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Reactions of Ammonia on Stoichiometric and Reduced TiO2(001) Single Crystal Surfaces J. N. Wilson and H. Idriss* Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received June 23, 2004. In Final Form: September 17, 2004 The reaction of NH3 on the surface of the {011}-faceted structure of the TiO2(001) single crystal is studied and compared to that on the O-defected surface. Temperature-programmed desorption (TPD) conducted after NH3 adsorption at 300 K shows only molecular desorption at 340 K. Modeling of TPD signals as a function of surface coverage indicated that the activation energy, Ed, and pre-exponential factor, veff, decrease with increasing coverage. Near zero surface coverage, Ed was found to be equal to 92 kJ/mol and veff to be close to 1013 /s. Both parameters decreased to ∼52 kJ/mol and ∼107 /s at saturation coverage. The decrease is due to a repulsive interaction of adsorbed NH3 molecules on the surface. Computing of the TPD results show that saturation is obtained at 1/2 monolayer coverage (referred to Ti atoms). Both the amount and shape of NH3 peak change on the reduced (Ar+-sputtered) surfaces. The desorption peak at 340 K is considerably attenuated on mildly reduced surfaces (TiO∼1.9) and has totally disappeared on the heavily reduced surfaces (TiO1.6-1.7), where the main desorption peak is found at 440 K. This 440-K desorption is most likely due to NHx + H recombination resulting from ammonia dissociation upon adsorption on Ti atoms in low oxidation states.
I. Introduction The ideal surface of a TiO2(001) single crystal contains Ti atoms 4-fold coordinated to oxygen atoms, and this makes them unstable (high surface energy). It facets to two thermodynamically stable structures: the {011}- and the {114}-faceted structures,1 obtained by annealing in a vacuum at 700-750 K and 900-950 K, respectively. The first structure, the {011}-faceted surface, contains Ti atoms in a 5-fold coordination environment, while the second one, the {114}-faceted surface, contains Ti atoms in 4-, 5-, and 6-fold coordination in equal proportions (maintaining an overall coordination number of Ti atoms ) 5). Surface defects play an important role in adsorption and like most other transition metal oxides defects occur as oxygen vacancies on TiO2. Defects can be created easily on TiO2 by several processes with increasing degrees of severity, namely: high-temperature annealing in Ultrahigh Vacuum (UHV)2 < electron irradiation3 < ion sputtering.4 It is thus considered an ideal material for the study of surface defects. NH3 being a polar molecule, like H2O, is ideally suited to study the variability of the surface states of oxide materials, and as such, several studies of NH3 have already been undertaken on TiO2 and will be described in detail below. Only one previous work was conducted by temperatureprogrammed desorption (TPD) on a TiO2(001) single crystal that has been annealed at 900 K in UHV conditions.5 One desorption domain for ammonia was observed at 338 K. A first-order desorption energy of 80 kJ/mol was calculated for ammonia on the basis of the temperature of desorption, assuming a frequency factor of 1013/s. In * Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: 64 9 373 7422. (1) Firment, L. E. Surf. Sci. 1982, 116, 205. (2) Hauffe, K.; Hupfeld, J.; Wetterling, T. Z. Physik. Chem. N. F. 1976, 103, 115. (3) Wang, L. Q.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1994, 320, 295. (4) Go¨pel, W.; Anderson, J. A.; Frankel, D.; Jaehning, M.; Phillips, K.; Schafer, J. A.; Rocker, G. Surf. Sci. 1984, 139, 333. (5) Roman, E.; de Segovia, J. L. Surf. Sci. 1991, 251, 742.
addition to this, two products, namely hydrogen (H2) and dinitrogen (N2), were detected desorbing at 364 and 343 K, respectively. HeI (hv ) 21.2 eV) Ultraviolet photoelectron spectroscopy (UPS) indicated that the surface is saturated after exposure of 1 L (1 L ) 1 × 10-6 Torr s). The features, 2 symmetrical bands, at 11.8 and 7.4 eV corresponding to 1e (N-H) and 3a1 (N) molecular orbitals, respectively, indicate molecular adsorption at room temperature. At temperatures between 300 and 340 K, ammonia desorption is seen by a significant decrease in intensity of the main spectral features at 11.8 eV. In addition, a lower saturation coverage is observed for the electron-reduced surface compared to the stoichiometric surface. A UPS synchrotron radiation study of NH3 adsorption on TiO2(110)6 found no significant differences in the electronic structure between adsorption at 300 or 185 K (The latter temperature was not cold enough to form NH3 multilayers) on the perfect surface (one with no Ti3+ point defects, annealed at 1000 K in 104 L O2) compared to the surface which contains a small concentration of point defects (∼0.1 ML Ti3+ point defects, annealed at 1000 K in UHV). The predominant species is molecularly adsorbed ammonia with the saturation coverage of 0.1 ML as evidenced by a rapid decrease in work function from the clean surface (∼0.3 eV) to this exposure and only a slow decrease (a farther 0.27 eV) from there up to 100 L. It was observed that the perfect surface adsorbs about 30% more ammonia than the defective surface. Possible dissociation of a small fraction of ammonia may occur on the defective surface upon heating above 300 K. A comprehensive study7 of ammonia (adsorbed at 160 K) by XPS on the “stoichiometric”, thermally treated and sputtered surfaces of a TiO2(110) single crystal was undertaken and several important observations resulted: (1) The adsorption behavior of ammonia is similar on all surfaces, i.e., molecular with saturation coverage of ∼0.16-0.19 ( 0.02 (6) Roman, E.; de Segovia, J. L.; Kurtz, R. L.; Stockbauer, R.; Madey Surf. Sci. 1992, 273, 40. (7) Diebold, U.; Madey, T. E. J. Vac. Sci. Technol. A 1992, 10, 2327.
10.1021/la0484422 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/04/2004
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monolayers (2.08 × 1015 atoms/cm2 forms one monolayer). (2) Ammonia is desorbed by ∼400 K on all surfaces, and no evidence of thermally dissociated ammonia molecules was observed. (3) Dissociation occurred only on the sputtered surface and, when irradiated by electrons from a W filament with the sample biased at + 325 V, was conducted. A recent study of ammonia on the stoichiometric and vacuum annealed, containing bridging oxygen vacancies, surfaces of TiO2(110)8 by Auger-photoelectron coincidence spectroscopy suggested that NH3 adsorbs at every other 5-fold-coordinated titanium site on the stoichiometric surface but on the vacuum annealed surface, NH3 adsorbs on the oxygen vacancies preventing adsorption on the neighboring 5-fold Ti atoms therefore reducing the saturation coverage. The adsorption of ammonia, pyridine, and water molecules on a TiO2(110) single crystal was studied by STM.9 This again revealed that ammonia is molecularly adsorbed (hydrogen adatoms10 on the bridging oxygen rows, 0.14 ML, prevent ammonia from dissociating to NH2 and H species) on the 5-fold titanium surface atoms with a coverage between 0.01 and 0.03 ML where a ML is defined as the density of the (1 × 1) units, 5.2 × 1018 /m2. Tightly bound species were found at step edges along the [11 h 1] and [11h 4] direction, but no species were observed along the [11 h 1] direction More recently, Farfan-Arribas and Madix11 have characterized the acid-base properties of the TiO2(110) surface by adsorption of amines (NH3, ethylamine, dimethylamine). NH3 was the only product detected desorbing from the stoichiometric and electron-irradiated (300 eV) surface during TPD (335 and 327 K respectively, 10 min irradiation time) after ammonia exposure at room temperature. Molecularly adsorbed ammonia was monitored by XPS and a slight shift in the B. E. of the N1s peak, when compared to the free molecule, was noticed on these surfaces. In addition, a slight decrease in coverage was detected upon prior e- irradiation. Using the Redhead analysis and assuming a pre-exponential factor of 1013/s, the desorption energy was found to be 86 kJ/mol. Hydrogen-reduced surfaces of TiO2 (a disk sliced parallel to the (001) face from a single crystal) that were exposed to ammonia showed no additional peaks in the N 1s region, other than the one due to molecularly adsorbed NH3 at ∼400 eV.12 The oxidized surface that had been exposed to H2O prior to NH3 adsorption13 showed an additional high binding energy peak assigned to NH4+. It was suggested that this species is formed by protonation from surface OH groups. Electron Stimulated Desorption of positive O+ from the surfaces of TiO2(110) was studied by Diebold et al.14 It was shown that a fractional monolayer of adsorbed ammonia, suppressed the desorption via a charge-transfer mechanism from the occupied orbitals of the adsorbed ammonia to the desorbing ions thus decreasing (linearly) the detected yield to ∼10% of their initial value. As seen from the above studies, most of NH3 adsorption was conducted on the (110) surface. Although the (110) surface is very well characterized, far less chemical (8) Siu, W. K.; Bartynski, R. A.; Hulbert, S. L. J. Chem. Phys. 2000, 113. (9) Suzuki, S.; Fukui, K-i.; Onishi, H.; Sasakai, T.; Iwasawa, Y. Stud. Surf. Sci. Catal. 2001, 132, 753. (10) Suzuki, S.; Fukui, K-i.; Onishi, H.; Sasakai, T.; Iwasawa, Y. Phys. Rev. Lett. 2000, 84, 2156. (11) Farfan-Arribas, E.; Madix, R. J. J. Phys. Chem. B 2003, 107, 3225. (12) Lazarus, M. S.; Sham, T. K. Chem. Phys. Lett. 1982, 92, 670. (13) Lazarus, M. S.; Sham, T. K. Chem. Phys. Lett. 1979, 68, 426. (14) Diebold, U.; Madey, T. E. Phys. Rev. Lett. 1994, 72, 1116.
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reactions were actually investigated when compared to the (011) faceted surface of a TiO2(001) single crystal. For example, this latter surface has been shown to be very active for aldolization of aldehydes and ketonization of carboxylic acids.15 These two reactions are particularly appealing because they involve carbon-carbon bond formations. On the other hand, TiO2 has recently been proposed as a candidate for catalytic reactions relevant to evolution theory. In other words, making nucleic acids such as purine from carbon- and nitrogen-containing compounds.16 It is thus the objective of this work to extract information relevant to the reaction of NH3 with the surfaces of TiO2 and see for (i) possible N-H bond dissociation and (ii) the effect of oxygen vacancies on the bonding. II. Experimental Section All TPD experiments were conducted in an ultrahigh vacuum (UHV) chamber described in detail previously.17,18 In brief, the UHV chamber is pumped with an ion, turbo molecular and titanium sublimation pump to a base pressure of ∼2 × 10-10 Torr and is equipped with a quadrupole mass spectrometer (up to 300 amu), with a Pyrex shroud (with an orifice smaller than the crystal), ion sputter gun, heating stage (x, y, r motion feed through), ion gauge and two dosing lines, one with a dosing needle positioned a few millimeters away from the crystal face. Surface characterization is carried out by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy in a separate stainless steel UHV chamber to the one described above for the TPD experiments. The main chamber is pumped with a Perkin-Elmer Ultek 220 L/s ion pump, titanium sublimation pump, and a sorption pump where the ultimate pressure is in the 10-10 Torr range. It is equipped with Perkin-Elmer; dual anode (magnesium and aluminum, KR 1253.6 and 1486.6 eV, respectively) X-ray source, angle resolved double pass cylindrical mirror analyzer (containing a coaxial electron gun for AES), and an auto-scaling ion gauge (Granville-Phillips series 271) for pressure reading. The figures were collected in pulse-counting mode at the following conditions, Mg KR, 0.1 eV/step, 350 ms time/eV, pass energy 25 eV). The Ti 2p 3/2 peak position (459.0 eV) is in good agreement with previous studies,19 and the full width half-maximum, ∼1.4 eV, is indicative of a stoichiometric surface (Ti4+). The single crystal of TiO2(001) is prepared with sputter and anneal cycles, typically Ar+ pressure 1 × 10-5 Torr, 5 kV beam voltage, 25 mA emission current, followed by several “flashes” at either 750 K ({011}-faceted surface) or 900 K ({114}-faceted surface) under oxygen pressure, 1 × 10-6 Torr, for 10 min intervals. The anhydrous NH3 (Matheson/BOC) is used without further purification. TPD experiments are conducted at a heating rate of 1 K/s. The correction factor of ammonia was calculated to be 1.7, using the standard method of Ko et al.20 ASTEK (Helix Science Applications, 1995), a software package for the analysis and simulation of thermal equilibrium and kinetics of gases adsorbed on solid surfaces written by Kreuzer and Payne, was used to acquire desorption kinetics from the TPD. In these experiments, the initial coverages were varied and the resulting TPD spectra analyzed via threshold analysis. Threshold analysis is based on the initial portion of each desorption trace up to the point where only a few percent of the initial coverage has been desorbed. The dependence of the resulting desorption energy and prefactor on the fractional depletion of the initial coverages corresponding to threshold desorption were monitored, and in our case, 5% is chosen. This method is used as the desorption energy and prefactor are generally weak functions of coverage; the desorption rate in a TPD experiment will change initially solely due to its temperature (15) Barteau, M. A. Chem. Rev. 1996, 96, 1413. (16) Saladino, R.; Ciambecchini, U.; Crestini, C.; Costanzo, G.; Negri, R.; Di Mauro, E. ChemBioChem 2003, 4, 514. (17) Wilson, J. N.; Idriss, H. J. Catal. 2003, 214, 46. (18) Wilson, J. N.; Titheridge, D. J.; Kieu, L.; Idriss, H. J. Vac. Sci. Technol. A 2000, 18, 1887. (19) Smentkowski, V. S. Prog. Surf. Sci. 2000, 64, 1. (20) Ko, E.; Benziger, J.; Madix, R. J. Catalysis 1989, 62, 264.
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dependence, as long as we consider only the rising portion of the TPD trace up to the point where only a small fraction of the initial coverage has desorbed.
III. Results A. Stoichiometric Surface. On the 750-K UHVannealed surface, the {011}-faceted surface, TPD following NH3 adsorption at room temperature, shows one clear desorption domain at ∼340 K, as shown in Figure 1. Masses 17 and 16 m/z are seen corresponding to ammonia desorption. No other products are seen to desorb including H2 (2 m/z), N2 (28 and 14 m/z), and nitrogen oxides: NO, N2O (30 m/z), or NO2 (46 m/z). Figure 2 shows the effect of coverage on the computed peak areas of ammonia from TPD runs at different initial surface exposures. As can be seen, saturation coverage is reached at ∼0.5 L (1 L ) 1 × 10-6 Torr s), the point where increasing the coverage has no further effect on increasing the desorption peak area of ammonia. Threshold analysis of the TPD experiments on this surface via ASTEK (see Experimental Section for more
Figure 4. Arrhenius plot, as calculated by ASTEK,40 following TPD on the {011}-faceted surface of TiO2(001). Table 1. Temperature-Programmed Desorption of NH3 on the {011}-Faceted TiO2 (001) Single Crystala coverage, θ
Ed (kJ/mol)
νeff (/s)
0 0.33 0.5 1.0
92 79 73 52
1013 1012 1010 107
a Effect of surface coverage, θ, of NH on the activation energy 3 for desorption, Ed and pre-factor, νeff.
Figure 1. TPD of ammonia on the {011}-faceted surface of a TiO2(001) single crystal at room temperature (exposure ) 1 L where 1 L ) 1 × 10-6 Torr s).
Figure 2. Peak area of ammonia desorption following TPD on the {011}-faceted surface of TiO2(001). (10% error bars shown).
Figure 3. Coverage curves as a function of surface temperature during ammonia TPD on the {011}-faceted surface of TiO2(001).
details), reveal valuable information about kinetics of desorption rd ) veff exp(-Ed/kBT). Figure 3 shows the coverage curves obtained from TPD as a function of surface temperatures, while Figure 4 shows the corresponding Arrhenius plots; both figures are needed to extract the preexponential factor and activation energy. As shown in Table 1, veff and Ed change with surface coverage. At near zero coverage, veff and Ed have values of 1013.5 /s and 92 kJ/mol, respectively. This is in good agreement with other experimental results, as detailed in the discussion section and listed in Table 2. NH3-TPD was also conducted on the {114}-faceted surface at saturation coverage. Since the results were very similar to those of the {011}-faceted surface, no coverage dependence was conducted. B. Sub-Stoichiometric Surface. Upon Ar+ bombardment, the surface of TiO2 is reduced and the presence of Ti cations in lower oxidation states than +4 are seen. We21,22 and other workers23 have extensively studied this by XPS and UPS, and the spectra will not be shown here for simplicity. In brief, increasing the sputtering time at a constant set of the other parameters (Ar pressure, voltage, emission current, and distance to the surface) results in increasing the fraction of Ti atoms in lower oxidation state than +4. In general, after an hour of sputtering time, the change in the relative concentration of the different ions becomes a minimum. On the Ar+ sputtered surface, the TPD following NH3 adsorption, at room temperature, shows two desorption domains, a low-temperature one at 370 K and a hightemperature domain appearing as a shoulder (above 400 K) with increasing sputtering time Masses 17 and 16 m/z are seen corresponding to ammonia desorption. As in the case of the {011}- and {114}-faceted surface, no other products are seen to desorb, including H2 and N2. Figure 5 shows the comparison of peak temperature as a function of sputter time where a shift to high temperature with increasing sputter time, (21) Idriss, H.; Barteau, M. A. Catal. Lett. 1994, 26, 123. (22) Idriss, H.; Pierce, K. G.; Barteau, M. A. J. Am. Chem. Soc. 1994, 116, 3063. (23) Diebold, U. Surf. Sci. Rep. 2003, 48, 53.
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Table 2. Desorption Energies of NH3 on a TiO2 Single Crystal from TPD Results surface {011}-faceted, 750 K annealed ASTEKb {114}-faceted, 900 K annealed 90 min Ar+ sputtered TiO2(001) single crystalc TiO2(110) single crystald
17 m/z desorption peak temp (K)
desorption energy (kJ/mol)
desorption peak area
340
89a 92 85a 100a 80a 86
5.0 × 10-7
340 440 338 335
5.7 × 10-7 1.2 × 10-6
a Assuming first-order desorption kinetics and pre-exponential 1013 /s. b Simulation of the {011}-faceted TPD data using ASTEK program, calculated pre-exponential 1013.5 /s. c From ref 5, 900 K annealed. d From ref 11.
Figure 5. TPD of ammonia on the TiO2(001) surface as a function of Ar+ sputter time.
Figure 7. (A) Unit cell of TiO2(001)-{011}-faceted surface.4 The black and white balls represent Ti and O atoms, respectively. (B) A schematic of a 0.5 monolayer of NH3 on Ti4+ (O atoms are omitted for clarity).
Figure 6. Peak area of ammonia on the TiO2(001) surface as a function of sputter time from TPD.
as a consequence of surface reduction, is clearly seen. In addition the TPD peak area on the sputtered surface is ∼2 times larger compared to the {011}-faceted surface (Figure 6). IV. Discussion From TPD, the peak area at saturation coverage on the stoichiometric surface corresponds to 3.66 × 1014 molecules/ cm2. There are 8 × 1014 titanium molecules/cm2 in 5-fold coordination on the {011}-faceted surface of TiO2(001); this equates to ∼50% of the available sites occupied. This figure correlates well with two previous studies where on anatase powder24 half of the theoretical concentration of coordinatively unsaturated Ti4+ ions where occupied by TPD peak area and an XPS study7 where 50% of the 5-fold titanium atoms on the (110) surface of TiO2 single crystal were found to be occupied. In addition, from the dimensions of the unit cell (4.59 Å × 5.46 Å) of this surface, it is apparent that only 1/2 of the total titanium atoms on the surface can be occupied at saturation coverage (there are 2 titanium atoms in the unit cell) as the next nearest (24) Bagnasco, G. J. Catal. 1996, 159, 249.
neighboring titanium atom (3.57 Å) is within the van der Waals radius of the ammonia atom25 (3.6 Å) Figure 7. The decreasing activation energy (and pre-exponential factor) for desorption with increasing coverage indicates repulsive interactions. The decrease of both terms (Ed and veff) is often seen as the surface coverage increases and is known as a compensation effect.26 (ln veff) is related to the entropy change by (S* - Ss)/k, where S* is the entropy of the activated complex and Ss is that of surface adsorbates. (ln veff) decreases as Ss increases, and the latter is known to increase with coverage in the case of lateral interaction because surface diffusion increases. Other workers have shown that ammonia is bonded to the surface titanium atoms via the 3a1 orbital, which is composed mainly of the nitrogen lone pair electrons.6 The adsorbed NH3 therefore has no additional lone-pair orbitals to take part in attractive lateral interactions via hydrogen bonding to neighboring NH3; the principal lateral interactions between neighboring NH3 molecules appear to be repulsive as the coverage increases. A simple estimation of this repulsive interaction can be obtained from plotting the full range of data such as those presented in Table 1. The decrease of the activation energy of desorption appears a linear function of θ, within 10-15%: E ) Eo - Wintθ (where Eo ≈ 92 kJ/mol, Wint is the interaction (25) Jacobi, K.; Jensen, E. S.; Rhodin, T. N.; Merrill, R. P. Surf. Sci. 1981, 108, 397. (26) Taylor, J. L.; Weinberg, W. H. Surf. Sci. 1978, 78, 259.
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energy, and θ is the coverage). Wint is found equal to ∼38 kJ/mol. This number represents an average interaction since it is taken here as a constant while it does change with surface coverage. The repulsive adsorbate interaction of molecular NH3 has been computed on a TiO2 polymer by Markovits et al.27 using periodic Hartree-Fock calculation (with a PS-31G** basis set) and was found changing from 55 to 39 kJ/mol, at θ ) 0.5 and θ ) 1, respectively. It has been seen that the work function, Φ, decreases upon NH3 adsorption on TiO2(110) single crystal,6 a decrease of 0.57 eV (55 kJ/mol) at surface saturation. One can estimate the decrease of the work function of the {011}faceted surface from the dipole moment, µ, of adsorbed NH3 using the Helmholtz equation28
µ × 2πnNH3 × 300 1018
) -∆Φ
where µ is in Debye, D and ∆Φ are in eV, and nNH3 in molecules/cm2. The dipole moment of NH3 in the gas phase is 1.527 D and has been found to increase upon adsorption (values range from 1.9 to 2.2 D29). If we consider a value of 2 D (we did not find the exact number for µNH3 on TiO2 surfaces) and using nNH3 ) 3.66 × 1014 molecules/cm2, ∆Φ ≈ 1.4 eV is obtained, which is reasonable. Upon sputtering, two factors are evident from the TPD’s. (1) There is a shift to higher desorption temperature and (2) an increase of the total peak area. Point 2 can be easily explained. The TiO2(001) stoichiometric single-crystal surface area (1 cm2) increases on the reduced “disordered” surface due to significant surface roughening during the sputtering process.19 Large conical features (100-nm high structures) are seen on the TiO2(110) single-crystal surface by atomic force microscopy (AFM) that has undergone Ar+ bombardment,30 suggesting preferential removal of oxygen atoms and surface morphology change. Values obtained from AFM31 indicate a large increase in RMS roughness upon Ar+ ion sputtering as compared to the stoichiometric surface, 0.45 and 0.28 nm ((0.02 nm), respectively. In contrast AFM of the faceted stoichiometric surfaces ({011} and {114}) of TiO2(001)32 showed that the surface roughness does not increase above 1.2% of the ideal flat surface, which is indicated by the similar peak area for NH3 desorption on these surfaces, see Table 2. Figure 5 shows that NH3 desorption shifts to higher temperature with increasing sputtering time. The presence of Ti atoms in low oxidation states associated with oxygen defects is behind the shift of the desorption temperature by about 100 K (or about 21-29 kJ/mol, assuming an unchanged pre-exponential factor ) 1013 /s), see Figure 8 for more details. The dissociation of ammonia is known to occur on several metals such as Ru,33,34 Ni,35 and more importantly Ti.36,37 Biwer and Bernasek36 studied NH3 on polycrystalline titanium metal via AES, (27) Markovits, A.; Ahdjoudj, J.; Minot, C. Surf. Sci. 1996, 365, 649. (28) Gordon, J.; Morgan, P.; Shechter, H.; Folman, M. Phys. Rev. B 1995, 52, 1852. (29) Benndorf, C.; Madey, T. E. Surf. Sci. 1983, 135, 164. (30) Antonik, M. D.; Lad, R. J. J. Vac. Sci. Technol. A 1992, 10, 669. (31) Pe´tigny, S.; Moste´fa-Sba, H.; Domenchini, B.; Lesniewska, E.; Steinbrunn, A.; Bourgeois, S. Surf. Sci. 1998, 410, 250. (32) Watson, B. A.; Barteau, M. A. Chem. Mater. 1994, 6, 771. (33) Sakaki, T.; Aruga, T.; Kuroda, H.; Iwasawa, Y. Surf. Sci. 1989, 224 L969. (34) Sax, T.; Aruga, T.; Kuroda, Ii.; Iwasawa, Y. Surf. Sci. 1990, 240, 223. (35) Chrysostomou, D.; Flowers, J.; Zaera, F. Surf. Sci. 1999, 439, 34. (36) Biwer, B. M.; Bernasek, S. L. Surf. Sci. 1986, 167, 207.
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Figure 8. Population of Ti4+ oxidation state on the TiO2(001) single-crystal surface as a function of Ar+ sputtering time, from ref 21. Changes in the (noncorrected) O-to-Ti AES ratio peak height as a function of Ar+ sputtering at the conditions given in the Experimental Section.
UPS, XPS, and ELS. The UPS shows atomic nitrogen, N 2p bands at 4.9 eV indicating dissociation upon adsorption at room temperature. The appearance of bonding levels at 7.9 and 10.1 eV are evidence of the NH3 3a1 orbital, primarily responsible for bonding to the surface (Ti-N bond) upon partial dissociation and N-H 1e orbital, respectively. Siew et al. also showed evidence for dissociation of NH3 on multilayer titanium films on Si(100) substrate with XPS and TDS.37 Upon adsorption at 120 K, three XPS N 1s peaks were observed at 397.8-398.1, 400.5-400.8, and 402.2-402.6 eV and are attributed to NHx (x ) 1 or 2), molecular NH3, and NH4δ+, respectively. Annealing the saturated ammonia surface of Timulti/Si(100) results in two pathways (1) TiN formation (evidenced by a N 1s peak at 396.9 eV) and (2) Ammonia desorption at elevated temperatures. The high-temperature domain at 640 K is thought to proceed via recombination of the NHx species and adsorbed hydrogen:
NHx(a) + (3 - x)H(a) f NH3(g) Evolution of N2(g) is not seen and is considered unlikely due to its high thermal stability (melting point ) 3223 K). XPS N(1s) following adsorption of NH3 on an oxide single crystal (at 300 K and above) is scarce in the literature; most of the studies are conducted using UPS. A recent work on a TiO2(110) single crystal has shown the XPS N(1s) after adsorption of NH3 and other derivatives using a conventional XPS machine. The main point is that NH3 adsorbs molecularly on the stoichiometric surface and that the signal is poor (see Figure 3 in ref 11). We have performed similar XPS studies on the {011}-faceted TiO2(001) single crystal (stoichiometric surface) and also found one peak (poorly resolved) at 400.2 eV attributed to molecular NH3. We have also conducted the same study on the highly defected surface (TiO1.65). Figure 9 shows the N(1s) obtained upon 10 L exposure to a surface that has been prior sputtered with Ar ions for 60 min. Although the signal is poor, a peak at ∼400.2 eV is clear and is attributed to adsorbed NH3. The expected signal of the N(1s) of TiN should be at ∼397.0 eV.7 The small signal at this binding energy is similar to that seen in Figure 3 of ref 11 (at 400 K). It is also at the same position as that observed after electron irradiation of an NH3-adsorbed TiO2 (110) single crystal.7 Several attempts to improve the signal were not successful, and we have thus judged it not to be worth collecting similar signals as a function of temperature since desorption of NH3 has further weakened the signal. From our results, Figures 5 and 9, and the works discussed above,36,37 it is highly likely that (37) Siew, H. L.; Qiao, M. H.; Chew, C. H.; Mok, K. F.; Chan, L.; Xu, G. Q. Appl. Surf. Sci. 2001, 173, 95.
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the nitrogen-oxygen bond of NO is dissociated on the Ar+-sputtered TiO2(110) surface.39 Thus, it appears that the defected TiO2 surface is capable of breaking the carbon-oxygen, nitrogen-oxygen, and nitrogen-hydrogen bonds. This is in addition to the fact that this surface is also capable of making carbon-carbon bonds,15,22,38 and probably nitrogen-nitrogen bonds,5 making it very attractive for studying fundamental bond-breaking and bond-making bonds relevant to almost all surface-gas processes. V. Conclusions
Figure 9. XPS N(1s) after 10-L exposure of NH3 on Ar+sputtered TiO2(001) single crystal at 300 K. The inset shows the corresponding XPS Ti 2p region; the wide shoulder at the low-binding-energy side of Ti4+ (459.0 eV) indicates the presence of multiple oxidation states, including +3 and +2. The line on the N(1s) peak is a guide to the eyes (because of the low signalto-noise) and not a curve fitting.
the peak at ∼440 K is due to recombination of NHx(a) + yH(a) (where x + y ) 3). The high sputtering yield in this work has created Ti atoms in low oxidation states capable of dissociating the N-H bond. The absence of H2 desorption indicates that H-H recombination is not favored on this surface, although traces of H2 desorption, beyond the detection of our experimental setup, cannot be ruled out. We have recently shown that CO2 (as well as CO) can couple to make C2 hydrocarbons on the highly defected surface.38 In other words, the dissociation of the carbonoxygen bond occurs. Recent work has also showed that
NH3 reactions were studied on both stoichiometric and reduced surfaces. Varying the coverage, 0 < θ e 1, and analyzing the TPD peaks resulted in a pre-factor and desorption energy of 1013.5 /s and 92 kJ/mol, respectively, at zero coverage. These parameters show a trend with increasing coverage to decreasing values indicating repulsive interactions between the ammonia molecules at high coverage on the surface. Saturation coverage is reached at ∼0.5 L. This is translated by a saturation coverage of one NH3 molecule per one Ti atom every second one (50% of surface Ti atoms) on the (011) surface in agreement with other reported studies using another TiO2 single crystal, the (110) surface. Considerable difference in peak desorption is seen between stoichiometric and reduced surfaces. The highly defected surfaces created in this work show evidence of higher desorption temperatures for ammonia. The higher the number of defects, the higher is the desorption temperature. A maximum temperature shift of about 100 K (TM ) 440 K) is obtained on TiO1.6, while that from TiO2 is seen at 340 K. LA0484422 (38) Wilson, J. N.; Senanayake, S. D.; Idriss, H. Surf. Sci. 2004, 562, L231. (39) Abad, J.; Bo¨hme, O.; Roma´n, E. Surf. Sci. 2004, 549, 134. (40) Kreuzer, H. J.; Payne, S. H. ASTEK, Version 2; Helix Science Applications: Armdale, Canada, 1995.
