1

HYDROTHERMAL SYNTHESIS OF HEMATITE

Clara Keng Hui Lin1, Chow Kit Mun

2, Deborah Ho Yee Ying

3, Koh Huan Kiat

3

1 Raffles Girls’ School (Secondary), 20 Anderson Road, Singapore 259978

2 River Valley High School, 6 Boon Lay Avenue, Singapore 649961

3 DSO National Laboratories, 20 Science Park Drive, Singapore 118230

ABSTRACT

Monodisperse Hematite nanostructures of varying shapes and sizes have been produced via

hydrothermal synthesis. The influence of the concentration of precipitating agent (NaOH) as

well as the presence of foreign anions, on the size and morphology of Hematite synthesised

were studied.

Under varying [NaOH], Hematite (α-Fe2O3) and Goethite (α-FeOOH) were obtained. At

higher supersaturation, Ferrihydrite, a less-stable intermediate, first nucleated. Hematite

converted from Ferrihydrite through an aggregation-dehydration-rearrangement mechanism;

Goethite converted from Ferrihydrite via dissolution-reprecipitation. At lower

supersaturation, it is proposed that the Hematite formed from forced hydrolysis of the ferric

nitrate solution.

It has been shown that anions control Hematite crystal growth by adsorbing preferentially on

certain crystallographic planes, resulting in anisotropic crystal growth. In the presence of

SO42-

, rhombohedral crystals were obtained at lower [SO42-

] while pseudocubic crystals were

obtained at higher [SO42-

]. In the presence of PO43-

, ellipsoidal crystals were obtained with

crystal growth clearly restricted in directions normal to the c-axis.

The magnetic properties of the crystals synthesised were also studied. Coercivity of the

particles was found to increase proportionally with size and vary with different morphologies.

High coercivity to particle size ratios suggest potential applications in media recording and

information storage devices.

INTRODUCTION

As a relatively low-cost, stable iron oxide phase, Hematite, α-Fe2O3, has been identified to be

environmentally-friendly as well as an n-type semiconductor with a small band gap (Eg =

2.1eV). Hematite is hence widely used in solar cells, catalysts, gas sensors, nanodevices and

drug delivery, amongst other applications. Due to its non-toxicity and corrosion-resistance,

Hematite has also been used in water splitting and environmental treatment [1,2]

.

The optical, magnetic, electrochemical and electrical properties of Hematite are largely

shape- and size-dependent, which necessitates the controlled synthesis of monodisperse

Hematite nanoparticles. There are a variety of techniques for Hematite synthesis, with those

most commonly used being the sol-gel method developed by Sugimoto et al. [3,4]

,

microemulsion, hydrothermal synthesis and chemical precipitation [5]

. In this project,

hydrothermal synthesis was studied as a means to effectively control the size and shape of the

Hematite nanostructures. Many studies suggest that Hematite is likely to be obtained through

either hydrolysis of iron (III) solutions or interconversions from other iron oxide-hydroxide

intermediates [6-9]

.

2

The morphology of Hematite particles has been shown to be controlled by factors such as pH,

synthesis temperature, duration of synthesis, and the presence of other ions or surfactants. In

particular, studies have shown that anions such as sulfate (SO42-

) and phosphate (PO43-

) play

significant roles in the control of the shape and size of Hematite [4,10,11]

. Previous research has

also highlighted the effect of precipitating agents on the morphology of Hematite crystals by

affecting the nucleation and growth stages of Hematite synthesis [1,12]

.

This paper aims to (i) determine the effects of the concentration of precipitating agent

Sodium Hydroxide (NaOH) and (ii) elucidate the effects of different anions and their

concentrations, in particular sulfate (SO42-

) and phosphate (PO43-

), on the crystal growth of

Hematite nanostructures obtained via hydrothermal synthesis. The magnetic properties of the

different Hematite nanostructures synthesised were also explored to shed light upon potential

real-world applications.

EXPERIMENTAL

Materials and preparation

Table 1. Summary of Experimental Conditions

All chemicals were of reagent grade and were used as received without further purification.

The experimental conditions for the various samples are tabulated in Table 1.

1.9g of Fe(NO3)3ᐧ 9H2O and various masses of NaOH pellets were dissolved separately in

deionised water and stirred to obtain separate homogeneous solutions. The NaOH solution

was then added dropwise to the Fe(NO3)3 solution contained in a 100ml Teflon container

under the constant stirring at 1000 rpm, and the mixture was dispersed for 15 minutes at room

temperature. For some of the experiments, anions (SO42-

and PO43-

) were introduced through

the addition of H2SO4 and H3PO4. Corresponding amounts of deionised water were added to

ensure a total mixture volume of 70ml. The Teflon container was then sealed in a

hydrothermal reactor and placed in an oven at 200°C for 8 hours.

3

Post-Processing

After the mixtures were heated in the oven, they were left overnight to cool to room

temperature. The supernatant liquid was decanted, resulting mixtures were neutralized with

either HCl or NaOH solutions and centrifuged with deionised water and acetone to remove

any impurities or residual ions. The powders were dried at 60°C and collected for analysis.

Characterization

To determine the morphology of the products, Scanning Electron Microscopy (SEM) analysis

was performed. Crystal phases and structures of the samples were identified through X-Ray

Diffraction (XRD) while magnetic properties were examined using a Vibrating Sample

Magnetometer (VSM). A list of equipment models can be found in the appendix.

RESULTS AND DISCUSSION

Effect of Concentration of Precipitating Agent (NaOH)

NaOH has multiple roles in Hematite synthesis. Firstly, NaOH acts as the precipitating agent

for Ferrihydrite, an intermediate iron oxide-hydroxide which precedes either the formation of

Hematite or Goethite at higher levels of supersaturation. In addition, NaOH influences the

level of supersaturation of the solution as well as provides nucleation sites for Hematite or

Goethite crystals. High [OH-] has also been shown to control crystal growth on the {001}

planes[11]

, resulting in hexagonal Hematite platelets.

Table 2. Summary of Results for Concentration of NaOH experiment set

Figure 1. SEM images of samples (a) S1 (b) S2 (c) S3 (d) S6 (e) S7 and (f) S9

4

Figure 2. XRD graphs of (a) S3, Hematite and Goethite (b) S6, Goethite (c) S7, Hematite and

(d) S9, Hematite

Hexagonal Hematite platelets were obtained for samples S1 and S2; acicular Goethite (α-

FeOOH) crystals were obtained for samples S3 - S6; small faceted Hematite particles were

obtained for samples S7 - S9 (Fig. 1, Table 2). The phases of the products were identified

using XRD (Fig. 2), and were in good accordance with database indexes for the pure phases

of Hematite (JCPDS Card No. 33-664) and Goethite (JCPDS Card No. 29-713), except for

S3, which showed both Hematite and Goethite peaks. This could be due to the presence of a

small proportion of Hematite in S3, which is further verified by an SEM image showing the

coexistence of acicular Goethite rods as well as larger Hematite polyhedrons (Fig. 3). The

crystal structures of Hematite and Goethite can be found in the appendix for reference.

Trends in the size of the products have also been observed with varying [NaOH]; with a

general increase in mean size of particles as [NaOH] increases (Fig. 4).

These results can be explained by studying the formation mechanisms of Hematite and

Goethite respectively at the varying NaOH concentrations. The concentration of NaOH plays

5

a large role in determining the extent of supersaturation of the solution - the higher the NaOH

concentration, the higher the level of supersaturation. NaOH, as a precipitating agent, binds

with the water molecules, thereby decreasing the available water content and resulting in a

higher level of supersaturation. Since different amounts of NaOH favour the formation of

different products, the trends in size will be studied individually in reference to the different

formation mechanisms.

At higher levels of supersaturation, Hematite and

Goethite are formed from a common iron oxide-

hydroxide intermediate, Ferrihydrite (Fig. 5) [13-16]

.

However, as a less stable phase, Ferrihydrite is often

eventually converted to the more stable phases

Hematite and Goethite. At lower levels of

supersaturation, it is proposed that the resultant iron

oxide forms through forced hydrolysis of the ferric

nitrate solution.

S1-S6 were considered to be at higher levels of

supersaturation due to the higher [NaOH]. As such,

precipitation of Ferrihydrite occurred. Hematite and

Goethite have been shown to convert from

Ferrihydrite from two competing mechanisms:

Hematite forms through an aggregation -

dehydration - rearrangement mechanism; Goethite forms via dissolution-reprecipitation [6]

.

Samples S1 and S2

At higher NaOH concentrations, Hematite is preferentially formed over Goethite. With high

[OH-], burst nucleation of Ferrihydrite occurs (in accordance with the von Weimarn Theory)

[1], and many small Ferrihydrite nuclei are formed. An unsustainable high-energy state is

formed due to the high surface energy of small particles with high surface area to volume

ratio (in accordance with the Gibbs-Thompson effect) [6,17]

and surface interactions between

particles of close proximity. The Ferrihydrite nuclei hence aggregate. After attaining a critical

mass, the Ferrihydrite aggregates then undergo dehydration, in which a proton is removed

from a hydroxyl group. The loss of a proton results in a local charge imbalance which is

resolved through the rearrangement of Fe3+

cations in the lattice, thus transforming into

Hematite [6, 17]

.

Samples S3 to S6

For samples S3-S6, at lower NaOH concentrations, the extent of supersaturation is lowered.

This promotes the dissolution of Ferrihydrite and reprecipitation as Goethite. Goethite growth

in the alkaline media of S3-S6 involves the incorporation of Fe(OH)4- complexes into the

growing crystal lattices [6]

. Higher [OH-] promotes growth of crystals by increasing the rate at

which Fe3+

from the solution is deposited onto the growing plane of the Goethite crystals.

Therefore, the length of Goethite rods increases from S6 to S3 as OH/Fe increases (Fig. 4).

6

Comparison of Hematite particles in samples S1/S2 and S3

Figure 6. SEM images of samples (a) S1 (b) S2 and (c) S3

To explain the differences in shape and size of the Hematite particles formed in S1/S2 and

S3, it is postulated that a different formation mechanism was involved in the reconstructive

transformation of Goethite to Hematite in S3, as compared to the topotactic transformation of

Ferrihydrite to Hematite in S1 and S2.

For S1 and S2, the rearrangement of Ferrihydrite particles to form Hematite crystals is

limited due to short-range diffusion in the solid phase. Consequently, these Hematite crystals

are irregularly shaped and show a large size distribution (Fig. 6).

Similar to S1 and S2, a high energy state system is formed in S3 due to high supersaturation

and the close packing of acicular Goethite with high surface area to volume ratio. This results

in the oriented aggregation of Goethite particles. However, unlike S1/S2, the Goethite

aggregates redissolve and recrystallize to form faceted hexagonal Hematite crystals which are

more energetically-stable. Greater ion mobility in liquid phase facilitates ion rearrangement at

longer length scales and thus favours the growth of bigger crystals.

Samples S7-S9

Lastly, samples S7-S9 were considered to be at lower levels of supersaturation due to the

lower NaOH concentrations. This prevented the nucleation of Ferrihydrite as an intermediate

phase, the reaction proceeding instead via the forced hydrolysis of the ferric nitrate solution

at high temperatures (200°C) [6]

. In an aqueous solution, ferric nitrate dissociates to form the

hexaaquairon(III) ion, as shown in equation 1:

Fe(NO3)3 + 6H2O → Fe(H2O)63+

+ 3(NO3)3 (1)

Being electropositive, the Fe3+

cation induces the H2O ligands to act as acids. This results in

hydrolysis, the deprotonation of the H2O ligands. Complete hydrolysis of the complex would

would form an iron oxide or oxide-hydroxide, as shown in equations 2.1 and 2.2 respectively:

2Fe(H2O)63+

→ Fe2O3 + 6H+ + 9H2O (2.1)

Fe(H2O)63+

→ FeOOH + 3H+ + 4H2O (2.2)

The resultant iron oxide or oxide-hydroxide nuclei then grow through the adsorption of Fe3+

cations from the solution onto the surfaces of growing crystals via Fe-OH complexes. For S8

and S9, in highly acidic media, Fe(H2O)63+

is the predominant complex. For S7, in mildly

7

alkaline media, it is thought that Fe(H2O)5(OH)2+

is the predominant complex. These higher-

valent complexes favour the formation of Hematite over Goethite [6]

, accounting for the

Hematite that was formed in S7-S9. The decreasing particle size from S7 to S9 can again be

explained by the decreased rate of transport of Fe3+

cations to the growing crystals by Fe-OH

complexes with the decrease in [OH-].

Effect of Anions

Effect of Sulfate Ions

Figure 7. SEM images of samples (a) A1, more-faceted rhombohedrons and (b) A4, less-

faceted pseudocubes

Sulfate ions have been reported to control the size and shape of Hematite crystals by

adsorbing on certain faces parallel to the c-axis of the growing crystal, resulting in ellipsoidal

crystals [11]

. However, in this study, ellipsoidal crystals were not obtained, possibly due to the

hydrothermal synthesis being conducted at a different temperature and with different solution

concentrations.

In the presence of sulfate ions, for A1 and A2, rhombohedral Hematite particles with clearly

defined edges were obtained (Fig. 7). These faceted particles suggest the adsorption of SO42-

ions on particular crystal planes, restraining crystal growth in particular crystallographic

directions and promoting growth on others.

Conversely, the Hematite particles synthesised in A3 and A4, at higher [SO42-

], are

comparatively more pseudocubic and less faceted (Fig. 7). It may be that at higher [SO42-

],

more faceted crystals with very sharp corners form. However, as these high-energy sites are

less stable, the atoms at the corners tend to redissolve into the solution to reach a more stable

energy state of equilibrium. This results in the pseudocubic, less faceted particles in A3 and

A4. This result has also been reported in other studies, where it is mentioned that the liquid

medium facilitates dissolution of atoms from high-energy sites such as corners and edges,

resulting in a change of Hematite morphology from rhombohedrons with well-defined edges

and surfaces into less-faceted polygonal structures [2]

.

8

Effect of Phosphate Ions

Figure 8. Schematic diagram of the adsorption of PO4

3- ions and resultant

anisotropic growth of Hematite crystals to form ellipsoidal particles

Figure 9. SEM images of samples (a) C1 and (b) C3, showing a decrease in the thickness of

the particles with increased [PO43-

] due to retardation of growth along crystal faces parallel to

the c-axis by phosphate ions

Phosphate ions have also been shown to adsorb preferentially onto certain planes of growing

Hematite particles, with studies often reporting the formation of ellipsoidal particles [4,10,18]

.

The phosphate and oxygen atoms are coordinated to two surface Fe3+

cations. Given that the

distance between two adjacent oxygen atoms in PO43-

is 2.50Å, it is expected that its adsorption is stronger on the faces parallel to the c-axis because of the better matching

between the O-O distance of PO43-

and the Fe-Fe distance on faces parallel to the c-axis

(2.29Å) than that on faces perpendicular to the c-axis (2.91Å) [4,5,19].

The stronger adsorption of the anion to faces parallel to the c-axis restricts growth on these

faces. A schematic diagram of the adsorption of PO43-

ions and resultant anisotropic growth

to form ellipsoidal Hematite particles is shown in Figure 8. Evidence of increasing adsorption

of PO43-

ions to Hematite crystal surfaces with increased [PO43-

] is presented in C1-C4.

Growth normal to c-axis is clearly retarded as [PO43-

] increases, as shown by the decreasing

thickness of the ellipsoidal particles (Fig. 9). Furthermore, a comparison of the peak

intensities of the XRD graphs of C1 and C3 shows restricted growth along the (104) plane

(Fig. 10).

9

Figure 10. XRD Graphs of C1 and C3

Magnetic Properties

Figure 11. Room temperature magnetization curves of (a) S7&S9 (b) A1&A4 (c) C1&C3

(d) A1,A4,C1,C3

Table 3. Summary of the Magnetic Parameters of Hematite nanoparticles synthesised

The magnetic properties of the synthesised hematite particles with different sizes (S7, S9) and

morphologies (A1, A4, C1, C3) were studied using a VSM at room temperature. Their

10

magnetization curves are plotted in Figure 11. The magnetic parameters of the particles are

summarized in Table 3, where Hc is coercivity and Mr is remanent magnetization.

The hysteresis loops show that saturation values are not reached at maximum applied

magnetic field of 15000 Oe, as such, no reliable conclusion on the saturation values of the

particles can be drawn. Most samples follow a general increase in coercivity proportional to

increase in particle size, with larger particles accounting for larger hysteresis loops (Fig. 11)

as prescribed in previous literature [1,20-23]

. However in the case where particle size of the

crystals are similar, crystal morphology plays a bigger part in determining coercivity

wherein; particles with more rounded morphologies and equidimensional shapes usually have

smaller coercive forces than more elongated particles [6]

.

This can be seen in the case of A1, A4 and C1. Though they are of similar sizes (61-68nm),

rhombohedrons of A1 possess greater magnetic anisotropy compared to pseudo cubes of A4

and ellipsoidal platelets of C1. It is thus more difficult to rotate magnetization vector of A1 to

demagnetize it as its magnetic moment has a greater tendency to favour the easy

magnetization axis of lower energy and align with the basal plane. Hence, A1 has a coercivity

of 998 Oe, almost twice that of A4 and 1.5 times that of C1. It is also observed that C3,

though much smaller than S7, possesses a much higher coercivity of 1566.6Oe, due to its

morphology as an ellipsoidal platelet (Fig. 9). This shape anisotropy limits magnetization

along the c-axis which will significantly increase the difficulty of magnetization and

demagnetization [20]

, thereby increasing coercivity and remanent magnetization.

Most particles have a proportional increase in remanent magnetization with coercivity except

for pseudocubes in A4. Its equidimensional structure causes it to possess lower magnetic

anisotropy, hence displaying higher remanence (0.1248 emu/g) than other particles with

higher coercivities; as shown by its narrow and steep hysteresis loop (Fig. 11(b))

The high coercivity of synthesised hematite nanoparticles suggests potential applications in

media recording and information storage devices. The demand for smaller devices with

greater data storage capacities calls for materials that balance size and coercivity [24]

, wherein

smaller particle sizes allow for higher density of data to be stored within a given amount of

space and high coercivity ensures that information stored is less likely to be erased due to

stray magnetic fields or excess heat [25]

. The hematite particles synthesised have a coercivity

to size ratio ranging from 6.45 to 17.4, which greatly exceeds those reported in previous

literatures from 0.0675 to 1.52 [1,20-23]

. Sample A1 in particular, displays especially

noteworthy properties due to the high anisotropy energy of the lengthened rhombohedron

particles; allowing it to possess high coercivity despite its small size.

CONCLUSION

Monodisperse Hematite particles were synthesised via the hydrothermal method. It has been

demonstrated that the products obtained and their morphologies can be controlled by

experimental parameters such as the concentration of precipitating agent (NaOH) and foreign

anions. At higher [NaOH], it is proposed that Hematite is formed via a topotactic

transformation from Ferrihydrite; at mid-range [NaOH], Goethite is formed from Ferrihydrite

via dissolution-reprecipitation; at lower [NaOH], Hematite is formed through the forced

hydrolysis of the Fe(III) solution. The foreign anions result in anisotropic crystal growth,

with rhombohedral and pseudocubic crystals obtained in the presence of SO42-

and ellipsoidal

particles obtained in the presence of PO43-

.

11

Despite their nanoscale size, the prepared hematite nanoparticles are shown to possess high

coercivities as magnetic anisotropy increases. Small rhombohedral particles at 66.4 nm with

relative uniformity in size displayed a high coercivity, suggesting promising applications of

hematite nanoparticles with specific morphologies as magnetic data storage materials for

media recording devices.

ACKNOWLEDGEMENTS

The authors would like to sincerely thank our mentors, Ms. Deborah Ho Yee Ying and Mr.

Koh Huan Kiat, for their guidance and advice on the development of our project. Equipment

for this project was provided by DSO National Laboratories and Temasek Labs @ National

University of Singapore.

12

REFERENCES

[1] Zhu, M., Wang, Y., Meng, D., Qin, X., & Diao, G. (2012). Hydrothermal Synthesis of

Hematite Nanoparticles and Their Electrochemical Properties. J. Phys. Chem. C, 116(30),

16276-16285.

[2] Lin, M., Tng, L., Lim, T., Choo, M., Zhang, J., Tan, H., & Bai, S. (2014). Hydrothermal

Synthesis of Octadecahedral Hematite (α-Fe2O3 ) Nanoparticles: An Epitaxial Growth from

Goethite (α-FeOOH). J. Phys. Chem. C, 118(20), 10903-10910.

[3] Sugimoto, T., Sakata, K., & Muramatsu, A. (1993). Formation Mechanism of

Monodisperse Pseudocubic α-Fe2O3 Particles from Condensed Ferric Hydroxide Gel.

Journal Of Colloid And Interface Science,159(2), 372-382.

[4] Sugimoto, T., Wang, Y., Itoh, H., & Muramatsu, A. (1998). Systematic control of size,

shape and internal structure of monodisperse α-Fe2O3 particles. Colloids And Surfaces A:

Physicochemical And Engineering Aspects, 134(3), 265-279.

[5] Almeida, T. (2010). Hydrothermal Synthesis and Characterisation of α-Fe2O3 Nanorods.

[6] Cornell, R., & Schwertmann, U. (2003). The Iron Oxides: Structure, Properties,

Reactions, Occurrences and Uses (2nd ed.). Weinheim: Wiley-VCH.

[7] Cornell, R., & Giovanoli, R. (1990). Transformation of Akaganéite into Goethite and

Hematite in Alkaline Media. Clays And Clay Minerals, 38(5), 469-476.

[8] Schwertmann, U., Friedl, J., & Stanjek, H. (1999). From Fe(III) Ions to Ferrihydrite and

then to Hematite.Journal Of Colloid And Interface Science, 209(1), 215-223.

[9] Misawa, T., Hashimoto, K., & Shimodaira, S. (1974). The mechanism of formation of

iron oxide and oxyhydroxides in aqueous solutions at room temperature. Corrosion Science,

14(2), 131-149.

[10] Lv, B., Xu, Y., Wu, D., & Sun, Y. (2011). Morphology evolution of α-Fe2O3

nanoparticles: the effect of dihydrogen phosphate anions. Crystengcomm, 13(24), 7293.

[11] Sugimoto, T., & Wang, Y. (1998). Mechanism of the Shape and Structure Control of

Monodispersed α-Fe2O3 Particles by Sulfate Ions. Journal Of Colloid And Interface Science,

207(1), 137-149.

[12] Barlow, D., Baird, J., & Su, C. (2004). Theory of the von Weimarn rules governing the

average size of crystals precipitated from a supersaturated solution. Journal Of Crystal

Growth, 264(1-3), 417-423.

[13] Schwertmann, U., & Murad, E. (1983). Effect of pH on the Formation of Goethite and

Hematite from Ferrihydrite. Clays And Clay Minerals, 31(4), 277-284.

[14] Cornell, R., & Giovanoli, R. (1985). Effect of Solution Conditions on the Proportion and

Morphology of Goethite Formed from Ferrihydrite. Clays And Clay Minerals, 33(5), 424-

432.

13

[15] Cudennec, Y., & Lecerf, A. (2006). The transformation of ferrihydrite into goethite or

hematite, revisited.Journal Of Solid State Chemistry, 179(3), 716-722.

[16] Lin, M., Tan, H., Tan, J., & Bai, S. (2013). Understanding the Growth Mechanism of α-

Fe2O3 Nanoparticles through a Controlled Shape Transformation. J. Phys. Chem. C, 117(21),

11242-11250.

[17] Jia, B., & Gao, L. (2008). Growth of Well-Defined Cubic Hematite Single Crystals:

Oriented Aggregation and Ostwald Ripening. Crystal Growth & Design, 8(4), 1372-1376.

[18] Sugimoto, T., Khan, M., & Muramatsu, A. (1993). Preparation of monodisperse peanut-

type α-Fe2O3 particles from condensed ferric hydroxide gel. Colloids And Surfaces A:

Physicochemical And Engineering Aspects, 70(2), 167-169.

[19] Ocaña, M., Morales, M., & Serna, C. (1995). The Growth Mechanism of α-Fe2O3

Ellipsoidal Particles in Solution. Journal Of Colloid And Interface Science, 171(1), 85-91.

[20] Supattarasakda, K., Petcharoen, K., Permpool, T., Sirivat, A., & Lerdwijitjarud, W.

(2013). Control of hematite nanoparticle size and shape by the chemical precipitation method.

Powder Technology, 249, 353-359.

[21] Tadic, M., Panjan, M., Damnjanovic, V., & Milosevic, I. (2014). Magnetic properties of

hematite (α-Fe2O3) nanoparticles prepared by hydrothermal synthesis method. Applied

Surface Science, 320, 183-187.

[22] Raming, T., Winnubst, A., van Kats, C., & Philipse, A. (2002). The Synthesis and

Magnetic Properties of Nanosized Hematite (α-Fe2O3) Particles. Journal Of Colloid And

Interface Science, 249(2), 346-350.

[23] Peng, D., Beysen, S., Li, Q., Sun, Y., & Yang, L. (2010). Hydrothermal synthesis of

monodisperse α-Fe2O3 hexagonal platelets. Particuology, 8(4), 386-389.

[24] Teja, A., & Koh, P. (2009). Synthesis, properties, and applications of magnetic iron

oxide nanoparticles.Progress In Crystal Growth And Characterization Of Materials, 55(1-2),

22-45.

[25] Altavilla, C., & Ciliberto, E. (2011). Inorganic nanoparticles (pp. 33-62). Boca Raton,

FL: CRC Press.

14

APPENDIX

List of Equipment used

Stirring of Mixture during addition of NaOH solution to Fe(NO3)3ᐧ 9H2O solution: Dispermat

VMA-GETZMANN GMBH d-51580 Reichshof

Scanning Electron Microscopy (SEM): ZEISS EVO MA10 Scanning Electron Microscope /

JEOL JSM-6701F Scanning Electron Microscope

X-Ray Diffraction (XRD): Rigaku Ultima IV X-Ray Diffractometer

Vibrating Sample Magnetometer (VSM): ADE Magnetic Model EV9

Crystal Structures of Hematite and Goethite

Hematite, α-Fe2O3

Hexagonal (Rhombohedral)

Space group: R3c

Goethite, α-FeOOH

Orthorhombic

Space group: Pnma

Reference: Cornell, R., & Schwertmann, U. (2003). The Iron Oxides: Structure, Properties,

Reactions, Occurrences and Uses (2nd ed.). Weinheim: Wiley-VCH.

Scherrer’s Equation

Scherrer’s equation (as given below) was used to determine the estimated average grain size

of the synthesised particles.

Where τ is the mean grain size, K is a dimensionless shape factor, with a value close to unity

(0.9), λ is the X-ray wavelength (0.154nm in this case for Cu K-α radiation), β is the full

width of the peak at half its maximum intensity (FWHM) in radians and θ is the Bragg angle.

The calculated dimensions are compared with the measured crystal size from the SEM

images and summarized in the table below:

https://en.wikipedia.org/wiki/X-rayhttps://en.wikipedia.org/wiki/X-rayhttps://en.wikipedia.org/wiki/Intensity_(physics)https://en.wikipedia.org/wiki/Full_width_at_half_maximumhttps://en.wikipedia.org/wiki/Full_width_at_half_maximumhttps://en.wikipedia.org/wiki/Radianhttps://en.wikipedia.org/wiki/Bragg_diffraction

15

Sample Measured Particle Size (nm) Calculated Grain Size (nm)

S3 593 37.1

S6 256 44.1

S7 152 176.8

S9 53 41.3

A1 66 32.2

A4 61 29.5

C1 68 53.4

C3 90 14.8

Von Weimarn Theory

The Von Weimarn theory shows the relationship between the initial supersaturation of a

metastable system undergoing a first-order phase transformation with the average size of the

aggregates of a newly evolving phase formed during nucleation in the system through the

equation below:

Where average size, is directly proportional to initial supersaturation. C0 and Ceq are the

initial and equilibrium concentrations of the solute component in the solvent at a temperature

T, KB is the Boltzmann constant, and is the difference of the chemical potential of the solute particle in the solvent and newly evolving phases.

The Von Weimarn Theory shows how high supersaturations obtained at high OH-

concentration causes burst nucleation of small nuclei as described in section 3.1.1.

Gibbs-Thompson Effect

The Gibbs-Thompson effect refers to variations in the chemical potential across a curved

surface, wherein the existence of a positive interfacial energy between different phases

increases the energy required to form small particles with high curvature, which in turn

causes these particles to possess high chemical potential. This effect is explained by the

Ostwald Freundlich equation below:

16

Where is the atomic volume, is Boltzmann constant, refers to surface tension (J

m−2

), is the equilibrium chemical potential, is the partial chemical potential and is

the absolute temperature.

The equation supports the Gibbs-Thompson effect by showing how the chemical potential of

a particle increases with a decrease in particle size and how the equilibrium solute

concentration near a small particle is higher than that near a larger one. This results in

concentration gradients that lead to the diffusion of molecular-scale species from smaller

particles to larger particles through solution, leading to the aggregation of ferrihydrite

particles as described in section 3.1.1.