The University of Manchester Fahad Saleh S Alotaibi Supervisor

The University of Manchester

The Synthesis of Teallite ( PbSnS2 ) From Molecular Precursors

A thesis submitted to the University of Manchester for the Degree of Master of Philosophy in the Faculty of Sciences and Engineering

2018

Fahad Saleh S Alotaibi

Supervisor: Prof. Paul O’Brien

Co- Supervisor: Dr. Sarah Haigh

School of Chemistry

The University of Manchester

Oxford Road, Manchester, M13 9PL, UK Table of Contents Abstract ...... 11

1. Introduction ...... 16

1.2. Aim of the Research ...... 16

1.3. Solids Classification ...... 17

1.3.1. Band Theory of Solids ...... 18

1.3.1.1. Energy Bands of Insulators ...... 20

1.3.1.2. Conductor Energy Bands ...... 20

1.3.1.3. Energy Bands for Semiconductors ...... 21

1.3.2. Semiconductors ...... 21

1.3.2.1. Extrinsic and Intrinsic Semiconductors ...... 22

1.3.3. Semiconductors Indirect and Direct Band Gap ...... 26

1.3.4. Effect of Bandgap on the Efficiency of Solar Cells ...... 27

1.3.5. Semiconductors and Regions of the Solar Spectrum...... 28

1.4. Solar Cells ...... 31

1.4.1. 1st Generation Solar Cells ...... 31

1.4.2. 2nd Generation Solar Cells ...... 32

1.4.3. 3rd Generation Solar Cells...... 32

1.4.4. Economic Considerations for Solar Cells ...... 33

1.5. Nanoparticles of Semiconductors...... 33

1.6. Methods for the Preparation of Semiconductor Nanoparticles ...... 34

1.6.1. Gas Condensation ...... 34

1.6.2. Chemical Vapour Deposition and Chemical Vapour Condensation ...... 35

1.6.3. Electrodeposition ...... 38

1.7. Single-source Precursors ...... 38

1.7.1. Dialkyldithiocarbamato Metal Complexes ...... 39

1.7.2. O-alkyl Dithiocarbomate Metal Complexes ...... 40

1.8. Thermogravimetric Analysis (TGA) of the Lead and Tin Complexes...... 42

1.9. Nanomaterials from Single-source Precursors ...... 43

1.9.1. Lead Sulphide ...... 43

1.9.2. Tin Sulphide ...... 44

1.9.3. Teallite ...... 44

2. Experimental ...... 45

2.1. Chemicals and Solvents Used ...... 45

2.2. Instrumentation...... 45

2.3. Synthesis of Ligands ...... 46

2.3.1. Synthesis of Potassium Alkyl Xanthate Ligands ...... 46

2.4. Synthesis of metal xanthate complexes ...... 46

2.4.1. Synthesis of bis(O-ethyldithiocarbomato)lead(II) ...... 46

2.4.2. Synthesis of bis(O-propyldithiocarbomato)lead(II) ...... 46

2.4.3. Synthesis of bis(O-ethyldithiocarbomato)tin(II) ...... 47

2.4.4. Synthesis of bis(O-propyldithiocarbomato)tin(II) ...... 47

2.5. Synthesis of PbS, SnS and Teallite PbSnS2 Nanomaterials in a Melt...... 47

3. Results and Discussion ...... 48

3.1. Pyrolysis of Pb(S2COEt)2 (1) and Sn(S2COEt)2(3)...... 48

3.1.1. The p-XRD of Solid State Pyrolysis of Pb(S2COEt)2 (1) ...... 48

3.1.2. Surface Morphology and Composition of PbS ...... 49

3.1.3. The p-XRD analysis of Solid State Pyrolysis Sn(S2COEt)2 Complex (3) ...... 51

3.1.4. Surface Morphology and Composition of SnS ...... 52

3.1.5. Phases Formed from Different Mole Fractions of Lead and Tin Precursors ...... 54

3.1.6. Surface Morphology and Composition of (PbSnS2) Particles ...... 64

3.1.7. Comparison of Data from Mixing Complexes (1) and (3) and Data from the

Literature ...... 65

3.2 Comparison between present study and other studies of Synthesis teallite (PbSnS2). .. 68

4. Conclusion ...... 72

5. Future Work ...... 73

6. References ...... 74

List of Figures

Figure 1.1: Bandgap diagrams of a conductor, semiconductor and insulator.14 ...... 18

Figure 1.2: Solids Energy Bands...... 19

Figure 1.3: Energy Bands Comments...... 19

Figure 1.4: intrinsic semiconductor electrons and Holes.15 ...... 23

Figure 1.5: The electrons' movement in the conduction band besides the valence band's holes of a semiconductor under the impact of an (E) Applied Electricity.15 ...... 24

Figure 1.6: Crystal lattice structure of (a) n-type and (b) p-type semiconductors.16 ...... 25

Figure 1.7: Energy band diagrams for (a) p-type and (b) n-type semiconductors.16 ...... 26

Figure 1.8: Graphical representation of direct and indirect band gaps.18 ...... 27

Figure 1.9.1: Electromagnetic spectrum showing all regions.24 ...... 29

Figure 1.9.2: Electromagnetic spectrum.25...... 29

Figure 1. 10: Solar radiation spectrum.26 ...... 30

Figure 1.11: Band gap of some selected solar cell materials of mixed generations.32 ...... 33

Figure 1.12: CVD Process Schematic utilizing a Single-Source Precursor.44 ...... 36

Figure 1.13: The Representing Schematic of the AACVD of Process's Stages.44 ...... 37

Figure 1.14: Three different coordination modes of dithiocarbamate groups: (a) monodentate,

(b) asymmetric bidentate and (c) symmetric bidentate.63 ...... 39

Figure 1. 15: Xanthate ligands are classified as (a) monodentate, (b) isobidentate and (c) anisobidentate and three more rare forms.77 ...... 40

Figure 1.16: Decomposition process of metal ethylxanthate complexes ...... 41

o Figure 1.17: TGA curves for [Pb(S2COEt)2] & [Pb(S2COnPr)2] from 30–600 C under N2. 42

o Figure 1.18: TGA curves for [Sn(S2COEt)2] & [Sn(S2COnPr)2] from 30–600 C under N2. 43

113 Figure 1.19: Crystal structure of PbSnS2...... 45

o Figure 3.1: The p-XRD patterns for PbS was prepared by melting [Pb(S2COEt)2] at 200 C,

300oC and 400oC under nitrogen...... 49

Figure 3.2: SEM image (a) and EDX spectrum (b) of PbS particles after 60 min. at

200oC …... 50

Figure 3.3: SEM image (a) EDX spectrum (b) of PbS particles after 60 minutes at 300oC..50

Figure 3.4: SEM image (a) XDS spectrum (b) of PbS particles after 60 minutes at 400oC....51

o Figure 3.5: The p-XRD patterns for SnS from [Sn (S2COEt)2] at 200, 300 and 400 C under nitrogen...... 52

Figure 3. 6: SEM image of SnS particles after 60 minutes at 200oC...... 53

Figure 3. 7: SEM image of SnS particles after 60 minutes at 300oC...... 53

Figure 3. 8: SEM image of SnS particles after 60 minutes at 400oC...... 54

Figure 3. 9: The p-XRD pattern for PbSnS from 0.25:0.25 mole ratio of a Pb(S2COEt)2 +

o Sn(S2COEt)2 at 300 C...... 56

Figure 3. 10: The p-XRD pattern for PbSnS from 0.50:0.25 mole ratio of a Pb(S2COEt)2 +

0 Sn(S2COEt)2 at 300 C...... 57

Figure 3.11: The p-XRD pattern for PbSnS from 0.75:0.25 mole ratio of a Pb(S2COEt)2 +

0 Sn(S2COEt)2 at 300 C...... 58

Figure 3. 12: The p-XRD pattern for PbSnS from 0.90:0.25 mole ratio of a Pb(S2COEt)2 +

o Sn(S2COEt)2 at 300 C ...... 59

Figure 3. 13: The p- XRD pattern for PbSnS2 from 0.25:0.40 mole ratio of a Pb(S2COEt)2 +

o Sn(S2COEt)2 at 300 C...... 61

Figure 3. 14: The p-XRD pattern for PbSnS2 from 0.25:0.50 mole ratio of a Pb(S2COEt)2 +

o Sn(S2COEt)2 at 300 C...... 61

Figure 3. 15: The p-XRD pattern for PbSnS2 from 0.25:0.85 mole ratio of a Pb(S2COEt)2 +

o Sn(S2COEt)2 at 300 C...... 62

Figure 3. 16: The p-XRD pattern for PbSnS2 from 0.25:0.95 mole ratio of a Pb(S2COEt)2 +

o Sn(S2COEt)2 at 300 C...... 63

Figure 3. 17: The p-XRD pattern for PbSnS2 from 0.25:1.00 mole ratio of a Pb(S2COEt)2 +

o Sn(S2COEt)2 at 300 C...... 64

Figure 3. 18: SEM image of PbSnS2 particles after 60 minutes at 300oC; b) 0.25:0.50 mole

o ratio for Pb(S2COEt)2 + Sn(S2COEt)2 at 300 C...... 65

Figure 3. 19: Variations in the d-spacing 111 and the plotted composition of PbS and PbSnS based on mole fraction...... 65

Figure 3. 20: Variations in (a) values for PbS and PbSnS compared to the metal precursor. ….. 66

Figure 3. 21: Variations in d-spacing (004) and the plotted composition of PbSnS2 and SnS based on mole fraction...... 67

Figure 3. 22: Variation in cell parameter (a) from thermal decomposition of mixture of complexes (1) and (3) at different mole ratios...... 70

Figure 3. 23: Variation in cell parameter (b) from thermal decomposition of mixture of precursors (1) and (3) at different mole ratios ...... 70

Figure 3. 24: Variation in cell parameter (c) from thermal decomposition of mixture of complexes (1) and (3) at different mole ratios...... 71

List of Tables

Table1. 1: Material classification depending on carrier density (n), an energy gap (Eg) and a resistivity at room temperature.14…………………………………………………………….17

Table1. 2: The Variance between the 2 Semiconductor Kinds ...... 22

Table1. 3: Cost versus efficiency of solar cell generation ...... 30

Table 3. 1: The table shows unit cell parameter for PbS powder prepared by melt reaction at

200, 300 and 400 °C for 1 hour, which is a good match with the value found in the literature value (a=5.930 Å) 118 ...... 49

Table 3. 2: Details of SnS preparation...... 52

Table 3. 3: Details of PbSnS preparation: Unit cells from complexes (1) and (3) and unit cells from the literature and galena (PbS)...... 55

Table 3. 4: Details of PbSnS2 preparation: Unit cells for mixing complexes (1) and (3), a unit cell from the literature and herzenbergite, SnS and orthorhombic teallite (PbSnS2) ...... 60

Table 3 .5: The number of millimoles in Pb(S2COEt)2 and Sn(S2COEt)2; the mole fractions of the present samples; a, b and c lattice constants of the present samples; mole fractions in the literature and a, b and c lattice constants in the literature...... 66

Table 3.6: Comparison of the mole fraction of this study and literatures, lattice constants and the obtained phase compared with the other studies of Brice 122 and Sugaki 123...... 69

List of Abbreviations

AACVD Aerosol-Assisted Chemical Vapor Deposition DNA Deoxyribonucleic acid UV Ultraviolet CZTS Copper zinc tin sulfide SRSS Semiconductors and regions of solar spectrum FGSC First generation of solar cells SGSC Second generation of solar cells TGSC Third generation of solar cells ESSC Economic considerations for solar cells QD Quantum dots MPSN Methods for the preparation of semiconductor nanoparticles CVD Chemical vapor deposition

MOCVD Metal-Organic CVD

UHVCVD Ultrahigh Vacuum CVD

APCVD Atmospheric Pressure CVD

HPCVD Hybrid Physicochemical Vapor Deposition

LPCVD Lower-pressure CVD

AACVD Aerosol-assisted CVD

DLICVD Direct liq. injection CVD CVC Chemical vapor condensation GC Gas condensation VHV Ultra-high vacuum VDV Vacuum deposition and vaporization TEOS Tetraethoxysilane TMOS Tetramethoxysilanes HPPD Hypersonic plasma particle deposition SSP Single source precursors CIGS Copper indium gallium sulfide SEM Scanning electron microscopy/ microscope DI De-ionized IR Infrared TGA Thermogravimetric analysis

EDX Energy dispersive X-Ray spectroscopy CM Centimeter N (cm-2) Newton per square centimeter E.g. Example BG Band Gap Å Angstrom LED Light-emitting diode Fig Figure Nm Nanometer NMR Nuclear magnetic resonance Mpa Mega Pascal Mmol millimol Rpm Revolutions per min PH Power of hydrogen W/cm2 Watt per square centimeter °C Celsius S Second KV Kilovolt Ma Mass MHz Megahertz G Gram X-RD X-ray diffraction PX-RD Powder X-ray diffraction Lit literature Ref Reference

Abstract

Recently there is growing interest in the production of cheap nanomaterial or thin films for photovoltaic applications. Lead and tin chalcogenides are cheapest materials available for solar cell applications. The aim of conducting this research is to produce teallite (PbSnS2) from single source precursors. This paper began by introducing the solids classification which encompasses semiconductors and conductors then the solids band theory was discussed. After that, semiconductors was described through it types; Extrinsic and Intrinsic besides Indirect and Direct Band Gap in details. Further headings were described in details for complete realizing of this research like solar cell and its generations, semiconductors nanoparticles and ways for preparing them and finally, the foremost subject; single source precursors and metal complexes prepared through this technique.

The work presented here contains the synthesis of lead and tin chalcogenides Nanocrystals to get teallite (PbSnS2) by melt method from single source precursors. In addition, a comprehensive literature review of teallite Nanocrystals and thin films is presented. Several new lead and tin complexes belonging to xanthate have been synthesized. Bis(O- ethyldithiocarbomato)lead(II), bis(O-propyldithiocarbomato)lead(II), bis(O- ethyldithiocarbomato)tin(II) and bis(O-propyldithiocarbomato)tin(II) complexes of general

n formula [M(S2COR)2] where M= Pb and Sn, R = Et and Pr and. The cubic and orthorhombic phases and morphology were obtained by Single source precursor. Cubic Lead sulfide (PbS) partials were obtained after pyrolysis using a furnace [Pb(S2COR)2] at 200, 300 and 400 °C, as well as Orthorhombic tin sulfide (SnS). Orthorhombic teallite (PbSnS2) was achieved after mixing lead and tin diethyl xanthate [Pb(S2COEt)2] and [ Sn(S2COEt)2] complexes at the different molar ratio and heating them using the furnace.

Declaration

I hereby declare that no portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of the University of Manchester or any other University or other Institute of Learning.

……………………..

Fahad Alotaibi

1. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the “Copyright”) and s/he has given The

University of Manchester certain rights to use such Copyright, including for administrative purposes.

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Dedication

To my family

Acknowledgement

First and foremost, I praise to God, the Almighty, who has made it possible for me to research the world of science and specifically chemistry so that I may pursue my passion and my dreams. I have also been supported by many good people to whom I would like to express my deepest gratitude.

I am sincerely grateful to my supervisor, Prof. Paul O‟Brien, for giving me the opportunity to research at the University of Manchester; he has guided me throughout my research. I am also thankful to Saudi Arabian Culture Bureau in London for giving me the full scholarship support.

I am grateful to all the staff in the School of Chemistry for their support and especially to all technical staff in School of Chemistry and the School of Material for their assistance on the NMR, Element Analysis, XRD, SEM and Raman.

I greatly acknowledge good company and support of my all, present and past, members of the POB group for their help during my study.

Without material and spiritual support of my parents this journey I could not have completed my MPhil; I am forever grateful for their kindness.

Fahad Alotaibi

1. Introduction

The consumption of global energy is expected to upsurge at least twice by the present century's middle due to economic and population growth. This request can be attained via fossil sources of energy, commonly coal. Nonetheless, the accumulative nature of carbon dioxide emissions in the atmosphere needs the development, invention and deployment of non-carbon-neutral energy production schemes. Amid renewable sources of energy, solar energy is the largest source of exploitation, offering more energy per hour of land than all the consumed energy by humans in a year.1 The solar energy significance has been realized via a growing body of literature as the most applicable, large-scale source of renewable energy that will meet the rising requests for energy prospectively.2 Lately, the metal chalcogenide nanoparticles synthesis has acquired significance due to their unique characteristics at the Nanoscale when matched to their bulk counterparts. Such unique properties of the semiconductor nanoparticles make them an ideal candidate for photovoltaic applications.3 From semiconducting constituents, their metal sulfides are investigated in plenty due to their distinctive band gap, elevated extinction coefficient making them a supreme candidate for solar cells applications as light absorbers,4 besides their outstanding luminescent efficiency.5 Metal sulfide nanoparticles also have decent electric and magnetic characteristics and have been used for electrochemical DNA sensors.6

1.2. Aim of the Research

Recently, a considerable effort has been invested to gain a better and deeper understanding of physical properties of the lead-tin chalcogenide semiconductors because of their potential application in electrical and photonic devices.7-9 The most interesting candidates are (Sn, Pb) S mixed crystals. SnS has an orthorhombic structure and consists of double layers weakly bound to each other, while PbS has a cubic structure.10 Both SnS (bandgap amid 1.1–1.7 eV) and PbS (bandgap = 0.4 eV) semiconductors are promising constituents in IR detection, photovoltaic and further optoelectronic apparatus.11 At this time, there are growing concerns with regard to the disadvantages of metal sulfides, such as limitations of the properties of metal sulfide materials. Therefore, further improvements and technological advancements are required to make such materials suitable for market use. It is vital that the performance of these material sulfides be comparable to if not better than the competition in order for them to be viable replacements for the presently used, less efficient materials currently on the

16 market.12 In this mission, metal sulfides were created and examined because they are promising constituents – as mentioned earlier – for solar cell. For instance, PbSnS2 (teallite) is a promising metal sulfide solid for thin film solar cells.

1.3. Solids Classification

Matter could be classified by a diversity of characteristics, and with regard to electronic potential the classifications, as shown in figure 1.1.13 Conductors; these constituents have partly filled and entirely unoccupied scales with comparable energies, which encompass a huge number of movable electrons. These electrons could be stimulated easily when applying an electric current. This is factual for metals. Semiconductors; the matter owns in this condition a valence band that is occupied with electrons and the vacant conductivity band is detached via a lesser BG (Eg). When these electrons absorb sufficient thermal energy, superior to the gauged BG, they are capable of jumping to the conduction band from the valence one. Insulator; in these constituents, the valence band is entirely occupied with electrons leaving the conduction band vacant, as in semiconductors, nonetheless, the band gap is too large (greater than 4.0 eV) to allow thermally or optically excited electrons to be excited to this level.

The semiconductors' valence band entirely occupied via electrons which detached from a vacant conduction band via a lesser BG (Fig. 1.1). Electrons could be moved to the conduction band from the valence one when applying an adequate energy. The semiconductors electrical conductivity is sensitive to a magnetic field, temperature and illumination. The classification of inorganic solids on the basis of the energy gap, resistivity and density is shown in Table 1.1.14

Table1. 1: Material classification depending on carrier density (n), an energy gap (Eg) and a resistivity at room temperature.14 Resistivity Solid Kind Eg (eV) cm n (cm-3) Instances (Ohm-cm)

Conductor …….. 10-5 to 10-6 1022 Tungsten, Ag, Cu

lead sulfide, lead Semiconductor 0 < Eg < 4 10-2 to 109 < 1017 telluride

Insulator 4 < Eg < 1012 to 1022 < 1 Rubber, Sulfur

Figure 1.1: Band gap diagrams of a conductor, semiconductor and insulator.14

The majority of important semiconducting materials are isoelectronic with elemental silicon since silicon (Si) and germanium (Ge) are examples of elemental semiconductors. Compound semiconductors are solids consisting of two or more elements which display semiconducting properties and can be classified as follows: (i) 2-elements, binary system (ii) 3-elements, ternary system (iii) 4-elements, quaternary system.

1.3.1. Solids Band Theory

A beneficial mean to visualize the modification amid conductors, insulators and semiconductors is to scheme the electrons' attainable energies in the constituents. Rather than attaining discrete energies as in the free atoms' case, the obtainable power situates form bands. Significance to the conduction operation is the absence or presence of electrons in the conduction range. For insulators, the valence band's electrons are divided by a big gap from the conductivity band. In conductors like metals, the valence band interferes with the conductivity band, while in the semiconductors there is a minor gap amid conduction and valence bands that thermal or further excitations which could raise the gap barriers. With such a slight gap, the existence of a small % of a doping material could upsurge conductivity intensely. Fermi level is a significant parameter in the band theory, the highest of the existing electron energy scales at little temperatures. The Fermi scale's site with the relative to the conduction band is a substantial aspect in crucial electrical characteristics.

Figure 1.2: Solids Energy Bands

Figure 1.3: Energy Bands Comments.

1.3.1.1. Energy Bands of Insulators

Utmost solids are insulators, which imply that there is a huge prohibited gap between the valence electrons energies and that at which the electrons could be moved easily through the conduction band. The Corundum is Al2O3 with a lesser Cr amount (0.05%) which correlates it its characteristic red/pink color via absorbing blue/green light. Whereas the doping of insulators could alter their optical aspects, it is not adequate to overcome the extensive BG to be decent electrical conductors. However, the doping of semiconductors has a further dramatic impact on its electrical conductivity which is the base of the solid electronics.

1.3.1.2. Conductor Energy Bands

In terms of the solids' band theory, metals are distinctive as decent electrical conductors. This could be considered to be an outcome of their valence electrons existence in essence free. In the theory, this is represented as an interference of the conduction and valence bands so as to at least a portion of the valence electrons could be moved across the material.

1.3.1.3. Energy Bands for Semiconductors

For intrinsic kind like Ge and Si, the Fermi level is in essence intermediate between the conduction and valence bands. While no conduction arises at 0 K, at elevated temperatures a restricted electrons' figure could reach the conduction band and offer little current. In doped semiconductors, further energy scales are added. The rise in conductivity could be demonstrated with temperature in terms of the Fermi role, which permits a person to gauge the conduction band's population.

1.3.2. Semiconductors

The utmost significant solid semiconductor materials exhibit electrical conductivity somewhere between insulators and conductors. Semiconductors could be recognized in accordance with their conduction mechanisms. At the absolute zero temperature, semiconductors act as insulators because of deficient thermal energy for the electrons' promotion from the valence band to the conduction band. Nonetheless, semiconductors at 21 usual temperature act as conductors due to the valence band obtains adequate energy to transfer electrons to the conductivity one.13 Semiconductors have an Eg (band gap) of amid 0.3 – 3.8 eV. In this valance band, electrons obtain energy at a high temperature and are promoted to the conduction band, the higher energy leaving a positive „hole‟ in the valence band. Semiconductors have a number of holes which can essentially control their conductivity.

1.3.2.1. Extrinsic and Intrinsic semiconductors

Intrinsic Semiconductor is considered being a pure form kind where there is no an impurity's addition occurs. 2 elements are an instance of this type which is Ge (Germanium) and Si (Silicon). Conversely, when a lesser amount of Tetravalent or Pentavalent impurity like Aluminum (Al), Arsenic (As), Phosphorus (P), Indium (In), Gallium (Ga), Antimony

(Sb) etc. is added to the pure form, an Extrinsic kind is attained.

The differentiation between these 2 kinds could be attained through considering numerous aspects like doping (impurity's addition), holes and electrons densities in the semiconductor substance, electrical conductivity and its reliance on diverse further factors.

Table 1.2: The variance between the 2 semiconductor kinds:14

Difference Intrinsic Kind Extrinsic Kind impurity Doping Doping does not occur in an An impurity's lesser quantity is

intrinsic semiconductor. doped in a pure semiconductor for

formulating extrinsic semiconductor.

The density of The holes (in valence band) The holes and electrons' numbers are electrons and and electrons (conduction not equivalent. holes band) numbers are equivalent.

Electrical Lower in conductivity. Higher in conductivity conductivity

Electrical EC is a function of the only EC rest on temperature besides on

22 conductivity temperature. the impurity doping quantity in the

Dependency pure semiconductor.

Instance The crystalline form of pure Impurities like Sb, As, P, In, Al, Bi,

Ge and Si. etc. are doped with Si & Ge atom.

15 Figure 1.4: intrinsic semiconductor electrons and Holes.

Figure 1.5: The electrons' movement in the conduction band besides the valence band's holes of a semiconductor under the impact of an (E) applied electricity.15

Because of doping, extrinsic semiconductors can be classified into two types, depending on which carrier type has raised density: (i) the n-type, which occurs when the electron density is increased, and (ii) the p-type, which occurs when the hole's density is increased. For an n-type semiconductor, for example, the addition of phosphorus pentavalent to silicon, only four electrons are involved in the bonding of the crystal and the 5th electron from the external shell of P-orbits surrounding the nucleus (Fig. 1.6(a)). This electron is loosely bonded to the nucleus to move the conduction band and only requires a small amount of energy. As the electron is donated by an impurity atom to the lattice of the crystal, it can be called a donor. Therefore, in the lattice of a crystal, a large number of electrons can be observed, which makes the concentration of electrons higher than that of holes. Hence, this

24 semiconductor can be called an n-type semiconductor because it is a carrier of negative donor charges. These donors create a new level of energy below the conduction band (Fig.1.7 (b)). When a boron atom is introduced into a silicon crystal, a covalent bond is formed between all three valence boron electrons and the four silicon electrons (Fig. 1.6(b)). One electron will be missing in one of the bonds, which creates a hole. Furthermore, the boron atom will remove the nearest electron from the silicon atom, leaving silicon with a negative charge. Thus, the B-ion is attracted to the hole, and an empty energy state or hole is created in the valence band of the silicon structure. Therefore, the boron atom becomes an electron acceptor. This guarantees that a huge holes number could be presented in the crystal lattice, raising their concentration over the electrons' concentration. These semiconductors are identified as p-type semiconductors because of the acceptor-type dopants, which are positive. These acceptors create a new energy level above the valence band (Fig.1.7 (a)).

Figure 1.6: Crystal lattice structure of (a) n-type and (b) p-type semiconductors.16

Figure 1.7: Energy band diagrams for (a) p-type and (b) n-type semiconductors.16

Other factors, such as defects in a crystal lattice, for example, empty sites in the lattice, an atom‟s presence in a wrong lattice site and lower and higher valence atoms existing on a lattice site, can determine extrinsic conduction.

1.3.3. Semiconductors Indirect and Direct Band Gap

Semiconductors could be also recognized in accordance with the electron transformation mechanism to the conduction band from the valence one. DBG semiconductor (direct bandgap) is where the max energy scale in the valence band corresponds to the min energy scale in the conductivity band regarding momentum. Whereas, IBG semiconductor is where the valence band's supreme power scale misaligned the conductivity band's least power scales concerning momentum. In DBG, direct recombination occurs with the emitting of energy equivalent to the energy variance between the recombined particles. While, in IBG, because of the relative momentum variance, 1st, the momentum is maintained via the energy- releasing, and then the recombination occurs with the energy release afterward the 2 momenta are aligned. The radiative recombination possibility is high and the DBG efficiency factor became higher Consequently, DBG type is continually favored over IBG for optical source make. The radiative recombination possibility is relatively low and the IBG efficiency factor

26 is lower. Instance for DBG; GaAs (Gallium Arsenide) and for IBG, Ge (Germanium) and Si (Silicon).17

Figure1. 8: Graphical representation of direct and indirect band gaps.18

1.3.4. Effect of Bandgap on the Efficiency of Solar Cells

It is largely believed that the future demand for energy will be filled by narrow bandgap semiconductor materials.19 This is because only specific materials allow devices to operate at higher temperatures and voltages, allowing for higher energy efficiencies; these reaction efficiencies are of great interest to all chemists. There is a continual search for efficient, cheap and environmentally friendly sources of energy. Examples of such semiconductor materials include dye-sensitized solar cells,20 polymer solar cells,21 organic solar cells22 and quantum dots.23 It needs to be determined whether these materials can convert the vacant photons efficiently. New literature focuses on the significance of how efficiently a material converts the photon carrying the energy of the sun. In light of photons with different wavelengths possessing different energies, a positive correlation was found between the efficiency of band gaps. The search for a material that can absorb the highest amount of photons continues and materials with higher proton absorption capabilities are in high demand. Solar cell materials act in response to UV/visible and other radiation charges of generated carriers. All initial considerations were on silicon semiconductor materials, however; as newer technologies are being developed, silicon has nearly arrived at its theoretical limits. As silicon is an indirect band gap material, it is likely

27 not a viable solution for the future. High conversion and absorption of solar radiation are necessary to fabricate the third generation of solar cells; however, due to the limitations posed by the properties of current materials, they will have to be replaced by other materials, such as copper zinc tin sulfide (CZTS) that have narrow band gaps.

1.3.5. Semiconductors and Solar Spectrum Areas Solar cells can be composed of electromagnetic radiations whose wavelengths range between nearly 250 nm and 25 000 nm. Three potentially useful segments in solar cells that cause photovoltaic effects: the visible, ultraviolet and infrared regions. The near-infrared region comprises nearly 52% and the visible region comprises 43%, while the infrared region comprises a mere 5% of the total photovoltaic effects. In such cells, the energy desired for electrons shifting to the conduction band from the valence band is fixed. The wavelength of the photons can identify the energy that is included in them. For commercial Si solar cells, the wavelength of light corresponds to 1 100 nm, which will be thrown out of an electron from the valence band to the conduction band and falls within the solar spectrum's IR area. This means that any wavelength longer than 1 100 nm, for instance, a photon with a wavelength of 1 120 nm, cannot cause electron excitation in silicon solar cells because they do not contain sufficient energy to cause excitation. Moreover, light photons that have shorter wavelengths, for example, a photon carrying a wavelength of 1 080 nm, will cause excitation of the electrons and any excess will be lost as heat. The photons that have longer wavelengths have far less energy than those with shorter wavelengths. As a result, radiation on the left of the spectrum, like X-rays, tends to have more energy than that on the right side of the spectrum, such as that in the infrared region. UV radiation consists of wavelengths that are less than 400 nm; wavelengths between 400 nm and 800 nm typically compose the visible region, and wavelengths greater than 800 nm belong to the infrared region.

Figure 1.9 1: Electromagnetic spectrum showing all regions24

Figure 1.9.2: Electromagnetic spectrum25

Figure1. 10: Solar radiation spectrum26

Using photovoltaic cells, solar cells can be used to convert light energy from the sun into electrical energy. The ability of these materials for photons' absorption and convert them efficiently into excitants (electron-hole pairs) is exceedingly desirable. Teallite (PbSnS2) is an example of a common material used in solar cells.27 Table1. 3: Cost versus efficiency of solar cell generation

Generation of Example(s) Cost Efficiency Solar Cells 1st generation Silicon wafers High High Amorphous Si, CIGS 2nd generation Low Low and CdTe Nanotubes, organic 3rd generation dyes and silicon Low High wires

1.4.1. 1st Generation Solar Cells

First generation solar cells (FGSC) can be made from germanium and silicon mixed with boron and phosphorus. In general, silicon solar cells are long lasting and efficient compared to thin films; however, on sunny days at higher temperatures, there is a higher risk of these cells losing efficiency. Although silicon is often preferred due to its higher efficiency, due to an intensive purifying process, it is more expensive than other materials, resulting in the end product being more costly. Currently, four types of silicon solar cells are on the market, as discussed below.

1.4.1.1. Mono-crystalline silicon solar cells: Efficiency up to 24.2%

Mono-crystalline solar cells are meticulously extracted from a single crystal. Made from wafers of silicon, these are some of the earlier solar cells. Although these cells are highly efficient, they require a high degree of precision and too much energy to cut a cell from a single crystal. These single crystals are then joined together to form a solar panel which yields an efficiency of 24.2%. Apart from the cost, another drawback of these types of solar cells is the loss of efficiency at a rate of 0.5% per unit rise in temperature at degrees above 25°C. In order to prevent this efficiency loss, a cooling system must be put in place which increases the cost of the final product.

1.4.1.2. Polycrystalline silicon solar cells: Efficiency up to 19.3%

Polycrystalline silicon solar cells can be made from poly-crystals of silicon grown together. When comparing their growth to single crystal, polycrystalline cells are easier to extract when using the Czochralski method.28 Because of the simpler extraction method, these types of cells tend to have a lower efficiency at 19.3%. However, higher efficiency in converting light into electricity can be achieved by controlling the crystal structure.

1.4.1.3. Hybrid Si solar cells: Efficacy up to 20.2%

These types of cells can be made by combining various materials to make more efficient solar cells compared to those made from single individual materials. The deposition of amorphous silicon and monocrystalline silicon is the first example.

1.4.1.4. Amorphous Si solar cells: Efficacy up to 10%

This kind of solar cells comprise of a thin film of Si deposited on a substrate like plastic or metal. Rarely, multiple layers of silicon can respond to various light wavelengths to further increase its efficiency, reaching a maximum of 10%. These types of cells, however, are still

31 desirable for low-light situations and can be used in products such as calculators and other low-power electronics.

1.4.2. 2nd Generation Solar Cells

Second generation solar cells (SGSCs) come at a higher cost but also provide higher efficiency when compared to first generation solar cells. They could be formed utilizing thin films composed of a few films of semiconducting constituents decreased to a thickness of a few micrometers to lessen the price by using less material. They can be made from cadmium sulfide/telluride (CdS/CdTe), copper indium gallium diselenide (CIGS), silicon and polycrystalline.

1.4.3. 3rd Generation Solar Cells:

Third generation solar cells (TGSCs) are hailed as the „smart‟ generation of solar cells. The aim of this generation is to produce solar cells that can absorb more wavelengths from sunlight, producing solar cells that can accommodate a wider solar spectrum band, so they are considered to be more effectual, cost-effective and environmentally friendly. Laboratory research continues in order to deliver third generation materials for commercially applicable efficiencies. These materials consist of organic dyes,29 nanotubes,30 semiconducting polymers31 and solar inks.32 The absorption of a few selected material used for third generation cells with respect to the solar spectrum is explained by Lewis et al., as shown in Fig. 1.11.

Figure 1.11: Band gap of some selected solar cell materials of mixed generations.32

1.4.4. Economic Considerations for Solar Cells

The majority of the population has relied on and continues to rely heavily on fossil fuels. The fossil fuels' usage is 1 of the prime climate change's causes. As fossil fuel is a non-renewable resource, produced over millions of years,33 sources are depleting. This has led to a shift towards more renewable energy sources, one of them being solar energy, for their theoretically limitless supply and cheaper production in the future.

1.5. Nanoparticles of Semiconductors

The literature calls semiconductors that cover the domain of 1–20 nm Nanocrystals, nanoparticles, Nanoclusters, artificial atoms, quantum dots (QDs) and Q-particles.34–38 The particles in this size range have unique properties because their dimensions in connection to the excitonic radius of the bulk material have been reduced. As the size of the particle is reduced, the surface ratio of atoms connecting the crystal lattice raises, and consequently, a significant amount of the surface determines the material‟s properties. A rise in the Nanocrystalline material bandgap with corresponding particle size reduction is identified via the phenomenon described as the quantum size effect. This effect causes charge carrier

33 confinement within the Nano crystal's dimensions, which results in them acting quantum mechanically as particles in a box.39

1.6. Semiconductor Nanoparticles Preparation Means

There is raised knowledge, realizing and enhancement of the physical world and its constituents from the prime to least mass extents, length and time. The slightest entity to have distinct characteristic aspect has been recognized as the element's atom. Until recently, however, the physical recognition of a single atom form was only a concept explored in science fiction films. Such recognition has been achieved through the development of Nanocrystalline materials, the finding of quantum concept limited atoms and the creation of doped Nanocrystalline materials. The growth mechanism of nanoparticles has excited great scientific and practical interest, as the specifications for their design, such as size and characteristics, become more demanding in nanotechnology. The growth mechanism of nanoparticles, determining the function distribution of nanoparticles on size, physical- chemical properties of nanoparticle mediums, to obtain nanoparticles with specified parameters (standard deviation, coefficient polydispersity, mean diameter and extra) and their features (magnetic moment) could be recognized, which provides a high level of precision and control over the creation of these nanoparticles. The nanoparticles' progression mechanism is a complicated operation and is dependent on diverse factors as viscosity, medium content and temperature. The determination of nanoparticle growth has evolved and is highly dependent on the method of preparation. Material engineers and doctors of material sciences have made crucial advancements in methods for synthesizing solid nanomaterials.40–42 Non-materials creation is categorized as bottom-up industrial, which is linked with the molecular constituents/atoms development as antagonistic to the top-down technique, which is linked with producing lesser structures etched from the entire substance as demonstrated by the semiconductor manufacturing. There are many types of synthesis methods for solid nanomaterial such as gas condensation (GC), vacuum deposition and vaporization, chemical vapor deposition (CVD) and chemical vapor condensation (CVC), mechanical attrition, chemical precipitation, sol-gel techniques and electrodeposition, as discussed below.

1.6.1. Gas Condensation

GC is utilized to create monocrystalline metals and alloys. In this practice, inorganic metals are vaporized utilizing a thermal evaporation source like an electron beam evaporation apparatus and Joule excited crucibles in a 1–50 mbar atmosphere. A high residual gas

34 pressure creates ultra-fine particles (100 nm) through gas phase collision in an evaporation of gas. The evaporated atoms collide with remaining molecules of gas producing too fine particles. Pressures gases with >3 mPa are vital. Vaporization sources might be resistance warming and low/ high energy electron rays prompting heating. The atoms combination in the gas stage could lead to the clusters' formation by homogeneous nucleation near the source in this phase. This demands UHV scheme with an evaporation source and a cluster apparatus' gathering of liq. N-filled cold scrapper and apparatus compaction. The atoms condensed in the hypersaturation zone during warming nearby the Joule apparatus. The nanoparticles are disposed in the metallic plate's phase by the scrapper. Evaporation is accomplished employing W/Mo or Ta refractory vessels.42 If the metal interacts with vessels, the electron beam's sputtering practice should be implemented. The operation is tremendously slow and suffers from boundaries like temperature ranges, source-precursor unsuitability and evaporation rates contradictory in an alloy. Over the years, further sources were developed. As Fe evaporates into an inert gas atmosphere (He), Fe atoms lose kinetic energy through collision with other atoms and condense in the form of small crystallite crystals that accumulate like a loose powder. Laser e sputtering or evaporation might be utilized for thermal evaporation.43 Sputtering is a non-thermal mean in which superficial atoms are physically insupportable at the surface via momentum move from an enthusiastic bombardment through an atomic-sized/ molecular kinds. Characteristically, a glow release or Fe beam is utilized in sputtering. The contact actions that arise at the desired surface and close to it waiting for this process, in evaporation magnetron have a pro over diode/triode sputtering. In magnetron spray, utmost of the plasma is restricted close to the target area. Ultra-fine/ Clusters particles could be created by further energy sources like sputtering electron warming beams and plasma practices. In lower- pressure surroundings, Sputtering has been utilized to create a diversity of clusters comprising Ag, Si and Fe.

1.6.2. CVC and CVD

These operations involve the solid deposition on a hot surface by a chemical reaction from the gas or vapor phase. Chemical vapor condensation (CVC) involves activation power, which could be created utilizing diverse means. In thermal CVD, the reaction is boosted by temperature exceeding 900°C. The distinctive utilized apparatus comprises a deposition chamber, a gas supply and exhaust schemes. CVD technique could be divided into different categories by the reactants used and the type of reaction environment employed. In CVD, the central area of the reactor is heated to a pre- 35 set reaction temperature and flushed with the reaction atmosphere. The reactants are then heated to a temperature high enough to vaporize the compound under the carrier/reaction atmosphere. The vaporized reagents are carried to the central area of the reactor; react and the product deposits on a substrate or plate are analyzed. The vaporization separates the molecules (or atoms in elemental reagents), which are then allowed to deposit in a carefully controlled and orderly manner on the substrate or receptor to form nanoparticles. This process is shown overleaf in figure 1.12.44

Figure 1.12 – CVD process schematic utilizing a single-source precursor. The volatile precursors are transferred by a carrier gas to the reactor's substrate (1). The precursor adsorbs to the substrate (2) and reacts (3) to liberate the by-products that subsequently desorb (6) and return to the carrier gas stream and transported out of the reactor (7). The target atoms then diffuse (4) to form nuclei of the materials, where subsequent growth occurs (5).44

There is a variety of CVD techniques which are normally differentiated by the reaction conditions, precursor systems and method of delivery used.  MOCVD (Metal Organic CVD) incorporates the use of organometallic compounds as precursors, namely (but not exclusively) metal carbonyls. This method is exemplified

in the production of MoS2 nanoparticles by reaction of Mo (CO)6 and S at 450 °C for 2 hrs, under Ar at 200 mL min-1 in a graphite receptor.45

 UHVCVD (Ultrahigh Vacuum CVD); operations are performed at a very little pressure, characteristically ≈ 10-8 torr (below 10-6 Pa). This procedure has been used -6 with binary systems (MoS2, H2S at 673 K, 15 min, 10 mbar on gold substrate) 46 forming triangular single-layer MoS2 nanoclusters, and single-source precursor -10 systems (Mo(Et2NCS2)4, Ar at >400 °C, 10 Torr) which produced MoS2 Nanocones and particles.47  APCVD (Atmospheric Pressure CVD) is as specified in the name. A given example is

the formation of MoS2 nanotubes, flowers and particles from the reaction of MoCl5 and S at 850 °C for 1 hr. under Ar at 20 sccm on silicon substrates.48  HPCVD (Hybrid Physicochemical Vapor Deposition); operations are encompassed both the precursor gas reaction and solid vaporization as a source. This variety of

CVD is sometimes used for two-stage reactions e.g. MoO3 forms nanoparticles and 49 then the particles undergo sulfurization to form MoS2.  LPCVD (Lower-pressure CVD); are operations at sub-atmospheric pressures, that could enhance equivalence through the product and lessen unwanted gas-phase reactions.  AACVD (Aerosol-assisted CVD); an operation in which, the precursors are transferred to the substrate by gas/liquid aerosol that could be created ultrasonically. This is best for in-volatile precursors and is illustrated in the figure 1.13.44

For example, MoS2 formed hexagonal thin films, from the decomposition of Mo(S2CNEt2)4 in THF at 400-475 °C for 90 min under Ar 180 sccm on glass substrates.50

Figure 1.13 – The representing schematic of the AACVD process's stages.44

 DLICVD (Direct liq. injection CVD); an operation in which, the precursors are presented in liq. form. Liq. reagents are inserted into a vaporization cavity towards injectors. These precursor vapors are transferred to the substrate as in broad CVD operation. This practice is appropriate for utilization on solid or liq. precursors. Elevated growth rates could be gotten utilizing this technique.

Further operation entitled CVC comprises the vapors pyrolysis of metal organic precursors in a lessened pressure atmosphere. ZrO2, Y2O3 particles and Nanowhiskers have been produced using the CVC method.51–54 Precursors of metal-organic could be presented by a warm zone of the reactor applying a mass current controller. For instance, (CH3)3SiNHSi-(CH3) 3hexamethyldisilazane could be utilized to create SiCxNyOz powder utilizing the CVC practice. The reactor allows the nanoparticle making combination of doped nanoparticles' 2 stages via offering 2 precursors at the front-end of the container, and a n-ZrO2 nanoparticles glazed with n alumina are delivered in the 2nd stage via a 2nd precursor; this process yields 20 g/hr. is yield quantity of the process. The product could be enhanced via expanding the warm wall reactor's diameter and the fluid flowing's mass within the reactor.

1.6.3. Electrodeposition

This method produces nanostructured materials. These films are identical and mechanically robust. Considerable progress has been achieved in nanostructured coverings applied utilizing CVC or CVD. Composite nanoparticles could be manufactured and deposited using further unconventional operations, like HPPD (hypersonic plasma particle deposition). Practically unexplored are the important perspective for the synthesis and using of nanomaterial and remain challenging. More effectively designed materials require further study and understanding. The properties required for deposits with nanostructures like resistance to wear, stiffness and electrical resistance seem to be significantly affected by grain size. The superior coating performance results in a combination of rose up wear resistance and rigidity.

1.7. Single-source Precursors

The best all-round precursor system appears to be single-source precursors; a single compound containing all of the elements required in the ultimate yield which thermally decomposes cleanly. There is a large range of compounds that meet these requirements and are classified as metal-organic compounds (organic ligands and no metal-carbon bonds).55 The specific advantages of using single source precursors are specified as; increased control

38 over the core structure of the precursor molecule‟s phase during growth, reduction of the number (amount) of potential impurities in the deposition, avoidance of pyrophoric, toxic or volatile precursor routes and the ability to engineer the precursor to be both air and moisture stable even if the nanoparticles that they form are not. These advantages allow cleaner synthesis and easier purification of the nanomaterials.56

1.7.1. Dialkyldithiocarbamato Metal Complexes

Dithiocarbamates [M(S2CNRR‟)n] have numerous applications in technology such as in anti- microbial agents, anti-cancer agents,57 in chemistry to detect nitric oxide,58 in fungicides59 and in metal sulfide deposition thin films.60 There are continuous concerns over these complexes. Dithiocarbamates indicate high thermal stability at room temperature,61 while their thermal decomposition begins at approximately 200 °C.62 Moreover, the advantage of using dithiocarbamates is that they act as a monodentate or as a bidentate and chelating ligand.63 The crystal structure of these complexes shows that the metal atoms are surrounded by monomeric complexes and the octahedral structure of four sulphur atoms.64-65 The coordination of dithiocarbamate ligands is shown in Fig. 1.14.

Figure 1.14: Three different coordination modes of dithiocarbamate groups: (a) monodentate, (b) asymmetric bi-dentate and (c) symmetric bidentate.63

This ligand can be synthesized via the reaction between carbon disulfide and a primary or secondary dialkyl amine with sodium or potassium hydroxide as follows:66-67

R2NH + NaOH → R2NNa + H2O (1.1)

R2NNa + CS2 → S2CNR2Na (1.2) where R = alkyl group and M = metals. According to Equation 1.3, the reaction between a solution of sodium dithiocarbamate ligand and an aqueous transition metal salt produces a dithiocarbamate complex. + + M (aq) + 2S2CNR2Na(aq) → M(S2CNR2)2(s) + 2Na (aq) (1.3)

1.7.2. O-alkyl Dithiocarbomate Metal Complexes

68 [M(S2COR)n] is the general chemical formula for xanthates. They are used in the mining industry69-70 and to attack cancer cells in cancer research.71 In addition, O-alkyl dithiocarbomate has been used as a collector precursor in the flotation process to deposit metal sulfide nanoparticles; indeed, xanthates have been successfully applied as single-source precursors72-73due to their typically low decomposition temperatures.74-75 Dithiocarbomate similar to dithiocarbamate groups can exhibit three coordination modes76 (Fig. 1.15): monodentate, isobidentate and anisobidentate. They can be divided into three more unusual forms, which can be achieved through metal-oxygen interaction or when bimetallic bonding occurs between sulfur and oxygen atoms or sulfur atoms.77

Figure 1. 15: Xanthate ligands are classified as (a) monodentate, (b) isobidentate and (c) anisobidentate and three morerare forms.77

One of the first reports on vibration frequencies of C-O and S-C-S bond angles of nickel, iron, zinc, copper and lead ethyl xanthate was by Winter et al., who advised on the frequency counts of the coordination around the oxygen atom and a „single or double bridging alkoxy group‟, the type of bond that occurs with alkoxy groups.78 The O-alkyl dithiocarbomate ligand can be made via a reaction between potassium or sodium hydroxide, alcohol and carbon disulphide as follows:

¯ + ROH + NaOH → RO Na + H2O (1.4)

+ ¯ + RO ¯ Na + CS2 → S2COR Na (1.5)

A reaction within a metal salt and a solution of sodium ethylxanthate yield the metal complex. + + + M (aq) + + 2S2COR¯ Na (aq)→ M(S2COR)2(s) + 2Na (aq) (1.6)

Thermal decomposition processes of metal xanthates have been intensively investigated; this process is derived from the Chugaev reaction.79 In this reaction, a xanthate complex is decomposed into an alkene, metal thioalkyl and carbonyl sulfide. Hydrogen sulfide and carbonyl sulfide elimination processes produce metal sulphide.80 The decomposition mechanisms of the metal xanthate, lead, zinc, copper, iron and nickel ethylxanthates were analyzed by Butler et al. They investigated the decomposition reaction of zinc and iron complexes and found that the triatomic species produced are eliminated to form a metal alkoxide or thioalkyl species and a constant ratio of CS2 to COS (Fig. 1.16). They explained similar results for lead, nickel and copper ethylxanthates, but the rate of decomposition was extremely low compared to zinc and iron ethylxanthates; moreover, oxygenated metals were more readily generated by the reactions.80

Figure 1.16: Decomposition process of metal ethylxanthate complexes

Reaction (1) describes an ethoxy group transformation from xanthate to the metal and would leave a CS2 and the metal alkoxide species as the major products of this reaction. Reaction (2) represents the alkyl group movement from oxygen to sulfur followed by a producer that is 80 similar to that for CS2 and leads to COS and the major product, which is the metal thioalkyl. 41

1.8. Thermogravimetric Analysis (TGA) of the Lead and Tin Complexes.

Thermogravimetric analysis (TGA) is a technique that monitors the changes in the chemical and physical properties of a material via a steady increase in temperature. The stability of the complexes was studied by performing TGA using nitrogen as the inert medium. A flow rate of 100 ml/minute was employed. The temperature was varied from room temperature to 600oC over 10 minutes. The TGA graphs of all four complexes are shown in Fig. 1.17 and Fig 1.18. These indicate a single sharp decomposition step at a temperature between 140oC and 168oC for all four complexes.

o Figure 1.17: TGA curves for [Pb(S2COEt)2] and [Pb(S2COnPr)2] from 30–600 C under

N2.

Complex (1) began decomposing below 140°C, with decomposition ending at 167°C. The final residue was 53.36%, which agreed with the calculated value for PbS (i.e., 53.04%). Complex (2) began decomposing at 150°C with a loss of mass of approximately 49.61%. The final product mass was 50.39%, which agreed with the calculation for PbS (i.e., 50.13%).

o Figure 1.18: TGA curves for [Sn(S2COEt)2] and [Sn(S2COnPr)2] from 30–600 C under

N2.

The TGA curves for complexes (3) and (4) are shown in Fig. 1.18. Complex (3) began to decompose between 90°C and 145°C, while complex (4) began to decompose between 105°C and 147°C. The final residue for complex (3) was 45.00%, which confirmed to calculations for SnS (i.e., 44.77%), while the final residue of the complex (4) was 41.00%, corresponding to calculations for SnS (40.60%).

1.9. Nanomaterial from Single-source Precursors

1.9.1. Lead Sulfide

Lead sulfide (PbS), also known as galena, is an important binary IV-VI semiconductor material. It has attracted a great deal of attention in the field of materials due to its direct narrow gap, with an approximate energy band of 0.4 eV at room temperature and a comparatively huge excitation Bohr radius of 18 nm.81 The nanostructure of lead sulfide is one of the most promising candidates for optical devices in infrared detection applications, optical switches and light emitting due to its exceptional third-order nonlinear.82 This material has also been applied in many areas such as photography,83 Pb2+ ion solar absorption84 and selective sensors.85 In addition, PbS has been utilized for diode lasers, solar control coatings, photo-resistance, humidity and temperature sensors and decorative purposes.86-87 The single-source precursors to Galena that have been investigated include

43 dithiocarbamate complexes,88 photo accelerated chemical deposition,89 spray pyrolysis,90 chemical bath deposition (CBD)90–94 and microwave heating.95-96

1.9.2. Tin Sulfide Tin sulfide belongs to the IV-VI family of semiconductors that have shown promise in optoelectronic applications and photovoltaics.97–100 Tin(II) sulfide (SnS) is a potential candidate as an absorber of sunlight layer in photovoltaic cells due to its 1.4 eV direct band gap that can harvest the visible and near-IR regions of the electron magnetic, its lower toxicity and cost as compared to other potential materials (e.g., CdS and PbS) as well as its binary system simplicity compared to materials multicomponent materials such as CZTS and CIGS.101-104

1.9.3. Teallite

Mineral teallite (PbSnS2), an IV–VI-layered ternary semiconductor, is an example of a layered material that has not yet been fully explored. Teallite forms a Pnma space set (orthorhombic assembly) as demonstrated in fig. 1.19. The sulfur (S) atoms are bonded to lead (Pb) or tin (Sn) atoms and together form puckered bilayers, approximately 0.6-nm thick.105 PbS (BG 0.37 eV)106 besides SnS (BG 1.3 eV)107 are promising constituents for IR recognition photovoltaic and further optoelectronic devices. Many research groups have intensively studied these two binary compounds. Moreover, the ternary semiconductor 108-109 PbSnS2 shows complete miscibility with SnS, and consequently, lattice parameter dissimilarity according to chemical composition.110 Teallite materials have a number of promising potential applications, among them, due to its 1.4 eV band gap, as a material for thin-film solar cells.111-112

113 Figure 1.19: Crystal structure of PbSnS2.

2. Experimental

2.1. Utilized Solvents and Chemicals

All solvents and chemicals were bought from Sigma-Aldrich deprived of further purification, comprising ethanol (99.8%, Aldrich), 1-propanol (99.8%, Aldrich), chloroform (99.8%, Aldrich), diethyl ether (99.8%, Aldrich), toluene (99.7%, Aldrich), tetrahydrofuran (99%, Aldrich), hexane (95%, Sigma-Aldrich), potassium hydroxide (99%, Sigma-Aldrich), tin(II) chloride (99.9%, sigma Aldrich), lead acetate trihydrate (99.9%, Aldrich) and carbon disulfide (99%, Aldrich).

2.2. Instrumentation

Nuclear magnetic resonance (NMR) spectra were recorded using a Bruker operating at the frequency of 500.0 MHz and 400.0 MHz FT-NMR Spectrometer with CDCl3 solvent. Infrared (IR) spectra were recorded using a Specac single reflectance attenuated total reflectance (ATR) instrument with wavelengths from 4000 cm-1 to 400 cm-1 and 4 cm-1 resolution. The fusion point was gauged utilizing a Gallenkamp apparatus. Elemental analysis of complexes was achieved using Flash 2000 Thermo Scientific elemental analyzer from School of Chemistry at the University of Manchester. Thermogravimetric analysis (TGA) is a technique that monitors the changes in the chemical and physical properties of a material via a steady increase in temperature. This technique was performed along with the University of Manchester micro50 analytical team. Gauges were achieved on a Seiko SSC/S200 at a warming scale of 10°C/min under N2 gas. Powdered X- ray diffraction was conducted using an X-Pert Diffractometer (Cu-Kα, λ=1.5418 Å); the samples were scanned between 5 and 80° in step sizes of 0.04, with a voltage of 40 kV and the current set at 30 mA. A Philips XL 30FEG Scanning Electron Microscope (SEM) was used for all the metal sulphides particles, and atomic and molar ratios were determined using a DX4 instrument. Carbon coating was identified using an Edwards E306A coating unit prior to SEM analysis. The experiment work consisted of firstly synthesizing xanthate ligands and secondly investigating the products using several experimental analysis techniques. In this section the synthesis procedures and analysis techniques will be discussed. The results of the analysis will be presented in the next section.

2.3. Synthesis of Ligands

2.3.1. Synthesis of Potassium Alkyl Xanthate Ligands

The Process followed for the synthesis of Potassium ethyl xanthate was as follows 114. 11.29 g, 0.201 mole of potassium hydroxide was placed in 75 ml of ethanol. 15.267 ml, 0.2 mole of carbon disulfide was added in controlled drops while the solution was stirred in an ice bath for 1 hr. The Ethanol evaporated, leaving potassium ethyl xanthate. The preparation of potassium n-propyl xanthate ligands required a 2 hour reaction. To a 5.645 g, 0.1 mole of potassium hydroxide a 7.733 ml, 0.10025 mole of CS2 were added in controlled drops, creating an orange solution containing potassium n-propyl xanthate. The alcohol was evaporated, and the solid yellow product was dried to collect the solid ligand of potassium n- propyl xanthate.

2.4. Synthesis of metal xanthate complexes

2.4.1. Synthesis of bis(O-ethyldithiocarbomato)lead(II)

115 Pb(S2COEt)2 was synthesized as follows . A potassium ethyl xanthate quantity, 5 g, 0.0311 mole was dissolved in 50 mL of ethanol and cooled to 0°C while stirring. Deionized water (50 mL) was added to the solution of lead acetate trihydrate (5.9 g, 0.0316 mol) in controlled drops. The mixture was stirred for another 30 min. The colorless precipitate that formed was filtered and washed with distilled ionized water and ethanol. The product was dried in a vacuum overnight. The Infrared (IR) spectrum for the Pb(S2COEt)2 indicated absorption bands at 1141.56-1108.18-1010.81-993.35 cm-1 and other bands at 1461.11, 1431.99, 1357.33, 1190.65 and 2986.59 cm-1, corresponding to the vibrations of connections C=S, O-

CS and C-H. An element analysis conducted for Pb(S2COEt)2 (C: 16.04; H: 2.24; S: 28, 47) found C: 16.02, H: 2.13, S: 28.24 and MP: 133°C.

2.4.2. Synthesis of bis(O-propyldithiocarbomato)lead(II)

Compound 2.4.2 was synthesized as for Pb(S2COEt)2 with 1-propanol (2.40 g, 0.04 mole) n used in place of ethanol. The IR spectrum of the Pb(S2CO Pr)2 revealed absorption bands at 1186.46-1127.31-1016.36-945.61 cm-1 and other bands at 1460.83 and 2963.21-2873.03 cm- 1, corresponding to vibrations of connections C=S, O-CS and C-H. Element analysis n conducted for Pb(S2CO Pr)2 (C: 20.13; H: 2.96; S: 26, 80) found C: 20, 26; H: 2.87; S: 26.63 and MP: 85.0°C.

2.4.3. Synthesis of bis(O-ethyldithiocarbomato)tin(II) 116 Sn(S2COEt)2 was synthesised as follows . A 5 g., 0.0623 moles of potassium ethyl xanthate ligand was dissolved in 50 ml of DI H2O, and a 2.9572 gm, 0.0311 moles of tin(II) chloride was added. The reaction was mechanically stirred for 30 min until a yellow precipitate formed. This was filtered using a vacuum DI H2O filtration method. The precipitate was washed three times in DI H2O and dried in a vacuum oven at room temperature for between 1 to 2 hours to evaporate the remaining solvent. The IR for Sn(S2COEt)2 was FTIR: 2986.8 (w), 2930.23 (w), 1998.80(w), 1470.50 (w), 1192.77 (s), 1125.53 (s), 1015.02 (s), 854.91 (w),

791.07 (w) and 659.58 (w). Element analysis conducted for Sn(S2COEt)2 (C: 19.98; H: 2.79; S: 35, 45) found C: 20.02; H: 2.74; S: 35.02 and MP: 51–52°C.

2.4.4. Synthesis of bis(O-propyldithiocarbomato)tin(II)

Compound 2.4.5 was synthesised as for Sn(S2COEt)2 with a 2.40 g, 0.04 mole of 1-propanol n used in place of ethanol. The IR analysis for Sn (S2CO Pr)2 gave the following results, FTIR: 2930.34 (w), 1470.67 (w), 1192.63 (s), 1125.96 (w), 1015.00(s) and 758.25 (w). Element n analysis conducted of Sn(S2Co Pr)2 (C: 24.71; H: 3.63; S: 32, 89) found C: 24.64; H: 3.61; S: 32.77 and MP: 66–69°C.

2.5. Synthesis of PbS, SnS and Teallite PbSnS2 Nanomaterial in a Melt.

The xanthate precursor PbS (0.3 g) was spread equally in a boat. The boat was positioned in the quartz tube's center and heated in presence of an Ar current of 200 SCCM. The Carbolite furnace was heated to the required temperatures of 200°C, 300°C and 400°C for PbS and SnS nanoparticles and 300°C for doping the PbS and SnS to obtain teallite PbSnS2. The quartz tube was inserted into the heated furnace for 60 minutes.

3. Results and Discussion

n Two lead xanthates [Pb(S2COEt)2] (1), [Pb(S2CO Pr)2] (2), and two tin xanthates n [Sn(S2COEt)2] (3) and [Sn(S2CO Pr)2] (4) with different alkyl groups were successfully synthesised and characterised.

3.1. Pyrolysis of Pb(S2COEt)2 (1) and Sn(S2COEt)2 (3).

Pyrolysis of these complexes was conducted at different temperatures (200°C, 300°C and 400°C), which were estimated from the results of the TGA discussed before. The lattice parameters were determined using the following equations: 117 2 2 2 2 4 sin θ = ( h + k + l ) (3-1) ,

λ2 a2 2 2 2 2 4 sin θ = h + k + l (3-2) , 2 2 2 2 λ a b c

Where θ is the diffraction angle; λ is the x-ray wavelength; h, k and l are the Miller indices and a, b and c are the lattice parameters.

The lattice parameters of cubic PbS and PbSnS were determined by using equation (3-1), while, orthorhombic SnS and PbSnS2 were identified by using equation (3-2).

Following the decomposition of the complexes the following analyses were performed on the products, viz. p-XRD, Surface morphology and phase analysis.

3.1.1. The p-XRD of Solid State Pyrolysis of Pb(S2COEt)2 (1)

The p-XRD patterns of the solid residue obtained from pyrolysis of the complex (1) at 200oC, 300 °C and 400°C are shown in Fig. 3.1. At these temperatures, a pure cubic phase (PbS, database No: 96-901-3404; Fm3m, a = 5.930 Å),118 known as a Galena, halite structure, was produced and identified by using equation (3-1). The Miller indices diffraction peaks for the Galena levels were (422), (420), (331), (400), (222), (311), (220), (200) and (111). A comparison of this result with results from the literature is provided in Table 3.1.

Figure 3.1: The p-XRD patterns for PbS was prepared by melting [Pb(S2COEt)2] at 200 °C, 300oC and 400°C under nitrogen.

Table 3.1: The table shows unit cell parameter for PbS powder prepared by melt reaction at 200, 300 and 400 °C for 1 hour, which is a good match with the value found in the literature value (a=5.930 Å)118

Annealing Unit cell Matching Precursor Method temperature Calc. for phase (°C) PbS 200 cubic galena Melt 300 PbS(ICDD Pb(S2COEt)2 5.930 reaction No:96-901- 400 3404)

3.1.2. Surface Morphology and Composition of PbS

Figures (3.2(a), 3.3(a) and 3.4(a)) shows images obtained from a Scanning Electron Microscope (SEM) images of PbS crystalline material with a cubic shape prepared at 200oC, 300oC and 400oC at 2 μm. A uniform particle surface structure showed is the primary morphology of the particles which are cubic (halit). Energy Dispersion X-ray (EDX) using a 20 keV energy source confirmed that the crystals were composed of PbS. Figs. (3.2.(b) 3.3.(b) and 3.4.(b)). Show the spectra obtained from EDX analysis of typical PbS particles. The peaks provide strong evidence that the particles are composed of PbS. Figs. 3.2 (b), 3.3(b) and 3.4(b) show that the atomic ratios of Pb and S were 54:46, 57:43 and 58:42, respectively, indicating cubic PbS.

(a) (b)

Figure 3.2: SEM image (a) and EDX spectrum (b) of PbS particles after 60 min. at 200oC.

(a) (b)

Figure 3.3: SEM image (a) EDX spectrum (b) of PbS particles after 60 minutes at 300oC

(a) (b)

Figure 3.4: SEM image (a) XDS spectrum (b) of PbS particles after 60 minutes at 400oC.

3.1.3. The p-XRD analysis of Solid State Pyrolysis Sn(S2COEt)2 Complex (3)

The p-XRD patterns for the solid residue obtained after pyrolysis of complex (3) at 200°C, 300°C and 400°C are shown in Fig. 3.5. The preparation at 200°C provided low-intensity peaks, while preparation at 300°C and 400°C revealed higher intensity peaks. The phase determined by using equation (3-2) agreed with the values quoted in the literature for orthorhombic herzenbergite and SnS (Database code: 1011253)119. The diffraction peaks confirmed tin sulfide at 26.22o, 27.60, 30.67o, 31.56o, 32.26o, 39.23o, 48.71o and 56.80o, corresponding to Miller indices (012), (102), (110), (111), (004), (104), (023) and (204), respectively. The lattice parameters were measured at 300oC and 400oC, giving lattice constants a = 3.978 Å, b = 4.328 Å and c = 11.178 Å. This agreed with the literature on orthorhombic herzenbergite and SnS (Database code: 1011253), which gave lattice constants a = 3.978 Å, b = 4.330 Å and c = 11.180 Å. A comparison of the current results to those from the literature can be seen in Table 3.2. Finally, the tin precursor or SnS orthorhombic phase of SnS herzenbergite was achieved at 200°C, 300°C and 400°C after annealing for 60 minutes under nitrogen.

Table 3.2: Details of SnS preparation

Annealing Annealing Unit cells from complex Unit cells from the Matching Precursor temperature time (3) (Å) literature112 (Å) phase (°C) a b c a b c orthorhombic 200 4.071 4.081 11.615 herzenbergite

Sn(S2COEt)2 300 1 hr. 3.978 4.328 11.178 (SnS; 3.978 4.330 11.180 database_code: 400 3.700 4.288 11.204 (1011253)

o Figure 3.5: The p-XRD patterns for SnS from [Sn (S2COEt)2] at 200, 300 and 400 C

under nitrogen.

3.1.4. Surface Morphology and Composition of SnS

Fig (3.6(a), 3.7(a) and 3.8(a)) shows SEM images of SnS crystalline material in an orthorhombic shape prepared at 200oC, 300oC and 400oC respectively and 2μm. A uniform particles surface structure comprises the primary morphology of the orthorhombic particles. EDX with a 20 keV power source confirmed that the crystals collected were SnS and Fig. (3.6.(b) , 3.7(b) and 3.8.(b)) shows the EDX spectra of a typical SnS particle. Peaks

52 correlated to Sn and S provided strong evidence that the particles were composed of SnS. Fig. 3.6, 3.7 and 3.8 show that the atomic ratios of Sn to S were 51:49, 53:47 and 54:46, respectively, indicating that the SnS is orthorhombic.

(a) (b)

Figure 3.6: SEM image of SnS particles after 60 minutes at 200oC.

(a) (b)

Figure 3.7: SEM image of SnS particles after 60 minutes at 300oC.

(a) (b)

Figure 3.8: SEM image of SnS particles after 60 minutes at 400oC.

3.1.5. Phases Formed from Different Mole Fractions of Lead and Tin Precursors

Lead sulfide and tin sulfide were mixed in different mass ratios using a mortar and pestle. Depending on the lead to tin complex ratio, cubic lead-tin sulfide (PbSnS) (ICDD No: 03- 065-8324)120 known by using equation (3-1) or the desired molecule lead-tin disulfide,

PbSnS2 (teallite) was obtained. The cubic phase is important for the photovoltaic, infrared detection of orthorhombic teallite 121 (PbSnS2) (ICDD No: 00-044-1437) determined by using equation (3-2). The orthorhombic phase provides promising material for thin-film solar cells and can be formed according to p- XRD patterns that appear after thermal annealing for 60 minutes at 300oC.

3.1.5.1. Cubic lead-tin sulfide (PbSnS) composition ranges

Cubic lead-tin sulfide (PbSnS) can be grown using the same molar ratio as that used for complex (3), or 0.25 mill moles, while increasing the molar ratio of the complex (1) and thermal annealing at 300 °C for 60 minutes. The p-XRD patterns of the residue from mixing complexes (1) and (3) were measured at 300°C and showed higher peak intensity (Fig. 3.9). This agrees with the literature values for cubic lead-tin sulfide (PbSnS, ICDD No: 04-001- 4833); the space group is (Fm-3m). The diffraction peaks were confirmed as lead-tin sulfide at 26.04o, 30.00o, 43.22o, 51.10o, 53.68o, 62.79o, 69.12o, 71.20o and 79.13o, which correspond to Miller indices (111), (200), (220), (311), (222), (400), (331), (420) and (422), respectively. There was a small signal for orthorhombic (herzenbergite) tin sulfide (Database code: 1011253), which was SnS signal labeled as (*). The diffraction peaks were 31.69o and 39.00o,

54 corresponding to Miller indices (013) and (014), respectively. The lattice parameters of PbSnS were obtained at 300 °C, with the lattice constant of (a) being 5.927 Å. This agrees with the literature for cubic PbSnS (database code: 04-001-4833), which gives the space group as (Fm-3m) and the lattice constant (a) as 5.922 Å1120 (Table 3.3).

Table 3.3: Details of PbSnS preparation: Unit cells from complexes (1) and (3) and unit cells from the literature and galena (PbS).

Complex Unit cells Unit cells Mole d- Mole d-spacing Literature millimole from from the fractions of spacing fractions (111) from reference 120 ratios: (1) + (3) complexes literature the present 118 from from the the (1) and (3) samples the this literature literature 118,120 (Sn/Sn + Pb) work (Sn/Sn + Pb) a / Å a / Å

0.25:0.25 5.927 0 3.4240 0 3.4242 111

0.22 3.4220

5.922 0.50:0.25 5.930 0.25 3.4216 0.5 3.4190 113

0.75:0.25 5.925 0.33 3.4207

0.90:0.25 5.922 0.50 3.4219

Figure 3.9: The p-XRD pattern for PbSnS from 0.25: 0.25 mole ratio of a Pb(S2COEt)2 + Sn(S2COEt)2 at 300 °C.

The p-XRD patterns of the residue from mixing complexes (1) and (3) were prepared at 300oC and showed higher peaks intensity (Fig. 3.10). This agrees with the literature values for cubic lead-tin sulfide (PbSnS, ICDD No: 04-001-4833) with a space group of (Fm-3m). The diffraction peaks confirmed lead-tin sulfide at 26.04o, 30.04o, 43.24o, 51.10o, 53.68o, 62.79o, 69.14o, 71.05o and 79.14o, corresponding to Miller indices (111), (200), (220), (311), (222), (400), (331), (420) and (422), respectively. There was a small signal for orthorhombic (herzenbergite) tin sulfide (Database_code:1011253), which was an SnS signal labeled as (*). The diffraction peaks were 31.55o and 38.85°, corresponding to Miller indices (013) and (014), respectively. The lattice parameter of PbSnS was calculated at 300 °C, with the lattice constant (a) being 5.927 Å. This agrees with the literature value for lattice constant (a), which is 5.922 Å120.

Figure 3.10: The p-XRD pattern for PbSnS from 0.50:0.25 mole ratio of a Pb(S2COEt)2 0 + Sn(S2COEt)2 at 300 C.

The p-XRD patterns of the residue from mixing complexes (1) and (3) were prepared at 300°C and showed higher peaks intensity (Fig. 3.11), which agreed with the literature values for cubic lead-tin sulfide (PbSnS, ICDD No: 04-001-4833) with a space group of (Fm-3m). The diffraction peaks confirmed lead-tin sulphide at 26.05o, 30.06o, 43.26o, 51.12o, 53.69o, 62.79o, 69.14o, 71.05o and 79.14o, corresponding to Miller indices (111), (200), (220), (311), (222), (400), (331), (420) and (422), respectively. There was a small signal for orthorhombic (herzenbergite) tin sulfide (Database code: 1011253), with an SnS signal labeled as (*). The diffraction peaks were at 31.69o and 38.93o, corresponding to Miller indices (013) and (014), respectively. The lattice parameters of PbSnS were calculated at 300oC, with the lattice constant (a) being 5.925 Å. This agrees with the literature value for lattice constant (a), which is 5.922 Å120.

Figure 3.11: The p-XRD pattern for PbSnS from 0.75:0.25 mole ratio of a Pb(S2COEt)2 0 + Sn(S2COEt)2 at 300 C.

The p-XRD patterns of the residue from mixing complexes (1) and (3) were prepared at 300°C and showed higher peaks intensity (Fig. 3.12), which agreed with the literature values for cubic lead-tin sulfide (PbSnS, ICDD No: 04-001-4833), with a space group of (Fm-3m). The diffraction peaks confirmed lead-tin sulfide at 26.06o, 30.08o, 43.28o, 51.13o, 53.71o, 62.81o, 69.19o, 71.13o and 79.19o, corresponding to Miller indices (111), (200), (220), (311), (222), (400), (331), (420) and (422), respectively. There was a small signal for orthorhombic (herzenbergite) tin sulfide (Database_code: 1011253), with an SnS signal labeled as (*). The diffraction peaks were at 31.69o and 38.85o, corresponding to Miller indices (013) and (014), respectively. The lattice parameters of PbSnS were calculated as described in 117, at 300oC, with the lattice constant (a) being 5.922 Å. This value is in agreement with that quoted in the literature 120.

Figure 3.12: The p-XRD pattern for PbSnS from 0.90:0.25 mole ratio of a Pb(S2COEt)2 o + Sn(S2COEt)2 at 300 C .

3.1.5.2. Teallite (PbSnS2) composition ranges

Orthorhombic teallite is obtained using the same molar ratio as that for Pb(S2COEt)2 (i.e.,

0.25 millimoles), increasing the molar ratio of Sn(S2COEt)2 and thermally annealing at 300 °C for 60 minutes. The p-XRD patterns of the solid residue gained from pyrolysis of the complex (1) with complex (3) at 300°C are shown in Fig 3.13. The measurement at 300°C provided higher intensity peaks, which agrees with the literature values for orthorhombic teallite (PbSnS2, ICDD No: 00-044-1437). The diffraction peaks from the p-XRD indicate teallite at 26.03o, 30.37o, 31.56o, 38.69o, 44.40o, 45.38o, 50.82o and 53.58o with Miller indices of (012), (110), (004), (104), (114), (015), (122) and (024), respectively. The lattice parameters for this phase were calculated by using equation (3-2) at 300oC, with lattice constants of a = 4.049 Å, b = 4.282 Å and c = 11.346 Å. This agrees with the literature on orthorhombic teallite (PbSnS2, ICDD No: 00-044-1437), which gave lattice constants of a= 4.047 Å, b= 4.286 Å and c= 11.341 Å.121

Table 3.4: Details of PbSnS2 preparation: Unit cells for mixing complexes (1) and (3),

unit cell from the literature and herzenbergite, SnS and orthorhombic teallite (PbSnS2) Millimole Unit cells from complexes Unit cells from the Mole fraction of D-spacing (004) Ref. 121 ratios of (1) and (3).(Å) literature .(Å) (Sn/Sn + Pb) Complexes : a b c a b c This Lit. This Lit. (1) + (3) work work

A) 0.25:0.40 4.049 4.282 11.346 4.084 4.270 11.417 0.615 0.5 2.8348 2.8350 114

B) 0.25:0.50 4.034 4.287 11.328 4.078 4.272 11.398 0.666 2.8348

C) 0.25:0.85 4.017 4.293 11.300 4.037 4.295 11.320 0.772 2.8347 None None

D) 0.25:0.95 4.011 4.293 11.281 4.016 4.306 11.270 0.791 2.8346

E) 0.25:1.00 4.010 4.296 11.263 4.000 4.320 11.240 0.800 2.8346

1 1 2.5345 2.8345 112

Figure 3.13: The p- XRD pattern for PbSnS2 from 0.25:0.40 mole ratio of a o Pb(S2COEt)2 + Sn(S2COEt)2 at 300 C.

The p-XRD ratio patterns for solid residue gained from the pyrolysis of the complex (1) with complex (3) were obtained at 300°C (Fig. 3.14); the values obtained agree with those found in 114 the literature for orthorhombic teallite (PbSnS2) (ICDD No: 00-044-1437). The diffraction peaks from the p-XRD indicate teallite at 26.01o, 30.35o, 31.56o, 38.86o, 44.59o and 50.86o and correspond to Miller indices (012), (110), (004), (104), (114) and (122), respectively. The lattice parameters of the phase were calculated and obtained at 300oC, providing lattice constants of a = 4.040 Å, b = 4.287 Å and c = 11.328 Å. This agrees with the literature on orthorhombic teallite (PbSnS2) that provide lattice constants of a = 4.078 Å, b = 4.272 Å and c = 11.398 Å122.

Figure 3.14: The p-XRD pattern for PbSnS2 from 0.25:0.50 mole ratio of a Pb(S2COEt)2 o + Sn(S2COEt)2 at 300 C.

Fig 3.15 shows the p-XRD ratio patterns of the solid residue obtained from the pyrolysis of the complex (1) with complex (3) at 300°C. These agree with the values for orthorhombic 121 teallite (PbSnS2) (ICDD No: 00-044-1437) quoted in the literature . The diffraction peaks from the p-XRD indicate teallite at 26.38o, 30.32o, 31.56o, 39.01o, 44.64o and 50.72°. These correspond to Miller indices of (012), (110), (004), (104), (015) and (122), respectively. The lattice parameters for this phase were calculated and obtained at 300oC.The lattice constants were a = 4.017 Å, b = 4.293 Å and c = 11.300 Å. These agree with those quoted in the

122 literature on orthorhombic teallite (PbSnS2), where the value of lattice constants are given as a= 4.037 Å, b = 4.295 Å and c = 11.320 Å.

Figure 3.15: The p-XRD pattern for PbSnS2 from 0.25:0.85 mole ratio of a Pb(S2COEt)2 o + Sn(S2COEt)2 at 300 C.

The p-XRD ratio patterns of the solid residue obtained from pyrolysis of the complex (1) with complex (3) at 300°C are shown in Fig. 3.16 and agree with the values given in the 121 literature of orthorhombic teallite (PbSnS2) (ICDD No:00-044-1437). The diffraction peaks from the p-XRD indicated teallite at 26.44o, 30.28o, 31.56o, 39.26o, 44.69o and 50.71o, which correspond to Miller indices (012), (110), (004), (104), (114) and (122), respectively. The lattice parameters of this phase were calculated at 300oC, with lattice constants of a = 4.011 Å, b = 4.293 Å and c = 11.281 Å. This agrees with the literature on orthorhombic teallite (PbSnS2), which gives lattice constants of a = 4.016 Å, b = 4.306 Å and c = 11.270 Å122.

Figure 3.16: The p-XRD pattern for PbSnS2 from 0.25:0.95 mole ratio of a Pb(S2COEt)2 o + Sn(S2COEt)2 at 300 C.

The p-XRD ratio patterns of the solid residue obtained from the pyrolysis of the complex (1) with complex (3) at 300°C are shown in Fig. 3.17 and agree with the literature values for 121 orthorhombic teallite (PbSnS2) (ICDD No:00-044-1437) . The diffraction peaks from the p- XRD indicate teallite at 26.43o, 30.27o, 31.56o, 39.32o, 44.72o and 50.71o, which correspond to Miller indices (012), (110), (004), (104), (114) and (122), respectively. The lattice parameters for this phase were obtained at 300oC, with lattice constants of a = 4.010 Å, b =

4.320 Å and c = 11.240 Å. This agrees with the literature on orthorhombic teallite (PbSnS2), which gives lattice constants of a = 4.00 Å, b = 4.3200 Å and c = 11.240 Å122.

Figure 3.17: The p-XRD pattern for PbSnS2 from 0.25:1.00 mole ratio of a Pb(S2COEt)2 o + Sn(S2COEt)2 at 300 C.

3.1.6. Surface Morphology and Composition of (PbSnS2) Particles

SEM images of the PbSnS2 particles at the same magnification at a temperature of 300 °C are shown in Fig. 3.18 (a), while Fig. 3.18 (b) shows the EDX Spectrum. A uniform nanoparticle surface structure comprises the primary morphology of the particles. EDX was conducted with a 20 keV energy source, which confirmed the analysis of the PbSnS2 particles. Fig. 3.18 shows that the atomic ratio of Pb:Sn:S was 21:34:44, which indicates primarily orthorhombic

PbSnS2.

Figure 3.18: SEM image of PbSnS2 particles after 60 minutes at 300oC; b) 0.25:0.50 o mole ratio for Pb(S2COEt)2 + Sn(S2COEt)2 at 300 C.

3.1.7. Comparison of Data from Mixing Complexes (1) and (3) and Data from the Literature

3.1.7.1 Crystallography studies of the variations in (PbS) rich complexes and those found in the literature

The plots below correspond to five molecules involved in mixing complexes (1) and (3) with different mole fractions than those given in the literature.

11 9

11 8

Figure 3.19: Variations in the d-spacing 111 and the plotted composition of PbS and PbSnS based on mole fraction.

Fig. 3.19 compares d-spacing (111) values of the height intensity peaks of PbS and PbSnS achieved by mixing complexes (1) and (3) with those given in the literature118,120 The d- spacing (111) values of this study declined which in an agreement with the values given in the literature118,120

Mole fraction: Present Lattice constants: Mole fraction: Lattice constant: samples (Sn/Sn + Pb) Present samples Literature 118,120 Literature (Sn/Sn + Pb)

0 5.930 0 5.930

0.22 5.9270

0.25 5.9268 0.5 5.922

0.33 5.9255

0.50 5.9225

12 0

11 8

Figure 3.20: Variations in (a) values for PbS and PbSnS compared to the metal precursor.

Fig. 3.20 compares variations of (a) unit cell values obtained by mixing complexes (1) and (3) with (a) lattice consistent which given in the literature.118,120 The values of the present sample declined and agreed with the value given in the literature118,120

3.1.7.2 Crystallography studies of the variation of (SnS) rich complexes.

The plots correspond to molecules involved in mixing complexes (1) and (3) with different mole fractions compared to values given in the literature119,121

119

121

Figure 3.21: Variations in d-spacing (004) and the plotted composition of PbSnS2 and SnS based on mole fraction.

Fig. 3.21 compares the d-spacing (004) values of the highest intensity peak in SnS and 119,121 PbSnS2 gained by mixing complexes (1) and (3) with the values found in the literature. The d-spacing (004) values of this work still agreed with the values given in the literature.119,121

3.2 The present study and further ones Comparison of teallite (PbSnS2) Synthesis.

Teallite produces continuous PbSnS2 solid solutions to SnS, from the elementals' powder and implemented in sealed silica tubes and warmth conduct via utilizing a furnace. Former work stated the teallite preparation via solid solutions produced at 700 °C via untreated blends prepared from annealed elements at 250 °C for a 7 months period, a grinding being attained each 2 utilized temperature 700 °C. An extensive two-phase area occurs between PbSnS2 and PbS, and the solid solution sequences extension beyond the teallite composition into the PbS- rich part of the scheme PbS-SnS is very restricted even at nearby the teallite's melting point temperatures of around 700°C. In another study Sungari, et al. 123 created the samples at different compositions. Simple mono-sulfides, PbS and SnS were created via the reaction amid sulfur and metals in a silica glass tube utilizing metallic Pb and Sn. Teallite was prepared at temperatures < 450 ° C via the hydrothermal recrystallization technique. Synthetic PbS and PbSnS2 mixtures, which had been produced in the glass tube, were utilizied for the nutrient substance. Gold tubes with interior diameter of 5m mm and 5 mm lengths were utilized as the reaction containers.

Powdered nutrient with a PbSnS2/ PbS molar ratio = 1.0 was positioned in the gold tube's bottom with 5M NH4Cl aq. solution. The sealed Au capsules were positioned in the test tube- kind pressure container kept upright and were heated for 9 to 20 days to 250 °C – 450 °C. An organized temperature incline of ~38 °C/cm was sustained in the pressure container. The sulfides' nutrient were dissolved at the, inferior, hotter capsule end, conveyed by convection, and deposited at the cooler, superior end. The crystallization degree at the gold capsule's top was gauged by thermocouples (chromel alumel). Conversely, the current study utilized a molecular precursor's melt to create tealite, by blending precursors (1) and (3) by diverse molar ratio and implementing a pyrolysis for 1 hr. at 300°C. However, intense interest has arisen in metal dichalcogenides MX2, (X is S or Se) over the past few decades as a result of their multipurpose structure and properties. The layered structure contains X–M–X sandwich layers Metals disulfide are important in the 2D material. For illustration, tin disulfide (SnS2) is an important candidate for the optoelectronic and electronic application. The interaction of its crystals makes the huge potential for Nanoelectronics applications owing to its high carrier mobility.124 It was found that a & c axes length were decreased gradually, whereas the c axis length increase. Extension of the solid solution to the Pb-rich portion of the PbS SnS system at high temperatures was determined Pb1.06Sn0.94S2–SnS as the solid solution area for herzenbergite– teallite minerals at 700°C. However, the solid solution area reduced to Pb1.02Sn0.98S2 SnS at 68

400°C. Chemical compositions of hydrothermal teallite were examined by EPMA, and the results fall within a relatively narrow range at each temperature and mean values are 116 Pb1.14Sn0.86S2 at 300°C, Pb1.14Sn0.886S2 at 400°C, and Pb1.124Sn0.876S2 at 450°C. Diverse chemical analyses were performed and estimated a Pb range 19.7 – 40.8 %, an Sn range 23.3 – 44.2 %, and an S range I4.1 to 20.9 % with lesser quantities of Fe, Zn, Cu, etc122

Table 3.6: Comparison of the mole fraction of this study and literatures, lattice constants and the obtained phase compared with the other studies of Brice 12 and Sugaki 123.

Mole Fraction: (Sn/Sn+Pb) lattice constants ( Å )

122 123 This Brice et Sugaki et This work Brice et al Sugaki et al

work al 122 al 123 a b c a b c a b c

0.615 0.5 0.5 4.0491 4.2816 11.3461 4.084 4.270 11.418 4.0878 4.2698 11.4244

0.666 0.525 0.55 4.0398 4.2870 11.3280 4.078 4.272 11.398 4.0781 4.2775 11.4053

0.6 0.60 4.055 4.283 11.360 4.0655 4.2810 11.3849

0.772 4.0167 4.2928 11.3004

0.7 0.7 4.037 4.295 11.320 4.0435 4.2928 11.3415

0.791 0.8 0.8 4.0115 4.2930 11.2810 4.016 4.306 11.270 4.0212 4.3059 11.2891

0.800 0.9 0.9 4.0100 4.2964 11.2630 4.000 4.320 11.240 4.0039 4.3188 11.2414

1 1 1 3.9780 4.3280 11.1750 3.978 4.330 11.180 3.9833 4.3302 11.1892

There are 2 approaches for creating teallite. The present study has used a melt of a molecular precursor while a melt of sulfur has been used by Brice et al122 and Sugaki et al.123

Orthorhombic phase has been determined for teallite (PbSnS2) in this work and literature.

Figure 3.22: Variation in cell parameter (a) from thermal decomposition of a mixture of complexes (1) and (3) at different mole ratios.

Fig.3.22. Compares variations in (a) values from mixing complexes (1) and (3) with those given in the literature.115,116 The values of the present samples decreased and still agreed with the lattice constant (a) values which are given in the literature119,122

Figure 3.23: Variation in cell parameter (b) from thermal decomposition of a mixture of precursors (1) and (3) at different mole ratios.

Fig.3.23. Compares variations in (b) values from this work with literature.122,123 The values of the present samples agreed with the lattice constant (b) values which given in the literature.119,122 but are slightly lower than those given in reference123.

Figure 3.24: Variation in cell parameter (c) from thermal decomposition of a mixture of complexes (1) and (3) at different mole ratios.

Fig.3.24 Compares variations in (c) values from this work with that in the literature.122,123 The values of the present samples declined and still agreed with the lattice constant (c) values which given in the literature 119,122

Solar cell energetics demand direct band gap materials as an alternative to the current dominant technology based on Si,Cu(In,Ga)Se2 and CdTe . This is because of the indirect band gap of crystalline silicon, the high toxicity of cadmium and the limited availability of indium in nature. Many sulfosalt materials show optical band gaps around 1.4 eV and electrical properties suitable for thin films photovoltaic applications.125-127 Both PbS (band gap 0.37 eV)106 and SnS (band gap 1.3 eV)107 are promising materials for infrared detection, photovoltaic and other optoelectronic devices. So, by melting precursors (1) and (3) teallite

(PbSnS2), an orthorhombic phase can be obtained. Teallite is important for optoelectronic applications such as thin film solar cell due to its 1.4 eV band gap.111,112

4. Conclusion

In this research, Sn and Pb xanthate complexes were created and described via elemental analysis TGA, NMR, IR and the fusion points. The complexes [Pb(S2COEt)2 ] (1) and

[Sn(S2COEt)2] (3) were used to form materials by melt reactions. At 200 °C, 300 °C and 400oC, precursor (1) yielded cubic PbS (galena with a halite structure), while precursor (3) yielded orthorhombic SnS (hezengebentite). Complexes (1) and (3) were ground together in various mole ratios and melted in a furnace at 300°C for 1 hour under a nitrogen atmosphere.

Lead-tin sulfide (PbSnS) and orthorhombic teallite (PbSnS2) were characterized using useful p-XRD, SEM and EDX.

5. Future Work

In the short term:  Complete a composition phase map based on the initial work.  Prepare a series of lead alkyl xanthates and tin alkyl xanthates, for example, propyl, isopropyl, butyl, isobutyl, hexyl and pentyl, use them as precursors.  Further, characterize teallites by elementals analysis (ICP) and Raman spectroscopy. Then :  Synthesize teallite by an aerosol-assisted chemical vapor deposition (AACVD) using

[Pb(S2COEt)2] and [Sn(S2COEt)2] at different temperatures with argon as the carrier gas.  Study the optical properties of the thin films and nanoparticles of teallite to measure the band gaps.  Extend this work to a related phase with other complex sulphide minerals such 2+ 2+ 4+ 3+ as cylindrite (Sn31.52Sb6.23Fe3.12S59.12) and franckeite (Pb, Sn ) 6Fe Sn2 Sb2 S14.

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