Layered Cobalt Hydroxide Nanocones: Microwave-Assisted Synthesis, Exfoliation, and Structural Modification

Hydroxide-Source-Dependent Polymorphism and Phase Stability of Cobalt(II) Hydroxides in Diffusion-Driven Systems

Hydroxides of cobalt(II) exist predominantly in two polymorphic forms, namely, the blue-green α-form [α-Co(OH)2] and reddish β-form [β-Co(OH)2]. These hydroxides have a layered structure with interlayer galleries of around 7 and 4 Å, respectively, for α- and β-Co(OH)2. In most of the previous studies, both the polymorphs were synthesized separately, and a few of them showed that the α-form gets converted to a thermodynamically more stable β-form via physical processes. In the present work, we have optimized the conditions for the simultaneous synthesis of both polymorphs under identical conditions in the same reactor using the 1D reaction-diffusion framework by employing different outer electrolytes. We found that the polymorph chemistry of Co(OH)2 depends on the source and concentration of OH- rather than other reaction conditions or later physical transformation. The products are characterized to confirm their morphology, structure, and chemical environment. We observed that the use of NaOH and NH4OH as the OH- precursor leads to α-Co(OH)2 only; however, with NaOH, a continuous precipitate is formed, and with NH4OH, periodic precipitation is formed. On the other hand, with hydrazine (HYZ) as the OH- source, Liesegang bands of α-Co(OH)2 and β-Co(OH)2 as granules are formed throughout the diffusion reactor. Another intriguing observation on the HYZ system is that at its high concentration, the bands of α-Co(OH)2 get converted to β-Co(OH)2. We articulate the reasons and mechanism of those observations.

Synthesis of ethane-disulfonate pillared layered cobalt hydroxide towards the electrocatalytic oxygen evolution reaction

We report the synthesis of a hybrid layered cobalt hydroxide sample and its redox behaviors in the electrochemical oxygen evolution reaction (OER). Compound Co₇(OH)₁₂(C₂H₄S₂O₆)·1.6H₂O was synthesized via a homogeneous alkalization reaction using Co(SO₃C₂H₄SO₃) and hexamethylenetetramine. This compound comprises cationic host layers of {[Co₇(OH)₁₂]²⁺}_∞, which comprise octahedrally (Co_Oh) and tetrahedrally (Co_Td) coordinated Co cations at a Co_Oh : Co_Td ratio of 5 : 2. The ethane-disulfonate ions are combined with the cationic host layers by electrostatic attractions and hydrogen bonding as a hybrid pillared layered framework. This hybrid sample can promote the OER in 1 M KOH with an overpotential as low as ∼410 mV (at a current density of 10 mA cm^-2). In situ Raman spectroscopy showed that the sample first evolved into Co(III)-based phases comprising a mixture of layered CoOOH and spinel Co₃O₄, and the Co(III)-based compounds were converted into Co(IV)-O intermediates containing [CoO₆] units at the onsite of the OER. The structural evolution behaviors suggest that the catalyst prefers a topotactic phase transition and the Co_Oh and Co_Td units exhibit different activities in the electrochemical reaction. The electron transfer events involved in the electrochemical reaction were identified by Fourier-transformed alternating current voltammetry.

Exfoliated Single Layers of Layered Cobalt Hydroxide for Ultrafine Co3 O4 Nanoparticles on Graphene Nanosheets as an Efficient Electrocatalyst for Oxygen Reduction

A highly effective way to produce an oxygen reduction electrocatalyst was developed through the self-assembly of exfoliated single layers of cobalt hydroxide (Co(OH)₂ ) and graphene oxide (GO). These 2D materials have complete contact with one another because of their physical flexibility and the electrostatic attraction between negatively charged GO and positively charged Co(OH)₂ layers. The strong coupling induces transformation of the Co(OH)₂ single layer into a discrete nanocrystal of spinel Co₃ O₄ with an average size of 8 nm on reduced GO (RGO) during calcination, which could not be obtained with bulk-layered cobalt hydroxide because of its rapid layer collapse. The ultrafine Co₃ O₄ /RGO hybrid exhibited not only comparable performance in the oxygen reduction reaction but also higher durability compared with the commercial 20 wt % Pt/C catalyst.

Advanced electrocatalysts based on two-dimensional transition metal hydroxides and their composites for alkaline oxygen reduction reaction

The electrocatalytic oxygen reduction reaction (ORR) is a crucial part in developing high-efficiency fuel cells and metal-air batteries, which have been cherished as clean and sustainable energy conversion devices/systems to meet the ever-increasing energy demand. ORR electrocatalysts currently employed in the cathodes of fuel cells and metal-air batteries are mainly based on high-cost and scarce noble metal elements. It is thus of great importance to develop cheap and earth-abundant ORR electrocatalysts. In this aspect, redox-active transition metal hydroxides, a class of multifunctional inorganic layered materials, have been proposed as prospective candidates on account of their abundance and high ORR activities. In this article, the preparation and structural evolution of transition metal hydroxides, in particular their exfoliation into two-dimensional (2D) nanosheets, as well as compositing/integrating with catalytic active and/or conductive components to overcome the insulating nature of hydroxides in alkaline ORR, are summarized. Recent advances have demonstrated that 2D transition metal hydroxides with carefully tuned compositions and elaborately designed nanoarchitectures can achieve both high activity and high pathway selectivity, as well as excellent stability comparable to those of commercial Pt/C electrocatalysts. To realize the dream of renewable electrochemical energy conversion, new strategies and insights into rational designing of 2D hydroxide-based nanostructures with further enhanced electrocatalytic performance are still to be vigorously pursued.

Synthesis of an Ultrafine CoP Nanocrystal/Graphene Sandwiched Structure for Efficient Overall Water Splitting

A CoP/graphene composite was synthesized through a coprecipitation and in situ phosphorization protocol using α-Co(OH)₂ and graphene oxide as precursors. The similar two-dimensional layered structures ensured evenly attached α-Co(OH)₂ nanosheets on the graphene oxide support and the formation of a sandwich-like structure. The sequential in situ phosphorization strategy not only generated a high density of ultrafine CoP nanocrystals but also simultaneously reduced the graphene oxide support. The enough exposed active sites combined with a highly conductive matrix resulted in an excellent electrochemical catalyst for overall water splitting. The overpotential is only 125 mV at 10 mA·cm² in 0.5 M H₂SO₄. Good electrocatalytic performance was also exhibited in alkaline conditions for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The overpotential is 119 mV for HER and 374 mV for OER at 10 mA·cm² in 1 M KOH. More importantly, the composite exhibited much higher exchange current densities during HER processes (1.64 × 10^-4 A·cm^-2 in 0.5 M H₂SO₄ and 2.93 × 10^-4 A·cm^-2 in 1 M KOH) when compared with similar materials reported before. This low-cost, simple, and efficient approach is suitable for mass production and practical applications.

Alternate Restacking of 2 D CoNi Hydroxide and Graphene Oxide Nanosheets for Energetic Oxygen Evolution

Morphology and composition tuning of layered materials is evaluated to influence their electrochemical performance for energy storage and conversion applications. Layered Co_1-x Ni_x hydroxides (x=0, 1/2, 1/3, 1/4, 1) of three different morphologies-nanocones, 2 D nanosheets obtained by the rapid exfoliation of nanoconical counterparts, and 2 D superlattice-like nanostructures alternately restacked by the oppositely charged hydroxide and graphene oxide (GO) nanosheets-have been systematically investigated for electrocatalytic oxygen oxidation. High activity is obtained with the 2 D Co_2/3 Ni_1/3 hydroxide nanosheets/GO superlattice (Co_2/3 Ni_1/3 NS-GO), achieving a current density of 10 mA cm^-2 at a low overpotential of 259 mV accompanied by a small Tafel slope of 35.7 mV dec^-1 , surpassing nanocones and 2 D nanosheets, as well as the congeneric heterostructured Co_1-x Ni_x hydroxide nanosheets/GO nanoarchitectures (Co_1-x Ni_x NS-GO; x=0, 1/2, 1/4, 1) and the commercial RuO₂ electrocatalyst. The outstanding activity of Co_2/3 Ni_1/3 NS-GO superlattice uncovers the combined merits of 2 D superlattice-like structure and composition optimization for electrocatalysis, providing a strategy for developing high-performance electrochemical materials by rational morphology and composition design.

Pyrolysis of a self-supported dodecyl sulfate anion-intercalated Co(OH)2 nanosheet with enlarged amorphous phase content towards enhanced activity for alkaline water oxidation

Highly active electrocatalysts made of earth-abundant elements are vital for efficient and cost-effective energy storage and conversion systems. In this communication, we report the further amorphization of a solvothermally synthesized dodecyl sulfate anion-intercalated cobalt hydroxide nanosheet array on nickel foam (DS-Co(OH)2/NF) via pyrolysis. Owing to the greatly enlarged interlayer distance of the DS-Co(OH)2/NF precursor (2.4 nm), and the more exposed active sites due to the enlarged amorphous phase content, the resulting P-DS-Co(OH)2/NF exhibits boosted activity as a 3D catalyst electrode for alkaline water oxidation. In 1.0 M KOH, an overpotential of only 266 mV is needed to drive a geometrical catalytic current density of 70 mA cm-2, which is 74 and 121 mV lower than the overpotentials for the DS-Co(OH)2/NF precursor and for Co(OH)2 without DS anion intercalation (Co(OH)2/NF), respectively. Impressively, this catalyst also displays superior long-term stability with a high turnover frequency value of 0.055 O2 s-1 at an overpotential of 340 mV.

Edge/Defect Sites in α-Co1-m Fem (OH)x Nanoplates Responsible for Water Oxidation Activity

Fe-doped transition metal (oxy)hydroxides are regarded as the most efficient oxygen evolution reaction (OER) electrocatalysts in alkaline conditions. The incorporation of Fe effectively enhances the OER activity of Co-/Ni-based materials, but the corresponding role of Fe in Co-based (oxy)hydroxide materials still remains unresolved. Herein, α-Co_1-m Fe_m (OH)_x is synthesized and systematically engineered to study the effect of Fe content on the morphology, crystalline structure, electronic structure, and OER activity. As the Fe content is changed, the basic crystalline phase of α-Co_1-m Fe_m (OH)_x is consistent whereas the micromorphology changes. Much smaller and thinner nanoplates with more edge/defect sites are fabricated because of increased Fe incorporation. When the Fe content is more than 0.1, twin nanoparticles emerge at the edge/defect sites of the sister nanoplate. Additionally, the OER activity of α-Co_1-m Fe_m (OH)_x against Fe content can be plotted as a volcano curve. These data thus support a hypothesis that the edge/defect sites in α-Co_1-m Fe_m (OH)_x are responsible for the OER performance. The incorporation of Fe leads to not only the accelerated intrinsic reactivity of each active site, which is attributed to the strong electronic interaction between Co and Fe but also changes the number of edge/defect sites.

Post-synthesis isomorphous substitution of layered Co-Mn hydroxide nanocones with graphene oxide as high-performance supercapacitor electrodes

Layered metal hydroxides are promising materials for electrochemical energy conversion and storage. Generally, compared with layered monometallic hydroxides, layered bimetallic hydroxides have more excellent electrochemical performance due to abundant redox reactions. Unfortunately, layered bimetallic hydroxides cannot be usually achieved through coprecipitation and/or homogeneous precipitation. Herein, we demonstrate that layered Co-Mn hydroxide nanocones (NCs) can be successfully fabricated via post-synthesis isomorphous substitution under mild conditions. In particular, the specific capacity and cycling stability of layered Co-Mn hydroxide NCs are remarkably enhanced in comparison with those of layered Co hydroxide NCs. Furthermore, the resulting layered Co-Mn hydroxide NCs and graphene oxide (GO) composite (GO/Co-Mn NCs) exhibits a high specific capacity of 677 C g-1 at 3 A g-1 and an excellent capacity retention of 95% after 2000 cycles. Asymmetric supercapacitor cells employing GO/Co-Mn NCs as the positive electrode and activated carbon (AC) as the negative electrode can achieve a high specific capacity of 189 C g-1 at 3 A g-1. This method provides a viable protocol for constructing efficient electrodes of layered bimetallic hydroxides for sustainable electrochemical energy storage.

Activity enhancement of layered cobalt hydroxide nanocones by tuning interlayer spacing and phosphidation for electrocatalytic water oxidation in neutral solutions

We explored the electrocatalytic performance of cobalt hydroxide nanocones in neutral solutions via tuning the interlayer spacing and phosphidation.

High-performance double ion-buffering reservoirs of asymmetric supercapacitors based on flower-like Co3O4-G>N-PEGm microspheres and 3D rGO-CNT>N-PEGm aerogels

Novel 3D flower-like Co3O4-G>N-PEGm composites have been synthesized by employing a solvothermal method, in which the incorporating graphene nanosheets are modified with methoxypolyethylene glycol (mPEG) via nitrene chemistry to form 2D macromolecular brushes. In Co3O4-G>N-PEGm, the flower-like Co3O4 microspheres can anchor on the G>N-PEGm nanosheets, corresponding to the coordination bonds between the lone pair of electrons on the mPEG polymer chains of the G>N-PEGm macromolecular brushes and cobalt ions. Owing to the novel structure, a high specific capacitance value of 1625.6 F g-1 at a current density of 0.5 A g-1 can be achieved in KOH solution. Meanwhile, 3D rGO-CNT>N-PEGm aerogels (GCA), as the negative electrode of electrical double-layer capacitor materials, exhibit a high reversible specific capacitance of 313.8 F g-1 at a current density of 2 A g-1. Based on the high electrochemical performance of both electrode materials, the double ion-buffering reservoirs of asymmetric supercapacitors configured with the Co3O4-G>N-PEGm as the positive electrode and 3D GCA as the negative electrode can deliver a high energy density of 34.4 W h kg-1 at a power density of 400 kW kg-1.

Freestanding CoSeO3·H2O nanoribbon/carbon nanotube composite paper for 2.4 V high-voltage, flexible, solid-state supercapacitors

The integration of high flexibility, high energy density and wide voltage window for solid-state supercapacitors remains a big challenge to date. Herein, ultrathin CoSeO3·H2O nanoribbons (thickness: ∼14 nm) with typical pseudocapacitive behavior were synthesized in a high yield by a solution-based refluxing process. Freestanding CoSeO3·H2O ribbon/hydroxylated multi-walled carbon nanotube (HWCNT) paper could be fabricated through a vacuum-assisted filtration strategy owing to its ultrathin nature, ribbon-like morphology and inherent flexibility. Unexpectedly, an asymmetric supercapacitor constructed from this as-prepared CoSeO3·H2O/HWCNT hybrid paper exhibits a high 2.4 V voltage window as well as excellent rate capability and cycle performance. The energy density of this device is 132.3 W h kg-1 at 960 W kg-1 with a stable cycling ability of up to 10 000 cycles, which is superior to those of almost all previously reported asymmetric supercapacitors based on freestanding paper. Furthermore, this supercapacitor shows outstanding bendability and mechanical stability at different bending degrees from 0° to 180° with no changes in capacitive behavior. Our work provides new opportunities for developing high-performance asymmetric supercapacitors with high energy density, wide voltage window, and high flexibility in a novel CoSeO3·H2O system for potential applications including flexible displays, collapsible mobile phones, and wearable equipment.

B, V. G, D. S, Kulal N, A. S. Framework of ruthenium-containing nickel hydrotalcite-type material: preparation, characterisation, and its catalytic application

The framework ruthenium-containing nickel (NiRu) hydrotalcite (HT)-type materials were prepared by a co-precipitation method for the first time in this study. Fourier-transform infrared spectroscopy and X-ray diffraction analysis revealed the formation of a layered hydrotalcite-type phase. DRUV-Vis and X-ray photoelectron spectroscopic studies revealed the presence of nickel in the +2 and +3 oxidation states along with the presence of ruthenium as Ru³⁺ ions. Temperature-programmed desorption studies of the NiRu-HT-type materials indicated a two-stage reduction with a decrease in T_max, supporting the presence of Ni²⁺ and Ru³⁺ in the framework of hydrotalcite. The obtained NiRu-HT-type materials proved to be promising catalysts for the reduction of aromatic nitro compounds in the presence of hydrazine as a hydrogen source under ambient conditions. The NiRu-HT-type material demonstrated enhanced activity and selectivity during the reduction of nitrobenzene and its derivatives due to the synergistic effect of nickel and ruthenium ions.

The framework ruthenium-containing nickel (NiRu) hydrotalcite (HT)-type materials were prepared for the first time without organics and the resultant materials were found to be promising catalysts for nitro-benzene reduction.

Rare-earth-doped yttrium oxide nanoplatelets and nanotubes: controllable fabrication, topotactic transformation and upconversion luminescence

Tetragonal platelets and tubular precursors can be selectively produced with the absence and presence of the surfactant SDS. The platelet-like and tubular precursors can be topotactically converted into oxides.

Layered rare-earth hydroxide nanocones with facile host composition modification and anion-exchange feature: topotactic transformation into oxide nanocones for upconversion

Conical structures with hollow interiors, namely, nanocones (NCs), may exhibit better carrier transport properties than nanorods or nanotubes, which make them promising candidates for potential applications in optical/display devices, electronics and optoelectronics. Generally, conical structures belong to a metastable state between lamellar and tubular forms due to the extreme curvature causing the increase of internal strain energy. Therefore, it is very difficult to prepare NCs in high yield and purity under mild conditions. Here we firstly demonstrate a general strategy for the synthesis of layered rare-earth hydroxide (LRH) NCs intercalating dodecyl sulfate anions (C₁₂H₂₅SO₄^-, DS^-) using hexamethylenetetramine (C₆H₁₂N₄, HMT) hydrolysis. The rare-earth cations (RE³⁺) in the host layer can be conveniently modified and/or doped, resulting in a large family of monometallic (Y, Tb, Er), bi- (Y-Tb, Y-Er) and even tri-metallic (Y-Yb-Er) LRH NCs with adjustable ratios. Moreover, the DS^--intercalated LRH NCs can be readily modified with various inorganic or organic anions (e.g., NO₃^-, Cl^-, and CH₃COO^-, etc.) through a conventional anion-exchange procedure, and the original conical morphology can be perfectly maintained. The anion-exchanged product, for example, NO₃^--intercalated NCs, can be more easily and topotactically transformed into oxide NCs than the original DS^--intercalated form, exempt from the formation of rare-earth oxysulfates induced by the combustion of interlayer DS anions. Taking advantage of this protocol, tri-metallic (Y-Yb-Er) LRH NCs were anion-exchanged into the NO₃^--intercalated form and subsequently calcined into Y₂O₃:Yb,Er oxide NCs, which showed efficient upconversion photoluminescence properties. The current strategy may become a general method for the designed synthesis of other related hydroxide and oxide NCs for a wide range of potential applications.

A bottom-up strategy for exfoliation-free synthesis of soluble α-Ni(OH)2 monolayer nanosheets on a large scale

α-Ni(OH)₂ monolayer nanosheets were successfully synthesized in pure ethanol solution with high yield using NaOH as precipitant, based on a facile one-step “bottom-up” strategy.

Radiation-induced formation of Co3O4 nanoparticles from Co(2+)(aq): probing the kinetics using radical scavengers

The effects of the Co(2+) content and different radical scavengers on the kinetics of γ-radiation-induced Co3O4 nanoparticle formation and growth were investigated. There are four distinct stages of particle formation with different oxidation rates. Scavengers and [Co(2+)]0 affect the oxidation kinetics in the different stages and consequently the final size of the particles formed. Radiolysis model calculations were performed to obtain the time-evolution of the concentrations of key oxidants and reductants, and the effect of scavengers on those concentrations. Based on the model results and experimental data a reaction mechanism for Co3O4 particle formation by γ-irradiation of solutions containing Co(2+)(aq) is proposed. The main cobalt oxidation reaction changes with time. Oxidation of Co(2+)(aq) to Co(3+)(aq) by radiolytically produced ˙OH occurs first in the solution phase. This is followed by spontaneous co-precipitation of mixed Co(II)/Co(III) hydroxide nucleate particles. Adsorption of Co(II)(ad) followed by surface oxidation of Co(II)(ad) to CoOOH(ad) by H2O2 grows particles with a solid CoOOH(s) phase. In parallel, the solid-state transformation of CoOOH(s) and Co(II)(ad) to form Co3O4(s) occurs.

Synthesis and Adsorption Property of SiO₂@Co(OH)₂ Core-Shell Nanoparticles

Silica nanoparticles were directly coated with cobalt hydroxide by homogeneous precipitation of slowly decomposing urea in cobalt nitrate solution. The cobalt hydroxide was amorphous, and its morphology was nanoflower-like. The BET (Brunauer-Emmett-Teller) surface area of the core-shell composite was 221 m²/g. Moreover, the possible formation procedure is proposed: the electropositive cobalt ions were first adsorbed on the electronegative silica nanoparticles surface, which hydrolyzed to form cobalt hydroxide nanoparticles. Then, the cobalt hydroxide nanoparticles were aggregated to form nanoflakes. Finally, the nanoflakes self-assembled, forming cobalt hydroxide nanoflowers. Adsorption measurement showed that the core-shell composite exhibited excellent adsorption capability of Rhodamine B (RB).

Controllable fabrication and magnetic properties of double-shell cobalt oxides hollow particles

Double-shell cobalt monoxide (CoO) hollow particles were successfully synthesized by a facile and effective one-pot solution-based synthetic route. The inner architecture and outer structure of the double-shell CoO hollow particles could be readily created through controlling experimental parameters. A possible formation mechanism was proposed based on the experimental results. The current synthetic strategy has good prospects for the future production of other transition-metal oxides particles with hollow interior. Furthermore, double-shell cobalt oxide (Co3O4) hollow particles could also be obtained through calcinating corresponding CoO hollow particles. The magnetic measurements revealed double-shell CoO and Co3O4 hollow particles exhibit ferromagnetic and antiferromagnetic behaviour, respectively.

Growth mechanism of curved Mg–Al–CO3 layered double hydroxide nanostructures in a one-pot assembly procedure under ambient pressure

The participation of peroxide in Mg–Al-LDH assembly causes hydroxyl point defects and carbonate compensation, leading to curved morphologies.

Layered zinc hydroxide nanocones: synthesis, facile morphological and structural modification, and properties

Layered zinc hydroxide nanocones intercalated with DS(-) have been synthesized for the first time via a convenient synthetic approach, using homogeneous precipitation in the presence of urea and sodium dodecyl sulfate (SDS). SDS plays a significant role in controlling the morphologies of as-synthesized samples. Conical samples intercalated with various anions were transformed through an anion-exchange route in ethanol solution, and the original conical structure was perfectly maintained. Additionally, these DS(-)-inserted nanocones can be transformed into square-like nanoplates in aqueous solution at room temperature, fulfilling the need for different morphology-dependent properties. Corresponding ZnO nanocones and nanoplates have been further obtained through the thermal calcination of NO3(-)-intercalating zinc hydroxide nanocones/nanoplates. These ZnO nanostructures with different morphologies exhibit promising photocatalytic properties.

Phase transformation guided single-layer β-Co(OH)₂ nanosheets for pseudocapacitive electrodes

It is known that Co(OH)2 can be crystallized into a layered structure with two polymorphs: α and β. The single-layer α-Co(OH)2 nanosheet has been prepared by exfoliating directly α phase layered Co(OH)2. However, due to theoretical barriers, a single-layer β-Co(OH)2 nanosheet has not been achieved so far. In this article, phase transformation during exfoliation of layered Co(OH)2 from α to β is observed and a single-layer β-Co(OH)2 nanosheet with a thickness of ∼1.1 nm is prepared through phase transition of layered α-Co(OH)2 nanocones in a mild wet chemical process for the first time, with a nearly 100% yield. The as-prepared single-layer β-Co(OH)2 nanosheets are assembled with graphene oxide to form an all-two-dimensional materials-based composite for use as an electrode for the pseudocapacitor. The reduced graphene oxide/β-Co(OH)2 composite exhibits a high specific capacitance up to 2080 F/g scaled to the total mass of the electrode or 3355 F/g scaled to the active mass of β-Co(OH)2 nanosheets at the current density of 1 A/g. The electrode also demonstrates the excellent rate performance and long cycle life.

Interface chemistry engineering in electrode systems for electrochemical energy storage

In this review, we introduce two powerful strategies for well-controlled interface. Interface chemistry engineering in electrode systems for electrochemical energy storage needs to integrate individual materials components to interface design and optimization.

Recent development of metal hydroxides as electrode material of electrochemical capacitors

Recent research on electrochemical capacitors using transition metal hydroxides as electrode materials is reviewed.

Tunable supercapacitor performance of potentiodynamically deposited urea-doped cobalt hydroxide

The influence of the basal length of Co(OH)₂ on supercapacitor performance was examined in detail by adding simple urea molecules during deposition. It was observed that the specific capacitance of Co(OH)₂ could be tuned from 700 to 1200 F g⁻¹. This high specific capacitance was attributed to the better access of electrolyte.

One pot synthesis of Mg2Al(OH)6Cl·1.5H2O layered double hydroxides: the epoxide route

Pure Mg2Al(OH)6Cl·1.5H2O layered double hydroxide (LDH) has been synthesized at room temperature by a one-pot method, homogeneously driven by chloride-assisted glycidol rupture (epoxide route). Well-defined nanoplatelet texture was achieved and the LDH crystallization mechanism discussed. Nanoplatelets self-assemble in the form of highly oriented films with excellent optical properties. LDH films exhibited stability toward detaching in aqueous solutions and allowed a fast anionic exchange preserving a high transparency.

General insights into structural evolution of layered double hydroxide: underlying aspects in topochemical transformation from brucite to layered double hydroxide

The topochemical transformation from transition-metal brucite hydroxide (Co(1-x)Fe(x)(OH)(2), Co(OH)(2), Co(1-x)Ni(x)(OH)(2)) to corresponding (Co(2+)-(Co(3+))-Fe(3+), Co(2+)-(Ni(2+))-Co(3+)) LDH under oxidizing halogen agents (iodine, bromine) exhibits different staging phenomena depending on the metallic composition/ratio in starting brucite. A plausible charge hopping mechanism based on valence interchange between redoxable charge center (Fe(3+)/Co(3+)) and neighboring divalent sites in the host sheet is proposed to understand the restoration of electron donor sites at the interface between brucite crystallites and halogen agents, which ensures a continual oxidative reaction, and a staged intercalation/diffusion of in situ reduced halide anions into the interlayer gallery commensurate with the host charge propagation. The discussion on the correlation between staging product and metallic composition/ratio offers a general perspective and new insights into M(2+)/M(3+) ratio and cation ordering, host layer charge, and phase evolution in LDH structure.

A general strategy to layered transition-metal hydroxide nanocones: tuning the composition for high electrochemical performance

A general and facile strategy for the synthesis of a large family of monometallic (Co, Ni) and bimetallic (Co-Ni, Co-Cu and Co-Zn) hydroxide nanocones (NCs) intercalated with DS ions is demonstrated. The basal spacing of the NCs can be varied by adjusting the intercalated DS amount. Especially, electrochemical characterizations reveal that bimetallic Co-Ni hydroxide NCs have a higher specific capacitance than their monometallic counterpart. These results suggest the importance of rational designing layered hydroxide NCs with tuned transition-metal composition for high-performance energy storage devices.

Bottom-up preparation of porous metal-oxide ultrathin sheets with adjustable composition/phases and their applications

A facile bottom-up synthesis approach is developed to prepare porous metal-oxide ultrathin sheets, e.g., SnO(2), Fe(2)O(3), and SnO(2)-Fe(2)O(3), with thicknesses of ∼5 nm. Graphene sheets are used as the sacrificing template. Such a process can be extended to the synthesis of multiphased porous metal-oxide thin sheets. These porous thin sheets show interesting applications as gas sensors, effective platforms for matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry, and supercapacitors.