Dimension dependency of tungsten oxide for efficient gas sensing

Abstract

Air pollution is a serious concern worldwide as it affects living beings and the environment due to the release of hazardous gases and particulate matter. The major cause of pollution is toxic gases which are released by industries, automobiles, waste and stubble burning, etc. In order to detect and monitor toxic gases like CO, NO_x, H₂S, SO₂, NH₃etc., metal oxide-based gas sensors are developed owing to their exotic properties such as high sensitivity, rapid response, and recyclability as well as long stability. Among these metal oxides, tungsten oxide (WO₃) is a transition metal oxide having enormous potential in gas-sensing applications. It is perceived that the morphology of tungsten oxide plays a significant role in the effective detection of these hazardous gases. The low dimension (0D, 1D, 2D) and hierarchical structures of WO₃ exhibit enhanced sensitivity and are utilized for efficient sensing of various gases. In this review, we have comprehensively discussed the effect of the morphology of tungsten oxide nanostructures and their application in detection of gases such as H₂, NH₃, H₂S, CO, O₃, VOCs etc. Further, the sensing mechanism along with the impact of surface functionalization and doping has been discussed. The challenges in the field and future prospects have also been discussed.

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Mr. Aditya Yadav

Mr. Aditya Yadav is a research scholar pursuing his Ph.D. at the Academy of Scientific and Innovative Research (AcSIR). He obtained his master's degree in Physics from Maharishi Dayanand University, Rohtak, India. His current research activity is focused on the fabrication of metal oxide thin films and nanostructure-based efficient gas sensors for detection of pollutant gases.

Preetam Singh

Dr. Preetam Singh is Principal Scientist at the CSIR-National Physical Laboratory, New Delhi, India. He received his Ph.D. degree in Condensed Matter Physics (Experimental) from Indian Institute of Technology Roorkee (IIT Roorkee), India, in 2008. He has worked as a Postdoctoral Fellow at Inha University, Incheon, Republic of Korea, during 2008–2009. He has expertise in the growth of high-quality thin films of metals, metal oxide semiconductors by PVD and CVD techniques and their characterization. His current research activity is focused on the fabrication of metal oxide thin films and nanostructures for the application of solid-state gas sensors.

Govind Gupta

Prof. Dr. Govind Gupta is Senior Principal Scientist and Professor, Academy of Scientific and innovative Research (AcSIR), Head, Sensor Devices and Metrology Group, CSIR-National Physical Laboratory (National Measurement Institute of India), New Delhi, India, and Professor, Academy of Scientific and Innovative Research (AcSIR). He received his Ph.D. in Solid State Physics from G. B. Pant Univ. Ag. and Tech., Pantnagar, India, in 2000. His core area of expertise is growth of III-nitrides, metal oxide and 2D materials, and fabrication of smart optical and gas sensors. He has published >250 research articles in SCI journals. He is a Senior Member of the IEEE (USA) and Associate Academician of Asia Pacific Academy of Materials (APAM). He has received numerous awards and fellowships including the MRSI medal, Young Scientist Medal, National Academy of Sciences, India, BOYSCAST fellowship, etc.

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Environmental significance

Air pollution is a serious concern worldwide as it affects living beings and the environment due to the release of hazardous gases and particulate matter. The major cause of pollution is toxic gases which are released by industries, automobiles, waste and stubble burning, etc. In order to detect and monitor toxic gases like CO, NO_x, H₂S, SO₂, NH₃etc., metal oxide-based gas sensors are developed owing to their exotic properties such as high sensitivity, rapid response, and recyclability as well as long stability. We have focused on the dimensionality of WO₃, which is one of the best suited for the detection of gases such as H₂, NH₃, H₂S, CO, O₃, VOCs etc. The low dimension (0D, 1D, 2D) and hierarchical structures of WO₃ exhibit enhanced sensitivity and are utilized for efficient sensing of various gases. The impact of structure dimensionality and surface morphologies on the gas-sensing behaviour is discussed. Further, the influence of surface functionalization and doping has also been discussed. The challenges in the field and future perspectives have also been discussed which can be useful for development of faster and efficient gas sensors.

1. Introduction

Several industrial and domestic activities like automobile discharge, burning fossil fuels etc. emit gases which are harmful, affect the environment and cause air pollution.¹ The majority of the gases emitted as industrial waste are poisonous like CO, NO_x, SO₂, NH₃, H₂S, etc. and to determine the concentration of these harmful/poisonous gases, special devices called gas sensors have been explored.² Among the various types of gas sensors, metal oxide (SnO₂, ZnO, WO₃, MoO₃etc.)-based gas sensors have been utilized to accurately detect these hazardous gases (CO, NO_x, etc.) in parts per million (ppm) or in sub-parts per billion (sub ppb) range. These metal oxide gas sensors are portable, highly sensitive, fast responsive, stable, inexpensive and offer accurate and reliable data.^3,4 Their smaller size and high sensitivity in identifying extremely low concentrations of a variety of gaseous chemical species are the significant factors which make them promising candidates in the field of gas sensors.^5,6 Among various available metal oxide semiconducting materials (i.e., ZnO, SnO₂, CuO, TiO₂, NiO etc.), tungsten oxide (WO₃) emerges as one of the best sensing materials due to its chemical and thermal stability at room temperature. WO₃ has a wide bandgap which varies between 2.6 and 3.2 eV.⁷ It is an intrinsically n-type bulk semiconductor having a cubic perovskite-like structure with a tungsten atom at the centre of each octahedron and oxygen atoms at the corners. Tungsten oxide is also available in various structures which are temperature dependent; for example, it is in the tetragonal phase for temperatures >740 °C, orthorhombic in the temperature range 330–740 °C, monoclinic in the temperature range 17–330 °C and triclinic in the temperature range −50 to 17 °C.⁷ In addition, it can easily be synthesized into nanoparticles and nanofilms by simple and inexpensive methods like solvothermal, hydrothermal etc.⁸ Diverse sizes and shapes have been attained from 0D to 3D which have shown incredible enhancement in the sensing response towards various gases. It was observed that the sensing response of the fabricated device is strongly dependent on the morphology of the grown structure. In this review, we focus on the size- and shape (morphology)-dependent sensing response of tungsten oxide nanostructures towards efficient detection of various gases. In addition, the growth process of tungsten oxide structures using different techniques has also been summarized. The impact of morphology on the parameters like selectivity, sensitivity, response time and stability of the fabricated device has also been discussed which highlights the influence of dimensionality and surface morphology on the functioning of these gas sensors. Further, to enhance the performance of the gas-sensing devices, strategies such as heterojunctions, doping and surface functionalization using noble metals are discussed. The challenges and future prospects related to tungsten oxide-based gas sensors have also been discussed.

2. Morphology-dependent growth of WO₃

It has been described that the dimensionality and surface morphology of the sensing film plays an important role in enhancement of the sensing performance of the device. As of now, diverse shape- and size-based tungsten oxide has been grown using different techniques such as thermal evaporation,^9,10 magnetron sputtering,¹¹ chemical vapour deposition,^12,13 hydrothermal,^14,15 template-assisted growth,^16,17etc. Each of the above-mentioned techniques has a set of parameters that control the morphology of the synthesized WO₃. Further, nanoscale structures are highly desired due to the high surface-to-volume ratio which plays a significant role in sensing. On the basis of dimension in the nanoscale, structural dimensions are sorted into various kinds: zero-dimensional (0D) structures (i.e., nanoparticles,¹⁸ quantum dots,¹⁹ nanoclusters²⁰), one-dimensional (1D) structures (i.e., nanowires,¹³ nanofibers,²¹ nanorods,²² nanotubes^10,23), two-dimensional (2D) structures (i.e., nanosheets,²⁴ nanobelts,²⁵ superlattices²⁶) and three dimensional (3D) and hierarchical structures (i.e., nanoflowers²⁷etc.). Composite structures which are created by one or more than one low-dimensional structures are known as 3D structures. It is perceived that tungsten oxide-based gas sensing is significantly dependent upon the method of preparation and the dimensionality of the grown material. Hence, it is essentially required to discuss the dimensionality-dependent sensing of WO₃-based structures for various gases. In this review, we have discussed the dimension-dependent tungsten oxide structures (0D, 1D, 2D, and 3D) and their response towards various gases.

2.1. Zero-dimensional (0D) structure

Zero-dimensional structures are spherical or near-spherical structures and also the simplest. Gas adsorption and diffusion are not favoured by the low specific surface area and porous structure of 0D tungsten oxide. Thereby, such 0D structures like quantum dots and tiny nanoparticles lead to unsatisfactory performance in gas sensing but are popular in the application in electronic and optical devices and catalysis.^28,29 The detection of the targeted gas may be increased due to the agglomeration and coarsening of such nanoparticles while fabricating the gas sensor. Recently, Yu et al. prepared colloidal quantum dots of WO₃ using a solvothermal method with a precursor of WCl₆ mixed with ethanol, oleyl amine and oleic acid,³⁰ which were utilized as a sensing layer to detect H₂S gas in chemiresistive gas sensors. By adjusting the reaction temperature, the grain size distribution of WO_3−x quantum dots was pre-controlled from 1.3 nm to 4.5 nm. The fabricated WO_3−x quantum dot-based sensors showed high stability, high sensitivity and the lowest detection limit at room temperature towards formaldehyde due to rich oxygen vacancy defects and their very small size.³¹ For the appropriate use of rich oxygen vacancies of nanoparticles or quantum dots of WO₃, other sensing materials such as graphene or reduced graphene are used to disperse these quantum dots.³² Recently, Li et al.¹⁵ utilized a hydrothermal method to synthesize highly dispersible nanoparticles of WO₃ with varying grain sizes between 10 and 50 nm. Similarly, Santos et al.³³ synthesized WO₃ nanoparticles with a size varying from 46 nm to 234 nm using a hydrothermal technique for biosensing applications. Zhang et al.³⁴ synthesized microspheres of WO₃ by using a hydrothermal method and found that uniform microspheres have better dispersion (Fig. 1).

Fig. 1 Tungsten oxide with different 0D nanostructures: (a) microspheres,³⁴ (b) nanoparticles,¹⁵ and (c) quantum dots.²⁹ The figure is adapted with permission from ref. 15, 29 and 34. Copyright (2021) Elsevier.

2.2. One-dimensional (1D) structures

One-dimensional structures are those which have two dimensions in the nanoscale, with the other one being of non-nanoscale size, such as nanowires, nanotubes etc. These one dimensional structures are also under goes effective transformation of chemical species on the surface which makes them suitable for gas sensing applications.³⁵ 1D structures also have a unique feature, i.e., specific electron flow in a particular dimension, as their length is larger than their diameter. For most of the gas sensor devices, the preferred size of the structure is in the millimetre (mm) range; therefore, a single 1D structure does not meet the required size and joining multiple 1D structures will hinder the electron flow which could be solved by using an ultralong 1D structure. Recently, Saidi et al. have grown WO₃ nanowires¹³ by an aerosol-assisted chemical vapour deposition technique and showed that they could be the best to detect volatile organic compounds (VOCs). Another simple and versatile technique considered for the development of several fibres and nanotubes over a wide variety of applications is electrospinning.³⁶ Leng et al. synthesized WO₃ nanofibers by an electrospinning technique for ammonia sensors operable at 500 °C.³⁷ Long et al. have synthesized substrate-free WO₃ nanorod arrays for NH₃ (ref. 38) sensing using a hydrothermal process which showed a high response of 8.3 for NH₃ (50 ppm) at 200 °C. Also, Punetha et al. synthesized WO₃ nanorods¹⁴ using a hydrothermal process which at 250 °C showed a response of 4.64 for a concentration of 5 ppm NH₃ gas. San et al.¹⁰ fabricated WO₃ nanotube arrays using a thermal evaporation method onto a Si substrate which showed a maximum response of 7.3 at 150 °C towards 10 ppm H₂S gas (Fig. 2).

Fig. 2 Tungsten oxide with different 1D nanostructures: (a) nanowires,¹³ (b and c) nanofibers.^37,39 The figure is adapted with permission from ref. 13, 37 and 39. Copyright (2021) Elsevier.

2.3. Two-dimensional (2D) structures

Two-dimensional structures are those which have only one dimension in the nanoscale such as thin films, nanosheets etc. Such structures offer a high surface-to-volume ratio, modulated surface activities, rich oxygen vacancies and surface polarisation which are useful for giving a high sensor response.⁴⁰ Due to strong interconnection and abundant active sites, they have more channels for electron transfer which can enhance the reaction ratio between the sensing material and the target gas.⁴¹ In addition, porous sub-microstructures play a major role in enhancing the gas-sensing performance of the thin films. Recently, thickness-dependent NO₂ sensing of tungsten oxide film prepared by reactive ion magnetron sputtering has been reported by Prajapati et al.¹¹ The fabricated sensors display high sensitivity and selectivity towards NO₂ gas, and a detection limit of 16 ppb is achieved. In addition, Preiß et al.⁴² has prepared porous WO₃ thin films by picosecond laser deposition which exhibits an increase in the porosity on increasing the oxygen pressure that leads to enhanced sensitivity. WO₃ nanoflake (vertically oriented) array films were also synthesized by an electrochemical technique which has been utilized for room-temperature H₂S sensing.⁴³ Various other techniques such as solvothermal⁴⁴ and hydrothermal⁴⁵ have also been employed for synthesizing WO₃ structures. The hydrothermal technique is used to develop various kinds of structures. WO₃ nanoplates with thickness varying from 90 to 150 nm grown by a hydrothermal technique have been deposited directly onto a soda lime glass substrate for NO₂ gas detection.⁴⁵ Similarly, Zhang et al.⁴⁶ synthesized WO₃ nanoplates with a length of 100–170 nm and thickness of 30–50 nm with thickness varying from tens to hundreds of nanometers.^47–49 Also, under the assistance of the surface templates polyethylene oxide–polypropylene oxide–polyethylene oxide, 2D tungsten oxide nanosheets having a thickness of 10 nm and rich in defects were synthesized for the detection of NO₂ gas.⁵⁰ Further, 2D WO₃ nanosheets were prepared by utilizing a facile wet-chemical synthesis process, i.e. sonication of tungsten particles in an acidic environment followed by thermal treatment.⁵¹ In addition, Rahmani et al. prepared WO₃ nanosheets by anodization of tungsten film for H₂ sensing in the range of 0.06% to 1%.⁵² Recently, Yuan⁵³ used a porous silicon (PS) surface as a template to grow 2D WO₃ hollow nanoscale structures by using a spin coating technique; the fabricated device displays high sensitivity and high response characteristics towards NO₂ gas. Huang et al.⁵⁴ synthesized Ru-loaded nanosheets of tungsten oxide using a facile impregnation technique for hydrogen gas-sensing application (Fig. 3).

Fig. 3 Tungsten oxide with different 2D nanostructures: (a) nanosheets,⁵⁰ (b and c) nanoplates.^45,46 The figure is adapted with permission from ref. 45, 46 and 50. Copyright (2021) Elsevier.

2.4. Three-dimensional (3D) and hierarchical structures

Three-dimensional WO₃ is an assembly of nanoplates, nanoparticles, nanosheets, and nanorods to form structures like microflowers, microspheres, urchin-like structures and other irregular structures. Such structures help in achieving better gas-sensing performance due to their high porosity, larger specific surface area and unique morphology.⁵⁵ 3D structures mainly consist of polyhedra and hierarchical structures which do not exhibit anisotropic growth and usually have complicated structures with diverse properties. The synthesis of WO₃ polyhedra mainly includes octahedral structures which are typically enclosed by surfaces with relatively high energy. Hence, such types of surfaces tend to be unstable during the reaction and thus are converted into low-energy surfaces which are more stable.⁵⁶ Mesoporous tungsten oxide (M-WO₃) can be functionalized by noble metal (Au, Ag, Pt etc.) particles or heterojunction formation, i.e., coupling with other sensing materials, thereby enhancing the performance. Ma et al. have used homogeneously distributed Pt nanoparticles (4 nm) onto a WO₃ matrix for the detection of CO.⁵⁷ Wu et al.⁵⁸ synthesized palladium-doped M-WO₃ for H₂ gas sensors using a hard template method which showed a sensor response of 11.78 for 5000 ppm H₂ gas at room temperature. Hydrothermal, microwave-assisted hydrothermal and sonochemical methods are convenient techniques to prepare 3D microspheres of WO₃,^59,60 urchin-like structures,^61,62 microflowers^63,64 and other WO₃ structures.^65,66 Recently, Li et al. used a 1-pot hydrothermal method to synthesize polyethyleneimine-mediated WO₃ nanoparticles,⁶⁷ which showed a sensing response of 251.7 for 5 ppm NO₂ at 100 °C. Also, Wei et al.⁶⁸ developed cauliflower-like WO₃ nanostructures using a hydrothermal synthesis process and the developed sensor exhibits good selectivity towards CO gas at 270 °C. Behera et al.⁶⁹ synthesized WO₃ nanorods by a thermal oxidation technique for NO₂ gas sensing with a lower detection limit of 2 ppm. Recently, Ding et al.⁷⁰ prepared pure WO₃ urchin-like hollow spheres using a hydrothermal method with diameter ranging between 1.5 and 2 μm.

Hierarchical structures are those which are formed by assembling 0D, 1D and 2D structures. Song et al., using a solvothermal method,⁷¹ synthesized WO₃ nanotubes. Adhikari et al.⁷² developed cauliflower-like hierarchical tungsten oxide structures using a microwave hydrothermal method. Tong et al.⁷³ prepared nanoporous WO₃ nanorod bundles. Wang et al.,⁷⁴ using a microwave-assisted solvothermal method, synthesized WO₃ with a flower-like architecture which was developed by inter-crossing nanorods, where the flower-like structure has a high surface area and more interspacing which is significantly useful for gas-sensing applications. Also, Wang et al.,⁷⁵ by calcining the acid-treated hydrothermal precursor, fabricated flower-like WO₃ which displayed a high response and good selectivity towards NO₂ gas. You et al.⁷⁶ used an acid-treated CaWO₄ hollow microsphere precursor to synthesize hierarchical WO₃ hollow microspheres with a porous structure of size 1 nm to 170 nm, which were found to be highly sensitive for NO₂ gas. Utilizing the hydrothermal method, Shen et al.⁷⁷ developed WO₃ flocky microspheres having a particle size of 3–4 μm. A facile template-free method was used by Zhang et al.⁷⁸ to synthesize WO₃ core–shell microspheres where the core was made up of a group of nanoparticles enclosed by a shell layer which are self-assembled by nearly 15 mm thick ultrafine nanoplates (Fig. 4).

Fig. 4 Tungsten oxide with various hierarchical structures: (a and b) Nanotubes⁷¹ and cauliflower-like structure⁷² (group of 0D nanostructures), (c and d) nanorod bundles⁷³ and flower-like architectures⁷⁴ (group of 1D nanostructures), (e and f) hollow microspheres⁷⁶ and flower-like structures⁷⁵ (group of 2D nanostructures), (g) flocky microspheres (group of 0D and 1D nanostructures),⁷⁷ (h) core–shell microspheres (group of 0D and 2D nanostructures).⁷⁸ The figure is adapted with permission from ref. 71–78. Copyright (2021) Elsevier.

3. Gas-sensing mechanism of WO₃

The concept of the gas-sensing mechanism of tungsten oxide can be explained by the variation in resistance of the sensing film on contact with the target gas. Initially, in the oxygen environment, atmospheric oxygen will be adsorbed on the surface of tungsten oxide as a material by the process of physical adsorption normally at temperatures above 200 °C. The oxygen will capture the electrons from the surface of tungsten oxide, becoming an oxygen ion (like O₂⁻, O⁻, O²⁻)⁷⁹ and thereby creating an electron depletion region in WO₃ grains. Hence, the potential barrier for electron transport across WO₃ grains increases, which will determine the initial resistance of the tungsten oxide thin film, as shown in Fig. 5(a). Then, the target gas adsorbed on the WO₃ surface will interact with the oxygen ions and change the resistance of the metal oxide.

In an oxidizing gas⁸⁰ environment, for example, NO gas will react with oxygen ions to emit N₂ and the gas gives oxygen atoms to the material, decreasing the oxygen vacancies in the material. Therefore, the potential barrier will increase and thereby increase the electrical resistance of the n-type semiconductor as a function of time.

NO_(gas) + e⁻ → NO⁻_(ads) (1)

2NO⁻_(ads) → N_2(gas) + 2O⁻_(ads) (2)

In a reducing gas environment, for example, CO (ref. 81) gas will react with oxygen ions to emit CO₂ and the captured electrons will be released back to the tungsten oxide semiconductor, as shown in Fig. 5(b). Therefore, the potential barrier will decrease, thereby decreasing the electrical resistance of the tungsten oxide semiconductor as a function of time.

CO_(g) + O⁻_(ad) → CO_2(g) + e⁻, (3)

CO + 2O⁻ → CO₃²⁻ → CO₂ + 1/2 O₂ + 2e⁻. (4)

The performance of the tungsten oxide-based gas sensor is classified by various performance parameters such as sensitivity, selectivity, response and recovery time, and stability.

Sensitivity is the comparison of resistance of the sensing element in the presence of gas to that in air. Mathematically, sensitivity is R_a/R_g and R_g/R_a for reducing and oxidizing gases, respectively. The resistance of the gas sensor in the reference gas is represented by R_a and the resistance of the gas sensor in the target gas is represented by R_g.⁸² Percentage sensitivity⁸³ is given by

S (%) = [(R_a − R_g)/R_a] × 100. (5)

The property by which the gas sensor senses only the specific or targeted gas among other gases is known as selectivity.⁸⁴ Stability⁸⁵ accounts for the overall repeatable capacity of the sensor device. With successive measurements, if the device is able to recover back to its base value and reaches the same peaks in the same time frame, then the device is said to be fit for further use. Response time⁸⁶ is the time (total) taken by the gas sensor to reach the 90% value of its change in resistance after introducing the target gases. Recovery time⁸⁶ is the total time taken by the gas sensor to recover back to 90% of the initial resistance value upon removal of the target gases. Temperature and humidity are also important factors influencing the performance of metal oxide gas sensors.⁸⁷ The conduction mechanism of WO₃ humidity sensor depends on H⁺ or H₃O⁺ resulting from the dissociation of adsorbed water on the metal oxide surface, and these H⁺ or H₃O⁺ ions hops between adjacent hydroxyl groups. In this process water adsorbing on the metal oxide surface will not donate electrons to the sensing layers which will reduce the sensitivity and enhance the recovery time of WO₃ sensors. Further, the adsorption of water will lead to less chemisorption of oxygen species on the surface due to the decrease of surface area which is responsible for the sensor response. Moreover, at high temperature, the sensing response initially increases and will reach a maximum value and then decreases rapidly with increasing temperature. In addition, at high temperatures, small grains tend to agglomerate into large entities, decreasing both the surface area and catalytic properties of the material.⁸⁸ Thus, it is requisite to keep a balance between decreasing grain sizes and stability.

3.1. Gas sensing with tungsten oxide structures

Tungsten oxide is an important material to detect hazardous gases. These gases are mainly of two types: oxidizing and reducing gases. Gases that have the capability of accepting electrons from the surface of metal oxide are called oxidizing gases (O₂, NO₂, CO₂etc.). Oxygen adsorbs swiftly on the surface of the metal oxide. Gases that have the capability to donate electrons and which can desorb the chemisorbed oxygen ions and physisorbed hydroxyl ions from the metal oxide surface are called reducing gases (CO, H₂, NH₃, H₂S etc.). WO₃ can sense various kinds of gases: (1) flammable gases that have a strong possibility of exploding on mixing with oxygen, e.g., H₂; (2) toxic gases, which are the by-product of industries, can cause a prompt response in human beings, e.g., CO, NH₃, H₂S; (3) inorganic gases, the environment has a constant concentration of gases and variation in their concentration will cause a series of changes, e.g., O₃; and (4) organic gases that can be subdivided into the above three types and are known as volatile organic compounds (VOCs).

3.1.1. Hydrogen (H₂). Tungsten oxide is considered as the most promising material to detect hydrogen gas.⁸⁹ H₂ is an odourless, tasteless and non-toxic gas in nature. However, the high concentration of H₂ gas in the atmosphere can cause a lack of oxygen in the environment, which may affect human health with symptoms including headache, vomiting, unconsciousness etc. It is hazardous to health if the concentration surmounts the threshold limit value (TLV) of 500–700 ppm (causes death after 30–60 min) and causes nearly instant death in concentrations of 1000–2000 ppm. Therefore, monitoring of the concentration of H₂ gas is necessary, which can be done by a hydrogen gas sensor. Rahmani et al.⁵² synthesized WO₃ by anodization of tungsten film which showed an optimum response of about 80% for 1% H₂ exposure at the temperature of 250 °C; the response time and recovery time were 120 s and 235 s respectively. Wu et al.⁵⁸ perceived that bare WO₃ powder is not appropriate to detect even 5000 ppm H₂ gas at room temperature (RT) due to inconsistent sensing. They utilized a hard template method and used Pd-loaded mesoporous WO₃ (Pd/M-WO₃) as the sensing material, which exhibited the highest hydrogen sensing ability in comparison to WO₃ powder and synthetic M-WO₃. Different types of structures were utilized to fabricate sensors and the performance was compared to analyse the hydrogen sensitivity, as shown in Fig. 6(a). It was discussed that the doping in mesoporous WO₃ with Pd effectively improves the specific surface area, which leads to absorption of large amounts of oxygen on the surface as well as inside the material, improving the sensing efficiency (from 1.01 to 11.78) with an extremely short recovery time (10 s) and making the fabricated sensor an ideal device highly selective for H₂, as shown in Fig. 6(b).

Fig. 5 Mechanism of semiconductor gas sensors for an n-type material in (a) an oxygen environment and (b) a reducing gas (CO) environment.

Fig. 6 (a) Variation in resistance of WO₃, MWO₃, and Pd/MWO₃ to hydrogen (5000 ppm) at RT. (b) Response of WO₃ towards various gases at RT.⁵⁸ The figure is adapted with permission from ref. 58. Copyright (2021) Elsevier.

Fig. 7 (a) Sensor response curves of a WO₃ film sensor towards various concentrations of NO₂ at 100 °C, (b) responses of WO₃ film for different gases (for 100 ppm) at 100 °C.⁴⁵ The figure is adapted with permission from ref. 45. Copyright (2021) Elsevier.

Fig. 8 (a) Sensor response curve of WO₃ arrays towards NH₃ for different concentrations at a temperature of 200 °C.³⁸ The figure is adapted with permission from ref. 38. Copyright (2021) Elsevier.

Fig. 9 (a) Comparison of various tungsten oxide-based NH₃ sensors. (b) The ammonia-sensing mechanism of WO₃ nanorods.³⁸ The figure is adapted with permission from ref. 38. Copyright (2021) Elsevier.

Fig. 10 (a) Response curves for mesoporous WO₃/NiO_−x (x = 0, 1.0, 2.0, 3.0, and 4.0) samples for H₂S at 250 °C. (b) Responses of the mesoporous WO₃/NiO_−x-based sensor for different gases (50 ppm) at 250 °C.¹¹⁰ The figure is adapted with permission from ref. 114. Copyright (2020) American Chemical Society.

Fig. 11 (a) Response curve for CO gas at different temperatures, (b) sample response towards different concentrations of CO, (c) selectivity for CO at 270 °C.¹¹³ The figure is adapted with permission from ref. 117. Copyright (2021) Elsevier.

Fig. 12 Band bending of WO₃ and SnO₂ in air (a) and TEA gas (b). (c) Response of the sensors based on single SnO₂, WO₃ and WO₃–SnO₂ composites to TEA gas at 220 °C (the inset shows the calibration curve from 1 to 50 ppm) and selectivity towards 50 ppm interference gases (d).¹⁶⁴ The figure is adapted with permission from ref. 164. Copyright (2021) Elsevier.

Recently, Chang et al.⁹⁰ used WO₃-based structure decorated with Pt nanoparticles and the sensor is fabricated via rapid thermal evaporation process. The performance of the device was tested for H₂ gas and it showed a very high response of 1.41 × 10⁶ with the lowest detection limit of 1 ppm H₂ at 200 °C and a short response/recovery time of 201/26 s. Kadir et al.⁹¹ reported a nanoporous WO₃ Schottky diode-based gas sensor which was fabricated by depositing a platinum catalytic contact and applying H₂ gas (anodic oxidation). The I–V characteristics and dynamic responses of the sensor shows a larger voltage shift in the presence of H₂ gas at 200 °C. The observed voltage shift is attributed to the reduction of the Schottky barrier height which can be easily estimated by the thermionic emission equation. However, Schottky diode-based gas sensors are more sensitive towards hydrogen gas. Yaacob et al.⁹² have also reported the gasochromic response of WO₃ films coated with catalytic Pt towards H₂ gas. The Pt/WO₃ thin films exhibited gasochromic characteristics in the visible–NIR (400–900 nm) spectral range where the total absorbance was increased by approximately 15% when exposed to 0.06% H₂ gas, which is further enhanced to 60% in the presence of 1% H₂ gas in air. However, the observed response was reduced by ∼ 40% when the same concentration of H₂ gas in air with humidity of 6% was exposed. The sensor exhibits low recovery time at room temperature, but at 100 °C the fabricated device displays a highly sensitive, stable and repeatable response towards low concentrations of H₂ gas. A detailed overview of the gas-sensing performance for H₂ gas using WO₃ structures is summarized in Table 1.

Table 1 Gas-sensing performance of H₂ using various WO₃ structures

Sensing materials	Synthesis method	Target gas	Concentration/sensor response	Operating temp.	Response/recovery time	Ref.
Pd-loaded mesoporous WO3	Hard template	H2	5000 ppm/11.78	RT	80 s/10 s	58
Nanoparticle WO3	Thermal evaporation	H2	1 ppm/1.41 × 10^6	200 °C	201 s/26 s	90
PdO-decorated WO3 nanospindle	Hydrothermal	H2	50 ppm/76	150 °C	1 s/214 s	119

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3.1.2. Nitrogen oxides (NO_x). Nitrogen oxides exist in various forms such as N₂O, NO, NO₂, N₂O₃, N₂O₄ and N₂O₅. Except for nitrogen dioxide, all these gases are unstable and convert into NO₂ easily. Nitrogen dioxide is a reddish-brown coloured toxic gas and also a pronounced air pollutant. It also affects humans and animals. Detection of NO₂ is important for environmental protection and human health. It is problematic for humans when its concentration is over the TLV of 3 ppm.⁹³ Unloaded WO₃ is described as the most capable metal oxide for NO₂ gas sensing from most reported data in the literature.^94–96 To achieve this, the gas-sensing measurements were performed in a relatively high-temperature environment.⁹⁷ Akiyama et al.⁹⁸ detected 80 ppm NO₂ at 300 °C and found that WO₃ is highly sensitive for NO₂ gas. Behera et al.⁶⁹ fabricated WO₃ nanorods using thermal oxidation which showed a response of 2.02 for 10 ppm NO₂ at 225 °C and a lower detection level of 2 ppm NO₂ when operated at an optimal working temperature of 250 °C. By using a hydrothermal method, Li et al.⁶⁷ prepared WO₃ nanoparticles which showed a sensing response of 251.7 at 100 °C and the response/recovery time of ∼11/124 s for a concentration of 5 ppm NO₂. Also, Shendage et al. utilized a similar method to grow WO₃ nanoplates (with average thickness ∼110 nm and length ∼750 nm) and directly deposited on a soda lime substrate, which exhibited a gas response of ∼10 for 5 ppm of NO₂ gas at 100 °C.⁴⁵ The sensing response of WO₃ nanoplates for NO₂ is shown in Fig. 7(a) along with a comparison of the sensing response with other gases at 100 °C in Fig. 7(b).

Recently, tungsten oxide nanosheets have also been utilized to detect an ultra-low concentration of NO₂ (40 ppb) at 150 °C with an extraordinary response of 30.⁵¹ Yuan et al.⁵³ used a spin coating technique to synthesize hollow nanoscale structures of WO₃ on the surface of porous silicon (PS), which displayed a distinguished response and high sensitivity up to the ppb level of NO₂. Prajapati et al. have shown precise measurement up to 39 ppb at 150 °C by utilizing an 85 nm thick WO₃ thin film, while the fabricated device can detect up to 16 ppb.¹¹ In recent studies, researchers have used 1D and 2D WO₃ structures for detecting various concentrations of NO₂ gas with different sensor response¹⁰⁰ at operating temperature, as summarised in Table 2.

Table 2 Gas-sensing performance of NO₂ using various WO₃ structures

Sensing materials	Synthesis method	Target gas	Concentration/sensor response	Operating temp. (°C)	Response/recovery time	Ref.
WO3 nanoplates	Hydrothermal	NO2	5 ppm/10	100	—	45
WO3 nanoparticles	Hydrothermal	NO2	5 ppm/251.7	100	11 s/124 s	67
WO3 nanorods	Thermal oxidation	NO2	10 ppm/2.02	250	—	69
WO3 nanosheet	Hydrothermal	NO2	50 ppb/6	140	140 s/75 s	50
Unloaded WO3	Atmospheric plasma spraying	NO2	450 ppb/77	130	—	94
Au@WO3 CSNSs	Hydrothermal and calcination processes	NO2	1 ppm/20	100	4 s/218 s	99
Polyaniline−WO3 thin-film	Hydrothermal	NO2	2 ppm/1.18	50	—	101

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3.1.3. Ammonia (NH₃). NH₃ is the most alkaline hazardous gas in the environment; it is colourless and lighter than air as well. Besides the vast usability of NH₃ in areas such as fertilizers, chemical industries, medical diagnosis, food processing and environmental protection, it is also hazardous for humans as it causes lung disease, poisoning, and a burning sensation when its concentration exceeds the TLV of 25 ppm.¹⁰² In an earlier study towards NH₃, Au-doped WO₃ was used by Maekawa et al.¹⁰³ in 1992. A better response of 18.2 for 10 ppm of NH₃ vapour at 200 °C (ref. 104) was shown by using WO₃ films prepared by thermal deposition than the sensors synthesized by other methods like hard template and electrospinning methods.^105,106 A linear relationship for NH₃ gas detection was realized using WO₃ nanowires, i.e., the sensitivity increased linearly with increasing temperature.⁵³ The sensitivity for 1500 ppm NH₃ at 200–300 °C (the optimum working temperature for NH₃ detection) was found to be 9.67 with a lowest response and recovery time of 7 s and 8 s, respectively. Long et al.³⁸ proposed a substrate-free fabrication of WO₃ nanoarrays using a hydrothermal method and showed a high response of 8.3 for 50 ppm NH₃ at 200 °C. The response of WO₃ towards different concentrations of NH₃ is illustrated in Fig. 8. Further, the comparison of various WO₃ nanostructures and mechanism of NH₃ sensing using tungsten oxide nanoarrays are shown in Fig. 9, where the orderly arrangement of nanorods was exploited. The flow of NH₃ gas through nanorod arrays in a zigzag way caused frequent contact with WO₃ which led to an enhanced response.

Wang et al.¹⁰⁷ used a flame melting biconical taper method to prepare WO₃ nanorods which exhibited very good sensitivity and selectivity towards NH₃ gas. The spectrum shift of tungsten oxide nanorods is found to be 16.23 nm when the concentration of NH₃ gas varies from low (0) to very high (11 640 ppm) concentrations. Recently, Fan et al.¹⁰⁸ have grown WO₃ nanoplates using an ultrasonic spray method and showed a high response of 34 at room temperature when exposed to 100 ppm NH₃; the fabricated device was found to be sensitive even to 5 ppm NH₃ (Table 3).

Table 3 Gas-sensing performance of NH₃ using various WO₃ structures

Sensing materials	Synthesis methods	Target gas	Concentration/sensor response	Operating temp. (°C)	Response/recovery time	Ref.
WO3 nanoarrays	Hydrothermal	NH3	50 ppm/8.3	200	—	38
WO3 film	Thermal deposition	NH3	10 ppm/18.2	200	—	104
WO3 nanoplates	Ultrasonic spray	NH3	100 ppm/34	RT	—	108
rGO/WO3 nanowire composites	Hydrothermal	NH3	500 ppm/35	300	37 s/711 s	109
Pd loaded WO3 film	Photochemical	NH3	50 ppm	300	10 s/20 s	110

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3.1.4. Hydrogen sulphide (H₂S). H₂S is a colourless, toxic, corrosive, flammable and extremely hazardous gas. It is released by the biological (natural) decomposition of organic waste and occurs naturally in crude petroleum, hot springs and natural gas. It is used in large quantities to extract heavy water. H₂S can cause paralysis in humans if exposed to 100 ppm and can cause death at concentrations higher than 250 ppm.¹¹¹ In an earlier study, the sensitivity of WO₃ towards H₂S was reported by Barrett et al.¹¹² in 1990. Tungsten oxide nanocrystalline films were grown with a Pt sensitizer which enhances the active sites, thereby enhancing the response. It was observed that the nanocrystalline WO₃ thick films displayed an excellent response towards H₂S gas at room temperature.¹¹³ Yu et al.³⁰ used quantum dots of WO₃ as a sensing layer in a chemiresistive H₂S gas sensor, which displayed a response of ∼57 towards 50 ppm H₂S at 80 °C with the response/recovery time of 47/126 s. Recently, Xiao et al.¹¹⁴ used p–n heterojunction-based WO₃/NiO sensors, which exhibited a response of 50 for a concentration of 50 ppm H₂S at 250 °C with response and recovery times of 4 s and 66 s, respectively. Nickel concentration-dependent response is shown in Fig. 10(a) and the sensing response of H₂S (50 ppm) and other gases is shown in Fig. 10(b).

San et al.¹⁰ synthesized tungsten oxide thin films onto silicon substrates by using a sputtering technique and these thin films were covered with WO₃ nanotubes grown by a thermal evaporation method which exhibited a maximum response of 7.3 at 150 °C towards 10 ppm H₂S gas. Also, Pt–Na₂W₄O₁₃-loaded WO₃ nanotubes were prepared by using the electrospinning method which unveiled the sensing response of >203 for 1 ppm H₂S with response and recovery time of <10 s and <30 s, respectively, under 95% humidity.¹¹⁵ Poongodi et al.⁴³ used vertically oriented WO₃ nanoflakes with an electrodeposition technique to detect H₂S (0.1–10 ppm), which revealed that the response of the device is directly correlated with temperature, i.e., 2.8% to 9.8% (at 100 °C), 12.8% to 68.9% (at 200 °C) and 26.2% to 85% (at 300 °C) (Table 4).

Table 4 Gas-sensing performance of H₂S using various WO₃ structures

Sensing materials	Synthesis method	Target gas	Concentration/sensor response	Operating temp. (°C)	Response/recovery time	Ref.
WO3 quantum dots	Colloidal synthesis	H2S	50 ppm/57	80	47 s/126 s	30
WO3 nanotubes	Thermal evaporation	H2S	10 ppm/7.3	150	—	10
WO3 nanoflakes	Electrodeposition	H2S	0.1–10 ppm/2.8% up to 85%	100–300	—	43
Pt–Na2 W4O13 loaded WO3 nanotubes	Electrospinning	H2S	1 ppm/203.5	450	<10s/<30s	115
WO3/NiO	Evaporation-induced multicomponent co-assembly	H2S	50 ppm/—	250	4 s/66 s	114
PdO loaded microspheres WO3 nanoplates	Solvothermal	H2S	25 ppm/1029	190	1 s/−	44

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3.1.5. Carbon monoxide (CO). CO is a colourless and odourless toxic gas which is produced due to incomplete combustion of fuels (automobile exhaust emission). This gas is toxic, as large amounts of this gas such as 600 ppm can cause coma in human beings by blocking the oxygen transport of blood in haemoglobin and higher levels of CO exposure can cause death. The threshold limit value for CO is at a concentration of 10 ppm for 8 hours of exposure.¹¹⁶ However, a concentration below the immediately dangerous to life and health (IDLH) level of CO can cause negative health effects on humans. The sensitivity of WO₃ towards CO appears to be relatively low. Wei et al.¹¹⁷ used cauliflower-like WO₃ (CFL-WO₃) structures obtained using a hydrothermal process and reported the response of 16.6 at 270 °C for 50 ppm CO gas. The gas-sensing properties of CFL-WO₃ sensor were depending on the operating temperature and it was observed that 270 °C is the optimal temperature for detection of 200 ppm CO gas, as shown in Fig. 11(a). Furthermore, the dynamic responses of CFL-WO₃ towards distinct concentrations of CO gas were examined, which is shown in Fig. 11(b). Subsequently, the gas sensor response towards the different concentrations of gases at 270 °C was examined and it was found that the sensor displayed high selectivity towards CO gas in the presence of other gases like HCHO, CH₄, H₂, and LPG (Fig. 11(c)).

Ma et al.⁵⁷ synthesized mesoporous WO₃ nanocomposites which revealed a good response to low concentrations of CO at low working temperature with a very short response/recovery time (16 s/1 s), good sensitivity and high selectivity. Using density functional theory calculation, Tian et al.¹¹⁸ indicated that tungsten oxide has high sensitivity towards CO gas. The theoretical analysis of gas interaction with the sensing film can abridge the research in achieving new results with completely different perspectives; however, these calculations are beyond the scope of the present review (Table 5).

Table 5 Gas-sensing performance of CO using various WO₃ structures

Sensing materials	Synthesis method	Target gas	Concentration/sensor response	Operating temp.	Response/recovery time	Ref.
Cauliflower-like WO3 structure	Hydrothermal	CO	50 ppm/16.6	270 °C	9–15 s/5–9 s	117
Mesoporous WO3 nanocomposites	Chemical route	CO	100 ppm/10.1 ± 1.0	125 °C	16 s/1 s	57

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3.1.6. Ozone (O₃). The most significant component of the ozone layer consists of O₃. This component protects the earth from harmful and excess ultraviolet rays that can harm human beings and other species. The fluctuation in the concentration of this gas can cause a number of changes. O₃ is highly toxic towards organisms, which is little known. A very small amount of O₃ like 10 ppm can cause a decline in oxygen level in the human blood which can stimulate an increase in respiratory rate. Cantalini et al.¹²⁰ fabricated WO₃ thin films using various methods and the grown films were capable of measuring O₃ in a range of 10 ppb to 160 ppb. In the recent past, the sensitivity/response towards O₃ is no longer the focus of researchers as the high sensitivity/response to O₃ has been investigated in earlier studies.

3.1.7. Volatile organic components. Volatile organic gases which come from combustion and human breathing are also polluting the enviornment. These organic gases can present a flammability, toxicity and asphyxiation risk. They can cause headache and fatigue while high concentrations of gases like CH₄ can even cause death due to suffocation. To detect these harmful organic gases, WO₃-based nanostructures are in the focus of research nowadays. The availability of a large number of gases makes it unrealistic to discuss all of them; here we have compiled some of the recent studies. A direct comparison of the performance of WO₃ structures toward various VOCs is not possible; therefore Table 6 indicates WO₃ structures with target gases without mentioning the specific parameters.

Table 6 Various studies on the sensitivity of WO₃ structures towards volatile organic gases

Sensing materials	Target gases	Ref.
Films of WO3	Liquid petroleum gas (LPG)	121
Flower-like WO3	Toluene, ethanol, n-butanol acetone, methanol	122
Porous WO3	Acetone, ethanol, formaldehyde, methanol	123
Nanoplates of WO3	Cyclohexene	124
WO3 spheres	Ethyl acetate, ethanol methanol, isopropanol, toluene	125
WO3 octahedral	Benzene	126

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4. Effort to enhance the gas-sensing properties of WO₃

We have discussed that the gas-sensing properties of WO₃-based gas sensors can effectively be enhanced on the basis of dimensionalities. For further improving the gas-sensing behaviour, WO₃ nanomaterials were optimized which improves the performance of the fabricated sensing devices. The resistive-type gas sensor response is determined by the change of resistivity which is derived after the reaction on the surface of the sensing material between the adsorbed oxygen species and the target gas molecules. Therefore, it can be concluded that the sensing property of WO₃ is enhanced by the transfer of induced electrons. For further improving the gas-sensing properties, basic strategies such as crystal facet engineering, noble metal functionalization and elemental doping might be adopted. Also, the electron transfer rate could be regulated by the formation of a heterojunction though coupling with the sensing material, thereby enhancing the sensing performance.

More recently, to achieve higher selectivity, rapid response and high reliability of WO₃ nanomaterials, special attention has been paid towards the crystal facet engineering^127,128 of metal oxide semiconductor (MOS) nanocrystals due to their surface atomic arrangement, active sites and dangling bonds. Two different hierarchical structures of pure WO₃ were synthesized by Wei et al.¹²⁹via a hydrothermal process: daisy-like hexagonal WO₃ (Dh-WO₃) and rose-like monoclinic WO₃. The Dh-WO₃ sensor exhibits outstanding acetone sensing at 260 °C from the dominant (002) facets and the intracrystalline tunnel structure of the hexagonal phase. Both WO₃ nanocrystals have the same matrix material but differ in crystalline phase.

4.1. Impact of doping

Doping in WO₃ has been a crucial technique to increase the gas-sensing performance. The creation of defects in the lattice, change in the bandgap and increase in the oxygen vacancies are done by doping with many non-metallic elements (Si,¹³⁰ C,¹³¹ N (ref. 132)) and metallic elements (Fe,¹³³ Cr,^134,135 Co,¹³⁶ Sb,¹³⁷ Sn,¹³⁸ Gd,¹³⁹ Cu,¹⁴⁰ and alkali metal,¹⁴¹ Pd (ref. 142)) into WO₃, resulting in enhanced sensing properties. Due to the similarity in the radius of Fe (0.0645 nm) and W (0.0620 nm) ions, Fe is easily doped in WO₃, giving a highly stable structure without having any distortion.¹⁴³ Fe-doped mesoporous WO₃ hollow nanospheres were observed to be selective towards NO₂ gas¹³³ and also a slight distortion was induced that created active sites as a result of a number of defects which acted as active sites. Detection of even 10 ppb of NO₂ at low operating temperature (100–140 °C) could be achieved with a good signal-to-noise ratio by these active sites.

4.2. Impact of surface functionalization

Surface functionalization is an important process for enhancing the performance of WO₃-based gas devices. Functionalization can be attained by using noble metals such as Pt, Ag, Pd, Ru, and Au, resulting in improved sensitivity, lowering the operating temperature and tuning selectivity. The role of ‘noble metal nanoparticles’ in enhancing the gas-sensing performance can be explained by catalytic oxidation and electronic sensitization.^144,145 In catalytic oxidation, the noble metal catalyst dissociates molecular oxygen in order to enrich the quantities of adsorbed oxygen molecules; the active oxygen species are thus deposited onto the WO₃ surface. Hence, the depletion layer is increased with more band electrons. As a result, the variation in resistance is apparently better than that of the pure counterparts. The gas-sensing performance can also be improved due to the interaction between the noble metals and WO₃ (called electronic sensitization) as it leads to shifting of the Fermi level of WO₃. Au/WO₃ core–shell nanospheres were prepared by using Au nanoparticles (20–25 nm diameter) and shells of WO₃ with a thickness of 30–50 nm (ref. 146) which evidenced four times higher response than pure WO₃ nanospheres, i.e., 136 towards 5 ppm NO₂ at an operating temperature of 100 °C with excellent NO₂ selectivity and long-term stability. Zhang et al. have fabricated an on-chip bilayer Au nanoparticle-decorated WO₃ nanoporous thin film by using polystyrene which is stacked layer by layer with periodic Au sputtering deposition.¹⁴⁷ A fast response and recovery time of 9 and 16 s, respectively, was observed at 150 °C that showed a sensitivity of 96.0 for 1 ppm NO₂. Also, Yang et al.¹⁴⁸ have prepared Au-functionalized WO₃ nanofibers by an electrospinning method. The film showed excellent sensitivity towards n-butanol with a sensor response of ∼152.7 for 100 ppm at 250 °C and response and recovery time varying between 5–43 s and 10–122 s, respectively. Zhang et al.¹⁴⁹ synthesized bare and PtAg-loaded WO₃ nanorods for CO detection. PtAg nanoparticles having a diameter of 20–30 nm were loaded on the surface of WO₃ nanorods, which resulted in improved performance for CO sensing. The sensor response was found to be 2.79 toward 100 ppm CO with response and recovery time of 75 s and 24 s, respectively. Moreover, first-principles analysis based on density functional theory (DFT) showed that the best adsorption sites of CO on the surface of Pt (111) and Ag (111) are the top sites and the adsorption energy of Pt (111) is better than that of Ag (111) on each site. Gas sensitivity improved due to the special sensitization mechanism of Pt and Ag and their strong interaction. Furthermore, a Pt-sensitized mesoporous WO₃ crystalline framework was synthesized by Ma et al.¹⁵⁰ by co-assembly of hydrophobic platinum precursor, amphiphilic poly-b-polystyrene and hydrophilic tungsten precursor. This device showed excellent sensitivity for CO gas for the concentration ranging from 100 to 500 ppm at a temperature of 125 °C and humidity level of 60% RH. By loading 1% Pt nanoparticles, Wang et al. have displayed the high sensing properties of mesoporous WO₃ for high concentrations of NH₃ (50–1500 ppm).¹⁵¹ It was observed that due to the catalytic behaviour of Pt nanoparticles, Pt-loaded WO₃ identified NH₃ at a significantly lower operating temperature (75 °C) than that of a pure WO₃ sensor (200 °C).

4.3. Impact of heterojunctions

The gas-sensing performance can be significantly upgraded by coupling various composites with WO₃, i.e., creating a heterojunction interface. This interface is created between the two different kinds of materials where the Fermi levels could equilibrate to the same energy.^152,153 Hence, the sensing performance is enhanced because of synergistic effects and also due to the interfaces derived from the diverse components. Furthermore, an extended region of charge depletion and charge transfer will ensue. WO₃-based heterojunctions have been reported by the coupling of WO₃ with various materials like metal oxides,^154–158 polymers,^67,159 carbon nanomaterials,^160,161 and also one other important approach to enhance the sensing performance of WO₃ is to establish ternary heterojunctions with liquid metals¹⁶²etc. Taking SnO₂ as an example from n-type metal oxides, the heterojunction of WO₃ and SnO₂ was formed where the electron transport mechanism of the heterojunction was found to be different from that of individual WO₃ or SnO₂ (where the bandgap and work function of WO₃ is 2.6 eV and 4.41 eV, respectively, while for SnO₂, it is 3.6 eV and 4.18 eV, respectively). The energy band structure of the heterojunction is configured in Fig. 12(a and b), which demonstrates that the position of the Fermi level in WO₃ is relatively higher than in SnO₂. Therefore, electrons will be transferred from WO₃ to SnO₂, forming a potential barrier across the sensor surface which hinders the movement of electrons. Thus, more active electrons are available for adsorbed oxygen molecules, increasing the conductivity of the sensor and improving the sensor response and selectivity in return towards TEA (triethylamine) as compared to the single WO₃ and SnO₂ (Fig. 12(c and d)). To increase the sensing properties, a similar mechanism is followed by the other WO₃ heterojunctions as well.

Recently, a simple ultrasonic method has been employed for the preparation of rGO/WO₃ nanocomposites at room temperature which showed a detection limit of 1.14 ppm NH₃, excellent selectivity, very high response of 680 and good response/recovery time (18/24 s).¹⁶¹ Surface engineering techniques can also further increase the selectivity of WO₃ gas sensors. The microwave-assisted gas–liquid interfacial solvothermal method was adopted by Gui et al.¹⁶³ to synthesize flower-like WO₃ with active (−112) facets, which showed high sensitivity towards TEA gas at room temperature with a response as high as 180.7 times for 100 ppm TEA along with response and recovery time of 264 and 72 s, respectively. Recently, Han et al. synthesized a WS₂–WO₃ heterojunction via a sonochemical oxidation process at room temperature, which showed a detection limit of 50 ppb for NO₂ gas and unveiled an enhanced response of 16.7 for 5 ppm NO₂ gas with a quick response time of 34 s; the fabricated device showed good recoverability compared to bare WS₂ sensor.¹⁶⁵Table 7 summarizes recent results of WO₃-based heterostructures and nanomaterials towards various gases and their sensing performance.

Table 7 Gas-sensing performance of WO₃ heterostructures and nanomaterials

Material	Synthesis technique	Targeted gas and concentration	Operating temp. (°C)	Response	Response and recovery time	LOD	Ref.
a Gas response S = Ra/Rb. b Gas response S = (Ra − Rb)/Rg.
WO3–NiO hollow nanoflower	Hydrothermal	Xylene/50 ppb–100 ppm	300	354.7^a	51 s/57 s	0.05 ppm	154
WO3–SnO2 nanospheres	Hydrothermal	Acetone 1000 ppm	240	16.9^a	51 s/11 s	—	155
WO3–ZnO ordered mesoporous	Hydrothermal	NO2 200 ppm	200	186^a	—	250 ppb	156
WO3–Fe2O3 nanoplates	Solution-based method	Acetone 100 ppm	260	105.8^a	—	1 ppm	157
WO3–CuO composite	Hydrothermal	H2S/5 ppm	100	223^a	70 s/450 s	—	158
GO(rGO)/WO3 nanofiber	Electrospinning	Acetone/100 ppm	375	35.9^a	4 s/10 s	—	160
GO(rGO)/WO3 porous composite	Ultrasonication	NH3/100 ppm	RT	14.53^a	18 s/24 s	1.14 ppm	161
PPy/WO3 nanoparticles	Chemical oxidation polymerization	TEA/100 ppm	RT	680^b	49 s/−	5.4 ppm	159
PEI/WO3 nanoparticles	Hydrothermal	NO2 5 ppm	100	251.7^a	>10 s/>120 s	50 ppb	67
WS2/WO3 composites	Drop casting	NH3	150	—	—	1 ppm	166
Rh-decorated WO3 nanorods	Glancing angle deposition	Acetone/0.2–1000 ppm	300	75.9	12 s/8 s	131 ppt	167
Au-loaded mesoporous WO3	Nanocasting	n-Butanol 100 ppm	300	5.67^a	10 s/35 s	—	168
Au-impregnated WO3 nanorods	Modified precipitation/impregnation	NO2 0.125–5 ppm	250	836.6	64.2 s/−	—	169
Ag/WO3 nanosheets	Precipitation	HCHO 100 ppm	300	20.8^a	5 s/5 s	10 ppm	170
Pt/WO3 mesoporous	Nanocasting method	NH3 200 ppm	125	13.61^a	43 s/272 s	—	171
Pd-loaded mesoporous WO3	Hard-template method	H2 5000 ppm	RT	11.78^a	80 s/10 s	1000 ppm	58

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5. Challenges and future perspectives

In the present review, we have summarized the impact of structure dimensionality and surface morphology on the gas-sensing behaviour of WO₃-based sensors. Substantial efforts have been carried out to address the challenges associated with the structure and performance of WO₃-based devices; however, there is still scope for further improvement, especially in the area of sensitivity and response time. We have systematically discussed the role of WO₃-based sensors in the detection of various gases from industrial/natural waste such as NO_x, NH₃, H₂S, CO and other volatile organic compounds. The advancement in sensor technology is useful in detecting different pollutant gases which will be beneficial to restrict the level of various toxic gases in the environment. To detect environmental pollutant gases, the role of surface morphology and structural changes has been discussed in the case of gas sensing using WO₃ as a base material. Various nanostructures or morphologies including 0D, 1D, 2D, and 3D such as nanoparticles, nanosheets, nanofibers, microspheres, microflowers, etc. (having high surface area which provides more active sites for the reaction between absorbed oxygen and target gas molecules⁵⁵) have been evaluated for detection of different gases. It was observed that the surface morphology and structure depend on the growth/synthesis techniques and deposition conditions. In addition, the annealing temperature also plays a significant role in controlling the morphology and the surface roughness which can alter the performance of a fabricated sensor. In addition, the gas-sensing mechanism for various nanostructure-based sensors developed by using WO₃ is discussed. However, the stability, selectivity, sensitivity and response/recovery time of the gas-sensing device still require further attention. Moreover, the role of surface functionalization, heterostructure, and doping has been discussed which can be utilized to enhance the sensitivity, stability, selectivity, and detectivity of the device and may be exploited for the commercialization of the technology. The other significant issue related to WO₃-based devices is their high operating temperature; efforts are essentially required to develop systems/devices which produce better sensor response at or slightly above room temperature. A lower operating temperature will result in less power consumption and will lead to the development of energy-efficient devices.

More recently, 2D materials¹⁷² including graphene, transition metal dichalcogenides, etc. are playing a significant role in the field of gas sensing and are potential competitors to these metal oxide sensors. These 2D material are efficiently detecting various gases at low or even room temperature; however the recovery time is a serious issue in these devices. Hence, efforts are essentially required to develop different kinds of structures specifically designed to enhance the sensing properties of WO₃ with a large surface area (vertically aligned, flower-like, and hierarchical dendrites etc.) with lower operating temperatures.

It is established that most target gases (except a few) are detected at a higher operating temperature which requires circuital complications due to the use of a heater. However, it could be disastrous if there is an existence of flammable gases such as H₂ along with the target gas. WO₃-based gas sensors have proven to be very efficient at higher temperature; however there is a requirement to substantially reduce the operating temperature without compromising the sensing performance.

In addition, there is huge scope of these metal oxide-based sensors in the area of healthcare monitoring where the growth of metal oxides on flexible/wearable substrates needs to be exploited.¹⁷³ Further, for industrial and space applications, the necessity is to develop sensors that are capable of operating under harsh environmental conditions. In addition, surface modification (loading of nanoparticles and doping with different elements), binary and ternary heterostructures for efficient gas detection with state-of-the-art performance parameters at lower operating temperature should be utilized.

Additionally, the synthesizing techniques of these sensor films play an important role in their performance; however, this dependency is not universal. Several methods are technically mature, yet they use harmful agents with energy consumption which is harmful for the environment and goes against “green science”. Hence, it is expected that researchers will focus on the designability during synthesis and consider all the effects including the economic and environmental effects. Enormous efforts have been invested to understand the surface morphology/dimensionality of WO₃ materials and their correlation with the detection of target gas; however, there is still a possibility for improvement. Hence, there is no doubt that the enhancement in the sensing performance will continue to play a vital role in controlling global worsening conditions in the future and would prove to be a great help to provide a clean environment.

Conflicts of interest

The authors declare no financial interest.

Acknowledgements

We acknowledge the Director, CSIR-NPL, New Delhi, for his encouragement and support. AY would like to acknowledge UGC for financial assistance via a Junior Research Fellowship.

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