Resistive nanostructured W₁₈O₄₉ gas sensors: an overview

Abstract

The detection of toxic gases by resistive gas sensors, which are mainly fabricated using semiconducting metal oxides, is of importance from a safety point of view. These sensors have outstanding electrical and sensing properties as well as are inexpensive. W₁₈O₄₉ (WO_2.72), which is a non-stoichiometric tungsten oxide, possesses abundant oxygen vacancies, which are beneficial for the adsorption of oxygen gas molecules and act as sites for sensing reactions. Thus, through the rational design of W₁₈O₄₉-based gas sensors using strategies such as morphology engineering, doping, decoration, formation of composites or their combination, the fabrication of high-performance W₁₈O₄₉ gas sensors is feasible. Herein, we present the gas-sensing features of pristine W₁₈O₄₉, doped W₁₈O₄₉, decorated W₁₈O₄₉ and composite-based W₁₈O₄₉ sensors. Moreover, focusing on the sensing mechanism of W₁₈O₄₉ sensors, this review provides an in-depth understanding on the working principles of the sensing of toxic gases using W₁₈O₄₉.

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1. Introduction to resistive gas sensors

Owing to the rapid industrialization and extensive emission of toxic and dangerous gases from automobiles, chemical plants, food industry, mines, etc., air pollution is becoming one of the main causes of human death in most countries.¹ Air pollution is linked to COVID-19 severity and mortality² and causes dizziness, reduced oxygen content in the blood, difficulty in breathing, as well as cardiovascular and respiratory issues.³

Traditionally, the presence of toxic gases can be demonstrated by techniques such as gas and ion chromatography.^4,5 However, these methods have the disadvantage of being expensive, bulky and complicated, which makes their application difficult for real-time and low power consumption gas detection.⁶ Thus, the development of sensitive and reliable devices to detect toxic and harmful gases is essential. Gas sensors are sensitive electrical devices that can detect the presence of surrounding gases via the generation of an electrical signal due to a change in one of their physical properties such as capacitance or resistance. Gas sensors should have merits over traditional detection methods, such as portability and small size, low power consumption, on-line response, ease of fabrication and low cost.⁷ To date, various types of sensors such as piezo-resistive,⁸ electrochemical,⁹ catalytic combustion,¹⁰ thermal conductivity,¹¹ infrared absorption¹² and resistive gas sensors¹³ based on different materials and principles have been introduced. Each type of gas sensor has its own merits and drawbacks. Among them, resistive sensors are the most widely used owing to their high stability, good response, cost-effectiveness, simple operation, fast dynamics, and ease of fabrication.¹⁴ However, high sensing temperature and weak selectivity are the main drawbacks of this type of sensors.^15,16

There are two types of sensor configurations: planar and tubular.¹ In both cases, resistive gas sensors are fabricated via the deposition of a sensing layer over a substrate equipped with electrons. Generally, alumina and SiO₂-coated Si are used as the substrate, and electrodes are fabricated using Pt, Au, and Pd–Ag. Moreover, on the back side of a planar sensor, a microheater is attached to the substrate, which is used for heating the sensor to the desired temperature, whereas in the case of tubular sensors, a resistive nichrome wire within the tubular substrate is used for heating the sensor.¹ Fig. 1(a)–(c) display a schematic of a planar sensor.

Fig. 1 Schematic of (a) the front side, (b) back side and (c) fabricated resistive gas sensor. General gas sensing mechanism of (d) n-type and (e) p-type gas sensors.

Various factors such as chemical composition, surface area, morphology, structure, and number of active sites significantly affect the sensing characteristics of gas sensors.¹⁷ Therefore, the choice of sensing material is very important to bring about a high performance sensor. For the realization of resistive gas sensors, semiconducting metal oxides such as SnO₂ and ZnO are extensively employed,¹⁸ owing to their abundance and high stability and electron mobility.¹⁹ However, they have a high sensing temperature and relatively weak selectivity, especially in pristine form, similar to most other semiconducting-based sensors.²⁰ In this regards, the development of metal oxide gas sensors using other materials such as non-stoichiometric tungsten oxides (WO_3−x) can overcome some disadvantages associated with resistive gas sensors and may open some new doors in the exploration and design of high-performance gas sensors.

1.1 General gas sensing mechanism of resistive gas sensors

Resistance modulation is the key factor for the generation of a sensing signal in resistive gas sensors. Fig. 1(d) and (e) schematically illustrates the general gas sensing mechanism of n-type and p-type gas sensors, respectively. When a resistive gas sensor is exposed to fresh air, oxygen molecules with high electron affinity take electron from the sensor surface and become adsorbed on the surface. The abstraction of electrons by oxygen changes the resistance of the gas sensor relative to vacuum conditions, where there is no air. In the case of an n-type gas sensor, in which the main charge carriers are electrons, the abstraction of electrons causes the formation of an electron depletion layer, where the concentration of electrons is less than that in the inner parts, resulting in a high electrical resistance in air. In contrast, in a p-type gas sensor, the abstraction of electrons causes the formation of a hole accumulation layer (HAL), where the concentration of holes is much higher than that in the core region, resulting in a decrease in electrical resistance in air. When an n-type gas sensor is exposed to a reducing gas, electrons are released on the sensor surface, resulting in a decrease in the thickness of the EDL, which decreases the electrical resistance. However, in an oxidizing gas, further abstraction of electrons causes an increase in the thickness of the EDL, resulting in an increase in the sensor resistance. In the case of p-type gas sensors, the trends of resistance change are inverse, where the resistance increases and decreases in the presence of reducing and oxidizing gases, respectively.^21–23

2. W₁₈O₄₉: an introduction

Tangiest oxide (WO₃) is a semiconductor with unique electrical properties and multifunctional applications.²⁴ Besides stoichiometric WO₃, non-stoichiometric WO_3−x, including WO_2.72 (W₁₈O₄₉),²⁵ WO_2.8 (W₅O₁₄)²⁶ and WO_2.9 (W₂₀O₅₈),²⁷ with abundant oxygen vacancies, is becoming an attractive candidate for the development of gas sensors owing to their abundance, low cost, nontoxicity, suitable band gap and high chemical stability.²⁸ In WO_3−x, the presence of oxygen vacancies results in the formation of shallow donors, increasing the number of adsorption sites and the electrical conductivity.²⁹ In particular, owing to the high conductivity³⁰ and abundant oxygen vacancies in W₁₈O₄₉ (WO_2.72), it has attracted significant attention compared to other WO_3−x materials.^31,32 In addition to gas sensors,^33–38 W₁₈O₄₉ has applications in energy storage,³⁹ medicine,⁴⁰ supercapacitors,⁴¹ sonodynamic therapy,⁴² and photocatalysts.⁴³

Bhavani et al.,²⁴ Yan et al.,³⁹ and Zhou et al.⁴⁴ discussed the use of W₁₈O₄₉-based materials for energy and environmental use, energy storage conversion and photocatalysts, respectively. Although there are many reviews in the field of semiconductor gas sensors,^1,35,45,46 a review on W₁₈O₄₉ as a gas sensor is lacking. Inspired by this lack of review papers related to the gas sensing features of W₁₈O₄₉, herein, we discuss the sensing properties and mechanisms of pristine, doped and composite W₁₈O₄₉-based gas sensors.

W₁₈O₄₉ has the most reduced form among the WO_3−x oxides.²⁸ The crystal structure of W₁₈O₄₉ as a sub-stoichiometric type of WO₃ is formed by crystallographic shearing based on WO₃. WO₃ has a perovskite structure (ABO₃), in which the “A” site is missing.³¹ In fact, it represents a repeated network consisting of WO₆ octahedra arranged across corners. The oxygen and W ions are placed in the center and corners of the octahedra, respectively, and each oxygen ion connects two octahedra (Fig. 2(a)).⁴⁷ Fig. 2(b) exhibits the corner- and edge-sharing modes as well as pentagonal columns mode. The ordered and coordinated structure may result in arrangement possibilities in the WO₃ lattice (Fig. 2(b)).²⁹ The band structure of a semiconductor can be altered due to the formation of oxygen deficiencies. The conduction band (CB) of WO₃ is formed by empty W 5d orbitals, while the valence band (VB) is formed by filled O 2p orbitals (Fig. 2(e)).⁴⁰ In W₁₈O₄₉, due to the formation of oxygen vacancies and subsequent ionization, new energy bands will be formed in its band gap, leading to higher conductivity relative to WO₃. Moreover, due to the formation of oxygen vacancies, the reduction of some W⁶⁺ ions occurs, resulting in the formation of shallow states below the CB due to formation of W⁵⁺/W⁴⁺ energy levels (Fig. 2(e)). W⁵⁺/W⁴⁺ would capture the electrons excited by the oxygen vacancies, leading to polarization of the surrounding lattice and producing polarons.³¹ The polarons will lead to the localized surface plasmon resonance (LSPR) effect. Owing to the special band structure of W₁₈O₄₉, it possesses photosensitive capability, and thus it can respond to almost the whole spectrum because of (i) the direct transition of electrons between its VB and CB; (ii) VB to W⁵⁺/W⁴⁺ state transition, and (iii) LSPR effect. The special characteristics of W₁₈O₄₉ include the LSPR effect, photochromic effect, and narrow band gap. In particular, as stated before, the presence of oxygen vacancies leads to the generation of more free electrons, resulting in high electrical conductivity of W₁₈O₄₉, and thus it is a potential material for the fabrication of gas sensing devices.¹⁴ The performance of W₁₈O₄₉-based gas sensors can be boosted by doping, decoration, formation of composites and morphological engineering. In the next sections, we discuss the gas sensing features of W₁₈O₄₉ gas sensors.

Fig. 2 (a) Structure of WO₃. (b) Corner- and (c) edge-sharing modes. (d) Pentagonal column mode. (e) Electronic band structure of WO₃ and W₁₈O₄₉.⁴⁴ Reproduced with permission from ref. 34. Copyright 2023, Elsevier.

3. Pristine W₁₈O₄₉ gas sensors

In the solvothermal or hydrothermal methods, high temperature and pressure are used to accelerate the rate of reactions. Thus, they are highly promising synthesis methods for generating various nanostructures such as one-dimensional (1D) nanostructures. In this context, 1D W₁₈O₄₉ nanoneedles were produced via the solvothermal route at 160–220 °C for 3–9 h. Also, the solvent type (ethanol, propanol, butanol and cyclohexanol) and WCl₆ amount (60, 80, 100 and 120 mg) were changed to find the optimal conditions to achieve the highest response to NO₂ gas.⁴⁸ Regarding the solvent type, the highest sensing output was obtained when cyclohexanol was used due to the formation of uniform 1D nanoneedles with excellent crystallinity and substantial surface area of ∼31 m² g⁻¹, which were beneficial for surface redox reactions, hence enhancing the sensing performance. The response of W₁₈O₄₉ nanostructures was also affected by the WCl₆ dosage. Small WCl₆ dosages led to the accelerated the growth of W₁₈O₄₉ nanostructures along the (010) direction. Finally, this resulted in the generation of thin W₁₈O₄₉ nanoneedles, where the sample prepared using 100 mg WCl₆ possessed outstanding crystallinity and the best sensing properties. At a high WCl₆ dosage (120 mg), not only short/thick W₁₈O₄₉ nanostructures were formed but also their crystallinity decreased, which resulted in a decrease in their gas response. Furthermore, the solvothermal temperature affected the gas response. At 160 °C and 180 °C, thin W₁₈O₄₉ nanoneedles with agglomeration and poor crystallinity were synthesized. At 200 °C, almost perfect W₁₈O₄₉ nanoneedles with high crystallinity were formed, which exhibited an enhanced gas response. At 220 °C, W₁₈O₄₉ nanorods (NRs) or even block-shaped nanostructures were synthesized because of the accelerated growth rate at high temperature and the NO₂ response decreased. Regarding the solvothermal time, after 3 h reaction, long and thin W₁₈O₄₉ nanoneedles with poor crystallinity were synthesized. A longer solvothermal time to 5 h led to the formation of nanoneedles with high crystallinity, which eventually caused the highest response to NO₂ gas. A further increase in solvothermal time to 7 h resulted in the formation of short rod-like nanostructures with low crystallinity and low response to NO₂ gas. Overall, the sample solvothermally synthesized at 200 °C for 5 h showed that the use of cyclohexanol and WCl₆ dosage of 100 mg resulted in an enhanced gas response (R_g/R_a) of ∼17 to 10 ppm NO₂ gas at 160 °C. Also, it exhibited excellent selectivity to NO₂ gas, which was related to the low bond dissociation energy of NO₂ gas (305 kJ mol⁻¹), which facilitated reactions with the absorbed oxygen ions. The improved response to NO₂ gas was related to following reasons: (i) due to the nonstoichiometric nature of W₁₈O₄₉, plenty of oxygen vacancies were generated in sensing materials, which acted as powerful adsorption sites for oxygen gas and (ii) due to the 1D nature of the W₁₈O₄₉ nanoneedles, not only a high surface area was created, but also the mobility of the charge carriers increased, while the recombination rate of charge carriers decreased. Fig. 3 schematically exhibits the sensing mechanism of the W₁₈O₄₉ sensor, in which initially an electron depletion layer (EDL) is formed in the presence of air by the adsorption of oxygen species and abstraction of electrons. Upon subsequent exposure to NO₂ gas, more electrons are extracted on the sensor surface, resulting in the expansion of the EDL and increase in the sensor resistance.

Fig. 3 Schematic of the NO₂ gas sensing mechanism of W₁₈O₄₉ nanoneedles.⁴⁸ Reproduced with permission from ref. 48. Copyright 2023, Elsevier.

The detection of ammonia at low concentrations and RT is of high importance for clinical diagnosis and food safety.⁴⁹ In this regard, ultra-thin (less than 5 nm) W₁₈O₄₉ nanowires (NWs) with a high surface area of 151 m² g⁻¹ were produced via the solvothermal method. Also, based on the photoluminescence (PL) study, a large amount of oxygen vacancies was formed during the hydrothermal synthesis of NWs. The sensor revealed a conductivity-type change with a change in the NH₃ amount. The n-to p-type behavior of the sensor was explained as follows: when the bulk donor density (N_D) was neither too high nor too low, then the surface state density (N_A) could reach the minimum with a variation in the gas concentration and a change in sign with a variation in conductivity was expected. Given that the diameter of the NWs was smaller than the Debye length of W₁₈O₄₉ at RT, when the bulk donor density of the NWs was in the appropriate range, an abnormal conductivity change was recorded. This sensor could detect sub ppm concentrations of NH₃ gas. The high response of the W₁₈O₄₉ NW sensor was due to the small diameter of the W₁₈O₄₉ NWs, creating a high surface area and non-stoichiometric nature due to the presence of a large amount of oxygen vacancies, which facilitated the chemisorption of oxygen at RT, resulting in a good response to NH₃ gas at RT.⁵⁰

The reliable sensing of volatile organic compounds (VOCs) at RT is vital due to the toxic effect of some VOCs.⁵¹ In this sense, 1D single crystalline W₁₈O₄₉ NRs with an aspect ratio of 20 were produced using a colloidal synthesis method for the detection of VOCs at RT.⁵² The response ([ΔR/R_a] × 100) to 130 ppm ethanol was 3.5% at RT. However, the t_rec was too long, which was related to the RT sensing temperature. They used UV irradiation during the recovery period of the sensor and the t_rec significantly decreased. Also, the effect of the thickness of the sensing layer (1.2, 4.6, and 7.0 μm) was investigated and the 4.6 μm-thick sensor manifested the highest response. Also, the response time (t_res) and recovery time (t_rec) were not significantly different, which was related to the highly porous nature of W₁₈O₄₉ NRs.

Given that various crystal planes lead to various atomic surface structures and chemical and physical features, the sensing features of W₁₈O₄₉ with well-defined crystalline planes should be investigated. In this regards, Zhang et al.,⁵³ synthesized ultra-fine W₁₈O₄₉ NWs via a chemical synthesis approach, using WCl₆ and ethanol as the solvent. When the content of WCl₆ was 0.4 g, ultra-fine NW bundles with a large surface area of 194.7 m² g⁻¹ and good crystallinity together with exposed (010) plane were formed. By increasing the content of WCl₆, sea urchin-like nanostructures were formed with a lower surface area and poor crystallinity. The response (R_a/R_g) of the ultra-fine NW bundle sensor to 50 ppm acetone was 48.6 at 280 °C, which was higher than that of other gas sensors, mainly owing to its high surface area and presence of abundant oxygen vacancies. According to DFT studies, the total adsorption energies for acetone gas are 80.60, 31.53 and 38.29 kJ mol⁻¹ on the (010), (100) and (001) crystal planes, respectively. Thus, the (010) crystal plane had the greatest adsorption ability for acetone. Furthermore, acetone had the highest adsorption energy on the W₁₈O₄₉ (010) surface relative to other VOCs, which justified its good selectivity for acetone.

Mesocrystals can be formed by the oriented growth of pre-synthesized, well-defined building blocks, where the fusion of the building blocks results in the formation of crystalline structures with a porous nature,⁵⁴ which are highly promising for sensing studies. In an interesting study, W₁₈O₄₉ mesocrystals with a narrow size distribution (the diameter of ∼250 nm and length of ∼200 nm) were prepared by dissolving 200 mg WC₆ in 30 mL 1-butanol and subsequent solvothermal synthesis at 200 °C for 24 h. Alternatively, W₁₈O₄₉ NWs were prepared by dissolving the same amount WCl₆ in double the amount of 1-butanol, followed by the same solvothermal process.⁵⁵ The W₁₈O₄₉ mesocrystals had an average diameters of ∼250 nm and average length of 200 nm. The mesocrystal sensor exhibited the maximum response (R_g/R_a) of 24.5 to 1 ppm NO₂, which was 5-times that of W₁₈O₄₉ NWs. The surface area of W₁₈O₄₉ mesocrystals was ∼79 m² g⁻¹, while that of W₁₈O₄₉ NWs was 71 m² g⁻¹. Also, according to PL studies, more oxygen vacancies were present in the W₁₈O₄₉ mesocrystals than in the W₁₈O₄₉ NWs. Thus, although no significant difference between their surface areas was recorded, the higher amount of oxygen vacancies led to a noticeable difference in their sensing properties.

Given that in resistive gas sensors, gas molecules need to be absorbed on their surface, increasing the surface area of W₁₈O₄₉ is a good strategy to improve its properties as a sensor. Hence, it is expected that different morphologies of W₁₈O₄₉ will lead to different sensing properties due to their different surface areas.⁵⁶ In this regards, Qin et al.⁵⁷ solvothermally synthesized short and thick 1D NRs W₁₈O₄₉ using cyclohexane as the solvent at 200 °C/6 h. Also, long and thin W₁₈O₄₉ NWs were produced using 1-propanol as the solvent at 200 °C/9 h (Fig. 4).

Fig. 4 SEM and TEM micrographs of the W₁₈O₄₉ (a) and (b) NRs and (c) and (d) NWs.⁵⁷ Reproduced with permission from ref. 57. Copyright 2011, Elsevier.

The response (R_g/R_a) of W₁₈O₄₉ NWs and NRs were 140.7 and 131.5 to 5 ppm NO₂ at 150 °C, respectively. Also, W₁₈O₄₉ NWs exhibited faster dynamics relative to W₁₈O₄₉ NRs. The NWs had a larger surface area (∼83.60 m² g⁻¹) than the NRs (∼69.25 m² g⁻¹), which led to the availability of more adsorption sites on NWs for gases. Therefore, the NWs showed higher resistance modulation when exposed to NO₂ relative to W₁₈O₄₉ NRs.

Gu et al.⁵⁸ reported that isolated tungsten oxide NWs can be directly grown on a metallic W layer. In this context, aligned arrays of W₁₈O₄₉ NWs were directly synthesized via the in situ oxidation of a sputtered W layer on a substrate equipped with Pt electrodes (Fig. 5).⁵⁹ After the deposition of a tungsten film with a thickness of 150 nm, they were thermally oxidized at 550–750 °C under a continuous flow of argon and oxygen gases. At 550 °C, complete W₁₈O₄₉ NWs were not formed, while at 650 °C well-formed W₁₈O₄₉ NWs with small diameters (10 to 20 nm) were formed. Also at 750 °C, much thicker and longer NWs were formed.

Fig. 5 Schematic of the synthesis of W₁₈O₄₉ NW arrays.⁵⁹ Reproduced with permission from ref. 59. Copyright 2016, Elsevier.

Also, the effect of oxygen exposure time (0–60 min) during thermal oxidation at 650 °C was explored (Fig. 6(a)–(d)) and it was found that after exposure to oxygen for 60 min, the NWs possessed a length of 500 nm and diameter of 10–20 nm.

Fig. 6 SEM micrographs of W₁₈O₄₉ grown at 650 °C for various oxidation times under (a) no oxygen and oxygen flow time for (b) 10, (c) 20, (d) 40 and (e) 60 min and (f) in Ar/O₂ flow during growth.⁵⁹ Reproduced with permission from ref. 59. Copyright 2016, Elsevier.

The sensor fabricated using W₁₈O₄₉ NW arrays at 650 °C for 60 min revealed a response (R_g/R_a) of 4.4 to 1 ppm NO₂ gas at 150 °C, with relatively fast t_res and t_rec of 78 and 32 s, respectively, which was attributed to the good diffusion of gas molecules in the NWs owing to their structural porosity. The rough alignment of the NWs resulted in formation of numerous NW/NW junctions, which served as electrical bridges between the electrodes. Also, in contact areas between NWs, potential barriers were formed in air, and upon subsequent exposure to NO₂ gas, great resistance modulation occurred.

As explained in this section, pristine W₁₈O₄₉ gas sensors are being used for detection of toxic gases; however, their performance can be further improved by noble metal decoration, which is explained in the next section.

4. Noble metal decorated W₁₈O₄₉ gas sensors

Decoration of the sensing layer using noble metals is a good technique to boost the sensing performance, where the improved performance is related to the electronic and chemical sensitization impacts of noble metals.⁶⁰ In this regards, Pd-decorated flower-like W₁₈O₄₉ with an average diameter of 500 nm was prepared by Wang et al.⁶¹ for the detection of formaldehyde (HCHO). Firstly, WCl₆ was added to an ethanol solution and sonicated. Then, PdCl₂ (0, 20, 40, and 60 mg) was added, and after ultra-sonication for 0.5 h, the final products were synthesized by hydrothermal reaction at 180 °C/10 h (Fig. 7(a)). Both W₁₈O₄₉ and Pd (20 mg)-decorated W₁₈O₄₉ showed a flower-like morphology (Fig. 7(b)–(d)). The Pd (20 mg)-decorated W₁₈O₄₉ sensor displayed a high response (R_a/R_g) of 25.1 to 15 ppm formaldehyde at 180 °C, while the pristine sensor displayed a response of 12. The t_res was 16 s for the W₁₈O₄₉ sensor, while it was only 1 s for the Pd-decorated W₁₈O₄₉ sensor. Also, DFT calculations confirmed that the adsorption energy of formaldehyde on the Pd-sensitized W₁₈O₄₉ was −1.362 eV, which was stronger than that of W₁₈O₄₉ (−0.835 eV), implying that the adsorption of HCHO on the surface of W₁₈O₄₉ increased after Pd decoration. Furthermore, the superior sensor capacity of the Pd-decorated sensor was mainly related to the fact that this sensor had the highest amount of the oxygen vacancies, which acted as active sites for oxygen gas. In addition, the catalytic and electronic effects of Pd NPs, together with the flower-like morphology of W₁₈O₄₉ contributed to the sensing response.

Fig. 7 (a) Synthesis procedure and SEM images of (a) W₁₈O₄₉ nanoflowers and (b–d) Pd (20 mg)-decorated W₁₈O₄₉ nanoflowers.⁶¹ Reproduced with permission from ref. 61. Copyright 2023, Elsevier.

Due to the highly explosive nature of H₂ gas, there is an urgent need to monitor the leakage of hydrogen to ensure human safety.⁶² In this case, Pd is an excellent noble metal for H₂ sensing. It can not only easily dissociate H₂ gas molecules, but also adsorb up to 900-times its own volume of H₂ gas.⁴⁵ In this respect, urchin-like W₁₈O₄₉ was hydrothermally synthesized at 160 °C/24 h (Fig. 8(a) and (b)). Then, Pd NPs were decorated on urchin-like W₁₈O₄₉ using a chemical method, and subsequently the samples were annealed at 300 °C and 400 °C.⁶³ Based on the thermogravimetry analysis, at 400 °C, owing to the oxidation of W₁₈O₄₉ in the air, W₁₈O₄₉ was oxidized to WO₃ and Pd-decorated WO₃ was obtained. However, annealing at 300 °C resulted in the formation of Pd-decorated urchin-like W₁₈O₄₉ (Fig. 8(c) and (d)). The Pd-decorated W₁₈O₄₉ sensor had a high response (R_a/R_g) of 1600 to 0.1 vol% H₂ at 100 °C, while the Pd-decorated WO₃ sensor revealed a response of ∼30. The boosted H₂ sensing performance was related to the 3D urchin-like hierarchical structure, which was beneficial for the easy and fast diffusion of H₂ gas in the in-depth parts of the sensor furthermore, W₁₈O₄₉ had many oxygen vacancies, which enhanced the adsorption of oxygen on the sensing layer. Finally, the spillover effect of Pd was responsible for the enhanced response. Firstly, in the presence of Pd NPs, H₂ molecules were dissociated into H species, and then they moved to W₁₈O₄₉, and finally reacted with the adsorbed oxygen ions on W₁₈O₄₉ (Fig. 8(e)),⁶⁴ as follows:

2H_(ads) + O₂⁻ → H₂O + e⁻ (2)

Fig. 8 TEM micrographs of (a) and (b) W₁₈O₄₉ and (c) Pd–W₁₈O₄₉. (d) HRTEM image of Pd–W₁₈O₄₉. (e) Schematic of the spillover effect.⁶³ Reproduced with permission from ref. 63. Copyright 2018, Elsevier.

Networked W₁₈O₄₉ NWs with a diameter of ∼50 nm and length of 2 μm were produced via the thermal evaporation of W powder at 1400–1450 °C for 10 min in the presence of oxygen.⁶⁵ Then, a Pt layer was sputtered on the W₁₈O₄₉ NW. The fabricated sensor was used in self-heating mode, and to minimize the power consumption, the samples were suspended with the bonding wires to reduce the dissipation of heat (Fig. 9(a)). Upon the application of a voltage, heat was generated inside the sensor due to the Joule heating effect (Fig. 9(b)). Accordingly, the response increased by applying a voltage due to the large amount of heat generated inside the sensor at high voltages. The selectivity of the sensor to H₂ gas was due to the dissociation of H₂ into H species by catalytic effect of Pt, and then their spill-over to the sensing layer.

Fig. 9 (a) Schematic of the electrical set up for gas sensing measurement together with digital photo of the sensor. (b) Induced temperature of networked Pt–W₁₈O₄₉ NW versus the applied voltage.⁶⁵ Reproduced with permission from ref. 65. Copyright 2011, Elsevier.

Urchin-like W₁₈O₄₉ microstructures were synthesized via the hydrothermal route at 160 °C for 24 h. Later, Pt NPs were deposited on W₁₈O₄₉ via the UV illumination of an H₂PtCl₆·6H₂O solution. Under UV illumination, some W–O bonds were broken and the number of oxygen vacancies increased relative to the pristine W₁₈O₄₉. The surface area of W₁₈O₄₉ and Pt/W₁₈O₄₉ was ∼48.90 and 53.70 m² g⁻¹, respectively. Therefore, the surface area slightly increased after Pt decoration, which is beneficial for the sensing of gases. The pristine W₁₈O₄₉ displayed a response (R_a/R_g) of 7 to 10 ppm ethanol at 325 °C, while the Pt/W₁₈O₄₉ sensor displayed a response of 23 to the same amount of ethanol at 300 °C. Also, both sensors showed good selectivity to ethanol. According to DFT studies, the adsorption energies of ethanol on the W₁₈O₄₉ and Pt/W₁₈O₄₉ sensors (−0.85 and −1.18 eV) were higher than that for interfering gases, resulting in the better adsorption of ethanol relative to interfering gases. Furthermore, the higher adsorption energy for ethanol on W₁₈O₄₉ indicated the superior adsorption ability of the Pd-decorated sensor for ethanol. Moreover, the adsorption energy for O₂ molecules on W₁₈O₄₉ and Pt/W₁₈O₄₉ was −2.31 and −3.41 eV, respectively, implying that Pt/W₁₈O₄₉ had higher adsorption ability for oxygen than W₁₈O₄₉. Due to the difference in work function between W₁₈O₄₉ and Pt, the Ohmic junctions were formed at the interface between W₁₈O₄₉ and Pt. Thus, in comparison with the W₁₈O₄₉ sensor, the flow of charge carriers in the Pt-decorated sensor was more favorable. To further study the behavior of the sensors, a fixed concentration of gas was extracted, and then injected into a gas chromatograph. It was revealed that the Pt/W₁₈O₄₉ sensor consumed more ethanol and generated more CO₂ relative to the W₁₈O₄₉ sensor. This was related to the catalytic effect of Pt towards the easy oxidation of ethanol.⁶⁶

The major gases in a coal mines are CO, H₂S and CH₄.⁶⁷ Thus, it is important to realize gas sensors for the reliable sensing of these gases. In this context, Zhang et al.,⁶⁸ prepared W₁₈O₄₉ NWs using the solvothermal approach at 180 °C/24 h. Subsequently, Pd@Au core–shell bimetallic NPs (BNPs) were decorated on W₁₈O₄₉ NWs (Fig. 10) for sensing coal mine gases. The concentration of HAuCl₄ was set to 0.8, 1.3 and 1.8 mM, while that of Pd was fixed and the samples with the above-mentioned Au precursor concentrations were labelled as NWs/BNPs-1, NWs/BNPs-2, NWs/BNPs-3, respectively.

Fig. 10 Schematic representation of the synthesis of W₁₈O₄₉ NWs and W₁₈O₄₉ NWs/BNPs.⁶⁸ Reproduced with permission from ref. 68. Copyright 2022, Elsevier.

Owing to the interaction between Pd and W₁₈O₄₉ NWs, a small shift in the XPS spectrum was observed, which led to the regulation of the surface electronic characteristics of the W₁₈O₄₉ NW, and enhanced sensing features. The response (R_a/R_g) of the W₁₈O₄₉ NWs/BNP-2 sensor to H₂S (50 ppm) at 100 °C was about 55.5 with a weak response to CH₄ gas (Fig. 11(a)), while at 320 °C it had a response (R_a/R_g) of ∼7.8 to 1000 ppm CH₄ (Fig. 11(b)). Thus, it was possible to tune the selectivity of the sensor to these gases by changing the temperature. In particular, CH₄ is a stable molecule because of its high molecule symmetry, accordingly its oxidation is difficult at low temperatures. With an increase in the working temperature, and due to the higher catalytic activity of BNPs, the oxidation of CH₄ molecules was accelerated. Furthermore, the sensor could successfully detect H₂S and CH₄ gases in mixed gases (50 ppm H₂S, NO, CO, and NH₃ as well as 1000 ppm CH₄) at their optimum temperature. The enhanced features of the sensor were related to (i) the ultra-fine 1D NWs with diameters less than 1 nm, which provided more adsorption sites and fast transport channels, and the presence of a large amount of oxygen vacancies, (ii) catalytic effects of Au and Pd NPs towards the target gases and (iii) formation of potential barriers between the bimetallic NPs and sensing materials and modulation of the potential barriers in an atmosphere containing target gases.

Fig. 11 Selectivity of W₁₈O₄₉ NWs/BNPs-2 at (a) 320 °C and (b) 100 °C to various gases.⁶⁸ Reproduced with permission from ref. 68. Copyright 2022, Elsevier.

Hollow nanostructures offer a higher surface area relative to bulk materials due to the availability of both inner and outer walls for gas adsorption. Therefore, they should possess a high response to target gases. Xu et al.⁶⁹ constructed hollow W₁₈O₄₉ spheres via the hydrothermal route at 180 °C/16 h. Subsequently, Co₃O₄/hollow W₁₈O₄₉ spheres were prepared via the chemical route by adding a Co precursor to hollow W₁₈O₄₉ spheres. Finally, Au was decorated on Co₃O₄/hollow W₁₈O₄₉ spheres using a chemical method. The hollow structure consisted of lamellar flakes, which formed porous and large spheres (Fig. 12).

Fig. 12 (a) and (b) SEM micrographs of W₁₈O₄₉ hollow spheres. TEM images of (c) W₁₈O₄₉, (d) Co₃O₄/W₁₈O₄₉, and (e) Au-decorated Co₃O₄/W₁₈O₄₉ hollow spheres.⁶⁹ Reproduced with permission from ref. 69. Copyright 2019, Elsevier.

The response (R_a/R_g) of the Au-decorated Co₃O₄/W₁₈O₄₉ sensor to 2 ppm TEA at 270 °C was to 16.7, which was significantly higher than that of Co₃O₄/W₁₈O₄₉ (11.3) and W₁₈O₄₉ (3.9). Firstly, the porous and hollow structure provided a high surface area (76.12 m² g⁻¹), which led to the availability of more active sites for the adsorption of oxygen molecules. Besides, the porous and hollow nature contributed to fast gas diffusion (Fig. 13(a)). Secondly, the Au NPs were a highly active catalyst for oxygen dissociation due to the spillover effect (Fig. 13(b)). Therefore, TEA easily reacted with the adsorbed oxygen ions and electrons were released to the sensor surface (Fig. 13(c)). Thirdly, the formation of p–n heterojunctions at the interface between p-type Co₃O₄ and n-type W₁₈O₄₉ contributed to the enhancement in the sensing performance. The work function of Co₃O₄ and W₁₈O₄₉ was 6.1, and 4.6 eV, respectively.^70–72 Hence, their Fermi levels were equal due to the flow of electrons from W₁₈O₄₉ to Co₃O₄. This resulted in the widening of the EDL, and at the interface of heterojunctions, potential barriers were created with band bending. Therefore, owing to the large baseline resistance, a remarkable reduction in resistance occurred upon exposure to TEA gas.

Fig. 13 Schematic of (a) pristine W₁₈O₄₉ in air, (b) and (c) Au-decorated Co₃O₄/W₁₈O₄₉ in air and TEA gas.⁷⁰ Reproduced with permission from ref. 70. Copyright 2019, Elsevier.

Sun et al.,⁷³ initially prepared W₁₈O₄₉ NRs via a solvothermal reaction at 200 °C for 8 h, and then Ag/AgCl NPs were decorated on the W₁₈O₄₉ NRs using AgNO₃ as the precursor and irradiation with an Xe lamp. The response (R_a/R_g) of the sensor was 115.5 to 100 ppm H₂S at 300 °C and t_res and t_rec were 21 and 26 s, respectively. The improved gas-sensing capacity relative to the pristine sensor was attributed to catalytic effect of Ag NPs owing to the spillover effect,⁷⁴ as well as the electronic effect due to the formation of potential barriers in the contact areas between Ag/AgCl and W₁₈O₄₉.

Alloy nanocrystals not only have the unique characteristics of each metal, but also additional features due to the synergy between different atoms. In particular, they provide boosted catalytic properties for gases, which can significantly enhance the catalytic performance of the sensor. In the study conducted by Bai et al.,⁷⁵ initially urchin-like W₁₈O₄₉ was hydrothermally prepared at 180 °C/24 h. Then, the Au₃₉Rh₆₁ alloy was synthesized via the co-precipitation method, and finally Au₃₉Rh₆₁ alloy-decorated W₁₈O₄₉ was prepared using a chemical method for n-butanol sensing. At 260 °C, the response (R_a/R_g) of the Au₃₉Rh₆₁–W₁₈O₄₉ sensor was ∼11 to 50 ppm n-butanol, which was 4-fold higher than that of Au–W₁₈O₄₉ (∼2.2) and 2-times higher than Rh–W₁₈O₄₉ (∼5.1) sensor. The increased gas sensing properties of the Au_xRh_1−x-decorated W₁₈O₄₉ was related to (i) its large surface area due to its urchin-like morphology, (ii) spillover effect of Au₃₉Rh₆₁ alloy, where the dissociation of molecular oxygen due to the catalytic effect of the noble metals resulted in activated oxygen species, which were spilt over on the sensing layer and (iii) formation of Schottky barriers between the noble metals and sensing layer.⁷⁶

5. Doped W₁₈O₄₉ gas sensors

Doping with metal atoms is an efficient approach to modify the electronic characteristics of materials via the introduction of additional energy levels, creation of oxygen vacancies and increasing the surface adsorption/desorption capability.^77,78 Doping of W₁₈O₄₉ with metals such as Mo, Fe, Pd and Ni led to the generation of oxygen vacancies, which eventually led to an enhanced gas response.^63,79–82

Acetone is a biomarker gas and the amount of acetone in exhaled breath can be used for the diagnosis of diabetic patients, where its content in healthy people is ∼0.3–0.9 ppm and increases to 1.8 ppm in diabetic patients. In this regards, Pd_xW₁₈O₄₉ (x = 1.83%, 3.60%, 7.18%, and 11.69%) NWs, were synthesized via the hydrothermal method. All the Pd-doped samples showed an NW morphology without a significant difference in surface morphology. Also, based on the XPS study, the Pd_7.18%W₁₈O₄₉ NWs had the largest oxygen vacancy content among the samples. Also, this sample displayed the highest response (R_a/R_g) of ∼146 to 50 ppm acetone at 175 °C, with the fast t_res and t_rec of 5 and 10 s, respectively.⁸¹ The short dynamics of the optimized gas sensor was related to its relatively high sensing temperature, which provided more energy to facilitate the flow of electrons. Also, this sensor exhibited a reliable response to acetone in a mixed gas comprised of ethanol and acetone, which is important for medical diagnosis applications. The improved gas sensing was related to the formation of Schottky barriers between Pd and W₁₈O₄₉ NWs and existence of the highest amount of oxygen vacancies, which favored the adsorption of oxygen on the sensor surface. Accordingly, the reactions between acetone and adsorbed oxygen ions lead to the generation of a higher sensing signal.

In an interesting study, rambutan-like Ni-doped W₁₈O₄₉ was fabricated via a one-step solvothermal route. The specific surface area of W₁₈O₄₉ and Ni_0.05W₁₈O₄₉ was ∼76 m² g⁻¹ and 130.5 m² g⁻¹, respectively. The Ni_xW₁₈O₄₉ sensor with x = 0.05 Ni-doping displayed a response (R_g/R_a) of 182 to 50 ppm n-butanol at 160 °C with a short t_res and t_rec (14 s/241 s), respectively, while the response of the pristine sensor was 26.7. The improved response of the doped sensor was correlated with the increase in the amount of oxygen vacancies due to Ni-doping and high surface area (130.5 m² g⁻¹).⁸³

Simultaneously doping two metals in tungsten oxides has rarely been reported. The introduction of two dopants with different sizes and valences into host materials can change their electronic features, create more oxygen vacancies, and improve their catalytic activity.⁸⁴ In this context, the impact of Co and Ni co-doping in W₁₈O₄₉ was investigated for the detection of TEA.⁸⁵ Co and Ni-doped porous W₁₈O₄₉ nano-urchins with a surface area of ∼165.85 m² g⁻¹ were achieved using the solvothermal method at 180 °C/24 h. At 250 °C, the sensor with 1 wt% Co and 2 wt% Ni W₁₈O₄₉ displayed a response (R_a/R_g) of 114 to 50 ppm TEA, which was higher than that of the single-doped and undoped W₁₈O₄₉ sensors. The enhanced response was related to the hierarchical urchin-like structure of the sensor, its high surface area and effects of the co-doped metal ions.⁸⁶ As p-type dopants, Co and Ni replaced W in the W₁₈O₄₉ lattice and created oxygen vacancies. Thus, more O₂ molecules were adsorbed on the surface, and therefore more reactions with TEA occurred. Besides, the oxidization of Co and Ni dopants reversibly transferred electrons at the interface of W₁₈O₄₉ with NiO or Co₂O₃ as (Ni ⇋ NiO and/or Co ⇋ Co₂O₃), forming a p–n heterojunction, resulting in high electrical resistance, and upon exposure to TEA, the release of electrons resulted in a remarkable change in resistance. Furthermore, dopants lowered the band gap and promoted the charge flow kinetics, and hence increased the reactivity to TEA molecules. Also, the C–N bond in TEA had the lowest bonding energy and easily broke and participated in redox reactions at the sensing temperature. Extra doping of Co and Ni (>3 wt%) resulted in intensive contraction/deformation in the W₁₈O₄₉ lattice and reduced the electrical conductivity or even caused the segregation of the dopant cations and formation of NiO and Co₃O₄. Accordingly, the sensing performance decreased when high amounts of dopant were used.

Prior studies revealed that the d-band center of materials is related to their adsorption activity, where a higher energy level of the d-band center results in enhanced activity.⁸⁷ Based on DFT results, the density of state (DOS) indicated that the doping of Ni and Co led to a shift in DOS from a low to high energy level, indicating an upward shifting of the d-band center (Fig. 14(a) and (b)). Thus, the adsorption ability of the sensor increased after co-doping. Also, based on the DFT analysis, the adsorption energy of TEA on Ni and Co co-doped W₁₈O₄₉ (E_ad = −1.53 eV) was much higher than the pristine W₁₈O₄₉ (E_ad = −0.79 eV) (Fig. 14(c) and (d)), which was consistent with the experimental results.

Fig. 14 DOS of (a) the W₁₈O₄₉ and (b) Co and Ni co-doped W₁₈O₄₉ (010) surfaces. Charge density difference of the adsorption configuration of TEA on (c) W₁₈O₄₉ and (d) Co and Ni co-doped W₁₈O₄₉ (010) surface.⁸⁵ Reproduced with permission from ref. 85. Copyright 2023, Elsevier.

Ti was added to W₁₈O₄₉ after and during the growth of W₁₈O₄₉ NWs. Before the hydrothermal growth of the W₁₈O₄₉ NWs, Ti (2 at%) was added as TiCl₄ to the final solution. Also, after the hydrothermal growth of the W₁₈O₄₉ NWs, using physical impregnation, Ti (0.6, 2, and 6 at%) was added to NWs using TiCl₄.⁸⁸ The pristine W₁₈O₄₉ NWs had an average diameter of ∼80 nm and average length of 900 nm (Fig. 15(a)). The Ti-added W₁₈O₄₉ NWs prepared via the physical impregnation method retained the morphology of the pristine W₁₈O₄₉ NWs, whereas the addition of Ti during solvothermal growth inhibited the growth of the W₁₈O₄₉ NWs, and finally W₁₈O₄₉ bundles were formed (Fig. 15(b)). Both Ti-added W₁₈O₄₉ sensors showed a p- to n-type transition with an increase in the temperature to 90 °C (Fig. 15(c)). The abnormal p-type feature observed at low temperature was due to the generation of an inversion layer at the surface of the n-type W₁₈O₄₉ NWs, originating from the intensive surface adsorption of oxygen due to the high density of surface states. In fact, the surface of nonstoichiometric W₁₈O₄₉ NWs was very active owing to the existence of a large amount of oxygen vacancies. When the additive was added to the W₁₈O₄₉ NWs a new energy level was created inside the band gap. Thus, the gas adsorption improved because of the presence of additional surface states. The intensive adsorption of oxygen on the NW surface caused a change in the Fermi level energy of the W₁₈O₄₉ NWs, resulting in the appearance of an inversion layer. Among the sensors, the sensor with 2% Ti prepared via physical impregnation displayed the highest response to NO₂ gas at RT (Fig. 15(d)) because the physical impregnation method led to the formation of a random dispersion of small TiO₂ particles, which increased the adsorption activity for oxygen on the surface of the sensing material. Also, the Ti-doped sample prepared via chemical synthesis revealed a lower response due to the fact that the W₁₈O₄₉ bundles inhibited the fast diffusion of NO₂ to the in-depth parts of the bundles.

Fig. 15 SEM images of (a) pristine W₁₈O₄₉ NWs and (b) Ti-added W₁₈O₄₉ NWs prepared using a chemical synthesis method. (c) Sensor responses vs. temperature for pristine and Ti-added W₁₈O₄₉ NWs to 2 ppm NO₂ and (d) corresponding calibration curves.⁸⁸ P stands for physical impregnation and C stands for chemical addition of Ti. Reproduced with permission from ref. 88. Copyright 2012, Elsevier.

6. Composite-based W₁₈O₄₉ gas sensors

In composite gas sensors, numerous heterojunctions between different materials can be formed, which cause a remarkable modulation in their electrical resistance.¹³ 2D graphene, which consists of C atoms with sp² hybridization, together with abundant defects and functional groups, high carrier mobility and high electrical conductivity,⁸⁹ is a promising material for sensing studies. Qiu et al.⁹⁰ prepared W₁₈O₄₉/graphene nanocomposites (with the graphene concentration of 0.005, 0.01, and 0.015 mg L⁻¹) using the hydrothermal technique at 180 °C/24 h. The composites were comprised of interlaced NWs with diameters of 30–60 nm and lengths of 1–2 μm. The response (R_a/R_g) of the W₁₈O₄₉/graphene (0.01 mg L⁻¹) sensor was 16.47 to 100 ppm ethanol at 340 °C, with t_res and t_rec of 1 and 11 s, respectively. A higher amount of graphene in the composite resulted in high conductivity, which eventually suppressed the variation in resistance in the fabricated sensor. The high sensing performance of the optimal sensor was related to its high active surface area (∼42.35 m² g⁻¹), which provided abundant active sites for ethanol molecules. Also, due to the n- and p-type nature of W₁₈O₄₉ and graphene, p–n heterojunctions were formed at the interfaces between the two semiconductors, resulting in the significant modulation of the resistance. Furthermore, double Schottky barriers were created in the contact areas between the W₁₈O₄₉ grains, where the flow of electrons was difficult between the grains in air (Fig. 16(a)). In an ethanol atmosphere and due to the release of electrons, the height of the double Schottky barriers decreased and caused a remarkable change in resistance (Fig. 16(b)). Also, because of the difference between the work functions of graphene and W₁₈O₄₉, localized p–n heterojunctions were created at the interface of the composite materials, with formation of potential barriers in air, which are considered powerful sources of resistance modulation,^91,92 remarkably modulating the resistance in an ethanol atmosphere. Furthermore, some structural defects were produced in the interface areas between the two materials, which acted as potential sites for the adsorption of ethanol.

Fig. 16 (a) Schematic of the sensing mechanism of W₁₈O₄₉/graphene in air (b) and (c) ethanol.⁹⁰ Reproduced with permission from ref. 90. Copyright 2021, Elsevier.

Transition metal dichalcogenides (TMDs) with the general formula of MX₂ (M = transition metal; X= S, Se, and Te) have a small band gap, good conductivity, and edge-exposed sites, leading to good binding interactions with gas molecules.⁹³ In this respect, a WS₂/W₁₈O₄₉ heterojunction was synthesized via the pyrolysis of WO₃ in the presence of sulfur (S) powder, thiourea (CH₄N₂S), and a mixture of “S” and CH₄N₂S at 850 °C for 1–4 h in an Ar atmosphere. The pyrolysis of WO₃ using a mixture of “S” and CH₄N₂S yielded WS₂ and W₁₈O₄₉ heterojunctions and exhibited superior gas-sensing properties. After 1 h of pyrolysis, agglomerated WS₂ covered the W₁₈O₄₉ nanorods. By performing pyrolysis for 2 h, the amount of NRs increased because of the diffusion of “S” vapor into the core parts of WO₃. After pyrolysis for 3 and 4 h, the formation of W₁₈O₄₉ NRs increased, with less agglomeration of WS₂. The sensor fabricated from the sample after 3 h pyrolysis exhibited an enhanced response [(ΔR/R_a) × 100] of 25.7% and 35.67% to 5 ppm NH₃ and NO_x gases, respectively, at RT. The sensor displayed p-type behavior because of the p-type nature of WS₂. The edge-exposed W atoms in WS₂ were highly reactive to gases due to the high electron density of the d-orbitals in W atoms. Furthermore, the formation of p–n heterojunctions between W₁₈O₄₉ and WO₃ resulted in the modulation of the electrical resistance of the sensor in the presence of gases.⁹⁴

MXenes are 2D transition metal carbides/nitrides with the formula of M_n+1X_nT_x, where M is a transition metal, X is a carbon or N atom and T_x stands for surface functional groups. Due to their high conductivity, high surface area and presence of functional groups, they are very promising sensing materials.⁹⁵ In this case, W₁₈O₄₉/Ti₃C₂T_x MXene nanocomposites with 1, 1.5,2 and 2.5 wt% MXene were fabricated via the in situ growth of 1D W₁₈O₄₉ NRs on the surface of 2D Ti₃C₂T_x MXene NSs through the solvothermal route at 150 °C/24 h.⁹⁶ The W₁₈O₄₉/Ti₃C₂T_x composite containing 2 wt% Ti₃C₂T_x displayed a response (R_g/R_a) of 11.6 to 20 ppm acetone gas at 300 °C with fast t_res and t_rec of ∼10.5 and ∼26 s, respectively. Furthermore, it could detect 170 ppb acetone, which was lower than the concentration of acetone in the exhaled breath of diabetic people.⁹⁷ The enhanced gas sensing performance was related to the presence of MXene with a high amount of surface groups such as –O and –OH, which are considered potential sites for the adsorption of acetone molecules. Also, Schottky junctions were formed between MXene and W₁₈O₄₉, and therefore the sensor experienced a larger resistance variation than that of the W₁₈O₄₉ sensor when it was exposed to acetone. The decrease in response with a higher content of MXene was due to the presence of –F groups on the MXene surface, which had negative effects on the gas sensing. Furthermore, a high amount of MXene resulted in stacking of the MXene NSs, which decreased the surface area for the adsorption of gas molecules.

Conducting polymers (CPs) have high conductivity, good flexibility, tunable properties and can be used for the detection of gases at RT.⁹⁸ In this regards, polypyrrole (PPy)@W₁₈O₄₉ C–S NRs were fabricated via the in situ polymerization of pyrrole monomer on solvothermally synthesized W₁₈O₄₉ NRs.⁹⁹ The thickness of PPy was 5, 10 and 18 nm over W₁₈O₄₉ NRs with a diameter of ∼60 nm. The response (R_a/R_g) of the fabricated sensor (Fig. 17(a)) with a PPy shell thickness of 5 nm was 4.1 to 20 ppm NH₃ at 15 °C. Furthermore, it could also detect 1 ppm NH₃, which is below the human toxicity level, i.e. 25 ppm.¹⁰⁰ Three types of junctions were present in the sensor, i.e., homojunctions between PPy in the contact areas between C–S NRs, PPy shell boundary and PPy-W₁₈O₄₉ heterojunctions (Fig. 17(b)). Owing to the modulation of the potential barriers in these junctions in the presence of NH₃, and particularly heterojunctions (Fig. 17(c) and (d)), a high response was observed. In fact, when NH₃ was adsorbed on the PPy shell, proton transfer and electron transfer occurred between the PPy and NH₃ molecules, leading to a decrease of the hole concentration in the p-type PPy shell, which resulted in an increase in the height of the potential barriers and increase in the resistance of the PPy shell. The larger response of the sensor with a thinner shell was related to the complete depletion of the shell layer from charge carriers, and upon subsequent exposure to NH₃, significant modulation of the resistance occurred (Fig. 17(e)).

Fig. 17 (a) Schematic of the C–S NR sensor. (b) Various potential barriers in the PPy-W₁₈O₄₉ C–S NR sensor. (c) Energy band levels of the PPy-W₁₈O₄₉ C–S NRs before contact, (d) change in energy band upon exposure to NH₃ and (e) influence of thinner PPy shell on the energy levels.⁹⁹ Reproduced with permission from ref. 99. Copyright 2017, Elsevier.

The fabrication of 1D C–S arrays with a uniform shell layer and high alignment over their entire surface is attractive for realizing gas sensors with an excellent performance. In the study by Qin et al.,¹⁰¹ a W layer with a thickness of ∼100 nm was first sputtered on a substrate. Then, roughly aligned W₁₈O₄₉ NWs (15 to 20 nm) were selectively grown on the substrate via thermal oxidation at 650 °C for 1 h. The intercrossing of NWs formed numerous NW–NW junctions, which acted as potential barriers and were considered powerful sources of resistance modulation. Then, a Ti film with thicknesses of 3 and 6 nm was sputter-deposited on W₁₈O₄₉. Upon annealing at 450 °C for 1 h, crystalline W₁₈O₄₉–TiO₂ C–S NWs were synthesized. The sensor with a TiO₂ shell thickness of 6 nm revealed a higher response (R_g/R_a) of 36.5 to 5 ppm NO₂ gas at RT. Interestingly, both C–S sensors exhibited a p-type feature at RT, despite the n-type semiconductor behavior of both the W₁₈O₄₉ and TiO₂ materials, which was observed in other metal oxides.^102,103 In fact, the significant adsorption of oxygen occurred due to (i) the presence of unstable surface states and (ii) catalytic activity of the TiO₂ shell. Hence, the EDL inside the shell significantly expanded and upward band bending occurred, causing the formation of an inversion layer, in which holes were the main charge carriers. Thereby, the sensors exhibited p-type behavior. The resistance of the sensor was determined by the presence of TiO₂–TiO₂ homojunctions, TiO₂–W₁₈O₄₉ heterojunctions and potential barriers as the grain boundaries of the TiO₂ shell. However, the effect of the grain boundary potential barrier was negligible in the thin shell layers. Fig. 18(b) and (c) exhibit the energy band levels of the W₁₈O₄₉–TiO₂ heterojunction before and after contact, respectively. The electron affinity, work function and band gap of TiO₂ are 4.6, and 4.7 eV, 3.6, and that of W₁₈O₄₉ are 6.5, 6.7 and 2.7 eV, respectively.¹⁰⁴ Thus, the electrons moved from TiO₂ to W₁₈O₄₉ to balance their Fermi levels, leading to the formation of heterojunctions at the interface of TiO₂ and W₁₈O₄₉ (Fig. 18(b)). In an NO₂ atmosphere, more electrons were extracted from the sensor, and the resistance increased.

Fig. 18 (a) Schematic of the W₁₈O₄₉/TiO₂ C–S NW gas sensor mechanism. Energy band levels of the W₁₈O₄₉/TiO₂ C–S NW before (b) and after (c) contact.¹⁰¹ Reproduced with permission from ref. 101. Copyright 2017, Elsevier.

Xiong et al.¹⁰⁵ synthesized a WO₃–W₁₈O₄₉ composite via a hydrothermal reaction at 200 °C for 24 h. Pristine W₁₈O₄₉ consisted of spindle-like particles (150–200 nm) (Fig. 19(a) and (c)), while the WO₃–W₁₈O₄₉ samples had a loose spindle-shaped structure composed of numerous well-aligned NWs with a diameter of 10–20 nm and length of 150–200 nm in (Fig. 19(b) and (d)), which were more suitable for gas adsorption.

Fig. 19 TEM images of (a) the W₁₈O₄₉ and (b) WO₃–W₁₈O₄₉ composites and HRTEM images of (c) W₁₈O₄₉ and (d) WO₃–W₁₈O₄₉ composites (insets reveal the corresponding SAED patterns).¹⁰⁵ Reproduced with permission from ref. 105. Copyright 2018, Elsevier.

The response (R_a/R_g) of WO₃–W₁₈O₄₉ was 23.3 to 500 ppm NH₃ at 250 °C, whereas the maximum response of the W₁₈O₄₉ sensor to 500 ppm NH₃ was ∼5 at 200 °C. The selectivity of the composite sensor to NH₃ was related to the different optimal operating temperatures for gases. Given that the work functions of the two materials were different (Fig. 20(a)), in the contact areas and to match their Fermi levels, electrons moved from WO₃ to W₁₈O₄₉, resulting in band bending and the formation of an expanded depletion layer on WO₃ (Fig. 20(a) and (b)). Upon the injection of NH₃ gas and by release of electrons, the thickness of the EDL decreased, leading to a change in the resistance of the sensor. Secondly, the surface areas of WO₃–W₁₈O₄₉ and pristine W₁₈O₄₉ were ∼95 and ∼37 m² g⁻¹, with pore volumes 0.2501 and 0.071 cm³ g⁻¹, respectively. Thus, the composite sensor offered a greater surface area and larger pores for gas molecules. Also, based on the XPS study, more oxygen ions were adsorbed on WO₃–W₁₈O₄₉ relative to the pristine W₁₈O₄₉, indicating the probability of higher reactions between them and gas molecules.

Fig. 20 Schematic of the energy band levels of WO₃ and W₁₈O₄₉ (a) before contact and after junction formation in (b) vacuum, (c) air and (d) NH₃ gas.¹⁰⁵ Reproduced with permission from ref. 105. Copyright 2018, Elsevier.

Hierarchical structures with a 3D morphology are very promising for the fabrication of gas sensors. In this regards, hierarchical WO₃ NS-W₁₈O₄₉ NW composites were prepared through the hydrothermal technique at 150 °C for 4 h using WCl₆ as the precursor, polyethylene–polypropylene glycol (P123) as the surfactant, and H₂O and ethanol as solvents.¹⁰⁶ In a fixed amount of ethanol (27.4 mL), WCl₆ (0.666 g) and P123 (0.167 g), depending on the amount of water, urchin-like W₁₈O₄₉ (no water), hierarchical WO₃/W₁₈O₄₉ (0.175 mL), and flower-like WO₃ (0.25 mL) were synthesized. The response (R_g/R_a) of the hierarchical WO₃/W₁₈O₄₉ to 1 ppm NO₂ gas was 1687 at 160 °C, which was much higher than that of the urchin-like W₁₈O₄₉ (826) and flower-like WO₃ (94). Furthermore, the t_res and t_rec of the hierarchical WO₃/W₁₈O₄₉ sensor were 36 s and 13 s, respectively. The fast dynamics of the sensor was related to the presence of abundant channels, leading to rapid diffusion and fast gas sensing reactions. Based on PL and electron paramagnetic resonance (EPR) studies, the sensors had almost the same amount of oxygen vacancies. Therefore, the difference between the sensing results was related to the synergistic nature of the NWs and NSs due to the fast electron separation and transport by the W₁₈O₄₉ NWs and abundant adsorption sites offered by the WO₃ NSs. Also, the NSs acted as a current collector, receiving electrons from the NWs to react with oxygen or NO₂ gas.

W₁₈O₄₉ NWs grown on SnO₂ NWs with hierarchical structures were prepared for sensing application. Initially SnO₂ NWs were grown via the vapor–liquid–solid (VLS) mechanism at 900 °C for 1 h. The SnO₂ NWs had diameters in the range of ∼70–300 nm and lengths of ∼5–100 μm. Next, W₁₈O₄₉ NWs with a diameter of ∼50 nm and length of more than 2 μm were grown on the SnO₂ NWs via the thermal evaporation of W. The effect of the growth temperature (1850 °C and 2050 °C) and growth pressure (5 × 10⁻⁵ mbar and 7 × 10⁻⁴ mbar) on the nature of the W₁₈O₄₉ NWs was investigated. Given that higher pressure led to a higher oxidation rate and increased evaporation rate, the density, diameter and length of the W₁₈O₄₉ NWs increased with an increase in pressure. At a fixed pressure, an increase in temperature resulted in the same trend as mentioned above.

The single wire SnO₂/W₁₈O₄₉ heterojunction sensor revealed a response (I_g/I_a) of 11.0 to 6 ppm Cl₂ gas, while the t_res and t_rec were long, namely, 4.6 min and 17 min, respectively. Abnormally, the resistance decreased on exposure to Cl₂ gas which is an oxidizing gas. Based on the comparison of the sensing behavior of the networked wire and single wire gas sensors, it was reported that the grain boundaries had a normal response to Cl₂ gas, whereas the intragrain contribution was anomalous. Also, in the single wire gas sensor, the sensing reactions mainly occurred with lattice oxygen and oxygen adsorbed on its surface, resulting in an anomalous response, and additional sites for adsorption were not available due to the low defect density of the single wire gas sensor. The networked sensor had a large number of defect sites between the grains, on which additional Cl₂ was adsorbed, leading to a normal response to Cl₂.¹⁰⁷

W₁₈O₄₉ hollow spheres were hydrothermally prepared at 180 °C for 16 h. Later, different amounts of FeCl₃ (0.01, 0.02, 0.04, and 0.06 g) were added to the W₁₈O₄₉ suspension and the mixed suspension was heated at 120 °C/1 h under hydrothermal conditions. After annealing at 400 °C for 2 h, the final W₁₈O₄₉-α-Fe₂O₃ composites with 3.99, 6.3, 13.98, and 18.98 wt% α-Fe₂O₃, were prepared.¹⁰⁸ The nanocomposites had a spherical hollow structure with an average diameter of ∼450 nm. The hollow shells were thicker and solid after the addition of α-Fe₂O₃ to W₁₈O₄₉. However, the composite with the highest amount of α-Fe₂O₃ lost the hollow spherical morphology. The surface area of the W₁₈O₄₉ hollow spheres was 78 m² g⁻¹, while that for the composite with 6.3 wt% α-Fe₂O₃ slightly decreased to 67 m² g⁻¹ due to the coverage of the pores in the W₁₈O₄₉ hollow spheres. However, both samples showed a mesoporous nature, in which the target gas could penetrate the in-depth parts of the sensor easily (Fig. 21(a)–(c)). The sensor fabricated using the composite with 6.3 wt% α-Fe₂O₃ exhibited a response (R_a/R_g) of 5.61 to 100 ppm acetone at 260 °C, while the pristine sensor revealed a much lower response. Also, its t_res and t_rec were 10 and ∼30 s, respectively. The selectivity to acetone was related to the oxygen-deficient nature of W₁₈O₄₉, where the W atoms in W₁₈O₄₉ had unsaturated coordination to oxygen, manifesting dangling bonds and bringing about the highly polarized state of W₁₈O₄₉. Given that acetone has a larger dipole moment (2.88 D) than the interfering gases, it was easily adsorbed on the surface of W₁₈O₄₉. In air, the thickness of the EDL further increased because of the adsorption of chemisorbed oxygen species (Fig. 21(b)) and subsequent exposure to acetone decreased the thickness of the EDL (Fig. 21(c)). The creation of heterojunctions between different materials is one of the most powerful sources of resistance modulation in resistive gas sensors. Due to the difference in the work function of α-Fe₂O₃ (5.88 eV)¹⁰⁹ and W₁₈O₄₉ (4.6 eV) (Fig. 21(d)), electrons flowed from W₁₈O₄₉ to α-Fe₂O₃ to match the Fermi levels at both sides of the interfaces (Fig. 21(e)).

Fig. 21 Schematic of the W₁₈O₄₉/α-Fe₂O₃ hollow sphere (a) in vacuum, (b) in air and (c) in acetone. (d and e) Energy band diagram of the W₁₈O₄₉/α-Fe₂O₃ heterojunction before and after contact.¹⁰⁸ Reproduced with permission from ref. 108. Copyright 2019, Elsevier.

The effect of plasma treatment on the gas sensing features of W₁₈O₄₉ was investigated. Initially, W₁₈O₄₉ NWs were hydrothermally synthesized at 180 °C for 5 h. Then, some of the powder was treated with H₂ plasma at a fixed flow rate for 30 and 50 min, and the remaining powder was subjected to the same treatment conditions with Ar plasma. The 1D W₁₈O₄₉ along the (010) direction had accelerated anisotropic growth behavior, which provided surface defects as active sites for gas adsorption. Then, PANI with different thicknesses adjusted by amount of aniline monomer was applied on W₁₈O₄₉ via a chemical polymerization process. Oxygen vacancies were created at the interface of W₁₈O₄₉-PANI during plasma treatment, which acted as electron acceptors and promoted PANI to generate more protons to react with the target NH₃ (Fig. 22(a)).¹¹⁰ Based on the EPR analysis, the concentration of oxygen vacancies in the Ar plasma-treated sample was lower than that in the H-plasma treated sample owing to their different atomic mass. Hence, Ar plasma could seriously destroy the surface lattice, resulting in the creation of various lattice defects, while the H₂ plasma mostly generated oxygen vacancies. The response (R_g/R_a) of the W₁₈O₄₉-PANI sensor after 50 min exposure to H₂ plasma was 35 to 100 ppm NH₃ at 25 °C, which was the highest response among the sensors. In fact, at a fixed plasma treatment time, the hydrogen plasma-treated sensor exhibited a better performance than the Ar-treated sensor due to the higher concentration of oxygen vacancies generated by hydrogen plasma treatment, consistent with another study.¹¹¹ Also with an increase in the plasma treatment time, the sensor showed a higher response due to the generation of more defects with longer times. At the same time, the H₂ plasma-treated sample had a better performance than the Ar plasma-treated sample, which may be due to the greater concentration of oxygen vacancies generated by the H₂ plasma treatment, in agreement with previous studies Also, the t_rec of the hydrogen plasma-treated sensor was shorter than that of the Ar plasma-treated sensor, which was again related to the presence of more oxygen defects in this sensor, accelerating the flow of electrons during the recovery period. Upon the interaction between PANI and NH₃ molecules, PANI lost an H atom, thus gaining an electron left by the H atom (Fig. 22(b)), causing the low response by the sensor. The free electrons were adsorbed by oxygen vacancies, permitting the occurrence of gas sensing reactions at a deeper level. Notably, due to the higher potential difference between the introduced oxygen vacancy energy level and the CB of PANI, the electron transfer in path 2 was more efficient than path 1 (Fig. 22(c)). Obviously, the presence of more oxygen vacancies in the hydrogen plasma-treated sensor led to better efficiency and a higher gas response.

Fig. 22 (a) Schematic of W₁₈O₄₉-PANI C–S, (b) interaction of NH₃ molecules with PANI and (c) heterojunction band structure and electron flow paths.¹¹⁰ Reproduced with permission from ref. 110. Copyright 2020, Elsevier.

7. Other sub-stoichiometric WO_3−x gas sensors

In comparison with W₁₈O₄₉, other sub-stoichiometric tungsten oxide nanostructures have been less studied for sensing purposes. Sub-stoichiometric WO_2.9 nanostructures have also been used for the detection of gases both experimentally and theoretically.^112,113 For example, Zhang et al.¹¹⁴ prepared WO_2.9 flowers via the acid etching of W/Cu alloy powder. The sensor with the W/Cu ratio of 1/9 displayed an enhanced sensing performance to trimethylamine at 220 °C. The improved sensing output was related to the presence of high amounts of oxygen vacancies, a high surface area owing to its hierarchical flower structure and presence of energetic reaction sites on its surface. In another study,¹¹⁵ button-shaped porous CeO₂/WO_2.9 composites with different amounts of Ce/W were synthesized via a hydrothermal reaction at 170 °C for 12 h. The optimal sensor displayed the highest response (R_a/R_g) of 23.68 to 100 ppm n-butanol at RT, which was related to the formation of double oxygen defects on the surface of the CeO₂/WO_2.9 heterostructure, the large surface area, and unique surface/interface conduction modes of the sensor. However, there are no reports on the fabrication of high-performance gas sensors based on WO_2.8 (W₅O₁₄). Overall, among the sub-stoichiometric WO_3−x, W₁₈O₄₉ is more favorable for sensing studies owing to its simple synthesis together with excellent semiconducting features.

Table 1 summarizes the gas sensing properties of the gas sensing performance of W₁₈O₄₉-based gas sensors. It can be seen that various toxic gases and VOCs can be detected by W₁₈O₄₉-based gas sensors. Also, their sensing temperatures are in the range of RT to 340 °C.

Table 1 Gas sensing performance of W₁₈O₄₉-based gas sensors

Sensing material	Synthesis method	Gas and	Conc. (ppm)	T (°C)	Response (Ra/Rg) or (Rg/Ra)	Ref.
Pristine sensors
W18O49 nanoneedles	Solvothermal	NO2	10	160	17	48
W18O49 NWs	Solvothermal	NH3	45	RT	1.13 (Ia/Ig)	50
Single crystalline W18O49 NRs	Colloidal synthesis method	Ethanol	130	RT	3.5% [ΔR/Ra] × 100	52
W18O49 NWs	Chemical synthesis	Acetone	50	280	48.6	53
W18O49 mesocrystals	Solvothermal	NO2	1	90	24.5	55
W18O49 NRs	Solvothermal	NO2	5	150	131.5	57
W18O49 NWs	In situ oxidation of sputtered W layer	NO2	1	150	4.4	59
Noble metal-decorated sensors
Pd-decorated flower-like W18O49	Ultrasonic-solvothermal method	HCHO	15	180	25.1	61
Pd-decorated urchin-like W18O49	Hydrothermal and impregnation	H2	1000	100	1600	63
Pt-decorated networked W18O49 NWs	Thermal evaporation and sputtering	H2	1000	∼48	1.6	65
Pt-decorated urchin-like W18O49 microstructures	Hydrothermal and chemical reduction	Ethanol	10	23	300	66
Pd@Au core–shell bimetallic NPs-decorated W18O49 NWs	Solvothermal and chemical reduction	H2S	50	100	55.5	68
Au-decorated Co3O4/W18O49 hollow spheres	Hydrothermal and chemical reduction	TEA	2	270	16.7	69
Ag/AgCl-decorated W18O49 NRs	Solvothermal and UV reduction	H2S	100	300	115.5	73
Au39Rh61-decorated urchin-like W18O49	Hydrothermal and co-precipitation	n-Butanol	50	260	11	75
Doped sensors
Pd-doped W18O49 NWs	Hydrothermal	Acetone	50	175	146	81
Ni-doped rambutan-like W18O49	Solvothermal	n-Butanol	50	160	182	83
Co and Ni-doped porous W18O49 nano-urchins	Hydrothermal	TEA	50	250	114	86
0.6 at% Ti-doped W18O49 NWs	Hydrothermal	NO2	4	25	23	88
Composite sensors
W18O49/graphene nanocomposites	Hydrothermal	Ethanol	100	340	16.47	90
WS2/W18O49 heterojunction	Pyrolysis of WO3 in sulfur atmosphere	NH3	5	RT	25.7% [ΔR/Ra]×100	94
W18O49/Ti3C2Tx MXene nanocomposite	Solvothermal	Acetone	20	300	11.6	96
PPy@W18O49 C–S NRs	Solvothermal and in situ polymerization	NH3	20	15	4.1	100
W18O49–TiO2 C–S NWs	Thermal oxidation and sputtering	NO2	5	RT	36.5	101
WO3–W18O49	Hydrothermal	NH3	500	250	23.3	105
WO3 NS-W18O49 NW composite	Hydrothermal	NO2	1	160	1687	106
SnO2/W18O49 heterojunction	VLS growth	Cl2	6	RT	11 (Ig/Ia)	107
W18O49-α-Fe2O3 composite	Hydrothermal	Acetone	100	260	5.61	108
W18O49-PANI	Hydrothermal and in situ polymerization	NH3	100	RT	35	109
CeO2/WO2.9 composite	Hydrothermal	n-Butanol	100	RT	23.68	115

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8. Conclusion and outlooks

Herein, the gas sensing characteristics of W₁₈O₄₉-based sensors were explained. There are various strategies for the synthesis of W₁₈O₄₉, among which the most common are hydrothermal synthesis and VLS growth. Hydrothermal synthesis has advantages such as the possibility of the synthesis of various morphologies, simple preparation procedures, and good control of the process variables such as temperature and time. However, generally it needs high synthesis pressures and long synthesis times. Alternatively, VLS growth is a simple synthesis method for the growth of 1D nanostructures such as NWs with good control of their diameters and length by control of the synthesis temperature and time. However, it often needs high temperatures for the growth of W₁₈O₄₉.

Pristine W₁₈O₄₉ gas sensors with various morphologies such as NWs have been successfully used for the detection of various toxic gases. However, their performance can be improved using different techniques such as doping, decoration with noble metals and composite formation. In all cases, optimization of the amount of added materials is required to achieve the highest sensing properties. Doping is often used with the addition of a small amount of metal cations into the W₁₈O₄₉ lattice. This results in the formation of structural defects and expansion or contraction of the lattice, which lead to changes in the surface and adsorption properties of doped W₁₈O₄₉. Decoration with noble metals such Pd, Pt and Au is another popular strategy to enhance the gas sensing features of W₁₈O₄₉ due to the catalytic and electronic effects of noble metals. Also, due to the catalytic effect of noble metals to some gases, the selectivity of W₁₈O₄₉ for a particular gas can be significantly increased. However, more studies are necessary to further explore the performance of W₁₈O₄₉ when it is decorated with noble metals. For example, no study related to Ag decoration on W₁₈O₄₉ has been reported to date. Another approach is bimetal decoration such as AuPt, AuPd, and PdPt on W₁₈O₄₉ to further increase its selectivity and response, owing to the synergistic effects of noble metals.

Composite formation is one the most reliable strategies to boost the sensing features of W₁₈O₄₉ because of the generation of a huge amount of heterojunctions at the interface areas of two different components. Composites of W₁₈O₄₉ with metal oxides, MXenes, TMDs and CPs have been reported for enhancing its gas sensing performance. Although MXenes, TMDs and CPs have lower sensing properties relative to metal oxides, they can work at lower temperatures, and therefore the sensing temperature of the fabricated sensors can be significantly decreased. In future works, composites of W₁₈O₄₉ with g-C₃N₄ and a combination of three sensing materials such as W₁₈O₄₉/TMD/MXene should be explored for gas sensing purposes.

Sensitivity is defined as the slope of the “response versus concentration” plot.¹¹⁶ Obviously, a higher response to a certain concentration of gas will lead to higher sensitivity. W₁₈O₄₉ gas sensors have relatively good response and sensitivity. However, compared with SnO₂ and ZnO gas sensors, their performances are weaker. Also, a common shortage of resistance gas sensors is their poor selectivity. In this regards, W₁₈O₄₉ gas sensors also have poor selectivity to gases. However, their selectivity can be improved by noble metal decoration and formation of composites. The limit of detection is the minimum concentration of gas that can produce a sufficiently different resistance from the base resistance of a gas sensor.¹¹⁷ Obviously, a lower LOD is better for practical application. Unfortunately, most papers related to W₁₈O₄₉ gas sensors did not report the LOD of the fabricated sensors. In some cases, they can have an LOD in the range of ppb.

Often W₁₈O₄₉ gas sensors work at high temperatures, resulting in significant power consumption in the range of mW or W. This limits their applications in remote area or areas with energy shortage. Accordingly, in future studies, the power consumption of W₁₈O₄₉ sensors can be significantly decreased by (i) illumination of W₁₈O₄₉ with UV light during exposure to gas, (ii) employing of a gas sensor in self-heating mode by directly applying an external voltage to the sensor electrodes and generation of heat within the sensor and (iii) fabrication of single NW W₁₈O₄₉ gas sensor, which needs only negligible power for working at the sensing temperature.

Also, to increase the sensitivity of W₁₈O₄₉ sensors, irradiation with high energy beams including an electron beam or gamma rays can be applied to W₁₈O₄₉, resulting in the generation of defects. The created defects act as favorable sites for gas adsorption, resulting in an enhanced sensing response. Furthermore, the implantation of W₁₈O₄₉ by high energy ions not only generates structural defects in W₁₈O₄₉ but also simultaneously dope them in the W₁₈O₄₉ lattice. Another approach is to increase the SSA of W₁₈O₄₉ using the nanotube morphology or synthesis of highly porous morphologies derived from metal organic frameworks (MOFs). In particular, MOF-derived W₁₈O₄₉ has a highly porous structure, which allows the easy diffusion of gas molecules inside the sensing layer, resulting in a high response to gas.

Data availability

The data used to support the findings of this study are available from the corresponding author upon request.

Author contributions

M. Hjiri: formal analysis, writing and editing; I. Najeh: data curation; Fatemah M. Barakat: methodology; G. Neri: reviewing; supervision.

Ethical statement

All authors declare that the presented work was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.

Conflicts of interest

Authors have a responsibility to disclose interests that might appear to affect their ability to present data objectively. Readers will benefit from transparency, including knowing authors' and contributors' affiliations and interests. Sources of funding for research were disclosed.

Acknowledgements

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2502).

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