Post-deposition annealing influences of gas adsorption on semi-vertical β-FeOOH nanorods at room temperature: A scanning kelvin probe analysis

Abstract

An investigation on gas adsorption of β-FeOOH at room temperature was performed through kelvin probe analysis. The microscopic images revealed the semi vertically grown nanorods on conductive fluorine doped tin oxide (FTO) substrate prepared by hydrothermal method. The annealing temperature strongly influenced the crystallite size, lattice stress, grain size and boundaries estimated through X-ray diffraction analysis. The crystallite size increased from 57 nm to 67 nm with a proportional increase in annealing temperature from 350 °C to 450 °C. The UV absorption spectra indicated a red shift towards visible wavelength region with increasing annealing temperature elucidating a visible-light enhanced response for the activation of gas adsorption. Correspondingly, the energy band gap decreased with increasing annealing temperature. The adsorption behaviour of different types of volatile organic compounds (VOCs) were studied using scanning kelvin probe (SKP) to measure the contact potential difference (CPD) on the surface of the samples. The CPD changes for ethanol were higher (−264 mV) at lower temperatures (350 °C) and vice versa. However, an intriguing behaviour was found for other types of gases. This peculiar behaviour with respect to the annealing temperature was understood by SKP and attributed to the presence of numerous grain boundaries interfaces which were absent when annealed at low temperature due to higher crystallite size. These adsorption studies provides notion on the fundamental mechanism of gas interaction for the fabrication of selective gas sensors.