Fabrication of a reusable carbon quantum dots (CQDs) modified nanocomposite with enhanced visible light photocatalytic activity

Abstract

In awareness of industrial dye wastewater, carbon quantum dots (CQDs) and cobalt zinc ferrite (CZF) nanocomposites were synthesised for the making of carbon quantum dots coated cobalt zinc ferrite (CZF@CQDs) nanophotocatalyst using oxidative polymerization reaction. The results of TEM, zeta potential value, and FTIR confirm highly dispersed 1–4 nm particles with the − 45.7 mV carboxylic functionalized surface of CQDs. The results of the synthesised CZF@CQDs photocatalyst showed an average particle size of ~ 15 nm according to TEM, SEM, and XRD. The photocatalyst showed a 1.20 eV band gap, which followed the perfect visible light irradiation. TGA and DTA revealed the good thermal stability of the nanophotocatalyst. VSM was carried out, and the saturation magnetisations for CZF and CZF@CQDs were 42.44 and 36.14 emu/g, respectively. A multipoint study determined the BET-specific surface area of the CZF@CQDs photocatalyst to be 149.87 m²/g. Under visible light irradiation, the final CZF@CQDs nanophotocatalyst demonstrated remarkable efficiency (~ 95% within 25 min) in the photocatalytic destruction of Reactive Blue 222 (RB 222) and Reactive Yellow 145 (RY 145) dyes, as well as mechanical stability and recyclability. Even after the recycling of the degradation study, the nanophotocatalyst efficiency (~ 82%, 7th cycles) was predominantly maintained. The effects of several parameters were also investigated, including initial dye concentration, nanophotocatalyst concentration, CQD content, initial pH of the dye solution, and reaction kinetics. Degradation study data follow the first-order reaction rate (R² > 0.93). Finally, a simple and low-cost synthesis approach, rapid degradation, and outstanding stability of the CQD-coated CZF nanophotocatalyst should make it a potential photocatalyst for dye wastewater treatment.

Introduction

Water pollutants, such as contamination from domestic residues, industrial chemicals, fertilisers, and dyes, are among the most significant environmental issues, and pollution treatment is a global and national priority¹. Each year, several substances with industrial and consumer uses are released into natural streams. Although many of these contaminants are highly toxic and can lead to major illnesses in people, they are nevertheless present in low amounts². Many of these organic contaminants are harmful and cannot be biologically purified³, while others may be biologically purified in the natural aquatic environment⁴. These pollutants have a long environmental shelf life and can harm humans and ecosystems. A million metric tonnes or more of dye are reportedly produced globally each year and used in the textile, leather, paper, cosmetic, and pharmaceutical industries. Due to its utilisation, significant amounts of coloured effluent are generated during industrial processes⁵. The coloured effluents of azo dyes are discharged worldwide in the paper, textile, and dye industries. Examples include reactive blue 222 (RB 222) and reactive yellow 145 (RY 145)⁶. In recent years, the removal of these organic dye pollutants from water and wastewater has been studied using a variety of strategies, such as chemical oxidation, biological treatment, coagulation and flocculation, photocatalytic degradation, membrane processes, electrochemical treatment, ion exchange, and adsorption processes^7–9. Despite a broader perspective, though the mentioned methods have all the prerequisites for wastewater remediation, they have several drawbacks, such as process complexity, high energy consumption, and inadequate efficiency for eliminating contaminated pollutants from wastewater¹⁰. In the development of advanced technologies, advanced oxidation processes (AOPs) have shown great interest in new research fields due to the formation of reactive oxygen species (ROS), which accelerate the degradation of toxic industrial dyes. However, one of the techniques of AOPs is photocatalyst, which has numerous advantages over other techniques (e.g., the Fenton process, the O₃/UV technique, the ozone water system, and H₂O₂ photolysis), which entail strict pH control, secondary waste formation, a small molar extinction coefficient, etc.¹¹. The mechanism of photocatalysis involves the following steps: i.e., (i) semiconductor photocatalyst absorbs the photons energy from certain light sources (ii) photocatalyst forms charge carriers (iii) the charges move in between the valence band (VB) and the conduction band (CB) (iv) charges create ROS, mainly hydroxyl radicals (^·OH) and superoxide radicals (^·O₂⁻) (v) ROS species degrade the dye pollutants that are adsorbed on catalyst surfaces¹². As a result, photocatalysts are one of the most effective techniques to decolourize wastewater to enable oxidation processes¹³. Moreover, Titanium dioxide (TiO₂)¹⁴, iron oxide (Fe₂O₃)¹⁵, cobalt oxide (Co₂O₃)¹⁶, zinc oxide (ZnO)¹⁷, cerium oxide (CeO₂)¹⁸, and other nanocomposites are a few of the substances used as photocatalysts¹⁹. Numerous compounds have been created in recent years by utilising the enhanced photocatalytic activity of nanocomposites²⁰. However, the spinel nanoferrite photocatalysts revealed a significant degradation rate for their low band gap energy, rapid generation of ROS, and high regeneration capacity compared to the other photocatalysts under visible light. Moreover, in an AB₂O₄ spinel structure, the tetrahedral (A–) and octahedral (B–) sites are occupied by the divalent and trivalent cations of a unit cell, respectively²¹. The distribution of cations between two sites depends on cation radii, the preparation technique, and the type of bonding. As a result, the optical and magnetic properties of ferrites depend on the distribution of cations; for example, Zn²⁺ affects magnetic properties. Both remanent magnetization (M_r) and coercivity (H_c) reduce with the increasing concentration of Zn²⁺²². Besides, the increase in volume in a cell causes a decrease in photon energy due to the substitution of Zn²⁺ by Co²⁺ because it is smaller than Zn²⁺²³. For the above reasons, cobalt zinc ferrite photocatalyst has drawn considerable attention for wastewater treatment. It has been observed that heterostructures of metal oxide nanocomposites with carbon quantum dots (CQDs) promote charge separation and transportation, which ultimately results in increased photocatalytic activity.

CQDs have drawn a lot of interest in recent times due to their unique chemical, physical, surface, and optical properties²⁴. However, there is still disagreement regarding their precise structure²⁵. Although they have structural contradictions, carbon dots exhibit excellent photoresponsiveness, photostability, biocompatibility, nontoxic characteristics, and facile tunability²⁴. Considering their remarkable characteristics, CQDs have seen remarkable opportunities for a variety of applications for water purification and environmental remediation²⁶. In recent times, natural waste materials, particularly green waste have been transformed into the most promising sources of carbon for the synthesis of water-soluble carbon quantum dots²⁷. According to the literature, most of the approaches for facile synthesis of CQDs from natural sources involved different types of techniques involving hydrothermal, solvothermal, chemical oxidation, carbonisation, and microwave-assisted synthesis due to simple fabrication and quick synthesis processes²⁸. Conventional chemical procedures discharge a vast amount of toxic chemicals that can have diverse effects on the environment as well as human health. Green nanotechnology reduces the excess use of solvents, hazardous chemicals, and energy, decreasing the damaging impact on the environment by reducing waste production and utilising renewable resources²⁹. This technology has diverse applications, such as nanomedicine, wound healing, engineering, electronics, energy, and environment^30,31. For the above circumstances of environmental problems and considerations of spinal nanoferrite as photocatalyst and advantageous CQDs as co-catalyst, we collected mango peel waste and used mango peels as carbon sources for the synthesis of CQDs via the hydrothermal method, which has a promising approach for the synthesis of nanomaterials due to its advantages such as ease of fabrication, low cost, scalability, and being environment-friendly over other methods³².

Recent articles revealed that the cobalt zinc ferrite-based photocatalysts have a significant dye degradation rate as compared to other metal oxide-based photocatalysts under UV, visible light, and ultrasonic waves^33–36. We synthesised cobalt substitute zinc ferrite nanoparticles (CZF) as a hetero-structured metal oxide composite by hydrothermal technique in our previous research work, which showed high levels of capacity and stability for degrading reactive azo dyes from industrial wastewater³⁷. In this new approach, the CZF nanophotocatalyst was coated with cost-effectively synthesised CQDs to enhance its photodegradation performance for the azo dye solutions. The synthesised nanophotocatalyst was systematically characterised by TEM, FTIR, XRD, TGA, FESEM, EDX, DLS, VSM, and UV–visible spectroscopy. The photocatalytic performance of the synthesised nanophotocatalyst was evaluated under visible light irradiation for RY 145 and, RB 222 azo dye degradation as a model of wastewater pollutants. Besides, the effects of various parameters on dye removal like CQD concentration, initial pH, initial dye concentration, photocatalyst dose, recyclability, and kinetic studies, were also evaluated.

Experimental

This section covers the materials used, the synthesis, schematic process diagram, and characterization techniques for carbon quantum dots and photocatalysts, as well as the various methods involved in dye degradation experiments.

Materials

Waste mango peels were taken in-house at Kushtia in Bangladesh and used after being thoroughly cleansed with water. Hydrochloric acid (HCl), sulfuric acid (H₂SO₄), sodium hydroxide pellets (NaOH), and rectified spirit (RS) were purchased from Sigma-Aldrich and Carew & Co. (Bangladesh) Ltd., respectively. The following chemicals were acquired from Merck Germany: cobalt chloride hexahydrate (CoCl₂·6H₂O), anhydrous ferric chloride (FeCl₃), anhydrous zinc chloride (ZnCl₂), and ammonium persulfate [(NH₄)₂S₂O₈]. Loba-Chemie Pvt. Ltd. was the manufacturer of RY 145 and RB 222. The study used double-distilled water and analytical-grade reagents.

Methods

Hydrothermal synthesis of carbon quantum dots (CQDs)

Mango peels were initially washed with water and dried for about 7 days in the sun. 2 g of dried mango peel were kept in the oven at 80 °C for 6 h. The heated mango peels were crushed with the help of a dry grinder. The powder sample was added to a 150 mL 0.1 N H₂SO₄ solution. After 10 min, mango peel powder was washed with distilled water multiple times and filtered, followed by 6 h of oven drying at 100 °C. After that, the dried sample was kept in 100 mL of sodium hypochlorite solution for 4 h and then filtered. Additionally, the sample was repeatedly washed in water until the pH reached 7. After being cleaned, the samples were placed in a hydrothermal autoclave lined with Teflon and kept at 180 °C for 10 h in an oven. It was assisted in bringing the autoclave down to room temperature. Samples were taken out and cleaned with dichloromethane to get rid of any unreacted organic molecules. The final product of CQDs was obtained by lyophilising (freeze drying) the frozen product at − 56 °C and 0.01 mbar pressure for 20 h after the solution was frozen for 48 h in a low-temperature laboratory refrigerator. The short synthesis process of CQDs is shown in Fig. 1a.

Figure 1.

Synthesis diagram of (a) carbon quantum dots and (b) carbon quantum dots coated cobalt zinc ferrite composite (CZF@CQDs).

Synthesis of cobalt zinc ferrite (CZF) nanocomposite

Zinc chloride (ZnCl₂), cobalt chloride hexahydrate (CoCl₂.6H₂O), and ferric chloride (FeCl₃) were dissolved in 25 mL of distilled water and agitated in stoichiometric molar quantities in a beaker. A uniform dark brown slurry solution was formed after an hour of stirring and placed in a 200 mL hydrothermal autoclave. Following the completion of the hydrothermal reaction, the composite was filtered and washed numerous times with distilled water and rectified spirit. After drying at 80 °C for 6 h, the CZF nanodot was ground into powder in a mortar. According to M.A. Bashar et al.³⁷, the modified hydrothermal co-precipitation process was applied to create CZF magnetic nanodots.

Synthesis of carbon quantum dots coated cobalt zinc ferrite (CZF@CQDs) nano photocatalyst

By in-situ oxidatively combining CZF nanoparticles with CQDs, a CZF@CQDs photocatalyst was developed. The photocatalyst synthesis process is shown in Fig. 1b. First, CZF was dispersed in a 1 M NaOH aqueous solution while being mechanically stirred for 30 min. CZF and CQDs were taken to a beaker at a ratio of 1:10 (w/v) dropwise and stirred for 20 min in an ice bath. Ammonium peroxydisulfate (AP), equal weight to CZF nanoparticles, was dissolved in 1 M HCl aqueous solution and added dropwise to the aforementioned mixture for about 5 h. Then, the mixture was kept overnight at room temperature and then filtered. The filtered composite was washed with rectified sprite and distilled water and then dried at 70 °C to create a consistent mass.

Characterisations

After the synthesis of CQDs, CZF, and CZF@CQDs nanocomposite were used to characterise its surface functional group, crystal size, crystal structure, surface morphology, surface areas, elemental composition, thermal behaviour, magnetic properties, surface potential, band gaps, and light absorption properties.

The Fourier transform infrared (FTIR) spectrophotometer (3000 Hyperion Microscope Vertex 80) determined the functional groups of synthesised CQDs, CZF nanocomposite, and CQDs coated on nanocomposite surfaces. With a resolution of 1 cm⁻¹, the FTIR spectra of KBr-pressed pellets were measured in the 3800–400 cm⁻¹ range. Briefly, Smart Lab, Rigaku, Japan, performed X-ray diffraction (XRD) on the produced CZF and CZF@CQDs crystalline structure using Kβ radiation (= 1.54056 Å) and scanning from 5 to 90 °C (2θ) with a 0.01 °C step size and a 20°/min speed. Using a Transmission Electron Microscope (TEM) operating at 300 kV at magnifications of 310 K and 190 K, the size of the nanoparticles and their morphologies were measured for CQDs, CZF, and CZF@CQDs nanocomposite respectively. The accelerating voltage was 40 kV and the emission current was 50 mA. JEOL, the JSM-7610F Field Emission Scanning Electron Microscope (FE-SEM), and Energy-dispersive X-ray (EDS) were used to determine the shapes and elemental compositions of CZF and CZF@CQDs nanocomposite, respectively. Using a JEOL-JEC-3000FC/Auto Fine Coater ion sputtering coating system, gold was applied to the SEM specimens. BET-specific surface areas were calculated from Belsorp Mini II (BEL Japan Inc.) nitrogen adsorption and desorption isotherms at 70 °C in vacuum for 12 h. A Perkin Elmer thermal analyser was used to do thermal analysis at 20–800 °C at 10 °C/min. The thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) curves for all materials were obtained in an inert nitrogen environment at 20 mL/min. The Vibrating Sample Magnetometer (VSM) measured the magnetic properties of CZF and its CQDs-coated nanocomposite at room temperature under a 20 kOe magnetic field. Diffuse reflectance spectroscopy (DRS) was performed using a Shimadzu UV-2401 spectrophotometer to investigate the optical properties of the CZF and CZF@CQDs. Finally, the T60 UV–Vis spectrophotometer measured UV–Vis’s absorption in the range of 190–1100 nm to evaluate the photocatalytic performance of the CZF and CZF@CQDs under different conditions.

Photocatalytic degradation

250 mL of 1000 ppm azo dye solutions (RB 222 and RY 145) were prepared separately and used as stock solutions. The required ppm of model dye pollutant solutions was made from the stock dilution. In various beakers, 50 mL solutions of 10 ppm, 30 ppm, and 50 ppm RB 222 and RY 145 were mixed with the needed amount of photocatalysts. The dye removal experiments were conducted with a catalyst under a 500 W halogen light lamp as steady sunlight for dye removal trials, 20 inches from the stirring solution for 25 min. At 5-min intervals, sampling was done, and the catalyst was separated and collected from the aqueous phase using an Nd–Fe–B magnet. After a certain contact period, the UV spectrophotometer reported dye solution absorbances and estimated photocatalytic degradation efficiency (Efficiency %) using³⁷ Eq. 1.

$$\text{Efficiency}\text{\%}=\frac{C_i-C}{C_i}$$ 1

where C_i is the initial concentration of dye and C is the equilibrium concentration of the solution after the dye removal process. Investigations were performed under different conditions of photocatalytic degradation for dye removal from aqueous solutions. Different conditions such as times, concentration of dyes, photocatalyst content values, CQD and AP content, dye concentration, recyclability, pH, and content were investigated in dye removal.

Photocatalytic kinetic study

The kinetic studies of the CZF@CQDs photocatalyst were performed and repeated three times with different initial dye concentrations against the degradation of RB 222 and RY 145 dyes. The reaction kinetics for the degradation of dyes can be assessed as a pseudo-first-order and pseudo-second-order reaction according to the Langmuir Hinshelwood³⁸ and Ernawati group’s³⁹ kinetics models, which are expressed as Eqs. 2 and 3, respectively.

$$ln\left(\frac{{\text{C}}_i}{{\text{C}}_t}\right)=k_{1}t$$ 2

$$\frac{1}{C_t}-\frac{1}{C_i}=k_{2}t$$ 3

where, C_i is the initial dye concentration, C_t is the concentration at time t, and k₁ and k₂ are the pseudo-first-order and second-order rate constants (min⁻¹), respectively.

Results and discussion

Characterisation of carbon quantum dots

The TEM pictures show clearly in Fig. 2 that CQDs are dispersed naturally and have a spherical shape. As shown in the inset of Fig. 2b, the size distribution curves of the CQDs that have been made are all in a small size range of 1–4 nm.

Figure 2.

(a) large and (b) short scale TEM images of CQDs.

The presence of polar functional groups on the surface of CQDs was investigated using FTIR in Fig. 3a. The spectrum reveals –OH stretching, C-O vibration, and a C-H peak at 3431 cm⁻¹, 1075 cm⁻¹, and 2938 cm⁻¹, respectively. Also peaking at 1638 cm⁻¹ and 1415 cm⁻¹ are declarations of the existence of –COO⁻⁴⁰. The spectrum shows hydrophilic surface functional groups on CQDs, which improve water solubility.

Figure 3.

(a) FTIR spectrum of CQDs, (b) UV–Vis’s absorption spectrum of CQDs in aqueous solution; (Inset) photographic image of water (left) and dropwise CQDs in water (right) on UV light (c) TGA spectrum of CQDs and (d) Zeta potential spectrum of CQDs.

The figure described as Fig. 3b demonstrates the standard UV–vis absorption spectrum of CQDs. From 800 to 200 nm, absorption steadily increased, and a wide absorption band at 272 nm was detected. According to previous reports for graphitic nanocarbons, this band is due to the n–π* and π–π* transitions of the –C=O and conjugated C=C bonds^41,42. As shown in the inset of Fig. 3b, the aqueous solution of CQDs exhibited vivid bluish luminescence when irradiated with UV light. As shown in Fig. 3c the thermal stability of as-synthesised CQDs was analysed by TGA in an inert atmosphere up to 812 °C. As CQDs, physically absorbed water caused an initial weight loss of ~ 12% between 30 and 100 °C. After that, CQDs lost weight gradually, and 78% weight loss was noticed at 530 °C. The breakdown of hydrophilic functional groups on the surface of CQDs caused weight loss⁴³. CQDs were burned out at about 812 °C. The aqueous solution of CQDs indicated a zeta potential value (δ) of − 45.75 mV at pH 10 in Fig. 3d, which shows that their surfaces have a high density of negative charge. The aqueous solution remained stable for a long time without precipitation or agglomeration. TEM, FTIR, UV–Vis’s absorption, TGA, and zeta potential values confirm the perfectly synthesised carbon quantum dots (CQDs) with the hydrophilic surface carboxylic functional group.

XRD results

Crystal structure, average crystallite size, and phase purity of the as-synthesised CZF and CZF@CQDs were determined by X-ray diffraction, and both diffraction patterns are presented in Fig. 4. The range of 2θ set for the XRD spectra was 15 to 80°. The peaks observed at 2θ values are 29.91°, 35.37°, 42.87°, 53.43°, 56.87°, and 62.44°. Bragg’s law was used to compute values in interplaner space (d-space) in Eq. (4);

$$2dsin\theta =n\lambda$$ 4

where, $\lambda$ is the x-ray wavelength (0.15406 nm), d is the spacing value of the crystal layers in nm, $\theta$ is the incident angle in radian, and n is the order of diffraction (n = 1). These peaks (2θ) are indicative of a single-phase cubic cobalt ferrite replaced with zinc nanocomposite with a spinel structure and are assigned to the (220), (311), (400), (422), (511), and (440) diffracted planes with the d-space values of 0.30, 0.25, 0.21, 0.17, 0.16, and 0.14 nm, respectively. The confirmation of the FCC phase of both of the composites was achieved using XRD plane analysis.

Figure 4.

X-ray diffraction patterns of CZF and CZF@CQDs nanocomposite.

The average sizes of the crystallites were determined by employing the Scherrer Eq. (5),

$$D=\frac{K\lambda }{\beta cos\theta }$$ 5

where D is the crystallite size in nanometres (nm), K is the Scherrer constant, which is 0.9, and $\beta$ is the full width at half maximum (FWHM) in radians. The average crystallite size was calculated to be ~ 8.50 nm and ~ 18.60 nm for CZF and CZF@CQDs, respectively. Due to the low crystallinity of CQDs⁴⁴, it is not possible to find a carbon peak in the CZF@CQDs indicating that the CQDs were spread out over the surface of CZF.

It was also found that the intensity of the peaks in all the XRD patterns of the CZF@CQDs was lower for the coating of the CQDs with CZF nanocomposite. The peaks moved slightly toward the smaller angles (right) with no significant planes change in the XRD pattern, indicating the successful coating of CQDs in the cobalt zinc ferrite structure.

SEM results

The microstructure and morphology of the synthesised CZF and the CZF@CQDs were determined by SEM, as shown in Fig. 5. Smaller filamentous nanoparticles with a very narrow size distribution were formed from the particles through agglomeration, as shown in Fig. 5a. The agglomerated particles’ surface morphology exhibited coalescence behaviour, which may be carried on by surface tension at the interfaces or by magnetic interactions between the individual nanoparticles, each of which functions as a magnet. That has also been confirmed by the VSM analysis of both composites. The surface’s significant aggregation suggests small pore crystallites.

Figure 5.

SEM micrographs of (a) CZF and (b) CZF@CQDs nanocomposite.

The SEM image in Fig. 5b shows a CQD-coated CZF photocatalyst. The SEM images present the same morphology in the CZF and CZF@CQDs. It also shows large and more agglomerated particles compared to CZF because of the polymerization reaction of CQDs with CZF, which is granular and agglomerated in shape.

EDS analysis

The EDS spectra of CZF and CZF@CQDs are shown in Fig. 6. The Fe, Zn, and Co peaks for Co_1-xZn_xFe₂O₄ (x = 0.5) are shown in Fig. 6a. The percentage of Co, Zn, and Fe values almost exactly matches the precursor levels, indicating no element loss during synthesis. In good agreement with their initial levels is the observed atomic percentage of Co, Zn, and Fe. It suggests that they are indeed cobalt zinc ferrites with the chemical formula of Co_1−xZn_xFe₂O₄(x = 0.5).

Figure 6.

EDS spectra of (a) CZF composite; (insert) the individual atom and mass percentages of CZF and (b) CZF@CQDs nanophotocatalyst; (insert) the individual atom and mass percentages of CZF@CQDs.

The elemental composition of CZF@CQDs is also provided in Fig. 6b. It shows the elemental percentage of carbon and oxygen is increased among the CZF EDS data, which could be obtained in carbon quantum dots for the perfectly coated cobalt zinc ferrite composite.

TEM results

Figure 7 depicts transmission electron micrographs of CZF and CZF@CQDs to further confirm the existence of uniform particle size. In actuality, the sample is composed of nanoparticles; the average crystallite size of CZF is approximately 9.45 nm, which is consistent with the CZF XRD results.

Figure 7.

TEM micrographs of (a) CZF and (b) CZF@CQDs nanocomposite.

The TEM image of the CZF@CQDs nanophotocatalyst indicated that CQDs were effectively coated onto the CZF nanodot surface by increasing the average particle size by approximately 15 nm, which was also consistent with the CZF@CQDs XRD result. The above results confirm the nanocrystalline character of CZF and CZF@CQDs as synthesised.

The TEM image of both samples showed that the majority of particles are round and that the synthesised nanodot has a crystalline character, but that there are also some irregular-shaped particles present. Dimensions of coherent diffraction extrapolated from SEM and XRD analyses and average crystallite size derived from TEM results agree well, proving that the nanodots are monodisperse nanocrystals.

FTIR analysis

The composite CZF’s molecular fingerprints and a confirmation of CZF@CQDs, as illustrated in Fig. 8, were all investigated through FTIR analysis. Cobalt zinc ferrite samples have shown two characteristic absorption bands in the spinel structure, with wave number ranges 560–594 cm⁻¹ and 399–420 cm⁻¹ respectively, corresponding to the vibrations of the M–O bonds in octahedral and tetrahedral sites⁴⁵. The FTIR spectra of the generated cobalt zinc ferrite nanodots showed characteristic peaks around 407 cm⁻¹ and 583 cm⁻¹, which were attributed to the intrinsic vibration of the octahedral and tetrahedral sites, respectively. Besides, the strong broad peak at ∼ 3400 cm⁻¹ corresponds to the vibrations of water molecules on CZF surfaces.

Figure 8.

FTIR spectra of CQDs, CZF and CZF@CQDs.

The FTIR spectra of CZF@CQDs show shifting of metal oxides band frequencies noted for 583 cm⁻¹ and 407 cm⁻¹ to 607 cm⁻¹. The sharp peaks of CZF@CQDs at 1073 cm⁻¹ and 1639 cm⁻¹ compared to the CZF FTIR spectrum indicate the existence of –COO⁻ on the composite. These shifts of the characteristic peaks can be attributed to the interaction between –COO⁻ of CQDs and metal-oxide linkages of CZF which delocalized the bond energy and electron density of the CQDs after the implementation of CZF@CQDs during in-situ polymerization.

TGA and DTA analysis

Thermogravimetric analysis of the thermal stability of CZF and CZF@CQDs composites was performed under air conditions at a heating rate of 10 °C/min, as shown in Fig. 9a. For CZF, weight loss was only 7.28% between 30 and 780 °C. Comparing the thermal behaviour of the CZF@CQDs, it was found that the total weight loss was 15.5% over the temperature range between 30 and 812 °C. Initially, the weight loss of CZF@CQDs is 6.5%, whereas that of the CZF nanocomposite is 1.83% between 30 and 100 °C, so it could be owing to the greater loss of water molecules for the coated CQDs with the CZF composite. In the range of 100–780 °C, a weight loss of 5.45% was observed due to the elimination of hydroxide species and chloride residues on the surface of CZF. Whereas a weight loss of 9% was observed for the CZF@CQDs, it could be due to extra weight loss (3.56%) for the decomposition of hydrophilic functional groups. In the DTA curve in Fig. 9b, the endothermic peaks start at 135.5 °C and 220 °C for CZF@CQDs and CZF due to the water loss of the compound. The endothermic peaks drastically increased to 625 °C and 560 °C of CZF and CZF@CQDs respectively due to the decomposition of metal hydroxides. The TGA and DTA curves of CZF@CQDs are lower than the CZF curve due to the low metal oxide bond for the CQDs coatings. As a result, CZF@CQDs have good thermal stability though CQDs coated on CZF surfaces.

Figure 9.

(a) TGA and (b) DTA curves of CZF and CZF@CQDs.

DRS analysis

The band gap energy is an important part of how well the nanocomposite works as a photocatalyst. The calculation of the energy levels of the CB and VB depends on the specific material and its crystal structure. In general, this band gap energy can be determined experimentally using optical spectroscopy such as DRS analysis. To find the band gap energy of the CZF and CZF@CQDs, the UV-DRS was used to record the reflectivity at different wavelengths. The formula below shows how to figure out the band gap energy numbers using the Kubelka–Munk transformation⁴⁶ in Eq. 6:

$${\left(\alpha h\vartheta \right)}^{\frac{1}{n}}=A\left(h\vartheta -E_g\right)$$ 6

where α is the linear absorption coefficient, h is the Planck constant, and $\vartheta$ represent the light frequency. The power of the parenthesis, n depends on whether it is a direct transition or an indirect transition semiconductor⁴⁷. A is the proportionality constant, and E_g is the band gap energy.

As shown in Fig. 10a and b, it is noted that the CZF and CZF@CQDs exhibit the band gap energy value of 1.16 eV and 1.20 eV, respectively. On proceeding with the CQD coating, it was observed that there was an increase in the band gap of 0.04 eV. The breakdown of band gap energy observed for the synthesised semiconductor nanophotocatalyst is in the visible region, and therefore, the photocatalytic reaction was performed under visible light.

Figure 10.

UV–vis DRS spectra and band gap of (a) CZF and (b) CZF@CQDs.

VSM analysis

The magnetic properties of the CZF and CZF@CQDs nanocomposite were investigated by VSM, as shown in Fig. 11. Magnetization was enhanced as magnetic field strength increased for both composites. The saturation mass magnetizations of the CZF and CZF@CQD were 42.44 and 36.14 emu/g, respectively. CZF@CQD possessed excellent magnetic properties, even though its saturation magnetization was marginally lower than that of CZF. CZF and CZF@CQD have coercivities of 0.45 and 0.34 Oe, respectively. As is typical for superparamagnetic particles, both compounds exhibit a very small hysteresis loop and low coercivity, as demonstrated by the results⁴⁸. Notably, this analysis reveals that the saturation magnetization of the magnetic CZF@CQDs composites was distinct and considerably greater in CQDs-coated samples. In addition, the magnetic property of the CQD-coated CZF indicates efficient magnetic separation for recycling and reuse.

Figure 11.

Magnetization curves of CZF and CZF@CQDs at room temperature; (Inset) photographic image of magnetic separation.

Surface area of CZF@CQDs photocatalyst

The adsorbate for the BET study was nitrogen gas, and the outgassing occurred at a temperature of 70 °C over a period of 12 h. The nitrogen adsorption and desorption isotherms of CZF@CQDs are shown in Fig. 12. The BET-specific surface area of the CZF@CQDs photocatalyst was estimated to be 149.87 m²/g using multiple-point analyses. The average pore volume and pore diameter were determined to be 0.2070 cm³/g and 55.24 Å, respectively. Based on the skeletal density of 1.00 g/cm³, the porosity was estimated to be 0.1715 per gramme of sample. Estimated high surface area and pore size are essential characteristics of the synthesised photocatalyst, as they provide increased active sites and enhanced capacity for the degradation of synthetic dyes in wastewater.

Figure 12.

BET surface area analysis of N₂ adsorption and desorption isotherm of CZF@CQDs.

Mechanism of CQDs coating on CZF nanocomposite

The procedure for coating carbon quantum dots on cobalt zinc ferrite for the CZF@CQDs nanocomposite is shown in Fig. 13. It is well known that metal oxide’s surface charge is positive below the pH of the point of zero charge (PZC) and negative above it. The ferrite composite’s surface has a PZC of pH 6⁴⁹, thus negatively charged in the basic conditions. As a result, some positive charges from the basic solution may be adsorbed, making up for the negative charges on the surface of the ferrite. In addition, this specific adsorption on the ferrite surface may create a positive charge on its surface.

Figure 13.

Proposed mechanism of synthesis of CQDs coated with CZF nanophotocatalyst.

In this approach, CQDs are migrated to anionic carboxylate ions in basic conditions that confirm the δ value of CQDs. Thus, electrostatic interactions happen between cations adsorbed on the CZF surface and anionic carboxylate ions from CQDs. AP acts as an oxidising agent to polymerize the CQDs electrostatically complex to the CZF surface.

Mechanism of dye removal

A photocatalytic experiment on CZF composite showed no significant changes in azo dye colour when exposed to visible light within a short time. CZF nanocomposites were then coated with CQDs and irradiated. The two photocatalytic reaction processes proposed for dye degradation are illustrated in Fig. 14.

Figure 14.

Proposed photodegradation mechanism of CZF@CQDs nanophotocatalyst.

The enlarged catalytic dot of CQDs at the CZF@CQDs interface increased photocatalytic activity. As illustrated in Fig. 14a, visible light irradiation formed excitons (e- and h + pairs) over CZF nanodots, transferring electrons from the CB to CQDs while leaving holes in the VB. Another up-conversion causes CQDs to absorb longer wavelengths (450 to 700 nm) and emit shorter wavelengths of light⁵⁰, which stimulates CZF to create e and h + pairs in Fig. 14b. This rapid excitation and recombination enhance photocatalysis. The azo dye molecule degrades when e/h + pairs react with adsorption oxidants or reducers to form ^·O₂⁻, ^·OH, breaking the azo bond (–N=N–).

Dye removal investigation

FTIR, EDS, SEM, and XRD analyses show that the synthesised CZF@CQDs photocatalyst has been successfully made, and TGA analysis demonstrates the high stability of the nanocomposite at high temperatures. According to DRS data, the estimated band gap for the CZF@CQDs photocatalyst is 1.20 eV. This demonstrates that in the presence of visible light, the prepared nanocomposite functions effectively as a photocatalyst. To investigate the decolourization of RY 145 and RB 222 dyes in an aqueous solution, the use of a CZF@CQDs photocatalyst in visible light was studied. The results are shown in Fig. 15. From the graph, it can be seen that for 10 mg/L, 30 mg/L, and 50 mg/L dye solutions, almost 95% of the colour was removed within 25 min for every dye. This shows that the decolourisation of (a) RY 145 and (b) RB 22 are attributed to photocatalytic reactions that can decolourize as well as degrade these dyes.

Figure 15.

Photocatalytic degradation of (a) RY 145; (Inset) photographic image of before and after treatment, and (b) RB 222 dyes; (Inset) photographic image of before and after treatment.

The electrophilic breakage of dyes’ chromophore azo bond (–N=N–) by the CZF@CQDs photocatalyst could accelerate decolourization. This photocatalyst degrades quickly due to its high surface area. The small crystal size and large number of active sites on the surface of the catalyst, combined with the CQD’s surface coating area, require lighter irradiation and high-energy light to excite the composite. This process generates more ^·OH and ^·O₂⁻ radicals that can oxidise or reduce azo bonds. Studies indicate that the more chromophore group RB 222 has in its chemical structure, the slower it degrades than RY 245.

Effect of photocatalyst dosage on dye degradation

The effects of catalyst dosage on RY 145 and RB 222 degradation were observed by adding 0.5, 0.75, 1, and 1.25 g/L of CZF@CQDs photocatalyst in the beaker to 50 mL of solutions with initial dye concentrations of 30 mg/L and pH of 7. Increasing catalyst dosage increases the number of active sites on the catalyst surface, leading to faster creation of radicals (^·OH/^·O₂⁻) to break down dye molecules, as shown in Fig. 16. The half-life of the dye concentration decreased as the catalyst dosage increased, implying that the intermediates were rapidly degraded due to the mineralization of the dye molecules⁵¹. However, because of the influence of light scattering and enhanced particle aggregation, increasing the catalyst loading above a certain point can result in reduced degradation efficiency⁵². For this reason, a 1.25 g/L photocatalyst revealed approximately the same outcome as a 1 g/L photocatalyst. It showed that 1 g/L is the best photocatalyst dosage for maximum dye degradation.

Figure 16.

Effect of photocatalyst dosage on the degradation of (a) RY 145 and (b) RB 222 dyes (volume of dye = 50 ± 1 mL, dyes concentration = 30 ± 1 mg/L, pH = 7 and time = 10 min).

The effect of pH on dye degradation

Because of its many roles, interpreting pH impacts on dye photodegradation efficiency is a difficult study⁵³. Two considerations must be made while researching how pH affects the photocatalytic degradation of dyes: first, industrial effluents may not be neutral, and second, the pH of the reaction mixture affects the surface-charge properties of the photocatalysts⁵⁴. As has been noted elsewhere, some azo dyes degrade more quickly at lower pH⁵⁵. Thus, pH changes can affect the adsorption of dye molecules onto the surfaces of CZF@CQDs, which is necessary for photocatalytic oxidation to occur. Furthermore, the mechanism may rely on the degraded compound’s adsorption capacity on the catalyst’s surface. At low pH, RB 222 and RY 145 degrade more quickly, while at neutral and high pH, hydroxyl radicals are thought to be the main species. For these circumstances, 4–6 is the optimum pH range for better RB 222 and RY 145 dye degradation, as shown in Fig. 17.

Figure 17.

Effect of pH on dye solution (volume of dye = 50 ± 1 mL, photocatalyst dosage = 1 ± 0.01 g/L, dyes concentration = 30 ± 1 mg/L, and time = 10 min).

The effect of CQDs and ammonium persulfate (AP) content in photocatalyst for dye degradation

A noticeable difference in the sample exhibited a degrading capacity with different molar ratios of CZF, CQDs, and AP. To analyse that, the degradation of RY 145 was carried out in the presence of different molar ratios of CQDs and AP in photocatalysts at a 30 mg/L dye solution. The results are shown in Fig. 18. According to the figure, CZF had about 12% degradation in 10 min. When adding CQDs without any oxidising polymerization agent, the dye degradation rate increased because CQDs were adsorbed inherently in CZF. When considering solely AP in CZF, the degradation rate equalled that of CZF. By increasing CQD volume with AP, the dye degradation rate drastically increased due to the optimum ratio of CQDs and AP. The excess content of CQDs and AP ratio showed insignificant degradation rates due to the scarcity of CZF surfaces and overlapping clusters to prevent CQD coating, respectively. The maximum photocatalytic degradation was noticed at the 1:10:1 (w: v: w) molar ratio between CZF, CQDs, and AP.

Figure 18.

CQDs and AP content in CZF nano photocatalyst for dye degradation.

Photodegradation kinetic study

Figure 19 depicts the plot of concentration ratios ln (C/C_o) and (1/C–1/C_o) with respect to irradiation time. By a linear fit of the data, the slope of the fitted line gives the value of the rate constants k₁ and k₂ according to first-order and second-order kinetic models. The rate constant and regression correlation coefficient (R²) values for the degradation of RY 145 and RB 222 in the presence of CZF@CQDs photocatalyst are given in Table 1. The first-order rate kinetics were shown to have greater R² values than the pseudo-second-order rate kinetics, which confirms the first-order kinetics determines the photocatalytic degradation of dyes. It also indicates that photocatalysts degrade the dyes without breaking the dye molecule structure itself.

Figure 19.

The plots of first-order kinetics for degradation of (a) RY 145 and (b) RB 222 and second-order kinetics for degradation of (c) RY 145 and (d) RB 222 by CZF@CQDs photocatalyst.

Table 1.

Kinetic parameters of CZF@CQDs photocatalyst for RY 145 and RB 222 dyes degradation (Fig. 19’s experimental data).

Initial dye concentration (mg/L)	RY-145				RB-222
	First-order		Second-order		First-order		Second-order
	Rate, k1 (min−1)	R2	Rate, k2 (min−1)	R2	Rate, k1 (min−1)	R2	Rate, k2 (min−1)	R2
10	0.1060	0.8437	0.3850	0.6423	0.1015	0.8479	0.1921	0.6622
30	0.1282	0.9474	0.1370	0.7117	0.0995	0.9837	0.0295	0.8618
50	0.0761	0.9995	0.0124	0.9545	0.1091	0.9721	0.0106	0.8070

Recyclability test

An important consideration in assessing industrial applications is the catalyst’s long-term stability and reusability because it significantly lowers operating costs. For the oxidative degradation of RY 145, the stability of the photocatalyst was determined in continuous operation for up to seven cycles under optimal reaction conditions (volume of dye pollutant = 50 ± 1 mL, photocatalyst dosage = 1 ± 0.01 g/L, RY 145 dye concentration = 30 ± 1 mg/L, initial pH = 7, and time = 10 min). The photocatalysts were separated magnetically for the recyclability runs; after that, without washing the used photocatalyst, a fresh solution of RY 145 dye was added to the photocatalyst. This procedure was carried out seven times, and it can be seen from Fig. 20 that even after seven cycles, about 72% of RY 145’s degradation efficiencies were still attained. During the consecutive reusability tests, slight deviations in the degradation rate of the photocatalytic process due to active sites on the catalyst surface may become deactivated or blocked by the previous dye mole.

Figure 20.

Recyclability of CZF@CQDs nanophotocatalyst (volume of dye = 50 ± 1 mL, photocatalyst dosage = 1 ± 0.01 g/L, dye concentration = 30 ± 1 mg/L, initial pH = 7, and time = 10 min).

This suggests that the photocatalyst is quite stable during the photocatalytic degradation of the organic dye molecules and might have played an important role in industrial applications for its excellent stability and activity.

Comparison of results with other photocatalysts

The photocatalytic performances of some photocatalysts previously reported are provided to confirm the advantage of utilising CZF@CQDs nanophotocatalyst. The as-synthesised CZF@CQDs photocatalyst had the greatest photocatalytic performance in terms of photodegradation, degradation time, and cyclic stability. The comparative study is given in Table 2.

Table 2.

Comparative photocatalytic study of CZF@CQDs nanophotocatalyst and other photocatalysts.

Photocatalyst name	Source of CQDs	Pollutant(dyes) & Conc. (mg/L)	Catalyst dose (g/L)	Using light source	Light irradiation time (min)	Degradation efficiency (%)	Cyclic stability runs/efficiency (%)	References
CZF	–	MB & 10	0.5	Xe lamp (300 W)	480	95.4	–	56
TiO2@CQDs	Lemon peel	MB & 10	–	Xe lamp (500 W)	120	90	–	40
CQD-S1	Urea and polyethyleneimine	RhB	–	Osram Lamp (300 W)	100	98.4	–	57
C-QDs/TiO2	Citric acid	RhB, 4	–	Sylvania lamps		79.78	5/75	58
Fe3O4@CQDs	Glucose	MB & 2	1.25	Xe lamp (400 W)	30	83	10/83	59
CeO2@CQDs	Wood powder	MB	1	Xe lamp (380 W)	120	85	–	60
Bi2MoO6@CQDs	Glucose	RhB & 10	1	Xe lamp (300 W)	120	91	10/75	61
KNbO3@CQD	L-ascorbic acid	Crystal Violet & 10	1	Xe lamp (300 W)	300	70	–	62
CZF@CQDs	Mango peel	RB and RY & 10, 30 and 50	1	Halogen lamp (500 W)	25	95	7/72	This study

Conclusion

Carbon quantum dots were synthesised successfully from mango waste peels using a facile hydrothermal process. Previously synthesised cobalt-zinc ferrite was perfectly coated with CQDs using a new approach in-situ oxidative method. The prepared nanocomposites were systematically characterised by XRD, FESEM, EDS, TEM, FTIR, DRS, VSM, BET, DTA, DLS, and TGA. These characteristics are consistent with the reported literature and also with each other for the synthesised nanocomposites. Importantly, the band gap energy observed for the synthesised nanoparticles is in the visible region; therefore, photocatalytic activity was performed under visible light. VSM revealed the paramagnetic properties of the synthesised nanocomposites for best recyclability. The removal of the azo dyes was observed to be dependent on pH, dye concentration, catalyst dosages, and various ratios for the CQDs and AP with CZF. Also, the samples were found to follow pseudo-first-order kinetics for the degradation of RY 145 and RB 222 dyes. After seven cycles, about 72% of RY 145’s photodegradation efficiencies were still attained. Thus, the findings show that CZF@CQDs can be employed as an excellent new photocatalyst for the treatment of dyes in industrial wastewater.

Author contributions

M.D.M.: Conceptualization, methodology, investigation, written original draft; M.T.H.M.: Commissioning of the experimental equipment and the criticism of the article; M.A.B.: methodology, review, and editing; D.C.: Commissioning of the experimental equipment and the criticism of the article; and M.S.A.: Conceptualization investigation, supervision, review, and editing of the manuscript.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.