Thermochromic VO2 nanorods and other vanadium oxides nanostructures

Thermochromic VO₂ nanorods were prepared via thermal conversion of the metastable VO₂–B phase synthesized by hydrothermal methods. We observe an increased thermochromic transition temperature to ∼75–80 °C by variable-temperature infrared spectroscopy. Nano- and sub-micron structures of other vanadium oxides (V₃O₇, (NH₄)_0.5V₂O₅, and V₂O₅) were obtained simply by varying the starting materials in the hydrothermal synthesis. We also obtained nanostructures of the high temperature tetragonal rutile phase of VO₂ by thermolysis of single-source vanadium (IV) precursors.     

Hydrothermal synthesis of mesoporous VO2·½(H2O) nanosheets and study of their electrical properties

Layered sheet-like nanocrystalline VO₂·½(H₂O) has been synthesized by hydrothermal process using V₂O₅ as vanadium source and 2-phenylethylamine as a reducing agent and a structure-directing template. Techniques X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and nitrogen adsorption/desorption isotherms have been used to characterize the structure, morphology and composition of the materials. Electrical conductivity measurements showed that the as synthesized VO₂·½(H₂O) nanosheets has a conductivity value which goes from 75 × 10^?6 Ω^?1 cm^?1 at 298 K, to 68 10^?5 Ω^?1 cm^?1 at 386 K with activation energy of 0.24 eV.     

Thermally driven sign switch of static dielectric constant of VO2 thin film

Smart multifunctional materials exhibiting phase transition and tunable optical and/electrical properties provide a new direction towards engineering switchable devices. Specifically, the reversible, tunable and sign switch dielectric constants via external temperature stimuli observed in vanadium dioxide (VO₂) make it a candidate of choice for tunable and switchable technologies devices. Here we report new aspect of the metal-insulator transition (MIT) through the sign switch of the static dielectric constant ε_S of pure VO₂. As it is shown, the static dielectric constant showed an abrupt change from positive at T < 70 °C to negative at T > 70 °C. ε_S > 0 confirms the insulating phase where charges are localized while ε_S < 0 confirms the metallic phase of VO₂ where charges are delocalized. We report for the first time the tunability of the dielectric constant from a negative sign for the static dielectric constant of VO₂ thin film rarely found in real physical systems. We also demonstrate the tunability and switchability of the real and imaginary part of the dielectric constant (ε) via external temperature stimuli. More specifically, the real (ε) and Imaginary (ε) showed an abrupt thermal hysteresis which clearly confirms the phase transition.     

3D hierarchical Zn3(OH)2V2O7·2H2O and Zn3(VO4)2 microspheres: Synthesis,characterization and photoluminescence

Via a simple glycine-assisted hydrothermal route, large-scale 3D hierarchical Zn₃(OH)₂V₂O₇·2H₂O microspheres have been fabricated. Their purity, crystalline phase, morphologies and thermal stability were characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Fourier transform IR (FTIR), scanning electron microscopy (SEM) and thermogravimetry-differential scanning calorimetry (TG-DSC). The SEM results indicate that the microspheres are self-assembled by numerous nanoflakes with mean thickness of 100 nm. Some factors influencing the morphologies of the Zn₃(OH)₂V₂O₇·2H₂O micro-/nanostructures have been systematically investigated, as well as quantity of glycine and the reaction time. The possible mechanism of the crystal growth and assembled procedure were also proposed. The as-prepared Zn₃(OH)₂V₂O₇·2H₂O can be transformed into Zn₃(VO₄)₂ with the similar morphologies by calcination in air at 600 °C. Furthermore, the photoluminescent properties of both Zn₃(OH)₂V₂O₇·2H₂O and Zn₃(VO₄)₂ were studied and exhibited different spectra.     

Hydrothermal synthesis,characterization and electrical investigation of poly(para-phenylenediamine)/vanadium oxide nanocomposite nanosheets

Sheet-like mesoporous poly(paraphenylenediamine)/vanadium oxide nanocomposite has been synthesized by the hydrothermal process using the vanadium oxide V₂O₅ as inorganic precursor and paraphenylendiamine as reducing and a structure-directing agent. Such techniques as X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal analysis (TG-DTA), Fourier transform infrared spectroscopy (FTIR), Raman Spectroscopy, X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption/desorption isotherms (BET) have been used to characterize the structure, morphology and the texture of the material. The conductivity of the material was measured by complex impedance spectroscopy which increases from 72 × 10^?5 Ω^?1 cm^?1 at 298 K to 95 × 10^?4 Ω^?1 cm^?1 at 493 K. The Arrhenius diagram is not linear, it presents a rupture situated at 407 K and the activation energies’ average values are 0.044 eV and 0.13 eV.     

A novel reduction–hydrolysis method of preparing VO2 nanopowders

A novel and convenient reduction–hydrolysis method was developed to make high-purity VO₂ nanopowders. The products have been studied with XRD, IR, XPS, and DTA. The nanopowders exhibited a strong crystallographic transition to the rutile-type structure around 68–72 °C. The XRD patterns and TEM images of products showed that the average size of samples was in the range of 30–40 nm. The XPS results demonstrated that the vanadium exists as V⁴⁺ ions in the products. We also found that the phase transition temperature of products decreased by tungsten doping.     

Growth of VO2 films with metal-insulator transition on silicon substrates in inductively coupled plasma-assisted sputtering

Single-phase monoclinic vanadium dioxide (VO₂) films were grown on a Si(100) substrate using inductively coupled plasma (ICP)-assisted sputtering with an internal coil. The VO₂ film exhibited metal-insulator (M-I) transition at around 65 °C with three orders of change in resistivity, with a minimum hysteresis width of 2.2 °C. X-ray diffraction showed structural phase transition (SPT) from monoclinic to tetragonal rutile VO₂. For conventional reactive magnetron sputtering, vanadium oxides with excess oxygen (V₂O₅ and V₃O₇) could not be eliminated from stoichiometric VO₂. Single-phase monoclinic VO₂ growths that are densely filled with smaller crystal grains are important for achieving M-I transition with abrupt resistivity change.     

Phase transition in (1 − x)Bi(Mg1/2Ti1/2)O3-xPbTiO3 ceramics

The phase structure and phase transition of (1 ? x)Bi(Mg_1/2Ti_1/2)O₃-xPbTiO₃ (BMT-PT) ceramics with x = 0.0–0.42 were investigated. It was found that pure perovskite phases were achieved for x ≥ 0.28, while Bi₄Ti₃O₁₂ or Bi₁₂TiO₂₀ phase existed for x ≤ 0.15. The anomaly dielectric peaks were observed around 620 °C for BMT-(0.28–0.38)PT samples, thus phase transition in (1 ? x)Bi(Mg_1/2Ti_1/2)O₃-xPbTiO₃ ceramics was studied using thermal expansion. It was found that dielectric anomalies at ~ 620 °C were resulted from the phase transition of the second phase and defects inside samples.     

Template-free hydrothermal derived cobalt oxide nanopowders: Synthesis,characterization, and removal of organic dyes

Pure spinel cobalt oxide nanoparticles were prepared through hydrothermal approach using different counter ions. First, the pure and uniform cobalt carbonate (with particle size of 21.8–29.8 nm) were prepared in high yield (94%) in an autoclave in absence unfriendly organic surfactants or solvents by adjusting different experimental parameters such as: pH, reaction time, temperature, counter ions, and (Co²⁺:CO₃^2?) molar ratios. Thence, the spinel Co₃O₄ (with mean particle size of 30.5–47.35 nm) was produced by thermal decomposition of cobalt carbonate in air at 500 °C for 3 h. The products were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM), and thermal analysis (TA). Also, the optical characteristics of the as-prepared Co₃O₄ nanoparticles revealed the presence of two band gaps (1.45–1.47, and 1.83–1.93 eV). Additionally, adsorption of methylene blue dye on Co₃O₄ nanoparticles was investigated and the uptake% was found to be >99% in 24 h.     

Low temperature synthesis,crystal structure and thermal stability studies of nanocrystalline VN particles

The simultaneous thermal decomposition and nitridation of [VO(NH₂O)₂Gly]·H₂O complex in NH₃ atmosphere at 723–973 K gives the nanocrystalline vanadium nitride (VN) having crystallite size of 8–32 nm. It shows cubic NaCl structure with lattice parameter of a = 4.137 nm. XRD pattern Rietveld analysis program for crystal structure of VN shows the space group-Fm3m. The particle sizes measured by BET and SEM techniques are in the range of 26–100 nm. The particles are spherical and distributed homogeneously and found larger than XRD crystallite size because of agglomeration of crystallites. The fundamental IR absorption of VN material is found at 995 cm⁻¹ which gives the force constant of 634.3 Nm⁻¹. The electrical resistivity and magnetic studies show the superconducting to normal transition (T_c) at 9.2 K. Thermal decomposition of VN is carried out in O₂ atmosphere which goes through the formation of an oxynitride (V–N_p–O_q) intermediate phase up to 913 K. Finally, nanocrystalline V₂O₅ is formed at 973 K. The V₂O₅ has orthorhombic structure with lattice parameters of a = 11.537, b = 3.568 and c = 4.380 Å and the XRD crystallite size of 10 nm.     

Facile synthesis,phase transition,optical switching and oxidation resistance properties of belt-like VO2(A) and VO2(M) with a rectangular cross section

Belt-like VO₂(A) with a rectangular cross section (VA-RCS) was successfully synthesized using V₂O₅, H₂C₂O₄·2H₂O and H₂O as the starting materials by a facile hydrothermal approach. Some synthetic parameters, such as, the reaction time, reaction temperature and concentration of H₂C₂O₄·2H₂O, were systematically investigated to control the fabrication of belt-like VA-RCS. The formation mechanism of belt-like VA-RCS was proposed. Subsequently, belt-like VO₂(M) with a rectangular cross section (VM-RCS) was prepared by the irreversible transformation of VA-RCS at 700 °C for 2 h under the inert atmosphere. The phase transition temperature (T_c) of VA-RCS and VM-RCS was evaluated by DSC test. The optical switching properties of VA-RCS and VM-RCS were studied by the variable-temperature infrared spectra, and it was found that the as-obtained VA-RCS and VM-RCS could be used as the optical switching materials. Furthermore, the oxidation resistance properties of VA-RCS and VM-RCS were investigated by TGA, indicating that they have good thermal stability and oxidation resistance below 400 °C in air.     

Initial phase hot corrosion mechanism of gas tunnel type plasma sprayed thermal barrier coatings

The hot corrosion resistance of the top layer in TBC is one of the main constructive factors which determines the lifetime of the coatings under critical operating environments. In the present study, 8 wt% yttria stabilized zirconia (8YSZ), lanthanum zirconate (La₂Zr₂O₇) and equal weight percentage of its composite (50%8YSZ + 50% La₂Zr₂O₇) coatings were prepared by using gas tunnel type plasma spray torch at optimum spraying conditions. The hot corrosion performances of the above thermal barrier coatings were examined against 40 wt%V₂O₅–60 wt%Na₂SO₄ corrosive ash at 1173 K for 5 h in open air atmosphere. After hot-corrosion testing, the coating surface was studied using a scanning electron microscope to observe the microstructure and X-ray diffraction techniques were used to identify the phase compositions. The results showed that LaVO₄ and YVO₄ are the main hot corrosion products along with the ZrO₂ phase transformation from tetragonal to monoclinic phases in La₂Zr₂O₇ and 8YSZ coatings respectively. The microstructure and phase formation mechanism of the hot corrosion products varied with each coating and among these, composition of 50%8YSZ + 50%La₂Zr₂O₇ coating exhibited least degradation against V₂O₅–Na₂SO₄ corrosive environment compared to the other coatings.     

Microwave-assisted synthesis and characterization of Ti1 − xVxO2 (x = 0.0–0.10) nanopowders

Ti_1 ? xV_xO₂ (x = 0.0–0.10) nanopowders were successfully synthesized by a microwave-assisted sol–gel technique and their crystal structure and electronic structure were investigated. The products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and UV–Vis spectroscopy. The results revealed that TiO₂ powders maintained the anatase phase for calcination temperature below 600 °C, but gradually changed to the rutile phase above 800 °C. The formation of the rutile phase was completed at 1000 °C. For Ti_1 ? xV_xO₂ (x = 0.05) powders, the phase transformation appeared at 600 °C. The absorption edge of Ti_1 ? xV_xO₂ (x > 0) powders broadened to the visible region with increasing V concentration and a strong visible light absorption was obtained with 10% V doping. V doping and subsequent coexistence of both anatase and rutile phases in our Ti_1 ? xV_xO₂ nanoparticles are considered to be responsible for the enhanced absorption of visible light up to 800 nm.     

Synthesis of lanthanum zirconium oxide nanomaterials through composite-hydroxide-mediated approach

A novel thermal barrier coating material, lanthanum zirconium oxide (La₂Zr₂O₇) has been synthesized through the composite-hydroxide-mediated method at low temperature. The phase structures, morphology, thermal stability and thermal conductivity of the as-synthesized La₂Zr₂O₇ were investigated systematically. The X-ray diffraction (XRD) patterns revealed a single phase with cubic pyrochlore structure for La₂Zr₂O₇ after treated at 1300 °C for 100 h. The transmission electron microscope (TEM) and scanning electron microscope (SEM) analyses showed that the sample was made up of sphere-like nanoparticles with the size between 50 and 100 nm. Furthermore, the thermal analysis result demonstrated the La₂Zr₂O₇ sample had high thermal stability even at 1300 °C. As the temperature increased to 1200 °C, the thermal conductivity value could be as low as 1.75 W m^?1 K^?1. Due to the high-temperature stability and lower thermal conductivity, the La₂Zr₂O₇ material is expected to be a promising candidate for the use of thermal barrier coatings.     

Chemical bath deposition and characterization of electrochromic thin films of sodium vanadium bronzes

Thin yellow-orange films of sodium vanadium oxide bronzes have been prepared from a sodium–vanadium solution (1:1) at 75 °C and pH = 3. The composition, structure and morphology of the films have been studied by XRD, IR spectroscopy, TG and SEM–EDX analyses. It has been established that the prepared films are a phase mixture of hydrated NaV₆O₁₅ (predominant component) and Na_1.1V₃O_7.9 with total water content of 10.58%.The sodium vanadium bronze thin films exhibit two-step electrochromism followed by color change from yellow-orange to green, and then from green to blue. The cyclic voltammetry measurements on the as-deposited and annealed vanadium bronze films reveal the existence of different oxidation/reduction vanadium sites which make these films suitable for electrochromic devices. The annealing of the films at 400 °C changes the composition, optical and electrochemical properties     

Synthesis and characterization of MFe2O4 (M = Mg,Mn, Ni) nanoparticles

MFe₂O₄ nanoparticles were obtained in the presence of natural compounds as carboxymethylcellulose (CMC). The CMC polymer had a double function as a capping agent and as a protecting agent in the growth process of nanoparticles. The synthesized nanoparticles were characterized using thermal analysis (TG, DTA, DTG), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and a vibrating sample magnetometer (VSM). The XRD patterns indicate that all the samples were formed in single phase spinel structure. The results also show that the samples calcinated at 500 °C for 6 h have the best crystallinity and the calculated crystallite size was in the range of 6–13 nm. The thermal analysis and FTIR spectra indicate a core–shell structure of the MFe₂O₄ nanoparticles obtained.     

Synthesis of Co3(OH)2V2O7·1.7H2O nanosheets and its application in lithium ion batteries

Cobalt vanadium oxide hydroxide hydrate (Co₃(OH)₂V₂O₇·nH₂O) nanosheets are successfully synthesized by a simple hydrothermal method. The composition of Co₃(OH)₂V₂O₇·nH₂O is studied by thermal gravity (TG) analysis in N₂ atmosphere and subsequent X-ray powder diffraction (XRD) characterization of the sample obtained via annealing Co₃(OH)₂V₂O₇·nH₂O nanosheets in N₂ atmosphere at 800 °C for 6 h. The results indicate that there are 1.7 water molecules in a Co₃(OH)₂V₂O₇·nH₂O molecular formula. Electrochemical properties of Co₃(OH)₂V₂O₇·1.7H₂O nanosheets as negative electrode of lithium ion batteries are studied by conventional charge/discharge test, which show an initial capacity of 730 mAh g^?1 with steady plateau near 0.9 V at a current density of 0.05 mA cm^?2.     

Effect of annealing conditions and concentration of oxygen vacancies on vanadium doped SrBi2Ta2O9

This paper studied the annealing effects on the dielectric characteristics of vanadium doped SrBi₂Ta₂O₉ (SBT). SBT was synthesized at 1000 °C and vanadium doped SBT at 900 °C by solid-state reaction. Crystallization structure and phase purity of the prepared ceramic samples was observed by X-ray diffraction analysis. XRD analysis indicated a single layered perovskite structure without any secondary phases up to 15% of vanadium doping in SBT ceramics. Detailed dielectric study on vanadium doped SBT ceramics indicated that post-sinter annealing enhances the peak dielectric permittivity, which is attributed to the increased homogeneity in the system at atomic scale upon annealing. Annealing for larger time interval suppresses the permittivity growth beyond transition temperature which gives a direct evidence for the existence of lower valance state of vanadium (V⁺⁴) in as-sintered SBTV ceramics and also the permittivity growth is related to the oxygen ions or oxygen vacancies created during sintering. UV–Vis spectroscopy was also performed to confirm the lower valance state of the vanadium ions in the ceramics.     

Structural and other physical properties of barium vanadate glasses

XRD, IR spectra, DTA, density, oxygen molar volume and dc conductivity of barium vanadate glasses of compositions xBaO(100 − x)V₂O₅, where x = 30, 35, 40, 45 and 50 mol%, are reported. The IR studies of the glasses suggest the glass network is built up of mainly VO₄ polyhedra. The glass transition temperatures are observed to increase with an increase of BaO content in the compositions. The cross-linking density decrease with increasing BaO content in the compositions. Introduction of BaO into the V₂O₅ matrix changes the 2D layer structure of the crystalline V₂O₅ into a more complicated 3D structure. Analysis of the electrical properties has been made in the light of small polaron hopping model. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with glass composition. The conduction is attributed to non-adiabatic hopping of small polaron.     

Porous nano-structured VO2 films with different surfactants: synthesis mechanisms, characterization, and applications

Porous nano-structured vanadium dioxide (VO₂) thin films have been prepared on mica substrates via sol–gel process using surfactant cetyltrimethyl ammonium bromide, nonionic surfactant polyethylene glycol, and anionic surfactant sodium dodecyl sulfate as nano-structure directing agents. Models concerning the structure forming were proposed to explain the synthesis mechanisms between V₂O₅ colloid and different surfactants. Porous nano-structured VO₂ films with sphere-shaped, island-shaped and strip-shaped nanocrystals are synthesized in the experiments, and the optical properties and thermochromic properties of these films are compared. The porous nano-structured VO₂ films showed excellent infrared transmittance (nearly 70 %), low transition temperature (59.7 °C without doping), wide hysteresis width (37.8 °C), and different optical transmittance difference before and after the phase transition (39–67 %). The results suggest that these porous nano-structured VO₂ films have significant importance in practical application in VO₂-based optical and electronic devices.