Facile synthesis of ZnFe2O4@RGO nanocomposites towards photocatalytic ciprofloxacin degradation and H2 energy production

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Abstract

The production of Hydrogen energy through the splitting of water is a promising pathway for clean environment and sustainability. Herein we have synthesized a series of ZnFe2O4 (ZFO)@Reduced Graphene Oxide (RGO) nanocomposites by hydrothermal followed by calcination method and studied its application towards hydrogen energy production and ciprofloxacin degradation. Powder X-ray diffraction (XRD) study and X-ray photoelectron spectroscopy (XPS) analysis indicate the good crystallinity and suitable chemical environment for the photocatalytic process. Among all the samples, ZFO@3%RGO showed 73.4% of CIP degradation under solar irradiation of 1 h, which is 1.67 times higher than that of pure ZFO nanoparticles. CIP degradation process follows first order kinetics with a good rate constant of 0.021 min−1 which is 2.3 times greater than ZFO. The photocatalyst ZFO@3%RGO illustrated maximum H2 energy production i.e. 410.32 μmol/h, which is 1.35 times more than that of neat ZFO nanoparticles. ZFO@3%RGO demonstrates the highest photocurrent density of 0.6 mA/cm2 under light illuminations, which is 250 times superior to that of the pristine photocatalyst. Bode phase analysis confirmed that ZFO@RGO shows 13 times higher charge separation efficiency in comparison to neat ZFO. The best photocatalytic activity of ZFO@3%RGO nanocomposite is due to its high light absorption capacity, low photogenerated exciton recombination, high electron-hole separation, and high photocurrent density.

Graphical abstract

Mechanistic path way of ZFO@RGO nanocomposites for Ciprofloxacin degradation and H2 energy production under visible light illumination.

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Introduction

Nowadays, Photocatalysis is considered as an energetic area of research in the field of heterogeneous catalysis due to its potential applications in environmental pollution abatement and Hydrogen energy production since the pioneering work by Honda and Fujishima in 1972 [1]. To date, various semiconductor-based materials like metal oxides, sulfides, nitrides, and heterostructured nanocomposites have been developed for the production of H2 energy [2], [3]. However, practical applications of the developed photocatalysts are still quite limited mainly due to the low absorption efficiency and low quantum efficiency. Now a day, antibiotics are the most wildly used medicine in human society. The excess of antibiotics causes many harmful problems in aquatic systems due to its non-biodegradable nature and threat to water supplies. So many methods have been used to remove the antibiotics in aquatic systems, out of all photocatalysis is one of them. Semiconductor-based photocatalysis shows tremendous growth in wastewater treatments. Iron-based ferrites, particularly zinc ferrites (ZnFe2O4) themselves shows a promising photocatalyst due to high activity, recyclability, and stability in various aquatic mediums. Efficient exploitation of solar light by narrow bandgap [4] ferrite semiconductor like ZnFe2O4 [5] have drawn tremendous attractiveness in photocatalysis on account of absorption of a wide range of sunlight, appropriate bandgap potential for photocatalytic application and advanced optoelectronic properties [6], [7]. Ferrites have the general chemical formula MFe2O4 (M = Mg2+, Ca2+and Zn2+etc.) Generally, n-type ZnFe2O4 (franklinite) act as photoanode in photoelectrolysis of water [8], promising adsorbent and photocatalyst for pollutant degradation [9], because of its narrow bandgap (Eg ≈ 1.9 eV) and prolonged stability towards the exposure of light irradiation. It is a great challenge to rectify the bottlenecks like low charge separation efficiency, low absorption efficiency, etc. of ZnFe2O4.Researchers have been focused on various methods for the development of such photocatalysts like doping of cations or anions [10], fabrication of heterojunction based materials and also loaded with noble metal co-catalysts [11], [12], [13] to enhance its activity under solar light and control the charge recombination property. Wu and co-workers synthesized reduced graphene decorated ZnFe2O4, which shows 99.3% of 12 ppm MB degradation within 5 h [14]. Hao et al prepared MoS2 quantum dots and graphene on metal-organic frameworks by solvothermal route and it shows 186.37 μmol of hydrogen evolution rate after 3 h [15]. Duanduan and co-workers synthesized rGO/MOF/Co-Mo-S photocatalyst by a facile two-step photocatalytic reduction method and it displays 339 μmol of H2 evolution after 5 h [16]. Zhang et al. modified g-C3N4/rGO/Ni2P ternary photocatalyst by simple hydrothermal methods and also reaches maximum H2 evolution rate after 5 h [17]. Although some successful results have been obtained from the above-said techniques still the development of efficient photocatalysts is still limited as the modification technique creates structural deformation of the pristine photocatalysts and crystal defects. Although noble metal loading minimizes the rate of recombination of excitons to some extent, still its own disadvantages like high cost, perniciousness to the environment, etc, retard its achievements. On the contrary, the exercise of carbon scaffolds is one of the ways to overcome the disadvantages of ferrites [18]. In this regard, graphene is considered as an efficient co-catalyst for photocatalytic H2 evolution because of its maximum specific surface area (2600 m2/g), outstanding electron mobility (15000 m2 v−1 s−1 at normal temperature), high mechanical strength and thermal conductivity. These emblematic properties are the driving force for the formation of nanocomposites, heterojunctions with various metal oxides, carbon-based material and polymers to obtain enhanced photocatalytic activity [19], [20]. Graphene oxide (GO) based nanocomposites with various metal ferrites have been reported earlier because GO has the property of high electron mobility, extended π-conjugation which acts as an electron sink [21]. So, the electron-hole recombination rate is significantly quenched and consequently increases the photocatalytic activity.

On this contest, we have developed a ZFO@RGO nanocomposite by single-step hydrothermal followed by calcination method from low-cost precursors. The efficient photocatalytic activity of this material is proved by ciprofloxacin degradation and H2 energy production under visible light irradiation. The efficient charge separation and transfer properties of the photocatalyst were proved by PL and EIS analysis. The existence of a graphene sheet and its composites with ZFO are confirmed by FTIR and XPS study. The detail photocatalytic degradation procedure was performed by studying the kinetics, scavenger test and reusability experiment in detail. The current study may build a new path in the direction of ZFO@RGO nanocomposites for environmental sustainability.

Section snippets

Materials and Methods

Zn(NO3)2·6H2O (purity-98%, Loba Chemie), Fe(NO3)3·9H2O (purity-98%, Merck), potassium permanganate and graphite powder were purchased from Sigma Aldrich. Sulfuric acid (H2SO4), sodium nitrate (NaNO3), hydrogen peroxide (H2O2), and sodium hydroxide (NaOH) were obtained from Finar Chemicals. All are analytical grade and were used for the synthesis of the ZFO@RGO nanocomposites.

PXRD analysis

The structural conformation and phase identification of pure ZFO and ZFO@RGO nanocomposites were confirmed by Powder X-ray Diffraction Analysis. The PXRD pattern of neat ZFO and ZFO@RGO nanocomposites are represented in Fig. 1. The neat ZFO demonstrate simple cubic structural arrangement having diffraction peaks positioned at 2θ = (68.1°, 62.8°, 56.6°, 47.5°, 42.7°, 35.5°, 31.8°) representing crystal planes such as (4 4 2), (4 4 0), (5 1 1), (3 1 1), (4 0 0), (3 3 1) and (2 2 0) respectively.

LSV measurement

The linear sweep voltammeter (LSV) study is performed to know the generation and separation of the excitons as the current is directly related to the concentration of electrons. The photocurrent density of the ZFO and ZFO@RGO electrodes are shown in Fig. 8. In the current Vs potential curve, photocurrent density of as-synthesized photocatalyst was seen in anodic direction with applied bias, which indicates a typical n-type behavior of the semiconductor. The current density of the photocatalysts

Photocatalytic hydrogen production using RGO modified ZFO nanoparticle

The designed RGO modified ZFO nanoparticle have proper band edge position to carry out photocatalytic H2 evolution and was performed in presence of the sacrificial agent. The photocatalytic H2 evolution rate of the as-prepared photocatalyst is depicted in Fig. 11(a). According to Fig. 11a, it was confirmed that ZFO@3%RGO shows maximum H2 evolution rate i.e. 410.32 μmol/h, which is 1.35 times more than that of the neat photocatalyst. The photocatalytic hydrogen evolution of the as-prepared

Conclusion

In summary, a series of ZFO@RGO nanocomposites were used as a catalyst for efficient clean H2 energy production by a water reduction reaction and photocatalytic antibiotics (CIP) degradation. The ZFO@RGO photocatalyst shows significant functions due to following reasons, such as easy synthesis method; highly robust which is active to water reduction reaction and highly effective for photocatalytic CIP degradation. The main cause for the highest photocatalytic degradation of the composites is

Acknowledgments

The authors are very much thankful to the management of Siksha ‘O’ Anusandhan, (Deemed to be University) for their encouragement and support.

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