Formation Of Layered Double Hydroxides Research Articles

One-pot synthesis of Mg[sbnd]Al layered double hydroxide (LDH) using MgO and metakaolin (MK) as precursors

Layered double hydroxides (LDH) are a class of basic inorganic layered compounds, which are widely used in the fields of adsorption and catalysis owing to their unique structure and properties. In previous studies, costly soluble Al salts or Al(OH)3 were used as the Al sources for the synthesis of MgAl LDH. The present work proposes a facile and one-pot method to synthesize MgAl LDH in a water bath with MgO and MK as precursors, which are both obtained from abundant natural resources. The effects of liquid/solid ratio (30:1 or 60:1), synthesis temperature (55–90 °C) and alkali concentration (3 or 6 mol/L) were investigated on the composition and characteristics of the synthetic products. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Thermogravimetric analysis (TGA), Scanning electron microscope (SEM) and Nuclear magnetic resonance (NMR). Findings showed that a highly alkaline solution (6 M NaOH) promoted the dissolution of MK, resulting in the release of trivalent Al for the formation of LDH. Increasing temperature also led to faster dissolution of MK and released more Al and Si, which resulted in the formation of both LDH and zeolite. More importantly, it was found that Mg2+ released during the hydrolysis of MgO combined with Al-containing substances directly to precipitate LDH, whereas its hydration product, i.e., brucite, remained stable in the alkaline condition and released little Mg2+ to participate in the formation of LDH. Zeolites may form depending on the Si/Al ratio of the solution which is governed by the various synthesis parameters. The findings not only shed lights on the reaction mechanism between the MgO and MK and the role of key synthetic conditions in the formation of LDH and zeolitic phases, but also demonstrated the feasibility of using widely-available, low-cost natural minerals to produce commercial adsorbents/catalysts.

Exploring the Influence of Milling Parameters on the Wet Mechanochemical Synthesis of Mg-Al Layered Double Hydroxides

The synthesis of Mg-Al layered double hydroxide (LDH) was explored, through a one-step wet mechanochemical route, with the use of a NETZSCH LME 1 horizontal bead mill. Raw materials selected comprised of a mixture of metallic oxides/hydroxides promoting green synthesis. The research aims to expand on the understanding of the wet mechanochemical synthesis of Mg-Al LDH through variation in milling and synthesis parameters. The selected parameters investigated were rotational speed, retention time, solids loading, bead size and jacket water inlet temperature. Samples were collected, filtered and dried at 60 °C for 12 h. Unless stated otherwise, or under investigation, parameters were kept constant at pre-selected conditions adapted from existing literature. LDH synthesis was deemed to occur successfully at elevated jacket water temperatures of 50 °C and longer retention times. It was noted that Al(OH)3 XRD peak reduction occurred readily for increased rotational speeds and residence times, regardless of system temperature. MgO was deemed to react more readily at elevated temperatures. It was proposed that the amorphitisation and mechanochemical activation of Al(OH)3 contributed to its dissolution providing the relevant Al3+ ions necessary for Mg2+ isomorphic substitution. Increasing the system temperature promoted the hydration of MgO, with the absence of Mg(OH)2 attributed to its contribution as an intermediate phase prior to LDH formation.

Solvothermal synthesis of carbonate-type layered double hydroxide monolayer nanosheets: Solvent selection based on characteristic parameter matching criterion

Monolayer nanosheets of CO32−-type layered double hydroxides (LDHs) have many special applications, but their fabrication is challenging. Herein, Co2Al-CO3 and Co2Fe-CO3 LDH nanosheets were synthesized via a solvothermal method. 31 solvents with different characteristic parameters, including the surface free energy (γ) and solubility (δ) parameters were chosen, to explore the correlation between the formation of monolayer LDHs (ML-LDHs) and the characteristic parameters of solvents. The results reveal that when the solvents used have the characteristic parameters matching to those of the LDHs, CO32−-type ML-LDHs with a thickness of ca. 1 nm can be obtained. The mixed-solvent strategy can provide the effective solvents for the synthesis of ML-LDHs. The dispersions of CO32−-type ML-LDHs can be stable for at least six months without obvious precipitation. In addition, it is demonstrated that the δ parameters of LDHs can be calculated from the γ parameters via the molar volume-free γ–δ equations developed previously. Furthermore, a new parameter called “surface free energy distance” is introduced, which can be used for screening effective solvents for the synthesis of ML-LDHs. To the best of our knowledge, this is the first time to investigate the applicable of the characteristic parameter matching principle for the bottom-up synthesis of ML-LDHs. This work deepens the understanding on the feature of CO32−-type LDHs and provides a solvent selection strategy for the synthesis of CO32−-type ML-LDHs.

Effects of particle size and mineral crystallinity on formation of Zn[sbnd]Al layered double hydroxides (LDH) on aluminum (oxyhydr)oxides

The widespread usage of zinc oxide nanoparticles (ZnO NPs) inevitably leads to its release in nature and interaction with environmental minerals. To reveal ZnO NP retention by natural aluminum (Al) minerals as layered double hydroxide (LDH) in the environments, the effects of Al (oxyhydr)oxide particle size and crystallinity on ZnAl LDH formation were investigated during ZnO NP interaction with seven different Al (oxyhydr)oxides at neutral and alkaline pHs. Dissolution kinetics displayed a decrease of dissolved Zn2+ and Al(OH)4− concentrations in a dual-solid system (ZnO and Al minerals) relative to a single-solid one (ZnO or Al minerals), suggesting a potential transformation of mixture solids. Diffraction, spectroscopic, and microscopic analyses indicated that Al (oxyhydr)oxide particle size and crystallinity had obviously impacted on ZnAl LDH formation. The composition percentage of ZnAl LDH in solid products followed the order of amorphous Al hydroxides (89.90% and 86.41% Zn species as ZnAl LDH at pH 7 and 10, respectively) > 5 nm γ-Al2O3 (41.62% and 49.62%) > 35 nm γ-Al2O3 (14.77% and 19.03%) > large size Al (oxyhydr)oxides (below 13.00%), including 70 nm γ-Al2O3, corundum, boehmite, and gibbsite. ZnAl LDH formation was regulated mainly by Al (oxyhyro)oxide crystallinity and particle size via changing Al oxide dissolution at pH 10 and Zn2+ adsorption at pH 7, whereas solution pH controlled the crystallinity of the formed ZnAl LDH. A dissolution-interaction-coprecipitation pathway was proposed to elucidate the mechanism of ZnAl LDH formation. The findings highlight the importance of substrate mineral size and crystallinity for ZnAl LDH formation in environmental settings.

Quantification by Powder X-ray Diffraction of Metal Oxides Segregation During Formation of Layered Double Hydroxides

A simple, easily applicable method is proposed to quantify the amount of ZnO segregated during synthesis and post-synthesis treatments of Zn-Al layered double hydroxides with the hydrotalcite structure. The method provides a feasible, easy, fast, and reliable way to get values for ZnO content, and the method can be extended to other compounds.

Quantification by Powder X-ray Diffraction of Metal Oxides Segregation During Formation of Layered Double Hydroxides

A simple, easily applicable method is proposed to quantify the amount of ZnO segregated during synthesis and post-synthesis treatments of Zn-Al layered double hydroxides with the hydrotalcite structure. The method provides a feasible, easy, fast, and reliable way to get values for ZnO content, and the method can be extended to other compounds.

Towards the Continuous Hydrothermal Synthesis of ZnO@Mg2Al-CO3 Core-Shell Composite Nanomaterials.

Core-shell Zinc Oxide/Layered Double Hydroxide (ZnO@LDH) composite nanomaterials have been produced by a one-step continuous hydrothermal synthesis process, in an attempt to further enhance the application potential of layered double hydroxide (LDH) nanomaterials. The synthesis involves two hydrothermal reactors in series with the first producing a ZnO core and the second producing the Mg2Al-CO3 shell. Crystal domain length of single phase ZnO and composite ZnO was 25 nm and 42 nm, respectively. The ZnO@LDH composite had a specific surface area of 76 m2 g−1, which was larger than ZnO or Mg2Al-CO3 when produced separately (53 m2 g−1 and 58 m2 g−1, respectively). The increased specific surface area is attributed to the structural arrangement of the Mg2Al-CO3 in the composite. Platelets are envisaged to nucleate on the core and grow outwards, thus reducing the face–face stacking that occurs in conventional Mg2Al-CO3 synthesis. The Mg/Al ratio in the single phase LDH was close to the theoretical ratio of 2, but the Mg/Al ratio in the composite was 1.27 due to the formation of Zn2Al-CO3 LDH from residual Zn2+ ions. NaOH concentration was also found to influence Mg/Al ratio, with lower NaOH resulting in a lower Mg/Al ratio. NaOH concentration also affected morphology and specific surface area, with reduced NaOH content in the second reaction stage causing a dramatic increase in specific surface area (> 250 m2 g−1). The formation of a core-shell composite material was achieved through continuous synthesis; however, the final product was not entirely ZnO@Mg2Al-CO3. The product contained a mixture of ZnO, Mg2Al-CO3, Zn2Al-CO3, and the composite material. Whilst further optimisation is required in order to remove other crystalline impurities from the synthesis, this research acts as a stepping stone towards the formation of composite materials via a one-step continuous synthesis.

Investigation of the structure-luminescence relationship in ZnAlEu layered double hydroxides intercalated with nitrate and benzenecarboxylates

Rare Earth-containing materials exhibit interesting luminescence properties, such as a spectral profile with narrow emission lines and a well understood crystal field dependence. These features make them appealing materials for molecular probing. Here, the structural properties governing the luminescence of Eu3+ ions hosted in layered double hydroxides (LDH) are investigated. Eu3+-substituted LDH have been shown to present photoluminescence properties dependent on the anionic species intercalated between the hydroxide layers. This feature is shown by the characterization and comparison between LDH intercalated with NO3− and a set of benzenecarboxylates: trimesate (benzene-1,3,5-tricarboxylate, BTC), trimellitate (benzene-1,2,4-tricarboxylate, TLA) and terephthalate (benzene-1,4-dicarboxylate, TERE). Successful LDH formation and intercalation with the respective anion was confirmed by powder X-ray diffraction (PXRD). The coordination structure of Eu3+ has been investigated by Extended X-ray Absorption Fine Structure (EXAFS) experiments, showing the incorporation of this activator in the hydroxide layers of the LDH in an 7/8-fold Oxygen-rich environment. The systems here investigated present very similar coordination, except for the presence of a water related Oxygen at around 3.0 Å in the nitrate-intercalated LDH, absent in the benzenecarboxylate-intercalated materials. The photoluminescence properties of all samples were analysed on the basis of the Judd-Ofelt theory, showing that the polycarboxylate- and NO3−-intercalated LDH can be grouped into two distinct classes with different sets of Judd-Ofelt intensity parameters (Ω2 and Ω4). This has been rationalized based on the EXAFS results. Symmetry distortions in the first coordination shell of Eu are also expected from the observed changes in Ω2 and Ω4. This distinctive feature allows one to probe interlayer modifications in LDH through changes in the luminescence properties of lanthanide activators hosted in their hydroxide layers, opening up new opportunities for in-situ investigation of anion exchange reactions in these materials.

Synthesis of NiFeAl LDHs from electroplating sludge and Their excellent supercapacitor performance

This work demonstrated that electroplating sludges (EPS) of specific composition may be used for the synthesis of layered double hydroxide (LDH) materials for energy applications after appropriate treatment. The unique composition and structure of EPS render it with good electrochemical energy storage performance. The EPS containing Ni, Fe, and Al was dissolved by acid and added with urea precipitator. The LDH material was prepared by a facile hydrothermal method. The increase of urea in a certain range is conducive to the formation of intact LDH. However excessive urea levels promoted the transformation from LDH to Ni(HCO3)2. Various active Ni bridged by N in ‒O‒CN promoted electron transfer, ‒O‒CN content in LDHs was proportional to the urea amount. The prepared LDHs exhibited a specific capacitance of 1652.20 F g−1 at 0.5 A g−1, and the value remained at 766.69 F g−1 after 1000 cycles. The prepared LDH has excellent supercapacitor performance, which is closely related to its structure. Therefore, the proposed recycling strategy of EPS resources can be used to prepare LDH supercapacitors, paving the way for new applications of EPS in the field of energy storage.

A two-step pH control method to remove divalent metals from near-neutral mining and metallurgical waste drainages by inducing the formation of layered double hydroxide

A neutral M2+-rich and M3+-poor (M = metal) metallurgical waste drainage was used to test a metal removal method based on the precipitation of layered double hydroxide (LDH). The LDH precipitation was induced by adding a salt of Al3+ (trivalent metal missing in the drainage) and maintaining or restoring the pH to a circum-neutral value. The precipitates were characterized by chemical analysis, XRD, ESEM, HRTEM and XAS. The main parameter controlling the removal of metals and the type of precipitate appeared to be the pH. As a function of pH variation during the experiments, analyses of precipitates and solutions showed either the formation of poor crystalline LDH combined with very high removal of Zn, Ni and Pb (92–100%), more variable removal of Mn (46–98%) and less Cd (33–40%), or the formation of more crystalline LDH combined with lower removal of Zn (62%), Mn (43%), Ni (88%), Pb (64%) and especially Cd (1%). The different metal removal efficiency in the two cases is only indirectly due to the different LDH crystallinity, and it is clearly affected by the following factors: 1) the two pH steps of the method; 2) the direction of pH variation within each step. In particular, the highest removal of metals is obtained when the first pH step goes towards acidic conditions, as a consequence of Al salt addition, and precipitation of a quasi-amorphous hydrated hydroxysulfate of Al (probably a precursor of felsӧbányaite Al4(SO4)(OH)10 · 4H2O) occurs. This first acidic pH step removes little or no metals (just 0–3%) but it is essential so that the second pH step towards slightly alkaline conditions, as a consequence of NaOH addition, can be highly efficient in removing divalent metals as the quasi-amorphous hydrated hydroxysulfate of Al gradually turns into an LDH incorporating Zn, Mg and other metals. On the contrary, when both pH steps remain in the neutral-alkaline range, only LDH precipitation occurs and a lower metal removal is observed. These results encourage further investigations on the removal of metals by inducing LDH precipitation as a simple and effective method for the treatment of circum-neutral polluted drainages.

Use of synergistic mixture of chelating agents for in situ LDH growth on the surface of PEO-treated AZ91

The principal possibility to grow layered double hydroxide (LDH) at ambient pressure on plasma electrolytic oxidation (PEO) treated magnesium alloy AZ91 in the presence of chelating agents is demonstrated for the first time. It avoids hydrothermal autoclave conditions, which strongly limit wide industrial application of such coating systems, and the presence of carbonate ions in the electrolyte, which lead to the formation of “passive” non-functionalizable LDH. A combination of chelating agents (sodium diethylenetriamine-pentaacetate (DTPA) and salicylate) were introduced to the treatment solution. The role of each additive and the influence of treatment bath composition on the LDH formation processes are discussed. A synergistic effect of DTPA and salicylate during LDH formation is discovered and its possible explanation is proposed.

Comprehensive study of the formation of stable colloids of Cu[sbnd]Al layered double hydroxide assisted by double hydrophilic block copolymers

The formation of stable aqueous colloidal suspensions of CuAl layered double hydroxide (LDH) assisted by double-hydrophilic block copolymers was investigated. It was of utmost importance since the direct preparation LDH colloids of controlled size is still a major challenge; moreover, regarding the CuAl system, the mechanism of formation of the LDH phase is not well understood yet. CuAl LDH nanoparticles were obtained by complexing Cu2+ and Al3+ cation mixture (Cu2+/Al3+ = 2) with an asymmetric poly(acrylamide)-b-poly(acrylic acid) block copolymer, followed by metal cation hydroxylation. The mechanism of formation of the CuAl LDH was studied and compared to that of MgAl LDH. ICP-MS analysis of the hybrid polyion complex (HPIC) micelles obtained at increasing complexation ratio, i. e. the ratio of the molar concentration of complexing acrylate group of DHBC to M2+ and Al3+ cations (R = AA/(M2+ + Al3+)), showed higher selectivity of DHBC toward Al3+ and, among M2+ cations, for Cu2+ than for Mg2+. Hydroxylation of the HPIC micelles led to macroscopic precipitates at R values below flocculation thresholds, i. e. R1 = 0.43 for DHBC/Cu-Al and 0.13 for DHBC/Mg-Al suspensions, while stable suspensions were formed above R1. The hydrodynamic diameter (Dh) of the colloids determined by DLS decreased from 160 to 50 nm and from 350 to 50 nm in the DHBC/Cu-Al and DHBC/Mg-Al colloids, respectively, when R increased from R1 to 1. XRD and TEM analyses of the dried colloidal suspensions (R > R1) revealed the presence of CuAl LDH and MgAl LDH phases and elemental analyses confirmed that M(II)/Al = 2. Particle sizes were lower in CuAl LDH than in MgAl LDH. Titration curves corresponding to progressive hydroxylation of DHBC/Cu(or Mg)-Al solutions at fixed complexation ratio R allowed establishing that formation of CuAl and MgAl LDH obeyed to a similar sequential mechanism in two steps but involving different aluminum hydroxide precursors according to the different pH values of precipitation. CuAl LDH was obtained by combination of aluminum poly(hydr)oxide species with dissolved copper-based species at pH ~ 5.5, while MgAl LDH resulted from combination of Al(OH)3 and dissolved Mg2+ at pH ~ 8.

Facile method for treating Zn, Cd, and Pb in mining wastewater by the formation of Mg–Al layered double hydroxide

Heavy metal pollution threatens aquatic systems worldwide, and mining activities are an important pollution source. Currently, the treatment of polluted water using a cost-effective technology that can purify multiple pollutants and is sustainable, environmentally friendly, and simple is a major challenge. The in situ preparation of Mg–Al layered double hydroxides (LDHs) and the concurrent treatment of Zn, Cd, and Pb from mining wastewater are important for preventing multiple steps and increasing adsorption sites. This study focused on mining wastewater containing high concentrations of Mg2+ and Al3+, with anion chemistry controlled by SO42−, which facilitates the formation of LDHs. The required amounts of Mg2+ and Al3+ ions were added to the wastewater, and the conditions for the creation of Mg–Al LDHs were controlled. The heavy metals in the experimental wastewater were effectively removed after the treatment via Mg–Al LDH formation. The X-ray diffraction of the post-treatment products suggested the formation of Mg–Al LDHs. The Mg/Al molar ratio (2.3:1) in the product approached the initial ratio (2:1), which meets the general limits of Mg–Al LDH formation. Scanning electron microscopy revealed that the products had a sheetlike stacked morphology, providing evidence for the formation of Mg–Al LDHs. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated that SO42− might be the intercalated anion in the Mg–Al LDH layers. Consequently, SO42− was removed from the mining wastewater, as it was captured between the LDH layers during the formation reaction of Mg–Al LDHs.

Performance of Halloysite-Mg/Al LDH Materials for Aqueous As(V) and Cr(VI) Removal.

This research focused on the investigation of layered double hydroxide (LDH)/halloysite materials’ adsorption efficiency and mechanisms in reactions with aqueous As(V) and Cr(VI) in a broad pH range. The materials consisting of Mg/Al LDH and halloysite were synthesized using both direct precipitation and physical mixing methods. The XRD, FTIR, DTA, SEM and XPS methods were used to evaluate the quality of the obtained materials and get insight into removal mechanisms. The XRD, FTIR and DTA confirmed LDH formation and showed the dominating presence of intercalated carbonates in the LDH structure. The SEM of the materials revealed characteristic agglomerates of layered LDH particles deposited on halloysite tubular forms. The raw LDH phases showed high removal efficiency of both As(V) and Cr (VI) for initial pH in the range of 3–7. In the studied concentration range the materials containing 25 wt % of LDH exhibited a removal efficiency very similar to the raw LDH. In particular, the halloysite presence in the materials’ mass had a positive effect in the reactions with As(V), which was removed by chemisorption. At a low pH the LDH component underwent partial dissolution, which lowered the adsorption efficiency. Apart from the anion exchange mechanism at a low pH the Cr(VI) was removed via formation of MgCrO4 with Mg (II) being released from the LDH structure. The XPS spectra for As(V) did not show changes in oxidation state in the reactions. In turn, a partial reduction of Cr(VI) to Cr(III) was observed, especially at a high pH. The use of materials composed of two different minerals is promising due to reduction of costs as well as prevention of adsorbent swelling. This opens the possibility of its use in dynamic adsorption flow through systems.

Characterization of LDHs prepared with different activity MgO and resisting Cl− attack of concrete in salt lake brine

Mg-Al layered double hydroxides (LDHs) were synthesized by roasted bauxite and MgO. The ability of concrete with LDHs synthesized by dead-burned MgO to resist Cl− erosion was investigated in a solution reminiscent of a salt lake. The results indicate that greater MgO activity results in faster LDH formation. With an increase in both the curing temperature and the curing time, the LDH content prepared by dead-burned MgO increased gradually. The formation of LDHs is controlled by the activity of bauxite in the bauxite-MgO-water system. Because CO32− and Cl− both enter the LDH intercalation, the Cl− permeability coefficient of the concrete with LDHs was greatly decreased.

Removal Mechanism of Concrete Deterioration Factor Using Expanded-Cafe Layered Double Hydroxides

The main deterioration factor ions cause corrosion and expansion of concrete with chloride, sulfate and carbonate ions. It is a major problem in field of construction resulting in a huge economic loss, also inducing serious safety disasters frequently. Layered double hydroxides (LDHs) has been successfully used for various purposes owing to its anion-exchange property. Among them, the calcium-based LDHs can improve immobilizing the migration of chloride, sulfate and carbonate ions in cement-based materials. In this study, we will focus on the description of the structural transformation of CaFe LDHs and anion exchange mechanism of immobilizing deterioration factors. The formation of expanded-CaFe LDHs occurred under the calcination conditions (400℃, 550℃ and 700℃, respectively). The characterizations of the expanded-CaFe-LDH after the adsorption was investigated using ion chromatography, x-ray fluorescence, fourier transform infrared spectroscopy, x-ray diffraction, field emission scanning electron microscopy and transmission electron microscope. The adsorption mechanism of ions by the CaFe LDHs related with structure property variation that happens during calcination process.

In-situ growth of layered double hydroxide films on biomedical magnesium alloy by transforming metal oxyhydroxide

Layered double hydroxide (LDH) films have been widely used for corrosion protection and medical applications. The most common synthesis methods for in-situ growth of LDH films on magnesium (Mg) alloys are hydrothermal treatment and steam coating. However, both the methods are mainly used for preparing MgAl LDH films and have limitations to in-situ fabricate other Mg-based LDH films which are suitable for biomedical application. Herein, a novel synthesis strategy for in-situ growth of LDH films on Mg alloys is proposed. Firstly, amorphous ferric oxyhydroxide (FeOOH) film was developed on Mg-Nd-Zn-Zr (JDBM) alloy by electrodeposition technique. Then, the as-prepared FeOOH films were transformed into MgFe LDH film via hydrothermal treatment in water. In the LDH formation process, Mg2+ would enter into amorphous FeOOH and replace the Fe3+ quickly to form LDH. The thickness of the LDH films can be easily controlled by adjusting electrodeposition time. Electrochemical tests and cells culture experiments indicated that the as-prepared MgFe LDH films improved corrosion resistance and biocompatibility of JDBM alloy. In addition, MgMn LDH can also be prepared from amorphous manganese oxyhydroxide (MnOOH) via this newly developed two-step method. The present work provides a new strategy for preparation of LDH films on Mg alloys surface.

Sonication accelerated formation of Mg-Al-phosphate layered double hydroxide via sol-gel prepared mixed metal oxides

Single-phase magnesium-aluminium layered double hydroxide (LDH) intercalated with dihydrogen phosphate was successfully produced by hydration of nanopowder of the respective mixed metal oxide (MMO) obtained using sol-gel based method followed by a two-step anion exchange hydroxide-to-chloride and chloride-to-phosphate. The MMO with the metal cation ratio of Mg/Al = 2:1 was prepared using the aqueous sol-gel method. Processes of the parent Mg2Al-OH LDH formation and the successive anion-exchanges, ОН− → Cl− and Cl− → H2PO4−, were considerably accelerated via the application of high-power (1.5 kW) ultrasound. The crystalline phases formed at all stages of the Mg2Al-H2PO4 LDH production were characterized using X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, inductive coupled plasma optical emission spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Based on the data of chemical analysis and the XRD data, the type of the intercalated phosphate anion was determined and the arrangement of this anion in the interlayer was modelled.

Layered double hydroxide based active corrosion protective sealing of plasma electrolytic oxidation/sol-gel composite coating on AA2024

This work reports a novel approach for growing layered double hydroxide (LDH) films on any plasma electrolytic oxidation (PEO) coated AA2024 independently of the nature of the PEO coating. The specific PEO coating chosen to carry out this work is considered to be not suitable for direct LDH growth because of phase composition and morphological features. In this paper, we describe a new methodology that consists of covering the PEO coating with a thin layer of aluminum oxide based xerogel as the source of aluminate ions for subsequent in-situ LDH growth. X-ray diffraction (XRD) and scanning electron microscope (SEM) images showed a successful formation of LDHs on the surface. An improvement in terms of active corrosion protection was also demonstrated by electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET).

In-line monitoring of layered double hydroxide synthesis and insights on formation mechanism and kinetics

Anionic clays such as layered double hydroxides (LDH) are promising materials for pharmaceutical applications, electrochemistry, engineering, bioremediation, and agriculture. The understanding of the mechanisms involved in the formation of LDH is an important step for their safe application and reproducible production. In this study, we in-line monitored the synthesis of Zn2Al-Cl and Mg2Al-Cl LDH using near infrared spectroscopy to explain the formation mechanism and kinetics of these materials when employing the coprecipitation method in organic environments. The data were evaluated chemometrically and principal component analysis (PCA) was used to investigate existing components related to the reaction path. The zeta potential (ζ) values indicated that, initially, unstable particles formed (ζ < +30 mV), which nucleated fast, and this was followed by a second, slower reaction stage, owing to the crystallization of the LDH. These processes were confirmed using the multivariate curve resolution-alternating least squares method, which demonstrated that the synthesis reaction followed a second-order kinetics. These findings were corroborated using microscopic and diffraction analysis.