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Determination of vanadium valence in hydrated compounds
Journal of Alloys and Compounds 382 (2004) 239–243
Determination of vanadium valence in hydrated compounds V. Bondarenka a,∗ , S. Grebinskij a , S. Mickeviˇcius a , H. Tvardauskas a , S. Kaˇciulis b b
a Semiconductor Physics Institute, A. Goštauto 11, LT-2600 Vilnius, Lithuania Institute for the Study of Nanostructured Materials (ISMN-CNR), P.O. Box 10, I-00016 Monterotondo Scalo (RM), Italy
Received 16 September 2003; received in revised form 16 February 2004; accepted 15 March 2004
Abstract X-ray photoelectron spectroscopy was used to determine the chemical shift and full-width at half-maximum intensity of the V4+ and V5+ cations peaks in the vanadium pentoxide matrix. It was found that the binding energy of V4+ ions shifts to the lower energy side of about 1.3 eV as compared with the main V5+ ions in the matrix. The V 2p3/2 line width for tetravalent vanadium ions in xerogels is actually the same as for pentavalent ions. The fitting procedure of X-ray photoelectron spectra permits determination of the relative concentrations of the reduced vanadium ions with an accuracy of about 3%. © 2004 Elsevier B.V. All rights reserved. Keywords: Surfaces and interfaces; Sol–gel synthesis; Photoelectron spectroscopy
1. Introduction The mixed valence metal speciation of solid samples is not a trivial problem. In fact, chemical analytical methods often fail due to the relative instability of the various oxidation states of metals in common dissolution media. X-ray photoemission spectroscopy (XPS) is a very useful technique for determining such important parameters as the chemical state of different element species and their relative concentration. The accurate binding energy (BE) has to be measured in order to determine the chemical state from the chemical shift (CS). Quantitative information on the relative surface amount of element species may be obtained from the relative areas of the peaks, corresponding to the single components of the envelope spectra. This may be easily done, if the individual peaks are well resolved, i.e. the full-width at half-maximum intensity (FWHM) of the peak is lower than CS for corresponding species. Unfortunately, the BE of V 2p3/2 line for various valence vanadium oxides is in a relatively low-energy range of about 515.5–517.5 eV [1], while the FWHM of the corresponding peaks varies from 1.2 eV (V2 O5 ) [2] to 4.8 eV (V2 O3 ) [3]. Moreover, the CS of V 2p doublet for V3+ and V4+ species compared with the V5+ doublet in vanadium oxides is lower ∗
Corresponding author. Tel.: +3702 619466; fax: +3702 627123. E-mail address: [email protected] (V. Bondarenka).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.005
than the FWHM of corresponding peaks [4] and the resulting spectra cannot be straightforwardly separated into individual components. Thus, the quantitative estimation of vanadium oxidation states is not a trivial task, and suitable standards for the calibration of V5+ , V4+ and V3+ BE and FWHM in vanadium oxides are required. The general aim of this work was to determine the chemical shift between V4+ and V5+ ions and the FWHM of the respective XPS peaks in the vanadium compounds with the different valence states of vanadium.
2. Literature analysis The relatively full collection (of about 50 references) of V 2p3/2 line BE for the main valence oxides (V2 O5 , VO2 and V2 O3 ) may be obtained from the literature analysis. Statistical methods then may be applied to obtain the mean values of BE for the corresponding valence vanadium species and to estimate the inaccuracy of the data presented in literature. Fig. 1 presents the BE distribution function fBE (E) for main valence vanadium oxides BE and defined as fBE (E) =
1 N Nt 2E
(1)
where N is the number of references, for which the V 2p3/2 line BE is within the interval (BE − E) < E ≤ (BE + E),
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1. The V 2p3/2 line BE measurements permit separation of vanadium species in the adjacent oxidation states; 2. The accuracy of such a BE determination is insufficient for precise deconvolution of the XPS spectra into the individual components; 3. The same care may be taken in the use of data, obtained from the main valence vanadium oxides reference samples study, in the mixed valence compounds XPS spectra analysis. Moreover, the parameters deduced from the analysis of XPS spectra of reference samples may be used only as a rough approximation to those in the mixed valence compounds because of: Fig. 1. Distribution functions of the V 2p3/2 binding energy for the main valence vanadium oxides found in the literature. The points correspond to reference data. The dashed lines present the Gaussian distribution for different oxides and the solid line is the distribution envelope.
we use E = 0.2 eV for the convolution of the literature data, and Nt the total number of references. Three fairly well-resolved peaks may be seen in Fig. 1, indicating that at least three different data sets (corresponding to V2 O5 , V2 O4 and V2 O3 oxides) are involved in the distribution function. Further analysis was done assuming that the reference data for individual oxide are distributed by a normal Gaussian law (corresponding to a random distribution of BE for each set of data): 1 1 BE − BEm 2 G FBE = √ exp − (2) 2 w w 2π where BEm is the average value of BE and w the statistical deviation of BE. It is clear from Fig. 1 that the V 2p3/2 line of the BE distribution function may be deconvoluted into three Gaussian peaks with centroids located at 517.3, 516.5 and 515.6 eV with a statistical deviation of w = 0.25 eV. These peaks may be attributed to V5+ , V4+ and V3+ species in V2 O5 , VO2 and V2 O3 oxides, respectively. So it may be concluded that use of the data reported in literature for the main valence vanadium oxides permits separation of the vanadium species into different oxidation states. On the other hand, the statistical error δBE of BE determination obtained from the literature analysis (δBE 0.7 = w = 0.25 eV and δBE = 2w = 0.5 eV for the 70 and 95% 0.95 probability levels, respectively) may be compared with an estimated value of CS for adjacent valence vanadium oxides CS34 = −0.9 eV and CS54 = 0.8 eV (see Fig. 1). Thus, the accuracy in CS obtained from BE analysis (δCS 0.7 = 2w = 0.5 eV and δCS = 4w = 1.0 eV) is insufficient to use 0.95 these data for the successful deconvolution of the spectra envelope containing spectra of the different oxidation states vanadium species. From the literature analysis it may be concluded that:
1. CS depends not only on the vanadium ion charge (V4+ , V5+ or V3+ ) but also on the value of the crystal field long-range Madelung potential [5]. The last correction may be of the same order of magnitude as the first one. 2. In the vanadium oxides, the FWHM of the V 2p3/2 line depends on the density of the population of the narrow d-band [4], thus this line width for V4+ species may differ significantly in V2 O4 crystals and in V4+ -ions containing xerogels.
3. Experimental It seems that a better way to determinate the relative concentrations of vanadium ions in various valence states is to compare the samples with a same host matrix but a different concentration of the vanadium ions of reduced valence. This comparison may be accomplished in two different ways: 1. by the intercalating of additional V4+ ions in the vanadium pentoxide based matrix. 2. by varying the concentration of V4+ ions by some external treatment. The advantage of this technique is that the same sample can be used to study the comparison spectra and the additional (structure-related effects) may be neglected. We try to use both these approaches to clarify the V4+ spectral line parameters in V2 O5 -based xerogels H2 V12 O31 ·nH2 O. The layered structure of these compounds allows to intercalate various elements that change the concentration of V4+ ions [5]. Moreover, the relative concentration of tetravalent vanadium ions may be changed by a thermal treatment, UV irradiation and ions etching [6–8]. First of all, the reference powder samples of V2 O5 and V2 O4 were used to determine the CS and the FWHM in clean oxides. The thin films of H2 V12 O31 ·nH2 O and (VO)V12 O31 ·nH2 O were used to compare the materials with identical crystal structure. Finally, (VO)V12 O31 ·nH2 O xerogels before and after heat treatment were used to separate the additional V4+ ions influence on the XPS spectrum. The XPS experiment has been carried out with an Escalab MkII (VG Scientific) spectrometer, equipped with an Al K␣
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three lines should be introduced to describe the shape of this peak for the studied samples. According to the chemical composition, these lines may be attributed to the following oxygen species: 1. the main ‘oxide’ peak at BE = 529.9 ± 0.3 eV, belongs to an oxide matrix; 2. the ‘H2 O’ peak at BE = 533.2 ± 0.3 eV, which may be related to adsorbed water; 3. hydroxide, carbonate and ‘bonded’ water species could contribute to the ‘hydro’ peak at BE = 531.7 ± 0.3 eV.
Fig. 2. XPS core-level spectra of V 2p and O 1s region for different samples: s1 – V2 O4 reference powders; s2 – V2 O5 reference powders; s3 – H2 V12 O31 ·nH2 O xerogel; s4 – (VO)V12 O31 ·nH2 O xerogel ‘as-grown’; s5 – (VO)V12 O31 ·nH2 O xerogel after annealing at 510 K.
(1486.6 eV) excitation source, a five-channeltron detection system and a hemispherical analyzer, which was set to 20 eV pass energy. The binding energy scale was corrected for charging effects by assigning a value of 284.6 eV to the C 1s peak. Photoemission data have been collected and processed by using a VGX-900 data system. After Shirley background subtraction, a nonlinear least-squares curve-fitting routine with a Gaussian/Lorentzian product function has been used for the analysis of the XPS spectra. Fig. 2 presents the original XPS core-level spectra of the V 2p and O 1s region for all treated samples. All the spectra have been corrected for X-ray source satellites. Preliminary information may be obtained directly from the view of the presented spectra. First of all it can be noted that the spectral lines of the corresponding regions overlap, hence the attempt to analyze each region independently may cause a significant error in the estimation of the areas of related peaks. So one needs to study the whole V 2p and O 1s region in order to obtain the reliable results. On the other hand, V 2p3/2 and O 1s lines are considerably separated in the BE scale and the main particularities of corresponding peaks may be revealed using the common methods (such as the second derivative and the comparison spectra) of the spectrum analysis [9]. It is clear that the O 1s peak exhibits a complex structure, indicating that various oxide species are presented. At least
As to vanadium lines, it may be stated that V 2p3/2 peak is actually symmetric for V2 O5 and V2 O4 samples, while the obvious widening at the lower BE wing of this spectra may be seen when going from s3 to s5 samples (Fig. 2). This dependency becomes more evident from the second derivation plots (see Fig. 3). For the s3 sample, the derivative spectra are actually symmetric, indicating that V5+ (BE ≈ 517.2 eV) ions dominate in this H2 V12 O31 ·nH2 O xerogel. The evident shoulder is, however, seen at BE ≈ 515.8 eV for the s4 and s5 samples. The appearance of this feature may be attributed to the significant increase of V4+ ions in xerogels after (H2 )2+ ions substitution for (VO)2+ cations and the following annealing. Unfortunately, the lower intensity main negative peak from V4+ the ions actually overlaps with the secondary positive peak of higher intensity, so the CS ≈ 1.4 eV, obtained from the second derivative plots may be regarded only as a rough estimation.
Fig. 3. The second derivatives of XPS spectra for V2 O5 -based xerogels: (1) H2 V12 O31 ·nH2 O xerogel; (2) (VO)V12 O31 ·nH2 O xerogel ‘as-received’; (3) (VO)V12 O31 ·nH2 O xerogel ‘after baking’ at 510 K.
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Fig. 4. Comparison and difference spectra for V 2p3/2 line in V2 O5 -based xerogels: (a) H2 V12 O31 ·nH2 O xerogel; (b) (VO)V12 O31 ·nH2 O xerogel ‘as-grown’; (c) (VO)V12 O31 ·nH2 O xerogel after annealing at 510 K; α = b − 0.93a; β = c − 0.85b.
The spectra comparison technique then was used for the further examination of the core-level states of V4+ ions in V2 O5 -based xerogels. Fig. 4 presents the comparison and the difference spectra for H2 V12 O31 ·nH2 O and (VO)V12 O31 ·nH2 O samples, as well as for the (VO)V12 O31 ·nH2 O sample before and after annealing at 240 ◦ C. The optimal weight procedure was used to extract the additional, low-intensity, V4+ 2p3/2 peaks from the background of the main, high-intensity, V5+ 2p3/2 peak [9]. It is clear from Fig. 4A and B that the replacement of (H2 )2+ cations by (VO)2+ cation, like the annealing of (VO)V12 O31 ·nH2 O sample, leads to the appearance of the additional peak, shifted by 1.1–1.2 eV to a lower energy side, compared with the main peak position. This peak may be attributed to the V4+ species and the CS of about 1.2 eV may be expected for V4+ -ions in V2 O5 based xerogels, in good agreement with a value estimated from the second derivative plots (Fig. 3). It should be noted that this value agrees with an average CS = 1.1 eV, obtained from the literature analysis (see Fig. 1). The more significant conclusion, which may be derived from the comparison plots (Fig. 4), is that V 2p3/2 line width for tetravalent vanadium ions in xerogels is actually the same as for pentavalent ions. It seems that this result contradicts the results obtained from the comparison of the reference V2 O5 and V2 O4 oxides XPS measurements (Fig. 1 [2,4]). However this may be explained, taking into account that in vanadium oxides the FWHM of V 2p line depends on the degree of population of the narrow d-band [4]. For the further quantitative analysis of the different species concentration in V2 O5 -based xerogels, the XPS spectra fitting procedure was used. After the Shirley background subtraction, the XPS spectra have been fitted to the whole V 2p and O 1s region to obtain the position, width and intensity of spectral components for various vanadium and oxygen species. According to the results obtained above, the following assumptions about the corresponding peak parameters were made:
1. the FWHM of V 2p3/2 and V 2p1/2 peaks are the same for both V5+ and V4+ ions; 2. for V4+ and V5+ ions, the doublet splitting of V 2p line DS = 7.4 eV; 3. for V4+ and V5+ ions, the V 2p1/2 and V 2p3/2 peak intensity ratio IR = 0.52; 4. the FWHM of O 1s peak is the same for ‘oxide’ and ‘hydro’ species. The fixed values of CS = 1.2 eV, DS = 7.4 eV and IR = 0.52 for both V5+ and V4+ ions were used in fitting procedure in order to get the initial approximation. The positions of O 1s peaks, corresponding to ‘oxide’, ‘hydro’ and ‘H2 O’ species, were fixed at 530, 532 and 533 eV, respectively. After that, these parameters for pentavalent vanadium and oxygen species were unfrozen, and the fitting procedure was repeated. As an example, the typical deconvolution spectra for s4 sample, exhibiting the main features related with the simultaneous presence of the ‘V5+ ’, ‘V4+ ’, ‘oxide’, ‘hydro’ and ‘H2 O’ species are presented in Fig. 5. Note that the additional peak, arising at BE ≈ 519.4 ± 0.4 eV, i.e. shifted to about 10 eV to a lower energy from the main O 1s peak, may be attributed to the errors of satellite subtraction procedure. Intensity of this peak never exceeds 1% of the O 1s line intensity, which corresponds to the 10% error in the satellite subtracting procedure. For a further check, the average vanadium valence (i.e. x in the chemical formulae V2 Ox ) of the tested samples was derived: (a) from the reduction ratio RR = V4+ /(V4+ + V5+ ), assuming that only tetravalent and pentavalent vanadium are presented in all tested samples; (b) from the relative intensities of vanadium and oxygen peaks, assuming that the Oox peak corresponds to the host V2 Ox oxide matrix.
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analysis. The accuracy of the reduction ratio determination of about 0.03 may be expected from the comparison of difference spectra (see Fig. 4) and data presented in Table 1.
4. Conclusions The binding energy of V 2p3/2 line of V4+ ions in V2 O5 -based xerogels has shifted about 1.3 eV to the lower energy side, compared to the basic V5+ line. The difference in the observed chemical shift in xerogels and the reference powders may be attributed to the difference of the Madelung potential in these compounds. In contrast with reference samples of stoichiometric powders, the V 2p3/2 line width for V4+ and V5+ ions in xerogels is actually the same, in agreement with a proposed model of the peak widening [4].
Acknowledgements
Fig. 5. Fitted X-ray photoelectron spectrum of V 2p and O 1s region for the s4 sample (VO)V12 O31 ·nH2 O xerogel ‘as-grown’. Assignment of spectral components is discussed in the text. Table 1 Comparison of the chemical composition of the reference oxides and V2 O5 -based xerogels and the reference powders obtained from the XPS spectrum by using different methods Sample
s3 s4 s5 s2 s1
x in V2 Ox estimated from Reduction ratio
Peak intensity
4.96 4.87 4.74 5.00 4.00
5.00 4.73 4.64 4.91 4.15
The results, presented in Table 1, are in good agreement, confirming that the oxygen deficit is responsible for the occurrence of V4+ ions in the xerogels. Note that the accuracy in the determination of V 2p3/2 and Oox 1s peak areas is about 5%, so a significant error of about 10% may be expected in the chemical composition obtained from peak area
This work was supported in part within: European Community project “The Centre in Processing, Research and Application of Advanced Materials (PRAMA)”, contract No. G5MA-CT-2002-04014.
References [1] J.F. Moulder, W.F. Stricle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., Eden Prairie, MN, USA, 1995. [2] M. Demeter, M. Neumann, W. Reichelt, Surf. Sci. 454–456 (2000) 41. [3] J. Medialdua, R. Casanova, Y. Barbaux, J. Electron. Spectrosc. Rel. Phenom. 71 (1994) 249. [4] G.A. Sawatzky, D. Post, Phys. Rev. B 20 (1979) 1546. [5] J. Livage, Coord. Chem. Rev. 178180 (1998) 999. [6] J. Bullot, P. Cordier, O. Gallais, M. Gauthier, J. Livage, J. Non-Cryst. Solids 68 (1984) 123. [7] V. Bondarenka, Z. Martunas, S. Kaciulis, L. Pandolfi, S. Grebinskij, S. Mickevicius, Lithuanian J. Phys. 42 (2002) 281. [8] V. Bondarenka, H. Tvardauskas, S. Grebinskij, S. Mickevicius, Z. Martunas, V. Volkov, G. Zakharova, Nucl. Instrum. Meth. Phys. Res. B 178 (2001) 323. [9] D. Briggs, M.P. Seach, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester, 1983.
Determination of vanadium valence in hydrated compounds V. Bondarenka a,∗ , S. Grebinskij a , S. Mickeviˇcius a , H. Tvardauskas a , S. Kaˇciulis b b
a Semiconductor Physics Institute, A. Goštauto 11, LT-2600 Vilnius, Lithuania Institute for the Study of Nanostructured Materials (ISMN-CNR), P.O. Box 10, I-00016 Monterotondo Scalo (RM), Italy
Received 16 September 2003; received in revised form 16 February 2004; accepted 15 March 2004
Abstract X-ray photoelectron spectroscopy was used to determine the chemical shift and full-width at half-maximum intensity of the V4+ and V5+ cations peaks in the vanadium pentoxide matrix. It was found that the binding energy of V4+ ions shifts to the lower energy side of about 1.3 eV as compared with the main V5+ ions in the matrix. The V 2p3/2 line width for tetravalent vanadium ions in xerogels is actually the same as for pentavalent ions. The fitting procedure of X-ray photoelectron spectra permits determination of the relative concentrations of the reduced vanadium ions with an accuracy of about 3%. © 2004 Elsevier B.V. All rights reserved. Keywords: Surfaces and interfaces; Sol–gel synthesis; Photoelectron spectroscopy
1. Introduction The mixed valence metal speciation of solid samples is not a trivial problem. In fact, chemical analytical methods often fail due to the relative instability of the various oxidation states of metals in common dissolution media. X-ray photoemission spectroscopy (XPS) is a very useful technique for determining such important parameters as the chemical state of different element species and their relative concentration. The accurate binding energy (BE) has to be measured in order to determine the chemical state from the chemical shift (CS). Quantitative information on the relative surface amount of element species may be obtained from the relative areas of the peaks, corresponding to the single components of the envelope spectra. This may be easily done, if the individual peaks are well resolved, i.e. the full-width at half-maximum intensity (FWHM) of the peak is lower than CS for corresponding species. Unfortunately, the BE of V 2p3/2 line for various valence vanadium oxides is in a relatively low-energy range of about 515.5–517.5 eV [1], while the FWHM of the corresponding peaks varies from 1.2 eV (V2 O5 ) [2] to 4.8 eV (V2 O3 ) [3]. Moreover, the CS of V 2p doublet for V3+ and V4+ species compared with the V5+ doublet in vanadium oxides is lower ∗
Corresponding author. Tel.: +3702 619466; fax: +3702 627123. E-mail address: [email protected] (V. Bondarenka).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.005
than the FWHM of corresponding peaks [4] and the resulting spectra cannot be straightforwardly separated into individual components. Thus, the quantitative estimation of vanadium oxidation states is not a trivial task, and suitable standards for the calibration of V5+ , V4+ and V3+ BE and FWHM in vanadium oxides are required. The general aim of this work was to determine the chemical shift between V4+ and V5+ ions and the FWHM of the respective XPS peaks in the vanadium compounds with the different valence states of vanadium.
2. Literature analysis The relatively full collection (of about 50 references) of V 2p3/2 line BE for the main valence oxides (V2 O5 , VO2 and V2 O3 ) may be obtained from the literature analysis. Statistical methods then may be applied to obtain the mean values of BE for the corresponding valence vanadium species and to estimate the inaccuracy of the data presented in literature. Fig. 1 presents the BE distribution function fBE (E) for main valence vanadium oxides BE and defined as fBE (E) =
1 N Nt 2E
(1)
where N is the number of references, for which the V 2p3/2 line BE is within the interval (BE − E) < E ≤ (BE + E),
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1. The V 2p3/2 line BE measurements permit separation of vanadium species in the adjacent oxidation states; 2. The accuracy of such a BE determination is insufficient for precise deconvolution of the XPS spectra into the individual components; 3. The same care may be taken in the use of data, obtained from the main valence vanadium oxides reference samples study, in the mixed valence compounds XPS spectra analysis. Moreover, the parameters deduced from the analysis of XPS spectra of reference samples may be used only as a rough approximation to those in the mixed valence compounds because of: Fig. 1. Distribution functions of the V 2p3/2 binding energy for the main valence vanadium oxides found in the literature. The points correspond to reference data. The dashed lines present the Gaussian distribution for different oxides and the solid line is the distribution envelope.
we use E = 0.2 eV for the convolution of the literature data, and Nt the total number of references. Three fairly well-resolved peaks may be seen in Fig. 1, indicating that at least three different data sets (corresponding to V2 O5 , V2 O4 and V2 O3 oxides) are involved in the distribution function. Further analysis was done assuming that the reference data for individual oxide are distributed by a normal Gaussian law (corresponding to a random distribution of BE for each set of data): 1 1 BE − BEm 2 G FBE = √ exp − (2) 2 w w 2π where BEm is the average value of BE and w the statistical deviation of BE. It is clear from Fig. 1 that the V 2p3/2 line of the BE distribution function may be deconvoluted into three Gaussian peaks with centroids located at 517.3, 516.5 and 515.6 eV with a statistical deviation of w = 0.25 eV. These peaks may be attributed to V5+ , V4+ and V3+ species in V2 O5 , VO2 and V2 O3 oxides, respectively. So it may be concluded that use of the data reported in literature for the main valence vanadium oxides permits separation of the vanadium species into different oxidation states. On the other hand, the statistical error δBE of BE determination obtained from the literature analysis (δBE 0.7 = w = 0.25 eV and δBE = 2w = 0.5 eV for the 70 and 95% 0.95 probability levels, respectively) may be compared with an estimated value of CS for adjacent valence vanadium oxides CS34 = −0.9 eV and CS54 = 0.8 eV (see Fig. 1). Thus, the accuracy in CS obtained from BE analysis (δCS 0.7 = 2w = 0.5 eV and δCS = 4w = 1.0 eV) is insufficient to use 0.95 these data for the successful deconvolution of the spectra envelope containing spectra of the different oxidation states vanadium species. From the literature analysis it may be concluded that:
1. CS depends not only on the vanadium ion charge (V4+ , V5+ or V3+ ) but also on the value of the crystal field long-range Madelung potential [5]. The last correction may be of the same order of magnitude as the first one. 2. In the vanadium oxides, the FWHM of the V 2p3/2 line depends on the density of the population of the narrow d-band [4], thus this line width for V4+ species may differ significantly in V2 O4 crystals and in V4+ -ions containing xerogels.
3. Experimental It seems that a better way to determinate the relative concentrations of vanadium ions in various valence states is to compare the samples with a same host matrix but a different concentration of the vanadium ions of reduced valence. This comparison may be accomplished in two different ways: 1. by the intercalating of additional V4+ ions in the vanadium pentoxide based matrix. 2. by varying the concentration of V4+ ions by some external treatment. The advantage of this technique is that the same sample can be used to study the comparison spectra and the additional (structure-related effects) may be neglected. We try to use both these approaches to clarify the V4+ spectral line parameters in V2 O5 -based xerogels H2 V12 O31 ·nH2 O. The layered structure of these compounds allows to intercalate various elements that change the concentration of V4+ ions [5]. Moreover, the relative concentration of tetravalent vanadium ions may be changed by a thermal treatment, UV irradiation and ions etching [6–8]. First of all, the reference powder samples of V2 O5 and V2 O4 were used to determine the CS and the FWHM in clean oxides. The thin films of H2 V12 O31 ·nH2 O and (VO)V12 O31 ·nH2 O were used to compare the materials with identical crystal structure. Finally, (VO)V12 O31 ·nH2 O xerogels before and after heat treatment were used to separate the additional V4+ ions influence on the XPS spectrum. The XPS experiment has been carried out with an Escalab MkII (VG Scientific) spectrometer, equipped with an Al K␣
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three lines should be introduced to describe the shape of this peak for the studied samples. According to the chemical composition, these lines may be attributed to the following oxygen species: 1. the main ‘oxide’ peak at BE = 529.9 ± 0.3 eV, belongs to an oxide matrix; 2. the ‘H2 O’ peak at BE = 533.2 ± 0.3 eV, which may be related to adsorbed water; 3. hydroxide, carbonate and ‘bonded’ water species could contribute to the ‘hydro’ peak at BE = 531.7 ± 0.3 eV.
Fig. 2. XPS core-level spectra of V 2p and O 1s region for different samples: s1 – V2 O4 reference powders; s2 – V2 O5 reference powders; s3 – H2 V12 O31 ·nH2 O xerogel; s4 – (VO)V12 O31 ·nH2 O xerogel ‘as-grown’; s5 – (VO)V12 O31 ·nH2 O xerogel after annealing at 510 K.
(1486.6 eV) excitation source, a five-channeltron detection system and a hemispherical analyzer, which was set to 20 eV pass energy. The binding energy scale was corrected for charging effects by assigning a value of 284.6 eV to the C 1s peak. Photoemission data have been collected and processed by using a VGX-900 data system. After Shirley background subtraction, a nonlinear least-squares curve-fitting routine with a Gaussian/Lorentzian product function has been used for the analysis of the XPS spectra. Fig. 2 presents the original XPS core-level spectra of the V 2p and O 1s region for all treated samples. All the spectra have been corrected for X-ray source satellites. Preliminary information may be obtained directly from the view of the presented spectra. First of all it can be noted that the spectral lines of the corresponding regions overlap, hence the attempt to analyze each region independently may cause a significant error in the estimation of the areas of related peaks. So one needs to study the whole V 2p and O 1s region in order to obtain the reliable results. On the other hand, V 2p3/2 and O 1s lines are considerably separated in the BE scale and the main particularities of corresponding peaks may be revealed using the common methods (such as the second derivative and the comparison spectra) of the spectrum analysis [9]. It is clear that the O 1s peak exhibits a complex structure, indicating that various oxide species are presented. At least
As to vanadium lines, it may be stated that V 2p3/2 peak is actually symmetric for V2 O5 and V2 O4 samples, while the obvious widening at the lower BE wing of this spectra may be seen when going from s3 to s5 samples (Fig. 2). This dependency becomes more evident from the second derivation plots (see Fig. 3). For the s3 sample, the derivative spectra are actually symmetric, indicating that V5+ (BE ≈ 517.2 eV) ions dominate in this H2 V12 O31 ·nH2 O xerogel. The evident shoulder is, however, seen at BE ≈ 515.8 eV for the s4 and s5 samples. The appearance of this feature may be attributed to the significant increase of V4+ ions in xerogels after (H2 )2+ ions substitution for (VO)2+ cations and the following annealing. Unfortunately, the lower intensity main negative peak from V4+ the ions actually overlaps with the secondary positive peak of higher intensity, so the CS ≈ 1.4 eV, obtained from the second derivative plots may be regarded only as a rough estimation.
Fig. 3. The second derivatives of XPS spectra for V2 O5 -based xerogels: (1) H2 V12 O31 ·nH2 O xerogel; (2) (VO)V12 O31 ·nH2 O xerogel ‘as-received’; (3) (VO)V12 O31 ·nH2 O xerogel ‘after baking’ at 510 K.
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Fig. 4. Comparison and difference spectra for V 2p3/2 line in V2 O5 -based xerogels: (a) H2 V12 O31 ·nH2 O xerogel; (b) (VO)V12 O31 ·nH2 O xerogel ‘as-grown’; (c) (VO)V12 O31 ·nH2 O xerogel after annealing at 510 K; α = b − 0.93a; β = c − 0.85b.
The spectra comparison technique then was used for the further examination of the core-level states of V4+ ions in V2 O5 -based xerogels. Fig. 4 presents the comparison and the difference spectra for H2 V12 O31 ·nH2 O and (VO)V12 O31 ·nH2 O samples, as well as for the (VO)V12 O31 ·nH2 O sample before and after annealing at 240 ◦ C. The optimal weight procedure was used to extract the additional, low-intensity, V4+ 2p3/2 peaks from the background of the main, high-intensity, V5+ 2p3/2 peak [9]. It is clear from Fig. 4A and B that the replacement of (H2 )2+ cations by (VO)2+ cation, like the annealing of (VO)V12 O31 ·nH2 O sample, leads to the appearance of the additional peak, shifted by 1.1–1.2 eV to a lower energy side, compared with the main peak position. This peak may be attributed to the V4+ species and the CS of about 1.2 eV may be expected for V4+ -ions in V2 O5 based xerogels, in good agreement with a value estimated from the second derivative plots (Fig. 3). It should be noted that this value agrees with an average CS = 1.1 eV, obtained from the literature analysis (see Fig. 1). The more significant conclusion, which may be derived from the comparison plots (Fig. 4), is that V 2p3/2 line width for tetravalent vanadium ions in xerogels is actually the same as for pentavalent ions. It seems that this result contradicts the results obtained from the comparison of the reference V2 O5 and V2 O4 oxides XPS measurements (Fig. 1 [2,4]). However this may be explained, taking into account that in vanadium oxides the FWHM of V 2p line depends on the degree of population of the narrow d-band [4]. For the further quantitative analysis of the different species concentration in V2 O5 -based xerogels, the XPS spectra fitting procedure was used. After the Shirley background subtraction, the XPS spectra have been fitted to the whole V 2p and O 1s region to obtain the position, width and intensity of spectral components for various vanadium and oxygen species. According to the results obtained above, the following assumptions about the corresponding peak parameters were made:
1. the FWHM of V 2p3/2 and V 2p1/2 peaks are the same for both V5+ and V4+ ions; 2. for V4+ and V5+ ions, the doublet splitting of V 2p line DS = 7.4 eV; 3. for V4+ and V5+ ions, the V 2p1/2 and V 2p3/2 peak intensity ratio IR = 0.52; 4. the FWHM of O 1s peak is the same for ‘oxide’ and ‘hydro’ species. The fixed values of CS = 1.2 eV, DS = 7.4 eV and IR = 0.52 for both V5+ and V4+ ions were used in fitting procedure in order to get the initial approximation. The positions of O 1s peaks, corresponding to ‘oxide’, ‘hydro’ and ‘H2 O’ species, were fixed at 530, 532 and 533 eV, respectively. After that, these parameters for pentavalent vanadium and oxygen species were unfrozen, and the fitting procedure was repeated. As an example, the typical deconvolution spectra for s4 sample, exhibiting the main features related with the simultaneous presence of the ‘V5+ ’, ‘V4+ ’, ‘oxide’, ‘hydro’ and ‘H2 O’ species are presented in Fig. 5. Note that the additional peak, arising at BE ≈ 519.4 ± 0.4 eV, i.e. shifted to about 10 eV to a lower energy from the main O 1s peak, may be attributed to the errors of satellite subtraction procedure. Intensity of this peak never exceeds 1% of the O 1s line intensity, which corresponds to the 10% error in the satellite subtracting procedure. For a further check, the average vanadium valence (i.e. x in the chemical formulae V2 Ox ) of the tested samples was derived: (a) from the reduction ratio RR = V4+ /(V4+ + V5+ ), assuming that only tetravalent and pentavalent vanadium are presented in all tested samples; (b) from the relative intensities of vanadium and oxygen peaks, assuming that the Oox peak corresponds to the host V2 Ox oxide matrix.
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analysis. The accuracy of the reduction ratio determination of about 0.03 may be expected from the comparison of difference spectra (see Fig. 4) and data presented in Table 1.
4. Conclusions The binding energy of V 2p3/2 line of V4+ ions in V2 O5 -based xerogels has shifted about 1.3 eV to the lower energy side, compared to the basic V5+ line. The difference in the observed chemical shift in xerogels and the reference powders may be attributed to the difference of the Madelung potential in these compounds. In contrast with reference samples of stoichiometric powders, the V 2p3/2 line width for V4+ and V5+ ions in xerogels is actually the same, in agreement with a proposed model of the peak widening [4].
Acknowledgements
Fig. 5. Fitted X-ray photoelectron spectrum of V 2p and O 1s region for the s4 sample (VO)V12 O31 ·nH2 O xerogel ‘as-grown’. Assignment of spectral components is discussed in the text. Table 1 Comparison of the chemical composition of the reference oxides and V2 O5 -based xerogels and the reference powders obtained from the XPS spectrum by using different methods Sample
s3 s4 s5 s2 s1
x in V2 Ox estimated from Reduction ratio
Peak intensity
4.96 4.87 4.74 5.00 4.00
5.00 4.73 4.64 4.91 4.15
The results, presented in Table 1, are in good agreement, confirming that the oxygen deficit is responsible for the occurrence of V4+ ions in the xerogels. Note that the accuracy in the determination of V 2p3/2 and Oox 1s peak areas is about 5%, so a significant error of about 10% may be expected in the chemical composition obtained from peak area
This work was supported in part within: European Community project “The Centre in Processing, Research and Application of Advanced Materials (PRAMA)”, contract No. G5MA-CT-2002-04014.
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