## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors  
Generally, substrate temperature affects the crystallinity and morphology of as-grown films, which eventually leads to differences in their gasochromic coloration. [PERSON] et al. [71] studied the gasochromic performance of epitaxial WO\({}_{3}\) sensors deposited by pulsed laser deposition (PLD) at different substrate temperatures. Changes in the optical density (OD) of the WO\({}_{3}\) sensor at \(\lambda\) = 645 nm between the initial and colored states when exposed to 1% H\({}_{2}\)/Ar for 20 min, i.e., optical density (\(\Delta\)OD) = \(-\)log(T\({}_{\mathrm{colored}}\)/T\({}_{\mathrm{initial}}\)), were obtained. The \(\Delta\)OD of the film deposited at 432 \({}^{\circ}\)C rapidly changed when exposed to hydrogen gas. With an increase in the substrate temperature, the coloration reduced, and in the film deposited at 538 \({}^{\circ}\)C (with better crystallinity), almost no coloration was observed. This was in contrast to previous reports on the enhancement in gasochromic coloration due to structural changes, where highly crystalline WO\({}_{3}\) nanowires (NW) films with a monoclinic structure transformed into the tetragonal phase upon hydrogen gasochromic coloration. Thus, the researchers highlighted that controlling the free space between WO\({}_{3}\) grains for the relaxation of mechanical stresses during structural transformation is essential when a sensor is exposed to hydrogen gas. [PERSON] et al. [72] prepared WO\({}_{3}\)/glass thin film gas sensors by PLD at various substrate temperatures (25-400 \({}^{\circ}\)C) for the gasochromic investigation of hydrogen. Figure 2a shows images of different colors the gas sensors exhibited at various substrate temperatures. Coloring was initiated from the boundary edge of the PdCl\({}_{2}\) drop as a tiny blue ring and gradually extended to the center, suggesting that hydrogen initially entered at the triple points of the substrate, liquid, and gas. In hydrogen atmosphere, Pd nanoparticles (NPs) were produced over the surface as well as at the triple points. The formation of Pd was accompanied by the gasochromic coloring of tungsten oxide. Hydrogen, with its reducing power, helps the accumulation of a layer of Pd NPs, as shown in Figure 2b. In the samples deposited at temperatures below 200 \({}^{\circ}\)C, a uniform coloring was observed. At 200 \({}^{\circ}\)C, a very deep blue color was obtained due to the high surface porosity of the sensor film. However, the uniformity of coloring decreased significantly with an increase in the deposition temperature. Pd, Pt, and Au catalysts are known to be highly reactive to activation by hydrogen, and they are often used along with WO\({}_{3}\) to increase gasochromic efficiency by the dissociation of hydrogen atoms. [PERSON] et al. [73] prepared Pt/WO\({}_{3}\) and Pd/WO\({}_{3}\) films by PLD and studied the effect of operating temperature (30-200 \({}^{\circ}\)C) on their gasochromic properties. coloration was evaluated by measuring the optical density (\(\Delta\)OD) at \(\lambda\) = 632.8 nm as follows,  
\[\Delta\mathrm{OD}=-\ln\left(\frac{\mathrm{I(t)}}{\mathrm{I(t_{0})}}\right) \tag{3}\]where I(t) and I(t\({}_{0}\)) represent the optical transmission intensities of the colored and bleached states, respectively. Pristine WO\({}_{3}\) did not show any noticeable response to hydrogen gas, while, due to the spillover effect of Pt, Pt/WO\({}_{3}\) films showed a good response to hydrogen gas. The response of the Pt/WO\({}_{3}\) sensor was almost constant in the range of 85-200 \({}^{\circ}\)C. At lower temperatures, its response was negligible, and its dynamics were slow due to the slow dissociation of hydrogen gas and low diffusivity of the hydrogen atoms. At 200 \({}^{\circ}\)C, the effect of humidity on the gas response was negligible. Further, the risk of catalyst poisoning with contaminants was lower at this temperature when compared to those at lesser temperatures. Therefore, this temperature was reported to be the optimal sensing temperature.