Hydrophilic Interaction Between Low-Coordinated Au and Water: H2O

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Hydrophilic Interaction Between Low-Coordinated Au and Water: HO/Au(310) Studied With TPD and XPS 2

Matthijs André Van Spronsen, Kees-Jan Weststrate, Angela den Dunnen, Maarten van Reijzen, Christine Hahn, and Ludo B F Juurlink J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00912 • Publication Date (Web): 07 Apr 2016 Downloaded from http://pubs.acs.org on April 15, 2016

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The Journal of Physical Chemistry

Hydrophilic Interaction Between Low-coordinated Au and Water: H2O/Au(310) Studied with TPD and XPS Matthijs A. van Spronsen†1,2 , Kees-Jan (C. J.) Weststrate3 , Angela den Dunnen2 , Maarten E. van Reijzen‡2 , Christine Hahn2 , and Ludo B. F. Juurlink∗2 1

Huygens-Kamerlingh Onnes Laboratory, Leiden University, P.O. box 9504, 2300 RA, Leiden, the Netherlands 2 Institute of Chemistry, Leiden University, P.O. box 9502, 2300 RA, Leiden, the Netherlands 3 Syngaschem BV, Eindhoven University of Technology, P.O. box 513, 5600 MB Eindhoven, the Netherlands

April 4, 2016

Abstract In this work, we study the relatively weak H2 O-Au interaction on the highly stepped and anisotropic (310) surface with temperature programmed desorption and X-ray photoelectron spectroscopy. Compared to Au(111), we report an enhanced adsorption energy of H2 O-Au(310) as observed from the (sub)monolayer desorption peak. This peak shows zero-order desorption kinetics, which we do not explain with a typical two-phase coexistence model but rather by desorption from the ends of onedimensional structures. These could cover both the steps and (part of) the terraces. We do not observe crystallization of ice clusters as was observed on Au(111). This leads to the conclusion that this stepped surface forms a hydrophilic template for H2 O adsorption. We also notice that the precise orientation of the steps determines the H2 O binding strength. Despite the surface’s enhanced H2 O interaction, we do not observe any significant H2 O dissociation. This indicates that the presence of low-coordinated Au atoms is not enough to explain the role of H2 O in Au catalysis.

1

Introduction

resulting inertness to corrosion. However, modern research shows that this inertness is not guaranteed and significant chemical reactivity can be ascribed to Au 3–5 .

Gold is one of few examples of transition metals with only minor industrial or technical application. In fact, only ∼12 % of newly mined Au goes into industry, while the majority ends up as either jewelry or financial investment 1,2 . The industrial-used fraction is utilized because of its nobility and its

The reactivity of metal surfaces is the central focus of heterogeneous catalysis. Its purpose is to facilitate the formation of many chemical products and to control the emission of pollutants. In this field, Au’s remarkable reactivity was discovered by † Present address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts observing a high activity at low temperatures for CO oxidation 6 . In addition, interesting applica02138, USA ‡ Present address: Laboratoire Chimie Physique Molecutions for Au as catalyst have been found in the laire, Ecole Polytechnique Federale de Lausanne, Lausanne, water-gas shift reaction (WGSR) 7 and the selecSwiss Confederation ∗ Corresponding author, +31 (0)71 527 4221, tive oxidation of alcohols 8 . All examples show that [email protected] Au must be dispersed into nano-sized structures to 1

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heptagonal rings 35–37 . Even more interesting, one-dimensional (1D) structures can form when the considered surface is anisotropic. This is well illustrated on Cu(110) on which intact H2 O forms chains from pentagons in the [001] direction 38,39 . Furthermore, partially dissociated H2 O forms chains in the [110] direction 40 . This anisotropy can be strengthened by going from flat to vicinal surfaces. The interaction between H2 O and stepped surfaces was the focus of a number of studies. Steps generally show a stronger interaction with H2 O as determined with Temperature Programmed Desorption (TPD) on Pt 41–44 , Ru 45,46 , Ni 47 , and Co 48 . The experimental work was supported by computational studies on Pt 42,49–52 , Cu 53,54 , and Ni 55,56 . Although the precise orientation of the steps play an important role, details have not been fully understood yet 42–44,52,57–59 . In addition to binding H2 O stronger, steps also lower the dissociation barrier on Pt 49–52 , Cu 54 , Ni 47,56,60 , Ru 46 , Co 48 , and Re 61 . Interestingly, the steps of Pt were observed to be covered by 1D chains 57,62,63 . The focus of the present work is to study how anisotropy influences the binding of H2 O with the much nobler Au and to see if also a weakly interacting surface can show enhanced binding and 1D structures. Of especial interest is the question whether steps are able to dissociate H2 O to any observable extent. To study these effects, care was taken to choose one of the most open and expectedly reactive surfaces. This led to the Au(310) surface, which can be considered as a highly stepped (100) surface with steps forming (110) planes. The adsorption and desorption was studied both with TPD and high-resolution X-ray Photoelectron Spectroscopy (XPS).

show reactivity. The enhanced reactivity of low-coordinated Au atoms is one of the explanations for the need of nano-sized structures. These atoms can be found in steps and kinks, which are increasingly abundant in smaller nanoparticles. To test this hypothesis, we used a stepped Au single crystal for this work. This model catalyst has a high density of steps and, therefore, low-coordinated Au atoms. So, it can be considered a good model system for a nanoparticle catalyst in this respect. Because it is a single crystal, it does not suffer from complicating details (such as support effects and electronic effects due to nano-confinement) and is perfectly suitable to probe just the effect of low coordination on the reactivity of Au. A key aspect of gold catalysis is the role of H2 O. By adding H2 O vapor, it was found that the reactivity in CO oxidation can be enhanced 9 . The role of H2 O is even more important in the WGSR, in which it is one of the reactants. The intimate relation between H2 O and Au’s reactivity stimulated research in understanding the details of this interaction on well-defined, single-crystal model catalysts both experimentally 10–21 and theoretically 17,22–29 and on well-defined supported Au clusters, for example 30 . This effort fits into a wider framework to understand the interaction between H2 O and solid surfaces. This interaction has become the subject of one of the most widely studied fields in surface science. Motivation for these studies are almost as numerous as the number of studies themselves. For example, it is also crucial to environmental chemistry, interstellar nucleation of ice particles, material corrosion, and electrochemistry in which H2 O is the most-used solvent. The interaction of H2 O with transition metals is reviewed multiple times 31–34 . The early results were believed to widely support an extendedbilayer model. This frequently-proposed bilayer model was not able to explain all results obtained with newly developed tools, most notably Scanning Tunneling Microscopy (STM), combined with Density Functional Theory (DFT). These studies led to interesting cases in which the bilayer model was simply incorrect and the structures formed were more complex and beautiful. An important case is the extended overlayer formed on Pt(111). This layer contained both pentagonal, hexagonal, and

2

Experimental

Two different Au single crystals were used and polished to the (310) surface with an accuracy of < 0.1◦ 64 (for TPD measurements) and ∼2.3◦ (for XPS measurements). To obtain a well-defined and clean surface, the crystals were prepared by Ar+ sputtering with an energy of 500 eV for the TPD experiments and 1 keV for the XPS experiments for a few minutes. Sputtering was followed by annealing in vacuum at 860 K. Multiple cy2


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havior was corrected using a hyperbolic tangent to describe the change in background level 44 . The XPS measurements were performed at the SuperESCA beamline, Elettra synchrotron, Trieste, Italy 67 . This beamline is designed to give a high intensity, monochromated X-ray beam tunable between roughly 0.1–2 keV. XP spectra were collected at normal emission and at a beam angle of 70◦ . The Au 4f7/2 signals were recorded with a photon energy of 170 eV and a dwell time of 50 ms and 650 eV with 30 ms for the O 1s signal. Both the Au 4f7/2 and O 1s signals were measured in a constant energy analyzer mode with a pass energy of 4.0 and 15.0 eV, respectively. Moreover, the hemispherical analyzer was set to medium area mode. The obtained XP spectra were compensated for changes in beam flux and differences in dwell time by dividing the spectra by a linear background. ˇ The XP peaks were fitted with a Doniach-Sunji´ c 68 function convoluted with a Gaussian line shape. This fitting function required the following fitting parameters: binding energy, intensity, Lorentzian line width, Gaussian line width, and asymmetry factor. Fitting parameters were accepted if a timeresolved data set could be successfully fitted with only the intensities as free fitting parameters. The XP binding energies are reported with respect to the Fermi level. This was measured every time after switching to a different photon energy. The uncertainty in the fitted binding energies are estimated to be around 50 meV. The O 1s signal was quantified in two separate ways. First, it was calibrated by the O 1s signal from a CO-saturated surface at 105 K. CO adsorption saturates when half of the step sites are covered, which occurs at 0.167 ML 19,69 . This calibration was performed both for a photon energy of 650 and 1205 eV, which agreed within 11–12 %. Second, the surface concentration ratio between the D2 O layer and the Au surface was calculated with 70 :

cles resulted in a contaminant-free and well-ordered surface confirmed by Low-Energy Electron Spectroscopy (LEED) and XPS. High-purity1 H2 O was used for the TPD experiments. It was degassed by multiple freeze-pumpthaw cycles until a Quadrupole Mass Spectrometer (QMS) confirmed the absence of O2 . Small amounts of N2 were considered to be irrelevant, due to the expected inertness of Au to N2 . The degassed H2 O was admitted via a capillary-array doser 65 . It was codosed with high-purity2 He at ∼1.5 bar at room temperature. This resulted in a mixture containing ∼1.5 % H2 O. Codosing of He was used to create reliable and repeatable pressure readings. For the XPS experiments, high-purity3 D2 O was used, which was prepared and dosed in the same way. The dose was reported in Langmuir (L), defined as 1×10–6 Torr s. The pressure was measured with a hot-filament ionization gauge. No gas-specific corrections were applied, because for H2 O these correction factors are close to and scattered around 1 66 . Experimental work was performed using two different Ultra-High Vacuum (UHV) systems. The TPD setup was equipped with a LEED4 setup and a QMS5 mounted directly on the main chamber. The main chamber was evacuated with two Turbomolacular Pumps (TMP) in series. These pumps ensured a base pressure of 1–2×10–10 mbar. The sample was mounted on a multistage manipulator containing a liquid-nitrogen cryostat, cooling the sample down to 88 K. The sample was heated by a filament mounted close to the rear of the crystal, combining thermal radiation with electronbeam heating. The temperature was measured by a chromel/alumel thermocouple laser spot welded to the side of the crystal. The temperature was controlled with a PID controller6 to create linear temperature ramps needed for the TPD experiments. All TPD traces reported in this work were obtained with a heating rate of ∼0.9 K/s. Adsorption of H2 O and subsequent desorption from the walls of the chamber resulted in enhanced background levels during a measurement. This unavoidable be-

)] σAu4f λAu [1 – exp(– λdAu ND2 O,surf IO1s Au = d D O NAu,surf σO1s λD2 O [1 – exp(– λD2 O )] IAu4f, surf 2

1 milliQ

Ultrapure N purity 3 99.96 atom % D, Aldrich 4 VG RVL 900 5 Baltzers Prisma 200 6 Eurotherm 2416 2 6.6

in which σ are the respective ionization cross sections, which were linearly interpolated for the correct photon energy from the values reported by Yeh 3



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sorbates stay trapped in a metastable phase. This phase can transform into a more stable phase upon heating the surface. This newly formed phase is probed with TPD and not the original metastable adsorption phase. This mechanism is believed to explain the discrepancy on Au(111) between desorption and adsorption measurements. On one hand, TPD experiments showed fractional desorption, indicating desorption from ice crystallites and non-wetting behavior 12,13 . On the other hand, several adsorption experiments resulted in an extended wetting layer 14,17,21 . During the temperature increase, a wetting to non-wetting phase transition occurs, similar to H2 O/Cu(111) 84,85 . The XP spectra recorded during the temperature ramp (Figure 13) indicated no sign of any such transition up to the onset of desorption. However, there was a slight increase in the O 1s ‘monolayer’ signal and the Au 4f7/2 D2 O-Au signal. This increase of O 1s ‘monolayer’ was correlated with decreased shielding of desorbing multilayers. In conclusion, we observe no changes in adsorption between ∼100 K and the onset of desorption. Therefore, the TPD experiments can be trusted to yield reliable information on the adsorption of H2 O on Au(310).

4.3

Monolayer adsorption

Next, we will discuss two interesting features of the TPD traces for (sub)monolayer desorption. The first one is the manifestation in a single peak (in addition to the peak ascribed to bulk desorption). In other words, we will explain the absence of separate terrace and step contributions in the TPD spectra. Second, we will explain the zero-order nature of this peak. 4.3.1

Single desorption peak

When we compare the TPD spectra of Au(111) 12,13 with the ones obtained from Au(310), the differences are obvious. On Au(111), desorption lacked a distinct (sub)monolayer peak and the desorption order was ∼0.6. Both features demonstrated that 3D ice crystallites were thermodynamically more stable than an extended overlayer. In contrary, there was distinct

(sub)monolayer desorption on Au(310) and it desorbed at higher temperatures than ice multilayers. This indicated that an extended (sub)monolayer was more stable than ice crystallites. The difference in desorption from Au(310) and Au(111) can be related to the presence of steps. These steps directly increase the bonding strength of H2 O on Au as discussed before. In addition, the steps may play a more subtle role in protecting the surface from reconstructing, thus keeping the coordination of the surface atoms lower. Interestingly, the precise step orientation is very important in stabilizing H2 O on Au. Desorption from the stepped Au(997) surface did not result in a separate contribution from (sub)monolayer desorption 83 . Desorption from this surface is remarkably similar to desorption from Au(111). However, both the Au(310) and the Au(997) surface have steps forming a small (110) facet. The difference between the two steps is the direction of the step with respect to the (110) plane: the steps of the (310) surface run along the [001] direction but the steps of the (997) surface are in the [110] direction. Surprisingly, no separate contribution from steps and terraces was observed in the TPD experiments, in contrast to desorption from stepped Pt surfaces 43,44 . This single (sub)monolayer desorption peak on Au(310) was attributed to desorption from (but not limited to) steps, since steps generally bind H2 O stronger than terraces. This interpretation is supported by a coadsorption study of H2 O and CO 19 . In this work, van Reijzen, et al., preadsorbed CO on Au(310), which is believed to occupy steps 19,69 . Additional H2 O adsorption competes with CO for step sites. This compresses the CO molecules and forces them to occupy closer-spaced step sites. The absence of a peak originating from terracebonded H2 O can be explained by either hydrophobic terraces, which are non-wetting. This would result into nucleation of ice crystallites between steps from which desorption would be similar to desorption on Au(111). A second possibility, however, is desorption from an H2 O structure covering both step and terrace. To explain the single desorption feature in this case, desorption needs to have a single rate-limiting step. This step could be the release of H2 O bonded to steps after which nearby terrace-bonded H2 O instantaneously desorbs.

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4.3.2

Zero-order kinetics

The zero-order kinetics of the (sub)monolayer peak is also rather remarkable. This can occur in two separate cases. The first one is adsorbate-geometry dependent and occurs when desorption takes place from the end points of 1D structures or from the surface of flat multilayers. The other situation of zero-order desorption results from an equilibrium between condensed island and isolated monomers. If desorption is restricted to the isolated monomers and as long as this equilibrium is maintained, the loss of monomers by desorption is replenished by deattachment from the islands. If the size of the islands does not change too much, the concentration of monomers remains (nearly) constant 86 . In both cases, the number of desorption sites is constant, which results in a desorption rate effectively independent of coverage. Zero-order desorption of water is frequently observed for multilayers and monolayers on flat surfaces 32 . But on stepped surfaces, examples show that desorption from terraces changes to first order 43,44 . This occurs when the step density is high enough and terraces too narrow to allow for phase coexistence 87 . In addition, desorption from steps was reported to follow first-order kinetics 42–44 . In our case, the surface had very small terraces and a very high step density but still exhibited zeroorder desorption. Instead of a two-phase coexistence model, we propose that desorption occurred from the ends of 1D structures covering either only a step or both a step and (part of a) terrace. This model would also yield zero-order desorption. The difference between H2 O desorption from stepped Pt and Au surfaces can be explained as follows. For the Pt steps, H bonding between adsorbed H2 O molecules is weaker compared to flat Pt 49,58 and the Pt-H2 O is relatively strong. This means that desorption could occur from every step site, resulting in first-order desorption. The AuH2 O interaction, however, is much weaker 88–90 and lateral interactions play a more important role and hinder first-order desorption. The distance between step atoms in Au(310) is larger than on the stepped Pt(111) surfaces (408 pm for Au(310) and 278 pm for steps on Pt(111)). To have strong lateral interaction and explain the zero-order desorption, it will be necessary that more H2 O is incorporated in the 1D structures

on Au(310). The binding of these H2 O molecules could predominantly be with step-bonded H2 O and less with the Au surface. This connective H2 O will most likely directly desorb together with desorption from the step sites. This is consistent with the appearance of a single desorption feature. 4.3.3

XPS data

The O 1s (Figure 7) and Au 4f7/2 (Figure 11) XP spectra taken during the uptake of D2 O shed more light on the relative step/terrace coverage of the (sub)monolayer structure. Remarkably, the D2 O ‘multilayer’ peak was populated from the start of adsorption (Figure 8), which seems completely counterintuitive. Even when molecules were kinetically trapped on top of the first layer, this growth rate at low D2 O dose would not be expected. Therefore, we propose to ascribe the O 1s ‘multilayer’ peak to weakly bonded D2 O. This D2 O could reside both in multilayers and in the (sub)monolayer. For D2 O in the (sub)monolayer, this could be D2 O bonded to the 9-fold coordinated Au atoms. Similarly coordinated atoms are also present on Au(111) and are the most predominant atoms on Au(997). On these surfaces, H2 O adsorption shows single, symmetric peaks with a binding energy of 533.2–532.6 81,91,92 and 532.7 eV 83 , respectively. These values are very close to the peak at 532.8 eV, which we assign to D2 O bonded to 9-fold coordinated Au atoms. A contribution from D2 O bonded to 9-fold coordinated Au atoms was absent in the TPD measurements. This could indicate that they desorbed at the same temperature as multilayer D2 O or that they diffused to 6 and 8-fold binding sites during the temperature ramp. The initial growth rate of weakly bonded D2 O was half that of the ‘monolayer’ O 1s peak (Figure 8). The latter could be ascribed to D2 O bonded to both the 6 and 8-fold coordinated Au atoms and reached a maximum of 0.6 ML. This was a bit lower than the expected value of 0.67 ML, which could be explained by shielding by the second layer of D2 O. The second-layer D2 O was indistinguishable with XPS from that of the weakly bonded D2 O in the (sub)monolayer. It started growing at a coverage of 0.9 ML after dosing 0.4 L. At this point, around 80 % of the 9-fold coordinated Au atoms were populated.

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Inspired by previous research 26,39 , we overlaid several possible adsorption geometries with the (310) surface (Figure S2). These models agree with our findings, however, without structural information we will not speculate further. To sum up, both Au 4f7/2 and O 1s XP spectra suggest that the (sub)monolayer adsorption consisted of differently bonded species, but the ratio between the species remained difficult to determine exactly.

2. The precise orientation is very important in stabilizing H2 O: (110) steps parallel to [001] as found on Au(310) bind H2 O significantly stronger than (110) steps in the [110] direction. 3. The observed desorption behavior can be explained by 1D adsorption structures.

The presence of steps led to a distinct (sub)monolayer desorption peak. The observed zero-order kinetics was rationalized by desorption 4.4 Multilayer desorption occurring only at the ends of 1D structures. During desorption, no separate contribution from Finally, we discuss the adsorption and desorption of steps and terraces was detected. In addition, XPS multilayer H2 O from this surface. This desorption occurred at typical temperatures for multilayer des- results suggested that H2 O occupies multiple adorption. Furthermore, the observed zero-order be- sorption sites. To explain these observation, we havior was not unexpected (Figure 3b). However, propose that 1D structures covered the steps and the calculated binding energy (47.8 ± 1.4 kJ/mol) also (part of) the terrace. These structures were (Figure 4) was somewhat higher than for desorp- stabilized by step-bonded H2 O. Desorption of the step-bonded H2 O was rate-limiting and resulted in tion of ice clusters on Au(111) (43.9 kJ/mol) 12 . Starting from the second layer, the adsorbed wa- a single desorption peak. In addition, XPS measurements showed no ter could form 3D clusters on the surface. In this case, a two-phase coexistence with monomers ad- dewetting transition while heating the surface, as sorbed on the first layer would explain the zero- was found on Au(111) and Cu(111). Furthermore, XP spectra showed no significant interaction beorder behavior. An alternative view can be derived from the ad- tween additional H2 O layers and the Au surface. sorption of H2 O on Au(115) 18,20 . On this surface, These additional layers could either have formed vibrational features remained nearly constant up clusters or 1D structures, dictated by the monolayto at least the tenth layer. These vibrational sig- ers structures. natures suggested that the monolayer structure extended in the following layers. A similar argument could apply on Au(310). If Supporting Information the high step density increased the formation energy of ice clusters, it could be energetically favorTPD simulation of desorption of various orders and able for the multilayers to adapt to the monolayer proposed models for H2 O adsorption on Au(310). structures. The multilayer structures could be stabilized by H bonding to the monolayer structures. These H bonds could originate from the upwardly Acknowledgments sticking O-H bonds in the first layer.

5

Summary and conclusion

The interaction between H2 O and the stepped Au(310) surface differed significantly from the hydrophobic Au(111) surface. Our findings obtained with TPD and XPS are summarized as follows: 1. Steps did not dissociate H2 O. They did make the surface hydrophilic.

We gratefully acknowledge Dr. Matteo Dalmiglio, Dr. Fabrizio Orlando, Dr. Paolo Lacovig, Prof. Dr. Alessandro Baraldi, and Dr. Silvano Lizzit of the SuperESCA beamline for their help and advice and Elettra synchrotron, Trieste, Italy for the allocation of beam time. This work is supported by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners. 13



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Hydrophilic Interaction Between Low-Coordinated Au and Water: H2O

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