Eco-friendly synthesis of NiO and Ag/NiO nanoparticles: applications in photocatalytic and antibacterial activities

Abstract

Herein, NiO and Ag/NiO NPs were produced via the solution combustion method using nickel nitrate and silver nitrate as oxidizers and Cocos nucifera water as a fuel at 450°C. The study also explores their applications in photocatalytic dye degradation, H₂ production and antibacterial properties. The primary advantage of using C. nucifera water as a green fuel in the solution combustion method is that it serves a dual purpose-both as a fuel and as a solvent. This eliminates the need for additional water to create a homogeneous redox mixture of fuel and oxidant in the experimental procedure. X-ray diffraction confirmed the existence of Ag in the bunsenite form of rhombohedral structure with a simple cubic system, with particles sized at 31-44 nm. Energy-dispersive X-ray spectroscopy revealed Ni, O and Ag weight percentages of 48.2, 44.5 and 7.3%, respectively. X-ray photoelectron spectroscopy confirmed the formation of Ag in NiO nanostructure. UV-visible spectrometry showed reduced band gap energy of Ag/NiO NPs (3.03-2.87 eV) compared to the bare NiO NPs (3.21 eV), red shift of the optical response towards the visible region after doping Ag into the NiO. The 0.3 wt% Ag/NiO NPs showed the highest quantum efficiency (0.781) among the other synthesized NPs. Fourier-transform infrared spectroscopy revealed absorption bands in the range of 460-900 cm^-1 stretching vibrations of Ni-O and Ag-O. Photoluminescence spectroscopy indicated that a doping concentration of 0.3 wt% Ag effectively introduces donor levels, defect levels and surface trap states within the NiO nanocrystalline structure, enhancing charge carrier separation and reducing recombination. Scanning electron microscopy revealed a voluminous, porous surface morphology characterized by numerous voids, resulting from the release of various combustible gases during the combustion process. Transmission electron microscopy images showed that most particles were spherical, irregular in size and well-distributed, with minimal aggregation with an average particle size of 25.8 nm. BET analysis of both NiO and 0.3 wt% Ag/NiO NPs exhibited type IV adsorption isotherms, indicating mesoporous structures and a clear monolayer-multilayer adsorption process, 0.3 wt% Ag/NiO NPs showed the highest surface area (170 m² g^-1) compared to the NiO (130 m² g^-1) NPs. Ag/NiO NPs has demonstrated a promising H₂ evolution rate of 1212 μmol g⁻¹ under visible light illumination in a water/ethanol system. The trypan blue dye degradation reaches up to 98% and has moderate stability for the reusable photocatalysis process. The synthesized NPs exhibited significantly enhanced antibacterial activity against a range of bacterial strains.

Keywords: Ag/NiO NPs; Cocos nucifera; H2 production; NiO NPs; dye; photocatalysis.

Figure 1.

PXRD patterns of (a) undoped NiO NPs, (b) 0.1 wt% Ag/NiO NPs, (c) 0.2 wt% Ag/NiO NPs, (d) 0.3 wt% Ag/NiO NP, (e) 0.4 wt% Ag/NiO NPs and (f) 0.5 wt% Ag/NiO NPs.

Figure 2.

FTIR spectra of (a) undoped NiO NPs, (b) 0.1 wt% Ag/NiO NPs, (c) 0.2 wt% Ag/NiO NPs, (d) 0.3 wt% Ag/NiO NP, (e) 0.4 wt% Ag/NiO NPs and (f) 0.5 wt% Ag/NiO NPs.

Figure 3.

(a) UV–Vis spectra of bulk NiO, NiO and 0.1–0.5 wt% Ag/NiO NPs and (b) Tauc plots of bulk NiO, NiO and 0.1–0.5 wt% Ag/NiO NPs.

Figure 4.

PL spectra of bulk NiO particles, NiO NPs and 0.1–0.5 wt% Ag/NiO NPs.

Figure 5.

Photocatalytic H₂ production using various photocatalysts.

Figure 6.

Photocatalytic dye degradation using various photocatalysts.

Figure 7.

XPS spectra of (a) survey spectrum of 0.3 wt% Ag/NiO NPs, (b) Ni 2p, (c) O 1s and (d) Ag 3d.

Figure 8.

(a) TEM image, (b) HRTEM image (c) SAED pattern and (d) particle size distribution histogram of 0.3 wt% Ag/NiO NPs.

Figure 9.

(a) BET measurements of NiO and 0.3 wt% Ag/NiO NPs. (b) Pore size distributions of NiO and 0.3 wt% Ag/NiO NPs.

Figure 10.

(a) Effect of concentration of dye, (b) effect of catalytic load, (c) effect of pH, (d) effect of different lights, (e) effect of recyclability, (f) scavenger studies and (g) detection of OH radicals of 0.3 wt% Ag/NiO photocatalyst on dye degradation.