## 3 Chromogenic: Definition, Materials and Basics  
Chromogenics is a Greek word with the stem \"chromo\" for color. It refers to the study of materials whose optical properties (color) change as a function of external ambient conditions [31]. Chromogenic materials generally have wide bandgaps and are transparent in the visible range,  
Figure 1: The perovskite lattice of WO\({}_{3}\) (Reprinted with permission from [28]).  
but they reversibly change from being transparent to a dark color in the presence of an electric field (electrochromic coloration), light (photochromic coloration), or when they are exposed to a gas (gasochromic coloration) [34]. Therefore, gasochromism refers to reversible changes in optical properties or color when a material is exposed to a gas [3; 35]. Gasochromic materials exhibit a promising potential for use as gas sensors. WO\({}_{3}\), which is light yellow in color, is one of the most important chromogenic materials known thus far. It exhibits a deep blue color upon exposure to hydrogen gas [36]. In addition to WO\({}_{3}\), other materials reported for gasochromic applications include V\({}_{2}\)O\({}_{5}\)[37; 38; 39; 40], VO\({}_{\mathrm{x}}\)[41], MO\({}_{\mathrm{x}}\)[42], MoO\({}_{3}\)[43], (MoO\({}_{3}\))\({}_{1-\mathrm{x}}\) (V\({}_{2}\)O\({}_{5}\))\({}_{\mathrm{x}}\)[44], mixed silver/nickel ammonium phosphomolybate [45], (Ti-V-Ta)O\({}_{\mathrm{x}}\)[35], Ni(OH)\({}_{2}\)[46], peroxoplytungstic acid [47; 48], and metals like Y [49]. This effect has also been exploited for the detection of other gases such as volatile organic compounds [50], NO\({}_{2}\)[51], H\({}_{2}\)S, SO\({}_{2}\)[52], NH\({}_{3}\)[53], XeF\({}_{2}\)[54], cyclohexane [55], CO, and Cl\({}_{2}\)[46]. Among the different gasochromic materials available, the most important ones are WO\({}_{3}\) and MoO\({}_{\mathrm{x}}\). However, due to its weak color change properties and the existence of several phases whose formation depends on the growth method, molybdenum oxide has received less attention for gasochromic studies [56].  
The ability of WO\({}_{3}\) to undergo reversible changes in its optical properties when exposed to an electric field was first reported by [PERSON] in 1973 [57]. Nineteen years later, [PERSON] [58] reported the potential of WO\({}_{3}\) for gasochromic studies. Thus far, the optical properties of WO\({}_{3}\) nanostructures have been modulated by applying an electric field (electrochromism), UV irradiation (photochromism), or a gas (gasochromism) [59]. Gasochromic coloration of WO\({}_{3}\) is mostly associated with hydrogen gas [35]. In contrast to the electrochromic response, the presence of catalytic noble metals on the surfaces of WO\({}_{3}\) nanostructures is necessary to induce an acceptable gasochromic effect. The most common catalysts used are Pd [60; 61], Au [30], and Pt [62; 63]. They promote chemical reactions by reducing the activation energy between WO\({}_{3}\) and hydrogen gas. Color changes occur in gasochromic WO\({}_{3}\) sensors when H\({}^{+}\) ions intercalate with the WO\({}_{3}\) layer after the dissociation of gas molecules (H\({}_{2}\)) into atoms by the action of noble metals. The optical properties of WO\({}_{3}\) films can be reversibly changed with the insertion and extraction of H\({}^{+}\) ions and electrons into the WO\({}_{3}\) films, which is accompanied by redox changes leading to the formation of W\({}^{5+}\) ions [64; 65]. Gasochromic measurements are often carried out by monitoring optical properties, such as absorbance/transmittance/reflectance in convenient wavelength ranges (visible-NIR) [66]. Such measurements offer simple, low-cost, and highly selective analytical methods for detecting specific gases [30]. In addition, the stability of the gas sensor can be enhanced as measurements are most often conducted at low or room temperatures.