Chapter 1 Introduction
2.3 Characterization Methods
Table 2.8. List of equipment used for characterization.
Name Company Model
number
Key parameter
Scanning electron micro- scope
Zeiss 1550 VP Backscattered electrons at 5-10 kV, secondary elec- trons at 2-5 kV
Transmission electron mi- croscope
FEI TF30UT Forward scattered electrons at 300 kV
Contact angle goniometer rame-hart Model 190 Water droplet volume of 5 - 25µL
Raman spectrometer Renishaw M1000 Excitation energy at 2.41 eV
Fourier transform infra red spectrometer
Nicolet 6700 Wavenumber range of 600 - 2000 cm−1
Xray photoelectron spec- trometer
Surface science
M-Probe Binding energy range of 0 - 1000 eV
Electrochemical impedance spectrometer
Biologic SP-200 Frequency range 10 mHz - 5 MHz
2.3.2 Transmission Electron Microscopy
Transmission electron microscopy is performed using a high performance transmis- sion electron microscope (FEI Tecnai F30UT). All images are taken under forward scattered electrons mode with acceleration voltage of 300 kV. Selected area diffraction patterns (SADP) are also collected from each set of CNT to verify their crystalline structure. Prior to imaging, a small amount of sample is collected from the CNT arrays by scraping them physically with tweezers or razor blades. These CNT sam- ples are then dispersed in DI water with the help of ultrasonication for about 5-15 minutes. For CNT samples that are difficult to disperse, 200 proof ethanol is used as the solvent, replacing the DI water. Subsequently, a small amount of the dispersion is pipetted while the dispersion is still homogeneous. This pipetted dispersion is then dropped on a holey carbon TEM grid (EMS CF213) and dried in ambient air. After the solvent is fully evaporated, this TEM grid is mounted into the TEM.
2.3.3 Contact Angle
In this thesis, the wettability of CNT arrays is mainly quantified by their static contact angle. All contact angle measurements are conducted using a contact angle goniometer at standard room pressure and temperature. Samples of CNT arrays are placed on the contact angle goniometer sample table. This sample table has to be perfectly level and not tilted toward one direction. A 5µL deionized water droplet was dispensed slowly on the top surface of each CNT array using a 5µL syringe (Hamilton 7105KH) equipped with a 31 gauge flat-tipped needle (Hamilton KF731).
Once a water droplet has come to rest on the surface of the CNT array, images of the water droplet are then taken 10 seconds afterwards to ensure the equilibrium condition was achieved and to avoid the excessive evaporation of the droplet. The contact angles are then measured by processing the captured images with LBADSA software (Stalder et al., 2006).
2.3.4 Raman Spectroscopy
Raman spectroscopy characterization is performed using micro Raman spectrometer system (Renishaw M1000) equipped with an Ar ion laser at an excitation wave length of 514.5 nm. Each CNT array sample is characterized as is without prior sample preparation. Each sample is then placed on the sample table of the optical microscope attached to the Raman spectrometer system. Raman spectroscopy characterizations are performed at an optical magnification of 50x or 100x in a non-polarized mode.
2.3.5 Fourier Transform Infra Red Spectroscopy
Fourier Transform Infra Red (FTIR) spectroscopy characterization is performed using an infrared spectrometer (Nicolet 6700) equipped with liquid nitrogen cooled mercury cadmium telluride (MCT) detector. Prior to characterization, a small amount of sam- ple is collected from the CNT arrays by scraping them physically with tweezers or razor blades. These CNT samples are then dispersed in deuterated dichloromethane (Sigma-Aldrich, 99.9 % atom) with the help of ultrasonication for about 5-15 minutes.
Subsequently, this dispersion is quickly drop-cast onto an IR transmitting KBr win- dow while the dispersion is still homogeneous. A second KBr window is then placed on top, sandwiching the CNT between two KBr windows. These KBR windows are then turned for a quarter of a rotation and pressed against each other to obtain an even film. This KBr-CNT sandwich is dried overnight under low vacuum (10 Torr) prior to characterization to remove the solvent leftover. A pair of blank KBr windows are used as a baseline correction.
2.3.6 X-ray Photoelectron Spectroscopy
In this study, surface chemistry characterizations are typically assessed using x- ray photoelectron spectroscopy (Surface Science M-Probe XPS). A monochromatic 1486.6eV Al Kα is used as the x-ray source and is directed at an angle of 35◦ to the sample surface. The emitted photoelectrons are collected by a hemispherical an- alyzer positioned at an angle of 35◦ to the sample surface. The oxygen to carbon
atomic ratio (O/C ratio) is calculated from low resolution survey spectra acquired at a binding energy of 1-1000eV, with a resolution of 1eV and a spot size of 800 µm. Higher resolution detailed scans are collected in C 1s region with a resolution of 0.065eV and a spot size of 100 µm. The C 1s spectra are acquired at a binding energy of 280-294 eV. Multiple low resolution survey spectra and high resolution C 1s spectra are collected from different regions of each CNT array sample. During the data collection, the sample chamber is maintained at an ultra high vacuum condition with a base pressure below 5x10-9 torr. All photoelectron spectra are obtained using ESCA25 Capture software (Service Physics, V5.01.04), and analyzed using CasaXPS software (Casa Software Ltd, V2.3.15). Deconvolution of the C 1s high resolution spectra are performed using a Gaussian-Lorentzian peak shape fitting with Shirley baseline correction.
2.3.7 Cyclic Voltammetry, Galvanostatic Charge-Discharge Cycle, and Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy characterization is conducted in three-electrode configuration using a potentiostat (Biologic SP-200) at a frequency of 10 mHz - 7 MHz. The CNT arrays are set as the working electrode, a platinum wire is used as the counter electrode, and a saturated calomel electrode is used as the reference electrode. For cyclic voltammetry and galvanostatic charge-discharge cycle charac- terizations, a two electrodes configuration is used. In a two electrodes configuration, CNT arrays serve as both anode and cathode. A separator film is then sandwiched between the anode and cathode to prevent direct electrical contact between them.