Chapter 3 Experimental methods
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 +0.02mV and 1ms for the positive pulse, respectively. The size of Pt nanoparticles and mass loading of platinum on SNCs were controlled by the applied potential and pulse duration.
Chloride anions in the electrolyte solution tend to adsorb on Pt particles, making Pt nanoparticles better separated from each other due to electrostatic repulsion between negatively charged surface layers [88]. The size of Pt nanoparticles and mass loading of platinum on SNCs were controlled by the applied potential and pulse duration. The Pt/SNC electrode was rinsed thoroughly with deionized water to remove residual chlorine ions after Pt electrodeposition, and allowed to dry before use in the measurement cell. The Pt/SNC electrode was rinsed thoroughly with deionized water to remove residual chlorine ions after Pt electrodeposition, and allowed to dry before use in the measurement cell.
4.4 Structural characterization of SNC array
Figures 4-2 shows the side-view SEM images of the fabrication process of SNC arrays.
The side-view SEM image in Fig.4-2 (a) reveals that the TiOx nanomask arrangement was compliant with the pore arrangement of the AAO template and had a nanomask density of
~1x1010 cm-2. To etch the TiN layer, a gas mixture of BCl3 and Cl2 was used as the plasma source in the ICP-RIE system. TiOx and Si could also be etched by the Cl2-based plasma
Figure 4-2 shows the side-view SEM images of the fabrication of SNC arrays: (a) TiOx
nanomasks after the removal of the AAO layer by wet etch, (b) the TiOx/TiN/SNC arrays after the RIE etch by SF6, Cl2 and O2 for 40 sec. and (c) removal of the oxide layer by wet chemical etch and the formation of highly ordered SNC arrays.
(a)
(b)
(c)
Figure 4-3 shows (a) TEM image of the SNC array and (b) high-resolution TEM image of the SNC array in which the inset reveals the selected area diffraction pattern.
source, but the relatively large height of the TiOx nanomask provided sufficient masking thickness to the underlying TiN and Si substrate so that the arrangement pattern of nanopillars could be successfully transferred to the Si substrate. Then, gas mixture of Cl2, SF6 and O2
(a)
(b)
were used as the plasma source to etch Si substrates and form SNCs of high aspect ratio, as shown in Fig. 4-2 (b). The SNC array had an ordered hexagonal arrangement as AAO as well.
The surface silicon oxide of the Si nanocones was removed by wet etching. Si nanocones were uniform in size and shape as shown in Fig. 4-2 (c). Most SNCs had a height of ~200 nm and a base width of 100 nm. The geometric shape and size of nanocones could be tailored by varying the RIE time.
Transmission electron microscope (TEM) was used to further characterize the morphology of an individual SNC. A bright-field TEM image of typical SNC is shown in Fig. 4-3 (a). The bright-field high resolution TEM image of SNC is shown in Fig. 4-3 (b) in which the nanocones are clearly evident as the final stage. Sample for TEM was made by simply scratching the specimen surface using sharp forceps. The inset to Fig. 4-3 shows corresponding selected area diffraction pattern. According to electron diffraction study (Fig.
4-3 inset), the SNCs is crystalline silicons. These observations indicated that formation of Si nanocones was a result of plasma etches of the Si substrate.
4.5 Structural characterization of Pt/SNC array
Pulse potentiostatic method was used for the depositing Pt nanoparticles on ACNC electrode. Pulse potentiostatic electrodeposition has advantages over the conventional direct current potentiostatic electrodeposition because of the possible beneficial effects on the morphology and compositional uniformity of the deposit [89]. Those effects are a consequence of the free choices of several variables independently, e.g., type of periodic waveform, potential, and duration of cathodic and anodic pulses for a particular deposition solution rather than simply the potential. The particles size control by applying the alternate positive and the negative potential pulses on the working electrode. The positive potential pulse was applied to avoid particle coalescence so that nanosized Pt nanoparticles could be
deposited on the Si nanocones.
Figure 4-4 shows the scanning electron microscopy (SEM) image of the SNC array with electrodeposited Pt nanoparticles. The well ordered SNCs, with a hexagonal arrangement, was
~300 nm high and had a base diameter of ~100 nm. X-ray photoelectron spectroscopy (XPS) indicated the presence of Pt on the SNCs after the electrodeposition (shown in Fig. 4-5).
According to the cross-sectional SEM image, Pt particles seemed to slightly accumulate on the tip of SNCs, but nanoparticles agglomeration on the sidewall of the nanocones was insignificant. Due to the very small size, Pt nanoparticles are hardly observed to adhere on the sidewall of the SNCs from the SEM images.
Figure 4-4 shows SEM images of the SNC array with electrodeposited Pt nanoparticles; (a) a side view, and (b) cross-sectional view.
The Pt nanoparticles had a size smaller than ~5 nm according to transmission electron (b)
(a)
microscopy (TEM) study. Figure 4-6 shows the high-resolution TEM image of a silicon nanocone with well dispersed Pt nanoparticles on it. The selected area electron diffraction (SAED) pattern is shown in the inset of Fig. 4-6. Four diffraction rings can be indexed as (111), (110), (200) and (311) orientations for the Pt face center cubic (FCC) lattice structure.
Electron energy loss spectroscopy (EELS) was also used to map the elemental distribution of Pt on the SNC, and the mapping clearly shows that Pt nanoparticles distributed on the tip and sidewall of the SNC (shown in Fig. 4-7).
Figure 4-5 shows the XPS spectrum of the Pt/SNC arrays.
Figure 4-6 shows HRTEM image of the Pt/SNC array in which the inset reveals the selected area diffraction pattern of the nanocone.
800 600 400 200
Pt(4f) Si(2p) Si(2s) Pt(4d)
C(1s) O(1s)
Relative XPS Intensity
Binding Energy (eV)
Pt/SNC
Figure 4-7 EELS mapping images of (a) Silicon, and (b) Pt for the Pt nanoparticles loaded SNCs.
4.5 Electrochemical characterization
The electroactive surface area (ESA) of the Pt loaded SNC electrode can be determined by CO stripping cyclic voltammetry. The CO stripping voltammogram for the Pt/SNC electrode in a CO saturated 1MH2SO4 aqueous solution is shown in Fig. 4-8 (a). A high ESA of ~317m2 g-1 was obtained for the Pt/SNC electrode by integrating the CO electro-oxidation peak area, assuming an oxidation charge of 420 µC cm-2 for one monolayer of CO on a smooth Pt surface [90]. For comparison, the ESA of the Pt film electrodeposited on a flat Si substrate (hereafter abbreviated as Pt/Si) was calculated to be 38.9 m2 g-1 from Fig. 4-8 (b).
The electrocatalytic stability of the Pt/SNC electrode was evaluated by repeating the cyclic voltammetric (CV) scan from -0.45 to 1.2V in the 1M H2SO4 solution for more than 1000 cycles. From Fig. 4-9, the CV curve of the 1000th cycle shows that the hydrogen adsorption/desorption peak had a peak current reduction by ~20% as compared with that of the second cycle, indicating possible Pt nanoparticle loss from the SNCs after 1000 cycles of hydrogen oxidation and reduction. Fig. 4-10 is the plots of the ESA as a function of the number of CV cycles for the Pt/SNC and the Pt/Si electrodes. The plots are normalized
(a) (b)
against the ESA of the first cycle. The ESA of the Pt/SNC electrode had a moderate drop for the first 100 CV cycles, and then progressively decreased to 75% of the initial ESA after 1000 CV cycles. On the other hand, the Pt/Si electrode shows a dramatic drop in the ESA in the first 200 cycles, followed by continuous and significant ESA decrease. The much smaller ESA loss of the Pt/SNC electrode suggests that the electrodeposited Pt nanoparticles were well adhered to the Si nanocones.
Figure 4-8 CO stripping cyclic voltammograms of (a) Pt nanoparticles electrodeposited on the ordered SNCs and (b) the Pt film electrodeposited on the flat silicon substrate in a CO saturated 1M H2SO4 solution. The scan rate is 20 mVs-1.
-0.4 -0.2 0.0 0.2 0.4
Figure 4-9 Cyclic voltammograms for Pt/SNC electrode in 1M H2SO4 aqueous solution at room temperature and with a scan rate of 25 mVs-1: (a) the 2nd cycle and (b) the 1000th cycle.
Figure 4-10 shows the plot of electroactive surface area (ESA) as a function of the number of the cyclic voltammetric scan for (a) the Pt film electrodeposited on the flat Si substrate and, (b) the ordered Pt/SNC electrode.
The Pt nanoparticles had a very high catalyst specific activity (current per ESA) for electro-oxidation of methanol. Figure 4–11 shows the cyclic voltammogram of the Pt/SNC
0 200 400 600 800 1000
electrode in 1 M methanol/1 M H2SO4 aqueous solution. The unit of the y-axis of the CV plot is labeled by the current per ESA. The CV curve shows that the methanol oxidation peak had the maximum around 0.83 V vs. SCE and a very low onset potential of ~0.08 V.
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Figure 4-11 shows the cyclic voltammograms of (a) the Pt catalyst nanoparticles on the ordered SNC array and (b) the Pt film electrodeposited on the flat silicon substrate in an argon saturated 1 M CH3OH/1 M H2SO4 aqueous solution. The scan rate is 25 mVs-1.
Also shown in Fig. 4-11 is the CV curve of the Pt/Si electrode, which shows a much smaller specific activity with a high onset potential. The negative onset potential shifting indicated that the Pt nanoparticles on the ordered SNC array can effectively reduce overpotentials in the methanol electrooxidation reaction [91–94]. Because of the very large ESA, the Pt/SNC electrode had a better electrocatalytic mass activity compared with many previously reported electrodes made from nanostructured materials. The mass activity of the Pt/SNC electrode at 0.4V was ~0.53 A mg-1 at room temperature, that was about six folds higher than that of the Pt loaded carbon nanocoils [84], and two folds higher than the carbon-coated anatase TiO2 nanocomposite [85].
Figure 4-12 shows the chronoamperogram of electroactivity of the Pt/SNC electrode at the oxidation potential ~0.3V in the 1 M methanol/1 M H2SO4 aqueous solution at 25○C. The chronoamperogram showed that electrocatalytic oxidation of methanol maintained a high activity and was very stable during the measurement for more than 2 h. The observation implied that most CO adspecies could be oxidized and removed from the Pt catalyst nanoparticles so that the catalytic oxidation of methanol could be kept proceeding efficiently on the Pt/SNC electrode. The argument was supported by the CO stripping cyclic voltammetry study. In Fig. 4-8, the CO stripping voltammogram of the Pt-SNC electrode was compared with that of the Pt/Si electrode. For the Pt/SCN electrode, the onset of electro-oxidation of CO took place below ~0.3V vs. SCE and the peak potential was ~0.6 V. On the other hand, the Pt/Si electrode had an onset potential for CO electrooxidation ~0.5V vs. SCE.
The observation that the Pt/SNC electrode had a much lower onset potential in the CO stripping voltammogram indicated that electrooxidation reaction of CO adspecies on Pt sites could be efficiently performed on the Pt/SNC electrode.
0 20 40 60 80 100 120 140
Figure 4-12 shows the chronoamperometric response of (a) Pt nanoparticles electrodeposited on the ordered SNCs, and (b) the Pt film electrodeposited on the flat Si substrate in a saturated 1 M CH3OH/1 M H2SO4 aqueous solution at 0.3V (SCE) for 2 h.
The scale of curve b is multiplies by 100.
The excellent electrocatalytic performance of the Pt-SNC electrode can be explained as follows. First, the SNCs provided a large surface area for Pt nanoparticles loading, resulting in a very high catalytic mass activity. With the present nanocone geometry, the SNCs provided a surface area applicable for Pt loading six times (the ratio of the cone surface area to the cone base area) larger than a flat Si surface, and thus increased the mass activity accordingly.
Second, nanosized and highly dispersed Pt catalyst nanoparticles were deposited on the SNCs.
Unlike many reported porous electrodes, which had a very large surface area as well, the ordered SNCs may provide better electrodeposition conditions for well dispersed Pt nanoparticles. Because the arrangement of the SNCs was highly ordered, the potential distribution over the SNC array should be even and, therefore, the deposited Pt particles were uniform in size. Besides, compared with many porous supports, the SNC array had a larger open volume, allowing ionic species to diffuse more freely between Si nanocones, and thus Pt particles could be readily electrodeposited on the SNCs without significant agglomeration. By carefully tuning the pulse voltage and pulse duration, we can limit the Pt particle size to the nanometer range and mitigate particle coalescence, thereby increasing the electroactive area.
Third, silicon oxide grown on the Si nanocones can play an important role in enhancing CO tolerance of the electrode via the bifunctional mechanism [12]. A nature SiO2 layer is usually terminated with silanol groups. The bifunctional model describes that hydroxyl surface groups are able to oxidize and remove adjacent CO adspecies from the Pt catalyst surface, thus avoiding CO poisoning. Because the Pt nanoparticles were well dispersed on the SNCs, CO adspecies bound on the periphery of Pt nanoparticles can be readily oxidized by surrounding silanol groups. Moreover, due to the nanoscaled size of Pt catalyst, OH adspecies can migrate over the Pt nanoparticle without much difficulty and react with adsorbed CO via a Langmuir – Hinshelwood type reaction mechanism, in which adsorbed reactants diffuse, collide and form products on the surface, thereby facilitating a better efficiency of CO oxidation.
4.6 Summary
We have electrodeposited Pt catalyst nanoparticles on highly ordered SNCs fabricated by AAO templation, and electrocatalytic oxidation of methanol on the Pt/SNC electrode was studied. Because of the large surface area of the Si nanocones and the highly dispersed nanosized Pt catalyst particles, the Pt/SCN electrode demonstrated a very high mass activity and a very low onset potential. CV measurements, CO stripping cyclic voltammogram and chronoamperometric study indicated that the Pt nanoparticles were well adhered to the Si
We have electrodeposited Pt catalyst nanoparticles on highly ordered SNCs fabricated by AAO templation, and electrocatalytic oxidation of methanol on the Pt/SNC electrode was studied. Because of the large surface area of the Si nanocones and the highly dispersed nanosized Pt catalyst particles, the Pt/SCN electrode demonstrated a very high mass activity and a very low onset potential. CV measurements, CO stripping cyclic voltammogram and chronoamperometric study indicated that the Pt nanoparticles were well adhered to the Si