Copper(II) oxide nanocatalyst preparation and characterization: green chemistry route

Recently, green chemistry routes of preparation of different metal/metal oxide nanomaterials have been attracting increasing attention due to their advantages being operating under mild conditions, environmentally safe, and assist in manipulating the reaction condition toward the production of desired engineered nanomaterials. These green synthesis routes could be achieved by using microbial microorganism, plant-related materials, and plant extract. As it is well known, nanomaterials are effectively existed in the scientific research market due to their marvelous wide range of applications as well as their outstanding optical, chemical, and physical properties which affect to a great extent their activity. Preparation of metal oxide nanoparticles has been extensively investigated using conventional methods as sonochemical method (Chen and Mao 2007; Xu et al. 2013), hydrothermal/solvothermal synthesis (Khan et al. 2011), sol-gel method (Macwan et al. 2011), combustion (Patil et al. 1997; Patil et al. 2002), chemical precipitation (Rakhshani 1986), and metal reduction (Dang et al. 2011; Phoka et al. 2009). Also, green synthesis methods of preparation of metal oxides have been investigated extensively (Parsons et al. 2009; Klaus et al. 1999; Phumying et al. 2013; Munshi et al. n.d.). Green chemistry synthesis involves two distinguished principle roles which served as a reaction medium (could be stabilizers or capping agents) and environmentally and biologically safe reducing agents (could help in medical and biological applications). Among metal oxide nanomaterials, copper oxide represents a very important and potent candidate in a wide range of applications as magnetic storage media (Li et al. 2004), sensors (Shishiyanu et al. 2006; Kim et al. 2008), catalytic applications (de Jongh et al. 1999; Zhang et al. 2014), and many other fields of application. Cu₂O is considered as a p-type semiconductor with a narrow bandgap of 1.4–1.9 eV (Grozdanov 1994), so it has a potent role in photoconductive and photothermal applications (Rakhshani 1986). In this work, CuO preparation—in a green chemistry method with the assistance of spinach extract—was studied. The obtained CuO nanoparticles were characterized using scanning electron microscopy, energy dispersive X-ray, and X-ray diffraction.

An aqueous solution of fresh spinach extract in deionized (DI) water (Munshi et al. n.d.) 80 ml together with 0.2 g copper metal (powder) was placed in a stainless-steel Teflon-lined metallic autoclave of 100 ml capacity and sealed under ambient conditions. A blank sample with copper metal aqueous solution free from spinach extract was processed at the same conditions. The autoclaved reaction mixture is heated to 180 °C (2 °C/min) in a preheated furnace and left for 72 h under the same conditions. After being cooled, the produced powder was centrifuged, washed, and vacuum dried.

Spinach green extract—like any green plant—contains chlorophyll which is considered as the first known photocatalyst from the beginning of the universe. Chlorophyll in photosynthesis process catalyzes the splitting of water molecules into hydroxyl ions OH⁻ and hydrogen ions H^+ 13. The first step in chlorophyll photocatalytic action is the oxidation of chlorophyll to produce electron which carried out the catalytic process through the attack of water molecules. The hydroxyl ions in subsequent step react with copper metal to produce copper II hydroxide which transformed into copper(II) oxide by the effect of heat as it was illustrated in Scheme 1.

The particle size of the sample was calculated using the above formula, and the small average grain size of copper oxide nanoparticles was 0.58 nm.

From SEM, it was obvious that CuO nanoparticles appear as irregular spherical particles, with predominant tetrahedron structure with dimensions ranging from 60 to 100 nm.

Chlorophyll as the main constituent of any green plant represents a potent photocatalyst to initiate and catalyze the preparation of metal oxide nanoparticles under milder conditions than that of the conventional method of preparation. Herein, it was concluded that chlorophyll that exists in spinach aqueous catalyzed the preparation of CuO NPs in a process simulating the way of action of chlorophyll in the photosynthesis process. The produced CuO NPs were found to have a particle size ranging from 60 to 100 nm.