Structure and Ordering in Solvents and Solutions of Carbon Nanotubes
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Structure and Ordering in Solvents and Solutions of Carbon Nanotubes Nadir S. Basma A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy Department of Physics and Astronomy University College London October 2018 Dedicated to my parents ii Declaration I, Nadir Basma, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated, and that appropriate credit has been given where reference has been made to the work of others. Copyright © 2018 by Nadir S. Basma “The copyright of this thesis rests with the author. No quotations from it should be published without the author’s prior written consent and information derived from it should be acknowledged.” iii Abstract Our lack of understanding of interactions between nanoparticles in liquids is impeding our ability to controllably produce and manipulate nanomaterials. Traditionally, nanoparticle dispersions are treated using classical colloidal theories originally developed for micron-scale particles. These theories have recently been called into question as significant deviations from micro-scale models are routinely seen as dimensions of the dispersed particle are reduced. The deviations are prominent in the liquid phase where low-dimensional nanomaterials are often processed to produce individualised species with which desirable properties are associated. In this context, a few solvents have proven to more effective than others. Yet, their liquid structures, which ultimately underpins their solvation properties, has not been established. In the first part of this work, advanced neutron scattering methods was used to probe the structure of three solvents: N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF) and dimethylacetamide (DMA). Monte Carlo molecular modelling was used to analyse the neutron data. In NMP, an unusually well-developed mix of interactions is uncovered and its dipole moment was found to have a profound effect on its structure, inducing molecular organisation extending into the nanometer range. In the case of DMF and DMA, structural information pertaining to spatial and orientational correlations in each of the solvents was revealed. The results demonstrate a variety of differences between these two structurally-analogous solvents. Most notably, a higher degree of ordering was found in DMF, dictated by its polar moment, though stronger hydrogen bonding interactions were found in DMA. The intrinsic order in all of these solvents enables a range of local solvent-solute interactions, rationalising their ability to coordinate to and solvate a variety of species. The influence of a nanoparticle on both local and system solvent structures is then investigated in the second part of this work. Solutions of nanomaterials provide a unique system to explore local solvent ordering effects at smaller particle dimensions, and examine the crossover from colloidal to solution behaviour, allowing comparison to classical models. The investigation represents the first atomistically-resolved neutron scattering measurement of a nanomaterial solution. Single-Walled Carbon Nanotubes (SWCNTs) have been shown to thermodynicamlly dissolve to high concentrations in aprotic solvents, though the underpinning mechanisms to their dissolution are unknown. Thus, the system studied was of a concentrated solution of SWCNTs in DMF. The findings provide experimental evidence of enhanced solvent ordering near the nanoparticle’s surface, and contributes crucial insights that differ from conventional standpoints in understanding nanoparticle dissolution. iv Impact Statement Nanomaterials promise to have a huge impact on many technologies, in areas as diverse as healthcare, energy infrastructure, and multifunctional composites. However, their widespread use has been historically hampered by issues of synthesis, agglomeration, and manipulation. Processing can be facilitated by dispersion of the nanomaterials in a liquid; such dispersions can be used to apply the nanomaterials over large areas at low costs. However, while promising, little is known about the underpinning forces and mechanisms at the atomic level for such dispersions to form and stabilise. This work uses neutron scattering methods to uncover local order in such liquids and finds far more developed solvent ordering and structure than as is proposed in conventional models and traditional understandings. The work not only points us a new direction in understanding nanoparticle dispersions, but develops a versatile methodology which is applicable to a wide range of similar systems, which range from electrochemical to biological devices. v Acknowledgements I take the opportunity to express my gratitude to many whom have in some way or another contributed to this thesis. First and foremost, to Chris Howard, for his supervision, support and guidance that have been paramount over the course of the project. His positive outlook and his confidence in my research have truly inspired me. His enthusiasm was contagious and motivational, and translated into my immense enjoyment of the process. To Milo Shaffer, for his supervision and counsel, and his commendable knowledge of all chemistry things which was invaluable to me from the beginning till the end of the project. I am thankful for many of his ideas and suggestions at various stages. To Tom Headen, with whom I collaborated with for most of the work in this thesis. I owe Tom a debt of gratitude for sharing his expertise so willingly and lending me his intuition. To Neal Skipper too, for his counsel and valuable input during the entire process. To Tristan Youngs, Daniel Bowron and Alan Soper whom during beam-time experiments at the Rutherford Appleton Laboratory provided a great deal of help. To post-doctoral researchers, David Buckley for showing me the ropes at the beginning of the journey, and Adam Clancy for timely advice, as well as thoughtful and detailed feedback on the thesis. To colleagues at the London Centre for Nanotechnology, and to members of both the Howard and the Shaffer groups at UCL and Imperial College. To many at the ACM doctoral training centre, and indeed the centre’s directors, Neil Curson and Stephen Skinner, for their management and direction. Finally, to my parents. I’m indebted to you both. I am so much of what I learned from you. Thank you the for the values you instilled in me. I dedicate this thesis to you. vi Contents Title page . i Dedication . ii Declaration . iii Abstract . iv Impact Statement . v Acknowledgements . vi Table of Contents . vii Contributions . xi List of Figures . xii List of Tables . .xxiii Chapter 1 Introduction 1 Chapter 2 Background and Literature 5 2.1 Introduction . 5 2.2 On Liquids and Liquid Structure . 6 2.2.1 The Liquid State . 6 2.2.2 Structure in Liquids . 7 2.2.3 Liquid Structure as Probed Experimentally . 10 2.3 On Interactions Within Liquids . 12 2.3.1 The Electrostatic Interaction . 12 2.3.2 Ion-Dipole Interaction . 12 2.3.3 van der Waals (vdW) Interactions . 13 2.3.4 Hydrogen Bonding . 16 2.4 On Single-Walled Carbon Nanotubes (SWCNTs) . 19 2.4.1 History of SWCNTs . 19 2.4.2 Geometry of SWCNTs . 19 2.4.3 Electronic Properties . 24 2.4.4 Applications and Challenges . 25 2.4.5 Dispersion of SWCNTs . 27 2.4.6 Charging SWCNTs . 28 2.4.7 Suitable Solvents for SWCNT Solubilisation . 30 2.5 Understanding SWCNTs in Solution . 31 2.5.1 Dissolution at the Nanoscale . 31 vii Contents Page viii 2.5.2 DLVO and Polyelectrolyte Theories . 34 2.5.3 The Breakdown of DLVO-OM . 37 2.5.4 Thermodynamic Considerations . 38 2.5.5 SWCNT-Solvent Systems in the Literature . 42 Chapter 3 Neutron Diffraction and Computational Modelling 46 3.1 Introduction . 46 3.2 Basics of Diffraction . 47 3.2.1 Wave Interference and Bragg’s Law . 47 3.2.2 Reciprocal Space and the Fourier Transform . 47 3.2.3 The Scattering Vector . 49 3.2.4 Small-Angle and Wide-Angle Diffraction . 51 3.2.5 Probes and Sample Interaction . 51 3.3 Neutron Diffraction Theory . 53 3.3.1 The Neutron . 53 3.3.2 Scattering Geometry . 53 3.3.3 Scattering by a Single Nucleus . 55 3.3.4 Scattering from a system of particles . 56 3.3.5 The Time-Dependent Pair-Correlation function . 57 3.3.6 Self and Distinct Scattering . 58 3.3.7 Coherent and Incoherent Scattering . 58 3.3.8 The Neutron Correlation Function . 59 3.3.9 Pair Correlation Functions and Structure Factors . 60 3.3.10 Isotopic Substitution . 61 3.4 The Neutron Diffraction Experiment . 62 3.4.1 Neutron Sources . 62 3.4.2 Instrumentation . 63 3.4.3 Experimental Details . 64 3.4.4 Correction and Calibration of Raw Data . 64 3.5 Empirical Potential Structure Refinement . 67 3.5.1 Computational Modelling and Simulations . 67 3.5.2 Constructing an EPSR Simulation Box . 67 3.5.3 Interatomic Potentials . 69 3.5.4 Running the Simulation . 71 3.5.5 The Empirical Potential . 74 3.5.6 The Case for EPSR . 75 Chapter 4 Results I: The Liquid Structure of NMP 77 4.1 Introduction . 77 4.2 About NMP . 77 Contents Page ix 4.3 Experimental Details . 79 4.4 Details of EPSR Simulation Runs . 80 4.5 Results and Analysis . 82 4.5.1 Data Fitting . 82 4.5.2 Radial Distribution Functions in NMP . 86 4.5.3 Positional Arrangement of NMP Molecules . 90 4.5.4 Orientation in the First Solvation Shell . 91 4.5.5 Orientational Ordering due to Dipole Moment . 94 4.5.6 Extent of Hydrogen Bonding in NMP . 97 4.6 Discussion . 98 Chapter 5 Results II: The Liquid Structures of DMF and DMA 101 5.1 Introduction . 101 5.2 About DMF and DMA . 101 5.3 Experimental Details . 105 5.4 Details of EPSR Simulations . 106 5.5 Results and Analysis . 109 5.5.1 Data Fitting .