Discussion

Although there is no direct indication from the CTR modelling that CO is adsorbed on the surface, the presence of CO can be inferred from:

1) Voltammetry – there is clear evidence of CO adsorption on Ag(hkl) as there are peaks observed in the CV on Ag(111) and Ag(001) which must

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be due to CO electroxidation, and CO appears to suppress oxidation on Ag(110).

2) Changes to the electrolyte structure.

3) Changes to the Ag(001) and Ag(110) metal layer relaxations.

From the cyclic voltammetry it is clear that CO must be adsorbed on the surface, as there are oxidation peaks observed in the CV measured on Ag(111) and Ag(001). On Ag(110) the oxidation peak is not visible over the extended potential range, despite this there is strong evidence for CO on the surface as the oxidation of the surface is suppressed. This implies that CO is strongly adsorbed on the Ag(110) surface as it seems to protect the surface from oxidation, and indicates that CO oxidation occurs at more positive potential which would result in a mix of current from both oxidation processes in the peak between 0.4-0.45 V, as the two charge transfer processes occur over the same potential region. This is supported by similar behaviour which has previously been reported [159] for polycrystalline silver where the electrooxidation of silver is inhibited by CO by 0.13 V. Orozco et al. report this is due to chemisorption of CO poisoning the Ag surface, which is supported by previous studies by Cuesta et al. on a number of other electrodes for Fe [157], Co [163] and Ni [164]. The effect of suppressing Ag oxidation was only observed for pH 13 indicating that the underpotential oxide formation (adsorption of OH-) is key to the electrooxidation of CO. Unfortunately, due to the cell arrangement, we were unable to hold the electrode at negative potential whilst switching the CO-free electrolyte, and do not have the stripping voltammetry so we are unable to determine whether CO was still absorbed.

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Figure 6.32 Schematic representation of the Ag(hkl)/alkaline interface in CO

saturated solution, highlighting the structural differences between the two potential limits. At both potentials there is no evidence of any ordered CO structures – however the presence is inferred from changes to the interface structure. At negative potential CO is adsorbed on the surface. At positive potential CO oxidation occurs on Ag(111) and Ag(001), however on Ag(110) it is proposed that CO is still adsorbed on the surface – poisoning the surface from oxidation.

In a study by Oda et al. [165] on Ag(polycrystalline) and Ag layers on Pt(110), the coverage of CO on the surface could not be determined, suggesting that the coverage of adsorbed CO is very low. A low coverage of CO could be the reason why no ordered structure was determined in the CTR analysis. The effect of CO in electrolyte can be indirectly determined through changes to the relaxation of the

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Ag(111) Ag(110) Ag(001)

Parameter -1.0 V -0.2 V -1.0 V -0.2 V -1.0 V -0.2 V Me tal laye rs ε34 (Å) 0.006(3) 0.02(5) ε23 (Å) -0.014(2) -0.037(6) 0.016(1) ε12 (Å) -0.019(1) -0.037(1) -0.043(2) 0.075(5) -0.019 -1.912 σ3 (Å) 0.08(7) σ2 (Å) 0.17(1) 0.016(1) σ1 (Å) 0.117(2) 0.147(2) 0.312(5) 0.423(8) 0.118(2) 0.123(4) O θ(ML) 0.23(3) 0.65(3) 0.76(6) 1.0(1) d(Å) 2.75(7) 2.27(3) 2.17(4) 1.48(3) σ(Å) 0.15 0.15 0.15 0.3 Na+ θ(ML) 0.39(2) 0.36(2) 0.49(5) 0.50(6) 0.21(5) d(Å) 4.08(2) 3.09(3) 2.86(4) 3.16(9) 2.939(1) σ(Å) 0.15 0.15 0.15 0.15 0.219(7) Error func dE (Å) 5.0(1) 5.1(2) Red χ2 1.79 1.71 1.60 2.30 2.02 1.82

Table 6.11 Summary of best fit parameters to data on all three low-index Ag(hkl)

surfaces in CO saturated 0.01 M NaOH + 0.09 M NaF electrolyte.

metal layers and of the electrolyte layering. A schematic representation of the interface structures is show in Figure 6.32 and a summary of the fit parameters is given in Table 6.11. The presence of CO has a significant effect on the relaxation of the metal layers on Ag(110). In comparison to the data in the absence of CO on Ag(110), at negative potential the presence of adsorbed CO induces a larger inward relaxation of the top metal layer, and at positive potential the outwards relaxation is slightly reduced. There are also small changes to the relaxation of the Ag(001) metal layers. The change in relaxation is due to the adsorption of CO at negative potential and at positive potential the difference in relaxation arises from the difference in the adsorbate metal bonding. A similar effect is reported on platinum surfaces, where the adsorption of CO on Pt(110) is inferred from the changes to the metal [166]. On Ag(111) there is no observed effect on the relaxation of the metal layers, although the roughness of the surface increases as the potential is scanned positive. CO seems

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to have to have a larger effect on the electrolyte structure of Ag(111) and Ag(001) surfaces at positive potential. The most significant difference is the height of the adlayers at positive potential; in both cases the adlayers are much closer to the surface suggesting that CO is still adsorbed on the surface at this potential and has an effect on the nature of the bonding.

Oxygen

The effect of oxygen on the Ag(hkl)/alkaline interface was very subtle. At negative potentials on all three surfaces there was no effect due to the presence of oxygen. This is not surprising considering there is no adsorption of oxygen species at the negative limit. A schematic representation of the interface structures is presente in Figure 6.33 and a summary of the fit parameters to the structural models are given in Table 6.12. At positive potential there are some subtle changes to the interface structure. On Ag(001) and Ag(111) there are some changes to the electrolyte layering; the Ag-OH/O layer height is reduced and the cation layer is closer to the surface; although the relative distance between the OH and Na+ layers remain the same. This implies that the OH-/O layer is more strongly bound to the surface or that the adsorption sites are different. On Ag(110) there is almost no change to the electrolyte layering, only a slight increase in the OH-/O layer which can be correlated to the bigger dip in intensity on the specular CTR. The presence of oxygen induces some small changes to the metal side of the interface on both Ag(001) and (110), small changes to the relaxation of the metal layers are observed, the relaxations of

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Figure 6.33 Schematic representation of the Ag(hkl)/alkaline interface in oxygen

saturated solution, highlighting the structural differences between the two potential limits. At negative potential the double layer structure is governed by a field effect attraction; positively charged Na+ cations redistribute at the interface to maintain electroneutrality at the interface. At positive potential OH is adsorbed on the surface, maintaining a negative charge, the Na+ are stabilised through non-covalent interactions forming a compact double layer at the Ag(111) and Ag(110) interface. At the Ag(001) interface, OH is adsorbed, but no evidence of Na+ ordering was identified by the structural model.

the top metal layers are reduced on both surfaces. On Ag(111) there is no disturbance to the metal, which is similar to the effect of CO. This is perhaps due to the close packed nature of the Ag(111) surface. Blizanac et al. find the kinetics of the ORR to increase in the order (001) ≤ (111) ≤ (110) [162], correlating this to the results presented in this chapter the presence of oxygen perturbs the (001) interface the most, both the metal and electrolyte layering, only the electrolyte layering changes in

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Ag(111) Ag(110) Ag(001)

-0.7 V -0.1 V -1.0 V -0.2 V -0.8 V -0.2 V Me tal laye rs ε34 (Å) 0.009(2) 0.021(3) ε23 (Å) -0.025(2) -0.025(3) -0.019(2) 0.001(1) ε12 (Å) 0.00 -0.039(4) -0.029(2) 0.08(2) -0.931 -0.025(1) σ3 (Å) 0.145(5) 0.00 σ2 (Å) 0.209(2) 0.00 0.051(5) σ1 (Å) 0.114(4) 0.088(6) 0.33 0.48 0.092(3) 0.14(1) O θ(ML) 0.9(1) 0.87(6) 0.87(5) d(Å) 2.45(7) 2.25(3) 1.53(3) σ(Å) 0.15 0.15 0.3 Na+ θ (ML) 0.35(3) 0.14(2) 0.66(5) 0.60(5) 0.32(3) d(Å) 3.33(4) 3.16(2) 2.89(4) 3.29(6) 3.02(1) σ(Å) 0.15 0.15 0.15 0.15 0.2 Error func dE (Å) 5.04(9) 5.29(1) Red χ2 2.1 1.65 1.89 1.75 1.9 1.88

Table 6.12 Summary of best fit parameters to data on all three low-index Ag(hkl)

surfaces in O2 saturated 0.01 M NaOH + 0.09 M NaF electrolyte.

the case of Ag(111) and on Ag(110) the changes are marginal. The largest effect of O2 in solution corresponds to the surface with the least ORR activity, and

interestingly the role of the cation does not appear to be significant in the structure of the Ag(001)/alkaline interface.This seemingly goes against the mechanism proposed by Stmcnik et al. [117] that the non-covalent interaction between cation and hydroxide leads to blocking of ORR active sites.

Conclusions 6.3

The double layer structure of the Ag(hkl) surfaces in alkaline electrolyte, in the underpotential oxide region, has been examined using SXRD measurements. The analysis has shown the layering in the electrolyte has a strong potential dependence; on all three surfaces there is a decrease in intensity on the specular CTR at potentials

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where OH adsorption occurs. The effect is most pronounced on the Ag(110) surface where the specular CTR has a prominent oscillation. It is clear that scattering from electrolyte layering significant and it is necessary to include in the modelling of these systems in order to obtain a good fit and structural model. The results indicate that there is evidence of non-covalent bonding at the electrode/electrolyte interface on the Ag(111) and Ag(110) surfaces. This was not the case on the Ag(001) surface, the data was fit with one adlayer at both potentials indicating that the cations are not stabilised on this surface at positive potential, or the coverage is so small that the contribution to the scattering is negligible. Interestingly at negative potential on Ag(001) there is evidence of some in-plane ordering in the double layer which has not previously been observed, this shows the importance of in situ structural studies and in particular surface X-ray diffraction which does not disturb the double layer structure during measurements.

As the underpotential oxide region is where the ORR, CO adsorption and oxidation occur the study was extended to consider the effects of CO and O2 on the

interface structure. Although the effects of these gases were very subtle on all surfaces, especially in the presence of O2, there were some interesting results to

consider. Mainly, the presence of CO and O2 in electrolyte affects the layering at the

interface, and perturbs the relaxations of the metal surfaces. CO is adsorbed on all three (hkl) surfaces, although the only surface to exhibit the suppression of electrooxidation of Ag is the Ag(110) surface, indicates that adsorption of CO is also dependent upon the geometry of the surface and highlights the need for single crystals to determine the origin of this process.

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To gain a thorough understanding of the interface it would be advantageous to apply other techniques which have a chemical sensitivity to give greater confidence to the models, and determine the nature of the changes identified by the structural models. For example, spectroscopic techniques such as FTIRS which could determine the element and the type of bond present at the interface. It would also be advantageous to repeat the CV measurements in a specially designed cell.

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7 Dynamics of potential-driven

structural changes at the

electrochemical interface

Introduction 7.1

The dynamics of processes at the electrochemical interface are vitally important to a number of technologies such as energy conversion devices - fuel cells and batteries, and electrodeposition for nanotechnology and corrosion. Understanding the dynamics of an electrochemical system on a fundamental level allows for predictions scaling up to industrial applications. Single crystal electrodes, which have a well-defined atomic arrangement and specific adsorption sites, are ideal candidates for fundamental studies and the field of electrochemistry has greatly

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profited from their study. Despite this progression the dynamics of potential-induced structural rearrangement are still not well understood.

The development of modern electrochemical methods to obtain a dynamic picture of events at the solid/liquid interface represents one of the most important contemporary frontiers of research in electrochemistry; in situ structural techniques must be employed to correlate electrochemical processes with structural changes at the electrochemical interface. In situ STM has been used to determine the kinetics of the Au surface reconstructions [150–153], electrodeposition [169], and desorption processes [50]. However STM is time resolution limited by the scan rate of the tip, and mass transport limited due to shielding effects from the tip which can affect the local growth rates in the scan area [51]. The problem of time resolution can be overcome by using high speed STM [170], however the interference of the tip is an integral problem with STM. These issues can be overcome by using SXRD techniques, as the probe does not interfere with the measurements – in this case (although for some systems the X-rays have been shown to interfere, such as in reference [52]). In the last few decades the improved brilliance of synchrotron sources has enabled such work and several time resolved studies of electrochemical systems have been reported [158-168]. In this technique the scattered intensity is monitored, at a specific reciprocal lattice point, as a function of time during a potential step measurement whilst simultaneously measuring the current response (current transients). The point in reciprocal space is chosen for its sensitivity to a particular surface property, such as relaxation of the surface, commensurate adlayer adsorption or the fact that new scattering arises due to the formation of a superstructure. From the change in intensity, so called ‘intensity transients’, the kinetics of specific structural changes can be determined. These can then be linked

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with electrochemical processes determined from the current response. Initial studies which utilised this technique were carried out in the conventional X-ray electrochemical cell, which was later found to be limited due to the thin layer configuration which was rate limiting due to the RC drop [60]. Development of a ‘droplet cell’, in which all three electrodes are in close proximity, allowed greater potential response of 50 s [57]. The focus of the studies has been metal deposition [161,165,166] and dissolution [13,163], and the results have demonstrated that the charge transfer process is much faster than the development of long range crystalline order.

Although the metal electrode side of the interface has been the focus of attention, recently there has been progress in understanding the electrolyte side of the interface [23,24,117,119,170], as discussed in chapter 6. Nakamura et al. [58] presented the first time resolved X-ray diffraction study to focus on the electrolyte side of the interface. The study determined the structural dynamics of the double layer during capacitive charging and discharging of Cs+ at the Ag(001)/c(2x2)-Br interface. The double layer charging is specifically due to changes in the reorganisation of ionic species and reorientation of water molecules at the interface, i.e. no electron transfer between the electrode and electrolyte. The capacitive charging/discharging processes occurred on a millisecond timescale comparable to the complementary current transients.

SXRD measurements in chapter 6 showed sensitivity to the ordering in the electrolyte layers at the Ag(hkl)/0.01 M NaOH + 0.09 M NaF interface and confirmed the cation participation in the so-called ‘double layer’ structure, which is consistent with the study by Lucas et al. in 0.1 M KOH. In this chapter time dependent studies of the electrolyte layering process are presented, complimented by

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metal reconstruction and metal deposition systems as a comparison of timescales for different structural processes. The Ag(111) electrode in alkaline electrolyte system offers a unique insight into the electrochemical interface as there is no significant restructuring of the metal surface (i.e. reconstruction), only a small inward relaxation, thus it is possible to separate structural changes in the metal from those that occur in the electrolyte. The Au(111) surface does, however, reconstruct; and so the kinetics of mass transport of the surface atoms and the subsequent structural ordering can be probed. In contrast to this, studying the underpotential deposition (UPD) process of Ag on Au(111) enables us to study the mass transport of ions through the electrolyte to the surface.

Experimental 7.2

The Ag(111), and Au(111) crystals (99.999%) obtained from Mateck (miscut < 0.1º) were prepared by Ar+ sputtering and thermal annealing cycles in UHV until a sharp LEED pattern was obtained, Ag(111)-(1 × 1) and Au(111)-(𝑝 × √3). The Ag(111) sample was transferred from UHV to the electrochemical cell in a glove bag under inert atmosphere to protect the surface from oxidising. After UHV preparation, the Au(111) crystal was flame annealed prior to transfer to the electrochemical droplet cell. The electrochemical cell consists of a Teflon cross-piece and quartz capillary, a droplet of electrolyte is suspended from the capillary forming a droplet covering the sample surface (setup is discussed in detail in chapter 3). Platinum wire is used as the counter electrode for Ag(111) and Au wire for Au(111), which formed a ring around the droplet, this additionally acted to stabilise the droplet. The outer chamber was continuously purged with nitrogen to keep the electrolyte drop