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.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-17684-w.

Introduction

Rapid industrialization, population growth and urbanization are the main causes of environmental pollution in the developing countries^1,2. Discharge of industrial effluents such as antibiotics, pesticides, pharmaceuticals³, inorganic salts⁴, synthetic dyes⁵, commercial colorants and oil spills from processing units cause water pollution⁶. Among these pollutants, 15–20% is released to water bodies in form of dyes, that badly affect the aquatic ecosystem^7–9. The random and untreated discharge of these dyes into the environment have a direct impact on human health as well. Therefore, water pollution remediation¹⁰ is of vital concern for the scientific community and had been given significant attention to conserve natural water resources¹¹.

Rhodamine B (RhB) is a basic synthetic cationic dye, commonly used in printing, dying textile, paper, photographic, paints and leather industries^12–14. The potential threats of RhB are nausea, vomiting, respiratory problem and gastritis in human. Due to greater solubility in water and organic solvents, high chemical stability, low biodegradability¹⁵ and high concentration in waste water, the RhB dye removal is a challenging issue¹⁶. During waste water treatment, for the degradation/mineralization of toxic organic dyes like RhB different techniques such as adsorption¹⁷, advance oxidation process (AOP)¹⁸, ultrasonic¹⁹, sonochemical degradation²⁰, membrane filtration²¹, coagulation/flocculation ²², ozonation²³, complexation²⁴, biodegradation²⁵, reverse osmosis²⁶, reduction²⁷, electrolysis²⁸ and impregnation²⁹ are used. However, these methods are nondestructive and produce secondary pollutants which are more toxic than the corresponding primary pollutants³⁰.

Photodegradation process could be considered as an effective method for the complete pollutants mineralization during waste water treatment process^31,32. Photodegradation has advantages over other treatment methods due to its simplicity, low cost, no harmful byproduct formation and ambient pressure and temperature operation³³. In this process, pollutants are decomposed into simple, small and stable molecules such as CO₂, H₂O and inorganic acids³⁴. Many semiconducting photocatalysts were used for the degradation of dyes such as TiO₂³⁵, ZrO₂²⁸, ZnO³⁶, boron carbonitride (BCN)³⁷, NiO-MnO₂³⁸, TiO₂-rGO^39,40, NiO-SiO₂⁴¹, CdS/ZnS⁴², TiO₂–ZrO₂⁴³, Cd(OH)₂/CdS³⁰, PbCrO₄/TiO₂⁴⁴, TiO₂/CoFe₂O₄¹², Au-ZnO⁴⁵, Cu₂O⁴⁶, CoC and CoC/Zn nano-catalysts⁴⁷, GO/ZnO⁴⁸, Ag@Cu/TiO₂⁴⁹ polyacrylamide/gelatin–iron lanthanum oxide P-G-ILO nanohybrid ⁵⁰, carbon-Doped ZnO ⁵¹, Cu–ZnO⁵¹, NiO–reduced graphene oxide (rGO)¹⁰, zinc oxide sheets (ZN–S)⁵², red phosphorus/ZnO nanohybrid (Z-Rp)⁵³, Zinc stannate (ZnSnO₃)@ reduced graphene oxide⁵⁴, S-doped ZnO⁵⁵, La–ZnO⁵⁶, PVDF/Zn0.5Cd0.5S/Ti-NT⁵⁷ and Zn/MWCNTs¹⁷. Zinc oxide has been observed as an eco-friendly photocatalyst⁵³ due to its non-toxicity ⁵⁸, photo-stability ⁵⁹, reusability⁶⁰. Low price, good thermal and chemical stability⁶¹. However, as a photocatalyst, ZnO is associated with some shortcomings as well. A wide band gap of 3.7 eV⁶², high exciton binding-energy (60 meV)⁶⁰, greater electron–hole pair recombination rate, photo-corrosion and photo-instability under UV illumination limit the application of ZnO as a photocatalyst^62–64. To overcome these shortcomings, different approaches were reported in literature such as metal and non-metal doping⁶⁵, co-doping, dye sensitization⁶⁶, coupling with semiconductor and supporting on a medium⁶⁷. Photocatalytic efficacy of ZnO⁴⁷ NPs doped with Gd towards methylene blue dye degradation was found to be superior in comparison to the undoped ZnO NPs⁵⁶. In a study reported by Zhuang et al. the synthesized ZnO/Co₃O₄ heterojunctions have shown much better photocatalytic efficiency as compared to the individual metal oxides⁵⁷. Malik et al. reported Lanthanide ions doped-SnO₂ as an efficient photocatalyst for the decontamination of RhB dye⁵⁸. The current investigation entails the synthesis of doped NiZnO and co-doped CdNiZnO NPs using co-precipitation method, followed by an assessment of their photocatalytic performance in the photodegradation of RhB dye in the presence of UV–visible light irradiation. The doped NiZnO and co-doped CdNiZnO NPs are not reported in the literature as photocatalysts for the degradation of RhB dye to the best of our knowledge. These materials were prepared from cost effective precursors by simple approach and they have shown enhanced photodegradation of the RhB dye in aqueous medium. The RhB is selected as a model dye because it can affect human health and environment significantly due to its carcinogenic and mutagenic nature^59,60,68.

Experimental design

Chemicals and reagents

Sodium hydroxide (NaOH, 98%), and RhB dye (C₂₈H₃₁ClN₂O₃, 99%) were procured from Merck (Germany). Cadmium(II) chloride hemi(pentahydrate) (CdCl₂.2.5H₂O, 98%), nickel(II) chloride hexahydrate (NiCl₂.6H₂O, 98%), and zinc(II) chloride dihydrate (ZnCl₂.2H₂O, 99%) were purchased from Tianjin Chemical reagents company (China). All these chemicals were used as such without any further purification treatment. Deionized water was used as a solvent. Zeta potential measurements were performed using a dynamic light scattering (DLS) instrument (Malvern Zetasizer) to determine the surface charge of nanoparticles dispersed in deionized water at neutral pH.

Synthesis of ZnO nanoparticles

A simple precipitation method was used for the preparation of ZnO nanoparticles (NPs). Zinc(II) chloride dihydrate (0.04 mol, 5.8 g) (0.012 M), was added to 250 mL of deionized water and the mixture was magnetically stirred (1000 rpm) at room temperature for 30 min to get a homogenous solution. The pH (10) of the mixture was adjusted by dropwise (1 drop/sec) addition of 1 M NaOH solution. The precipitates formed during the process were centrifuged at 10,000 rpm, separated and washed with deionized water followed by ethanol washing. These precipitates were oven dried at 80 °C for 5 h and annealed at 500 °C for 4 h to get white powder of ZnO NPs.

Synthesis of doped NiZnO NPs

Co-precipitation method was employed to obtain Ni doped ZnO NPs (NiZnO). About 50 mL aqueous solutions of NiCl₂.6H₂O (0.01 M) and ZnCl₂.2H₂O (1 M) were prepared separately in borosilicate beakers. The two solutions were mixed and then stirred magnetically at room temperature for 150 min. After this, an aqueous solution of NaOH (0.1 M) was added dropwise (1 drop/sec) to the reaction mixture with continuous stirring (1000 rpm) until the pH of the mixture was 10. The resulting white precipitate were allowed to settle down overnight. The precipitates were separated by centrifugation at 10,000 rpm for 10 min, and washed repeatedly with alternating rinses of DI-water and ethanol, for a total of 3–5 cycles. The precipitates were dried overnight at 80 °C in the hot air oven for 5 h and then calcined at 450 °C for 5 h.

Synthesis of co-doped CdNiZnO NPs

Co-precipitation method was employed to obtain cadmium and nickel co-doped ZnO NPs. Aqueous solutions (30 mL each) of cadmium(II) chloride hemi(pentahydrate) (0.01 M), nickel(II) chloride hexahydrate (0.1 M) and zinc chloride dihydrate (1 M) were made separately and then mixed together with continuous magnetic stirring (1000 rpm) at room temperature for 150 min. The pH of the mixture was raised to 10 by drop wise addition of aqueous NaOH solution (0.1 M) with continuous stirring at the rate of 1 drop/sec. The resulting precipitates were allowed to settle down overnight. The precipitates were separated by centrifugation at 10,000 rpm for 10 min, and washed repeatedly with alternating rinses of DI water and ethanol, for a total of 3–5 cycles. The precipitates were dried overnight at 80 °C in the hot air oven for 5 h and then calcined at 450 °C for 5 h to get CdNiZnO NPs.

Radical trapping experiment

The mechanism of RhB degradation by ZnO, doped NiZnO and co-doped CdNiZnO NPs was confirmed by performing trapping experiments. Active radicals responsible for the photocatalytic degradation of RhB was identified by adding three different scavengers such as benzoquinone, para benzoquinone and EDTA. Under the optimized setup conditions, three experimental runs were performed for hydroxyl radical, super oxide radicals, and hole (h⁺). The spectrophotometric analysis has shown the degraded products of each experiment.

Characterization

The prepared photocatalysts were characterized using different analytical techniques. SEM (Model No. 5910, company Japan) and EDX (model INCA 200/oxford instrument, UK) were used to study the morphology and elemental composition, respectively. XRD (Model JOEL-300) was used to study phase purity and crystallite size, while TEM (Tecnia F-20 FEI, USA) determined particle size and shape of the photocatalysts. The Brunauer–Emmett–Teller (BET) was used to analyze the surface area and porosity of the as-synthesized photocatalysts by using nitrogen adsorption desorption instrument (Micrometric ASAP-2020). The photocatalytic degradation of RhB dye was monitored by using UV–visible spectrophotometer (Model UV-1800 SHIMDZU).

Degradation of Rhodamine B dye in UV light

The photocatalytic reactions were carried out in a UV chamber equipped with 15 W lamp (254 nm/365 nm). The aqueous suspension containing 0.2 g sample of ZnO, NiZnO and CdNiZnO in separate 5 mL beakers along with 10 mL of Rhodamine B solution (30 ppm) were first stirred in dark for 1 h to establish the adsorption desorption equilibrium, and then solution mixtures were irradiated with a pre-warmed UV lamp for specific time. The catalysts were removed from solutions via centrifugation (12,000 rpm, 10 min). Small aliquots were withdrawn to monitor the decrease in intensity of the absorption peak (544 nm) in supernatant solution by using UV–visible spectrophotometer. Degradation efficacy was calculated using the following equation.

Initial concentration is shown by C_o while C_t represents concentration after a distinct reaction time under UV-lamp.

Results and discussion

Morphology and elemental analysis

Scanning electron microscopy (SEM) was used to study the morphology of as-synthesized nanomaterials. Figure 1a–i depicts the SEM images of ZnO, NiZnO and CdNiZnO NPs respectively. Figure 1a–c represents pristine ZnO dispersed/agglomerated NPs having cylindrical symmetry with a homogenous growth pattern. Figure 1d–f indicates the external morphology, orientation and crystalline structure of doped NiZnO NPs. The result demonstrated that particles of doped NiZnO NPs are quite uniform in size and very clear with a roughly spherical appearance making a nano flower configuration. The agglomerates are fused showing highly porous phase that enhances the photocatalytic activity of doped NiZnO NPs. Figure 1g–i reveals that the co-doped CdNiZnO NPs are well mixed having irregular shape, and mostly present in aggregated and dispersed orientations.

Fig. 1.

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

The Cd and Ni having solubility limits of 3% are distributed superficially on ZnO matrix having a rough surface in the doped NiZnO and co-doped CdNiZnO NPs. The particle size have crucial role in the aggregation of NPs, smaller the particles size, greater will be the rate of nucleation to decrease the energy of the system. Interestingly, the Ni and Cd co-doped ZnO NPs with average particles diameter of 19.36 nm upon calcination got agglomerated thus creating grain boundary and oxygen vacancy in crystal lattice⁶³. Ni and Cd co-doping effectively enhances the photocatalytic performance of ZnO for RhB dye degradation. This improvement is driven by reduced bandgap energy and suppressed electron–hole recombination. Additionally, Ni and Cd play a critical role in influencing the morphological evolution and particle growth dynamics of ZnO, contributing to improved photocatalytic activity. The study underscores the potential of CdNiZnO as a highly efficient photocatalyst for environmental remediation applications. Cadmium-doped ZnO (CdZnO) NPs have been extensively investigated in previous studies due to their unique optoelectronics⁶⁹ and photocatalytic properties⁷⁰. Incorporation of Cd²⁺ ions into the ZnO lattice is known to narrow the band gap⁷¹, enhance visible-light absorption⁷², and improve charge separation efficiency, thereby promoting higher photocatalytic activity⁷³. Several reports have demonstrated that Cd doping introduces defect states⁷⁴ and modifies the electronic structure of ZnO⁷⁵, which facilitates more effective utilization of solar irradiation and reduces electron–hole recombination⁷⁶. These reports support the interpretation that the enhanced activity observed in our CdNiZnO sample arises from the combined and cooperative effects of Cd and Ni co-doping. The improvements observed from ZnO → NiZnO → CdNiZnO are consistent with the reported individual contributions of Cd and Ni in literature, thereby supporting our conclusion regarding a synergistic enhancement.

SAED patterns of co-doped CdNiZnO NPs are shown in Fig. 2a. The crystallinity, purity, and morphology of co-doped CdNiZnO NPs was further observed at different magnifications using TEM analysis as depicted in Fig. 2b–e. The TEM images reveal that co-doped CdNiZnO NPs are extremely crystalline having microscopic variations in particle size (below 50 nm) and shape. The TEM study strongly supports SEM analysis. It is clearly observed that the Cd and Ni are well dispersed and also intercalated on the surface of ZnO NPs as depicted in Fig. 2f The HRTEM analysis of CdNiZnO nanoparticles confirms their high crystallinity, as evidenced by well-defined lattice fringes and indexed FFT patterns as shown in Fig. 3a. The observed diffraction spots (Fig. 3b–d) correspond to the hexagonal wurtzite ZnO structure, with lattice planes (100, 002, 101, 102, 110) and measured d-spacing’s of 0.28 nm, 0.245 nm, 0.261 nm, and 0.193 nm, indicating phase purity as shown in Table 1. Variations in d-spacing suggest slight lattice distortions due to Ni and Cd co-doping⁷⁷. The inverse FFT (IFFT) (Fig. 3e–h) reconstructions further validate these lattice parameters, reinforcing the structural integrity of the synthesized nanoparticles. These findings confirm the successful synthesis of CdNiZnO with a well-defined crystalline structure⁷⁸, which is essential for its enhanced photocatalytic performance.

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.

Table 1.

Experimental d-Spacing values of CdNiZnO NPs obtained from HRTEM analysis and their comparison with JCPDS Reference Data (Card No. 36-1451).

Plane (hkl)	Measured d-spacing (nm) (HRTEM)	Standard d-spacing (nm) JCPDS 36–1451	Remarks
(100)	0.28	0.281	Matches well with JCPDS
(002)	0.261	0.260	Slight deviation due to doping
(101)	0.245	0.247	Close agreement with JCPDS
(102)	0.193	0.197	Minor shift due to lattice distortion
(110)	0.16	0.161	Matches JCPDS

Raman spectroscopy analysis for phase identification and defect analysis

Raman spectroscopy was performed to further confirm the phase composition and assess defect states in ZnO, Ni-doped ZnO (NiZnO), and Cd-Ni co-doped ZnO (CdNiZnO) NPs, as shown in Fig. 4. The undoped ZnO spectrum exhibits characteristic Raman modes of the wurtzite hexagonal ZnO phase, with the dominant E2 (high) mode at ~ 437 cm⁻¹, which is a fingerprint of ZnO crystallinity. The presence of A1​(TO) and E1(TO) modes further confirms the hexagonal wurtzite structure of ZnO⁷⁹. Upon Ni doping⁸⁰, the E2 (high) mode shifts slightly to a lower wavenumber, indicating lattice distortion due to the substitution of Zn²⁺ by Ni²⁺. Additionally, the appearance of an intense E1​(LO) mode at ~ 580 cm⁻¹ suggests an increase in oxygen vacancies, which act as charge carrier traps and play a crucial role in photocatalytic activity⁸¹. For Cd-Ni co-doped ZnO, further broadening and shifting of the E2 (high) mode are observed, signifying enhanced structural disorder⁸². The pronounced intensity of the LO phonon modes further supports the presence of oxygen vacancies and defect states, which can facilitate improved charge separation in photocatalytic applications⁸². The Raman results, in conjunction with XRD and HRTEM findings, confirm the successful doping of Ni and Cd into the ZnO lattice, leading to modifications in crystal structure and defect concentration⁷⁸. These changes are critical for tuning the electronic and optical properties of the nanoparticles, further validating their photocatalytic efficiency.

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.

Energy dispersive X-ray spectroscopy analysis

The composition of ZnO, NiZnO and CdNiZnO NPs were further characterized by EDX analysis. Well-defined peaks for zinc, oxygen, cadmium and nickel can be seen in Fig. 5a–c. The analysis demonstrates that Ni is present in both doped (NiZnO) and co-doped (CdNiZnO) NPs while Cd in present only in the co-doped CdNiZnO NPs. The percentage composition of all the elements in photocatalysts are mentioned in tabulated form in the EDX spectra that confirm the purity of the synthesized materials. The elemental mapping of constituents elements of co-doped CdNiZnO NPs was further executed that clearly reveals that Ni and Cd are well distributed throughout the surface of ZnO NPs as depicted in Fig. 5d–h.

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.

X-rays diffraction (XRD) analysis

XRD analysis was performed to study the crystalline phase and purity of the materials. The XRD diffraction peaks of ZnO, NiZnO and CdNiZnO are depicted in Fig. 6. All the observed signals correspond to (100), (002), (101), (102), (110), (103), and (200) crystal plane of ZnO⁷⁷ with the JCPDS data (card number 36-1451). All the observed major peaks are well-defined and match to the standard Bragg position of hexagonal wurtzite crystal structure with the space group P63mc. FWHM from XRD analysis shows crystallite growth⁶⁴. Debye Scherrer equation was used to estimate the average crystallite size of neat ZnO NPs (48 nm), doped NiZnO (34 nm) and co-doped CdNiZnO (29 nm) NPs. The results revealed decrease in average crystallite size of co-doped NPs. No clear peaks were observed for Cd and Ni, as these are present in extremely small amount as depicted in EDX analysis. The atomic weights (%) of dopant elements were less than 1%. No peak related to Ni and Cd or the derivatives of Ni is seen in the product, which manifests that successful substitution of Ni²⁺ and Cd²⁺ ions at Zn²⁺ sites in ZnO matrix occurred. The minute quantity of dopant elements (Cd and Ni) cannot be identified due to the limitation of XRD analysis. So the synthesized nanocomposites displayed diffraction peaks similar to neat ZnO NPs. The exact solubility limits were not experimentally determined. However, limits of the Ni and Cd in the ZnO matrix were related to the literature⁶⁵. The small amounts (≈ 3%) of Ni and Cd dopants may enter the ZnO lattice and form substitutional or interstitial sites, which affect the particle growth and morphology⁶⁵.

Fig. 6.

XRD pattern of ZnO, NiZnO and CdNiZnO.

BET analysis

Surface area and optimum porosity have a promising role in the catalytic activity of nanomaterials. Larger the surface area, higher will be the efficiency of photocatalysts due to greater number of active sites available for the photodegradation. To examine the surface area and porosity of doped NiZnO and co-doped CdNiZnO NPs, Brunauer–Emmett–teller (BET) analysis was executed in nitrogen environment. The N₂ adsorption–desorption isotherm inset BJH plot of ZnO, NiZnO and CdNiZnO NPs are shown in the Fig. 7a–c, respectively. The adsorption isotherms of the samples were considered as type-IV according to the IUPAC classification and H3-type hysteresis loops. Sharp increase in the adsorption volume at 0.8 to 1 P/P₀ due to capillary condensation and multi-layer adsorption, indicated the existence of the mesopores host in the as-synthesized photocatalysts. The shifting of hysteresis loop to higher value reveals that microporous phase also exist in the samples. The H3 type hysteresis loop indicates the presence of plate like particles having non rigid aggregate giving rise to silt like pores in the photocatalysts. The results also demonstrated that the surface area of pure ZnO, doped NiZnO and co-doped CdNiZnO is 9.1302, 40.507 and 60.709 m²/g, respectively. The greater surface area of the doped and co-doped NPs is due to the introduction of Cd and Ni ions to the ZnO framework which enhanced the strain of catalysts network. The incorporation of Cd and Ni into the ZnO lattice may lead to the formation of smaller crystallites and increased surface roughness, resulting in a higher surface area. The presence of Cd and Ni dopants can introduce defects and lattice distortions, which may contribute to enhanced porosity and higher adsorption capacity. The pores diameter of the synthesized mesoporous materials is found within the IUPAC recommended range of 2–50 nm. The corresponding values of surface area, particles size, total pore volume and BJH pore width for the synthesized materials are shown in Table 2. Although NiZnO shows a lower total pore volume than pure ZnO, its significantly higher BET surface area (42 m2/g vs. 26 m2/g) results in enhanced dye adsorption, as demonstrated in dark adsorption experiments (Figure S1a–d). The dye uptake for NiZnO reached 50 mg/g compared to 35 mg/g for ZnO, confirming that surface area plays a dominant role in adsorption behavior over pore volume in this system. Moreover, the adsorption kinetic study follows a pseudo-first-order model, indicating that surface adsorption is the primary mechanism, rather than intra particle diffusion. The photocatalytic degradation kinetics (Figure S2a) show a higher pseudo-first-order rate constant for NiZnO as compared to ZnO, reinforcing the conclusion that enhanced performance is primarily driven by increased surface area/surface activity and not by the total pore volume. Langmuir isotherm plot (Figure S2b) further confirms monolayer adsorption and stronger dye affinity for NiZnO, supported by a higher Langmuir constant. Table S1 summarizes the kinetic rate constants and Langmuir isotherm parameters, confirming that NiZnO exhibits superior photocatalytic activity and dye adsorption capacity as compared to pure ZnO.

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.

Table 2.

Textural properties of pristine ZnO, doped NiZnO and co-doped CdNiZnO NPs.

Samples	BET (m2/g)	Pore volume (cm3/g)	Pore size (nm)	BJH average pore width Ads/Des (nm)	Photocatalytic efficacy (%)
ZnO	9.1302	0.028299	12.39819	15.0887	68
NiZnO	40.507	0.0233	15.027	4.0851	89
CdNiZnO	60.709	0.035	19.362	7.2419	98

Photoluminescence (PL) and photoluminescence excitation (PLE) analysis

Photoluminescence (PL) spectroscopy was employed to investigate the charge carrier recombination dynamics of ZnO, Ni-doped ZnO (NiZnO), and CdNi co-doped ZnO (CdNiZnO) NPs. The obtained spectra provide insights into defect states, electronic transitions, and photocatalytic efficiency. Figure 8a–b presents the PL spectra of ZnO, NiZnO, and CdNiZnO at excitation wavelengths of 254 nm and 365 nm, respectively. ZnO exhibits the highest PL intensity, indicating rapid electron–hole recombination. Ni doping reduces the PL intensity, suggesting improved charge separation due to the introduction of defect states. The lowest PL intensity is observed for CdNiZnO, signifying effective suppression of charge carrier recombination. The diminished emission intensity confirms that Cd and Ni co-doping enhances the formation of charge trapping centers, thereby facilitating better charge transfer, which is beneficial for photocatalysis. The photoluminescence excitation (PLE) spectra, recorded at 418 nm (Fig. 8c) and 520 nm (Fig. 8d), reveal the excitation-dependent absorption characteristics of the synthesized nanoparticles. The peak intensities follow the order: ZnO > NiZnO > CdNiZnO, aligning with the trend observed in PL spectra. The redshift in excitation peaks and reduced intensity in CdNiZnO confirm the modification of energy levels due to co-doping, leading to increased charge carrier lifetime. This further supports the hypothesis that co-doping induces defect states, which act as charge carrier trapping sites, reducing radiative recombination and improving photocatalytic activity.

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.

Implications for photocatalytic performance

The lower PL intensity of CdNiZnO indicates suppressed electron–hole recombination, making it the most promising photocatalyst among the studied materials. The introduction of Cd and Ni into the ZnO lattice creates defect sites, which contribute to enhanced charge separation and reduced non-radiative recombination pathways. This behavior is crucial for photocatalytic applications, as it leads to prolonged charge carrier lifetimes and improved photocatalytic degradation efficiency.

UV–Visible and band gap analysis

The optical band gap and absorption wavelength of ZnO, NiZnO and CdNiZnO were determined by using UV–visible spectroscopy. Figure 9 displays the UV–visible absorption spectra of ZnO, NiZnO and CdNiZnO NPs. The strong absorption band at 372 nm is attributed to ZnO. The maximum absorbance wavelength for NiZnO and CdNiZnO represent slight red shifts which make the doped and co-doped NPs better photocatalysts compared to ZnO NPs. The coupling of minute quantity of Ni and Cd to the ZnO enhanced the visible light absorption and extended the absorption peaks of doped NiZnO (418 nm) and co-doped CdNiZnO (438 nm) NPs. It means that heterojunction formation in NiZnO and CdNiZnO NPs increased the absorption in the UV–visible region.

Fig. 9.

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

Tauc’s plots for assessing the band gaps of ZnO, NiZnO, and CdNiZnO are shown in Fig. 10a–c. Following equation is used to determine the band gap energy of the photocatalysts.

where α is the absorption coefficient, h is plank’s constant and υ in the frequency of photon. A is constant of proportionality and E_g represents band gap energy and n is an electronic transition which may be 2 or 1/2 for direct allowed and indirect allowed transitions, respectively. The band gap energy calculated for ZnO, NiZnO and CdNiZnO are 3.1, 2.62, and 2.33 eV, respectively. The comparative band gap energies for the synthesized materials are shown in Fig. 10d. The improved photocatalytic activity of doped NiZnO and co-doped CdNiZnO NPs is due to the incorporation of Ni²⁺ and Cd²⁺ ions into the ZnO lattice, which leads to the formation of heterojunction thus reducing the band gap energy of NiZnO and CdNiZnO NPs. This shift in band gap energy is due to quantum non-confinement effect⁶³, and electronic structure modification⁵² of NPs. The observed reduction in the optical band gap is attributed not to quantum confinement effects given the particle size remains well above the Bohr excitons radius of ZnO but rather to dopant‐induced modification of the electronic structure and the formation of defect states within the ZnO lattice. Co-doping with Cd²⁺ and Ni²⁺ introduces localized donor and acceptor levels inside the forbidden gap due to their different ionic radii and electronic configurations compared to Zn²⁺, leading to perturbation of the periodic potential and enhanced hybridization between the dopant orbitals and the host lattice states. This interpretation is strongly supported by our experimental data: Photoluminescence (PL) spectra exhibit pronounced quenching of the near-band-edge (NBE) emission along with the emergence and intensification of deep-level emissions (DLEs), signifying an increased density of non-radiative recombination centers and dopant-related trap states, which effectively suppress electron–hole pair recombination. Furthermore, Raman spectra show systematic broadening and frequency shifts of the characteristic E₂ (high) mode, consistent with lattice strain, local symmetry breaking, and increased phonon scattering arising from substitutional incorporation of Cd²⁺ and Ni²⁺. Taken together, these converging lines of evidence confirm that the apparent narrowing of the band gap originates from dopant-induced band tailing and defect-mediated band structure modification, rather than from size-quantization effects. The substitution of Zn²⁺ with larger Cd²⁺ and smaller Ni²⁺ ions introduces significant lattice strain and modifies the crystal field, shifting the conduction and valence band edges. Such co-doping also alters the carrier concentration; the interplay of donor- and acceptor-type defects can counteract the typical Burstein–Moss band-filling effect, leading instead to an effective redshift. Furthermore, dopant incorporation enhances the formation of oxygen vacancies and zinc interstitials, which generate shallow donor levels and tail states within the forbidden gap.

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.

Photocatalytic degradation of Rhodamine B (RhB) dye

To investigate the photocatalytic activity of ZnO, NiZnO and CdNiZnO samples, RhB is considered as a model pollutant. The effect of different parameters such as irradiation time, catalysts amount, and pH was investigated.

Impact of irradiation time on photodegradation of RhB

The change in absorption value of the dye solutions before and after UV–visible light irradiation (intensity 254 and 365 nm) for different time intervals in aqueous medium was measured by UV–visible spectroscopy and the data obtained is shown in Fig. 11a–c. The maximum absorption peak in the absence of photocatalysts at 544 nm is attributed to the chromophore present in RhB. A steady reduction in the intensity of absorption peak at 544 nm in the presence of photocatalysts is due to photocatalytic oxidation of chromophore present in the dye. The photodegradation of RhB effectively increased with irradiation time. Figure 11d displays the %degradation of RhB dye versus change in irradiation time. The neat ZnO showed the lowest photocatalytic efficiency, followed by the doped NiZnO, while the co-doped CdNiZnO exhibited the highest efficiency under the same experimental conditions. The %degradation results revealed that ZnO, NiZnO, and CdNiZnO degraded 25%, 45%, and 84%, respectively, within 10 min. The degradation rate effectively increased to 68%, 89%, and 98% for ZnO, NiZnO, and CdNiZnO, respectively, after 50 min of irradiation under UV–visible light in an aqueous medium. The enhanced photocatalytic activity of co-doped CdNiZnO is attributed to the coupling of Ni and Cd with ZnO that inhibits the faster recombination rate of separated charges and presence of more active sites duet to its largest surface area and high dispersion in dye solution.

Fig. 11.

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

Pseudo first order kinetics

The kinetic study of photocatalytic degradation of RhB dye under UV-light was performed several times with same experimental conditions for ZnO, NiZnO and CdNiZnO photocatalysts using Langmuir–Hinshelwood model⁶⁷. Following equation was used to investigating the order of reaction.

where C is the concentration of RhB after time t under UV light irradiation and C_o is the initial concentration before the reaction, contact time is denoted with t (min) and K_app is rate constant for the photodegradation of RhB. The results demonstrate linear relationship between ln C/C₀ versus time as shown in Fig. 12. The apparent rate constant can be calculated from the slope of the plot. It is investigated that even at a higher concentration of the dye, a straight line graph is obtained. It is examined that the photodegradation of RhB compiled with the pseudo-first order kinetic model. Our results are in close agreement with an earlier reported pseudo-first order kinetic model⁶⁸ as depicted in Table 3. The rate constant K_app decreased from 0.0185 to 0.0052 and 0.00112 min⁻¹ as the concentration of RhB was increased from 20 mg/L to 30 mg/L and 50 mg/L, respectively. When the concentration of the dye is high, UV light penetration and generation of the active species decrease that make the degradation process less efficient The degradation of RhB over ZnO, NiZnO, and CdNiZnO followed pseudo-first-order kinetics, and the quality of fit was assessed using the coefficient of determination (R²), root mean square error (RMSE), and Akaike Information Criterion (AIC) metrics. The statistical metrics (R² = 1.000, RMSE = 6.41 × 10⁻¹⁷, AIC = –21.72) validate the pseudo-first-order kinetic model applied to the RhB degradation using CdNiZnO (Table S2). The residual data (Table S3, Figure S3) present the residual plot for the pseudo-first-order kinetic modeling of RhB degradation. The random scatter of residuals around zero confirms the validity of the pseudo-first-order kinetic model for RhB degradation. The plot complements the statistical parameters (R², RMSE, AIC) and reinforces the reliability of the model used in this study.

Fig. 12.

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

Table 3.

Comparison of kinetic models for the photocatalytic degradation of RhB dye using ZnO, NiZnO, and CdNiZnO Photocatalysts.

Catalysts	Kinetic model	Rate constant (k) min−1	Regression coefficient (R2)	Best fit model	References
ZnO	Pseudo-first-order	0.00112	0.98907	Best fit	69
	Pseudo-second-order			Not fit	70
	Langmuir–Hinshelwood	0.0225	0.98589	Best fit	71
NiZnO	Pseudo-first-order	0.0052	0.99366	Best fit	72
	Pseudo-second-order			Not fit	73
	Langmuir–Hinshelwood	0.0205	0.99366	Best fit	74
CdNiZnO	Pseudo-first-order	0.0185	0.98589	Best fit	75
	Pseudo-second-order			Not fit	76
	Langmuir–Hinshelwood	0.0214	0.99366	Best fit	77

Reaction mechanism of photocatalytic degradation of RhB

It is evident that Cd-Ni co-doping almost has no significant influence on the morphology and crystal structure of the samples (as seen in TEM and XRD results). Although Cd-Ni co-doping influences the specific surface area (BET) and crystal size of the samples to a certain extent, yet photocatalytic activity of the catalysts does not follow the trends in the variations of crystal size and BET, completely. In order to explain the changes in photocatalytic activity under UV–visible light irradiation, the excitation and transfer process of charges between (Cd-Ni) and ZnO NPs is schematically illustrated in Fig. 13. When Cd and Ni species are implanted in to the ZnO NPs, an interaction between dye molecules and doped ZnO surface may takes place which serve a photodynamic mechanism of photo-oxidation. Therefore, it is believed that presence of Ni²⁺/Cd²⁺ results in an increased electron capture capacity for the ZnO NPs. Initially, under the influence of UV–visible light, the surface dye molecules may absorb the energy, and then introduce electrons into the conduction bands of ZnO NPs. The surface photocurrent results show the ZnO cannot be excited by visible light. Therefore, self–sensitive decomposition of dye molecules is responsible for the photocatalytic activity of pure ZnO NPs. The catalytic degradation of RhB takes place in several steps under UV–visible light irradiation. When photons of specific frequency equivalent or greater than the band gap energy of the photocatalyst are absorbed, electrons are excited from the valance band to the conduction band, leaving holes in the valance band. The photocatalysts (ZnO, NiZnO and CdNiZnO) generate electron–hole pairs under UV–visible light illumination. The electrons react with oxygen⁶⁹ on the surface of photocatalysts to generate superoxide radical (·O²⁻)⁷⁰. While the hole interact with H₂O/H₂O₂ to generate hydroxyl radicals (·OH) in the medium⁷¹. Both superoxide (·O²) and hydroxyl (·OH) radicals⁷² degrade the RhB dye into simple and small molecules such as CO₂ and H₂O. The lower photocatalytic activity of neat ZnO is due to the faster recombination rate of electron hole pairs⁷³. While in case of doped NiZnO and co-doped CdNiZnO NPs, the electrons present in the valance band are captured by coupled Ni and Cd. This electron capture significantly reduces the electron–hole pair recombination rate⁷³ and as a result the photocatalytic efficiency of the dope and co-doped NPs for the degradation of RhB dye increases. This is consistent with the production mechanism of O²⁻ via O₂, as previously reported⁷⁴. Figure 13 shows proposed mechanism of photocatalytic degradation of RhB. In the process of photocatalytic oxidation, significant contributions are made by various key oxidative species, namely superoxide radicals (·O²⁻), holes (h⁺), and hydroxyl radicals (·OH). To gain deeper insights into the fundamental photocatalytic mechanism, extensive investigations were performed to identify the active species involved. To accomplish this, a series of experiments were conducted to scavenge and capture the specific species. Specifically, benzonquinone (BQ) [C₆H₄O₂, (97%)], SigmaAldrich, Germany, p-benzonquinone (p-BQ) [C₆H₄(=O)₂, (98%)], SigmaAldrich, Germany, and disodiumethylenediaminetetraacetate [EDTA-2NaC₁₀H₁₄N₂Na₂O₈.2H₂O, (97%)], Sigma-Aldrich, Germany, were introduced into the RhB dye solution. These substances were utilized to selectively trap and identify the superoxide radicals (·O²⁻), holes (h⁺), and hydroxyl radicals (·OH), respectively. The degradation efficiency of RhB on pure ZnO, doped and co-doped NiZnO, CdNiZnO NPs significantly decreased upon the addition of p-BQ/ or BQ to the photocatalytic reaction solution. These data provided confirmation that the superoxide radicals (·O²⁻) and holes (h⁺) indeed exerted a key influence in RhB’s photocatalytic degradation. Additionally, the inclusion of EDTA in the reaction solution inhibit the degradation efficiency of RhB indicating that the hydroxyl radicals (·OH) were the primary reactive species involved within the process to degrade the RhB dye. The free radicals trapping tests suggest that the h⁺ shows strong response for the reaction process of photoelectrocatalytic RhB degradation. Radical trapping experiments demonstrated the importance of ROS in photocatalysis. Table 4 revealed that the major contributing radical for photo-degradation of organic pollutants is hydroxyl radical (·OH) as shown in Fig. 14a. Further confirmation of dye mineralization was obtained through post-photocatalysis FTIR analysis of the RhB solution, as shown in Figure S4. The FTIR spectrum of non-degraded RhB dye shows bands for C=O, C=C (aromatic), and C–N stretching vibrations. These bands disappear in the spectrum after degradation, confirming the effective mineralization of the dye into simple inorganic molecules such as CO₂ and H₂O.

Fig. 13.

Proposed mechanism of photodegradation of RhB dye by CdNiZnO NPs.

Table 4.

Radical trapping experiment.

S. No	Radicals	Scavengers	Photo-degradation
1	Hydroxyl	Benzoquinone [BQ]	Decreased
2	Superoxide	p-Benzoquinone [p-BQ]	Decreased
3	Hole (h+)	Ethylenediaminetetraacetic acid [EDTA]	Inhibited

Fig. 14.

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

Fluorescence-based ROS assays

The fluorescence spectra in Fig. 14b reveal that the generation of reactive oxygen species (ROS) progressively increases with irradiation time for all photocatalysts, confirming their ability to produce oxidative species under UV light excitation. Among the samples, CdNiZnO exhibits the highest fluorescence intensity, followed by NiZnO and ZnO, indicating that co-doping with Cd and Ni significantly enhances the separation of photo-generated charges and promotes the formation of abundant ROS such as ·OH and O₂^·−. This superior ROS production by CdNiZnO can be attributed to the synergistic effect of dopants, which introduce additional defect sites and extend light absorption, thereby facilitating more efficient electron–hole pair utilization. The observed trend directly correlates with improved photocatalytic activity, as a higher ROS yield is critical for accelerating the degradation of pollutants.

Impact of photocatalysts dosage on photodegradation of RhB dye

The rate of degradation of RhB dye is considerably altered by changing the amount of a catalyst. Increasing the photocatalyst dosage will significantly enhance the photodegradation of RhB due to maximum surface-active sites available for degradation. The RhB dye is first adsorbed on the active sites of the photocatalyst followed by degradation. Keeping the irradiation time (20 min) and dye amount (10 mL, 30 ppm) constant, different amounts of photocatalysts (0.005, 0.010, 0.015, 0.020, 0.025 and 0.030 g) were taken in separate beakers to evaluate the impact of catalysts dosage on the rate of degradation of dye using VU-visible spectroscopy. The data obtained is presented in Fig. 15a–c for ZnO, NiZnO and CdNiZnO, respectively. The results demonstrate that enhancing the catalysts dosage, significantly rise the photodegradation of RhB dye in aqueous medium. Figure 15d represents the %degradation of RhB dye photodegraded by different amounts of ZnO, NiZnO and CdNiZnO. The findings reveal that maximum photodegradation of RhB dye occurred at 0.030 g of the catalysts. The degradation efficiency of 0.030 g of ZnO, NiZnO and CdNiZnO was 64, 86 and 98.5%, respectively. Further increasing the catalysts amount beyond the optimum limit (0.030 g) had no significant effects on the photodegradation of RhB dye. At higher dosage, the agglomeration of photocatalyst particles and interception of light by suspension reduce the active surface area of photocatalyst and amount of light and hence the degradation efficiency decreases⁴⁶.

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.

Impact of pH on photodegradation of RhB dye

The photocatalytic activity is greatly affected by pH of the medium. It is important parameter because various industries discharge effluents into water bodies at different pH levels. Keeping the irradiation time (20 min), catalysts dosage (0.030 g) and concentration of RhB dye (10 mL, 30 ppm) constant, the effect of pH variations (1, 3, 6, 9 and 12) on photodegradation of RhB was evaluated and the results are shown in Fig. 16a–c. The point of zero charge (PZC) for pure ZnO NPs is 8.5 to 9.5. The surfaces of ZnO are positively charged at pH < PZC and negatively charged at pH > PZC. Ni²⁺ doping introduces defect sites, oxygen vacancies, and changes in surface hydroxylation thus enhancing surface acidity of the doped NiZnO NPs having PZC value of 7.5 to 8.5. Similarly, in co-doped CdNiZnO NPs, combination of Cd and Ni create a surface with increased defect sites, which shift the PZC to a more acidic range (6.5 to 7.5).

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.

The degradation of RhB was low in acidic media as at pH 1, 19, 21 and 61% of the dye was degraded by ZnO, NiZnO and CdNiZnO, respectively. However, the photodegradation efficacy in basic media become quite high and at pH 12 about 63, 86 and 98% of RhB dye was photodegraded by ZnO, NiZnO and CdNiZnO, respectively. The %degradation of RhB dye at varying pH by the synthesized photocatalysts are compared in Fig. 16d. The enhanced photodegradation in basic media might be due to the formation of greater hydroxyl (•OH) and superoxide (·O²⁻) radicals which are responsible for the photodegradation of RhB dye.

To further clarify the effect of surface charge on pH-dependent photocatalytic performance, zeta potential measurements were carried out for ZnO, NiZnO, and CdNiZnO NPs (Figure S5). All samples exhibited positive surface charges, indicating the presence of protonated hydroxyl groups on the particle surfaces in aqueous media, with values of approximately + 8 mV (ZnO), + 10 mV (NiZnO), and + 15 mV (CdNiZnO). The higher positive charge observed for CdNiZnO suggests stronger electrostatic attraction toward anionic dye molecules, promoting their adsorption on the catalyst surface and facilitating subsequent photodegradation. This confirms that the enhanced degradation efficiency at specific pH values arises from a combination of favorable electrostatic interactions and improved photocatalytic activity, rather than from adsorption alone.

Recycling and stability of photocatalysts

The catalysts were recycled from the reaction mixture separately, undergoing multiple cleaning cycles with water and ethanol before drying to assess stability and cost-effectiveness. Subsequently, the reused catalysts were employed in successive degradation reactions. In Fig. 17a–d, the degradation efficiency of the catalysts remained consistent for up to five cycles. This behavior shows the effectiveness of the synthesized NPs as photocatalysts in the photodegradation of RhB dye in the present study. A minimal drop in efficiency indicates good reusability. Prolonged exposure to UV or visible light can lead to photo-corrosion, particularly in ZnO-based systems, resulting in the loss of lattice oxygen and catalyst degradation. A comparison between the present study and previously reported results on photocatalytic degradation of RhB and some other dyes is presented in Table 5. To evaluate the stability and reusability of the photocatalyst, FTIR analysis was conducted after five consecutive photocatalytic cycles. The spectra (Figure S6) show no significant structural changes, indicating excellent stability and resistance to photo-corrosion under UV light. The degradation efficiency remained nearly consistent across all cycles, indicating good recyclability. The retention of key vibrational bands confirmed that the crystal structure and functional groups of the photocatalysts remained largely unchanged, further validating their structural stability. These results confirm that the synthesized materials maintain both photocatalytic activity and structural integrity upon reuse, supporting their practical applicability in environmental remediation. Error bars derived from three independent trials were included to assess the statistical reliability of the observed trends as depicted in Figure S7. All catalysts retained significant activity, with CdNiZnO showing the highest stability and minimal decline in efficiency. These observations support the claim of sustained catalytic performance over repeated use.

Fig. 17.

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

Table 5.

Comparison of the current study with previously reported NPs for their photocatalytic dyes degradation.

Photocatalysts	Light source	Method	Dye	Degradation	References
Ni/ZnO	Visible	Impregnation	Malachite green	99% for 40 min	75
Mg–ZnO	Sun	Co-precipitation	Methylene blue	97% for 100 min	76
Ag–ZnO	UV–VIS	Co-precipitation	Methyl orange	96% for 90 min	77
TiO2/ZnO	Electrochemical	Sol–gel method	Crystal violet	85% for 6.5 h	78
Ni doped ZnO	Sun	Sol–gel method	Methyl orange	94% for 80 min	79
ZnO Al + Mn	UV	Co-precipitations	Congo red	97% for 120 min	80
Zr-ZnO	UV	Sol–gel method	Reactive yellow 105	92% for 60 min	81
Sr-ZnO	UV	Hydrothermal	RhB	98% for 90 min	82
NiZnO	UV–VIS	Co-precipitation	RhB	86% for 50 min	Present study
CdNiZnO	UV–VIS	Co-precipitation	RhB	98% for 50 min

Comparison of photocatalysts

Compared to the neat ZnO and doped NiZnO NPs, co-doped CdNiZnO was found the most efficient photocatalyst in the photodegradation of RhB dye as shown in Fig. 17d. The higher efficiency of co-doped CdNiZnO NPs is due to the coupling of Cd and Ni on ZnO matrix. This coupling reduces the band gap energy and slow down the recombination rate of electron–hole pairs in co-doped CdNiZnO NPs. So, the photocatalytic efficiency of CdNiZnO NPs significantly increases.

Figure 18 illustrates the photocatalytic degradation efficiency of ZnO, NiZnO, and CdNiZnO NPs under different pollutant loads (RhB concentrations of 50 ppm, 100 ppm, and 150 ppm. The efficiency of ZnO NPs decreases significantly from 68% at 50 ppm to 20% at 150 ppm, indicating that ZnO is less effective at higher dye concentrations due to increased adsorption competition and light scattering effect. A relatively higher and more stable photocatalytic performance is observed for doped NiZnO, with efficiency dropping from 85% (50 ppm) to 65% (150 ppm). This suggests that Ni doping enhances charge separation and catalytic activity. The co-doped CdNiZnO exhibits the highest photocatalytic efficiency, with 98% degradation at 50 ppm and still maintaining 80% at 150 ppm. This superior performance may be attributed to the synergistic effects of Cd and Ni doping, which improve light absorption and charge carrier dynamics. Table 6 presents the photocatalytic efficiency of ZnO, NiZnO, and CdNiZnO NPs under varying RhB dye concentrations, demonstrating their performance at different pollutant loads.

Fig. 18.

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

Table 6.

Different pollutant load vs photocatalytic degradations efficiency.

Photocatalysts	Concentration of RhB dye
	50 ppm	100 ppm	150 ppm
ZnO	68	45	20
NiZnO	85	74	65
CdNiZnO	98	87	80

Conclusion

This study employed co-precipitation method to synthesize doped NiZnO NPs and co-doped CdNiZnO NPs. Band edge potential calculations for NiZnO and CdNiZnO suggested significant photocatalytic efficiency in RhB (30 mgL⁻¹) dye degradation with irradiation time of 50 min for the doped and co-doped NPs. CdNiZnO NPs displayed remarkable efficiency, achieving 98% degradation over 50 min, surpassing NiZnO NPs (89%) and ZnO NPs (68%) with a calculated reaction rate of 4.632 × 10⁻⁴ s⁻¹. The catalysts exhibited good recyclability, maintaining activity for up to 5 cycles, showing no phase change even after the 5th cycle. In particular, co-doped CdNiZnO NPs could be considered as effective photocatalyst for the photodegradation of harmful organic pollutants like RhB dye.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

All authors contributed to the study; Conceptualization, methodology, experimental work, writing original draft and characterization were performed by Shakeel khan, Dae-Sung Kim, Mahboob Ullah and Momin Khan. Investigation and supervision was made by Niaz Muhammad and Muhammad Sadiq. Funding acquisition, writing review and editing was made by Awal Noor and Sadaf Qayyum. All authors have read and agree to the submitted version of the manuscript.

Data availability

All the data generated during this study is provided in the manuscript.

Declarations

Competing interests

The authors declare no competing interests.