## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors
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]. [PERSON] 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\({}^{5+}\)  
Figure 2: (**a**) Photographs of coloration of PdCl\({}_{2}\)(aq)/WO\({}_{3}\)/glass sensors deposited at different temperatures and exposed to hydrogen gas; (**b**) An illustration of the gasochromic coloration effect. (Reprinted with permission from [72]). 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, [PERSON] 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. 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.