Pt thin film temperature sensors (Pt T sensors) are embedded in micro gas sensors to measure and control the working temperature. We characterized electrical resistances of Pt T sensors and micro heaters with temperature changing in the oven and by Joule heating. In order to enhance the accuracy of temperature measurement by the Pt T sensors, we investigated the correlation among the Pt T sensor, micro heater, and the working temperature, which was linear proportional relation expressed as the equation: T2 = 6.466R1–642.8, where T2 = temperature of the Pt micro heater and R1 = the electrical resistance of the Pt T sensor. As the error by physically separated gap between Pt T sensor and micro heater calibrated, measuring and controlling temperature of micro heater in micro gas sensors were possible through the Pt T sensors. For the practical use of Pt T sensor in micro gas sensor, the gas sensing properties of fabricated micro gas sensors to 25 ppm CO and 1 ppm HCHO gases were characterized.

A catalytic combustible type single-chip micro gas sensor was fabricated by MEMS technology and responses with input powers and methane and hydrogen gas concentrations were characterized. The ranges of responses at Pt thickness of 450 nm and input power of 128 mW were 1.076–2.433 mV for methane concentrations of 2315–5787 ppm, and 0.965–2.514 mV for hydrogen concentrations of 282–706 ppm, respectively. The ranges of responses at Pt thickness of 150 nm and input power of 112 mW were 0.192–0.438 mV for methane concentrations of 2315–5787 ppm and 0.949 mV to 2.496 ppm for hydrogen concentrations of 282–706 ppm, respectively. The responses to H₂ concentration ratios were 3.65 mV/10³ ppm for a micro gas sensor with a 450 nm thick heater and 3.81 mV/10³ ppm for a micro gas sensor with a 150 nm thick heater. But in the case of methane gas response, the response to concentration ratios of the micro gas sensor using the 150 nm thick Pt heater was remarkably different from the case of the 450 nm thick Pt heater. The ratios for CH₄ were 3.51 mV/10⁴ ppm for the micro gas sensor with a 450 nm thick heater and 0.6 mV/10⁴ ppm for the micro gas sensor with a 150 nm thick heater, respectively. From these results, the micro gas sensor that has the thicker heater with a thickness of 450 nm showed higher sensitivity to methane gas than the micro gas sensor with a thinner heater with a thickness of 150 nm.

Substitutional doping and different nanostructures of ZnO have rendered it an effective sensor for the detection of volatile organic compounds in real-time atmosphere. However, the low selectivity of ZnO sensors limits their applications. Herein, hafnium (Hf)-doped ZnO (Hf-ZnO) nanostructures are developed by the hydrothermal method for high selectivity of hazardous NO_X gas in the atmosphere, substantially portraying the role of doping concentration on the enhancement of structural, optical, and sensing behavior. ZnO microspheres with 5% Hf doping showed excellent sensing and detected 22 parts per billion (ppb) NO_X gas in the atmosphere, within 24 s, which is much faster than ZnO (90 s), and rendered superior sensing ability (S = 67) at a low temperature (100 °C) compared to ZnO (S = 40). The sensor revealed exceptional stability under humid air (S = 55 at 70% RH), suggesting a potential of 5% Hf-ZnO as a new stable sensing material. Density functional theory (DFT) and other characterization analyses revealed that the high sensing activity of 5% Hf-ZnO is attributed to the accessibility of more adsorption sites arising due to charge distortion, increased oxygen vacancies concentration, Lewis acid base, porous morphology, small particle size (5 nm), and strong bond interaction amidst NO₂ molecule with ZnO-Hf-O_vacancy sites, resulting from the substitution of the host cation (Zn²⁺) with doping cation (Hf⁴⁺).