Chlorpyrifos Sensing Behavior of Metal Incorporated Zinc Oxide Nanowires Grown by Limited Volume Heating Technique

Abstract

In the modern agricultural world, extensive usage of pesticides to preserve crops from pests has become one of the major concerns due to their neurotoxic property. Organophosphates (OP) are the most widely used pesticide due to their low cost and a broad spectrum of applications. Moreover, chlorpyrifos (CP), which belongs to the OP family, get readily accumulated in the ecosystem and as a result, their traces are remain in the environment due to their low solubility in water and high soil sorption coefficient. This residual amount of CP causes severe health problems for humans and other livestock. In order to detect these toxic compounds, there is a need for real-time, portable, low-cost sensors, where the response can be recorded by the variation in current-voltage behavior. Nanostructured metal oxide (Zinc oxide) have been found to exhibit interesting properties such as large surface-to-volume ratio, low cost, low temperature growth, non-toxic nature, which made ZnO a potential material for the development of chemical sensors. In this research work, high aspect ratio zinc oxide (ZnO) nanowires are grown using the custom-designed limited volume heating (LVH) technique, where the optimization of various growth parameters is carried out systematically. The morphological and structural characterizations have revealed the evolution of c-axis oriented ZnO NWs by LVH technique. The length and aspect ratio of the NWs, grown by LVH technique, is found to be around 9 μm and 300, respectively, which is nearly five times higher than that of the conventional growth technique. In addition, an effort has been made to further improve the CP sensing response of LVH grown NWs by incorporation of various metallic dopants like Al, Ti, Mn, Cu, and Ag. The LVH grown ZnO NWs are coated with metal thin film by RF sputtering (pre-deposition) technique followed by the heat treatment (drive-in) in inert ambient for thermal diffusion of metal dopants. The drive-in process is conducted at various temperatures below the melting point of respective metals. The metal film thickness is also varied as 15 nm, 30 nm, and 45 nm to study the effect of film thickness. Post-growth morphological and structural investigations of the NWs have been carried outby FESEM, HRTEM, and XRD techniques. Besides, the distribution of dopant in NWs is verified from elemental mapping images obtained by energy-dispersive X-ray spectroscopy measurement. Furthermore, microstructural and photoluminescence properties of metal incorporated ZnO NWs are investigated with various process parameters. Finally, the CP sensing behavior of metal incorporated ZnO NWs is carried out by studying the current-time response at room temperature for various concentrations of CP varying from 500 mg/kg to 6000 mg/kg. The as-grown ZnO NWs have shown the CP in/out current ratio around 5 and 38 for CP lethal dosage of 500 mg/kg and 6000 mg/kg, respectively. The CP sensing behavior of metal incorporated ZnO NWs is found to be significantly improved with metal type, thickness, and drive-in temperature. Among various metallic dopants, the highest CP sensor response has been observed for Cu incorporated NWs with a noteworthy improvement in current in/out ratio of around 768, for CP dosage of 6000 mg/kg. Besides, the response and recovery time of Cu incorporated ZnO NWs are found to be around 14 s and 5 s, respectively for CP concentration of 6000 mg/kg. The above research work opens the path for the fabrication of metal incorporated LVH grown nanostructures for low cost, portable, real-time CP sensors.