Synthesis of nanostructured Co3O4 by a surfactant-free hydrothermal method and its application to the hydrogen evolution reaction

We report the synthesis of nanostructured Co(3)O(4 )using the hydrothermal method, without the need for any additives, such as surfactants or reducing agents. The hydrothermal synthesis was conducted at a temperature of 180 C-degrees over a 5-h period. The obtained results confirmed the presence of both fl-Co(OH)(2) and Co(3)O(4 )phases in the as-synthesized material, referred to here as the precursor. Upon thermal analysis, it was observed that /3-Co(OH)(2) underwent complete decomposition into Co(3)O(4 )at temperatures above 300 C-degrees. After subjecting the precursor to a heat treatment at 500 C-degrees for 3 h, we successfully obtained pure-phase Co(3)O(4 )exhibiting a cubic spinel-like structure. The optical behavior of Co(3)O(4 )was characterized by the presence of electronic transitions, specifically O2- -> Co3+ and O2- -> Co2+, which occurred at wavelengths of 388 and 689 nm, respectively. Additionally, the results suggest that manipulating the synthesis duration and the concentration of Co2+ in the chemical solution can lead to the formation of various Co3O4 morphologies, including nanospheres, microspheres, or a mixture of nanoparticles with both spherical and octahedral shapes. All Co(3)O(4 )samples were effectively employed as catalysts for the hydrogen evolution reaction in an alkaline environment. These catalysts demonstrated remarkable stability, sustaining continuous gas evolution for over 5 h at a current density of -10 mA & sdot;cm(-2), indicating their promising potential for large-scale H-2 generation. Notably, the samples featuring octahedral-shaped nanoparticles exhibited superior electrocatalytic activity for H-2 production, attributable to the exposure of (111) crystallographic planes on their surfaces. These samples initiated gas evolution at a low potential of 84 mV and reached the desired current density of -10 mA & sdot;cm(-2) at 320 mV. Finally, the mechanism for H 2 formation followed the Volmer-Heyrovsky reaction pathway.