Surface morphology-driven gas sensing performance of CdS-modified TiO₂ thick film: a study based on fractal concepts

In recent years, gas sensors have played a crucial role in gas detection technologies, largely due to the increased surface area of their sensing films. The concept of fractals was applied to quantify surface characteristics such as roughness, surface area, and fractal dimension. This approach was employed to enhance sensing performance by informing the design of sensor microstructures. In this study, we investigate how CdS incorporation into TiO₂ affects the fractal characteristics of the fabricated films and analyse the correlation between these properties and performance in detecting volatile organic compounds (VOCs). Thick films based on TiO₂ and CdS-TiO₂ composites were characterised using atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM) to analyse their surface morphology. The AFM images were used to estimate parameters such as fractal dimension, Hurst exponent, and surface roughness. The study reveals that these fractal parameters significantly influence the gas sensing properties. The study evaluated the sensing performance of both TiO₂ and CdS-TiO₂ thick films for test VOC gases (LPG, ethanol, toluene, and acetone) at concentrations of 1000-5000 ppm and low operating temperatures. Also, the fabricated sensor studies the effect of relative humidity on sensing properties. Among the tested gases, the sensor based on 2 wt% CdS-TiO₂ exhibited the highest response of 71%, with more selectivity (47.7%) towards acetone over LPG, toluene, and ethanol. The experimental findings are in good agreement with theoretical models, confirming the validity of the approach.