Chapter 4 Investigation of Formation Mechanisms for PtRu Core-Shell Nanostructures during
4.3.4 EQCM analysis
Electrochemical quartz crystal microbalance (EQCM) measurements were performed to further investigate mass variation occurring during PtRu galvanic displacement reaction. The quartz crystal, with an Au film on surface for electrical conductivity, was immersed in the Pt or PtRu plating baths. The Pt bath was 10 mM H2PtCl6 + 100 mM H2SO4 aqueous solution, and the PtRu bath was 5 mM H2PtCl6 + 5 mM Ru(NO)(NO2)4(OH)2- + 100 mM H2SO4 aqueous solution.
Periodic pulse currents were imposed at repeated sequences of 50 mA/cm2 for 1 sec and rested for 50 sec. The plot of current density vs. time is shown in Fig. 4.15(a) and Fig. 4.15(d). The mass of electrodeposited species on the Au surface as a function of time was recorded for Pt and PtRu baths, respectively (shown in Fig. 4.15(b) and Fig. 4.15(e)). At a fixed pulse current, the mass increment in each pulse for the Pt bath was nearly identical (Fig. 4.15(b)). In contrast, the mass increment in each pulse for the PtRu bath was notably different (Fig. 4.15(e)). The open circuit potential for the metal deposited on the Au surface as a function of time was also recorded. As shown in Fig. 4.15(c), during the pulse current deposition in Pt bath, the potential remained unchanged during resting time of 50 sec, indicating that the surface state for the deposited Pt was rather stable. In contrast, as shown in Fig. 4.15(f), during the pulse current deposition in PtRu bath, the potential changed significantly during resting time, suggesting that a continuous surface rearrangement was taking place. This surface rearrangement was attributed to the displacement reaction between the freshly deposited Ru metal and Pt cations during resting time of 50 sec.
87 bath; (d), (e), and (f). The (a) and (d) are the current profile during pulse deposition. The (b) and (e) are their respective mass variation in each pulse. The (c) and (f) are the voltage reading during plating time and open circuit voltage during resting time.
The enlarged EQCM profiles for a single pulse are presented in Fig. 4.16 in which the blue vertical line defines both the ending point of the pulse current of 50 mA/cm2 and the starting point of resting time. As shown in Fig. 4.16(b), in the Pt bath the mass change at the blue vertical line was associated entirely with pulse current electrodeposition, designated as the “electrodeposition mass change”, while the mass change after the blue vertical line was significantly smaller as compared to the “electrodeposition mass change”. This reduced amount was attributed to the Pt surface oxidation, which is designated as the “surface oxide mass change”. Also the potential for the deposited Pt after the blue line, as shown in Fig. 4.16(c), stabilized rapidly and arrived at a value that is rather close to the thermodynamic potential of oxide formation on the Pt surface (E0 = 0.43 V vs. Ag/AgCl)[30]. In contrary, in PtRu bath, the mass change after the blue line was considerably larger than the “electrodeposition mass change” before the blue line, as shown in Fig. 4.16(e). Also the potential for the deposited PtRu after the blue line shown in Fig. 4.16(f) stabilized slowly as compared to that shown in Fig. 4.16(c). We attribute such notable mass and potential variations on the samples from PtRu bath to galvanic displacement reaction. When the current was resting, a severe displacement reaction took place between the metallic Ru atoms electrodeposited on the Au surface and the Pt ions in the plating bath, resulting in the observed changes in both mass and potential.
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Figure 4.16 EQCM profiles in a single pulse; (a), (b), and (c) for Pt plating bath, and (d), (e), and (f) for PtRu plating bath. The (a) and (d) are the current profile during pulse deposition. The (b) and (e) are their respective mass variation in each pulse. The (c) and (f) are the voltage reading during plating time and open circuit voltage during resting time.
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Fig. 4.17 provides the ratio for the surface oxide mass change (for the sample from Pt bath) or the displacement mass change (for the sample from PtRu bath) to the electrodeposition mass change, i.e. the ratio of mass change when the current was resting to that when the current was on.
As shown in Fig. 4.17(a), in Pt bath, the ratio of mass change was merely 30-40 %. In contrast, in PtRu bath, as shown in Fig. 4.17(b), the mass change when the current was resting was 50-400 %, suggesting that a substantial amount of mass increment was resulted from the galvanic displacement reaction when the current was turned off.
0 2 4 6 8 10
20 30 40 50
(oxide / electrodepositon)
Ratio (%)
Pulse number (a)
0 2 4 6 8 10
100 200 300
400 (diplacement / electrodeposition)
Ratio (%)
Pulse number (b)
Figure 4.17 Ratio of (a) the surface oxide mass change (for the sample in Pt bath) or (b) the displacement mass change (for the sample in PtRu bath) to the electrodeposition mass change.
These data were obtained for the first ten pulses from EQCM measurements in Fig. 4.15.
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4.4 Conclusions
The mechanism of Pt displacement reaction on the Ru to form Ru@Pt nanoparticles was investigated by immersing the carbon-supported Ru nanoparticles in hexachloroplatinic acids with various pH values, followed by hydrogen reduction. XRD patterns demonstrated that the Ru hcp lattice was expanded slightly after Pt incorporation. Results from ICP-MS suggested that the dissolution of Ru was mostly caused by the reduction of Pt cations. TEM images demonstrated a uniform distribution of Ru@Pt in size of 3–5 nm. Analysis from XANES and EXAFS indicated that the pH value of hexachloroplatinic acids determined the type of ligands around the Pt cations that led to different stages of displacement reaction. The oxidation state of Ru ions after displacement was determined to be +3 by Ru K-edge XANES. After hydrogen reduction, samples from pH=1 bath revealed a desirable core–shell structure that displayed a reduced onset potential in CO stripping measurements and stable catalytic performances for H2 oxidation with negligible performance degradation. The further exploration on the displacement reaction mechanism was carried out by EQCM. The unusual magnificent mass increment could not be explained unless it was ascribed to the galvanic displacement reaction.
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