Amorphous molybdenum trioxide thin films for gas sensing applications

Abstract

Uniform and smooth substoichiometric Molybdenum trioxide (MoO3−x) films have been achieved via thermal evaporation technique. MoO3 thin films were deposited onto glass and fluorine doped tin oxide (FTO) coated glass slides. The field-emission scanning electron microscopy micrographs reveal the amorphous behavior for films deposited either on glass or FTO substrates. The two thicknesses 500 and 650 nm of MoO3−x films which are deposited on glass and FTO coated glass slides were electrically examined as sensors towards NO,CO2 and HCl gases. The sensing process have been explained based on the electrical dipole layer formation consisting of the two unlike charged ion- layers and causing an energy band bending at the surface region of the film as a result of the gas adsorption. MoO3−x films on glass substrate is a perfect sensor towards oxidizing CO2 and NO gasses. The sensors are very sensitive towards low gas flow rate (20 standard cubic centimeters per minute (sccm)) and manifest the highest sensitivity values at relatively low temperatures compared with the other MOS with short response and recovery times (4 min for CO2 and 2 min for NO). The recorded sensitivity of MoO3−x sensor towards CO2 and NO gases at temperature 200 0 C shows that these samples are more sensitive towards CO2 gas, so this sensor is selective to CO2 gas more than NO. These results confirm that MoO3−x film deposited on glass of 500 nm thick can solve a large section of detecting CO2 gas problems such as selectivity, sensitivity to low gas concentrations, short response, recovery time, reversibility and low working temperature. Finally, the MoO3−x films (with thickness of 500 nm) deposited on FTO substrate was examined as gaseous HCl sensor at room temperature. It manifests a very high sensitivity towards gaseous HCl but cannot be applied for detecting it due to irreversibility and no stability of their properties after HCl vapor adsorption.