Synthesis, enhanced optical and photocatalytic study of Cd–Zn ferrites under sunlight

Abstract

Nanocrystalline Cd_xZn_1−xFe₂O₄ (where x = 0.0, 0.3, 0.7, 1.0) were prepared successfully by a soft chemical synthesis method and were characterized by XRD, SEM, XEDS, FTIR and UV–vis techniques. X-Ray diffraction studies revealed the formation of single phase spinel structure of the samples. Light absorption properties of the Cd–Zn ferrite nanocrystals were studied by a UV–Vis spectrophotometer. It was found that the energy band gap of cadmium ferrite is 1.46 eV, which is smaller than that of prepared zinc ferrite (1.95 eV). Thus cadmium substitution into zinc ferrite results in substantial shifting of the absorption edge of zinc ferrite to red and enhancement of visible light absorption. The photocatalytic results for degradation of methyl orange (MO) indicated that the cadmium substitution dramatically enhances the catalytic activity of zinc ferrite. This means that cadmium substituted zinc ferrite shows the highest photocatalytic activity under solar light irradiation. Thus cadmium substituted zinc ferrite utilizes the possibility of solar energy in the solar spectrum.

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1. Introduction

In recent years, preparations and properties of transition metal ferrites with the molecular formula MFe₂O₄ (M = Zn, Cd, Ni, Mn, Co, etc.) have been largely studied, as they are technologically important materials. Of the many transition metal ferrites, the semiconductor zinc ferrite has drawn a lot of attention due to its potential applications in gas sensors, semiconductor photocatalysis,¹ magnetic data storage, ferrofluid, medical imaging,^2–5 drug delivery, and pigments due to its magnetic properties.⁶ ZnFe₂O₄ (franklinite) belongs to the normal spinel structure with a tetrahedral A-site occupied by Zn(II) ions and a octahedral B-site occupied by Fe(III) ions. This is an anomalous antiferro-magnetic material with a Neel temperature of T_N ≈ 10 K and Curie temperature p ≈ 0 K.⁷

As a low-cost, environmentally friendly, and sustainable treatment technology, semiconductor photocatalysis has been proven to be effective in treating wastewater pollution, hydrogen generation and air purification and has received much attention.^8,9 Photocatalysis is based on the reactive properties of photogenerated electron–hole pairs;¹⁰ thus, the performance of the used photocatalyst is the most important factor influencing catalytic efficiency. Among all photocatalysts, nano-titanium dioxide (TiO₂) is the most studied because of its nontoxicity, photo- and chemical stability, low production cost, and superior photocatalytic reactivity.¹¹ However, these oxides have no effective absorption in the visible light region (λ > 420 nm) and can only operate within the UV light region due to their band gaps being higher than 3.0 eV. On the one hand, UV light only accounts for ca. 4% of the incoming sunlight, and it leads to low light utilization efficiency.¹² Its practical applications seem limited for several reasons, among which one is the low photon utilization efficiency; another is the need to use the ultraviolet (UV) as an excitation source.

To make full use of solar energy, many attempts have been made to prepare the narrow band gap semiconducting material that utilizes the much larger visible region. Among those, zinc ferrite (ZnFe₂O₄) is regarded as a promising visible-light photocatalyst with a band gap of 1.9 eV that makes it possible to utilize solar energy.¹³ In addition to its photochemical stability and low toxicity, ZnFe₂O₄ has been applied to degrade organic pollutants.¹⁴ However, the poor quantum efficiency of ZnFe₂O₄ results in low photocatalytic activity. In order to solve these problems, the modification of these catalysts has also been attempted by doping them with various transition metals. This can be improved by loading Ag on the ZnFe₂O₄ surface.¹⁵ However, the content of Ag is as high as 22.7%, which is impractical for application.

It has been reported that Somenath et al.¹⁶ investigated the conductivity properties of Cd–Zn ferrite, Mahuya et al.¹⁷ studied the Mossbauer and positron annihilation properties of Cd–Zn ferrite, Siddique et al.¹⁸ reported the composition dependence of quadrupole splitting in Cd–Zn ferrites. To the best of our knowledge no report has been cited in the literature on the catalytic properties of Cd–Zn ferrite nanoparticles.

Considering the importance of Cd–Zn ferrite nanoparticles and their wide applications, an improvement of the synthesis appears to be of great interest. Thus in the present study we prepared Cd–Zn ferrites by the co-precipitation process nearly at 100 °C. This is well below the temperature reported, Chinasamy et al. employed milling techniques for the conventionally prepared bulk CdFe₂O₄ sample, Mostafa et al. employed a ceramic technique at 1000 °C, Mazen et al. employed a ceramic technique at 1150 °C.¹⁹ UV–vis absorption studies of the samples were performed and enabled us to calculate the band gap energy of the nanoparticles. Methyl orange (MO) was selected as a model pollutant, the photocatalytic property of cadmium–zinc ferrite nanoparticles under real solar light irradiation was investigated and reported.

2. Experimental details

2.1. Materials used

The starting materials such as ferric chloride (FeCl₃·6H₂O), zinc chloride (ZnCl₂), cadmium chloride (CdCl₂·H₂O) and sodium hydroxide (NaOH) are used. Methyl orange was used as a target compound for degradation. All chemicals and solvents were of analytical grade purchased from Himedia laboratory. All the compounds were used as received without further purification.

2.2. Catalyst preparation

Synthesis of Cd–Zn ferrite nanoparticles was done by a co-precipitation method.^20,21 According to the chemical formula Cd_xZn_1−xFe₂O₄ (where x = 0.0, 0.3, 0.7, 1.0), each starting material was weighed separately and added suitable quantity of de-ionized water to make them 0.5 M solutions and then mixed all these cationic solutions while stirring to complete dissolution. The NaOH solution was prepared in sufficient quantity at 0.2 M concentration, heated to 60 °C and poured into the cationic solution in a thin flow while maintaining the stirring and heating till the precipitation occurs. Heating of the precipitate under alkaline conditions (pH = 8.5 to 9.5) was continued to a soaking temperature of 100 °C for 40 minutes in order to complete the reaction. Stirring was maintained further for 7 h for ageing and then the precipitated particles were washed and filtered before drying them at 60 °C for 48 hours. The co-precipitated ferrite agglomerates were then ground for few minutes using an agate mortar and pestle to have very fine particles. These particles were subsequently heat treated at 360 °C for further crystallization.

2.3. Characterization techniques

The phase compositions and structures of the Cd–Zn ferrite samples were determined by powder X-ray diffraction (PXRD, PANalytical Xpert Pro X-ray Diffractometer) with Cu-Kα radiation (k = 0.15406 nm) over the 2θ range of 10–80°. The morphology of Cd–Zn ferrite samples was observed by scanning electronic microscopy (SEM) with a JSM-6700 LV electron microscope operating at 5.0 kV, and the chemical compositions were examined by X-ray energy dispersive spectroscopy (XEDS). The structural characterization of the nanocrystalline Cd–Zn ferrite samples were examined by Fourier transform infrared spectroscopy (FTIR) (using a Nicolet IR200 FT-IR spectrometer). Light absorption properties of the Cd–Zn ferrite nanocrystals dispersed in ethylene glycol were studied by a UV–Vis spectrophotometer (Shimadzu, UV-1650 PL model).

2.4. Measurement of photocatalytic activity

Photocatalytic activities of the synthesized Cd–Zn ferrites were measured under solar light irradiation by the decomposition of methyl orange in an aqueous solution at ambient temperature. In each experiment, 0.05 g of Cd–Zn ferrite nanoparticles were added into 50 ml of methyl orange solution with a concentration of 10 mg l⁻¹. The suspension was magnetically stirred in the dark for 30 min to establish the adsorption/desorption equilibrium at room temperature, then the solution was directly irradiated under sunlight. During irradiation, stirring was maintained to keep the mixture in suspension. At regular intervals, samples were taken from the suspension and then centrifuged to remove the Cd–Zn ferrite nanoparticles. The change in the concentration of each degraded solution was monitored on a UV–Vis spectrophotometer (Shimadzu, UV-1650 PL model) by measuring the absorbance in the range of 200–800 nm wavelength. Distilled water was used as the reference sample.

3. Results and discussion

3.1. XRD and SEM analysis

In order to investigate the crystal structure of the obtained powder material XRD analysis was performed and the resultant pattern of the samples is presented in Fig. 1. The analysis of the diffraction pattern by using (111), (220), (311), (222), (400), (422), (333), (440), (620), (533), (622) and (444) reflection planes confirms the formation of cubic spinel structure. All diffraction peaks can be indexed in a single cubic lattice, and the positions along with relative intensity of peaks match well with standard ZnFe₂O₄ and CdFe₂O₄ powder diffraction data (JCPDS card no. 82-1049) and (JCPDS card no. 22-1063), respectively, indicating that the obtained products have an Fd3m(227) cubic spinel structure. All the samples have been analyzed in the same way; no diffraction peaks of other structures were detected in the sample, indicating that there were no secondary phases or precipitates in the samples. The full width at half maximum (FWHM) of the XRD peaks was used to calculate the crystallite size D of Cd–ZnFe₂O₄ using Scherrer's relation.²²where β is the broadening of the diffraction line measured at half maximum intensity (radians) and λ = 1.5406 Å, the wavelength of Cu-Kα. The average crystallite sizes were found to be 55, 44, 30, and 49 nm with respect to compositions x = 0.0, 0.3, 0.7, and 1.0 respectively. The lattice constant of the Cd–ZnFe₂O₄ nanostructure was determined using the following relationship.²³

Fig. 1 XRD spectra of the Cd_xZn_1−xFe₂O₄ nanoparticles synthesized by a co-precipitation method.

The lattice parameter a varies with Cd²⁺ concentration x. It was observed that the lattice parameter increases linearly with concentration x. The calculated lattice parameters a for ZnFe₂O₄ and CdFe₂O₄ samples were found to be 0.844 and 0.869 nm, respectively, which are in good agreement with the literature values of 0.844 and 0.869 nm for ZnFe₂O₄ and CdFe₂O₄ samples.²⁴ The larger value of lattice parameter a for CdFe₂O₄ compared to ZnFe₂O₄ is due to the larger ionic radius of Cd²⁺ than Zn²⁺.¹⁷

The scanning electron microscopy studies were undertaken for the samples with x = 0.0 (Fig. 2a) and x = 1.0 (Fig. 2b) and images are shown in Fig. 2(a) and (b). It is evident from the SEM micrographs that these samples have uniform, almost spherical structural, morphology with a narrow size distribution of the particles. XEDS analysis of the as-prepared samples shows that Fe, Zn, Cd and O atoms are the main components present in the samples, as shown in Fig. 3(a) and (b). As prepared ZnFe₂O₄ samples (Fig. 3(a)) contain Fe = 68%, Zn = 32%, suggesting that the atomic ratio of Zn : Fe is (very close) 1 : 2 and CdFe₂O₄ samples (Fig. 3(b)) contain Fe = 83%, Cd = 17%, suggesting that atomic ratio of Cd : Fe is 1 : 5, which confirms the formation of ZnFe₂O₄ and CdFe₂O₄ nanoparticles.

Fig. 2 SEM micrographs of x = 0.0 (a) and x = 1.0 (b) samples.

Fig. 3 X-Ray energy-dispersive spectroscopy of x = 0.0 (a) and x = 1.0 (b) samples.

3.2. FTIR analysis

FTIR spectra were recorded in the range of 400–4000 cm⁻¹. The IR bands of solids are usually assigned to the vibrations of ions in the crystal lattice.²⁵ Two main broad metal–oxygen bands are seen in the FTIR spectra of all spinels and ferrites in particular. The highest one, ν₁, is generally observed in the range 600–500 cm⁻¹ and it corresponds to intrinsic stretching vibrations of the metal at the tetrahedral site (T_d) M_tetra ↔ O [A site], whereas the ν₂ lowest band usually observed in the range 450–385 cm⁻¹ is assigned to octahedral-metal stretching (O_h) M_octa ↔ O [B site].²⁶ The FTIR spectrum of nanocrystalline Cd–Zn ferrite is shown in Fig. 4. In the FTIR spectrum of the products, a band appears at around 549 cm⁻¹ which is assigned as ν₁ (M_tetra ↔ O) and another band at around 416 cm⁻¹ is assigned as ν₂. The absorption bands observed at ∼3437 cm⁻¹ and ∼1639 cm⁻¹ suggested the presence of adsorbed water on the surface of ferrite nanoparticles. The bands occurring at ∼2925–2854 cm⁻¹ are assigned to antisymmetric and symmetric CH₂-vibrations of the carbon chains respectively. The ν₁ and ν₂ band values of the Cd_xZn_1−xFe₂O₄ nanoparticles are summarized in Table 1. In the present spectrum the ν₁ band that appeared between 543 and 559 cm⁻¹ is shifted towards smaller wavenumbers with increasing Zn content or in other words the ν₁ band continues to shift towards the higher values with increasing Cd concentration. This is because the prominent ν₁ band is due to the Fe³⁺–O²⁻ bond. The larger atomic radius of the Cd²⁺ (1.40 Å) when substitutes Zn²⁺ (1.19 Å) or displaces the Fe³⁺ ion to the B-site; results in the firmness of Cd²⁺ ion at the tetrahedral configuration. The Fe³⁺ and O²⁻ ions need lower wavenumber or higher energy for vibration. The resonant frequency of this group increases with the decrease in the Fe³⁺–O²⁻ bond length.^27–30 These strong absorption bands are typical of inverse spinel ferrite nanostructures. This FTIR spectrum results were quite similar to that given in the literature.³¹

Fig. 4 FTIR analysis of the Cd_xZn_1−xFe₂O₄ nanoparticles.

Table 1 Values of absorption bands (ν₁ and ν₂) for Cd_xZn_1−xFe₂O₄ nanoparticles

Composition, x	Absorption band
	ν 1/cm^−1	ν 2/cm^−1
0.0	543	415
0.3	549	426
0.7	555	406
1.0	559	418

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3.3. Optical absorption spectra studies

The analysis of optical absorption spectra is one of the most productive tools for understanding and developing the band structure and energy gap of both crystalline and amorphous materials.³² The optical properties of the Cd–Zn ferrite nanoparticles were studied with the help of optical absorption data. The optical absorption spectra of the Cd–Zn ferrite samples x = 0.0, 0.3, 0.7, 1.0 are shown in Fig. 5. The result shows that the absorption band slightly shifted to the visible region with the increase in the cadmium concentration. The absorption bands of Cd–Zn ferrites has no structures such as shoulders which possess relatively steep edges, indicating that the absorption in the visible light region may not be due to surface states but to an intrinsic band transition.³³ It is well known that in the normal spinel-type compound Cd–Zn ferrite, tetrahedral and octahedral sites are occupied by (Zn²⁺, Cd²⁺) and Fe³⁺ cations, respectively.³⁴ The band structure of Cd–Zn ferrite is generally defined by taking the O-2p orbital as the valence band and the Fe-3d orbital as the conduction band.³⁵ The absorption of Cd–Zn ferrite in the visible light region may be due to the electron excitation from the O-2p level into the Fe 3d level.³⁶ In Zn ferrite band gap between the O-2p level and the Fe-3d level is 1.95 eV, whereas with increasing Cd concentration, the band gap between the O-2p and Fe-3d levels decreases as 1.85, 1.7 and 1.46 eV w.r.t x = 0.3, 0.7 and 1.0 as shown in Fig. 6. This shows that in Cd substituted samples, metastable energy levels are formed within the energy gap. Therefore, the energy required to excite the electron from the O-2p into the Fe-3d level goes on decreasing as band gap decreases. So Cd–Zn ferrite shows a red shift with increasing Cd concentration.³⁷ This makes it possible to utilize more percent of solar energy. So for Cd–Zn ferrite samples, a better photocatalytic capability under solar light was expected.

Fig. 5 Optical absorption spectra of Cd_xZn_1−xFe₂O₄ nanoparticles.

Fig. 6 Plot of (αhγ)² as a function of photon energy (eV) for Cd_xZn_1−xFe₂O₄ compositions.

3.4. Optical studies

The energy band gap and optical absorption of a material are the most important parameters when referring to optical materials. In some cases the band diagram remains unchanged for materials having the same constituents but the band gap energy changes according to the proportions of the elements in the composition.

The absorption coefficient α of the Cd–Zn ferrite nanoparticles has been determined from the absorption data by using the fundamental relationships:^38,39

I = I₀e^αt (3)

A = log(I₀/I) (4)

and

α = 2.303(A/t) (5)

where A is the absorption and t is the thickness of the Cd–Zn ferrite samples. To estimate the optical absorption edge for these nanoparticles, (αhγ)^1/n was plotted as a function of the photon energy hγ for different n values (n = 1/2, 3/2, 2, 3) (Tauc plots).⁴⁰ The best linear fit was obtained in the case of n = 1/2, which indicates a direct allowed optical transition in Cd–Zn ferrite nanoparticles. The Tauc plot is presented in Fig. 6. The straight line fit to the (αhγ)^1/nvs. hγ plot is obtained by using linear regression software with only very small standard deviation. The intercept of the line at α = 0 gives the value of the optical absorption edge. Fig. 6 shows the variation of optical band gap with different composition x. It is observed that the energy band gap decreases with increase in Cd concentration. The estimated energy band gap for zinc ferrite was found to be approximately 1.95 eV. This band gap value is nearly equal to experimental values reported in the literature.³⁶ Increase in cadmium concentration x = 0.3, 0.7 and 1.0 results in the red shift of the apparent optical band gap 1.85, 1.7 and 1.46 eV respectively. Red shift effects have been observed for ZnFe₂O₄ nanoparticles capped by a surfactant⁴¹ as well as for other systems.⁴² In our coprecipitation nanoparticulated samples, since the size of the agglomerated particles is larger than the exciton Bohr radius of bulk ferrite nanoparticles, the sample has a weak quantum confinement. And similarly,⁴² the interface of nanoparticles obviously contains many oxygen vacancies and may be a source of trapped exciton states that form a series of metastable energy levels within the energy gap, resulting in the red shift of the apparent optical band gap.⁴³ The samples x = 0.7 and 1.0 do not have a single distinct absorption onset, there is a broad absorption as shown in Fig. 5 which may be the reason for the nonlinearity at higher energy curves (for x = 0.7 and 1.0) in Fig. 6.⁴⁴ These band gap energies are greater than the theoretical energy required for water splitting (λ > 1.23 eV),⁴⁵ thus, they are suitable for the role of solar light photocatalysts.

3.5. Photocatalytic activities

The photocatalytic activities of the Cd–Zn ferrite samples were evaluated by the photocatalytic degradation of methyl orange solution under sunlight irradiation as shown in Fig. 7. As can be clearly seen in Fig. 7, the methyl orange in aqueous solution could be hardly degraded without the catalyst under illumination. Whereas, the methyl orange aqueous solution was degraded by the addition of Cd–Zn ferrite catalysts. Curve [B] shows that Zn-ferrite has a little photocatalytic activity under sunlight irradiation, despite single Zn-ferrite having a good absorption of visible light. In comparison Cd ferrite (Curve [E]) shows highest photocatalytic activities among other samples. This may be the reason that the samples x = 0.3, 0.7 and 1.0 are semiconductors having a relatively narrow band gap (1.85 eV, 1.7 eV, 1.46 eV, respectively), which shows the more ability to absorb the visible light and in comparison with Cd substituted ferrites, Zn ferrite has the wide band gap (1.923 eV).^36,46 The higher efficiency of Cd ferrite in comparison with other samples (x = 0.0, 0.3, 0.7) can be correlated to its ability to absorb a larger fraction of visible light,⁴⁵ as demonstrated in Fig. 5. Considering the other ferrite samples (band gap 1.7 to 1.95 eV), the Cd ferrite sample has narrow band gap energy (1.46 eV), metastable energy levels are formed within the energy gap, this shifts the absorption to visible region. Because of the lower band gap, the number of electrons reaching the conduction band increases, that is, the electron density in the conduction band is relatively high. Consequently, the number of holes in the valence band also increases. Both these electrons and holes interact with surface bound H₂O or OH⁻ to produce OH˙ radicals. This favors the formation of more OH˙ free radicals, which are main active species in the photocatalytic degradation process.^47–49 Collectively, the optical property (small band gap, large visible light absorption) and crystallinity factors make Cd ferrite a more efficient photocatalyst than the other samples under solar light irradiation.

Fig. 7 Degradation of methyl orange (10.0 mg L⁻¹) with different Cd_xZn_1−xFe₂O₄ photocatalysts under solar light irradiation: [A] without photocatalyst; [B] x = 0.0; [C] x = 0.3; [D] x = 0.7; [E] x = 1.0.

4. Conclusion

We have shown that Cd_xZn_1−xFe₂O₄ nanoparticles with narrow size distribution have been successfully prepared by a co-precipitation technique at low temperature compared to the conventional ceramic route of synthesis reported in the literature. The absorption band of Cd_xZn_1−xFe₂O₄ is wider, which shifts to the visible light regions with increase in the concentration of Cd²⁺. The structural and optical properties of the nanoparticles point to a direct allowed transition in the nanoparticles. Cd ferrite shows a good photocatalytic degradation of methyl orange under solar light irradiation. In an attempt to modify the optical and photocatalytic properties of zinc ferrite, we were successful in substituting cadmium into zinc ferrite structure, which shifted the absorption into the visible region and shows the obvious enhancement of photo-catalytic activity under solar light irradiation in contrast to zinc ferrite without cadmium substitution. Therefore, Cd ferrite material may be used as an active component of a single or a composite system for the decomposition of toxic or hazardous gases under solar light irradiation.

Acknowledgements

One of the authors, K. N. Harish, wishes to thank Kuvempu University for providing junior research fellowship and the St. John College, Bangalore, for providing spectral data.

Notes and references

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