Polycrystalline gold electrode redox behavior in an ammoniacal electrolyte

Journal of Electroanalytical Chemistry 461 (1999) 131 – 142

Polycrystalline gold electrode redox behavior in an ammoniacal electrolyte Part I. A parallel RRDE, EQCM, XPS and TOF-SIMS study of supporting electrolyte phenomena1 Xiangqun Zeng, Stanley Bruckenstein * Department of Chemistry, State Uni6ersity of New York, Buffalo, NY 14260 -3000, USA Received 21 October 1997; received in revised form 12 January 1998; accepted 13 January 1998

Abstract The redox behavior of a gold electrode in an ammoniacal electrolyte (0.1 M NH3 and 0.1 M NaClO4) was investigated using four techniques. These were the rotating ring disk electrode (RRDE), the electrochemical quartz crystal microbalance (EQCM), X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS). Two different gold surface compounds, AuO and AuNH2, are formed in relative amounts that depend on the holding time at + 0.6 V (versus SCE). XPS and SIMS studies were used to distinguish the surface species AuO and AuNH2 from AuO and AuOHNH2. When the potential is held at + 0.6 V for longer times, other oxidized gold surface species may form, e.g. AuO, AuOHNH2, [OH(NH2)Au]2NH, AuNH2, Au2O3 · 2NH3 and Au2O3 · 3NH3. During the gold electrode oxidation process, NH3 is probably oxidized to N2. During the surface gold species reduction process, some soluble Au(III) species form that can be reduced to an adsorbed Au(I) species at − 0.4 V. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Polycrystalline gold electrode; Ammoniacal electrolyte; Electrolyte phenomena

1. Introduction Underpotential deposition of many metals at more negative potential cannot be studied in acidic media because of hydrogen evolution. The electrochemistry of metals with multiple oxidation states stabilized by complex formation in alkaline media, e.g. with NH3, is of especial interest to us. Such studies require detailed knowledge about the ‘inert’ electrode at which the redox processes occur. This study was motivated by complications we encountered during an investigation of copper underpotential deposition (upd) in a 0.1 M NH3 +0.1 M NaClO4 medium [1]. The latter study, * Corresponding author. Fax.: + 1 716 6456963; e-mail: [email protected] 1 Dedicated to Professor W. Vielstich on the occasion of his 75th birthday.

Part II, was undertaken as a model system to develop strategies to study some deposition baths for copper (and other metals) in complexing alkaline media. A major complication in copper upd studies with this electrolyte arises because the copper upd processes overlaps the reduction processes of the oxidized gold surface. Consequently, it becomes necessary to understand these gold surface reduction processes. The oxidation and reduction of gold electrodes in acidic media has been investigated by many authors [2]. However, few fundamental studies of gold electrode redox behavior in alkaline media exist. Such studies in alkaline media, e.g. containing NH3, are much more demanding than those in acid media. In this work, the redox behavior of a gold electrode in 0.1 M NH3 +0.1 M NaClO4 was studied using four techniques. These were the rotating ring-disk electrode (RRDE), electro-

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Table 1 Experimental conditions for XPS and TOF-SIMS studies Electrochemical treatment before each surface experimenta

XPS Sample 1 Sample 2 Sample 3 TOF-SIMS Sample 4 Sample 5 Sample 6 Sample 7

a

Electrochemistry

Emersion potential

Cycled between +0.6 and −0.8 V for 10 min, held at −0.4 V for 2 min and then jumped to +0.6 V for 5 min in pH 10.6 NaOH+NaHCO3 buffer. Sample 1 electrode after XPS experiment, immediately immersed in 0.1 M NH3+0.1 M NaClO4 for 10 min. Cycled between +0.6 and −0.8 V for 10 min, held at −0.4 V for 2 min and then jumped to +0.6 V in 0.1 M NH3+0.1 M NaClO4 for 5 min.

+0.6 V

Held at −0.7 V in pH 10.6 NaOH+NaHCO3 buffer for 2 min, then stepped to −0.1 V for 5 min. Sample 4 electrode after TOF-SIMS experiment, immediately immersed in pH 10.6 NaOH+NaHCO3 buffer, step potential from 0.0 to +0.6 V and held at +0.6 V for 10 min. Sample 5 electrode after TOF-SIMS experiment, immediately immersed in 0.1 M NH3+0.1 M NaClO4 electrolyte for 10 min. Sample 6 electrode after TOF-SIMS experiment, immediately immersed in 0.1 M NH3+0.1 M NaClO4 electrolyte, step potential from 0.0 to +0.6 V and held at +0.6 V for 10 min.

Open circuited +0.6 V

−0.1 V +0.6 V Open circuited +0.6 V

See Section 2.3 for a description of how the sample was transferred from the electrochemical cell to the XPS and TOF-SIMS instruments.

chemical quartz crystal microbalance (EQCM), X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (TOF-SIMS) techniques. Complementary information obtained from these techniques successfully clarified the gold electrode redox mechanisms.

for easy connection to a gold wire. The area of the flag was 0.12 cm2 and the roughness factor of this electrode was  1.63. Reference electrode: An SCE was used and all electrode potentials are reported versus the SCE.

2.3. XPS and TOF-SIMS experimental protocols 2. Experimental

2.1. Reagents All solutions were prepared using Milli-Q deionized water. Reagent grade NH3, NaClO4, NaOH and NaHCO3 were used to prepare all solutions.

2.2. Electrodes EQCM electrode: One of the evaporated gold electrodes on the 10 MHz AT-cut quartz crystal served as a working electrode in the EQCM experiments. RRDE: The rotating gold disk-gold ring electrode had the following dimensions: disk electrode radius (R1) = 0.3010 cm, ring electrode inner radius (R2)= 0.3150 cm and ring electrode outer radius (R3)= 0.4055 cm. These geometric quantities yield the following theoretical value for the collection efficiency (Nc)=0.4037 and b 2/3 =1.190. The gold electrode roughness factor was 2.8 [3]. XPS and TOF-SIMS: A rectangular gold plate electrode (1.0× 1.2 cm) was used, with a small flag

XPS and TOF-SIMS techniques were used to identify the gold nitrogen compound formed by conditioning gold electrodes at + 0.6 V in pH =10.6 electrolyte. A carbonate buffer of pH= 10.6 was used as a blank. Electrode emersion experiments were carried out at + 0.6 V in both the pH=10.6 carbonate buffer and a pH= 10.6, 0.1 M NH3 +0.1 M NaClO4 electrolyte using the same gold electrode. After emersion under potential control, the electrode was rinsed with deionized water and dried with nitrogen. The film samples were then transferred from the laboratory where the electrochemical experiments were performed to the laboratory containing the XPS and TOF-SIMS instruments. The samples were in direct contact with the ambient atmosphere during this transfer which took about 5–10 min. Table 1 lists the experimental electrochemical conditions used for the XPS and TOF-SIMS experiments. Note that Samples 1, 2 and 3 used in the XPS experiments and Samples 5, 6 and 7 used in the TOF-SIMS experiments were, respectively, emersed at the same potentials, but had slightly different electrochemical preparations.

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3. Results and discussion

3.1. RRDE experiments in the 0.1 M NH3 +0.1 M NaClO4 electrolyte 3.1.1. Soluble Au species Fig. 1a gives the gold disk current-disk potential curve in the 0.1 M NH3 +0.1 M NaClO4 supporting electrolyte obtained at a rotation speed of 2500 rpm and a potential scan rate of 100 mV s − 1. Two current peaks occurred in the negative potential scan, peak A at − 0.04 V and peak B at −0.28 V. The height of these two peak currents did not change with rotation speed; however, they increased with increasing potential scan rates. This behavior is characteristic of the redox behavior of a surface immobilized species. First, the positive potential region that produces these two reduction peaks was identified. The gold electrode was then held at selected positive potentials for 1 min each, after which the electrode potential was scanned from the holding potential to −0.8 V. We found that the two current peaks appeared only if the holding potential was more positive than + 0.2 V. Holding the potential at +0.1, 0.0 and − 0.1 V respectively, for 5 min, did not produce these two peaks or a

Fig. 1. Au ring-Au disk electrode current-potential curves in the 0.1 M NH3 + 0.1 M NaClO4. Curve (a) disk current-disk potential curve; Curve (b) ring current-ring potential curve. v=2500 rpm, Ering = − 0.8 V, scan rate 100 mV s − 1. Solid curves are first scan, dotted curves are second scan. Disk electrode roughness factor = 2.8. Peak A occurs at − 0.04 V and peak B occurs at −0.28 V.

Fig. 2. Au ring current-time curve in the 0.1 M NH3 +0.1 M NaClO4 when jumping Au disk electrode potential from +0.6 to 0.0 V. The Au disk electrode was held at +0.6 V for 15 min before jumping. (a) Ering = +0.4 V; (b) Ering = −0.8 V.

detectable mass of surface bound species on the EQCM electrode. To determine whether the two cathodic current peaks were caused by the same species, the electrode potential was scanned from −0.8 V to positive potentials between + 0.15 and +0.52 V and then held there until the current became constant. Next, the potential was scanned back to − 0.8 V and the I–E curve was recorded. When the electrode was held at +0.15 V, the current dropped to zero and only the current peak at −0.04 V appeared on scanning to −0.8 V. When the electrode potential was held more positive than + 0.15 V, the current did not drop completely to zero and the second peak at −0.28 V appeared. The latter peak became

Fig. 3. Potential scan rate effect on cathodic disk peak currents in the 0.1 M NH3 +0.1 M NaClO4 electrolyte. Open circles, peak A in first scan; open triangles, peak B in first scan; solid circles, peak A in second scan; solid triangles , peak B in second scan.

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Fig. 4. EQCM experiments in the 0.1 M NH3 + 0.1 M NaClO4 electrolyte. (a) mass-potential curve, scan rate = 50 mV s − 1; (b) current-potential curves at different scan rates.

more prominent as the holding potential became more positive. These results prove that these two peaks are caused by two different surface species. Possibly, the − 0.04 V reduction peak produces the species reduced at − 0.28 V, e.g. Au(III) “ Au(I) “ Au(0). Alternatively, two different surface compounds form at + 0.6 V and are reduced at −0.04 and − 0.28 V. The more positive the holding potential, the larger the two cathodic peaks became.

Fig. 5. Mass-charge curves at different scan rates in the 0.1 M NH3 + 0.1 M NaClO4 electrolyte. Potential ranges for A– F are defined in Table 8.

3.1.2. RRDE detection of soluble species in the 0.1 M NH3 +0.1 M NaClO4 electrolyte RRDE electrode experiments were conducted with a rotating gold disk-gold ring electrode to learn if any soluble product could be detected either during the oxidation or the reduction of the gold disk electrode. Fig. 1b shows the RRDE ring current-disk potential curve obtained in the 0.1 M NH3 + 0.1M NaClO4 electrolyte while the disk electrode was scanned from + 0.6 to −0.8 V. A ring electrode collection current peak, A, shown in Fig. 1b, occurs concurrently with the reduction of the gold oxide at the disk electrode. Both this ring collection peak and the associated disk reduction peak are much bigger in the first disk potential scan than those in the second and subsequent potential scans. Therefore, holding the disk electrode at +0.6 V for 5 min not only produces a more oxidized disk electrode surface, it also generates much more soluble species than in subsequent potential scans. Experiments show that the height of the ring current peak is independent of the ring electrode potential when cycling Edisk between the potential limits of + 0.60 and −0.80 V; in these experiments, ring electrode potentials, Ering, between + 0.6 and − 0.8 V, in 0.1 V intervals were used. This result implies that only one soluble, electroreducible species is produced during the disk surface reduction process. When more positive Ering’s were used, odd ring current-disk potential curves occurred, probably because the ring electrode itself was undergoing oxidation. It seems likely that the species causing the ring current peak is a soluble gold species produced during reduction of the oxidized gold surface [4]. This hypothesis was tested by jumping Edisk from + 0.6 to 0.0 V to reduce the gold oxide formed at +0.6 V. Here, we take advantage of the fact that Au(III) and Au(I) can be reduced to Au(0) at a gold ring electrode. Two experiments were carried out: in one, the ring potential was held at Ering = − 0.8 V and in the other it was held at Ering = + 0.4 V. At Ering = − 0.8 V, the ring electrode can detect both Au(III) and Au(I) by reducing either species to gold. At Ering = + 0.4 V, the ring electrode detects only Au(I) by oxidizing it to Au(III). Fig. 2 gives the experimental ring current-time curves for these two experiments. When Ering = + 0.4 V, there is no ring collection current, while there is a ring collection peak current when Ering = − 0.8 V. An analogous experiment was carried out by jumping the disk potential from +0.6 to −0.34 V. The observed ring current-time curves were the same as in the previous experiment. Also, the longer Edisk is held at + 0.6 V before the disk potential jumps from + 0.6 to 0.0 V, the larger is the observed ring collection current peak. Therefore, Au(III) escapes into the solution during reduction of the oxidized gold surface.

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Table 2 Potential range (V) for different cathodic processes at different scan rates Scan rate/mV s−1

AB

BC

CD

DE

50 100 200

+0.6 to −0.03 +0.6 to −0.02 +0.6 to −0.1

−0.03 to −0.38 −0.02 to −0.35 −0.1 to −0.35

−0.38 to −0.6 −0.35 to −0.8 −0.35 to −0.8

−0.6 to +0.06 −0.8 to +0.01 −0.8 to 0.0

When Edisk is held at +0.6 V for 15 min, the ring collection charge, Qring =6 mC and the corresponding disk charge, Qdisk = 15 mC. Therefore, the total Au(III) produced during this process at the disk is 10.2 ng cm − 2. The RRDE experiments in this section show that an oxidized gold surface is formed by holding the electrode potential at +0.6 V for 5 min. The two reduction current peaks seen on the negative scan could be due to the same surface species formed at two different surface gold crystal sites; alternatively the peaks could be caused by two different surface species. During the surface reduction process, some soluble Au(III) species are generated. EQCM experiments described below help to determine whether one or two surface species exist.

3.2. EQCM experiments 3.2.1. Different potential scan rates (i) Potential scan rate effect on peak current: The gold electrode of an EQCM was held at +0.6 V for 5 min and cycled to −0.80 V and back twice at three different potential scan rates. As found from Fig. 3, the second peak current is proportional to the potential scan rate, as is characteristic of faradaic surface processes. The first peak current increases less than linearly with the potential scan. One reason for the deviation from linearity is that soluble Au(III) species are gener-

ated primarily in the potential region of the first peak. Less deviation from linearity was seen in later scans. This result was undoubtedly caused by holding the electrode potential at + 0.6 V for 5 min before the first potential scan. At this potential, more of the precursor species that produce soluble Au(III) form at longer holding times, so that on subsequent potential scans smaller amounts of this precursor exist and less Au(III) forms. (ii) Potential scan rate effect on equivalent masses, mequiv = FDm/Q: Fig. 4 gives the Dm –E curves at a scan rate 50 mV s − 1 and I–E curves at different scan rates for the gold electrode redox behavior in the 0.1 M NH3 + 0.1 M NaClO4 electrolyte. The mass decreases during the reduction of gold surface compounds and then starts to increase at more negative potentials. When the potential scan is reversed, the mass decreases between − 0.4 and − 0.8 V where no obvious redox processes can be seen in the I–E curves. The processes occurring between − 0.4 and − 0.8 V are more obvious in the Dm –Q curves given in Fig. 5 for different potential scan rates. The slope, mequiv = FDm/Q, yields the equivalent mass of the species arriving or departing from the electrode surface and can be obtained as a function of the electrode potential; linear Dm –Q regions are observed. Slopes AB and BC (Table 2) correspond to potential regions where the two cathodic reduction peaks occur for the reduction of surface gold compounds. Slopes CD and DD% caused by the reduc-

Table 3 Equivalent masses by cyclic voltammetry when the the potential was scanned between +0.6 and −0.8 V First scan/mV s−1

Slope Negative scan

Positive scan

AB

BC

CD

DE

EF

50 100 200

−8.8 −7.3 −7.0

−22 −13.9 −12.9

+103 +115 +126

— — —

+4.2 +2.9 +2.6

Second scan/mV s−1

FG

GH

HI

IJ

JK

50 100 200

−8.4 −5.1 −3.9

−18.0 −11.2 −5.8

+133 +133 +122

— — —

+3.3 +2.7 +2.7

The scan rate is in (mequiv/g equiv−1).

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Table 4 Equivalent masses for linear scan potential programing, scan rate =50 mV s−1 mequiv(slope)/g equiv−1 Potential scan range/V

0.6“ 0.0

0.6 “−0.2

Cathodic processes

AB

AB

BC

AB

BC

CD

DE

−10.9 −8.01 −9.5

−10 −8.0 −7.4

−28 −14 −13

−8.8 −8.5

−22 −18

+103 +133

— —

EF

EF

Negative scan

First scan Second scan Third scan

Anodic processes Positive scan

First scan Second scan Third scan

+1.2 +1.7 +1.8

0.6 “−0.8

EF

+2.5 +2.7 +2.8

tion of soluble Au(III) species generated at more positive potentials during the reduction of gold surface compounds. These Au(III) species diffuse back from the solution to the gold electrode surface and are reduced to an adsorbed Au(I) species. This adsorbed species is (electrostatically) desorbed when the scan direction is reversed (slope: D%E). Slope EF corresponds to the oxidation of the gold electrode. As seen in Fig. 5, the anodic charge exceeds the cathodic charge; the extra anodic charge is probably associated with the oxidation of NH3. Table 3 shows that the slopes (mequiv/g equiv − 1), decrease with increasing the scan rates in nearly all potential regions.

3.2.2. Potential programs Two different kinds of experiments were used to study the two cathodic processes occurring in regions AB and BC. The electrode potential was either scanned or jumped from the conditioning potential of +0.6 V to different more negative potentials. The Q – Dm (Escanning) and Dm – t (E-jump) curves were recorded. Table 4 gives the results for the E-scan experiments and Table 5 gives those for the E-jump experiments. When the electrode potential was cycled between + 0.6 and 0.0 V and back, the average value of the mequiv (Table 4 for the negative scan, slope AB) is 9.5 and for the positive scan (slope EF) is 1.6. If part of the BC process also occurred, its contribution to the mequiv is not detectable.

+4.2 +3.3 —

When the electrode potential was scanned to a more negative potential, − 0.2 V, a second slope associated with BC is seen. The value of mequiv = 28/g equiv − 1 for the first scan and deceases to half this value on subsequent scans. Finally, on scanning the potential to −0.8 V, processes CD (average mequiv = 122/g equiv − 1) and DE (mequiv = variable) occur. Process CD is consistent with the reduction of the soluble Au(III) species to Au(I) (mequiv = 98.5/g equiv − 1). The reduction product desorbs during the reverse potential scan. The value of mequiv for region DE could not be determined because the charge in this region is too small to measure accurately. Short periods after a potential jump to 0.0 V (Table 5), the early mequiv values are close to the one obtained from the corresponding first potential scanning experiment (Table 4). The magnitude of the mequiv increases with time after a potential jump because the reduction of the oxidized gold surface formed at + 0.6 V is not instantaneous, probably because the oxidized ‘surface’ consists of more than one layer. Also, while holding the potential at 0.0 V, small amounts of soluble Au(III) are produced during the reduction of the surface and the underlying oxidized layer(s). This is reflected as an apparent gradual increase of equivalent mass with time. The observed mequiv differences seen for the potential jump and scanning potential experiments are the result of their different times scales.

Table 5 Equivalent masses from potential jumps mequiv(slope)/g equiv−1 for specified time after potential jump (s)

Potential jump/V From

To

1

2

3

50

(constant mass)

+0.6 +0.6 0.0 −0.4

0.0 −0.4 +0.6 +0.6

−8.2 −8.0 +4.4 +3.9

−10.6 −8.5 +4.7 +3.5

−11.8 −8.7 +5.0 +3.5

−12.5 −8.6 +4.5 +3.8

−15.7 −8.5 +5.5 +3.5

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Table 6 Effect of NH3 on gold electrode redox behavior in pH =10.6 buffers Held at +0.6 V for 5 min, then scanned to −0.8 V and back.

NaHCO3+NaOH+0.1 M NH3 buffer

NaHCO3+NaOH buffer

Cathodic peak

Cathodic peak

Anodic peak

First peak

Second peak

1 min Ep/V Ip/mA

+0.03 7.8

−0.24 1.6

25 min Ep/V Ip/mA

−0.02 9.0

−0.27 2.1

Jumping the electrode potential from + 0.6 to − 0.4 V rapidly produces an equivalent mass-value, mequiv = 8.5/g equiv − 1, that does not change with time. At this potential, the reduction of the oxidized gold surface formed at + 0.6 V proceeds rapidly to Au(0) and Au(III) adsorbate, without Au(III) leaving the electrode. Also, within the experimental error, the equivalent mass at very long periods is characteristic of the reduction of O2 − to OH − , or NH2− to ammonia (Eqs. (1) – (3)) in Table 8. The small calculated values for the equivalent masses found on jumping the potential back to +0.6 are caused by the additional amount of charge associated with the oxidation of ammonia to nitrogen at + 0.6 V.

3.2.3. NH3 adsorption from NaClO4 solution The working gold electrode on the EQCM was held at +0.6 V in a 0.1 M NaClO4 solution while NH3 was injected to produce an NH3 concentration of 0.1 M. The I–E and Dm –E curves were recorded before and after NH3 injection. During NH3 injection, both the oxidation current and the mass increased. However, injecting NH3 while holding the potential at 0.0 V did not cause any mass change and no current was observed. These two experiments show that an electrode

Anodic peak

First peak

Second peak

+0.6 5.63

+0.15 7.3

−0.04 1.26

+0.6 3.3

+0.6 19.3

−0.03 10.1

−0.04 1.81

+0.6 3.3

reaction between Au and NH3 or adsorption of NH3 takes place at E= +0.6 V, but not at 0.0 V. We ascribe the origination of the oxidation current at +0.6 V to three possible sources: (a) oxidization of NH3 (to N2); (b) because of the high pH of the solution, the oxidation of the gold surface and immediate sublayers forms an oxygen; and/or (c) a nitrogen compound. The total mass increase would be the sum of surface and subsurface oxygen and nitrogen gold compound masses produced by (b) and (c). The electrode mass increases the longer the holding time at + 0.6 V, e.g. the mass increased by 46.21 ng cm − 2 ( 6 min holding time) to 130 ng cm − 2 ( 10 min holding time). We attribute the mass increase with longer holding time to thickening of the oxidized surface gold films, possibly to form compounds such as [OH(NH2)Au]2NH, AuNH2, AuOHNH2, Au2O3 · 2NH3 and Au2O3 · 3NH3.

3.2.4. Effect of NH3 6ersus the effect of pH The pH of the 0.1 M NH3 + 0.1 M NaClO4 solution is 10.6. It was possible that the electrochemical response of the gold electrode was only a pH effect, rather than being due to nitrogen compound formation. Consequently, experiments were performed in two NaHCO3 buffers of identical pH. One was free of NH3 but

Table 7 Charge and mass difference for the negative and positive scan Qcathodic/mC

Qanodic/mC

DQ(anodic-cathodic)/mC

Dm/ng cathodic-anodic (Exp.)

Exp. 1 First scan Second scan Third scan

539.4 377.9 377.9

811.0 818.9 818.9

271.6 441.0 441.0

31.5 10.5 5.71

Exp. 2 First scan Second scan Third scan

531.5 385.8 377.9

866.1 866.1 866.1

334.6 480.3 488.2

22.5 7.62 9.0

Scan potential range: +0.6 to −0.8 V; scan rate 50 mV s−1.

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Table 8 Mechanism for the cathodic and anodic process in 0.1 M NH3+0.1 M NaClO4 mequiv/g equiv−1

Region (Fig. 5)

Redox process

Potential range/V 6 =10 mV s−1

Reactions

AB

Gold oxide reduction

+0.60 to +0.03

AuO–(H2O)ads+2e−+H2O “

8.0

−

Au−(H2O)ads+2OH BC

Gold nitrogen compound reduction

+0.03 to −0.34

(1)

AuNH2–(H2O)ads+e−+H2O “ NH3+Au–(H2O)ads+OH−

16.5 (2)

or Au(OH)NH2–(H2O)ads+2e−+H2O “ −

NH3+Au–(H2O)ads+2OH CD and DD%

Soluble Au(III) species reduction and adsorption

−0.3 to −0.70

D%E EF

Desorption of Au+ adsorbed Oxidation of Au electrode and NH3

−0.54 to −0.10 −0.10 to +0.60

adjusted to pH= 10.6 with sodium hydroxide. The other had 0.1 M NH3 and was also adjusted to pH = 10.6. We used buffers of pH= 10.6 because this is the pH of the 0.1 M NH3 +0.1 M NaClO4 supporting electrolyte used in the experiments. In both buffers, the gold electrode potential was held at + 0.6 V, first for 1 min and then for 25 min; the potential was then scanned between + 0.6 and − 0.8 V at a scan rate of 50 mV s − 1. The current-potential curves were recorded. Table 6 lists the difference in the peak potentials and currents for these two holding times in both buffers. The major change was the larger anodic peak current at +0.6 V found in the ammoniacontaining buffer. This larger current is ascribed to the oxidation of ammonia to nitrogen. Another significant effect observed on comparing the ammonia-containing and ammonia-free buffers are the potentials of the peak currents. For example, the second cathodic peak current occurs +0.2 V more negatively in the ammoniacontaining buffer. This result is strong evidence for the formation of a nitrogen-containing species in the oxidized gold surface. The data in Table 6 also show that in both buffers, longer potential holding times at +0.6 V cause higher cathodic peak reduction currents. This result is ratio-

(3)

− Au(NH3)3+ 4 +2e “

Au(NH3)+ 2 adsorbed+2NH3 −0.70 to −0.80 to −0.54

17

133 (4)

or

99

Au3++2e− “Au adsorbed +

(5) — 3.2

Au–(H2O)ads+2OH− “ AuO–(H2O)ads+H2)+2e−

(6)

NH3+3OH− “1/2N2+3H2O+3e−

(7)

—

nalized by continued growth of subsurface oxidized layers. We propose that introducing NH3 to the buffer may form, at a holding potential of + 0.6 V, one or more of the following compounds: [OH(NH2)Au]2NH, AuNH2, AuOHNH2, Au2O3 · 2NH3, Au2O3 · 3NH3; their formation under our conditions is apparently slow. Similar compounds have been reported in the literature [5]. The data in Sections 3.2.5 and 3.4 (below) suggest that mainly AuNH2 forms when the potential is held at +0.6 V for less than 5 min. However, other species form if the potential is held at + 0.6 V for longer periods. Therefore, conditioning at + 0.6 V for 5 min was chosen prior to each subsequent experiment.

3.2.5. Electrolyte reduction and oxidation mechanism Based on the above studies, we propose the mechanism of Au electrode redox behavior in an ammoniacal electrolyte given in Table 8. As shown in Fig. 1a and explained in Table 8, holding a gold electrode at +0.6 V for 5 min forms two kinds of oxidized gold surface species. We ascribe the differences as being associated with two different polycrystalline gold surface sites. These are most probably (111) and (110) single crystal orientations as described by Adzic and Yeager [6] in

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Table 10 Concentration of Au, N, O, and C on the gold plate electrode Sample

Atomic concentration of element/%

1 2 3 Sensitivity factor

Au

N

O

C

23.1 20.5 17.6

1.38 5.39 3.24

12.0 15.8 17.3

63.5 58.4 61.8

4.95

0.42

0.66

0.20

adsorption process causes the observed increase of electrode mass. As the potential becomes more positive, the + adsorbed Au(I)adsorbed or Au(NH3)2adsorbed species desorbs from the electrode surface for electrostatic reasons. These small effects of Au(III) species were neglected in our mequiv calculations. Considering the experimental error, we believe this is justified. The processes given in Eqs. (2) and (3) are plausible and are consistent with the experimental data in Table 2. The oxidation process EF, Eq. (8) is the sum of Eqs. (6) and (7). Au−(H2O)ads + NH3 + 5OH − “ AuO− (H2O)ads + 1/2N2 + 4H2O+ 5e −

Fig. 6. XPS high resolution multiplex spectra in nitrogen 1 s binding energy region. Sample 1: oxidized gold at +0.6 V in pH =10.6 NaHCO3 + NaOH buffer. Sample 2: oxidized gold open circuited in the pH= 10.6, 0.1 M NH3 + 0.1 M NaClO4 electrolyte. Sample 3: oxidized gold at +0.6 V in the pH = 10.6, 0.1 M NH3 +0.1 M NaClO4 electrolyte.

Table 11 Interpretation of XPS experimental results Sample

Reactions

Surface compounds

1

Au–(H2O)ads+2OH− “

AuO

−

AuO–(H2O)ads+2e +H2O

their studies of evaporated polycrystalline gold electrodes. The gold electrodes of the EQCM are of this kind. During processes AB and BC, a small amount of soluble Au(III) species is generated, causing a small mass decrease given in Table 4. At process CD, Au(III) species diffuse back from the solution to the gold electrode surface and are reduced to either Au(I)adsorbed + or Au(NH3)2adsorbed . This simultaneous reduction and

2

NH3+OAu−(H2O)ads “

Au(OH)NH2

or NH3+OAu–(H2O)ads “

or AuNH

HNAu–(H2O)ads+H2O Au–(H2O)ads+2OH− “

AuO

−

AuO–(H2O)ads+2e +H2O

Sample

Binding energy/eV Au

1 2 3

83.8 83.6 84.0

87.6 87.3 87.6

NH3+Au–(H2O)ads+OH− “ AuNH2–(H2O)ads+e +H2O

O

399.0 400.0 400.3

532.0 532.4 532.5

(15)

and

−

N

(15)

Au(OH)NH2–(H2O)ads

3

Table 9 Binding energies of Au, O and N on the gold plate electrode

(8)

AuNH2 (16)

and NH3+OAu–(H2O)ads “ Au(OH)NH2–(H2O)ads

Au(OH)NH2 (14)

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X. Zeng, S. Bruckenstein / Journal of Electroanalytical Chemistry 461 (1999) 131–142

Au+NH3 “ Au− NH3(ads) Au–NH3(ads) + OH “ Au–NH2 + H2O+ e −

Fig. 7. TOF-SIMS positive survey spectra at m/z = 145–150: (a) Sample 4: reduced gold electrode in pH = 10.6 NaHCO3 + NaOH buffer; (b) Sample 5: oxidized gold electrode at + 0.6 V in pH =10.6 NaHCO3 +NaOH buffer; (c) Sample 6: oxidized electrode open circuited in pH = 10.6 0.1 M NH3 + 0.1 M NaClO4 electrolyte; (d) Sample 7: oxidized gold electrode at +0.6 V in pH= 10.6 0.1 M NH3 + 0.1 M NaClO4 electrolyte.

where mmolar = 16/g mol − 1, mequiv =(16/5)/g equiv − 1 = 3.2/g equiv − 1. An analogous mechanism was proposed by Oswin and Salomon. who reported the electrochemical oxidation of ammonia to nitrogen on a Pt-black electrode [7]. Another experimental fact supporting oxidation of ammonia to nitrogen is shown in Table 7 and Fig. 5. First, the anodic charge consumed for all the positive potential scans is the same at the same scan rates. Second, the anodic charge is always larger than the cathodic charge used in all of the potential scans. We attribute the additional charge in the positive scan to the kinetically limited oxidation of NH3 to a soluble product, i.e. nitrogen. If nitrogen oxides had been formed under our conditions, we would expect to have observed ring collection currents for these species. Using potential data for nitrogen couples [8], we calculate the E 0 for reaction (7) to be +0.486 V (versus SCE.) Taking into account the composition of our supporting electrolyte, the equilibrium potential for reaction (6) would be + 0.266 V versus SCE. Consequently, our assignment of reaction (8) to Process EF ( +0.55 V) is thermodynamically possible. This reaction has been confirmed by Wasmus et al. [9] who studied the electro-oxidation and -reduction of 0.05 M ammonia in 0.5 M NaOH at Pt-black electrodes using a variation of the original electrochemical mass spectrometric technique [10]. They showed that ammonia was oxidized to nitrogen at ca. +0.7 V and to nitrogen oxides at potentials higher than +0.8 V. We propose the following mechanism for the electrochemical oxidation of ammonia to nitrogen at a gold electrode.

(9) −

(10)

Au–NH2 + OH − “ Au= NH+ H2O+ e −

(11)

AuNH + OH − “ Au− N+H2O+ e −

(12)

2AuN “ 2Au+N2

(13)

Current-potential curves in Fig. 1a show a much bigger increase in the first peak current than in the second peak current during the first scan compared to the second scan. We interpret this as having been caused by the faster growth of the oxide film species— the precursor to AB (first peak region)—than by the oxidized film that is the precursor to BC (second peak region). As seen in Table 7, conditioning the gold electrode at +0.6 V for 5 min causes more cathodic charge to be used during the first scan. EQCM experiments show that process BC could be described by either Reaction (2) or (3). Reaction (2) would require that Au(NH2)OH, rather than AuNH2, forms on the gold surface at + 0.6 V via a reaction such as: AuO–(H2O)ads + NH3 “ Au(OH)NH2 –(H2O)ads (14) Resolving this question by EQCM and RRDE techniques is difficult. Consequently, XPS and TOF-SIMS experiments were used to study this issue further.

3.3. XPS Experiments The low resolution XPS survey spectra obtained for Samples 1–3 show that carbon, oxygen and gold are present in all samples. Carbon and oxygen signals have their origin mainly in atmosphere contamination caused during transfer from the electrochemical cell to the XPS instrument. Fig. 6 gives the XPS high resolution multiplex spectra in the nitrogen binding energy region for Samples 1–3. A nitrogen binding energy peak was found in Samples 2 and 3. However, there is obvious peak broadening of the nitrogen binding energy peak found in Sample 3 compared with Sample 2. Table 9 lists the binding energies and Table 10 the atomic concentrations, for Au, N, O and C. The nitrogen binding energy shift and nitrogen atomic concentration data show that the nitrogen present in Sample 2 is different from that in Sample 3. The broadening of the nitrogen binding energy peak implies that two kinds of nitrogen may be present on the gold surface in Sample 3. The increase of oxygen AC% in Sample 2 compared with Sample 1 was probably caused via the variable of oxygen contamination in these two samples. The proposed mechanisms summarizing the XPS results are listed in Table 11. XPS experiments show that

X. Zeng, S. Bruckenstein / Journal of Electroanalytical Chemistry 461 (1999) 131–142

gold nitrogen compounds form on an electrode surface conditioned at a potential +0.6 V. However, these XPS experiments are unable to characterize gold nitrogen compounds. Consequently, TOF-SIMS experiments were carried out to study this question further.

3.4. TOF-SIMS experiments Secondary ion mass spectrometry (SIMS) with a Cs + primary beam can form and detect secondary ions of the type MCs + or MCs2+ [11], where M is any element in the sample. MCs + or MCs2+ ions are suited for quantitative analysis, as their yield is relatively insensitive to the matrix from which they are emitted. This approach also overcomes uncertainties in peak assignment, e.g. mequiv =16/g equiv − 1, can be either NH2− or O − ; m/z =17 can be NH3− or OH − ; m/z = 19 can be NH4+ or H3O + ; m/z= 213 can be AuO + or AuNH2+ , etc. However, resolving CsN + and CsO + is simple. As shown in Fig. 7, the reduced gold electrode in a bicarbonate buffer (Sample 4) has only weak CsN + and CsO + signals. When the electrode is oxidized at +0.6 V in the bicarbonate buffer (Sample 5), there a large CsO + signal and small CsN + signal result. The large CsO + signal originated from AuO in the oxidized gold film. The small CsN + signal is caused by the nitrogen contamination that occurred while transferring the electrode through air to the TOF-SIMS instrument. When the oxidized gold electrode was dipped into the 0.1 M NH3 +0.1 M NaClO4 electrolyte at an open circuit (Sample 6), the CsO + signal decreased and the CsN + signal increased, compared with Sample 5. This decreased CsO + signal rules out the possibility of formation of AuO(NH3)ads and suggests the occurence of reaction (14). For the gold electrode oxidized in the 0.1 M NH3 + 0.1 M NaClO4 electrolyte at + 0.6 V, Sample 7, both the CsO + and CsN + signals increased compared with Sample 5. Also, the CsO + signal increased more than the CsN + signal. This result shows that most of the nitrogen-containing surface compounds of gold, formed at potentials occurring prior to the ammonia oxidation process, can be oxidized at more positive potentials to AuO. In Sample 7, the CsN + signal decreases and the CsO + signal increases compared to Sample 6. This rules out the possibility of formation of Au(OH)NH2. The latter would cause an increase of the Sample 7 CsN + signal compared with Sample 6. However, the nitrogen peak broadening in Sample 3 in the XPS study implies that two possible kinds of nitrogen are produced on the gold electrode surface when it is oxidized at +0.6 V in the 0.1 M NH3 + 0.1 M NaClO4 electrolyte. This result suggests that small amounts of AuO might be converted to AuOHNH2.

141

4. Conclusions The RRDE, EQCM, TOF-SIMS and XPS techniques have been successfully used to study the gold electrode redox process in a 0.1 M NH3 +0.1 M NaClO4 electrolyte. The mechanism is proposed as follows. (1) Electrolyte reduction process: The proposed mechanism of gold amine compound and gold oxide reduction is (a) OAu–(H2O)ads + 2e − + H2O“ Au–(H2O)ads + 2OH − mequiv = (16/2)/g equiv − 1 = 8/g equiv − 1

(1)

(b) NH2 –Au–(H2O)ads + e − “ Au–(NH3)ads + OH − mequiv=(17/1)/g equiv−1 = 17/g equiv−1

(3)

(2) Thicker gold film formation: When holding at + 0.6 V for a longer period, place exchange reactions occur, forming thick oxide films [12] and gold nitrogen compounds. The proposed mechanism is: Au+ OAu − (H2O)ads + OH − “ Au–O–Au–OH+H2O+ e −

(17)

Au–O–Au–OH+ OH − “ OAu–O–Au–(H2O)ads + e −

(18)

OAu–O–Au–(H2O)ads + OH − “ OAu–O–Au–OH + H2O+ e −

(19)

OAu–O–Au–OH + OH − “ OAu–O–AuO + H2O+ e −

(20)

OAu–O–AuO + H2O“ OAu–O–AuO–(H2O)ads

(21)

and Au+ NH2 –Au–(H2O)ads + OH − “ Au–NH2 –Au–OH+ H2O+ e −

(22)

Au–NH2 –Au–OH+ OH “ −

NH2 –Au–O–Au–(H2O)ads + e −

(23)

RRDE studies show production of a few percent of a soluble Au(III) species when gold oxide is reduced. (3) The supporting electrolyte oxidation process produces a soluble species, probably nitrogen, according to the overall electrode process: Au–(NH3)ads + 5OH − “ OAu + 1/2N2 + 4H2O+ 5e −

(4)

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X. Zeng, S. Bruckenstein / Journal of Electroanalytical Chemistry 461 (1999) 131–142

Acknowledgements We thank the University at Buffalo/SUNY and the National Science Foundation (Grant CHE-9616641) for financial support.

References [1] X. Zeng, S. Bruckenstein, J. Electroanal. Chem. 461 (1999) 143. [2] K. Ogura, S. Haruyama, K. Nagasaki, J. Electrochem. Soc. 118 (1971) 531. [3] S.B. Brummer, A.C. Makrides, J. Electrochem. Soc. 111 (1964) 1122.

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[4] S.H. Cadle, S. Bruckenstein, Anal. Chem. 46 (1974) 16. [5] N.V. Sidgewick, Chemical Elements and Their Compounds, Oxford University Press, London, 1950, p. 191. [6] R. Adzic, E. Yeager, B.D. Cahan, J. Electrochem. Soc. 121 (1974) 474. [7] H.G. Oswin, M. Salomon, Can. J. Chem. 41 (1963) 1686. [8] W.M. Latimer, Oxidation Potentials, II, Prentice – Hall, Englewood Cliffs, NJ, 1952, p. 90. [9] S. Wasmus, E.J. Vasini, M. Krausa, H.T. Mishima, W. Vielstich, Electrochim. Acta 39 (1994) 23. [10] S. Bruckenstein, R.R. Gadde, J. Am. Chem. Soc. 93 (1971) 793. [11] M. Haag, H. Gnaser, H. Oechsner, in: A. Benninghoven, Y. Nihei, R. Shimizu, H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS IX, Wiley, Chichester, 1994, p. 390. [12] P.M. Shay, Ph.D thesis, Ch. 5, State University of New York, Buffalo, NY, 1990.