Advanced NH3 Detection by 1D Nanostructured La:ZnO Sensors with Novel Intrinsic p-n Shifting and Ultrahigh Baseline Stability.

Due to its stability, transportability, and ability to be produced using renewable energy sources, NH3 has become an attractive option for hydrogen production and storage. Detecting NH3 is then essential, being a toxic and flammable gas that can pose dangers if not properly monitored. ZnO chemiresistive sensors have shown great potential in real NH3 monitoring applications; yet, research and development in this area are ongoing due to reported limitations, like baseline instabilities and sensitivity to environmental factors, including temperature, humidity, and interferent gases. Herein, we suggest an approach to obtain sensors with competitive performance based on ZnO semiconducting metal oxides. For this purpose, one-dimensional nanostructured pure and La-doped ZnO films were synthesized hydrothermally. Incorporating large rare earth ions, like La, into the bulk lattice of ZnO is challenging and can lead to surface defects that are influential in gas-sensing reactions. The sensors experienced a temperature-induced p-n shifting at about 100 °C, verified by the Hall effect and AC impedance measurements. The doped sensor showed exceptional stepwise baseline stability and outstanding performance at a relatively low operating temperature (150 °C) with a sensing response of 91 at best (@ 50 ppm NH3) and recorded a tolerance to water vapor up to 70% RH. Alongside p-n shifting, the enhanced performance was discussed in correlation with La doping-triggered changes in the nanostructural and surfacial properties of the films. We validated the proposed technique by producing similar sensors and performing multiple replicates to ensure consistency and reproducibility. We also introduced the fill factor concept into the gas sensor field as a new trustworthy parameter that could improve sensor performance assessment and help rate sensors based on deviation from ideality.

Quick NO Gas Sensing by Ti-Doped Flower-Rod-like ZnO Structures Synthesized by the SILAR Method.

In this study, Ti-doped ZnO films with flower-rod-like nanostructures were synthesized by the successive ionic layer adsorption and reaction (SILAR) method for enhanced NO gas-sensing applications. The stoichiometric ratio of Ti in the host ZnO lattice was confirmed by atomic absorption and energy-dispersive X-ray spectroscopies. All of the synthesized films exhibited a pure wurtzite hexagonal structure that seemed to deteriorate at high Ti doping contents as was manifested by the measured X-ray diffraction patterns. Scanning electron microscopy images of ZnO revealed the coexistence of porous flower- and rod-like structures, which became finer, denser, and more compact with Ti doping. By UV-vis measurements, the transmittance of the synthesized pure ZnO thin film in the visible region (∼75%) increased by about 10% with Ti doping, and the energy band gap seemed to decrease up to some limit of Ti content. Among the fabricated sensors (based on pure ZnO, 1% Ti-doped, 3% Ti-doped, and 5% Ti-doped ZnO films), the best sensing performance was observed for the 1% Ti-doped ZnO film. At first, this was associated with its high density of oxygen vacancies present on the surface of the film and ionized oxygen vacancies present in the ZnO lattice (confirmed, respectively, by X-ray photoelectron and photoluminescence spectroscopies). Nonetheless, this may also be due to its increased crystallinity (confirmed by X-ray diffraction and photoluminescence spectroscopy), high area-to-volume ratio (confirmed by scanning electron microscopy images), high specific surface area (confirmed by Brunauer-Emmett-Teller measurements) as well as high mobility and carrier concentration (confirmed by Hall measurements). The sensor was highly selective to NO gas and showed notable stability as well as very short response and recovery times, which makes it eligible for the early detection of any indoor or outdoor NO gas leakages.