Highly [001]-oriented N-doped orthorhombic Nb2O5 microflowers with intercalation pseudocapacitance for lithium-ion storage

Orthorhombic Nb₂O₅ (T-Nb₂O₅), a typical intercalation pseudocapacitor, is favorable for realizing high power and energy density for lithium-ion batteries; furthermore, the 2D layered channels perpendicular to the [001] direction facilitate fast Li⁺ intercalation in T-Nb₂O₅. Herein, N-doped T-Nb₂O₅ microflowers (N-Nb₂O₅) assembled from highly [001]-oriented nanoflakes are rationally synthesized using NH₄F as the nitrogen source and capping agent. It is found that NH₄⁺ can adsorb on the O-terminated (010) plane of T-Nb₂O₅via N-H⋯O hydrogen bonds, which is highly conducive to the generation of 1D nanorods and the subsequent fusion of the nanorods into highly [001]-oriented nanoflakes. The special growth orientation of the T-Nb₂O₅ nanoflakes endows them with abundant available Li⁺ intercalation channels; moreover, the bandgap of N-Nb₂O₅ is narrowed (∼2.91 eV) owing to the doping of N atoms, and the intrinsic electronic conductivity is improved. Accordingly, the intercalation pseudocapacitive behavior of N-Nb₂O₅ is notably promoted and N-Nb₂O₅ exhibits superior Li⁺ storage properties, including large discharge capacity (214.7 mA h g^-1 at 1C), excellent rate capability (203.7 and 174.6 mA h g^-1 at 1 and 20C), and superior cyclic stability (150.7 mA h g^-1 at 10C after 1000 cycles). In addition, the LiNi_0.5Mn_0.3Co_0.2O₂//N-Nb₂O₅ full cell delivers outstanding Li⁺ storage performance, especially in terms of long-term cycling (126.2 mA h g^-1 at 10C after 3500 cycles).

Facile synthesis of Mn-doped NiCo2O4 nanoparticles with enhanced electrochemical performance for a battery-type supercapacitor electrode

We report the synthesis of manganese-doped nickel cobalt oxide (Mn-doped NiCo2O4) nanoparticles (NPs) by an efficient hydrothermal and subsequent calcination route. The material exhibits a homogeneous distribution of the Mn dopant and a battery-type behavior when tested as a supercapacitor electrode material. Mn-doped NiCo2O4 NPs show an excellent specific capacity of 417 C g-1 at a scan rate of 10 mV s-1 and 204.3 C g-1 at a current density of 1 A g-1 in a standard three-electrode configuration, ca. 152-466% higher than that of pristine NiCo2O4 or MnCo2O4. In addition, Mn-doped NiCo2O4 NPs showed an excellent capacitance retention of 99% after 1000 charge-discharge cycles at a current density of 2 A g-1. The symmetric solid-state supercapacitor device assembled using this material delivered an energy density of 0.87 μW h cm-2 at a power density of 25 μW h cm-2 and 0.39 μW h cm-2 at a high power density of 500 μW h cm-2. The cost-effective synthesis and high electrochemical performance suggest that Mn-doped NiCo2O4 is a promising material for supercapacitors.

Perovskite Oxides Prepared by Hydrothermal and Solvothermal Synthesis: A Review of Crystallisation, Chemistry, and Compositions

Perovskite oxides with general composition ABO₃ are a large group of inorganic materials that can contain a variety of cations from all parts of the Periodic Table and that have diverse properties of application in fields ranging from electronics, energy storage to photocatalysis. Solvothermal synthesis routes to these materials have become increasingly investigated in the past decade as a means of direct crystallisation of the solids from solution. These methods have significant advantages leading to adjustment of crystal form from the nanoscale to the micron-scale, the isolation of compositions not possible using conventional solid-state synthesis and in addition may lead to scalable processes for producing materials at moderate temperatures. These aspects are reviewed, with examples taken from the past decade's literature on the solvothermal synthesis of perovskites with a systematic survey of B-site cations, from transition metals in Groups 4-8 and main group elements in Groups 13, 14 and 15, to solid solutions and heterostructures. As well as hydrothermal reactions, the use of various solvents and solution additives are discussed and some trends identified, along with prospects for developing control and predictability in the crystallisation of complex oxide materials.

Low-temperature wet chemistry synthetic approaches towards ferrites

Solution chemistry allows the crystallisation of range of iron oxides, including MFe₂O₄ spinels, MFeO₃ perovskites and hexaferrites, such as BaFe₁₂O₁₉, with nanoscale crystallinity and properties suitable for fields such as catalysis and electronics.

Molten Salt Flux Synthesis, Crystal Facet Design, Characterization, Electronic Structure, and Catalytic Properties of Perovskite Cobaltite

We present a simple and cost-effective molten salt synthetic route toward phase-pure perovskite cobaltite microcrystallines and successfully regulate different crystal facets for perovskite LaCoO₃ by the strong interaction between Cl^- anions and Sr²⁺ cations in molten salt system and polar plane. We then take LaCoO₃ (100 and 110), LaCoO₃ (111), and La_0.7Sr_0.3CoO₃ (111) as comparison models, and we characterize their crystal structure, morphology, composition, electronic state, and catalytic properties. X-ray photoelectron spectroscopy (XPS) shows that the prepared samples with high-energy (111) crystal facets contain more surface oxygen species and active Co ions than La enrichment perovskite LaCoO₃ (110 and 100) on the surface. Furthermore, combining with ambient-pressure XAS, valence band spectroscopy, and density functional calculations, we find that exposed high-energy (111) crystal facets and doping Sr ions can enhance the hybridization between Co cations and O anions and their O p-band center is closer to the Fermi level, compared with that of LaCoO₃ (100 and 110). As expected, the samples with high-energy (111) crystal facets show better CO oxidation activity than LaCoO₃ (100 and 110), and La_0.7Sr_0.3CoO₃ (111) exhibits the highest catalytic activity. Our findings provide a new avenue to prepare high-energy facet perovskite catalysts and we also clearly reveal the relationship between surface electronic structure and intrinsic CO oxidation activity of perovskite cobaltite.

Kinetic Growth Regimes of Hydrothermally Synthesized Potassium Tantalate Nanoparticles

A general mathematical kinetic growth model is proposed on the basis of observed growth regimes of hydrothermally synthesized KTaO₃ nanoparticles from electron microscopy studies on the surface morphology and surface chemistry. Secondary electron imaging demonstrated that there are two dominant growth mechanisms: terrace nucleation, where the surfaces are rough, and terrace growth, where surfaces are smooth. In the proposed model based upon standard step-flow growth, the rates of both mechanisms are established to be dependent on the chemical potential change of the growth environment-terrace nucleation dominates with larger negative chemical potential, and terrace growth dominates with smaller negative chemical potential. This analysis illustrates the importance of ending a synthesis in a regime of low negative chemical potential in order to achieve smooth well-faceted nanoparticles.

Hydrothermal shape controllable synthesis of La0.5Sr0.5MnO3 crystals and facet effect on electron transfer of oxygen reduction

Controllable growth of perovskite structure oxide crystals with well-defined facets is challenging, especially in the mixed-valence state manganites with both rare-earth and alkaline-earth cations in their A-site.

Mineralizer effect on facet-controllable hydrothermal crystallization of perovskite structure YbFeO3 crystals

Mineralizer is a key factor in the hydrothermal crystallization of most metal oxide materials.

Shape tuneable synthesis of perovskite structured rare-earth chromites RECrO3via a mild hydrothermal method

Controllable synthesis of perovskite structured oxides with well-defined facets is a challenging but important route for crystal shape and facet dependent applications.

Crystal facet tailoring arts in perovskite oxides

This review highlights various facet tailoring arts in perovskite structure oxides.

Engineering the surface of perovskite La0.5Sr0.5MnO3 for catalytic activity of CO oxidation

A simple treatment of La_0.5Sr_0.5MnO₃ with diluted HNO₃ creates more B-sites (rich) on the terminated perovskite surface and improves its catalytic activity toward CO oxidation, and the perovskite catalyst possesses a higher ratio of Mn⁴⁺/Mn³⁺ and thus enhances the O₂ adsorption capability, favourable for CO oxidation and catalytic activity.