CoFe₂O₄–ZnS nanocomposite: a magnetically recyclable photocatalyst

Abstract

A new synthesis of a CoFe₂O₄–ZnS magnetic nanocomposite has been achieved and used as an efficient photocatalyst in the degradation of methyl orange in water under UV irradiation. The photocatalyst is magnetically recoverable from the reaction medium and suitable for multiple uses with sustained catalytic activity.

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Introduction

Magnetic nanocomposites (NCs) are gaining importance in recent years for their potential applications in varied fields of fundamental science and technology such as biomedical engineering, chemical sensing, magneto-optical devices, solar cells and catalysis.^1,2 These hybrid structures have been extensively investigated because they can provide the combined advantages of the constituent materials relative to their single-component counterparts. They exhibit improved chemical and physical properties such as thermal and chemical stability, better solubility, less or no cytotoxicity, etc.

Semiconductor nanocatalysts having high surface area, strong UV absorption and long life-span have been widely used for the degradation of organic pollutants in water.^3–5 Among these, ZnS (bandgap energy: 3.6–3.8 eV) has served as one of the most efficient photocatalysts for wastewater treatment such as in the photodegradation of dyes, p-nitrophenol, and halogenated benzene derivatives, reduction of heavy metals and water-splitting for H₂ evolution.^6–11 Recently, Suslick developed a nanostructured Ni²⁺-doped ZnS photocatalyst for the evolution of H₂ under visible light.¹² ZnS nanoparticles (NPs) are a good photocatalyst due to the rapid generation of electron–hole (e−/h+) pairs by photoexcitation and high negative reduction potentials of excited electrons. However, they have the serious limitation of easy recombination of e−/h+ pairs which in turn reduce the photocatalytic activity significantly.^13,14 Moreover, these types of suspended photocatalysts suffer a noticeable disadvantage in ease of separation, recovery, recycling and high cost in large scale production.¹⁵ In addition, they tend to agglomerate in aqueous solution and hinder the penetration of lights.

Consequently, design and development of magnetic NCs with high catalytic activity and easy separation can heighten their potential for practical applications. Magnetic materials such as Fe₃O₄,^16,17 BaFe₂O₄,^18,19 NiFe₂O₄^20,21 and γ-Fe₂O₃²² have recently been utilized as core materials in making magnetic NCs with TiO₂ or ZnO semiconductor shells, which can be easily separated from a solution by applying a magnetic field.

It is also noteworthy that the fabrication of a magnetic-nanocomposite with a uniform coating over the core surface in nano-scale dimensions is a challenging task due to difficulties in complete dispersion of the magnetic core in solution. Moreover, the commonly used core materials such as Fe₃O₄ or γ-Fe₂O₃ are not chemically or thermally stable.²³ Therefore, a new approach for preparing a stable and highly dispersible magnetic core is desirable for effective coating with catalytically active species in nano-scale dimensions.

Recently, we have reported a new magnetic nanocomposite using ferromagnetic CoFe₂O₄ as the core material with multi-functional properties.^24–27 CoFe₂O₄ is well-known to have large magnetic anisotropy, moderate saturation magnetization, remarkable chemical stability and mechanical hardness,²⁸ which makes it a good candidate as a core material for the fabrication of NCs. Zhang reported a new CoFe₂O₄-ZnO nanocomposite with multiple properties.²⁹ Very recently, Mourão reported the synthesis of a CoFe₂O₄-TiO₂ nanocomposite for photodegradation of dye in water.³⁰ The photocatalytic activity of these NCs might be related to the electronic and absorption properties of the core materials which could restrain the recombination of e−/h+ pairs by absorbing photo-generated electrons from the conduction band of the semiconductor. Thus, the avenue of CoFe₂O₄ as a core material can be explored for the development of a conjugate of CoFe₂O₄ and ZnS NPs. However, no study on photocatalytic activity of CoFe₂O₄–ZnS NCs has been reported so far. Therefore, a study on such conjugates might pave the way for developing recyclable photocatalysts for the degradation of organic contaminants in water and environmental remediation.

2. Experimental section

2.1 Synthesis of CoFe₂O₄–ZnS nanocomposite

Initially, a magnetic core of CoFe₂O₄ NPs was prepared by ultrasonication-assisted co-precipitation and then a layer of ZnS NPs was structured over it forming the CoFe₂O₄–ZnS nanocomposite. The CoFe₂O₄ magnetic core was synthesized following our previously reported method^24,25 except a 1 : 1 adduct of oleic acid-dodecylamine was used for higher stability and better compatibility with ZnS. Then, ZnS NPs were prepared by a modified method reported by Saravanan et al.³¹ and added in to the dispersion of CoFe₂O₄ NPs in ethanol to obtain CoFe₂O₄–ZnS core-shell NCs (CoFe₂O₄–ZnS NCs). The detail of the synthetic protocol is described in the supplementary information.†

2.2. Characterization

The X-ray diffraction (XRD) patterns of the as-synthesized samples were measured using an X-ray diffractometer (Bruker AXS D8) with Cu-Kα radiation (λ = 1.54178 Å) and a graphite monochromator from 20° to 70° (2θ) at a scanning rate of 5° min⁻¹. The particle size and morphology of the samples were characterized by transmission electron microscopy (TEM, JEOL JEM2100) using an accelerating voltage of 200 kV with the potential of performing selected area electron diffraction (SAED). Field emission scanning electron microscopy (FESEM, Carl Zeiss, ΣIGMA) was also performed to observe the surface morphology of the samples. For TEM measurement, a drop of a dispersion of the NPs in ethanol was placed on a carbon coated copper grid (400 mesh size) and evaporated to dryness. The absorption spectra of methyl orange (MO) solutions were obtained by a UV-Vis spectrophotometer (Varian, Cary 50). The Raman spectra of the samples were recorded using a 633 nm line of He-Ne laser as the excitation source in a Laser Micro-Raman Spectrometer (HORIBA Jobin Yvon, Lab RAM HR) at room temperature. The magnetic properties of the CoFe₂O₄–ZnS NCs and CoFe₂O₄ NPs were measured in a vibrating sample magnetometer (VSM, Lakeshore 7410) with a maximum applied field of ±2 T. The specific surface area and porosity were measured by the application of the Brunauer-Emmett-Teller (BET) equation and the Barrett-Joyner-Halanda (BJH) method.³² The sample was first degassed for 2h at 200 °C and then analyzed by a Backman Coulter (SA 3100) Surface Area and Pore Size Analyzer. Liquid chromatography–mass spectrometry (LCMS, Waters, Q-Tof Premier LCMS) analysis was performed to observe the degradation products of MO.

2.3. Photocatalytic activity measurement

The photocatalytic activities of the as-synthesized samples were measured on degradation of MO in aqueous medium under UV illumination at different time intervals. The samples (0.02 g of each) of CoFe₂O₄–ZnS, ZnS and CoFe₂O₄ NPs were placed separately into a 200 mL tubular quartz vessel containing 100 mL of 1 × 10⁻⁵ M MO aqueous solutions and mixed by ultrasonication for 10 min. Then, the mixtures were stirred in the dark (30 min) to obtain adsorption–desorption equilibrium until the concentration of MO was constant. The mixtures were then illuminated with a UV lamp (12 mW, main wavelength of 400 nm), and were put into the bottom of the quartz vessel and stirred continuously. All experiments were carried out at ambient temperature (25 °C) with circulation of water through the outer jacket of the reaction vessel. During the course of the reaction, the mixtures were sampled at different times and separated by a magnet to discard any residue. Then, an absorption spectrum of each sample was recorded through a wavelength scan from 300 to 600 nm on a UV-Vis spectrophotometer. The characteristic absorption of MO at 460 nm was chosen as the monitoring parameter for the photocatalytic degradation process. The photocatalytic degradation percentage of MO was calculated using eqn (1) where A₀ is the initial absorbance of MO before degradation and A is the absorbance after time t.

Results and discussion

In the present article, we report the synthesis and characterization of a core–shell CoFe₂O₄–ZnS NC and its photocatalytic property. The composite was found to exhibit excellent performance in photodegadation of MO in water under UV irradiation. To the best of our knowledge, this is a new magnetic NC which has been used for the photodegradation of a dye.

The Powder XRD measurement was done individually for the ZnS, CoFe₂O₄ and CoFe₂O₄–ZnS NPs to characterize the phase and crystallization (Fig. 1a–c). It can be seen that cubic ZnS NPs (JCPDS card 5-0566)³³ have been prepared and the broadening of diffraction peaks is in accordance with the nanocrystalline nature of the ZnS particles. The diffraction pattern of the as-synthesized CoFe₂O₄–ZnS particles (Fig. 1a) shows that all the peaks are in good agreement with the cubic structure of spinel CoFe₂O₄ (JCPDS cards: 3-864 and 22-1086)^34,35 and of ZnS NPs. These results reveal that the nanocomposite is composed of cubic-structured CoFe₂O₄ and ZnS NPs. The average size of the crystalline particles for the primary CoFe₂O₄ NPs and ZnS NPs was estimated to be 30 ± 2 nm and 7 ± 1 nm, respectively, based on the Debye–Scherrer formula.³⁶

Fig. 1 XRD pattern of (a) CoFe₂O₄–ZnS (*contribution from ZnS NPs), (b) CoFe₂O₄ and (c) ZnS NPs.

Fig. 2 shows the representative TEM image, with the SAED pattern in the inset, of the as-prepared ZnS NPs showing the uniform size-distribution with the size in between 5–10 nm, which is in good agreement with the size determined from the Debye–Scherrer relation.³⁶ The diffraction rings have been identified as the (111), (220) and (311) planes of the cubic ZnS phase.³³ The morphology of the CoFe₂O₄–ZnS NCs was analyzed by FESEM (Fig. S1 in ESI†) and TEM measurement as shown in Fig. 2 (c, d) which show the uniform size distribution of NPs with cubic morphology. The corresponding SAED pattern of the composite is shown in the inset of Fig. 2(e). The crystalline sizes were found to be about 30 ± 5 nm, which were close to the average crystallite size obtained from the Debye–Scherrer formula based on XRD patterns. A close observation of the FESEM micrographs indicates that the primary CoFe₂O₄ nanocrystals are covered with ZnS NPs. The TEM image of CoFe₂O₄–ZnS NCs shows the core-shell nanostructure, where the dark particles of CoFe₂O₄ are shielded by a thin layer of bright ZnS particles (Fig. 2c–d).

Fig. 2 TEM image of (a) ZnS NPs and (c, d) CoFe₂O₄–ZnS NCs with the corresponding SAED patterns (b, e).

The lattice image of the nanocomposite (Fig. S3–S5 of ESI†) marked with the (111) lattice plane of spinel CoFe₂O₄ and the (220) lattice plane of cubic ZnS NPs revealed the highly crystalline nature of the NCs. The high resolution TEM (HRTEM) image allowed us, by means of Gatan Digital Micrograph^TM software, to obtain the fast Fourier transform (FFT) from which lattice spacings of 4.59 A° and 2.7 A° were obtained (experimental error, <5%) for the CoFe₂O₄ and ZnS NPs, respectively.

The as-synthesized NPs were further characterized by laser micro Raman spectroscopy which is a useful tool for investigating NPs and their surrounding media.³⁷ Strong evidence for the core–shell structure of CoFe₂O₄–ZnS NCs was also observed in the Raman spectra. The inset in Fig. 3 shows the Raman spectra of ZnS NPs with the characteristic peaks at around 259 and 345 cm⁻¹ which indicates the cubic structure of ZnS NPs.³⁸ The spinel CoFe₂O₄ shows five Raman active modes (A_g + E_g + 3E_2g).³⁹ Three modes of CoFe₂O₄ NPs at 458 cm⁻¹, 600 cm⁻¹ and 665 cm⁻¹ can be seen in Fig. 3(b) along with the 258 cm⁻¹ and 345 cm⁻¹ modes from ZnS NPs. We did not observe any significant shifts in bands of Raman spectra, which indicates that there is no substitution in the CoFe₂O₄–ZnS nanocomposite.⁴⁰ Therefore, it may be inferred that the core–shell structured CoFe₂O₄–ZnS NC has been effectively synthesized.

Fig. 3 Raman spectra of (a) CoFe₂O₄ NPs and (b) CoFe₂O₄–ZnS NCs and the inset is of ZnS NPs (*Raman scattering peaks from ZnS NPs).

We further evaluated the magnetic properties of the CoFe₂O₄ core and the CoFe₂O₄–ZnS NCs by VSM measurement at room temperature as shown in Fig. 4. The NCs give the coercivity (Hc) and saturation magnetization (Ms) of 1.2 kOe and 17.5 emu g⁻¹ respectively which is only a negligible change from the CoFe₂O₄ core (Hc: 1.6 kOe and Ms: 18 emu g⁻¹) and that is an indication of the formation of nano-scale coating at the core-surface. The band-gap energy, E_g of the as-synthesized nanocomposite was determined based on the Tauc plot⁴¹ and was found to be 3.2 eV from the absorbance spectra (Fig. S6 in ESI†).

Fig. 4 Magnetic hysteresis loops for (a) CoFe₂O₄ NPs and (b) CoFe₂O₄–ZnS NCs.

The specific surface area and porosity were measured by the application of the BET equation and the BJH method. Fig. 5 shows the N₂ adsorption–desorption isotherms of the CoFe₂O₄–ZnS NC. The BET surface area of the NCs was found to be 30.269 m² g⁻¹ as calculated by the linear part of the BET plot, which is much higher than that of the CoFe₂O₄ NPs prepared by the co-precipitation method (17.97 m² g⁻¹).⁴² The total pore volume at P/P_o = 0.98 is 0.18 cm³ g⁻¹. The BET isotherm is of type II and H3 hysteresis loop (BDDT/IUPAC classification), characteristic of mesoporous adsorbents.⁴³

Fig. 5 BET isotherm and pore size distribution (inset) of the as-synthesized CoFe₂O₄–ZnS magnetic NPs.

To investigate the potential application of the magnetic NC as a recyclable photocatalyst for the removal of contaminants from wastewater, we performed the degradation of MO in an aqueous medium driven by UV-irradiation as a probe reaction. Fig. 6 (I) and (II) show the absorption spectra of an aqueous solution of MO irradiated under UV-light for different time intervals in the presence of CoFe₂O₄–ZnS NCs and ZnS NPs, respectively. The typical absorption peak at 460 nm gradually disappears as the UV-irradiation time increases, and completely diminishes after 20 min, meaning complete photodegradation of MO by the CoFe₂O₄–ZnS NCs. The photocatalytic degradations of MO over different time intervals using the as-synthesized samples as the catalyst and with no catalyst are shown in Fig. 6 (III). We have observed that the CoFe₂O₄–ZnS NCs exhibit high photocatalytic efficiency with a degradation of 98% in 20 min of UV light exposure. The photocatalytic degradation of MO was further investigated by LCMS analysis as described in the supplementary information (Fig. S10–13 in ESI†) which revealed its appreciable degradation under the photocatalytic conditions applied. Thus, a NC with enhanced photocatalytic activity has been achieved that is capable of faster degradation of MO than any other reported method.^44–46

Fig. 6 Changes of UV-Vis spectra of MO with irradiation time in the presence of (I) CoFe₂O₄–ZnS NCs and (II) ZnS NPs and (III) degradation curves of MO with (a) CoFe₂O₄–ZnS NCs, (b) no catalyst, (c) ZnS NPs, (d) CoFe₂O₄ NPs and (e) TiO₂ NPs under UV light irradiation.

For comparison, we also checked the photocatalytic activity of the as-synthesized CoFe₂O₄ core and ZnS NPs shell employing entirely the same photocatalytic conditions. While keeping all the reaction conditions the same, the ZnS NPs showed low photocatalytic activity and no apparent photocatalytic activity was also detected for CoFe₂O₄ NPs under the identical UV-irradiation. We also tested the photocatalytic activity of TiO₂ NPs (TEM and XRD are shown in Fig. S7 and S8, respectively, in ESI†) under the same reaction conditions and observed very low degradation of MO in 20 min of UV light exposure. The good photocatalytic activity of the CoFe₂O₄–ZnS NCs with a larger rate (rate constant: 0.0718 min⁻¹) in spite of much larger particle sizes as compared to ZnS NPs, which have considerably low photocatalytic activity (rate constant: 0.0041 min⁻¹), may be associated with small crystallites and poor crystallinity of small NPs. In addition, the high photocatalytic efficiency of the NCs as compared to ZnS NPs is due to the electrostatic interaction between the core and shell components of the conjugate nanostructure of CoFe₂O₄ and ZnS NPs.

The corresponding mechanism can be interpreted as follows: under UV light irradiation, electrons in the valence band (VB) of ZnS are excited to its conduction band (CB) leaving the same amount of holes in the VB forming the e−/h+ pairs. Due to the strong interfacial connection between ZnS and CoFe₂O₄, the excited electrons of the CB of ZnS can transfer to that of CoFe₂O₄. So, the e−/h+ pairs are separated at the CoFe₂O₄–ZnS interfaces, which decreased their recombination probability. Meanwhile, the O₂ absorbed on the CoFe₂O₄ surface can accept electrons and enhance the efficiency of generating hydroxyl radicals, which are very reactive towards the degradation of organic molecules and so enhanced photocatalytic activity is observed. A plausible mechanistic scheme of the charge separation and the photocatalytic activity for the photocatalyst is shown in Scheme 1.

Scheme 1 Illustration of photocatalysis by CoFe₂O₄–ZnS NCs.

The photocatalytic decomposition of MO by CoFe₂O₄–ZnS NCs follow a pseudo first-order kinetic law, and can be expressed as

where C and C_o are the reactant concentration at time t = t and t = 0, respectively, and k and t are the pseudo-first order rate constant and time respectively.⁴⁷ The relationships between ln(C_o/C) and irradiation time (t) for both the CoFe₂O₄–ZnS and ZnS samples are shown in Fig. 7.

Fig. 7 Kinetics of MO degradation in the presence of ZnS NPs and CoFe₂O₄–ZnS NCs.

It can be seen that there exists a linear relationship between ln(C_o/C) and irradiation time. The pseudo-first-order rate constant and linear regression coefficients (R) for photo-degradation of MO with CoFe₂O₄–ZnS NCs and ZnS NPs are mentioned in the graph. It has been observed that the photo-degradation rate by CoFe₂O₄–ZnS NCs (k = 0.0718 min⁻¹) is much faster than that by ZnS NPs (k = 0.0041 min⁻¹).

To extend the scope of the nanocomposite for photodegradation of other dyes, we checked the degradation of methylene blue and phenolphthalein using the CoFe₂O₄–ZnS photocatalyst under the same reaction conditions (Fig. S16 and S17 in ESI†). The degradation of methylene blue was comparable to that of MO. However, in case of phenolphthalein, the degradation was lower (49% in 60 min) under the identical reaction conditions. This difference in degradation might be due to the difference in redox potentials of different dyes along with their difference in adsorption capacities.

We further observed that the magnetic photocatalyst could be easily separated in the liquid-phase reaction with the aid of an external magnet. To ensure complete removal of the catalyst we measured the energy-dispersive X-ray spectrum of the water residue after centrifugation of the treated water at very high speed and no elemental contribution present in the nanocomposite was detected. The recyclability of the photocatalyst was tested upon separation of the photocatalyst magnetically and then performance of photocatalysis for multiple cycles (Fig. 8). We have not observed any significant loss of activity up to five catalytic cycles, which indicates that the as prepared magnetic photocatalyst is stable and very effective for the removal of organic contaminants from water.

Fig. 8 Recyclability of photocatalytic degradation of MO.

Conclusions

In conclusion, we have developed a CoFe₂O₄–ZnS nanocomposite by a facile method. The enhanced photocatalytic activity, easy magnetic separation and multiple reuses make possible the practical utility of the new magnetic nanocomposite in photocatalysis.

Acknowledgements

Financial support from DST (India) (grant no. SR/S1/RFPC- 07/2006 and SR/NM/NS-18/2011) is gratefully acknowledged. The authors would also like to acknowledge the support from IIT Guwahati for analytical facilities viz. TEM, VSM, FESEM, XRD and Laser micro-Raman analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20400b

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