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An AES, UPS and HREELS study of the oxidation and reaction of Nb(110)
surface s c i e n c e ELSEVIER
Surface Science 372 (1997) L285-L290
Surface Science Letters
An AES, UPS and HREELS study of the oxidation and reaction
of Nb(ll0) Yinsheng
W a n g a, X u m i n g W e i a, Z h i j i a n T i a n a, Y u m i n g C a o a, R u n s h e n g Z h a i a,,, T. U s h i k u b o b, K . S a t o b, S h u x i a n Z h u a n g °
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian 116023, P.R. China b Mitsubishi Chemical Corporation, Japan Department of Chemical Physics, The University of Science and Technology of China, Hefei 230026, P.R. China
Received 15 February 1996;accepted for publication 10 October 1996
Abstract
The oxidation of Nb(ll0) upon oxygen adsorption has been investigated using AES, UPS and HREELS. For low and medium exposures of oxygen, two new loss features at 720 and ~ 950 cm- 1 develop, which are assigned to Nb-O and Nb-O-Nb stretching vibrations, respectively.When oxygenexposure is high, a broad and strong loss peak at 600-900cm- 1 appears, which is due to bulklike Nb-O-Nb stretching vibrations, the changes in the vibrational spectroscopy suggest that, with increasing oxidation of the surface, NbO4 tetrahedra transform into N b O 6 octahedra. Experiments involvingmethanol adsorption further confirm these results concerning the process of oxidation. Keywords: Electron energy loss spectroscopy;Niobium; Oxidation
Niobium pentoxide and niobic acid, as a kind of new catalytic material, have attracted great attention in recent years, especially for solid superacidity of niobic acid (H0 = --5.6) [1]. However, the origin of the acidity is not known. Details of oxidation and the structure of the oxide are essential for this purpose. The oxidation of niobium has been investigated with surface science techniques such as AES, UPS, XPS and SIMS. But the answer to this question is still far from clear. H R E E L S is a powerful tool for obtaining vibrational information on surface species and proved very useful in our exploration of the structure and properties of niobium oxide. *Corresponding author. Fax: +86 411 469 1570.
In this Letter we report the adsorption of oxygen and the interaction between oxygen and methanol on N b ( l l 0 ) by means of AES, U P S and HREELS. The experiments were carried out in an ELS-22 spectrometer with a base pressure of about 1-3 x 10 -1° mbar. The spectra were taken in the specular direction at a primary energy of 5 eV with an energy resolution of 72 to 100 cm -1. Cooling of the sample was achieved through liquid nitrogen in the sample holder. Temperature measurement was performed via a chromel-alumel thermocoupie, which was mounted directly onto the back of the sample. Methanol was directed into the vacuum through a leak valve after cycles of freezepump-thaw.
0039-6028/97/$17.00 Copyright© 1997Elsevier ScienceB.V. All rights reserved PII S0039-6028 (96) 01253-8
Y. Wang et al./Surfaee Science 372 (1997) L285-L290
The Nb(110) single crystal was cleaned by cycles of Ar+-sputtering and annealing until no impurity other than oxygen could be detected by AES and HREELS. Oxygen contamination could only be removed by heating to 2400°C [2-4]. However, the highest temperature our resistive heating device could attain was about 850°C and we could only obtain (by AES) a surface O/Nb ratio of 0.2. Under these circumstances the LEED showed an indistinct (1 x 1) pattern. All the work reported in this Letter began with such a surface, which we define as a "clean" surface. Fig. 1 shows AES of Nb(ll0) upon oxygen
,U
(f)
s
(e)
adsorption. The peak at 167.8 eV corresponds to the transition of Nb(MNV). When the oxygen exposure was 2 L(1 L=1.33 × 10 - 6 m b a r - s ) , a shoulder at about 5 eV in the low energy region of the Nb(MNV) developed. This is due to the role of O(2p) electrons in the new Nb(MN)O(2p) Auger transition, which suggests the formation of high valence niobium oxide (Nb2Os). This is consistent with what has been reported in the literature [ 5]. Fig, 2 shows the oxygen uptake on the Nb(ll0) surface, as measured by the AES O(KLL)/Nb (MNV) peak-to-peak intensity ratio during different stages of oxygen adsorption. The intensity ratio of O(KLL) to Nb(MNV) increases with increasing oxygen exposure. When the oxygen exposure exceeds 2 L, it is clear that the increase of the ratio becomes lower. This indicates that the oxide film formed at the surface inhibits the transport of surface oxygen into the bulk. He(I) UPS (not shown) of "clean" Nb(ll0) shows a narrow peak at 5.8 eV. This corresponds to the ~ hybrid orbital of O(2px, ~)/Nb(4d) derived from O(2p) and indicates that a kind of single
(d) 1.6
1.4
~
(b)
1.2
I --
1
0.8
0.6
511.7
0.4 I
16Z8
f KINETIC
0
ENERGY(eV)
Fig. 1. Lineshape of N b ( M N V ) and O ( K L L ) transition of N b ( l l 0 ) at different exposures of oxygen: (a) 0, (b) 2, (c) 5, (d~ l o n (e~ 45130 nnd (f'~ 101300 I.
1
I
I
2 3 LOG ( 02 exposure)
4
Fig. 2. AF,S peak-to-peak intensity ratio of O ( K L L ) / N b ( M N V ) as a function of the logarithm of oxygen ~ynn~nr~
Y. Wang et aL /Surface Science 372 (1997) L285-L290
valence niobium oxide has been formed on the surface (probably as NbOo.z [6]). The other derivative energy level of O(2p) is a weak peak at 8.2 eV. When oxygen exposure was increased to 1000 L, the valence band becomes narrower. This indicates that the work-function increases upon oxygen adsorption, suggesting electron transfer from the substrate to the adsorbate. At higher exposures, the shift of the tr and rc states to lower binding energies at 7.8 and 5.2 eV, respectively, may indicate the formation of a more ionic oxide species [7] as the spectrum is characteristic of transition metal oxides [7]. Fig. 3 shows the H R E E L S results of N b ( l l 0 ) upon oxygen adsorption at both 140 and 298 K. Fig. 3a is the H R E E L S of the "clean" surface. There is a strong loss feature at 536 cm -1. This, together with the gain peak at 552 c m - 1 at the left side of the elastic peak, corresponds to a phonon vibration, which indicates that the surface of our "clean" N b ( l l 0 ) contains a niobium oxide species. That the phonon loss is relatively weak compared to the p h o n o n losses of ZnO [8], MgO [9] and NbzO5 on Pt(111) [ 10], and also that the overtone at 1088 c m - ~ is subtle, permits us to conclude that the amount of oxide formed on the surface is small. This is consistent with the O / N b ratio of 0.20, obtained by AES. When a 2 L exposure of oxygen is performed, a peak at 944 c m - ~ appears and a shoulder peak at 720 cm -1 could also be discerned, simultaneously the gain peak weakened. Fig. 3c shows the H R E E L S of N b ( l l 0 ) upon adsorption of 100 L oxygen. Now the 720 c m - ~ peak has strengthened to become a prominent peak and the feature at 944 cm - t has also intensified to a strong loss feature at 952 cm -~. In addition, the phonon loss feature weakened, relatively, and the gain peak was so weak that it could hardly be detected, which indicated that the ordered surface had begun to break down. When the oxygen exposure was increased to 4500 L, a strong and broad feature centered at about 600-900 cm -~ appeared. Recently, there have been many studies on SiO2supported Nb2Os. Even though these studies differed somewhat in the support and precursor used and the experimental technique, they have led to a clearer picture of the structure of surface
X128571~
S
x2.56 x
/L___ (e)
~
/QL.._
I
500
--I
0
I
I
I
500 1000 1500 ENERO/LOSS(cq) m
Fig. 3. HREELS of different exposures of oxygen on N b ( l l 0 ) at 140 and 298 K: (a) 0 L, 298 K; (b) 2 L, 298 K; (c) 100 L,
298 K; (d) 4500L, 298 K; (e) 0.2 L, 140 K; (f) 5 L, 298 K. phase Nb2Os. Using extended X-ray adsorption fine structure (EXAFS), Yoshida et al. [11] concluded that, at low loadings, the surface phase Nb205 exists as a tetrahedral species with a N b - O double bond; at high loadings of surface phase
Y. Wang et aL /Surface Science 372 (1997) L285-L290
±'~t~2~5, mey reported a square pyramidal species in an aggregated state, which has also been referred to as a highly distorted octahedron by Jehng and Wachs [12]. Burke and Ko [13] attributed this to the lower heat of adsorption and weak Bronsted acid site for Nb2Os/SiO 2 with higher NbeOs species. They also used diffuse reflectance infrared spectroscopy ( D R I F T ) and assigned the features at 720 and 920cm -~ to the absorption by N b - O - N b and N b - O respectively. With increasing Nb2Os concentration, they found that the relative dominance of the N b - O - N b absorption peak increased, which they thought was because of the extensive linking of octahedra in the sample. We thus assign the 940-960 c m - ~ and 720 c m - ~ loss to the N b - O stretching vibration and N b - O - N b stretching vibrations respectively in NbO4 tetrahedra. Ibach et al. [14] investigated the adsorption and photoreactions of N O on N b ( l l 0 ) and assigned a 935 cm -1 loss feature to the atop bonded atomic oxygen (Nb-O); Raman study [ 15] of Nb2Os showed a sharp, narrow peak at 985 cm -~, which was assigned to the N b - O stretching vibration, providing further confirmation of our assignment. From the results of Yoshida et al. [11] and Burke and K o [13] and from the results of our vibrational spectroscopy we can tentatively assign the oxidation species of niobium under U H V as follows: at low oxygen exposure, Nb205 formed on the surface is composed of NbO4 (with N b - O upright) tetrahedrally coordinated as in the low Nb205 loading case; with increasing oxygen exposure, the amount of N b 2 0 5 increases and the tetrahedra link together though the N b - O - N b bridge bond; when the oxygen exposure is as high as 4500 L, the tetrahedra are transformed into octahedra and bulk-like Nb205 forms, (this is similar to the high Nb205 loading case.) The H R E E L S of N b ( l l 0 ) upon adsorption of oxygen at 140 K (Fig. 3e-f) indicate that, at low temperature, oxygen exposures as low as 0.2 and 5 L give nearly the same spectra as those of 2 and 100 L respectively, at room temperature, suggesting that at low temperature the sticking probability of oxygen on Nb(110) greatly increases. The absence of a p h o n o n gain peak is due to the Boltzmann h~hnvic~r nf th~ rntln n£ ¢rnln-tn-ln~l int~nqltv F 1(iq
Fig. 4 is the H R E E L S of the adsorption of 5 L of CH3OH on N b ( l l 0 ) , which was preadsorbed with different exposures of oxygen. In Fig. 4a-d, two peaks at 1440 and 2920 cm-1 are undoubtedly assigned to the deformation and stretching vibrations of CHx(x = 1 to 3)(&(C-H) and v(C-H)). On the 2 and 100 L 0 2 preadsorbed surfaces the peak at 944(952)cm -1 is greatly attenuated because the Nb cations in N b - O groups at the surface coordinate with methanol through the formation of Lewis-acid-base adducts [15]. This, together with the absence of any feature of v(O-H), indicates that methanol decomposes to methoxy (CH30) groups on the surface. On the 100 L 02 preadsorbed surface, the peak at 720 c m - ~ weakens
X64
292O X1024i~ 40
s6,o A s4i x2o I
I
0
1000
I
I
2000 3000 E N V y LOSS(cm-'I)
Fig. 4. HREELS of 5L methanol adsorbed on Nb(ll0) preadsorbed with different exposures of oxygen: (a) 0, (b) 2, [ c 4 1£11-1 ~ d
¢rl'l ~ i £ 1 f l l
1 ~ 1 ~ . ,.t-. I l l i . F ~ B I a ~ 3
of-a-phonon gain peak is due to the Boltzmann h~hnvinr nf the rntln n£ ¢rnln-tn-ln~l int~nqltv F 1(iq
I
UI
J la
III~Ulil, UUI
I:I,U S U I L l i ; U
UII
lltl)~.llU)
preadsorbed with different exposures of oxygen: (a) O, (b) 2,
Y. Wang et al. /Surface Science 372 (1997) L285-L290
and shifts to 6 8 0 c m -~ when methanol was adsorbed. This is due to the weakening of N b - O - N b bond upon the adsorption of methanol, which suggests that the N b - O - N b bridge bond formed at this oxygen exposure case is confined to the surface, and the N b 2 0 5 formed was isolated. On the surface preadsorbed with 4500 L of 02, the strong, b r o a d peak centered at 600-900 c m - 1 was not disturbed upon methanol adsorption. This further indicated that the Nb2Os formed is bulklike. With increasing exposures of preadsorbed oxygen, the C - O stretching vibration frequency of methoxy increased from 960 to 1096 cm -1. Fig. 5a is the H R E E L S of N b ( l l 0 ) upon the adsorption of 5 L of CH3OH. Fig. 5b is the H R E E L S of (a) followed by 100 L oxygen adsorption. Comparing Fig. 5a with 5b, we find that two strong peaks, at 736 and 984 crn -1, develop on
57~
5~I0 ×2048~k ~
oxygen adsorption; the changes in th are quite similar to those of 100 L oxygen adsorption on the " d e a n " N b ( l l 0 ) . There is also a very interesting large frequency shift of C - O stretching vibration of m e t h o x y from 960 to 1104 cm -1. On a N i ( l l 0 ) surface [17], a frequency shift of C - O stretching vibration in methoxy from 1040cm -1 at 1 8 0 K to 9 7 5 c m -1 at 3 0 0 K was found. The author thought this related directly to the a m o u n t of methoxy present, but the exact nature of this was not shown. From our results, we can conclude that there is a close relationship between the amount of oxygen on the surface and the frequency of v(CO) in methoxy: the more oxygen present, the higher the frequency. The reason for this is not clear, and so further work is necessary. Fig. 5c is the H R E E L S of a further 5 L methanol adsorption on the surface defined for Fig. 5b. We find that the loss features at 736 and 9 8 4 c m -1 weaken a lot, which is also due to the formation of Lewisacid-base adducts. In summary, we found that Nb2Os formed upon oxygen adsorption. At low and medium exposures of oxygen, two loss features appeared at 960 and 720 c m - 1 , which were assigned to the stretching vibrations of N b - O and N b - O - N b , respectively; Nb205 formed in this case was isolated and mostly confined on the surface, At high exposures of oxygen, a broad, strong feature at 600-900 c m - a developed and bulk-like Nb205 formed. A structure changing from tetrahedral to octahedral has been postulated for the oxidation process. The adsorption of methanol further confirms our conclusion.
Acknowledgement
l(a)
Financial support by Chinese National Science Foundation is gratefully acknowledged.
976
0
1000
~
ENEr~Y LOSS(era-I)
30~0
Fig. 5. HREELS of cycles of oxygen and methanol adsorption on Nb(ll0): (a) 5L methanol; (b) 100L oxygen after 5 L methanol; (c) a further 5 L methanol after (b).
References [1] Z. Hen, T. Iizuka and K. Tanabe, Chem. Lett. (1984) 1085. [2] R. Pantel and M. Bujor, Surf. Sci. 62 (1977) 589. [-3] Shang-Lin Weng and O.F. Kammerer, Phys. Rev. B 26 (1982) 2281.
Y. Wang et al. /Surface Science 372 (1997) L285-L290
L~J [5] [6] [7] [8] [9] [10]
. . . . . . d J.L. Erskine, Phys. Rev. B 34 (1986) 5951. K.H. Rieder, Appl. Surf. Sci. 4 (1980) 183. J. Halbritter, Appl. Phys. A 43 (1987) 1. K. Wandelt, Surf. Sci. Rep. 2 (1982) 1. W.T. Petrie and J.M. Vohs, Surf. Sci. 245 (1991) 315. M.C. Wu and D.W. Goodman, Catal. Lett. 15 (1992) 1. L. Xie, D.Z. Wang, C.M. Zhong, X.X. Guo, T. Ushikubo and K. Wada, Surf. Sci. 320 (1994) 62. [11] S. Yoshida, Y. Nishimura, T. Tanaka, T. Kanai and T. Funabiki, Catal. Today 8 (1990) 67.
[12] J.M. Jehng and I.E. Wachs, Catal. Today 8 (1991) 37. [13] P.A. Burke and E.I. Ko, J. Catal. 129 (1991) 38. [14] T.U. Bartke, R. Franchy and H. Ibach, Surf. Sci. 272 (1992) 299. [15] R.M. Pittman and A.T. Bell, J. Phys. Chem. 97 (1993) 12178. [t6] H. Ibaeh, H. Wagner and D. Brunehmann, Solid State Commun. 42 (1982) 457. [17] S.R. Bare, J.A. Stroseio and W. Ho, Surf. Sei. 150 (1985) 399.
Surface Science 372 (1997) L285-L290
Surface Science Letters
An AES, UPS and HREELS study of the oxidation and reaction
of Nb(ll0) Yinsheng
W a n g a, X u m i n g W e i a, Z h i j i a n T i a n a, Y u m i n g C a o a, R u n s h e n g Z h a i a,,, T. U s h i k u b o b, K . S a t o b, S h u x i a n Z h u a n g °
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian 116023, P.R. China b Mitsubishi Chemical Corporation, Japan Department of Chemical Physics, The University of Science and Technology of China, Hefei 230026, P.R. China
Received 15 February 1996;accepted for publication 10 October 1996
Abstract
The oxidation of Nb(ll0) upon oxygen adsorption has been investigated using AES, UPS and HREELS. For low and medium exposures of oxygen, two new loss features at 720 and ~ 950 cm- 1 develop, which are assigned to Nb-O and Nb-O-Nb stretching vibrations, respectively.When oxygenexposure is high, a broad and strong loss peak at 600-900cm- 1 appears, which is due to bulklike Nb-O-Nb stretching vibrations, the changes in the vibrational spectroscopy suggest that, with increasing oxidation of the surface, NbO4 tetrahedra transform into N b O 6 octahedra. Experiments involvingmethanol adsorption further confirm these results concerning the process of oxidation. Keywords: Electron energy loss spectroscopy;Niobium; Oxidation
Niobium pentoxide and niobic acid, as a kind of new catalytic material, have attracted great attention in recent years, especially for solid superacidity of niobic acid (H0 = --5.6) [1]. However, the origin of the acidity is not known. Details of oxidation and the structure of the oxide are essential for this purpose. The oxidation of niobium has been investigated with surface science techniques such as AES, UPS, XPS and SIMS. But the answer to this question is still far from clear. H R E E L S is a powerful tool for obtaining vibrational information on surface species and proved very useful in our exploration of the structure and properties of niobium oxide. *Corresponding author. Fax: +86 411 469 1570.
In this Letter we report the adsorption of oxygen and the interaction between oxygen and methanol on N b ( l l 0 ) by means of AES, U P S and HREELS. The experiments were carried out in an ELS-22 spectrometer with a base pressure of about 1-3 x 10 -1° mbar. The spectra were taken in the specular direction at a primary energy of 5 eV with an energy resolution of 72 to 100 cm -1. Cooling of the sample was achieved through liquid nitrogen in the sample holder. Temperature measurement was performed via a chromel-alumel thermocoupie, which was mounted directly onto the back of the sample. Methanol was directed into the vacuum through a leak valve after cycles of freezepump-thaw.
0039-6028/97/$17.00 Copyright© 1997Elsevier ScienceB.V. All rights reserved PII S0039-6028 (96) 01253-8
Y. Wang et al./Surfaee Science 372 (1997) L285-L290
The Nb(110) single crystal was cleaned by cycles of Ar+-sputtering and annealing until no impurity other than oxygen could be detected by AES and HREELS. Oxygen contamination could only be removed by heating to 2400°C [2-4]. However, the highest temperature our resistive heating device could attain was about 850°C and we could only obtain (by AES) a surface O/Nb ratio of 0.2. Under these circumstances the LEED showed an indistinct (1 x 1) pattern. All the work reported in this Letter began with such a surface, which we define as a "clean" surface. Fig. 1 shows AES of Nb(ll0) upon oxygen
,U
(f)
s
(e)
adsorption. The peak at 167.8 eV corresponds to the transition of Nb(MNV). When the oxygen exposure was 2 L(1 L=1.33 × 10 - 6 m b a r - s ) , a shoulder at about 5 eV in the low energy region of the Nb(MNV) developed. This is due to the role of O(2p) electrons in the new Nb(MN)O(2p) Auger transition, which suggests the formation of high valence niobium oxide (Nb2Os). This is consistent with what has been reported in the literature [ 5]. Fig, 2 shows the oxygen uptake on the Nb(ll0) surface, as measured by the AES O(KLL)/Nb (MNV) peak-to-peak intensity ratio during different stages of oxygen adsorption. The intensity ratio of O(KLL) to Nb(MNV) increases with increasing oxygen exposure. When the oxygen exposure exceeds 2 L, it is clear that the increase of the ratio becomes lower. This indicates that the oxide film formed at the surface inhibits the transport of surface oxygen into the bulk. He(I) UPS (not shown) of "clean" Nb(ll0) shows a narrow peak at 5.8 eV. This corresponds to the ~ hybrid orbital of O(2px, ~)/Nb(4d) derived from O(2p) and indicates that a kind of single
(d) 1.6
1.4
~
(b)
1.2
I --
1
0.8
0.6
511.7
0.4 I
16Z8
f KINETIC
0
ENERGY(eV)
Fig. 1. Lineshape of N b ( M N V ) and O ( K L L ) transition of N b ( l l 0 ) at different exposures of oxygen: (a) 0, (b) 2, (c) 5, (d~ l o n (e~ 45130 nnd (f'~ 101300 I.
1
I
I
2 3 LOG ( 02 exposure)
4
Fig. 2. AF,S peak-to-peak intensity ratio of O ( K L L ) / N b ( M N V ) as a function of the logarithm of oxygen ~ynn~nr~
Y. Wang et aL /Surface Science 372 (1997) L285-L290
valence niobium oxide has been formed on the surface (probably as NbOo.z [6]). The other derivative energy level of O(2p) is a weak peak at 8.2 eV. When oxygen exposure was increased to 1000 L, the valence band becomes narrower. This indicates that the work-function increases upon oxygen adsorption, suggesting electron transfer from the substrate to the adsorbate. At higher exposures, the shift of the tr and rc states to lower binding energies at 7.8 and 5.2 eV, respectively, may indicate the formation of a more ionic oxide species [7] as the spectrum is characteristic of transition metal oxides [7]. Fig. 3 shows the H R E E L S results of N b ( l l 0 ) upon oxygen adsorption at both 140 and 298 K. Fig. 3a is the H R E E L S of the "clean" surface. There is a strong loss feature at 536 cm -1. This, together with the gain peak at 552 c m - 1 at the left side of the elastic peak, corresponds to a phonon vibration, which indicates that the surface of our "clean" N b ( l l 0 ) contains a niobium oxide species. That the phonon loss is relatively weak compared to the p h o n o n losses of ZnO [8], MgO [9] and NbzO5 on Pt(111) [ 10], and also that the overtone at 1088 c m - ~ is subtle, permits us to conclude that the amount of oxide formed on the surface is small. This is consistent with the O / N b ratio of 0.20, obtained by AES. When a 2 L exposure of oxygen is performed, a peak at 944 c m - ~ appears and a shoulder peak at 720 cm -1 could also be discerned, simultaneously the gain peak weakened. Fig. 3c shows the H R E E L S of N b ( l l 0 ) upon adsorption of 100 L oxygen. Now the 720 c m - ~ peak has strengthened to become a prominent peak and the feature at 944 cm - t has also intensified to a strong loss feature at 952 cm -~. In addition, the phonon loss feature weakened, relatively, and the gain peak was so weak that it could hardly be detected, which indicated that the ordered surface had begun to break down. When the oxygen exposure was increased to 4500 L, a strong and broad feature centered at about 600-900 cm -~ appeared. Recently, there have been many studies on SiO2supported Nb2Os. Even though these studies differed somewhat in the support and precursor used and the experimental technique, they have led to a clearer picture of the structure of surface
X128571~
S
x2.56 x
/L___ (e)
~
/QL.._
I
500
--I
0
I
I
I
500 1000 1500 ENERO/LOSS(cq) m
Fig. 3. HREELS of different exposures of oxygen on N b ( l l 0 ) at 140 and 298 K: (a) 0 L, 298 K; (b) 2 L, 298 K; (c) 100 L,
298 K; (d) 4500L, 298 K; (e) 0.2 L, 140 K; (f) 5 L, 298 K. phase Nb2Os. Using extended X-ray adsorption fine structure (EXAFS), Yoshida et al. [11] concluded that, at low loadings, the surface phase Nb205 exists as a tetrahedral species with a N b - O double bond; at high loadings of surface phase
Y. Wang et aL /Surface Science 372 (1997) L285-L290
±'~t~2~5, mey reported a square pyramidal species in an aggregated state, which has also been referred to as a highly distorted octahedron by Jehng and Wachs [12]. Burke and Ko [13] attributed this to the lower heat of adsorption and weak Bronsted acid site for Nb2Os/SiO 2 with higher NbeOs species. They also used diffuse reflectance infrared spectroscopy ( D R I F T ) and assigned the features at 720 and 920cm -~ to the absorption by N b - O - N b and N b - O respectively. With increasing Nb2Os concentration, they found that the relative dominance of the N b - O - N b absorption peak increased, which they thought was because of the extensive linking of octahedra in the sample. We thus assign the 940-960 c m - ~ and 720 c m - ~ loss to the N b - O stretching vibration and N b - O - N b stretching vibrations respectively in NbO4 tetrahedra. Ibach et al. [14] investigated the adsorption and photoreactions of N O on N b ( l l 0 ) and assigned a 935 cm -1 loss feature to the atop bonded atomic oxygen (Nb-O); Raman study [ 15] of Nb2Os showed a sharp, narrow peak at 985 cm -~, which was assigned to the N b - O stretching vibration, providing further confirmation of our assignment. From the results of Yoshida et al. [11] and Burke and K o [13] and from the results of our vibrational spectroscopy we can tentatively assign the oxidation species of niobium under U H V as follows: at low oxygen exposure, Nb205 formed on the surface is composed of NbO4 (with N b - O upright) tetrahedrally coordinated as in the low Nb205 loading case; with increasing oxygen exposure, the amount of N b 2 0 5 increases and the tetrahedra link together though the N b - O - N b bridge bond; when the oxygen exposure is as high as 4500 L, the tetrahedra are transformed into octahedra and bulk-like Nb205 forms, (this is similar to the high Nb205 loading case.) The H R E E L S of N b ( l l 0 ) upon adsorption of oxygen at 140 K (Fig. 3e-f) indicate that, at low temperature, oxygen exposures as low as 0.2 and 5 L give nearly the same spectra as those of 2 and 100 L respectively, at room temperature, suggesting that at low temperature the sticking probability of oxygen on Nb(110) greatly increases. The absence of a p h o n o n gain peak is due to the Boltzmann h~hnvic~r nf th~ rntln n£ ¢rnln-tn-ln~l int~nqltv F 1(iq
Fig. 4 is the H R E E L S of the adsorption of 5 L of CH3OH on N b ( l l 0 ) , which was preadsorbed with different exposures of oxygen. In Fig. 4a-d, two peaks at 1440 and 2920 cm-1 are undoubtedly assigned to the deformation and stretching vibrations of CHx(x = 1 to 3)(&(C-H) and v(C-H)). On the 2 and 100 L 0 2 preadsorbed surfaces the peak at 944(952)cm -1 is greatly attenuated because the Nb cations in N b - O groups at the surface coordinate with methanol through the formation of Lewis-acid-base adducts [15]. This, together with the absence of any feature of v(O-H), indicates that methanol decomposes to methoxy (CH30) groups on the surface. On the 100 L 02 preadsorbed surface, the peak at 720 c m - ~ weakens
X64
292O X1024i~ 40
s6,o A s4i x2o I
I
0
1000
I
I
2000 3000 E N V y LOSS(cm-'I)
Fig. 4. HREELS of 5L methanol adsorbed on Nb(ll0) preadsorbed with different exposures of oxygen: (a) 0, (b) 2, [ c 4 1£11-1 ~ d
¢rl'l ~ i £ 1 f l l
1 ~ 1 ~ . ,.t-. I l l i . F ~ B I a ~ 3
of-a-phonon gain peak is due to the Boltzmann h~hnvinr nf the rntln n£ ¢rnln-tn-ln~l int~nqltv F 1(iq
I
UI
J la
III~Ulil, UUI
I:I,U S U I L l i ; U
UII
lltl)~.llU)
preadsorbed with different exposures of oxygen: (a) O, (b) 2,
Y. Wang et al. /Surface Science 372 (1997) L285-L290
and shifts to 6 8 0 c m -~ when methanol was adsorbed. This is due to the weakening of N b - O - N b bond upon the adsorption of methanol, which suggests that the N b - O - N b bridge bond formed at this oxygen exposure case is confined to the surface, and the N b 2 0 5 formed was isolated. On the surface preadsorbed with 4500 L of 02, the strong, b r o a d peak centered at 600-900 c m - 1 was not disturbed upon methanol adsorption. This further indicated that the Nb2Os formed is bulklike. With increasing exposures of preadsorbed oxygen, the C - O stretching vibration frequency of methoxy increased from 960 to 1096 cm -1. Fig. 5a is the H R E E L S of N b ( l l 0 ) upon the adsorption of 5 L of CH3OH. Fig. 5b is the H R E E L S of (a) followed by 100 L oxygen adsorption. Comparing Fig. 5a with 5b, we find that two strong peaks, at 736 and 984 crn -1, develop on
57~
5~I0 ×2048~k ~
oxygen adsorption; the changes in th are quite similar to those of 100 L oxygen adsorption on the " d e a n " N b ( l l 0 ) . There is also a very interesting large frequency shift of C - O stretching vibration of m e t h o x y from 960 to 1104 cm -1. On a N i ( l l 0 ) surface [17], a frequency shift of C - O stretching vibration in methoxy from 1040cm -1 at 1 8 0 K to 9 7 5 c m -1 at 3 0 0 K was found. The author thought this related directly to the a m o u n t of methoxy present, but the exact nature of this was not shown. From our results, we can conclude that there is a close relationship between the amount of oxygen on the surface and the frequency of v(CO) in methoxy: the more oxygen present, the higher the frequency. The reason for this is not clear, and so further work is necessary. Fig. 5c is the H R E E L S of a further 5 L methanol adsorption on the surface defined for Fig. 5b. We find that the loss features at 736 and 9 8 4 c m -1 weaken a lot, which is also due to the formation of Lewisacid-base adducts. In summary, we found that Nb2Os formed upon oxygen adsorption. At low and medium exposures of oxygen, two loss features appeared at 960 and 720 c m - 1 , which were assigned to the stretching vibrations of N b - O and N b - O - N b , respectively; Nb205 formed in this case was isolated and mostly confined on the surface, At high exposures of oxygen, a broad, strong feature at 600-900 c m - a developed and bulk-like Nb205 formed. A structure changing from tetrahedral to octahedral has been postulated for the oxidation process. The adsorption of methanol further confirms our conclusion.
Acknowledgement
l(a)
Financial support by Chinese National Science Foundation is gratefully acknowledged.
976
0
1000
~
ENEr~Y LOSS(era-I)
30~0
Fig. 5. HREELS of cycles of oxygen and methanol adsorption on Nb(ll0): (a) 5L methanol; (b) 100L oxygen after 5 L methanol; (c) a further 5 L methanol after (b).
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Y. Wang et al. /Surface Science 372 (1997) L285-L290
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