Prolonged exposure to NO₂ can cause lung tissue inflammation, bronchiolitis fibrosa obliterans, and silo filler’s disease. In recent years, nanostructured semiconducting metal oxides have been widely used to fabricate gas sensors because of their unique structure and surface-to-volume ratio compared to layered materials. In particular, the different morphologies of ZnO-based nanostructures significantly affect the detection property of NO₂ gas sensors. However, because of the large interaction energy of chemisorption (1–10 eV), metal oxide-based gas sensors are typically operated above 100 °C, overcoming the energy limits to attain high sensitivity and fast reaction. High operating temperature negatively affects the reliability and durability of semiconductor-based sensors; at high temperature, the diffusion and sintering effects at the metal oxide grain boundaries are major factors causing undesirable long-term drift problems and preventing stability improvements. Therefore, we demonstrate NO₂ gas sensors consisting of ZnO hemitubes (HTs) and nanotubes (NTs) covered with TiO₂ nanoparticles (NPs). To operate the gas sensor at room temperature (RT), we measured the gas-sensing properties with ultraviolet illumination onto the active region of the gas sensor for photoactivation instead of conventional thermal activation by heating. The performance of these gas sensors was enhanced by the change of barrier potential at the ZnO/TiO₂ interfaces, and their depletion layer was expanded by the NPs formation. The gas sensor based on ZnO HTs showed 1.2 times higher detection property than those consisting of ZnO NTs at the 25 ppm NO₂ gas.

UV-assisted room temperature operated NO₂ gas sensors were prepared from ZnO/TiO₂ core–shell nanorods decorated with Au nanoparticles synthesized by a simple low-temperature aqueous solution process.

TeO₂-nanostructured sensors are seldom reported compared to other metal oxide semiconductor materials such as ZnO, In₂O₃, TiO₂, Ga₂O₃, etc. TeO₂/CuO core-shell nanorods were fabricated by thermal evaporation of Te powder followed by sputter deposition of CuO. Scanning electron microscopy and X-ray diffraction showed that each nanorod consisted of a single crystal TeO₂ core and a polycrystalline CuO shell with a thickness of approximately 7 nm. The TeO₂/CuO core-shell one-dimensional (1D) nanostructures exhibited a bamboo leaf-like morphology. The core-shell nanorods were 100 to 300 nm in diameter and up to 30 μm in length. The multiple networked TeO₂/CuO core-shell nanorod sensor showed responses of 142% to 425% to 0.5- to 10-ppm NO₂ at 150°C. These responses were stronger than or comparable to those of many other metal oxide nanostructures, suggesting that TeO₂ is also a promising sensor material. The responses of the core-shell nanorods were 1.2 to 2.1 times higher than those of pristine TeO₂ nanorods over the same NO₂ concentration range. The underlying mechanism for the enhanced NO₂ sensing properties of the core-shell nanorod sensor can be explained by the potential barrier-controlled carrier transport mechanism. PACS 61.46. + w; 07.07.Df; 73.22.-f

Because the oxides of nitrogen (NO_x) cause detrimental effects on not only the environment but humans, developing a high-performance NO₂ gas sensor is a crucial issue for real-time monitoring. To this end, metal oxide semiconductors have been employed for sensor materials. Because in general, semiconductor-type gas sensors require a high working temperature, photoactivation has emerged as an alternative method for realizing the sensor working at room temperature. In this regard, titanium dioxide (TiO₂) is a promising material for its photocatalytic ability with ultraviolet (UV) photonic energy. However, TiO₂-based sensors inevitably encounter a problem of recombination of photogenerated electron-hole pairs, which occurs in a short time. To address this challenge, in this study, TiO₂ nanorods (NRs) and Pt nanoparticles (NPs) under a UV-LED were used as an NO₂ gas sensor to utilize the Schottky barrier formed at the TiO₂-Pt junction, thereby capturing the photoactivated electrons by Pt NPs. The separation between the electron-hole pairs might be further enhanced by plasmonic effects. In addition, it is reported that annealing TiO₂ NRs can achieve noteworthy improvements in sensing performance. Elucidation of the performance enhancement is suggested with the investigation of the X-ray diffraction patterns, which implies that the crystallinity was improved by the annealing process.

2,6-Diaminoanthracene (AnDA)-functionalized graphene oxide (GO) (AnDA-GO) was prepared and used to synthesize a graphene oxide-based polyimide (PI-GO) by the in-situ polymerization method. A PI-GO nanocomposite thin film was prepared and characterized by infrared (IR) spectroscopy, thermogravimetric analysis (TGA) and UV-visible spectroscopy. The PI-GO film was used as a memory layer in the fabrication of a resistive random access memory (RRAM) device with aluminum (Al) top and indium tin oxide (ITO) bottom electrodes. The device showed write-once-read-many-times (WORM) characteristics with a high ON/OFF current ratio (I_on/I_off = 3.41 × 10⁸). This excellent current ratio was attributed to the high charge trapping ability of GO. In addition, the device had good endurance until the 100th cycle. These results suggest that PI-GO is an attractive candidate for applications in next generation nonvolatile memory.