Introduction of Gas Sensing

            Clean energy such as hydrogen has been attracting much attention. Non- renewable hydrogen gas is presently obtained from fossil fuels although it can still be generated from renewable resources such as water. Different techniques of hydrogen production and sensing are currently documented. However, concerns over the suitability of the gas for the application given that it is highly flammable and explosives have been raised. Additionally, the gas is associated with safety threats in production, usage, and transportation due to its inflammable and explosive features especially when is concentration is above 4%.  Traditional gas sensors such as gas chromatographs and catalytic combustion type sensors are limited by the high temperature of use, high cost, slow response, large size and potential safety hazards.  The amperiometric gas sensors has several advantages such as high miniaturization potential, simple in operation, low power, fast measurement, portable, and inexpensive technique that can detect gas species at ppm levels3-4. amperiometrichydrogen gas sensing is the most extensively studied area when employing solid state sensor platforms with semiconducting oxides sensitive layers such as titanium dioxide (TiO2), zinc oxide (ZnO), and tungsten trioxide (WO3)8-9 or some metal sulfides such as Molybdenum disulfide (MoS2) and Bismuth sulfide (Bi2S3). amperiometric gas sensor works on a simple principle where there is change in resistance when gas species interact with the sensitive layer. However, due to insufficient selectivity and sensitivity, the real time measurement of the biomarker molecules in complex gas mixture like the human breath is found to be challenging order to employ them in healthcare industry. For preparing semiconductor gas sensors there are many methods can be used such as CVD (chemical vapor deposition), sol- gel method, hydrothermal synthesis and sputtering. 9- 11. The unique structures which uniform and high surface area such as hollow sphere and nanorod able to make the observation of the gas better and can regulated the band gap of SMOs by excite the ground state electrons under UV-light. Light assisted is one of the most method that used to decrease the working temperature to ambient temperature. Among the various methods to improve the H2 gas sensing performance and enhanced the selectivity of the amperiometric H2 gas sensor, Nobel metals such as Pd, Pt, and Au are commonly used as catalytic material. Pd nanoparticle (NPs) is one of the best catalytic materials for amperiometric hydrogen sensing and it is widely regarded as one of the most effective techniques in order to decrease the operating temperature, as well as the response/recovery time of H2 gas sensor.

        The review focus on hydrogen working principles and sensing mechanisms of using metal-oxide semiconductors and Pd (NPs) decoration metal-oxide semiconductors. Some challenges of the future application of the two are also discussed.