## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Mixed Metal Oxides and Other Systems  
Mixed metal oxides (MMOs) represent an advanced class of materials in gas sensing, combining two or more metal oxides to leverage synergistic effects that enhance their sensitivity, stability, and selectivity. By forming heterojunctions between the different metal oxides, MMOs can achieve tailored electronic and surface properties, resulting in superior gas sensing performance compared to single metal oxides. For instance, ZnO-CuO, SnO\({}_{2}\)-TiO\({}_{2}\), and Fe\({}_{2}\)O\({}_{3}\)-SnO\({}_{2}\) composites are popular MMO combinations, each demonstrating a marked improvement in response to specific target gases due to optimized band alignment, electron transfer efficiency, and selective adsorption capabilities [106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118]. One key advantage of MMOs is their ability to support heterojunctions, which form at the interface of two metal oxides with different energy band structures. These heterojunctions facilitate electron transfer by creating built-in electric fields that enhance the sensor's response to target gases. For example, in ZnO-CuO heterojunctions, the p-n junction created between p-type CuO and n-type ZnO leads to a modulation in resistance upon gas adsorption. When exposed to reducing gases like hydrogen (H\({}_{2}\)), electrons from the ZnO layer flow into CuO, increasing resistance and enabling the detection of low-concentration gases. It is observed that H\({}_{2}\) sensitivity of the heterojunction is increased with increases in  
Figure 11: SEM images of (**a**) as-grown \(\alpha\)-MoO\({}_{3}\) nanoribbon network on SiO\({}_{2}\)/Si interdigitated substrate and (**b**) sulfurized \(\alpha\)-MoO\({}_{3}\) nanoribbons. Insets in (**a**,**b**) show corresponding optical micrographs. High-resolution SEM images of (**c**) as-grown \(\alpha\)-MoO\({}_{3}\) and (**d**) sulfurized \(\alpha\)-MoO\({}_{3}\). The inset in (**d**) is a magnified view of an individual sulfurized MoO\({}_{3}\) nanoribbon (reproduced from [102], MDPI, 2022). temperature, as well as the thickness of CuO film. A sensitivity value as high as 266.5 is observed at 300 \({}^{\circ}\)C when biased at 3 V in the presence of approximately 3000 ppm of H\({}_{2}\). Similarly, SnO\({}_{2}\)-TiO\({}_{2}\) heterojunctions improve the sensor's response to oxidizing gases such as nitrogen dioxide (NO\({}_{2}\)) by enhancing charge carrier mobility and creating additional adsorption sites [106]. Response (recovery) times to 400 ppb NO\({}_{2}\) were determined as a function of the operating temperature and indicated a significant decrease from 62 (42) s at 123 \({}^{\circ}\)C to 12 (19) s at 385 \({}^{\circ}\)C. A much smaller sensitivity to H\({}_{2}\) was observed, which might be advantageous for the selective detection of nitrogen oxides. The influence of humidity on NO\({}_{2}\) response was demonstrated to be significant below 150 \({}^{\circ}\)C, and systematically decreased upon increasing the operating temperature up to 400 \({}^{\circ}\)C [107]. Doping in MMO systems adds further versatility by fine-tuning properties such as carrier density, selectivity, and response time. Dopants can modify the electronic structure, induce oxygen vacancies, and increase active sites for gas adsorption, significantly improving sensitivity and selectivity. For instance, doping ZnO-CuO composites with palladium (Pd) increases sensitivity to gases like methane (CH\({}_{4}\)) and carbon monoxide (CO) due to the enhanced catalytic properties Pd provides. Sensitivity improved by 17.1 times and 327.8 times compared with the pristine CuO- and ZnO-based gas sensors, respectively. Moreover, the detection limit to H\({}_{2}\)S of such sensors could be reduced as low as 300 ppb [105].