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Oxidation of nanostructured Ti films produced by low energy cluster beam deposition: An X-ray Photoelectron Spectroscopy characterization
Thin Solid Films 520 (2012) 4803–4807
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Oxidation of nanostructured Ti films produced by low energy cluster beam deposition: An X-ray Photoelectron Spectroscopy characterization Monica de Simone a,⁎, Elena Snidero a, Marcello Coreno b, e, Gero Bongiorno c, Luca Giorgetti d, Matteo Amati e, Cinzia Cepek a a
CNR-IOM Laboratorio TASC, Area Science Park Basovizza, 34149 Trieste, Italy CNR-IMIP, c/o Laboratorio TASC Area Science Park Basovizza, 34149 Trieste, Italy Fondazione Filarete, v.le Ortles 22/4, 20139 Milano, Italy d Istituto Europeo di Oncologia, Dip. di Oncologia Sperimentale, Via Adamello 16, 20139, Milano, Italy e Sincrotrone Trieste ScpA, Area Science Park Basovizza, 34149 Trieste, Italy b c
a r t i c l e
i n f o
Available online 29 October 2011 Keywords: Nanostructures Low energy cluster beam deposition X-ray Photoelectron Spectroscopy (XPS) Oxidation
a b s t r a c t We used in-situ X-ray Photoelectron Spectroscopy (XPS) to study the oxidation process of a clusterassembled metallic titanium film exposed to molecular oxygen at room temperature. The nanostructured film has been grown on a Si(111) substrate, in ultra high vacuum conditions, by coupling a supersonic cluster beam deposition system with an XPS experimental chamber. Our results show that upon in-situ oxygen exposure Ti3 + is the first oxidation state observed, followed by Ti4 +, whereas Ti2 + is practically absent during the whole process. Our results compare well with the existing literature on Ti films produced using other techniques. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide is a versatile material, with a wide range of important technological applications [1, 2]. The (110) surface of rutile is recognized as a bulk model oxide and has been widely used as a starting point to explore phenomena on oxides. However, the behavior of this system is rather complex, and the precise experimental conditions used in sample preparation are crucial to define the geometric and defect structure, which can affect in a decisive way the physical and chemical properties of the surface [1]. In the last decade interest has largely drifted to study of the properties of other surfaces and crystalline phases of titania, to the study of the relationship between preparation procedure and physico-chemical properties, and, finally, to nanostructured titania, which is stable both in amorphous and in anatase form. Cluster-assembled nanostructured titanium dioxide (ns-TiO2) films have been produced and intensely studied because of their widespread industrial use, their electronic, optical, and catalytic properties, and their biocompatibility [3–5]. Such ns-TiO2 films are produced with a high-intensity Pulsed Microplasma Cluster Source (PMCS) [6] in two sequential steps. First a nanostructured film of metallic titanium is grown in vacuum, and it is then exposed to dry air. In this way we have obtained a highly defective and porous surface with ⁎ Corresponding author at: CNR-IOM, Laboratorio, TASC, Area Science Park, Strada Statale 14, Km. 163.5 Basovizza, 34149 Trieste, Italy. Tel.: + 39 040 3758419; fax: + 39 040 226767. E-mail address: [email protected] (M. de Simone). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.10.075
greatly enhanced chemical reactivity relative to bulk TiO2. The asdeposited films, after exposure to air, are stoichiometric TiO2 [7, 8] and have the appealing property of being optically transparent [4]. The PMCS technique, does not involve the use of chemicals, like Chemical Vapor Deposition, sol-gel deposition or anodic oxidation, and is rather similar to magnetron sputtering, but it allows a better control of the reactive gas and avoids the inclusion of inert carrier gas in the body of the deposit. In addition, the use of a PMCS proved to be very effective in the synthesis and manipulation of metal nanoparticles [6] and therefore in the production of nanostructured materials. These have different electronic, mechanical and optical properties which can be easily tailored by varying the size distribution of the constituent clusters [9]. In particular one can vary the nano and meso structure of cluster-assembled ns-TiO2 by controlling the mass distribution, structure and chemical composition of cluster prior to deposition [8, 10–13]; in the end chemical and physical properties of the film are mainly determined by the reactivity to the gaseous species it is exposed to after deposition. The high reactivity of metallic titanium to atmospheric components (O2, N2, CO2, H2O, H2, and so on), is responsible for the stoichiometry of its surface, which in turn determines most of its physical and chemical properties. The present study specifically addresses the oxidation process of the nanostructured metallic titanium surface after interaction with molecular oxygen. This topic has been the subject of a large number of works in the last 30 years [14–27], mostly limited to smooth metallic surfaces. We describe the experimental set-up we assembled for this purpose by coupling a roaming UHV-compatible version of the PMCS cluster source with an X-ray Photoelectron Spectroscopy
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(XPS) facility. With this apparatus, cluster assembled materials can be grown and analyzed in-situ by standard surface science techniques. Results about oxidation of Ti films, with controlled quantity of molecular oxygen are compared with related experimental findings available in the literature.
2. Experiment The synthesis and characterization of the cluster-assembled titanium films before and after exposure to oxygen have been carried out at the XPS facility available at Lab. TASC-Analytical Division. An Ultra High Vacuum (UHV)-compatible roaming Pulsed Microplasma Cluster Source (PMCS, from TETHIS s.r.l.) has been coupled to the XPS preparation chamber to allow in-vacuum sample deposition of cluster assembled films, which can then be transferred under UHV conditions into the principal analysis chamber, equipped with a concentric hemispherical analyzer and a non-monochromatic Mg X-ray source. In the present experiment titanium clusters in a mass range up to several thousand atoms per cluster were produced by the PMCS as described in Refs. [6] and [28]. Briefly the source consists of a ceramic cavity hosting two electrodes, the cathode being a titanium rod (Advent Research Material, purity 99.6%). The cavity is closed on one side by a pulsed solenoid valve facing, on the opposite side, a nozzle (1 mm diameter) through which the supersonic expansion takes place. A pulsed jet of He (purity 99, 9995%, O2 1 ppm V, pulse duration ≈ 300 μs, ν ≈ 5 Hz), expanding from a 4 × 10 6 Pa high pressure reservoir, is directed toward the titanium rod inside the ceramic cavity. After a delay of ≈0.6 ms an electric discharge is triggered by applying a high voltage pulse (around 0.9 kV, with pulse duration of ≈80 μs) to the electrodes. Intense and localized plasma develops at the cathode surface where stagnation conditions provide a sufficiently high pressure. The cathode material is locally ablated via a sputtering process and the vaporized atoms thermalize with the buffer gas and condense to form clusters. The He/Ti cluster mixture expands through the nozzle into a vacuum chamber to form a supersonic beam. A skimmer admits the central part of the beam into the chamber where the cluster deposition takes place onto a Si(111) substrate at a base pressure of 7 × 10 − 8 Pa, producing, after 2 h deposition, a circular film (≈1 cm radius, ≈1 μm thickness). The sample was morphology characterized ex-situ, after growth, by using a ZEISS Scanning Electron Microscope (SEM; mod. Supra 40, energy range 0.1–30 keV, lateral resolution up to 1 nm). Due to air exposure, the sample analyzed ex-situ is completely oxidized. Fig. 1 shows ex situ SEM micrographs of the cluster-assembled
titanium films we used for the present experiment and that are characterized by a granular structure with a mean grain size of roughly 10 nm as also confirmed by atomic force microscopy analysis [4] on samples deposited using a similar procedure. The fine substructure is due to the small average size of clusters (1–2 nm) that coalesce in grains after landing on the substrate, as observed by Tunneling Electron Microscopy measurements on sub monolayer coatings of metallic titanium clusters [8, 11]. The as-deposited film has been characterized in-situ by means of X-ray photoemission spectroscopy (XPS), measuring the Ti 2p and O 1s core level spectra of the film as-deposited, and after exposure to controlled doses of oxygen. All the photoemission spectra have been acquired in normal emission geometry at room temperature, with an overall instrumental energy resolution of ≈1.2 eV. Binding energies were calibrated by fixing the Ti 0 2p2/3 level to 454.1 eV [29]. Oxygen was introduced by a leak valve, and a quadrupole mass spectrometer was used to check gas-line cleanness and to monitor the presence of any possible contaminant. No background oxygen pressure was detected by the quadrupole mass spectrometer in, deposition nor in the analysis chambers, implying a residual partial pressure b1.5 × 10 − 10 Pa. Initially oxygen was dosed at a pressure of 2.0 × 10 − 6 Pa (0–175 L, Fig. 2a), increasing progressively the exposure time from 60 s to 3600 s. The last three exposures were performed at a pressure of 2.0 × 10 − 4 Pa (7400–30,500 L, Fig. 2b). We have to note that, the residual O2 traces in the carrier gas are equivalent to an exposure of the sample to a dose of less than 0.5 Langmuir. This well agrees with part of the low O 1s signal we observe in the photoemission spectra of the as-deposited sample (as discussed in the next session). XPS results have been analyzed by performing a non-linear least square fit of the data in the energy range of the Ti 2p photoemission peaks. We used a Shirley background, and we reproduced the photoemission intensity by using, as fit function, four doublets with Doniach–Sunjic line shapes, corresponding to the different titanium oxidation states (Ti 0, Ti 2 +, Ti 3 + and Ti 4 +). The binding energy of the 2+, 3 + and 4 + components was fixed, respectively, at +1 eV, +3.4 eV and + 4.7 eV above the metallic component, in agreement with literature values [18 and references therein]. Asymmetry was set to zero for Ti 2 +, Ti 3 + and Ti 4 +, and to 0.41 ± 0.03 for the metallic component, as extracted by analyzing the thick metallic film. We fixed the Lorentzian line-width at 0.55 ± 0.05 eV for all the doublets, consistent with the literature. The fit results give spin-orbit splitting values of 6.0 ± 0.2 eV and 5.7 ± 0.2 eV for the metallic and oxide components, respectively, while the branching ratio is 0.49 ± 0.05, again compatible with literature [18]. The O 1s photoemission data have been analyzed by performing a non-linear least square fit procedure using two Gaussian peaks, superimposed to a linear background. The Ti 0, + 2, + 3, + 4 and O concentrations have been evaluated from the photoemission spectra by using as sensitivity factors for the Ti 2p and O 1s levels, 1.66 and 0.61, respectively.
3. Results and discussion
Fig. 1. Scanning Electron Microscope images of surface morphology of as deposited nanostructured film of titanium after full oxidation due to its exposure to air at room temperature. Grain structure and voids are visible on the nanometric scale. The main grain size is around 10 nm.
Fig. 2 shows Ti 2p in situ spectra of the as deposited ns-Ti film and after the progressive exposure to oxygen at room temperature, together with the results of our fit analysis, which include all the possible Ti oxidation states: Ti 0, Ti + 2, Ti + 3, Ti + 4. The corresponding O 1s photoemission spectra are shown in Fig. 3, while the concentrations of the titanium oxidation states and oxygen, resulting from the data analysis, are shown in Fig. 4. The reported concentrations (Table 1) have been calculated under the assumption of a material with homogeneous stoichiometry, and without taking into account any other possible dis-homogeneity due, for example, to adsorption of contaminants on the film surface, whose absence
M. de Simone et al. / Thin Solid Films 520 (2012) 4803–4807
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Fig. 2. a) Ti 2p XPS spectra of sample as-deposited sample and exposure to 3.6, 11 and 175 L of O2, p(O2) = 2 × 10− 6 Pa. First point, corresponding to the as-deposited ns-Ti, can be considered as exposed at a dose around 0.5 L, as explained in the text. b) Ti 2p XPS spectra of sample following exposure 7400 L, 2200 L and 30,500 L of O2, p(O2) = 2 × 10− 4 Pa. The spectrum at is the same in Fig. 2a. Spectra have been acquired in normal emission geometry, and have been normalized at to have the same value at high binding energy intensity.
was assured by wide range XPS performed on a series of ns-Ti film prepared in similar experimental conditions. From our data we see that already the as-deposited film shows an oxygen contribution (O 2 − + OH −) of ≈32% that, as mentioned in the Experimental part, may be partly due to contaminants present in the helium. However we note that the O 1s photoemission peak, both of the as-deposited film and after exposures to molecular oxygen, always consists of at least two different components, one centered at 530.2 ± 0.5 and the other at 532.3 ± 0.6 eV (BE1 and BE2 respectively). Both the binding energy absolute values and their energy shifts, are compatible with the value obtained in Ti + 3, + 4 oxides (BE1) and
Fig. 3. O 1s XPS spectra of as-deposited sample and exposure to 3.6, 11 and 175 L of O2 p(O2) = 2 × 10− 6 Pa and 7400 and 30,500 L of O2, p(O2) = 2 × 10− 4 Pa. The spectra have been acquired in normal emission geometry, and have been normalized to have the same intensity at the O2 − peak. In the picture are displayed experimental data (round marks), and the results of the Gaussian fit, both as whole (double line) and the two Gaussian components (black dot lines).
Fig. 4. Percentage of contribution from Ti0, Ti2 +, Ti3 +, Ti4 + and from O2 −. An almost constant fraction (~ 20%) of the signal comes from OH− groups. First point, corresponding to the as-deposited ns-Ti, can be placed at a dose around 0.5 L, as explained in the text.
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Table 1 Percentage of contribution from Ti0, Ti2 +, Ti3 +, Ti4 + and from O2 −. An almost constant fraction (~20%) of the signal comes from OH− groups. First point, corresponding to the as-deposited ns-Ti, can be placed at a dose around 0.5 L, as explained in the text. O2 dose[L]
Ti0%
Ti2 + %
Ti3 + %
Ti4 + %
O2 − %
OH− %
As-deposited film 1.2 3.6 11 102 175 7400 22,000 30,500
63.1 40.7 31.6 22.9 19.1 11.5 8.9 8.9 7.2
1.5 0.4 0.3 0.9 0.4 1.7 1.5 0.9 1.9
3.7 8.1 8.2 8.2 9.2 11.3 13.9 14.1 14.5
0.0 2.8 6.7 9.6 10.2 11.5 11.3 11.0 11.7
14.2 21.8 29.8 35.0 39.2 41.0 44.2 43.3 43.9
17.5 26.2 23.3 23.4 21.9 23.0 20.2 21.7 20.8
hydroxyl oxygen (BE2) [18, 30]. So, part of the initial oxygen contribution can be ascribed to the hydroxyl group (OH −) that may originate from a layer of chemisorbed water. The partial pressure of water in the preparation chamber was 1.3 × 10 − 8 Pa (RGA measure performed immediately before and after deposition of the film). This water partial pressure was indeed constant during all the experiment and, consistently the XPS signal due to the OH group was constant in all the studied samples (≈20%, see Fig. 4 and Table 1) and compatible with a sticking coefficient on the fresh deposited ns-Ti film close to unit. However, since the oxidation efficiency of water is much lower than that of oxygen [30], we can follow the evolution of the titanium oxidation as a function of O2 dose, simply subtracting the hydroxyl contribution. Surface oxidation starts immediately (Fig. 2a and Table 1), as reflected in the growth of the Ti 3 + component. Already at 3.6 L, the concentration of Ti 4 + is comparable to that of Ti 3 +, and the ratio became slightly favorable to Ti 4 + already at 175 L. After dosing oxygen at high pressure, passing from 175 L to 7400 L, we observed a decrease of the metallic component, up to saturation at higher exposures. As we are mindful of the relatively poor resolution of our XPS data, in order to test the robustness of our fits, we performed systematic variations that may influence the results (i.e., Lorentzian and Gaussian line width asymmetry, background shape). In the end, even if the Ti + 2/Ti 3 + intensity ratio does slightly change, none of these fits show any significant amount of Ti + 2. Our results suggest that exposure at room temperature of a ns-Ti film to molecular oxygen causes a progressive oxidation of the metal surface, initially with the prevalent formation of Ti 3 +, followed by slower formation of Ti 4 +, while the Ti 2 + component is practically absent. This result is in good agreement with the observations of Burrell et al. [24] and Kurahashi et al. [25]. Both works study the oxidation of polycrystalline metallic films, obtained by evaporation of ultra pure titanium whose sole further treatment was annealing [25]. The work of Burrell et al. explicitly states that the choice not to purify the sample by sputtering was meant to avoid the reduction of titanium layers involved in the sputtering process, which is the main source of the high level of Ti 2 + previously observed in previous literature regarding the oxidation of metallic titanium surfaces [14– 16]. A recent work on the oxidation on a single-crystal Ti(0001) surface at 200 °C, reports an initial formation of Ti 2 + [27], but in this case too, the surface was cleaned using several cycles of annealing and sputtering. Debated point in the literature is the effect of oxygen pressure in the dynamics of oxidation and on its saturation point. Indeed, both the penetration depth of the oxidation process and the final obtained stoichiometry seem to depend on the oxygen pressure during exposure [14]. In some works saturation has been observed already for exposures below 100 L [15, 16, 18], and the formation of a passivated film of TiO2 on the external surface was invoked to explain the observed lack of any further oxidation inside the metal. In works,
where a higher oxygen pressure was used, pressure effects appear to be less important [17, 20, 21]. This may be an artifact due to a lower sensitivity of the chosen experimental techniques and/or to the difficulty in following the process on a rather large range of doses and pressures, or may be a genuine independence of the process from the pressure [17]. From our experiment we can infer that, regardless of the dosing pressure, the oxidation process is taking place over the whole dosing range, as witnessed by progressive decrease of the metallic titanium signal in XPS and simultaneous increase of the signal from O 2 − (see Fig. 4). The rates of change of these signals do actually slow down with increasing dose, but the overall behavior does not appear to indicate a saturation of the surface. It is indeed still compatible with a progressive oxidation of the inner layers of the bulk, with migration of the oxygen in the form of oxide or sub-oxides of titanium. 4. Conclusions In this study we have characterized by in-situ XPS the oxidation processes occurring to a cluster assembled titanium film exposed to a controlled quantity of molecular oxygen at room temperature. A PMCS was used to grow a nanostructured metallic film of ca. 1 μm thickness onto a Si(111) substrate by means of low energy supersonic cluster beam deposition technique, by coupling a supersonic cluster beam deposition system with an XPS experimental chamber. The progressive oxidation of the metallic nanostructured surface started with the prevalent formation of Ti 3 +, followed by production of Ti 4 +, whereas the Ti 2 + component is practically absent during the whole process. Our results are consistent with the previous observation on samples prepared by evaporation of ultra pure metallic titanium, whose sole further treatment was annealing. Acknowledgments The authors gratefully thank Dr.Carlo Callegari for the critical and constructive discussion. References [1] U. Diebold, Surf. Sci. Rep. 48 (2002) 53. [2] A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008). [3] E. Barborini, A.M. Conti, I. Kholmanov, P. Piseri, A. Podestà, P. Milani, C. Cepek, O. Sahko, R. Macovez, M. Sancrotti, Adv. Mater. 17 (2005) 1842. [4] R. Carbone, I. Marangi, A. Zanardi, L. Giorgetti, E. Chierici, G. Berlanda, A. Podesta, F. Fiorentini, G. Bongiorno, P. Piseri, P.G. Pelicci, P. Milani, Biomaterials 27 (2006) 3221. [5] A. Podestà, G. Bongiorno, P.E. Scopelliti, S. Bovio, P. Milani, C. Semprebon, G.J. Mistura, J. Phys. Chem. C 113 (2009) 18264. [6] E. Barborini, P. Piseri, P. Milani, J. Phys. D: Appl. Phys. 32 (1999) L105. [7] I.N. Kholmanov, E. Barborini, S. Vinati, P. Piseri, A. Podestà, C. Ducati, C. Lenardi, P. Milani, Nanotechnol. 14 (2003) 1168. [8] E. Barborini, I.N. Kholmanov, P. Piseri, C. Ducati, C.E. Bottani, P. Milani, Appl. Phys. Lett. 81 (2002) 3052. [9] R. Wegner, P. Piseri, H. Vahedi Tafreshi, P. Milani, J. Phys. D: Appl. Phys. 39 (2006) R439. [10] C. Chiappino, P. Piseri, S. Vinati, P. Milani, Rev. Sci. Instrum. 78 (2007) 066105. [11] T. Mazza, E. Barborini, I.N. Kholmanov, P. Piseri, G. Bongiorno, S. Vinati, P. Milani, C. Ducati, D. Cattaneo, A. Li Bassi, C.E. Bottani, A.M. Taurino, P. Siciliano, Appl. Phys. Lett. 87 (2005) 103108. [12] C. Ducati, E. Barborini, G. Bongiorno, S. Vinati, P. Milani, Appl. Phys. Lett. 87 (2005) 201906. [13] T. Mazza, E. Barborini, P. Piseri, P. Milani, D. Cattaneo, A. Li Bassi, C.E. Bottani, C. Ducati, Phys. Rev. B 75 (2007) 045416. [14] C. Oviedo, J. Phys. Condens. Matter 5 (1993) A153. [15] I. Vaquila, M. Passeggi Jr., J. Ferron, J. Phys. Condens. Matter 5 (1993) A157. [16] L.I. Vergara, M.C.G. Passeggi Jr., J. Ferron, Appl. Surf. Sci. 187 (2002) 199. [17] A. Azoulay, N. Shamir, E. Fromm, M.H. Mintz, Surf. Sci. 370 (1997) 1. [18] G. Lu, S. Bernasek, J. Schwartz, Surf. Sci. 458 (2000) 80. [19] A. Platau, L.I. Johansson, A.L. Hagstrom, S.-E. Karlsson, S.B.M. Hagstrom, Surf. Sci. 63 (1977) 153. [20] L.I. Johansson, A.L. Hagstrom, A. Platau, S.-E. Karlsson, Phys. Status Solidi B 83 (1977) 77. [21] J.B. Bignolas, M. Bujor, J. Bardolle, Surf. Sci. 108 (1981) L453. [22] B.M. Biwer, S.L. Bernasek, Surf. Sci. 167 (1987) 207.
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E. Roman, M. Sanchez-Avedillo, J.L. de Segovia, Appl. Phys. A 35 (1984) 35. M.C. Burrell, N.R. Armstrong, J. Vac. Sci. Technol., A 1 (1983) 1831. M. Kurahashi, Y. Yamauchi, J. Vac. Sci. Technol., A 17 (1999) 1047. E. Bertel, R. Stockbauer, T.E. Madey, Surf. Sci. 141 (1984) 355. Y. Takakuma, S. Ishidzuka, A. Yoshigoe, Y. Teraoka, Y. Yamauchi, Y. Mizuno, H. Honda, T. Homma, Appl. Surf. Sci. 216 (2003) 395.
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[28] P. Milani, P. Piseri, E. Barborini, A. Podestà, C. Lenardi, J. Vac. Sci. Technol., A 19 (2001) 2025. [29] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Boben, in: J. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Praire, MN, 1992, p. 72. [30] A. Azoulay, N. Shamir, V. Volterra, M.H. Mintz, Surf. Sci. 422 (1999) 141.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Oxidation of nanostructured Ti films produced by low energy cluster beam deposition: An X-ray Photoelectron Spectroscopy characterization Monica de Simone a,⁎, Elena Snidero a, Marcello Coreno b, e, Gero Bongiorno c, Luca Giorgetti d, Matteo Amati e, Cinzia Cepek a a
CNR-IOM Laboratorio TASC, Area Science Park Basovizza, 34149 Trieste, Italy CNR-IMIP, c/o Laboratorio TASC Area Science Park Basovizza, 34149 Trieste, Italy Fondazione Filarete, v.le Ortles 22/4, 20139 Milano, Italy d Istituto Europeo di Oncologia, Dip. di Oncologia Sperimentale, Via Adamello 16, 20139, Milano, Italy e Sincrotrone Trieste ScpA, Area Science Park Basovizza, 34149 Trieste, Italy b c
a r t i c l e
i n f o
Available online 29 October 2011 Keywords: Nanostructures Low energy cluster beam deposition X-ray Photoelectron Spectroscopy (XPS) Oxidation
a b s t r a c t We used in-situ X-ray Photoelectron Spectroscopy (XPS) to study the oxidation process of a clusterassembled metallic titanium film exposed to molecular oxygen at room temperature. The nanostructured film has been grown on a Si(111) substrate, in ultra high vacuum conditions, by coupling a supersonic cluster beam deposition system with an XPS experimental chamber. Our results show that upon in-situ oxygen exposure Ti3 + is the first oxidation state observed, followed by Ti4 +, whereas Ti2 + is practically absent during the whole process. Our results compare well with the existing literature on Ti films produced using other techniques. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide is a versatile material, with a wide range of important technological applications [1, 2]. The (110) surface of rutile is recognized as a bulk model oxide and has been widely used as a starting point to explore phenomena on oxides. However, the behavior of this system is rather complex, and the precise experimental conditions used in sample preparation are crucial to define the geometric and defect structure, which can affect in a decisive way the physical and chemical properties of the surface [1]. In the last decade interest has largely drifted to study of the properties of other surfaces and crystalline phases of titania, to the study of the relationship between preparation procedure and physico-chemical properties, and, finally, to nanostructured titania, which is stable both in amorphous and in anatase form. Cluster-assembled nanostructured titanium dioxide (ns-TiO2) films have been produced and intensely studied because of their widespread industrial use, their electronic, optical, and catalytic properties, and their biocompatibility [3–5]. Such ns-TiO2 films are produced with a high-intensity Pulsed Microplasma Cluster Source (PMCS) [6] in two sequential steps. First a nanostructured film of metallic titanium is grown in vacuum, and it is then exposed to dry air. In this way we have obtained a highly defective and porous surface with ⁎ Corresponding author at: CNR-IOM, Laboratorio, TASC, Area Science Park, Strada Statale 14, Km. 163.5 Basovizza, 34149 Trieste, Italy. Tel.: + 39 040 3758419; fax: + 39 040 226767. E-mail address: [email protected] (M. de Simone). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.10.075
greatly enhanced chemical reactivity relative to bulk TiO2. The asdeposited films, after exposure to air, are stoichiometric TiO2 [7, 8] and have the appealing property of being optically transparent [4]. The PMCS technique, does not involve the use of chemicals, like Chemical Vapor Deposition, sol-gel deposition or anodic oxidation, and is rather similar to magnetron sputtering, but it allows a better control of the reactive gas and avoids the inclusion of inert carrier gas in the body of the deposit. In addition, the use of a PMCS proved to be very effective in the synthesis and manipulation of metal nanoparticles [6] and therefore in the production of nanostructured materials. These have different electronic, mechanical and optical properties which can be easily tailored by varying the size distribution of the constituent clusters [9]. In particular one can vary the nano and meso structure of cluster-assembled ns-TiO2 by controlling the mass distribution, structure and chemical composition of cluster prior to deposition [8, 10–13]; in the end chemical and physical properties of the film are mainly determined by the reactivity to the gaseous species it is exposed to after deposition. The high reactivity of metallic titanium to atmospheric components (O2, N2, CO2, H2O, H2, and so on), is responsible for the stoichiometry of its surface, which in turn determines most of its physical and chemical properties. The present study specifically addresses the oxidation process of the nanostructured metallic titanium surface after interaction with molecular oxygen. This topic has been the subject of a large number of works in the last 30 years [14–27], mostly limited to smooth metallic surfaces. We describe the experimental set-up we assembled for this purpose by coupling a roaming UHV-compatible version of the PMCS cluster source with an X-ray Photoelectron Spectroscopy
4804
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(XPS) facility. With this apparatus, cluster assembled materials can be grown and analyzed in-situ by standard surface science techniques. Results about oxidation of Ti films, with controlled quantity of molecular oxygen are compared with related experimental findings available in the literature.
2. Experiment The synthesis and characterization of the cluster-assembled titanium films before and after exposure to oxygen have been carried out at the XPS facility available at Lab. TASC-Analytical Division. An Ultra High Vacuum (UHV)-compatible roaming Pulsed Microplasma Cluster Source (PMCS, from TETHIS s.r.l.) has been coupled to the XPS preparation chamber to allow in-vacuum sample deposition of cluster assembled films, which can then be transferred under UHV conditions into the principal analysis chamber, equipped with a concentric hemispherical analyzer and a non-monochromatic Mg X-ray source. In the present experiment titanium clusters in a mass range up to several thousand atoms per cluster were produced by the PMCS as described in Refs. [6] and [28]. Briefly the source consists of a ceramic cavity hosting two electrodes, the cathode being a titanium rod (Advent Research Material, purity 99.6%). The cavity is closed on one side by a pulsed solenoid valve facing, on the opposite side, a nozzle (1 mm diameter) through which the supersonic expansion takes place. A pulsed jet of He (purity 99, 9995%, O2 1 ppm V, pulse duration ≈ 300 μs, ν ≈ 5 Hz), expanding from a 4 × 10 6 Pa high pressure reservoir, is directed toward the titanium rod inside the ceramic cavity. After a delay of ≈0.6 ms an electric discharge is triggered by applying a high voltage pulse (around 0.9 kV, with pulse duration of ≈80 μs) to the electrodes. Intense and localized plasma develops at the cathode surface where stagnation conditions provide a sufficiently high pressure. The cathode material is locally ablated via a sputtering process and the vaporized atoms thermalize with the buffer gas and condense to form clusters. The He/Ti cluster mixture expands through the nozzle into a vacuum chamber to form a supersonic beam. A skimmer admits the central part of the beam into the chamber where the cluster deposition takes place onto a Si(111) substrate at a base pressure of 7 × 10 − 8 Pa, producing, after 2 h deposition, a circular film (≈1 cm radius, ≈1 μm thickness). The sample was morphology characterized ex-situ, after growth, by using a ZEISS Scanning Electron Microscope (SEM; mod. Supra 40, energy range 0.1–30 keV, lateral resolution up to 1 nm). Due to air exposure, the sample analyzed ex-situ is completely oxidized. Fig. 1 shows ex situ SEM micrographs of the cluster-assembled
titanium films we used for the present experiment and that are characterized by a granular structure with a mean grain size of roughly 10 nm as also confirmed by atomic force microscopy analysis [4] on samples deposited using a similar procedure. The fine substructure is due to the small average size of clusters (1–2 nm) that coalesce in grains after landing on the substrate, as observed by Tunneling Electron Microscopy measurements on sub monolayer coatings of metallic titanium clusters [8, 11]. The as-deposited film has been characterized in-situ by means of X-ray photoemission spectroscopy (XPS), measuring the Ti 2p and O 1s core level spectra of the film as-deposited, and after exposure to controlled doses of oxygen. All the photoemission spectra have been acquired in normal emission geometry at room temperature, with an overall instrumental energy resolution of ≈1.2 eV. Binding energies were calibrated by fixing the Ti 0 2p2/3 level to 454.1 eV [29]. Oxygen was introduced by a leak valve, and a quadrupole mass spectrometer was used to check gas-line cleanness and to monitor the presence of any possible contaminant. No background oxygen pressure was detected by the quadrupole mass spectrometer in, deposition nor in the analysis chambers, implying a residual partial pressure b1.5 × 10 − 10 Pa. Initially oxygen was dosed at a pressure of 2.0 × 10 − 6 Pa (0–175 L, Fig. 2a), increasing progressively the exposure time from 60 s to 3600 s. The last three exposures were performed at a pressure of 2.0 × 10 − 4 Pa (7400–30,500 L, Fig. 2b). We have to note that, the residual O2 traces in the carrier gas are equivalent to an exposure of the sample to a dose of less than 0.5 Langmuir. This well agrees with part of the low O 1s signal we observe in the photoemission spectra of the as-deposited sample (as discussed in the next session). XPS results have been analyzed by performing a non-linear least square fit of the data in the energy range of the Ti 2p photoemission peaks. We used a Shirley background, and we reproduced the photoemission intensity by using, as fit function, four doublets with Doniach–Sunjic line shapes, corresponding to the different titanium oxidation states (Ti 0, Ti 2 +, Ti 3 + and Ti 4 +). The binding energy of the 2+, 3 + and 4 + components was fixed, respectively, at +1 eV, +3.4 eV and + 4.7 eV above the metallic component, in agreement with literature values [18 and references therein]. Asymmetry was set to zero for Ti 2 +, Ti 3 + and Ti 4 +, and to 0.41 ± 0.03 for the metallic component, as extracted by analyzing the thick metallic film. We fixed the Lorentzian line-width at 0.55 ± 0.05 eV for all the doublets, consistent with the literature. The fit results give spin-orbit splitting values of 6.0 ± 0.2 eV and 5.7 ± 0.2 eV for the metallic and oxide components, respectively, while the branching ratio is 0.49 ± 0.05, again compatible with literature [18]. The O 1s photoemission data have been analyzed by performing a non-linear least square fit procedure using two Gaussian peaks, superimposed to a linear background. The Ti 0, + 2, + 3, + 4 and O concentrations have been evaluated from the photoemission spectra by using as sensitivity factors for the Ti 2p and O 1s levels, 1.66 and 0.61, respectively.
3. Results and discussion
Fig. 1. Scanning Electron Microscope images of surface morphology of as deposited nanostructured film of titanium after full oxidation due to its exposure to air at room temperature. Grain structure and voids are visible on the nanometric scale. The main grain size is around 10 nm.
Fig. 2 shows Ti 2p in situ spectra of the as deposited ns-Ti film and after the progressive exposure to oxygen at room temperature, together with the results of our fit analysis, which include all the possible Ti oxidation states: Ti 0, Ti + 2, Ti + 3, Ti + 4. The corresponding O 1s photoemission spectra are shown in Fig. 3, while the concentrations of the titanium oxidation states and oxygen, resulting from the data analysis, are shown in Fig. 4. The reported concentrations (Table 1) have been calculated under the assumption of a material with homogeneous stoichiometry, and without taking into account any other possible dis-homogeneity due, for example, to adsorption of contaminants on the film surface, whose absence
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Fig. 2. a) Ti 2p XPS spectra of sample as-deposited sample and exposure to 3.6, 11 and 175 L of O2, p(O2) = 2 × 10− 6 Pa. First point, corresponding to the as-deposited ns-Ti, can be considered as exposed at a dose around 0.5 L, as explained in the text. b) Ti 2p XPS spectra of sample following exposure 7400 L, 2200 L and 30,500 L of O2, p(O2) = 2 × 10− 4 Pa. The spectrum at is the same in Fig. 2a. Spectra have been acquired in normal emission geometry, and have been normalized at to have the same value at high binding energy intensity.
was assured by wide range XPS performed on a series of ns-Ti film prepared in similar experimental conditions. From our data we see that already the as-deposited film shows an oxygen contribution (O 2 − + OH −) of ≈32% that, as mentioned in the Experimental part, may be partly due to contaminants present in the helium. However we note that the O 1s photoemission peak, both of the as-deposited film and after exposures to molecular oxygen, always consists of at least two different components, one centered at 530.2 ± 0.5 and the other at 532.3 ± 0.6 eV (BE1 and BE2 respectively). Both the binding energy absolute values and their energy shifts, are compatible with the value obtained in Ti + 3, + 4 oxides (BE1) and
Fig. 3. O 1s XPS spectra of as-deposited sample and exposure to 3.6, 11 and 175 L of O2 p(O2) = 2 × 10− 6 Pa and 7400 and 30,500 L of O2, p(O2) = 2 × 10− 4 Pa. The spectra have been acquired in normal emission geometry, and have been normalized to have the same intensity at the O2 − peak. In the picture are displayed experimental data (round marks), and the results of the Gaussian fit, both as whole (double line) and the two Gaussian components (black dot lines).
Fig. 4. Percentage of contribution from Ti0, Ti2 +, Ti3 +, Ti4 + and from O2 −. An almost constant fraction (~ 20%) of the signal comes from OH− groups. First point, corresponding to the as-deposited ns-Ti, can be placed at a dose around 0.5 L, as explained in the text.
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Table 1 Percentage of contribution from Ti0, Ti2 +, Ti3 +, Ti4 + and from O2 −. An almost constant fraction (~20%) of the signal comes from OH− groups. First point, corresponding to the as-deposited ns-Ti, can be placed at a dose around 0.5 L, as explained in the text. O2 dose[L]
Ti0%
Ti2 + %
Ti3 + %
Ti4 + %
O2 − %
OH− %
As-deposited film 1.2 3.6 11 102 175 7400 22,000 30,500
63.1 40.7 31.6 22.9 19.1 11.5 8.9 8.9 7.2
1.5 0.4 0.3 0.9 0.4 1.7 1.5 0.9 1.9
3.7 8.1 8.2 8.2 9.2 11.3 13.9 14.1 14.5
0.0 2.8 6.7 9.6 10.2 11.5 11.3 11.0 11.7
14.2 21.8 29.8 35.0 39.2 41.0 44.2 43.3 43.9
17.5 26.2 23.3 23.4 21.9 23.0 20.2 21.7 20.8
hydroxyl oxygen (BE2) [18, 30]. So, part of the initial oxygen contribution can be ascribed to the hydroxyl group (OH −) that may originate from a layer of chemisorbed water. The partial pressure of water in the preparation chamber was 1.3 × 10 − 8 Pa (RGA measure performed immediately before and after deposition of the film). This water partial pressure was indeed constant during all the experiment and, consistently the XPS signal due to the OH group was constant in all the studied samples (≈20%, see Fig. 4 and Table 1) and compatible with a sticking coefficient on the fresh deposited ns-Ti film close to unit. However, since the oxidation efficiency of water is much lower than that of oxygen [30], we can follow the evolution of the titanium oxidation as a function of O2 dose, simply subtracting the hydroxyl contribution. Surface oxidation starts immediately (Fig. 2a and Table 1), as reflected in the growth of the Ti 3 + component. Already at 3.6 L, the concentration of Ti 4 + is comparable to that of Ti 3 +, and the ratio became slightly favorable to Ti 4 + already at 175 L. After dosing oxygen at high pressure, passing from 175 L to 7400 L, we observed a decrease of the metallic component, up to saturation at higher exposures. As we are mindful of the relatively poor resolution of our XPS data, in order to test the robustness of our fits, we performed systematic variations that may influence the results (i.e., Lorentzian and Gaussian line width asymmetry, background shape). In the end, even if the Ti + 2/Ti 3 + intensity ratio does slightly change, none of these fits show any significant amount of Ti + 2. Our results suggest that exposure at room temperature of a ns-Ti film to molecular oxygen causes a progressive oxidation of the metal surface, initially with the prevalent formation of Ti 3 +, followed by slower formation of Ti 4 +, while the Ti 2 + component is practically absent. This result is in good agreement with the observations of Burrell et al. [24] and Kurahashi et al. [25]. Both works study the oxidation of polycrystalline metallic films, obtained by evaporation of ultra pure titanium whose sole further treatment was annealing [25]. The work of Burrell et al. explicitly states that the choice not to purify the sample by sputtering was meant to avoid the reduction of titanium layers involved in the sputtering process, which is the main source of the high level of Ti 2 + previously observed in previous literature regarding the oxidation of metallic titanium surfaces [14– 16]. A recent work on the oxidation on a single-crystal Ti(0001) surface at 200 °C, reports an initial formation of Ti 2 + [27], but in this case too, the surface was cleaned using several cycles of annealing and sputtering. Debated point in the literature is the effect of oxygen pressure in the dynamics of oxidation and on its saturation point. Indeed, both the penetration depth of the oxidation process and the final obtained stoichiometry seem to depend on the oxygen pressure during exposure [14]. In some works saturation has been observed already for exposures below 100 L [15, 16, 18], and the formation of a passivated film of TiO2 on the external surface was invoked to explain the observed lack of any further oxidation inside the metal. In works,
where a higher oxygen pressure was used, pressure effects appear to be less important [17, 20, 21]. This may be an artifact due to a lower sensitivity of the chosen experimental techniques and/or to the difficulty in following the process on a rather large range of doses and pressures, or may be a genuine independence of the process from the pressure [17]. From our experiment we can infer that, regardless of the dosing pressure, the oxidation process is taking place over the whole dosing range, as witnessed by progressive decrease of the metallic titanium signal in XPS and simultaneous increase of the signal from O 2 − (see Fig. 4). The rates of change of these signals do actually slow down with increasing dose, but the overall behavior does not appear to indicate a saturation of the surface. It is indeed still compatible with a progressive oxidation of the inner layers of the bulk, with migration of the oxygen in the form of oxide or sub-oxides of titanium. 4. Conclusions In this study we have characterized by in-situ XPS the oxidation processes occurring to a cluster assembled titanium film exposed to a controlled quantity of molecular oxygen at room temperature. A PMCS was used to grow a nanostructured metallic film of ca. 1 μm thickness onto a Si(111) substrate by means of low energy supersonic cluster beam deposition technique, by coupling a supersonic cluster beam deposition system with an XPS experimental chamber. The progressive oxidation of the metallic nanostructured surface started with the prevalent formation of Ti 3 +, followed by production of Ti 4 +, whereas the Ti 2 + component is practically absent during the whole process. Our results are consistent with the previous observation on samples prepared by evaporation of ultra pure metallic titanium, whose sole further treatment was annealing. Acknowledgments The authors gratefully thank Dr.Carlo Callegari for the critical and constructive discussion. References [1] U. Diebold, Surf. Sci. Rep. 48 (2002) 53. [2] A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008). [3] E. Barborini, A.M. Conti, I. Kholmanov, P. Piseri, A. Podestà, P. Milani, C. Cepek, O. Sahko, R. Macovez, M. Sancrotti, Adv. Mater. 17 (2005) 1842. [4] R. Carbone, I. Marangi, A. Zanardi, L. Giorgetti, E. Chierici, G. Berlanda, A. Podesta, F. Fiorentini, G. Bongiorno, P. Piseri, P.G. Pelicci, P. Milani, Biomaterials 27 (2006) 3221. [5] A. Podestà, G. Bongiorno, P.E. Scopelliti, S. Bovio, P. Milani, C. Semprebon, G.J. Mistura, J. Phys. Chem. C 113 (2009) 18264. [6] E. Barborini, P. Piseri, P. Milani, J. Phys. D: Appl. Phys. 32 (1999) L105. [7] I.N. Kholmanov, E. Barborini, S. Vinati, P. Piseri, A. Podestà, C. Ducati, C. Lenardi, P. Milani, Nanotechnol. 14 (2003) 1168. [8] E. Barborini, I.N. Kholmanov, P. Piseri, C. Ducati, C.E. Bottani, P. Milani, Appl. Phys. Lett. 81 (2002) 3052. [9] R. Wegner, P. Piseri, H. Vahedi Tafreshi, P. Milani, J. Phys. D: Appl. Phys. 39 (2006) R439. [10] C. Chiappino, P. Piseri, S. Vinati, P. Milani, Rev. Sci. Instrum. 78 (2007) 066105. [11] T. Mazza, E. Barborini, I.N. Kholmanov, P. Piseri, G. Bongiorno, S. Vinati, P. Milani, C. Ducati, D. Cattaneo, A. Li Bassi, C.E. Bottani, A.M. Taurino, P. Siciliano, Appl. Phys. Lett. 87 (2005) 103108. [12] C. Ducati, E. Barborini, G. Bongiorno, S. Vinati, P. Milani, Appl. Phys. Lett. 87 (2005) 201906. [13] T. Mazza, E. Barborini, P. Piseri, P. Milani, D. Cattaneo, A. Li Bassi, C.E. Bottani, C. Ducati, Phys. Rev. B 75 (2007) 045416. [14] C. Oviedo, J. Phys. Condens. Matter 5 (1993) A153. [15] I. Vaquila, M. Passeggi Jr., J. Ferron, J. Phys. Condens. Matter 5 (1993) A157. [16] L.I. Vergara, M.C.G. Passeggi Jr., J. Ferron, Appl. Surf. Sci. 187 (2002) 199. [17] A. Azoulay, N. Shamir, E. Fromm, M.H. Mintz, Surf. Sci. 370 (1997) 1. [18] G. Lu, S. Bernasek, J. Schwartz, Surf. Sci. 458 (2000) 80. [19] A. Platau, L.I. Johansson, A.L. Hagstrom, S.-E. Karlsson, S.B.M. Hagstrom, Surf. Sci. 63 (1977) 153. [20] L.I. Johansson, A.L. Hagstrom, A. Platau, S.-E. Karlsson, Phys. Status Solidi B 83 (1977) 77. [21] J.B. Bignolas, M. Bujor, J. Bardolle, Surf. Sci. 108 (1981) L453. [22] B.M. Biwer, S.L. Bernasek, Surf. Sci. 167 (1987) 207.
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