Structural characterizations of PtRu nanoparticles by galvanostatic pulse electrodeposition

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Structural characterizations of PtRu nanoparticles by galvanostatic pulse

electrodeposition

Yu-Chi Hsieh

a

_{, Li-Chung Chang}

b

_{, Yuan-Chieh Tseng}

a,⇑

_{, Pu-Wei Wu}

a,⇑

_{, Jyh-Fu Lee}

c a

Department of Materials Science and Engineering, National Chiao Tung University, Hsin-Chu 30010, Taiwan, ROC

b

Graduate Program for Science and Technology of Accelerator Light Source National Chiao Tung University, Hsin-Chu 30010, Taiwan, ROC

c

National Synchrotron Radiation Research Center, Hsin-Chu 30076, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 23 April 2013

Received in revised form 13 August 2013 Accepted 24 August 2013

Available online 5 September 2013 Keywords:

PtRu

Galvanostatic pulse electrodeposition Electrocatalyst

Nanoparticle EXAFS XAS

a b s t r a c t

We demonstrate that the compositions and structures of PtRu nanoparticles can be altered via adjusting plating variables during galvanostatic pulse electrodeposition. During current-on time (Ton), both Pt and

Ru are deposited according to their respective diffusion-limiting current. However, during current-off time (Toff), due to difference in the redox potentials, a displacement reaction is occurring that leads to

the reduction of Pt ions from the electrolyte while the Ru atoms in the PtRu nanoparticles experience oxi-dation reaction and corrosive dissolution simultaneously. Therefore, the duty cycle, deﬁned as Ton/(Ton+

-Toff), serves as an indicator for the severity of displacement reaction that affects the PtRu makeup.

Inductively-coupled mass spectrometry determines the composition as Pt83Ru17, Pt64Ru36, and Pt53Ru47

for duty cycles of 0.077, 0.111, and 0.333, respectively. Images from transmission electron microscope exhibit nanoparticles in sizes of 4–11 nm. Analysis from extended X-ray absorption ﬁne structure (EXAFS) suggests that Pt atoms tend to segregate outward and reside at the alloy’s surface upon the dis-placement reaction. Composition proﬁles from line scans of energy dispersive X-ray spectroscopy are consistent with atomic structure revealed by EXAFS.

1. Introduction

PtRu nanoparticles have captured great research attentions for their impressive catatlyic activities as an anode electrocatalyst for direct methanol and reformate hydrogen fuel cells[1–9]. To further enhance electrocatalytic performance, it is necessary to fabricate PtRu nanoparticles in desirable compositions or surface Pt/Ru ratios[8–17]. However, in synthesis, it is rather challenging to prepare nanoparticles with different compositions/structures from the same bath as the formulation and processing protocol need to be adjusted accordingly. Earlier, in our study of pulse depo-sition to form PtRu nanoparticles on carbon clothes, we observed that the resulting PtRu composition can be controlled by varying the duty cycle imposed during the plating process[18]. We real-ized that both Pt and Ru were electrodeposited during current-on time (Ton) while during current-off time (Toff), a displacement reac-tion was effective where the Pt ions were reduced from the electro-lyte while the Ru atoms on the PtRu surface underwent oxidation reaction and corrosive dissolution simultaneously. Thus, the duty cycle, deﬁned as Ton/(Ton+ Toff) in a single pulse, indicates the

severity of the displacement reaction that affects the resulting PtRu composition.

In literatures, the displacement reaction, also known as redox-transmetalation or spontaneous deposition, has been employed to prepare binary deposits such as CuNi, AgGe, or AuGe[19–22]. In a displacement reaction between Ru atom and Pt ions during pulse plating, during Toffthe corrosive dissolution of Ru atom is a straightforward electrochemical step forming Ru cations in the electrolyte. An alternative route is the oxidation reaction known as cementation process where the Ru atoms become Ru(OH)xon surface and those Ru(OH)xare reduced in Tonof subsequent pulse. Therefore, the severity of displacement reaction and the reaction routes taken by the Ru atoms are expected to result in PtRu nano-particles with varying compositions after multiple plating cycles. In practice, the displacement reaction can also be engaged when the PtRu nanoparticles are in contact with electrolytes containing Pt ions. For example, surface modiﬁcation of commercially avail-able PtRu nanoparticles has been demonstrated by immersing them in a concentrated hexachloroplatinic acid solution, so a Pt-enriched surface is obtained with a substantial increment in meth-anol electro-oxidation activities[23].

X-ray absorption spectroscopy (XAS) acquires the energy range near and above the core-level binding energy of the selected atom, which can probe chemical and physical states, sensitive to the coordination number, bonding distances surrounding around the

⇑Corresponding authors. Tel.: +886 3 5731898 (Y.-C. Tseng).

E-mail addresses: yctseng21@mail. nctu. edu.tw (Y.-C. Tseng), ppwu @mail.nctu.edu.tw(P.-W. Wu).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

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PtRu nanoparticles were deposited on a carbon cloth via a galvanostatic pulse electrodeposition technique. The plating solution was prepared by mixing 99.9 wt% RuCl3(Sigma–Aldrich) and 97 wt% NaNO2(Showa) in an aqueous solution at

100 °C for 1 h, followed by dissolution of 99.9 wt% H2PtCl6. Subsequently, the

mix-ture was cooled to 25 °C and 97 wt% H2SO4(Showa) was added. The concentrations

for the H2PtCl6, RuCl3, NaNO2, and H2SO4were 0.005, 0.005, 0.050, and 0.250 M,

respectively. Afterward, the mixture was aged for 2 weeks to obtain a homogeneous solution. To plate PtRu nanoparticles, rectangular pulses were imposed with ﬁxed Ton(50 ms), Ja(50 mA cm2), and total coulomb charge (8 C cm2). With varying Toff

of 100, 400, and 600 ms, the resulting samples were designated TF100, TF400, and TF600, respectively. More details about the sample preparation can be found else-where[18].

The exact compositions of the PtRu nanoparticles were determined by an Induc-tively Coupled Plasma Mass Spectrometry (ICP-MS; Agilent 7500CE). Transmission Electron Microscope (TEM; Philips Tecnai-20) was utilized to observe their mor-phologies and sizes. High-resolution images and atomic proﬁles from line scans of Energy Dispersive X-ray Spectroscopy (STEM-EDS) were obtained using a Scan-ning Tunneling Electron Microscope (STEM; JEOL ARM200F). XAS measurements over Pt LIII-edge (11,564 eV) and Ru K-edge (22,117 eV) were performed at beam

lines of BL01C1 and BL17C1, respectively at Taiwan Light Source, National Synchro-tron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The measurements were carried out in a ﬂuorescence detection mode at room temperature. A Pt foil, H2PtCl6

electrolyte, as well as Ru and RuO2powders were served as references for Pt LIII

-edge and Ru K--edge XAS spectra.

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data analysis and fitting were processed by an IFEFFIT 1.2.11c data analysis package (Athena, Artemis, and FEFF6). The recorded profiles were calibrated by aligning against the reference in each scan. During measure-ments, multiple spectra for a single absorption edge were collected and averaged to achieve better signal quality. Each raw spectrum was pedge subtracted to re-move the background and then was normalized with respect to the edge jump. The normalized spectrum was converted into k-space and was k-weighted properly, and finally was Fourier transformed (FT) into r-space. Specific ranges in k-space for the Fourier transformation were selected from 3.32 to 12.74 Å1

for the Pt LIII-edge and

from 4.01 to 13.42 Å1

for the Ru K-edge. The ranges of r-space for the curve fitting were established without phase correction from 1.90 to 3.17 Å for Pt and from 1.70 to 2.73 Å for Ru. Relevant structural characteristics were fitted by Artemis with the-oretical standards generated by FEFF6 code. The fitting parameters include coordi-nation number (N), bond distance (R), Debye–Waller factor Dr2

j

, and inner potential shift (DE0). Analysis details have been published previously[26].

3. Results and discussion

Fig. 1 demonstrates the TEM images for samples of TF100,

TF400, and TF600, respectively. The size distribution for the PtRu nanoparticles appeared relatively broad as their average values

were estimated to be 7.4 ± 3.9, 5.1 ± 2.6, and 12.7 ± 5.6 nm, respec-tively. A wider size distribution was anticipated since in pulse elec-trodeposition, every single pulse engendered independent events of nucleation and growth simultaneously. Table 1 presents the composition variation for the PtRu nanoparticles and their respec-tive coulomb efﬁciency in electrodeposition. As listed, the coulomb efﬁciency was rather subdued at 1.83–3.83% because most of the currents were not responsible for the formation of PtRu nanoparti-cles during Ton. Instead, the majority of coulomb charge was con-sumed in parasitic reactions including proton reduction, water electrolysis, and reduction of surface oxides and oxyhydroxides formed during Toff. In addition, we would like to point out that the contribution of capacitive current was negligible in our work as the Ton and Toff are considerably longer than the typical Ton and Toffof 1 ms where the capacitive current becomes a tangible factor[18].

Composition analysis from ICP-MS indicated Pt53Ru47, Pt64Ru36, and Pt83Ru17for TF100, TF400, and TF600, respectively. It is note-worthy that these values represent the average PtRu makeup since the ICP-MS dissolves the entire sample in atomic states. In princi-ple, a displacement reaction between Ru atoms in the PtRu nano-particles and Pt ions in the electrolyte during Toff allowed the deposition of excess Pt atoms on the PtRu surface, so the average composition was expected to move toward a Pt-rich one. This behavior was consistent with our ﬁnding where a smaller duty cy-cle produced Pt83Ru17while a larger duty cycle rendered Pt53Ru47 instead. To further determine relative locations of Pt and Ru atoms within our samples, we carried out composition analysis using STEM-EDS on individual PtRu nanoparticles, and the resulting line scan proﬁles for TF100, TF400, and TF600 are displayed inFig. 2. As shown, their respective widths were 7.12, 6.21, and 16.2 nm, and the relative atomic percentages (vertical axis) of Pt (black) and Ru (red) are plotted with respect to the probed position (horizontal axis). The line scans suggested that our samples displayed a homo-geneous state, and the atomic percentages are consistent with re-sults from ICP-MS.

X-ray absorption spectroscopy (XAS) was conducted to explore the element-speciﬁc local structures of Pt and Ru for the three investigated samples, by probing Pt LIIIand Ru K edges. Fig. 3(a)

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demonstrates TF-dependent Pt LIII-edge XANES and EXAFS spectra. The XANES corresponded to the Pt 2p ? 5d photo-excitation pro-cess with the white-line intensity reflecting the available vacancy in the 5d orbital, as well as the binding energy of the 2p core elec-trons as highlighted by the inset figure. The binding energy of Pt has been reported to decrease while alloying with Ru[27]. This is because a Ru–Pt bond contributed a binding energy different than a Pt–Pt bond with respect to Pt, which can be reflected by the white-line intensity. On the contrary, inFig. 3(b) the TF-depen-dency was imperceptible in Ru XANES, due to the intrinsically weak white-line intensity of the K-edge that corresponded to 1s ? 4p photo-excitation. However, in the extended energy range the Ru spectra of the investigated samples were less oscillatory than that of the reference Ru. This is expected to give rise to a dis-ordered atomic environment around the Ru that would smear the local ordering of the Ru–Pt bond.

Fig. 4(a and b) present k-space EXAFS for the Pt LIIIand Ru K

edge, respectively, together with the reference Pt and Ru. In

Fig. 4(a) the reference Pt featured a superposition of two

frequen-cies/wavelengths particularly around 9 and 10 Å1_{, meaning that it} is necessary to take into account more than the nearest-neighbor shell. In contrast, such phenomenon became obscure in the inves-tigated samples, which mainly featured one dominant frequency contributed from the nearest-neighbor shell, due to reduced coor-dination number and a smaller Debye–Waller factor in the nano-particle structure. These two factors also accounted for the discrepancy in spectral amplitude. Nevertheless, the phase of oscil-lation and spectral amplitude appeared to approach to those of the Pt reference with increasing Toff. The frequency and wavelength of TF600 were in a close proximity to those of the reference Pt, imply-ing that in this particular sample the metallic nature of Pt was more dominant in comparison with TF100 and TF400. This is con-sistent with the ICP-MS results (Table 1) where the highest Pt/Ru ratio was obtained in TF600. InFig. 4(b), the EXAFS amplitudes of all the Ru were largely suppressed as a result of the less oscillatory

0 2 4 6 8 10 12 14 Pt Ru Distance (nm) TF100 0 2 4 6 8 10 12 14 Pt Ru Distance (nm) TF400 0 10 20 30 40 50 60 Pt Ru Distance (nm) TF600

Fig. 2. STEM EDS line spectra for (a) TF100, (b) TF400, and (c) TF600 PtRu nanoparticles. In the bottom are the re-plotted spectra corresponding to the upper spectra, where Pt and Ru compositional distributions are presented by black and red colors, respectively. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

11600 11700 11800 11900 0.0 0.4 0.8 1.2 1.6 11560 11565 11570 11575 1.0 1.2 1.4 TF100 TF400 TF600 Pt foil Pt LIII-edge

Normalized absorption

Energy (eV)

TF100 TF400 TF600 Pt foil

Pt L

III

-edge

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22100 22200 22300 22400 0.0 0.4 0.8 1.2 22100 22120 22140 0.0 0.4 0.8 1.2 TF100 TF400 TF600 Ru metal Ru K-edge

Normalized absorption

Energy (eV)

TF100 TF400 TF600 Ru metal

Ru K-edge

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Fig. 3. The XAS spectra for (a) Pt LIII-edge and (b) Ru K-edge, of TF100, TF400, TF600

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natures in the extended energy range of E-space EXAFS (Fig. 3(b)). It came to our attention that all the phase shifts of Ru EXAFS were towards a lower k range in comparison with the reference Ru, likely due to the formation of Pt–Ru bonds with a longer bond-distance than Ru–Ru[28–31].

We Fourier transformed (FT) the k3 _weighted

_v

_{(k) data into} r-space, as provided inFig. 5(a and b) for Pt and Ru, respectively, to place a more quantitative footing for the Pt (Ru) local structures. In FT-EXAFS, the resulting peaks corresponded to the position of successive shells of the centered Pt (Ru). InFig. 5(a), the Pt FT-EX-AFS of TF600 was nearly identical to that of the reference Pt, indic-ative of a major phase of metallic Pt. Despite that the 1st atomic shells of TF100 and TF400 were correctly positioned, the shells were rather smeared and overlapped with the 2nd shells. This sug-gested that considerable fraction of the Pt–Ru bonds have smeared the FT-EXAFS generated by the Pt–Pt bonds. Interestingly, in

Fig. 5(b) the Ru FT-EXAFS of the investigated samples were

funda-mentally different than that of the reference Ru, but rather similar to that of the Pt, especially TF600. Obviously a picture where Pt dominated the PtRu’s structural properties was preserved micro-scopically, with Pt-domination increased upon displacement reaction.

Fig. 4. The k-space EXAFS spectra for (a) Pt LIII-edge and (b) Ru K-edge, of TF100,

TF400, TF600 samples, in comparison with reference Pt foil and Ru metal.

0

R (angstrom)

0 2 4 6 0 2 4 6 0 5 10 15 20 25 TF600 TF400

FT Magnitiude

R (angstrom)

TF100 Ru- K-edge Ru metal x1/5

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Fig. 5. The Fourier-transformed EXAFS Spectra for (a) Pt LIII-edge and (b) Ru K-edge,

of TF100, TF400, TF600 samples, in comparison with reference Pt foil and Ru metal.

Table 2

EXAFS ﬁtting parameters gained from Pt LIII-edge and Ru K-edge for TF100, TF400 and TF600 PtRu nanoparticles. Details about the notations are described in content.

NPt–Ru NPt–Pt RNPt NRu–Pt NRu–Ru RNRu PPt PRu Pt (at.%) Ru (at.%)

TF100 4.16 5.91 10.07 4.69 3.54 8.23 0.41 0.57 51.9 48.1 TF400 2.25 5.9 8.15 6.2 2.98 9.18 0.28 0.68 65.5 34.5 TF600 1.0 8.98 9.98 9.01 1.52 10.53 0.1 0.85 83.4 16.6 22080 22100 22120 22140 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Normalized ab

sorption

Energy (eV)

Ru metal Ru/C Ru ion RuCl₃(aq) RuO2(s) Ru K-edge XANES

Fig. 6. The Ru K-edge XANES spectra of Ru ions after displacement reaction (marked as Ru ion, blue color), along with reference Ru metal, Ru/C, RuCl3(aq) and

RuO2(s). (For interpretation of the references to colour in this ﬁgure legend, the

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The FT-EXAFS spectra were subsequently fitted by IFEFFIT Arte-mis program to analyze the coordination number of the selected atoms, using fixed Pt–Ru (RPt–Ru= 2.65 A) and Pt–Pt (RPt–Pt= 2.70 A) bond distances. The fitting results were summarized inTable 2. Here, NPtreferred to the total coordination number of the cen-tered Pt, but NPt–Rurepresented the coordination number of the Pt only surrounded by the Ru, and the same principle was applied to all notations. Furthermore, a term of PPt(PRu) was created to esti-mate the possibility for the centered Pt (Ru) encountering a differ-ent neighboring atom of Ru (Pt), which can be mathematically expressed as the following relations, according to Refs.[30–32]. PPt¼ NPtRu

R

NPt ; PRu¼ NRuPt

R

NRu

The reciprocal of PPt(PRu) revealed the tendency for assembling the same atomic species in statistics. By summarizing the ﬁtting results inTable 2, we observe opposite trends for Pt and Ru depo-sition with increasing Toff, where Pt continuously deposited onto the surface whereas Ru was removed. This provided a conclusive perspective to the structural evolution of the displacement reaction in addition to the analyses above. To acquire the chemical state of the removed Ru, an XANES comparison for the Ru of dis-placement reaction (DR), Ru on a carbon substrate (Ru/C), Ru of RuCl3 and of RuO2, is presented inFig. 6. The spectral shape of the Ru in DR was close to those of RuCl3 and RuO2. Since the absorption rising energy of the Ru in DR was almost overlapped with that of RuCl3, it is reasonable to assign +3 state to the re-moved Ru ions for the displacement reaction.

As the displacement reaction can be characterized by a Pt-on, Ru-off mechanism, tracking structural evolution upon the reaction was of great interest especially concerning the PtRu’s technological application. For EXAFS, it was collected by a transmission mode with an X-ray fully penetrating the PtRu, so the probed coordina-tion numberRN can reveal whether the selected atom segregates outward (lowerRN) or stays at the core (higherRN) of the alloy. We noticed thatRNPtwas larger thanRNRuin TF100 while became smaller thanRNRuin TF600, which indicated a tendency of segre-gating outward for the Pt upon displacement reaction. By summa-rizing the results from EXAFS, STEM-EDS and ICP-MS, a picture can be illustrated where TF 600 featured a modest segregation of Pt at surface, but TF100 was rather a homogeneous PtRu solid solution. A scheme illustrating the structural evolution with the displace-ment reaction is therefore depicted inFig. 7.

4. Conclusion

Galvanostatic pulse electrodeposition was employed to fabri-cate PtRu nanoparticles on carbon clothes. With ﬁxed Tonand cou-lomb charge, we varied the Toffand the severity of displacement

reaction was altered accordingly. Results from ICP-MS determined that with a smaller duty cycle, a severe displacement reaction occurring during Toffallowed excess Pt deposition from the electro-lyte to form Pt83Ru17 (TF600) nanoparticles, while with a larger duty cycle the composition became Pt53Ru47 (TF100) instead. XANES and EXAFS analyses revealed structural evolution upon dis-placement reaction and indicated a modest segregation of Pt at surface for TF600. Equipment assistances from Professor George Tu and Professor Pang Lin are greatly appreciated. Financial sup-port from the National Science Council of Taiwan (98-2221-E-009-040-MY2; 98-2112-M-009-022-MY3) is acknowledged.

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Fig. 7. Schematic illustrations for the structural evolution of PtRu nanoparticles with displacement reaction, showing samples of TF100 (left, high duty cycle), TF400 (middle, medium duty cycle), and TF600 (right, low duty cycle).

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