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AES study of electron beam induced damage on TiO2 surfaces
ELZVIER
Applied Surface Science 99 ( 1996) 133- 143
AES study of electron beam induced damage on TiO, surfaces Ad J.M. Mens, Onno L.J. Gijzeman Debye Institute. Surjace Science Diuision, Utrecht Uniaersity, Padualaan 8, 3584 CH Utrecht, The Netherlands Received 9 August 1995; accepted
IO December
1995
Abstract The damage induced by an electron beam on several TiO, surfaces has been studied with Auger Electron Spectroscopy. We studied single crystal specimens of TiO, (rutile) of the (100) and (110) orientation as well as pressed powder specimens consisting of rutile and a 65%-35% mixture of anatase and rutile. All surfaces acquired a (sometimes extremely high) negative charge upon electron irradiation. All surfaces did loose a significant amount of oxygen due to electron stimulated desorption. In the presence of background oxygen this process did not occur. Carbon (graphite) was deposited on the samples from residual gases. This is due to the decomposition of adsorbed carbonaceous species on the damaged surface and to the decomposition of positive gas phase ions on the negatively charged (irradiated) region of the surface. Significantly less carbon was deposited on the powder samples as compared to the single crystals. Carbon deposition is inhibited by the presence of oxygen in gas phase.
1. Introduction Titanium dioxide is a compound that is widely used as a support for catalytic materials [l]. Probably part of its success is due to its non-stoichiometry, the valence of the Ti4+ ions can be reduced rather easily to a lower oxidation state. This is accompanied by a loss of oxygen from the surface. Surface defects can be introduced deliberately by ion bombardment [2,3], thermal treatments [2,4] or by electron beam irradiation [5-71. Electron stimulated desorption of oxygen has been demonstrated for high beam currents, lo-50 A/cm* [7], and beam currents as low as 10 mA/cm* [5] down to 1.2 PA/cm2 [8]. These defects lead to pronounced differences in the electronic structure of the material for both the rutile [9] and anatase [lo] crystal structure and also influence their adsorptive properties [ 1 I]. 0169-4332/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00603-6
Titanium dioxide is also used as an insulating material in several technical applications such as cathode ray tubes. In this case, as in the case of catalysis, poly-crystalline material is used. Degradation of the insulating material by electrons may occur as well as changes in the electrical properties due to the creation of electron induced defects, which increase the conductivity. For applications as an insulator a low electrical conductivity is of course required. In this paper we report on the electron induced loss of oxygen from rutile single crystals with the (100) and (110) orientation. Although these crystals can be prepared with a high conductivity, we have deliberately chosen to prepare badly conducting, but stoichiometric, specimens. We did this in order to compare the results to less well defined, but equally badly conducting, specimens obtained by pressing
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commercial TiOz into suitable samples. These samples were either pure i-utile or a 65%/35% mixture of anatase and rutile.
2. Experimental The experiments were carried out in a Fisons XPS/AES system (MT 500) with facilities for XPS and AES, using a CLAM 2 hemispherical analyser for electron detection. The electron gun (LEG 62) could be focused according to the manufacturer specification to a spot size of down to 3 pm. This ensured a high current density on the target. Using a measured value of up to 2 PA for the target to earth current the estimated current density becomes of the order of 10 A/cm’, or around 6 X lOI electrons cm-’ SK’. Due to secondary electron emission this value must be considered a lower limit. The base pressure in the main chamber was lo- lo mbar, but rose during the actual experiments to 1O-8 mbar when these high current densities were used. At a lower current density (by decreasing the emission current from the electron gun) the pressure remained in the lo- lo mbar region. The electron current from the gun was used both as the source for electron induced damage and to obtain Auger spectra as well from the same irradiated spot on the sample. In all experiments described here the electron gun was switched on with a fixed emission current (as measured on the electron gun power supply) and kept on while data acquisition took place at certain time intervals. This procedure ensures that the continuously irradiated area on tbe sample is in fact investigated and that slight displacements of the electron beam due to the switching on and off of the power supply are avoided. The single crystal samples, consisting of two TiO, single crystals with (110) and (100) orientation with dimension of approximately 1 X 1 cm2 and thickness of 2 mm, were mounted on a stainless steel sample holder. These crystals had the rutile structure. A sample of a more commercial type of titanium dioxide was also used in the form of a TiO, powder (Degussa 7702, particle diameter 100 nm). Tbe powder was pressed into a stainless steel disk shaped sample holder for easy measurement. This compound consists of approximately 65% anatase and 35%
Surfhce Science 99 (1996) 133-143
t-utile as verified by X-ray diffraction. This sample will be referred to as TiO,(anatase). A second sample of TiO, powder was prepared by heating the pressed pellet to 900°C in flowing air for 24 h. After this treatment the sample consists exclusively of rutile, as shown by X-ray diffraction. This sample will be referred to as TiO,(rutile). All samples could be transferred between a the main chamber and a sample preparation chamber by means of a linear transfer device and several wobble sticks. Sample preparation and cleaning could be done in this separate chamber, attached to the main vessel, with facilities for argon ion sputtering, sample heating (25 to 600°C) and gas exposures of up to 1 atmosphere. After initial removal of impurities on the specimens by mild sputtering (2000 eV and 50 PA at an angle of 30”) samples were routinely treated with either 1 atmosphere of flowing oxygen at 500°C or 0.1 mbar oxygen at 500°C during 15 minutes before an experiment. XPS spectra showed that this treatment was sufficient to remove all carbon impurities on the surface, which was otherwise clean. Tbe samples prepared in this way were always bad conductors, as can be expected, but do have a stoichiometric surface and show reproducible results. Auger spectra were taken in the direct, i.e. N(E) versus E mode and shown here in the ‘as recorded’ state without background subtraction and/or data smoothing. Due to the charging of the samples only single scan spectra could be obtained, which resulted in a sometimes poor signal to noise ratio.
3. Results and discussion
3.1. Surface charging A severe problem with the measurement of Auger spectra from the TiO, samples is, as could be expected, their poor electrical conductivity. Fig. 1 shows the G(KLL) spectra from a TiO,(l 10) sample taken in single scan at approximately 5 minutes intervals at a beam energy of 2 keV, while keeping the electron gun switched on all the time. A negative charging of the analysed region of up to 20 volts can be seen clearly. This charge was only present in the area irradiated by the electron beam as displacing the crystal slightly from its original position caused the
A.J.M. Mens. O.L.J. Gijzeman/Applied
103 c/s
t
12
TlO,( 110) 45 3
Fig. 1. Consecutive oxygen Auger spectra of a TiOJl 10) surface taken with a low beam current and beam energy of 2 keV. Spectra were taken at 5 min intervals and took about 1 min to record. During the intervals the sample remained exposed to the electron beam.
peak to be restored to its original value, after which charge built-up took place again. The pressure in the chamber during measurements was below 10m9 Tot-r. The rate of charge built-up was dependent on the incoming electron flux. Fig. 2 shows the position of the oxygen KLL and titanium LMM Auger peaks, normally located at 5 10 and 380 eV respectively as a function of time. After each experiment (with a constant beam energy of 2 keV) data collection was stopped for five minutes while keeping the electron gun switched on. At certain points, indicated by the arrows in Fig. 2, the emission current of the gun was increased and a further set of data collected. The rate of charging increases with increasing electron flux. Also the oxygen and titanium peaks move simultaneously, keeping their separation constant during charging of the sample. This indicates that the negative charge is distributed rather homogeneously in the irradiated region, which is also corroborated by the fact that the Auger peaks showed no noticeable broadening in the charged state. The amount of charge on the specimen could be decreased at a given beam current by admitting any foreign gas into the chamber. Since in this case the incoming electron beam will cause ionisation of gas phase molecules, the positive ions formed will be attracted towards the negatively charged region and neutralise the negative charge present there, a process that occurs for instance in any ionisation gauge. The effect is unlikely to be caused be neutral gas molecules as the effect
Surface Science 99 (1996) 133-143
135
occurred readily with argon. A process in which a thermal Ar atom acquires a negative change upon collision with a charged surface is rather unlikely. The maximum charge that could be imparted on the sample varied considerably between several experiments. Also the reproducibility of the rate of charge was poor, although a negative charge was always observed. Samples prepared by pretreating the crystal in one atmosphere of oxygen at 500°C appeared to have a lower electrical conductivity than samples that were treated with 0.1 mbar of oxygen at the same temperature, although their Auger spectra could not be distinguished. As both the actual electrical connection between the sample and the sample holder and the position of the electron beam on the sample (near the edge or in the middle of the sample) may affect the observed electrical properties no definite conclusions can be drawn from these measurements apart from the observation that, as the halfwidths of the peaks did not change upon charging, the charge resides in a rather homogeneous region. This fact is further illustrated in Fig. 3 where we show a single scan spectrum, using 4 keV electrons as excitation source. Although all Auger peaks visible, carbon, titanium and oxygen, have shifted by as much as 650 V all peaks remain clearly identifiable and can be used (apart from signal to noise problems) for further analysis.
450
E,(Tl)
TIO,( 110)
cd0
Fig. 2. The peak position of the titanium (m, left hand axis) and oxygen ( l , right hand axis) of TiO,(l 10) as a function of time (experiment number). At the points indicated by the downward pointing arrows the beam current was increased (keeping the beam energy at 2 keV). The rate of charging the surface increases with beam current. The relative position of the oxygen and titanium peaks remains the same.
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A.J.M. Mew O.L.J. Gijzeman/Applied
The severe charge may of course affect the observed peak intensities. The effect may, however, be expected to be small. For a 4000 eV beam as used to collect the data in Fig. 3 the actual impinging electron energy would be 3350 eV. As the ionisation cross-section changes rather slowly with the excitation energy at this energy [12], the effect, especially when considering ratio’s, should be small. We performed a simple test to verify this hypothesis by measuring the palladium MNN and oxygen KLL signals on an oxidised palladium foil, biasing the sample up to - 800 V. No effect on either the absolute peak heights or their ratio’s was found within experimental error. So, using the fact that in the TiO, system the observed halfwidths of the peaks do not change we believe that our data are in fact reliable. 3.2. Electron induced loss of oxygen 3.2.1. TiO,(llO) The loss of oxygen from the surface under the influence of electron irradiation can be studied from data as those shown in Fig. 3, if a reliable way to determine the O/Ti ratio can be found. Since also carbon built-up is evident from this figure the amount of carbon must be quantified in some way as well. The oxygen and titanium peak heights can be obtained rather reliably from these and similar data. As said before, data were taken in a single scan, so occasionally noise prevented an accurate estimate.
TlO,( 110)
Fig. 3. The Auger spectrum of TiO,(llOl taken after severely charging the sample by electron irradiation with a beam energy of 4 keV for 16 h. During this treatment substantial carbon built-up on the surface occurs.
Surface Science 99 (1996) 133-143
TIO,( 110)
I(O)/I(Tl)
I(C)/I(Tl) .
. .
ssJJ-_
.
c...!! . .
* 8
ii . . .=.
*
‘i”
.
.
Fig. 4. The oxygen to titanium ( n , left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(llO) as a function of time during continuous irradiation with electrons of 4 keV.
Carbon, located on a fairly steep sloping background and showing a rather broad Auger feature is more difficult to quantify. We have chosen to subtract a simple linear background from the data in order to obtain the peak intensity of the carbon peak. Fig. 4 shows a measurement for the TiO,(llO> surface where the intensity ratio of the oxygen and the titanium peak and the intensity ratio of the carbon and titanium peak is plotted against time. The O/Ti ratio remains constant at a value of 0.59, as shown by the dotted line, whereas the C/Ti ratio increases continuously. This may point to a stable surface where no oxygen is lost, a surface where all oxygen that can be desorbed has in fact been removed, or it may represent the situation where oxygen is continuously desorbed under the influence of the electron beam and replenished by bulk diffusion to the surface. As this figure was obtained with a fairly high beam current (the current to earth was measured to be around 400 nA during this experiment, so that the actual specimen current may be higher due to secondary electron emission) a separate determination of the ‘true’ O/Ti ratio was attempted using a beam current as low as possible and defocusing the electron beam as much as possible. The results for the TiO,(llO) surface yielded 1.OOf 0.07 in one set of experiments and 0.96 + 0.03 in another. Thus it seems likely that loss of oxygen from the surface does indeed occur, but on a timescale much faster than used in Fig. 4.
A.J.M. Mens, O.L.J. Gijzeman/Applied
I(O)II(Tl)
120 -
TlO,( 110)
wu~) t
t 8_... ..__ !! *..*___* LLn-
,,.....F....!
i
.y.
om-
r 0
20
40
60
80
,o(
Fig. 5. The oxygen to titanium (m, left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(llO) as a function of time during continuous irradiation with electrons of 4 keV. During the first 25 min of this experiment a background pressure of 10e6 mbar of oxygen was present. Pumping away the oxygen leads to an immediate and drastic change in the O/Ti intensity ratio. Almost simultaneously carbon built-up on the surface starts.
In order to verify this hypothesis we performed the same experiment on a smaller timescale. In order to keep experimental parameters as constant as possible we started the experiment on a clean surface in the presence of an oxygen pressure of 10m6 mbar. This procedure, as tested in several independent experiments, yields a constant O/Ti intensity ratio over a prolonged period of time without any carbon deposition on the surface. After about 25 minutes the oxygen was pumped away and the Ti, 0 and C Auger peaks were monitored as a function of time. The result is shown in Fig. 5. As can be seen the O/Ti ratio drops almost instantaneously from the initial value of 1.07 to 0.63 after removal of the gas phase oxygen. At the same time carbon built-up starts at the same rate as that shown in Fig. 4. Note, however, that no carbon deposition has taken place in the presence of gaseous oxygen in a similar period of time. Surprisingly, the O/Ti ratio increases again after 60 minutes of experiment. The first conclusion to be drawn from this figure is that electron stimulated desorption of oxygen does indeed take place and even very fast with the current density used (the measured current to earth was in this case 300 nA, corresponding to a minimum current density of 4.2 A cme2), in agreement with previous findings [7,8]. The increase in the oxygen signal after 60 minutes of
137
Surface Science 99 (1996) 133-143
irradiation must point the reappearance of oxygen in the surface region. Diffusion of oxygen to the surface of TiO, at room temperature has in fact been reported by Heise and Courths [13], so the process can indeed occur. Also Mayer et al. [14] found a similar effect, even at 150 K. Evaporation of titanium onto a TiO, surface leads to the formation of a (defect) oxide structure with an intermediate oxidation state for the titanium. In our case we could say that, in stead of evaporating titanium onto the crystal, we have in fact removed oxygen, leading to a titanium-rich surface. This is expected to behave similarly to a Ti-TiO, interface, so that oxygen will diffuse to the surface. In the intermediate state in Fig. 4 the amount of oxygen in the surface region is determined by the mass balance between diffusion from the bulk to the surface and electron stimulated desorption. After sufficient carbon has been deposited on the surface, however, the process of electron stimulated desorption stops, as the carbon layer physically prohibits the desorption of oxygen ions or neutrals. Now the bulk-to-surface diffusion process will continue to replenish the amount of oxygen at the surface. The same trend is found for an experiment with a lower beam current by decreasing the emission current from the electron gun. Fig. 6 shows the data for a surface that was irradiated during 16 h. Again a sizable carbon built-up and an increasing O/Ti ratio
IDI
It wKw
TiO,( 110)
tr
I(C)/I(Tl)
y)
Fig. 6. The oxygen to titanium ( W, left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(llO) as a function of time during continuous irradiation with electrons of 4 keV. As the beam current was much lower (current to earth below 10 nA) than that used in Fig. 5 the timescale of the experiment is now much longer.
138
A. J.M. Mens, 0. L. J. Gijzemun /Applied
is observed. There seems, however, a limiting value to the amount of carbon deposited on the surface. The amount of oxygen lost from the (110) surface is considerable. Assuming the sampling depth of the Auger electrons to be of the order of 10 A and a perfectly flat (1101 surface with oxygen and titanium atoms in the outermost layer they will probe the oxygen concentration in the topmost two unit cells. These contain 8 oxygen atoms, so that a decrease of almost 40%, as found here, would correspond to a net loss of three of those oxygen atoms. Since the TiO,(l 10) surface is known to contain rows of oxygen atoms as the outermost species [2,11], the situation is somewhat less drastic. These atoms will be removed first, two for each surface unit cell, so that the net loss from the conventional unit cell will only be 1 oxygen atom. 3.2.2. TiO,(lOO) The results for the TiO,(lOO) surface show broadly speaking the same effects. The initial O/Ti ratio, as determined from measurements with a defocused electron beam and a beam current as low as possible was found to be 0.97 f 0.05 from nine independent sets of data, each consisting of 10 determinations of this ratio. It must be stated here, though, that the scatter within one data set was occasionally quite large. Values of 1.lO or 0.90 did occur. A
Fig. 7. The oxygen to titanium (a, left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(lOO) as a function of time during continuous irradiation with electrons of 4 keV, During the first 40 min of this experiment a background pressure of low6 mbar of oxygen was present. Pumping away the oxygen leads to a gradual change in the O/Ti intensity ratio. Almost simultaneously carbon built-up on the surface starts.
Surface Science 99 C1996) 133-I 43
I(O)/I(Tl)
TlO,( 100)
Fig. 8. The oxygen to titanium (D, left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(lOO) as a function of time during continuous irradiation with electrons of 4 keV with the same beam current as that used in Fig. 7. Note that the initial value of the O/Ti ratio is lower than that shown in Fig. 7 leading to a lower steady state value for this quantity as well.
similar experiment as that shown in Fig. 5 for the TiO,(llO) surface is presented in Fig. 7. Although the current to earth was higher (900 nA) as compared to 300 nA in Fig. 5 the loss of oxygen from the surface proceeds much slower. Note again, that no carbon is deposited onto the surface in the presence of a background of oxygen. After prolonged irradiation under these conditions an enormous amount of carbon was amassed on the surface, as evident from Fig. 8. Its saturation value appears to be the same for the (100) and (1101 surface shown in Fig. 6. However, no increase in the steady state O/Ti ratio could be observed for this surface. The steady state O/Ti ratio in this figure is lower than that shown in Fig. 7. However, also the inial value of this ratio was lower, so that the net decrease is still about the same. The loss of oxygen from the (100) surface appears to be lower than for the (110) surface. A decrease of 10 to 25% seems to be the limit that can be reached. Following the same argument as given above for a perfect, titanium terminated, (100) surface this will correspond again to a net loss of one oxygen atom per unit cell. The fact that the decrease in steady state oxygen concentration seems to be rather independent of the electron current density can be rational&d by assuming that the oxygen coverage is in fact determined by the equilibrium between oxygen in the bulk and at the surface and that electron
A.J.M. Mew
OLJ.
Gijzeman/Applied
stimulated desorption is a fast process. These ideas may be written in the form of a reaction scheme as:
I(O)/I(Tl)
TlO,(anatase)
I(C)/I(Tl)
-lo i -8
k
4
0, PO,
139
Surface Science 99 (1996) 133-143
I;”
kz
where 0, represent the bulk oxygen concentration, 0, the surface concentration and the rate constants k, and k2 describe the transport process to and from the surface and kesd accounts for electron stimulated desorption. The steady state oxygen concentration at the surface will simply be given by: 0, =
4 0, k* + ksd
(2)
As long as kesd is much larger than k, the surface oxygen concentration will be negligible and the Auger signal will only be caused by the bulk oxygen atoms. The observed rate of oxygen removal will then be: d0 s dt
k,
= kesdOs = kesd
0, = k,O,
k, + Li
and is only determined oxygen to the surface.
by the rate of diffusion
(3) of
3.2.3. TiO,(rutile) and TiOJanatase) The results for the TiO,(anatase) pellet, consisting of a mixture of 65% anatase and 35% rutile are similar to those for the single crystals. The initial O/Ti ratio, as determined from measurements with a defocused electron beam and a beam current as low as possible was found to be 0.82 + 0.03. This change, as compared to the single crystal results of almost unity for this ratio, may be attributed to a change in crystal structure for the sample. Electron induced loss of oxygen did, however, occur in a similar way as on the single crystal surfaces. Fig. 9 shows a representative curve, obtained by irradiating the sample with 4 keV electrons during 18 h with two different beam currents (current to earth 1 PA and 70 nA respectively). A gradual loss of oxygen is seen. the O/Ti ratio dropping to almost half of its original value for the highest bean current used. Somewhat surprisingly not much carbon was deposited on the surface, the C/Ti ratio reaching a value of only 3, in contrast to the single crystal surfaces where values of 30 to 35 for this ratio were reached, as shown in Figs. 6 and 8.
Fig. 9. The oxygen to titanium (m, 0 left hand axis) and carbon to titanium (a, + right hand axis) Auger ratio for TiO,(anatase) as a function of time during continuous irradiation with electrons of 4 keV. The open symbols refer to a high beam current (current to ground 1 PA cm-’ 1, the filled symbols to a low beam current (current to ground 70 nA cm-* ). The data n have been shifted upwards by 0.1 for clarity.
Similar experiments were tried with the TiO,(rutile) pellet. Although this sample had the same crystal structure as the TiO, single crystal specimens the initial O/Ti ratio, as measured with as low a beam current as could be achieved turned out to be less reproducible on different positions of the surface. Values between 1.0 and 2.0 could be obtained for the O/Ti ratio. This is not caused by electron stimulated desorption, even at this low beam current, as measuring this ratio at a fixed spot on the crystal showed no effects of the exciting electron beam. In one example the O/Ti remained constant at a value 1.64 * 0.05 in the course of time. The electron beam induced loss of oxygen from this surface at higher beam currents is shown in Fig. 10. This curve was obtained with the same beam current as used in Fig. 9 to obtain the data represented by the closed symbols. The loss of oxygen from the surface is seen to occur much faster on this TiO&utile) surface, compared to the TiO,(anatase). Again carbon built-up is found, but to a much lesser extent than on the single crystal surfaces under the same experimental conditions. The maximum decrease in the oxygen content for the two pellet samples (50% for the anatase and even some 65% for the rutile sample) seems to be larger than that found for the single crystal surfaces. This can be understood if we assume that the pellets consist of small TiOz particles (as they do) which
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A.J.M. Mew, O.LJ. Gijzeman/Applied
IM
I(O)iI(Tt)
TlO,(rutlel)
I(C)II(Ti)
t
’
.
14 120 i
DLW P
5
10
IS
20
+ z)(
Fig. 10. The oxygen to titanium (m left hand axis) and carbon to titanium (A right hand axis) Auger ratio for TiO,(rutile) as a function of time during continuous irradiation with electrons of 4 keV. The beam current was the same as that used in Fig. 9 for TiO,(anatase).
behave more or less independently from one another. Now the source of (subsurface) oxygen will be limited by the volume of one such particle. Thus if all or most of the oxygen in a particle which is located at the surface of the specimen has been removed by electron stimulated desorption a deeper lying particle must provide its oxygen to replenish the surface concentration. This will require diffusion across grain boundaries, which could very well be slower than bulk diffusion in an otherwise perfect lattice leading to a larger net loss of oxygen in the steady state as compared to the single crystal surfaces. The data obtained for the single crystal specimens of (110) and (100) orientation and the pressed pellets of the rutile and anatase structure are thus quite similar. Despite the sometimes severe negative charge on the surfaces oxygen can be removed by incident electrons. This process can be avoided by admitting 10m6 mbar of oxygen to the system during the electron irradiation. 3.2.4. Carbon deposition On all surfaces and under all conditions of electron beam current (some) carbon was deposited on the surface, a higher beam current in general leading to a faster deposition of carbon. This deposition must be assisted or influenced by the presence of the electron beam as no carbon was observed on unirradiated places on the sample. It should be noted that carbon deposition can be completely suppressed on all surfaces studied by admitting an oxygen pressure
Surface Science 99 (19%) 133-143
of lop6 mbar during irradiation. In this case also no loss of oxygen from the surface occurs. A first question is obviously about the nature of the carbonaceous species. For the highest coverages obtained (C/Ti 2 10) a lineshape analysis was attempted by numerical differentiation of the spectra. Comparison with the standard reference spectra [15] made it most likely that the deposit is in this case graphite. The initial deposit at low coverage cannot be identified by this method. We do believe it to be carbidic on the ground that it can be made to react with oxygen under the influence of the exciting electron beam. The small carbon peak observed in this case did disappear completely when the surface was exposed to lop6 mbar of oxygen whilst keeping the electron gun on. The graphitic carbon present in a later stage did not react with oxygen under similar experimental conditions. As graphite is well known to be inert to oxidation this observation confirms the assignment of the ‘thick’ carbon overlayer to graphite. The fact that no carbon is deposited on a surface when irradiation takes place in the presence of 10m6 mbar of oxygen in the background gas indicates that any carbon, if deposited at all, is readily removed by reaction before a significant amount of graphite can be formed. A second question that might be asked is about the actual amount of carbon deposited. The standard procedure to answer this question would be to measure the absolute peak heights of the titanium peak as a function of the carbon coverage and use the well known relation: Z(d) = I(O)exp - (d/h)
(4) were A is the inelastic mean free path of titanium Auger electrons in the overlayer to determine the l?yer thickness d. The value of h was taken as 13.1 A [16] assuming the overlayer to be graphite. For those few data sets with sufficiently accuracy in the absolute peak heights (of course, ratio’s as reported thus far are less susceptible to random fluctuations) we found the estimates for the (110) and (100) surface: d(i)
= (0.19~0.05);,
TiO,( lOO), TI (5)
d(i)
= (0.14~0.01)4,
TiO,( 110). TI
A.J.M. Mew O.L.J. Gijzeman/Applied Surface Science 99 (19%) 133-143
The agreement between these results is encouraging and may indicate that the estimates are at least reasonable. It implies, however, that for the highest C/Ti ratio found (= 35) the layer thickness is only some 5 to 6 A or, in other words, 2 to 3 monolayers of graphite. In hindsight this may not be too surprising. Graphite is a rather inert substance, once a more or less closed layer has accumulated on the surface the surface ceases to be reactive for further adsorption and carbon deposition stops. The different amounts of carbon that can be deposited on the single crystal surfaces (2 to 3 monolayers) as compared to the pressed pellets (around 0.2 monolayers according to the estimate given above) may be explained by the roughness of the surfaces. The single crystal surfaces are expected to be flat, ideally even atomically flat. A graphite layer on the surface then needs not to be strained in order to keep contact with the underlying surface. For the rough surfaces of the pellets on the other hand, an extended graphite layer cannot be formed due to the high stress that must accumulate in this layer if it were to fit closely on the underlying surface. Finally we may wonder about the mechanism of the carbon deposition. Experimentally it is found that carbon deposition only takes place under the influence of the irradiating electron beam, as no carbon is deposited on unirradiated places of the sample. The rate of carbon deposition could, of course, be increased by deliberately admitting a carbon containing gas such as carbon monoxide or propylene. Denoting the carbon containing species by C, two different mechanisms may be envisaged. One is the case where this species first adsorbs reversibly on the surface and is decomposed by the incoming electrons, schematically represented by:
141
Accepting our previous estimate of the carbon concentration the rate of carbon built-up is seen to be 3 x lop5 monolayers per second or 3 X lo-” molecules cm-’ s- ‘. Assuming reversibly adsorbed gas phase molecules to be non-reversibly adsorbed under the influence of the electron beam we may estimate the rate of adsorption as: ?-= #%_olv,,
(8)
where the last step requires the presence of the electron beam. An alternative mechanism could be the adsorption of gas phase (positive) ions of C,, formed by ionisation of gas phase C,:
where Nad is the number of adsorbed molecules per unit area and cr is now the cross-section for electron stimulated decomposition. Inserting the experimental values for the rate of adsorption and an estimated value of lo-l6 cm2 for the cross-section the number of adsorbed molecules is found to be 5 X lo6 cme2. This implies, however, that at a reasonable pressure of 1 mbar the number of adsorbed molecules must be in the order of 5 X lOi cmW2 , or almost one monolayer. As no measurable adsorption on undamaged TiO, from residual gas molecules such as carbon monoxide or methane has been reported [I], we may exclude this mechanism for carbon deposition if the surface was indeed free of defects. Experimentally, however, we found that the surface under electron irradiation does not have the normal TiO, stoichiometry, a sizeable amount of oxygen being absent. So we may expect the surface to exhibit many Ti sites, where adsorption could be much enhanced. The mechanism would then effectively be the adsorption of residual gas on titanium, a situation that exists only at the irradiated part of the surface. The alternative mechanism, adsorption of positive ions on the negatively charged surface, requires an effective ion flux of around 3 X lOlo cm-’ SK’. The flux caused by gas phase positive ions may be estimated in two ways. Assuming a reported ionisation efficiency of 2 ions formed per Torr gas pressure per centimeter path length per incident electron [ 171 an incident electron beam of 6 X 1019 electrons per square centimeter per second will produce 1.2 X 10” ions cm-* ss’ at a pressure of lo- I0 mbar. Alternatively we may estimate the ion flux &,ns as:
C, -+ C:
&I,, = 4ecapL
C,(g)
+ C ,(ad)
+ xC(ad)
+ xC(ad)
(6)
(7)
where now the first step requires the presence of the impinging electron beam and the positive ions formed will be accelerated toward the negatively charged part of the crystal.
(9)
where +e is the incident electron flux, u the ionisation cross-section, p the gas density and L the length of the irradiated volume. At a pressure of lo-” mbar and using an ionisation cross-section of
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A.J.M. Mens, O.L.J. G&eman/Applied
1O- I6 cm2 [ 181 we obtain for the ion flux a value of 1.4 X 10” cm-’ s- ‘. This implies a rate of carbon deposition of 1.4 X 10” cm-’ SC’ at a background mbar. Experimentally we found pressure of lo-” this rate to be 3 X 10” cmm2 se2. As the pressure during a measurement was always at least one order of magnitude higher than the figure of lo-” mbar used in our estimate in Eq. (9), the mechanism involving carbon deposition through gas phase ions is certainly possible. In order to corroborate this idea an experiment was performed were the parameters of the electron gun were kept unchanged at a beam energy of 2 keV and fixed (but rather low) emission current as measured on the spectrometer power supply during the following sequence of events: - measure the 0 and Ti Auger peaks ten times (the peaks shifted upwards); - admit propylene as a carbon containing impurity in the residual gas at a pressure of 1O-5 mbar during ten minutes (no measurements were possible at this pressure); - decrease the propylene pressure to 10e7 mbar and measure the spectra again. We found that now the oxygen (and titanium) Auger peaks has shifted to lower energy and continued to shift continuously to lower energy. This suggests that the surface becomes neutralised by positive propylene ions, which must leave carbon on the surface. This trend continued when the background pressure was reduced to 5 X lo-* mbar. Only after switching on the ion pump, which decreased the pressure to below 10e9 mbar was this trend reversed and the usual negative charging of the surface reestablished. This experiment shows unequivocally that gas phase ions do adsorb at the surface, so the assumption that they do so also at a much lower pressure, where they cannot neutralize the surface, is by no means unrealistic.
4. Conclusions . Different kind of samples of TiO, (single crystals, rutile and anatase pellets) show electron stimulated desorption of oxygen. The amount of oxygen lost is substantial for all samples.
Surface Science 99 (1996) 133-143
+ The samples are always negatively charged by the incoming electron beam, surface charges may be as high as 800 V, without preventing the measurement of Auger spectra. This charge resides homogeneously in the irradiated area only. - Surface contamination by carbon (graphite) can occur via a mechanism of adsorption of carbonaceous species on the defect surface followed by electron induced decomposition. Decomposition of positively charged gas phase ions on the negatively charged surface does also take place. - Both the loss of oxygen from the surface and the deposition of carbon can be suppressed by the addition of 10m6 mbar oxygen to the background gas.
Acknowledgements The authors would like to thank Mr. Jan van de Loosdrecht for the preparation and characterisation of the TiO, pellet samples. Helpful discussions with lr. Jan Verhoeven and Dr. Henk Hopman (AMOLF, Amsterdam) contributed to the final version of this work.
References [II J.P.S. Badyal, in: The Chemical Physics of Solid Surfaces, Vol. 6, Eds. D.A. King and D.P. Woodruff (Elsevier, Amsterdam, 1993). [21 W. Giipel, J.A. Anderson, D. Frankel, M. Jaehnig, K. Philips, J.A. S&tier and G. Rocker, Surf. Sci. 139 (1984) 333. [31 H. Idriss and M.A. Barteau, Catal. Lett. 26 (1994) 123. 141 G. Lu. A. Linsebigler and J.T. Yates, Jr., J. Phys. Chem. 98 (1994) 11733. 151 S. Eriksen and R.G. Egdell, Surf. Sci. 180 (1987) 263. [d M.R. McCartney and D.J. Smith, Surf. Sci. 250 (1991) 169. [71 M.R. McCartney and D.J. Smith, Surf. Sci. 221 (1989) 214. Dl M.C. Torquemada and J.L. de Segovia, J. Vat. Sci. Technol. A 12 (1994) 2318. 191 M. Ramamoorthy. R.D. King-Smith and D. Vanderbilt, Phys. Rev. B 49 (1994) 7709. [lOI R. Sanjin&, H. Tang, H. Berger, F. Gozzo, G. Margaritondo and F. L&y, J. Appl. Phys. 75 (1994) 2945. 1111 T. Bredow and K. Jug, Surf. Sci. 327 (1995) 398. iI21 M.P. Seah, in: Practical Surface Analysis, Eds. D. Briggs and M.P. Seah (Wiley. New York, 1990) ch. 5. [I31 R. Heise and R. Courths, Surf. Sci. 331 (1995) 1460.
A.J.M. Mens, 0L.J.
Gijzeman/Applied
[14] J.T. Mayer, U. Diebold, T.E. Madey and E. Garfunkel, J. Electron Spectrosc. Rel. Phen. 73 (1995) 1. [15] T.W. Haas, J.T. Grant and G.J. Dooley, J. Appl. Phys. 43 (1972) 1853. [16] S. Tanuma, C.J. Powell and D.R. Penn, Surf. Interf. Anal. 11 (1988) 577.
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143
[17] A. Roth, Vacuum Technology (North-Holland. Amsterdam, 1982) ch. 6. [18] L.C. Feldman and J.W. Mayer, Fundamentals of Surface and Thin Film Analysis (North-Holland, New York, 1986).
Applied Surface Science 99 ( 1996) 133- 143
AES study of electron beam induced damage on TiO, surfaces Ad J.M. Mens, Onno L.J. Gijzeman Debye Institute. Surjace Science Diuision, Utrecht Uniaersity, Padualaan 8, 3584 CH Utrecht, The Netherlands Received 9 August 1995; accepted
IO December
1995
Abstract The damage induced by an electron beam on several TiO, surfaces has been studied with Auger Electron Spectroscopy. We studied single crystal specimens of TiO, (rutile) of the (100) and (110) orientation as well as pressed powder specimens consisting of rutile and a 65%-35% mixture of anatase and rutile. All surfaces acquired a (sometimes extremely high) negative charge upon electron irradiation. All surfaces did loose a significant amount of oxygen due to electron stimulated desorption. In the presence of background oxygen this process did not occur. Carbon (graphite) was deposited on the samples from residual gases. This is due to the decomposition of adsorbed carbonaceous species on the damaged surface and to the decomposition of positive gas phase ions on the negatively charged (irradiated) region of the surface. Significantly less carbon was deposited on the powder samples as compared to the single crystals. Carbon deposition is inhibited by the presence of oxygen in gas phase.
1. Introduction Titanium dioxide is a compound that is widely used as a support for catalytic materials [l]. Probably part of its success is due to its non-stoichiometry, the valence of the Ti4+ ions can be reduced rather easily to a lower oxidation state. This is accompanied by a loss of oxygen from the surface. Surface defects can be introduced deliberately by ion bombardment [2,3], thermal treatments [2,4] or by electron beam irradiation [5-71. Electron stimulated desorption of oxygen has been demonstrated for high beam currents, lo-50 A/cm* [7], and beam currents as low as 10 mA/cm* [5] down to 1.2 PA/cm2 [8]. These defects lead to pronounced differences in the electronic structure of the material for both the rutile [9] and anatase [lo] crystal structure and also influence their adsorptive properties [ 1 I]. 0169-4332/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00603-6
Titanium dioxide is also used as an insulating material in several technical applications such as cathode ray tubes. In this case, as in the case of catalysis, poly-crystalline material is used. Degradation of the insulating material by electrons may occur as well as changes in the electrical properties due to the creation of electron induced defects, which increase the conductivity. For applications as an insulator a low electrical conductivity is of course required. In this paper we report on the electron induced loss of oxygen from rutile single crystals with the (100) and (110) orientation. Although these crystals can be prepared with a high conductivity, we have deliberately chosen to prepare badly conducting, but stoichiometric, specimens. We did this in order to compare the results to less well defined, but equally badly conducting, specimens obtained by pressing
134
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commercial TiOz into suitable samples. These samples were either pure i-utile or a 65%/35% mixture of anatase and rutile.
2. Experimental The experiments were carried out in a Fisons XPS/AES system (MT 500) with facilities for XPS and AES, using a CLAM 2 hemispherical analyser for electron detection. The electron gun (LEG 62) could be focused according to the manufacturer specification to a spot size of down to 3 pm. This ensured a high current density on the target. Using a measured value of up to 2 PA for the target to earth current the estimated current density becomes of the order of 10 A/cm’, or around 6 X lOI electrons cm-’ SK’. Due to secondary electron emission this value must be considered a lower limit. The base pressure in the main chamber was lo- lo mbar, but rose during the actual experiments to 1O-8 mbar when these high current densities were used. At a lower current density (by decreasing the emission current from the electron gun) the pressure remained in the lo- lo mbar region. The electron current from the gun was used both as the source for electron induced damage and to obtain Auger spectra as well from the same irradiated spot on the sample. In all experiments described here the electron gun was switched on with a fixed emission current (as measured on the electron gun power supply) and kept on while data acquisition took place at certain time intervals. This procedure ensures that the continuously irradiated area on tbe sample is in fact investigated and that slight displacements of the electron beam due to the switching on and off of the power supply are avoided. The single crystal samples, consisting of two TiO, single crystals with (110) and (100) orientation with dimension of approximately 1 X 1 cm2 and thickness of 2 mm, were mounted on a stainless steel sample holder. These crystals had the rutile structure. A sample of a more commercial type of titanium dioxide was also used in the form of a TiO, powder (Degussa 7702, particle diameter 100 nm). Tbe powder was pressed into a stainless steel disk shaped sample holder for easy measurement. This compound consists of approximately 65% anatase and 35%
Surfhce Science 99 (1996) 133-143
t-utile as verified by X-ray diffraction. This sample will be referred to as TiO,(anatase). A second sample of TiO, powder was prepared by heating the pressed pellet to 900°C in flowing air for 24 h. After this treatment the sample consists exclusively of rutile, as shown by X-ray diffraction. This sample will be referred to as TiO,(rutile). All samples could be transferred between a the main chamber and a sample preparation chamber by means of a linear transfer device and several wobble sticks. Sample preparation and cleaning could be done in this separate chamber, attached to the main vessel, with facilities for argon ion sputtering, sample heating (25 to 600°C) and gas exposures of up to 1 atmosphere. After initial removal of impurities on the specimens by mild sputtering (2000 eV and 50 PA at an angle of 30”) samples were routinely treated with either 1 atmosphere of flowing oxygen at 500°C or 0.1 mbar oxygen at 500°C during 15 minutes before an experiment. XPS spectra showed that this treatment was sufficient to remove all carbon impurities on the surface, which was otherwise clean. Tbe samples prepared in this way were always bad conductors, as can be expected, but do have a stoichiometric surface and show reproducible results. Auger spectra were taken in the direct, i.e. N(E) versus E mode and shown here in the ‘as recorded’ state without background subtraction and/or data smoothing. Due to the charging of the samples only single scan spectra could be obtained, which resulted in a sometimes poor signal to noise ratio.
3. Results and discussion
3.1. Surface charging A severe problem with the measurement of Auger spectra from the TiO, samples is, as could be expected, their poor electrical conductivity. Fig. 1 shows the G(KLL) spectra from a TiO,(l 10) sample taken in single scan at approximately 5 minutes intervals at a beam energy of 2 keV, while keeping the electron gun switched on all the time. A negative charging of the analysed region of up to 20 volts can be seen clearly. This charge was only present in the area irradiated by the electron beam as displacing the crystal slightly from its original position caused the
A.J.M. Mens. O.L.J. Gijzeman/Applied
103 c/s
t
12
TlO,( 110) 45 3
Fig. 1. Consecutive oxygen Auger spectra of a TiOJl 10) surface taken with a low beam current and beam energy of 2 keV. Spectra were taken at 5 min intervals and took about 1 min to record. During the intervals the sample remained exposed to the electron beam.
peak to be restored to its original value, after which charge built-up took place again. The pressure in the chamber during measurements was below 10m9 Tot-r. The rate of charge built-up was dependent on the incoming electron flux. Fig. 2 shows the position of the oxygen KLL and titanium LMM Auger peaks, normally located at 5 10 and 380 eV respectively as a function of time. After each experiment (with a constant beam energy of 2 keV) data collection was stopped for five minutes while keeping the electron gun switched on. At certain points, indicated by the arrows in Fig. 2, the emission current of the gun was increased and a further set of data collected. The rate of charging increases with increasing electron flux. Also the oxygen and titanium peaks move simultaneously, keeping their separation constant during charging of the sample. This indicates that the negative charge is distributed rather homogeneously in the irradiated region, which is also corroborated by the fact that the Auger peaks showed no noticeable broadening in the charged state. The amount of charge on the specimen could be decreased at a given beam current by admitting any foreign gas into the chamber. Since in this case the incoming electron beam will cause ionisation of gas phase molecules, the positive ions formed will be attracted towards the negatively charged region and neutralise the negative charge present there, a process that occurs for instance in any ionisation gauge. The effect is unlikely to be caused be neutral gas molecules as the effect
Surface Science 99 (1996) 133-143
135
occurred readily with argon. A process in which a thermal Ar atom acquires a negative change upon collision with a charged surface is rather unlikely. The maximum charge that could be imparted on the sample varied considerably between several experiments. Also the reproducibility of the rate of charge was poor, although a negative charge was always observed. Samples prepared by pretreating the crystal in one atmosphere of oxygen at 500°C appeared to have a lower electrical conductivity than samples that were treated with 0.1 mbar of oxygen at the same temperature, although their Auger spectra could not be distinguished. As both the actual electrical connection between the sample and the sample holder and the position of the electron beam on the sample (near the edge or in the middle of the sample) may affect the observed electrical properties no definite conclusions can be drawn from these measurements apart from the observation that, as the halfwidths of the peaks did not change upon charging, the charge resides in a rather homogeneous region. This fact is further illustrated in Fig. 3 where we show a single scan spectrum, using 4 keV electrons as excitation source. Although all Auger peaks visible, carbon, titanium and oxygen, have shifted by as much as 650 V all peaks remain clearly identifiable and can be used (apart from signal to noise problems) for further analysis.
450
E,(Tl)
TIO,( 110)
cd0
Fig. 2. The peak position of the titanium (m, left hand axis) and oxygen ( l , right hand axis) of TiO,(l 10) as a function of time (experiment number). At the points indicated by the downward pointing arrows the beam current was increased (keeping the beam energy at 2 keV). The rate of charging the surface increases with beam current. The relative position of the oxygen and titanium peaks remains the same.
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The severe charge may of course affect the observed peak intensities. The effect may, however, be expected to be small. For a 4000 eV beam as used to collect the data in Fig. 3 the actual impinging electron energy would be 3350 eV. As the ionisation cross-section changes rather slowly with the excitation energy at this energy [12], the effect, especially when considering ratio’s, should be small. We performed a simple test to verify this hypothesis by measuring the palladium MNN and oxygen KLL signals on an oxidised palladium foil, biasing the sample up to - 800 V. No effect on either the absolute peak heights or their ratio’s was found within experimental error. So, using the fact that in the TiO, system the observed halfwidths of the peaks do not change we believe that our data are in fact reliable. 3.2. Electron induced loss of oxygen 3.2.1. TiO,(llO) The loss of oxygen from the surface under the influence of electron irradiation can be studied from data as those shown in Fig. 3, if a reliable way to determine the O/Ti ratio can be found. Since also carbon built-up is evident from this figure the amount of carbon must be quantified in some way as well. The oxygen and titanium peak heights can be obtained rather reliably from these and similar data. As said before, data were taken in a single scan, so occasionally noise prevented an accurate estimate.
TlO,( 110)
Fig. 3. The Auger spectrum of TiO,(llOl taken after severely charging the sample by electron irradiation with a beam energy of 4 keV for 16 h. During this treatment substantial carbon built-up on the surface occurs.
Surface Science 99 (1996) 133-143
TIO,( 110)
I(O)/I(Tl)
I(C)/I(Tl) .
. .
ssJJ-_
.
c...!! . .
* 8
ii . . .=.
*
‘i”
.
.
Fig. 4. The oxygen to titanium ( n , left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(llO) as a function of time during continuous irradiation with electrons of 4 keV.
Carbon, located on a fairly steep sloping background and showing a rather broad Auger feature is more difficult to quantify. We have chosen to subtract a simple linear background from the data in order to obtain the peak intensity of the carbon peak. Fig. 4 shows a measurement for the TiO,(llO> surface where the intensity ratio of the oxygen and the titanium peak and the intensity ratio of the carbon and titanium peak is plotted against time. The O/Ti ratio remains constant at a value of 0.59, as shown by the dotted line, whereas the C/Ti ratio increases continuously. This may point to a stable surface where no oxygen is lost, a surface where all oxygen that can be desorbed has in fact been removed, or it may represent the situation where oxygen is continuously desorbed under the influence of the electron beam and replenished by bulk diffusion to the surface. As this figure was obtained with a fairly high beam current (the current to earth was measured to be around 400 nA during this experiment, so that the actual specimen current may be higher due to secondary electron emission) a separate determination of the ‘true’ O/Ti ratio was attempted using a beam current as low as possible and defocusing the electron beam as much as possible. The results for the TiO,(llO) surface yielded 1.OOf 0.07 in one set of experiments and 0.96 + 0.03 in another. Thus it seems likely that loss of oxygen from the surface does indeed occur, but on a timescale much faster than used in Fig. 4.
A.J.M. Mens, O.L.J. Gijzeman/Applied
I(O)II(Tl)
120 -
TlO,( 110)
wu~) t
t 8_... ..__ !! *..*___* LLn-
,,.....F....!
i
.y.
om-
r 0
20
40
60
80
,o(
Fig. 5. The oxygen to titanium (m, left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(llO) as a function of time during continuous irradiation with electrons of 4 keV. During the first 25 min of this experiment a background pressure of 10e6 mbar of oxygen was present. Pumping away the oxygen leads to an immediate and drastic change in the O/Ti intensity ratio. Almost simultaneously carbon built-up on the surface starts.
In order to verify this hypothesis we performed the same experiment on a smaller timescale. In order to keep experimental parameters as constant as possible we started the experiment on a clean surface in the presence of an oxygen pressure of 10m6 mbar. This procedure, as tested in several independent experiments, yields a constant O/Ti intensity ratio over a prolonged period of time without any carbon deposition on the surface. After about 25 minutes the oxygen was pumped away and the Ti, 0 and C Auger peaks were monitored as a function of time. The result is shown in Fig. 5. As can be seen the O/Ti ratio drops almost instantaneously from the initial value of 1.07 to 0.63 after removal of the gas phase oxygen. At the same time carbon built-up starts at the same rate as that shown in Fig. 4. Note, however, that no carbon deposition has taken place in the presence of gaseous oxygen in a similar period of time. Surprisingly, the O/Ti ratio increases again after 60 minutes of experiment. The first conclusion to be drawn from this figure is that electron stimulated desorption of oxygen does indeed take place and even very fast with the current density used (the measured current to earth was in this case 300 nA, corresponding to a minimum current density of 4.2 A cme2), in agreement with previous findings [7,8]. The increase in the oxygen signal after 60 minutes of
137
Surface Science 99 (1996) 133-143
irradiation must point the reappearance of oxygen in the surface region. Diffusion of oxygen to the surface of TiO, at room temperature has in fact been reported by Heise and Courths [13], so the process can indeed occur. Also Mayer et al. [14] found a similar effect, even at 150 K. Evaporation of titanium onto a TiO, surface leads to the formation of a (defect) oxide structure with an intermediate oxidation state for the titanium. In our case we could say that, in stead of evaporating titanium onto the crystal, we have in fact removed oxygen, leading to a titanium-rich surface. This is expected to behave similarly to a Ti-TiO, interface, so that oxygen will diffuse to the surface. In the intermediate state in Fig. 4 the amount of oxygen in the surface region is determined by the mass balance between diffusion from the bulk to the surface and electron stimulated desorption. After sufficient carbon has been deposited on the surface, however, the process of electron stimulated desorption stops, as the carbon layer physically prohibits the desorption of oxygen ions or neutrals. Now the bulk-to-surface diffusion process will continue to replenish the amount of oxygen at the surface. The same trend is found for an experiment with a lower beam current by decreasing the emission current from the electron gun. Fig. 6 shows the data for a surface that was irradiated during 16 h. Again a sizable carbon built-up and an increasing O/Ti ratio
IDI
It wKw
TiO,( 110)
tr
I(C)/I(Tl)
y)
Fig. 6. The oxygen to titanium ( W, left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(llO) as a function of time during continuous irradiation with electrons of 4 keV. As the beam current was much lower (current to earth below 10 nA) than that used in Fig. 5 the timescale of the experiment is now much longer.
138
A. J.M. Mens, 0. L. J. Gijzemun /Applied
is observed. There seems, however, a limiting value to the amount of carbon deposited on the surface. The amount of oxygen lost from the (110) surface is considerable. Assuming the sampling depth of the Auger electrons to be of the order of 10 A and a perfectly flat (1101 surface with oxygen and titanium atoms in the outermost layer they will probe the oxygen concentration in the topmost two unit cells. These contain 8 oxygen atoms, so that a decrease of almost 40%, as found here, would correspond to a net loss of three of those oxygen atoms. Since the TiO,(l 10) surface is known to contain rows of oxygen atoms as the outermost species [2,11], the situation is somewhat less drastic. These atoms will be removed first, two for each surface unit cell, so that the net loss from the conventional unit cell will only be 1 oxygen atom. 3.2.2. TiO,(lOO) The results for the TiO,(lOO) surface show broadly speaking the same effects. The initial O/Ti ratio, as determined from measurements with a defocused electron beam and a beam current as low as possible was found to be 0.97 f 0.05 from nine independent sets of data, each consisting of 10 determinations of this ratio. It must be stated here, though, that the scatter within one data set was occasionally quite large. Values of 1.lO or 0.90 did occur. A
Fig. 7. The oxygen to titanium (a, left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(lOO) as a function of time during continuous irradiation with electrons of 4 keV, During the first 40 min of this experiment a background pressure of low6 mbar of oxygen was present. Pumping away the oxygen leads to a gradual change in the O/Ti intensity ratio. Almost simultaneously carbon built-up on the surface starts.
Surface Science 99 C1996) 133-I 43
I(O)/I(Tl)
TlO,( 100)
Fig. 8. The oxygen to titanium (D, left hand axis) and carbon to titanium (A, right hand axis) Auger ratio for TiO,(lOO) as a function of time during continuous irradiation with electrons of 4 keV with the same beam current as that used in Fig. 7. Note that the initial value of the O/Ti ratio is lower than that shown in Fig. 7 leading to a lower steady state value for this quantity as well.
similar experiment as that shown in Fig. 5 for the TiO,(llO) surface is presented in Fig. 7. Although the current to earth was higher (900 nA) as compared to 300 nA in Fig. 5 the loss of oxygen from the surface proceeds much slower. Note again, that no carbon is deposited onto the surface in the presence of a background of oxygen. After prolonged irradiation under these conditions an enormous amount of carbon was amassed on the surface, as evident from Fig. 8. Its saturation value appears to be the same for the (100) and (1101 surface shown in Fig. 6. However, no increase in the steady state O/Ti ratio could be observed for this surface. The steady state O/Ti ratio in this figure is lower than that shown in Fig. 7. However, also the inial value of this ratio was lower, so that the net decrease is still about the same. The loss of oxygen from the (100) surface appears to be lower than for the (110) surface. A decrease of 10 to 25% seems to be the limit that can be reached. Following the same argument as given above for a perfect, titanium terminated, (100) surface this will correspond again to a net loss of one oxygen atom per unit cell. The fact that the decrease in steady state oxygen concentration seems to be rather independent of the electron current density can be rational&d by assuming that the oxygen coverage is in fact determined by the equilibrium between oxygen in the bulk and at the surface and that electron
A.J.M. Mew
OLJ.
Gijzeman/Applied
stimulated desorption is a fast process. These ideas may be written in the form of a reaction scheme as:
I(O)/I(Tl)
TlO,(anatase)
I(C)/I(Tl)
-lo i -8
k
4
0, PO,
139
Surface Science 99 (1996) 133-143
I;”
kz
where 0, represent the bulk oxygen concentration, 0, the surface concentration and the rate constants k, and k2 describe the transport process to and from the surface and kesd accounts for electron stimulated desorption. The steady state oxygen concentration at the surface will simply be given by: 0, =
4 0, k* + ksd
(2)
As long as kesd is much larger than k, the surface oxygen concentration will be negligible and the Auger signal will only be caused by the bulk oxygen atoms. The observed rate of oxygen removal will then be: d0 s dt
k,
= kesdOs = kesd
0, = k,O,
k, + Li
and is only determined oxygen to the surface.
by the rate of diffusion
(3) of
3.2.3. TiO,(rutile) and TiOJanatase) The results for the TiO,(anatase) pellet, consisting of a mixture of 65% anatase and 35% rutile are similar to those for the single crystals. The initial O/Ti ratio, as determined from measurements with a defocused electron beam and a beam current as low as possible was found to be 0.82 + 0.03. This change, as compared to the single crystal results of almost unity for this ratio, may be attributed to a change in crystal structure for the sample. Electron induced loss of oxygen did, however, occur in a similar way as on the single crystal surfaces. Fig. 9 shows a representative curve, obtained by irradiating the sample with 4 keV electrons during 18 h with two different beam currents (current to earth 1 PA and 70 nA respectively). A gradual loss of oxygen is seen. the O/Ti ratio dropping to almost half of its original value for the highest bean current used. Somewhat surprisingly not much carbon was deposited on the surface, the C/Ti ratio reaching a value of only 3, in contrast to the single crystal surfaces where values of 30 to 35 for this ratio were reached, as shown in Figs. 6 and 8.
Fig. 9. The oxygen to titanium (m, 0 left hand axis) and carbon to titanium (a, + right hand axis) Auger ratio for TiO,(anatase) as a function of time during continuous irradiation with electrons of 4 keV. The open symbols refer to a high beam current (current to ground 1 PA cm-’ 1, the filled symbols to a low beam current (current to ground 70 nA cm-* ). The data n have been shifted upwards by 0.1 for clarity.
Similar experiments were tried with the TiO,(rutile) pellet. Although this sample had the same crystal structure as the TiO, single crystal specimens the initial O/Ti ratio, as measured with as low a beam current as could be achieved turned out to be less reproducible on different positions of the surface. Values between 1.0 and 2.0 could be obtained for the O/Ti ratio. This is not caused by electron stimulated desorption, even at this low beam current, as measuring this ratio at a fixed spot on the crystal showed no effects of the exciting electron beam. In one example the O/Ti remained constant at a value 1.64 * 0.05 in the course of time. The electron beam induced loss of oxygen from this surface at higher beam currents is shown in Fig. 10. This curve was obtained with the same beam current as used in Fig. 9 to obtain the data represented by the closed symbols. The loss of oxygen from the surface is seen to occur much faster on this TiO&utile) surface, compared to the TiO,(anatase). Again carbon built-up is found, but to a much lesser extent than on the single crystal surfaces under the same experimental conditions. The maximum decrease in the oxygen content for the two pellet samples (50% for the anatase and even some 65% for the rutile sample) seems to be larger than that found for the single crystal surfaces. This can be understood if we assume that the pellets consist of small TiOz particles (as they do) which
140
A.J.M. Mew, O.LJ. Gijzeman/Applied
IM
I(O)iI(Tt)
TlO,(rutlel)
I(C)II(Ti)
t
’
.
14 120 i
DLW P
5
10
IS
20
+ z)(
Fig. 10. The oxygen to titanium (m left hand axis) and carbon to titanium (A right hand axis) Auger ratio for TiO,(rutile) as a function of time during continuous irradiation with electrons of 4 keV. The beam current was the same as that used in Fig. 9 for TiO,(anatase).
behave more or less independently from one another. Now the source of (subsurface) oxygen will be limited by the volume of one such particle. Thus if all or most of the oxygen in a particle which is located at the surface of the specimen has been removed by electron stimulated desorption a deeper lying particle must provide its oxygen to replenish the surface concentration. This will require diffusion across grain boundaries, which could very well be slower than bulk diffusion in an otherwise perfect lattice leading to a larger net loss of oxygen in the steady state as compared to the single crystal surfaces. The data obtained for the single crystal specimens of (110) and (100) orientation and the pressed pellets of the rutile and anatase structure are thus quite similar. Despite the sometimes severe negative charge on the surfaces oxygen can be removed by incident electrons. This process can be avoided by admitting 10m6 mbar of oxygen to the system during the electron irradiation. 3.2.4. Carbon deposition On all surfaces and under all conditions of electron beam current (some) carbon was deposited on the surface, a higher beam current in general leading to a faster deposition of carbon. This deposition must be assisted or influenced by the presence of the electron beam as no carbon was observed on unirradiated places on the sample. It should be noted that carbon deposition can be completely suppressed on all surfaces studied by admitting an oxygen pressure
Surface Science 99 (19%) 133-143
of lop6 mbar during irradiation. In this case also no loss of oxygen from the surface occurs. A first question is obviously about the nature of the carbonaceous species. For the highest coverages obtained (C/Ti 2 10) a lineshape analysis was attempted by numerical differentiation of the spectra. Comparison with the standard reference spectra [15] made it most likely that the deposit is in this case graphite. The initial deposit at low coverage cannot be identified by this method. We do believe it to be carbidic on the ground that it can be made to react with oxygen under the influence of the exciting electron beam. The small carbon peak observed in this case did disappear completely when the surface was exposed to lop6 mbar of oxygen whilst keeping the electron gun on. The graphitic carbon present in a later stage did not react with oxygen under similar experimental conditions. As graphite is well known to be inert to oxidation this observation confirms the assignment of the ‘thick’ carbon overlayer to graphite. The fact that no carbon is deposited on a surface when irradiation takes place in the presence of 10m6 mbar of oxygen in the background gas indicates that any carbon, if deposited at all, is readily removed by reaction before a significant amount of graphite can be formed. A second question that might be asked is about the actual amount of carbon deposited. The standard procedure to answer this question would be to measure the absolute peak heights of the titanium peak as a function of the carbon coverage and use the well known relation: Z(d) = I(O)exp - (d/h)
(4) were A is the inelastic mean free path of titanium Auger electrons in the overlayer to determine the l?yer thickness d. The value of h was taken as 13.1 A [16] assuming the overlayer to be graphite. For those few data sets with sufficiently accuracy in the absolute peak heights (of course, ratio’s as reported thus far are less susceptible to random fluctuations) we found the estimates for the (110) and (100) surface: d(i)
= (0.19~0.05);,
TiO,( lOO), TI (5)
d(i)
= (0.14~0.01)4,
TiO,( 110). TI
A.J.M. Mew O.L.J. Gijzeman/Applied Surface Science 99 (19%) 133-143
The agreement between these results is encouraging and may indicate that the estimates are at least reasonable. It implies, however, that for the highest C/Ti ratio found (= 35) the layer thickness is only some 5 to 6 A or, in other words, 2 to 3 monolayers of graphite. In hindsight this may not be too surprising. Graphite is a rather inert substance, once a more or less closed layer has accumulated on the surface the surface ceases to be reactive for further adsorption and carbon deposition stops. The different amounts of carbon that can be deposited on the single crystal surfaces (2 to 3 monolayers) as compared to the pressed pellets (around 0.2 monolayers according to the estimate given above) may be explained by the roughness of the surfaces. The single crystal surfaces are expected to be flat, ideally even atomically flat. A graphite layer on the surface then needs not to be strained in order to keep contact with the underlying surface. For the rough surfaces of the pellets on the other hand, an extended graphite layer cannot be formed due to the high stress that must accumulate in this layer if it were to fit closely on the underlying surface. Finally we may wonder about the mechanism of the carbon deposition. Experimentally it is found that carbon deposition only takes place under the influence of the irradiating electron beam, as no carbon is deposited on unirradiated places of the sample. The rate of carbon deposition could, of course, be increased by deliberately admitting a carbon containing gas such as carbon monoxide or propylene. Denoting the carbon containing species by C, two different mechanisms may be envisaged. One is the case where this species first adsorbs reversibly on the surface and is decomposed by the incoming electrons, schematically represented by:
141
Accepting our previous estimate of the carbon concentration the rate of carbon built-up is seen to be 3 x lop5 monolayers per second or 3 X lo-” molecules cm-’ s- ‘. Assuming reversibly adsorbed gas phase molecules to be non-reversibly adsorbed under the influence of the electron beam we may estimate the rate of adsorption as: ?-= #%_olv,,
(8)
where the last step requires the presence of the electron beam. An alternative mechanism could be the adsorption of gas phase (positive) ions of C,, formed by ionisation of gas phase C,:
where Nad is the number of adsorbed molecules per unit area and cr is now the cross-section for electron stimulated decomposition. Inserting the experimental values for the rate of adsorption and an estimated value of lo-l6 cm2 for the cross-section the number of adsorbed molecules is found to be 5 X lo6 cme2. This implies, however, that at a reasonable pressure of 1 mbar the number of adsorbed molecules must be in the order of 5 X lOi cmW2 , or almost one monolayer. As no measurable adsorption on undamaged TiO, from residual gas molecules such as carbon monoxide or methane has been reported [I], we may exclude this mechanism for carbon deposition if the surface was indeed free of defects. Experimentally, however, we found that the surface under electron irradiation does not have the normal TiO, stoichiometry, a sizeable amount of oxygen being absent. So we may expect the surface to exhibit many Ti sites, where adsorption could be much enhanced. The mechanism would then effectively be the adsorption of residual gas on titanium, a situation that exists only at the irradiated part of the surface. The alternative mechanism, adsorption of positive ions on the negatively charged surface, requires an effective ion flux of around 3 X lOlo cm-’ SK’. The flux caused by gas phase positive ions may be estimated in two ways. Assuming a reported ionisation efficiency of 2 ions formed per Torr gas pressure per centimeter path length per incident electron [ 171 an incident electron beam of 6 X 1019 electrons per square centimeter per second will produce 1.2 X 10” ions cm-* ss’ at a pressure of lo- I0 mbar. Alternatively we may estimate the ion flux &,ns as:
C, -+ C:
&I,, = 4ecapL
C,(g)
+ C ,(ad)
+ xC(ad)
+ xC(ad)
(6)
(7)
where now the first step requires the presence of the impinging electron beam and the positive ions formed will be accelerated toward the negatively charged part of the crystal.
(9)
where +e is the incident electron flux, u the ionisation cross-section, p the gas density and L the length of the irradiated volume. At a pressure of lo-” mbar and using an ionisation cross-section of
142
A.J.M. Mens, O.L.J. G&eman/Applied
1O- I6 cm2 [ 181 we obtain for the ion flux a value of 1.4 X 10” cm-’ s- ‘. This implies a rate of carbon deposition of 1.4 X 10” cm-’ SC’ at a background mbar. Experimentally we found pressure of lo-” this rate to be 3 X 10” cmm2 se2. As the pressure during a measurement was always at least one order of magnitude higher than the figure of lo-” mbar used in our estimate in Eq. (9), the mechanism involving carbon deposition through gas phase ions is certainly possible. In order to corroborate this idea an experiment was performed were the parameters of the electron gun were kept unchanged at a beam energy of 2 keV and fixed (but rather low) emission current as measured on the spectrometer power supply during the following sequence of events: - measure the 0 and Ti Auger peaks ten times (the peaks shifted upwards); - admit propylene as a carbon containing impurity in the residual gas at a pressure of 1O-5 mbar during ten minutes (no measurements were possible at this pressure); - decrease the propylene pressure to 10e7 mbar and measure the spectra again. We found that now the oxygen (and titanium) Auger peaks has shifted to lower energy and continued to shift continuously to lower energy. This suggests that the surface becomes neutralised by positive propylene ions, which must leave carbon on the surface. This trend continued when the background pressure was reduced to 5 X lo-* mbar. Only after switching on the ion pump, which decreased the pressure to below 10e9 mbar was this trend reversed and the usual negative charging of the surface reestablished. This experiment shows unequivocally that gas phase ions do adsorb at the surface, so the assumption that they do so also at a much lower pressure, where they cannot neutralize the surface, is by no means unrealistic.
4. Conclusions . Different kind of samples of TiO, (single crystals, rutile and anatase pellets) show electron stimulated desorption of oxygen. The amount of oxygen lost is substantial for all samples.
Surface Science 99 (1996) 133-143
+ The samples are always negatively charged by the incoming electron beam, surface charges may be as high as 800 V, without preventing the measurement of Auger spectra. This charge resides homogeneously in the irradiated area only. - Surface contamination by carbon (graphite) can occur via a mechanism of adsorption of carbonaceous species on the defect surface followed by electron induced decomposition. Decomposition of positively charged gas phase ions on the negatively charged surface does also take place. - Both the loss of oxygen from the surface and the deposition of carbon can be suppressed by the addition of 10m6 mbar oxygen to the background gas.
Acknowledgements The authors would like to thank Mr. Jan van de Loosdrecht for the preparation and characterisation of the TiO, pellet samples. Helpful discussions with lr. Jan Verhoeven and Dr. Henk Hopman (AMOLF, Amsterdam) contributed to the final version of this work.
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Gijzeman/Applied
[14] J.T. Mayer, U. Diebold, T.E. Madey and E. Garfunkel, J. Electron Spectrosc. Rel. Phen. 73 (1995) 1. [15] T.W. Haas, J.T. Grant and G.J. Dooley, J. Appl. Phys. 43 (1972) 1853. [16] S. Tanuma, C.J. Powell and D.R. Penn, Surf. Interf. Anal. 11 (1988) 577.
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[17] A. Roth, Vacuum Technology (North-Holland. Amsterdam, 1982) ch. 6. [18] L.C. Feldman and J.W. Mayer, Fundamentals of Surface and Thin Film Analysis (North-Holland, New York, 1986).