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 [1] . Among one-dimensional structures, the synthesis of vanadium pentoxide (V₂O₅) and their derivative compounds has been researched intensively due to their redox activity and layered structures [2] - [11] . The layered crystal structure of V₂O₅ allows the insertion of small ions, for example Li⁺ and has potential applications in lithium batteries [12] - [14] , electric field-effect transistors [15] and chemical sensors or actuators [16] .

Vanadium forms a variety of binary compounds with the general formula, VO_2+x (−0.5 ≤ x ≤ 0.5), such as V₂O₃, VO₂, V₂O₅, V₃O₇, V₄O₉, V₆O₁₃ [17] . Among them vanadium pentoxide presents the polymorphs α-V₂O₅ (orthorhombic) [18] , β-V₂O₅ (monoclinic or tetragonal) and γ-V₂O₅ (orthorhombic) structures [19] [20] . The polymorph α-V₂O₅ is one of the most widely studied transition metal oxides in the V-O system, because it is the most stable phase at atmospheric pressure and room temperature. It has a band gap of ∼2.3 eV and shows a semiconductor-metal transition at about 250˚C. The α-V₂O₅ is lightly soluble in water (0.08 g/ml, 20˚C) but extremely soluble in acids. In particular it is physically and chemically stable in hot acid solutions. By increasing pressure and temperature, it is possible to improve the contact between the phases in the orthorhombic structure formed by VO₅ layers forming square pyramids that share edges and corners. In addition, the interaction between these layers is based on Van der Waals chemical bonds which favour growth of the planes along the [001] direction [21] - [23] .

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 [24] - [33] . In this work we synthesized by a solvothermal method at low temperature monocrystalline α-V₂O₅ nanowires with preferential [200] growth direction. In the solvothermal synthesis the oxidation states V⁵⁺, V⁴⁺ and V³⁺ were present. The presence of V⁴⁺ ions in the material, promotes the growth of V₁₀O₂₄∙12H₂O/V₃O₇∙H₂O nanobelts in metastable phase. However the metastable phases present in the V₁₀O₂₄∙12H₂O/V₃O₇∙H₂O template are stabilized after applying a thermal process, which encourages and promotes formation of α-V₂O₅ nanowires in an orthorhombic phase. With the aid of XRD, SEM and HRTEM we have studied the morphological changes and structural behavior during the formation of α-V₂O₅ nanowires. The solvothermal chemical route seems optimal, since it is suitable for the synthesis of materials that decompose at elevated temperatures, which are poorly soluble, reactive, or for species with multiple oxidation states, as in the case of α-V₂O₅ nanowires. A benefit of this method of synthesis is that one obtains very pure products at low temperature using only V₂O₅ as starting material and hydrogen peroxide (H₂O₂) as solvent-oxidant agent. Furthermore, these α-V₂O₅ nanowires can be doped with small ions, for example Li⁺ ions by electrochemical methods, and subsequently can be used as electrodes in rechargeable lithium-ion nanobatteries.

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 Figure 2, the SEM images correspond to each of the patterns of X-ray diffraction of the samples in different reaction times. As shown by SEM, the V₂O₅ reagent (Aldrich) is formed by agglomerates of morphologically heterogeneous grains with sizes 62.46 nm - 816.73 nm. In this case, the reflections in the XRD pattern (Figure 2(a)), are indexed as V₂O₅ in orthorhombic phase (PDF 41-1426). The precipitates dried at room temperature for 30 h obtained after solvothermal synthesis at 180˚C for 48 h, XRD patterns show that the metastable film green-blue (V³⁺/V⁴⁺) is a mixture of vanadium oxides in different phases (Figure 2(b)).

The XRD patterns refined by Rietveld (Figure 3), determine that the metastable film is constituted by 66% of V₁₀O₂₄∙12H₂O in monoclinic phase (Figure 3(a)) with lattice parameters a = 11.70 Å, b = 3.63 Å and c = 29.06 Å (PDF 25-1006). In addition this analysis shows that the other 34% present in the metastable film, corresponds to the V₃O₇∙H₂O oxide in orthorhombic phase (Figure 3(b)) with lattice parameters a = 9.34 Å, b = 17.00 Å and c = 3.62 Å (PDF 28-1433). SEM images shown that the bi-com- pound template of V₁₀O₂₄∙12H₂O/V₃O₇∙H₂O is conformed by overlapping nanobelts, growing both on the surface and in the inside of the film. These one-dimensional structures have an average width of 209.55 nm and hundreds of micrometers long.

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 (Figure 2(c)) shows reflections corresponding to a 92.8% V₂O₅ in orthorhombic phase (α-V₂O₅), with lattice parameters a = 11.51 Å, b = 3.56 Å and c = 3.22 Å, indexed in agreement to the PDF 41-1426. However, we see the reflections due to the remnant of the V₁₀O₂₄∙12H₂O in monoclinic phase (PDF 25-1006). The smaller interplanar spacing is caused by the removal of water from the interlamellar region of V₁₀O₂₄∙12H₂O/V₃O₇∙H₂O structure. SEM images show homogeneous α-V₂O₅ nanowires measuring in average 111.90 nm in cross section and hundreds of micrometers long. After increasing the thermal process to 12 h, the color in the metastable film change from green-blue to orange, indicating the progress of oxidation state V⁵⁺ in the material. The intensity of the diffractions peaks associated α-V₂O₅ in stable orthorhombic phase increased with 95.8% of (Figure 2(d)) and clearly decreased by 4.2% the reflections associated to V₁₀O₂₄∙12H₂O in metastable monoclinic phase. The crystal structures of the α-V₂O₅ nanowires are similar to that of the commercial V₂O₅ powders (Figure 2(a)).

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 [35] . The molecule is dissociating by the acid hydrolysis caused by the addition of H₂O₂. Therefore, the degree of acid hydrolysis is controlled by pH 0.5 in the precursor phase solution (Equation (2)).

The electron transfer increases the charge on the molecule and weakens the OH bonds [36] - [39] . Applying pressure and temperature, monoperoxo and dimer species are then progressively formed as peroxo groups are decomposed (Equation (3)).

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 (Figure 4), show the variety of one-dimensional structures forming the metastable phase V₁₀O₂₄∙12H₂O/V₃O₇∙H₂O template. These bi-com- pound nanobelts exhibit an average width of 125 - 250 nm and in some cases are core- shell types. On the other hand it is determined that the shell is not uniform in all cases; furthermore, it varies along the same nanobelt, which clearly indicates a non- uniform oxidation.

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 (Figure 5).

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 (Figure 6). These results are consistent with those established by the XRD patterns corresponding to the analysis in the metastable phase.

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 (Figure 7).

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 [200] growth direction. Into the solvothermal synthesis, the formation of α-V₂O₅ nanowires is promoted by a template conformed of V₁₀O₂₄∙12H₂O/V₃O₇∙H₂O

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 N^o 311298 and PAPIIT N^o 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