## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Copper Oxide (CuO)  
Copper oxide (CuO) has gained significant attention as a gas sensing material due to its relatively narrow bandgap of approximately 1.2 eV, which allows for the sensitive detection of reducing gases, such as hydrogen (H\({}_{2}\)), carbon monoxide (CO), and methane (CH\({}_{4}\)). CuO's p-type semiconducting behavior plays a critical role in its gas sensing properties, as exposure to reducing gases reduces the number of holes (the primary charge carriers)  
Figure 6: (**A**) FE-SEM images of the gas sensors are shown. (**a,b**) are the results of the hydrothermal synthesis. (**c,d**) are TiO\({}_{2}\) NRs and Pt NPs of Sensor A. (**e,f**) are TiO\({}_{2}\) NRs and Pt NPs of Sensor B. (**g,h**) are TiO\({}_{2}\) NRs and Pt NPs of Sensor C. The inset in (**f**) shows an enlarged image to clearly display the Pt NPs on the TiO\({}_{2}\) NRs. The red arrows indicate Pt NPs. The green arrows in (**c**) indicate chunks of Pt agglomerated together on the top surface of TiO\({}_{2}\) NRs. (**B**) Proposed sensing mechanisms are depicted. Photoelectrochemical activities for sensing NO\({}_{2}\) with respect to time of annealing TiO\({}_{2}\) are shown. (**a**) is the schematic of Sensor A, which did not have an annealing process to TiO\({}_{2}\). (**b,c**) suggest the working mechanisms of Sensor B and Sensor C, of which TiO\({}_{2}\) NRs were annealed for 1 h and 2 h, respectively. e\({}^{-}\) and h\({}^{+}\) indicate electron and hole, respectively (reproduced from [57], MDPI, 2021). in the material, resulting in an increase in resistance that can be measured. This p-type behavior, combined with its narrow bandgap, enables CuO to operate effectively at lower temperatures than many n-type oxide sensors, making it especially valuable in applications where low-power operation and sensitivity to hazardous gases are required [64]. The nanostructuring of CuO into forms like nanoparticles, nanowires, and thin films has led to substantial improvements in sensitivity and response times. CuO nanoparticles, due to their high surface-to-volume ratio, provide a large number of active sites for gas adsorption, facilitating quicker and more effective interactions with target gases. Nanowires offer a unique advantage in charge transport, as their one-dimensional structure supports efficient electron transfer, leading to improved sensitivity and reduced response and recovery times. Thin films, which can be precisely engineered to control thickness and crystallographic orientation, exhibit stable response characteristics and are ideal for applications requiring long-term stability and repeatability [65; 66]. Doping CuO with elements such as zinc (Zn) or tin (Sn) has further enhanced its selectivity and sensitivity to specific hazardous gases. Zn doping, for example, has been shown to improve CuO's sensitivity to carbon monoxide by increasing the density of active sites and modifying the electronic structure to favor CO adsorption. Tin-doped CuO has demonstrated improved selectivity and response to hydrogen and ammonia, which are critical in industrial monitoring and safety applications. These dopants function by altering the carrier density and oxygen vacancy concentration in the CuO lattice, which enhances the charge transfer process and creates preferential adsorption sites for target gases [64; 67]. Additionally, the presence of dopants can adjust the response temperature range of CuO, allowing for greater flexibility in sensor operation under varying environmental conditions. CuO's interaction with reducing gases involves efficient electron transfer processes, where the gas molecules donate electrons to the CuO surface, leading to a reduction in hole concentration and causing a measurable resistance increase. For instance, when exposed to hydrogen, CuO undergoes a reduction reaction where hydrogen molecules dissociate on the CuO surface, resulting in electron transfer that alters the material's conductivity.