Studied by Electron Spectroscopy in Ultrahigh Vacuum - American

J. Phys. Chem. C 2007, 111, 3459-3466

3459

Li Insertion in Sol-Gel Prepared Mn-Doped TiO2 Studied by Electron Spectroscopy in Ultrahigh Vacuum J. H. Richter,† P. G. Karlsson,† G. Westin,‡ J. Blomquist,§ P. Uvdal,§ H. Siegbahn,† and A. Sandell*,† Department of Physics, Uppsala UniVersity, P.O. Box 530, SE-751 21 Uppsala, Sweden, Department of Materials Chemistry, Uppsala UniVersity, P.O. Box 538, SE-751 21 Uppsala, Sweden, and Chemical Physics, Department of Chemistry, Lund UniVersity, P.O. Box 124, SE-221 00 Lund, Sweden ReceiVed: September 21, 2006; In Final Form: January 3, 2007

The properties of a sol-gel prepared Mn-modified TiO2 film have been studied with X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The chemical composition and oxidation state of the elements have been determined. The manganese content estimated by PES of the Mn-modified film is about 10% and both Mn2+ and Mn3+ are observed. Addition of Mn is found to modify the valence band edge. The Mn 3d states are found to extend about 1 eV into the TiO2 band gap region. It is demonstrated that lithium insertion into the sol-gel film can be performed in a stepwise fashion in situ under ultrahigh vacuum (UHV) conditions. Lithium is distributed evenly throughout the entire film and leads to reduction of Mn3+ to Mn2+ followed by reduction of Ti4+ to Ti3+. The XAS and PES measurements give fully consistent results regarding the amount of inserted lithium.

1. Introduction Titanium dioxide (TiO2) is a material that has a wide range of applications. One of its important properties is that it is highly efficient in inducing heterogeneous photocatalytic reactions.1 It has been shown that doping TiO2 with foreign cations can enhance the photocatalytic properties and improve the efficiency of dye-sensitized solar cells.2-4 Doping of TiO2 has attracted considerable attention lately for another reason: Anatase TiO2 with the addition of a few percent of Co has been found to exhibit ferromagnetic properties.5,6 Due to the semiconducting nature of TiO2, Co:TiO2 has been considered as a possible material in systems for spin-selective electron transport, so-called spintronic devices. However, the properties of TiO2 doped with ferromagnetic elements such as Co, Fe, and Mn are still heavily debated.7 There is consequently a clear need for systematic investigations of this type of materials to elucidate the properties and refine the methods for synthesis. Furthermore TiO2 can, in its anatase form, accommodate small ions such as H+ or Li+ and displays electrochromic properties upon ion insertion. The ability to effectively and reversibly store a substantial amount of lithium makes it an interesting candidate for cathode material in rechargeable lithium ion batteries.8 Due to the accompanying color change it is also considered for electrochromic devices.9-11 Lithium insertion into anatase TiO2 has been successfully achieved by various methods, where the maximum amount of storable lithium varies with the chosen method. A typical value for maximum lithium uptake into anatase TiO2 via electrochemistry is 0.5 Li per Ti.12-15 The general view is that a phase * Address correspondence to this author. E-mail: [email protected]. Phone: +46-18-4713548. Fax: +46-184713524. † Department of Physics, Uppsala University. ‡ Department of Materials Chemistry, Uppsala University. § Lund University.

separation into an Li-poor and an Li-rich phase occurs upon Li insertion.16-18 The Li-poor phase is very similar to the original anatase structure, whereas the Li-rich phase is described as an orthorhombic distortion of the atomic positions.17,19 Additionally, ion insertion into TiO2 significantly alters (reduces) the band gap, which makes it an interesting material for photocatalysis and solar cells.20 Specifically, the insertion process is believed to involve population of states in the band gap of mainly Ti 3d character.21 The examples above illustrate that studies on Mn-modified anatase TiO2 are highly motivated. A careful mapping of the chemical state of the elements and the frontier electronic structure has a clear relevance to the development of photocatalytic (and possibly spintronic) materials. Moreover, the modifications of the electronic structure imposed by Li insertion deserve attention; apart from its relevance to rechargeable battery systems this process also serves as a tool to probe the unoccupied states as an extra electron is donated to the host oxide. In fact, on the basis of calculations it has been proposed that Mn-modified TiO2 films can even develop magnetic properties upon exposure to small amounts of lithium.22,23 Photoelectron spectroscopy (PES) of core electronic levels can be used to determine the chemical composition of the sample as well as the oxidation state of the elements. In addition, photoelectron spectroscopy of the valence electronic levels gives information about the valence electronic structure. For the present systems, this means that modifications of the band gap can be observed. X-ray absorption spectroscopy (XAS) constitutes a valuable complement to PES. XAS provides a local probe of the unoccupied electronic states since it selects one atomic species at a time. The XAS spectra are furthermore sensitive to the local geometric structure of the probed element. From this it follows that core and valence PES in conjunction with XAS forms a powerful tool to elucidate changes in the electronic and geometric structure associated with ion insertion and metal doping.

10.1021/jp066192n CCC: $37.00 © 2007 American Chemical Society Published on Web 02/03/2007

3460 J. Phys. Chem. C, Vol. 111, No. 8, 2007 We have previously employed PES and XAS to show that Li-inserted TiO2 can be realized in a pure in situ approach in ultrahigh vacuum (UHV).24-26 With this method of insertion the modifications imposed by lithium can be monitored in a stepwise fashion under well-defined conditions. This is not possible in systems with less controlled experimental conditions, e.g., electrochemical ion insertion, where exposure to atmosphere as well as the electrolyte exclude the unambiguous identification of the origin of changes. A system prepared in UHV is thus excellent for benchmarking of theoretical models. By combining ab initio calculations and experimental data a very detailed picture of the electronic structure of the Li:TiO2 system has been obtained.26 The purpose of this study is twofold: (1) to use PES and XAS to examine the properties of a sol-gel prepared Mn-doped TiO2 film and (2) to demonstrate that lithium insertion into a sol-gel prepared film can be accomplished in situ through evaporation in UHV. Comparisons are made to TiO2 films prepared by sol-gel and UHV-CVD. We report on the Mn content and oxidation state of the Mn:TiO2 film. A modification of the valence band edge as compared to pure TiO2 is observed. Mn 3d states are found to extend by about 1 eV into the TiO2 band gap region. Lithium is distributed evenly throughout the entire film upon UHV exposure and leads to reduction of Mn3+ to Mn2+ followed by reduction of Ti4+ to Ti3+. XAS and PES are used independently to estimate the amount of inserted lithium. 2. Experimental Section All spectra were recorded at beamline D1011 at the Swedish National Synchrotron Facility MAX-II.27 XAS spectra were recorded using a multichannel plate for detection of secondary electrons. The photon energy scale of the XAS spectra has been established by comparing the binding energy of the Ti 3p feature for excitation by first- and second-order light from the monochromator. The photon energy resolution of the Ti 2p and Mn 2p XAS spectra was set to 100 and 250 meV, respectively. A 200 mm hemispherical electron energy analyzer of Scienta type was used to obtain PES spectra. The binding energy of all peaks was calibrated by relating the energy of the Ti 3p peak to the Fermi level of the sample holder clips. The high-resolution core level photoemission spectra (Ti 2p, O 1s and Mn 2p) were recorded with a photon energy resolution of 250-350 meV and an electron energy resolution of 400 meV. This gives a typical total energy resolution of 500 meV, assuming quadratic addition. The corresponding values for the valence photoemission spectra were 60 (photons) and 100 meV (electrons), giving a total energy resolution of 120 meV. The films under investigation are as follows: (1) An ex situ sol-gel prepared film grown by spin-coating with a titanium tetraisopropoxide, Ti(OPri)4, solution to yield a gel-film and subsequent heat treatment to 600 °C. The films consisted of ca. 5 nm sized crystallites, were 80-100 nm thick, and were of the anatase modification. (2) A manganese modified ex situ sol-gel prepared film grown in the same way with Mn19O12(moe)14(moeH)14 (moeH ) 2-methoxyethanol) as the Mn-oxide source. The 7% Mn film was purely of the anatase phase and of approximately the same film thickness, crystallite size, and thickness as the pure sol-gel derived TiO2 film. (3) An in situ ultrahigh vacuum (UHV) chemical vapor deposition (CVD) grown anatase TiO2 film of about 44 Å. This UHV film serves as a reference as it has previously proven to be a good model system for lithium insertion into clean anatase TiO2.25 Sol-gel preparation employs the transition of a colloidal liquid (the sol, containing solid precursor material) from liquid

Richter et al. into solid (gel) phase. The resulting porous gel is then chemically purified and fired at high temperatures to form high-purity oxide materials. The sol-gel derived films were prepared with an allalkoxide route, using the commercial titanium tetraisopropoxide, Ti(OPri)4, (Aldrich) precursor and a novel Mn-alkoxide precursor, Mn19O12(moe)14(moeH)14 (moeH ) 2-methoxyethanol), prepared by metathesis of anhydrous MnCl2 and 2Na or 2Kmoe in toluene-moeH solvent.28 This novel, reactive Mn precursor provides facile removal of all organic components in the gel formed by hydrolysis by the moisture in the air on spin-coating. The purely inorganic gel-films provide an advantage in the homogeneous doping and more controlled conversion of the gel to oxide on heat treatment, since there are no organic groups left that cause uncontrolled local heating and reduction of ions in the gel and subsequent reoxidation by air-oxygen. This difference has been shown for the corresponding Co:TiO2 system29 where gels with only minor amounts of organic groups showed phase impurities while films free of organic became phase pure anatase. On heating the corresponding powders of the Ti and TiMn-gel films to 600 °C, water was removed endothermically up to 200 °C yielding essentially an oxide film, which showed a rather sharp exothermic peak in the differential scanning calorimeter at about 490 °C for the Mn-doped films. This peak might be associated with crystallization of the anatase phase, which was the sole phase at 600 °C for these samples. The UHV prepared film was formed by CVD TiO2 from titanium tetraisopropoxide (TTIP) on Si(111)-(7×7) at 500 °C sample temperature under ultrahigh vacuum conditions.24 This results in an ultrathin (44 Å) TiO2 film made up of nanocrystallites about 10 nm in diameter. This film has previously been studied extensively and has proven to be a good model system for pure anatase TiO2. Thus this film can serve as a valuable reference for the sol-gel prepared films, where we try to establish similarities and differences between the films prior to and under lithium insertion. Lithium was inserted into these films in situ by controlled exposure to lithium vapor at room temperature. To this end a getter source (SAES Getters SpA) has been used. The Li source was degassed for several hours. The Li evaporation rate was calibrated by deposition on a clean Si(111)-(7×7) surface. For this system, the Li coverage can be related to changes in the work function and in the shape of the Si 2p core level spectrum.30 The Li coverage as read by a thickness monitor has furthermore been related to the work function changes.31 We found that the Li film thickness estimated from the attenuation of the Si 2p signal under the assumption of layerby-layer growth gives consistent values with those obtained from the changes in the work function and the Si 2p spectral shape. Regarding the presence of contaminants, a weak oxygen signal could be detected after the calibration runs but it was not ascertained whether its origin was due to the Li source or to rest gas adsorption. The base pressure in the preparation chamber was 1 × 10-10 mbar. During Li exposure to the Mn:TiO2 film, the pressure quickly rose to about 8 × 10-10 mbar. The pressure increased further with evaporation time, reaching about 5 × 10-9 mbar after the longest exposures (60 min). It can finally be noted that the Mn:TiO2 film was probed in remanence with X-ray magnetic circular dichroism (XMCD) in Mn L-edge absorption before and after Li insertion. However, no ferromagnetic response was detected. 3. Results and Discussion 3.1. Characterization of the Sol-Gel Prepared Mn:TiO2 Film. To estimate the manganese content in the Mn:TiO2 film

Li Insertion in Sol-Gel Prepared Mn-Doped TiO2

J. Phys. Chem. C, Vol. 111, No. 8, 2007 3461

Figure 1. Ti 2p3/2 (a) and O 1s (b) core level photoemission spectra for the sol-gel prepared Mn:TiO2 film (top), the sol-gel prepared TiO2 film (middle), and the UHV prepared TiO2 film (bottom).

the Mn 2p and Ti 2p features in an overview spectrum recorded at 758 eV photon energy have been analyzed. The integrated intensities of the Mn 2p and the Ti 2p features were compared taking into account the different differential cross-sections for photoionization.32 The estimated Mn content in the film is estimated to (9.6 ( 0.4)%. The PES results thus suggest an approximate molar composition of Mn0.1Ti0.9O2. This is in reasonable agreement with the Mn content of 7% predicted by the preparation method and an SEM-EDS analysis of the powder. The Mn 2p spectrum does not show any evidence for metallic Mn (cf. Section 3.2). The Mn oxidation state is discussed in detail in connection with the XAS results below. Figure 1a shows high-resolution, surface-sensitive Ti 2p core level spectra for all films recorded with use of 610 eV photons. In all cases the Ti 2p signal is identified to arise from a single spin-orbit split feature. The Ti 2p state is attributed to Ti4+.33 The Ti4+ 2p3/2 binding energies (BEs) for the Mn:TiO2 film, the sol-gel, and the UHV prepared TiO2 films are 458.6, 459.3, and 459.2 eV respectively. That is, the Ti4+ 2p3/2 BE value for the Mn-modified film is significantly smaller. Figure 1b depicts the corresponding O 1s core level spectra recorded with use of 640 eV photons. Here we observe the main O 1s peak at 530.0 eV for the Mn:TiO2 film, at 530.6 eV for the sol-gel TiO2 film, and at 530.7 eV for the film prepared in UHV. Thus, the O 1s BE downshift for the Mn-modified film as compared to the O 1s state of the TiO2 films is very similar to that observed for the Ti 2p state. The cleanliness of the UHV prepared TiO2 film is evident as one feature clearly dominates. Only a very weak additional structure is found, of 532 eV BE. This shoulder can be due to surface hydroxyl groups or SiO2. The signal from SiO2 confined within the interfacial region may be visible even after large deposits of TiO2. The reason is that TiO2 particles form, giving rise to discontinuities in the film.34 The sol-gel prepared films show, apart from the main feature, an additional high binding energy component of considerable intensity (about 50% of the main peak). This component is due to contamination after exposure to atmosphere. The sol-gel prepared samples are also characterized by a significant amount of surface carbon (spectra not shown). An upper limit of 5 Å is found for the thickness of the contamination layer by comparing survey spectra for a solgel prepared TiO2 film to a UHV prepared TiO2 film after normalizing to the number of scans and the photon flux. However, it is noteworthy that there is no distinct difference in the Ti 2p and the O 1s spectral shape between the sol-gel

prepared films. Thus the addition of Mn to the film does not lead to the formation of any new Ti or O species detectable by PES. Photoelectron spectra covering the valence region of the asprepared films are shown in Figure 2a. The spectra are dominated by the O 2p states (with a small contribution from hybridized Ti 3d states21,35,36). The spectra for the two TiO2 samples display small differences in the relative intensities of states in the BE region 6-8 eV. These are explained by the surface contamination present after sol-gel preparation and exposure to atmosphere, giving rise to additional O 2p derived states (from, e.g., -OH groups) and C 2p and C 2s derived states from carbonaceous species. By using the linear approach as described by Chambers et al.,37 the valence band edge (VBE) position for the sol-gel prepared TiO2 film is estimated to about 3.5 eV relative to the (sample holder) Fermi level. The same value is obtained for the UHV prepared TiO2 film. This value is in very good agreement with optical measurements for anatase; a direct transition has been observed at 3.3 eV whereas an indirect transition is observed at 3.8 eV.38 From this follows that the VBE position reflects the magnitude of the band gap, that is, the conduction band edges (CBEs) for the two TiO2 films are aligned to the Fermi level. The Fermi level alignment to the CBE is commonly attributed to defects, mainly in the form of oxygen vacancies, which give rise to population of band gap states.39,40 In the UHV prepared film a band gap state at about 1 eV is somewhat more pronounced. This state has previously been identified as localized Ti 3d states originating from Ti3+ species.16,21,41 It can be surmised that since the sol-gel prepared films have been exposed to atmosphere and are thus more heavily oxidized the intensities of the band gap states are lower. The valence band structures for the Mn:TiO2 film are shifted in BE by about -0.7 eV as compared to the sol-gel prepared TiO2 film. This is the same shift as observed for the Ti 2p and O 1s levels, which clearly indicates that the electronic levels of the Mn:TiO2 film are shifted in a rigid fashion as compared to the TiO2 films. The rigid BE shift can be due to variations in the valence band alignment relative to the Fermi level (our reference level). The core level BE can be sensitive to small changes in the valence occupation, caused by, e.g., oxygen vacancies, Mn doping, and Li insertion. This means that the core level BE depends on the density of states (DOS) of the material. The importance of the DOS is evident from the observation that the Ti4+ 2p BE offset for the Mn:TiO2 sample relative to the other

3462 J. Phys. Chem. C, Vol. 111, No. 8, 2007

Richter et al.

Figure 2. (a) Valence region photoemission spectra of the three different films. (b) Photoemission spectra showing the valence band edge for the sol-gel prepared Mn:TiO2 film (solid line) and TiO2 film (dashed line). The estimated positions of the valence band maxima are indicated. The spectrum for the Mn:TiO2 film has been aligned in binding energy to the spectrum for the sol-gel prepared TiO2 film so that the Ti 3p binding energies are the same. (c) Photoemission spectra showing the valence band edge for the sol-gel prepared Mn:TiO2 film (solid line) and TiO2 film (dashed line). The spectra are aligned in binding energy as in part b. The thick solid line represents the difference spectrum, in which the valence band edge is indicated.

samples is eliminated to within 0.1 eV upon insertion of 1-2% of Li. In detail, the Mn:TiO2 states shift rigidly by +0.8 eV, whereas the corresponding shifts for the sol-gel and UHV prepared TiO2 films are 0.1 and 0.2 eV, respectively. The variations in the band alignment can also be discussed in terms of Fermi level offsets due to poor conductivity of the samples. As a result there are rigid shifts in the valence band edge and core level peak positions depending on the Fermi level offset. Lithium insertion results in a population of empty states derived from the conduction band, generating free carriers. Consequently, an alignment of the sample Fermi level to the spectrometer Fermi level occurs upon lithium doping. However, there is no clear indication that the shift between the two solgel prepared films should be explained in terms of sample charging effects. The film thickness is similar, as is the level of contamination and the individual line widths. Clearly, to elucidate the details on the shifts and the DOS effects a systematic study supported by theoretical calculations is required. To simplify the discussion about Mn-induced effects in the valence and band gap region the spectrum for the Mn:TiO2 film has been aligned to the spectrum for the sol-gel prepared TiO2 film. That is, the Mn:TiO2 spectrum has been shifted by +0.7 eV. The valence band edges of the two sol-gel prepared films are compared in Figure 2b, normalized as to have the same maximum intensity of the valence band. When aligned in this way, it stands clear that the valence band for the Mn:TiO2 film displays additional intensity on the low binding energy side. This intensity is not present in the two TiO2 films and thus must be a direct consequence of the presence of manganese in the film. Applying the linear method on the shifted raw spectrum of the Mn:TiO2 film gives a VBE position of approximately 3.0 eV (Figure 2b). Estimating the VBE location for the Mn:TiO2 film is, however, more uncertain, due to the mixed valence band edge containing O 2p (with an admixture of Ti 3d) and Mn 3d states. By subtracting the spectrum of the sol-gel prepared TiO2 film from that of the Mn-modified TiO2 film, as shown in Figure 2c, it is found that the additional intensity in the Mn-modified film is centered at around 3.5 eV. This is in reasonable agreement with the Mn 3d binding energy found in MnO of 4 eV.42,43 If we apply the linear method on the difference spectrum in Figure 2c, this yields a value for the valence band edge of

Figure 3. Mn L-edge (2pf3d) XAS spectra for the as-prepared film (Mn:TiO2) and the situation after lithium insertion (Mn2+). The intensity of the spectrum of Mn2+ is chosen to represent the contribution of the Mn2+ species to the spectrum of Mn:TiO2. The difference spectrum of Mn:TiO2 - Mn2+ is characteristic for Mn3+.

about 2.5 eV. Thus, the results suggest that the Mn 3d derived states extend about 1 eV into the band gap region of TiO2. To gain insight into the geometric structure and to identify the Mn chemical state, XAS spectra of the manganese and titanium L-edge have been recorded. The Mn L-edge spectrum of the as-prepared Mn:TiO2 film shown in Figure 3 can be described in terms of a superposition of spectra from Mn2+ and Mn3+ species. To illustrate this we have employed a subtraction process. After exposure to lithium the film only contains Mn2+. This is discussed in detail below and confirmed by a comparison to previously reported XAS spectra for Mn2+.44 Subtraction of the spectrum characteristic for Mn2+ from the spectrum for the as-prepared film yields a difference spectrum that closely resembles a spectrum characteristic for Mn3+.45 From the relative contributions of the Mn3+ and Mn2+ spectra the ratio is estimated to be 0.6 Mn3+ to 0.4 Mn2+. However, it has to be noted that PES and XAS are surface-sensitive techniques (the data are strongly dominated by the outermost 10% of the film). Thus we cannot exclude segregation of Mn to the surface region and a different stoichiometry in the bulk.

Li Insertion in Sol-Gel Prepared Mn-Doped TiO2

J. Phys. Chem. C, Vol. 111, No. 8, 2007 3463 As described in the previous study,24 the expected x-value as a function of lithium exposure time can be estimated from the following formula:

x(t) )

Figure 4. Ti L-edge (2pf3d) XAS spectra for the three different films.

The anatase crystal phase of TiO2 can accommodate a significant concentration of small ions such as H+ and Li+. The rutile phase is, however, much less susceptible to ion insertion. For facile ion insertion it is therefore important that the films consist of anatase TiO2. To gain insight into the crystal structure of the films XAS spectra of the Ti L-edge have been recorded (Figure 4). The XAS spectra for all films closely resemble that of the anatase TiO2.46 There seems, however, to be a difference in the spectral form for the L3 eg state of the Mn:TiO2 film. This spectral anomaly disappears after lithium insertion. As will be discussed later on, Li effectively reduces Mn3+ to Mn2+, suggesting that this spectral difference is connected to the presence of Mn3+. 3.2. UHV Lithiation of the Sol-Gel Prepared Anatase Mn:TiO2 Film. Ti 2p core level spectra for the Mn:TiO2 film recorded upon Li exposure in UHV are shown in Figure 5a. It is observed that reduction from Ti4+ to Ti3+ occurs. The behavior is thus directly comparable to that observed upon Li exposure of the thin film grown in UHV.24 Upon successful insertion, lithium donates charge so that the concentration of Li can be approximated by the relative amount of Ti3+ to the total Ti signal. This quantity is commonly denoted the x-value, which thus corresponds to x in LixTiO2. However, the sol-gel film is thicker than the UHV prepared film by about a factor of 20 and it also has a degree of contamination. It is therefore highly motivated to show that Li penetrates the entire film and is not confined to the surface region. A clear indication that Li-penetration occurs for the sol-gel prepared film is the possibility for real-time observation of diffusion into the film. Figure 6 shows a series of Ti 2p spectra for the same situation but at different times after insertion was completed. A slow decrease of the relative Ti3+ contribution to the Ti 2p signal (i.e., the x-value) is observed. This is interpreted as lithium accumulating in the surface region and slowly diffusing into deeper regions of the film with lower lithium concentration. Keeping in mind the surface sensitivity of the measurement this means that lithium that is visible in the immediate surface region becomes invisible upon diffusion into the film. The results and quantitative estimates discussed below refer to situations close to steady state, i.e., after about 1 h.

FLi C(t) FTi N

(1)

In this expression FLi and FTi are the densities of Li and Ti atoms within a layer, respectively. C is the nominal number of Li monolayers, which is the lithium deposition rate in monolayers per minute times the exposure time in minutes. The lithium deposition rate as estimated from a calibration run Li on Si (111)-(7×7) is about 2.5 ML per minute, thus C is given by 2.5 times the exposure time in minutes. Finally, N is the number of anatase TiO2 layers. Anatase TiO2 crystallites and islands mainly expose the (101) face, implying that this termination has the lowest surface energy.47 Approximating a dense anatase film 100 nm thick, growing in the (101) direction, yields an N value of 290. The density of Ti atoms in the anatase (101) layer is 1.03 × 1015 atoms/cm2. The density of Li in a nominal layer is 1.16 × 1015 atoms/cm2. Assuming that the x-value directly corresponds to the amount of reduced Ti and thus the formation of Ti3+, we can calculate a line that represents the expected behavior for a homogeneous distribution of lithium in the film. Figure 7a depicts the titanium reduction curves for the Mn: TiO2 film and the linear development of the Mn:TiO2 film given by (1). The x-scale has been calculated so it represents the ratio of Li to Ti atoms rather than the lithium exposure time, thereby allowing also for a direct comparison to the UHV case. Figure 7a convincingly demonstrates that lithium must penetrate the film and be distributed throughout a substantial part of the film. The lithium cannot be confined to the surface region, as we otherwise would observe a much higher rate of reduction of Ti in the Mn:TiO2 case compared to the UHV prepared film and the calculated average concentration. In the case of surface agglomeration of lithium a strong attenuation of the Ti 2p signal would be observed, which is not the case. Furthermore, Figure 7b shows that the Li 1s signal displays a linear increase with exposure time. This lends strong support for Li penetration since the formation of a Li film on top of the Mn:TiO2 film would not lead to a strict linear increase of the Li 1s signal after completion of the first Li layer. Instead we would observe an exponential decrease of the linear growth of the Li 1s feature as new lithium deposited on the film would attenuate the lithium signal from underlying regions. The evolution of the Ti 2pf 3d XAS spectra for the Mn: TiO2 film under lithium insertion is shown in Figure 8. We have previously demonstrated for the UHV prepared TiO2 film that the evolution of the spectra can be interpreted in terms of a changeover from anatase (x ) 0) to lithium titanate (x ) 0.5).26 Each intermediate situation can be described as a superposition of the contributions from the two different phases. A best-fitof-form procedure has been tried to delineate the Ti XAS spectra and thus identify the relative contribution of the two phases and with it the amount of lithium contained in the film. Setting the x-value to 0.5 for the lithium titanate phase, the x-value derived this way is given by the percentage of the lithium titanate phase needed to recreate the spectrum divided by two. In this case we combine the XAS spectrum taken after 0 min, representing the anatase phase, and the fully inserted spectrum taken after 172 min, which equals the lithium titanate phase. It is found that the x-values derived from the XAS decomposition are very similar to those derived from the PES results, as indicated in Figure 8. However, the fitting procedure was more

3464 J. Phys. Chem. C, Vol. 111, No. 8, 2007

Richter et al.

Figure 5. Ti 2p (a) and Mn 2p (b) core level photoemission spectra of the Mn:TiO2 film for increasing amounts of inserted lithium.

Figure 6. Ti 2p core level photoemission spectra of the Mn:TiO2 film for an intermediate Li concentration recorded directly after Li exposure (solid line) and 38 min after Li exposure (dashed line). The x-value at “steady state” is 0.33. Figure 8. Series of Ti L-edge (2pf3d) XAS spectra for the Mn:TiO2 film for increasing Li concentration (x-value in LixTiO2). The values are obtained both from XAS decompositions and from a delineation of the corresponding Ti 2p PES spectra. The x-values obtained from the PES results are given in parentheses.

Figure 7. (a) The evolution of the relative contribution of the Ti3+ component to the Ti 2p signal as a function of relative lithium content. The circles represent the values for insertion into Mn:TiO2, while the crosses represent the values for insertion into the TiO2 film formed in UHV. The solid line represents semiempirical values, calculated from the estimated film thickness and Li evaporation rate. (b) Integrated intensity of the Li 1s signal of the Mn:TiO2 film as a function of lithium exposure time.

difficult than that for the UHV prepared film, due to the aforementioned change in the L3 eg state. This discrepancy in the spectral shape of the L3 eg as seen in Figure 4 is attributed to the presence of Mn3+ in the TiO2 matrix as it is observable only where both Mn3+ and Mn2+ are present in the film (i.e., the as-prepared Mn:TiO2 film), but not in any other case.

After having established that proper Li insertion takes place, the process can be discussed in more detail. The addition of Mn (present as Mn3+ and Mn2+) to the TiO2 matrix complicates the denomination to some degree due to the presence of two reducible species. The Mn L-edge XAS spectrum exhibits a significant change of spectral shape after the first lithium insertion step. In the as-prepared film the spectrum contains contributions from both Mn3+ and Mn2+, whereas all spectra with inserted lithium are comprised solely of Mn2+. As shown in Figure 3, a subtraction procedure has been tried on the Mn XAS spectra, giving an estimate of the Mn3+ to Mn2+ ratio of 0.6/0.4 for the as-prepared film. Figure 5b shows the Mn 2p PES spectra before and after Li insertion, recorded at 790 eV photon energy. Noteworthy is that there is no evidence for metallic manganese species in the film, which would show up as a distinctive state on the low binding energy side.48 That is, there is no agglomeration or clustering of metallic Mn. The XAS results (see Figure 3) suggest a reduction of the Mn3+ species to Mn2+ upon Li insertion. Reduction is typically associated with a decreased binding energy in the core level PES spectrum. However, the binding energy shift upon changes in the oxidation state is not as

Li Insertion in Sol-Gel Prepared Mn-Doped TiO2 pronounced in the core level PES spectra for mid-transition metals like V, Fe, Co, and Mn as for, e.g., Ti. The binding energy difference between Mn3+ in Mn2O3 and Mn2+ in MnO is less than 0.5 eV.49 Hence the chemical shift of the Mn 2p peak upon Li insertion and reduction from Mn3+ to Mn2+ is expected to be small and difficult to discern, especially since the pristine Mn:TiO2 film already contains 40% Mn2+. It is observed in Figure 5b that the Mn 2p binding energy shifts by about +0.7 eV after the first lithium insertion, very similar to the rigid shift found for the other electronic levels (+0.8 eV). That is, the BE shift attributed to the reduction of Mn is only of the order of 0.1 eV. Notable is also that the Mn 2p spectrum exhibits a slightly increased relative intensity of the satellite at 645 eV upon Li insertion. This is in agreement with the previous study, where the satellite in the Mn 2p spectrum of Mn2+ is stronger than that in the Mn 2p spectrum of Mn3+.49 Tabulated redox potentials suggest that the reduction of Mn3+ to Mn2+ takes place well ahead of Ti4+ to Ti3+ reduction (Mn3+ f Mn2+ +1.76 vs Ag/AgCl compared to Ti4+ f Ti3+ (in TiO2) -0.7 vs Ag/AgCl). To directly compare the Ti reduction for the Mn:TiO2 film to the UHV prepared TiO2 film the lithium deposition rate (in ML/min) in relation to the film thickness has to be considered. The lithium deposition rate for the UHV prepared film has been found to be 0.08 ML/min with a film thickness of 4.4 nm. The lithium deposition rate to film thickness ratio is thus very similar in both cases and we would expect both films to “fill up” under comparable times. The x-value derived from the relative contribution of the Ti3+ component to the total Ti 2p signal is compared to the in situ film in Figure 7a. Preferential reduction of Mn3+ to Mn2+ would be observed in Figure 7a as a delay in the onset for the Ti4+ to Ti3+ reduction. That is, during the first stages of the experiment an exclusive reduction of manganese takes place and only after all Mn3+ has been reduced does the reduction of titanium begin. It is found that there is no apparent Li-induced formation of Ti3+ in the Mn:TiO2 film for the first 6 min of lithium exposure corresponding to 6% of Li atoms per Ti atom. However, the results are not conclusive; the discrepancy for at least the first point is within the error bars and there is also a possibility that some Li atoms initially react with the contamination layer. In spite of these uncertainties, it is instructive to discuss a presumed 6-min offset in the titanium reduction due to manganese reduction. In the case of preferential reduction of manganese the formation of Ti3+ no longer is a direct measure of the amount of lithium in the film as charge from lithium is transferred to Mn and thus should be determined by estimating the relative amounts of Mn3+ and Mn2+. Furthermore, we cannot exclude segregation of Mn to the surface leading to an overestimation of the Mn content in the film or the formation of Mn3+ being a surface effect. Thus we will keep the notation of x being the relative contribution of Ti3+ to the Ti 2p spectra; however, we will keep in mind the presence of the additional lithium associated with the reduction of the Mn3+ species. The Mn3+ content can be estimated indirectly from the rate of titanium reduction. From the development of the Ti 2p PES spectra we derived an initial reduction rate of (1.3 ( 0.3)% of atoms reduced per minute. Under the assumption that all Mn reduction takes place during the first 6 min the amount of reducible Mn (i.e., Mn3+) that must have been present in the as-prepared film amounts to 6-10%. This is in good agreement with the estimate of 6% based on the PES survey spectrum and the XAS subtraction process (60% Mn3+ in Mn0.1Ti0.9O2). It has to be noted that the PES and XAS estimation could only

J. Phys. Chem. C, Vol. 111, No. 8, 2007 3465

Figure 9. (a) Valence region photoemission spectra recorded after Li insertion (x ) 0.4). Top: The sol-gel prepared Mn:TiO2 film. Middle: The sol-gel prepared TiO2 film. Bottom: The UHV prepared TiO2 film. (b) Photoemission spectra showing the valence band edge and band gap states after lithium insertion. Solid line: the sol-gel prepared Mn:TiO2 film. Dashed line: the sol-gel prepared TiO2 film. The spectra are appropriately shifted and normalized. The thick solid line represents the difference spectrum, emphasizing the Mn 3d derived states.

yield information about the immediate surface region, whereas this indirect method provides information on the total Mn3+ content within the film due to lithium diffusion throughout the entire film. Finally, the valence band region after lithium insertion is shown in Figure 9a. A state in the band gap region around 1 eV binding energy is formed upon lithium insertion. This state arises from the formation of Ti3+ and is attributed to Ti 3d states.16,21,41 Furthermore, lithium insertion induces a narrowing of the O 2p band, which has been observed previously for the Li:TiO2 system.25 In the case of the Mn-modified film, the valence band narrowing leads to an enhanced detachment of the Mn 3d states from the O 2p band. It is found that the low binding energy edge of the Mn 3d states does not shift appreciably toward higher binding energies as found for the O 2p band and the core levels. A comparison of the difference spectra obtained before (Figure 2c) and after lithium insertion (Figure 9b) rather suggests that the Mn 3d states encompass a wider binding energy range. An increased width of the Mn 3d derived states is most probably the result of the increased Mn 3d population. Due to the proximity in binding energy, the population of Ti 3d states is also expected to influence the Mn 3d states. It is observed in Figure 9b that there is a continuous distribution of states all the way up to the Fermi level. This indicates that the Mn 3d states become hybridized with the Ti 3d band gap states. 4. Conclusions A Mn-modified TiO2 film prepared by the sol-gel technique has been characterized and its behavior upon lithium insertion studied. Using PES and XAS the identification and quantification of manganese species in the Mn modified film was possible. We find that the film under study contains about 10% manganese present as Mn3+ (60%) and Mn2+ (40%). The Mn 3d states were found to extend about 1 eV into the TiO2 band gap region. We demonstrate the feasibility of lithium insertion in UHV even into ex situ prepared films. In the Mn:TiO2 film a preferential reduction of Mn3+ to Mn2+ upon lithium insertion is observed. In every other respect the insertion behavior of the sol-gel prepared films agrees well with the previously established behavior for the Li:TiO2 system fully prepared in UHV. Lithium is successfully inserted with a final x-value (about 0.4 Li/Ti). The formation of the two different phases (anatase-like

3466 J. Phys. Chem. C, Vol. 111, No. 8, 2007 and lithium titanate) is observed. PES and XAS have been used independently to establish the x-values of the insertion series. The results from both methods are fully consistent and show the expected behavior. Confirming that insertion is achieved into the entire film, we find that the insertion velocity matches the one predicted from an estimate of the Li deposition rate in relation to the film thickness. We conclude that lithium must penetrate the film and be distributed evenly throughout the entire film. However, it is observed that the insertion process takes a certain amount of time so care has to be exercised to ensure a stable situation during measurements. References and Notes (1) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (2) Borgarello, E.; Kiwi, J.; Gra¨tzel, M.; Pelizetti, E.; Visca, M. J. Am. Chem. Soc. 1982, 104, 2996. (3) Kim, S.; Hwang, S.-J.; Choi, W. J. Phys. Chem. B 2005, 109, 24260. (4) Ko, K. H.; Lee, Y. C.; Jung, Y. J. J. Colloid Interface Sci. 2005, 283, 482. (5) Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikuow, T.; Koshihara, S.-Y.; Koinuma, H. Science 2001, 291, 854. (6) Chambers, S. A.; Thevuthasan, S.; Farrow, R. F. C.; Marks, R. F.; Thiele, J. U.; Folks, L.; Samant, M. G.; Kellock, A. J.; Ruzycki, N.; Ederer, D. L.; Diebold, U. Appl. Phys. Lett. 2001, 79, 3467. (7) Jeong, Y. H.; Han, S.-J.; Park, J.-H.; Lee, Y. H. J. Magn. Magn. Mater. 2004, 272-276, 1976. (8) Huang, S. Y.; Kavan, L.; Exnar, I.; et al. J. Electrochem. Soc. 1995, 142, L142. (9) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (10) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague, J.; Gregg, B. A. Nature 1996, 383, 608. (11) van de Krol, R.; Goossens, A.; Meulenkamp, E. A. Appl. Phys. 2001, 90, 2335. (12) Bonino, F.; Busani, L.; Lazzari, M.; Mastretta, M.; Rivolta, B.; Scrosati, B. J. Power Sources 1981, 6, 261. (13) Zachau Christiansen, B.; West, K.; Jacobsen, T.; Atlung, S. Solid State Ionics 1988, 28, 1176. (14) Henningsson, A.; Rensmo, H.; Sandell, A.; Siegbahn, H.; So¨dergren, S.; Lindstro¨m, H.; Hagfeldt, A. J. Chem. Phys. 2003, 118, 5607. (15) Lindstro¨m, H.; So¨dergren, S.; Solbrandt, A.; Rensmo, H.; Hjelm, J.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 1997, 101, 7717. (16) Wagemaker, M.; van de Krol, R.; Kentgens, A. P. M.; van Well, A. A.; Mulder, F. M. J. Am. Chem. Soc. 2001, 123, 11454. (17) van de Krol, R.; Goossens, A.; Meulenkamp, E. A. J. Electrochem. Soc. 1999, 146, 3150. (18) Wagemaker, M.; Kentgens, A. P. M.; Mulder, F. M. Nature 2002, 418, 397. (19) Cava, R. J.; Murphy, D. W.; Zahurak, S.; Santoro, A.; Roth, R. S. J. Solid State Chem. 1984, 53, 64. (20) Bickley, R. I.; Jayanty, R. K. M. Chem. Soc. 1975, 58, 194. (21) Koudriachova, M. V.; de Leeuw, S. W. Phys. ReV. B 2004, 69, 054106.

Richter et al. (22) Park, M. S.; Min, B. I. Phys. ReV. B 2003, 68, 033202. (23) Park, M. S.; Yu, U.; Min, B. I. J. Magn. Magn. Mater. 2004, 1487, 272-276. (24) Henningsson, A.; Andersson, M. P.; Uvdal, P.; Siegbahn, H.; Sandell, A. Chem. Phys. Lett. 2002, 360, 85. (25) Richter, J. H.; Henningsson, A.; Karlsson, P. G.; Andersson, M. P.; Uvdal, P.; Siegbahn, H.; Sandell, A. Phys. ReV. B 2005, 71, 235418. (26) Richter, J. H.; Henningsson, A.; Sanyal, B.; Karlsson, P. G.; Andersson, M. P.; Uvdal, P.; Siegbahn, H.; Eriksson, O.; Sandell, A. Phys. ReV. B 2005, 71, 235419. (27) Andersen, J. N.; Bjo¨rneholm, O.; Sandell, A.; Nyholm, R.; Forsell, J.; Thånell, L.; Nilsson, A.; Mårtensson, N. Synchrotron Radiat. News 1991, 4, 15. (28) Pohl, I. A. M.; Westin, L. G.; Kritikos, M. Chem. Eur. J. 2001, 7, 3438. (29) Westin, G.; Jansson, K.; Pohl, A.; Leideborg, M. J. Sol-Gel Sci. Technol. 2004, 31, 25. (30) Grehk, T. M.; Larsson, C. U. S.; Prince, N. P.; Flodstro¨m, S. A. Surf. Sci. 1993, 284, L384. (31) Fan, W. C.; Ignatiev, A. Surf. Sci. 1993, 296, 352. (32) Yeh, J. J.; Lindau, I. At. Nucl. Data Tables 1985, 32, 1. (33) Sanjines, R.; Tang, H.; Berger, H.; Gozzo, F.; Margaritondo, G.; Levy, F. J. Appl. Phys. 1994, 75, 2945. (34) Sandell, A.; Andersson, M. P.; Alfredsson, Y.; Johansson, M. K.J.; Schnadt, J.; Rensmo, H.; Siegbahn, H.; Uvdal, P. J. Appl. Phys. 2002, 92, 3381. (35) Asahi, R.; Taga, Y.; Mannstadt, W.; Freeman, J. Phys. ReV. B 2000, 61, 7459. (36) de Groot, F. M. F.; Faber, J.; Michiels, J. J. M.; Czyzyk, M. T.; Abbate, M.; Fuggle, J. C. Phys. ReV. B 1993, 48, 2074. (37) Chambers, S. A.; Liang, Y.; Yu, Z.; Droopad, R.; Ramdani, J.; Eisenbeiser, K. Appl. Phys. Lett. 2000, 77, 1662. (38) Boschloo, G. K.; Goossens, A.; Schoonman, J. J. Electrochem. Soc. 1997, 144, 1311. (39) Westermark, K.; Henningsson, A.; Rensmo, H.; So¨dergren, S.; Siegbahn, H.; Hagfeldt, A. Chem. Phys. 2002, 285, 157. (40) Karlsson, P. G.; Bolik, S.; Richter, J. H.; Mahrov, B.; Johansson, E. M. J.; Blomquist, J.; Uvdal, P.; Rensmo, H.; Siegbahn, H.; Sandell, A. J. Chem. Phys. 2004, 120, 11224. (41) Henningsson, A.; Rensmo, H.; Sandell, A.; Siegbahn, H.; So¨dergren, S.; Lindstro¨m, H.; Hagfeldt, A. J. Chem. Phys. 2003, 118, 5607. (42) Fujimori, A.; Kimizuka, N.; Akahane, T.; Chiba, T.; Kimura, S.; Minami, F.; Siratori, K.; Taniguchi, M.; Ogawa, S.; Suga, S. Phys. ReV. B 1990, 42, 7580. (43) Audi, A. A.; Sherwood, M. A. Surf. Interface Anal. 2002, 33, 274. (44) de Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A. Phys. ReV. B 1990, 42, 5459. (45) Cramer, S. P.; de Groot, F. M. F.; Ma, Y.; Chen, C. T.; Sette, F.; Kipke, C. A.; Eichhorn, D. M.; Chan, M. K.; Armstrong, W. H.; Libby, E.; Christou, G.; Brooker, S.; McKee, V.; Mullins, O. C.; Fuggle, J. C. J. Am. Chem. Soc. 1991, 113, 7937. (46) Ruus, R.; Saar, A.; Aarik, J.; Aidla, A.; Uustare, T.; Kikas, A. J. Electron Spectrosc. Relat. Phenom. 1998, 93, 193. (47) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (48) Sandell, A.; Jaworowski, A. J. J. Electron Spectrosc. Relat. Phenom. 2004, 135, 7. (49) Oku, M.; Wagatsuma, K.; Konishi, T. J. Electron Spectrosc. Relat. Phenom. 1999, 277, 98-99.