## 3 Segmentation of Nanostructured Materials for Gas Sensing
### Mixed Metal Oxides and Other Systems
In summary, MMOs are valuable for their adaptability, improved sensitivity, and selectivity across a range of gases, making them highly suitable for complex environmental applications. Advances in nanostructuring, doping, and photoactivation continue to expand their potential, promising enhanced performance for the next generation of gas sensors (Table 1). \begin{table}
\begin{tabular}{c c c c c} \hline
**Metal** & **Sensitive** & **Sensitivity** & **Limit of Detection** & **Operating** & **Light Use** \\
**Oxides** & **Gases** & & **(LOD)** & **Temperature Range** & **During Test** \\ \hline ZnO & CO, H\({}_{2}\), & High sensitivity to NO\({}_{2}\) and NH\({}_{3}\), enhanced by nanostructures (nanowires, nanoparticles) and doping with Al, Ga, and In. & -7 ppb for NO\({}_{2}\)[43] & 200–400 \({}^{\circ}\)C & Not typically used \\ TiO\({}_{2}\) & VOCs, & Strong sensitivity to VOCs and NO\({}_{2}\), improved by UV activation; doping with Pt or N enhances recovery time. & -15 ppb for NO\({}_{2}\)[53] & RT-30 \({}^{\circ}\)C, UV & UV light enhances recovery time. \\ CuO & CO, CH\({}_{4}\), & Effective for reducing gases like H\({}_{2}\) and CO; p-type behavior enables selective detection with Zn or Sn doping. & -8 ppb for CO [61] & -8 ppb for CO [61] & Not typically used \\ Fe\({}_{2}\)O\({}_{3}\), & H\({}_{2}\), CO, & Chemically stable; high sensitivity to CO and NO\({}_{2}\) & -1 ppm for H\({}_{2}\)S [69] & 200–400 \({}^{\circ}\)C & Not typically used \\ Fe\({}_{3}\)O\({}_{4}\) & NO\({}_{x}\) & when Sn- or Ni-doped; nanostructuring enhances response rate. & -1 ppm for H\({}_{2}\)S [69] & 200–400 \({}^{\circ}\)C & Not typically used \\ SnO\({}_{2}\) & H\({}_{2}\), CO, & Highly sensitive to reducing gases like CO and H\({}_{2}\); doping with Pt or Pd improves selectivity and response time. & -10 ppb for H\({}_{2}\)[74] & 200–400 \({}^{\circ}\)C & Not typically used \\ In\({}_{2}\)O\({}_{3}\) & NO\({}_{2}\), O\({}_{3}\), & Sensitive to oxidizing gases like NO\({}_{2}\) and O\({}_{3}\); Sn and Zn doping improves response to low-concentration gases. & -10 ppb for NO\({}_{2}\)[83] & 145 \({}^{\circ}\)C & Not typically used \\ MoO\({}_{3}\) & NH\({}_{3}\), H\({}_{2}\), & Versatile for oxidizing and reducing gases; V and Cr doping enhances selectivity; nanostructuring yields faster response. & -20 ppb for H\({}_{2}\)[90] & 200–400 \({}^{\circ}\)C & Not typically used \\ ZnO- & H\({}_{2}\), CO, & High sensitivity to reducing gases; the p-n junction CuO & -1 ppm for H\({}_{2}\)[109] & 15 ppb for H\({}_{2}\)[109] & 200–400 \({}^{\circ}\)C & Not typically used \\ SnO\({}_{2}\)- & NO\({}_{2}\), & Enhanced sensitivity due to improved charge transfer; effective for detecting low concentrations of NO\({}_{2}\) & -50 ppm for VOCs & -50 ppm for VOCs & Not typically used \\ TiO\({}_{2}\) & VOCs & transfer; effective for detecting low concentrations of NO\({}_{2}\) & -1 ppm for VOCs & -50 ppm for VOCs & -50 ppm for VOCs \\ FeO\({}_{2}\)- & NO\({}_{2}\), & Higher sensitivity to reducing gases; the p-n junction CuO & -1 ppm for VOCs & -1 ppm for VOCs & -1 ppm for VOCs \\ TiO\({}_{2}\) & VO\({}_{3}\) & increase sensitivity; effective under low-temperature conditions. & -1 ppm for VOCs & -1 ppm for VOCs & -1 ppm for VOCs \\ In\({}_{2}\)O\({}_{3}\)- & NO\({}_{2}\), CO, & High sensitivity to oxidizing gases; doping with Sn improves selectivity and lowers detection limits for environmental monitoring. & -1 ppm for VOCs & -1 ppm for VOCs & -1 ppm for VOCs \\ CuO- & H\({}_{2}\), CO, & Good response to reducing gases; the combination offers improved selectivity/stability due to the electronic properties of both oxides. & -2 ppm for H\({}_{2}\)[106] & 200–400 \({}^{\circ}\)C & Not typically used \\ Fe\({}_{2}\)O\({}_{3}\) & CH\({}_{4}\) & increase sensitivity; effective under low-temperature conditions. & -1 ppm for VOCs & -1 ppm for VOCs & -1 ppm for VOCs & -1 ppm for VOCs & -1 ppm for VOCs \\ TiO\({}_{2}\)- & NH\({}_{3}\), NO\({}_{x}\), & & Particularly in detecting NH\({}_{3}\) doping further optimizes selectivity. & -1 ppm for VOCs