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XPS studies of V2O5, V6O13, VO2 and V2O3

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XPS studies of V2O5, V6O13, VO2 and V2O3 ELSEVIER Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249-261 JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena XPS studies of V205, V6013 , VO 2 and V203 J. Mendialdua a, R. Casanova a'*, Y. Barbaux b aLaboratorio de Super...

XPS studies of V2O5, V6O13, VO2 and V2O3
ELSEVIER Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249-261 JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena XPS studies of V205, V6013 , VO 2 and V203 J. Mendialdua a, R. Casanova a'*, Y. Barbaux b aLaboratorio de Superficies, Departamento de Fisica, Facultad de Ciencias, Universidad de Los Andes, M&ida, Venezuela bLaboratorie de Catalyse H&&ogbne et homogbne, URA CNRS # 402, UniversitO des Sciences et Techniques de Lille, 59655 Villenueve D'Ascq Cedex, France First received 30 March 1994; in final form 4 November 1994 Abstract A detailed XPS study of several oxides of vanadium is reported in this work, in an attempt to characterize clearly the surface of these oxides. Several parameters, such as the FWHM of the V2p3/2 and Ols XPS peaks, their shape and binding energy difference, have been utilized. The characterization is extended to these oxides following different sample treatments. The effect of the presence or absence of adventitious carbon on the behaviour of the V205 sample is investigated when the sample is heated in UHV. It is found that the X-ray beam has some effect on the properties of both crystalline and polycrystalline V205 covered with Cd. Keywords: Vanadium oxide; X-ray photoelectron spectroscopy 1. Introduction In the field of heterogeneous catalysts, it is of fundamental importance to know the state of the surface of a given catalyst with high accuracy. Vanadium pentoxide V205 is a widely used cata- lyst in a variety of chemical reactions, for example, the oxidation of orthoxylene to phthalic anhydride [1,4]. There is great controversy about the surface characterization of this catalyst, such that the nature of its surface remains obscure. The surface of this catalyst could be V205 or a lower vanadium oxide. In this work, we present a detailed XPS study of several vanadium oxides in an attempt to identify clearly the different oxidation states present on the surface of these oxides. For this purpose we have * Corresponding author. analysed several parameters, including the FWHM of the peaks V2p3/2 and O 1 s, their peak shape, and the binding energy difference of these peaks. 2. Experimental Four oxides, V205, V6013, VO2 and V203, were studied in the present investigation. V203 was studied in a polycrystalline form, prepared by hydrogen reduction of polycrystalline V205 at 500°C for 24 h. The V203 stoichiometry was determined from thermogravimetric and X-ray diffraction studies. The polycrystalline V205 employed in the preparation of V203 was supplied by the Catalysis Institute of Cracovia (Poland) and was prepared by decomposition, in air at 500°C, of ammonium metavandadate [1]. The specific surface area of the polycrystalline V203 0368-2048/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02291-7 250 J. Mendialdua et al./Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249-261 was 4.2m 2 g-l. This oxide was subjected to a reduc- tion treatment at 500°C in a flow of H2/He gas mixture, in the proportion 1:3, for 24 h. The thermo- gravimetric measurements reveal that the weight loss of this oxide proceeds to a stationary state and cor- responds to the transformation V205 ~ V203. In air and at room temperature, this sample regains weight (approximately 0.5% of the total weight loss) indicating a surface re-oxidation. Our thermogravimetric studies of V205 at 550°C in a mixture of O2/N2 reveal that this oxide crystal- lizes with many non-stoichiometric defects, since this sample continues to gain weight even after 60h of treatment. V205 was studied as a single crystal (010) face and also in a polycrystalline form. Taking into account the thermogravimetric results, the V205 samples were subjected to an oxi- dation treatment, in a stainless steel furnace, at 550°C under oxygen for 16 h in order to guarantee a well-oxidized surface; the samples were transfered to the spectrometer, under an inert atmosphere, immediately after treatment. V6013 and VO 2 were studied in polycrystalline form without being sub- jected to any previous treatment. The V203 samples were left in the XPS spectrometer in UHV for 14 h in order to get rid of excess oxygen on the surface. The thermogravimetric measurements were per- formed in a Sartorius microbalance. The XPS spec- tra were taken with two spectrometers: an AEI ES200 spectrometer and a Leybold-Heraeus spec- trometer, both operated in the fixed retarding ratio mode. Non-monochromatic A1 Kc~ radiation was used as the X-ray source, with a constant power of 300 W during the XPS measurements. The Leybold spectrometer was fitted with Ar ion etching facil- ities. The measured FWHM of Au4f7/2 in our spec- trometers is 1.4 eV for the Leybold and 1.3 eV for the AEI ES200. The V205, V2013 and VO2 were provided by the Crystallography Laboratory of Gand University (Belgium). The surface of our samples looked very uniform and we found that our sample surface charged homogeneously in the XPS experiment. 3. Results and discussion The wide use of the C ls level associated with adventitious carbon as energy reference in XPS studies of the oxides of vanadium, has contributed to the scattering in the binding energy values of some atomic energy levels in these materials, as given in the literature, making an accurate identification of their oxidation state difficult. It has been shown [5] that for vanadium oxides the energy reference mentioned above is not a good reference, the best energy reference being that of the Ols level at 529.8 eV for V205 and at 530 eV for those oxides whose FWHM is larger than that of V205. A comparison among the different results found in the literature can be established if we correct the energy reference and adopt as such that of the Ols level. Mendialdua et al. [6] have shown that in vanadium oxides the ratio No/N v between the oxygen concentration No and the vanadium concentration Nv does not allow differ- entiation between the different oxidation states at the surface of these oxides. This difficulty in obtain- ing the degree of oxidation of these materials has led us to utilize three parameters, with promising results. These parameters are: the energy difference (A) between the binding energy of the Ols level and that of V2p3/2, the full width (A) at half maximum (FWHM) of each of the former XPS peaks, and their peak shapes. In Table 1 we present our results and the corresponding spectra are shown in Figs. 1 8. 3.1. V:05 The XPS spectra of a polycrystalline V2O 5 sample, obtained by grinding a single crystal, are presented in Fig. 1, in both the as-received con- dition and after it had experienced a sequence of sample treatments in UHV. Spectrum l(a) corre- sponds to the as-received polycrystalline sample: the full width at half maximum for the Ols and V2p3/2 peaks are 1.8 and 1.5 eV respectively. After the sample was subjected to Ar ion bombard- ment (20 min, 10 mA, 2 keV), the XPS spectrum reveals that the FWHM of the Ols peak is 2.2 eV while that of V2p3/2 is 3.8 eV (see Fig. l(b)). Sub- sequently, the sample was re-oxidized in oxygen (1 atm in situ) at 400°C for 14 h; the corresponding XPS spectrum is shown in Fig. l(c); in this case the full widths at half maximum for the peaks under J. Mendialdua et al./Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249-261 Table 1 Result of XPS studies of vanadium oxides 251 Bulk phase Sample treatment Surface phase E Ols/eV A E V2p3/2/eV ), A V205 (single crystal) 16 h in 02 at 500°C V205 529.8 1.55 517 1.3 12.8 V205 (single crystal) Heating at 360°C in UHV V409 529.8 1.75 516.75 2.5 13.05 V205 (polycrystalline) 16 h at 1 atm 02 at 500°C V205 529.8 1.60 517 1.4 12.8 V205 (polycrystalline) Heating at 300°C in UHV V409 529.8 1.85 516.8 2.25 13.0 V205 (polycrystalline) Without contamination and State between V307 and V409 529.8 1.9 516.9 1.8 12.9 heating at 550°C in UHV V205 (polycrystalline) With contamination and V6013 529.8 2.0 516.2 3.3 13.6 heating at 500°C in UHV V6013 (polycrystalline) Short annealing at 200°C V6OI3 530.0 2.2 516.5 3.4 13.5 in UHV V6013 (polycrystalline) Oxidized in 02 at 300°C V3079. 529.8 1.8 516.9 1.8 12.9 for3h VO 2 (polycrystaUine) Ion etching and annealing VO2 530.0 2.8 515.65 4.0 14.35 at 200°C in UHV VO2 (polycrystalline) Oxidized in 02 at 300°C V307? 529.8 1.8 517.1 1.6 12.7 for5h V203 (polycrystalline) Heating at 550°C for 14 h V203 530.0 2.0 515.15 4.8 14.84 in UHV study, i.e. the Ols and V2p3/2, were 1.7 and 1.4 eV respectively. We consider spectrum 1 (c) to be repre- sentative of V205 fully oxidized at its surface (entry 3 in Table 1). From the XPS spectrum of Fig. l(d) which corresponds to this sample after a short annealing at 550°C (2min), we found that the FWHM of the Ols peak changes very little (it goes to 1.8 eV); A for the V2p3/2 peak is now 1.7 eV. After the temperature of the sample was kept at 550°C for 20 min, we obtained spectrum l(e) where A(Ols) = 1.9 eV and (V2p3/2) -- 1.8 eV. (It is inter- esting to note, by comparing spectra l(b) and l(c), that in the reduced state the intensity of the shake-up satellite associated with O ls tends to decrease notably [7].) Spectrum l(f) corresponds to the sample heated to 500°C for 2 h; in this case the full widths at half maximum for the O ls and V2p3/2 peaks were 2 and 3.3 eV respectively. As we can see, it is possible to obtain an FWHM for the V2p3/2 peak that is smaller than that reported in the literature (see Table 2) when a well-defined oxidized surface of V205 is prepared. 3.2. V6013 The XPS spectra of polycrystalline V6013 sample are shown in Fig. 2. The sample was prepared by grinding a single crystal. Spectrum 2(a) corre- sponds to the sample after a short annealing at 200°C in UHV. The obtained values of A for the Ols and V2p3/2 peaks are 2.2 and 3.4 eV respectively, which are notably higher than those of V205. We consider this spectrum to be representative of surface V6013 (entry 7 in Table 1). After the sample was subjected to Ar ion etching (20 min, 10 mA, 2 keV) the FWHMs of the Ols and V2p3/2 change to 2.4 and 4.2 eV respec- tively (this is shown in Fig. 2(b)); these widths become 2.6 and 4.6 eV respectively, once the sample is maintained at 300°C for 13 h in UHV followed by Ar ion etching (20 min, 10 mA, 2 keV); this is represented in spectrum 2(c)). Sub- sequently, the sample was annealed in oxygen at 300°C for 3 h; the corresponding spectrum is pre- sented in Fig. 2(d); in this case, the widths of both peaks are reduced to 1.8 eV. We can also observe in this spectruman increase in the intensity of the Ols shake-up. It is important to note how easy it is to over-oxidize the surface of V6013. 3.3. V02 Fig. 3 shows several XPS spectra of a poly- crystalline VO2 sample in both the as-received con- dition and after being subjected to sample treatments. 252 J. Mendialdua et al./Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249-261 c: o v >.. I.... z I.U l..- z IIII ............. • . . . . . . . . . . . " . I I .... I I I I 550 540 530 520 \ 510 BINDING ENERGY(eV) Fig. 1. XPS spectra of polycrystalline V205 showing the Ols and V2p3/2 peaks after a sequence of several sample treatments: (a) as-received condition; (b) after Ar ion bombardment for 20 min; (c) after oxidation in 02 at 400°C for 14 h; (d) after a short annealing (2 min) at 550°C; (e) sample temperature kept at 550°C for 20 min; (f) sample heated to 500°C for 2 h. Spectrum 3(a) corresponds to the sample in the as-received condition; in this case A(Ols) = 1.9 eV and A(V2p3/2 ) ---- 3.1 eV. Fig. 3(b) gives the XPS spectrum for this sample after being subjected to Ar ion etching (20 min, 10 mA, 2 keV). The obtained values of FWHM for the Ols and V2p3/2 peaks are 2.8 and 4.2 eV respectively. The temperature of the sample was raised from ambient up to 200°C; in this condition the corresponding spectrum is Fig. 3(c); it is found that A for the V2p3/2 peak increases slightly up to 4.4 eV. The sample temperature was kept at 200°C for 15 h (Fig. 3(d)); the FWHM of V2p3/2 increases up to 4.6 eV. The sample was cooled to 35°C (Fig. 3(e)); in this case, the FWHMs of the Ols and V2p3/2 peaks were 2.6 and 4 eV respectively. We consider spectrum 3(e) to be representative of surface VO2 (entry 9 in Table 1). Subsequently, the sample was reoxidized in 02 at 300°C for 5 h; the correspond- ing XPS spectrum is presented in Fig. 3(f) and the determined full widths at half maximum for the Ols and V2p3/2 are 1.8 and 1.6 eV, respectively. We can observe, in the last spectrum, an increase in the intensity of the shake-up satellite associated with O ls. It can also be verified how readily the surface of this oxide over-oxidizes, which has led us to think that spectrum 3(a) does not correspond to a surface state that is characteristic of VO2. J. Mendialdua et al./Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249-261 253 b,. 0 I - . N Z w F- Z // .... c) .......................... " __2bL-- - ~- - . _ J Vz P3,'z /i j, .... • .................... '""" " '" '" "": / ~: \\._1- 5,50 540 L r 550 520 510 BINDING ENERGY(eV) Fig. 2. XPS spectra of polycrystalline V6013. Ols and V2p3/2 peaks are shown after the sample has experienced a sequence of sample treatments: (a) the sample was given a short annealing at 200°C in UHV; (b) after Ar ion etching; (c) sample maintained at 300°C for 13 h followed by Ar ion etching (20 min); (d) sample was annealed in 02 at 300°C for 3 h. 3•4• 1/203 The XPS spectra of a polycrystalline V203 sample prepared as described in the experimental section are presented in Fig. 4. In order to counter- act the reoxidation effect of this sample (see Experi- mental section), the sample temperature was kept at 550°C for 14 h in UHV. The XPS spectrum for this condition is shown in Fig. 4(a). The obtained full widths at half maximum for the O 1 s and V2p3/2 peaks were 2 and 4.8 eV respectively. Spectrum 4(a) is representative of surface V20 3 (entry 11 in Table 1). Spectrum 4(b) corresponds to this sample after being subjected to Ar ion etching (15 min, 10 mA, 2 keV); in this case the determined values of FWHM for the above-mentioned peaks are 2.6 and 6 eV respectively• From the former results and from Table 1, we can observe that the FWHM for the V2p3/2 peak increases as the degree of oxidation decreases, and this is correlated to larger values of the binding energy difference. According to Tables 1 and 2 we can see that our binding energy values agree quite well with those reported by Sawatzki and Post [8], although their values for the FWHM of V2p3/2 are larger than ours, which could indicate a slightly reduced state for their surface. The values given by Rao et al. [9] would correspond to even more reduced states, and their results for VO2 would correspond to a higher 254 J. Mendialdua et al./Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249 261 E 0 >- F- Z LiJ I-- Z hi OIs II II I I V2p312 ,50 540 550 520 510 BINDING ENERGY (eV) Fig. 3. Ols and V2p3/2 XPS region for polycrystalline VO 2 after a sequence of several treatments: (a) as-received condition; (b) after Ar ion etching (20 min); (c) sample temperature raised from ambient to 200°C; (d) sample temperature kept at 200°C for 15 h; (e) sample cooled to 35°C; (f) oxidation in 02 at 300°C for 5 h. oxidation state; in fact once we have made the energy corrections using their quoted reference, we obtain 516.3 eV for the binding energy of the V2p3/2 level, which actually corresponds to V2013. The results reported by Werfel et al. [10] for V205 are very close to ours, if the energy dif- ference between the binding energies attributed to the O ls level is taken into account. How- ever, their values for the parameter A for V6013 and VO2 are larger that ours, which indicates that their samples are very reduced at the surface. The values reported by Anderson [1] for V205 correspond to reduced surfaces; the results for V203, taking into account the width of the V2p3/2 level and the corrected value for its binding energy (516.7 eV), indicate an oxidized surface. The results obtained by Larsson et al. [11] and Blauw et al. [12] correspond to reduced surfaces. Our binding values for V203 are very close to those of Rao et al. [9] who have used the same sample preparation procedure as in this work. The results of Werfel et al. [10] indicate a very reduced surface (A = 17.2 eV) with a corrected binding energy for the V2p3/2 level at 512.8 eV which is very close to the binding energy of metallic J. Mendialdua et al./Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249-261 255 .,=_ c b. o >- Z hl I"- Z O|S L I I I L 550 540 530 520 510 BINDING ENERGY(eV) Fig. 4. Ols and V2p3/2 XPS spectral peaks ofV205 after several consecutive sample treatments: (a) sample temperature kept at 550°C for 14 h in UHV; (b) after Ar ion bombardment (15 min). vanadium reported by Sawatzki and Post [8]. These authors reported values for V203 which would cor- respond to oxidized surfaces (A = 14.4 eV) and a corrected binding energy for V2p3/2 of 515.6 eV. For the case of V205, in which vanadium exhib- its the highest degree of oxidation, we can see that the use of FWHM and the parameter A (or the corrected binding energy for the V2p3/2 level), allows us to differentiate easily between well- oxidized and reduced states of the surface. How- ever, to distinguish between V6013. VO2, V203, and their reduced or oxidized surfaces, it is neces- sary to study in detail the structure and width of the peaks. After making a detailed comparison of the XPS spectra of V205, V6013, VO 2 and V203, after dif- ferent treatments we can observe that the FWHM values of the Ols and V2p3/2 peaks increase in the above order. However, the broadening experienced by the O 1 s peak is less that than of V2p3/2, which can reach up to three or four times the correspond- ing width in V205. We should point out that in V205 oxygen occupies three different sites whose Madelung potentials are very different [13]; consequently, according to the ionic model, three oxygen peaks should be observed in the XPS spectrum. This is not the case, and what is found experimentally is that the width of the Ols peak in V205 is even smaller than in V6013, VO2, and V203. This sug- gests a more covalent model and the presence of mechanisms in addition to those of Madelung. 256 J. Mendialdua et al./Journal of Electron Spectroscopy and Related Phenomena 71 (1995) 249-261 L ._ 0 m @ C H V2p3/2 \ 01s vep3/2 BINDING ENERGY (eV) Fig. 5. Comparison between the XPS spectra of (a) V6013 after a short heating at 200°C in UHV; and (b) V205 at 500°C in UHV. Vanadium occupies three different sites in V6013 with charges [2] 4.13(e), 4.65(e) and 4.36(e), and seven different types of oxygen whose charges range from 1.94(e) to 2.17(e), i.e. a range of variation of 0.23(e) (in contrast in V205 where the range is only 0.1(e)). Taking into account these differences we should expect that the Ols peak width in V203 is larger than in V205. VO2 has two different types of oxygen, but the differences in the V-O bond lengths are smaller and their Madelung potentials differ only by 0.3 eV [2]. However, it is found experimentally that the O ls peak is wider than the corresponding peak in V205. We believe that the broadening of the Ols peak is not due to the presence of contami- nants (H20, CO, etc.)
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