[en] The analysis of exhaled breath is important for health vs. disease diagnostics. Diagnosis and monitoring methods based on metal-oxide gas sensors are highly attractive, owing to their portability and ease of operation. These methods are non-invasive and affordable, and allow early disease diagnosis. Metal-oxide gas sensors are heat-resistant and can easily be combined with various nanostructures. We review concurrent research into metal-oxide gas sensors used to detect various biomarkers (e.g., acetone, nitric oxide, hydrogen sulfide, and ammonia) in the exhaled breath of humans. We also review efforts to improve the properties of metal-oxide gas sensors, especially structures, morphologies, the control of impurities in semiconductor materials, and changes in the specific surface area of metal-oxide materials that have been used in gas sensors to analyze exhaled breath. Various approaches to the humidity resistance characteristics and nonlinear responses of metal-oxide semiconductor gas sensors are addressed and should be applicable in real life. (topical review)

[en] In this paper, the fabrication of ZnO tetrapod was investigated. It was synthesized by the thermal oxidation technique using metal zinc powder mixed with oxidizing agents such as hydrogen peroxide (H₂O₂) and ammonium persulfate ((NH₄)₂S₂O₈). The furnace heating temperature reached at 1000 °C in the air. The average diameter and length of a tetrapod leg for mixture with H₂O₂ from SEM were 45.3 nm and 1.57 μm, respectively. The oxygen vacancy (36%) of ZnO tetrapod with H₂O₂ was higher than 33% of ZnO tetrapod with only Zn. Growth mechanism of ZnO tetrapod was processed via the formation of Zn nucleus and growing the wurtzite structure. The growing directions of ZnO crystal conformed with the [0001] direction. ZnO tetrapod showed up the high resolution TEM image with the lattice spacing 0.252 nm. From these results, this work was indicated that H₂O₂ solution was a better oxidizing reaction helper to make ZnO tetrapod nanostructures than anything else.

[en] Herein we reported the effect of doping and addition of surfactant on SnO₂ nanostructures for enhanced photocatalytic activity. Pristine SnO₂, Zn–SnO₂ and SDS-(Zn–SnO₂) was prepared via simple co-precipitation method and the product was annealed at 600 °C to obtain a clear phase. The structural, optical, vibrational, morphological characteristics of the synthesized SnO₂, Zn–SnO₂ and SDS-(Zn–SnO₂) product were investigated. SnO₂, Zn–SnO₂ and SDS-(Zn–SnO₂) possess crystallite size of 20 nm, 19 nm and 18 nm correspondingly with tetragonal structure and high purity. The metal oxygen vibrations were present in FT-IR spectra. The obtained bandgap energies of SnO₂, Zn–SnO₂ and SDS-(Zn–SnO₂) were 3.58 eV, 3.51 eV and 2.81 eV due to the effect of dopant and surfactant. This narrowing of bandgap helped in the photocatalytic activity. The morphology of the pristine sample showed poor growth of nanostructures with high level of agglomeration which was effectively reduced for other two samples. Product photocatalytic action was tested beneath visible light of 300 W. SDS-(Zn–SnO₂) nanostructure efficiency showed 90% degradation of RhB dye which is 2.5 times higher than pristine sample. Narrow bandgap, crystallite size, better growth of nanostructures paved the way for SDS-(Zn–SnO₂) to degrade the toxic pollutant. The superior performance and individuality of SDS-(Zn–SnO₂) will makes it a potential competitor on reducing toxic pollutants from wastewater in future research.