Oxidation and Reduction of TiOx Thin Films on Pd(111) and Pd(100

Article pubs.acs.org/JPCB

Oxidation and Reduction of TiOx Thin Films on Pd(111) and Pd(100) M. H. Farstad,† D. Ragazzon,‡ M. D. Strømsheim,† J. Gustafson,§ A. Sandell,‡ and A. Borg*,∥ †

Department Department § Department ∥ Department ‡

of of of of

Chemical Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway Physics and Astronomy, Uppsala University, P.O. Box 530, SE-75121 Uppsala, Sweden Physics and Astronomy, Uppsala University, P.O. Box 530, SE-75121 Uppsala, Sweden Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway

ABSTRACT: Thin films of TiOx on Pd(100) and Pd(111) have been investigated with respect to their properties after oxidation and reduction cycles. High-resolution photoemission spectroscopy (HRPES) and low energy electron diffraction (LEED) have been applied to characterize the thin film oxidation states and structure before and after oxidation and reduction under ultrahigh vacuum conditions. Fully oxidized TiO2 films were formed on both surfaces. These structures display Moiré patterns in LEED, in one dimension for Pd(100) and in two dimensions for Pd(111), and they have previously not been reported for TiO2/Pd. The oxidation process causes strong reduction in the interaction between the oxide thin film and the Pd substrate, most significantly for Pd(111). Reversible oxidation/reduction cycling of TiOx thin films on Pd(111) and Pd(100) was possible.

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INTRODUCTION TiO 2 is a remarkable metal oxide finding numerous applications, among others due to its photocatalytic properties. Water splitting and purification are examples of reactions photocatalyzed by TiO2.1,2 Titania also finds applications in heterogeneous catalysis, as oxide support material for metallic particles but also in the form of titania particles on metal substrates. An interesting combination is Au/TiO2, where TiO2 supported Au nanoclusters are active for CO oxidation3,4 as well as for H2 production from ethanol−water mixtures.5 The inverse system, with Au serving as support for TiO x nanostructures, has been reported to be active toward the water−gas shift reaction, and the boundary between the materials ascribed a significant role for the catalytic behavior.6 Compared to Au, Pd is a more versatile catalyst, among others because it more readily adsorbs and splits molecules, such as O2 and H2. TiO2 supported Pd particles have been considered as a catalyst for the water−gas shift reaction7−9 and also for hydrogenation of CO2 to methanol.10 Unlike Au/TiO2, the Pd/TiO2 system displays strong metal−support interaction (SMSI).11−15 Heating Pd particles supported on TiO2 in a reducing atmosphere has been reported to produce a reduced titania layer (TiOx, x < 2) on the metal particles.14,15 The thin film termed “zigzag” on the Pd(111) particles strongly resembles the TiOx phase observed on the Pd(100) single crystal.16 This TiOx overlayer may potentially suppress the catalytic activity if it persists during catalytic reaction conditions. Thus, understanding the behavior of TiOx thin films on Pd model surfaces during oxidation and reduction conditions may provide valuable insight in this context and is the target of the present work. © XXXX American Chemical Society

Previously, we have investigated the structure and oxidation state of TiOx thin films grown on Pd(111) and Pd(100) using chemical vapor deposition (CVD) with titanium(IV) isopropoxide (TTIP) as the precursor. Figure 1 summarizes the results obtained by high-resolution photoelectron spectroscopy (HRPES) and low energy electron diffraction (LEED).16 For each surface, two different TiOx phases were distinguished, in addition to a Pd−Ti alloy phase which formed during the initial stages of growth. The lower spectra in Figure 1 display a Ti 2p3/2 peak at about 455 eV binding energy (BE) (marked in yellow), which is assigned to the alloy phase. Further deposition yields a wetting layer for which the Ti 2p3/2 BE is about 456 eV (marked in green). The BE is consistent with Ti2+ species and the composition was determined as TiOx with x = 0.75 ± 0.05, henceforth labeled TiO0.75. The corresponding LEED patterns exhibit an incommensurate structure on Pd(111), while a mixture of structures with (3 × 5) and (4 × 5) periodicities was observed for deposition on Pd(100). Increasing the coverage even more results in a fully oxidized (TiO2) phase on both Pd surfaces, displaying a Ti 2p3/2 peak at 458.6 eV (blue). The LEED patterns suggest the formation of the TiO2(B) polymorph. The TiO2(B) structures reside on a partially oxidized layer (characterized by a Ti3+ peak with a Ti 2p3/2 BE around 457.2 eV, in red) on top of the TiO0.75 wetting layer. In the present work, we continue the study by addressing the oxidation and reduction behavior of the TiO0.75 wetting layer Special Issue: Miquel B. Salmeron Festschrift Received: June 27, 2017 Revised: August 17, 2017 Published: August 21, 2017 A

DOI: 10.1021/acs.jpcb.7b06282 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. LEED images and Ti 2p3/2 spectra of different phases discovered by growth of TiOx by TTIP on (a) Pd(111) and (b) Pd(100) as reported in ref 16.

The Pd 3d5/2 photoemission spectra have been fitted with Doniach−Sunijc line profiles and a linear background. In order to compensate for the rapidly changing density of states around the Fermi level, a peak shifted relative to the bulk peak with 0.43 eV for Pd(100) and 0.53 eV for Pd(111) has been included. The intensity of this peak was locked at 10−12% of the bulk peak for Pd(100) and 15% for Pd(111) following the procedure of Göthelid et al.20,21 The Ti 2p3/2 spectra have been fitted with a Shirley background and Voigt line shapes. The surface coverage was calculated using the intensity of the surface contribution of the Pd 3d5/2 spectra recorded at 400 eV, and the TiOx film thickness was calculated using the attenuation of the bulk Pd 3d5/2 peak recorded at 460 eV in combination with a correction for the estimated surface coverage. This procedure works quite well for higher coverages, but preparations consisting mostly of the alloy phase and/or very low coverages, up to about 20− 30%, suffer from quite high uncertainties. The reported coverages and thicknesses have uncertainties estimated to ±2% and ±0.1 nm, respectively.

grown by CVD on Pd(111) and Pd(100). The effects of oxidation and reduction were monitored by HRPES in combination with LEED. The oxidation and reduction processes of the TiOx thin films were found to be reversible, demonstrated by successful cycling between oxidized and reduced states. Notably, the change in the oxidation state of the film entailed a correlated change in the interaction between the TiOx thin film and the Pd substrate.

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EXPERIMENTAL SECTION The experiments were performed at the I311 beamline17 of the no-longer-existing MAX II ring at the MAX IV Laboratory. The UHV chamber had a base pressure in the low 10−10 mbar range. The chamber was equipped with a sputter gun, sample heating possibilities, and LEED. The HRPES spectra were recorded using the following photon energies: 650 eV for the O 1s region, 590 eV for the Ti 2p region, and both 400 and 450 eV for the Pd 3d region. Fermi edges were recorded after every spectrum for BE calibration. The TiOx thin films were prepared by CVD using TTIP (Sigma-Aldrich, purity 99.999%) as a precursor. TTIP, with the chemical formula Ti[OCH(CH3)2]4, decomposes upon contact with heated surfaces and forms TiOx.18,19 In the present work, depositions were made with the sample kept at 500 °C. The dose was selected to maximize the coverage of TiO0.75 on the surface. Previous growth studies using TTIP as a precursor revealed that for TTIP decomposition at 500 °C there are no C residues on the surface once the Pd surfaces are covered by the TiOx thin film.16 Oxidation of the film was achieved by heating the sample to 500 °C in O2 for 10 min; 5 × 10−7 mbar was used on Pd(111), and 5 × 10−6 mbar was used on Pd(100). The reduction was performed by heating the sample to 500−550 °C in UHV for 5 min unless otherwise stated.

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RESULTS Pd(111). The starting point chosen for the oxidation− reduction study was the TiO0.75 wetting layer. Figure 2 shows Ti 2p3/2 and Pd 3d5/2 spectra of the as-deposited film on Pd(111) as well as the film development after oxidation and reduction. The clean Pd(111) spectra are included for comparison. The Ti2+ peak from the TiO0.75 wetting layer (green) is the dominating contribution in the Ti 2p3/2 spectrum, but small contributions from more reduced Ti (i.e., the surface alloy, yellow component) and more oxidized Ti species (red and blue components) were also present. The corresponding Pd 3d5/2 spectrum shows three different contributions: the bulk Pd peak at BE 334.9 eV (dark gray), B


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disappeared, suggesting a significantly reduced chemical interaction between thin film and Pd(111) substrate. With respect to geometrical structure changes, oxidation leads to a change in the LEED pattern from the one representing the TiO0.75 phase upon growth, shown in Figure 1, to the Moiré pattern shown in Figure 3. The Moiré

Figure 2. Ti 2p3/2 (left) and corresponding Pd 3d5/2 (right) spectra of the TiO0.75 on Pd(111). Starting from the bottom with the clean Pd(111) substrate, second is the initial preparation, third is after oxidation, fourth after reduction, and at the top after reoxidation. Surface coverage (in %) and thin film thickness (in nm) are indicated for each step.

Figure 3. LEED image of the pattern occurring after oxidation of the TiO0.75 phase. The simulated (13 × 13) Moiré pattern is overlaid on the bottom right quarter of the LEED image.

superstructure has a 13 × 13 periodicity with respect to the substrate. On the basis of the LEED pattern, this phase will in the following be referred to as the oxidized Moiré phase, and the corresponding contribution in the Ti 2p3/2 has been given a purple color in order to distinguish it from the as-prepared TiO2 phase grown by CVD, Figure 1. When the oxidized Moiré phase is annealed in a vacuum, it gradually reduces. After 20 min of annealing at temperatures between 500 and 550 °C, the Ti 2p3/2 spectrum (Figure 2) is comparable to the as-prepared TiO0.75 film. Consistently, the LEED pattern has transformed to the pattern of the as-prepared film. In the Pd 3d5/2 spectrum, the surface peak has almost disappeared and the interaction peak has reappeared. The measured film thickness is comparable to the thickness of the as-prepared film, 0.4 nm. The surface covered by the thin film is slightly less (92%) compared to the initial coverage of 97%. This finding indicates a minor decrease in the amount of Ti as a result of the treatment. In order to verify the reversibility of the oxidation/reduction process, the preparation was oxidized once more. The LEED pattern and HRPES spectra all show the same results as after the first oxidation. The only apparent difference is slightly broader peaks in the Ti 2p3/2 spectrum, and a film covering a smaller fraction of the Pd(111) surface which in addition has a smaller apparent height. This adds to the observation of a minor decrease in the amount of Ti observed during the oxidation/reduction cycle. A low coverage (∼10%) preparation was also used to investigate the full reduction sequence from the oxidized Moiré phase to the alloy phase. The as-prepared film, consisting predominantly of the alloy phase, was subsequently oxidized. Next, the film was reduced by gradual heating to higher temperatures, while the Ti 2p3/2 region was monitored. The

the surface contribution at BE 334.6 eV (light gray), and a component at BE 335.3 eV (white fill). The latter component originates from the interaction between Pd atoms in the outermost Pd layer and the TiOx thin film, as determined during the growth studies based on correlation with coverage and photon energy.16 This component will henceforth be referred to as the “interaction component”. Pd surfaces are known to exhibit core level shifts in this energy range as a result of adsorbed species; shifts due to CO and O have been particularly well characterized.22−24 The last peak is the correction for the rapidly changing density of states around the Fermi level (patterned fill). In total, 97% of the Pd surface was initially covered by the TiO0.75 film, as judged by the attenuation of the surface component from the clean Pd surface. The average film thickness was estimated to 0.5 nm. This preparation was then oxidized at 5 × 10−7 mbar of O2 pressure, reduced by heating in UHV, and subsequently reoxidized. The resulting Ti 2p3/2 and Pd 3d5/2 spectra are presented in Figure 2, in chronological order, starting from the bottom. The HRPES spectra reveal that the oxidation procedure produced a thin film almost entirely consisting of stoichiometric TiO2. The alloy component has almost completely vanished. In the Pd 3d5/2 spectrum, the intensity of the Pd surface component has increased to a level corresponding to a film coverage of 49%. The thin film thickness has increased from 0.5 to 2.0 nm, consistent with a decrease in the fraction of surface covered, that is, a rougher morphology. Another change in the Pd 3d5/2 spectrum is that the interaction component (marked in white) has practically C


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The Journal of Physical Chemistry B resulting map of the Ti 2p3/2 spectrum as a function of annealing temperature is displayed in Figure 4. Line spectra for

Figure 4. Intensity map (left side) of the Ti 2p3/2 region recorded during annealing in UHV of a TiOx thin film prepared by CVD on Pd(111) followed by an oxidation. Line spectra for indicated temperatures are shown on the right side. The blue line spectrum is recorded after reoxidation of the reduced alloy phase formed by the annealing experiment.

indicated temperatures are shown on the right side in the figure. At 550 °C, only the alloy component remained in the spectra and the annealing was stopped. Reoxidation of this phase resulted in a Ti 2p3/2 spectrum practically identical to the spectrum taken after the first oxidation. LEED shows traces of the oxidized Moiré pattern after oxidation, while the alloy phases (as-prepared and after oxidation−reduction) both display only the 1 × 1 pattern characteristic of the Pd(111) substrate. Thus, reversible oxidation and reduction cycles can be performed for different TiOx thin film thicknesses prepared by CVD on Pd(111). Pd(100). The behavior of the TiO0.75 film on the Pd(100) surface is similar to the behavior on the Pd(111) surface. In Figure 5, the Ti 2p3/2 and Pd 3d5/2 spectra after oxidation and subsequent reduction are displayed, together with the asdeposited film and for comparison clean Pd(100). In the Pd 3d5/2 spectrum of the as-prepared film, the same type of peaks are observed as those for Pd(111), namely the bulk peak (in dark gray) at 334.9 eV, the surface contribution (light gray) shifted by −0.5 eV relative to the bulk peak, the correction for the rapidly changing electronic density (patterned fill), and a peak shifted by 0.4−0.5 eV (white fill) relative to the bulk peak due to the interaction between Pd and the TiOx film. The Ti 2p3/2 spectra are delineated into the same components as described for the TiOx film deposited on Pd(111). The main contribution in the Ti 2p3/2 spectrum originates from TiO0.75. The film covers the entire Pd(100) surface and is 0.6 nm thick. The LEED image observed for this preparation was the same as that previously observed for the TiO0.75 wetting layer (Figure 116). In the Pd 3d5/2 spectrum, the surface component is almost gone and the interaction peak has emerged. In order to fully oxidize the TiOx thin film on Pd(100), a O2 pressure of 5 × 10−6 mbar was required, as compared to 5 ×

Figure 5. Ti 2p3/2 (left) and corresponding Pd 3d5/2 (right) core level spectra including curve fittings for oxidation and reduction of the TiOx thin film group on Pd(100). Starting at the bottom, the spectra are from the clean Pd(100), next is the as-deposited film, then after oxidation, and on top after subsequent annealing to 500 °C in UHV. Surface coverage and thin film thickness are indicated for each step.

10−7 mbar for Pd(111). Upon oxidation, a new LEED pattern compared to those previously observed for TiOx on Pd(100) emerges and changes in the oxidation state of the thin film can be clearly observed by the appearance of a peak at BE 458.3 eV in the Ti 2p3/2 spectrum, characteristic of fully oxidized TiO2. The Ti 2p3/2 and Pd 3d5/2 core level spectra at this stage are displayed in the middle panel of Figure 5. It can be noted that the Ti 2p3/2 peak of TiO2 formed by oxidation of the film is shifted 0.5 eV to lower energy compared to the Ti 2p3/2 BE observed for TiO2 formed during growth.16 The interaction peak in the Pd 3d5/2 spectrum disappears, similar to what was found when using the Pd(111) substrate. The thickness of the film increases slightly upon oxidation, reaching 1.1 nm, while the surface coverage shows only a minor reduction to 95%. This behavior a quite different compared to the observations for the Pd(111) surface where the thickness increased significantly at the same time as the surface coverage was reduced by a factor of 2. In Figure 6a, the LEED pattern of the fully oxidized thin film on Pd(100) is displayed. The pattern indicates a structure with (12 × 4/3) periodicity relative to Pd(100), which is differing from the 1−2 11 LEED structure for TiO2 observed during the growth studies.16 Compared with Pd(111), which displayed a two-dimensional Moiré pattern, the oxidation of the thin film on Pd(100) yields a Moiré structure in one dimension.

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Figure 6. LEED patterns with suggested interpretation overlaid on the right side. One unit cell is drawn. (a) After oxidizing the as-prepared phase, which results in a (12 × 4/3) structure and (b) after subsequent annealing in UHV yielding a reduced phase with a (4 × 10) structure.

After the oxidation, the film was annealed to 500 °C in UHV in order to reduce it. The resulting Ti 2p3/2 and Pd 3d5/2 core level spectra are shown in the upper panel of Figure 5. After 5 min of annealing, the Ti 2p3/2 spectrum reveals that the TiO0.75 related component is clearly present again, while the TiO2 related component has been reduced and shifted to the position observed for TiO2 formed during growth (Figure 116). In the Pd 3d5/2 spectrum, the interaction peak has reappeared, but the relative intensity is lower compared to the spectrum for the asprepared thin film. Both Pd 3d5/2 and Ti 2p3/2 spectra indicate that the film is only partially reduced. The fact that the entire TiO2 peak shifts once the reduction starts and the interaction peak emerges in the Pd 3d5/2 spectrum indicates that the thin oxidized film changes state once the reduction starts. The coverage has increased again and the film thickness decreased to a slightly higher thickness than the initial preparation, which are consistent with the finding that the film is only partly reduced. In Figure 6b, the corresponding LEED pattern is displayed. It shows a (4 × 10) structure, that is, a larger unit cell than the (3 × 5) and (4 × 5) structures observed for the asprepared phase.16 Finally, a preparation consisting of mostly the alloy phase was prepared to test the full reversibility from this phase on Pd(100) to an oxidized film and back to the alloy phase. The full reduction back to the alloy phase was achieved with an annealing temperature of 690 °C, which is significantly higher than that required for the corresponding experiment on Pd(111). This may partly be due to a steeper temperature ramp used for Pd(100). Otherwise, the reduction process proceeded in the same stepwise manner as observed for the Pd(111) surface.

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lepidocrocite-like structures on Pt(110) are the most similar ones to the findings in the present work, consisting of 1−2 atomic layers and exhibiting Moiré LEED patterns.30,31 The thicknesses of the oxidized Moiré phases on Pd exceed 2 atomic layers. Further insight into the exact structure of the TiO2 thin film phases on Pd(100) and Pd(111) requires complementary investigations with other structure sensitive techniques. The oxidation process leads to extraction of alloyed Ti from the Pd substrate for both Pd(111) and Pd(100), witnessed by the disappearance of the alloy component in the Ti 2p3/2 spectra. The interaction between the Pd substrate and the TiOx thin film is clearly altered, much more so for Pd(111) compared to Pd(100). On the Pd(111) substrate, a significant morphological change (2D → 3D) of the TiOx film is observed upon oxidation, consistent with a reduced interaction between the TiOx film and the substrate. On the Pd(100) surface, however, the tendency for such a morphological change is much weaker. The interaction component in the Pd 3d5/2 core level spectrum is significantly reduced, but it still has an appreciable intensity after the oxidation procedure. This finding may be related to the shorter annealing time as well as lower maximum temperature; that is, there are kinetic limitations in the experiments. Still, a different nature of the TiO2 film on Pd(100) compared to Pd(111) cannot be ruled out from the current experiments. Similar investigations of transitions between TiOx and TiO2 phases have been reported on Pt(111).32 Through structural information based on DFT, they are able to propose a structural change at the interface between the Pt substrate and the TiOx thin films, alternating between Pt−Ti bonds and Pt− O bonds.32 This type of bond switching is a very good candidate for explaining the behavior of the interaction component observed in the Pd 3d5/2 spectra. The perimeter between the metal substrate and the oxide thin film can be an important parameter when it comes to catalytic performance.33−35 For the Pd(111) surface, it is clear that the TiOx thin film is dynamic and allowing structural changes in both shape and surface coverage when oxidized and reduced. This behavior also makes it possible to create a situation where a perimeter between pure Pd and TiOx thin film is present. The behavior of Pd(100) is at this point less clear, though it cannot be ruled out that it is possible to achieve a similar oxide/substrate perimeter by annealing at higher

DISCUSSION

On both Pd(100) and Pd(111), the oxidation of a thin TiO0.75 film results in a fully oxidized TiO2 film displaying a Moiré interference pattern in LEED, in one and two directions, respectively. To the best of our knowledge has the formation of such TiO2 structures on Pd single crystal surfaces not been reported previously. Thin TiOx films are known to form numerous structures on various surfaces depending on preparation conditions and thickness.25−29 However, there is a limited number of fully oxidized TiO2 thin film structures reported. Out of those which have been observed, the E


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temperatures and/or for longer times. The challenge lies in finding an optimum balance between restructuring and alloying. In a dynamic material like TiOx thin films where oxygen is alternately released and stored, it is important that the oxidation and reduction process is reversible and maintaining the same reduction and oxidation properties. On both Pd surfaces, the TiOx thin film displayed similar properties with respect to oxidation and reduction. The oxidation/reduction cycling reveals a reversible behavior. For the Pd(111) surface, a minor decrease in the amount of Ti was observed; this may be due to Ti segregating toward the bulk of the Pd crystal.16,36 Compared to Pd(111), the Pd(100) surface requires a higher O2 pressure to achieve the structural change into a TiO2 thin film. This could be a result of the higher capacity for alloying observed for this surface during the growth studies.16 After reduction, the periodicity of the LEED pattern displays a (4 × 10) structure, which differs from the (4 × 5) structure observed for a CVD grown TiOx film with similar oxidation state on this surface.16 This difference may be an indication either that the thin film resulting from reduction of the fully oxidized phase is forming larger domains or that there is an actual change in the structure. As the CVD grown film displaying the (3 × 5)/(4 × 5) structure has more diffuse LEED spots, indicating a less welldefined or strained structure, we suggest that the (4 × 10) structure observed in the present work is a relaxed variation of the CVD grown structure.

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CONCLUSIONS The oxidation and reduction properties of TiOx thin films on Pd(100) and Pd(111) have been investigated by HRPES and LEED. The thin films were found to be reversibly oxidized and reduced. The thin films alter their structure as well as the chemical composition during the oxidation/reduction cycles. New fully oxidized Moiré TiO2 structures were observed on both surfaces, with the Moiré pattern in one dimension for Pd(100) and in two dimensions for Pd(111). These structures have previously not been reported for TiO2 thin films grown on these surfaces. The oxidation process results in a strong reduction in the interaction between the oxide thin film and the Pd substrate, most significantly for Pd(111). Reversible oxidation/reduction cycling of TiOx thin films on Pd(111) and Pd(100) was possible.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

M. H. Farstad: 0000-0002-4078-8408 A. Borg: 0000-0001-7911-0349 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was received from the Swedish Energy Agency (STEM), the Swedish Research Council (VR), the Trygger foundation, the Crafoord foundation, NordForsk, and the Göran Gustafsson foundation. We thank the staff at the MAX IV Laboratory for their support.

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REFERENCES

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Oxidation and Reduction of TiOx Thin Films on Pd(111) and Pd(100

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