Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V2O3, V3O5, VO2, V3O7, V2O5, V2O2, V6O13, and V4O9. We begin with a tutorial on the phase diagram of the V–O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

Vanadium was first discovered by Andreś Manuel del Rio in Mexico City from Pb 5 (VO 4 ) 3 Cl in 1801. 1 However, it was wrongly identified as a form of chromium by Hippolyte Victor Collet-Descotils in 1805. 2 Until 1831, Swedish chemist Nil Gabriel Self-strom in Stockholm named the element vanadium, which is from the Norse Goddess Vanadis and means beauty and fertility. 3 In the Earth's crust, vanadium is the 20th most abundant element and the sixth most abundant element among the transition metals. 3−5 However, some literature indicates that vanadium is the fourth most abundant transition metal after iron, titanium, and manganese. 5−8 High purity vanadium (about 99.7%) was first produced in 1925 by reducing vanadium pentoxide (V 2 O 5 ) with calcium metal. 1 Pure vanadium exhibits a transition metal feature, which shows a high melting point and good corrosion resistance at low temperatures. Vanadium can be dissolved in nitric and sulfuric acids but is insoluble in hydrochloric acid. 9 In nature, vanadium is difficult to exist in metal form because it easily reacts with oxygen, even nitrogen and carbon at elevated temperatures. 2,10 Vanadium is an important component of specific steel alloys, which provides additional tensile strength and extra protection against rust and corrosion of these materials.

Vanadium Oxides
Vanadium has the electronic configuration [Ar]4s 2 3d 3 . Therefore, the oxidation state of vanadium can range from +5 to −3, and the valences of +5, +4, +3, and +2 are most commonly observed. 11,12 Four vanadium oxides feature single oxidation states (+2 for VO, +3 for V 2 O 3 , +4 for VO 2 , and +5 for V 2 O 5 ), and others have mixed oxidation states. Different oxidation states exhibit various colors: +5 (orange to yellow), +4 (blue), and +3 (green). 9 The vanadium oxides exhibit crystalline structures with different oxygen coordinations, which result in the formation of octahedral, pentagonal bipyramids, square pyramids, and tetrahedral sharing corners, edges, or faces. 12 The oxidation state of the vanadium cations dramatically affects the physicochemical properties of the vanadium oxides with different phases. Due to the multioxidation states and various crystalline structures, the vanadium oxides exhibit excellent intercalation properties to host−guest molecules or ions, 5 giving excellent catalytic activities, 4 strong electron−electron correlations, 13 outstanding phase transitions (metal−insulator transition), 14 and high electrical conductivity. 5 Furthermore, the abundant nanostructures of vanadium oxides can be achieved by different preparation methods, which not only shorten the transportation distance of ions or electrons and yield a faster solid-state diffusion in electrochemical energy conversion systems, 15,16 but also provide more active positions for the interaction with other molecules or ions and more exposed active crystal facets for catalysis applications. 17,18 Thus, the vanadium oxides provide promising applications in energy conversion/saving fields, 19,20 such as ion batteries, 21−25 water splitting, 26 smart windows, 27,28 supercapacitors, 29,30 sensors, 31 and so on.
A series of vanadium oxides with strong electron−electron correlations exhibit metal−insulator transition (MIT). The V 2 O 3 , VO 2 , and V 2 O 5 with single oxidation undergo MIT at 160 K, 32 340 K, 33 and 530 K, 34 respectively. These phase transitions are reversible and accompanied by a change of crystallographic, magnetic, optical, and electrical properties. The mixed-valence vanadium oxides belong to either Magneĺi series (V n O 2n-1 ) or Wadsley series (V n O 2n+1 ). For the Wadsley series, V 3 O 7 and V 6 O 13 exhibit the phase transition at 5.2 and 155 K, respectively. 35,36 Except for V 7 O 13 (metallic), all the Magneĺi series show a transition from a paramagnetic to an antiferromagnetic state and consequently exhibit an antiferromagnetic ground state at low temperatures, including V 3 O 5 (430 K), V 4 O 7 (250 K), V 5 O 9 (135 K), V 6 O 11 (170 K), and V 8 O 15 (70 K) with different phase transition temperatures, respectively. 37

Scope of the Review
Vanadium oxides have a long history and rapid development in recent years as they are one of the most promising candidates in versatile applications in batteries, energy-saving smart windows, sensors, catalysts, optoelectronic devices, and so on. Therefore, many research papers and reviews have been published. Pioneering reviews on the chemistry of oxovanadium were published in 1965. 38 The synthesis of vanadium oxides through hydrothermal and gas phase was reviewed by Whittingham, 39 Livage, 40 and Bahlawane. 41 The atomic layer deposition of vanadium oxides was summarized by Papakonstantinou. 12 The synthesis, properties, and applications of vanadium oxide nanotube were described by Kianfar. 6 The catalytic applications of vanadium oxides have been described by Delferro, 3 Hess, 42 Carrero, 43 and Granozzi. 44 The Raman spectroscopy of vanadium oxides was recently reviewed by Shvets. 45 The sensing properties of vanadium oxide nanostructures were described by Sheikhi 46 and Madanagurusamy. 31 The energy-related applications of vanadium oxides were reviewed by Xie,5,20 Jiang,4 and Streb. 47 A large number of reviews described the progress in metal ions batteries, including those by Chen, 25 O'Dwyer, 22 Mai, 21,24,48 Lowe, 23 Whittingham, 49 Rashada, 50 Lee, 51 Yang,52 Zheng, 53 Kim, 54 Liang, 55 and Cao. 56 The vanadium oxides in supercapacitors were reviewed by Chen,30 Dutta, 57 and Li. 29 Furthermore, our group also reviewed the multistimuli responsive properties and energy applications of some vanadium oxides. 19,27,58,59 The great potential of vanadium oxides for new applications and accelerated industrialization led to the dramatically increase of the importance of vanadium oxides over the last 5−10 years. Furthermore, due to the complexity of various oxidation states of vanadium, vanadium oxides show a large variety of stable and metastable structures, which pose an inevitable challenge to synthesize vanadium oxides with high purity, well controlled stoichiometry, and meticulously designed nanostructures, a must for high performance devices. Even though lots of reviews have been published, most of them focus on a specific kind or application of vanadium oxides, such as batteries, spectra, supercapacitors, and so on. No comprehensive reviews illustrate the different vanadium oxides with different applications. In this review, we focus on the most recent progress on the structure, synthesis, and applications of five thermodynamically stable vanadium oxides (V 2 O 3 , V 3 O 5 , VO 2 , V 3 O 7 , V 2 O 5 ) and some metastable vanadium oxides (V 2 O 2 , V 6 O 13 , V 4 O 9 , etc.), which can provide a better understanding of a specific vanadium oxide phase and their process-structure−property interrelationships. Figure 1 summarizes the main contents of this review. This review begins with the phase diagram of the V−O system to show the different vanadium oxide phases, followed by the vanadium oxides with different stoichiometries. For each vanadium oxide, the structures, synthesis methods, and applications in several fields will be covered. The last section presents the future prospects and a summary of this review.

PHASE DIAGRAM OF THE V−O SYSTEM
The V−O binary phase diagram was compiled according to previously reported experimental data in 1989. 60 The oxygenrich phases are well-defined, which reveal more than 20 compounds. 41 However, the vanadium-rich phases exhibit broad homogeneity ranges and high nonstoichiometry. 61,62 Several groups have calculated the V−O binary phase diagram. 61−64 Figure 2a shows a calculated phase diagram of the V−O system in the entire composition range at 1 atm. In the V-rich range, four types of solid solutions exist. The α and β solid solutions are formed by a certain amount of oxygen dissolved in the vanadium. The maximum solubilities of oxygen in α-V and β-V phase are up to 17.9 atom % and 27.4 atom %, respectively. The β-phase exhibits a wide range of homogeneities. With the increase of oxygen content, the γ and δ solid solutions phase can be formed. The γ-phase is monoclinic and δ-phase has the stoichiometry of VO with NaCl-type structure.
There are two types of vanadium oxides with a mixed valence of vanadium. One is the Magneĺi series, which is defined by the general stoichiometric formula: 3 9 n n 2 1 2 3 2 This type of homologous series has been reported for molybdenum oxides for the first time by Magneĺi. 65 The other homologous series is the Wadsley series, which has the general formula of V n O 2n+1 (n ≥ 3). All the Magneĺi phases maintain a triclinic symmetry (P1) and are metastable. They are expected to yield VO 2 and V 3 O 5 after lowering the system entropy. For the Wadsley series, V 3 O 7 exhibits lower formation entropy compared with other Wadsley phases, which indicates the phase is more stable. Other Wadsley phases, such as V 4 O 9 and V 6 O 13 , can be decomposed into a mixture of VO 2 and V 3 O 7 . 41 V 4 O 9 and V 6 O 13 show multiple metastable crystalline structures due to the close formation energies. Therefore, these compounds are candidates for polymorphism, where three crystalline structures were identified for V 6 O 13 and two for V 4 O 9 . 60

Structures and Synthesis
V 2 O 3 has a typical corundum-type hexagonal structure (space group: R3̅ c) with lattice parameters of a = b = 4.9492(2) Å, c = 13.988(1) Å at room temperature. 66,67 The crystal structures of V 2 O 3 viewed in different directions are illustrated in Figure 3. It is interesting that after being calcined at 600°C for several hours, vanadium vacancies (red circle in Figure 3a) formed, which is  suitable for aqueous zinc metal batteries. 67 From another viewing direction, V 2 O 3 possesses an open tunnel structure consisting of a 3D V−V framework (Figure 3b). 68 Such tunnel structures could efficiently facilitate the insertion of alkali metal ions, which provide potential application in metal ion batteries. V 2 O 3 with different morphologies can be obtained by several methods, including reduction, oxidation, and hydrothermal approaches. For the reduction pathway, V 2 O 5 was employed as initial materials, and hydrogen or ammonia gas was used as reducing agents, whereas vanadium metal was used as a starting material for the oxidation method. In the hydrothermal route, vanadium alkoxides or sulfides with some small organic molecules (e.g., thiourea, benzyl alcohol, etc.) were added together to the autoclave to grow V 2 O 3 .
Li et al. 69 designed a plasma hydrogen reduction system to synthesize V 2 O 3 nanocrystals via a single precursor of V 2 O 5 powders. The coarse-grained V 2 O 5 powders are injected into the hydrogen plasma by a powder feeder, reducing the V 2 O 5 powders into V 2 O 3 by hydrogen. Such single crystalline V 2 O 3 nanocrystals have a spherical shape with average sizes in the range of 35−50 nm (Figure 4a). The morphology of the raw materials determines the V 2 O 3 morphology. Seshadri et al. 70 first synthesized the V 2 O 5 nanorods via a hydrothermal reaction followed by reducing in 5% H 2 :95% N 2 (reduction time = 3 h and reduction temperature = 600°C) to obtain V 2 O 3 nanorods (Figure 4b). The ammonia gas (NH 3 ) is another agent to reduce V 2 O 5 to V 2 O 3 . 71 V 2 O 3 shows a much larger size with several micrometers, and the morphologies of the V 2 O 3 particles were micrometer layered structures that were assembled by nanometer or micrometer sheets. Tao et al. 72 reported that the V 2 O 3 nanoparticles had been synthesized by supercritical ethanol fluid reduction of VOC 2 O 4 with an average size of 50 μm. Madanagurusamy et al. employed the oxidation way to obtain the V 2 O 3 nanosheets. 73 High-density vertically aligned V 2 O 3 nanosheets on glass substrates were obtained via a simple onestep sputtering technique. Vanadium metal was first deposited on the well-cleaned glass substrates followed by oxidation in the argon and oxygen mixture gas with a ratio of 3:1. Well-ordered, ultrathin, vertically aligned V 2 O 3 nanosheets with voids were obtained (Figure 4c).  Hydrothermal is one of the most popular methods for crystal growth, which is conducted under moderate temperature and a high vapor pressure environment in sealed containers with the unique advantages of being easy to handle and environmentally friendly. Compared with the reduction and oxidation methods, the hydrothermal reaction is more convenient for synthesis the V 2 O 3 with various morphologies. Niederberger et al. 74 adopted vanadium alkoxides and benzyl alcohol as precursors to synthesize V 2 O 3 nanocrystals through hydrothermal reaction sizes ranging from 20 to 50 nm with good yields (Figure 4d). Without using any surfactant and template, Su et al. 75 successfully synthesized dandelion-like V 2 O 3 microspheres with core−shell structures. With increasing reaction time, the morphology of V 2 O 3 can be tuned from a solid sphere to dandelion-like. If the reaction time is further increased, some broken V 2 O 3 microspheres with core−shell structures could be observed with an average diameter of the core−shell microspheres of 2 μm.

Batteries.
As the low valence of V 3+ in vanadium oxide, corundum-type V 2 O 3 with a metallic behavior shows that the electrons in the V-3d orbital travel along the V−V chains.
The 3D V−V framework provides an intrinsic tunnel structure, which is suitable for ion transport and intercalation. Mai et al. reported uniform nitrogen-doped carbon-confined V 2 O 3 (V 2 O 3 @NC) hollow spheres (Figure 5a). The in situ carbonization hollow structure can provide high ion/electron conductivity, short diffusion distance, and excellent structure adaptability, which is beneficial for lithium-ion storage ( Figure  5b). The V 2 O 3 @NC delivers an average discharge capacity of 785, 599, 528, and 361 mAh g −1 at a current density of 100, 500, 1000, and 5000 mA g −1 , respectively ( Figure 5c). Furthermore, 811 mAh g −1 can be maintained after 120 cycles at a current density of 200 mA g −1 (Figure 5d). 76 In addition, V 2 O 3 can also be used as other ions (Na + /K + /Zn 2+ ) storage material. Jiao's group 68 fabricated a flexible and self-standing electrode of V 2 O 3 nanoparticles embedded in porous N-doped carbon nanofibers (V 2 O 3 @PNCNFs) through electrospinning assisted hightemperature sintering method, which can directly be used as a KIB anode material (Figure 5e). V 2 O 3 @PNCNFs deliver a capacity of 240 and 134 mAh g −1 at a current density of 50 and 1000 mA g −1 , respectively. High-capacity retention of 94.5% can be maintained after 500 cycles (Figure 5f). The density functional theory (DFT) results demonstrate that 1 mol K + can insert into the V 2 O 3 crystal forming KV 2 O 3 , with the K +  occupying the 6e sites of the KV 2 O 3 crystal. The main capacity of V 2 O 3 in KIB is main contribution from capacitance (surface or near-surface redox reactions), which is very different from the conversion reaction of conventional transition metal oxides for ion storage. When K + inserts into the tunnels of V 2 O 3 @ PNCNFs, the material exhibits no structural phase transitions, which indicates that V 2 O 3 @PNCNFs can exhibit excellent K + rate/cycling performance. This work suggests that the pseudocapacitive electrode materials could be suitable for a large-scale energy storage system. Generally, the conventional low valent V 2 O 3 cannot effectively accommodate Zn 2+ intercalation during the discharging process due to its inherently unsuitable structure and inferior physicochemical properties. Luo et al. 77 attempted to utilize V 2 O 3 in aqueous ZIBs by the in situ anodic oxidation strategy, and the hierarchical microcuboid structure V 2 O 3 can accommodate nearly 2 electrons intercalation. Moreover, they found H 2 O is a reactant that participates in the first charge oxidation process of V 2 O 3 . Furthermore, the porous structure with a higher specific surface area of the V 2 O 3 leads to more reaction sites and a faster phase transition from V 2 O 3 to V 2 O 5−x ·nH 2 O (Figure 5h). Meanwhile, the high surface and the small size of V 2 O 3 nanoparticles benefit the first charge oxidation reaction process. The V 2 O 3 delivers a Zn 2+ discharging capacity of 625 mAh g −1 at 0.1 A g −1 , corresponding to 1.75-electron intercalation ( Figure 5g). Specifically, the capacities can maintain 87% and 78% when the current increases to 10 and 20 A g −1 , respectively. The V 2 O 3 can maintain 100% after 10000 cycles at 10 A g −1 , which is better than some previously reported ZIB cathode materials.  82 reported a self-supported MoS x /V 2 O 3 heterostructure for the HER. Two steps were involved in the synthesis process ( Figure 6a). V 2 O 3 / carbon cloth (CC) was first obtained by a hydrothermal method, and the dense V 2 O 3 was uniformly distributed on CC fibers. The as-prepared V 2 O 3 /CC was immersed in an ammonium thiomolybdate solution and dried under a vacuum. Then, followed by a thermal decomposition process, the final MoS x / V 2 O 3 /CC was successfully achieved with the same morphology as V 2 O 3 /CC. The XRD pattern (Figure 6b) shows the composite contains the MoS x and hexagonal V 2 O 3 . MoS x / V 2 O 3 /CC displayed an overpotential of 146 mV to achieve a 10 mA cm −2 HER current density (Figure 6c), which is lower than that of MoSx/CC (221 mV). The Tafel slope of MoS x /V 2 O 3 / CC is around 45 mV dec −1 , suggesting the HER process obeys the Volmer-Heyrovsky mechanism. Meanwhile, the MoS x / V 2 O 3 /CC presents a higher double-layer capacitance (C dl : 85 mF cm −2 ) than MoS x /CC (4 mF cm −2 ), indicating V 2 O 3 can create more active sites for the HER (Figure 6d). Furthermore, the MoS x /V 2 O 3 /CC electrocatalyst has great stability in the acid electrolyte for the HER. Two reasons for the MoS x /V 2 O 3 /CC electrocatalyst were given to understand the improved HER activity: (1) V 2 O 3 enhanced the electrochemically active surface area with more active sites for the HER; (2) better electron transfer between MoS x and V 2 O 3 . Qiu et al. 80 synthesized V 2 O 3 nanosheets anchored with NiFe nanoparticles as a bifunctional electrode for overall water splitting. A self-templated strategy was employed to synthesize the NiFe@V 2 O 3 (Figure 6e). By a calcined reduction process, ZnO@NiFe@V 2 O 3 nanosheets can be obtained. In-situ alkaline media corrosion was performed to dissolve the ZnO NPs and produce the porous NiFe@V 2 O 3 nanosheets, which exhibit a clear V 2 O 3 porous matrix and NiFe nanoparticles ( Figure 6f). The porous NiFe@V 2 O 3 exhibits good OER performance in an alkaline medium with an overpotential of 255 mV at 10 mA cm −2 and good stability (Figure 6g,h). Meanwhile, the NiFe@V 2 O 3 also gave good HER performance in the same alkaline medium. It shows an overpotential of 84 mV at 10 mA cm −2 and good stability (Figure 6i,j). Furthermore, the porous NiFe@V 2 O 3 delivers a small Tafel slope of 51 mV dec −1 for OER and a Tafel slope of 85.4 mV dec −1 for the HER, respectively. Considering the excellent HER and OER performance, two identical NiFe@ V 2 O 3 electrodes are integrated into a two-electrode cell to investigate the water splitting performance. The catalyst shows a cell potential of 1.56 V to reach 10 mA cm −2 with an ignorable cell voltage increase during 20 h measurements (Figure 6k).
The stability of the catalyst is a key parameter for practical applications, which is crucial to determine whether the catalyst can be commercialized. 83 Several factors limit the catalytic stability, such as poor chemical/electrochemical stability of the catalysts under operation, abandoned gas evolution leading to the physical detachment of catalysts, dissolution of the catalysts in the electrolytes with different pHs, poor mechanical stability, and so on. 84−86 For example, Schmidt et al. 87 proved that the metal atoms are thermodynamic unstable during the OER process due to metal oxide released lattice oxygen, which leads to the low stability of the catalysts. From the theoretical perspective, there are three critical criterion of the catalysts that should be considered: excellent water dissociation performance, suitable Gibbs adsorption energy of H* (ΔG H *), and faster H 2 desorption. 83 Such properties enable the fast release of the active site without destroying the active site and further improve the stability of the catalysts.

Supercapacitors and Electromagnetic Wave
Absorber. Supercapacitors (SCs) have attracted attention in recent years to bridge between a classic electrolytic capacitor and rechargeable battery, which are characterized by high power density and good cyclic stability. SCs are capable of storing electrical energy via two mechanisms: the electrochemical double layer capacitors having a nonfaradaic charge character and the pseudocapacitors based on faradaic electrochemical redox reactions. 88 The theoretical specific capacitance of a pseudocapacitive electrode is proportional to the number of electrons involved in a specific redox reaction, and vanadium oxides possess four readily accessible valence states, making vanadium oxides especially promising for high pseudocapacitance. 89 The conductivity of V 2 O 3 (∼10 3 Ω −1 cm −1 ) is higher than monoclinic VO 2 (∼4 Ω −1 cm −1 ) and comparable with Ru (∼10 4 Ω −1 cm −1 ). 90 Meanwhile, V 2 O 3 is stable in both acid and base mediums. 91 Thus, V 2 O 3 is a suitable material for energy storage, especially for SCs. However, the reported specific capacitances of V 2 O 3 -based materials are not good enough, suggesting that design and synthesis of new structured V 2 O 3based materials with high performance are required. 92 Cao et al. 93 synthesized V 2 O 3 nanoparticles highly dispersed in amorphous carbon composites through the calcination of the (NH 4 ) 2 V 3 O 8 /C precursor, which was fabricated through the hydrothermal reaction by using commercial NH 4 VO 3 and glucose ( Figure 7a). The as-prepared V 2 O 3 -based composites exhibit a specific capacitance of the electrode of 458.6 F g −1 at a current density of 0.5 A g −1 (Figure 7b), which is higher than other reported V 2 O 3 -based composites. 94 Meanwhile, the asymmetric supercapacitors assembled by the as-prepared V 2 O 3 -based composites display good flexibility properties ( Figure 7c). The capacitances are almost constant with the bending from 180°to 30°. The V 2 O 3 -based composites are one kind of high-efficiency electromagnetic wave absorber. Yan et al. 95 produced the hierarchical Co/C@V 2 O 3 hollow spheres by hydrothermal, aging, and annealing methods (Figure 7d). The VO 2 spheres were first fabricated through a hydrothermal method followed by coating with ZIF-67 by precipitation. After that, the ZIF-67@VO 2 composite was calcined in H 2 /Ar to form the hierarchical Co/C@V 2 O 3 hollow spheres. The Co/C@ V 2 O 3 hollow spheres exhibited excellent electromagnetic wave absorption performance with a reflection loss of @40.1 dB and a broad bandwidth of 4.64 GHz at a small thickness of only 1.5 mm (Figure 7e). The good performance is mainly due to the impedance matching and low density, which come from the combination of hollow V 2 O 3 spheres and porous Co/C.

Structures and Synthesis
V 3 O 5 crystal structure was first reported in 1954, 96 which possesses a monoclinic symmetry (space group P2/c). 97 The crystal structure is shown in Figure 8. The oxygen atoms are occupied in the octahedral sites, and vanadium atoms are located in the center of octahedron. There are two types of octahedra chains along the c axis: face-shared octahedra via edge-sharing and corner-sharing octahedra. Such chains formed a framework with many large open spaces, which is capable of accommodating lithium ions. 98 It is difficult to stabilize the V 3 O 5 phase by using solid-state chemistry and to control the stoichiometry between oxygen and vanadium by using a solution method, which makes V 3 O 5 an uncommon phase used for electrochemical and other applications. 98,99 Therefore, the reported synthetic method for V 3 O 5 is very limited. Reduction of V 2 O 5 by different reductants is commonly used to obtain the V 3 O 5 polycrystalline powders. Reisner et al. 100 selected vanadium metal as a reductant to  reduce V 2 O 5 , and the V 3 O 5 polycrystalline powders were synthesized according to the following chemical reaction: V + V 2 O 5 → V 3 O 5 . Vanadium powder and V 2 O 5 powder were mixed together and pressed into bars, which were sealed in a quartz ampule and heated for 24 h at 870 K and for 100 h at 1220 K. The V 3 O 5 polycrystalline powders exhibit a size around 10 μm. Alternatively, Yu et al. 98 used sulfur powders as a reductant. The mixture of sulfur and V 2 O 5 powders were vigorously grounded and calcined at 1023 K in a tube furnace under a vacuum for 2 h. The V 3 O 5 microcrystals can be obtained by washing the calcined powders with nitrogen tetrasulfide several times. The obtained V 3 O 5 microcrystals range from 1 to 3 μm. The large size single crystal of V 3 O 5 was grown by chemical vapor transport in the 1970s. 101,102 V 3 O 5 powders or V 2 O 3 −VO 2 mixed powders were used as starting materials, and TeCl 4 was used as a transport agent. Both starting materials and transport agent were sealed in a quartz tube with low pressure (∼1 × 10 −5 mbar), and then the tube was put into a two-zone furnace. Generally, the source zone has higher temperature, while the crystallization zone has a lower temperature, and the temperature gradient is around 100 to 150 K. The size of final V 3 O 5 single crystal is as large as 1 cm.

Applications
4.2.1. Batteries. V 3 O 5 is relatively less-studied though it exhibits a three-dimensional open-framework structure, which is due to the strict synthesis condition. Yu's group 98 successfully synthesized V 3 O 5 microcrystals via vacuum calcination and first employed it as an LIB anode material. The oxygen atoms are closely arranged in a hexagonal shape, and the vanadium atoms take up three-fifths of the octahedral interstices (Figure 9a). The  3D open-framework structure of V 3 O 5 is formed with chain connections, which contain distorted, sharing corners, edges, and faces of VO 6 octahedral, respectively ( Figure 9b). The 3D framework of V 3 O 5 with much large open space endows the capacity of Li + intercalation/deintercalation. No impurity peaks from the XRD pattern are observed (Figure 9c), which indicates that the V 3 O 5 powder shows a single-phase nature with a monoclinic structure within the P2/c space group (JCPDS card 72-0977). It delivers a high capacity of 628 mAh g −1 at 100 mA g −1 , a good rate (125 mAh g −1 at 50 A g −1 ), and a long stable cycling performance (117 mAh g −1 after 2000 cycles) ( Figure  9d,e). There is no obvious crystal structure change of the V 3 O 5 with Li + intercalation, which is the main reason for the good rate and cycling performance (Figure 9f).

Other
Applications. V 3 O 5 thin films exhibit a photoinduced insulator-to-metal phase transition, which results in a strong nonlinear optical response. 103−105 Fernańdez et al. 104 deposited the V 3 O 5 directly on the SiO 2 substrates by DC magnetron sputtering to form thin films. The ultrafast nonlinear optical response was probed by using a pump−probe scattering technique. A reduction in the transient relative scattered light signal was observed, which showed an ∼10% decrease within 800 fs. Such a response is due to the changes in the material's optical constants and very likely related to the photoinduced insulator-to-metal phase transition. 106 The photoinduced screening of electron correlations followed by melting of polaronic Wigner crystal and coalescence of V−O octahedra is the main reason for the order−disorder structural transition. 103 Figure 10a shows the crystal structure of V 3 O 7 . The unit cell contains 36 vanadium atoms (12 vanadium atoms are inside the octahedra and 24 vanadium atoms are five-coordinated). 107 The V 3 O 7 consists of VO 6 octahedra and VO 5 polyhedra, which are linked by corners and edges to form a three-dimensional framework. The crystal structure of V 3 O 7 ·H 2 O shows a twodimensional structure compared with V 3 O 7 (Figure 10b). Each V 3 O 8 layer consists of corner-or edge-shared VO 6 octahedra and VO 5 polyhedra. 108 The water molecules are located at the sides of the V 3 O 8 layer, where the hydrogen atoms are directly bonded to the vanadium atom of a VO 5 polyhedron and hydrogen bonds connect two neighboring V 3 O 8 layers. 7,109 Bulk V 3 O 7 single crystal can be grown by a typical chemical vapor transport method using V 3 O 7 polycrystalline powders as starting materials and NH 4 Cl as a transport agent. 110 The mixed V 3 O 7 polycrystalline powders and NH 4 Cl were pressed into a pellet and heated at 823 K in an evacuated silica tube for 7 days. The black needle-like single crystals of V 3 O 7 were formed with a length of 2 mm (Figure 11a). Nanostructured V 3 O 7 was generally obtained by a typical hydrothermal method. The precursor solution dramatically affects the morphology of the V 3 O 7 nanostructures. Wen et al. combined a soft chemical topotactic synthesis and hydrothermal process to prepare V 3 O 7 nanobelts. 111 In the beginning, layered structured KV 3 O 8 platelike particles were first prepared as the precursor by a hydrothermal method of V 2 O 5 and KOH. The H + -form vanadate (HVO) nanobelt colloidal solution was subsequently obtained by reacting KV 3 O 8 plate-like particles with HNO 3 . Finally, the one-dimensional (1D) pure single crystal V 3 O 7 nanobelts were successfully prepared after hydrothermally treating the colloidal solution at 180°C for 12 h (Figure 11b). By using NH 4 VO 3 and HCl as starting materials, the nest-like V 3 O 7 self-assembled by porous V 3 O 7 nanowires on Ti foil was also synthesized through a hydrothermal method. 112 In the beginning, a layer of V 3 O 7 nanosheets was deposited on the Ti substrate, which was subsequently placed in the NH 4 VO 3 −HCl solution and kept at 160°C for 10 h for a hydrothermal method. With the increase of hydrothermal time, the nest-like V 3 O 7 is self-assembled by nanowires. The schematic synthesis process and SEM images for nest-like V 3    The V 3 O 7 fibers can be obtained by thermal treatment of the electrospun NH 4 VO 3 /PVP nanofibers in the presence of reductant. 113 The thermal treatment condition dramatically affects the final products, and V 2 O 5 may be obtained together with V 3 O 7 .

Structures and Synthesis
The hydrothermal reaction is also a facile way to synthesize V 3 O 7 ·H 2 O nanostructures. The V 3 O 7 ·H 2 O nanobelts can be achieved by using V 2 O 5 , phenolphthalein and distilled water as starting materials through a hydrothermal method for 4 days of reaction at 180°C. 114 By changing phenolphthalein to ethanol or glucose, V 3 O 7 ·H 2 O nanobelts were also successfully synthesized at 180°C, while the reaction time was shortened to 12 h. 115,116 In above-mentioned the methods, phenolphthalein, ethanol, or glucose is used as the reductant. Without the reductant, the V 3 O 7 ·H 2 O nanobelts or nanowires can be obtained by a hydrothermal reaction of only V 2 O 5 or NH 4 VO 3 . 117,118 The key point is the pH of the precursor solution. After adjusting the pH to 3 by adding concentrated HCl, V 3 O 7 ·H 2 O nanobelts and nanowires were obtained by a reaction at 190°C for 24 h and 160°C for 3 h, respectively. Furthermore, ultrafine V 3 O 7 ·H 2 O nanogrids can be obtained through electrochemical oxidation. 119 In the beginning, the nsutite-type VO 2 black powder was synthesized by a hydrothermal method. Then, a three electrode system was employed to electrochemically transform the VO 2 precursor to V 3 O 7 ·H 2 O. The slurry, consisting of VO 2 nanoplates, was coated on the working electrode. With a constant current density of 50 mA cm −2 and a cutoff potential of 1.7 V, V 3 O 7 ·H 2 O nanogrids were obtained ( Figure 12).

Applications
5.2.1. Batteries. V 3 O 7 is a mixed-valence vanadium oxide for metal-ion storage. Yan et al. 120 designed and synthesized a V 3 O 7 nanowire templated graphene scroll (VGS) via an "oriented assembly" and "self-scroll" strategy. They used joint experimental-MD simulation to investigate the construction and formation mechanisms of VGS. The systemic energy, the curvature of nanowires, and the reaction time determined the length and formation process of the semihollow bicontinuous structure. Through this strategy, the VGS with a length up to 30 μm has interior cavities between the nanowire and scroll ( Figure  13a). The unique structure of VGS with the nanowire templated graphene scroll offers a continuous Li + /ion transfer channel and free volume expansion space during Li + de/intercalation ( Figure  13b). The VGSs exhibit a high capacity of 321 mAh g −1 and good cycle stability (87.3% after 400 cycles), which is better than the pure V 3 O 7 nanowire and V 3 O 7 nanowire/graphene structure ( Figure 13c).
In addition, Nazar et al. 121 (Figure 13d). The V 3 O 7 ·H 2 O delivers a capacity of 400 mAh g −1 (>2 mol Zn 2+ insertion) with an average voltage of ∼0.65 V at the first discharge process, and maintains 375 mAh g −1 at the subsequent charge process (Figure 13e), while in the Zn(CF 3 SO 3 ) 2 /acetonitrile nonaqueous electrolyte, the V 3 O 7 ·H 2 O exhibits a poor Zn 2+ storage performance (59 and 175 mAh g −1 for the first and 50th cycles at 5 mA g −1 , respectively) ( Figure 13f).
Magnesium-ion battery (MIB) as another multivalent ion battery has been attracting more attention due to its high abundance in the Earth and low redox potential (−2.37 V vs. SHE). V 3 O 7 ·H 2 O with high electronic conductivity (V +4.67 ) has been widely used as a cathode material in LIB/NIB and hybrid Li + /Mg 2+ batteries. Hong et al. 108 synthesized V 3 O 7 ·H 2 O nanowires via a one-step hydrothermal method and applied them to a high-energy MIB cathode (Figure 13g). The electrochemical tests and structural characterization results demonstrate that the structured water in V 3 O 7 ·H 2 O will remain stable during the cycling. 0.97 mol Mg 2+ inserts into V 3 O 7 ·H 2 O, accompanying the formation of Mg 0.97 H 2 V 3 O 8 at the first discharged state. V 3 O 7 ·H 2 O exhibits an initial discharge capacity of 231 mAh g −1 at 10 mA g −1 with an average discharge voltage of ∼1.9 V, and the energy density can reach 440 Wh kg −1 ( Figure  13h). Meanwhile, V 3 O 7 ·H 2 O delivers a 171 mAh g −1 and maintains 132 mAh g −1 (77%) after 100 cycles at 40 mA g −1 (Figure 13i). The excellent Mg 2+ storage performance is attributed to the unique crystal structure with direct bonding. This strategy of applying water-metal bonding and hydrogen bonding provides a new idea to search for new oxide-based MIB materials with stable and high energy density.

Ammonium Perchlorate Decomposition.
Ammonium perchlorate, a common oxidizer, plays a key role in the combustion of composite solid propellants. Furthermore, the catalyst greatly affected the performance of composite solid propellants by the thermal decomposition of ammonium perchlorate. 122,123 Huang et al. 124 found that the thermal decomposition temperatures of ammonium perchlorate in the presence of V 3 O 7 ·H 2 O nanobelts and V 3 O 7 ·H 2 O@C core−shell structures can be dramatically reduced. Both V 3 O 7 ·H 2 O nanobelts and V 3 O 7 ·H 2 O@C core−shell structures were synthesized by a hydrothermal method. Especially, the core− shell structures are synthesized by using V 3 O 7 ·H 2 O nanobelts as the cores and glucose as the source of carbon. A well-defined nanobelt morphology with a length up to several micrometers can be observed (Figure 14a), which consists of a V 3 O 7 ·H 2 O core and carbon shell ( Figure 14b). The thermogravimetric analysis (TGA) indicated that the addition of V 3 O 7 ·H 2 O or V 3 O 7 ·H 2 O@C in ammonium perchlorate exhibited a significant reduction in the decomposition temperature of ammonium perchlorate (Figure 14c). The thermal decomposition temperature was lowered by 70 and 89°C by adding V 3 O 7 ·H 2 O or V 3 O 7 ·H 2 O@C, respectively. The V 3 O 7 ·H 2 O@C core−shell structures exhibited a higher catalytic activity than V 3 O 7 ·H 2 O, with two possible mechanisms proposed. First, the partially filled 3d orbit in the vanadium atom promoted the electrotransfer process by accepting electrons from ammonium perchlorate and further accelerated the thermal decomposition of ammonium perchlorate. Second, the amorphous carbon shell possessed lots of active groups (such as C�C, C�O), which could facilitate the thermal decomposition of ammonium perchlorate. . Even at a higher current density of 50 A g −1 , the N-doped carbon -V 3 O 7 electrode still exhibits a better performance (187.72 F g −1 ) than V 3 O 7 (33.18 F g −1 ), as shown in Figure 15a. Furthermore, the N-doped carbon coated nestlike V 3 O 7 electrode also delivers better stability (80.47% capacitance retention after 4000 cycles) compared with pure V 3 O 7 (23.16% capacitance retention after 4000 cycles) . The superior performance of N-doped carbon coated nest-like V 3 O 7 is mainly due to the unique three-layer structure: V 3 O 7 core/ carbon/nitrogen doped carbon (Figure 15b). Such a unique three-layer structure can not only stabilize V 3 O 7 , but also provide high-speed ionic and electronic transmission channels, which is responsible for the good supercapacitor performance of N-doped carbon coated nest-like V 3 O 7 . Yu et al. 125 reported the growth of V 3 O 7 nanowires on a carbon fiber cloth through a hydrothermal method. The obtained V 3 O 7 /carbon fiber cloth composites show a spider web-like morphology, which exhibits robust adhesion. The composite electrode gives a maximum specific capacitance of 151 F g −1 at a current density of 1 A g −1 with ultrahigh cycling stability of 97% (after 100000 cycles) in a full cell configuration ( Figure 15c). Meanwhile, the V 3 O 7 / carbon fiber cloth composites reveal maximum power and energy densities of 5.128 kW kg −1 and 24.7 Wh kg −1 , respectively by using 1-ethyl-3-methylimidazolium trifluoromethanesulfonate as the electrolyte. Furthermore, coin cell-type configuration with the V 3 O 7 -carbon fiber cloth composites electrode was assembled. The symmetric supercapacitors successfully and effectively power light-emitting diodes to produce blue light (Figure 15d). Huang et al. 115 reported that V 3 O 7 ·H 2 O nanobelts exhibited a capacitance of 447.6 F g −1 . However, the cycling performance is limited by the poor conductivity and high solubility in an aqueous electrolyte. Therefore, composing with another conductive phase could be an alternative way to fabricate high-performance V 3 O 7 ·H 2 O based materials. When the V 3 O 7 ·H 2 O nanobelt is incorporated with carbon nanotube and reduced graphene, the formed 3D hierarchical porous composites exhibit outstanding electrochemical performance with a high specific capacitance (685 F g −1 at 0.5 A g −1 ) and excellent cycle stability (99.7% after 10,000 cycles) (Figure 15e,f). 126 Meanwhile, the composites also give relatively high energy densities and power densities of 34.3 Wh kg −1 and 150 W kg −1 , respectively. The better electrochemical performance can be attributed not only to the highly conductive carbon materials, but also to the 3D hierarchical porous structure. The carbon materials offer the transport pathway bridges, leading to the rapid transfer of charges. Meanwhile, the 3D porous structure minimizes the diffusion distance and supplies a large surface area with abundant active sites.
In general, vanadium oxides have received massive interest as supercapacitor electrodes that exhibit high theoretical specific capacity than most of the other transition metal oxides due to their variable valence state from V 2+ to V 5+ . In addition, the layered structure of vanadium oxides facilitates the intercalation/deintercalation of electrolyte ions during the charging/ discharging process. However, vanadium oxides-based supercapacitor electrode materials still suffer from poor long-term cycling stability, which is usually caused by the collapse of a layered crystal structure, severe agglomeration of particles, and low electrical conductivity. The electrochemical stability of vanadium oxide-based supercapacitors can be improved by material modification, optimization of the structure, or combining with other materials with excellent electrical conductivity. Developing vanadium oxide nanomaterials with suitable micro-/nanostructures is an important factor to improve the cycling stability. 3D vanadium oxides with micro-/nanostructures are often employed, including microspheres and hollow spheres, which can inherit the superior high surface area characteristics of nanobuilding blocks, and simultaneously possess a decent structural stability. 127−130 Furthermore, the 3D structure can effectively reduce the agglomeration of particles, which is beneficial for the cycling and rate performance. 131 Integrating vanadium oxides with carbon materials has been demonstrated to be effective to suppress the structural degradation upon cycling. The carbon materials, such as porous carbons and graphene, can serve as elastic buffering layers to release the strain within metal oxides during cycling. 132 To some extent, carbon materials can avoid loose attachment between the electrode material and current collector, which helps improve both the conductivity and the stability of the supercapacitor.

Electrochromism.
Nanostructured V 3 O 7 thin films showed electrochromic properties by using lithium perchlorate as the electrolyte, which was prepared by a nebulizer spray pyrolysis technique. 133 The color of the films is changed from yellow to pale blue by applying an external potential of 1.5 V for intercalation of Li + ions, while the color is reversed by applying an external potential of −1.5 V for deintercalation of Li + ions. Such results indicate that the nanostructured V 3 O 7 thin films could be effectively used for smart window applications. In general, several models are proposed to explain the electrochromic phenomenon, such as the color center model (Deb model), electrochemical redox model, and so on. 134 The "Deb's  color center model" is in nature related to the defects (such as oxygen vacancies) induced change of the visible light absorption, which is generally independent of the external electric field, 135 while in the "electrochemical redox model" it is believed that the injection and trapping of a large density of electron and hole lead to coloration. 136 The mechanism of electrochromism for V 3 O 7 thin films is the variation of the band structure caused by intercalation/deintercalation of Li + ions, which belongs to the electrochemical redox model. During the application of negative bias, Li + ions are absorbed onto the surface of V 3 O 7 and diffuse into the lattice of V 3 O 7 . The intercalated Li + ions react with O 2− ions to introduce oxygen vacancies in the lattice and reduce the V 5+ in the mixed state V 3 O 7 to V 4+ . As the result, the optical transparency of the film changes. 137 Moreover, the intercalation of Li + ions upshifts the Fermi level close to the conduction band, which leads to an increased transmittance of V 3 O 7 . The process can be reversed by deintercalating of Li + ions through applying positive bias. Furthermore, the V 3 O 7 ·H 2 O thick films also show good electrochromic properties (good reversibility and good color switching between reduced and oxidized states). 138 By using lithium bis-trifluoromethanesulfonimide (LiTFSI) as the electrolyte, the color of the V 3 O 7 ·H 2 O films changes from green to orange by applying a positive potential of 1.9 V, while it changes from green to blue by applying a negative potential of −1.2 V (Figure 16a). Such film presented a maximum optical reflectance modulation of 29% at 590 nm ( Figure 16b). The V 3 O 7 ·H 2 O films still exhibited color changes by using a Nabased electrolyte ( Figure 16c) and a maximum optical reflectance modulation of 10% at 590 nm ( Figure 16d). The maximum optical reflectance modulation of Na-based electrolyte is lower than that in the Li-based electrolytes, which is mainly due to a larger faradaic contribution resulting from the larger cation size of Na + ions.

Structures and Synthesis
Vanadium dioxide (VO 2 ) can exist in various polymorphic phases, including but not limited to VO 2 (B), VO 2 (A), VO 2 (M), VO 2 (R), VO 2 (D), and VO 2 (P). 139 Some of these phases and their corresponding lattice parameters are shown in Figure  17. The discussion will focus more on VO 2 (R) and its reversible MIT to VO 2 (M) as well as on VO 2 (B) due to its application as cathode materials in electrochemical devices. VO 2 was first demonstrated to undergo MIT in 1959 by Morin. At the critical temperature (τ C ) of about 340 K, VO 2 transforms from high-temperature, conducting rutile VO 2 (R) to  low-temperature, insulating monoclinic VO 2 (M). There are two mechanisms that have been used to describe this ultrafast phase transition phenomenon in VO 2 , namely the Peierls model and Mott-Hubbard model. The Peierls model describes the nature of MIT as the change from a shared d-orbital between all vanadium atoms to localized d-orbital in the V−V dimer, which is a result of the change in the V−V distance from 2.88 Å in VO 2 (R) to 2.65 and 3.12 Å in VO 2 (M). 20 Therefore, the Peierls model stated that the structural distortion is the cause of MIT. Wentzcovitch et al. applied the local-density approximation to study the electronic and structural change of VO 2 during the MIT. 140 From the view of band theory, a monoclinic distorted state is in good agreement with the experiment result. Meanwhile, the structural distortion enhances the bonding between neighboring V atoms, which is expected in the Peierls model. Moreover, Baum et al. utilized four-dimensional (4D) femtosecond electron diffraction to visualize the phase transformation process of VO 2 . 141 They pointed out that during the MIT, the displacement of atoms happened within picoseconds, and followed by the sound wave shear motion of the crystal in the time scale of nanoseconds. The observation indicates the occurrence of fast structural distortion during the MIT.
On the other hand, the Mott-Hubbard model states that MIT would occur when the electron density (n e ) and Bohr radius (a H ) satisfy n e 1/3 a H ≈ 0.2. 142 Compared with the Peierls model, the Mott-Hubbard model has the advantage in explaining the phenomenon such as the anomalously low conductivity in the metallic phase. 143 Whittaker et al. summarized multiple experiment cases and pointed out that the metallic phase of VO 2 might be introduced without the structural phase transformation if the excitation of carriers reaches a threshold density. 144 Their observation provides key evidence in revealing the nature of MIT. While there remains a debate on which mechanism best describes the MIT, the usage of both models is strongly encouraged due to the transition kinetics 145 as well as the stimuli involved during the transition. Shao et al. 146 reported recent progress in understanding the mechanism and kinetics of MIT, including the lattice distortion and electron correlations (Peierls phase transition, Mott phase transition) and modulation methods (elemental doping, external electric field, light irradiation, and strain engineering).
Upon application of suitable external stimuli (i.e., photons, heat, electric, magnetic, electrochemical, and stress) to initiate the MIT, physical properties of VO 2 , such as electrical resistance, optical transmittance, and thermal conductivity, are reversibly and drastically changed. Long et al. 27 have summarized the connections between the stimuli and responses of VO 2 -based devices in detail. This flexibility in external stimuli and corresponding responses is the main reason why VO 2 is a material of interest in multiple novel devices spanning from thermally, 139,148 electrically, 149−151 to optically 152,153 activated devices. The functional performance of these devices is thus highly dependent on, but not limited to, the physical attributes, such as dimension, morphology, doping level, and crystallinity, of the fabricated VO 2 . Multiple techniques have been used to fabricate functional VO 2 devices, each with their own strengths and weaknesses. Some of these fabrication processes are reviewed with polymer-assisted deposition methods, 154 hydrothermal, 139 sol−gel, 155 chemical vapor deposition (CVD), 19 and physical vapor deposition (PVD) 156 methods discussed in detail regarding the controlled synthesis of VO 2 for thermochromic application.

Applications
In recent years, multiple omnibus reviews 157−159 have been done in attempts to give the most encompassed view of VO 2 research progress, often including a combination of the following topics: MIT mechanism, kinetics, fabrication techniques, and applications. While these reviews could offer wide coverage of VO 2 research progress in multiple topics, indepth discussion of each topic, applications in this case, is in much needed demand. In subsequent sections, reported applications of VO 2 in functional devices both based and not based on MIT in recent years, especially in the last three years, are classified and discussed in different categories: optical, electrical, and mechanical applications.
6.2.1. Optical Applications. The operations of these VO 2based optical devices are often based on the changes in refractive index, n, and extinction coefficient, k of VO 2 upon crossing the MIT. The changes to n and k at different temperatures are shown in Figure 18. These optical functional devices are further divided into two main groups: infrared regulators and optical switches.

Infrared Regulators.
Since its first introduction in 1985, 161 the smart window has attracted much attention due to rising energy consumption in the commercial building sector, which contributes up to 40% of total consumption globally and leads to 30% of global greenhouse gas emissions today. 162 VO 2 is the prime candidate for smart window materials due to its ability to seamlessly and rapidly regulate the amount of infrared (IR) across MIT with miniscule side effects to the visible transmission. However, intrinsic limitations (high τ C ≈ 68°C, low luminous transmittance (T lum ) < 40%, poor solar modulation (ΔT sol ) < 10%, and poor durability) prevent pristine VO 2 from meeting the requirements for commercial smart window applications. The most common way to reduce τ C is by elemental (i.e., W, Mo, Ti, F, Mg, etc.) doping as summarized by Cui et al. 163 Different approaches exist to enhance T lum and ΔT sol via advanced device morphological engineering such as multilayered VO 2 , 164 biomimetic structure, 165−168 nanothermochromism, 169 porous, 170 and gridded structure. 171−173 Zhou et al. recently reported a new customized VO 2 composite structure in which a new factor, the incident angle, was considered in the development of smart window devices. Figure 19a shows a schematic of how different incident angles in the summer and winter can be taken advantage of with the reported customizable composite structure. The VO 2 composite structure was  Figure 19b. The incident-angle dependency properties of this structure are shown in Figure 19c,d in which modulating performance of the 3D printed device improves significantly, and ΔT sol improves from 9.7% to 25.8% with respect to an increase in the incident angle from 0°to 45°. 173 Aside from NIR transmittance regulation in smart windows, VO 2 also has an unusual ability to change its long-wave infrared emissivity (ε LWIR ) upon crossing the MIT. An ideal smart window should have high transparency in the visible region (380−780 nm), while having a transparent state in the winter and an opaque state in the summer ( Figure 20a). Moreover, the ideal smart window should have a high ε LWIR at a high temperature to promote radiative cooling (RC) and a low ε LWIR at a low temperature to suppress RC. Based on this concept, Long et al. 174 fabricated a VO 2 -based multilayer structure which was able to regulate NIR transmittance and ε LWIR spontaneously ( Figure 20b). Through forming a Fabry−Perot resonator, the passive RC regulating thermochromic (RCRT) smart window possessed an ε LWIR of 0.21 at 20°C, while the ε LWIR increased to 0.61 above τ c . In addition, the RCRT window kept a promising T lum of 27.8% and a ΔT sol of 9.3% (Figure 20c). With the actual building energy consumption simulation conducted with a 12story building, the RCRT window yielded a higher energy savings compared with a commercial low-E window across different climate zones ( Figure 20d). Meantime, Wu et al. 175 designed a flexible temperature-adaptive radiative coating (TARC) through embedding lithographically patterned Wdoped VO 2 in a dielectric BaF 2 layer on top of a reflective gold layer ( Figure 20e). The TARC film had a low ε LWIR (∼0.2) in the insulation state and a high ε LWIR (∼0.9) at the metallic state (Figure 20f), and the observation agreed with the simulation (Figure 20g). Long et al. 176 further expanded the concept of RC regulation from window to wall by preparing a switchable interwoven structure. As shown in Figure 20h, through pulling the block of interwoven structure, the original exposed block on the top side becomes concealed, and the underneath block becomes exposed. As a result, the structure switches its phase from phase 0 to phase 1. Taking into account different requirements of windows and walls, Long's group designed on-demand interwoven structures. Figure 20i shows the interwoven structure for window and wall applications. As discussed in Figure 20a, a window requires high visible transmittance and dual-band regulation for NIR and LWIR ranges. An ITO/VO 2 /PVC combination was employed for windows. In this structure, VO 2 was used to regulate NIR, while the ε LWIR was regulated by alternatively exposing low-E ITO and high-E PVC. On the other hand, an ideal wall has high solar absorption and low ε LWIR in the winter, and low solar absorption and high ε LWIR in the summer. An ITO/black paint/PVDF-HFP combination was utilized to cater to this demand. On cold days, visible transparent ITO is exposed, and sunlight will be absorbed by black paint underneath, while on hot days, the highly solar reflective high-E PVDF-HFP is exposed to prompt RC. Hence, compared with a conventional performance index T lum and ΔT sol , the newly proposed ε LWIR needs to be included to gauge the real energy-saving performance. 177 Moreover, VO 2 is notoriously known for poor durability, which is the bottleneck for the applications in smart windows. There are two recent reports to embed VO 2 in the V 2 O 5 matrix, and such a strategy could increase the lifetime up to 33 years. 178,179 Besides the application in building, the unique property of emissivity switching makes VO 2 the material of interest for IR camouflage and passive radiator for military and aerospace applications because VO 2 -based devices can function entirely on the thermal trigger with no additional sources, electrical or otherwise, required. VO 2 -based camouflage devices work by reducing the amount of IR emitted into the environment, shrouding the user from being detected with an IR detector such as most night-vision technologies. Examples include VO 2 / graphene/CNT heterostructure by Xiao et al., 180 VO 2 /carbon hybrid by Wang et al., 181 and VO 2 /ZnS core−shell structure ( Figure 21a) by Ji et al. 182 As seen in Figure 21b, under a similar IR detector and temperature, the VO 2 /ZnS core−shell pallets exhibit the ability to control their IR radiation intensity and lower their detected temperature as compared to V 2 O 5 pallets with constant emissivity.
Different from a camouflage device, a VO 2 -based passive radiator requires modification to the device structure to counter the lower emissivity at the higher temperature. This intrinsic problem could be overcome by depositing VO 2 on a highly reflective metal substrate with 183,184 or without 185 a spacer layer. Figure 22a is a multilayer Si/VO 2 /BaF 2 /Au structure reported by Kim et al., 184 which was designed and tested specifically for simulated space (vacuum) applications. Figure 22b demonstrates the radiated thermal power of the multilayer device. Experimental data were compared with simulated ones for both high and low-temperature operations. The measured radiated power at 300 and 373 K was 72 W/m 2 and 552 W/m 2 , respectively, showing a massive jump in emitted radiation upon crossing the MIT threshold. 184 Due to the typical multilayer design of a VO 2 -based passive radiator, factors such as functional emissivity difference between low and high temperature as well as the wavelength of emitted light can be further fine-tuned by adjusting the substrate/spacer/film combination. The electrochromic setup has also been shown to also result in IR regulating behavior in VO 2 -based devices. 186 Figure 22c is the schematic of a three-terminal thin-film-transistor-type electrochromic device by Katase et al. 186 Upon application of external voltage (+12 V according to literature), the VO 2 channel undergoes protonation, and the device becomes IR opaque, similar to smart window applications. When a reversed voltage is applied, deprotonation happens, and the device becomes IR transparent once again. Figure 22d shows optical transmittance spectra measured during this transition with +12 V stimulus. The optical transmittance modulation ratio at λ of 3000 nm was 49%. 186 Electrical input into a VO 2 -based device can also be utilized as a Joule heating source for MIT. To realize this, VO 2 can be combined with transparent conductive electrode materials such as ITO, Ag NWs, CNT, etc. 187,188 to become an electro-optic modulator. An example of such a modulator is a VO 2 +Au/GaN/ Al 2 O 3 device by Fan et al., 189 which has the ability to change the transmission step-by-step according to the applied voltage.
6.2.1.2. Optical Switches. Based on the sudden change in the optical constants n and k of VO 2 across MIT, radio frequency (RF) switches or waveguides can be fabricated to control the  flow of electromagnetic waves (i.e., microwave and radiowave). 190 Even though the design of each device is largely dependent on whether it is thermally 191 or electrically activated, 120 the mechanism for turning from the ON to OFF state is still entirely based on the transition from VO 2 (M) to VO 2 (R) respectively. Examples of RF-switches are the thermally configurable hybrid Al nanoholes/VO 2 photonic switch by Sun et al.,192 metamaterial design by Ding et al. which can act as an absorber from 0.562 to 1.232 THz at room temperature and a high-efficiency halfwave plate at high temperature, 193 and temperature controlled asymmetric optical switch by Liu et al. 194 Figure 23a shows the working principal of this design in which different output of the same polarized electromagnetic wave input can be achieved at low temperature by physically reverting the device by 180°while achieving similar output at high temperature. This asymmetrical mechanism is demonstrated in Figure 23b,c. A large asymmetry exceeding 20% was detected at 23°C, while it disappeared almost entirely at 87°C (Figure 23b). With incident x-polarized waves, the device gave y-polarized waves output at 23°C, and this can be considered the ON state. Upon heating to 87°C, output waves returned to approximately x-polarized, similar to input waves, turning the switch OFF (Figure 23c). 194 While the thermal-and electricalactivated optical switches mainly depend on the change of optical constant upon MIT, the optical-activated switches of VO 2 focus on the speed of the transition as the defining factor. However, the details of the ultrafast induced phase transition of VO 2 are not the focus of this review; it can be found in a summary and discussion by Wegkamp et al. 195 VO 2 ability to switch between the insulator and metal state within picoseconds is promising for the field of nanophotonics as well as all-optical integrated circuits (i.e., switches, modulators, and data-storage devices). VO 2 integrated metamaterials have been reported to exhibit nonlinear transmittance by Liu et al. 153 in the THz range as well as broadband responses spanning from the visible to midinfrared range by Guo et al. 196 VO 2 /Au nanoplate memory device was also reported by Lei et al., 197 giving a stepwise tuning ability with the use of successive laser pulses.
6.2.1.3. Plasmonic Applications. The reversible crystal phase transition makes VO 2 very unique among plasmonic materials. It undergoes a crystal phase transition from the monoclinic semiconductor state to rutile metallic state with significantly promoted conductivity and free carrier density, 27 leading to a significant difference in its plasmonic property. Recently, VO 2 nanoparticles (NPs) have been reported with thermalresponsive localized surface plasmonic resonance (LSPR) in the NIR region. 168,198 Based on the colloidal lithography method, 59 Long's group successfully produced the hexagonally patterned VO 2 NPs on quartz with controllable average diameters from ∼70 to ∼280 nm. 198 It was observed that the LSPR position of metallic VO 2 shifts to the longer wavelength on the larger NPs (∼1120 to ∼1220 nm) or under the increasing reflective index of the surrounding medium (∼1120 to ∼1360 nm) (Figure 24a). Besides, the NIR LSPR is temperatureresponsive that is quenched on a low-temperature semiconductor state and can be gradually switched on from 20 to 100°C (Figure 24b). They further investigated the LSPRinduced absorbance and scattering effects of VO 2 plasmonics through a finite-difference time-domain method. 168 On a single VO 2 NP, it is revealed that both the absorbance and scattering are low at the semiconductor state (monoclinic, M), while a strong absorbance emerges in the metallic state (rutile, R) ( Figure 24c). This result suggests that the LSPR in metallic VO 2 is characterized as a strong absorbance enhancement and a relatively weak scattering effect. Moreover, they reported the dispersity-and strain-induced LSPR response on VO 2 NPs in the polydimethylsiloxane (PDMS) elastomer matrix ( Figure  24d,e). 28 The dispersity-induced LSPR position can be attributed to the changes in average gaps among VO 2 NPs in the matrix, which is consistent with the simulation result ( Figure  24d), while the strain-dependent LSPR position change can be explained by the local reflective index change induced by the delamination between the NP and matrix under applied strains as being demonstrated by the finite element method ( Figure  24e,f). A more recent report used a similar approach to tailor the VO 2 surface plasmon by manipulating its atomic defects and establishing a universal quantitative understanding. 199 Record high tunability is achieved for LSPR energy from 0.66 to 1.16 eV and a transition temperature range from 40 to 100°C. The Drude model and DFT calculation reveal that the charge of cations plays a dominant role in the numbers of valence electrons to determine the free electron concentration. It is believed the investigation of VO 2 LSPR is still in its early stage. The reversible crystal transition makes VO 2 an intrinsic active plasmonic material, which is unique among the plasmonic field. It is expected for researchers to further understand the VO 2 plasmonic and to explore its potential applications.

Electrical Applications.
Not only optical constants, but the electrical conductivity of VO 2 is also altered dramatically upon transitioning from insulating VO 2 (M) to metallic VO 2 (R). This measurable electrical response to various external stimuli makes VO 2 the prime candidate for electrical applications such as sensors or transistors. The following section discusses the electrical applications of VO 2 and their corresponding devices.
6.2.2.1. Sensors. A sensor is defined by its ability to measure physical input and translate these measurements into interpretable data. Based on the significant conductivity changing of VO 2 across the MIT, it is possible to convert physical environmental input into a readable electrical signal. Some examples of VO 2based sensors include temperature sensors, photodetectors, flexible strain sensors, and gas sensors. Intrinsically, VO 2 is not suitable for temperature sensing applications because the change in its electrical conductivity only happens at 68°C, even though Kim et al. 200 managed to fabricate a programmable VO 2 critical temperature sensor. VO 2 was deposited on an Al 2 O 3 (1010) substrate and between two nickel (Ni) electrodes. A voltage can be applied across these electrodes to cause the τ C of VO 2 to decrease, causing the VO 2 (M) film to go into an intermediate phase before fully transitioning into VO 2 (R). During this intermediate phase, the measured current through the device was found to be linear with the change in temperature. At a voltage of 20 V, the τ C is found to be ∼20°C, enabling full range sensing capabilities from 20 to 68°C. Another approach, which is based on the sensing ability resulting from an abrupt change of the dielectrically constant of VO 2 during MIT instead of conductivity, was done by Yang et al. 201 As mentioned in the previous section, optically stimulated applications of VO 2 are promising due to the ultrafast transition mechanics, stability, as well as the broadband optical response of VO 2 -based devices. Hou et al. 202 demonstrated the device stability and speed of response using a VO 2 (M) nanowire on Au electrode setup (Figure 25a). It was reported that the device needs less than 1.6 s to detect IR (980 nm) and <1.0 s to recover (Figure 25b). The device was also reported to maintain responsivity for more than 500 cycles. Another design from Takeya et al. 203 combined the photoresponsivity of VO 2 film and the localized surface plasmon resonance of silver nanorods. The results indicated a correlation between the incident light transmission and resistivity within a wavelength of 400−900 nm. While the VO 2 acted as a photosensitive component, the nanorod array introduced a wavelength and polarization sensitivity to the photodetector. Because the MIT of VO 2 results in the change in the lattice structure and constant, it is also possible to induce MIT by causing changes to the lattice through mechanical force, which serves as the basis for VO 2 flexible strain sensor application. Hu et al. 204 showed that a VO 2 strain sensor device could be fabricated by bonding one single VO 2 nanobeam to a polystyrene (PS) substrate with silver paste and measuring the resistivity of the nanobeam as tensile and compressive stress is applied along the length of the nanobeam. In this study, VO 2 (R) Chemical Reviews pubs.acs.org/CR Review was not formed, and only the VO 2 (M 2 ) phase was formed due to the constraint of force applied (only 0.25% as compared to the required 2% at room temperature). However, the device showed remarkable potential when exhibiting a stepwise response to as small as 0.05% tensile or compressive strain. A typical gas sensor design is demonstrated in Figure 25c. For gas sensor application, the semimetallic VO 2 (B) phase is more commonly used than the insulating VO 2 (M) phase to maintain sensing capability at room temperature. VO 2 , regardless of the phase, responds to humidity, ammonia (NH 3 ), and nitrogen dioxide (NO 2 ). 205,206 Compositing VO 2 with carbon species such as single-or multiwalled carbon nanotubes (SWCNTs or MWCNTs) was reported by Evans et al. 206 The setup was effective in creating a stable, responsive VO 2 −CNT gas sensor. Figure 25d shows excellent response and good recovery of VO 2 −SWCNT to different humidity levels. The resistive response was increased dramatically from 0.5 for pure VO 2 (B) to 2.7 for VO 2 −SWCNT and 7.1 for VO 2 -MWCNT at 50% humidity. This p-type gas sensing response was also reported for NH 3 in the same study despite the longer and lower recovery level recorded. Depending on the applications, property change across the MIT is not the only viable option to use VO 2 in a functional device.
6.2.2.2. Electrical Switches, FETs, Oscillators, and Memristors. Different from sensing applications, the MIT of VO 2 can be deliberately triggered with programmable duration and patterns to great advantage in electrical switching, FET, oscillator, and memory devices. Similar to optical switches in the previous section, by toggling VO 2 across the MIT, it is possible to create an ON/OFF switching mechanism based on the difference in electrical resistance of VO 2 (M) and VO 2 (R). It has been demonstrated by Zhou et al. 207 that a two-terminal VO 2 -based switching device can have ultrafast, reliable 2 orders of magnitude ON/OFF toggling ability within 2 ns. While the MIT in this report was induced by an applied current, an electrical switch activated by Joule heating was also reported in a separate study by Li et al. 208 Mott FET is a gated FET device in which the conventional semiconductor channel is swapped with a Mott insulator, a material with the ability to switch from insulator to metal through external voltage to the gate. VO 2 , as a Mott insulator, is the prime material for novel Mott FET studies. An example of a typical Mott FET setup was reported by Yajima et al. 209 in which a large current modulation can be observed at 315 K, indicating a positive-bias gate-controlled MIT near τ C of VO 2 . Another novel Mott FET design was also fabricated by Shukla et al. 210 Figure 26a). The novel FeFET device was reported to achieve up to 85% resistance change under a gate voltage of 18 V (Figure 26b). Interestingly, the presence of the ferroelectric materials created a polarization effect after the applied voltage was removed, in which the channel resistance could attain up to 50%. Through this mechanism, it is possible to achieve multiple resistive states by the sweeping suitable gate voltage. To overcome the disadvantage of solid-gate oxide dielectric FET, such as current leakage, which might interfere with the MIT of VO 2 , ionic liquid (IL) and solid-state electrolyte gating have been the research interest for VO 2 FET devices in recent years. 212 However, the mechanism in which IL drives the MIT of VO 2 is still a debate between different studies. Nanako et al. 151 suggested that the underlying mechanism is the bulk carrier delocalization caused by the electrostatic effect. On a different train of thought, Jeong et al. 213 attributed the transition to the field-induced creation of oxygen vacancies, rather than the purely electrostatic effect. Ji et al. 214 and Shibuya et al., 215 however, suggested that electrochemical protonation was the origin of the modulation of electrical property in VO 2 , similar to what was observed in the electrochromic setup in the previous section. An electronic oscillator is a common component in modern electronic circuitry which can produce periodic signals such as a square wave or a sine wave. Due to the periodicity of the output, it is often used to convert a direct current (DC) input into an alternate current output. The two main types of electronic oscillators are the linear (harmonic) and nonlinear (relaxation) oscillator. Because of the ability to undergo a nonlinear MIT, VO 2 can be used as the basis for a nonlinear oscillator with a relaxation behavior stimulated by external electricity input. A VO 2 -based oscillator design by Leroy et al. 216 and its I−V characteristic curve is shown in Figure 26c. The inset shows the schematic of the oscillator circuit in which a resistor (R s ) is connected to the VO 2 device to produce a currentcontrolled negative differential resistance (NDR). The NDR portion happens when VO 2 enters the transitive state between VO 2 (M) and VO 2 (R). It was reported that by controlling R s , the VO 2 -based oscillator circuit can become self-sustaining, and the frequency can range from kHz to 1 MHz. 152 Aside from the standard setup as shown in Figure 26c, studies have also been Chemical Reviews pubs.acs.org/CR Review done in which two oscillators are coupled with a resistor, a capacitor, or FET in between. 217 A memristor is a nonvolatile electronic memory device which has a programmable resistance. The resistance of a memristor is retained even after removal of the power and is dependent on the original applied voltage. It is crucial that the resistance can be reversed or reprogrammed. Thus, a two-terminal VO 2 electrical device with the nonvolatile switching of resistance across the MIT can also be adapted into memristors. 149 Bae et al. 150 demonstrated a two-terminal memristor based on a single VO 2 nanobeam. The nanobeam undergoes MIT when a bias of 3 and 5 V was applied for 0.25 s; the resistance of the device goes from an initial 10 11 Ω to 10 9 and 10 8 Ω respectively. The resistance change can be reset with a zero-voltage bias. VO 2 has also been utilized in other memory devices including a multistate free-standing VO 2 /TiO 2 cantilever, 218 resistive random-access memory (ReRAM) devices. 219 and 3D memory array. 189 Not included in the above discussion is the minor application of VO 2 in field emitter and spintronic devices which are based on the abrupt drop in resistance across thermal-and magnetical-activated MIT, respectively. Studies on VO 2 /ZnO core−shell nanotetrapod thermal-activated field emitters were reported by Yin et al. in 2014. 220 On the other hand, VO 2 -based spintronic devices and the behavior of the magnetoresistance of VO 2 were reported in detail by Li et al., 221 Choi et al., 222 and Singh et al. 223

Mechanical Applications.
The actuator is typically a component in a machine or a system which converts provided energy into mechanical motion. The concept of the actuator has been widely adapted into novel scientific research, especially in the field of microrobotics or micro-/nanoelectromechanics. 224 VO 2 , which has a high theoretical work density (≈ 7 J cm −3 ) and fast response rate to external stimuli, is suitable for actuator applications. It offers the ability to offset disadvantageous low work density and the slow response rate of common actuator materials such as piezoelectric ceramics or polymers and CNT, respectively. 225 In device fabrication, single crystal VO 2 or composite bimorph of VO 2 can be designed to respond to specific external stimuli such as light, heat, or electrical current. An example of a photodriven VO 2 bimorph design was reported by Ma et al. 226 in 2018. The VO 2 /CNC device was conceived by combining the carbon nanocoil (nanosprings twisted by hollow carbon nanofibers) core with a VO 2 shell. When exposed to 980 nm radiation, the temperature of the spring increases unevenly, forming a temperature gradient from tip to end. This results in a transition gradient in which the tip becomes VO 2 (R) first and shrinks, creating the curvature. The VO 2 /CNC actuator delivers a large displacement-to-length ratio (∼0.4), fast response rate (9400 Hz), and long durability (>10 7 cycles). More recently, Shi et al. 227 fabricated thermal-activated single-crystalline VO 2 actuators (SCVAs) which were designed so that the τ C of a single VO 2 nanobeam is a gradient along the radial direction. When exposed to heat, one side of the fabricated W-doped VO 2 nanobeam with lower τ C would undergo MIT first and shrink, creating a bending as seen in Figure 27a. It was reported that this SCVAs design performed competitively with other reported VO 2 bimorph actuator designs with an extremely high displacement-to-length ratio (∼1), high energy efficiency (∼0.83%), fast response rate in the order of kHz, and long durability (>10 7 cycles) (Figure 27b). VO 2 electrothermal devices with joule heating activation for oscillator 228 and microelectromechanical systems (MEMS) 229 have also been fabricated with variable degrees of success. A resonator is a device that exhibits resonance at its eigenfrequency. With VO 2 , the eigenfrequency of a resonator can be dynamically controlled  by thermally triggering the MIT. This specific frequency is positively related to Young's modulus of VO 2 , which is widely different between the monoclinic phase (151 ± 2 GPa) and the rutile phase (218 ± 3 GPa). 230 Studies have been made to   231 compared the Cr-doped VO 2 resonator with the undoped one and concluded that the doped sample had a higher frequency change due to a lower Young's modulus. Other strategies to improve performance such as changing the shape from a simple cantilever have also been done by Manca et al. 232 6.2.4. Supercapacitors. VO 2 (B) with its layered structure and multioxidation states is ideal as an electrode material with charge storage through insertion and fast surface Faradaic reaction. 233 However, being in common with all metastable VO 2 phases, VO 2 (B) structural instability is not suitable for cyclic stability of SC application. Multiple studies have been done to combine VO 2 (B) with various carbon composites to create stable electrode materials. 234 An example of a VO 2 and reduced graphene oxide (VO 2 (B)/rGO) device by Liu et al. 235 is shown in Figure 28. The schematic diagram of the all-solid-state sandwich-structured supercapacitor design is shown in Figure  28a, where symmetrical VO 2 (B)/rGO and PVA/LiCl gel are used as electrodes and electrolytes, respectively. The performance of this design was reported to have a superior specific capacitance of 353 F g −1 at 1 A g −1 and a maximum power density of 7152 W kg −1 at an energy density of 3.13 Wh kg −1 . By compositing VO 2 (B) with rGO, 78% capacitance was retained after 10000 cycles, improving the stability of the device immensely (Figure 28b).

Magnetic Refrigeration.
Magnetic refrigeration is a cooling technique that is based on the magnetocaloric effect (MCE). The MCE describes the phenomenon in which a suitable material can be heated up or cooled down when exposed to a changing magnetic field. Due to the changing magnetization when crossing the MIT, VO 2 was first shown to be suitable for magnetic refrigeration application by Wu et al. 236 in 2011 with a single crystalline nanorod fabrication technique. Although the potential was shown for VO 2 in magnetic refrigeration applications, studies to further improve this are still limited.
6.2.6. Batteries. VO 2 formed by edge-sharing VO 6 octahedra with a unique bilayer structure exhibits a large lattice spacing that can accommodate Li + (0.76 Å), Na + (1.02 Å), K + (1.38 Å), and Zn 2+ (0.74 Å) insertion/extraction. Fan et al. 237 designed and synthesized a binder-free VO 2 cathode via biface VO 2 arrays directly growing on a graphene foam (GF) network ( Figure 29a). They constructed a geometric model of bilayered VO 2 nanobelts through the growth direction and lattice spacings (Figure 29b). The relatively high stacking rate of the "steplike" VO 6 octahedra along the [010] direction determines the preferred growth direction. As a result, the (001) facet of the VO 2 nanobelt is the thinnest, and the interlayers between the (200) crystal planes provide a facile channel for Li + and Na + diffusion. Meanwhile, the graphene quantum dots (GQDs) coating on the VO 2 surfaces can act as highly efficient surface protection to further enhance the Li + /Na + storage. When asprepared GF-supported GQD-anchored VO 2 arrays (GVGs) are directly used as a LIBs/NIBs cathode, it exhibits two advantages: high ion diffusion sensitization and charge transport kinetics are beneficial to obtain high-rate capacities, and the homogeneous GQDs suppress VO 2 dissolution which is in favor of retaining long-term cycles. The GVG electrode delivers a high specific capacity of 421 mAh g −1 at 1/3 C for Li + , which is much higher than that of the uncoated GF@VO 2 electrode (391 mAh g −1 ). It can maintain 151 mAh g −1 even at 120 C,and 94% of the initial capacity can be retained after 1500 cycles (Figure 29c). When it is used as an NIB cathode, it exhibits a specific capacity of 306 mAh g −1 at 1/3 C and good capacity retention (Figure 29d). These results demonstrate that the GVG is an excellent electrode material for Li + /Na + storage.
Zhang et al. 238 first designed and synthesized a surface amorphized VO 2 (B) nanorod (SA-VO 2 ) with a crystalline core and a surface-amorphized shell heterostructure by an interfacial engineering strategy. The crystalline/amorphous heterointerface in SA-VO 2 substantially narrows the bandgap, lowers the surface energy, and reduces the K + diffusion barrier of VO 2 (B) via DFT calculations. Therefore, the as-obtained SA-VO 2 electrode exhibits a higher reversible capacity of 288.3 mAh g −1 (at 50 mA g −1 ), superior rate capacity (141.4 mAh g −1 ), and long-term cyclability (86% after 500 cycles at 500 mA g −1 ) (Figure 29e), while the VO 2 only delivers a specific capacity of 147.2 mAh g −1 at 50 mA g −1 and maintains 16.5% capacity after 200 cycles at 500 mA g −1 (Figure 29f). Compared with oxygenrich defect amorphous shell VO 2 , the crystalline/amorphous heterointerface SA-VO 2 enhances the K + storage capacity and enables rapid K + /electron transfer, which results in large capacity and outstanding rate capability (Figure 29g).
Mai et al. 240 reported VO 2 hollow microspheres with a high surface area and excellent structural stability via a facile and controllable ion-modulating approach. VO 2 hollow microspheres deliver the best Li + storage performance compared to six-armed microspindles and random nanowires. The highest surface area of VO 2 hollow microspheres can provide efficient self-expansion, self-shrinkage buffering, and self-aggregation during lithiation/delithiation, which delivers 3 times higher capacity than that of random nanowires. In addition, they also synthesized highly homogeneous VO 2 nanorods by a rapid and simple hydrothermal method for aqueous ZIBs cathode material (Figure 29h). 239 The in situ XRD and ex-situ XPS/TEM results demonstrate that the VO 2 undergoes a single-phase reaction during the discharge process, accompanying a phase transition process of VO 2 −Zn 0.07 VO 2 −Zn 0.29 VO 2 −Zn 0.54 VO 2 with a unit cell volume expansion of 6.69%. On the contrary, the evolution of Zn 0.54 VO 2 −Zn 0.25 VO 2 −Zn 0.09 VO 2 −Zn 0.04 VO 2 occurs during the Zn 2+ deintercalation from the Zn 0.54 VO 2 . Meanwhile, detailed qualitative analysis verified that the VO 2 unit cell expands in the a, b, and c directions sequentially during the discharge/charge processes. Satisfactorily, the VO 2 nanorods deliver a high specific capacity of 325.6 mAh g −1 and excellent long cycle performance (86% after 3000 cycles), which is outstanding performance among the reported cathode materials of the aqueous ZIBs (Figure 29i,j). 6.2.7. HER, OER, and Water Splitting. VO 2 is a wellknown semiconductor material with a band gap of 0.7 eV, which is seldom considered as a candidate material as a catalyst or photocatalyst for the production of hydrogen. 241 VO 2 can be used as a photocatalyst for hydrogen evolution through phase engineering. Ajayan et al. 242 synthesized the body-centeredcubic nanostructured VO 2 , which shows excellent photocatalytic activity with a hydrogen production rate up to 800 mmol m −2 h −1 from a mixture of water and ethanol under UV light at a power density of ∼27 mW cm −2 . Furthermore, vanadium oxide composites have the great potential to accelerate water dissociation kinetics and reduce charge-transfer resistance. 243,244 Tao et al. 245 synthesized MoS 2 /VO 2 hybrids by using a two-step hydrothermal method. The phase transition of VO 2 exhibits a significant effect on hydrogen evolution properties of the heterostructures (an onset potential of 99 mV and a Tafel slope of 85 mV dec −1 ). The enhanced performance is mainly due to the faster electron transport as well   249 The outstanding characteristics of V 2 O 5, such as a layered structure, a direct band gap in the visible-light region, high chemical and thermal stability, electrochemical safety, low cost, and easy preparation, make V 2 O 5 a suitable material for electrochemical energy conversion and storage, 250 catalysis, 251−253 solar cells, 254 gas sensors, 31 electrochromic devices, 255 and optoelectronic devices. 256 Compared with bulk V 2 O 5 , the nanostructured ones have higher surface to volume ratios, which is beneficial to improve various performances. Over the past few years, a variety of methods, such as sol−gel, hydrothermal, chemical vapor deposition, magnetron sputtering, and atomic layer deposition, have been developed to prepare V 2 O 5 nanostructures.
The sol−gel method has been used to fabricate V 2 O 5 thin films and nanopowders through V 2 O 5 sols, which were prepared by ion exchange, alkoxide hydrolysis, peroxide-assisted hydrolysis, and melt-quenching. The disadvantage of using an ion exchange is the difficulty to control the vanadium concentration as it varies throughout the whole process. In addition, some foreign ions such as Na + may remain in the gel after ion exchange. 257 The alkoxide hydrolysis always involves some expensive raw materials, and the molten V 2 O 5 quenching process will produce toxic gas.
Hence the synthesis of V 2 O 5 sol by peroxide-assisted hydrolysis stands out due to its advantages of being environmental friendly, inexpensive, and requiring simple fabrication. Vanadium metal or commercial V 2 O 5 powders are commonly used as a vanadium source that can be dissolved vigorously in a solution of hydrogen peroxide. According to Alonso 258 Etman et al. also found [H 2 V 10 O 28 ] 4− is the main species via real-time nuclear magnetic resonance. 259 It is noted that, not related to the preparation method, the V 2 O 5 sols are comprised of a fibrous structure, dissimilar from many inorganic sols that are typified by a random aggregate of particle structure. 260 These fibrous structures can self-assemble into V 2 O 5 nanofibers upon long-term aging through a coagulation mechanism. 261 The obtained V 2 O 5 sol was applied onto substrates via coating techniques, including spin coating, dip-coating, and spray process, and the subsequent drying and heat treatment are necessary to obtain V 2 O 5 films. 262−264 Figure 31a highlight the evaporation, hydrolysis, and subsequent solidification procedure during the formation of V 2 O 5 thin films by dip-coating. 265 Figure 31b,c shows TEM diffraction patterns and corresponding FFT of V 2 O 5 thin films formed from low concentration dilution (LCP) and high concentration dilution (HCP) using PEG-400 as an additive (HCP-PEG), respectively. V 2 O 5 thin films formed from an LCP precursor show the formation of grains of orthorhombic V 2 O 5 with defined grain boundaries (Figure 31b), while the HCP-PEG samples have a polycrystalline structure without uniformed grain (Figure 31c). Meanwhile, as shown in AFM images in Figure 31d−g, thin films formed from higher concentration precursors have a larger surface roughness (R S ). Thermal treatment can further increase the R S due to the formation of crystallites on the thin film surface. The thin film formed from LCP-PEG precursor has the lowest R S of 0.2 nm. Liu et al. studied the substrate effect on the structure and electrical properties of nanocrystalline V 2 O 5 thin films prepared by the sol−gel method. 266 They found the annealed V 2 O 5 film on the Si substrate exhibited more uniform rod-like morphology, and electrical measurements indicated the typical n-type semiconducting behavior. Senapati et al. prepared nanoscale V 2 O 5 films having thicknesses ranging from 92 to 137 nm by spin coating V 2 O 5 sol at different stages of aging. 267 They reported the decrease of strain in the films with aging, and the electrical conductivity increased with aging due to the improved crystallinity of the films.    Figure 32a illustrates the fabrication process and images of VO x nanofibers and graphene oxide sheets. First, graphene oxide (GO) aqueous solution was added to the V 2 O 5 sol under vigorous stirring to induce hydrolysis and in situ recombination of the GO sheets and VO x oligomers. Then, a dark red VO x /GO hybrid gel was obtained after about 5 min because of the rapid formation of intermediate vanadium phases and the growth of nanofibers. After aging for 2 days, the gel gradually changed to deep green, and the V 2 O 5 nanofibers are anchored and in situ grown on the graphene surfaces. The V 2 O 5 /graphene hybrid aerogel is so light that it can be lifted by a feather, but it is strong enough to support the weight of 100 g (Figure 32b). The hydrothermal method has been widely used for the synthesis of a vast range of V 2 O 5 nanostructures with a desired size and morphology, such as nanoparticles, 274 nanowires, 275 nanotube, 276 nanosheets, 277 and micro-/nanostructures. 278,279 The importance of solubility of precursors, the pH value, the surfactant, as well as the hydrothermal temperature, reaction time, and solution filling factor are highlighted in many references. Li's research group conducted extensive research on the hydrothermal treatment of V 2 O 5 sol. 280 They prepared V 2 O 5 nanoparticles and ultralong nanobelts with the usage of an inorganic V 2 O 5 sol precursor (Figure 33a,b). 261,274 The obtained single-crystalline V 2 O 5 nanobelts have a large specific surface area, with width and thickness of 30−200 nm and length in millimeters or even longer (Figure 33b). Strong evidence suggested that the oriented attachment growth mechanism was responsible for the formation of V 2 O 5 nanobelts. Pan et al. reported a one-step solvothermal method to form V 2 O 5 hollow spheres without adding surfactants. 281,282 As shown in Figure  33c, the SEM image of the V 2 O 5 hollow spheres has a uniform size of around 1 μm in diameter. They investigated the timedependent interior structural evolution by TEM and gave the possible growth mechanism of the V 2 O 5 microspheres ( Figure  33d): vanadium oxide nanoparticles are first generated by the hydrolysis of VOC 2 O 4 and then aggregation to form solid microspheres in stage I. The solid spheres undergo the initial inside-out Ostwald ripening process and transform to the yolk− shelled structure (stage II). With extended solvothermal reaction, secondary Ostwald-ripening takes place on the preformed solid cores, resulting in the formation of a multishelled structure (stage III). Finally, completely hollow microspheres are obtained as a result of the thorough dissolution and recrystallization of the less stable interior architectures (stage IV). Hence, the interior structure of the VO 2 microspheres could be effectively tailored by simply controlling the reaction duration and concentration of the precursor.
The fabrication of two-dimensional (2D) V 2 O 5 nanosheets has been studied by Cao et al. 283 As cathode materials for lithium-ion batteries, the resulting 2D V 2 O 5 nanosheets ( Figure  33e) exhibit remarkable electrochemical performances, including high reversible capacity, good cyclic stability, and great rate capability. 3D hierarchical vanadium oxide microstructures, including urchin-like microflowers (Figure 33f), have been successfully synthesized by Lou et al. through a solvothermal method. 284 The morphologies of the microstructures can be easily tailored by varying the concentration of the vanadium oxalate solution, and the obtained V 2 O 5 microflowers are highly porous with a surface area of 33.64 m 2 g −1 giving high lithium storage capacity, and enhanced cycling stability and rate  capability. V 2 O 5 nanorings and microloops are rarely reported, but they are very interesting morphologies (Figure 33g). The cation-induced asymmetric strain is the main driving force in making a layered V 2 O 5 coil into a ring structure. 285 CVD is widely used for depositing high-quality and highperformance solid materials. As shown in Figure 34a, CVD involves the transfer of precursor molecules, which are either liquid or gaseous to a reaction chamber by a carrier gas (step 1), and then it is followed by the reaction and/or decomposing on the surface of the substrate to produce the desired films (steps 2a, 3, and 4) or powders (step 2b), and the byproducts and unreacted precursor are transported out from the chamber at the end of the process (step 5a, 5b). 19 There are several types of CVD systems such as atmospheric pressure CVD, aerosolassisted chemical vapor deposition (AACVD), atomic layer CVD, plasma enhanced CVD, metal−organic CVD, and so forth. 286 The morphology, size, crystal phase, and specific surface area of V 2 O 5 can be affected by various parameters, namely the reaction time, substrate temperature, pressure, precursor properties, and reaction position during the CVD method. SEM images in Figure 34b display the effect of growth temperature on the morphological characteristics of V 2 O 5 coatings. V 2 O 5 grown at 350 and 375°C has rod-like structures of nonuniform thickness and width, while at 400°C pellet-like features of V 2 O 5 are observed, and the morphology evolution could be due to the coexistence of both α-V 2 O 5 and β-V 2 O 5 . 287 Chun et al. prepared V 2 O 5 nanosheets via the reaction of VCl 3 vapor with oxygen in the CVD system without a vacuum system. 288 Figure 34c shows representative optical images and their corresponding SEM images of the obtained V 2 O 5 nanosheets and three distinguished shapes: hexagons, triangles, and truncated triangles. Wang et al. used the CVD method to control the morphologies of V 2 O 5 by changing the reaction distance from the source position using vanadyl acetylacetonate (VO(acac) 2 ) as the vanadium precursor. 289 They found that VO x vapor and VO(acac) 2 vapor existed simultaneously during the growth process, and the different supersaturation distributions of these two vapors led to three main growth areas. 1. V 2 O 5 thin-films were formed at a high concentration and supersaturation of VO x in the region near the source material ( Figure  34d); 2. nanowires with a length of about 10 μm and width about 200 nm were formed at a distance of 18 cm from the source due to the low vapor concentration of VO x (Figure 34e); 3. nanospheres with a diameter of about 200−500 nm were obtained when the source material was far away (about 30 cm) due to the high concentration and supersaturation of VO(acac) 2 that was oxidized to V 2 O 5 nanospheres (Figure 34f). 289 ALD is considered a specific type of CVD, which was first introduced in the 1960s and is currently receiving ever-growing attention as a method of choice for the growth of conformal coatings on nanostructures with high aspect ratios. 12 Figure 35a depicts the typical ALD process, in which the precursor gases   Chemical Reviews pubs.acs.org/CR Review sequentially react with a surface to form an ultrathin film through a self-limiting process, and all byproducts and unreacted precursor molecules are purged out of the reactor. 290 The primary advantages of ALD lie in subnanometer film thickness and conformality control that profit from the cyclic, selfsaturating nature of ALD processes. Moreover, the ALD is unique as it is able to coat complex 3D structures with a high degree of uniformity and smoothness. 291,292 Chen et al. successfully fabricated a multiwall carbon nanotube (MWCNT)/V 2 O 5 core/shell sponge by ALD. 291 Figure 35b shows the experimental flow schematically: the MWCNT sponge structure exhibits a very low density (∼7 mg/cm 3 ) and high porosity (>99%), allowing for a high amount of active material loading; the V 2 O 5 layer of about 16 nm is subsequently deposited on the MWCNT sponge by 1000 cycles of H 2 O-based ALD; finally, the MWCNT/V 2 O 5 sponge is compressed and assembled in a coin cell battery, which enables de/lithiation in active material within a very short time. Figure 35c, (Figure 35e). 293 Østreng et al. prepared V 2 O 5 films by ALD using the βdiketonate VO(thd) 2 and ozone as precursors. 294 They found that the crystallographic orientation, optical properties, band gap, and surface roughness of the V 2 O 5 films were correlated and could be varied by controlling the deposition temperature and film thickness. PVD techniques (Figure 36a) involve evaporation and many different modes of physical sputter deposition, in which the primary source of the depositing species is a solid or liquid, as opposed to generally gaseous precursors in CVD. However, chemical reactions can and do occur in PVD systems, such as in the reactive sputtering deposition. The presence of the reactive gas (oxygen or nitrogen) in the chamber can significantly alter the PVD source. 295 PVD possesses some unique advantages for the creation of uniform and dense solid thin films that strongly adhere to the substrates. Meanwhile, the thickness, composition, crystallinity, and crystal orientation of the thin film can be well controlled by changing the growth conditions with minimal risk of contamination due to the absence of organic reactants. Another advantage is to sequentially deposit several materials to form well-defined multilayer systems as well as special alloy compositions and structures. 296 Magnetron sputtering is one of the most used PVD methods to fabricate a large range of materials, including metal oxides. The application of a negative voltage to the cathode will generate positively charged argon ions that can bombard the target ions to be ejected toward the substrate to form a film. Magnets are used in order to increase ion bombardment. This technique has been developed on an industrial scale to make large surface deposits with a wide variety of materials. V 2 O 5 film consisted of fine long strip particles deposited by radio frequency magnetron sputtering, and the thickness of the V 2 O 5 film was determined to be approximately 150 nm according to the cross-sectional SEM image ( Figure  36b). V 2 O 5 films underwent four different thermal transition behaviors to other vanadium oxides that were closely related to the oxygen proportion of the annealing ambient. 297 Amorphous V 2 O 5 film can be used as a hole injection layer in quantum dot light-emitting diodes, which exhibited a maximum luminance of 198.5 cd/m 2 , a turn-on voltage of 1.7 V, and a max external quantum efficiency of about 8.3%. 298 Pulsed laser deposition (PLD) is another PVD method, which has been preferred to grow different structures ranging from high-quality epitaxial thin films to various nanostructured layers. During PLD, a high-power pulsed laser beam is focused on a target of the desired composition. Material vaporized from the target is deposited as a thin film on a substrate that faces the target in an ultrahigh vacuum (UHV) environment. Polycrystalline V 2 O 5 thin film in the desired orientation can be prepared by PLD, which has aligned nanorod morphology on a flexible stainless steel substrate (Figure 36c). 299 Huotari et al. found the film surface morphology varied largely according to oxygen partial pressure: lower O 2 partial pressures resulted in a denser and thinner film, while higher O 2 partial pressures gave a film surfaces formed with randomly agglomerated nanoparticles or agglomerates with pillar-like morphology. 300 Electron-beam deposition (EBD) is another form of PVD where a target anode is bombarded with a high-energy electron beam that is given off from a charged tungsten filament under a high vacuum. The electron beam leads to joule heating and converts the target into the gaseous phase, which subsequently precipitates into the solid form on the desired substrate. Han et al. 301 reported the growth of nanocolumnar V 2 O 5 molecules that were aggregated with each other and collapsed after annealing treatment (Figure 36d). Most of the nanosized V 2 O 5 columns' structure could retain its original shape during the annealing process by changing the source from V 2 O 5 to VO 2 . Highly oriented V 2 O 5 thin films with nanosized grains were grown by EBD, and the film thickness was found to be in the range of 800− 1200 nm that varied by adjusting the substrate position. 302 Meanwhile, the mobility and carrier concentration of the oriented V 2 O 5 thin films increased with the increase of V 2 O 5 film thickness. Thermal evaporation is the vaporization of a material by heating to a temperature that the vapor pressure becomes appreciable, and the materials are sublimated from the target surface in a vacuum. By this method, heterojunctions, 303 nanorods, 304 nanoparticles, 305 or highly crystalline V 2 O 5 films 306 have been studied in several reports. Wang et al. synthesized Ga-doped V 2 O 5 nanorods by thermal evaporation at 850°C and found interstitial Ga and Ga−O phases influence the photoluminescence properties of V 2 O 5 nanorods. 304 Berouaken et al. used thermal evaporation to prepare V 2 O 5 nanoplatelets on the quartz crystal microbalance, followed by rapid thermal annealing. 307 The obtained V 2 O 5 nanoplatelets exhibited good sensing performance toward NH 3 vapor at room temperature: a fast response time, a short recovery time, good stability, reproducibility, reversibility, and linearity. Velmurugan et al. prepared highly crystalline V 2 O 5 films with a controlled thickness of about 530 nm and an average particle size of around 560 nm using a thermal evaporation process ( Figure  36e). 306 The films were fabricated in electrochemical microcapacitors and subjected to various electrochemical characterizations, which display improved reliability and excellent capacitance retention. crystalline V 2 O 5 through calcination (Figure 37a). Compared with crystalline V 2 O 5 (V 2 O 5 -Ms) and V 2 O 5 nanowires (V 2 O 5 − NWs), the V 2 O 5 −HMs exhibit the best Li + storage performance (145.3 and 94.8 mAh g −1 at 0.67 and 65 C, respectively), which is due to the 3D hierarchical microstructure with intertangled nanowires (Figure 37b). This 3D hierarchical microstructure not only inherits fast electrolyte penetration, and short ionic and electronic transport pathways, but also significantly alleviates the strain during the Li + intercalation/deintercalation. Meanwhile, compared to disordered V 2 O 5 nanowires, a unique V 2 O 5 −HM structure effectively helps improve the tap density, which is more suitable for commercial applications (Figure 37c). V 2 O 5 can be used as other metal ions (Na + /K + /Zn 2+ ) electrode material except for LIB storage material. Chung group synthesized a nanosized V 2 O 5 /C composite cathode by ball milling the nanosized V 2 O 5 with acetylene black and investigating the reaction mechanism in the NIB system. 309 Generally, compared with other vanadium oxides (VO 2   the square-based pyramid with the highly distorted environment and exhibits the highest pre-edge intensity from the X-ray absorption near edge structure (XANES) result. Thus, the average vanadium oxidation state of the calcinated V 2 O 5 /C sample is +4.83 (Figure 37d), which may be due to the calcined carbon reduction (a slight difference with standard V 2 O 5 in the pre-edge peak position). They used ex-situ XRD to demonstrate the major NaV 2 O 5 with a minor Na 2 V 2 O 5 phase formed at the first discharge state, accompanying a c lattice parameter increase by 9.09% and unit cell volume increase by 9.2%. At the subsequent charge, the NaV 2 O 5 + Na 2 V 2 O 5 will transform into NaV 2 O 5 + V 2 O 5 along with a V 4+ → V 5+ change (Figure 37e). The V 2 O 5 /C delivers an initial discharge capacity of 195 mAh g −1 , and increases to 255 mAh g −1 at the 10th cycle corresponding to 1.7 Na + inserts into per unit formula ( Figure  37f). Besides LIB/NIB cathodes, V 2 O 5 can be investigated as an aqueous zinc-ion battery (ZIB) cathode material, and the intercalation of water into the vanadium oxide can increase the interlayer distance, which is in favor of expanding the gallery for Zn 2+ intercalation. Yang et al. 310 reported V 2 O 5 ·H 2 O/graphene (VOG) synthesized via a freeze-drying method. They investigated the critical role ("lubricating" effect) of structural H 2 O on the Zn 2+ intercalation into bilayer V 2 O 5 ·nH 2 O, and H 2 O-solvated Zn 2+ possesses a largely reduced effective charge and improves electrochemical performance. VOG delivers a high capacity of 381 mAh g −1 at 60 mA g −1 and maintains 248 mAh g −1 at a high current density of 30 A g −1 , which are much higher than those of most aqueous ZIB cathode materials (Figure 37g,h). The interlayer distances of VOG are 12.6, 13.5, and 10.4 Å at initial, discharge, and charge states by ex-situ XRD and MAS NMR (Figure 37i). These results demonstrate that water in vanadium oxide layers plays an important role in the performance of an aqueous ion battery.
To investigate the electrical conductivity and structural mechanism during lithium insertion/deinsertion of V 2 O 5 , Yoon et al. 311 developed a 3D V 2 O 5 /rGO/CNT with short Li + diffusion, and high continuous 3D conductive network, and investigated its structural mechanism during Li + intercalation/ deintercalation by in situ XRD/XANES analysis (Figure 38a). The 3D V 2 O 5 /rGO/CNT delivers a high discharge capacity of 100 mAh g −1 at 20 C, which is much higher than 2D V 2 O 5 /rGO (68 mAh g −1 ). There are numerous metastable phases of form in turn during the first discharge to 3.4, 3.3, 3.19, 2.28, and 2.01 V, respectively. In addition, the most reflection will return to the same position at a pristine state during the subsequent charge process, which can confirm the high structural reversibility of V 2 O 5 in the ternary composite upon Li + intercalation/deintercalation (Figure 38b). The 3D V 2 O 5 / rGO/CNT delivers a high discharge capacity of 100 mAh g −1 at 20 C, which is higher than 2D V 2 O 5 /rGO (68 mAh g −1 ) (Figure 38c). The preintercalation interlayer metal ions can act as pillars to increase the electronic conductivity, ion diffusion rate, and stability of layered vanadium oxides. Mai et al. 312 designed and assembled Na 0.33 V 2 O 5 (NVO) and V 2 O 5 single nanowire devices to investigate the effect on the intrinsic electrical conductivity of Na + intercalation. The conductivity of NVO is 5.9 × 10 4 S m −1 , while the conductivity of V 2 O 5 is 7.3 S m −1 , which indicates that the electronic conductivity of V 2 O 5 is greatly improved by the Na + intercalation (Figure 38d). Stucky et al. 313 fabricated a series of nanostructured Mn-doped V 2 O 5 cathode materials and found that the larger Mn doping in the modified V 2 O 5 structure can increase the cell volume, which facilitates high Li + diffusion and improves the electronic conductivity (Figure 38e). The Mn 0.01 V 1.99 O 5 delivers a high discharge capacity (251 mAh g −1 at 1 C) and excellent cycling stability (80% after 50 cycles), which is much higher than V 2 O 5 (215 mAh g −1 vs. 70%) (Figure 38f,g). Fan et al. 314 reported lightweight, freestanding V 2 O 5 nanoarray-based positive electrodes (UGF-V 2 O 5 /PEDOT), which were prepared by growing a V 2 O 5 nanobelt array directly on 3D ultrathin graphite foam (UGF), followed by coating the V 2 O 5 with a mesoporous thin layer of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) (Figure 38h). In addition, the PEDOT coating constructs an integrated conductive network for the V 2 O 5 , providing decreased electrode polarization, improved charge transfer kinetics, and a prolonged discharge plateau of V 2 O 5 (Figure 38i,j), and therefore it can lead to an increased proportion of high-voltage capacity and energy density than that without PEDOT.

Supercapacitors.
Among all types of vanadium oxides, V 2 O 5 has attracted attention for the application of SCs due to its broad oxidation states, high specific capacitance, and low acquisition cost. Palani et al. fabricated RuO 2 nanoparticledecorated V 2 O 5 nanoflakes by a solvothermal method. 315 Figure  39a illustrates the corresponding schematic of the fabricated asymmetric cell that exhibited a high specific capacitance of 421 F g −1 at a current density of 1 A g −1 with excellent cyclic retention of 94.6% over 10000 cycles (Figure 39b). The symmetric device of V 2 O 5 ||PVA-KOH||V 2 O 5 was fabricated using thin flexible substrate by Velmurugan et al. (Figure 39c), where both annealed (A-500) and as-prepared (RT) V 2 O 5 films were used as electrode material separately. 306 A schematic  Figure 39d. The CV curves in Figure 39e denote that the symmetric A-500 gives a larger area under the curve than the symmetric-RT, suggesting the improved performance of the annealed sample. The A-500 devices display a maximum energy density of 0.68 μWh cm −2 , which is obviously much higher than that of the RT electrodes (0.05 μWh cm −2 , Figure 39f). Moreover, the symmetric A-500 shows excellent cycle life up to 30000 cycles with a Coulombic efficiency of 99%. As shown in Figure 39g, the practical feasibility of the as-fabricated devices was demonstrated by lighting blue light-emitting diodes. Several groups reported the hybrid structure of V 2 O 5 with carbon materials fabricated by different strategies for enhancing the SC property. For example, Sahu et al. 316  exhibited good OER performance due to the multivalent states of the V element, which can enrich active intermediates (*OH, *O, and *OOH) by regulating the valence electron structure of the V element. 275,320 The OER performance could be enhanced by fabricating the composites with other materials. Lan et al. 320 synthesized CoV 2 O 6 -V 2 O 5 /nitrogen-doped reduced graphene oxide composites (CoV 2 O 6 -V 2 O 5 /NRGO) by a one-pot hydrothermal method integrating polyoxovanadate, ethylenediamine (EN), and graphene oxide (GO) for the precursor and postcalcined process (Figure 40a). Without V 2 O 5 , the CoV 2 O 6 / NRGO delivered a relatively acceptable OER performance with an overpotential of 379 mV at a current density of 10 mA cm −2 , which is comparable to that of IrO 2 (337 mV). By adding V 2 O 5 , the OER performance could be enhanced with an overpotential of 239 mV at a current density of 10 mA cm −2 (Figure 40b). Furthermore, the CoV 2 O 6 −V 2 O 5 /NRGO exhibits a higher current density (47.08 mA cm −2 ) at an overpotential of 300 mV compared with CoV 2 O 6 /NRGO (0.45 mA cm −2 ) and IrO 2 (3.95 mA cm −2 ) (Figure 40c). Meanwhile, the CoV 2 O 6 −V 2 O 5 / NRGO shows the fastest reaction kinetics with a Tafel slope of 49.7 mV dec −1 , which could be due to the enhanced charge transport (Figure 40d). The CoV 2 O 6 −V 2 O 5 /NRGO gives good stability from the polarization curves, which are almost overlapping before and after 1000 cycles (Figure 40e). The theoretical calculation found that the existence of the hydrogen bond between V 2 O 5 and intermediate HOO* of OER decreases the adsorption energy, which may be responsible for the low overpotential.

HER.
In general, V 2 O 5 is generally considered as an HER-inactive material due to the weak H* adsorption on V sites, which limits the formation of H* at the active site. 321,322 Three main methods can be adopted to promote the HER performance of V 2 O 5 : defects creation, interfacial engineering, and forming composites. First of all, creating defects, especially vacancies, is regarded as an efficient way to enhance the HER performance. 323 (Figure 41a). Meanwhile, the catalyst shows a negligible potential drop at different current densities, indicating long-time stability (Figure 41b). The Co− V 2 O 5 −H is employed as both a cathode and anode to establish a two-electrode alkaline electrolyzer for overall water splitting, which can maintain a steady output current at a cell voltage of 1.6 V for 24 h (Figure 41c). Second, the interfacial engineering between V 2 O 5 and transition metal also provides a route to improve the HER performance. Kim et al. 329 directly grew the V 2 O 5 particles on Ni foam via a one-step hydrothermal method. The as-prepared V 2 O 5 /Ni(OH) 2 @NF catalyst shows a low overpotential of 39 mV at a current density of 10 mA cm −2 , which is comparable to Pt (35 mV @ 10 mA cm −2 ). The Ni(OH) 2 @NF samples show an overpotential of 188 mV at 10 mA cm −2 , which indicates that the V 2 O 5 plays an important role in the HER performance (Figure 41d). DFT calculation was performed to investigate the active sites of the V 2 O 5 / Ni(OH) 2 @NF catalyst. The exposed facet of V 2 O 5 is the (010), (001), and (310) planes, which is confirmed by TEM (Figure 41e). The H adsorption energy of the V 2 O 5 (001) and (310) surfaces is larger than that of V 2 O 5 (010), which indicates that V 2 O 5 (010) is the active surface. Furthermore, the ΔG H* values of the Ni-sites at the edges of interfaces in Ni(OH) 2 @Ni and V 2 O 5 @Ni and the O-site of V 2 O 5 (corresponding to 2, 4, and 5, respectively, as shown in Figure 41f) are close to zero, which is nearly equal to that of the Pt(111)-surface. The calculation indicated the edges of the interfaces in V 2 O 5 / Ni(OH) 2 @NF play a significant role in the HER process. Moreover, the hierarchical V 2 O 5 @Ni 3 S 2 hybrid nanoarray also exhibited a good overpotential of 95 mV at 10 mA cm −2 , 330 which is also due to the interfaces between V 2 O 5 and Ni 3 S 2 . Third, V 2 O 5 is also used as an additive with other materials to form a composite and further enhance the HER performance. By doping phosphorus and adding V 2 O 5 into Pt/graphene, the prepared catalysts exhibit a good HER performance of the initial potential of 32 mV and a Tafel slope of 23 mV dec −1 . 331

Photocatalysis.
It is well documented that V 2 O 5 has a typical narrow band gap (∼2.3 eV) and wide optical absorption range and is a high electron mobility semiconductor, which exhibits good photoresponsive properties by capturing visible light and is widely used in the electro-photocatalytic field, such as for hydrogen production, environmental pollutant degradation, etc. 332−334 Garcia et al. 335 found that the morphology of V 2 O 5 nano-/microparticles dramatically affected the hydrogen production by photocatalysis. The V 2 O 5 with microbelts, nanoplates, and nanopillars morphology can be obtained by using sunflowers' petals and the center of the sunflower as biodegradable templates during the synthesis. 335 The nanopillar  (Figure 42a). The high hydrogen generation rate is attributed to the large surface area, high absorbance in the UV−vis range, high photocurrent, and high content of defects of the V 2 O 5 nanopillars. By fabricating the nanocomposite with V 2 O 5 containing 1 or 2 heterojunctions, the hydrogen generation rate will be further improved, which is due to the delayed electron−hole recombination. 335 For example, graphitic carbon nitride nanosheets/V 2 O 5 composites (578 μmol g −1 h −1 ), 336 Na 2 Ti 3 O 7 /V 2 O 5 /g-C 3 N 4 composites (11000 μmol g −1 h −1 ), 337 Na 2 TiO 3 /V 2 O 5 /g-C 3 N 4 composites (567 μmol g −1 h −1 ), 337 and Nb-doped SnO 2 /V 2 O 5 (1346 μmol g −1 h −1 ). 338 Transition metal oxides (TiO 2 , ZnO, etc.) have been applied in the removal of environmental pollutants in water, which could completely decompose organic pollutants into CO 2 and H 2 O by photocatalytic oxidation. 339,340 Hollow V 2 O 5 microspheres consisting of randomly packed platelets showed an enhanced UV light absorption compared to the commercial V 2 O 5 powder, which led to the highest activity for degrading rhodamine B under UV light. 341 Composites, consisting of 1D V 2 O 5 nanorod and 2D carbon-based materials, are promising photocatalysts for environmental pollutant degradation. The V 2 O 5 nanorods/graphene oxide and V 2 O 5 nanorods/graphene nanocomposites showed good degradation performance of Victoria blue dye and methylene blue dye (>95% degradation within 90 min), respectively. 342,343 Especially, the nanocomposites exhibited the best degradation performance under direct sunlight irradiation compared to UV and visible light. The g-C 3 N 4 is also used to construct the heterojunctions with V 2 O 5 for a high-performance degradation catalyst. 344 The photoexcited electron in the conduction band of g-C 3 N 4 shows a strong reducing ability, while the photoexcited hole on the valence band of V 2 O 5 exhibits a strong oxidizing ability. Thus  cycling life, switching time, and so on. 353−358 Recently, several reviews were published on the electrochromic application of V 2 O 5 film, which provide more specific and detailed information on this topic. 256,352,359,360 For V 2 O 5 , the discoloration mechanism is explained as the result of injection/extraction of electrons and electrolyte cations and variation of valence change of vanadium ions, which is widely accepted. 361,362 In order to improve the performance of V 2 O 5 as an electrochromic device, many strategies have been designed. Especially, V 2 O 5 nanostructures with small sizes and large specific surface areas are expected to facilitate the ion intercalation/deintercalation process, thereby enhancing the electrochromic properties. Panagopoulou et al. 363 successfully prepared Mg-doped V 2 O 5 thin films using RF sputtering, and found the 15 atom % Mg-doped films displayed optimal electrochromic properties with the fastest switching time of t c = 10/4 s (intercalation/deintercalation), the best coloration efficiency of 71.3 cm 2 C 1− at 560 nm, higher visible transmittance of 85%, and the highest contrast value between the coloration states of ΔT (34.4% @ 560 nm). Qi et al. 364 fabricated flexible V 2 O 5 nanosheets/graphene oxide films, which exhibit ultrafast coloring response time (1.6 s) and bleaching time (2 s) attributed to the reduced charge transport distances of the ultrathin nanosheet structure (4−40 nm). They also display an excellent transmittance contrast of 57.5% at 425 nm and reversible yellow/green/blue-gray multicolor changes. Tong et al. 365 fabricated a 3D crystalline V 2 O 5 nanorod architecture on ITO substrates by a colloidal crystal-assisted electrodeposition method. Such architecture exhibits a highly reversible Li-ion insertion/extraction process (Columbic efficiency up to 96.9%), five distinct color change, good transmittance modulation ΔT (38.48% @ 460 nm), and acceptable response times (8.8 s for coloration and 9.3 s for bleaching), making it a promising film electrode for electrochromic devices. Kim et al. 366 prepared highly crystalline 2D V 2 O 5 nanosheets by using single-layer V 2 CT x film as a sacrifice template (Figure 43a). The mean thickness and lateral size of V 2 CT x are 2.38 nm and 0.78 μm, respectively (Figure 43b). Figure 43c shows the SEM image of V 2 O 5 nanosheets film after annealing of V 2 CT x at 350°C. The 2D V 2 O 5 nanosheets based electrochromic device has sharp multicolor transformations with a robust optical contrast from yellow to green to blue (Figure 43d). The corresponding optimal electrochromic performance shows a high optical contrast (53.98% @ 700 nm) and a fast response time (6.5 s for coloration and 5.0 s for bleaching).

V 2 O 2
V 2 O 2 is generally used as a tool to investigate the nature of metal−oxygen bonds, which is crucial to provide a proper rationalization of the relationship between structure and properties at an atomic scale. 367 Stable V 2 O 2 could not be synthesized by a traditional solid-state reaction or solution methods. However, it can be observed by IR spectroscopy on V, Ne, and O 2 codeposited matrices. 368 The calculation results show two possible vanadium bonding situations: 1) no bonds between the two vanadium atoms; 2) a short distance between the two vanadium atoms, which indicated multiple bonding (Figure 44a). 368

V 4 O 9
V 4 O 9 has an orthorhombic structure with a space group of Cmcm, which processes three types of VO polyhedra ( Figure  45a). The VO 5 pyramids and VO 6 octahedra make pairs, which are connected by corner oxygen atoms from the VO 4 tetrahedra. 371 The valence of the vanadium ion in the octahedron and pyramid is +4, while that in the tetrahedron is +5. V 4 O 9 is difficult to synthesize by a solid-state reaction from the mixture of binary V 2 O 5 and V 2 O 3 (or VO 2 ), but it can be synthesized by the reduction of V 2 O 5 using reducing agents of carbon, SO 2 , and sulfur. 371,372 The amount of reducing agents dramatically affects the final products, which may consist of other vanadium oxides, such as V 6 O 13 and VO 2 . Consequently, reducing V 2 O 5 by the solvothermal method is a facile way to obtain V 4 O 9 . Different solvents (tetraethylene glycol, 2propanol, and tetrahydrofuran) are used to synthesize V 4 O 9 with different morphologies, such as nanoflakes, nanosheets, and so on. 373−375 Liang et al. 375 investigated the aqueous zinc ion batteries of V 4 O 9 . It is found that the V 4 O 9 exhibits fast zinc ion and electron diffusion, which is due to the unique tunnel structure and the V 5+ /V 4+ mixed-valences induced metallic behavior. The V 4 O 9 cathode shows a high reversible discharge capacity (420 mA h g −1 at 0.5 C). Even at a high current density of 50 C, it also exhibits an impressive discharge capacity of 234.4 mA h g −1 , suggesting a fast Zn 2+ storage ability of V 4 O 9 ( Figure  45b). Furthermore, V 4 O 9 delivers an energy density of 175.8 W h kg −1 at a high power of 17625 W kg −1 , which gives a high power density compared with other vanadium-based cathode materials in aqueous zinc ion batteries, such as V 2 O 5 , 376 V 6 O 13 , 377 LiV 3 O 8 , 378 Na 3 V 2 (PO 4 ) 3 , 379 VO 2 , 380 and VS 2 . 381 The 2D single layered V 4 O 9 nanosheet assembled 3D microflowers exhibit good supercapacitor performance with a specific capacitance of 392 F g −1 at a current density of 0.5 A g −1 and 75% retained capacitance after 2000 cycles (Figure 45c). 373 The flower-like structure assembled from ultrathin and wellseparated nanosheets was unchanged during the charge− discharge cycles, which is responsible for the high capacitance and stability. Meanwhile, the V 4 O 9 flower demonstrates a good

V 6 O 13
The mixed-valence V 6 O 13 attracts extensive attention because it can be used in a variety of ion batteries, such as Li + , Na + , Mg 2+ , Zn 2+ , and so on. As shown in Figure 46a, V 6 O 13 is composed of alternating single and double vanadium oxide layers. There are two types of VO 6 octahedra: V 4+ occupied the yellow octahedra, and V 5+ occupied the blue octahedra. All the octahedra are connected with corner O atoms, which form a tunnel-like structure. 49,382 The solvothermal reaction is widely used to obtain V 6 O 13 nanostructures, particles, and related composites, such as V 6 O 13 nanogrooves, 383 388 and so on (Figure 46b). Cao et al. 389 recently developed a new strategy to synthesize V 6 O 13 nanosheets by microwaves, which could reduce the thickness of V 6 O 13 nanosheets greatly compared to that prepared by a hydrothermal method (Figure 46c,d).
Due to the tunnel-like structure and mixed-valence, V 6 O 13 exhibits a metallic character at room temperature, which is beneficial for high-rate charge and discharge. When it is used as an LIB electrode material, 8 mol Li + intercalated per formula unit endowing a high theoretical specific capacity of 417 mAh g −1 and energy density of 900 Wh kg −1 . The Yu group 383 synthesized a 3D V 6 O 13 nanotextile with interconnected 1D nanogrooves via a facile solution-redox-based self-assembly route at room temperature (Figure 47a). They confirmed that the precursor concentration affected the mesh size in the textile structure. The 3D V 6 O 13 delivers a high capacity of 326 mAh g −1 at 20 mA g −1 and maintains 80% capacity after 100 cycles at 500 mA g −1 . The energy density can reach 780 Wh kg −1 , which is much higher than those of commercialization cathodes (LiFePO 4 and LiCoO 2 ) (Figure 47b). The excellent electro-  Chemical Reviews pubs.acs.org/CR Review chemical performance is due to the unique structure of 3D textiles, which can be maintained upon cycling and are beneficial for ion transport and cycle stability (Figure 47c). In addition, Mai et al. 391 synthesized a novel ultrathin prelithiated V 6 O 13 nanosheet by a secondary hydrothermal prelithiation process (Figure 47d). A single-nanosheet device was employed to in situ probe the intrinsic advantages of prelithiated nanosheets. Compared with nonlithiated V 6 O 13 nanosheets, the ultrathin prelithiated V 6 O 13 nanosheets exhibit a higher electrical conductivity and maintain the same conductance level after the Li + intercalation (Figure 47e). Meanwhile, the specific capacity of the ultrathin prelithiated V 6 O 13 nanosheets can be Chemical Reviews pubs.acs.org/CR Review maintained at 98% after 150 cycles at a current density of 1000 mA g −1 , which is much higher than 46% capacity of nonlithiated V 6 O 13 nanosheets (Figure 47f). These results demonstrate that prelithiation is a strategy to obtain high-energy and long-cycling energy storage cathode materials. Doping of various metal ions can improve the electrochemical performance and achieve a good capacity of the battery. The V 6 O 13 with Al/Ga, Al/Fe, and Al/Na doping delivered an initial discharge specific capacity of 411.5 mA h g −1 , 426.9 mA h g −1 , and 514 mA h g −1 at 0.1 C, respectively. 388 However, the electrochemical performances were poor at a high current density. V 6 O 13 was also employed in multivalent ion (Zn 2+ , Mg 2+ ) batteries. 384,385,389,392,393 Choi et al. 392 applied V 6 O 13 as the ZIB cathode material and investigated its electrochemical behavior for Zn 2+ . In particular, they analyzed the effect of water in the electrolyte on the Zn 2+ storage to investigate the physicochemical characteristics of V 6 O 13 . DFT calculation results demonstrate that the coordination environments of Zn show a big difference with/without water (Figure 47g,h). It will form octahedral coordination with water, but undercoordination without water. The Zn 2+ storage of V 6 O 13 increases with increasing water content in the electrolyte, and it can deliver a high capacity of 360 mAh g −1 and be maintained at 92% after 2000 cycles in an aqueous Zn(CF 3 SO 3 ) 2 (∼1 M) electrolyte (Figure 47i). Even at a high current density of 24 A g −1 , it maintains a relatively high capacity of 145 mAh g −1 . 16 This work highlights that cointercalating water molecules play a vital role in enhancing the electrochemical performance of the aqueous ion storage system. Zhao et al. 394 found that the K + intercalated V 6 O 13 exhibited a specific capacity of 367 mAh g −1 at 0.5 A g −1 and 198.8 mAh g −1 at 10 A g −1 (Figure 47j). Meanwhile, the capacity could be maintained at 90% after 2000 cycles at 10 A  Chemical Reviews pubs.acs.org/CR Review The DFT calculation suggested that K + intercalation could significantly contribute to the reduction of the Zn 2+ diffusion energy barrier (from 0.94 to 0.33 eV), which enables Zn ions to migrate away from the intercalation sites more easily ( Figure  47k). Thus, K + intercalated V 6 O 13 showed good battery performance. Furthermore, by growing the V 6 O 13 on carbon cloth and using the ZnSO 4 as the electrolyte, the V 6 O 13 cathode showed a capacity of 520 mAh g −1 (at a current density of 0.5 A g −1 ) and good cycle life (a stable capacity of 335 mAh g −1 over 1000 cycles) (Figure 47l). 385  (Figure 48a,b). They also demonstrated that the Pt additive makes no contribution, and is even counterproductive to conductivity, but facilitates a significant enhancement of pseudocapacitance. Therefore, it is clear that there is a strong relationship between Pt and the new phase Zn 4 SO 4 (OH) 6 · 5H 2 O. The in situ XRD patterns show that obvious characteristic peaks shifted to lower angles during the discharge process and returned to the pristine state during charging, indicating the interlayer spacing gradual enlargement and recovery upon Zn 2+ intercalation/deintercalation. Meanwhile, the formation/disappearance of the zinc hydroxyl complex is accompanied by Zn 2+ insertion/extraction (Figure 48c). Mai et al. 396 artificially constructed a VO x cluster/reduced graphene oxide (rGO) cathode material with interfacially inserted Zn 2+ repelling the pristine-bonded C atom into the plane of the rGO and constructing interfacial V−O−C bonds ( Figure 49a). Meanwhile, the VO x consists of subnanoclusters (less than 1 nm per dimension) and partial nanoclusters (slightly larger than 1 nm). In combination with the electrons transferred to the rGO during the discharge process (Figure 49b−d), the reduced degree of defect is additional proof of the interfacial Zn 2+ storage. As a result, they have discovered a new mechanism in which Zn 2+ ions are stored mainly at the interface between VO x and rGO, which leads to anomalous valence changes compared to conventional mechanisms and exploits the storage capacity of the nonenergy storing active but highly conductive rGO. The obtained VO x -G heterostructure delivers a superior rate performance with a capacity of 174.4 mAh g −1 at an ultrahigh current density of 100 A g −1 , with capacity retention of 39.4% for a 1000-fold increase in current density (Figure 49e). To the best of our knowledge, the rate performance is one of the best among ZIBs.

Other Vanadium Oxides for Energy-Related Application
Besides the battery application, the nanostructured VO x materials/composites have been considered as a good catalyst for the HER due to the creation of the more active site and modification of the adsorption and desorption of H atoms. 397 Guan et al. 398 prepared 2D Zn-VO x -Co ultrathin nanosheets on carbon fiber paper by an electrodeposition method. The Zn-VO x -Co electrocatalysts contain the amorphous Co metal phase and crystalline Zn−Co alloy phase, which gives the materials a good HER performance (an overpotential of 46 mV at 10 mA cm −2 and a Tafel slope of 75 mV dec −1 ). Furthermore, Zhao et al. 397 adopted the same method to fabricate a Ni(Cu)VO x catalyst by changing Zn and Co to Ni and Cu (Figure 50a). The Ni(Cu)VO x electrode displays a small overpotential of 21 mV at a current density of 10 mA cm −2 and a Tafel slope of 28 mV dec −1 (Figure 50b), which is comparable to the commercial 20% Pt/C catalyst (15 mV @ j = 10 mA cm −2 and 25 mV dec −1 ). Meanwhile, the Ni(Cu)VO x electrode also remains relatively stable for more than 100 h HER at 100 mA cm −2 (Figure 50c). The good HER performance is due to the existence of Ni−O− VO x sites, which promote the formation of highly disordered     (Tables 1, 2, and 3). While vanadium oxides are important materials for many applications, more detailed, mechanistic and systematical studies are needed to fully explore their potential as the bottleneck is increasingly related to the materials' quality and device fabrications. We propose a few future works in this field  Chemical Reviews pubs.acs.org/CR Review which could be further developed in the following aspects ( Figure 51): (1) Searching the applications of vanadium oxides in underexplored areas. One example is biological thermal imaging by leveraging its thermal phase transition characteristics as the insulator-to-metal phase transition of some vanadium oxides gives sharply enhanced optical absorption above their critical temperature. It has potential applications in biochemistry, especially in the fields of bioimaging. Another example is that some nanostructured vanadium oxides also possess optimal physicochemical properties (e.g., optical, thermal, magnetic properties), which give the opportunity to be applied in intelligent medicine based on micro-/nanorobots.
(2) Development of high-level theoretical calculations to guide the rational design of vanadium oxides and their composites and to better understand the fundamentals of high-performance devices. The use of advanced computational tools and theories enables researchers to understand the origin of complex and interacting phenomena at multiple scales, which could accelerate our understanding of the fundamentals of high-performance devices and improve the operation and design of new materials systems. Additionally, machine learning is a useful toolkit for designing and exploring new vanadium oxides with desired properties. It can also aid the understanding of the complex correlations between structures and properties in vanadium oxides.
(3) Fabrication of ultrathin or two-dimensional vanadium oxides, which could be applied in some new electronic devices. Owing to the atomic-scale thickness of single layers, the 2D materials exhibit tunable electrical properties and bandgaps. Therefore, the 2D vanadium oxides may hold promise for a wide range of applications in low-power electronics, flexible electronics, optoelectronics, catalysis, batteries, and so on. Furthermore, the 2D vanadium oxides may also form novel 2D heterostructures with other ultrathin 2D nanomaterials, which would be of certain interest.
(4) Exploration of new approaches to stabilize the vanadium oxides related devices, especially thermal, light, moisture, and oxygen environmental stability. Long-term device stability is one of the most important challenges for all devices. Most vanadium oxides are sensitive to oxygen and moisture, which may lead to the degradation of the device performances. Furthermore, most stability measurements of the devices are performed under ambient conditions, which limits their application in high temperature, high humidity, and high light intensity conditions. Meanwhile, it is also important to understand the degradation mechanisms in the different types of devices based on vanadium oxides, which could be the key to improving the device stabilities.
(5) Exploring new green chemistry synthesis methods to eliminate the toxicity of vanadium oxides. Vanadium oxides cause a variety of toxic effects such as biochemical changes, neurobehavioral injury, and functional lesions in the liver and bones. Especially, vanadium oxides in breathing air can cause pulmonary problems and DNA damage in leukocytes. The toxicity is more related to the phase structure, stoichiometric ratio, concentration, particle size, and crystalline degree, which could be considered in all the processes of application of vanadium oxides. Therefore, exploring new green synthesis methods and avoiding risks to humans and the environment during vanadium oxides' production, use, and disposal processes deserve more systematic and comprehensive studies. First, it is important to develop new synthesis protocols with minimum steps to prepare vanadium oxides, such as one-pot synthesis, completely enclosed-system synthesis, and so on. Second, the encapsulation of the devices to confine the toxicity needs to be considered when designing a new device. Third, the release of vanadium oxides to the atmosphere should be controlled during normal operation. For example, the marked decrease in toxicity is confirmed via silica coating on vanadium oxides due to the perception that the toxicity of vanadium oxide is closely related to the solubility and the robust silica barrier can isolate air and water to reduce the solubility. Chemical Reviews pubs.acs.org/CR Review (6) Realization of the bifunctional, trifunctional, or even multifunctional vanadium oxides to achieve integrated functionality. Different device integrations based on the same materials is expected to greatly reduce the cost and the incompatibility of different materials. Meanwhile, multifunctional materials would reduce the complexity of designing devices and promote the application in designated situations. Overall, we believe that vanadium oxides are great candidates for future applications in related fields and help solve key challenges in the global warming crisis. Moreover, the integration of vanadium oxides with multidisciplinary fields such as material science, device physics, civil engineering, mechanical design, and bioscience would continue to attract the interest of many scientists from different disciplines for new fascinating fields.