Anion-exchange properties of nickel–aluminum layered double hydroxide prepared by liquid phase deposition

The optimum coordination structure of Ni–fluoro complexes for the preparation of Ni–Al LDH by LPD process and the diverse anion-exchange properties of as-deposited Ni–Al on α-alumina powder were quantitatively evaluated for the industrial application of new positive material for alkali secondary batteries. The [NiF_6−x−y(NH₃)_x(OH⁻)_y]ⁿ⁺ was more suitable than [NiF₆]⁴⁻ as the precursor of the deposition of Ni–Al LDH in the LPD reaction, and the improved LPD reaction achieved the synthesis of high purity and high crystallinity Ni–Al LDH. All anion-exchanged Ni–Al LDHs for OH⁻–, Cl⁻–, SO₄²⁻–, and CH₃COO⁻–forms kept the high crystallinity and showed the enlargement of interlayer distances. The tilting angle of the intercalated CH₃COO⁻ anions was about 15°. Anion-exchange capacity remained constant at a minimum of 0.8 meq g⁻¹ in pH >10, increased as pH decreased, and reached a maximum of 8 meq g⁻¹ at pH 2. Anion-exchange of OH⁻–form of Ni–Al LDH was accelerated by the neutralization of hydroxide ions in interlayers, in addition, the anion-exchange capacity and the crystallinity of Ni–Al LDH could be controlled by the amount of doped aluminum ions.     

Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.     

High rate performances of the cathode material LiNi1/3Co1/3Mn1/3O2 synthesized using low temperature hydroxide precipitation

A low-temperature reaction route is introduced based on hydroxide precipitation method to synthesize the cathode material LiNi_1/3Co_1/3Mn_1/3O₂. The crystal structure and morphology of the prepared powder have been characterized by X-ray diffraction and Scan electron microscope, respectively. The charge–discharge tests were performed between 2.5 and 4.5 V. The discharge capacity of the material is strongly impacted by the reaction temperature. The powders sintered at 850 °C show the best electrochemical performance and the initial discharge capacity is about 160 mAh g⁻¹ at 5 C. Powder X-ray diffraction and Scan electron microscope results reveal that the excellent electrochemical performances should be ascribed to the lower precursor reaction temperature, the lower degree of cation mixing and analogous spherical small particles, which can improve the transfer of Li ions and electrons. All these results indicate that this material has potential application in lithium-ion batteries.     

Fabrication and capacitance of Ni-Fe LDHs/MnO2 layered nanocomposite via an exfoliation/reassembling process

Ni²⁺-Fe³⁺ layered double hydroxides (LDHs)/MnO₂ layered nanocomposite has been fabricated by using both layer-by-layer self-assembly method and flocculated technology, based on electrostatic interaction of positively charged Ni²⁺-Fe³⁺ LDHs nanosheets and negatively charged MnO₂ nanosheets. Ultraviolet-visible spectroscopy is used to probe the dynamic growth of the multilayer film, exhibiting progressive enhancement of optical absorption due to the assembly of Ni²⁺-Fe³⁺ LDHs nanosheets and MnO₂ nanosheets. The assembled Ni²⁺-Fe³⁺ LDHs/MnO₂ nanocomposite has been characterized by XRD, SEM and TEM. The electrochemical property of the synthesized Ni²⁺-Fe³⁺ LDHs/MnO₂ layered nanocomposite has been studied using cyclic voltammetry in a mild aqueous electrolyte. The Ni²⁺-Fe³⁺ LDHs/MnO₂ nanocomposite exhibits a relative good capacitive behavior in a neutral electrolyte system, and its initial capacitance value is 104 F g⁻¹.     

Synthesis of layered Li1.2+x[Ni0.25Mn0.75]0.8−xO2 materials (0 ≤ x ≤ 4/55) via a new simple microwave heating method and their electrochemical properties

Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) was prepared by a new simple microwave heating method and the effect of extra Li⁺ content on electrochemistry of Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) was firstly revealed. X-ray diffraction identified that they had layered α-NaFeO₂ structure (space group R-3m). Linear variation of lattice constant as a function of x value supported the formation of solid solution, that is, extra Li⁺ is possibly incorporated in structure of layered Li_1.2Ni_0.2Mn_0.6O₂ (x = 0), accompanying oxidization of Ni²⁺ to Ni³⁺ to form Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55). This was confirmed by X-ray photoelectron spectroscopy that Ni³⁺ appeared and increased in content with increasing x value. Charge–discharge tests showed that Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂ (0 ≤ x ≤ 4/55) truly displayed different electrochemical properties (different initial charge–discharge plots, capacities and cycleability). Li_1.2Ni_0.2Mn_0.6O₂ (x = 0) in this work delivered the highest discharge capacity of 219 mAh g⁻¹ between 4.8 and 2.0 V. Increasing Li content (x value in Li_1.2+x[Ni_0.25Mn_0.75]_0.8−xO₂) reduced charge–discharge capacities, but significantly enhancing cycleability.     

Effect of intercalated aromatic sulfonates on uptake of aromatic compounds from aqueous solutions by modified Mg-Al layered double hydroxide

In this study, we utilized Mg-Al layered double hydroxide (Mg-Al LDH) modified by intercalation with three aromatic sulfonates—2,7-naphthalene disulfonate (2,7-NDS²⁻), benzenesulfonate (BS⁻), and benzenedisulfonate (BDS²⁻)—for the uptake of two aromatics—1,3-dinitrobenzene (DNB) and anisole (AS)—from aqueous solution and determined the effect of the aromatic sulfonates on the uptake of these aromatics. We found that the electron-rich aromatic ring of the intercalated aromatic sulfonates such as 2,7-NDS²⁻ undergoes strong π-π stacking interactions with the electron-poorer benzene ring of DNB in aqueous solution, and these interactions result in a higher uptake of DNB by the modified Mg-Al LDHs. In contrast, the electron-poor aromatic ring of the aromatic sulfonates such as BDS²⁻ undergoes weak π-π stacking interactions with the electron-poorer benzene ring of DNB, and these interactions result in a lower uptake of DNB by the modified Mg-Al LDHs.     

Facile and rapid synthesis of multiferroic TbMnO3 single crystalline

The pure single phase of multiferroic material TbMnO₃ powders were successfully synthesized by one-step molten salt synthesis (MSS) method in the NaCl–Na₂SO₄ eutectic salts at the temperature as low as 800 °C for 1 h. The temperature of synthesized high purity TbMnO₃ is limited in a very narrow range. Prolonging the sintering time will not have an effect on the purity of samples, and either lower or higher salt concentration is not conducive to form pure TbMnO₃. The obtained TbMnO₃ was indexed to an orthorhombically distorted perovskite phase. The as-prepared crystals exhibit uniform and regular rhombic-like morphology with an average size of about 2 μm in edge length and 1–2 μm in thickness. The elements Mn and Tb in TbMnO₃ exist dominantly as Mn³⁺ and Tb³⁺, respectively. The magnetic measurements of the TbMnO₃ powders exhibit antiferromagnetism. Because of the simplicity and generalizability of the MSS method, it is reasonable to expect that the MSS method could also be exploited in future works which involves the nanoscale investigation of ferroelectric, ferromagnetic and multiferroic materials.     

A facile method to prepare layered manganese oxides with large interplanar spacing

Layered manganese oxides with basal spacings up to 38.1 Å have been synthesized by a facile novel method in which cationic surfactant CTAB directly reacts with MnSO₄·H₂O and NaOH. The as-synthesized samples have expanded birnessite-type layered structures characterized by powder XRD and TEM. Synthetic parameters that are important to the formation and the basal spacing of the layered mesostructures are investigated, including the surfactant concentration, OH⁻/Mn²⁺ ratio, CTAB/Mn²⁺ ratio and aging time. Mechanisms based on self-assembling are discussed.     

Synthesis and photoluminescence properties of MgAl2O4:Mn hexagonal nanoplates

MgAl₂O₄:Mn²⁺ hexagonal nanoplates have been synthesized via a simple two-step method. The nanoplates have uniform hexagonal morphology with an average edge length of 1 μm and thickness of 30 nm. X-ray diffraction and various microscopic techniques indicate that MgAl₂O₄:Mn²⁺ nanoplates are single-crystal with multilayered morphology. The formation mechanism has also been discussed. Photoluminescence (PL) spectrum of the MgAl₂O₄:Mn²⁺ nanoplate shows a broad green emission band centered at 568 nm, which is assigned to the ⁴T₁ → ⁶A₁ transition of Mn²⁺ ion. The MgAl₂O₄:Mn²⁺ nanoplate is a promising candidate for efficient nanoscale optical material.     

Influence of isovalent ion substitution on the electrochemical performance of LiCoPO4

LiCo_1−xM_xPO₄ (M = Mg²⁺, Mn²⁺ and Ni²⁺; 0 ≤ x ≤ 0.2) compounds have been synthesized by solid-state reaction method and studied as cathode materials for secondary lithium batteries. LiCoPO₄ exhibits a discharge plateau at ∼4.7 V with an initial discharge capacity of 125 mAh/g and on cycling capacity falls. Substitution of Co²⁺ with Mg²⁺/Mn²⁺/Ni²⁺ in LiCoPO₄ has an influence on the initial discharge capacity and on cycling behaviour. The capacity retention of LiCoPO₄ is improved by manganese substitution. Among the manganese substituted phases, LiCo_0.95Mn_0.05PO₄ shows good reversible capacity of ∼50 mAh/g.     

Morphology-controlled hydrothermal synthesis of MnCO3 hierarchical superstructures with Schiff base as stabilizer

MnCO₃ with hierarchical superstructures such as chrysanthemum, straw-bundle, dumbbell and sphere-like were synthesized in water/ethanol system under environment-friendly hydrothermal condition. In the synthesis process, the CO₂ in atmosphere was used as the source of carbonate ions and Schiff base was used as shape guiding-agent. The different superstructures of MnCO₃ could be obtained by controlling the hydrothermal temperature, the molar ratio of manganous ions to the Schiff base, or the volume ratio of water to ethanol. A tentative growth mechanism for the generation of MnCO₃ superstructures was proposed based on the rod–dumbbell–sphere model. Furthermore, the MnCO₃ as precursor could be further successfully transferred to Mn₂O₃ microstructure after heating in the atmosphere at 500 °C, and the morphology of the Mn₂O₃ was directly determined by that of the MnCO₃ precursor.     

Synthesis of hierarchical Ni11(HPO3)8(OH)6 superstructures based on nanorods through a soft hydrothermal route

In this paper, we reported the successful synthesis of hierarchical Ni₁₁(HPO₃)₈(OH)₆ superstructures based on nanorods via a facile hydrothermal route, employing NiCl₂·6H₂O and NaH₂PO₂·H₂O as the reactants in the presences of polyvinylpyrrolidone (PVP) and CH₃COONa·3H₂O. The reaction was carried out at 170 °C for 10 h. HPO₃²⁻ ions were provided via the dismutation reaction of H₂PO₂⁻ ions in a weak basic solution. The as-obtained products were characterized by X-ray powder diffraction (XRD), energy dispersive spectrometry (EDS), field emission scanning electron microscopy (SEM), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). Some factors influencing the morphology of the hierarchical Ni₁₁(HPO₃)₈(OH)₆ nanorods, such as the reaction temperature, time, the amounts of PVP and CH₃COONa, and the initial concentration of Ni²⁺ ions, were systematically investigated. A possible growth mechanism was proposed based on experimental results.     

Structural transformation and photoluminescence behavior during calcination of the layered europium-doped yttrium hydroxide intercalate with organic-sensitizer

We report, for the first time, structural transformation and photoluminescence behavior by calcination of layered europium-doped yttrium hydroxide (LYH:Eu) intercalate with organic sensitizer terephthalate (TA). The calcined samples displayed tunable luminescent performance dependent on calcined temperatures. Calcination under low temperatures (200 and 300 °C) retained layered structure, while high temperatures (>400 °C) yielded oxide phase. The optimal fluorescence occurred in 200 °C-calcination, possibly resulting from an optimal arrangement of TA. Above 200 °C, the luminescence intensity was first weakened and then enhanced, due to gradual departure of TA and following occurrence of oxide phase. The energy transfer presented an intrinsic transition from TA-to-Eu³⁺ in the organic intercalate to O²⁻-to-Eu³⁺ charge transfer in as-transformed oxide. The predominant luminescence property of the hybrid material can provide valuable guide for developing tunable luminescent materials, especially flexible materials resulting from the containing organic component.     

The double hydroxide of Al with Li: mechanism of formation and reversible thermal behavior

All the polymorphic modifications of Al(OH)₃ and the oxide-hydroxide of Al, boehmite, on hydrothermal treatment in Li⁺ containing solutions transform into the layered double hydroxide of Al with Li having the nominal composition [LiAl₂(OH)₆](CO₃)_1/2·1.5H₂O, suggesting that these compounds form via a dissolution-reprecipitation mechanism. The oxide residue obtained by the thermal decomposition of this layered double hydroxide also partially reconstructs the layered double hydroxide on soaking in water lending further evidence to the dissolution-reprecipitation mechanism. Under hydrothermal conditions, in water, the layered double hydroxide undergoes delithiation to yield aluminum hydroxide phases other than gibbsite, suggesting again that delithiation too proceeds by a dissolution-reprecipitation mechanism.     

Synthesis and characterization of layered double hydroxides (LDHs) with intercalated chromate ions

Chromate intercalated layered double hydroxides (LDHs) having the formula M^II₆M′^III₂(OH)₁₆CrO₄·4H₂O (M^II = Ca, Mg, Co, Ni, Zn with M′^III = Al and M^II = Mg, Co, Ni with M′^III = Fe) have been prepared by coprecipitation. The products obtained are replete with stacking disorders. DIFFaX simulations show that the stacking disorders are of three kinds: (i) turbostratic disorder of an originally single layered hexagonal (1H) crystal, (ii) random intergrowth of polytypes with hexagonal (2H) and rhombohedral (3R) symmetries and (iii) translation of randomly chosen layers by (2/3, 1/3, z) and (1/3, 2/3, z) leading to stacking faults having a local structure of rhombohedral symmetry. IR spectra show that the CrO₄²⁻ ion is incorporated either in the T_d or in the C_3v symmetry. The interlayer spacing in the latter case is 7.3 Å characteristic of a single atom thick interlayer showing that the CrO₄²⁻ ion is grafted to the metal hydroxide slab. On thermal treatment, the CrO₄²⁻ ion transforms into Cr(III) and is incorporated into the spinel oxide or phase separates as Cr₂O₃. In the LDH of Mg with Al, Cr(III) remains in the MgO lattice as a defect and promotes the reconstruction of the LDH on soaking in water. In different LDHs, 18-50% of the CrO₄²⁻ ion is replaceable with carbonate anions showing only partial mineralization of the water-soluble chromate. The extent of replaceable chromates depends upon the solubility of the corresponding LDH, which in turn is determined by the solubility of the MCrO₄. These studies have profound implications for the possible use of LDHs for chromate amelioration in green chemistry.     

Electrochemical behaviors of Li[Li(1−x)/3Mn(2−x)/3Nix/3Cox/3]O2 cathode series (0 < x < 1) synthesized by sucrose combustion process for high capacity lithium ion batteries

A mixed cathode material between Li₂MnO₃ and Li[Mn_1/3Ni_1/3Co_1/3]O₂ for high capacity lithium secondary batteries was introduced in this study. It was prepared using the sucrose combustion process because this is a simple process. The oxidation states of Mn, Co and Ni ions in the pristine Li[Li_(1−x)/3Mn_(2−x)/3Ni_x/3Co_x/3]O₂ compounds were confirmed to be tetravalent, trivalent and divalent, respectively, via XANES measurements. Electrochemical charge/discharge studies showed that the highest first discharge capacity of 224 mAh/g was obtained in composition of x = 0.5 at a 0.2 C rate. The oxidation state of the Co and Ni ions in the Li[Li_1/6Mn_1/2Ni_1/6Co_1/6]O₂ changed to higher oxidation states, but that of the Mn ions did not change.     

Synthesis and electrochemical behavior of a new layered cathode material LiCo1/2Mn1/3Ni1/6O2

A new layered type lithium nickel manganese cobalt oxide with the composition of LiCo_1/2Mn_1/3Ni_1/6O₂ was synthesized by using a layered double hydroxides (LDHs) as precursor and solid state reaction method. Phase-pure LiCo_1/2Mn_1/3Ni_1/6O₂ was obtained when the mixed precursors of NiMnCo–LDHs and LiOH·H₂O were calcined at 750 °C for 12 h. It showed discharge capacity of 180 and 148 mAh/g in the first cycle, corresponding to the discharge voltage ranges of 2.5–4.5 and 2.5–4.2 V, respectively, and still delivered 173 and 140 mAh/g after 60 cycles at room temperature, which represented favorable capacity retention upon cycling. This material was expected as a potential alternative of cathode material to be used for Li-ion secondary battery because of its good electrochemical performance and lower synthesis cost.     

Design of magnetic and fluorescent Mg-Al layered double hydroxides by introducing Fe3O4 nanoparticles and Eu ions for intercalation of glycine

We describe a novel route for the preparation of magnetic and fluorescent magnesium-aluminum layered double hydroxides by introducing Fe₃O₄ nanoparticles and Eu³⁺ ions. From the powder X-ray diffraction results, it was found that the Fe₃O₄ nanoparticles were highly dispersed in the inner void of octahedral lattice, and the Eu³⁺ ions substituted for the Al³⁺ ions and entered into hydrotalcite lattice through isomorphous replacement. Moderate introduction of Fe₃O₄ nanoparticles and Eu³⁺ ions did not change the lamellar structure of magnesium-aluminum layered double hydroxides. Glycine can also be intercalated into this magnetic and fluorescent layered double hydroxides by ion-exchange method. After intercalation of glycine, the basal spacing of magnetic and fluorescent layered double hydroxides increased from 7.6 to 8.8 Å, indicating that glycine was successfully intercalated into the interlayer space of layered double hydroxides. Magnetic measurements reveal that these novel layered double hydroxides possess paramagnetic property at room temperature, and the emission and excitation spectra indicate the layered double hydroxides exhibit fluorescent property.     

Theoretical studies on the defect structure for Mn in KTaO3

The defect structure for Mn²⁺ in KTaO₃ is theoretically studied by using perturbation formulas of the spin Hamiltonian (SH) parameters for 3d⁵ ions in tetragonal symmetry based on the strong-field scheme. By analyzing the electron paramagnetic resonance (EPR) data of the studied system, we suggest that the impurity Mn²⁺ ion occupy the dodecahedral K⁺ site, rather than the octahedral Ta⁵⁺ site. Based on the studies, it is found that the Mn²⁺ impurity undergoes an off-center displacement away from the ideal K⁺ site by about 0.60 Å along the C₄ axis. The above displacement is qualitatively consistent with the recent result based on the generalized gradient approximation (GGA) and that obtained from EPR and dielectric spectroscopy studies.     

Fabrication of TiO2 hollow fibers with surface nanostructure

Polyvinyl alcohol-TiO₂ (PVA-TiO₂) core sheath nanofibers were fabricated by electrospinning an aqueous solution of PVA and introducing the thread-like droplets directly into a titanium tetraisopropoxide (TTIP)/hexane solution. Rod-like and sheet-like structures of lepidocrocite-type layered titanate formed on the surface of the TiO₂ sheath of the nanofibers by alkaline treatment in 1 mol L⁻¹ aqueous NaOH solution at 363 K. The nanofibers were converted to hollow TiO₂ nanofibers with surface nanostructure and anatase crystallinity by acid treatment to remove sodium ions and heat treatment at 773 K. The surface nanostructures enhanced the crystallinity and external surface area of the nanofiber and contributed to the improvement of photocatalytic oxidation activity.