Gasochromic WO 3 Nanostructures for the Detection of Hydrogen Gas : An Overview

Hydrogen is one of the most important gases that can potentially replace fossil fuels in the future. Nevertheless, it is highly explosive, and its leakage should be detected by reliable gas sensors for safe operation during storage and usage. Most hydrogen gas sensors operate at high temperatures, which introduces the risk of hydrogen explosion. Gasochromic WO3 sensors work based on changes in their optical properties and color variation when exposed to hydrogen gas. They can work at low or room temperatures and, therefore, are good candidates for the detection of hydrogen leakage with low risk of explosion. Once their morphology and chemical composition are carefully designed, they can be used for the realization of sensitive, selective, low-cost, and flexible hydrogen sensors. In this review, for the first time, we discuss different aspects of gasochromic WO3 gas sensor-based hydrogen detection. Pristine, heterojunction, and noble metal-decorated WO3 nanostructures are discussed for the detection of hydrogen gas in terms of changes in their optical properties or visible color. This review is expected to provide a good background for research work in the field of gas sensors.


Hydrogen Gas
Hydrogen is a gas with no color, odor, or taste and cannot be detected by human senses [1,2].Due to its efficiency, renewability, and green nature, it can replace fossil fuels in the near future [3].Nevertheless, because of its small size, high diffusion coefficient, and consequently easy permeation through most materials, it is difficult to store hydrogen.Furthermore, hydrogen is highly explosive with a broad flammability range (4-75 vol%) and possesses an extremely low ignition energy [4,5].Therefore, it is important to develop reliable and safe gas sensors for detecting hydrogen gas.So far, several gas sensors based on different mechanisms have been reported for sensing hydrogen, including fiber-optic [6], catalytic [7], electrochemical [8], acoustic [9], resistive [10], thermoelectric [11], and gasochromic gas sensors [12].Each of these sensors has its own merits and shortages [13].For example, resistive gas sensors are inexpensive, simple in design and operation, highly responsive, and exhibit good stability [14][15][16].However, they can only work efficiently at high temperatures [17], which increases the risk of hydrogen explosion during detection.Gasochromic sensors have the advantage of working at low temperatures, which can significantly decrease the risk of hydrogen explosion.Furthermore, in some cases, they can be fabricated on flexible substrates, with eye-readable color changes, which remarkably facilitate the detection of hydrogen in different places.In addition, the removal of electrical power from ambient atmosphere, high resistance to electromagnetic noise, and compatibility with optical fibers make them advantageous in hydrogen gas detection [18].In this review, we will focus on gasochromic gas sensors based on WO 3 .First, we introduce one of the most widely-used materials for fabricating gasochromic sensors, i.e., WO 3 , after which we discuss different gasochromic WO 3 gas sensors.

WO 3 and Its Crystal Structure
Tungsten trioxide (WO 3 ) is a very promising metal oxide with diverse properties.It has an n-type (E g = 2.60-3.25 eV) [19,20] semiconducting nature and unique electrical properties.Further, it is transparent to visible and infrared light.Therefore, it has been used in different applications including smart windows [21], photocatalysts [22], solar cells [23], humidity sensors [24], and gas sensors [25].Moreover, due to its excellent coloration efficiency [26], WO 3 is the most used chromogenic material in photochromic, electrochromic, and gasochromic applications [27].
Tungsten oxide has a perovskite-type WO 6 octahedral crystal structure (Figure 1).In its structure, W 6+ ions occupy the corners of the octahedra, and oxygen ions are located at mid-crystal edge.In the ideal form, the octahedra are connected at the corners.The central atom (C) is absent and this defective perovskite configuration is often referred to as the ReO 3 structure [28].
Appl.Sci.2019, 9, x FOR PEER REVIEW 2 of 22 places.In addition, the removal of electrical power from ambient atmosphere, high resistance to electromagnetic noise, and compatibility with optical fibers make them advantageous in hydrogen gas detection [18].In this review, we will focus on gasochromic gas sensors based on WO3.First, we introduce one of the most widely-used materials for fabricating gasochromic sensors, i.e., WO3, after which we discuss different gasochromic WO3 gas sensors.

WO3 and Its Crystal Structure
Tungsten trioxide (WO3) is a very promising metal oxide with diverse properties.It has an n-type (Eg = 2.60-3.25 eV) [19,20] semiconducting nature and unique electrical properties.Further, it is transparent to visible and infrared light.Therefore, it has been used in different applications including smart windows [21], photocatalysts [22], solar cells [23], humidity sensors [24], and gas sensors [25].Moreover, due to its excellent coloration efficiency [26], WO3 is the most used chromogenic material in photochromic, electrochromic, and gasochromic applications [27].
Tungsten oxide has a perovskite-type WO6 octahedral crystal structure (Figure 1).In its structure, W 6+ ions occupy the corners of the octahedra, and oxygen ions are located at mid-crystal edge.In the ideal form, the octahedra are connected at the corners.The central atom (C) is absent and this defective perovskite configuration is often referred to as the ReO3 structure [28].
Similar to the behavior of most perovskites and ceramics, depending on the temperature, WO3 crystals can structurally transform in the following order: Monoclinic (ε-WO3, < −43 °C), triclinic (δ-WO3, −43 to 17 °C), monoclinic (γ-WO3, 17-330 °C), orthorhombic (β-WO3, 330-740 °C), and tetragonal (α-WO3 > 740 °C) [29,30].The monoclinic crystal structure is the most stable at room temperature [30].The large voids generated in WO6 octahedral networks in the WO3 structure induce some variations in the position of W and in the WO6 octahedron orientation.Thus, displacement of tungsten from the center of the octahedron and tilting of the WO6 octahedra are two kinds of distortions [26], which lead to 11 different structures of WO3 [31].The gasochromic coloration of crystalline WO3 is associated with changes in its structure from monoclinic to tetragonal and cubic [32].For example, Inouye et al. [33] reported crystal structure transition from monoclinic to tetragonal in RF-sputtered WO3 films upon exposure to hydrogen gas.However, the structure of hydrated WO3•xH2O sensors does not change during gasochromic detection of hydrogen gas [32].

Chromogenic: Definition, Materials and Basics
Chromogenics is a Greek word with the stem "chromo" for color.It refers to the study of materials whose optical properties (color) change as a function of external ambient conditions [31].The perovskite lattice of WO 3 (Reprinted with permission from [28]).
Similar to the behavior of most perovskites and ceramics, depending on the temperature, WO 3 crystals can structurally transform in the following order: Monoclinic (ε-WO 3 , <−43 • C), triclinic (δ-WO 3 , −43 to 17 • C), monoclinic (γ-WO 3 , 17-330 • C), orthorhombic (β-WO 3 , 330-740 • C), and tetragonal (α-WO 3 > 740 • C) [29,30].The monoclinic crystal structure is the most stable at room temperature [30].The large voids generated in WO 6 octahedral networks in the WO 3 structure induce some variations in the position of W and in the WO 6 octahedron orientation.Thus, displacement of tungsten from the center of the octahedron and tilting of the WO 6 octahedra are two kinds of distortions [26], which lead to 11 different structures of WO 3 [31].The gasochromic coloration of crystalline WO 3 is associated with changes in its structure from monoclinic to tetragonal and cubic [32].For example, Inouye et al. [33] reported crystal structure transition from monoclinic to tetragonal in RF-sputtered WO 3 films upon exposure to hydrogen gas.However, the structure of hydrated WO 3 •xH 2 O sensors does not change during gasochromic detection of hydrogen gas [32].

Chromogenic: Definition, Materials and Basics
Chromogenics is a Greek word with the stem "chromo" for color.It refers to the study of materials whose optical properties (color) change as a function of external ambient conditions [31].Chromogenic materials generally have wide bandgaps and are transparent in the visible range, but they reversibly change from being transparent to a dark color in the presence of an electric field (electrochromic coloration), light (photochromic coloration), or when they are exposed to a gas (gasochromic coloration) [34].Therefore, gasochromism refers to reversible changes in optical properties or color when a material is exposed to a gas [3,35].Gasochromic materials exhibit a promising potential for use as gas sensors.WO 3 , which is light yellow in color, is one of the most important chromogenic materials known thus far.It exhibits a deep blue color upon exposure to hydrogen gas [36].In addition to WO 3 , other materials reported for gasochromic applications include V 2 O 5 [37][38][39][40], VO x [41], MO x [42], MoO 3 [43], (MoO 3 ) 1−x (V 2 O 5 ) x [44], mixed silver/nickel ammonium phosphomolybdate [45], (Ti-V-Ta)O x [35], Ni(OH) 2 [46], peroxopolytungstic acid [47,48], and metals like Y [49].This effect has also been exploited for the detection of other gases such as volatile organic compounds [50], NO 2 [51], H 2 S, SO 2 [52], NH 3 [53], XeF 2 [54], cyclohexane [55], CO, and Cl 2 [46].Among the different gasochromic materials available, the most important ones are WO 3 and MoO x .However, due to its weak color change properties and the existence of several phases whose formation depends on the growth method, molybdenum oxide has received less attention for gasochromic studies [56].
The ability of WO 3 to undergo reversible changes in its optical properties when exposed to an electric field was first reported by Deb in 1973 [57].Nineteen years later, Ito [58] reported the potential of WO 3 for gasochromic studies.Thus far, the optical properties of WO 3 nanostructures have been modulated by applying an electric field (electrochromism), UV irradiation (photochromism), or a gas (gasochromism) [59].Gasochromic coloration of WO 3 is mostly associated with hydrogen gas [35].In contrast to the electrochromic response, the presence of catalytic noble metals on the surfaces of WO 3 nanostructures is necessary to induce an acceptable gasochromic effect.The most common catalysts used are Pd [60,61], Au [30], and Pt [62,63].They promote chemical reactions by reducing the activation energy between WO 3 and hydrogen gas.Color changes occur in gasochromic WO 3 sensors when H + ions intercalate with the WO 3 layer after the dissociation of gas molecules (H 2 ) into atoms by the action of noble metals.The optical properties of WO 3 films can be reversibly changed with the insertion and extraction of H + ions and electrons into the WO 3 films, which is accompanied by redox changes leading to the formation of W 5+ ions [64,65].Gasochromic measurements are often carried out by monitoring optical properties, such as absorbance/transmittance/reflectance in convenient wavelength ranges (visible-NIR) [66].Such measurements offer simple, low-cost, and highly selective analytical methods for detecting specific gases [30].In addition, the stability of the gas sensor can be enhanced as measurements are most often conducted at low or room temperatures.

General Sensing Mechanism of Gasochromic WO 3
Currently, there are four models related to the gasochromic coloration of WO 3 sensors, namely charge transfer of electrons between (i) W 6+ /W 5+ states, (ii) W 5+ /W 4+ states, (iii) W 6+ /W 5+ and W 5+ /W 4+ states, and (iv) the oxygen vacancy model.In the first three models, the gasochromic coloration of WO 3 is explained by charge transfer-electron transitions between different states and the composition of the colored state changes to tungsten bronze (H x WO 3 ) due to the double injection of hydrogen ions and electrons into WO 3 lattice as follows [67]: The above reaction is comprised of following steps (i) absorption and dissociation of the H 2 on the surface of noble metal, (ii) spillover of the H from the noble metal onto the WO 3 surface, (iii) diffusion of H along the grain surface of WO 3 , (iv) injection of H into the outer surfaces of WO 3, (v) diffusion of H into the interior of WO 3 lattice and formation of H x WO 3 [68].The magnitude of coloration in H x WO 3 is closely related to the amount of hydrogen insertion (x), which usually varies with the structure, morphology, and crystallinity of the WO 3 sensing layer [67,69].The oxygen vacancy model supposes the formation of a sub-stoichiometric WO 3−x structure due to the formation of oxygen vacancies [67].
In fact, in the oxygen vacancy model, H 2 is dissociated on the noble metal and then transferred to WO 3 , where it reacts with oxygen on the surface of WO 3 , leading to the formation of water and oxygen vacancies and a blue color as follows [70]: The oxygen vacancies diffuse into the interior of the WO 3 and lead to the coloration, while the H 2 O slowly desorbs from the WO 3 [68].
A challenging task in gasochromic sensor science lies in improving sensor dynamics, which is important for the rapid detection of hydrogen leakage.Reduction of sensor size from micro to nano-scale increases its surface area significantly, which means an increase in the number of available surface sites for the adsorption of atomic hydrogen, leading to faster sensor dynamics.Furthermore, nanostructured gas sensors are expected to exhibit an improved response to H 2 gas even at low concentrations [30].In the following sections, different WO 3 nanostructured gasochromic sensors are presented and discussed.

Gasochromic WO 3 Nanostructures as Hydrogen Gas Sensors
Generally, substrate temperature affects the crystallinity and morphology of as-grown films, which eventually leads to differences in their gasochromic coloration.Yamamoto et al. [71] studied the gasochromic performance of epitaxial WO 3 sensors deposited by pulsed laser deposition (PLD) at different substrate temperatures.Changes in the optical density (OD) of the WO 3 sensor at λ = 645 nm between the initial and colored states when exposed to 1% H 2 /Ar for 20 min, i.e., optical density (∆OD) = −log(T colored /T initial ), were obtained.The ∆OD of the film deposited at 432 • C rapidly changed when exposed to hydrogen gas.With an increase in the substrate temperature, the coloration reduced, and in the film deposited at 538 • C (with better crystallinity), almost no coloration was observed.This was in contrast to previous reports on the enhancement in gasochromic coloration due to structural changes, where highly crystalline WO 3 nanowires (NW) films with a monoclinic structure transformed into the tetragonal phase upon hydrogen gasochromic coloration.Thus, the researchers highlighted that controlling the free space between WO 3 grains for the relaxation of mechanical stresses during structural transformation is essential when a sensor is exposed to hydrogen gas.
Behbahani et al. [72] prepared WO 3 /glass thin film gas sensors by PLD at various substrate temperatures (25-400 • C) for the gasochromic investigation of hydrogen.Figure 2a shows images of different colors the gas sensors exhibited at various substrate temperatures.
Coloring was initiated from the boundary edge of the PdCl 2 drop as a tiny blue ring and gradually extended to the center, suggesting that hydrogen initially entered at the triple points of the substrate, liquid, and gas.In hydrogen atmosphere, Pd nanoparticles (NPs) were produced over the surface as well as at the triple points.The formation of Pd was accompanied by the gasochromic coloring of tungsten oxide.Hydrogen, with its reducing power, helps the accumulation of a layer of Pd NPs, as shown in Figure 2b.In the samples deposited at temperatures below 200 • C, a uniform coloring was observed.At 200 • C, a very deep blue color was obtained due to the high surface porosity of the sensor film.However, the uniformity of coloring decreased significantly with an increase in the deposition temperature.
Pd, Pt, and Au catalysts are known to be highly reactive to activation by hydrogen, and they are often used along with WO 3 to increase gasochromic efficiency by the dissociation of hydrogen atoms.Garavand et al. [73] prepared Pt/WO 3 and Pd/WO 3 films by PLD and studied the effect of operating temperature  • C) on their gasochromic properties.Coloration was evaluated by measuring the optical density (∆OD) at λ = 632.8nm as follows, where I(t) and I(t 0 ) represent the optical transmission intensities of the colored and bleached states, respectively.Pristine WO 3 did not show any noticeable response to hydrogen gas, while, due to the spillover effect of Pt, Pt/WO 3 films showed a good response to hydrogen gas.The response of the Pt/WO 3 sensor was almost constant in the range of 85-200 • C. At lower temperatures, its response was negligible, and its dynamics were slow due to the slow dissociation of hydrogen gas and low diffusivity of the hydrogen atoms.At 200 • C, the effect of humidity on the gas response was negligible.Further, the risk of catalyst poisoning with contaminants was lower at this temperature when compared to those at lesser temperatures.Therefore, this temperature was reported to be the optimal sensing temperature.In addition, the higher response of the Pt/WO 3 sensor when compared to that of the Pd/WO 3 sensor was attributed to the hydrogen spillover effect of Pt and Pd.As the dissolution rate of hydrogen in Pd is higher than that in Pt, the spillover range of hydrogen on the Pt surface is longer.Therefore, the number of injected species into the WO 3 lattice in Pt/WO 3 was higher than that in Pd/WO 3 .This implies that in the Pt/WO 3 gas sensor, more color centers were activated, leading to a higher gasochromic response.In contrast, the Pd-WO 3 gas sensor was capable of detecting H 2 gas at lower temperatures [73].
Appl.Sci.2019, 9, x FOR PEER REVIEW 5 of 22 Pt/WO3 sensor was almost constant in the range of 85-200 °C.At lower temperatures, its response was negligible, and its dynamics were slow due to the slow dissociation of hydrogen gas and low diffusivity of the hydrogen atoms.At 200 °C, the effect of humidity on the gas response was negligible.Further, the risk of catalyst poisoning with contaminants was lower at this temperature when compared to those at lesser temperatures.Therefore, this temperature was reported to be the optimal sensing temperature.In addition, the higher response of the Pt/WO3 sensor when compared to that of the Pd/WO3 sensor was attributed to the hydrogen spillover effect of Pt and Pd.As the dissolution rate of hydrogen in Pd is higher than that in Pt, the spillover range of hydrogen on the Pt surface is longer.Therefore, the number of injected species into the WO3 lattice in Pt/WO3 was higher than that in Pd/WO3.This implies that in the Pt/WO3 gas sensor, more color centers were activated, leading to a higher gasochromic response.In contrast, the Pd-WO3 gas sensor was capable of detecting H2 gas at lower temperatures [73].(Reprinted with permission from [72]).
Lee et al. [74] investigated the gasochromic mechanism of amorphous Pd/WO3 films using Raman scattering tests.Firstly, when a Pd/WO3 gas sensor was exposed to hydrogen, hydrogen dissociation occurred on the Pd surface, and these atoms diffused across the Pd layer.Subsequently, atomic hydrogen and electrons were inserted at the Pd/WO3 interface.Later, W 6+ ions were reduced to W 5+ by procuring one electron from the hydrogen atom, resulting in a change in the color of WO3.For gasochromic bleaching in the presence of oxygen, there occurred a recombination reaction between atomic hydrogen (on Pd) and oxygen gas, leading to the formation of water molecules and oxidation of W 5+ to W 6+ .Lee et al. [74] investigated the gasochromic mechanism of amorphous Pd/WO 3 films using Raman scattering tests.Firstly, when a Pd/WO 3 gas sensor was exposed to hydrogen, hydrogen dissociation occurred on the Pd surface, and these atoms diffused across the Pd layer.Subsequently, atomic hydrogen and electrons were inserted at the Pd/WO 3 interface.Later, W 6+ ions were reduced to W 5+ Appl.Sci.2019, 9, 1775 6 of 21 by procuring one electron from the hydrogen atom, resulting in a change in the color of WO 3 .For gasochromic bleaching in the presence of oxygen, there occurred a recombination reaction between atomic hydrogen (on Pd) and oxygen gas, leading to the formation of water molecules and oxidation of W 5+ to W 6+ .
In another study related to amorphous WO 3 films, Lee et al. [75] deposited amorphous WO 3 thin films with thicknesses of 270, 540, and 760 nm by RF sputtering, and subsequently Pd nanoparticles (NPs) layers of different thicknesses (<5 nm) were deposited by e-beam evaporation.Figure 3 shows a schematic of the developed gas sensing system.To quantitatively measure visible color changes in the Pd-WO 3 sensor, measurements were carried out in the wavelength range of 300 to 1100 nm at room temperature.
In another study related to amorphous WO3 films, Lee et al. [75] deposited amorphous WO3 thin films with thicknesses of 270, 540, and 760 nm by RF sputtering, and subsequently Pd nanoparticles (NPs) layers of different thicknesses (<5 nm) were deposited by e-beam evaporation.Figure 3 shows a schematic of the developed gas sensing system.To quantitatively measure visible color changes in the Pd-WO3 sensor, measurements were carried out in the wavelength range of 300 to 1100 nm at room temperature.The optimal thickness of Pd on WO3 films was found to be 3-4 nm.The transmittance spectrum obtained after exposure to 1% H2 gas for 10 min showed a 50% difference in the transmittance before and after exposure to H2 gas.The influence of temperature on coloration was investigated in the range of 298 to 473 K. Transmittance was found to increase with an increase in temperature.Color degradation occurred due to annealing, where grain growth led to a decrease in the grain boundary diffusion of protons.Furthermore, surface water molecules, which provided channels for oxygen vacancies, evaporated, and the evacuated cavities were covered with some oxygen.
Color difference measurements showed that a thick gas sensor provided better coloration difference.The structure of the thinnest WO3 film was dense, while, at higher thicknesses, the films exhibited a porous structure.Further, different gas compositions, including pure H2 (10 sccm), H2 + CH4 (5:5 sccm), pure CH4 (10 sccm), and H2 + CO (5:5 sccm), were tested (Figure 4a).Pure CH4 gas did not result in any changes in transmittance, while H2 + CO resulted in a slightly decreased transmittance due to Pd poisoning by CO gas.On the other hand, the H2 + CH4 mixture showed the lowest transmittance and greatest coloration.Enhanced coloration with the H2 + CH4 mixture may be related to the oxidation of CH4 on Pd, which produced CH3OH and H2O.As a result, additional hydrogen atoms were produced.They diffused into the WO3 lattice, reacted to form an intermediate W=O-H state, and produced H2O, thereby creating oxygen vacancies.Consequently, the amount of W 5+ sites inducing coloration increased.A Pd-WO3 flexible gas sensor was also prepared on a polyester substrate (Figure 4b,c).No annealing was performed during the fabrication of the flexible gas sensor.The sensor exhibited coloration without cracks or failure upon bending, which is important for some applications.For example, joints in supply lines are potential leakage points for hydrogen gas.A flexible and bendable sensor can conform to the morphology of the joint.Accordingly, low cost and flexible gasochromic H2 sensors can facilitate H2 detection at leakage sites with unusual shapes.
In another study related to the thickness of the sensing film, Shanak et al. [76] deposited WO3 thin films with different thicknesses in a mixed atmosphere of O2 and Ar gas using a reactive DC-sputtering technique.Subsequently, Pt was deposited using a sputtering method-films of different thicknesses showed different response times.Thin films (20-170 nm) exhibited long coloring and bleaching times between coloration and bleaching states.Thicker films showed shorter switching times.The shortest switching time was found for the thickest sample (~1000 nm) with a coloration rate of 1.64%/s.Additionally, the thickest film showed good saturation, reversibility, and stability.The optimal thickness of Pd on WO 3 films was found to be 3-4 nm.The transmittance spectrum obtained after exposure to 1% H 2 gas for 10 min showed a 50% difference in the transmittance before and after exposure to H 2 gas.The influence of temperature on coloration was investigated in the range of 298 to 473 K. Transmittance was found to increase with an increase in temperature.Color degradation occurred due to annealing, where grain growth led to a decrease in the grain boundary diffusion of protons.Furthermore, surface water molecules, which provided channels for oxygen vacancies, evaporated, and the evacuated cavities were covered with some oxygen.
Color difference measurements showed that a thick gas sensor provided better coloration difference.The structure of the thinnest WO 3 film was dense, while, at higher thicknesses, the films exhibited a porous structure.Further, different gas compositions, including pure H 2 (10 sccm), H 2 + CH 4 (5:5 sccm), pure CH 4 (10 sccm), and H 2 + CO (5:5 sccm), were tested (Figure 4a).Pure CH 4 gas did not result in any changes in transmittance, while H 2 + CO resulted in a slightly decreased transmittance due to Pd poisoning by CO gas.On the other hand, the H 2 + CH 4 mixture showed the lowest transmittance and greatest coloration.Enhanced coloration with the H 2 + CH 4 mixture may be related to the oxidation of CH 4 on Pd, which produced CH 3 OH and H 2 O.As a result, additional hydrogen atoms were produced.They diffused into the WO 3 lattice, reacted to form an intermediate W=O-H state, and produced H 2 O, thereby creating oxygen vacancies.Consequently, the amount of W 5+ sites inducing coloration increased.A Pd-WO 3 flexible gas sensor was also prepared on a polyester substrate (Figure 4b,c).No annealing was performed during the fabrication of the flexible gas sensor.The sensor exhibited coloration without cracks or failure upon bending, which is important for some applications.For example, joints in supply lines are potential leakage points for hydrogen gas.A flexible and bendable sensor can conform to the morphology of the joint.Accordingly, low cost and flexible gasochromic H 2 sensors can facilitate H 2 detection at leakage sites with unusual shapes.
In another study related to the thickness of the sensing film, Shanak et al. [76] deposited WO 3 thin films with different thicknesses in a mixed atmosphere of O 2 and Ar gas using a reactive DC-sputtering technique.Subsequently, Pt was deposited using a sputtering method-films of different thicknesses showed different response times.Thin films (20-170 nm) exhibited long coloring and bleaching times between coloration and bleaching states.Thicker films showed shorter switching times.The shortest switching time was found for the thickest sample (~1000 nm) with a coloration rate of 1.64%/s.Additionally, the thickest film showed good saturation, reversibility, and stability.In their investigation, Nishizawa et al. [77] fabricated WO3 amorphous films decorated with Pt using UV irradiation.Changes in the optical transmittance of the films were studied by measuring the light intensity of a laser (λ = 670 nm) with a silicon photodiode (Figure 5).The optical transmittance of Pt-WO3 sensors at λ = 670 nm was measured as a function of time during exposure to a 60 s cycle of hydrogen injection and removal.Optical changes in the presence of H2 gas or air were quite large (~70%).In the case of the gas sensor with a film thickness of 400 nm, the response time was small (~5 s), while a thicker film required more time due to the slow diffusion In their investigation, Nishizawa et al. [77] fabricated WO 3 amorphous films decorated with Pt using UV irradiation.Changes in the optical transmittance of the films were studied by measuring the light intensity of a laser (λ = 670 nm) with a silicon photodiode (Figure 5).In their investigation, Nishizawa et al. [77] fabricated WO3 amorphous films decorated with Pt using UV irradiation.Changes in the optical transmittance of the films were studied by measuring the light intensity of a laser (λ = 670 nm) with a silicon photodiode (Figure 5).The optical transmittance of Pt-WO3 sensors at λ = 670 nm was measured as a function of time during exposure to a 60 s cycle of hydrogen injection and removal.Optical changes in the presence of H2 gas or air were quite large (~70%).In the case of the gas sensor with a film thickness of 400 nm, the response time was small (~5 s), while a thicker film required more time due to the slow diffusion The optical transmittance of Pt-WO 3 sensors at λ = 670 nm was measured as a function of time during exposure to a 60 s cycle of hydrogen injection and removal.Optical changes in the presence of H 2 gas or air were quite large (~70%).In the case of the gas sensor with a film thickness of 400 nm, the response time was small (~5 s), while a thicker film required more time due to the slow diffusion of hydrogen into the film.The gas sensors could maintain their stability for over 1500 switching cycles.Their high gasochromic performance was attributed to the presence of many voids in the film and the catalytic activity of Pt NPs.
Gasochromic eye-readable H 2 sensors are in high demand as they do not require electrical or additional contacts.Furthermore, there is no risk of explosion in high hydrogen gas concentrations [78].In this regard, amorphous Pd/TiO 2 /WO 3 /glass thin films were prepared using a PLD technique.Using a laser (λ = 248 nm) with different incident energies (110, 140, 170, and 200 mJ), thin top layers of anatase TiO 2 (a-TiO 2 ) with a high photocatalytic activity were generated and their gasochromic performance was studied at room temperature.Figure 6 shows photographs of different coloring steps over 1 h after exposure to hydrogen gas.
Appl.Sci.2019, 9, x FOR PEER REVIEW 8 of 22 of hydrogen into the film.The gas sensors could maintain their stability for over 1500 switching cycles.Their high gasochromic performance was attributed to the presence of many voids in the film and the catalytic activity of Pt NPs.Gasochromic eye-readable H2 sensors are in high demand as they do not require electrical or additional contacts.Furthermore, there is no risk of explosion in high hydrogen gas concentrations [78].In this regard, amorphous Pd/TiO2/WO3/glass thin films were prepared using a PLD technique.Using a laser (λ = 248 nm) with different incident energies (110, 140, 170, and 200 mJ), thin top layers of anatase TiO2 (a-TiO2) with a high photocatalytic activity were generated and their gasochromic performance was studied at room temperature.Figure 6 shows photographs of different coloring steps over 1 h after exposure to hydrogen gas.(Reprinted with permission from [79]).
Coloring in the sensors started from the edges.The as-synthesized sample (TW) and the 110 TW sample were fully colored after 1 h of exposure to hydrogen gas, however, the former showed a long switching time.This is because the continuous TiO2 layer decreased the interlayer diffusion of hydrogen gas.The poor coloring effect in the samples 140 TW to 200 TW is attributed to their crystalline nature [79].
In another study related to naked-eye gas sensors, rapid gasochromic hydrogen gas sensors were realized using 2D nanoplates of WO3, which were synthesized by a sol-gel method.Subsequently, Pd NPs were deposited by a photochemical deposition method [78].In a hydrogen atmosphere, the sensors exhibited a visible color change from brown to dark blue-all H2 concentrations could be detected according to an eye-readable gasochromic scheme.Figure 7 shows photographs of 2D WO3-Pd sensors deposited on filter papers and exposed to different concentrations of H2 gas at room temperature.A complete color change occurred at different H2 gas exposure times and concentrations.The 2D WO3/Pd sensors showed fast coloration and bleaching times at all hydrogen concentrations.At 100% H2 concentration, complete color change occurred in 4 s, with 30 s of recovery time.The fast response of the sensor was attributed to the rapid and complete diffusion of H atoms throughout the WO3 structure, which caused an increase in the access of W atoms to H atoms in a shorter period of time when compared to 1D materials.Coloring in the sensors started from the edges.The as-synthesized sample (TW) and the 110 TW sample were fully colored after 1 h of exposure to hydrogen gas, however, the former showed a long switching time.This is because the continuous TiO 2 layer decreased the interlayer diffusion of hydrogen gas.The poor coloring effect in the samples 140 TW to 200 TW is attributed to their crystalline nature [79].
In another study related to naked-eye gas sensors, rapid gasochromic hydrogen gas sensors were realized using 2D nanoplates of WO 3 , which were synthesized by a sol-gel method.Subsequently, Pd NPs were deposited by a photochemical deposition method [78].In a hydrogen atmosphere, the sensors exhibited a visible color change from brown to dark blue-all H 2 concentrations could be detected according to an eye-readable gasochromic scheme.(Reprinted with permission from [78]).
The limit of detection for the above described gas sensors was 4%, which is considered the lower limit for H2 explosion in air.Further, the gas sensors exhibited good reproducibility and high stability-their gas sensing mechanism was related to the formation of hydrogen tungsten bronzes (HxWO3, x = 0-1) [78].
Yamaguchi and co-workers [80] synthesized Pt/WO3 thin films using a sol-gel method for eye-readable gasochromic hydrogen sensors.When the sensors were exposed to 1% hydrogen, their transmittance reduced and, in the presence of 100% hydrogen gas, they changed from transparent to blue.The gasochromic phenomenon of these sensors was attributed to the formation of HxWO3, which was different in color when compared to WO3.Hydrogen molecules were split onto hydrogen atoms and, upon subsequent spillover to Pd, tungsten ions were reduced (changing W 6+ to W 5+ ) and H WO was formed.The relevant reactions are as follows [80]: Kalanur et al. [81] synthesized WO3-Pd nanocomposites and subsequently deposited them on filters for hydrogen gas sensing at room temperature.Figure 8 shows images of WO3-Pd nanocomposites when exposed to different concentrations of hydrogen gas.In a H2 atmosphere, the WO3-Pd nanocomposite changed its color to blue-gray, which could be easily detected by a naked eye.Color change was strongly dependent on both hydrogen concentration and exposure time.Hydrogen tungsten bronze (HxWO3) formation is held to be responsible for color changes in these gas sensors.At 1% and 100% hydrogen concentration, the full coloration times were only 240 s and 45 s, respectively.However, the recovery time was long (~30 min) at all concentrations of hydrogen gas.The limit of detection for the above described gas sensors was 4%, which is considered the lower limit for H 2 explosion in air.Further, the gas sensors exhibited good reproducibility and high stability-their gas sensing mechanism was related to the formation of hydrogen tungsten bronzes (H x WO 3 , x = 0-1) [78].
Yamaguchi and co-workers [80] synthesized Pt/WO 3 thin films using a sol-gel method for eye-readable gasochromic hydrogen sensors.When the sensors were exposed to 1% hydrogen, their transmittance reduced and, in the presence of 100% hydrogen gas, they changed from transparent to blue.The gasochromic phenomenon of these sensors was attributed to the formation of H x WO 3 , which was different in color when compared to WO 3 .Hydrogen molecules were split onto hydrogen atoms and, upon subsequent spillover to Pd, tungsten ions were reduced (changing W 6+ to W 5+ ) and H x WO 3 was formed.The relevant reactions are as follows [80]: xH Kalanur et al. [81] synthesized WO 3 -Pd nanocomposites and subsequently deposited them on filters for hydrogen gas sensing at room temperature.Figure 8 shows images of WO 3 -Pd nanocomposites when exposed to different concentrations of hydrogen gas.In a H 2 atmosphere, the WO 3 -Pd nanocomposite changed its color to blue-gray, which could be easily detected by a naked eye.Color change was strongly dependent on both hydrogen concentration and exposure time.Hydrogen tungsten bronze (H x WO 3 ) formation is held to be responsible for color changes in these gas sensors.At 1% and 100% hydrogen concentration, the full coloration times were only 240 s and 45 s, respectively.However, the recovery time was long (~30 min) at all concentrations of hydrogen gas.
Generally, gas sensors with nanowire (NW) morphology exhibit good self-heating effects [82].In this context, Chen et al. [83] studied the effect of self-heating on the gasochromic response of Pt/WO 3 NW sensors.Changes in the optical properties of Pt-WO 3 NW sensors at λ = 632.8nm indicated that their coloration response was greatly improved by the self-heating effect, resulting in a high coloration contrast and fast response time (less than 10 ms).The ultrafast gasochromic response time is related to the temperature of WO 3 NWs, which increased to above 200 • C during self-heating cycles.In a hydrogen atmosphere, the absorbed H 2 molecules on the surface of Pt NPs dissociated into H ions and electrons (Figure 9a).Later, they were transferred to the surfaces of WO 3 NWs by the spillover effect and were subsequently injected into WO 3 NWs.Due to the reaction between injected hydrogen atoms and oxygen atoms in the lattice structure of WO 3 , WO 3−x xH 2 O was formed, which caused coloration in the gas sensor.When the NWs were exposed to oxygen gas or air (Figure 9b), due to the spillover effect of Pt on oxygen gas, the density of free electrons in NWs decreased to a great extent (Figure 9c) which led to instability in the localized water molecules in the body of the WO 3 NW.These localized water molecules decomposed into atomic hydrogen and oxygen species, which subsequently diffused to the NW surface and reacted with foreign oxygen ions to form water vapor, while oxygen species re-filled oxygen vacancies and the sensor regained its initial structure and was bleached [83].Generally, gas sensors with nanowire (NW) morphology exhibit good self-heating effects [82].In this context, Chen et al. [83] studied the effect of self-heating on the gasochromic response of Pt/WO3 NW sensors.Changes in the optical properties of Pt-WO3 NW sensors at λ = 632.8nm indicated that their coloration response was greatly improved by the self-heating effect, resulting in a high coloration contrast and fast response time (less than 10 ms).The ultrafast gasochromic response time is related to the temperature of WO3 NWs, which increased to above 200 °C during self-heating cycles.In a hydrogen atmosphere, the absorbed H2 molecules on the surface of Pt NPs dissociated into H ions and electrons (Figure 9a).Later, they were transferred to the surfaces of WO3 NWs by the spillover effect and were subsequently injected into WO3 NWs.Due to the reaction between injected hydrogen atoms and oxygen atoms in the lattice structure of WO3, WO3−x xH2O was formed, which caused coloration in the gas sensor.When the NWs were exposed to oxygen gas or air (Figure 9b), due to the spillover effect of Pt on oxygen gas, the density of free electrons in NWs decreased to a great extent (Figure 9c) which led to instability in the localized water molecules in the body of the WO3 NW.These localized water molecules decomposed into atomic hydrogen and oxygen species, which subsequently diffused to the NW surface and reacted with foreign oxygen ions to form water vapor, while oxygen species re-filled oxygen vacancies and the sensor regained its initial structure and was bleached [83].Generally, gas sensors with nanowire (NW) morphology exhibit good self-heating effects [82].In this context, Chen et al. [83] studied the effect of self-heating on the gasochromic response of Pt/WO3 NW sensors.Changes in the optical properties of Pt-WO3 NW sensors at λ = 632.8nm indicated that their coloration response was greatly improved by the self-heating effect, resulting in a high coloration contrast and fast response time (less than 10 ms).The ultrafast gasochromic response time is related to the temperature of WO3 NWs, which increased to above 200 °C during self-heating cycles.In a hydrogen atmosphere, the absorbed H2 molecules on the surface of Pt NPs dissociated into H ions and electrons (Figure 9a).Later, they were transferred to the surfaces of WO3 NWs by the spillover effect and were subsequently injected into WO3 NWs.Due to the reaction between injected hydrogen atoms and oxygen atoms in the lattice structure of WO3, WO3−x xH2O was formed, which caused coloration in the gas sensor.When the NWs were exposed to oxygen gas or air (Figure 9b), due to the spillover effect of Pt on oxygen gas, the density of free electrons in NWs decreased to a great extent (Figure 9c) which led to instability in the localized water molecules in the body of the WO3 NW.These localized water molecules decomposed into atomic hydrogen and oxygen species, which subsequently diffused to the NW surface and reacted with foreign oxygen ions to form water vapor, while oxygen species re-filled oxygen vacancies and the sensor regained its initial structure and was bleached [83].Optical heating using a laser can also be used to attenuate the gasochromic properties of a sensor.In general, sensor sensitivity decreases, and its response time increases in harsh environments, such as cold areas.In such conditions, heating a gas sensor may increase its performance.In this context, Okazaki et al. [84] reported the hydrogen (4 vol% H 2 /N 2 gas) gasochromic performance of sol-gel-prepared Pt/WO 3 films at different temperatures using optical transmittance measurements.They used optical heating with a high-power laser diode to improve the response rate of the films at low temperatures.Sensitivity was defined as the percentage of change in the transmitted light in the presence of the target gas.More than 60% change was observed in the transmittance at room temperature with a response time of ~10 s.The sensor exhibited lower sensitivity at lower temperatures, even though an acceptable sensitivity (>40%) was still obtained at −40 • C.During the optical heating of sensors with laser diodes, reaction kinetics are enhanced, resulting in their good sensitivity.
In another study on NWs, Luo et al. [85] studied the gasochromic phenomenon in Pt/WO 3 NWs.When H 2 gas was injected, it dissociated on Pt into hydrogen species and subsequently reacted with absorbed oxygen ions to form H 2 O molecules.The hydrogen species could also react with the oxygen atoms of WO 3 to form localized water molecules.Therefore, coloration could be attributed to the formation of a new structure, namely WO 3−x .xH 2 O, which indicates the coexistence of localized water molecules and oxygen vacancies.Furthermore, coloration was related to photon absorption involving both the defect band in the bandgap and the two tails of the valence and conduction bands of the sensing layer.
Noble metals not only increase gas sensitivity but also significantly reduce the response and recovery times [30] of gas sensors.However, surface poisoning of noble metals can decrease the coloring performance of WO 3 gasochromic sensors.Poisoning may be ascribed to the adsorption of some gases, such sulfur and CO, leading to a reduction in the number of sites available for hydrogen dissociation and therefore a poor gasochromic performance.Luo et al. [86] reported a decrease in the gasochromic coloration of Pt-coated WO 3 NW films due to the poisoning of Pt NPs when held in air for long time periods.W 18 O 49 NW films were prepared by thermal evaporation.The samples were then annealed in air at 600 • C for 5 h to obtain WO 3 NW films.Finally, Pt NPs were sputter-deposited onto the surfaces of WO 3 NWs.When Pt/WO 3 NWs were exposed to air for a long time, the Pt and WO 3 surfaces were covered by oxygen atoms, resulting in a drastic decrease in the number of surface sites available for the adsorption and dissociation of H 2 molecules.Accordingly, the response and response times decreased.The poisoned Pt catalyst led to a degraded response, which resulted in a concomitant increase in the response time.To restore the gasochromic properties of the NWs, the samples were subjected to annealing.After annealing, the samples exhibited an increasingly deep blue color (Figure 10a-c).The change in transmittance (∆T) at λ = 632.8nm increased from about 50% to more than 70% after annealing (Figure 10d).Therefore, maintaining a high vacuum space or oxygen-free atmosphere with Ar and N 2 gases is suggested for prolonging the lifetime of gas sensors.
sensor.In general, sensor sensitivity decreases, and its response time increases in harsh environments, such as cold areas.In such conditions, heating a gas sensor may increase its performance.In this context, Okazaki et al. [84] reported the hydrogen (4 vol% H2/N2 gas) gasochromic performance of sol-gel-prepared Pt/WO3 films at different temperatures using optical transmittance measurements.They used optical heating with a high-power laser diode to improve the response rate of the films at low temperatures.Sensitivity was defined as the percentage of change in the transmitted light in the presence of the target gas.More than 60% change was observed in the transmittance at room temperature with a response time of ~10 s.The sensor exhibited lower sensitivity at lower temperatures, even though an acceptable sensitivity (>40%) was still obtained at −40 °C.During the optical heating of sensors with laser diodes, reaction kinetics are enhanced, resulting in their good sensitivity.
In another study on NWs, Luo et al. [85] studied the gasochromic phenomenon in Pt/WO3 NWs.When H2 gas was injected, it dissociated on Pt into hydrogen species and subsequently reacted with absorbed oxygen ions to form H2O molecules.The hydrogen species could also react with the oxygen atoms of WO3 to form localized water molecules.Therefore, coloration could be attributed to the formation of a new structure, namely WO3−x.xH2O, which indicates the coexistence of localized water molecules and oxygen vacancies.Furthermore, coloration was related to photon absorption involving both the defect band in the bandgap and the two tails of the valence and conduction bands of the sensing layer.
Noble metals not only increase gas sensitivity but also significantly reduce the response and recovery times [30] of gas sensors.However, surface poisoning of noble metals can decrease the coloring performance of WO3 gasochromic sensors.Poisoning may be ascribed to the adsorption of some gases, such sulfur and CO, leading to a reduction in the number of sites available for hydrogen dissociation and therefore a poor gasochromic performance.Luo et al. [86] reported a decrease in the gasochromic coloration of Pt-coated WO3 NW films due to the poisoning of Pt NPs when held in air for long time periods.W18O49 NW films were prepared by thermal evaporation.The samples were then annealed in air at 600 °C for 5 h to obtain WO3 NW films.Finally, Pt NPs were sputter-deposited onto the surfaces of WO3 NWs.When Pt/WO3 NWs were exposed to air for a long time, the Pt and WO3 surfaces were covered by oxygen atoms, resulting in a drastic decrease in the number of surface sites available for the adsorption and dissociation of H2 molecules.Accordingly, the response and response times decreased.The poisoned Pt catalyst led to a degraded response, which resulted in a concomitant increase in the response time.To restore the gasochromic properties of the NWs, the samples were subjected to annealing.After annealing, the samples exhibited an increasingly deep blue color (Figure 10a-c).The change in transmittance (ΔT) at λ = 632.8nm increased from about 50% to more than 70% after annealing (Figure 10d).Therefore, maintaining a high vacuum space or oxygen-free atmosphere with Ar and N2 gases is suggested for prolonging the lifetime of gas sensors.Not only NWs but also nanofibers (NFs) can be used for gasochromic studies.Tavakoli et al. [87] reported the gasochromic properties of PdCl 2 -decorated WO 3 nanofibers fabricated by electrospinning.Figure 11 reveals the changes in the color of NFs with time in the presence of different concentrations of hydrogen gas.The initial pale yellow color of the sensor changed to blue, and hydrogen was detected in ~60-90 s.With longer exposure times, the color intensified to some extent and the blue parts covered most of the surfaces.When hydrogen reaches the PdCl 2 solution, due to interaction between hydrogen and PdCl 2 , Pd ions gradually begin to reduce to Pd.Then, in a spillover process, H 2 dissociated into two H + atoms and two free electrons, which diffused into the WO 3 lattice and occupied the available sites.As a result, tungsten oxide bronze (H x W 6+  1−x W 5+ x O 3 ), which is a mixture of W 6+ and W 5+ , was formed.W 5+ is blue in color, while WO 3 (W 6+ ) is pale yellow in color (Figure 12).with permission from [86]).
Not only NWs but also nanofibers (NFs) can be used for gasochromic studies.Tavakoli et al. [87] reported the gasochromic properties of PdCl2-decorated WO3 nanofibers fabricated by electrospinning.Figure 11 reveals the changes in the color of NFs with time in the presence of different concentrations of hydrogen gas.The initial pale yellow color of the sensor changed to blue, and hydrogen was detected in ~60-90 s.With longer exposure times, the color intensified to some extent and the blue parts covered most of the surfaces.When hydrogen reaches the PdCl2 solution, due to interaction between hydrogen and PdCl2, Pd ions gradually begin to reduce to Pd.Then, in a spillover process, H2 dissociated into two H + atoms and two free electrons, which diffused into the WO3 lattice and occupied the available sites.As a result, tungsten oxide bronze (HxW 6+ 1−xW 5+ xO3), which is a mixture of W 6+ and W 5+ , was formed.W 5+ is blue in color, while WO3 (W 6+ ) is pale yellow in color (Figure 12).Not only NWs but also nanofibers (NFs) can be used for gasochromic studies.Tavakoli et al. [87] reported the gasochromic properties of PdCl2-decorated WO3 nanofibers fabricated by electrospinning.Figure 11 reveals the changes in the color of NFs with time in the presence of different concentrations of hydrogen gas.The initial pale yellow color of the sensor changed to blue, and hydrogen was detected in ~60-90 s.With longer exposure times, the color intensified to some extent and the blue parts covered most of the surfaces.When hydrogen reaches the PdCl2 solution, due to interaction between hydrogen and PdCl2, Pd ions gradually begin to reduce to Pd.Then, in a spillover process, H2 dissociated into two H + atoms and two free electrons, which diffused into the WO3 lattice and occupied the available sites.As a result, tungsten oxide bronze (HxW 6+ 1−xW 5+ xO3), which is a mixture of W 6+ and W 5+ , was formed.W 5+ is blue in color, while WO3 (W 6+ ) is pale yellow in color (Figure 12).Gas sensors with nanorod (NR) morphology have been used for the gasochromic detection of hydrogen gas.Wisitsoorat et al. [88] used Pd/WO 3 NRs prepared by RF magnetron sputtering for the gasochromic sensing of hydrogen gas.The optical absorbance of the gas sensors was continuously monitored over a wavelength range of 650 to 1000 nm.Upon H 2 exposure at 150 • C, the absorbance of the WO 3 NRs increased over the entire visible-NIR region.Changes in absorbance correspond to the gasochromic phenomenon, in which transparent W 6+ cations in WO 3 were reduced to W 5+ by the electrons generated during the catalyzed hydrogen reduction reaction.
Most gasochromic WO 3 sensors show slow coloring/bleaching times, which is a limitation for their practical application.Chan et al. [89] prepared nanostructured WO 3 films by a sol-gel method and spin coating.Pt was then sputtered onto the WO 3 films.The transmission of the produced films at λ = 550 nm was measured when they were exposed to hydrogen and air.The gas sensors exhibited excellent gasochromic performance with good coloration changes and fast response dynamics at room temperature.The response times of the coloring/bleaching processes of the Pt/WO 3 film were very fast and less than 5 s.The rapid dynamics of the gas sensors are related to their structural features and morphology.In a separate study, Hsu et al. [90] fabricated Pt/WO 3 thin films by PLD and sputtering methods.Their gasochromic performance at room temperature was determined from optical transmittance measurements at λ = 550 nm at different H 2 gas concentrations.Upon injecting H 2 gas, the color of the film changed to dark blue very fast.In fact, the film response time to 5% H 2 was less than 1 s, which clearly demonstrates its very fast dynamics.
Chen et al. [91] designed a novel Pd-WO 3 /graphene/Si gasochromic sensor with a tandem structure for detecting hydrogen at room temperature.A laser (λ = 980 nm) was used as the light source to study the transmittance of the sensor in the presence of hydrogen and air.Based on changes in its transmittance, the gas sensor was able to detect hydrogen gas at concentrations as low as 0.05 vol%, with response time and recovery time less than 13 s and 43 s, respectively.Figure 13a shows photographs of the Pd-WO 3 gas sensor on glass substrates in the bleached state and after exposure to 4 vol% H 2 /Ar gas. Figure 13b shows the transmittance spectra of the sensor film in the bleached and colored states.The difference in transmittance between the bleached and colored states increased with an increase in the wavelength.The maximum reduction of 72% (∆T/T 0 = 72%) was observed at a wavelength of ~1000 nm.
monitored over a wavelength range of 650 to 1000 nm.Upon H2 exposure at 150 °C, the absorbance of the WO3 NRs increased over the entire visible-NIR region.Changes in absorbance correspond to the gasochromic phenomenon, in which transparent W 6+ cations in WO3 were reduced to W 5+ by the electrons generated during the catalyzed hydrogen reduction reaction.
Most gasochromic WO3 sensors show slow coloring/bleaching times, which is a limitation for their practical application.Chan et al. [89] prepared nanostructured WO3 films by a sol-gel method and spin coating.Pt was then sputtered onto the WO3 films.The transmission of the produced films at λ = 550 nm was measured when they were exposed to hydrogen and air.The gas sensors exhibited excellent gasochromic performance with good coloration changes and fast response dynamics at room temperature.The response times of the coloring/bleaching processes of the Pt/WO3 film were very fast and less than 5 s.The rapid dynamics of the gas sensors are related to their structural features and morphology.In a separate study, Hsu et al. [90] fabricated Pt/WO3 thin films by PLD and sputtering methods.Their gasochromic performance at room temperature was determined from optical transmittance measurements at λ = 550 nm at different H2 gas concentrations.Upon injecting H2 gas, the color of the film changed to dark blue very fast.In fact, the film response time to 5% H2 was less than 1 s, which clearly demonstrates its very fast dynamics.
Chen et al. [91] designed a novel Pd-WO3/graphene/Si gasochromic sensor with a tandem structure for detecting hydrogen at room temperature.A laser (λ = 980 nm) was used as the light source to study the transmittance of the sensor in the presence of hydrogen and air.Based on changes in its transmittance, the gas sensor was able to detect hydrogen gas at concentrations as low as 0.05 vol%, with response time and recovery time less than 13 s and 43 s, respectively.Figure 13a shows photographs of the Pd-WO3 gas sensor on glass substrates in the bleached state and after exposure to 4 vol% H2/Ar gas. Figure 13b shows the transmittance spectra of the sensor film in the bleached and colored states.The difference in transmittance between the bleached and colored states increased with an increase in the wavelength.The maximum reduction of 72% (ΔT/T0 = 72%) was observed at a wavelength of ~1000 nm.In hydrogen atmosphere, one hydrogen molecule can be dissociated into H atoms (on Pd) and hydrogen atoms react with WO3, producing HxWO3 (x ~ 0-1).Thus, the color of the film changed to dark blue.The relevant reaction can be written as follows [92]: The reaction of the bleaching process is as follows: In hydrogen atmosphere, one hydrogen molecule can be dissociated into H atoms (on Pd) and hydrogen atoms react with WO 3 , producing H x WO 3 (x ~0-1).Thus, the color of the film changed to dark blue.The relevant reaction can be written as follows [92]: The reaction of the bleaching process is as follows: Sensors with reversible behavior, good selectivity to hydrogen gas, and a low working temperature can potentially be used to detect hydrogen gas in practical scenarios.
The effect of water vapor on the gasochromic phenomenon has also been investigated.E-beam evaporated WO 3 films covered by Pt on glass substrates were used for gasochromic studies.In a humid atmosphere, the films contained large amounts of water, and the coloration response rate was strongly dependent on the relative humidity of the atmosphere.This was due to site blocking by H 2 O vapor.Therefore, a decrease in the coloration rate was observed with an increase in the H 2 O vapor pressure [93].
Ranjbar et al. [94] fabricated WO 3 films by PLD, and Pd NPs were coated on WO 3 using an electrodeless method.The effect of Pd growth time (1-40 min) on the response and recovery times of the gas sensors was studied.Optical changes in the presence of H 2 gas were so complex when two different mechanisms, including bleaching due to the formation of PdH x and the coloring of WO 3 itself, occur together.As a result, with the injection of 10% H 2 /Ar gas, the optical density of the samples decreased rapidly and then gradually increased during the process of coloration, which was attributed to the rapid formation of the PdH x complex by hydrogen absorption on Pd NPs.The shortest coloring/bleaching rates were observed with the sample prepared at a growth time of 2 min.At shorter growth times, the number of Pd NPs formed was too little to indicate good catalytic activity.Meanwhile, in the sample prepared over longer growth times, large-sized Pd particles were formed.They exhibited a limited catalytic activity as compared to those of small Pd NPs, resulting in longer reaction dynamics-accordingly, only for a critical growth time of 2 min, as-grown Pd particles had enough sizes and concentrations.Gasochromic studies demonstrated that that the samples deposited in an oxygen environment exhibited good coloring response, which was due to the amorphous structure and near stoichiometric nature of these samples.
Yamamoto et al. [95] investigated the effect of composition and structure of WO 3 sensing films on the gasochromic coloration of sputter deposited WO 3 films with various O/W atomic ratios from 1.5 to 3.0.In a hydrogen atmosphere, stoichiometric WO 3 films with an amorphous structure showed enhanced gasochromic coloration when compared to those of non-stoichiometric WO 3 films with an amorphous structure or with a crystalline structure.Therefore, it was concluded that gasochromic coloration strongly depends on the composition and structure of WO 3 films.In another study on the composition of gasochromic sensors, Pt-coated (WO 3 ) 1−x (Nb 2 O 5 ) x thin films were prepared by PLD [96] and their gasochromic coloration properties were studied using an UV-Vis spectrophotometer at room temperature in H 2 /N 2 gas mixtures containing 1-100 mol % H 2 .Because Nb itself exists in various oxidation states from +5 to +1, it is expected that Nb 2 O 5 improves tungsten oxidation and thus enhances the gasochromic characteristics of WO 3 .The results showed that the (WO 3 ) 0.85 (Nb 2 O 5 ) 0.15 /Pt gas sensor exhibited the shortest response time (30 s)-this gas sensor exhibited the highest transmittance change (∆T) of 30%.Ranjbar et al. [97] prepared (WO 3 ) 1−x (V 2 O 5 ) x (x = 0, 0.09, 0.17, 0.23, 0.29, and 0.33) mixed powders by PLD.It was reported that the ratio of W 5+ /W 6+ increased upon increasing the amount of vanadium in the produced films.The sample with x = 0.09 showed both deeper and faster color changes.The weak responses of samples with x > 0.09 are related to the low amount of W 5+ in the films and also to the low porosity of the films.W 5+ ions are generated due to the interaction between hydrogen ions and electrons with the initial W 6+ ions.Therefore, a primarily high concentration of W 6+ is important for switching on the gasochromic response upon hydrogen exposure [97].
The sensor preparation method also affects the performance of gas sensors.Ranjbar et al. [98] prepared two types of WO 3 films by PLD and sol-gel methods.Subsequently, using PdCl 2 , Pd-decorated WO 3 nanostructures were prepared.Gasochromic investigations showed that the level of coloring depends on the amount of PdCl 2 as well as the synthesis method.Sol-gel-prepared Pd-decorated WO 3 showed weak gasochromic properties.In fact, it exhibited a compact and dense structure, owing to which Pd ions hardly diffuse into the WO 3 layer and remain on the surface.Furthermore, the diffusion of hydrogen atoms is greatly limited at the top of Pd-decorated WO 3 sol-gel samples.In contrast, Pd-decorated WO 3 gas sensors prepared by PLD exhibited a highly porous nature with a columnar open structure, which improved the penetration of the PdCl 2 solution into their pores, leading to the growth of Pd NPs over pore walls.Furthermore, such sensors exhibited a large surface area and provided more adsorption sites for hydrogen gas, resulting in better gasochromic characteristics [98].
In an interesting study, Garavand et al. [99] investigated the gasochromic properties of colloidal WO 3 NPs synthesized by a PLD technique in deionized water using a laser (λ = 1064 nm) at different pulse energies of E 1 = 160, E 2 = 370 and E 3 = 500 mJ/pulse.A PdCl 2 solution was added to colloidal tungsten oxide NPs, after which a laser was used to reduce Pd 2+ to metallic Pd.The reduction of Pd NPs to Pd 2+ did not occur effectively at E 1 , while at E 3 , all Pd 2+ ions were effectively reduced to metallic Pd.As more unreduced Pd 2+ ions are available in the sample prepared at E 1 , it exhibited greater irreversibility as compared to those of samples prepared at higher laser pulse energies.Gasochromic coloration in the sensors was attributed to the formation of W 5+ ions in the presence of hydrogen gas.Optical absorption by W 5+ ions resulted in the colloidal solution changing from a bleached state to a blue-colored state.The optical properties of W 5+ centers are dependent on oxygen-tungsten bonds.The absorption peaks at 977 and 630 nm were attributed to different oxygen-tungsten bonds that led to the formation of different energy levels for W 5+ centers.
Gasochromic sensors with Au decoration are rarely reported for hydrogen gas detection.Ahmad et al. [30] used nanostructured Au/WO 3 thin films for the gasochromic detection of hydrogen gas.At 200 • C, it was observed that adsorption increased by ~2.5% in the presence of 0.06% H 2 .The response times to 0.06% and 1% H 2 were 180 s and 120 s, respectively, and the recovery time was ~5 min at both concentrations.The gasochromic reaction occurred due to the reduction of W 6+ ions (transparent) to W 5+ ions (blue).Repeated gasochromic cycles and long-term storage led to structural collapse and catalyst (Au) poisoning, resulting in an increase in the response and recovery times and also partial gasochromic irreversibility.
Sometimes, sensor surfaces can also affect their gasochromic properties.Li et al. [100] synthesized Pt/WO 3 nanosphere films by a sol-gel method.The color of the nanospheres changed rapidly in 5 to 10 s from light yellow to blue in the presence of hydrogen.Upon removal of hydrogen gas, the films were bleached quickly (10 to 20 s).In the second cycle, the response time was ~40 s.The WO 3 nanosphere films maintained their crystal structure after hydrogen gasochromic testing.The high response of the film was attributed to the high surface area of WO 3 nanospheres, which favors coloration.Further, it was reported that the WO 3 /Pt films were inert to other gases such as NH 3 , alcohols, and acetone at room temperature, which demonstrates their good selectivity.
Composite films with nanoporous structures can exhibit good gasochromic performance.Li et al. [101] prepared with Pd-loaded nanoporous WO 3 /SiO 2 composite films by sol-gel dip coating.Their coloring (R c ) and bleaching (R b ) rates were calculated as linear slopes of transmittance (T) as functions of time (t) as follows [ The subscripts "c" and "b" refer to colored and bleached states, respectively.The coloring rate of WO 3 films was very fast.The bleaching rate of Pd/WO 3 -SiO 2 (44.1%/s) was significantly higher than that of pristine WO 3 films (7.18%/s), demonstrating remarkably faster reaction kinetics in the former sensor.The coloring rate of Pd/WO 3 -SiO 2 films was 14.8%/s, while that of pristine WO 3 was only ~2.84%/s.Furthermore, the Pd/WO 3 -SiO 2 sensor showed good reversibility.The gasochromic characteristics of these sensors are related to the diffusion of H atoms, which are strongly dependent on the microstructure of WO 3 films and presence of the Pd catalyst.Upon adding SiO 2 to a WO 3 matrix, a nanoporous structure with a high surface to volume ratio was obtained, which greatly enhanced the diffusion of hydrogen atoms.In fact, in the nanoporous structure, WO 3 particles were loaded with Pd catalyst around the SiO 2 network, where SiO 2 acted as a supporting framework to form a nanoporous and stable composite structure.
In another study on porous structures, WO 3 thin films were coated with Pt by reactive sputtering, and Pt NPs were coated on WO 3 by spin coating [102].Pt/WO 3 layers with a porous nature showed high transmittance.Upon exposure to hydrogen, a deep blue coloration was observed.A similar study was conducted by Chan et al. [67].The gasochromic coloration mechanism of these sensors is related to the reduction of W 6+ (transparent) centers in the WO 3 crystal lattice to W 5+ (blue color) and the formation of blue tungsten bronze (H x WO 3 ) in the presence of hydrogen gas.The relevant reactions for the reduction of W 6+ to W 5+ can be illustrated as follows:

Conclusions and Outlook
In this review, we discussed nanostructured gasochromic WO 3 for the detection of hydrogen gas.The effects of substrate, crystallinity, preparation method, and morphology on the sensing properties of these sensors were discussed and explained.Gasochromic WO 3 gas sensors need noble metals such as Pd, Pt, and Au for the catalytic dissociation of hydrogen molecules and rapid fast detection of hydrogen gas.Most gasochromic sensors work at low or room temperatures and, therefore, significantly decrease the risk of explosion of hydrogen gas.Furthermore, they can be fabricated on flexible substrates and, hence, can be used for detecting the leakage of hydrogen gas from pipelines.However, gasochromic sensors generally exhibit a poor performance at low H 2 concentrations, under which conditions the color change is very slow or cannot be detected by a naked eye.Therefore, for very low concentrations of hydrogen gas, resistivity-based gas sensors may be a better choice.For practical applications of gasochromic sensors, color changes in gasochromic WO 3 gas sensors should be visible to the naked eye.To achieve this objective, high-performance gasochromic sensors with a high surface area, engineered design, and appropriate amount of noble metals should be synthesized using novel protocols.Such gas sensors not only exhibit color changes in the visible light range, but also show fast dynamics, which are vital for the detection of hydrogen, an explosive gas.

22 Figure 4 .
Figure 4. (a) Transmittance spectra in the presence of various gases; (b) Digital images of flexible Pd-WO3 sensors before and after exposure to 1% H2; (c) A gasochromic flexible Pd-WO3 sensor on a gas pipeline carrying pure H2. (Reprinted with permission from [75]).

Figure 4 .
Figure 4. (a) Transmittance spectra in the presence of various gases; (b) Digital images of flexible Pd-WO 3 sensors before and after exposure to 1% H 2 ; (c) A gasochromic flexible Pd-WO 3 sensor on a gas pipeline carrying pure H 2 .(Reprinted with permission from [75]).

22 Figure 4 .
Figure 4. (a) Transmittance spectra in the presence of various gases; (b) Digital images of flexible Pd-WO3 sensors before and after exposure to 1% H2; (c) A gasochromic flexible Pd-WO3 sensor on a gas pipeline carrying pure H2. (Reprinted with permission from [75]).

Figure 13 .
Figure 13.(a) Photographs of the Pd-WO3/graphene/Si sensor before and after exposure to 4 vol% H2/Ar gas; (b) Transmittance spectra of the Pd-WO3/graphene/Si sensor in the bleached and colored states.(Reprinted with permission from [91]).

Figure 13 .
Figure 13.(a) Photographs of the Pd-WO 3 /graphene/Si sensor before and after exposure to 4 vol% H 2 /Ar gas; (b) Transmittance spectra of the Pd-WO 3 /graphene/Si sensor in the bleached and colored states.(Reprinted with permission from [91]).