CoFe2O4 nanocomposite for sunlight driven photocatalytic dye degradation and bactericidal application

CoFe2O4 nanocomposite for sunlight driven photocatalytic dye degradation and bactericidal application

Journal Pre-proof Magnetically recoverable multifunctional ZnS/Ag/CoFe2O4 nanocomposite for sunlight driven photocatalytic dye degradation and bacteri...

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Journal Pre-proof Magnetically recoverable multifunctional ZnS/Ag/CoFe2O4 nanocomposite for sunlight driven photocatalytic dye degradation and bactericidal application G. Palanisamy, K. Bhuvaneswari, A. Chinnadurai, G. Bharathi, T. Pazhanivel PII:

S0022-3697(19)32030-X

DOI:

https://doi.org/10.1016/j.jpcs.2019.109231

Reference:

PCS 109231

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 8 September 2019 Revised Date:

11 October 2019

Accepted Date: 17 October 2019

Please cite this article as: G. Palanisamy, K. Bhuvaneswari, A. Chinnadurai, G. Bharathi, T. Pazhanivel, Magnetically recoverable multifunctional ZnS/Ag/CoFe2O4 nanocomposite for sunlight driven photocatalytic dye degradation and bactericidal application, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/j.jpcs.2019.109231. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Magnetically recoverable multifunctional ZnS/Ag/CoFe2O4 nanocomposite for sunlight driven Photocatalytic dye degradation and bactericidal application G. Palanisamy a, K. Bhuvaneswari a, A. Chinnadurai a, G. Bharathi b and T. Pazhanivel *a a

Smart Materials Interface Laboratory, Department of Physics, Periyar University, Salem-636 011, Tamilnadu, India.

b

Key Laboratory of Optoelectronic Devices and Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong Province 518060, P.R.China.

*Corresponding author Dr. T. Pazhanivel E-mail: [email protected]

Graphical abstract

Abstract Sunlight driven photocatalysis is the next step of environmental remediation, which can utilize the renewable energy to cure environmental problems. Herein we report a magnetically recoverable ZnS/Ag/CoFe2O4 nanocomposite photocatalyst for sunlight driven photocatalytic dye degradation. The composition, structure, morphology, magnetic and photophysical properties of the as prepared samples were investigated by a wide range of characterization methods. The ZnS/Ag/CoFe2O4 nanocomposite demonstrated sunlight driven photocatalytic activity towards the degradation of two different organic dye pollutants. The radical scavenging tests were conducted to examine the role of different radicals in degrading the dye pollutants. With demonstrated ferromagnetic behavior, the ZnS/Ag/CoFe2O4 nanocomposite was magnetically recoverable using a commercial bar magnet. The antibacterial activity of the nanocomposite were also examined against both G+ (Staphylococcus aureus) and G− (Escherichia coli) bacteria’s. Relatively, the ZnS/Ag/CoFe2O4 nanocomposite showed better antibacterial activity as compared to CoFe2O4 NPs and Ag/CoFe2O4 samples. Keywords: ZnS/Ag/CoFe2O4 nanocomposite; Sunlight; Organic Pollutants; Antibacterial activity;

INTRODUCTION In the current scenario, it is necessary to turn towards the renewable energy resource based wastewater treatment strategies [1]. Solar energy has been extensively used for clean energy production and environmental remediation [2]. However, the use of solar energy for semiconductor photocatalysis based wastewater treatment requires a lot more advancement. Most of the reported semiconductor based photocatalysts exhibited high photocatalytic efficiencies only under the UV-light [3]. In general, scientist use artificial/commercial lamps as UV-light source, because the solar spectrum contains only a small fraction of the ultraviolet radiation (4-5%). But the visible light covers nearly 46% of the solar spectrum, which could be effectively utilized for photocatalytic applications [4]. As stated above, the widely investigated semiconductor photocatalysts perform poorly under visible light. This poor performance has been equally affected by the poor visible light absorption and large trap states that hinder the effective utilization of photoseparated charge carriers in oxide semiconductors. Recently, chalcogenide based semiconductors are experimented for the photocatalytic water treatment, which have the advantage of good light absorption and comparatively reduced

trap state issues. The toxic nature chalcogenide semiconductors kept them away from the photocatalytic applications for a while. The strategy of using comparatively less-toxic chalcogenides such as ZnS and complete recovery of chalcogenide photocatalysts after the photocatalytic reaction, give hope for their application in the field of semiconductor photocatalysis based organic pollutant degradation. For example, Kula Kamal Senapati. et.al. reported the uses magnetically recoverable CoFe2O4–ZnS nanocomposite for degradation of methyl orange dye [5]. While addressing the trap state short comings, such less toxic chalcogenides also suffer from poor light absorption in the visible region. By preparing composite with a non/less-toxic and good visible light absorbing material, the photocatalytic performance of ZnS can be greatly enhanced. Noble metals are known to have excellent absorption in the visible region. Amongst Ag nanoparticles exhibit good visible light absorption owing to the interband transition (4d band to 5sp band) [6]. In addition the local surface plasmon resonance (SPR) effect makes it a better candidate for the visible light driven applications [7]. There are several reports on using the Ag nanoparticle-semiconductor nanocomposites, such as Ag/NiFe2O4 [8], Ag/CuO/WO3 [9] and gC3N4/Ag/TiO2 [10] in order to achieve better photocatalytic performances. In the present study, Ag nanoparticles has been used to enhance the visible light absorption of ZnS. Another challenge in using the chalcogenide semiconductors is the recovery of the used photocatalysts after the photodegradation experiments. While the aqueous-route-prepared chalcogenides are mostly soluble in water, their recovery is a difficult process, even using the high speed centrifugation. One alternate is the magnetic field assisted separation. The used photocatalysts can be separated by using a commercial magnet, provided the photocatalysts are highly magnetic and attractable. Unfortunately, chalcogenides possess weak magnetic behavior and will not be attracted by a magnet. Only the magnetic nanoparticles (MNPs) will respond to the commercial magnets by showing attraction/repulsion. Among several categories of MNPs, spinel ferrites are unique and versatile due to their wide range of applications, viz., supercapacitor [11], photocatalytic [12], magnetic hyperthermia [13], sensors [14], and photovoltaic application [15] Cobalt ferrite (CoFe2O4) is a well-known ferromagnetic material due to its remarkable physical properties, such as high coercive field, high magnetocrystalline anisotropy, chemical stability, high resistivity and good mechanical hardness [16]. The CoFe2O4 spinel structure is equally distributed and it has predominant superexchange interaction between octahedral (Co2+ ions) site to tetrahedral sites (Fe3+ ions) via Oxygen ions [17]. CoFe2O4

nanoparticles can be easily recovered through a commercial bar magnet and therefore by preparing a composite containing CoFe2O4, the chalcogenide semiconductor photocatalysts can be completely recovered for the reaction medium. In the present study, the magnetically separable semiconductor photocatalysts has been prepared by using the Ag/CoFe2O4 with ZnS quantum dots (QDs). The wastewater not only contains the industrial waste pollutants, but also several microorganisms, which can harm human beings. While the bacterial infections can create diseases like typhoid, cholera, and paratyphoid fever, bacillary dysentery and skin diseases. Several materials exhibit antibacterial/antimicrobial activity, i.e., the materials can stop the growth and/or can kill the harmful bacterias [18]. Scientists have always been keen to develop multifunctional materials, which can be used for more than one application. For example, M. Madhukara Naik et. al [19]., prepared zinc doped cobalt ferrite nanoparticles and used it for the degradation of Congo red and antibacterial applications. Recently, Jing Xu et.al [20] have prepared porous g-C3N4 by loading Ag nanoparticles used for both pollutant degradation and disinfection under visible light irradiation. In the present work, we have prepared the magnetically-recoverable ZnS/Ag/CoFe2O4 nanocomposite by simple precipitation and wet impregnation methods. The as-prepared ZnS/Ag/CoFe2O4 nanocomposite exhibited enhanced sun-light photocatalytic degradation of methylene blue (MB) and Rhodamine B (RhB) dyes. The possible reaction mechanism for the enrichment of photocatalytic activity was investigated. The antibacterial activities of the asprepared

ZnS/Ag/CoFe2O4 nanocomposite

was

also

studied

against

gram

positive

(Staphylococcus aureus) and gram negative (Escherichia coli) bacteria’s. The experimental results were discussed in detail. 2. EXPERIMENTAL 2.1. Materials Chemicals used for experiments are Cobalt (II) nitride hexahydrate (Co (NO3)2.6H2O), Iron(III) nitrate nonahydrate (Fe (NO3)2.9H2O), Zinc chloride (ZnCl2), sodium sulfide (Na2S), Silver nitrate (AgNO3), Sodium hydroxide (NaOH), Trisodium citrate (Na3C6H5O7), Ethanol (CH3CH2OH). All the chemicals are of analytical grade purchased from Merck and Aldrich. MB and RhB dye powders were purchased from Merck chemical company. All the substances were used without further purification. Deionized water (DI) was employed as the solvent. 2.2. Preparation of ZnS QDs

In a typical synthesis, 0.1 M zinc chloride was dissolved in DI water, then an equimolar solution of newly prepared 0.1 M of sodium sulfide solution was added dropwise to the above solution. The product was precipitated and the sample was subjected to purification process by washing with ethanol and water, in order to remove the byproducts in the reaction mixture. The obtained mixture was heated at 60 °C for 12 hours to get the sample. 2.3. Preparation of Pure CoFe2O4 Pure CoFe2O4 nanoparticles were synthesized through a simple wet precipitation method [21]. A 2 M of Fe (NO3)3. 9H2O and 1 M of Co (NO3)2.6H2O were dissolved in DI by magnetic stirring for 30 min. to obtain the homogeneity. After that, 1 M of NaOH (precipitating agent) was added dropwise into the above solution until the pH reached 12 and the solution was then kept at 85 °C for 60 min, in order to form the spinel ferrite from hydroxides [17]. The obtained precipitate was collected by centrifugation and washed with ethanol and DI for several times to remove the byproducts in the reaction mixture. The resulting product was dried at 60 °C for overnight. The dried powder was then, calcinated at 900 °C for 3 hours in order to obtain the well crystallized sample. 2.4. Preparation of Ag/CoFe2O4 nanocomposites The same procedure (pure CoFe2O4) was carried out with AgNO3 to prepare the Ag/CoFe2O4 nanocomposite and also 0.4 M of Na3C6H5O7 was used to reduce the silver nitrate in to silver. 2.5. Preparation of ZnS/Ag/CoFe2O4 nanocomposites ZnS/Ag/CoFe2O4 nanocomposites were prepared through wet impregnation technique [22]. The equal weight percentages of the as-prepared ZnS QDs and Ag/CoFe2O4 nanocomposite were added into 30 mL of ethanol. The obtained solution was continuously stirred and subsequently heated at 70 °C to evaporate the solvent. Then, the obtained ZnS/Ag/CoFe2O4 nanocomposites were dried for overnight, collected and stored for further processes. 2.6 Characterization method and instrumentation The crystal structure and phase purity of all prepared materials were tested by X-ray diffraction (XRD, Rigaku Mini Flex II powder X-ray diffractometer) with Cu Kα radiation (1.541 Å). The vibrational spectroscopic measurement was carried out using a Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer spectrometer). The surface morphology, chemical composition and elemental mapping of the Ag/CoFe2O4and ZnS/Ag/CoFe2O4 nanocomposite were examined by field emission scanning electron microscopy (FESEM, FEI Quanta FEG 200).

The detailed morphology of ZnS QDs was carried out using a high resolution transmission electron microscope (HRTEM, JEOL JEM-2000EX electron microscope). Optical absorption spectra were recorded by SHIMADZU 3600 UV-Vis model spectrophotometer in the wavelength range between 200 to 800 nm. The room temperature photoluminescence (PL) spectra were carried out on a Horiba Jobin Yvon Spectro Fluromax 4. The strain state distribution of the as prepared samples were carried out by Enwave Optronics Field-type EZ Raman spectrometer. To determine ferromagnetic properties of the synthesized samples were measured with Lakeshore VSM 7410 vibrating sample magnetometer. The surface chemical bonding characterized by X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000 VersaProbe III. 2.7. Photocatalytic Activity test The photocatalytic behavior of the as-synthesized samples was evaluated by the photodegradation of MB and RhB dye under the direct natural sunlight irradiation.

The

photocatalytic experiments were executed on the days of bright sunny light in between 10 am to 2 pm in the summer session of Salem city in India. 0.04 g of ZnS/Ag/CoFe2O4 nanocomposite was added to 100 mL of an aqueous solution of dyes (50 ppm). The solution was placed in the darkroom for 20 min under constant stirring to achieve the adsorption-desorption equilibrium of dye on the photocatalyst surface. Photocatalytic degradation was performed by illuminating the suspension under natural sunlight. The degradation of dyes was examined by UV-Vis spectroscopy at different time intervals. For comparison, the photocatalytic activity of other nanocatalyst was also investigated under the same conditions. The dye (MB & RhB) degradation efficiency was calculated by using the following equation [23] Efficiency (%) =

C0 − C × 100 - - - - - - - - - - (1) C0

Where, C0 and C are the dye concentrations before and after photo-irradiation, respectively. ZnS/Ag/CoFe2O4 nanocomposite photocatalytic activity stability and reusability was assessed with the same sunlight irradiation and after each degradation cycle, the catalyst was separated from the MB and RhB solutions through bar magnet.

2.8. Detection of Reactive Oxygen Species test

One of the important themes in the photocatalytic degradation process was the detection of reactive species. The different scavengers were added into the RhB aqueous solution prior to the addition of ZnS/Ag/CoFe2O4 catalyst such as p-benzoquinone (BQ), triethanolamine (TEOA) and isopropyl alcohol (IPA). The same above mentioned process was repeated with different scavengers under the same sunlight radiation to detect the active species[24].

2.9. Antibacterial activity evolution The antibacterial activity of as-prepared samples were studied by Agar well diffusion method to determine their ability as a potential antimicrobial agent[25]. The human pathogenic strain such as Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were cultured on nutrient agar plates. 150 mg of as-prepared samples were pressed at ~20 MPa to form a disc of 13 mm diameter and 1 mm thickness. The presence and the extent of the inhibition zone around the samples were studied after incubation for 24 hours at 37 °C. The same experiments were repeated in triplicate and zone of inhibition (width) was measured in all the experiments.

3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared samples. As shown in fig. 1 (a), the ZnS QDs show three strong and distinct peaks at 2θ of 28.9º, 47.6 º and 57.5º corresponding to the (111), (220) and (311) diffraction planes, reflecting the standard ZnS (JCPDS card # 050566) [22]. For the pure CoFe2O4 (Fig. 1 (b)), the diffraction peaks at 30º, 35.4º, 43.02º, 53.4º, 57.2º and 62.2º correspond to the (220), (311), (400), (422), (411) and (440) diffraction confirming the formation of spinel cubic structured CoFe2O4 nanoparticles (JCPDS card # 221086)[26]. Fig. 1 (c) clearly indicates the contribution of the cubic Ag NPs diffraction peaks (JCPDS card # 65-2871) [27] which are encompassed by the broad peaks of CoFe2O4 spinel ferrite (cubic phase, Fd3m) [28]. The recorded diffraction pattern of ZnS/Ag/CoFe2O4 nanocomposite contains the contributory peaks corresponding to ZnS, Ag and CoFe2O4, which indicates the formation of the ZnS/Ag/CoFe2O4 ternary nanocomposite (Fig.1 (d)). By using the Debye-Scherer formula, we have calculated the average crystallite size values which are about 2.45, 21.72, 28.08 and 36 nm for ZnS, CoFe2O4 and Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposites, respectively.

(d) (c)

(422)∗ (411)∗



(b)

(311) •

(400)∗

(311)∗

(200) •

(111) •

(220)∗





(111)♦







∗ ∗ ∗ ∗ (440)∗ ∗ ∗ (220)♦ ♦





-CoFe2O4 NPs

• Intensity (arb. unit)

-Ag



•-ZnS QDs

(a) 10

20

30

40 50 2θ θ (degree)

60

70

80

Fig. 1. XRD pattern of (a) ZnS QDs (b) CoFe2O4 NPs (c) Ag/CoFe2O4 and (d) ZnS/Ag/CoFe2O4 nanocomposites The detailed size and structure of ZnS QDs were analyzed through HRTEM and the results are displayed in Fig.2 (a).

Fig. 2. (a) HRTEM image and (b) EDAX spectra of ZnS QDs and insert of figure 2 (a) is SAED pattern of ZnS QDs

Fig. 3. (a, c and e) FESEM images and (b, d and f) EDAX spectra of CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposites

The ZnS QDs exhibit spherical like morphology with an average particle size of 2.9 nm, which is in correspondence with the XRD result. The insert of Fig. 2 (a) demonstrates the SAED pattern of the ZnS QDs, from this ring formation reveals that the particles are in the polycrystalline in nature. The EDAX spectrum of ZnS QDs presented in Fig. 2 (b) indicate the presence of Zn and S content and the sample purity as well. The morphologies of CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite were analyzed by FESEM. The bare CoFe2O4 displayed spherical like morphology (presented in Fig. 3. (a)) in which the larger spheres are the agglomeration of small spherical nanoparticles. In the case of Ag/CoFe2O4 sample, the electron microscopic images show the formation of micron sized Ag plates (Fig. 3 (c)), which are irregularly covered with the CoFe2O4 nanoparticles (The elemental mapping analysis presented in Fig. S1 show the distribution of individual elements across the Ag/CoFe2O4 sample). The addition of ZnS quantum dots lead to even coverage of these Ag plates by ZnS and CoFe2O4. As shown in Fig. 3 (e), the ZnS/Ag/CoFe2O4 nanocomposite is evenly covered with nearly spherical shaped particles. The elemental mapping analysis (Fig. S2) indicate the homogeneous distribution of all corresponding elements across the ZnS/Ag/CoFe2O4 nanocomposite sample. The recorded EDAX spectra of all the samples demonstrated the purity and the atomic and weight percentages of the corresponding samples reveal the composition (Fig. 3 (b, d and e). FTIR spectroscopy is a useful tool to identify the functional group of organic and inorganic samples. Fig. 4 shows the FTIR spectra of the as-prepared samples. In all the samples a strong band at 3410 cm-1 and a broad band at 1621 cm-1 are attributed to O–H stretching and H– O–H bending vibrations, respectively, arising from the physically adsorbed water molecules. The strong peaks appearing at 470-670 cm-1 are due to the characteristic Zn-S stretching vibrations of ZnS QDs[29]. The two bands observed at 1550 and 1417 cm−1 are due to the asymmetrical and symmetrical stretching of the zinc carboxylate (COO−), respectively[30]. The FTIR spectra of pure CoFe2O4 nanoparticles exhibit peaks at 2921 and 2850 cm-1 which are attributed to the asymmetric and symmetric stretching vibration of the C-O groups, respectively. Ferrites exhibited two main metal-oxygen bands in the range of 400-600 cm-1. The peak corresponding to high frequency band vibrations of the metal-oxygen at the tetrahedral site (ν1) appeared in the frequency range of 600–500 cm-1 and low frequency band vibrations of metaloxygen at the octahedral site (ν2) frequency range appeared at 490–400 cm-1 [31]. The addition of Ag to the CoFe2O4 sample has resulted a shift in the Fe–O peak position from 587 cm-1 to 583

cm-1. This shift is attributed to the interaction of Ag nanoparticles with the octahedral lattice of CoFe2O4 [32]. The peak appearing at 1018 cm-1 is attributed to the cobalt ion alloy vibration, as

1384 1114

597 467

(d) (c)

3000

2000 Wavenumber (cm-1)

1018

583 615 472

(a)

4000

1621

1550 1417 1114 914

(b)

2922 2851

587 3410

Transmittance (%)

472

reported elsewhere [33].

1000

Fig. 4. FTIR spectra of (a) ZnS QDs (b) CoFe2O4 NPs (c) Ag/CoFe2O4 and (d) ZnS/Ag/CoFe2O4 nanocomposites The FTIR spectra of ZnS/Ag/CoFe2O4 nanocomposites showed all the characteristic peaks of individual samples, thereby indicating the formation of the ZnS/Ag/CoFe2O4 nanocomposites. The strain state the cation distribution of CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposites are shown in Fig. S3 (The detailed description of the Raman results are presented in supporting information section). In order to further confirm the surface composition and chemical state, the ZnS/Ag/CoFe2O4 nanocomposites were analysis by using XPS and results are depicted in Fig. 5. The surface of ZnS/Ag/CoFe2O4 nanocomposites consists of Co, Fe, S, Ag, Zn and O, which is in good agreement with the EDAX analysis results. The survey spectrum of the ZnS/Ag/CoFe2O4 nanocomposite is displayed in Fig. S4. The Zn 2p spectrum (Fig. 5 (a)) shows two different peaks at binding energies of 1021.5 and 1045.2 eV corresponding to Zn 2p3/2 and Zn 2p1/2, respectively; the typical binding energies of Zn2+ states in ZnS QDs [22,34]. Further, the peak at 23.0–23.1 eV is attributed for zinc ions binding to sulfur ions in ZnS lattices [35] are

demonstrated in Fig. S4. The S 2p spectrum (shown in Fig. 5 (b)) displayed two distinct peaks at 160.95 and 162.75 eV, araising from S 2p3/2 and S 2p1/2 of metal-sulfur binding, as reported elsewhere [22].

Fig. 5. XPS spectra of ZnS/Ag/CoFe2O4 nanocomposites (a) Zn 2P (b) S 2P (c) Ag 3d (d) Co 2P (e) Fe 2P (f) O 1s

As shown in Fig. 5 (c), the two eminent peaks at 368.2 and 374.2 eV are ascribed to characteristic Ag 3d5/2 and Ag 3d3/2 of Ag0, respectively. The Ag 3d5/2 peak clearly shows the presence of Ag metal hybrid into the ferric matrix, which is in good agreement with the other analytical results. As shown in Fig. 5 (d), the Co 2p XPS spectrum contains four peaks which can be described as, (i) peaks at 781.5 and 786.4 eV corresponding to Co 2p3/2 and its shakeup satellites, respectively, and (ii) at 796.06 and 803.26 eV corresponding to Co 2p1/2 and its shakeup satellites, respectively[36]. Based on the above observation, we conclude that, cobalt mainly exists in the Co2+ state in the sample, since the intense shakeup Co 2p satellites are derived from the unpaired valence 3d electron orbitals of the high spin Co2+oxidation state. The Fe 2p high resolution XPS spectrum presented in Fig. 5 (e) displays two major peaks centered at 712.6 eV and 725.16 eV, which are assigned to Fe 2p3/2 and Fe 2p1/2, respectively, which indicates the presence of Fe3+ oxidation state[37]. The peaks at 715.96 and 733.66 eV are ascribed to Fe 2p3/2 and Fe 2p1/2 shakeup satellites, respectively[36]. The O1s spectrum presented in Fig. 5 (f) was deconvoluted into two peaks at 529.81 and 531.81 eV are attributed to O2present in the lattices and the chemisorbed oxygen species, respectively[38]. Therefore, from the XRD, FTIR, Raman EDAX and XPS investigation, the analytical results confirmed the formation of ZnS QDs/Ag metal/CoFe2O4 nanocomposite. The optical absorption measurement is the powerful tool for understanding the absorption behavior and band structure of both crystalline and non-crystalline materials.

Fig. 6 (A) UV-Visible spectra and (B) Band gap energy of (a) ZnS QDs (b) CoFe2O4 NPs (c) Ag/CoFe2O4 and (d) ZnS/Ag/CoFe2O4 nanocomposites

The optical properties of the as-prepared nanoparticles were investigated by UV-Visible spectrometer and the results are shown in Fig. 6 A. It can be clearly seen that, the pure ZnS QDs have strong absorption peak at 308 nm, which is in correspondence with previous reports [39]. Moreover, the reported absorption value of bulk ZnS is 350 nm and it is blue shifted in our pristine ZnS QDs (~308 nm). As reported elsewhere, this blue shift is due to quantum confinement effect that comes into play when the particle is comparable or less the exciton Bohr radius[40]. Our ZnS QDs have the size value of 2.45 nm, and therefore it is obvious to observe the quantum confinement effect [41]. The Pure CoFe2O4 sample have strong and broad absorption peak at 409 nm which indicates strong absorption in the visible light region (Fig. 6 A (b)) and the incorporation of Ag, further enhanced the absorption (Fig. 6A (c)). In the case of ZnS/Ag/CoFe2O4 nanocomposite sample, the ZnS QD incorporation enhanced the absorption in the UV region and it is clear that the as prepared ZnS/Ag/CoFe2O4 nanocomposite can absorb both visible and UV light. This enhanced optical absorption lead to improved charge carrier separation, which is discussed in the application part. The optical band gap energy (Eg) for the synthesized samples was calculated using the Tauc’s relation[42]. Fig. 6 B shows the plots of (αhν)2 versus absorption energy hν in eV for all the synthesized samples. The obtained Eg value of pure ZnS QDs is 3.8 eV, which is larger than the reported value of the ZnS nanoparticles, indicating the role of quantum confinement effect [22]. The CoFe2O4 nanoparticles show a direct bandgap of about 1.62 eV and the addition of Ag content in CoFe2O4 nanoparticles, increased the band gap value from 1.62 to 2.02 eV. Further, the bandgap of ZnS/Ag/CoFe2O4 nanocomposites is increased to 2.3 eV. The optical absorption and the measured bandgap values indicate the electronic interaction between ZnS QDs, Ag and CoFe2O4. Since, the recombination rate and separation efficiency is an important tool in the wastewater degradation process, the PL spectra were recorded at room temperature in order to investigate the charge carrier trapping and recombination phenomena[43]. Fig. 7 (a-d) shows the PL spectra of the as-prepared samples. The ZnS QDs exhibited a broad peak around 440 nm with a shoulder-like peak around 360 nm. While the peak around 360 nm is produced by the band edge recombination transition, the peak around 440 nm was originated from the sulphur vacancy trap state mediated radiative recombination process [44].

443

Intensity (arb.unit)

355

(a) (b) (c) (d)

367 360

350

400 Wavelength (nm)

450

Fig. 7 PL spectra of (a) ZnS QDs (b) CoFe2O4 NPs (c) Ag/CoFe2O4 and (d) ZnS/Ag/CoFe2O4 nanocomposites The CoFe2O4 and Ag/CoFe2O4 samples display the emission band at around 355 nm, originating from the radiative defects related to the interface traps existing at the grain boundaries [45]. In the case of ZnS/Ag/CoFe2O4 nanocomposite, the emission was similar to that of ZnS QDs with decreased intensity in the visible region around 440 nm. This observed decrease indicates the decreased trap state mediated recombination in ZnS QDs, which means that the photoexcited electrons trapped at the sulphur vacancies might be transferred to other components, thereby creating a charge separated state. Such charge separation process will have good effect in the photocatalytic degradation process by enhancing the radical generation. The room temperature (300 K) magnetic measurements of pure CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite were carried out by applying an external magnetic field of 1.5 Tesla. From the hysteresis loop, it can be observed that all the as-prepared samples exhibit ferromagnetic behavior, as shown in Fig. 8. The measured saturation magnetization, coercivity and residual magnetization values of Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite (0.5407 emu/g, 1139.6 Oe and 0.17 emu/g and 0.4338 emu/g, 1050.46 Oe, 0.12 emu/g), are lower than that of the bare CoFe2O4 nanoparticles (0.8670 emu/g, 1175.1 Oe and 0.22 emu/g). This decrease is ascribed to the incorporation of non-magnetic ZnS and Ag nanoparticles to CoFe2O4.

CoFe2O4 NPs

Magnetization (emu/g)

0.9 0.6

Ag/CoFe2O4

0.3

ZnS/Ag/CoFe2O4 nanocomposite

0.0 -0.3 -0.6 -0.9 -15000 -10000 -5000 0 5000 10000 15000 Magnetic Field (Oe)

Fig. 8 Magnetic hysteresis loop of as-prepared samples This type of reduction may be due to the quenching of surface moment and small particle size effect [46,47]. These experimental results confirm the magnetically recoverable nature of the ZnS/Ag/CoFe2O4 nanocomposite photocatalyst. The widely used MB and RhB dyes were chosen as model organic pollutants to check photocatalytic degradation efficiency of the as prepared photocatalysts. The photocatalytic degradation of RhB was carried out in the presence of ZnS QDs, CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite under the sunlight irradiation. The treated dye samples were collected at regular intervals and subjected to optical absorption measurements. The measured absorption spectra showed the linear decrease in the characteristic absorption of RhB dye (at around 550 nm), which indicates the photocatalytic degradation under visible light. The ZnS/Ag/CoFe2O4 nanocomposites exhibited the excellent photocatalytic activity for the degradation of RhB under the sunlight irradiation, which is about 96.54% of RhB photodegraded after 150 min (Fig. 9 (a) & (b)). While, the control dye sample treated without photocatalysts showed only a 2% degradation, the other samples exhibited the efficiency of 50-60%.

Fig. 9 (a) Concentration changes of RhB as a function of irradiation time. (b) First order kinetics in RhB dye degradation of as prepared photocatalyst (c) Degradation efficiency of asprepared nanocatalyst (d) Cycling runs for the photocatalytic degradation of RhB over ZnS/Ag/CoFe2O4 nanocomposite under sun light irradiation Therefore, it is clear that the ZnS/Ag/CoFe2O4 nanocomposite photocatalyst exhibited better performance in degrading the RhB dye under visible light. To quantitatively study the reaction kinetics, the RhB degradation was fitted to the first-order kinetic equation [24]. The fitting model indicates the better photocatalytic activity of ZnS/Ag/CoFe2O4 nanocomposite, which was improved by the incorporation of ZnS QDs and Ag with CoFe2O4 nanoparticles. The bar graph in fig. 9 (c) represents the comparative % RhB dye degradation for all prepared samples.

Fig. 10 (a) Concentration changes of MB as a function of irradiation time. (b) First order kinetics in MB dye degradation of as prepared photocatalyst (c) Degradation efficiency of asprepared nanocatalyst (d) Cycling runs for the photocatalytic degradation of MB over ZnS/Ag/CoFe2O4 nanocomposite under sun light irradiation In Fig.10 (a), similar behavior was observed for photocatalytic degradation of MB dye in the presence of as-prepared samples under direct sunlight irradiation. It was obvious from (Fig. S5, Supporting Information) that the absorption peak of MB at 664 nm decreased significantly as time increases, indicating the efficient degradation of this cationic dye in the presence of the ZnS/Ag/CoFe2O4 nanocomposite. Fig. 10 (b) shows the high value first-order rate constant (k=0.0256 min-1) values for ZnS/Ag/CoFe2O4 nanocomposites curves exhibited good linear performance compared to other prepared samples. The degradation efficiency (C/C0) of MB over ZnS QDs, CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposites was found to be 50, 62.12, 74.19 and 95.78 %, respectively, after 90 min sunlight irradiation (Fig. 10 (c)).

Recyclability of Photocatalysts is a very important parameter of the catalytic degradation process, because the use of recyclable and reusable photocatalysts lead to a significant cost reduction of the treatment. The reusability and photostability of a photocatalyst are shown in Fig. 9 (d) and 10 (d). The ZnS/Ag/CoFe2O4 nanocomposites were further investigated through four successive runs under the same experimental conditions. Each run was conducted for 150 and 90 min for RhB and MB dye respectively, after every run the reaction system was re-evaluated. These results indicated that the ZnS/Ag/CoFe2O4 nanocomposites remained effective for first two cycle and later it was slightly decreased due to the slight inactivation of the photocatalyst caused by surface adsorption of small fragmented species during the degradation process.

Fig. 11 (a) Powder XRD patterns and (b) FTIR spectra for ZnS/Ag/CoFe2O4 nanocomposite before and after 4 cyclic runs. The stability of the ZnS/Ag/CoFe2O4 nanocomposite was examined by XRD and FTIR [48]. Fig. 11 (a) shows the XRD pattern of ZnS/Ag/CoFe2O4 nanocomposite before and after four photocatalytic reactions, which are very similar, thereby indicating the stability of the photocatalysts. Fig. 11 (b) shows the FTIR Spectra of Ag/CoFe2O4 nanocomposite before and after four cycling trials. The characteristic vibration peaks of ZnS/Ag/CoFe2O4 nanocomposite do not noticeably change, indicating that it was stable after recycling.

Fig. 12 Effects of many active scavengers on the degradation of RhB over ZnS/Ag/CoFe2O4 nanocomposite under irradiation with sunlight. In order to identify which active species played important role in dye degradation when using ZnS/Ag/CoFe2O4 under sunlight irradiation, a series of experiments on quenching active species by adding individual scavengers to the photocatalytic reaction system were carried out. •



The photodegradation of RhB with the addition of TEOA (h+ quencher), BQ ( O2 quencher), and IPA ( •O H quencher) were evaluated and the results are shown in Fig.12. The extent of decrease caused by scavengers in the conversion indicated the importance of the corresponding reactive •



species. The ZnS/Ag/CoFe2O4 nanocomposite was greatly inhibited after adding BQ ( O2 •



quencher), therefore it is clear that the ( O2 ) is the main reactive species in the photocatalytic degradation of RhB under direct sunlight irradiation. While the •O H , e- and h+ reactive species were playing a minor role, the synergistic effect could have affected the overall better photocatalytic degradation of the RhB dye. The photocatalytic activity of the as-prepared ZnS/Ag/CoFe2O4 nanocomposites was compared to the previously reported experiments under the visible light irradiation and presented in Table 1 [49–57].

Table 1 Comparison of degradation efficiency for various photocatalysts S.No.

Photocatalyst

Dye

1. 2.

AgCl/Ag NPs CoFe2O4-graphene composite

3.

Zn0.6Co0.4Fe2O4

MB MB and RhB MB

4.

CuFe2O4graphene composite P25-CoFe2O4PANI CoFe2O4− rGO nanocomposite Ag–TiO2 nanofibers. TiO2/CoFe2O4 nanocomposite Ag/ZnO nanocomposite ZnS/Ag/CoFe2O4 nanocomposite

5. 6. 7. 8. 9. 10.

Catalyst amount (mg/L) 50 20

Irradiation Degradation time efficiency (min.) (%) 240 ~100 240 ~93 and 94

Ref.

10

480

~90

[51]

MB

25

240

~96

[52]

MO

40

420

~90

[53]

MB

10

180

~95

[54]

MB

50

260

100

[55]

MO

20

120

75

[56]

MB

~20

480

~93

[57]

MB and RhB

~40

90 and 150

~95.78 and 96.54

This work

[49] [50]

The band structure (valance and conduction band) of ZnS/Ag/CoFe2O4 nanocomposite was calculated by using Mulliken electronegativity theory [24]. The absolute electronegativity value of ZnS QDs and CoFe2O4 nanoparticles were 5.26 [58] and 5.47 eV [21]. The calculated valance band (VB) and conduction band (CB) edge positions of ZnS and CoFe2O4 are 2.66, -1.14 eV and 1.78, 0.16 eV, respectively. When the photocatalyst was irradiated, the sunlight generates electron-hole pairs. The CB edge position of ZnS QDs is more negative than that of CoFe2O4 NPs. Thus the electrons were transferred from CB of ZnS QDs into the CB of CoFe2O4 NPs, while the holes are transferred into VB of ZnS QDs. Silver nanoplates might have worked as an electron emitter to send out electrons (e-) to the CB of CoFe2O4 NPs[59]. These electrons react with oxygen molecules in water to generate the superoxide radical anions ( O2•− ) for the degradation of dye molecules [60]. The RhB or MB dye pollutant was oxidized (active species) to produce carbon dioxide and water [22]. The photocatalytic separation and transfer of

photogenerated charge carriers on the ZnS/Ag/CoFe2O4 nanocomposite under sunlight irradiation were schematically represented in fig.13.

Fig. 13 Schematic illustration of the photocatalytic separation and transfer of photogenerated charge carriers on the ZnS/Ag/CoFe2O4 nanocomposite under sun light irradiation A possible photocatalytic mechanism of the ZnS/Ag/CoFe2O4 nanocomposite under sunlight irradiation was proposed. The ZnS/Ag/CoFe2O4 nanocomposite absorbs the visible light photons and produce the electron-hole pairs. Silver nanoparticles immigrated (h+) from the VB of CoFe2O4 into VB of ZnS QDs, which reacted with OH- or water molecules to form to •

OH radicals. The electrons transferred from CB of ZnS to CB of CoFe2O4 are available on the

ZnS/Ag/CoFe2O4 nanocomposite surface to get reacted with absorbed oxygen, thereby forming the superoxide radicals ( O2•− ). The generated active species ( O2•− , •OH , h+) react with dye molecule to produce CO2 , NO3− , H 2O, etc... Generally, Ag nanoparticles and Ag based composites have been widely used as an effective antibacterial agent [61]. The antibacterial properties of CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite are studied to explore their convenience as a prospective material for antibacterial applications. The excellent activity of the antibacterial effect of the asprepared samples against S. aureus (G+) and E. coli (G-) shown in Fig. 14. The activity towards

the G+ bacteria are more because of G- bacteria cell wall is more probable than the G+ bacteria, and the composite nanoparticles may get trapped into easily [62] by the electrostatic interactions between the appreciated ions and the bacteria’s. The ZnS/Ag/CoFe2O4 nanocomposite showed better antibacterial activity against G+ and G- compared to the bare samples.

Fig. 14 Inhibition zone formed around the samples disks against E.coli and S. aureus cultures (a) CoFe2O4 NPs (b) Ag/CoFe2O4 and (c) ZnS/Ag/CoFe2O4 nanocomposites The ZnS/Ag/CoFe2O4 nanocomposites showed a clear zone of inhibition around the disc indicating the bacterial inhibition. The zone of inhibition exhibited by CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposites are given in Table 2.

Table 2 Antibacterial activity of pure CoFe2O4 and Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite towards E. coli and S. aureus

Samples

Diameter of Zone of inhibition (mm) E. coli

S. aureus

Positive Control

18±0.50

19±0.25

CoFe2O4 NPs

6±0.25

6±0.50

Ag/CoFe2O4 nanocomposite

11±0.80

12±0.30

ZnS/Ag/CoFe2O4 nanocomposite

26±0.10

21±0.60

The zones of inhibition were then evaluated so that their sizes could be averaged from three small dishes on the same testing plate. The catalytic process of ZnS/Ag/CoFe2O4 generates

different reactive oxygen species including superoxide and hydroxyl radicals. The superoxide radical’s directly involved and damage to the outer surface of the bacteria. The electromagnetic attraction between the positive charge of the ZnS/Ag/CoFe2O4 nanocomposite and the negative charge of the bacteria cell wall lead to the detriment of the microorganism. It is clear that, the ZnS/Ag/CoFe2O4 nanocomposite is more effective against Gram-positive bacteria; so the zone of inhibition was larger for S. aureus than that for E. coli. The above study showed that the antibacterial activity of the ZnS/Ag/CoFe2O4 nanocomposite is better than those of the bare samples.

Conclusion In summary, pure ZnS QDs, CoFe2O4 NPs, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite samples have been successfully fabricated via a simple precipitation method. The crystal structure, functional group, morphology, chemical composition, and photophysical properties were revealed by XRD, FTIR, FESEM, EDAX with mapping, UV, and PL analyses. The ZnS/Ag/CoFe2O4 nanocomposite exhibited superior photocatalytic activity for the degradation of MB and RhB under sunlight irradiation. The added Ag content provided the visible light absorption capability and the CoFe2O4 provided ferromagnetic behavior to the ZnS/Ag/CoFe2O4 nanocomposite sample. The enhanced visible light absorption resulted the efficient photocatalytic degradation and the ferromagnetic behavior helped to recover the sample with ease. The antibacterial activity of the nanocomposite was examined against both G+ and G− bacteria’s. The ZnS/Ag/CoFe2O4nanocomposite showed better antibacterial activity as compared to other samples. With all the experimental results, we conclude that, the ZnS/Ag/CoFe2O4 nanocomposite serves as an overall better candidate in terms of degradation efficiency as well as recyclability.

Supporting Information The Supporting Information contains the element mapping of Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite (Fig. S1 and S2), the detailed description Raman spectra of CoFe2O4, Ag/CoFe2O4 and ZnS/Ag/CoFe2O4 nanocomposite (Fig. S3), XPS survey spectrum (Fig. S4) and the photocatalytic degradation of MB and RhB dyes under visible light irradiation (Fig. S5 and S6).

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Highlights Magnetically

recoverable

ZnS/Ag/CoFe2O4

nanocomposite

photocatalyst

were

synthesized by simple wet precipitation method. ZnS/Ag/CoFe2O4 nanocomposite exhibit high stability for successive cycles with enhanced photocatalytic performance against MB & RhB dyes. The antibacterial activity of the ZnS/Ag/CoFe2O4 nanocomposite showed better antibacterial activity as compared to CoFe2O4 NPs and Ag/CoFe2O4 samples.

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