Chapter 2 Literature Review
2.8 Structure of the thesis
The remainder of this thesis consists of eight chapters and is organized as follows: In chapter 3, gives a detailed description of the experimental methods and procedures used in this study, and the materials and chemical chosen to accomplish the research work. Chapter 4 shows silicon nanocone (SNC) array supported Pt nanoparticles as anode catalyst for direct
methanol fuel cells (DMFCs). A high current density, mass activity and fairly good stability achieved for Pt/SNC anode in DMFCs. Chapter 5 reports an amorphous carbon coated silicon nanocone (ACNCs) array supported Pt nanoparticles for DMFCs applications. The Pt-ACNC electrode exhibited excellent electrocatalytic activity and stability towards both methanol oxidation reaction (MOR) and oxygen reduction reaction. Chapter 6 presents a novel new method to prepare 3D nanoporous graphitic carbon (g-C) supported Pt and Pt-Ru alloy catalysts as anode for DMFCs. The Pt-Ru/3D nanoporous g-C has better electrocatalytic activity towards CO and methanol oxidation in cyclic voltammetry measurements. Chapter 7 and 8 reports a novel 3D Pt nanofloweres structure and 2D contentious island Pt networks as a catalyst to improve the catalytic activity and durability for DMFCs. Chapter 9 presents the new approach to synthesis of the Pt nanocubes catalyst to enhance the CO tolerance, ethanol oxygen reaction (EOR) and MOR for fuel cells applications. In addition, chapter 9 shows the surface dependent study such as catalytic activity in an aqueous solution of ethanol-H2SO4
and methanol-H2SO4, respectively. Finally, Chapter 10 gives an overviews and conclusion of the present study, and future works.
Chapter 3
Experimental Methods
We have developed a several convenient techniques for the synthesis of nanostructured materials. The processes include the anodic aluminum oxides (AAO) template-assisted, flame-assisted, electrochemical assisted and fasten-silicon-assisted methods. The experimental flowcharts, the experimental procedures and structure property analysis methods for each process is discussed separately in more detail in the following sections.
3.1 Experimental flowcharts
Figure 3-1 shows the experiment flowchart of fabrications and analyses of Pt nanoparticles supported on the highly ordered Si nanocones (SNCs) and on the amorphous carbon coated silicon nanocones (ACNCs), which were fabricated by AAO templation method. During the preparation of highly ordered pore channel arrays, underlying TiN layers were anodically oxidized as well in the late stage of the AAO anodization to form TiOx
nanodots. TiOx nanodots were then used as nanomasks to etch the TiN layer and the underlying Si substrate in an inductively coupled plasma reactive ion etch (ICP-RIE) system.
Figure 3-2 illustrates experiment flowchart of synthesis and analyses of Pt-Ru alloy catalyst supported on the nanoporous graphitic carbon (g-C)/Si. The nanoporous g-C materials have been synthesized using the flame of adamantane (C10H16). Additionally, the experimental flowchart in Fig. 3-2 shows the electrochemical synthesis and analyses of the two-dimensional (2D) continuous Pt island networks and the three-dimensional (3D) Pt nanoflowers.
Finally, Fig. 3-3 shows the experiment flowchart for the synthesis and analysis of shape-controlled Pt nanoparticles and their electrochemical properties are measured.
The surface morphology, bonding structure and chemical composition were analyzed by
scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman spectroscopy, Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS), etc. The electrochemical measurements of specimens were performed at room temperature using a conventional three-electrode cell, in which specimens was used as a working electrode, Pt wire served as counter electrode and the reference electrode was an aqueous saturated calomel electrode (SCE).
Figure 3-1 Experimental flowchart of fabrications and analyses of Pt/SNC and Pt/ACNC arrays.
Figure 3-2 Experimental flowchart of fabrications and analyses of Pt-Ru/g-C/Si, 2D Pt networks/Si and 3D Pt nanoflowers/Si.
3.2 Deposition of TiN and Al films
Titanium nitride (TiN) films were deposited on a (100)-oriented p-type 4-inch silicon wafer of low resistivity (0.002 Ω-cm) in an ultrahigh vacuum reactive dc magnetron sputtering system (MRC PRIMUS 2500TM), with a base pressure of 5×10-9 Torr. The titanium target used was 99.9999% pure. TiN films were prepared in a gas mixture of argon and nitrogen. During the deposition, the dc power was set at 8 kW and followed by the deposition of an Al film in ~3 μm thickness on the TiN layer by a high vacuum thermal evaporator with the base pressure of 5×10-7 Torr. Aluminum ingots with a purity of 99.999%
were used as the aluminum source. Tungsten boats were used to melt and evaporate aluminum
ingots.
Figure 3-3 Experimental flowchart of synthesis and analyses of shape-controlled Pt nanoparticles.
3.3 Anodic aluminum oxide
The anode was the aluminum film specimen and a Pt foil was used as the cathode. The O-ring was clipped between the specimen and the tank fixed by a jig. Anodic oxidation of the Al film was carried out in 0.3 M oxalic acid (H2C2O4) at 25oC under a constant polarization
voltage of 40 V. The anodic current was measured by a source -measure unit (Keithley Model 2400). Lab-view for GPIB interface with Keithley 2400 was employed to force voltage and measure current simultaneously. The program could also set up the anodic oxidation of Al film end current point. The schematic diagram of experimental setup for Al anodization is shown in Fig. 3-4.
Figure 3-4 Schematic diagram of experiment setup for aluminum anodization.
3.4 Reactive-ion-etch system
The schematic diagram of the inductively coupled plasma (ICP) chamber is shown in Fig.
3-5. TiOx nanodots were then used as nanomasks to etch the TiN layer and the underlying Si substrate in an ICP reactive-ion-etch (RIE) system (Duratek). An inductively coupled plasma source is also called a transformer-coupled plasma source because of its similar mechanism to a transformer. The inductive coils serve just like the initial coils of a transformer. When an RF current flows in the coils, it generates a changing magnetic field, which in turn generates a changing electric field through inductive coupling. The inductively coupled electric field accelerates electrons and initiates ionization collisions. Since the electric field is in the
angular direction, electrons are accelerated in the angular direction and travel a long distance without collision with the chamber wall or electrode. This is why an ICP system can generate density plasma at low pressure (a few 10 mTorr). The ICP design that includes high-density plasma (HDP) dielectric CVD system, silicon, metal, and dielectric HDP etching system, native oxide sputtering clean system and ionized metal plasma PVD system is very popular in the semiconductor industry.
Figure 3-5 schematic diagram of ICP chamber
A bias RF system is added to the ICP chamber to generate self-bias and to control ion bombardment energy. Since ion bombardment from high-density plasma generates a lot of heat, a helium back-side cooling system with E-chuck is required for better wafer temperature control. In the ICP system, ion flux, manly determined by the plasma density, is controlled by the source RF power and ion bombardment energy is controlled by the bias RF power.
3.5 MPCVD system
Fabrication of α-C coated Si nanotips was then carried out in a microwave plasma CVD (MPCVD) system. The overall MPCVD system layout is shown schematically in Fig.3-6. A quartz tube is vertically attached to a rectangular waveguide used as deposition chamber. The microwave from a magnetron source (model IMG 2502-S, IDX Tokyo, Japan) is supplied to the quartz tube through an isolator and a power meter.
Figure 3-6 Schematic diagram of the bias assisted microwave plasma chemical vapor deposition system.
Then the microwave power is coupled to the quartz tube through an aluminum waveguide with a hole drilled through from top to bottom face. Aluminum tubes extend out from both holes.
Tube extensions are water-cooled as well. A sliding short circuit is then attached at the end of the waveguide. The lower position of the quartz tube is connected with a stainless steel multi-port
chamber equipped with a rotary pump. Substrates are positioned in the middle of the quartz tube waveguide intersection and held vertically by a substrate holder of ~1 cm2 in diameter and made of molybdenum.
3.6 Electrodeposition of Pt nanostructures
Electrodeposition of Pt nanostructure films were carried out by using bipolar pulse electrodeposition (BPPE) in a three electrode cell system (Jiehan 5000 Electrochemical Workstation). Figure 3-7 shows the setup of an electrochemical deposition facility. As shown in Fig. 3-7, a thin Pt wire, saturated calomel electrode (SCE) and Si substrate (Si nanocones, amorphous carbon coated Si nanocones, and flat silicon), as counter, reference and working electrodes, respectively, were used.
Figure 3-7 Schematic diagram of the electrochemical deposition setup.
In the BPPE, there are four operation parameters influencing the depositing of nanoparticles on the substrate: The high potential (VH), the lower potential (VL), the potential on time (Ton), and the potential off time (Toff). By applying specific potential pulses with time interval for the total experimental time (texp), Pt nanostructure films were deposited on the working electrode (Figure 3-7). All specimens used in this test had an area of 1.2 cm2.
3.7 Material analysis methods
3.7.1 Raman spectroscopy
While photons illuminate molecules or crystals, reaction between photons and atoms is always accompanied with momentum change or energy exchange. By collecting the scatter photons, sequence of spectrum, including Raman scattering (inelastic scattering) and Rayleigh scattering (elastic scattering) can be obtained. Photons of Raman scattering can be classified into two kinds, the Stoke side where photons loss energy or molecules gain energy and the anti-Stoke side where photons gain energy or molecules loss energy. Stoke side is generally used to characterize the material. As Raman spectrum provides the information of crystallinity and bonding, it has become the most direct and convenient way to identify carbon related materials. The Raman spectrum peak of sp3 and sp2 bonds in crystalline graphite are 1380 (D peak) and 1580 cm-1(G-peak), respectively. The instrument in operation is a Renishaw’s Raman microscope, Model 2000. The source available is He-Ne laser with the wavelength of 632.82nm and the power of 200mW. The spectral slit width is 0.4cm-1.
3.7.2 SEM
Scanning electron microscopy (SEM) is used to examine the surface morphology of the samples at a high magnification. The high magnification range of the SEM is achieved due to its resolving power of approximately 3-6 nm. The SEM is useful and popular for many reasons. One of the great advantages of SEM is its large depth of field (the amount of sample that is in sharp focus at one time). This makes it possible to examine surfaces with a relatively high level of surface variability (and at much higher magnifications). This is because the depth of field of the SEM can be up to four hundred times greater than that of a light microscope. Moreover, it has the advantage of easy sample preparation, and high image resolution. A common SEM is equipped with an electron gun to generate electron beams to be
accelerated under 0.4-40 kV voltage. By deflecting the incident beams with focusing coils, a two dimensional image can be obtained by detecting the reflected secondary electrons and the backscatter electrons.
The model mainly engaged is field emission type SEM JEOL-6500. Accelerating voltage is 15 kV with current of 10μA. Working distance is 10mm under 9.63x10-5Pa. The Fig. 3.7 shows a simple schematic drawing of the basic principle of the SEM.
Figure 3-8 Schematic drawing of the basic principle of the SEM.
3.7.3 TEM
Transmission electron microscopy (TEM) (JEOL JEM-2010F) is operating at 200 kV accelerating voltage. This is the most important instrument to study defects in detail. Transmitted and diffracted electrons are recombined by objective lens to form a diffraction pattern in the back focal plane of that lens and a magnified image of the sample in its image plane. A number of
intermediate lenses are used to project either the image or the diffraction pattern onto a fluorescent screen for observation. The screen is usually lifted and the image is formed on photographic film for recording.
Figure 3-9 Schematic diagram of the TEM image modes (a) Bright field image (b) Dark field image. [http://www.microscopy.ethz.ch/TEM_imaging.htm].
The image modes in TEM are shown in Fig. 3-9. The TEM specimens were prepared by either mechanically scratching or ultrasonic agitation using dissecting forceps in the presence of a small drop of ethanol. The scratched specimens or ultrasonic agitation solution was put onto a holey-copper grid and dried in air at room temperature. Energy-dispersive X-ray
(a)
(b)
analysis (EDX) was employed to study the chemical composition of the specimens. Moreover, electron energy loss spectroscopy (EELS-Gatan GIF 2000) analyses were also performed in the TEM system to map the elemental distribution.
3.7.4 AES and XPS
Auger electron spectroscopy (AES) analysis technique employs an electron beam (2-30 keV) to irradiate the specimen surface and excite Auger electrons which possess specific energy. Through assaying the kinetics energy of Auger electrons by an electron energy analyzer, the element composition and the specimen chemical state are ready to be acquired.
Because incident electrons with low-energies (1-3 keV) have very short inelastic mean free paths (5-20 Å) inside the solid phase materials, AES technique is usually used to obtain the information within 50 Å from the surface. In this study, AES was employed to investigate the chemical composition of specimens. The AES analyses were performed by using a VG Micro-lab 310F Auger system with a Schottky field emission electron source.
Surface analysis by X-ray photoelectron spectroscopy (XPS) involves irradiating a solid in vacuum with monoenergetic soft X-rays and analyzing the emitted electrons by energy.
The spectrum is obtained as a plot of the number of detected electrons per energy interval versus their binding energy (or kinetic energy). Since the mean free path of the electrons is very small, the electrons which are detected originate from only the top few atomic layers.
Quantitative data can be obtained from peak heights or peak areas, and the identification of chemical states often can be made from exact measurements of peak positions and separations.
In this study, XPS was used to analyze the chemical states of Pt and Pt-Ru alloy catalysts.
XPS analyses were performed on a VG Microlab 310F system with Al-Kα (1486.6 eV) excitation. X-ray emission energy was 400 W with 15 kV accelerating voltage. Argon ion with ion energy of 5 keV was used for sputter profiling.
3.8 Electrochemical measurements
The electrochemical tests were performed with a model Jiehan 5000 Electrochemical Workstation System at 25°C. Figure 3-10 shows the setup of an electrochemical measurements facility. In these tests, a standard three-electrode electrochemical cell was used.
In the experiments of all cyclic voltammetry (CVs), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS), the counter electrode was used as a thin Pt wire (99.99%), saturated calomel electrode (SCE) was used as the reference electrode, and a specimen were used as the working electrode. The thin Pt wire was connected to a high-quality screened cable. A high conductivity connection was made to the specimen for the working electrode. The electrodes of the probe were connected to a Jiehan 5000 Electrochemical Workstation system that controlled all experiments. Before each test, the Pt wire electrode was immersed in HCl (3:1, v/v) solution for about 1 min and then washed by distilled water. All specimens used in this test had an area of 1.2 cm2.
Figure 3-10 Schematic diagram of the electrochemical measurements setup.
Chapter 4
Pt Nanoparticles Supported on Ordered Si Nanocones as Catalyst for Methanol Oxidation
4.1 Introduction
Extensive studies have been devoted to the development of various nanostructured catalysts for energy-related technologies such as direct methanol fuel cells (DMFCs) for small portable electronic applications, such as the supplementary rechargeable battery for laptops and cellophanes etc. DMFCs fuel cell attracts such a wide attention is due to simple system design, low operation temperature, convenient fuel storage, high energy density and long life, as compared to conventional rechargeable power sources, such as Li ion batteries. In the fuel cells, nanostructured Pt are normally used as the catalyst in DMFCs because of the high electrocatalytic activity for methanol oxidation. A major issue in such applications is to increase the catalytic activity by increasing the surface area of the catalyst deposited on its support. In addition, In order to increase the electrocatalytic mass activity and reduce usage of the precious Pt catalyst, most methods of fabricating Pt catalyst-supporting electrodes tried to disperse Pt nanoparticles on the electrode support so that creating a high catalytic surface area and reducing Pt consumption could be achieved together. Much effort was done to improve catalyst mass activity by different approaches, such as nanoporous graphite with Pt nanoparticles [79], Pt particles supported on polymeric nanocones [80], carbon nanotubes (CNTs) with Pt-Ru [81, 82], MnO2/CNT supported Pt-Ru [83], carbon nanocoils with Pt-Ru catalyst alloys [84] and carbon-coated anatase TiO2 composite [85]. However, many complications of using Pt catalyst nanoparticles for DMFCs still exist and need to be solved for better utilization of the Pt catalyst. In particular, CO poisoning effect and catalyst loss during electrocatalytic reactions in DMFCs are among the major difficulties frequently
addressed and widely studied.
In this chapter, Pt nanoparticles were electrodeposited on a highly ordered Si nanocone (SNC) array, which was fabricated by means of anodic aluminum oxide (AAO) templation.
The SNCs provided a high surface area for Pt catalyst loading, and the well ordered arrangement of the nanocones allowed a relatively uniform electric potential distribution over the nanocones during the Pt electrodepositon, thereby Pt nanoparticles could be well dispersed on the Si support and uniform in size. In addition, the SNCs were fabricated from a Si substrate of low resistivity, and thus the SNC support was very suitable for the use as the Pt electrocatalytic electrodes in respect of electrical conductivity. Moreover, the surface oxide formed on the SNC surface could enhance the CO tolerance of the Pt catalyst via bifunctional mechanism.
4.2 Fabrication procedure of the SNC array
The fabrication procedures of Si nanocones had been described elsewhere[86, 87] and illustrated in Figure 4-1. A p-type 4-inch Si wafer of low resistivity (0.002 Ω-cm) was use used as the substrate. A TiN thin film 30 nm thick was first sputter-deposited on the Si surface, followed by thermal evaporation of an aluminum thin film 1 μm thick. The TiN layer was used as an adhesion layer between the Si substrate and the Al thin film, and would be later used for preparation of the nanodot mask for fabrication the SNCs. The as-deposited Al/TiN film stack was then oxidized by electrochemical anodization, which was performed in 0.3 M oxalic acid (H2C2O4) at 25oC under a constant polarization voltage of 40 V for 20 min. Anodic oxidation of the Al thin film under the anodization conditions would produce hexagonally arranged AAO nanopore channels. As the anodization reaction approached the interface between the Al and TiN thin films, local anodization of the underlying TiN layer occurred.
Because the TiN oxidation reaction was confined in the nanosized AAO pore channels,
dome-shaped TiOx nanodots were produced on the TiN layer. The AAO was then removed by the aqueous solution of 6 wt % H3PO4 and 1.5 wt % CrO3 at 60oC for 40 min., thereby the TiO2
nanomask was formed. To fabricate Si nanocones, the TiN capped Si substrate with the TiO2
nanomask was etched in an inductively-coupled-plasma reactive-ion-etch (ICP-RIE) system for 50 sec., using a gas mixture of BCl3 and Cl2 as the plasma source. The RIE process was performed under the following working conditions: plasma power 400 W, substrate bias power 120 W, working pressure 10 mtorr with a flow rate of 35 sccm for the plasma gas source.
Figure 4-1 shows the fabrication scheme of SNC arrays: (a) deposition of TiN and Al thin film on the Si wafer by sputter deposition and thermal evaporation, respectively, (b) anodic oxidation of the Al film and formation of TiOx nanodots, (c) removal of the AAO by wet etch, (d) reactive ion etch (RIE) of the remaining TiN and the Si substrate, and (e) formation of the SNC arrays.
4.3 Electrodeposition of Pt nanoparticles on SNC array
The Pt nanoparticles were electrodeposited on SNCs in an aqueous solution of 1M K2PtCl6-1M HCl at 25○C by potentiostatic pulse plating in a three electrode cell system with SNCs as the working electrode, thin Pt wire as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. Prior to electrodeposition of Pt catalyst on SNCs, the ordered SNC array was immersed in a deoxygenated aqueous solution of 1 M H2PtCl6 at 35oC for 3 hours. All solutions were prepared in deionized water (>18 MΩ). Bipolar pulses were used and the pulse height and duration were .0.09mV and 7ms for negative pulse, and
The Pt nanoparticles were electrodeposited on SNCs in an aqueous solution of 1M K2PtCl6-1M HCl at 25○C by potentiostatic pulse plating in a three electrode cell system with SNCs as the working electrode, thin Pt wire as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. Prior to electrodeposition of Pt catalyst on SNCs, the ordered SNC array was immersed in a deoxygenated aqueous solution of 1 M H2PtCl6 at 35oC for 3 hours. All solutions were prepared in deionized water (>18 MΩ). Bipolar pulses were used and the pulse height and duration were .0.09mV and 7ms for negative pulse, and