There is an increasing need for the development of low-cost and highly sensitive gas sensors for environmental, commercial, and industrial applications in various areas, such as hazardous gas monitoring, safety, and emission control in combustion processes. Considering this, resistive-based gas sensors using metal oxide semiconductors (MOSs) have gained special attention owing to their high sensing performance, high stability, and low cost of synthesis and fabrication. The relatively low final costs of these gas sensors allow their commercialization; consequently, they are widely used and available at low prices. This review focuses on the important MOSs with different morphologies, including quantum dots, nanowires, nanofibers, nanotubes, hierarchical nanostructures, and other structures for the fabrication of resistive gas sensors.

Nanostructured semiconducting metal oxides (SMOs) are among the most popular sensing materials for integration into resistive-type gas sensors owing to their low costs and high sensing performances. SMOs can be decorated or doped with noble metals to further enhance their gas sensing properties. Ag is one of the cheapest noble metals, and it is extensively used in the decoration or doping of SMOs to boost the overall gas-sensing performances of SMOs. In this review, we discussed the impact of Ag addition on the gas-sensing properties of nanostructured resistive-based gas sensors. Ag-decorated or -doped SMOs often exhibit better responsivities/selectivities at low sensing temperatures and shorter response times than those of their pristine counterparts. Herein, the focus was on the detection mechanism of SMO-based gas sensors in the presence of Ag. This review can provide insights for research on SMO-based gas sensors.

H₂S gas is a toxic and hazardous byproduct of the oil and gas industries. It paralyzes the olfactory nerves, with concentrations above 100 ppm, resulting in loss of smell; prolonged inhalation may even cause death. One of the most important semiconducting metal oxides for the detection of H₂S is Cu_xO (x = 1, 2), which is converted to Cu_xS upon exposure to H₂S, leading to a remarkable modulation in the resistance and appearance of an electrical sensing signal. In this review, various morphologies of Cu_xO in the pristine form, composites of Cu_xO with other materials, and decoration/doping of noble metals on Cu_xO nanostructures for the reliable detection of H₂S gas are thoroughly discussed. With an emphasis to the detection mechanism of Cu_xO-based gas sensors, this review presents findings that are of considerable value as a reference.

Quantum dots (QDs) are used progressively in sensing areas because of their special electrical properties due to their extremely small size. This paper discusses the gas sensing features of QD-based resistive sensors. Different types of pristine, doped, composite, and noble metal decorated QDs are discussed. In particular, the review focus primarily on the sensing mechanisms suggested for these gas sensors. QDs show a high sensing performance at generally low temperatures owing to their extremely small sizes, making them promising materials for the realization of reliable and high-output gas-sensing devices.

This study investigates the optimization of mechanical milling parameters to enhance the recycling of Ti6Al4V machining chips, addressing a significant challenge in sustainable materials processing. The influence of ball-to-powder ratio (BPR) and ball size distribution on powder characteristics, including crystallite size, particle size, and phase composition, was systematically examined. Key findings include a 30% reduction in crystallite size, with the smallest crystallite size of 51.6 nm achieved at a BPR of 10:1, as determined by Rietveld refinement. Dynamic light scattering (DLS) measurements revealed the smallest average particle size of 220.09 nm for a 20:1 BPR with a 25:75 wt% ball size ratio. Energy-dispersive X-ray analysis (EDAX) confirmed the highest Ti content (76.62 wt%) in the 10:1 BPR sample, highlighting the correlation between milling parameters and chemical purity. Electron microscopy showed that ball size distribution significantly influenced particle morphology, with a higher fraction of smaller balls producing a more uniform particle distribution and spherical morphology. Additionally, annealing-induced phase transformations were analyzed, revealing the conversion of TiO into TiO₂ under specific conditions. This study demonstrates that optimized milling parameters can reduce crystallite size and improve particle morphology while achieving high chemical purity, laying the groundwork for practical applications in materials recycling and advanced manufacturing. The findings also show the potential for producing single-phase TiO₂ powders for use in paint and cosmetic products through tailored heat treatment processes.