Response to VOCs stimuli by triphenylamine derivatives functionalized zinc oxide nanorods: A promising material for food freshness monitoring

In this work, we have examined the photo-induced volatile organic compounds (VOCs) sensing ability of triphenylamine (TPA) derivatives functionalized zinc oxide nanorods (ZnO NRs) using the scanning Kelvin probe (SKP). We have explored the effect of contact potential difference variance and surface photovoltage in both dark and visible light illumination under different VOCs with reference to the ambient conditions. The ZnO NRs are synthesized by hydrothermal method followed by functionalization with three different TPA organic molecules namely (i) 4-formyltriphenylamine-acetophenone, (ii) 4,4'-diformyltriphenylamine-acetophenone, and (iii) 4,4',4"-triformyltriphenylamine-acetophenone (TFTPA). The pure and functionalized ZnO NRs are characterized by XRD, FESEM, HR-TEM, UV-visible absorption spectroscopy, Raman spectroscopy, XPS, and, SKP system. Three different VOCs such as ethanol, nonanal, and triethylamine are exposed on pure and functionalized ZnO NRs. In dark condition, high surface potential changes have been observed for nonanal in all the samples. Noteworthy, TFTPA_ZnO NRs show high adsorption towards nonanal during visible light illumination too. The irradiation of light has tuned the energy levels favorable for enhanced charge transfer between nonanal and TFTPA_ZnO. In parallel, the VOCs adsorption mechanisms are understood through density functional theory calculations. The computational adsorption energy and Mulliken charge analysis results are in accordance with the experimental ones. Overall, this combined study suggests that the ZnO NRs functionalized with the TPA derivatives are highly sensitive to nonanal. This can be used to monitor the freshness of edible oils (Peanut, soybean, and linseed), virgin olive oil, yak meat, etc. under visible light illumination.