1. Introduction

Vanadium is sparsely distributed in the earth's crust. It is found at an average concentration of 150 mg kg^–1 in mineral ores and in the range of 3–300 mg kg^–1 in soil. Vanadium pentoxide, the highest oxidation state 5+ of vanadium, has been known for more than a century. In December 1867, as a part of his Bakerian lecture to the Royal Society, the British chemist Sir Henry Roscoe outlined the multiple oxidation states of binary oxides of vanadium and set the end-member "vanadic acid" V₂O₅ ^[1]. The first V₂O₅ gel was described by the French chemist Ditte in 1885 ^[2]. The natural occurrence (50%) of V₂O₅ is found in flue-gas deposits from oil-fired furnaces and residues from elemental phosphate plants. However, V₂O₅ mineral is rare; shcherbinaite V₂O₅ (orthorhombic crystal system) is found on the walls of volcanic fissures ^[3] and navajoite is a mineral trihydrate V₂O₅·3H₂O (monoclinic structure). The titanoferous magnetite ore in lump form contains approximately 1.5–1.7% vanadium pentoxide.

The structure of crystal isolated from fused V₂O₅ was first elucidated by Katelaar ^[4] as a rhombic unit cell containing two molecules of V₂O₅. Refinements were further done by Byström et al. ^[5] who established the orthorhombic layered structure of V₂O₅ with Pmmn space group. In 1961, Bachmann et al. ^[6] redetermined the crystal structure of V₂O₅ and reported its lattice parameters a = 11.510 Å, b = 4.369 Å, c = 3.563 Å. Their structural description is as follows: "the structure is built up from distorted trigonal bipyramidal coordination polyhedral of O around V, which share edges to form zigzag double chains along [001] and are cross-linked along [100] through shared corners, thus forming sheets in the zz plane". More recently, Enjalbert and Galy ^[7] proposed the bonding interactions of van der Waals type between two-dimensional V₂O₅ slabs and stated the presence of "square pyramid" instead of trigonal bipyramid. Ramana et al. ^[8] and Julien et al. ^[9] have described the easy cleavage along the (001) plane due to the weak interaction between V₂O₅ layers. Three V₂O₅ polymorphs have been identified as α-, β-V₂O₅, which crystallize with the orthorhombic and tetragonal structure, respectively, and the rutile-type δ-V₂O₅, which is a modification of β-V₂O₅ (space group C12/c1) ^[10]. Recently, Parija et al. ^[11] evaluated the metastable polymorphs including γ'-, δ'- and ρ'- phase of V₂O₅ using density functional theory calculations.

Here, we review the properties of V₂O₅ thin films employed in energy storage and conversion systems, which were prepared with a variety of deposition options. Numerous works prior 2001 have been devoted to the studies of synthesis, physical characterizations and electrochemical performances of films used as either cathode in all-solid-state batteries ^{[12,13,14,15]} or electrode of electrochromic devices ^[16]. Recently, Mo-doped V₂O₅ thin film has been studied as an electrode of supercapacitors ^[17]. This is an illustration of the increasing interest in this material in the recent years for many applications, sustained by numerous works that are reviewed here.

This review paper is organized as follows. In Section 2, we summarize the techniques used for the preparation of films and discuss the structure and morphology of V₂O₅ related to the synthesis conditions (temperature, partial pressure, substrate, etc.). Section 3 is devoted to the physical properties of V₂O₅ thin films; structure, morphology, vibrational spectroscopy, elemental analysis, electrical properties and intercalation process are considered. In the following Sections 4 and 5, doped V₂O₅ thin films and composite films are treated, respectively. Finally, Section 6 is devoted to the applications in the field of energy, for which V₂O₅ thin films are used as electrode materials in lithium microbatteries, electrochromic devices, sensors and supercapacitors.

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2. Thin film synthesis

The structure and morphology of vanadium oxide films are intimately related to the deposition method and the operating conditions. As the direct growth of crystalline V₂O₅ films is very difficult except in the cases of some sub-stoichiometric VO_x oxides, an annealing process of the as-prepared films is required in air and high temperature. In this section we describe the different evaporation methods and provide some typical examples for each technique used. Note that the choice of a deposition method depends of the thin film application. For instance, electrodeposition ^[18], reactive sputtering ^[19], sol-gel ^[20], hydrothermal method ^[21], doctor-blade route ^[22] were carried out for electrochromic V₂O₅ films that requests large surface coatings. The reader will find supplementary data in the review by Beke ^[23]. During the deposition process in vacuum or in a reducing atmosphere, the removal of oxygen atoms occurs from the film network when V₂O₅ is heated above its melting point that induces the formation of defects or reduced VO_x phases. Consequently, the structural and morphological disorders and phase instability can be controlled using appropriate deposition parameters.

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2.1. Thermal deposition

The thermal evaporation technique is the simplest method based in the production of flux of vapor in a high vacuum chamber (pressure 100 mPa) to form thin films without the presence of catalyst. In this method, the substance is evaporated by means of resistive heating. Generally, the raw powder is placed in a molybdenum boat heated at T_boat. The modifications of the structure, stoichiometry and morphology of the deposited films are obtained by varying the substrate temperature T_s, the flow of reactive gases (in cm³ min^–1 at standard pressure and temperature, expressed as "sccm" hereafter) and the duration of the target evaporation. For example, thermally evaporated polycrystalline V₂O₅ films (10–20 nm grain size) were fabricated using T_boat = 650 ℃ for 6 h with gas flow 13 sccm Ar + 50% O₂ ^[24]. The nature of the substrate is an important factor to obtain films with preferential orientation ^[25]. V₂O₅ films deposited on silicon (111) wafers by vacuum thermal evaporation were amorphous when deposited at T_s ≤ 200 ℃, while polycrystalline at T_s ≥ 300 ℃. This later temperature is optimum for V₂O₅ films strongly oriented with (001) planes parallel to the substrate ^[26]. Nanostructured V₂O₅ thin films (25 nm grain size) thermally deposited onto Ni substrates at T_s = 300 ℃ show preferential (001) orientation. These films have pseudocapacitance of 730 mF cm^–2 at current density of 1 mA cm^–2 and charge transfer resistance of 7.5 Ω ^[27]. The thermally evaporated V₂O₅ films at T_s = 25 ℃, 100-nm thick, crystallized after annealing process at 500 ℃ with grain size 26 nm and exhibited an electrical conductivity of 5.5 S cm^–1 (activation energy of 0.16 eV) ^[28] and optical bandgap of 2.8 eV.

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2.2. Flash-evaporation

In the flash-evaporation method, the powders are placed in a reservoir and poured drop by drop into the boat heated at T_boat to ensure the vaporization. This process allows the control of the vaporization rate and preserves any decomposition of the starting material before evaporation ^[29]. It was demonstrated that V₂O₅ flash-evaporated films are more homogeneous ^[30]. Flash-evaopration was also used for depositing Li_xV₂O₅ ^[31]. Polycrystalline V₂O₅ flash-evaporated films have been investigated as a function of the deposition conditions: various substrates, substrate temperature (T_s), oxygen partial pressure (pO₂) and post-annealing treatment (T_a) ^[9]. Figure 1 presents the evolution of the O/V ratio as a function of the temperature of the molybdenum boat ^[32]. Films with the best stoichiometry O/V = 2.497 were obtained for T_boat in the range 910–980 ℃. However, properties of flash-evaporated films are strongly dependent on the deposition "quenching rate" ΔT, which is the difference in temperature between the melt and the substrate (ΔT = T_a – T_s).

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2.3. Chemical vapor deposition (CVD)

Chemical vapor deposition (CVD) is a versatile technique that consists of an evaporator and a deposition system. The precursor solution is vaporized into a reactor which acts as a flash evaporation system. The vapor condenses in a cold-wall reactor in which the substrate placed in the central zone is maintained at constant temperature T_s ^[33]. This technique has been successfully carried out to grow V₂O₅ films using pure or diluted triisopropoxyvanadium oxide VO(OC₃H₇)₃ precursor ^[34]. Typical V₂O₅ film 240 nm thick was formed at T_s = 300 ℃ in the total pressure p_t = 2400 Pa with a flow of O₂ of 100 sccm. V₃O₇ and V₄O₇ films are deposited when p_t decreases to 1200 Pa, while V₆O₁₃ films are formed at T_s = 350 ℃ and O₂ flow rate of 120 sccm. Highly crystallized films (94 nm crystallite size) were obtained after post-annealing at 500 ℃ for 2 h in O₂ atmosphere with a slow cooling process to insure a good adherence on substrate. Plasma-enhanced chemical vapor deposition (PE-CVD) is a method for large scale film deposition. In this technique, glow discharge creating high-energy electrons ionize gaseous molecules and generate chemically reactive ions. Heat-treated (500 ℃ for 2 h) films deposited by CVD technique onto Pt foil are highly structured with homogeneous and smooth surfaces as shown in Figure 2a. The coherence domains along the a-and c-axis are L₂₀₀ = 71 nm and L₀₀₁ = 36 nm, respectively. The cross-section image (Figure 2b) displays the compactness of annealed V₂O₅ films ^[35].

Barreca et al. ^[36] prepared V₂O₅ thin films by PE-CVD using VO(hfa)₂·H₂O (Hhfa = 1, 1, 1, 5, 5, 5-hexafluoro-2, 4-pentanedione) as precursor. This precursor was vaporized at 70 ℃ at the rate of 2 × 10^–4 mmol m^–2 s^–1 in a reactor in which argon (constant flow rate ϕ = 40 sccm) and oxygen (flow rate 5 ≤ ϕ ≤ 20 sccm) were plasma sources. V₂O₅ thin films deposited at T_s = 200 ℃ and 10 sccm of O₂ are highly textured (14 nm crystallite size) and grow with the (001) preferential orientation. In the metal organic chemical vapor deposition (MOCVD) the organometallic vapor phase takers place at moderate pressure (10–1000 hPa). Watanabe et al. ^[37] prepared V₂O₅ thin films by means of microwave plasma MOCVD on ITO-coated fused silica substrate. Bis-acetylacetonatovanadyl, VO(acac)₂, VO(C₅H₇O₂)₂ heated at ~600 ℃ was selected as vanadium precursor. Its vapor was injected into the oxygen plasma generated by the microwave discharge. Other experimental conditions were as follows: the substrate was maintained at T_s ≈ 300 ℃, the O₂ flow was ϕ = 1.2 dm³ h^–1 under pressure of 650 Pa. Typical polycrystalline film, 120 nm thick, was deposited after 15 min. Vanadyl(Ⅳ) β-diketonate has been also used with water in a low-pressure reactor under different conditions ^[38]. A novel vanadium(Ⅲ) precursor such as vanadium(Ⅲ) alkoxide [V(OCMe₂CH₂OMe)₃] was also used, which presents an appreciable volatility at 55 ℃ under pressure of 200 Pa ^[39]. Strongly (00l)-oriented film composites of V₂O₅–V₆O₁₃ were grown at temperatures ≥560 ℃ onto fused quartz substrate using vanadyl acetylacetonate as precursor. Single phase was obtained for substrate maintained at T_s = 580 ℃ due to the reentrant-type growth behavior ^[40]. V₂O₅ electrochromic films were grown on flexible polymer substrates using plasma-enhanced chemical vapor deposition (PECVD). Films were deposited at high rate of 50 nm min^–1 from the decomposition of vanadium oxytrichloride (VOCl₃) and O₂ ^[41]. Nandakumar ^[42] studied the growth rate r_G of CVD V₂O₅ films as a function of T_s and reported an Arrhenius behavior with 0.14 eV activation energy; r_G = 50 nm min^–1 at T_s ≈ 230 ℃.

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2.4. Magnetron sputtering

Sputtering deposition method is the most popular technique to grow metal-oxide films, since it allows faster deposition rates. Its main advantage comes from the production of good surface uniformity of as-deposited films. It includes radio-frequency (r.f.) magnetron sputtering ^{[43,44,45,46]}, direct current (dc) magnetron sputtering ^[47,48,49] and ion beam sputtering ^[50]. The stoichiometry of V₂O₅ films can be tuned by this evaporation method ^[51,52]. Typical r.f.-magnetron sputtered V₂O₅ films are formed in a plasma sputtering chamber using V₂O₅ target bombarded by argon ions in a plasma of power P_w = 150–300 W. For specific application, low sputtering power P_w < 100 W can be used ^[53]. The reactive deposition is realized by injection of pure argon and oxygen gases, at flow rate ϕ < 10 sccm. Silversmit et al. ^[51] obtained V₂O₅ films on Si (100) wafers by reactive dc magnetron sputtering and studied the influence of deposition parameters on their stoichiometry. These experiments were carried out in a vacuum of 5 × 10^–5 Pa with a magnetron discharge voltage at ~550 V and a plasma constant current of 150 mA. The O₂ flow was maintained at ϕ = 3.5 sccm. Fateh et al. ^[54] deposited V₂O₅ films onto (100) oriented Mg substrates and established detailed synthesis-structure relationships showing the best crystallinity for the film deposited at 80 ℃. Lourenço et al. ^[55] investigated the effect of different oxygen flow rates on the structure of VO_x films and determined that V₂O₅ films are obtained at high oxygen flows (ϕ > 9 sccm). Benmoussa et al. ^[56,57] sputtered V₂O₅ thin films on Corning glass and ITO-coated glass substrates for electrochromic application. The films deposited on both substrates are c-axis preferred oriented showing that texture is substrate independent.

Yoon et al. ^[58] compared the structure of V₂O₅ thin films grown on (100) Si wafers by d.c. and r.f. reactive sputtering at room temperature; d.c.-sputtered films are amorphous, while those prepared from reactive r.f.-sputtering crystallized with large grain size that is due to the self-bias effect. Gianneta et al. ^[46] deposited VO_x films using a base pressure 1 mPa, a r.f. plasma power of 200 W and an Ar flow ϕ = 20 sccm. As-deposited films are almost amorphous and exhibit broad XRD reflections matching those of VO_0.9, while annealing at 475 ℃ in air for 3 h promotes the well-crystallized V₂O₅ phase with well-developed orthorhombic shape and grain size > 100 nm (see Figure 3). Ottaviano et al. ^[59] reported the improved electrochromic properties of rf reactive sputtered films deposited at low O₂ flow (2%) showing the highest value of injected charge (49.8 mC cm^–2) and the greatest differences in optical density. Lin and Tsai ^[19,60] prepared V₂O_5–z films on flexible PET(polyethylene terephthalate)/ITO (indium tin oxide) substrates. Oxygen deficient V₂O_5–z films (z = 0.13) were formed in a chamber pressure of 6 Pa with Ar and O₂ flow rate of 4 sccm that show a transmittance variance od 36.5% after 200 cycles. Kang et al. ^[61] prepared nanorod-like V₂O₅ film by electron-beam irradiated amorphous film, which were obtained by r.f. sputtering at power of 200 W at 1 nm min^–1 growth rate on alumina substrate.

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2.5. Pulsed-laser deposition (PLD)

Extensive works using the PLD technique have been successful for the growth of V₂O₅ thin films. Advantages include an easily control of the film composition by tuning the deposition parameters and a good reproducible stoichiometry of the target material in the films ^{[62,63,64,65,66,67,68,69,70,71,72,73]}. In the PLD technique, a pulsed laser beam (10 ns duration) is focused by a lens to ablate the V₂O₅ target. The energy of the beam is in the range 100–500 mJ per pulse (laser fluence of 10 J cm^–2) with a laser pulse repetition of 10 Hz. Target and substrates are placed inside the deposition chamber evacuated to ~1 mPa. To avoid depletion of deposit at any given spot, the target rotates at 10 rpm. For reactive synthesis, pure oxygen gas is introduced into the chamber with a typical partial pressure (pO₂) in the range 0.1–10 Pa to obtain single-phased and stoichiometric V₂O₅ films. Typical deposition rate is in the range 2–5 Å s^–1. Various laser wavelengths are reported in the literature: 532 nm line of a doubled frequency pulsed Nd:YAG laser ^[63], 248 nm line of a KrF excimer UV laser ^[62,64] or the 266 nm line of the quadrupled Nd:YAG laser. Zhang et al. ^[62] prepared PLD crystalline oriented V₂O₅ films deposited at 200 ℃ in different partial pressure of O₂ + Ar mixed gases. The films grown in oxygen-rich, pO₂ = 0.1 Pa, environment at low substrate temperature T_s ≈ 200 ℃ crystallizes in the orthorhombic structure. Iida et al. ^[65] suggested that films deposited at T_s = 350 ℃ in a chamber under partial pressure pO₂ = 13.3 Pa using the 238-nm line of a KrF excimer laser have a surface morphology suitable for electrochromic application. The same process was used to grow doped films, with various elements Nb, Ce, Nd, Dy, Sm, Ag ^[66]. Fang et al. ^[74] prepared orientated V₂O₅ electrochromic thin films on glass by pulsed excimer laser ablation. Nanocrystalline films highly oriented along the c-axis were obtained at lower temperature T_s (200 ℃) that that of thermal vacuum deposition (300–500 ℃). McGraw et al. ^[67] found that the crystallographic texture of PLD V₂O₅ films depends mainly on temperature and oxygen pressure rather than on the choice of substrate. Films deposited on SnO₂/glass substrate are dense and phase pure orthorhombic V₂O₅. For T_s = 200 ℃ and pO₂ = 2.7 Pa, the films are well (200)-textured, while they are (001) and (101) dual-oriented for T_s = 500 ℃ and pO₂ > 12 Pa. Ramana et al. ^[75] investigated the structural patterns of PLD V₂O₅ films which revealed that stoichiometric specimens are well-crystallized with the layered orthorhombic structure even onto amorphous substrates at T_s = 200 ℃ under partial oxygen atmosphere pO₂ = 10 Pa. Atomic force microscopy (AFM) analysis gave a consistent picture of the surface morphology and microstructure: grains of small size, spherical in shape. Figure 4 shows the variation of the grain size as a function of the growth temperature for various substrate materials. The evolution of the grain size follows an exponential law (dashed lines) as ^[76]:

${L_c} = {L_0}\exp \left( { - {Q_d}/{k_B}T} \right)$

(1)

where Q_d is the activation energy, k_B is the Boltzmann constant, T is the absolute temperature, and L_o is a preexponential factor that depends on the physical properties of the substrate-deposit. The grain size appears to be 50–300 nm for films deposited onto glass, 50–400 nm for films on ITO-coated glass and 60–600 nm for films evaporated on Si wafer. Q_d of the PLD V₂O₅ film in the range 0.43–0.55 eV reflects the nucleation rate ^[64]. AFM images show that films deposited onto Si (8 nm) and ITO-coated glass (10 nm) exhibited lower surface roughness. The dependence of the substrate temperature on the gross microstructure of PLD V₂O₅ films is presented in Figure 5. The domain of optimum controlled morphology is illustrated by the shaded region of substrate temperature range 200 ≤ T_s ≤ 300 ℃.

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2.6. Electron-beam (e-beam) evaporation

The e-beam evaporation method implies a high vacuum coating unit with vacuum better that 0.1 mPa, in which an electron beam accelerated by a voltage of several kV (typically 6 kV) and power density of ~1.5 kW cm^–2 is scanned on the surface of the target. It is a low-cost technique, which allows high deposition rate (30–50 Å s^–1) with good control of the structure and morphology, and allows for sequential and co-deposition with minimum contamination ^{[8,77,78,79,80,81,82,83,84]}.

In their early work, Ramana et al. ^[81] reported the growth of non-stoichiometric V₂O₅ films deposited on Corning 7059 glass substrates maintained at T_s = 280 ℃ in a vacuum of 0.05 mPa, while the injection of pure oxygen (partial pressure of 0.1 mPa) produced near-stoichiometric films with pale orange yellow color. The XPS experiments revealed that films formed at room temperature are nearly stoichiometric, while the films formed at elevated temperatures are oxygen deficient. These results are supported by the shift of the vanadyl mode in the FTIR and Raman spectra. However, the films deposited at T_s ≈ 420 ℃ in oxygen partial pressure of 10^–4 mbar led to vanadium ions in their highest oxidation state ^[8,79]. Alumina-doped V₂O₅ thin films, (V₂O₅)_100–x(Al₂O₃)_x with x = 5, 10, 15, 20 wt.%, were fabricated by e-beam evaporation method ^[85]. Thiagarajan et al. ^[84] studied the physical properties of highly oriented V₂O₅ films (0.8–1.2 µm thick, 31–45 grain size) grown by electron beam evaporation at 200 ℃ throughout all depositions. It was shown that the microstrain decreases with the decrease of the film thickness, due to the reduced dislocation density.

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2.7. Spray pyrolysis

Spray pyrolysis is considered as a cost-efficient technique and has been widely employed to fabricate V₂O₅ thin films. Currently, the pyrolysis process is performed at relatively low temperature > 500 ℃ ^{[86,87,88,89,90]}. The V₂O₅ films are deposited through thermal decomposition of vanadium precursor such as VCl₃ in deionized water ^[91]. By increasing the spray deposition rates, the crystallite size increases, while the microstrain decreases. Nanostructured flower-like V₂O₅ (crystallite size of ~25 nm) were prepared by spray pyrolysis from 0.1 mol L^–1 ammonium vanadate in aqueous solution. Polycrystalline films deposited at T_s = 300 ℃ exhibit the largest carrier density 3.6 × 10¹⁸ cm^–3 and the lowest electrical resistivity 5.56 Ω cm^–1, which allow to use them as xylene sensors ^[92]. 10% Mo-doped films (~28 nm crystallite size) prepared by spraying VCl₃ and MoCl₅ in aqueous solution on substrate maintained at 380 ℃ crystallize with a tetragonal structure ^[93]. Wei et al. ^[90] prepared V₂O₅ films using ultrasonic spray method that revealed a small amount of V⁴⁺ species. These films heat-treated at 350 ℃ show good charge storage stability over 10000 electrochromic cycles. A d.c. plasma torch was used to atomize the solution VOCl₃ precursor, which is injected externally into the plasma flame and deposited on the substrate ^[88]. Nanostructural V₂O₅ thin films deposited by spray pyrolysis technique exhibit the highest transmittance when deposited at 550 ℃. A shift of the absorption edge from 2.5 to 2.8 eV is observed for T_s > 450 ℃ due to modified chemical bonds at the film/substrate interface ^[89]. The influence of the molarity of the spray pyrolysis precursor on the structure and morphology of V₂O₅ films shows that vanadium nitrate V(NO₃)₅·5H₂O with concentration ≥0.1 mol L^–1 provides orthorhombic structured films with (001) preferred orientation ^[94].

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2.8. Sol-gel

Sol-gel method is a low-temperature wet-chemistry method in which the raw materials are mixed at the molecular level. Wang et al. ^[95] investigated the structural modifications of spray deposited V₂O₅·nH₂O xerogel films as a function of post-annealing temperatures. Films sprayed at 150 ℃ showed broad peaks in their ⁵¹V NMR spectra for heat-treatment at T_a ≈ 120 ℃, which gives evidence of a distortion of the square pyramids. The structural feature of V₂O₅·nH₂O (with structural water) favors the lithium intercalation between the slabs with respect to contracted dried films at T_a > 250 ℃. The effect of deposition conditions and post-annealing treatment of xerogel V₂O₅ films was studied by several groups. Najdoski et al. ^[96] examined the electrochromic properties of (NH₄)_0.3V₂O₅·1.25H₂O nanostructured films deposited onto SnO₂:F (FTO) as a function of the temperature of preparation. Elongated aggregates 250–500 nm length were obtained at 50 ℃, while shorter particles (50–300 nm) were obtained at 85 ℃. Chen et al. ^[97] prepared thin films by spin coating of the sol-gel composed by the mixture of V₂O₅ powders benzyl alcohol (BA) and isobutanol (IB) (molar ratio 1:40:4) heated at 110 ℃ for 5 h. Wang et al. ^[98] fabricated V₂O₅·nH₂O xerogel films by reacting V₂O₅ and H₂O₂ under ambient conditions. V₂O₅·⅓H₂O film exhibited the best Li⁺ intercalation properties with a stabilized specific capacity of 185 mAh g^–1 at 0.1 mA cm^–2 current density after 50 cycles. Cazzanelli et al. ^[99] analyzed the Raman spectra of xerogel films intercalated with lithium. V₂O₅·nH₂O xerogel films (1.8 ≤ n ≤ 0) were spin-coated at angular velocity of 1500 rpm. Spectral features of Li-intercalated samples show significant effect of the water content in the interlayer space inducing new Raman bands at 800 and 950 cm^–1. V₂O₅ films deposited by the doctor-blade route using polyol dispersed in waterand acetyl acetone crystallized with the C2/c structure, which transformed to orthorhombic Pmmn (86 nm crystallite size) after calcination at 170 ℃ for 3 h ^[22]. Dip-coating method was used to produce V₂O₅ xerogel films from organic (vanadium tri-isopropoxide into isopropyl alcohol) and inorganic (V₂O₅ powers in 15 wt.% H₂O₂) precursors ^[100,101]

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2.9. Electrodeposition

A typical electrodeposition experiment takes place in an aqueous solution of vanadium precursor such as vanadyl sulfate hydrate (VOSO₄·nH₂O) or V₂O₅ powers dissolved in hydrogen peroxide as electrolyte using a constant potential (1.7 V vs. Ag/AgCl) ^[18,102,103]. Liu et al. ^[104] prepared nanostructured V₂O₅ films by means of cathodic deposition from V₂O₅ and H₂O₂ aqueous solution. As-deposited samples contained 14% V⁴⁺, while annealing at 500 ℃ in air oxidized all vanadium cations. Electrochromic V₂O₅ films were synthesized by electrophoretic deposition of a sol formed by the dissolution of V₂O₅ powers in H₂O₂ solution with the H₂O₂/V₂O₅ ratio of 8:1. The deposition occurred in a Teflon vessel at the voltage of 5 V using a Pt rod as counter electrode ^[102]. The electrodeposition experiments carried out by Vernardou et al. ^[18] consisted in the coating of V₂O₅ powders dissolved in a solution of methanol and water as electrolyte to form an orange suspension. Pt rod was the counter electrode. Experiments were performed using deposition current density of 0.25 mA cm^–2.

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2.10. Atomic layer deposition (ALD)

The atomic layer chemical vapor deposition (ALCVD) was successfully applied to grow V₂O₅ films ^{[105,106,107,108,109,110,111,112]}. This method provides uniform and compact films with a high control of phase formation and the film thickness that include atomic layer epitaxy (ALE) and atomic layer deposition (ALD). ALD process consists in alternative exposition of a substrate to the precursor fluxes, which are timely separated in space. In principle this process is self-limiting as the monomolecular layers are absorbed with molecular excess flushed away by evacuation of the system. The vapor pressure of the precursor must agree with the deposition temperature chosen. Groult et al. ^[105,106] prepared nanometric V₂O₅ thin films deposited onto Si wafers (100) by means of ALCVD.

The atomic layer deposition (ALD) technique is proven suitable for the growth of uniform and pin-hole free V₂O₅ thin films. ALD is becoming an attractive process to prepare thin films for energy storage, because it allows the control of deposit films of thickness in the range 1–100 nm with remarkable uniformity and control of the thickness. V₂O₅ has been synthesized by this process by several authors ^{[113,114,115,116,117,118,119]}. This scalable method, which allows to do bath processes, is based on self-limited reactions between gas phase precursors and active sites on surfaces. This technique leads to monolayer thickness control. The growth of V₂O₅ thin films consists of the successive cycles of deposition of atomic layers from the reaction of a vanadium precursor with oxygen. Various vanadium precursors have been proposed such as vanadyl tri-isopropoxide VO(OC₃H₇)₃ (VO(OPrⁱ)₃, VTIP), vanadyl triethoxide (VO(OEt)₃), vanadyl triisobutoxide (VO(OBui)₃), vanadyl acetylacetonate (VO(acac)₂) and vanadium acetylacetonate (V(acac)₃ (acac = C₅H₇O₂^–)), VTIP being the most popular due to its high vapor pressure (38 Pa at 45 ℃) and its rapid reaction with water ^{[107,108,109]}. Examination of the literature shows that the choice of ALD precursors has a significant impact on the crystallinity and morphology of V₂O₅ films. One ALD cycle is composed of four pulses in the sequence t₁/t₂/t₃/t₄, where t_i is the duration (in s) of injection of V precursor, purge, reactive gas, purge, respectively. For instance, the global ALD reaction of VTIP precursor with water is:

${\rm{VO(O}}{{\rm{C}}_{\rm{3}}}{{\rm{H}}_{\rm{7}}}{{\rm{)}}_{\rm{3}}}{\rm{ + 3}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}{\rm{ + 6HO}}{{\rm{C}}_{\rm{3}}}{{\rm{H}}_{\rm{7}}}$

(2)

The pressure of (VTIP) in the reactor is adjusted to 5 mPa and the water pressure to 0.3 Pa. Typical cycle to grow V₂O₅ films lasts 17 s (2 s VTIP, 5 s pumping, 5 s reactive gas and 5 second pumping) that makes a growth rate of 0.7 Å per cycle. Several authors demonstrated that the rate-determining step is the oxidant in O₃-based or H₂O-based ALD reaction ^[109,110]. Keränen et al. ^[107] prepared ALD V₂O₅ films deposited onto various metal oxides for catalytic applications using VO(acac)₂ as V precursor with a N₂ flow of 3 dm³ h^–1. Chen et al. ^[111] used a water-free system to prepare crystalline as-deposited V₂O₅ films from VTIP and ozone, using an Al₂O₃ template at low temperature (170–180 ℃). With this process, a deposition rate of 0.27 Å per cycle onto Si wafer was achieved. Badot et al. ^[110] reported a growth rate of ~20 ng cm^–2 per cycle at 170 ℃ using VTIP + water sequence. With these growth conditions and annealing at 400 ℃, the ALD V₂O₅ films were well-crystallized and preferentially oriented with the (a, b) planes parallel to the SnO₂-coated glass substrate. ALD V₂O₅ films deposited onto Ti foil using the VTIP-water vapor route in the pulsing sequence—VTIP/N₂ purge/H₂O vapor/N₂ purge of 200/400/2400/1000 ms and annealed at 500 ℃ for 2 h showed a small-polaron drift mobility of 1.84 × 10^–2 cm² V^–1 s^–1 at room temperature (activation energy of 0.14 eV) ^[112]. Ostreng et al. ^[113] prepared nano-structured ALD V₂O₅ films from vanadyl 2, 2, 6, 6-tetramethylhepta-3, 5-dione (VO(thd)₂) and ozone at 215 ℃ with N₂ as carrier gas (500 sccm flow) applied as cathode films for lithium microbatteries. 86-nm thick films oriented along a (001)/(010) orientation were thus obtained after 2000 cycles. Chen et al. ^[114] employed VTIP precursor with both O₃ and H₂O as oxidant. For ALD cycles operating in the 170–185 ℃ window, the water-VTIP route (growth rate 0.4 Å per cycle) produces amorphous films, while crystalline films are formed with the ozone-VTIP reaction (growth rate 0.20 Å). The transition from amorphous to crystalline structure was characterized by rectangular V₂O₅ nanograins (20 nm size) with a monomodal distribution starting at 400 ℃.

A comparison of thermal and plasma-enhanced ALD/CVD has been carried out by Musschoot et al. ^[109]. Chen et al. ^[114] determined that areal energy and power density is optimized with V₂O₅ thickness round 60 nm. Larger thickness results in a decrease of the rate capability due to the small intrinsic diffusion coefficient of lithium, while thinner films suffer from a decrease of material loading. The authors estimated that the device would outperform the gravimetric power density of the current Li-ion batteries by one order of magnitude. Song et al. ^[114] adopted to deposit V₂O₅ thin films by ALD using vanadyl tri-isopropoxide and water and showed remarkable electrochemical response and voltage-induced insulator-to-metal transition. Ultrathin V₂O₅ films (10–90 nm thick) were prepared by ALD using vanadyl acetylacetonate as the vanadium precursor ^[119]. Crystalline films were obtained below 200 ℃ yielding a growth rate of 4.5 pm per cycle. Heterojunction diodes based on TiO₂(p)-(n)V₂O₅ were fabricated as humidity sensor.

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3. Characterizations of V₂O₅ films

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3.1. Structure and morphology

V₂O₅ is the highest oxidation compound of the vanadium oxide system which crystallizes with a lamellar orthorhombic structure in space group Pmmn (D_2h¹³, No. 59) with lattice parameters a = 11.512 Å, b = 3.564 Å, c = 4.368 Å and V = 179.17 ų (see Table 1). It has a layered structure with atomic layers in the [100] × [010] plane and van der Waals-type interlayer coupling. The structure is formed of three types of oxygen atoms: apical-vanadyl (O_van), bridge (O_bri) and chain (O_ch) (see Figure 6). Oxygen vacancies in V₂O_5–δ are predominantly of the O_van type, forming the vanadyl bonds. Atomic positions in V₂O₅ are summarized in Table 1. Pure V₂O₅ is a diamagnetic semiconductor with an energy gap (E_g) of 2.2–2.3 eV ^[120], while oxygen-deficient material is ferromagnetic with spin moment ~2 µ_B per vacancy ^[121]. A density functional calcualtion of the electronic structure is given in Refs. ^[122,123]. The role of octahedral deformations has been explored in ^[124].

Figure 7 shows the typical XRD patterns of V₂O₅ films deposited by PLD (a) and (b) by electron beam (EBD) techniques. The spectra of samples deposited at low T_s are rather amorphous, while they can be obtained crystallized by raising the substrate temperature and/or increasing p(O₂). The significant feature is the growth of XRD reflections at ca. 2θ = 20.26° and around 2θ = 41.4° that correspond to the (001) and (002) lattice planes of the orthorhombic structure, respectively. The (001) reflection is the dominant line for well-crystallized films showing the preferential c-axis orientation lying perpendicular to the substrate with stacking ab planes ^[125]. Gradual increase in intensity of the (101) reflection at high T_s is attributed to a re-organization process of the in-plane V–O–V chains in flash-evaporated film. This crystallographic behavior seems to be the general trend for the growth of V₂O₅ films; it has been found for various deposition techniques: as-prepared r.f. sputtered films under a gas mixture of Ar and O₂ ^[56,126], sol-gel spin-coated films annealed at 400 ℃ ^[127], pulsed-laser deposited V₂O₅ films ^[63], plasma-enhanced CVD method ^[128] and vapor deposition in high vacuum ^[26,129] on heated substrates. The c-axis preferred growth was also found for films prepared by ALD ^[110,113] and EBD ^[79] processes. Well-ordered V₂O₅-(001) films were obtained by the oxidation of vacuum deposited vanadium layers with oxygen partial pressure of 50 hPa ^[130]. Due to the imperfect stacking of ab planes built up of O_van, the (001) orientation for crystalline V₂O₅ films deposited by PECVD from a vanadyl(IV) β-diketonate compound appears at T_s as low as 150 ℃ ^[38], while higher temperature T_s = 240 ℃ for films prepared by CVD of VOCl₃ with water ^[131]. Note that films prepared from sol-gel precursor display complex behavior due to the large fraction of water content. For instance, the XRD spectrum of spin-coated films obtained from V₂O₅ + H₂O₂ sol-gel (crystallized at T_a = 150 ℃) displayed the dominant (110) Bragg line, which disappeared and was replaced at T_a = 300 ℃ by the (00l) reflections showing the nucleation parallel to the substrate ^[121]. Other workers found that spin-coated films crystallize above 350 ℃ ^[132]. The texture of V₂O₅ films grown by ALD technique varies with the number of deposition cycles (n_c). For n_c < 1000, the predominant orientation is along the (001) plane, which is modified for n_c ≈ 2000 with the appearance of sharp (0k0) Bragg reflections. Finally (021) and (061) reflections are observed for n_c ≈ 5000. This is attributed to the overgrowth of nuclei on the surface of platelets with their edges orientated normal to the substrate ^[113]. Similar thickness (d_f) effect was observed on e-beam deposited films ^[84]. Moreover, the increase of the peak broadening of the (001) lattice place with d_f is attributed to microstrains inversely proportional to the crystallite size that linearly decreases with the increase in d_f.

Rajendra-Kumar et al. ^[133] deposited V₂O₅ films at various substrate temperatures (25 ≤ T_s ≤ 400 ℃) using vacuum evaporation technique. The orthorhombic lattice parameters "a" and "c" of polycrystalline films determined by Nelson-Rieley function were found to decrease when the deposition temperature increases (Figure 7a). According to the model proposed by Gilles and Boesman ^[134] from EPR spectroscopy, these structural changes are assigned to the increase of non-stoichiometry (oxygen vacancies) leading to a contraction of the (001) interplanar spacing.

The crystallite size L_c of composites is calculated using the Scherrer's formula by considering the (002) reflection line:

${L_c} = \frac{{K\lambda }}{{B\cos \theta }}$

(3)

where K is the shape factor, which varies with the crystallite shape (0.89 for spherical crystallites), λ is the wavelength of X-ray radiation (0.15406 nm), B is the full-width at half maximum (FWHM) expressed in radians and θ is the Bragg angle. Generally, the peak intensity and the full-width at half-maximum (FWHM) vary with the deposition conditions, which indicates differences in crystallinity, particle sizes and ordering of local structure between samples. The analysis is obtained by the combination of the Scherrer and Bragg formulæ:

${B^2}{\cos ^2}\theta = 16{e^2}{\sin ^2}\theta + \frac{{{K^2}{\lambda ^2}}}{{L_c^2}}$

(4)

where < e² > denotes local strain (defined as ∆d/d with d the interplanar spacing). From the slope 16 < e² > and intercept K²λ²/L_c² of the plot of B²cos²θ as a function of sin²θ, one can estimate the strain < e² > and coherence length L_c. The variation of the local strain as a function of the substrate temperature for V₂O₅ films deposited by PLD method is reported in Figure 7b.

The surface morphology and surface topography are currently investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) as a function of at various T_s. The AFM images recorded at the scan area of 2 × 2 μm² for thermally evaporated V₂O₅ films are displayed in Figure 8 ^[82]. These patterns show that both the surface roughness and the grain size increase with the increase in T_s. Surface roughness of 27 nm and grain size of 130 nm are found for films deposited at 300 ℃. It was demonstrated that the morphology, structure and optical and electrical properties of dip-coated V₂O₅ films are affected by the sol aging ^{[135,136,137,138]}. Senapati and Panda ^[131] investigated nanoscale films prepared by spin coating of V₂O₅·nH₂O sol at different stages of aging. Microstrains of films 92–137 nm thick were found to decrease from 4.93 × 10^–3 for fresh sol to 2.62 × 10^–3 for 192 h for aged sol in relation to the water loss, while the optical bandgaps decrease from 2.66 to 2.36 eV suggesting a decrease in the localized states with aging.

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3.2. Optical properties

Optical properties of V₂O₅ thin films were investigated by means of absorption measurements for the determination of the optical bandgap, dispersion parameters and spectral window of electrochromic films. Optical absorption coefficient α is calculated by the relation:

$T = \frac{{{{\left( {1 - R} \right)}^2}\exp \left( { - {\rm{ \mathsf{ α} }}d} \right)}}{{1 - {R^2}\exp \left( { - {\rm{ \mathsf{ α} }}d} \right)}}$

(5)

where T and R are the spectral transmittance and reflectance and d the film thickness. The interference effect due to internal reflection at normal incidence is negligible for higher optical density (αd > 1) that reduce Eq 1 to:

$T \approx \exp \left( { - {\rm{ \mathsf{ α} }}d} \right)$

(6)

and the optical absorption coefficient becomes:

${\rm{ \mathsf{ α} }} = - \left( {1/d} \right)\ln \left( T \right)$

(7)

The optical bandgap E_g^opt of a semiconductor is calculated using the interband absorption theory:

${\rm{ \mathsf{ α} }}h{{\rm{V}}_i} = A{\left( {h{{\rm{V}}_i} - E_g^{opt}} \right)^n}$

(8)

where ν_i is the incident photon energy, A is a probability parameter and n is the transition coefficient with n = 2 for indirect transition and n = 3/2 for direct bandgap that is the case of V₂O₅.

Overall, optical absorption spectra, electrical conductivity and electron paramagnetic resonance (EPR) spectra of V₂O₅ show a polaronic character. The polaron originates from the oxygen vacancies that result in an excess of electrons localized in the empty 3d orbitals of vanadium atoms. Consequently, V⁵⁺/V⁴⁺ pairs are closed to oxygen vacancies and result in an absorption in the infrared spectrum. The optical transmittance T_opt of V₂O₅ prepared by e-beam deposition at different oxygen partial pressure revealed a sharp increase of T_opt in the spectral range 550–600 nm. This is due to the fundamental absorption edge that shifts towards the higher energy with the increase in oxygen partial pressure. Figure 9 shows the plots of (αhν_i)^2/3 vs. photon energy for V₂O₅ deposited at T_s = 280 ℃ under various oxygen partial pressure. The high value of the optical bandgap E_g = 2.32 eV determined by extrapolation of the linear part of the graph to zero (α = 0) indicates that the films are stoichiometric for p(O₂) = 20 mPa, while oxygen vacancies are created at lower pressure.

The partial filling of oxygen vacancies results in a decrease of the concentration of electronic localized states and an increase of E_g. The overall process of oxygen loss can be expression by the Kroger notation:

$\begin{array}{l} {\rm{O}}_o^x \leftrightarrow {\rm{V}}_o^x + \frac{1}{2}{{\rm{O}}_2}\\ {\rm{V}}_o^x \leftrightarrow {\rm{V}}_o^ \bullet + e' \leftrightarrow {\rm{V}}_o^{ \bullet \bullet } + 2e' \end{array}$

(9)

Lourenço et al. ^[55] studied the effect of oxygen flow rate ϕ on the optical properties of r.f. sputtered V₂O₅ films. The highest E_g = 2.56 eV was obtained at low ϕ = 1 sccm while higher f produced films with an optical bandgap in the range 2.41–2.45 eV. Optical bandgap data of the literature for r.f. sputtered films are in the range 2.15–2.40 eV ^[56,139,140]. The evolutions of both the width of electronic localized states E_els and the bandgap E_g as a function of T_s and as a function of p(O₂) for e-beam evaporated V₂O₅ films are shown in Figure 10a, b respectively. These results show that stoichiometric films were obtained at p(O₂) > 5 mPa and T_s < 100 ℃ ^[79].

Song et al. ^[141] determined the work function of the annealed V₂O₅ films synthesized by VTOP-based ALD method at 135 ℃ using valence-band ultraviolet photoelectron spectroscopy (UPS). The results showed that the valence band maximum (E_v) is located at ∼2.45 eV below the Fermi level (E_F) and gave evidence of an indirect bandgap (E_g) of ~2.63 eV for V₂O₅ film annealed at 500 ℃ for 1 h in air. Irani et al. ^[89] investigated the structural and optical properties of nanostructured V₂O₅ thin films deposited by spray pyrolysis technique. The substrate temperature T_s appears to be the key growth along the (001) direction with an orthorhombic structure. It was pointed out that crystallites increased with elevating T_s, reached a maximum at T_s = 450 ℃ and decreased for T_s > 450 ℃. Optical characterizations showed that the highest transmittance occurs for T_s = 550 ℃ and the absorption edge shifts from 2.5 to 2.8 eV due to the formation of chemical bonds at the interface between V₂O₅ film and substrate.

Beke et al. ^[73] determined a bandgap of 2.52 eV for PLD V₂O₅ films prepared onto amorphous glass substrate with p(O₂) = 13.3 Pa and T_s = 220 ℃ but did not distinguish between indirect allowed (n = 1/2) and direct forbidden (n = 3/2) transitions. Luo et al. ^[142] reported the indirect allowed transition (n = 2) that leads a decrease of E_g from 2.29 to 2.02 eV with increasing T_s from 275 to 320 ℃ for d.c. magnetron sputtered films. Similar trends were obtained on PVD films ^[143]. Ramana et al. ^[72] investigated the effect of the grain size on the optical properties of PLD films. The increase of grain size from 0 to 300 nm produces a red shift of the absorption edge (decrease of E_g) from 2.47 to 2.12 eV following a parabolic behavior that is attributed to the imperfections at grain-boundary regions. The optical absorption was explored on e-beam deposited V₂O₅ films grown at 200 ℃ on glass substrate with various thicknesses (~0.8–1.2 µm) ^[84]. The authors elucidated the role of film thickness; the indirect optical bandgap E_g = 2.36 eV is found for thicker films. Optical constants of V₂O₅ films, i.e., refractive index n_opt, extinction coefficient k, and plasma frequency ω_p have been investigated by Akl ^[144]. Determination of the optical bandgap of 2.25 eV was obtained after roughness correction for d.c. magnetron reactive-sputtered α-V₂O₅ films after heating at 500 ℃ for 1 h in oxygen atmosphere ^[145]. Ellipsometry measurements were performed as a function of incident angles on (001) oriented sputtered films yield the values of the refractive index n = 2.67, optical absorption coefficient k = 0.0011 and thickness d = 165 nm ^[146].

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3.3. X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is the common technique to investigate the composition of the V₂O₅ surface and the oxidation states of elements ^{[79,97,147,148,149,150]}. The typical V 2p and O 1s XPS spectra recorded in the energy range of 510–540 eV for V₂O₅ thin films deposited by EBD technique at various T_s are displayed in Figure 11. The clear evolution of the spectral response corresponds to the V/O ratio in VO_x lattices. Analysis of the XPS spectrum of a V₂O₅ film deposited at T_s = 400 ℃ under oxidizing atmosphere p(O₂) = 0.01 mPa is presented in Figure 12. The binding energies of the V 2p levels are 516.9 and 524.5 eV for V 2p_3/2 and V 2p_1/2, respectively, with a splitting of 7.6 eV, whereas the binding energy of the O 1s level is 529.6 eV. For more detailed information, the background corrected spectrum was fitted by Gaussian profiles. Different V 2p and O 1s components are detected upon deconvolution by the fit by six Gaussian functions at binding energies of 514.8, 516.9, 520.8, 524.2, 529.7 and 532.5 eV that correspond to V⁴⁺ 2p_3/2, V⁵⁺ 2p_3/2, O 1s' satellite of O^2–, V⁵⁺ 2p_1/2, O 1s of O^2–, and O^–, respectively, as shown in Figure 12. From these results, the relative amounts of ions in different chemical states could be calculated by taking into account the atomic sensitivity factors: 1.41 for V 2p_3/2 and 0.63 for O 1s. Note that the energy splitting between V 2p_3/2 and V 2p_1/2 levels is Δ = 7.3 eV for stoichiometric VO_2.5 film.

The chemical composition and topography of V₂O₅ thin films at different steps of Li insertion/de-insertion have been investigated by quantitative XPS and AFM analyses ^{[151,152,153,154]}. From the first cycle, a solid electrolyte interface (SEI) layer has grown on the whole electrode surface, which consists of Li₂CO₃ aggregates. XPS analysis shows that various chemistries appeared during the cycle of charge/discharge such as partial dissolution of the initial Li₂CO₃ layer, residual deposit of ROLi (R = radical), PEO and oxalates. The overall dissolution/residue deposit process explains the capacity loss noticed at the end of the first cycle. Time flight secondary ion mass spectroscopy (ToF-SIMS) was carried out to analysize the ion distribution in V₂O₅ thin films ^[150,155].

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3.4. Vibrational spectroscopy

Among the local probes, Raman scattering (RS) and Fourier transform infrared (FTIR) spectroscopy are techniques sensitive to the short-range environment of oxygen coordination around the cations ^[156]. As a first approximation, the spectra consist of a superposition of the components of all local entities present in the same material in contrast to diffraction data, which give a weighted average. As a rule, the frequency and relative intensity of the bands are sensitive to coordination geometry and oxidation states ^[157]. Both FTIR and RS spectroscopies are also probing the stoichiometry of films (Figure 13). The vibrational spectra of V₂O₅, which crystallizes in the orthorhombic system (Pmmn space group) with D_2h¹³ spectroscopic symmetry, are composed of internal modes of the V₂O₅ layer in the ab crystal plane and the external mode in the low-frequency region (~144 cm^–1) ^[158]. V₂O₅ has the spectroscopic D₁₃^2h symmetry with the Wyckoff 4f position for the vanadium, vanadyl and chain oxygen atoms whereas the bridge oxygen occupies the Wyckoff 2a position ^[124]. The vibration modes associated to the D_2h factor group decomposed as: Γ_opt = 7A_g + 7B_1g + 3B_2g + 4A_3g + 3A_u + 3B_1u + 6B_2u + 6B_3u. According this decomposition, 21 modes of vibration are Raman active (7A_g + 7B_1g + 3B_2g + 4A_3g) and 18 modes are infrared active (3A_u + 3B_1u + 6B_2u + 6B_3u) ^[158,159].

Ex situ and in situ FTIR spectra of V₂O₅ crystalline films prepared by dip-coating from V-oxoisopropoxide sols have been investigated as a function of the degree of Li⁺ intercalation and analyzed in terms of dispersion analysis (LO and TO phonons) ^[160]. A diagnostic test to differentiate the safe region upon intercalation has been established by the red-shift of the V–O_A stretching frequency at 1016 cm^–1 and the disappearance of the bridging V–O_B–V stretching mode at 795 cm^–1.

Numerous studies of the Raman spectroscopy of V₂O₅ films have been performed on as-deposited, heat-treated, and Li-intercalated samples ^[9,99,161]. The typical Raman spectrum exhibits more or less well-defined internal and external modes, indicative of the film purity and crystallization (Figure 13b). The internal modes (in the high-frequency region) are the V = O bending vibration at 283 and 405 cm^–1 and the bending vibration of the bridging V–O–V bonds at 485 cm^–1, the V–O stretching mode at 525 cm^–1 that results from the corner-shared oxygen atoms, and the vanadyl terminal oxygen stretching mode at 993 cm^–1. The external modes are generated by the motions of two V₂O₅ layers of the elementary unit cell; these translational vibrations occur at 144 and 196 cm^–1 (skeleton vibrational modes) ^[71,76]. The RS spectra of V₂O₅ films deposited by flash-evaporation onto silicon substrate show frequency shift of the vanadyl mode at 995 cm^–1 (due to the shortest (1.58 Å) V = O double chemical bond stoichiometry) in relation with the stoichiometry of the film ^[9,161]. Ramana et al. ^[71] reported a frequency shift of the vanadyl vibration at 993 cm^–1 with the substrate temperature for V₂O₅ films prepared by PLD technique at 200 ≤ T_s ≤ 500 ℃, which is attributed to the stress developed in the film. As XRD patterns indicate that the films deposited at T_s > 200 ℃ are polycrystalline with a preferred orientation along the c-axis, the shift of the vanadyl mode suggests a structural organization that introduces a stress associated to a compression in the layered structure. Raman spectroscopy was utilized to characterize the microstructure of V₂O₅ films prepared by d.c. magnetron sputtering on Si (111) wafer. Optimized films with a mere desirable layer-like lattice were obtained for a gas flow ratio O₂/Ar of 3/2 ^[162]. Several works investigated the microstructure of amorphous V₂O₅ thin films by means of Raman ^[30,163,164] and FTIR ^[75] spectroscopy. Lee et al. ^[163] reported the spectral fingerprint of as-deposited amorphous V₂O₅ films, which exhibit two broad bands at ca. 520 and 650 cm^–1 assigned to the stretching modes in a disordered V–O–V lattice. The absence of the skeleton mode at 144 cm^–1 is due to the lack of long-range order. Flash-evaporated V₂O₅ thin films at 25 ≤ T_s ≤ 200 ℃ display a poor crystallization observed by the Raman features, which result in the appearance of the intense external mode at 144 cm^–1 and ill-resolved internal Raman modes in the high-frequency region ^[30].

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3.5. Electrical properties

V₂O₅ is a n-type semiconductor. Its semiconducting properties are mainly due to the oxygen non-stoichiometry, which are the consequence oxygen vacancies V₂O_5–δ. The electrical conductivity of V₂O₅ thin films have been investigated by means of d.c. four-point probe method and a.c. impedance spectroscopy as a function of the deposition parameters and post-annealing treatment ^[165]. The temperature dependence of the conductivity follows an Arrhenius law in a large range of temperatures. The general condition for semiconducting behavior is that the transition-metal ion could exhibit several valence states such as V⁴⁺ to V⁵⁺ in V₂O₅, so that electron hopping takes place between these two levels. If C = V⁴⁺/(V⁴⁺ + V⁵⁺) is the ratio of the concentration of low valence ions with the total concentration of transition metal ions and A is the average hopping distance, one can estimate the number of charge carriers N = CA^–3. Using C ~ 0.02 and A = 0.384 nm, the carrier mobility is quite low lying between 10^–7 and 10^–2 cm² V^–1 s^{–1 [166]}. Murawski et al. showed that the electrical transport arises from thermally activated hopping that takes place between the electronic states for V₂O₅ films made by PVD at 840 ℃ ^[167] and for amorphous V₂O₅ films obtained by flash evaporation ^[31]. In V₂O₅, the V⁴⁺/V⁵⁺ ionic pair is bound to positively charged defects, i.e., oxygen vacancies. The removal of an electron from the vacancy and produce a free carrier requests an energy e²/κd. The electrical conductivity is usually described in terms of Mott's theory for electronic transport in transition-metal glasses ^[168]:

$\sigma (T) = C(1 - C)\frac{{{\nu _0}{e^2}}}{{A{k_B}T}}\exp \left( { - 2\alpha A} \right)\exp \left( {\frac{{ - W}}{{{k_B}T}}} \right)$

(10)

where ν_o is a phonon frequency, and α is the rate of the wave function decay. The thermal activation energy can be expressed as

$W = {W_h} + {W_d}/2$

(11)

where W_h corresponds to the small polaron formation and W_d to the disorder energy arising from the energy difference of neighboring sites. The separation of W into a polaron and a disorder term is a rather difficult task, which can be partly solved using data taken at low temperatures. Julien et al. ^[166] reported that oxygen-deficient V₂O₅ films grown at high substrate temperature (T_s ≈ 250 ℃) are highly conductive with σ_e = 10^–2 S cm^–1 due to the quenching rate ΔT of the films.

Figure 14a, b present the Arrhenius plots of the electrical conductivity σ and the activation energies of flash-evaporated films as a function of the substrate temperature and annealing conditions. These results show that (ⅰ) the σ of films deposited at T_s = 25 ℃ is always lower than that of films deposited at higher temperature even after a heat treatment, and (ⅱ) there is a hierarchy in the conductivity such as σ_ad < σ_arg < σ_ox, where σ_ad, σ_arg and σ_ox are the conductivities of films as-deposited, annealed in Ar, and annealed in O₂, respectively. The resistivity of an as-grown film at T_s = 25 ℃ is ρ = 5 × 10⁴ Ω cm with an activation energy E_a = 0.43 eV, whereas that of a film heat-treated in O₂ atmosphere is ρ = 0.62 Ω cm with E_a = 0.15 eV. We expect that C is a function of T_s and T_a due to the preferable desorption of oxygen during the growth. The increase in σ is attributed to the primary consequence of the structural modification from amorphous to polycrystalline state, and from oxygen deficiency of films grown with smaller O/V ratio at high substrate temperature. Similar effect was observed on the activation energy of the electrical conductivity which decreases with the increase of T_s. Sanchez et al. ^[167] reported a conductivity of 1.4 × 10^–5 S cm^–1 at 25 ℃ with an activation energy of W = 0.41 eV and a disordered activation energy W_d = 0.16 eV for flash-evaporated films. Kumar et al. ^[82] reported that thermally deposited films at T_s = 300 ℃ exhibit an electrical resistivity of 3.6 Ω cm. Luo et al. ^[142] studied the impact of the substrate temperature on the electrical properties of sputtered nanostructured films having semiconducting character. Four-point probe measurements show that the square resistance of film decreases exponentially from 46 to 33 kΩ/□ with the increase of T_s from 230 to 320 ℃.

Rosaiah et al. ^[83] reported the effect of T_s on the electrical conductivity of e-beam deposited V₂O₅ films onto ITO-coated flexible Kapton substrate. They showed that σ increases from 2 × 10^–6 to 3 × 10^–2 S cm^–1 by the increase of T_s from 30 to 300 ℃. Crystalline V₂O₅ thin films (26 nm grain size) deposited by thermal evaporation and annealed at 500 ℃ exhibited thermoelectric properties with a maximum Seebeck coefficient of –218 µV K^–1 and electrical conductivity of 0.055 S cm^{–1 [28]}. The electrical conductivity was ~10^–7 S cm^–1 with a high activation energy E_a = 0.9 eV for sputtered V₂O₅ films (85 nm thick) obtained in O₂/Ar ratio of 20% ^[169]. Wang et al. ^[170] demonstrated that a rather thick (200 nm) crystalline V₂O₅ films can work properly as hole-extraction slab for organic photovoltaic devices. V₂O₅ thin films grown via r.f. reactive sputtering on fused silica substrates showed good electrical response for use as sensors methane and propane (50–3000 ppm) with an activation energy of 0.153 eV and a CPE exponent of n = 1.046, which approximates a CPE due to Debye-like capacitor ^[171].

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3.6. Intercalation of V₂O₅ films

The Li⁺ intercalation into V₂O₅ films has been widely studied on specimens grown on various substrates using all deposition techniques described above. The insertion/de-insertion process occurs in a large potential window 3.5–1.5 V vs. Li⁺/Li according the reaction:

${{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}\left( {{\rm{orange}}} \right) + x{\rm{L}}{{\rm{i}}^ + } + x{{\rm{e}}^ - } \leftrightarrow {\rm{L}}{{\rm{i}}_x}{{\rm{V}}_2}{{\rm{O}}_5}\left( {{\rm{green}}} \right)$

(12)

where x is the molar fraction of ions and electrons inserted in the host lattice. About 3e^– per mole of V₂O₅ could be reached at the end of the discharge (1.5 V), providing a theoretical specific capacity of 442 mAh g^–1. The insertion of 3Li⁺ implies the formation of several lithiated phases during discharge with the transitions α→ε (0 < x < 0.5), ε→δ (0 < x < 0.5), δ→γ (0 < x < 0.5) that are characterized by voltage plateaus at 3.4, 3.2 and 2.3 V vs. Li⁺/Li, respectively. This is illustrated in Figure 15a for film formed by 50-nm grain size cycled at C/20 rate (20 mA g^–1 current density). The corresponding cyclic voltammogram is shown in Figure 15b. The irreversible ω-Li₃V₂O₅ is formed at the end of the discharge (1.85 V). Subsequent charge appears as a smooth S-shaped voltage profile with a mid-voltage at 2.67 V. As displayed in Figure 15b, four distinctive peaks are observed at 3.30, 3.05, 2.15 and 1.85 V vs. Li⁺/Li during a cathodic scan, which is a multi-step Li⁺-ion intercalation process. The corresponding phase transforms from α-V₂O₅ to ε-Li_0.5V₂O₅ (3.30 V), δ-Li₁V₂O₅ (3.05 V), γ-Li₂V₂O₅ (2.15 V) and ω-Li₃V₂O₅ (1.85 V) represented by the relations:

${{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}} + 0.5{\rm{Li}} + 0.5{{\rm{e}}^ - } \to {\rm{L}}{{\rm{i}}_{0.5}}{{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}$

(13)

${\rm{L}}{{\rm{i}}_{0.5}}{{\rm{V}}_{\rm{2}}}{{\rm{O}}_5} + 0.5{\rm{L}}{{\rm{i}}^ + } + 0.5{{\rm{e}}^ - } \to {\rm{Li}}{{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}$

(14)

${\rm{Li}}{{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}} + 1{\rm{L}}{{\rm{i}}^ + } + 1{{\rm{e}}^ - } \to {\rm{L}}{{\rm{i}}_{\rm{2}}}{{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}$

(15)

${\rm{L}}{{\rm{i}}_{\rm{2}}}{{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}} + 1{\rm{L}}{{\rm{i}}^ + } + 1{{\rm{e}}^ - } \to {\rm{L}}{{\rm{i}}_{\rm{3}}}{{\rm{V}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}}$

(16)

In the following anodic scan, one peak is recorded at 2.67 V vs. Li⁺/Li corresponding to the delithiation of the ω-Li₃V₂O₅ phase.

The early work of Park et al. ^[172] showed that spin-coated V₂O₅ xerogel films can accommodate up to 3.3 moles Li/mole V₂O₅ in the potential range 3.8–1.9 V vs. Li⁺/Li. The films are highly reversible hosts and deliver a specific energy density 1137 Wh kg^–1 and specific capacity 1682 C g^–1. The Li⁺ insertion into V₂O₅ films deposited by CVD, rf-sputtered and ALD techniques onto various substrates such as Pt, Ti, stainless steel, glass and F-doped SnO₂ was studied by Groult et al. ^{[34,35,126,106,108]}. Films grown on stainless steel and heat treated at 350 ℃ show a discharge capacity of 115 mAh g^–1 at C/23 rate in the potential range 3.8–2.8 V vs. Li⁺/Li. The best performances (200 mAh g^–1 at C/23 rate) were obtained in the electrochemical window 3.8–2.2 V for V₂O₅ deposited onto Ti foil. Jourdani et al. ^[173] reported an increase of the optical absorption edge from 2.3 to 2.8 eV and the (110) Bragg reflection shift from 2θ = 24.8° to 25.2° for spin coated V₂O₅ intercalated at –0.4 V vs. SCE.

The structural evolution of Li-intercalated V₂O₅ films has been investigated by in situ and ex situ XRD, FTIR and Raman spectroscopy ^{[174,175,176]}. Meulenkamp et al. ^[177] investigated the different phases of lithiated sol-gel V₂O₅ films in the range 0 ≤ x ≤ 1.4 using in situ X-ray diffraction measurements and compared the structural changes with those of V₂O₅ films prepared by other techniques. Spectroscopic studies of Li-intercalated V₂O₅ polycrystalline films have been reported by several groups ^{[71,174,178,179,180]}. Cazzanelli et al. ^[99] analyzed the Raman spectra of xerogel films intercalated with lithium that showed a significant effect of the water content in the interlayer space inducing new Raman bands at 800 and 950 cm^–1. Julien et al. ^[174] investigated the vibrational modifications of Li intercalated V₂O₅ films deposited by flash evaporation. They proposed that the tensile stress in the Li_xV₂O₅ film affects the Raman skeleton mode which shifts from 145 to 154 cm^–1 due to an increase in the restoring force (Figure 16). Jung et al. ^[175] confirmed this hypothesis by the correlation of the spectral shift of the Raman mode at 145 cm^–1 with the stress change as a function of the degree of intercalation in the Li_xV₂O₅ lattice.

Micro-Raman spectroscopy was used to study of electrochemically intercalation Li_xV₂O₅ (0 ≤ x ≤ 1.8) samples obtained from V₂O₅ films deposited by various techniques, i.e., ALD and r.f. sputtering. Results showed the phase transformation from α-Li_xV₂O₅ to γ-Li_xV₂O₅ via ε and δ phases. A linear correlation exists between the V–O₁ stretching (vanadyl) mode frequency and the V–O₁ bond length in V₂O₅ ALD film ^[179,180]. Careful examination of the high-frequency region of RS spectra of Li_xV₂O₅ shows that, for the Li-rich phase (0 ≤ x ≤ 0.5), the vanadyl stretching mode at 984 cm^–1 splits into two components, the first one at 975 cm^–1, the second at 957 cm^–1 for x ≥ 0.7. This is due to the evolution of the local environment of oxygen atoms, i.e., the continuous increase of the interlayer distance with x and it reflects the existence of two different lithium sites ^[159]. Ramana et al. ^[71] analyzed the Raman features of the Li_xV₂O₅ films prepared by PLD technique in the range 0.00 ≤ x ≤ 1.25. Crystalline V₂O₅ thin films synthesized by means of r.f. sputtering without post-heating treatment were obtained with different crystallographic preferential orientations by changing the growth parameters (T_s and deposition rate) as follows: (ⅰ) h00-oriented films (ab planes perpendicular to the substrate) were prepared at T_s = 300 ℃ for short time t_d, (ⅱ) longer deposition time at low T_s leads to 110-oriented films and (ⅲ) the 001-oriented films were obtained at high T_s irrespective on t_d ^[45]. Electrochemical tests of the h00-oriented films in Li/EC-DEC–LiPF₆/V₂O₅ cells showed specific discharge capacities > 300 mAh g^–1 in the potential range 3.8–1.5 V, with a capacity fading of 3% at C rate over 70 cycles.

Numerous studies of the electrochemical insertion reactions of Li⁺ ions in the V₂O₅ film lattice were tested in Li cells with aprotic electrolyte such as 1 mol L-1 LiClO₄ in propylene carbonate (PC) solution or 1 mol L^–1 LiPF₆ in ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1) solution ^{[58,181,182,183,184,185,186]}. Park et al. ^[187] investigated the discharge capacity of V₂O₅ film cathode as a function of the thickness. Films (230 nm thick) obtained by sputter deposition at power of 100 W in working pressure of 1.3 Pa delivered a specific capacity of ~105 µAh cm^–2 s^–1 at current density 500 µA cm^–2 s^–1. Lithium microcells with optimized V₂O₅ films sputtered in O₂/Ar (50/50) atmosphere were investigated by Yoon et al. ^[58]. When cycled in the range 1.5–3.7 V vs. Li⁺/Li, the cell delivered a specific capacity of 125 µAh cm^–2 µm^–1 after a first cycle of formation. Nano-porous crystallized V₂O₅ films (~35 nm crystallite size) fabricated by gel electrodeposition with the addition of block copolymer Pluoronic P123 delivered a specific discharge capacity of 240 mAh g^–1 at 300 mA g^–1 current density after 40 cycles ^[188]. Electrochemical studies of V₂O₅ films deposited by spin coating of vanadium-tri(isopropoxide)oxide gel show that the specific capacity and Li⁺ diffusion coefficient increased from 4.2 × 10^–12 to 6.8 × 10^–10 cm² s^–1 in the non-stoichiometric films ^[189]. Nanostructured V₂O₅ thin films made by anodic deposition from V₂O₅/H₂O₂ solution displayed a large discharge capacity of 596 mAh g^–1 at a current density of 1.08 A g^–1 and a fading rate of 1% per cycle in a cell using 1 mol L^–1 LiClO₄ in PC as the electrolyte and a Pt plate as the counter electrode ^[190]. The V₂O₅ thin film cathode with thickness of ~30 nm fabricated using biological templates delivered specific capacities of ~12 µAh cm^–2 which is 7–8 times higher than that of planar V₂O₅ cathodes ^[191]. The effects of ageing on Li intercalation in V₂O₅ films, i.e., the evolution of the SEI, were investigated by a combination of techniques such as cyclic voltammetry, XPS, RBS and AFM for more than 3000 cycles ^[192].

Kinetics of Li⁺ ions in the lattice of V₂O₅ films has been widely studied. Li-ion transport, i.e., ionic conduction (σ_ion) and diffusion coefficient (D_Li) have been investigated using cyclic voltammetry, galvanostatic titration and electrochemical impedance spectroscopy (EIS) ^{[106,193,194]}. D_Li varies in the range 6 × 10^–10–2 × 10^–11 cm² s^–1 in sol-gel crystalline V₂O₅ thin films (1–3 µm thick) obtained by impedance spectroscopy ^[195]. Miyazaki et al. ^[196] investigated D_Li of sputtered films oriented along the a-and b-axis using the chronoamperometry method. Kinetics along the a-axis D_Li ≈ 10^–11 cm² s^–1 was larger than along the b-axis 10^–13 ≤ D_Li ≤ 10^–14 cm² s^–1. McGraw et al. ^[197] obtained D_Li vs. x(Li) in amorphous V₂O₅ films by PITT measurements on an h00 and 110 oriented films made by PLD method. D_Li was initially 5 × 10^–13 cm² s^–1 and decreased steadily from 1.2 × 10^–13 cm² s^–1 at x = 0.4 to 5.52 × 10^–14 cm² s^–1 at x = 1.5 in Li_xV₂O₅. Lantelme et al. ^[106] analyzed the phase transition process controlled by diffusion in V₂O₅ films deposited by CVD method. The component diffusion was estimated as a function of the film thickness: D_Li = 0.48 × 10^–12 cm² s^–1 for a semi-infinite configuration. D_Li was also estimated from both PITT and EIS measurements using a semi-infinite diffusion model ^[198]. Evolution of the diffusion coefficient D_Li in V₂O₅ film (160 nm thick) as a function of the electrode potential is shown in Figure 17. This figure indicates a good agreement between the two techniques.

Pyun and Bae reported a decrease of the diffusion coefficient of xerogel films from 10^–10 to 10^–12 cm² s^–1 as the electrode potential fell from 3.0 to 2.2 V vs. Li⁺/Li ^[193]. The transport mechanism was described as a transition from the diffusion-controlled insertion to an interfacial-controlled reaction due to a change of the lattice versus water in V₂O₅·nH₂O xerogel. Ultrathin (17 nm) V₂O₅ thin were fabricated by spin coating and annealed at at 250 ℃ for 3 h in a muffle furnace. In the potential range, these films deliver a specific capacity of 262 mAh g^–1 corresponding to Li_1.78V₂O₅ when discharge in the potential range from +1 V to –1 V vs. SCE. The capacity loss is 0.03% per cycle over 600 cycles ^[199]. The effect of porosity on anomalous behavior of the Li intercalation into V₂O₅ film prepared by the polymer (Tergitol from Sigma-Aldrich Co.) surfactant templating method has been studied by AFM and EIS ^[200]. The diffusion impedance Z_d, which displays a constant phase element (CPE) response can be expressed by:

${Z_d} = \frac{L}{{nFA{D^{1/2}}}}\left( {\frac{{\partial E}}{{\partial c}}} \right){\left( {j\omega } \right)^{ - \alpha }}$

(17)

where L is the film thickness, n the valence of inserted ion, A the active surface, F the Faraday constant, ∂E/∂C the slope of the coulometric titration and α_d the CPE exponent. Analysis showed that α_d is constant in the frequency range 0.63–0.10 Hz is significantly affected by the surface roughness and the formation of pores with a large size. The chemical diffusion coefficient of 1 × 10^–11 cm² s^–1 was suggested. Thin films fabricated by electrostatic spray deposition (ESD) followed by heat treatment at 350 ℃ in air demonstrated a high energy efficiency. Ultra-high rate capability is obtained at 56C rate with a specific capacity of 86.7 mAh g^–1 in the potential range 2.5–4.0 V vs. Li⁺/Li corresponding to the transitions of three phases: α ↔ ε ↔ δ. When cycled below 2.5 V, these ESD films have poor electrochemical performance due to irreversible structure degradation of the γ-phase ^[201,202]. Mui et al. ^[203] established a relationship between microstructure and kinetics in as-deposited and heat-treated sputtered V₂O₅ thin-film cathodes. D_Li was determined by the GITT method for two submicrometric films: finer-grained 80 nm size and coarsed-grained 250 nm size. Both samples display similar values of D_Li of 1 × 10^–11 for the α-Li_0.03V₂O₅ phase and 5 × 10^–12 cm² s^–1 for the ε-Li_0.35V₂O₅ and δ-Li_0.9V₂O₅ phases, whereas D_Li in finer-grain film decreases to 1 × 10^–13 and 1 × 10^–15 cm² s^–1 for the α–ε and ε–δ two-phase systems, respectively. Li⁺ diffusion coefficients of electrochemically inserted V₂O₅ films (0.6–3.6 µm thick) deposited by r.f. sputtering with h00 or 110 preferred orientation was studied using EIS measurements ^[194,204]. The easy diffusivity of Li⁺ ions (2 × 10^–11≤ D_Li ≤ 3 × 10^–10 cm² s^–1) along the b-direction is due to the lattice expansion along the c-axis. The plot D_Li vs. x(Li) displays two minima at x = 0.14 and 0.6 that confirm the results of Lu et al. ^[198]. An energy barrier of 0.98 eV along the b-direction was found from the Arrhenius plot of D_Li vs.1/T.

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4. Doped V₂O₅ films

The poor electrochemical performance of V₂O₅ films during long term cycling attributed to low electronic conductivity can be improved by replacing a dopant for one of the V atoms forming V⁴⁺ cations ^[205,206]. Grégoire et al. ^[207] showed that octahedral chains are formed with metal dopant atoms making stable the structure during electrochemical process. The EXAFS analysis of Cu-and Zn-doped V₂O₅ showed that the environment of vanadium atoms is not altered by the doping atoms (M), while new V–O–M interactions are triplets ^[208]. The enhanced stability of M-doped vanadium pentoxide films used as electrode for lithium microbatteries and electrochromic devices comes from the distorted V₂O₅ layered structure. For example, the introduction of Ti dopant in spin-coated V₂O₅ films results in a disturbance of the diffraction peaks of (001) and (002) planes. The formation of V–O–Ti bond opens the interlayer space, distorts the lamellar structure and reduces the oxidation state of V ^[209]. Numerous metallic ions have been investigated as dopants, including Ag, Fe, Mo, Ta, Mg, Mn, Ti ^{[17,66,93,202,209,210,211,212,213,214]}.

Dip-coated Ag-doped V₂O₅ xerogel films with a molar ratio Ag/V in the range 0.005–0.5 showed an increase of the electronic conductivity by 2–3 orders of magnitude that allowed an insertion capacity of 4 Li per mole of Ag_xV₂O₅ with an excellent reversibility ^[215]. Ag_yV₂O₅ thin films were prepared by rf magnetron co-sputtering in 14% partial oxygen pressure pO₂ = 14% from metallic Ag and V₂O₅ targets. Ag_0.26V₂O₅ films were amorphous and showed improved intercalation properties due to the porosity and the additional influence of Ag⁺ ions to the redox reaction ^[205]. Nam et al. ^[216] investigated the electrochemical properties of copper-doped V₂O₅ thin film prepared by d.c. reactive magnetron co-sputtering within atmosphere O₂:Ar ratio of 10:90. These films were tested as cathode materials in Cu_xV₂O₅/Lipon/Li cell and showed stable cycleability beyond 500 cycles with average capacity of 50 µAh cm^–2 µm^–1. Mo-doped V₂O₅ films were grown by thermal evaporation technique onto Ni substrates maintained at T_s = 250 ℃ and applied as electrode in supercapacitor with 1 mol L^–1 KCl solution electrolyte. A maximum specific capacity of 175 mF cm^–2 was obtained for 4 at.% Mo-doped V₂O₅ films ^[17]. The effect of the annealing temperature on the structure of Mo-doped V₂O₅ films electrochemically deposited resulted in the increase of the interlayer distance from 1.16 nm for as-deposited to 1.38 nm for film annealed at 250 ℃ that enhanced the Li⁺ ion motion in the electrochromic matrix ^[217].

Tantalum-doped V₂O₅ thin films were deposited by sol-gel dip-coating method using anhydrous isopropyl alcohol as solvent ^[211]. 5 mol% Ta doped films as counter-electrode for electrochromic device exhibited a charge density of 70 mC cm^–2 with high stability up to 1500 cycles. Mg-doped V₂O₅ films deposited by RF magnetron reactive sputtering onto SnO₂-coated glass substrates were studied for various amount of dopant. Low Mg content (2 at.%) enhanced the specific discharge capacity and the diffusion coefficient of Li⁺ inserted ions, while high Mg content (15 at.%) favored the electrochromic window and the switching response time ^[212].

Molybdenum acts as a n-type dopant in V₂O₅ that results in an increase of the carrier concentration and a shift of the absorption edge towards shorter wavelengths ^[93]. Several techniques were used to prepare V₂O₅ doped with Mn such as electrodeposition ^[218], e-beam evaporation ^[219], and dip-coating ^[213].

Mn-doped V₂O₅ films were fabricated by dip-coating route with post-annealing at 250 ℃ in air for 3 h using ~1.67% of Mn(Ac)₂·4H₂O as Mn²⁺ source. Structural and elemental analyses showed that: (ⅰ) Mn doping suppresses the orthorhombic symmetry, (ⅱ) V⁴⁺ and V⁵⁺ coexist, (ⅲ) the formation of oxygen vacancies is due to the increase in the Mn²⁺ cation size. The suggested formula was (Mn, V)₂O_4.74(V_Ö)_0.26. The intercalation/deintercalation process is enhanced by the oxygen vacancies that eliminate the phase transition and reduce the irreversible capacity ^[182,213].

Sn-doped V₂O₅ films were obtained by drop-casting and further heat treatment at 450 ℃ in air for 2 h. As measured by XPS experiments, the 10% Sn⁴⁺ ions accommodation is compensated by the reduction of V⁺⁵ to V⁴⁺ ions. The introduction of Sn atoms causes a slight expansion of the unit volume due to the small lattice expansion along a-and c-direction, which indicates the presence of Sn⁴⁺ ions between the VO₅ layers, forming SnO₆ octahedra ^[220]. Ce-doped V₂O₅ films prepared by sol-gel route and annealed at 400 ℃ showed a phase transition from α-V₂O₅ orthorhombic to β-V₂O₅ tetragonal structure. Film doped with 1 mol.% Ce exhibited the best ion storage capacity of ~207 mC cm^{–2 [221]}. Titanium doped V₂O₅ films have been widely investigated as intercalation hosts for electrochromic devices ^{[209,214,222,223,224,225]}. Sol-gel dip-coated (100 – x)V₂O₅·xTiO₂ films as electrochromic electrodes were formed with Ti content up to 20 mol.% by reacting vanadium and titanium tetra-isopropoxide as precursors ^[214]. The best results were obtained for the proper amount of Ti dopant (5 mol.%) showing that 550 nm-thick films switched with a coloration from pale yellow to a greenish state when reduced with 5.4 mC cm^–2 of lithium. Wei et al. ^[209] reported a remarkable improved stability of the WO₃-based electrochromic devices using Ti-doped V₂O₅ films. The device utilizing the optimized V₂O₅:Ti electrode (V:Ti = 2:1) lasted 200,000 cycles in the transmittance range 2–62% without degradation. An increase of the Li intercalation capacity from 14 to 27 mC cm^–2 for 5% Ti-doped V₂O₅ films prepared by spin coating of inorganic sol-gel precursors was attributed to the non-stoichiometry and the smaller grain size ^[222]. Lee and Cao ^[223] demonstrated an Li⁺ intercalation performance improved by 100% in 20 mol.% Ti-doped V₂O₅ polycrystalline films that results from interaction force between adjacent V₂O₅ slabs. Highly stable V₂O₅–TiO₂ films fabricated by sol-gel electrodeposition ^[224,225] were proposed as electrochromic windows using PProDOT-Me₂ that is poly (3, 3-dimethyl-3, 4-dihydro-2H thieno[3, 4b]^[1,4]-di-oxepine). Tungsten-doped V₂O₅ films were deposited by rf magnetron sputtering on non-alkali glass substrate. Due to their good thermal insulation function, these films were applied as self-cleaning windows or anti-fogging glasses. The optimal thermal insulation temperature is 19.3 ℃ for 3 wt.% W-doped V₂O₅ thin films ^[226].

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5. Nanocomposite films

V₂O₅-based nanocomposite films have been fabricated for several applications, such as electrochromic devices and gas sensors. Such a nanocomposite was obtained with V₂O₅ obtained from an aqueous solutions of equimolar vanadium chloride and ammonium tungstate and F-doped SnO₂-coated glass substrates maintained at T_s = 400 ℃. Coloration efficiency (CE) of ~49 cm² C^–1 was obtained for a film mixed with 15% WO₃ which is stable up to 1000 cycles ^[227]. Thin (240 nm) and thick (485 nm) V₂O₅–MoO₃ films were deposited on conducting (SnO₂:F) glass by e-beam (3 kV) and thermal vacuum deposition (EBD and TVD, respectively) for opto-electronic applications ^[228]. A specific capacity of 80 µAh cm^–2 s^–1 after 70 cycles in the potential range 1.5–3.9 V vs. Li⁺/Li was delivered by MoO₃–V₂O₅ nanocomposite thin film electrodes deposited by r.f. sputtering from dual targets ^[229]. Mixed V₂O₅–WO₃ thin films were deposited by pulsed spray pyrolysis technique onto glass ^[227]. Sol-gel deposited (1 – x)WO₃·xV₂O₅ films at x = 0.035 showed the best electrochromic performance with a brownish blue ^[230]. Thin films of (V₂O₅)_1–x·(MoO₃)_x (0 ≤ x ≤ 1) were fabricated by e-beam evaporation in p(O₂) = 2 × 10^–2 mPa and T_s = 150 ℃. The optical bandgap increases and the electrical conductivity decreased with the increase of composition x ^[231]. V₂O₅–28 at.% MoO₃ TVD thick films deposited onto borosilicate glass were amorphous, while thin films were crystalline (crystallite size 38 nm) with strong reflections along (131), (160) and (270) direction. The bandgap varies linearly with the atomic wt.% of MoO₃ in V₂O₅–MoO₃ EBD thin films, i.e., from 2.35 eV for 10 wt.% to 2.70 eV for 90 wt.%. Composite V₂O₅ xerogel films assembled with polypyrrole (PPy) showed high electrical conductivity and fast electrochemical response that makes them candidates for redox sensor in flow injection analysis ^[232]. Graphene/V₂O₅/MoO₃ nanocomposite film was synthesized by dip-coating via sol-gel preparation of the precursors. This composite shows fast switching electrochromic performances with bleaching time of 1.25 s and the coloration time of 1.40 s ^[20]. V₂O₅/graphene nanocomposite films prepared by direct intercalation method using V₂O₅ sol and graphene were developed for their use in fast switching electrochromic devices. The intercalation of graphene improves the stability and the optical reversibility of the films, i.e., 1.5 times larger than that of V₂O₅ xerogel, due to the fast electron mobility of 1.5 × 10⁴ cm² V^–1 s^–1 in graphene ^[233]. Thin film electrodes for supercapacitors were made of V₂O₅/carbon nanotubes composites. A symmetric capacitor formed by identical V₂O₅/CNT electrodes operates in a wide potential range of 1.6 V in aqueous electrolyte delivering a volumetric energy density of 41 Wh dm^{–3 [234]}. V₂O₅ xerogel films were modified by poly(ethylene-oxide) (PEO). The inserted structure is modified the interaction of hydrogens with oxygens of the V–O bonds that shields the interaction between Li⁺ ions and V₂O₅ slabs resulting in an improved anodic electrochromic property ^[235].

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6. Applications

The early potential applications of nanostructured V₂O₅ were found in the field of lithium-ion batteries ^[236,237], thin-film microbatteries ^[12,108,238], actuators ^[239], catalysis ^[240], humidity sensors ^[241], electrochromic display devices ^[139,242], optical switching memories ^[243], antirelection coating ^[81].

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6.1. Li microbatteries

Since the work of Bates et al. ^[12], V₂O₅ thin films have been extensively investigated as positive electrodes (so-called cathodes) in rechargeable lithium microbatteries constituted by a lithium metal anode, an amorphous inorganic electrolyte and amorphous or crystalline V₂O₅ ^[244]. The most popular solid-electrolyte thin film Li_2.9PO_3.3N_0.46 (named Lipon) was deposited by r.f. magnetron sputtering of Li₃PO₄ in N₂ ambient. Its ionic conductivity is 2 × 10^–6 S cm^–1 at 25 ℃ ^[245,246]. A typical solid-state battery (SSB) is built by 1 µm thick amorphous V₂O₅ film deposited at 25 ℃, a 1 µm thick solid electrolyte and 5 µm thick lithium film (5 times in excess) ^[247,248]. Currently, thin-film cells are evaluated at different current densities and their delivered capacity is expressed by surface and thickness in mC cm^–2 µm^–1 or µAh cm^–2 µm^–1. Considering the theoretical specific capacity of 440 mAh g^–1 for Li_xV₂O₅ with uptake of 3 Li per formula and the density d = 3.36 g cm^–3 of V₂O₅, the theoretical specific capacity is Q_th = 147.8 µAh cm^–2 µm^–1 for a film as dense as the crystalline material. The schematic cross-section of a thin-film lithium microbattery is presented in Figure 18.

Prior Li//V₂O₅ cell cycled in the potential window 3.6–1.5 V delivered a capacity ~100 µAh cm^–2 µm^–1 at 10 µA cm^{–2 [12]}. Vanadium oxide films (VO_x) prepared by several techniques such as PECVD, PLD, rf-sputtering at the National Renewable Energy Lab. (USA) exhibit high discharge capacity and are highly stable during electrochemical cycling. Both crystalline and amorphous PLD V₂O₅ films (T_s = 200 ℃) deliver a discharge specific capacity of 380 mAh g^–1, while for the most stable films prepared by PECVD and exhibiting an O/V ratio close to that of V₆O₁₃ (0.5 μm thick) the discharge capacity exceeds 408 mAh g^–1 over 4400 cycles. These results indicate that the PECVD technology is very attractive for manufacturing rechargeable lithium microbatteries ^[62,67]. Typical experimental conditions for the deposition of a thin-film battery made by r.f. sputtering are summarized in Table 2 ^[249,250]. A solid-state film cell including a crystalline V₂O₅ cathode (2.4 µm thick), a Lipon electrolyte film (1.4 µm thick) and a lithium metal film (3.5 µm thick) operated in the potential 3.8–2.15 V vs. Li⁺/Li. Figure 19 displays the discharge-charge profiles of the SSB at 10 µA cm^–2 current density. After the first cycle of formation (SEI formation), a stable specific capacity of 30 µAh cm^–2 was obtained with a cathode film 2.4 µm thick ^[250].

Optimized conditions for rf-sputtering deposition (T_s = 250 ℃, P_w = 50 W) provide discharge capacity of 30.2 µAh cm^–2 µm^–1 with two potential plateaus at 3.5–3.6 V that corresponds to the Li uptake 0 < x < 1 in Li_xV₂O₅. Assuming a gravimetric density of 2.5 g cm^–3 for rf-sputtered films, the discharge capacity is 121 mAh g^–1. A capacity loss of 0.0076% per cycle was reported over 2000 cycles ^[251]. V₂O₅ thin films obtained by electrostatic spray-deposition (ESD) onto Pt substrates from triisopropoxy-vanadium oxide in 0.05 mol L^–1 ethyl alcohol solution were tested in Li cells with 1 mol L^–1 LiClO₄/propylene carbonate (PC) as electrolyte. The crystallized films annealed at 275 ℃ exhibit a good cycleability and high capacity of 270 mAh g^–1 at a current rate of 0.2C. After 25 cyles at 1C rate the capacity remains stable at 260 mAh g^{–1 [252]}. Navone et al. ^[253] reported the fabrication of all-solid-state lithium microbatteries using crystalline sputtered V₂O₅ thin film (1.2 µm thick) as cathode and lithium phosphorus oxynitride (Lipon) (1 µm thick) as solid electrolyte. The electrochemical performances are strongly affected by the cathode morphology; its porosity depends on the oxygen flow during the film deposition; thus, a 0.6 µm thick porous film was optimized using an oxygen flow of 12.5 sccm (p(O₂) = 0.05 Pa). Such a microcell film showed a stable discharge capacity of 35 µAh cm^–2 at a current density of 10 µA cm^–2 between 3.8 and 2.15 V vs. Li⁺/Li. Electrochemical performance of 50 µAh cm^–2 was obtained with the use of a 1 µm thick dense film. All-solid-state thin-film lithium cells with the configuration Li/Lipon/Li_xV₂O₅/Cu were fabricated on stainless steel (SS) substrate using sequential deposition techniques. The amorphous V₂O₅ cathode film was lithiated by vacuum evaporation of pure Li metal to form the Li_1.3V₂O₅ cathode film (500 nm thick). The microbattery was formed during the first charging cycle, where lithium anode was electroplated in between the Lipon electrolyte layer and the stainless-steel substrate. These "Li-free" cells (1 cm² active area) delivered a specific capacity of 43 mAh cm^–2 µm^–1 at a current density of 0.1 mA cm^–2 in the potential range 3.8–1.8 V vs. Li⁺/Li and showed a good stable cycleability up to 770 cycles at 0.1 mA cm^–2 current density ^[254]. All-solid-state microbatteries Li_1.5V₂O₅/Lipon/Li were fabricated by the successive deposition of active elements on Si/Ti substrate as follows: (ⅰ) the substrate was first coated with 0.2 m thick gold film to form a current collector; (ⅱ) crystalline Li_xV₂O₅ films with x = 0.8 and 1.5 (1 µm thick) were prepared by thermal evaporation of metallic lithium (0.2 µm thick) deposited on a magnetron sputtered pristine material. This cathode element was keep at rest for 5 days to ensure the diffusion of Li ions into the V₂O₅ matrix; (ⅲ) the in situ solid electrolyte film was obtained by reactive sputtering of Li₃PO₄ in N₂ atmosphere (p(N₂) = 2 Pa) at a power of 350 W to ensure a deposition rate of 034 µm h^–1; (iv) the Li anode (3.5 µm thick) was obtained by thermal evaporation at growth rate of 150 nm min^–1. The all-solid-state microbattery delivered a typical specific capacity of 50 µAh cm^–2 in the potential range 2.15–3.80 V vs. Li⁺/Li ^[255].

V₂O₅ thin films prepared by means of cathodic deposition from V₂O₅ and H₂O₂ in aqueous show excellent electrochemical properties as cathodes. The films had a specific discharge capacity of 240 mA h g^–1 retained after 200 cycles with a 1.3C rate (200 mA g^–1) that corresponds to an energy density of 723 Wh kg^–1. Even at high 70C rate, V₂O₅ film electrodes retained a good discharge capacity of 120 mAh g^{–1 [104]}. Ostreng et al. ^[113] reported remarkable electrochemical properties of strongly structured ALD V₂O₅ films as cathodes in Li microbatteries. The best performance was obtained a discharge rate of 120C for more than 1500 cycles for 10-nm nanostructured film. McGraw et al. ^[197] investigated the intercalation mechanism in Li_xV₂O₅ thin films deposited by by pulsed laser deposition and estimated the chemical diffusion coefficients D̃ of Li⁺ ions from potentiostatic intermittent titration technique (PITT) measurements. D̃ varies in the range from 1.7 × 10^–12 cm² s^–1 to 5.8 × 10^–15 cm² s^–1 in crystalline PLD films, while D̃ = 5 × 10^–13 cm² s^–1 in amorphous V₂O₅ films. The general trend is a decrease of D̃ upon Li intercalation in the host lattice reaching the final value of 5.5 × 10^–14 cm² s^–1 at Li_1.5V₂O₅ (δ phase). The specific discharge capacity 334 mAh g^–1 of 10% Sn-doped V₂O₅ film is twice that of pure V₂O₅ film ^[220]. This superior electrochemical performance is attributed the stabilization of VO₅ slabs by Sn⁴⁺ ions that produces higher diffusion coefficient of Li⁺ ions. High-rate V₂O₅-based Li-ion thin film polymer cells show outstanding long-term cyclability. Cathode thin film growth was performed by r.f. sputtering (power 100 W) in high vacuum (1 mPa) at the 1.3 Pa partial pressure of the Ar/O₂ gas mixture (ratio 2:1) ^[256]. The V₂O₅/solid polymer/Li cells were electrochemically tested in the potential range 2.5–3.8 V at current regimes ranging from 1.5C to 10C. Stable specific capacity of 100 mAh g^–1 was maintained up to 5C after 300 cycles.

Recent developments of V₂O₅ thin films as cathode elements of rechargeable microbatteries shows the general trends of amorphous films. Zeng et al. ^[257] reported that films prepared via sol-gel and liquid deposition method show a gradual activation upon cycling that yields higher capacity (~200 mAh g^–1 after 200 cycles) and faster kinetics. This good performance was attributed to the novel nanostructure obtained from mild synthesis conditions. Mattelaer et al. ^[258,259] prepared amorphous films by ALD technique based on tetrakis[ethylmethylamino]vanadium in combination with water and ozone. Galvanostatic charging and discharging performed at 1C in the potential range 3.5–2.9 V (LiV₂O₅) and 4.0–1.5 V (Li₃V₂O₅), the thin films cells deliver volumetric capacities of 488 and 810 mAh cm^–3, respectively. Chae et al. ^[260] compared the electrochemical features of amorphous (a-V₂O₅) and crystalline (c-V₂O₅) films. The high performance of a-V₂O₅ with a reversible specific capacity > 600 mAh g^–1 at 13C rate was attributed to the absence of irreversible phase transitions and lithium ions trapping. This remarkable rate property for a-V₂O₅ films is due to the vacant sites that open the Li⁺ diffusion pathways ^[261]. Zhang et al. ^[238] made a all-solid-state battery V₂O₅/Lipon/LiCoO₂ prepared by r.f. magnetron sputtering in which the V₂O₅ (200 nm thick) acts as anode film. This microbattery (1.03 µm thick) delivered a volumetric capacity of 9.5 µAh cm^–2 µm^–1 after 40 cycles.

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6.2. Supercapacitors

Electrochemical capacitors (ECs) known as "supercapacitors" employ the electrical double-layer phenomenon to store energy. There are two subclasses of ECs: the electrochemical double-layer capacitors (EDLCs) having a non-faradaic charge character and the pseudocapacitors based on faradaic electrochemical redox reaction. Among transition-metal oxides used as ECs electrode, V₂O₅ has attracted attention because of its pseudocapacitive features, broad oxidation states and high specific capacitance 294 F g^–1. Reports on the properties of V₂O₅ thin films as electrodes for supercapacitors are rather sparse ^{[17,234,262,263]}. Films deposited onto Ni substrates by dc-magnetron sputtering in O₂:Ar (1:8) atmosphere and grown with predominant (001) orientation are composed of 32-nm grain size and surface roughness of 14 nm. These films exhibit high specific capacitance of 238 F g^–1 at current density of 1 mA cm^{–2 [263]}. 4 at.% MoO₃-doped V₂O₅ films obtained by thermal evaporation on Ni substrates, in which Mo atoms substitute for V, exhibit a specific capacitance of 175 F g^{–1 [17]}. Wu et al. ^[234] fabricated V₂O₅/CNT films in three steps: (ⅰ) mixture of carbon nanotubes (CNTs) and V₂O₅ with ethyl cellulose in ethanol and terpilenol as uniform and viscous slurry, (ⅱ) evaporation of ethanol and "doctor blade" coating onto alumina substrate and (ⅲ) annealing at 350 ℃. Reversible redox reaction was evidenced by cyclic voltammetry providing a specific capacitance of 216 F g^–1 at 5 mV s^–1 that is 5.2 times of that of CNTs (41 F g^–1) and a volumetric capacitance of ≈460 F cm^–3. This electrode maintains a power density of 22.2 kW L^–1.

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6.3. Electrochromic devices

A typical electrochromic device (ECD) includes an electrochromic layer, an electrolyte layer and an ion storage layer. Both electrochromic and storage layers are deposited on transparent substrates coated by transparent conductive film. ECDs operate by a change in optical properties between different colors. V₂O₅ is utilized as a counter electrode material in ECDs ^{[22,65,96,264,265,266,267,268,269]}. During the optical switching, it provides electrochemical redox reactions that balance charge transfer at the electrochromic working electrode. The typical electrochromic reaction of V₂O₅ can be expressed by the double injection of ions and electrons (Eq 12). The transmittance variance (ΔT) is defined by ^[270]:

$\Delta T\left( \% \right) = {T_b} - {T_c}$

(18)

where T_b and T_c are the transmittances (in %) of bleached and colored states, respectively. The coloration efficiency (CE) is defined as the change in the optical density (ΔOD) per unit of electrode area (A) at a given wavelength, as follows:

$CE = \Delta OD/\left( {q/A} \right) = {\rm{log}}({T_b}/{T_c})/\left( {q/A} \right)$

(19)

where q is the inserted charge capacity and the color efficiency (η) is defined by:

${\rm{ \mathsf{ η} }} = \Delta OD/q$

(20)

Following the work of Sarminio et al. ^[271] there is a close correlation between the stress change in the crystalline film and the electrochromic properties. Tensile stress varies with the x(Li) composition, the largest obtains with the δ-LiV₂O₅ phase. V₂O₅ thin film electrochemically deposited onto ITO glass from a poly-vanadic acid sol were used in ECD cell ^[272]. The cell was tested more than 8 × 10⁴ times and its response time was 2–20 s, depending upon the cell voltage. Shimizu et al. ^[273] developed an electrochromic display device using V₂O₅ thin film deposited by spin coating of V₂O₅ powder dissolved into a mixed solution of benzyl alcohol and iso-butanol. The films coated onto ITO glass followed by annealing at T_a > 300 ℃ are crystalline and exhibit excellent reversibility of the electrochromism. Mesoporous V₂O₅ films prepared by hydrolysis of vanadium tri-isopropoxide in the presence of polyethylene glycol (PEG) have shown that the surfactant promotes the formation of micrometric crystallites highly suitable for the development of advanced ECDs ^[274]. V₂O₅ thin films deposited by means of microwave plasma MOCVD have a transmittance of 70% at 400 nm absorption edge. The reversible Li intercalation reaction occurs with a change in the film color from yellow (V₂O₅) to blue (LiV₂O₅) ^[37]. The addition of the organic–inorganic template 3-isocyanatopropyltriethoxysilane (ICS) and poly(propylene glycol) bis-2(ammino-propyl-ether) (2-APPG) in sol-gel deposited V₂O₅ electrochromic thin film implements the storage capacity and increases the diffusivity of Li⁺ ions by modification of the surface structure of the film ^[275]. Magnetron sputtered V₂O_5–z thin films onto flexible polyethylene terephthalate/indium tin oxide (PET/ITO) substrates at oxygen flow rate of 2 sccm were oxygen deficient with z = 0.169 and offered a light modulation of 43.3%. After 200 cycles, at wavelength of 400 nm, an optical density change and color efficiency of 0.38 and 102.5 cm² C^–1 were obtained, respectively ^[276]. Similar ECD devices were fabricated at 23 ℃ by Lin and Tsai ^[19], showing an oxygen deficiency z = 0.13 and a transmittance modulation of 36.5% at wavelength of 400 nm.

Recent studies show that high switching speed and coloration efficiency are obtained for titanium-doped V₂O₅ thin film electrochromic devices ^[277] and spray-deposited lithium-doped V₂O₅ ^[278]. The ammonium intercalated V₂O₅ xerogel thin film prepared at 50 and 85 ℃ show two-step color changes: yellow/green and green/blue related to the redox process of V⁴⁺/V⁵⁺ states ^[96].

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6.4. Gas sensors

Because of their excellent catalytic properties, V₂O₅ thin films have attracted great interest as gas sensors. The adsorbtion of gaseous molecules and catalytic reactions are due to the active sites formed by V⁵⁺ ions with d⁰ electronic configuration ^[279,280]. Especially in thin film form, V₂O₅ is used in sensor of volatile organic compounds (VOCs) emitted by industries such as benzene, toluene, acetone, methanol, xylene, etc. that are hazardous substances to human health ^{[70,281,282,283,284,285,286,287]}. For a short review of V₂O₅-based resistive gas sensors, see Ref. ^[171]. Nanocrystalline V₂O₅ thin films deposited by a spray pyrolysis technique at T_s = 300 ℃ had a preferential orientation along the (001) direction and revealed the highest sensing response in the presence of ethanol vapor ^[280]. Nanocrystalline V₂O₅ thin films synthesized by the ethanol suspension were developed as solid-state sensors. A reversible reaction occurs with ammonia with the formation of ammonium metavanadate (bleaching), which is turn back to the starting state after annealing in air at 350 ℃ ^[279]. The polycrystalline V₂O₅ films prepared by spray pyrolysis were found to be highly sensitive towards VOCs. The selective nature of the films deposited at T_s = 300 ℃ are shown in Figure 20 ^[92].

V₂O₅ films fabricated by PLD technique were characterized as ammonia sensors even in the presence of NO used in diesel engine exhaust ^[70]. V₂O₅ thin films 200–600 nm thick were fabricated at 290 ℃ on fused silica and alumina by means of rf reactive sputtering from a metallic vanadium target for use as gas sensor. Thin films exhibit good response towards hydrogen (5–300 ppm), methane and propane (50–3000 ppm) ^[171]. Ultrathin crystalline epitaxial V₂O₅ films (10–90 nm thick) were deposited onto c-Al₂O₃ by ALD method with oxygen plasma. The films exhibiting a (001) preferred orientation are used as humidity sensors ^[119]. Nanostructured V₂O₅ films deposited by dc reactive magnetron sputtering have also been proposed as 2-propanol vapor sensors with an excellent response towards 5–200 ppm under ambient atmosphere ^[287].

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6.5. Healthy and safety issues

A very detailed review (210 pages) on the toxicity of V₂O₅ can be found on the web site ^[287]. In short, V₂O₅ is toxic, and it is specified on the U.S. National Library of Medecine ^[288] that probable oral lethal dose for humans is between 5 and 50 mg/kg or between 7 drops and 1 teaspoonful for a 70 kg (150 lb.) person. In practice, however, there is little occasion to eat such an amount of vanadium pentoxide and absorption can occur in humans following inhalation exposure. In this case, V₂O₅ is readily absorbed through the lungs. The effects from exposure may include burns to the skin and eyes, tracheitis, bronchitis, emphysema, pulmonary edema, or bronchial pneumonia. The American Conference of Governmental Industrial Hygienists (ACGIH) has established a threshold limit value time-weighted average (TLV TWA) of 0.05 mg/cu m for respirable dust. When absorbed, V₂O₅ is eliminated, mainly by urine. In practice, the protection against this product thus concerns people who have to manipulate or prepare it in factories since they are placed in situation where over-exposure may occur. The hygiene and safety policy of laboratories and institutions that are concerned is given in ^[288].

While hygiene and safety policy practice when handling or dealing with V₂O₅, this material plays an increasing role to make progress in environmental pollution and industrial safety issue, owing to its use for gas sensing application, reviewed in ^[289]. The list of targeted gas is constantly increasing ^[286].

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7. Concluding remarks

V₂O₅ is one of the most important material for many applications, including optical switching devices, gas and humidity sensors, smart widows, window for solar cell, photocatalysis, for electrochromic devices ^[82], color filters ^[146], reflectance mirrors, gas sensors, surfaces with tunable emittance for temperature control of space vehicles ^[81,147], thermochromic thin films for optoelectronic applications ^[93]. Nanocrystal V₂O₅ thin film used as hole-extraction layer in normal architecture organic solar cells demonstrated the potential of this material for photovoltaic applications ^[170], and W-doped V₂O₅ film can be used for thermal insulation ^[226].

So many applications explain the huge amount of works devoted to the preparation of V₂O₅ thin films. The difficulty is that their properties are strongly dependent of the mode of preparation, the film thickness, the orientation of the film, the crystallinity, the purity, the substrate ^[84], the nature and amount of dopants, the packing density of the film ^[85], temperature of the substrate ^[142], deposition rate, in short, deposition conditions ^[96], post-annealing treatment ^[217], the choice of precursors ^[100], porosity ^{[188,201,262]}, doping.

As electrochromic device, Ti-doped V₂O₅ electrode has lasted 200,000 cyclic switching times between the lowest (2%) and highest (62%) transmittance with no significant degradation of performance ^[209]. Nevertheless, V₂O₅ alone cannot be commercialized as an electrochromic device, because of its low electrical conductivity, poor coloration efficiency and narrow color variation. However, these drawbacks can be overcome by combining V₂O₅ with other compounds. A recent example is the graphene/V₂O₅/MoO₃ film, which combines the richer colors of MoO₃, the high conductivity of graphene and the remarkable electrochromic properties of V₂O₅. Such films present multi-electrochromic behavior (yellow → green → blue → gray-brown), excellent electrochromic performance, with tranmittence variation of 25.35%, bleaching time of 1.25 seconds, coloration time 1.40 seconds. In addition, it demonstrates good cycling ^[20]. Such films are thus very good fast switching electrochromic materials. Recently, the growth of large-scale electrodes by atmospheric pressure chemical vapor deposition has been made possible, with good performance for electrochromic devices with a change in optical density per unit inserted charge density reaches 336 cm² C^–1 at 630 nm, with a diffusion coefficient for the lithium as large as 9.19 × 10^–11 cm² s^{–1 [33]}.

For use as a supercapacitors, nano-structured V₂O₅ is needed to increase the surface area. In particular, a grain size of 148 nm of the deposited film exhibited a high rate pseudo capacitance of 730 mF cm^–2 at 1 mA cm^–2 of current density ^[27]. Note the result was also obtained under such synthesis conditions that led to a c-axis oriented structure, giving better results than a (201) orientation. A film electrode composed of intertwined V₂O₅ nanowires and carbon nanotubes (CNTs) demonstrated a volumetric capacitance of ≈460 F cm^{–3 [234]}. A capacity of 397 F g^–1 was obtained after ultrasonic weltering ^[262]. V₂O₅ thin films with 4 at.% of Mo doping exhibited a maximum specific capacitance of 175 mF/cm² at current density of 1 mA/cm^{2 [17]}.

Regarding the electrochemical properties, high energy density (900 W h kg^–1 at 200 mA g^–1), power density (28 kW kg^–1 at 10.5 A g^–1), good cyclic stability over 200 cycles have been obtained with nanostructured V₂O₅ thin-film electrodes prepared by cathodic deposition ^[104,190]. These results, as well as results obtained by atomic layer deposition ^{[111,113,114]} show so that V₂O₅ nano-electrodes can now be used with high power and energy densities for thin film Li-ion batteries.

V₂O₅ is also increasingly used as sensor. Fit has been tested for longer time periods up to 100 h as ammonia sensors. Very stable behavior was found with detection limit of 80 ppb for NH₃ both in dry and humid air. The detection limits for NO and CO were 20 ppm and 50 ppm, respectively. The results suggest that the vanadium oxide thin-film sensing layers are good candidates for a Selective Catalytic Reduction (SCR) process control application ^[70]. The room temperature gas sensing response of V₂O₅ films was found to be highly selective towards xylene ^[92]. They also exhibited good response towards hydrogen (5–300 ppm), methane and propane (50–3000 ppm) ^[171]. Recently, intrinsic thermochromism of V₂O₅ films was reported as a perceptible thermally induced color change from bright yellow to deep orange ^[290].

All these recent results show the continuous improvement in the synthesis of the films and the performance for tehes applications. In most of the works that have been published, however, the optimization of the film is made within one deposition technique, for one application. Therefore, further works should address two questions. The first one is to determine which deposition technique and synthesis parameters are best suited for each application. For instance, electrochemical properties of vanadium oxide are reported to be optimized for highly crystallized V₂O₅ ^[21], but better kinetics and higher volumetric capacities were observed for the amorphous vanadium oxides compared to their crystalline counterparts in Ref. ^[259]. Therefore, further works are needed to clarify the conditions that optimize the films for electrochemical applications. The second question is which kind of film should be used to optimize the film as a function of the application that is targeted. For instance, low Mg-doping (2%) is preferred for lithium ion battery applications, but 15% Mg-doping is preferred for electrochromic applications ^[212]. Only very few such reports have been made to relate the synthesis to applications, and further works along these lines would be very useful in the near future.

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Conflict of interest

The authors declare that there is no conflict of interest regarding the publication of this manuscript.

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