Article pubs.acs.org/JPCC
Carbothermal Transformation of TiO2 into TiOxCy in UHV: Tracking Intrinsic Chemical Stabilities Laura Calvillo,† Diego Fittipaldi,‡ Celine Rüdiger,§,∥ Stefano Agnoli,† Marco Favaro,† Carlos Valero-Vidal,§,∥ Cristiana Di Valentin,‡ Andrea Vittadini,⊥ Nathalie Bozzolo,# Suzane Jacomet,# Luca Gregoratti,∇ Julia Kunze-Liebhaü ser,§,∥ Gianfranco Pacchioni,‡ and Gaetano Granozzi*,† †
Department of Chemical Sciences, Universit di Padova, Via Marzolo 1, 35131 Padova, Italy Department of Material Science, Università Milano-Bicocca, Milano, Italy § Physics Department E19, Technical University of Munich, James-Franck-Str., D-85747 Garching, Germany ∥ Institute of Physical Chemistry, University of Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria ⊥ CNR-IENI, Via Marzolo 1, 35131 Padova, Italy # MINES ParisTech, PLS - Research University, CEMEF - Centre de mise en forme des matériaux, CNRS UMR 7635, CS 10207 rue Claude Daunesse 06904 Sophia Antipolis Cedex, France ∇ Elettra − Sincrotrone Trieste SCpA, SS14-Km163.5 in Area Science Park, 34149 Trieste, Italy ‡
S Supporting Information *
ABSTRACT: The conversion of anodic TiO2 films into TiOxCy in ultrahigh-vacuum (UHV) has been traced by photoemission spectroscopy in order to optimize the process parameters and study the different phase stabilities. In addition, density functional theory (DFT) calculations have been performed in order to elucidate the main questions about TiOxCy composition and stability. The experimental data indicate that the anodic TiO2 film is stable both in UHV and ethylene background up to ca. 600 K, and at this temperature, it starts to reduce leading to suboxide TiOx species. Above ca. 750 K, the formation of TiOxCy starts, since the oxygen vacancies begin to be replaced by carbon atoms. A surface enrichment in TiO2 and elemental carbon has been detected on the converted TiOxCy film at room temperature. Realtime measurements have shown that this phenomenon takes place during the cool down process and DFT calculations suggest a possible explanation: as the temperature decreases below ca. 750 K (temperature at which the formation of TiOxCy starts), the TiOxCy phase is not thermodynamically stable, and it decomposes into TiO2 and elemental carbon. The comparison of the experimental valence band data with DFT results has also allowed to establish that the film surface is not homogeneous and that segregation of TiO and TiC systems may take place. On the other hand, the local compositional study carried out by scanning photoelectron microscopy has shown that the conversion of the film is not homogeneous but depends on the grain orientation, in particular crystallites with an orientation close to and planes show a higher grade of conversion. Both experimental and DFT data validate the use of TiOxCy as an innovative support for electrocatalysis.
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INTRODUCTION
alternative support materials to the standard carbon-based ones,13,14,17 especially for high temperature applications. One major issue regarding their use as electrode material is associated with their stability under electrochemical conditions: in fact, their transformation into corresponding oxycarbides can occur when operating in air and in solutions. On the other hand, also early transition metal oxides are studied as alternative supports for electrocatalysts.18−21 Therefore, a rational approach to optimize their properties relies on a detailed
Since some early transition metal carbides can mimic the electronic structure of noble metals, they have recently attracted much interest for their possible use in catalysis1−3 and electrocatalysis,4−7 both as catalysts8,9 and supports.7,10−14 Actually, they are being studied for different electrochemical reactions, such as the oxidation of hydrogen, CO and alcohols, and the oxygen reduction reaction (ORR), since they are potential substitutes for Pt as electrode materials for low temperature fuel cells (FCs). The characteristics that make carbide based electrocatalysts promising materials for FCs are their high tolerance to CO and electrochemical stability to corrosion.15,16 Recently, carbides have been also presented as © 2014 American Chemical Society
Received: July 6, 2014 Revised: September 9, 2014 Published: September 10, 2014 22601
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After that the samples were washed with deionized (DI) water (Millipore-Milli-Q system, resistivity 18.2 MΩ). This procedure was repeated 3 times. Prior to the EBSD measurements, a scratch was placed as a marker in the center of the electropolished area by using a tungsten wire. EBSD maps were acquired in proximity of the marker so that the scanned areas could be retrieved. EBSD measurements were performed in a FEI XL30 LaB6 ESEM operated under high vacuum and 20 kV accelerating voltage, and equipped with an EDAX-TSL Digiview camera. The full map size was 300 μm × 500 μm area, and only a part of it will be shown here. The microstructure of the as-received material was fully equiaxed with an average grain size of about 15 μm, so that a step size of 2 μm was chosen for EBSD mapping and provided a suitable resolution of the grain structure of the substrate. It is worth mentioning here that the scanned area was slightly contaminated by carbon atoms after EBSD mapping. Such contamination after SEM observation is a very common phenomenon, which is enhanced when doing EBSD because of the requested high electron beam current. Efforts have been made to remove that contamination before running the subsequent experiments, but it was not possible to fully remove it without etching the sample, with a highly detrimental risk of losing the measured grains. After the EBSD measurements, the samples were rinsed with and/or ultrasonicated in ethanol (technical grade), isopropanol (high purity) and DI water before being submitted to anodization. Preparation of the Anodic TiO2 Film. Compact amorphous TiO2 films of a mean thickness of ∼60 nm were produced on electropolished Ti by potentiostatic electrochemical anodization at 20 V for 10 min using a DC power supply (PE 1540, 40 V/3A, Philips) controlled by a multimeter (Voltcraft Dual Display M-3650D). The process was carried out at room temperature (RT) in a Teflon electrochemical cell with a conventional two-electrode configuration in a 0.1 M H2SO4 (H2SO4, analytical grade, 95−97%, Merck, Germany) electrolyte, using a platinum mesh as counter electrode.7 After anodization, the samples were thoroughly rinsed with DI water and dried in an Ar stream. Two sets of samples were prepared: the former on the EBSD aligned substrates for the SPEM experiments, while the latter on Ti disks without marker and EBSD characterization for the standard photoemission ones. Conversion and in Situ Characterization of Anodic TiO2 Films into TiOxCy in UHV. The conversion of the anodic TiO2 film into TiOxCy was followed in two separate experimental set-ups, a home-lab UHV chamber in Padova and an experimental chamber located at the Elettra Synchrotron Radiation Facility (Trieste, Italy). The first set of experiments carried out in the home-lab was preliminary and preparatory for the second set, where the transformation was tracked in real-time. Home-Lab Experiments. The home-lab UHV system consists of two interconnected UHV chambers: a preparation and an analysis chamber. In the preparation chamber, the sample is subjected to a carbothermal treatment and, subsequently, it is transferred to the analysis chamber and characterized by using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Two types of anodic TiO2 films were subjected to consecutive carbothermal treatments in ethylene at different temperatures and partial pressures of ethylene. The conditions used for the treatments are reported in Table 1. Prior to the
study of the transformation between oxides and carbides, including their oxycarbide mixed systems. Taking into account that the activity of both catalysts and electrocatalysts depends on the nature of their surface, the most appropriate approach to study the activity and stability of these materials is based on the premises of surface science. From this point of view, much can be learned by using in situ surface science methods in ultrahigh-vacuum (UHV) conditions to study model catalysts, i.e. films rather than powders, to discriminate between intrinsic stabilities and environment induced transformations. Very recently, the use of titanium oxycarbides (hereafter TiOxCy) as alternative supports for intermediate temperature FCs has been suggested.7 However, the problem of the intrinsic stability of TiOxCy in ambient and under electrochemical conditions present in a FC is still an open issue.16 Ex situ investigations have already been reported,7,16 but the main reasons for the oxidation of TiOxCy materials are not fully understood. Such an oxidation can be accompanied by the formation of carbon, but using ex situ methods it is hard to discern the role of contamination and air exposure. In addition, an interesting phenomenon has been recently reported:22 the activity and stability toward the ethanol oxidation reaction (EOR) on Pt/TiOxCy model systems that are prepared on anodized polycrystalline titanium (Ti) are affected by the crystallographic orientation of the individual grains of the subjacent Ti substrate. Thence, following in situ the transformation of each single-grain of the anodic TiO2 film into TiOxCy allows for further understanding of the complex interdependence of activity, stability and surface orientation. In this paper we report a fundamental study where the transformation of anodic TiO2 films into TiOxCy is traced in situ by core-level and valence band photoemission spectroscopies, in a standard lab equipment and in a synchrotron radiation facility using the scanning photoelectron microscopy (SPEM) tool. The former investigation gives an area-integrated information, while the latter gives a local information which allows single-grain tracking. The characterization of grain orientation has been obtained using electron backscatter diffraction (EBSD) measurements. Unique interesting phenomena are disclosed as a function of temperature. As a further source of information and as an aid for the experimental data interpretation, we have also carried out density functional theory (DFT) calculations. These data give basic insights on the composition, stability and electronic structure of TiOxCy, which is mainly a solid solution of TiC/TiO.
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EXPERIMENTAL SECTION Crystal Orientation Mapping of the Ti Substrate (EBSD). Disks of 1 mm thickness and 10 mm diameter were cut from a 20 mm diameter polycrystalline Ti rod (99.6% purity, temper annealed, Advent Ltd., England). One side of the samples was mechanically polished using SiC-grinding paper (grits: P1200, P2500 and P4000, Buehler, ITW Test & Measurement GmbH, Germany) and ethanol (technical grade). A disc of 8 mm diameter on the polished side was then electrochemically polished in a solution of 40% perchloric acid, methanol, and butoxyethanol,23 which was cooled in a bath of liquid nitrogen to −35 °C ± 2 °C. For this treatment we employed a two-electrode setup consisting of two parallel samples as working electrode and a sufficiently large Ti sheet as counter electrode. An anodic potential of 60 V was applied for 5 min using a power supply (in combination with a voltmeter). 22602
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providing one of the 48 images simultaneously acquired. By using the entire set of maps the reconstruction of the spectra corresponding to this energy window from a selected microarea is possible too. The better spectral resolution (0.2 eV) of the microspot PES mode (μ-PES) provides more precise information on chemical states and local potentials with lateral resolution down to 120 nm.25 DFT Based Calculations. All calculations were performed with the PBE26 functional as implemented in the Quantum Espresso code.27 Vanderbilt ultrasoft pseudopotential with energy cutoff of 30 and 240 Ry (for kinetic and charge density grids) were used. Some preliminary calculations with both GGA and hybrid functionals were performed also with the CRYSTAL09 code28,29 to determine the most suitable method for the accurate description of the materials under study. From the comparison with available experimental data on structural parameters and electronic properties of the bulk parent TiC and TiO cubic systems (see Table S1.1 and Figure S1.1 and S1.2 in Section S1 of the Supporting Information),30,31 we concluded that PBE functional is a reliable method in this context. Different configurations for bulk TiOxCy systems were obtained for 2 × 2 × 2 or 4 × 4 × 4 supercell models (16 or 128 atoms) by applying the CRYSTAL09 feature for configurations counting and cluster expansion. Brillouin zone integrations used a 10 × 10 × 10 or 2 × 2 × 2 k-points grids for the small and larger supercells, respectively. Optimization runs of both atomic positions and lattice parameters were carried out until all components of the residual forces were less than 0.25 eV Å−1. To model TiOxCy (100) surfaces we used a four-layer thick 2 × 2 supercell slab model of 64 atoms and a 4 × 4 × 1 kpoints grid. Atomic positions in the slab models were relaxed, keeping the cell parameters fixed at the TiC optimized geometry (4.32 Å), which is larger than that of TiO (4.28 Å). A vacuum region of 15 Å was employed to avoid spurious interaction between repeated images along the z-axis. Denser kpoints grids of 20 × 20 × 20 and 8 × 8 × 1 were used for the calculation of the total and projected density of states (tDOS and pDOS) of the 2 × 2 × 2 bulk supercell and of the 2 × 2 slab supercell, respectively. Theoretical valence band photoemission spectra were obtained by computing weighted DOS (wDOS) curves. This was made by weighting the pDOS of each atomic subshell by the appropriate photoionization cross section, as tabulated in literature.32 The simulated spectra where finally obtained by summing all the wDOS curves after applying a 1 eV Lorentzian broadening in order to match the resolution of the experiment.
Table 1. Conditions Used for the Carbothermal Treatment of the TiO2 Films in the Home-Lab Experimentsa sample reference
temp/K
time/min
1 2 3 4 5 6 7 8 9
473 573 673 723 773 773 823 823 823
5 5 5 5 5 5 2 2 2
PC2H4/mbar 5 5 5 5 5 5 5 5 5
× × × × × × × × ×
10−7 10−7 10−7 10−7 10−7 10−6 10−6 10−6 10−6
Note: For the film subjected to sputtering and annealing, treatments 6−8 were at 773 K for 5 min, and treatment 9 at 773 K for 10 min. a
carbothermal treatment, one film was annealed in UHV at 573 K for 15 min in order to remove the adsorbed water but not the carbon contamination, whereas the other one was sputtered at 1 keV at RT for 10 min using a partial pressure of argon of 2 × 10−6 mbar in order to remove the carbon contamination and, subsequently, it was annealed in oxygen atmosphere (PO2 = 2 × 10−6 mbar) at 573 K for 30 min to reoxidize the surface of the film. The chemical surface changes of the film were investigated by XPS after each treatment, when the sample was cooled down to RT and transferred into the analysis chamber. Photoemission data were acquired in a custom designed UHV system equipped with a VG MK II Escalab electron analyzer, working at a base pressure of 10−10 mbar. Core level photoemission spectra (C 1s, Ti 2p and O 1s regions) were taken at RT in normal emission using a non monochromatized Mg Kα X-ray source (1253.6 eV). Some selected samples were also investigated at different emission angles to increase the surface sensitivity. The spectrometer energy calibration was carried out by using a gold sample (Au 4f at 84 eV). Single spectral regions were collected using 0.1 eV steps, 0.5 s collection time and 20 eV pass energy. UPS data were collected using a He discharge lamp (HIS 13, Omicron) exploiting the He II radiation line (40.81 eV). Synchrotron Based Experiments. The conversion of an anodic TiO2 film into TiOxCy in UHV was monitored by the SPEM operated at the ESCAmicroscopy beamline of Elettra.24 The conversion process was performed in the microscope chamber, according to the following in operando procedure: first, the sample was heated in UHV until a slight reduction was observed and, at this point, ethylene was dosed in the chamber (PC2H4 = 10−7 mbar), while the temperature was increasing. When the formation of TiOxCy was detected, the pressure was raised to 10−6 mbar. After the carbothermal treatment, chemical maps of areas as wide as 100 × 100 μm2 and spatially resolved photoemission spectra (120 nm size spot) of selected regions of the film surface were recorded covering the C 1s and Ti 2p core level energies (kinetic energy (KE) = 473 and 300 eV, respectively). A photon energy of 756 eV was used for the SPEM measurements carried out during and after the carbothermal treatment. The SPEM imaging mode can map the elemental species and their chemical state and charge distribution by collecting photoelectrons emitted within a selected KE window, covered by the 48 channels of the detector, while rasterscanning the specimen with respect to the microprobe. Each detector channel measures electrons with a specific KE,
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RESULTS AND DISCUSSION Preliminary Tests of Conversion of the TiO2 Film into TiOxCy in the Home-Lab UHV Chamber. Figure 1 shows the C 1s and Ti 2p XPS spectra obtained in the home-lab XPS equipment before and after the carbothermal treatments (see Table 1) at different temperatures for the TiO2 film subjected to the annealing treatment at 573 K for 15 min. The corresponding XPS spectra for the carbon-free film subjected to the sputtering and annealing treatment can be found in the Supporting Information (Figure S2.1). These measurements represent an average of processes occurring on different grains of the sample, since the lateral resolution is limited to a surface area of ∼3 mm2. The C 1s spectrum for the pristine TiO2 anodic film, after an annealing treatment at 573 K, shows two main components at binding energy (BE) of 285.7 and 289.2 eV, which are assigned 22603
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starts to decrease (not shown), and the newly created vacancies are replaced by carbon atoms (i.e., an uptake of carbon is occurring). This is well evident from the new component in the C 1s peak that starts to appear at a BE of 282 eV, associated with Ti−C bonds,33,34 whose intensity is increasing with time. After the last treatment (sample 9), the C 1s peak only shows a very small component at 285 eV, which can be attributed either to remaining carbon contamination or to the extra carbon on the surface coming from the decomposition of the ethylene. Regarding the Ti 2p region, a new component at 455 eV, attributed to TiC and/or TiO,33 starts to appear at 823 K, but the characteristic peaks of oxidized TiO2 are still predominant. However, after 6 min of treatment (sample 9), the amount of TiOx phases decreases considerably, whereas the amount of TiC/TiO phases increases. At this point, the stoichiometry of the film (estimated to be ca. 6 nm thick) was calculated from the spectra and resulted to be TiO0.42C0.58. In the case of the sample subjected to the sputtering and annealing treatment, the XPS spectra for the sample before the carbothermal treatments confirmed the effective removal of the contamination carbon layer on the surface of the film, since no peaks in the C 1s are observed (see Figure S2.1). Regarding the conversion products and temperatures, the same results as for the film containing the layer of contaminating carbon were obtained. However, it takes longer to convert the film that does not contain the contamination carbon layer (the same final conversion was achieved after a sum time of 25 min instead of 6 min), even if the temperature at which the formation of TiOxCy starts is the same in both cases. This indicates that the layer of contamination carbon present on the surface of the film can be used as a carbon source to form TiOxCy. The final TiO0.42C0.58 sample was investigated by changing the emission angle in order to obtain a qualitative depth profile of the different components (Figure 2). The spectra show that there is a slight enrichment of carbon (peak at 284.5 eV in the C 1s photoemission line) and TiOx species (component at 458 eV in the Ti 2p photoemission line) on the surface (spectra acquired at θ = 25°, which are more sensitive to the surface). Similarly, the Ti 2p and O 1s data demonstrate a slight surface enrichment of TiOx. In order to study if the TiO0.42C0.58 film is chemically stable, it was exposed to air at RT and then reanalyzed by XPS. Figure 3 shows the comparison of the
Figure 1. C 1s and Ti 2p XPS spectra obtained in the home-lab for the anodic TiO2 film before and after the carbothermal treatments in ethylene at different temperatures described in Table 1. The spectra for the TiO2 film before the carbothermal treatment (referred as TiO2 RT) correspond to the XPS measurement carried out at RT after a preannealing treatment in UHV at 573 K.
to carbon contamination and C−O bonds, respectively. When ethylene is dosed in the UHV chamber, the main C 1s peak suffers a shift toward lower BE that might be associated with the sticking of ethylene on the surface, which superimposes with the one coming from the pristine contamination. Up to 773 K (samples 1−6), no significant changes occur in the C 1s region, only a slight intensity decrease of the main C 1s peak due to the reduced sticking coefficient of ethylene at higher temperatures is observed. From the C 1s XPS data of Figure 1, it is well evident that there is no formation of Ti−C bonds up to 823 K (no other C 1s peak up to sample 7). A similar conclusion can be inferred from the Ti 2p spectra: the Ti 2p XPS data for the pristine TiO2 film (Ti 2p3/2 and Ti 2p1/2 peaks at 459.1 and 464.9 eV) are unchanged up to 573 K. Above this temperature, a gradual reduction of TiO2 as a function of temperature occurs, leading to TiOx suboxides, as evidenced by the appearance of a distinct shoulder at lower values of BE in the Ti 2p3/2 peak at 458 eV. At 823 K, however, the gradual conversion of TiOx into TiOxCy starts; therefore, consecutive treatments of 2 min were carried out at this temperature (samples 7−9, see Table 1). After 2 min of treatment, the amount of oxygen in the sample
Figure 2. C 1s, Ti 2p, and O 1s XPS spectra obtained in the home-lab for the TiO0.42C0.58 film at different emission angles measured with respect to the surface plane. 22604
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Figure 3. XPS spectra obtained in the home-lab for the TiO0.42C0.58 film before and after the exposure to air and oxygen atmosphere (in UHV). (a) C 1s region; (b) Ti 2p region; (c) O 1s region.
spectra obtained before and after air exposure. The results show that an extensive reoxidation of the surface takes place. This fact is evidenced by (i) the increase of the amount of oxygen at BE of 530.2 eV, associated with Ti−O bonds in TiO2 (Figure 3c); (ii) the appearance of a new Ti 2p component at BE of 458.8 eV, corresponding to TiO2, and the diminution of the TiO/TiC peak (Figure 3b); and (iii) the decrease of the amount of carbidic carbon (282.2 eV), the increase of the graphitic/amorphous carbon (285.5 eV), and the presence of oxidized carbon (around 289 eV) (Figure 3a). In order to ascertain if this latter is just due to the usual environmental contamination or derives from the decomposition of the oxycarbide to TiO2 and C, a UHV experiment, where the TiOxCy film was exposed at RT to 10−3 mbar of oxygen, was performed. Also in this case we observed the clear formation of stoichiometric TiO2 as well as a slight increase in the elemental carbon component. This experiment therefore supports the hypothesis that the TiOxCy film reacts with oxygen forming TiO2 and elemental carbon. This result is particularly significant for the possible use of TiOxCy as an electrocatalyst support: it indicates that under ambient conditions it is not possible to obtain a pure TiOxCy surface, since it tends to oxidize in contact with air, and that eventually a composite surface formed by a mixture of TiOxCy, TiO2 and carbon is unavoidable. Such chemical instability toward oxygen will be further discussed on the basis of the electronic structure derived by DFT calculations (see below). Real-Time Conversion of the TiO2 Film into TiOxCy in Front of SPEM. The conversion of an anodic TiO2 film into TiOxCy was monitored in real-time in front of SPEM at the C 1s and Ti 2p core level energies (KE = 473 and 300 eV, respectively). The goal of this experiment was to determine accurately the temperatures associated with the different processes during carburization of the TiO2 film into TiOxCy. Figure 4 shows the spectra obtained at some key temperatures in the C 1s and Ti 2p regions, whereas the whole set of spectra obtained during the experiment can be found in Figure S3.1. The film was first heated up in UHV in front of the SPEM to determine its stability, and the changes were followed by XPS (measuring the Ti 2p region). Figures 4 and S3.1a show that the TiO2 film is stable up to 603 K in UHV, since no changes are observed in the Ti 2p photoemission region. At 608 K, however, a shoulder in the Ti 2p3/2 peak at lower BE (∼458
Figure 4. C 1s and Ti 2p XPS spectra at key temperatures obtained in front of SPEM during the conversion of TiO2 into TiOxCy (red) and the cooling process (blue). The spectra in black represent the TiOxCy obtained before cooling.
eV) starts to appear, indicating that this is the temperature at which the TiO2 reduction starts in UHV, with the relative formation of TiOx suboxides. At this T, we started to dose ethylene. In presence of ethylene, the component associated with TiOx species in the Ti 2p photoemission region grows progressively with temperature (Figures 4 and S3.1b). At 748 K, a new component at 455 eV in the Ti 2p region, attributed to TiC/TiO, starts to appear, indicating the formation of TiOxCy. At this point, we started to follow the formation of TiOxCy by measuring both the C 1s and Ti 2p regions, which are reported in Figures 4 and S3.1c and d. The C 1s spectrum shows one component at a BE of 284.5 eV, associated with amorphous/graphitic carbon on the surface of the film. In addition, a second component at a BE of 282 eV, attributed to Ti−C bonds, starts to appear and increases as a function of time and temperature, indicating that the carbon atoms start to replace the oxygen vacancies created during the reduction of the TiO2 film. In the Ti 2p region, the component associated with TiC/TiO (at 455 eV) grows with temperature and time, 22605
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Table 2. Summary of the Conditions and Phases of the Conversion of TiO2 into TiOxCy in UHV temp/K preliminary
real-time
carbon
titanium
oxygen −7
TiO2 film is stable at PC2H4= 5 × 10
