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Titanium and reduced titania overlayers on titanium dioxide(110)
JOURNAL OF ELECTRON SPECTROSCOPY and RelatedPhenomena
ELSEVIER
Journal of Electron Spectroscopyand Related Phenomena 73 (1995) 1-11
Titanium and reduced titania overlayers on titanium dioxide(110) J . T . M a y e r a, U . D i e b o l d b, T . E . M a d e y ~, E. G a r f u n k e l a'* aDepartments of Chemistry and Laboratory for Surface Modification, Rutgers, the State University of New Jersey, Piscataway, NJ 08855-0939, USA bDepartment of Physics, Tulane University, New Orleans, LA 70118-5698, USA CDepartment of Physics and Laboratory for Surface Modification, Rutgers, the State University of New Jersey, Piscataway, NJ 08855-0939, USA
First received 22 July 1994; in final form 9 August 1994
Abstract
The adsorption of titanium on titanium dioxide TiO2(110) has been studied by X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). The XPS data for Ti overlayers are interpreted using peak fitting based on experimental standard spectra. 4 A_of Ti deposited at 150 K reacts with the substrate to produce ~ 12 A of intermediate oxidation state Ti. Adsorption of neutral metal begins on top of this interface oxide film, but 20 A of deposited Ti are needed to cover the oxide completely. LEIS data indicate a tendency for clustering of Ti on top of the interface oxide. Ar + sputtering of stoichiometric TiO2 leads to preferential loss of O from the near surface region. This reduction of the clean, annealed oxide surface by Ar + ion bombardment starts immediately and does not reach a steady state until 3 × 1017 ions cm -2, at which point the reduced overlayer is 17 A thick. " Keywords: Low energy ion scattering; TiO2(110); Titania; Titanium; X-ray photoelectron spectroscopy
I. Introduction
Titanium dioxide has received extensive attention in the surface science community, a result of both its technological importance [1-3] and its relative ease of preparation and examination. Perfect bulk TiO2 is a 3.1 eV bandgap insulator; however, annealing in ultra-high vacuum (UHV) creates bulk and surface n-type defects, oxygen vacancies, with a density of 10 ]8 to 1019 cm -3 [4]. A stoichiometric surface can be restored by annealing the U H V processed crystal in 02 a t 10 - 6 Torr. The relatively high defect (oxygen * Corresponding author.
vacancy) mobility in the TiO2 crystal makes this system different from many other important oxides such as MgO or A1203; any analysis of Ti overlayers must be made with this in mind. Electronically, TiO2 is neither a purely covalent nor an ionically bonded crystal but shows properties of both. It is probably best classified as containing highly polar covalent bonds. In addition to the rutile and anatase phases of TiO2, bulk oxides of lower Ti valence also exist (Ti20, TiO and Ti203). A model for the rutile(110) surface was first proposed by Henrich [5]. A neutral surface requires that half the surface Ti cations be covered with O; this leaves the surface with six-fold (as in the bulk) and five-fold coordinated Ti.
0368-2048/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02258-5
J.T. Mayer et al./Journal o f Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
Oxygen vacancies in the titania lattice cluster in the bulk to form crystallographic shear (CS) planes. A high density of CS planes can form ordered reduced phases, which are reviewed in the literature on higher titanium oxides (TiOx for 1.66 < x < 2.0) [6]. The formation of oxygen vacancies deep in the bulk is evidence for the relatively high vacancy mobility in the TiO2 rutile crystal. Several of the applications of TiO2 thin films (e.g. as sensors) depend on the conductivity change of TiO2 as the stoichiometry of the surface or the concentration of surface adsorbates is changed slightly. There have been many attempts to analyze suboxide (chemically reduced) TiO2_x surfaces using X-ray photoelectron spectroscopy (XPS) [7-23]. This task is complicated by the number of stable intermediate oxidation states for Ti in the Ti/O system. Currently there is confusion and disagreement about the treatment of the XPS data. As a result, we still lack a good understanding of several key aspects of this system, including the thickness of the suboxide overlayer and its composition. One purpose of this paper is to review published analyses of Ti core-level XPS data and to propose a peak fitting routine based on experimental standard spectra. We have applied XPS and low energy ion scattering (LEIS) with the aim of understanding the near surface structure and composition of stoichiometric TiO2 dosed with ultrathin films of Ti and of stoichiometric TiO2 sputtered with Ar ÷ ions. The XPS data are used for compositional information and charge state identification in the near surface regime (~ 0-20 A). Application of our peak fitting technique identifies a reduced layer 12 A thick that is composed of Ti in a distribution of oxidation states. LEIS gives the relative Ti and O concentrations in the outermost surface layer. LEIS data as a function of Ti overlayer thickness show that, after the formation of the reduced oxide interface, the metallic Ti film does not grow uniformly. Approximately 20 A of Ti deposition is required before the oxide is completely covered.
2. Experimental All experiments are performed in a UHV
chamber with a base pressure of 6 x 10-11 Torr. The peak pressure is 2 × 10-1° Torr during Ti evaporation and 1 x 10-9 Torr during sample annealing. Rutile TiO2(I10) crystals from Commercial Crystal Laboratories are cleaned with methanol and mounted on a sample holder. Each crystal is held in tantalum foil supported by tungsten wire. The crystals are heated by electron bombardment and cooled with liquid nitrogen. We find that uniform heating is very important to avoid fracture of the crystals. To accomplish this, the heater filament is roughly the same size as the l cm 2 sample and is in close proximity to the back of the crystal. A chromel-constantan (type-E) thermocouple is spot welded to the Ta foil holding the TiO2 crystal. After insersion of a fresh crystal into the vacuum chamber, it is sputtered with 500 eV Ar-- ions at an impact angle 20 ° off normal for 10 min at 5 #A. This is followed by annealing to 1100 K for 10 min. This thermal processing forms enough oxygen vacancies (bulk and surface defects) to turn the initially transparent and colorless sample blue. The surface order is monitored by low energy electron diffraction (LEED). Ti is deposited from a water cooled source by heating a W filament wrapped with Ti wire. The overlayer thickness is measured in situ with a quartz crystal oscillator calibrated by Rutherford backscattering spectroscopy (RBS). The Ti exposure is expressed as a thickness as measured by the quartz crystal monitor. The Ti thickness corresponds to the amount of pure Ti added, independent of the actual overlayer morphology or chemical composition. Therefore, a 10/~ Ti overlayer corresponds to a dosage of 5.5 x l0 Is atoms per cm2 (based on the bulk density of Ti metal), rather than a uniform film that is 10 A thick. XPS is performed using A1 K a radiation and a hemispherical analyzer mounted near normal to the surface and operating in the fixed analyzer transmission mode. The low energy ion scattering (LEIS) experiments are performed with the same hemispherical analyzer operating in the fixed retarding ratio mode and opposite polarity with a scattering angle of 144°. The He + ion current to the sample is ~ 100 nA at 1000 eV, with spectral acquisition time of 400 s. Deposition of Ti, and
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
3
(---615eV--~
Io_n_nFluence
_J (1) ¢¢1_
J
I
2.3A
Clean TiO2 !
450
I
455
450
II
460
465
Binding Energy (eV) Fig, 1. XPS spectra of the Ti2p region for different Ti overlayer thicknesses. The spin-orbit splittings for clean TiO 2 (bottom curve) and metallic Ti (top curve) are indicated, as is the chemical shift of the Ti2p3/2 peak.
all subsequent spectroscopy, is performed at a sample temperature of ~ 150 K. Each Ti overlayer experiment that involves multiple sequences of deposition followed by XPS and LEIS is also checked by discrete experiments that start with a clean, annealed surface.
3. Results
In Fig. 1 we present the X-ray photoelectron spectra in the region of the Ti2p peaks for increasing Ti exposure on TiO2. The Ti° state of the metallic thick film in the upper curve is shifted by 5.2 eV to lower binding energy relative to the Ti4+ state of the clean TiO2 surface spectrum. Titanium in intermediate oxidation states can be observed in the first few A of deposition (see the 2.3 A spectrum). The 22 A and 43 /~ (not shown) spectra look almost identical and show little trace of the oxide substrate; this indicates the formation of a continuous metallic Ti (zero valent) overlayer. The Ar + bombardment of transition metal oxides is known to sputter oxygen preferentially until an oxygen deficient surface is obtained under steady state conditions [24]. The Ti2p XPS data in Fig. 2 show the effect of Ar + sputter damage on the stoichiometric TiO2 surface at
I
I
460
470
Binding Energy (eV) Fig. 2. XPS spectra of the Ti2p region after Ar + ion bombardment. Ion fluences are given in ions cm-2
room temperature. Broadening on the lower binding energy side is due to the reduction of surface Ti4+ species to intermediate oxidation states. This correlates well with previous work by other authors [13,25,26]. Oxygen diffusion via defect migration is well known for reduced TiO2 oxides [27-29]. We are able to identify the onset of near surface diffusion necessary to oxidize Ti overlayers on the nearly stoichiometric TiO2 crystal. Fig. 3 shows the chemical change of 14 A of Ti deposited on TiO2 as determined by XPS and analyzed using the peak fitting routine discussed below. The sample, initially at 150 K, is annealed for 3 min periods at
I
t-
+
•
!
Ti
TiO2 ]
r-
0
lO0
I
I
I
300
I
500
I
I
700
900
Temperature (K) Fig. 3. Contributions of metallic Ti and Ti 4+ from a 14 A thick Ti overlayer on TiO2(110) are plotted as a function of temperature. Ti is deposited at 150 K. The sample is annealed for 3 min at each temperature.
4
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1 11
30 E20 "-'I
o
¢-
/
3
~2 300
400
500 600 Energy (eV)
700
800
Fig. 4. LEIS of 1 keV He+ ions on TiO2 with various Ti coverages.
X
0
1 0
~
0
-
10
20
-
30
1_
40
Ti thickness (A)
temperatures increasing in intervals of 50 K. The onset temperature for oxidation of the Ti overlayer by oxygen migration from the TiO2 crystal is 800K. This corresponds favorably to results indicating the onset of bulk cation and cationic vacancy mobility at 865 K [30]. This diffusion mechanism is differentiated from large scale mass transport associated with sintering, which occurs at a higher temperature (973 K) [31]. In Fig. 4 we show LEIS spectra for various overlayer coverages of Ti on TiO2. LEIS is extremely surface sensitive. A large shadow cone radius for ions of this energy means that He + does not directly scatter off second layer atoms, and the high neutralization rate of He + makes detection of double scattering events improbable [32]. Several interesting features can be noted by examining the O (375 eV) and Ti (700 eV) peaks as a function of overlayer exposure, as presented in Fig. 5. As seen from the XPS results considered above, chemical reduction of the oxide surface begins immediately upon Ti deposition. This is seen in LEIS as an attenuation in the O signal intensity relative to the stoichiometric surface. However, Ti peak growth and O attenuation do not reach limiting values until the deposit is 20 A thick. The O scattering intensity is almost completely attenuated by 20 A Ti dosage. The Ti scattering intensity does not mirror the O attenuation; rather, it grows linearly before leveling out at ~ 20/~. The difference between the exponential decrease in O
Fig. 5. LEIS peak areas from Ti overlayer experiments are plotted as a function of Ti dosage. Smooth lines are drawn through the data points. intensity and the linear increase in Ti intensity is discussed below.
4. X P S curve fitting
Our data indicate that deposition of Ti on TiO2 leads to an interfacial layer between the oxide and the metallic overlayer. The interface is amixture of reduced oxide phases. Ar + ion bombardment of the TiO2 surface also forms a chemically reduced surface layer through the preferential sputtering of oxygen. In both cases the changed chemical nature of the reduced oxide is seen in XPS by the shifting of the Ti2p peaks as the Ti valency is reduced from a + 4 charge state. Ti atoms in this reduced oxide have a valency between 0 and + 4, as is seen in the core level photoemission peak position. Quantitative XPS analysis of this system presents several problems. The form or shape of the Ti2p XPS curve varies from a broadened Ti 4+ doublet at Ti overlayers of a fractional monolayer or lightly sputtered surfaces to a near continuum between the Ti 4+ and the Ti ° peaks for 3-12 A Ti overlayers or the more heavily sputtered surfaces. Distinct peaks due to Ti in intermediate oxidation states are usually not observed. Two approaches to
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11 , 1
Ti02 "\
~1600
o
~
1200
E 800
400 0
0.4
0.8
1.2
1.6
2.0
O/Ti ratio
Fig. 6. The main elements of the Ti/O phase diagram are presented with some details omitted for clarity, a and b refer to the hexagonal and cubic metal phases of Ti respectively;It and ht refer to low and high temperature phases respectively.Adapted from P.G. Wahlbeck and P.W. Gilles, J. Am. Ceram. Soc., 49 (1966) 180. curve fitting are detailed in the existing literature dealing with TiO2 [10,19]. Traditionally, XPS data are fitted to a convolution of analytical functions after subtraction of a suitable secondary electron background. Adjustable parameters for the fit are peak position, width and intensity and the number of peaks. Several parameters may be constrained based on a priori information. Numerical optimization of the initial parameters results in the peak energy and intensity for all the contributing species, including the intermediate oxidation states of Ti. An alternate approach may also be used. Since the XPS peak shape of the clean, annealed oxide is known (most studies start with this), the Ti 4+ contribution to the reduced oxide XPS spectrum can be subtracted. This leaves the contribution of intermediate oxidation states. In special cases, discrete peaks in the remaining curve can be associated with individual oxidation states, and therefore the fit of this curve to Gaussian broadened Lorentzians is simplified [10]. In practice, both of the above methods are inadequate for this system. First, an optimized fit for the reduced oxide usually does not converge to unique values because of the large number of adjustable parameters. Second, in many cases the XPS signal due to the intermediate oxides (Ti 4+ subtracted) cannot be visually resolved into
5
discrete peaks, and therefore suffers from the same problem of non-convergence. Third, the peak shapes of TiO2 and metallic Ti are distinctly different (see Fig. 1), necessitating different fitting functions. To date, the peak shapes and positions of the intermediate oxides remain undetermined. Nonetheless, there is an extensive body of work on resolving the intermediate oxides of TiO 2 into separate oxidation states using the above two methods to interpret XPS data. Table 1 compares the literature results for the absolute energy Ti 4+, and the shift of the other oxidation states relative to Ti 4+. Fitting methodologies and descriptive comments are also noted. There is good agreement on the chemical shift between metallic Ti and stoichiometric TiO 2, 5.2 eV. The only significant deviation from this value comes from the PerkinElmer XPS Handbook, which reports 4.7 eV [23]. Since Ti and TiO2 samples are easily obtained, there is little experimental difficulty in doing the spectroscopy, although discrepancies may arise from band bending or from charging of the oxide sample. The intermediate oxides are available as stable bulk compounds; however, the surface is generally oxidized to TiO2. The usual method of sputtering to clean the surface will leave a mixture of oxidation states, as shown in this paper. Therefore literature values for these oxides are more scattered than for Ti and TiO2. There is not enough independent information to fit the XPS data of reduced TiO2 unambiguously. However, curve fitting can still be quite useful if it illustrates general trends without artifacts or distortions induced by the method applied. Our peak fitting technique is based on the following three premises: (1) The oxygen-titanium phase diagram (Fig. 6) shows regions with stable Ti20, TiO and Ti203 phases in addition to TiO2 and pure Ti. The reduced surface oxide does not show long-range order and may in fact have no distinct and segregated phase structure, but the bulk stability of the above oxides implies the existence of titanium in at least the + 1, + 2, and + 3 intermediate oxidation states. Therefore, we fit our XPS data to five oxidation states for Ti (0 through + 4). (2) The five oxidation states of Ti are fitted to experimentally determined lineshapes for Ti and
6
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
Table 1 Comparison of XPS peak fitting results and methodologies for the Ti2p data of reduced titania oxides ATi
ATi1+
-5.5 -5 -5.1
-4.1
ATi2+
ATi3+
Ti4+ (eV)
Authors
-3.6 -2.9 -3.5 -3.5 -3.4
-2.1 - 1.6 -2.1 - 1.5 -1.5 -2.0 -1.7 - 1.5 -1.8 -2.5
461.8 457.9 459.0 459.0 458.7 458.6 458.5 459.1 457.0 459.4 459.8 458.7 455.5 459.1 458.6 458.8 458.8 459.3
Armstrong [7] Asensio [8] Bardi [9] Carley [10] Choudhury [11] Dwyer [12] G6pel [13] Idriss [14] Pan [15] Paul [16] Plateau [17] Ramqvist [18] Rocker [19] Saha [20] Sayers [21] Simon [22] Handbook of XPS [23] This paper
-3.1 -3.0 -3.6
-5.1
-4.2
-3.8 -4.9 -4.9 -5.2
-4.4
-3.9 -3.3
-3.9
-3.5 -3.5 -3.8 -2.6
-5.5 -5.4 -4.7 -5.2
- 1.9 -2.3 - 1.9
- 1.3
Notes
d 1 2 d 1,2,a 5,c 1,3 d
2,4 1,a
3,6
Methods are referenced as follows: 1. Subtraction of known data (such as Ti4+ data from the stoichiometric surface). 2. Fit optimized Gaussian curves. 3. Fit internal standard spectra (such as chemically shifted Ti4+ spectrum). 4. Fit Doniach-Sunjic function (accounts for non-symmetrical peaks). 5. Methodology not stated; some references to other authors. 6. Even spacing of oxidation states; initial state approximation. Comments are referenced as follows: a. Does not assign peaks to discrete oxidation states. b. Some fitting parameters taken from referenced sources. c. TiO2/Pt. d. Oxidation of Pt3Ti sample. TiO2 in a 40 eV region that includes the 2p3/2 a n d 2pl/2 peaks of all o x i d a t i o n states. N o t e that this fitting procedure does n o t necessitate a n y b a c k g r o u n d s u b t r a c t i o n a p p r o x i m a t i o n . The use of i n t e r n a l s t a n d a r d spectra reduces the n u m b e r o f fitting parameters a n d improves the accuracy a n d confidence level of the resultant fit. The TiO2 substrate a n d the Ti overlayer are fitted directly using the i n t e r n a l s t a n d a r d spectra, a n d intermediate o x i d a t i o n states are fitted using the chemically shifted metallic Ti w a v e f o r m spaced evenly (at 1.3 eV intervals) between the energy o f Ti a n d Ti 4+. The even spacing o f the intermediate oxidation states is a n a p p r o x i m a t i o n based o n the different effective charge s u r r o u n d i n g Ti nuclei (an initial state effect), a l t h o u g h it clearly ignores final state a n d other complicating effects. The shape a n d
position of XPS peaks from the reduced oxide are a p p r o x i m a t i o n s necessary to reduce the n u m b e r o f u n c o n s t r a i n e d fitting parameters. W e have n o a priori reason to believe that these fitting f u n c t i o n s represent true chemical shifts a n d peak shapes. However, the c o n s t r a i n t of these parameters leads to results that are at least systematic, can be used to illustrate trends, a n d in some cases can be used quantitatively. (3) T o allow for changes in the core levels with respect to the F e r m i level ( b a n d b e n d i n g ) [19], the fitting functions can be shifted as a group (maintaining c o n s t a n t relative spacing). The shift is always less t h a n 0.4 eV a n d can readily be checked, as errors o f even 0.1 eV are easily observed. The fitting is optimized by the G a u s s - N e w t o n m e t h o d i m p l e m e n t e d by a p r o g r a m written in
11
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
, ,
/_~1~
jt,
-
:
' \~
__
m -........ --
Data Fit Ti Ti+l
--
7!+3
7
0.8
~;~ 0.6
~'
°0"!i
+1 + 2 ~
0.
.m c/) t-
_c
0
10 20 3() 40 Thickness of deposited Ti (,&,)
Fig. 8. The substrate oxide, interface oxide and metal overlayer intensities, as obtained from curve fitting, are plotted as a function of Ti exposure. The calculated overlayer thickness (intermediate oxide plus metal overlayer) as a function of deposited Ti is shown as an inset. 455
460
465
470
Binding Energy (eV) Fig. 7. Fitted XPS data from (a) 0.3 •, (b) 1.5 A and (c) 5.4 A of Ti on TiO2. Each spectrum is fitted using contributions from metallic Ti, the oxide substrate Ti4+, and Ti 3+, Ti2+, Tit+ from the interfaceregion. house [33]. Examples of three specific Ti overlayer thicknesses are shown in Fig. 7. Five oxidation states for Ti are used for each curve fit; for the 0.3 A example, however, the Ti l+ and Ti ° curves are not shown, as their contribution is small. A Shirley-type background is subtracted only to display the results more clearly, and was not used in the fitting. We obtained satisfactory curve fits for slightly reduced (light sputtering or thin Ti overlayers) or for heavily metallized surfaces; the fit is not quite as good for heavily reduced surfaces (e.g. heavy sputtering) where a large contribution of intermediate oxide species is present.
5. Discussion
5.1. X-ray photoelectron spectroscopy The contributions from substrate, interface, and
metallic overlayer are separated using the peak fitting routine described above. The results for increasing Ti coverage are plotted in Fig. 8. The important points are (1) the formation of zero valent Ti starting after the first 4 A, of deposition, and (2) the attenuation of the + 4 valent state (the TiO2 substrate) much faster than would be expected if it were simply shielded by the deposited Ti. The mean free path of Ti2p photoelectrons through the oxide is ~ 12 fi~ [34]. But after only 4 A, of Ti deposition, the intensity of photoemission from the substrate is reduced by 1/e from the initial value before any Ti deposition. There is thus ~ 12 A, of interface which is composed of intermediate Ti oxides formed upon deposition of only 4 A. of Ti (i.e., we assume no Ti 4+ in the interface region). At these temperatures, no significant bulk diffusion of O is observed. Initially, Ti reacts with O available in the near-surface region, probably through localized O migration. Starting after 4 A, of deposition, metallic Ti (zero valence state) begins to accumulate. Chemical reduction of an oxide substrate by a metallic overlayer has been demonstrated previously in a study of Cr/TiO2(ll0), Hf/TiO2(ll0) and Al/TiO2, where the chemical shifts of the substrate Ti x+ and the overlayer signals are well separated in XPS [15,35,36]. Cr and H f are highly
8
J.T. Mayer et aL/Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
12
0.8 ~
~ 8t "--¢'-- Ti+3 I ..... I
0.6 t-"
, 0.2
\
I
~
/¢"
/
1~,'---'-~'6~18 Log (Ion fluence)
\
~ |
0
--v
....... ~
.......
1,~
, 16
........ ,
......
, 18
Log (Ion fluence/cm2)
Fig. 9. The XPS data from Ar + sputtered TiO2 (see Fig. 4) are fitted to give the contribution from each of five oxidation states: Ti °, Ti 1+, Ti 2+, Ti 3+ and Ti 4+ as a function of ion fluence. The calculated overlayer thickness (reduced oxide) as a function of
ion fluenceis shown as an inset. reactive toward the oxide surface in a manner similar to Ti overlayers. The oxidation of these metals at the interface and concomitant reduction of TiO2 were observed. A plot similar to Fig. 8 is used in Fig. 9 to illustrate the surface reduction caused by Ar + ion bombardment. The ion fluence is varied through several orders of magnitude. The onset of reduction occurs at 2.7 x 1014 incident ions, much less than the surface number density of O atoms. This indicates the efficient sputtering of oxygen by heavy ion bombardment. It is well known that the sputter cross section for O is larger than that for Ti in this lattice [37]. Eventually, however, the surface number density of O decreases and that of Ti increases to a point where the scattering probabilities and the sputter cross sections are balanced, yielding a steady state surface concentration. This condition is reached after 2 x 1017 incident ions. Shown as an inset is the thickness of the sputter-reduced overlayer as a function of ion fluence. The calculated final thickness of 17 A is based only on the attenuation of the Ti 4+ peak.
5.2. Low energy ion scattering In the simplest case, the detected ion fraction in LEIS is proportional to the concentration of each species on the surface. This implies that no "matrix
effects" exist; i.e. that the He + neutralization probability during a binary collision with a surface atom is independent of its environment. For binary compounds this concept can be tested by observing a linear relationship between the scattering signals of the two surface constituents for various known surface stoichiometries [38]. For the work presented in this paper, a deviation from linearity between the O and Ti scattering signals is observed (based on Fig. 5). This could be due to a change in neutralization probability, as the electronic structure of the TiO2 surface undergoes drastic changes from an insulating state to a slightly reduced state, and finally to a conducting state. Both metallization and sputtering of the TiO2 surface populate states in the bandgap region. The redistribution of electronic energy levels may activate or deactivate ion neutralization channels [32,39] and it has been shown that resonant neutralization of He + into the He2s state is sensitive to work function changes [40,41]. It has also been demonstrated that the oxidation of Ti on a TiC(ll 1) surface has a profound effect on the yield of directly scattered and reionized He + [42]. These arguments would suggest that a change in neutralization probability occurs, as reflected in the non-complementary behavior of Ti and O scattering signals in Fig. 5. However, we cannot completely rule out the possibility that adsorption of water or hydrogen from the residual gas affects the scattering yield of observed ions. Fresh Ti surfaces are highly reactive and dissociative adsorption of water occurs on sputtered TiO2 [37]. The presence of H and OH on the surface would decrease observed scattering yields owing to blocking effects. In spite of these complications, our LEIS data clearly indicate that, after the first 4 ,& of deposition, additional Ti overlayers do not cover the reduced interface oxide, either due to clustering or to diffusion of O through the film during deposition [15]. It takes ~ 20/k of deposited Ti to attenuate fully the He + scattering from O and to form a continuous metallic (zero valent) Ti film.
5.3. Overlayer growth and morphology A comparative analysis of XPS and LEIS results
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
A
o-4AT,. Clean, annealed 4-20A Ti
B ~
f
Interface formation J
C >20ATi
Tioverlayer
Continuous
growth
Ti film
Fig. 10. Reactive adsorption is characterized by strong interaction between the overlayer and the substrate. Ti overlayer growth on TiO2 proceeds in three stages. A, 0-4/k Ti exposure: localized migration of oxygen in near surface region leads to a thick interface composed of intermediate Ti oxides; B, 4-20 /k Ti exposure: growth of Ti overlayer on interface oxide; C, > 20 /k Ti exposure: continuous Ti overlayer formation. can help to build a coherent picture of both the Ti/ TiO 2 system and the sputter reduced films. Both systems can be thought of as non-equilibrium overlayer structures. Given a source of O, Ti is more stable as an oxide, with TiO2 being the most stable oxide compound at atmospheric partial pressures of oxygen. It is also known that under UHV conditions and at sufficiently high temperature O will diffuse from a bulk TiO2 substrate and oxidize reduced surface Ti, with bulk defects coalescing as planar structures extending through the crystal. We have found that a 14 /~ Ti overlayer is completely oxidized to TiO2 by bulk migration of O when annealed to 800 K for 3 min. This is a kinetically limited process and diffusion over shorter distances at lower temperature is possible. Given the non-equilibrium and reactive nature of the Ti/TiO2 system, it is misleading to think in terms of conventional or equilibrium growth modes. Initial deposition of Ti on the TiO2 surface will lead to reaction at the interface between terminal O and overlayer Ti, consistent with a model in which the newly adsorbed Ti bonds to the surface O. We characterize this process as reactive adsorption, illustrated schematically in Fig. 10. A recent
9
comparison o f C u , Fe and Cr on TiO2(110) [43] has correlated the metal-oxygen bond strength with the overlayer growth behavior, concluding that a large heat of formation for the overlayer oxide (e.g. Cr203) leads to better "coverage" of the substrate. In other words, at least for the first 1-2 monolayers of incident metal atoms, the reaction of overlayer Fe or Cr with substrate oxygen leads to a thin, low surface energy overlayer rather than metal cluster formation on the oxide as "conventional" models predict. Extensive chemical change occurs upon initial Ti deposition; the formation of a 12 A interface oxide layer is completed in the first 4 .~ of deposition. Subsequent Ti exposure results in the accumulation of metallic titanium. The adsorption process at this point is very different and can result in (1) reactive adsorption by pulling more oxygen to the surface, (2) quasi-uniform metal layer growth by wetting the existing suboxide, or (3) clustering on top of the suboxide layer induced by strain and free energy effects. We observe that the reactivity between deposited Ti and the interface oxide is greatly reduced compared with the clean annealed substrate. The result is a rather inefficient coverage by Ti that requires 20/k of additional exposure to completely attenuate the O signal in LEIS. Once again, this is a kinetically limited process. Given a slightly higher deposition temperature, Ti could perhaps form much larger clusters on the interface oxide. At still higher temperatures, bulk diffusion of O will dominate, resulting in complete oxidation of the overlayer. Although we never reach an equilibrium Ti overlayer, we can form metastable structures stable for a limited time at a given temperature. The stability is such that it is probably inappropriate to speak of well defined TiOx phases; X-ray and/or other diffraction methods are needed to confirm the existence of such phases and their microcrystalline size and structure. The growth of the sputtered surface is difficult to model. XPS shows that at an ion fluence of 3 x 1014 cm -2 there is significant reduction of the surface Ti. Eventually, at a fluence of 4 x 1017 cm -2, the Ti 4+ photoemission intensity reaches 25% of the initial value. This attenuation corresponds to an overlayer thickness of 17 A. The
10
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
overlayer is composed of Ti 3+, Ti 2+, Ti 1+ and Ti ° species. The sequential onset of each reduced species may indicate that Ti 2+ is not stable in proximity to Ti 4+ at room temperature, but rather exhibits a tendency to form a gradient for the Ti oxidation state. There is not yet a consensus in the literature on the presence of Ti ° at high ion fluences. One prior study identified metallic Ti after an Ar + fluence of 4.5 x 1017 cm -2 at 500 eV [44]. Our data corroborate these results. However, in other experiments where the ion fluence is not quantified, the authors explicitly state that no evidence of zero valent Ti is found [45,46].
6. Summary Reactivity at the Ti/TiO2 interface makes its analysis both a very interesting and a complex problem. A curve fitting algorithm for XPS data using experimental standard spectra is proposed. Application of this algorithm shows that the deposition of 4 A of Ti results in the formation of an interface layer ~ 12 A thick. This interface between the TiO2 and thicker layers of metallic Ti is composed of reduced titanium oxides. The same chemical analysis can be applied to the reduced surface layer formed by Ar + sputtering of the annealed TiO 2 crystal. The near surface elemental composition reaches a steady state at an ion fluence of 4 x 1017 cm 2, at which point the reduced layer is 17/~ thick. At this ion fluence the presence of metallic Ti is observed. LEIS data for the Ti overlayer experiments clearly show that the interface oxide is not completely covered until > 20 A of Ti exposure. The initial deposition of Ti is characterized as reactive adsorption forming a progressively more reduced Ti suboxide layer; subsequent Ti exposure inefficiently covers the oxide surface.
References [1] [2] [3] [4] [5]
G.L. Hailer and D.E. Resasco, Adv. Catal., 36 (1989) 173. V. Henrich and R.L. Kurtz, Phys. Rev. B, 23 (1982) 6280. A. Fujishima and K. Honda, Nature, 238 (1972) 37. D.C. Cronemeyer, Phys. Rev., 87 (1952) 876. V.E. Henrich, Prog. Surf. Sci., 14 (1983) 175.
[6] L.A. Bursill and B.J. Hyde, Prog. Solid State Chem., 7 (1972) 177. [7] N.R. Armstrong and R.K. Quinn, Surf. Sci., 67 (1979) 451. [8] M.C. Asensio, M. Kerkar, D.P. Woodruff, A.V. de Carvalho, A. Fermindez, A.R. Gonz~lez-Elipe, M. Fern~ndezGarcia and J.C. Conesa, Surf. Sci., 273 (1992) 31. [9] U. Bardi, P.N. Ross and G. Rovida, Proc. Eur. Conf. Structure and Reactivity of Surfaces, Trieste, 1988. [10] A.F. Carley, P.R. Chalker, J.C. Riviere and M.W. Roberts, J. Chem. Soc., Faraday Trans. 1, 83 (1987) 351. [11] T. Choudhury, S.O. Saied, J.L. Sullivan and A.M. Abbot, J. Phys. D: Appl. Phys., 22 (1989) 1185. [12] D.J. Dwyer, S.D. Cameron and J. Gland, Surf. Sci., 159 (1985) 430. [13] W. G6pel, J.A. Anderson, D. Frankel, M. Jaehnig, K. Phillips, J.A. Sch~ifer and G. Rocker, Surf. Sci., 139 (1984) 333. [14] H. Idriss, K.S. Kim and M.A. Barteau, Surf. Sci., 262 (1992) 113. [15] J.-M. Pan, Thesis, Rutgers University, New Brunswick, NJ, 1993. [16] J. Paul, S.D. Cameron. D.J. Dwyer and F.M. Hoffmann, Surf. Sci., 77 (1986) 121. [17] A. Plateau, L.I. Johansson, A.L. Hagstr6m, S.-E. Karlsson and S.B.M. Hagstr6m, Surf. Sci., 63 (1977) 153. [18] L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman and C. Nordling, J. Phys. Chem. Solids, 30 (1969) 1835. [19] G. Rocker and W. G6pel, Surf. Sci., 181 (1987) 530. [20] N.C. Saha and H.G. Tompkins, J. Appl. Phys., 72 (1992) 3072. [21] C.N. Sayers and N.R. Armstrong, Surf. Sei., 77 (1978) 301. [22] D. Simon, C. Perrin, and J. Bardolle, C. R. Acad. Sci., Ser. C, 283 (1976) 299. [23] J.F. Moulder, W.F. Stickel, P.E. Sobol and K.D. Bomben, in J. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Norwalk, CT, 1992. [24] V.E. Henrich, G. Dresselhaus and H.J. Zeiger, J. Vac. Sci. Technol., 15 (1978) 534. [25] S. Eriksen and R.G. Egdell, Surf. Sci., 180 (1987) 263. [26] U. Bardi, K. Tamura, M. Owari and Y. Nihei, Appl. Surf. Sci., 32 (1988) 352. [27] D.L. Drobeek, A.V, Virkar and R.M. Cohen, J. Phys. Chem. Solids, 51 (1990) 977. [28] P.F. Dennis and R. Freer, J. Mater. Sci., 28 (1993) 4804. [29] G.I. Golodets, L.G. Svintsova, I.T. Chashechnikova, and V.V. Shimanovskaya, Kinet. Katal., 31 (1990) 997; Kinet. Catal. (Engl. Transl.), 31 (1990) 887. [30] A. Davidson and M. Che, J. Phys. Chem., 96 (1992) 9909. [31] H.J. H6fler, H. Hahn and R.S. Averback, Diffus. Defect Data, Part A, 75 (1991) 195. [32] H. Niehus, W. Heiland and E. Taglauer, Surf. Sci. Rep., 17 (1993) 213. [33] Code written in Asyst programming language. Keithley Instruments, Inc., Keithley Data Acquisition Division, Taunton, MA. [34] U. Diebold, J.-M. Pan and T.E. Madey, Phys. Rev. B, 47 (1993) 3868.
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11 [35] L. Zhang, U. Diebold and T.E. Madey, to be published. [36] L.S. Dake and R.J. Lad, Surf. Sci., 289 (1993) 297. [37] J.-M. Pan, B.L. Maschhoff, U. Diebold and T.E. Madey, J. Vac. Sci. Technol. A, 10 (1992) 2470. [38] L.C.A. van den Oetelaar, J.-P. Jacobs, M.J. Mietus, H.H. Brongersma, V.N. Semenov and V.G. Glebovsky, Appl. Surf. Sci., 70/71 (1993) 79. [39] R. Souda and M. Aono, Nucl. Instrum. Methods, Sect. B, B15 (1986) 114. [40] M.J. Ashwin and D.P. Woodruff, Surf. Sci., 244 (1991) 247. [41] R. Souda, T. Aizawa, C. Oshima, S. Otani and Y. Ishizawa, Phys. Rev. B, 41 (1990) 803.
11
[42] R. Souda, T. Aizawa, K. Miura, C. Oshima, S. Otani and Y. Ishizawa, Nucl. Instrum. Methods, Sect. B, B33 (1988) 374. [43] J.-M. Pan, U. Diebold, L. Zhang and T.E. Madey, Surf. Sci., 295 (1993) 411. [44] Q. Zhong, J.M. Vohs and D.A. Bonnell, Surf. Sci., 274 (1992) 35. [45] G.B. Hoflund, H.-L. Yin, A.L. Grogan Jr. and D.A. Asbury, Langmuir, 4 (1988) 346. [46] H. Idriss, K. Pierce and M.A. Barteau, J. Am. Chem. Soc., 113 (1991) 715.
ELSEVIER
Journal of Electron Spectroscopyand Related Phenomena 73 (1995) 1-11
Titanium and reduced titania overlayers on titanium dioxide(110) J . T . M a y e r a, U . D i e b o l d b, T . E . M a d e y ~, E. G a r f u n k e l a'* aDepartments of Chemistry and Laboratory for Surface Modification, Rutgers, the State University of New Jersey, Piscataway, NJ 08855-0939, USA bDepartment of Physics, Tulane University, New Orleans, LA 70118-5698, USA CDepartment of Physics and Laboratory for Surface Modification, Rutgers, the State University of New Jersey, Piscataway, NJ 08855-0939, USA
First received 22 July 1994; in final form 9 August 1994
Abstract
The adsorption of titanium on titanium dioxide TiO2(110) has been studied by X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). The XPS data for Ti overlayers are interpreted using peak fitting based on experimental standard spectra. 4 A_of Ti deposited at 150 K reacts with the substrate to produce ~ 12 A of intermediate oxidation state Ti. Adsorption of neutral metal begins on top of this interface oxide film, but 20 A of deposited Ti are needed to cover the oxide completely. LEIS data indicate a tendency for clustering of Ti on top of the interface oxide. Ar + sputtering of stoichiometric TiO2 leads to preferential loss of O from the near surface region. This reduction of the clean, annealed oxide surface by Ar + ion bombardment starts immediately and does not reach a steady state until 3 × 1017 ions cm -2, at which point the reduced overlayer is 17 A thick. " Keywords: Low energy ion scattering; TiO2(110); Titania; Titanium; X-ray photoelectron spectroscopy
I. Introduction
Titanium dioxide has received extensive attention in the surface science community, a result of both its technological importance [1-3] and its relative ease of preparation and examination. Perfect bulk TiO2 is a 3.1 eV bandgap insulator; however, annealing in ultra-high vacuum (UHV) creates bulk and surface n-type defects, oxygen vacancies, with a density of 10 ]8 to 1019 cm -3 [4]. A stoichiometric surface can be restored by annealing the U H V processed crystal in 02 a t 10 - 6 Torr. The relatively high defect (oxygen * Corresponding author.
vacancy) mobility in the TiO2 crystal makes this system different from many other important oxides such as MgO or A1203; any analysis of Ti overlayers must be made with this in mind. Electronically, TiO2 is neither a purely covalent nor an ionically bonded crystal but shows properties of both. It is probably best classified as containing highly polar covalent bonds. In addition to the rutile and anatase phases of TiO2, bulk oxides of lower Ti valence also exist (Ti20, TiO and Ti203). A model for the rutile(110) surface was first proposed by Henrich [5]. A neutral surface requires that half the surface Ti cations be covered with O; this leaves the surface with six-fold (as in the bulk) and five-fold coordinated Ti.
0368-2048/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02258-5
J.T. Mayer et al./Journal o f Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
Oxygen vacancies in the titania lattice cluster in the bulk to form crystallographic shear (CS) planes. A high density of CS planes can form ordered reduced phases, which are reviewed in the literature on higher titanium oxides (TiOx for 1.66 < x < 2.0) [6]. The formation of oxygen vacancies deep in the bulk is evidence for the relatively high vacancy mobility in the TiO2 rutile crystal. Several of the applications of TiO2 thin films (e.g. as sensors) depend on the conductivity change of TiO2 as the stoichiometry of the surface or the concentration of surface adsorbates is changed slightly. There have been many attempts to analyze suboxide (chemically reduced) TiO2_x surfaces using X-ray photoelectron spectroscopy (XPS) [7-23]. This task is complicated by the number of stable intermediate oxidation states for Ti in the Ti/O system. Currently there is confusion and disagreement about the treatment of the XPS data. As a result, we still lack a good understanding of several key aspects of this system, including the thickness of the suboxide overlayer and its composition. One purpose of this paper is to review published analyses of Ti core-level XPS data and to propose a peak fitting routine based on experimental standard spectra. We have applied XPS and low energy ion scattering (LEIS) with the aim of understanding the near surface structure and composition of stoichiometric TiO2 dosed with ultrathin films of Ti and of stoichiometric TiO2 sputtered with Ar ÷ ions. The XPS data are used for compositional information and charge state identification in the near surface regime (~ 0-20 A). Application of our peak fitting technique identifies a reduced layer 12 A thick that is composed of Ti in a distribution of oxidation states. LEIS gives the relative Ti and O concentrations in the outermost surface layer. LEIS data as a function of Ti overlayer thickness show that, after the formation of the reduced oxide interface, the metallic Ti film does not grow uniformly. Approximately 20 A of Ti deposition is required before the oxide is completely covered.
2. Experimental All experiments are performed in a UHV
chamber with a base pressure of 6 x 10-11 Torr. The peak pressure is 2 × 10-1° Torr during Ti evaporation and 1 x 10-9 Torr during sample annealing. Rutile TiO2(I10) crystals from Commercial Crystal Laboratories are cleaned with methanol and mounted on a sample holder. Each crystal is held in tantalum foil supported by tungsten wire. The crystals are heated by electron bombardment and cooled with liquid nitrogen. We find that uniform heating is very important to avoid fracture of the crystals. To accomplish this, the heater filament is roughly the same size as the l cm 2 sample and is in close proximity to the back of the crystal. A chromel-constantan (type-E) thermocouple is spot welded to the Ta foil holding the TiO2 crystal. After insersion of a fresh crystal into the vacuum chamber, it is sputtered with 500 eV Ar-- ions at an impact angle 20 ° off normal for 10 min at 5 #A. This is followed by annealing to 1100 K for 10 min. This thermal processing forms enough oxygen vacancies (bulk and surface defects) to turn the initially transparent and colorless sample blue. The surface order is monitored by low energy electron diffraction (LEED). Ti is deposited from a water cooled source by heating a W filament wrapped with Ti wire. The overlayer thickness is measured in situ with a quartz crystal oscillator calibrated by Rutherford backscattering spectroscopy (RBS). The Ti exposure is expressed as a thickness as measured by the quartz crystal monitor. The Ti thickness corresponds to the amount of pure Ti added, independent of the actual overlayer morphology or chemical composition. Therefore, a 10/~ Ti overlayer corresponds to a dosage of 5.5 x l0 Is atoms per cm2 (based on the bulk density of Ti metal), rather than a uniform film that is 10 A thick. XPS is performed using A1 K a radiation and a hemispherical analyzer mounted near normal to the surface and operating in the fixed analyzer transmission mode. The low energy ion scattering (LEIS) experiments are performed with the same hemispherical analyzer operating in the fixed retarding ratio mode and opposite polarity with a scattering angle of 144°. The He + ion current to the sample is ~ 100 nA at 1000 eV, with spectral acquisition time of 400 s. Deposition of Ti, and
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
3
(---615eV--~
Io_n_nFluence
_J (1) ¢¢1_
J
I
2.3A
Clean TiO2 !
450
I
455
450
II
460
465
Binding Energy (eV) Fig, 1. XPS spectra of the Ti2p region for different Ti overlayer thicknesses. The spin-orbit splittings for clean TiO 2 (bottom curve) and metallic Ti (top curve) are indicated, as is the chemical shift of the Ti2p3/2 peak.
all subsequent spectroscopy, is performed at a sample temperature of ~ 150 K. Each Ti overlayer experiment that involves multiple sequences of deposition followed by XPS and LEIS is also checked by discrete experiments that start with a clean, annealed surface.
3. Results
In Fig. 1 we present the X-ray photoelectron spectra in the region of the Ti2p peaks for increasing Ti exposure on TiO2. The Ti° state of the metallic thick film in the upper curve is shifted by 5.2 eV to lower binding energy relative to the Ti4+ state of the clean TiO2 surface spectrum. Titanium in intermediate oxidation states can be observed in the first few A of deposition (see the 2.3 A spectrum). The 22 A and 43 /~ (not shown) spectra look almost identical and show little trace of the oxide substrate; this indicates the formation of a continuous metallic Ti (zero valent) overlayer. The Ar + bombardment of transition metal oxides is known to sputter oxygen preferentially until an oxygen deficient surface is obtained under steady state conditions [24]. The Ti2p XPS data in Fig. 2 show the effect of Ar + sputter damage on the stoichiometric TiO2 surface at
I
I
460
470
Binding Energy (eV) Fig. 2. XPS spectra of the Ti2p region after Ar + ion bombardment. Ion fluences are given in ions cm-2
room temperature. Broadening on the lower binding energy side is due to the reduction of surface Ti4+ species to intermediate oxidation states. This correlates well with previous work by other authors [13,25,26]. Oxygen diffusion via defect migration is well known for reduced TiO2 oxides [27-29]. We are able to identify the onset of near surface diffusion necessary to oxidize Ti overlayers on the nearly stoichiometric TiO2 crystal. Fig. 3 shows the chemical change of 14 A of Ti deposited on TiO2 as determined by XPS and analyzed using the peak fitting routine discussed below. The sample, initially at 150 K, is annealed for 3 min periods at
I
t-
+
•
!
Ti
TiO2 ]
r-
0
lO0
I
I
I
300
I
500
I
I
700
900
Temperature (K) Fig. 3. Contributions of metallic Ti and Ti 4+ from a 14 A thick Ti overlayer on TiO2(110) are plotted as a function of temperature. Ti is deposited at 150 K. The sample is annealed for 3 min at each temperature.
4
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1 11
30 E20 "-'I
o
¢-
/
3
~2 300
400
500 600 Energy (eV)
700
800
Fig. 4. LEIS of 1 keV He+ ions on TiO2 with various Ti coverages.
X
0
1 0
~
0
-
10
20
-
30
1_
40
Ti thickness (A)
temperatures increasing in intervals of 50 K. The onset temperature for oxidation of the Ti overlayer by oxygen migration from the TiO2 crystal is 800K. This corresponds favorably to results indicating the onset of bulk cation and cationic vacancy mobility at 865 K [30]. This diffusion mechanism is differentiated from large scale mass transport associated with sintering, which occurs at a higher temperature (973 K) [31]. In Fig. 4 we show LEIS spectra for various overlayer coverages of Ti on TiO2. LEIS is extremely surface sensitive. A large shadow cone radius for ions of this energy means that He + does not directly scatter off second layer atoms, and the high neutralization rate of He + makes detection of double scattering events improbable [32]. Several interesting features can be noted by examining the O (375 eV) and Ti (700 eV) peaks as a function of overlayer exposure, as presented in Fig. 5. As seen from the XPS results considered above, chemical reduction of the oxide surface begins immediately upon Ti deposition. This is seen in LEIS as an attenuation in the O signal intensity relative to the stoichiometric surface. However, Ti peak growth and O attenuation do not reach limiting values until the deposit is 20 A thick. The O scattering intensity is almost completely attenuated by 20 A Ti dosage. The Ti scattering intensity does not mirror the O attenuation; rather, it grows linearly before leveling out at ~ 20/~. The difference between the exponential decrease in O
Fig. 5. LEIS peak areas from Ti overlayer experiments are plotted as a function of Ti dosage. Smooth lines are drawn through the data points. intensity and the linear increase in Ti intensity is discussed below.
4. X P S curve fitting
Our data indicate that deposition of Ti on TiO2 leads to an interfacial layer between the oxide and the metallic overlayer. The interface is amixture of reduced oxide phases. Ar + ion bombardment of the TiO2 surface also forms a chemically reduced surface layer through the preferential sputtering of oxygen. In both cases the changed chemical nature of the reduced oxide is seen in XPS by the shifting of the Ti2p peaks as the Ti valency is reduced from a + 4 charge state. Ti atoms in this reduced oxide have a valency between 0 and + 4, as is seen in the core level photoemission peak position. Quantitative XPS analysis of this system presents several problems. The form or shape of the Ti2p XPS curve varies from a broadened Ti 4+ doublet at Ti overlayers of a fractional monolayer or lightly sputtered surfaces to a near continuum between the Ti 4+ and the Ti ° peaks for 3-12 A Ti overlayers or the more heavily sputtered surfaces. Distinct peaks due to Ti in intermediate oxidation states are usually not observed. Two approaches to
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11 , 1
Ti02 "\
~1600
o
~
1200
E 800
400 0
0.4
0.8
1.2
1.6
2.0
O/Ti ratio
Fig. 6. The main elements of the Ti/O phase diagram are presented with some details omitted for clarity, a and b refer to the hexagonal and cubic metal phases of Ti respectively;It and ht refer to low and high temperature phases respectively.Adapted from P.G. Wahlbeck and P.W. Gilles, J. Am. Ceram. Soc., 49 (1966) 180. curve fitting are detailed in the existing literature dealing with TiO2 [10,19]. Traditionally, XPS data are fitted to a convolution of analytical functions after subtraction of a suitable secondary electron background. Adjustable parameters for the fit are peak position, width and intensity and the number of peaks. Several parameters may be constrained based on a priori information. Numerical optimization of the initial parameters results in the peak energy and intensity for all the contributing species, including the intermediate oxidation states of Ti. An alternate approach may also be used. Since the XPS peak shape of the clean, annealed oxide is known (most studies start with this), the Ti 4+ contribution to the reduced oxide XPS spectrum can be subtracted. This leaves the contribution of intermediate oxidation states. In special cases, discrete peaks in the remaining curve can be associated with individual oxidation states, and therefore the fit of this curve to Gaussian broadened Lorentzians is simplified [10]. In practice, both of the above methods are inadequate for this system. First, an optimized fit for the reduced oxide usually does not converge to unique values because of the large number of adjustable parameters. Second, in many cases the XPS signal due to the intermediate oxides (Ti 4+ subtracted) cannot be visually resolved into
5
discrete peaks, and therefore suffers from the same problem of non-convergence. Third, the peak shapes of TiO2 and metallic Ti are distinctly different (see Fig. 1), necessitating different fitting functions. To date, the peak shapes and positions of the intermediate oxides remain undetermined. Nonetheless, there is an extensive body of work on resolving the intermediate oxides of TiO 2 into separate oxidation states using the above two methods to interpret XPS data. Table 1 compares the literature results for the absolute energy Ti 4+, and the shift of the other oxidation states relative to Ti 4+. Fitting methodologies and descriptive comments are also noted. There is good agreement on the chemical shift between metallic Ti and stoichiometric TiO 2, 5.2 eV. The only significant deviation from this value comes from the PerkinElmer XPS Handbook, which reports 4.7 eV [23]. Since Ti and TiO2 samples are easily obtained, there is little experimental difficulty in doing the spectroscopy, although discrepancies may arise from band bending or from charging of the oxide sample. The intermediate oxides are available as stable bulk compounds; however, the surface is generally oxidized to TiO2. The usual method of sputtering to clean the surface will leave a mixture of oxidation states, as shown in this paper. Therefore literature values for these oxides are more scattered than for Ti and TiO2. There is not enough independent information to fit the XPS data of reduced TiO2 unambiguously. However, curve fitting can still be quite useful if it illustrates general trends without artifacts or distortions induced by the method applied. Our peak fitting technique is based on the following three premises: (1) The oxygen-titanium phase diagram (Fig. 6) shows regions with stable Ti20, TiO and Ti203 phases in addition to TiO2 and pure Ti. The reduced surface oxide does not show long-range order and may in fact have no distinct and segregated phase structure, but the bulk stability of the above oxides implies the existence of titanium in at least the + 1, + 2, and + 3 intermediate oxidation states. Therefore, we fit our XPS data to five oxidation states for Ti (0 through + 4). (2) The five oxidation states of Ti are fitted to experimentally determined lineshapes for Ti and
6
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
Table 1 Comparison of XPS peak fitting results and methodologies for the Ti2p data of reduced titania oxides ATi
ATi1+
-5.5 -5 -5.1
-4.1
ATi2+
ATi3+
Ti4+ (eV)
Authors
-3.6 -2.9 -3.5 -3.5 -3.4
-2.1 - 1.6 -2.1 - 1.5 -1.5 -2.0 -1.7 - 1.5 -1.8 -2.5
461.8 457.9 459.0 459.0 458.7 458.6 458.5 459.1 457.0 459.4 459.8 458.7 455.5 459.1 458.6 458.8 458.8 459.3
Armstrong [7] Asensio [8] Bardi [9] Carley [10] Choudhury [11] Dwyer [12] G6pel [13] Idriss [14] Pan [15] Paul [16] Plateau [17] Ramqvist [18] Rocker [19] Saha [20] Sayers [21] Simon [22] Handbook of XPS [23] This paper
-3.1 -3.0 -3.6
-5.1
-4.2
-3.8 -4.9 -4.9 -5.2
-4.4
-3.9 -3.3
-3.9
-3.5 -3.5 -3.8 -2.6
-5.5 -5.4 -4.7 -5.2
- 1.9 -2.3 - 1.9
- 1.3
Notes
d 1 2 d 1,2,a 5,c 1,3 d
2,4 1,a
3,6
Methods are referenced as follows: 1. Subtraction of known data (such as Ti4+ data from the stoichiometric surface). 2. Fit optimized Gaussian curves. 3. Fit internal standard spectra (such as chemically shifted Ti4+ spectrum). 4. Fit Doniach-Sunjic function (accounts for non-symmetrical peaks). 5. Methodology not stated; some references to other authors. 6. Even spacing of oxidation states; initial state approximation. Comments are referenced as follows: a. Does not assign peaks to discrete oxidation states. b. Some fitting parameters taken from referenced sources. c. TiO2/Pt. d. Oxidation of Pt3Ti sample. TiO2 in a 40 eV region that includes the 2p3/2 a n d 2pl/2 peaks of all o x i d a t i o n states. N o t e that this fitting procedure does n o t necessitate a n y b a c k g r o u n d s u b t r a c t i o n a p p r o x i m a t i o n . The use of i n t e r n a l s t a n d a r d spectra reduces the n u m b e r o f fitting parameters a n d improves the accuracy a n d confidence level of the resultant fit. The TiO2 substrate a n d the Ti overlayer are fitted directly using the i n t e r n a l s t a n d a r d spectra, a n d intermediate o x i d a t i o n states are fitted using the chemically shifted metallic Ti w a v e f o r m spaced evenly (at 1.3 eV intervals) between the energy o f Ti a n d Ti 4+. The even spacing o f the intermediate oxidation states is a n a p p r o x i m a t i o n based o n the different effective charge s u r r o u n d i n g Ti nuclei (an initial state effect), a l t h o u g h it clearly ignores final state a n d other complicating effects. The shape a n d
position of XPS peaks from the reduced oxide are a p p r o x i m a t i o n s necessary to reduce the n u m b e r o f u n c o n s t r a i n e d fitting parameters. W e have n o a priori reason to believe that these fitting f u n c t i o n s represent true chemical shifts a n d peak shapes. However, the c o n s t r a i n t of these parameters leads to results that are at least systematic, can be used to illustrate trends, a n d in some cases can be used quantitatively. (3) T o allow for changes in the core levels with respect to the F e r m i level ( b a n d b e n d i n g ) [19], the fitting functions can be shifted as a group (maintaining c o n s t a n t relative spacing). The shift is always less t h a n 0.4 eV a n d can readily be checked, as errors o f even 0.1 eV are easily observed. The fitting is optimized by the G a u s s - N e w t o n m e t h o d i m p l e m e n t e d by a p r o g r a m written in
11
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
, ,
/_~1~
jt,
-
:
' \~
__
m -........ --
Data Fit Ti Ti+l
--
7!+3
7
0.8
~;~ 0.6
~'
°0"!i
+1 + 2 ~
0.
.m c/) t-
_c
0
10 20 3() 40 Thickness of deposited Ti (,&,)
Fig. 8. The substrate oxide, interface oxide and metal overlayer intensities, as obtained from curve fitting, are plotted as a function of Ti exposure. The calculated overlayer thickness (intermediate oxide plus metal overlayer) as a function of deposited Ti is shown as an inset. 455
460
465
470
Binding Energy (eV) Fig. 7. Fitted XPS data from (a) 0.3 •, (b) 1.5 A and (c) 5.4 A of Ti on TiO2. Each spectrum is fitted using contributions from metallic Ti, the oxide substrate Ti4+, and Ti 3+, Ti2+, Tit+ from the interfaceregion. house [33]. Examples of three specific Ti overlayer thicknesses are shown in Fig. 7. Five oxidation states for Ti are used for each curve fit; for the 0.3 A example, however, the Ti l+ and Ti ° curves are not shown, as their contribution is small. A Shirley-type background is subtracted only to display the results more clearly, and was not used in the fitting. We obtained satisfactory curve fits for slightly reduced (light sputtering or thin Ti overlayers) or for heavily metallized surfaces; the fit is not quite as good for heavily reduced surfaces (e.g. heavy sputtering) where a large contribution of intermediate oxide species is present.
5. Discussion
5.1. X-ray photoelectron spectroscopy The contributions from substrate, interface, and
metallic overlayer are separated using the peak fitting routine described above. The results for increasing Ti coverage are plotted in Fig. 8. The important points are (1) the formation of zero valent Ti starting after the first 4 A, of deposition, and (2) the attenuation of the + 4 valent state (the TiO2 substrate) much faster than would be expected if it were simply shielded by the deposited Ti. The mean free path of Ti2p photoelectrons through the oxide is ~ 12 fi~ [34]. But after only 4 A, of Ti deposition, the intensity of photoemission from the substrate is reduced by 1/e from the initial value before any Ti deposition. There is thus ~ 12 A, of interface which is composed of intermediate Ti oxides formed upon deposition of only 4 A. of Ti (i.e., we assume no Ti 4+ in the interface region). At these temperatures, no significant bulk diffusion of O is observed. Initially, Ti reacts with O available in the near-surface region, probably through localized O migration. Starting after 4 A, of deposition, metallic Ti (zero valence state) begins to accumulate. Chemical reduction of an oxide substrate by a metallic overlayer has been demonstrated previously in a study of Cr/TiO2(ll0), Hf/TiO2(ll0) and Al/TiO2, where the chemical shifts of the substrate Ti x+ and the overlayer signals are well separated in XPS [15,35,36]. Cr and H f are highly
8
J.T. Mayer et aL/Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
12
0.8 ~
~ 8t "--¢'-- Ti+3 I ..... I
0.6 t-"
, 0.2
\
I
~
/¢"
/
1~,'---'-~'6~18 Log (Ion fluence)
\
~ |
0
--v
....... ~
.......
1,~
, 16
........ ,
......
, 18
Log (Ion fluence/cm2)
Fig. 9. The XPS data from Ar + sputtered TiO2 (see Fig. 4) are fitted to give the contribution from each of five oxidation states: Ti °, Ti 1+, Ti 2+, Ti 3+ and Ti 4+ as a function of ion fluence. The calculated overlayer thickness (reduced oxide) as a function of
ion fluenceis shown as an inset. reactive toward the oxide surface in a manner similar to Ti overlayers. The oxidation of these metals at the interface and concomitant reduction of TiO2 were observed. A plot similar to Fig. 8 is used in Fig. 9 to illustrate the surface reduction caused by Ar + ion bombardment. The ion fluence is varied through several orders of magnitude. The onset of reduction occurs at 2.7 x 1014 incident ions, much less than the surface number density of O atoms. This indicates the efficient sputtering of oxygen by heavy ion bombardment. It is well known that the sputter cross section for O is larger than that for Ti in this lattice [37]. Eventually, however, the surface number density of O decreases and that of Ti increases to a point where the scattering probabilities and the sputter cross sections are balanced, yielding a steady state surface concentration. This condition is reached after 2 x 1017 incident ions. Shown as an inset is the thickness of the sputter-reduced overlayer as a function of ion fluence. The calculated final thickness of 17 A is based only on the attenuation of the Ti 4+ peak.
5.2. Low energy ion scattering In the simplest case, the detected ion fraction in LEIS is proportional to the concentration of each species on the surface. This implies that no "matrix
effects" exist; i.e. that the He + neutralization probability during a binary collision with a surface atom is independent of its environment. For binary compounds this concept can be tested by observing a linear relationship between the scattering signals of the two surface constituents for various known surface stoichiometries [38]. For the work presented in this paper, a deviation from linearity between the O and Ti scattering signals is observed (based on Fig. 5). This could be due to a change in neutralization probability, as the electronic structure of the TiO2 surface undergoes drastic changes from an insulating state to a slightly reduced state, and finally to a conducting state. Both metallization and sputtering of the TiO2 surface populate states in the bandgap region. The redistribution of electronic energy levels may activate or deactivate ion neutralization channels [32,39] and it has been shown that resonant neutralization of He + into the He2s state is sensitive to work function changes [40,41]. It has also been demonstrated that the oxidation of Ti on a TiC(ll 1) surface has a profound effect on the yield of directly scattered and reionized He + [42]. These arguments would suggest that a change in neutralization probability occurs, as reflected in the non-complementary behavior of Ti and O scattering signals in Fig. 5. However, we cannot completely rule out the possibility that adsorption of water or hydrogen from the residual gas affects the scattering yield of observed ions. Fresh Ti surfaces are highly reactive and dissociative adsorption of water occurs on sputtered TiO2 [37]. The presence of H and OH on the surface would decrease observed scattering yields owing to blocking effects. In spite of these complications, our LEIS data clearly indicate that, after the first 4 ,& of deposition, additional Ti overlayers do not cover the reduced interface oxide, either due to clustering or to diffusion of O through the film during deposition [15]. It takes ~ 20/k of deposited Ti to attenuate fully the He + scattering from O and to form a continuous metallic (zero valent) Ti film.
5.3. Overlayer growth and morphology A comparative analysis of XPS and LEIS results
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
A
o-4AT,. Clean, annealed 4-20A Ti
B ~
f
Interface formation J
C >20ATi
Tioverlayer
Continuous
growth
Ti film
Fig. 10. Reactive adsorption is characterized by strong interaction between the overlayer and the substrate. Ti overlayer growth on TiO2 proceeds in three stages. A, 0-4/k Ti exposure: localized migration of oxygen in near surface region leads to a thick interface composed of intermediate Ti oxides; B, 4-20 /k Ti exposure: growth of Ti overlayer on interface oxide; C, > 20 /k Ti exposure: continuous Ti overlayer formation. can help to build a coherent picture of both the Ti/ TiO 2 system and the sputter reduced films. Both systems can be thought of as non-equilibrium overlayer structures. Given a source of O, Ti is more stable as an oxide, with TiO2 being the most stable oxide compound at atmospheric partial pressures of oxygen. It is also known that under UHV conditions and at sufficiently high temperature O will diffuse from a bulk TiO2 substrate and oxidize reduced surface Ti, with bulk defects coalescing as planar structures extending through the crystal. We have found that a 14 /~ Ti overlayer is completely oxidized to TiO2 by bulk migration of O when annealed to 800 K for 3 min. This is a kinetically limited process and diffusion over shorter distances at lower temperature is possible. Given the non-equilibrium and reactive nature of the Ti/TiO2 system, it is misleading to think in terms of conventional or equilibrium growth modes. Initial deposition of Ti on the TiO2 surface will lead to reaction at the interface between terminal O and overlayer Ti, consistent with a model in which the newly adsorbed Ti bonds to the surface O. We characterize this process as reactive adsorption, illustrated schematically in Fig. 10. A recent
9
comparison o f C u , Fe and Cr on TiO2(110) [43] has correlated the metal-oxygen bond strength with the overlayer growth behavior, concluding that a large heat of formation for the overlayer oxide (e.g. Cr203) leads to better "coverage" of the substrate. In other words, at least for the first 1-2 monolayers of incident metal atoms, the reaction of overlayer Fe or Cr with substrate oxygen leads to a thin, low surface energy overlayer rather than metal cluster formation on the oxide as "conventional" models predict. Extensive chemical change occurs upon initial Ti deposition; the formation of a 12 A interface oxide layer is completed in the first 4 .~ of deposition. Subsequent Ti exposure results in the accumulation of metallic titanium. The adsorption process at this point is very different and can result in (1) reactive adsorption by pulling more oxygen to the surface, (2) quasi-uniform metal layer growth by wetting the existing suboxide, or (3) clustering on top of the suboxide layer induced by strain and free energy effects. We observe that the reactivity between deposited Ti and the interface oxide is greatly reduced compared with the clean annealed substrate. The result is a rather inefficient coverage by Ti that requires 20/k of additional exposure to completely attenuate the O signal in LEIS. Once again, this is a kinetically limited process. Given a slightly higher deposition temperature, Ti could perhaps form much larger clusters on the interface oxide. At still higher temperatures, bulk diffusion of O will dominate, resulting in complete oxidation of the overlayer. Although we never reach an equilibrium Ti overlayer, we can form metastable structures stable for a limited time at a given temperature. The stability is such that it is probably inappropriate to speak of well defined TiOx phases; X-ray and/or other diffraction methods are needed to confirm the existence of such phases and their microcrystalline size and structure. The growth of the sputtered surface is difficult to model. XPS shows that at an ion fluence of 3 x 1014 cm -2 there is significant reduction of the surface Ti. Eventually, at a fluence of 4 x 1017 cm -2, the Ti 4+ photoemission intensity reaches 25% of the initial value. This attenuation corresponds to an overlayer thickness of 17 A. The
10
J.T. Mayer et al./Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 1-11
overlayer is composed of Ti 3+, Ti 2+, Ti 1+ and Ti ° species. The sequential onset of each reduced species may indicate that Ti 2+ is not stable in proximity to Ti 4+ at room temperature, but rather exhibits a tendency to form a gradient for the Ti oxidation state. There is not yet a consensus in the literature on the presence of Ti ° at high ion fluences. One prior study identified metallic Ti after an Ar + fluence of 4.5 x 1017 cm -2 at 500 eV [44]. Our data corroborate these results. However, in other experiments where the ion fluence is not quantified, the authors explicitly state that no evidence of zero valent Ti is found [45,46].
6. Summary Reactivity at the Ti/TiO2 interface makes its analysis both a very interesting and a complex problem. A curve fitting algorithm for XPS data using experimental standard spectra is proposed. Application of this algorithm shows that the deposition of 4 A of Ti results in the formation of an interface layer ~ 12 A thick. This interface between the TiO2 and thicker layers of metallic Ti is composed of reduced titanium oxides. The same chemical analysis can be applied to the reduced surface layer formed by Ar + sputtering of the annealed TiO 2 crystal. The near surface elemental composition reaches a steady state at an ion fluence of 4 x 1017 cm 2, at which point the reduced layer is 17/~ thick. At this ion fluence the presence of metallic Ti is observed. LEIS data for the Ti overlayer experiments clearly show that the interface oxide is not completely covered until > 20 A of Ti exposure. The initial deposition of Ti is characterized as reactive adsorption forming a progressively more reduced Ti suboxide layer; subsequent Ti exposure inefficiently covers the oxide surface.
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