## 3 Chromogenic: Definition, Materials and Basics
### Gasochromic WO\({}_{3}\) Nanostructures as Hydrogen Gas Sensors
[99] investigated the gasochromic properties of colloidal WO\({}_{3}\) NPs synthesized by a PLD technique in deionized water using a laser (\(\lambda\) = 1064 nm) at different pulse energies of E\({}_{1}\) = 160, E\({}_{2}\) = 370 and E\({}_{3}\) = 500 mJ/pulse. A PdCl\({}_{2}\) solution was added to colloidal tungsten oxide NPs, after which a laser was used to reduce Pd2+ to metallic Pd. The reduction of Pd NPs to Pd2+ did not occur effectively at E1, while at E3, all Pd2+ ions were effectively reduced to metallic Pd. As more unreduced Pd2+ ions are available in the sample prepared at E1, it exhibited greater irreversibility as compared to those of samples prepared at higher laser pulse energies. Gasochromic coloration in the sensors was attributed to the formation of W5+ ions in the presence of hydrogen gas. Optical absorption by W5+ ions resulted in the colloidal solution changing from a bleached state to a blue-colored state. The optical properties of W5+ centers are dependent on oxygen-tungsten bonds. The absorption peaks at 977 and 630 nm were attributed to different oxygen-tungsten bonds that led to the formation of different energy levels for W5+ centers. Gasochromic sensors with Au decoration are rarely reported for hydrogen gas detection. [PERSON] et al. [30] used nanostructured Au/WO3 thin films for the gasochromic detection of hydrogen gas. At 200 \({}^{\circ}\)C, it was observed that adsorption increased by \(\sim\)2.5% in the presence of 0.06% H2. The response times to 0.06% and 1% H2 were 180 s and 120 s, respectively, and the recovery time was \(\sim\)5 min at both concentrations. The gasochromic reaction occurred due to the reduction of W6+ ions (transparent) to W5+ ions (blue). Repeated gasochromic cycles and long-term storage led to structural collapse and catalyst (Au) poisoning, resulting in an increase in the response and recovery times and also partial gasochromic irreversibility. Sometimes, sensor surfaces can also affect their gasochromic properties. [PERSON] et al. [100] synthesized Pt/WO3 nanosphere films by a sol-gel method. The color of the nanospheres changed rapidly in 5 to 10 s from light yellow to blue in the presence of hydrogen. Upon removal of hydrogen gas, the films were bleached quickly (10 to 20 s). In the second cycle, the response time was \(\sim\)40 s. The WO3 nanosphere films maintained their crystal structure after hydrogen gasochromic testing. The high response of the film was attributed to the high surface area of WO3 nanospheres, which favors coloration. Further, it was reported that the WO3/Pt films were inert to other gases such as NH3, alcohols, and acetone at room temperature, which demonstrates their good selectivity. Composite films with nanoporous structures can exhibit good gasochromic performance. [PERSON] et al. [101] prepared with Pd-loaded nanoporous WO3/SiO2 composite films by sol-gel dip coating. Their coloring (\(R_{c}\)) and bleaching (\(R_{b}\)) rates were calculated as linear slopes of transmittance (\(T\)) as functions of time (\(t\)) as follows [101]:  
Figure 2:8  
Figure 2:9  
The subscripts \"c\" and \"b\" refer to colored and bleached states, respectively. The coloring rate of WO3 films was very fast. The bleaching rate of Pd/WO3-SiO2 (44.1%/s) was significantly higher than that of pristine WO3 films (7.18%/s), demonstrating remarkably faster reaction kinetics in the former sensor. The coloring rate of Pd/WO3-SiO2 films was 14.8%/s, while that of pristine WO3 was only \(\sim\)2.84%/s. Furthermore, the Pd/WO3-SiO2 sensor showed good reversibility. The gasochromic characteristics of these sensors are related to the diffusion of H atoms, which are strongly dependent on the microstructure of WO3 films and presence of the Pd catalyst. Upon adding SiO2 to a WO3 matrix, a nanoporous structure with a high surface to volume ratio was obtained, which greatly enhanced the diffusion of hydrogen atoms.