Chapter 1
1
CHAPTER 1
Existing information on covellite Copper Sulphide
(CuS)
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
1.1
2
Introduction
Materials science plays a vital role in this modern age of science and
technology. Various kinds of materials are used in industries, housing, agricultures,
transportations, etc. to meet the plants and individual requirements. The rapid
developments in the field of quantum theory of solids have opened vast opportunities
for better understanding and utilization of various materials. The spectacular success
in the field of space is primarily due to the rapid advances in high-temperature and
high-strength materials. In the 21st century many researchers are working on various
kinds of materials for technology application such as metal oxides, metal
chalcogenides, polymers, organic dyes, etc. Among them transition metal
chalcogenides (TMCs) semiconductor materials play an important role in the solar
cell and other technological applications.
Transition metal chalcogenides (TMCs) occur with many stoichiometry and
many structures. The most common and the most important, from the point of view of
technological applications are the chalcogenides having simple stoichiometry, such as
1:1 and 1:2. Extreme cases include metal-rich phases (e.g. Sn2S, Cu2S, Ta2S), which
exhibit extensive metal-metal bonding [1] and chalcogenide-rich materials such as
Re2S7, WS2, WSe2, MoSe2, MoS2, TaSe2, TaS2, etc. which features extensive
chalcogen-chalcogen bonding. For the purpose of classifying these materials, the
chalcogenide is often viewed as a di-anion, i.e., S2-, Se2- and Te2-. In fact, transition
metal
chalcogenides are highly covalent, not
ionic, as
indicated by their
semiconducting properties.
Metal monochalcogenides have the formula MX, where M = a transition metal
such as Mn, Fe, Co, Ni, Cu, Zn, etc. and X= S, Se, Te. They typically crystallize in
one of two motifs, named after the corresponding forms of zinc sulphide. In the zinc
blende structure, the sulphide atoms pack in a cubic symmetry and the Zn2+ ions
occupy half of the tetrahedral holes. The result is a diamondoid framework. The main
alternative structure for the monochalcogenides is the wurtzite structure wherein the
atoms connectivity is similar to tetrahedral, but the crystal symmetry is hexagonal. A
third motif for metal monochalcogenide is the nickel arsenide lattice, where the metal
and chalcogenide each have octahedral and trigonal prismatic coordination,
respectively. This motif is commonly subject to non-stoichiometry.
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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Important monochalcogenides include some pigments, notably cadmium
sulphide. Many minerals and ores are monosulfides [2] like CuS, ZnS, CdS, NiS,
MnS, CoS, etc. They are the members of the transition metal chalcogenides. These
materials have attracted increasing attention in recent years due to their excellent
physical and chemical properties [3-5]. Among all semiconductor materials, copper
sulphide (CuS) is an important IB-VIA semiconductor material.
Copper is a transition metal of group IB, occupy column 11 of the periodic
table. It has orthorhombic crystalline structure. Copper (Cu); atomic weight
63.546(3); atomic number 29; freezing point 1084.62 °C; boiling point 2562 C;
density 8.96 gm.cm-3 (20C); valence 1 or 2. The discovery of copper dates back to
prehistoric time. It is said to have been mined for more than 5000 years. It is one of
man‟s most important metal. Copper is reddish in colour, takes on a bright metallic
luster, and is malleable, ductile, and a good conductor of heat and electricity (second
only to silver in electrical conductivity). The electrical industry is one of the greatest
user of copper. Copper occasionally occurs native, and is found in many minerals
such as cuprite, malachite, azurite, chalcopyrite, and bornite. The most important
copper ores are the sulphides, oxides, and carbonates. From these, copper is obtained
by smelting, leaching, and by electrolysis. Its alloys, brass and bronze, long used, are
still very important; all American coins are now copper alloys; monel and gun metals
also contain copper. The most important compounds are the oxide and the sulphate,
blue vitriol; the latter has wide use as an agricultural poison and as an algicide in
water purification. Copper compounds such as Fehling‟s solution are widely used in
analytical chemistry in tests for sugar. High-purity copper (99.999) is readily available
commercially. The price of commercial copper has fluctuated widely. Natural copper
contains two isotopes. Twenty-six other radioactive isotopes and isomers are known
[6].
Sulphur is the one of the chalcogenides, or “ore-formers”, oxygen (O), sulphur
(S), selenium (Se) and tellurium (Te), which occupies the group VIA of the periodic
table. Sulphur (S); atomic weight 32.066(6); atomic number 16; melting point
115.21C; boiling point 444.60C; tc (critical temperature) 1041C; density (rhombic)
2.07gm.cm-3, (monoclinic) 1.957gm.cm-3 (20°C); valence 2, 4, or 6. Sulphur is a pale
yellow, odourless, brittle solid, which is insoluble in water but soluble in carbon
disulfide. In every state, whether gas, liquid or solid, elemental sulphur occurs in
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
4
more than one allotropic forms or modification; these present a confusing multitude of
forms whose relations are not yet fully understood. Amorphous or “plastic” sulphur is
obtained by fast cooling of the crystalline form. X-ray studies indicate that amorphous
sulphur may have a helical structure with eight atoms per spiral. Crystalline sulphur
seems to be made of rings, each containing eight sulphur atoms, which fit together to
give a normal X-ray pattern. Twenty-one isotopes of sulphur are now recognized.
Four occur in natural forms, none of which is radioactive. A finely divided form of
sulphur, known as flowers of sulphur, is obtained by sublimation. Sulphur readily
forms sulphides with many elements. Sulphur is a component of black gunpowder,
and is used in the vulcanization of natural rubber and as fungicide. It is also used
extensively in making phosphate fertilizers. A tremendous tonnage is used to produce
sulphuric acid, the most important manufactured chemical. It is used in making
sulphite paper and other papers, as a fumigant, and in the bleaching of dried fruits.
The element is a good electrical insulator. Organic compounds containing sulphur are
very important. The material has unusual optical and electrical properties [6].
In transition metal chalcogenides (TMCs), copper sulphide (CuS) is binary
chemical compound of the elements copper and sulphur. It occurs in the nature as the
dark indigo blue mineral. Copper sulphide vary widely in the composition with 0.5 ≤
Cu/S ≤ 2, including numerous non stoichiometric compounds with the formula CuxSy
such as CuS2 –Villamanitite [7], CuS-Covellite [7], Cu9S8 (Cu1.12S) - Yarrowite[8],
Cu39S28 (Cu1.39S) - Spionkopite [8], Cu8S5 (Cu1.6S)- Geerite [9], Cu7S4 (Cu1.75S) Anilite [7], Cu9S5 (Cu1.8S) - Dignenite [7], Cu31S16 (Cu1.96S) - Djurleite [7], and Cu2S
- Chalcocite [7]. In addition to the technological interest, copper sulphide is an
important material from the point of view of fundamental research. Because of the
effect of the 3d electrons, this transition-metal compound has the ability to form
various stoichiometries, of which at least five phases are stable at room temperature. It
is a promising material with potential application in Lithium ion rechargeable
batteries [10], gas sensors [11], photovoltaic applications [12] and catalysts [13].
Copper monosulphide crystallize in the hexagonal crystal system in the form
of the mineral covellite [14-16] and whilst these studies are in general agreement on
assigning the space group P63/mmc, there are small discrepancies in the bond lengths
and angles between them. The structure was described as “extraordinary” by Wells [7]
and is quite different from copper (II) oxide but similar to copper selenide (CuSe)
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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(Kockmannite). The crystal structure of CuS has been studied under applied
hydrostatic pressure up to 33 kbar (Takeuchi et al., 1985). The main effect on the
crystal structure is a considerable increase of the S-S distances, whereas the Cu-S
separations are correspondingly shortened.
The covellite unit cell contains 6 formula units (12 atoms) in which:

4 Cu atoms have tetrahedral coordination (Figure 1).

2 Cu atoms have trigonal planar coordination (Figure 2 a-b).

The two pairs of S atoms are only 2.07 Å apart indicating the existence of an S-S
bond (a disulfide unit).

The remaining two S atoms form trigonal planar triangles around the copper
atoms, and are surrounded by five Cu atoms in a pentagonal bipyramid
(Figure 2 c).

The S atoms at each end of a disulfide unit are tetrahedrally coordinated to 3
tetrahedrally coordinated Cu atoms and the other S atom in the disulfide unit
(Figure 2d)
Figure 1 One ball-and-stick model of the crystal structure of covellite.
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
Figure 2
(a)
(b)
(c)
(d)
(a)
(b)
6
(c)
(d)
trigonal coordination of copper
tetrahedral coordination of copper
trigonal bipyramindal coordination of sulphur
tetrahedral coordination of sulphur-note disulfide unit
Figure 3 and 4 present a phase diagram of the Cu-S system though some changes
have been proposed by D. J Chakraborti et al. and R. Blachnik et al. [17, 18]. This
phase diagram shows the wide diversity of compound composition and structural
phases that have been found in the system. Also some basic properties of copper
sulphide are listed in Table 1.
Figure 3 The Cu-S equilibrium phase diagram [17].
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
7
Figure 4 Temperature ranges of phases and compounds during reactions in powders
of (2Cu+S) according to the Cu-S phase diagram [18].
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
8
Table 1 Basic Properties of Copper Sulphide.
Nos.
Basic Properties of Mineral Copper Sulphide
1
Formula
CuS
2
Molar Mass
95.62 g.mol-1 [6]
3
Colour
Indigo-blue
4
Density
4.76 gm.cm-3 [6]
5
Crystal Structure
Hexagonal [6]
6
Space group
P63/mmc [6]
7
Unit cell
8
Bond length
a =3.79 Ǻ,
c=16.34 Ǻ, Z=6 [6]
Cu-Cu =2.19 Ǻ
S-S =2.07 Ǻ
Cu-S= 2.30 Ǻ [16]
9
Melting Point
10
Solubility
11
Refractive Index
12
Magnetic susceptibility (ᵡm)
13
Hardness
14
Thermodynamic Parameters
 Standard molar enthalpy (heat) of formation at 298.15
K (∆H)
 Standard molar Gibbs energy of formation at 298.15 K
(∆G)
 Standard molar entropy at 298.15 K (S)
 Molar heat capacity(CP) at constant pressure at 298.15
K
15
16
17
Solubility Product Constant
Superconductivity
Electric conductivity
transition 507 ᵒC
[6]
Soluble=HNO3,
NH4OH, HCN
Insoluble=HCl,
H2SO4 [6]
1.45 [6]
- 2.0 ×10-6
cm3 mol-1[6]
1.8 Mohs Scale [6]
∆H=-53.10
kJ.mol-1
∆G=-53.60
kJ.mol-1
S= 66.50
J.mol-1 K-1
CP= 47.8
J.mol-1 K-1 [6]
ĸsp= 6×10-16 [6]
1.6 K [19]
Metallic
hole
conduction [20]
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
1.2
9
Single Crystals
The subject of single crystal growth has held a high level of interest both
scientifically and technologically since long time. Nearly all basic solid materials of
modern technology are made of crystals. Hence an understanding of how crystals are
grown and study of their properties are important aspect of the science of materials.
Crystals are the most important constituent in the modern technology. Without
materials in the crystal forms, electronic industry, photonic industry and fibre optic
communication would have been not possible. The crystals used in these sectors are
semiconductor, metal, insulator, superconductor, non-linear, magnetic, etc. materials.
Crystal growth is an interdisciplinary topic covering physics, chemistry, materials
science, chemical engineering, metallurgy, crystallography, mineralogy, etc. In the
past few decades, most of the focus is on crystal growth processes due to increasing
demand of materials for technology application. It is very difficult to grow single
crystal materials compare to the polycrystalline materials because single crystals are
regular and repeated periodic arrangement of atom in three dimensions. The effects of
grain boundaries in single crystals are responsible for the important changes in
physical, optical and electrical properties. The main significance is the anisotropy,
uniformity of composition and the absence of boundaries between individual grains,
which are certainly present in polycrystalline materials. Single crystals play important
role in the optoelectronic devices but to achieve high performance from the
optoelectronic devices, good quality single crystals are needed. Growth of single
crystals and their characterization towards device fabrication have assumed great
movement due to their importance for both academic as well as applied research field.
In the past few years, studies of materials with layered structures such as
graphite [21], transition metal chalcogenides/ dichalcogenides [22, 23], metal
oxychlorides [24], clay minerals [25-27] and A2X3-M2X3-M‟X (A = Ga, In; M =
trivalent metal; M̕ = divalent metal; X=S, Se) [28, 29] etc. have received increasing
attention. But among all these, most of the studies have been focused on transition
metal chalcogenides/ dichalcogenides. Reason being they are layered semiconductors
having anisotropic and corrosion resistive properties.
The transition metal chalcogenides (TMCs) has general formula MX, where M
is transition metal (M = Zn, Cd, Cu) from IB to VIII B group of periodic table and X
(S, Se, Te) is one of the chalcogen. This makes the material extremely interesting,
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
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because within layer, the bonds are strong while between the layers remarkably weak.
Some of the materials are ZnS, ZnSe, ZnTe, CdS, CdSe, CuS, CuSe, WS2, WSe2,
MoS2, MoSe2, etc. having unique and unusual properties based on the extreme
degrees of anisotropy in their structure [30, 31].
They find application as high pressure-high temperature lubricants, catalysts,
as electrode material for solar energy conversion purposes and in the development of
primary and secondary batteries [32-34]. It is well established that physical properties
of materials in single crystal forms are largely influenced by the nature and extent of
the defects present in their atomic arrangements [35-39]. Prominent among these
defects are crystallite size, strain, dislocation, stacking fault and different layer
disorder parameters in case of layered compounds and their combination. These
defects develop partly during their growth as crystal and partly during the mechanical
and thermal treatments, which the sample are subjected to. Small concentration of
these defects gives rise to striking changes in various properties of the materials.
Electrical and thermal conduction are controlled by scattering of electrons and
phonons by defects. Localized energy levels, which lie in the energy bandgap between
the valence and conduction bands and which arise due to impurities are responsible
for the electrical properties of the semiconductors. The strength of materials is found
to dependent on the size and angular misorientation, stacking faults [40], etc.
1.2.1
Crystal Growth methods
Growth of crystal ranges from a low cost technique to a complex sophisticated
expensive procedure and crystallization time ranges from minutes, hours, days to
months. Single crystals may be produced by the transport of crystal constituents in the
solid, liquid or vapour phase [41]. On the basis of this, crystal growth may be
classified into following categories given below,
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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Single Crystal
The techniques for crystal growth are not limited to the one presented above. Small changes
in the growth parameters, variation in the procedure, etc. gives rise to different growth
techniques.
1.2.2
Literature Survey of bulk CuS Single Crystal growth
1. CuS single crystals of size 1.5 × 1.5 × 0.1 mm3 were grown by a high-temperature
solution growth method, using the KCl–LiCl eutectic as solvent [42]. The starting
materials were Cu (Goodfellow, 99.99+%), S (Aldrich, 99.99+%), KCl (Merck,
99.5+%) and LiCl (BDH, 99.5+%). KCl and LiCl were dried at 200 ᵒC under
vacuum for 2 h, before being used. The eutectic composition was prepared from a
mixture of KCl and LiCl with a 42:58 molar ratio, which was heated up to 650 ᵒC
inside a quartz ampoule sealed under vacuum. The elemental constituents, with a
Cu:S ratio of 1:1.01, were sealed inside a quartz ampoule (10mm of inner
diameter and 100mm in length) together with the eutectic mixture, under a
vacuum atmosphere of 10-5 mbar. The proportion between the (Cu+S) mixture and
the (KCl–LiCl) eutectic was 1:60. The ampoule was heated up to an average
temperature of 480 ᵒC and held at this temperature for 170 h. A temperature
gradient of 10 ᵒC was applied between the top (hot junction) and the bottom (cold
junction) in order to minimize the S evaporation from the solution. The ampoule
was then cooled down to 400ᵒC at 2ᵒC.min-1 maintaining at this temperature for 5 h
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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before being removed from the furnace. After cooling down to room temperature
the ampoule was broken and the mixture was removed. The eutectic was removed
from the CuS crystals by washing the mixture several times with de-ionized water.
Surface microtopographic studies of the crystals indicated that the growth was
made by the lateral spreading of the layers. Electrical resistivity measurements
clearly show an anomaly at T~55 K, which was related to the low-temperature
structural transition. Also showed high residual resistivity ratio of ~400 with a
sharp superconducting transition at T~1.7K confirming the very good quality of
the crystals.
2. H. J. Scheel [43] has grown CuS single crystal by using sodium polysulfide
fluxes. The only disadvantage was the grown single crystals have 450 ppm of
sodium as impurity. The structural characterization showed that as grown crystal
had hexagonal structure with space group P63/mmc and match with the standard
ASTM No. 6-464.
3. The CuS samples were prepared by standard solid state reaction, mixing Cu and S
in a 1:1 M ratio. In the case of CuS, the mixture was pressed in the form of
rectangular bars that were placed in an alumina finger, sealed in a silica tube under
vacuum and heated up to 800 ᵒC for 24 h with an intermediate heating at 600 ᵒC
for 12 h. After that, the mixture was pulverized and pressed in the form of
rectangular bars which were heated in alumina/ silica tubes at a lower temperature
of 600 ᵒC for 24 h. Finally, the samples were slowly cooled down to room
temperature [44] to give single crystals. The investigation of these crystals
exhibited a sharp diamagnetic transition and resistivity drop around 40K.
1.2.3
Properties of bulk CuS Single Crystals
Nos.
1
2
3
4
Structural
Optical
Thermal stability
Electrical
5
Magnetism
Properties
Hexagonal , Space group: P63/mmc [43]
Indirect bandgap:1.21 eV [45]
Decompose at 507±2 ᵒC [46]
Resistivity: 210 μΩ•cm (at room temperature) [42,47]
p-type metallic conduction[20]
Diamagnetic [48]
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
1.2.4
13
Application of CuS Single Crystals
The most common and the potential application of CuS in single crystals form
are as below. It is not limited to these applications only.
1
Cathode materials in lithium rechargeable batteries [49].
2
High temperature superconductors [50].
3
Thermo- and photoelectric transformers and high temperature thermistors [51].
1.3
Thin Films
The technology of thin films deposition has advanced significantly during the
past few decades. This development was driven primarily by the requirements for new
products and devices in the electronics and optical industries. The rapid progress in
solid-state electronic devices would not have been possible without the improvement
of new thin film deposition processes, improved film characteristics and superior film
qualities. Thin film deposition technology is still undergoing speedy changes which
will lead to even more complex and advanced electronic devices in future.
The phenomenal rise in thin film researches is, no doubt, due to their extensive
application in the diverse field of electronics, optics, space science, aircrafts, defence
and other industries. These investigations have led to numerous invention in the forms
of active devices and passive components, piezo-electric devices, microminiaturisation of power supply, rectification and amplification, sensor element,
storage of solar energy and its conversation to other forms, magnetic memories,
superconducting films, interference filters, reflecting and antireflection coating and
many others. The present development trend is towards newer types of devices,
monolithic and hybrid circuits, field effect transistors (FET), metal oxide
semiconductor transistor (MOST), sensors for different applications, switching
devices, cryogenic applications, high density memory systems for computers, etc.
Further, because of compactness, better performance and reliability coupled with the
low cost of production and low package weight, thin films devices and components
are preferred over their bulk counterparts. There has been a phenomenal increase in
their applications which have outpaced the technology of production, development of
new types of materials and better processes for semiconducting, dielectric and other
films needed by various industries. Intensive investigation are going on not only in the
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
14
field of basic thin films physics, but also in the materials science, thin films circuit
designs, production engineering concerning thin films, etc. to cope up the demand of
industries. Film properties are also sensitive not only to their structures but also to
many other parameters including their thickness especially in the thin films regions.
Hence a stringent control of the latter is imperative for reproducible electronics,
dielectric, optical and other properties [52- 55].
1.3.1
Deposition Methods
A solid material is said to be in thin film form when it is grown as a thin layer
on a solid substrate by controlled condensation of the individual atomic, molecular, or
ionic species either by physical process or chemical reactions. There are many dozens
of deposition techniques for materials formation in thin films form [56, 57]. Since, the
concern here is with thin-film deposition methods for forming layers in the thickness
range of a few nano-meters to about tens of micrometers, the task of classifying the
techniques is made simpler by limiting the number of techniques to be considered.
Basically, thin-film deposition techniques are either purely physical, such as
evaporative methods, or purely chemical, such as gas- and liquid-phase chemical
processes. Thin films deposition techniques are broadly classified under two heading
as listed in below flowchart.
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
1.3.2
16
Literature Survey of the last two decades on CuS Thin Films deposition
1. Physical vapour deposition method [58] was used for synthesis of CuS films.
In this method constituent elements (Cu and S, both with 99.999% purity) was
evaporated on soda-lime glass substrates (50 mm×50 mm×2 mm) by thermal
co-evaporation. A self designed evaporation chamber was used for this
purpose. The substrate temperature was kept constant at 450 ᵒC during
deposition. Achievement of constant temperature from ambient temperature
was obtained within 25 min by using a combination of halogen lamps placed
inside of the chamber and measured by thermocouples. Films were deposited
to cover a broad thickness range having values of 100, 150, 200, 225 and 250
nm. The deposited thin films were studied in details.
2. Highly oriented crystalline film of copper sulfide (CuS) have been grown on
glass substrates by low-pressure metal-organic chemical vapor deposition (LPMOCVD) and by aerosol assisted chemical vapour deposition (AACVD)
using
the
novel
air
stable
n
(asymmetric
carbamato)
ᵒ
compound
ᵒ
[Cu(S2CNMe Hex)2] at high temperatures of 450 C to 500 C [59]. A
comparative study of the two method, LP-MOCVD and AACVD, deposited
thin films were made in this paper.
3. Thin films of CuS have been deposited via electrodeposition in a [EMIm]TFSI
(1-ethyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)
imide)
electrolytic bath and studied [60]. A closed electrochemical cell was used to
synthesis CuS thin films with an Autolab PGSTAT 30 potentiostat (Eco
Chemie BV) in air. The electrolytic bath consists of different molar ratios of
Cu(TFSI)2 (99.5%, Solvionic) and sulfur powder (99.5%, Alfa Aesar) in the
ionic liquid [EMIm] TFSI (99.5%, Solvionic). Platinum disk (1.25 cm2) acted
as a working electrode in the synthesis. A platinum foil (400 mm2) and a silver
wire were used as counter and reference electrodes, respectively.
The
deposited CuS thin film by this technique has potential application in
photovoltaic or lithium ion batteries.
4. Synthesis of CuS thin films by successive ionic layer adsorption and reaction
(SILAR) method was done on cleaned and polished n-type Si semiconductor
with (111) orientation having 1–10Ωcm resistivity [61]. The Si wafer was
dipped in boiled NH3+H2O2+6H2O solution for 10 min and followed by a 10
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
17
min dip in HCl+H2O2+6H2O at 60 ᵒC. The native oxide on the front surface of
the substrate was removed by HF+H2O2 (1:10) solution and then followed by
rinse in de-ionized water. Copper sulphide thin films were deposited using
CuCl2 as cationic solutions. The anionic solution was the freshly prepared
sodium sulphide (Na2S). The cationic and anionic precursor solutions
characteristics: adsorption, reaction and rinsing times were set to optimum
conditions for thin film deposition. One SILAR cycle contained four steps: (1)
the substrate was first immersed into aqueous cation precursor (2) rinsed with
water (3) immersed into the anion solution and (4) rinsed with water. The
obtained film was polycrystalline having preferred orientation. The scanning
electron microscopy study showed that the surface morphology of these films
looked relatively smooth and homogeneous.
5. Y. Lu et al. [62] prepared CuS thin films by chemical bath deposition (CBD)
method. The functionalized substrates were immersed in prepared precursor
solution consisted of CuSO4.5H2O (copper source), EDTA (complexing agent)
and Na2S2O3 (sulphur source). The solution temperature was maintained at 70
ᵒ
C using a thermostatically controlled water bath. The pH of the bath solution
was adjusted to 2.2–2.3 by adding H2SO4 solution (1 M). The substrates were
placed vertically to the bottom of the beakers to avoid the effect of gravity.
After deposition, the deposited films were rinsed in deionized water and
ultrasonically washed to remove the leftover copper sulphide precipitates and
dried with nitrogen gas. The deposition mechanisms of the CuS thin film on
the functionalized self-assembled monolayers were investigated and discussed
based on the morphology and crystallinity analysis of CuS using FESEM,
XRD
and
XPS.
The
investigation
of
optical
properties
and
photoelectrochemical response were also carried.
6. Spray pyrolysis method was used by M. Adelifard et al. [63] for deposition of
CuS thin films on glass substrates. The spray solution consist of
Cu(CH3COO)2·H2O (99.9%, Merck) and thiourea CS(NH2)2 (99.9%, Merck),
having two variations of Cu to S molar ratios; 1: 3 (group a, Cu-poor), and
2.28: 1 (group b, Cu-rich). Here the substrate temperature was varied from
260C to 285C and 310C for both options. The concentration of
Cu(CH3COO)2·H2O in the precursor solution was 0.02 M. The glass substrates
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
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were cleaned by boiling in hydrochloric acid followed by ultrasonication in
acetone bath for 15 min and then dried in a nitrogen flow. Other deposition
parameters such as spray solution volume, spray deposition rate, nozzle to
substrate distance and hot plate rotation speed were maintained at: 100 ml, 7
ml.min-1, 30 cm and 50 rpm, respectively. They had achieved CuS thin films
having high absorption coefficient and degenerate p-type conductivity.
7. Thin films of CuS were deposited by microwave assisted chemical bath
deposition (MA-CBD) [64]. In this method, 10 ml of copper acetate
(1.0 mol.l-1) solution was placed in a 100 ml laboratory beaker with constant
stirring to which 10 ml ethylenediamine tetra acetate acid disodium salt
(EDTA-2Na) solutions (1.0 mol.l-1) was added successively. After stirring for
several minutes, the solution became homogenous and clear navy-blue with
purplish colour. The pH value of the mixed solution was adjusted to a certain
range by NH3.H2O (6.0 mol.l-1). Along with continuous stirring, 10 ml
thioacetamide solution (1.0 mol.l-1) was mixed in the solution which became
olive-drab in colour suddenly. Deionized water was added to make the volume
up to 80 ml rather than 100 ml, so that the solution could not spill over from
the beaker during the reaction thus decreasing any kind of errors. Then the
pre-cleaned substrates were floated on the surface of the above solution to
nucleate heterogeneously instead of particles accumulate on the substrate
surface. The beaker was then placed in a microwave oven of 2.45 GHz and a
maximum power of 700 W, and the reaction was performed under ambient air
for different times with only 17% power. In order to avoid the loss of the
liquid, circumfluence equipment was added to keep the volume of solution
invariable. All experiments were carried out initially at room temperature
(about 20 ᵒC) without any further heat treatment. After duration of time of
deposition, the coated substrates were separated from the solution and washed
by deionized water and dried in air for further studies. The variations on film
thickness, morphology, optical and electrical properties brought by the change
of reaction time and microwave radiation in the treatment process were
investigated.
8. C. N. R. Rao et al. [65] synthesized CuS nanocrystalline thin film by liquidliquid interface using copper cupferronate (Cu(cup)2) as the copper source and
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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Na2S as the sulphur source. In a typical preparation, 75 ml of 0.12 mM
Cu(cup)2 solution in toluene was slowly added to 75 ml 0.5 mM of Na2S taken
in a crystallization dish (diameter 10 cm). An excess of Na2S was required in
order to prevent the formation of Cu2S. The interface gradually turns green
and the CuS film is formed at the interface after 12 h, while the two liquid
phases remained colorless. On completion of the reaction, the toluene layer
was replaced with fresh toluene. The film could be lifted onto various
substrates for characterization. This method helped not only for generating
nanocrystalline thin films but also to study processes occurring at the liquid–
liquid interface.
9. The CuxS thin film depositions were carried out in a commercial flow-type F120 ALE reactor manufactured by ASM Microchemistry Ltd (Espoo, Finland)
[66]. The precursor vapors were alternately introduced into the reactor while
nitrogen (purity 99.999%) was used as a carrier and purging gas. The
precursor materials for copper and sulfur were the volatile copper (II) bdiketonate Cu(thd)2 (thd=2,2,6,6- tetramethyl-3,5-heptanedione) and hydrogen
sulfide (Messer, Krefeld, Germany, no. 30335, purity class 5.0), respectively.
The copper precursor Cu(thd)2 was synthesized from analytical grade
Cu(NO3)2.3H2O (Merck) and Hthd (Merck-Schuchardt) and purified by
vacuum sublimation. The Cu(thd)2 precursor was evaporated at 115 ᵒC and the
H2S gas was delivered into the reactor at a flow rate of 10 cm3.min-1 with an
absolute pressure of about 800 mbar. The total reactor pressure was
approximately 2 mbar during the deposition of thin films. The Cu(thd)2 pulse
time was varied between 0.8 and 2.0 s and the H2S pulse time between 0.1 and
2.0 s. Nitrogen gas pulses of 1.5 s duration were used for purging the reactor
between the successive precursor pulses. One growth cycle is thus determined
as the sequence of a Cu(thd)2-pulse, a first purge pulse, an H2S-pulse and a
second purge pulse. The CuxS films were deposited onto fine polished soda
lime glass (Grade LCD, Tosoh Corp., Japan) and Si(100) substrates (Okmetic,
Finland) measuring 5×5 cm2 at deposition temperatures of 125 to 250 ᵒC. Four
substrates were used in each deposition and in most of the experiments both
silicon (100) and glass substrates were used in the same batch. This enabled to
evaluate the effect of substrate under strictly identical conditions as well as to
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check the deposition profile over a total length of 10 cm. The total number of
cycles was varied between 500 and 9000 leading to thin films of
approximately 25 to 220 nm thicknesses, respectively. The deposited CuS thin
films were characterized by XRD, AFM and four point resistivity.
10. Thin films of CuS were deposited by solution growth technique (SGT) [67].
The CuCl2 was used as source material for the Cu+2, Na2S2O3, and
dimethylthiourea (AR grade) was used as source material for the S-2 ions. An
aqueous solution of 0.3M CuCl2.5H2O, 0.3M (CH3) NHCSNH (CH3), and
0.3M Na2S2O3 were prepared in deionized water. These solutions were mixed
in 100 ml beaker and its pH was maintained at 2.3. The optimized bath
temperature of 70 ᵒC and deposition time 3.5 h were kept constant throughout
the experiment. For a particular composition of the films, the volumes of
source solutions were changed according to the atomic weight calculation.
Prior to the deposition of the films, the glass substrates were cleaned using
chromic acid and degreased with acetone. These cleaned substrates were
placed in the bath, vertically supported on the wall of the beaker. The
deposition was carried out without stirring at different temperatures on
magnetic heater. After a period of 3.5 h, the deposited films were taken out of
the bath, washed well with deionized water and dried to be used for further
studies. Study of the growth parameters on structural, morphological, and
optical bandgap of the CuS thin films was made in this paper
11. CuxS thin films were deposited on ITO coated glass substrates by
photochemical deposition (PCD) [68] from the aqueous solution of 50 ml
containing CuSO4 in the range of 0.0025–0.05 mol.l-1 and Na2S2O3 in the
range of 0.025–0.1 mol.l-1. The solution was prepared using deionized water.
The ITO-coated glass substrate, substrate holder, magnetic stirrer, etc., were
ultrasonically cleaned and purged with N2 gas prior to immersion into the
solutions. In PCD, degreased ITO-coated glass substrate was immersed in the
solution and illuminated by a high-pressure mercury lamp through a spherical
lens from top of the substrate. The distance from the solution surface to the
substrate was maintained about 1–3 mm. The diameter of the illumination
region was approximately 10 mm. The power density of the UV region was of
the order 100 mW.cm-2. The PCD was carried out with different deposition
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parameters such as concentration, pH and deposition time variation. The
deposition period was varied from 1 to 2 h. The pH of the solution was varied
from 6 to 3 by adding few drops of H2SO4. The study of different properties of
film states them to be suitable for solar control coatings and photovoltaic
devices.
12. L. Chen [69] and his coworkers deposited CuS thin films by Hydrothermal
method. All the chemicals were of reagent grades and were used without
further purification. In a Hydrothermal process, 0.005 mol.l-1 of glutathione
and 1 mmol of thiourea were dissolved in 40 mL of de-ionized water. Then 2
mmol of CuCl was introduced to the solution. After stirring for 1 min, the
suspension was transferred into a polytetrafluoroethylenelined autoclave. ITO
substrates that were washed with toluene, isopropanol, acetone, ethanol, and
de-ionized water were arranged vertically in the bottom of the vessel. The
autoclave was then sealed and maintained at 160 ᵒC for 4 h. After deposition,
the resulting films were rinsed with de-ionized water and dried naturally. The
growth mechanisms, optical and electrical properties of the thin films were
studied in detail.
13. Y. Lei et al. [70] fabricated copper sulfide nanosheet thin films by a very
facile, low temperature, one-step route. In a typical synthesis, a piece of
copper foil (Tianjin Dengfeng Chemical Reagent Factory, China; purity,
99.9%; thickness: 0.2 mm; 1.5 cm × 0.5 cm) and 0.01 g of sulfur powder were
placed separately in a 20 ml Teflon-lined autoclave, and then, 15 mL of DMF
was added. Before being used, the Cu foil was cleaned by ultrasonication in
diluted HCl solution to remove the copper oxide on the surface of Cu foil and
rinsed by DMF several times. The temperature of the autoclave was
maintained at 60 ᵒC (or less) for 24 h. The Cu foil coated with product was
taken out of the solution, washed with ethanol several times, and finally dried
at room temperature. The resulting CuS films were characterized by XRD,
SEM, TEM, SAED and UV–vis spectrometer, etc.
14. Patterned copper sulfide (CuS) microstructures were successfully fabricated
by a relatively simple solution growth method [71]. The copper precursor
solution was prepared by dissolving CuSO4, EDTA and Na2S2O3 (mole ratio,
1: 1: 1) into Milli-Q water. The concentration of each constituent was adjusted
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to 10mM. Droplets of diluted H2SO4 were slowly added to make the pH ~2.5.
The patterned APTES-SAM wafers were then immersed in the fresh copper
solution at 70◦C for about 2h. Finally, the samples were taken out and washed
with Milli-Q water by ultrasonication and dried with a stream of dry N2. The
optical microscopy and AFM results of the synthesized thin films were carried
out. The cyclic votammograms studies showed good electrical conductivity of
the films.
15. Copper sulfide thin films were deposited by aqueous solution method [72], the
TiO2 thin film surfaces with pre-patterned Self-assembled monolayer (SAMs)
were immersed in an aqueous solution of CuSO4, Na2S2O3 (precursors) and
EDTA (complexing agent) in acidic medium. The deposited films were then
rinsed in deionized water and ultrasonically washed to remove the leftover
copper sulfide precipitates, and dried with nitrogen gas. The substrates were
placed vertically to the bottom of the beakers to avoid the effect of gravity. In
this way, positive and negative CuxS microarray patterns were produced on
TiO2 thin films. Meanwhile, the positive, negative and un-patterned thin films
were deposited synchronously in same aqueous solution and deposition time in
one pot to avoid the film thickness differences. The resultant CuxS/ TiO2
composite films were investigated using SEM, XRD, XPS and a 3D Surface
Profiler.
16. The copper sulfide thin films were deposited by chemical deposition onto
microscopic glass slides as substrates, by using 5 ml of 0.5 M solution of
CuCl2·5H2O mixed with 9 mL of 1 M solution of Na2S2O3, 10 ml of 0.5 M
dimethylthiourea, and the remainder was distilled water to make it 100 ml. By
stirring all the reagents were mixed, and for the deposition the substrates were
placed vertically in the solution at 70°C for 1 h without stirring. The initial and
final pH of the solution was 5.50 and 3.43, respectively. The average thickness
obtained for the thin films were approximately 0.1 μm [73]. The effect of
alternating current (AC) plasma in air on the chemically deposited CuS thin
films and comparison in performance of thermal annealing treatment was also
analyzed in this paper.
17. A p-type transparent conducting CuS thin film was deposited „layer by layer‟
method [74]. The glass substrates were first treated in a piranha solution
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(H2SO4–H2O2, 4:1 vol. vol-1) and then immersed in 3-(trimethoxysilyl)-1propanethiol benzene solution (0.25 wt %). A few drops of acetic acid were
added as hydrolysis catalyst. After surface modification, the substrate was
washed by benzene and dried in vacuum. Two precursor solutions, thiourea
(0.1 mol.l−1) and copper dichloride (CuCl2, 0.09 mol.l−1) with chelating
reagent NH4OH and tetra ethanolamine were mixed at the volume ratio of 1:2.
In the deposition process, each substrate was placed at a 60 ° angle to the
horizontal line. The upward side of the substrates was covered by an adhesive
tape, and film deposition took place only on the downward side. Using p-type
CuS film as front contact layers, a dye-sensitized solar cell was fabricated with
a significant photoelectric response.
18. S. Y. Wang et al. [75] synthesized CuS thin film by asynchronous-pulse
ultrasonic spray pyrolysis deposition technique. In this method N2 gas was
introduced into the reaction chamber at relatively slow and steady flow rate for
about 30 min to purge the ambient and let flowing during the entire process.
The nebulized solution of thiourea and CuCl2 was delivered to the substrates
in 3 s spray pulses through the two nozzles, respectively. After the pulse spray
of thiourea was conducted lasting 3 s, a delay of 2 s was employed to ensure
that the introduced thiourea was completely decomposed before conveying a
pulse spray of CuCl2. The deposition was carried out by repeatedly performing
these spray processes. It has been known that an appropriate interval between
the pulse sprays of thiourea and CuCl2 is necessary for obtaining CuS
crystalline film. The films were characterized by XRD, SEM, XPS and Raman
spectroscopy, etc. The XRD studies indicated that the films were
polycrystalline in nature.
19. Semiconducting stoichiometric copper sulfide thin films were deposited using
modified chemical deposition method by H. M. Pathan at al. [76] The
modified chemical method is mainly based on immersion of the substrate into
separate cation and anion precursor solutions and rinsing between every
immersion with ion exchange water to avoid homogeneous precipitation. The
cationic precursor for thin film deposition was copper (II) sulphate
pentahydrate (CuSO4.5H2O) solution complexed with mixture of 2N
triethanolamine (TEA) and 2N hydrazine hydrate (HH). The pH of this
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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solution was adjusted to ~5. The anionic precursor was sodium sulphide
(Na2S.H2O) solution with pH ~ 12. The concentration of sodium sulphide
solution was 0.05 M throughout the experiment. For rinsing purpose, highly
purified deionised water was used. For the deposition of thin films, glass
substrate was immersed in cationic precursor solution of copper (II) sulphate
pentahydrate for 30 s in which copper ions are adsorbed on the surface of the
substrate. The substrate was rinsed with ion exchange water for 50 s to remove
loosely bounded ions. The substrate was then immersed in an anionic
precursor of sodium sulphide for 30 s in which sulphur ions are reacted with
pre-adsorbed copper ions on the glass substrate. This was followed by rinsing
again in ion exchange water for 50 s to remove unreacted sulphur ions. This
completes one deposition cycle for the deposition of Cu2S thin films. By
repeating such deposition cycles for 60 times, a Cu2S thin film on glass
substrate was obtained. The deposition was carried out at room temperature
(27 ᵒC). The film was found to be nanocrystalline. Optical absorbance of the
film was high (104 cm-1) with optical band gap of 2.35 eV. The electrical
resistivity was of the order of 10-2 ῼ cm with p-type electrical conductivity.
20. Y. B. He et al. [77] deposited CuxS films on bare float glass substrates by a
reactive sputtering (RF) process. High-purity (99.999%) argon was used to
provide the plasma at a base pressure of 10-6 torr, and H2S (purity: 98.0%) was
injected as reactive gas during the sputtering. A metallic (99.999%) Cu
circular plate with a diameter of 10.16 cm was used as the sputter target. The
RF power was in the range between 50 and 300W (0.62–3.70Wcm-2), while
the H2S flow was varied from 2.0 to 10 sccm. The substrate temperature was
varied from room temperature to 500ᵒC. The film thickness was obtained in
the range between 50 and 600 nm mainly depending on the sputtering power
and time. Comparative studied of different stoichiometric CuxS thin films
were carried out in this paper.
21. D. J. Elliot et al. [78] reports the fabrication of copper sulfide in Langmuir–
Blodgett films. First, Langmuir monolayers of arachidic acid on a subphase of
0.3 mM CuSO4, 17 mM NH3 were transferred to hydrophobic glass and mica
substrates to give Langmuir-Blodgett films of cupric arachidate (CuAr) after
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that the films were exposed to H2S for the formation of copper sulfide. The
obtained film was characterized by XPS, UV-Vis spectroscopy and AFM.
22. I. Grozdanov et al. [79] deposited CuS thin film by a simple and low-cost
technique of electroless deposition. In this method, 8-10 ml 0.5M aq.sol.
CuSO4 were placed into a beaker and 8-10 ml 1.0 M aq.sol. Na2S2O3 were
added and the final solution was made 100 ml. The blue solution turned green
at this point, due to the reduction of Cu(II) to Cu(I) by the thiosulphate.
Deionized water was added to make the volume up to 80 - 100 ml. The pH of
the bath was about 5 and can be adjusted with diluted acetic acid if necessary.
Previously cleaned and activated substrates were then inserted into the beaker
and the bath was heated and kept at 40-45 ᵒC. No stirring was applied. At this
temperature, the solution turned yellow and soon a brown precipitate began to
form in the beaker and golden-yellow films were deposited on the activated
sides of the substrates. Once the precipitation began, the reaction at this
temperature was completed within 25-35 minutes. The substrates were then
taken out, rinsed with distilled water, dried in air and preserved for optical and
electrical characterization. The green polycrystalline thin film had thickness
0.1 μm, optical bandgap of 2.20 eV and showed p-type electrical conductivity
with sheet resistance 105 ῼ/square was obtained.
23. M. Kundu et al. [80] grew copper sulfide films on Si (001) substrates in an
UHV deposition system. In the deposition firstly N-type Si (001) samples
(20×20×0.5 mm3) were chemically cleaned in a H2SO4:H2O2 solution and
rinsed in de-ionized water. After that the sample was introduced into the
treatment chamber of the deposition system. The sample was then transferred
into the metal deposition chamber of the system, which was equipped with a
Knudsen cell that served as a source of copper. A 70nm thick Cu film was
deposited on the clean Si (001) substrate at room temperature, where the film
thickness was monitored by using a quartz crystal microbalance. The sample
was finally transferred into the sulphur deposition chamber that was connected
to sulphur VCC (valved cracker effusion cell). The bulk evaporator of the
VCC that held the sulphur source crucible was heated at 135 °C. The cracking
zone of the VCC was kept at 900 °C in order to convert sulphur from a
polyatomic form to simpler species through thermal pyrolysis and therefore,
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enhance the reactivity of the sulphur species with the Cu film. Sulphur was
introduced into the UHV chamber at a pressure of 2×10−6 torr by using a
needle valve located between the crucible and the cracking zone. The substrate
was kept at 75°C. The effect of various sulfurization conditions on structural
and electrical properties of CuS thin films was studied.
24. Thin films of CuxS were deposited by the thermal evaporation of highly pure
cuprous sulphide powder from a molybdenum boat onto thoroughly cleaned
and vapour-degreased glass substrates maintained at 300, 400 and 475 K in a
vacuum of about l × 10-5 torr. The substrates were held directly above the boat
at a distance of about l8cm. The rate of evaporation was maintained at about l0
- 15Ǻmin-1. The thicknesses of the films were obtained in range between 650
and 1000 Ǻ [81]. The structure, phase transitions and electrical conductivity of
CuxS films deposited by vacuum evaporation at different substrate
temperatures were studied.
1.3.3
Properties of CuS Thin Films
Nos.
1
Properties
Structure
Hexagonal, a=b= 3.768-3.800 Å, c= 16.270-16.344 Å
[58,64,67,82-84]
2
Optical
High transmittance = 36% and low reflectance = 15% in the
visible region, low transmittance = 10% and high transmittance =
45% in near infrared region.[ 58]
Optical bandgap = 1.67-2.88 eV (Direct and Indirect bandgap)
[58,62,64,67,85-90]
3
4
Solid –
Refractive index (n) = 2.05, Real dielectric constant (εr) = 4.22
State
Optical conductivity (σo) = 1.32×10-13 sec-1 [91]
Electrical
Semi metallic, Sheet resistance (Rs) = 154 Ω/□
transport
Electrical conductivity (σ) = ~2×103 S cm−1
properties
P-type conductivity
Carrier concentration = ∼1.8×1020 to 1.7×1021 cm−3
Hall Mobility(μH) = 12-25 cm2.V-1.s-1 [63,74]
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Mechanical
1.3.4
27
Data at 0.5 mN Load
Data at 1 mN Load
Elastic modulus (EП)= 96.8
Elastic modulus (EП)= 91.4 GPa
GPa
Hardness (HП)= 7.3 GPa
Hardness (HП)= 9.9 GPa
Hardness (HV)=676.5 Vickers
Hardness (HV)=915.5 Vickers
[92]
Application of CuS Thin Films
The copper sulphide thin films have wide usefulness like, as gas sensors, as
absorbing layer in solar devices, active layer in devices, etc. Some of the applications
are listed below, but not limited to the list.
1. Solid state electrolytic memory devices [93].
2. Solar controller and solar radiation absorber [77, 86, 94, 95].
3. Electro conductive coatings [58].
4. Lithium ion batteries [60].
5. Solar energy conversions [96].
6. Nonlinear optical material [97].
7. As selective radiation filters on architectural windows for solar control in
warm climates [98].
8. Optical filter [99].
9. Architectural glazes [98].
10. Supersonic materials [100].
11. CdS/CuS and CuS/CdS Heterojunction solar cell [101].
12. Optically transparent light emitting diodes (LEDs) [102-104].
13. Photovoltaic application [105,106].
14. As polarizer of infrared radiation [107].
15. CuS-Sb2S3 heterojunction solar cell [108].
16. Active absorbents of radio waves [109].
17. Photoelectrochemical solar cell (PEC) [110].
18. Ammonia gas sensor [111].
19. Dye-sensitized solar cell [74].
20. Solid-state solar cell [112].
21. Flat panel display [113].
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Chapter 1
1.4
28
Nanomaterials
More or less in the last three decades, new terms having prefix `nano‟ rapidly
intruded into the scientific vocabulary. The terms were nanoparticles, nanostructures,
nanotechnology,
nanomaterials,
nanoclusters,
nanochemistry,
nanocolloids,
nanoreactor and so on. Books and new scientific journals entirely devoted to nano,
even having corresponding names appeared on the scientific horizon. The `nano'specialized institutes, laboratories and establishments cropped up; numerous
conferences are held the world over. In most of the cases, the new names having word
nano were applied to long known objects or phenomena. This objects and phenomena
remained inaccessible earlier due to lack of sophisticated analytical techniques. With
the development of new sophisticated techniques that can view phenomena at the
atomic or sub-atomic levels made it possible to study this unknown phenomena or
objects. These include fullerenes, quantum dots, nanotubes, nanofilms and nanowires,
i.e., the objects having at least one dimension in nanometer range.
The enhanced interest of the researchers in nano objects is due to discovery of
unusual physical and chemical properties of these objects, which is related to
manifestation of so-called `quantum size effects‟. These arise in the case where the
size of the system is commensurable with the de-Broglie wavelengths of the electrons,
phonons or exciton propagating in them.
A key reason for the change in the physical and chemical properties of small
particles as their size decreases is the increased fraction of the `surface' atoms, which
differs from those of the bulk. From the energy stand point, a decrease in the particle
size results in an increase in surface energy with respect to its chemical potential.
Currently, unique physical properties of nanoparticles are under intensive
research [114]. A special place belongs to the magnetic properties in which the
difference between a massive (bulk) material and a nanomaterial is especially
pronounced. The magnetic properties of nanoparticles are determined by many factors,
the key of these includes the chemical composition, the type and the degree of
defectiveness of the crystal lattice, the particle size and shape, the morphology (for
structurally inhomogeneous particles), the interaction of the particles with the
surrounding matrix and the neighbouring particles. By changing the nanoparticles size,
shape, composition and structure, one can control the magnetic characteristics of the
material [115-120].
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Chapter 1
1.4.1
29
Synthesis Methods
In order to explore novel physical/ chemical 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. Nanostructure materials are those with at least one dimension falling
in nanometer scale, and include nanoparticles (including quantum dots, when
exhibiting quantum effects), nanorods and nanowires, thin films, and bulk materials
made of nanoscale building blocks or consisting of nanoscale structures. Many
technologies have been explored to fabricate nanostructures and nanomaterials.
Generally, top-down and bottom-up approaches [121] are the two basic synthesis or
fabrication pattern accepted for nanostructure materials. Brief details are shown in
below chart.
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1.4.2
31
Literature Survey on CuS Nanomaterials of the last decade
In nanometer scale, copper sulfide (CuS) exhibited various forms such as
nanocomposite, nanocones, nanobelts, nanoslice, nanofluids, nanocages, nanocrystals,
nanoflowers/flakes, nanoplates, nanoparticles, nanotubes, nanowalls, nanosheets,
nanowhiskers, nanoribbons, nanospheres, etc. All forms are synthesized by different
methodology with the copper to sulfur molar ratio remaining same. Few of the
synthesis methods used in last decade are listed below.
CuS nanocomposite
1. Nanocomposites of CuS coated with polyvinyl alcohol (PVA) are synthesized
by sonochemical irradiation of a 10% ethylenediamine- water solution of
sulfur and copper acetate in presence of PVA [122]. The synthesis procedure
followed is as follows: firstly 500 mg of sulphur are dissolved in 10 ml of
ethylenediamine. This prepared solution along with 1 g of copper (II) acetate
monohydrate
(Aldrich) and 250 mg of polyvinyl alcohol
(Aldrich 98%
hydrolyzes Mw = 90,000) are dissolved in 90 ml of water. These two solutions
are well mixed and irradiated with a high intensity ultrasonic horn (Ti-horn, 20
kHz, 100 W.cm-2) under the flow of argon at room temperature for 1 h. During
the sonication of reaction mixture the temperature is increased to ~80 °C. The
products obtained are washed thoroughly with double distilled water and
finally with absolute ethanol and then dried in vacuum at room temperature by
keeping it overnight. This nanocomposite CuS was characterized using
analytical techniques such as X-ray diffraction, transmission electron
microscopy, thermo gravimetric analysis, and diffuses reflection spectroscopy.
CuS nanocone /nanobelts
2. Nanocones and nanobelts of copper sulfide were hydrothermally fabricated
using arcrylamide and sodium dodecyl benzene sulfonate (SDBS) as
surfactants. In a typical experimental procedure, firstly 0.35 g of Cu powder
and 1.80 g of Na2S2O4 were dissolved in 20 ml of distilled water. After that
second solution was prepared by dispersing 0.8 g of surfactant (acrylamide or
SDBS) in 20 ml of distilled water. Then the two solutions were loaded into a
50ml Teflon-lined stainless steel autoclave under vigorous stirring, which was
then filled with distilled water up to 90% of the total volume. The autoclave
was sealed and maintained at 140 ᵒC for 24 h. After the reaction was
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completed, the resulting black solid products were filtered, washed with
absolute ethanol and distilled water for several times, and then finally dried in
vacuum at 60 ᵒC for 4 h [123]. Several factors such as the temperature,
surfactant, and reaction time, which influence the final samples, have been
investigated. The surfactant was found to be vital to the final morphology of
the sample.
CuS nanoslice
3. S. Xu et al. [124] did intergrowth of CuS polycrystalline nanoslices by facile
method. In a typical procedure, a mixture of ethylene glycol (A. R.) and acetyl
acetone (A. R.) with the volume ratio of 3:1 was put into a beaker. Then 1.3
mmol cupric chloride (CuCl2·2H2O, A.R) was added under stirring at room
temperature to ensure well dispersion of the reactant. Afterward, the mixture
was transferred into a Teflon-lined autoclave which was filled with 0.04 g of
sulfur powder. The autoclave was sealed into a stainless steel tank and
maintained at 120 ᵒC for 12 h without shaking or stirring. After the autoclave
had been cooled to room temperature naturally, the product was washed three
times using distilled water and absolute ethanol. Finally, the products were
dried at 80 ᵒC in an oven for further characterization. On the basis of the
experimental results, the current–voltage characteristic under different gas
atmospheres shows that the as prepared CuS polycrystalline nanoslices were
sensitive to ammonia at ppm level and the electrical conductivity was found to
be weaker in ammonia than that in air.
CuS nanofluids
4. Synthesis of nanofluids by the chemical solution method (CSM) was carried
out by X. Wei et al. [125], the used solution amount is 5 ml, 20 ml, 25 ml and
4 ml for CuSO4, PVP, NaOH and N2H4, respectively. The PVP and NaOH
mass fractions in the solution are fixed at 25 g.L-1 and 0.004 g.L-1,
respectively. The pH value of NaOH solution and the molar concentration of
N2H4 solution are 10 and 0.1 mol.L-1, respectively. The added amount of
C2H5NS is determined by keeping its molar mass as 5 times as that of CuSO 4
which is varied from 0.005 mol.L-1 to 0.025 mol.L-1. The chemical reaction
after adding C2H5NS lasts for 30 min. The study showed that fluid
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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conductivity can be either increased or decreased by the presence of
nanoparticles.
CuS nanocages
5. Octahedral CuS nanocages were synthesized via a solid-liquid reaction [126].
Octahedral Cu2O particles were prepared first. In a typical procedure, 0.852 g
CuCl2·H2O was dissolved in 100 ml of deionized water. Then 2 ml of NH3
(30%) solution was added to the CuCl2 solution under constant stirring.
Cu(OH)2 precipitate was produced when 10 ml NaOH solution (1 M) was
added. Octahedral Cu2O particles with an average size of 360–400 nm were
obtained when reducing the above suspension with 1 ml hydrazine hydrate
(85%). The Cu2O precipitate was collected and washed several times. Then,
0.143 g Cu2O was redispersed in 100 ml deionized water followed by the
addition of 0.114 g thiourea to the suspension. After heating the suspension at
90ºC for 2 h, the black precipitate of CuS was centrifuged and washed
sequentially with deionized water and ethanol, then dried at 50 ºC for 5 h
under vacuum. The mechanism for the formation of the hollow structure was
investigated with the assistance of TEM, SEM and EDX analyses. It is
suggested that both mass diffusion and Ostwald ripening play important roles
in the transformation process.
CuS nanocrystal
6. W. P. Lim and his co-workers [127] have described a simple strategy for
preparing phase selective CuS nanocrystal. The copper (I) thiobenzoate
(CuTB) precursor was first prepared. All procedures for the preparation of
copper sulfide faceted nanocrystals were carried out using standard techniques
under a nitrogen atmosphere. Dodecanethiol (DDT) was carefully degassed
before use. Faceted nanocrystals were prepared using a tributylphosphite
(TBPT). A degassed solution of CuTB (0.04 g) in tributylphosphite (TBPT;
0.2 mL) was injected into a hot solution (135/160/180 °C) of DDT. After 20
min, the reaction mixture was cooled to room temperature, and toluene was
then added. The precipitate was centrifuged and dried in a vacuum overnight.
No size sorting was performed for any of the samples. In the experiment, the
reaction temperature and the DDT concentration were varied. The molar ratio
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between CuTB and DDT (denoted as [DDT]/ [CuTB]) was kept between 30
and 50. Controlled experiments using different surfactants (oleylamine) were
carried out using the same procedure. The study sates that it appears that the
precursor undergoes two competitive pathways, leading to seeds, and thus the
growth of different crystal phases becomes possible.
7. A simple biomolecule-assisted hydrothermal approach was developed to
synthesize
one-dimensional
copper
sulfide
self-assembly
having
nanocrystallites size [128]. The CuS nanocrystal synthesis details of the
typical experiment are as follows: CuSO4·5H2O (2 mmol) and L-cysteine
(C3H7NO2S, 3 mmol) were dissolved in 20 ml distilled water, respectively,
and then transferred into a 50 ml Teflon-lined stainless steel autoclave. The
autoclave was sealed and maintained at 120 °C for 12 h, and further cooled to
room temperature naturally. The precipitate was filtered off, washed with
distilled water and absolute ethanol for several times, and then dried in
vacuum at 60 ºC for 4 h. The approach presented in the synthesis was the
application of L-cysteine, acting not only as complexing agent but also as
sulfur source.
8. P. Bere et al.[129] synthesized nanocrystalline CuS of varying morphologies
and stoichiometry in a low temperature solvothermal process using a new
single source molecular precursor. In a typical synthesis, 0.128 g (0.5 mmol)
of as-prepared [Cu(SMDTC)Cl2] was taken in 10 mL solvent in a 50 mL twonecked flask equipped with a condenser and thermocouple adapter. The flask
was degassed at room temperature for 10 min and then filled with inert argon
gas. The resultant solution was then gradually heated up to desired
temperature and maintained at this temperature for 1h under argon
atmosphere. The black precipitate formed was collected by centrifugation
followed by decantation of the supernatant liquid and then the isolated solid
was dispersed in ethanol. The nanocrystallites were initially purified by
precipitating the dispersed particles with excess ethanol and discarding the
supernatant liquid after centrifugation. The above centrifugation and isolation
procedure was repeated four times with aqueous ethanol (75%) for the
purification of the product and redispersed in spectrograde ethanol for further
characterization. Dry powder of the copper sulphide nanocrystallites were
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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collected by evaporating the ethanol at 90 ºC for 1 h in vacuum. It was
demonstrated that solvent plays an important role to control the stoichiometry
and morphology of copper sulfide by forming a metastable intermediates with
copper ion.
9. Micro-emulsion directed synthesis of different CuS nanocrystals was carried
out by
L. Gao et al. [130]. The microemulsion system used is composed of
water, cyclohexane, cetyltrimethylammonium bromide (CTAB) and ethanol as
co-surfactant. By keeping the volume ratio between water (20 ml) and
cyclohexane (10 ml) equal to 2 and varying the amount of CTAB (0, 0.9
mmol, 1.8 mmol, 3.7 mmol and 9.2 mmol), different micro-emulsion systems
were obtained when adding an appropriate volume of ethanol (except when the
amount of CTAB = 0) to render the system totally homogeneous. Then
corresponding amount of thioacetamide (CH3CSNH2) and copper chloride
(CuCl2·2H2O) (the molar ratio approximately equals 1:1) were added. The
solution soon turned turbid and a yellow precipitate was formed. With
vigorous stirring, the resulting mixture was maintained at 60 °C under the
atmospheric condition for about 30 min. Then the mixture was left undisturbed
at 60 °C until the black products get formed. Dumping out the upper
homogeneous solution, the surfactant dissolved in it was at the same time
disposed. The products collected were further washed with distilled water and
ethanol and dried under vacuum at 50 °C for 12 h. The XRD, TEM, etc.
techniques were used to characterize the properties of the final products.
10. X. H. Liao at al. [131] reported metal sulfides nanocrystalline by microwave
irradiation using formaldehyde solution. The starting materials for the
synthesis of metal (CuS) sulfide nanocrystals were copper acetate
monohydrate (Cu(CH3COO)2·H2O) and thioacetamide (TAA). Distilled water
was used throughout the experiments. In a typical procedure, an appropriate
amount of metal salt was dissolved in 100 ml formaldehyde. Then, an
appropriate amount of TAA was added into the solution. Finally, a flask of
250 ml was filled with the mixture solution. The mixture solution was reacted
in a microwave refluxing system for 20 min with power 20% (meaning of
20% power is that microwave operates at 30 s cycle, on for 6 s, off for 24 s
having total power of 650W). After cooling to room temperature naturally, the
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precipitate was centrifuged, washed with distilled water, and dried in the air.
The advantage of this process was that it was simple, fast and efficient for
producing nanocrystalline metal sulfides.
11. Study of thermal behavior of mechanochemically synthesized nanocrystalline
CuS by high-energy milling in an industrial mill using copper acetate as
source of Cu+2 and sodium sulfide as source of S-2 was done by E. Godocikova
et al. [132]. Here nanocrystalline CuS particles were synthesized in an
industrial eccentric vibratory mill ESM654 (Siebtechnik, Germany). In this
work the following conditions were applied: time of milling in an air
atmosphere was 6–48min; loading of the mill with steel balls of 30mm
diameter in total amount of 17 kg; and rotation speed of the milling chamber
960 rpm. Study of structure and thermal properties of the synthesized copper
sulphide from copper acetate and sodium sulphide in the industrial eccentric
vibratory mill were carried out.
12. Shape-controlled synthesis of copper sulfide nanocrystals via a soft solution
route was carried out by K. Tang and his co-workers [133]. In a typical
synthesis procedure, newly prepared 1 mmol CuO and 1 mmol thiourea (Tu)
were put into a Teflon-lined autoclave of 50 ml capacity. Then the autoclave
was filled with distilled water up to 80% of the total volume. After being
sealed, the autoclave was maintained at 100 ºC for 48 h. Cooled to room
temperature, the dark precipitates were filtered, washed several times with
absolute alcohol and distilled water, respectively, and then vacuum dried at 60
º
C for 4 h. The experimental procedure for the preparation of flower-like CuS
nanocrystals is similar to the above procedure except that ethylene glycol (EG)
was used as the reaction medium instead of distilled water and the reaction
temperature was 180 ºC instead of 100 ºC. Study found that the copper source
and reaction time also have important influence on the morphology of the final
products.
CuS nanoflowers/nanoflakes
13. Micrometer-scaled hierarchical tubular structures of CuS assembled by
nanoflake-built microsphere were first synthesized in high yield via a one pot
intermediate crystal templating process without surfactant or added templates
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[134]. In this intermediate complex, Cu3(TAA)3Cl3, was formed insitu and
subsequently served as a self-sacrificed template. In the typical experiment,
CuCl2.2H2O (2.4 mmol) was dissolved in distilled water (40 mL) to form a
blue solution. Then, thioacetamide (TAA) (2.4 mmol) was dissolved in
distilled water (30 mL) to form a colorless solution. Then prepared TAA
solution was added gradually into the jar with CuCl2 solution without stirring
or vibration at room temperature. The mixture gradually turned into a yellow
suspension in a few minutes. Then the jar was covered and maintained at 60 °C
for about 24 h, and then was allowed to cool to room temperature naturally.
The black precipitate that formed at the bottom of the jar was filtered, washed
with distilled water and absolute ethanol in sequence, and then dried in a
vacuum at 60 °C for 4 h. The study showed that the final products had
potential in the catalyst industry and hydrogen storage.
14. Highly ordered hexagonal prism microstructures of copper sulfide (CuS) by
assembling nano-flakes were synthesized with high yield via a facile one-step
route [135]. Formation of nanoflakes was a simple process, here
Na2S2O3.5H2O solution (0.1 mol.l-1, 10 ml) was added into CuSO4.5H2O
solution (0.2 mol.l-1, 10 ml) under stirring at room temperature. The colour of
the solution changed from blue to light yellow. Then C6H12N4 (HMT) solution
(0.4 mol.l-1, 10 ml) was added into the above solution. The final solution was
transferred into a flask. After that, the flask was placed into a water bath and
maintained at 60°C for about 35 h and then cooled to room temperature
naturally. The black precipitate was collected by centrifugation. The sample
was washed with absolute alcohol and distilled water at room temperature,
respectively, and then was dried at 50 °C in atmosphere. The obtained products
were characterized by XRD, SEM, EDAX and TEM.
15. CuS nanoflowers with a specific surface area of 18.8 m2.g-1 were prepared
through a rapid polyol route by T. Y. Ding et al. [136]. A 40mL glycol (EG)
solution of 4mmol CuCl2·2H2O was heated to 120 °C in a three-neck flask,
and then another 40mL EG solution of 16mmol (NH2)2CS (Tu) was injected
into the flask under strong stirring. The mixture was further heated to 140ºC
and refluxed for 90 min, and then cooled to room temperature naturally. The
black precipitates were washed several times with distilled water and absolute
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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alcohol. The final product was obtained (0.344 g, 88.9% yield based on
CuCl2·2H2O) after drying at 60 °C for 3 h in the air. The higher photocatalytic
activity of CuS nanoflowers indicated that it could be used as a potential
application for purification of polluted water.
16. Nanostructured CuS (hcp) flowers were produced using a transient solid-state
reaction by the direct flow of electricity through solids, containing 1:1 molar
ratio of Cu:S powders, in a high vacuum system for different lengths of time
[137]. To produce flower, 1:1 molar ratio of Cu:S (2g dried powders each)
was put in a bottle, mixed by rotation for 1 h at ambient temperature, loaded to
fill a silica tube (11 mm I.D.×10 mm long), and connected with two electrical
stainless steel electrodes in a tightly closed chamber. Evacuation was done for
removal of air to 2×10−2mbar absolute pressure, and argon was gradually fed
into the chamber for replacement. Subsequently, argon in this chamber was
evacuated to a constant absolute pressure of 2×10−4 mbar. To produce copper
sulphide at the rapid rate, each solid mixture was heated by the direct flow of
electricity (25 DC V and 20 A) through it for 1 s, 3 s, 5 s and 3min, and left to
cool down in the vacuum to room temperature. There are two reasons to use
the current of 20 A for the present process. (a) The limitation of DC power
supply, which was set for working at 20–200 A. The minimum current of 20 A
was chosen, such that the formation process was long enough to be measured
by the processing intervals. (b) The electrical property of the samples, which
were measured to be 1–2 Ω or 400–800 W. These powers were high enough to
produce the sulphide. Thus it is not necessary to use a higher current.
Contrarily, the processing time will be longer when the electrical current is
less than 20 A. For the 1s, 3s, and 5s heating samples, the powders were filled
in the silica tubes without the use of a compressive force (CPF). But for the 3
min heating sample, the 103 kg CPF was used to press the powder for 1 min.
Finally, the products were intensively characterized to determine their phase,
morphologies, vibrations and emissions.
17. CuS:Ni flowerlike morphologies composed of nanosheets were fabricated by
the solvothermal route with polyvinyl pyrrolidon as surfactant and ethylene
glycol as solvent [138]. A mixture of 2.5mmol Cu(NO3)2·3H2O and
NiCl2·6H2O, and 5mmol sulphur powder with 0.075 g polyvinyl pyrrolidon
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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(PVP) was added to 10 ml ethylene glycol (EG) in a teflon lined stainless steel
autoclave, and then the mixture was vigorously stirred. The autoclave was
sealed and maintained at 140◦C for 24h. The Ni concentration was varied by
changing the amount of NiCl2·6H2O. Black powder of CuS:Ni nanoflower
was obtained by centrifuging the mixture after cooling the solution down to
room temperature. At low reaction temperatures, the sulphur reacts with
ethylene glycol to form S-2 ions slowly, which gives the Ni+2 ions enough time
to substitute for Cu+2 ions to form Ni doped CuS. Finally, the powder was
washed several times with carbon disulfide and pure ethanol and dried in
vacuum at 60 ºC for 4 h. The analysis results indicated that the concentration
of doped Ni influences the morphology of CuS.
CuS nanoplates
18. Hexagonal copper sulfide (CuS) nanoplates were successfully prepared by
mild hydrothermal method by L. Chu et al. [139]. In a typical synthesis, 50 ml
of aqueous 20mM CuCl2 solution was drop wise added to aqueous 80mM
Na2S2O3 solution (50 ml). The resulting complex solution was rapidly loaded
into a 150ml flat–bottom flask and mixed with 6mmol CTAB. After the
resulting mixture was heated in a 45◦C water bath for 30 min to ensure the
complete dissolution of CTAB, 0.5ml of aqueous 1.40M HNO3 was
immediately injected into the resulting clear solution. All the above steps were
under continuous magnetic stirring. The final concentration of CuCl2,
Na2S2O3, and CTAB in aqueous solution was 10, 40 and 60mM, respectively.
Here 70ml of the above solution was transferred into a 100 ml Teflon-lined
auto-clave, which was sealed, heated at 100 ºC for 7 h, and cooled to room
temperature naturally. After all reactions were completed, the resulting
product was collected, washed several times with CS2 and absolute ethanol,
centrifuged, and dried under vacuum at room temperature for 4 h. They found
that CTAB play an important role in the formation of hexagonal nanoplates
and exploring the construction of nanodevices with these attractive, promising,
and abundant building blocks.
19. Y. Liu et al. [140(i)] reported a facile solution route for the synthesis of single
crystalline and hexagonal CuS nanoplatelets by thermolysing single precursor
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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copper ethylxanthate [Cu(exan)2] in hexadecylamine (HDA) at moderate
temperature. Copper nitrate, potassium methylxanthate and ethanol used in
synthesis of CuS plates were reagents of analytical grade. All chemicals were
used without further purification. [Cu(exan)2] was prepared according to the
method described by Nair et al. [140(ii)] In a typical synthesis process, 4g of
HDA loaded in a three-necked flask was heated to 120ºC, and cooled down to
60ºC; then 0.4g of [Cu(exan)2] was added. The mixture was then heated to a
desired temperature and reacted for a desired period of time. Subsequently, the
mixture was cooled to 70ºC followed by the addition of ethanol for
flocculation, and then was centrifuged and washed with ethanol several times.
The final deposit was stored in the dark. The reaction conditions have great
influence on the size and morphology of the products. XRD, TEM, HRTEM,
SAED and UV–vis absorption results revealed that the obtained products
prepared below 200 ºC were discrete, hexagonal single crystalline CuS
nanoplatelets.
20. Copper sulfide (CuS) superstructure composed of intersectional nanoplates
was synthesized by a micro-interfaced reaction method. In a typical synthesis,
0.01 g of sulphur S was dissolved in 10ml 1, 2- dichlorobenzene and a
transparent yellow solution was formed. After that 0.0754 g Cu (NO3)2.3H2O
was dissolved in 40mL ethylene glycol and a green solution was formed. Then
these two solutions were mixed together. Under vigorous stirring microinterface was formed because 1, 2-dichlorobenzene and ethylene glycol, which
is similar to that of oil, dispersed in water. The mixture was then kept at 160
º
C for 2 h under stirring condition. After it was cooled to room temperature,
the black precipitate was centrifuged, washed with absolute ethanol for several
times and dried in a vacuum oven at 40 ºC for 24 h [141]. The covellite CuS
were formed by the growth of hexagonal plates along the diagonal directions
of the basal plate with an average edge length of ca. 350nm and thickness of
ca. 20 nm.
21. The controlled synthesis of copper sulfide (CuS) nanoplates-based
architectures by simple reaction of Cu(NO3)2.3H2O and S under solvothermal
conditions without the use of any templates was carried out. In a typical
synthesis, 1 mmol Cu(NO3)2.3H2O was dissolved in 40 ml ethanol and a green
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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solution was formed. Then 2 mmol sulfur was added into above-mentioned
solution under vigorous stirring for 30 min. Afterwards, the solution was
transferred into a 60 ml Teflon-lined stainless steel autoclave, sealed, and
maintained at 150 ºC for 24 h and then cooled naturally to room temperature.
Finally, the black precipitates were centrifuged and washed with distilled
water and ethanol several times and dried under vacuum at 60 ºC for 4 h. To
investigate the effect of solvent on the growth of CuS architectures, parallel
experiments were also carried out in H2O, ethylene glycol (EG) and
dimethylformamide (DMF) [142].
22. Single crystalline CuS nanoplates with average sizes of about 20-40 nm was
synthesized without any surfactant by a sonochemical approach under ambient
condition [143]. In a typical procedure, 0.0852 g CuCl2·2H2O was dissolved in
100 ml deionized water. Then 30 ml NH3 (0.15 M) solution was added to the
CuCl2 solution under constant stirring. A blue precipitate of Cu(OH)2 was
produced when NaOH (1 M) was added drop wise to the above solution to
adjust the pH value to 13–14. After being stirred for 15 min, the precipitate
was separated by centrifugation and washed with deionized water for several
times. The precipitate was then redispersed in 100 ml deionized water. Excess
thiourea was added to the suspension. The suspension was then sonicated for
40 min by an ultrasonicator. During the sonication, Cu(OH)2 precipitate
gradually turned into brown then black. The black precipitate was centrifuged
and washed sequentially with deionized water and ethanol, then dried at 50ºC
for 5 h under vacuum. The experiment results found that ultrasonic irradiation
and Cu(OH)2 play important roles in the fabrication of CuS nanoplates.
CuS nanoparticles
23. Z. Y. Xu et al. [144] synthesized metal sulfide nanoparticles in air liquid-solid
phase using metal acetates and thiourea. In a typical synthesis of CuS
nanoparticles, 0.005 mol of metal (Copper) acetates and thiourea powders
were separately grounded in a carnelian mortar, then mixed thoroughly in a
corundum crucible. The crucible containing the reactants was heated at 190ºC
for 3 h in an electric oven, and then allowed to cool to room temperature
naturally. The resultant powders were collected directly as the products. The
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structure, composition and optical property of the resultant product were
characterized.
24. Y. J. Yang [145] demonstrated a new approach for the preparation of
nanoparticles metal sulfides by the thioglycerol catalyzed reaction of metal
salts and elemental sulfur. He used ethylene glycol and thioglycerol (TG) as
organic solvent and surface capping agent, respectively. CuS nanoparticles
were prepared by the reaction between metal salts (CuSO4) and elemental
sulphur. All chemicals used in this experiment were of analytical grade and
used as received. They dissolved 0.016 g elemental sulphur, 5×10−4 mol metal
salts and 5×10−3 mol TG in 50 ml ethylene glycol at 70 ºC. Then, heat the asprepared solution at 70–80 ºC for 30 min under stirring. Centrifuge the
solution and wash the precipitate with deionised water and absolute ethanol
for several times. CuS nanoparticles were obtained after drying in vacuum
oven for 4 h. The study stated that this method was suitable to synthesis
spherical shape nanoparticles because thioglycerol not only acts as the capping
agent of the produced metal sulfide nanoparticles but also remarkably
improves the reactivity of the elemental sulfur in the synthesis of the metal
sulfide nanoparticles.
25. L. Xu et al. [146] prepared facile CuS nanoparticles from perovskite templates
containing bromide anions. Decylamine (98%, GC), dodecylamine (99%,
GC), hexadecylamine (AR), octadecylamine(AR), copper bromide(AR), ethyl
alcohol (AR), hydrobromic acid (AR), sulfuric acid (GR) and sodium sulfide
(AR) were used as template in preparation of CuS nanoparticles. The samples
of nalkylammonium bromides CnH2n+1NH3Br (abbreviated as C Br, n = 10, 12,
16, 18) are prepared from their corresponding nalkylamines and hydrobromic
acid. Because the hydrobromic acid is easier to be oxidized than hydrochloric
acid care has to be taken during preparation of hydrobromide materials.
CnCuBr perovskites are synthesized by reacting corresponding CnBr with the
stoichiometric amount of CuBr2 in absolute ethanol solution. After solvent
evaporation, the obtained solid is crystallized triply with the absolute ethanol.
Purple black CnCuBr lamellar crystals are then obtained. The sulphide
nanoparticles are directly fabricated within CnCuBr at room temperature by
exposing their spin casting films to H2S gas, which is produced by reacting
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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Na2S with dilute H2SO4. The obtained results indicated an important effect of
template anions on size control of the formed particles.
26. Intercalation of semiconductor copper sulfide nanoparticles was carried out by
solid-solid reactions of Cu(II)-montmorillonite with sodium at room
temperature. Montmorillonite, Kunipia F, was used as the host material. The
cation exchange capacity (CEC) was 1.19 meq.g-1. Here sodium sulfide
(Na2S·H2O) was purchased from Aldrich and copper chloride (CuCl2·2H2O)
was from Univar and BDH. All chemicals were of analytical grade and were
used without further purification. Cu (II)- montmorillonites were prepared by
conventional ion exchange. The reactions of sulfide ions (from Na2S) with
Cu(II)- montmorillonites were carried out by solid–solid reactions. The molar
ratio of sulphide ions to Cu(II) was 1:1. After the reactions, all samples were
heated at 200°C for 1 h in air and allowed to be in desiccator with silica gel at
room temperature [147]. The intercalation compounds were characterized by
X-ray diffraction, transmission electron microscopy, Raman spectroscopy,
UV–visible, photoluminescence spectroscopy and thermal analysis.
27. A stable colloidal dispersion of CuS nanoparticles in water was prepared by
employing copper acetate monohydrate (CuAc)(Cu(CH3COO)2). Here H2O
and thiourea (NH2CSNH2) was the starting material in the presence of sodium
dodecyl sulphate (SDS), poly vinyl pyrrolidone (PVP), sodium (bis2ethylhexyl) sulfosuccinate (Na-AOT) as stabilizing agents. Double distilled
water was used in all reactions. The procedure employed was as follows: 0.2 g
of SDS in 10ml water was taken in a 100ml three necked round bottom flask
equipped with a condenser and the whole system was placed over a magnetic
stirrer. After that 0.099 g (0.0005mol) cupric acetate monohydrate was
dissolved in 10 ml of double distilled water and added slowly to the aqueous
solution of the stabilizer. The temperature was raised slowly to 80ºC and
mixing was continued for 1 h. Then 0.076 g (0.001 mol) of thiourea in 10 ml
of double distilled water was added drop wise to the above solution under
vigorously stirred condition. During this process the colour changes from blue
to white then colourless, followed by green was observed over a period of 24h
indicating the formation of CuS nanoparticles [148]. The average diameter of
the particles was ~76 nm. The influence of thiourea concentration on
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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conversion of golden brown copper sulfide solution to green form was also
studied.
28. K. Iwahori et al. [149] designed a slow chemical reaction system for
semiconductor nanoparticles in the apoferritin cavity. They optimized
synthesis of CuS nanoparticles in the apoferritin cavity. The synthesis was
performed with a basic reaction solution (3 mL) with 0.3 mg.mL-1 HsAFr, 40
mM ammonia sulfate, 5–75 mM ammonia water, 1 mM thioacetic acid, and 1–
10 mM copper acetate and incubated overnight at room temperature. All of
reaction solutions were adjusted to pH of 4.5 by acetic acid. After the
overnight reaction, each reaction solution was centrifuged at 12,000 rpm for 5
min. to remove the bulk precipitate.
29. Copper monosulfide (CuS) nanoparticles was prepared via a sonochemical
route from an aqueous solution containing copper acetate (CH3COO)2 and
thioacetemide (TAA) in the presence of triethenolamine (TEA) as complexing
agent under ambient air. In a typical procedure, 0.01 mol Cu(CH3COO)2,
0.012 mol TAA and 5 ml TEA were mixed into 100 ml distilled water by
taking it in a 150ml round-bottom flask. Then the mixture solution was
exposed to high-intensity ultrasound irradiation under ambient air for 50 min.
Ultrasound irradiation was accomplished with a high-intensity ultrasonic
probe (Xinzhi, China; 0.6 cm diameter; Ti horn, 20 kHz, 60 W/cm2) immersed
directly in the reaction solution. At the end of the reactions, a great amount of
black precipitates occurred. After cooling to room temperature, the precipitates
were centrifuged, washed by distilled water, absolute ethanol and acetone in
sequence, and dried in the air at room temperature [150]. The study of this
method as-prepared nanoparticles states that they have regular shape, narrow
size distribution and high purity.
30. Low temperature growth of CuS nanoparticles by reflux condensation method
was done by K. Mageshwari et al. [151] They used analytical grade copper
nitrate trihydrate (Cu(NO3)2.3H2O), thioacetamide (TAA, CH3CSNH2),
sodium sulfide (Na2S), ethylenediamine (EDA) and ethanol without further
purification. In a typical synthesis, 0.1 M of Cu(NO3)2.3H2O was dissolved in
50 ml of water under constant stirring until a homogeneous blue colour
solution was obtained. Then, 50 ml of 0.2M aqueous thioacetamide solution
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
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was injected to the above suspension at 80 ºC. The resulting mixture was
refluxed for 12 h in argon atmosphere under rigorous stirring. After cooling to
room temperature naturally, the black product was collected by filtration and
washed repeatedly with deionized water and ethanol several times to remove
the impurities and by products. Finally the product was dried in oven at 60 ◦C
for 2 h. They have successfully characterized obtained product and conclude
that CuS nanoparticles is a suitable candidate in photocatalysis application.
31. CuS nanoparticles were prepared by 1mL of 1% (w/w) SDS and 3μl of 2
aminoethanethiol added to 50ml of 0.4MCu (NO3)2 solution. After bubbled
with N2 for 30 min, 50ml of 1.3×10−3 M Na2S solution was added drop wise to
the solution. The reaction was carried out for 24 h under N2 bubbled, and a
brown colloid was formed. The synthesized CuS nanoparticles had an average
diameter of 5–10 nm [152].
32. Room temperature sulfidation of 100 nm sized copper nanoparticles with
powderous elemental sulfur in chloroform results in fast formation of irregular
nanostructure covellite (CuS) particles containing nanoplates. A single-pot
reaction between sublimed sulphur powder (Lachema) and copper
nanopowder (100 nm, 99.8% purity, Aldrich) in chloroform (2 ml, RiedeldeHaen for HPLC, better than 99.8%) under Ar blanket was carried out by
homogenizing the suspension through magnetic stirring or immersion in
ultrasonic bath for 30 min. The used amounts of Cu nanopowder (0.30 g) and
sulphur powder (0.15 g) corresponded to 1:1 atomic mass ratio, and the 30
min reaction time was sufficient for an almost complete reaction. Thereafter,
chloroform was evaporated and the obtained solid dark ultrafine powder was
dried under low pressure [153]. This powder was characterized by Raman and
UV- Vis spectroscopy, X-ray diffraction, scanning and transmission electron
microscopy.
33. J. N. Solanki et al. [154] reported copper sulfide nanoparticles synthesis by
microemulsion method. Here, reducing agent was sodium borohydride
(NaBH4, 95%) and copper chloride (CuCl2, 99%) purchased from Merck
Specialties, Mumbai, India. The nonionic surfactant polyoxyethylene octyl
phenyl ether (Triton X-100), dioctyl sodium sulphosuccinate (AOT, 99%),
cyclohexane, copper acetate, thiourea and ammonia solution (25 wt.%) all
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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were of analytical grades and purchased from S.D. fine chemicals, Mumbai,
India. Gamma-alumina powder, Al2O3 of 98% purity and 100 mesh size was
purchased from National Chemicals, Vadodara, India. All the chemicals were
used without further purification. Distilled water was used for preparing all the
aqueous solutions.
The nonionic surfactant, Triton X-100 (TX-100), is used for the preparation of
water-in-oil
(W/O)
microemulsion.
Microemulsion-I
composed
of
cyclohexane as solvent, TX-100 as surfactant and aqueous solution of copper
ammonia complex. Solution of surfactant TX-100 (0.2 mol/L) was prepared
by dissolving required amount of Triton X-100 in cyclohexane and vigorously
stirring by high-speed blender at 12,000 rpm. High-speed blender (Boss,
India) containing turbine type of agitator was used for stirring purpose.
Ammonia solution (25 wt.%) was added drop wise to the copper acetate
aqueous solution (0.6 M) and pH variation was monitored, until pH of 11 was
obtained. Required quantity of the prepared aqueous solution was then added
to definite quantity of organic mixture, TX-100 in cyclohexane, to get desired
water-to- surfactant molar ratio (w) of 2. Vigorous stirring was used for proper
emulsification. Similarly, microemulsion-II of same water-to-surfactant molar
ratio (w) value was prepared simply by replacing solution of copper ammonia
complex by that of thiourea (0.6 M) solution.
The microemulsion-I and microemulsion-II were then mixed in equal
quantities via magnetic stirring for 5 min; during the mixing the color turns to
be greenish due to the precipitation. The precipitation settled after 10 h and the
yellow color supernatant solution having nanoparticles of copper sulfide was
then separated by simple filtration and used for further analysis. The effect of
most crucial operating parameter, water-to-surfactant molar ratio (w), on the
product specification including size as well as size distribution and
morphology were investigated.
34. The aggregation of CuS nanoparticles was synthesised by a hassle-free
aqueous route under microwave irradiation giving remarkable spherical shape
by utilizing Cu(CH3COO)2.H2O as source of copper and Na2S2O3.5H2O as
source of sulfur.
Solutions were prepared for copper and sulfur sources of
required molarities. Then, solution of copper source was added to solution of
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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sulfur source drop wise under sturdy stirring condition. The mixed solution
was then treated under microwave irradiation (2.45 GHz) at 160 and 320 W
for 30 and 15 min by a domestic microwave oven. After the treatment of
solution, CuS (black ppt) was formed which were collected. These CuS (black
ppt) was washed by distilled water and ethanol several times and dried at 60ºC
in air [155]. CuS nanoparticles have great stability in inert atmosphere and no
phase change was observed in thermal analysis.
35. Copper sulfide nanoparticles (CuS) were successfully synthesized by the
pulsed plasma liquid method, using two copper rods as electrodes submerged
in molten sulfur. In this method, low electrical energy and no high temperature
was applied for synthesis. Experimental setup for copper sulphide
nanoparticles synthesis consists of power source and glove box, containing the
Pyrex beaker with sulphur, which needs to be heated (120 °C) in order to be in
a liquid state. Two copper electrodes were submerged in the molten sulphur,
and connected to a power source. After the sulphur powder was melted,
another 150 gm of sulphur was added and heated to 140°C to melt, and was
kept at this temperature by a temperature controller throughout the experiment.
Copper rod electrodes with diameter of 5 mm and 150 mm in length were used
(purity of 99.98%). Electrical voltage of 180V, current of 3 A, and frequency
of 60 Hz were applied for the synthesis. Single pulse duration was equal to 10
microseconds (μs). Nitrogen gas (N2) was blown into the glow box, in order to
keep the oxygen content below 5% for safety purpose [156]. The obtained
product was analyzed by XRD, HRTEM, FESEM, XPS, and Raman
spectroscopy.
36. Various kind of copper sulfides were synthesized by simply adjusting the
amount of copper chloride and sodium sulfide in a solvothermal process [157].
The typical copper sulphide powder synthesis procedure is as follows: 12
mmol of CuCl2 were dissolved in a beaker containing 28 mL of deionized
water and 14 mL of ethanol, which is named copper precursor solution A. And
12 mmol of Na2S were also dissolved in a beaker containing 28 mL of
deionized water and 14 mL of ethanol, which is named sulphur precursor
solution B. Then, solution A was slowly added to solution B under a vigorous
stirring condition (the molar ratio of Cu2+/S2- =1/1). Immediately the mixing
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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solution changed to a black suspension. Then, the black suspension was
transferred into a 100 mL Teflon-lined autoclave maintained at 140 °C for 12
h. After cooling to room temperature, the black precipitate was collected,
washed with deionized water and ethanol, and dried at 60 °C in air. The
difference in stoichiometry resulted in different morphologies and different
optical properties of the products were observed.
CuS nanorods
37. X. H. Liao et al. [158] reported a microwave assisted heating method for
preparation of copper sulfide nanorods. In a typical procedure, 0.005 mol
analytical grade Cu(NO3)2.3H2O was dissolved in 100 ml 1.5% (w/v) sodium
dodecyl sulfate (SDS) aqueous solution. Then, 0.01 mol thioacetamide (TAA)
was added into the solution, primrose yellow precipitation was observed,
which may be a precursor containing Cu–SDS–TAA composition. Finally, a
flask of 250 ml was filled with the mixture solution. The reaction was carried
out in a microwave refluxing system for 20 min with power 20% (the means
of 20% power is that of microwave operates in 30 s cycle, on for 6 s, off for 24
s. The total power is 650 W). After cooling to room temperature, the
precipitate was centrifuged, washed with distilled water a few times. Then it
was dried in air. The final product was characterized by the TEM and XPS.
38. K. P. Kalyanikutty et al. [159] did Hydrogel-assisted synthesis of CuS
nanorods showing some evidence for oriented attachment. A sol of the
hydrogel was obtained by dissolving 5 mg (0.0075 mmol) in 100 μL of acetic
acid and 400 μL of water. In a typical reaction, for the preparation of CuS
nanorods, a gel was formed by adding 8 mg (0.04 mmol) of copper acetate to a
solution obtained by dissolving 250 mg of KOH in 250 μL water and 25 μL
distilled ethanol. To this gel was added a sol of the hydrogel. This was
thoroughly mixed under sonication, and warmed slightly to form a blue sol.
When the blue sol containing Cu(OH)2 was mixed with aqueous solution of
6.25 mg (0.08 mmol) of Na2S, then obtained a black CuS gel. A black gel so
obtained was allowed to be at 30 °C for 24 h. In order to remove the hydrogel
template, the products containing CuS nanorods were washed several times
with ethanol. Generally the product was polycrystalline suggesting that the
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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templating role of the hydrogel fiber was possibly responsible for occurrence
of oriented attachment.
39. CuS nanorods of length 60-100 nm have been synthesized by simple wet
chemical method using copper chlorides as source of Cu and carbon disulfide
as source of sulfur along with ethylenediamine as the attacking reagents. In a
round bottom flask, 2 ml of ethylenediamine (SRL, India) and 1.8 ml of CS2
(Merck, India) were added into 20 ml of distilled water and stirred for 15 min.
After that, 0.253 gm of CuCl2·2H2O (Merck, India) was added into the
solution and stirred for another 15 min at room temperature and the colourless
solution turns to green indicating the formation of [Cu(en)2]2+ complex. The
temperature of the solution was increased slowly and the green solution
becomes yellow, red and finally colourless when the temperature attains 60ºC
and the temperature was maintained for 4 h. Then, the whole solution was
refluxed at 105°C for 12 h and the black product was collected and washed by
distilled water and ethanol and finally dried in vacuum at 60°C for 4 h [160].
The CuS nanorods were studied by structural, morphological and optical
analysis.
40. A precursors decomposition route [161] to polycrystalline CuS nanorods
synthesis follow as, 0.005mol analytical pure grade CuSO4·5H2O was
dissolved in 25ml water. Then, 25ml alcohol was added into the solution.
After that, 2ml acetylacetone was added into the system under vigorous
stirring and a uniform blue white precipitate of Cu(acac)2 was formed.
Afterwards, 1ml CS2 was added into the above solution under stirring. In the
end, the reaction system was transferred into a 50ml Teflon-lined autoclave
and maintained at 120ºC for 48 h. After cooling to room temperature naturally,
the indigo blue products were obtained and filtered, washed with distilled
water and absolute ethanol several times and dried in a vacuum at 60ºC for 8 h.
The study states that the formation of the micron rods of the Cu(acac)2
precursor and its decomposition into CuS nanorods structures played crucial
role in the formation of the products.
41. The procedure employed by W. Wang et al. [162] for preparing CuS nanorods
was via room temperature one-step, solid-state route. In a typical synthesis,
2.180 g of CuCl2·2H2O and 3.072 g of Na2S·9H2O were ground for 5 min
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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each before mixing together. Then 3 ml of C18H37(CH2CH2O)10H (C18EO10)
was added to the mixture. After 30 min of grinding, the green mixtures were
washed in an ultrasonic bath several times with distilled water to remove
surfactant C18EO10, NaCl and the unreacted precursors. Finally, the product
was dried in an oven at 100ºC for 3 h. The experimental result indicated that
the surfactant played a key role for the formation of CuS nanorods.
42. Solventless synthesis of copper sulfide nanorods by thermolysis of a single
source thiolate-derived precursor has been demonstrated by T. H. Larsen et al.
[163]. The copper precursor is made by combining an aqueous Cu(NO3)2
solution (0.21 g in 36 mL) with 24.5 mL of chloroform, and then adding
sodium octanoate (0.18 g, Aldrich, 98%) as a phase transfer catalyst to
solubilize the copper cations in the organic phase. After the blue copper
octanoate complex transfers into the organic phase, the aqueous phase is
discarded. Dodecanethiol (240 μL, Aldrich, 98%) is added to the organic
solution, which changes colour from blue to green as dodecanethiol displaces
octanoate bound to the copper species. The green colour results from the
mixture of copper complexed with thiol (which produces a yellow color) and
carboxylated ligands. Evaporation of the organic solvent leaves a waxy residue
consisting of the copper precursor species. The solid residue is heated to 148
°
C for 140 min to produce a brown solid material. This material is re-dispersed
in chloroform for precipitation with ethanol to remove unreacted surfactant
and by products. A typical preparation yields 10-20 mg of purified nanorods
(yield = 10-20%). Study suggests that during the synthesis process dipoledipole interaction was responsible for this long stand of nanorods reported by
this method.
43. Template-assisted electrochemical synthesis of nanorods was done by the use
of electrolyte for electrodeposition prepared by dissolving Na2S2O3 (400 mM)
and CuSO4 (60 mM) in de-ionized water. Tartaric acid (75 mM) was used to
maintain pH of the solution below 2.5, as required. For the nanorod synthesis,
polycarbonate (PC) templates (nominal pore sizes: 200, 100, and 50 nm) were
used as working electrodes. A conductive coating of liquid paste of metallic
GaIn was applied on the backside of the template. The use of liquid metal is
beneficial in two ways; first, it can be easily removed by applying nitric acid
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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and second, it eliminates the expensive and time-consuming step of metallic
layer sputtering. PC templates are advantageous, as they can be easily
dissolved in chloroform to liberate nanorods. A platinum spiral rod was used
as a counter electrode. Nanorods were prepared by depositing copper sulfide
in the template pores at constant potential. The whole electrochemical cell was
immersed in ultrasonicator (Bransonic 2510) containing water. After the
nanorods were formed, they were liberated by dissolving the template in
chloroform.
The
solution
containing
nanorods
was
cleaned
by
ultracentrifugation [164]. Nanorods in the range of 50-200 nm in diameter
were produced and were found to be p-type semiconductors.
44. In the electrodeposition synthesis method, equimolar (0.1M) copper sulphide
(CuSO4) and sodium thiosulphate (Na2S2O3) were used as source of copper
and sulphur and 0.10M triethanolamine was used as complexing agent.
Solutions are prepared in double distilled water. The ultrasonically cleaned
stainless steel and ITO substrate are used to prepare samples. Copper sulphide
nanorods were prepared on stainless steel and ITO substrate by electro
deposition technique. Electrolytic bath containing 12 ml CuSO4 and 12ml
Na2S2O3 as sources of Cu and S ions and 6ml TEA as complexing agent with
deposition time of 15, 20, 25 and 30min. Using Cyclic Voltammetry (CV),
cyclic voltamograms of aqueous acidic bath were scanned with a scan rate of
50 mVs-1 using potentiostat (Princet on Perkin–Elmer, Applied Research
Versa-stat-II; Model250/270) in three electrode configuration. The reference
electrode was a Saturated Calomel Electrode (SCE). Deposition potential was
determined by Cyclic Voltammetry (CV) for a material deposition. Orange
colored Cu layer got deposited on the substrate at reduction potential of 0.65V. The film deposited at reduction potential of -0.6V gives blackish
sulphur layer. The electro deposition of CuS nanorods were carried out at the
deposition potential of -0.7V/SCE which gives greenish CuS nanorods. After
deposition the films were washed with double distilled water and preserved in
desiccator to avoid oxidation. Preparative parameters such as deposition time
and concentration of precursor were optimized [165].
The obtained CuS
nanorods were having diameter of 30-35nm and length of 10-15 μm at room
temperature.
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
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CuS Nanotubes
45. C. Tan et al. [166] reported a novel method for the preparation of CuS
nanotube using hydrogel based on N-lauroylalanine as template under mild
condition. The formation of N-lauroylalanine (LAA) gel in water was
described as follow: 0.0271 g LAA was mixed with 2ml 5% aqueous acetic
acid and 0.5ml ethanol in a sealed test tube and the mixture was heated until
the solid dissolved. The resulting solution was cooled at room temperature for
1 h, LAA gel (translucent) was formed. The typical processes of preparation of
the CuS nanotube are described below. LAA was dissolved in acetic acid,
ethanol and water (v/v = 1:5:19), then stoichiometric proportion of copper (II)
acetate was added at 70 ºC under stirring and ethanol was added to dissolve the
precipitate. After cooling to room temperature, the translucent gel (Cu–LAA
gel) was formed, and then double thioacetamide (TAA) which was dissolved
in 0.5ml water was added into the gel. After 2 days, CuS precipitate was
obtained. The resulting sample was examined by TEM, FTIR spectroscopy,
XRD, UV–vis absorption spectroscopy. The as-prepared copper sulfide
nanotubes were hollow with diameters ranging from 150 to 500 nm and
lengths of 1–10μm.
46. CuS nanotubes assemble with nanoparticles were successfully synthesized by
microwave-assisted solvothermal method using Cu(OH)2 nanowires in the
solvent of ethylene glycol. In a typical experimental procedure for the
preparation of CuS nanotubes assembled with nanoparticles, 0.22 g thiourea
was dissolved into 20 mL ethylene glycol under magnetic stirring at room
temperature, and the resulting solution was added into the above 10 mL
precursor (Cu(OH)2 nanowires) ethylene glycol solution under stirring. Then,
the mixed solution was loaded into a 60ml Teflon-lined autoclave, sealed,
microwave-heated to 80°C and kept at this temperature for 60 min. The
microwave oven used for the sample preparation was microwave-solvothermal
synthesis system (MDS-6, Sineo, China). After cooling to room temperature
naturally, the product was separated by centrifugation and washed with
deionized water and absolute ethanol several times. Finally, the product was
dried at 60°C [167]. This method has the advantages of the simplicity and low
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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53
cost, and no surfactant was needed. The method reported herein may be
extended to the synthesis of nanotubes of other copper-containing compounds.
47. C. Wu et al. [168] synthesized CuS nanotubes using Cu nanowire as source of
Cu+2 and thiourea as source of S-2. Before synthesis of nanotubes they
synthesized Cu nanowires, 12g NaOH was dissolved in 20 mL distilled water
to form a homogeneous solution. 1 mL Cu(NO3)2 aqueous solution (0.1 M)
was then added under magnetic stirring, followed by 150 mL ethylenediamine
(EDA, 99 wt%) and 25 mL hydrazine (35 wt%). After a thorough mixing, the
reactor was kept at 60°C for 2 h. Cu nanowires were obtained after washing
with distilled water and absolute ethanol for several times and collected. In a
typical experimental procedure for the synthesis of CuS nanotubes, 0.1 mmol
of the as-prepared Cu nanowires were dispersed by sonication in 20ml
ethylene glycol, in which 0.2 mmol thiourea was previously dissolved. The jar
was then sealed and kept at 80°C for 12 h. The obtained black solid product
was collected by centrifuging the mixture, then washed with absolute ethanol
for several times and dried in a vacuum at 60°C for characterization. The shape
evolution process and the formation mechanism of CuS nanotubes as well as
the thermal stability of these nanotubes were investigated.
48. The CuS nanotubes were solvothermally prepared by reduction of copper
nitrate and sodium thiosulfate at 150°C for 12 h in a Teflon lined stainless steel
autoclave with a capacity of 60 mL using a microemulsion system. The yield
can reach up to 90 wt% [169]. The as-prepared CuS nanotube modified
electrode was used as an enzyme-free glucose sensor.
CuS nanowalls/nanowires
49. Solution growth of copper sulfide nanowalls were prepared by immersing
cleaned and polished Cu substrates (5% NaOH at 70 °C for 5 min and 10%
HNO3 for 20 s) in an aqueous solution containing Na2S (1 M) and HCl (1 M)
for 5 min and 40 min at around 4 °C. After the above immersion process, the
samples were dried in air for the characterizations [170].
50. Vertically oriented CuS nanowalls supported on a copper substrate was
synthesized through a facile method involving an inorganic vapor-solid phase
reaction by X. Feng et al. [171]. In a typical procedure, sulfur powder and
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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copper foil (1.5 cm × 3 cm × 0.5 mm) were kept in two separate ceramic boats
(the distance between the copper foil and the sulphur powder is about 4 cm)
and placed at the centre of a quartz tube which was inserted into a horizontal
tube furnace along the argon gas flow direction in sequence. When the flow
rate of argon gas was kept constant, the furnace was heated to an appropriate
temperature within 1 h and kept at that temperature for another 1 h. Finally, it
was cooled to room temperature. After reaction, the products grew on the
surface of the copper foil as dark blue films. The as-prepared CuS nanowalls
exhibit excellent field emission properties, which suggest that the CuS
nanowalls may have potential applications in the vacuum microelectronics
industry.
51. Spontaneous growth of copper sulfide nanowires from elemental sulfur in
carbon-coated Cu grids has been reported by Q. Han et al. [172]. Here 1.58 g
Na2S2O3was added to 14 mL distilled water under stirring, then 2 mL
concentrate hydrochloric acid (HCl, 36%) was added. The resulting mixture
was poured into the Teflon-liner autoclave of 20 mL capacity and was
maintained at 140°C for 12 h. When the reaction was completed, the product
was filtered and washed with water and absolute alcohol for several times, and
dried under vacuum for 12 h. They have demonstrated a simple solid-state
approach for the synthesis of nanowires from elemental sulfur on TEM Cu
grids under ambient conditions.
52. Y. C. Chen et al. [173] successfully fabricated CuS nanowires by sulfuring
method and studied the optical properties of it. For fabrication, high-purity
(99.9995%) aluminum foil was used as the starting material. Anodized
aluminum oxide (AAO) was prepared by a two-step anodizing process. The
alumina template was formed by anodizing an Al plate in H2SO4 solution
under constant voltage of 25V. After anodization for several hours, the
alumina membrane was immersed in an etching solution of H3PO4 to remove
the alumina layer. Then, the aluminum foil was anodized again. After the
anodization, the remaining aluminum was etched by HgCl2. To widen the pore
diameter, the alumina template was immersed in a solution of H3PO4. After
this process, the diameter of the holes of the alumina membrane was adjusted
to about 30 nm. Arrays of Cu nanowires were fabricated by electrochemical
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
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deposition into the nanometer-sized pores. To prepare Cu nanowires, a layer of
Pt film was sputtered onto one side of the through-hole AAO template to serve
as the working electrode in a two-electrode electrochemical cell. The
electrodeposition was carried out at appropriate voltage conditions, using an
electrolyte containing CuSO4·5H2O and H3BO3. Then the samples together
with the sulfur powder were annealed in vacuum sealed quartz tube for several
hours at 400, 450, and 500°C, respectively.
53. The hydrothermal synthesis [174] of copper sulfide from Cu-DTO as a singlesource precursor was carried out by taking 0.3 g of this complex dispersed in
35 mL of distilled water in a Teflon-lined stainless steel autoclave and
maintained at 120°C for 24 h. After completion of the reaction, the reactor was
allowed to cool to room temperature naturally. The black product obtained
was filtered, washed thoroughly using distilled water and ethanol, and finally
dried in a vacuum at 60ºC for 4 h and characterized. The nanowires were 4080 nm in diameter and up to a few microns long, and a possible reaction
mechanism of their formation was proposed. The effects of reaction
temperature, duration, and solvents also were studied.
54. One step template-free electro synthesis [175] of 300 μm long copper sulfide
nanowires were grown from a solution consisting of 1.0 mM CuSO4 and 4.0
mM thiourea (TU) as the source for copper and sulfur, respectively. Copper
sulfate and 18 mL of concentrate hydrochloric acid were dissolved in 700 mL
of dionized water. Afterwards, TU was added to the solution and mixed. When
the resulting mixture turned clear, the total volume of the solution was filled to
1 liter by adding deionized water and finally the pH of the solution was
adjusted to 1.8 by adding HCl. Each experiment was conducted with 800 mL
of fresh electrolyte. The electro deposition of copper sulfide nanowires was
carried out by pulse potential (Voff=0.0 V and Von=−0.85 V) with on-time of
10 ms and off-time of 20 ms (duty cycle 33%). The deposition time was
between 30 and 210 min. Depending on the deposition duration, the nanowires
have diameters between 40 and 600 nm and lengths up to 300 μm.
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
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CuS Nanosheet/ Nanowhishkers/ Nanoribbons
55. To synthesize ultrathin CuS nanostructures, 1.5mmol of copper (I) chloride
(CuCl) was added to a mixture of 5 ml oleylamine (OM) and 5ml octylamine
(OTA) in a three-necked flask (100 ml) at room temperature. The slurry was
heated to 100°C with vigorous magnetic stirring under vacuum for ~30 min in
a temperature-controlled electro mantle to remove water and oxygen. The
temperature was maintained at 130°C for 4 h and the solution became
transparent. Then, the sulphur (S) dispersion formed by ultra-sonication of 4.5
mmol of S powder in the mixture of 2.5 ml OTA and 2.5ml OM at room
temperature was quickly injected into the resulting solution at 95°C. The
resulting mixture was kept at 95°C for 18 h, and it became dark. After it cooled
to room temperature, the CuS nanosheets were precipitated by adding the
excess absolute ethanol (~40 ml) into the solution [176]. CuS nanosheets
synthesized by this method were used for fabrication of an electrode for a
lithium-ion battery. They exhibited a large capacity and good cycling stability,
even after 360 cycles.
56. S. H. Chaki et al. [177] synthesized CuS nanowhiskers by simple wet
chemical route. In the synthesis, 10 ml of 0.5 M copper (II) chloride solution
was rigorously mixed with 5 ml of 4 M triethanolamine (TEA) solution in a
100 ml glass beaker for 5 minutes. Then, 16 ml of 2 M ammonia followed by
10 ml of 1 M sodium hydroxide solutions were added under constant stirring
of 5 minutes each respectively. Finally 6 ml of 0.5 M thiourea was added and
stirred for 5 minutes. The final volume of the solution was made 100 ml by
adding 53 ml double distilled water. After 2 hours, greenish-black precipitates
settled at the bottom of the glass beaker were filtered and washed with double
distilled water and absolute methanol for several times. The final precipitates
were dried in oven at 45 °C for 2 hours to get the final CuS nanowhiskers
yield.
The
synthesized
CuS
nanowhiskers
were
characterized
for
stoichiometry, structure, optical absorption, etc.
57. Synthesis of CuS nanoribbons by hydrogel was done by C. Tan et al. [178]. In
the synthesis, initially CuS mineralization template by gel C12-Glu in ethanol–
water was done, the procedure is described as follows. The mixture of
compound C12-Glu (37.1mg) and ethanol–water (2.0 mL, v/v = 1/4) in a sealed
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
57
vial was heated until the solid disappeared, then the stable gel formed after it
was cooled to room temperature. After the gels were aged for approximately 1
day, water (5.0mL) was added and stirred for 5 h at room temperature. After
that, aqueous solution (1.0mL) containing 0.1mol L−1 Cu(OAc)2 was added
into the above solution and stirred for 2 h at room temperature. After that 40
mg thioacetamide was added into the solution containing the gels and a black
product appeared soon. After that, additional ethanol (5 mL) was added and
the inorganic product was isolated by centrifugation. Finally, the CuS was redispersed in ethanol. The as prepared copper sulfides showed a nanoribbon
structure with diameter of 30–70nm and lengths of 1–10μm.
CuS Spheres
58. Copper sulfide hollow spheres were prepared via a solvothermal technique
[179] in a Teflon-lined stainless steel autoclave. In a typical synthesis of CuS
hollow spheres, 2mmol of Cu(NO3)23H2O was dissolved in 25 ml of absolute
ethanol to form a clear solution, and then 4 mmol of thioacetamide (TAA) was
added to this solution under vigorous stirring. Afterwards, this solution was
transferred into a 30mL Teflon-lined stainless steel autoclave. The autoclave
was sealed and maintained at 120°C for 16 h. After the solution was cooled to
room temperature, the obtained black solid products were collected by
centrifuging the mixture, and were then washed with absolute ethanol and
deionized water several times and dried at 60°C for 6h for further
characterization. For solid CuS spheres, 2mmol of Cu(NO3)2.3H2O and 4
mmol of NH4SCN were dissolved in 25mL deionized water, and then 0.4g
poly(vinypyrrolidone) (PVP, Mw = 80,000) was added to this solution under
vigorous stirring. Afterwards, this solution was transferred into 30mL Teflonlined stainless steel autoclave and maintained at 210°C for 10h. Due to the
unique optical property, these hollow structures were envisaged to be used in
applications such as novel building blocks for the advanced materials,
catalysis, solar cell devices, and drug delivery system.
59. CuS hollow spheres were synthesized through a facile microemulsiontemplate-interficial-reaction route [180] using copper naphthenate as metal
precursor and thioacetamide as the source of sulfur. Starting with, 15 mg
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
58
thioacetamide dissolved in 2 ml deionized water was kept for 5 min at 50 °C
and then was dropped into the microemulsion. Meanwhile, the microemulsion
solution became brown immediately, indicating the formation of CuS. After 5
min, a dark green powder was obtained. The product was collected by
centrifugation, washed several times with deionized water and ethanol. The
final products were dried in a vacuum furnace at 80°C for 2 h. The analysis of
the final product concludes that the size of the hollow spheres can be tailored
by changing the content of oil phase. The reaction conditions that can control
interfacial reaction rate were important factors for forming hollow spheres.
60. CuS hollow spheres synthesis [181] was carried out using reagents of
analytical grade and used without further purification. Here 25 ml, 2 mmol.L-1
CuSO4 solution (0.05 mmol CuSO4) and 0.24 g of poly-(vinylpyrrolidone)
(PVP-K30) were added into a conical flask under magnetic stirring at room
temperature. Then, 25mLof NaOH solution with pH value of 9.0 (prepared by
dropping 0.01 mol.L-1 fresh NaOH solution into distilled water until the pH
value of the mixture reached 9.0) was added into the above mixture. After
being stirred for 2 min, 2.0 mL of 0.10 mol.L-1 N2H4.3H2O solution was
added. A suspension of Cu2O spheres was obtained after a reaction of 5min.
Then 0.266 mmol thioacetamide was added into the above suspension and the
temperature of the mixture was heated to 40°C. After a further reaction of 1h at
40°C under magnetic stirring, the product was obtained, centrifuged, washed
with distilled water and ethanol, and then dried under vacuum at room
temperature. By repeating the experiment for 10 times, the total product yields
of Cu2O and CuS spheres were about 82% and 76%, respectively. The
experiment results revealed that the formation of loose aggregates of Cu2O
nanoparticles was the key to the fast synthesis of hollow spheres at low
temperature. The thickness of the shell can be controlled easily by adjusting
the aggregation degree of the Cu2O nanoparticles.
61. Nanoplate-based copper sulfide (CuS) hierarchical hollow spheres were
synthesized using the spontaneous oil droplets as the templates in two-phase
system [182]. In a typical synthesis, 0.03 g thioacetamide was added to 18.5
ml of deionized water and stirred for several minutes at room temperature (15
°
C). Then 0.08 g of copper naphthenate (CNC) was dissolved in 1 ml
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
59
dimethylbenzene which was in the oil phase. This oil phase 1 ml was added to
the above aqueous solution without stirring, and then the cyan oil layer was
formed. Meanwhile, the interfacial reaction had started. The mixture was kept
still at room temperature. After 24 h, the colour of the cyan oil layer
disappeared, and a dark brown film was formed at the oil/ water interface,
indicating the formation of poorly crystallized CuS hollow spheres. In
addition, the water phase had become brown. The dark brown film was
collected, washed several times with dimethylbenzene (DMB) and ethanol,
and then was transferred into stainless steel autoclaves and maintained at 60°C
for 96h in 10ml ethanol, resulting in the formation of hierarchical CuS hollow
spheres. Finally, a dark green powder was obtained. The product was washed
several times with deionized water and ethanol. The final products were dried
in a vacuum furnace at 60°C for 2h. The photocatalytic activity of the
hierarchical CuS hollow spheres has been evaluated by the degradation of
methylene blue solution in the presence of hydrogen peroxide under natural
light, showing that the as-prepared hierarchical CuS hollow spheres exhibit
high photocatalytic activity for the degradation of methylene blue (MB).
62. W. Wang et al. [183] synthesized CuS hollow nanospheres in aqueous solution
at room temperature. Typically, 0.24 g Cu(NO3)2·2H2O and 0.05 g sodium
dodecyl sulfate (SDS) were dissolved in 100 ml distilled water to form a
transparent solution. Then, the solution was mixed with 50ml 1M
thioacetamide (CH3CSNH2). The colour of the system changed gradually from
light blue to milk white, then to light orange and brown. At last, the colour
turned to black after 15h, indicating the formation of copper sulphide. The
black deposition was collected (over 98% yield, based on the amount of
Cu(NO3)2·2H2O input) and washed with distilled water and anhydrous ethanol
for several times, and then dried in a vacuum at 60°C for 6h. The products
were characterized by XRD, EDAX, FESEM and TEM. The study showed
that SDS played a key role in the synthesis process.
63. Reagents of analytical grade without further purification were used for the
synthesis of CuS spheres by a hydrothermal method [184]. Here 50 ml of
CuSO4 with a concentration of 0.01M, 50 μL of thioglycolic acid (TGA) and
50 ml of thioacetamide (CH3CSNH2) with a concentration of 0.02M were
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
60
mixed slowly under stirring. After 10 min stirring, the final solution was put
into a Teflon-lined stainless steel autoclave and then sealed. The autoclave
was maintained at 200°C for 20h and then cooled to room temperature
naturally. The mixture turned black due to the formation of CuS precipitates.
The product was filtered out, washed with alcohol and deionized water for
several times, and then dried at 60°C for 30 min in air. According to the
analysis of this product it was found that TGA –assisted hydrothermal process
offers great opportunity for scale-up preparation of other morphology
chalcogenides.
1.4.3
Properties of CuS nanomaterials
Nos
1
Properties
Structure
Hexagonal, a=b= 3.760-3.802 Å and c= 16.210-16.430 Å
[155,176,182,185-187]
Optical
1.46 -3.32 eV (Direct and Indirect bandgap)[177,188-191]
Thermal
Decompose temperature between 230-250 ᵒC to 300-330 ᵒC [168,
157]
Electrical
Semiconductor, Resistivity=16-41 Ω.cm (room temperature),
activation energy =0.14 -0.29 eV [151]
Mechanical yield strength (YS)= 445MPa, tensile strength (TS)=554 MPa
[192]
Magnetic
ᵡm=1.198×10-3 emu.mol-1 (Weak Paramagnetic) [138]
Chemical
Change the morphology of the CuS nanomaterials due to the
copper to sulphur molar ratio [138,193]
2
3
4
5
6
7
1.4.4
Application of CuS nanomaterials
1.
Catalyst [13].
2.
Photocatalyst [194].
3.
Ultrasensitive nonenzymatic glucose sensor [169].
4.
Electrocatalytic activity [195].
5.
Nonenzymatic amperometric sensor of hydrogen peroxide [193].
6.
Nanoswitches [196].
7.
Lithium ion battery [186].
8.
Biological application [197].
9.
Solar cell and electronic circuit [198].
10. Electrochemical sensor for detecting cyteine, ascorbic acid [199] and methyl
orange [195].
11. Gas sensitivity/ Gas sensor [122, 200].
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
61
12. Drug delivery system [179].
13. Environmental pollution control [201].
14. Photovoltaic application [202].
15. DNA biosensor [203].
16. Vacuum microelectronic industry [171].
17. Electrochemical storage materials and resistive switching devices [204].
18. Optoelectronic devices [205].
19. Optical recording materials [206].
20. LED [207].
21. Thermoelectric generator [208].
1.5
Conclusions
A member of the transition metal chalcogenides, covellite copper sulfide (CuS)
belonging to IB-VIA group has received much attention in recent time [10-13] due to
its potential technological applications.
The literature survey showed very little work reported on CuS single crystals
(Chapter 1, Section 1.2), so the author thought of growing CuS in single crystals form.
The CuS single crystals were grown by chemical vapour transport (CVT) technique
using iodine as a transporting agent. The as grown single crystals were thoroughly
characterized for their structural, optical, electrical, thermal, etc. properties
(Chapter - 2).
The author synthesized CuS in thin films form by dip coating technique, since
literature showed no report of CuS thin films synthesized by this technique. The
author carried out comparative study of the synthesized dip coated CuS thin films
with chemical bath deposited (CBD) CuS thin films (Chapter – 3).
Literature survey reveals that properties and characterstics of CuS nanomaterials
improves and changes on doping with different elements such as Zn [209], Fe [210]
and Ni [138]. But there is no report of Mn doped CuS nanoparticles. Therefore
undoped and Mn doped CuS nanoparticles were synthesized by simple wet chemical
route at ambient temperature. The comparative study of undoped and Mn doped CuS
nanoparticles were done for structural, morphological, optical, photoluminescence,
thermal, magnetic, electrical transport, etc. properties (Chapter - 4). Also study of the
catalytic activity of synthesized CuS nanoparticles was carried out for cellulose
pyrolysis (Chapter – 5).
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
62
The CuS single crystal growth, synthesis of thin films, synthesis of nanoparticles
and use of CuS as thermal catalyst, together with all the obtained characterization
results are deliberated in details in the next subsequent chapters of this thesis.
Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014
Chapter 1
1.6
63
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