## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors
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, [PERSON] 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 \(\lambda\) = 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. [PERSON] 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 (\(\lambda\) = 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% (\(\Delta\)T/T\({}_{0}\) = 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 WO\({}_{3}\), producing H\({}_{x}\)WO\({}_{3}\) (x \(\sim\) 0-1). Thus, the color of the film changed to dark blue. The relevant reaction can be written as follows [92]:  
Figure 2:5  
The reaction of the bleaching process is as follows:  
Figure 2:6  
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  
Figure 2:7 (**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]). vapor. Therefore, a decrease in the coloration rate was observed with an increase in the H\({}_{2}\)O vapor pressure [93]. [PERSON] 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\({}_{\mathrm{x}}\) 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\({}_{\mathrm{x}}\) 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.