4.2.1 Atomic Force Microscopy (AFM)
AFM is a method of measuring surface topography on a scale from angstroms to 100 microns. The technique involves imaging a sample through the use of a probe, or tip, with a radius of 20 nm. The tip is held several nanometers above the surface using a feedback mechanism that measures surface–tip interactions on the scale of nanoNewtons. Variations in tip height are recorded while the tip is scanned repeatedly across the sample, producing a topographic image of the surface. AFM measurements were performed using a Nanoscope IIIa Scanning Probe Microscopy (Digital Instruments, Santa Barbara, CA) in tapping mode. The software was equipped with a surface roughness determination and peak counting function, which allowed the investigation of the impact of modification on the surface of the membrane.
38 4.2.2 X- Ray Photoelectron Spectroscopy (XPS)
XPS, also known as Electron Spectroscopy for Chemical Analysis (ESCA), was used to determine the quantitative atomic composition and chemistry of the membrane surfaces.
XPS utilizes photo-ionization and energy-dispersive analysis of the emitted photoelectrons to study the composition and electronic state of the surface region of a sample. The photon is absorbed by an atom on the surface, leading to ionization and the emission of a core (inner shell) electron. The kinetic energy distribution of the emitted photoelectrons can be measured using any appropriate electron energy analyzer and a photoelectron spectrum can thus be recorded. For each element, there is a characteristic binding energy associated with each core atomic orbital. The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample under study. XPS analyses were performed on a Kratos AXIS Ultra DLD X-ray Photoelectron Spectrometer located at Electron Microbeam Analysis Laboratory (EMAL) in The University of Michigan, Ann Arbor. Membranes samples were mounted on a sample holder with the use of an adhesive tape and kept overnight at high vacuum in the preparation chamber before they were transferred to the analysis chamber of the spectrometer for analysis. Each spectral region was scanned until a good signal to noise ratio was observed. Survey scan spectra, from which the overall atomic composition was computed, were acquired in 300 s using a pass energy of 160 eV and a 0.5 eV step.
4.2.3 Fourier Transform Infrared (FTIR) Spectroscopy
Infrared (IR) spectroscopy is widely used to assess the chemical nature of a substance including chemical bonds, molecular orientations, molecular energy levels and molecular interactions. This technique can be used to determine a sample’s static properties as well
39
as time resolved information for dynamic events that occur in the sample. In a standard experiment, light with a broad range of frequencies is directed though a sample while the transmittance of the light through the sample is measured. By using a pulsed light source, dynamic events in the sample can be observed resulting in additional information on chemical behavior of the sample. Experiments can be performed to gain information regarding a sample’s atomic geometry and molecular interactions. This kind of analysis is called time resolved spectroscopy (TRS).
Two different techniques are used in IR spectroscopy. The first method uses a diffraction grating to spatially separate the spectrum. The frequency resolution of this spectrometer is a function of the fineness of the grating. A scanning mirror is used to channel one wavelength of light into the detector. The second method involves using an interferometer to encode the spectrum in the measured signal. This method is called Fourier transform infrared spectroscopy (FTIR). An FTIR spectrometer uses a Michelson interferometer to modulate the optical signal and encode the spectrum information. A Fourier transform is performed on the resulting signal to retrieve the frequency/magnitude information. The basic operation of FTIR equipment depends on four important parts. They are source, interferometer, sample and detector, which are interconnected.
FTIR was used in attenuated total reflectance (ATR) mode to study the chemical nature of the membrane surfaces prior to and after modification. Digilab UMA 600 FT-IT microscope with a Pike HATR adapter and an Excalibur FTS 400 spectrometer (Randolph, MA) was used for all the analyses experiments conducted in this study.
40
4.2.4 Scanning Electron Microscopy (SEM) / Energy Dispersive X-ray Spectroscopy (EDS)
Scanning electron microscope (SEM) uses an electron beam produced by high voltage (15-20KeV) to visualize the sample as the light microscope uses the visible light produced by illuminating the source. Due to the application of electron optics, SEM results in a much higher resolution and greater depth of field in imaging a sample surface.
In SEM, an electron gun produces a beam of monochromatic electrons. This beam passes through the first and second condenser lens resulting in a thinner and coherent beam, which is focused onto the sample surface through an objective lens. The finer electron beam thus focused on the specimen is scanned across the specimen surface. Sharpness of the image produced depends on the fineness of the beam diameter.
Figure 4.2: Specimen interaction with electron beam.
10e-beam
2oelectrons Characteristic X-rays
Backscattered electrons
Cathodoluminescence
Auger electrons
Transmitted electrons Specimen
Current
Specimen
41
When an electronic beam strikes the specimen, due to various interactions a variety of signals are generated as shown in Figure 4.2. Of these generated signals, the strongest region of the electron energy spectrum is due to secondary electrons. The secondary electron yield depends on many factors, and is generally higher for high atomic number targets, and at higher angles of incidence. Secondary electrons are produced when an incident electron excites an electron in the sample and loses some of its energy in the process. The excited electron moves towards the surface of the sample undergoing elastic and inelastic collisions until it reaches the surface, where it can escape if it still has sufficient energy. Secondary electrons, by convention, are those emitted with energies less than 50 eV. This is only a small fraction of the electrons emitted from the sample. These secondary electrons are used for imaging in SEM.
SEM was used to visually observe the degree of polymer grafting and membrane fouling. SEM analyses were also conducted to obtain cross section images of the membranes to determine the thickness of the top selective layer. The samples were coated with a thin layer of gold under an argon atmosphere using a SPI Module Sputter Coater with Etch mode, and were placed in the scanning electron microscope for analysis. Two electron microscopes were used in this project to study the modification and fouling associated with membrane filtration. A Philips XL 30 FEG SEM, located at EMAL at The University of Michigan, Ann Arbor, and a Hitachi S – 4800 SEM, located at Center for Material and Sensor Characterization at The University of Toledo, Toledo.
EDS or EDX is a chemical microanalysis technique used in conjunction with SEM. The EDS technique detects x-rays emitted from the sample during bombardment by the primary electron beam to characterize the elemental composition of the analyzed
42
surface. The emitted X-ray has an energy characteristic of the parent element allowing for it to be specifically identified. The pattern obtained by this method contains peaks in various positions displaying the energy levels of each scattered X-ray corresponding to a certain material/element. EDS data was obtained by Ultra Thin Window (UTW) Si-Li Solid State X-ray detector attached to XL 30 FEG SEM.