Tio2–Cu Thin Film Material for Optical Hydrogen Gas Sensor Applications
Oluoch, Okowa Benard
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Several scientific researches are underway to investigate the possibility of using various green energies. Hydrogen gas is a candidate of such research, since its use as a fuel in automobiles releases pure water, a recyclable biproduct. But, the leakages of the gas is detrimental to its application due to its low auto ignition energy of 20 μj, wider air flame limit of 4-75 % and high flame velocity of 3.46 ms-1. This study involved fabrication of an optical gas sensor for sensing the leakage levels of hydrogen gas to a surrounding. This work explored titanium dioxide-copper (TiO2-Cu) thin films as the gas sensing layers. Titanium dioxide thin films of thicknesses 47.7, 56.2, 82.3, 100.4 and 120.5 nm were deposited on both microscope and FTO glass slides using DC magnetron sputtering technique and characterised as primate and annealed at 400 and 500 oC. Copper (Cu) catalytic layers of 5.6, 10.2, 17.3 and 21.0 nm were deposited using EDWARDS AUTO 306 Magnetron sputtering system on an optimised 100.4 nm TiO2 sample, annealed at 400 oC. Optical properties were deduced from transmittance and absorbance spectra measured using 1800 Shimadzu spectrophotometer in the optimum range of 280-800 nm through simulation. The optical behavior of the films were generated using SCOUT software and analysed using ORIGIN 9.1 64 bit software. The energy band gap decreased with material thickness from 4.2±0.05 eV for 47.7 nm film to 3.9±0.05 eV for 100.4 nm films. This was attributed to increased crystallinity as the film approaches bulk state. 120.5 nm films showed higher energy gap of 4.0±0.05 eV. This was attributed to Moss-Burstein effect. Transmittance decreased with increase in thickness probably due to agglomeration of film particles. The energy gap of the 100.4 nm, TiO2 thin films annealed at 400 oC was 3.9±0.05 eV. This is a material quality of the anatase phase. The copper surface layer increased absorption in the higher wavelength region. The energy band gaps were reduced from 3.9 to 3.8±0.05 eV with increased coverage. Self-limiting at 17.3 nm copper overlayer realised increased energy gap to 4.1±0.05 eV. A lower energy band gap range of 3.9-3.8±0.05 eV was realised when FTO substrates were used. The transmittance decreased with increased H2 gas concentration. This probably resulted from reduction in interface oxide layer formation, or inhibition of diffusion of cations from substrate towards the TiO2 layer, hence increasing photocatalytic efficiency under UV radiation. The optical energy gap reduced from 4.1±0.05 eV in 0 ccm to 3.9±0.05 eV in 50 ccm of hydrogen gas concentration. This probably resulted from chemisorption processes between the analyte gas and the fabricated sensor material. The sensitivity increased from 0.3 % in 0ccm to 3.9 % in 50 ccm hydrogen gas concentration. An average sensitivity of 2.0 % was realised for films fabricated on FTO substrate. This is higher than 1.7 % reported earlier. The material gas sensing potential was done at room temperature. The fabricated sensor material showed higher sensitivity and lower temperature operation and is furthermore, expected to be cheaper and safer.