Synthesis of copper-silver-zinc oxide nanocomposites for 4-nitrophenol reduction: doping and heterojunction

Abstract

The charge transfer and visible-light absorption capacities of stable materials are crucial in several applications, such as catalysis, absorption, sensors, and bioremediation. Copper-silver-zinc oxide nanocomposites (NCs) were synthesized using PVA as a capping agent and urea as a stabilizing agent. DTG analysis confirmed 500 °C was the optimum temperature for the total decomposition of PVA after capping the nanoparticles (NPs) to yield a pure composite. The XRD analysis showed the presence of copper inclusions in the ZnO lattice and the formation of Ag and CuO heterojunctions with ZnO. The photoluminescence (PL) analysis confirmed the more significant visible light absorption and charge transfer properties of the composite compared to those of single ZnO NPs. Foam-type porosity occurred during gas evolution at many of the points shown in the SEM/TEM images. Slight lattice fringe differences between the composite and ZnO NPs due to copper inclusion were confirmed from the HRTEM image and XRD pattern analysis. The crystallinity of the NPs and NCs was confirmed by the XRD pattern and SAED analysis. The diffusion-controlled charge transfer process was witnessed through CV electrochemical analysis. Thus, the energy- and time-efficient solution combustion synthesis (SCS) approach has a crucial future outlook, specifically for an industrial, scalable application. The NCs demonstrated more potential than ZnO NPs in an organic catalytic reduction reaction of 4-nitrophenol to 4-aminophenol.

Fig. 1. TGA-DTA thermograms: the thermal decomposition behavior of poly(vinyl alcohol)/fuel–zinc nitrate complex; the two decomposition points are due to the surface/crystal adsorbed water molecules and total decomposition of PVA polymer.

Fig. 2. The XRD pattern: (a) the XRD pattern for raw PVA polymer, zinc oxide, silver, copper oxide, and silver–copper–zinc composites, (b and c) the magnified view of 2(a), except the PVA XRD pattern. Herein, the occurrence of independent copper and silver peak on the composite XRD pattern indicates the presence of their independent crystal. The higher 2θ shift for the composite confirm inclusion of copper in the ZnO lattice, (d) the crystal structure of silver, copper oxide, and zinc oxide was developed from the AMCSD, CIF data, using VESTA software.

Fig. 3. PL spectra: (a) the optical properties analysis spectra for zinc oxide and silver–copper–zinc composite, (b and c) the deconvoluted zinc oxide and silver–copper–zinc composite spectra, respectively. The intensity reduction for the composites than ZnO indicates a reduction in the electron–hole recombination process. The deconvoluted spectra for the composite show intensity reduction than zinc oxide (see the ellipse curve), which confirms its greater visible light absorption capacity.

Fig. 4. SEM images: SEM morphological analysis images of (a) zinc oxide and (b) silver–copper–zinc composite. The composite SEM image showed greater porosity, probably the enhanced combustion and gas evolution ability of copper and silver precursors in the presence of fuel/surfactant.

Fig. 5. TEM/HRTEM/SAED images: (a) TEM image, (b) HRTEM image (the inset is the fast Fourier transform pattern), (c) SAED ring (the inset is its XRD pattern) for zinc oxide. Some pores were observed on the TEM image, probably due to gas evolution (see the red arrows). The d-spacing value is corresponding to (002) plane of zinc oxide. The circle and bright spots on the SAED ring indicates the crystal plane and crystallinity of zinc oxide, respectively.

Fig. 6. TEM/HRTEM/SAED images: (a) TEM image, (b) HRTEM image (the inset is the fast Fourier transform pattern), (c) SAED ring (the inset is its XRD pattern) for silver–copper–zinc composite. Pores were observed on the TEM image, probably due to gas evolution (see the red arrows). The d-spacing value here also corresponds to the (002) plane of zinc oxide. The circle and bright spots on the SEAD ring indicate the crystal plane and crystallinity of the silver–copper–zinc composite, respectively.

Fig. 7. The CV voltammogram: the CV analysis for (a) zinc oxide and (b) silver–copper–zinc composite: the inset is peak current versus square root of potential linear plot. No clear redox peak was detected for zinc oxide, and there is a redox peak for the composites, indicating proton intercalation and de-intercalation due to porosity. The fitting of the linear plot indicates diffusion-controlled charge transfer processes.

Fig. 8. 4-NP catalytic reduction plots of (a) ZnO NPs and (b) silver–copper–zinc composite: The NCs show complete reduction of the 4-nitrophenol to the 4-aminophenol within 210 seconds.

Fig. 9. Possible understanding of the conversion of 4-nitrophenol to 4-aminophenol using NaBH₄ and metal NPs as an H₂ source and catalyst, respectively. Reproduced/Adapted from ref. with permission from The Royal Society of Chemistry.