## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors
W\({}_{18}\)O\({}_{49}\) NW films were prepared by thermal evaporation. The samples were then annealed in air at 600 \({}^{\circ}\)C for 5 h to obtain WO\({}_{3}\) NW films. Finally, Pt NPs were sputter-deposited onto the surfaces of WO\({}_{3}\) NWs. When Pt/WO\({}_{3}\) NWs were exposed to air for a long time, the Pt and WO\({}_{3}\) surfaces were covered by oxygen atoms, resulting in a drastic decrease in the number of surface sites available for the adsorption and dissociation of H\({}_{2}\) molecules. Accordingly, the response and response times decreased. The poisoned Pt catalyst led to a degraded response, which resulted in a concomitant increase in the response time. To restore the gasochromic properties of the NWs, the samples were subjected to annealing. After annealing, the samples exhibited an increasingly deep blue color (Figure 10a-c). The change in transmittance (\(\Delta\)T) at \(\lambda\) = 632.8 nm increased from about 50% to more than 70% after annealing (Figure 10d). Therefore, maintaining a high vacuum space or oxygen-free atmosphere with Ar and N\({}_{2}\) gases is suggested for prolonging the lifetime of gas sensors. Not only NWs but also nanofibers (NFs) can be used for gasochromic studies. [PERSON] et al. [PERSON] et al. [87] reported the gasochromic properties of PdCl\({}_{2}\)-decorated WO\({}_{3}\) nanofibers fabricated by electrospinning. Figure 11 reveals the changes in the color of NFs with time in the presence of different concentrations of hydrogen gas. The initial pale yellow color of the sensor changed to blue, and hydrogen was detected  
Figure 2:2 (**a**) The bleached state; (**b**) colored state before poisoning, and (**c**) colored state after annealing of Pt/WO\({}_{3}\) NW sensors; (**d**) Light transmittance spectra of Pt/WO\({}_{3}\) NW sensors. (Reprinted with permission from [86]). in \(\sim\)60-90 s. With longer exposure times, the color intensified to some extent and the blue parts covered most of the surfaces. When hydrogen reaches the PdCl\({}_{2}\) solution, due to interaction between hydrogen and PdCl\({}_{2}\), Pd ions gradually begin to reduce to Pd. Then, in a spillover process, H\({}_{2}\) dissociated into two H\({}^{+}\) atoms and two free electrons, which diffused into the WO\({}_{3}\) lattice and occupied the available sites. As a result, tungsten oxide bronze (H\({}_{x}\)W\({}^{6+}\)1-\({}_{x}\)W\({}^{5+}\)\(\lambda\)O\({}_{3}\)), which is a mixture of W\({}^{6+}\) and W\({}^{5+}\), was formed. W\({}^{5+}\) is blue in color, while WO\({}_{3}\) (W\({}^{6+}\)) is pale yellow in color (Figure 12). Gas sensors with nanorod (NR) morphology have been used for the gasochromic detection of hydrogen gas. [PERSON] et al. [88] used Pd/WO\({}_{3}\) NRs prepared by RF magnetron sputtering for the gasochromic sensing of hydrogen gas. The optical absorbance of the gas sensors was continuously monitored over a wavelength range of 650 to 1000 nm. Upon H\({}_{2}\) exposure at 150 \({}^{\circ}\)C, the absorbance of the WO\({}_{3}\) NRs increased over the entire visible-NIR region. Changes in absorbance correspond to the gasochromic phenomenon, in which transparent W\({}^{6+}\) cations in WO\({}_{3}\) were reduced to W\({}^{5+}\) by the electrons generated during the catalyzed hydrogen reduction reaction. Most gasochromic WO\({}_{3}\) sensors show slow coloring/bleaching times, which is a limitation for their practical application. [PERSON] et al. [89] prepared nanostructured WO\({}_{3}\) films by a sol-gel method  
Figure 2:3 Color changes in WO\({}_{3}\) nanofibers (NFs) when exposed to 1%, 2%, 3%, and 4% hydrogen. (Reprinted with permission from [87]). Figure 2:4 Illustration of the gasochromic mechanism of PdCl\({}_{2}\)-decorated WO\({}_{3}\) nanofibers. (Reprinted with permission from [87]). and spin coating. Pt was then sputtered onto the WO\({}_{3}\) films. The transmission of the produced films at \(\lambda\) = 550 nm was measured when they were exposed to hydrogen and air.