Unveiling the Mechanisms Leading to the ‘Green’ Synthesis of Metal Oxide Quantum Dots By Plasma Induced Non-Equilibrium Electrochemistry (PINE)

Abstract

The synthesis of a range of metal oxide quantum dots (QDs) has been demonstrated in the past few years utilizing plasma-induced non-equilibrium electrochemistry (PiNE).1 - 4These reports have also shown important features of these QDs with benefits for energy and other applications. The synthesis process has shown a generalized approach for metal oxide QDs that include Cu, Mo, Zn, Ni, Mn and Co. In this contribution we will first overview the opportunities offered by this technique for metal oxides. We will then report on an in-depth investigation to understand the mechanisms and reaction pathways leading to the formation of QDs. This will be mainly based on the synthesis of CuO QDs (< 5 nm diameter), however we will also attempt to extrapolate our findings to understand the synthesis of other metal oxides, highlighting any differences. In particular, we carried out extensive characterization of both the QDs as well as the electrolyte. In addition to materials characterization techniques, we used Fourier transform infra-red spectroscopy (FTIR), ultraviolet-visible (UV-Vis) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and gas chromatography-mass (GC-MS) spectroscopy. Further, measurements for pH and other trace products were also carried out as well as optical emission spectroscopy. Overall our work discloses important general aspects of plasma-liquid interactions, in particular when ethanol is used. The specific synthesis of CuO QDs follows a ‘green’ synthesis cycle that only produces water as by-product with traces of hydrogen peroxide and isopropanol. Reference (1) Velusamy, T.; Liguori, A.; Macias-Montero, M.; Padmanaban, D. B.; Carolan, D.; Gherardi, M.; Colombo, V.; Maguire, P.; Svrcek, V.; Mariotti, D. Plasma Process. Polym. 2017, 14 (7), 1600224.(2) Ni, C.; Carolan, D.; Rocks, C.; Hui, J.; Fang, Z.; Padmanaban, D. B.; Ni, J.; Xie, D.; Maguire, P.; Irvine, J. T. S.; Mariotti, D. Green Chem. 2018, 20 (9), 2101–2109.(3) Ni, C.; Carolan, D.; Hui, J.; Rocks, C.; Padmanaban, D.; Ni, J.; Xie, D.; Fang, Z.; Irvine, J.; Maguire, P.; Mariotti, D. Cryst. Growth Des. 2019, 19 (9), 5249–5257.(4) Chakrabarti, S.; Carolan, D.; Alessi, B.; Maguire, P.; Svrcek, V.; Mariotti, D. Nanoscale Adv. 2019.