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 1000^th 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 2^nd cycle and (b) the 1000^th 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 nanocones and the Pt/SNC electrode had a very good CO tolerance. We ascribed the good CO tolerance to the efficient CO oxidation by the abundant silanol groups surrounding the Pt nanoparticles via bifunctional mechanism.

Chapter 5

Fabrication and Electrocatalytic Properties of Pt Nanoparticles Electrodeposited on Amorphous Carbon

Coated Silicon Nanocones 5.1 Introduction

Since direct methanol fuel cells (DMFCs) can operate at relatively low temperatures, and are lightweight and power efficient, they are very attractive for mobile power source applications, such as automotive systems and portable electronics. Significant progress has been made over the past decade to enhance the electrochemical performance of DFMCs.

However, low methanol oxidation reaction (MOR) activity, poor kinetics of the oxygen-reduction reaction (ORR), and high cost of Pt-based electrocatalysts still pose a great challenge for commercialization. Different approaches have attempted to improve electrocatalytic activity to reduce precious Pt catalyst usage and increase electrocatalytic activity for MOR and ORR. To achieve this goal, well dispersed Pt catalyst nanoparticles are usually deposited on the catalyst support with a large surface area. Because carbon materials can usually be prepared to have a large surface area for Pt loading, they have been widely used as the support for the Pt catalyst, such as mesoporous graphite with Pt nanoparticles [96], graphene nanosheets decorated Pt nanoparticles [97], carbon nanocoils with Pt–Ru catalyst alloys [84] and carbon-coated anatase TiO2 composites [85]. Carbon materials generally have a high content of oxygen surface groups, which act as anchoring centers for the Pt precursor and thus improve wetability of the Pt precursor on the carbon support, resulting in better Pt dispersion [98]. The large support surface and highly dispersed nanosized Pt catalyst create a large effective electrocatalytic surface area and, therefore, greatly enhance ORR and MOR activities. Moreover, π-bonding site on the carbon support improves Pt-C adhesion strength,

alleviating agglomeration and loss of Pt catalyst particles during electrochemical reactions [99-101]. Many studies have shown that carbon support significantly enhances MOR and ORR activities and Pt catalyst stability [102-104].

This study prepares amorphous carbon (α-C) coated nanostructured supports for Pt loading, and studies electrocatalytic performance of the nanostructured electrode in MOR and ORR for DMFC applications. The electrocatalyst support is a highly ordered silicon nanocone (SNC) array coated by an ultrathin α-C surface layer, which provides a large surface area for Pt loading. Nanometer scale Pt catalyst particles were deposited on the nanocones by potentiostatic bipolar pulse electrodeposition. Because the α-C coated Si nanocone (ACNC) array is fabricated directly on a low resistivity Si substrate (0.002 Ω-cm), acting as the current collector, the ACNC support is very suitable for use as the Pt electrocatalytic electrode for electrical conductivity. The Pt nanoparticle/ACNC electrode exhibits high electrocatalytic activity and stability for MOR and ORR.

5.2 Fabrication procedure of the ACNC array

The SNC array fabrication procedure has been reported in detail previously [87, 105]. Figure 5-1 shows schematically the fabrication procedure. A p-type 4-inch Si wafer of low resistivity (0.002 Ω-cm) was 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 the adhesion layer between the Si substrate and the Al thin film, and later used for preparing the nanodot mask for fabricating the SNC array. The as-deposited Al/TiN film stack was then oxidized by electrochemical anodization, performed in a 0.3 M oxalic acid (H2C2O4) aqueous solution at 25^○C under a constant polarization voltage of 40 V for 20 min. Anodic oxidation of the Al thin film under the anodization conditions produced hexagonally arranged anodic aluminum oxide (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, forming the TiO2 nanodot mask on the TiN layer. The AAO was then removed by the aqueous solution of 6 wt % H³PO⁴ and 1.5 wt % CrO³ at 70^○C for 40 min. Figure 5-2 shows the side-view SEM image of TiOx nanopillars after the removal of AAO template, had well ordered hexagonal arrangement as shown in the inset of Fig. 5-2. Since TiOx nanopillars were only formed

Figure 5-1 shows the fabrication processes of the ordered α-C coated SNC array: (a) deposition of TiN and Al thin films on the Si wafer by sputter deposition and thermal evaporation, respectively, (b) formation of the porous AAO template (c) formation of TiOx nanodots in the AAO pore channels, (d) removal of AAO by wet etch, (e) RIE of the remaining TiN and silicon substrate, forming Si nanocones, and (f) deposition of the α-C layer on SNCs by MPCVD.

under the AAO pore bottom, it was necessary to remove the remaining TiNs before TiOx

pillars were used as nanomasks to fabricate Si nanocones. In order to etch the TiN layer, a gas mixture of BCl3 and Cl2 was used for 40 sec as plasma source in the inductively-coupled-plasma reactive-ion-etch (ICP-RIE) system. 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. Deposition of the α-C on the SNC array was carried out by microwave plasma chemical vapor deposition (MPCVD) with the following deposition conditions: the CH4:H2 gas flow rate ratio 20:80 sccm, plasma power 300 W, bias power 200 W, working pressure 1 torr, and deposition time 25 min.

Figure 5-2 shows the side-view SEM image of TiOx nanopillars after the removal of the AAO layer by wet etch. The inset shows the plane-view SEM image of TiOx nanopillar arrays.

5.3 Electrodeposition of Pt nanoparticles

The potentiostatic pulse electrodeposition (PPE) has more advantages over the direct potential electrodeposition and the conventional physical vapor deposition. Because of the easy experimental setup, deposition at room temperature in a large surface area, control the distribution of electrodeposits, control of particles size, surface morphology, and time consuming. The desired size of nanoparticles was adjusted by properly tuning the anode and cathode potential parameter. In the PPE, 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). However, in our experiments, Pt nanoparticles were electrodeposited on the ACNC array in the aqueous solution of 1 M K2PtCl6 – 0.5 M HCl at room temperature by potentiostatic pulse plating in a three electrode cell system with a saturated calomel reference electrode (SCE). The highly ordered ACNC array was the working electrode and the counter electrode was a thin Pt wire. The particle size and dispersion of Pt nanoparticles was controlled by tuning the pulse height of the applied potential and the pulse duration. The time durations for the high potential pulse (-0.08 mV) and the low potential pulse (+0.01 mV) were 7 and 1 ms, respectively. The particles size control by applying the alternate positive and the negative potential on the working electrode. The Pt nanoparticles uniformly deposited on α-C coated Si nanocones array because of following reasons: The rate of nuclei formation increases during the negative potential pulse because overpotential is lower and this causes that new nuclei are formed and crystallized before the deposited metal ion diffuses to the stable places. During the second positive potential pulse, the overpotential is higher and the growth of existing crystals is superior to the formation of nuclei. The amorphous carbon used to produce the homogeneous deposition of Pt particles. The small curvature on amorphous carbon nanocones is produced the proper electric fields, which help to uniform deposition of nanoparticles.

5.4 Material properties of the Pt-ACNC electrode

The α-C coated SNCs fabricated by the AAO templation method had a highly ordered hexagonal arrangement. Figure 5-3 (A) and (B) show SEM images of the ordered ACNC array before and after the electrodeposition of Pt nanoparticles. The well ordered nanocones were ~250 nm in height and had a base diameter of ~100 nm. The ACNCs provided a surface area applicable for Pt loading about five times (the ratio of the cone surface area to the cone base area) that of a flat Si substrate. Pt loading on the nanocone electrode by potentiostatic bipolar pulse electrodeposition is an effective approach to deposit crystalline nanoparticles of well-controlled quality on the cathode. The highly ordered ACNC arrangement allowed a relatively uniform potential distribution over the nanocone array in the electrolyte, crucial for electrodeposition of well dispersed Pt nanoparticles on the ACNC cathode. Because the Pt nanoparticles on the ACNC support were very small and without obvious agglomeration, the SEM image of Fig. 5-3 (B) hardly observed the Pt nanoparticles. Figure 5-4 (A) shows the bright-field TEM image of the ACNC with the electrodeposited Pt catalyst. The figure clearly shows that Pt nanoparticles were well dispersed on the nanocones. According to the high-resolution TEM image (Fig. 5-4 (B)), most of the deposited Pt nanoparticles had a size smaller than 5 nm. Figure 5-4 (C) shows the selected area electron diffraction (SAD) pattern.

The distinct diffraction spots in the SAD image were due to the crystalline Si nanocone substrate. Two diffraction rings of discernible intensity were indexed as the (111) and (200) lattice planes of the Pt face center cubic (FCC) structure. The diffraction rings corresponding to the (220) and (311) lattice planes could also be perceived but with a very weak intensity.

The TEM analysis indicated that crystalline Pt nanoparticles were uniformly electrodeposited on the ACNCs.

Figure 5-3 shows the side-view SEM images of the α-C coated SNC array (A) before and (B) after the electrodeposition of Pt nanoparticles.

Figure 5-5 (A) shows the XPS spectrum of the Pt loaded ACNC array. In addition to XPS

Figure 5-5 (A) shows the XPS spectrum of the Pt loaded ACNC array. In addition to XPS