29

2. CADMIUM AND MANGANESE BASED SULPHIDES

This Chapter gives a brief review (though not comprehensive) of various

studies made on cadmium and manganese based sulphides in the recent past.

2.1. Cadmium Sulphide (CdS) Transition metal chalcogenides CdS, MnS, ZnS, CuS, CdSe, CdTe, ZnSe,

MnSe, etc (MX; M = Metal, X = S, Se, Te) represent a class of II-VI compound

semiconducting materials. Metal chalcogenides have attracted considerable interest

due to their intriguing properties and structural diversity. The small dimensions of

these chalcogenide nanoparticles result in different physical properties from those

observed in the corresponding macrocrystalline bulk materials. As particle sizes

become smaller, the ratio of surface atoms to those in the interior increases, leading to

the surface properties playing an important role in the properties of the materials.

Chalcogenide nanoparticles also exhibit a change in their electronic properties relative

to those of the bulk materials, as the dimensions of the solids get smaller the bandgap

becomes larger. This allows chemist and materials scientists to change the electronic

properties of the materials simply by controlling their sizes, progress in the synthesis of

chalcogenide nanoparticles will promote the research on their applications. Meanwhile,

advances in the applications will bring new challenges in the synthesis of these

materials to both synthetic chemists and materials scientists [12].

The wide-bandgap chalcogenide semiconductors are of current interest for

optoelectronic application, light-emitting diodes and optical devices. CdS has wide

bandgap of 2.4eV. MnS and ZnS also have wide bandgaps of 3.7 and 3.6eV

30

respectively [60-62]. Semiconductor nanocrystals displaying interesting electronic

and optical properties attributed to the quantum confinement effect and the large ratio

of surface atoms are of both fundamental and technological interest. In particular,

chalcogenides have received much attention for potential applications due to the

tunable electronic bandgaps depending on the size and shape of nanocrystals [63].

Diluted magnetic semiconductors (DMS) are very important materials in

current research. Group II-VI semiconductors with Mn doping such as (Zn, Mn) and

(Cd, Mn) chalcogenide mixed crystals are typical and most extensively studied DMS.

DMS has great application in blue green light emitters [64, 65].

The dielectric constant ( εs and ε∞ ), refractive index, effective mass (in units of

the free electron rest mass) of charge carriers and energies (eV) related to the optical

properties of some of the metal chalcogenides [66] are provided in Table. 2.

Table 2 : Dielectric constants (εεεεs and εεεε∞) refractive index, effective mass (in units

of the free electron rest mass) and band gap energy (eV) for some of the metal chalcogenides

Compound Structure Dielectric constant Refractive

index, n

Effective masses Eg

(eV) εεεεs ε∞ *

nm *pm

CdS Wurtzite

(ε || c) 8.64

(ε⊥c) 8.28

5.24 2.30 0.11 0.30 2.58

CdSe Wurtzite

(ε || c) 9.25

(ε⊥c) 8.75

6.4 2.55 0.13 0.45 1.84

ZnS Zincblende 8.1 5.13 2.26 0.27 0.58 3.91

ZnSe Zincblende 8.66 5.90 2.43 0.16 0.60 2.80

31

In most studies nanocrystalline semiconductors belonging to the II-VI group

have been considered. These are relatively easy to synthesize and generally prepared

as particulates or in the form of clusters. Among II-VI compounds CdS is one of the

most studied materials. Others like ZnS, AlSb and GaAs have also been studied for

their interesting properties. There are various techniques to synthesize nanoparticles

such as chemical bath, sol-gel, gas evaporation, magnetron sputtering and

precipitation [67].

This part is an integral part of continued investigation of cadmium sulphide

(CdS). It deals with various aspects of synthesis and characterization.

Cadmium sulphide (CdS) is one of the transition metal chalcogenides. It is an

important inorganic II-VI semiconductor compound with a direct bulk phase band gap

of 2.4eV at room temperature [33,68]. It is also an n-type semiconductor. With the

emergence of CdS nanocrystals which demonstrate properties lying between the

molecular and bulk limits, a number of striking effects such as size quantization,

nonlinear optical behaviours and unusual fluorescence have also been explored [69].

Hexagonal CdS belongs to P63 mc space group [70].

Cadmium sulphide (CdS) is one of the most important nano-structural

semiconductors that have been widely studied due to their potentiality in the possible

application in optoelectronic devices and photocatalysis. CdS in a nanocrystalline

form can be prepared by a variety of methods (both physical and chemical) like sol-

gel, electrostatic depostiion, gas evaporation, micelles solvent growth, etc[11].

32

Cadmium sulphide has three types of crystal structure: Cubic zincblende

(Hawleyite), hexagonal wurtzite (Greenockite) and orthorhombic structure. Many

synthesis methods for CdS nanoparticles have been reported.

Murray et al [71] have prepared high quality CdE (E = S, Se, Te)

semiconductor nanocrystals. The average sizes of the prepared crystallites were 12 to

115Å. Khiew et al [72] have prepared 5.1 nm sized nano CdS by nonionic water-in-oil

microemulsion method [73, 74]. Chamarro et al [75] have prepared CdS doped with

Mn by inverted micelle technique. The average size of the nanocrystals was equal to

15Å. Liu et al [76] have synthesized CdS nanoclusters by hydrothermal

microemulsion method. The mean diameter of the synthesized CdS cluster at 30°C

was 20 nm and 120°C was 80 nm.

Yu et al [77] has prepared CdS thin films comprising nanoparticles by a

solution growth technique. Solution of cadmium acetate ammonia and triethanalamine

were mixed slowly in deionized water at room temperature. A glass substrate was

mounted vertically in a beaker containing this solution. Thiourea solution was added

at 80°C. The average size of the particles was obtained from 5 to 20 nm. Henglein

et al [78] have prepared 3 nm size particles by pulse radiolysis method.

Chen et al [79] have synthesized metal sulphide nanoparticles by one step

reaction between metal salts and thiourea in ethylene glycol under the microwave

irradiation method. In this synthesis ethylene glycol acting as both reaction media

and dispersion media can efficiently absorb and stabilize the surface of the particles

with good dispersivity [80]. Pattabi and Uchil [81] have synthesized nanoparticle CdS

films on biological membrane substrate. Double layer chicken-egg membrane was

33

used as the host matrix. Chemical reaction of aqueous solutions of cadmium acetate

and thiourea were allowed to diffuse across the membrane for different periods to

control the deposition time. Mandal et al [82] have fabricated Schottky diode. 3-5 nm

sized CdS layer was deposited by magnetron sputtering technique. Othamani et al

[83] have prepared pure and porous silica xerogels doped CdS nanocrystals by a sol-

gel process.

Saviot et al [84] obtained CdS nanoparticles in a borosilicate glass matrix

during a thermal annealing process based on diffusion controlled phase

decomposition. Zou et al [85] have prepared CdS nanoparticles in sodium

bissulfosuccinate by reverse micelles technique. The synthesized particles are capped

with Cd(OH)2, ZnS and CdO. The average size was about 5 nm. Romeo et al [86]

have fabricated CdTe/CdS thin film solar cells using the magnetron RF sputtering

method. Xu et al [87] have synthesized coated CdSe/CdS nanocrystals by a new

reaction routine in micelle solution.

Artemyev et al [88] synthesized CdS nanocrystals by exchange reaction

between cadmium thioglicerate and sodium sulphide by both dissolving in dimethyl

formamide [89]. The average size of the prepared CdS was 1.6-1.8 nm. Abdulkhadar

and Thomas [90] have prepared 4 nm size particles by a chemical method and EDTA

was a stabilizer. Thomas and Abdulkhadar [91] have synthesized 4 nm sized particles

by a chemical method. They reported that their elastic moduli and density were found

to be significantly lower than the corresponding values of the bulk crystals.

Lianos and Thomas [92] have synthesized small dimensional CdS about 5 Å

radius by inverted micelles method. Cizeron and Pileni [93] have synthesized

34

CdyZn1-yS nanosized particles by reverse micelles method. They have reported that

the CdyZn1-yS particle showed the quantum size effect for various compositions. The

size effect was more pronounced for cadmium rich particles.

Matsuzawa and Suzuki [94] have prepared nanosized semiconductor CdS

covered with a carboxyl group in an aqueous solution. Jinesh et al [95] have

fabricated bulk-nanojunction CdS using chemical bath deposition method at room

temperature. They have prepared bulk CdS in the form of thin film using Narayanan

et al [96] method. Zhu et al [97] have prepared nano CdS and ZnS by a simple

microwave irradiation method. The size of the prepared nano CdS was 5-10 nm and

2-3 nm for ZnS. Rossett et al [98] described a synthetic process which involves the

controlled nucleation of CdS on mixing of dilute aqueous solution of CdSO4 and

(NH4)2S. The size of the prepared nanoparticles was about 34Å.

Gupta et al [99] have prepared CdS film by high pressure magnetron

sputtering technique on the glass and NaCl substrate. Korgel and Monbouquette [100]

have synthesized size quantized CdS of dimension ranging from 20 to 60Å by using

phosphatidylcholine vesicles as true reaction compartments. Liu et al [101] have

synthesized Mn doped nanoparticles in aqueous solution by using mercapto acetate as

capping reagents. Ningzhong Bao et al [102] have synthesized CdS nanocrystals with

air-insensitive reagents in the water-rich P123 aqueous solution at room temperature

under air condition. The particle size was 25.1nm.

Murugan et al [103] have synthesized nanocrystalline CdS by a combination

of microwave and hydrothermal techniques. They used to prepare nano CdS with

different solvents like water, ethanol, polyamines and ethylene glycol. Mo et al [104]

35

have synthesized CdS nanocrystallites with different morphologies by an ultraviolet

irradiation technique. The solution of thioacetamide and CdSO4.8H2O were allowed

to ultraviolet irradiation; in this solution polyvinyl alcohol was added as protecting

material to the morphology controlled synthesis of CdS nanocrystals.

Fan et al [105] have fabricated CdS nanowires by sulfurizing metal Cd that

was electrodeposited in the nanochannels of porous anodic alumina template. They

have reported that the prepared CdS nanowires had hexagonal phase and having

uniform diameter of about 50 nm. Lee et al [106] have prepared CdS-ZnS thin film

doped by thermal diffusion of vacuum evaporated indium films. The conductivity of

CdS-ZnS films improves and the lowest resistivity of 0.3Ω cm was attained for CdS-

ZnS films with 40 nm indium coating and annealed at 450° C. The optical bandgap

energy of 2.94 eV was observed in the case of CdS-ZnS film with 40 nm indium

coating and annealed at 550°C.

Bhattacharjee et al [107] have prepared Cd1-xZnxS nanoparticles with different

compositions (0 ≤ x ≤ 1) embedded in silica matrix by the sol-gel technique. They

have reported a structural change from hexagonal to cubic at x ~ 0.74. The PL peak

position shifted to higher energy with increasing Zn content (x value) in the films. Lu

et al [108] have synthesized porous CdS/silica nanocomposites prepared through sol-

gel and gas reaction processes. For CdO/SiO, the average pore radii examined by Hg

pressing method were ∼5.6 nm, and the average crystallite sizes of CdS were about

3.5 to 4.5 nm. The 3rd nonlinear optical susceptibility was obtained to be 4.5×10-11

esu (CW laser) and 2.33×10-11 esu (pulse laser), respectively.

36

Dutta and Fendler [109] have synthesized cadmium sulphide nanoparticles by

the self-reproducing reversed micelles mehod. Taneja et al [110] have prepared a thin

film on quartz plates and a silicon wafer using RF-sputtering technique. They have

obtained 2 nm size particles. Zhao et al [111] have synthesized CdyZn1-yS

nanoparticles in salt-induced block copolymer micelles using polystyrene-block-2-

vinylpyridine as the stabilizer. The sizes of the particles were calculated as 1.5 - 2 nm

and the sizes were controlled by adjusting the concentration of metal ions in solution.

Kumar et al [112] have prepared CdS-ZnS films by the vacuum evaporation

method. They have reported that the prepared film was of direct bandgap which

varies from 3.50 eV for ZnS to 2.44 eV for CdS. Lee et al [113] have prepared

Cd1-xZnxS (0 ≤ x ≤ 1) thin films by the co-operation of CdS and ZnS. The optical

bandgap of Cd1-xZnxS films varied from 2.41 eV for CdS to 3.48 eV for ZnS. They

have reported that the open circuit voltage of Cd 1-x ZnxS/CdTe solar cells increased

with x due to reducing of the electron affinity difference between Cd1-xZnxS and CdTe

films, having approximately 830 mV of the maximum value at x = 0.35.

Co-evaporation of CdS and ZnS lead to films of Cd 1-x ZnxS with a blue-shifted

absorption edge

Singh et al [114] have fabricated CdS films by sonochemical, microwave and

solution methods. Xun Fu et al [115] have prepared sheet-like CdS nanocrystal using

the liquid crystal template formed in organophosphinic acid extractant (Cyanex 272 or

Cyanex 302) - hexane - NaOH aqueone solution systems. Vasa [116] has prepared

CdS-ZnO nanocomposite thin films by sputter deposited method. She has reported

that the nanocomposite films have very high PL yield and enhanced coherence

37

compared to individual nanoparticles of CdS and ZnO. Raji et al [117] have

synthesized nano crystalline CdS from CdSO4, thiourea and ethylene glycol by

precipitation method.

Tang et al [118] have synthesized water soluble CdS nanocrystals by surface

modification of ethylene diamine. Zhang et al [119] have synthesized CdS nanorods

by the hydrothermal process. Babu et al [120] have reported the effect of organic

solvent on the formation and stabilization of CdS and PbS nanoclusters. Sedaghat et

al [121] have synthesized nanosized CdS by using CdSO4 and Na2S2O3 as the

precursors and thioglycerol as the capping agent. They have reported that the

crystalline phase of the nano CdS was temperature dependent. Pattabi and Amma

[122] have synthesized thiophenol-capped CdS nanoparticles prepared by a non-

aqueous chemical method. The crystallite sizes varied from 2.8 to 3.6 nm with

increasing stabilizer concentration.

Hiie et al [123] have synthesized CdS films deposited on glass and ITO-

covered glass substrates by two techniques: Chemical bath deposition (CBD) and

spray pyrolysis techniques. CdCl2 and thiocarbamide were used as basic precursors in

both the cases. They have reported that the optical bandgap (Eg) for CBD deposited

CdS was shifted from 2.51 eV down to 2.42 eV, whereas the Eg of sprayed films (2.46

eV) did not change. Murali et al [124] have synthesized CdS films deposited by the

brush plating technique on titanium and conducting glass substrates using a current

density of 80 mA/cm2. They have reported that the crystallite size was found to

increase with the increase of substrate temperature; also the bandgap varied (3.00 -

2.60 eV) with respect to the increased substrate temperature.

38

Wang et al [125] have synthesized CdS nanocomposites using macroporous

ion-exchange resins. The average size of the CdS crystallite was 9 nm. Ma et al

[126] have synthesized CdS nanowire by the chemical vapour deposition method.

Hota et al [127] have synthesized nano-sized CdS-Ag2S core-shell nanoparticles using

sodium bis (2-ethylhexyl) sulfosuccinate (AOT)/n-heptane/water microemulsion

technique. Iacomi et al [128] have synthesized Mn, Se and Sb doped nano CdS thin

films deposited by the thermal evaporation method. They have reported that the

prepared thin films were hexagonal phase.

Yang et al [129] have reported a simple and convenient organic synthesis

method for the preparation of CdS and Se-doped CdS semiconductor nanocrystals.

The particle sizes were 2 and 2.5 nm respectively. Borse et al [130] have synthesized

alloy Cd1-xZnxS thin films by using solution growth technique. They have reported

that the compound showed the hexagonal crystal structure. The lattice parameters

changed with respect to composition ‘x’ and the optical bandgap varied from 2.34 to

3.43 eV as Cd, Zn concentration changed from x = 0.0 to x = 1.0.

Akyuz et al [131] have synthesized Cd1-xZnxS semiconductor films by the

ultrasonic spray pyrolysis (USP) technique. The bandgaps of as deposited thin films

varied from 3.513 and 2.486 eV. Chavhan et al [132] have synthesized Cd1-xZnxS

thin films grown on ITO substrates using the chemical bath deposition technique.

They have reported that the synthesized film was of hexagonal phase and the

bandgaps of as deposited thin films varied from 2.46 to 2.62 eV, whereas in the

annealed films these varied from 2.42 to 2.59 eV.

39

Gautam et al [133] synthesized thiophenol capped CdS nanocrystals by

chemical precipitation technique. The size range was 1.2-4.3 nm. Osipyonok et al

[134] synthesized CdS nanoparticles by chemical bath deposition method. The

particle size was 10-20 nm. Qing Xia et al [135] prepared cadmium sulphide (CdS)

3D polycrystalline walnut - like nanocrystals by solvothermal method with PVP as

stabilizer. The obtained CdS nanoparticles were of hexagonal polycrystalline phase.

Nanoparticles of CdS were prepared by Unni et al [136] at 303K by aqueous

precipitation method in the presence of the stabilizing agent thioglycerol. The particle

size was ~ 3nm. Andrea Pucci et al [137] prepared CdS nanoparticles stabilized by

mercaptoethanol layers in solution and successively dispered into different poly (vinyl

alcohol) - based polymer matrices. The average crystallite size was within 1.5 - 2.4 nm.

The synthesis of CdS nanoparticles in sodium citrate solutions was made by Thelma

Serrano [138] using microwave radiation. The particle size was smaller than 20 nm.

Jung- chul Lee et al [139] synthesized high density and single - crystalline

CdS nanowires at 450°C by chemical vapour transport technique. Owing to the low

synthesis temperature CdS NWs were successfully grown on transparent conducting

oxide (TCO) - coated glass substrates. CdS NWs were (defect) free single crystalline

wurtzite crystals and they were 50-100 nm and 2-5 µm in diameter and length

respectively.

Shancheng Yan et al [140] synthesized CdS nanowires with uniform diameter

of about 26nm and length upto several micrometers by the solvothermal method. The

synthesis of hexadecylamine capped CdS nanoparticle was made by Thandeka

Mthethwa et al [141] using single source precursors. The XRD patterns confirm the

hexagonal phase of CdS. Nanoparticles of cadmium sulphide (nano-CdS) were

40

successfully prepared by Yandan Wu et al [142] using novel sonochemical method by

adding polyvinyl pyrrolidone k30 (PVP) as the dispersant. The size of the prepared

nanoparticles was about 3-5 nm.

The synthesis of water - soluble luminescent CdxZn1-xS nanocrystals was made

by Jia Zhu et al [143] in aqueous solution with TGA as the surface modifier. The

average diameter of CdS nanocrystals characterized by TEM image is about 4nm.

Puja Chawla et al [144] synthesized CdS (cubic + hexagonal) nanoparticles by the

chemical co-precipitation method. The grain size was 1.7nm. CdS nanoparticles were

synthesized by Seoudi et al [145] making use of simple fabrication steps at ambient

conditions (room temperature and atmospheric pressure). The cubic CdS nanoparticle

was obtained. The particle size was 5.3 nm.

Liu et al [146] adopted microwave assisted chemical bath deposition (MA-

CBD) method to fabricate CdS thin films. Microwave irradiation plays a crucial role

in the film growth and the formation of micro/nano-binary structure. The SEM picture

shows leaf like structure. Titipun Thongtem et al [147] synthesized nano –and micro-

crystalline CdS from cadmium (CdCl2.2H2O, Cd(NO3)2.4H2O, Cd(CH3COO)2.2H2O]

and sulphur [CH3 CSNH2, CH5N3S, CH6N4S] sources in ethylene glycol assisted by

cyclic microwave radiation at different conditions. Different phases (hcp and cubic)

and morphologies (rose- shaped particles, spikes in clusters, cauliflowers and

nanoparticles in cluster) were detected

Anukorn Phuruangrat et al [148] synthesized CdS (hcp) solvothermally from

different Cd and S sources (Cd(NO3)2, CdCl2, Cd(CH3COO)2, (NH4)2S, CH3CSNH2,

NH2CSNH2 and CH5N3S) at 200°C in mixed solvents of ethylenediamine and

butylamine. SEM and TEM revealed the presence of different products consisting of

41

nano-wire, nano-rods and tetra pods controlled by Cd and S sources. Baocheng Zhang

et al [149] prepared CdS nanostructure by combine gas diffusion and hydrothermal

method. Flower like structure can be seen in the nanostructure.

Dongre and Ramrakhiani [150] successfully synthesized flower- like CdS

nanostructures by a simple chemical bath deposition and wet chemical etching

method. The average particle size was 15.6 nm. Zinki Jindal and Verma [151]

synthesized one-dimensional (1D) CdS nanostructures, including micro/nanorods, and

nanostructures resembling flowers and cactus by electrochemical template deposition

technique, using polycarbonate membranes and by controlling various reaction

parameters. The optical bandgap of the CdS nanostructures has been found to be

Eg = 2.62eV (473nm). The PL of the CdS nanostructures have green emissions

centered at 531 and 545nm, attributed to the surface donar-acceptor pair

recombinations.

Sonia Arora and Manoharan [152] prepared single phase solid solution

CdxZn1-yMnYS in nano- dimension by microwave assisted synthesis. A broad

emission with FWHM of ~ 155nm leading to a broad band emission can be harvested

in a nominal composition such as Cd0.54 Zn0.43 Mn0.03 S by a compound effect of CdS,

ZnS and ZnS: Mn responsible for blue, green and red emissions respectively.

Qi Xiao et al [153] prepared CdS nanocrystals by a simple one –step aqueous

synthesis method using TGA as the capping molecule. The PL intensity was enhanced

with the increase of Cd precursor concentration and shifted to longer wavelength side.

In the PL spectra band-edge emission was noticed at about 470nm and surface – trap

state emission at about 510nm respectively.

42

Yao-hai Zhang et al [154] prepared L-cysteine –coated CdSe/CdS core - shell

QDs. The maximal fluorescence wavelengths of L-cysteine-coated CdSe/CdS QDs

are 585nm. The line width of fluorescence spectra is very narrow (with the FWHM of

about 30nm) symmetric, monodisperse and homogeneous. The synthesis of CdS

nanocrystals was made by Baoyou Geng et al [155] using a new nonhydrolytic single-

source molecular method. The particle size was 17nm. The absorption edge was

480nm and PL emission at 462 nm.

Burcu Girginer et al [9] prepared CdS nanoparticles in aqueous solution using

cop olymers of DADMAC with NVP as stabilizer. The UV absorption spectrum of

the nanoparticle showed one maximum at 340 and a shoulder around 460nm. The

shoulder could be ascribed to the surface plasmon absorption of the CdS particles.

The absorption edge was observed at 520 nm. The particle size was 50-70 nm. Jiebo

Chen et al [156] prepared Cd1-xZnxS solid solutions by coprecipitation method with

PAMAM as a template. The particle size of CdS was 4.6nm. The bandgap of CdS was

2.09 eV.

Vineet Singh and Pratima Chauhan [10] synthesized CdS nanoparticles by

chemical precipitation method using thioglycerol as the capping agent. The maximum

intensity peak crystallite size was 3.2 nm. The bandgap of CdS nanoparticle was

2.9eV. PL spectrum exhibits peaks centered at 485 and 515 nm because of band -

edge emission and interstitial sulphur sites. Yadav et al [157] synthesized CdS

nanoparticles by sonochemical method together with aminoacid histidine as an

organic chelating agent. The particle size was ~ 2.59nm. It has band edge emission at

~ 481nm and has enhanced photoluminescence intensity.

43

Table 3 : Some preparation methods, particle sizes, optical bandgap energies and photoluminescence emission wavelengths of CdS nanocrystals

Sl. No Preparation

Particle size using XRD (nm)

Optical bandgap energy

(eV)

Photolumi -nescence emission

wavelength (nm)

References

1.

2.

3.

4.

5.

6.

7.

8.

9.

Microwave irradiation (CdS

nanocrystals)

Microwave irradiation

(CdS/PVK nanocomposite)

Hydrothermal (PVP – capped

CdS nanoparticles)

Hydrothermal (PSV/CdS

nanocomposite)

Solvothermal (CdS

nanocrystals)

Surfactant –assisted

sonochemical route (CdS

hollow nanosphere)

Micro-emulsion under

ultrasound (CdS

nanocrystals)

Biomimetic synthesis method

(Chitosan capped CdS

composite nanoparticles)

γ - irradiation method (CdS /

dendrimer nanocomposite)

9

16

8

15

-

9

2.19

27

Using

TEM

3.1nm

2.7

-

2.7

2.6

2.6

2.49

-

-

-

500

378 & 530

530

530

530

-

-

-

510

[159]

[160]

[161]

[162]

[163]

[164]

[165]

[166]

[167]

44

Thangadurai et al [158] prepared CdS nanorods by wet chemical method. The

crystal structure of the nanorods is in the hexagonal phase of CdS. UV-Vis

spectroscopy shows a blue- shifted absorption at 493nm because of the quantum

confined excitonic absorption. Titipun Thongtem et al [70] synthesized CdS nanowire

solvothermally with PEG as a template. Hexagonal CdS was obtained.

Photoluminescence peak was deteeted at 518nm. Table 3 shows some preparation

methods, particle sizes, optical bandgap energies and photoluminescence emission

wavelengths of CdS nanocrystals.

2.2. Manganese Disulphide (MnS2) Transition metal dichalcogenides are MnS2, FeS2, CoS2, NiS2, MnSe2, CoSe2,

NiSe2, etc with the pyrite structure. Many of the dichalcogenides of the 3d transition

metals MX2 (X=S, Se, Te) crystallize in pyrite structure. This structure is cubic, space

group Th6 (Pa3) and can be considered as a NaCl-like grouping of metal atoms and

chalcogen atom pairs X2. The distance in an X2 pair is short, due to the presence of a

covalent bond. Several of the MX2 compounds with the pyrite structure can be

prepared only under high pressure [28].

The pyrite-type crystals on the other hand, possess cubic symmetry and are thus

relatively isotropic. Like the layered crystals however, the pyrite crystals display a

wide range of physical characteristics, ranging from semiconductors to ferromagnetic

metals to superconductors [19]. Table 4 shows pyrite compounds, lattice constants

and their interatomic distances [23].

45

Table 4 : Pyrite compounds, lattice constants and interatomic distances

Compound Lattice constant a(Å)

Atomic distances

A-B (Å) B-B (Å) MnS2

MnSe2

MnTe2

FeS2

CuS2

ZnS2

CoS2

RuS2

RuSe2

RuTe2

OsS2

OsSe2

OsTe2

6.101

6.417

6.943

5.418

5.791

5.954

5.524

5.6095

5.935

6.3906

5.9196

5.941

6.3968

2.59

2.71

2.90

2.26

2.45

2.53

2.32

2.351

2.473

2.647

2.352

2.478

2.647

2.09

2.33

2.74

2.14

2.05

2.04

2.12

2.179

2.44

2.791

2.210

2.43

2.826

Table 5 : Dielectric constants, oscillator strength weighted ν (cm-1)s, and mass weighted f (Ncm-1) mean TO phonon frequencies of pyrites

Compound Dielectric constant

ν f εεεε∞ εεεε0

MnS2

MnSe2

MnTe2

FeS2

RuS2

RuSe2

RuTe2

OsS2

OsSe2

OsTe2

5.36

8.34

12.74

19.80

11.56

11.96

14.04

10.38

12.50

24.00

8.11

12.38

17.55

24.11

13.62

14.60

17.84

12.44

15.00

31.80

202

170

147

388

367

256

215

362

239

189

0.71

0.69

0.58

2.65

3.12

2.38

1.97

3.70

2.90

2.29

46

Table 5 shows the dielectric constants, oscillator strength weighted ν (cm-1)s

and mass weighted f (Ncm-1) mean TO phonon frequencies of pyrite type

compounds [168].

Table 6 shows the absolute electronegativities (X), bandgaps (Eg) and energy

levels of calculated conduction band edge (ECB) for metal sulphide minerals [169].

Table 6 : Absolute electronegativities (X) band gaps (Eg) and energy levels of

calculated conduction band edge (ECB) for metal sulphide minerals

Mineral X (eV)

Eg (eV)

ECB (eV)

CdS

MnS

MnS2

CoS2

CuS2

FeS2

NiS2

OsS2

RuS2

ZnS

ZnS2

5.18

4.81

5.24

5.49

5.57

5.39

5.54

5.74

5.58

5.26

5.56

2.40

3.00

0.50

0.00

0.00

0.95

0.30

2.00

1.38

3.60

2.70

-3.98

-3.31

-4.99

-5.49

-5.57

-4.92

-5.39

-4.74

-4.89

-3.46

-4.21

The Neel temperatures were determined from magnetic susceptibility and

Mossbauer effect measurements. Electrical data show that the manganese pyrites are

semiconductors. The bandgaps were also measured. Table 7 shows the Neel

temperatures and bandgaps of manganese dichalcogenides [21].

47

Table 7 : Neel temperature and band gap of manganese dichalcogenides

Compound Neel temperature TN(K)

Band gap (eV)

MnS2

MnSe2

MnTe2

≈ 48

≈ 90

≈ 83

≈ 1

>0.2

0.11

Table 8 : Core levels of anion pairs of metal dichalcogenides

Compound 2P½ 2P3/2 )3( Sgσ )3(*Suσ ∆

(eV)

X- X distance

R(Å)

MnS2

FeS2

CoS2

NiS2

CoSe2

NiSe2

162.9

163.4

163.7

163.7

161.9

162.4

162.6

162.7

16.0

16.4

16.9

16.6

)4( Sgσ

16.7

16.3

12.4

13.3

13.6

12.9

)4(*SUσ

14.2

14.0

3.6

3.1

3.3

3.7

2.5

2.3

2.088

2.132

2.124

2.065

2.435

2.417

Table 9 : Core levels of cation pairs of metal dichalcogenides

Compound 2P 3/2 <3S>

Binding energy (eV)

Width (eV)

Binding energy (eV) Width (eV)

MnS2

FeS2

CoS2

NiS2

MnSe2

CoSe2

NiSe2

640.4

706.7

778.1

853.6

640.5

778.3

853.2

∼4.0

1.6

2.6

2.7

∼4.0

2.0

2.2

82.6 (5S)

88.1(7S)

92.0

101.9

111.9

82.6 (5S)

88.8 (7S)

112.0

3.5

4.2

4.3

3.6

48

Tables 8 and 9 show the core levels of anion and cation pairs of metal

dichalcogenides. Binding energies (eV) are given relative to the Fermi level[28].

This part is an integral part of continued investigations of physical and

chemical properties of manganese sulphides. It deals with various aspects of synthesis

and characterization of MnS2 and MnSO3.xH2O.

Bulk manganese disulphide (MnS2) is a magnetic semiconductor crystallizing

with the cubic pyrite structure (C2, Pa3) with a = 6.104 (2) Å at room temperature.

The Mn2+ in MnS2 is in the high spin state and carries a magnetic moment

corresponding to five unpaired electrons (6S5/2). MnS2 undergoes paramagnetic to

antiferromagnetic phase transition near 48K. The antiferromagnetic phase transition in

MnS2 has been found to be of first order with a thermal hysteresis at about 0.5K. The

magnetic semiconductor MnS2 orders with the so-called type –III antiferromagnetic

structure [170-175].

The structure of MnS2 is cubic as shown in Figure 1(a). There is a close

relationship between the structure of α-MnS and MnS2. Both phases crystallize with a

NaCl-like structure. In α-MnS (Figure 1(b)) the arrangement of manganese and

sulphur atoms is the same as for sodium and chlorine atoms in the rocksalt structure,

whereas in the pyrite type structure of MnS2 (Figure 1(a)) pairs of S atoms take the

positions of the S atoms in α-MnS. It will be noticed that the centre of gravity of the

S2 pairs in Figure 1(a) lies at the position of the S atoms in Figure 1(b). The axes of

the S2 pairs are paralled to the various body diagonals [176].

49

(a) MnS2 (b) αααα - MnS

Figure 1 : The crystal structures of (a) MnS2 and (b) αααα - MnS

MnS2 is a magnetic semiconductor with a band gap of about 1eV [21,170].

MnS2 is also a Mott-semiconductor [169]. The optical band gap of MnS2 is around 3.1

eV. The electrical resistivity of the film is of the order of 106 – 107 Ωcm with p-type

electical conductivity [22].

Many of the researchers used hauerite (MnS2) pyrite-type natural crystal [13,

19, 20, 23, 28, 170, 177-179] for characterization.

Van Der Heide et al [28] have not been able to prepare a good sample of

MnS2. So they used natural single crystal hauerite (MnS2).

MnS2 pyrite can rarely be found in nature and synthesized only under elevated

pressure because of high spin state of Mn2+. Higher repulsive forces of the ligand

generate a Me-S distance of 2.59Å which is close to the sum of the ionic radii of Mn2+

(0.8 Å) and S2-(1.8 Å) as discussed by Ennaoui et al [23].

Chattopadhyay et al [14] observed that MnS2 undergoes a pressure – induced

first order structural phase tranisition from the cubic pyrite – type (C2, Pa3) structure

with unit cell parameter a = 5.86 Å to the orthorhombic marcasite-type (C18, Pnnm)

50

structure with unit cell parameters a = 4.50 and b = 5.61 Å at about 140kbar with

about 15% volume contraction. Mn2+ in the pyrite type MnS2 is in the high spin state.

Mn2+ in the marcasite –type MnS2 is in the low spin state. The Mn-S bond distance in

the pyrite type MnS2 at ambient pressure is 2.592 Å whereas that in the marcasite type

MnS2 at 170kbar is 2.25 Å.

Pure MnS2 was prepared by Ahuja et al [24] by the aqueous solution method.

At room temperature MnS2 was formed. The crystallite size of the MnS2 is 348 Å. At

about 200°C both MnS and MnS2 are present. The particle size is 270 Å. At various

temperatures, viz. 300, 400 and 500°C MnS is formed.

Synthetic MnS2 was prepared hydrothermally by Blitz and Wiechmann [180].

MnS2 (a = 6.1021 Å) contained minor impurities of other phases.

Jun Liu and Dongfeng Xue [181] prepared MnS2 by the bubble template

solvothermal method. The diameter of MnS2 sphere is in the range of 1-5µm observed

from low magnification SEM image. The high magnification SEM image shows that

many spheres are broken and have a hollow interior.

Nigrey et al [182] prepared MnS2 using hydrothermal conditions (~1400C and

365 kPa). XRD revealed that the particle size was ~120nm. The XRD particle size

results were somewhat inconsistent with SEM investigations that revealed ~100nm

cubic crystals on micron - size cubo - octahedral crystals.

Arinov and Depasquali [183] and Auroux et al [184] prepared MnS2 crystals

by the hydrothermal method. Schnaase [185] prepared ZnS- MnS and MnS – CdS

51

systems from dilute aqueous solutions. Schnaase [185] observed that the MnS2

present in the investigated samples decomposed with increasing temperature.

Sharma et al [186] prepared MnIn2 S4 thin films by spray pyrolysis method.

They found that MnS2 phases are present in films A, B and C only but not in films D,

E and F as per the reported nomenclature.

Chao Zhang et al [187] prepared MnS nanorods by using an anodized

aluminum oxide (AAO) template under a hydrothermal condition. In this method they

found that α- and γ-MnS were mainly present. Very weak peaks of Mn(OH)2 and

MnS2 were also present.

Borse et al [188] synthesized Mn doped ZnS nanoparticles by the chemical

method in aqueous solution. Higher concentration of Mn was used and found that the

sample has ZnS and MnS2 phases.

Sharma et al [189] prepared CdS : Mn alloys by chemical bath - deposition

method. MnS2 (cubic), α-MnS, Mn (cubic), CdS (cubic and hexagonal) phases were

present. The particle size was ~26nm.

The Raman spectrum of MnS2 has been recorded by Verble and Humphrey

[19] for an undefined scattering configuration. Raman spectra reveal MnS2 vibrations

at 489, 488, 246, 226 and 257 cm-1 [170].

Prabhakara Rao et al [190] synthesized manganese sulphite of the composition

Mn5(OH)4(SO3)3.2H2O using the hydrothermal method. The colourless needle shaped

crystal was obtained.

52

The pyrite starts to decompose at T = 573K [23]. The decomposition

temperature of MnS2 was 260°C in the experiment done by Sigrid Furuseth and Arne

Kjekshus [176] and 304°C by Blitz and Wiechmann [180]. Nigrey et al [182] show

that the thermal decomposition of MnS2 starts at ~410°C and is complete by 450°C.

2.3. Applications of CdS and MnS2

CdS is extensively used for optoelectronic devices because of its tuning

emission in the visible - light range with different sizes and shapes. CdS has the great

potential applications as nanoelectronics and photocatalytic materials [33].

CdS is an important semiconducting material that has attracted much interest

owing to their unique electronic and optical properties, and their potential applications

in solar energy conversion, photoconducting cells, nonlinear optics and heterogeneous

photo catalysis [161].

CdS is of special interest because it exhibits high photosensitivity and its

band-gap energy (2.4eV) appears in the visible spectrum [191]. CdS is useful in

applications such as optoelectronics, photo catalysis and photo degradation of water

pollutants [166].

CdS semiconductor nanoparticles have attracted intense interest due to their

unique photochemical and photo physical properties [167].

CdS is an excellent n-type window material in hetero-junction solar cells

[114]. Solar cells of efficiencies higher than 16% were fabricated using n-type CdS

window [192].

53

CdS nanoparticle is of great interest for many optoelectronic applications

including solar cells, photodiodes, light emitting diodes, nonlinear optics,

heterogeneous photocatalysis, photovoltaic cells and photonic switches. In recent

times novel biomedical applications of the fluorescent nanocrystals, i.e., biosensing,

biolabeling and drug delivery have also come up [193,167].

Manganese disulphide (MnS2) might be useful as a cathode-active material for

thermal batteries at the lower end of their operating temperature range, i.e, 300 to

400°C. This temperature range is ideal for thermal batteries used in geothermal

applications such as borehole data logging devices since it in above that for ambient -

temperature lithium batteries and below that for standard thermal batteries [182].

Thermal batteries are mainly used for military purposes that require a high level of

reliability and whose performance is not compromised after lengthy storage times

[194].

Mn2+ in MnS2 has the high-spin 2

56S ground state with a magnetic moment

of Bµ5 [174]. High - Spin → Low–spin transition involves volume contraction [195]

and therefore application of hydrostatic pressure in MnS2 is likely to induce high -

Spin → low-spin transition. MnS2 is a magnetic semiconductor with a band gap of

about l eV [171]. Hydrostatic pressure might produce an insulator → metal transition

and convert isolated magnetism in MnS2 into the itinerant one [14].

Hauerite (MnS2) was the only mineral that showed appreciable leaching of the

metal because of the high Mn2+ content in the hauerite suspension [179].

54

The role of semiconducting minerals may play as catalysts of redox reactions

in natural environments and engineered systems designed to degrade hazardous

chemicals [169].

Metal sulphides may also have a technological use. Pyrite has been shown to

catalyze the breakdown of polychlorinated molecules. Their semiconductor properties

also make them candidates for solar energy conversion applications [179].

Manganese disulphide (MnS2) might find applications in hydrostatic pressure,

catalysis, solar energy conversion, thermal batteries and lithium batteries

[14,169,179,182]. Manganese sulphite hydrate is used as the hydrogen storage

material [190].