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Reconstruction and electrochemical oxidation of Au(110) surface in 0.1 M H2SO4
Electrochimica Acta 139 (2014) 281–288
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Reconstruction and electrochemical oxidation of Au(110) surface in 0.1 M H2 SO4 Koji Yoshida, Akiyoshi Kuzume 1 , Peter Broekmann 1 , Ilya V. Pobelov ∗,1 , Thomas Wandlowski 1 Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
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Article history: Received 23 May 2014 Received in revised form 19 June 2014 Accepted 21 June 2014 Available online 15 July 2014 Keywords: Au(110) Electrochemical scanning tunneling spectroscopy SHINERS Surface reconstruction Surface oxidation
a b s t r a c t Variations of the surface structure and composition of the Au(110) electrode during the formation/lifting of the surface reconstruction and during the surface oxidation/reduction in 0.1 M aqueous sulfuric acid were studied by cyclic voltammetry, scanning tunneling microscopy and shell-isolated nanoparticle enhanced Raman spectroscopy. Annealing of the Au(110) electrode leads to a thermally-induced reconstruction formed by intermixed (1×3) and (1×2) phases. In a 0.1 M H2 SO4 solution, the decrease of the potential of the atomically smooth Au(110)-(1×1) surface leads to the formation of a range of structures with increasing surface corrugation. The electrochemical oxidation of the Au(110) surface starts by the formation of anisotropic atomic rows of gold oxide. At higher potentials we observed a disordered structure of the surface gold oxide, similar to the one found for the Au(111) surface. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction The understanding of fundamental processes on low-index metal electrodes, such as surface reconstruction, surface oxidation/reduction and adsorption of atoms or molecules, is an essential foundation for studies of the surface reactivity. Thanks to their high stability and relatively easy handling, basic planes of gold, a metal with a face-centered cubic crystallographic structure, are probably the most characterized low-index single-crystal electrodes. Experiments employing scanning tunneling microscopy (STM), cyclic voltammetry and various types of X-ray and electron spectroscopy created a consistent picture of the structural changes associated with electrochemical processes on Au(111) and Au(100) electrodes [1–6]. In comparison with these surfaces, even the basic structural properties of the Au(110) surface in electrolyte solutions are still poorly known. An ideal Au(110) surface is characterized by a rectangular unit cell with the dimensions a = 0.408 nm and b = 0.289 nm. In comparison with the Au(100) and Au(111) surfaces, Au(110) is a rather anisotropic one. It may form a range of reconstruction phases (1×n), n = 2, 3, . . ., by removing the atomic rows in the [110] direction [7],
∗ Corresponding author. Tel.: +41316314254 E-mail address: [email protected] (I.V. Pobelov). 1 ISE member. http://dx.doi.org/10.1016/j.electacta.2014.06.162 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
i.e. along the crystallographic axis with the shorter period b (Fig. 1). Theoretical studies of the reconstruction phases demonstrated that their surface energy is rather comparable at least up to n = 3 [8–10]. In agreement with this conclusion, different Au(110)-(1×n) structures were observed experimentally depending on various factors. Multiple sputter-annealing cycles were required to produce a defect-free Au(110) surface with an extensive (1×2) reconstruction in ultra-high vacuum (UHV) at room temperature [11]. According to high-temperature electron diffraction [12,13] and STM [14,15] studies in UHV, the reconstructed Au(110) surface with mostly (1×2) structure is stable at low temperatures. At T ≈ 655 K it transforms in an unreconstructed one and then roughens at T > 710 K. The structure of Au(110) in UHV was further found to be affected by the rate of cooling after the surface annealing [16,17] as well as by the deposition of alkali metals [18], molecules [19,20] or an ionic liquid [21]. Under electrochemical conditions, the electrode potential is the primary parameter controlling surface processes. The electrochemical (EC)-STM experiments in 0.1 M H2 SO4 [22] and 0.1 M HClO4 [22–24] solutions found mostly the (1×2) phase interrupted by (1×n), n ≥ 3, elements. This observation was also supported by surface x-ray scattering studies in 0.1 M HClO4 [25] and by reflection anisotropy spectroscopy in 0.1 M H2 SO4 [26]. On the other hand, the (1×3) phase was identified upon reconstruction of the Au(110) surface in neutral and basic solutions [25,27,28]. It was suggested that the presence of solvated alkali metal cations promotes the
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meniscus configuration. All potentials are quoted with respect to the saturated calomel electrode (SCE). 2.3. Electrochemical STM
Fig. 1. Schematics of unreconstructed and reconstructed phases of the Au(110) surface (side view perpendicular to the [110] direction). The darkened atoms lie the second layer.
formation of the (1×3) phase as compared to acidic solutions. A step-wise transition (1 × 1) → (1 × 2) → (1 × 3) upon the decrease of the electrode potential in 0.1 M Na2 SO4 [29] was also proposed. Variations of the structure and chemical composition of Au(110) during its electrochemical oxidation is even less known. Magnussen et al. [22] mention “large-scale surface roughening” upon the oxidation of Au(110) in 0.1 M H2 SO4 , but did not explore this phenomena further. A c(2 × 2) pattern was found at the pre-oxidation potentials of Au(110) in 0.01 M HClO4 by ex-situ X-ray photoelectron spectroscopy and low energy electron diffraction experiments [30]. It was assigned to the formation of surface hydroxide (Au-OH) species. In the present work, we studied the reconstruction and the oxidation/reduction of an atomically-smooth Au(110) surface in 0.1 M H2 SO4 . We explored potential-dependent structural transitions by electrochemical STM and probed chemistry at a solid/liquid interface by shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS). SHINERS is an in-situ spectroscopic technique, recently developed to characterize adsorbed or deposited species at solid/liquid interfaces based on their vibration response [6,31,32]. In this technique, gold nanoparticles (NPs) coated with a chemically inert, pinhole-free thin SiO2 film are deposited on single crystal surfaces and act as surface plasmon antennas enhancing the Raman signal. This approach allows to obtain high quality Raman spectra from atomically smooth metal surfaces. 2. Experimental details 2.1. Chemicals and glassware The electrolyte solution was prepared from H2 SO4 (98%, Merck, suprapure) and Milli-Q water (18.2 M cm, 2 ppb TOC) and deaerated by Ar (5N, Carbagas). Glassware, STM/SHINERS cells and O-rings were cleaned by soaking in a caroic acid solution (3:1 v/v mixture of H2 SO4 and H2 O2 , CAUTION: extremely strong oxidant) overnight followed by three cycles of boiling and rinsing with MilliQ water. All measurements were carried out at room temperature. 2.2. Cyclic voltammetry The electrochemical measurements were carried out in a 3-compartment glass cell equipped with a platinum counter electrode and a platinum trapped hydrogen reference electrode using a -AutoLab type III potentiostat/galvanostat (Metrohm AutoLab). A half-bead Au(110) single crystal electrode was employed as a sample. Prior to experiments, the latter was flame-annealed to red heat and cooled in an argon atmosphere. Contact with the deoxygenated electrolyte was established under potential control in a hanging
Electrochemical STM experiments were carried out with PicoPlus 5500 SPM system (Agilent Technologies, Inc.) equipped with an STM scanner (10 m scanning range). A Au(110) disk crystal (diameter 10 mm, height 2 mm, MaTeck) was employed as a sample. Prior to the measurements, the Au(110) sample was mounted in an Ar-filled glass tube, annealed at a slightly red color for 30 minutes using an inductive heating system (Himmel), and then cooled to the room temperature during 5 to 10 minutes. The Au(110) sample and two Pt wires employed as counter and quasireference electrodes were mounted into a lab-build EC-STM cell, which was filled with the electrolyte under potential control. STM tips were prepared by electrochemically etching tungsten wires in a 2 M KOH solution and subsequently coated with polyethylene. Sample and tip potentials E respectively ET were controlled by the internal bipotentiostat. Tunneling current setpoint IT employed for imaging is indicated in the figure captions. WSxM software [33] was employed for the data analysis. 2.4. SHINERS The SHINERS experiments were carried out with a LabRAM HR800 confocal Raman microscope (Horiba Jobin Yvon). The excitation wavelength from a He-Ne laser operating with the power of ≈1 mW was 632.8 nm. An objective with a long working distance (magnification 50, focal length 8 mm) was used to focus the laser onto the sample surface. The Raman signal was collected in a backscattering geometry. The radius of the laser spot was 2 m, the grating 600 lines/mm, the size of the confocal hole 1 mm, and the slit size 100 m. A half-bead Au(110) single crystal electrode was employed as a sample. Gold NPs (diameter ≈55 nm) were synthesized and coated with a 2 to 3 nm thick SiO2 shell as described previously [6,31,32]. 1.5 l of the NPs solution in Milli-Q water was drop-cast on a flame-annealed half-bead Au(110) electrode and dried under an argon atmosphere. The concentration of NPs was chosen so that they form a submonolayer of small twodimensional islands composed of 5 to 15 individual particles. The modified Au(110) electrode was then polarized in 0.1 M KClO4 at E = −2.0 V vs. Ag/AgCl (sat. KCl) for 200 s and rinsed by MilliQ water. This treatment was repeated six times. The final surface coverage of NPs estimated by AFM measurements [32] was 10 to 15%. This surface modification procedure was demonstrated not to alter electrochemical responses of single-crystal surfaces significantly [32]. The electrode covered with a sub-monolayer of NPs was then mounted into a lab-made spectroelectrochemical cell equipped with a Pt counter electrode and a Ag/AgCl (sat. KCl) reference electrode [32]. All in-situ Raman experiments were carried out in a 0.1 M H2 SO4 solution in the strict absence of oxygen. The deoxygenated electrolyte was added under potential control. The electrode potential was controlled by a lab-built potentiostat [34]. Each Raman spectrum was measured at a fixed potential at least 30 s after a potential step. At every potential we recorded 10 SHINER spectra during 10 s each, averaged them and applied a base line correction. 3. Results 3.1. Cyclic voltammetry Fig. 2 shows representative cyclic voltammograms (CVs) of a freshly flame-annealed Au(110) electrode in 0.1 M H2 SO4 . The sample was brought in contact with the electrolyte at E = −0.250 V.
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Fig. 2. Cyclic voltammograms (sweep rate 0.02 V·s−1 ) and potentiostatic STM images of Au(110) in 0.1 M H2 SO4 . The main curve shows peaks of surface oxidation and reduction P1 and P1 . CVs measured in the potential range -0.250 V
Under these conditions the thermally-induced surface reconstruction remains stable [22]. During the anodic potential scan, a split peak system P1 assigned to the surface oxidation appeared at E > 1.050 V. The corresponding reduction peak system P1 is observed during the reverse potential scan in the potential range 0.650 V
3.2. EC-STM study of thermally-induced reconstruction The thermally-induced reconstruction of Au(110) as imaged by STM in 0.1 M H2 SO4 after contacting the sample with the electrolyte at E = −0.215 V is shown in Figs. 3a and 3b. We observed 10 to 20 nm wide terraces covered by trenches of two types: those with the width of ≈0.8 nm and the deeper ones with the width of ≈1.2 nm, the latter type being predominant. The width values correspond well to 2a and 3a of the Au(110) surface, therefore we assign these trenches to the (1×2) and (1×3) reconstruction structures (c.f. Figs. 1b and 1c). Cross-sections measured along the lines indicated in Figs. 3a and 3b (Fig. 3c and 3d) demonstrate two trenches with the (1×2) structure, separated and surrounded by (1×3) trenches. Furthermore, we observe branching of a (1×3) trench from a (1×2) trench, as indicated by a circle in Fig. 3a. We note that the investigated Au(110) sample was annealed at a temperature well below that corresponding to the surface roughening [12–15] and cooled down with a rate of 30 to 60 K·min−1 . Our results are in agreement with those by Speller et al. [16,17], who
Fig. 3. (a,b) STM images of a thermally-reconstructed Au(110) surface in 0.1 M H2 SO4 at E = −0.215 V, size 80 × 80 nm2 (a) and 30 × 30 nm2 (b), IT = 3 nA, ET = −0.015 V. (c,d) Cross-sections measured perpendicularly to atomic rows as shown by lines in the panels (a) and (b), respectively. The vertical lines indicate positions corresponding to the centers of the upper atomic rows. Numbers 2 and 3 indicate trenches with width 2a and 3a.
demonstrated the predominant (1×3) reconstruction after annealing and a fast cooling with the rate of ≈50 K·min−1 , while a slower (≈1 K·min−1 ) cooling was required to obtain a Au(110) surface with a well-ordered (1×2) reconstruction. 3.3. EC-STM study of electrochemically-induced reconstruction To avoid any effects due to the thermally-induced surface reconstruction, we employed in further experiments unreconstructed Au(110) electrodes. The thermally-induced reconstruction created during the flame annealing was lifted by immersing the Au(110) sample into a 10 mM HCl solution for 20 minutes. To remove the adsorbed chloride, the sample was rinsed with a copious amount of Milli-Q water. EC-STM experiments were started at E = 0.585 V corresponding to the stability of the unreconstructed phase [22]. Fig. 2a
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Fig. 4. (a-e) A sequence of STM images showing the electrochemically-induced reconstruction of the Au(110) surface in 0.1 M H2 SO4 during a potential sweep from E = 0.585 V to E = −0.215 V (rate 0.001 V·s−1 ), size 80 × 80 nm2 , IT = 5 nA, ET = −0.315 V acquisition time 140 s. The arrows show the slow scan directions, the scale bars indicate the scanning distance corresponding to the potential change of 0.030 V. Electrode potentials were 0.585 V (a), 0.315 V to 0.175 V (b), 0.175 V to 0.035 V (c), 0.035 V to -0.105 V (d), -0.105 V to -0.215 V (e). (f) Cross-sections measured perpendicularly to atomic rows as shown by lines in the panels (b-e). The vertical lines indicate the positions corresponding to the centers of the upper atomic rows. Numbers 2 and 3 label trenches with widths 2a and 3a.
demonstrates a high-resolution STM image recorded under these conditions. The obtained dimensions of the rectangular unit cell, 0.40 nm and 0.26 nm, correspond well to the theoretical unit cell for Au(110)-(1×1). The formation of the electrochemically-induced reconstruction was monitored in a potentiodynamic STM experiment. We continuously recorded STM images while the electrode potential was swept from E = 0.585 V to E = −0.215 V with a rate of 0.001 V·s−1 (Fig. 4). The first image (Fig. 4a) shows a well-ordered Au(110)(1×1) surface with 10 to 40 nm wide terraces and monoatomic steps as imaged at E = 0.585 V. At E = 0.225 V we detected a modification of steps as well as an occasional appearance of trenches on the terraces (Fig. 4b). A circle in Fig. 4b marks atomic rows that were attached to a step (c.f. Fig. 4a) during the initial steps of the reconstruction process. The distance between these atomic rows (Fig. 4f, curve b) as well as between trenches (Fig. 4f, curve c) seen on terraces at 0.175 < E < 0.205 V corresponds to 3a. At E < 0.175 V we observed adislands growing on top of the original terraces (Fig. 4c) starting from single rows seen in Fig. 4b. Trenches with a period
Fig. 5. A sequence of STM images showing electrochemical oxidation and reduction of the Au(110) surface during a potential sweep from E = 0.585 V to E = 1.385 V and back (rate 0.001 V·s−1 ), size 100 × 100 nm2 , IT = 0.2 nA, ET = −0.015 V, acquisition time 140 s, the slow scan direction is from top to bottom. Electrode potentials were 0.585 V (a), 0.995 V to 1.185 V (b), 1.365 V to 1.175 V (c), 0.945 V to 0.765 V (d), 0.585 V (e). (f) A high-resolution image showing the formation of a single-atom row on the Au(110) surface during the potential sweep from 0.995 V to 1.045 V, size 7 × 7 nm2 , IT = 3 nA, ET = −0.015 V. The scale bars in (b-e) and (f) indicate the scanning distance corresponding to the potential change of 0.050 V and 0.010 V.
of 3a were identified on top of the adislands. After the potential decrease to E = 0.050 V (Fig. 4d), trenches with the width of 2a started to form on terraces and adislands (Fig. 4f, curve d) that previously exhibited 3a periodicity. Only 2a trenches were seen after the potential reached E = −0.020 V. Finally, at E = −0.070 V (Fig. 4e) we observed the formation of trenches with the width 3a which appear deeper than those seen at higher potentials (Fig. 4f, curve e). This process continues at lower potentials, but the trenches with the width of 2a did not disappear completely even after 20 min at E = −0.215 V (Fig. 2b). The unreconstructed surface (Fig. 2c) was recovered during the reverse potential sweep. We note that it displays many defects even after one cycle of formation and lifting of the reconstruction. 3.4. EC-STM study of the surface oxidation and reduction The electrochemical oxidation of the Au(110)-(1×1) surface was studied in another potentiodynamic STM experiment. We recorded STM images (Fig. 5) while the electrode potential was swept from E = 0.585 V to E = 1.385 V and back with a rate of 0.001 V·s−1 . The
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4. Discussion 4.1. Chloride contaminations
Fig. 6. SHINER spectra of Au(110) in 0.1 M H2 SO4 as a function of the electrode potential. The directions of the potential change are indicated by the arrows.
image of the original Au(110)-(1×1) surface (Fig. 5a) obtained at E = 0.585 V is similar to that in Fig. 4a. The surface morphology shown in Fig. 5a was preserved up to E = 1.005 V. We neither found a rearrangement of surface defects, nor a presence of ordered adlayers. At slightly higher potentials we observed trenches that propagate from steps into the terraces and atomic rows on top of the terraces (upper part of Fig. 5b). The trenches and the atomic rows are oriented in one direction but, unlike in the case of the reconstruction, their appearance is irregular. Fig. 5f shows an atomic row formed at E = 1.025 V on a terrace, which is oriented in the [110] direction of the underlying Au(110) surface. With the further potential increase the amount of trenches and atomic rows increased. They were distinguishable up to E ≈ 1.180 V. The potential increase above 1.110 V led to the formation of nm-sized Au oxide clusters very similar to ones observed after the electrooxidation of the Au(111) surface [6]. They are observed up to E = 1.385 V (Fig. 5c). During the reverse potential scan, the surface smoothens at E = 0.830 V (Fig. 5d) and further reorganizes at lower potentials. The surface morphology similar to the initial one is recovered upon return to E = 0.585 V (Fig. 5e).
3.5. SHINERS study of the surface oxidation and reduction The chemical nature of species formed on the Au(110) surface during its oxidation and reduction were characterized in SHINERS experiments under potential control. Fig. 6 shows typical SHINER spectra of a freshly annealed Au(110) surface prepared as described above in 0.1 M H2 SO4 . Each spectrum was measured at a fixed potential that was varied from 0.5 V to 1.2 V and back with an increment of 0.1 V. At 610, 616 and 980 cm−1 (the dashed lines in Fig. 6), although the intensities of these bands are comparable to the noise level, we identified bands which did not change during the potential cycle and assigned them to the bulk sulfate [35]. A broad peak located in the range of 400 to 700 cm−1 appeared at E > 1.0 V during the anodic cycle. In agreement with our recent results obtained for Au(111) in 1 M H2 SO4 , we assign it to a stretching mode of gold oxide [36,37]. During the reverse potential scan, the gold oxide peak becomes indistinguishable at E ≤ 0.9 V. We tentatively assign a sharp peak at 520 cm−1 observed during the surface reduction at E = 1.0 V (the dotted line in Fig. 6) to an unknown intermediate of the Au(110) reduction.
As our procedure employed to prepare samples for EC-STM experiments included thermal annealing and a chemical lifting of the reconstruction, we would like to explicitly exclude the possibility of chloride contaminations in our experiments. Magnussen et al. [22] compared the behavior of Au(110) electrodes in “pure” 0.1 M H2 SO4 , where the chloride concentration is determined by the quality of the chemical used and was estimated to be less than 10−7 M, and in 0.1 M H2 SO4 containing 10−6 to 10−5 M Cl− . Similar to a behavior observed for other low-index gold surfaces, the presence of even this trace amount of chloride leads to the smoothening of surface defects, which are otherwise stable “for more than half an hour”, and create a smooth Au(110)-(1×1) surface at 0.4 V
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Magnussen et al. [22]. They reported that the electrochemicallyinduced reconstruction is identical in 0.1 M HClO4 and in 0.1 M H2 SO4 , and described it in detail for 1 M H2 SO4 with a trace amount of chloride. The latter leads to the smoothening of defects created after lifting of the thermally-induced reconstruction. In this work, we studied an intrinsic electrochemically-induced reconstruction of an atomically-smooth Au(110)-(1×1) surface with wide terraces and a low amount of defects (Fig. 4a) in pure 0.1 M H2 SO4 . It starts by the removal of gold atoms from the terraces and steps and their attachment to other steps (Fig. 4b) at E = 0.225 V, in agreement with Ref. 22. This potential corresponds rather well to the onset of the cathodic current peak P2 , attributed to the surface reconstruction (Fig. 2). At slightly lower potentials we observed a formation of trenches growing along the [110] direction from step edges or on the terraces. Ref. 22 states that the former is a predominant reconstruction mechanism on surfaces with many defects. Adislands observed on top of the original terraces at E < 0.175 V (Fig. 4c) grow from single rows, also extending in [110] direction, by adding parallel rows of atoms removed from steps and terraces. Current peak P2 in Fig. 2 is located in the potential of the adisland growth. The latter therefore can be identified as the fastest step of the reconstruction. Ref. 22 describes formed “reconstruction elements” as “not very stable” ones. The authors reported on considerable fluctuations of step edges and shape of adislands even in the absence of chloride, while the total area of the reconstructed surface appears to be constant at a fixed electrode potential. They therefore concluded that there is a “dynamic equilibrium between a reconstructed and an unreconstructed surface”. Our potentiodynamic images, on the other hand, demonstrate only an increase of adislands with the decrease of potential. While our observations described in the previous paragraph are similar to that reported by Magnussen et al. [22], we were able to observe more details. Namely, at 0.050 V 0.050 V (c.f. curves b and e in Fig. 4f), and therefore we identified them as a standard two-layers deep (1×3) reconstruction phase (Fig. 4c). The observed phase transitions can be rationalized as a sequential increase of the surface corrugation upon the decrease of the electrode potential. We also note that the (1×3) phase was observed over the entire Au(110) surface at lower potentials in neutral and basic solutions [25,27,28].
4.4. Electrochemical surface oxidation The electrochemical oxidation of the Au(110) surface in 0.1 M H2 SO4 was comprehensively studied in this work employing cyclic voltammetry, EC-STM and SHINERS techniques. Using STM, we did not find any ordered adlayer before the surface oxidation. Therefore we conclude that an ordered adlayer of adsorbed sulfate anions, typical for Au(111) [6] and Au(100) [38], does not form on Au(110). The CVs of the latter also do not exhibit a sharp peak characteristic for a disorder-order transition in a sulfate adlayer on Au(111) [6]. The formation of trenches and atomic rows extending in the [110] direction of Au(110) was observed in potentiodynamic STM experiments at E = 1.005 V (Fig. 5). The latter corresponds well to the onset of the oxidation peak found in CVs (Fig. 2). The anisotropic structure of the oxidized Au(110) surface remains visible up to E ≈ 1.180 V. Comparison with the CVs allows us to associate the first oxidation peak located at E = 1.080 V with the formation of the latter structure. At E > 1.110 V we observed the formation of nmsized clusters (Fig. 5c and 2d). The corresponding second oxidation peak is located at E = 1.140 V. We conclude that the second oxidation step proceeds in the same way as for Au(100) [39] and Au(111) [6]. It was described as a “replacement-turnover” process [40,41], in which adsorbed oxide species exchange their position with surface gold atoms. On the other hand, the first “anisotropic” oxidation step, which can be described as adsorption of oxygen species, is rather unique for Au(110). An oxidation peak observed before the main surface oxidation peak on CVs of Au(111) and Au(100) in sulfuric acid is associated with the oxidation of steps and other defects. We therefore conclude that two intrinsic oxidation peaks of Au(110) reflect the intrinsic anisotropy of this surface. The main potential-dependent feature of SHINER spectra of the Au(110) surface during its oxidation is represented by a broad peak located between 400 to 700 cm−1 with a center at 575 cm−1 (Fig. 6). We assign it to the Raman response of gold oxide based on the following arguments. The vibrational band of gold oxide was determined at 645 cm−1 by high resolution electron energy loss (HREEL) spectroscopy [42]. Ref. 37 describes HREEL and complementary simulation studies of Au(111) covered by atomic oxygen in UHV. These results indicate that oxygen chemisorbed on gold produces a vibrational peak at 380 cm−1 , while surface gold oxide is characterized by a broad peak at 580 cm−1 . A peak very similar to that in Fig. 6 was observed by surface-enhanced Raman spectroscopy during gold oxidation in 1 M HClO4 [36]. Our recent SHINERS studies of the Au(111) surface [6] demonstrated only one broad peak centered around 590 cm−1 in 0.1 M H2 SO4 , and an additional peak at 810 cm−1 in 0.1 M Na2 SO4 . Their assignment to Au-O and Au-OH species was confirmed by the absence respectively the presence of an isotopic effect in complimentary SHINERS experiments carried out in deuterated water. Neither a band at 810 cm−1 , nor bands at 635 and 677 cm−1 determined respectively for gaseous Au(OH)2 [43] and solid Au(OH)3 [44] were observed in our spectra. We therefore conclude that only Au-O species are formed during the oxidation of Au(110) in 0.1 M H2 SO4 . Finally we would like to note that the gold oxidation in sulfuric and perchloric acids is typically assumed to proceed via an intermediate formation of Au-OH species [30,40], which were not observed in SHINERS experiments carried out by our group. We will extend the SHINERS studies of the gold oxidation process to other low-index gold surfaces and other electrolytes in upcoming publications.
5. Conclusion In the present work, we have explored the reconstruction and oxidation/reduction of an atomically-smooth Au(110)-(1×1)
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surface in 0.1 M H2 SO4 . In general, we found that its structural properties are determined by its intrinsic anisotropy. In agreement with previous reports, thermal annealing of a Au(110) surface and its spontaneous cooling in a gaseous atmosphere creates a surface with a mixture of structural elements corresponding to (1×2) (Fig. 1b) and (1×3) (Fig. 1c) phases. Under electrochemical conditions, the decrease of the electrode potential causes a steady increase of the surface corrugation. Starting from an atomically-flat Au(110)-(1×1) surface (Fig. 1a), we subsequently observed a missing/added row (1×3) phase (Figs. 1e and 1f), a (1×2) phase (Fig. 1b), and elements of a “deep” (1×3) phase (Fig. 1c) that did not replace the (1×2) phase completely. The first reconstruction phase was observed before only in a CsI solution [27]. Unlike for other low-index gold surfaces, at positive potentials we did not observe on Au(110) any ordered sulfate layer until the onset of the electrochemical oxidation. The latter starts by the formation of atomic rows and trenches extending in a [110] direction and attributed to adsorbed oxygen. At higher potentials, they transform via a “replacement-turnover” process into disordered isotropic clusters, similar to the ones observed for other gold surfaces. Based on the Raman spectra obtained during the oxidation of the Au(110) surface, we identified created structures as Au-O species and excluded the presence of Au-OH species during the whole course of oxidation. Acknowledgements This work was supported by the Swiss National Science Foundation (200020 144471, NFP 62, Sinergia CRSII2 126969/1), the Swiss Commission for Technology and Innovation (project 13696.1), COST Action TD1002 and the University of Bern. AK acknowledges a MC fellowship (project ELCAMI). References [1] A. Hamelin, Cyclic voltammetry at gold single-crystal surfaces. Part 1. Behaviour at low-index faces, J. Electroanal. Chem. 407 (1996) 1, doi:10.1016/00220728(95)04499-X. [2] L.A. Kibler, Preparation and characterization of noble metal single crystal electrode surfaces (2003). [3] D.M. Kolb, Reconstruction phenomena at metal-electrolyte interfaces, Prog. Surf. Sci. 51 (1996) 109–173, doi:10.1016/0079-6816(96)00002-0. [4] W. Schmickler, Electronic effects in the electric double layer, Chem. Rev. 96 (1996) 3177–3200, doi:10.1021/cr940408c. [5] B. Conway, Electrochemical oxide film formation at noble metals as a surfacechemical process, Prog. Surf. Sci. 49 (1995) 331–452, doi:10.1016/00796816(95)00040-6. [6] U. Zhumaev, A.V. Rudnev, J.-F. Li, A. Kuzume, T.-H. Vu, T. Wandlowski, Electro-oxidation of Au(111) in contact with aqueous electrolytes: new insight from in situ vibration spectroscopy, Electrochim. Acta 112 (2013) 853–863, doi:10.1016/j.electacta.2013.02.105. [7] G. Binnig, H. Rohrer, C. Gerber, E. Weibel, (111) facets as the origin of reconstructed Au(110) surfaces, Surf. Sci. 131 (1983) L379–L384, doi:10.1016/0039-6028(83)90112-7. [8] M. Landmann, E. Rauls, W.G. Schmidt, First-principles calculations of clean Au(110) surfaces and chemisorption of atomic oxygen, Phys. Rev. B 79 (2009) 045412, doi:10.1103/PhysRevB.79.045412. [9] A.Y. Lozovoi, A. Alavi, Reconstruction of charged surfaces: general trends and a case study of Pt(110) and Au(110), Phys. Rev. B 68 (2003) 245416, doi:10.1103/PhysRevB.68.245416. [10] K.-P. Bohnen, K. Ho, Surface structure of gold and silver (110)-surfaces, Electrochim. Acta 40 (1995) 129–132, doi:10.1016/0013-4686(94)00246-W. [11] J.K. Gimzewski, R. Berndt, R.R. Schlittler, Observation of the temporal evolution of the (1×2) reconstructed Au(110) surface using scanning tunneling microscopy, J. Vac. Sci. Technol. B 9 (1991) 897–901, doi:10.1116/1.585491. [12] U. Romahn, P. von Blanckenhagen, C. Kroll, W. Göpel, Step-induced deconstruction and step-height evolution of the Au(110) surface, Phys. Rev. B 47 (1993) 12840–12851, doi:10.1103/PhysRevB.47.12840. [13] B. Lin, H.E. Elsayed-Ali, Reflection high-energy electron diffraction study of surface morphology of vicinal Au(110) throughout the (2×1) to (1×1) phase transition, Surf. Sci. 538 (2003) 89–100, doi:10.1016/S0039-6028(03)00663-0. [14] M. Sturmat, R. Koch, K.H. Rieder, Real space investigation of the roughening and deconstruction transitions of Au(110), Phys. Rev. Lett. 77 (1996) 5071–5074, doi:10.1103/PhysRevLett.77.5071.
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Reconstruction and electrochemical oxidation of Au(110) surface in 0.1 M H2 SO4 Koji Yoshida, Akiyoshi Kuzume 1 , Peter Broekmann 1 , Ilya V. Pobelov ∗,1 , Thomas Wandlowski 1 Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 19 June 2014 Accepted 21 June 2014 Available online 15 July 2014 Keywords: Au(110) Electrochemical scanning tunneling spectroscopy SHINERS Surface reconstruction Surface oxidation
a b s t r a c t Variations of the surface structure and composition of the Au(110) electrode during the formation/lifting of the surface reconstruction and during the surface oxidation/reduction in 0.1 M aqueous sulfuric acid were studied by cyclic voltammetry, scanning tunneling microscopy and shell-isolated nanoparticle enhanced Raman spectroscopy. Annealing of the Au(110) electrode leads to a thermally-induced reconstruction formed by intermixed (1×3) and (1×2) phases. In a 0.1 M H2 SO4 solution, the decrease of the potential of the atomically smooth Au(110)-(1×1) surface leads to the formation of a range of structures with increasing surface corrugation. The electrochemical oxidation of the Au(110) surface starts by the formation of anisotropic atomic rows of gold oxide. At higher potentials we observed a disordered structure of the surface gold oxide, similar to the one found for the Au(111) surface. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction The understanding of fundamental processes on low-index metal electrodes, such as surface reconstruction, surface oxidation/reduction and adsorption of atoms or molecules, is an essential foundation for studies of the surface reactivity. Thanks to their high stability and relatively easy handling, basic planes of gold, a metal with a face-centered cubic crystallographic structure, are probably the most characterized low-index single-crystal electrodes. Experiments employing scanning tunneling microscopy (STM), cyclic voltammetry and various types of X-ray and electron spectroscopy created a consistent picture of the structural changes associated with electrochemical processes on Au(111) and Au(100) electrodes [1–6]. In comparison with these surfaces, even the basic structural properties of the Au(110) surface in electrolyte solutions are still poorly known. An ideal Au(110) surface is characterized by a rectangular unit cell with the dimensions a = 0.408 nm and b = 0.289 nm. In comparison with the Au(100) and Au(111) surfaces, Au(110) is a rather anisotropic one. It may form a range of reconstruction phases (1×n), n = 2, 3, . . ., by removing the atomic rows in the [110] direction [7],
∗ Corresponding author. Tel.: +41316314254 E-mail address: [email protected] (I.V. Pobelov). 1 ISE member. http://dx.doi.org/10.1016/j.electacta.2014.06.162 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
i.e. along the crystallographic axis with the shorter period b (Fig. 1). Theoretical studies of the reconstruction phases demonstrated that their surface energy is rather comparable at least up to n = 3 [8–10]. In agreement with this conclusion, different Au(110)-(1×n) structures were observed experimentally depending on various factors. Multiple sputter-annealing cycles were required to produce a defect-free Au(110) surface with an extensive (1×2) reconstruction in ultra-high vacuum (UHV) at room temperature [11]. According to high-temperature electron diffraction [12,13] and STM [14,15] studies in UHV, the reconstructed Au(110) surface with mostly (1×2) structure is stable at low temperatures. At T ≈ 655 K it transforms in an unreconstructed one and then roughens at T > 710 K. The structure of Au(110) in UHV was further found to be affected by the rate of cooling after the surface annealing [16,17] as well as by the deposition of alkali metals [18], molecules [19,20] or an ionic liquid [21]. Under electrochemical conditions, the electrode potential is the primary parameter controlling surface processes. The electrochemical (EC)-STM experiments in 0.1 M H2 SO4 [22] and 0.1 M HClO4 [22–24] solutions found mostly the (1×2) phase interrupted by (1×n), n ≥ 3, elements. This observation was also supported by surface x-ray scattering studies in 0.1 M HClO4 [25] and by reflection anisotropy spectroscopy in 0.1 M H2 SO4 [26]. On the other hand, the (1×3) phase was identified upon reconstruction of the Au(110) surface in neutral and basic solutions [25,27,28]. It was suggested that the presence of solvated alkali metal cations promotes the
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meniscus configuration. All potentials are quoted with respect to the saturated calomel electrode (SCE). 2.3. Electrochemical STM
Fig. 1. Schematics of unreconstructed and reconstructed phases of the Au(110) surface (side view perpendicular to the [110] direction). The darkened atoms lie the second layer.
formation of the (1×3) phase as compared to acidic solutions. A step-wise transition (1 × 1) → (1 × 2) → (1 × 3) upon the decrease of the electrode potential in 0.1 M Na2 SO4 [29] was also proposed. Variations of the structure and chemical composition of Au(110) during its electrochemical oxidation is even less known. Magnussen et al. [22] mention “large-scale surface roughening” upon the oxidation of Au(110) in 0.1 M H2 SO4 , but did not explore this phenomena further. A c(2 × 2) pattern was found at the pre-oxidation potentials of Au(110) in 0.01 M HClO4 by ex-situ X-ray photoelectron spectroscopy and low energy electron diffraction experiments [30]. It was assigned to the formation of surface hydroxide (Au-OH) species. In the present work, we studied the reconstruction and the oxidation/reduction of an atomically-smooth Au(110) surface in 0.1 M H2 SO4 . We explored potential-dependent structural transitions by electrochemical STM and probed chemistry at a solid/liquid interface by shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS). SHINERS is an in-situ spectroscopic technique, recently developed to characterize adsorbed or deposited species at solid/liquid interfaces based on their vibration response [6,31,32]. In this technique, gold nanoparticles (NPs) coated with a chemically inert, pinhole-free thin SiO2 film are deposited on single crystal surfaces and act as surface plasmon antennas enhancing the Raman signal. This approach allows to obtain high quality Raman spectra from atomically smooth metal surfaces. 2. Experimental details 2.1. Chemicals and glassware The electrolyte solution was prepared from H2 SO4 (98%, Merck, suprapure) and Milli-Q water (18.2 M cm, 2 ppb TOC) and deaerated by Ar (5N, Carbagas). Glassware, STM/SHINERS cells and O-rings were cleaned by soaking in a caroic acid solution (3:1 v/v mixture of H2 SO4 and H2 O2 , CAUTION: extremely strong oxidant) overnight followed by three cycles of boiling and rinsing with MilliQ water. All measurements were carried out at room temperature. 2.2. Cyclic voltammetry The electrochemical measurements were carried out in a 3-compartment glass cell equipped with a platinum counter electrode and a platinum trapped hydrogen reference electrode using a -AutoLab type III potentiostat/galvanostat (Metrohm AutoLab). A half-bead Au(110) single crystal electrode was employed as a sample. Prior to experiments, the latter was flame-annealed to red heat and cooled in an argon atmosphere. Contact with the deoxygenated electrolyte was established under potential control in a hanging
Electrochemical STM experiments were carried out with PicoPlus 5500 SPM system (Agilent Technologies, Inc.) equipped with an STM scanner (10 m scanning range). A Au(110) disk crystal (diameter 10 mm, height 2 mm, MaTeck) was employed as a sample. Prior to the measurements, the Au(110) sample was mounted in an Ar-filled glass tube, annealed at a slightly red color for 30 minutes using an inductive heating system (Himmel), and then cooled to the room temperature during 5 to 10 minutes. The Au(110) sample and two Pt wires employed as counter and quasireference electrodes were mounted into a lab-build EC-STM cell, which was filled with the electrolyte under potential control. STM tips were prepared by electrochemically etching tungsten wires in a 2 M KOH solution and subsequently coated with polyethylene. Sample and tip potentials E respectively ET were controlled by the internal bipotentiostat. Tunneling current setpoint IT employed for imaging is indicated in the figure captions. WSxM software [33] was employed for the data analysis. 2.4. SHINERS The SHINERS experiments were carried out with a LabRAM HR800 confocal Raman microscope (Horiba Jobin Yvon). The excitation wavelength from a He-Ne laser operating with the power of ≈1 mW was 632.8 nm. An objective with a long working distance (magnification 50, focal length 8 mm) was used to focus the laser onto the sample surface. The Raman signal was collected in a backscattering geometry. The radius of the laser spot was 2 m, the grating 600 lines/mm, the size of the confocal hole 1 mm, and the slit size 100 m. A half-bead Au(110) single crystal electrode was employed as a sample. Gold NPs (diameter ≈55 nm) were synthesized and coated with a 2 to 3 nm thick SiO2 shell as described previously [6,31,32]. 1.5 l of the NPs solution in Milli-Q water was drop-cast on a flame-annealed half-bead Au(110) electrode and dried under an argon atmosphere. The concentration of NPs was chosen so that they form a submonolayer of small twodimensional islands composed of 5 to 15 individual particles. The modified Au(110) electrode was then polarized in 0.1 M KClO4 at E = −2.0 V vs. Ag/AgCl (sat. KCl) for 200 s and rinsed by MilliQ water. This treatment was repeated six times. The final surface coverage of NPs estimated by AFM measurements [32] was 10 to 15%. This surface modification procedure was demonstrated not to alter electrochemical responses of single-crystal surfaces significantly [32]. The electrode covered with a sub-monolayer of NPs was then mounted into a lab-made spectroelectrochemical cell equipped with a Pt counter electrode and a Ag/AgCl (sat. KCl) reference electrode [32]. All in-situ Raman experiments were carried out in a 0.1 M H2 SO4 solution in the strict absence of oxygen. The deoxygenated electrolyte was added under potential control. The electrode potential was controlled by a lab-built potentiostat [34]. Each Raman spectrum was measured at a fixed potential at least 30 s after a potential step. At every potential we recorded 10 SHINER spectra during 10 s each, averaged them and applied a base line correction. 3. Results 3.1. Cyclic voltammetry Fig. 2 shows representative cyclic voltammograms (CVs) of a freshly flame-annealed Au(110) electrode in 0.1 M H2 SO4 . The sample was brought in contact with the electrolyte at E = −0.250 V.
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Fig. 2. Cyclic voltammograms (sweep rate 0.02 V·s−1 ) and potentiostatic STM images of Au(110) in 0.1 M H2 SO4 . The main curve shows peaks of surface oxidation and reduction P1 and P1 . CVs measured in the potential range -0.250 V
Under these conditions the thermally-induced surface reconstruction remains stable [22]. During the anodic potential scan, a split peak system P1 assigned to the surface oxidation appeared at E > 1.050 V. The corresponding reduction peak system P1 is observed during the reverse potential scan in the potential range 0.650 V
3.2. EC-STM study of thermally-induced reconstruction The thermally-induced reconstruction of Au(110) as imaged by STM in 0.1 M H2 SO4 after contacting the sample with the electrolyte at E = −0.215 V is shown in Figs. 3a and 3b. We observed 10 to 20 nm wide terraces covered by trenches of two types: those with the width of ≈0.8 nm and the deeper ones with the width of ≈1.2 nm, the latter type being predominant. The width values correspond well to 2a and 3a of the Au(110) surface, therefore we assign these trenches to the (1×2) and (1×3) reconstruction structures (c.f. Figs. 1b and 1c). Cross-sections measured along the lines indicated in Figs. 3a and 3b (Fig. 3c and 3d) demonstrate two trenches with the (1×2) structure, separated and surrounded by (1×3) trenches. Furthermore, we observe branching of a (1×3) trench from a (1×2) trench, as indicated by a circle in Fig. 3a. We note that the investigated Au(110) sample was annealed at a temperature well below that corresponding to the surface roughening [12–15] and cooled down with a rate of 30 to 60 K·min−1 . Our results are in agreement with those by Speller et al. [16,17], who
Fig. 3. (a,b) STM images of a thermally-reconstructed Au(110) surface in 0.1 M H2 SO4 at E = −0.215 V, size 80 × 80 nm2 (a) and 30 × 30 nm2 (b), IT = 3 nA, ET = −0.015 V. (c,d) Cross-sections measured perpendicularly to atomic rows as shown by lines in the panels (a) and (b), respectively. The vertical lines indicate positions corresponding to the centers of the upper atomic rows. Numbers 2 and 3 indicate trenches with width 2a and 3a.
demonstrated the predominant (1×3) reconstruction after annealing and a fast cooling with the rate of ≈50 K·min−1 , while a slower (≈1 K·min−1 ) cooling was required to obtain a Au(110) surface with a well-ordered (1×2) reconstruction. 3.3. EC-STM study of electrochemically-induced reconstruction To avoid any effects due to the thermally-induced surface reconstruction, we employed in further experiments unreconstructed Au(110) electrodes. The thermally-induced reconstruction created during the flame annealing was lifted by immersing the Au(110) sample into a 10 mM HCl solution for 20 minutes. To remove the adsorbed chloride, the sample was rinsed with a copious amount of Milli-Q water. EC-STM experiments were started at E = 0.585 V corresponding to the stability of the unreconstructed phase [22]. Fig. 2a
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Fig. 4. (a-e) A sequence of STM images showing the electrochemically-induced reconstruction of the Au(110) surface in 0.1 M H2 SO4 during a potential sweep from E = 0.585 V to E = −0.215 V (rate 0.001 V·s−1 ), size 80 × 80 nm2 , IT = 5 nA, ET = −0.315 V acquisition time 140 s. The arrows show the slow scan directions, the scale bars indicate the scanning distance corresponding to the potential change of 0.030 V. Electrode potentials were 0.585 V (a), 0.315 V to 0.175 V (b), 0.175 V to 0.035 V (c), 0.035 V to -0.105 V (d), -0.105 V to -0.215 V (e). (f) Cross-sections measured perpendicularly to atomic rows as shown by lines in the panels (b-e). The vertical lines indicate the positions corresponding to the centers of the upper atomic rows. Numbers 2 and 3 label trenches with widths 2a and 3a.
demonstrates a high-resolution STM image recorded under these conditions. The obtained dimensions of the rectangular unit cell, 0.40 nm and 0.26 nm, correspond well to the theoretical unit cell for Au(110)-(1×1). The formation of the electrochemically-induced reconstruction was monitored in a potentiodynamic STM experiment. We continuously recorded STM images while the electrode potential was swept from E = 0.585 V to E = −0.215 V with a rate of 0.001 V·s−1 (Fig. 4). The first image (Fig. 4a) shows a well-ordered Au(110)(1×1) surface with 10 to 40 nm wide terraces and monoatomic steps as imaged at E = 0.585 V. At E = 0.225 V we detected a modification of steps as well as an occasional appearance of trenches on the terraces (Fig. 4b). A circle in Fig. 4b marks atomic rows that were attached to a step (c.f. Fig. 4a) during the initial steps of the reconstruction process. The distance between these atomic rows (Fig. 4f, curve b) as well as between trenches (Fig. 4f, curve c) seen on terraces at 0.175 < E < 0.205 V corresponds to 3a. At E < 0.175 V we observed adislands growing on top of the original terraces (Fig. 4c) starting from single rows seen in Fig. 4b. Trenches with a period
Fig. 5. A sequence of STM images showing electrochemical oxidation and reduction of the Au(110) surface during a potential sweep from E = 0.585 V to E = 1.385 V and back (rate 0.001 V·s−1 ), size 100 × 100 nm2 , IT = 0.2 nA, ET = −0.015 V, acquisition time 140 s, the slow scan direction is from top to bottom. Electrode potentials were 0.585 V (a), 0.995 V to 1.185 V (b), 1.365 V to 1.175 V (c), 0.945 V to 0.765 V (d), 0.585 V (e). (f) A high-resolution image showing the formation of a single-atom row on the Au(110) surface during the potential sweep from 0.995 V to 1.045 V, size 7 × 7 nm2 , IT = 3 nA, ET = −0.015 V. The scale bars in (b-e) and (f) indicate the scanning distance corresponding to the potential change of 0.050 V and 0.010 V.
of 3a were identified on top of the adislands. After the potential decrease to E = 0.050 V (Fig. 4d), trenches with the width of 2a started to form on terraces and adislands (Fig. 4f, curve d) that previously exhibited 3a periodicity. Only 2a trenches were seen after the potential reached E = −0.020 V. Finally, at E = −0.070 V (Fig. 4e) we observed the formation of trenches with the width 3a which appear deeper than those seen at higher potentials (Fig. 4f, curve e). This process continues at lower potentials, but the trenches with the width of 2a did not disappear completely even after 20 min at E = −0.215 V (Fig. 2b). The unreconstructed surface (Fig. 2c) was recovered during the reverse potential sweep. We note that it displays many defects even after one cycle of formation and lifting of the reconstruction. 3.4. EC-STM study of the surface oxidation and reduction The electrochemical oxidation of the Au(110)-(1×1) surface was studied in another potentiodynamic STM experiment. We recorded STM images (Fig. 5) while the electrode potential was swept from E = 0.585 V to E = 1.385 V and back with a rate of 0.001 V·s−1 . The
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4. Discussion 4.1. Chloride contaminations
Fig. 6. SHINER spectra of Au(110) in 0.1 M H2 SO4 as a function of the electrode potential. The directions of the potential change are indicated by the arrows.
image of the original Au(110)-(1×1) surface (Fig. 5a) obtained at E = 0.585 V is similar to that in Fig. 4a. The surface morphology shown in Fig. 5a was preserved up to E = 1.005 V. We neither found a rearrangement of surface defects, nor a presence of ordered adlayers. At slightly higher potentials we observed trenches that propagate from steps into the terraces and atomic rows on top of the terraces (upper part of Fig. 5b). The trenches and the atomic rows are oriented in one direction but, unlike in the case of the reconstruction, their appearance is irregular. Fig. 5f shows an atomic row formed at E = 1.025 V on a terrace, which is oriented in the [110] direction of the underlying Au(110) surface. With the further potential increase the amount of trenches and atomic rows increased. They were distinguishable up to E ≈ 1.180 V. The potential increase above 1.110 V led to the formation of nm-sized Au oxide clusters very similar to ones observed after the electrooxidation of the Au(111) surface [6]. They are observed up to E = 1.385 V (Fig. 5c). During the reverse potential scan, the surface smoothens at E = 0.830 V (Fig. 5d) and further reorganizes at lower potentials. The surface morphology similar to the initial one is recovered upon return to E = 0.585 V (Fig. 5e).
3.5. SHINERS study of the surface oxidation and reduction The chemical nature of species formed on the Au(110) surface during its oxidation and reduction were characterized in SHINERS experiments under potential control. Fig. 6 shows typical SHINER spectra of a freshly annealed Au(110) surface prepared as described above in 0.1 M H2 SO4 . Each spectrum was measured at a fixed potential that was varied from 0.5 V to 1.2 V and back with an increment of 0.1 V. At 610, 616 and 980 cm−1 (the dashed lines in Fig. 6), although the intensities of these bands are comparable to the noise level, we identified bands which did not change during the potential cycle and assigned them to the bulk sulfate [35]. A broad peak located in the range of 400 to 700 cm−1 appeared at E > 1.0 V during the anodic cycle. In agreement with our recent results obtained for Au(111) in 1 M H2 SO4 , we assign it to a stretching mode of gold oxide [36,37]. During the reverse potential scan, the gold oxide peak becomes indistinguishable at E ≤ 0.9 V. We tentatively assign a sharp peak at 520 cm−1 observed during the surface reduction at E = 1.0 V (the dotted line in Fig. 6) to an unknown intermediate of the Au(110) reduction.
As our procedure employed to prepare samples for EC-STM experiments included thermal annealing and a chemical lifting of the reconstruction, we would like to explicitly exclude the possibility of chloride contaminations in our experiments. Magnussen et al. [22] compared the behavior of Au(110) electrodes in “pure” 0.1 M H2 SO4 , where the chloride concentration is determined by the quality of the chemical used and was estimated to be less than 10−7 M, and in 0.1 M H2 SO4 containing 10−6 to 10−5 M Cl− . Similar to a behavior observed for other low-index gold surfaces, the presence of even this trace amount of chloride leads to the smoothening of surface defects, which are otherwise stable “for more than half an hour”, and create a smooth Au(110)-(1×1) surface at 0.4 V
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Magnussen et al. [22]. They reported that the electrochemicallyinduced reconstruction is identical in 0.1 M HClO4 and in 0.1 M H2 SO4 , and described it in detail for 1 M H2 SO4 with a trace amount of chloride. The latter leads to the smoothening of defects created after lifting of the thermally-induced reconstruction. In this work, we studied an intrinsic electrochemically-induced reconstruction of an atomically-smooth Au(110)-(1×1) surface with wide terraces and a low amount of defects (Fig. 4a) in pure 0.1 M H2 SO4 . It starts by the removal of gold atoms from the terraces and steps and their attachment to other steps (Fig. 4b) at E = 0.225 V, in agreement with Ref. 22. This potential corresponds rather well to the onset of the cathodic current peak P2 , attributed to the surface reconstruction (Fig. 2). At slightly lower potentials we observed a formation of trenches growing along the [110] direction from step edges or on the terraces. Ref. 22 states that the former is a predominant reconstruction mechanism on surfaces with many defects. Adislands observed on top of the original terraces at E < 0.175 V (Fig. 4c) grow from single rows, also extending in [110] direction, by adding parallel rows of atoms removed from steps and terraces. Current peak P2 in Fig. 2 is located in the potential of the adisland growth. The latter therefore can be identified as the fastest step of the reconstruction. Ref. 22 describes formed “reconstruction elements” as “not very stable” ones. The authors reported on considerable fluctuations of step edges and shape of adislands even in the absence of chloride, while the total area of the reconstructed surface appears to be constant at a fixed electrode potential. They therefore concluded that there is a “dynamic equilibrium between a reconstructed and an unreconstructed surface”. Our potentiodynamic images, on the other hand, demonstrate only an increase of adislands with the decrease of potential. While our observations described in the previous paragraph are similar to that reported by Magnussen et al. [22], we were able to observe more details. Namely, at 0.050 V
4.4. Electrochemical surface oxidation The electrochemical oxidation of the Au(110) surface in 0.1 M H2 SO4 was comprehensively studied in this work employing cyclic voltammetry, EC-STM and SHINERS techniques. Using STM, we did not find any ordered adlayer before the surface oxidation. Therefore we conclude that an ordered adlayer of adsorbed sulfate anions, typical for Au(111) [6] and Au(100) [38], does not form on Au(110). The CVs of the latter also do not exhibit a sharp peak characteristic for a disorder-order transition in a sulfate adlayer on Au(111) [6]. The formation of trenches and atomic rows extending in the [110] direction of Au(110) was observed in potentiodynamic STM experiments at E = 1.005 V (Fig. 5). The latter corresponds well to the onset of the oxidation peak found in CVs (Fig. 2). The anisotropic structure of the oxidized Au(110) surface remains visible up to E ≈ 1.180 V. Comparison with the CVs allows us to associate the first oxidation peak located at E = 1.080 V with the formation of the latter structure. At E > 1.110 V we observed the formation of nmsized clusters (Fig. 5c and 2d). The corresponding second oxidation peak is located at E = 1.140 V. We conclude that the second oxidation step proceeds in the same way as for Au(100) [39] and Au(111) [6]. It was described as a “replacement-turnover” process [40,41], in which adsorbed oxide species exchange their position with surface gold atoms. On the other hand, the first “anisotropic” oxidation step, which can be described as adsorption of oxygen species, is rather unique for Au(110). An oxidation peak observed before the main surface oxidation peak on CVs of Au(111) and Au(100) in sulfuric acid is associated with the oxidation of steps and other defects. We therefore conclude that two intrinsic oxidation peaks of Au(110) reflect the intrinsic anisotropy of this surface. The main potential-dependent feature of SHINER spectra of the Au(110) surface during its oxidation is represented by a broad peak located between 400 to 700 cm−1 with a center at 575 cm−1 (Fig. 6). We assign it to the Raman response of gold oxide based on the following arguments. The vibrational band of gold oxide was determined at 645 cm−1 by high resolution electron energy loss (HREEL) spectroscopy [42]. Ref. 37 describes HREEL and complementary simulation studies of Au(111) covered by atomic oxygen in UHV. These results indicate that oxygen chemisorbed on gold produces a vibrational peak at 380 cm−1 , while surface gold oxide is characterized by a broad peak at 580 cm−1 . A peak very similar to that in Fig. 6 was observed by surface-enhanced Raman spectroscopy during gold oxidation in 1 M HClO4 [36]. Our recent SHINERS studies of the Au(111) surface [6] demonstrated only one broad peak centered around 590 cm−1 in 0.1 M H2 SO4 , and an additional peak at 810 cm−1 in 0.1 M Na2 SO4 . Their assignment to Au-O and Au-OH species was confirmed by the absence respectively the presence of an isotopic effect in complimentary SHINERS experiments carried out in deuterated water. Neither a band at 810 cm−1 , nor bands at 635 and 677 cm−1 determined respectively for gaseous Au(OH)2 [43] and solid Au(OH)3 [44] were observed in our spectra. We therefore conclude that only Au-O species are formed during the oxidation of Au(110) in 0.1 M H2 SO4 . Finally we would like to note that the gold oxidation in sulfuric and perchloric acids is typically assumed to proceed via an intermediate formation of Au-OH species [30,40], which were not observed in SHINERS experiments carried out by our group. We will extend the SHINERS studies of the gold oxidation process to other low-index gold surfaces and other electrolytes in upcoming publications.
5. Conclusion In the present work, we have explored the reconstruction and oxidation/reduction of an atomically-smooth Au(110)-(1×1)
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surface in 0.1 M H2 SO4 . In general, we found that its structural properties are determined by its intrinsic anisotropy. In agreement with previous reports, thermal annealing of a Au(110) surface and its spontaneous cooling in a gaseous atmosphere creates a surface with a mixture of structural elements corresponding to (1×2) (Fig. 1b) and (1×3) (Fig. 1c) phases. Under electrochemical conditions, the decrease of the electrode potential causes a steady increase of the surface corrugation. Starting from an atomically-flat Au(110)-(1×1) surface (Fig. 1a), we subsequently observed a missing/added row (1×3) phase (Figs. 1e and 1f), a (1×2) phase (Fig. 1b), and elements of a “deep” (1×3) phase (Fig. 1c) that did not replace the (1×2) phase completely. The first reconstruction phase was observed before only in a CsI solution [27]. Unlike for other low-index gold surfaces, at positive potentials we did not observe on Au(110) any ordered sulfate layer until the onset of the electrochemical oxidation. The latter starts by the formation of atomic rows and trenches extending in a [110] direction and attributed to adsorbed oxygen. At higher potentials, they transform via a “replacement-turnover” process into disordered isotropic clusters, similar to the ones observed for other gold surfaces. Based on the Raman spectra obtained during the oxidation of the Au(110) surface, we identified created structures as Au-O species and excluded the presence of Au-OH species during the whole course of oxidation. Acknowledgements This work was supported by the Swiss National Science Foundation (200020 144471, NFP 62, Sinergia CRSII2 126969/1), the Swiss Commission for Technology and Innovation (project 13696.1), COST Action TD1002 and the University of Bern. AK acknowledges a MC fellowship (project ELCAMI). References [1] A. Hamelin, Cyclic voltammetry at gold single-crystal surfaces. Part 1. Behaviour at low-index faces, J. Electroanal. Chem. 407 (1996) 1, doi:10.1016/00220728(95)04499-X. [2] L.A. Kibler, Preparation and characterization of noble metal single crystal electrode surfaces (2003). [3] D.M. Kolb, Reconstruction phenomena at metal-electrolyte interfaces, Prog. Surf. Sci. 51 (1996) 109–173, doi:10.1016/0079-6816(96)00002-0. [4] W. Schmickler, Electronic effects in the electric double layer, Chem. Rev. 96 (1996) 3177–3200, doi:10.1021/cr940408c. [5] B. Conway, Electrochemical oxide film formation at noble metals as a surfacechemical process, Prog. Surf. Sci. 49 (1995) 331–452, doi:10.1016/00796816(95)00040-6. [6] U. Zhumaev, A.V. Rudnev, J.-F. Li, A. Kuzume, T.-H. Vu, T. Wandlowski, Electro-oxidation of Au(111) in contact with aqueous electrolytes: new insight from in situ vibration spectroscopy, Electrochim. Acta 112 (2013) 853–863, doi:10.1016/j.electacta.2013.02.105. [7] G. Binnig, H. Rohrer, C. Gerber, E. Weibel, (111) facets as the origin of reconstructed Au(110) surfaces, Surf. Sci. 131 (1983) L379–L384, doi:10.1016/0039-6028(83)90112-7. [8] M. Landmann, E. Rauls, W.G. Schmidt, First-principles calculations of clean Au(110) surfaces and chemisorption of atomic oxygen, Phys. Rev. B 79 (2009) 045412, doi:10.1103/PhysRevB.79.045412. [9] A.Y. Lozovoi, A. Alavi, Reconstruction of charged surfaces: general trends and a case study of Pt(110) and Au(110), Phys. Rev. B 68 (2003) 245416, doi:10.1103/PhysRevB.68.245416. [10] K.-P. Bohnen, K. Ho, Surface structure of gold and silver (110)-surfaces, Electrochim. Acta 40 (1995) 129–132, doi:10.1016/0013-4686(94)00246-W. [11] J.K. Gimzewski, R. Berndt, R.R. Schlittler, Observation of the temporal evolution of the (1×2) reconstructed Au(110) surface using scanning tunneling microscopy, J. Vac. Sci. Technol. B 9 (1991) 897–901, doi:10.1116/1.585491. [12] U. Romahn, P. von Blanckenhagen, C. Kroll, W. Göpel, Step-induced deconstruction and step-height evolution of the Au(110) surface, Phys. Rev. B 47 (1993) 12840–12851, doi:10.1103/PhysRevB.47.12840. [13] B. Lin, H.E. Elsayed-Ali, Reflection high-energy electron diffraction study of surface morphology of vicinal Au(110) throughout the (2×1) to (1×1) phase transition, Surf. Sci. 538 (2003) 89–100, doi:10.1016/S0039-6028(03)00663-0. [14] M. Sturmat, R. Koch, K.H. Rieder, Real space investigation of the roughening and deconstruction transitions of Au(110), Phys. Rev. Lett. 77 (1996) 5071–5074, doi:10.1103/PhysRevLett.77.5071.
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