with that for neat EP. The flame retardant mechanism is systematically explored focusing on the interfacial interactions of different hybrids within the epoxy matrix, ushering in a new stage of study of nanostructural design-guided interface manipulation in MOF-based polymer nanocomposites.

Peanut Shell Derived Carbon Combined with Nano Cobalt: An Effective Flame Retardant for Epoxy Resin

Biomass-derived carbon has been recognised as a green, economic and promising flame retardant (FR) for polymer matrix. In this paper, it is considered that the two-dimensional (2D) structure of carbonised peanut shells (PS) can lead to a physical barrier effect on polymers. The carbonised sample was prepared by the three facile methods, and firstly adopted as flame retardants for epoxy resin. The results of thermal gravimetric analysis (TGA) and cone calorimeter tests indicate that the carbon combined with nano Cobalt provides the most outstanding thermal stability in the current study. With 3 wt.% addition of the FR, both peak heat release rate (pHRR) and peak smoke production rate (PSPR) decrease by 37.9% and 33.3%, correspondingly. The flame retardancy mechanisms of the FR are further explored by XPS and TG-FTIR. The effectiveness of carbonised PS can be mainly attributed to the physical barrier effect derived by PS's 2D structure and the catalysis effect from Cobalt, which contribute to form a dense char layer.

Morphology, Thermal Stability, and Flammability Properties of Polymer-Layered Double Hydroxide (LDH) Nanocomposites: A Review

The utilization of layered nanofillers in polymer matrix, as reinforcement, has attracted great interest in the 21st century. This can be attributed to the high aspect ratios of the nanofillers and the attendant substantial improvement in different properties (i.e., increased flammability resistance, improved modulus and impact strength, as well as improved barrier properties) of the resultant nanocomposite when compared to the neat polymer matrix. Amongst the well-known layered nanofillers, layered inorganic materials, in the form of LDHs, have been given the most attention. LDH nanofillers have been employed in different polymers due to their flexibility in chemical composition as well as an adjustable charge density, which permits numerous interactions with the host polymer matrices. One of the most important features of LDHs is their ability to act as flame-retardant materials because of their endothermic decomposition. This review paper gives detailed information on the: preparation methods, morphology, flammability, and barrier properties as well as thermal stability of LDH/polymer nanocomposites.

Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron

To reduce the toxicity of vanadium(V) [V(V)] and inhibit the desorption of adsorbed vanadium in groundwater, we synthesized nanoscale zerovalent iron (nZVI) dispersed on layered double hydroxide (LDH) composites (nZVI@LDH) to remove V(V) from simulated groundwater. We found that nZVI@LDH could reduce high-valence vanadium to low-valence vanadium, then forming vanadium-containing precipitation to reduce the toxicity and inhibiting vanadium from returning to groundwater. SEM and XRD characterizations exhibited the uniform dispersal of nZVI on the surface of LDH. nZVI@LDH with nZVI/LDH at a mass ratio of 1:2 provided the maximum adsorption capacity of 93.7 mg g^-1 at pH 3.0. Coexisting anions and dissolved oxygen in groundwater have little effect on V(V) removal. nZVI@LDH performed well across a wide pH range (3.0-8.0). The surface characterizations and XPS analysis revealed that LDH as supporting materials inhibited the aggregation and passivation of nZVI. The adsorbed V(V) was reduced to V(IV) and V(III) by nZVI and spontaneously transformed into insoluble VO₂ and V₂O₃. The DFT calculations indicated the strong complexation and better stability of the V(IV) and V(III) species with nZVI@LDH than V(V). This work suggests that nZVI@LDH has the potential to serve as an efficient material for the immobilization of V(V) in groundwater.

Surface modified colloidal silica nanoparticles: Novel aspect for complete identification of explosive materials

Terrorism by means of explosives has become a crucial threat. Nanoparticles with distinctive properties can offer novel aspects for instant detection of explosive materials. Common explosives are organic compounds that contain nitro group (NO₂) along with carbon and hydrogen elements. This study demonstrates complete identification of nitramine explosives (RDX & HMX) using colloidal silica nanoparticles. Sustainable fabrication of colloidal silica was conducted via hydrothermal processing technique. Explosive identification involves a digestion of the tested material using strong acid. The digestion process results in the development of nitro group and corresponding formaldehyde segment. The identification of the nitro group was performed using colloidal silica nanoparticles functionalized with secondary amine to develop a characteristic dark blue colour. Simultaneous identification of formaldehyde segment was performed using colloidal silica functionalized with aromatic phenol to develop a red colour. This robust explosive detection technology can find wide applications on site where instant identification to assess potential threat is a crucial demand. Thanks to hydrothermal processing, sustainable fabrication and surface modification of colloidal silica particles can be obtained.

The Influence of Environmentally Friendly Flame Retardants on the Thermal Stability of Phase Change Polyurethane Foams

To improve thermal insulation, microencapsulated phase change materials (micro-PCMs), expandable graphite (EG), and ammonium polyphosphate (APP) were introduced into polyurethane foam (PUF) to enhance the thermal stability and improve the thermal insulation behavior. The morphology of the PUF and micro-PCM was studied using a scanning electronic microscope (SEM), while the thermophysical properties of the PUF were investigated using a hot disk thermal constants analyzer and differential scanning calorimetry (DSC). The thermal stability of the PUF was investigated by thermogravimetric analysis (TGA), and the gas products volatilized from the PUF were analyzed by thermogravimetric analysis coupled with Fourier transform infrared spectrometry (TGA-FTIR). The results revealed that the thermal conductivities of the PUF were reduced because micro-PCM is effective in absorbing energy, showing that the PUF functions not only as a thermal insulation material but also as a heat sink for energy absorption. Moreover, the EG and APP were found to be effective in improving the thermal stabilities of the PUF, and the optimized formulation among EG, APP, and micro-PCMs in the PUF showed a significant synergistic effect.

Modification and Compounding of CaMgAl-Layered Double Hydroxides and Their Application in the Flame Retardance of Acrylonitrile-Butadiene-Styrene Resin

CaMgAl-layered double hydroxides (CaMgAl-LDHs) were synthesized by a co-precipitation method to prepare sodium oleate-modified, borate-intercalated CaMgAl-LDHs (O-CaMgAl-LDHs) using in-situ intercalation and modification, and the LDHs samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), and thermal gravimetric analysis (TGA). The FESEM observations showed that the as-prepared CaMgAl-LDHs had a lamellar structure with a particle size of 200~500 nm, while the O-CaMgAl-LDHs had a plate-like structure with a particle size of about 100 nm. TGA showed that O-CaMgAl-LDHs resulted in higher thermal stability at high temperature compared to CaMgAl-LDHs. O-CaMgAl-LDHs/ABS composites were prepared by adding O-CaMgAl-LDHs to acrylonitrile-butadiene-styrene resin (ABS) to test the resulting flame retardancy and mechanical properties, and the results showed that the limiting oxygen index (LOI) could increase from 18% to 26%, while the mechanical properties decreased significantly when the added fraction was 40% (relative to ABS). O-CaMgAl-LDHs, ammonium polyphosphate (APP) and expandable graphite (EG) were added into the ABS to prepare ABS composites, and the effects of different compositions on the flame retardancy and mechanical properties of the ABS composites were investigated. The results showed that, when adding 5 g of O-CaMgAl-LDHs, 1 g of APP, and 14 g of EG into 40 g of ABS, the LOI of the ABS composite reached 28.8%, and the composite prepared could meet the V-0 grade requirements of the UL-94 combustion test, while the flexural strength decreased only 21.9% compared to pure ABS, the smallest decrease compared to all of the other composites.

Effects of Aluminum Hydroxide and Layered Double Hydroxide on Asphalt Fire Resistance

When a fire occurs in a tunnel, the instantaneous high temperature and smoke cause great danger to people. Therefore, the asphalt pavement material in the tunnel must have sufficient fire resistance. In this study, the effects of aluminum hydroxide and layered double hydroxide on the fire resistance of styrene-butadiene-styrene (SBS) polymer-modified asphalt was investigated. The fire resistance of the asphalt was evaluated by using a limiting oxygen index (LOI). The impact of aluminum hydroxide (ATH), layered double hydroxide (LDHs), and mixed flame retardant (MFR) on LOI was studied. The synergistic fire resistance mechanism of ATH and LDHs in asphalt binder was analyzed by using an integrated thermal analyzer‒mass spectrometry combined system (TG-DSC-MS) and Fourier transform infrared spectrometer (FTIR). The experimental results indicated that the main active temperature range of these flame retardants was 221–483 °C. The main components of smoke were methane, hydroxyl, water, carbon monoxide, aldehyde, carbon dioxide, etc. The addition of flame retardants could inhibit the production of methane, carbon monoxide, and aldehyde. Moreover, due to the good synergistic effects of ATH and LDHs, 20 wt % MFR had the best fire resistance.

Biobased polyelectrolyte multilayer-coated hollow mesoporous silica as a green flame retardant for epoxy resin

Here, we describe a multifunctional biobased polyelectrolyte multilayer-coated hollow mesoporous silica (HM-SiO₂@CS@PCL) as a green flame retardant through layer-by-layer assembly using hollow mesoporous silica (HM-SiO₂), chitosan (CS) and phosphorylated cellulose (PCL). The electrostatic interactions deposited the CS/PCL coating on the surface of HM-SiO₂. Subsequently, this multifunctional flame retardant was used to enhance thermal properties and flame retardancy of epoxy resin. The addition of HM-SiO₂@CS@PCL to the epoxy resin thermally destabilized the epoxy resin composite, but generated a higher char yield. Furthermore, HM-SiO₂ played a critical role and generated synergies with CS and PCL to improve fire safety of the epoxy resin due to the multiple flame retardancy elements (P, N and Si). This multi-element, synergistic, flame-retardant system resulted in a remarkable reduction (51%) of peak heat release rate and a considerable removal of flammable decomposed products. Additionally, the incorporation of HM-SiO₂@CS@PCL can sustainably recycle the epoxy resin into high value-added hollow carbon spheres during combustion. Therefore, the HM-SiO₂@CS@PCL system provides a practical possibility for preparing recyclable polymer materials with multi-functions and high performances.

Preparation and characterization of polystyrene-MgAl layered double hydroxide nanocomposites using bulk polymerization

Polymer/mineral filler nanocomposites are more and more used for diverse applications. As mineral fillers, layered double hydroxides (LDHs) present a great advantage as flame retardants from an environmental point a view (reduction of smoke and toxic gases). This article deals with the use of LDH as flame retardants as compared to montmorillonite (MMT). In situ bulk polymerization of styrene was carried out in the presence of MgAl LDH modified with dodecyl sulfate (DDS) and dodecylbenzene sulfonate (DBS) surfactants. LDH concentrations used were lower than 10 wt.%. X-ray diffraction analysis of the LDH-styrene suspensions revealed the monomer intercalation into the DDS-LDH galleries and a slight decrease in the DBS-LDH basal spacing. Transmission electron microscopy analysis showed that the polymerization occurred outside the DBS-LDH galleries, leading to exfoliation of the layers on the outer surface of LDH platelets. DDS-LDH particles were trapped in the PS polymer. The thermal stability effect was observed for all LDH nanocomposites by thermogravimetric analysis. Cone calorimetry measurements revealed that only the DBS-LDH nanofiller resulted in a reduction of the peak heat released rate (PHHR) and a decrease of smoke released. DBS-LDH/PS exhibited fire properties close to those of clay-PS nanocomposite at 7 wt.% montmorillonite. The PHRR reduction remained small and the total heat release rate constant at 7 wt.% DBS-LDH loading.