Photocatalytic degradation of Rhodamine B dye over Ni-Cd doped and co-doped ZnO nanoparticles

Abstract

This study demonstrates the photocatalytic degradation efficiency of doped NiZnO and co-doped CdNiZnO NPs. Initially, ZnO NPs with a unique mesoporous ellipsoidal morphology were synthesized by simple precipitation and calcination. Powder X-ray diffraction revealed the formation of a hexagonal phase of the wurtzite structure. The average crystallite size of pristine ZnO NPs is 48 nm. The NPs possess higher thermal stability with the surface area, pore volume, and pore size of 9.1302 m²/g, 0.028299 cm³/g, and 12.39819 nm, respectively. Furthermore, different mesoporous doped NiZnO and co-doped CdNiZnO NPs in the range of 34 and 29 nm were synthesized by co-precipitation method and characterized by XRD, EDX, SEM, TEM, PL, Raman, BET analyses and UV-Vis spectroscopy. The calculated optical bandgaps for pure ZnO, doped NiZnO and co-doped CdNiZnO NPs were found to be 3.1, 2.62 and 2.33 eV, respectively. The photocatalytic activity of co-doped CdNiZnO was significantly higher than that of pure ZnO. After 50 min of irradiation, approximately 98% of rhodamine B was degraded by CdNiZnO, compared to 65% with pure ZnO. This enhancement is attributed to the synergistic effects of Ni and Cd, which trap electrons and holes, reducing recombination and extending charge carrier lifetimes. The photocatalytic efficiency of the synthesized materials to decompose the RhB dye (30 mgL^-1) in aqueous media was tested via Langmuir-Hinshelwood model under UV-visible light. The degradation process followed pseudo-first order kinetic model. The synthesized NPs were re-used for five cycles without any significant decrease in the photodegradation ability. The mechanistic concept of generating reactive oxygen species by electron and hole charge (e‾/h⁺ ) carriers seems to be responsible for the photocatalytic degradation of the dye by CdNiZnO NPs. Zeta potential analysis revealed positive surface charges for all catalysts, with co-doping significantly increasing charge and colloidal stability, thereby supporting the pH-dependent photocatalytic performance.

Keywords: CdNiZnO; Co-precipitation; Photocatalysts; Pseudo first order kinetics; Rhodamine B.

Fig. 1

SEM images of (a–c) ZnO NPs, (d–f) doped NiZnO and (g–i) co-doped CdNiZnO NPs.

Fig. 2

(a) SAED patterns of co-doped CdNiZnO (b–e) TEM images of co-doped CdNiZnO NPs at different magnifications (f) HRTEM image of co-doped CdNiZnO.

Fig. 3

(a) HRTEM image of co-doped CdNiZnO (b) FFT pattern (c, d) lattice fringe measurements using HRTEM (e–h) IFFT analysis of selected regions, confirming the presence of distinct lattice fringes and d-spacing’s supporting the high crystallinity and successful co-doping in CdNiZnO NPs.

Fig. 4

Raman spectra of ZnO, NiZnO, and CdNiZnO NPs confirming the wurtzite phase. The peak shifts and broadening in doped samples indicate lattice distortion and defects. Co-doping with Ni and Cd creates a synergistic effect by enhancing charge separation and reducing recombination, leading to improved photodegradation performance of CdNiZnO.

Fig. 5

EDX spectra of (a) ZnO NPs (b) doped NiZnO (c) co-doped CdNiZnO NPs and (d–h) mapping of the co-doped CdNiZnO NPs.

Fig. 6

XRD pattern of ZnO, NiZnO and CdNiZnO.

Fig. 7

(a) BET and N₂ adsorption desorption isotherm of ZnO NPs and BJH pore size distribution curve (inset) of pure ZnO NPs. (b) BET and N₂ adsorption desorption isotherm of NiZnO and BJH pore size distribution curve (inset) of NiZnO (c) BET and BJH plots for CdNiZnO NPs.

Fig. 8

Photoluminescence (PL) spectra of photocatalysts at (a) 254 nm excitation (b) 365 nm excitation and Photoluminescence Excitation (PLE) spectra at (c) 418 nm and (d) 520 nm.

Fig. 9

UV–Visible absorption spectra of pure ZnO, doped NiZnO and co-doped CdNiZnO NPs.

Fig. 10

Tauc plot: (αhυ)² versus energy for the (a) ZnO (b) NiZnO (c) CdNiZnO (d) optical band gap variation of ZnO, NiZnO and CdNiZnO (inset); comparison of band gap energy.

Fig. 11

UV–Visible absorption spectra of RhB dye photodegraded by (a) ZnO (b) NiZnO (c) CdNiZnO (d) comparison of %degradation of RhB dye.

Fig. 12

Kinetic model for the photodegradation of RhB under UV–visible light irradiation.

Fig. 13

Proposed mechanism of photodegradation of RhB dye by CdNiZnO NPs.

Fig. 14

(a) Scavenging experiment for the RhB degradation upon UV–visible light irradiation (b) Fluorescence-based ROS assays of photocatalysts.

Fig. 15

UV–Visible absorption spectra of RhB dye photodegraded at different dosage of (a) ZnO (b) NiZnO (c) CdNiZnO (d) comparison of %degradation of RhB dye by different dosage of photocatalysts.

Fig. 16

UV–Visible absorption spectra of RhB dye photodegraded in different pH media by (a) ZnO (b) NiZnO (c) CdNiZnO (d) Comparison of %degradation of RhB dye photodegraded at different pH in the presence of photocatalysts.

Fig. 17

%degradation efficiency of (a) ZnO (b) NiZnO (c) CdNiZnO for five consecutive cycles (d) Degradation efficiency of ZnO, NiZnO and CdNiZnO photocatalysts.

Fig. 18

Photocatalytic efficiency of ZnO, NiZnO, and CdNiZnO NPs at different RhB concentrations.