Examples of phases and structures of V-O phase diagram.
The first synthesis of pentoxide vanadium (V2O5) as gel completed 135 years in 2020. Since its first synthesis, the V2O5 has attracted attention over the years in different areas in science and technology. There are several possibilities to obtain V2O5 resulting in different structures. Among these methods, it is possible to mention the sol–gel, hydrothermal/solvothermal synthesis, electrospinning, chemical vapor deposition (CVD), physical vapor deposition (PVD), template-based methods, reverse micelle techniques, Pechini method and electrochemical deposition that can be considered as the great asset for its varied structures and properties. Progress towards obtaining of different structures of V2O5, and phases have been resulted in lamellar structure with wide interlayer spacing, good chemical and thermal stability and thermoelectric and electrochromic properties. Throughout this advancement, its performance for industrial applications have made a strong candidate in electrochromic devices, photovoltaic cell, reversible cathode materials for Li batteries, supercapacitor, among others. This chapter will be to assist an updated review since the first synthesis up to current development.
- obtaining methods
1. Historical and sources
The discovery of vanadium was marked by uncertainty and confusion due to its chemical similarity with some elements. In 1801, the Spanish mineralogist, Andrés Manuel Del Rio, discovered an element with the atomic number 23, in Mexico, in a lead mineral. Due to the similarity of its colors to those of chrome, Del Rio called this element as panchrome. Later, after noting that the color of these salts turned red when heated, he renamed it as erythron. However, Del Rio withdrew his claim when, four years later, it was suggested by the French chemist, Hippolyte Victor Collett-Desotils, that the mineral was really an impure chromium, provoking the retraction of Andrés Manuel Del Rio . In 1830, Swedish chemist Nils Gabriel Sefström rediscovered the element in an oxide that it was found while working at an iron mine and gave it the name by which it is known today. A year later, in 1831, Friedrich Woehler confirmed that this element was the same already discovered by Del Rio in 1801. In 1867, Henry Enfield Roscoe, an English chemist, isolated it almost purely by reducing the chloride with hydrogen . The name vanadium refers to the goddess of beauty in Scandinavian mythology Vanadis, also known as Freya, due to the beautiful variation in the color of its compounds. Vanadium is the nineteenth most abundant element in the earth’s crust (136 ppm), and the fifth among transition metals. Despite being a metal considered abundant, it is not found in its elemental form, but it is present in approximately 65 different minerals, among which stand out vanadinite, PbCl2.3Pb3 (VO4)2, carnotite, K2(UO2)2(VO4)2.3H2O, roscoelite K(V3AlMg)2(SiAl)4O10(OH)2 and patronite, V2S3 . Of the world’s vanadium resources, most are present in magma, located in the Bushveld volcanic complex in South Africa, which has the world’s largest reserves of iron/vanadium, followed by Russia, the United States and China. In 2019, about 90% of vanadium was obtained from magnetite and titanomagnetite ores. Regarding vanadium production, China led the largest global production of 2019 through slag. Asia China is the world’s largest producer of vanadium, with 59%, followed by Russia accounting for 17% and South Africa with 7% of the global supply of vanadium. Most of its vanadium was derived from the primary production of Bushveld Minerals and Glencore. The most commercially available vanadium products are vanadium pentoxide and iron-vanadium. Vanadium pentoxide is obtained by treating magnetite iron ores and slag.
2. Structures of vanadium oxides
The system of V-O has different oxidation states with V2O5 being the most stable. This system occurs from V2+ to V5+ such as vanadium monoxide (VO), vanadium sesquioxide (V2O3), vanadium dioxide (VO2) and vanadium pentoxide (V2O5). Besides, it is possible to obtain mixed valence oxides that present several of oxides containing V5+/V4+ mixture (in V3O7, V4O9, and V6O13) and V4+/V3+ mixture (in V6O11, V7O13, and V8O15 . From these mixing phases is possible to form two phases called as Magnéli phase (VnO2n-1) and Wadsley phase (VnO2n + 1). A schematic V-O phase diagram calculated by Kang  presented the Magnéli phase as being V6O13, V3O7, V2O5 as well as Wadsley phase V3O5, V4O7, V5O9, V6O11, V7O13 and V8O15. The phases and structures in the V-O phase diagram is depicted in Table 1.
From the Table 1, it is possible to observe that the V-O system can exhibits multiples crystalline structures. These crystalline structures can be modified considering the oxygen fractions in the range 0.5–0.75 and decrease of formation energy (eV.atom−1). It is worth mentioning that formation energy between the stable and metastable phases ranges, for example, between 4 meV in V2O5 and 35 meV in V2O3, making possible a reversible structural transition . The Magnéli phase (VnO2n-1, with
On the other hand, Wadsley phase, (VnO2n + 1, with
The most famous and stable of the layered VnO2n + 1 is V2O5. Along the xyz axis (3D) V2O5 presents a V chains forming a network with oxygen which results as VO5 pyramids . X-ray diffraction (XRD) pattern of orthorhombic V2O5 and a layered crystalline structure has a standard pattern number JCPDS No. 41–1426 . That way, its structure is orthorhombic with parameters a = 1.151 nm, b = 0.356 nm and c = 0.437 nm. From xy axis, V-O layer-like structure with two oxygen in z axis forming a distorted trigonal bipyramidal coordination polyhedral. Each combination of VO5 pyramids has planes (00 l) and V is linked with five oxygen atoms by single bonds being four oxygen in xy axis and one oxygen in z axis. Then, in series of planes of VO5 are connected with alternating oxygen position in z axis (perpendicular) according to the sequence two up and two down. Therefore, the V-O single bond in perpendicular position presents a weak interaction compared to oxygen located in adjacent layer [7, 11]. This layered characteristic makes enable an introduction of several ions into the lamellar spacing which bringing change of the crystalline structure resulting in different properties (Figure 1).
V-O bonds from V2O5 have different distance caused by spontaneous deformation of the geometry to reduce the energy of the system. Then, vanadyl bond with four oxygen from the plane present a value of 0.178 nm. The bond of the extension along the z axis has 0.279 nm and the vertical axis opposite to the V–O bond has 0.158 nm.
Depending on the conditions, vanadium oxidation states might range of V2+ to V5+ as well as changes in coordination geometrics. Structural evolution in function of pH and concentration of V2O5 precursors are responsible by different oxidation states. Whenever a decrease of pH (13 to 1) and increase of H+/V (1 to 3 and concentration range from 10−3 to 10−1 mol.L−1) there are predominant species such as pyrovanadate, metavanadates, decavanadates, respectively. The main reaction that occurs during the formation of V2O5 are called olation and oxolation which will be detailed in Section 3.
It is possible to find V2O5 as α, β, and γ polymorphs (Figure 2) and among them have four orthorhombic, two monoclinic and one triclinic phase. α-V2O5 is a stable phase with an interlayer spacing of 0.452 nm. After a distortion forming by VO6 in the V2O5 structure, V atoms are dislocated to the corner from the middle forming β-V2O5. γ-V2O5 presents oxygen alternated up and down pyramids connected with vanadium in the center and each VO5 square pyramids forming zig-zag structure. There is a V=O bond along z axis that presents a weak bond and a covalent bond along x and y axis, creating double layers of O-V-O which is considered as short and strong. These torsion on structure makes γ-V2O5 more flexible and results in a structure metastable [12, 13].
The double layers of O-V-O are separated by van der Waals bond being a weak one. The different bonds along the coordinates of V2O5 creating a strong anisotropic in the V2O5 as 2-D layered material. Then, the basal plane (010) presents lower surface energy compared to (100) and (001) planes. As V2O5 presents plane as stacking of playing cards with weak interlayer force, it is possible to provides an intercalation reaction with several substances .
3. Structure of V2O5 based on its obtaining methods
In this section various conditions of synthesis and preparation methodology will be approached as well as their structural influences on V2O5. The interesting of V2O5 is focused on its versatility in several applications based on obtaining methods. The most used preparation technique is sol–gel but other methods such as hydrothermal/solvothermal synthesis, electrospinning, chemical vapor deposition (CVD), physical vapor deposition (PVD), template-based methods, reverse micelle techniques, Pechini method and electrochemical deposition can be used as well. Often, combinations of these methods can be found to obtain different structures of V2O5. The control of reaction conditions allows the formations of V2O5 as powder, nanomaterial, thin films, porous materials, among others.
4. Properties and applications
V2O5 is a versatile material in terms of properties and importance in technological applications. This versatility is designed by rich morphologies and structures, various synthetic methods giving excellent properties. It is good to highlight that commercial V2O5 does not present electrochromic properties, for example, because of its low electrical conductivity, poor coloration efficiency and narrow color variation. Then, since the first synthesis of V2O5 several many studies have been proposed with the objective of obtaining this same oxide, however, with different syntheses. Thus, it was found that the change in preparation to obtain V2O5 had several important characteristics that result in structural change and, consequently, in the final properties and applications. Among them, it is observed that each type of synthesis causes variation in the band gap resulting in the variation of semiconductivity of V2O5. Generally, from the Tauc plot is possible to obtain the band gap value of V2O5. The value of the V2O5 band gap may shift, but always within the range of semiconductive materials. This result directly impacts in many final properties of this material. Then, with the innumerous possibilities of synthesis for obtaining V2O5 it became possible to make a structural and morphological manipulation resulting in a wide and varied potential in applications. Studies using V2O5 are mainly focused on characteristics and properties such as, electronic, magnetic, conductive, electrochemical, optical, mechanical, catalytic chemisoption among others.
More specifically, V2O5 can exhibits wide interesting and useful properties including metal–insulator transitions and electron. As addressed above, a layered structure like orthorhombic α-V2O5 can be a host for cation intercalation and potential application in Li-ion batteries. However, other metastable polymorph ζ-V2O5 is found to opens up the possibility of controlling the charge ordering of the network, and makes a prime candidate for applications in the next generation of Li- and multivalent-ion cathode materials . The insertion/deinsertion of Li+ occurs according to the reaction (1):
There are other phases that occurs as lithiated vanadium oxide LixV2O5 during the redox reaction. The amount of lithium, x, presents a variation according to the structure: α-V2O5 (x < 0.01), ε-LixV2O5 (0.35 < x < 0.7), δ-LixV2O5 (x = 1), γ-LixV2O5 (x < 2) and υ-LixV2O5 (x > 2) . Structure manipulation to remodel V2O5 as nanosheet-assembled hollow microflowers by solvothermal method exhibits high specific capacity and remarkable cycling stability due to hierarchical structure with nanosheet subunits and hollow interior . Regarding to electrodeposition of V2O5, this nanostructured material obtained can presents high energy density, power density, good cyclic stability over 200 cycles as high power and energy densities for thin film Li-ion batteries  and it has 5 times higher current density than sol–gel-derived film and can intercalate up to 5 times higher concentration of Li+ . Nanostructures can promote a rapid and facilitate the electron transfer ensuring satisfactory capacity retention even at high current densities, reduced ion diffusion distance and improved surface area. In theoretical terms, the specific capacity is much greater than those commercialized . In general, using electrochemical studies to application as supercapacitor, V2O5 presents high capacitance and great energy density. A supercapacitor is an energy storage component, and comparing with battery, a supercapacitor presents more long lifetime, high power density, eco-friendly material and high efficiency . It was found that V2O5 presents a range of 140 to 704.17 F.g−1  using different synthesis and, consequently, different structures. It was found that the stability was 94.3% after 10,000 cycles indicating the potential of the V2O5 electrode for supercapacitor . In general, V2O5 compound presents advantages in energy storage devices as ease to prepare even different method of synthesis, wide lithiation/delithiation potential and abundant storage.
V2O5 can be used as counter electrode (CE) in dye-sensitized solar cells (DSSC). Different methods to prepare V2O5 was demonstrated in literature [36, 37, 38]. The presence of V2O5 as component in DSSC contributed with power conversion efficiency in range of 2.04 to 3.80% using different method of obtaining as well as exhibited longer lifetime in an ambient environment, ease of film preparation at room temperature, low production cost and high optical transmittance over a wide range of solar spectrum. [36, 37].
A gas sensor has a possibility to detect several gases in different atmospheres transforming the chemical reactions to analytically useful detectable signals. Then, the efficiency of a gas sensor depends on the materials present in sensor and the interaction of the gas with the material. The temperature can be an important factor between interaction of gas with materials on the surface of the sensor. Therefore, the choose of the material is particularly important. Consequently, V2O5 is found in several studies being a sensing material for several gas, such as, ammonia, ethanol, pH-sensor, EGFET pH-sensor, phenylhydrazine, NO2, H2O2 and among others. In all cases, V2O5 demonstrated to be highly sensitivity in several temperature and under both dark and illumination conditions [18, 39, 40, 41, 42, 43, 44, 45, 46, 47]. Similar to the sensor, a biossensor turn an analytical response into a measurable chemical signal and detects only a certain biological product as a target analyte. The use of different structures of V2O5 as sensitive component in biosensor device has been explored in several target analyte such as urea, glucose, gene sequence, EGFET-biosensor, methylglyoxal in rice and among others [48, 49, 50, 51, 52, 53].
V2O5 presents optical property upon charging/discharging resulting in the color change. This ability to change its color by redox reaction is denominated electrochromic effect . During the charge insertion into the vanadium is observed that the transmittance increases in the ultraviolet in a short wavelength of the spectrum, while the transmittance drops in the long-wavelength part of this spectrum near in infrared region. The optical and multicolor characteristics in V2O5 generate several applications such as in transmittance smart windows in energy efficient buildings, displays, in reflectance mirrors and emittance surfaces for temperature control of space vehicles. During the Li+ extraction from V2O5 in range potential from −0.6 to +0.6 V the color of the film changes the deep blue to green and, finally, to yellow due to the oxidation of V4+ to V5+. Initially, only partial V4+ ions change to V5+ at −0.3 V. In region more positive of potential, the remaining V4+ ions turn to V5+. The reduction reaction occurs when the shift of potential to region more negative is applied makes the V2O5 a reversible capacity after several cycling [11, 55].
V2O5 has traditionally been used in various applications based on its obtained methods and properties of final structure. This chapter has summarized these obtaining methods using vanadium pentoxide with emphasis in different structure for wide applications. Since the first publication of V2O5 using sol–gel method, many reports have been found. The versatility and stability of V2O5 generated studies about structural changes according to the property of interest. Then, based on different obtaining techniques, it was possible to find structures of V2O5 such as nanostructures, lamellar, among others. Therefore, growth techniques have contributed to the extensive range of V2O5 applications. Additionally, depending on the conditions and methods, V2O5 films can have considerably different structural, optical, conductivity and electrical properties. The characteristic of V2O5 in offers a wide possibility of synthesis, low cost, easy to be obtained, revels that the material has high potential in several application areas being technological or innovation. Besides, in all application, the use of V2O5 has been demonstrated a promisor response and in a near future, the technology of designing new devices will have, as one of the components, the V2O5 presents on scale range.
The authors thank to INEO, FAPEMIG, RQ-MG/FAPEMIG, CNPq, UFSJ and CAPES.
Conflict of interest
The authors declare no conflict of interest.