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.     

Synthesis,characterization, and thermodynamic parameters of vanadium dioxide

A novel process was developed for synthesizing pure thermochromic vanadium dioxide (VO₂) by thermal reduction of vanadium pentoxide (V₂O₅) in ammonia gas. The process of thermal reduction of V₂O₅ was optimized by both experiments and modeling of thermodynamic parameters. The product VO₂ was characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TG), and differential scanning calorimetry (DSC). The experimental results indicated that pure thermochromic VO₂ crystal particles were successfully synthesized. The phase transition temperature of the VO₂ is approximately 342.6 K and the enthalpy of phase transition is 44.90 J/g.     

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.     

Microstructures and thermochromic properties of tungsten doped vanadium oxide film prepared by using VOX-W-VOX sandwich structure

Tungsten doped vanadium oxide (VO_X) thin films were prepared by oxygen annealing VO_X-W-VO_X sandwich layers. X-ray photoelectron spectroscopy, X-ray diffraction and field emission scanning electron microscope were employed to characterize the compositions, crystal structures and surface morphologies, respectively. It was demonstrated that sandwich structure suppressed the crystallization of VO_X, and that V⁵⁺ was reduced by diffused W atom to V⁴⁺. The results of surface morphologies indicated that the grain arrangement of W doped vanadium dioxide film exhibited some regular patterns compared with the random grain distribution of undoped film. Electrical measurements showed that the square resistance of V₂O₅ film and semiconductor-metal transition temperature of VO₂-V₂O₅ film decreased obviously after W doping. In addition, thermal hysteresis loop was observed in W doped V₂O₅ film with thick W middle layer. The investigation of optical properties indicated that the optical band gap of W doped V₂O₅ film decreased with the increase of thickness of W middle layer, and the optical switching performance in the near-infrared range of VO₂-V₂O₅ slightly weakened after W doping.     

Effects of thermal cycling and subsequent heat treatment on the electrical conductivity of a VO2-based ceramic

Thermal cycling through the temperature of the metal-semiconductor transition (T _tr = 341 K) was found to have an adverse effect on the electrical behavior of a VO₂-based ceramic (80 wt % VO₂ + 20 wt % glass; glass composition, 80 wt % V₂O₅ + 20 wt % P₂O₅). The observed increase in resistivity and reduction in resistivity jump at T_tr are attributable to the formation of microcracks in VO₂ crystallites. Heat treatment at 1170 K in an inert atmosphere eliminates the irreversible changes produced by thermal cycling, presumably owing to microcrack healing through growth of VO₂ crystallites from the vanadium phosphate melt filling the microcracks     

Solid-state reactions in V x O y (NH4VO3)-P2O5 and V x O y (NH4VO3)-(NH4)2HPO4 closed systems

Solid-state reactions in V _x O _y (NH₄VO₃)-P₂O₅ and V _x O _y (NH₄VO₃)-(NH₄)₂HPO₄ closed systems can be used to synthesize vanadyl hydrogen phosphate at 300°C and a variety of ammonium vanadium phosphates at lower and higher temperatures.     

Oxidation potential control of VO2 thin films by metal oxide co-sputtering

For metal-to-insulator transition (MIT) in vanadium oxide thin film, a thermodynamically stable vanadium dioxide (VO₂) phase is essential. In VO₂ films sputter-deposited on a quartz substrate from a V₂O₅ target, a radio-frequency (RF) magnetron sputter system at working pressure of 7 mTorr is used. Due to the lower sputtering yield of oxygen compared to vanadium leading to oxygen-ion deficiency, the reduction of V ions is resulted to compensate charge with the oxygen ions. Under lower working pressures, the deposition rate increases, but a simultaneous oxygen-ion deficiency causes the destabilization of VO₂. To prevent this, titanium oxide co-deposition is suggested to enrich the oxygen source. When TiO₂ is used, it is found that the Ti ion has a stable +4 charge state so that the use of extra oxygen in sputtering prevents the destabilization of VO₂. However, this is not the case for TiO. For the latter, Ti ions are oxidized from the +2 state to the +3 and +4 states, and V ions with less oxidation potential are reduced to +3 or so. Pure VO₂ thin film exhibits MIT at 66 °C and a large resistivity ratio of four orders of magnitude from 30 to 90 °C. The (V₂O₅ + TiO₂) system under working pressure as low as 5 mTorr yields fairly good films comparable to pure VO₂ deposited at 7 mTorr, whereas the use of TiO yields films with MIT absent or considerably weakened.     

Microstructure and microhardness of vanadium oxides in the range VO<Subscript>0.57</Subscript>-VO<Subscript>1.29</Subscript>

X-ray diffraction and optical microscopy data are presented which demonstrate that substoichiometric vanadium oxide (VO_0.57-VO_0.97) consists of a cubic phase with the B1 structure (sp. gr. Fm \(\bar 3\) m) and an ordered monoclinic phase of composition V₁₄O₆ (sp. gr. C2/m). The content of the latter phase decreases with increasing oxygen content. The superstoichiometric vanadium oxide VO_1.29 is shown to contain trace amounts of V₅₂O₆₄. Vickers microhardness data for nonstoichiometric vanadium oxides in the range VO_0.57-VO_1.29 show that, with increasing oxygen content, their H _V has a tendency to decrease, from 18 to 12 GPa. Their microhardness is shown for the first time to have a maximum near the stoichiometric composition VO_1.00.     

Phase evolution during synthesis of vanadium carbide (V8C7) nanopowders by thermal processing of the precursor

The phase and structure evolution during synthesis of vanadium carbide (V₈C₇) nanopowders by thermal processing of the precursor were investigated using X-ray diffraction (XRD). The morphology of the reactant was characterized by transmission electron microscopy (TEM). Simultaneous thermogravimetric and differential thermal analysis (TG-DTA) were made on the precursor. The results indicate phase evolution sequences are NH₄VO₃→V₂O₅→VO₂→V₅O₉+V₄O₇→V₂O₃→VC_1−X→V₈C₇. The single phase V₈C₇ powders can be prepared at ∼1100 °C for 1 h, and the powders show good dispersion and are mainly composed of uniformly sized spherical particles with the majority diameters of 20-50 nm.     

The effect of vanadium oxides on the crystallisation of silicate glasses

The influence of vanadium oxides as catalysts for nucleation and crystal growth in CaO-MgO-Al₂O₃-SiO₂ glasses has been investigated. The effect of varying the total vanadium content and the ratio of oxidised to reduced vanadium ions has been observed. No internal nucleation was observed, but the rate of growth of crystals of anorthite and wollastonite from the surface was increased by additions of V₂O₅ and V₂O₃. The values of the growth rates and the analysed V⁵⁺ content in both oxidised and reduced glasses suggest that V⁵⁺ ions are the most active species. Increasing concentrations of V₂O₅ in the glass gave maxima in the growth rates between 2 and 4 wt % for crystallisation temperatures between 900 and 980° C.     

A XANES study of the valence state of vanadium in lithium vanadate phosphate glasses

Valence states of vanadium in Li₂O-V₂O₅-P₂O₅ electronic-ionic conducting glasses have been studied by X-ray absorption spectroscopy. The composition dependence of the XANES range of the spectra has been used to estimate relative abundance of the vanadium atoms in V⁴⁺ or V⁵⁺ charge states. This information is important for analysis of electronic transport in these glasses, which occurs via electronic hopping between aliovalent vanadium centers. It was found that in samples with the lowest V₂O₅ contents (10 mole% of V₂O₅) a vast majority of vanadium ions are in V⁴⁺ charge state. At higher V₂O₅ contents the proportion of V⁴⁺ ions decreases at the expense of V⁵⁺ ones, but remains substantial even in glasses with the highest contents of V₂O₅.     

Controlled hydrothermal synthesis of VO2(B) nanobelts

Large-scale VO₂(B) nanobelts have been synthesized by hydrothermal strategy via one-step method using V₂O₅ as vanadium source and C₆H₅-(CH₂)_n-NH₂ with n = 2 and 4 (2-phenylethylamine and 4-phenylbutylamine) as structure-directing templates. The composition and morphology of the nanobelts were established by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The as-obtained VO₂(B) nanobelts have a length of 3-10 μm, a wideness of 100-375 nm and a thickness of 30-66 nm.     

Facile synthesis of single crystalline vanadium pentoxide nanowires and their photocatalytic behavior

High quality single crystalline vanadium pentoxide (V₂O₅) nanowires were grown on sapphire and ITO coated glass substrates using spin coating followed by annealing process. The nanowires formed by this method are found to be approximately 5 μm long with an average diameter of 100 nm. The thickness of the spin coated vanadium precursor film played a vital role to form uniform seed layers which are essential for the growth of high quality V₂O₅ nanowires. The growth mechanism was investigated with respect to temperature and thickness of the precursor film. The synthesized nanowires have been proven to be a potential photocatalyst for the degradation of toluidine blue O dye under ultraviolet irradiation.     

Enhanced hydrophilicity of the Si substrate for deposition of VO2 film by sol–gel method

We have illustrated the role of hydrophilic nature of Si substrate played in the improvement of the contact performance between the vanadium dioxide (VO₂) film and Si substrate. The VO₂ films were fabricated by sol–gel method on single crystal Si substrate, which was pre-treated with hydrophilic solution and obtained a quite improved hydrophilicity. The bonding of Si substrate with precursor V₂O₅ gel was interpreted. The morphology and crystalline structure of the films were investigated by field-emission scanning electron microscopy, atomic force microscopy and X-ray diffraction. It is shown that the surface of the film on Si substrate with enhanced hydrophilicity is quite homogeneous and uniform. The film exhibits the formation of VO₂ phase with (011) preferred orientation. Moreover, the optical pump induced phase transition property of the film was studied by terahertz time-domain spectroscopy, which revealed around 70% reduction of transmission at 0.1–1.5?THz in the VO₂ film across the phase transition.     

Controlled synthesis and electrochemical properties of vanadium oxides with different nanostructures

Vanadium oxides (V₃O₇·H₂O and VO₂) with different morphologies have been selectively synthesized by a facile hydrothermal approach using glucose as the reducing and structure-directing reagent. The as-obtained V₃O₇·H₂O nanobelts have a length up to several tens of micrometers, width of about 60?C150?nm and thickness of about 5?C10?nm, while the as-prepared VO ₂ (B) nanobelts have a length of about 1·0?C2·7???m, width, 80?C140?nm and thickness, 2?C8?nm. It was found that the quantity of glucose, the reaction temperature and the reaction time had significant influence on the compositions and morphologies of final products. Vanadium oxides with different morphologies were easily synthesized by controlling the concentration of glucose. The formation mechanism was also briefly discussed, indicating that glucose played different roles in synthesizing various vanadium oxides. The phase transition from VO₂(B) to VO₂(M) were investigated and the phase transition temperature of the VO₂(M) appeared at around 68 °C. Furthermore, the electrochemical properties of V₃O₇·H₂O nanobelts, VO₂(B) nanobelts and VO₂(B) nanosheets were investigated and they exhibited a high initial discharge capacity of 296, 247 and 227 mAh/g, respectively.     

A High‐Rate V2O5 Hollow Microclew Cathode for an All‐Vanadium‐Based Lithium‐Ion Full Cell

V₂O₅ hollow microclews (V₂O₅‐HMs) have been fabricated through a facile solvothermal method with subsequent calcination. The synthesized V₂O₅‐HMs exhibit a 3D hierarchical structure constructed by intertangled nanowires, which could realize superior ion transport, good structural stability, and significantly improved tap density. When used as the cathodes for lithium‐ion batteries (LIBs), the V₂O₅‐HMs deliver a high capacity (145.3 mAh g^‐1) and a superior rate capability (94.8 mAh g^‐1 at 65 C). When coupled with a lithiated Li₃VO₄ anode, the all‐vanadium‐based lithium‐ion full cell exhibits remarkable cycling stability with a capacity retention of 71.7% over 1500 cycles at 6.7 C. The excellent electrochemical performance demonstrates that the V₂O₅‐HM is a promising candidate for LIBs. The insight obtained from this work also provides a novel strategy for assembling 1D materials into hierarchical microarchitectures with anti‐pulverization ability, excellent electrochemical kinetics, and enhanced tap density.     

V<Subscript>3.047</Subscript>O<Subscript>7</Subscript>, a new high-pressure oxide with the simpsonite structure

Reaction between α-V₂O₅ and NaN₃ has been studied at pressures from 5.0 to 6.0 GPa and temperatures from 600 to 800°C using Toroid high-pressure chambers. A new oxide, V_3.047O₇ (VO_2.297), isostructural with simpsonite, Al₄Ta₃O₁₃(OH), has been detected in samples with the initial composition 0.2NaN₃ · V₂O₅ after high-temperature, high-pressure processing at p = 5.0 GPa and t = 800°C for 2 min. The crystal structure of the oxide has been refined by the Rietveld method using X-ray powder diffraction data: a = 7.35136(2) Å, c = 4.51462(2) Å, V = 211.294(1) ų, Z = 2, sp. gr. P3. Each vanadium atom in this structure is coordinated by six oxygens in the form of a [VO₆] octahedron. The synthesized oxide is a second compound with the simpsonite structure. We have measured the infrared transmission and Raman spectra of V_3.047O₇. Electrical measurements have demonstrated that the material is a semiconductor.     

Structural and electrochemical behaviour of sputtered vanadium oxide films: oxygen non-stoichiometry and lithium ion sequestration

Structural and electrochemical aspects of vanadium oxide films recently reported from ICMCB/ ENSCPB have been examined using appropriate structural models. It is shown that amorphous films are nonstoichiometric as a result of pre-deposition decomposition of V₂O₅. It is proposed that the structure of amorphous films corresponds to a nanotextured mosaic of V₂O₅ and V₂O₄ regions. Lithium intercalation into these regions is considered to occur sequentially and determined by differences in group electronegativities. Open circuit voltages (OCV) have been calculated for various stoichiometric levels of lithiation using available thermodynamic data with approximate corrections. Sequestration of lithium observed in experiments is shown to be an interfacial phenomenon. X-ray photoelectron spectroscopic observation of the formation of V³⁺ even when V⁵⁺ has not been completely reduced to V⁴⁺ is shown to be entirely consistent with the proposed structural model and a consequence of initial oxygen nonstoichiometry. Based on the structural data available on V₂O₅ and its lithiated products, it is argued that the geometry of VO_n polyhedron changes from square pyramid to trigonal bipyramid to octahedron with increase of lithiation. A molecular orbital based energy band diagram is presented which suggests that lithiated vanadium oxides, Li_xV₂O₅, become metallic for high values ofx.     

Phase Separation Derived Core/Shell Structured Cu11V6O26/V2O5 Microspheres: First Synthesis and Excellent Lithium‐Ion Anode Performance with Outstanding Capacity Self‐Restoration

Novel amorphous vanadium oxide coated copper vanadium oxide (Cu₁₁V₆O₂₆/V₂O₅) microspheres with 3D hierarchical architecture have been successfully prepared via a microwave‐assisted solution method and subsequent annealing induced phase separation process. Pure Cu₁₁V₆O₂₆ microspheres without V₂O₅ coating are also obtained by an H₂O₂ solution dissolving treatment. When evaluated as an anode material for lithium‐ion batteries (LIBs), the as‐synthesized hybrid exhibits large reversible capacity, excellent rate capability, and outstanding capacity self‐recovery. Under the condition of high current density of 1 A g^?1, the 3D hierarchical Cu₁₁V₆O₂₆/V₂O₅ hybrid maintains a reversible capacity of ≈1110 mA h g^?1. Combined electrochemical analysis and high‐resolution transmission electron microscopy observation during cycling reveals that the amorphous V₂O₅ coating plays an important role on enhancing the electrochemical performances and capacity self‐recovery, which provides an active amorphous protective layer and abundant grain interfaces for efficient inserting and extracting of Li‐ion. As a result, this new copper vanadium oxide hybrid is proposed as a promising anode material for LIBs.     

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.