## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors
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, [PERSON] 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  
Figure 3: In situ gasochromic sensing by UV-Vis transmittance. (Reprinted with permission from [75]). 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, [PERSON] 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 (\(\lambda\) = 670 nm) with a silicon photodiode (Figure 5). 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]). Figure 5: H\({}_{2}\) gas switching system for Pt-WO\({}_{3}\) films. (Reprinted with permission from [77]). The optical transmittance of Pt-WO\({}_{3}\) sensors at \(\lambda\) = 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 (\(\lambda\) = 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. 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].