Visible light photocatalytic activities of ZnFe₂O₄/ZnO nanoparticles for the degradation of organic pollutants

Highlights

• Co-precipitation method is proposed to synthesize magnetic nanoparticles.

• Modifications in the molar concentration lead to the shift in absorption edge.

• Superparamagnetic property is demonstrated for the nanoparticles.

• Two dye pollutants are utilized to demonstrate the photocatalytic activity.

Abstract

ZnFe₂O₄/ZnO nanoparticles have been synthesized by co-precipitation method using polyvinyl alcohol (PVA) as surfactant. The phase formation of synthesized products was systematically investigated from powder X-ray diffraction. Cubic ZnFe₂O₄ and hexagonal ZnO were identified in accordance with different molar concentrations of Fe³⁺ ions. The morphology and functionality were analyzed using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. The optical properties and change in the band gap from UV to visible region upon increasing molar concentration of Fe³⁺ ions were analyzed from diffuse reflectance spectra (DRS). Superparamagnetic property was observed for synthesized ZnFe₂O₄/ZnO nanoparticles using vibrating sample magnetometer (VSM). The methylene blue and methyl orange were taken as model dyes to illustrate the photocatalytic activity of synthesized products under visible light irradiation. Maximum degradation of 99% for methyl orange (MO) was achieved by the use of 13 nm sized ZnFe₂O₄/ZnO nanoparticles as catalyst and a minutely less activity was observed for the methylene blue (MB) degradation (98%), when the photocatalytic processes were carried out for 5 h and 6 h, respectively.

Introduction

Methylene blue (MB) and Methyl orange (MO) are frequently used dyes in textile industries, which are highly hazardous in nature. Expulsion of these dyes to water bodies (lakes, rivers, oceans, groundwater etc.) by industries causes severe health issues [1], [2]. These textile dyes are very complex in structures, which creates difficulties in their degradation [3], [4], [5]. In recent times, various semiconductor based photocatalysts have been designed to achieve high photocatalytic efficiency for the degradation of these dyes [6]. However, the major drawback is that their photocatalytic activity requires irradiation/activation in the UV region. Accounting, the low availability of UV light in the solar spectrum, various catalysts have been developed to exhibit photocatalytic activity in visible region of the electromagnetic spectrum. The improvement of visible light induced photocatalysts, to counter global energy crisis and environmental problems, has become one of the most significant topics in current photocatalysis research [7], [8], [9].

For the remediation of hazardous wastes under stimulated light condition many efforts have been taken in recent times by the use of semiconductor materials like TiO₂ and ZnO as catalyst for the photodegradation of toxic dye pollutants [10], [11], [12], [13]. ZnO and TiO₂ having band gaps of 3.37 eV and 3.2 eV, respectively at room temperature offer several advantages over other catalytic materials such as higher quantum efficiency, environmental friendly, low cost and superior photocatalytic activity [14], [15], but unfortunately they have no effective absorption in the visible light region (λ > 420 nm). They can only operate within the UV light region of solar radiations due to their wide band gap. Since the photocatalytic activity of TiO₂ is influenced by the crystal size, crystal structure and crystallinity. Synthesis of pure nanocrystalline anatase TiO₂ is challenging [2]. The major hindrances for enhancing the photocatalytic efficiency of pure semiconductor photocatalysts are the fast recombination of photo generated electron-hole pairs [16]. In order to improve the photocatalytic efficiency of these materials, doping with rare and/or transition metals [17] and hetero structures have been reported [18]. Electron tends to trap inside the quasi-stable energy states (active sites) created due to doping which significantly increases the photocatalytic efficiency even when the electron-hole pair recombination rate is lower than the rate of electron transfer to adsorbed molecule [19].

Some spinel materials including ZnFe₂O₄ form a class of semiconductors with narrow band gaps and exhibit characteristics of excellent visible-light response and good photochemical stability. Zhang et al., reported the magnetic composite photocatalyst ZnFe₂O₄/BiVO₄ under visible-light and demonstrated that the photocatalytic activity of BiVO₄ is improved by adding a narrow band gap ZnFe₂O₄ [20]. Fan et al. [21], reported the synthesis of cobalt-doped zinc ferrite nanocrystals acting as visible light photocatalyst. Fu et al., reported about the synthesis and photocatalytic analysis of magnetically separable ZnFe₂O₄-graphene catalyst [22]. Magnetic materials have an added advantage that it can be easily separated and recovered from the medium with the help of external magnetic field.

Nanomaterials have large surface area to volume ratio, which results in high surface reactivity, thus increasing the chemical reaction dynamics and photon absorption efficiency. Several methods have been proposed to synthesize ZnFe₂O₄ nanoparticles such as sol-gel [23], hydrothermal [24], combustion [25], ball milling [26], microemulsion [27], spray pyrolysis [28], sol-gel auto combustion [29], electrospinning [30], solid-state combustion [31] and co-precipitation technique [32]. Among the above mentioned techniques, co-precipitation technique is very simple, cost effective and non toxic. Furthermore, it offers the possibility to deal with a great number of experimental strategies by the modification of parameters such as solvent, precipitant agent concentration or temperature. For example, Jadhav et al., investigated the effect of sintering at different temperatures on photocatalytic degradation of methyl orange under UV-light irradiation using zinc ferrite prepared by co-precipitation method [33]. Harish et al., have reported about the synthesis of Cd-Zn ferrites nanoparticles by co-precipitation method and its enhanced photocatalytic activity under solar light irradiation [34].

In the present work, ZnFe₂O₄/ZnO nanoparticles are synthesized by co-precipitation method using polyvinyl alcohol (PVA) as surfactant. The structural, vibrational, morphological, optical and magnetic studies are carried out by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), diffuse reflectance spectra (DRS) and vibrating sample magnetometer (VSM) techniques. The photocatalytic activity is investigated by the degradation of methylene blue and methyl orange under visible light irradiation.