Partículas de Hidróxido de Níquel [Ni(OH)2]: Efeito dos Precursores, Estudo Espectroscópico e Eletroquímico

Abstract

In this paper, the synthesis, characterization and electrochemical behavior of nickel hydroxide (Ni(OH)2) particles have been described. These particles were obtained from a combination of different precursor salts via a direct precipitation method using alkaline agents. Initially, the nickel precursor salts (nickel acetate tetrahydrate, nickel chloride hexahydrate, and nickel nitrate hexahydrate) were characterized. The crystalline lattice and the network parameter of each precursor were identified. The electronic spectra of these precursor salts showed regions of absorption assigned to the electron transitions in Ni2+. A CIELab colorimetric analysis classified the precursor salts as a function of green color intensity (nitrate > chloride > acetate). Thermogravimetric studies helped quantify the degrees of hydration of these precursor salts. As-synthesized Ni(OH)2 particles are solids of different cores and tons, varying in the region of the green to the side. Structural characterization by X-ray diffraction analysis helped identify the phases and the degree of crystallinity of the particles. The β-Ni(OH)2 polymorph predominates when synthesized from chloride and nitrate salts (degree of crystallinity 68.2–76.2%). On the other hand, nickel acetate generated mixed phases containing the both α and β polymorphs (degree of crystallinity 32.9–62.9%). Large surface areas were obtained for the particles of the NaOH group: AcNa, 416.5 m2g-1; ClNa, 11.43 m2g-1; NitNa, 103.7 m2g-1. The mixed particles α/β-Ni(OH)2 present low index of luminosity (CIELab), while pure β-Ni(OH)2 particles present high index of luminosity. The morphology of the particles is revealed to be aggregates of irregular sizes and rough surface. The electrochemical measurements using carbon paste electrode modified with nickel hydroxide particles (Ni(OH)2/EPC) allowed the definition of three electrodes with the best responses in terms of peak current: AcNa/EPC, ClNa/EPC, and NitNa/EPC. Current stability was achieved with 50 successive cycles as pre-treatment of the electrode. The best responses were obtained in KOH (1 mol L-1). The scanning speed study shows that the charge transfer process is diffusion-limited. The chronoamperometry measurements helped identify the limits of detection and quantification of ethyl alcohol: AcNa/EPC [LD = 0.331 mmol L-1; LQ = 1.00 mmol L-1]; NitNa/EPC [LD = 0.427 mmol L-1; LQ = 1.29 mmol L-1] and ClNa/EPC [LD = 0.444 mmol L-1; LQ = 1.35 mmol L-1]. Finally, preliminary tests showed promising applications of the AcNa/EPC electrode as a sensor for analytes such as benzyl alcohol, cyclohexanol, glycerol and urea.