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Investigation of chemical and adsorption properties of carbon nanotubes: Building a bridge for technological applications of carbon nanotubes

ProQuest Dissertations and Theses, 2009
Dissertation
Author: Dmitry Kazachkin
Abstract:
In the present work, the results of investigations of the chemical, adsorption and optical properties of carbon nanotubes (CNTs) will be presented. A brief introduction describes CNTs, how they are produced, and how they are purified. Experimental investigations of the effect of air/HCl purification on the introduction of oxygen functionalities will be reported. It was established that air/HCl purification results in the introduction of oxygen containing functionalities to single wall carbon nanotubes (SWCNTs) produced by the HiPco (high pressure carbon monoxide) method. The introduced oxygen functionalities decompose at ∼670 K detected as masses 18 (H 2 O), 28 (CO) and 44 amu (CO2 ) in the mass spectrum. The exact chemical nature of those functionalities requires more detailed investigation. Low-temperature (100 K) adsorption of acetone on carbon black, as-produced and air/HCl purified SWCNTs allowed the accessibility of different adsorption sites in SWCNTs to be established. A key variable was the vacuum-annealing temperature. The energetics of interaction of acetone with different adsorption sites was determined. The most energetic adsorption sites were found to be endohedral adsorption sites. The interaction of solvents with carbonaceous materials was studied under different conditions: sonication, reflux, and exposure to solvent vapors over a range of pressures. It was shown that the binding energy of molecules with SWCNTs depends on the interaction conditions: the higher the temperature and pressure during the contact of molecules with SWCNTs, the higher the adsorption energy of molecules on/in SWCNTs. This finding suggests a "pressure gap" effect for nanoporous carbonaceous materials. Infrared studies of CNTs suggest that molecules adsorbed inside of endohedral channels are invisible to IR. This result is in contrast with experimental findings by other authors. Additional research, both experimental and theoretical, must be done to identify factors responsible for the screening of molecules adsorbed inside SWCNTs.

v TABLE OF CONTENTS GLOSSARY..............................................................................................................................XIII

ACKNOWLEDGEMENTS....................................................................................................XIV

1.0 INTRODUCTION........................................................................................................1

1.1 IMPORTANCE OF THE WORK.....................................................................1 1.2 THE GOAL AND SCOPE OF THE WORK....................................................2 2.0 EQUIPMENT AND EXPERIMENTAL SETUP......................................................4

2.1 EQUIPMENT.......................................................................................................4 2.1.1 Temperature Programmed Desorption (TPD) with Mass Spectrometry (MS) detection system.................................................................................................. 4 2.1.2 Infrared system (FTIR)................................................................................6 2.1.3 Volumetric adsorption system.....................................................................7 2.1.4 Thermo-gravimetric analysis (TGA)...........................................................7 2.2 PRECAUTIONS: EXPERIMENTAL CONCERNS AND SAFETY IS SUES8 3.0 WHAT ARE CARBON NANOTUBES?...................................................................9 3.1 PRODUCTION OF CARBON NANOTUBES...............................................11 3.2 PURIFICATION OF CARBON NANOTUBES.............................................12

vi 4.0 EFFECT OF AIR/HCL PURIFICATION OF SINGLE WALL CARBON NANOT UBES PRODUCED BY HIPCO METHOD ON THE NATURE OF SURFACE FUNCTIONALITIES.................................................................................................................14

4.1 INTRODUCTION.............................................................................................15 4.2 EXPERIMENTAL.............................................................................................18 4.2.1 Sample purification.....................................................................................18 4.2.2 Sample preparation for vacuum studies...................................................18 4.2.3 Thermo-gravimetric analysis (TGA).........................................................19 4.2.4 Temperature programmed desorption studies........................................20 4.2.5 Infrared studies...........................................................................................21 4.3 RESULTS...........................................................................................................21 4.3.1 TGA Results................................................................................................21 4.3.2 TPD-MS Results..........................................................................................23 4.3.3 FTIR results.................................................................................................28 4.4 DISCUSSION.....................................................................................................31 4.5 SUMMARY AND CONCLUSIONS................................................................34 5.0 INTRODUCTION INTERACTION OF ACETONE WITH SINGLE WALL CARBON NANOTU BES AT CRYOGENIC TEMPERATURES: A COMBINED TEMPERATURE PROGRAMMED DESORPTION AND THEORETICAL STUDY.....36

5.1 INTRODUCTION.............................................................................................38 5.2 EXPERIMENTAL AND THEORETICAL APPROACH .............................39 5.2.1 Materials......................................................................................................39 5.2.2 Experimental setup.....................................................................................40 5.2.3 Sample pretreatment and gas exposure procedures................................41

vii 5.2.4 Theoretical Studies - Computational Methodology.................................42 5.3 EXPERIMENTAL RESULTS.........................................................................43 5.4 DISCUSSION OF EXPERIMENTAL RESULTS.........................................48 5.5 THEORETICAL RESULTS AND DISCUSSION.........................................54 5.5.1 Comparison of DFTB-D with MP2 Energetics: Benchmark Results.....54 5.5.2 DFTB-D Interaction Energies for Acetone─SWCNT Complexes ..........58 5.6 CONCLUSIONS................................................................................................63 6.0 TEMPERATURE AND PRESSURE DEPENDENCE OF SOLVENT MOLECULE ADSORPTION O N SINGLE WALL CARBON NANOTUBES AND THE EXISTENCE OF A "PRESSURE GAP".................................................................................66

6.1 INTRODUCTION.............................................................................................67 6.2 EXPERIMENTAL.............................................................................................69 6.2.1 Materials purification.................................................................................69 6.2.2 Sample preparation....................................................................................70 6.2.3 TPD-MS measurements..............................................................................71 6.3 RESULTS...........................................................................................................72 6.3.1 Interaction of acetone with SWCNTs.......................................................72 6.3.2 Interaction of ethanol with SWCNTs........................................................80 6.3.3 Quantum chemical modeling of acetone binding to individual SWCNTs and SWCNT bundles .................................................................................................82 6.4 DISCUSSION.....................................................................................................88 6.5 SUMMARY........................................................................................................93

viii 7.0 “STEALTH” MOLECULES INSIDE SINGLE WA LL CARBON NANOTUBES: CAN FTIR DETECT MOLECULES ADSORBED IN CARBON NANOTUBE BUNDLES? ......................................................................................................................................95

7.1 INTRODUCTION.............................................................................................96 7.2 EXPERIMENTAL.............................................................................................97 7.2.1 Materials......................................................................................................97 7.2.2 Sample preparation and acetone adsorption............................................97 7.3 RESULTS AND DISCUSSION........................................................................98 8.0 CONCLUSIONS AND FUTURE DIRECTIONS.................................................105

APPENDI X A............................................................................................................................107

APPENDIX B............................................................................................................................110

APPENDIX C............................................................................................................................113

APPENDIX D............................................................................................................................117

BIBLIOGRAPHY.....................................................................................................................118

ix LIST OF TABLES

Table 1: Proposed assignme nt of IR bands observed in the spectra of SWCNTs......................30 Table 2: Counterpoise corrected interaction energies ΔE (MP2 BSSE counterpoise corrections in parentheses) [kJ mo l -1 ] and for acetone-coronene complexes shown in Figure 14 MP2 energies were obtained at MP2/SVP geometries........................................................................................55

Table 3: DFTB-D interaction energy ΔE and its components [kJ mol -1 ] for acetone-SWCNT complexes. N/A means that optimization from this conformation resulted in a different confirmation..................................................................................................................................60

Table 4: Interaction energies ΔE in [kJ mol -1 ] for dimers of 10 Å-long, hydrogen-terminated (11,9) (L) and (6,5) (S) tube fragments, and for acetone in the groove site relative to the tube dimers, averaged over six optimized geometries..........................................................................85

x LIST OF FIGURES

Figure 1: Model of nanotube (10, 10) bundle with different adsorption sites shown – endohedral, grooves, external walls, and interstitial...........................................................................................3

Figure 2: Vacuum cham ber for TPD studies (A). The vacuum chamber (B) with a sample holder (C) inside. The part B shows the sample holder with W-grid heated to 1400 K. In the part C the sample holder with CNTs deposited on a CaF 2 pellet pressed into the W-grid is shown.....5

Figure 3: Vacuum IR system . Custom made IR vacuum cell is inserted into sample compartment of FTIR spectrometer......................................................................................................................6

Figure 4: The chiral vector R is defined by unit vectors [a 1 , a 2 ] on the honeycomb lattice of a graphene sheet...............................................................................................................................10

Figure 5: Schema tic of experimental setups used for different nanotube production methods...11

Figure 6: Results of therm o-gravimetric analysis in air flow of as-produced and purified SWCNTs. .....................................................................................................................................22

Figure 7: TPD profiles of 18 amu [H 2 O], 28 amu [CO], and 44 amu [CO 2 ] extracted from full TPD spectra of A. As-produced SWCNTs deposited by direct pressing; B. As-produced SWCNTs sonicated in H 2 O deposited by drop-and-dry, and C. Air/HCl purified SWCNTs sonicated in H 2 O and deposited by drop-and-dry.........................................................................23

Figure 8: TPD profiles for masses 20, 19, 18, and 17 amu for the as-produced SWCNTs and purified SWCNTs sonicated in H 2 O and D 2 O..............................................................................26

xi Figure 9: TPD following H-D exchange on the purified SWCNTs with molecules dosed from the gas phase. ................................................................................................................................28

Figure 10: FTIR spectra of the as-p roduced (A) and air/HCl purified SWCNTs (B) annealed to 500, 700, 900, and 1400 K............................................................................................................29

Figure 11: Desorption of acetone from purified SW CNTs annealed to 500, 700, 900, and 1400 K. (A) Acetone exposure 1-100 L. (B) Acetone exposure 100-1000 L.......................................45

Figure 12: Comp arison of acetone adsorption on as-produced SWCNTs (A) and carbon black (B) annealed to different temperatures.........................................................................................47

Figure 13: Proposed evolution of endohedral sites access ibility based on SWCNT (8,8) model. .......................................................................................................................................................51

Figure 14: Relative or ientations of the acetone molecule on the central hexagon ring of coronene. Only the central coronene ring is shown, and is positioned underneath the acetone molecule. From left to right: planar parallel 1 (PP1) and 2 (PP2), up-perpendicular 1 (UP1) and 2 (UP2), and down-perpendicular 1 (DP1) and 2 (DP2)..............................................................55

Figure 15: DFTB-D interaction en ergy ΔE plotted versus sidewall curvature for averaged series of PP and UP complexes..............................................................................................................63

Figure 16: TPD profiles of acetone-d6 related fragments evolving from the air/HCl purified SWCNTs (A.) sonicated in acetone-d6; (B.) refluxed in acetone-d6; (C.) sonicated in ultra-pure water..............................................................................................................................................73

Figure 17: (A.) Full TPD spectrum collected from purified SWCNTs annealed to 900 K and exposed to acetone-d6 (7.6 Torr, 300 K, 10 min); (B.) Most abundant acetone-d6 related fragments extracted from the full TPD spectrum are plotted as versus temperature....................75

Figure 18: Temp erature profiles of acetone-d6 fragments - 46 amu (CD 3 CO) and 64 amu ((CD 3 ) 2 CO). Profiles for acetonde-d6 related fragments coincide, suggesting that acetone desorbs from SWCNTs as an intact molecule..............................................................................76

xii Figure 19: Exchange experime nt. SWCNTs with pre-adsorbed acetone-h6 were exposed to acetone-d6 and after evacuation signal from acetone-h6 and acetone-d6 were collected simultaneously with TPD-MS. The results suggest that acetone-h6 that is strongly bound to the surface can be partially replaced with acetone-d6........................................................................77

Figure 20: Comp arison of TPD profile of acetone-d6 (46 amu – CD 3 CO) adsorbed at ~10 -6

Torr-100 K to profile of acetone adsorbed at 7.6 Torr-300 K. (A.) Adsorption of acetone on carbon black annealed to 900 K (0.5 h); (B.) Adsorption of acetone on the purified SWCNTs annealed to 900 K (0.5 h)..............................................................................................................79

Figure 21: Temperature profiles of etha nol related fragments extracted from full TPD spectra of purified SWCNTs sonicated in (A) ethanol, and (B) H 2 O. The SWCNTs sonicated in ethanol evolve significant amount of fragments that can be assigned to ethanol: [CH 2 OH] - 31 amu, [C 2 H 5 O] - 45 amu, [C 2 H 5 OH] - 46 amu........................................................................................80

Figure 22: TPD-MS spectra of ethanol desorbing from SWCNTs af ter adsorption from the gas phase on SWCNTs at 100 K-10 -6 Torr and 300 K-7.6 Torr (A.). Ethanol related fragments have similar desorption profiles either after exposure SWCNTs to ethanol vapors (B.) or sonication of SWCNTs (C.) in ethanol. Multiplication coefficients 2.7 and 5.8 were determined experimentally by introducing ethanol vapors into the TPD vacuum chamber............................81

Figure 23: Representative optimized geom etries for acetone adsorbed in LL, LS, and SS groove sites. (a) top view, (b) skewed front view....................................................................................86

Figure 24: Optim ized geometry for acetone adsorbed inside the interstitial site of an LLS bundle............................................................................................................................................87

Figure 25: Model describing activated adsor ption/desorption mechanism of molecules inside interstitial channels of SWCNT bundle........................................................................................92

Figure 26: Infrared spectra of acetone adsorbed on SWCNTs annealed to 500 K. .....................99

xiii GLOSSARY CNT – Carbon Nanotubes S(D, M)WCNT – Single (Double, Multi) Walled Carbon Nanotubes FTIR – Fourier Transform Infrared HOPG – Highly Oriented Pyrolitic Graphite MS – Mass Spectroscopy RGA – Residual Gas Analyzer; the same as a Mass Spectrometer in given context SSA – Specific Surface Area (m 2 /g) TGA – Thermo Gravimetric Analysis TPD – Temperature Programmed Desorption XPS – X-ray Photoelectron Spectroscopy UV-vis-NIR – Ultraviolet-visible-near infrared absorption spectroscopy

xiv ACKNOWLEDGEMENTS First, I would like to acknowledge the help of my research advisor - Professor Eric Borguet. For me Professor Borguet was more than advisor. His help and constructive critique allowed making me a good progress. I appreciate the help and support of my family: my parents who supported me all these years, my spouse, Miraslava, who believed in me and sacrificed the best years of her life helping me to pass this part of the way. My son, Alexander, who inspired me by his joy, energy, and ability to make discoveries every day! I acknowledge the help of all Borguet group members. The support of Professor Radisav Vidic and Professor J. Karl Johnson, who backed me up at the University of Pittsburgh, is greatly appreciated. I would like to say thank you to all dissertation committee members. I appreciate your patience and flexibility. The help of our collaborators, theoretical chemists - Professor Stephan Irle and PhD student Yoshifumi Nishimura from Nagoya University - is difficult to underestimate. The explanation of experimental findings made in this work would not be complete without their thoughtful input. Financial support from National Science Foundation (NSF), Department of Energy (DOE), and Applied Sensor Research & Development Corporation (ASRD) is acknowledged.

1 1.0 INTRODUCTION It is possible that ancient steelmakers knew the secret of CNT synthesis - traces of CNTs have been found in the legendary Damascus steel. 1 Perhaps the presence of CNTs was the origin of the unique properties of Damascus steel. 1 The first experimental detection of CNTs with electron microscopy was in 1952. 2 Nowadays, CNTs are attracting increasing attention. 3-9

Carbon nanotubes are promising materials for the development of CNT based devices and materials – chemical sensors, 10-17 materials for storage of chemicals, 5, 18-20 membranes for gas separation, 21-23 composite materials with unique properties (electrically conductive, thermally stable, mechanically strong), 5, 8, 9, 24 and electronic devices (nanowires, electron emitters, etc.) 5 . The unique mechanical properties of CNTs made the concept of a space elevator feasible. 25 The author is confident that the advanced properties of CNTs will allow their use for more applications in the near future. 1.1 IMPORTANCE OF THE WORK

Technological applications of CNTs are not possible without a detailed knowledge of their chemistry. The development of CNT based chemical detectors that are sensitive to specific molecules requires information on the interaction of CNTs with these molecules. 10-13, 15-17 The

2 synthesis of composite materials (e.g., modified polymers) and incorporating CNTs often requires the use of organic solvents. Sonication is often used to facilitate dispersion of CNTs in solvents. 8, 26-29 However, sonication can induce chemical interaction of CNTs with solute molecules 30, 31 due to temperature and pressure changes. 32 Understanding of the interaction of CNTs with solvents under different conditions will benefit future applications of solvents for CNT processing. 1.2 THE GOAL AND SCOPE OF THE WORK In the present work, the results of investigations of simple molecule interactions (e.g., acetone, ethanol, etc.) with CNTs and other carbonaceous materials will be reported. Experiments on the interaction of different solvents with SWCNTs under cryogenic conditions provide information on the energetics of adsorption of simple molecules on different sites: external walls (exohedral), grooves, interstitial channels, and internal walls (endohedral sites) (Figure 1). This information is important to understand the mechanism of molecular binding to different adsorption sites. Theoretical consideration for molecules adsorbed on different CNT sites will be provided. Interactions, especially reversible interactions, of molecules with different carbon nanotubes sites is important for understanding CNT based sensors, and for the creation of storage materials based on CNTs (capturing and release of molecules).

3 The interaction of CNTs with solvents under reflux, sonication, and exposure to solvents vapors at elevated pressures and temperatures is helpful to understand the mechanisms of interaction of CNTs with molecules. Such investigations may reveal chemical reactivity of CNTs to different solvents. Information of this kind is important for technological processing of CNTs. Figure 1: Model of nanotube (10, 10) bundle with different adsorption sites shown – endohedral (1), grooves (2), external walls (3), and interstitial (4) (red – oxygen, white – hydrogen, grey – carbon atoms).

2 3 4 1

4 2.0 EQUIPMENT AND EXPERIMENTAL SETUP 2.1 EQUIPMENT In this section the equipment used for the experimental work and its technical characteristics will be described. The reader should address individual chapters of the thesis to find out more details about procedures used for preparation of samples, pretreatments and experimental conditions. 2.1.1 Temperature Programmed Desorption (TPD) with Mass Spectrometry (MS) detection system The Temperature Programmed Desorption with Mass Spectroscopy detection (TPD-MS) system 33 in the present work is used primarily for studying the chemical species leaving the surface of materials upon heating. The TPD technique can address the following issues: a. Identify molecular fragments leaving the surface of materials upon thermal decomposition; b. Detect multiple adsorption sites for adsorbed molecules and determine the binding energy for each adsorption site (multiple peaks in TPD spectrum); c. Establish the kinetics of desorption processes (simulation of the desorption profiles); d. Ascertain whether irreversible chemical transformations took a place upon adsorption.

5

Figure 2: Vacuum chamber for TPD studies (A). The vacuum chamber (B) with a sample holder (C) inside. The part B shows the sample holder with W-grid heated to 1400 K. In the part C the sample holder with CNTs deposited on a CaF 2 pellet pressed into the W-grid is shown. IR transparent materials (e.g., CaF 2 , NaCl, KBr) were used in some FTIR experiments.

The TPD-MS experiments are performed in a stainless steel ultrahigh vacuum chamber with a base pressure better than 10 -9 Torr (Figure 2). Samples are deposited either by drop-and- dry me thod on a W-grid (AlfaAesar, 100 mesh, 0.002” wire diameter) or by pressing the sample directly into the W-grid. The W-grid is fixed by nickel clamps connected to copper wires cooled via a liquid nitrogen Dewar. The overall design of the sample holder allows cooling of the sample to cryogenic temperatures (<100 K) and heating up to 1400 K. Gases are dosed to the sample, preconditioned to the desired temperature, by backfilling the chamber through a leak valve (Varian). After the desired dose is achieved and the base pressure is recovered, the TPD spectrum is recorded while heating the sample at a constant ramp (typically 2 K/sec). The total

6 pressure is monitored with an ion gauge. To heat the sample, direct current from a power supply (Kepco 10-100) is driven through the W-grid. Temperature is monitored by means of a K-type thermocouple (Omega) spot-welded to the W-grid. The residual gas analyzer (RGA 300, Stanford Research Systems) is used to monitor the desorption species. Custom Labview programs are used to control the temperature, monitor the dosing, and to record TPD-MS spectra. 2.1.2 Infrared system (FTIR) Infrared spectroscopy is used to study surface functionalities introduced to CNTs during materials preparation stages. Also, adsorbed species dosed from the gas phase to the surface of CNTs are studied with FTIR technique. Infrared spectra are recorded with an infrared spectrometer (Tensor 27, Bruker) equipped with a Mercury Cadmium Telluride (MCT) detector. A vacuum stainless steel system was used for FTIR measurements (Figure 3). The design of sample holder used in FTIR studies was analogous to the design of the sample holder used in TPD studies and allowed cooling of samples to ~100 K

Figure 3: Vacuum IR system. Custom made IR vacuum cell is inserted into sample compartment of FTIR spectrometer (Tensor 27, Bruker).

7 and heating to 1400 K. Typical base pressure before the FTIR experiments was better then 10 -8

Torr. The vacuum FTIR cell was equipped with differentially pumped KBr windows. Spectra were recorded by collecting 900-10,000 scans (depending on S/N ratio required) in the range of 400-6000 cm -1 at a resolution of 4-8 cm -1 .

2.1.3 Volumetric adsorption system A surface area analyzer (Micrometrics, ASAP 2020) was used for characterization of the porous structure and total surface area of samples under investigation. The Brunauer, Emmett and Teller (BET) method provided with instrument was used for the data processing. 34

2.1.4 Thermo-gravimetric analysis (TGA) The objective of the TGA experiment was the evaluation of residual catalyst content (Fe, Ni, Co). The TGA analysis was performed using a Pyris 6 Thermogravimetric Analyzer (PerkinElmer) or a Hi-Res 2950 (TA Instruments). During the analysis pre-purified air (Airgas), controlled by mass-flow controller, was metered at 20 cc min -1 through the TGA sample compartment while ramping the temperature from ambient to 1173 K at 10 K min -1 .

8 2.2 PRECAUTIONS: EXPERIMENTAL CONCERNS AND SAFETY ISSUES There are certain precautions should be taken while working with materials and equipment described in this thesis. The effect of CNTs on living organisms are still being debated. 35, 36 For the present moment, it is rational to avoid penetration of CNTs into the body. Avoid skin contact with CNTs by using gloves. Avoid inhaling of CNTs - work under the hood, try to operate CNTs in solutions, use respirators and safety dust goggles. Address Materials Safety Data Sheets (MSDS) before using any chemicals. The use of electrical equipment requires a common sense and an understanding of the principles described in manuals provided by equipment producers. Laboratory procedures should be consulted to understand the operation of each piece of equipment better.

9 3.0 WHAT ARE CARBON NANOTUBES? Carbon nanotubes are an allotropic modification of carbon that can be represented as a sheet of graphene (single layer of graphite) rolled into a cylinder. 3, 4, 7, 8 The carbon atoms of carbon nanotubes are in the sp 2 hybridization state. Depending on the number of concentric walls in a carbon nanotube the following classification is used: single (SWCNTs), double (DWCNTs) and multi-wall (MWCNTs). The layers in DW and MWCNTs are attracted to each other due to van der Waals forces (similar to layers of graphene in graphite). Depending on coefficient n and m of the chiral vector (R=na 1 +ma 2 ), where a 1 and a 2 are the unit cell vectors (Figure 4), carbon nanotubes can be armchair (n=m), zigzag (n, m=0 or n=0, m), or achiral (all other cases). Knowing the coefficients of the chiral vector (n, m) it is possible to calculate the diameter, d, of carbon nanotubes: 0 2 2 1/2 2 2 1/2 3 ( )/~ 7.8 ( ) C C d a m nm n m nm n        , where a C-C the length of C-C bond (1.42 Å). By knowing the coefficients of the chiral vector (n, m) it is possible to predict the electronic properties of carbon nanotubes: if 2n + m = 3q, where q is an integer the nanotube is metallic and if 2n+m ≠ 3q then the nanotube is semiconducting. 3, 4 According to calculations based on statistically equal probability to synthesize carbon nanotube with any chiral vector (n, m), it is found that only ~1/3 of all nanotubes is metallic. 4 Currently, there is no known way to

10 synthesize selectively carbon nanotubes of uniform electronic properties (metallic or semi- conducting). Methods of nanotubes separation are being actively investigated. 37, 38

The mechanical properties of carbon nanotubes surpass the properties of many known structural materials. For example, the Young’s modulus of SWCNTs is ~ 1 TPa (10 12 Pa) 39 and exceeds the Young’s modulus of steel ~0.2 TPa 40 . The thermal conductivity of individual SWCNT (10, 10) was predicted to be 6600 W/m K at room temperature and 37000 W/m K at 100 K 41 . For comparison, the thermal conductivities of Cu and Ag are 401 and 429 W/m K, respectively. 42

Typical diameters of SWCNTs produced either by HiPco method 43 , laser ablation, or arch- discharge are on the order of 1-2 nm, with a typical length of several microns (1-5 µm) 43 that corresponds to the aspect ratio (length/diameter) of ~1000. Nanotubes of several millimeters in length were reported. 44 The specific surface area (SSA) depends on pretreatment and for SWCNTs varies in ~400-1600 m 2 /g range. 45-47

Carbon nanotubes have unique electronic properties. It was established that conductance of MWCNTs is quantized. 48 MWCNTs conduct current ballistically and do not dissipate heat; the current density was measured to be 10 7 A/cm 2 . 48

For SWCNTs experimentally the maximum measured current density was 10 9 A/cm 2 . 49

physicsworld.com Figure 4: The chiral vector R is defined by unit vectors [a 1 , a 2 ] on the honeycomb lattice of a graphene sheet. 4 The coefficients (n, m) define how many times the corresponding unit vector contribute to the chiral vector R.

11 3.1 PRODUCTION OF CARBON NANOTUBES There are several principal methods for carbon nanotube production: arc-discharge, 50 laser ablation, 51 chemical vapor deposition (CVD), 52 and catalytic growth in the gas phase (e.g., HiPco method) 43 . Some exotic methods of carbon nanotubes production were reported such as production of MWCNTs from grass, 53 or non-catalytic method of SWCNTs production from silicon carbide. 54

In the arc-discharge method, CNTs are produced from graphite electrodes that are evaporated by plasma (usually inert gas, e.g., He or Ar) (Figure 5). Plasma is produced by running high currents (~100 Amp) through electrodes separated by an inert atmosphere. During the arc-discharge process, material from the anode evaporates producing CNTs that deposit on the cathode. Both single- 55 and multi-wall 56 CNTs can be produced by the arc-discharge method. For SWCNT production a catalyst 1 is required, 55

1 NASA researchers claim development of arc-discharge method of SWCNT production without using catalyst. Details of the process are not disclosed (http://ipp.gsfc.nasa.gov/ft-tech-nanotech.html ). Arc-Discharge (adapted from www.nec.co.jp)

Laser Ablation

(adapted from ipn2.epfl.ch)

Chemical Vapor Deposition (CVD)

(adapted from ipn2.epfl.ch) Figure 5: Schematic of experimental setups used for different nanotube production methods (see text for details).

12 while there procedures exist for production of MWCNTs that do not require catalyst (MER corporation ). In the laser ablation method CNTs are produced by ablation of preheated (~1200 °C) carbon target with an intense laser pulses (Figure 5). 51 For the production of SWCNTs by the laser ablation method a catalyst is required (Co, Ni, Y). 51, 57

In the method of chemical vapor deposition (CVD) for CNT production, a feedstock based on carbon containing chemicals (e.g., C 2 H 2 , CH 3 OH, etc.) passes over preheated (~1000 °C) heterogeneous catalyst (Co, Ni supported over, e.g., silica or alumina). Carbon containing chemicals decompose over catalyst particles producing CNTs. 58, 59

The High Pressure carbon monoxide (HiPco) decomposition process 43 is somewhat similar to CVD. In the HiPco process metal carbonyls (Fe(CO) 5 or Ni(CO) 4 ) are injected into the reactor along with CO in the gas phase. The reaction mixture has a pressure of ~30-50 atm. and temperature of ~900-1100 °C. 60 Formation of CNTs proceeds through a thermal disproportionation of CO on metal clusters formed from metal carbonyls. Single wall carbon nanotubes produced by HiPco process have a narrow diameter distribution (maximum at ~1.0 nm). 43, 61, 62

3.2 PURIFICATION OF CARBON NANOTUBES One of the concerns of production, and subsequent applications, of CNTs is the purity of the final product. As-produced carbon nanotubes contain admixtures of carbonaceous impurities (fullerenes, graphitic impurities) and catalytic particles. 63 For many applications, such as

13 electronics, and biological applications (drug delivery), it is highly desirable to have CNTs of high purity. Hence, the development of CNT purification methods is very important. Fullerenes and polyaromatic carbons can be removed by rinsing in CS 2 , followed by filtration. 64 Air oxidation was suggested to remove non soluble carbonaceous impurities, on the basis that carbon nanotubes are more stable towards oxidation than carbonaceous impurities. 63, 65

Several methods for liquid phase oxidative purification of CNTs were proposed using HNO 3 , 66

H 2 O 2 /H 2 SO 4 , 67 Fe/H 2 O 2 /HCl (“Fenton”) 68 . Oxidative purification in the presence of acids can allow removal of carbonaceous impurities and metal impurities in one step. In the case when air oxidation is used for removal of carbonaceous impurities, a second step is required (acid treatment) for removal of catalyst particles. From the technological point of view single step processes are more advantageous. Challenges to purification of CNTs exist. One of these challenges is how to prevent the loss of CNTs during purification of carbon nanotubes, especially oxidative purification. Oxidative treatment of CNTs in air in the presence of metal particles leads to combustion of CNTs along with carbonaceous impurities. 61, 62 Another problem is how to avoid cutting of carbon nanotubes in oxidative media; oxygen can attack the side walls of CNTs, destroying the tubular structure of CNTs and finally lead to the shortening of CNTs. 67

In this work the effect of air/HCl purification on the introduction of oxygen functionalities will be considered. Air/HCl purification was chosen for the study, because results reported in the literature on the nature of chemical functionalities introduced are scarce and inconsistent. 69-71

14 4.0 EFFECT OF AIR/HCL PURIFICATION OF SINGLE WALL CARBON NANOTUBES PRODUCED BY HIPCO METHOD ON THE NATURE OF SURFACE FUNCTIONALITIES Results reported in this chapter require additional research to be done before submitting for publication.

Abstract Oxygen functionalities formed during air/HCl purification of single wall carbon nanotubes (SWCNTs) were studied by a combination of temperature programmed desorption (TPD-MS) and infrared spectroscopy (FTIR). The air/HCl purification introduces oxygen functionalities to SWCNTs. According to TPD-MS the dominant species desorbing below 600 K are H 2 O and CO 2 . These species are observed both for the as-produced and purified SWCNTs and might be associated with desorption of species adsorbed from ambient. Oxygen functionalities introduced decompose at ~670 K in TPD-MS spectra with release of H 2 O, CO, and CO 2 as the dominant species. A second maximum for CO release at 1130-1180 K is observed for both the as- produced and purified SWCNTs. Hydrogen-deuterium exchange experiments carried out in the liquid phase upon sonication and in the gas phase upon exposure of SWCNTs to water (H 2 O or D 2 O) vapors show easy H-D proton exchange for species desorbing below 600 K, while for species desorbing with a maximum at 670 K there is only limited H-D proton exchange suggesting limited accessibility of those species for water molecule protons. FTIR spectroscopy

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Abstract: In the present work, the results of investigations of the chemical, adsorption and optical properties of carbon nanotubes (CNTs) will be presented. A brief introduction describes CNTs, how they are produced, and how they are purified. Experimental investigations of the effect of air/HCl purification on the introduction of oxygen functionalities will be reported. It was established that air/HCl purification results in the introduction of oxygen containing functionalities to single wall carbon nanotubes (SWCNTs) produced by the HiPco (high pressure carbon monoxide) method. The introduced oxygen functionalities decompose at ∼670 K detected as masses 18 (H 2 O), 28 (CO) and 44 amu (CO2 ) in the mass spectrum. The exact chemical nature of those functionalities requires more detailed investigation. Low-temperature (100 K) adsorption of acetone on carbon black, as-produced and air/HCl purified SWCNTs allowed the accessibility of different adsorption sites in SWCNTs to be established. A key variable was the vacuum-annealing temperature. The energetics of interaction of acetone with different adsorption sites was determined. The most energetic adsorption sites were found to be endohedral adsorption sites. The interaction of solvents with carbonaceous materials was studied under different conditions: sonication, reflux, and exposure to solvent vapors over a range of pressures. It was shown that the binding energy of molecules with SWCNTs depends on the interaction conditions: the higher the temperature and pressure during the contact of molecules with SWCNTs, the higher the adsorption energy of molecules on/in SWCNTs. This finding suggests a "pressure gap" effect for nanoporous carbonaceous materials. Infrared studies of CNTs suggest that molecules adsorbed inside of endohedral channels are invisible to IR. This result is in contrast with experimental findings by other authors. Additional research, both experimental and theoretical, must be done to identify factors responsible for the screening of molecules adsorbed inside SWCNTs.