Synthesis of vanadium dioxide thin films from vanadium alkoxides (original) (raw)
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Thin Solid Films, 2013
A simple and cost effective sol-gel process for producing vanadium dioxide thin films was developed via thermolysis of V 2 O 5 ·nH 2 O (n≈ 2) V V precursors prepared by dissolving vanadium powder or V 2 O 5 powder in 30% hydrogen peroxide solutions. After spin-coating on fused silica substrates and annealing at 750°C in vacuum, without any intermediate gas reducing step, the major phase VO 2 (M, monoclinic phase) was found in both of the films based on V-H 2 O 2 and V 2 O 5 -H 2 O 2 precursor, exhibiting large transmittance changes (>40%) in the IR region (>2000 nm) and small hysteresis loop width (b 5°C) which were comparable to reported epitaxial VO 2 films. The two films have similar metal-to-insulator transition temperature τ C =62.5°C, lower than the classical value of 68°C for VO 2 thin films. In addition, the method enables simple doping, as found for 0.56 at.% W-doped VO 2 films. This intrinsically simple solution process followed by one-step annealing makes it potentially useful in smart window applications.
Low-Temperature Synthesis of Vanadium Dioxide Thin Films by Sol-Gel Dip Coating Method
Journal of Nanotechnology
The vanadium dioxide (VO2) thin films were synthesized by sol-gel dipping on a glass slide substrate at low temperature of 500°C in a vacuum tube furnace at a pressure of 2 × 10−3 mbar by 2-step calcination without an intermediate gas purging. Synthesis conditions, including temperature, vacuum pressure, and calcination steps in the vacuum tube furnace, were investigated to find the optimum condition that promoted the formation of VO2 phase. It was found that the 2nd calcination step was very important in realizing the monoclinic vanadium dioxide (VO2 (M)). The results of the valence electron analysis revealed the outstanding phase of VO2 and a small amount of V2O5 and V2O3 phases. The small crystallites of the VO2 were homogeneously distributed on the surface, and the grain was of an irregular shape of ∼220−380 nm in size. The film’s thickness was in a range of 69−74 nm. The film exhibited a metal-to-insulator transformation temperature of ∼68oC and good thermochromic property. Vis...
Optical properties of sol-gel derived vanadium oxide films
Journal of Sol-gel Science and Technology, 1997
Vanadium oxide gels can be made from vanadate aqueous solutions or from vanadium alkoxides. The condensation of vanadic acid gives long ribbon-like oxide particles which macroscopically orient in the same direction in aqueous sols when their concentration is larger than 0.12 mol·l−1. These anisotropic sols and gels should be considered as lyotropic nematic liquid crystals. Thick films in which ribbons align along the same direction can be deposited. These oriented coatings exhibit improved electrochemical properties as reversible cathodes for lithium batteries. Amorphous oxo-polymers are formed via the controlled hydrolysis of vanadium alkoxides. They allow the deposition of optically transparent thin films that exhibit interesting electrochromic properties and turn reversibly from yellow to green upon electrochemical reduction. Moreover these alkoxide derived films can be easily reduced into vanadium dioxide. These VO2 thin films exhibit thermochromic properties and could be used as optical switches in the infrared. The transition temperature of these VO2 films can be modified by doping the vanadium oxide with other cations.
Optical and structural studies of vanadium pentoxide thin films
Nanosystems: Physics, Chemistry, Mathematics, 2016
Recently, transition metal oxides like Vanadium pentoxide have become a subject of intensive studies. The particular physical and chemical properties of these materials allow a wide range of practical applications such as electrochromic devices, cathode electrodes for lithium batteries, humidity sensors. The V 2 O 5 film was prepared by an electrodeposition technique. The structural and optical properties were studied by X-Ray Diffraction (XRD), scanning electron microscopy (SEM), UV-Visible and Fourier Transform Infrared Spectroscopy (FT-IR). XRD spectra recorded has been observed and compared with the JCPDS values. SEM images showed very smooth surface morphology and the elemental compositions of the film were confirmed by EDAX. The transmittance of the V 2 O 5 films showed 75 % at 425 nm for the as-deposited substrate. The energy band gap of the films was found to be 2.45 eV and the band assignments of the V 2 O 5 film are comparable with the reported values.
Vanadium oxide thin films and fibers obtained by acetylacetonate sol–gel method
Thin Solid Films, 2015
Vanadium oxide films and fibers have been fabricated by the acetylacetonate sol-gel method followed by annealing in wet nitrogen. The samples are characterized by X-ray diffraction and electrical conductivity measurements. The effects of a sol aging, the precursor decomposition and the gas atmosphere composition on the annealing process, structure and properties of the films are discussed. The two-stage temperature regime of annealing of amorphous films in wet nitrogen for formation of the well crystallized VO2 phase is chosen: 1) 25-550°C and 2) 550-600°C. The obtained films demonstrate the metal-insulator transition and electrical switching. Also, the effect of the polyvinylpyrrolidone additive concentration and electrospinning parameters on qualitative (absence of defects and gel drops) and quantitative (length and diameter) characteristics of vanadium oxide fibers is studied.
Journal of The Electrochemical Society, 1999
Vanadium oxides have been extensively used in a large number of scientific and technological applications. 1 Among the many compounds present in the V-O system, 2 VO 2 and V 2 O 5 are undoubtedly the most studied because of their electrical, optical, and catalytic properties. Stoichiometric vanadium (V) oxide has the d band unoccupied; however, the oxygen vacancies usually present in the lattice 3 make this oxide an n-type semiconductor with an optical bandgap of about 2.6 eV. The main technical uses of V 2 O 5 , when deposited as thin film, can be found in electrochromic devices, oxidation catalysts, and optical switches. 5 Due to the semiconductor-metal phase transition, corresponding to a distortion in the crystal structure, that results in an increase in the electrical conductivity, 6 VO x thin films can find important applications, such as thermochromic devices and variable-reflectance mirrors. Actually VO 2 can be considered as a prototype of transition metal oxides showing the first-order metalinsulator transition (MIT). Abrupt changes in electrical, optical and magnetic properties are detected across the transition temperature, which is around 68ЊC for bulk VO 2 . 7 These dramatic changes in the physical properties are reversible, but affected by hysteresis cycles depending on the microstructure and stoichiometry of the films used. 8 In order to optimize functional properties of this material it is very important to control the crystal structure and stoichiometry during thin film, growth. To this purpose it is important to remember that three different crystal phase of VO 2 are currently known: the most stable one is the rutile structure, while there are two metastable phases, A and B, characterized by a layered structure and a higher oxygen content. 9 V 2 O 5 thin films have been prepared by vacuum evaporation, by the sol-gel method, and by spin-coating from organic vanadium solutions, 10 while VO 2 films have been deposited by reactive ion beam sputtering, magnetron sputtering, chemical vapor deposition (CVD), 11 and, recently, by the inorganic sol-gel technique. 12 Oxides in the range VO 2 -V 2 O 5 can also be grown by thermal oxidation of metallic vanadium substrates. CVD of vanadium oxides thin films has been performed starting from vanadyl trichloride, 14a vanadium(III) 14d and vanadium(IV) acetylacetonate, 14f vanadyl triisopropoxide, 14b and triethoxide. 14c,e In this study we report results about the synthesis and characterization of vanadium oxide thin films, obtained by the CVD method using a series of vanadyl precursors of formula VO(L) 2 (X) where L is hfa, acac, dpm, and fod (Hhfa ϭ 1,1,1-5,5,5-hexafluoro-2,4-pentanedione; Hacac ϭ 2,4-pentanedione; Hdpm ϭ 2,2-6,6-tetramethyl-3,5-heptanedione; Hfod ϭ 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione; X ϭ H 2 O when L ϭ hfa). These complexes have the advantage, with respect to the other precursors previously used, to be easily prepared and purified (all manipulations are carried out in air and in aqueous solvents). They are characterized by different vapor pressures, the VO(hfa) 2 (H 2 O) being the most volatile, 15 and by different stability and reactivity depending on the ligand nature. The aim of the present work is to examine how different ligands, in the framework of vanadyl(IV) complexes, can influence the composition, microstructure, and morphology of the grown films. For instance, it is well known that the replacement of methyl groups by trifluoromethyls strengthens the C=O and C=C bonds, while the M-O bonds weaken. As reported in previous works, the thermal decomposition of fluorinated -diketonato VO(L) 2 complexes is different from that of nonfluorinated ones. 15 It is also expected that the reactivity toward H 2 O of the M-O bonds depends on the values of K b s of the ligands used.
Optical Properties of Vanadium Pentoxide Thin Films Prepared by Thermal Evaporation Method
Jordan Journal of Pharmaceutical Sciences
The Vanadium Pentoxide (V 2 O 5) thin films were obtained using thermal evaporation technique on a glass substrate. Many films of different thickness were prepared. Deposition rate was controlled by using quartz monitor connected with the system and the rate was about 5 nm per second for all films. The transmittance and reflectance of each film, in the spectral range 300 to 900 nm, were measured from which the optical constants (Refractive index, Absorption coefficient, Extinction coefficient, and Energy gap) were determined. The energy gap was calculated for two films (d1=320 nm and d2=700 nm) and found to be 1.87 and 2.1 eV for the direct allowed transitions respectively. All measured values were in consistent with other previous studies. All our films found to have an amorphous structure as was shown by the XRD patterns of the films.
Effect of solution molarity on the characteristics of vanadium pentoxide thin film
Applied Surface Science, 2006
Vanadium pentoxide (V 2 O 5) thin films have been prepared by spray pyrolysis technique. The influence of solution molarity on the characteristics of the V 2 O 5 has been investigated. X-ray diffraction analysis (XRD) showed that, the films deposited at !0.1 M were orthorhombic structure with a preferential orientation along h0 0 1i direction. Moreover, the crystallinity was improved by increasing solution molarity. The microstructure parameters have been evaluated by using a single order Voigt profile method. The optical band gaps, determined by using Tauc plot, have been found to be 2.50 AE 0.02 and 2.33 AE 0.02 eV for the direct and indirect allowed transition, respectively. Also the complex optical constants for the wavelength range 300-2500 nm are reported. At room temperature, the dark conductivity as a function of solution molarity showed the range of 5.74 Â 10 À2 AE 0.03 to 3.36 Â 10 À1 AE 0.02 V À1 cm À1. While at high temperature, the behaviour of electrical conductivity dominated by grain boundaries. The values of activation energy and potential barrier height were 0.156 AE 0.011 and 0.263 AE 0.012 eV, respectively.
Thermochromic vanadium oxide thin films: Electronic and optical properties
Journal of Physics: Conference Series
Vanadium dioxide, VO 2 , is a widely studied thermochromic material with potential applications in energy efficient window technology. It undergoes a first-order metal-toinsulator transition, accompanied by a crystal structure transformation from monoclinic to tetragonal rutile, at a critical temperature of 68 o C. Below this temperature, VO 2 is semiconducting and infrared transmitting whereas it is metallic and infrared reflecting above the transition temperature. However, in order to achieve significant thermochromic switching, the luminous transmittance of thin films will typically be less than 50%. Here we report on recent research to improve the luminous transmittance as well as the transmittance change at the transition temperature. We systematically evaluate the effect of antireflection coatings, doping with Mg and the performance of coatings comprising thermochromic nanoparticles in a transparent matrix. The last option is shown to give the best performance and holds great promise for practical applications.
Nanocrystalline vanadium dioxide: synthesis and mid-infrared properties
2020
This work describes the ®rst successful synthesis of nanocrystalline thermochromic VO 2 powder using the low temperature irreversible structural transformation of the metastable VO 2 (B). At this step, the transformation is associated with a total rearrangement of VO 6 octahedra, and a strong increasing of density. The reversible metal±insulator phase transition (MIPT) of vanadium dioxide (T t 68°C) is associated with strong changes in electrical, magnetic and optical properties. The contrast of the optical transition in mid-infrared (MIR) region and the optical transparency are remarkably increased for these nanosized particles. Modi®cations in coloration are also observed. Ó