Elsevier

Surface Science

Volume 512, Issues 1–2, 20 June 2002, Pages 37-47
Surface Science

Electrochemical and in situ STM studies of anomalous phosphate adsorption induced on Zn UPD at Au(111) in the presence of halide ions in aqueous phosphate solutions

https://doi.org/10.1016/S0039-6028(02)01558-3Get rights and content

Abstract

Underpotential deposition (UPD) of Zn2+ at Au(1 1 1) was studied in the phosphate solutions with and without halide ions (Cl, Br, and I) by cyclic voltammetry and in situ STM for the evaluation of the anion adsorption induced on the Zn UPD. The order of the strength of anion adsorption induced on Zn UPD was found to be different from the order of specific adsorption strength of I>Br>Cl>PO43−⩾SO42− at an Au substrate electrode. This anomalous tendency of anion adsorption induced on UPD Zn at Au seems to be identical with the cases of sulfate and phosphate adsorption induced on UPD Zn at a Pt electrode in the presence of chloride, being confirmed by radiotracer method. Such anomalous phosphate adsorption induced on Zn UPD was found to take place at Au(1 1 1) not only in the presence of chloride or bromide, but also even in the presence of iodide in the phosphate solution. The in situ STM images of Zn UPD were observed at Au(1 1 1) in the phosphate solution with iodide ions. At −200 mV (saturated calomel electrode; SCE) where the iodide adsorbs at substrate Au(1 1 1), the STM image was c(p×3R30°), and in the Zn UPD region, the STM image became (3×3)R30°. The latter image is discussed to be assigned to the image in the absence of iodide, i.e., that the adsorbed anions induced at Zn UPD are phosphate species even in the solution containing iodide ions.

Introduction

The underpotential deposition (UPD) is a surface monolayer formation of metal (M) from Mn+ on a substrate metal (M′) at potentials more positive to the equilibrium potential of Mn+/M. As a fundamental electrochemical process, various works have been carried out, as firstly recognized in 1912, which were reviewed by Kolb in 1978 [1] and references therein. After the preparation method of a single crystal electrode was established for Au by Hamelin [2], and for Pt by Clavilier [3], the study on the metal UPD at a single crystal electrode was advanced as a monolayer structure science. In situ ordered STM images of Cu UPD on Au(1 0 0) and Au(1 1 1) were first observed in sulfate solution [4] and later in Ref. [5]. The STM images showed simple (3×3)R30° structure of which coverage is equal to 1/3 with respect to a substrate Au. The sulfate adsorption induced on UPD Cu at Au had been confirmed by radiotracer method of Horányi and Rizmayer [6] and with AES in UHV by Kolb and co-workers [7]. Tadjeddine et al. reported that Cu interatomic distance was identical to that of Au at Au(1 1 1) surface in sulfate solution by EXAFS, i.e., this result proposed that the Cu UPD structure is not simple (3×3)R30°, because of the difference of the interatomic distance [8]. Kolb and co-workers concluded for in situ STM through the assistance of previous reports that the UPD induces the adsorption of sulfate, and that Cu UPD phase consists of bi-layer of UPD copper and co-adsorbed chloride [9]. Later, electrochemical QCM study showed that the Cu and sulfate coverages with respect to Au atoms of quasi-Au(1 1 1) surface were 2/3 and 1/3, respectively [10], and Itaya and co-workers suggested that the in situ STM image of (3×3)R30° was assigned to be due to the image of the adsorbed sulfate induced on UPD Cu on Pt(1 1 1) and the structure of Cu adlayer was honeycomb (3×3)R30° of which coverage is equal to 2/3 [11].

With respect to the degree of the potential shift of UPD potential of M on substrate M′ to equilibrium potential of Mn+/M, ΔEUPD, various considerations were given and came to the relation of ΔEUPD against the difference of work functions of M and M′, Δφ as eΔEUPD≅(1/2)Δφ, where e is the charge of free electron [1]. Later, Trasatti gave eΔEUPDΔφ from the above data of Ref. [1], since the Born–Haber cycle treatment gives eΔEUPD=Δφ as a first approximation [12], where no effect of the anion induced on the UPD metal is taken into account, as the anion adsorption on UPD metal was not known at that time. Zinc ion UPD (Zn UPD) is quite interesting, since the UPD shift potentials ΔEUPDZn are nearly identical to the difference of work function of Zn and Pt (or Au) substrate in a potential scale; 1.2 and 0.7 V for Pt(1 1 1) and Au(1 1 1) at ≈pH 4 in the phosphate solution, i.e., eΔEUPDZnΔφ was observed on Zn UPD.

Much works of metal UPD on metal electrodes have been carried out in the presence of various anions, for examples, by means of cyclic voltammetry (CV) [13], [14], radiotracer method [6], [15], [16], X-ray diffraction [17], [18], thermodynamic analysis [19], and STM [4], [9], [20], [21]. Those works suggest that the anions always co-adsorb on UPD metal at substrate metal electrodes [21]. Particularly, the Cu UPD has been intensively investigated in the presence of halide ions in the solution. The co-adsorption of Cl on Cu UPD at an Au electrode was studied with a radiotracer method by Horányi and Rizmayer [6], and Lipkowski and co-workers discussed Cl adsorption at the Au(1 1 1) electrode in the presence of Cu UPD by chronoamperometry [22]. Behm and co-workers suggested that the STM image is taken to be due to Cl on UPD Cu by means of in situ STM [20]. Abruña and co-workers proved Br adsorption on Cu UPD by X-ray diffraction [17], [18], showing that the anion adsorption is induced on UPD Cu at Au(1 1 1) in a bi-layer form. It seems likely that the order of induced anion adsorption strength on Cu UPD is identical to the adsorption strength order on substrate Au as I>Br>Cl>SO42−.

In the case of Zn UPD, we have reported the case at single crystal and polycrystalline electrodes of Au and Pt in acidic solutions in correlation with the induced anion adsorption [13], [14], [15], [16], [19]. We have found among them that the induced anion adsorption on Zn UPD at Pt does not obey a specific adsorption strength order on the substrate Pt; in the presence of Cl in the solution, adsorbed sulfate or phosphate was observed to be induced on Zn UPD at Pt by radiotracer method [15], [16]. The Zn UPD at Pt(1 1 1) in the presence of halide in the phosphate solution shows the decrease of the degree of the Zn UPD shift potential [14]. This also reveals a great importance of the role of anion for the formation of the UPD at the substrate metal. With assistance of much knowledge on specific adsorption of anion at Au(1 1 1), we report the induced anion adsorption on the Zn UPD at Au(1 1 1) by means of CV and in situ STM.

Section snippets

Experimental

All electrochemical experiments were performed on Au(1 1 1) electrodes prepared by the Clavilier method [2], [3]. Three-compartment electrolytic cell was used. Reference electrode was a saturated calomel electrode (SCE) in the reference electrode compartment of the cell. The counter electrode was a Pt mesh in the counter electrode compartment. Water was purified in millipore water purification apparatus. Base solutions were 0.1 M KH2PO4 of pH=4.2–4.6, and 0.1 M KClO4+0.1 mM HClO4 of pH=4.0–4.2.

The effect of halide ions on the Zn UPD

Fig. 1 shows the cyclic voltammograms (CV) observed at the potential sweep rate of 5 mV s−1 on Au(1 1 1) in a 0.1 M KH2PO4 solution with and without 1 mM Zn2+ where the ordinate is expressed by capacitance (current density/potential sweep rate). Fig. 1(a) shows phosphate adsorption peaks at potentials E>200 mV, as discussed in Ref. [13]. By the addition of 1 mM Zn2+ ions in the solution of Fig. 1(a), CV of Fig. 1(b) was observed. The CV feature at E>200 mV remained nearly identical, but cathodic

Conclusion

The order of adsorption strength at substrate Au electrode was confirmed to be I>Br>Cl>PO43−⩾SO42−>ClO4. However, the order of the anion adsorption strength induced on UPD Zn at Au(1 1 1) was found to be different from the above order. In the phosphate solution, chloride and bromide ions gave no influence to the Zn UPD, and an iodide ion inhibited the Zn UPD formation. We found that in all the cases, co-adsorbed anions induced on Zn UPD at Au(1 1 1) are phosphate anions in the present report,

Acknowledgements

The financial support (HIT. MD. 2000. 22) of Harbin Institute of Technology, China is acknowledged.

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