Template-Assisted Growth of Ultrathin Single Crystalline IrO2

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Template-Assisted Growth of Ultrathin Single Crystalline IrO(110) Films on RuO(110)/Ru(0001) and its Thermal Stability 2

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Marcel Abb, Benjamin Herd, and Herbert Over J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04375 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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The Journal of Physical Chemistry

Template-Assisted Growth of Ultrathin Single Crystalline IrO2(110) Films on RuO2(110)/Ru(0001) and its Thermal Stability Marcel J. S. Abb, 1,# Benjamin Herd, 1,# Herbert Over1*

1)

Physical Chemistry Department, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany

*corresponding author: E-mail: [email protected] # contributed equally to the present study

Abstract: A template-assisted growth of a flat, covering and single crystalline IrO2(110) film with controlled film thickness is reported that is suitable for the use in model catalysis and as model electrodes in electrocatalysis. The template consists of a single crystalline covering RuO2(110) layer grown on Ru(0001). In a first step, we formed IrO2 seeds on the RuO2(110) layer which then continue to grow by deposition of Ir in an oxygen atmosphere of 3·10-7 mbar at a sample temperature of 700 K. The IrO2 seeds are prepared by depositing nanometer size metallic Ir particles on RuO2(110) (in total 0.3-0.5 ML of Ir) at room temperature. Subsequently the Ir particles are oxidized in 10-5 mbar of O2 at a sample temperature at 700 K. The techniques of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED) are employed to characterize the morphology, crystallinity and electronic structure of the prepared ultrathin IrO2(110) films. Thermal desorption spectroscopy (TDS), LEED and STM provide strong evidence that the IrO2(110) films start already to thermally decompose at 500 K under UHV conditions.

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1. Introduction IrO2 is well-known to be a promising catalyst material in heterogeneous catalysis and in electrocatalysis, where IrO2 is utilized as stable and relatively active electrocatalyst for both the chlorine (CER) and the oxygen evolution reaction (OER). In general, not pure IrO2 but rather IrO2 is added to the RuO2 anode coating, mostly in a molar 1:1 ratio of the oxides, to improve the service lifetime and the activity in the industrial large-scale chlor-alkali process.1,2 Both IrO2 and RuO2 crystallizes in the rutile structure so that RuxIr1-xO2 solid solution can be formed. Previous density functional theory (DFT) studies predicted that the rutile IrO2(110) surface strongly binds a variety of molecules such as NH3, N2, and CH4 and should therefore be catalytically active in oxidative dehydrogenation reactions.3,4 Indeed, efficient low-temperature activation of methane was reported recently to proceed on the IrO2(110) surface supported on Ir(100).5 Theoretical studies indicated further that IrO2(100) and IrO2(110) are covered by Oot up to thermal decomposition of the oxide.6 For the gas phase HCl oxidation (Deacon process)7 theory predicted that both oxygen and chlorine are more strongly bound on IrO2(110) than on RuO2(110), thus rationalizing the inferior catalytic activity of IrO2 compared to RuO2. In order to gain experimentally a molecular understanding on the activity and stability of IrO2, it is beneficial to start from a single crystalline IrO2-based model system. Of course, one can use IrO2 single crystals of IrO2 and properly align them along specific surface orientations. However, IrO2 single crystals are hardly available and very small in size.8 Therefore, the growth of highquality single-crystalline ultrathin IrO2 layers with specific orientation provides a promising alternative. The formation of single crystalline RuO2(110) layers has been extensively studied over the past decade9,10 and is well-understood both on the microscopic level10-12 and on the mesoscale.13,14 RuO2(110) layers are formed by simple high temperature oxidation of Ru(0001) single crystals in an O2 atmosphere of 10-5 mbar under ultraclean UHV conditions, while keeping the sample temperature at 650 K. Therefore, the most obvious way to produce single crystalline IrO2(110) layers could be the surface oxidation of Ir(111).6,15,16 However, besides chemisorbed phase, O-IrO trilayer15,17, IrO2(100)6,15 and IrO2(110)15 or Ir2O3(100)16 coexists on the Ir(111) surface, making this approach less suitable. The achieved crystallinity of the IrO2(100) film on Ir(111) is by far not convincing as indicated in a recent STM study.6 Alternatively, Ir(100) can be oxidized


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forming IrO2(110) layers as identified with LEED, but has so far not been studied by STM. The streaky LEED pattern reveals, however, substantial disorder of IrO2(110) film.5 The main objective of the present study is devoted to the preparation and in-depth characterization of a well-defined single-crystalline IrO2-based model system for the potential application in model electrocatalysis (as model electrode)18 and heterogeneous gas phase catalysis. Highly crystalline IrO2(110) films are prepared by growing them on a template consisting of RuO2(110)/Ru(0001). The epitaxial growth of IrO2(110) is facilitated by the small lattice mismatch of IrO2(110) on RuO2(110) of less than 2% (∆a/a = 0.3% and ∆c/c = 1.9%). Helpful for the preparation of a covering IrO2(110) layer is also that the surface energy of RuO2(110) (66 meV/Å2) is substantially higher than that of IrO2(110) (41 meV/Å2) as determined by DFT calculations.19 Therefore, IrO2(110) will presumably be wetting the RuO2(110)/Ru(0001) substrate surface. We develop a dedicated preparation recipe by optimizing the process parameters (sample temperature, post annealing, step by step deposition and others) to obtain a flat, covering and single crystalline IrO2(110) film with controlled film thickness.

2. Experimental Details The experiments were conducted in a custom-built three-chamber ultrahigh vacuum (UHV) system; details have been described elsewhere.10 Briefly, the main chamber is furnished with a mass spectrometer and a dual x-ray source together with a hemispherical analyser (PSP Vacuum Technology) to perform x-ray photoelectron spectroscopy (XPS) experiments (Mg-Kα: 1253.6 eV). XP spectra are acquired at normal photoemission. The sample can be transferred from the analysis chamber to the STM chamber (VT-STM, Omicron). For the STM experiments we used home-made tungsten tips. All STM images presented in this paper were taken in the constant current mode at 298 K. Typical sample voltage and tunneling current used for STM imaging were 1.0 V and 1.2 nA, respectively. The Ru(0001) sample was cleaned by Ar-ion sputtering for 60 minutes (p(Ar) = 5·10−7 mbar, 1.5 kV, 18 mA) and annealing the sample at 950 K in 10−7 mbar of oxygen in order to remove carbon contamination segregating from the bulk Ru. The highest sample temperature achieved with the present sample holder is 1100 K. The sample temperature was measured with an infrared (IR) pyrometer, which was pre-calibrated with a K type thermocouple. -3ACS Paragon Plus Environment


The Ru(0001) was oxidized by exposing the sample to 5·10-5-2·10-4 mbar of O2 for 140 minutes, while keeping the sample at 680 K. This procedure leads to a well-defined and crystalline RuO2(110) layer that fully covers the Ru(0001) surface (cf. Figure 1a).10,20 Iridium was deposited on the fully RuO2(110)-precovered Ru(0001) sample by physical vapour deposition (PVD), utilizing a well-outgassed electron beam evaporator (EMF 3, Omicron). With STM and XPS the deposition rate of Ir was calibrated to be approximately one monolayer (ML) per eight minutes. Iridium was oxidized at 700 K either during deposition by a background pressure of 10-7 mbar of O2 or post-oxidized by 10-5 mbar of O2. The growth process of the IrO2(110) film was deliberately separated in a nucleation followed by a growth step. For the nucleation step we deposited first 0.3-0.5 ML of Ir at room temperature and subsequently oxidized the Ir particles in 10-5 mbar of O2 at a sample temperature at 700 K. Starting from the IrO2 nuclei, we grew the film by continuous deposition of Ir in an oxygen atmosphere of 3·107

mbar and a sample temperature of 700 K.

3. Experimental Results Deposition of 0.3-0.5 ML of Ir in an O2 atmosphere of 3·10-7 mbar keeping the RuO2(110)/Ru(0001) sample temperature of 700 K does not lead to sub-monolayer IrO2(110) film but instead results in a heterogeneous surface with many particles in one region of the surface and flat IrO2 islands in other regions (cf. Figure 1b). Further deposition of Ir in an oxygen atmosphere leads then inevitably to a rough IrO2 film.

Figure 1: STM images (500nm x 500nm) of the submonolayer IrO2(110) deposition; a) U = 1 V, I = 0.9 nA, b) U = 1.1 V, I = 1 nA. a) Fresh prepared RuO2(110) surface (b) Typical surface regions after the deposition of 0.5 ML Ir in an oxygen atmosphere p(O2)=3·10-7 mbar at a sample temperature of 700 K with many clusters covering the RuO2(110)/Ru(0001) surface.

Since we were not able to suppress the cluster-type deposition of Ir/IrO2 by varying the deposition temperature and the O2 background pressure, we pursued a different approach, in which we separated the nucleation step from the actual layer-by layer growth of IrO2(110). In -4ACS Paragon Plus Environment

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doing so, we deposited first 0.3 ML of metallic Ir on the RuO2(110)-Ru(0001) surface at room temperature. Subsequently we post-oxidized the metallic Ir particles at 700 K with molecular oxygen p(O2) = 5·10-5 mbar. Quite to our surprise, this time flat IrO2(110) islands were formed all over the RuO2(110)-Ru(0001) surface and no clustering occurred (cf. Figure 2a).

Figure 2: STM study of the “nucleation” process of IrO2(110) seeds on RuO2(110)-Ru(0001) after 0.3 ML Ir deposition at room temperature and subsequent oxidation at 700 K in p(O2) = 5·10-5 mbar. STM: U = 1.2 V, I =1 nA. a) Overview STM image (150 nm x 150 nm); b) Line scan along the blue line in a) crossing various islands; c) Magnified STM image (20 nm x 20 nm) indicating the internal row structure (bridging O rows) of the IrO2(110) islands.

Nucleation of small (size about 10 nm across) square shaped IrO2(110) islands takes place on the RuO2(110) terrace and also at step edges. The height of these islands is about 3 Å as determined by a line scan (cf. Figure 2b), consistent with a single monolayer IrO2 island. The internal structure on the islands is shown in Figure 2c. The row-like structure and the separation of the rows of 6-7 Å evidence the formation of one-monolayer IrO2(110) islands. A quite similar behavior was observed with the submonolayer growth of RuO2(110) on TiO2(110), although there the RuO2(110) islands were mostly three monolayers high.21,22 The actual growth of larger islands up to multilayer IrO2(110) was studied by STM and is summarized in Figure 3. A STM image of the starting RuO2(110)-Ru(0001) surface is shown in Figure 3a, whereas in Figure 3b the surface is shown after the deposition of 0.3 ML of Ir that was post-oxidized in 10-5 mbar of O2 at a sample temperature of 700 K. Depositing additional -5ACS Paragon Plus Environment


0.3 ML of Ir in an O2 atmosphere of 3·10-7 mbar led the islands grow laterally in size (cf. Figure 3c). Adding further 0.4 ML of Ir in an O2 atmosphere, the IrO2(110) islands coalesced forming partly single monolayer IrO2(110) terraces (cf. Figure 3d).

Figure 3: STM images (300 nm x 300 nm) of the “nucleation separated from the growth” preparation of IrO2(110) grown on the RuO2(110)-Ru(0001) template. a) Clean RuO2(110)-Ru(0001) surface (starting point); b) Deposition of 0.3 ML of Ir at room temperature and post-oxidation at 700 K in p(O2) = 1·10-5 mbar; c-f) Resulting surfaces after additional deposition of 0.3 ML (c), 0.7 ML (d), 1.2 ML (e) and 1.7 ML (f) of Ir in an oxygen atmosphere of p(O2) = 3·10-7 mbar and a sample temperature of 700 K. All STM images were taken at U = 1.1 V and I = 0.9 nA.

With the deposition of additional 0.5 ML (cf. Figure 3e) and 1 ML (cf. Figure 3f) of Ir the second layer of IrO2(110) started to grow. About 2 ML of Ir were required to form a covering IrO2(110) film on RuO2(110)/Ru(0001) template. The IrO2(110) terraces are elongated in specific direction as also observed with the RuO2(110) layer on Ru(0001). In Figure 3f one can recognize two rotational domains of IrO2(110), rotated by 60°. The topmost IrO2(110) islands grows preferentially along the bridging O rows. With SEM we imaged at a 3 ML thick IrO2(110) layer on RuO2(110)-Ru(0001). The original RuO2(110)-Ru(0001) surface did not show any contrast in SEM. The SEM image in Figure 4 was taken by the Inlense detector and reveals that the whole surface is covered with large IrO2(110) domains. The domain boundaries are clearly visible as dark lines. We should recall that RuO2(110) forms already three rotation domains on Ru(0001) and therefore the overgrowing


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IrO2(110) film adopts this rotational structure. From the internal texture of the domains we can infer that they consist only of one type of rotational domain of IrO2(110).

Figure 4: SEM image after the deposition of 3 ML Ir in oxygen atmosphere starting from a RuO2(110)Ru(0001) surface that is covered by nucleation IrO2(110) islands. Domain boundaries (black) are clearly visible. From the internal texture of the domains it is possible to discriminate between the three rotational domains of IrO2(110) (white arrows).

The as-prepared IrO2(110) film of 4 ML thickness is depicted in Figure 5a. In order to improve the crystallinity of the IrO2 film, a post-oxidation step with p(O2)=5·10-5 mbar was conducted at a sample temperature of 700 K for 5 minutes. The resulting IrO2(110) films indicates now a much better ordering (cf. Figure 5b). Even the internal structure on the terraces in the form of bridging O-rows can now be discerned (zoom in Figure 5b). A well-ordered IrO2(110) film of 15 ML is depicted in Figure 5c, together with high-resolution STM image revealing the row-like structure (zoom in Figure 5c).

Figure 5: a) Resulting surfaces after the deposition of 4 ML in oxygen atmosphere at p(O2) = 3·10-7 mbar and 700 K. b) Surface (a) after an additional post-oxidation step at 700 K and p(O2)=5·105 mbar for 5 min. c) Obtained surface after the deposition of 15 ML Ir in cyclic deposition steps of Ir deposition in oxygen atmosphere (3·10-7 mbar)) and subsequent oxidation steps at p(O2)=5·10-5 mbar -7ACS Paragon Plus Environment


and 700 K for 5 min. All STM images: 200 nm x 200 nm, zoom in b) and c): 40 nm x 40 nm. U = 1.0 V, I = 1.2 nA.

The growth of IrO2(110) on RuO2(110)/Ru(0001) was also followed by XPS (cf. Figure 6). We started from the clean RuO2(110)/Ru(0001) with characteristic Ru3d and O1s features known from the literature.10 For the nucleation step 0.5 ML of Ir was deposited onto the RuO2(110) surface and then post-oxidized in p(O2) = 5·10-5 mbar at 700 K. The Ru-3d spectrum (cf. Figure 6a) is hardly affected, while the Ir-4f spectrum shows a small duplet Ir-4f7/2 and Ir-4f5/2 at 62.0 eV and 65.0 eV (cf. Figure 6b) that is assigned to Ir+4 consistent with previous XPS experiments of bulk IrO2.16,23 In the next step, 2.5 ML of Ir were deposited in an oxygen atmosphere of p(O2)=3·10-7 mbar and 700 K.

Figure 6: Following the oxidation of Ir layer of various thickness with XPS: a) Ru-3d; b) Ir-4f, and c) O-1s. For clarity each spectrum is offset. After the nucleation process (0.5 ML of Ir) and post-oxidation at p(O2) = 5·10-5 mbar at 700 K, 2-4 ML of Ir were deposited in each step at a sample temperature of 700 K in an oxygen atmosphere of p(O2) = 3·10-7 mbar. Subsequently the layer was additionally postoxidized with p(O2) = 5·10-5 mbar at 700 K for 5 minutes. In the lower part of the panels the integral intensities of (a) Ru-3d and (b) Ir-4f are shown as a function of the thickness of IrO2. The thickness -8ACS Paragon Plus Environment

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dependence was determined from the attenuation of the Ru3d signal by using the effective mean free path of photoelectron through the IrO2 layer of 14.2 Å taken from NIST (red line).

In order to improve the crystallinity, the resulting layer was post-oxidized in p(O2) = 5·10-5 mbar at 700 K for 5 minutes. The Ir-4f signal increases substantially, while the Ru-3d feature decreases. The O-1s signal (Figure 6c) is slightly shifted to higher binding energies. Depositing additional 3 ML of Ir in an oxidative environment (a total of 6 ML Ir) followed by a postoxidation step led to a further increase of Ir-4f and decrease of Ru-3d. The O1s emission shifts now to 530.0 eV und does not shift further with continuing Ir deposition. The binding energies of O1s at 530 eV and of Ir4f at 62.0 eV and 65.0 eV are ascribed to IrO2;16,24 the apparently wrong intensity ratio of Ir4f7/2 to Ir4f5/2 (not reflecting a nominal ratio of 4:3) is due to electron hole excitations forming long range tails at the high binding energy side of the peaks.24 With increasing Ir deposition the Ru3d signal is attenuated exponentially and the Ir4f grows exponentially (cf. Figures 6a,b, lower panels). Finally between 14 ML und 18 ML of IrO2(110) the Ru3d signal vanishes completely, evidencing a covering film of IrO2(110). The saturation behavior of Ir3d signal and the attenuation of the Ru3d signal fit well the layer-by-layer growth the covering IrO2(110) layer (in total 18 ML). For studying the thermal stability of the resulting IrO2(110) films on RuO2(110)-Ru(0001), we increased the temperature of the sample under UHV conditions and imaged with STM the morphological changes at the surface (cf. Figure 7a). Already at 600 K (but this process starts actually at 500 K) the IrO2(110) layer change considerably by the formation of small roundshaped particles (lateral size = 7 nm, thickness = 0.6-0.7 nm) that mostly decorate the step edges but are also located on the terraces. We presume that these particles consist of metallic Ir particles as corroborated by XPS experiments. A similar reduction behavior was encountered with IrO2 single crystals oriented along the (110) direction. Pai et al.8 reported that the temperature-induced decomposition already starts at 500 K. Increasing the annealing temperature to 700 K leads to a IrO2(110) layer that is fully covered by larger particles (size = 7 nm, thickness = 0.7 nm) (cf. Figure 7b). The rectangular domain structure of IrO2(110) is completely lost. Upon further increase of the annealing temperature to 900 K and 1000 K (cf. Figures 7c,d), the number of particles does not change but their size increases (900 K: size = 11 nm, thickness = 0.9 nm, 1000 K: size = 11 nm, thickness = 1 nm). At 1000 K the particle form hexagons and combined particles are akin to small terraces with well-defined step edges. The thermally induced -9ACS Paragon Plus Environment


decomposition process is quite different from that of RuO2(110) in that pits are not formed as observed on RuO2(110).25

Figure 7: STM-images of an IrO2(110)/RuO2(110)-Ru(0001) surface (10 ML Ir) after thermal treatment for 10 s at a) 600 K, b) 700 K, c) 800 K, d) 1000 K in UHV. All STM images 150 nm x 150 nm. U = 1.0 V, I = 1.2 nA.

Further information about the decomposition of 10 ML thick IrO2(110) layer on RuO2(110)Ru(0001) can be gained by XPS experiments as a function of the annealing temperature (cf. Figure 8). Annealing a IrO2(110) sample to 600 K leads to the development of a low binding energy shoulder of Ir-4f at 61.3 eV (Ir-4f7/2) and 64.3 eV (Ir-4f5/2) (cf. Figure 8a). With increasing annealing temperature this shoulder becomes more pronounced and above 800 K this pair of shifted Ir-4f peaks dominates the Ir-4f spectrum. The energy position of Ir-4f (61.3 eV (Ir4f7/2) and 64.3 eV (Ir-4f5/2)) is ascribed to metallic iridium, thus being consistent with the interpretation of STM images in Figure 7. The Ru-3d spectra (cf. Figure 8b) indicate no changes of the buried RuO2(110) templates up to 700 K. A slight increase in intensity of Ru-3d is observed after annealing at 800 K, indicating that Ru may have segregated to the surface. For an anneal at 900 K, the Ru-3d duplet shifts by 0.6 eV to lower binding energies, pointing to the decomposition of RuO2(110) and the formation of metallic ruthenium. The O-1s spectra (cf. Figure 8c) do not change very much with increasing annealing temperature, neither the intensity nor the binding energy. Only for an anneal above 1000 K the O-concentration decreases steeply, pointing to the decomposition of RuO2(110). -10ACS Paragon Plus Environment

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Figure 8: XP spectra of an 10 ML thick IrO2(110) layer on RuO2(110)-surface (10 ML Ir) after thermal treatment for 10 s at 600 K, 700 K, 800 K and 1000 K in UHV. a) Ir-4f; b) Ru-3d and c) O-1s.

With thermal desorption spectroscopy (TDS) of oxygen we examined the thermal decomposition of the IrO2(110)-RuO2(110)/Ru(0001) (cf. Figure 9a). Oxygen desorption sets in at 700 K that is also the temperatures where morphology changes of IrO2(110) are readily visible in STM. The maximum in the TD spectrum at 900 K corresponds to the decomposition of IrO2(110) and the release of oxygen, while the shoulder at 947 K may be assigned to the decomposition of the underlying RuO2(110) template layer. At even higher temperatures oxygen that is chemisorbed on Ir(111) (900-1150 K)26 and Ru(0001) (1200-1500 K)27 desorbs from the surface. We followed the initial decomposition of the 10 ML thick IrO2(110) layer also by LEED. In Figure 9b we took a LEED pattern of the as-prepared IrO2(110) film that was annealed to 600 K (the LEED pattern is identical to that of the freshly prepared IrO2(110) film). Ru(0001)-related LEED reflections are not visible in the pattern, indicating fully covering IrO2(110) film on RuO2(110)/Ru(0001). However, the LEED pattern degrades substantially after annealing the sample to 725 K (cf. Figure 9c) in that the intensity of all LEED reflections are much lower than those of the freshly-prepared IrO2(110) surface. This observation is consistent with STM where the IrO2(110) surface is partly covered by Ir islands (cf. Figure 7a).



Figure 9: a) O2 thermal desorption spectrum of a 10 ML thick IrO2(110) layer on RuO2(110)/Ru(0001). Heating rate was 4 K/s. b) LEED pattern of a 10 ML thick IrO2(110) films (as prepared and annealed to 600 K) in comparison with a sample that was heated to 725 K (c) in UHV and therefore partly reduced.

Figure 10 depicts a STM and SEM image of a 30 ML-IrO2(110) film that is about 10 nm thick. The 30 ML thick IrO2(110) film roughens on the mesoscale, forming arrays of “roofs”, with terraces being atomically flat. This is quite different from the growth of RuO2(110) on Ru(0001) that shows for layer thicker than 3 nm faceting of the surface.28,29 These “roofs” consists of few nm wide (110) terraces that are separated by single atomic steps (STM) (cf. Figure 10a). The “roof” structure can readily be seen in SEM (cf. Figure 10b) with 200-600 nm long roofs that are oriented along the three high symmetry direction of the Ru(0001) substrate and are separated by 80 nm. XPS proves that the 30 ML thick IrO2(110) film is completely covering the RuO2(110)Ru(0001) template.


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Figure 10: STM (a) and SEM (b) image of 30 ML-IrO2(110)-RuO2(110)/Ru(0001). Three rotational domains of IrO2(110) are indicated by white arrows in the SEM micrograph. STM images 150 nm x 150 nm. U = 1.0 V, I = 1.2 nA.

Discussion We are able to prepare fully-covering IrO2(110) layers with high crystallinity on a well-ordered RuO2(110)/Ru(0001) template as evidenced by STM, LEED and XPS experiments. Even atomically resolved STM images can be acquired from the supported IrO2(110) layer. The RuO2(110) buffer layer structure direct the deposited IrO2(110) film. The preparation of IrO2(110) films requires a separate well-designed nucleation step, in which 0.3-0.5 ML of metallic Ir is deposited on RuO2(110) first that is post-oxidized in an oxygen atmosphere of 10-5 mbar and a sample temperature of T = 700 K. This nucleation step leads to the formation of 10 nm wide square-shape monoatomic IrO2(110) islands. Only when these IrO2(110) islands are formed on RuO2(110) the further growth proceeds quasi layer-by-layer during deposition of Ir in an oxygen atmosphere of 3·10-7 mbar. From LEED we conclude that IrO2(110) grows pseudomorphically, i.e., with the same dimensions of the surface unit cell as RuO2(110).



Ir and Ru may form mixed oxides. From segregation energies, Ir is not expected to dissolve into RuO2, but Ru may dissolve into IrO2(110).19 Intermixing energy of IrO2 and RuO2 can be found in Ref.19 However, Ir-Ru alloying was not observed in XPS experiments, at least not for thick IrO2(110) layers beyond 15 ML and annealing temperatures below 800 K. The IrO2(110) films are thermally only stable up to 500-600 K under UHV conditions and therefore much less stable than RuO2(110) layers that start to decompose only above 800 K. A similar behavior in thermal stability was reported for IrO2 single crystals.8 The free formation energy of IrO2 is lower by 0.55 eV than that of RuO2 consistent with observed lower thermal stability of IrO2.30,31 For the oxidation of Ir(111) Peuckert reported that the decomposition temperature of such a surface oxide takes place at 800-900 K.32 Also Conrad et al.26 reported that the decomposition of the surface Ir-oxide is found to be at 900 K under UHV conditions. These findings are not conflicting with our present results since both studies with a decomposition temperature of up to 900 K are consistent with the observed maximum O2 release during the TDS of IrO2(110) at 900 K in Figure 9a. The low thermal stability of the morphology of the IrO2(110) layer is still quite surprising, since IrO2(110) films have been shown to electrochemically very stable in the OER region up to an anodic potential of 2-3 V versus standard hydrogen electrode (SHE) (unpublished results). In future experiments we will employ these single crystalline IrO2(110) films as model catalyst for the methane activation. From the low thermal stability of IrO2(110) we expect that not only IrO2(110), but likely the interplay of Ir nano-islands on IrO2(110) may be responsible for the observed low temperature methane activation.5 In another project IrO2(110) films will be used as model electrodes to study the microscopic details of activity and more importantly the stability in the oxygen evolution reaction (OER). The kinetics of the OER over IrO2(110) will be studied with extended Tafel plots in order to determine the free energy surface along the reaction coordinate of the OER.33

Conclusions We report here a successful preparation of single crystalline IrO2(110) films with variable thickness of 1 nm up to 10 nm and high quality in crystallinity as evidenced by STM, employing RuO2(110)-Ru(0001) as a structuring template for the subsequent growth of IrO2(110). The strategy behind this approach is that RuO2(110) will be directing the growth of IrO2 in the same orientation as the underlying RuO2(110) film. The growth of smooth IrO2(110) layer requires -14ACS Paragon Plus Environment

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small IrO2(110) monolayer islands as nucleation seeds on RuO2(110)/Ru(0001). These islands are prepared by depositing 0.3-0.5 ML of metallic Ir on the RuO2(110) surface and its post-oxidation at 700 K with 5·10-5 mbar of O2. The crystallinity and flatness of the grown IrO2(110) layer can be further improved by a post-oxidation step at 700 K with 5·10-5 mbar of O2 for 5 minutes. With STM we found that IrO2(110) layers partly decompose under UHV conditions, forming metallic Ir islands, when rising the sample temperature to 500 K and above. Single crystalline IrO2(110) films are promising model systems in electrocatalysis and heterogeneous gas phase catalysis. The template assisted technique may open the door for the preparation of many other rutile oxides in (110) direction, including PtO2, TiO2 and SnO2. Acknowledgement: We thank financial support from the BMBF (project: 05K2016 HEXCHEM).



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TOC Graphics


Template-Assisted Growth of Ultrathin Single Crystalline IrO2

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