Equilibrium pH Values Research Articles

Insights Into the Adsorption Behavior of Polyethylene Microplastics Towards Lead(II) Ions

ABSTRACTMicroplastics and metals represent widespread contaminants which can cause significant damage to aquatic ecosystems and organisms. However, the complex interactions between microplastics and various metals in water environments remains to be understood. This study aims to investigate the interaction dynamics between lead(II) ions and polyethylene in freshwater environments. Adsorption trials were conducted systematically by adjusting operational parameters such as initial Pb(II) concentration, equilibrium pH value, and residence times. An in‐depth characterization study using scanning electron microscopy (SEM), x‐ray diffraction and Fourier transform infrared (FTIR) analysis was conducted to explain the adsorption mechanism of polyethylene microplastics (PEM). The data indicated the porous surface of PEM, highlighting the presence of diverse functional groups. According to the Langmuir model, the PEM exhibited maximum Pb(II) uptake of 3.69 mg/g at pH 4.5. The pseudo‐first‐order model demonstrated superior fitting to Pb(II)‐PEM kinetics. Desorption trials were performed to assess the release of Pb(II) from Pb(II)‐bounded PEM using various chemical agents. It was observed that 0.01 M HNO₃ desorbed Pb(II) ions optimally, achieving a desorption efficiency exceeding 99.9%. Consequently, desorption trials provided evidence that Pb(II)‐bounded PEM may release Pb(II) ions in acidic environments, facilitating the transfer of Pb(II) to the digestive tracts of aquatic organisms.

Separation of heavy metals from dilute solutions using D2EHPA carrier fixed onto the outer surface of coaxially electrospun core-sheath fibers

In this study, porous membranes based on core-sheath fibers covered with fixed carriers were prepared by coaxial electrospinning for the separation of heavy metal ions from dilute solutions. Cellulose triacetate (CTA) and an extractant di(2-ethylhexyl)phosphoric acid (D2EHPA) were mixed to prepare a sheath-layer solution and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was adopted to prepare a core-layer solution for electrospinning. The morphological and textural properties as well as chemical composition of the fabricated core-sheath fibers were first analyzed to determine the optimal preparation conditions. The ability and selectivity of such fixed D2EHPA carriers were evaluated via the adsorption of Zn2+, Cu2+, and Ni2+ ions from dilute solutions. Dynamic tests revealed that the adsorption of metal ions on the core-sheath fibrous membrane made of 52 wt% PVDF-HFP, 18 wt% D2EHPA, and 30 wt% CTA (on a dry basis) reached equilibrium within 8 h. As expected, all adsorption isotherms at specific equilibrium pH values were well described by the Langmuir equation, ascribed to the chemical reaction nature between metal ions and D2EHPA carrier. The crossflow permeation-adsorption tests of a multi-metal solution in a total recycling mode revealed the adsorption selectivity of Zn2+ over Cu2+ and Ni2+, as observed in conventional liquid–liquid extraction. Four repeated adsorption-regeneration experiments revealed an adsorption efficiency beyond 95 % relative to the first route, demonstrating the satisfactory reusability and stability of the as-prepared core-sheath fibrous membranes for this purpose.

Transformation mechanism and selective leaching of nickel and cobalt from limonitic laterite ore using sulfation-roasting-leaching process

The sulfation-roasting-leaching process holds promise as an environmentally friendly method of production but has low recovery efficiency and poor selectivity for Ni and Co in ores containing impurity metals, including Fe, Al, and Cr. In this study, the transformation mechanism and leaching behavior of metals in a limonitic laterite ore during the sulfation-roasting-leaching process were investigated. Results suggest that the transformation of ferrous ion can reduce leaching efficiency for impurity metals in air atmosphere. The leaching efficiencies for Ni, Co, and Al were 91.1%, 91.5%, and 3.3%, respectively, during roasting at 725 °C for 1.5 h. The leaching solution equilibrium pH value was 3, the concentration of the added H2O2 was 0.5 wt%, and Fe and Cr were not detected in the leaching solution. Simulation and characterization results indicated that the conversion of Fe(II) into Fe(III) reduced the amount of FeSO4 in the roasting product and inhibited iron dissolution, demonstrating that the sulfation-roasting-leaching process is a feasible and environmentally friendly method that selectively leaches Ni and Co in ores containing trivalent metals.

Synergistic Extraction of Ga(III) from Chloride Solution by Mixture of Cyanex 272 and Alamine 336

Aming to select a suitable system for the synergistic solvent extraction of Ga(III), solvent extraction (SX) experiments were carried out by using acidic organo-phosphorous extractants, namely P204, P507 and Cyanex 272, and their mixtures with neutral extractants (TBP, N503) or amine extractants (Alamine 336). Among the studied mixtures, a remarkable synergistic effect for Ga(III) extraction was resulted by the combination of Cyanex 272 and Alamine 336 and the maximal synergistic enhancement coefficient reached 3.11. Compared to the single Cyanex 272, higher equilibrium pH values were observed when mixture of Cyanex 272 and Alamine 336 was employed. The extraction of hydrogen ions by Alamine 336 during the reaction was considered to be responsible for the synergistic extraction, and thus the the extraction of Ga(III) by Cyanex 272 was promoted. Finally, the synergistic extraction stoichiometry of mixture of Cyanex 272 and Alamine 336 for Ga(III) was projected by slope analysis. Ga(OH)A2 was resulted to be the composition of extracted complexes.

Thymol-menthol-based deep eutectic solvent as a modifier in reactive liquid–liquid extraction of carboxylic acids from pretreated sweet sorghum silage press juice

Reactive liquid–liquid extraction has been widely investigated as a means of isolating carboxylic acids from fermentation broths. However, commonly applied fossil-based solvent phases should be replaced by green alternatives. In this work, we investigate a thymol-menthol-based deep eutectic solvent (tmDES) as a green alternative to the commonly applied modifier 1–octanol for the extraction of lactic acid, acetic acid, and oxalic acid from a model solution and pre-treated sweet sorghum silage press juice (pSPJ); tri–n–octylamine (TOA) was used as a reactive extractant. Additionally, NMR spectroscopy was used to detect interactions between menthol and thymol molecules, as well as among these and TOA and lactic acid. NMR measurements of tmDES revealed the existence of multiple intermolecular contacts between thymol and menthol. Furthermore, nuclear Overhauser effects (NOEs) between the tmDES, TOA, and lactic acid molecules were found. Extraction experiments showed that tmDES improves the extraction efficiency when using pSPJ as the feed phase. The maximum extraction efficiency for lactic acid increased from 35.0 ± 0.8 % to 48.7 ± 0.7 % and for acetic acid from 52.4 ± 0.2 % to 82.2 ± 0.6 % when using a tmDES instead of 1–octanol at 50 °C. Furthermore, the use of tmDES resulted in higher extraction efficiencies as compared to the use of 1–octanol at equilibrium pH values ranging from five to seven, which is a common pH range for fermentation. Besides having superior biocompatibility and extraction efficiency as compared to 1–octanol, tmDES allow carboxylic acid extraction processes to be carried out at higher pH levels.

Cradle-to-cradle recycling of spent NMC batteries with emphasis on novel Co2+/Ni2+ separation from HCl leached solution and synthesis of new ternary precursor

Herein, a cradle-to-cradle recycling of post-consumer NMC batteries has been demonstrated with a focused separation of cobalt over nickel by applying the ionic liquid, trihexyl(tetradecyl)phosphonium bis-2,4,4-(trimethylpentyl)phosphinate. At the optimized condition of: 0.8 mol·L−1 ionic liquid, 3.0 mol·L−1 Cl− ions, equilibrium pH value ∼5.0, and organic-to-aqueous ratio 2/3, > 99% cobalt was extracted in the organic phase with a separation factor of 1097. Approximately 99% cobalt from the loaded organic was stripped back in 2.0 mol·L−1 HCl solution, yielding high-pure CoCl2·xH2O crystals after crystallizing the stripped solution. Subsequently, ∼99% of nickel from the Co-depleted raffinate was extracted over lithium using 0.32 mol·L−1 acetophenone at an organic-to-aqueous ratio of 1 and equilibrium pH ∼5.3. Nickel stripped in 2.0 mol·L−1 H2SO4 was crystallized to yield high-pure NiSO4·6H2O crystals. Further, Li-bearing raffinate was subjected to carbonate precipitation at a higher pH (∼12) and CO32−:Li+ ratio of 1.2. All the recycled products were further employed to the stepwise synthesis of a new ternary precursor, exhibiting similar electrochemical behaviour (with 149 mAh·g−1 capacity) and found compatible with the precursor prepared using virgin materials. The sustainable process index value determined to be 0.0006 cap·t−1 for the overall recycling process indicated that the process is suitable for sustainable development.

Ecological treatment of El Kriymat boiler ash for recovering vanadium, nickel and zinc from sulfate leach liquor

The primary goal of this work is to develop a technology that allows for the recovery of metal values from waste products, thereby promoting the wise and efficient use of our nation's resources. To achieve this goal, an industrial waste of El Kriymat boiler fly Ash was used for recovering its content of vanadium, nickel and zinc. About 97, 95 and 99% respectively of these economic elements were first dissolved from boiler fly ash magnetic concentrate (after physical concentration). Leaching experiments using optimum conditions include: 180 g/L sulfuric acid concentration and 4% solid/solid proportion manganese dioxide acts as an oxidant at 80 °C. The recovery of vanadium (V) metal ions was carried out using 3% Alamine 336 in kerosene at an equilibrium pH value of 0.9. Subsequently, 15% sodium sulfide solution was used for co-precipitation of nickel and zinc metal ions in the raffinate solution at pH value of 3.5.Graphical

Recovery of critical metals from spent Li-ion batteries: Sequential leaching, precipitation, and cobalt–nickel separation using Cyphos IL104

This study presents a novel recycling scheme for spent Li-ion batteries that involves the leaching of lithium in hot water followed by the dissolution of all transition metals in HCl solution and their separation using the ionic liquid Cyphos IL104. The parametric studies revealed that >84 % Li was dissolved while the cathode material was leached at 90 °C for 2 h. Approximately 98 % Li from the non-acidic solution was directly precipitated as Li2CO3 at a Li+:CO32− ratio of 1:1.5. The transition metals from the Li-depleted cathode mass were efficiently (>98 %) dissolved in 3.0 mol·L−1 HCl at 90 °C for a 3 h leaching process. Manganese from the chloride leach liquor was selectively precipitated by adding KMnO4 at a 1.25-fold higher quantity than the stoichiometric ratio, pH value 2.0, and temperature 80 °C. The remaining co-existing metals (Ni and Co) were separated from the chloride solution by contacting it with a phosphonium-based ionic liquid at an equilibrium pH value of 5.4 and an organic-to-aqueous phase ratio of 2/3. The loaded ionic liquid was quantitatively stripped in 2.0 mol·L−1 H2SO4 solution, which yielded high-purity CoSO4·xH2O crystals after evaporation of the stripped liquor. Subsequently, ∼99 % nickel was recovered as nickel carbonate [NiCO3·2Ni(OH)2] from the Co-depleted raffinate by the precipitation performed at Ni2+:CO32− ratio of 1:2.5, pH value of 10.8, and temperature of 50 °C. Finally, a process flow with mass and energy balances yielding a high recovery rate of all metals in the exhausted cathode powder of spent LiBs was proposed.

Remediation of uranium-contaminated acidic red soil by rice husk biochar.

Uranium (U) in the U-contaminated acidic red soil exhibits high mobility. In the present study, rice husk was used to produce biochar to remediate U-contaminated red soil under acid precipitation. Firstly, batch adsorption experiments showed that the dissolution of alkaline substance in biochar could buffer the pH value of acidic solution. The equilibrium pH value had a crucial influence on biochar adsorption capacity of U, and the neutral equilibrium pH value was favorable for adsorption. Then, the incubation experiments of red soil with biochar were performed, and the Synthetic Precipitation Leaching Procedure (SPLP) extraction of amended red soil showed that the short-term leachability of U was decreased from 26.53% in control group (without biochar) to 1.40% in 10% biochar-amended red soil. Subsequently, the sequential extraction showed that the fraction of U was mainly transformed from exchangeable and Fe/Mn oxide fraction to carbonate fraction after biochar amendment, and the total amount of exchangeable U and carbonate fraction U in soil was increased slightly. Finally, simulated acid rain leaching experiments showed that the capability of amended red soil to resist acid rain acidification was enhanced. And the long-term leachability of U in amended red soil was decreased from 26.37% in control group to 3.18% in the 10% biochar-amended red soil under the simulated acid rain leaching conditions. In conclusion, biochar has passivation effect on U in U-contaminated red soil, which can reduce the long-term and short-term mobility of U in acidic environments. This study provided an experimental basis for the application of biochar in remediation and improvement of U-contaminated acidic red soil.

Reactive liquid–liquid extraction of bio-based 3-hydroxypropionic acid using a biocompatible organic phase containing a tertiary amine: A model-based approach to elucidate dominant mechanisms

This work studies reactive liquid–liquid extraction of 3-hydroxypropionic acid (3-HP) from aqueous solutions using a biocompatible organic phase consisting in N,N-didodecylmethylamine (DDMA) (20% v/v) diluted in 1-dodecanol (40% v/v) and dodecane (40% v/v). The objective was to propose an equilibrium model based on the law of mass action accounting for the main phenomena occurring in the system. A set of equilibrium extraction experiments was performed in the initial acid concentration range 0.0028 – 1 mol L-1 in order to determine the extraction yield, equilibrium pH and to collect infrared spectra of the loaded organic phases. FT-IR spectra allowed to identify and elucidate the main mechanisms of acid–amine interaction, which were taken into account in the model. The formation of 1:1 acid–amine stoichiometry complex through ion-pairing was determined as the dominant mechanism in all the concentration range. For initial acid concentrations above 0.11 mol L-1, the formation of 2:1 acid–amine complexes through H-bonding of acid molecules with 1:1 complexes was also observed and determined to be almost as important as ion-pair formation at 1 mol L-1. The presence of impurities in the organic phase was taken into account by introducing a side reaction essential to represent low extraction yields and high equilibrium pH values observed at low initial acid concentrations (< 0.02 mol L−1). A global sensitivity analysis of the model’s parameters pointed out three concentration regions based on the relative contribution of each phenomenon: (i) for initial acid concentrations below 0.02 mol L−1, the main phenomena are 1:1 ion-pair formation and the side reaction; (ii) between 0.02 mol L−1 and 0.11 mol L−1, mainly 1:1 ion-pair formation; and (iii), for concentrations above 0.11 mol L−1 both considered complexation reactions i.e., 1:1 ion-pair formation and 2:1 H-bonding. The proposed model allowed thus to evaluate the relative importance of the phenomena occurring in the system in all the studied acid concentration range.

Relationship between Surface Hydroxyl Complexation and Equi-Acidity Point pH of MnO2 and Its Adsorption for Co2+ and Ni2.

MnO2 has shown great potential in the field of adsorption and has a good adsorption effect on heavy metal ions in aqueous solution, but there have been problems in the adsorption of heavy metal ions in high-concentration metal salt solutions. In this paper, different crystal forms of MnO2 (α-MnO2, β-MnO2, γ-MnO2, δ1-MnO2, δ2-MnO2, and ε-MnO2) were prepared and characterized by XRD, SEM, EDS, XPS, ZETA, and FT-IR. The reasons for the equi-acidity point pH change of MnO2 and the complex mechanism of surface hydroxylation on metal ions were discussed. The results showed that the equi-acidity point pHs of different crystalline MnO2 were different. The equi-acidity point pH decreased with the increase of reaction temperature and electrolyte concentration, but the reaction time had no effect on it. The equi-acidity point pHs of MnO2 were essentially equal to the equilibrium pH values of adsorption and desorption between surface hydroxyl and metal ions on them. The change of equi-acidity points was mainly due to the complexation of surface hydroxyl, and the equi-acidity point pHs depended on the content of surface hydroxyl and the size of the complexation ability. According to the equi-acidity point pH characteristics of MnO2, more hydroxyl groups could participate in the complexation reaction by repeatedly controlling the pH, so that MnO2 could adsorb heavy metals Co2+ and Ni2+ in high-concentration MnSO4 solution, and the adsorption rates of Co2+ and Ni2+ could reach 96.55 and 79.73%, respectively. The effects of MnO2 dosage and Mn2+ concentration on the adsorption performance were further investigated, and the products after MnO2 adsorption were analyzed by EDS and FT-IR. A new process for MnO2 to adsorb heavy metals Co2+ and Ni2+ in high-concentration MnSO4 solution was explored, which provided a reference for the deep purification of manganese sulfate solutions.

Aqueous Processed Ni-Rich Li(Ni0.8Co0.1Mn0.1)O2 Cathodes Along with Water-Based Binders and a Carbon Fabric as 3-D Conductive Host

A carbon fabric with three-dimensional carbon fiber network is proposed as a conductive framework instead of Al current collector, in order to fabricate a LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode using an environmentally friendly water-based process. Water-based NCM811 slurries prepared without binder, with poly(acrylic acid) (PAA) binder, and with poly(acrylonitrile) (PAN) binder have the equilibrium pH values of 12.4, 6.7, and 12.6, respectively. Without a binder, the cathode materials are poorly dispersed in the slurry, which hinders smooth casting on the carbon fabric. The addition of either PAA or PAN improves the uniform dispersion of particles and the loading capability of slurries, leading to a lower impedance of the cast cathodes. Compared to PAA, PAN performs better with the carbon fabric as a conductive framework, due to the better dispersion of electrode materials, lower impedance in the fabricated electrode, and good chemical stability of PAN to the electrolyte. The cell constructed with PAN-added cathode exhibits a high specific capacity of 150–160 mAh g−1 at a charge/discharge rate of 0.5 C, and nearly 100% capacity retention after 100 cycles. Nevertheless, PAA is soluble in the electrolyte and has some negative effects on the capacity performance of the constructed cell.

Separation of lithium and transition metals from the leachate of spent lithium-ion battery by extraction-precipitation with p-tert-butylphenoxy acetic acid

Unlike common solvent extraction and chemical precipitation, a novel extraction-precipitation process for the separation of Li and transition metals using p-tertylphenoxyacetic acid (POAA) is developed. Extraction-precipitation method refers to a method in which the extraction-precipitant can quantitatively extract the metal ions to form solid extracted complex without the requirement of organic solvent or carrier, and the extraction-precipitant can be stripped and recycled. In this study, the extraction-precipitation mechanism and thermodynamic parameters between POAA and manganese (Mn), cobalt (Co), nickel (Ni) were analyzed. Subsequently, the equilibrium time and pH value of the separation process were optimized. Finally, the leach solution of LIB having Li, Ni, Co, Mn with concentration of 3.52 g/L, 7.93 g/L, 5.72 g/L, and 14.16 g/L, respectively, was studied for the separation at pH 4.5. The results indicate that efficient separation of Li and transition metals was possible by one-step precipitation using 0.2 mol/L POAA solution (degree of saponification = 100%) when the ratio of nPOAA/nMn+Co+Ni was 2. The separation factor for the transition metals over lithium (βCoNiMn/βLi) reached as high as 133,014. After the transition metal-loaded POAA was stripped with 2 mol/L sulfuric acid, the stripped solution could be used for the production of ternary material precursor, and the regenerated POAA could be recycled for the extraction-precipitation process. The lithium in the solution was recovered in the form of lithium carbonate (Li2CO3) with a purity level of 97.7%. This high efficiency and sustainable process based on extraction-precipitation has shown potential application for spent LIB recovery.

Sorption of Silver Cations from Solutions by Titanium Phosphate Sorbents

This paper examines the feasibility of using titanyl hydrogen phosphate-based sorbents for removing silver radioisotopes from high-salt liquid radioactive waste. We have studied the sorption activity of ion exchangers for silver cations as a function of time and found equilibrium pH values for effective use of titanium phosphate sorbents. Titanyl hydrogen phosphate-based materials have been shown to have increased affinity for silver cations and be capable of ensuring effective removal of radionuclides from liquid radioactive waste.

Chemical characterization of Wood and Bark biomass of the invasive species of Tree-of-heaven (Ailanthus altissima (Mill.) Swingle), focusing on its chemical composition horizontal variability assessment

ABSTRACT Aim of this study is to evaluate the potential of a rational utilization of wood and bark of the invasive species of tree-of-heaven (Ailanthus altissima (Mill.) Swingle), through their thorough chemical characterization. Extractives, celluloses, hemicelluloses, lignin, and ash contents of wood and bark material of different tree sections were examined, using wet chemical analyses/extractions. The horizontal variability of the chemical composition of the stems from pith to cambium was investigated, as well as the differences between early- and late-wood, different annual rings, sapwood and heartwood, and differences between wood and bark. The material’s equilibrium moisture content, density, pH, colour, and calorific value were also determined. Bark material presented higher extractives, slightly higher lignin, much higher ash, and slightly lower holocellulose content than wood. As the tree-age increases, the wood and bark present lower lignin, but a linear increase of holocellulose and extractives. The heartwood demonstrated more extractives, lignin and ash, and less holocellulose than sapwood. Very low ash contents and high calorific values constitute features highly crucial for its utilization in solid biofuel production. The optimum harvesting time for the highest possible utilization of extractives and holocellulose is after the 21st year, while for lignin after the 5th year.

Thermodynamic Analysis of Trace Cobalt(II) Extraction from Cobalt Chloride Aqueous Solution Using Cyanex 272 in Kerosene

In this study, it was researched that the extraction of Co(II) from CoCl2 aqueous solutions using Cyanex 272 (bis(2,4,4-trimethylpentyl) phosphonic acid) in kerosene and determined the effects of the equilibrium pH value, extractant concentration and chloride ion concentration on the extraction of cobalt at different temperatures. The results indicate that the distribution ratio of cobalt increases as the equilibrium pH value, extractant concentration and extraction temperature increase. However, the cobalt distribution ratio decreases as the chloride ion concentration increases. We calculated the equilibrium constants (AH9, AG9, and AS9) for a range of temperatures and investigated the relationship between AG9 and T. The extraction reaction of cobalt was determined to be an endothermic reaction; thus, raising the temperature was beneficial to the reaction.

Evaluation of ionic equilibria in mixed-buffer isothermal titration calorimetry and continuously stirred tank reactors

Determination of an equilibrium pH value in complex aqueous solution and deconvolution of this equilibrium to evaluate phenomena related to mixing, dilution, or progress of reaction is increasingly important in areas ranging from water quality to pharmaceutical formulations and manufacturing. Linearization of pH problems by simple algebraic substitution enables equilibria within complex buffered aqueous solutions to be modeled as an eigenvalue problem. This formulation approach makes rigorous determination of equilibrium pH values and reactor dynamics more accessible than with previous calculation methods, even when activity coefficients and non-ideality are considered. This work demonstrates how such calculations can enable detailed modeling of enthalpic changes in an isothermal titration calorimeter. In support of this work, the acid dissociation constants for three furancarboxylic acids (2-furancarboxylic acid, FA; 5-formyl-2-furancarboxylic acid, FFA; and 2,5-furandicarboxylic acid, FDCA), two of them novel, were determined and compared with multi-wavelength ultraviolet–visible spectrophotometry. The thermodynamic pKa values were determined to be 3.1 for FA, 2.2 for FFA, and 2.1 and 3.4 for the first and second ionization steps of FDCA, respectively.

Crucial Role of Water in the Formation of Basic Properties of Living Matter

A relation between the water properties and the behavior of aqueous solutions of albumin, the main protein component of human blood plasma, has been analyzed. The dependence of the pH index of acid-base balance in aqueous albumin solutions on the albumin concentration is experimentally studied. It is shown that the temperature dependences of pH in biological solutions are determined by the properties of water, and the concentration ones by the concentration of a protein component. It is albumin that makes the main contribution to the pHs of blood and blood plasma, and it should be considered as a factor that maintains the equilibrium pH value. It is shown that the most characteristic changes in the concentration dependences of the density and shear viscosity of human plasma occur at a protein concentration corresponding to the percolation threshold. A characteristic dimerization of albumin macromolecules is assumed to take place at the percolation threshold, which corresponds to the superposition on one another of heart-shaped medallions representing the spatial forms of albumin. The dependences of the effective radii of polyvinyl alcohol and albumin macromolecules on the solution temperature and concentration are demonstrated to be an indicator that water plays a decisive role in the formation of basic properties of biosolutions. In particular, it is responsible for the presence of an upper temperature limit of 42 ∘C for the existence of living matter. The universal nature of the water influence manifests itself in that the water properties affect the behavior of both the classic PVA polymer and protein biomolecules.

Low temperature synthesis of Сu0.3IrOx·nH2O nanocrystals by successive ionic layer deposition and their electrocatalytic properties in oxygen evolution reaction during water splitting in acidic medium

Ultrafine Сu0.3IrOx·nH2O nanocrystals with rutile-like structure were synthesized for the first time on the surface of titanium foil through facile and precise Successive Ionic Layer Deposition (SILD). An aqueous solution of Cu(OAc)2 with the equilibrium pH value and a colloidal solution containing IrOx∙nH2O nanoparticles and Na2Ir(OH)6 with pH = 10.5 obtained by hydrolysis of H2IrCl6 in a NaOH solution were used as reagents. The nanocrystals were investigated by SEM, EDX, TEM, HRTEM, XPS, XRD, and FTIR spectroscopy. As a result of a sequential and repeated treatment of titanium foil according to the SILD method with the reagent solutions, a nanolayer of Cu0.3Ir(OH)x nanocrystals is formed on its surface, which, after heating at a temperature of 200°C, transforms into a porous nanolayer consisting of ultrafine Сu0.3IrOx·nH2O nanocrystals. They have a distorted crystal structure similar to that of rutile with sizes of 2–3 nm and form polycrystals with a thickness of 4–8 nm and lateral sizes of 40–60 nm on the substrate surface. The porous nanolayers of such polycrystals exhibit electrocatalytic properties in the oxygen evolution reaction during acidic water electrolysis. In a series of samples synthesized as a result of 10–30 SILD cycles, the best electrochemical characteristics are demonstrated by a sample acquired after 20 synthetic cycles: the overpotential value is 256 mV and the Tafel slope is 41.4 mV/dec. These results are an experimental validation of a new approach in the search for highly efficient electrocatalysts based on the study of electrochemical properties in a series of samples synthesized as a result of a varying number of SILD cycles and the selection of samples with the best properties.

Direct successive ionic layer deposition of nanoscale iridium and tin oxide on titanium surface for electrocatalytic application in oxygen evolution reaction during water electrolysis in acidic medium

Abstract In this paper, we propose a novel promising route for the direct synthesis of nanoscale iridium and tin oxide nanolayers by a successive ionic layer deposition method (SILD). The precursors for the synthesis included an aqueous solution of SnF2 with an equilibrium pH value and the colloidal solution of IrOx∙nH2O nanoparticles and Na2[Ir(OH)6] mixture with pH = 10.5, obtained by the alkaline hydrolysis of the H2IrCl6 solution. The synthesized nanoscale oxide exhibited electrocatalytic properties in the oxygen evolution reaction (OER) upon water splitting in the acidic medium. A series of samples were obtained, synthesized as a result of 10, 30, and 50 SILD processing cycles. It was found that the films obtained as a result of 30 SILD cycles and heated in argon at a temperature of 300 °C had the best electrocatalytic properties in the reaction of oxygen evolution in the HClO4 solution. For these films, the overpotential value at a current density of 10 mA/cm2 was 288 mV and the Tafel coefficient value totaled 56 mV/dec. Investigations by SEM, TEM, EDX, XRD, XPS and FTIR spectroscopy revealed that the samples were built of Ir0.7SnOx∙nH2O amorphous nanoparticles with a size of 10–20 nm. For this kind of electrode, estimation of the specific mass of iridium showed that its amount is about 0.02 mg/cm2. It is suggested that the new synthetic method may serve as the basis for creating a wide range of coatings for electrodes in electrochemical devices, including electrochemical sensors, electrical stimulators, electrochromic devices, etc.