## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors
At shorter growth times, the number of Pd NPs formed was too little to indicate good catalytic activity. Meanwhile, in the sample prepared over longer growth times, large-sized Pd particles were formed. They exhibited a limited catalytic activity as compared to those of small Pd NPs, resulting in longer reaction dynamics--accordingly, only for a critical growth time of 2 min, as-grown Pd particles had enough sizes and concentrations. Gasochromic studies demonstrated that that the samples deposited in an oxygen environment exhibited good coloring response, which was due to the amorphous structure and near stoichiometric nature of these samples. [PERSON] et al. [95] investigated the effect of composition and structure of WO\({}_{3}\) sensing films on the gasochromic coloration of sputter deposited WO\({}_{3}\) films with various O/W atomic ratios from 1.5 to 3.0. In a hydrogen atmosphere, stoichiometric WO\({}_{3}\) films with an amorphous structure showed enhanced gasochromic coloration when compared to those of non-stoichiometric WO\({}_{3}\) films with an amorphous structure or with a crystalline structure. Therefore, it was concluded that gasochromic coloration strongly depends on the composition and structure of WO\({}_{3}\) films. In another study on the composition of gasochromic sensors, Pt-coated (WO\({}_{3}\))\({}_{1-x}\) (Nb\({}_{2}\)O\({}_{5}\))\({}_{x}\) thin films were prepared by PLD [96] and their gasochromic coloration properties were studied using an UV-Vis spectrophotometer at room temperature in H\({}_{2}\)/N\({}_{2}\) gas mixtures containing 1-100 mol % H\({}_{2}\). Because Nb itself exists in various oxidation states from +5 to +1, it is expected that Nb\({}_{2}\)O\({}_{5}\) improves tungsten oxidation and thus enhances the gasochromic characteristics of WO\({}_{3}\). The results showed that the (WO\({}_{3}\))\({}_{0.85}\)(Nb\({}_{2}\)O\({}_{5}\))\({}_{0.15}\)/Pt gas sensor exhibited the shortest response time (30 s)--this gas sensor exhibited the highest transmittance change (\(\Delta\)T) of 30%. [PERSON] et al. [97] prepared (WO\({}_{3}\))\({}_{1-x}\) (V\({}_{2}\)O\({}_{5}\))\({}_{x}\) (\(x\) = 0, 0.09, 0.17, 0.23, 0.29, and 0.33) mixed powders by PLD. It was reported that the ratio of W\({}^{5+}\)/W\({}^{6+}\) increased upon increasing the amount of vanadium in the produced films. The sample with \(x\) = 0.09 showed both deeper and faster color changes. The weak responses of samples with \(x\) > 0.09 are related to the low amount of W\({}^{5+}\) in the films and also to the low porosity of the films. W\({}^{5+}\) ions are generated due to the interaction between hydrogen ions and electrons with the initial W\({}^{6+}\) ions. Therefore, a primarily high concentration of W\({}^{6+}\) is important for switching on the gasochromic response upon hydrogen exposure [97]. The sensor preparation method also affects the performance of gas sensors. [PERSON] et al. [98] prepared two types of WO\({}_{3}\) films by PLD and sol-gel methods. Subsequently, using PdCl\({}_{2}\), Pd-decorated WO\({}_{3}\) nanostructures were prepared. Gasochromic investigations showed that the level of coloring depends on the amount of PdCl\({}_{2}\) as well as the synthesis method. Sol-gel-prepared Pd-decorated WO\({}_{3}\) showed weak gasochromic properties. In fact, it exhibited a compact and dense structure, owing to which Pd ions hardly diffuse into the WO\({}_{3}\) layer and remain on the surface. Furthermore, the diffusion of hydrogen atoms is greatly limited at the top of Pd-decorated WO\({}_{3}\) sol-gel samples. In contrast, Pd-decorated WO\({}_{3}\) gas sensors prepared by PLD exhibited a highly porous nature with a columnar open structure, which improved the penetration of the PdCl\({}_{2}\) solution into their pores, leading to the growth of Pd NPs over pore walls. Furthermore, such sensors exhibited a large surface area and provided more adsorption sites for hydrogen gas, resulting in better gasochromic characteristics [98]. In an interesting study, [PERSON] et al.