Synthesis of ZnO Based Metal Oxide Nanomaterials for Multifunctional Application Studies

Abstract

The application of metal oxides is one of the most promising practices in a diverse field. Zinc oxide (ZnO) is one of the novel metal oxides utilized for various applications such as sorption, photocatalysis, sensing, and antimicrobial. The electron-hole recombination that occurs due to the aggregation/agglomeration is the major limitation in employing ZnO for photocatalysis. Among several efforts made to enhance the charge transfer capability of ZnO, forming a heterojunction is a simple and economical way. The sol-gel followed by the auto-combustion method was utilized for synthesizing a high surface area to volume ratio and porous ZnO-based metal oxide nanocomposites. The effects of the synthesis techniques, type of precursors, amount of poly (vinyl alcohol) loading, and precursor percentage were studied and optimized. The synthesized nanoparticles and nanocomposites were characterized by DTG/DSC, UV-vis-DRS, XRD, FT-IR, BET, SEM/EDX, XPS, TEM/HRTEM/SAED, and CV/EIS/amperometric analytical techniques. The characterization results revealed the surface area to volume ratio, porosity, and charge transfer property improvement on the ternary nanocomposite (Zn/Fe/Mn oxide), compared to single ZnO and binary (Zn/Fe and Zn/Mn) oxide counterparts. From the DTG/DSC result, the calcination temperature of 500 oC was found to be ideal for degrading impurities and poly (vinyl alcohol) polymer after its role as a capping agent. Using the XRD pattern and TEM image analysis, the crystallite size of the nanoparticles and nanocomposites was confirmed to be in the nanometer range (7nm - 70 nm). The porous nature of the optimized nanocomposites was understood from the SEM image and BET analysis; consistent results were also noted from the HRTEM (IFFT) and SAED pattern analysis. The EDX, XPS, HRTEM analysis confirmed the presence of a predictable composition of the nanocomposites. As calculated from the BET analysis, the specific surface area (SSA) of the porous ternary nanocomposite was found to be 15 times greater than that of ZnO. The presence of greater charge transfer property for ternary nanocomposites, compared to ZnO and binary nanocomposites were evidenced from CV/EIS analysis. The electron transfer resistance values, as determined using Nyquist plot for ZnO/Fe2O3/Mn2O3, ZnO/Fe2O3, ZnO/Mn2O3, and ZnO were found to be 7, 25, 61, and 65 Ω, respectively. The adsorption test on methylene blue dye showed the domination of a chemisorption type of adsorption. The Langmuir, Freundlich, Dubinin-Radushkevich, Temkin, Flory-Huggins, and Fowler-Guggenheim isotherm models were applied to understand the adsorption process. Among them, the Langmuir and Flory-Huggins models showed better fitting. From the Langmuir model, the adsorption efficiency of ternary nanocomposites was determined to be 7.75 mg g-1 . Compared to the other nanoparticles and nanocomposites, ZnO/Mn2O3 nanocomposite showed better photocatalytic activity only on the acid orange-8 dye. However, the ZnO/Fe2O3/Mn2O3 nanocomposite is effective on both Congo red and Acid orange 8 dyes degradation. The ZnO/Fe2O3/Mn2O3 and ZnO/Fe2O3 nanocomposites showed superior ascorbic acid detection capacity. Compared to ZnO, the synthesized binary and ternary nanocomposites exhibited an enhanced antibacterial activity on both Gram-positive and Gram-negative bacteria. The highest zone of inhibition for ternary nanocomposite is 28 and 29 mm on E. coli and S. aureus bacteria, respectively. From these results, it is possible to conclude that the synthesized nanocomposites have good potential for multifunctional applications including photocatalysis, sensor, and antibacterial activities.