XPS study and electrochemical behaviour of the nickel hexacyanoferrate film electrode upon treatment in alkaline solutions

EISWIER

Journal of Electroanalytical Chemistry 417 (1996) 83-88

XPS study and electrochemical behaviour of the nickel hexacyanoferrate film electrode upon treatment in alkaline solutions Tommaso R.I. Cataldi * , Rachele Guascito, Anna Maria Salvi Dipnrtimento

di Chimica,

lJniuersiti2

degli Studi della Basilicata,

Via N. Saurn 85, I-85100

Potenza,

Italy

Received 7 March 1996; revised 30 April 1996

Abstract This paper describes the X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of nickel hexacyanoferrate (NiHCF) film electrodes. Glassy carbon was used as a substrate and was modified by cycling the potential between -0.1 and + 0.9 V vs. SCE in fresh 0.5 M KC1 solutions containing 0.25 mM NiCl, and 0.25 mM K ,Fe(CN),. Subsequent electrochemical cycling of the NiHCF film in alkaline solutions induces significant changes, yielding a modified electrode that is primarily composed of hydrated Ni(II) oxide/hydroxide with a single set of well-defined Ni(II)/(III) voltammetric peaks. Such induced changes are irreversibIe, and the XPS investigation was very useful for verifying the electrode modifications occurring. Iron present in the NiHCF film suffered the most changes. The absolute intensity of the Fe2p and N:K elemental ratio were especially useful in evaluating the modifications occurring, providing a clear indication of the extent to which NiHCF film was converted. The implications of these findings are discussed. Keywords:

Nickel

hexacyanoferrate; XPS; Film electrodes

1. Introduction Thin

films

of nickel

hexacyanoferrate

(NiHCF)

can

readily be prepared by repetitive potential cycling on a number of different conducting substrates,like nickel [l-9], graphite [ 10,111, and gold [12,13]. The robust chemical nature of these inorganic films makes them good candidates for the modification of electrode surfaces, having potential applications in several different areas such as electrocatalysis, amperometric sensors,display technology, photochemical energy conversion, etc. Although most reports have dealt with the electrochemical activity of NiHCF film electrodes, little attention has been devoted to their characterisation by surface science techniques. Besidesthe potential practical utility of the NiHCF deposit, a detailed structural investigation may have general significance in the study of such an inorganic film. Extensive studies have already been carried out on its electrochemical behaviour in several supporting electrolytes [3,7,8,10], * Corresponding author. Phone: + 39 971 474237; Fax: t39 474223.

0022.0728/96/$15.00 PII

SOO22-0728(96)04749-3

971

and some models have been proposed to explain the presence of two redox transitions. Recently, Prabhakara Rao and coworkers [ 111reported the electrocatalytic capability

of an Ni(I1)

oxide/hydroxide

film,

obtained

upon

conversion of the NiHCF deposit in alkaline electrolytes (i.e. 1M KOH or 1 M NaOH). As previously proposedfor other nickel-based electrodes [ 14-231, this film on glassy carbon (referred to as Ni(OH),(hyd)) exhibits a powerful electroactivity towards the oxidation of mono- and polyhydric compounds. In this report, X-ray photoelectron spectroscopy (XPS) in ultra high vacuum (UHV) and cyclic voltammetry were employed to characterise the NiHCF deposit on glassy carbon electrodes before and after treatment in high pH solutions. Special attention was devoted to its structural and chemical modifications, and a detailed XPS analysisof the Ni2p, Nls, K2s, K2p and Fe2p regions is provided, allowing a comprehensive description of changes occurring. Since the chemical modifications investigated were effected by using electrochemical techniques, the modified electrode surfaces showed an inherent high degree of reproducibility.

Copyright 0 1996 Elsevier Science S.A. All rights reserved.

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2. Experimental 2.1. Reagents and materials

Nickel chloride hexahydrate, potassium hexacyanoferrate(II1) 99 + % and potassium hexacyanoferrate(I1) trihydrate were purchased from Aldrich Chemical Co. and used as received. Other chemicals employed were of reagent grade and were used without further purification. All solutions were prepared just prior to use with deionized and twice distilled water. 2.2. Electrochemical

instrumentation

Cyclic voltammetry was performed with an EG&G Princeton Applied Research Model 273 potentiostat/galvanostat equipped with a Linseis X-Y recorder, Model LY 18 100. All experiments were carried out at room temperature (20 + 2°C) in a standard threeelectrode glass cell employing an SCE reference electrode and a platinum gauze as counter electrode. The glassy carbon working electrode (3 mm diameter) used in cyclic voltammetry was purchased from Amel (Milan, Italy), Model 493. Glassy carbon plates (Sigradur K) used for the XPS investigation were obtained from HTW Hochtemperatur-Werkstoffe GmbH (Meitingen, Germany). 2.3. Electrode preparation

Before modification the glassy carbon surfaces were polished with 0.05pm o-alumina slurry on a microcloth polishing pad, washed with water and sonicated for a few minutes in twice distilled water. The electrode modification was accomplished with freshly prepared solutions using a procedure similar to that reported by Prabhakara Rao and coworkers [IO,1 l]. A preliminary characterisation of the modified glassy carbon electrodes was carried out by cyclic voltammetry.

Chemistry

417 (1996)

83-88

constant pass energy of 50eV and a channel width of 1.0 and 0.1 eV respectively. Binding energies were assigned by referring the energy scale to that of the glassy carbon Cls peak, which was arbitrarily set to 284.6eV. Data acquisition and spectra analysis were accomplished with a data processing program [24]. Elemental surface stoichiometries were obtained from peak area ratios corrected by sensitivity factors derived by Wagner [25]. Pressed pellets of K,Fe(CN), and K,Fe(CN), powder samples, as well as electrochemically produced inorganic films deposited onto glassy carbon as a substrate, were fixed onto the sample rod by using a conductive-adhesive copper tape. XP spectra were recorded only after the wide scan showed no extra features arising either from the copper tape or the sample rod. 3. Results and discussion 3.1. Nickel hexacyanoferrate

films

Cyclic voltammograms (CVs) obtained during the modification of a glassy carbon electrode immersed in a solution containing 0.25 mM Ni2+ and 0.25 mM Fe(CN)z- in 0.5M KC1 are shown in Fig. l(A). Upon deposition the modified electrode was rinsed with pure water and immersed in the blank supporting electrolyte; the resulting CV curves, recorded at 50mV s-t, are shown in Fig. l(B). Two redox transitions, Ia/Ic and IIa/IIc, were observed, with mid-point potentials (EPA + E,,)/2 at + 0.45 and +0.61 V vs. SCE respectively. Notably, the voltammetric peaks at higher positive potentials are sharp and narrow, whereas the set at +0.45 V is significantly broader. It is well known that the redox behaviour of ferrocyanide is strongly perturbed by its incorporation into an inorganic lattice. This behaviour is particularly evident in the NiHCF

50 LA

2.4. XPS instrumentation

XPS measurements were made using a Leybold LH Xl spectrometer interfaced to an IBM compatible personal computer for data acquisition. Spectra were obtained using either the achromic Mg Ka radiation (12536eV) or Al Ka radiation (1486.6eV) operating at 260 W (13 kV, 20mA) as excitation sources, and were recorded in fixed analyser transmission mode (FAT) to achieve maximum instrumental resolution. The instrument operated at a pressure below 5 X 10e9 Torr in the analysis chamber. The binding energy (BE) scale was calibrated with respect to the Cu2p,,, (932.7 eV, full-width at half-maximum (FWHM) 1.75 eV> and Au4f,,, (84.0eV, FWHM 1.20 eV> signals using spectroscopically pure metals (Johnson Matthey). Wide and detailed spectra were recorded with a

0.0

POTENTIAL/V

vs. SCE

Fig. I. (A) CVs for 0.25mM NiCI, and 0.25 mM K,Fe(CN), in 0.5 M KC1 at a glassy carbon electrode (area 12.6mm’). The potential was scanned 25 times at SOmVs-’ between -0.10 and + IOV. (B) Voltanmetric profiles of the NiHCF fiIm electrode in the blank supporting electrolyte.

T.R.I. Cataldi

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I

I

POTENTIAL/V

vs. SCE

Fig. 2. (A) Initial voltammetric behaviour corresponding to the first nine cycles of an NiHCF film deposited on glassy carbon, recorded continuously in 0.10M potassium hydroxide solution at IOOmVs~ ’ between 0.0 and +0.9V vs. SCE. (B) Subsequent potential cycling of the same film. (C) Steady state obtained after about 250 scans in the potential range from 0.0 to +0.7V. The current scale S is equal to 200t.cA for (A) and (Cl, and 400 )LA for (B).

film, since within the investigated range of potentials only the ferro/ferricyanide couple is electroactive. Several authors have invoked the involvement of non-electroactive counterions in the redox behaviour of NiHCF films [3,5,8lo]. Indeed, these changes in the oxidation state are accompanied by incorporation or release of hydrated alkali ions into and from the inorganic film to maintain electrical charge neutrality. As has been pointed out previously [9,10], when the NiHCF film is grown in K+-containing supporting electrolytes, the major part of the charge transport is accomplished by the Kf ion, and the presenceof two redox transitions seems to be associated with the existence of two forms of ferrocyanide more or lesspaired with potassiumions. 3.2. NiHCF films in alkaline solutions During potential cycling in 0.10 M KOH, the NiHCF film deposited on a glassy carbon substrate undergoes profound chemical and structural alterations [l 11. In Fig. 2(A) the initial voltammetric profiles with the appearance of two redox couples are illustrated; these gradually increaseas the electrode potential is repeatedly cycled. Peaks IA at + 0.49 V and IC at + 0.43 V most likely correspond to surface-confined nickel(I1) hydroxide/oxyhydroxide centres, that is Ni(OH),/NiOOH, whereasthe anodic (IIA) and cathodic (IIC) peaks, observed at + 0.64 and + 0.59 V respectively (Fig. 2(B)), may be ascribed to the redox

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system of hexacyanoferrate(II/III). As the potential cycling continues, the intensity of this last redox couple progressively decreasesto a negligible value, which probably leads to rupture of the Fe-CN-Ni bonds and releaseof iron in solution, suggestingsignificant structural changes taking place in the polynuclear inorganic lattice. In contrast, peaksIA and IC exhibit a marked rise until a steady state voltammetric profile is reached. This is particularly apparent upon prolonged cycling of the same modified electrode, as illustrated in Fig. 2(C). That the voltammetric pattern IA/IC observedis associatedwith an Ni(OH),(hyd) film will be corroborated by the XPS analysis describedin the next section. One of the most important characteristics of this modified electrode is its impressiveelectrochemical stability; the redox process was sustained for at least 4h during which time the potential was continuously cycled at 50 mV s- ’ between 0.0 and + 0.7 V. The meaning of this behaviour is that Ni(I1) species remain favourably adsorbed on the glassy carbon surface. The major modification observed after a few hours of continuous potential cycling is a modest onset of background current at more positive potentials than the anodic peak IA. The electrochemically induced structural and chemical modifications in basic media were found to be irreversible, as judged by the results obtained when cycling the modified electrode in 0.5 M KCl, where a featureless voltammetric profile was recorded. The observation that the redox couple IIA/IIC completely disappears leads to the conclusion that no presenceof iron should be found on the electrode surface. From an electrochemical point of view, the surface modified glassy carbon obtained upon potential cycling in alkaline electrolytes of the NiHCF film behaves like that observed at polycrystalline nickel electrodes [ 14- 171, nickel hydroxide films obtained by anodic electrodeposition [26], nickel oxide-based glassy carbon electrodes electropolymerized nickel complexes [18,201, [19,21,27,28], and NiO dispersedin a composite matrix of graphite and polyvinylchloride [22]. Generally, all these Ni(I1) oxide/hydroxide films exhibit good electrocatalytic capabilities, allowing the oxidation of several hydroxylcontaining compounds[ 11,14-24,27,28]. It shouldbe noted that the chemical modification under consideration is performed by using electrochemical techniques, therefore the films formed with the methodology described here are subject to a high degree of reproducibility. 3.3. XPS investigations To understandthe effects of electrochemical conditioning in basic media on the chemistry and composition of NiHCF films, XPS was employed as a surface technique. Although the electrochemical behaviour of NiHCF films has been well studied and several reports exist, no photoelectron spectroscopicinvestigation has been reported. The Ni2p spectrum of the NiHFC film grown on a glassy

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417 (19961 83-88 I

Fe2p h/2

2Pl/2

a90

I

I

am

a70

BINDING

,

I

I

I

a50

660

J

-

-d

ENERGY 1 eV

Fig. 3. X-ray photoelectron spectra of the Ni2p,,, region: (a) NiHCF film of a glassy carbon substrate; (b) Ni(OH),(hyd) deposit obtained upon electrochemical cycling of the NiHCF film. The NiHCF was deposited and treated as described in Fig. 1.

carbon electrode is shown in Fig. 3(a). An intensestructure at high binding energy and adjacent to both peaks, 2p,,, and 2p,,,, is present, which is ascribedto a multi-electron excitation and is referred to as a shake-upsatellite [29]. In Fig. 3(b) the Ni2p spectrumof the Ni(OH),(hyd) on glassy carbon obtained after 2 h of continuous potential cycling in O.lOM KOH is also illustrated. XPS examination of the Ni2p,,, peak (not shown) reveals the presence of two components,which on the basis of their BEs at 855.8 and 857.3eV were assignedto Ni(I1) and Ni(II1) respectively [30]. The binding energies of the Ni2p,,, level and the spin-orbit splitting value are collected in Table 1. Since the overall area exposed to XPS analysis was not fully covered by the inorganic film, the C 1s peak coming from the glassy carbon substratewas taken as an internal reference and its BE value set at 284.6eV. The spin-orbit splitting of the Ni2p level in the NiHFC film is 17.6eV, that is the same value obtained for the Ni(OH),(hyd). These results are virtually identical to those recently reported in the case of an Ni(II)-tetraazaannulene film de-

730

720

710

BINDING

700

ENERGY/e”

Fig. 4. XP spectra of the Fe2p region. The samples are: (a) K,Fe(CN), pellet; (b) NiHCF film deposited on glassy carbon, (c) same as (b) but after potential cycling in 0.1 M KOH; (d) obtained after prolonged cycling in KOH. In this last sample the presence of iron is barely discernible. Mg Ku was used as an X-ray source.

posited on glassy carbon upon cycling in alkaline solutions [21]. Hence, as mentioned, a hydrated film of nickel(U) oxide/hydroxide is thought to composethe glassy carbon surface. Evidence of significant structural and chemical composition changesis given in Fig. 4, where the high resolution XP spectra of the Fe2p are compared. Indeed, the following data provide a very simple way to elucidate the modifications occurring. Spectra were recorded with Mg Kol X-rays in order to avoid the overlap of the X-ray induced Auger signal nickel LMM. The striking similarity between the Fe2p spectrum of NiHCF (b) and the spectrum of K,Fe(CN& (a> with narrow and symmetric peaks positively identifies the character of Fe(I1) in the inorganic film (see Table 1). Moreover, both spectra exhibit binding

Table 1 XPS data Sample

Binding

energy

“/eV

Ni%,,

K,Fe(CN), powder ’ NiHCF/GC as grown * Ni(OH),/NiOOH on GC e

+ 0.2 ABE(2p,,,

Ni”

Ni”’

857.5 855.8

857.3

f

17.6 17.6

- 2~,,,)

FeGw2

Ni:Fe

K~P,,,

K2s

Nls

N:K

709.8 709.6 -

1

294.4 294.1

379.2 379.2

399.1 398.8 398.8 to 399.7

4

b

1 -

a The BEs were calibrated with the graphic Cls peak taken at 284.6eV. b Molar ratio estimated with the Nl s and K2s levels. ’ Powder pellet in which the calibration was done with the Cls peak of cyan0 groups taken at 286.4eV. d NiHCF film grown on a glassy carbon sample by continuous potential scanning (25 cycles) in 0.5 M KC1 containing 0.25 mM NiCl, and 0.25 mM K,Fe(CN), at 50mVs-’ between -0.1 and +0.9V vs. SCE. ’ The same film as (c) electrochemically cycled in alkaline solution, 0.1 M KOH, and washed accurately with pure water before XPS analysis. Each BE value was averaged from at least three replicate samples. f The main peak is accompanied by several satellites.

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energies of the Fe2p,,Z and 2p,,, that correspond closely to those reported by Wertheim and Rosencwaig for the same iron complex [31]. On the basis of the relevant signal areas, the atomic ratio of Ni:Fe after cross-section correction was found equal to 1, in good agreement with that reported by Kulesza et al. [32], and that obtained by electrochemical methods. It is worthwhile mentioning that, despite the consistent atomic ratio of 41 between K and Fe for the K ,Fe(CN),, as calculated from the integrated signals, the N:Fe elemental ratio is only 4:l rather than 6: 1, as expected from the stoichiometric formula. We recently reported an analogous result with K,Ru(CN&, that is 4:1:4 for the atomic ratios of K:Ru:N [33]. However, what is probably more important is that, after electrochemical treatment of the NiHCF in an alkaline solution, the presence of iron in the surface-confined deposit is drastically decreased (Fig. 4, spectrum c>. It is rather remarkable that after 2 h of potential cycling the Fe2p peaks were hardly discernible from the background signal (spectrum d). These results strongly support the hypothesis that cleavage of Fe-CN-Ni bonds occurs during potential cycling in basic media, followed most likely by dissolution of iron species as hydroxide species. Further information on the film conversion was inferred from the Nl s and K2s signals. Typical Nls and K2s spectra for the K,Fe(CN), pressed powder (a), NiHCF film grown in a supporting electrolyte containing K+ (b), and the relevant nickel(B)-based films in the oxide/hydroxide form (c and d) are shown in Fig. 5. The high resolution spectra of the Nl s and K2s were chosen because of their presence in the ferrocyanide complex and NiHCF film, and because they allow an immediate correlation of changes in the film composition. The spectrum of NiHCF positively confirms the incorporation of potassium in the

Chemistry

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film, and on the basis of the relevant signal areas the atomic ratios N:K and N:Fe, equal to 4 in both cases,were estimated. Note that all sampleswere washed thoroughly with pure water before executing the spectroscopic analysis. A comparison with the spectra of Ni(OH),(hyd) films, e.g. spectrum c and d, provides a precisely conclusive indication of the extent to which NiHCF is completely converted in hydrated nickel specieson the glassy carbon surface. Remarkably, the last spectrum reflects the complete absenceof potassium,while the Nls peak, although greatly decreasedin intensity, is still clearly distinguishable. At least two components of the Nls signal were identified (not shown), with BEs at 398.8 and 399.7eV, which may be assignedto residual nitrogen of the NiHCF film and that of surface functional groups of the glassy carbon substraterespectively. The observations taken together lead to the conclusion that the NiHCF films undergo profound and chemically irreversible alterations in alkaline solutions. Even though slight alterations were observed in the Ni2p signals, the iron present in the film suffered the most changes, with final dissolution in the supporting electrolyte. Interestingly, these results have demonstrated the ability to follow the changesin chemical composition by XPS methodology.

Acknowledgements We gratefully acknowledge the dedicated assistanceof Mr. A. Galassoin taking the XP spectra. We are grateful for the financial support of the Italian National Research Council (CNR, Rome) and the Minister0 dell’Universit’a e della Ricerca Scientifica e Tecnologica, Rome.

References : :.

[l]

K2s

:: Bii~rL

[2] [3] [4]

a :: .:.. i: i i

[5]

--b [6] [7] [8]

-d [9] 420

410

400

BINDING

390

380

370

360

ENERGY / eV

Fig. 5. XP spectra of the Nls and K2s levels reported in Fig. 4, using an Al Ka X-ray source.

[IO] [ 1 l]

of the same samples [ 121

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