Vanadium Oxide Nanotubes Research Articles

Surveying and Comparing Thermal Conductivity and Physical Properties of Oil Base NanoFluids Containing Carbon and Metal Oxide Nanotubes

In this research, nano materials with tubular structures are added to SAE 20W50 engine oil to study the rate of their effects on the properties of engine oil. Multi-walled carbon nanotubes (MWCNTs) and vanadium oxide nanotubes (VONTs) has been used as two different additive materials, one of them is carbonic and the other is metallic oxides and their effect on different parameters containing viscosity, thermal conductivity coefficient, flash point and pour point of engine oil as the quality properties of engine oil has been studied and compared. The samples of two concentrations 0.1 and 0.2 wt% with using planetary ball mill were made. The obtained results show that MWCNTs in all cases, which have been evaluated, had better functionality with respect to vanadium oxide nanotubes. In the 0.1 wt% concentration, flash point of MWCNTs/oil and VONTs/oil increased about 9.3% and 5.8% respectively. In addition, thermal conductivity of them increased 13.2% and 10.2% respectively.

Ex Situ X-ray Diffraction, X-ray Absorption Near Edge Structure, Electron Spin Resonance, and Transmission Electron Microscopy Study of the Hydrothermal Crystallization of Vanadium Oxide Nanotubes: An Insight into the Mechanism of Formation

The nucleation and growth of vanadium oxide nanotubes (VOx-NT) have been followed by a combination of numerous ex situ techniques. long the hydrothermal process. Intermediate solid phases extracted at different reaction times have been characterized by powder X-ray diffraction, scanning and transmission electron microscopy, electron spin resonance, and V-K edge :X-ray absorption near-edge structure spectroscopy. The supernatant vanadate solutions extracted during the hydrothermal treatment have been studied by liquid V-51 NMR and flame. spectroscopy. For short durations of the hydrothermal synthesis, the initial V2O5-surfactant intercalate. is progressively transformed into VOx-NT whose crystallization starts to be detected after a hydrothermal treatment of 24 h. Upon heating from 24 h to 7 days, VOx-NT are obtained in larger amount and with an improved crystallinity. The detection of soluble amines and cyclic metavanadate V4O12](4-) in the supernatant solution along the hydrothermal process suggests that VOx-NT result from a dissolution precipitation mechanism. Metavanadate species V4O12](4-) could behave as molecular precursors in the polymerization reactions leading to VOx-NT.

Synthesis of Mo, W, and Mo‐ and W‐Doped Multiwall VONTs via Sol‐Gel and Hydrothermal Methods

Mo, W, and Mo and W were doped into multiwall vanadium oxide nanotubes. The syntheses were performed using sol‐gel method followed by hydrothermal method. The synthesized samples were characterized by XRD, SEM, EDX, and TEM techniques. The XRD patterns of the synthesized samples indicated that Mo and W could be doped into VONTs totally up to 50%. The SEM and TEM images showed that the prepared samples have tubular and multiwall morphology and open ends.

Electrochemical Performance Improvement of Vanadium Oxide Nanotubes as Cathode Materials for Lithium Ion Batteries through Ferric Ion Exchange Technique

Mixed-valence vanadium oxide nanotubes (VOx-NTs) are successfully prepared with dodecylamine as the templates intercalated between VOx layers. A novel ferric ion exchange technique is carried out to remove the amine templates which have no contribution to electrochemical performance of VOx-NTs. TEM observation, XRD study, and FTIR analysis reveal that a large proportion of the amine templates within VOx-NTs can be exchanged by ferric ions in the reaction process, and the tubular morphology and multiwalled structure are not damaged by this exchange technique. Moreover, XPS results indicate that the ferric ion exchange process is also accompanied by the increase of vanadium valence, which is conducive to the improvement of electrochemical capacity of Fe-VOx-NTs. The final synthesized Fe-VOx-NTs exhibit superior specific capacity (311 mAh g–1 at 50 mA g–1 discharge current density in the potential range of 1.5–4 V versus Li/Li+) and cycling performance (178 mAh g–1 after 50 cycles) compared to congeneric mod...

The local environment of Co2+ ions intercalated in vanadium oxide/hexadecylamine nanotubes

Vanadium oxide nanotubes constitute promising materials for applications in nanoelectronics as cathode materials, in sensor technology and in catalysis. In this work we present a study on hybrid vanadium oxide/hexadecylamine multiwall nanotubes doped with Co ions using state of the art x-ray diffraction and absorption techniques, to address the issue of the dopant location within the nanotubes’ structure. The x-ray absorption near-edge structure analysis shows that the Co ions in the nanotubes are in the 2 + oxidation state, while extended x-ray absorption fine structure spectroscopy reveals the local environment of the Co2+ ions. Results indicate that Co atoms are exchanged at the interface between the vanadium oxide’s layers and the hexadecylamines, reducing the amount of amine chains and therefore the interlayer distance, but preserving the tubular shape. The findings in this work are important for describing Co2+ interaction with vanadium oxide nanotubes at the molecular level and will help to improve the understanding of their physicochemical behavior, which is desired in view of their promising applications.

Thermal stability and the magnetic properties of hybrid vanadium oxide-tetradecylamine nanotubes

Vanadium oxide nanotubes (NTs) were synthesized by the sol-gel method followed by a long-term hydrothermal treatment. The obtained nanotubes have a multiwall structure, and 70% of vanadium ions are in the V4+ state. This percentage was derived by evaluating three components of the magnetic susceptibility; namely, (i) the paramagnetic Curie-Weiss behavior, (ii) antiferromagnetic dimers, and (iii) magnetic trimers. The as-made NTs were annealed in situ in the cavity of the electron paramagnetic resonance (EPR) spectrometer. The line shape changes irreversibly at 390 K, and the EPR susceptibility presents an anomaly at 425 K. These changes are interpreted as a partial oxidation of the V4+ ions and consequently a decrease in the concentration of the magnetic species. The quantification of the V4+ ions of the annealed NTs reveals a diminution to 39% of V4+, a weakening of the Curie-Weiss and antiferromagnetic dimers contributions, and the suppression of magnetic trimers. Vibrational studies confirm the decrease of V4+ amount.

Research on the electrochemical performance of nanocomposites of vanadium oxide and carbon nanotubes as cathode materials

The vanadium oxide (VOx)/carbon nanotubes (CNTs) nanocomposites were synthesized by a hydrothermal method. The morphologies and structures of composites were characterized by XRD, SEM and TEM. The results indicated that the VOx/CNTs nanocomposites have a crossing network structure. The electrochemical cycling stability of the nanocomposites as cathodes could be improved with the increase of CNTs content. Specially, when the CNTs content was 20wt%, the initial discharge capacity of the nanocomposites was about 190mAh/g in 2.0–4.0V vs. Li+/Li, and the capacity could remain 93.1% after fiftieth cycle.

Deconvolution of the EPR spectra of vanadium oxide nanotubes

In this work we report results of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy of vanadium oxide nanotubes. The observed EPR spectra are composed of a weak well-resolved spectrum of isolated V4+ ions on top of an intense and broad structure-less line shape, attributed to spin–spin exchanged V4+ clusters. With the purpose to deconvolute the structured weak spectrum from the composed broad line, a new approach based on the Krylov basis diagonalization method (KBDM) is introduced. It is based on the discrimination between broad and sharp components with respect to a selectable threshold and can be executed with few adjustable parameters, without the need of a priori information on the shape and structure of the lines. This makes the method advantageous with respect to other procedures and suitable for fast and routine spectral analysis, which, in conjunction with simulation techniques based on the spin Hamiltonian parameters, can provide a full characterization of the EPR spectrum. Results demonstrate and characterize the coexistence of two V4+ species in the nanotubes and show good progress toward the goal of obtaining high fidelity deconvoluted spectra from complex signals with overlapping broader line shapes.

Gas sensing properties of p-type semiconducting vanadium oxide nanotubes

Vanadium oxide nanotubes (VONTs) were synthesized using the V2O5 powder as precursor with dodecylamine as structure-directing template via one-step hydrothermal way. The prepared VONTs sensors exhibit excellent sensing sensitivity and superior recovery property in detecting ethanol vapor. The response to the ethanol vapor and NO2 gases confirms that the samples are of p-type behavior at 80°C, which is associated with the presence of the protonated amine molecules in the VONTs. The sample changes to n-type behavior at 260°C arising from the decomposition of the protonated amine molecules. The p-type VONTs have potential for development of a novel gas sensor.

Structural and Electrochemical Characterization of Thermally Treated Vanadium Oxide Nanotubes for Li-Ion Batteries

not Available.

Time influence factor of vanadium oxide nanotube on Si substrate and initial gas sensing test

Vanadium oxide nanotubes are prepared on a Si substrate with hydrothermal method in this experiment with dodecylamine serving as template. X-ray diffraction, Raman spectroscopy and scanning electron microscope (SEM) are employed to characterize the structural and electronic properties of these nanotubes. The results show that the growth of VOX nanotubes and their gas sensing properties are affected by the hydrothermal reaction time rather than mixing time. With longer reaction time, the VOX nanotubes display better properties with smoother walls, stronger gas sensitivity and faster response time (15 s), than with shorter reaction time. SEM results reveal that VOX nanotubes have inner diameters between 25-35 nm and outer diameters between 65-100 nm. The sample inner diameters vary between 25 and 35 nm. The outer diameters are also quite similar in all tubes and lie between 65 and 100 nm.

V2O5/Polypyrrole Core–Shell Nanotubes for Gas Sensor

V2O5/polypyrrole core-shell nanotubes have been synthesized through an in situ polymerization of pyrrole monomers in the presence of prepared vanadium oxide nanotubes. Formation of polypyrrole in core-shell structure was endorsed by FTIR spectral analyses. Transmission electron microscopy images revealed that one dimensional morphology of vanadium oxide nanotubes remained undisturbed upon coating of polypyrrole, and polypyrrole was uniformly coated on the surface of vanadium oxide nanotubes, with a thickness of 30 nm. XRD suggested certain synergetic interaction existed in the core-shell structure, probably resulting in the enhanced thermal stability of polypyrrole coatings. The gas tests showed that the core-shell structure exhibited a high response, good stability and a short response/recovery time in the detection of different concentrations of ethanol at room temperature, implying its potential application for gas sensors. The enhanced gas sensing properties due to the synergic interaction between intimately contacted polypyrrole and vanadium oxide nanotubes lead to the formation of a hetero p-n junction with a depletion region, which is similar to a field effect transistor. The gas effects a change of the depletion region and thus modulates the conductivity of the junction.

Vanadium Oxide Nanotube Spherical Clusters Prepared on Carbon Fabrics for Energy Storage Applications

Vanadium oxide nanotubes have shown great promise as electrode materials for energy storage devices. In this study, we report the synthesis of V(2)O(5) nanotube (VNT) clusters, which form densely packed radial arrangements of VNTs on high-surface-area carbon fiber fabrics (CF). This was achieved by coating the CF with V(2)O(5) by pulsed laser deposition (PLD). Hydrothermal treatment of the PLD films in the presence of excess intercalated V(2)O(5) results in formation of well-adhered VNT clusters on the CF. The densely packed VNTs have inner and outer diameters and interlayer distances of ~24, ~70, and ~2.4 nm, respectively. Coin cell type supercapacitors (CR2032) were assembled using VNT-CF as the anode material and electrochemical properties were evaluated.

Synthesis and Characterization of Layered Vanadium Oxide Nanotubes for Rechargeable Lithium Batteries

A range of vanadium oxide nanotubes consisting of scrolled layers of primary amine functionalized xerogel have been produced by a hydrothermal treatment of a xerogel/amine mixture. The nanotubes consist of a hollow core flanked by parallel layers of scrolled vanadium oxide layers. The interlayer spacing is varied on the sub-nm scale by intercalated amines templates of various molecular lengths. High resolution transmission electron microscopy and X-ray diffraction were used to characterize the resulting nanotubes. When the molar ratio of xerogel:amine is 1:2, a maximum yield of nanotubes is produced and the interlayer spacing can be controlled. For a molar ratio of xerogel:amine of 2:1, the product contains a low yield of nanotubes and the corresponding interlayer spacing is independent of the length of the amine template. A comparison of measured interlayer distances with the actual physical length of the primary amine chains shows that interdigitation of the amines occurs.

Vanadium Oxide-Based Composite Catalysts for the Oxidation of Styrene: A Comparative Study

Vanadium oxide-based composite catalysts were prepared, and their activities in a styrene oxidation reaction were tested. These catalysts included (i) vanadium oxide (VxOy) supported on multi-walled carbon nanotubes (CNTs) (CNTs-VxOy), (ii) a carbon nanotube CNTs-VxOy template coated with γ-Al2O3 (CNTs-VxOy/Al2O3), (iii) γ-Al2O3/VxOy nanoparticles and (iv) vanadium oxide nanotubes (VxOy-NTs). The structure of these catalysts was determined by TEM, SEM, nitrogen adsorption (BET surface area), TGA and FTIR. The oxidation process was examined between styrene and anhydrous hydrogen peroxide which was catalyzed using the above VxOy-based composite catalyst. The vanadium particles embedded on the wall of CNTs have a satisfactory catalytic activity for oxidation. The effect of the operating parameters, including the reaction temperature, solvent type, reaction time, oxidant concentration and catalyst loading, on styrene conversion and benzaldehyde selectivity was studied. Keywords: Carbon nanotubes (CNTs), Composite materials, Nanostructure materials, Transition metal alloys and compounds, Sol-gel processes, Vanadium oxide, Styrene oxidation, Benzaldehyde, Anhydrous hydrogen peroxide, Nano catalyst

Magnetic properties of vanadium oxide nanotubes and nanolayers

We report results of magnetic measurements of VO${}_{x}$ nanolayers (stacks of four to eight distorted layers of vanadium oxide separated by an organic template) obtained at the early stages of a hydrothermal synthesis of VO${}_{x}$ nanotubes (VO${}_{x}$ NTs), and VO${}_{x}$ NTs themselves, which are the product of scrolling of VO${}_{x}$ nanolayers (VO${}_{x}$ NLs). Static and dynamic magnetic properties of VO${}_{x}$ NLs and VO${}_{x}$ NTs have been studied by means of a high-frequency (60 GHz) ESR and SQUID magnetometry in a temperature range 1.8--300 K. The temperature dependence of the magnetic susceptibility $\ensuremath{\chi}$ of the V${}^{4+}$ quasifree spins in VO${}_{x}$ NTs and VO${}_{x}$ NLs shows a transition from the Curie-Weiss laws with close paramagnetic temperatures ${\ensuremath{\Theta}}_{\mathrm{AFM}}=\ensuremath{-}25$ K (VO${}_{x}$ NTs) and ${\ensuremath{\Theta}}_{\mathrm{AFM}}=\ensuremath{-}19$ K (VO${}_{x}$ NLs) to the low-temperature power law $\ensuremath{\chi}\ensuremath{\propto}1/{T}^{\ensuremath{\xi}}$ with $\ensuremath{\xi}=0.75$ (VO${}_{x}$ NTs) and $\ensuremath{\xi}=0.85$ (VO${}_{x}$ NTs). The observed $\ensuremath{\chi}(T)$ dependence for quasifree spins was adequately described within an analytical model of an antiferromagnetic system with a disorder-driven quantum critical behavior. Comparison of static and dynamic susceptibilities reveals that the total magnetic susceptibility in VO${}_{x}$ NLs and VO${}_{x}$-NTs consists of several contributions. Apart from the oscillating susceptibility, there is a nonoscillating background, likely, consisting of the Van Vleck-type and Pauli-type terms. In the case of VO${}_{x}$ NTs, a correlated change of the Curie constant for the nondimerized V${}^{4+}$ quasifree spins and a variation of the background are observed: the decrease in temperature in the interval $70\phantom{\rule{4.pt}{0ex}}\text{K}<T<120$ K induces 1.8-fold growth of the Curie constant followed by a nonoscillating background decrease along with a change of its sign. The estimated concentration of magnetic sites indicates that a characteristic feature of VO${}_{x}$ NLs and VO${}_{x}$ NTs is a discrepancy between the maximal possible concentration of V${}^{4+}$ magnetic ions that is allowed by a chemical composition, and a concentration of the localized magnetic moments. The number of electrons in the V${}^{4+}$ state is always less than the total electron concentration, and the concentration of the ``excessive'' electrons may vary in the range 0.11--0.46 per vanadium site. At the same time, the concentration of V${}^{4+}$-V${}^{4+}$ antiferromagnetic dimers in VO${}_{x}$ NTs is rather high and reaches 49% of all vanadium sites. The effects of scrolling and Mott-Hubbard-type models of unusual magnetism in the VO${}_{x}$ nanomaterials are discussed.

Synthesis and Characterization of Re-, Mo-, and W-Doped Vanadium Oxide Nanotubes

In this research study for the first time, Re-, (Re + W)-, and (Re + Mo)-doped vanadium oxide nanotubes (VONTs) were synthesized. Doping of these elements into vanadium oxide nanotubes up to 10 at% was performed by using a sol–gel method. The structure and morphology of the nanotubes were investigated by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The obtained samples were found to have a tubular morphology, be nanosized, and be multiwalled with open ends. The outer diameter of the nanotubes varied from 50 to 100 nm and the inner diameter of them varied from 20 to 40 nm with a length of larger than 1 μm. The results showed that interlayer distances of the nanotubes for Re-doped VONTs were larger than those of (Re + W)-doped VONTs and (Re + Mo)-doped VONTs. In addition, interlayer distances of the (Re + W)-doped VONTs were larger than those of (Re + Mo)-doped VONTs.

Strong reduction of V4+ amount in vanadium oxide/hexadecylamine nanotubes by doping with Co2+ and Ni2+ ions: Electron paramagnetic resonance and magnetic studies

In this work we present a complete characterization and magnetic study of vanadium oxide/hexadecylamine nanotubes (VOx/Hexa NT’s) doped with Co2+ and Ni2+ ions. The morphology of the NT’s has been characterized by transmission electron microscopy, while the metallic elements have been quantified by the instrumental neutron activation analysis technique. The static and dynamic magnetic properties were studied by collecting data of magnetization as a function of magnetic field and temperature and by electron paramagnetic resonance. At difference of the majority reports in the literature, we do not observe magnetic dimers in vanadium oxide nanotubes. Also, we observed that the incorporation of metallic ions (Co2+, S = 3/2 and Ni2+, S = 1) decreases notably the amount of V4+ ions in the system, from 14−16% (nondoped case) to 2%−4%, with respect to the total vanadium atoms (fact corroborated by XPS experiments) anyway preserving the tubular nanostructure. The method to decrease the amount of V4+ in the nanotubes improves considerably their potential technological applications as Li-ion batteries cathodes.

The effect of surface morphology on the response of Fe 2O 3-loaded vanadium oxide nanotubes gas sensor

The effect of surface morphology on the response of an ethanol sensor based on vanadium nanotubes surface loaded with Fe 2O 3 nanoparticles (Fe 2O 3/VONTs) was investigated in this work. The particle size of Fe 2O 3 loaded on VONTs was varied by using novel citric acid-assisted hydrothermal method. In the synthesis progress, citric acid was used as a surfactant and chelate agent, which ensured the growth of a uniform Fe 2O 3 loading on the nanotubes surface. The ethanol sensing properties was then measured for these Fe 2O 3/VONTs at 230–300 °C. The results showed that the sensor response increased with the particles size and the loading amount of Fe 2O 3. It appears that the load of Fe 2O 3 on the VONTs surface increases the concentration of oxygen vacancies and decreases the concentration of free electrons. The effects of morphology on the sensor resistance were interpreted in terms of the Debye length and the difference in the number of active sites.

Electrochemical Behavior and Magnetic Properties of Vanadium Oxide Nanotubes

The electrochemical behavior and magnetic properties of electrochemically lithium-doped mixed valence vanadium oxide nanotubes (VOx-NTs) have been studied. The materials were synthesized hydrothermally. Primary amines (CH3−(CH2)11−NH2) were used as spacer molecules aiding to retain the nanotubular shape of the VOx layers. Cyclic voltammograms and potentiostatic measurements indicate that the electrochemical behavior of this material is fairly sensitive to the preparation conditions. Specific capacity values for this material are higher than similar template VOx-NTs. Lithium doping level of x = 0.1 in two different VOx-NTs gives rise to room-temperature ferromagnetism in LixVOx-NTs.