Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1348–1356

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Magnetically recyclable Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05Onano-photocatalyst: Structural, optical, magnetic and photocatalyticproperties

http://dx.doi.org/10.1016/j.saa.2014.09.0391386-1425/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 40 23134454; fax: +91 40 23011087.E-mail address: ddse@uohyd.ernet.in (D. Das).

1 Current address: Department of Metallurgical Engineering and Materials Science, IIT Mumbai, Mumbai, India.

Mohd Qasim a, Khushnuma Asghar a,b, Braj Raj Singh b, Sateesh Prathapani a,1, Wasi Khan b, A.H. Naqvi b,Dibakar Das a,⇑a School of Engineering Sciences and Technology (SEST), University of Hyderabad, Hyderabad 500 046, Andhra Pradesh, Indiab Centre of Excellence in Materials Science (Nanomaterials), Department of Applied Physics, ZHCET, Aligarh Muslim University, Aligarh 202 002, Uttar Pradesh, India

h i g h l i g h t s

� Egg albumen assisted synthesis ofnovel visible light activeNi0.5Zn0.5Fe2O4/Zn0.95Ni0.05O nanophotocatalyst.� Structural, optical, magnetic and

photocatalytic properties have beenstudied.� Enhanced photo-decoloration of the

Rhodamine B dye molecules undersolar light irradiation.� Photo-decoloration through the

production of reactive oxygen species(ROS).� The enhancement has been explained

by reduced electrons–holesrecombination.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 May 2014Received in revised form 3 September 2014Accepted 19 September 2014Available online 28 September 2014

Keywords:NanocompositeStructural propertyMagnetic propertyOptical propertyPhotocatalysis

a b s t r a c t

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 albumenassisted sol gel technique. The structural, optical, magnetic, and photocatalytic properties have beenstudied by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scan-ning electron microscopy (FESEM), fourier transform infrared spectroscopy (FTIR), UV–visible (UV–Vis)spectroscopy, and vibrating sample magnetometry (VSM) techniques. Powder XRD, TEM, FTIR and energydispersive spectroscopic (EDS) analyses confirm coexistence of Ni0.5Zn0.5Fe2O4 and Zn0.95Ni0.05O phases inthe catalyst. Crystallite sizes of Ni0.5Zn0.5Fe2O4 and Zn0.95Ni0.05O in pure phases and nanocomposites,estimated from Debye–Scherrer equation, are found to be around 15–25 nm. The estimated particle sizesfrom TEM and FESEM data are �(22 ± 6) nm. The calculated energy band gaps, obtained by Tauc relationfrom UV–Vis 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)) showall samples to be super-paramagnetic in nature and saturation magnetizations (Ms) decrease withdecreasing ferrite content in the nanocomposites. These novel nanocomposites show excellentphotocatalytic activities on Rhodamin Dye.

� 2014 Elsevier B.V. All rights reserved.

M. Qasim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1348–1356 1349

Introduction

The increasing extent of waste water generation from textileand other industries is posing a serious threat to environmentalremediation [1]. Constant disposal of noxious organic pollutantslead to unrealizable side effects, since the mineralization efficiencyof these pollutants by conventional mineralization methods isinadequate [2]. The use of nanoparticles as catalysts in organictransformations has attracted considerable interest in recent years,because of their larger surface area-to-volume ratio [3]. Amongthem multifunctional fluorescent magnetic nanocomposites havebecome the subject of intensive research due to their interestingmulti physical–chemical properties and potential applications inphotocatalysis [4], antimicrobial activities [5], magnetic resonanceimaging (MRI), hyperthermia, bioseparation, drug delivery, and celllabeling [6–9]. Semiconductor nanoparticles offer photocatalyticproperties, which are used to degrade organic pollutants in waterunder UV or solar light. Among them ZnO is a well known photo-luminescent semiconductor with wide band gap (3.4 eV), and largeexcitonic binding energy (60 meV) at room temperature. It haswidely been used as photocatalyst due to its biocompatibility, easeof preparation, and stability. Unfortunately, ZnO, being a wideband gap semiconductor, requires ultraviolet irradiation for itsband gap excitation [1,2]. The band gap of the photocatalyst deter-mines 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 (UVlight). It is worth mentioning here that solar light contains �50%visible light and only �5% UV light. Thus, to utilize the maximumsolar light a photocatalyst that can absorb visible solar energyshould be used [10]. The band gap of ZnO can be engineered bysuitable doping in the ZnO lattice making them suitable for visiblelight absorption [11–13]. However, recovery of these ZnO catalystparticles after the photocatalytic process is difficult. Fastrecombination of the generated electron–hole pairs is anothermajor drawback of pure semiconductor photocatalysts, whichcan be avoided by combining P-type and N-type semiconductors,as it facilitates charge migration [14–17]. Thus commonly usednon magnetic semiconductor photocatalysts suffer from threemain drawbacks, viz. less visible light activity, poor recovery, andfast recombination of the generated electron–hole pairs. Ferritenanoparticles having band gap �2 eV offer several advantagesincluding visible light absorption, magnetic separability, andenhanced photocatalytic efficiency due to the presence of extracatalytic sites in their crystal structures [18]. Independently ferriteshave rarely been used in photolocatalysis due to its lower valenceband potential and poor photocatalytic conversion efficiency [19].Nanocomposite photocatalysts made up of semiconductor andmagnetic materials, such as ZnFe2O4/ZnO, CoFe2O4/ZnO, Fe2O3/ZnO,and Fe3O4/ZnO, are gaining increasing importance because of theirrecyclability and higher photocatalytic activity than pure ferritesor ZnO [8]. Incorporation of ferrite in ZnO helps in improving thequantum yield of ZnO by slowing down the recombination ofphotogenerated electrons and holes [20]. Since pure ZnO is notsuitable for absorption and utilization of visible region of the solarspectrum, in this study Ni doped ZnO (Zn0.95Ni0.05O) with a bandgap of �2.95 eV and Ni0.5Zn0.5Fe2O4 (band gap �2.2 eV) have beenchosen to prepare nanophotocatalysts for visible light active,magnetically separable, and recyclable photocatalytic activity.Numerous surfactants and stabilizing agents, including ethyleneglycol, sodium dodecyl sulfate (SDS), and citric acid, have beenused to control the size and shape of ZnO NPs during synthesis [21].These chemicals have varying degree of toxicity and are difficultto remove from nanoparticle surfaces even after repeated washing.Recently, natural bioresources with excellent biocompatibility areincreasingly being used as templates for synthesis of nanomaterials.

Egg albumen offers several advantageous including gelling,foaming, emulsifying, water solubility, heat setting and goodbinding 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 isenvironment friendly and cost effective. In this study freshlyextracted egg albumen was used because of its good watersolubility, metal binding ability, simple processing condition,amphiphilic nature, easy availability and low cost. No externalsurfactant or stabilizing agent has been used to preparenanocomposites. The proteins of egg albumen, which havedifferent functional groups, provide not only stable suspension offerrite in water but also may enhance selective deposition ofcoating materials by virtue of its metal binding capacity. It playsan active role in ZnO synthesis process by forming complexwith Zn precursor and can provide nanoparticles with specificmorphologies and high surface area [21–23]. The structural,optical, magnetic, and photocatalytic properties of the compositeshave been studied by XRD, TEM, FESEM, FTIR, UV–Vis spectroscopy,and VSM techniques.

Materials and methods

Materials

All reagents used in this synthesis were of analytical grade. Zincnitrate (>98% Zn(NO3)2.6H2O), nickel nitrate (>98% Ni(NO3)2.6H2O),iron nitrate (>98% Fe(NO3)3.9H2O) from Sigma Aldrich and citricacid, ammonia from SRL, India were used in this synthesis withoutany purification. Egg white was obtained from fresh egg availablein the market.

Ni0.5Zn0.5Fe2O4 NPs synthesis

Ni0.5Zn0.5Fe2O4 NPs were synthesized by gel-combustionmethod reported earlier [4]. Nickel nitrate, zinc nitrate and ironnitrate with a molar ratio of 1:1:4 were dissolved in 100 ml ofwater. Citric acid was added to the above nitrate precursor solutionwith citric acid: nitrate molar ratio of 1:1. The resultant sol wascontinuously stirred at 90 �C for 1 h. The gel so formed was sub-jected to combustion at �300 �C. A reddish brown powder wasobtained, which was grounded in mortar and pestle for subsequentuse.

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

Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O nanocomposites were synthesizedin two steps. First, a stable dispersion of Ni0.5Zn0.5Fe2O4 nanoparti-cles in egg albumen solution, which served as seeding materials,was obtained. Secondly, coating of Zn0.95Ni0.05O on Ni0.5Zn0.5Fe2O4

nanoparticle surfaces was realized on the basis of chemicalprecipitation method. A typical synthesis procedure is as below.Appropriate amount of Ni0.5Zn0.5Fe2O4 nanoparticles were dispersedin 20 ml of water taken in a beaker and 30 ml freshly extracted eggalbumen was added to it (beaker 1). In another beaker appropriateamount of zinc nitrate and nickel nitrate were dissolved in 50 mlwater (beaker 2). Both beakers were sonicated separately for15 min to ensure proper mixing. Stable dispersion of Ni0.5Zn0.5Fe2O4

nanoparticles was obtained in beaker 1. Aqueous solution of zincnitrate and nickel nitrate, from beaker 2, was added drop wise tothe stable dispersion of Ni0.5Zn0.5Fe2O4 (NZF) in beaker 1 withcontinuous stirring. 30 min vigorous stirring of the resultantsolution followed by addition of 2–3 ml ammonia resulted inbrown color precipitate. Obtained precipitate was centrifuged,washed with alcohol & water and dried at 50 �C in oven. The driedNZF@Z precursor was calcined at 600 �C for 3 h. Pure Zn0.95Ni0.05O

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nanoparticles were prepared by the same method except theaddition of ferrite nanoparticles. Different nanocomposites wereprepared by adding different percentages of ferrite (Ni0.5Zn0.5Fe2O4)nanoparticles, 15%, 40% and 60% with Zn0.95Ni0.05O and labeled as15%NZF@Z, 40%NZF@Z and 60%NZF@Z respectively. Obtainednanocomposites were stored in glass vials at room temperaturefor further characterization.

Characterization

The X-ray diffraction (XRD) patterns of the powder sampleswere recorded with a MiniFlexTM II benchtop XRD system (RigakuCorporation, Tokyo, Japan) operating at 40 kV. Particle sizes andmorphology of the samples were measured by Transmission elec-tron microscope (FEI Tecnai T20G2 S TWIN TEM) and field emis-sion scanning electron microscope (Carl Zeiss Ultra 55 FESEM).For both TEM and FESEM a pinch of nanoparticles were dispersedand sonicated in alcohol and a drop of suspension was placed oncarbon coated copper grid (for TEM) and carbon tape pasted onstub (for FESEM) respectively. The elemental analysis of the samplewas carried out by Energy dispersive spectroscopy (EDS) attachedwith the FESEM. The electronic absorption behavior at roomtemperature was analyzed using a UV–VIS spectrophotometer(Perkin Elmer Lambda 35). A suspension was prepared bydispersing �1 mg of nanopowder in 5 ml distilled water. Waterwas used as reference. The spectra was recorded in the wavelengthrang of 200–800 nm from the aqueous suspension of thesample. Fourier transformed infrared (FT-IR) analysis of thesamples was conducted using Perkin-Elmer 2000 FT-IR spectrome-ter in the wavenumber range of 500–4000 cm�1. The magneticcharacterization of the sintered (800 �C) pellets was performedusing 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 waswithin ±1%.

Fig. 1. XRD pattern of Ni0.5Zn0.5Fe2O4, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z andZn0.95Ni0.05O {Ni0.5Zn0.5Fe2O4 (Marked with ⁄), Zn0.95Ni0.05O (marked with #)}.

Photocatalytic activity measurement

The photocatalytic activities of the samples were estimatedusing Rhodamine B (RhB) dye under solar light irradiation. All pho-tocatalytic experiments were carried out under similar conditionson sunny days between 11 am and 3 pm. In the photocatalyticexperiment, 50 lg/ml of Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z,60%NZF@Z, and Ni0.5Zn0.5Fe2O4 catalysts were added to 50 ml dyesolution (of concentration 20 lg/ml). Before irradiation thesuspensions containing RhB dye and Zn0.95Ni0.05O, 15%NZF@Z,40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4 were stirred in darkfor 60 min to ensure the establishment of an adsorption/desorption equilibrium. 5 ml aliquots were magnetically filtratedat a fixed time interval (60 min) and was analyzed for the variationin maximum absorption band (kmax � 553 nm) using a UV–visspectrophotometer. The photo-decoloration of the RhB dye viathe photocatalytic activities of Zn0.95Ni0.05O, 15%NZF@Z,40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4 was calculatedfollowing the formula:

Photo-decoloration efficiency ð%Þ ¼ Co� CCo

� 100

where Co is the RhB dye initial concentration before photo-decolor-ation and C is the absorbance after different time intervals. The roleof active reactive oxygen species (ROS) generated in the photocata-lytic conversion was confirmed by trapping them with tert-butylalcohol (C4H10O) and disodium ethylenediaminetetraacetatedehydrate (EDTA-Na2; C10H14N2Na2O8�2H2O) [4].

Results and discussion

XRD analysis

Powder XRD patterns of Ni0.5Zn0.5Fe2O4, 15%NZF@Z, 40%NZF@Z,60%NZF@Z, and Zn0.95Ni0.05O recorded in the 2h range 20–80� areshown in Fig. 1.

Diffraction peaks (marked with ⁄) centered around 30�, 35�, 37�,43�, 53�, 57�, 62�, and 74� can be assigned to reflections obtainedfrom (220), (311), (222), (400), (422), (511), (440), and (533)planes, respectively, of crystalline Ni0.5Zn0.5Fe2O4 nanoparticles.The observed diffraction pattern is found to be similar to the char-acteristics of the spinel cubic structure of Ni0.5Zn0.5Fe2O4 {spacegroup: Fd3 m (227), JCPDS card No. 520278}. The diffraction peaks(marked with #) at 2h = 31.1�, 33.7�, 35.5�, 46.8�, 55.9�, 62.2�, 65.7�,67.3�, 68.4�, 72�, and 76.3�Corresponding to reflections from (100),(002), (101), (102), (110), (103), (200), (112), (201), (004) and(202) planes, respectively, are of crystalline Zn0.95Ni0.05O nanopar-ticles, which is representative of the hexagonal wurtzite structure{space group: P63mc9(186), JCPDS card No. 790207}. XRD patternsof nanocomposites, 15%NZF@Z, 40%NZF@Z and 60%NZF@Z showcharacteristic peaks of both materials, which confirmed theformation of crystalline nanocomposites. The X-ray intensity of theferrite peaks increases with increasing weight fraction of ferritesin the composites, similar to results reported by Roychowdhuryet al. [6]. No impurity peak is detected in the XRD patterns of thesecomposites. The average crystallite sizes of Ni0.5Zn0.5Fe2O4 andZn0.95Ni0.05O were estimated from the full width at half maximum(FWHM) of the corresponding most intense diffraction peaks,((311) for ferrite and (101) for Zn0.95Ni0.05O) using the Debye–Scherrer equation, D = 0.9k/bcosh, where k is the wavelength ofX-ray (Cu Ka = 1.540598 A�), b is the broadening of the diffractionline measured at half of its maximum intensity in radians and h isthe Bragg’s diffraction angle. The crystallite sizes of Ni0.5Zn0.5Fe2O4

and Zn0.95Ni0.05O were found to be �16–25 nm and �22–27 nmrespectively. XRD peaks broadening may also be due to latticestrain. The contributions of lattice strain and crystallite size todiffraction peak broadening have been separated out usingWilliamson–Hall equation, bcosh = 0.9k/D + 4eSinh, [24], where Dis effective crystallite size and e is effective strain. TheWilliamson–Hall plot (bcosh vs Sinh) for a representative sample,

Fig. 2. Williamson–Hall plot of 40%NZF@Z sample.

M. Qasim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1348–1356 1351

40%NZF@Z, is shown in Fig. 2. From the linear fit to the experimen-tal data the crystalline size was estimated from the y-intercept,and the strain e from the slope of the fitted line. Lattice strains, e,in Zn0.95Ni0.05O and Ni0.5Zn0.5Fe2O4 phases were found to be��0.006 to 0.004, both in pristine phases and nanocomposites.Similar strain values have been reported in the literature forFe3O4/ZnO nanocomposite and ZnO nanoparticle [6,24]. A shift in(101) peak position in the diffraction pattern of Zn0.95Ni0.05O canbe attributed to the presence of strain in the crystal lattice. Theeffective crystallite sizes, calculated by Williamson–Hall method,were 17–27 nm and 20–30 nm for Ni0.5Zn0.5Fe2O4 and Zn0.95Ni0.05Orespectively. The estimated crystallite sizes obtained from both themethods (Debye–Scherrer’s and Williamson–Hall) were in goodagreement. The obtained lattice parameters, estimated by PowderX-ray software, are a = b = 3.27 A�, c = 5.24 A� for Zn0.95Ni0.05O anda = b = c = 3.387 A� for Ni0.5Zn0.5Fe2O4.

Analysis of microstructure and phase composition

The FESEM images of NZF (a) and 40%NZF@Z (b) with corre-sponding EDS spectra (c and d) and TEM images of NZF (a) and40%NZF@Z (b) with corresponding diffraction patterns are shownin Figs. 3 and 4 respectively. It has been observed from FESEMand TEM images that particle sizes in NZF are more uniform thanthose in 40%NZF@Z nanocomposite. NZF nanoparticles are nearlyspherical with particle sizes �22 ± 6 nm measured both by FESEMand TEM, as shown in Figs. 3(a) and 4(a) respectively. The distribu-tion of particle sizes has been shown in the insets of the respectiveFigs. 3(a) and 4(a). The distribution is broader in pure NZF com-pared to that in 40%NZF@Z. The bigger particle sizes in compositescompared to those in pure Ni0.5Zn0.5Fe2O4 could be due to the coat-ing of Zn0.95Ni0.05O around the surface of Ni0.5Zn0.5Fe2O4 nanopar-ticles. The average particle sizes for 40%NZF@Z was �70 ± 17 nm,measured by FESEM & TEM. Fig. 3(b) shows the formation ofagglomerates of 40%NZF@Z nanoparticles but the distribution ofparticle sizes is narrower (as shown in inset of Fig. 3(b)) than thatin pure NZF, as shown in inset of Fig. 3(a). The smaller particle sizes(�22 nm) of 40%NZF@Z in Fig. 3(b) could be due to uncoating ofNi0.5Zn0.5Fe2O4 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 ismagnetic in nature it absorbs more electron than Zn0.95Ni0.05Oand hence appear darker than ZnO in the TEM image (Fig. 4(b))[26].

The EDS spectra (Fig. 3(c) and (d)) of Ni0.5Zn0.5Fe2O4 and40%NZF@Z samples show only the presence of Fe, Ni, Zn, and Oin stoichiometric ratio, without any impurity peak, confirmingthe purity of the phases. In 40%NZF@Z the mass ratio of Fe to Znis �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 Ni0.5Zn0.5Fe2O4 and 40%NZF@Z samples. TheSAED pattern of Ni0.5Zn0.5Fe2O4 shows distinct dotted ring pattern,which confirms the polycrystalline nature of the cubic spinel Ni0.5-

Zn0.5Fe2O4 nanoparticles. The SAED pattern was indexed for cubicstructure by estimating the d-spacing from the ring pattern andcomparing with the JCPDS card No. 520278 for NZF. 40%NZF@Zsample was also observed to be polycrystalline in nature. The SAEDpattern of 40%NZF@Z shows a complex and mixed dotted ringpattern, which is due to the presence of both the phases, cubicNi0.5Zn0.5Fe2O4 and hexagonal Zn0.95Ni0.05O [6]. Presence of bothphases have also been confirmed by indexing the SAED patternsof other nanocomposites investigated in this study.

Role of egg albumen

The mechanistic aspect of the synthesis of NZF@Z nanocompos-ites using egg albumen can be explained as follows.

Egg albumen contains different types of proteins such as �60%ovalbumin, �12% Ovotransferrin, and �11% Ovomucoid. Theseproteins have polar-COOH (hydrophilic) and nonpolar-alkyl(hydrophobic) groups. It is difficult to prepare stable dispersionof bare ferrite nanoparticles due to its hydrophobic nature. Whenferrite nanoparticles are sonicated along with albumen thesepolymeric proteins wrap the surface of nanoparticles and thehydrophilic parts of the protein interact with water giving rise tostability of the suspension. Ovotransferrin is well known for itsiron binding property. Ovotransferrin folds into two globular lobes,each containing an iron binding site located within theinterdomain cleft of each lobe [27]. These iron binding sites mayalso provide affinity toward iron of ferrites and help in decorationof ovotransferrin on ferrite nanoparticles leading to improved sta-bility of the resultant suspension. Ni0.5Zn0.5Fe2O4 nanoparticlesdispersed in water with egg albumen (water:albumen ratio �2:3and ferrite concentration �10 mg/ml) shows improved stabilitythan only in water, as shown in digital photograph in Fig. 5(a).Ni0.5Zn0.5Fe2O4 nanoparticles in water are seen to settle within12 min of dispersion, whereas with egg albumen it is stable up to3 h. To find out the role of albumen in the synthesis process TEManalysis of albumen treated Ni0.5Zn0.5Fe2O4 nanoparticles wascarried out after repeated washing with water. Formation of5–6 nm thick amorphous layer of albumen around the crystallineNi0.5Zn0.5Fe2O4 nanoparticles has been confirmed by HRTEManalysis as shown in Fig. 5(b)–(d). Ni0.5Zn0.5Fe2O4 nanoparticlesdecorated with albumen on its surface provide nucleation sitesfor Zn0.95Ni0.05O deposition [20]. Metal binding tendency of albumenmay attract zinc precursor to selectively nucleate on it [28].Zn(OH)2 is formed when zinc nitrate reacts with H2O. Functionalgroups of egg protein chelate with Zn atom and form protein-Zn(OH)2 complex, which subsequently converted to hyderozinciteintermediate. On calcination at �500 �C the hyderozinciteintermediate decomposes to ZnO [21]. In our previous report themechanistic aspect of synthesis of pure ZnO nanoparticles usingegg albumin as biotemplate has been discussed in detail [21].

Optical properties

UV–visible absorption spectroscopy is a powerful technique toexplore the optical properties of semiconducting nanoparticles. Theabsorbance of a material depends on several factors such as bandgap, oxygen deficiency, surface roughness and impurity centers[29]. Fig. 6 shows UV–visible absorption spectra of Ni0.5Zn0.5Fe2O4,15%NZF@Z, 40%NZF@Z, 60%NZF@Z and Zn0.95Ni0.05O.

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).

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A sharp absorption peak at �373–380 nm has been observed forZn0.95Ni0.05O and all nanocomposites. This characteristic absorp-tion peak can be assigned to the intrinsic band-gap absorption ofZn0.95Ni0.05O nanoparticles due to the electron transitions fromvalence band to the conduction band (O2p–Zn3d) [21,22]. Presenceof absorbance peak corresponding to Zn0.95Ni0.05O in all samplesconfirms the formation of nanocomposites. Decrease in the absor-bance after lamda max (at �380 nm) in Fig. 6 or a dip after 3.3 eVin Fig. 7, may most likely be due to lesser number of availablestates (low density of states) to absorb photons of thosewavelengths. No sharp absorption peak corresponding toNi0.5Zn0.5Fe2O4 was observed in the wavelength range studied inthis investigation.

Optical band gaps of all the samples were estimated using theTauc relationship, ahm = A(hm � Eg)n, where a is the absorptioncoefficient, A is a constant, h is the Planck’s constant, m is the pho-ton frequency, and Eg is the optical band gap [30]. The value of ncould be 1/2, 3/2, 2 or 3 depending on the nature of electronic tran-sition responsible for absorption and n = ½ is for direct band gapsemiconductor. An extrapolation of the linear region of plot(ahm)2 vs hm gives the value of the optical band gap, Eg, as shownin Fig. 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, and2.24 eV respectively. The observed energy band gap of Zn0.95Ni0.05Onanoparticles is much lower than that reported for pure ZnO nano-particles (3.5 eV for ZnO with 16 nm crystallite size) [31]. Energyband gaps of the nanocomposites are found to decrease withincreasing Ni0.5Zn0.5Fe2O4 content in the nanocomposites. This isdue to the reduced band gap energy of Zn0.95Ni0.05O in the nano-composites in presence of Ni0.5Zn0.5Fe2O4.This reduction in bandgap energy has been explained in the literature in terms of mixingof the 4s orbital of Fe and Zn and formation of the conduction band

of Zn0.95Ni0.05O at lower energy [6,32]. It may also be due to theformation of sub band/s within the band gap of Zn0.95Ni0.05O dueto doping of metal ions from Ni0.5Zn0.5Fe2O4 resulting in loweringof band gap energy.

FT-IR spectra of all the samples are shown in Fig. 8. The IR spec-tra of Ni0.5Zn0.5Fe2O4 nanoparticles shows two principle absorptionbands in the wavelength range 400–600 cm�1, the first band isaround 422 cm�1 and the second around 577 cm�1. These twovibration bands can be attributed to the intrinsic lattice vibrationsof octahedral and tetrahedral coordination complexes in the spinelstructure, respectively [33]. The broad absorption band�400–600 cm�1 in all samples could arise from the simultaneouspresence and overlapping of Zn-O vibration mode at 453 cm�1 inZnO and two above mentioned absorption bands i.e. at 577 cm�1

and 422 cm�1 in ferrite [6]. In IR spectra of 15%NZF@Z, 40%NZF@Zand 60%NZF@Z nanocomposites, presence of Fe–O vibration modeat 577 cm�1 as shoulder has been observed, which increases withincrease in ferrite content in these samples. Therefore, IR resultsalso confirm successful formation of nanocomposites. In additionto these absorption peaks, the absorption band centered at�1121 cm�1 may be attributed to the vibration mode of C–O,which could indicate the presence of decomposition products ofalbumen as impurities in the samples [34]. The absorption bandat�2364 cm�1 can be assigned to trace of adsorbed or atmosphericCO2 [35].

Magnetic properties

The field dependent magnetic behavior of all the samples isshown in Fig. 9. Different magnetic properties such as, saturationmagnetization (Ms), remanent magnetization (MR) and coercivity(Hc) have been derived from the magnetization curves. The Ms

Fig. 4. (a) TEM image of Ni0.5Zn0.5Fe2O4 nanoparticles with inset shows the particles size distribution, (b) TEM image of 40%NZF@Z nanocomposite. (c) Indexed SAED patternof spinel cubic Ni0.5Zn0.5Fe2O4 (JCPDS card No. 520278). (d) Indexed SAED pattern of 40%NZF@Z nanocomposite which shows both spinel cubic Ni0.5Zn0.5Fe2O4 and hexagonalZn0.95Ni0.05O present in the sample (JCPDS card No. 790207).

M. Qasim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1348–1356 1353

values for Ni0.5Zn0.5Fe2O4, 60%NZF@Z, 40%NZF@Z, and 15%NZF@Zwere 56, 33, 23, and 8 emu/gm respectively. It is found that Ms

decreases linearly with decrease in ferrite content in these com-posite samples (Fig. 9 inset). The decreasing saturation magnetiza-tion (Ms) is consistent with the increasing non-magneticZn0.95Ni0.05O content in these samples [6]. Magnetization behaviorat lower field shows all the samples to be super-paramagnetic innature with very low remanent magnetization and coercivity.Remanent magnetizations of all the samples are found to be�0.4–1.4 emu/gm. The coercivities are 37, 44, 48 and 54 Oe forNi0.5Zn0.5Fe2O4, 60%NZF@Z, 40%NZF@Z and 15%NZF@Z respec-tively. The increasing coercivity of the samples with increasingZn0.95Ni0.05O content could be attributed to increasing domain wallpining by the non-magnetic Zn0.95Ni0.05O phase in these compos-ites. The superparamagnetic behavior of Ni0.5Zn0.5Fe2O4 nanoparti-cles could be attributed to very small particle sizes (�22 nm) ofthis sample. The smaller size particles may be equivalent to singledomain (magnetic) particles, where thermal vibration surpass theenergy barrier for its spin reversal leading to superparamagneticbehavior. Magnetic separation ability of the dispersed nanocom-posites/photocatalysts in water has also been demonstrated andshown as a digital image in the other inset of Fig. 9 for 15%NZF@Z(10 mg/ml), (a) without magnet, and (b) near the magnet. It isobserved that prepared nanocomposite photocatalysts have gooddispersibility in water and can be easily separated out from thesolution by applying an external magnetic field even though the

magnetization of the sample is not too strong (8 emu/gm for15%NZF@Z, for example). Thus, the photocatalysts could be usedrepeatedly and opens up a possibility of recyclability.

Analysis of photocatalytic activity

Enhanced visible-light driven photo-activity has been observedfor some heterostructure semiconductor nanocomposites previ-ously [1,35], which motivated us to study the photocatalytic activ-ity of NZF@Z nanocomposites. The high photocatalytic activity ofmetal oxide nanocomposites was attributed to the enhanced sepa-ration efficiency of photoinduced carriers (electrons and holes)through electronic interaction. Thus, photo-decoloration study ofRhB dye was performed under solar light irradiationusing Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z, andNi0.5Zn0.5Fe2O4 as photocatalysts. The photo-decolorationexperiments were performed after proper adsorption of RhB dyeon the surface of Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z,and Ni0.5Zn0.5Fe2O4 photocatalysts. The extent of photo-decolor-ation of RhB dye by Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z,60%NZF@Z, and Ni0.5Zn0.5Fe2O4 under solar light irradiation, at25 �C, is shown in Fig. 10A. The obtained photo-decoloration data,under solar light irradiation, shows the promise of photocatalyticactivity of Zn0.95Ni0.05O, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z, andNi0.5Zn0.5Fe2O4. The control (RhB dye solution without nanocom-posites) does not exhibit significant photo-decoloration under

Fig. 5. (a) The digital photograph shows the stability of dispersed Ni0.5Zn0.5Fe2O4 inwater without-a and with egg albumen-b with respect to time.(b) TEM image of eggalbumen coated Ni0.5Zn0.5Fe2O4 nanoparticles.(c and d) HRTEM image of crystallineNi0.5Zn0.5Fe2O4 nanoparticle, covered with �5 nm thick amorphous albumen layer.Inverse Fast Fourier transform from the framed part in (c) is shown in the (d).

Fig. 6. UV–Vis absorption spectra of all the samples (Ni0.5Zn0.5Fe2O4, 15%NZF@Z,40%NZF@Z, 60%NZF@Z, and Zn0.95Ni0.05O).

Fig. 7. Tauc plot depicting energy band gap of Ni0.5Zn0.5Fe2O4, 15%NZF@Z,40%NZF@Z in the main panel and 60%NZF@Z and Zn0.95Ni0.05O in the inset.

Fig. 8. FTIR spectra of Ni0.5Zn0.5Fe2O4 nanoparticles, 15%NZF@Z, 40%NZF@Z and60%NZF@Z nanocomposites.

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solar light irradiation up to 120 min. This suggests that the photo-decoloration of RhB dye takes place only by photocatalysis and not byphotosensitization [4]. All the nanocomposites have shown betterphotocatalytic activity than pure Zn0.95Ni0.05O or Ni0.5Zn0.5Fe2O4.The enhanced photo-decoloration rates of RhB dye, observed inthis study, may be attributed to the lager surface area of thephotocatalysts and reduced carriers (photo-induced) recombina-tion 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, anelectron is excited from VB to CB and an e�/h+ pair is formed.The generated e� and h+ may travel to the surface of the particleand 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 hydroxylgroups (OH�) and H2O, to form highly reactive �OH radicals, whichdegrade organic dye molecules. The undesired high e�/h+ recombi-nation slows down the photocatalysis process, especially, in case ofpure semiconductor photocatalysts. To increase the photocatalyticefficiency the e�/h+ recombination has to be minimized. A schemeto reduce the recombination of e� and h+ is to prepare heterostruc-ture/nanocomposite with hetrojunction [36]. The band potentialsof the component materials in the nanocomposite form a hetero-junction with a straddling gap, which may facilitate the transferof charge carriers and retard the electron hole recombination,resulting in improved photocatalytic performance [35]. In otherword, combining two photocatalysts with different band gap posi-tions effectively causes a greater separation of e�/h+ pairs, allowingmore of the species to be available for surface reactions leading todegradation of the organic species [10,37]. Thus, as a magneticsemiconductor material, NZF might not only add recyclabilityand visible light activity to the catalyst nanoparticles, but also offersome synergetic enhancement of the catalytic activity by formingthe hybrid structure [38].

The identification of the main active oxidant (reactive oxygen spe-cies) in the photocatalytic reaction is of great importance to under-stand the mechanism of the photocatalytic conversion process. Thechemical interactions between the photocatalysts (Zn0.95Ni0.05O,15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4) and the

Fig. 9. Room temperature magnetization behavior (M–H) of all the samples in anapplied field upto15000 Oe. The inset at the top shows linear variation of Ms as afunction of varying weight percent (wt%) of Ni0.5Zn0.5Fe2O4 in the nanocomposites.Digital image, in the other inset shows dispersed 15%NZF@Z nanocomposite inwater (10 mg/ml), (a) without magnet and (b) near the magnet.

Zn0.95 Ni0.05O 15%NZF@Z 40%NZF@Z 60%NZF@Z Ni 0.5Zn0.5Fe2O4

Pro

tect

ion

RhB

Dye

deco

lora

tion

(%

)

96.2493.51 91.96 90.84 89.9

93.2790.11

87.22 85.66 84.87

0

20

40

60

80

100

EDTA-Na2

C4H10O

Fig. 10B. Photodecoloration analysis shows the protective effect of disodiumethylenediaminetetraacetate dehydrate (EDTA-Na2; C10H14N2Na2O8 2H2O) (holescavenger) (Q1) and tert-butyl alcohol (C4H10O) (radical scavenger) (Q2) on RhB dyein presence of Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O nanocomposites (Q1-Column1 andQ2- Column2).

M. Qasim et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1348–1356 1355

charged groups of RhB molecules lead to significant adsorption fol-lowed by high photo-decoloration. The role of the active oxidantsgenerated in the photocatalytic reactions involving Zn0.95Ni0.05O,15%NZF@Z, 40%NZF@Z, 60%NZF@Z, and Ni0.5Zn0.5Fe2O4 was ascer-tained by quenching the reaction mixture in disodium ethylenedi-aminetetraacetate dehydrate (EDTA-Na2; C10H14N2Na2O8�2H2O)(hole scavenger) and tert-butyl alcohol (C4H10O) (radical scavenger)[4]. The photo-decoloration of RhB under ultraviolet light irradiationwas supressed after the addition of t-BuOH and EDTA-Na2, as shownin Fig. 10B, which suggests that both radicals and holes are active spe-cies 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 thenecessary condition for photochemical reactions to proceed on thephotocatalyst surface. The band edges positions (VB and CB) ofthe AB2O4 type ferrite (ZnFe2O4/NiFe2O4) lies above the correspond-ing band edges positions of ZnO respectively [1,10,39,40]. ReportedCB and VB potential of ZnFe2O4 is nearly at �1.54 eV (vs NHE) and+0.38 eV (vs NHE) respectively [38,41]. The CB and VB potentials of

Fig. 10A. RhB dye photo-decoloration efficiency under solar light irradiation byZn0.95O0.05, 15%NZF@Z, 40%NZF@Z, 60%NZF@Z and Ni0.5Zn0.5Fe2O4 photocatalysts.

ZnO is nearly at �0.76 eV (vs NHE) and �+2.7 eV (vs NHE) respec-tively [39,42]. Fig. 10C shows the schematic of the possible mecha-nism in this photocatalysis process. Under solar light irradiation,the electrons (e�) from the filled valence bands (VB) of Ni0.5Zn0.5Fe2O4

and Zn0.95Ni0.05O will be excited to the respective empty conductionbands (CB), separately giving an equal number of holes (h+) in thecorresponding VBs. The presence of Ni0.5Zn0.5Fe2O4 nanoparticleswill favor the utilization of visible region of the spectrum due toits narrow band gap (2.2 eV) thereby enhancing the photocatalyticactivity under visible light irradiation [10,35]. Moreover, thedifference in band structures of Ni0.5Zn0.5Fe2O4 and Zn0.95Ni0.05Owill facilitate photoinduced electrons transfer from the CB ofNi0.5Zn0.5Fe2O4 to that of Zn0.95Ni0.05O and holes transfer from theVB of Zn0.95Ni0.05O to that of Ni0.5Zn0.5Fe2O4, respectively [1]. Theseprocesses will efficiently hinder the recombination of photogenerat-ed electron–hole pairs and considerably enhance the photocatalyticactivity under solar light irradiation. The resulted electron–holepairs recombine or migrated to the surface of the particles and retorteither with H2O or OH� to form OH�, which have strong oxidationability and can destroy the dye molecules completely. The electronswill also react with adsorbed molecular oxygen to form superoxideradical anion, O��2 ions, which will further react with water to giveOH� [1].

Fig. 10C. Schematic diagram illustrating the mechanism of photo-decoloration ofRhB dye on Ni0.5Zn0.5Fe2O4/Zn0.95Ni0.05O surface under exposure to solar light.

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Conclusions

A novel, visible light active and magnetically separableNi0.5Zn0.5Fe2O4/Zn0.95Ni0.05O nanocomposites have been synthesizedsuccessfully by a simple and cost effective sol gel technique usingegg albumen as biotemplate. Powder XRD, TEM, FESEM, FTIR,UV–Vis spectrophotometer, and VSM techniques have been usedto characterize their structural, optical, and magnetic properties.The photocatalytic activity of these nanocomposites has beeninvestigated by photo-decoloration study of Rhodamine B dyemolecules. Enhanced photo-decoloration of the Rhodamine B dyemolecules by NZF@Z photocatalysts, compare to that by pureNi0.5Zn0.5Fe2O4 or Zn0.95Ni0.05O, have been observed under solarlight irradiation, through the production of reactive oxygen species(ROS). Narrow band gap of Ni0.5Zn0.5Fe2O4 (2.2 eV), relatively lowband gap of Zn0.95Ni0.05O (2.95 eV), and reduced electrons-holesrecombination through electronic interactions have contributedsignificantly to this visible light active photocatalysis process. Thisnovel visible light active and magnetically separable photocatalystmay immensely contribute to environmental remediation.

Acknowledgement

M. Qasim greatly acknowledges the financial support obtainedfrom University Grants Commission (UGC) in the form of MANFfellowship 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 inMaterials Science (Nanomaterials) Aligarh Muslim University,School of Engineering Sciences & Technology (SEST), Centre forNanotechnology, and School of Physics at the University ofHyderabad is greatly appreciated.

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