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

Abstract

The synthesis of ruthenium IV oxide (RuO2) was carried out by the hydrothermal method assisted by controlled anionic surfactant at 90 ºC for 24 h and was heat treated between 100 ºC and 500 ºC for 1 h. The samples obtained were characterized structurally, morphologically and optically by X-ray diffraction (XRD), Rietveld refinement, field emission scanning electron microscopy (SEM-EC) and diffuse reflectance spectroscopy in the ultraviolet-visible region (UV -Vis). Furthermore, the electronic structure was investigated by first-principles quantum mechanical calculations based on density functional theory at the B3LYP level to obtain the electronic band structure and density of states and finally its photocatalytic and electrochemical properties were analyzed. Structural analysis by XRD and Rietveld refinement data indicated that the samples heat treated at 500 ºC for 1 h exhibit a tetragonal rutile-type structure and P42/mnm space group. The morphology of RuO2 crystals was analyzed by SEM-EC images and showed large clusters formed by several RuO2 nanocrystals with a spherical shape and average size distribution estimated between 12 and 30 nm. The optical properties were obtained experimentally and theoretically through UV-Vis spectra and theoretical computational calculations, which indicate an optical bandgap 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 methyl orange and rhodamine B dyes, a discoloration of 93 and 92% was obtained after 120 minutes of irradiation under visible light, respectively. RuO2 nanocrystals as film showed good properties as electrodes in supercapacitors. This result was confirmed by cyclic voltammetry analyzes and charge-discharge tests that show 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 environment, the RuO2 nanocrystals/film maintained the voltammetric profile expected for supercapacitors even at high scanning speeds, such as 1000 mV s-1 and with a specific capacitance of 290 F g-1. Furthermore, significant results of cyclic efficiency of 98.9 and 91.1% were obtained for acidic and basic media, respectively. Therefore, this study proves the use of RuO2 nanocrystals as a promising oxide for efficient use as a catalyst and as an electrode material in supercapacitors.