A stable one-dimensional system in an orthorhombic α-V 2O 5 nanowires monocrystalline structure was obtained by a solvothermal method from a polymorphic V 2O 5 structure. The starting material was firstly submitted to acid hydrolysis in H 2O 2 followed by a solvothermal treatment. The outcome of this procedure, a metastable phase of the one-dimensional system V 10O 24·12H 2O/V 3O 7·H 2O, was subsequently reoxidized by controlled heating in an open air system. The final product was an orange crystalline solid mainly formed by monocrystalline nanowires of α-V 2O 5 having lengths of tens of micrometers and widths of about 75 nm with a preferential [200] growth direction. It was found that the pH value of the initial solution plays an important role in the formation of the crystalline phase in the final products. Characterization was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). This study offers an alternate route for the synthesis of vanadium oxides and related compounds.
One-dimensional (1D) nanostructured materials, including nanotubes, nanowires, nanobelts, nanoribbons and nanorods, often exhibit specific physical and chemical properties due to their nanometer dimensions, which differ greatly from those of their bulk counterparts [
Vanadium forms a variety of binary compounds with the general formula, VO2+x (−0.5 ≤ x ≤ 0.5), such as V2O3, VO2, V2O5, V3O7, V4O9, V6O13 [
Nowadays, several research groups have reported the partial or total use of the solvothermal method of synthesis because it is an easy route to the formation of vanadium oxides with different morphologies [
The synthesis of α-V2O5 nanowires was performed using a solvothermal method similar to the one reported by Guicun Li et al. [
X-ray diffraction analysis (XRD) was performed using a D8-Advanced Bruker
diffractometer with CuKα monochromatic radiation (λ = 1.5405 Å, 2θ) using a Bragg- Brentano geometry. The samples for this analysis were subjected to slight mechanical milling. The XRD was refined using the Rietveld method and the FULLPROF program. The size of the structure was determined by the Debye Scherrer method. The morph- ological characterization of the products was studied by Scanning Electron Microscopy (SEM) with a microscope JEOL model JSM 7600F equipped with tungsten filament, operating at 20 kV and a pressure of 20 Pa and using the backscattered electron signal. For SEM analysis, the samples were directly placed on a specimen carrier and without adding any conductive layer. The High-Resolution Transmission Electron Microscopy (HRTEM) was done with a TITAN 80 - 300 microscope operating in the 80 - 300 kV range and a Tecnai G2-F30 operating at 300 kV. For the HRTEM observations, mechanically milled samples were deposited on the surface of a copper grid, previously coated with carbon and fomvar films.
XRD patterns and SEM images obtained from the samples show the structural and morphological changes during the synthesis of the α-V2O5 nanowires. In the
The XRD patterns refined by Rietveld (
The color of the metastable V10O24∙12H2O/V3O7∙H2O film (monoclinic and orthorhombic, respectively) changes from green-blue to yellow-pale after a thermal process at 80˚C at atmospheric pressure for 4 h, which causes the oxidation of the products to V5+ and the stabilization partial of the phases. The XRD pattern (
A probable process for the observed evolution of morphologies and crystal structures can be understood as follows: the process begins when vanadium salts are dissolved partially in distilled water; the metal cations (V5+) are solvated by molecules of water and the proposed reaction is (Equation (1)):
For transition metal cations, charge transfer occurs from the σ orbitals of the water molecule to the empty metal d orbitals, this causes an increase in the acidity of the water [
The electron transfer increases the charge on the molecule and weakens the OH bonds [
The system redox efficiently promotes a molecular rearrangement due to the condensation of vanadic acid via a homogeneous nucleation into the reduction of V5+ ions to V3+ ions, resulting in a metastable crystalline structure V10O24∙12H2O (hydrated bariandite) in monoclinic phase with one-dimensional growth (Equation (4)).
The partial stability of these products in metastable phase is achieved naturally when oxidized by exposure to air in the process of washing and drying (partially oxidation of V3+ to V4+ ions), obtaining as final products, a mixture of vanadium oxide in different phase but with an one-directional growth (Equation (5)).
With the thermal process of 4 and 12 h the phases in the metastable products V10O24∙12H2O/V3O7∙H2O are stabilized removing the un-coordinated water molecules with metal centers V-O, giving rise to structures of V2O5 nanowires in stable orthorhombic phase (Equation (6)).
The HRTEM micrographs (
It is determined that the shell is constituted by the V3O7∙H2O oxide in orthorhombic phase with families of planes {200}, {200}, {101}, {230}, {120} and interplanar distances of 0.850 nm, 0.467 nm, 0.339 nm, 0.360 nm, 0.630 nm respectively (
In the same way is established that the core corresponds to V10O24∙12H2O in monoclinic phase which exhibits the families of planes {004}, {317} and {313} with d-spacing of 0.708 nm, 0.236 nm and 0.245 nm respectively (
With the thermal processes of 4 and 12 h the metastable phases in the one-dimen- sional V10O24∙12H2O/V3O7∙H2O template are stabilized. The crystal structure corresponds to an orthorhombic phase, obtaining α-V2O5 nanowires monocrystalline that are morphologically stable at atmospheric pressure and room temperature. As result the α-V2O5 nanowires are monocrystalline, they have families of planes {200}, {310}, {110}, with d-spacing of 0.576 nm, 0.340 nm and 0.261 nm, respectively (
We have obtained monocrystalline nanowires of α-V2O5 in orthorhombic phase by a low temperature using a solvothermal synthesis, inducing a redox process controlled in the V2O5 reagent grade and morphologically heterogeneous. These one-dimensional α-V2O5 structures have lengths of tens of micrometers and widths of about 75 nm, with a preferential [
nanobelts in metastable phase. In addition, formation of V10O24∙12H2O/V3O7∙H2O nanobelts in metastable phase depends strongly on the reaction conditions, like pH of the phase precursor solution, temperature into the acid digestion vessel, along with the time of the solvotermal reaction. It is determined that the bi-compound system in metastable phase may present core-shell type structures with average widths 209.55 nm and hundreds of micrometers long. Furthermore, the analysis presented identified the V10O24∙12H2O in monoclinic phase as the core, and the orthorhombic phase V3O7∙H2O as the shell. A thermal process stabilized metastable phases in the products in periods of 4 and 12 h. The influence of the time in the thermal process has a direct impact on the morphologies of the monocrystalline α-V2O5 nanowires, which have wide applications in lithium-ion batteries.
We acknowledge the financial support CONACYT No 311298 and PAPIIT No IN113411. The authors are also thankful to Laboratory of Microscopy facilities of the Mexican Petroleum Institute (IMP) and Crystal Structures Refinement Laboratory (LAREC) facilities of the Institute of Physics, UNAM and particularly to M. C. Manuel Aguilar for the XRD measurements.
Tafoya Ronquillo, M.L., Santiago Jacinto, P., Ovalle, P., Rendón Vázquez, L., Chavira Martínez, E., Marinero, E. and Garibay, V. (2016) Synthesis and Structural Characterization of Monocrystalline α-V2O5 Nanowires. Materials Sciences and Applications, 7, 484-495. http://dx.doi.org/10.4236/msa.2016.79042