Journal of Electroanalytical Chemistry 660 (2011) 80–84

Contents lists available at ScienceDirect

Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Study on the electrodissolution and roughening of a palladium electrodein chloride containing solutions

Shu Chen a,b, Wei Huang b, Jufang Zheng c, Zelin Li b,⇑a Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education of China, School of Chemistry and Chemical Engineering,Hunan University of Science and Technology, Xiangtan 411201, Chinab Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education of China, College of Chemistry and Chemical Engineering,Hunan Normal University, Changsha 410081, Chinac Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China

a r t i c l e i n f o

Article history:Received 29 January 2011Received in revised form 21 May 2011Accepted 7 June 2011Available online 15 June 2011

Keywords:PalladiumIn situ Raman spectroscopyElectrodissolutionMicro-nanostructureElectrocatalysis

1572-6657/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jelechem.2011.06.008

⇑ Corresponding author. Tel.: +86 731 88871533; faE-mail address: lizelin@hunnu.edu.cn (Z. Li).

a b s t r a c t

Potential-dependent surface electrooxidation processes of a Pd electrode in chloride containing solutionshave been investigated in detail by means of in situ confocal Raman spectroscopy for the first time. In asolution of HCl, characteristic Raman bands for the oxidative coordination of Pd with Cl�, the transforma-tion of soluble Pd(II) to Pd(IV) complexes, the electrooxidation of Cl� into Cl2, and the redox between Cl2

and Pd were all detected unambiguously during the potential ascending. While in a solution of KCl, insol-uble salt films of K2PdCl4 and K2PdCl6 were found on the electrode surface due to their poor solubility.Moreover, electrochemical roughing of Pd electrode by electrodissolution–electrodeposition cycling inthe HCl solution with a surfactant have been utilized to construct micro-nanostructures of Pd, which dis-play high activity toward the electrooxidation of ethanol in an alkaline medium and obvious surfaceenhanced Raman spectroscopy (SERS) signals for pyridine probe molecules.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Electrodissolution of metals and alloys involves complicated sur-face processes like adsorption/desorption, film formation of oxidesor insoluble salts, pitting corrosion, coordination, and gas evolution.Raman spectroelectrochemistry is a promising tool to understandthe physicochemical processes that occur at these metal/solutioninterfaces on the molecular level, and has been employed to investi-gate the electrodissolution course of many metals like Cu [1,2], Au[3–5], Co [6], Hg [7], Fe [8–10], Fe–P [11], carbon steel [12–15],Au–Sn [16], Pt–Ni alloy [17] and so on.

Electrooxidation of Pd has been intensively investigated innoncomplex acids (e.g. perchloric or sulfuric acid) and in alkalinesolutions [18–21], emphasizing the formation and growth of insolu-ble oxide/hydroxide layers in mild electrolyte. Their oxidation stateand chemical composition at different potential limit are deter-mined by diversiform surface analytical and materials characteriza-tion techniques [18]. Comparatively, in strong acid/basic mediumand in the presence of anions (e.g. Cl� or I�), the electrooxidationof Pd facilitate the formation of kinds of soluble intermediates andproducts [22–30], which pronounces the electrodissolution of Pdby hydration or complexation of Pd cations. The electrooxidation

ll rights reserved.

x: +86 731 88872531.

of Pd in chloride containing electrolyte has received less attentionup to the present and the understanding of electrochemicallyformed species only derived from electrochemical measurements[31–33]. Therefore, further investigation especially using in situspectroscopy is required to confirm the possible species and inter-mediates involved in the electrodissolution.

In this paper, in situ potential-dependent Raman spectroscopyhas been performed to clarify the species in the electrooxidationof Pd in chloride containing solutions. With the knowledge of spe-cies at different potentials, we construct micro-nanostructures ofPd for electrocatalysis on the electrode surface by electrodissolu-tion–electrodeposition cycling.

2. Experimental

Raman measurements were performed with a RenishawRM1000 confocal Raman spectrometer (Gloucestershire, UK) in aself-designed spectroscopic cell made by Teflon with a quartz win-dow. The objective equipped on a DMLM Leica microscope was oflong working distance (ca. 8 mm) with 50� magnifications. Theexciting wavelength was 785 nm from a diode laser with a powerof ca. 18.5 mW on the sample. More detailed descriptions on theinstrument can be found elsewhere [5]. Normal electrochemicalmeasurements were carried out with three electrodes using CHI660C electrochemical station (Chenhua, Shanghai, China). Theworking electrode was a Pd disk made from a 1 mm diameter wire

S. Chen et al. / Journal of Electroanalytical Chemistry 660 (2011) 80–84 81

(purity P99.99%) procured from Tianjin Aida Co., Ltd. (China). Thecurrent density is related the geometric area of the electrode un-less indicated otherwise. Prior to use the electrode was polishedwith 1000# metallographic paper and cleaned with ultrasonicwaves in ultra-pure water (from a water purification system ofMillipore Corp., USA). Then, the electrode subjected potential cy-cling in the range of �0.6 to 0.9 V in 0.5 M H2SO4 until the curvesrepeated. The roughness factor of the polished electrode was 20with the conversion factor of 424 lC cm�2 for the reduction ofmonolayer Pd oxides. The potential quoted here is versus the sat-urated mercurous sulfate electrode (SMSE). Cyclic voltammetrywas applied to roughen the Pd electrode in 1 M HCl solution(M = mol dm�3) containing a certain amount of cetyltrimethylammonium bromide (CTAB). Surface morphology of the Pd elec-trode was obtained with a JSM-6380LV scanning electron micros-copy (SEM). SERS spectra were measured under the excitationlaser of 633 nm with ca. 8.2 mW power and the collection timewas 60 s. Pyridine was chosen as the probe molecule, which wasdetected by immersing the roughened Pd electrode in the solutionof 0.01 M pyridine + 0.1 M KClO4. All experiments were performedat room temperature (�20 �C) and solutions were freshly preparedwith analytical grade chemicals and ultra-pure water.

3. Results and discussion

3.1. Electrochemical behaviors of Pd in HCl

The effect of Cl� ions on the electrodissolution of Pd is presentedin Fig. 1 with different HCl concentrations. The cyclic voltammo-grams in the blank solution of aqueous perchloric acid in Fig. 1a (dot-ted and dashed lines) resembled those reported in numerousinvestigations for polycrystalline Pd [18–21], where adsorption/absorption–desorption of hydrogen and the formation–reduction

Fig. 1. (a) Cyclic voltammograms of the Pd electrode in 1 M HClO4 (dotted anddashed line) and in 1 M HClO4 + 0.01 M HCl (solid line) within different potentialranges at a scan rate of 100 mV s�1. (b) Linear sweep voltammograms at 100 mV s�1

for the Pd electrode by changing the HCl concentration from 0.1 to 1 M as indicated.(c) Series of anodic sweep voltammograms in 0.1 M HCl at different scan rate andthe relationship between first anodic peak current and square root of scan rate.

of surface Pd (hydro)oxides proceeded in the lower and higher po-tential ranges, respectively. In order to exclude the interference fromthe oxidation of absorbed hydrogen into bulk Pd [30,34,35], the low-er potential limit was set such that it was not less than�0.3 V in theelectrochemical experiments for anodic dissolution. In 1 M HClO4,the oxidation of Pd started at around 0.2 V in the positive-going po-tential scan and the reduction of the oxides began at �0.1 V in thenegative-going scan. In the presence of 0.01 M Cl� ions (the solid linein Fig. 1a), two pairs of new redox peaks appeared, which are attrib-uted to the formation and reduction of two possible complex ions ofPd with chloride [31–33]:

Pdþ 4Cl� ¼ PdCl2�4 þ 2e� ðiÞ

PdCl2�4 þ 2Cl� ¼ PdCl2�

6 þ 2e� ðiiÞ

Evidence for the involvement of such soluble complexes will beresolved latter by Raman spectra (Section 3.2). Notably, the twoanodic peak currents increased remarkably (Fig. 1b) by increasingthe concentration of HCl. Moreover, the first anodic peak current(Ip,a) varied linearly with the square root of the scan rates (m1/2)and the numbers of electron transferred were estimated closedto 2, as shown in Fig. 1c [36].

3.2. In situ Raman spectroscopy for the electrooxidation of Pd in 1 MHCl

Fig. 2 shows the Raman spectra of Pd in 1 M HCl for a sequence ofanodic potentials applied. There were no Raman peaks below 0 V be-fore the electrodissolution of Pd. The onset of surface oxidation wassignaled near 0.1 V with a discernible band at 304 cm�1, locating atthe lower end of the first anodic climbing branch in Fig. 1b. This bandis assigned to the Pd–Cl stretching (v1) in PdCl2�

4 [37] from Reaction(i) which gained in intensity with the increase of potential. Note thatthe stretching of Pd—Cl�ads for the monolayer of adsorbed Cl� is tooweak to be detected. At 0.7 V, which situates at the lower end ofthe second ascending branch in Fig. 1b, the peak at 304 cm�1 wassubstituted by a double band of Pd–Cl stretching (v1, v2) at293/315 cm�1 for the species of PdCl2�

6 [38] from Reaction (ii).Immediately following this doublet, three more new peaks appearedat 535 (shoulder), 540 and 547 cm�1 while E P 0.8 V and their inten-

Fig. 2. Potential-dependent in situ Raman spectra for the Pd electrode in 1 M HClduring increasing the potential. The collection time for each spectrum was 10 s witha 785 nm laser.

Fig. 4. In situ Raman spectra at selected potentials for the Pd electrode in 1 M KClsolution. Its anodic sweep voltammogram at 100 mV s�1 was shown in the inset.

82 S. Chen et al. / Journal of Electroanalytical Chemistry 660 (2011) 80–84

sities got stronger till 1.1 V. The bands appeared as three peaks canbe ascribed to the fundamentals and the hot bands of the isotopicCl2 molecules [39,40]. The produced Cl2 (g) at 0.8 V and above agi-tated the solution near the electrode surface and dispersed somePdCl2�

6 into the bulk solution, and that is the reason why the spec-trum intensity of PdCl2�

6 was the strongest at 0.7 V. WhenE P 1.2 V where oxygen released, the triplet and the doubletremarkably weakened and strengthened, respectively. We supposethat these three peaks are related with the formation of Cl2:

2Cl� ! Cl2 þ 2e� ðiiiÞ

To confirm the supposition, a Cl2 containing nonaqueous solu-tion was prepared by bubbling chlorine gas through the cyclohex-ane (C6H12). The triplet at 535, 540, 547 cm�1 was observed indeed(Fig. 3a) for the solution. More interestingly, we also confirmedthat the produced Cl2 can react with Pd chemically formingPdCl2�

6 very likely by the following reaction:

Pdþ 2Cl2 þ 2Cl� ¼ PdCl2�6 ðivÞ

And it might undergo two steps like

Pdþ Cl2 þ 2Cl� ! PdCl2�4 ðvÞ

PdCl2�4 þ Cl2 ! PdCl2�

6 ðviÞ

We confirmed Reaction (iv) by setting the circuit open afterpolarizing the electrode at 1 V (Fig. 3b). The intensities of the dou-blet for PdCl2�

6 and of the triplet for Cl2 went oppositely upon open-ing the circuit because the moieties of Cl2 and PdCl2�

6 wereconsumed and produced, respectively, as expressed in Reaction(iv). The exhaustion of Cl2 and the diffusion of PdCl2�

6 account forthe vanishing and diminishing of the triplet and doublet, respec-tively, in 30 s (Fig. 3b). Moreover, the opposite tendency in inten-sity for Cl2 (triplet) and for PdCl2�

6 (doublet) at 1.2 V in Fig. 2 canalso be explained by Reaction (iv) because the convection inducedby oxygen evolution (vii) can speed Reaction (iv).

H2O! ð1=2ÞO2 þ 2Hþ þ 2e� ðviiÞ

3.3. In situ Raman spectroscopy for the electrooxidation of Pd in 1MKCl

Raman bands for Pd(II) and Pd(IV) species also occurred with1 M KCl (Fig. 4) at 0.3 V and 0.9 V, respectively, corresponding tothe oxidative current peaks (the inset of Fig. 4). Again, the doubletat 293/324 cm�1 for the PdCl2�

6 got stronger in intensity uponopening the circuit then. However, the intensity of the doubletdid not decrease for a long period of time, and the small peak at

Fig. 3. (a) Raman spectra for the sample of liquid cyclohexane before and afterabsorbing chlorine gas. (b) In situ Raman spectra for the Pd electrode when the cellwas set at 1 V and then let the circuit open. The exposure time for CCD was 10 s.

305 cm�1 for PdCl2�4 was always there. These facts indicate that salt

films of K2PdCl4, K2PdCl6 precipitated on the surface:

PdCl2�4 þ 2Kþ ¼ K2PdCl4ðsÞ ðviiiÞ

PdCl2�6 þ 2Kþ ¼ K2PdCl6ðsÞ ðixÞ

Although the triplet for Cl2 did not show up for some reasons,Reactions (iv)–(vi), incorporating (viii) and (ix), should be stillthe reason for the increase in intensity after opening the circuit.We observed that the anodic peak current related K2PdCl4 (the firstpeak) was larger in 1 M HCl (Fig. 1b) than in 1 M KCl (the inset ofFig. 4). However, opposite situation appeared for the anodic peakcurrent related K2PdCl6 (the second peak). These facts suggest thatthe film of K2PdCl4 rather than K2PdCl6 blocked the electrochemi-cal reaction on the surface in some extent.

3.4. A potential-dependent species sketch for the electrooxidation of Pd

Based on the spectroelectrochemical results and discussion, theelectrooxidation processes of Pd are summarized in Fig. 5. In theHCl solution, the Pd electrode is electrooxidized into two solublecomplexes PdCl2�

4 (aq) and PdCl2�6 (aq) successively with the poten-

tial increase; Cl� can be then oxidized into gaseous Cl2 (g) parallelto the formation of PdCl2�

6 (aq), so the oxidation of Pd can also pro-ceed chemically by reacting with the produced Cl2 (g); and the con-vection induced by the O2 (g) evolution at still higher potentialscan accelerate the chemical reactions of Pd with Cl2 into PdCl2�

6

(aq). Nevertheless, insoluble salts K2PdCl4 (s) and K2PdCl6 (s) pre-cipitate on the electrode in the solution of KCl. Thus, we have got-ten an entire sketch for the species involved in the electrooxidationprocess of Pd in chloride containing solutions.

Fig. 5. A mechanistic scheme for the anodic processes of Pd in chloride containingsolutions with the potential increase.

Fig. 7. Cyclic voltammograms at 100 mV s�1 on the electrodes of the roughed (realline) and the smooth Pd electrode (dotted line) in a solution of (a) 1 M NaOH and (b)1 M NaOH + 1 M ethanol. The dash line in (a) indicated the CVs of Pd electrodetreated in a solution of 1 M HCl without CTAB for comparison. The current densityin (b) was normalized by the real surface areas.

Fig. 8. In situ SERS spectra of pyridine on the roughed Pd electrode in 0.01 Mpyridine + 0.1 M KClO4. The peak at 937 cm�1 is attributed to ClO�4 .

S. Chen et al. / Journal of Electroanalytical Chemistry 660 (2011) 80–84 83

3.5. Electrochemical roughening of Pd and the application inelectrocatalysis and SERS

Pd electrodes have been roughened previously in 1 M H2SO4, anoncomplex medium, involving formation and reduction of Pdoxides [41,42]. We performed roughening of Pd electrodes herein 1 M HCl involving electrodissolution and electrodeposition ofPd under the guidance of above species sketch (Fig. 5). In 1 MHCl, the upper potential limit in the cyclic voltammetry played akey role in the resultant morphology. As shown in Fig. 6a and b,microspheres consisting of small aggregated Pd nanoparticles wereobtained with an upper potential limit of 0.6 V. The Pd nanoparti-cles were formed by the repeated electrodissolution of bulk Pd andelectrodeposition of the dissolved Pd(II) during the potential cy-cling. The produced nanoparticles can stick on the rough surfaceaggregating into microparticles under the action of strong electro-lyte (1 M HCl). Since these particles were protected by the surfac-tant CTAB, they were not as active as the bulk electrode which keptelectrodissolution. Therefore, assisted with the CTAB surfactant(the solid line), much higher surface area was obtained than rough-ening the Pd electrode in the blank HCl solution (the dash line inFig. 7a). However, seriously etched macroporous structure oc-curred without the aggregated nanoparticles while the upper po-tential was changed to 1.0 V (Fig. 6c and d), and the solutionturned into orange. The surface area of these etched pores is muchlower than that of the microspheres. These phenomena can be as-cribed to massive anodic dissolution of Pd into Pd(IV) complex andstrong chemical etching by Cl2 evolution in this region as discussedin Section 3.2.

Compared with the polished Pd electrode, the roughened Pd elec-trode in Fig. 6a possess very high surface area, as seen in Fig. 7a in1 M NaOH background solution, and displayed superior catalyticactivity for the electrooxidation of ethanol, as shown in Fig. 7b.Apparently, the onset potential for the oxidation of ethanol wasabout 100 mV lower and the anodic current density was about 5times higher. Fig. 8 shows the in situ potential-dependent Ramanspectra of pyridine adsorbed on the surface of roughed Pd. The Pd

Fig. 6. Typical SEM images with different magnifications for the Pd electrode treated in different potential range (a and b) �0.6 to 0.6 V for 300 cycles, (c and d) �0.6 to 1.0 Vfor 100 cycles in a solution of 1 M HCl + 10 mM CTAB.

84 S. Chen et al. / Journal of Electroanalytical Chemistry 660 (2011) 80–84

micro/nanostructure exhibits an obvious SERS activity, as evidencedby the typical band at 1004 cm�1 for the typical ring breath vibration(v1), other peaks located at 629, 1561, 1591 cm�1 are all the charac-teristic bands (v6a, v12, v8a) related to absorbed pyridine [42].

4. Conclusions

Electrodissolution processes of Pd in chloride containing solu-tions have been elucidated by in situ Raman spectroscopy. Thespecies involved in a wide anodic potential window have beenidentified, including different soluble complexes and gas evolution.By potential cycling treatments of the Pd electrode in a given po-tential range, we can get micro-nanostructured surface with higharea through the electrodissolution–electrodeposition of Pd. Itaffords us an efficient way to prepare high performance electrodematerials of Pd for applications in electrocatalysis and SERS.

Acknowledgments

We are grateful for the financial supports of this research fromNatural Science Foundation of Zhejiang Province of China (GrantNo. Y4090658), the Open Foundation of Key Laboratory of the Min-istry of Education for Advanced Catalysis Materials & Zhejiang KeyLaboratory for Reactive Chemistry on Solid Surfaces (Grant No.DH201001), Ph.D. Programs Foundation of the Education Ministryof China (Grant No. 20104306110003), and National Natural Sci-ence Foundation of China (Grant Nos. 20673103 and 21003045).

References

[1] S.T. Mayer, R.H. Muller, J. Electrochem. Soc. 139 (1992) 426–434.[2] G.L. Chen, H. Lin, J.H. Lu, L. Wen, J.Z. Zhou, Z.H. Lin, J. Appl. Electrochem. 38

(2008) 1501–1508.[3] B.H. Loo, J. Phys. Chem. 86 (1982) 433–437.[4] B. Bozzini, A. Fanigliulo, J. Appl. Electrochem. 32 (2002) 1043–1048.[5] Z.L. Li, T.H. Wu, Z.J. Niu, W. Huang, H.D. Nie, Electrochem. Commun. 6 (2004)

44–48.[6] J.A. Calderón, O.R. Mattos, O.E. Barcia, S.I. Córdoba de Torresi, J.E. Pereira da

Silva, Electrochim. Acta 47 (2002) 4531–4551.[7] A.G. Brolo, M. Odziemkowski, J. Porter, D.E. Irish, J. Raman Spectrosc. 33 (2002)

136–141.

[8] D. Larroumet, D. Greenfield, R. Akid, J. Yarwood, J. Raman Spectrosc. 38 (2007)1577–1585.

[9] D. Larroumet, D. Greenfield, R. Akid, J. Yarwood, J. Raman Spectrosc. 39 (2008)1740–1748.

[10] E.-S.M. Sherif, R.M. Erasmus, J.D. Comins, Electrochim. Acta 55 (2010)3657–3663.

[11] H. Tanabe, T. Shibuya, N. Kobayashi, T. Misawa, ISIJ Int. 37 (1997) 278–282.[12] M. Odziemkowski, J. Flis, D.E. Irish, Electrochim. Acta 39 (1994) 2225–2236.[13] S. Simard, M. Odziemkowski, D.E. Irish, L. Brossard, H. Ménarte, J. Appl.

Electrochem. 31 (2001) 913–920.[14] C.T. Lee, M.S. Odziemkowski, D.W. Shoesmith, J. Electrochem. Soc. 153 (2006)

B33–B41.[15] M. Reffass, R. Sabot, M. Jeannin, C. Berziou, Ph. Refait, Electrochim. Acta 52

(2007) 7599–7606.[16] S. Chen, Y.P. Chu, J.F. Zheng, Z.L. Li, Electrochim. Acta 54 (2009) 1102–1108.[17] S. Chen, S. Wu, J.F. Zheng, Z.L. Li, J. Electroanal. Chem. 628 (2009) 55–59.[18] M. Grden, M. Łukaszewski, G. Jerkiewicz, A. Czerwin, Electrochim. Acta 53

(2008) 7583–7598.[19] K. Gossner, E. Mizera, J. Electroanal. Chem. 125 (1981) 347–358.[20] L.D. Burke, D.T. Buckley, J. Electrochem. Soc. 143 (1996) 845–854.[21] C.C. Hu, T.C. Wen, Electrochim. Acta 41 (1996) 1505–1514.[22] K. Juodkazis, J. Juodkazyte, B. Šebeka, G. Stalnionis, A. Lukinskas, Russ. J.

Electrochem. 39 (2003) 954–959.[23] R. Schumacher, W. Helbig, I. Hass, M. Wünsche, H. Meyer, J. Electroanal. Chem.

354 (1993) 59–70.[24] A.E. Bolzán, M.E. Martins, A.J. Arvía, J. Electroanal. Chem. 172 (1984) 221–233.[25] T. Solomun, J. Electroanal. Chem. 302 (1991) 31–46.[26] S.H. Cadle, J. Electrochem. Soc. 121 (1974) 645–648.[27] M. Grden, J. Kotowski, A. Czerwinski, J. Solid State Electrochem. 3 (1999)

348–351.[28] M. Lukaszewski, A. Czerwinski, J. Electroanal. Chem. 589 (2006) 38–45.[29] M. Lukaszewski, A. Czerwinski, J. Alloys Compd. 473 (2009) 220–226.[30] M. Lukaszewski, T. Kedra, A. Czerwinski, Electrochem. Commun. 11 (2009)

978–982.[31] J.A. Harrison, T.A. Whitfield, Electrochim. Acta 28 (1983) 1229–1236.[32] J. Genescá, L. Victori, Platinum Met. Rev. 30 (1986) 80–83.[33] J. Genescá, R. Durán, Electrochim. Acta 32 (1987) 541–544.[34] M.W. Breiter, J. Electroanal. Chem. 81 (1977) 275–284.[35] C.C. Hu, T.C. Wen, J. Electrochem. Soc. 141 (1994) 2996–3001.[36] A.J. Bard, L.R. Faulkner, Electrochemical Methods, second ed., J. Wiley & Sons,

New York, 2001.[37] P.L. Goggin, J. Mink, J. Chem. Soc., Dalton Trans. (1974) 1479–1483.[38] M. Debeau, H. Poulet, Spectrochim. Acta. 25A (1969) 1553–1562.[39] G. Hochenbleicher, H.W. Schrötter, Appl. Spectrosc. 25 (1971) 360–362.[40] H. Chang, D.M. Hwang, J. Raman Spectrosc. 7 (1978) 254–260.[41] T. Kessler, A. Visintin, A.E. Bolzan, G. Andreasen, R.C. Salvarezza, W.E. Triaca,

A.J. Arvia, Langmuir 12 (1996) 6587–6596.[42] Z. Liu, Z.L. Yang, L. Cui, B. Ren, Z.Q. Tian, J. Phys. Chem. C 111 (2007)

1770–1775.