## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors
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. Figure 7 shows photographs of 2D WO\({}_{3}\)-Pd sensors deposited on filter papers and exposed to different concentrations of H\({}_{2}\) gas at room temperature. A complete color change occurred at different H\({}_{2}\) gas exposure times and concentrations. The 2D WO\({}_{3}\)/Pd sensors showed fast coloration and bleaching times at all hydrogen concentrations. At 100% H\({}_{2}\) 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  
Figure 6: Images of Pd/WO\({}_{3}\) sensors during gasochromic coloration (TW is incident energy). (Reprinted with permission from [79]). WO\({}_{3}\) 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. 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]. [PERSON] 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]:  
\[\mathrm{H_{2}}\overset{\mathrm{Pt}}{\rightarrow}2 \tag{4}\]  
\[2\mathrm{H}\to 2\mathrm{H^{+}}+2\mathrm{e^{-}} \tag{5}\]  
\[\mathrm{xH^{+}}+\mathrm{xe^{-}}+\mathrm{WO_{3}}\to\mathrm{H_{x}WO_{3}} \tag{6}\]  
[PERSON] 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 (\(\sim\)30 min) at all concentrations of hydrogen gas. Generally, gas sensors with nanowire (NW) morphology exhibit good self-heating effects [82]. In this context, [PERSON] 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 \(\lambda\) = 632.8 nm 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 \({}^{\circ}\)C during self-heating cycles.