ISSN 1318-0010 KZLTET 32(3-487)165(1998) IR AND X-RAY PHOTOELECTRON SPECTROSCOPY OF V2O5, TiO2 AND V/Ti-OXIDE SOL-GEL DERIVED FILMS INFRARDEČA IN RENTGENSKA FOTOELEKTRONSKA SPEKTROSKOPIJA V2O5, TiO2 IN V/Ti-OKSIDNIH SOL-GEL FILMOV ĐORĐE MANDRINO1, M. JENKO1*, A. ŠURCA2, B. OREL2 institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana 2National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana Prejem rokopisa - received: 1998-12-06; sprejem za objavo - accepted for publication: 1998-12-14 V/Ti-oxide films were prepared by dip-coating from sols made by mixing of V-oxoisopropoxide and Ti-propoxide in V:Ti molar ratio in precursors 3:1, 1:1 and 1:3. Amorphous films were obtained after annealing at 300°C (1 h). IR spectroscopic analysis revealed that V=O stretching modes appeared at 1020 cm-1 in the IR spectrum of powder (V:Ti=1:1). However, in the IR spectra of a film with the same molar ratio the bands at 1008 and 914 cm-1 appeared suggesting the presence of V4+-O bonds. The new band at 790 cm-1 signalled the V-O-Ti bridging bonds connecting V-O and Ti-O polyhedra. XPS measurements confirmed that initial-state films contained V5+, V4+ and Ti species, but at the film surface only V5+ species existed. Sequential depth analysis of films performed with Ar+ sputtering showed that the vanadium is reduced to 3+ oxidation state with progressive exposure while titanium is not affected. Key words: electrochromic (EC) devices, oxide films, IR spectroscopy, XPS, oxidation states, sputtering, sol-gel V/Ti-oksidne filme smo pripravili iz koloidnih raztopin V-oksoizopropoksida in Ti-propoksida v 2-propanolu s tehniko potapljanja. Molska razmerja V:Ti v prekurzorjih so znašala 3:1, 1:1 in 1:3. Amorfni filmi so nastali po segrevanju na 300°C (1h). V IR spektru praškastega vzorca (V:Ti = 1:1) so se valenčna nihanja V=O pojavila pri 1020 cm-1. V IR spektru filma z istim molskim razmerjem pa se pojavita trakova pri 1008 cm-1 in 914 cm , kar kaže na prisotnost V4+-O vezi v filmih. Novi trak pri 790 cm-1 kaže na V-O-Ti vezi, ki povezujejo poliedre V-O in Ti-O. XPS meritve so potrdile, da filmi vsebujejo V4+, V5+ in Ti , medtem ko je na njihovem površju prisoten le V5+. Profilna analiza filma s pomočjo Ar+ ionskega jedkanja je pokazala, da se vanadij tekom jedkanja reducira do oksidacijskega stanja 3+, medtem ko jedkanje ne vpliva na oksidacijsko stanje titana. Ključne besede: elektrokromne (EC) naprave, oksidni filmi, IR spektroskopija, XPS, oksidacijska stanja, ionsko jedkanje, sol-gel 1 INTRODUCTION Sol-gel processing is a rapidly growing field of material chemistry with applications in synthesis of glasses, ceramics, organic-inorganic hybrid materials and thin films1. Thin films for window glazings were one of the first commercial products of sol-gel processing and can be easily made by using a dip-coating deposition1. The main advantage of the dip-coating is the possibility of the preparation of large-area films with only a fraction of cost needed for the production of sputtered or evaporated films. Dip-coating is performed by pulling a substrate slowly from aqueous or alcoholic sols1. During pulling solvent evaporates from the deposited thin layer of the sol on the glass substrate allowing the reactions between colloidal species. These condense in larger aggregates which results in a formation of a compact xerogel film. Its annealing at elevated temperatures (up to 500°C) brings about metal oxide film with an amorphous or crystalline structure, desired surface texture and porosity. The deposition of multicomponent films is easy to perform because precursors are mixed at the molecular level, i.e. in solution. High homogeneity of films is therefore obtained. * To whom the correspondance should be adressed. In this paper we report investigations of V/Ti-oxide films which were made from alkoxide precursors of vanadium and titanium after annealing at 300°C. Sputter-deposited V/Ti-oxide films with an amorphous structure exhibit intercalation properties and are currently used in electrochromic (EC) devices2. In EC devices they function as counter-electrodes assuring the balance of the charge which is transported from the active colouring films (WO3 in most cases) through the lithium ionic conductor (electrolyte) which glues both parts of the EC device together to the counter-electrode film3. To achieve electronically balanced EC device ion-storage capacity of the counter-electrode should be nearly the same as that of WO3 film (40 - 50 mCcm-2). Sputtered V/Ti-oxide exhibit ion-storage capacity in the range 30 - 40 mCcm-2 (4), i.e. close to that of WO3 films. Additionally, the optical modulation of V/Ti-oxide counter-electrodes is small, they are highly transparent (>70%) in both charged and discharged states contributing to the total transmittance of the EC device. This property is particularly important for "smart" window applications3. Preliminary studies of sol-gel derived V/Ti-oxide films (300°C) revealed that films were amorphous and formed solid solutions in the whole concentration range KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 3-4 165 Đ. MANDRINO ET AL.: IR AND X-RAY PHOTOELECTRON SPECTROSCOPY... (0 - 100% of V or Ti)5. IR spectroscopy combined with electrochemical (CV)6 and in-situ UV-VIS spectroelec-trochemical measurements2,5 revealed that in initial-state films vanadium could be either in 5+, 4+ or both oxidation states, while the oxidation state of titanium was proposed to be 4+. Accordingly, in this work we undertook X-ray photo-electron spectroscopy (XPS) investigations of V/Ti-oxide films aiming to evaluate the oxidation states of V and Ti ions and to compare these results to those gathered from our previous IR spectroscopic and electrochemical inves-tigations2,5. We then made a sequential depth analysis of the same films by using Ar+ sputtering. Correct assignment of XPS spectra of films containing V and Ti is not trivial because both elements show tendency to become reduced due to the Ar+ sputtering during XPS measurements. We explain the XPS results on the basis of known binding energies of various V and Ti compounds and by using a standard deconvolution procedure available at our XPS spectrometer. 2 EXPERIMENTAL Amorphous V/Ti-oxide films with V:Ti molar ratios in precursors 3:1, 1:1 and 1:3 were prepared via a sol-gel alkoxide route7. A titanium (IV) propoxide and vanadium (V) oxoisopropoxide were added into 2-propanol while atmospheric water was sufficient to initiate the hydrolysis reactions. The V2O5 and TiO2 sols were prepared by 542 _i_i_i_i-.-1— 2000 1500 1000 500 v (cm-1 ) Figure 1: IR absorbance spectra of powders: a) V2O5, 200°C (1h), b-d) V/Ti-oxide, 300°C (1h) and e) TiO2, 300°C (1h). Molar ratios V:Ti in precursors are: b) 3:1, c) 1:1 and d) 1:3 Slika 1: IR absorbcijski spektri prahov: a) V2O5, 200°C (1h), b-d) V/Ti-oksid, 300°C (1h) in e) TiO2, 300°C (1h). Molska razmerja V:Ti v prekurzorjih so: b) 3:1, c) 1:1 in d) 1:3 488 adding the appropriate amount of corresponding alkoxide into 2-propanol. The films were deposited by dip-coating on SnO2:F glass substrates or silicone wafers and annealed at 300°C for 1 hour in air. The approximate thickness of V/Ti-oxide films was 80 nm. The V2O5, TiO2 and V/Ti-oxide powders were prepared by first drying the sol in air and annealing the residue at various temperatures for 1 h5. FT-IR spectroscopic measurements were obtained using a Perkin Elmer System 2000 spectrometer. The resolution was 4 cm-1. V/Ti-oxide powders were pressed in KBr pellets and the IR spectra were recorded between 4000 and 400 cm-1. The V/Ti-oxide films were deposited on silicon wafers for FT-IR absorbance measurements8. Films to be examined by XPS were deposited on SnO2:F glass substrates with the area of about 1 cm2. This was enough to ensure that all signal in XPS spectra was due to X-ray irradiation of the film under investigation since the irradiated area of the sample is about 3x6 mm2. XPS measurements were obtained using a VG Microlab 310-F system equipped with an XPS facility. Mg Ka radiation (1253.6 eV) was used. Carbon 1s peak at 284.8 eV was used as an internal standard. Pressure in the analysis chamber was below 10-9 mbar. During the sputtering the pressure did not exceed 10-7 mbar due to the differentially pumped ion gun. Sputtering rate (p 1 nm/min) was partly controlled by argon influx into the ion gun and partly by focusing of the ion beam. XPS spectra were processed by Eclipse Data System v 2.1 software package. SIMPLEX fitting algorithm with linear combination of Gaussian and Lorentzian peak shapes was used in deconvolution subroutine of the software. 3 RESULTS AND DISCUSSION 3.1 FT-IR spectra of powders and films Bands at 1018, 990, 806 and 535 cm-1 in the IR spectrum of a V/Ti-oxide powder with the molar ratio V:Ti = 3:1 (300°C, 1 h) resemble those observed in the IR spectrum of a V2O5 powder (200°C, 1 h), (Figure 1). Certain bands, i.e. 1018, 806 and 535 cm-1, also coincide with transversal (TO) optical modes in the IR absorbance spectrum of the crystalline V2O5 films annealed at 300 or 400°C8 9: the V=O (vanadyl) stretching at 1016 cm-1, the bridging V-Ob-V stretching at 795 cm-1 and the 3V-OC stretching mode at 475 cm-1. In the IR spectrum of V/Ti-oxide powder (V:Ti=1:3) in Figure 1 the Ti-O stretching mode is predominant, however, the band at 1018 cm-1 confirms the presence of V=O groups (V5+-O bonds). The Ti-O stretching (601 cm-1) can neither be ascribed to anatase nor rutile. This agrees with the fact that crystalline TiO2 (anatase) does not appear below 500°C. The IR spectrum of the V/Ti-oxide powder (V:Ti = 1:1; 300°C) differs considerably from the IR spectra of V2O5 (200°C), TiO2 and V/Ti-oxide powders (V:Ti=3:1 and 1:3), (Figure 1). The presence of a strong band at 823 cm-1 suggests that V/Ti-oxide at 300°C forms a solid KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 6 Đ. MANDRINO ET AL.: IR AND X-RAY PHOTOELECTRON SPECTROSCOPY... solution. Weak 1020, 772 and 556 cm-1 bands indicate the presence of V=O and 3V-Oc modes typical of edge-sharing V-O pyramids8. The strong band at 823 cm-1 undoubtedly reveals the V-O-Ti groups which link the poly-hedra of V and Ti together. FT-IR absorbance spectra of TiO2 and V/Ti-oxide film with a molar ratios of V:Ti = 3:1 and 1:1 (300°C) are depicted in Figure 2. The IR spectra of an initialstate V/Ti-oxide films (Figure 2) and powders (Figure 1 b, c) reveal similar features. However, the bands in the IR absorbance spectrum of the initial-state film are systematically shifted to lower frequencies. The reason is the polarisation effect10, which shifts the IR bands of powders in the frequency range between the transversal (TO) and longitudinal (LO) optical modes. The former are observed in IR absorbance spectra of films while the latter are detected in near-grazing incidence angle (NGIA) IR spectra7 (not shown here). In the IR spectrum of an initial-state V/Ti-oxide film with the V:Ti molar ratio 1:1 (Figure 2) - contrary to the strong band at 823 cm-1 in the IR spectrum of the powder in Figure 2 (V-O-Ti stretching) - a less distinct band at 790 cm-1 is observed. We did not detect any band around 500 cm-1 due to the 3V-Oc mode. Obviously Ti modifies the structure of the V/Ti-oxide film by forming V-O-Ti bonds. However, the 3V-Oc mode appears in the IR spectrum of the V/Ti-oxide film (V:Ti = 1:1) prepared at 400°C2 and signals the segregation to V2O5 and TiO2 _i_i_i_i_i_i_i_ 4000 3000 2000 1000 v (cm-1 ) Figure 2: FT-IR absorbance (TO) spectra of: a, b) V/Ti-oxide films and c) TiO2 film annealed at 300°C for 1h. The molar ratios V:Ti are: a) 3:1 and b) 1:1. *and **denote CO2 and water absorption band due to insufficient purging Slika 2: FT-IR absorbcijski (TO) spektri: a, b) V/Ti-oksidnih filmov in c) TiO2 filmov segrevanih 1h na 300°C. Molska razmerja V:Ti so: a) 3:1 in b) 1:1. *in **označujeta absorpcijske trakove CO2 in vode zaradi nezadostnega čiščenja 489 KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 6 phases. The 658 cm-1 band in Figure 2 is an artefact from the Si substrate. 3.2 XPS measurements XPS spectra of V2O5, TiO2 and V/Ti-oxide films (V:Ti=1:1) annealed at 300°C are shown in Figure 3. Peaks at 458.4 eV observed in XPS spectra of TiO2 and V/Ti-oxide films correspond to Ti 2p3/2 peaks of Ti4+. The V 2p3/2 peaks corresponding to V5+ appeared at 517.2 eV in XPS spectra of V2O5 and V/Ti-oxide films. Surprisingly, XPS spectrum of initial-state V/Ti-oxide film did not show the presence of V4+ phase as we expected from the IR spectra of V/Ti-oxide films (Figure 2). However, IR spectrum of the powder (Figure 1c) showed the V=O stretching band (V5+ sites). At 1020 cm-1 which occur due to the oxidation of the large surface of the powder grains. To prove the presence of the surface rich V5+ states the argon sputtering with sputtering rate of 1 nmmin-1 was used for the removal of the surface layer of the film. Corresponding XPS spectra measured after 3, 4, a: Ti02 I I I I Ti 2p3/2 458.4 eV A - b: V/Ti-oxide before sputtering - _ Ti 2p1/2 - - 464 462 460 4S8 456 454 452 450 Binding Energy (eV) — a: V/Ti-oxide before sputtering b: V205 V 2p3/2 517.2 eV I B V 2p1/2 - - b// - 526 524 522 520 516 516 514 512 Binding Energy (eV) Figure 3: XPS spectra of the initial-state of: A) V/Ti-oxide and TiO2 films (Ti 2p3/2) and B) V/Ti-oxide and V2O5 films (V 2p3/2). The V/Ti-oxide film with a V:Ti molar ratio 1:1 was annealed at 300°C for 1 h Slika 3: XPS spektri za~etnih stanj: A) V/Ti-oksidnih in TiO2 filmov (Ti 2p3/2) in B) V/Ti-oksidnih in V2O5 filmov (V 2p3/2). V/Ti-oksidni film z V:Ti molskim razmerjem 1:1 je bil 1 h segrevan na 300°C Đ. MANDRINO ET AL.: IR AND X-RAY PHOTOELECTRON SPECTROSCOPY... Figure 4: XPS spectra of V/Ti-oxide film: A) Ti 2p3/2 and B) V 2p3/2 peaks. The XPS spectra were taken: a) before sputtering and after sputtering for: b) 3 min, c) 4 min, d) 5 min and e) 10 min. V/Ti-oxide film with a V:Ti molar ratio 1:1 was annealed at 300°C for 1 h Slika 4: XPS spektri V/Ti-oksidnih filmov: A) Ti 2p3/2 in B) V 2p3/2 vrhov. XPS spektri so bili posneti: a) pred ionskim jedkanjem in po: b) 3 min jedkanja, c) 4 min jedkanja, d) 5 min jedkanja in e) 10 min jedkanja. V/Ti-oksidni film z V:Ti molskim razmerjem 1:1 je bil 1 h segrevan na 300°C Figure 5: Deconvolution of V 2p3/2 peaks in XPS spectra of V/Ti-oxide film obtained after sputtering for: A) 3 min, B) 4 min, C) 5 min and D) 10 min. V/Ti-oxide film with a V:Ti molar ratio 1:1 was annealed at 300°C for 1 h Slika 5: Dekonvolucija vrhov V 2p3/2 v XPS spektrih V/Ti-oksidnega filma po ionskem jedkanju: A) 3 min, B) 4 min, C) 5 min in D) 10 min. V/Ti-oksidni film z V:Ti molskim razmerjem 1:1 je bil 1 h segrevan na 300°C 490 KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 6 Đ. MANDRINO ET AL.: IR AND X-RAY PHOTOELECTRON SPECTROSCOPY... 5 and 10 minutes of sputtering showed that Ti 2p3/2 peaks shifted 0.4 to 0.5 eV towards higher binding energies with respect to the non-sputtered V/Ti-oxide film (Figure 4a). In addition, the shift of this peak towards higher binding energies ruled out the expected change in oxidation state of Ti from 4+ to 3+ which would result in a shift towards lower binding energies. This could be due to the presence of V-O-Ti bonds in the structure of V/Ti-oxide film. Contrary to Ti 2p3/2 peak V 2p3/2 peaks of Ar+ sputtered V/Ti-oxide film (V:Ti=1:1) were shifted 1.4 to 1.6 eV towards lower binding energies (Figure 4b). Nevertheless, this energy shift is somewhat too large to be attributed unequivocally solely to a difference between V5+ (517.2 eV) and V4+ (516.3 eV) phases. It was therefore assumed that the observed V 2p3/2 peaks in Ar+ sputtered V/Ti-oxide film consist of two overlapping peaks, the first one belonging to the V4+ component and the additional one originating from V species in oxidation states lower than 4+. To verify this idea we made the deconvo-lution analysis of V 2p3/2 peaks in XPS spectra obtained after different sputtering times (Figure 5). Deconvolu-tion analysis show that the 2p3/2 peaks are actually composed of two components: the first located at higher binding energy of 516.3 - 516.4 eV undoubtedly corresponds to V4+ species while the second with the binding energy of 515.1 - 515.2 eV could be assigned to V3+ species. However, the position of the latter peak is about 0.3 eV too low to be due solely to the V3+ species. It is likely that the additional component stemming from the V reduction to oxidation states lower than 3+ is present in the second peak. However, any attempt to deconvolute the V 2p3/2 peaks into more than two components resulted in worse fitting (higher %2 values) indicating that the energy peak of the second component is the characteristic feature of the V/Ti-oxide structure. Without knowing the binding energy values of 2p3/2 peaks of V/Ti-oxide more detailed deconvolution analysis of the experimental XPS spectra could not be done. We also tried to deconvolute the V 2p3/2 peak of non-sputtered V/Ti-oxide film to check the possible presence of the V4+ component in the surface layer. However, very high %2 values were obtained suggesting that such a fit was unrealistic. The de-convolution analysis proved that the structure of the inner part of the V/Ti-oxide films consists also of V4+ species and that the sputtering increases the amount of V3+. The latter was also judged from the increase in the ratio of the respective areas of V3+ and V4+ peaks with sputtering time (Figure 5). 4 CONCLUDING REMARKS XPS spectra of V/Ti-oxide films revealed the binding peak of V 2p3/2 at 517.2 eV11 which undoubtedly con- firmed the presence of V5+ species in the upper layer of initial-state films. This agrees with the fact that the upper part of the V/Ti-oxide film has been completely oxidised 491 KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 6 during the annealing at 300°C (1 h). Like the binding energy peak of V5+ also Ti4+ peak at 458.4 eV observed in XPS spectra of the upper part of the films did not show considerable deviation12 from the binding energy expected for Ti in 4+ oxidation state. In Ar+ sputtered film Ti peak shifts to higher energies contrary to the expected shift of Ti in 3+ oxidation state12. The positive energy shift of the Ti 2p3/2 peak clearly signalled the electron band structure around titanium in the V/Ti-oxide film and therefore could not be ascribed to a simple sum of XPS spectra of segregated V2O5 and TiO2 phases. This result agrees with results of IR spectra analysis exhibiting the V4+-O mode at 914 cm-1 contrasting the V=O stretching mode which was found in IR spectra of V2O5 films8. The presence of the 790 cm-1 band gave evidence of the edge and/or corner-shared V- and Ti-octahedra with at least one well defined V-O-Ti connecting bond. The behaviour of the V 2p3/2 binding energy peak at 517.2 eV changed drastically with Ar+ sputtering. Already after first 3 nm of etching the peak is shifted to 515.6 eV. The observed energy shift is quite large and is higher than that which could be expected for the pure V4+ states. Deconvolution of V 2p3/2 peaks revealed two well defined components: the first one appearing at 516.4 eV and the second one at much lower energies (515.1 eV). The first was assigned to V4+ states11 while the second very likely corresponds to the mixed V3+ and lower V states. This proved that amorphous V/Ti-oxide films prepared via the sol-gel synthesis are not homogeneous with respect to the composition of V ions. Annealing at 300°C is sufficient to oxidise to 5+ state only those vanadium ions which are present in the upper layer of the film while the rest of the film interior contains V4+ and V5+ ions together with Ti4+ species. Slower sputtering rates with very low Ar+ current density is planned to be done to decrease the sputtering-induced reduction of V. This is prerequisite before the XPS measurements of electro-chemically charged and discharged films will be performed. ACKNOWLEDGEMENT This work was financed by the Ministry of Science and Technology as a part of Joule III "Smartglass" project (Pilkington plc., United Kingdom). 5 REFERENCES 1C. J. Brinker, G. W. Scherer, Sol-Gel Science, Academic Press, Boston, 1990 2 A. [urca, S. Ben~i~, B. Orel, B. Pihlar, accepted in Electochim. A. 3 C. G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier Science, Amsterdam, 1995 4M. G. Hutchins, N. S. Butt, P. Ageorbes: Optical Bandgap Shifts in Vanadium Titanium Oxide Thin Films. In: Abstracts, International Meeting on Electrochromics, London, September 7-9, 1998 5A. [urca, S. Ben~i~, B. Orel: Spectroelectrochemical Studies of V/Ti, V/Ti/Zr and V/Ti/Ce Oxide Counter Electrodes. In: Abstracts, International Meeting on Electrochromics, London, September 7-9, 1998 Đ. MANDRINO ET AL.: IR AND X-RAY PHOTOELECTRON SPECTROSCOPY... 6 A. Davies, R. J. Hobson, M. J. Hudson, W. J. Macklin, R. J. Neat, J. Mater. Chem., 6 (1996) 49 7 J. Livage, .Solid State Ionics, 86-88 (1996) 935 8A. Surca, B. Orel, G. Dražič, B. Pihlar, J. Electrochem. Soc., 146 (1999) xxx 9 A. Talledo, C. G. Granqvist, J. Appl. Phys., 77 (1995) 4655 10 M. Ocana and C. J. Serna, Spectrochim. Acta, 47A (1991) 765 11 N. Kumagai, K. Tanno, T. Nakajima, N. Watanabe, Electrochim. Acta, 28 (1983) 17 12 C. Trapalis, V. Kozhukharov, B. Samuneva, P. Stefanov, J. Mater. Sci. 28 (1993) 1276 492 KOVINE, ZLITINE, TEHNOLOGIJE 32 (1998) 6