Optimization of the Catalytic Activity of Ag@Cu2O Decorated on rGO for Organic Pollutant Degradation

1. Introduction

Recently, as a result of industrial development, environmental pollution by nitro compounds in water has become increasingly serious, attracting widespread attention from all sectors of society [1,2]. 4-Nitrophenol (4-NP) and other phenolic pollutants are the most representative pollutants in industrial wastewater [3,4]. The carcinogenic and mutagenic and pose a considerable threat to the environment and human [5,6]. Catalytic hydrogenation has the advantages of being simple and resulting in less environmental pollution during the treatment of phenolic pollutant compounds. This method has potential development prospects. At present, the key to the catalytic hydrogenation process for treating nitrophenol compounds is to develop efficient, stable, and low-cost catalysts. Therefore, many kinds of inorganic catalysts have been designed and fabricated for the catalysis of phenolic pollutants [7,8,9,10].

Traditional catalysts include noble metals (such as Pt, Pd, Au, and Ag) were widely used in 4-NP catalytic hydrogenation [11,12,13,14]. These catalysts exhibit high catalytic activity under mild reaction conditions. Extensive studies on these types of catalysts have been conducted worldwide. However, the challenge is to develop a new type of catalyst that is stable, highly active, and reusable. Therefore, metal/semiconductor composites were developed. The interface of the metal/semiconductor heterojunctions has outstanding advantages in catalytic applications. Due to the strong interaction between metal and semiconductor at the interface, charge transfer (CT) and transfer between the components in the composite region are the main features leading to significant changes at the heterojunction interface. More importantly, the stability of the nanocomposite is better than that of the individual components. In addition, to change the morphology, composition, and geometric structure of the material, the researchers can easily control the catalytic properties. Metal/semiconductor composites are excellent functional materials and are widely used in various materials [15,16,17,18]. In addition, core-shell nanostructures have been designed and fabricated to solve the problems of a single mechanism and limited material properties [19,20]. Therefore, Ag@Cu₂O nanoparticles were used for the chemical catalysis of phenolic pollutants. The difference in energy between Ag nanoparticles and Cu₂O induces electron transfer between the core and shell [21,22].

Two-dimensional (2D) materials show extraordinary mechanical, optical, electronic, and thermal properties, enabling expansion of their applications. Graphene, because of its high charge carrier mobility, has demonstrated excellent performance and has received extensive research attention since 2004, when it was developed as a novel 2D material [23,24]. The mechanical and electronic properties of monolayer or multilayer 2D graphene have made it a hot research topic in nanoelectronics. Therefore, graphene was widely used in the different catalysis systems. Graphene is also widely used in batteries, supercapacitors, solar cells, detectors, and sensors due to its excellent performance [25,26]. Ag@Cu₂O core-shell decorated on reduced graphene oxide (rGO) nanocomposites (ACRN) exhibited excellent catalytic activity because of the Cu₂O shell was in-situ growing on the Ag-rGO surface. After the introduction of rGO onto the Ag@Cu₂O core-shell, the material exhibited excellent catalytic performance and carrier transport [20].

In this study, ACRN were fabricated for catalytic studies. Ag-rGO nanocomposites were synthesized throw the sol-gel method, and Ag@Cu₂O nanocomposites were obtained by reducing cupric nitrate (Cu(NO₃)₂) in the presence of the hydrazine hydrate (N₂H₄). By tuning the concentration of Cu(NO₃)₂, the thickness of the Cu₂O shell of the Ag@Cu₂O can be controlled. These materials exhibited different catalytic properties and are expected to solve water pollution problems. The number of available free electrons increased, causing the catalytic target to randomly adsorb ACRN. More importantly, the ACRN show the high adsorption capacity, thus, the catalytic activity for 4-NP, methyl orange (MO), and trinitrophenol (TNP) were improved significantly.

2. Experimental section

For experimental details, please refer to the supplementary information.

3. Results and discussion

3.1. Morphological characterization of the ACRN

To obtain ACRN, we first fabricated Ag decorated on rGO (Ag-rGO) using the wet chemical method (Figure 1A). The Ag on the rGO acted as a nucleation site for the Cu₂O shell coating. By decreasing the volume of the Ag-rGO composite solution, the density of the Ag@Cu₂O decorated on the rGO was significant increase. Meanwhile, the size of the Ag@Cu₂O spheres increased significantly with the increase of the Cu₂O shell thickness. SEM images were obtained to characterize the morphology of the ACRN. As shown in Figure 2, after surface modification, the composite material maintained the 2D structure of rGO and spherical nanoparticles was arranged on the surface, indicating the successful fabrication of Ag@Cu₂O composites on the rGO platform. To confirm the core-shell structure, we obtained TEM images. As shown in Figure 3, the composites (Figure 3 B-D) we fabricated possessed a clear Ag-Cu₂O core-shell structure decorated on the rGO substrate. Figure 3A shows Ag decorated on the rGO substrate. By introducing 15 mL of Ag-rGO composite, Cu₂O formed and covered the Ag core. However, the Ag core was not fully covered by the Cu₂O shell (ACRN-1, Figure 3B). As the volume of the Ag-rGO composite decreased, the thickness of the Cu₂O shell increased significantly, resulting in complete coverage of the Ag core (ACRN-2 and ACRN-3, Figure 3C and 3D). Comparative observation showed that as the amount of Ag-rGO composite decreased, the shell thickness of Cu₂O also gradually increased until the thicknesses were approximately 18 nm (ACRN-2) and 30 nm (ACRN-3). Therefore, by controlling the amount of Ag-rGO composite and then controlling the thickness of the composite shell, composites with different structural properties were prepared. In the TEM images, we observed that the surface of the Cu₂O shell was rough and porous, making the material conducive to molecular adsorption. Because of the loose structure on the surface of Cu₂O, the catalytic and active sites were increased for catalytic reduction of the target.

To study the light response of the different ACRN, we obtained the UV‒Vis spectra of different ACRN (Figure 4). The spectra of ACRN with different shell thicknesses exhibited plasmon resonance absorption peaks. Comparative observation showed that the plasmon resonance absorption peak gradually weakened as the amount of Ag-rGO increased. The peak at approximately 250-300 nm and 300-500 nm were assigned to the C=C and C=O bands of rGO, and the surface plasmon resonance (SPR) of the Ag decorated on rGO [27]. In addition, the peak at 500-700 nm was corresponded to the SPR of the core-shell structure, the optical absorbance increased significantly with increasing thickness of the Cu₂O nanoshell. The slight redshift of the peak at 500-700 nm was due to the increase in the local dielectric constant of Cu₂O [28].

3.2. Catalytic performance analysis

The harm of organic pollutants has received widespread attention. Developing methods to catalytically degrade these pollutants is of great importance. 4-NP was widely used in the synthesis of pesticides, drugs, and dyes, showing their potential hazards. Therefore, various catalytic materials were designed and fabricated for the reduction of it [3,4].

We employed the 4-NP as a probe molecule to evaluate the catalytic performance of ACRN. The catalytic rate for 4-NP (0.005 M) was increased significantly by adding the ACRN. We observed the absorption peak of 4-NP at approximately 320 nm (Figure 5 A-D). After the addition of NaBH₄, 4-NP was ionized, resulting in the red-shifted peak at approximately 400 nm (Figure 5 A-D), and the solution was changed to light yellow (Figure 1B-a). After complete degradation of 4-NP, the solution in the cuvette became colorless (Figure 1B-b), mainly due to the formation of ionic 4-NP.

In Figure 5 A-D, we observed the characteristic peak at 400 nm was gradually decreased and disappeared, and new peaks at approximately 310 nm gradually increased, indicating the generation of p-aminophenol (4-AP). As we know, the work function of Ag was smaller than Cu₂O, so electrons were transferred to Ag core from the Cu₂O shell. There was a large amount of positive charge on the surface of Cu₂O, which attracted 4-NP to the Ag@Cu₂O and accelerated BH₄⁻ catalytic degradation. In addition, rGO had an efficient electron transfer platform, allowing rapid electron transfer and resulting in a high catalytic efficiency.

Figure 5. UV‒Vis absorption spectra of the reduction of 4-NP by the introduction of (A) Ag-rGO (t = 24 min), (B) ACRN-1 (t = 20 min), (C) ACRN-2 (t = 14 min) and (D) ACRN-3 (t = 18 min).

Figure 5. UV‒Vis absorption spectra of the reduction of 4-NP by the introduction of (A) Ag-rGO (t = 24 min), (B) ACRN-1 (t = 20 min), (C) ACRN-2 (t = 14 min) and (D) ACRN-3 (t = 18 min).

The ACRN exhibited porous structures and provided many active sites. During the catalytic reactions, the electrons on the Ag@Cu₂O surface were transferred to BH₄⁻, and 4-NP was reduced to 4-AP. As shown in Figure 5 A-D, the complete degradation of 4-NP by Ag-rGO and three different ACRN occurred in less than 24 min, indicating the excellent performance of the fabricated catalytic materials. Figure 5C indicates that the ACRN-2 exhibited the best catalytic efficiency, and the catalytic time was approximately 14 min. As shown in Figure 6, the absorption logarithm [ln (C_t/C₀)] and time (t) exhibited the linear relationship, which indicated catalytic reduction of 4-NP was related to the ACRN following a first-order kinetic relationship. The catalytic reaction rate constants for 4-NP were 0.16, 0.16, 1.29, and 0.23 min^-1, indicating that ACRN-2 exhibited excellent catalytic activity, as the reaction rate was proportional to the reaction rate constant.

MO and TNP are abundant in soil and water, and the catalytic degradation of MO and TNP in water is required for environmental protection [29,30]. Therefore, the optimal ACRN-2 was utilized for the catalytic degradation of MO and TNP, which are toxic and explosive pollutants, and the catalytic process was observed by UV‒Vis absorption spectroscopy (Figure 7A and 7B). The absorption peaks attributed to MO and TNP at 464 and 390 nm, respectively, gradually decreased, indicating that MO and TNP were catalytically degraded, and the ACRN-2 exhibited higher catalytic efficiency for MO than TNP. Furthermore, the linear relationship between the absorption logarithm [ln (C_t/C₀)] and time (t) in Figure 7C and 7D indicated that the reduction process of MO and TNP was associated with the ACRN-2 and followed a first-order kinetic relationship. The catalytic reaction rate constants were 0.34 and 0.08 min^-1 for MO and TNP, respectively.

The high absorption capacity of the ACRN was due to the abundant active sites on Cu₂O that improve the catalytic performance. This also clearly indicated that the ACRN had excellent catalytic efficiency, trace catalytic activity, and high electron transfer efficiency, so the material exhibited excellent catalytic performance.

Figure 7. UV‒Vis absorption spectra during the catalytic reduction of (A) MO and (B) TNP by introducing the ACRN-2. Logarithm of C_t/C₀ versus catalytic reduction time for (C) MO and (D) TNP.

Figure 7. UV‒Vis absorption spectra during the catalytic reduction of (A) MO and (B) TNP by introducing the ACRN-2. Logarithm of C_t/C₀ versus catalytic reduction time for (C) MO and (D) TNP.

4. Conclusions

To develop low-consumption and environmentally friendly organic pollutant degradation technologies, we designed ACRN for the chemical reduction of potential pollutants. The morphology and properties of the ACRN were analysized by UV‒Vis spectroscopy, TEM, and SEM. To study the catalytic activity of ACRN, we selected 4-NP as a probe molecule and revealed the excellent chemical catalytic activity for the degradation of 4-NP. Notably, among the four catalytic materials with different Cu₂O shell thicknesses, the ACRN-2 exhibited excellent catalytic performance due to its superior electron transfer ability after the introduction of the rGO platform. More importantly, the fabricated ACRN exhibited excellent adsorption properties and significantly improved the degradation rate of 4-NP. Moreover, the ACRN demonstrated the potential for the improvement for reduction of phenolic pollutants. These ACRN have significant potential for applications in catalytic systems for the removal of the harmful substances from water. The graphene-based material in catalytic study is of great significance for the development of nanomaterial science.