Synthesis of cuprous oxide nanocubes combined with chitosan nanoparticles and its application to p-nitrophenol degradation

12 For the first time, cuprous oxide nanocubes (Cu2O NCBs) were successfully combined with chitosan nanoparticles (CS NPs) to generate Cu2O NCBs/CS NPs composites material with highly optical property and photocatalytic activity using a simple and eco15 friendly synthetic approach at room temperature for 30 min. The synthesized Cu2O NCBs/CS NPs were characterized by Ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FTIR), X – ray Diffraction (XRD), Transmission 18 Electron Microscope (TEM) and Energy-dispersive X-ray spectroscopy (EDX). Results show that the Cu2O NCBs/CS NPs composites possessed an average particle size of ~3-5 nm; in which, Cu2O had the form of nanocubes with a size of ~3-4 nm and CS NPs 21 had spherical shape with a size of ~4-5 nm. In addition, the composition percentages of elements presented in Cu2O NCBs/CS NPs composites material were: Cu (23.99%), O (38.18%), and C (33.61%). Moreover, Cu2O NCBs/CS NPs composites material was 24 also investigated for photocatalytic activity applied in p-nitrophenol degradation. The obtained results showed that the catalytic capability of Cu2O NCBs/CS NPs for pnitrophenol reduction reached the highest efficiency of >55% in the treatment time of 25 27 min, and this efficiency was higher than that of the ZnO@chitosan nanoparticles catalyst under the same conditions. 30

had spherical shape with a size of ~4-5 nm. In addition, the composition percentages of elements presented in Cu2O NCBs/CS NPs composites material were: Cu (23.99%), O (38.18%), and C (33.61%). Moreover, Cu2O NCBs/CS NPs composites material was 24 also investigated for photocatalytic activity applied in p-nitrophenol degradation. The obtained results showed that the catalytic capability of Cu2O NCBs/CS NPs for pnitrophenol reduction reached the highest efficiency of >55% in the treatment time of 25 27 min, and this efficiency was higher than that of the ZnO@chitosan nanoparticles catalyst under the same conditions. Keywords: Cuprous oxide nanocubes, chitosan nanoparticles, cuprous oxide nanocubes/chitosan nanoparticles composites, photocatalytic activity, p-nitrophenol.

INTRODUCTION
As a side effect of industrialization, many polluting compounds were discharged into the 36 environment, especially the water environment. To this end, p-Nitrophenol (p-NP) is one of the most refractory and stable nitroaromatic compounds due to its resistance to chemical and biological degradation [1]. This organic toxic is widely used in the 39 synthesis of plenty of industrial and agricultural products such as pesticides, herbicides, petrochemicals, explosives, pharmaceuticals, and dyes [2][3][4]. p-NP contamination has significant effects on human and animal health. Thus, p-NP was listed as one of the 42 priority hazardous and toxic pollutants by the U.S. Environmental Protection Agency (EPA) [5]. Various studies have been performed to reduce the content of p-NP in solution, such as photocatalytic hydrogenation [1,6], photoelectrocatalytic [7,8], 45 biological degradation [9], and metal-free catalyst [10]. In recent years, the reduction of p-NP to p-aminophenol (p-AP) has gained attention from the scientific community, since  [16], and 60 nanowires [17]. The catalytic ability of Cu2O is influenced by its morphology since the structure-related band gap energy is essential to their photocatalytic performance [7].
The photocatalytic activity of pure Cu2O is very low because of the easy recombination 63 between photo-generated electrons and holes, be oxidized or easy to aggregate to micron-size [11]. Several methods including the incorporation of Cu2O composites with noble metal [18] and other semiconductors [19] have been developed to enhance the that is able to disperse and stabilize the Cu2O NPs and enhance its photocatalytic performance has been proposed. Chitosan (CS) is one of the most commonly used natural biopolymers [20]. CS is an abundant low-cost raw material with high reproducibility and biodegradability [21]. CS NPs possess surface and interface effect, small size and quantum size effect [22]. In addition, CS NPs can be easily combined 72 with metal ions or metal oxides to form composites (due to the existence of -NH2 and -OH functional groups in the molecule). These composites have a stable structure and shape and higher photocatalytic activity than the original metal oxide properties [23].   Figure 1 shows The morphology and size of the product was studied by TEM, Figure 4(a). The result 216 shows that Cu2O NCBs have the cubic structure with an average particle size ~3-4 nm;

Characterization and morphology of Cu2O NCBs/CS NPs composites:
and CS NPs have spherical shape with an average particle size ~4-5 nm. Thus, the synthesized Cu2O NCBs/CS NPs composites have an average particle size of ~3-5 nm 219 (Figure 4(b)). Additionally, the Cu2O NCBs/CS NPs composites particles were uniformly distributed and dispersed without agglomeration.

Photocatalytic activity of Cu2O NCBs/CS NPs composites for p-nitrophenol degradation:
The reduction of p-NP by NaBH4 without using photocatalytic is 237 presented in Figure 6. Firstly, the UV-vis spectrum (Figure 6(a)) indicated that the absorption wavelength of the initial p-NP was shifted from 317 to 400 nm immediately upon the addition of NaBH4, corresponding to a significant change in solution color from 240 light yellow to yellow-green due to the formation of 4-nitrophenolate ion. The reduction of p-NP by NaBH4 is thermodynamically feasible but possesses a high kinetic impediment between negative ions that repel each other between p-nitrophenolate and 243 BH4in the absence of an effective catalyst [29,30]. As shown in Figure 6(c), the yellowgreen decolorization of the p-NP solution after 6 h has no significant change compared to that at the initial time. The decomposition percentage of p-NP after 6 h in the absence 246 of catalyst was only 3.68%. This represented an extremely slow reaction rate in the absence of a catalyst. Fig. 6. UV-vis spectra of p-nitrophenol (p-NP) reduction without photocatalysts.
Using different catalysts in this reaction could yield disparate results due to the fact that 252 many oxides are also unable to degrade p-NP under different reaction conditions [29].
In the presence of Cu2O NCBs/CS NPs composites catalyst, the results of p-NP degradation (Figure 7) are significantly different from those of NaBH4 without catalytic 255 materials. The absorption peak at 400 nm of p-nitrophenolate ions was gradually decreased in intensity. At the same time, there was the formation and intensification of another absorption peak at 300 nm, which is the characteristic absorption peak of p- NCBs made the electron transfer occur faster. This result was completely consistent with previous studies [29].