Faraday Discussions Cite this: Faraday Discuss., 2015, 180, 81
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Solid/fluid interface: general discussion Janine Mauzeroll, Geoffrey Thornton, Trevor Rayment, Vincent Maurice, David Williams, Frank Heberling, Philippe Marcus, Clara Wren, Kirsi Yliniemi, Rob Lindsay, Stephen Lyth, Tom Majchrowski, Hadeel Hussain, Gregory Hunt, Frank Renner, Geraint Williams, Roger Newman, Gerald Frankel, Johannes Lu ¨ tzenkirchen and Hendrik Bluhm
DOI: 10.1039/c5fd90044a
Stephen Lyth opened a general discussion of the paper by G. Frankel by asking: The corrosion chemistry community has made interesting insights into the role of hydrogen evolution as a degradation mechanism in metals. Can these ndings be turned around to design more efficient catalysts for electrochemical hydrogen production? Gerald Frankel replied: Thank you for this interesting question. There is no doubt that the combination of a Mg anode and any other cathode forms a potent hydrogen production engine. Mg alloys such as Mg–Fe generate even more H2 than pure Mg.1 However, a catalyst should be a non-consumable material. The energy that goes into making Mg or another active metal like Al is lost during the process, so it is an expensive way to create hydrogen. I doubt that understanding of the situation at dissolving metal anodes can be translated into non-consumable catalysts. 1 A. Samaniego, N. Birbilis, X. Xia and G. S. Frankel, Hydrogen Evolution During Anodic Polarization of Mg Alloyed with Li, Ca, or Fe, Corrosion, 2015, 71, 224–233.
Roger Newman asked: Regarding the enhanced evolution of hydrogen during anodic polarization of magnesium, the relevance, if any, of the chloride ion was not mentioned. This seems to be universally present in the testing solutions. One can imagine that a Mg atom with an adsorbed anion might be more able to manifest its high reactivity, including for hydrogen evolution, than if it has an adsorbed OH. Does the effect exist over a wide range of anions? If it does, then indeed the anion adsorption idea would not be correct and the most likely explanation is simply that a fast-dissolving metal has more (albeit momentarily) bare metal sites which are the sites of hydrogen evolution (I believe this was suggested by Professor Williams). As a footnote to the latter suggestion, this is somewhat related to a concept for activation of aluminum by low melting point (LMP) alloying elements that I
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developed with a student in the 90s. There too we appealed to the momentary presence of bare Al sites, which in that case were occupied by diffusing LMP.
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1 F. Sato and R. C. Newman, Mechanism of Activation of Aluminum by Low Melting Point Elements: Part 1 – Effect of Zinc on Activation of Aluminum in Metastable Pitting, Corrosion, 1998, 54, 955–963. 2 F. Sato and R. C. Newman, Mechanism of Activation of Aluminum by Low-Melting Point Elements: Part 2 – Effect of Zinc on Activation of Aluminum in Pitting Corrosion, Corrosion, 1999, 55, 3–9.
Gerald Frankel responded: In a recent paper, anodic HE was collected on HP Mg in chloride-free buffer solutions over a range of pH values.1 Citrate (pH 3), borate (pH 7) and carbonate (pH 10.5) solutions were used. Anodic HE was observed in all of the solutions. However, in the citrate solution the HE rate increased only at higher applied current densities when the buffer capacity was seemingly overwhelmed by local pH changes. The bottom line is that chloride is not needed, but pH change is important. Please see my response to Prof. Williams regarding the issues associated with explaining the effect solely by an increase in the number of active sites. 1 L. Rossrucker, A. Samaniego, J.-P. Grote, A. M. Mingers, C. A. Laska, N. Birbilis, G. S. Frankel and K. J. J. Mayrhofer, The pH Dependence of Magnesium Dissolution and Hydrogen Evolution During Anodic Polarization, J. Electrochem. Soc., 2015, 162 C333– C339.
David Williams asked: Is it possible that, under the extreme dissolution conditions prevailing (which could be considered as kink sites moving extremely rapidly across the surface) that Al atoms at the kink sites could react with water either to form the aluminium hydroxide anodically or to split water to form hydrogen and aluminium hydroxide in a chemical process – in which case there would not be any current owing through the external circuit. The balance between the two reactions could depend on the surface density and orientation of water molecules. Gerald Frankel answered: This is an interesting notion that might be rephrased as follows: Assuming that Mg or Al dissolution occurs at active sites on the surface such as kink sites, it is possible that the number of active sites per unit exposed area increases as the dissolution rate increases, and these active sites can also reduce water. The result would be more water reduction with more dissolution. This perspective describes how the active sites for both the anodic dissolution and hydrogen evolution reactions would be linked. However, as the applied anodic current or potential increases, the rate of the HER should decrease exponentially. Therefore, an explanation would still be needed to explain why the hydrogen evolution rate remains about the same percentage of the applied anodic current. One would have to assume that the rate of HER on these sites is independent of potential. Janine Mauzeroll asked: Would it not be a good idea to also consider the complex equilibria that occurs in the magnesium system to account for some of the negative difference effect observed?
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Gerald Frankel answered: I suppose you are referring to solution equilibria in the electrolyte near the dissolving surface. It is a complex situation owing to the rapid dissolution and the copious hydrogen evolution. It is generally understood that the pH will increase because of weak hydrolysis and strong water reduction. However, there is evidence of initial pH decrease because of a short dominance of hydrolysis.1 And pH changes seem to be critical for the anodic HE effect on Mg. So, yes we need to understand details of both the surface and the near solution phase.
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1 L. Rossrucker, A. Samaniego, J.-P. Grote, A. M. Mingers, C. A. Laska, N. Birbilis, G. S. Frankel and K. J. J. Mayrhofer, J. Electrochem. Soc., 2015, 162, C333.
Geraint Williams queried: Are there any local effects observed during the anodic polarisation of the Mg specimens, especially at high imposed current density values? For example, is the entirety of the “black” surface strongly evolving hydrogen, or are HE sites limited to a few regions on the exposed Mg. If the latter best represents observed behaviour, is there evidence that the number of sites strongly evolving hydrogen increases with potential or applied current density? Maybe the increase in “anodic” HE with anodic current density is an area effect, where the rise in current density progressively produces more regions of “nascent”, totally lm-free Mg which are the principal sites of both anodic dissolution and cathodic HE? Gerald Frankel answered: Thanks for your question. This is a good point as simple optical observation should be able to help sort out the location of the HER. Unfortunately, this location is difficult to assess, particularly at high applied anodic currents when the HE is also large. Because of the copious HE at the surface, the entire surface is masked so it is impossible to determine the exact origin of the bubbles. This was especially difficult in our case because the sample surface was vertically oriented to allow for easy viewing of the surface through the
Fig. 1
Image of the surface during polarization at 25 mA cm2
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cell wall. The accompanying gure (Fig. 1) shows an image of the surface during polarization at 25 mA cm2. Images of the surfaces of electrodes aer passage of a xed charge but at different current densities exhibit increased coverage of dark corrosion product with increasing current density. This might indicate the important role of the dark regions. However, the variations are not orders of magnitude as are the increases in HE rate. Regarding the increase in active sites, please see my response to David Williams. Clara Wren opened a general discussion of the papers by Hendrik Bluhm: What happens to the electrons ejected from the solid? The energy of the electrons would be the energy of the photoelectrons minus the binding energy and work function and, hence, relatively low, on the order of ~100 eV. This is not sufficient to penetrate the adsorbed water layer but is sufficient to ionize water molecules close to the metal or metal hydroxide surface. Could the ionization by the secondary electrons affect the water chemistry? Hendrik Bluhm answered: The kinetic energy of the photoelectrons is the energy of the incident X-rays, minus the electron binding energy and work function of the electron analyzer. The incident photon energy in these experiments is 3100 eV, which leads to photoelectron kinetic energies of ~2570 eV (O 1s), ~2240 eV (Ni 2p), and 2820 eV (C 1s), sufficient to penetrate solution layers with thicknesses in the 10 nm range (see Table 1 in our paper). The inuence of secondary electrons on the water chemistry has to be evaluated for each experiment to avoid artifacts in the experiments. This can be done by monitoring the spectral features as a function of sample irradiation and time. We have no indication of secondary electron-induced chemistry in the present experiments. Geoffrey Thornton asked: In the paper by Springell et al.1 it is claimed that Xrays of energy 17 keV cause radiolysis of water. Do you have any evidence one way or another for radiolysis at the lower energies that you employ? 1 Springell et al., Faraday Discuss., 2015, DOI:10.1039/c4fd00254g.
Hendrik Bluhm answered: We do not have evidence for X-ray induced radiolysis in the present experiments and neither in past experiments carried out resonantly at the O K-edge (incident photon energy ~530 eV) of bulk ice samples,1 where radiolysis would manifest itself in the appearance of a OH-related peak in oxygen K-edge NEXAFS data, which was not observed. 1 D. E. Starr, D. Pan, J. T. Newberg, M. Ammann, E. G. Wang, A. Michaelidis, H. Bluhm, Phys. Chem. Chem. Phys., 2011, 13, 19988.
Philippe Marcus said: I have two questions: (1) Looking at your Ni 2p spectra, it appears that the nickel surface is already oxidized prior to exposure to water. Do you have a way to prepare clean surfaces? (2) A problem encountered with ultrathin water layer is the dissolution of the metal, leading to saturation and redeposition on the surface. Is this an issue in your experiments?
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Hendrik Bluhm answered: (1) Upon introduction into the measurement chamber, a Ni sample will always be oxidized since it was exposed to air during transfer into the measurement chamber. Specialized experimental set-ups with in situ Ni lm preparation and introduction into the measurement chamber without exposure to air or water vapor before the start of the experiments are conceivable. The oxide can also be reduced electrochemically inside the solution. Another option for the preparation of a clean Ni lm could be the direct (galvanic) deposition of Ni onto the multilayer substrate. (2) To the second question: We did not see any evidence for dissolution of the Ni sample in these experiments. The solubility is very low under basic conditions, e.g. for NiO of the order of 107 mole L1 (at pH ¼ 13), with dissolution rates of 1012 to 1015 moles/(cm2 s).1 The solubility of NiOH2 in neutral conditions is in the range of 105 moles L1 and will be even smaller at pH ¼ 13.2 1 P. R. Tremaine and J. C. LeBlanc , J. Chem. Thermodynamics, 1980, 12, 521. 2 W. M. Haynes (ed), Physical Constants of Chemical Compounds, in CRC Handbook of Chemistry and Physics, 95th Edn, 2014.
Vincent Maurice asked: The thickness of 27 nm found for the hydroxide/oxyhydroxide layer appears quite large. This is about one order of magnitude larger than for the passive lm produced in similar conditions of passivation at pH 13 (from ~1.5 to ~4.5 nm with potential).1,2 How would you explain such a difference? 1 H.-W. Hoppe and H.-H. Strehblow, Surf. Interface Anal., 1989, 14, 121. 2 A. Seyeux et al., J. Solid State Electrochem., 2005, 9, 337–346.
Hendrik Bluhm replied: We believe that the complete oxidation of the Ni lm in the present case as compared to partial oxidation in the case of a passive lm at the same pH is due to the fact that the present measurements are performed under active oxidation conditions, i.e. at a sustained potential of +0.6 V. Geoffrey Thornton asked: The signal-to-noise level of the standing wave data are rather limited. What are the prospects for signicant improvement based on instrument development? Hendrik Bluhm replied: The main causes for the signal attenuation in these measurements are the scattering of electrons by the liquid lm and the gas phase surrounding the sample. These scattering processes depend on the given experimental conditions; for instance, for measurements at lower background vapor pressures and thinner liquid lms the signal-to-noise ratio is greatly improved, while in the present case, for thicker liquid lms at a RH close to 100% at room temperature scattering is stronger. The signal-to-noise in the measurement can be improved through either a better detection efficiency of the electron spectrometer or the use of incident X-rays with higher brilliance, such as will become available in future ultimate storage ring sources. In the experiments presented here we used a state-of-the-art ambient pressure photoelectron spectrometer with a theoretical maximum electron collection angle of 22 . The greatest room for improvement lies in a stronger horizontal focusing of the incident X-ray beam. Due to the small entrance aperture of the differentially-pumped photoelectron spectrometer, and shallow incident angle in standing wave experiments, the illuminated area of the This journal is © The Royal Society of Chemistry 2015
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sample is in general much larger than the detected area in SW-APXPS (here by a factor of ~8). Thus, improved horizontal focusing will lead to a higher signal-tonoise-ratio in the experiments.
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Trevor Rayment enquired: Does the presence of an organic contaminant layer at the surface of the electrolyte have any effect upon the wetting properties of the electrolyte since this might affect the thickness of the wetting layer which is a critical component of the system? Hendrik Bluhm replied: The presence of the organic contaminant layer at the liquid/vapor interface may have an inuence on the condensation/evaporation rates of the liquid,1 or on the concentration of ions at the liquid/vapor interface.2 We do not expect a direct effect of the contamination layer at the liquid/vapor interface on the wetting properties of the liquid/solid interface since the substrate surface is electrochemically cleaned inside the bulk solution before the sample is pulled out of the solution (always staying in contact with the bulk solution in the beaker). 1 J. F. Davies, R. E. H. Miles, A. E. Haddrell and J. P. Reid, PNAS, 2013, 110, 8807. 2 M. J. Krisch, R. D’Auria, M. A. Brown, D. J. Tobias, J. C. Hemminger, M. Ammann, D. E. Starr and H. Bluhm, J. Phys. Chem. C, 2007, 111, 13497.
Trevor Rayment remarked: It would be helpful to have more information about the opportunities and limitations which result from the use of a standing wave method for the use of ambient pressure XPS in the study of corrosion. For example how does the method constrain the thickness and smoothness of the electrode? Hendrik Bluhm replied: The roughness of the electrode interfaces should not be more than a fraction of the standing wave period for an accurate characterization of the liquid/solid interface properties. However, the period of the multilayer can be increased beyond the few nm in this study so as to look at greater roughness. The total thickness of the electrode is limited by the effective attenuation of the X-ray standing wave eld by the electrode material, as the Xrays must penetrate signicantly into the substrate multilayer mirror; e.g. an upper value for the case of Ni and incident X-rays with an energy of 3.1 keV is about 100 nm at an incident angle of 3.5 . Frank Heberling commented: The sample surface investigated in your study, deposited as a thin lm on a multilayer mirror, appears very special. What kinds of surface can be or have already been investigated by the standing-wave in situ XPS method you describe in your paper? Would it be feasible to investigate the surface speciation on single crystal samples if a total external reection standing wave (TERXSW) eld would be used for the excitation of the photoelectrons? Hendrik Bluhm answered: So far only measurements on Ni (present case) and iron oxide lms1 have been performed using SW-APXPS, but we expect that the method is applicable to a wide range of substrate types due to the progress that
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has been made in the preparation of a wide variety of thin lm systems using, e.g., PLD or MBE. Near-total-reection (NTR equivalent to TERXSW measurements have recently been demonstrated for solid-solid oxide interfaces.2 However, measurements with harder X-rays may be difficult due to the very large beam spot along the beam propagation direction for the extremely shallow angles of incidence needed in these experiments. This is due to the very small effective sample area imposed by the entrance aperture of the ambient pressure photoelectron spectrometer. At so X-ray energies, this should be less of a problem, however.
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1 S. Nemˇs´ ak, A. Shavorskiy, O. Karslioglu, I. Zegkinoglou, P. K. Greene, E. C. Burks, K. Liu, A. Rattanachata, C. S. Conlon, A. Keqi, F. Salmassi, E. M. Gullikson, S.-H. Yang, H. Bluhm and C. S. Fadley, Nat. Commun. 2014, 5, 5441. 2 M. Marinova, et al., Nano Lett., 2015, 15(4), 2533.
Clara Wren asked: Could you specify conditions in the solution? Was it aerated? What was the reason for the 0.1 M KOH? Nickel could irreversibly oxidize in an aerated basic solution and cathodic cleaning may not be able to reduce the resultant nickel oxide/hydroxide back to the metallic state. A surface of nickel oxide immersed in water is always covered with a nickel hydroxide. How do you ensure that the hydroxide being monitored is the real-time layer as claimed? Hendrik Bluhm replied: The solution was degassed before use through evacuation in a separate vacuum chamber. Our past experiments have shown that hydroxide solutions are less prone to beam-induced changes (compared to solutions containing, e.g., halide anions), hence the choice of KOH. All experiments in this study were performed at a potential of +0.6 eV, due to the limited amount of beamtime in this proof-of-principle study. We will explore the reduction of NiOx under cathodic conditions in a next set of experiments that will help to answer these questions. Roger Newman asked: How low does the current in the electrochemical cell have to be in order for adequate potential control to be maintained in the thin liquid layer? It seems that a fairly easy calculation would give the answer, which I imagine will be very low indeed. In that case the method would only seem to work for reactions that have gone to completion, like UPD. For growth of an oxide lm there will be a residual current. Hendrik Bluhm replied: The potentials measured by XPS at a location of the thin lm ~2 cm above the bulk liquid surface in the beaker are in agreement with the applied potentials to the bulk liquid, i.e. the currents along the thin liquid lm are sufficiently low under our experimental conditions. A simple estimate for a 20 nm thick 0.1 M KOH lm with a specic conductance of 20 mS cm1 yields an upper limit for the current of around 20 nA. At this current a full potential drop occurs over the whole lm length. We stress, however, that, since at least part of the sample is immersed into the electrolyte, most of the current measured during cyclic voltammetry originates from the part of the sample in contact with the bulk solution. The current passing through the thin lm is limited by ion diffusion, and therefore could be signicantly lower than the total current. In other measurements we found that, as the current in cyclic voltammetry approaches 10 mA, one observes potential drops across the thin lm length. This journal is © The Royal Society of Chemistry 2015
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Frank Renner said: You described the thin liquid lm as very stable. Can you also pull the specimen surface out of the solution and study the subsequent changes of the dry surface versus the wetted one?
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Hendrik Bluhm replied: Yes, that is possible and would simulate a more commonly used ex-situ characterization of an electrochemically-treated sample, provided that the liquid reservoir is also removed from the vacuum chamber to provide a vacuum environment. Tom Majchrowski stated: In Figure 7 of your paper, XP spectra, there is a signicant shi in the position of the O 1s signal under humid conditions, however there is no shi in the position of the corresponding Ni 2p3/2 signal, does that suggest that Ni is not oxidised and oxygen reacts with C to form carbonate species? Hendrik Bluhm replied: The apparent shi in the O 1s signal when going from dry to humid conditions is due to the appearance of the adsorbed water peak at around 534 eV and the gas phase water peak at 536 eV. The Ni 2p signal is dominated by the metallic component due to the thin oxide/hydroxide layer at the Ni surface, combined with the reduced surface sensitivity when using high kinetic energy photoelectrons. There is no evidence for an increase in the carbonate species coverage under humid conditions, based on the C 1s spectrum, where carbonates would appear at a binding energy of ~290 eV. Gregory Hunt asked: Have you considered how this method could be applied to non-aqueous systems? What would be the challenges that need overcoming to do this with an oil and an organic matrix? Hendrik Bluhm responded: This method is applicable to various types of liquid/solid interfaces and is not limited to aqueous solutions. The challenge that needs to be overcome is the preparation of a sufficiently thin and stable liquid lm at the solid/liquid interface. In fact, systems like those you mention would be more amenable, in the sense that their vapor pressures are lower, such that the entrance aperture to the ambient pressure photoelectron spectrometer could be somewhat larger to increase intensities. Rob Lindsay opened a general discussion of the paper by Johannes L¨ utzenkirchen: Inspecting the AFM images in Fig. 8 of your paper, I was somewhat surprised by the apparent quality of your diffraction data, i.e. small error bars on structure factors. Given the small apparent terrace sizes, I would have expected that it would have been difficult to obtain signicant intensity away from Bragg peaks. Could you comment on the width of surface reections in the region of an anti-Bragg position, and perhaps show examples of reections from the datasets acquired. Frank Heberling and Johannes L¨ utzenkirchen replied: The total height scale of the relevant AFM image (Fig. 8 upper image, the height scale is depicted on the right) is 570 pm. The surface heterogeneity is obvious, but the 10 nm to 20 nm wide islands (by which the surface is covered) have a height of no more than about ˚ Therefore, even though it is obvious that the surface is showing a distinct 2 A. heterogeneity, it may be considered extremely at. To show this more clearly we 88 | Faraday Discuss., 2015, 180, 81–96
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Fig. 2 3D projection of the AFM data presented in the upper image in Fig. 8 of our paper. Despite the impression that might be generated by the strong contrast chosen in Fig. 8, this projection shows that the surface is actually very flat. In this projection the roughness would hardly be visible if it was not highlighted by the color scheme.
have generated a new representation of the AFM data of the fresh hematite sample in air, which is shown in Fig. 2. The different colors in Fig. 1 correspond roughly to the expected terrace heights on hematite (001). This indicates that the roughness of the sample corresponds quite nicely with the concept of the “b model” used to account for roughness during CTR modelling. Fig. 3 presents a detail of an image measured by the PILATUS pixel array detector at the 1, 1, L CTR at L ¼ 7.53. The shape of the X-ray reection is largely determined by the beam footprint on the sample surface. The minor reections at the lower le, and the two fold splitting of the main reection, are due to the fact that the hematite crystal is not a perfect single crystal as oen observed for oxide minerals. Geoffrey Thornton commented: The main difference between CTR's obtained from the as-prepared substrate and that exposed to solution is the specular rod, i.e. 0,0,L. This is very sensitive to roughness. Have you analysed the data without including the specular rod to test for consistency. Frank Heberling and Johannes L¨ utzenkirchen responded: No, we have not performed such an analysis. However, according to the “b” model1 which is widely
Fig. 3 X-ray reflection at the Anti-Bragg position at L ¼ 7.53 on the 1, 1, L CTR measured at the fresh hematite sample. This journal is © The Royal Society of Chemistry 2015
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used to describe the effect of roughness on CTR-data, roughness affects specular and off-specular CTRs equally, i.e. it decreases the scattered intensity in the antiBragg regions, between the Bragg peaks. Comparing the specular CTR-data obtained with our two samples (Fig. 4) it is obvious that there is no systematic difference between the two datasets concerning the scattered intensities in the anti-Bragg regions, i.e. around L ¼ 3, 9, or 15. Therefore, we may conclude that differences in the data must be related to changes in the structure and not to changes in roughness. These differences are certainly most pronounced on the specular data, but they do occur throughout the whole data set. Other CTRs with obvious signicant differences are for example the negative side of the 21L CTR and the positive side of the 20L CTR. From the CTR data analysis we obtain b values of 0.17 and 0.18 for the fresh and aged hematite samples, respectively. These values are relatively small, compared to previous studies on the hematite (001) surface. Thus Tanwar et al.2 ˚ obtained b ¼ 0.18 – 0.23 and Catalano3 b ¼ 0.23 (reported as s(rms) ¼ 1.4 A). For the fresh hematite sample we had the possibility to compare the roughness obtained from the CTR analysis with the roughness measured by high-resolution ˚ and AFM and found an extraordinarily good agreement, i.e. s(rms, CTR) ¼ 1.1 A ˚ s(rms, AFM) ¼ 1.2 A. One should not be misled by a rst impression of the highresolution AFM image, which appears quite heterogeneous. The total height range of the image at comparable conditions to the CTR measurements covers no ˚ i.e. the surface is heterogeneous, but still very at. more than 6 A, In summary roughness should not be a major issue in the presented CTR study, and we see no reason for a special treatment of the specular CTR or for its exclusion from the analysis. 1 I. K. Robinson, Phys. Rev. B, 1986, 33, 3830–3836. 2 K. S. Tanwar, S. C. Petitto, S. K. Ghose, P. J. Eng and T. P. Trainor, Geochim. Cosmochim. Acta, 2009, 73, 4346–4365. 3 J. G. Catalano, Geochim. Cosmochim. Acta, 2011, 75, 2062–2071.
Fig. 4 Direct comparison of specular CTR data measured on the fresh (circles) and aged (diamonds) hematite (001) samples.
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Kirsi Yliniemi commented: When modelling the surface potentials of hematite samples (for example in Fig. 7 in your paper) have you taken into account the interaction term between the neighbouring surface sites? If not, could you comment on how much the interaction between the neighbouring sites may inuence the protonation/de-protonation behaviour. Johannes L¨ utzenkirchen and Frank Heberling answered: The models used in our paper do not consider next-neighbor interaction. Coupling between protonation and deprotonation reactions on the surface sites within the model occurs via mean-eld electrostatic terms. Ising models consider those kinds of next-neighbor effects which you refer to and have been used by Borkovec and Koper to describe charging of polyelectrolytes.1 Borkovec has also shown for oxides, that the mean-eld models we are using are equivalent to the Ising models.2 The relation between the two approaches involves some interesting aspects. For one thing, unlike polyelectrolytes, it is almost impossible to completely protonate or deprotonate an oxide surface within the normal pH range although some publications seem to show saturation, i.e. a plateau which has been interpreted as site density, e.g. for goethite.3 Subsequent work has shown that the saturation data are doubtful.4 The maximum degree of protonation/deprotonation is expected to be much smaller on oxides than on polyelectrolytes (like polyacrylic acid, which can be completely deprotonated). From this point of view next-neighbor effects on oxides might be expected to be less pronounced than on polyelectrolytes. Also, when dealing with next neighbors the situation is likely to become highly complex on oxides which is shown in Fig. 5 for the (110) face of goethite. On this plane various sites occur. They have different proton affinities. These can be predicted within the MUSIC model framework.5 The sites have a given structural arrangement, which requires in an Ising model the appropriate interaction parameters. The MUSIC approach probably requires a smaller number of parameters. Fig. 6 shows the individual contributions of the different sites to the overall charging of this plane based on the MUSIC model.
Fig. 5
Surface structure of the ideal (110) face of goethite.
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Fig. 6
Contributions of the different sites to the overall charging of goethite (110) as a function of the pH (here we plot the logarithm of the proton concentration, which for a given ionic strength is proportional to the pH).
The overall point of zero charge occurs at pH 9.2. The pH-dependent charge mainly arises from the reaction of the singly co-ordinated group. As in our paper the doubly co-ordinated groups within this model do not show signicant charging within the normal pH range. The triply co-ordinated groups have a constant charge which hardly changes with pH in the pH range of interest. Actually the model can be further complicated by involving two types of triply coordinated groups (one with high and one with low proton affinity) resulting in a structurally even more complex situation. The model can also be simplied by excluding the doubly co-ordinated groups. The inuence of the different sites on the overall protonation is best discussed by considering the singly co-ordinated sites. Their intrinsic pKa value is 7.7. The conditional pKa is between 3 and 4, which means a shi of about 4 pH units caused by the triply co-ordinated sites. In other words here the interaction among the different sites occurs via the model inherent electrostatic term(s). It would probably be possible to design an Ising model for this surface as well. As pointed out before one would need to consider the interactions between the different sites under the given structural constraints. Addition of site-interaction parameters to the present model would increase the number of parameters. However, with the present number of adjustables within the bare MUSIC model we already obtain a good t to the data. Following the principle of parsimony, we try to keep the model to a minimum of adjustable parameters. The next-neighbour issue and its inuence on pKa values of surface groups has been raised previously by Bickmore in a sense.6 He has heavily criticized the MUSIC model (which we apply in our paper) for neglecting close range effects. His argument was that the close-range effects would affect next-neighbor pK values. Since bond distances on a given site would be affected due to protonation or deprotonation of adjacent sites and because it is known that the MUSIC model is highly sensitive to bond-distances7 such effects would be substantial. From our point of view again such close-range effects are covered in the common model by the smeared-out charge and the resulting electrostatic factors supplemented by the restricted degree of protonation/deprotonation. Whether the close-range features should be combined with larger scale features is an interesting question. 92 | Faraday Discuss., 2015, 180, 81–96
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The electrostatic factors are sometimes also interpreted as the ratios of activity coefficients of the surface species involved in a given reaction. Overall, many aspects of the surface reactions could be improved based on detailed knowledge, but it usually involves new parameters that have to be adjusted or assumed. As pointed out previously we apply the principle of parsimony. 1 M. Borkovec and G. J. M. Koper, J. Phys. Chem., 1994, 98, 6038–6045. 2 M. Borkovec, Langmuir, 1997, 13, 2608–2613. 3 L. Lovgren, S. Sjoberg and P. W. Schindler, Geochim. Cosmochim. Acta, 1990, 54, 1301–1306. 4 J. L¨ utzenkirchen, J.-F. Boily, L. Lovgren and S. Sjoberg, Geochim. Cosmochim. Acta, 2002, 66, 3389–3396. 5 W. H. v. Riemsdijk and T. Hiemstra, in Interface Science and Technology, ed. L. Johannes, Elsevier, 2006, vol. 11, pp. 251–268. 6 B. R. Bickmore, K. M. Rosso and S. C. Mitchell, in Interface Science and Technology, ed. L. Johannes, Elsevier, 2006, vol. 11, pp. 269–283. 7 J.-F. Boily, J. L¨ utzenkirchen, O. Balmes, J. Beattie and S. Sjoberg, Colloid Surf. A-Physicochem. Eng. Asp., 2001, 179, 11–27.
Rob Lindsay said: Regarding your approach to tting the SXRD data, I was wondering if you had tried to model the experimental data by adopting partial site occupancies? Frank Heberling and Johannes L¨ utzenkirchen responded: As described in the paper, a preliminary approach to t the data included one domain and partial occupancy of the top most iron site. The two domain model is to some extent equivalent to such an approach. The difference is that the consideration of two distinct surface domains allows us to address different local structural relaxations in the two domains. Thus regions where the topmost iron site is occupied are modeled separately from regions where the site is unoccupied. These additional degrees of freedom in the model turned out to be crucial to obtain a satisfactory t to the data. Philippe Marcus said: Oxides are extremely important in the eld of corrosion chemistry because several metals and alloys are protected against corrosion by a thin (or ultra-thin) oxide layer. However thin oxide layers on metals may differ in many ways from bulk oxides that are not supported by metals (for example the work function can be different). According to you, can we take the values of the zeta-potential for bulk oxides and use them for oxide lms on metals? Johannes L¨ utzenkirchen and Frank Heberling replied: This is an intriguing question. We believe there are several aspects to this question. One concerns how the chemistry of the oxide lm on a metal relates to the surface of a bulk oxide. If one tends to believe that the reactivity is quite similar for both, the difficult part will be to choose the correct data from the literature. The major part of the available charging data is for oxide-particles. Data for such oxide particles can vary a lot (i.e. IEPs can cover ranges of several pH units). So the question would be which data to choose. The answer would be largely arbitrary. A second more interesting aspect concerns the actual structure of the oxide lm on a metal. If it can be characterized in terms of bond distances and nature
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and number of surface functional groups, then the available models1 can in principle predict at least the point of zero charge and maybe more than that. This would involve the assumption that the supporting metal does not affect the solid– liquid interface additionally (i.e. beyond constraining bond distances in the lm). If bond distances in the oxide lm (as affected by the underlying metal) are different from a nominally identical bulk oxide surface, the charging properties between the two samples will probably differ. The proton affinity constants are actually very sensitive to the bond-distances.2 A nal aspect has been discussed by McCafferty.3 According to him the relation between IEP and the potential of zero charge is quite complex. In the models as we use them and for the systems we are interested in we are not considering space charge issues. Experience has shown that one should measure the points of zero charge and the zeta-potentials for unknown samples or samples that are not sufficiently characterized for application of MUSIC type models or where any other uncertainties exist under the conditions of interest. This is also true for bulk oxide samples. 1 W. H. v. Riemsdijk and T. Hiemstra, in Interface Science and Technology, ed. L. Johannes, Elsevier, 2006, vol. 11, pp. 251–268. 2 J.-F. Boily, J. L¨ utzenkirchen, O. Balmes, J. Beattie and S. Sjoberg, Colloid Surf. A-Physicochem. Eng. Asp., 2001, 179, 11–27. 3 E. McCafferty, Electrochim. Acta, 2010, 55, 1630–1637.
David Williams asked: Could you describe the surface changes as a slow, hydration-driven surface reconstruction that is spatially heterogeneous (and thus perhaps also susceptible to spatially heterogeneous surface contamination)? Johannes L¨ utzenkirchen and Frank Heberling replied: The question about the process behind the changes is crucial. Unfortunately we know very little about the mechanisms. We are condent that surface contamination does not play a major role because the cleaning procedures we apply have been previously elaborated very carefully for single crystals.1 Cleaning substrates in this way also led to simultaneous observation of the very same features on the companion surface sapphire (001) which we report here for hematite (001). On sapphire (001) we observed them on one kind of sample, i.e. at surface potential curve and steep zeta potential feature. What makes us condent is that those two features had been independently observed in a substantial number of studies.2 Furthermore, the cleaning procedure we use has been applied by others in a paper that came out simultaneously as ours3 and in subsequent work.4,5 In particular these latter studies involve highly surface specic techniques that would detect minor amounts of adventitious carbon compounds. As for the major difference in the surface properties with the changes over time, the outcome namely just a change in the extent of hydration was certainly disappointing. A simple and far more appealing cause would have been the change from a surface that is initially oxygen terminated (for which we can explain the low isoelectric point, IEP) to one that involves singly co-ordinated groups (i.e. which has the bi-domain properties) and allows a straightforward explanation for the high IEP.
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It would also agree with independent work, where we have found that the presence of dissolved aluminum can increase the IEP of sapphire (001)6 to the same extent as we report here hematite (001) on simple aging. In that paper6 we also cite previous work on hematite (001) where the formation of adatoms had been reported. So there is some consistency there. We are also currently re-evaluating the CTR data with new soware in the hope of obtaining more satisfactory relationships between the surface structure and the charging. We were very happy to see the high IEP appearing on the aged sample, because in the previous work on single crystals it has been very rare that IEPs of bare oxide surfaces above say pH 5 were measured (either by streaming potential or in force distance measurements). Even on aged steel samples usually low IEPs were obtained,7,8 while one would tend to expect the IEP of iron oxides (particles), which are usually around pH 9. We agree that the study of the actual transformation would be highly interesting. Furthermore, we believe that future studies should involve a number of different techniques over the experimentation period. Such multi-technique studies have the potential to shed light on the processes. 1 T. Rabung, D. Schild, H. Geckeis, R. Klenze and T. Fanghanel, J. Phys. Chem. B, 2004, 108, 17160–17165. 2 J. L¨ utzenkirchen, R. Zimmermann, T. Preocanin, A. Filby, T. Kupcik, D. Kuttner, A. Abdelmonem, D. Schild, T. Rabung, M. Plaschke, F. Brandenstein, C. Werner and H. Geckeis, Adv. Colloid Interface Sci., 2010, 157, 61–74. 3 M. M. Gentleman and J. A. Ruud, Langmuir, 2010, 26, 1408–1411. 4 E. Anim-Danso, Y. Zhang, A. Alizadeh and A. Dhinojwala, J. Am. Chem. Soc., 2013, 135, 2734–2740. 5 E. Anim-Danso, Y. Zhang and A. Dhinojwala, J. Am. Chem. Soc., 2013, 135, 8496–8499. 6 J. L¨ utzenkirchen, A. Abdelmonem, R. Weerasooriya, F. Heberling, V. Metz and R. Marsac, Geochem. Trans., 2014, 15, 9. 7 N. Kallay, D. Kovacevic, I. Dedic and V. Tomasic, Corrosion, 1994, 50, 598–602. 8 B. M. Cabanas, J. L¨ utzenkirchen, S. Leclercq, P. Barboux and G. Lefevre, J. Nucl. Mater., 2012, 430, 150–155.
Hadeel Hussain asked: In your paper it was not mentioned which model you have used to simulate surface roughness.I assume it is the beta roughness model? Have you tried using different models, as I’m sure you know, it is a relatively simple model and so may not accurately represent the rough surface you have as evident from the AFM images. Frank Heberling and Johannes L¨ utzenkirchen replied: In the electronic supplementary information (ESI) we mention that we use the beta model to account for the inuence of roughness on the model. We did not test other models. As already mentioned in the previous discussion for the fresh hematite sample we were able to compare the roughness derived from the beta model (b ¼ ˚ to the rms roughness measured by AFM, which 0.17 is equivalent to srms ¼ 1.1 A) ˚ Such agreement, and the type of roughness as mentioned in the is srms ¼ 1.2 A. answer to the earlier question from Robert Lindsay, supports the applicability of the beta model to our data. Concerning the “rough surface as evident from the AFM image”, as already mentioned in the previously, we think it is important to point out that the total height scale of the relevant AFM image (as depicted on the right of Fig. 8 in our paper) is 570 pm. The surface heterogeneity is obvious, but the 10 nm to 20 nm wide islands (by which the surface is covered) have a height of This journal is © The Royal Society of Chemistry 2015
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˚ Therefore, even though it is obvious that the surface is showing a only about 2 A. distinct heterogeneity, it may be considered as extremely at.
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Hadeel Hussain queried: To me it seems your c2 is quite high (usually with CTR data an acceptable model has a c2 less than 2 and in many cases less than 1.5) and could possibly be due to the error bars you have on your CTR data. How have you calculated these? Frank Heberling and Johannes L¨ utzenkirchen replied: For a model renement on a full set of CTRs including specular and off-specular data, and considering the constraints on the degrees of freedom within the model due to symmetry constraints at hematite (001), it is our notion that a c2 < 4 is an acceptable t. We do admit that there is some space for improvement of our structural models. As pointed out previously we are currently re-evaluating the data for other reasons. As for the use of the goodness-of-t parameter, we prefer not to put too much weight on absolute values of c2. In our opinion c2 is mainly useful to compare various ts to one specic dataset. Discussing the goodness-of-t parameter concerns, however, an important and critical issue regarding the development of structural models based on CTR data. The determination of the error bars on the structure factor amplitudes have a direct impact on c2. Therefore the uncertainties associated with structure factor amplitudes are a key issue when deciding how many degrees of freedom should be allowed in a structural model. Our standard approach to estimate error bars on CTR data is based on counting statistics. As such the error of the integrated intensity, Ierr, is estimated as the square root of the total intensity, Itot, plus the background intensity, Ibgr: Ierr ¼ (Itot + Ibgr)0.5. To calculate structure factor amplitudes, |F|, the integrated intensity, I, is multiplied by a correction factor, c, that depends on geometry and normalized by the beam intensity, I0. Subsequently the square root of the resulting expression is taken. This leads to the following equation for the error, Ferr, associated with |F|: Ferr ¼ 1/2 (c/I0 Ierr/I)0.5. For the two datasets presented in the manuscript we were in the comfortable situation that several symmetry equivalent CTRs were available. By comparing these equivalent CTRs we can calculate standard deviations for the structure factors and compare them to error estimates based on counting statistics. Doing so we found excellent agreement. For the fresh hematite sample the average uncertainty based on counting statistics is 19% and the average standard deviation based on symmetry equivalent CTRs is 17%. For the aged sample the agreement is even better and we found 18% average uncertainty with both methods. Consequently, for the datasets presented here we are condent that the errors associated with the structure factors are reasonable.
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DISCUSSIONS
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Solid/fluid interface: general discussion Janine Mauzeroll, Geoffrey Thornton, Trevor Rayment, Vincent Maurice, David Williams, Frank Heberling, Philippe Marcus, Clara Wren, Kirsi Yliniemi, Rob Lindsay, Stephen Lyth, Tom Majchrowski, Hadeel Hussain, Gregory Hunt, Frank Renner, Geraint Williams, Roger Newman, Gerald Frankel, Johannes Lu ¨ tzenkirchen and Hendrik Bluhm
DOI: 10.1039/c5fd90044a
Stephen Lyth opened a general discussion of the paper by G. Frankel by asking: The corrosion chemistry community has made interesting insights into the role of hydrogen evolution as a degradation mechanism in metals. Can these ndings be turned around to design more efficient catalysts for electrochemical hydrogen production? Gerald Frankel replied: Thank you for this interesting question. There is no doubt that the combination of a Mg anode and any other cathode forms a potent hydrogen production engine. Mg alloys such as Mg–Fe generate even more H2 than pure Mg.1 However, a catalyst should be a non-consumable material. The energy that goes into making Mg or another active metal like Al is lost during the process, so it is an expensive way to create hydrogen. I doubt that understanding of the situation at dissolving metal anodes can be translated into non-consumable catalysts. 1 A. Samaniego, N. Birbilis, X. Xia and G. S. Frankel, Hydrogen Evolution During Anodic Polarization of Mg Alloyed with Li, Ca, or Fe, Corrosion, 2015, 71, 224–233.
Roger Newman asked: Regarding the enhanced evolution of hydrogen during anodic polarization of magnesium, the relevance, if any, of the chloride ion was not mentioned. This seems to be universally present in the testing solutions. One can imagine that a Mg atom with an adsorbed anion might be more able to manifest its high reactivity, including for hydrogen evolution, than if it has an adsorbed OH. Does the effect exist over a wide range of anions? If it does, then indeed the anion adsorption idea would not be correct and the most likely explanation is simply that a fast-dissolving metal has more (albeit momentarily) bare metal sites which are the sites of hydrogen evolution (I believe this was suggested by Professor Williams). As a footnote to the latter suggestion, this is somewhat related to a concept for activation of aluminum by low melting point (LMP) alloying elements that I
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developed with a student in the 90s. There too we appealed to the momentary presence of bare Al sites, which in that case were occupied by diffusing LMP.
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1 F. Sato and R. C. Newman, Mechanism of Activation of Aluminum by Low Melting Point Elements: Part 1 – Effect of Zinc on Activation of Aluminum in Metastable Pitting, Corrosion, 1998, 54, 955–963. 2 F. Sato and R. C. Newman, Mechanism of Activation of Aluminum by Low-Melting Point Elements: Part 2 – Effect of Zinc on Activation of Aluminum in Pitting Corrosion, Corrosion, 1999, 55, 3–9.
Gerald Frankel responded: In a recent paper, anodic HE was collected on HP Mg in chloride-free buffer solutions over a range of pH values.1 Citrate (pH 3), borate (pH 7) and carbonate (pH 10.5) solutions were used. Anodic HE was observed in all of the solutions. However, in the citrate solution the HE rate increased only at higher applied current densities when the buffer capacity was seemingly overwhelmed by local pH changes. The bottom line is that chloride is not needed, but pH change is important. Please see my response to Prof. Williams regarding the issues associated with explaining the effect solely by an increase in the number of active sites. 1 L. Rossrucker, A. Samaniego, J.-P. Grote, A. M. Mingers, C. A. Laska, N. Birbilis, G. S. Frankel and K. J. J. Mayrhofer, The pH Dependence of Magnesium Dissolution and Hydrogen Evolution During Anodic Polarization, J. Electrochem. Soc., 2015, 162 C333– C339.
David Williams asked: Is it possible that, under the extreme dissolution conditions prevailing (which could be considered as kink sites moving extremely rapidly across the surface) that Al atoms at the kink sites could react with water either to form the aluminium hydroxide anodically or to split water to form hydrogen and aluminium hydroxide in a chemical process – in which case there would not be any current owing through the external circuit. The balance between the two reactions could depend on the surface density and orientation of water molecules. Gerald Frankel answered: This is an interesting notion that might be rephrased as follows: Assuming that Mg or Al dissolution occurs at active sites on the surface such as kink sites, it is possible that the number of active sites per unit exposed area increases as the dissolution rate increases, and these active sites can also reduce water. The result would be more water reduction with more dissolution. This perspective describes how the active sites for both the anodic dissolution and hydrogen evolution reactions would be linked. However, as the applied anodic current or potential increases, the rate of the HER should decrease exponentially. Therefore, an explanation would still be needed to explain why the hydrogen evolution rate remains about the same percentage of the applied anodic current. One would have to assume that the rate of HER on these sites is independent of potential. Janine Mauzeroll asked: Would it not be a good idea to also consider the complex equilibria that occurs in the magnesium system to account for some of the negative difference effect observed?
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Gerald Frankel answered: I suppose you are referring to solution equilibria in the electrolyte near the dissolving surface. It is a complex situation owing to the rapid dissolution and the copious hydrogen evolution. It is generally understood that the pH will increase because of weak hydrolysis and strong water reduction. However, there is evidence of initial pH decrease because of a short dominance of hydrolysis.1 And pH changes seem to be critical for the anodic HE effect on Mg. So, yes we need to understand details of both the surface and the near solution phase.
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1 L. Rossrucker, A. Samaniego, J.-P. Grote, A. M. Mingers, C. A. Laska, N. Birbilis, G. S. Frankel and K. J. J. Mayrhofer, J. Electrochem. Soc., 2015, 162, C333.
Geraint Williams queried: Are there any local effects observed during the anodic polarisation of the Mg specimens, especially at high imposed current density values? For example, is the entirety of the “black” surface strongly evolving hydrogen, or are HE sites limited to a few regions on the exposed Mg. If the latter best represents observed behaviour, is there evidence that the number of sites strongly evolving hydrogen increases with potential or applied current density? Maybe the increase in “anodic” HE with anodic current density is an area effect, where the rise in current density progressively produces more regions of “nascent”, totally lm-free Mg which are the principal sites of both anodic dissolution and cathodic HE? Gerald Frankel answered: Thanks for your question. This is a good point as simple optical observation should be able to help sort out the location of the HER. Unfortunately, this location is difficult to assess, particularly at high applied anodic currents when the HE is also large. Because of the copious HE at the surface, the entire surface is masked so it is impossible to determine the exact origin of the bubbles. This was especially difficult in our case because the sample surface was vertically oriented to allow for easy viewing of the surface through the
Fig. 1
Image of the surface during polarization at 25 mA cm2
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cell wall. The accompanying gure (Fig. 1) shows an image of the surface during polarization at 25 mA cm2. Images of the surfaces of electrodes aer passage of a xed charge but at different current densities exhibit increased coverage of dark corrosion product with increasing current density. This might indicate the important role of the dark regions. However, the variations are not orders of magnitude as are the increases in HE rate. Regarding the increase in active sites, please see my response to David Williams. Clara Wren opened a general discussion of the papers by Hendrik Bluhm: What happens to the electrons ejected from the solid? The energy of the electrons would be the energy of the photoelectrons minus the binding energy and work function and, hence, relatively low, on the order of ~100 eV. This is not sufficient to penetrate the adsorbed water layer but is sufficient to ionize water molecules close to the metal or metal hydroxide surface. Could the ionization by the secondary electrons affect the water chemistry? Hendrik Bluhm answered: The kinetic energy of the photoelectrons is the energy of the incident X-rays, minus the electron binding energy and work function of the electron analyzer. The incident photon energy in these experiments is 3100 eV, which leads to photoelectron kinetic energies of ~2570 eV (O 1s), ~2240 eV (Ni 2p), and 2820 eV (C 1s), sufficient to penetrate solution layers with thicknesses in the 10 nm range (see Table 1 in our paper). The inuence of secondary electrons on the water chemistry has to be evaluated for each experiment to avoid artifacts in the experiments. This can be done by monitoring the spectral features as a function of sample irradiation and time. We have no indication of secondary electron-induced chemistry in the present experiments. Geoffrey Thornton asked: In the paper by Springell et al.1 it is claimed that Xrays of energy 17 keV cause radiolysis of water. Do you have any evidence one way or another for radiolysis at the lower energies that you employ? 1 Springell et al., Faraday Discuss., 2015, DOI:10.1039/c4fd00254g.
Hendrik Bluhm answered: We do not have evidence for X-ray induced radiolysis in the present experiments and neither in past experiments carried out resonantly at the O K-edge (incident photon energy ~530 eV) of bulk ice samples,1 where radiolysis would manifest itself in the appearance of a OH-related peak in oxygen K-edge NEXAFS data, which was not observed. 1 D. E. Starr, D. Pan, J. T. Newberg, M. Ammann, E. G. Wang, A. Michaelidis, H. Bluhm, Phys. Chem. Chem. Phys., 2011, 13, 19988.
Philippe Marcus said: I have two questions: (1) Looking at your Ni 2p spectra, it appears that the nickel surface is already oxidized prior to exposure to water. Do you have a way to prepare clean surfaces? (2) A problem encountered with ultrathin water layer is the dissolution of the metal, leading to saturation and redeposition on the surface. Is this an issue in your experiments?
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Hendrik Bluhm answered: (1) Upon introduction into the measurement chamber, a Ni sample will always be oxidized since it was exposed to air during transfer into the measurement chamber. Specialized experimental set-ups with in situ Ni lm preparation and introduction into the measurement chamber without exposure to air or water vapor before the start of the experiments are conceivable. The oxide can also be reduced electrochemically inside the solution. Another option for the preparation of a clean Ni lm could be the direct (galvanic) deposition of Ni onto the multilayer substrate. (2) To the second question: We did not see any evidence for dissolution of the Ni sample in these experiments. The solubility is very low under basic conditions, e.g. for NiO of the order of 107 mole L1 (at pH ¼ 13), with dissolution rates of 1012 to 1015 moles/(cm2 s).1 The solubility of NiOH2 in neutral conditions is in the range of 105 moles L1 and will be even smaller at pH ¼ 13.2 1 P. R. Tremaine and J. C. LeBlanc , J. Chem. Thermodynamics, 1980, 12, 521. 2 W. M. Haynes (ed), Physical Constants of Chemical Compounds, in CRC Handbook of Chemistry and Physics, 95th Edn, 2014.
Vincent Maurice asked: The thickness of 27 nm found for the hydroxide/oxyhydroxide layer appears quite large. This is about one order of magnitude larger than for the passive lm produced in similar conditions of passivation at pH 13 (from ~1.5 to ~4.5 nm with potential).1,2 How would you explain such a difference? 1 H.-W. Hoppe and H.-H. Strehblow, Surf. Interface Anal., 1989, 14, 121. 2 A. Seyeux et al., J. Solid State Electrochem., 2005, 9, 337–346.
Hendrik Bluhm replied: We believe that the complete oxidation of the Ni lm in the present case as compared to partial oxidation in the case of a passive lm at the same pH is due to the fact that the present measurements are performed under active oxidation conditions, i.e. at a sustained potential of +0.6 V. Geoffrey Thornton asked: The signal-to-noise level of the standing wave data are rather limited. What are the prospects for signicant improvement based on instrument development? Hendrik Bluhm replied: The main causes for the signal attenuation in these measurements are the scattering of electrons by the liquid lm and the gas phase surrounding the sample. These scattering processes depend on the given experimental conditions; for instance, for measurements at lower background vapor pressures and thinner liquid lms the signal-to-noise ratio is greatly improved, while in the present case, for thicker liquid lms at a RH close to 100% at room temperature scattering is stronger. The signal-to-noise in the measurement can be improved through either a better detection efficiency of the electron spectrometer or the use of incident X-rays with higher brilliance, such as will become available in future ultimate storage ring sources. In the experiments presented here we used a state-of-the-art ambient pressure photoelectron spectrometer with a theoretical maximum electron collection angle of 22 . The greatest room for improvement lies in a stronger horizontal focusing of the incident X-ray beam. Due to the small entrance aperture of the differentially-pumped photoelectron spectrometer, and shallow incident angle in standing wave experiments, the illuminated area of the This journal is © The Royal Society of Chemistry 2015
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sample is in general much larger than the detected area in SW-APXPS (here by a factor of ~8). Thus, improved horizontal focusing will lead to a higher signal-tonoise-ratio in the experiments.
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Trevor Rayment enquired: Does the presence of an organic contaminant layer at the surface of the electrolyte have any effect upon the wetting properties of the electrolyte since this might affect the thickness of the wetting layer which is a critical component of the system? Hendrik Bluhm replied: The presence of the organic contaminant layer at the liquid/vapor interface may have an inuence on the condensation/evaporation rates of the liquid,1 or on the concentration of ions at the liquid/vapor interface.2 We do not expect a direct effect of the contamination layer at the liquid/vapor interface on the wetting properties of the liquid/solid interface since the substrate surface is electrochemically cleaned inside the bulk solution before the sample is pulled out of the solution (always staying in contact with the bulk solution in the beaker). 1 J. F. Davies, R. E. H. Miles, A. E. Haddrell and J. P. Reid, PNAS, 2013, 110, 8807. 2 M. J. Krisch, R. D’Auria, M. A. Brown, D. J. Tobias, J. C. Hemminger, M. Ammann, D. E. Starr and H. Bluhm, J. Phys. Chem. C, 2007, 111, 13497.
Trevor Rayment remarked: It would be helpful to have more information about the opportunities and limitations which result from the use of a standing wave method for the use of ambient pressure XPS in the study of corrosion. For example how does the method constrain the thickness and smoothness of the electrode? Hendrik Bluhm replied: The roughness of the electrode interfaces should not be more than a fraction of the standing wave period for an accurate characterization of the liquid/solid interface properties. However, the period of the multilayer can be increased beyond the few nm in this study so as to look at greater roughness. The total thickness of the electrode is limited by the effective attenuation of the X-ray standing wave eld by the electrode material, as the Xrays must penetrate signicantly into the substrate multilayer mirror; e.g. an upper value for the case of Ni and incident X-rays with an energy of 3.1 keV is about 100 nm at an incident angle of 3.5 . Frank Heberling commented: The sample surface investigated in your study, deposited as a thin lm on a multilayer mirror, appears very special. What kinds of surface can be or have already been investigated by the standing-wave in situ XPS method you describe in your paper? Would it be feasible to investigate the surface speciation on single crystal samples if a total external reection standing wave (TERXSW) eld would be used for the excitation of the photoelectrons? Hendrik Bluhm answered: So far only measurements on Ni (present case) and iron oxide lms1 have been performed using SW-APXPS, but we expect that the method is applicable to a wide range of substrate types due to the progress that
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has been made in the preparation of a wide variety of thin lm systems using, e.g., PLD or MBE. Near-total-reection (NTR equivalent to TERXSW measurements have recently been demonstrated for solid-solid oxide interfaces.2 However, measurements with harder X-rays may be difficult due to the very large beam spot along the beam propagation direction for the extremely shallow angles of incidence needed in these experiments. This is due to the very small effective sample area imposed by the entrance aperture of the ambient pressure photoelectron spectrometer. At so X-ray energies, this should be less of a problem, however.
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1 S. Nemˇs´ ak, A. Shavorskiy, O. Karslioglu, I. Zegkinoglou, P. K. Greene, E. C. Burks, K. Liu, A. Rattanachata, C. S. Conlon, A. Keqi, F. Salmassi, E. M. Gullikson, S.-H. Yang, H. Bluhm and C. S. Fadley, Nat. Commun. 2014, 5, 5441. 2 M. Marinova, et al., Nano Lett., 2015, 15(4), 2533.
Clara Wren asked: Could you specify conditions in the solution? Was it aerated? What was the reason for the 0.1 M KOH? Nickel could irreversibly oxidize in an aerated basic solution and cathodic cleaning may not be able to reduce the resultant nickel oxide/hydroxide back to the metallic state. A surface of nickel oxide immersed in water is always covered with a nickel hydroxide. How do you ensure that the hydroxide being monitored is the real-time layer as claimed? Hendrik Bluhm replied: The solution was degassed before use through evacuation in a separate vacuum chamber. Our past experiments have shown that hydroxide solutions are less prone to beam-induced changes (compared to solutions containing, e.g., halide anions), hence the choice of KOH. All experiments in this study were performed at a potential of +0.6 eV, due to the limited amount of beamtime in this proof-of-principle study. We will explore the reduction of NiOx under cathodic conditions in a next set of experiments that will help to answer these questions. Roger Newman asked: How low does the current in the electrochemical cell have to be in order for adequate potential control to be maintained in the thin liquid layer? It seems that a fairly easy calculation would give the answer, which I imagine will be very low indeed. In that case the method would only seem to work for reactions that have gone to completion, like UPD. For growth of an oxide lm there will be a residual current. Hendrik Bluhm replied: The potentials measured by XPS at a location of the thin lm ~2 cm above the bulk liquid surface in the beaker are in agreement with the applied potentials to the bulk liquid, i.e. the currents along the thin liquid lm are sufficiently low under our experimental conditions. A simple estimate for a 20 nm thick 0.1 M KOH lm with a specic conductance of 20 mS cm1 yields an upper limit for the current of around 20 nA. At this current a full potential drop occurs over the whole lm length. We stress, however, that, since at least part of the sample is immersed into the electrolyte, most of the current measured during cyclic voltammetry originates from the part of the sample in contact with the bulk solution. The current passing through the thin lm is limited by ion diffusion, and therefore could be signicantly lower than the total current. In other measurements we found that, as the current in cyclic voltammetry approaches 10 mA, one observes potential drops across the thin lm length. This journal is © The Royal Society of Chemistry 2015
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Frank Renner said: You described the thin liquid lm as very stable. Can you also pull the specimen surface out of the solution and study the subsequent changes of the dry surface versus the wetted one?
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Hendrik Bluhm replied: Yes, that is possible and would simulate a more commonly used ex-situ characterization of an electrochemically-treated sample, provided that the liquid reservoir is also removed from the vacuum chamber to provide a vacuum environment. Tom Majchrowski stated: In Figure 7 of your paper, XP spectra, there is a signicant shi in the position of the O 1s signal under humid conditions, however there is no shi in the position of the corresponding Ni 2p3/2 signal, does that suggest that Ni is not oxidised and oxygen reacts with C to form carbonate species? Hendrik Bluhm replied: The apparent shi in the O 1s signal when going from dry to humid conditions is due to the appearance of the adsorbed water peak at around 534 eV and the gas phase water peak at 536 eV. The Ni 2p signal is dominated by the metallic component due to the thin oxide/hydroxide layer at the Ni surface, combined with the reduced surface sensitivity when using high kinetic energy photoelectrons. There is no evidence for an increase in the carbonate species coverage under humid conditions, based on the C 1s spectrum, where carbonates would appear at a binding energy of ~290 eV. Gregory Hunt asked: Have you considered how this method could be applied to non-aqueous systems? What would be the challenges that need overcoming to do this with an oil and an organic matrix? Hendrik Bluhm responded: This method is applicable to various types of liquid/solid interfaces and is not limited to aqueous solutions. The challenge that needs to be overcome is the preparation of a sufficiently thin and stable liquid lm at the solid/liquid interface. In fact, systems like those you mention would be more amenable, in the sense that their vapor pressures are lower, such that the entrance aperture to the ambient pressure photoelectron spectrometer could be somewhat larger to increase intensities. Rob Lindsay opened a general discussion of the paper by Johannes L¨ utzenkirchen: Inspecting the AFM images in Fig. 8 of your paper, I was somewhat surprised by the apparent quality of your diffraction data, i.e. small error bars on structure factors. Given the small apparent terrace sizes, I would have expected that it would have been difficult to obtain signicant intensity away from Bragg peaks. Could you comment on the width of surface reections in the region of an anti-Bragg position, and perhaps show examples of reections from the datasets acquired. Frank Heberling and Johannes L¨ utzenkirchen replied: The total height scale of the relevant AFM image (Fig. 8 upper image, the height scale is depicted on the right) is 570 pm. The surface heterogeneity is obvious, but the 10 nm to 20 nm wide islands (by which the surface is covered) have a height of no more than about ˚ Therefore, even though it is obvious that the surface is showing a distinct 2 A. heterogeneity, it may be considered extremely at. To show this more clearly we 88 | Faraday Discuss., 2015, 180, 81–96
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Fig. 2 3D projection of the AFM data presented in the upper image in Fig. 8 of our paper. Despite the impression that might be generated by the strong contrast chosen in Fig. 8, this projection shows that the surface is actually very flat. In this projection the roughness would hardly be visible if it was not highlighted by the color scheme.
have generated a new representation of the AFM data of the fresh hematite sample in air, which is shown in Fig. 2. The different colors in Fig. 1 correspond roughly to the expected terrace heights on hematite (001). This indicates that the roughness of the sample corresponds quite nicely with the concept of the “b model” used to account for roughness during CTR modelling. Fig. 3 presents a detail of an image measured by the PILATUS pixel array detector at the 1, 1, L CTR at L ¼ 7.53. The shape of the X-ray reection is largely determined by the beam footprint on the sample surface. The minor reections at the lower le, and the two fold splitting of the main reection, are due to the fact that the hematite crystal is not a perfect single crystal as oen observed for oxide minerals. Geoffrey Thornton commented: The main difference between CTR's obtained from the as-prepared substrate and that exposed to solution is the specular rod, i.e. 0,0,L. This is very sensitive to roughness. Have you analysed the data without including the specular rod to test for consistency. Frank Heberling and Johannes L¨ utzenkirchen responded: No, we have not performed such an analysis. However, according to the “b” model1 which is widely
Fig. 3 X-ray reflection at the Anti-Bragg position at L ¼ 7.53 on the 1, 1, L CTR measured at the fresh hematite sample. This journal is © The Royal Society of Chemistry 2015
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used to describe the effect of roughness on CTR-data, roughness affects specular and off-specular CTRs equally, i.e. it decreases the scattered intensity in the antiBragg regions, between the Bragg peaks. Comparing the specular CTR-data obtained with our two samples (Fig. 4) it is obvious that there is no systematic difference between the two datasets concerning the scattered intensities in the anti-Bragg regions, i.e. around L ¼ 3, 9, or 15. Therefore, we may conclude that differences in the data must be related to changes in the structure and not to changes in roughness. These differences are certainly most pronounced on the specular data, but they do occur throughout the whole data set. Other CTRs with obvious signicant differences are for example the negative side of the 21L CTR and the positive side of the 20L CTR. From the CTR data analysis we obtain b values of 0.17 and 0.18 for the fresh and aged hematite samples, respectively. These values are relatively small, compared to previous studies on the hematite (001) surface. Thus Tanwar et al.2 ˚ obtained b ¼ 0.18 – 0.23 and Catalano3 b ¼ 0.23 (reported as s(rms) ¼ 1.4 A). For the fresh hematite sample we had the possibility to compare the roughness obtained from the CTR analysis with the roughness measured by high-resolution ˚ and AFM and found an extraordinarily good agreement, i.e. s(rms, CTR) ¼ 1.1 A ˚ s(rms, AFM) ¼ 1.2 A. One should not be misled by a rst impression of the highresolution AFM image, which appears quite heterogeneous. The total height range of the image at comparable conditions to the CTR measurements covers no ˚ i.e. the surface is heterogeneous, but still very at. more than 6 A, In summary roughness should not be a major issue in the presented CTR study, and we see no reason for a special treatment of the specular CTR or for its exclusion from the analysis. 1 I. K. Robinson, Phys. Rev. B, 1986, 33, 3830–3836. 2 K. S. Tanwar, S. C. Petitto, S. K. Ghose, P. J. Eng and T. P. Trainor, Geochim. Cosmochim. Acta, 2009, 73, 4346–4365. 3 J. G. Catalano, Geochim. Cosmochim. Acta, 2011, 75, 2062–2071.
Fig. 4 Direct comparison of specular CTR data measured on the fresh (circles) and aged (diamonds) hematite (001) samples.
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Kirsi Yliniemi commented: When modelling the surface potentials of hematite samples (for example in Fig. 7 in your paper) have you taken into account the interaction term between the neighbouring surface sites? If not, could you comment on how much the interaction between the neighbouring sites may inuence the protonation/de-protonation behaviour. Johannes L¨ utzenkirchen and Frank Heberling answered: The models used in our paper do not consider next-neighbor interaction. Coupling between protonation and deprotonation reactions on the surface sites within the model occurs via mean-eld electrostatic terms. Ising models consider those kinds of next-neighbor effects which you refer to and have been used by Borkovec and Koper to describe charging of polyelectrolytes.1 Borkovec has also shown for oxides, that the mean-eld models we are using are equivalent to the Ising models.2 The relation between the two approaches involves some interesting aspects. For one thing, unlike polyelectrolytes, it is almost impossible to completely protonate or deprotonate an oxide surface within the normal pH range although some publications seem to show saturation, i.e. a plateau which has been interpreted as site density, e.g. for goethite.3 Subsequent work has shown that the saturation data are doubtful.4 The maximum degree of protonation/deprotonation is expected to be much smaller on oxides than on polyelectrolytes (like polyacrylic acid, which can be completely deprotonated). From this point of view next-neighbor effects on oxides might be expected to be less pronounced than on polyelectrolytes. Also, when dealing with next neighbors the situation is likely to become highly complex on oxides which is shown in Fig. 5 for the (110) face of goethite. On this plane various sites occur. They have different proton affinities. These can be predicted within the MUSIC model framework.5 The sites have a given structural arrangement, which requires in an Ising model the appropriate interaction parameters. The MUSIC approach probably requires a smaller number of parameters. Fig. 6 shows the individual contributions of the different sites to the overall charging of this plane based on the MUSIC model.
Fig. 5
Surface structure of the ideal (110) face of goethite.
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Fig. 6
Contributions of the different sites to the overall charging of goethite (110) as a function of the pH (here we plot the logarithm of the proton concentration, which for a given ionic strength is proportional to the pH).
The overall point of zero charge occurs at pH 9.2. The pH-dependent charge mainly arises from the reaction of the singly co-ordinated group. As in our paper the doubly co-ordinated groups within this model do not show signicant charging within the normal pH range. The triply co-ordinated groups have a constant charge which hardly changes with pH in the pH range of interest. Actually the model can be further complicated by involving two types of triply coordinated groups (one with high and one with low proton affinity) resulting in a structurally even more complex situation. The model can also be simplied by excluding the doubly co-ordinated groups. The inuence of the different sites on the overall protonation is best discussed by considering the singly co-ordinated sites. Their intrinsic pKa value is 7.7. The conditional pKa is between 3 and 4, which means a shi of about 4 pH units caused by the triply co-ordinated sites. In other words here the interaction among the different sites occurs via the model inherent electrostatic term(s). It would probably be possible to design an Ising model for this surface as well. As pointed out before one would need to consider the interactions between the different sites under the given structural constraints. Addition of site-interaction parameters to the present model would increase the number of parameters. However, with the present number of adjustables within the bare MUSIC model we already obtain a good t to the data. Following the principle of parsimony, we try to keep the model to a minimum of adjustable parameters. The next-neighbour issue and its inuence on pKa values of surface groups has been raised previously by Bickmore in a sense.6 He has heavily criticized the MUSIC model (which we apply in our paper) for neglecting close range effects. His argument was that the close-range effects would affect next-neighbor pK values. Since bond distances on a given site would be affected due to protonation or deprotonation of adjacent sites and because it is known that the MUSIC model is highly sensitive to bond-distances7 such effects would be substantial. From our point of view again such close-range effects are covered in the common model by the smeared-out charge and the resulting electrostatic factors supplemented by the restricted degree of protonation/deprotonation. Whether the close-range features should be combined with larger scale features is an interesting question. 92 | Faraday Discuss., 2015, 180, 81–96
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The electrostatic factors are sometimes also interpreted as the ratios of activity coefficients of the surface species involved in a given reaction. Overall, many aspects of the surface reactions could be improved based on detailed knowledge, but it usually involves new parameters that have to be adjusted or assumed. As pointed out previously we apply the principle of parsimony. 1 M. Borkovec and G. J. M. Koper, J. Phys. Chem., 1994, 98, 6038–6045. 2 M. Borkovec, Langmuir, 1997, 13, 2608–2613. 3 L. Lovgren, S. Sjoberg and P. W. Schindler, Geochim. Cosmochim. Acta, 1990, 54, 1301–1306. 4 J. L¨ utzenkirchen, J.-F. Boily, L. Lovgren and S. Sjoberg, Geochim. Cosmochim. Acta, 2002, 66, 3389–3396. 5 W. H. v. Riemsdijk and T. Hiemstra, in Interface Science and Technology, ed. L. Johannes, Elsevier, 2006, vol. 11, pp. 251–268. 6 B. R. Bickmore, K. M. Rosso and S. C. Mitchell, in Interface Science and Technology, ed. L. Johannes, Elsevier, 2006, vol. 11, pp. 269–283. 7 J.-F. Boily, J. L¨ utzenkirchen, O. Balmes, J. Beattie and S. Sjoberg, Colloid Surf. A-Physicochem. Eng. Asp., 2001, 179, 11–27.
Rob Lindsay said: Regarding your approach to tting the SXRD data, I was wondering if you had tried to model the experimental data by adopting partial site occupancies? Frank Heberling and Johannes L¨ utzenkirchen responded: As described in the paper, a preliminary approach to t the data included one domain and partial occupancy of the top most iron site. The two domain model is to some extent equivalent to such an approach. The difference is that the consideration of two distinct surface domains allows us to address different local structural relaxations in the two domains. Thus regions where the topmost iron site is occupied are modeled separately from regions where the site is unoccupied. These additional degrees of freedom in the model turned out to be crucial to obtain a satisfactory t to the data. Philippe Marcus said: Oxides are extremely important in the eld of corrosion chemistry because several metals and alloys are protected against corrosion by a thin (or ultra-thin) oxide layer. However thin oxide layers on metals may differ in many ways from bulk oxides that are not supported by metals (for example the work function can be different). According to you, can we take the values of the zeta-potential for bulk oxides and use them for oxide lms on metals? Johannes L¨ utzenkirchen and Frank Heberling replied: This is an intriguing question. We believe there are several aspects to this question. One concerns how the chemistry of the oxide lm on a metal relates to the surface of a bulk oxide. If one tends to believe that the reactivity is quite similar for both, the difficult part will be to choose the correct data from the literature. The major part of the available charging data is for oxide-particles. Data for such oxide particles can vary a lot (i.e. IEPs can cover ranges of several pH units). So the question would be which data to choose. The answer would be largely arbitrary. A second more interesting aspect concerns the actual structure of the oxide lm on a metal. If it can be characterized in terms of bond distances and nature
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and number of surface functional groups, then the available models1 can in principle predict at least the point of zero charge and maybe more than that. This would involve the assumption that the supporting metal does not affect the solid– liquid interface additionally (i.e. beyond constraining bond distances in the lm). If bond distances in the oxide lm (as affected by the underlying metal) are different from a nominally identical bulk oxide surface, the charging properties between the two samples will probably differ. The proton affinity constants are actually very sensitive to the bond-distances.2 A nal aspect has been discussed by McCafferty.3 According to him the relation between IEP and the potential of zero charge is quite complex. In the models as we use them and for the systems we are interested in we are not considering space charge issues. Experience has shown that one should measure the points of zero charge and the zeta-potentials for unknown samples or samples that are not sufficiently characterized for application of MUSIC type models or where any other uncertainties exist under the conditions of interest. This is also true for bulk oxide samples. 1 W. H. v. Riemsdijk and T. Hiemstra, in Interface Science and Technology, ed. L. Johannes, Elsevier, 2006, vol. 11, pp. 251–268. 2 J.-F. Boily, J. L¨ utzenkirchen, O. Balmes, J. Beattie and S. Sjoberg, Colloid Surf. A-Physicochem. Eng. Asp., 2001, 179, 11–27. 3 E. McCafferty, Electrochim. Acta, 2010, 55, 1630–1637.
David Williams asked: Could you describe the surface changes as a slow, hydration-driven surface reconstruction that is spatially heterogeneous (and thus perhaps also susceptible to spatially heterogeneous surface contamination)? Johannes L¨ utzenkirchen and Frank Heberling replied: The question about the process behind the changes is crucial. Unfortunately we know very little about the mechanisms. We are condent that surface contamination does not play a major role because the cleaning procedures we apply have been previously elaborated very carefully for single crystals.1 Cleaning substrates in this way also led to simultaneous observation of the very same features on the companion surface sapphire (001) which we report here for hematite (001). On sapphire (001) we observed them on one kind of sample, i.e. at surface potential curve and steep zeta potential feature. What makes us condent is that those two features had been independently observed in a substantial number of studies.2 Furthermore, the cleaning procedure we use has been applied by others in a paper that came out simultaneously as ours3 and in subsequent work.4,5 In particular these latter studies involve highly surface specic techniques that would detect minor amounts of adventitious carbon compounds. As for the major difference in the surface properties with the changes over time, the outcome namely just a change in the extent of hydration was certainly disappointing. A simple and far more appealing cause would have been the change from a surface that is initially oxygen terminated (for which we can explain the low isoelectric point, IEP) to one that involves singly co-ordinated groups (i.e. which has the bi-domain properties) and allows a straightforward explanation for the high IEP.
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It would also agree with independent work, where we have found that the presence of dissolved aluminum can increase the IEP of sapphire (001)6 to the same extent as we report here hematite (001) on simple aging. In that paper6 we also cite previous work on hematite (001) where the formation of adatoms had been reported. So there is some consistency there. We are also currently re-evaluating the CTR data with new soware in the hope of obtaining more satisfactory relationships between the surface structure and the charging. We were very happy to see the high IEP appearing on the aged sample, because in the previous work on single crystals it has been very rare that IEPs of bare oxide surfaces above say pH 5 were measured (either by streaming potential or in force distance measurements). Even on aged steel samples usually low IEPs were obtained,7,8 while one would tend to expect the IEP of iron oxides (particles), which are usually around pH 9. We agree that the study of the actual transformation would be highly interesting. Furthermore, we believe that future studies should involve a number of different techniques over the experimentation period. Such multi-technique studies have the potential to shed light on the processes. 1 T. Rabung, D. Schild, H. Geckeis, R. Klenze and T. Fanghanel, J. Phys. Chem. B, 2004, 108, 17160–17165. 2 J. L¨ utzenkirchen, R. Zimmermann, T. Preocanin, A. Filby, T. Kupcik, D. Kuttner, A. Abdelmonem, D. Schild, T. Rabung, M. Plaschke, F. Brandenstein, C. Werner and H. Geckeis, Adv. Colloid Interface Sci., 2010, 157, 61–74. 3 M. M. Gentleman and J. A. Ruud, Langmuir, 2010, 26, 1408–1411. 4 E. Anim-Danso, Y. Zhang, A. Alizadeh and A. Dhinojwala, J. Am. Chem. Soc., 2013, 135, 2734–2740. 5 E. Anim-Danso, Y. Zhang and A. Dhinojwala, J. Am. Chem. Soc., 2013, 135, 8496–8499. 6 J. L¨ utzenkirchen, A. Abdelmonem, R. Weerasooriya, F. Heberling, V. Metz and R. Marsac, Geochem. Trans., 2014, 15, 9. 7 N. Kallay, D. Kovacevic, I. Dedic and V. Tomasic, Corrosion, 1994, 50, 598–602. 8 B. M. Cabanas, J. L¨ utzenkirchen, S. Leclercq, P. Barboux and G. Lefevre, J. Nucl. Mater., 2012, 430, 150–155.
Hadeel Hussain asked: In your paper it was not mentioned which model you have used to simulate surface roughness.I assume it is the beta roughness model? Have you tried using different models, as I’m sure you know, it is a relatively simple model and so may not accurately represent the rough surface you have as evident from the AFM images. Frank Heberling and Johannes L¨ utzenkirchen replied: In the electronic supplementary information (ESI) we mention that we use the beta model to account for the inuence of roughness on the model. We did not test other models. As already mentioned in the previous discussion for the fresh hematite sample we were able to compare the roughness derived from the beta model (b ¼ ˚ to the rms roughness measured by AFM, which 0.17 is equivalent to srms ¼ 1.1 A) ˚ Such agreement, and the type of roughness as mentioned in the is srms ¼ 1.2 A. answer to the earlier question from Robert Lindsay, supports the applicability of the beta model to our data. Concerning the “rough surface as evident from the AFM image”, as already mentioned in the previously, we think it is important to point out that the total height scale of the relevant AFM image (as depicted on the right of Fig. 8 in our paper) is 570 pm. The surface heterogeneity is obvious, but the 10 nm to 20 nm wide islands (by which the surface is covered) have a height of This journal is © The Royal Society of Chemistry 2015
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˚ Therefore, even though it is obvious that the surface is showing a only about 2 A. distinct heterogeneity, it may be considered as extremely at.
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Hadeel Hussain queried: To me it seems your c2 is quite high (usually with CTR data an acceptable model has a c2 less than 2 and in many cases less than 1.5) and could possibly be due to the error bars you have on your CTR data. How have you calculated these? Frank Heberling and Johannes L¨ utzenkirchen replied: For a model renement on a full set of CTRs including specular and off-specular data, and considering the constraints on the degrees of freedom within the model due to symmetry constraints at hematite (001), it is our notion that a c2 < 4 is an acceptable t. We do admit that there is some space for improvement of our structural models. As pointed out previously we are currently re-evaluating the data for other reasons. As for the use of the goodness-of-t parameter, we prefer not to put too much weight on absolute values of c2. In our opinion c2 is mainly useful to compare various ts to one specic dataset. Discussing the goodness-of-t parameter concerns, however, an important and critical issue regarding the development of structural models based on CTR data. The determination of the error bars on the structure factor amplitudes have a direct impact on c2. Therefore the uncertainties associated with structure factor amplitudes are a key issue when deciding how many degrees of freedom should be allowed in a structural model. Our standard approach to estimate error bars on CTR data is based on counting statistics. As such the error of the integrated intensity, Ierr, is estimated as the square root of the total intensity, Itot, plus the background intensity, Ibgr: Ierr ¼ (Itot + Ibgr)0.5. To calculate structure factor amplitudes, |F|, the integrated intensity, I, is multiplied by a correction factor, c, that depends on geometry and normalized by the beam intensity, I0. Subsequently the square root of the resulting expression is taken. This leads to the following equation for the error, Ferr, associated with |F|: Ferr ¼ 1/2 (c/I0 Ierr/I)0.5. For the two datasets presented in the manuscript we were in the comfortable situation that several symmetry equivalent CTRs were available. By comparing these equivalent CTRs we can calculate standard deviations for the structure factors and compare them to error estimates based on counting statistics. Doing so we found excellent agreement. For the fresh hematite sample the average uncertainty based on counting statistics is 19% and the average standard deviation based on symmetry equivalent CTRs is 17%. For the aged sample the agreement is even better and we found 18% average uncertainty with both methods. Consequently, for the datasets presented here we are condent that the errors associated with the structure factors are reasonable.
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