Green synthesis of cobalt oxide nanoparticles from spent coffee

Abstract

Nanomaterials have emerged as a technology with limitless potential in a broad spectrum of fields due to their exceptional magnetic, electrical, optical, mechanical, and catalytic properties compared to their bulk counterparts. This is brought about by their significantly larger surface areas achieved through the rational design of the material, which can be calibrated as desired by precisely controlling the synthesis conditions and appropriate functionalisation. Despite this, conventional techniques do not promote sustainability as they necessitate toxic chemicals and high energy consumption. Recently, green chemistry has been presented as a competent replacement for traditional nanoparticle synthesis methods by using biological precursors such as plants, bacteria, fungi, algae, and yeast. However, this approach faces complications as the physio-chemical properties of the nanoparticles are considerably unpredictable and challenging to control due to the diverse and complex array of biomolecules present in each material. Clean water has always been essential for social development, and pollution of these systems poses one of the substantial issues in the world today. One of the major contributors to this pollution is dyes, as existing wastewater treatment facilities suffer immense stress due to dye-stained industrial effluents, which inevitably accumulate in large water bodies and damage delicate ecosystems. Several counteracting techniques have been developed, with nanoparticle-induced oxidation processes proving to be an efficient and low-cost path for the degradation of organic pollutants. However, there is much exploration needed for full-scale application. The successful synthesis of Co3O4 nanoparticles using spent coffee extract was demonstrated. The Fourier-Transform Infrared (FTIR) spectrum confirmed the presence of the nanomaterial, while the X-ray Diffraction (XRD) pattern and Transmission Electron Microscopy (TEM) presented the cubic crystal phase of cobalt oxide nanoparticles that possessed spherical and irregular morphologies with average diameters of 29.01 nm. The optical absorption data indicated that the direct band gap energy was 3.09 eV. Cobalt oxide nanoparticles with these specifications illustrated remarkable catalytic efficiencies, with an 89.27% degradation of Tartrazine dye in 30 minutes. No significant concentration of dissolved cobalt was detected in the system, as an accumulation of only 0.0001% was perceived throughout the entirety of the reaction. Furthermore, the introduction of simulated solar light resulted in a 37.60% increase in catalytic degradation rate, thereby expressing the excellent photoactivity of the nanoparticles. However, to minimise operating costs, the artificial light was replaced with natural sunlight illumination, where dye removal efficiencies were further improved, resulting in an overall Tartrazine degradation of 97.11% and a 45.41% increase in reaction rate compared to the standard oxidation process.

Therefore, spent coffee was presented as an abundant and environmentally-benign raw material for a highly efficient Co3O4 nanocatalyst with excellent photocatalytic properties.