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The adsorption of liquid and vapor water on TiO2(110) surfaces: the role of defects
surface science ELSEVIER
Surface Science 344 (1995) 237-250
The adsorption of liquid and vapor water on TiO2(110) surfaces: the role of defects Li-Qiong Wang
a,*,
D.R. Baer
b, M.H.
Engelhard
b, A,N.
Shultz c
Materials and Chemical Sciences Center, Pacific Northwest National Laboratory, Riehland, WA 99352, USA b Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA c Department of Physics, Oregon State University, Corvillas, OR 97331, USA
Received 13 March 1995; accepted for Publication4 August 1995
Abstract The adsorption of liquid and vapor water on defective and nearly defect-free TiO2(110) surfaces has been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). The study focuses on examining electronic defects as created in vacuum and after exposure to both liquid and vapor water. Defective surfaces were prepared by electron-beam exposure and Ar + bombardment. With exposure up to 104 L low vapor pressure ( < 10 -s Torr) water to defective surfaces, little change on Ti 3d defect intensity was observed. However, defect intensities were greatly reduced after exposing defective surfaces to ~ 108 L higher vapor pressure (0.2-0.6 Torr) water for 5 min. More significantly, XPS and UPS spectra showed that electron-beam induced defects were completely removed upon liquid water exposure, while defects created by Ar ~- bombardment were only partially removed. Surface defects created by Ar + bombardment were removed more readily than sub-surface defects. Water adsorption on the surface has been quantified using the OH signal from the O ls photopeak. For a nearly defect-free surface, water coverage was ~ 0.02 ML at 104 L exposure to low vapor pressure water, ~ 0.07 ML at 10 s L exposure to higher vapor pressure water, and ~ 0.125 ML with liquid water exposure, respectively. Keywords: Chemisorption;Electron bombardment;Ion bombardment;Low index single crystal surfaces; Surface electronic phenomena;
Titanium oxide; Visible and ultravioletphotoelectron spectroscopy; Water; X-ray photoelectronspectroscopy
1. Introduction There has been considerable interest in the study of surfaces of rutile TiO 2 since the discovery of the photocatalytic behavior of rutile TiO 2 and its role in the photodecomposition of water [1]. Numerous ultrahigh vacuum (UI-IV) studies of the interaction of water with TiO 2 single-crystal surfaces have been reported [2-17]. Much work has focused on TiO 2
* Correspondingauthor. Fax: +7 509 375 2186.
surface defects and their role in the adsorption and dissociation of vapor water. S e v e r a l studies [2-5] have found little change in defect density upon exposure to vapor water. In particular, Henrich et al. [2] have reported that defect density was unaffected even by exposure to yapor water up to 10 s L. However, small amounts of Ti 3d defect states were observed to become depopulated after water expOsure in other studies [6-11]. In contrast, Kurtz et al. [12] have reported that the dissociative adsorption of water results in the increased Ti 3d defect intensity
0039-6028//95//$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00859-4
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L.-Q. Wang et aL / Surface Science 344 (1995) 237-250
on a nearly perfect surface, while for slightly defective surfaces, the defect intensity varied little upon water exposure. A coherent picture of the stability of defect states after water exposure is still missing. Surface defects are believed to strongly influence the reactivity of oxide surfaces. The defects observed or created in vacuum may not be present in solution. ff surface defects (Ti 3d defect states) are completely removed by water exposure, these electronic defects may not play an important role in determining the electrochemical and photocatalytical properties of rutile TiO 2 since most of these reactions are carded out in aqueous media. Thus, it is important to determine the stability of defects upon water exposure in order to have a better understanding of the role of defects in electrochemical and photocatalytical reactivities. Interesting questions include: the stability of Ti 3d state defects upon exposure to vapor and liquid water, the comparison of the adsorption of the vapor water with the liquid water, and the role of defects in chemical reactivity. A primary objective of this work was an exploration of defect stability in liquid water. However, this also required us to address the above questions by examining electronic defects as created in vacuum and after exposure to both low-pressure ( < 10 -5 Torr) and higher-pressure (0.2-0.6 Torr) vapor water. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) were used to monitor the presence of electronic defects. The defective surfaces were prepared by Ar + bombardment and low-density ( ~ ~ A / c m 2) electron-beam exposure described in the previous work [18]. The results of adsorption at low vapor exposure have been compared with the literature and with results obtained from exposure to higher vapor and liquid water. To our best knowledge, there has been no study on interactions of liquid water with TiO2(110) using XPS and UPS. In an early paper by Lazarus et al. [13], TiO2(001) reduced and scraped surfaces were studied by exposure to liquid water in air. Because of the surface preparation, their " c l e a n " surface's spectra [14], unlike the spectrum of clean TiO2(ll0) in Ref. [15], had a very strong high energy shoulder on the O ls peak, indicating the presence of impurity or defect sites. Previous studies [6,12] have shown that the amount of water adsorbed as OH species on TiO2(110) surfaces at 300 K was well below one
monolayer ( < 0.1 ML) and water saturated at exposure of < 10 3 L. In those studies, water exposure was often carried out in a main vacuum chamber, allowing exposure only to the low-pressure ( < 10 -5 Torr in most cases) vapor water. A relatively low exposure range was probed since it was difficult to achieve the high exposure in reasonable short time. Since liquid exposure will inherently involve some high vapor pressure exposure, it is of interest to know if higher-pressure vapor water (or liquid water) would increase the water coverage on the surface. Thus, in this study, we examine defective and nearly perfect surfaces by exposing not only to low-pressure vapor water but also to higher-pressure vapor and liquid water.
2. Experimental An 8 × 8 × 1 m m 3 polished TiO2(ll0) crystal from Princeton Scientific was used in this study. The TiO2(110) sample was reduced through a long period of heating in UHV at ~ 1000 K so that there was no charging problem during XPS and UPS measurements. The surface was cleaned by 15 min of 4 keV Ar + bombardment (current density of ~ 10 /~A/cm2). The nearly defect-free surface was then obtained by annealing the sample at 1000 K for 15 min in 1 × 10 -7 Tort 02, then slowly cooling down to room temperature in the same oxygen atmosphere for 30 min. The highly defective surfaces were prepared by Ax + bombardment at 4 keV for 10 min, while a relatively low concentration of defects were created by low-density electron-beam exposure. Electron beams were provided by an Auger electron gun coexisting with a cylindrical mirror analyzer (CMA). In this study, a 10 /xA 1 kV electron beam was rastered over an area of 0.25 cm 2 for 30 rain. For comparison, approximately 90% of the XPS signal comes from an area of ~ 0.1 cm 2 [19]. Thus most of the XPS signals arise from beam exposed material. The XPS measurements were made on a Physical-Electronics 560 multiprobe system that uses a double pass cylindrical mirror analyzer (CMA). Non-monochromatic M g K a X-rays were used to generate the spectra. During these experiments the base pressure of the system was about 8 × 10 -a°
L.-Q. Wang et al. / Surface Science 344 (1995) 237-250
UPS experiments ( H e I radiation at 21.2 eV) were also performed in the 560 system. As also discussed in Section 4.1, water exposure were made in more than one way and produced
Torr. The multiplex data were collected with a pass energy of 25 eV. The collected data were referenced to an energy scale with binding energies for Au 4f at 8 4 _ 0.03 eV and Cu2p3/2 at 9 3 2 . 6 7 _ 0.03 eV.
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L.-Q. Wang et al./ Surface Science 344 (1995) 237-250
consistent results. An ultrahigh vacuum (UHV) compatible transfer system connected to the 560 system allowed transfer of the specimen prepared in vacuum for low-pressure, higher-pressure ( > 10 -3 Torr) vapor or liquid water and return the specimen to the spectrometer for analysis. For exposures at 10 - 6 Torr or less, the dosing could be conducted either in the main vacuum chamber or in the transfer system. The consistency of the low pressure results in both systems and the similarity of both results with much of that reported in the literature demonstrate the reliability of the transfer system used for high pressure and liquid exposures. To minimize contamination for liquid water exposure in this study, the distilled and deionized liquid water was contained in a small high purity Teflon cup then put inside a 2.75 inch diameter standard UHV vacuum " T e e " with a window attached. The system was processed with many freeze-pump-thaw cycles before sample exposure. After the desired UHV preparation, a sample was transferred to the liquid compartment which was evacuated slightly, allowing the water splash from the cup to uniformly wet the whole surface. After exposing the sample to liquid water for about 2 - 3 min without pumping, we then transferred it out of
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3. Results 3.1. Water interaction with nearly defect-free surfaces Ti 2p and O 1s XPS spectra and UPS spectra for nearly defect-free surfaces exposed to both vapor
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the liquid compartment. It normally takes about 1 h to pump out before the sample is returned to the main chamber. The whole setup used UHV parts and connections and was thoroughly baked before adding the water. Although the transfer system could not be maintained at UHV conditions during water exposure, the background gas was water vapor. To arrange water to contact only TiO 2 crystal surface, the TiO2(110) crystal was mounted on a Ta plate with a small amount of molten indium spread over the interface. Due to the limitations of the spectrometer and having specimen transport system, the front of crystal used for this work was heated by an electron filament, rather than by resistance heating used in the previous study [18]. Nearly defect-free surfaces were successfully prepared by using this heating method and no contamination was detected by AES or XPS.
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L.-Q. Wang et al. / Surface Science 344 (1995) 237-250 • Low Vapor Pressure (5E-6 Torr) • HighVapor Pressure(0.2-0.6 Torr) • Li, aid Water
ing at higher binding energy (BE) was observed after additional exposure to 0.6 Torr H 2 0 for 5 min ( ~ 10 8 L). Furthermore, significantly more broadening was observed after liquid water exposure. In contrast, both Ti 2p XPS and UPS spectra showed minimal changes in defect intensity even after liquid water exposure. However, close examination of the Ti2p3/2 XPS and UPS spectra shows that the nearly defect-free surface became closer to perfect after exposure to higher vapor pressure water and liquid water. Previous studies [5,6,12] have shown that water dissociatively adsorbed on TiO2(110) surfaces at 300 K. Based on these previous studies, we assume that the O ls peak broadening after water exposure on defect-free surfaces is solely due to the OH species adsorbed on the surface (we also assume that the OH served to " h e a l " the small number of defects present after the initial surface preparation). The OH area ratios have been obtained from the fits to the O ls XPS spectra. A symmetric Gaussian-Lorentzian function was used to fit the O ls spectrum for a nearly defect-free surface before water exposure, while two symmetric GaussianLorentzian functions were used for a nearly defect-
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and liquid water are shown in Figs. la, lb, and 2, respectively. For nearly defect-free surfaces, we observed symmetric Ti 2p and O 1s peaks in XPS spectra and a small amount of Ti 3d defect intensity in UPS spectra. From Fig. lb, little change in the O ls peak was observed after exposure to 4.5 × 10 -6 Torr H 2 0 for 5 rain ( ~ 103 L). An obvious broaden-
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L-Q. Wang et al. / Surface Science 344 (1995) 237-250
242
ML at 108 L exposure to higher-pressure vapor water, and ~ 0.25 ML with liquid water exposure. Based on K u r r s model [12] where one water molecule is dissociated into one OH bonded to the five-fold Ti atom and one H which is then attached to the bridging oxygen to form another OH, the water coverage was 0.02 ML, 0.07 ML, and 0.125 ML for exposure to 104 L low vapor water, 108 L high vapor water and liquid water, respectively. Approximately half of the five-fold Ti atoms and half of the bridging oxygen atoms become hydroxylated upon liquid water exposure (water coverage of 0.125 ML). A previous study [6] has reported a water saturation coverage of 0.07 ML for a nearly defectfree TiO2(110) surface. This value is the same as we obtained at 10 s L exposure to higher vapor pressure water, but is lower than that of liquid water exposure.
free surface after water exposure (using parameters identified below). The part of contribution from the clean TiO 2 for a water exposed surface was fitted by fixing FWHM (full width at half maximum) and percentage of Gaussian (%G) to the values obtained for a nearly defect-free surface before water exposure. Typical FWHM and %G for a nearly defect-free surface before water exposure are 1.6 eV and 90%, respectively, while the energy shifts from the bulk oxygen, FWHM, and %G for the part of contribution from the OH species are 1.6, 1.1-1.6 eV and 100%, respectively. Two-function fits for water exposed surfaces are very good. The area ratio of OH versus the log of the H 2 0 exposure (L) is shown in Fig. 3. At low exposure to low-pressure vapor water, the OH concentration increases slowly with the exposure, while at higher exposure to higher-pressure vapor water, the OH concentration continuously goes up and reaches up to ~ 12% for the liquid water exposure. In this study, we define that 1 ML coverage is equivalent to the surface atomic density of 2 × 1015/cm 2. Using a similar method described in a previous work [18], we estimate that for a nearly defect-free surface, OH coverage was ~ 0.04 ML at 10 4 L exposure to low vapor pressure water, ~ 0.14 160
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L.-Q. Wang et al. / Surface Science 344 (1995) 237-250
centration of 3.3 × 1014/cm 2 Ti 3+ corresponds to 1.7 × 1014/cm 2 oxygen vacancies if we assume that one missing bridging oxygen introduces one oxygen vacancy and two Ti 3+. Fig. 4 shows little change in
previous work [18], we estimate that the number of Ti 3+ defects induced by electron-beam exposure in the present study is about 1/3 of the top layer Ti (0.33 × 1 × 10]5/cm 2) on the (110) surface. A con-
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244
L.-Q. Wang et al./ Surface Science 344 (1995) 237-250
the Ti2p3/2 peak after exposure to 4.5 X 10 -6 Torr H 2 0 for 5 min ( ~ 10 3 L), while less broadening at lower BE was observed after additional exposure to 0.6 Torr H 2 0 ( ~ 108 L) for 5 min. A symmetric peak was obtained upon liquid water exposure. This peak (after liquid water exposure) is nearly identical to or sometimes slightly narrower than that of "nearly defect-free" data (similar to Fig. 1). In previous work [2,18], broadening of the Ti2p3/2 peak at lower BE has been attributed to surface defects. In agreement with previous studies [2-5], our current work shows that defect intensity varies little at lowpressure water exposure. However, electron-beam induced defects were removed substantially by higher vapor water exposure and almost completely by liquid water exposure. UPS spectra shown in Fig. 5 confirmed the above XPS results. The Ti 3d defect intensity was reduced significantly by higher vapor exposure, while no defects were observed after liquid water exposure. Fig. 6 gives Ti2p and O ls XPS spectra for a nearly defect-free surface, an electron-beam exposed surface, and an electron-beam exposed surface after 0 2
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exposure, respectively. The spectra for the nearly defect-free surface and the electron-beam exposed surface after O 2 exposure are well overlapped, indicating that electron-beam induced defects were completely healed by 02 exposure (10 -7 Torr, 30 min) at 300 K. A previous study [6] has reported that oxygen at 300 K and at much lower exposure ( ~ 10 L) can heal most of defects on slightly defective surfaces prepared by thermal annealing. Similarly, our experiment showed that electron-beam induced defects were completely healed by much lower 02 exposure than the water exposure. To determine the thermal stability of healed surfaces, both 0 2 and H 2 0 healed surfaces were heated to several different temperatures and then cooled down to room temperature for acquiring data. For 02 healed surfaces, both Ti2p and O ls XPS spectra (not shown here) did not change upon heating to temperatures, 420, 670 and 870 K. For H 2 0 healed surfaces, Ti 2p XPS spectra did not change when the surface was heated to temperatures: 320, 550, 720, 800, and 870 K, while O Is spectra did not change only up to 550 K. The O ls spectra showed less
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-454
245
L.-Q. Wang et aL / Surface Science 344 (1995) 237-250
broadening at high BE when the temperature was increased to above 550 K (probably d u e to loss of H 2 0 or H 2)
were only partially removed: the surface defects are removed more readily than sub-surface defects. A previous study [6] has also shown that 0 2 at saturation coverage only partially healed the Ar + bombarded surface. This was explained by the formation of localized reduced sub-surface oxide structures such as Ti20 3 and TiO due to the oxygen vacancies in the sub-surface region [6]. The current study has shown that at sufficiently high exposure, water can heal electron-beam induced defects and part of the wider variety of surface defects produced by Ar + bombardment [18]. The diffusion of water or oxygen to the sub-surface may be the main controlling factor for the removal of Ar + bombardment defects, although there may be other explanations. It is difficult to accurately calculate the amount of OH species on the defective surface because defects also contribute to the broadening at higher BE in the O ls peak. However, based on a rough estimation, we found no obvious difference in OH concentrations for a nearly defect-free surface and a defective surface prepared by electron-beam exposure, but a slightly higher OH concentration was observed for a defective surface prepared by Ar + bombardment.
3.3. Water interaction with defective surfaces prepared by A r + bombardment
Ti 2p XPS and UPS spectra for a defective surface prepared by Ar + bombardment before and after water exposure are given in Figs. 7 and 8, respectively. The XPS spectra show that after liquid water exposure the Ti2p3/2 peak at lower BE becomes narrower as compared with the Ar + bombarded surface before water exposure, but is still much broader than the nearly defect-free surface. This shows that many defects remained near the surface after water exposure. However, UPS spectra shown in Fig. 8 present a different picture: almost no Ti 3d defect states were observed after liquid water exposure. Since UPS (He I 21.2 eV) is more surface sensitive than Ti2p XPS, and Ar + bombardment (4 keV) creates substantially amounts of sub-surface defects, we conclude that defects created by Ar + bombardments
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L.-Q. Wang et al. / Surface Science 344 (1995) 237-250
4. D i s c u s s i o n
The following discussion focuses on comparison of vacuum and solution experiments, water coverage for a nearly defect-free surface, the kinetics of water interaction with TiO2(110) surfaces, water saturation coverage versus initial defect concentration, " h e a l , i n g " of defects after water exposure, and surface defects versus sub-surface defects. 4.1. Comparison o f vacuum and solution experiments A n y comparison of vapor and solution exposure introduces issues regarding the nature and type of exposure the surface has seen. Before questions of coverage and defect healing can be addressed, some consideration must be given to the nature of the exposures, impurities and vacuum conditions. Exposure to liquid water introduces the possibilities of interactions not possible during simple vapor work. In addition to allowing movement of ions and dissolved materials away from the specimen, the surface is exposed to any ions in the solution, including the ~ 10 -7 molar concentration of H + and O H - ions. Since water is ~ 55M, approximately 2 out of every 10 9 molecules in solutions is an O H - . Since the density of water is 3 × 10 22 molecules cm -3, this means that a monolayer equivalent of O H - ions (assuming atomic density of 1015 m o l e c u l e s / c m 2 for one monolayer) are within 17
cm of the surface. Since the diffusion coefficient for O H - ions in water is 5 × 10 -5 cm 2 s -1, only < 1% of a monolayer coverage could come from interaction of O H - ions with TiO 2 in solution in 200 s. As mentiQned in the experimental section, we went to considerable effort to minimize impurities of liquid water. Hbwever, due to the system processing and the nature of sample exposure, it is not easy to examine the exact water purity as used during the experiment. However, conductivity and other indications suggest that concentrations of impurities in liquid water were less than ~ 1 ppm. Although the exact impurity concentrations were not known, several statements can be made with confidence. First, no build up of ions from solution were observed on the sample. Thus, no evidence of F - , C I - , Ca 2+ or CO 2- was observed after liquid water exposure. Second, as noted above, at the low concentrations, diffusion in water and the times of exposure do not allow much impurity interactions with the surface in solution. Special care was taken to make vacuum conditions as best as possible when working with the water. The water was pumped, frozen and otherwise processed in U H V compatible equipment. Although some preliminary studies were made using the "altered" standard electrochemical corrosion cell, concern about 0 2 contamination in the N 2 cover-transport gas used caused us to devise an alternate system in which no such gas was used. Thus, for the results
Table 1 Comparison of water coverage Reference This work
Coverage (ML) (< 10-5 Ton')
( > 10 - 2
Tort)
(Liquid)
Exposure (L)
Temperature (K) 300
0.125 -
104 108 (liquid) 102 102
300 300
-
NA
NAb
-
103
300
106
300
0.02 0.07
Ref. [3] Ref. [15] Ref. [12] Ref. [17]
0.07 0.15 a 0.07 b 0.1 (0.03-0.05) c 0.125
a The coverage of 0.15 ML is for a highly defective surface. b The coverage of 0.07 ML was obtainedfrom an estimated OH coverage assumingthe high temperature tail in the 275 K TPD peak due to hydroxyl disproportionation. c The coverage was estimatedby assumingthat water adsorbed at 160 K corresponds to about one to two water moleculesper unit cell [15].
L.-Q. Wang et al. / Surface Science 344 (1995) 237-250
reported here, the sample was exposed to only the water vapor (vapor of the pumped water source) and ambient gases in the transfer system. A check of the transfer process showed that exposure to the gases in the transfer system did not heal the defects. Furthermore, XPS spectra have shown that defects prepared in UHV were stable in our vacuum condition for at least several hours. We have also done similar experiments in slightly different ways to ensure consistency. Several different types of doses used in the experiments are: (1) low-pressure vapor water (exposure up to 103 L) in UHV chamber with glass water source as does in previous studies [3,12]; (2) olow-pressure and high-pressure vapor water in transfer chamber with the glass source; (3) high pressure in transfer chamber with Teflon beaker source, and (4) liquid water in transfer chamber with Teflon beaker source. Similar results of defects "healing" after water exposure were found for different chambers and possibly different "clean" water sources. Furthermore, low exposure (almost no impurity), high exposure and liquid water exposure showed the same results: Ti 3+ defects were healed upon water exposure while the OH concentration was increased as increasing water exposure. 4.2. Water coverage f o r a nearly defect-free surface
Different groups have measured and reported water coverages in different ways. Although comparisons are not easily understandable, they are reported here in Table 1 to provide background and context for current work. This study has reported that water coverage was 0.02, 0.07, and 0.125 ML and percentages of hydroxylated five-fold Ti atoms were 8, 25, and 50% for exposure to 10 4 L low vapor water, 108 L high vapor water and liquid water, respectively. Water coverage of 0.125 ML (we defined 1 ML as 2 × 1015/cm2 or 4 molecules per unit celI)- upofi liquid water exposure for a nearly defect-free surface is equivalent to one water molecule for every two unit cells. For a defect-free or a slightly defective surface, the liquid exposure coverage of 0.125 ML from this study is larger than the saturation coverage of 0.07 ML upon vapor exposure reported by Pan et al. [3]. Pan et al. [3] have reported the coverage of 0.15 ML for a highly defective surface prepared by Ar + bombardment. Hugenschmidt et al. [15] have
247
estimated the OH coverage of 2.6 × 1014/cm 2 (equivalent to ~ 0.07 ML water coverage in our definition) by assuming the high-temperature tail in the 275 K TPD peak due to hydroxyl disproportionation. Kurtz et al. [12] have reported a saturation coverage of 0.1 ML based on a comparison of the intensity of the 10.8 eV O H - 30- feature in the UPS spectrum with the intensity of the 13 eV lb 2 level of 1 monolayer (ML) of molecular water at 160 K. They defined 1 ML in terms of an adsorbed H 2 0 monolayer on the TiO2(ll0) surface at 160 K and assumed that water adsorbed at 160 K corresponds to about four water molecules per surface unit cell, whereas XPS data taken by Hugenschmidt et al. [15] showed amount of water to be closer to one and no more than two per unit cell. The water saturation coverage observed by Kurtz et al. [12] would be ~ 0.025-0.05 ML assuming that water adsorbed at 160 K corresponds to about 1-2 water molecules per surface unit cell [15]. Patthey et al. [17] have also studied hydroxylated TiO2(110) surfaces in relating to the biocompatibility of Ti implants and reported a water coverage of 0.125 ML upon exposure to 10 6 L vapor water. This coverage is higher than that obtained from this work for the same amount of vapor water exposure. However, they used a different method to calculate the water coverage than was used in this work. In the study by Patthey et al. [17], the water coverage was calculated from the intensity ratio of the two surface shifted core levels (O ls). One of these, at a shift of 1.18-1.33 eV compared to the bulk oxygen, was attributed to the surface bridging oxygen. The other, at a 2.4 eV shift, was attributed to the basic OH species bonded to the five-fold coordinated Ti atoms. In this study, the OH area ratios were obtained by fitting the O 1s peak with one additional symmetric Gaussian-Lorentzian function for the OH contribution. The fits are good without additional peaks at lower binding energy. Since Patthey et al. [17] have taken XPS spectra at a glancing electron-emission angle, in contrast to the angle integrated analyzer as used in this study, their clean TiO2(ll0)spectrum has a high BE shoulder which was attributed to the surface bridging oxygen. They also did not use the same set of parameters (FWHM and the core lever shifts) to fit the bulk oxygen contribution for both clean and hydroxylated TiO 2 surfaces. Furthermore,
248
L.-Q. Wanget al. / Surface Science344 (1995)237-250
the contribution from the acidic OH species which may appear at BE near the surface oxygen region was ignored in their calculations. Since the exact binding energies for basic OH, acidic OH, or surface bridging oxygen are not known, fitting the O ls spectrum with three peaks signifies uncertainty. Thus, we can't directly compare our results with those obtained by Patthey et al. [17]. 4.3. Kinetics of water interaction with Ti02(110) surfaces
The fact that water did not saturate at 109 L exposure as shown in Fig. 3 suggests that the interaction of water with TiO2(l10) surfaces may be an activated (or kinetically controlled) process, depending on the water vapor pressure and length of exposure. It is possibly that previous studies [3,12] observed only a kinetic saturation of the state due to the low exposure range probed. The true saturation coverage of hydroxyls is likely much higher, as indicated by the present study. As suggested by Kurtz et al. [12], the kinetic barrier in the process may be the interaction which requires substantial molecular tilt and displacement from an atop Ti site. This study has shown that higher water coverage can be achieved by increasing water exposure. The simple reason for this is that at higher water exposure, more molecules are able to overcome the activation barrier and become dissociated OH adsorbed on the surface. Although water saturation coverage is increased substantially upon liquid water exposure, a coverage of 0.125 ML is still low. As suggested by Kurtz et al. [12], the low saturation coverage seems to imply that the occupation of one adsorption site by dissociated OH species quenches the activity of neighboring sites with respect to additional HzO adsorption and dissociation. This effect could result from OH dipole-dipole interactions combined with subtle changes induced in the surface electronic structure by the adsorbate. However, Pan et al. [3] have concluded that the low saturation coverage for defect-free and slightly defective surfaces is not controlled predominantly by OH dipole-dipole interactions based on a higher uptake of H 2 0 on the highly defective surface. Thus, a combination of kinetic and structural studies is required to have a better understanding of water adsorption and dissociation on TiO2(110) surfaces.
4.4. Saturation coverage versus initial defect concentration
Kurtz et al. [12] have found that the water saturation coverage on TiO2(110) at 300 K was independent of the initial defect concentration. In agreement with Kurtz's work, Pan et al. [3] have also reported that water saturation coverage is independent of the oxygen vacancy concentration before water adsorption for defect-free and slightly defective surfaces. However, water coverage and the rate of dissociative adsorption were found to be higher for highly defective surfaces (by Ar + bombardmen0 than those for slightly defective (by thermal annealing) surfaces [3]. Our study has also observed a similar water coverage for nearly defect-free surface and defective surfaces prepared by electron-beam exposure, but a slightly higher water coverage for an Ar ÷ bombarded surface. It was found in a previous study [18] that defects created by electron-beam exposure and by thermal annealing have similar electronic structures, mainly composed of Ti 3+, while defects produced by Ar + bombardment are complex, with a variety of different local environments where oxygen and titanium surface atoms coexist. The defect concentration for electron-beam exposure in this study is larger than for thermal annealing, but smaller than for Ar + bombardment. Based on the previous and current studies [3,12,18], it appears that relatively simple defects created by electron-beam exposure and thermal annealing did not change the water coverage, but an increased adsorption observed for a higher concentration of more complex defects. This effect could be due to different electronic structure, a physically enhanced surface area or increased defect concentration. The fact that liquid water can remove not only defects induced by electron-beam exposure but also the (surface exposed) wider variety of surface defects produced by Ar + bombardment [18] indicates no obvious difference in defect reactivity for defects created by electron-beam exposure and Ar ÷ bombardment at high water exposure. 4.5. "Healing" o f defects
Additional questions arise concerning the nature of the oxygen- or water-" healed" surfaces. If the electronic signal for the defect is removed upon the
L.-Q. Wang et al./ Surface Science 344 (1995) 237-250
adsorption of a loosely bound molecule, the surface may be chemically " h e a l e d " but might be very different from a nearly defect-free surface which may be more stable than a " h e a l e d " surface. If water or oxygen were loosely bonded on the surfaces to heal the defects, the defects would reappear when we heated the surface to desorb some loosely bonded molecules. For O 2 healed surfaces, experiments showed that defects did not reappear upon heating to 870 K. The O ls XPS spectra did not change upon heating, indicating that oxygen healed surfaces are stable upon heating and that 0 2 is strongly bonded to the surface. For H 2 0 healed surfaces, experiments also showed that defects did not reappear upon heating to 870 K, while O 1s spectra did not change only up to 550 K. However, the O ls spectra showed less broadening at higher BE as the temperature was increased to above 550 K, indicating desorption (or disproportionation) of the hydroxyl species from the surface. TPD data given by Hugenschmidt et al. [15] have shown that surface hydroxyl species started to desorb from the defective TiO2(l10) surface at ~ 500 K. However, it is not clear why the defect intensity did not change upon heating to above 550 K while the hydroxyl species desorbed from the surface. Nevertheless, results obtained from the temperature at 320 and 550 K indicate that water healed surfaces are stable up to ~ 550 K and that water is chemically bonded to the surface in healing the defects. Recently, Shultz [20] et al. have reported that low-energy (4.7 eV) ultraviolet (UV) photons create stable Ti 3+ defects on TiO2(l10) surfaces in UHV. The adsorption and photodesorption of oxygen on the TiO2(110) surface have reported recently by Lu et al. [21,22]. The UV created defects are similar to the electron-beam induced defects, mainly composed of Ti 3+, and can be healed by exposing to 0 2 gas. This observation was interpreted as due to photo-desorption and re-adsorption of molecular oxygen bound loosely to Ti 3+ defects on the surface as Ti4+:O~ complex. Shultz [20] et al. assumed that UV photons at energy just above the band gap should not be energetic enough to directly remove a surface oxygen atom. However, the true nature of oxygen-healed surface is not clear and needs further study. Although electronic defects are healed by exposure, the surface with adsorbed hydroxyl species
249
is a chemically and structurally different surface from the surface without water exposure. Structural defects including physical defects or other factors may also played an important role in determining chemical reactivity for this surface. Thus it is important to study both electronic and geometric structures and their relationship using a combination of several techniques including STM, LEED, XPS, UPS, HREELS, IR, etc. As already stated, this paper deals with defects that perturb the chemical or electronic structure of the surface and are observable by XPS measurements. 4.6. Surface versus sub-surface defects
The difference between surface and sub-surface defects (as well as " c h e m i c a l " versus "physical" healing) raises many interesting questions. If surface defects (Ti 3d defect states) are completely removed by water exposure, these electronic defects (exposed on the surface) may not play an important role in determining electrochemical and photocatalytical properties of ruffle TiO 2 since most of these reactions are carded out in aqueous media. Our study has shown that surface electronic defects on TIO2(110) created by electron-beam exposure and thermal annealing can be completely removed upon liquid water exposure, while the sub-surface defects created by Ar + bombardment are still present after water exposure. Because photons can penetrate hundreds of nm in our study and electrochemistry generation of holes may occur over a high field region (more depth), in electrochemical and photocatalytical reactions, there are still sub-surface defects that can be photo or electrochemical active. Thus the sub-surface defects may play an important role in electrochemical and photocatalytical reactions when surface defects are removed.
5. Conclusions
The adsorption of liquid a n d vapor water on defective and nearly defect-free TiO2(ll0) surfaces has been examined using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Defective surfaces were prepared by
250
L.-Q. Wang et a l . / Surface Science 344 (1995) 237-250
low-density ( ~ 10 / ~ A / c m 2) electron-beam exposure and Ar+-ion bombardment. With exposure up to 10 4 L low-pressure ( < 10 -5 Torr) vapor water to defective surfaces, little change on Ti 3d defect intensity was observed, in agreement with previous studies [2-5]. However, defect intensity was greatly reduced by exposing to ~ 10 8 L higher-pressure ( 0 . 2 - 0 . 6 Torr) vapor water or to liquid water. More significantly, electron-beam induced defects completely disappeared upon liquid water exposure, while defects created by Ar + bombardments were only partially removed. Surface defects created by Ar + bombardments were removed more readily than sub-surface defects. For a nearly defect-free surface, the water coverage was ~ 0.02 ML at 10 4 L exposure to low-pressure vapor water, ~ 0.07 ML at 10 8 L exposure to high-pressure vapor water, and ~ 0.125 ML for the liquid water exposure. The fact that water did not saturate at 10 9 L exposure suggests that the interaction of water with TiO 2 surfaces may be a kinetic controlled process, depending on the water vapor pressure and length of exposure.
Acknowledgements The authors would like to thank Dr. V. Henrich, Dr. T. Madey, and Dr. M.A. Henderson for many discussions. This work has been supported by the Division of Materials Sciences Office of Basic Energy Sciences US Department of Energy (USDOE). Pacific Northwest Laboratory is a multiprogram national laboratory operated for the U S D O E by Battelle Memorial Institute under Contract DE-AC0676RLO 1830.
References [1] A. Fujushimaand K. Honda, Nature 238 (1972) 37. [2] V.E. Henrich, G. Dresselhaus and H.J. Zeiger, Solid State Commun. 24 (1977) 623. [3] J.M. Pan, B.L. Maschhoff, U. Diebold and T.E. Madey, J. Vac. Sci. Technol. A 10 (1992) 2470. [4] F.J. Bustillo, E. Roman and J.L. de Segovia, Vacuum 41 (1990) 19. [5] F.J. Bustillo, E. Roman and J.L. de Segovia, Vacuum 39 (1989) 659. [6] S. Eriksen, P.D. Naylor and R.G. Egdell, Spectrochim. Acta 43A (1987) 1535. [7] W.J. Lo, Y.W. Chung and G.A. Somorjai, Surf. Sci. 71 (1978) 199. [8] Y.W. Chung, W.J. Lo and G.A. Somorjai, Surf. Sci. 64 (1977) 588. [9] P.B. Smith and S.L. Bernasek, Surf. Sci. 188 (1987) 241. [10] C.A. Muryn, P.J. Hardman, J.J. Crouch, G.N. Raiker, G. Thornton and D.S.-L. Law, Surf. Sci. 251/252 (1991) 747. [11] (a) V.E. Henrich, Prog. Surf. Sci. 9 (1979) 143; (b) Rep. Prog. Phys. 48 (1986) 1481. [12] R.L. Kurtz, R. Stockbauer, T.E. Madey, E. Roman and J.L. de Segovia, Surf. Sci. 218 (1989) 178. [13] M.S. Lazarus and T.K. Sham, Chem. Phys. Lett. 92 (1982) 670. [14] T.K. Sham and M.S. Lazarus, Chem. Phys. Lett. 68 (1979) 426. [15] M.B. Hugenschmidt, L. Gamble and C.T. Campbell, Surf. Sci. 302 (1994) 329. [16] M. Henderson,Surf. Sci. 319 (1994) 315. [17] L. Patthey, E.L. Bullock, S.G. Steinemann,J. Redinger, D. Vogtenhuber and R. Podloucky,ECS (1994). [18] L.Q. Wang, D.R. Baer and M.H. Engelhard, Surf. Sci. 320 (1994) 295. [19] N.E. Erickson and C.J. Powell, J. Vac. Sci. Technol. A 4 (1986) 1551. [20] A. Shultz, W. Jang, W.M. HetheringtonHI, D.R. Baer, L.-Q. Wang and M.H. Engelhard, Surf. Sci. 339 (1995) 114. [21] G. Lu, A. Linsebiglerand J.T. Yates, Jr., J. Chem. Phys. 102 (1995) 4657. [22] G. Lu, A. Linsebiglerand J.T. Yates, Jr;, J. Chem. Phys. 102 (1995) 3005.
Surface Science 344 (1995) 237-250
The adsorption of liquid and vapor water on TiO2(110) surfaces: the role of defects Li-Qiong Wang
a,*,
D.R. Baer
b, M.H.
Engelhard
b, A,N.
Shultz c
Materials and Chemical Sciences Center, Pacific Northwest National Laboratory, Riehland, WA 99352, USA b Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA c Department of Physics, Oregon State University, Corvillas, OR 97331, USA
Received 13 March 1995; accepted for Publication4 August 1995
Abstract The adsorption of liquid and vapor water on defective and nearly defect-free TiO2(110) surfaces has been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). The study focuses on examining electronic defects as created in vacuum and after exposure to both liquid and vapor water. Defective surfaces were prepared by electron-beam exposure and Ar + bombardment. With exposure up to 104 L low vapor pressure ( < 10 -s Torr) water to defective surfaces, little change on Ti 3d defect intensity was observed. However, defect intensities were greatly reduced after exposing defective surfaces to ~ 108 L higher vapor pressure (0.2-0.6 Torr) water for 5 min. More significantly, XPS and UPS spectra showed that electron-beam induced defects were completely removed upon liquid water exposure, while defects created by Ar ~- bombardment were only partially removed. Surface defects created by Ar + bombardment were removed more readily than sub-surface defects. Water adsorption on the surface has been quantified using the OH signal from the O ls photopeak. For a nearly defect-free surface, water coverage was ~ 0.02 ML at 104 L exposure to low vapor pressure water, ~ 0.07 ML at 10 s L exposure to higher vapor pressure water, and ~ 0.125 ML with liquid water exposure, respectively. Keywords: Chemisorption;Electron bombardment;Ion bombardment;Low index single crystal surfaces; Surface electronic phenomena;
Titanium oxide; Visible and ultravioletphotoelectron spectroscopy; Water; X-ray photoelectronspectroscopy
1. Introduction There has been considerable interest in the study of surfaces of rutile TiO 2 since the discovery of the photocatalytic behavior of rutile TiO 2 and its role in the photodecomposition of water [1]. Numerous ultrahigh vacuum (UI-IV) studies of the interaction of water with TiO 2 single-crystal surfaces have been reported [2-17]. Much work has focused on TiO 2
* Correspondingauthor. Fax: +7 509 375 2186.
surface defects and their role in the adsorption and dissociation of vapor water. S e v e r a l studies [2-5] have found little change in defect density upon exposure to vapor water. In particular, Henrich et al. [2] have reported that defect density was unaffected even by exposure to yapor water up to 10 s L. However, small amounts of Ti 3d defect states were observed to become depopulated after water expOsure in other studies [6-11]. In contrast, Kurtz et al. [12] have reported that the dissociative adsorption of water results in the increased Ti 3d defect intensity
0039-6028//95//$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00859-4
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L.-Q. Wang et aL / Surface Science 344 (1995) 237-250
on a nearly perfect surface, while for slightly defective surfaces, the defect intensity varied little upon water exposure. A coherent picture of the stability of defect states after water exposure is still missing. Surface defects are believed to strongly influence the reactivity of oxide surfaces. The defects observed or created in vacuum may not be present in solution. ff surface defects (Ti 3d defect states) are completely removed by water exposure, these electronic defects may not play an important role in determining the electrochemical and photocatalytical properties of rutile TiO 2 since most of these reactions are carded out in aqueous media. Thus, it is important to determine the stability of defects upon water exposure in order to have a better understanding of the role of defects in electrochemical and photocatalytical reactivities. Interesting questions include: the stability of Ti 3d state defects upon exposure to vapor and liquid water, the comparison of the adsorption of the vapor water with the liquid water, and the role of defects in chemical reactivity. A primary objective of this work was an exploration of defect stability in liquid water. However, this also required us to address the above questions by examining electronic defects as created in vacuum and after exposure to both low-pressure ( < 10 -5 Torr) and higher-pressure (0.2-0.6 Torr) vapor water. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) were used to monitor the presence of electronic defects. The defective surfaces were prepared by Ar + bombardment and low-density ( ~ ~ A / c m 2) electron-beam exposure described in the previous work [18]. The results of adsorption at low vapor exposure have been compared with the literature and with results obtained from exposure to higher vapor and liquid water. To our best knowledge, there has been no study on interactions of liquid water with TiO2(110) using XPS and UPS. In an early paper by Lazarus et al. [13], TiO2(001) reduced and scraped surfaces were studied by exposure to liquid water in air. Because of the surface preparation, their " c l e a n " surface's spectra [14], unlike the spectrum of clean TiO2(ll0) in Ref. [15], had a very strong high energy shoulder on the O ls peak, indicating the presence of impurity or defect sites. Previous studies [6,12] have shown that the amount of water adsorbed as OH species on TiO2(110) surfaces at 300 K was well below one
monolayer ( < 0.1 ML) and water saturated at exposure of < 10 3 L. In those studies, water exposure was often carried out in a main vacuum chamber, allowing exposure only to the low-pressure ( < 10 -5 Torr in most cases) vapor water. A relatively low exposure range was probed since it was difficult to achieve the high exposure in reasonable short time. Since liquid exposure will inherently involve some high vapor pressure exposure, it is of interest to know if higher-pressure vapor water (or liquid water) would increase the water coverage on the surface. Thus, in this study, we examine defective and nearly perfect surfaces by exposing not only to low-pressure vapor water but also to higher-pressure vapor and liquid water.
2. Experimental An 8 × 8 × 1 m m 3 polished TiO2(ll0) crystal from Princeton Scientific was used in this study. The TiO2(110) sample was reduced through a long period of heating in UHV at ~ 1000 K so that there was no charging problem during XPS and UPS measurements. The surface was cleaned by 15 min of 4 keV Ar + bombardment (current density of ~ 10 /~A/cm2). The nearly defect-free surface was then obtained by annealing the sample at 1000 K for 15 min in 1 × 10 -7 Tort 02, then slowly cooling down to room temperature in the same oxygen atmosphere for 30 min. The highly defective surfaces were prepared by Ax + bombardment at 4 keV for 10 min, while a relatively low concentration of defects were created by low-density electron-beam exposure. Electron beams were provided by an Auger electron gun coexisting with a cylindrical mirror analyzer (CMA). In this study, a 10 /xA 1 kV electron beam was rastered over an area of 0.25 cm 2 for 30 rain. For comparison, approximately 90% of the XPS signal comes from an area of ~ 0.1 cm 2 [19]. Thus most of the XPS signals arise from beam exposed material. The XPS measurements were made on a Physical-Electronics 560 multiprobe system that uses a double pass cylindrical mirror analyzer (CMA). Non-monochromatic M g K a X-rays were used to generate the spectra. During these experiments the base pressure of the system was about 8 × 10 -a°
L.-Q. Wang et al. / Surface Science 344 (1995) 237-250
UPS experiments ( H e I radiation at 21.2 eV) were also performed in the 560 system. As also discussed in Section 4.1, water exposure were made in more than one way and produced
Torr. The multiplex data were collected with a pass energy of 25 eV. The collected data were referenced to an energy scale with binding energies for Au 4f at 8 4 _ 0.03 eV and Cu2p3/2 at 9 3 2 . 6 7 _ 0.03 eV.
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240
L.-Q. Wang et al./ Surface Science 344 (1995) 237-250
consistent results. An ultrahigh vacuum (UHV) compatible transfer system connected to the 560 system allowed transfer of the specimen prepared in vacuum for low-pressure, higher-pressure ( > 10 -3 Torr) vapor or liquid water and return the specimen to the spectrometer for analysis. For exposures at 10 - 6 Torr or less, the dosing could be conducted either in the main vacuum chamber or in the transfer system. The consistency of the low pressure results in both systems and the similarity of both results with much of that reported in the literature demonstrate the reliability of the transfer system used for high pressure and liquid exposures. To minimize contamination for liquid water exposure in this study, the distilled and deionized liquid water was contained in a small high purity Teflon cup then put inside a 2.75 inch diameter standard UHV vacuum " T e e " with a window attached. The system was processed with many freeze-pump-thaw cycles before sample exposure. After the desired UHV preparation, a sample was transferred to the liquid compartment which was evacuated slightly, allowing the water splash from the cup to uniformly wet the whole surface. After exposing the sample to liquid water for about 2 - 3 min without pumping, we then transferred it out of
[
3. Results 3.1. Water interaction with nearly defect-free surfaces Ti 2p and O 1s XPS spectra and UPS spectra for nearly defect-free surfaces exposed to both vapor
tl
i
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the liquid compartment. It normally takes about 1 h to pump out before the sample is returned to the main chamber. The whole setup used UHV parts and connections and was thoroughly baked before adding the water. Although the transfer system could not be maintained at UHV conditions during water exposure, the background gas was water vapor. To arrange water to contact only TiO 2 crystal surface, the TiO2(110) crystal was mounted on a Ta plate with a small amount of molten indium spread over the interface. Due to the limitations of the spectrometer and having specimen transport system, the front of crystal used for this work was heated by an electron filament, rather than by resistance heating used in the previous study [18]. Nearly defect-free surfaces were successfully prepared by using this heating method and no contamination was detected by AES or XPS.
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0
L.-Q. Wang et al. / Surface Science 344 (1995) 237-250 • Low Vapor Pressure (5E-6 Torr) • HighVapor Pressure(0.2-0.6 Torr) • Li, aid Water
ing at higher binding energy (BE) was observed after additional exposure to 0.6 Torr H 2 0 for 5 min ( ~ 10 8 L). Furthermore, significantly more broadening was observed after liquid water exposure. In contrast, both Ti 2p XPS and UPS spectra showed minimal changes in defect intensity even after liquid water exposure. However, close examination of the Ti2p3/2 XPS and UPS spectra shows that the nearly defect-free surface became closer to perfect after exposure to higher vapor pressure water and liquid water. Previous studies [5,6,12] have shown that water dissociatively adsorbed on TiO2(110) surfaces at 300 K. Based on these previous studies, we assume that the O ls peak broadening after water exposure on defect-free surfaces is solely due to the OH species adsorbed on the surface (we also assume that the OH served to " h e a l " the small number of defects present after the initial surface preparation). The OH area ratios have been obtained from the fits to the O ls XPS spectra. A symmetric Gaussian-Lorentzian function was used to fit the O ls spectrum for a nearly defect-free surface before water exposure, while two symmetric GaussianLorentzian functions were used for a nearly defect-
14
12 10
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~" 6 O
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t
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4
6
8
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Log [ exposure (L) ] Fig. 3. OH area ratio versus the logarithm of H 2 0 exposure (L).
and liquid water are shown in Figs. la, lb, and 2, respectively. For nearly defect-free surfaces, we observed symmetric Ti 2p and O 1s peaks in XPS spectra and a small amount of Ti 3d defect intensity in UPS spectra. From Fig. lb, little change in the O ls peak was observed after exposure to 4.5 × 10 -6 Torr H 2 0 for 5 rain ( ~ 103 L). An obvious broaden-
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-454
L-Q. Wang et al. / Surface Science 344 (1995) 237-250
242
ML at 108 L exposure to higher-pressure vapor water, and ~ 0.25 ML with liquid water exposure. Based on K u r r s model [12] where one water molecule is dissociated into one OH bonded to the five-fold Ti atom and one H which is then attached to the bridging oxygen to form another OH, the water coverage was 0.02 ML, 0.07 ML, and 0.125 ML for exposure to 104 L low vapor water, 108 L high vapor water and liquid water, respectively. Approximately half of the five-fold Ti atoms and half of the bridging oxygen atoms become hydroxylated upon liquid water exposure (water coverage of 0.125 ML). A previous study [6] has reported a water saturation coverage of 0.07 ML for a nearly defectfree TiO2(110) surface. This value is the same as we obtained at 10 s L exposure to higher vapor pressure water, but is lower than that of liquid water exposure.
free surface after water exposure (using parameters identified below). The part of contribution from the clean TiO 2 for a water exposed surface was fitted by fixing FWHM (full width at half maximum) and percentage of Gaussian (%G) to the values obtained for a nearly defect-free surface before water exposure. Typical FWHM and %G for a nearly defect-free surface before water exposure are 1.6 eV and 90%, respectively, while the energy shifts from the bulk oxygen, FWHM, and %G for the part of contribution from the OH species are 1.6, 1.1-1.6 eV and 100%, respectively. Two-function fits for water exposed surfaces are very good. The area ratio of OH versus the log of the H 2 0 exposure (L) is shown in Fig. 3. At low exposure to low-pressure vapor water, the OH concentration increases slowly with the exposure, while at higher exposure to higher-pressure vapor water, the OH concentration continuously goes up and reaches up to ~ 12% for the liquid water exposure. In this study, we define that 1 ML coverage is equivalent to the surface atomic density of 2 × 1015/cm 2. Using a similar method described in a previous work [18], we estimate that for a nearly defect-free surface, OH coverage was ~ 0.04 ML at 10 4 L exposure to low vapor pressure water, ~ 0.14 160
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L.-Q. Wang et al. / Surface Science 344 (1995) 237-250
centration of 3.3 × 1014/cm 2 Ti 3+ corresponds to 1.7 × 1014/cm 2 oxygen vacancies if we assume that one missing bridging oxygen introduces one oxygen vacancy and two Ti 3+. Fig. 4 shows little change in
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244
L.-Q. Wang et al./ Surface Science 344 (1995) 237-250
the Ti2p3/2 peak after exposure to 4.5 X 10 -6 Torr H 2 0 for 5 min ( ~ 10 3 L), while less broadening at lower BE was observed after additional exposure to 0.6 Torr H 2 0 ( ~ 108 L) for 5 min. A symmetric peak was obtained upon liquid water exposure. This peak (after liquid water exposure) is nearly identical to or sometimes slightly narrower than that of "nearly defect-free" data (similar to Fig. 1). In previous work [2,18], broadening of the Ti2p3/2 peak at lower BE has been attributed to surface defects. In agreement with previous studies [2-5], our current work shows that defect intensity varies little at lowpressure water exposure. However, electron-beam induced defects were removed substantially by higher vapor water exposure and almost completely by liquid water exposure. UPS spectra shown in Fig. 5 confirmed the above XPS results. The Ti 3d defect intensity was reduced significantly by higher vapor exposure, while no defects were observed after liquid water exposure. Fig. 6 gives Ti2p and O ls XPS spectra for a nearly defect-free surface, an electron-beam exposed surface, and an electron-beam exposed surface after 0 2
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exposure, respectively. The spectra for the nearly defect-free surface and the electron-beam exposed surface after O 2 exposure are well overlapped, indicating that electron-beam induced defects were completely healed by 02 exposure (10 -7 Torr, 30 min) at 300 K. A previous study [6] has reported that oxygen at 300 K and at much lower exposure ( ~ 10 L) can heal most of defects on slightly defective surfaces prepared by thermal annealing. Similarly, our experiment showed that electron-beam induced defects were completely healed by much lower 02 exposure than the water exposure. To determine the thermal stability of healed surfaces, both 0 2 and H 2 0 healed surfaces were heated to several different temperatures and then cooled down to room temperature for acquiring data. For 02 healed surfaces, both Ti2p and O ls XPS spectra (not shown here) did not change upon heating to temperatures, 420, 670 and 870 K. For H 2 0 healed surfaces, Ti 2p XPS spectra did not change when the surface was heated to temperatures: 320, 550, 720, 800, and 870 K, while O Is spectra did not change only up to 550 K. The O ls spectra showed less
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L.-Q. Wang et aL / Surface Science 344 (1995) 237-250
broadening at high BE when the temperature was increased to above 550 K (probably d u e to loss of H 2 0 or H 2)
were only partially removed: the surface defects are removed more readily than sub-surface defects. A previous study [6] has also shown that 0 2 at saturation coverage only partially healed the Ar + bombarded surface. This was explained by the formation of localized reduced sub-surface oxide structures such as Ti20 3 and TiO due to the oxygen vacancies in the sub-surface region [6]. The current study has shown that at sufficiently high exposure, water can heal electron-beam induced defects and part of the wider variety of surface defects produced by Ar + bombardment [18]. The diffusion of water or oxygen to the sub-surface may be the main controlling factor for the removal of Ar + bombardment defects, although there may be other explanations. It is difficult to accurately calculate the amount of OH species on the defective surface because defects also contribute to the broadening at higher BE in the O ls peak. However, based on a rough estimation, we found no obvious difference in OH concentrations for a nearly defect-free surface and a defective surface prepared by electron-beam exposure, but a slightly higher OH concentration was observed for a defective surface prepared by Ar + bombardment.
3.3. Water interaction with defective surfaces prepared by A r + bombardment
Ti 2p XPS and UPS spectra for a defective surface prepared by Ar + bombardment before and after water exposure are given in Figs. 7 and 8, respectively. The XPS spectra show that after liquid water exposure the Ti2p3/2 peak at lower BE becomes narrower as compared with the Ar + bombarded surface before water exposure, but is still much broader than the nearly defect-free surface. This shows that many defects remained near the surface after water exposure. However, UPS spectra shown in Fig. 8 present a different picture: almost no Ti 3d defect states were observed after liquid water exposure. Since UPS (He I 21.2 eV) is more surface sensitive than Ti2p XPS, and Ar + bombardment (4 keV) creates substantially amounts of sub-surface defects, we conclude that defects created by Ar + bombardments
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246
L.-Q. Wang et al. / Surface Science 344 (1995) 237-250
4. D i s c u s s i o n
The following discussion focuses on comparison of vacuum and solution experiments, water coverage for a nearly defect-free surface, the kinetics of water interaction with TiO2(110) surfaces, water saturation coverage versus initial defect concentration, " h e a l , i n g " of defects after water exposure, and surface defects versus sub-surface defects. 4.1. Comparison o f vacuum and solution experiments A n y comparison of vapor and solution exposure introduces issues regarding the nature and type of exposure the surface has seen. Before questions of coverage and defect healing can be addressed, some consideration must be given to the nature of the exposures, impurities and vacuum conditions. Exposure to liquid water introduces the possibilities of interactions not possible during simple vapor work. In addition to allowing movement of ions and dissolved materials away from the specimen, the surface is exposed to any ions in the solution, including the ~ 10 -7 molar concentration of H + and O H - ions. Since water is ~ 55M, approximately 2 out of every 10 9 molecules in solutions is an O H - . Since the density of water is 3 × 10 22 molecules cm -3, this means that a monolayer equivalent of O H - ions (assuming atomic density of 1015 m o l e c u l e s / c m 2 for one monolayer) are within 17
cm of the surface. Since the diffusion coefficient for O H - ions in water is 5 × 10 -5 cm 2 s -1, only < 1% of a monolayer coverage could come from interaction of O H - ions with TiO 2 in solution in 200 s. As mentiQned in the experimental section, we went to considerable effort to minimize impurities of liquid water. Hbwever, due to the system processing and the nature of sample exposure, it is not easy to examine the exact water purity as used during the experiment. However, conductivity and other indications suggest that concentrations of impurities in liquid water were less than ~ 1 ppm. Although the exact impurity concentrations were not known, several statements can be made with confidence. First, no build up of ions from solution were observed on the sample. Thus, no evidence of F - , C I - , Ca 2+ or CO 2- was observed after liquid water exposure. Second, as noted above, at the low concentrations, diffusion in water and the times of exposure do not allow much impurity interactions with the surface in solution. Special care was taken to make vacuum conditions as best as possible when working with the water. The water was pumped, frozen and otherwise processed in U H V compatible equipment. Although some preliminary studies were made using the "altered" standard electrochemical corrosion cell, concern about 0 2 contamination in the N 2 cover-transport gas used caused us to devise an alternate system in which no such gas was used. Thus, for the results
Table 1 Comparison of water coverage Reference This work
Coverage (ML) (< 10-5 Ton')
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Exposure (L)
Temperature (K) 300
0.125 -
104 108 (liquid) 102 102
300 300
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300
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300
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Ref. [3] Ref. [15] Ref. [12] Ref. [17]
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a The coverage of 0.15 ML is for a highly defective surface. b The coverage of 0.07 ML was obtainedfrom an estimated OH coverage assumingthe high temperature tail in the 275 K TPD peak due to hydroxyl disproportionation. c The coverage was estimatedby assumingthat water adsorbed at 160 K corresponds to about one to two water moleculesper unit cell [15].
L.-Q. Wang et al. / Surface Science 344 (1995) 237-250
reported here, the sample was exposed to only the water vapor (vapor of the pumped water source) and ambient gases in the transfer system. A check of the transfer process showed that exposure to the gases in the transfer system did not heal the defects. Furthermore, XPS spectra have shown that defects prepared in UHV were stable in our vacuum condition for at least several hours. We have also done similar experiments in slightly different ways to ensure consistency. Several different types of doses used in the experiments are: (1) low-pressure vapor water (exposure up to 103 L) in UHV chamber with glass water source as does in previous studies [3,12]; (2) olow-pressure and high-pressure vapor water in transfer chamber with the glass source; (3) high pressure in transfer chamber with Teflon beaker source, and (4) liquid water in transfer chamber with Teflon beaker source. Similar results of defects "healing" after water exposure were found for different chambers and possibly different "clean" water sources. Furthermore, low exposure (almost no impurity), high exposure and liquid water exposure showed the same results: Ti 3+ defects were healed upon water exposure while the OH concentration was increased as increasing water exposure. 4.2. Water coverage f o r a nearly defect-free surface
Different groups have measured and reported water coverages in different ways. Although comparisons are not easily understandable, they are reported here in Table 1 to provide background and context for current work. This study has reported that water coverage was 0.02, 0.07, and 0.125 ML and percentages of hydroxylated five-fold Ti atoms were 8, 25, and 50% for exposure to 10 4 L low vapor water, 108 L high vapor water and liquid water, respectively. Water coverage of 0.125 ML (we defined 1 ML as 2 × 1015/cm2 or 4 molecules per unit celI)- upofi liquid water exposure for a nearly defect-free surface is equivalent to one water molecule for every two unit cells. For a defect-free or a slightly defective surface, the liquid exposure coverage of 0.125 ML from this study is larger than the saturation coverage of 0.07 ML upon vapor exposure reported by Pan et al. [3]. Pan et al. [3] have reported the coverage of 0.15 ML for a highly defective surface prepared by Ar + bombardment. Hugenschmidt et al. [15] have
247
estimated the OH coverage of 2.6 × 1014/cm 2 (equivalent to ~ 0.07 ML water coverage in our definition) by assuming the high-temperature tail in the 275 K TPD peak due to hydroxyl disproportionation. Kurtz et al. [12] have reported a saturation coverage of 0.1 ML based on a comparison of the intensity of the 10.8 eV O H - 30- feature in the UPS spectrum with the intensity of the 13 eV lb 2 level of 1 monolayer (ML) of molecular water at 160 K. They defined 1 ML in terms of an adsorbed H 2 0 monolayer on the TiO2(ll0) surface at 160 K and assumed that water adsorbed at 160 K corresponds to about four water molecules per surface unit cell, whereas XPS data taken by Hugenschmidt et al. [15] showed amount of water to be closer to one and no more than two per unit cell. The water saturation coverage observed by Kurtz et al. [12] would be ~ 0.025-0.05 ML assuming that water adsorbed at 160 K corresponds to about 1-2 water molecules per surface unit cell [15]. Patthey et al. [17] have also studied hydroxylated TiO2(110) surfaces in relating to the biocompatibility of Ti implants and reported a water coverage of 0.125 ML upon exposure to 10 6 L vapor water. This coverage is higher than that obtained from this work for the same amount of vapor water exposure. However, they used a different method to calculate the water coverage than was used in this work. In the study by Patthey et al. [17], the water coverage was calculated from the intensity ratio of the two surface shifted core levels (O ls). One of these, at a shift of 1.18-1.33 eV compared to the bulk oxygen, was attributed to the surface bridging oxygen. The other, at a 2.4 eV shift, was attributed to the basic OH species bonded to the five-fold coordinated Ti atoms. In this study, the OH area ratios were obtained by fitting the O 1s peak with one additional symmetric Gaussian-Lorentzian function for the OH contribution. The fits are good without additional peaks at lower binding energy. Since Patthey et al. [17] have taken XPS spectra at a glancing electron-emission angle, in contrast to the angle integrated analyzer as used in this study, their clean TiO2(ll0)spectrum has a high BE shoulder which was attributed to the surface bridging oxygen. They also did not use the same set of parameters (FWHM and the core lever shifts) to fit the bulk oxygen contribution for both clean and hydroxylated TiO 2 surfaces. Furthermore,
248
L.-Q. Wanget al. / Surface Science344 (1995)237-250
the contribution from the acidic OH species which may appear at BE near the surface oxygen region was ignored in their calculations. Since the exact binding energies for basic OH, acidic OH, or surface bridging oxygen are not known, fitting the O ls spectrum with three peaks signifies uncertainty. Thus, we can't directly compare our results with those obtained by Patthey et al. [17]. 4.3. Kinetics of water interaction with Ti02(110) surfaces
The fact that water did not saturate at 109 L exposure as shown in Fig. 3 suggests that the interaction of water with TiO2(l10) surfaces may be an activated (or kinetically controlled) process, depending on the water vapor pressure and length of exposure. It is possibly that previous studies [3,12] observed only a kinetic saturation of the state due to the low exposure range probed. The true saturation coverage of hydroxyls is likely much higher, as indicated by the present study. As suggested by Kurtz et al. [12], the kinetic barrier in the process may be the interaction which requires substantial molecular tilt and displacement from an atop Ti site. This study has shown that higher water coverage can be achieved by increasing water exposure. The simple reason for this is that at higher water exposure, more molecules are able to overcome the activation barrier and become dissociated OH adsorbed on the surface. Although water saturation coverage is increased substantially upon liquid water exposure, a coverage of 0.125 ML is still low. As suggested by Kurtz et al. [12], the low saturation coverage seems to imply that the occupation of one adsorption site by dissociated OH species quenches the activity of neighboring sites with respect to additional HzO adsorption and dissociation. This effect could result from OH dipole-dipole interactions combined with subtle changes induced in the surface electronic structure by the adsorbate. However, Pan et al. [3] have concluded that the low saturation coverage for defect-free and slightly defective surfaces is not controlled predominantly by OH dipole-dipole interactions based on a higher uptake of H 2 0 on the highly defective surface. Thus, a combination of kinetic and structural studies is required to have a better understanding of water adsorption and dissociation on TiO2(110) surfaces.
4.4. Saturation coverage versus initial defect concentration
Kurtz et al. [12] have found that the water saturation coverage on TiO2(110) at 300 K was independent of the initial defect concentration. In agreement with Kurtz's work, Pan et al. [3] have also reported that water saturation coverage is independent of the oxygen vacancy concentration before water adsorption for defect-free and slightly defective surfaces. However, water coverage and the rate of dissociative adsorption were found to be higher for highly defective surfaces (by Ar + bombardmen0 than those for slightly defective (by thermal annealing) surfaces [3]. Our study has also observed a similar water coverage for nearly defect-free surface and defective surfaces prepared by electron-beam exposure, but a slightly higher water coverage for an Ar ÷ bombarded surface. It was found in a previous study [18] that defects created by electron-beam exposure and by thermal annealing have similar electronic structures, mainly composed of Ti 3+, while defects produced by Ar + bombardment are complex, with a variety of different local environments where oxygen and titanium surface atoms coexist. The defect concentration for electron-beam exposure in this study is larger than for thermal annealing, but smaller than for Ar + bombardment. Based on the previous and current studies [3,12,18], it appears that relatively simple defects created by electron-beam exposure and thermal annealing did not change the water coverage, but an increased adsorption observed for a higher concentration of more complex defects. This effect could be due to different electronic structure, a physically enhanced surface area or increased defect concentration. The fact that liquid water can remove not only defects induced by electron-beam exposure but also the (surface exposed) wider variety of surface defects produced by Ar + bombardment [18] indicates no obvious difference in defect reactivity for defects created by electron-beam exposure and Ar ÷ bombardment at high water exposure. 4.5. "Healing" o f defects
Additional questions arise concerning the nature of the oxygen- or water-" healed" surfaces. If the electronic signal for the defect is removed upon the
L.-Q. Wang et al./ Surface Science 344 (1995) 237-250
adsorption of a loosely bound molecule, the surface may be chemically " h e a l e d " but might be very different from a nearly defect-free surface which may be more stable than a " h e a l e d " surface. If water or oxygen were loosely bonded on the surfaces to heal the defects, the defects would reappear when we heated the surface to desorb some loosely bonded molecules. For O 2 healed surfaces, experiments showed that defects did not reappear upon heating to 870 K. The O ls XPS spectra did not change upon heating, indicating that oxygen healed surfaces are stable upon heating and that 0 2 is strongly bonded to the surface. For H 2 0 healed surfaces, experiments also showed that defects did not reappear upon heating to 870 K, while O 1s spectra did not change only up to 550 K. However, the O ls spectra showed less broadening at higher BE as the temperature was increased to above 550 K, indicating desorption (or disproportionation) of the hydroxyl species from the surface. TPD data given by Hugenschmidt et al. [15] have shown that surface hydroxyl species started to desorb from the defective TiO2(l10) surface at ~ 500 K. However, it is not clear why the defect intensity did not change upon heating to above 550 K while the hydroxyl species desorbed from the surface. Nevertheless, results obtained from the temperature at 320 and 550 K indicate that water healed surfaces are stable up to ~ 550 K and that water is chemically bonded to the surface in healing the defects. Recently, Shultz [20] et al. have reported that low-energy (4.7 eV) ultraviolet (UV) photons create stable Ti 3+ defects on TiO2(l10) surfaces in UHV. The adsorption and photodesorption of oxygen on the TiO2(110) surface have reported recently by Lu et al. [21,22]. The UV created defects are similar to the electron-beam induced defects, mainly composed of Ti 3+, and can be healed by exposing to 0 2 gas. This observation was interpreted as due to photo-desorption and re-adsorption of molecular oxygen bound loosely to Ti 3+ defects on the surface as Ti4+:O~ complex. Shultz [20] et al. assumed that UV photons at energy just above the band gap should not be energetic enough to directly remove a surface oxygen atom. However, the true nature of oxygen-healed surface is not clear and needs further study. Although electronic defects are healed by exposure, the surface with adsorbed hydroxyl species
249
is a chemically and structurally different surface from the surface without water exposure. Structural defects including physical defects or other factors may also played an important role in determining chemical reactivity for this surface. Thus it is important to study both electronic and geometric structures and their relationship using a combination of several techniques including STM, LEED, XPS, UPS, HREELS, IR, etc. As already stated, this paper deals with defects that perturb the chemical or electronic structure of the surface and are observable by XPS measurements. 4.6. Surface versus sub-surface defects
The difference between surface and sub-surface defects (as well as " c h e m i c a l " versus "physical" healing) raises many interesting questions. If surface defects (Ti 3d defect states) are completely removed by water exposure, these electronic defects (exposed on the surface) may not play an important role in determining electrochemical and photocatalytical properties of ruffle TiO 2 since most of these reactions are carded out in aqueous media. Our study has shown that surface electronic defects on TIO2(110) created by electron-beam exposure and thermal annealing can be completely removed upon liquid water exposure, while the sub-surface defects created by Ar + bombardment are still present after water exposure. Because photons can penetrate hundreds of nm in our study and electrochemistry generation of holes may occur over a high field region (more depth), in electrochemical and photocatalytical reactions, there are still sub-surface defects that can be photo or electrochemical active. Thus the sub-surface defects may play an important role in electrochemical and photocatalytical reactions when surface defects are removed.
5. Conclusions
The adsorption of liquid a n d vapor water on defective and nearly defect-free TiO2(ll0) surfaces has been examined using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Defective surfaces were prepared by
250
L.-Q. Wang et a l . / Surface Science 344 (1995) 237-250
low-density ( ~ 10 / ~ A / c m 2) electron-beam exposure and Ar+-ion bombardment. With exposure up to 10 4 L low-pressure ( < 10 -5 Torr) vapor water to defective surfaces, little change on Ti 3d defect intensity was observed, in agreement with previous studies [2-5]. However, defect intensity was greatly reduced by exposing to ~ 10 8 L higher-pressure ( 0 . 2 - 0 . 6 Torr) vapor water or to liquid water. More significantly, electron-beam induced defects completely disappeared upon liquid water exposure, while defects created by Ar + bombardments were only partially removed. Surface defects created by Ar + bombardments were removed more readily than sub-surface defects. For a nearly defect-free surface, the water coverage was ~ 0.02 ML at 10 4 L exposure to low-pressure vapor water, ~ 0.07 ML at 10 8 L exposure to high-pressure vapor water, and ~ 0.125 ML for the liquid water exposure. The fact that water did not saturate at 10 9 L exposure suggests that the interaction of water with TiO 2 surfaces may be a kinetic controlled process, depending on the water vapor pressure and length of exposure.
Acknowledgements The authors would like to thank Dr. V. Henrich, Dr. T. Madey, and Dr. M.A. Henderson for many discussions. This work has been supported by the Division of Materials Sciences Office of Basic Energy Sciences US Department of Energy (USDOE). Pacific Northwest Laboratory is a multiprogram national laboratory operated for the U S D O E by Battelle Memorial Institute under Contract DE-AC0676RLO 1830.
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