Recently one dimensional (1-D) nanostructured metal-oxides have attracted much attention because of their potential applications in gas sensors. 1-D nanostructured metal-oxides provide high surface to volume ratio, while maintaining good chemical and thermal stabilities with minimal power consumption and low weight. In recent years, various processing routes have been developed for the synthesis of 1-D nanostructured metal-oxides such as hydrothermal, ultrasonic irradiation, electrospinning, anodization, sol-gel, molten-salt, carbothermal reduction, solid-state chemical reaction, thermal evaporation, vapor-phase transport, aerosol, RF sputtering, molecular beam epitaxy, chemical vapor deposition, gas-phase assisted nanocarving, UV lithography and dry plasma etching. A variety of sensor fabrication processing routes have also been developed. Depending on the materials, morphology and fabrication process the performance of the sensor towards a specific gas shows a varying degree of success. This article reviews and evaluates the performance of 1-D nanostructured metal-oxide gas sensors based on ZnO, SnO₂, TiO₂, In₂O₃, WO_x, AgVO₃, CdO, MoO₃, CuO, TeO₂ and Fe₂O₃. Advantages and disadvantages of each sensor are summarized, along with the associated sensing mechanism. Finally, the article concludes with some future directions of research.

In this research work, the sensitivity of TiO₂ nanoparticles towards C₂H₅OH, H₂ and CH₄ gases was investigated. The morphology and phase content of the particles was preserved during sensing tests by prior heat treatment of the samples at temperatures as high as 750 °C and 1000 °C. Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis were employed to characterize the size, morphology and phase content of the particles. For sensor fabrication, a film of TiO₂ was printed on a Au interdigitated alumina substrate. The sensing temperature was varied from 450 °C to 650 °C with varying concentrations of target gases. Results show that the sensor has ultrahigh response towards ethanol (C₂H₅OH) compared to hydrogen (H₂) and methane (CH₄). The optimum sensing temperature was found to be 600 °C. The response and recovery times of the sensor are 3 min and 15 min, respectively, for 20 ppm C₂H₅OH at the optimum operating temperature of 600 °C. It is proposed that the catalytic action of TiO₂ with C₂H₅OH is the reason for the ultrahigh response of the sensor.

An ethanol gas sensor with enhanced sensor response was fabricated using Ni-doped SnO ₂ nanorods, synthesized via a simple hydrothermal method. It was found that the response ( R = R ₀ / R _g ) of a 5.0 mol% Ni-doped SnO ₂ (5.0Ni:SnO ₂ ) nanorod sensor was 1.4 × 10 ⁴ for 1000 ppm C ₂ H ₅ OH gas, which is about 13 times higher than that of pure SnO ₂ nanorods, (1.1 × 10 ³ ) at the operating temperature of 450 °C. Moreover, for 50 ppm C ₂ H ₅ OH gas, the 5.0Ni:SnO ₂ nanorod sensor still recorded a significant response reading, namely 2.0 × 10 ³ with a response time of 30 s and recovery time of 10 min. To investigate the effect of Ni dopant (0.5–5.0 mol%) on SnO ₂ nanorods, structural characterizations were demonstrated using field emission scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, x-ray diffraction (XRD) analysis, x-ray photoelectron spectroscopy and an ultraviolet–visible spectrometer. XRD results confirmed that all the samples consisted of tetragonal-shaped rutile SnO ₂ nanorods. It was found that the average diameter and length of the nanorods formed in 5.0Ni:SnO ₂ were four times smaller (∼6 and ∼35 nm, respectively) than those of the nanorods formed in pure SnO ₂ (∼25 and 150 nm). Interestingly, both samples had the same aspect ratio, ∼6. It is proposed that the high response of the 5.0Ni:SnO ₂ nanorod sensor can be attributed to the particle size, which causes an increase in the thickness of the charge depletion layer, and the presence of oxygen vacancies within the matrix of SnO ₂ nanorods.