Chapter 8 Synthesis of 3D Platinum Nanoflowers and Their Electrochemical
8.4 Electrochemical characteristics
The electroactive surface area of the catalyst was evaluated from charges associated with hydrogen adsorption/desorption on the electrode surface from cyclic voltammograms (CVs) in 1 M H2SO4 aqueous solution at a scan rate of 20 mV s-1 [172]. Figure 8-5 shows CVs of the 3D Pt nanoflower/Si and the Pt thin film/Si electrodes. In the potential range characteristic of hydrogen electrosorption in the sulfuric acid solution (-0.4 – 0.0 V vs. SCE), the Pt thin film/Si electrode shows a very weak and featureless broad peak, while two peaks (at ca. -0.22 and -0.02 V vs. SCE) can be clearly observed on the CV curve of the Pt nanoflower/Si electrode. The peak feature in a cyclic voltammogram of Pt in sulfuric acid solutions strongly depends on the crystallographic orientation of the Pt surface. For the Pt(111) surface, the
cyclic voltammogram is characterized by a broad and flat hydrogen electrosorption peak in the potential range ~ -0.4 V - 0.0 V (vs. SCE) [173]. On the other hand, in the same potential range, the Pt(100) surface gives two distinct hydrogen electrosorption peaks, and a single peak can be found for the Pt(110) surface in the same potential range. Thus, from the CV curves of Fig. 8-5, we suggest that the (100) and (110) lattice planes prevailed over the (111) plane on the 3D Pt nanoflower surface.
The hydrogen adsorption charge (QH) evaluated from Fig. 7-5 is ~0.27 mC cm-2 and
~31.17 mC cm-2 for the Pt thin film/Si and the 3D Pt nanoflower/Si electrodes, respectively.
QH is usually used to quantify active sites for hydrogen adsorption/desorption on the Pt catalyst. The measured QH values show that the active surface area of the 3D Pt nanoflowers/Si electrode is much higher than that of the Pt thin film/Si electrode by a factor of >110.
Figure 8-5 shows the CVs of the 3D Pt nanoflower/Si and the Pt thin film /Si electrodes in the 1 M H2SO4 solution. The scan rate was 20 mV s-1.
Electrocatalytic activity of the 3D Pt nanoflower/Si and the Pt thin film/Si electrodes toward methanol oxidation reaction (MOR) was studied by cyclic voltammetry in a nitrogen-saturated 1 M CH3OH/1 M H2SO4 solution at a scan rate of 25 mV s-1, and the CV curves are shown in Fig. 8-6. The onset potential for methanol oxidation of the 3D Pt nanoflower catalyst was ~0.38 V, which was ~0.13 V lower than that of the Pt thin film catalyst, indicating faster electrode kinetics [174]. In addition, the oxidation current peak density of the 3D Pt nanoflower catalyst in the forward scan was higher than that of the blanket Pt thin film catalyst. This implies that the 3D Pt nanoflower catalyst has a better electrocatalytic activity toward the MOR compared with the blanket Pt thin film catalyst. In the CV scan, the anodic peak in the reverse scan might be attributed to the removal of CO-like poisoning species formed on the Pt catalyst in the forward scan. The catalyst tolerance against CO adsorption may be estimated by the ratio of the forward current density (If) to the reverse anodic peak current density (Ib), (If/Ib) [143]. A high If/Ib ratio suggests efficient electrooxidation of methanol during the forward scan and less accumulation of residues on the electrodes, whereas a low ratio indicates incomplete electrooxidation of methanol and excessive accumulation of carbonaceous residues on the electrode surface. The (If/Ib) ratio of the 3D Pt nanoflower/Si electrode and the Pt thin film/Si electrode are ~2.5 and ~0.93 respectively. This ratio for the 3D Pt nanoflower/Si electrode is ~2.68 times larger than that for the Pt thin film/Si electrode, indicating that the 3D Pt nanoflowers electrode had a higher electrocatalytic activity toward MOR and thus a better CO tolerance.
To study CO tolerance of the 3D Pt nanoflower/Si electrode, CO-stripping CV measurement was carried out. Figure 8-7 shows the CO stripping CV curves of the 3D Pt nanoflower/Si and the Pt thin film/Si electrodes in 1 M H2SO4 aqueous solution. For the CO stripping analysis, CO adsorption on the Pt catalyst was conducted by flowing a 10% CO/N2
gas mixture into the (1 M H2SO4) electrolyte for 35 min at 0.1 V (vs. SCE), followed by purging with nitrogen gas for 30 min to remove any residual CO from the solution.
Figure 8-6 shows the CVs of the 3D Pt nanoflower/Si and the Pt thin film /Si electrodes in the 1 M CH3OH/1 M H2SO4 solution. The scan rate was 25 mV s-1.
As shown in Fig. 8-7, during the first cycle, the CV curve was flat in the potential range between −0.3 to 0.3 V indicating that the hydrogen adsorption was suppressed due to the complete coverage of available active Pt sites by CO adspecies. In the first scan, a broad anodic peak appeared between 0.4-0.8 V, which was absent in the subsequent scan, indicating that CO adspecies were effectively oxidized during the first scan. The CO oxidation current peaks of the 3D Pt nanoflower/Si and the thin film Pt/Si electrodes were centered at ~0.58 V and ~0.64 V, respectively. The peak current density of CO electrooxidation on the 3D Pt nanoflower/Si electrode was much larger than that on the Pt thin film electrode, further indicating that the 3D Pt nanoflower/Si electrode had a much larger electroactive surface area.
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Figure 8-7 shows the CO stripping CVs of the 3D Pt nanoflower/Si and the Pt thin film /Si electrodes in the CO saturated 1 M H2SO4 solution. The scan rate was 25 mV s-1.
The above electrochemical analysis results indicate that the 3D Pt nanoflower/Si electrode possessed a much higher electrocatalytic activity towards CO oxidation and MOR compared to the Pt thin film/Si electrode. This may be ascribed to that Pt catalysts on the two electrodes had different preferential surface lattice orientations. It is known that methanol oxidation reaction catalyzed by Pt in acidic aqueous solutions is sensitive to the surface structure of the Pt catalyst [175]. According to previous reports, the electrocatalytic activity of Pt toward MOR in H2SO4 aqueous solution increases in the order Pt(100)> Pt(110)> Pt(111) [176]. As discussed above about the hydrogen eletrosorption peaks (Fig. 8-5), the Pt nanoflower/Si electrode had more surface areas with the Pt(100) and Pt(110) lattice orientations than the blanket Pt thin film electrode, of which the catalyst surface was Pt(111) preferentially oriented.
Therefore, as shown in Fig. 8-6, a lower onset potential and a much higher current density
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
were obtained during the forward scan in the methanol oxidation CV measurement for the Pt nanoflower/Si electrode. In addition, the oxidation rate of CO adspecies on Pt also varies with the Pt surface orientation, with the electrocatalytic activity increasing in the order Pt(111)
<Pt(110) <Pt(100) [177]. As a result, CO like adspecies on the Pt(100) and (110) surfaces can be oxidized more effectively than on the (111) surface, leaving more active sites for methanol adsorption. The better CO tolerance observed for the Pt nanoflower/Si electrode is likely due to the preferential (100) and (111) surface orientation on the Pt catalyst as well.
Figure 8-8 shows the chronoamperograms of the 3D Pt nanoflower/Si and the Pt thin film /Si electrodes in the 1 M CH3OH/1 M H2SO4 solution at the polarization potential of 0.4 V.
Chronoamperometry was used to compare the stability of the electrocatalytic activity toward MOR of the 3D Pt nanoflower/Si electrode with that of the Pt thin film/Si electrode.
Figure 8-8 shows the chronoamperograms of the two electrodes, which were obtained by measuring the steady-state reaction current density at the electrode potential of 0.4 V. After the polarization of 350 s, the current density of methanol electrooxidation for the Pt thin
0 50 100 150 200 250 300 350
film/Si and the 3D Pt nanoflower/Si electrode were ~0.02 mA/cm2, and ~6.2 mA/cm2, respectively. Compared with the blanket Pt thin film, the 3D Pt nanoflower/Si had a much higher activity with a steady-state currents density about 310 times that of the thin film Pt/Si electrode. These results are very consistent with the CV measurements shown in Figs. 8-5, 8-6, and 8-7.
Figure 8-9 shows the Nyquist plot of electrochemical impedance spectra (EIS) of the 3D Pt nanoflowers and the Pt thin film catalysts in the 1 M CH3OH + 1 M H2SO4
solution at the potential of 0.3 V. Insert is the equivalent circuit model used to fit the impedance spectra.
Electrochemical impedance spectroscopy (EIS) is a powerful analysis technique, which can provide a wealth of information on the charge transfer resistance and capacitance. The Nyquist plots of the 3D Pt nanoflower/Si and the Pt thin film/Si electrodes at the potential of 0.3 V in 1 M CH3OH/1 M H2SO4 aqueous solution are shown in Fig. 8-9, where Zr and Zi are the real and imaginary parts of the impedance. The thin lines are fitting results according to
the equivalent circuit shown as the inset in Fig. 8-9. Rs denote the solution resistance, Rct
represents the charge-transfer resistance and CPE is defined as the constant phase element, which takes into account methanol adsorption and oxidation [142]. As shown in Fig. 8-9, the proposed model fits well to the obtained EIS data points. The charge-transfer resistance at the 3D Pt nanoflower/Si and the Pt thin film/Si electrodes is ~9 and ~50 Ω-cm-2, respectively. The very low Rct obtained with the Pt nanoflower/Si electrode suggests that this type of electrode could be suitable for direct methanol fuel cell applications.
8.4 Summary
We have demonstrated a facile and reproducible method to synthesize 3D Pt nanostructures on the silicon substrate at room temperature by potentiostatic pulse plating.
The Pt nanostructure is made up of bamboo-leaf like nanopetals, and has a geometric shape of roses. Electrochemical analysis show that the 3D Pt nanoflower had a much larger active surface area than the Pt thin film by a factor of >110, and were likely preferentially oriented in the (100) and (110) surface planes. Due to the preferential surface orientations and high surface area, the 3D Pt nanoflower catalyst had an excellent electrocatalytic activity toward methanol oxidation and a high CO tolerance as compared with the Pt thin film catalyst.
Chapter 9
Controlled Synthesis and Growth of Perfect Platinum Nanocubes by Fasten Silicon at NTP
9.1 Introduction
Currently, the synthesis and growth mechanism of the shape-controlled Pt nanostructures is fundamentally interesting, and potentially very useful because of their wide range of applications in photocatalytic activity [178], hydrogen production [179], liquid feed fuel cells catalyst [43, 105, 156], surface enhanced Raman scattering [180], fine chemical synthesis [181], reduction of pollutant gases emitted from automobiles and the synthesis of nitric acid, oil cracking [44]. The synthesis of well-controlled sizes, facets and shapes of metal nanoparticles can also effectively influence their optical, electrical, thermal, magnetic, chemical and catalytic properties [44, 47, 182]. Thus, different Pt nanoparticle shapes were synthesized in high yields, such as Pt nanocubes, [42, 183, 184] nanorods, [185] nanotubes, [185] and dendritic nanoparticles [186-188]. Among these shapes, well-controlled Pt nanocubes are particularly interesting due to their high catalytic activity in different catalytic reactions. For instance, Pt nanocubes with the high energy {100} facets are highly active and selective than that of conventional NO catalysts [189]. The oxidation of ammonia, almost exclusively takes place on the Pt {100} plane [190]. Many previous studies have also shown that Pt {100} plane enhanced the catalytic activity for hydrogen oxidation reactions and the oxygen reduction reaction [191, 192]. In the last few years several chemical methods have been developed for the synthesis of Pt nanocubes but most of the chemical methods are either tedious or difficult to controlled, making exceedingly challenging for the researchers to the synthesis of Pt nanocubes.
In this chapter, we describe a new, simple and efficient method for the synthesis and growth of well-dispersed perfect Pt nanocubes, employing a pair of low-resistivity silicon samples at normal temperature and pressure (NTP) in the absence of surfactant, additives and capping materials. Moreover, the perfect Pt nanocubes catalyst shows excellent electrocatalytic activity and better stability toward methanol and ethanol oxidation.
Figure 9-1 shows the synthesis procedure of Pt nanostructures: (A) the Si-wafer is cut into 1.5 x 2 cm2 samples, (B) dipped into HF aqueous solution for 5 minutes in order to remove any native oxide layer on the Si samples, (C) then the polished side of cut two identical piece of the Si-wafer was fasten together and dipped into stirred 1 M H2PtCl6 + 1 M H2SO4 aqueous solutions, {after stirring for (a) 3 hr and (b) 5 hr to form the truncated (cubes + tetrahedron) and truncated cubes respectively}, (D) furthermore, the perfect Pt nanocubes produced by mixing 1 M HCl into 1 M H2PtCl6 + 1 M H2SO4 aqueous solutions (vigorous stirring for 5 hr).
9.2 Synthesis process of Pt nanostructures
The synthesis process of Pt nanocubes is illustrated in Fig. 9-1. The p-type 4-inch Si wafer of low resistivity (0.002 Ω-cm) was used as the substrate. A pair of Si wafer was dipped in 10 wt. % HF aqueous solution for 5 min. in order to remove any native SiO2 layer from the silicon sample. Two kinds of aqueous solution were prepared for the synthesis of Pt
10 wt%
nanostructures, (a) 2 M H2PtCl6 + 1 M H2SO4,and (b) 2 M H2PtCl6 + 1 M H2SO4 + 1 M HCl.
Aqueous solution (a) was prepared by constant stirring for 3 h and 5 h at normal temperature and pressure (NTP). Solution (b) was prepared by constant stirring for 5 h at NTP.
Subsequently, for the typical synthesis of truncated tetrahedron, truncated nanocubes, truncated nanorods and perfect nanocubes, a pair of silicon sample was fasten together and dipped in a solution of (a) and solution (b).
The possible chemical reaction in the solution may be summarized as follows:
First half cell reactions
SiO2 + HF!→!H2.SiF6 + H2O → SiF6 + 2H2O + H2↑ Second half cell reactions
H2PtCl6 + 2H2SO4 → Pt+4 + 6Cl– + 6H+ + 2SO4-2 H2PtCl6 + 2H2SO4 + HCl → Pt+4 + 7Cl– + 7H+ + 2SO4-2
The two half-cell electrochemical reactions may be involved in the Pt deposition process.
After Pt nanostructures deposition, the sample was washed by DI water to removed contamination from the sample surface and dried in the ambient. All solutions were prepared from Milli-Q water (~18 MΩ) and analytical grade chemicals.
9.3 Structural characterization of Pt nanostructures
The synthesis and growth of Pt nanocubes were affected by physical and chemical conditions. For example, stirring time of the solution and dipping time of FS sample affects physically while the concentration of H2PtCl6, H2SO4 and HCl affect chemically. In our perfect Pt nanocubes synthesis, we optimized the concentration of HCl, H2PtCl6, H2SO4 and stirring time of aqueous solution.
Scanning electron microscopy (SEM) study was carried out to investigate the surface morphology of the synthesized Pt nanostructures onto the surface of fasten silicon (FS)
sample. Figure 9-2 (A to C), show the plane-view SEM images of the truncated Pt (cubes + tetrahedron), obtained by varying the dipping times 10 min, 13 min and 15 min respectively, in the aqueous solution of 2 M H2PtCl6 + 1 M H2SO4 at NTP. As shown in Fig. 9-2 (A to C), the Pt nanostructure sized is increased as the dipping time of FS sample increased. The Fig. 9-2 (C) clearly indicates that the products were mainly composed of the truncated Pt nanocubes with the truncated Pt tetrahedron, after 15 min dipping in an aqueous solution. The morphology of the truncated Pt tetrahedron nanostructures was further investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Figure 9-2 (D) shows a bright field TEM image of individual truncated Pt tetrahedron nanostructure. It can be clearly seen that the truncated Pt tetrahedron had a size smaller than ~80nm. Figure 9-2 (E) shows a HRTEM image taken from the corner of truncated Pt tetrahedron. The measured atomic spacing from the HRTEM image (Fig. 9-2 (E)) is ~0.23 nm, which corresponds to the set of {111} lattice plane. The above results confirm the truncated Pt tetrahedron grew along the {111} direction. The inset of Fig. 9-2 (E) shows the fast Fourier transform (FFT) image and confirmed that the truncated Pt tetrahedron is single crystalline.
In addition, to investigate the influence of stirred time on the shape of the Pt nanostructure, 5 h stirred solution (2 M H2PtCl6 + 1 M H2SO4) was used for the experiment. Figure 9-3 (A to C), display the plane-view SEM images of the truncated Pt nanocubes deposited on the FS sample in 13, 15, and 20 min, respectively. The obtained Pt nanostructured were 98%
truncated nanocubes (Fig. 9-3, A and B). In the case of 20 min dipping, the nanostructure obtained was well dispersed truncated Pt nanocubes with nanorods (Fig. 9-3 (C)). The morphological evolution of the nanocubes indicates that the larger nanocubes or nanorods originated because of the surface energy minimization and Ostwald repining (Fig. 9-5).
Figure 9-2 shows the SEM images of truncated Pt (cubes + tetrahedron) deposited on the FS sample at NTP for (A) 10, (B) 13, and (C) 15 min; (solution stirring for 3 hr). (D) Low-magnification TEM image of individual truncated Pt tetrahedron synthesized with the FS sample. (E) High-resolution TEM lattice image of the square region in single truncated Pt tetrahedron. The d-spacing between the fringes was ~0.23 nm, which was identified {111} plane of Pt; the inset shows the FFT of the lattice image gives the optical diffractogram, which clearly shows the presence of {111} plane.
100 nm 100 nm
100 nm
C
A B
D E
100 nm 100 nm
B A
1 μm
C
100 nm 100 nm
B A
1 μm
C
Figure 9-3 shows the SEM images of truncated Pt nanocubes deposited on the FS sample at NTP for (A) 13, (B) 15, and (C) 20 min; (solution stirring for 5 hr).
Figure 9-4 (A) Low-magnification TEM image of individual truncated Pt nanocubes and (B) High-resolution TEM lattice image of the square region in single truncated Pt nanocubes. The d-spacing between the fringes was ~0.20 nm, which was identified {200} plane of Pt.
20 nm
A B
Figure 9-5 Shows the SEM images of Pt nanostructures formed by immersing FS samples into aqueous solutions of H2SO4- H2PtCl6 for 20 min at NTP (stirring time = 5 h).
The images clearly show the presence of Pt nanocrystals with cube and rod shapes. The mechanism of rod growth is Ostwald ripening, define as the dissolution of the fine particles and their redeposition on large particles [198].
According to the well-known Gibbs–Thomson law, there is energy difference between large particles and small particles result in vanishing of smaller particles and formation of longer nanorods with the reaction in progress [199].
Figure 9-6 FFT image obtained from fig. 3E. The FFT pattern exhibits only a diffraction spots, revealing a good single-crystal property.
Figure 9-7 SAED image obtained from fig. 9-5 (A). SAED pattern also confirms the single crystal structure of the truncated Pt nanocubes.
As shown in Fig. 9-2 (C), when the stirring time was 3 h, a large number of the truncated Pt tetrahedron with truncated Pt nanocubes was formed. While on the other hand, when the stirring time was 5 h, only the Pt nanocubes were observed (Fig. 9-3(B)). This might be partly due to the homogeneity of aqueous solution controlled by stirred time. Figure 9-4 (A and B), shows the representative TEM and HRTEM images of an individual truncated Pt nanocube.
The size of truncated Pt nanocubes from TEM image was estimated ~80nm. The HRTEM image in Fig. 9-4 (B) recorded from a Fig. 9-3 (A) shows continuous lattice fringes with lattice spacing of ~0.20 nm, which corresponds to the {100} planes of Pt. The FFT image (Fig.
9-6) indicates that each truncated Pt nanocubes was a single crystalline. Furthermore, the selected area electron diffraction (SAED) pattern obtained by directing the electron beam perpendicular to individual truncated Pt nanocube (Fig. 9-7). The SAED pattern also confirmed that the truncated Pt nanocube was a single crystalline.
However, for the synthesis and growth of perfect Pt nanocubes, we used the same aqueous solution (2 M H2PtCl6 + 1 M H2SO4) mixed with 1 M HCl. To explore the synthesis and growth mechanism of the Pt nanocubes, again time-dependent experiments were carried out at NTP.
Figure 9-8 shows the SEM images of perfect Pt nanocubes deposited on the FS sample at NTP for (A) 10, (B) 13, and (C) 15 min; (solution stirring for 5 hr).
Figure 9-9 shows the typical XPS survey spectrum of perfect Pt nanocubes deposited on the surface of FS sample. XPS clearly show the presence of Pt nanocube catalyst.
100 nm 100 nm
100 nm
B
C
A
800 600 400 200 0
Si(2p) C(1s)
Pt(4d) Si(2s) Pt(4f) O(1s)
Rel at iv e XP S I nt en si ty
Binding Energy (eV)
Pt Nanocubes/Si
A series of SEM images taken at different dipping times (t) are presented in (Fig. 9-8, A to C). As shown in Fig. 9-8 (A), at t = 10 min, the Pt nanocubes were very hard to observed by SEM image. When t = 13 min, the Pt nanocubes were start to develop on the both side of the FS sample (Fig. 9-8 (B)). At t = 15 min, the Pt nanocubes were start to grow in all directions, which is considered as evidence for a growth of Pt nanocubes (Fig. 9-8 (C)).
The x-ray photoelectron spectroscopy (XPS) data were collected to further confirm the deposition of perfect Pt nanocubes onto the silicon. Figure 9-9 shows an XPS survey analysis of a Pt nanocube deposited onto a FS sample. Two distinct Pt (4f7/2) and Pt (4f5/2) peaks at
The x-ray photoelectron spectroscopy (XPS) data were collected to further confirm the deposition of perfect Pt nanocubes onto the silicon. Figure 9-9 shows an XPS survey analysis of a Pt nanocube deposited onto a FS sample. Two distinct Pt (4f7/2) and Pt (4f5/2) peaks at