Magnetically Recyclable Ni0

8/10/2019 Magnetically Recyclable Ni0

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A novel visible light active and magnetically separable nanophotocatalyst,

Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O (denoted as NZF@Z), with varying amount of Ni0.5Zn0.5Fe2O4, has

been synthesized by egg albumen assisted sol gel technique. The structural, optical, magnetic,

and photocatalytic properties have been studied by powder X-ray diffraction (XRD),

transmission electron microscopy (TEM), field emission scanning electron microscopy(FESEM), fourier transform infrared spectroscopy (FTIR), UVvisible (UVVis) spectroscopy,

and vibrating sample magnetometry (VSM) techniques. Powder XRD, TEM, FTIR and energy

dispersive spectroscopic (EDS) analyses confirm coexistence of Ni 0.5Zn0.5Fe2O4and Zn0.95Ni0.05O

phases in the catalyst. Crystallite sizes of Ni 0.5Zn0.5Fe2O4and Zn0.95Ni0.05O in pure phases and

nanocomposites, estimated from DebyeScherrer equation, are found to be around 1525 nm.

The estimated particle sizes from TEM and FESEM data are (22 6) nm. The calculated

energy band gaps, obtained by Tauc relation from UVVis absorption spectra, of Zn0.95Ni0.05O,

15%NZF@Z, 40%NZF@Z and 60%NZF@Z are 2.95, 2.72, 2.64, and 2.54 eV respectively.

Magnetic measurements (field (H) dependent magnetization (M)) show all samples to be super-

paramagnetic in nature and saturation magnetizations (M s) decrease with decreasing ferrite

content in the nanocomposites. These novel nanocomposites show excellent photocatalytic

activities on Rhodamin Dye.

Graphical abstract

Figure options

Keywords

Nanocomposite;

Structural property;

Magnetic property;

Optical property;
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Photocatalysis

Introduction

The increasing extent of waste water generation from textile and other industries is posing a

serious threat to environmental remediation[1].Constant disposal of noxious organic pollutants

lead to unrealizable side effects, since the mineralization efficiency of these pollutants by

conventional mineralization methods is inadequate[2].The use of nanoparticles as catalysts in

organic transformations has attracted considerable interest in recent years, because of their

larger surface area-to-volume ratio[3]. Among them multifunctional fluorescent magnetic

nanocomposites have become the subject of intensive research due to their interesting multi

physicalchemical properties and potential applications in photocatalysis[4], antimicrobial

activities[5], magnetic resonance imaging (MRI), hyperthermia, bioseparation, drug delivery,and cell labeling[6],[7],[8]and[9].Semiconductor nanoparticles offer photocatalytic properties,

which are used to degrade organic pollutants in water under UV or solar light. Among them

ZnO is a well known photoluminescent semiconductor with wide band gap (3.4 eV), and large

excitonic binding energy (60 meV) at room temperature. It has widely been used as

photocatalyst due to its biocompatibility, ease of preparation, and stability. Unfortunately, ZnO,

being a wide band gap semiconductor, requires ultraviolet irradiation for its band gap

excitation[1]and[2].The band gap of the photocatalyst determines the particular wavelength

of light that can be absorbed. Many commonly used photocatalysts have wide band gaps

(>3.1 eV) and can absorb only small portion of the solar light (UV light). It is worth mentioning

here that solar light contains 50% visible light and only 5% UV light. Thus, to utilize the

maximum solar light a photocatalyst that can absorb visible solar energy should be used[10].

The band gap of ZnO can be engineered by suitable doping in the ZnO lattice making them

suitable for visible light absorption[11],[12]and[13].However, recovery of these ZnO catalyst

particles after the photocatalytic process is difficult. Fast recombination of the generated

electronhole pairs is another major drawback of pure semiconductor photocatalysts, which can

be avoided by combining P-type and N-type semiconductors, as it facilitates charge

migration[14],[15],[16]and[17]. Thus commonly used non magnetic semiconductor

photocatalysts suffer from three main drawbacks, viz. less visible light activity, poor recovery,

and fast recombination of the generated electronhole pairs. Ferrite nanoparticles having band

gap 2 eV offer several advantages including visible light absorption, magnetic separability,

and enhanced photocatalytic efficiency due to the presence of extra catalytic sites in their

crystal structures[18]. Independently ferrites have rarely been used in photolocatalysis due to

its lower valence band potential and poor photocatalytic conversion efficiency [19].
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Nanocomposite photocatalysts made up of semiconductor and magnetic materials, such as

ZnFe2O4/ZnO, CoFe2O4/ZnO, Fe2O3/ZnO, and Fe3O4/ZnO, are gaining increasing importance

because of their recyclability and higher photocatalytic activity than pure ferrites or ZnO[8].

Incorporation of ferrite in ZnO helps in improving the quantum yield of ZnO by slowing down the

recombination of photogenerated electrons and holes[20].Since pure ZnO is not suitable forabsorption and utilization of visible region of the solar spectrum, in this study Ni doped ZnO

(Zn0.95Ni0.05O) with a band gap of 2.95 eV and Ni0.5Zn0.5Fe2O4(band gap 2.2 eV) have been

chosen to prepare nanophotocatalysts for visible light active, magnetically separable, and

recyclable photocatalytic activity. Numerous surfactants and stabilizing agents, including

ethylene glycol, sodium dodecyl sulfate (SDS), and citric acid, have been used to control the

size and shape of ZnO NPs during synthesis[21]. These chemicals have varying degree of

toxicity and are difficult to remove from nanoparticle surfaces even after repeated washing.

Recently, natural bioresources with excellent biocompatibility are increasingly being used as

templates for synthesis of nanomaterials. Egg albumen offers several advantageous including

gelling, foaming, emulsifying, water solubility, heat setting and good binding capacity with metal

ions[22]. In the present investigation, Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O nanocomposites have been

prepared, for the first time, using egg albumen as biotemplate, which is environment friendly

and cost effective. In this study freshly extracted egg albumen was used because of its good

water solubility, metal binding ability, simple processing condition, amphiphilic nature, easy

availability and low cost. No external surfactant or stabilizing agent has been used to prepare

nanocomposites. The proteins of egg albumen, which have different functional groups, provide

not only stable suspension of ferrite in water but also may enhance selective deposition of

coating materials by virtue of its metal binding capacity. It plays an active role in ZnO synthesis

process by forming complex with Zn precursor and can provide nanoparticles with specific

morphologies and high surface area[21],[22]and[23]. The structural, optical, magnetic, and

photocatalytic properties of the composites have been studied by XRD, TEM, FESEM, FTIR,

UVVis spectroscopy, and VSM techniques.

Materials and methods

Materials

All reagents used in this synthesis were of analytical grade. Zinc nitrate (>98% Zn(NO 3)2.6H2O),

nickel nitrate (>98% Ni(NO3)2.6H2O), iron nitrate (>98% Fe(NO3)3.9H2O) from Sigma Aldrich and

citric acid, ammonia from SRL, India were used in this synthesis without any purification. Egg

white was obtained from fresh egg available in the market.

Ni0.5Zn0.5Fe2O4NPs synthesis
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Ni0.5Zn0.5Fe2O4NPs were synthesized by gel-combustion method reported earlier[4]. Nickel

nitrate, zinc nitrate and iron nitrate with a molar ratio of 1:1:4 were dissolved in 100 ml of water.

Citric acid was added to the above nitrate precursor solution with citric acid: nitrate molar ratio

of 1:1. The resultant sol was continuously stirred at 90 C for 1 h. The gel so formed was

subjected to combustion at 300 C. A reddish brown powder was obtained, which wasgrounded in mortar and pestle for subsequent use.

Synthesis of Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O nanocomposites

Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O nanocomposites were synthesized in two steps. First, a stable

dispersion of Ni0.5Zn0.5Fe2O4 nanoparticles in egg albumen solution, which served as seeding

materials, was obtained. Secondly, coating of Zn0.95Ni0.05O on Ni0.5Zn0.5Fe2O4nanoparticle

surfaces was realized on the basis of chemical precipitation method. A typical synthesis

procedure is as below. Appropriate amount of Ni0.5Zn0.5Fe2O4nanoparticles were dispersed in

20 ml of water taken in a beaker and 30 ml freshly extracted egg albumen was added to it

(beaker 1). In another beaker appropriate amount of zinc nitrate and nickel nitrate were

dissolved in 50 ml water (beaker 2). Both beakers were sonicated separately for 15 min to

ensure proper mixing. Stable dispersion of Ni0.5Zn0.5Fe2O4nanoparticles was obtained in beaker

1. Aqueous solution of zinc nitrate and nickel nitrate, from beaker 2, was added drop wise to

the stable dispersion of Ni0.5Zn0.5Fe2O4(NZF) in beaker 1 with continuous stirring. 30 min

vigorous stirring of the resultant solution followed by addition of 23 ml ammonia resulted in

brown color precipitate. Obtained precipitate was centrifuged, washed with alcohol & water and

dried at 50 C in oven. The dried NZF@Z precursor was calcined at 600 C for 3 h. Pure

Zn0.95Ni0.05O nanoparticles were prepared by the same method except the addition of ferrite

nanoparticles. Different nanocomposites were prepared by adding different percentages of

ferrite (Ni0.5Zn0.5Fe2O4) nanoparticles, 15%, 40% and 60% with Zn0.95Ni0.05O and labeled as

15%NZF@Z, 40%NZF@Z and 60%NZF@Z respectively. Obtained nanocomposites were

stored in glass vials at room temperature for further characterization.

Characterization

The X-ray diffraction (XRD) patterns of the powder samples were recorded with a MiniFlexTM II

benchtop XRD system (Rigaku Corporation, Tokyo, Japan) operating at 40 kV. Particle sizes

and morphology of the samples were measured by Transmission electron microscope (FEI

Tecnai T20G2 S TWIN TEM) and field emission scanning electron microscope (Carl Zeiss Ultra

55 FESEM). For both TEM and FESEM a pinch of nanoparticles were dispersed and sonicated

in alcohol and a drop of suspension was placed on carbon coated copper grid (for TEM) and

carbon tape pasted on stub (for FESEM) respectively. The elemental analysis of the sample

was carried out by Energy dispersive spectroscopy (EDS) attached with the FESEM. The
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electronic absorption behavior at room temperature was analyzed using a UVVIS

spectrophotometer (Perkin Elmer Lambda 35). A suspension was prepared by dispersing

1 mg of nanopowder in 5 ml distilled water. Water was used as reference. The spectra was

recorded in the wavelength rang of 200800 nm from the aqueous suspension of the sample.

Fourier transformed infrared (FT-IR) analysis of the samples was conducted using Perkin-Elmer2000 FT-IR spectrometer in the wavenumber range of 5004000 cm1. The magnetic

characterization of the sintered (800 C) pellets was performed using a Lakeshore (Model

7407) Vibrating Sample Magnetometer (VSM) in magnetic fields up to 1.5 T at ambient

temperature (298 K). The accuracy of the magnetization measurement was within 1%.

Photocatalytic activity measurement

The photocatalytic activities of the samples were estimated using Rhodamine B (RhB) dye

under solar light irradiation. All photocatalytic experiments were carried out under similar

conditions on sunny days between 11 am and 3 pm. In the photocatalytic experiment, 50 g/ml

of Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4catalysts were

added to 50 ml dye solution (of concentration 20 g/ml). Before irradiation the suspensions

containing RhB dye and Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and

Ni0.5Zn0.5Fe2O4were stirred in dark for 60 min to ensure the establishment of an

adsorption/desorption equilibrium. 5 ml aliquots were magnetically filtrated at a fixed time

interval (60 min) and was analyzed for the variation in maximum absorption band (

max553 nm) using a UVvis spectrophotometer. The photo-decoloration of the RhB dye via the

photocatalytic activities of Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and

Ni0.5Zn0.5Fe2O4was calculated following the formula:

Turn MathJaxon

where Cois the RhB dye initial concentration before photo-decoloration and Cis the

absorbance after different time intervals. The role of active reactive oxygen species (ROS)

generated in the photocatalytic conversion was confirmed by trapping them with tert-butyl

alcohol (C4H10O) and disodium ethylenediaminetetraacetate dehydrate (EDTA-Na2;C10H14N2Na2O82H2O)[4].

Results and discussion

XRD analysis
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been shown in the insets of the respectiveFigs. 3(a) and4(a). The distribution is broader in

pure NZF compared to that in 40%NZF@Z. The bigger particle sizes in composites compared

to those in pure Ni0.5Zn0.5Fe2O4could be due to the coating of Zn0.95Ni0.05O around the surface of

Ni0.5Zn0.5Fe2O4nanoparticles. The average particle sizes for 40%NZF@Z was 70 17 nm,

measured by FESEM & TEM.Fig. 3(b) shows the formation of agglomerates of 40%NZF@Znanoparticles but the distribution of particle sizes is narrower (as shown in inset ofFig. 3(b))

than that in pure NZF, as shown in inset ofFig. 3(a). The smaller particle sizes (22 nm) of

40%NZF@Z inFig. 3(b) could be due to uncoating of Ni 0.5Zn0.5Fe2O4 nanoparticles as also

observed by other researcher[25]. In TEM image of 40%NZF@Z the NZF particles (appear

dark) are seen to be surrounded by Zn0.95Ni0.05O particles (appear grey) confirming the coating of

the former by the later. Since ferrite is magnetic in nature it absorbs more electron than

Zn0.95Ni0.05O and hence appear darker than ZnO in the TEM image (Fig. 4(b))[26].

Fig. 3.

FESEM images with particles size distribution histogram of Ni0.5Zn0.5Fe2O4(a), 40%NZF@Z nanocomposite (b) and

corresponding EDS spectra (c and d).

Figure options
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Fig. 4.

(a) TEM image of Ni0.5Zn0.5Fe2O4nanoparticles with inset shows the particles size distribution, (b) TEM image of

40%NZF@Z nanocomposite. (c) Indexed SAED pattern of spinel cubic Ni 0.5Zn0.5Fe2O4(JCPDS card No. 520278).

(d) Indexed SAED pattern of 40%NZF@Z nanocomposite which shows both spinel cubic Ni 0.5Zn0.5Fe2O4and

hexagonal Zn0.95Ni0.05O present in the sample (JCPDS card No. 790207).

Figure options

The EDS spectra (Fig. 3(c) and (d)) of Ni0.5Zn0.5Fe2O4and 40%NZF@Z samples show only the

presence of Fe, Ni, Zn, and O in stoichiometric ratio, without any impurity peak, confirming thepurity of the phases. In 40%NZF@Z the mass ratio of Fe to Zn is 0.37, which is close to the

expected value 0.365. In 40%NZF@Z the weight% of O, Fe, Ni, and Zn are found to 33.05,

16.65, 5.14, and 45.15 respectively.

Fig. 4(c) and (d) shows the selected area electron diffraction (SAED) patterns of

Ni0.5Zn0.5Fe2O4and 40%NZF@Z samples. The SAED pattern of Ni0.5Zn0.5Fe2O4 shows distinct
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dotted ring pattern, which confirms the polycrystalline nature of the cubic spinel

Ni0.5Zn0.5Fe2O4nanoparticles. The SAED pattern was indexed for cubic structure by estimating

the d-spacing from the ring pattern and comparing with the JCPDS card No. 520278 for NZF.

40%NZF@Z sample was also observed to be polycrystalline in nature. The SAED pattern of

40%NZF@Z shows a complex and mixed dotted ring pattern, which is due to the presence ofboth the phases, cubic Ni0.5Zn0.5Fe2O4and hexagonal Zn0.95Ni0.05O[6].Presence of both phases

have also been confirmed by indexing the SAED patterns of other nanocomposites investigated

in this study.

Role of egg albumen

The mechanistic aspect of the synthesis of NZF@Z nanocomposites using egg albumen can be

explained as follows.

Egg albumen contains different types of proteins such as 60% ovalbumin, 12%Ovotransferrin, and 11% Ovomucoid. These proteins have polar-COOH (hydrophilic) and

nonpolar-alkyl (hydrophobic) groups. It is difficult to prepare stable dispersion of bare ferrite

nanoparticles due to its hydrophobic nature. When ferrite nanoparticles are sonicated along

with albumen these polymeric proteins wrap the surface of nanoparticles and the hydrophilic

parts of the protein interact with water giving rise to stability of the suspension. Ovotransferrin is

well known for its iron binding property. Ovotransferrin folds into two globular lobes, each

containing an iron binding site located within the interdomain cleft of each lobe [27].These iron

binding sites may also provide affinity toward iron of ferrites and help in decoration of

ovotransferrin on ferrite nanoparticles leading to improved stability of the resultant suspension.

Ni0.5Zn0.5Fe2O4nanoparticles dispersed in water with egg albumen (water:albumen ratio 2:3

and ferrite concentration 10 mg/ml) shows improved stability than only in water, as shown in

digital photograph inFig. 5(a). Ni0.5Zn0.5Fe2O4nanoparticles in water are seen to settle within

12 min of dispersion, whereas with egg albumen it is stable up to 3 h. To find out the role of

albumen in the synthesis process TEM analysis of albumen treated Ni 0.5Zn0.5Fe2O4nanoparticles

was carried out after repeated washing with water. Formation of 56 nm thick amorphous layer

of albumen around the crystalline Ni0.5Zn0.5Fe2O4nanoparticles has been confirmed by HRTEM

analysis as shown inFig. 5(b)(d). Ni0.5Zn0.5Fe2O4nanoparticles decorated with albumen on its

surface provide nucleation sites for Zn0.95Ni0.05O deposition[20]. Metal binding tendency of

albumen may attract zinc precursor to selectively nucleate on it [28].Zn(OH)2 is formed when

zinc nitrate reacts with H2O. Functional groups of egg protein chelate with Zn atom and form

protein-Zn(OH)2complex, which subsequently converted to hyderozincite intermediate. On

calcination at 500 C the hyderozincite intermediate decomposes to ZnO[21].In our previous
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report the mechanistic aspect of synthesis of pure ZnO nanoparticles using egg albumin as

biotemplate has been discussed in detail[21].

Fig. 5.

(a) The digital photograph shows the stability of dispersed Ni 0.5Zn0.5Fe2O4in water without-a and with egg albumen-b

with respect to time.(b) TEM image of egg albumen coated Ni 0.5Zn0.5Fe2O4nanoparticles.(c and d) HRTEM image of

crystalline Ni0.5Zn0.5Fe2O4nanoparticle, covered with 5 nm thick amorphous albumen layer. Inverse Fast Fourier

transform from the framed part in (c) is shown in the (d).

Figure options

Optical properties

UVvisible absorption spectroscopy is a powerful technique to explore the optical properties of

semiconducting nanoparticles. The absorbance of a material depends on several factors such

as band gap, oxygen deficiency, surface roughness and impurity centers[29].Fig. 6shows

UVvisible absorption spectra of Ni0.5Zn0.5Fe2O4, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z and

Zn0.95Ni0.05O.
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Fig. 7.

Tauc plot depicting energy band gap of Ni 0.5Zn0.5Fe2O4, 15%NZF@Z, 40%NZF@Z in the main panel and

60%NZF@Z and Zn0.95Ni0.05O in the inset.

Figure options

Optical band gaps of all the samples were estimated using the Tauc relationship, h=A(h

Eg)n, where is the absorption coefficient,Ais a constant, his the Plancks constant, is the

photon frequency, and Egis the optical band gap[30].The value of n could be 1/2, 3/2, 2 or 3

depending on the nature of electronic transition responsible for absorption and n= is for

direct band gap semiconductor.An extrapolation of the linear region of plot (h)2vs hgives

the value of the optical band gap, Eg, as shown inFig. 7. Energy band gaps of Zn0.95Ni0.05O,

15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4 were 2.98, 2.72, 2.64, 2.54, and

2.24 eV respectively. The observed energy band gap of Zn0.95Ni0.05O nanoparticles is much

lower than that reported for pure ZnO nanoparticles (3.5 eV for ZnO with 16 nm crystallite

size)[31]. Energy band gaps of the nanocomposites are found to decrease with increasing

Ni0.5Zn0.5Fe2O4content in the nanocomposites. This is due to the reduced band gap energy of

Zn0.95Ni0.05O in the nanocomposites in presence of Ni0.5Zn0.5Fe2O4.This reduction in band gap

energy has been explained in the literature in terms of mixing of the 4s orbital of Fe and Zn and

formation of the conduction band of Zn0.95Ni0.05O at lower energy[6]and[32].It may also be due

to the formation of sub band/s within the band gap of Zn 0.95Ni0.05O due to doping of metal ions

from Ni0.5Zn0.5Fe2O4resulting in lowering of band gap energy.

FT-IR spectra of all the samples are shown inFig. 8. The IR spectra of

Ni0.5Zn0.5Fe2O4nanoparticles shows two principle absorption bands in the wavelength range

400600 cm1, the first band is around 422 cm1and the second around 577 cm1. These two

vibration bands can be attributed to the intrinsic lattice vibrations of octahedral and tetrahedral

coordination complexes in the spinel structure, respectively[33].The broad absorption band
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400600 cm1in all samples could arise from the simultaneous presence and overlapping of Zn-

O vibration mode at 453 cm1in ZnO and two above mentioned absorption bands i.e. at 577 cm

1and 422 cm1in ferrite[6]. In IR spectra of 15%NZF@Z, 40%NZF@Z and 60%NZF@Z

nanocomposites, presence of FeO vibration mode at 577 cm1as shoulder has been

observed, which increases with increase in ferrite content in these samples. Therefore, IRresults also confirm successful formation of nanocomposites. In addition to these absorption

peaks, the absorption band centered at 1121 cm1may be attributed to the vibration mode of

CO, which could indicate the presence of decomposition products of albumen as impurities in

the samples[34].The absorption band at 2364 cm1can be assigned to trace of adsorbed or

atmospheric CO2[35].

Fig. 8.

FTIR spectra of Ni0.5Zn0.5Fe2O4nanoparticles, 15%NZF@Z, 40%NZF@Z and 60%NZF@Z nanocomposites.

Figure options

Magnetic properties

The field dependent magnetic behavior of all the samples is shown inFig. 9.Different magnetic

properties such as, saturation magnetization (Ms), remanent magnetization (MR) and coercivity

(Hc) have been derived from the magnetization curves. The M svalues for Ni0.5Zn0.5Fe2O4,

60%NZF@Z, 40%NZF@Z, and 15%NZF@Z were 56, 33, 23, and 8 emu/gm respectively. It is

found that Ms decreases linearly with decrease in ferrite content in these composite samples

(Fig. 9inset). The decreasing saturation magnetization (Ms) is consistent with the increasing

non-magnetic Zn0.95Ni0.05O content in these samples[6]. Magnetization behavior at lower field

shows all the samples to be super-paramagnetic in nature with very low remanent

magnetization and coercivity. Remanent magnetizations of all the samples are found to be

0.41.4 emu/gm. The coercivities are 37, 44, 48 and 54 Oe for Ni 0.5Zn0.5Fe2O4, 60%NZF@Z,

40%NZF@Z and 15%NZF@Z respectively. The increasing coercivity of the samples with

increasing Zn0.95Ni0.05O content could be attributed to increasing domain wall pining by the non-
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magnetic Zn0.95Ni0.05O phase in these composites. The superparamagnetic behavior of

Ni0.5Zn0.5Fe2O4nanoparticles could be attributed to very small particle sizes (22 nm) of this

sample. The smaller size particles may be equivalent to single domain (magnetic) particles,

where thermal vibration surpass the energy barrier for its spin reversal leading to

superparamagnetic behavior. Magnetic separation ability of the dispersednanocomposites/photocatalysts in water has also been demonstrated and shown as a digital

image in the other inset ofFig. 9for 15%NZF@Z (10 mg/ml), (a) without magnet, and (b) near

the magnet. It is observed that prepared nanocomposite photocatalysts have good dispersibility

in water and can be easily separated out from the solution by applying an external magnetic

field even though the magnetization of the sample is not too strong (8 emu/gm for 15%NZF@Z,

for example). Thus, the photocatalysts could be used repeatedly and opens up a possibility of

recyclability.

Fig. 9.

Room temperature magnetization behavior (MH) of all the samples in an applied field upto15000 Oe. The inset at

the top shows linear variation of Ms as a function of varying weight percent (wt%) of Ni 0.5Zn0.5Fe2O4in the

nanocomposites. Digital image, in the other inset shows dispersed 15%NZF@Z nanocomposite in water

(10 mg/ml), (a) without magnet and (b) near the magnet.

Figure options

Analysis of photocatalytic activity

Enhanced visible-light driven photo-activity has been observed for some heterostructure

semiconductor nanocomposites previously[1]and[35], which motivated us to study the

photocatalytic activity of NZF@Z nanocomposites. The high photocatalytic activity of metal

oxide nanocomposites was attributed to the enhanced separation efficiency of photoinduced
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carriers (electrons and holes) through electronic interaction. Thus, photo-decoloration study of

RhB dye was performed under solar light irradiation using Zn0.95Ni0.05O, 15%NZF@Z,

40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4as photocatalysts. The photo-decoloration

experiments were performed after proper adsorption of RhB dye on the surface of Zn 0.95Ni0.05O,

15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4photocatalysts. The extent ofphoto-decoloration of RhB dye by Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and

Ni0.5Zn0.5Fe2O4under solar light irradiation, at 25 C, is shown inFig. 10A.The obtained photo-

decoloration data, under solar light irradiation, shows the promise of photocatalytic activity of

Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4. The control (RhB dye

solution without nanocomposites) does not exhibit significant photo-decoloration under solar

light irradiation up to 120 min. This suggests that the photo-decoloration of RhB dye takes

place only by photocatalysis and not by photosensitization[4]. All the nanocomposites have

shown better photocatalytic activity than pure Zn0.95Ni0.05O or Ni0.5Zn0.5Fe2O4.The enhanced

photo-decoloration rates of RhB dye, observed in this study, may be attributed to the lager

surface area of the photocatalysts and reduced carriers (photo-induced) recombination through

electronic interaction in these nanocomposites[4].The reduced carrier recombination can be

understood as follows. When light is absorbed by a pure semiconductor particle, an electron is

excited from VB to CB and an e/h+pair is formed. The generated eand h+may travel to the

surface of the particle and react with adsorbed species resulting in the desired process

(degradation of dye for example), or they may recombine, which is an undesired process. The

holes react with surface hydroxyl groups (OH) and H2O, to form highly reactive OH radicals,

which degrade organic dye molecules. The undesired high e/h+recombination slows down the

photocatalysis process, especially, in case of pure semiconductor photocatalysts. To increase

the photocatalytic efficiency the e/h+recombination has to be minimized. A scheme to reduce

the recombination of eand h+is to prepare heterostructure/nanocomposite with

hetrojunction[36].The band potentials of the component materials in the nanocomposite form a

heterojunction with a straddling gap, which may facilitate the transfer of charge carriers and

retard the electron hole recombination, resulting in improved photocatalytic performance [35].In

other word, combining two photocatalysts with different band gap positions effectively causes a

greater separation of e/h+pairs, allowing more of the species to be available for surface

reactions leading to degradation of the organic species[10]and[37]. Thus, as a magnetic

semiconductor material, NZF might not only add recyclability and visible light activity to the

catalyst nanoparticles, but also offer some synergetic enhancement of the catalytic activity by

forming the hybrid structure[38].
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Fig. 10A.

RhB dye photo-decoloration efficiency under solar light irradiation by Zn 0.95O0.05, 15%NZF@Z, 40%NZF@Z,

60%NZF@Z and Ni0.5Zn0.5Fe2O4photocatalysts.

Figure options

The identification of the main active oxidant (reactive oxygen species) in the photocatalytic

reaction is of great importance to understand the mechanism of the photocatalytic conversion

process. The chemical interactions between the photocatalysts (Zn0.95Ni0.05O, 15%NZF@Z,

40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4) and the charged groups of RhB molecules lead

to significant adsorption followed by high photo-decoloration. The role of the active oxidants

generated in the photocatalytic reactions involving Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z,

60%NZF@Z, and Ni0.5Zn0.5Fe2O4was ascertained by quenching the reaction mixture in disodium

ethylenediaminetetraacetate dehydrate (EDTA-Na2; C10H14N2Na2O82H2O) (hole scavenger)

and tert -butyl alcohol (C4H10O) (radical scavenger)[4].The photo-decoloration of RhB under

ultraviolet light irradiation was supressed after the addition of t-BuOH and EDTA-Na2, as shown

inFig. 10B,which suggests that both radicals and holes are active species in this system. The

absorption of photons with sufficient energy (2.98, 2.72, 2.64, 2.54, and 2.24 eV of

Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4, respectively) is the

necessary condition for photochemical reactions to proceed on the photocatalyst surface. The

band edges positions (VB and CB) of the AB 2O4type ferrite (ZnFe2O4/NiFe2O4) lies above the

corresponding band edges positions of ZnO respectively[1],[10],[39]and[40]. Reported CBand VB potential of ZnFe2O4is nearly at 1.54 eV (vs NHE) and +0.38 eV (vs NHE)

respectively[38]and[41].The CB and VB potentials of ZnO is nearly at 0.76 eV (vs NHE) and

+2.7 eV (vs NHE) respectively[39]and[42].Fig. 10Cshows the schematic of the possible

mechanism in this photocatalysis process. Under solar light irradiation, the electrons (e ) from

the filled valence bands (VB) of Ni0.5Zn0.5Fe2O4and Zn0.95Ni0.05O will be excited to the respective

empty conduction bands (CB), separately giving an equal number of holes (h+) in the
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corresponding VBs. The presence of Ni0.5Zn0.5Fe2O4nanoparticles will favor the utilization of

visible region of the spectrum due to its narrow band gap (2.2 eV) thereby enhancing the

photocatalytic activity under visible light irradiation[10]and[35]. Moreover, the difference in

band structures of Ni0.5Zn0.5Fe2O4and Zn0.95Ni0.05O will facilitate photoinduced electrons transfer

from the CB of Ni0.5Zn0.5Fe2O4to that of Zn0.95Ni0.05O and holes transfer from the VB of Zn 0.95Ni0.05Oto that of Ni0.5Zn0.5Fe2O4, respectively[1]. These processes will efficiently hinder the

recombination of photogenerated electronhole pairs and considerably enhance the

photocatalytic activity under solar light irradiation. The resulted electronhole pairs recombine

or migrated to the surface of the particles and retort either with H 2O or OHto form OH, which

have strong oxidation ability and can destroy the dye molecules completely. The electrons will

also react with adsorbed molecular oxygen to form superoxide radical anion, ions, which

will further react with water to give OH[1].

Fig. 10B.

Photodecoloration analysis shows the protective effect of disodium ethylenediaminetetraacetate dehydrate (EDTA-

Na2; C10H14N2Na2O82H2O) (hole scavenger) (Q1) and tert-butyl alcohol (C4H10O) (radical scavenger) (Q2) on RhB

dye in presence of Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O nanocomposites (Q1-Column1 and Q2- Column2).

Figure options
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Fig. 10C.

Schematic diagram illustrating the mechanism of photo-decoloration of RhB dye on Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O

surface under exposure to solar light.

Figure options

Conclusions

A novel, visible light active and magnetically separable Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O

nanocomposites have been synthesized successfully by a simple and cost effective sol gel

technique using egg albumen as biotemplate. Powder XRD, TEM, FESEM, FTIR, UVVis

spectrophotometer, and VSM techniques have been used to characterize their structural,

optical, and magnetic properties. The photocatalytic activity of these nanocomposites has been

investigated by photo-decoloration study of Rhodamine B dye molecules. Enhanced photo-

decoloration of the Rhodamine B dye molecules by NZF@Z photocatalysts, compare to that by

pure Ni0.5Zn0.5Fe2O4or Zn0.95Ni0.05O, have been observed under solar light irradiation, through the

production of reactive oxygen species (ROS). Narrow band gap of Ni0.5Zn0.5Fe2O4(2.2 eV),

relatively low band gap of Zn0.95Ni0.05O (2.95 eV), and reduced electrons-holes recombination

through electronic interactions have contributed significantly to this visible light active

photocatalysis process. This novel visible light active and magnetically separable photocatalyst

may immensely contribute to environmental remediation.

Acknowledgement

M. Qasim greatly acknowledges the financial support obtained from University Grants

Commission (UGC) in the form of MANF fellowship in carrying out this research work. BRS

thanks to CSIR, India for awarding Scientists Pool Scheme (13(8595-A) 2012-Pool). The

technical support received from the Centre of Excellence in Materials Science (Nanomaterials)

Aligarh Muslim University, School of Engineering Sciences & Technology (SEST), Centre for

Nanotechnology, and School of Physics at the University of Hyderabad is greatly appreciated.
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http://www.sciencedirect.com.sci-hub.org/science/article/pii/S1386142514013973#b0045http://www.sciencedirect.com.sci-hub.org/science/article/pii/S1386142514013973#b0045http://www.sciencedirect.com.sci-hub.org/science/article/pii/S0928493112004353http://www.sciencedirect.com.sci-hub.org/science/article/pii/S0928493112004353http://www.sciencedirect.com.sci-hub.org/science/article/pii/S0928493112004353/pdf?md5=b4900a7b8e47b9fe82da9da458e54b97&pid=1-s2.0-S0928493112004353-main.pdfhttp://www.sciencedirect.com.sci-hub.org/science/article/pii/S0928493112004353/pdf?md5=b4900a7b8e47b9fe82da9da458e54b97&pid=1-s2.0-S0928493112004353-main.pdfhttp://www.scopus.com.sci-hub.org/inward/record.url?eid=2-s2.0-84869092457&partnerID=10&rel=R3.0.0&md5=0d0250d2d9b7cf7bd786143c5915d19ahttp://www.scopus.com.sci-hub.org/inward/record.url?eid=2-s2.0-84869092457&partnerID=10&rel=R3.0.0&md5=0d0250d2d9b7cf7bd786143c5915d19ahttp://dx.doi.org/10.1155%2F2013%2F187024http://dx.doi.org/10.1155%2F2013%2F187024http://www.scopus.com.

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