Synthesis of cuprous oxide via ion exchange route for photocatalytic degradation of methylene blue

Metal and metallic oxide nanoparticles have been extensively studied because of their extraordinary properties. Among metal oxide nanoparticles, cuprous oxide (Cu₂O), in particular, has received a great deal of attention in the field of photocatalysis. Cu₂O is a narrow gap, nontoxic, p-type semiconducting oxide that has been widely used in applications such as photocatalysis, self-cleaning pigments, antibacterial agents and solar cells because of its excellent photocatalytic activity under visible light. Cu₂O has been prepared by a variety of methods, including sonochemistry (Muthukumaran et al. 2020), liquid phase reduction (Yu et al. 2018), electro deposition (Sun et al. 2021), microwave irradiation (Kuo et al. 2014) and sol-gel methodology (Karthikeyan et al. 2014). Liquid phase reduction is one of the more common methods for the preparation of Cu₂O and, here, selection of the reductant is a key factor; the most commonly used reductants are glucose (Kumar et al. 2016), hydrazine hydrate (Wang et al. 2010), ascorbic acid (Bai & Dang 2015) and maltose (Wu et al. 2015). Since the photocatalytic activity of Cu₂O nanoparticles is closely dependent upon particle size, surfactants or templates have typically been used to suppress agglomeration of the Cu₂O nanoparticles. The most commonly used templates are cetyltrimethyl ammonium bromide (CTAB; Duan et al. 2012), sodium dodecyl sulfate (SDS; Yu et al. 2012) and anodic aluminum oxide (AAO; Musselman et al. 2020). Zhang et al. synthesized differently shaped Cu₂O nanoparticles using poly(vinyl pyrrolidone) (PVP) as the surfactant (Zhang et al. 2007), while Luo et al. synthesized hollow Cu₂O spheres, with a diameter of ∼5 nm, using aerosol OT/sodium dodecyl benzene sulfonate (AOT/SDBS) vesicles with Cu²⁺-4-ethylpyridine as the soft template (Luo et al. 2018). These methods have several disadvantages, including low yield, high cost and complex synthetic techniques. Template-free synthesis of Cu₂O should produce a relatively clean surface and we now report the preparation of Cu₂O nanoparticles using an ion exchange method, without the need for surfactants. The Cu₂O nanoparticles were synthesized from cupric hydroxide (Cu(OH)₂) gel, which was prepared by treatment of cuprous chloride (CuCl₂) with anion exchange resin (AER). The main component of AERs is polystyrene, which can modify the Cu(OH)₂ precursor through a mechanochemical reaction and, consequently, suppress agglomeration of the Cu(OH)₂ particles. During the reaction, hydroxide is released slowly to produce a uniform precipitate, and the shear forces between the resins beads promote uniform dispersion of the nanoparticles. Mechanical and chemical effects can also modify particles and suppress nanoparticle agglomeration and, for comparison, Cu(OH)₂ precursor was also prepared by reaction of sodium hydroxide with CuCl₂. We term this method of preparation of Cu₂O a chemical precipitation method.

The crystal structure and morphology of the Cu₂O nanoparticles were investigated by X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM). The photocatalytic activity of the Cu₂O was determined by measuring the rate of degradation of methylene blue (MB) under visible light irradiation. Photocorrosion of Cu₂O was shown to be inhibited by hydrogen peroxide as a sacrificial agent, and the mechanism of this protection is discussed.

All chemicals used in this work were analytical grade and were used as received.

Cupric chloride solution was added to AER (201 × 7) under continuous stirring at room temperature, producing an immediate blue gel. Once the reaction was complete, the gel was separated and added to a vigorously stirred alkaline (pH = 11) solution of glucose at 75 °C. After 1 h, the reaction mixture was filtered and the precipitate was washed several times with distilled water to pH 7. The precipitate was then dried in a furnace at 40 °C for 12 h to provide Cu₂O nanoparticles (Sample 1).

Aqueous sodium hydroxide solution was added to aqueous cuprous sulfate solution under constant stirring, producing an immediate flocculent blue precipitate. The reaction mixture was then stirred vigorously at 75 °C during the addition of aqueous glucose solution. After 1 h, the mixture was filtered and the precipitate was washed several times with distilled water to pH 7. The precipitate was then dried in a furnace at 40 °C for 12 h to provide Cu₂O particles (Sample 2).

The surface morphology of the Cu₂O samples was investigated using a Sigma scanning electron microscope (Zeiss Co., Germany), operating at 10 kV. ATR-FTIR spectra of the samples were recorded with a Tensor 27 spectrometer (Bruker Co., Germany) over the range 4,000–400 cm⁻¹, with a resolution of 4 cm⁻¹, using KBr discs prepared with spectral grade KBr pellets. XRD patterns were recorded using an Empyrean X-ray diffractometer (PANalytical Co., Netherlands) over the range 2θ = 10–80°, with Cu Kα radiation and a scan rate of 0.02 s⁻¹.

The SEM images of Sample 1 (Figure 3(b, b1)) show that the Cu₂O is in the form of uniform spheres, with smooth surfaces and no obvious aggregation. The SEM images of Sample 2 (Figure 3(c, c1)), by contrast, show that the particle size distribution is not uniform, due to aggregation of the Cu₂O particles.

Different methods of preparation of Cu₂O nanomaterials are shown in Table 1. Compared with other synthetic strategies, the work described in this paper is low cost and easy to operate. The Cu₂O nanoparticles prepared by our method do, however, have both larger size and wider particle size distribution than most of those produced using other methods, which means that our method requires further refinement.

During the photocatalytic degradation of MB by Cu₂O, strongly oxidizing hydroxyl radicals (·OH) are formed on the surface of the Cu₂O nanoparticles. During the reaction, not only is MB degraded by the ·OH radicals, but the Cu₂O itself is also partially oxidized, eventually resulting in nanoparticles with reduced photocatalytic activity and poor stability, a phenomenon known as photocorrosion (Cui et al. 2022).

H₂O₂ inhibits photocorrosion mainly because superoxide radicals (·O₂⁻) generated in the presence of Cu₂O under visible light irradiation (Equations (2) and (3) react with H₂O₂ and produce ·OH radicals (Cheng et al. 2022; Meng et al. 2022; Equation (4)). These ·OH radicals can then oxidize and degrade MB, increasing photocatalytic degradation (Equation (5)). The H₂O₂ also acts as a sacrificial agent by directly reducing interactions between the Cu₂O and the strongly oxidizing, newly generated superoxide radicals.

In summary, Cu₂O nanoparticles, with an average particle size <800 nm were successfully prepared by an ion exchange method, using Cu(OH)₂ gel as a precursor. The polystyrene in the ion exchange resin modifies the Cu₂O nanoparticles and reduces or eliminates agglomeration. The Cu₂O nanoparticles are effective catalysts for the photodegradation of MB, and hydrogen peroxide effectively inhibits photocorrosion of the Cu₂O nanoparticles during the photocatalytic reaction.

This work was supported by the Foundation of Key Laboratory of Pulp and Paper Science and Technology of the Ministry of Education, Shandong Province of China (KF201826), the NEPU Scientific Research Foundation (rc201726) and the National Natural Science Foundation of China (201606042).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.