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Influence of argon ion beam etching and thermal treatment on polycrystalline and single crystal gold electrodes Au(100) and Au(111)
Journal of Electroanalytical Chemistry 832 (2019) 233–240
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Influence of argon ion beam etching and thermal treatment on polycrystalline and single crystal gold electrodes Au(100) and Au(111)
T
Paula Ahrensa, Manfred Zanderb, Dietmar Hirschc, Ulrich Hassea, Harm Wulffd, Frank Frostc, ⁎ Fritz Scholza, a
University of Greifswald, Institute of Biochemistry, Felix-Hausdorff-Straße 4, D-17487 Greifswald, Germany University of Greifswald, Institute of Geography and Geology, Friedrich-Ludwig-Jahn-Straße 6, D-17487 Greifswald, Germany c Leibniz Institute of Surface Engineering (IOM), Permoserstraße 15, D-04318 Leipzig, Germany d University of Greifswald, Institute of Physics, Felix-Hausdorff-Straße 6, D-17487 Greifswald, Germany b
ARTICLE INFO
ABSTRACT
Keywords: Gold Ion beam etching Thermal treatment Surface morphology Cyclic voltammetry Pb-UPD
The effect of argon ion beam etching and subsequent annealing on the surface morphology and electrochemical response of poly- and monocrystalline gold electrodes was studied with the help of scanning electron microscopy, cyclic voltammetry and measurements of under-potential deposition of lead. While the samples were cleaned and restructured by the argon ion beam the following thermal treatment produced stable and very smooth highly ordered surfaces. The specific behaviour of polycrystalline and monocrystalline gold surfaces is discussed.
1. Introduction Essentially two well-established methods exist for preparing single crystal electrodes [1] of gold or other noble metals for laboratory use. One is the so-called Clavilier method [2] that involves melting and crystalizing the end of a platinum or gold wire to monocrystalline beads while the other one uses the Czochralski method [3] for crystal growth. Usually, both methods are followed by a series of different procedures that includes cutting the grown crystal along desired crystallographic orientation [4,5], mechanical, chemical or electrochemical polishing [4,6], chemical etching [7], annealing [2,7] and occasionally repetitive oxidation-reduction cycles [8]. For scanning probe microscopic investigations very often annealed gold electrodes made of thin films evaporated on (glass) substrates are used [9]. The common preparation process for polycrystalline gold electrodes is less laborious since it merely involves mechanical polishing or electropolishing, surface cleaning by ultrasound and very frequently repetitive formation and reduction of gold oxide layers [10]. The electrochemistry of gold, especially its oxidation, is a complex topic that still holds many unsolved questions despite the many decades of research. Whereas the reduction of the gold oxides proceeds mostly at one single voltammetric peak, the oxidation process comprises several anodic peaks and involves anion replacement, adsorption and place-exchange of oxygen species and the formation of a so called
⁎
“monolayer oxide” [11,12]. There are various interpretations of the nature of the oxidation peaks, e.g., intermediate steps in the formation of a gold oxide layer [13], stepwise coordination and formation of several monolayers of adsorbed OH−-species [14] as well as effects of the quantity of specific low index crystal planes [7,15,16] and defect structures [4,17–20] at the surface of polycrystalline and monocrystalline gold electrodes. To clarify the background of the macroscopic electrochemical signals, structure sensitive non-faradaic doublelayer current features [16,21–23] and structure sensitive probes are used. Underpotential deposition [24] (UPD) of foreign metals on metal substrates is such an appropriated tool for characterization and investigation of the surface structure of a metal electrode. Herrero et al. [25] gave a broad overview of the different types of systems that have been frequently investigated. Among the methods used for gold electrodes, underpotential deposition of lead is a favoured procedure, and the painstaking studies of Hamelin et al. [26–29] played next to others [30–34] an outstanding role in elucidating this phenomenon: The UPDCV of a crystal face gives the specific energy spectrum of that face with respect to the effects of present terraces and their widths, steps, kinks etc., resulting in several peaks. Only faces with just one type of adsorption site would show a single UPD-peak [28]. Ion beam erosion or sputtering is the removal of surface atoms caused by momentum and energy transfer from the incoming ions (or atoms). Due to its atomistic nature ion beam sputtering founds a
Corresponding author. E-mail address: [email protected] (F. Scholz).
https://doi.org/10.1016/j.jelechem.2018.10.066 Received 10 July 2018; Received in revised form 29 August 2018; Accepted 30 October 2018 Available online 05 November 2018 1572-6657/ © 2018 Elsevier B.V. All rights reserved.
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widespread application for surface patterning, depth profiling or surface cleaning, in particular in the area of high-end optics fabrication it becomes indispensable [35]. Beside the actual removal of material, the surface erosion processes often give rise to a pronounced topography evolution, generally accomplished by a kinetic roughening of the surface and occasionally accompanied by the formation or self-organized nanostructures [36–38]. Different self-organized nano-pattern, like ripple or dot pattern, were observed for various materials ranging from insulators to semiconductors or even metals [39] At present, it is assumed that a lot of atomistic processes contribute to the nanopatterning process making it rather complex. Furthermore, in the case of metals the surface evolution is additionally affected by surface atom diffusion mechanisms biased by Ehrlich-Schwoebel barriers and therefore depends on the surface orientation [39]. This was recently also shown for Au(001) surfaces [40]. In this work, we combined ion beam etching and subsequent annealing in an uncommon pretreatment process for poly- and monocrystalline gold electrodes and investigated the effects on their particular surface morphologies and electrochemical responses.
850 °C (Nabertherm P330 furnace) in a quartz glass reaction vessel pervaded with nitrogen, in order to record the SEM images. After this they were again annealed at the same temperature for 26 h to see whether further changes are observable in the SEM images: no further changes occurred. The furnace annealing was repeated prior to each investigational step and the electrodes were cooled and kept in argon atmosphere to avoid contamination of the surface. 2.4. Electrochemistry
2. Experimental
The electrochemical setup and procedures for cyclic voltammetry and Pb-UPD measurements are reported elsewhere [20]. Gold electrodes, prepared as depicted in Section 2.3, served as working electrodes. Unless otherwise stated, all potentials given in the text refer to the used reference electrode Ag|AgCl (3 M KCl, E = 0.210 V vs. SHE). Sulphuric acid ampullae were from VEB Feinchemie Sebnitz. All solutions of HNO3 (65%, Suprapure, Roth), KNO3 (Merck), NaClO4 (Alfa-Aesar), Pb(ClO4)2 (Sigma-Aldrich), HClO4 (70%, 99.999% trace metals basis, Sigma-Aldrich) and H2SO4 were used as-purchased from the respective companies or prepared by using Millipore water (18,2 MΩ cm, Sartorius arium® 600UV, Sartorius AG, Germany).
2.1. Ion beam etching
3. Results and discussion
Three different gold samples were processed simultaneously by argon ion beam etching at low ion energy of 700 eV in a home-build ion beam equipment in 2 steps for 15 and 20 min. The samples were mounted on a water-cooled substrate holder in a high vacuum chamber with a base pressure of 10−6 mbar. The substrate holder rotated with 10 rpm and with an ion incidence angle of 0° with respect to the surface normal. The ion source was a home-built broad beam source of Kaufman-type with a two grid ion optics system. The ion broad beam had a nearly top hat intensity distribution with a uniformity of ± 5% over a diameter of 100 mm. The working distance between ion source and sample was 400 mm. During processing, the current density was kept constant at 200 μA cm−2 which resulted in an ion flux of ca. 1.25 × 1015 cm−2 s−1 in a plane perpendicular to the ion beam.
3.1. Polycrystalline gold Ion beam etching with argon ions has a massive impact on the surface morphology of polycrystalline gold as can be seen in Fig. 1. The initial, untreated surface (cf. Fig. 1a) was rough and very non-uniform as a result of manufacturing and abrasive ‘polishing’. These surface structures were obviously removed within the first 15 min of argon ion beam treatment (cf. Fig. 1b). Simultaneously, this led to the formation of many cones on the electrode surface. As can be seen in Fig. 1c, the number of cones increased with treatment time, although some small areas remained free of cones. It is interesting to note that argon ion beam treatment produced similar cones on the surface of InP [41], a material very different to gold. This may be a hint that for the cone formation, the material is not as important as the etching technique. After thermal treatment (cf. Fig. 1d) all cones have disappeared and the entire surface appeared considerably smoothed. The new surface still showed valleys, cavities, grooves and ridges, however, the main part of the surface was transformed into extended terraces with numerous steps. In addition, some salient corners of several terraces are decorated with round-shaped gold particles while angular particles (possibly impurities?) are found all over the surface. The surface transformation due to thermal treatment also resulted in changes of the electrochemical behaviour: The black curve in Fig. 2a shows the voltammetric signal of the polycrystalline gold electrode after 15 min of argon ion beam etching. The gold oxidation signal is a single broad peak, centred at ca. 1.31 V and comparable to the signal of an ‘amorphous’, ordinary polished gold electrode. The associated UPDCV (black curve in Fig. 2b) is undifferentiated as well and shows only broad peaks 1a, 1b and 3a that are related to Au(110), Au(110) orientated steps and Au(111) respectively [28,31]. All this indicates either a poor crystallinity, or a surface at which all possible crystalline planes are statistically distributed as to exhibit a complete mixture of the behaviour of the single planes. After thermal treatment, the overall current, i.e. of the redox signals and the capacitive background, (red curve in Fig. 2a) dropped considerably due to surface smoothing, leading to a strong decrease of the real electrode surface. The real electrode surface limits the oxidation and reduction currents caused by the formation of the gold oxide layer and its reduction back to metallic gold. Furthermore, the annealing resulted in a splitting of the single oxidation peak into 3 distinguishable features at 1.19 V, 1.26 V and 1.38 V with the last one being a prominent spike. According to literature, these peaks are related to gold oxidation at the low-index crystal
2.2. Scanning electron microscopy (SEM) SEM-imaging of the gold electrode surfaces was performed using the field emission scanning electron microscopes Carl Zeiss Ultra 55 and Carl Zeiss Auriga® de03 with GEMINI® column and operating software ZEISS SmartSEM (Carl Zeiss NTS Ltd). Everhart-Thornley-type detectors were used for secondary electron signal detection. 2.3. Electrode preparation and thermal treatment Gold single crystal electrodes Au(100) and Au(111) (Au 99,99%, orientation ± 3°, 10 mm diameter) were obtained unpolished from GoodFellow and an untreated polycrystalline gold (pc-Au) sample of same size (Au 99,99%) was purchased from m&k gmbh (Germany). 2.3.1. Preliminary treatment of the single crystal electrodes The Au(100) and Au(111) specimen were cleaned and polished as reported earlier [20]. They were subsequently annealed several times at red heat for 5 min in a propane-butane flame and cooled down to room temperature in a stream of argon. To get equal heat distribution, the samples were then annealed for several hours at 850 °C (Nabertherm P330 furnace) and subsequently subjected to prolonged potential cycling in sulphuric acid [20]. After sonication and washing with acetone, isopropanol, nitric acid and Millipore water the single crystal electrodes were used for ion beam etching. The polycrystalline gold sample was cleaned by abrasion on 9 μm abrasive paper (3M, Germany), sonicated and washed with Millipore water before ion beam etching. After the etching procedure all gold electrodes were again annealed for 24 h at 234
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Fig. 1. SEM images of a polycrystalline gold electrode before and after argon ion beam and heating treatment; a) untreated pc-Au before argon ion beam etching, sample tilted 45°; b) after 15 min of argon ion beam etching, sample tilted 45°; c) after 35 min of argon ion beam etching, sample tilted 45°, d) after annealing for 24 h at 850 °C, sample not tilted.
Fig. 2. Electrochemical measurements of an argon ion beam etched (15 min) pc-Au electrode before and after annealing (80 h at 850 °C); a) cyclic voltammograms (first scan) in 0.1 M H2SO4 from −0.25 V to +1.6 V at 0.1 Vs−1; b) stripping (anodic) part of Pb-UPD curves recorded at 0.02 Vs−1 in 0.1 M NaClO4 + 0.01 M HClO4 solution containing 1 mM Pb(ClO4)2. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 3. SEM images of an Au(100) electrode before and after argon ion beam and heating treatment; a) before argon ion beam etching, sample tilted 45°; b) after 15 min of argon ion beam impact, sample tilted 45°; c) after 35 min of argon ion beam impact, sample tilted 45°, d) after annealing for 24 h at 850 °C, sample not tilted.
planes Au(100), Au(110) and Au(111) [15,16] and to the oxidation of variably coordinated gold atoms located in various defect structures or long-range ordered crystal faces [20]. The reduction current at the start of the CV (Fig. 2a, black curve) at −0.25 V, is most probably the result of reduction of gold oxides formed in the time between etching and electrochemical measurements. However, a higher crystallinity and greater diversity of crystal planes on the annealed surface is incontestable as it is indicated by the SEM images and the Pb-UPD results (cf. red curve in Fig. 2b): After thermal treatment, the broad Pb-UPD peak 3 splits up into peaks 3a and 3b and two new peaks, 2a and 2b arose to their sides. These new peaks were related to Au(100) [27,29] while peak 3b is associated with extended terraces of Au(111) [28,34].
treatment showed a broad main oxidation peak, which is more copped than in case of the polycrystalline sample. The peak is situated at ca. 1.26 V with small shoulders at ca. 1.15 V and 1.37 V. This is indicative that no monocrystalline Au(100) plane has formed, as the main oxidation peak of the Au(100) plane is situated around 1.15 V with an additional peak for steps and defects at slightly lower potential [4,16]. The absence of a clean Au(100) plane is further supported by the PbUPD peaks, as the peaks 2a and 2b are missing (cf. Fig. 4b, black curve) [29]. Instead, the stripping curve indicated a rather large contribution of Au(111) domains (peak 3a) and even long-range ordered areas with Au(111)-orientation (spike 3b), that both might be ascribable to the cones. The thermal treatment then resulted in an enormous smoothing of the surface as can be seen in SEM Fig. 3d. All conical structures have disappeared and were replaced by extended terraces with round-shaped particles decorating the steps. The smoothing effect of annealing is reflected in the cyclic voltammogram (cf. Fig. 4a, red curve) as the overall current intensity is remarkably smaller. Moreover, a heat induced change of the oxidation signal and the Pb-UPD behaviour (cf. Fig. 4b, red cure) was detectable: The main oxidation peak shifted from ca. 1.26 V negatively towards approx. 1.19 V with a small peak at its left side at 1.13 V and shoulders at ca. 1.28 V and 1.37 V. This new profile is now in line with findings for stepped Au(100) single crystals [1,4]. The first, small oxidation peak was probably related to the oxidation of atoms with lower coordination number as in steps or defects while the main peak was ascribable to the reaction at less convenient
3.2. Au(100) Fig. 3a shows a SEM of the Au(100) electrode recorded after the preliminary treatment described in the experimental part. After argon ion beam etching, the resulting microscopic surface structures on an Au (100) surface are comparable to those on polycrystalline material (cf. Fig. 3b): cones of predominantly uniform size were formed but here they are densely packed all over the surface with no uncovered areas. Furthermore, no difference between the first (cf. Fig. 3b) and second etching (cf. Fig. 3c) was detectable, as the surface was already completely covered after the first process step. The corresponding CV in Fig. 4a (black line) for the ion beam etched surface before thermal 236
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Fig. 4. Electrochemical measurements of an argon ion beam etched Au(100) electrode before and after annealing (80 h at 850 °C); a) cyclic voltammograms (first scan) in 0.1 M H2SO4 from −0.25 V to +1.6 V at 0.1 Vs−1; b) stripping (anodic) part of Pb-UPD curves recorded at 0.02 Vs−1 in 0.1 M NaClO4 + 0.01 M HClO4 solution containing 1 mM Pb(ClO4)2. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 5. SEM images of an Au(111) electrode before and after argon ion beam and heating treatment; a) before argon ion beam etching, sample tilted 45°; b) after 15 min of argon ion beam impact, sample tilted 45°; c) after 35 min of argon ion beam impact, sample tilted 45°, d) after annealing for 50 h at 850 °C, sample not tilted.
sites as flat, long-range ordered Au(100) faces. The shoulders at more positive potentials then might be assigned to oxidative reactions at the other low index crystal planes Au(110) (1.28 V) and Au(111) (1.37 V) and similar coordinated steps and defects. The Pb-UPD stripping curve
supports this assumption. The previously dominating peaks 3a and 3b, related to Au(111), as well as 1a and 1b, assigned to Au(110), decreased, while the formerly absent peaks 2a and 2b, corresponding to Au(100), remarkably evolved. We therefore conclude, that the ion 237
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Fig. 6. SEM images of an Au(111) electrode before and after argon ion beam and heating treatment; a) before argon ion beam etching, sample tilted 45°; b) after 15 min of argon ion beam impact, sample tilted 45°; c) after 35 min of argon ion beam impact, sample tilted 45°, d) after annealing for 50 h at 850 °C, sample not tilted.
Fig. 7. Electrochemical measurements of an argon ion beam etched Au(111) electrode before and after annealing (80 h at 850 °C); a) cyclic voltammograms (first scan) in 0.1 M H2SO4 from −0.25 V to +1.6 V at 0.1 Vs−1; b) stripping (anodic) part of Pb-UPD curves recorded at 0.02 Vs−1 in 0.1 M NaClO4 + 0.01 M HClO4 solution containing 1 mM Pb(ClO4)2. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
beam treated and annealed surface is highly crystalline and predominantly orientated towards {100}. Steps and those round-shaped gold particles might be accountable for the signals of Au(110) and Au (111).
3.3. Au(111) Fig. 5a shows a SEM of the Au(111) electrode recorded after the preliminary treatment described in the experimental part. The surface 238
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Fig. 8. Comparative presentation of electrochemical data of different argon ion beam etched gold samples after annealing (80 h at 850 °C); a) cyclic voltammograms (first scan) in 0.1 M H2SO4 from −0.25 V to +1.6 V at 0.1 Vs−1; b) stripping (anodic) part of Pb-UPD curves recorded at 0.02 Vs−1 in 0.1 M NaClO4 + 0.01 M HClO4 solution containing 1 mM Pb(ClO4)2.
morphology, UPD and CV signals as it was shown for the ion beam etched and subsequently thermally treated Au(111) and Au(100) single crystals. On the other hand, as indicated by the results for the merely ion beam etched Au(111) sample, there can be discrepancies between CV and UPD results. This leads to the conclusion that not all of these signals are caused by electrochemical reactions exclusively on definite crystal planes but other energetic phenomena, for instance differently coordinated atoms, have to be considered as well. In our earlier studies on the behaviour of polycrystalline gold electrodes we could generate a great variety of crystal planes on electrochemically grown gold crystals [20]. Nevertheless, the resulting CV signal was expanded rather than a combination of sharp peaks one would expect for a combined signal of specific values. Therefore, and as summarised in Fig. 8, the electrochemical response of polycrystalline gold is not equally the sum of the individual low-index crystal planes present on an electrode surface but has particular behaviour and properties. In this context, polycrystalline gold may be described as polyenergetic. The role of padding material between the individual crystals has to be surveyed in future research.
was mainly structured by embedded triangular shaped, staged pyramids and very flat areas with stepped terraces. In contrast to the polycrystalline and the Au(100) sample, the resulting surface of the argon ion beam treated Au(111) sample (cf. Fig. 5b and c) was not covered with cones. Instead, the ion beam lead to a smoothing and clearer appearance of the stratified structure. Besides, some pyramid shaped crystallites appeared either separately embedded in the surrounding environment or bunched at certain locations. Following the plasma etching, the microstructure of the surface was considerably better resolved, showing regularly orientated crystal columns with their head faces of about 20 nm radius in plane with the surface. The columns stay perpendicular to the surface and seem to be very long. This is best seen in Fig. 6b at higher magnification than Fig. 5b. As in the case of the Au (100) specimen, there was no qualitative difference between the first and second etching process step. Although the surface morphologies of the argon ion treated Au(100) and Au(111) appeared entirely different, the resulting CVs and especially Pb-UPD stripping curves were almost identical: In the Pb-UPD signal (Fig. 7b, black curve) merely the intensity of peaks 1a and 1b (corresponding to Au(110)) were decreased and of peak 3b (extended terraces of Au(111)) slightly increased. This makes the sharp peaks 3a and 3b the dominating features and indicates a predominant orientation of the surface atoms towards {111}. However, this cannot be concluded from the CV (Fig. 7a, black curve) as the oxidation signal showed a rather broad peak centred at ca. 1.25 V with shoulders at 1.16 V and 1.36 V. Assuming, that the majority of surface atoms is orientated towards {111}, the main oxidation peak would be expected at more positive potential, around 1.37 V with smaller features for steps and defects at its left side [1,4]. After annealing, the resulting CV (cf. Fig. 7a, red curve) was in accordance with this. The main oxidation peak became rather sharp and shifted towards more positive potentials (ca. 1.38 V) while the intensities of the other features decreased. However, the Pb-UPD curve after annealing (Fig. 7b, red curve) did not change qualitatively and also surface transformations due to thermal treatment (cf. Fig. 5d) were low. The ion beam induced microstructure smoothed and the horizontally stratified surface structure was even more emphasised as it was reflected in increased intensities of UPD-peaks 3a and 3b.
4. Conclusion The combination of ion beam etching and thermal treatment seems to be a time-efficient method to produce clean, stable and very smooth highly ordered surfaces of poly- and monocrystalline gold electrodes. The ion beam leads next to a cleaning effect to a restructuring of the electrode surface different to the original orientation of the base material. A follow-up thermal treatment restores the original orientation back, accompanied by a significant surface smoothing. The present study further corroborates the perception of polycrystalline gold electrodes as a material in its own, and their properties not just being the summation of the properties of the individual single crystal planes. Acknowledgement P. A. was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) as associated PhD student of the Research Training Group (RTG) 1947 “BiOx”. We are thankful to Dr. C. Jeyabharathi for having introduced the Pb-UPD to our laboratory. Furthermore we appreciate the support by Katrin Ohndorf from IOM Leipzig, who performed the ion beam etching.
3.4. Discussion As mentioned in the introduction, UPD is an indisputable structure sensitive probe that detects even smallest amounts of the low-index crystal planes. Often, there is a good correlation between surface 239
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Influence of argon ion beam etching and thermal treatment on polycrystalline and single crystal gold electrodes Au(100) and Au(111)
T
Paula Ahrensa, Manfred Zanderb, Dietmar Hirschc, Ulrich Hassea, Harm Wulffd, Frank Frostc, ⁎ Fritz Scholza, a
University of Greifswald, Institute of Biochemistry, Felix-Hausdorff-Straße 4, D-17487 Greifswald, Germany University of Greifswald, Institute of Geography and Geology, Friedrich-Ludwig-Jahn-Straße 6, D-17487 Greifswald, Germany c Leibniz Institute of Surface Engineering (IOM), Permoserstraße 15, D-04318 Leipzig, Germany d University of Greifswald, Institute of Physics, Felix-Hausdorff-Straße 6, D-17487 Greifswald, Germany b
ARTICLE INFO
ABSTRACT
Keywords: Gold Ion beam etching Thermal treatment Surface morphology Cyclic voltammetry Pb-UPD
The effect of argon ion beam etching and subsequent annealing on the surface morphology and electrochemical response of poly- and monocrystalline gold electrodes was studied with the help of scanning electron microscopy, cyclic voltammetry and measurements of under-potential deposition of lead. While the samples were cleaned and restructured by the argon ion beam the following thermal treatment produced stable and very smooth highly ordered surfaces. The specific behaviour of polycrystalline and monocrystalline gold surfaces is discussed.
1. Introduction Essentially two well-established methods exist for preparing single crystal electrodes [1] of gold or other noble metals for laboratory use. One is the so-called Clavilier method [2] that involves melting and crystalizing the end of a platinum or gold wire to monocrystalline beads while the other one uses the Czochralski method [3] for crystal growth. Usually, both methods are followed by a series of different procedures that includes cutting the grown crystal along desired crystallographic orientation [4,5], mechanical, chemical or electrochemical polishing [4,6], chemical etching [7], annealing [2,7] and occasionally repetitive oxidation-reduction cycles [8]. For scanning probe microscopic investigations very often annealed gold electrodes made of thin films evaporated on (glass) substrates are used [9]. The common preparation process for polycrystalline gold electrodes is less laborious since it merely involves mechanical polishing or electropolishing, surface cleaning by ultrasound and very frequently repetitive formation and reduction of gold oxide layers [10]. The electrochemistry of gold, especially its oxidation, is a complex topic that still holds many unsolved questions despite the many decades of research. Whereas the reduction of the gold oxides proceeds mostly at one single voltammetric peak, the oxidation process comprises several anodic peaks and involves anion replacement, adsorption and place-exchange of oxygen species and the formation of a so called
⁎
“monolayer oxide” [11,12]. There are various interpretations of the nature of the oxidation peaks, e.g., intermediate steps in the formation of a gold oxide layer [13], stepwise coordination and formation of several monolayers of adsorbed OH−-species [14] as well as effects of the quantity of specific low index crystal planes [7,15,16] and defect structures [4,17–20] at the surface of polycrystalline and monocrystalline gold electrodes. To clarify the background of the macroscopic electrochemical signals, structure sensitive non-faradaic doublelayer current features [16,21–23] and structure sensitive probes are used. Underpotential deposition [24] (UPD) of foreign metals on metal substrates is such an appropriated tool for characterization and investigation of the surface structure of a metal electrode. Herrero et al. [25] gave a broad overview of the different types of systems that have been frequently investigated. Among the methods used for gold electrodes, underpotential deposition of lead is a favoured procedure, and the painstaking studies of Hamelin et al. [26–29] played next to others [30–34] an outstanding role in elucidating this phenomenon: The UPDCV of a crystal face gives the specific energy spectrum of that face with respect to the effects of present terraces and their widths, steps, kinks etc., resulting in several peaks. Only faces with just one type of adsorption site would show a single UPD-peak [28]. Ion beam erosion or sputtering is the removal of surface atoms caused by momentum and energy transfer from the incoming ions (or atoms). Due to its atomistic nature ion beam sputtering founds a
Corresponding author. E-mail address: [email protected] (F. Scholz).
https://doi.org/10.1016/j.jelechem.2018.10.066 Received 10 July 2018; Received in revised form 29 August 2018; Accepted 30 October 2018 Available online 05 November 2018 1572-6657/ © 2018 Elsevier B.V. All rights reserved.
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widespread application for surface patterning, depth profiling or surface cleaning, in particular in the area of high-end optics fabrication it becomes indispensable [35]. Beside the actual removal of material, the surface erosion processes often give rise to a pronounced topography evolution, generally accomplished by a kinetic roughening of the surface and occasionally accompanied by the formation or self-organized nanostructures [36–38]. Different self-organized nano-pattern, like ripple or dot pattern, were observed for various materials ranging from insulators to semiconductors or even metals [39] At present, it is assumed that a lot of atomistic processes contribute to the nanopatterning process making it rather complex. Furthermore, in the case of metals the surface evolution is additionally affected by surface atom diffusion mechanisms biased by Ehrlich-Schwoebel barriers and therefore depends on the surface orientation [39]. This was recently also shown for Au(001) surfaces [40]. In this work, we combined ion beam etching and subsequent annealing in an uncommon pretreatment process for poly- and monocrystalline gold electrodes and investigated the effects on their particular surface morphologies and electrochemical responses.
850 °C (Nabertherm P330 furnace) in a quartz glass reaction vessel pervaded with nitrogen, in order to record the SEM images. After this they were again annealed at the same temperature for 26 h to see whether further changes are observable in the SEM images: no further changes occurred. The furnace annealing was repeated prior to each investigational step and the electrodes were cooled and kept in argon atmosphere to avoid contamination of the surface. 2.4. Electrochemistry
2. Experimental
The electrochemical setup and procedures for cyclic voltammetry and Pb-UPD measurements are reported elsewhere [20]. Gold electrodes, prepared as depicted in Section 2.3, served as working electrodes. Unless otherwise stated, all potentials given in the text refer to the used reference electrode Ag|AgCl (3 M KCl, E = 0.210 V vs. SHE). Sulphuric acid ampullae were from VEB Feinchemie Sebnitz. All solutions of HNO3 (65%, Suprapure, Roth), KNO3 (Merck), NaClO4 (Alfa-Aesar), Pb(ClO4)2 (Sigma-Aldrich), HClO4 (70%, 99.999% trace metals basis, Sigma-Aldrich) and H2SO4 were used as-purchased from the respective companies or prepared by using Millipore water (18,2 MΩ cm, Sartorius arium® 600UV, Sartorius AG, Germany).
2.1. Ion beam etching
3. Results and discussion
Three different gold samples were processed simultaneously by argon ion beam etching at low ion energy of 700 eV in a home-build ion beam equipment in 2 steps for 15 and 20 min. The samples were mounted on a water-cooled substrate holder in a high vacuum chamber with a base pressure of 10−6 mbar. The substrate holder rotated with 10 rpm and with an ion incidence angle of 0° with respect to the surface normal. The ion source was a home-built broad beam source of Kaufman-type with a two grid ion optics system. The ion broad beam had a nearly top hat intensity distribution with a uniformity of ± 5% over a diameter of 100 mm. The working distance between ion source and sample was 400 mm. During processing, the current density was kept constant at 200 μA cm−2 which resulted in an ion flux of ca. 1.25 × 1015 cm−2 s−1 in a plane perpendicular to the ion beam.
3.1. Polycrystalline gold Ion beam etching with argon ions has a massive impact on the surface morphology of polycrystalline gold as can be seen in Fig. 1. The initial, untreated surface (cf. Fig. 1a) was rough and very non-uniform as a result of manufacturing and abrasive ‘polishing’. These surface structures were obviously removed within the first 15 min of argon ion beam treatment (cf. Fig. 1b). Simultaneously, this led to the formation of many cones on the electrode surface. As can be seen in Fig. 1c, the number of cones increased with treatment time, although some small areas remained free of cones. It is interesting to note that argon ion beam treatment produced similar cones on the surface of InP [41], a material very different to gold. This may be a hint that for the cone formation, the material is not as important as the etching technique. After thermal treatment (cf. Fig. 1d) all cones have disappeared and the entire surface appeared considerably smoothed. The new surface still showed valleys, cavities, grooves and ridges, however, the main part of the surface was transformed into extended terraces with numerous steps. In addition, some salient corners of several terraces are decorated with round-shaped gold particles while angular particles (possibly impurities?) are found all over the surface. The surface transformation due to thermal treatment also resulted in changes of the electrochemical behaviour: The black curve in Fig. 2a shows the voltammetric signal of the polycrystalline gold electrode after 15 min of argon ion beam etching. The gold oxidation signal is a single broad peak, centred at ca. 1.31 V and comparable to the signal of an ‘amorphous’, ordinary polished gold electrode. The associated UPDCV (black curve in Fig. 2b) is undifferentiated as well and shows only broad peaks 1a, 1b and 3a that are related to Au(110), Au(110) orientated steps and Au(111) respectively [28,31]. All this indicates either a poor crystallinity, or a surface at which all possible crystalline planes are statistically distributed as to exhibit a complete mixture of the behaviour of the single planes. After thermal treatment, the overall current, i.e. of the redox signals and the capacitive background, (red curve in Fig. 2a) dropped considerably due to surface smoothing, leading to a strong decrease of the real electrode surface. The real electrode surface limits the oxidation and reduction currents caused by the formation of the gold oxide layer and its reduction back to metallic gold. Furthermore, the annealing resulted in a splitting of the single oxidation peak into 3 distinguishable features at 1.19 V, 1.26 V and 1.38 V with the last one being a prominent spike. According to literature, these peaks are related to gold oxidation at the low-index crystal
2.2. Scanning electron microscopy (SEM) SEM-imaging of the gold electrode surfaces was performed using the field emission scanning electron microscopes Carl Zeiss Ultra 55 and Carl Zeiss Auriga® de03 with GEMINI® column and operating software ZEISS SmartSEM (Carl Zeiss NTS Ltd). Everhart-Thornley-type detectors were used for secondary electron signal detection. 2.3. Electrode preparation and thermal treatment Gold single crystal electrodes Au(100) and Au(111) (Au 99,99%, orientation ± 3°, 10 mm diameter) were obtained unpolished from GoodFellow and an untreated polycrystalline gold (pc-Au) sample of same size (Au 99,99%) was purchased from m&k gmbh (Germany). 2.3.1. Preliminary treatment of the single crystal electrodes The Au(100) and Au(111) specimen were cleaned and polished as reported earlier [20]. They were subsequently annealed several times at red heat for 5 min in a propane-butane flame and cooled down to room temperature in a stream of argon. To get equal heat distribution, the samples were then annealed for several hours at 850 °C (Nabertherm P330 furnace) and subsequently subjected to prolonged potential cycling in sulphuric acid [20]. After sonication and washing with acetone, isopropanol, nitric acid and Millipore water the single crystal electrodes were used for ion beam etching. The polycrystalline gold sample was cleaned by abrasion on 9 μm abrasive paper (3M, Germany), sonicated and washed with Millipore water before ion beam etching. After the etching procedure all gold electrodes were again annealed for 24 h at 234
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Fig. 1. SEM images of a polycrystalline gold electrode before and after argon ion beam and heating treatment; a) untreated pc-Au before argon ion beam etching, sample tilted 45°; b) after 15 min of argon ion beam etching, sample tilted 45°; c) after 35 min of argon ion beam etching, sample tilted 45°, d) after annealing for 24 h at 850 °C, sample not tilted.
Fig. 2. Electrochemical measurements of an argon ion beam etched (15 min) pc-Au electrode before and after annealing (80 h at 850 °C); a) cyclic voltammograms (first scan) in 0.1 M H2SO4 from −0.25 V to +1.6 V at 0.1 Vs−1; b) stripping (anodic) part of Pb-UPD curves recorded at 0.02 Vs−1 in 0.1 M NaClO4 + 0.01 M HClO4 solution containing 1 mM Pb(ClO4)2. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Fig. 3. SEM images of an Au(100) electrode before and after argon ion beam and heating treatment; a) before argon ion beam etching, sample tilted 45°; b) after 15 min of argon ion beam impact, sample tilted 45°; c) after 35 min of argon ion beam impact, sample tilted 45°, d) after annealing for 24 h at 850 °C, sample not tilted.
planes Au(100), Au(110) and Au(111) [15,16] and to the oxidation of variably coordinated gold atoms located in various defect structures or long-range ordered crystal faces [20]. The reduction current at the start of the CV (Fig. 2a, black curve) at −0.25 V, is most probably the result of reduction of gold oxides formed in the time between etching and electrochemical measurements. However, a higher crystallinity and greater diversity of crystal planes on the annealed surface is incontestable as it is indicated by the SEM images and the Pb-UPD results (cf. red curve in Fig. 2b): After thermal treatment, the broad Pb-UPD peak 3 splits up into peaks 3a and 3b and two new peaks, 2a and 2b arose to their sides. These new peaks were related to Au(100) [27,29] while peak 3b is associated with extended terraces of Au(111) [28,34].
treatment showed a broad main oxidation peak, which is more copped than in case of the polycrystalline sample. The peak is situated at ca. 1.26 V with small shoulders at ca. 1.15 V and 1.37 V. This is indicative that no monocrystalline Au(100) plane has formed, as the main oxidation peak of the Au(100) plane is situated around 1.15 V with an additional peak for steps and defects at slightly lower potential [4,16]. The absence of a clean Au(100) plane is further supported by the PbUPD peaks, as the peaks 2a and 2b are missing (cf. Fig. 4b, black curve) [29]. Instead, the stripping curve indicated a rather large contribution of Au(111) domains (peak 3a) and even long-range ordered areas with Au(111)-orientation (spike 3b), that both might be ascribable to the cones. The thermal treatment then resulted in an enormous smoothing of the surface as can be seen in SEM Fig. 3d. All conical structures have disappeared and were replaced by extended terraces with round-shaped particles decorating the steps. The smoothing effect of annealing is reflected in the cyclic voltammogram (cf. Fig. 4a, red curve) as the overall current intensity is remarkably smaller. Moreover, a heat induced change of the oxidation signal and the Pb-UPD behaviour (cf. Fig. 4b, red cure) was detectable: The main oxidation peak shifted from ca. 1.26 V negatively towards approx. 1.19 V with a small peak at its left side at 1.13 V and shoulders at ca. 1.28 V and 1.37 V. This new profile is now in line with findings for stepped Au(100) single crystals [1,4]. The first, small oxidation peak was probably related to the oxidation of atoms with lower coordination number as in steps or defects while the main peak was ascribable to the reaction at less convenient
3.2. Au(100) Fig. 3a shows a SEM of the Au(100) electrode recorded after the preliminary treatment described in the experimental part. After argon ion beam etching, the resulting microscopic surface structures on an Au (100) surface are comparable to those on polycrystalline material (cf. Fig. 3b): cones of predominantly uniform size were formed but here they are densely packed all over the surface with no uncovered areas. Furthermore, no difference between the first (cf. Fig. 3b) and second etching (cf. Fig. 3c) was detectable, as the surface was already completely covered after the first process step. The corresponding CV in Fig. 4a (black line) for the ion beam etched surface before thermal 236
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Fig. 4. Electrochemical measurements of an argon ion beam etched Au(100) electrode before and after annealing (80 h at 850 °C); a) cyclic voltammograms (first scan) in 0.1 M H2SO4 from −0.25 V to +1.6 V at 0.1 Vs−1; b) stripping (anodic) part of Pb-UPD curves recorded at 0.02 Vs−1 in 0.1 M NaClO4 + 0.01 M HClO4 solution containing 1 mM Pb(ClO4)2. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 5. SEM images of an Au(111) electrode before and after argon ion beam and heating treatment; a) before argon ion beam etching, sample tilted 45°; b) after 15 min of argon ion beam impact, sample tilted 45°; c) after 35 min of argon ion beam impact, sample tilted 45°, d) after annealing for 50 h at 850 °C, sample not tilted.
sites as flat, long-range ordered Au(100) faces. The shoulders at more positive potentials then might be assigned to oxidative reactions at the other low index crystal planes Au(110) (1.28 V) and Au(111) (1.37 V) and similar coordinated steps and defects. The Pb-UPD stripping curve
supports this assumption. The previously dominating peaks 3a and 3b, related to Au(111), as well as 1a and 1b, assigned to Au(110), decreased, while the formerly absent peaks 2a and 2b, corresponding to Au(100), remarkably evolved. We therefore conclude, that the ion 237
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Fig. 6. SEM images of an Au(111) electrode before and after argon ion beam and heating treatment; a) before argon ion beam etching, sample tilted 45°; b) after 15 min of argon ion beam impact, sample tilted 45°; c) after 35 min of argon ion beam impact, sample tilted 45°, d) after annealing for 50 h at 850 °C, sample not tilted.
Fig. 7. Electrochemical measurements of an argon ion beam etched Au(111) electrode before and after annealing (80 h at 850 °C); a) cyclic voltammograms (first scan) in 0.1 M H2SO4 from −0.25 V to +1.6 V at 0.1 Vs−1; b) stripping (anodic) part of Pb-UPD curves recorded at 0.02 Vs−1 in 0.1 M NaClO4 + 0.01 M HClO4 solution containing 1 mM Pb(ClO4)2. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
beam treated and annealed surface is highly crystalline and predominantly orientated towards {100}. Steps and those round-shaped gold particles might be accountable for the signals of Au(110) and Au (111).
3.3. Au(111) Fig. 5a shows a SEM of the Au(111) electrode recorded after the preliminary treatment described in the experimental part. The surface 238
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Fig. 8. Comparative presentation of electrochemical data of different argon ion beam etched gold samples after annealing (80 h at 850 °C); a) cyclic voltammograms (first scan) in 0.1 M H2SO4 from −0.25 V to +1.6 V at 0.1 Vs−1; b) stripping (anodic) part of Pb-UPD curves recorded at 0.02 Vs−1 in 0.1 M NaClO4 + 0.01 M HClO4 solution containing 1 mM Pb(ClO4)2.
morphology, UPD and CV signals as it was shown for the ion beam etched and subsequently thermally treated Au(111) and Au(100) single crystals. On the other hand, as indicated by the results for the merely ion beam etched Au(111) sample, there can be discrepancies between CV and UPD results. This leads to the conclusion that not all of these signals are caused by electrochemical reactions exclusively on definite crystal planes but other energetic phenomena, for instance differently coordinated atoms, have to be considered as well. In our earlier studies on the behaviour of polycrystalline gold electrodes we could generate a great variety of crystal planes on electrochemically grown gold crystals [20]. Nevertheless, the resulting CV signal was expanded rather than a combination of sharp peaks one would expect for a combined signal of specific values. Therefore, and as summarised in Fig. 8, the electrochemical response of polycrystalline gold is not equally the sum of the individual low-index crystal planes present on an electrode surface but has particular behaviour and properties. In this context, polycrystalline gold may be described as polyenergetic. The role of padding material between the individual crystals has to be surveyed in future research.
was mainly structured by embedded triangular shaped, staged pyramids and very flat areas with stepped terraces. In contrast to the polycrystalline and the Au(100) sample, the resulting surface of the argon ion beam treated Au(111) sample (cf. Fig. 5b and c) was not covered with cones. Instead, the ion beam lead to a smoothing and clearer appearance of the stratified structure. Besides, some pyramid shaped crystallites appeared either separately embedded in the surrounding environment or bunched at certain locations. Following the plasma etching, the microstructure of the surface was considerably better resolved, showing regularly orientated crystal columns with their head faces of about 20 nm radius in plane with the surface. The columns stay perpendicular to the surface and seem to be very long. This is best seen in Fig. 6b at higher magnification than Fig. 5b. As in the case of the Au (100) specimen, there was no qualitative difference between the first and second etching process step. Although the surface morphologies of the argon ion treated Au(100) and Au(111) appeared entirely different, the resulting CVs and especially Pb-UPD stripping curves were almost identical: In the Pb-UPD signal (Fig. 7b, black curve) merely the intensity of peaks 1a and 1b (corresponding to Au(110)) were decreased and of peak 3b (extended terraces of Au(111)) slightly increased. This makes the sharp peaks 3a and 3b the dominating features and indicates a predominant orientation of the surface atoms towards {111}. However, this cannot be concluded from the CV (Fig. 7a, black curve) as the oxidation signal showed a rather broad peak centred at ca. 1.25 V with shoulders at 1.16 V and 1.36 V. Assuming, that the majority of surface atoms is orientated towards {111}, the main oxidation peak would be expected at more positive potential, around 1.37 V with smaller features for steps and defects at its left side [1,4]. After annealing, the resulting CV (cf. Fig. 7a, red curve) was in accordance with this. The main oxidation peak became rather sharp and shifted towards more positive potentials (ca. 1.38 V) while the intensities of the other features decreased. However, the Pb-UPD curve after annealing (Fig. 7b, red curve) did not change qualitatively and also surface transformations due to thermal treatment (cf. Fig. 5d) were low. The ion beam induced microstructure smoothed and the horizontally stratified surface structure was even more emphasised as it was reflected in increased intensities of UPD-peaks 3a and 3b.
4. Conclusion The combination of ion beam etching and thermal treatment seems to be a time-efficient method to produce clean, stable and very smooth highly ordered surfaces of poly- and monocrystalline gold electrodes. The ion beam leads next to a cleaning effect to a restructuring of the electrode surface different to the original orientation of the base material. A follow-up thermal treatment restores the original orientation back, accompanied by a significant surface smoothing. The present study further corroborates the perception of polycrystalline gold electrodes as a material in its own, and their properties not just being the summation of the properties of the individual single crystal planes. Acknowledgement P. A. was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) as associated PhD student of the Research Training Group (RTG) 1947 “BiOx”. We are thankful to Dr. C. Jeyabharathi for having introduced the Pb-UPD to our laboratory. Furthermore we appreciate the support by Katrin Ohndorf from IOM Leipzig, who performed the ion beam etching.
3.4. Discussion As mentioned in the introduction, UPD is an indisputable structure sensitive probe that detects even smallest amounts of the low-index crystal planes. Often, there is a good correlation between surface 239
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References
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