Sensing Performances of Hierarchical Nano-Layered V₂O₅ Structures and Ab Initio Calculation of Their Gas-Adsorption Properties

1. Introduction

The release of these toxic gases (NO, SO₂, CO, CO₂, SO₂, H₂S, and NH₃) from chemical industries, mines, and power plants has put a strain on the health of human beings, resulting in cardiovascular illnesses such as asthma, arrhythmia, and heart failure etc.), especially in pregnant women and elderly people. Moreover, these toxic gases and volatile gases have also contributed immensely to climate change resulting in extreme weather occurrences. The demand for highly efficient, reliable, and low-cost gas sensors has increased tremendously all over the world in recent years. With the advances of the fourth industrial revolution in bridging gaps between the field of science and technology, nanotechnology has been marked to be a major player among 4.0 emerging technologies. Nano gas sensors have gained extensive use in the medical field, to diagnose diseases and identify the nature of illness through monitoring the exhaled breath of patients. However, the most reliable and highly recommended technique for gas detection nanoparticle-based materials is to be considered, especially in the medical field for long-term health purposes.

The release of toxic gases (e.g., NO, SO₂, CO, CO₂, H₂S, and NH₃) from chemical industries, mines, and power plants poses a significant threat to human health, leading to cardiovascular illnesses such as asthma, arrhythmia, and heart failure, particularly among pregnant women and the elderly. Furthermore, these toxic and volatile gases contribute immensely to climate change and extreme weather events. Consequently, global demand for highly efficient, reliable, and low-cost gas sensors has increased tremendously in recent years.

Advancements linked to the Fourth Industrial Revolution, which bridges science and technology, have positioned nanotechnology as a major player among emerging Industry 4.0 technologies. Nano gas sensors are now extensively used in medicine to diagnose diseases and identify illnesses by monitoring patients' exhaled breath [5,6,7,8]. For reliable, long-term health monitoring, nanoparticle-based materials are considered a highly recommended technique for gas detection.

The use of metal oxide semiconductors (MOS) materials for the development of gas sensors can be dated back to 1954 and has continued to be one of the most effective techniques in monitoring the concentration of gases in the atmosphere. Nano MOS materials like SnO₂, ZnO, WO₃, V₂O₅, Fe₂O_3, and TiO₂ are widely investigated for gas sensor applications because of their high surface to volume ratio, low manufacturing cost, and capability of detecting a large number of toxic gases at different temperature. The sensing mechanism of these MOS materials lies in the changes in electrical conductivity upon exposure to a concentration of gases due to catalytic reduction/oxidation reactions occurring at the oxide surface. In recent years, gas sensors based on 2-dimensional structures (2D)- such as vanadium pentoxide (V₂O₅)_, molybdenum disulphide (MoS₂₎, tungsten disulphide (WSe₂), hexagonal boron nitride (h-BN), Carbon materials, and graphene nanostructures have been attracting tremendous attention for gas sensor fabrication.

The use of metal oxide semiconductor (MOS) materials for gas sensor development dates back to 1954 and remains one of the most effective techniques for monitoring atmospheric gas concentrations. Nano-MOS materials such as SnO₂, ZnO, WO₃, V₂O₅, Fe₂O_3, and TiO₂ are widely investigated for gas sensing due to their high surface-to-volume ratio, low manufacturing cost, and capability of detecting numerous toxic gases at different temperatures. The sensing mechanism of these materials relies on changes in electrical conductivity upon exposure to target gases, driven by catalytic reduction/oxidation reactions at the oxide surface. Recently, gas sensors based on two-dimensional (2D) structures—such as vanadium pentoxide (V₂O₅), molybdenum disulphide (MoS₂), tungsten disulphide (WSe₂), hexagonal boron nitride (h-BN), carbon materials, and graphene nanostructures—have attracted tremendous attention for sensor fabrication.

Among various 2D-based structures, V₂O₅ is an upcoming gas-sensing material because of its low optical bandgap of 2.5 eV, and excellent thermal and thermoelectric properties relative to other semiconducting metal oxides. This material is among the top 10 most researched oxides. Moreover, this material is considered the most stable among other vanadium oxide members such as vanadium monoxide (VO), vanadium dioxide (VO₂), and vanadium sesquioxide (V₂O₃). V₂O₅ possesses different polymorphs namely α-V₂O₅ (orthorhombic), metastable β-V₂O₅ (tetragonal or monoclinic), γ-V₂O₅ (orthorhombic), and δ-V₂O₅ (monoclinic) with each being stable at different temperature and pressure. α-V₂O₅ is the most commonly used material for gas sensing applications compared to the other polymers. This material exhibits Mott Hubbard’s metal-to-insulator (MTI) at a given temperature (375°C). Its unique crystallographic unit cell structure, electronic properties, high surface area to volume ratios, and several reaction sites enable it to exhibit high selectivity and response towards different gases.

Among various 2D-based structures, V₂O₅ is an emerging gas-sensing material owing to its low optical bandgap (~2.5 eV) and excellent thermal and thermoelectric properties compared to other semiconducting metal oxides [1,2,3]. It ranks among the top ten most researched oxides [4] and is considered the most stable member of the vanadium oxide family, which includes vanadium monoxide (VO), vanadium dioxide (VO₂), and vanadium sesquioxide (V₂O₃) [5]. V₂O₅ exhibits several polymorphs, including α-V₂O₅ (orthorhombic), metastable β-V₂O₅ (tetragonal or monoclinic), γ-V₂O₅ (orthorhombic), and δ-V₂O₅ (monoclinic), each stable under different temperature and pressure conditions [6,7]. The α-V₂O₅ phase is the most commonly used for gas sensing and exhibits a Mott–Hubbard metal-to-insulator transition (MTI) at approximately 375 °C [8,9]. Its unique crystallographic structure, electronic properties, high surface-area-to-volume ratio, and abundant reaction sites enable high selectivity and sensitivity toward various gases [10,11,12,13,14].

Research has shown that different atomic arrangements or structures of semiconducting materials called hierarchical structures, often result in different chemical, optical-electronic, magnetic, etc. properties [15,16]. This has also been demonstrated in the case of gas and chemical sensing where hierarchical structures of materials exhibited different adsorption properties (surface areas, pores volumes, and pore diameters) which consequently yielded different sensitivity, response time, recovery time, and gas selective ability [17,18,19]. For instance, the large surface area of WO₃ hierarchical structures has been said to provide more channels which not only make gas diffusion great but also allow more active sites for adsorbing gases, thus leading to high gas sensing performance [19]. Higher gas response of SnO₂ nanoneedles with quicker response and recovery of the same material when in nanosheets assemble was reported [20]. The high response property of the nanoneedles structures is attributed to the large surface area from the Brunauer-Emmett-Teller (BET) test. More adsorption and desorption sites were also reported for nanoneedle structures which allowed gas molecules to easily diffuse across the surface. Spacing among the nanosheets is large (compared to an open room) relative to nanoneedles which give high pores for quicker response and recovery. Other reports have also shown that nanowires are the most researched morphology with 40%, followed by nanobelts and nanorods with 20%, nanotubes with 20%, and nanobelts with 20% [4].

Research has shown that hierarchical structures—different atomic arrangements or assemblies of semiconducting materials—can lead to distinct chemical, optoelectronic, and magnetic properties [15,16]. This principle also applies to gas and chemical sensing, where hierarchical morphologies influence adsorption properties (e.g., surface area, pore volume, pore diameter), thereby affecting sensitivity, response time, recovery time, and selectivity [17,18,19]. For instance, the large surface area of WO₃ hierarchical structures provides more channels for gas diffusion and additional active sites for adsorption, leading to enhanced sensing performance [19]. Similarly, SnO₂ nanoneedles have demonstrated higher gas response, while SnO₂ nanosheet assemblies enable quicker response and recovery [20]. The improved performance of nanoneedles is attributed to their large BET surface area, which offers abundant adsorption/desorption sites and facilitates gas molecule diffusion on the materials ‘surface. In contrast, the wider spacing between nanosheets creates more open pores, promoting faster response and recovery. Reports indicate that nanowires are the most researched morphology (≈40%), followed by nanobelts, nanorods, and nanotubes [4].

It has been reported that the hierarchical hollow structure of Zinc stannate (ZnSnO₃ ) composed of ultra-thin nanorods as building blocks has shown fast response and recovery capacities, and good repeatability in the detection of ethanol. This is related to the hollow structure having efficient surface area and surface accessibility [21]. Due to these exceptional properties, ZnSnO₃ shows permeable surfaces that allow easy absorption of ethanol gas molecules [22] resulting in a fast response. Zinc oxide (ZnO) nanorod is another unique material that exhibits a quicker response and recovery speed towards ethanol detection, as compared to nanosheets which display a larger ethanol response. The quicker response of ZnO nanorod is due to its better conductivity and lower potential barrier, while the larger response of nanosheets results from its higher specific surface area [23]. Other studies have also reported that hierarchical and hollow Indium oxide (In₂O₃) material has shown higher response and recovery times as compared to its agglomerated counterparts (powder) in the detection of carbon monoxide (CO) [24]. This is a result of its structural assembly which allows effective and rapid gas diffusion toward the entire sensing surface. Metal oxide nanostructures/Porous Silicon (PS) composites, such as PS/WO₃ [25] and PS/ZnO [26,27] have been reported to show good sensing abilities towards different gases. PS/V₂O₅ nanorod composite material exhibits high response and good selectivity towards NO₂ at room temperature [28]. The porous silicon provides a high surface-to-volume ratio in composites (PS/V₂O₅) while the nanorods have specific surface areas and dimensions comparable to Debye length. The heterojunction between the two helps the material in achieving good sensing performance [28].

Hierarchical hollow structures also show promise. For example, zinc stannate (ZnSnO₃) composed of ultrathin nanorod building blocks exhibits fast response/recovery, good repeatability, and efficient ethanol detection due to its high surface area and accessibility [21,22]. Zinc oxide (ZnO) nanorods demonstrate quicker response and recovery toward ethanol compared to nanosheets, owing to better conductivity and a lower potential barrier, whereas nanosheets show a larger response due to their higher specific surface area [23]. Hierarchical and hollow indium oxide (In₂O₃) structures exhibit higher response and faster recovery times than their agglomerated powder counterparts in carbon monoxide (CO) detection, a result of their assembly promoting effective and rapid gas diffusion [24].

Metal oxide nanostructure/porous silicon (PS) composites, such as PS/WO₃ [25] and PS/ZnO [26,27], have also demonstrated good sensing abilities. Notably, a PS/V₂O₅ nanorod composite exhibits high response and good selectivity toward NO₂ at room temperature [28]. In such composites, porous silicon provides a high surface-to-volume ratio, while the nanorods offer specific surface areas and dimensions comparable to the Debye length. The heterojunction between the two materials further enhances sensing performance [28].

Having established that the morphologies or structures of metal oxide nanomaterials have a relationship with the chemical, physical, and gas-sensing properties of a material, it is obvious that crucial information about the choice of gas-sensing materials can be obtained. It is then essential to critically review the structures of V₂O₅ for the benefit of exploring them for gas sensor development. Thus, this paper presents a systematic review of the sensing properties of V₂O₅ nanomaterials in hierarchical structure assembly.

Given the established relationship between morphology (hierarchical structure) and the chemical, physical, and gas-sensing properties of metal oxide nanomaterials, crucial insights can be gained for selecting optimal sensing materials. Therefore, a critical review of V₂O₅ nanostructures is essential to explore their full potential for gas sensor development. This paper presents a systematic review of the sensing properties of V₂O₅ nanomaterials with a focus on hierarchical structure assemblies.

Figure 1. Summary of V₂O₅ morphologies with their possible detectable gases.

Figure 1. Summary of V₂O₅ morphologies with their possible detectable gases.

2. Hierarchical Nanostructures of V₂O₅

2.1. V₂O₅ Nanobelts

Vanadium pentoxide (V₂O₅) nanobelts are distinct metal oxide semiconductor nanoparticles that are in the form of a belt [29]. They are distinguishable from other morphologies such as nanotubes and nanowires by their rectangular cross-sectional area, which allows a complete understanding of dimensionally limited transport events at the nanoscale scale [30]. The V₂O₅ single crystalline nanobelts are often synthesised through the hydrothermal technique, which is a versatile and convenient process that allows full control of size, shape, and crystallinity [31,32]. This process often results in nanobelts that are highly uniform in size and shape. Their high porosity also makes them more suitable for the absorption of different species. Nanobelts possess good electrical conductivity, which allows them to be more useful in various applications such as solid-state batteries, supercapacitors, and gas sensors. However, in gas sensor applications, the fabricated gas sensors based on pristine V₂O₅ nanobelts have shown moderate response towards detecting gases such as ethanol and butylamine at different temperatures (Table 1). [33]. Many studies have suggested surface modification, through coating the surface of V₂O₅ nanobelt with noble metal nanoparticles as well as doping with transition metals, as an effective method to enhance the material sensitivity and stability [34].

Vanadium pentoxide (V₂O₅) nanobelts are distinct metal oxide semiconductor nanoparticles with a belt-like morphology [29]. They are distinguished from other morphologies, such as nanotubes and nanowires, by their rectangular cross-section, which facilitates the study of dimensionally confined transport phenomena at the nanoscale [30]. Single-crystalline V₂O₅ nanobelts are often synthesised via the hydrothermal technique, a versatile and convenient process that allows full control over size, shape, and crystallinity [31,32]. This method typically yields nanobelts with high uniformity in size and shape. Their high porosity enhances their suitability for absorbing various species. Nanobelts also possess good electrical conductivity, making them useful in applications such as solid-state batteries, supercapacitors, and gas sensors. However, in gas sensing, sensors fabricated from pristine V₂O₅ nanobelts have shown only a moderate response to gases such as ethanol and butylamine at different operating temperatures (Table 1) [33]. Consequently, many studies suggest that surface modification—through coating with noble metal nanoparticles or doping with transition metals—is an effective method to enhance sensitivity and stability [34].

The surface of the V₂O₅ nanobelt has been reported to be significantly enhanced when coated with metal oxide nanoparticles such as SnO₂, TiO₂, and Fe₂O₃. This has been observed to increase the width and thickness of the sensing material by 60-100 nm and 10-20 nm, respectively, compared to the pristine V₂O₅ nanobelt. The sensitivity of the coated V₂O₅ nanobelt has been reported to be three times that of the pristine V₂O₅ nanobelt. This is attributed to the difference in sensing mechanism [34]. The adsorption process of the coated V₂O₅ nanobelt occurs primarily at the surface of the metal oxide. The smaller-sized nanocrystal grains of the coating metal oxide enable the adsorbed species to interact with all the atoms, resulting in the formation of a space-charged region that covers the entire metal oxide surface. The depth of the space-charged region is influenced by the oxygen ionosorption when the sensor is exposed to air. Upon exposure to a reducing gas such as ethanol, the adsorbed species react with ethanol, and electrons are released back into the surface of the sensor. These electrons travel through the metal oxide into the V₂O₅ nanobelt, resulting in conduction [35].

The surface properties of V₂O₅ nanobelts are reported to improve significantly when coated with metal oxide nanoparticles such as SnO₂, TiO₂, and Fe₂O₃. This coating increases the width and thickness of the sensing material by approximately 60–100 nm and 10–20 nm, respectively, compared to pristine V₂O₅ nanobelts. The sensitivity of the coated nanobelts can be up to three times higher, a difference attributed to a change in the sensing mechanism [34]. In the coated system, gas adsorption occurs primarily on the metal oxide surface. The small nanocrystal grains of the coating allow adsorbed species to interact with most atoms, forming a space-charge region that covers the entire metal oxide surface. The depth of this region is modulated by oxygen ionosorption when the sensor is in air. Upon exposure to a reducing gas like ethanol, the adsorbed species react, releasing electrons back to the sensor surface. These electrons then travel through the metal oxide into the V₂O₅ nanobelt, thereby increasing conduction [35].

2.2. V₂O₅ Thin-Films

The nanostructured vanadium oxide thin films are one of the metal oxides that possess variable optical properties and have been applied in a range of devices, including optical, catalysis, electrical switching, and gas sensing applications [40]. The functional properties of these thin films are highly influenced by several parameters such as surface area, crystallinity and the method or technique used for preparation [41]. There are several reported methods used for V₂O₅ thin film preparation, such as chemical vapour deposition [42], sol-gel method [43], and pulsed laser deposition [44,45]. Among them, pulsed laser deposition (PLD) has been reported to be an inexpensive and less energy-consuming process. This technique is capable of operating at a low temperature and has demonstrated advantages in the preparation of V₂O₅ films. These include easy control of the thickness and composition of the film by tuning the deposition parameters, as well as a good reproducible stoichiometry of the target material in the films for gas sensing applications [46]. In addition, this flexible and powerful method usually gives V₂O₅ thin films in various spherical sizes, with an average grain size of 50 nm and the surface roughness ranging from 9-20nm [47]. Reports have shown that V₂O₅ thin film demonstrates a good response towards oxidizing gases such as NO₂, organic amines, ethanol, and ammonia at different temperatures (Table 2).

Nanostructured vanadium oxide thin films are metal oxides with tunable optical properties, applicable in optical devices, catalysis, electrical switching, and gas sensing [40]. Their functional properties are highly influenced by parameters such as surface area, crystallinity, and the preparation method [41]. Several techniques are used to prepare V₂O₅ thin films, including chemical vapour deposition [42], the sol–gel method [43], and pulsed laser deposition (PLD) [44,45]. Among these, PLD is often reported as an inexpensive and low-energy process. It operates at low temperatures and offers advantages such as easy control over film thickness and composition through deposition parameters, as well as good reproducible stoichiometry from target to film, which is crucial for gas sensing [46]. This flexible and powerful method typically yields V₂O₅ thin films with various spherical grain sizes, averaging 50 nm, and surface roughness ranging from 9 to 20 nm [47]. Reports indicate that V₂O₅ thin films demonstrate a good response to oxidising gases such as NO₂, organic amines, ethanol, and ammonia at different temperatures (Table 2).

Other studies have revealed that coupling V₂O₅ thin-film material with another composite material, such as V₇O_16, improves its response time. Table 2 shows that V₂O₅ coupled with V₇O₁₆ results in improved response time compared to pure V₂O₅. Further, doping V₂O₅ thin films with titanium (Ti) leads to an n-p transition, which shows a good response towards hydrogen (5-300 ppm) as compared to the undoped n-type V₂O₅ thin [55]. In energy storage applications, doping V₂O₅ thin films with manganese (Mn) improves their cyclic stability [56]. For example, at a current density of 68mAg^-1, Mn-doped V₂O₅ thin films have a discharge capacity of 283mAhg^-1, which is significantly higher than pure V₂O₅ thin films 271mAhg^-1 [57].

Other studies have shown that coupling V₂O₅ with another composite material, such as V₇O₁₆, improves its response time. As shown in Table 2, the V₂O₅/V₇O₁₆ composite exhibits a faster response than pure V₂O₅. Furthermore, doping V₂O₅ thin films with titanium (Ti) induces an n-to-p transition, resulting in a good response to hydrogen (5–300 ppm) compared to undoped n-type V₂O₅ [55]. In energy storage applications, doping with manganese (Mn) improves cyclic stability [56]. For example, at a current density of 68 mA g⁻¹, Mn-doped V₂O₅ thin films achieve a discharge capacity of 283 mAh g⁻¹, significantly higher than the 271 mAh g⁻¹ of pure V₂O₅ thin films [76].

2.3. V₂O₅ Nanorods

Vanadium pentoxide nanorods (Figure 4) exhibit nanoscale particles with a rod-like nanostructure. They are synthesised through versatile techniques, namely electrospinning, reverse-micelle, hydrothermal, DC magnetron deposition, etc. Their diameter and length are often measured between 150–200 nm and 1-10 μm [58] respectively, using the TEM. The morphology and size of these nanorods can be effectively manipulated by altering the calcination temperature during synthesis, with lower temperatures promoting the formation of smaller, more uniform nanorods [59]. V₂O₅ nanorods possess high reactivity and thermal stability, making them suitable for various applications, including catalysis, energy storage, fuel cells, and gas sensing. In particular, their large surface area and porous structure are conducive to efficient gas diffusion in the sensing layer, resulting in a quick response to the detection of various gases (Table 3) [60].

Vanadium pentoxide nanorods (Figure 4) are nanoscale particles with a rod-like morphology. They are synthesised through versatile techniques such as electrospinning, reverse-micelle, hydrothermal, and DC magnetron deposition. Their diameter and length typically range from 150–200 nm and 1–10 μm, respectively, as measured by TEM [78]. The morphology and size can be effectively tuned by altering the calcination temperature, with lower temperatures promoting smaller, more uniform nanorods [59]. V₂O₅ nanorods possess high reactivity and thermal stability, making them suitable for catalysis, energy storage, fuel cells, and gas sensing. Their large surface area and porous structure facilitate efficient gas diffusion in the sensing layer, leading to a quick response to various gases (Table 3) [60].

Doping the surface of V₂O₅ nanorods with Tellurium (Te) through a hydrothermal method has been reported to be an effective method to enhance its sensitivity. Te is reported to have a nanorod-like structure, and doping it into V₂O₅ nanorods tends to produce a greater number of reactive sites and also increase the surface-to-volume ratio, which improves the sensitivity of the sensor [63]. Other reports have also revealed that doping the V₂O₅ nanorod’s surface with palladium (Pd) noble metal tends to improve the selectivity of the sensor and decrease the response-recovery times. Pds act as a catalyst and agitate the surface interaction between the sensing layer and the targeted gas [48]. Pd-sensitised V₂O₅ sensor has high porosity nanostructure and greater surface activities. The absorption of the gas takes place on the Pd and then spills over into V₂O₅.

Doping V₂O₅ nanorods with tellurium (Te) via a hydrothermal method has been reported to enhance sensitivity. Te itself has a nanorod-like structure, and its incorporation is believed to increase the number of reactive sites and the surface-to-volume ratio, thereby improving sensor sensitivity [63]. Other reports indicate that doping with palladium (Pd) improves selectivity and reduces response and recovery times. Pd acts as a catalyst, promoting surface interactions between the sensing layer and the target gas [48]. Pd-sensitised V₂O₅ sensors exhibit high porosity and greater surface activity, where gas absorption occurs first on Pd before spilling over to V₂O₅.

The Pd-sensitised V₂O₅ nanorod sensor exhibits a different sensing mechanism from the pristine V₂O₅ nanorod sensor, which follows the general mechanism of an n-type metal-semiconductor. Upon exposure of the Pd-sensitised V₂O₅ nanorods sensor to atmospheric air. The dissociation of the oxygen molecule takes place in the Pd particle, and the adsorbed (O₂ ⁻, O⁻, and O) species diffuses into the surface of the V₂O₅ nanorods, resulting in a space charge region with a high potential barrier [64]. This leads to high resistance on the surface of the nanorods. When the Pd-sensitised V₂O₅ nanorods sensor is then set to a reducing gas atmosphere such as ethanol, the catalytic effect of Pd facilitates the reaction between the surface of the material and adsorbed species, which results in a release of an electron back to the conduction band [61]. The potential barrier reduces, resulting in a decrease in resistance and an increase in conductance. [51,65,66].

The sensing mechanism of Pd-sensitised V₂O₅ nanorods differs from that of pristine V₂O₅, which follows the general n-type metal oxide mechanism. When exposed to air, oxygen molecules dissociate on Pd particles, and the resulting adsorbed species (O₂⁻, O⁻, O) diffuse onto the V₂O₅ nanorod surface, creating a space-charge region with a high potential barrier [64]. This leads to high surface resistance. Upon exposure to a reducing gas like ethanol, the catalytic effect of Pd facilitates a reaction between the gas and adsorbed oxygen, releasing electrons back into the conduction band [61]. This reduces the potential barrier, decreasing resistance and increasing conductance [51,65,66].

2.4. V₂O₅ Nanofibers

Nanofibers are nanoparticle fibres that are often used in energy storage, drug delivery, biometric materials, and gas sensors due to their high surface-to-volume ratio [67,68]. V₂O₅ nanofibers are distinguished from other nanostructures (nanowires, nanoribbons, nanoneedles, nanorods, and nanotubes) by their regular structure (1.5 nm × 10 nm cross-section), [69,70]. These nanofibers are synthesised by combined methods (electrospinning and the sol-gel) which have proven to produce small-diameter fibers that require no expensive purification [71,72]. Both electrospinning and sol-gel methods are well-known for their low cost, and combining the two results in long, continuous, uniform nanofibers with a very significant surface area and reduced diameter [73]. V₂O₅ nanofibers are often prepared as an n-type semiconductor with an electronic conductivity of 0.5 cm⁻¹ at room temperature, which makes it an ideal material for novel devices that can operate at room temperature [74]. These unique electronic conductivity properties allow V₂O₅ nanofibers to be used as an ideal gas sensor material and have shown a good response time when detecting different gases. (Table 4). The addition (doping) of a small amount of silver (Ag) to the surface of V₂O₅ nanofibers increases the selectivity and sensitivity of the material tremendously. When V₂O₅ nanofiber is doped with Ag (Ag-V₂O₅), the width of the nanofiber formed ranges from 8 nm to 15 nm, which is similar to a pure V₂O₅ nanofiber (width of ∼10 nm) [70].

Nanofibers are nanoparticle fibres widely used in energy storage, drug delivery, biometric materials, and gas sensors due to their high surface-to-volume ratio [67,68]. V₂O₅ nanofibers are distinguished from other nanostructures (nanowires, nanoribbons, nanoneedles, nanorods, and nanotubes) by their regular structure, typically with a cross-section around 1.5 nm × 10 nm [69,70]. They are often synthesised by combined methods such as electrospinning and sol–gel, which produce small-diameter fibers without requiring expensive purification [71,72]. Both techniques are low-cost, and their combination yields long, continuous, uniform nanofibers with a high surface area and reduced diameter [73]. V₂O₅ nanofibers are typically n-type semiconductors with an electronic conductivity of ~0.5 Ω⁻¹ cm⁻¹ at room temperature, making them suitable for novel room-temperature devices [74]. These conductive properties make V₂O₅ nanofibers an ideal gas sensing material, and they have shown good response times to various gases (Table 4). Doping with a small amount of silver (Ag) significantly increases both selectivity and sensitivity. Ag-doped V₂O₅ nanofibers have a width ranging from 8 to 15 nm, similar to that of pure V₂O₅ nanofibers (~10 nm) [70].

2.5. V₂O₅ Nanoflowers

Vanadium pentoxide nanoflowers are nanostructured particles that resemble a flower pattern [77]. They are made up of ultrathin nanowires and nanoribbons. The large surface-to-volume ratio of ultrathin particles provides a strong surface effect, which is very beneficial for gas adsorption [78]. The space between the lateral dimensions of the sheets in the nanoflower structure enhances the surface area, providing additional reactive sites for the target chemical [79]. Flower-like V₂O₅ hierarchical structure exhibits unusual p-type semiconductor features below 100^◦C which is the result of the ultrathin structure of the nanowire and nanoribbon [80]. V₂O₅ nanoflower-like structures can be synthesised using various methods such as hydrothermal process [81], water bath method [82], and electrochemical process [83]. The diameter of each petal in the nanoflower usually consists of a thickness less than 100 nm, and the diameter ranges between 400–700 nm [84]. V₂O₅ nanoflowers are heavily explored in the gas sensing application because of their high surface-to-volume ratio, higher surface activity, and high thermal stability [85]. Different studies have reported on using V₂O₅ nanoflowers for the detection of different gases. (Table 5).

Vanadium pentoxide nanoflowers are nanostructured particles with a flower-like pattern, composed of ultrathin nanowires and nanoribbons [77]. The large surface-to-volume ratio of these ultrathin components provides a strong surface effect, which is highly beneficial for gas adsorption [78]. The spacing between the sheets in the nanoflower structure further increases the surface area, providing additional reactive sites for target chemicals [79]. Flower-like V₂O₅ hierarchical structures exhibit unusual p-type semiconductor behaviour below 100 °C, a result of their ultrathin nanowire and nanoribbon building blocks [80]. They can be synthesised via various methods, including hydrothermal processing [81], the water bath method [82], and electrochemical processes [83]. Each petal typically has a thickness below 100 nm and a diameter between 400–700 nm [84]. V₂O₅ nanoflowers are extensively explored for gas sensing due to their high surface-to-volume ratio, surface activity, and thermal stability [85]. Various studies have reported their use in detecting different gases (Table 5).

The gas sensor based on a nanoflower V₂O₅ has demonstrated a significantly quicker response towards the detection of ethanol and methane gas at different concentration levels compared to other morphologies, including honeycomb-like and nano-chain, especially [90]. Moreover, this material has also demonstrated an increased sensitivity towards ethanol gas at a concentration of 1.37–25 ppm, which is substantially lower than the breath analyser limit (200 ppm) at room temperature [91]. The superior performance of nanoflower is highly attributed to its porous and high crystalline nature [91].

Gas sensors based on V₂O₅ nanoflowers demonstrate a significantly faster response to ethanol and methane at various concentration levels compared to other morphologies such as honeycomb-like and nanochain structures [90]. Moreover, they exhibit increased sensitivity to ethanol at concentrations of 1.37–25 ppm, substantially below the typical breath analyser limit of 200 ppm, and can operate at room temperature [91]. This superior performance is attributed to their porous and highly crystalline nature [91].

2.6. V₂O₅ nanowires

Nanowires are often referred to as one-dimensional (1D) or quasi-materials that resemble a wire. These materials are synthesised by a variety of techniques, including hydrothermal, chemical vapour deposition (CVD), sol-gel, and electrospinning [92,93]. Electrospinning is the most commonly used technique due to its ability to produce nanowires with high porosity and high surface area. The average diameter of each nanowire synthesised by electrospinning is found to be 200-250 nm [94]. Over the years, V₂O₅ nanowires have been used in a wide range of applications such as electrodes in supercapacitors, components in lithium-ion batteries, and catalysis in organic synthesis. V₂O₅ nanowires have also been widely used in electrochemical sensors as a sensing material due to their unique properties, such as a large surface-to-volume ratio, superior stability owing to the high degree of crystallinity, and dimensions comparable to the extension of the surface charge region [95,96]. V₂O₅ nanowires have shown good response towards different gases at different concentrations and temperatures.

Nanowires are one-dimensional (1D) or quasi-1D materials with a wire-like morphology. They are synthesised by various techniques, including hydrothermal, chemical vapour deposition (CVD), sol–gel, and electrospinning [92,93]. Electrospinning is the most common due to its ability to produce nanowires with high porosity and surface area; the average diameter of electrospun nanowires is typically 200–250 nm [94]. V₂O₅ nanowires have been used in supercapacitor electrodes, lithium-ion battery components, and catalysis in organic synthesis. They are also widely used in electrochemical sensors due to their unique properties: a large surface-to-volume ratio, superior stability from high crystallinity, and dimensions comparable to the extent of the surface charge region [95,96]. V₂O₅ nanowires show good responses to various gases at different concentrations and temperatures.

However, studies have indicated that materials of nanowire structure are not commonly available due to the fact that they show non-uniform growth, poor control over density and distribution, reproducible contact between nanowires and electrodes, and sensor geometry [102]. Modification of the V₂O₅ nanowire surface via decorating with other oxides such as tin oxide (SnO₂) and copper oxide (CuO) (Table 6) has been reported to be an effective method to enhance its response time and sensitivity towards the detection of different gases. Previous reports have indicated that SnO₂-decorated V₂O₅ exhibits a heterojunction interface structure that possesses a more enhanced response toward ethanol gas compared to pristine V₂O₅. Further, using a p-type CuO (work function of 5.3 eV) to decorate n-type V₂O₅ (work function of 4.7 eV) has been found to create a p-n junction structure with enhanced sensitivity towards acetone [103].

However, nanowire-based materials face challenges, including non-uniform growth, poor control over density and distribution, difficulty in achieving reproducible contact between nanowires and electrodes, and constraints in sensor geometry [102]. Modifying the V₂O₅ nanowire surface by decorating it with other oxides, such as tin oxide (SnO₂) and copper oxide (CuO), has been reported as an effective method to enhance response time and sensitivity (Table 6). Previous reports indicate that SnO₂-decorated V₂O₅ forms a heterojunction interface with an enhanced response to ethanol compared to pristine V₂O₅. Furthermore, decorating n-type V₂O₅ (work function ~4.7 eV) with p-type CuO (work function ~5.3 eV) creates a p–n junction structure with enhanced sensitivity to acetone [103].

2.7. V₂O₅ Nanotubes

Vanadium pentoxide (V₂O₅) nanotubes are nanoparticles with a tubular morphology composed of three distinct regions: inner surface, outer surface, and tube ends [104,105,106]. These nanotubes are often synthesised by the sol-gel technique followed by hydrothermal [107,108]. Their inner and outer diameters usually range between 20-40 nm and 80-100 nm, respectively [104,109]. V₂O₅ nanotubes have been found to be efficient for electrochemical and catalytic applications due to their large surface area and various ionic transport channels. Further, these nanotubes have been identified as viable materials for gas sensing applications, demonstrating good response towards the detection of different gases at different temperatures (Table 7) [104,110,111].

Vanadium pentoxide (V₂O₅) nanotubes are nanoparticles with a tubular morphology composed of three distinct regions: the inner surface, outer surface, and tube ends [104,105,106]. They are typically synthesised by a sol–gel technique followed by a hydrothermal step [107,108]. Their inner and outer diameters usually range from 20–40 nm and 80–100 nm, respectively [104,109]. V₂O₅ nanotubes are efficient for electrochemical and catalytic applications due to their large surface area and multiple ionic transport channels. They are also promising for gas sensing, demonstrating good responses to various gases at different temperatures (Table 7) [104,110,111].

The surface doping of V₂O₅ nanotube with transition metals has been reported to effectively enhance their electric and magnetic properties [114]. The openings at the endpoints of the nanotubes are especially suitable for introducing noble metals, and the intercalation of Au into V₂O₅ nanotubes has been reported to provide more active sites for gas adsorption and allow good sensitivity [112].

Surface doping of V₂O₅ nanotubes with transition metals effectively enhances their electrical and magnetic properties [114]. The open ends of the nanotubes are particularly suitable for introducing noble metals. For example, intercalating gold (Au) into V₂O₅ nanotubes provides more active sites for gas adsorption and improves sensitivity [112].

The sensing mechanism of a V₂O₅ nanotube sensor follows the general sensing mechanism of an n-type semiconductor and can be explained by the space charge region model [115]. However, a different sensing mechanism is observed for a doped Au/V₂O₅ nanotube sensor. Upon exposure to air, electrons are transferred from the surface of the V₂O₅ nanotube to the Au particle due to the Schottky junction between the metal and the semiconductor. [50]. Au acts as a catalytic activator in promoting the dissociation of an oxygen atom into O₂ ⁻, O⁻, and O-adsorbed species, which are transported into the nanotube surface [116]. When the Au/V₂O₅ nanotube sensor is subjected to ethanol gas, the gas molecules react with the adsorbed oxygen species, and the captured electrons are released into the V₂O₅ conduction band, thereby decreasing the resistance [117].

The sensing mechanism of a pristine V₂O₅ nanotube sensor follows the general n-type semiconductor model, explained by the space-charge region model [115]. However, a different mechanism operates for Au-doped V₂O₅ nanotubes. Upon air exposure, electrons transfer from the V₂O₅ nanotube surface to Au particles due to the Schottky junction between the metal and semiconductor [50]. Au acts as a catalytic activator, promoting the dissociation of oxygen into O₂⁻, O⁻, and O adsorbed species, which then migrate to the nanotube surface [116]. When exposed to ethanol, the gas molecules react with these adsorbed oxygen species, releasing captured electrons into the V₂O₅ conduction band and thereby decreasing resistance [117].

3. Density Functional Theory (DFT) Perspective

This review article has so far intensively discussed the impact of hierarchical structures towards gas sensing using the experimental approach. However, to explore the full extent of the analyses, we need to consider the quantum mechanical aspect based on density functional theory (DFT) calculations. Previous studies have used DFT to investigate different properties of hierarchical structures, including band structure, density of states, elastic constants, absorption energy, and optical properties. For example, first principles calculations have been used to explore the energetics, electronic and geometric structures of zinc sulphide (ZnS) nanotubes, nanorods, nanosheets, and nanowires based on their thickness or diameter, and its effect on the band gap. Results revealed that ZnS nanowires and double-wired nanotube with higher thickness diameter were the most energetically favoured hierarchical structures, compared to single-wired ZnS nanotubes [118]. The band gap size of these hierarchical structures was found to decrease with an increase in diameter, resulting in high conductivity. Other works also used DFT to estimate the band gap of copper oxide (CuO) thin films, which was found to be 1.66 eV, in agreement with experimental results [119]. In addition, this tool has also been used to gain an atomic-level understanding of how hierarchical structures interact with adsorbate gas molecules. It was observed that ZnS nanotubes showed a high response to ammonia and phosphine, due to the structural orientation of the gas molecules. The sensitivity of a tube was found to be more favourable towards the chemisorption mode [120].

This review has thus far intensively discussed the impact of hierarchical structures on gas sensing from an experimental perspective. To explore the full extent of the analysis, we must also consider the quantum mechanical aspect based on density functional theory (DFT) calculations. Previous studies have used DFT to investigate various properties of hierarchical structures, including band structure, density of states, elastic constants, adsorption energy, and optical properties. For example, first-principles calculations have been employed to explore the energetics and the electronic and geometric structures of zinc sulphide (ZnS) nanotubes, nanorods, nanosheets, and nanowires as a function of their thickness or diameter, and the consequent effect on band gap. Results revealed that ZnS nanowires and double-walled nanotubes with larger diameters were the most energetically favorable hierarchical structures compared to single-walled ZnS nanotubes [118]. The band gap of these structures was found to decrease with increasing diameter, resulting in higher conductivity. DFT has also been used to estimate the band gap of copper oxide (CuO) thin films, yielding a value of 1.66 eV which agrees with experimental results [119]. Furthermore, this computational tool provides an atomic-level understanding of how hierarchical structures interact with adsorbate gas molecules. For instance, ZnS nanotubes showed a high response to ammonia and phosphine due to the structural orientation of the gas molecules, with sensitivity found to be more favorable toward the chemisorption mode [120].

Other studies have also employed the DFT to simulate stable surfaces of various hierarchical structures. Reports have demonstrated that (010) V₂O₅ nanobelts have a good response to the adsorption of ethanol gas. Atomic Mulliken population analysis revealed a transfer of 0.18e electrons from the ethanol gas into the nanobelt's conduction band, resulting in increased conductivity. Furthermore, the reaction's enthalpy change was reported to be −2.84 eV, signifying an exothermic reaction, which is in agreement with experimental findings [121]. Similar effects were observed when (001) nanorods were loaded with NH₃ molecules, wherein the band gap of the nanorods reduced with increasing loading of NH₃ molecules, indicating electron transfer from NH₃ to the nanorod and corresponding shift of the Fermi level towards the conduction band [122].

Other studies have employed DFT to simulate stable surfaces of various hierarchical structures. Reports demonstrate that the (010) surface of V₂O₅ nanobelts exhibits a strong response to ethanol adsorption. Atomic Mulliken population analysis revealed a charge transfer of 0.18 e from ethanol to the nanobelt's conduction band, increasing conductivity. The reaction enthalpy change was reported to be −2.84 eV, signifying an exothermic process consistent with experimental findings [121]. A similar effect was observed when (001) nanorods were exposed to NH₃ molecules; the band gap decreased with increasing NH₃ loading, indicating electron transfer from NH₃ to the nanorod and a corresponding shift of the Fermi level toward the conduction band [122].

Further, DFT has been found to play a significant role in determining the type of transition metal dopants likely to improve the adsorption energies and electronic properties of materials. Reports have shown that doping carbon nanotubes (CNT) with Sc, Ti, V and Cr can drastically alter the electronic properties, resulting in enhanced NH₃, PH₃ and AsH₃ adsorption energies. It was reported that doping CNT with Cr shows a band gap of 0.707 eV, resulting in higher adsorption energy to NH₃ in comparison to the other dopants [123]. The natural bond orbital that indicates the charge transfer of NH₃ to the CNT was found to be 0.195e, indicating a strong covalent bond [123]. Additionally, it has also been reported that intercalation of metal atoms such as Rhodium (Rh) into (001) phase of V₂O₅ reduces the band gap of the material from 2.2 eV to 0.3 eV [159]. Rh becomes the centre of attraction when the material is exposed to some gases such as CO, PH₃, and H₂S. In the case of CO exposure, the most stable adsorption is when the C atom is adsorbed on Rh, yielding an energy of -296 meV as compared to the pristine V₂O₅, which gives the adsorption energy of -137 meV. However, polar molecules such as SO₂, and CO₂ prefer to adsorb on the V atom, which is supported by charge contour plots analysis that shows polar molecules bond via the oxygen atom to the zone near the V and bridging O₂ atom [123]. Thus, the pre-adsorption of Rh on the surface of (001) V₂O₅ results in a weaker interaction between (001) V₂O₅ and CO₂ [123]_.

DFT also plays a significant role in screening transition metal dopants to improve adsorption energies and electronic properties. For example, doping carbon nanotubes (CNTs) with Sc, Ti, V, and Cr can drastically alter electronic properties, enhancing the adsorption energies for NH₃, PH₃, and AsH₃. Specifically, Cr-doped CNTs exhibit a band gap of 0.707 eV and higher adsorption energy for NH₃ compared to other dopants [146]. The natural bond orbital analysis indicated a charge transfer of 0.195 e from NH₃ to the CNT, suggesting a strong covalent interaction [123]. Additionally, intercalating metal atoms such as rhodium (Rh) into the (001) phase of V₂O₅ reduces the band gap from 2.2 eV to 0.3 eV [159]. Rh acts as an active centre when the material is exposed to gases such as CO, PH₃, and H₂S. For CO exposure, the most stable configuration involves adsorption of the C atom on Rh, with an adsorption energy of −296 meV, compared to −137 meV for pristine V₂O₅. In contrast, polar molecules such as SO₂ and CO₂ prefer to adsorb on V atoms, supported by charge contour plots showing that these molecules bond via an oxygen atom near the V and bridging O atoms [123]. Thus, pre-adsorption of Rh on the (001) V₂O₅ surface weakens the interaction with CO₂ [123].

This current study employed DFT to investigate the effect of doping vanadium pentoxide (V₂O₅) with transition metals including tungsten (W), copper (Cu), manganese (Mn), tin (Sn), and silver (Ag) to enhance its adsorption energy towards nitrogen dioxide (NO₂). Incorporating foreign metals in the surface of V₂O₅ has been previously reported to improve the electromechical properties of the material. For instance, doping V₂O₅ with Ag and Cu have shown to improve the electric conductivity properties by two order magnitude. This dopant has been believed reduce the oxidation of vanadium ion state from V⁵⁺ to V⁴⁺ resulting in oxygen vacancies in the material which are more beneficial for gas adsorption, especially to reducing gases. As such in this present work we intensively reviewed the sensing performances of nano layered hierarchical V₂O₅ structures and comprehensively study the gas molecule adsorption properties of pristine and doped V₂O₅ from ab initio approach. The different transition metals were incorporated using the substitution methods.

In the current study, DFT was employed to investigate the effect of doping vanadium pentoxide (V₂O₅) with transition metals—tungsten (W), copper (Cu), manganese (Mn), tin (Sn), and silver (Ag)—to enhance its adsorption energy toward nitrogen dioxide (NO₂). Incorporating foreign metals into V₂O₅ has been previously reported to improve its electrochemical properties. For instance, doping with Ag and Cu can improve electrical conductivity by two orders of magnitude. These dopants are believed to reduce the oxidation state of vanadium from V⁵⁺ to V⁴⁺, creating oxygen vacancies that are beneficial for gas adsorption, particularly for reducing gases. Therefore, this work comprehensively reviews the sensing performance of nano-layered hierarchical V₂O₅ structures and provides a detailed ab initio study of gas molecule adsorption on pristine and doped V₂O₅. The different transition metals were incorporated using substitutional doping methods.

3.1. Density Functional Theory Study of Transition-Metal-Doped V₂O₅ for Enhanced Adsorption of NO₂

Bulk alpha vanadium pentoxide (α-V₂O₅) is classified as an orthorhombic crystal structure with a space group 59 (Pmmn). The basis (Figure 9a) consists of identical vanadium atoms and three nonequivalent oxygen atoms, namely: vanadyl oxygen (O_v), bridging oxygen (O_b), and Chain oxygen (O_c). This structure is composed of a distorted octahedra with one central vanadium atom bonded to six oxygen atoms (VO₆). (See Figure 9b) Each VO₆ is linked together at the edges by the chain (O_c) and at the corners by the bridging (O_b). The vanadyl (V=O_v) is double-bonded to a single vanadium atom, with a bond distance of 1.610 Å, while the bridging (V-O_b) is bonded to two adjacent vanadium atoms with a bond distance of 1.806 Å, and chain (V-O_c) atoms are bonded to three vanadium atoms with a bond distance of 1.88 Å, respectively.

Bulk alpha vanadium pentoxide (α-V₂O₅) has an orthorhombic crystal structure with space group Pmmn (No. 59). Its unit cell (Figure 9a) contains identical vanadium atoms and three nonequivalent oxygen atoms: vanadyl oxygen (Oᵥ), bridging oxygen (O_b), and chain oxygen (O_c). The structure consists of distorted VO₆ octahedra (Figure 9b), each with a central vanadium atom bonded to six oxygen atoms. Adjacent VO₆ units are linked at edges by chain oxygen (O_c) and at corners by bridging oxygen (O_b). The vanadyl oxygen (V=Oᵥ) is double-bonded to a single vanadium atom with a bond distance of 1.610 Å, while the bridging oxygen (V–O_b) bonds to two adjacent vanadium atoms at 1.806 Å, and the chain oxygen (V–O_c) bonds to three vanadium atoms at 1.88 Å.

Figure 2. (a) Crystal structure of V₂O₅ and (b) Coordination of a vanadium and oxygen atom in V₂O₅. The red balls represent oxygen, and the grey represents the vanadium atom.

Figure 2. (a) Crystal structure of V₂O₅ and (b) Coordination of a vanadium and oxygen atom in V₂O₅. The red balls represent oxygen, and the grey represents the vanadium atom.

In order to determine the ground state properties of V₂O_5, Full geometry optimisation of the V₂O₅ unit cell was performed using plane pseudopotential wave density functional theory calculations, as implemented in the CASTEP. Vanderbilt ultrasoft pseudopotentials were used to describe the valence-core interactions, while the Perdew-Burke-Ernzerhof generalized gradient approximation functional was used to approximate the electronic exchange correlation effects. A plane wave cut-off of 1000 eV and a k-point sampling of 4×14×11 was found to be sufficient to converge the bulk V₂O₅. The optimised lattice constants and the bond distances were compared with the experiment and literature as summarised in Table 8. Our calculated lattice parameters and bond lengths are in good agreement with experimental and literature values.

To determine the ground-state properties of V₂O₅, full geometry optimisation of the unit cell was performed using plane-wave pseudopotential density functional theory as implemented in CASTEP. Vanderbilt ultrasoft pseudopotentials described the valence–core interactions, and the Perdew–Burke–Ernzerhof generalized gradient approximation functional was used for the exchange–correlation effects. A plane-wave cut-off energy of 1000 eV and a *k*-point mesh of 4 × 14 × 11 were sufficient to converge the bulk V₂O₅ properties. The optimised lattice constants and bond distances were compared with experimental and literature values, as summarised in Table 8. Our calculated parameters show good agreement with previous data.

3.2. Adsorption of NO₂ Molecule V₂O₅ (011) Surface

The adsorption of NO₂ molecule in this study was modeled using a V₂O₅ (011) surface supercell. The (011) plane is proven to be the most thermodynamically stable compared to other V₂O₅ such as (100), (200), and (400). V₂O₅ surface was constructed by cleaving the optimised bulk V₂O₅ in the (011) direction and building a 2×2 supercell slab of four repeat units. A vacuum space of 20 Å was allowed to ensure negligible interaction with image slabs. Adsorption of the NO₂ molecule was performed for both the clean and transition metals (Cu, Mn, Ag, Sn) doped V₂O₅ surfaces, with the metal atom substituting the atom located on the top layer of the surface slab, as shown in Figure 3. The adsorption energies in each case were calculated using the equation

The adsorption of NO₂ was modeled on a V₂O₅ (011) surface supercell, chosen for its thermodynamic stability relative to other surfaces such as (100), (200), and (400) [reference]. The surface was constructed by cleaving the optimised bulk V₂O₅ along the (011) direction to create a 2 × 2 supercell slab of four repeat units. A vacuum space of 20 Å was added to avoid interactions between periodic images. Adsorption was studied on both pristine and transition-metal-doped (Cu, Mn, Ag, Sn) V₂O₅ surfaces, with the dopant atom substituting a top-layer vanadium atom (Figure 3). The adsorption energy (Eₐ) for each case was calculated using the equation:

$${\mathit{E}}_{\mathit{a}}={\mathit{E}}_{\mathit{s}\mathit{y}\mathit{s}\mathit{t}\mathit{e}\mathit{m}}-{(\mathit{E}}_{\mathit{s}\mathit{u}\mathit{r}\mathit{f}\mathit{a}\mathit{c}\mathit{e}}+{\mathit{E}}_{\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{a}\mathit{t}\mathit{e}})$$

where ${\mathit{E}}_{\mathit{s}\mathit{y}\mathit{s}\mathit{t}\mathit{e}\mathit{m}}$is the total energy of the optimised (undoped or doped)V₂O₅ adsorbed surfaces whereas ${\mathit{E}}_{\mathit{s}\mathit{u}\mathit{r}\mathit{f}\mathit{a}\mathit{c}\mathit{e}}$ and ${\mathit{E}}_{\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{a}\mathit{t}\mathit{e}}$ denote the total energy of the doped and clean surfaces and energy of the NO₂ molecule, respectively. A negative value of ${\mathit{E}}_{\mathit{a}}$ indicates that the process is exothermic, whereas a positive value indicates an endothermic process.

Where ${\mathit{E}}_{\mathit{s}\mathit{y}\mathit{s}\mathit{t}\mathit{e}\mathit{m}}$ is the total energy of the optimised (doped or undoped) V₂O₅ surface with adsorbed NO₂, ${\mathit{E}}_{\mathit{s}\mathit{u}\mathit{r}\mathit{f}\mathit{a}\mathit{c}\mathit{e}}$ is the total energy of the clean (doped or undoped) surface, and ${\mathit{E}}_{\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{a}\mathit{t}\mathit{e}}$ is the energy of an isolated NO₂ molecule. A negative ${\mathit{E}}_{\mathit{a}}$ indicates an exothermic process, while a positive value indicates an endothermic one.

Figure 4 presents the adsorption energy of NO₂ on clean and transition metal-doped V₂O₅ (011) surface calculated for different numbers of NO₂ molecules. It is seen that all the calculated adsorption energies are negative, indicating that the reaction between the surface and the adsorbate occurs spontaneously. Increasing the load of NO₂ molecules leads to a reduction in the adsorption energy in both the clean and doped V₂O₅ surface, but while remaining in the negative regime, indicating a possibility of sensing application over a wide range of concentrations. Clearly, Ag-doped V₂O₅ presents the most negative adsorption energy as compared to the clean and the other dopants (see Figure 5). However, at concentrations beyond four NO₂ molecules, the adsorption energy of the clean surface becomes more negative, indicating that Ag doping may play a critical role in increasing the sensing effectiveness of V₂O₅ at low concentrations of NO₂ molecules.

Figure 4 presents the adsorption energies of NO₂ on pristine and transition-metal-doped V₂O₅ (011) surfaces for varying numbers of NO₂ molecules. All calculated adsorption energies are negative, indicating spontaneous adsorption. Increasing the NO₂ load reduces the adsorption energy (though remaining negative) for both pristine and doped surfaces, suggesting potential for sensing over a wide concentration range. Notably, Ag-doped V₂O₅ exhibits the most negative adsorption energy compared to the pristine surface and other dopants (see Figure 5). However, beyond four NO₂ molecules, the adsorption energy of the pristine surface becomes more negative, indicating that Ag doping may be particularly effective in enhancing V₂O₅ sensitivity at low NO₂ concentrations.

Similar results have been reported experimentally when comparing Ag_0.35 V₂O₅ with undoped V₂O_5. The Ag_0.35 V₂O₅ material tends to show high sensitivity and selectivity toward different gases (including ammonia, acetone, and amines) at 100 ppm concentration. This is due to the decrease in electronegativity of vanadium (V) in Ag_0.35 V₂O_5, which results in the high absorbing site [126]. The other dopants, such as Cu, Ag, and Mn, have also enhanced the structural stability of V₂O₅ at higher concentrations of NO₂ molecules. The graph tends to increase beyond the saturation phase (four NO₂ molecules) on the undoped surface.

Similar results have been reported experimentally for Ag₀.₃₅V₂O₅, which shows higher sensitivity and selectivity toward gases such as ammonia, acetone, and amines at 100 ppm concentration. This is attributed to decreased electronegativity of vanadium in Ag₀.₃₅V₂O₅, leading to more favorable adsorption sites [126]. Other dopants (Cu, Mn, Sn) also enhance the structural stability of V₂O₅ at higher NO₂ concentrations, as the adsorption energy trend rises beyond the saturation point observed for the undoped surface.

4. Conclusions

This paper presents a review of hierarchical nano-layered V₂O₅ structures for application in gas sensing and gas sensing technology. Various morphologies and atomic structures, ranging from nanowires to nano thin films, were reviewed in terms of their gas detection capabilities, such as target gas, concentration, response, and recovery times were discussed. The work also successfully analysed the temperature of detection of those nanorod structures that have been studied more at room temperature, since these are more likely to be energy efficient in practical applications. It was further found that most structures and morphologies in many studies explored ethanol and NO₂ as preferred gases for detection, with nanotubes as the structure standing out as the most sensitive and most selective to ethanol. This paper also added data of Density Functional Theory calculations of the adsorption energies of NO₂ on α-V₂O₅ (011) clean and 3d transition metal (Cu, Mn, Ag, Sn) surfaces, revealing that incorporation of Ag could play a critical role in increasing the sensing effectiveness of V₂O₅ at low concentrations of NO₂ molecules.

This paper has reviewed hierarchical nano-layered V₂O₅ structures for gas sensing applications. Various morphologies and atomic structures, ranging from nanowires to thin films, were examined in terms of their gas detection capabilities, with discussion focused on target gases, operating concentrations, response, and recovery times. The analysis also highlighted that, among the reviewed structures, nanorods have been studied more extensively at room temperature—a desirable feature for energy-efficient practical applications.

It was found that ethanol and NO₂ are the most commonly targeted gases for these V₂O₅ nanostructures, with nanotubes emerging as a particularly sensitive and selective morphology for ethanol detection. Furthermore, this review incorporates original DFT calculations of the adsorption energies and adsorption energy per number of chemical molecules [145] of NO₂ on pristine and 3d transition metal (Cu, Mn, Ag, Sn, W)-doped α-V₂O₅ (011) surfaces. The results reveal that silver (Ag) doping significantly enhances adsorption energy, suggesting a critical role for Ag in improving the sensing effectiveness of V₂O₅, especially at low concentrations of NO₂.