Vanadium (V) is a well-known transition metal which can form different oxides. The principal oxides of vanadium are vanadium monoxide (VO, violet color), vanadium sesquioxide (V₂O₃, green color), vanadium dioxide (VO₂, blue color), and vanadium pentoxide (V₂O₅, yellow color). Presence of oxygen vacancies leads to formation other oxides such as V₃O₇, V₄O₉, and V₆O₁₃ (a mixture of V⁵⁺ and V⁴⁺), and a series of oxides such as V₆O₁₁, V₇O₁₃, and V₈O₁₅ (a mixture of V⁴⁺ and V³⁺). In general, the mixing phases can be categorized into two series of phases namely the Magnéli phase (V_nO_2n-1) and the Wadsley phase V_nO_2n+1 ^[1][2][1,2]. Among different phases, V₂O₅ is thermodynamically the most stable oxide and it exist in different polymorphs namely the most stable α-V₂O₅ (orthorhombic), metastable β-V₂O₅ (tetragonal or monoclinic), γ-V₂O₅ (orthorhombic) and δ-V₂O₅ (monoclinic) [3]. Each polymorph is stable at a temperature and pressure range. For example, β-V₂O₅ is stable at high pressure and temperatures [4].

The orthorhombic V₂O₅ has a layered structure and it is comprised of the distorted polyhedra of six oxygen atoms which form octahedral polyhedra with central V atoms [5]. There are three different oxygen positions in the V₂O₅ crystal structure. The VO₆ octahedra are linked, sharing edges through the chain (O_c) and corners via the bridging oxygens (O_b). Two vanadyl oxygen atoms (O_v) form the vertices of the octahedra along the c-axis [6].

V₂O₅ is highly abundant in nature, it has low price and has several oxidation states ^[7][8][7,8]. Accordingly, due to its relatively open layered structure and its unique properties, V₂O₅ has been used in different applications such as water splitting [9], field effect transistors ^[10][11][10,11], supercapacitors [12], IR detectors [13], photodetectors [14], UV sensors [15], optical sensors [16], amprometric gas sensors [17], potentiometric sensors [18] electrochemical sensors ^[19][20][19,20], cataluminescence sensors [21], resistance gas sensors ^[22][23][22,23], gasochromic sensors [24] and humidity sensors [25].

V₂O₅ with an n-type semiconducting behavior, has a relatively high conductivity (0.5 S·cm⁻¹) at room temperature [26]. At high temperatures, stoichiometric V₂O₅ spontaneously converts to V₂O_5−x as follows:

Values of “x” vary depending on annealing temperature and oxygen partial pressure during synthesis. As a result, oxygen vacancies form in the oxygen sub-lattice and n-type semiconducting is induced. To be neutral, some V⁵⁺ ions will be reduced to V⁴⁺ ions. Electrical conductivity in V₂O_5−x is due to jumping (hopping) of the electrons from V⁴⁺ ions to the neighboring V⁵⁺ ions [26]. Compared with bulk V₂O₅, nanostructured V₂O₅ materials have unique electrical and chemical properties and due to their ultrafine sizes, they offer a high surface area which is extremely beneficial for sensing studies ^{[27][28][29][30]}[27,28,29,30]. Nowadays gas sensors have become widely used in different areas for detection of toxic, hazardous, explosive and greenhouse gases and vapors ^[31][32][31,32]. There are many types of gas sensors. However, resistive-based gas sensors using metal oxides are the most widely used gas sensors due to their unique features such as low price, high sensitivity, high stability, fast dynamics and simple fabrication and operation ^[33][34][33,34]. In  a typical gas sensor,  the front side of substrate is equipped with conductive electrodes such as with Pt and its back side is equipped with a heater [35].

2. Pristine Nanostructured V

O

Gas Sensors

Pristine V₂O₅ gas sensors with different morphologies have been reported in the literature. The gas sensing characteristics of V₂O₅ hierarchical architectures, especially for hollow spheres are rarely reported in the literature. In this regard, V₂O₅ hollow spheres (500–550 nm in diameter and a shell thickness of 55 nm) were synthesized through a solvothermal route. The hollow spheres were comprised of nanoplates with thicknesses of 50–80 nm and lengths of 70–120 nm. Moreover, for comparison solid nanostructured spheres were fabricated. The maximum responses (R_a/R_g) to trimethylamine (TEA) at 370 °C were 9.7 for V₂O₅ hollow spheres and 3.08 for V₂O₅ solid spheres, respectively. In fact, the hollow sphere hierarchical architecture with a higher surface area offered more adsorption sites for TEA molecules and a higher response for hollow spheres resulted. The sensing mechanism of the sensor to TEA was related to the reduction of V⁵⁺ ions to V⁴⁺ ions in the presence of TEA. Furthermore, based on XPS studies there was a slight shift to lower binding energy values after exposure of the sensor to TEA gas, confirming formation of V⁴⁺ ions. In addition, a color change from yellow to dark blue was observed, further confirming the formation of V⁴⁺ ions. V₂O₅ is an acidic oxide, which is highly suitable for the adsorption of basic molecules, such as TEA and consequently a larger response of the V₂O₅ hollow spheres sensor to TEA resulted ^[36][37]. Generally, resistive based gas sensors work at high temperatures, which need external heaters and increase the power consumption. Therefore, development of room temperature gas sensors not only solves above problems, but also integration with flexible substrates become easier. Furthermore, possible risk of explosion during sensing of explosive gases such as H₂ gas significantly decreases. Hollow spheres comprising numerous nanocrystals of V₂O₅ as a shell were synthesized by a facile polyol approach for room temperature hydrogen gas sensing. The surface area of hollow spheres was about 356 m²/g and, therefore, it provided a large active surface area for adsorption of target gases. Furthermore, its porous structure led to further enhancement of gas reactions. Furthermore, the sharp corners as well as the edges of the tiny building blocks of hollow spheres were reported as highly active sites for enhancement of the sensing reactions during hydrogen sensing ^[37][38]. Another room temperature gas sensor was realized from V₂O₅ nanoneedles which were synthesized by a physical vapor deposition method ^[38][39]. The sensor exhibited a response (R_a/R_g) of 2.37 to 140 ppm acetone at room temperature. The relevant reaction was as follows: The most energetically favorable gas reaction is that a surface oxygen atom attacks the carbonyl carbon to form a C-O bond. Acetone contains the carbonyl group and because of the greater electronegativity of oxygen; a carbonyl group is a polar functional group and, therefore, it has a larger dipole moment (D = 2.88), relative to other tested gases, leading to the higher response of the gas sensor to acetone. Trimethylamine (TMA; (CH₃)₃N)) is generated from dead fish and, therefore, the concentration of TMA is an indicator of the freshness of fish ^[39][40]. Furthermore, exposure to TMA vapor can cause nausea, headaches, and irritation to the eyes ^[40][41]. In this regard, spherical V₂O₅ hierarchical nanostructures comprised of plenty of nanosheets were produced through a hydrothermal method. The optimal sensor based on spherical V₂O₅ hierarchical nanostructures displayed a response of ~2.8 to 100 ppm TMA along with fast response/recovery times (5/28 s) at 240 °C. The spherical V₂O₅ nanostructures were comprised of numerous monocrystalline nanosheets, and therefore they have a unique three-dimensional hierarchical structure which provided plenty of active sites for gas molecules. Accordingly, the reaction between chemisorbed oxygen ions and TMA molecules, resulted in a decrease in electrical resistance and contributed to the sensor signal. In addition, in a monocrystalline structure free electrons were able to transfer faster than in a polycrystalline structure, which results in fast response and recovery times of gas sensors and improvement of sensing properties ^[41][42]. Table 1 presents the gas sensing characteristics of pristine V₂O₅ gas sensors, where different morphologies of V₂O₅ prepared using various methods have been successful for sensing of different gases. Gas sensing properties of pristine VO gas sensors. 

V2O5 Morphology	Synthesis Method	Target Gas	Conc. (ppm)	Response (Ra/Rg) or (Rg/Ra)	Response (Ra/Rg) Or (Rg/Ra) T (°C)		T (°C) Response time/Recovery Time(s)	Response Time/Recovery Time(S) Ref.
		Ref.
Hollow spheres Solid spheres	Solvothermal	C3H
V2O5	9	N	/In2O3 core–shells	Hydrothermal 500 500	9.7 3.08		n-propylamine370	20/83 45/150		200	4	190	48/121 [36][37]
	[	86	]	[	92	]		Hollow spheres	Chemical synthesis	H2	200	2.8	RT	50/10	[37]
MoO3-V2O5 thin films	[	Chemical spray pyrolysis	38	]
	NO	2	120	80 *	200	118/1182	[87][93]	Nanoneedles
			(MoO3)0.4(V2O5)0.6 sheet composite	PVD		C3H6O	140	2.35	RT		Chemical spray pyrolysis 67/-	[	100	115	39/453	[8838][39]
]	[	94	]		Hierarchical nanostructures
Au/V2O5/CuWO	Hydrothermal	4 composite	C3	Hydrothermal	H9N	NH3 10–200	5 1.13-3.57	2.7 240		1505/28	[41][42]
	35/33					[	89	]	[98]	Flower-like	Hydrothermal	5
SnO2/V2O5 composite	Sol-gel	C6H6	2.25	200	13/13		200	10.5 [42	275 ][43]
	-/-			[	90	]	[99]	Nanorods
V2O5/polyvinyl acetate NF composite	CVD	Electrospinning NH3	NH3 100	0.8 235 *		6 *400	-/-		260[43][44]
		-/-			[	91	][100]	Nanorods	Solvothermal
V2O5/ZnV2O6 nanobelt composite	Chemical route	C2H5OH NH3	C2H5OH 500		2000 1.04 1.02	16.5	RT		240-/-		-/- [44][45]
	[	92		]	[	101		]		Spherical
TiO2/V2	Precipitation	O5 NF composite C	Electrospinning 2H5OH NH3	100 1.04 1.06	-/-	[	24.6 45]	350 [46]
	6/7			[	93	][102]	Flower-like
ZnO/V2O5 thin films	Hydrothermal		Spray pyrolysis	1-butylamine	C7H8 100	400 2.6		2.3140		27 9/49	23/28 [46][47]
	[	94	]		[	103	]		Nanofibers	Electrospinning	9.5	500 *	RT	-/-	[47][48]
Flower-like Sheet-like	Hydrothermal	100	3.6 2.8		300	25/14 17/14	[48][49]
Nanofibers	Sol–gel	NH3	2.1	11 *	200	50/350	[49][50]
Flower-like	DC sputtering	CH4	500		100	206/247	[50][52]
	Hydrothermal	C8H10		3	300	44/74	[51][54]
Nano stars	Hydrothermal	He	300	53 *	RT	9/10	[52][55]
Nanorods	Chemical spray pyrolysis	NO2	100	24.2 *	200	13/140	[53][56]
Nanofibers	Chemical spray pyrolysis	C8H10		27	RT	80/50	[54][57]
Nanowires	Melt quenching	C2H5OH	1000	9.09	330	-/-	[55][58]
Thin film	Plasma focus method	H2		50 *	275	-/-	[56][59]
	Chemical spray pyrolysis	NO2	100	41 *	200	20/150	[57][60]

* Response = Δ𝑅/𝑅𝑎×100; RT: Room temperature; PVD: Physical vapor deposition; CVD: Chemical vapor deposition.

3. Decorated/Doped V₂O₅ Gas Sensors

Based on the above section about pristine V₂O₅ gas sensors, pristine V₂O₅ nanostructures suffer from low sensitivity and selectivity, which hinder their applications for sensing applications ^[58][61]. Accordingly, different strategies, such as p-n heterojunction formation ^[59][62], n-n heterojunction formation ^[60][61][63,64] and noble metal decoration ^[62][65] have been proposed to enhance their sensing properties. V₂O₅ decoration is a good strategy to enhance the sensing properties of gas sensors. V₂O₅-decorated α-Fe₂O₃ composite NRs were prepared by an electrospinning technique and they had a high surface area of 30.5 m²/g. The composite exhibited a response (R_a/R_g) of 9 to 100 ppm diethylamine along with good selectivity to diethylamine gas and a fast response time of 2 s at 350 °C ^[63][66]. Improved sensing performance was related to the formation of V₂O₅-Fe₂O₃ heterojunctions and catalytic effect of V₂O₅ to diethylamine. Porous silicon (PS) is a good candidate for sensing studies due to its offering of a high surface area and a porous structure. In addition it can be simply prepared by a chemical etching process ^[64][68]. Therefore, composites between V₂O₅ and PS can be promising for gas sensing studies. In a relevant study, thin V films were decorated on the PS by sputtering at different times of 30 and 60 min and then, the samples were annealed at 600 °C ^[65][69]. The PS/V₂O₅ NRs structure provided a better response than pristine PS at 25 °C, and the sensor sputtered for 60 min exhibited the highest response of 7.4 to 2 ppm NO₂ gas. Both the PS and V₂O₅ NRs had plenty of dangling bonds, oxygen vacancies and defects, leading to high adsorption of oxygen molecules even at room temperature. In the interfaces between PS and V₂O₅, p-n heterojunctions formed and, upon exposure to NO₂ gas, the significant modulation of electrical resistance in the heterojunctions led to the appearance of a sensing signal. Graphene and its derivations such as graphene oxide and reduced graphene oxide have high surface areas and unique electrical properties which can be beneficial for gas sensing studies ^[66][67][70,71]. The initial resistance of the GO is high, limiting its practical applications in pristine form ^[68][69][72,73]. However, after the reduction GO to RGO, there are some defects, vacancies and functional groups which are useful for gas sensing applications ^[70][74]. In a relevant study, an RGO surface was decorated with Mn₃O₄ and V₂O₅ NOs via a hydrothermal method for detection of H₂ gas at 30 °C. The sensor showed a high response (ΔR/R_a × 100) of 174% to 50 ppm H₂ gas at room temperature. The sheet-like structure of RGO provided a large surface area for gas sensing reactions. In addition, because of the formation of p-n (RGO-V₂O₅) and p-p (RGO-Mn₃O₄) heterojunctions, significant modulation of resistance in the presence of H₂ occurred, resulting in the generation of a sensing signal ^[71][75]. In another study, a V₂O₅ film was prepared by a reactive sputtering technique and then, RGO was decorated over the V₂O₅ thin film by a drop casting method for NO₂ sensing studies. The sensor showed a response of 50.7% to 100 ppm NO₂ gas at 150 °C. However, its recovery time was long (778 s). Formation and modulation of the p-n heterojunction at the interface of RGO and V₂O₅ was the main reason for the detection of NO₂ gas. Moreover, the presence of active sites such as oxygen functional groups on the RGO surface improved the sensing response ^[72][76]. Not only can V₂O₅-decoration be a useful technique to enhance the gas sensing properties, but decoration of other metal oxides or noble metals on the surface of gas sensor can also be a good technique to improve the gas sensing properties of V₂O₅-based gas sensors. A P-type CuO with excellent catalytic activity is extensively used for sensing studies ^[73][77]. The work function of CuO is 5.3 eV, which is different to that of V₂O₅ (4.7 eV). Therefore, when heterojunctions form between the CuO and V₂O₅, enhanced gas response can be expected. In this context, hollow nanostructures using CuO decorated V₂O₅ nano-strings of pearls were fabricated through an electrospinning method. The V₂O₅/CuO sensors demonstrated a response (R_a/R_g ) of 8.8 to 500 ppm acetone at 440 °C, which was more than three times higher than that of bare V₂O₅ NFs. The improved performance was related to the generation of CuO/V₂O₅ p-n heterojunctions, which provided plenty of resistance modulation sources and upon exposure to acetone gas higher response was resulted ^[74][78]. Table 2 shows the gas sensing properties of decorated or doped V₂O₅-based gas sensors, where different synthesis methods along with different morphologies and various materials have been reported to realize gas sensors for the sensing of toxic gases.