Analysis Instruments

3.4.1 Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) is used to observe the surface morphology of wide range kinds of objects. There are many advantages including of easy sample preparation, high image resolution, large depth of field, and high magnification. [33]

The SEM image is that signals (secondary electrons and backscattered electrons) emit from the sample surface as the sample is bombarded by the high energy incident electrons. The fabricating CNT morphology and the dispersing Pt on CNT morphology could be observed by JEOL JSM6500F in NCTU with field emission electron source and 15kV accelerate voltage.

Fig.3.5 Diagram of a Scanning Electron Microscopy [http://www.le.imm.cnr.it/sito/laboratories/jsm6500f.html]

3.4.2 Transmission Electron Microscopy (TEM)

In a typical TEM a static beam of electrons at 100-400kV accelerating voltage illuminate a region of an electron transparent specimen which is immersed in the objective lens of the microscope. The transmitted and diffracted electrons are recombined by the objective lens to form a diffraction patter in the back focal plane of that lens and a magnified image of the sample in its image plane. [33]

The raw MWNTs and the modified MWNTs morphology could be compared by TEM using

a JEOL JEM 4000 system in NCTU operating at 200kV. And the particle morphology, size and size distribution of Pt nanoparticles dispersed on the surface of MWNTs are also characterized by TEM.

3.4.3 X-ray Photoelectron Spectroscopy (XPS)

The phenomenon is based on the photoelectric effect. The concept of the photon was used to describe the ejection of electrons from a surface when photons impinge upon it. The XPS technique is highly surface specific (< 5nm) due to the short range of the photoelectrons that are excited from the solid. The energy of the photoelectrons leaving the sample is determined using a Spherical Capacitor Analyzer (SCA) this gives a spectrum with a series of photoelectron peaks.

The binding energy of the peaks is characteristic of each element. The peak areas can be used (with appropriate sensitivity factors) to determine the composition of the materials surface. [33]

XPS could determine the difference of the raw CNT and the modified CNT due to the element C chemical shifts. Furthermore, it may ensure if Pt is reductive by the same way. In this study, XPS analysis is carried out on a ESCA PHI 1600 using an Mg Kα X-ray source in NTHU.

Fig.3.6 Diagram of a X-ray Photoelectron Spectroscopy [http://www.nscric.nthu.edu.tw/Other/augeresca/auesca.html]

3.4.4 Fourier Transform Infrared Spectrometer (FTIR)

Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify organic and inorganic materials. This technique measures the absorption of various infrared light wavelengths by the material of interest. These infrared absorption bands identify specific molecular components and structures. [33]

The functional groups on the surface of MWNTs modified by chemical solution could be determined by FTIR. The FTIR measurements are performed on a PROTEGE 460 series FTIR apparatus by transmission spectroscopy in NCTU.

Fig.3.7 Diagram of a Fourier Transform Infrared Spectrometer [http://www.forumsci.co.il/HPLC/FTIR_page.html]

3.4.5 Cyclic Voltammetry (CV) Potentiostat

A potentiostat is an electronic device that controls the voltage difference between a working electrode and a reference electrode. Both electrodes are contained in an electrochemical cell.

The potentiostat implements this control by injecting current into the cell through an auxiliary, or counter, electrode. In almost all applications, the potentiostat measures the current flow between the working and auxiliary electrodes. The controlled variable in a potentiostat is the cell potential

and the measured variable is the cell current.

The CHI Version 5.01 system in the potentiostat in NCTU is used to measure the electrochemical specific surface area of the dispersive Pt on the surface of MWNTs for the fuel-cell electrodes. From the CV, the charge equivalent to the area under the hydrogen desorption region is evaluated and the electrochemical specific surface area is calculated assuming that the charge is required for the adsorption-desorption of a monolayer of atomic hydrogen on the surface.

Fig.3.8 Schematic of a Cyclic Voltammetry (CV) Potentiostat [http://www.gamry.com/App_Notes/Potentiostat_Primer.htm#Workking]

3.4.6 Energy Dispersive X-ray (EDX)

It is a technique used for identifying the elemental composition of the specimen, or an area of interest thereof. The EDX analysis system works as an integrated feature of SEM (JEOL JSM6500F) in NCTU. An EDX spectrum plot not only identifies the element corresponding to each of its peaks, but the type of X-ray to which it corresponds as well. For example, a peak

corresponding to the amount of energy possessed by X-rays emitted by an electron in the L-shell going down to the K-shell is identified as a K-Alpha peak. The peak corresponding to X-rays emitted by M-shell electrons going to the K-shell is identified as a K-Beta peak. [33]

EDX measurements show the element on the MWNTs and the content of Pt/MWNTs. It appears that the difference content of Pt is on the raw MWNTs and the modified MWNTs.

Fig.3.9 Elements in an EDX spectrum are identified based on the energy content of the X-rays

[http://www.semiconfareast.com/edxwdx.htm]

3.4.7 X-Ray Diffraction (XRD)

X-ray Diffraction (XRD) is one of the primary techniques used by solid state chemists to characterize materials. XRD can provide information about crystalline structure and particle size in a sample even when the crystallite size is too small for single crystal x-ray diffraction. [33]

An X-ray beam hits a sample and is diffracted. We can observe the diffraction peaks when the distances between the planes of the atoms apply to Bragg's Law. Bragg's Law is:

nλ =2dsinθ (1) Where the integer n is the order of the diffracted beam, is the wavelength of the incident X-ray beam, d is the distance between adjacent planes of atoms (the d-spacings), and is the angle of incidence of the X-ray beam.

The broader diffraction peaks for the catalyst led to smaller average particle size as calculated by the Scherrer equation.

θB

λ

θ α

cos B L K

2 1

= K (2)

Where L is the average particle size, K is the constant, λ K^α1 is the X-ray wavelength

(1.54056 Ǻ for Cu Kα1 radiation), B2θ is the peak broadening, and θB is the angle corresponding to the peak maximum.

In this study, the particle size calculated by XRD is compared with the data measured by TEM. It is determined how the functional group (COOH) on the MWNTs affects the Pt particle size forming.

Fig.3.10 Schematic of X-Ray Diffraction

[http://pubs.usgs.gov/info/diffraction/html/index.html]