Spectroelectrochemical Investigations of the Interaction of Flotation Reagents with Gold Surfaces

Author Zhang, Kun

Published 2016

Thesis Type Thesis (PhD Doctorate)

School School of Natural Sciences

DOI https://doi.org/10.25904/1912/905

Downloaded from http://hdl.handle.net/10072/366588

Griffith Research Online https://research-repository.griffith.edu.au

Spectroelectrochemical investigations of the interaction of flotation reagents with gold surfaces

Kun Zhang

B.Eng., M.Sc.

School of Natural Sciences

Griffith Sciences

Griffith University

Submitted in fulfilment of the requirements of the degree of

Doctor of Philosophy

August 2016

Abstract

This thesis presents the results of a spectroelectrochemical investigation into the interactions of two flotation collectors (mercaptobenzothiazole (MBT) and dithiophosphate (DTP)) with gold electrode and nanorods surfaces. The relevant compounds and ligands of these two collectors were characterised and identified by

Raman spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Gold nanorods (Au NRs) with average aspect ratio of 4.04 ±

0.08 were synthesised and reacted with MBT and DTP. Gold particle flotation was investigated using controlled potential techniques. Electrochemical techniques have also been used to investigate the open circuit potential (OCP) and controlled potential oxidation of chalcopyrite.

The interaction of collectors with gold electrode surfaces was investigated using in situ surface-enhanced Raman scattering (SERS) spectroscopy. The SERS surface for a gold electrode was generated by electrochemically roughening using an oxidation-reduction cycling (ORC).

The SERS spectra of gold electrodes in 10-3 M sodium mercaptobenzothiazole (NaMBT) were obtained in situ by increasing the potential stepwise from −0.7 V to 0.5 V. A weak band at 311 cm-1 in the SERS spectra can be contributed by the Au−S stretching vibration, while no evidence was observed for the existence of the Au–N stretching vibration in the SERS spectra. These spectra are consistent with the bonding by the two

S atoms of MBT− with the gold electrode surface. When the SERS spectra of SERS spectra were recorded directly under a potential control of 0.3 V in 10-3 M and 10-4 M

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NaMBT, the dithiolate, 2,2’-dithiobisbenzothiazole ((MBT)2) was observed on the gold electrode surface. In flotation tests, gold metal particles floated in the cell when the potential reached 0.5 V. At that potential, (MBT)2 was observed on the gold particles using ATR-FTIR, and the particles floatability was attributed to the adsorption of that dithiolate species.

The SERS spectra of gold electrodes in 10-3 M sodium dibutyl dithiophosphate

(NaDBDTP) were obtained in situ by increasing the potential stepwise from −0.7 V to

0.3 V. Both the adsorption of DBDTP− and formation of butyl thioperoxydiphosphate

-1 -1 ((DBDTP)2) were observed in the SERS spectra. Two bands at 620 cm and 530 cm were observed in the SERS spectra at negative potentials, which were assigned to the

PS2 antisymmetric and symmetric stretch of the DBDTP molecule. This observation was consistent with DBDTP− bonding to the gold electrode surfaces through two S atoms. Three bands were observed at 660, 505 and 482 cm-1 , these can be assigned to the P=S, S−S and P−S stretch of (DBDTP)2. Compared to NaMBT, gold metal particles showed good floatability in the presence of NaDBDTP under various conditions in the flotation test.

Absorption and vibrational spectroscopies were applied for the ex situ investigation of the interaction between collectors and Au NRs. Au NRs solution was mixed with 10-3 M

NaMBT and NaDBDTP solution, respectively. Black precipitates (Au NRs/MBT complex) were obtained from the exposure of Au NRs to MBT−, while no precipitate was formed with the DBDTP− solution. A very weak band at 314 cm-1 was observed in the Raman spectra for the Au NRs/MBT complex, which was assigned to the formation of a semi-covalent Au−S bond or Au−N bond between Au NRs and MBT molecules. At

iv least two chemisorbed S environments were observed in the X-ray Photoelectron spectroscopy S 2p spectrum of the Au NRs/MBT complex. The N 1s spectrum of the

Au NRs/MBT complex may also be consistent with a bonding between the N atom and

Au NRs. It indicates that the exocyclic S atom of MBT could chemisorb on a gold nanorod growth surface through a Au−S bond, while the endocyclic S or N atom of the same MBT adsorbed on another gold nanorod surface.

In this study, a novel in-situ method for preparing fractured chalcopyrite surfaces was developed. It was demonstrated that the voltammograms obtained from the chalcopyrite electrode with in-situ fractured and externally abraded surfaces did not vary significantly. These observations demonstrated that abrasion can be an effective and sufficient method for making fresh mineral surfaces for electrochemical investigation. It was also observed that oxidised chalcopyrite may not respond to the collectors used in flotation process, and that deoxygenation using a nitrogen purge can minimise the oxidation reaction of the mineral surfaces.

Statement of Originality

This work has not previously been submitted for a degree or diploma in any university.

To the best of my knowledge and belief the thesis contains no material previously published or written by another person except where due reference is made in the thesis itself.

Kun Zhang

VI Table of contents

Abstract ...... iii

Statement of Originality ...... vi

Table of contents ...... vii

List of Figures ...... xi

List of Tables ...... xv

List of Abbreviations ...... xvi

Acknowledgements ...... xviii

Chapter 1 Introduction ...... 1

1.1 Preamble ...... 2

1.2 Aims and objectives ...... 4

1.3 Structure of thesis ...... 5

1.4 References ...... 6

Chapter 2 Literature review ...... 8

2.1 Gold flotation ...... 9

2.1.1 Gold minerals ...... 9 2.1.2 The effects of physical and chemical conditions on gold flotation ...... 11

2.2 Interaction of collectors with gold and gold-bearing sulfide minerals . 13

2.2.1 Xanthate ...... 14 2.2.2 MBT ...... 16 2.2.3 DTP ...... 18

2.3 Gold nanorods ...... 20

2.3.1 Synthesis and characterisation ...... 20 2.3.2 SERS ...... 24

2.4 References ...... 27

Chapter 3 Experimental and instrumental ...... 41

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3.1 Preamble ...... 42

3.2 Experimental materials and methods ...... 43

3.2.1 Reagents and materials ...... 43 3.2.2 Synthesis of MBT compounds ...... 44 3.2.3 Synthesis of DTP compounds ...... 45 3.2.4 Synthesis of Au NPs ...... 46 3.2.5 Flotation experiments ...... 47

3.3 Electrochemistry ...... 51

3.3.1 Background ...... 51 3.3.2 Instrumentation ...... 52

3.3.2.1 Working electrodes ...... 53 3.3.2.2 Reference electrodes ...... 55 3.3.2.3 Counter electrodes and potential control ...... 56

3.3.3 Investigations in this thesis ...... 56

3.4 Raman Spectroscopy ...... 57

3.4.1 Background ...... 57 3.4.2 Surface-enhanced Raman spectroscopy ...... 59 3.4.3 Instrumentation ...... 61 3.4.4 Investigation in this thesis ...... 61

3.5 Fourier transform infrared Spectroscopy ...... 63

3.5.1 Background ...... 63 3.5.2 Instrumentation ...... 63 3.5.3 Investigation in this thesis ...... 64

3.6 XPS ...... 65

3.6.1 Background ...... 65 3.6.2 Instrumentation ...... 65 3.6.3 Investigation in this thesis ...... 66

3.7 TEM and other instruments ...... 67

3.8 References ...... 68

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Chapter 4 Characterisation of MBT and DTP compounds ...... 71

4.1 Preamble ...... 72

4.2 Characterisation of MBT compounds ...... 73

4.2.1 HMBT ...... 73

4.2.1.1 Region 4000−2000 cm-1 ...... 81 4.2.1.2 Region 2000−1000 cm-1 ...... 82 4.2.1.3 Region below 1000 cm-1 ...... 84

4.2.2 (MBT)2 ...... 85

4.2.2.1 Molecular structure of (MBT)2 ...... 85

4.2.2.2 Characterisation of (MBT)2 ...... 88

4.2.3 Metal MBT compounds ...... 94

4.3 Characterisation of DTP compounds ...... 101

4.3.1 Molecule structures of dibutyl dithiophosphate compounds ...... 101 4.3.2 Assignments of dibutyl dithiophosphate compounds ...... 104

4.4 Conclusions ...... 110

4.5 References ...... 111

Chapter 5 Investigations of the interaction of collectors with gold electrode and nanorod surfaces ...... 119

5.1 Preamble ...... 120

5.2 Characterisation of gold nanorods ...... 121

5.3 SERS of collectors on Au NRs ...... 129

5.4 The interaction of collectors with gold nanorods ...... 132

5.4.1 Absorption and vibrational spectroscopy ...... 134 5.4.2 XPS ...... 141

5.5 The interaction of collectors with gold electrode surfaces ...... 148

5.5.1 Voltammetry ...... 148 5.5.2 SERS spectra ...... 151

5.5.2.1 NaMBT ...... 151

5.5.2.2 NaDBDTP ...... 159

5.6 Flotation...... 161

5.7 Conclusions ...... 165

5.8 References ...... 166

Chapter 6 Electrochemical study of chalcopyrite ...... 172

6.1 Preamble ...... 173

6.2 Voltammetry ...... 174

6.2.1 Voltammogram of chalcopyrite ...... 174 6.2.2 Fractured vs abraded surfaces ...... 176

6.3 Open Circuit Potential ...... 182

6.3.1 OCP of chalcopyrite from Messina, South Africa ...... 183 6.3.2 OCP of chalcopyrite from Mt. Isa, Australia ...... 186 6.3.3 OCP of chalcopyrite from Butte, USA ...... 188

6.4 Controlled potential oxidation of Chalcopyrite ...... 190

6.5 Conclusions ...... 193

6.5 References ...... 194

Chapter 7 Conclusions ...... 196

7.1 Concluding remarks ...... 197

7.2 Future work ...... 202

7.3 References ...... 203

List of Figures

Figure 2.1 The generic molecular structures of xanthate, dithiophosphate and mercaptobenzothiazole...... 13 Figure 2.2 The bonding types between MBT and gold surfaces (Beattie et al., 2009). . 17 Figure 2.3 The bonding types between DTP and gold surfaces (Beattie et al., 2009). .. 19 Figure 2.4 Three mechanisms for the role of Ag+ ions (Lohse and Murphy, 2013)...... 23 Figure 2.5 Schematic representation of LSPR excitation for Au NRs (a) and LSPR absorption bands of Au NRs (b) (Cao et al., 2014)...... 24 Figure 3.1 Hallimond tube flotation cell...... 48 Figure 3.2 Gold chips were cut off from the gold block...... 49 Figure 3.3 Gold chips were ground to ultra fine particles...... 49 Figure 3.4 Picture (a) and optical microscopy image (b) of gold particles...... 50 Figure 3.5 The multi-necked heart shaped glass cell. (a) working electrode; (b) counter electrode; (c) reference electrode; (d) nitrogen purge...... 52 Figure 3.6 The borosilicate electrochemical cell. (a) working electrode; (b) counter electrode; (c) reference electrode; (d) nitrogen purge...... 53 Figure 3.7 Potential regime for oxidation-reduction cycling of gold SERS electrode. .. 55 Figure 3.8 Diagram of the Rayleigh and Raman scattering processes. The frequency of the incident beam is v0 and the vibrational frequency of the molecule is vm...... 58 Figure 3.9 Raman spectroscopy...... 62 Figure 3.10 Attenuated total reflectance Fourier transform infrared spectroscopy...... 64 Figure 4.1 Molecular structures of the thione and thiol forms of HMBT...... 73 Figure 4.2 The dimer structure of HMBT...... 74 Figure 4.3 Raman and FTIR spectra of HMBT...... 76

Figure 4.4 Molecular structures of the two forms of (MBT)2...... 85

Figure 4.5 The N 1s (a) and S 2p (b) spectra of (MBT)2...... 87

Figure 4.6 Molecular structures of (MBT)2 and benzothiazole...... 88 Figure 4.7 Schematic skeletal vibrations of five-membered heteroaromatic ring compounds with two double bonds (after Colthup et al., 1990)...... 89

Figure 4.8 Raman spectra of (MBT)2 and benzothiazole...... 92 Figure 4.9 Raman spectra of metal MBT compounds...... 96

Figure 4.10 Raman spectra of CuMBT, AuMBT, (MBT)2 and their syntheses...... 99

Figure 4.11 Raman spectra of metal MBT compounds in the range of 300−350 cm-1. 100 Figure 4.12 Two resonance structures of dibutyl dithiophosphate anions...... 101 Figure 4.13 Cu 2p spectrum CuDBDTP corrected for energy of flood-gun electrons. 102

Figure 4.14 Molecular structures of CuDBDTP (I), Zn(DBDTP)2 (II) and Zn2(DBDTP)4 (III)...... 103 Figure 4.15 Raman spectra of dithiophosphate compounds...... 106 Figure 4.16 FTIR spectra of NaDBDTP, CuDBDTP and ZnDBDTP in the range of 750−500 cm-1...... 109 Figure 5.1 Schematic of the seed-mediated method for Au NRs synthesis...... 121 Figure 5.2 Zeta potential of Au NRs...... 122 Figure 5.3 The Ag 3d (a) and Br 3d (b) spectra of Au NRs...... 123 Figure 5.4 TEM images of Au NRs under different scale: (a) 50 nm; (b) 200 nm...... 125 Figure 5.5 The TEM image of Au NR showing the gold lattice spacing under the scale of 2 nm...... 126 Figure 5.6 TEM images of Au NPs under different scale: (a) 20 nm; (b) 50 nm...... 127 Figure 5.7 UV-vis absorption spectrum (a) and aspect ratio distribution (b) of Au NRs...... 128 Figure 5.8 SERS spectra of MBT− ions on Au NRs in the range of 300 to 1700 cm-1. 130 Figure 5.9 SERS spectra of DBDTP− ions on Au NRs...... 131 Figure 5.10 Schematic of the formation of nanochains (after Shibu Joseph et al., 2006 )...... 133 Figure 5.11 UV-vis absorption spectra of Au NRs solutions with different concentrations of NaMBT (a) and NaDBDTP (b) solutions after 30 and 120 min of the addition of the collector...... 136 Figure 5.12 Au NRs solution (a) and Au NRs mixed with 10-3 M NaMBT solution after 120 min (b) and 180 min (c)...... 137 Figure 5.13 Raman spectra of AuMBT, Au NRs/MBT and Au NPs/MBT precipitates...... 139 Figure 5.14 FTIR spectra of CTAB and Au NRs/MBT precipitates...... 140 Figure 5.15 The Au 4f spectra of Au NRs/MBT complex (a) and Au NRs (b)...... 144 Figure 5.16 The S 2p (a) and N 1s (b) spectra of Au NRs/MBT complex...... 145

Figure 5.17 The S 2p spectra of AuMBT (a) and (MBT)2 (b)...... 146 Figure 5.18 Schematic of the reaction between Au NRs and NaMBT...... 147

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Figure 5.19 Voltammograms of gold electrode in solution of pH 9.2 containing 0, 10-4 and 10-3 M of NaMBT (a) and NaDBDTP (b), respectively...... 150 Figure 5.20 SERS spectra of gold electrode in 10-3 M NaMBT under different potential control compared with Raman spectrum of AuMBT...... 153 Figure 5.21 SERS spectra of gold electrode in 10-3 M NaMBT under different potential control compared with Raman spectrum of AuMBT in the range of 300 to 450 cm-1 and 1300 to 1450 cm-1...... 154 Figure 5.22 Schematic diagram of the adsorption of MBT on Au surfaces...... 154 Figure 5.23 SERS spectra of gold electrode in 10-3 M and 10-4 M NaMBT under different potential control compared with Raman spectra of AuMBT and (MBT)2. ... 157 Figure 5.24 SERS spectra of gold electrode in 10-3 M and 10-4 M NaMBT under different potential control compared with Raman spectra of AuMBT and (MBT)2 in the range of 300 to 550 cm-1 and 1300 to 1500 cm-1...... 158 Figure 5.25 SERS spectra of gold electrode in 10-3 NaDBDTP under different potential -1 control compared with Raman spectrum of (DBDTP)2 in the range of 400 to 1500 cm ...... 160 Figure 5.26 Schematic diagram of the adsorption of DBDTP on Au surfaces...... 160 Figure 5.27 Floated gold particles in Hallimond tube...... 162

Figure 5.28 FTIR spectra of (MBT)2 and floated gold particles in the range of 500 to 1500 cm-1...... 164 Figure 6.1 Voltammogram of chalcopyrite electrode in pH 9.2 buffer solution...... 175 Figure 6.2 The chalcopyrite electrode prior to score and snap...... 176 Figure 6.3 The chalcopyrite electrode in cell post fracture...... 177 Figure 6.4 The fractured chalcopyrite electrodes post experiment...... 178 Figure 6.5 Voltammograms of fractured and abraded chalcopyrite electrodes...... 180 Figure 6.6 SEM images of fractured (a) and abraded (b) chalcopyrite electrodes by Dr. Kym Watling...... 181 Figure 6.7 The OCPs of chalcopyrite from South Africa under different conditions: (a) in borate solution; (b) in phosphate solution; (c) in carbonate solution...... 185 Figure 6.8 The OCPs of chalcopyrite from Australia under different conditions: (a) in borate solution; (b) in phosphate solution; (c) in carbonate solution...... 187 Figure 6.9 The OCPs of chalcopyrite from United States under different conditions: (a) in borate solution; (b) in phosphate solution; (c) in carbonate solution...... 189

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Figure 6.10 Chronoamperogram of abraded chalcopyrite in pH 9.2 buffer solution at 0.06 V (black) and 0.5 V (red)...... 190 Figure 6.11 Voltammogram of chalcopyrite electrodes under different oxidation conditions in solutions with 0 and 10-3 M NaDBDTP...... 192

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List of Tables

Table 2.1 Gold minerals and their composition and content (after Fuerstenau et al., 2007)...... 9 Table 3.1 List of reagents and materials...... 43 Table 4.1 Observed frequencies for HMBT...... 77

Table 4.2 Characteristic Raman bands for (MBT)2...... 93 Table 4.3 Characteristic Raman bands for metal MBT compounds...... 97 Table 4.4 Characteristic Raman bands for dithiophosphate compounds...... 107 Table 5.1 The assignment of Au 4f, S 2p and N 1s binding energies of Au NRs/MBT...... 141 Table 5.2 Flotation of gold particles tests in pH 9.2 solutions without potential control...... 162 Table 5.3 Flotation of gold particles tests in pH 9.2 solutions with potential control. . 163 Table 6.1 The OCP of fractured and abraded chalcopyrite electrodes...... 178

List of Abbreviations

Å Angstrom

AR Analytical Reagent

ATR attenuated total reflectance

°C Celsius

CCD charge-coupled device cm-1 wavenumber cm2 square metre

CTAB hexadecyltrimethylammonium bromide

CV cyclic voltammogram

DDI doubly de-ionized

DTP dithiophosphate eV electron volt

FT Fourier transform g gram h hour

ICP-OES inductively coupled plasma optical emission spectroscopy

IR infrared kV kilovolt

LSPRs localized surface plasmon resonances

M molar mbar millibar

MBT mercaptobenzothiazole min minute mL millilitre mM millimolar

xvi mm millimetre mV millivolt nM nanomolar nm nanometre

NPs nanoparticles

NRs nanorods

OCP open circuit potential

ORC oxidation-reduction cycling ppm parts per million s second

SERS surface-enhanced Raman scattering

TEM transmission electron microscopy

UV-Vis ultraviolet visible

µM micromolar

µm micrometre

V volt wt% weight percent

XPS X-ray photoelectron spectroscopy

xvii

Acknowledgements

I would like to thank many people for supporting me during the past four years. Without their help, I would not have accomplished this work today. First and foremost, I would like to express the deepest appreciation to my principal supervisor Professor Gregory

Hope for his guidance, support and encouragement. His motivation and advice have always inspired me to enjoy and complete my study in Australia. I am also very grateful for the wonderful example he has provided as an excellent supervisor and professor.

I would like to express my gratitude to my co-supervisor, Associate Professor Chris

Brown, who provided great assistance and advice for my experiments. I am very grateful to Professor Alan Buckley for X-ray photoelectron spectroscopy undertaken at

The University of New South Wales. I also want to thank his suggestion and contribution on my research and publication. Thanks also to Associate Professor

Haohua Li for transmission electron microscopy undertaken at Jiangsu University.

I would also want to acknowledge all the members of Hope group, Dr. Gretel Heber, Dr.

Kym Watling, Dr. Apisit Numprasanthai and Dr. Jianlan Cui. Thank them all for their collaboration and generous help. Great thanks to Dr. Gretel Heber and Dr. Jianlan Cui for helping me start my life and study in Australia. Thanks to Alan White, Sarah

Brookes, James Cameron and Scott Byrnes from the chemistry teaching laboratory of

Griffith University for their support and assistance. I also want to extend my thanks to the academic and technical staff from Griffith University School of Natural Sciences and Queensland Micro- and Nanotechnology Centre.

xviii

I must express my gratitude to Griffith University for the Griffith University

Postgraduate Research Scholarship and Griffith University International Postgraduate

Research Scholarship. Without these financial supports, I cannot conduct my life and study in Australia. I also want to thank Griffith University for the Completion

Assistance Scholarship which supported me in completing my thesis. Thanks to the

Conference Travel Grant of Griffith University for funding me to attend a conference in

Cape Town. Thanks to the Australian Research Council and the Australian National

Fabrication Facility for providing support for this study.

Special thanks to my parents for supporting me in seeking the degree of Doctor of

Philosophy abroad. Without their love and encouragement, this work would not have been possible. I am also deeply grateful to Elizabeth Hope and all the members of Hope family. Their kindness and hospitality have made me feel no longer lonely.

Finally and most importantly, I would like to express my wholehearted thanks to my wife, Hanzhi Zhang, who has always understood and supported me in my journey. Her patience, encouragement and love made this thesis possible.

xix

Publications

1. Kun Zhang, Gregory A. Hope, Alan N. Buckley, Haohua Li, 2016. The interaction of sodium mercaptobenzothiazole with gold electrode and nanorod surfaces, Minerals

Engineering, 96-97, 135-142.

2. Kun Zhang, Kym M. Watling, Gregory A. Hope, 2013. Chalcopyrite open circuit potential: Dependence on surface treatment, buffer and mineral source, Chemeca 2013:

Challenging Tomorrow, 238-241.

Chapter 1 Introduction

1.1 Preamble

Australia, as the world’s second largest gold producer, produced about 300 tonnes of gold and accounted for 10% of global gold production in 2015 (Ober, 2016). The recovery of gold from ore deposits is an important industrial practice, which usually consists of cyanidation, flotation, gravity separation, various leaching methods, or the combination of these processes (Allan and Woodcock, 2001). Cyanidation is the most commonly used method for gold recovery, however, it is not economic to treat refractory gold ores such as gold-bearing sulfide directly with cyanidation (Gül et al.,

2012; Zhou and Cabri, 2004). This study is mainly concerned with the flotation aspect of gold recovery.

Froth flotation is a concentrating process which has been widely used in mineral industry for over a century (Fuerstenau et al., 2007). During the flotation process, the valuable and gangue minerals are separated to produce two product streams such as concentrate and tailing (Agorhom et al., 2015). The flotation of gold was first observed by H.L. Sulman who obtained a patent for a process to prevent “float” gold escaping from the stamp battery milling of gold ores (Fuerstenau et al., 2007; Hoover, 1912). It was not until about 1930, following the introduction of xanthate and dithiophosphate

(DTP), the application of flotation to gold ores began to be widespread (Allan and

Woodcock, 2001; Fuerstenau et al., 2007).

A number of reagents are used in gold flotation such as collectors, frothers, depressants and activators. Collectors are probably the most important reagents as the recovery of gold mainly depends on the interaction between collectors and gold surfaces

(Fuerstenau et al., 2007; Teague et al., 1999). Xanthate, DTP and

2 mercaptobenzothiazole (MBT) are the most commonly used collectors for the gold flotation (Swaminathan et al., 1993). The interaction of xanthate with gold and gold- bearing sulfide minerals has been extensively studied (Chen et al., 2014; Leppinen et al.,

1991; Moreno-Medrano et al., 2013), while few studies have been done on DTP and

MBT collectors.

The particle sizes of gold in gold-bearing ores can be in the range of 10 nm to 10 mm

(Fuerstenau et al., 2007), which means the size scale of ultra fine gold particles can be at nanoscale. The interaction of collectors with these ultra fine gold particles might be subtly different from that with larger gold particles, and an understanding of the former could lead to higher recovery of the ultra fine gold particles which can be found in certain gold-bearing ores.

Gold nanoparticles (Au NPs) normally have different physical and chemical properties compared to their bulk counterpart. Gold nanorods (Au NRs) as anisotropic nanoparticles have drawn much attention leading to their applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices

(Chen et al., 2013; Pérez-Juste et al., 2005). Compared to other gold nanostructures, Au

NRs are excellent surface-enhanced Raman scattering (SERS) substrates due to the strong electric field enhancements and tenability of the plasmon resonances (Chen et al.,

2013). This property of Au NRs makes it possible for a SERS spectroscopic study of the interaction of collectors with ultra fine gold particles.

1.2 Aims and objectives

The aim of this study is to investigate the interaction of collectors with gold surfaces, which can provide a basis for understanding the mechanism of gold flotation and improving the recovery of gold.

Specifically, the objectives were as follows:

(a) To synthesise and characterise the relevant MBT and DTP compounds and Au NRs.

(b) To investigate the interaction of MBT and DTP with gold electrode surfaces.

(d) To undertake laboratory-scale flotation tests on gold metal particles.

(f) To undertake initial study on the electrochemical properties of gold-bearing minerals.

1.3 Structure of thesis

This thesis contains seven chapters. The introductory information and aims of this research are given in chapter 1.

Chapter 2 reviews the literature of the gold flotation and Au NRs. The interaction of different collectors with gold and gold-bearing sulfide minerals is reviewed. It also reviews the synthesis and SERS application of Au NRs.

Chapter 3 presents the experimental details and techniques used for this thesis. The principles, applications and experimental conditions of each technique used in this research are also provided.

Chapter 4 is devoted to the characterisation of MBT and DTP ligands and compounds prepared for this research using techniques including vibrational spectroscopy, X-ray

Photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP-OES).

Chapter 5 details the investigations of the interaction of collectors with gold electrode and nanorods surfaces. The flotation behaviour of gold particles is also presented and discussed. Chapter 6 presents the initial study on the electrochemical properties of chalcopyrite.

Finally, the conclusions of this research and suggestions relating to possible future work are presented in chapter 7.

1.4 References

Agorhom, E.A., Lem, J.P., Skinner, W., Zanin, M., 2015. Challenges and opportunities

in the recovery/rejection of trace elements in copper flotation-a review. Minerals

Engineering 78, 45-57.

Allan, G.C., Woodcock, J.T., 2001. A review of the flotation of native gold and

electrum. Minerals Engineering 14, 931-962.

Chen, H., Shao, L., Li, Q., Wang, J., 2013. Gold nanorods and their plasmonic

properties. Chemical Society Reviews 42, 2679-2724.

Chen, J.-H., Li, Y.-Q., Lan, L.-H., Guo, J., 2014. Interactions of xanthate with pyrite

and galena surfaces in the presence and absence of oxygen. Journal of Industrial

and Engineering Chemistry 20, 268-273.

Fuerstenau, M.C., Jameson, G., Yoon, R.-H., 2007. Froth flotation : a century of

innovation. Society for Mining, Metallurgy, and Exploration, Inc., Littleton,

Colorado, USA.

Gül, A., Kangal, O., Sirkeci, A.A., Önal, G., 2012. Beneficiation of the gold bearing ore

by gravity and flotation. International Journal of Minerals, Metallurgy, and

Materials 19, 106-110.

Hoover, T.J., 1912. Concentrating ores by flotation. Mining magazine, London.

Leppinen, J.O., Yoon, R.H., Mielczarski, J.A., 1991. FT-IR studies of ethyl xanthate

adsorption on gold, silver and gold—silver alloys. Colloids and Surfaces 61, 189-

203.

Moreno-Medrano, E.D., Casillas, N., Cruz, R., Larios-Durán, E.R., Lara-Castro, R.,

Barcena-Soto, M., 2013. Adsorption study of sodium isopropyl xanthate on

chalcopyrite. ECS Transactions 47, 69-75.

Ober, J.A., 2016. Mineral commodity summaries 2016, Mineral Commodities

Summaries, Reston, VA, p. 205.

Pérez-Juste, J., Pastoriza-Santos, I., Liz-Marzán, L.M., Mulvaney, P., 2005. Gold

nanorods: Synthesis, characterization and applications. Coordination Chemistry

Reviews 249, 1870-1901.

Swaminathan, C., Pyke, P., Johnston, R.F., 1993. Reagent trends in the gold extraction

industry. Minerals Engineering 6, 1-16.

Teague, A.J., Van Deventer, J.S.J., Swaminathan, C., 1999. A conceptual model for

gold flotation. Minerals Engineering 12, 1001-1019.

Zhou, J.Y., Cabri, L.J., 2004. Gold process mineralogy: Objectives, techniques, and

applications. JOM 56, 49-52.

Chapter 2 Literature review

2.1 Gold flotation

2.1.1 Gold minerals

Gold is one of the most precious metals on the earth. It is a relatively rare metal, and the average gold grade of a low grade deposit is about 3 ppm (O'Connor and Dunne, 1994).

Gold can occur in many ways in minerals and ores such as gold compounds, visible and submicroscopic gold particles (Allan and Woodcock, 2001). Table 2.1 presents the most common gold minerals with their composition and content.

Table 2.1 Gold minerals and their composition and content (after Fuerstenau et al., 2007).

Mineral name Nominal composition Nominal gold content (wt %)a

Metals

Native gold Au, Ag, Cu 80−99.9

Electrum Au, Ag 35−70

Native amalgam Au, Ag, Hg Variable

Tellurides

Calaverite AuTe2 43.6

Krennerite Au4AgTe16 26.8

Sylvanite AuAgTe4 24.2

Montbroyite Au2Te3 50.7

Petzite Ag3AuTe2 25.4

Nagyagite Au(Pb,Sb,Fe)8(S,Te)11 Variable

Kostovite CuAuTe4 25.6

Mineral name Nominal composition Nominal gold content (wt %)a

Minor Minerals

Aurostibite AuSb2 44.7

Auricupride AuCu3 50.8

Maldonite Au2Bi 65.3

Auriferous sulfides

Pyrite FeS2 Variable

Arsenopyrite FeAsS Variable

Chalcopyrite CuFeS2 Variable

Bornite Cu5FeS4 Variable

Covellite CuS Variable

Chalcocite Cu2S Variable

Sibnite Sb2S3 Variable

Other minerals

Carbonaceous aggregates C,Au Variable a Calculated for ideal composition.

The recovery of gold from ore deposits usually consists of crushing, grinding, cyanidation, flotation, gravity separation, various leaching methods, or the combination of these processes (Allan and Woodcock, 2001; Bulatovic, 2010; Gül et al., 2012;

Torres et al., 1999). The choice among these processes is predominantly determined by the mineralogy and gold content of the ore (Agorhom et al., 2013; O'Connor and Dunne,

1994). Generally, gold ores are classified into free-milling and refractory ores based on the gold recovery and process method required (Zhou and Cabri, 2004).

Most of the free-milling gold ores, such as oxide (or oxidized) ores and quartz vein gold ores, can be recovered by the gravity separation and cyanidation (Fuerstenau et al., 2007;

Zhou and Cabri, 2004). Some of the sulfide-containing free-milling gold ores, particularly those from porphyry deposits, however, are normally treated by flotation prior to cyanidation (Bulatovic, 1997; Fuerstenau et al., 2007).

Refractory gold mainly refers to submicroscopic gold in minerals such as auriferous sulfides and tellurides (Allan and Woodcock, 2001). It is usually uneconomic and difficult to recover gold directly from refractory gold ores by cyanidation. Thus, more complicated pre-concentration processes, such as flotation, roasting, pressure leaching and bacterial leaching, are required for satisfactory gold recovery (Fomchenko et al.,

2016; Greet and Bartle, 2008; Iglesias and Carranza, 1994; Sun et al., 2014).

2.1.2 The effects of physical and chemical conditions on gold flotation

The flotation recovery of gold is affected by the physical constraints and chemical conditions in the pulp (Allan and Woodcock, 2001; O'Connor and Dunne, 1994; Teague et al., 1999a). The physical constrains mainly include particle size and shape, surface coatings, pulp density and temperature (Forrest et al., 2001; Small and Grano, 2003).

Lins and Adamian (Lins and Adamian, 1993) investigated the influence of some physical variables on the flotation recovery of free gold in bench-scale tests using amyl xanthate as flotation collector. The grain size of the gold particles used in their study was in the range of 0.16 mm to 1.0 mm. They concluded that the upper size limit of gold for efficient flotation was 0.71 mm where 80% recovery of gold was obtained. It

11 was also observed that the finer gold particles floated more easily, and the recovery of gold was more than 90% at the size of 0.16 mm.

The chemical conditions in the gold flotation pulp mainly consist of reagent synergism, redox potential, aeration conditions and galvanic interaction (Teague et al., 1999a). A number of reagents are used in gold flotation such as collectors, frothers, depressants and activators. Generally, collectors are probably the most important reagents in the gold flotation process (Acarkan et al., 2010; Fuerstenau et al., 2007). The interaction of collectors and gold will be reviewed in the following section.

The effect of galvanic interaction on the recovery of free and refractory gold have been investigated by Teague and co-workers (Teague et al., 1999b). They found that the overall gold recovery increased as more metallic iron is added to the ore prior to milling regardless of collectors. The potential control in the flotation of sulfide minerals and precious metals was studied by Hintikka and Leppinen (Hintikka and Leppinen, 1995).

They concluded that potential can be accurately controlled by flotation gases and the recovery of gold can be maximised by controlling the potential between 250 and 300 mV vs standard hydrogen electrode. They also found that the recovery of sulfide minerals can be minimised in the flotation of gold ores.

2.2 Interaction of collectors with gold and gold-bearing sulfide minerals

The interaction of collectors with mineral surfaces plays a very important role in the flotation process. Collectors are mainly organic chemical substances which produce hydrophobicity on the desired mineral (Lotter and Bradshaw, 2010). Most of the collector molecules contain both a non-polar group and a polar group (Bulatovic, 2007).

The non-polar group is a hydrocarbon radical which does not react with water, while the polar group can interact with the mineral surface. Sulfur-based anionic collectors such as xanthate, DTP and MBT are the most commonly used collectors for the gold flotation

(Fuerstenau et al., 2007; Swaminathan et al., 1993). The generic molecular structures of these collectors are presented in Figure 2.1.

S R O S R O C M P M S R O S

Xanthate Dithiophosphate

C S M S

Mercaptobenzothiazole

Figure 2.1 The generic molecular structures of xanthate, dithiophosphate and mercaptobenzothiazole.

2.2.1 Xanthate

Xanthates were first used in flotation in 1924 (Bulatovic, 2007). The adsorption of xanthate on gold and gold-bearing sulfide minerals has been extensively studied (Hope et al., 2007; Lezna et al., 1988; Moreno-Medrano et al., 2013; Moreno-Medrano et al.,

2015; Talonen et al., 1991; Urbano et al., 2016).

In 1971, Woods studied the oxidation of ethyl xanthate on platinum, gold, copper and galena electrodes by potentiodynamic techniques (Woods, 1971). He explained the oxidation of xanthate by a mixed potential mechanism. The anodic oxidation of xanthate and the cathodic reduction of oxygen can be explained by the following equations:

− − Anodic reactions X → Xads + e (2.1)

2 Xads → X2 (2.2)

− − Xads + X → X2 + e (2.3)

− − Cathodic reaction ½ O2 + H2O + 2e → 2OH (2.4)

Where X is C2H5OCS2 and Xads refers to the chemisorbed xanthate ion on the gold surface. He also found that dixanthogen (X2) formed a physically absorbed layer at the electrode interphase and did not diffuse into the bulk solution. Leppinen and co-workers studied the ethyl xanthate adsorption on gold, silver and gold-silver alloys by Fourier transform infrared spectroscopy (FTIR) and cyclic voltammetry and XPS techniques

(Leppinen et al., 1991). They found that dixanthogen was the oxidation product on pure gold and the formation of unknown surface species at low potentials.

The investigation of the adsorption of ethyl xanthate on metallic gold surfaces was also carried out by Ihs and co-workers using FTIR and XPS (Ihs et al., 1993). They believed that the unknown surface species found by Leppinen and co-workers were actually chemisorbed xanthate. They also found the xanthate ions were coordinated to the gold surface through both sulfur atoms.

Ihs and co-workers’ result is consistent with Woods and co-workers’ research results obtained in 1995. It was found that the chemisorption of xanthate commenced below the potential at which dixanthogen deposited (Woods et al., 1995). It also suggested that the chemisorbed xanthate could be the key species in the flotation of gold with xanthate collectors. In 1998, Woods and co-workers confirmed that the chemisorbed xanthate was covalently bonded to gold surfaces by SERS spectroscopy (Woods et al., 1998).

The adsorption mechanism of xanthates on sulfide mineral surfaces is similar to that on gold surfaces (Leppinen, 1990; Moreno-Medrano et al., 2013; Szargan et al., 1992;

Urbano et al., 2016). The surface products of the chalcopyrite-sodium isopropyl xanthate (NaiPX) flotation system were investigated by Andreev and Barzev using

Raman spectroscopy (Andreev and Barzev, 2003). They concluded that both copper isopropyl xanthate (CuiPX) and diisopropyldixanthogen (iPX2) were present on the chalcopyrite surface when performing flotation at high NaiPX collector concentration.

They also found that only CuiPX was formed on the chalcopyrite surface under conditions near to real industrial froth flotation. The FTIR studies of sodium butyl xanthate adsorption on chalcopyrite, pentlandite and pyrite surfaces were carried out by

Zhang and co-workers (Zhang et al., 2013). They found that both metal xanthate and dixanthogen were formed on these mineral surfaces with different relative proportions.

2.2.2 MBT

MBT and derivatives are known to be very effective for the treatment of gold and gold- bearing sulfide minerals (Allan and Woodcock, 2001; Maier and Dobiáš, 1997). There are some studies of the adsorption of MBT on sulfide minerals (Buckley et al., 2016;

Contini et al., 1997; Flemmig et al., 2001; Numata et al., 1998), however, the investigation of the interaction between MBT and gold has been limited.

The monolayer of MBT on polycrystalline gold and silver films were investigated by

Sandhyarani and co-workers using SERS and XPS spectroscopies (Sandhyarani et al.,

1999). A band at 522 cm-1 was observed in their SERS spectrum of MBT monolayer on gold surfaces. They assigned this band as the N−H vibration of the thione form (MBT) and concluded that MBT adsorbed in the thione form on gold with its molecular plane perpendicular to the gold surface. They did not consider the chemisorption of MBT on a gold surface.

Woods and co-workers (Woods et al., 2000) studied the interaction of MBT with copper, silver and gold surfaces by SERS spectroscopy. They reported that attachment of the organic compound occurs through bonding between the exocyclic sulfur atom and metal atoms in the surface. Their XPS results also confirmed that the absorbed layer was of monolayer thickness. Their SERS spectra of a gold electrode at different controlled potentials were all similar to gold(I) mercaptobenzothiazole (AuMBT), and no dithiolate, 2,2’-dithiobisbenzothiazole ((MBT)2) was found in the positive potential region.

The adsorption and co-adsorption of MBT and DTP on gold surfaces was studied by

Beattie and co-workers using XPS (Beattie et al., 2009). In order to ensure the adsorption was limited to the monolayer regime, gold was immersed in pH 9.2 solution with 10-5 M MBT for 60 s. They proposed that there were two bonding types between

MBT and gold surfaces as presented in Figure 2.2. The first one was the bonding between exocyclic S and gold only, while the other one was caused by the interaction of both exocyclic S and N with gold in a chelating fashion.

Figure 2.2 The bonding types between MBT and gold surfaces (Beattie et al., 2009).

The electrochemical behaviour of MBT on a Au (111) surface has been investigated by

Cui and co-workers using electrochemical scanning tunnelling microscopy and cyclic voltammetry (Cui et al., 2011). Contrary to Beattie’s conclusion, they proposed that

MBT molecules adsorbed on Au (111) surfaces through the two S atoms from the thiol group and the heteroaromatic ring, respectively.

The adsorption of MBT on galena and pyrite was studied by Szargan and co-workers by

XPS (Szargan et al., 1999). They believed that the deprotonated MBT molecules could bond to the mineral surfaces via the thiol group. They also found the formation of

(MBT)2 on minerals surfaces attributed by the adsorption of MBT. Jiao and co-workers studied the adsorption of MBT on chalcopyrite and sphalerite surfaces by FTIR and density functional theory (Jiao et al., 2015). They found the adsorption product was copper (I) mercaptobenzothiazole on the chalcopyrite surfaces, while there were no characteristic peaks of MBT on the sphalerite surfaces.

2.2.3 DTP

DTP is also a very common collector for the flotation of gold and gold-bearing ores

(Fuerstenau et al., 2007; Swaminathan et al., 1993). There have been few studies of the adsorption of DTP on gold and gold bearing ores (Güler et al., 2006). The adsorption of potassium O,O’-di(para-fluorophenyl) dithiophosphate on gold, silver and copper were investigated by Persson and co-workers by FTIR, XPS and ellipsometry (Persson et al.,

1999). They found that both sulfur atoms of DTP molecule bonded to gold surfaces. As the immersion times and solution concentrations increased, the orientation of the molecules changed from a flat to a fully erected conformation.

This is consistent with the conclusion reached by Beattie and co-workers (Beattie et al.,

2009) who also studied the adsorption of potassium di-isoamyl dithiophosphate on gold surfaces using XPS. They concluded that DTP could have two chemisorbed geometries

(bidentate and monodentate) on gold surfaces as presented in Figure 2.3.

Figure 2.3 The bonding types between DTP and gold surfaces (Beattie et al., 2009).

Güler and co-workers studied the electrochemical behaviour of chalcopyrite in the absence and presence of dithiophosphate by cyclic voltammetry and diffuse reflectance infrared Fourier transformation spectroscopy (Güler et al., 2005). They found that the current density decreased proportionally as the concentration of DTP increased in the buffer solution. It indicated that the addition of DTP caused the formation of passive layer on the chalcopyrite electrode surface. They concluded that copper (I) DTP and

(DTP)2 were the major surface compounds formed on the chalcopyrite surfaces.

2.3 Gold nanorods

Au NRs have attracted a great deal of attention since their first discovery in 1990s

(Chen et al., 2013; Huang et al., 2009; Murphy et al., 2010; Pontifex et al., 1991). Since then, a number of synthesis methods and applications of Au NRs have been developed and studied (Gabudean et al., 2012; Lohse and Murphy, 2013; Nusz et al., 2008; Pérez-

Juste et al., 2005; Yu et al., 1997). In this review section, we will mainly focus on the seed-mediated growth method and SERS applications of the Au NRs.

2.3.1 Synthesis and characterisation

There are mainly two types of methods for the fabrication of Au NRs, which are so- called “top-down” and “bottom-up” methods (Chen et al., 2013). A combination of different physical lithography processes and Au deposition are normally used in the

“top-down” methods, while the “bottom-up” methods are based on the chemical reduction of Au salts, nucleation and subsequent overgrowth (Chen et al., 2013; Zhou et al., 2009).

The seed-mediated growth method is one of the most common “bottom-up” methods for

Au NRs fabrication (Lohse and Murphy, 2013). It was first developed by Murphy’s group in 2001, and independently by EI-Sayed’s group in 2003 (Jana et al., 2001a; Jana et al., 2001b; Nikoobakht and El-Sayed, 2003). In this method, a gold seed solution was first prepared by reducing Au3+ complex ions (from chloroauric acid) with sodium borohydride in hexadecyltrimethylammonium bromide (CTAB) solution. In the growth solution, Au3+ complex ions were reduced to Au+ complex ions by ascorbic acid in solution with CTAB and Ag+ ions (from silver nitrate). After the addition of gold seed

20 solution into the growth solution, the Au+ complex ions are further reduced to Au atom with the catalysis of the seed solution.

The cationic ammonium surfactants, such as CTAB, are widely used in the colloidal growth of Au NRs (Pérez-Juste et al., 2005). It is believed that CTAB molecules can form a partially-interdigitated bilayer on the surface of Au NRs. The two layers of the

CTAB bilayer react with each other through the carbon-chain tails of the CTAB molecules. The CTAB molecules with the ammonium head groups binding to the gold surface lead to the formation of inner layer, while the outer layer consists of the CTAB with ammonium head groups pointing outside (Chen et al., 2013; Gou and Murphy,

2005; Lee et al., 2011). This CTAB bilayer forms a soft template to make the growth of

Au NRs along the longitudinal surfaces (Jana et al., 2001a; Johnson et al., 2002;

Nikoobakht and El-Sayed, 2003).

Nikoobakht and El-Sayed studied the surface structure of Au NRs capped with CTAB by FTIR, thermogravimetric analysis and transmission electron microscopy (TEM)

(Nikoobakht and El-Sayed, 2001). They synthesised the Au NRs by an electrochemical method using CTAB and tetraoctylammonium bromide as surfactants. They suggested that a bilayer structure of CTAB was formed around Au NRs rather than a micellar form.

They also found that Au NRs transformed to gold nanospheres when the inner layer was released from the gold surfaces.

Gómez-Graña and co-workers also investigated the surfactant (bi)layers on Au NRs by

TEM, small-angle X-ray scattering and small-angle neutron scattering (Gómez-Graña et al., 2012). They synthesised the Au NRs by the seed-mediated growth method. They

21 observed a bilayer with the thickness of 32 ± 2 Å and the dominative alkyl chain density

(80−100%), which confirmed the presence of a CTAB bilayer on the surfaces of Au

NRs.

In 2008, the influence of the CTAB with different manufacturer and lot number on the colloidal seed-mediated synthesis of Au NRs was first studied by Smith and Korgel

(Smith and Korgel, 2008). They supposed that an impurity present in the different

CTAB stocks could either induce or disrupt the growth of Au NRs. In the following year, they confirmed that the iodide presented in some of the CTAB stocks could prevent the formation of Au NRs (Smith et al., 2009).

The Ag+ ions in the growth solution also play an important role in both controlling the aspect rations and improving the yields of Au NRs (Langille et al., 2012; Liu and

Guyot-Sionnest, 2005; Nikoobakht and El-Sayed, 2003). Jackson and co-workers have confirmed the presence of Ag+ ions on Au NRs, which showed no preference for a specific face or axis (Jackson et al., 2014).

Notwithstanding that the detailed mechanisms are still not really understood, three separate mechanisms have been proposed as presented in Figure 2.4 (Lohse and Murphy,

− + 2013). The first theory suggests that either AgBr2 or CTA−Br−Ag is formed on the surface of Au NRs and acts as a face-specific capping agent (Hubert et al., 2008;

Niidome et al., 2009). This silver bromide complex blocks the growth along the longitudinal faces, which leads to the growth at the two ends of the Au NRs (Murphy et al., 2010). The second theory proposes that Ag+ and Br− ions modify the CTAB

22 micelles from spherical to cylindrical shapes (Jana, 2005). Then the CTAB micelles act as soft templates for the formation of Au NRs.

In the last theory, it is suggested that Ag+ ions are reduced to Ag atoms and a submonolayer of silver metal preferentially deposits at the longitudinal faces of the Au

NRs (Grzelczak et al., 2008; Jain et al., 2008; Murphy et al., 2010). The deposition of the silver submonolayer prevents the further growth along the longitudinal faces and contributes to the anisotropic growth of Au NRs (Murphy et al., 2005), which is quite similar to the first theory.

Figure 2.4 Three mechanisms for the role of Ag+ ions (Lohse and Murphy, 2013).

2.3.2 SERS

Localized surface plasmon resonances (LSPRs) are collective electron charge oscillations confined to metallic nanoparticles, which are probably the most extraordinary properties of Au NRs (Hutter and Fendler, 2004; Zijlstra and Orrit, 2011).

The LSPRs properties of gold nanoparticles (Au NPs) can be tailored by tuning the sizes and shapes of Au NPs (Chen et al., 2013; Ni et al., 2008). In this aspect, Au NRs have significant advantages over the other nanoparticles due to their two plasmon modes (Pérez-Juste et al., 2005). These are the longitudinal and the transverse LSPR mode (Figure 2.5a) associated with the electron oscillations along the long axes and short axes, respectively (Cao et al., 2014). Two absorption bands (Figure 2.5b) can be observed in the ultraviolet-visible (UV-vis) absorption spectrum of Au NRs, which are assigned to the transverse and the longitudinal oscillation of the electrons, respectively.

Figure 2.5 Schematic representation of LSPR excitation for Au NRs (a) and LSPR absorption bands of Au NRs (b) (Cao et al., 2014).

SERS is one of the LSPRs applications of Au NRs (Santos Costa et al., 2009; Tebbe et al., 2014; Ukaegbu et al., 2016; Wang et al., 2010). The electromagnetic field enhancements, as the dominant contributors to most SERS processes, occur close to the metal nanostructures when LSPRs are excited (Le Ru and Etchegoin, 2009).Thus, Au

NRs with strong electric field enhancements can be excellent SERS substrates (Chen et al., 2013).

The detection of carbendazim by SERS using Au NRs as substrates was conducted by

Strickland and Batt (Strickland and Batt, 2009). They demonstrated the ability to accurately detect carbendazim at a concentration as low as 50 µM. Saute and co- workers also used Au NRs as solution-based SERS substrates for the detection of ultra- low levels of three dithiocarbamate pesticides (Saute et al., 2012). They reported that the limits of detection for thiram, ferbam, and ziram were 11.00 ± 0.95 nM, 8.00 ± 1.01 nM, and 4.20 ± 1.22 nM, respectively.

When two Au NRs are placed close to each other, their plasmon oscillations can couple with each other and form different collective plasmon modes (Prodan et al., 2003).

These newly formed plasmon modes can provide a much larger electric field enhancement than the individual LSPR (Chen et al., 2013). Thus, the plasmon coupling

Au NRs can offer much better SERS responses than the single Au NR.

A number of studies have been done on the assemblies of Au NRs (Pramod and Thomas,

2008; Shao et al., 2010; Shibu Joseph et al., 2006). Kumar and Thomas studied the

Raman signal enhancement of bipyridine and phenyl moieties by liking them onto the edges of Au NRs using monothiol derivatives and at the junctions of two Au NRs using

25 dithiol derivatives (Kumar and Thomas, 2011). They found that the dimerization of Au

NRs could lead to a spontaneous enhancement (enhancement factor of 1.4 × 105) in the intensity of Raman signals.

Jain and co-workers (Jain et al., 2006) proposed that the orientation between the Au

NRs can significantly affect the plasmon coupling of Au NRs. In 2011, the SERS properties of side-by-side, end-to-end and end-to-side assemblies of Au NRs have been studied by Zhong and co-workers (Zhong et al., 2011). They successfully achieved the orientation-controllable assemblies of Au NRs by bifunctional PEG molecules. They also found that different assembly orientations of Au NRs exhibited different SERS signals. Among the three orientations, the end-to-end Au NRs assembly presented the highest Raman enhancement. Thus, it suggests that the SERS performance of the plasmon coupling Au NRs mainly depends on the assembly orientation of the Au NRs.

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Chapter 3 Experimental and instrumental

3.1 Preamble

ATR-FTIR, Raman spectroscopy, XPS, TEM, ICP-OES, UV-vis spectroscopy, zetasizer and electrochemistry techniques have been applied to characterize and investigate the interactions between flotation collectors and gold in this research.

Raman Spectroscopy, as the primary technique in this research, provided information that enabled identification of the vibrational modes of molecules. It was used to characterize and identify both the flotation collectors and the relevant compounds.

SERS spectroscopy also provided useful information of the interactions that happened on gold electrode surfaces. The qualitative elemental and valence state information of compounds were obtained by XPS and the characterization of Au NPs was mainly achieved using TEM.

This chapter describes the experimental details and brief backgrounds of the used experimental techniques. The first half of the chapter provides the experimental materials and methods, synthesis of the relevant compounds and the Au NPs. The second half of the chapter presents an introduction and detailed information for the experimental techniques.

3.2 Experimental materials and methods

3.2.1 Reagents and materials

All reagents were of Analytical Reagent (AR) grade and used without further purification. Electrolyte and solutions were made with AR grade reagents and doubly de-ionized (DDI) water. Table 3.1 presents a list of the reagents and materials used in this research.

Table 3.1 List of reagents and materials.

Name Formula Source

Ascorbic acid C6H8O6 Chem-Supply

Benzothiazole C7H5NS Sigma-Aldrich

Chalcopyrite CuFeS2 Messina Mine, Limpopo, South Africa; Mt. Isa Mine, Queensland, Australia; Mountain Consolidated Mine, Butte, Montana, USA

Chloroform CHCl3 Chem-Supply

Copper (II) acetate Cu(CH3COO)2∙H2O Chem-Supply

Gold Au Johnson Matthey

Hexadecyltrimethylammonium (C16H33)N(CH3)3Br Sigma-Aldrich bromide

Hydrochloric acid 32% HCl Scharlau

Iodine solution KI/I2 Chem-Supply

Methanol CH3OH Chem-Supply

Name Formula Source

Nitric acid 70% HNO3 RCI Labscan

Nitrogen gas N2 BOC

Oxygen gas O2 BOC

Potassium chloride KCl Chem-Supply

Potassium dihydrogen KH2PO4 Merck phosphate

Potassium persulfate K2S2O8 Chem-Supply

Silver nitrate AgNO3 Sigma-Aldrich

Sodium borate Na2B4O7∙10H2O Chem-Supply

Sodium borohydride NaBH4 Sigma-Aldrich

Sodium carbonate Na2CO3 BDH

Sodium dibutyl C8H18O2PS2Na Orica dithiophosphate

Sodium hydrogen phosphate Na2HPO4 Merck

Sodium hydrogen carbonate NaHCO3 BDH

Sodium hydroxide NaOH Chem-Supply

Sodium mercaptobenzothiazole C7H4NS2Na Orica

3.2.2 Synthesis of MBT compounds

Sodium mercaptobenzothiazole (NaMBT) was supplied by Orica Australia Pty. Ltd. as an aqueous solution. The mercaptobenzothiazole compounds were prepared following the methods of Woods and co-workers (Woods et al., 2000) with some modifications.

The acid form, 2-mercaptobenzothiazole (HMBT), was precipitated from a 10-3 M

NaMBT solution by the addition of 1 M HCl solution. The precipitate was washed with

DDI water and evaporated in air to obtain pure HMBT.

The dithiolate, 2,2’-dithiobisbenzothiazole ((MBT)2) was obtained by oxidising an

NaMBT solution with a potassium persulfate solution. The precipitate was treated as for

HMBT. Potassium mercaptobenzothiazole (KMBT) was prepared by adding 50 mL of

0.1 M KOH solution into 50 mL of 0.1 M HMBT in methanol. KMBT was obtained after the evaporation of the mixed solution. Copper (I) mercaptobenzothiazole

(CuMBT), gold (I) mercaptobenzothiazole (AuMBT) and silver (I) mercaptobenzothiazole (AgMBT) were prepared by adding aqueous copper (II) acetate, gold chloride and silver nitrate, respectively, to HMBT in methanol. The precipitated compound was washed with DDI water and chloroform to remove any residual reactants and by-product of (MBT)2.

3.2.3 Synthesis of DTP compounds

Sodium dibutyl dithiophosphate (NaDBDTP) was supplied by Orica Australia Pty. Ltd. as an aqueous solution. The dibutyl dithiophosphate acid (HDBDTP) was prepared by adding 1 M HCl dropwise to 10-3 M NaDBDTP solution until no further reaction was observed. A pungent odorous oil was produced and it was separated from the solution using a pipette.

The butyl thioperoxydiphosphate ((DBDTP)2) was prepared by oxidising a NaDBDTP solution with a iodine solution. A milky oil layer was formed that was separated from the aqueous solution by centrifugation. Copper (I) dibutyl dithiophosphate (CuDBDTP)

45 was prepared by the addition of a copper (II) acetate solution to a solution of NaDBDTP.

The mixed solution was filtered through a glass microfiber filter into a reaction vessel and allowed to sit for 30 min, during which time a white precipitate was formed. The white precipitate was filtered and washed with DDI water and methanol.

3.2.4 Synthesis of Au NPs

Au NRs were fabricated following the seed-mediated growth methods described in the literature (Guo et al., 2009; Nikoobakht and El-Sayed, 2003) with some modifications.

Briefly, gold seed solution was prepared by mixing 5 mL of 0.2 M hexadecyltrimethylammonium bromide (CTAB) with 5 mL of 0.5 mM HAuCl4. Then

0.6 mL of an ice-cold solution of 0.01 M NaBH4 was added to the mixed solution.

Vigorous stirring of the solution was continued for 2 min. The colour of the solution changed from yellow to brown. The seed solution was held at 25 °C for 2 h.

The growth solution was made in a step-wise process: 5 mL of 0.2 M CTAB, 0.25 mL of 4 mM AgNO3 and 5 mL of 1 mM HAuCl4 were combined and mixed thoroughly.

Then addition of 0.08 mL of 1 M HCl solution, followed by 0.07 mL of 0.0788 M ascorbic acid. The addition of HCl solution was used to decrease the overall reduction rate of the Au3+ in the growth solution (Guo et al., 2009). Growth of nanorods was then initiated by addition of 0.012 mL of the seed solution to the growth solution with the resultant mixture held undisturbed in a water bath at 30 °C for 12 h. The Au NRs were recovered from the containing solution by centrifuging at 10000 rpm for 20 min, the supernatant was removed and the nanorods precipitate was redispersed in 5 mL deionized water for further investigation.

Au NPs with triangular, spherical and square outlines were produced after the method of Sau and Murphy (Sau and Murphy, 2004) with some modifications. The concentration of CTAB, AgNO3, HAuCl4 and ascorbic acid used in the growth solution were changed to 0.1 M, 1 mM, 0.4 mM and 0.05 M, respectively. The other steps were followed the seed-mediated growth methods as described above.

3.2.5 Flotation experiments

A Hallimond tube flotation cell (Figure 3.1) was used for the laboratory scale flotation testing in this research. Ultra fine gold particles were prepared from metal of 99.99% purity. Gold chips were cut off from the gold block using the teeth of a fine hacksaw blade (Figure 3.2) and ground (Figure 3.3) until the diameter of particles was reduced to

~100 μm (Figure 3.4). Gold particles were washed with HCl and DDI water after grinding, then heating at 450 °C for 16 h to remove any impurities. In each experiment, a flotation test was conducted for 15 min under oxygen, nitrogen and air gas flow, respectively. A platinum counter electrode, a Ag/AgCl reference electrode and a stainless steel grid working electrode were used for potential control. Borate buffer of pH 9.2 and 0.2 g of gold particles were used for each flotation experiment.

Figure 3.1 Hallimond tube flotation cell.

Figure 3.2 Gold chips were cut off from the gold block.

Figure 3.3 Gold chips were ground to ultra fine particles.

(a)

(b)

Figure 3.4 Picture (a) and optical microscopy image (b) of gold particles.

3.3 Electrochemistry

3.3.1 Background

Electrochemistry is the branch of chemistry that studies the interrelation of electrical and chemical effects (Bard and Faulkner, 2001). Electroanalytical techniques as used in this research mainly focused on cyclic voltammetry and spectroelectrochemistry.

Cyclic voltammetry is one the most widely used electroanalytical techniques for acquiring qualitative information about electrochemical reactions (Wang, 2000).

Spectroelectrochemistry is an in situ technique combining electrochemical and spectroscopic techniques, which acquires molecular information under potential control

(Zoski, 2007).

A conventional three-electrode configuration is used for electrochemical measurements, consisting of a working electrode, a reference electrode and a counter electrode (Zoski,

2007). All three electrodes are immersed in an inert supporting electrolyte with the target analyte. The potential of the working electrode is controlled by a potentiostat with respect to the reference electrode. When a certain potential is applied on the working electrode, a current passes between the working electrode and the counter electrode. The reaction of interest occurs on the surface of working electrode. No current passes through the reference electrode which is only used to monitor and control the potential of the working electrode surface.

3.3.2 Instrumentation

A multi-necked heart shaped glass cell (Figure 3.5) was used for voltammetry experiments, while spectroelectrochemical studies were conducted in a borosilicate electrochemical cell with an optically flat transparent window (Figure 3.6).

d b a

Figure 3.5 The multi-necked heart shaped glass cell. (a) working electrode; (b) counter electrode; (c) reference electrode; (d) nitrogen purge.

b d a c

Figure 3.6 The borosilicate electrochemical cell. (a) working electrode; (b) counter

electrode; (c) reference electrode; (d) nitrogen purge.

3.3.2.1 Working electrodes

Three sources of chalcopyrite working electrodes were prepared for this research.

Chalcopyrite samples were obtained from Messina Mine, Limpopo, South Africa; Mt.

Isa Mine, Queensland, Australia; Mountain Consolidated Mine, Butte, Montana, USA.

For production of these electrodes, the chalcopyrite mineral was connected to a copper wire with RS silver-loaded epoxy adhesive. The chalcopyrite and copper wire were then encased in Nuplex polyplex resin to insulate the sample apart from the exposed working electrode.

The electrode surface was abraded on P400 mesh SiC paper and rinsed with DDI water prior to immersion in the electrolyte. The oxidised chalcopyrite surface electrode was generated by boiling the electrode assembly in DDI water for 30 min. In order to obtain the fractured surface, the resin was scored to the chalcopyrite edge using a low-speed circular saw, leaving a small region intact to prevent premature breakage. The electrodes fractured spontaneously due to contact with the cell, or were fractured using a spatula under positive N2 pressure.

Gold working electrodes were prepared from metal of 99.99% purity with an exposed area of ~2 cm2. Fresh electrode surfaces were generated by grinding on P400 SiC paper, ultrasonicating in DDI water, then heating at 450 °C for 16 h followed by multiple rinsing in DDI water. The SERS surface for a gold electrode was generated by electrochemically roughening by an oxidation-reduction cycling (ORC) in 1 M KCl and

0.1 M HCl in equilibrium with air. As shown in Figure 3.7, the ORC procedure involved the application of 4−5 cycles between −0.7 and 1.0 V with a polarisation period of approximately 30 s before reversing the polarity.

1500

1000

500

-500 Potential (mV vs. Ag/AgCl) vs. (mV Potential

-1000 0 30 60 90 120 150 180 210 240 270 Time (s)

Figure 3.7 Potential regime for oxidation-reduction cycling of gold SERS electrode.

3.3.2.2 Reference electrodes

Ag/AgCl reference electrodes were used in this research. They were prepared in-house using fine Ag wire which was chloridised in 1 M KCl solution (acidified to pH 1 with

HCl) by applying 5 V vs. a fine Ag cathode wire. Then the AgCl-plated wire was immersed in 3 M KCl solution assembly comprising a glass tube with a Teflon shrink tube barrel and porous vycor frit. The vycor frit was cleaned at 100 °C in peroxide and soaked at room temperature in 2 M HNO3 acid solutions overnight before sealing the

Teflon shrink tube. The reference electrode was stored in 3 M KCl solution in a sealed container when not in use. The reference electrode was checked against the potential of a platinum electrode in saturated quinhydrone at pH 7 and rinsed with DDI water prior

55 to use in experiments. All potentials in this research are reported against the Ag/AgCl 3

M KCl reference electrode unless otherwise stated.

3.3.2.3 Counter electrodes and potential control

The counter electrode was prepare from a 1 mm diameter pure platinum wire and stored in 2 M HNO3 acid solutions when not in use. Potential control was maintained using a

Pine Instruments WaveNano potentiostat controlled by a computer running AfterMath software.

3.3.3 Investigations in this thesis

Cyclic voltammetry was used in this research to investigate the behaviour of flotation collectors on gold and chalcopyrite working electrodes. The open circuit potential (OCP) of chalcopyrite working electrodes under different conditions were also studied.

Spectroelectrochemical techniques were used to investigate the interaction of flotation collectors with gold working electrode surfaces.

3.4 Raman Spectroscopy

3.4.1 Background

Raman scattering, or the Raman Effect, as an inelastic scattering of light was first predicted by Smekal (Smekal, 1923) and first observed experimentally by Raman and

Krishnan (Raman and Krishnan, 1928). Raman spectroscopy rapidly increased in popularity when lasers replaced the mercury arc as the excitation source during the

1960’s (Pelletier, 1999).

Raman spectroscopy can be used to provide information on chemical structures, to identify substances from the characteristic spectral patterns and to determine quantitatively or semi-quantitatively the amount of a substance in a sample (Smith and

Dent, 2005a). When a sample is illuminated with a laser beam, the photons which make up the laser may be transmitted, absorbed or scattered. There are two types of scattered light: the strong one is called Rayleigh scattering and the weak one is called Raman scattering. Most of the scattered photons have the same wavelength as the initial photons and emit without energy transfer. This event is called Rayleigh scattering. If the energy of the photon is changed after scattering, this process is called Raman scattering.

The basic processes of Rayleigh and Raman scattering are present in Figure 3.8. Most molecules are present in ground state m, which is the lowest energy vibrational level at room temperature (Smith and Dent, 2005a). When the molecule absorbs energy from the photon, the molecule will be excited to a higher energy excited vibrational state (n).

The scattered (emitted) photon exhibits the energy difference between the ground and excited vibrational states. This scattering process is called Stokes scattering. Some

57 molecules may be present in the excited state n and the scattering from state n to state m is called anti-Stokes scattering. This anti-Stokes scattering is normally weaker than

Stokes scattering and only the energy (frequency) changes of photons in Stokes scattering are recorded as a Raman shift in spectra.

Virtual States

풗ퟎ

n Vibrational States m 풗풎

Stokes Rayleigh Anti-Stokes

Figure 3.8 Diagram of the Rayleigh and Raman scattering processes. The frequency of the incident beam is v0 and the vibrational frequency of the molecule is vm.

According to classical theory, the information about the frequency of the scattered light and the selection rules of the Raman scattering is shown in Equation 3.1 (Ferraro et al.,

2002).

1 휇 = 훼 휖 cos 2휋푣 푡 + 훽푞 휖 [cos{2휋(푣 + 푣 )푡} + cos{2휋(푣 − 푣 )푡}] (3.1) 0 0 0 2 0 0 0 푚 0 푚

Where: 휇 = the extent of distortion of the molecule,

훼0 = the polarizability at the equilibrium position,

푞0 = the vibrational amplitude,

훽 = the rate of change of 훼 with respect to the change in 푞, evaluated at the

equilibrium position.

The first term in the equation relates to an oscillating dipole which radiates light of frequency v0 (Rayleigh scattering), while the second term represents the Raman scattering of frequency v0 + vm (anti-Stokes) and v0 − vm (Stokes). If the rate of change of polarizability with the vibration is zero, no Raman scattering will be observed. This equation also shows that only vibrations that change the polarizability of the molecule will generate Raman scattering. The value of β is normally much smaller than the value of α, which means that the intensity of Rayleigh scattering is much higher than Raman scattering.

3.4.2 Surface-enhanced Raman spectroscopy

The vibrational Raman scattering cross-section of a material may be enhanced by a factor of 107 or more by the presence of a roughened surface or metal colloids (Pelletier,

1999). This effect is called surface-enhanced Raman scattering (SERS). The first SERS observation was made by Fleischmann and co-workers (Fleischmann et al., 1974) through their investigation of molecule pyridine absorbed on roughened silver electrode in 1974. The authors believed that the intense Raman signal was due to a large increase in the surface area of the electrode and then in the number of adsorbates. In 1977,

Jeanmaire and Van Duyne (Jeanmaire and Van Duyne, 1977) were the first to realize that the strong Raman signal of pyridine must be caused by an enhancement of Raman

59 scattering. Albrecht and Creighton (Albrecht and Creighton, 1977) also reported a similar result independently in the same year.

There are two major mechanisms that can contribute to SERS, which are electromagnetic effects and chemical effects (Campion and Kambhampati, 1998;

Moskovits, 2005). The electromagnetic effects focus on the enhanced electromagnetic fields which can be supported on metal surfaces with appropriate morphologies. The electromagnetic effects mainly depend on the large local field enhancements which occur close to metallic surfaces when localized surface plasmon resonances are excited

(Le Ru and Etchegoin, 2009). Surface plasmons are oscillations of conduction band electrons at a metal surface. The frequency of the surface plasmon relies on both the electronic structure of the metal and the size of the metal surface features such as surface roughness and colloid diameter (Pelletier, 1999). The most widely accepted explanation for chemical effects is called charge transfer mechanism. The charge transfer involves the formation of a bond between the analyte and the metal surface

(Smith and Dent, 2005b). This bond makes it possible to transfer charge from the metal surface into the analyte and increases the molecular polarizability of the molecule because of the interaction with the metal electrons. The chemical enhancement is produced via the new electronic states which arise from the formation of the bond.

The main advantage of Raman spectroscopy in this research is that samples can be analysed in aqueous solutions through transparent containers and windows. The Raman spectra of samples are not affected by water vibrations as water is a very weak Raman scatterer. This advantage also makes it possible for the combined application of SERS and electrochemistry, which is a powerful surface characterisation technique for in situ

60 investigation of adsorption and chemical changes at gold electrodes (Sharma et al., 2012;

Tian and Zhang, 2014).

3.4.3 Instrumentation

Raman spectra in this research were acquired on one of two Renishaw InVia Raman spectrometers using a 785 nm excitation from a diode laser and a 633 nm excitation from a HeNe source. SERS spectra were only recorded on the 785 nm laser Raman spectrometer (Figure 3.9). Raman spectra were calibrated using the 520 cm-1 silicon band and scattered light was detected with a Peltier-cooled CCD detector with spectral resolution ~2 cm-1. The laser and scattered light were usually focused through a ×50

Leica N Plan lens (NA = 0.75), while an ultra-long working distance ×20 Olympus Plan

Fl lens (NA = 0.4) was used for in situ SERS spectroelectrochemical studies. Spectral manipulations such as baseline adjustment, smoothing and normalisation were performed with the Renishaw WiRE 3.3 software.

3.4.4 Investigation in this thesis

Raman spectroscopy was used in this study to characterize and identify the flotation collectors and relevant compounds. SERS was used to investigate the interaction between flotation collectors and gold by both Au NPs and roughened gold surface.

Figure 3.9 Raman spectroscopy.

3.5 Fourier transform infrared Spectroscopy

3.5.1 Background

FTIR is a preferred method of infrared spectroscopy. FTIR and Raman spectroscopy are complementary techniques, which are both used to measure the vibrational modes of a molecule (Larkin, 2011). Infrared spectroscopy measures transitions between molecule vibrational energy levels. When a beam of infrared light passes through the sample, molecules can absorb the infrared radiation at specific wavelengths. Then the detector records those absorption energy regions in the spectrum of the infrared radiation and the final spectrum presents the data as a function of transmission against energy. Both FTIR and Raman Spectroscopy can be used to create a molecular fingerprint of the sample.

Attenuated total reflectance is a contact sampling method which was first developed by

Harrick in 1967 (Harrick, 1967). It involves an optically dense crystal with a high refractive index and excellent infrared transmission properties. The ATR-FTIR technique has become very popular because it is suitable for most solid and liquid samples, is non-destructive and requires little or no sample preparation.

3.5.2 Instrumentation

Fourier transform infrared (FTIR) spectra were measured on a PerkinElmer attenuated total reflectance Fourier transform infrared spectroscopy (Figure 3.10). The data were collected and processed under PerkinElmer Spectrum 10 spectroscopy software. A diamond crystal plate was used as a reflector. The plate was cleaned with isopropanol before and after each sample measurement. Samples were placed on the plate and

63 covered the diamond crystal area. Spectra were acquired in the range of 450−4000 cm-1 at 4 cm-1 resolution.

Figure 3.10 Attenuated total reflectance Fourier transform infrared spectroscopy.

3.5.3 Investigation in this thesis

ATR-FTIR was used in this study to characterize and identify the flotation collectors and relevant compounds. The gold particles used in the flotation experiments were also analysed using ATR-FTIR.

3.6 XPS

3.6.1 Background

XPS is one of the most widely used techniques for surface chemical analysis. It is used for the detection and energy analysis of photoelectrons produced by the irradiation of X- rays (Wagner, 2010). When the surface of sample is irradiated with X-rays of known energy, photoionization takes place in the sample surface and the emitted photoelectron will have a kinetic energy, Ek , given by the equation 3.2 (Settle, 1997):

퐸푘 = ℎ푣 − 퐸푏 – Φ (3.2)

Where hv is the energy of the X-ray photo, Eb is the binding energy of photoelectron with respect to the fermi level and Φ is the work function of the spectrometer. The binding energy of electron in a particular shell of an atom is unique to each element, which means elements present on a material’s surface other than hydrogen and helium can be identified and relative composition determined (Wagner, 2010).

3.6.2 Instrumentation

X-ray photoelectron spectroscopy data were obtained on an ESCALAB 250Xi spectrometer using monochromatised Al Kα X-rays focused to a spot size of 0.5 mm and an electron analyser pass energy of 20 eV for narrow range scans. Included in the binding energies employed for calibration were 83.96 eV for Au 4f7/2 of metallic gold and 932.6 eV for Cu 2p3/2 of Cu metal. The pressure in the analysis chamber was better than 5 × 10-9 mbar during spectral acquisition. Because of the poor electrical

65 conductivity of the materials investigated, photoelectron spectra had to be obtained while the specimen was under the influence of a low energy electron beam (4 V, 175

μA) from a charge-neutralisation flood gun. The observed binding energies, which were lower than the correct values because of over-compensation of charging to achieve minimum linewidth, were usually referenced to either the 285.0 eV level for aliphatic hydrocarbon C 1s photoelectrons or 284.6 eV for the fitted 1s component from aromatic

C atoms in MBT. The possibility of beam damage by the low energy electrons was monitored, and in order to minimise any damage, spectra were obtained as quickly as possible at the expense of signal-to-noise. The data were collected and processed under

Thermo Scientific Avantage 4.58 and 4.54 software.

3.6.3 Investigation in this thesis

XPS was used in this study to characterize the compounds of mercaptobenzothiazole,

Au NPs and relevant reaction products.

3.7 TEM and other instruments

Transmission electron microscopy is normally used to study the internal microstructure and crystal structure of samples. A beam of electrons is transmitted through the sample with relatively little loss of energy and an image is formed from the interaction between the transmitted electrons and the samples. The thickness of samples is required to be in the range of 20−300 nm depending on the average atomic number of the material

(O'Connor et al., 2003). In this research, a 200 kV TEM (JEOL JEM-2100 high resolution transmission electron microscope) was used to take the images of Au NPs with diameters less than 100 nm.

The element ratios of relevant compounds were obtained using a PerkinElmer Optima

8300 ICP-OES spectrometer. The UV-vis absorption spectra and zeta potential of Au

NPs solutions were measured on an Agilent 8453 UV-vis spectrometer and Malvern

Zetasizer Nano ZS, respectively.

3.8 References

Albrecht, M.G., Creighton, J.A., 1977. Anomalously intense Raman spectra of pyridine

at a silver electrode. Journal of the American Chemical Society 99, 5215-5217.

Bard, A.J., Faulkner, L.R., 2001. Electrochemical methods: Fundamentals and

applications, 2nd ed. John Wiley & Sons, Inc., New York, USA.

Campion, A., Kambhampati, P., 1998. Surface-enhanced Raman scattering. Chemical

Society Reviews 27, 241-250.

Ferraro, J.R., Nakamoto, K., Brown, C.W., 2002. Introductory Raman spectroscopy,

2nd ed. Academic Press, San Diego, USA.

Fleischmann, M., Hendra, P.J., McQuillan, A.J., 1974. Raman spectra of pyridine

adsorbed at a silver electrode. Chemical Physics Letters 26, 163-166.

Guo, Z.R., Gu, C.R., Fan, X., Bian, Z.P., Wu, H.F., Yang, D., Gu, N., Zhang, J.N., 2009.

Fabrication of anti-human cardiac troponin I immunogold nanorods for sensing

acute myocardial damage. Nanoscale Research Letters 4, 1428-1433.

Harrick, N.J., 1967. Internal Reflection Spectroscopy. John Wiley & Sons Inc., New

York, USA.

Jeanmaire, D.L., Van Duyne, R.P., 1977. Surface raman spectroelectrochemistry: Part I.

Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver

electrode. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry

84, 1-20.

Larkin, P., 2011. Infrared and raman spectroscopy: Principles and spectral interpretation.

Elsevier, USA.

Le Ru, E.C., Etchegoin, P.G., 2009. Principles of surface-enhanced Raman

spectroscopy and related plasmonic effects. Elsevier, Oxford, UK.

Moskovits, M., 2005. Surface-enhanced Raman spectroscopy: A brief retrospective.

Journal of Raman Spectroscopy 36, 485-496.

Nikoobakht, B., El-Sayed, M.A., 2003. Preparation and growth mechanism of gold

nanorods (NRs) using seed-mediated growth method. Chemistry of Materials 15,

1957-1962.

O'Connor, D.J., Sexton, B.A., Smart, R.S.C., 2003. Surface analysis methods in

materials science, 2nd ed. Springer-Verlag Berlin Heidelberg, New York, USA.

Pelletier, M.J., 1999. Analytical applications of Raman spectroscopy. Blackwell Science

Ltd., UK.

Raman, C.V., Krishnan, K.S., 1928. A new type of secondary radiation. Nature 121,

501-502.

Sau, T.K., Murphy, C.J., 2004. Room temperature, high-yield synthesis of multiple

shapes of gold nanoparticles in aqueous solution. Journal of the American Chemical

Society 126, 8648-8649.

Settle, F.A., 1997. Handbook of instrumental techniques for analytical chemistry.

Prentice Hall PTR, Upper Saddle River, NJ, USA.

Sharma, B., Frontiera, R.R., Henry, A.-I., Ringe, E., Van, D.R.P., 2012. SERS:

Materials, applications, and the future. Materialstoday 15, 16-25.

Smekal, A., 1923. The quantum theory of dispersion. Naturwissenschaften 11, 873-875.

Smith, E., Dent, G., 2005a. Introduction, basic theory and principles, Modern Raman

spectroscopy – A practical approach. John Wiley & Sons, Ltd, pp. 1-21.

Smith, E., Dent, G., 2005b. Surface-enhanced Raman scattering and surface-enhanced

resonance Raman scattering, Modern Raman spectroscopy – A practical approach.

John Wiley & Sons, Ltd, pp. 113-133.

Tian, Z.-Q., Zhang, X.-M., 2014. Electrochemical surface-enhanced Raman

spectroscopy (EC-SERS): Early history, principles, methods, and experiments,

Developments in electrochemistry. John Wiley & Sons, Ltd, Chichester, UK, pp.

113-135.

Wagner, J.M., 2010. X-ray photoelectron spectroscopy. Nova Science Publishers, New

York, USA.

Wang, J., 2000. Analytical electrochemistry, 2nd ed. John Wiley & Sons, Inc., New

York, USA.

Woods, R., Hope, G.A., Watling, K., 2000. A SERS spectroelectrochemical

investigation of the interaction of 2-mercaptobenzothiazole with copper, silver and

gold surfaces. Journal of Applied Electrochemistry 30, 1209-1222.

Zoski, C.G., 2007. Handbook of electrochemistry. Elsevier, The Netherlands.

Chapter 4 Characterisation of MBT and DTP compounds

4.1 Preamble

This chapter presents the characterisation of mercaptobenzothiazole and dithiophosphate ligands and compounds prepared for this research. The spectral characterisation of these compounds identifies characteristics of the compounds that can be used in the investigation of the interaction of the ligands with surfaces. The characterisation and identification of the compounds were mainly based on their vibrational spectra. Raman spectra were generally obtained from solid samples. These spectra may be slightly different to the spectra obtained from solvated species in solution. The characteristic Raman bands of each compound have been discussed and assigned based on the published literature and relevant compounds. XPS and ICP-OES were also used to provide supporting molecular information where necessary.

4.2 Characterisation of MBT compounds

4.2.1 HMBT

2-Mercaptobenzothiazole and derivatives are known to be very effective for the treatment of sulfide, precious metal and gold-bearing ores (Allan and Woodcock, 2001;

Maier and Dobiáš, 1997; Numata et al., 1998; Yekeler and Yekeler, 2006). Aqueous mixtures of its soluble sodium salt and dithiophosphate are usually used for the recovery of precious metals such as gold (Swaminathan et al., 1993).

2-Mercaptobenzothiazole is present in acid media as the unionised protonated form

(HMBT) which can exist in either the thione form (benzothiazoline-2-thione) with NH or the thiol form (2-mercaptobenzothiazole) with an SH group (Lee et al., 2007; Woods et al., 2000). The molecular structure of each form is presented in Figure 4.1. The equilibrium between the thione and thiol tautomers has been studied by computational and experimental methods, and most of the studies have found that HMBT exists predominantly as the thione form in the solid state (Dong et al., 1995; Mohamed et al.,

2008; Rai et al., 2006; Woods et al., 2000). The spectral investigation of HMBT can provide a basis for characterisation the of relevant mercaptobenzothiazole compounds.

H N N H C S C S

S S

benzothiazoline-2-thione 2-mercaptobenzothiazole

Figure 4.1 Molecular structures of the thione and thiol forms of HMBT.

Böhlig and co-workers (Böhlig et al., 1999) investigated the structure of benzothiazoline-2-thione by vibrational spectra analysis and quantum chemical calculations. They concluded that the vibrational spectrum of benzothiazoline-2-thione was consistent with the thione form alone. Woods and co-workers (Woods et al., 2000) confirmed that HMBT adopts the thione rather than the thiol structure by 13C NMR spectroscopy. The energy of thione molecule is also less than that of the thiol molecule according to the quantum chemical calculation results (Flemmig et al., 2001; Jiao et al.,

2015), a result that indicates the thione form is more stable than the thiol form.

HMBT exists as a cyclic dimer in condensed phases as shown in Figure 4.2. The two

HMBT molecules in thione form are connected with linear N−H∙∙∙S hydrogen bonds caused by mutual proton transfer (Böhlig et al., 1999; Chesick and Donohue, 1971).

H S C

N N C S H

Figure 4.2 The dimer structure of HMBT.

In Figure 4.3, the Raman and FTIR spectra of solid HMBT are presented for the range of 300−3500 cm-1 and 450−3500 cm-1, respectively. The assignments of the observed frequencies for HMBT in this study are mainly based on the assignment made by

Woods and co-workers (Woods et al., 2000). These assignments in this study and literature are listed and compared in Table 4.1.

Both Böhlig’s group (Böhlig et al., 1999) and Mohamed’s group (Mohamed et al., 2008) recorded the FTIR spectra of solid HMBT in the form of a KBr pellet, while the FTIR spectra of solid HMBT in this study was acquired directly from crystals by ATR-FTIR technique as described in chapter 3. The justifications for these spectral band assignments are discussed in the sections following the table.

FTIR

Raman

500 1000 1500 2000 2500 3000 3500 -1 Wavenumber (cm )

Figure 4.3 Raman and FTIR spectra of HMBT.

Table 4.1 Observed frequencies for HMBT.

Observed (this study) Observed (Böhlig et al., 1999)a Observed (Mohamed et al., 2008)

IR Raman Assignment IR Raman Assignment IR Raman Assignment

3408c w NH stretch 3402c w NH stretch

3195 vwb CH stretch

3111 w CH stretch 3115 m

3075 w 3072 vw CH stretch 3081 m 3081 vw CH stretch 3174 sh 3080 sh CH stretch

3039 sh CH stretch 3045 m 3069 w CH stretch 3112 w 3069 s CH stretch

2962 w CH stretch 2967 m 3060 sh CH stretch 3056 w 3040 sh CH stretch

2892 w CH stretch 2899 m 3042 sh CH stretch 3041 w 3040 sh CH stretch

1596 w 1600 w Bz ring stretch 1598 w 1599 vw CC ring stretch 1596 m 1599 w CC ring stretch

1582 vw 1584 m Bz ring stretch 1583 vw 1583 m CC ring stretch 1588 vw 1583 s CC ring stretch

1496 s 1497 w CN stretch 1498 s 1496 vw CC ring stretch 1497 vs 1496 w CC ring stretch (Thioamide I)

1456 m 1459 w Bz ring stretch 1458 m 1458 w CH bending 1457 s 1458 m CH in-plane deformation

Observed (this study) Observed (Böhlig et al., 1999)a Observed (Mohamed et al., 2008)

IR Raman Assignment IR Raman Assignment IR Raman Assignment

1424 s 1424 w N−C−S ring stretch 1429 vs 1423 vw CH bending 1426 vs 1432 w NH in- plane deformation

1379 vw 1374 vw CC ring stretch 1382 vw 1370 vw CC ring stretch

1319 s 1319 m NH stretch 1322 vs 1318 m CN stretch 1320 vs 1318 s CH in-plane (Thioamide II) deformation

1283 m 1271 s CH in-plane 1285 m 1270 vw CH bending 1284 s 1270 s CN stretch bending

1244 m 1254 vs N−C−S ring stretch 1248 m 1252 vs NH bending 1245 s 1253 vs CN stretch

1154 vw 1157 vw CH in-plane 1155 vw 1156 vw CH bending 1157 vw 1160 sh CH in-plane bending deformation

1128 w 1131 w CH in-plane 1129 vw 1130 w CH bending 1128 w 1131 w CH in-plane bending deformation

1075 sh 1075 w Bz ring stretch 1078 s 1074 w CCC bending 1076 s 1074 w CC ring stretch

1032 s 1030 w C=S stretch 1036 vs 1029 w CC ring stretch 1034 vs 1029 w CC ring stretch (Thioamide III)

Observed (this study) Observed (Böhlig et al., 1999)a Observed (Mohamed et al., 2008)

IR Raman Assignment IR Raman Assignment IR Raman Assignment

1012 sh 1013 w CH in-plane 1014 s 1013 w CH bending 1013 vs 1012 w C=S stretch bending

982 vw 977 vw CH bending 985 vw 974 vw CH wag

938 vw 934 vw CH out-of-plane 939 vw 933 vw CH bending 941 sh 931 vw CH wag bending

868 w 869 vw CH out-of-plane 858 vw 862 vw CCC bending 868 w 866 w CCC ring bending deformation

849 w CH out-of-plane 850 vw 848 vw CH bending 849 w CH wag bending

749 sh 757 vw CH out-of-plane 752 vs 756 vw CH bending 765 vs 754 vw CH wag bending

718 vw 720 vw CH out-of-plane 719 vw 719 vw CC torsion 720 w Butterfly torsion bending

707 sh 706 w CS stretch 707 vw 705 w CCC bending 706 w 705 w CS stretch

667 vs 663 w N−H wag 670 s 663 w CS stretch 669 s 661 w CS stretch

Observed (this study) Observed (Böhlig et al., 1999)a Observed (Mohamed et al., 2008)

IR Raman Assignment IR Raman Assignment IR Raman Assignment

603 s 607 m CS stretch 606 m 606 w CCC bending 604 s 607 m CCC ring deformation

568 m 573 vw N−H wag 570 w 572 vw NH bending 569 m 573 vw NH wag

524 w Bz ring 525 vw 524 vw C=S bending 524 w C=S wag deformation

501 w 500 w Bz ring 501 vw 500 w Side ring torsion 515 w 500 w CCC ring deformation deformation

424 vw Bz ring 425 vw 423 vw CC ring torsion 424 m 422 vw Butterfly torsion deformation

395 s Bz ring 396 w 394 vs Side ring torsion 394 s 395 s N−C−S scissors deformation

380 w Bz ring 383 w 380 vw CCC bending 382 s 380 sh CCC ring deformation deformation a Only the primary assignments are listed in the table. b Relative intensity. Vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder. c Observed in CCl4 solution.

4.2.1.1 Region 4000−2000 cm-1

In the FTIR spectrum, the broad dip around 3400 cm-1 represents the N−H symmetric stretch, which may be too broad to assign it. It is consistent with IR spectra observed by the two authors in Table 4.1. They observed a monomeric band of the N−H symmetric

-1 stretch at about 3400 cm , when HMBT was dissolved in CCl4 solutions (Böhlig et al.,

1999; Mohamed et al., 2008). The N−H symmetric stretch band is much weaker in

Raman than in FTIR spectra (Colthup et al., 1990), therefore, no corresponding band was observed in the Raman spectrum presented.

A range of bands were observed around 3000 cm-1 in the FTIR spectrum. This is consistent with the FTIR spectra observed by Böhlig’s group (Böhlig et al., 1999) and

Rai’s group (Rai et al., 2006), while they assigned these bands to the N−H stretch of the

HMBT cyclic dimer. The most reasonable explanation was proposed by Flakus (Flakus,

1981) who believed that these unexpected N−H stretch bands were due to the superposition of the vibrational progression of two N−H∙∙∙S hydrogen bonds in the

HMBT cyclic dimer.

In this research, similar bands around 3000 cm-1 were also observed in the FTIR spectra of other MBT species without N−H bond. It suggests that the assignment made by

Böhlig and Rai must be inaccurate. The hydrogen atoms in benzene ring normally have three types of vibrations which are; stretching, in-plane bending and out-of-plane bending. The C−H stretching can give rise to band near 3000 cm-1. Thus, these bands around 3000 cm-1 are assigned to the C−H stretch in this study. A corresponding band at

3072 cm-1 was observed in the Raman spectrum of HMBT, which can be assigned to the

C−H stretch. The other two types of vibrations will be presented in the following sections.

In this research, no band was observed in the expected region for the S-H stretch, which is from 2550 to 2600 cm-1 (Coates, 2006; Colthup et al., 1990). It confirmed that HMBT exists as the thione form in the solid state. Sandhyarani and co-workers (Sandhyarani et al., 1999) assigned a weak band at 2641 cm-1 in the Raman spectrum of HMBT to the

S−H stretch. It is inconsistent with the assignment made by Woods and co-workers

(Woods et al., 2000) that the band should be assigned to a vibration mode of the benzene ring of HMBT. Böhlig and co-workers (Böhlig et al., 1999) had concluded that the band at 2643 cm-1 is too high to be attributed as an S−H stretch and it could be an overtone of the very strong band at 1322 cm-1.

4.2.1.2 Region 2000−1000 cm-1

The bands observed in this region are mainly from the vibrations of benzene ring carbon-carbon and C−H bonds. The carbon-carbon bonds in benzene ring are equivalent and are approximately a “bond-and-a-half” (Lin-Vien et al., 1991). The ortho substituted benzene ring in HMBT may have two stretch pairs absorbed in the infrared spectrum. The semicircle stretch pair at about 1445 and 1500 cm-1, and the quadrant stretch pair at about 1575 and 1605 cm-1 (Colthup et al., 1990).

Correspondingly, four infrared bands at 1456, 1496, 1582 and 1596 cm-1 were observed in this research, which can be all assigned to the benzene ring stretch except the band at

1496 cm-1. According to Böhlig and co-workers’ calculation (Böhlig et al., 1999), the

C−C stretch bond which acts as the bridge bond of the two rings has the lowest C−C

82 stretch bond force constant in the benzene ring. Thus, vibrational frequency of this C−C stretch bond is lower than that of the other C−C stretch bonds (Ferraro et al., 2002). The weak band at 1075 cm-1 is assigned to this C−C stretch vibration.

HMBT contains a secondary thioamide group (H−N−C=S) which gives rise to four combination vibrations in the infrared spectrum (Desseyn and Herman, 1967; Singh et al., 1977). These vibrations are also known as thioamide І, ІI, III and IV bands, which are located at about 1500, 1400−1300, 1000 and 870 cm-1. The band observed at 1496 cm-1 can be assigned to C−N stretch as the thioamide band I. The thioamide II band in

HMBT was observed at 1319 cm-1 mainly attributed to the N−H stretch interacting with

C−N and C=S bonds. The thioamide III band is due to a complex vibration mode which is predominantly from a C=S stretch. Böhlig and co-workers (Böhlig et al., 1999) assigned three bands at 1014, 1036 and 1078 cm-1 as the C=S stretch, and they believed that it could not be an isolated exocyclic C=S stretch. Based on the literature (Chesick and Donohue, 1971; Flemmig et al., 2001; Rao and Venkataraghavan, 1989) and spectra of other MBT species, the band at 1032 cm-1 is assigned as the thioamide III band.

The bands at 1254 and 1424 cm-1 are both assigned to the N−C−S ring stretch, which is similar to the ring stretch in five-membered ring heteroaromatic compounds. In the infrared spectrum of ortho substituted benzene ring, there are usually several bands in the range of 1300−1000 cm-1 caused by the in-plane C−H bending vibrations (Colthup et al., 1990; Lin-Vien et al., 1991). These C−H bending vibrations sometimes strongly interact and mix with the benzene ring carbon-carbon vibrations. In this research, four bands at 1013, 1131, 1157 and 1283 cm-1 can be all assigned to the C−H in-plane bending that are predominantly C−H vibrations.

4.2.1.3 Region below 1000 cm-1

The C−H out-of-plane bending vibrations in substituted benzene ring compounds normally give rise to bands in the range of 1000−700 cm-1. The carbon and its hydrogen usually move out-of-plane in opposite directions in these vibrations (Colthup et al.,

1990). In the spectra of HMBT, the bands from 700 to 1000 cm-1 can be all assigned to the C−H out-of-plane bending except the band at 706 cm-1. The band at 868 cm-1 is not assigned to the thioamide IV band due to the observation of this band in the vibrational spectra of other MBT species.

The C−S stretch vibrations can present bands from 570 to 705 cm-1 for both cyclic and open-chain C−S compounds (Lin-Vien et al., 1991). The bond lengths of the two C−S stretch bonds in the HMBT molecule reported by Chesick and Donohue (Chesick and

Donohue, 1971) are 1.740 Å and 1.732 Å, respectively. Notwithstanding they exhibit the similar bond lengths, the force constants of these two bonds calculated by Böhlig and co-workers (Böhlig et al., 1999) are not close. Thus, the bands at 607 and 706 cm-1 in the Raman spectrum of HMBT can both be assigned to C−S stretch, which is consistent with the assignment made by Rai et al. (Rai et al., 2006).

The bands at 573 and 663 cm-1 are attributed to the out-of-plane N−H wagging vibration, which is inconsistent with assignments by the authors in Table 4.1. The 663 cm-1 band was absent in the spectra of other MBT species, which indicates that it may not from the vibration of C−S stretch. The other bands in the low frequency range can be assigned to the benzene ring deformation.

4.2.2 (MBT)2

4.2.2.1 Molecular structure of (MBT)2

Figure 4.4 presents the oxidation product structures found for NaMBT. 2,2’- thiobisbenzothiazole with monosulfide bridge (R−S−R) and 2,2’-dithiobisbenzothiazole with disulfide bridge (R−S−S−R) (Zajíčková and Párkányi, 2008).

N N

C S C

S S

2,2'-thiobisbenzothiazole

N N

C S S C

S S

2,2'-dithiobisbenzothiazole

Figure 4.4 Molecular structures of the two forms of (MBT)2.

Marconato and co-workers (Marconato et al., 1998) studied the inhibition of MBT on the surface of copper electrode in ethanolic solutions by a spectroelectrochemical method. They found the oxidation of MBT on the copper electrode surfaces, but they could not decide which form it is due to the similarity of their Raman spectra. The adsorption of MBT on galena (PbS) and pyrite (FeS2) was investigated by Szargan and

85 co-workers (Szargan et al., 1999) using XPS technique. They concluded that the oxidation product of MBT on the sulfide surfaces is 2,2’-dithiobisbenzothiazole.

Kazansky and co-workers (Kazansky et al., 2012) also reported that the oxidation product of MBT on copper surfaces is 2,2’-dithiobisbenzothiazole.

The N 1s and S 2p spectra of the (MBT)2 formed in this research are shown in Figure

4.5. The binding energy for N 1s signal is 398.9 eV, which is consist with the result obtained by Szargan et al. (Szargan et al., 1999) for (MBT)2. It is lower than the N 1s binding energy in MBT, which indicates the transformation of the amino group into an imino group (Szargan et al., 1999). The S 2p binding energy at 164.3 eV emanates from the bridging sulfur atoms (−S−). Both the forms of (MBT)2 have similar S 2p binding energies (~164.3 eV), while the atomic composition differs. The solubility in organic solvents of these two (MBT)2 compounds are also different (Szargan et al., 1999). The

(MBT)2 compound with monosulfide bridge (2,2’-thiobisbenzothiazole) can dissolve in both ethanol and chloroform, while the 2,2’-dithiobisbenzothiazole only dissolves in chloroform.

In this research, at least 45% of the S in the specimen would have been exocyclic S bonded to exocyclic S, and at least 45% would have been endocyclic S according to the

XPS results. The (MBT)2 samples dissolved in chloroform and insoluble in ethanol.

Based on the evidence, the (MBT)2 compounds prepared in this research are in the form of 2,2’-dithiobisbenzothiazole.

N1s

(a)

408 404 400 396 392 Binding Energy (eV)

S2p

(b)

172 168 164 160 Binding Energy (eV)

Figure 4.5 The N 1s (a) and S 2p (b) spectra of (MBT)2.

4.2.2.2 Characterisation of (MBT)2

The molecular structure of (MBT)2 is quite similar to benzothiazole (Figure 4.6). In general, (MBT)2 has two more bonds (S−S and C−Sexocyclic) than benzothiazole. Both of them have the five-membered heteroaromatic ring containing two double bonds in their molecular structures. Thus, the Raman spectrum of (MBT)2 is compared to the spectrum of benzothiazole in Figure 4.8 for band assignments. The assignment of the Raman bands of (MBT)2 are shown in Table 4.2. It can be seen that some of the bands are assigned to the same vibrations as in HMBT, because they also have similar groups in their structures.

N N N

CH C S S C

S S S

Benzothiazole 2,2'-dithiobisbenzothiazole

Figure 4.6 Molecular structures of (MBT)2 and benzothiazole.

A number of studies on five-membered heteroaromatic ring vibrations have been made based on compounds such as thiophene, furan and thiazoles (Katritzky, 1959; Rao and

Venkataraghavan, 1964; Sathyanarayanmoorthi et al., 2013; Taurins et al., 1957). The nine types of vibrations in five-membered heteroaromatic ring compounds identified by

Colthup and co-workers (Colthup et al., 1990) are presented in Figure 4.7. These vibrations are divided into the stretching modes (a, b, c and d) at about 1620−1250 cm-1, ring-breathing mode (e) at about 1150−800 cm-1, in-plane bending modes (f and g) at

88 about 900−600 cm-1 and out-of-plane bending modes (h and i) at about 600−450 cm-1

(Colthup et al., 1990). The carbons of the five-membered heteroaromatic ring are not identical, and the substituent on the ring can be varied. Thus, it is normal that not all types of these vibrations can be observed in a specific compound such as (MBT)2.

a b c

1615 - 1520 cm-1 1510 - 1404 cm-1 1450 - 1354 cm-1

d e f

-1 1418 - 1252 cm-1 1114 - 832 cm-1 872 - 647 cm

g h i

-1 724 - 604 cm 565 - 510 cm-1 605 - 453 cm-1

Figure 4.7 Schematic skeletal vibrations of five-membered heteroaromatic ring compounds with two double bonds (after Colthup et al., 1990).

The bands at 1240, 1431 and 1561 cm-1 can be all assigned to the N=C−S five- membered ring stretch, which is contributed by the stretch and contraction among the five atoms in the ring. All these three bands have corresponding bands in the Raman spectrum of benzothiazole. The band near 1431 cm-1 observed in the Raman spectrum of HMBT is assigned to a solitary N−C−S ring stretch. No bands are assigned to the ring-breathing and in-plane bending modes in the Raman spectrum of (MBT)2. Taurins and co-workers (Taurins et al., 1957) observed several very strong bands in the infrared spectrum of thiazole in this region, which are all assigned to the ring-breathing and in- plane bending vibrations. Compared to thiazole, these two vibration modes may be affected by the S−S and C−Sexocyclic bond vibrations for (MBT)2, so that they are not

-1 observable. The bands at 704 and 588 cm are assigned to the C−Sendocyclic stretch, which is consistent with the assignment for HMBT.

Leopold and co-workers (Leopold et al., 2005) reported two bands at 507 and 605 cm-1 as the out-of-plane deformation of S−C=N and C−N−C in the five-membered ring of thiamine. They are both attributed to the out-of-plane bending mode for the five- membered ring. In this research, only a band at 535 cm-1 in the Raman spectrum of

(MBT)2 can be assigned to a five-membered ring out-of-plane bending. This is inconsistent with the assignment made by Woods and co-workers (Woods et al., 2000).

They considered the band at 536 cm-1 was from the S−S bond since such a band was absent in the spectra of the other MBT species. However, as shown in Figure 4.8, a band at 532 cm-1 was also observed in Raman spectra of benzothiazole, a molecule which has no S−S bond present. Sathyanarayanmoorthi and co-workers

(Sathyanarayanmoorthi et al., 2013) assigned the theoretical wave number of benzothiazole at 532 cm-1 to a C−C−C out-of-plane bending in the benzene ring based

90 on their calculation. They did not observe this band in their Raman spectrum. Their assignment must be incorrect, however, for it should have been observed in Raman spectra of other MBT species if this assignment was true.

The S−S stretching vibration band position can be in the range of 400−650 cm-1 based on two main theories. The first one proposes that the band position of S−S stretching vibration is dependent upon the torsional angles in the C−C−S−S−C−C conformation

(Sugeta et al., 1972). It means that the S−S stretch band position is only dependent on the internal rotation of the C−C and C−S bonds. Based on this theory, the band position of S−S stretching vibration should be in the range of 510−540 cm-1 which is inconsistent with the experimental results obtained by Biswas and co-workers (Biswas et al., 2007). The other theory suggests that the S−S stretching vibration band position depends on the dihedral angle of the C−S−S−C group rather than the torsional rotation of the C−S bonds (Van Wart and Scheraga, 1976a, b). The band position can be from

400 to 650 cm-1 according to this theory. Parker and co-workers (Parker et al., 2008) also observed a band at 472 cm-1 due to the symmetric stretching vibration of S−S in

-1 cyclo-octasulfur (S8). Thus, a very weak band at 445 cm which was only observed in the Raman spectrum of (MBT)2 can be assigned to the S−S stretch in this research.

Benzothiazole

(MBT) 2

500 1000 1500 2000 2500 3000 3500 -1 Raman shift cm ( )

Figure 4.8 Raman spectra of (MBT)2 and benzothiazole.

Table 4.2 Characteristic Raman bands for (MBT)2.

(MBT)2 Assignment (MBT)2 Assignment

3065 wa CH stretch 1009 s CH in-plane bending

3032 w CH stretch 851 m CH out-of-plane bending

1593 m Bz ring stretch 763 w CH out-of-plane bending

1561 m NCS ring stretch 704 s C−Sendo stretch

1467 vs Bz ring stretch 672 w CH out-of-plane bending

1431 s NCS ring stretch 588 w C−Sendo stretch

1314 m CH in-plane bending 535 m NCS ring out-of-plane bending

1277 m CH in-plane bending 502 s Bz ring deformation

1240 s NCS ring stretch 445 vw SS stretch

1159 w CH in-plane bending 400 m Bz ring deformation

1125 m CH in-plane bending 366 m Bz ring deformation

1080 vw Bz ring stretch a Relative intensity. Vs, very strong; s, strong; m, medium; w, weak; vw, very weak.

4.2.3 Metal MBT compounds

The Raman spectra and band assignments of metal MBT compounds are shown in

Figure 4.9 and Table 4.3, respectively. Khullar and Agarwala (Khullar and Agarwala,

1975) believed that Cu(MBT)2 was obtained when CuCl2 and MBT were mixed in ethanol based on the combustion elemental analysis values of their samples. However, the elemental analysis value of Cu(MBT)2 is actually the same with specimen which contains CuMBT and (MBT)2 in the ratio of 2:1.

In this research, the copper MBT compound was made by adding aqueous copper (II) acetate to HMBT in methanol as mentioned in chapter 3. The first Raman spectrum of the product was taken after the precipitated was only washed with DDI water. Then the second Raman spectrum was taken after the precipitated was washed with DDI water and chloroform. As shown in Figure 4.10, the first Raman spectrum (green line) has the characteristic bands of both (MBT)2 and the second Raman spectrum (red line). It means that the reaction product of copper (II) acetate and HMBT is probably the synthesis of CuMBT and (MBT)2 as shown in Equation 4.1. The interaction of HMBT with gold chloride is similar to that observed with copper (II) acetate, and has AuMBT and (MBT)2 as products (Woods et al., 2000).

2+ − 2Cu + 4MBT = 2CuMBT + (MBT)2 (4.1)

The Raman spectra of these metal MBT compounds are similar. The band near 1400 cm-1, assigned to the NCS ring stretch, presented the major difference among these compounds indicative of the effects of bonding the metal cation(s) to this region of the ligand. It reflects the different bonding vibrations between metal atoms and the five-

94 membered ring. The absence of the band at 535 cm-1 in the Raman spectra of metal

MBT species could also consistent with the NCS ring out-of-plane bending being modified by the metal−sulfur stretch vibrations.

A number of studies (Dong et al., 1995; Jiao et al., 2015; Marconato et al., 1998; Wang et al., 2004; Woods et al., 2000) have been done on the adsorption mechanism of MBT on metal surfaces. Parker and co-workers assigned a band at about 310 cm-1 as the

Au−S stretch in the Raman spectrum of Au2S (Parker et al., 2008). Based on these studies, the metal−sulfur stretch vibration bands can be in the range of 225 to 350 cm-1.

The Raman spectra of metal MBT compounds in the range of 300 to 350cm-1 are presented in Figure 4.11. The metal-sulfur vibration is of weak intensity and determined by factors such as mass, coordination number and the electronic configuration of the cation (Adams and Cornell, 1968; Ferraro, 1971). The bands at 319, 316 and 311 cm-1 are assigned to the Cu−S, Ag−S and Au−S stretch, respectively. The metal−sulfur stretch vibration is shifted to lower wavelengths with the increase of the mass.

AgMBT

CuMBT

AuMBT

KMBT

NaMBT

500 1000 1500 2000 2500 3000 3500 -1 Raman shift (cm )

Figure 4.9 Raman spectra of metal MBT compounds.

Table 4.3 Characteristic Raman bands for metal MBT compounds.

NaMBT KMBT AuMBT CuMBT AgMBT Assignment

3057 vwa 3063 vw 3069 vw 3059 vw 3074 vw CH stretch

3028 vw 3027 vw 3012 vw 3028 vw 3021 vw CH stretch

1589 m 1588 m 1586 m 1587 m 1590 m Bz ring stretch

1560 m 1559 m 1562 m 1564 m 1563 m NCS ring stretch

1454 m 1455 w 1455 m 1455 m 1458 m Bz ring stretch

1398 vs 1402 sh 1384 vs 1399 vs 1394 vs NCS ring stretch

1312 w 1310 w 1316 w 1316 w 1317 w CH in-plane bending

1278 m 1276 m 1276 m 1281 m 1280 m CH in-plane bending

1248 m 1248 m 1241 s 1249 s 1242 s NCS ring stretch

1159 m 1160 m 1161 m 1161 w 1163 w CH in-plane bending

1129 m 1127 m 1131 m 1130 m 1133 m CH in-plane bending

1079 m 1077 m 1079 vw 1081 vw 1079 vw Bz ring stretch

1019 m 1018 m CH in-plane bending

1009 m 1004 w 1011 vs 1012 m 1010 vs CH in-plane bending

857 sh 854 sh 861 sh 871 sh 860 m CH out-of- plane bendind

NaMBT KMBT AuMBT CuMBT AgMBT Assignment

759 vw 763 vw 721 m CH out-of- plane bending

708 s 707 s 713 sh 701 w 716 m CS stretch

660 vw CH out-of- plane bending

600 sh 600 m 604 sh 608 m 602 sh CS stretch

506 m 506 s 508 m 508 m 507 m Bz ring deformation

392 sh 394 s 390 m 400 m 388 sh Bz ring deformation

310 vw 317 vw 311 vw 319 vw 316 vw Metal−S stretch a Relative intensity. Vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder.

CuMBT+(MBT) synthesis 2

AuMBT+(MBT) synthesis 2

(MBT)

CuMBT

AuMBT

500 1000 1500 2000 2500 3000 3500

-1 Raman shift (cm )

Figure 4.10 Raman spectra of CuMBT, AuMBT, (MBT)2 and their syntheses.

AgMBT CuMBT AuMBT KMBT

NaMBT

300 310 320 330 340 350 -1 Raman shift (cm )

Figure 4.11 Raman spectra of metal MBT compounds in the range of 300−350 cm-1.

100

4.3 Characterisation of DTP compounds

4.3.1 Molecule structures of dibutyl dithiophosphate compounds

The anions of dibutyl dithiophosphate compounds can exist in two resonance structures as shown in Figure 4.12 (Haiduc et al., 1995). The metal dibutyl dithiophosphate compounds normally present in the structure form II in which both the two metal−sulfur bonds and two phosphorus−sulfur bonds are nearly equivalent. The oxidation product

(DBDTP)2 seems to prefer the structure form I with only one sulfur−sulfur bridge, which is similar to the structure of bis(diisobutylthiophosphoryl)disulfide reported by

Hope and co-workers (Hope et al., 2003).

S S O O P P O S O S

(I) (II)

Figure 4.12 Two resonance structures of dibutyl dithiophosphate anions.

Based on the Cu 2p, S 2p, P 2p, O 1s and C 1s spectra of copper dibutyl dithiophosphate, the atomic ratios of relative elements were 1:2:1:2:8, and the Cu 2p binding energy was

932.7 eV (Figure 4.13). The atomic ratios result from XPS for Cu, S and P is consistent with the result obtained by ICP-OES which was 1:1.9:0.9. Moreover, as shown in

Equation 4.2, the cupric dibutyl dithiophosphate is unstable and can rapidly decompose to cuprous dibutyl dithiophosphate and (DBDTP)2 (Chander and Fuerstenau, 1974).

101

2Cu(DBDTP)2 = 2CuDBDTP + (DBDTP)2 (4.2)

Thus, the copper dibutyl dithiophosphate synthesised during this research is CuDBDTP.

The atomic ratios of zinc dibutyl dithiophosphate for Zn, S and P were roughly 1:1.9:0.9 according to the ICP-OES results. It indicates the molecular formula of zinc dibutyl dithiophosphate is either Zn(DBDTP)2 or Zn2(DBDTP)4, a result that is consistent with the two structures for zinc dithiophosphate reported in the literature (Armstrong et al.,

1997; Ito et al., 1969; Ivanov et al., 2000). The molecular structure of CuDBDTP,

Zn(DBDTP)2 and Zn2(DBDTP)4 are shown in Figure 4.14.

Cu2p 3 x 10 45

CPS 25

965 960 955 950 945 940 935 930 925 Binding Energy (eV)

Figure 4.13 Cu 2p spectrum CuDBDTP corrected for energy of flood-gun electrons.

102

O S S S O Cu O P P Zn P S O O O S S (II) (I)

O O P S S O S S O P Zn Zn P O S S O S S P O O

(III)

Figure 4.14 Molecular structures of CuDBDTP (I), Zn(DBDTP)2 (II) and Zn2(DBDTP)4

(III).

103

4.3.2 Assignments of dibutyl dithiophosphate compounds

The Raman spectra and band assignments of dibutyl dithiophosphate compounds are shown in Figure 4.15 and Table 4.4, respectively. The assignments of Raman bands were made on previous reports of vibrational spectra of dibutyl dithiophosphate compounds and of compounds containing the same basic elemental groups such as dithiophosphinate, dipara-fluorophenyl dithiophosphate and dicyclohexyl dithiophosphate (Hope et al., 2003; Larkin, 2011; Larsson and Holmgren, 2004; Persson et al., 1999).

As shown in Table 4.4, most of the Raman bands of dibutyl dithiophosphate compounds are contributed by the vibrations of the dibutyl groups. The CH3 and CH2 group give rise to bands in the range of 3000−2700 cm-1 due to the symmetrical and antisymmetrical carbon−hydrogen stretching vibrations, while the deformation vibrations normally appear in the range of 1400−700 cm-1 (Sheppard and Simpson,

-1 1953). The band at about 1458 cm can be caused by the CH2 deformation which may coincide with the antisymmetric CH3 deformation vibration.

The P−O−C group in dibutyl dithiophosphate compounds can give rise to bands in the range of 1100−700 cm-1 (Colthup et al., 1990; Lin-Vien et al., 1991). Compared to the

Raman spectrum of dithiophosphinate (without P−O−C group), the bands at about 1033,

963, 909 and 832 can be all assigned to the P−O−C stretch. The first two bands are belonged to the P−O−C antisymmetric stretch, while the last two bands are due to the symmetric stretch (Larsson and Holmgren, 2004; Persson et al., 1999).

104

The molecular strucures of the O,O’-dialklydithiophosphate derivatives of copper (II) and copper (I) have been strudied by Tripathi and wo-workers (Tripathi et al., 1992).

They assigned the bands in the regions 660−630 cm-1 and 580−510 cm-1 to the P=S and

P−S stretching vibrations, respectively. It is inconsistent with the molecular strucure as mentioned above.

Jiang and co-workers (Jiang et al., 1996) reported that the bands in these two regions were contributed by the antisymmetric and symmetric stretching vibrations of P−S in the PS2 group. They found the four phosphorus−sulfur bonds in zinc dithiophosphate are all equivalent for all cases. Thus, the high frequency bands at 625 and 657 cm-1 can

-1 be assigned to the PS2 antisymmetric stretch, while the bands at 560, 542 and 551 cm can be assigned to the PS2 symmetric stretch.

No band was observed around 660 cm-1 in the Raman spectrum of NaDBDTP. However, as shown in Figure 4.16, a very strong band at 674 cm-1 was obtained in the infrared spectrum of NaDBDTP, which is contributed by the PS2 antisymmetric stretch. In the

-1 Raman spectrum of (DBDTP)2, the bands at 663, 506 and 483 cm can be assigned to the P=S, S−S and P−S stretch, which are all absent in the Raman spectra of metal dibutyl dithiophosphate compounds.

105

(DBDTP)2

ZnDBDTP

CuDBDTP

NaDBDTP

500 1000 1500 2000 2500 3000 3500 -1 Raman shift (cm ) Figure 4.15 Raman spectra of dithiophosphate compounds.

106

Table 4.4 Characteristic Raman bands for dithiophosphate compounds.

NaDBDTP CuDBDTP ZnDBDTP (DBDTP)2 Assignment

a 2963 s 2958 m 2957 m 2962 w CH3 antisymmetric stretch

2939 s 2936 m 2937 m 2939 w CH2 antisymmetric stretch

2914 vs 2907 m 2910 m 2914 w CH2 antisymmetric stretch

2877 vs 2877 m 2874 m 2877 w CH3 symmetric stretch

2734 w 2749 vw 2728 w 2732 vw Overtone of CH3 or CH2 groups

1628 w OH2 deformation of H2O

1458 vs 1456 sh 1457 sh 1458 sh CH2 bending and CH3 antisymmetric deformation

1384 vw 1390 w 1388 w 1387 vw CH3 symmetric deformation

1302 m 1299 m 1300 m 1302 w CH2 wag

1262 w 1261 w 1261 w 1263 vw CH2 wag

1233 w 1229 w 1231 w 1233 vw CH3 rock

1149 w 1147 w 1147 w 1149 vw CH3 rock

1122 m 1123 m 1123 m 1123 w CH3 rock

1065 m 1064 m 1064 w 1062 w CCC stretch

1033 w 1029 sh 1026 w 1041 vw P−O−C antisymmetric stretch

107

NaDBDTP CuDBDTP ZnDBDTP (DBDTP)2 Assignment

963 w 964 sh 966 sh 962 vw P−O−C antisymmetric stretch

909 sh 908 sh 900 sh 910 vw P−O−C symmetric stretch

832 m 833 m 832 s 830 w P−O−C symmetric stretch

790 w 792 m 794 s CH2 rock

729 w 731 w 737 w CH2 rock

663 vs P=S stretch

625 w 657 w PS2 antisymmetric stretch

560 vs 542 vs 551 vs PS2 symmetric stretch

506 m S−S stretch

483 vs P−S stretch

414 m 411 sh 416 sh 365 w CCC deformation a Relative intensity. Vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder.

108

NaDBDTP

CuDBDTP

ZnDBDTP

500 600 700 -1 Wavenumber (cm )

Figure 4.16 FTIR spectra of NaDBDTP, CuDBDTP and ZnDBDTP in the range of

750−500 cm-1.

109

4.4 Conclusions

The Raman bands of mercaptobenzothiazole and dithiophosphate compounds have been recorded and assigned in this chapter. The characteristic Raman bands of these compounds were observed and assigned such as S−S stretch in (MBT)2, NCS ring stretch in metal MBT and P=S stretch in (DBDTP)2. The structure of certain compounds have been discussed and determined. All these compounds information provide a basic reference for the Raman and SERS investigation in the following chapters.

110

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118

Chapter 5 Investigations of the interaction of collectors with gold electrode and nanorod surfaces

119

5.1 Preamble

Au NPs normally have different physical and chemical properties compared to their bulk counterpart. Au NRs, as anisotropic nanoparticles, have drawn much attention resulting in their applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices (Chen et al., 2013; Pérez-Juste et al., 2005). Au NRs normally have stronger SERS enhancements than other Au NPs. The enhancements are further improved when two Au NRs are close to each other in a linear fashion (Kumar and Thomas, 2011), this makes it possible to investigate the interaction of collectors with Au NRs.

In this chapter, spectroelectrochemical and XPS techniques have been applied to characterise and investigate the interaction of NaMBT and NaDBDTP with Au NRs and gold electrode surfaces. The flotation behaviour of gold particles with diameter of ~100

μm was also studied utilising controlled potential techniques under different flotation conditions.

120

5.2 Characterisation of gold nanorods

Au NRs with various head shapes and sizes were obtained by using the seed-mediated growth method (Gou and Murphy, 2005; Nikoobakht and El-Sayed, 2003; Sohn et al.,

2009). The schematic of this method is presented in Figure 5.1. Well characterised Au

NRs are found to be capped by the CTAB bilayer, which prevents the further growth of

Au NRs along the longitudinal faces (Lohse and Murphy, 2013). The CTAB bilayer- capped Au NRs are positively charged and stabilised in aqueous solutions, which are supported by the zeta potential results (Figure 5.2).

CTAB

N Br

NaBH4 CTAB + HAuCl4

Gold seed solution

Gold seed solution CTAB + HAuCl4 + AgNO3 Ascorbic acid

CTAB bilayer-capped Au NR Au NR

Figure 5.1 Schematic of the seed-mediated method for Au NRs synthesis.

121

Figure 5.2 Zeta potential of Au NRs.

Based on the theoretical photoionisation cross-section, XPS data revealed that the

Au:Ag:N:Br atomic ratios of the Au NRs were approximately 1:0.1:0.34:0.1. This confirmed the presence of silver on the Au NR surface, and silver ion is used to significantly improve the yields of Au NRs and provide the control of their aspect ratios

(Chen et al., 2013; Liu and Guyot-Sionnest, 2005; Nikoobakht and El-Sayed, 2003).

Jackson and co-workers (Jackson et al., 2014) also reported the presence of silver ions on the Au NRs surface, which showed no preference for a specific face or axis.

In Figure 5.3, the Ag 3d5/2 peak could be fitted with two components of equal width at

368.3 and 369.3 eV, and the Br 3d5/2 binding energy was 68.8 eV. The Ag 3d5/2 and Br

3d5/2 binding energies reported by Hubert and co-workers (Hubert et al., 2008) were

367.7 and 67.8 eV, respectively. It suggests the mean charge-shift of Ag was 1.1 eV, a value that was consistent with the charge-shift observed (1.0 eV) for Br. The Ag and Br binding energies are close to the energies observed in CTASB which is a CTAB silver

122 complex reported by Hubert and co-workers (Hubert et al., 2008). It may indicate the presence of CTA-Ag-Br2 complex on Au NRs, which plays an important role in the formation of Au NRs (Chen et al., 2013).

Ag3d

(a)

380 376 372 368 Binding Energy (eV)

Br3d

(b)

80 76 72 68 64 Binding Energy (eV)

Figure 5.3 The Ag 3d (a) and Br 3d (b) spectra of Au NRs.

123

In this research, Au NRs with spherical heads and high yields (Figure 5.4) were obtained by the addition of HCl solution to the seed-mediated growth solution. The overall reduction rate of Au3+ in the growth solution is reduced by the HCl solution, which can increase the aspect ratio of Au NRs (Guo et al., 2009).

A TEM image of a single crystal Au NR is presented in Figure 5.5. The gold lattice spacing and patterns on the {111} and {110} faces are consistent with the results in the literature (Nikoobakht and El-Sayed, 2003; Pérez-Juste et al., 2005), which also suggests that the Au NRs with spherical heads were successfully synthesised.

Au NPs with inhomogeneous shapes were obtained by adjusting the reactants in the growth solution. The TEM images of Au NPs with triangular, spherical and square outlines are shown in Figure 5.6.

Figure 5.7a presents the UV-vis absorption spectrum of the Au NRs solution. Two distinct localised surface plasmon resonance (LSPR) bands are observed at 510 nm and

730 nm, which are assigned to the transverse and the longitudinal oscillation of the electrons, respectively (Nikoobakht and El-Sayed, 2003). Au NRs (~100 nanorods) in

Figure 5.4b were measured by ImageJ software to obtain the average size and aspect ratio. The aspect ratio distribution of the Au NRs was plotted as a histogram and fitted with a Gaussian distribution as shown in Figure 5.7b. Finally, the average aspect ratio and length of the longitudinal axis are calculated to be 4.04 ± 0.08 and 53.5 ± 1.5 nm, respectively.

124

(a)

(b)

Figure 5.4 TEM images of Au NRs under different scale: (a) 50 nm; (b) 200 nm.

125

Figure 5.5 The TEM image of Au NR showing the gold lattice spacing under the scale of 2 nm.

126

(a)

(b)

Figure 5.6 TEM images of Au NPs under different scale: (a) 20 nm; (b) 50 nm.

127

1.0 (a)

0.5 Absorbance (AU)

0.0 400 500 600 700 800 900 Wavelength (nm)

25 Average Aspect Ratio = 4.04 ± 0.08

20 (b)

Frequency 10

0 2 4 6 Aspect Ratio

Figure 5.7 UV-vis absorption spectrum (a) and aspect ratio distribution (b) of Au NRs.

128

5.3 SERS of collectors on Au NRs

SERS is a powerful analytical tool for highly sensitive structural detection of molecules adsorbed on noble metal nanostructures (Sharma et al., 2012). It is generally accepted that the electromagnetic effects are the dominant contributors to most SERS processes, which mainly depend on the electric field enhancements provided by the metal surfaces

(Stiles et al., 2008). The electric field enhancements occur close to metallic surfaces when localised surface plasmon resonances are excited (Le Ru and Etchegoin, 2009).

The greatest field enhancements can be obtained when the plasmon frequency is in resonance with the excitation lasers.

Compared to other nanostructures, Au NRs are excellent SERS substrates due to the strong electric field enhancements and tunability of the plasmon resonances (Chen et al.,

2013). The highly curved ends of Au NRs present stronger electric field enhancements than other areas of the Au NRs (Pramod et al., 2007). It can be further improved by making two Au NRs close to each other in a linear fashion. Then the longitudinal plasmon oscillations can couple each other, and enhanced electric fields (hot spots) are formed between the gap region (Kumar and Thomas, 2011).

Au NRs synthesised in this research are ideal SERS substrates as the longitudinal localised surface plasmon resonance bands at 730 nm is close to the excitation lasers

(785 nm) used in this research. The SERS spectra of different concentrations of MBT− and DBDTP− ions on Au NRs were measured during this research.

In Figure 5.8, the SERS spectra of MBT− ions on Au NRs improved as the concentration of NaMBT solution increased. The SERS spectra of MBT− ions showed

129 no obvious difference with the Raman spectrum of solid NaMBT. A very weak band at

315 cm-1 was observed in the SERS spectra obtained from 10-3, 10-4 and 10-5 M NaMBT solution. This band is assigned to a bonding interaction between MBT− and Au NRs, the nature of which will be discussed later in the chapter. The main band was observed as an NCS ring vibration band near 1400 cm-1. This band could still observed in the SERS spectrum of 10-10 M NaMBT. It demonstrates that MBT molecules have strong SERS signals on Au NRs, which can be applied to investigate the interaction between NaMBT and Au NRs.

10-3 M NaMBT

10-4 M NaMBT

-5

10 M NaMBT

10-6 M NaMBT

10-7 M NaMBT

10-8 M NaMBT

10-9 M NaMBT

10-10 M NaMBT

500 1000 1500 -1 Raman shift (cm )

Figure 5.8 SERS spectra of MBT− ions on Au NRs in the range of 300 to 1700 cm-1.

130

The SERS spectra of DBDTP− ions on Au NRs are presented in Figure 5.9. Compared to MBT− ions, the signal intensity of the SERS spectrum was very low until the concentration of DBDTP− ions reached 10-3 M. The lower intensity is a result of

DBDTP− generating much weaker SERS signals than MBT− does. In the SERS spectrum of 10-3 M NaDBDTP, the highest intensity band at 833 cm-1 can be assigned to the P−O−C symmetric stretch of DBDTP−. The band at 560 cm-1 in the Raman spectrum of solid NaDBDTP was replaced by two bands at 531 and 615 cm-1. These two bands are assigned to the PS2 antisymmetric and symmetric stretch, which may also indicate that covalent bonding between DBDTP− and Au NRs can occur.

10-3 M NaDBDTP

10-4 M NaDBDTP

10-5 M NaDBDTP

500 1000 1500 2000 2500 3000 3500 -1 Raman shift (cm )

Figure 5.9 SERS spectra of DBDTP− ions on Au NRs.

131

5.4 The interaction of collectors with gold nanorods

The surface of Au NRs can be modified by molecules such as thiols, disulfides and dithiocarbamates for various applications (Caswell et al., 2003; Huff et al., 2007; Kou et al., 2007). These molecules have been found to bond preferentially to the growth ends of Au NRs at low concentrations, which is attributed to the lower density of CTAB at the highly curved ends (Chang et al., 2005; Thomas et al., 2004). It is proposed that thiol molecules bond to gold surfaces by strong semi-covalent Au−S bonds (Love et al.,

2005). However, few studies have been done to observe this semi-covalent Au−S bond using vibrational spectroscopy. Small thiol molecules bonding to the Au NRs normally result in low dispersibility and irreversible aggregation of Au NRs. The reason for this is attributed to the attractive force between Au NRs overcoming the weak steric effect of small thiol molecules (Chen et al., 2013).

The mechanism of the end-to-end assembly of Au NRs to nanochains in the presence of dithiols has been studied by Shibu Joseph’s group using absorption spectroscopy and

TEM (Shibu Joseph et al., 2006). They reported a stepwise formation mechanism of nanochains (Figure 5.10). During the first incubation step, one of the thiol groups of dithiol preferentially bond to the ends of the Au NRs with the other thiol group free.

The incubation step occurs before the concentration of dithiols reaches a critical value.

When the concentration of dithiols is above the critical value, dimers of Au NRs are first formed in an end-to-end fashion and finally assembled as oligomers/nanochains.

This is called the dimerisation and oligomerisation step. After this period, the precipitation of nanochains can be observed by the addition of excess dithiols (Pramod and Thomas, 2008).

132

S H

Step 1 S

S S

S ( ( Dithiols n n ( Below critical (

concentration S S S

S H H

Step 2a Dithiols above

critical concentration

S SH S S

S (

H S S S S

S S

S SH S

( H S n/2

S Step 2b

H S S

S H S

S S

S S S S S S

S S S S

S S

Excess dithiols

Precipitation

Figure 5.10 Schematic of the formation of nanochains (after Shibu Joseph et al., 2006 ).

133

5.4.1 Absorption and vibrational spectroscopy

In order to investigate the interaction between Au NRs and collectors, Au NRs solutions were mixed with two different concentrations (10-3 M and 10-5 M) of NaMBT and

NaDBDTP solution, respectively. Au NPs (with inhomogeneous shapes) solution was also mixed with 10-3 M NaMBT solution as a comparison of the Au NRs solution.

The UV-vis absorption spectra were recorded at 30 min and 120 min after the addition of the collectors. In Figure 5.11, the longitudinal plasmon absorption bands in the absorption spectra of the mixed solutions were all observed at around 730 nm after 30 min of reaction, which indicates that Au NRs still remain isolated in all mixed solutions.

After 120 min of reaction, no significant changes were observed in the absorption spectra of the mixed solutions with lower collector concentrations (10-5 M). This indicates that the interaction between Au NRs and collectors stay at the incubation step in the low collector concentration solutions as mentioned above.

No plasmon resonance band was observed in the absorption spectrum of the Au NRs and 10-3 M NaMBT mixed solution after 120 min. The mixed solution turned from red to colourless and black precipitates were observed and finally settled from the solution

(Figure 5.12), which was consistent with the formation of Au nanochains that precipitated in the higher concentration (10-3 M) NaMBT solution. Thus, it indicates that the MBT molecules are similar to dithiol molecules in that they interlock the Au

NRs to form the Au nanochains.

134

Compared to NaMBT, no precipitate was formed in the mixed solution of Au NRs and

10-3 M NaDBDTP solution after 120 min. A decrease of the longitudinal plasmon absorption band was observed in the absorption spectrum, which may be a result of the formation of Au nanochains in the mixed solution (Shibu Joseph et al., 2006). As described previously, the addition of excess dithiols can result in the precipitation of nanochains. In this research, however, no precipitation was observed even when the Au

NRs solution was mixed with a relatively high concentration of NaDBDTP solution.

Thus, it may indicate that there are insignificant Au nanochains formed in the solution.

The dithiophosphate molecule may only bond to a single Au NR and be unable to play the role of dithiol molecules as a linkage for the formation of Au nanochains.

135

1.0

Au NRs (a) -3 Au NRs + 10 M NaMBT (30 min) -3 Au NRs + 10 M NaMBT (120 min) -5 Au NRs + 10 M NaMBT (30 min) -5 Au NRs + 10 M NaMBT (120 min)

0.5 Absorbance (AU) Absorbance

0.0 400 500 600 700 800 Wavelength (nm)

1.0

Au NRs (b) -3 Au NRs + 10 M NaDBDTP (30 min) -3 Au NRs + 10 M NaDBDTP (120 min) -5 Au NRs + 10 M NaDBDTP (30 min) -5 Au NRs + 10 M NaDBDTP (120 min)

0.5 Absorbance (AU) Absorbance

0.0 400 500 600 700 800 Wavelength (nm)

Figure 5.11 UV-vis absorption spectra of Au NRs solutions with different concentrations of NaMBT (a) and NaDBDTP (b) solutions after 30 and 120 min of the addition of the collector.

136

(a) (b) (c)

Figure 5.12 Au NRs solution (a) and Au NRs mixed with 10-3 M NaMBT solution after 120 min (b) and 180 min (c).

137

Black precipitates were also obtained when the Au NPs (with inhomogeneous shapes) solution was mixed with 10-3 M NaMBT solution. The Raman spectra of black precipitates and AuMBT do not vary substantially (Figure 5.13). One difference was observed for the NCS ring vibration band near 1400 cm-1. This band became a shoulder peak in the Raman spectrum of Au NPs/MBT precipitates. A very weak band at 314 cm-

1 was also observed in the Raman spectra, which has a shift of 3 cm-1 from the Au−S stretch vibration band at 311 cm-1 in the Raman spectrum of AuMBT. It could be a result of the semi-covalent Au−S bond or Au−N bond between Au NRs and MBT molecules. These differences are indicative that the bonding interactions between Au and other atoms in the precipitates are different with that in AuMBT.

The infrared spectra of Au NRs/MBT precipitates and CTAB are presented in Figure

5.14. Most of the characteristic bands of CTAB were observed in the infrared spectra of

Au NRs/MBT precipitates confirming the presence of CTAB on Au NRs surfaces. This observation is consistent with the CTAB-capped Au NRs theory mentioned above. No characteristic Raman band for CTAB was detected in the Raman spectrum of Au

NRs/MBT precipitates with the exception of a very weak band near 2900 cm-1 that may be attributable to a C−H stretch. That is probably because only the ammonium head group of the CTAB molecule was close enough to Au NRs for the generation of SERS signals. N−H of the ammonium group is a very poor Raman scatterer and could not be detected by Raman spectroscopy.

138

Au NPs/MBT Precipitates

Au NRs/MBT Precipitates

AuMBT

400 600 800 1000 1200 1400 1600 -1 Raman shift (cm )

Figure 5.13 Raman spectra of AuMBT, Au NRs/MBT and Au NPs/MBT precipitates.

139

CTAB

Au NRs/MBT Precipitates

500 1000 1500 2000 2500 3000 3500 -1 Wavenumber (cm )

Figure 5.14 FTIR spectra of CTAB and Au NRs/MBT precipitates.

140

5.4.2 XPS

The surface atomic ratios of this complex for Au+Ag:S:N were 1:16.6:12.3. The S atoms are predominantly contributed by MBT compounds in which the atomic ratio of

S:N is roughly 2:1 according to the photoelectron spectra via theoretical photoionisation cross-sections. In addition, the Au:N atomic ratio of Au NRs was approximately 1:0.34 indicating that the ratio of N atom was relatively high in the Au NRs/MBT precipitate.

No Br was found in the Au NRs/MBT precipitate and the relative ratio of Ag atoms was relatively low in the complex. It is apparent that CTA-Ag-Br2 was not observed in the complex in contrast to the result reported by Hubert and co-workers (Hubert et al.,

2008). Thus, the high ratio of N atom observed for complex could be attributed to the formation of either CTA-MBT or the MBT-CTA-Ag-MBT complex in the Au

NRs/MBT precipitate. The XPS spectra of Au NRs/MBT precipitate were more complicated than expected for a surface monolayer. The assignment of Au 4f, S 2p and

N 1s binding energies of Au NRs/MBT complex are presented in Table 5.1.

Table 5.1 The assignment of Au 4f, S 2p and N 1s binding energies of Au NRs/MBT.

Peak Binding Energy (eV) Assignment

Au 4f7/2 84.33 Au−CTA

Au 4f7/2 85.21 Au−CTA

Au 4f7/2 86.48 Au−MBT

2− S 2p3/2 161.14 S species

S 2p3/2 161.89 S in AuMBT or MBTCTA

S 2p3/2 163.57 S in AuMBT or MBTCTA

141

Peak Binding Energy (eV) Assignment

S 2p3/2 164.36 Non-interacting endocyclic S of MBT

N 1s 398.18 N in AuMBT or MBTCTA

N 1s 399.23 Non-interacting N of MBT

N 1s 402.32 Au−CTA

N 1s 403.43 Au−CTA

In Figure 5.15a, the Au 4f spectrum of the Au NRs/MBT complex required three doublets at the 4f7/2 binding energies of 84.33 eV (65%), 85.21 eV (33%) and 86.48 eV

(2%) for an adequate fit. The lowest 4f7/2 binding energy at 84.33 eV is slightly higher than that value for Au metal (84.0 eV), which may suggest strong interactions between

Au and other atoms. Compared to the Au 4f spectrum of Au NRs (Figure 5.15b), the highest 4f7/2 binding energy at 86.48 eV is probably caused by the interaction between

Au atoms and MBT species. None of the binding energies were the same as that observed for AuMBT (84.86 eV), which is consistent with the Raman result in this research.

The S 2p spectrum of the Au NRs/MBT complex is presented in Figure 5.16a. It is made up of four doublets with 2p3/2 peaks at 161.14 eV (23%), 161.89 eV (13%),

163.57 eV (44%) and 164.36 eV (20%). In Figure 5.17, the double at 164.36 eV was also observed in the S 2p spectra of AuMBT (164.24 eV) and (MBT)2 (164.26 eV).

Thus, it can be assigned to the non-interacting endocyclic S, which is consistent with the assignment made by Szargan and co-workers (Szargan et al., 1999).

142

Compare to the 2p3/2 peak of the exocyclic S in AuMBT (162.78 eV), the doublets at

161.89 eV and 163.57 eV suggest at least two chemisorbed S environments in the Au

NRs/MBT complex. They can be contributed by the Au−S bonding or the CTA−MBT complex. Beattie and co-workers (Beattie et al., 2009) also observed two chemisorbed S environments in the S 2p spectrum of MBT absorbed on gold surfaces. They concluded that the doublet at 162.0 eV was contributed by the bonding between exocyclic S and gold only, while the doublet at 162.6 eV was caused by the interaction of both exocyclic

S and N with gold in a chelating fashion. The doublet at 161.14 eV can be assigned to

2− S species due to the rupture of S–S bond in (MBT)2 (Schaufuß et al., 1998).

In Figure 5.16b, the N 1s spectrum required four main components to account for ~96% of the intensity with the binding energies of 398.18 eV (36.4%), 399.23 eV (13.6%),

402.32 eV (37.9%) and 403.43 eV (8.5%). The two higher binding energies can be contributed by the CTA+ capping the gold surface, which were also observed in the N 1s spectrum of Au NRs in this research. The component at 399.23 eV is assigned to the non-interacting N of MBT−, while the component at 398.18 eV can be caused by the

CTA-MBT complex or the interaction between MBT and Au NRs through N atom

(Beattie et al., 2009).

Based on the XPS results and literature (Beattie et al., 2009; Lee et al., 2007; Parham et al., 2015; Yang et al., 2008), a possible mechanism for the formation of Au NRs/MBT precipitates is presented in Figure 5.18. The exocyclic S of MBT could chemisorb on a gold nanorod surface, while the corresponding endocyclic S or N might adsorb on another gold nanorod surface to form the Au nanochain. Then Au NRs/MBT precipitates were formed with the addition of excess MBT−.

143

Au4f

(a)

92 88 84 80 Binding Energy (eV)

Au4f

(b)

92 88 84 80 Binding Energy (eV)

Figure 5.15 The Au 4f spectra of Au NRs/MBT complex (a) and Au NRs (b).

144

S2p

(a)

172 168 164 160 Binding Energy (eV)

N1s

(b)

408 404 400 396 Binding Energy (eV)

Figure 5.16 The S 2p (a) and N 1s (b) spectra of Au NRs/MBT complex.

145

S2p

(a)

172 168 164 160 Binding Energy (eV)

S2p

(b)

172 168 164 160 Binding Energy (eV)

Figure 5.17 The S 2p spectra of AuMBT (a) and (MBT)2 (b).

146

N S C S N C S

S S

C S N N C S Excess S Au NRs/MBT precipitates Au NR MBT MBT

S S C S N C S

N S

C N S

Figure 5.18 Schematic of the reaction between Au NRs and NaMBT.

147

5.5 The interaction of collectors with gold electrode surfaces

5.5.1 Voltammetry

The voltammograms of gold electrode in deoxygenated buffer solution (pH 9.2) containing 0 M, 10-4 M and 10-3 M of NaMBT and NaDBDTP are presented in Figure

5.19. All potentials are reported against the Ag/AgCl 3 M KCl reference electrode.

Experiments were conducted at an ambient temperature of 20 °C. The sweep rate was

20 mV/s and the area of working electrode was 0.5 cm2. In the solutions without collectors, the formation of gold oxide/hydroxide layer was observed at potentials above about 0.6 V and the corresponding reduction peak was observed at about 0.4 V (Woods,

1996).

With the addition of NaMBT, the formation of gold oxide/hydroxide layer is inhibited and the oxidation peak was observed at about 0.5 V in 10-3 M NaMBT. The anodic current was observed at potentials above about 0.2 V in both 10-4 M and 10-3 M of

NaMBT. It is consistent with the potential value observed in 10-2 M of HMBT (pH 9.2) by Woods and co-workers (Woods et al., 2000). The oxidation reaction is presented in

Equation 5.1.

− − 2 MBT = (MBT)2 + 2e (5.1)

-3 The oxidation peak of (MBT)2 in 10 M NaMBT solution is much higher than that in

10-4 M NaMBT solution, which indicates that more MBT− ions were oxidized in the solution with a higher NaMBT concentration. No cathodic current was observed for the reduction of (MBT)2. It suggests that a resistive coating of multilayer (MBT)2 was

148 formed on the surface of gold electrode, and this coating was not reduced during the voltammetric scanning.

No current can be assigned to the MBT− adsorption or the AuMBT formation. These reactions possibly start at potentials below the investigated potential region, which may lead to the coverage of MBT− on the gold surface before the scans were commenced

(Woods et al., 2000).

The voltammograms of gold electrode in NaDBDTP are similar to those in NaMBT. No obvious features can be attributed to the adsorption of DBDTP−, which can be caused by the same reason with NaMBT. The anodic current observed at potentials above 0.1 V was attributed to the formation of (DBDTP)2 on the gold surfaces.

149

0 M NaMBT 50 -4 10 M NaMBT -3

10 M NaMBT

)

A 

( 0

Current Current

-50

(a)

-0.5 0.0 0.5 1.0 Potential (V)

0 M NaDBDTP 50 -4 10 M NaDBDTP -3

10 M NaDBDTP

)

A 

( 0

Current Current

-50

(b)

-0.5 0.0 0.5 1.0 Potential (V)

Figure 5.19 Voltammograms of gold electrode in solution of pH 9.2 containing 0, 10-4 and 10-3 M of NaMBT (a) and NaDBDTP (b), respectively.

150

5.5.2 SERS spectra

5.5.2.1 NaMBT

SERS spectra that were recorded in situ under potential control in 10-3 M NaMBT solutions (pH 9.2) are presented in Figure 5.20. These spectra were obtained by increasing the potential stepwise from −0.7 V to 0.5 V, with the electrode potential being held for 5 min at each point before measurement. The SERS spectra for NaMBT exhibit no obvious change with potential. The spectra obtained are similar to the Raman spectrum of AuMBT with the NCS ring stretch near 1400 cm-1 being the major difference (Figure 5.21). The NCS vibration is shifted to lower wavenumbers by 5 cm-1 and another band at 1406 cm-1 is also observed. These are attributed to the presence of two different bonding types between MBT− and the gold electrode surface.

Compared to the normal Raman spectrum of AuMBT, the bands at 713 cm-1 (C−S stretch) and 1241 cm-1 (NCS ring stretch) are both enhanced and had higher relative intensities in the SERS spectra. According to the electromagnetic effects mechanism of

SERS (Le Ru and Etchegoin, 2009), it may indicate that the NCS ring of MBT− is bonded closely to the gold electrode surfaces through the endocyclic S or N atom.

It has been reported that these S and N atoms of MBT could bond to metals such as gold, silver and zinc (Lee et al., 2007; Yang et al., 2008). Beattie and co-workers (Beattie et al., 2009) concluded that MBT molecules could interact with gold surface through both the exocyclic S and N atoms based on their XPS results. The adsorption of MBT on the

Au (111) surface has been investigated by Cui and co-workers using electrochemical scanning tunnelling microscopy and cyclic voltammetry (Cui et al., 2011). Contrary to

151

Beattie’s conclusion, they believed that MBT molecules adsorbed on Au (111) surfaces through both the exocyclic and endocyclic S atoms.

Several SERS studies of organosulfur compounds adsorbed on gold surfaces have been reported by various authors (Hassan and Holze, 2012; Joo et al., 2000; Kang et al., 2009;

Szafranski et al., 1998). They all observed the presence of Au−S bond between organosulfur compounds and gold surfaces. The adsorption of MBT on SERS active gold and silver surfaces has been studied by Santa and co-workers (Santa et al., 2010).

They confirmed the presence of the covalent Au−S bond by the S 2p spectrum of MBT absorbed on gold surfaces. In this research, the weak band at 311 cm-1 in the SERS spectra can be contributed by the Au−S stretching vibration, which is consistent with that in the Raman spectrum of AuMBT.

No obvious evidence was observed for the existence of the Au–N stretching vibration in the SERS spectra, which may be at around 330 cm-1 as suggested by Jurkiewicz-

Herbich and co-workers (Jurkiewicz-Herbich et al., 2002). Thus, it is concluded that only the two S atoms of MBT can bond to the gold electrode surfaces (Figure 5.22), which is consistent with the interaction between MBT and Au NRs.

152

0.5 V

0.3 V

-0.1 V

-0.4 V

-0.7 V

AuMBT

500 1000 1500 2000 2500 3000 3500 -1 Raman shift (cm )

Figure 5.20 SERS spectra of gold electrode in 10-3 M NaMBT under different potential control compared with Raman spectrum of AuMBT.

153

0.5 V

0.3 V

-0.1 V

-0.4 V

-0.7 V

AuMBT

400 -1 1400 Raman shift (cm )

Figure 5.21 SERS spectra of gold electrode in 10-3 M NaMBT under different potential control compared with Raman spectrum of AuMBT in the range of 300 to 450 cm-1 and

1300 to 1450 cm-1.

N S

C C S S N S Au

Figure 5.22 Schematic diagram of the adsorption of MBT on Au surfaces.

154

As pointed out above, the oxidation of MBT− was observed at potentials above about

0.2 V in the voltammograms of gold electrode with 10-3 M NaMBT. In Figure 5.20, no spectrum of (MBT)2 was observed on gold electrodes, which is consistent with the studies of Woods and co-workers (Woods et al., 2000). They suggested that the investigated potential region in SERS experiments were below the potential region which could form (MBT)2. The final potential (0.5 V) in this research, however, is much higher than the initial oxidation potential mentioned above, and the spectra of

− (MBT)2 was unlikely to be concealed by the MBT spectra. It is possible that the

(MBT)2 molecule was not close enough to the gold electrode surface due to the

− coverage of MBT . Thus, no SERS signal of (MBT)2 was generated and observed during the stepped potential increasing process.

In order to eliminate the effect of MBT− coverage which formed rapidly under negative potential control, SERS spectra for a gold electrode were also recorded directly under potential control of 0.3 V in 10-3 M and 10-4 M NaMBT (Figure 5.23), respectively.

-1 -1 -1 Three distinctive bands of (MBT)2 at 535 cm , 1431 cm and 1467 cm were observed in the SERS spectra accompanied by the presence of SERS spectra of MBT− (Figure

5.24). No band can be assigned to the S−S stretch which was observed at 445 cm-1 in

-1 the Raman spectrum of (MBT)2. Meanwhile, the weak band at 311 cm contributed by the Au−S stretching vibration was absent in these SERS spectra.

When the potential was decreased to −0.7 V after holding at 0.3 V for 5 min, the SERS

-4 spectra showed an obvious decline of the bands of (MBT)2 in the 10 M NaMBT solution (Figure 5.24). This spectrum was similar to the SERS spectra of MBT−, and was indicative of the degradation of (MBT)2 on the gold electrode. However, the SERS

155 spectra did not differ significantly when the potential was decreased to −0.7 V in 10-3 M

NaMBT solution. This could be due to the formation of multilayer (MBT)2 on a gold electrode in high concentration NaMBT solution. No other SERS spectra were observed under the coverage of the (MBT)2.

156

0.3 V decrease to -0.7 V (10-3 M)

0.3 V (10-3 M)

0.3 V decrease to -0.7 V (10-4 M)

0.3 V (10-4 M)

(MBT) 2

AuMBT

500 1000 1500 2000 2500 3000 3500 -1 Raman shift (cm )

Figure 5.23 SERS spectra of gold electrode in 10-3 M and 10-4 M NaMBT under different potential control compared with Raman spectra

of AuMBT and (MBT)2.

157

0.3 V decrease to -0.7 V (10-3 M)

0.3 V (10-3 M)

0.3 V decrease to -0.7 V (10-4 M)

0.3 V (10-4 M)

(MBT) 2

AuMBT

500 1200 1400 -1 Raman shift (cm )

Figure 5.24 SERS spectra of gold electrode in 10-3 M and 10-4 M NaMBT under different potential control compared with Raman spectra of AuMBT and (MBT)2 in the range of 300 to 550 cm-1 and 1300 to 1500 cm-1.

158

5.5.2.2 NaDBDTP

In Figure 5.25, the SERS spectra of gold electrode in 10-3 M NaDBDTP under potential control were compared with Raman spectrum of (DBDTP)2 in the range of 400 to 1500 cm-1. Similarly, these spectra were obtained by increasing the potential stepwise from

−0.7 V to 0.3 V being held for 5 min at each potential before measurement.

Few bands attributable to NaDBDTP were observed in these SERS spectra. A band at

830 cm-1 was observed in the SERS spectra at negative potentials with the highest intensity. It can be assigned to the P−O−C symmetric stretch of DBDTP−, which is consistent with the SERS spectra of NaDBDTP on Au NRs. In addition, two bands at about 620 cm-1 and 530 cm-1 were also observed in the SERS spectra at negative potentials, which can be contributed by the PS2 antisymmetric and symmetric stretch.

Thus, it suggests that DBDTP− bonds to the gold electrode surfaces through two S atoms as presented in Figure 5.26.

Contrary to NaMBT, (DBDTP)2 was observed in the SERS spectra at positive potential

(0.3 V) during the stepped potential increase process. Three bands observed at 660, 505

-1 and 482 cm can be assigned to the P=S, S−S and P−S stretch of (DBDTP)2. These three distinctive bands of (DBDTP)2 declined after the potential decreased to −0.7 V, which indicates the degradation of (DBDTP)2 on the gold electrode.

159

0.3 V decrease to -0.7 V

482 660

0.3 V 505

-0.1 V

-0.3 V

-0.7 V 483 663

(DBDTP) 506 2

500 1000 1500 -1 Raman shift (cm )

Figure 5.25 SERS spectra of gold electrode in 10-3 NaDBDTP under different potential

-1 control compared with Raman spectrum of (DBDTP)2 in the range of 400 to 1500 cm .

O O

P S S

Figure 5.26 Schematic diagram of the adsorption of DBDTP on Au surfaces.

160

5.6 Flotation

The Au NRs experiments were utilized to investigate the surface behaviour of ultra fine gold particles that may be present in mineral flotation processing of fine gold containing ore. As pointed out above, both NaMBT and NaDBDTP molecules can bond to the Au

NRs and Au electrode surfaces. As the flotation test of ultra fine gold particles could not be undertaken, larger particulate gold was used in the flotation tests, an area that is also of interest in mineral processing.

Flotation behaviour of gold metal particles with diameter of ~100 μm was investigated in different concentrations of NaMBT and NaDBDTP solutions, respectively. The flotation tests were first conducted in pH 9.2 solutions without potential control, and the results were listed in Table 5.2. The flotation behaviour of gold metal particles was observed to vary to an insignificant extent under the different gases used in the flotation cell. This indicates that oxygen is not critically involved in reactions taking place on the gold surface. These results are in agreement with the observations of Woods and co- workers (Woods et al., 2000).

In the flotation tests without potential control, no gold metal particles were floated in

NaMBT solutions regardless of the concentrations. It is possible that the MBT coverage might have been too low to achieve a sufficiently hydrophobic surface of the gold metal particles for floatability. Contrary to NaMBT, the gold metal particles were all floated in NaDBDTP solutions under different conditions (Figure 5.27). It indicates that the coverage of DBDTP on the gold surfaces could lead to the floatability of the gold particles.

161

Table 5.2 Flotation of gold particles tests in pH 9.2 solutions without potential control.

Concentration NaMBT NaDBDTP (M) O2 N2 Air O2 N2 Air

10-3 ×a × × √b √ √

10-4 × × × √ √ √

10-5 × × × √ √ √

10-6 × × × √ √ √ a Gold particles non-floated. b Gold particles floated.

Figure 5.27 Floated gold particles in Hallimond tube.

The potential control flotation experiments involved increasing the potential stepwise from −0.7 V to 0.7 V (Table 5.3). The gold metal particles floated at potentials above

162 approximately 0.5 V in both 10-3 and 10-4 M NaMBT solutions. This was consistent with (MBT)2 beginning to be formed when the potential reached 0.3 V as observed in the SERS studies. The floated gold metal particles were analysed by FTIR spectroscopy, and Figure 5.28 presents a spectrum that exhibits the characteristic bands of (MBT)2 on the gold metal particle surface. Because the adsorption of (MBT)2 was much more extensive than a chemisorbed monolayer, it was concluded that the former was responsible for the gold particles becoming hydrophobic and being floated in the cell.

The gold particles were all floated in NaDBTP solutions under different conditions, which is consistent with the results obtained in flotation tests without potential control.

Compared to NaMBT, no FTIR spectra were observed from gold particles floated by

NaDBDTP. It is probably due to the low coverage and intensity of DBDTP molecules.

Table 5.3 Flotation of gold particles tests in pH 9.2 solutions with potential control.

Potential NaMBT NaDBDTP (V) 10-3 M 10-4 M 10-3 M 10-4 M

-0.7 ×a × √b √

-0.5 × × √ √

-0.3 × × √ √

0.1 × × √ √

0.3 × × √ √

0.5 √ √ √ √

0.7 √ √ √ √ a Gold particles non-floated.

b Gold particles floated.

163

(MBT)

Floated Gold Particles

600 800 1000 1200 1400 Wavenumber (cm-1)

Figure 5.28 FTIR spectra of (MBT)2 and floated gold particles in the range of 500 to

1500 cm-1.

164

5.7 Conclusions

The interaction of NaMBT and NaDBDTP with bulk gold surfaces, Au NPs and 100 micron gold particles has been investigated. The adsorption of MBT on Au NRs was observed and it was deduced that the exocyclic S atom of MBT could chemisorb on a gold nanorod growth surface through a Au−S bond, while the endocyclic S or N atom of the same MBT adsorbed on another gold nanorod surface. Similarly, the two S atoms of

MBT can bond to the gold electrode surfaces. No obvious evidence was found for the adsorption of DBDTP on Au NRs surfaces, while it is concluded that DBDTP can bond to the gold electrode surfaces through two S atoms.

The SERS spectroscopy showed the formation of (MBT)2 on the bulk gold electrode surface above applied potentials of 0.3 V. Degradation of (MBT)2 was observed in lower concentration (10-4 M) of MBT when the potential decreased to −0.7 V, while no change was seen in a higher concentration (10-3 M) of NaMBT. The adsorption of

− DBDTP and formation of (DBDTP)2 were observed in the SERS spectra of gold electrodes during the stepped potential increase process.

The flotation behaviour of gold metal particles was observed to vary to an insignificant extent under the different gases used in the flotation cell. This indicates that oxygen is not critically involved in reactions taking place on the gold surface. Gold metal particles were floated at potentials above approximately 0.5 V in both 10-3 and 10-4 M NaMBT solutions. (MBT)2 was observed on the gold particles, which was associated with the flotation of the gold metal particles. Gold metal particles showed good floatability with the presence of NaDBDTP under various conditions.

165

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Caswell, K.K., Wilson, J.N., Bunz, U.H.F., Murphy, C.J., 2003. Preferential end-to-end

assembly of gold nanorods by biotin−streptavidin connectors. Journal of the

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Chang, J.-Y., Wu, H., Chen, H., Ling, Y.-C., Tan, W., 2005. Oriented assembly of Au

nanorods using biorecognition system. Chemical Communications, 1092-1094.

Chen, H., Shao, L., Li, Q., Wang, J., 2013. Gold nanorods and their plasmonic

properties. Chemical Society Reviews 42, 2679-2724.

Cui, B., Chen, T., Wang, D., Wan, L.-J., 2011. In situ STM evidence for the adsorption

geometry of three N-heteroaromatic thiols on Au(111). Langmuir 27, 7614-7619.

Gou, L., Murphy, C.J., 2005. Fine-tuning the shape of gold nanorods. Chemistry of

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Guo, Z.R., Gu, C.R., Fan, X., Bian, Z.P., Wu, H.F., Yang, D., Gu, N., Zhang, J.N., 2009.

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Hassan, N., Holze, R., 2012. Surface enhanced Raman spectroscopy of self-assembled

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bromide complex as the capping agent of gold nanorods. Langmuir 24, 9219-9222.

Huff, T.B., Hansen, M.N., Zhao, Y., Cheng, J.-X., Wei, A., 2007. Controlling the

cellular uptake of gold nanorods. Langmuir 23, 1596-1599.

Jackson, S.R., McBride, J.R., Rosenthal, S.J., Wright, D.W., 2014. Where’s the silver?

Imaging trace silver coverage on the surface of gold nanorods. Journal of the

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Joo, S.W., Han, S.W., Kim, K., 2000. Adsorption characteristics of 1,3-propanedithiol

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Jurkiewicz-Herbich, M., Slojkowska, R., Zawada, K., Bukowska, J., 2002.

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Kang, H., Noh, J., Ganbold, E.-O., Uuriintuya, D., Gong, M.-S., Oh, J.J., Joo, S.-W.,

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Colloid and Interface Science 336, 648-653.

Kou, X., Zhang, S., Yang, Z., Tsung, C.-K., Stucky, G.D., Sun, L., Wang, J., Yan, C.,

2007. Glutathione- and cysteine-induced transverse overgrowth on gold nanorods.

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Kumar, J., Thomas, K.G., 2011. Surface-enhanced Raman spectroscopy: Investigations

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Liu, M., Guyot-Sionnest, P., 2005. Mechanism of silver(I)-assisted growth of gold

nanorods and bipyramids. The Journal of Physical Chemistry B 109, 22192-22200.

Lohse, S.E., Murphy, C.J., 2013. The quest for shape control: A history of gold nanorod

synthesis. Chemistry of Materials 25, 1250-1261.

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171

Chapter 6 Electrochemical study of chalcopyrite

172

6.1 Preamble

Macroscopic and submicron gold particles are dispersed in gold-bearing sulfide minerals such as pyrite, chalcopyrite and bornite (Allan and Woodcock, 2001; Mikhlin and Romanchenko,

2007; O'Connor and Dunne, 1994; Xie et al., 2016). The flotation behaviour of these minerals should also been taken into account when optimising the recovery of gold from ore deposits.

The electrochemical mechanism plays an important role in sulfide mineral flotation (Fuerstenau et al., 2007; Tolley et al., 1996), while the electrochemical properties of the minerals impact on the flotation behaviour.

In this chapter, electrochemical techniques have been used to investigate the open circuit potential (OCP) and controlled potential oxidation of chalcopyrite. The voltammetry of chalcopyrite electrodes with fractured and an abraded (modified) surface have also been studied.

173

6.2 Voltammetry

6.2.1 Voltammogram of chalcopyrite

The voltammogram of chalcopyrite from Messina with an abraded surface in deoxygenated buffer solution (pH 9.2) is presented as Figure 6.1. All potentials are reported against the Ag/AgCl 3 M KCl reference electrode. The electrochemical behaviour of the chalcopyrite is similar to those observed in the literature (Gardner and

Woods, 1979; Velasquez et al., 1998; Yin et al., 1995). In the positive-going sweep, an anodic pre-wave is observed at 0.07 V, which can be caused by the following reactions proposed by Yin and co-workers (Yin et al., 2000).

− − 2CuFeS2 + 6xOH → 2CuFe1− xS2 + xFe2O3 + 3xH2O + 6xe (6.1)

− − 2CuFeS2 + 6OH → 2CuS2* + Fe2O3 + 3H2O + 6e (6.2)

− − 2CuFeS2 + 3OH → CuS2* + Fe(OH)3 + 3e (6.3)

CuFe1− xS2 and CuS2* represent metastable phases with a limiting composition of Cu:2S, which can form a passivation film with Fe2O3/Fe(OH)3.

The main anodic current is observed at potentials above about 0.2 V with a maximum at

0.6 V. It could be a result of the decomposition of the CuS2* passivation film as presented in Equation 6.4 and 6.5 (Yin et al., 2000).

− − CuS2* + 2OH → Cu(OH)2 + 2S + 2e (6.4)

− − CuS2* + 2OH → CuO + 2S + H2O + 2e (6.5)

174

During the negative-going sweep, two cathodic waves are observed at −0.17 and −0.4 V, which are attributed to the reduction of the anodic oxidation products. The cathodic currents are much weaker than anodic currents, which is caused by the coverage of

Fe2O3/Fe(OH)3 and CuS2* on the exposed chalcopyrite surface (Yin et al., 2000).

)

 (

Current Current

-40 -0.5 0.0 0.5 Potential (V)

Figure 6.1 Voltammogram of chalcopyrite electrode in pH 9.2 buffer solution.

175

6.2.2 Fractured vs abraded surfaces

The reproducible surface of the chalcopyrite is normally obtained by fracture and abrasion in electrochemical studies. Although a fresh and unchanged surface can be obtained by fracture, the in situ fracture process is difficult to achieve and requires a lot of mineral. Compared to the fracture process, abrasion with silicon carbide paper is a common used method and easier to achieve. During the abrasion process, the surface of chalcopyrite is oxidised and the oxidation products would be lost as a result of rinsing

(Buckley et al., 2010). Thus, the surface chemical composition of an abraded chalcopyrite electrode can be different from that of a fractured chalcopyrite electrode.

In this study, three chalcopyrite electrodes were prepared to investigate the differences between the fracture and abrasion method. An in situ fracture experiment was carried out to directly compare the fractured chalcopyrite electrode surface (obtained in cell) with the abraded surface (obtained outside the cell).

The chalcopyrite electrodes (Figure 6.2) were fabricated by the method mentioned in chapter 3. Each of them was made from a different piece of Messina chalcopyrite. The resin was scored to the chalcopyrite edge using a low-speed circular saw, leaving a small region intact to prevent premature breakage.

Figure 6.2 The chalcopyrite electrode prior to score and snap.

176

The pH 9.2 buffer solution was deoxygenated with high purity nitrogen for 30 min before the introduction of the electrode. In Figure 6.3, the electrode was inserted in the cell and fractured spontaneously due to contact with the cell or was fractured using a spatula under positive N2 pressure. The OCP of the fractured chalcopyrite electrode was recorded, and the voltammogram was commenced from the OCP in the positive direction with a sweep rate of 20 mV/s. The scanning potential range is from −0.6 V to

0.6 V.

Figure 6.3 The chalcopyrite electrode in cell post fracture.

177

After the experiment of the fractured surface, the chalcopyrite electrode (Figure 6.4) was wet-abraded outside the cell in air using P400 mesh SiC paper. Then the abraded chalcopyrite electrode was rinsed with DDI water prior to immersion in the buffer solution. The voltammogram was started from the new OCP using the same sweep rate and scanning potential range with that for the fractured surface.

(a) (b) (c)

Figure 6.4 The fractured chalcopyrite electrodes post experiment.

The OCPs of the chalcopyrite electrode with fractured and abraded surfaces are presented in Table 6.1. The voltammogram of a chalcopyrite electrode obtained from with fractured and abraded surfaces in pH 9.2 buffer solution are compared in Figure

6.5, respectively.

Table 6.1 The OCP of fractured and abraded chalcopyrite electrodes.

Chalcopyrite electrode Fractured surface (mV) Abraded surface (mV)

a 143 0

b -57 -178

c 0 -88

178

It can be seen that measurable differences in the OCP were observed for the three electrodes, however, the voltammograms of each chalcopyrite electrode for fractured and then abraded surfaces did not vary significantly. The magnitudes of the current passed by the fractured and abraded surfaces were also similar. The current differences between the fractured and abraded surfaces are attributed to the inconsistent real surface area as the current of the electrochemical process is proportional to the real surface area of the electrode (Doña Rodríguez et al., 2000).

The real surface area of the electrode is related to the surface roughness of the electrode.

The scanning electron microscope (SEM) images of fractured (a) and abraded (b) chalcopyrite electrodes obtained by Dr. Kym Watling in our group are presented in

Figure 6.6. Compared to the abraded surface, the fractured surface shows more macroscopic roughness at the same magnification. The rough surface of the fractured chalcopyrite electrode may contribute to the similar current observed for the fractured and abraded surfaces.

The anodic pre-wave was only observed in the voltammogram of the abraded chalcopyrite electrode (b), which also had the lowest OCP. It was consistent with the abraded surface of chalcopyrite electrode (b) being less oxidised and that CuS2* was formed on the surface as one of the oxidation products. It may also indicate that the observance of an anodic pre-wave is related to the OCP of the chalcopyrite electrode.

179

200 (a)

100

)



(

Current Current

Fractured

-100 Abraded

-0.5 0.0 0.5 Potential (V)

(b)

100

)



(

Current Current

Fractured Abraded

-0.5 0.0 0.5 Potential (V)

200 (c)

) 100



(

Current Current

Fractured Abraded

-0.5 0.0 0.5 Potential (V)

Figure 6.5 Voltammograms of fractured and abraded chalcopyrite electrodes.

180

(a)

(b)

Figure 6.6 SEM images of fractured (a) and abraded (b) chalcopyrite electrodes by Dr.

Kym Watling.

181

6.3 Open Circuit Potential

In this study, the OCPs of the chalcopyrite electrodes with abraded and artificially oxidised surfaces have been studied in a range of buffer solutions of pH 9.2. Three chalcopyrite working electrodes were prepared using chalcopyrite samples from

Messina, South Africa; Mt. Isa, Australia; and Butte, USA, respectively.

In order to investigate the effects of different electrolytes at the same pH value (9.2),

borate (0.05 M Na2B4O7), phosphate (0.1 M Na2HPO4) and carbonate (0.009 M

NaHCO3 and 0.001 M Na2CO3) solution were prepared using analytical reagents with

DDI water. The OCP was also studied under a nitrogen atmosphere to observe the inhibition of oxidation at low oxygen concentration.

Each of the three chalcopyrite electrodes was abraded in air using P400 mesh SiC paper and rinsed with DDI water prior to immersion in the electrolyte. The OCP of the chalcopyrite electrode with abraded surface was first measured in borate solution under a nitrogen atmosphere for 60 min. Then the chalcopyrite electrode was abraded outside the cell in air again before the measurement was conducted in borate solution under an air atmosphere. The same process was repeated for each OCP measurement in the phosphate and carbonate solution under the nitrogen and air atmosphere, respectively.

After the experiment of the abraded surface, the chalcopyrite electrode was fractured and boiled in DDI water under air atmosphere for 30 min. Then the OCP of the artificially oxidised chalcopyrite electrode was recorded. Similarly, the chalcopyrite electrode was also re-fractured and re-boiled before each experiment in different solution and atmosphere.

182

The abraded electrodes reveal a fresh chalcopyrite surface, while the boiled method emulates the high humidity and temperature of the true environmental conditions which may contribute to oxidation of the surface of stockpiled chalcopyrite.

6.3.1 OCP of chalcopyrite from Messina, South Africa

The OCPs of chalcopyrite from South Africa, measured in borate, phosphate and carbonate solution, compared under a range of conditions, are presented in Figure 6.7.

There are no significant differences among the OCP in these three solutions.

In all three solutions, it is obvious that the initial potentials of oxidised surface are higher than the abraded surface, regardless of air or nitrogen atmosphere. The oxidised chalcopyrite electrodes measured in air atmosphere solutions (black curves) have the highest initial potentials. The difference between the initial potentials of abraded and oxidised surfaces is contributed to the oxidation reactions occurred during the boiled procedure. The oxidation reactions can be represented by the equations mentioned in section 6.2.

The blue curves in Figure 6.7 show the OCP trends of abraded surface in air atmosphere solutions. The OCP of abraded surface in air atmosphere solutions has the greatest increment in all three solutions. It suggests that the abraded surface undergoes the most oxidation reactions in air atmosphere solutions. All curves increase rapidly in the first

10 min, which may indicate a reaction of similar type to the oxidation reactions in boiled water. After the initial rapid increase, the curve trends become almost the same

183 as that of the oxidised surface. The black and blue curves in Figure 6.7 (b) provide the best example of this trend, overlapping after about 10 min.

The pink curves represent the OCP of abraded surface in nitrogen-purged solutions, which have the lowest of both initial and final potentials. It indicates that there are less oxidation reactions in nitrogen atmosphere solution, which should be caused by the lack of oxygen. The pink curves in phosphate and carbonate solutions drop at first, then rise again, which can be due to the reduction reactions on the electrode surface.

184

(a) Borate

0.10

0.05

0.00

Potential (V) Potential

-0.05

oxidised, air oxidised, nitrogen

-0.10 abraded, air abraded, nitrogen

0 10 20 30 40 50 60 Time (min)

(b) Phosphate

0.00

-0.05 Potential (V) Potential

oxidised, air oxidised, nitrogen -0.10 abraded, air abraded, nitrogen

0 10 20 30 40 50 60 Time (min)

0.05

0.00

Potential (V) Potential -0.05

oxidised, air oxidised, nitrogen -0.10 abraded, air abraded, nitrogen

0 10 20 30 40 50 60 Time (min)

Figure 6.7 The OCPs of chalcopyrite from South Africa under different conditions: (a) in borate solution; (b) in phosphate solution; (c) in carbonate solution.

185

6.3.2 OCP of chalcopyrite from Mt. Isa, Australia

The OCP results of chalcopyrite from Australia are presented in Figure 6.8. The oxidised surface also has the highest initial potentials, as seen in the chalcopyrite from

South Africa. All OCP curves of this chalcopyrite rise slowly, with the exception of the abraded surface in borate solution under an air atmosphere. As shown in Figure 6.8 (a), the blue curve (abraded surface in borate under air) increases sharply in the first 5 min, then plateaus to reach a stable potential. This indicates that the oxidation reaction on the abraded surface is very rapid in borate solution. When the curve becomes steady, the reaction reaches equilibrium on the electrode surface. The OCP of abraded surfaces under nitrogen atmosphere, as seen in the pink curves, are still lower than those under air atmosphere.

The OCP of oxidised surfaces has the least increment in solutions under an air atmosphere. This may indicate that, after the boiling procedure, there are very few new surfaces left for reaction. Their initial potentials are all higher than 100 mV, within the range of the second oxidation peak suggested by Yin and co-workers (Yin et al., 2000).

There is the possibility of the formation of Fe2O3/Fe(OH)3 and CuS2* on the electrode surface, which may prevent further reactions on the electrode surface. As shown in

Figure 6.8 (a), the curve of the oxidised surface (red curve) in borate solution under nitrogen fluctuates in the first 10 min, which may be caused by a cycling of oxidation and reduction reactions.

The initial potentials of the Australian chalcopyrite are all higher than 20 mV, which is very different from the chalcopyrite of South Africa, indicating the possibility of different mineral composition.

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(a) Borate

0.15

0.10

Potential (V) Potential

oxidised, air oxidised, nitrogen

0.05 abraded, air abraded, nitrogen

0 10 20 30 40 50 60 Time (min)

(b) Phosphate

0.15

0.10 Potential (V) Potential

oxidised, air 0.05 oxidised, nitrogen abraded, air abraded, nitrogen

0 10 20 30 40 50 60 Time (min)

0.25

0.20

0.15

Potential (V) Potential 0.10

oxidised, air 0.05 oxidised, nitrogen abraded, air abraded, nitrogen 0.00 0 10 20 30 40 50 60 Time (min)

Figure 6.8 The OCPs of chalcopyrite from Australia under different conditions: (a) in borate solution; (b) in phosphate solution; (c) in carbonate solution.

187

6.3.3 OCP of chalcopyrite from Butte, USA

The OCP results of chalcopyrite from United States are shown in Figure 6.9. The initial potentials of oxidised surfaces are higher than those of the abrade surfaces as expected.

The OCP curve trends and initial potentials of this chalcopyrite are almost the same in three different solutions, which is inconsistent with the first two chalcopyrite electrodes.

The biggest difference is that the OCPs of the oxidised surfaces decrease in all solutions regardless of atmosphere. It indicates that some reduction reactions occurred on the electrode surfaces. As mentioned above, Fe2O3/Fe(OH)3 and CuS2* could have formed on the electrode surface as a result of the boiling process. The reactions possibly consist of the reduction of these oxidation compounds.

The OCP curve trends of abraded surfaces in solutions under an air atmosphere (blue curves) are very similar to that under a nitrogen atmosphere (pink curves), and the gap between them is very small. It suggests that the effect of nitrogen is not significant for this chalcopyrite electrode.

188

(a) Borate 0.15

0.10

0.05

Potential (V) Potential

0.00 oxidised, air oxidised, nitrogen abraded, air abraded, nitrogen -0.05 0 10 20 30 40 50 60 Time (min)

(b) Phosphate

oxidised, air oxidised, nitrogen abraded, air 0.10 abraded, nitrogen

0.05

Potential (V) Potential

0.00

-0.05

0 10 20 30 40 50 60 Time (min)

0.15

0.10

0.05

Potential (V) Potential

0.00

oxidised, air oxidised, nitrogen -0.05 abraded, air abraded, nitrogen

0 10 20 30 40 50 60 Time (min)

Figure 6.9 The OCPs of chalcopyrite from United States under different conditions: (a) in borate solution; (b) in phosphate solution; (c) in carbonate solution.

189

6.4 Controlled potential oxidation of Chalcopyrite

The voltammetry of oxidised chalcopyrite obtained by controlled potential techniques has also been studied in this research. As mentioned above, the passivation film was formed on the surface of the chalcopyrite electrode in both the pre-wave and bulk potential regions. Thus, the potential was controlled at 0.06 and 0.5 V to determine the time required for the formation of the pre-wave and bulk layer on the surface of the chalcopyrite electrode. As shown in Figure 6.10, the passivation film was formed by 2 and 10 min for the pre-wave and bulk potential, respectively.

200

0.06 V 150

0.5 V

)

 (

100

Current Current

0 3 7 10 13 Time (min)

Figure 6.10 Chronoamperogram of abraded chalcopyrite in pH 9.2 buffer solution at

0.06 V (black) and 0.5 V (red).

190

The voltammogram of the chalcopyrite electrode under different oxidation conditions in pH 9.2 buffer solutions with 0 and 10-3 M NaDBDTP are presented in Figure 6.11. The new surface of the chalcopyrite electrode was obtained by abrasion. The oxidised chalcopyrite electrode surfaces were obtained by holding the potential at 0.06 and 0.5 V for 2 and 10 min, respectively. It is obvious that the voltammetry curves of the chalcopyrite electrodes with the same surface treatment do no vary significantly. The currents obtained in 10-3 M NaDBDTP solutions are slightly higher than that in solutions without NaDBDTP. These results indicate that the adsorption and oxidation currents of NaDBDTP may overlap with the oxidation current of chalcopyrite.

Compared to the abraded surface, the chalcopyrite surface oxidised at 0.5 V present a depressed current curve. It suggests that the bulk layer was formed during the controlled potential oxidation process, which inhibits the redox reactions on the surface of the chalcopyrite electrode. The current curve of this chalcopyrite in 10-3 M NaDBDTP solutions exhibits the lowest increment, which indicates that the bulk layer on the surface of the chalcopyrite electrode also inhibits the adsorption and oxidation of

NaDBDTP.

The chalcopyrite oxidised at 0.06 V shows a significant current increment regardless of

NaDBDTP. Compared to the bulk layer, the CuS2* passivation monolayer formed during the controlled potential oxidation process may be not thick enough to prevent further redox reactions (Yin et al., 2000). There should be more CuS2* presented on the surface of this oxidation chalcopyrite electrode than the abraded one. Thus, the decomposition of the additional CuS2* passivation film may contribute to the current increment of the oxidised chalcopyrite.

191

oxidised at 0.06 V in buffer solution -3 200 oxidised at 0.06 V in 10 M NaDBDTP abraded surface in buffer solution -3 abraded surface in 10 M NaDBDTP oxidised at 0.5 V in buffer solution

) -3

A 100 oxidised at 0.5 V in 10 M NaDBDTP



(

Current Current

-100 -0.5 0.0 0.5 Potential (V)

Figure 6.11 Voltammogram of chalcopyrite electrodes under different oxidation conditions in solutions with 0 and 10-3 M NaDBDTP.

192

6.5 Conclusions

The voltammograms of the chalcopyrite electrode with fractured and abraded surfaces did not vary significantly. It suggests that abrasion is an effective and sufficient method for making fresh mineral surfaces in electrochemical investigations.

The OCP results of all three chalcopyrite electrodes showed that their surfaces were oxidised after the boiling procedure, which makes their initial potentials higher than that of abraded surfaces. Comparison of the effects of three different solutions on the chalcopyrite OCP leads to the conclusion that there are no obvious differences between them, and no special effect on the OCP due to anions on electrode surfaces. Under the same surface treatment conditions, the OCP is low under a nitrogen atmosphere due to lack of oxygen. Nitrogen could be used to limit the oxidation reaction on the mineral surface, which increases the efficiency of mineral flotation.

During the controlled potential oxidation process, the bulk layer was formed at 0.5 V and inhibited both the redox reactions of chalcopyrite and the adsorption and oxidation of NaDBDTP on the electrode. This result indicates that the oxidised chalcopyrite may not respond to the collectors used in flotation process.

193

6.5 References

Allan, G.C., Woodcock, J.T., 2001. A review of the flotation of native gold and

electrum. Minerals Engineering 14, 931-962.

Buckley, A.N., Goh, S.W., Fan, L.-J., 2010. Ex situ surface chemical characterization in

minerals processing research: The effect of hydrophilic organic solvents on sulfide

mineral surfaces. ECS Transactions 28, 69-80.

Doña Rodríguez, J.M., Herrera Melián, J.A., Pérez Peña, J., 2000. Determination of the

real surface area of Pt electrodes by hydrogen adsorption using cyclic voltammetry.

Journal of Chemical Education 77, 1195.

Fuerstenau, M.C., Jameson, G., Yoon, R.-H., 2007. Froth flotation : a century of

innovation. Society for Mining, Metallurgy, and Exploration, Inc., Littleton,

Colorado, USA.

Gardner, J.R., Woods, R., 1979. An electrochemical investigation of the natural

flotability of chalcopyrite. International Journal of Mineral Processing 6, 1-16.

Mikhlin, Y.L., Romanchenko, A.S., 2007. Gold deposition on pyrite and the common

sulfide minerals: An STM/STS and SR-XPS study of surface reactions and Au

nanoparticles. Geochim. Cosmochim. Acta 71, 5985-6001.

O'Connor, C.T., Dunne, R.C., 1994. The flotation of gold bearing ores - a review.

Minerals Engineering 7, 839-849.

Tolley, W., Kotlyar, D., Van Wagoner, R., 1996. Fundamental electrochemical studies

of sulfide mineral flotation. Minerals Engineering 9, 603-637.

194

Velasquez, P., Gomez, H., Ramos-Barrado, J.R., Leinen, D., 1998. Voltammetry and

XPS analysis of a chalcopyrite CuFeS2 electrode. Colloids and Surfaces A:

Physicochemical and Engineering Aspects 140, 369-375.

Xie, Z.-J., Xia, Y., Cline, J.S., Yan, B.-W., Wang, Z.-P., Tan, Q.-P., Wei, D.-T., 2016.

Comparison of the native antimony-bearing Paiting gold deposit, Guizhou Province,

China, with Carlin-type gold deposits, Nevada, USA. Mineralium Deposita, 1-16.

Yin, Q., Kelsall, G.H., Vaughan, D.J., England, K.E.R., 1995. Atmospheric and

electrochemical oxidation of the surface of chalcopyrite (Cufes2). Geochimica Et

Cosmochimica Acta 59, 1091-1100.

Yin, Q., Vaughan, D.J., England, K.E.R., Kelsall, G.H., Brandon, N.P., 2000. Surface

oxidation of chalcopyrite (CuFeS2) in alkaline solutions. Journal of the

Electrochemical Society 147, 2945-2951.

195

Chapter 7 Conclusions

196

7.1 Concluding remarks

This thesis has described the spectroelectrochemical investigation into the interactions of two flotation collectors with gold electrode and nanorods surfaces. The relevant compounds and ligands of these collectors were characterised and identified by vibrational spectroscopies. The interaction of collectors with gold electrode surfaces was investigated using in situ spectroelectrochemical techniques, while vibrational spectroscopies were mainly used for the ex situ investigation of the interaction between collectors and Au NRs.

The SERS spectra of gold electrode in 10-3 M NaMBT were obtained in situ by increasing the potential stepwise from −0.7 V to 0.5 V. In each SERS spectrum, the bands at 713 cm-1 (C−S stretch) and 1241 cm-1 (NCS ring stretch) were both enhanced and had higher relative intensities than the normal Raman spectrum of AuMBT. The weak band at 311 cm-1 in the SERS spectra can be contributed by the Au−S stretching vibration, while no obvious evidence was observed for the existence of the Au–N stretching vibration in the SERS spectra. Thus, it is concluded that only the two S atoms of MBT− can bond to the gold electrode surfaces, which is consistent with Cui and co- workers study (Cui et al., 2011).

The oxidation of MBT− was observed at potentials above about 0.2 V in the voltammograms of gold electrode with 10-3 M NaMBT, however, no spectrum of

(MBT)2 was observed in the SERS spectra of gold electrodes obtained by increasing the potential stepwise from −0.7 V to 0.5 V. It is possible that the (MBT)2 molecule was not close enough to the gold electrode surface due to the coverage of MBT−. This result is consistent with the studies of Woods and co-workers (Woods et al., 2000).

197

When the SERS spectra of SERS spectra were recorded directly under potential control

-3 -4 -1 of 0.3 V in 10 M and 10 M NaMBT, three distinctive bands of (MBT)2 at 535 cm ,

1431 cm-1 and 1467 cm-1 were observed in these SERS spectra accompanied by the presence of SERS spectra of MBT−. Although no band can be assigned to the S−S stretch of (MBT)2, it is concluded that (MBT)2 was formed on the bulk gold electrode surface above applied potentials of 0.3 V. Degradation of (MBT)2 was also observed in lower concentration (10-4 M) of MBT when the potential decreased to −0.7 V, while no change was seen in a higher concentration (10-3 M) of NaMBT.

The adsorption of MBT− on Au NRs was also observed in this study. When the Au NRs solution was mixed with 10-3 M NaMBT solution, black precipitates (Au NRs/MBT complex) were obtained in the mixed solution. A very weak band at 314 cm-1 was observed in the Raman spectra of this Au NRs/MBT complex, which has a shift of 3 cm-1 from the Au−S stretch vibration band at 311 cm-1 in the Raman spectrum of

AuMBT. It could be a result of the semi-covalent Au−S bond or Au−N bond between

Au NRs and MBT molecules. At least two chemisorbed S environments were observed in the S 2p spectrum of the Au NRs/MBT complex. The N 1s spectrum of the Au

NRs/MBT complex may also reflect a bonding between the N atom and Au NRs.

It is deduced that the exocyclic S atom of MBT could chemisorb on a gold nanorod growth surface through a Au−S bond, while the endocyclic S or N atom of the same

MBT adsorbed on another gold nanorod surface. Au nanochains were formed and precipitated in the higher concentration (10-3 M) NaMBT solution due to this bonding type of MBT on Au NRs.

198

The flotation behaviour of gold metal particles with diameter of ~100 μm was observed to vary to an insignificant extent under the different gases used in the flotation cell. This indicates that oxygen is not critically involved in reactions taking place on the gold surface. Gold metal particles were floated at potentials above approximately 0.5 V in both 10-3 and 10-4 M NaMBT solutions. The floated gold metal particles were analysed by FTIR spectroscopy, and a spectrum that exhibited the characteristic bands of (MBT)2 was observed on the gold metal particle surface. Because the adsorption of (MBT)2 was much more extensive than a chemisorbed monolayer, it is concluded that the former was responsible for the gold particles becoming hydrophobic and being floated in the cell.

− The adsorption of DBDTP and formation of (DBDTP)2 were both observed in the

SERS spectra of gold electrodes with 10-3 NaDBDTP during the stepped potential increase process. Two bands at about 620 cm-1 and 530 cm-1 were observed in the SERS spectra at negative potentials, which can be contributed by the PS2 antisymmetric and symmetric stretch. It suggests that DBDTP− bonds to the gold electrode surfaces through two S atoms. Three bands observed at 660, 505 and 482 cm-1 can be assigned to the P=S, S−S and P−S stretch of (DBDTP)2. These three distinctive bands of (DBDTP)2 declined after the potential decreased to −0.7 V, which indicates the degradation of

(DBDTP)2 on the gold electrode.

Compared to NaMBT, no precipitate was formed in the mixed solution of Au NRs and

10-3 M NaDBDTP. It is supposed that DTP molecules only bond to a single Au NR and be unable to play the role of dithiol molecules as a linkage for the formation of Au nanochains. Thus, no precipitation was observed even when the Au NRs solution was mixed with a relatively high concentration of NaDBDTP solution.

199

Gold metal particles showed good floatability with the presence of NaDBDTP under various conditions. Compared to NaMBT, no FTIR spectra were observed from gold particles floated by NaDBDTP. It is probably due to the low coverage and intensity of

DBDTP molecules.

In this study, a novel method for preparing fractured chalcopyrite surfaces was established. It was observed that the voltammograms of the chalcopyrite electrode with fractured and abraded surfaces did not vary significantly. Thus, it suggests that abrasion is an effective and sufficient method for preparing fresh mineral surfaces for electrochemical investigation.

During the controlled potential oxidation process, the bulk layer was formed at 0.5 V and inhibited both the redox reactions of chalcopyrite and the adsorption and oxidation of NaDBDTP electrode. It indicates that the oxidised chalcopyrite may not respond to the collectors used in flotation process.

The OCP results of all three chalcopyrite electrodes showed that their surfaces were oxidised after a boiling procedure, and, that there was no effect on the OCP attributable to the anions in different buffer solutions. It is also concluded that nitrogen can limit the oxidation reaction on the mineral surfaces, which can be used to increase the efficiency of mineral flotation.

In summary, the main findings of this study are:

(a) Both MBT and DTP can chemisorb on the gold electrode surfaces through their S

atoms.

200

(b) (MBT)2 was observed on the gold electrode surfaces under potential control, and

gold metal particles were floated when (MBT)2 was formed on their surfaces. Gold

metal particles showed good floatability with the presence of DTP under various

conditions.

through a Au−S bond, while the endocyclic S or N atom of the same MBT

adsorbed on another gold nanorod surface.

(d) Abrasion can be used for making fresh mineral surfaces in electrochemical

investigations. The oxidised chalcopyrite may not respond to the collectors used in

flotation process, while nitrogen can limit the oxidation reaction on the mineral

surfaces.

201

7.2 Future work

Gold is one of the classic SERS substrates which also include silver and copper. This study has demonstrated that MBT molecules have strong SERS signals and Au NRs are excellent SERS substrates. It also shows that gold metal nanostructures allow the use of

SERS spectroscopy to investigate the interaction of MBT with finely divided gold metal particles. Thus, silver and copper nanoparticles can also be used for the study of the interaction of MBT with finely divided silver and copper metal particles.

An initial electrochemical study of chalcopyrite has been completed in this research.

The interaction of MBT and DTP with chalcopyrite surfaces can be further studied in the future. The electrochemical and flotation properties of other gold-bearing sulfide minerals are also an area of particular interest for future investigation.

202

7.3 References

Cui, B., Chen, T., Wang, D., Wan, L.-J., 2011. In situ STM evidence for the adsorption

geometry of three N-heteroaromatic thiols on Au(111). Langmuir 27, 7614-7619.

Woods, R., Hope, G.A., Watling, K., 2000. A SERS spectroelectrochemical

investigation of the interaction of 2-mercaptobenzothiazole with copper, silver and

gold surfaces. Journal of Applied Electrochemistry 30, 1209-1222.

203