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 signals due to Pt nanoparticles and the α-C overlayer, the XPS analysis also detected O(1s), Si(2p) and Si(2s) photoelectrons. The relatively strong O(1s) XPS signal suggested oxygen surface groups on the α-C layer. Curve fitting for the broad and asymmetric C(1s) XPS signal was performed, assuming a Gaussian-Lorentzian curve with a half-width-at-full-maximum of 1.9 eV for all the synthesized peaks. The curve fitting result (Fig. 5-5 (B)) revealed four chemical states with binding energies higher than that typically assigned to the C-C bonding

(A)

(B)

Figure 5-4 shows TEM images (A) Bright-field TEM image of the α-C coated SNC with electrodeposited Pt nanoparticles, (B) high resolution TEM image of the α-C coated SNC with Pt nanoparticles; (C) the SAD pattern, and (D) bright-field image of the α-C coated SNC without the Pt nanoparticle.

and/or graphitic structure (~284.6 eV) [106]. The curve-fitted peaks at 285.6, 286.8, 287.6 and 289.4 eV can be ascribed to C-O-C, C-OH, C=O and -COOH (RCOO-) functional groups, respectively [107, 108]. The Pt(4f) XPS spectrum shown in Fig. 5-5 (C) exhibited a broad doublet peak with the Pt(4f7/2) peak maximum at 70.5 eV, which negatively shifted from that for bulk Pt by ~0.8 eV. As discussed later, we ascribe the negative binding energy shift of the Pt(4f) doublet peak to a combination effect of the Pt particle size and charge transfer between Pt nanoparticles and the ACNC support. Detection of the Si(2s) and Si(2p) XPS signals due to the Si nanocone

20 nm 5 nm

111 200

220 311

10 nm

(B)

(C) (D)

(A)

Figure 5-5 shows the wide scan XPS of the Pt nanoparticles deposited on the ACNC array, (B) the C(1s) XPS spectrum with curve fitted peaks, and (C) the Pt(4f) XPS spectrum.

substrate indicated that the thickness of the α-C layer was smaller than the probe depth of the Si(2s) and Si(2p) photoelectrons excited by the Mg Kα source, which was roughly less than 10 nm. The TEM analysis did show an α-C layer ~5-10 nm thick deposited on the SNC (Fig.

5-4 (D)). Although thin, the α-C layer could greatly affect Pt electrodeposition and electrocatalytic characteristics of the Pt nanoparticles.

Figure 5-6 shows the Raman spectrum of the α-C coated SNCs without Pt nanoparticles.

Figure 5-6 shows the Raman spectrum in the range of 1150–1800 cm^-1 of the ACNC array without the Pt catalyst. The two characteristic Raman peaks for disordered graphitic carbon materials in this range, the G and D peaks, were situated at ~1605 and ~1336 cm^-1, respectively. Peak features of both the Raman modes, such as peak position and intensity, greatly depend on the sp² bonding structure of CVD deposited carbon materials. The G peak is due to sp² bond stretching and the D peak is considered to be a ring breathing mode in a disordered graphitic structure [109]. The G peak is particularly useful to reveal nanocrystallinity of the graphitic structure [110]. When the size of the nanocrystalline graphite cluster is smaller than 2 nm, the G peak shifts to a position higher than 1600 cm^-1[109]. The peak position of the G mode at ~1605 cm^-1 and the relative large D peak signal implied that a significant amount of nanosized graphitic structures were likely present in the α-C layer.

Because of the disordered nanosized graphitic carbon structure, oxygen containing species,

such as those revealed by the XPS analysis, effectively adsorbed on defect sites of nanocrystalline graphite clusters in the α-C layer. The oxygen containing adspecies acted as anchoring centers on the nanocone surface for the Pt precursor during Pt electrodeposition, thereby resulting in a better Pt dispersion [98]. As discussed latter, π-bondings on nanosized graphitic clusters of the α-C layer improved adhesion between Pt nanoparticles and the ACNC support, greatly alleviating agglomeration and loss of Pt nanoparticles during electrocatalytic reactions [99-101].