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Chapter I Introduction

1.1 Nanomaterials

1.2 Synthesis approach

1.2.1 Top-down synthesis

1.2.2 Bottom-up synthesis

1.3 Magnetic hematite (α-Fe2O3)

1.4 Literature review

1.4.3 Review of amino acid inspired approach

1.4.2 Review of heterostructure approach

1.4.3 Review of graphene based metal oxide approach

1.5 Dissertation Objectives

1.6 Dissertation research flow

1.7 Organization of the dissertation

References

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Chapter I

Introduction

1.1 Nanomaterials:

From the forms of materials, nanomaterials are classified as zero, one and two

dimensional nanostructures. Zero-dimensional nanostructures, also named as nanoparticles,

include single crystal, polycrystalline and amorphous particles with all possible

morphologies, such as spheres, cubes and platelets. In general, the characteristic dimension

of the particles is one hundred nanometres or below. Some other terminologies are zero-

dimensional nanostructures: If the nanoparticles are single crystalline, they are often referred

to as nanocrystals. When the characteristic dimension of the nanoparticles is sufficiently

small and quantum effects are observed, quantum dots are the common term used to describe

such nanoparticles.

One-dimensional (1D) nanostructures have been called by a variety of names including:

whiskers, fibres or fibrils, nanowires and nanorods. In many cases, nanotubes and nanocables

are also considered one-dimensional structures. In general, nanotechnology can be

understood as a technology of design, fabrication and applications of nanostructures and

nanomaterials, as well as fundamental understanding of physical properties and phenomena

of nanomaterials and nanostructures [1, 2]. A particle which exhibits one or more

dimensions on the nano-scale (< 100 nm) is considered as a nanoparticle (NP). The scientific

community is in general agreement with this definition. However, sub-micron sized particles

are often regarded, mistakenly, to fall within this particular group of materials. Fig.1.1

illustrates the context of the NP size domain compared to the size of some well known

objects [3]. Nanomaterials and nanotechnology have found the significant applications in

physical, chemical and biological systems. NPs have attracted considerable attention

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Fig.1.1 Schematic diagram showing the nano-scale domain (red) relative to some

common physical and biological objects [3].

because they act as an effective bridge between bulk materials and atomic or molecular

structures. Bulk materials exhibit constant physical properties, irrespective of their size and

mass. However, NPs possess unique size dependent properties due to the significant

proportion of atoms existing on their surface in relation to the bulk, resulting in a large

specific surface area, as demonstrated in Fig.1.2 [4].

Fig.1.2 Schematic diagrams showing the scaling of ‘surface to bulk’ atoms to

illustrate the size-related effect of NPs [4].

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In view of this, the electronic, optical and magnetic properties of materials are found to

change as their size decreases towards the nano-scale. Accordingly, the size control of NPs

is of particular interest due to the possibility of influencing their functional properties. The

importance of nanotechnology was pointed out by Feynman at the annual meeting of the

American Physical Society in 1959, in the classic science lecture entitled ‘‘There is plenty of

room at the bottom’’. Since 1980s, many inventions and discoveries in the fabrication of

nano-objects have been developed. The discovery of novel materials, processes, and

phenomena at the nanoscale, as well as the development of new experimental and theoretical

techniques for research provide plenty of new opportunities for the development of

innovative nanostructured materials devices.

1.2 Synthesis approach:

In order to explore novel physical properties and phenomena and realize potential

applications of nanostructures and nanomaterials, the ability to fabricate and process

nanomaterials and nanostructures is the first corner stone in nanotechnology. The common

nanomaterials synthesis procedures can generally be grouped in two different categories, the

top-down and the bottom-up approaches, as shown in Fig.1.3.

1.2.1 Top-down synthesis:

The top-down approaches usually utilize planar, lithographic, etching and deposition

techniques to transfer a pre-designed pattern to a substrate which can form complex high

density structures in well-defined positions on substrates and their integrated systems [6-7].

The top-down approach has been exceedingly successful in extensive applications, with

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Fig.1.3 Schematic representation of the ‘bottom-up’ and ‘top-down’ approaches of

nanomaterials [5].

microelectronics being perhaps the best example today. They can produce nanostructures

with very uniform shapes and electronic properties. While developments continue to push

the resolution limits of the top-down approach, these improvements in resolution are

associated with a near-exponential increase in cost associated with each new level of

manufacturing facility.

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1.2.2 Bottom-up synthesis:

The bottom-up approach, in which functional structures are assembled from well-

defined chemically and/or physically synthesized nanoscale building blocks, much like the

way nature uses proteins and other macromolecules to construct complex biological systems,

represents a powerful alternative approach to conventional top-down methods [8]. The

bottom-up approach has the potential to go far beyond the limits and functionality of top-

down technology by defining key nanometer-scale metrics through synthesis and subsequent

assembly. Moreover, it is highly likely that the bottom-up approach will enable entirely new

device concepts and functional systems and thereby create technologies that we have not yet

imagined. For example, it is possible to seamlessly combine chemically distinct nanoscale

building blocks that could not be integrated together in top-down processing and thus obtain

unique function and/or combinations of function in an integrated nanosystem. The bottom-

up approach necessitates nanostructured building blocks with precisely controlled and

tunable chemical composition, structure, size, and morphology, since these characteristics

determine their corresponding physical properties. Nanoparticles of various oxides can also

be synthesized by confining chemical reaction, nucleation and growth process. Various

synthesis methods can be grouped into two categories: thermodynamic approach or kinetic

approach. In the thermodynamic approach, synthesis process consists of (i) generation of

super saturation, (ii) nucleation, and (iii) subsequent growth. In kinetic approach, formation

of nanoparticles is achieved by either limiting the amount of precursors available for the

growth or confining the process in a limited space. Hydrothermal and solution mixing

approaches fall under the category of bottom-up synthesis, an elaborate description in this

regard is given in chapter II.

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1.3 Magnetic hematite (α-Fe2O3):

Iron is abundant in the Earth’s crust and an important element in our environment. It

is present in soils as oxides and easily mobilized in natural waters in the form of molecular

complexes and colloids. There are a number of iron oxides, hydroxides and oxy-hydroxides,

as summarized in table 1.1. For simplicity, this group of iron oxides and hydroxides are

Table 1.1 List of iron oxides, hydroxides and oxy-hydroxides.

often referred to as iron oxide. The structural chemistry of these compounds is diverse,

reflecting the large number of atomic structures [9]. Almost all these phases can be formed

from chemical solution, alluding to a complicated chemistry of formation [10,11]. In

particular, the diversity of physicochemical conditions present in the environment (e.g.

acidity, redox conditions, temperature, salinity, presence of organic or inorganic ligands, etc.)

suggests, practically, all the iron oxide phases can be found naturally. The high versatility of

iron chemistry in an aqueous medium originates from two factors: the occurrence of two

oxidation states, Fe2+ and Fe3+, which are stable over a large range of acidity, and the high

reactivity of iron complexes towards acid-base and condensation phenomena [9]. Mineral

hematite, α-Fe2O3, formerly spelt haematite, is derived from the Greek haimatite meaning

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‘blood-like’, referring to its powder’s shade of red [12]. In bulk compact form, it is coloured

black or silver-grey. Hematite is mined as the main ore for producing iron. The structure of

α-Fe2O3 (Fig.1.4), determined by Pauling and Hendricks in 1925, is isostructural with

corundrum, α-Al2O3 [13]. The space group is R3c (rhombohedral symmetry) and the lattice

parameters in the hexagonal cell are: a = b = 5.0346 Å and c = 13.752 Å [14].

Nanostructured α-Fe2O3 is the most thermodynamically stable iron oxide phase and is of

particular interest because of its high resistance to corrosion, low processing cost and non-

toxicity [15]. This multifunctional material has therefore been investigated extensively for a

variety of applications including photo-catalysis [16], gas sensing [17,18], magnetic

recording [18], drug delivery [19], tissue repair engineering [20] and magnetic resonance

imaging [21], along with its use in lithium-ion batteries [18], spin electronic devices [22] and

pigments [23]. In particular, the magnetic properties of α-Fe2O3 have attracted much interest

Fig.1.4 The crystal structure of α-Fe2O3 (hematite).

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over the past decades. In the 1950’s it was found to be canted antiferromagnetic (weakly

ferromagnetic) at room temperature, antiferromagnetic below the Morin transition

temperature of 263 K and paramagnetic above its Néel temperature of 950 K. Therefore

understanding the magnetic behaviors is of considerable interest for future device

fabrications.

1.4 Literature review:

1.4.1 Review of amino acid inspired approach:

Research on α-Fe2O3 powder synthesis was motivated by its numerous applications such

as, catalysts [24-26], lithium-ion batteries [27,28], water treatment [29,30] and gas sensors

[31-37]. The efficiency of the oxide in each of those applications depends on the phase, size,

and shape of the particles used. Hematite of varied morphologies have been fabricated,

including nanocrystals [38,39], particles [40-42], cubes [43], rods [44,45], wires [46], tubes

[47], flute-like [31], hollow spheres [48] and nanocups [49], and ordered mesoporous α-

Fe2O3 [46]. Amino acids, the protein building units, represent an interesting first step in the

bioinspired approach, as they display a variety of chemical functions [51]. They can

consequently interact with the metal precursors in solution or with the different oxide

surfaces. Bioinspired synthesis has attracted much attention due to the huge advantage of the

biomolecules in directing and assembling the superstructures [52-54]. However, it still

remains a big challenge for scientists to assemble nanostructures for the designed

applications. Amino acids like lysine and histidine were used for the synthesis of SnO2,

ZrO2 and ZnO [55-57]. Glycine-assisted hydrothermal synthesis is frequently used to prepare

specific structures [58,59]. For example, Prevot and co-workers prepared flower-like NiAl-

layered double hydroxides using Ni (II) glycinate complex as precursor [58]. Zhang and co-

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workers prepared hollow Ni(OH)2 microspheres using Ni (II) glycinate complex under strong

basic conditions. Glycine played an important role in the formation of the specific

morphologies and architectures [59]. Cao et al. [60] have controlled the size of α-Fe2O3

using the amino acid L-arginine by hydrothermal method. L-Arginine has also been used as

both reducing agent and stabilizing ligand for iron oxide NPs. L-arginine was reported as a

possible initiator for the nucleation process due to iron hydrolysis. The size of the NPs can

be controlled by adjusting Fe:arginine ratio. This synthetic approach was extended to the

synthesis of manganese and cobalt ferrite NPs [61]. In this work a simple and fast synthesis

of α-Fe2O3 nanoparticles using glycine assisted hydrothermal method is adopted. Glycine

with functional groups NH2 and COOH is considered as the key for controlled crystallization

of the α-Fe2O3 nanoparticles.

1.4.2 Review of heterostructure approach:

Heterostructures of α-Fe2O3/SnO2 are an interesting model for α-Fe2O3 and SnO2

crystallizing into different structures, hexagonal and tetragonal ones, respectively. The side

view diagrams of α-Fe2O3 and SnO2 are shown in Fig.1.5 (a) and (c) and the diagram near the

interface is presented in Fig.1.5 (b) [62]. Oxygen atoms in the α-Fe2O3 crystal present a

hexagonal close packed structure along the c-axis, and iron atoms fill in octahedral cavities.

In the SnO2 crystal, oxygen atoms present an approximate hexagonally close packed

structure along the a-axis and tin atoms fill in bigger octahedral cavities. The close packed

oxygen atoms act as the framework for extension crystal growth. If the frameworks on both

sides of the interface are similar, the corresponding heterostructure could be formed in a

feasible manner and would be sufficiently stable. With an ever increasing list of promising

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Fig.1.5 Diagram of side view of (a) and (c) planes of α-Fe2O3 and SnO2; (b) Interfacial

diagram of α-Fe2O3/SnO2 heterostructures. The interfacial layers are indicated

with a dotted frame [62].

applications, there is a surge in developing efficient and scalable strategies for synthesizing

nanostructures with diverse and tunable properties [63,64]. Often the recruitment of other

materials with new properties to form core-shell, hetero- and/or doped structures represents a

facile pathway to modify materials properties [65-71]. Heterostructures consisting of

chemically distinct materials have attracted intense research interest due to the fact that

desired properties and multifunctionality can be achieved by fine-tuning their compositions,

shapes, and controlled alignments of the primary nanobuilding blocks [67,72-75]. Another

key motivation underlying the preparation of heterostructures is the potential to realize the

integration of nanoscale devices at a level not possible with conventional top-down methods

[76-80].

SnO2 and α-Fe2O3, two of the most investigated functional semiconductors, have

attracted great attention due to their wide range of applications. For example, Zhou et al.

[81] fabricated a six-fold symmetry nanorod branches of α-Fe2O3 onto the SnO2 nanowires

and observed a superior electrochemical performance. Nu et al. [82] grew SnO2 nanorods

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epitaxially on the surface of α-Fe2O3 nanospindles and noticed an enhanced photocalytic

activity toward methylene blue. Xu et al. [83] fabricated SnO2 nanorods hierarchically on the

inner and outer surfaces of hollow α-Fe2O3 nanoprisms and studied their optical and

photocatalytic properties. Sun et al. [84] assembled short nanorod branches on the basal

surface of SnO2 nanosheets and demonstrated an enhanced gas sensing performance toward

acetone. Wang et al. [85] grew α-Fe2O3 nanorods on the surface of SnO2 nanosheets and

showed an enhanced lithium ion storage capacity and cycling stability. Zhu et al. [86]

fabricated SnO2/α-Fe2O3 heterojunction nanotubes and showed higher visible light

photocatalytic activity for the degradation of Rhodamine B dye. Wu et al. [87] prepared α-

Fe2O3/SnO2 core-shell heterostructures and demonstrated an enhanced photocatalytic

performance of Rhodamine B. All the above works are devoted to check the application

performance of the material. To explore the potential applications, however, architecturally

assembling primary nanobuilding blocks into expected geometric forms is required. To the

best of our knowledge, there have been no reports on the fabrication of SnO2 nanoparticles

(NP’s) on the α-Fe2O3 hexagonal discs (HD’s) and their detailed investigations on the

magnetic properties viz.; hysteresis behavior and Morin transition temperature.

The motivation of this work has been generated based on the results obtained by

Sytnyk et al. [88], Ma et al. [89] and Chen et al. [90], which are given below. Sytnyk et al.

[88] tuned the magnetic parameters of colloidal magnetite nanocrystal heterostructures by

cation exchange. They noticed a significant enhancement in coercivity and blocking

temperature when magnetite was modified with Co2+ ions. Ma et al. [89] studied the

magnetic characteristics of Fe3O4/α-Fe2O3 hybrid cubes and inferred a disappearance of

Morin transition temperature. Chen et al. [90] prepared Fe3O4/SnO2 core–shell nanorods and

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observed a significant enhancement of coercivity and reduction of saturation magnetization.

They concluded that the effective complementarities between the dielectric and magnetic loss

tangents played crucial role in electromagnetic wave absorption. Therefore integrating

diverse compositions and functionalities with enhanced properties could be attributed to the

synergetic strengths owing to the unique interfacial interaction of the binary components.

Recent studies have revealed that the physical performances of SnO2 or α-Fe2O3 can be

remarkably improved by forming α-Fe2O3-SnO2 heterostructures [82,87,91-93]. So far, there

are generally three kinds of, i.e. core-shell [87,94-98], branched [82,83,92,99,100], and

random [91,101,102], α-Fe2O3–SnO2 heterostructures. Heterostructures possess two

outstanding merits. One is a well-defined interface. Heterostructures with a simple physical

contact between α-Fe2O3 and SnO2 always have a definite epitaxial relationship

[82,83,92,99,100]. The novel properties distinct from individual constituents usually stem

from the interface within the heterostructure [103,104]. The other is greatly increased

surface area. In this work of α-Fe2O3 hexagonal disc/SnO2 nanoparticle semiconductor

nanoheterostructure is fabricated and their structural, formation mechanism, optical and

detailed magnetic properties like hysteresis behavior and Morin transition temperature are

investigated.

1.4.3 Review of graphene based metal oxide approach:

Graphene (GN), as a sparking rising star in materials science with well-defined two-

dimensional honeycomb-like network of carbon atoms, has initiated an explosive interest

owing to its prominent thermal stability [105], superior electronic conductivity [106,107],

remarkable structural flexibility [108,109], high specific surface area [110] and widespread

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potential applications [111] in nanoscience and nanotechnology. Therefore, by combining

fascinating merits and low costs, it suggests that GN sheets could be an ideal matrix for the

growing and anchoring of a large number of functional substances. A schematic sketch of

GNS-supported Fe2O3 nanoparticles is shown in the Fig.1.6 [112]. With this in mind,

numerous GN-based inorganic nanocomposites with metal [113-116], metal oxides [117-

Fig.1.6 Schematic illustration of Fe2O3/graphene nanocomposite [108].

-124] and sulfide [125] have been successfully synthesized and show enhanced properties of

these host materials. There has been several reports highlighting the variations in the

properties of graphene based iron oxide nanocomposites. Recently G. Pradhan et.al [126]

fabricated α-Fe2O3/graphene nanocomposite and have found that the absorption edge is larger

when α-Fe2O3 was modified with graphene. The obsorption band edge also shifted to the

visible region and attributed it to the presence of blackbody properties of graphene sheets that

modifies the fundamental process of electron-hole pair formation of α-Fe2O3 nanorods.

Similar kind of band edge shift is also noticed by F. Meng et al. [127]. Z. Jiang et al

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anchored Fe2O3 nanocrystals uniformly onto graphene nanosheets by a nanocasting technique

and the resulting composites applied as anodes of sodium-ion batteries exhibited excellent

cycling performance and rate capability [128]. F. Meng et al have performed

photoelectrochemical measurement and the results showed that coupling the hematite

nanoparticles with the reduced graphene oxide (RGO) greatly increases the photocurrent and

reduces the charge recombination rate [127]. L. Xiao et al have demonstrated outstanding

electrochemical performance of Fe2O3/graphene aerogel and attributed to the synergistic

interaction between uniformly dispersed Fe2O3 particles and graphene aerogel [129]. B.

Zhao et. al. have checked the electrochemical performance of Fe2O3 nanorod/graphene

composites and found that the composites exhibit a very large reversible capacity [130]. W.

Xiao et. al. have showed that Fe2O3/Graphene composite exhibits remarkable cycle

performance with highly reversible charge capacity due to the synergic effects between

Fe2O3 and graphene [131]. X. Zhu et al have demonstrated that the total specific capacity of

RGO/α-Fe2O3 was higher than the sum of pure RGO and nanoparticle α-Fe2O3, indicating a

positive synergistic effect of RGO and α-Fe2O3 on the improvement of electrochemical

performance [132]. Despite these promising results, there has been no reports on the

magnetic properties of these kinds of composite. Very few efforts have been taken to

understand the magnetic hysteresis behavior of graphene based iron oxides. For instance,

saturation magnetization of graphene/γ-Fe2O3 hybrid aerogels are smaller than that of the γ-

Fe2O3 powder due to incorporation of magnetically inactive graphene sheets into iron oxide

aerogels [133] and showed high biocatalytic activity and definite repeatability [133]. Similar

kind of observation is also noticed for Fe3O4/Carbon core-shell nanorods and this material

showed superior electromagnetic wave absorption properties [134]. Thus there is much room

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to investigate the magnetic properties like hysteresis behaviour and Morin transition

temperature. In this context, an attempt has been made to understand the magnetic

behaviours of the composite.

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1.5 Dissertation Objectives:

In this dissertation, the aim is to synthesize and characterize a new class of α-Fe2O3

based nanoarchitectures with disc/horse shoe shaped morphology, α-Fe2O3/SnO2

nanoheterostructure and α-Fe2O3/graphene nanocomposites. The following goals are pursued

toward the overall objective:

1. Synthesize of α-Fe2O3 nanoparicles with size less than 100 nm by glycine assisted

hydrothermal process.

2. Synthesis of α-Fe2O3/SnO2 nanoheterostructure employing hydrothermal method.

3. Fabrication of α-Fe2O3/graphene nanocomposite using solution mixing method and

annealing.

4. Crystal structure analysis of the nanoarchitectures with the assistance of X-ray

diffractometer.

5. Morphological studies employing TEM/HRTEM equipment.

6. Optical properties using Fourier transform infrared (FTIR) and diffuse reflectance

UV-Vis (DRUV) spectrophotometers.

7. Magnetic properties with the assistance of Vibrating sample magnetometer (VSM).

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1.6 Dissertation research flow:

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1.7 Organization of the dissertation:

This thesis is organized in the following manner. Chapter I provides an introduction

of research focus and an overview of the current activities by covering different reports on

the synthesis and characterization of α-Fe2O3 nanoarchitectures. Chapter II presents the

general experimental methodologies applied to synthesize nanoarchitectures. The possible

formation mechanism in terms of equations and schemes are also presented in this chapter.

Chapter III gives the details regarding X-ray diffraction technique employed to identify the

crystal structure of the fabricated nanoarchitectures. Chapter IV contains the morphological,

SAED and EDX details pertaining to different nanoarchitectures studied using

TEM/HRTEM. Chapter V contains the details of optical spectroscopies like FTIR and

DRUV. FTIR presents the confirmation of purity, while from DRUV the optical band gap of

the nanoarchitectures is determined. Chapter VI is based on the results obtained by the

magnetic measurements like hysteresis loop and magnetization versus temperature.

Magnetic parameters like coercivity, saturation magnetization, remanance and Morin

transition temperature of the nanoarchitectures are determined and they are discussed in this

chapter. Eventually a brief summary drawn from the results of the entire research work is

included at the end of the dissertation.

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