Water Adsorption on Different TiO2 Polymorphs Grown as Ultrathin

May 11, 2012 - Structure and thermal stability of fully oxidized TiO2/Pt(111) polymorphs. Emanuele Cavaliere , Luca Artiglia , Gian Andrea Rizzi , Luc...
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Water Adsorption on Different TiO2 Polymorphs Grown as Ultrathin Films on Pt(111) Luca Artiglia,† Alessandro Zana,† Gian Andrea Rizzi,† Stefano Agnoli,† Federica Bondino,§ Elena Magnano,§ Emanuele Cavaliere,‡ Luca Gavioli,‡ and Gaetano Granozzi*,† †

Department of Chemical Sciences, University of Padova, via Marzolo 1, I-35131 Padova, Italy Interdisciplinary Laboratories for Advanced Materials Physics (i-LAMP) and Dipartimento di Matematica e Fisica, Università Cattolica del Sacro Cuore, via dei Musei 41, I-25121 Brescia, Italy § Istituto Officina dei MaterialiCNR, Laboratorio TASC, Area Science Park-Basovizza, Strada Statale 14, Km.163.5 I-34149 Trieste, Italy ‡

ABSTRACT: By using reactive Ti evaporation, we have grown on Pt(111) stoichiometric TiO2 ultrathin (UT) films of increasing thickness, presenting different polymorphic structures and surface terminations, that is, the thickness-limited lepidocrocite-like nanosheet, TiO2(B) (001) and rutile-TiO2 (100), respectively. This allowed us to study comparatively for the first time the reactivity toward water of different polymorph surfaces in the form of ultrathin films by adopting several in situ surface science tools. There is no evidence for water interaction with lepidocrocite-TiO 2 nanosheets, in agreement with theoretical predictions found in the literature. For the first time evidence for some water dissociation on TiO2(B) surfaces is reported. A higher water interaction is observed on the high coverage films, and the observed behavior is in line with the previous literature data obtained on the rutile (100) bulk surface: our data confirm the common interpretation that rutile-TiO2 (100) surface termination is highly reactive toward water dissociation. In this paper we demonstrate that this ability is maintained also in the case of UT films.

1. INTRODUCTION Titania (TiO2) is a highly strategic material in technologically important areas, like heterogeneous catalysis (used both as active catalysts as well as support for metal catalysts),1,2 gas sensors,3 photoassisted oxidation,4 wastewater remediation,5 optical devices (optical filters and optical waveguides),6,7 antireflective coatings,8 and photovoltaic devices.9 Since most of the peculiar titania properties are surface-mediated, a detailed knowledge of the TiO2 surface properties10 is crucial to exploit the full potential in innovative devices. Moreover, a rapidly expanding subset of studies is focusing on the groundbreaking properties of nanodimensional TiO2 phases, where surface properties become predominant, for example, in ultrathin (UT) supported films11,12 or dispersed nanophases (i.e., nanosheets, nanotubes, nanorods, and nanoparticles) and structural and chemical properties different from the ones of the most common bulk polymorphs (rutile, anatase, and brookite) are often observed.13,14 As an example, TiO2(B), first synthesized in 1980,15 successively identified as a confined phase (lamellae) in natural anatase crystals16 and recently prepared as an UT film supported on Pt single crystals,17 is currently much investigated for a possible enhancement of the photocatalytic activity.18 For almost all the applications detailed above, the interaction of the TiO2 surfaces with water plays an important role. Actually, many applications in catalysis and electrochemistry © 2012 American Chemical Society

strongly depend on the water/TiO2 interaction. We can mention the photocatalytic devices to produce H2 as a source of green energy (the so-called water splitting)19,20 as an example of primary importance. Because of the topic relevance, since the first report on the photocatalytic activity of titania,19 many efforts have been undertaken to explain what are the key steps involved in the process.20 The adoption of model systems that can be studied by a rigorous surface science approach is a very important route, and large efforts have been spent on the investigation of water behavior on TiO2 bulk crystal surfaces. In this respect the most studied system is the rutile-TiO2 (110) surface:21,22 the long debate on the adsorption state of water (molecular vs dissociated) on a rutile (110) surface free from defects has been recently addressed by a high-resolution X-ray photoemission spectroscopy (HR-XPS) investigation demonstrating that the first water layer contains a significant fraction of dissociated species, even in a total absence of surface oxygen vacancies.23 A similar approach has been also recently applied to the TiO2 anatase (001) and (101) surfaces.24,25 However, in general, a multitechnique approach is needed to have a full description of the water interaction with the substrate of interest. Received: January 18, 2012 Revised: April 15, 2012 Published: May 11, 2012 12532

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cycle LHe cryostat and a XL25VH VG sample carrier, a VG sputtering gun, an Omicron EFM-3 water-cooled triple e-beam evaporator, a QMS, and a gas line. The XAS measurements were carried out in normal incidence and partial electron yield, measuring the secondary electrons with the analyzer set to a resolution of 0.08 eV. The valence band (VB) at 200 eV, the HR-XPS measurements (O 1s at 596 eV and Ti 2p at 660 eV) were obtained in normal emission configuration, with a total energy resolution of 0.15, 0.20, and 0.202 eV, respectively. The Res-PES measurements were obtained in normal emission with a total resolution of 0.27 eV. A possible role of the beam in the water dissociation process has been considered. After water exposure at low temperature, several scans of the O 1s core level photoemission peak have been acquired during a time of 20 min at increasing temperature. All the spectra (not reported herein) have been analyzed and the fitting results clearly show that there is no time-dependent increase of the −OH component and thus no radiation-induced water dissociation takes place. The STM data were acquired by Pt/Ir tips in constant current mode at room temperature (RT), using an Omicron multiscan system, equipped with XPS and Auger facilities to check the oxidation state of the film, and with tip-to-sample bias in the 1.5−2.6 V range and tunneling current in the 0.1−0.6 nA range. The same experimental procedures described here were used during all the experiments in the different chambers: to have a good reproducibility of the preparation of the different films, whose nature is strongly dependent on the actual Ti dose (and consequently film thickness) and deposition conditions, chemical compositions and structures were checked by photoemission and by a LEED scanning over the entire surface before each experiment. The adopted preparation procedure is a further refinement of the previously reported procedures:11 Ti was evaporated on the substrate held at RT in an oxygen partial pressure of 5.0 × 10−4 Pa, and subsequently a postannealing treatment in the temperature range of 800−900 K and cooling down in oxygen (same pressure) has been applied to improve the long-range order of the deposited layer, and to allow the complete oxidation of Ti. Then the samples were analyzed by LEED. The Ti evaporator was calibrated using the photoemission signal attenuation of the Pt 4f core level of the Pt(111) substrate. To maintain a coverage unit consistent with previous literature data, we have adopted the definition of one MLeq assuming the growth of an anatase TiO2 (001) bilayer with a distance between adjacent planes of 0.24 nm (one-fourth of the unit-cell dimension in the [001] direction, equal to 0.951 nm). By adopting such assumption, the growth of a full bilayer (i.e., the one representing the rect-TiO2 film) is achieved when a coverage of 2 MLeq is reached. Water was evaporated from a flanged glass test tube connected to a stainless steel gas line. Before each experiment, the gas line was pumped and water purified from air and other contaminants through freezing−pumping cycles. Its purity was determined by a QMS. Water was dosed on the samples at low temperature (close to LN2). All the TPD experiments herein presented have been performed with a 2 K/s heating rate.

The water dissociation (WD) process in a nondefective titania surface requires the concurrence of an acid (fivefold coordinated Ti ions) site, where the water molecule is adsorbed, and a base nearby site (commonly bridging O ions), which is able to extract a proton from the adsorbed water molecule, thus creating a bridging OH group.21,26 Several experimental results show a direct correlation between the oxide surface termination and its reactivity toward the WD reaction, explained by the distance between the active (acidic and basic) sites, that is changing as a function the single-crystal surface termination.26 Moreover, also defects can participate in the WD process, as shown by the effective role of oxygen vacancies (VO) and titanium interstitials (Tiin) revealed through surface science techniques, for example, thermal programmed desorption (TPD) and scanning tunneling microscopy (STM).21,27−29 Extending the study from bulk titania surfaces to UT films represents a step from ideal surfaces toward systems closer to reality, where morphological and structural defects could play a role. In addition, exploring the WD capabilities of supported titania films can be of importance to develop devices of interest in many applicative fields where titania-based WD reactions are exploited. In this work we investigated the water adsorption on UT TiO2 films of different thicknesses grown on the same substrate, Pt(111). By combining several experimental techniques (i.e., HR-XPS, TPD, STM, X-ray absorption, XAS, and resonant photoemission spectroscopy [Res-PES]) it has been possible to correlate the observed water/TiO2 interaction with the structure and defectivity of the different UT TiO2 films. Particularly effective is the complementary information gained by the synergic application of different techniques which adopt different probes (e.g., photons, HR-XPS and probe molecules, TPD). The aim is to provide a general overview on water/ TiO2−UT films interaction, and to verify whether UT films can somehow mimic their bulk (single crystals) counterparts. The investigated systems have been referred to as rect-TiO2/ Pt(111), rect′-TiO2/Pt(111), and quasi(2×1)-TiO2/Pt(111): the three different structures correspond to a thickness-limited lepidocrocite-like nanosheet,30 TiO2(B) (001)17 and the rutileTiO2(100)31 UT films, respectively. This study is part of a comprehensive research plan on TiOx/Pt UT films11 where different structures and innovative chemical properties have been investigated.

2. EXPERIMENTAL METHODS The experiments were carried out in three different ultra high vacuum (UHV) chambers. TPD experiments were performed in a multitechnique chamber (base pressure 2.0 × 10−8 Pa) equipped with a Hiden quadrupole mass spectrometer (QMS), a four-grid rear view low-energy electron diffraction (LEED), a Omicron DAR 400 conventional double-anode X-ray source, a VG CLAM 2 photoelectron analyzer, and an Omicron EMF-3 single e-beam water-cooled evaporator. The sample was fixed to the manipulator through Ta wires (diameter 0.3 mm) to avoid any external contribution to the sample desorption. A K-type thermocouple was spot-welded on the sample backside. HR-XPS, XAS, and Res-PES experiments were carried out at the Elettra Synchrotron Radiation (SR) facility in Trieste, beamline BACH (BL 8.2). The UHV chamber (base pressure ∼3.0 × 10−8 Pa) was equipped with a VSW CLASS WA 150 mm hemispherical analyzer, a four-grid rear-view OCI LEED, a 4-degree-of-freedom CREATEC manipulator with an open

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of the Different Ultrathin TiO2/Pt(111) Films. We have prepared and characterized different fully oxidized TiO2 UT films grown on the same Pt(111) substrate. In fact, just increasing the amount 12533

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been reported before; thus, these structural data represent the basis for the present study of their reactivity with water. Figure 1a−c shows the Ti L2,3 XAS data of the thicker S2, S3, and S4 films acquired with horizontally polarized incident beam. The absorption experiments have been repeated, changing both the beam polarization (from horizontal to vertical) and the incidence (from normal to grazing). No differences in the spectra were observed. After a comparison with the data reported in the literature for the most common TiO2 polymorphs,10 it is possible to find many similarities between the S2 spectrum (Figure 1a) and those reported for TiO2-anatase (101). In particular, the second feature at the Ti L3 edge (around 460 eV), corresponding to the eg orbitals, has almost the same shape reported for the TiO2-anatase (101) surface.10 It is known that XAS is very sensitive to local ion environment; thus, our data suggest a similarity between the S2 film, which was predicted to be a TiO2(B) (001) oriented slice,17 and TiO2-anatase (101) termination. Our experimental findings confirm the theoretical calculations of Vittadini et al.,35 demonstrating that the most stable structure for the TiO2(B) (001) (type-I) is almost identical to a very stable pentacoordinated nanosheet found for unsupported TiO2-anatase (101).36 Also, the XAS data relative to the S3 film (Figure 1b) seem to be anatase-like, although the two components present in the eg feature have different relative intensity compared to that of the S2 (Figure 1a). This difference could be in tune with a change in the crystalline structure. Another change is found in the S4 film (Figure 1c): in this case the two components of the eg feature around 460 eV have almost the same relative intensity. This absorption profile is in very good agreement with the one reported in the literature for the TiO2-rutile (100) single crystal,37 in agreement with previous interpretation of the XPD data.31 The Ti 2p HR-XPS data corresponding to the S2−S4 UT films are reported in Figure 1g−i. A Ti 2p peaks fitting (not reported) shows the presence of a reduced Ti component in all cases. In the case of S2 this component is clearly visible and is due to the presence of the reduced wetting layer of z-TiOx (456.5 eV) (see STM data in Figure 2a discussed below), while in the case of S3 and S4 is due to a small amount of Ti3+ defects (457.0 eV). VB Res-PES experiments (Figure 1d−f) have been performed across the Ti L3 absorption edge for the S2, S3, and S4 films, with the goal of obtaining more information about the structure and the degree of defectivity of the investigated films. From the analysis of the Res-PES data in the 4−10 eV BE range, where the Ti 3d−O 2p π-bonding (ca. 5 eV) and O 2p− Ti 3d σ-bonding (ca. 8 eV) bonding states are present, we find Res-PES behavior of the S2 film similar to that found in a TiO2anatase (101) single crystal.38 On the same basis, the S4 film shows Res-PES behavior in good agreement with that reported for TiO2-rutile (100) single crystal.37 Both these results are in agreement with the above-discussed XAS data. Hence the intermediate S3 film behaves more like the S4 sample. On the other hand, the Res-PES behavior in the lower BE region (1−3 eV) gives information on the defects. The Res-PES data reported in Figure 1d−f show a resonant feature at about 1 eV. Since the VB of S2 is the sum of two main contributions (zTiOx UT film and stoichiometric TiO2(B)), it is not possible to obtain any quantitative indication about the amount of defects contained in the fully oxidized film. In the case of the thicker S3 and S4 films, some information about the relative amount and vertical location of the Ti3+ defects within the two films can be

of Ti dose in the same conditions (see the Experimental Section) the following nanophases (for continuity with previous papers, we have adopted the same labeling), each of them showing a peculiar LEED pattern, can be synthesized: (i) rect-TiO2, obtained at a Ti coverage between 1.0 and 2.0 monolayer equivalent (MLeq, see definition in the Experimental Section). Previous experimental data and theoretical calculations have identified it as a lepidocrocite-like nanosheet, grown with an incommensurate rectangular unit cell (3.8 × 3.0 Å), rotated by 8.3° with respect to the Pt(111) high-symmetry directions.30,32,33 (ii) rect′-TiO2, obtained at a Ti coverage between 2.0 and 10 MLeq. In this case, the experimental results have been rationalized by density functional theory (DFT) calculations based on a (001) oriented slice of the TiO2(B) polymorph.17 The UT film is characterized by an incommensurate centered unit cell (3.7 × 12.2 Å). Here we will present the data collected on 4 and 8 MLeq thick films. (iii) quasi(2×1)-TiO2, obtained at a Ti coverage higher than 10 MLeq. This incommensurate UT film shows a typical quasi(2×1) LEED pattern.34 A preliminary comparison of LEED simulation with X-ray photoelectron diffraction (XPD) results31 suggests that its unit cell is quite similar to that of a TiO2-rutile (100) surface. On the basis of the LEED observations, this UT film is the only stable one for a TiO2 coverage higher than 10 MLeq (under the reported preparation conditions) and no other ordered structure has been observed up to 40 MLeq. The results obtained on 17 and 25 MLeq thick quasi(2×1) films will be presented in the following. For reader convenience, we will label the previously described UT films as reported in Table 1. In the same table the main conclusions on the film composition obtained by the following analysis are summarized. Table 1. Summary of the Different UT Films Studied in the Present Paper film thickness (MLeq) 2 8 17 25

nanophase lepidocrocite-TiO2 TiO2(B) + reduced wetting layer (zTiOx) TiO2-rutile (100) + precursor TiO2rutile TiO2-rutile (100)

UT film label S1 S2 S3 S4

The incommensurate nature of the UT films has to be traced back to the weak oxygen−Pt interaction.17 For the same reason, in a recent paper we proposed that the Pt support plays the role of an (almost) inert observer, where the TiO2 structures selfassemble on the basis of their intrinsic stability, giving the opportunity to grow 2D nanophases on the Pt(111) surface in the form of different polymorphs, depending on the film thickness.17 The low-coverage (up to 2 MLeq) S1 film has been recently studied by a complete set of XAS and HR- XPS measurements,32 pointing to the peculiar electronic properties of the lepidocrocite-like nanosheet. In the following we provide experimental data, that is, Ti L2,3 XAS, HR-XPS, and ResPES, on the S2, S3, and S4 films. Such information will be complemented with morphological data obtained by STM. It is important to point out that the structure of high-coverage stoichiometric TiO2 UT films grown on Pt(111) has never 12534

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Figure 1. Ti L 2,3 edge XAS, VB photoemission taken at different photon energy (456−460 eV, Res-PES) and Ti 2p core level photoemission data taken on the (a, d, g) S2, (b, e, h) S3, and (c, f, i) S4 UT films.

higher in S3. Moreover, from the same table 2 it turns out that the Ti3+ defects are located deeper in S3 than in S4. In fact, the photoelectrons from Ti 2p core levels are characterized by an escape depth of ca. 6 Å, while those from the VB is ca. 12 Å. The picture emerging from the spectroscopic XAS and ResPES measurements as a function of the coverage can be compared with the STM data reported in Figure 2, where we

obtained. The relative amount of defects in S3 is higher than that in S4, as well evident by the simple visual inspection of the resonant behavior. To quantify such analysis, it is correct to compare the Ti3+/Ti4+ peaks intensity ratio (from the Ti 2p spectra) with the (resonant feature)/(total VB) intensity ratio taken with the photon at 459 eV (from the Res-PES data) (see Table 2): it is evident that the total amount of Ti3+ defects is 12535

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Note that the islands are patching the surface assuming different orientations with respect to the substrate, in agreement with LEED data which show a set of rotationally equivalent domains, as indicated by the formation of characteristic bright circles.17 However, the island size and distribution and the observed wetting layer are different from what has been reported in a previous study:40 such differences might reflect the actual morphology (roughness and step density) of the Pt(111) substrates, which can bias the diffusion kinetics of the surface adatoms, and also the slightly different oxidative conditions used in the present experiment. At 18 MLeq (Figure 2b), the surface structure is largely modified, presenting the typical dendritic morphology of the layer-by-layer growth, with predominant areas covered by irregular islands with a granularity at the nanometer scale, coexisting with few patches of flat and regular islands. On the basis of the above-discussed spectroscopic data, we propose to associate the regular flat islands to TiO2-rutile (100) patches (these islands are more abundant, in agreement with the presence of the quasi(2×1) LEED pattern) and the irregular islands to a disordered TiO2precursor phase. Unfortunately, atomically resolved images of such thick films, which could support such an assignment, have not been attained so far. The presence of the TiO2-precursor phase in S3 films could provide a rationale to the high level of defectivity outlined by the Res-PES data analysis (Table 2). 3.2. Water/TiO2 Interaction. Once the actual structures of the films we are probing were clarified, the interaction with water was studied by using H2O-TPD and HR-XPS. Figure 3 illustrates the TPD data collected after different water exposures at low temperature for the four analyzed UT films. In Figure 3a we report the water desorption profiles from the low-coverage S1 film. Two clear peaks are found: a first one centered at ca. 210 K and a shoulder, already saturated at the lowest water exposure (0.05 L), at 265 K. Figure 3b refers to

Figure 2. STM morphological images (167 × 157 nm2) of TiO2/ Pt(111) UT films of different thicknesses: (a) 4 MLeq (I = 0.67 nA, V = 1.2 V). Inset: high-resolution STM image (I = 1.6 nA, V = 1.2 V) showing the reconstructions observed on the wetting layer (z-TiOx) and on the islands (rect′-TiO2); (b) at 18 MLeq (I = 0.1 nA, V = 2 V).

Table 2. Ti3+/Ti4+ and (Resonant Feature)/(Overall VB) Intensity Ratios Obtained from HR-XPS Ti 2p Data (see Figure 1h,i) and Res-PES Data (see Figure 1e,f)

a

UT film

Ti3+/Ti4+

(resonant feature)/(overall VB)a

S3 S4

0.028 0.019

0.146 0.043

Taken with a photon of 459 eV.

show a large-scale morphological view of two films at 4 MLeq (Figure 2a, comparable to the S2 film) and 18 MLeq (Figure 2b, comparable to the S3 film). In the former, the surface presents a morphology characterized by large and flat islands (similar to that previously observed and assigned to rect′-TiO2 patches)17 on a substrate covered by a wetting monolayer of a reduced TiOx phase (see inset of Figure 2a), thus in good agreement with Ti 2p photoemission data (Figure 1g). In particular, the wetting monolayer is present in the form of the z-TiOx film.39

Figure 3. H2O-TPD spectra collected after the low-temperature exposure of the TiO2/Pt(111) UT films of different thicknesses: (a) S1, (b) S2, (c) S3, and (d) S4 (see Table 1 for film labeling). 12536

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Figure 4. O 1s HR-XPS (top), VB (middle), and VB difference (bottom) spectra collected for TiO2/Pt(111) UT films before and after water dosing (2.0 L) and after multilayer water desorption (ca. 210 K): (a, d, g) S2, (b, e, h) S3, and (c, f, i) S4 (see Table 1 for film labeling).

the S2 film (8 MLeq) and, as observed for the previous film, shows two main desorption contributions: a first peak, centered at ca. 210 K, and a second one, appearing as a shoulder, at 265 K. Also in this case the higher temperature peak is very soon saturated. According to the literature on water adsorption on TiO2 single crystals,21,26,41 these two peaks relate to molecular desorption from saturated water multilayer (210 K) and monolayer (265 K). The low water dosing (up to 5.0 L) presented herein has allowed us to saturate the first adsorbed layer and to start to condensate the water multilayer on top of it. This is the reason why the multilayer desorption peak temperature is higher than those reported in literature26 (ca. 160 K). In fact, an increase in the water dosing would lead to a progressive multilayer peak maximum shift to lower temperatures. The desorption profiles plotted in parts (c) and (d) of Figure 3 refer to the S3 and S4 films, respectively. It is important to reiterate that the observed LEED patterns for both samples are similar, and, as reported in section 3.1, the distinctive quasi(2×1) pattern is compatible with a TiO2-rutile (100) surface termination. In the S3 film case (Figure 3c) two main peaks are clearly resolved: a first broad one, centered at about

300 K, and a second one at about 200 K, although both their shape and position change as a function of water exposures. In particular, a higher amount of water (between 0.5 and 1.0 L) is needed to saturate the monolayer and the peak temperature shifts to higher values by 30 K. In the S4 film case (Figure 3d), it is possible to detect three different contributions to the desorption profile: a first peak is found at about 330 K, a second one at about 265 K, and a third one at 200 K. The 330 K peak can be traced back to the water monolayer, while the peak at 200 K corresponds to the multilayer desorption. The intermediate peak can be associated with the second layer water desorption. Moreover, an amount of water between 1.0 and 2.0 L is needed to saturate the monolayer desorption peak, and its maximum temperature shifts by 30 K with respect to the S3 case. This behavior outlines a thickness-dependent interaction with water in the monolayer state (shifts of the monolayer desorption peak from 265 to 330 K). This stronger interaction, as will be shown later, leads to a more efficient dissociation of the H2O bond. It is also interesting to note that the amount of water in the monolayer peak desorption state, passing from the S2 to the S4 UT film, increases by about three times. Nevertheless, we cannot attribute such an increase to the 12537

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intrinsic film islands reactivity because, from an analysis of several STM images, it is possible to estimate an increase of the edge defects passing from the S2 to the S4 film (calculated as the length of the edges) of the same extent. We want also to point out that no other water desorption peak is present in the reported TPD spectra. In particular, the absence of a hightemperature (490 K) desorption contribution suggests that no exposed defects (Ti3+) participate in the reactions.26 These results, in good agreement with Res-PES and Ti 2p presented before, allow us to say that Ti3+ states are not active when the water/film interaction is considered. A detailed comparison with the literature data about water desorption from low-index TiO2-rutile single-crystals surfaces26,41 demonstrates that the TPD results obtained from the S4 film (Figure 3d) are in good agreement with those of the rutile-TiO2 (100) single crystal, thus confirming our previously reported structural assignment (section 3.1).31 In particular, the water desorption profile matches almost perfectly with the literature data, although the peak centered at about 265 K, and corresponding to the second layer water desorption, is not well resolved.41 If we compare the desorption peak corresponding to the water monolayer for the S3 and S4 films, it is possible to single out some differences. The peak is broader in the S3 film, in tune with the copresence of two phases (see section 3.1) and thus different adsorption sites. The observed 30 K temperature shift (from 300 to 330 K) when passing from S3 to S4, is in tune with the water dissociation observed on the rutile (100) surface,26 which implies the OH groups recombination before the desorption process. From the TPD data as a whole we can deduce that both the S1 and S2 films (i.e., lepidocrocite-like TiO2 and TiO2(B)) are less active than the rutile (100) one toward the interaction with water. Photoemission experiments, carried on at the Elettra SR facility, were useful to clarify the TPD results and to sharpen our description of the water/UT films interaction. The spectra corresponding to the clean UT films were acquired immediately after the reactive growth procedure (the sample was at a temperature higher than 500 K) to avoid any formation of −OH groups. In the case of the low coverage S1 film it results, both from the recorded O 1s core level peak and the VB (not reported), that there is no clear signature of dissociative interaction with water at low temperature, in agreement with the TPD data. This is also in agreement with a DFT analysis of the water/lepidocrocite-like nanosheet interaction.42 In contrast, some interesting results were obtained from the HR-XPS and VB photoemission experiments on the higher coverage films. The O 1s HR-XPS spectra reported in Figure 4 confirm the TPD data so far discussed for the S2, S3, and S4 UT films. In particular, it turns out that both the S3 and S4 films (parts (b) and (c) in Figure 4) are characterized by a stronger interaction with water. In fact, the core level O 1s spectra, normalized to the peak maximum and shown in Figure 4a−c, refer to the three different UT films before the water dosing (clean samples, black curves), after water dosing at low temperature (blue curves), and after the water multilayer desorption (collected keeping the samples at about 210 K, red curves). The black curves are characterized by a single main O 1s peak, centered at ca. 530.1 eV (S2) and at 530.5 eV (S3− S4), which refers to the oxygen belonging to the titania UT films lattice.33 The observed 0.4 eV shift can be drawn back both to the different UT films structure and to a more bulklike behavior shown by the thicker films. The spectra acquired immediately after water dosing at low temperature, plotted in

the upper part of the graphs together with their peaks deconvolution, clearly show the presence of both the water multilayer component (533.5 eV) and the one corresponding to −OH groups (531.5 eV) coming from water dissociation. After desorption of the water multilayer, the presence of −OH groups is maintained, as shown by red curves (shoulder on the main O 1s peak, centered at about 531.5 eV). From the deconvolution procedure results, the ratio between the −OH component and that from the oxide lattice is respectively 0.21 for S2 and 0.30 for S3 and S4. Therefore, we can say that S3 and S4 are more active in water dissociation than S2 (in agreement with TPD results). Similar results have been obtained on TiO2 single crystals during a study of their reactivity toward water dissociation.23,43−45 Since no OH groups are observed by photoemission before the water exposure, we can conclude that, based on O 1s HR-XPS, water dissociation takes place on all the analyzed UT films. From the comparison between the relative intensities of the 531.5 eV shoulder we can also hypothesize that the relative amount of OH groups on the S2 is smaller than that on the other thicker films. Also, VB spectra (Figure 4d−f), collected at the Pt Cooper’s minimum (hν = 200 eV) corresponding to the minimum cross section for the Pt 5d levels, confirm the presence of residual surface OH groups after the multilayer water desorption. On the basis of the work of Muryn et al.46 on water adsorption on TiO2-rutile (100) single crystal, it is possible to single out three different peaks in the VB obtained after water exposure, each of them corresponding to a water molecular orbital. The spectra show that after a 2.0 L water adsorption at ca. 100 K on the clean UT films, a broad peak is present in the 10−15 eV range. After the multilayer desorption a contribution, whose intensity depends again on the UT film substrate, is still present and is clearly due to the surface-bound OH groups. To have better insight on the VB modifications as a function of the temperature, we have reported in Figure 4g−i the spectra where each clean UT film contribution has been subtracted from that collected after 2.0 L water exposure and after the multilayer desorption respectively (black and red curves). The black difference spectra thus correspond to the total amount of water dosed on the surfaces and show two well-resolved peaks, named α (6−12 eV range) and β (12−15 eV range) in Figure 4h, in good agreement with the results obtained by Muryn et al.46 Once water multilayer desorption is performed, the β peak vanishes (red solid line). Hence, as expected, β component refers to the molecular water belonging to the multilayer. Moreover, the other component intensity reduces and it is possible to observe its shift to ca. 10.4 eV. All those findings are in perfect agreement with the literature46 so that the 10.4 eV component matches the characteristic position of the OHσ and OHπ orbitals,47 and thus confirm the dissociation of water to surface OH groups. It is important to outline that the VB difference analysis provides important information about the relative amount of OH groups on each UT TiO2 film. In fact, while in the case of S2 the 10.4 eV peak intensity is negligible (at least, it is difficult to detect a clear peak), both the S3 and S4 films present a clear contribution, which increases together with the film thickness. These data are in tune with the O 1s core level XPS (Figure 4a−c) trend, whic shows the highest amount of dissociated water (in terms of detected intensity associated with the OH groups) on the S3 and S4 films. To quantify the amount of dissociated water in the first adsorbed layer, we performed isotopic labeling TPD experi12538

dx.doi.org/10.1021/jp300614n | J. Phys. Chem. C 2012, 116, 12532−12540

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Figure 5. Isotopic labeling TPD spectra for TiO2/Pt(111) UT films of different thicknesses: (a) S2 and (b) S4 films after a 1.0 and 3.0 L water dosing, respectively. The m/e = 20 signals (red curves) have been normalized to a benchmark temperature of 260 and 290 K, respectively (see Table 1 for film labeling).

ments26,41 on both the S2 (Figure 5a) and the S4 (Figure 5b) films. During such an experiment, the films were grown in an 18 O2 environment, thus allowing us to obtain a labeled Ti18O2 sample. Hence, H216O was dosed on the sample and the m/e = 20 spectral line (corresponding to H218O, formed as a consequence of water dissociation and oxygen scrambling on the TiO2 surface) was monitored together with the m/e = 18 component, providing both a m/e = 18 (Figure 5, black) and m/e = 20 (Figure 5, red) desorption profiles. Subsequently, the latter have been compared to the former after a careful step by step normalization as a function of the desorption temperature of the relative intensities of each spectrum. This data elaboration has allowed us to distinguish between the H218O signal coming from the oxygen scrambling within the QMS and the one coming from sample surface OH groups recombination26,41 finding a benchmark temperature of 260 K (for S2) and 290 K (for S4). The TPD curves reported in Figure 5 show that, above the selected benchmark temperatures, the two desorption profiles are different so that the m/e = 20 signal maximum results to be positively shifted with respect to the m/ e = 18 signal (by ca. 10 K for S2 and 30 K for S4) as a consequence of recombinative desorption due to surface −OH groups. The temperature shift observed for S4 is in good agreement with other literature data reported for water dissociation on TiO2-rutile (100).26 Moreover, the overall m/ e = 20 peaks area, calculated after the normalization, is higher than the m/e = 18 one by 10% (for S2) and 30% (for S4), being the latter value in good agreement with the literature work published on bulk TiO2-rutile(100) single crystal.26 Therefore, the experimental data so far discussed allow us to conclude that all the analyzed UT films (apart from S1) are reactive toward water dissociation. On S4 there is a 30% contribution to the desorption curve of water due to OH recombination, which is found at a temperature above 290 K. At the same time it was possible to find a 10% contribution in the m/e = 20 TPD, due to the recombinative desorption process, for the S2 sample. Since for the same UT film a small O 1s core level peak shoulder at 531.5 eV was found, we can conclude that water dissociation takes place also on S2.

surfaces, that is, lepidocrocite-like nanosheet, TiO2(B) (001), and rutile-TiO2 (100), are the results of the self-organization of titania on Pt(111) in different thickness regimes and their structural differences are tracked by a series of surface science experiments.17,30−33 The XAS data reported in this paper evidence a trend from an anatase-like XAS pattern toward a rutile-like one by increasing the film thickness, eventually exposing the TiO2-rutile (100) surface termination.31 The ResPES and Ti 2p photoemission data indicate that, under the experimental conditions, the Ti3+ defects are mostly located inside the film. In good agreement, the TPD results demonstrate that Ti3+ defects are not involved in the water dissociation reaction. Taking into consideration the main outcomes of the TPD and HR-XPS experiments, it is possible to conclude that both the TiO2(B) (001) and the TiO2-rutile (100) terminations are able to dissociate water. This is actually the first experimental evidence of WD on TiO2(B) (001). On the other hand, our results are in agreement with the literature data on WD capability of the TiO2(100) surface.26 In this paper we demonstrate that this ability is maintained also in the case of UT films. In particular, the possibility to grow stoichiometric rutile-TiO2 films exposing the (100) termination on substrates characterized by hexagonal symmetry like Pt(111) opens interesting perspectives in the production of effective photocatalytic devices to produce H2 as a source of green energy on cheaper substrates like sapphire(0001).48

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work has been funded by the Italian Ministry of Instruction, University and Research (MIUR) through the FIRB Project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications”, and through the fund “Programs of national relevance” (PRIN-2009). We thank Dr. Andrea Vittadini (Padova) for helpful discussion.

4. CONCLUSION For the first time, a comparative study of the water/titania interaction in different polymorph surfaces obtained in the form of UT films has been reported. The different polymorphs 12539

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