Article pubs.acs.org/Langmuir
Cite This: Langmuir 2019, 35, 7720−7726
Growth of Ultrathin Single-Crystalline IrO2(110) Films on a TiO2(110) Single Crystal Marcel J. S. Abb,†,‡ Tim Weber,†,‡ Lorena Glatthaar,† and Herbert Over*,†,‡ †
Physikalisch-Chemisches Institut, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany Zentrum für Materialforschung, Justus Liebig University, Heinrich-Buff-Ring 16, 35392 Giessen, Germany
‡
Downloaded via NOTTINGHAM TRENT UNIV on August 6, 2019 at 23:17:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: The growth of a flat, covering, and single-crystalline IrO2(110) film with controlled film thickness on a single-crystalline TiO2(110) substrate is reported. The preparation starts with a deposition of metallic Ir at room temperature followed by a post-oxidation step performed in an oxygen atmosphere of 10−4 mbar at 700 K. On this surface, additional Ir can be deposited at 700 K in an oxygen atmosphere of 10−6 mbar to produce a IrO2(110) layer with variable thicknesses. To improve the crystallinity of the resulting IrO2(110) layer, the final film was post-oxidized in 10−4 mbar of O2 at 700 K for 5 min. The surface-sensitive 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 and how these films decompose upon annealing under ultrahigh vacuum (UHV) conditions. STM provides evidence that the IrO2(110) films start already to reduce at 465 K under UHV conditions. Upon annealing to 605 K under UHV the reduction of IrO2 intensifies (XPS), but the oxide film can readily be restored by re-oxidation in 10−4 mbar of O2 at 700 K. Thermal decomposition at 725 K leads, however, to severe reduction of the IrO2(110) layer (XPS, STM) that cannot be restored by a subsequent re-oxidation step. The utility of the IrO2(110)−TiO2(110) system as model electrodes is exemplified with the electrochemical oxygen evolution reaction in an acidic environment.
■
INTRODUCTION Iridium dioxide (IrO2) is considered to be the “gold standard” for the electrocatalysis of the oxygen evolution reaction (OER) in acidic environment1 owing to its high activity and even more importantly to its superior stability. IrO2 is commonly added to the RuO2 anode coating of dimensionally stable anodes, mostly in a 1:1 molar ratio of the oxides and diluted with 70% TiO2 to improve service lifetime,1 still keeping the activity high in the chlor-alkali electrolysis. IrO2 is not only applied in electrocatalysis but also has shown to be a promising catalyst for low-temperature activation of methane2 and a moderately efficient oxidation catalyst in the HCl oxidation reaction (Deacon process).3 To gain molecular insight into the activity and stability of IrO2-based catalysts within the surface science approach, first, we need to prepare IrO2 films with low structural complexity, that is, best with single crystallinity and fixed orientation. There are several ways to produce well-oriented singlecrystalline IrO2 films, either by direct oxidation of Ir(111)4−6 and Ir(100)2 single-crystal surfaces or by depositing Ir under an oxygen atmosphere on a dissimilar substrate, such as RuO2(110)/Ru(0001).7 The drawbacks of these IrO2 model systems are twofold. First, frequently both the substrate and the covering film are catalytically active, making it difficult to discriminate between the contributions from the film and that from the substrate (for instance from the backside of the substrate). Second is the formation of rotational domains of IrO2(110) on a substrate with higher surface symmetry, that is, © 2019 American Chemical Society
two domains on Ir(100) and three rotational domains on Ir(111). This leads inevitably to surface grain boundaries at the intersection of rotational domains. An alternative to the oxidation of a single-crystalline metal substrate is the use of TiO2(110)8,9 in order to deposit dissimilar oxide films with rutile structure. Recently, we accomplished to grow single-crystalline RuO2(110) layer on TiO2(110) with a high level of control on the atomic structure, the surface morphology, and the thickness of the RuO2(110) film.10,11 In principle, this strategy should equally work for the growth of single-crystalline IrO2(110) layers on TiO2(110), as the induced strain and involved surface energies are similar to the RuO2(110)/TiO2(110) system (cf. Table S1 in the Supporting Information). Indeed, it was reported that singlecrystalline IrO2(110) films can be grown on TiO2(110) single crystals by molecular beam epitaxy.12−15 The growth was in situ followed by reflection high-energy electron diffraction and ex situ characterized by transmission electron microscopy and X-ray diffraction. In this article, we are focusing on the preparation of singlecrystalline IrO2(110) films grown on single-crystal TiO2(110) that is partially reduced to improve its electronic conductivity.16 The growth of IrO2(110) on TiO2(110) on the atomic scale is followed with scanning tunneling microscopy (STM) Received: March 6, 2019 Revised: April 24, 2019 Published: May 24, 2019 7720
DOI: 10.1021/acs.langmuir.9b00667 Langmuir 2019, 35, 7720−7726
Article
Langmuir
Figure 1. STM images [300 nm × 300 nm; (a) U = 2 V, I = 1.2 nA; (b−f) U = 1 V, I = 1.2 nA] for IrO2 islands and films grown on TiO2(110) with increasing deposition time including some line scans: (a) clean TiO2(110), metallic iridium is deposited at room temperature for various deposition times (b) 1 min, (c) additional 2 min (total 3 min.), (d) additional 3 min (total 6 min), (e) additional 6 min (total 12 min), (f) additional 12 min (total 24 min), (g) additional 28 min (total 52 min), (h) additional 28 min (total 80 min), (i) additional 40 min, (total 120 min). One ML of iridium corresponds to a deposition time of about 4.8−5 min. (b−f) Deposition of metallic Ir at room temperature and post-oxidation at a temperature of 700 K in an O2 atmosphere of 1 × 10−4 mbar. (g−i) During further growth the sample was kept at a temperature of 700 K in an O2 atmosphere of 1 × 10−6 mbar and then additionally post-oxidized with 10−4 mbar for 20 min. sample to 950 K under UHV conditions for 3 h, leading to a light-blue TiO2−x(110) sample, a slightly bulk-reduced sample. A mild reoxidation in an oxygen atmosphere of 5 × 10−6 mbar at 950 K for 1 min allowed to re-oxidize the surface of the reduced TiO2(110) sample, while maintaining the degree of chemical reduction in the bulk material (cf. Figure 1). On such a prepared TiO2−x(110) sample, iridium was stepwise deposited and oxidized by physical vapor deposition using a well-outgassed electron beam evaporator at 700 K in an oxygen atmosphere of 10−6 mbar. With STM and XPS the deposition rate of iridium was calibrated to be approximately one monolayer (ML) per 5 min. In the low coverage regime, the island size and thickness were determined by STM for a few deposition times and translated to an IrO2 coverage. This calibration was counter-checked by the attenuation of the Ti 2p with the deposition time of Ir. All STM images presented in this paper were taken at room temperature in the constant current mode with the sample positively biased, representing empty states of the surface. Typical sample voltages and tunneling currents used were 0.5−4.5 V and 0.8−1.5 nA, respectively. Additionally, the IrO2(110) films grown on TiO2(110) were characterized by scanning electron microscopy (SEM) experiments (Zeiss Merlin). Those were conducted with an acceleration voltage of 2 kV and a probe current of 100 pA. The scanning electron micrographs were acquired with the inlens detector. For the electrochemical experiments the TiO2(110) substrate was reduced by annealing the sample under UHV conditions at 1170 K for 12 min followed by 4 min at 1220 K. The temperature was measured with a K type thermocouple which was spot-welded to a tantalum plate on which the sample was placed. Details of the home-built UHV system can be found elsewhere. 18 The thermally reduced
and X-ray photoelectron spectroscopy (XPS). It is shown that thermal decomposition at 725 K under UHV conditions leads to severe reduction of the IrO2(110) layer that cannot be restored by a subsequent re-oxidation step.
■
EXPERIMENTAL SECTION
The experiments were conducted in a home-built three-chamber UHV system.17 The sample can be introduced via the load lock chamber that contains a small sample manipulator and a magnetic rod for sample transfer to the long-traveling sample manipulator in the analysis chamber. This chamber houses a quadrupole mass spectrometer and a dual X-ray source together with a hemispherical analyzer (PSP Vacuum Technology) to conduct XPS experiments. In addition, the analysis chamber contains an electron beam evaporator (tectra e-flux). The scanning tunneling microscope chamber is separated from the analysis chamber by a CF150 gate valve and separately pumped by an ion getter pump (100 L/s). The sample can be transferred from the analysis chamber to the STM chamber just by translating the manipulator through the open CF150 gate valve and then placing the sample plate into the scanning tunneling microscope (VT-STM, Omicron) with a wobble stick. In general, we used homemade tungsten tips for the STM experiments. The sample temperature was measured with an infrared pyrometer, which was pre-calibrated with a K-type thermocouple. Prior to the film growth, the TiO2(110) crystal (hat-shaped disk, 4.7 mm diameter, MaTecK, Jülich, Germany) was cleaned by cycles of Ar-ion sputtering (p(Ar) = 10−6 mbar, U = 1.0−1.5 kV, Iemission = 20 mA) and subsequent annealing at 950 K in 10−7 mbar of oxygen. In order to increase the electronic conductivity of TiO2, we annealed the 7721
DOI: 10.1021/acs.langmuir.9b00667 Langmuir 2019, 35, 7720−7726
Article
Langmuir
Figure 2. Following the growth of IrO2 on TiO2(110) with XPS of various layer thicknesses (as defined in Figure 1): (a) Ti 2p; (b) Ir 4f, and (c) O 1s. For clarity each spectrum is offset. In the lower part of the panels the integral intensities of (a) Ti 2p and (b) Ir 4f are shown as a function of the thickness of IrO2. The film thickness was determined from the attenuation of the Ti 2p signal by using the effective mean free path of the photoelectron through the IrO2 layer of 14.2 Å taken from NIST (red line). TiO2−x(110) sample was subsequently transferred to the STM chamber to prepare the IrO2(110) film. Cyclic voltammetry experiments were conducted in an electrochemical glass cell utilizing a hanging-meniscus setup, so that only the IrO2(110) film was exposed to the electrolyte solution, an aqueous 0.5 M H2SO4 solution (pH = 0.4) prepared from H2SO4 Suprapur (Merck, Darmstadt, Germany) and high-purity water. Prior to the experiments the electrolyte was flushed with argon, whereas during the measurements the atmosphere above the electrolyte solution was kept in argon. A saturated Ag/AgCl electrode served as the reference electrode, the counter electrode consisted of a glassy carbon rod. All electrode potential values are given versus the standard hydrogen electrode. The employed potentiostat was a PGSTAT302N (AutolabMetrohm) equipped with modules enabling electrochemical impedance spectroscopy (EIS) and truly analog voltage sweeps.
slightly higher cluster of 11−12 Å height, whereas the lateral cluster size increased to 8−14 nm. The resulting islands are significantly flatter than those in Figure 1b,c. After additional deposition of for 6 min, the STM image in Figure 1e indicates flat islands (size 15−20 nm) that partly emerge into larger islands with no preferential growth direction. The underlying substrate morphology modulates the height of the islands. On this surface, additional Ir was deposited for 12 min at 700 K. The STM image in Figure 1f exhibits large flat islands (20−30 nm in size) with a slight preference of the growth along the [001] direction. Still the morphology of the TiO2(110) substrate can be recognized in the height distribution of the islands. With increasing Ir deposition (additional 28 min: Figure 1g) the preferential growth along the [001] becomes very clear, with domains strongly elongated along the [001] direction by several 100 nm, whereas the domain size in the [1−10] direction is limited to 10−20 nm. The morphology of the underlying TiO2(110) substrate is not reflected in the height modulation of the IrO2 film. The step height between the neighboring terraces is 3.2 Å. From STM, it seems that the IrO2(110) domains do not always merge along the [001] direction, thereby leaving 1.3−1.5 nm deep holes in the film (marked by black circles in Figure 1g). We should remember that these pits may be even deeper because of the finite size of the STM tip. With further Ir deposition and oxidation the overall surface morphology remains unchanged (cf. Figure 1h,i), whereas the density of the holes is reduced; the depth of the pits are still about 1.5 nm. The growth of IrO2(110) on TiO2(110) was also studied by XPS (cf. Figure 2). We started from the clean TiO2(110) with characteristic Ti 2p, Ti 3s, and O 1s features known from the literature.16,19−21 The O 1s peak appears symmetric. Of metallic Ir, 2.5 ML was deposited onto the TiO2(110) surface at room temperature and then post-oxidized in p(O2) = 1 × 10−4 mbar at 700 K. The intensity of the Ti 2p spectrum (cf. Figure 2a) decreases, whereas the Ir 4f spectrum shows a duplet Ir 4f7/2 and Ir 4f5/2 at 62.0 and 65.0 eV (cf. Figure 2b)
■
RESULTS AND DISCUSSION The growth of IrO2 on TiO2(110) was studied by STM (cf. Figure 1). The clean TiO2(110) surface in Figure 1a reveals large terraces which are separated by mono-atomic steps of a height of 3.2 Å. The round-shaped terraces are due likely to the repeated sputter and annealing cycles of TiO2(110) with a maximum annealing temperature of 1000 K being too low to allow for the formation of straight step edges. On this prepared TiO2(110) surface, Ir was deposited for increasing deposition times. To learn more about the growth of IrO2(110) on TiO2(110) we focus on the initial growth of IrO2(110). We deposited successively more Ir on the surface at room temperature and post-oxidized the layer at 700 K in an oxygen atmosphere of 10−4 mbar for 5 min. In Figure 1b, we show an STM image of IrO2 when Ir was deposited for 1 min. Most of the clusters visible in STM decorate the step edges of TiO2(110). The cluster height is about 8 Å, whereas the lateral size varies from 4 to 9 nm. Adding on this surface additional Ir (deposition of 2 min) followed by post-oxidation we observe in STM (cf. Figure 1c) that the clusters grow laterally (7−11 nm) without varying their height. Adding even more Ir (deposition of 3 min) followed by post-oxidation (cf. Figure 1d) leads to a 7722
DOI: 10.1021/acs.langmuir.9b00667 Langmuir 2019, 35, 7720−7726
Article
Langmuir that is assigned to Ir4+, consistent with previous XPS experiments of bulk IrO2.5,22 The apparently wrong intensity ratio of Ir 4f7/2 to Ir 4f5/2 (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.22 The O 1s signal (Figure 2c) at 529.4 eV is slightly shifted to higher binding energies and becomes asymmetric in shape. This asymmetry of the O 1s feature is due to both the superposition of O 1s from TiO2 (529.4 eV) with that of IrO2 (530 eV) and in addition because of the metallic nature of IrO2. The metallic nature of IrO2 is revealed in the valence band spectra as well (Figure S1a). Note that for metallic systems the O 1s (and also other photoemission lines) becomes asymmetric in shape because of electron−hole pair excitation with low excitation energy. This happens already for low IrO2 coverages (cf. O 1s spectra in Figure S1b), although for O 1s the observed asymmetry may be due partly to OH formation. In the next step, additional 2.5 ML of Ir were deposited in an oxygen atmosphere of p(O2) = 1 × 10−6 mbar at 700 K. In order to improve the crystallinity, the resulting layer was postoxidized in p(O2) = 1 × 10−4 mbar at 700 K for 20 min. With increasing Ir deposition, the Ti 2p signal is attenuated exponentially, and the Ir 4f grows linearly first and then saturates (cf. Figure 2a,b, lower panels). Here, we assume the growth of a homogeneous film; for the case of layer-by-layer growth one would expect to find an overall exponential decay plus break points after completion of the ML.23 The O 1s peak has shifted to 530 eV. Finally, between 16 ML and 24 ML of IrO2(110) the Ti 2p signal vanishes completely, evidencing a covering film of IrO2(110). The saturation behavior of the Ir 4f signal and the attenuation of the Ti 2p signal fit well the layerby-layer growth of a covering IrO2(110) film (in total 24 ML). Assuming that the 24 ML O 1s spectrum consists only of O from IrO2, the other O 1s spectra in Figure 2c can be approximated as a linear superposition of O 1s(TiO2) and O 1s(IrO 2 ) spectra (cf. Figure S2a in the Supporting Information). It turns out that the 2.5 ML O 1s spectrum consists of 50% O 1s(TiO2) that gradually decreases to 35% for the 5 ML O 1s spectrum and reaches 0% O 1s (TiO2) already for the 10.5 ML IrO2(110) film on TiO2(110). In Figure 3 we depict an scanning electron (SE) micrograph and the low energy electron diffraction (LEED) pattern of a 24 ML thick IrO2(110) layer grown on TiO2(110). Clearly, in the SE micrograph a texture of the film is visible that is likely to be caused be the elongated IrO2(110) terraces in the [001] direction and this mesoscopic feature can be advantageous in future electrochemical stability studies to recognize morphological changes under OER conditions with SEM. The corresponding LEED patterns at 50 and 120 eV indicate high surface crystallinity of the IrO2(110) film, which is consistent with the corresponding STM images in Figure 1. We prepared a 12 ML thick IrO2(110) film on TiO2(110) and studied with STM its thermal induced reduction (decomposition) with increasing annealing temperature under UHV conditions. In Figure 4a we present a highresolution STM image of the grown film, clearly indicating protruding rows along the [001] direction that are separated by 6.4 Å and likely to be ascribed to oxygen rows on IrO2(110). On the terraces there are few small clusters (height: 1.5−4 Å; lateral dimension: 13−15 Å) randomly distributed. The step edges are mostly straight with a step height of 3.2 Å. Closer inspection (zoom of Figure 4a) reveals that part of the surface exposes a c(2 × 2) structure rather than a bulk-
Figure 3. SE micrograph (a) of 24 ML-IrO2(110)−TiO2(110). The texture because of elongated IrO2(110) along the [001] direction (white double-headed arrow) is well visible. The LEED patterns at E = 50 eV(b) and 120 eV (c) indicate a single crystalline surface.
truncated IrO2(110)-(1 × 1) structure. The c(2 × 2) may be formed when every second O atom is removed from the Orows and neighboring rows are shifted by a surface lattice constant along the [001] direction. In LEED the c(2 × 2) reflections are not reconciled likely because of antiphase correlation of the c(2 × 2) islands together with a low concentration of these islands. Upon annealing to 465 K in UHV (Figure 4b), the concentration and the size of the clusters increase (height: 4−6 Å; lateral dimension: 24−35 Å). The morphology of the step edges is hardly affected. When increasing the annealing temperature to 605 K (Figure 4c), the concentration and size of the clusters increase significantly (height: 4.5−6 Å; lateral dimension: 36−40 Å). The step edges are roughened. Up to annealing temperatures of 605 K, the IrO2(110) can be recovered by oxidation at 700 K in an oxygen atmosphere of 10−4 mbar. The appearance of larger clusters is paralleled by a roughening of the step edges of IrO2(110). Very likely the observed clusters are made from Ir coming from the step edge. On annealing the surface at 725 K, round- and hexagonalshaped flat clusters, presumably metallic Ir, are discernible with a height of 5.5−6.5 Å and lateral dimension of 40−50 Å. Step edges of IrO2(110) are not discernible in STM. A zoom into the STM image (inset Figure 4d) reveals that the Ir domains are lying partly on a still intact IrO2(110) terrace. Simple post-oxidation is not able to restore the IrO2(110) film. When increasing the annealing temperature to 871 K (Figure 4e), the concentration of metallic Ir domains decreases and their lateral size increases (height: 5−6 Å; lateral dimensions: 80−120 Å). Finally, increasing the annealing temperature to 1015 K (Figure 4f), the flat Ir domains are 20−30 Å thick and 110−140 Å wide. As the thickness of the Ir domains is now compatible with the thickness of the original IrO2(110) film (about 30−40 Å with 60% Ir density), the supported IrO2(110) film is considered to be fully reduced (thermally decomposed). The thermally induced reduction of 12 ML-IrO2(110)− TiO2(110) was also studied by XPS as summarized in Figure 5. The chemical reduction of IrO2(110) can best be seen in the Ir 4f doublet, which shifts by 0.7 eV to lower binding energies, 7723
DOI: 10.1021/acs.langmuir.9b00667 Langmuir 2019, 35, 7720−7726
Article
Langmuir
Figure 4. STM images [(a) 100 nm × 50 nm, the zoom is 40 nm × 20 nm, (b−f) 100 nm × 100 nm; the zooms are 20 nm × 20 nm; U = 1 V, I = 1.2 nA] of the thermal reduction/decomposition of a 12 ML thick IrO2(110) film grown on TiO2(110) at various temperatures indicated under UHV conditions.
Figure 5. Following the thermal induced reduction of 12 ML IrO2(110) on TiO2(110) with XPS: (a) Ti 2p; (b) Ir 4f, and (c) O 1s. For clarity each spectrum is offset.
IrO2 with its asymmetric shape. All other spectra can be considered therefore as a linear combination of these two spectra. In this way the contribution of O 1s from TiO2 and IrO2 can be disentangled (cf. Figure S2c, Supporting Information). According to the linear combinations we conclude that the reduction of IrO2 to metallic Ir starts already below 605 K, fully consistent with the STM and XPS experiments in Figures 4 and 5. From the Ti 2p spectra in Figure 5a we can conclude that up to 725 K, the buried TiO2 is covered by the IrO2 or by a Ir/IrO2 coating. Dewetting of the Ir film sets in at 725 K and becomes quite remarkable at an annealing temperature of 1015 K. This observation is consistent with the corresponding STM images that show deep grooves between neighboring metallic Ir domains. The designed IrO2(110) films on TiO2(110) will be used as a model system in the electrocatalysis of water-splitting (OER). A 11 ML thick IrO2(110) film on TiO2(110) was prepared and then characterized by cyclic voltammetry. A cyclic voltammogram (CV) of the IrO2(110)−TiO2(110) model electrode in 0.5 M H2SO4 is shown in Figure 6. The
indicative of a transformation from IrO2 (as prepared) to metallic Ir (annealed to 1015 K). For temperatures between 465 and 871 K the Ir 4f spectra consist of a linear superposition of the IrO2 spectrum and the metallic Ir spectrum (Figure 5b, 1015 K). The same information is obtained when looking at the O 1s spectra. Assuming that the 1015 K Ir 4f spectrum in Figure 5b consists only of metallic Ir, the other Ir 4f spectra can be approximated as a linear superposition of Ir 4f(prepared) and Ir 4f(1150 K) (cf. Figure S2b, Supporting Information). Up to annealing temperature of 465 K no metallic feature is discernible in the Ir 4f spectrum. However, for annealing to 605, 725, and 871 K, the metallic Ir component increases to 10, 22, and 38%, respectively. The O 1s peak (Figure 5c) of IrO2 is asymmetric because of electron−hole pair excitation of metallic IrO2, whereas the O 1s peak of TiO2(110) is symmetric because of the semiconducting property of TiO2. Actually, the O 1s spectrum for 1015 K is symmetric, so that this O species must originate from the TiO2(110) substrate. As the IrO2 film is 12 ML thick, the O 1s spectrum of the “prepared” one is dominated by O in 7724
DOI: 10.1021/acs.langmuir.9b00667 Langmuir 2019, 35, 7720−7726
Article
Langmuir
the IrO2(110)−TiO2(110) model electrode is consistent with electrochemical data for other IrO2(110) films in acidic medium.34 With EIS the resistance of the thermally reduced TiO2−x(110) substrate plus the cell/electrolyte was determined to be ∼275 Ω, the main part of which is expected to be due to the TiO2 substrate. However, with a resistance in this range the TiO2−x(110) substrate is suitable for electrocatalytic studies. Flat IrO2(110) films grown on heavily reduced TiO2(110) will be employed in future experiments to study the electrochemical stability of such model electrodes under OER conditions.
■
CONCLUSIONS We successfully prepared IrO2(110) films on TiO2(110) by physical vapor deposition of Ir in an oxygen atmosphere of 10−6 mbar at 700 K with a high level of control on the crystallinity, the morphology, and the thickness. The growth was studied by STM, LEED, and XPS. The resulting IrO2(110) films are not thermally very stable. To improve the crystallinity of the resulting IrO2(110) film the surface was post-oxidized in 10−4 mbar of O2 at 700 K for 5 min. Already annealing to the temperature range of 465−605 K under UHV conditions leads to partial reduction of the IrO2(110) film, which can be recovered by post-oxidation at 700 K in 10−4 mbar of O2. Thermal decomposition of the IrO2(110) at 725 K leads to substantial decomposition that is not possible to recover by post-oxidation. In a model experiment we demonstrate that this IrO2(110)−TiO2(110) system can be utilized as a model electrode, for instance, in the OER.
Figure 6. CV of the 11 ML IrO2(110)−TiO2(110) surface in 0.5 M H2SO4 utilizing a hanging-meniscus setup. The scan rate was 50 mV· s−1, the potential was corrected for the ohmic drop.
potential was corrected for the ohmic drop because of cell/ electrolyte and substrate resistance (∼275 Ω as determined with EIS). The broad anodic feature in the potential region from 0.9 to 1.3 V can be attributed to Ir redox transitions at the surface.24−27 The rise in current density above 1.4 V is due to the beginning of OER at IrO2(110).24 Recently, we succeeded to grow single-crystalline IrO2(110) on a RuO2(110)/Ru(0001) template.7 However, three rotational domains of IrO2(110) form on the three rotational domains of RuO2(110) on Ru(0001) as RuO2(110) exhibits only a twofold axis, whereas the symmetry group of the Ru(0001) substrate contains a threefold axis. Therefore, the choice of TiO2(110) as the substrate for growing IrO2 films is motivated by two reasons. First, the IrO2(110) film will grow only as a single domain on TiO2(110), thus avoiding the formation of surface grain boundaries. Second, TiO2 alone is catalytically inactive so that the observed activity of IrO2(110)−TiO2(110) in heterogeneous gas phase catalysis or in electrocatalysis (such as OER) can solely be traced to the contribution of the IrO2 film. The thermal stability of IrO2(110)−TiO2(110) under UHV conditions is remarkably low. Already around 465 K decomposition of the IrO2(110) film sets in. This thermalinduced decomposition of IrO2(110) is facilitated by (or maybe even due to) chemical reduction with hydrogen from the residual gas phase forming water that starts to desorb at 400 K.28 However, the partially reduced IrO2(110) layer that is prepared by annealing the sample up to 605 K under UHV conditions can be restored by re-oxidation in an oxygen atmosphere of 10−4 mbar at 700 K. Quite in contrast, decomposition of IrO2(110) above 725 K will lead to a partially reduced IrO2(110) film that cannot be re-oxidized by excessive oxygen exposure at 700 K under typical UHV conditions. Similar results were reported for the IrO2(110) on Ir(100)2 and for single-crystalline IrO2(110) films grown on a directing RuO2(110)/Ru(0001) template.7 This low thermal stability contrasts the high electrochemical stability of IrO2(110) in the potential region of OER.29−31 Quite in contrast to IrO2(110), RuO2(110) films are thermally stable up to 700 K under UHV conditions, whereas their electrochemical stability under OER conditions is low.32,33 With cyclic voltammetry it is demonstrated that the present IrO2(110)−TiO2(110) model system is applicable for electrochemical experiments. The electrochemical characterization of
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00667. Dimensions of the surface unit cells of the rutile (110) orientation of various native rutile oxides; valence band spectra of IrO2(110) grown onTiO2(110); and decomposition of the spectra for varied MLs in pure TiO2 and fully covered 24 ML IrO2(110)−TiO2(110) spectra (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Herbert Over: 0000-0001-7689-7385 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We acknowledge financial support by the BMBF (project: 05K2016-HEXCHEM). We would like to thank the Center of Materials Research (LaMa) at Justus Liebig University Giessen for the support of this project.
■
REFERENCES
(1) Trasatti, S. Electrocatalysis: Understanding the Success of DSA. Electrochim. Acta 2000, 45, 2377−2385. (2) Liang, Z.; Li, T.; Kim, M.; Asthagiri, A.; Weaver, J. F. LowTemperature Activation of Methane on the IrO2(110) Surface. Science 2017, 356, 299−303 . Supporting Information Figure S2 . 7725
DOI: 10.1021/acs.langmuir.9b00667 Langmuir 2019, 35, 7720−7726
Article
Langmuir (3) Moser, M.; Mondelli, C.; Amrute, A. P.; Tazawa, A.; Teschner, D.; Schuster, M. E.; Klein-Hoffman, A.; López, N.; Schmidt, T.; Pérez-Ramírez, J. HCl Oxidation on IrO2-Based Catalysts: From Fundamentals to Scale-Up. ACS Catal. 2013, 3, 2813−2822. (4) He, Y. B.; Stierle, A.; Li, W. X.; Farkas, A.; Kasper, N.; Over, H. Oxidation of Ir(111): From O−Ir−O Trilayer to Bulk Oxide Formation. J. Phys. Chem. C 2008, 112, 11946−11953. (5) Chung, W.-H.; Tsai, D.-S.; Fan, L.-J.; Yang, Y.-W.; Huang, Y.-S. Surface Oxides of Ir(111) Prepared by Gas-Phase Oxygen Atoms. Surf. Sci. 2012, 606, 1965−1971. (6) Rai, R.; Li, T.; Liang, Z.; Kim, M.; Asthagiri, A.; Weaver, J. F. Growth and Termination of a Rutile IrO2(100) Layer on Ir(111). Surf. Sci. 2016, 652, 213−221. (7) Abb, M. J. S.; Herd, B.; Over, H. Template-Assisted Growth of Ultrathin Single-Crystalline IrO2(110) Films on RuO2(110)/Ru(0001) and Its Thermal Stability. J. Phys. Chem. C 2018, 122, 14725− 14732. (8) Diebold, U.; Pan, J.-M.; Madey, T. E. Ultrathin Metal-Film Growth on TiO2(110): An Overview. Surf. Sci. 1995, 331−333, 845− 854. (9) Chambers, S. A. Epitaxial Growth and Properties of Thin Film Oxides. Surf. Sci. Rep. 2000, 39, 105−180. (10) He, Y.; Langsdorf, D.; Li, L.; Over, H. Versatile Model System for Studying Processes Ranging from Heterogeneous to Photocatalysis: Epitaxial RuO2(110) on TiO2(110). J. Phys. Chem. C 2015, 119, 2692−2702. (11) Herd, B.; Abb, M.; Over, H. Photo-Induced Morphology Changes at the RuO 2(110)/TiO2(110) Surface: A Scanning Tunneling Microscopy Study. Top. Catal. 2017, 60, 533−541. (12) Kawasaki, J. K.; Uchida, M.; Paik, H.; Schlom, D. G.; Shen, K. M. Evolution of Electronic Correlations Across the Rutile, Pervoskite, and Ruddelsden-Popper Iridates with Octahedral Connectivity. Phys. Rev. B 2016, 94, No. 121104(R). (13) Kawasaki, J. K.; Baek, D.; Paik, H.; Nair, H. P.; Kourkoutis, L. F.; Schlom, D. G.; Shen, K. M. Rutile IrO2/TiO2 Superlattices: A Hyperconnected Analog to the Ruddelsden-Popper Structure. Phys. Rev. Mater. 2018, 2, 054206. (14) Hou, X.; Takahashi, R.; Yamamoto, T.; Lippmaa, M. Microstructure Analysis of IrO2 thin Films. J. Cryst. Growth 2017, 462, 24−28. (15) Uchida, M.; Sano, W.; Takahashi, K. S.; Koretsune, T.; Kozuka, Y.; Arita, R.; Tokura, Y.; Kawasaki, M. Field-Direction Control of the Type of Charge Carriers in Nonsymmorphic IrO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, No. 241119(R). (16) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (17) Herd, B.; Knapp, M.; Over, H. Atomic Scale Insights into the Initial Oxidation of Ru(0001) Using Molecular Oxygen: A Scanning Tunneling Microscopy Study. J. Phys. Chem. C 2012, 116, 24649− 24660. (18) Farkas, A.; Mellau, G. C.; Over, H. Novel Insight in the CO Oxidation on RuO2(110) by in Situ Reflection−Absorption Infrared Spectroscopy. J. Phys. Chem. C 2009, 113, 14341−14355. (19) Göpel, W.; Anderson, J. A.; Frankel, D.; Jaehnig, M.; Phillips, K.; Schäfer, J. A.; Rocker, G. Surface Defects of TiO2(110): A Combined XPS, XAES AND ELS Study. Surf. Sci. 1984, 139, 333− 346. (20) Mayer, J. T.; Diebold, U.; Madey, T. E.; Garfunkel, E. Titanium and Reduced Titania Overlayers on Titanium Dioxide(110). J. Electron Spectrosc. Relat. Phenom. 1995, 73, 1−11. (21) Handbook of X-ray Photoelectron Spectroscopy; Moulder, J. F., Stickle, W. F., Sobol, P. E., Bomben, K. D., Chastain, J., King, R. C., Jr., Eds.; Physical Electronics: Lake Drive East, Chanhassen, MN, 1995; pp 72−73. (22) Wertheim, G. K.; Guggenheim, H. J. Conduction-Electron Screening in Metallic Oxides: IrO2. Phys. Rev. B: Condens. Matter Mater. Phys. 1980, 22, 4680−4683. (23) Seah, M. P. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, 1983; p 211.
(24) Kuo, D.-Y.; Kawasaki, J. K.; Nelson, J. N.; Kloppenburg, J.; Hautier, G.; Shen, K. M.; Schlom, D. G.; Suntivich, J. Influence of Surface Adsorption on the Oxygen Evolution Reaction on IrO2(110). J. Am. Chem. Soc. 2017, 139, 3473−3479. (25) Mo, Y.; Stefan, I. C.; Cai, W.-B.; Dong, J.; Carey, P.; Scherson, D. A. In Situ Iridium LIII-Edge X-ray Absorption and Surface Enhanced Raman Spectroscopy of Electrodeposited Iridium Oxide Films in Aqueous Electrolytes. J. Phys. Chem. B 2002, 106, 3681− 3686. (26) Hüppauff, M.; Lengeler, B. Valency and Structure of Iridium in Anodic Iridium Oxide Films. J. Electrochem. Soc. 1993, 140, 598−602. (27) Pauporté, T.; Aberdam, D.; Hazemann, J.-L.; Faure, R.; Durand, R. X-ray Absorption in Relation to Valency of Iridium in Sputtered Iridium Oxide Films. J. Electroanal. Chem. 1999, 465, 88− 95. (28) Li, T.; Kim, M.; Liang, Z.; Asthagiri, A.; Weaver, J. F. Dissociative Chemisorption and Oxidation of H2 on the Stochiometric IrO2(110) Surface. Top. Catal. 2018, 61, 397−411. (29) Spöri, C.; Kwan, J. T. H.; Bonakdarpour, A.; Wilkinson, D. P.; Strasser, P. The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation. Angew. Chem., Int. Ed. 2017, 56, 5994−6021. (30) Siracusano, S.; Hodnik, N.; Jovanovic, P.; Ruiz-Zepeda, F.; Š ala, M.; Baglio, V.; Aricò, A. S. New Insights into the Stability of a High Performance Nanostructured Catalyst for Sustainable Water Electrolysis. Nano Energy 2017, 40, 618−632. (31) Kasian, O.; Grote, J.-P.; Geiger, S.; Cherevko, S.; Mayrhofer, K. J. J. The Common Intermediates of Oxygen Evolution and Dissolution Reactions during Water Electrolysis on Iridium. Angew. Chem., Int. Ed. 2018, 57, 2488−2491. (32) Kötz, R.; Stucki, S. Stabililsation of RuO2 by IrO2 for Anodic Oxygen Evolution in Acid Media. Electrochim. Acta 1986, 31, 1311− 1316. (33) Cherevko, S.; Geiger, S.; Kasian, O.; Kulyk, N.; Grote, J.-P.; Savan, A.; Shrestha, B. R.; Merzlikin, S.; Breitbach, B.; Ludwig, A.; Mayrhofer, K. J. J. Oxygen and Hydrogen Evolution Reactions on Ru, RuO2, Ir and IrO2 Thin Film Electrodes in Acidic and Alkaline Electrolytes: A Comparative Study on Activity and Stability. Catal. Today 2016, 262, 170−180. (34) Kuo, D.-Y.; Paik, H.; Nelson, J. N.; Shen, K. M.; Schlom, D. G.; Suntivich, J. Chlorine Evolution Reaction Electrocatalysis on RuO2(110) and IrO2(110) Grown Using Molecular-Beam Epitaxy. J. Chem. Phys. 2019, 150, 041726.
7726
DOI: 10.1021/acs.langmuir.9b00667 Langmuir 2019, 35, 7720−7726
