CdS nanorod–MFe₂O₄ (M = Zn, Co and Ni) nanocomposites: a heterojunction synthesis strategy to mitigate environmental deterioration

Abstract

A facile strategy to encrust MFe₂O₄ (M = Zn, Co and Ni) nanoparticles over CdS nanorods via a two-step solvothermal method has been reported. The ferrite–CdS nanocomposites (NCs) were characterized using powder X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. A shift in the peak corresponding to the (311) plane confirmed the presence of different metal ions in the spinel ferrite lattice. However, no variation in the peak of CdS was observed which indicates that the phase and morphology of the CdS nanorods remain unaltered after hydrothermal treatment. High resolution transmission electron microscopy (HR-TEM) analysis revealed the efficacious attachment of the nanoferrite on the CdS nanorods. Optical studies of the NC samples provided information about the fabrication of the visible light responsive photocatalyst and covered the solar spectrum from 525 nm to 737 nm. From magnetic studies, CdS–CoFe₂O₄ and CdS–NiFe₂O₄ were found to be ferromagnetic in nature with a saturation magnetization of 26.4 and 15.5 emu g⁻¹, respectively. Interestingly, a transition from ferromagnetic to super-paramagnetic was observed for the ZnFe₂O₄ loaded CdS nanorods. The photo-catalytic activities of the nanocomposites were studied by carrying out photo-degradation of rhodamine B and methylene blue dyes under visible-light irradiation. The maximum activity was observed in the case of the CdS–ZnFe₂O₄ nanocomposites.

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1. Introduction

In the last decade, multicomponent nanomaterials have gained considerable attention from the scientific community due to their unique electrical, optical and magnetic properties.^1–3 Consequently, such hybrid nanomaterials have shown caliber in diverse applications such as drug carriers, medical diagnosis, optoelectronic devices, lithium ion batteries and heterogeneous photocatalysis.^4–6

Various semiconductor nanomaterials such as TiO₂, SnO₂, BiVO₄, ZrO₂, ZnO, ZnS, CdS, WO₃, graphene and C₃N₄ have been coated with suitable metal nanoparticles (NPs) of Au, Ag, Ni, Pt, Rh and Cu which facilitates interfacial charge transfer.^7–9 These heterostructure nanomaterials accelerate the photo-degradation of noxious organic pollutants and the photocatalytic splitting of water to produce hydrogen gas. Anyhow, these materials limit their application due to their non-magnetic nature and deprivation of visible light responsive behavior. Secondly, these noble metals are expensive and scarce. Therefore, it is of great interest to replace them with highly efficient and economic green magnetic materials. But, magnetic NPs were found to be inactive in the visible region until the generation of Fenton’s reagent as previously reported.^10,11 So, in order to play on these magnetic materials in the visible light spectrum, such magnetite nanomaterials have been encrusted over visible solar spectrum sensitive luminescent moieties.

CdS is the most important II–VI semiconductor which has a band gap of 2.4 eV and absorbs in the visible region of the solar spectrum. However, the rapid recombination rate of the electron–hole pair retards the photocatalytic efficiency and enhances photo-corrosion.¹² In order to alleviate these limitations, visible light driven CdS/MFe₂O₄ (M = Fe, Zn, Co, Ni and Cu) luminescent magnetic materials have gained prominence as a frontline solution for the oxidation of toxic organic pollutants and dyes through photocatalysis.

The synthetic protocol of such heterostructure nanocomposites (NCs) deals with the consecutive growth of one component over the pre-synthesized domain or simultaneous growth of two different NP domains in a one pot reaction. The confined core–shell structure and layered distribution of one component over another distinctively shaped nanostructure (nanorod, nanosphere or nanocube) bestow them with extraordinary properties. Such multicomponent nanostructures behave as a single entity which retains the properties of all the components.^13,14 However, the nanorod (NR) core/shell heterostructure renders incessant electron transport pathways due to a high aspect ratio of NRs, minimal aggregation of NPs on the NR surface and good surface contact.¹⁵ Hence, this phenomenon mitigates the recombination of charges compared to other core/shell structures leading to enhanced efficacy under visible light irradiation.

However, tailoring of these different moieties is also the vanguard challenge so as to make effective contact between heterojunction materials. Roychowdhury et al.¹⁶ synthesized fluorescent-magnetic CdS–Fe₃O₄ NCs by a two step chemical route method. Magnetic measurements revealed the presence of super paramagnetic (SPM) behavior in the NCs with a smaller Fe₃O₄ NP size (5 to 7 nm), whereas the NC with 12 nm Fe₃O₄ was essentially ferrimagnetic. Liu et al.¹⁷ synthesized CdS–Fe₃O₄ NCs by a sonochemical method, which exhibited good luminescence and magnetic properties and photocatalytic activity in visible light spectrum. Wang et al.¹⁸ reported the epitaxial and non-epitaxial growth of α-Fe₂O₃ and Fe₃O₄ nanoparticles on the surface of CdS nanorods via a polymer wrapping technique. Photocatalytic studies revealed that α-Fe₂O₃–CdS completely degraded methylene blue dye in 7 h. However, only 62% of the dye was degraded using the Fe₃O₄–CdS nanocomposite in the same time interval; which could be due to the blocking of the active sites of the catalyst by polyethylene glycol and polyvinylpyrrolidone binders.

In order to overcome the above problems, this work reports a facile solvothermal method for the synthesis of MFe₂O₄ (M = Zn, Co and Ni) NP–CdS NR heterostructure NCs. The synthesized samples were characterized by powder X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. Transmission electron microscopy (TEM) was employed to analyze the shape and structure of the nanostructures, whereas high resolution transmission electron microscopy (HR-TEM) was used to scrutinize the heterojunction contact surface. Energy dispersive X-ray (EDX) analysis revealed information about the elemental stoichiometry. Scanning transmission electron microscopy (STEM) with EDX elemental mapping was used to analyze the Zn, Fe, O, Cd and S proportionality in the sample. Further optical and magnetic properties were investigated at room temperature. The comparative photocatalytic behavior of all the samples was also studied in the presence of visible light.

2. Experimental

2.1. Materials

Cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O, 99%), thiourea (NH₂CSNH₂, 99.9%), ethylenediamine (C₂N₂H₈, 98%), ferric nitrate (Fe(NO₃)₃·9H₂O, 98%), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, 97%), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O 98%), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 99%) sodium hydroxide (NaOH, 98%), rhodamine B (99.7%) and methylene blue (99.2%) were purchased from Loba Chemicals and used without further purification. Absolute ethanol (C₂H₅OH, 99.9%) and acetone (C₃H₆O, 98%) were purchased from Fisher Scientific. Deionized water was obtained using an ultrafiltration system (Milli-Q, Millipore) with a measured conductivity of 35 mho cm⁻¹ at 25 °C.

2.2. Physical measurements

The powder X-ray diffractometer (XRD) X’Pert PRO, PANalytical, with a locked coupled scan type, scan angle range of 20–80°, scan step of 0:02°, scan speed of 3° min⁻¹, maximum power of 40 kV/40 mA, a Cu tube and a T/T horizontal was used to analyze the phase purity of the synthesized samples. The scan time for each sample was about 20 minutes and the wavelength of the Cu-Kα target was 1.5406 Å. The Fourier-transform infrared (FT-IR) spectra for all the samples were recorded by an FTIR instrument (PERKIN ELMER) using KBr pellets in the range of 4000–400 cm⁻¹. High resolution transmission electron microscopy (HR-TEM) images, energy dispersive X-ray (EDX) analysis and scanning transmission electron microscopy (STEM) images were recorded using an FEI Tecnai G2 F20 operated at 200 keV with a magnification of 6 × 10⁶ times and a resolution of 0.2 Å. A BET surface area analyzer ((11-2370) Gemini, Micromeritics, USA) was used to obtain the surface area of the samples which was operated in the temperature range of 10–350 °C. Prior to nitrogen adsorption measurements the samples were preheated at a temperature of 100 °C for 1 h. Optical properties were analyzed using a UV-Vis spectrophotometer (Analytikjena SPECORD-205). Photo-irradiation was carried out using a 160 W mercury lamp (96 000 Lux) with a distance of 6 inches between the light source and the target surface.

2.3. Fabrication of CdS NRs

CdS NRs were fabricated by a solvothermal process. In a typical synthesis, Cd(NO₃)₂·4H₂O (4.66 g) and thiourea (3.45 g) were added to 72 mL of ethylenediamine solvent. This reaction mixture was magnetically stirred for 30 min at room temperature followed by high temperature autoclaving for 24 h at 160 °C. The obtained yellow colored precipitates were filtered and washed several times with acetone and distilled water. Then, this sample was dried overnight in vacuum oven at 60 °C.¹⁹

2.4. Fabrication of MFe₂O₄ (M = Zn, Co and Ni)–CdS NCs

For the synthesis of the CoFe₂O₄–CdS NCs, 400 mg of CdS NRs was dispersed in a 0.25 M NaOH (50 mL) solution using an ultrasonic bath sonicator for 30 minutes. Then, 20 mL of 0.05 M of a Co(NO₃)₂·6H₂O and 0.1 M Fe(NO₃)₃·9H₂O mixed solution was added drop wise to the above mixture. Brown colored precipitates appeared in the reaction mixture which was due to the formation of metal hydroxide. In order to obtain complete homogeneity, this mixture was magnetically stirred for 2 h. This mixture was further transferred into a 100 mL capacity Teflon lined stainless steel autoclave and kept in the furnace at 180 °C for 12 h. Then, this mixture was allowed to cool down at room temperature followed by cleansing and purification of the sample with distilled water and acetone.²⁰ Due to the attachment of the magnetic moiety to the luminescent CdS NRs, an external magnet was used to clean and purify the sample without using the complex filtration process. A similar procedure was followed for the synthesis of ZnFe₂O₄–CdS NCs and NiFe₂O₄–CdS NCs.

2.5. Photocatalytic activity

The photocatalytic activities of pure CdS NRs and their corresponding CdS–MFe₂O₄ nanocomposites were scrutinized for the degradation of rhodamine B (RhB) and methylene blue (MB) dyes. For a typical photo-catalysis reaction, 2.4 mg of RhB dye (100 mL) was degraded in the presence of visible light using 100 mg of the nanocatalyst. Before irradiation in visible light, the sample was kept in the dark for 30 minutes to achieve an adsorption/desorption equilibrium. Afterward, this dye solution was kept under the visible light lamp with continuous stirring. A 2 mL aliquot was withdrawn at regular time intervals and the catalyst was removed using an external magnet. Subsequently, the decolorization of the dye was investigated using UV-visible spectroscopy.

3. Results and discussion

3.1. Fourier transform infra-red (FT-IR) spectroscopy

The FT-IR spectra of pristine CdS NRs and their NCs with ferrite NPs are depicted in Fig. 1. All of the peaks in the spectra match well with the values reported in the literature.²¹ It is evident from the figure that the ferrite NPs garnered over the CdS NR surface display their fingerprint peaks at around 530–560 cm⁻¹, which is ascribed to the stretching vibration of the tetrahedral M–O cluster. However, this peak is absent in the pure CdS NR sample. The ZnFe₂O₄–CdS NCs show a Zn–O tetrahedral frequency at 536 cm⁻¹, whereas for the CoFe₂O₄–CdS NCs a peak appeared at 560 cm⁻¹ (see the inset of Fig. 1). The NiFe₂O₄–CdS NCs also displayed a peak at a vibrational frequency of 562 cm⁻¹. This shifting in frequency can be rationalized in terms of reduced mass.²²

ν² = k/4c²π²μ (1)

where ν is the frequency in cm⁻¹, k is the force constant, c is the velocity of light and μ is the reduced mass. So, according to above equation, a decrease in the reduced mass results in an increase in the vibrational frequency. Thus, the shifting of the peak towards a higher vibrational frequency for cobalt and nickel ferrite NCs is in good agreement with the above equation because of their lower molecular masses compared to zinc ferrite NCs.

Fig. 1 FT-IR spectra of CdS NRs and MFe₂O₄–CdS NCs (M = Zn, Co and Ni).

3.2. Powder X-ray diffraction (XRD) characterization

Fig. 2 shows the XRD pattern of CdS NRs and MFe₂O₄–CdS (M = Zn, Co and Ni) NCs. The peaks in the diffraction pattern of CdS NRs show the hexagonal phase (JCPDS-01-080-0006) of the nanostructure, and the planes of the CdS NRs match well with the previous literature.²³ The diffraction patterns of the CdS–MFe₂O₄ (M = Zn, Co and Ni) NCs displayed the signature peaks for pristine CdS and the corresponding spinel ferrite NPs, which were in good agreement with JCPDS-01-089-1009, JCPDS-00-001-1121 and JCPDS-01-074-208 for zinc, cobalt and nickel ferrite NCs respectively.²⁴ A shifting in the peak corresponding to the lattice plane (311) was observed in the NC samples. However, no variation in the lattice plane (102) of the CdS NRs was observed in any of the samples as shown in the inset of Fig. 2. This implies that the CdS NRs remain unaffected after the hydrothermal treatment. The shifting in the (311) plane of the nanocomposite samples was scrutinized using the Le-bail refinement method. It was observed that the lattice parameter of the NC samples decreased in order as follows:

CdS–ZnFe₂O₄ (0.844 Å) > CdS–CoFe₂O₄ (0.838 Å) > CdS–NiFe₂O₄ (0.833 Å)

Fig. 2 Typical powder XRD pattern of (a) hexagonal CdS NRs and their nanocomposites with (b) ZnFe₂O₄ (c) CoFe₂O₄ and (d) NiFe₂O₄ NPs synthesized by a two step hydrothermal process (inset showing the shifting of the peak corresponding to the (311) plane of different ferrite–CdS NCs without disturbing the (102) plane of the hexagonal CdS NRs).

This change in lattice parameter was attributed to the presence of metal ions of different ionic radii in the ferrite nanostructures:

Zn²⁺ (88 pm) > Co²⁺ (83.8 pm) > Ni²⁺ (73.1 pm)

The average crystallite size of the samples was calculated using the Debye–Scherrer equation:²⁵

where d is the average crystallite size, K is the shape factor approximated to 0.9, β is the line broadening at half the maximum intensity (full width at half maximum intensity, FWHM) and λ is the X-ray wave length. Broad and sharp peaks of pristine CdS NRs and their corresponding NC samples depict that the samples are completely crystalline. The average crystallite sizes of the ferrite nanoparticles were calculated using the most intense (311) peak of the nanocomposite samples and are presented in Table 1. As is clear from the table, the average crystallite size of the samples decreased as follows:

CdS–NiFe₂O₄ > CdS–CoFe₂O₄ > CdS–ZnFe₂O₄

Table 1 Average crystallite size and lattice parameter shifting of CdS nanorod–MFe₂O₄ (M = Zn, Co and Ni)

Nanocomposite	Angle (2θ)	Average crystallite size (nm)	Lattice parameter (Å)
CdS nanorods	26.804	44.166	a = 4.121, b = 6.682
ZnFe2O4 NPs	35.406	8.1	a = 0.844
CoFe2O4 NPs	35.661	18.7	a = 0.838
NiFe2O4 NPs	35.865	21.5	a = 0.833

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3.3. HR-TEM characterization

The size, shape and mode of attachment for the CdS–ferrite heterojunction domains were scrutinized using HRTEM and STEM-point EDX characterization techniques. Scattered area electron diffraction (SAED) patterns and lattice fringe widths were also calculated to probe the crystallinity of the samples. Prior to analysis, the annealed samples were dispersed in ethanol and sonicated for 10 minutes to obtain a clear dispersion. A drop of this suspension was put on the 400 mesh size carbon coated copper grid. The as prepared specimen was thoroughly investigated under the electron beam to substantiate the formation of ZnFe₂O₄ NP encrusted CdS NR heterostructure NCs. The HR-TEM images shown in Fig. 3(a and b) manifested that the CdS NRs have a diameter of around 50–70 nm and the size of the zinc ferrite NPs were around 5–6 nm. As is clear from the images, a very fine layer of zinc ferrite was coated on the CdS NRs, and high resolution of around 2 nm revealed information about the point of contact between the ferrite and CdS nano domains (see Fig. 3(c)). The lattice fringes were calculated from the portrayed image and the inter-planar distance between the two consecutive fringes was found to be around 0.25 nm for the (311) plane of the zinc ferrite NPs, whereas the CdS NRs exhibited a distance of 0.33 nm for the (002) plane. The SAED pattern of the nanocomposite sample demonstrated the structural crystallinity and revealed information about the (311) plane of zinc ferrite NPs along with the (002) plane of the CdS NRs (Fig. 3(d)). The profile of the frame portraying the inter-planar distance between consecutive fringes and the fringe width obtained so far matches well with the XRD results (Fig. 3(e and f)).

Fig. 3 HRTEM image showing (a and b) ZnFe₂O₄ NP loaded CdS NRs, (c) the high resolution image clearly interpreting the point of attachment and lattice fringes of both domains, (d) the SAED pattern of the CdS NR–ferrite NCs depicting the crystalline nature of the sample, and (e and f) the profile of frame describing the lattice fringe width.

3.4. STEM point EDX analysis

For additional support the STEM point EDX provided information about the photoluminescent core with a magnetite shell. Fig. 4(a) displays the 3-D vision of the CdS–ZnFe₂O₄ core–shell heterostructure in order to interpret the surface morphology and the thickness of shell. Point EDX spectroscopy acquired signals of Cd, S, Zn, Fe and O from the selected frame of the STEM image by positioning the electron beam on the center of the heterostructure. Additional signals for carbon and copper were also obtained due to the TEM grid. No additional peaks for any other impurities were observed, which concluded that the obtained samples were clean. Thus, the combinatorial effect of all the techniques lead to the unanimous conclusion that the nanorod shaped luminescence CdS was ensheathed with zinc ferrite nanoparticles and a core–shell structure was achieved through this synthetic methodology with a complete stoichiometric proportion of the nanocomposite sample.

Fig. 4 ZnFe₂O₄ coated CdS NRs showing the (a) 3-D view STEM image, (b) EDX pattern showing the elemental proportion, and (c) elemental mapping of Cd, S, Zn, Fe and O from the selected frame of the STEM image.

3.5. BET surface area analysis

The surface area of the nanocomposite samples was measured using the single point BET analyzer. All of the samples were preheated at 100 °C for 1 hour before N₂ adsorption. The zinc ferrite–CdS nanocomposite sample was found to have a maximum surface area value of 79.6 m² g⁻¹. The surface area values measured for the cobalt and nickel ferrite–CdS nanocomposite samples were found to be 54.5 and 24.4 m² g⁻¹, respectively.

3.6. Diffuse UV-visible reflectance spectra and band gap of MFe₂O₄–CdS NCs

The CdS NRs and ferrite NP ensheathed CdS NRs were analyzed by diffuse UV-visible reflectance spectroscopy to investigate their optical properties. Fig. 5 shows the band edge absorption spectra of the pristine CdS NRs and their corresponding CdS–ferrite NCs. The absorbance results demonstrate that CdS NR–MFe₂O₄ (M = Zn, Co and Ni) had significant absorbance in the 525–725 nm wavelength range of the visible region which is important for photo catalytic reactions. The band edge absorption onset for the CdS NRs was at 525 nm, whereas for the CdS NR–ZnFe₂O₄ NC the curve inclined towards a higher value of onset absorbance around 600 nm. Further, the encrustation of nickel and cobalt ferrite onto CdS NRs increased the band edge absorbance.

Fig. 5 Diffuse UV-visible spectra of CdS NRs and nanocomposites of CdS NR–MFe₂O₄ (M = Zn, Co and Ni) showing different band edge absorbance.

In order to estimate the corresponding band gap energy, the (αhν)² vs. photon energy (hν) was plotted (Fig. 6). The optical absorption coefficient near the band edge follows the equation:²⁶

αhν = A(hν − E_g)^1/2 (3)

where α, h, ν, E_g, and A are the absorption coefficient, Planck constant, light frequency, band gap and proportionality constant, respectively. The band gap energy was deduced from this plot by drawing a normal to the linear part of the curve. A clear estimate about the onset absorbance and band gap value was derived from Tauc’s equation. The band gap value for the CdS NRs was found to be 2.35 eV. However, zinc, cobalt and nickel ferrite nanoparticles encrusted on CdS NRs displayed band gap values of around 2.22, 1.71 and 2.29 eV, respectively. This indicated a significant change in the band energies due to the presence of a different spinel structure of ferrite, as well as the synergetic effect of attaching a small band gap material of ferrite NPs with large band gap CdS NRs. In comparison to the bulk band gap values for zinc, cobalt and nickel ferrite (1.92, 1.45 and 2.15 eV, respectively), a red shift in the absorption edge was observed for all the ferrites which might be due to interfacial defects and the formation of sub-band-gap energy level in the nanocomposite material.²⁷

Fig. 6 Plots of (αhν)² vs. hν showing the band gap of (a) CdS NRs, and their nanocomposites with (b) ZnFe₂O₄ (c) CoFe₂O₄ and (d) NiFe₂O₄ NPs.

3.7. Magnetic properties of CdS–MFe₂O₄ NCs (M = Zn, Co and Ni)

Hysteresis loops for the CdS–MFe₂O₄ NCs (M = Zn, Co and Ni) were investigated at 298 K using VSM by applying an external magnetic field between ±10 kOe and are shown in Fig. 7. From the plots of the applied magnetic field and saturation magnetization it is clear that the cobalt ferrite NPs wrapped on the CdS NRs show the maximum magnetization value (Table 2). This magnetization behavior can be explained by Neel’s two sub-lattice model. According to this model, the magnetic moment per formula unit in Bohr magneton n_B^N(χ) is expressed:²⁸

n_B^N(χ) = M_B(χ) − M_A(χ) (4)

where M_B and M_A are the magnetization of B and A sub-lattices, respectively.

Fig. 7 Magnetic hysteresis loops for CdS NR ensheathed ZnFe₂O₄, CoFe₂O₄ and NiFe₂O₄ NPs.

Table 2 Saturation magnetization, coercivity and remanence of CdS nanorod–MFe₂O₄ (M = Zn, Co and Ni) nanocomposites

Nanocomposite sample	Saturation magnetization Ms (emu g^−1)	Coercivity Hc (Oe)	Remanence Mr (emu g^−1)
CdS nanorod–ZnFe2O4	4.0	8.5	0.005
CdS nanorod–CoFe2O4	26.4	377	4.85
CdS nanorod–NiFe2O4	15.5	90	2.58

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This saturation magnetization behavior of CoFe₂O₄–CdS NCs is due to the strengthening of A–B interactions as the cobalt ion has a tendency to occupy an octahedral B site. So, some of the Fe³⁺ ions get transferred to the tetrahedral site and the magnetic moment of M_B gets concerted. So, the saturation magnetization for the CoFe₂O₄–CdS NRs increased. However, with the Zn²⁺ ion substituent, the A–B exchange interactions weaken as Zn²⁺ ions have the tendency to occupy the tetrahedral A sites. So the magnetic moment of M_A gets diluted.²⁹ Therefore, for ZnFe₂O₄–CdS NCs, the curve didn’t get saturated, indicating the fabrication of super-paramagnetic nanocomposites with negligible coercivity and remanence. However, NiFe₂O₄–CdS NCs displayed ferromagnetic behavior. This superparamagnetic behavior of ZnFe₂O₄–CdS NCs can be attributed to smaller particle size and the presence of non-magnetic ions in the spinel ferrite structure, which would have led to single domains.²⁹ These results are corroborated by the presence of a single domain structure in the HR-TEM images (Fig. 3).

A huge difference in the magnetic parameters is also conspicuous from the hysteresis loops of the NCs. It is evident from the figure that the saturation magnetization value for the CdS–ZnFe₂O₄ NCs is 4.0 emu g⁻¹. However, for the CdS–CoFe₂O₄ NCs and CdS–NiFe₂O₄ NCs the saturation magnetization is 26.4 and 15.5 emu g⁻¹, respectively. Similarly, a vast variation in the values of remanence and coercivity was observed. The coercivity for the CdS–ZnFe₂O₄ NCs was 8.5 Oe, which upsurged to 90 Oe and 377.2 Oe for the CdS–NiFe₂O₄ NCs and CdS–CoFe₂O₄ NCs. On a similar line, the remanence for the ZnFe₂O₄–CdS NCs was found to be 0.005 emu g⁻¹ and this value increased substantially for the NiFe₂O₄–CdS NCs and CoFe₂O₄–CdS NCs, respectively, as shown in Table 2.

3.8. Photoluminescence (PL) properties

The PL spectra of all the samples were recorded at an excitation wavelength of 260 nm, using 0.5 mg mL⁻¹ of sample. Acetone was used for dispersing the samples, due to its protic nature it manifested in high intensity emission peaks.³⁰ As is evident from Fig. 8, a strong PL peak at 535 nm appeared for all the samples, along with weak intensity bands at 595 nm and 652 nm. The appearance of the intense peak can be ascribed to excitonic fluorescence caused by the radiative electron–hole recombination of the detrapped electrons. The weak bands may be attributed to the trapped charge carrier at the defect site of the CdS NRs and CdS–MFe₂O₄ NCs.³¹

Fig. 8 Photoluminescence spectra of CdS NRs and their nanocomposites with zinc, cobalt and nickel ferrite nanoparticles.

It is also clear from the PL spectra that the band emission intensities decreased as CdS NRs were loaded with ferrite nanoparticles. Hence, ferrite nanoparticles serve as a p-type semiconductor whereas CdS nanorods work as an n-type semiconductor. The combination of these two distinctive moieties leads to the formation of a p–n type heterojunction, which facilitates the migration of photo-induced electrons and holes at the interface of the MFe₂O₄–CdS NCs.^32,33 The variation band emission intensity followed the order:

CdS NRs > CdS–ZnFe₂O₄ > CdS–NiFe₂O₄ > CdS–CoFe₂O₄.

The conduction band of MFe₂O₄ (M = Zn, Co, Ni) is more negative compared to CdS. Also, the valence band in CdS is more positive compared to MFe₂O₄ (M = Zn, Co, Ni).³⁴ The light irradiation results in the creation of photo-induced electrons and holes. The photo-induced holes move towards the MFe₂O₄ region and the photo-induced electrons drive into the CdS region resulting in electron–hole separation. So, the CdS–ferrite coupled system reduces the electron hole recombination, resulting in quenching emission intensity of the CdS–MFe₂O₄ as compared to pristine CdS NRs.

3.9. Photocatalytic activity measurements

In the photocatalytic degradation experiments, 100 mg of catalyst was added to 100 mL of RhB solution (24 mg/1000 mL) and MB solution (15 mg/1000 mL), which are considered as model pollutants. Before irradiation under the visible light lamp, the solutions were stirred for half an hour in the dark to achieve an absorption/desorption equilibrium between the surface of the catalyst and the dye molecules. Then, this solution was kept under visible light illumination and 2.5 mL aliquots were withdrawn after regular time intervals. Then, the catalyst was removed using an external magnetic field and the dye sample was analyzed using a UV-visible spectrophotometer. The control experiments, without any photo-catalyst, showed no noticeable decomposition of RhB indicating the good photo stability of the dye as shown in our previous reports.^35,36

Discrete ferrite nanoparticles were unable to oxidize the dye molecules in the visible region until the generation of Fenton’s reagent, which is homogeneous in the dye solution. So, this system faces difficulty in separation of the homogenous reagent and also has a drawback in quenching the effect of free radicals generated while analyzing the absorbance spectra of the dye solution. However, when these green magnetic nanoparticles were tailored with visible light responsive moieties via the creation of a heterojunction, these magnetite materials became photoactive heterogeneously, easily separable by an external magnet and the time consumption for the degradation of the dye molecules also reduces.

In the presence of the pure CdS NRs under visible light irradiation, the RhB dye was degraded in approximately 210 minutes. However, the CdS NR–ZnFe₂O₄ NCs degraded the RhB dye molecules completely in 150 minutes. Also, with the CdS NRs, MB dye was degraded in 180 minutes. However, with the zinc ferrite–CdS NR heterostructure, complete decolorization took place in 90 minutes. The percentage of photocatalytic degradation was calculated by applying following equation:³⁷

where A_o is the initial concentration of dye and A_t is the concentration of dye at time t. The percentage degradation of MB and RhB in the presence of all the synthesized composites is shown in Fig. 9(a) and (b) respectively.

Fig. 9 Percentage degradation of (a) methylene blue and (b) rhodamine B with time in the presence of CdS nanorods and their ferrite nanocomposites under visible light irradiation.

It is clear from the results that the ZnFe₂O₄–CdS NCs show maximum efficiency in comparison to the pristine CdS NRs and their corresponding CoFe₂O₄ and NiFe₂O₄ NC structures, which can be attributed to various factors.

The formation of a smooth p–n heterojunction interface and the synergetic effect of the CdS and ferrite nanostructure are also responsible for the uniform transfer of the charges. In such heterojunction coupled systems, photo-induced holes lean towards the photo-chemically stable MFe₂O₄ region and the photo-induced electrons lean towards the active CdS region resulting in electron–hole separation. Thus, photo-generated electrons and holes move in reverse directions, reducing the recombination probability and enhancing the charge separation efficiency, as can be seen from the decrease in the luminescence behavior with the heterojunction as compared to the pristine CdS NRs.³⁸

So, following this mechanism, produced electrons react with the dissolved oxygen molecules to generate the superoxide anion radicals, whereas the holes are scavenged by the adsorbed water to form hydroxyl radicals. The superoxide anions and hydroxyl radicals generated through this procedure decompose the chromophore of the dye molecules into other molecules which diminishes the intensity of the absorbance spectra.³⁹

It is clear from the comparative powder X-ray diffraction pattern of the zinc ferrite loaded CdS NRs with cobalt and nickel ferrite NCs. From the inset of Fig. 2 it is found that the (311) peak of the samples is widened as we move from nickel to cobalt followed by zinc ferrite. This peak broadening indicates a reduction in the particle size of the zinc ferrite nanoparticles, which ultimately increases the surface area of the samples and hence leads to activation of the catalyst at its maximum extent. Similar evidence is also observed from HRTEM and the superparamagnetic nature of the zinc ferrite loaded CdS NCs which could be due to single domain zinc ferrite NPs. BET surface area analysis also showed the maximum value of 79.6 m² g⁻¹ for the zinc ferrite–CdS nanocomposite. So, an increased surface area indicates a reduction in the particle size and enhanced photocatalytic activity of the zinc ferrite encrusted CdS nanorod samples.

The color of the samples shown in Fig. 2 changes from yellow to black with subsequent encrustation of the zinc, cobalt and nickel ferrite nanoparticles on the CdS NRs. Hence, as the color of the sample gets darker, the penetration of the visible light to the illuminated dye solution gets reduced due to the turbidity of the solution. Hence, the photo activity of the samples decreases as the color of the samples darkens.³⁵

The zinc ferrite–CdS NCs have proven to be significant in photocatalysis, in comparison to the other NC samples due to the combined optical, magnetic and structural properties favoring the enhanced activity of the nanocomposite.

4. Conclusion

CdS nanorod–MFe₂O₄ (M = Co, Ni and Zn) nanocomposites (NCs) have been successfully synthesized using a solvothermal route. The synthesized NCs have been characterized using powder XRD, FT-IR, HR-TEM, STEM-point EDX and optical spectroscopy. The surface area of CdS–ZnFe₂O_4, CdS–CoFe₂O₄ and CdS–NiFe₂O₄ NCs was found to be 79.6, 54.5 and 24.4 m² g⁻¹, respectively. The potential of the synthesized NCs as photocatalysts has been explored by studying the degradation of rhodamine B and methylene blue dyes. CdS nanorod–ZnFe₂O₄ NCs were found to be a better photocatalyst than pristine ferrite and CdS nanorods, and degraded rhodamine B and methylene blue dyes in 120 and 90 min, respectively.

Acknowledgements

The authors are grateful to Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR) for providing the necessary financial support. We are also highly thankful to Prof. K. B. Tikoo for the facilitating the HR-TEM Lab, National Institute of Pharmaceutical Education and Research (Mohali). The assistance of Kunash Instruments Pvt. Ltd., Thane (W), in performing surface area analysis of the samples is highly acknowledged. The authors are also grateful to SAIF, Panjab University, Chandigarh, for performing the required sample analysis.

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