J. Phys. Chem. B 1997, 101, 3087-3090
3087
Lithium Intercalation in Nanoporous Anatase TiO2 Studied with XPS Sven So1 dergren and Hans Siegbahn* Department of Physics, Uppsala UniVersity, P.O. Box 530, S-751 21 Uppsala, Sweden
Ha˚ kan Rensmo, Henrik Lindstro1 m, Anders Hagfeldt, and Sten-Eric Lindquist Department of Physical Chemistry, Uppsala UniVersity, P.O. Box 532, S-751 21 Uppsala, Sweden ReceiVed: NoVember 27, 1996X
Intercalation of lithium ions in nanoporous anatase TiO2 was studied with XPS using a novel electrochemical preparation technique. The electrolyte was 0.5 M LiClO4 in acetonitrile (not water-free). The electrochemical insertion of lithium ions creates reduced Ti ions (Ti3+), which was observed as a new well-resolved state in the Ti 2p core level spectrum shifted 2.1 eV to lower binding energy. The new Ti 2p state was reversible when the electrode was deintercalated. Changes in the O 1s core spectra were also observed and discussed in terms of hydrated lithium ions in the Helmholtz layer, or formation of Li2O/LiOH.
I. Introduction The electrochemistry of nanoporous electrodes is a quickly expanding research area since the demonstration of the usefulness of such electrodes in efficient photoelectrochemical solar cells.1 The electrode consists of interconnected nanocrystallites forming a nanoporous structure. A significant property is the extremely large inner surface, allowing for electrochemical reactions to take place in the entire volume of the electrode. The solar cell is only one of several potential applications of such electrodes. Recent papers in the area also report high charging capacities of nanoporous anatase titanium dioxide electrodes2,3 when lithium is intercalated into the nanocrystals. Lithium intercalation in titanium dioxide also exhibits electrochromic properties. The porous nanostructure can thus be employed both in battery and display applications. Schematically the electrochemical intercalation reaction is written as xLi+ + TiO2 + xe- h LixTiO2, where x is the mole fraction of lithium in the titanium dioxide. The reversibility of the above reaction is of prime importance for its utilization in technical devices. To maximize the reversibility and hence the longterm performance of such devices, it is of great interest to identify and monitor potential side reactions. In this study we have used a newly developed XPS/electrochemistry technique to focus on the surface chemistry behind the intercalation of lithium in anatase titanium dioxide and thus to detect possible side reactions. II. Experimental Section The use of a surface sensitive technique, such as XPS, for the study of electrochemical phenomena is a nontrivial experimental task for several reasons. First, to study the interface of interest, viz., the solid-liquid interface, the liquid phase has to be thinner than the sampling depth of the technique (e25 Å for XPS). Second, the liquid phase normally has a vapor pressure that far exceeds the levels tolerable in electron spectrometers. Third, to monitor the processes on the surfaces under different electrochemical conditions, it is important to be able to cycle the surface with repeated measurements on the same spot on the surface. We have developed a technique where electrochemistry is performed in a preparation chamber close to the XPS analysis X
Abstract published in AdVance ACS Abstracts, March 15, 1997.
S1089-5647(96)03939-9 CCC: $14.00
chamber.4 When desired, the electrode is subjected to the electrochemistry, then rinsed, and subsequently introduced to the analysis chamber for the electron spectroscopic measurement. The electrochemistry is performed in the atmosphere of the solvent without any exposure in air, and the surface is analyzed within minutes after the reaction. The same electrode is used for the different electrochemical treatments. The electrodes were prepared from a suspension of colloidal TiO2.1 The diameter of the colloids was about 7-8 nm. Application of the suspension on conducting glass followed by sintering in air at 450 °C for 30 min forms a porous film. The film, 4 µm thick with a porosity of about 50%, thus consists of interconnected TiO2 nanoparticles. The purpose of the sintering is to form a mechanically stable nanoporous anatase TiO2 film and to remove the hydrocarbons originating from the colloidal suspension. The electrochemical preparation was made in an ordinary three-electrode setup, with the TiO2 electrode as the working electrode using an Eco Chemie PGSTAT10 potentiostat. The reference electrode was Ag/AgCl in acetonitrile saturated with LiCl. The electrolyte was 0.5 M LiClO4 in acetonitrile. All chemicals were of reagent grade and used as received. To remove traces of the solute, the electrode was rinsed in acetonitrile before each XPS measurement. All electrochemical preparations were made at room temperature (20 °C) in the atmosphere of acetonitrile (originating from the electrolyte). The XPS spectrometer has been specifically designed to study high vapor pressure samples, in particular liquid interfaces. Excitation of the spectra was performed by means of monochromatized Al KR radiation. All spectra were recorded at a pressure in the 10-6 mbar region with an electron takeoff angle of 90°. The electron energy analyzer was a VSW CLASS100 with a resolution in the present spectra of 0.8 eV. III. Results and Discussion In Figure 1 the Ti 2p core level spectra are shown obtained from a nanoporous anatase TiO2 electrode for different electrochemical treatments. Spectrum a was recorded before any contact of the electrode with the electrolyte. The symmetric Ti 2p peaks indicate stoichiometric TiO2 with a low concentration of defects.5 The fwhm of Ti 2p3/2 was 1.22 eV, which is comparable with the value of 1.0 eV obtained by Sanjine´s et al.6 for an anatase single crystal. The difference in line widths © 1997 American Chemical Society
3088 J. Phys. Chem. B, Vol. 101, No. 16, 1997
So¨dergren et al. potential of -1.7 V was applied for as long as 300 s, the TiO2 nanocrystalline electrode did not saturate in terms of the Ti3+ states. The x-value of 0.32 indicates that the diffusion process for the lithium ion and the corresponding electron is slow. Since XPS mainly examines the surface layers where the highest concentration of Ti3+ is expected, the amount of Ti3+ determined is an upper limit of the x-value achieved after 300 s. On the basis of an approximate equation for spherical diffusion,12 the diffusion coefficient D can be determined for the following expression.
x(t) ) 6xmaxxDt/πr2
Figure 1. Ti 2p photoelectron spectra of anatase nanoporous TiO2 (a) as prepared directly from the oven, (b) lithium intercalated at -1.7 V vs Ag/AgCl for 5 min, (c) deintercalated at -0.15 V for 5 min. A new clearly resolved state is formed for the intercalated electrode (b); as can be seen, this state is electrochemically reversible (c). Solid lines are curve fits.
is due to the different experimental energy resolutions, 0.4 eV in ref 6 compared to our resolution, 0.8 eV. The spin-orbit splitting obtained was 5.74 eV, with an intensity of ratio of 0.46 between the Ti 2p1/2 and Ti 2p3/2. An applied potential at -1.7 V vs Ag/AgCl to the nanocrystalline electrode allows for intercalation of lithium ions (this process starts at potentials more negative than -1.0 V). Intercalating at this potential for 5 min turns the electrode dark blue. As can be seen in Figure 1b a new well-resolved Ti 2p peak now appears, shifted 2.1 eV to lower binding energy, containing 32% of the total Ti 2p intensity. In the curve fit the spin-orbit splitting and the intensity ratio was fixed using the values obtained from spectrum a. The appearance of this new Ti 2p peak is consistent with the picture that the electrons are trapped in localized states reducing formally Ti4+ to Ti3+. Similar results, but not as well resolved and with smaller relative intensity of the Ti3+ state, have been ordered in UHV experiments with evaporation of alkali metals on rutile single crystal, K,7,8 and Na.9 One question to be addressed by the present result is whether the intercalation process actually involves bulk TiO2 or if the high surface area of the nanocrystalline film is sufficient to house the total intercalation charge. Since as much as 32% of the Ti 2p signal resides in the Ti3+ state, we may conclude that the bulk of the nanocrystals is involved (about 10% of the atoms are estimated to be surface atoms in this case). This conclusion is corroborated by comparison with the result from the evaporated alkali metal films on TiO2 rutile single crystal, where monolayer coverages resulted in only weak Ti3+ peaks.7-9 As a consequence, to maintain charge neutrality and minimize electrostatic repulsion inside the anatase nanocrystals, the lithium ions also have to diffuse into the bulk of the crystal. The upper intercalation limit xmax at ambient temperature and potentials is 0.5; see refs 10, 11 and 13. Even though the
(1)
With a radius r of 4 nm, an intercalation time t of 300 s, and x, xmax given above, the diffusion coefficient is determined to 2 × 10-17 cm2/s. This value agrees well with what has been determined with electrochemical methods for lithium insertion into nanoporous anatase TiO2.13 An increase in the fwhm for the Ti 2p signal is also observed, 1.52 eV for Ti4+ and 1.57 eV for Ti3+. The broadening of the Ti 2p line profile, which was found both reproducible and reversible, may have several causes; for example, the double injection of electrons and lithium ions may lead to distortion of the TiO2 lattice, resulting in several slightly shifted Ti 2p components. Another explanation of this effect might be that the TiO2 is not homogeneously intercalated, i.e. that the stoichiometry of the lithium-intercalated TiO2 nanocrystals is not well-defined due to the low mobility of the Li ions. Edwards et al.14 have reported similar Ti 2p features from metallic LiTi2O4 with spinel structure. Our present interpretation of the Ti3+ peak differs from that suggested by these authors for this material, namely, with respect to the initial vs final state origin of this peak. Thus, metallic LiTi2O4 has itinerant character of its outermost conduction electrons, and therefore the occurrence of a Ti3+ peak in the Ti 2p spectrum is associated with the pulling-down in the final state of an occupied local level on the core-ionized site. In the present case, however, the intercalated material is not metallic and the outermost levels are thus not to be associated with itinerant electrons. Indeed, model calculations demonstrate the local band-gap character of the intercalation electron levels.15 Therefore, the presence of the Ti3+ peak in the present Ti 2p spectra is instead to be interpreted in terms of an initial state occupancy of local levels on Ti sites. Application of -0.15 V to the electrode leads to decoloration of the TiO2 film. As seen in Figure 1c, the Ti3+ states thus return to their original Ti4+ states, as expected from a completely reversible intercalation reaction. The reversibility of the intercalation is at least 96% in water-free electrolytes as determined by electrochemical methods.3 O 1s spectra corresponding to the Ti 2p spectra are shown in Figure 2. The peak at 530 eV is interpreted as due to bulk oxygen in the TiO2.16 The fwhm in all spectra are approximately equal, 1.35 eV. The tailing to the higher binding energy side of the peak in Figure 2a contains several components resulting from the hydroxylation of the TiO2 surface (the electrode is exposed to air when it is mounted). These are due to acidic and basic hydroxyl groups at the outermost surface as well as adsorbed water. The O 1s spectrum we observe in similar to those reported on untreated (as grown) anatase single crystals6 and rutile single crystals exposed to vapor and liquid water.5,17 When the electrode is intercalated (cf Figure 2b), a new broad peak appears in the O 1s spectrum at the higher binding energy side of the original bulk peak. At the same time, only a slight decrease of about 5% in the signal ratio between the bulk TiO2
Lithium Intercalation in Nanoporous Anatase TiO2
J. Phys. Chem. B, Vol. 101, No. 16, 1997 3089
Figure 2. O 1s XPS spectra of nanoporous anatase TiO2 (a) as prepared, (b) lithium intercalated at -1.7 V for 5 min, and (c) deintercalated at -0.15 V for 5 min. The peak at 530 eV is due to bulk TiO2, and the broad peak at 532.2 eV (b) is tentatively interpreted as hydrated lithium ions in the Helmoltz layer. The electrolyte was 0.5 M LiClO4 (not water-free).
Figure 3. Overview spectra of nanoporous anatase TiO2 (a) as prepared, (b) lithium intercalated at -1.7 V for 5 min, (c) deintercalated at -0.15 V for 5 min, (d) intercalated at -2.0 V for 5 min, and (e) deintercalated at -0.15 V for 5 min. When the electrode is intercalated as in d the titanium signals disappear completely due to the formation of a Li2O or LiOH surface layer.
O 1s and the total Ti 2p3/2 is obtained from the observed data. However, due to overlap with the new O 1s peak, this small decrease in the signal ratio is within the curve-fitting uncertainty. This means that the broader high binding O 1s peak basically constitutes additional intensity due to the applied negative potential and not transfer of intensity from the original bulk peak. Also, within the accuracy of the curve fit, the fwhm of the bulk TiO2 O 1s does not seem to change. These observations imply that the O 1s shift associated with the presence of the Li ions in the bulk seems to be very small. This may be seem surprising; one would perhaps expect a significant effect on the bulk oxygen as a result of a nearby positive lithium ion. However, in estimating possible O 1s shifts one must consider the combined electrostatic potential induced by a lithium ion and the concomitant electron transfer to the coordinating Ti4+ ion. Judging simply from the ionic radii (the Li+ and the Ti3+ ions have closely the same radii, ∼0.7 Å), these two electrostatic contributions felt by the oxygen atom may counterbalance each other quite accurately. Speculating further, this lack of O 1s shift and the electrostatic balance that this implies could be related to the diffusion of the Li ion/electron pair through the bulk lattice. Thus, if one electrostatic contribution strongly supersedes the other, the pair will either not enter the lattice or be strongly bound to one site in the lattice and not diffuse at all. A further account of the influence of the lithium intercalation on the bulk (and surface) O 1s will be made in conjunction with measurements under water-free conditions (at present in progress). When deintercalating the electrode the oxygen rich overlayer disappears almost completely (Figure 2c). The O 1s spectrum shows only a small difference in the high binding energy tail compared to the spectrum obtained prior to the intercalation of the electrode (cf. Figure 2a).
It is clear from the above that additional species are formed on the titanium oxide surface during the intercalation process, indicating side reactions that under the conditions of Figures 1 and 2 are almost but not completely reversible. To further elucidate the nature of these overlayer species, an analysis of the elements in terms of scan spectra of the TiO2 electrode is shown in Figure 3. The three topmost spectra show the overall behavior of a reversible intercalation cycle. On a fresh electrode, taken directly from the oven (spectrum a), we detect in addition to the titanium and oxygen signals also C 1s due to hydrocarbons. These hydrocarbons probably partly originate from the preparation of the electrodes, since the colloidal solution for the preparation of the electrodes contains hydrocarbons. The intercalation (spectrum b) is seen to result in a 40% attenuation of the total Ti 2p signal. This attenuation is most likely due to the formation of the overlayer of oxygencontaining species formed on the surface of the nanocrystals (cf. Figure 2b). Contributions to the overlayer of solute perchlorate is excluded since no chlorine signals were detected. Changes in the C 1s structure in terms of total intensity and appearance of oxidized states may also be noticed on the intercalated sample. Nitrogen is not detected in any of the spectra, which means that the solvent acetonitrile is not adsorbed on the electrode surface during the XPS measurement. The scan spectrum c of the deintercalated electrode is very similar to that of the electrode taken directly from oven, perhaps with a slight decrease of the relative intensity of the hydrocarbon C 1s signal. A detailed analysis of lithium is difficult due to its low photoelectric cross section with Al KR excitation. With further intercalation, now at -2 V for 5 min, the titanium signals disappear completely (spectrum d) along with the bulk O 1s signal of the TiO2. The remaining O 1s peak occurs at the same position as the broad peak in Figure 2b. Thus, a substantially thicker overlayer has been formed on the
3090 J. Phys. Chem. B, Vol. 101, No. 16, 1997 surface of the TiO2 nanocrystals under these conditions, containing oxygen, lithium, carbon, and small amounts of chlorine. The fact that the lithium signal is now substantially stronger (albeit small due to its low cross section) indicates that lithium is a major component in the overlayer. This suggests that lithium oxide or lithium hydroxide is formed on the surface at these intercalating potentials due to the presence of traces of water in the electrolyte. Visually it was confirmed that the nanoporous film was intact since the electrode retained its dark blue color. By applying -0.15 V for 5 min the electrode becomes transparent again. However, as can be seen in Figure 3e, this does not change the overlayer on the TiO2 within the 25-30 Å sampling depth. In view of the coincidence in position of the additional O 1s peak at -1.7 and -2.0 V intercalation potentials, it would be natural to interpret the overlayer formation also in the former case as due to lithium oxide/hydroxide. However, the absence, or very small intensity, of a lithium signal at -1.7 V (Figure 3c) indicates that the additional O 1s signal in this case originates rather from water on the electrode surface. This adsorbed water may be associated with an inner Helmholtz layer of hydrated lithium ions. This would explain both the reversibility and the much lower lithium to oxygen (532 eV peak) signal ratio in this case. It should be noted that an irreversible overlayer formed on the surface, as observed in the spectra of Figure 3d,e, did not noticeably deteriorate the electrochromic cycling of the film. It is clear, though, that an overlayer of this thickness on the TiO2 film constitutes the initial stages of a progressive loss of intercalation efficiency. IV. Conclusions Using a newly constructed XPS/electrochemical setup, we have studied the electronic structure of nanoporous anatase titanium dioxide electrodes due to lithium intercalation. The
So¨dergren et al. experimental technique is also a powerful tool for detailed studies of unwanted side reactions. A clearly resolved Ti3+ state is formed shifted 2.1 eV toward lower binding energy in the Ti 2p spectrum when the electrode is intercalated. The Ti3+ states are reversibly oxidized back to Ti4+ when deintercalating the electrode. Analysis of the O 1s spectra suggests that the electrostatic potential of the bulk oxygen in the TiO2 is only little affected by the lithium intercalation. A side reaction is detected, leading to the formation of overlayers on the TiO2 nanocrystals interpreted as due to lithium/hydroxide. Under certain electrochemical conditions these overlayers become irreversible on the depth scale of the XPS analysis, while the electrochromic properties of the films are essentially retained. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature (London) 1991, 353, 737. (2) Hagfeldt, A.; Vlachopoulos, N.; Gra¨tzel, M. J. Electrochem. Soc. 1994, 141, L82. (3) Kavan, L.; Kratochilova´, K.; Gra¨tzel, M. J. Electroanal. Chem. 1995, 394, 93. (4) So¨dergren, S.; Rensmo, H.; Siegbahn, H. Manuscript in preparation. (5) Li, Q. W.; Baer, D. R.; Engelhard, M. H.; Shultz, A. N. Surf. Sci. 1995, 344, 237. (6) Sanjine´s, R.; et al. J. Appl. Phys. 1994, 75, 2945. (7) Hardman, P. J.; et al. Surf. Sci. 1992, 269, 677. (8) Heise, R.; Courths, R. Surf. ReV. Lett. 1995, 2, 147. (9) Onishi, H.; Aruga, T.; Egawa, C.; Iwasawa, Y. Surf. Sci. 1988, 199, 1. (10) Murphy, D. W.; et al. ReV. Chim. Mine´r. 1982, 19, 441. (11) Zachau-Christiansen, B.; West, T.; Jacobsen, T.; Atlung, S. Solid State Ionics 1988, 28-30, 1176. (12) Crank, J. Mathematics at Diffusion; Oxford University Press: Amen House, London E.C. 4, 1956; Chapter 6. (13) Lindstro¨m, H.; et al. Manuscript in preparation. (14) Edwards, P. P.; et al. J. Solid State Chem. 1984, 54, 127. (15) Stashans, A.; et al. Phys. ReV. B 1996, 53, 159. (16) Go¨pel, W.; et al. Surf. Sci. 1984, 139, 333. (17) Bullock, E. L.; Patthey, L.; Steinemann, S. G. Surf. Sci. 1996, 352, 504.
