## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors
In a hydrogen atmosphere, the absorbed H\({}_{2}\) molecules on the surface of Pt NPs dissociated into H ions and  
Figure 7: Photographs of 2D WO\({}_{3}\)/Pd on filter papers during gasochromic hydrogen gas tests. (Reprinted with permission from [78]). 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]. 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, [PERSON] 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  
Figure 2:0 Photographs of WO\({}_{3}\)-Pd nanocomposites during gasochromic hydrogen gas tests. (Reprinted with permission from [81]). Figure 2:1 The surface of a Pt/WO\({}_{3}\) nanowire (NW) in (**a**) H\({}_{2}\) gas; (**b**) O\({}_{2}\) gas, and (**c**) during the bleaching process. (Reprinted with permission from [83]). 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 \(-40\)\({}^{\circ}\)C. During the optical heating of sensors with laser diodes, reaction kinetics are enhanced, resulting in their good sensitivity. In another study on NWs, [PERSON] 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. [PERSON] 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.