Journal of Electroanalytical Chemistry 628 (2009) 55–59

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Journal of Electroanalytical Chemistry

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Spectroscopic and morphological studies on the electrooxidation of Pt–Ni alloysin HCl solution

Shu Chen a, San Wu a, Jufang Zheng b, Zelin Li a,*

a Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and ChemicalEngineering, Hunan Normal University, Lushan Road, Changsha 410081, Chinab Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 November 2008Received in revised form 8 January 2009Accepted 12 January 2009Available online 22 January 2009

Keywords:Pt–Ni alloyIn situ Raman spectroscopyDealloyingSimultaneous dissolutionNanopores

0022-0728/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.jelechem.2009.01.005

* Corresponding author. Tel.: +86 731 8871533; faxE-mail addresses: wujina@hunnu.edu.cn, lizelin@z

Investigation on the potential-dependent anodic oxidation of a Pt11Ni89 alloy electrode has been per-formed in an HCl solution. Spectroscopic information of UV–vis absorption and in situ Raman scatteringshows that the alloy undergoes selective Ni dissolution and simultaneous Pt dissolution successively withthe increase of the applied potential. The measurements of SEM–EDX, AFM and XRD at selected potentialsreveal that Pt enrichment occurs in the alloy degradation accompanying diversiform morphological evo-lution such as cracks from stress corrosion, ultrafine pores by selective dissolution, and flaky nanoporousfilms involving simultaneous dissolution and potential-accelerated replacement reaction between Ni inthe alloy and the dissolved PtCl2�

6 . Moreover, the nanoporous films display high electrocatalytic activitytoward the methanol oxidation.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction alloyed components has not been direct probed by in situ spectro-

Investigations on dissolution mechanisms of metallic materialsare of great importance for corrosion science and practical applica-tions. The situation for alloys is much more complex than that forpure metals, and is far less well understood in fact. The inherentdifferences in dissolution potential and in consumption rate foreach component in alloys usually induce diverse structural, mor-phological and compositional variations of alloys during their elec-trooxidation processes. Alloys can be dissolved either selectively orsimultaneously depending on the conditions employed. In the for-mer case, the preferential dissolution of the less noble componentcan yield bicontinuous structures with porous networks for the no-ble component [1], e.g. the most typical nanoporous gold fabri-cated by dealloying of Au–Ag [2–4]. In the latter case, bothcomponents sustain active dissolution for the binary alloys, e.g.Fe–Cr, Cu–Ni and Cu–Zn, at quasi-steady state corroding conditions[5–7].

Dealloying of Pt alloys has ever been presented for these sys-tems like Pt–Co [8,9], Pt–Al [10], Pt–Si [11], Pt–Ag [12], Pt–Zn[13] and Pt–Cu [14–17], etc. Therein, conventional nanoporousstructures were formed by selective dissolution of active compo-nent and post heat coarsening. However, as a relative inert compo-nent, the obvious dissolution of Pt simultaneously with other

ll rights reserved.

: +86 579 82282595.jnu.cn (Z. Li).

scopic techniques as far as we know.In this study, we plan to analyze the electrodissolution behavior

of the Pt11Ni89 alloy by a combination of spectroscopic and mor-phological means. UV–vis absorption spectra/in situ Raman scat-tering spectra, SEM/AFM, EDX/XRD were employed to identifythe soluble species, the surface nanostructures and the surfacecomposition of the alloy, respectively, in the electrooxidation.

2. Experimental

Electrochemical measurements were performed on CHI 660Celectrochemical station (Chenhua, Shanghai, China) with a counterelectrode of platinum and a reference electrode of saturated mer-curous sulfate (SMS). The working electrodes were disks made ofmetal wires of pure Pt and Ni (both in a diameter of 2 mm, purityP99.99%), Pt11Ni89 (3 mm diameter) and Pt35Ni65 at.% (2 mmdiameter). The Pt–Ni alloy materials were prepared from pure met-als by melting above 1700 �C to homogenize and by rapidlyquenching in water to yield a single phase alloy [18,19]. Fig. 1shows the X-ray diffraction pattern for the Pt11Ni89 sample, whichconfirms a typical homogeneous, single phase solid solution. Thistest was performed on a Bruker D8-Advance type powder diffrac-tometer with Cu Ka1, under 40 kV and 40 mA. Prior to use theworking electrodes were polished with 1000# metallographic pa-per and successively cleaned with ultrasonic waves in ultra-purewater. Optical absorption spectra of the electrolyte were registeredby a TU-1901 UV–vis spectrophotometer from Beijing Purkinje

Fig. 1. The XRD pattern for the quenched Pt11Ni89 sample.

Fig. 2. Linear sweep voltammograms for electrodes of Pt, Ni, Pt11Ni89 and Pt35Ni65

in 2 M HCl.

Fig. 3. UV–vis absorption spectra for the electrolyte after polarizing the Pt11Ni89

alloy in 2 M HCl at 0 V for 1 h and at 1 V for 10 min, the Pt35Ni65 alloy in 2 M HCl at1 V for 10 min and for sample solutions of 2 M HCl + 5 mM H2PtCl6 or 0.1 M NiCl2.

56 S. Chen et al. / Journal of Electroanalytical Chemistry 628 (2009) 55–59

General Instrument Co. Ltd. In situ Raman measurements wereperformed with a Renishaw RM1000 confocal microscope underthe excitation of a 785 nm laser ca. 18.5 mW on the sample in aself-designed spectroscopic cell made by Teflon with a quartzwindow.

The morphology and composition measurements were per-formed with a Hitachi S-4800 field emission scanning electronmicroscopy (SEM) equipped with Energy-dispersive X-ray (EDX).Prior to test, the as-prepared electrodes were washed with waterand dried at 30 �C in ambient condition. The dealloying samplesby selective dissolution were annealed at 500 �C to coarsen theporous structure for SEM identification. The flaky nanoporous filmsthat were produced after polarization at 1 V were detached fromthe surface of the Pt–Ni alloy electrode by ultrasonication in etha-nol for 5 min. The collected nanoporous slices dispersed in ethanolwere dropped onto different substrates, such as HOPG flat, glassslide and glass carbon (GC) electrode, for the test of atomic forcemicroscopy (AFM), X-ray diffraction (XRD) and electrocatalyticperformance. AFM measurement was performed on a NanoscopeIII (Veeco) instrument in tapping mode in air. All images were flat-tened using the Nanoscope software and presented in the height-mode, where higher parts are brighter. XRD patterns of nanopor-ous materials were collected on a Philips PW 3040/60 powder dif-fractometer equipped with a Philips Analytical X’Celerator, usingCu Ka radiation in a 2h range from 30� to 90� with a scan rate of0.2 s�1. The working voltage of the instrument was 40 kV and thecurrent was 40 mA.

3. Results and discussion

3.1. Electrochemical behaviors

In Fig. 2, the linear sweep voltammogram (LSV) for Pt11Ni89 isshown in comparison to that of pure Pt, pure Ni and Pt35Ni65 anodein 2 M HCl solution. Markedly, pure nickel starts active dissolutionas low as �0.6 V, and it becomes diffusion limited at 0.4 V.Whereas pure Pt is inert until 0.6 V and a sharp increase of currentthen is from the electrolysis of H2O and the oxidation of Cl�. By vi-sual inspection, the Ni electrode was significantly etched and the Ptremained bright and reflecting, as the pure Pt is too resistive to beworn away [20]. The Pt35Ni65 alloy, with higher Pt content, pre-sents good anti-corrosion strength and an almost coincident LSVcurve with that of pure Pt in HCl solution.

However, the Pt11Ni89 shows unwonted active behaviors in HClsolution, noting the peak near �0.5 V and steep current increase atpotentials higher than 0 V. In view of the large scale of the anodiccurrent density (1000 mA cm�2), this alloy undergoes mass dis-solving and possible surface stress crack (SCC), which will be de-

scribed in Section 3.3. The electrode surface became dimimmediately as soon as the anodic current was conducted for thealloy’s surface was roughened by the active dissolution, and thesolution also turned yellow after long time operation.

3.2. UV–vis absorption and in situ Raman scattering spectra

After polarizing the Pt11Ni89 alloy in 2 M HCl solution at se-lected potentials for a period of time, the electrolyte was analyzedby UV–vis absorption spectra. As shown in Fig. 3, only absorptionpeaks (400, 650 and 710 nm) for Ni2+ were observed at 0 V, butboth PtCl2�

6 (band at 460 nm) and Ni2+ coexisted in the electrolyteat 1 V. These results indicate that Ni selectively dissolved firstly atlower potentials and Pt–Ni simultaneously dissolved at higherpotentials.

To better learn the potential-dependent dissolving behaviors ofPt in the alloy, in situ Raman scattering spectra have been adoptedin this system. Displayed in Fig. 4 are typical spectral sequences onPt11Ni89 anode in 2 M HCl, which were taken by potentiostatic con-trol with an increment step of 0.1 V. Two bands peaked at 317 and342 cm�1 are ascribed to the complex PtCl2�

6 [21,22] in solution.

Fig. 4. Sequential potential-dependent in situ Raman spectra for the Pt11Ni89 alloyin 2 M HCl. The collection time for each line is 20 s. The insets are the spectrum of asample H2PtCl6 solution for comparison and a typical spectrum for the Pt35Ni65

alloy in 2 M HCl in the whole potential range investigated.

S. Chen et al. / Journal of Electroanalytical Chemistry 628 (2009) 55–59 57

They appear at ca. 0.2 V, corresponding to the potential for the for-mation of monolayer Pt oxide in the chloride-free solution. Thedoublet grows in intensity up to 0.5 V, attenuates gradually, andthen vanishes upon 1 V, where O2/Cl2 bubbles interfere with themeasurement severely. It is unexpected that Pt does dissolveapparently in the Pt11Ni89 alloy by electrooxidation in HCl solu-tions at rather low potentials. However, no soluble species were

Fig. 5. Typical potential-dependent SEM images with different magnifications for the ele1 h, (b) 0 V for 600 s and coarsened at 500 �C for 1 h, (c) 0.6 V for 100 s, (d) 1 V for 10comparison of surface Ni content before and after anodic polarization by EDX determin

discernible (neither Ni2+ nor PtCl2�6 ) using UV–vis and Raman spec-

troscopy for the electrooxidation of Pt and Pt35Ni65 alloy under thesame conditions, as shown in Figs. 3 and 4. It is therefore evidentthat Pt dissolution tightly depends on the Ni content in the alloyand a high ratio of Ni is beneficial to promote the simultaneous dis-solution of Pt.

3.3. Evolution in morphology and composition

SEM–EDX study was performed at some selected potentials inorder to reveal the evolution of surface texture and compositionof the alloy. Fig. 5a–d show typical potential-dependent SEMimages of Pt11Ni89 at �0.5, 0, 0.6 and 1.0 V in 2 M HCl, which locatein the regions of selective dissolution (�0.5 to 0 V), simultaneousdissolution (P0.2 V) and O2/Cl2 evolution (P0.8 V), respectively,as deduced from the electrochemical and spectral analysis in theprevious Sections.

For the samples polarized at �0.5 V and 0 V, the enlarged mor-phological details present some ultrafine pores (Fig. 5a and b) andthe Pt ratio in the alloy gets rich from the initial Pt11Ni89 to Pt34Ni66

and Pt36Ni64 (A, B bars in Fig. 5f), respectively. The Ni component isselectively stripped partly in this potential region as expected, andthe alloy becomes inert with the Pt enrichment. Note that in orderto image the porous structure after dealloying, SEM measurementswere performed on coarsened samples by annealing at high tem-perature to enlarge the pores, as treated in previous Pt–M dealloy-ing studies [11–15]. Unlike in the dealloying of Au–M, no obviousbicontinuous-ligament porous structures can be observed for Pt–M, which could be attributed to the relatively low surface diffusionrate for Pt atoms, at least 3–4 orders of magnitude lower than thatof Au atoms [14,15]. Significant SSC was observed on the surface,which originated from the residual stress in material generatedfrom the quenching procedure, mechanical cutting or polishingprocesses [19]. The SEM–EDX results of Fig. 5e and E in Fig. 5f forPt35Ni65 clearly indicate a relatively flat surface without erodingeven at a potential as high as 1 V.

ctrooxidation of Pt11Ni89 in 2 M HCl at (a) �0.5 V for 1 h and coarsened at 500 �C for0 s, and of (e) Pt35Ni65, at 1 V for 600 s. (f) White and black bars (A–E) show the

ation, which correspond to the SEM images (a–e), respectively.

58 S. Chen et al. / Journal of Electroanalytical Chemistry 628 (2009) 55–59

As the potential rising, simultaneous dissolution occurs, anddistinct nanoporous films were obtained at 0.6 V and 1 V as shownin Fig. 5c, d without needing heat treatment. The polarization at0.6 V results in corroded surface with micrometer slices alongthe crack propagation of SCC. High magnification of the slice tipclearly reveals the nanoporosity. Successively, larger pieces of lay-ered nanoporous films were produced at 1 V (Fig. 5d). The evolu-tion of Cl2/O2 bubbles may accelerate the mass transfer ofchloride ions, propitious to the simultaneous dissolution and thesurface film peeling, so the underneath substrate dissolves contin-uously. These films contain much higher Pt content up to Pt49Ni51,Pt52Ni48 according to the EDX result of C, D in Fig. 5f. The larger ex-tent enrichment of Pt in the simultaneous dissolution than in theselective dissolution is ascribed to the incorporation of a replace-ment reaction between Ni in the alloy and the dissolved Pt(IV) spe-cies: PtCl2�

6ðaqÞ þNið0ÞðsÞ ! PtnanoðsÞ þNiðIIÞðaqÞ þ Cl�ðaqÞ, which isthermodynamically feasible.

In order to valid this hypothesis kinetically, a simulative exper-iment was conducted on pure Ni electrode by immerging it in a 2 MHCl solution containing H2PtCl6 and with an anodic potential of0.6 V for 100 s, where the anodic dissolution of Ni takes place simul-taneously: Ni ? Ni2+ + 2e�. Interestingly, similar layered films ap-pear on Ni in Fig. 6b but with smaller pore size. The replacementis confirmed with the familiar cyclovoltammogram in Fig. 6a (thesolid line). However, no electrochemical signal of platinum can beobserved in Fig. 6a (the dashed line) by dipping the Ni electrode un-der open circuit for 100 s. In other words, the applied anodic poten-tial can accelerate the above replacement reaction.

The XRD patterns for the nanoporous films obtained at 1.0 V byultrasonic collection are shown in Fig. 7b. The diffraction peak

Fig. 6. (a) CVs in 0.5 M H2SO4 for pure Ni electrodes that was immerged in 2 MHCl + 15 mM H2PtCl6 for 100 s under open circuit or with an applied potential of0.6 V. (b) The SEM images for the latter case.

(111) of Pt shifts slightly to higher Bragg angles, which indicatesa decrease in the lattice constant in the presence of Ni replacement[23]. The surface structure of the nanoporous film was also studiedin more detail by AFM. There are obviously ultrafine nanoclustersdispersed on the surface (the bright dots in the image of Fig. 7a).

Fig. 7. (a) The AFM image (1 � 1 lm2) and (b) XRD patterns for the nanoporousslices obtained at 1 V from the Pt11Ni89 alloy in 2 M HCl.

Fig. 8. CVs of the as-prepared nanoporous material at 1 V loaded on a 2 mmdiameter GC electrode in (a) 1 M HClO4 and (b) 1 M HClO4 + 1 M CH3OH with a scanrate of 100 mV s�1. The dotted curves refer those on a smooth Pt disk electrode(2 mm diameter). The current density in (b) is normalized on the basis of theelectrochemically active area of hydrogen desorption.

S. Chen et al. / Journal of Electroanalytical Chemistry 628 (2009) 55–59 59

3.4. Application of the porous films in the electrocatalytic oxidation ofmethanol

Nanoporous materials have potential applications in electroca-talysis. Electrooxidation of methanol has been carried out withthe as-prepared Pt–Ni nanoporous film. The electrochemically ac-tive area of the electrode was evaluated from the area of hydrogendesorption on Pt by using the charge factor 210 lC cm�2. The filmdisplays superior catalytic activity for the electrooxidation ofmethanol (Fig. 8b) with low onset potential and high oxidative cur-rent density against the active area, comparing with the polishedpure Pt electrode. Besides the nanostructure, the residual Ni alsoplays an enhancing role due to the change in electronic effect[24,25]. More detailed studies on it are beyond the scope of thispaper.

4. Conclusions

UV–vis absorption and in situ Raman scattering spectra in com-bination with morphological studies have provided direct informa-tion for the electrodissolution behavior of Pt–Ni alloys in HClsolution. The dissolution heavily depends on the Ni content. PurePt and Pt35Ni65 alloy are strongly corrosion-resistant. Selectiveand simultaneous electrodissolution occurs successively with thepotential ascending for the electrooxidation of Pt11Ni89 alloy,resulting in different Pt-enriched nanoporous structures. The for-mer produces ultrafine pores after post-dealloying heat treatment,and the latter gives rise to novel layered nanoporous films incorpo-rating a potential-accelerated galvanic replacement reaction be-tween Ni in the alloy and the dissolved PtCl2�

6 . It also affords us amethod to prepare stable nanoporous Pt–Ni materials for applica-tions in electrocatalysis.

Acknowledgments

Financial support of this research from National Natural ScienceFoundation of China (20673103, 20373063) is gratefullyacknowledged.

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