chapter 1 existing information on covellite copper...

Chapter 1 1

Jiten Tailor/Ph.D Thesis/P. G. Dept. of Physics/Sardar Patel University/May-2014

CHAPTER 1

Existing information on covellite Copper Sulphide

(CuS)

Chapter 1 2


1.1 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.

http://en.wikipedia.org/wiki/Sulfide

http://en.wikipedia.org/wiki/Selenide

http://en.wikipedia.org/wiki/Telluride

http://en.wikipedia.org/wiki/Covalent

http://en.wikipedia.org/wiki/Zinc_sulfide

http://en.wikipedia.org/wiki/Diamondoid

http://en.wikipedia.org/wiki/Wurtzite

http://en.wikipedia.org/wiki/Nickel_arsenide

http://en.wikipedia.org/wiki/Nonstoichiometry

Chapter 1 3


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

(20C); 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

115.21C; 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

http://en.wikipedia.org/wiki/Pigment

http://en.wikipedia.org/wiki/Cadmium_sulfide



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)

Chapter 1 5


(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.

http://en.wikipedia.org/wiki/Ball-and-stick_model

http://en.wikipedia.org/wiki/Covellite

Chapter 1 6


Figure 2 (a) (b) (c) (d)

(a) trigonal coordination of copper

(b) tetrahedral coordination of copper

(c) trigonal bipyramindal coordination of sulphur

(d) 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].

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].

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 a =3.79 Ǻ,

c=16.34 Ǻ, Z=6 [6]

8 Bond length

Cu-Cu =2.19 Ǻ

S-S =2.07 Ǻ

Cu-S= 2.30 Ǻ [16]

9 Melting Point transition 507 ᵒC

[6]

10 Solubility Soluble=HNO3,

NH4OH, HCN

Insoluble=HCl,

H2SO4 [6]

11 Refractive Index 1.45 [6]

12 Magnetic susceptibility (ᵡm) - 2.0 ×10-6

cm3 mol

-1[6]

13 Hardness 1.8 Mohs Scale [6]

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

∆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]

15 Solubility Product Constant ĸsp= 6×10-16

[6]

16 Superconductivity 1.6 K [19]

17 Electric conductivity Metallic hole

conduction [20]

Chapter 1 9


1.2 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; M ̕= 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,

Chapter 1 10


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,

Chapter 1 11


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 2

ᵒC.min

-1 maintaining at this temperature for 5 h

Chapter 1 12


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. Properties

1 Structural Hexagonal , Space group: P63/mmc [43]

2 Optical Indirect bandgap:1.21 eV [45]

3 Thermal stability Decompose at 507±2 ᵒC [46]

4 Electrical Resistivity: 210 μΩ•cm (at room temperature) [42,47]

p-type metallic conduction[20]

5 Magnetism Diamagnetic [48]

Chapter 1 13


1.2.4 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, micro-

miniaturisation 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

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.

Chapter 1 15


Chapter 1 16


1.3.2 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 (LP-

MOCVD) and by aerosol assisted chemical vapour deposition (AACVD)

using the novel air stable (asymmetric carbamato) compound

[Cu(S2CNMenHex)2] at high temperatures of 450

ᵒC to 500

ᵒC [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 1–10Ωcm resistivity [61]. The Si wafer was

dipped in boiled NH3+H2O2+6H2O solution for 10 min and followed by a 10

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

ᵒC 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

260C 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

Chapter 1 18


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 liquid-

liquid interface using copper cupferronate (Cu(cup)2) as the copper source and

Chapter 1 19


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 F-

120 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) b-

diketonate 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

Chapter 1 20


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

Chapter 1 21


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

Chapter 1 22


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

Chapter 1 23


(H2SO4–H2O2, 4:1 vol. vol-1

) and then immersed in 3-(trimethoxysilyl)-1-

propanethiol 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

Chapter 1 24


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

Chapter 1 25


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,

Chapter 1 26


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. Properties

1 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 Solid –

State

Refractive index (n) = 2.05, Real dielectric constant (εr) = 4.22

Optical conductivity (σo) = 1.32×10-13

sec-1

[91]

4 Electrical

transport

properties

Semi metallic, Sheet resistance (Rs) = 154 Ω/□

Electrical conductivity (σ) = ~2×103 S cm

−1

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]

Chapter 1 27


6 Mechanical Data at 0.5 mN Load

Elastic modulus (EП)= 96.8

GPa

Hardness (HП)= 9.9 GPa

Hardness (HV)=915.5 Vickers

Data at 1 mN Load

Elastic modulus (EП)= 91.4 GPa

Hardness (HП)= 7.3 GPa

Hardness (HV)=676.5 Vickers

[92]

1.3.4 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].

Chapter 1 28


1.4 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].

Chapter 1 29


1.4.1 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.

Chapter 1 30


Chapter 1 31


1.4.2 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 ᵒ

C for 24 h. After the reaction was

Chapter 1 32


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 CuSO4

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

Chapter 1 33


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

Chapter 1 34


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 two-

necked 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

Chapter 1 35


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

Chapter 1 36


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

ºC 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

Chapter 1 37


[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

Chapter 1 38


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 2×10−2

mbar 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

Chapter 1 39


(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

Chapter 1 40


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 micro-

interface 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

ºC 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

Chapter 1 41


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

Chapter 1 42


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 as-

prepared 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

Chapter 1 43


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 (bis-

2ethylhexyl) 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

Chapter 1 44


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

Chapter 1 45


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, Riedel-

deHaen 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

Chapter 1 46


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

Chapter 1 47


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

Chapter 1 48


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

Chapter 1 49


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

Chapter 1 50


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 dipole-

dipole 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

Chapter 1 51


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 mVs-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.

Chapter 1 52


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

Chapter 1 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

Chapter 1 54


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

Chapter 1 55


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 single-

source 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 40-

80 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.

Chapter 1 56


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

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 re-

dispersed 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)23H2O 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 Teflon-

lined 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 microemulsion-

template-interficial-reaction route [180] using copper naphthenate as metal

precursor and thioacetamide as the source of sulfur. Starting with, 15 mg

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

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

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 Properties

1 Structure Hexagonal, a=b= 3.760-3.802 Å and c= 16.210-16.430 Å

[155,176,182,185-187]

2 Optical 1.46 -3.32 eV (Direct and Indirect bandgap)[177,188-191]

3 Thermal Decompose temperature between 230-250 ᵒC to 300-330

ᵒC [168,

157]

4 Electrical Semiconductor, Resistivity=16-41 Ω.cm (room temperature),

activation energy =0.14 -0.29 eV [151]

5 Mechanical yield strength (YS)= 445MPa, tensile strength (TS)=554 MPa

[192]

6 Magnetic ᵡm=1.198×10-3

emu.mol-1

(Weak Paramagnetic) [138]

7 Chemical Change the morphology of the CuS nanomaterials due to the

copper to sulphur molar ratio [138,193]

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].

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).

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.

Chapter 1 63


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chapter 1 existing information on covellite copper...

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