NANOMATERIALS PREPARED BY SOL-GEL AND ELECTROSPINNING ROUTES

Weak ferrimagnetic behavior occurs in the sample upon crystallization of the orthorhombic phase at temperatures above 400 C with significant reduction in magnetization. CaFe2O4 exhibits superparamagnetic behavior and weak ferrimagnetic behavior appears in the sample upon crystallization of the orthorhombic phase at temperatures above 500°C.

Introduction

Investigations on Nanocrystalline ZnO obtained from PVA/ZnO nanofibers

INTRODUCTION

Potential DMS materials

Although the TC of (Ga,Mn)As could be raised up to 190 K, researchers do not hope to achieve a higher TC [HASS11, WANG13]. In order to find a correlation between FM and semiconductor properties, the predicted TC [PEAR03] of several promising semiconductors is plotted as a function of their band gap in Figure 1.1.

Figure 1.1: Predicted T C of prospective semiconductors as a function of bandgap [PEAR03]

ZnO based DMS

Crystal structure of ZnO
Defects in ZnO nanostructures
Optical properties of ZnO
Magnetic properties of ZnO
Some applications of nanostructured ZnO

The atomic configurations of various point defects in the ZnO crystal are shown in Figure 1.5. The dopant distribution in the crystallinity/grains of the matrix material can be homogeneous or heterogeneous.

Figure 1.3: ZnO crystal structures: (a) rock salt (B1), (b) zinc blende (B3) and (c) wurtzite (B4)

Classification of ferrites and their structures

Spinel ferrites
Garnets
Ortho-ferrites
Hexagonal ferrites

In inverse spinel, the moments of the Fe3+ cations at the A and B sites are antiparallel, so that the total moment of the sample is due only to the M2+ ions. In yttrium iron garnet (Y3Fe5O12), Y3+ has no magnetic moment (it is a 4d° element) and the moments of Fe3+ ions on the tetrahedral sites are antiparallel to those on the octahedral sites.

Figure 1.10: Typical spinel structure with unit cell of MgAl 2 O 4 as example [KITT05]

Magnetism in ferrites

Since the Curie constants CA and CB are not identical for the two different sites, the above equations are modified by introducing the term density of ferrimagnetic materials on the right-hand side. By now inserting the values of HmA and HmB into equations, the magnetization at the two different sites is obtained as.

Properties of ferrite nanostructures

From equation 1.12 it is clear that the relaxation time decreases or flip frequency increases as the particle size decreases. Higher magnetic saturation magnetization is the additional attractive property of superparamagnetic magnetic nanoparticles compared to paramagnetic materials when exposed to an external magnetic field [WALK12, ZELE10].

Spinel ferrite nanostructures

The characteristic time for switching from one moment to another is called the Neel relaxation time and is approximated by the Neel-Arrhenius relation [GRIM75]. The TC enhancement was described by a finite size scaling formula and attributed to the limited dimensions of the material in three dimensions.

Some applications of spinel ferrites

Potential applications of MgFe2O4 nanoparticles in ferrofluid, magnetic cooling and magnetic resonance imaging have been pointed out [CHEN98]. Many biomedical applications have been developed based on biogenic and synthetic magnetic micro- and nanoparticles [ZHAN98].

Focus of the Present Thesis

Soft magnetic ferrite films have been prepared by a combination of sol-gel and spin-coating techniques for use as inductors in microelectronics [YANG06]. Ferrite cores based on nanocrystalline MnxNi0.5−xZn0.5Fe2O4 have been developed with low eddy current for switched mode power supplies [VERM06, ZASP04].

Sample preparation

Sol-gel technique
Electrospinning technique
Post synthesis heat treatment

The positive electrode of the high voltage source is then connected to the metal capillary [TRAV08a]. Extrinsic parameters consist of the electric field strength, the distance between the nozzle (metallic needle, anode) and the collector (cathode), solution flow rate, shape and movement of the collector.

Figure 2.1: Schematic of sol-gel processes leading to nanoscale powders or a dense film

Characterization Techniques

Morphology, structure and composition

Field–Emission Scanning Electron Microscopy
Transmission Electron Microscopy
X–ray Diffraction

Optical characterization

Ultraviolet-visible-near infrared spectrophotometry
Micro-Raman Spectroscopy
Photoluminescence spectroscopy

Magnetic characterization

Vibrating Sample Magnetometer (VSM)
Electron paramagnetic resonance
Mössbauer Spectroscopy

Thermal analysis

Thermo-gravimetric analyzer

A photograph of the FESEM (Sigma Zeiss, Germany) used in the present study is shown in Figure 2.3(a). When a solid sample is placed in a uniform magnetic field, a dipole moment proportional to the product of the sample susceptibility and the applied field is induced in the sample. The sample cavity is mounted between the pole pieces of the electromagnet perpendicular to the magnetic field, H which can be varied in a controlled manner.

Figure 2.3: (a) Photograph of the Field Emission Scanning Electron Microscope (Sigma Zeiss, Germany)

Preparation

As mentioned in the first chapter, RTFM has been observed in undoped wide-bandgap semiconductors such as TiO2 [HONG06], ZnO [KHAL09] and MgO [BOUB10], making the phenomenon very interesting from both scientific and applied perspectives. Given that the dimensionality of nanomaterials and the preparation process affect its properties, it is appropriate to research new forms of nanocrystalline ZnO. In this chapter, new 1-dimensional ZnO nanostructures were prepared by electrospinning and their properties were investigated.

Thermo-gravimetric analysis

Structural characterization and morphology

FESEM micrograph (Figure 3.3(a)) of the as-spun composite nanofibrous membranes indicates homogeneous and interconnected fibrous network with no beading or rope formation. Average diameter of the individual fibers was estimated from several FESEM images using Image JTM software. Such a variation in the nanostructure is special and typical of the electrospun ZnO obtained in this work.

Table 3.1: Average crystallite size (d av ), microstrain (), lattice parameters (a, c) and unit cell volume (V) of ZnO nanostructure in heated treated nanofibres

Optical Properties

The as-spun nanofibers are smooth and bead-free due to the amorphous nature of the zinc acetate/PVA composite and the optimal electrospinning conditions, respectively. However, after annealing at 550 C, the surface of the nanofibers becomes rough and the fiber diameter is reduced due to the removal of PVA and the conversion of the Zn salt into ZnO with wurtzite structure as depicted in Figure 3.3(b,c) ). The inter-planar d-spacing of the crystal plane shown was calculated to be 0.28 nm which corresponds to the lowest order (100) plane of wurtzite ZnO.

Magnetic Properties

The field-dependent magnetization curves for the 500, 550, and 600 C annealed samples are shown in Figure 3.5(c), all samples exhibit a magnetic hysteresis loop at room temperature with saturated magnetization. The M-H curve of the sample annealed at 550 C for 90 minutes in a nitrogen atmosphere (cf. Figure 3.6) showed a higher saturation magnetic moment (0.08 emu/g) compared to the sample annealed in air. Further analysis of the state of defects in the samples could shed light on this proposal.

Figure 3.5: (a) ZFC-FC magnetization curve and (b) M-T curve of 550 C annealed sample (FC curved recorded under 1.5 kOe field)

Defects analysis

It is clear that magnetization in this sample is related to the oxygen defects in the samples. It is now important to see if any paramagnetic defects are present in the sample. Consequently, the EPR signal in the annealed samples is only related to the ZnO phase.

Figure 3.8 displays the PL spectra of air annealed samples. All the spectra exhibit a relatively sharp UV emission peak centered at 385 nm and a broad band in the visible range

Summary

This chapter presents the investigations carried out on the effect of Mg, Co substitution on the evolution of PVA/ZnO nanofibers and their characterization. In this chapter, the preparation and characterization of novel 1-dimensional ZnO nanostructures containing Mg and Co are reported.

Mg substituted ZnO nanostructures

Preparation
Thermo-gravimetric analysis
Structural characterization and morphology
Defect Analysis
Optical properties

Here, Fig. 4(e) shows the change in unit cell volume with Mg substitution in the ZnO structure. The Williamson-Hall method [CULL13] was used to calculate the average crystallite size (dav) and microstrains () present in the samples using Eq. This indicates that Mg2+ replaces Zn2+ thereby increasing the band gap energy of the system.

Figure 4.1 shows the TG curves of as-spun PVA/zinc acetate and 8 wt.% Mg doped PVA/zinc acetate nanofibres recorded from ambient temperature to 800 C under a constant heating rate at 10 C/min

Co substituted ZnO nanostructures

Preparation
Structure and morphology
Optical properties
Magnetic properties
Defect analysis

Furthermore, the decrease in the intensity of the diffraction peaks in Co-doped ZnO indicates that the Co2+ dopant ions are substituted in the Zn2+ inner lattice. Average crystallite size (dav), microstrain (), lattice parameters (a, c) and unit cell volume (V) of Co-doped ZnO nanowires. This indicates that oxygen defects are not present in an appreciable manner in the 1-d Co-doped ZnO samples.

Figure 4.9 shows TG curves of as-spun PVA/zinc acetate and 5 wt.% Co doped PVA/zinc acetate nanofibers recorded a constant heating rate at 10⁰ C/min

Summary

EPR spectra of 1 wt. % and 2 wt. % Co doped ZnO nanofibers are symmetrical with Hr below 300 mT. However, the EPR spectra of 3 wt. % and 5 wt. % Co doped ZnO nanofibers asymmetric with a larger shift in Hr. The lack of oxygen defects in Co-doped ZnO nanowires exhibiting RTFM suggests that alternative mechanisms can also cause RTFM in TM-doped ZnO nanostructures.

Nanocrystalline CaFe 2 O 4

Preparation
Magnetic properties

This shows that the spinel unit cell of the synthesized CaFe2O4 expands slightly when the annealing temperature is increased to 500 C. Heat treatment above this temperature induces a slow transformation from the cubic phase to the orthorhombic phase. It can be seen that the composition is very close to that of the compound CaFe2O4.

Figure 5.1: Thermogravimetric curve for as-synthesized CaFe 2 O 4 nanoparticles.

Preparation

As-synthesized Ca0.9Co0.1Fe2O4 powders were annealed at 1100C for 8 hours to obtain the orthorhombic phase in the Co-substituted CaFe2O4 nanoparticles. In fact, the Ms of Ca0.9Co0.1Fe2O4 is slightly lower than the synthesized (superparamagnetic) CaFe2O4. On the other hand, the Ca0.9Co0.1Fe2O4 annealed at 1100 C shows lower Ms than the synthesized sample.

Figure 5.11 shows the TG curve of as-synthesized Ca 0.9 Co 0.1 Fe 2 O 4 powder

Summary

It should be noted that small amounts of CoFe2O4 present as an impurity phase in both cubic and orthorhombic samples may also contribute to the observed magnetic properties. Remanent magnetization Mr, saturation magnetization Ms, coercive field Hc, and average crystallite size (Dav) of as-synthesized and heat-treated Ca0.9Co0.1Fe2O4 powders. This chapter presents the structural and magnetic properties of heat-treated PVA nanofibers containing calcium and iron salts.

Preparation

Thermo-gravimetric analysis

In the previous chapter, studies carried out on CaFe2O4 and Ca0.9Co0.1Fe2O4 prepared by sol-gel route were discussed. This could be attributed to the removal of water content and organic matter (by degradation of PVA) and the complete crystallization of cubic CaFe2O4 from the precursor salts. This large (90%) weight loss leads to a stable structure, as no further weight loss is observed in this system.

Structure and morphology

The dav of orthorhombic CaFe2O4 obtained by annealing at 1000 C for 8 hours is 28.6 nm, which is almost twice that of the spinel phase obtained by annealing at 500 C. The spun nanofibers are smooth and bead-free due to the amorphous nature of PVA or the optimal electrospinning conditions. The EDX spectrum of the sample is shown in Figure 6.3(d), confirming the presence of Ca, Fe and O, and the table given as an inset shows the elemental composition of the particles.

Figure 6.2: X-ray diffraction patterns of (a) as-spun and (b) 400 C, 500 C, 600 C and

Magnetic properties

Inset (b) shows an expanded view of the data on annealed samples near the origin. The ZFC magnetization curve shows a maximum value corresponding to the blocking temperature (TB) at a certain low temperature depending on Dav of the nanoparticle. The superparamagnetic properties of CaFe2O4 nanoparticles show that the magneto-crystalline anisotropy (EA) of the nanomaterial is a key parameter.

Figure 6.4: Room temperature M-H curves of annealed samples. Inset (a) shows M-H curve of as-spun sample

Summary

CONCLUSIONS AND SCOPE FOR FUTURE WORK

Conclusions

Here, undoped and TM-doped ZnO-based DMS materials are selected for study to (1) obtain DMS with RTFM and high TC that can be used in spintronic devices and (2) to unravel the mechanism(s) that lead to RTFM in this DMS. A combined analysis of Raman, PL, and EPR data revealed that the singlet ionized oxygen vacancy present in the annealed samples is responsible for the RTFM observed in 1-d undoped ZnO. Raman and PL spectrum analysis showed that the oxygen vacancy that was responsible for RTFM in the undoped sample was drastically decreased after Mg doping.

Scope for future work

As synthesized cubic CaFe2O4 nanoparticles are slowly transformed to orthorhombic phase when annealed above 300 C and the transformation completes at 1100 C. Superparamagnetic behavior is associated with the cubic phase and weak ferrimagnetic behavior with the orthorhombic phase. However, the superparamagnetic cubic phase could be stabilized up to 500 C and single phase orthorhombic could be obtained at 1000 C in this 1-d nanostructure.

Publications/communications originating from the thesis work

In journals

Publications not part of the thesis work

Arnab Kumar Das, Rajkumar Modak and Ananthakrishnan Srinivasan, “Structural and optical properties of electrospun MoO 3 nanowires” AIP Conference Proceedings