Synthesis and characterization of carbon quantum dots obtained from the pyrolysis of the cocoa bean husk and its impregnation in electrocatalysts based on Ti and Pt

Abstract Carbon quantum dots (CQDs) are nanomaterials characterized by their optoelectronic properties, their fluorescent activity and their good electrical conductivity. In addition, due to their small size of between 1 and 10 nm, they can form mesoporous surfaces when impregnated in thin films of interest. These characteristics are why they were synthesized in this research, to help enhance electrochemical properties. For this, agro-industrial waste was used. In this research, we worked with cocoa bean husks (CBH), a biomass rich in lignocellulose. The biomass was subjected to pyrolysis at 350 °C for 6 h, from which a biochar was obtained that was subjected to ultrasonic exfoliation where the CQDs were finally obtained. Morphological characterizations were made using scanning electron microscopy, high resolution transmission electron microscopy and spectroscopic characterizations using energy dispersive spectroscopy, Fourier transform infrared spectroscopy, fluorescence spectroscopy, UV–vis spectroscopy. The results give a yield of 4.32 ± 0.08 % in a synthesis that can be considered green, considering that no polluting or expensive reagents were used, since agro-industrial waste or residues and water was used as a solvent. High resolution transmission electron microscopy imaging allowed a size of 2.6 ± 0.7 nm of the CQDs to be obtained, consistent with the blue–violet fluorescence that shows the nanoparticles in solution. This was also observed in fluorescence spectroscopy where the emission range is 408–450 nm. When the CQDs were impregnated in Ti and Pt-based electrocatalysts, they were shown to enhance their electrochemical response. This confirmed the hypothesis regarding the improvement of electrical and morphological properties to improve electrocatalytic reactions such as oxygen evolution reaction, hydrogen adsorption and desorption.