Síntese de nanocristais/filmes de óxido rutênio (IV), investigação da estrutura eletrônica, propriedades fotocatalíticas e eletroquímicas para supercapacitores

Abstract

The synthesis of ruthenium oxide IV (RuO2) was performed by the hydrothermal method assisted by anionic surfactant controlled at 90 ºC for 24 h and then heat treated between 100 ºC and 500 ºC for 1 h. The obtained samples were characterized structurally, morphologically and optically by X-ray diffraction (XRD), Rietvelds refinement, field emission scanning electron microscopy (FE-SEM) and spectroscopy of diffuse reflectance in the ultraviolet-visible region (UV-Vis). In addition, the electronic structure was investigated by calculations of quantum mechanics of the first principles based on the functional theory of density at the B3LYP level to obtain the band structure and state density and finally analyzed its photocatalytic and electrochemical properties. Structural analysis by XRD and Rietveld refinement data indicated that samples heat treated at 500 ºC for 1 h exhibit a rutile-like tetragonal structure and space group P42/mnm. The morphology of the RuO2 crystals was analyzed by MEV-EC images and showed large agglomerates formed by several RuO2 nanocrystals with spherical shape and mean distribution of estimated size between 12 and 30 nm. The optical properties were obtained experimentally and theoretically through the UV−Vis spectra and computational theoretical calculations, which indicate an optical band gap energy governed by direct electronic transitions of 2.60 and 1.92 eV, respectively. In the study of these nanocrystals as a photocatalyst for the degradation of the methyl orange and rhodamine B dyes, obtained a discoloration of 93 and 92% was obtained after 120 minutes of irradiation under visible light, respectively. RuO2 nanocrystals as films showed good properties as electrodes in supercapacitors. This result was confirmed by cyclic voltammetry analyzes and load-discharge tests showing specific capacitance of 659 F g-1 and 195 F g-1 both at 10 mV s-1, in acidic and basic media, respectively. In an acidic medium the RuO2 nanocrystals/films remained with expected voltammetric profile for supercapacitors even at high scanning speeds, for example at 1000 mV s-1 and with specific capacitance of 290 F g-1. Moreover, significant cyclic efficiency results of 98.9 and 91.1% were obtained for acidic and basic media, respectively. Therefore, this study confirms the use of RuO2 nanocrystals as a promising oxide for efficient use as a catalyst and as electrode material in supercapacitors.