Abstracts

Fluorescent carbon nanomaterials have recently emerged as a competitor to conventional metal semiconductors for photocatalysis. The conversion of lignocellulosic biomass into value-added carbon nanomaterials has advantages compared to the metal ones due to the low toxicity, low cost, environmental friendliness, and simple synthetic routes; moreover, there are several applications regarding drug delivery, bioimaging, biosensors, and for photoelectrocatalytic processes such as oxygen and hydrogen evolution. In this study, we synthesized carbon quantum dots (Qdots) from sugarcane biomass through pyrolysis and acid treatment. The carbon dots were applied for the photoelectrocatalytic oxygen evolution reaction. Raman spectra of pyrolyzed biomass showed a D band related to carbon atoms breathing mode due to structure defects; a G band related to stretching of C-C bonds, common of sp² carbon; and an I_D/I_G= 1.00, suggesting high defect intensity (thermal treatment with the formation of oxygenated functional groups). The formation of functional groups was evidenced by Fourier-transform infrared spectroscopy. With fluorescence spectroscopy, we determined the maximum emission at 526 nm. This result is due to the Stokes shift, and it is easier to comprehend with the Jablonski diagram. Four lasers were used (wavelength excitation of 365, 405, 532, and 650 nm). Except for the 650 nm laser, the emission light of Qdots is seen to be at longer wavelengths i.e. lower energy. This result is explained by the Stokes shift, and it is better understood with the Jablonski diagram. The bandgap was determined through the Tauc plot method using UV-Vis data (4.79 eV). Also, the UV-Vis spectra show two bands at 265 and 360 nm, which are related to pi → pi* transitions within the carbon structure (sp² network) and n → pi* transition of C=O groups at the basal plane and the edge, respectively. The size of Qdots is 1.06 nm measured by dynamic light scattering. For the photoelectrochemical experiments, the working electrode was prepared with layer-by-layer (LbL) deposition, as follows: fifteen bilayers of Qdots were deposited at indium tin oxide (ITO)-covered glass electrode, immersing alternately in poly (allylamine hydrochloride) (PAH) and Qdots suspension. Cyclic voltammetry (CV) and linear voltammetry (LV) were done at pH = 1.0 (+0.2 V to +1.2 V). Voltametric analyses were done in the dark and with laser irradiation (365 and 405 nm). The current density increased from 0.4 µA cm^-2 to 0.67 and 1,18 µA cm^-2 with 405 and 365 nm laser irradiation, respectively – at potential 1.2 V vs Ag/AgCl(KCl_sat). In this system, Qdots could be performing light absorption and separation of electron-hole pairs, a fundamental process for photocatalytic charge transfer for water oxidation.