Introduction

V₂O₅ is a binary transitional metal oxide with the highest oxidation state (+V) among vanadium oxides. At room temperature, this oxide is an odorless solid crystalline powder whose color varies from pale yellow to dark orange depending on its state of division (Figure 1)^{[1, 2]}. V₂O₅ is hazardous for health and environment, but it has properties that are used in many industry applications, such as a cathode in lithium-ion batteries^{[2, 3]}, a component in special glasses^[4], and a catalyst in the production of various acids such as sulfuric^{[1, 5]}. And although this is only a part of all possible applications, vanadium pentoxide has found the greatest distribution (about 85% of all world vanadium) in the production of springs and cutting-tool steels, where it is mixed with Fe₂O₃ and Al to increase their toughening^[1].

Syntehesis

Vanadium pentoxide can be obtained in several ways, for example, by heating metallic vanadium in oxygen under pressure as shown in following equation:

4 V + 5 O₂ → V₂O₅

However, this method does not produce the purest vanadium pentoxide, since in addition to V₂O₅, other vanadium oxides are formed during this heating, which are difficult to separate from each other^[6].

Another method is to calcinate other vanadium oxides such as VO, V₂O₃ and VO₂ in oxygen as shown in the example below^[7]:

4 VO₂ + O₂ → 2 V₂O₅

The purest V2O5 is obtained by a multi-step reaction. First, vanadium slag is mixed with NaCl and roasted, which leads to the oxidation of vanadium to form V(V) and the formation of sodium vanadate. Further, with the help of sulfuric acid, the pH is lowered, which leads to the formation of sodium polyvanadate 6 Na₄H₂V₁₀O₂₈. An increase in temperature triggers the hydrolysis of sodium polyvanadate, which leads to the precipitation of a phase called "red cake" (Figure 2.) due to the characteristic reddish color:

6 Na₄H₂V₁₀O₂₈ + 7 H₂SO₄ + (n+13) H₂O → 5 Na₂V₁₂O₃₁ · n H₂O + 7 Na₂SO₄+13 H₂O

Next, the "red cake" is dissolved in sodium hydroxide, which leads to an increase in pH, and NH₄Cl is added, which leads to the precipitation of ammonium metavanadate:

VO₃^- + NH₄⁺ → NH₄VO₃

Finally, the ammonium metavanadate is calcined, resulting in V2O5 with a purity of about 98.5%^{[9, 10]}:

2 NH₄VO₃ → V₂O₅ + NH₃ + H₂O

Structure

Vanadium pentoxide occurs overwhelmingly in an orthorhombic crystal system^[11-13], however, there are also reports of the existence of V₂O₅ with a monoclinic system^[14]. The difference between the methods for producing an orthorhombic and monoclinic structure is that a much higher pressure (about 4-6 GPa) is required to obtain a monoclinic structure, which allows the orthorhombic structure to be reformed into a denser form. Below are images of V₂O₅ structures in the orthorhombic system (Figs. 3 and 4) and in the monoclinic system (Figs. 5 and 6). As can be seen from these figures, the difference between these systems, in addition to the unit cell parameters and crystal symmetry, lies in the fact that the polyhedra in the orthorhombic system have the shape of a square pyramid, and in the monoclinic system they have the shape of an octahedron. However, it should be noted that in the ICSD data, from which the pictures below were made, there is also a sixth V-O bond with a bond length significantly longer than the others (2.81 Å vs. 2.02 Å), making the polyhedra octahedral. Earlier studies did indicate the presence of this bond in the structure^[11], however, later studies have shown that this bond is really a weak interaction, and polyhedra are in fact square pyramids with common corners and edges^[16]. Thus, if we consider the orthorhombic structure from a larger distance (Fig. 4), we see that it forms two-dimensional nets.

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Figure 3. Close-up structure of orthorhombic V₂O₅ (Data from ICSD, visualized with VESTA. Figures: Nikita Jamkin).

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Figure 4. Distant orthorhombic V₂O₅ structure (Data from ICSD, visualized with VESTA. Figures: Nikita Jamkin).

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Figure 5. Close-up structure of monoclinic V2O5 (Data from ICSD, visualized with VESTA. Figures: Nikita Jamkin).

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Figure 6. Distant structure of monoclinic V2O5 (Data from ICSD, visualized with VESTA. Figures: Nikita Jamkin).

Properties

V₂O₅ is a diamagnetic compound and experiences rather weak repulsive forces in the presence of an external magnetic field. Can be used as an n-type semiconductor, i.e. with electrons as charge carriers because of its relatively small indirect band gap of approximately 2.3 eV^[17]. Moreover, due to its high oxidation state, it is an amphoteric compound and, depending on the medium, behaves like a base or an acid^[18]. It is insoluble in ethanol, but unlike most metal oxides it is slightly soluble in water and highly soluble in bases and acids^{[6, 8]}. Below are the XRD patterns for both V₂O₅ crystal systems (Figure 7) and band structure for orthorhombic V₂O₅ (Figure 8).

Figure 7. XRD patterns for orthorhombic (left, license: CC BY)^[19] and monoclinic (right; calculated data from ICSD, Figure: Nikita Jamkin) V₂O₅ systems.

Figure 8. V₂O₅  band structure (License: CC BY)^[17].

References

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