Magnetically addressable fluorescent Fe3O4/ZnO nanocomposites: Structural, optical and magnetization studies

doi:10.1016/j.jpcs.2013.01.012

Journal of Physics and Chemistry of Solids

Volume 74, Issue 6, June 2013, Pages 811-818

https://doi.org/10.1016/j.jpcs.2013.01.012 Get rights and content

Abstract

Fe₃O₄/ZnO nanocomposites (NCs) are prepared by a wet chemical route. X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy studies confirm the coexistence of Fe₃O₄ and ZnO phases in the NCs. The UV–vis absorption spectra show a red shift of the absorption peak with increase in Fe₃O₄ content indicating a modification of the band structure of ZnO in the NCs. Photoluminescence emission spectra of the NCs display strong excitonic emission in the UV region along with weak emission bands in the visible range caused by electronic transitions involving defect-related energy levels in the band gap of ZnO. Positron annihilation lifetimes indicate that cation vacancies in the ZnO structure are the strong traps for positrons and the overall defect concentration in the NCs decreases with increase in Fe₃O₄ content. Dc magnetization measurements reveal an anomalous temperature dependence of the coercivity of the NCs that is argued to be due to the anomalous variation of magnetocrystalline anisotropy at lower temperature. The irreversibility observed in the temperature dependent ZFC-FC magnetization points to the presence of a spin-glass phase in the NCs.

Graphical abstract

Highlights

► Fe₃O₄/ZnO nanocomposites are successfully prepared by the sonochemical method. ► Defects related to cation vacancies in ZnO are confirmed by PL and PALS studies. ► An anomalous variation of coercivity at low temperature is observed. ► The irreversibility in the MT measurements is due to the spin-glass like state.

Introduction

Multifunctional nanocomposites (NCs) have recently become the subject of intensive research due to their interesting physical properties and potential applications. Among all nanocomposites, fluorescent magnetic materials have attracted special attention of the scientific community due to their numerous vital applications e.g. in nanomedicine [1], efficient hyperthermia for cancer removal [2], clinical diagnosis [3], drug delivery systems (DDSs) [4], magnetic resonance imaging (MRI) [5], cancer therapy [6], labels or tags in specific targeting biological labeling [7], etc. Fluorescent magnetic NCs comprise of a luminescence material (e.g. semiconductors, silica, polymers, dye, quantum dots, etc) along with a magnetic component (e.g. transition metals and alloys, transition metal oxides, ferrites, etc). Various synthesis procedures have been reported for fluorescent magnetic multifunctional NCs. Vargas et al. synthesized core/shell Fe₃O₄/ZnSe fluorescent magnetic nanoparticles (NPs) using a chemical route [8]. Wang et al. reported the synthesis of CdS/Fe₃O₄ and CdS/α-Fe₂O₃ heterostructures having magnetic, optical and photocatalytic properties [9]. Hong et al. showed higher photocatalytic activity of Fe₃O₄/ZnO composite than that of ZnO [10]. More recently, Chiu et al. succeeded in making bifunctional magnetic-optical nanocrystals by growing luminescent ZnO shell on the magnetic Fe₃O₄ nanoparticle surface [11]. Apart from these, a wide range of other luminescent magnetic materials have been studied e.g., Fe/ZnO [12], FePt/CdS [13], Fe₃O₄/ZnS [14], Co/CdSe [7], etc.

In the present study, we have synthesized pure phase Fe₃O₄/ZnO NCs by a chemical route. Magnetite is a ferrimagnetic mineral belonging to the family of inverse spinels with chemical formula [Fe³⁺]_A[Fe²⁺Fe³⁺]_BO_4, A and B being the tetrahedral and octahedral sites, respectively. On cooling below 124 K, magnetite undergoes a metal–insulator transition (known as Verwey transition) driven by charge ordering that results a two order of magnitude increase in resistance [15]. At this low temperature, Fe₃O₄ exhibits many exotic properties, whose physical nature has not been fully understood till now [16]. ZnO is a wide band gap semiconductor (Eg. ∼3.37 eV) having high excitonic binding energy (60 meV). The high efficiency of luminescence in the UV to visible regions of the spectrum makes ZnO an attractive material for optoelectronic applications [17].

The aim of the present work is to make a systematic study on the dependence of optical and magnetic properties of Fe₃O₄/ZnO NCs on relative concentration of the constituents. To the best of our knowledge no such study has been reported in the literature. X-ray diffractometry (XRD) was used to check the phase purity of the samples. Transmission electron microscopy (TEM) was used to study the morphology of the samples. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were used to extract further structural information.

Section snippets

Materials

Iron(II) chloride tetrahydrate (99% FeCl₂·4H₂O, Sigma Aldrich), iron(III) nitrate nonahydrate (99% Fe(NO₃)₃·9H₂O, Sigma Aldrich), sodium hydroxide pellet (NaOH) and zinc nitrate tetrahydrate (99% Zn(NO₃)₂·4H₂O, Sigma Aldrich) were used as starting materials which were all of analytical grade. Acetone and propanol were of chemical grade. N₂ gas was of industrial grade. Polyvinyl alcohol (PVA) was used as a surfactant.

Procedure

Fe₃O₄ and ZnO nanoparticles were synthesized separately by the chemical routes.

Microstructural analysis

Fig. 1 shows x-ray diffraction (XRD) patterns of the pristine Fe₃O₄, pristine ZnO and all the NCs. A comparison of the 2θ values of the observed diffraction peaks with the standard JCPDS files indicates the presence of cubic magnetite (card no. 851436) and wurtzite ZnO (card no. 800075) phases in all the NCs. No other impurity phase could be detected indicating formation of pure Fe₃O₄/ZnO NCs. The average crystallite sizes of both Fe₃O₄ and ZnO components were calculated from the full width at

Conclusion

Fe₃O₄/ZnO NCs are synthesized successfully by ultrasonication of the constituents prepared separately by the chemical routes. XRD, TEM, SAED and FTIR studies show the presence of ZnO and Fe₃O₄ structures in the prepared NCs. UV–vis spectra show a red shift of the absorption peak with increase in Fe₃O₄ content in NCs, indicating the formation of a hybrid conduction band at lower energy due to the mixture of 4s orbitals of Zn and Fe. PL spectra of the NCs reveal defect-related emission bands due

Acknowledgments

The authors are grateful to Jadavpur University, Kolkata for providing the XRD and TEM facilities. The authors thank Dr. A. Saha, UGC DAE CSR, Kolkata for the optical measurements. The magnetization measurements were done at the high magnetic field facility at UGC-DAE CSR, Kolkata created under the DST project no. IR/S2/PU-0006/2006. A. Roychowdhury acknowledges University Grants Commission, New Delhi for providing the Junior Research Fellowship.

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Magnetically addressable fluorescent Fe3O4/ZnO nanocomposites: Structural, optical and magnetization studies

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials

Procedure

Microstructural analysis

Conclusion

Acknowledgments

J. Magn. Magn. Mater.

Biomaterials

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

J. Colloid Interface Sci.

J. Biosci. Bioeng.

J. Am. Chem. Soc.

J. Phys. Chem. C

Soc. Symp. Proc.

Appl. Surf. Sci.

Chem. Mater.

J. Appl. Phys.

J. Appl. Crystallogr.

Chin. Phys. Lett.

Polymer

J. Appl. Phys.

J. Phys. Chem. C

Mater. Res. Bull.

J. Phys. Chem. C

Nanoscale Res. Lett.

Physica (Utrecht)

Nature (London)

J. Phys.: Condens. Matter

J. Appl. Phys.

Comput. Phys. Comm

Comput. Phys. Commun.

Elements of X-Ray Diffraction

Langmuir

Magnetically addressable fluorescent Fe₃O₄/ZnO nanocomposites: Structural, optical and magnetization studies