The synthesis of iron sulfide nanocrystals from tris(O-alkylxanthato) iron(III) complexes

Article abstract of DOI:10.1039/c3ta12178j

Tris(O-alkylxanthato)iron(iii) complexes (alkyl = Me (1), Et (2), and ⁱBu (3)) were used as single source precursors for the synthesis of iron sulfide nanocrystals by thermolysis in oleylamine, hexadecylamine and octadecylamine at 170 to 300 °C. The morphology of the crystallites was strongly affected by the reaction conditions as well as the type of precursor used.

Full text of DOI:10.1039/c3ta12178j

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The synthesis of iron sulfide nanocrystals from
tris(O-alkylxanthato)iron(III) complexes
Cite this: J. Mater. Chem. A, 2013, 1,
8766
*
Masood Akhtar, Mohammad Azad Malik, Floriana Tuna and Paul O'Brien
i
Received 5th June 2013
Accepted 10th June 2013
Tris(O-alkylxanthato)iron(III) complexes (alkyl ¼ Me (1), Et (2), and Bu (3)) were used as single source
precursors for the synthesis of iron sulfide nanocrystals by thermolysis in oleylamine, hexadecylamine
and octadecylamine at 170 to 300 ꢀC. The morphology of the crystallites was strongly affected by the
reaction conditions as well as the type of precursor used.
DOI: 10.1039/c3ta12178j
www.rsc.org/MaterialsA
iron (FeS2),28 ash evaporation (FeS2),29 vacuum thermal evap-
oration (FeS2).30 vapour transport (FeS2),31 and chemical spray
Introduction
pyrolysis (FeS2).32 Soon et al.8 reported iron-rich FeS crystals
(Fe1+xS) by using iron(III) diethyldithiocarbamate as a precursor
by chemical vapour deposition (CVD) method. These iron-rich
FeS crystals (Fe1+xS) show interesting shape anisotropy as well
as ferromagnetic properties.
Iron containing nanocrystals are being extensively investigated
due to their potential application in: catalysis, imaging, sensors
and information storage devices.1–4 Magnetic nanocrystals of
iron sulde are of particularly important because of their use
and or potential for magnetic data storage devices and as
magnetic resonance imaging contrast agents.5–7 Iron suldes
have several phases with a complex phase diagram. The phases
include: pyrite (cubic-FeS2), greigite (cubic spinel-Fe3S4),
marcasite (calcium chloride structure-FeS2), pyrrhotite-1T
(Fe1ꢁxS), pyrrhotite-4M (Fe7S8), (Fe9S10), troilite-2H (FeS), and
mackinawite (Fe1+xS).8–11 Among these phases, pyrite phase is of
interest because of its potential application as an absorber
material for solar cells. Pyrite has a high absorption coefficient
(ꢂ105 cmꢁ1) and band gap (Eg ¼ 0.8–0.95 eV) suitable for pho-
tovolatic application; it is also a cheap and non-toxic material.12
Stoichiometric iron sulde (FeS) adopts the trollite structure,
with antiferromagnetic properties at room temperature and it
undergoes a transition into a NiAs-type structure at higher
temperature.13,14 Many pyrrhotites Fe1ꢁxS are known with
different compositions which exhibit superstructures and have
interesting magnetic and electrical properties.15,16 The magnetic
behaviour of the pyrrhotites is very sensitive to changes in
composition.17 FexS samples with x ¼ 0.87–0.88 are Weiss type
ferrimagnetics due to unbalanced ferromagnetic coupling. On
the other hand nonstochiometric Fe1ꢁxS shows a wide variety of
interesting morphologies, including whiskers like grains,18
nanowires,19 and U-shaped microslots19 and make interesting
ferromagnetic semiconductors.19,20
Meester et al.33 prepared iron disulde (FeS2) using iron(III)
acetylacetonate [Fe(acac)3], tert-butyl disulde, and hydrogen.
Single-source precursors including dithiocarbamato complexes
i
[Fe(S2CNRR')3] (R,R0 ¼ Et, Et, Me, Pr)34 and the sulfur-bridged
binuclear iron carbonyl complex [Fe2(CO)6(m-S2)]35 have been
used for the deposition of iron sulde as Fe1+xS, FeS2, and
Fe1ꢁxS thin lms. O'Brien and Vanitha36 reported the synthesis
cubane-type Fe–S cluster and thermolysed in octylamine at
180 ꢀC to give pyrrhotite (Fe7S8) and in dodecylamine at 200 ꢀC
to give greigite (Fe3S4) nanocrystals.
Gao and Han37 used tris(diethyldithiocarbamato)iron(III)
(Fe(S2CNEt2)3) and bis(diethyldithiocarbamato)Fe(II):1,10-phe-
nanthroline [Fe(S2CNEt2)2(phen) to prepare nanosheets of
Greigite (Fe3S4) and pyrrhotite (Fe7S8) by thermolysis in oleyl-
ꢀ
amine at 240–320 C. Thin lms of iron sulde were prepared
from a series of iron(III) thiobiurets complexes by Aerosol
Assisted Chemical Vapour Deposition (AACVD) method.38
Law et al.39 prepared colloidal pyrite nanocrystals by inject-
ing a solution of sulfur dissolved in diphenyl ether into a
solution of FeCl2 in octadecylamine at 220 ꢀC and stirring
for several hours. Pure phase pyrite (FeS2) nanocrystals were
synthesised from iron–oleylamine complexes with sulfur in
oleylamine. The nanodendrites or nanocubes were obtained by
adjusting the iron-oleylamine concentration.40
We reported the synthesis of iron sulde nanocrystals with
greigite, pyrrhotite and mixed phases, at different thermolysis
temperatures, from symmetrical- and unsymmetrical-dia-
lkyldithiocarbamatoiron(III) complexes. The unsymmetrical
alkyl groups gave mixed phases (greigite and pyrrhotite) iron
sulde nanocrystals at all temperatures. Whereas symmetrical
alkyl groups with longer chain alkyl groups gave pure greigite
Iron sulde thin lms have been prepared by different
techniques including atmospheric- or low-pressure metal–
organic chemical vapour deposition (AP-or LP-MOCVD),21–24
sulfurization of iron oxides (FeS2),25,26 ion beam and
reactive sputtering (FeS2),27 plasma assisted sulfurization of
The School of Chemistry and The School of Materials, The University of Manchester,
Manchester, UK. E-mail: paul.obrien@manchester.ac.uk; Fax: +44 (0)161 275 4598;
Tel: +44 (0)161 275 4653
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phase at lower thermolysis temperature of 170 ꢀC.41 We also while vigorously stirring. The mixture was then allowed to
used iron(III) complex of 1,1,5,5-tetra-iso-propyl-2-thiobiuret as return to room temperature. At this stage a freshly prepared
a single-source precursors for the synthesis of iron sulde solution of iron chloride (FeCl3) (16.76 g, 103.3 mmol) in water
nanoparticles in hot oleylamine, octadecene, or dodeca- was added drop wise to the reaction mixture by dropping fun-
nethiol.40 The thermolysis of the iron complex in oleylamine/ nel. A black precipitate started to form which was ltered aer
oleylamine produced Fe7S8 nanoparticles with different the completion of reaction. The crude prꢀoduct was washed with
morphologies (spherical, rods, or plates), depending on the water. Yield 10.8 g (28%), mp 253–255 C, elemental analysis:
growth temperature and precursor concentration.42 Most found: C, 18.8; H, 2.9; S, 50.4; Fe, 13.5%; calc. C, 19.1; H, 2.4; S,
recently we reported the use of symmetrical- and unsymmet- 51.0; Fe, 14.8%. IR (vmax/cmꢁ1): 2937(w), 1435(s), 1229(s),
rical-dialkyldithiocarbamato-iron(III) complexes as single 1162(s), 1027 (s), 939(s), mass (m/z): 355 (molecular ion peak),
source precursors and produced iron sulde thin lms by 270 (S2COCH3), 226 (Fe(S2COH3) and 164 (S2COCH3)2.
Aerosol Assisted Chemical Vapour Deposition (AACVD)
method.43
Herein, we report the use of tris(O-alkylxanthato)iron(III) Synthesis of Fe(S2COEt)3 – tris(O-ethylxanthato)iron(III) (2)
complexes as single source precursors to synthesise iron sulde
nanocrystals in oleylamine, hexadecylamine and octadecyl-
amine and investigate the effect of different reaction conditions
on the shape of nanocrystals.
Ethanol (excess) and sodium hydride (5.22 g, 217.4 mmol) were
mixed in a beaker and stirred for 2 hours. The mixture was
cooled to about 0 ꢀC in an ice bath while stirring. Then carbon
disulde (16.55 g, 217.4 mmol) was added to the reaction
mixture while vigorously stirring. The mixture was allowed to
Experimental
warm at room temperature aer addition of all carbon disul-
de. At this stage a freshly prepared solution of iron chloride
All synthesis was performed under an inert atmosphere of dry
(FeCl3) (11.75 g and 72.43 mmol) in water (50 ml) was added
nitrogen using standard Schlenk techniques. All reagents were
drop wise to the reaction mixture by dropping funnel. A black
precipitate started to form which was ltered aer the
completion of reaction. The crude product was washed with
water and dried under vacuum. Yield 8.7 g (83.4%), mp 117–
purchased from Sigma-Aldrich and used as received. Solvents
were distilled prior to use.
Elemental analysis was performed by the University of
Manchester micro-analytical laboratory. TGA measurements
118 ꢀC. Elemental analysis: found C, 23.6; H, 2.9; S, 43.5; Fe,
were carried out by a Seiko SSC/S200 model under a heating rate
15.30. Calc. C, 25.8; H, 3.6; S, 45.8; Fe, 13.3. IR (vmax/cmꢁ1):
of 10 ꢀC minꢁ1 under nitrogen. Mass spectra were recorded on a
2979(w), 1742(s), 1440(s), 1366(s), 1237 (s), 1106(s), 922(s),
Kratos concept 1S instrument. Infrared spectra were recorded
842(s), mass (m/z): 419 (molecular ion peak) 298 (S2COC2H5),
210 (FeS) and 270 (C2H5).
on a Specac single reectance ATR instrument (4000–400 cmꢁ1
,
resolution 4 cmꢁ1). Melting points were recorded on the Bar-
loworld SMP10 Melting Point Apparatus. XRD studies were
performed on a Bruker AXSD8 diffractometer using CuKa
radiation. The samples were mounted at and scanned between
20 and 70ꢀ in a step size of 0.05 with a varying count rate
depending upon the sample. TEM images were collected on
Philips CM200 transmission electron microscope using an
accelerating voltage 200 kV. TEM samples were prepared by
evaporating a drop of a dilute suspension of the sample in
toluene or hexane on carbon coated copper grid. The excess
solvent was allowed to dry completely at room temperature.
Some images were also collected on Teccnai microscope using
accelerating voltage of 300 kV.
Synthesis of Fe(S2COiBu)3 – tris(O-iButylxanthato)iron(III) (3)
Complex 3 was prepared by the same method as compound 2
i
was prepared using butanol. The crude product was washed
with water. Yield, 21.80 g (77%), mp 116–127 ꢀC. Elemental
analysis: found C, 32.5; H, 4.9; S, 36.3; Fe, 11.8, calc. C, 35; H,
5.4; S, 38.2; Fe, 11.1%. IR (vmax/cmꢁ1): 2979(w), 1738(s), 1448(s),
1348(s), 1264(s), 1144(s), 1080(s), 897(s), 793(s), mass (m/z):
503 (molecular ion peak) 447 (Fe), 298 (Fe(S2COC4H9)) and
242 (C4H9).
Synthesis of iron sulde nanocrystals
Precursor synthesis
In a typical reaction, 15 ml of oleylamine reuxed under vacuum
at 90 ꢀC for 30 minutes, then it was purged by nitrogen gas for
30 minutes. The precursor (0.5 g and 1.32 mmol) was then
added into hot oleylamine, and the reaction temperature was
slowly increased to a desired point (170, 230 and 300 C). The
temperature was maintained for one hour and the mixture was
The tris(O-alkylxanthato)iron(III) complexes were prepared by
adapting literature methods reported previously.5 A brief
description of each synthesis is given below.
ꢀ
Synthesis of Fe(S2COMe)3 – tris(O-methylxanthato)iron(III) (1)
Methanol (10 g, 312.5 mmol) and sodium hydroxide (12.50 g, allowed to cool at room temperature. Addition of 30 ml of
312.5 mmol) were mixed with water (30 ml) in a beaker and methanol produced a black precipitate which was separated by
stirred it for 2 hours. The mixture were cooled in an ice bath to centrifugation. The black residue was washed twice by meth-
ꢀ
0 C and carbon disulde (23.56 g and 312.5 mmol) in diethyl anol and redispersed in toluene or hexane for further
ether (20 ml) was added drop wise into the reaction mixture characterisations.
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Result and discussion
All three complexes are dark brown solids soluble in common
organic solvents such as acetonitrile, toluene or acetone. The
reason for the elemental analysis not matching 100% to the
calculated values of the corresponding complexes may be due
the decomposition of the complexes at lower temperatures to
give black iron sulde hence the dark colour of the samples. The
elemental analysis could not be improved even aer recrystall-
izing them several times. In fact the recrystallized samples
showed analysis worse than the non-recrystallized samples.
Thermogravimetric analysis (TGA)
Fig. 2 p-XRD pattern for predominantly greigite (Fe3S4) nanocrystals in oleyl-
amine from [Fe(S2COMe)3] (1), at (a) 170 (b) 230 and (c) 300 ꢀC. The diffraction
peaks for (220), (311), (400), (511) and (440) planes of greigite (Fe3S4) are
dominant. (#) shows the pyrrhotite (FeS) phase.
TGA of all the complexes (1–3) show two step decomposition
ꢀ
with a rapid weight loss between 92 and 162–230 C for [Fe(S2-
COMe)3] (1), 104–189–247 ꢀC for [Fe(S2COEt)3] (2) and, 58–200–
i
ꢀ
252 C for [Fe(S2CO But.)3] (3) respectively.
The solid decomposition residue amounts to 30.8% for
complex (1) which is close to the calculated value 31.8% for
FeS2. Also the solid decomposition residue amounts to 31%
for complex (2) which is also close to the calculated value
(28.64%) for FeS2. The nal residue amounts 48% for complex
(3) which is considerably less than the calculated value 59% for
Fe3S4. The TGA results are shown in Fig. 1.
Fig. 3. At 170 ꢀC (Fig. 3(a)) pure greigite (Fe3S4) was obtained but
as the growth temperature was increased from 170 to 230 to
300 ꢀC, a mixture of different phases of iron sulde was
obtained. At 230 ꢀC (Fig. 3(b)) greigite and pyrite was obtained.
Whereas at the higher growth temperature 300 ꢀC greigite,
pyrite (*) and pyrrhotite (#) were obtained (Fig. 3(c)). This
observation is consistent with the thermodynamic stability of
mixtures of pyrite and pyrrhotite over greigite.9,43
Powder X-ray diffraction of iron sulde nanocrystals
The complex (3) shows similar behaviour to the complex (2),
giving the pure phase of greigite at a growth temperature of 170
The p-XRD patterns of the nanocrystals obtained from
[Fe(S2COMe)3] (1) at different growth temperatures are shown in
Fig. 2. The growth at 170 ꢀC produced a pure greigite phase
(Fe3S4), (ICDD no.: 00-016-0713) with very low intensity peaks
(Fig. 2(a)).
The p-XRD pattern of the nanocrystals obtained at temper-
atures of 230 and 300 ꢀC from [Fe(S2COMe)3] (1), correspond to
a mixture of the cubic greigite (Fe3S4) (ICDD no.: 00-016-0713)
and hexagonal pyrrhotite (FeS) phase (ICDD no.: 00-002-1241)
(Fig. 2(b and c)).
ꢀ
and 230 C (Fig. 4(a and b)) but at the growth temperature of
ꢀ
300 C mixture of greigite, pyrite (*) and pyrrhotite (#) phases
were obtained (Fig. 4(c)). The diffraction peaks (Fig. 4(c)) for
(220), (311), (400), (422), (511) and (440) planes of greigite
(Fe3S4) were dominant as compared to other phases.
Transmission electron microscopy of iron sulde nanocrystals
TEM images of the iron sulde nanocrystal from (1) in oleyl-
amine at 230 and 300 ꢀC are shown in Fig. 5(a and b). The
The p-XRD patterns obtained for the samples from ther-
molysis of complex (2) at different temperatures are shown in
Fig. 3 p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from
[Fe(S2COEt)3] (2) at (a) 170 (b) 230 and (c) 300 ꢀC. The diffraction peaks for (220),
(311), (400), (511) and (440) planes of greigite (Fe3S4) are dominant. The (*)
shows pyrite and (#) pyrrhotite (FeS) phases.
Fig.
[Fe(S2COEt)3] (2), and [Fe(S2COiBu)3] (3) at heating rate of 10 ꢀC minꢁ1 under
nitrogen, flow rate 100 ml minꢁ1
1
Thermogravimetric analysis of complexes [Fe(S2COMe)3] (1),
.
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ranging from 14 nm to 75 nm in length and from 12 to 48 nm in
width (approximately). HRTEM image (Fig. 6(b)) show clear
˚
lattice fringes with a d-spacing of 2.98 A corresponding to (311)
reection plane of greigite phase.
Fig. 7(a) shows the TEM image of iron sulde nanocrystals
ꢀ
obtained with mixed morphologies from (2) at 230 C in oleyl-
amine. Lattice fringes in HRTEM image (Fig. 7(b)) with
˚
d-spacing 5.72 A correspond to (111) plane of greigite phase.
Almost similar shapes of nanocrystals are obtained at 300 ꢀC
(Fig. 7(c)). The SAED pattern (Fig. 7(d)) shows that the nano-
crystals are highly crystalline material and single crystal in
nature.
Fig. 8(a and b) shows the TEM images of nanocrystals grown
at 170 and 230 ꢀC from (3). The HRTEM (Fig. 8(c)) at 230 ꢀC
˚
showing lattice fringes with a d-spacing of 3.50 A corresponding
Fig. 4 p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from
[Fe(S2COiBu)3] (3) at (a) 170 (b) 230 and (c) 300 ꢀC. The peaks for (220), (311),
(400), (511) and (440) of planes are of greigite (Fe3S4) phase. The (*) shows pyrite
and (#) pyrrhotite (FeS) phase.
to the (220) reection of greigite phase. TEM image of nano-
crystals at 300 C (Fig. 8(d)) shows the different shapes (sheets
ꢀ
and pyramids) of crystallites. At lower temperature of 170 ꢀC
complex (3) produced large size (139 nm in length and 65 nm in
width) crystallites (Fig. 8(a)). Small size crystallites was
produced at higher temperature of 300 ꢀC (Fig. 8(d)). For these
nanocrystals size ranging from 25 nm to 75 nm in length and 12
to 38 in width. The broad peaks in p-XRD pattern at higher
temperature also support the evidence of small size crystallites.
The effect of different capping agents
Complexes (1–3) were also thermolysed in hexadecylamine and
ꢀ
octadecylamine at 230 C to see if the capping agent had any
effect on the nature of the growth of iron sulde phases.
Signicant differences were noticed from the thermolysis of the
complexes (1–3) in hexadecylamine at 230 C for 1 hour.
Fig. 5 Shows the TEM images of iron sulfide nanocrystals obtained from
[Fe(S2COMe)3] (1), in oleylamine at (a) 230 ꢀC and (b) 300 ꢀC.
ꢀ
The diffraction pattern for complexes (1) and (2) shows peaks
for (111), (200), (210), (211), (220) and (311) planes corre-
sponding to pure pyrite phase (Fig. 9(a and b)) (ICDD no.:
nanocrystals obtained show the mixed morphologies (poly-
hedra) at all temperatures. The inset in (Fig. 5(b)) shows a single
hexagonal nanocrystal obtained at 300 ꢀC from (1). The
approximate size of the crystallites obtained from (1) in oleyl-
amine ranging from 30 nm to 124 nm in length and 21 nm to
52 nm in width.
The iron sulde nanocrystals from [Fe(S2COEt)3] (2) at 170 ꢀC
also show different sizes and shapes (Fig. 6(a)). These nano-
crystal are relatively small in size as compared to the nano-
crystals obtained from complex (1). For the nanocrystals the size
Fig. 6 TEM images of iron sulfide nanoparticles obtained from thermolysis of
[Fe(S2COEt)3] (2), in oleylamine at 170 ꢀC. (a) Shows the different shapes of
Fig. 7 Shows the TEM images of iron sulfide nanocrystals obtained from
[Fe(S2COEt)3] (2) in oleylamine at (a and b) 230 ꢀC and (c) 300 ꢀC. The insets in (c)
show different shape of nanocrystals at 300 ꢀC, whereas (d) shows the SAED
pattern of the nanocrystals.
˚
nanocrystals, (b) HRTEM image showing lattice fringes with a d-spacing of 2.98 A
corresponding to the (311) plane reflection of greigite.
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These sheets like crystallites have a size range between
13.6 nm and 31 nm in length and 8 nm to 26 nm in width.
Sheets and cube like crystallites (Fig. 10(c)) were obtained from
the thrombolysis of complex (3) in hexadecylamine at 230 ꢀC in
1 hour reaction time. These nanocrystals have almost same size
as the nanocrystals from complex (1) and (2) obtained in hex-
adecylamine at same temperature.
The p-XRD pattern of the nanocrystals produced from the
thermolysis of complexes (1–3) in octadecylamine at 230 ꢀC
show that the complex (1) produced predominantly greigite
phase (ICDD no.: 00-016-0713) (Fig. 11(a)) with traces of pyrite
Fig.
8
Shows the TEM images of iron sulfide nanocrystals obtained from
[Fe(S2COiBu)3] (3), in oleylamine (a) at 170 ꢀC, (b) 230 ꢀC (c) HRTEM at 230 ꢀC
˚
showing lattice fringes with a d-spacing of 3.50 (A) corresponding to the (220)
reflection plane of greigite phase (d) TEM image of nanocrystals produced at 300 ꢀC.
00-042-1340) at 230 ꢀC in hexadecylamine. The complex (3)
produced pure pyrrhotite phase (ICDD no.: 00-002-1241)
(Fig. 9(c)). The broad peaks in the p-XRD pattern of the iron
sulde crystallites obtained from all three complexes in hex-
adecylamie indicates the small size of the crystallites.
TEM images of the nanocrystals obtained from complex (1)
in hexadecylamine at 230 ꢀC show a mixture of irregular and
rectangular crystallites (inset Fig. 10(a)). The size of these
nanocrystals are considerably smaller than the crystallites
produced by the same complex in oleylamine at the same
temperature of 230 ꢀC. The size of the crystallites ranging from
12 nm to 30 nm in length and 7 nm to 22 nm in width.
The complex (2) in hexadecylamine at 230 ꢀC in 1 hour
reaction time produced plate like crystallites (Fig. 10(b)). The
insets in (Fig. 10(b)) show a sheet like crystallites.
Fig. 10 Nanocrystals obtained from (a) complex (1) in hexadecylamine at
230 ꢀC, insets shows individual nanocrystals, (b) TEM images from complex (2) in
hexadecylamine, insets shows sheets like crystallites and (c) shows nanocrystals
obtained from complex (3) in hexadecylamine at 230 ꢀC.
Fig. 9 p-XRD pattern from complexes (1–3) in hexadecylamine at 230 ꢀC for 1 hour
reaction time, (a) and (b) show pattern for pyrite phase obtained from complexes
(1–2), (c) shows a pattern for pyrrhotite phase obtained from complex (3).
Fig. 11 p-XRD (a) from complex (1), (b) from complex (2), (c) from complex (3) in
octadecylamine at 230 ꢀC after 1 hour. The (*) shows the pyrite phase and (#)
shows pyrrhotite phase.
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(*). Complex (2) produced a mixture of greigite, pyrite and
pyrrhotite phases (Fig. 11(b)). Complex (3) produced a pure
pyrrhotite phase (ICDD no.: 00-002-1241).
The TEM images of the nanocrystals obtained from
complexes (1–3) in octadecylamine aer 1 hour reaction time
(Fig. 12(a–c)) show no well-dened shapes of the crystallites.
Law et al.39 reported that longer growth times in octadecylamine
produced pure phase material but our experiments for 3 hours
growth time showed no change in the phase of material. The
only difference was the larger crystallites and hence the sharper
p-XRD peaks (Fig. 13(a–c)). TEM images in Fig. 14 show the
larger size of crystallites as compared to those produced aer 1
hour. The approximate size ranging from 18 nm to 100 nm in
length and 10 to 80 nm in width.
Fig. 14 Nanocrystals obtained from (a) complex (2) in Octadecylamine after 3
hours at 230 ꢀC, (b) shows the lattice fringes correspond to greigite phase
obtained from complex (2).
Table 1 shows the summary of the different phases of iron
sulde nanocrystals produced from all three complexes under
various reaction conditions.
The magnetic properties of iron sulde nanocrystals
Changes in magnetisation across the samples were measured
by SQUID magnetometry and are illustrated in Fig. 15 and 16.
Magnetic characterisation reveals that all iron sulde nano-
crystals prepared from the thermolysis of complexes (1) and (3)
at various temperatures show room temperature magnetic
hysteresis loops with magnetization well saturated under
applied magnetic elds of 10–15 kOe (1 Oe ¼ 10ꢁ4 T).
It is known that magnetic properties of nanocrystals are
governed by the anisotropy energy that constrains the
nanocrystals magnetisation to align along a specic direction
known as the easy axis. The temperature above which
thermal energy inside a magnetic system is high enough to
enable the free alignment of the magnetisation in arbitrary
directions is called the blocking temperature (TB), and is
typically obtained from peaks in the zero-eld cooled (ZFC)
and eld-cooled (FC) magnetisation curves recorded at very
small magnetic elds. The blocking temperature is graphi-
cally determined to be the point at which the gradient of the
ZFC curve approaches zero.
Fig. 12 TEM images of the nanocrystals obtained from complex (1–3) in octa-
decylamine at 230 ꢀC after 1 hour (a) from (1), (b) from (2) and (c) from (3).
It can be seen that no peaks can be discerned in the ZFC–FC
magnetisation curves recorded at 100 Oe for nanocrystals
generated from (1) and (3) (Fig. 15(a) and 16(a)), suggesting that
the blocking temperature of these nanocrystals is well above the
room temperature. Indeed, the eld dependence of the mag-
netisation reveals hysteresis loops at 5, 50 and 300 K (Fig. 15(b)
and 16(b)) for all nanocrystals, suggesting that all of them are
already in their blocking regime at room temperature. In the
case of the iron sulde nanocrystals obtained by thermolysis of
complex (1) in oleylamine at 230 ꢀC, a magnetic hysteresis curve
with a coercive eld (Hc) (i.e. the magnetic eld needed for the
magnetization to return to zero) of ca. 230 Oe at 300 K,
increasing to 520 Oe at 50 K and to 590 Oe at 5 K, and a remnant
magnetization (Mr) (i.e. the magnetization retained by nano-
crystals when the magnetic eld is switched off) of 7.85 emu gꢁ1
at 300 K, 12.53 emu gꢁ1 at 50 K and 13.39 emu gꢁ1 at 5 K is
obtained. This result is in contrast with our previous results on
nanocrystals of Fe3S4 and Fe7S8, which shows super-
paramagnetism at room temperature.39
Fig. 13 p-XRD (a) from complex (1). (b) From complex (2) and (c) from complex
(3) in octadecylamine at 230 ꢀC in 3 hours reaction time. The (*) shows pyrite and
(#) shows pyrrhotite phase.
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Table 1 Summary of phases and morphology of iron sulfide nanocrystals obtained from all four precursors at different reaction conditions
Complex
Capping agent
Temp. (ꢀC)
Phase
Morphology of crystallites
[Fe(S2COMe)3]
Oleylamine
Oleylamine
Oleylamine
Hexadecylamine
Octadecylamine
Oleylamine
Oleylamine
Oleylamine
Hexadecylamine
Octadecylamine
Oleylamine
Oleylamine
Oleylamine
170
230
300
230
230
170
230
300
230
230
170
230
300
230
230
Greigite
Hexagonal, cubic and trigonal
Greigite + pyrrhotite
Greigite + pyrrhotite
Pyrite
Greigite + pyrite
Greigite
Greigite + pyrite
Greigite + pyrite + pyrrhotite
Pyrite
Greigite + pyrite + pyrrhotite
Greigite
Rectangular
Irregular
Hexagonal and trigonal
[Fe(S2COEt)3]
[Fe(S2COiBu)3]
Plates
Irregular
Sheets, cubes, hexagonal and trigonal
Greigite
Greigite + pyrite + pyrrhotite
Pyrrhotite
Hexadecylamine
Octadecylamine
Sheets and cubes
Irregular
Pyrrhotite
Fig. 15 (a) ZCF and FC (b) at 5 K, 50 K and at 300 K from iron sulfide (greigite
phase dominant with pyrrhotite phase) nanocrystals obtained from
[Fe(S2COMe)3] (1) in oleylamine at 230 ꢀC.
Fig. 16 (a) ZFC and cold field magnetization curves. (b) Room temperature
magnetic hysteresis loops of iron sulfide (greigite phase) nanocrystals at 5 K, 50 K
and 300 K obtained from [Fe(S2COiBut.)3] (3) in oleylamine at 230 ꢀC.
The remanent magnetisation of 7.85 emu gꢁ1 at room
temperature, is signicantly larger than that reported in liter-
ature for Fe1ꢁxS (pyrrhotite) nanopillars (2.5 emu gꢁ1 at 300 K),5
Comparison to pyrrhotite Fe7S8 and greigite Fe3S4 nano-
and slightly larger than our reported value of Mr ¼ 6.5 emu gꢁ1 sheets34 prepared from Fe(S2CNEt2)2(Phen) and Fe(S2CNEt2)3 in
(300 K).39 This difference may originate from the difference in oleylamine shows that the nanocrystals reported here have a
the shape of the nanocrystals and small variation in their FeS signicantly larger magnetisation saturation (MS ¼ 35.8 emu
content.
gꢁ1 at 300 K; 37.6 emu gꢁ1 at 5 K) as compared to 18.9 emu gꢁ1
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Journal of Materials Chemistry A
for the former. The remanence-to-saturation ratio R ¼ Mr/Ms
(R ¼ 0.22–0.36) is also larger, suggesting an either larger
anisotropy energy or a higher degree of crystallinity.
The remanence to saturation ratio R ¼ Mr/Ms ¼ 0.43
(14.78/34.46), 0.40 (13.9/34.52) and 0.30 (9.65/32.44) at 5, 50
and 300 K, while the coercive eld increases on cooling from
334 Oe at 300 K to 680 Oe at 50 K and 770 Oe at 5 K. Improved
magnetic properties were also obtained for nanocrystals
obtained from complex (3) in oleylamine at 230 ꢀC
(Fig. 16(b)).
Acknowledgements
M.A. Thanks the EPSRC for funding. POB wrote this manuscript
while a Visiting Fellow at IAS University of Durham. He would
like to thank the University for the Fellowship and Collingwood
College and its Fellows for being gracious hosts.
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