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X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems
NOC-16856; No of Pages 6 Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
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X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems M. Milanova a,⁎, R. Iordanova a, K.L. Kostov a, Y. Dimitriev b a b
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev”, str., bld. 11, 1113 Sofia, Bulgaria University of Chemical Technology and Metallurgy, 8 “Kl. Ohridski” blvd., 1756 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 4 September 2013 Received in revised form 8 January 2014 Available online xxxx Keywords: Non-traditional glasses; Glass structure; XPS
a b s t r a c t The local structure and the connectivity in the amorphous network of glasses from MoO3-Bi2O3 and MoO3Bi2O3-CuO systems have been studied by comparative analysis of crystalline and amorphous phases using X-ray photoelectron spectroscopy. According to the XPS data the amorphous networks of both glass systems consist mainly of MoO4 tetrahedra as well as of small amounts of MoO6 octahedra. Based on the O1s spectra analysis it was suggested that the MoO6 octahedra are connected by bridging Mo\O\Mo bonds, while the MoO4 tetrahedra participate in the formation of mixed type Bi\O\Mo bonds. The presence of mixed Bi\O\Mo bonds with a relatively significant covalent character is crucial for the glass formation. Mainly Mo6+ and Bi3+ ions were detected in MoO3-Bi2O3 glasses and additionally Cu+ and Cu2+ ions were observed in MoO3-Bi2O3-CuO amorphous samples. UV-visible diffuse reflectance analysis confirmed the XPS data, revealing the presence of Mo6+, Bi3+ and Cu2+ ions in the glasses. Formation of MoO4 tetrahedral units (absorption band at 260 nm) in the amorphous network was also found by optical absorption spectra. The role of MoO3 as glass former was confirmed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The properties of glass systems depend on their composition and to a considerable extent on their structure. In the search of new functional glass materials, the systematic investigations of composition-structureproperties relationships are both of fundamental and of technological importance. Among the other experimental methods, X-ray photoelectron spectroscopy (XPS) has been extensively applied to the interpretation of the structure and electronic state of glasses. A key concept in XPS is the so called chemical shift, which represents the change in the binding energy of certain electron core-level. The chemical shift could result from a difference in the actual oxidation state as well as a difference in the molecular environment [1,2]. The binding energy increases typically with an increase in the oxidation state of a metal atom [2]. However there is no straightforward correlation between binding energy and structural coordination. For example the Mo3d5/2 binding energy of octahedrally coordinated Mo6+ ions has been measured to be with 1 eV higher compared to that of the Mo ions in tetrahedral oxygen environment, existing in the network of MoO3-CuO glasses [3]. A change of Ti 2p3/2 binding energy in the opposite direction has been observed in the transition of Ti4+ ions from octahedral to tetrahedral coordination, respectively from 458.3 eV to 460.5 eV [4]. This complexity in the interpretation of XPS results can be significantly avoided by using standard crystalline compounds, whose structural and electronic properties of the building units have been previously clarified. Therefore in the ⁎ Corresponding author. E-mail address: [email protected] (M. Milanova).
present paper we study the local structure and the connectivity in the amorphous network of binary MoO3-Bi2O3 glasses by comparative analysis of appropriate crystalline and amorphous phases using X-ray photoelectron spectroscopy. The Bi4f and Cu2p photoelectron spectra of ternary MoO3-Bi2O3-CuO glasses with one and the same MoO3 content (60 mol%) are also discussed here in details in order to obtain more information about the valence state and structural role of these oxides in the network of ternary glasses. 2. Experimental procedure The glass formation in the MoO3-Bi2O3 and in the MoO3-Bi2O3-CuO systems was achieved at high cooling rates (104-105 K/s) by using a roller-quenching technique. The reference crystalline phases Bi2Mo3O12, and Bi2Mo2O9 were obtained by crystallization of glasses having the corresponding compositions (75MoO3·25Bi2O3 and 66MoO3·33Bi2O3). The detailed preparation procedures applied were already described in Refs. [5–7]. The X-ray photoelectron (XPS) measurements were carried out on an ESCALAB Mk II (VG Scientific) electron spectrometer with Al Kα excitation source and a total instrumental resolution of 1.16 eV (as it was measured with the FWHM of Ag3d5/2 photoelectron line). The energy scale was corrected with respect to the C1s - peak maximum at 285.0 eV for electrostatic charging. The processing of the measured spectra included a subtraction of X-ray satellites and Shirley-type background. The peak positions and areas were evaluated by a symmetrical Gaussian-Lorentzian curve fitting. The relative concentrations of the different chemical species were determined by normalization of the peak areas in regard to their
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Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
2
M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
photoionization cross-sections calculated by Scofield [8]. The absorption spectra of the MoO3-Bi2O3 and MoO3-Bi2O3-CuO samples were measured at room temperature using Evolution 300 UV–Vis Spectrophotometer in the 190–1100 nm wavelength range.
Table 1 Concentrations (in at.%) of Bi and Mo atoms in different chemical states. Additionally the total oxygen amount (at.%) is shown in the last column. The corresponding Bi4f7/2 and Mo3d5/2 binding-energies (in eV) are shown above the concentration values. Samples
3. Results
Crystal Bi2Mo2O9
3.1. MoO3-Bi2O3 glasses
Glass MoO3: Bi2O3 = 66:33
MoO3:Bi2O3 mol %
Mo3d
66:33
crystal
glass
66:33
glass
70:30
Glass MoO3: Bi2O3 = 75:25 Crystal Bi2Mo3O12
Bi (2)
Mo (1)
Mo (2)
O
159.6 19.5 159.5 18.5 159.7 17.0 159.7 14.5 159.7 14.5
157.7 0.5 157.8 1.0 158.0 1.0 157.8 0.5 157.8 0.5
232.5 18.0 232.4 17.0 232.5 17.0 232.7 19.0 232.8 20.0
-
62.0
233.3 0.5 233.2 1.0 233.3 1.5 -
63.0 64.0 64.5 65
established the existence of two types of oxygen atoms in the structure of Bi2Mo2O9 crystal marked as O(1) and O(3) in Table 2 with a concentration ratio of О(1):О(3) = 1:8. The O(1) involves oxygen surrounded by three Bi atoms (marked as Bi3O bonding configuration) and the O(3) involves oxygen participating in the mixed Bi\O\Mo bonds [9]. In addition to their fitting contributions a third peak denoted as O(6) at 532.1 eV was obtained and it was attributed to adsorbed OH groups on the surface (Fig. 3). Similarly, considering the crystallographic studies of Bi2Mo3O12 crystal [10,11] we suppose the existence of three kinds of oxygen atoms (denoted as O(2), O(3) and O(4)) in a concentration ratio of 2:7:3 participating in different chemical bonds namely short Mo\O, Bi\O\Mo and Mo2O2, respectively. The latter notification (Mo2O2) refers to edge bonds between two Mo polyhedra [10,11]. Note that both Bi2Mo2O9 and Bi2Mo3O12 crystals have a common type of oxygen atom O(3), involved in the Bi\O\Mo bonds. The O1s signal deconvolutions for both crystals gave similar binding energy of 530.4530.5 eV for this kind of oxygen, which is a convincing validation of the above described fitting approach. The O1s spectra of glasses were fitted not only on the basis of the O1s results of the crystalline samples of the same compositions but also having in mind the previous studies [3,6]. Table 2 summarizes the concentrations (at. %) and the О1s binding energies of oxygen atoms incorporated in different chemical bonds of the studied crystalline and glassy samples. The measured (actual) compositions of all samples were calculated using the total areas of the MoO3:Bi2O3 mol %
Bi4f
66:33
crystal
66:33
glass
70:30
glass
75:25
glass
75:25
crystal
glass
75:25
crystal
75:25 225
Glass MoO3: Bi2O3 = 70:30
Intensity (arb. units)
Intensity (arb. units)
The main approach, applied in the present study, consists of comparative analysis of X-ray photoelectron spectra of Bi2Mo3O12 and Bi2Mo2O9 crystalline standards and those of amorphous phases of the same MoO3-Bi2O3 compositions. The spectra deconvolutions were made on the basis of the crystallographic data analysis of Bi2Mo3O12 and Bi2Mo2O9 crystals, available in the literature, concerning the number of the different atoms in their unit cells, the type of bonding and the Me\O (Me_Mo; Bi) bond distances in the unit cell [9–11]. In this way the Mo3d spectra of both crystalline phases were well fitted using only one pair of 3d5/2 and 3d3/2 peaks (Fig. 1). The measured Mo3d5/2 binding energies of Bi2Mo2O9 at 232.5 eV and of Bi2Mo3O12 crystal at 232.8 eV, respectively are in good agreement with those found in the literature [12]. However the Mo3d spectra of the glasses with the identical compositions (Fig. 1) can be fitted with two pairs of 3d5/2 and 3d3/2 peaks, characteristic of Mo atoms in two different coordination states (see Discussion section). The binding energies of the Mo3d5/2 peak contributions and their corresponding concentrations, obtained by measuring the areas of Mo3d5/2 + 3/2 peaks, are shown in Table 1. The Bi4f spectral regions of all MoO3-Bi2O3 samples can be well fitted with two pairs of 4f7/2 and 4f5/2 peaks, evidencing the presence of Bi atoms in two different chemical states (Fig. 2). Their concentrations (in at. %), as well as the corresponding binding energies of Bi4f7/2 are listed in Table 1. Fig. 3 displays the O1s photoelectron spectra of all the studied crystalline and amorphous samples. These spectra were fitted taking into account the crystallographic data for both Bi2Mo3O12 and Bi2Mo2O9 crystals determining the number of different oxygen atoms involved in various bonds in the structures through analysis of the Me\O (Me_Mo; Bi) bond distances in the crystal unit cells [9–11]. Thus we
eV at.% eV at.% eV at.% eV at.% eV at.%
Bi (1)
230
235
240
Binding Energy (eV) Fig. 1. Mo3d-spectral regions of two crystal samples and glasses with different nominal MoO3:Bi2O3 ratios.
155
160
165
170
Binding Energy (eV) Fig. 2. Bi4f-spectral regions of two crystal samples and glasses with different nominal MoO3:Bi2O3 ratios.
Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
MoO3:Bi2O3 mol %
Intensity (arb. units)
Table 3 Comparison between nominal and measured MoO3: Bi2O3 compositions in mol%.
O1s crystal
66:33
glass
66:33
glass
70:30
Samples
Nominal composition MoO3: Bi2O3 mol%
Measured composition MoO3: Bi2O3 mol%
Crystal Bi2Mo2O9 Glass MoO3: Bi2O3 = 66:33 Glass MoO3: Bi2O3 = 70:30 Glass MoO3: Bi2O3 = 75:25 Crystal Bi2Mo3O12
66:33 66:33 70:30 75:25 75:25
64.3:35.7 64.2:35.8 66.7:33.3 73.2:26.8 72.7:27.3
incorporated in different chemical bonds in the investigated glasses. Their concentrations (in at.%) and the corresponding O1s binding energies (eV) are given in Table 5. In contrast to the binary MoO3-Bi2O3 glasses (see Section 3.1) now, in the presence of CuO, the nominal and measured MoO3:Bi2O3 ratios are not in such good agreement (Table 6).
glass
75:25
3
3.3. UV-visible diffuse reflectance analysis
crystal
Mo3d and Bi4f peaks normalized with respect to their photoionization cross-sections [8] and the results (in mol.%) shown in Table 3 are in good agreement with the nominal compositions.
Fig. 6 shows the UV–vis absorption spectra of crystalline Bi2Mo3O12 and glass with the same composition (75MoO3 · 25Bi2O3) and as well as that of ternary 60MoO3 · 30CuO · 10Bi2O3 glass. All spectra exhibit an absorption band at 260 nm. In the spectrum of the crystalline phase (Fig. 6(a)) not well resolved peak located above 300 nm is observed, that is shifted to longer wavelength (370 nm) in the spectrum of the glass sample with the same composition (Fig. 6(b)). Stronger absorption in the 450–700 nm range is observed in the spectrum of the 75MoO3 · 25Bi2O3 glass sample in comparison with the spectrum of its crystalline analog. The UV spectrum of CuO-containing glass (Fig. 6(c)) displays highly intensive and very broad absorption band in the visible region.
3.2. Comparison with MoO3-CuO-Bi2O3 glasses
4. Discussion
The Bi4f and Cu2p photoelectron spectra of ternary MoO3-Bi2O3CuO glasses with one and the same MoO3 content (60 mol%) are shown in Figs. 4 and 5, respectively, while the Mo3d and O1s regions have already been reported elsewhere [6]. Similar to the MoO3-Bi2O3 glasses (Fig. 2) a significantly dominating Bi 4f7/2 peak at about 159.2159.3 eV is observed with all ternary glasses. The Cu2p3/2 spectra show a two-peak structure (Fig. 5), indicating the presence of Cu+ and Cu2 + ions in these glasses [13]. The Cu2p3/2 binding energy of Cu+ ions is found to be ~ 932.1 eV while Cu2+ ions are characterized with a peak at about 934.2 eV as well as with a broad intensive satellite structure centered at ~942 eV (Fig. 5). The surface content of Cu+ ions is approx. 10-15% of the total Cu amount (Table 4). In the same table the corresponding Mo3d5/2, Bi4f7/2 and Cu2p3/2 binding energies of the different chemical states of Mo, Bi and Cu are represented. As it has been shown earlier [6] the O1s spectrum of MoO3-Bi2O3CuO glasses can be fitted with four peaks attributed to oxygen atoms,
4.1. MoO3-Bi2O3 glasses
75:25 525
530
535
Binding Energy (eV) Fig. 3. O1s-spectral regions of two crystal samples and glasses with different nominal MoO3:Bi2O3 ratios. The ratios of О(2):О(3):О(4) = 2:7:3 and О(1):О(3) = 1:8 are supposed for the Bi2Mo3O12 and Bi2Mo2O9 crystals, respectively.
As it was mentioned in the previous section the interpretation of the XPS results is based on the comparative analysis of the photoelectron spectra of Bi2Mo3O12 and Bi2Mo2O9 crystalline samples and that of amorphous phases with equivalent MoO3-Bi2O3 compositions. According to the crystallographic data the Bi2Mo2O9 crystal contains only Mo atoms in tetrahedral coordination against oxygen [9]. Their Mo3d5/2 binding energy was measured to be centered at 232.5 eV in the photoelectron spectrum (Fig. 1). Similar binding energy has been obtained earlier for the tetra-coordinated Mo atoms in MoO3-CuO glasses and in CuMoO4 crystal and has been attributed to the Mo6+ ions in tetrahedral oxygen environment (MoO4) [3]. Based on these earlier observations we can also propose the existence mainly of hexavalent molybdenum atoms in the glasses discussed in the present paper. However as it was shown in the literature, for example by Cozar et al. [14],
Table 2 Concentrations (in at.%) of oxygen atoms incorporated in different chemical bonds of studied crystal and glass samples. The corresponding O1s binding-energies (in eV) are shown above the concentration values. The ratios of О(2):О(3):О(4) = 2:7:3 and О(1): О(3) = 1:8 are supposed for the Bi2Mo3O12 and Bi2Mo2O9 crystals, respectively. Samples
Crystal Bi2Mo2O9 Glass MoO3: Bi2O3 = 66:33 Glass MoO3: Bi2O3 = 70:30 Glass MoO3: Bi2O3 = 75:25 Crystal Bi2Mo3O12
eV at.% eV at.% eV at.% eV at.% eV at.%
O(1)
O(2)
Bi3O
Mo\O
529.1 5.9 529.1 5.0 529.1 3.5 -
-
-
529.7 9.8 529.7 11.2 529.6 15.7 529.6 9.6
O(3)
O(4)
O(5)
O(6)
Bi\O\Mo
Mo2O2
Mo\O\Mo
ads.OH
530.4 47.1 530.4 37.7 530.5 35.7 530.5 30.3 530.5 33.8
-
-
-
531.2 2.4 531.2 4.6 531.2 9.4 -
532.0 9.1 532.1 8.0 532.1 9.1 532.1 9.2 532.1 7.1
530.9 14.5
Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
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M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
MoO3:CuO:Bi2O3 mol %
Intensity (arb. units)
Bi 4f
60:10:30
60:20:20
60:30:10 150
155
160
165
170
175
Binding Energy (eV) Fig. 4. Bi4f-spectral regions of glasses with different nominal MoO3:CuO:Bi2O3 ratios.
the low local symmetry of the Mo polyhedra in some glasses can be an indication for the presence of Mo5 +. Therefore we cannot exclude completely the existence of some amount of Mo atoms in valence state lower than Mo6+. The Mo3d5/2 binding energy of Bi2Mo3O12 crystal is measured to be 232.8 eV, which is higher than that found for Bi2Mo2O9, most probably because of the presence of oxygen ion in the vicinity of each MoO4 tetrahedron at a distance of ca. 2.2 Å. The presence of this fifth oxygen ion is often considered as evidence that molybdenum in the structure of Bi2Mo3O12 is in 5-fold coordination geometry (MoO5) [10,11]. However the Mo3d5/2 binding energy of Bi2Mo3O12 crystal still is in a range characteristic of tetrahedrally coordinated Mo6 + ions [3]. The deconvolution of the Mo3d core-level peak of MoO3-Bi2O3 glasses into two contributions indicates that the Mo atoms in glasses exist in two different coordination states (denoted as Mo(1) and Mo(2) in Table 1) in contrast to the reference crystalline
Cu 2p3/2
MoO3:CuO:Bi2O3 mol %
Intensity (arb. units)
60:10:30
60:20:20
60:30:10
925
930
935
940
945
950
Binding Energy (eV) Fig. 5. Cu 2p3/2 -spectral regions of glasses with different nominal MoO3:CuO:Bi2O3 ratios.
Table 4 Binding energies (in eV) of Mo3d5/2, Bi4f7/2 and Cu2p3/2 core-levels of Mo, Bi and Cu atoms in different chemical states. The Cu1+: Cu2+ ratio (in %) is shown in the last column. Samples
Mo(1) Mo(2) Bi(1)
60MoO3 · 30CuO · 10Bi2O3 60MoO3 · 20CuO · 20Bi2O3 60MoO3 · 10CuO · 30Bi2O3
232.0 232.1 232.3
233.1 233.3 233.4
Bi(2)
Cu1+
Cu2+
Cu1+: Cu2+
157.5 159.2 932.1 934.3 9:91 157.5 159.3 932.1 934.2 8:92 157.4 159.3 932.0 934.1 14:86
compounds. Similar to the crystalline phase results we suppose that the dominant Mo3d5/2 peak at 232.4-232.8 eV is characteristic of Mo6 + ions in tetrahedral oxygen environment (MoO4), whereas the higher binding energy contribution at 233.2-233.3 eV can be assigned to the Mo6+ ions in octahedral coordination (MoO6). Evidence for the latter supposition is that the Мo3d5/2 binding energy at 233. 6 eV was found for the pure MoO3 oxide, whose crystalline structure consists of MoO6 octahedra only [3]. The rising intensity of Mo3d5/2 peak characteristic for MoO6 polyhedra with increasing MoO3 concentrations (Fig. 1, Table 1), indicates the increase of the number of MoO6 units in the glass structure. This assumption is in agreement with the previous infrared spectral results of MoO3-Bi2O3 glasses reported by Iordanova et al. [5]. However, still a significantly dominant feature is that of tetrahedrally coordinated Mo6+ ions. The binding-energy values of the predominant Bi4f peak contribution (Fig. 2) reported in Table 1 and denoted as Bi(1) are in good agreement with those for other bismuth-oxide compounds and glasses indicating the presence of Bi3 + ions [12,15–18]. The low bindingenergy peak denoted as Bi(2) in Table 1 could be related to the existence of a small amount of bismuth, reduced to metallic state (Bio) most probably due to the thermal reduction of bismuth ions during meeting processes [19,20]. Several types of oxygen atoms incorporated in different chemical bonds of the studied crystals and glass samples are supposed, taking into account the analysis of crystallographic data for Bi2Mo2O9 and Bi2Mo3O12 (Section 3.1). Considering these results, represented in Table 2, no significant shift of O1s-peak positions in the spectra of glasses and their crystalline analogs is visible, which gives us the reason to claim that the type of bonding in the amorphous network is similar to that in the crystalline structure. However, changes in the concentrations of different O atoms are observable, indicating that some structural transformation is occurring during the transition from crystalline into amorphous state. From Table 2 it becomes obvious that O(4) atoms are absent in the glass network. The O(4) is attributed to the oxygen in edge shared MoO5 units existing in the structure of Bi2Mo3O12 [10,11]. Also the presence of O(5), related to oxygen from corner shared MoO6 [3], could be observed in the glass samples, but it is not found in the structure of crystalline analogs. The concentration of O(1) oxygen atoms connected with the Bi\O\Bi bonds is smaller in the glasses in comparison with the respective crystalline compound, while the amount of O(2), ascribed to the short Mo\O bonds, increases in the glasses simultaneously with the increase of MoO3 initial amounts. In both crystalline phases and glass samples, the highest amounts of oxygen atoms are involved in mixed type Bi\O\Mo bonds. These concentrations decrease slightly after the transition from crystalline into glassy state. Based on the abovementioned observations the following conclusions can be drawn: (i) the transition to an amorphous structure leads to the breaking of edge bonds (Mo2O2) between molybdenum polyhedra; (ii) the MoO3-Bi2O3 glass network consists mainly of MoO4 tetrahedra as well as of a small amount of MoO6 octahedra; (iii) the MoO6 are connected by bridging Mo\O\Mo bonds, while some MoO4 tetrahedra participate in the formation of mixed type Bi\O\Mo bonds, which are predominant in the MoO3-Bi2O3 glass network; and (iv) the presence of mixed Mo\O\Bi bonding is crucial for the glass formation. It should be pointed out that these heteroatomic Mo\O\Bi linkages cannot be regarded as the typical bridging bond (as for an example is Mo\O\Mo
Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
5
Table 5 Concentrations (in at. %) of oxygen atoms incorporated in different chemical bonds of studied glass samples. The corresponding O1s binding energies (eV) are shown above the concentration values. Samples
O(1)
O(2)
O(3)
O(4)
Me (Me′)\O\Me (Me′)
Me (Me′)\O\Mo
Mo\O\Mo
ads.OH
531.5 22.0 531.6 15.5 531.4 2.0
532.1 19.0 532.1 13.0 532.2 15.0
+
2+
Me_Cu ; Cu 60MoO3 · 30CuO · 10Bi2O3
eV at.% eV at.% eV at.%
60MoO3 · 20CuO · 20Bi2O3 60MoO3 · 10CuO · 30Bi2O3
+
; Me′_Bi
2+
Me_Cu ; Cu
529.0 8.0 529.2 10.4 529.1 11.0
; Me′_Bi
530.5 51.0 530.5 61.0 530.4 72.0
bonding) and there are no typical non-bridging bonds too. Following the classification of Dimitrov and Komatsu [21] these bonds can be considered as semi-covalent bonds, assuming the existence of strong interionic interaction. A good agreement between the nominal and measured compositions is found with all studied MoO3-Bi2O3 glasses (Table 3). Considering that XPS is a surface sensitive method giving information about the upper Mo\O\Bi layers within a depth of few nm, we suppose that Mo:Bi ratios on the surface and in the bulk of the glass samples do not differ significantly. This is in good agreement with earlier XPS study of bismuth molybdates, which found equal Bi:Mo ratios on the surface and in the bulk [22].
Considering the calculated concentrations, based on XPS data, it should be pointed out that the addition of CuO could change the MoO3:Bi2O3 ratio, although not so strongly expressed (Tables 3 and 6). This interesting fact can be explained having in mind that XPS is a surface-sensitive method and the main contribution to the signal is coming from the uppermost surface layer, whereas in the depth of the sample the signal is decaying exponentially depending on the mean free path of the emitted electrons. Therefore we can conclude that the results for the nominal and actual ratios (see Tables 3 and 6) represent a comparison between bulk and surface compositions, respectively. In this aspect the tables outline one apparent difference between both binary MoO3-Bi2O3 and ternary MoO3-Bi2O3-CuO glasses, namely a slightly different surface MoO3:Bi2O3 ratio, compared to the bulk ratio for CuOcontaining glass.
4.2. Comparison with MoO3-CuO-Bi2O3 glasses 4.3. UV-visible diffuse reflectance analysis The XPS results were also supported by UV-visible diffuse reflectance analysis performed on crystalline Bi2Mo3O12 and glass having the same composition (75MoO3 · 25Bi2O3). The absorption band at 260 nm, observed in both spectra, can be assigned to the isolated MoO4 tetrahedra [26,27] as they were also detected by XPS. The poorly resolved absorption peak above 300 nm, detected in the spectrum of crystalline phase, is difficult to be assigned precisely as charge transfer UV–vis bands of both Mo6+ and Bi3+ ions are also located in this spectral range [26–29]. The observed shift of the latter band to longer wavelengths in the spectrum of glass sample and the increasing absorption in the 450–700 nm range in comparison with the spectrum of its crystalline analog are due to the structural changes as a result of transformation into amorphous state and increasing structural disorder [30]. The presence of the absorption band at 260 nm also in the UV-visible 24 22 20 18
Kubelka-Munk units
According to our previous study on MoO3-Bi2O3-CuO glasses [6] there are several types of O atoms participating in different bonds. In this paper, we give the numerical results of the O1s fitting procedure (Table 5), which ensures more precise information about the bonding configuration and structure of the glasses. As it is seen from Table 5 the addition of CuO to the MoO3-Bi2O3 glass does not lead to any noticeable differences in the O1s peak positions of O(1), O(2) and O(3) oxygen atoms in comparison with those found for binary glasses. This indicates the presence of the same type of bonding in both groups of glasses (similar middle range order). However the contents of various oxygen atoms in the binary and ternary glasses are different. The concentrations of O(3) oxygen atoms grow up with the increase of CuO at the expense of Bi2O3 content. The observation evidences that the introduction of CuO to binary MoO3-Bi2O3 glasses with the same MoO3 content (60 mol%) does not change the way of glass network formation, but inhibits MoO6 → MoO4 transformation. Clear preference for the formation of heteroatomic bonds is also observable (Table 5) indicating the crucial role of this type of bonding in the MoO3-CuO-Bi2O3 glass network. It is mainly the Bi3+ ions (denoted as Bi(2) in Table 4) that are detected in the ternary glasses. A small amount of Bio (indicated as Bi(1)) is observed most probably as a consequence of the thermal reduction during the melting of the glasses [19,20]. The observed asymmetry and satellite peaks in the Cu2p spectra of all studied glass samples (Fig. 5) are indications of the presence of both Cu+ and Cu2+ ions. This result is in good agreement with previous XPS measurements of other glasses containing CuO, where monovalent and divalent copper ions were also supposed to exist [3,23–25].
260
16 14 12
340 370
10
c
8 6 4
Table 6 Comparison between nominal and measured MoO3: Bi2O3 ratios. Samples
60MoO3 · 30CuO · 10Bi2O3 60MoO3 · 20CuO · 20Bi2O3 60MoO3 · 10CuO · 30Bi2O3
b
2
Nominal ratio
Measured ratio
MoO3: Bi2O3
MoO3: Bi2O3
6 3 2
4.8 2.5 1.5
a
0 200
400
600
800
1000
Wavelength, nm Fig. 6. Diffuse-reflectance UV–vis spectra of: a) crystalline Bi2Mo3O12; b) 75MoO3 · 25Bi2O3 glass; and c) 60MoO3 · 30CuO · 10Bi2O3 glass.
Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
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M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
absorption spectrum of ternary glass (Fig. 6(c)) is most probably due to the MoO4 groups, which are the predominant structural units building up the amorphous network of these glasses [5]. The broad absorption band in the visible region, noticed in the spectrum of 60MoO3 · 10Bi2O3 · 30CuO glass, is most probably due to the presence of copper ions because a broad visible band characteristic for Cu2+ ions is detected in the optical spectra of copper doped bismuth-borate glasses [31]. 5. Conclusions Using X-Ray photoelectron spectroscopy it has been found out that the local structure and the connectivity in the amorphous network of binary MoO3-Bi2O3 glasses are similar to those of crystalline Bi2Mo2O9 and Bi2Mo3O12 because no chemical shifts of the Mo3d and Bi4f binding energies have been found in the glass and crystalline phases. The MoO4 tetrahedral units have been identified as the main building blocks of the amorphous network of MoO3-Bi2O3 glasses. This fact has been confirmed by UV-visible diffuse reflectance measurements, where an absorption band at 260 nm attributed to MoO4 tetrahedra has been observed. Additionally the existence of a small amount of MoO6 octahedra has also been indicated by XPS. The O1s spectra analysis suggests that the MoO6 octahedra are connected by bridging Mo\O\Mo bonds, while MoO4 tetrahedra participate in the formation of mixed Bi\O\Mo bonds. The presence of mixed Bi\O\Mo bonds with a relatively significant covalent character is crucial for the glass formation. The addition of CuO to the MoO3-Bi2O3 glass does not lead to the noticeable differences in the Mo3d, Bi4f and O1s peak positions, compared with those found for binary glasses, which indicates similar short and middle range order. Besides Mo6+ and Bi3+ both Cu+ and Cu2+ ions are also observed in the case of MoO3-Bi2O3-CuO amorphous samples. In all studied glasses the role of MoO3 as glass former has been confirmed. Acknowledgment This research work has been funded by the Bulgarian Ministry of Education, Project No. BG051PO001-3.3-05/0001 “Science and Business”, within the Operational Program “Development of Human Resources”.
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Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
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X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems M. Milanova a,⁎, R. Iordanova a, K.L. Kostov a, Y. Dimitriev b a b
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev”, str., bld. 11, 1113 Sofia, Bulgaria University of Chemical Technology and Metallurgy, 8 “Kl. Ohridski” blvd., 1756 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 4 September 2013 Received in revised form 8 January 2014 Available online xxxx Keywords: Non-traditional glasses; Glass structure; XPS
a b s t r a c t The local structure and the connectivity in the amorphous network of glasses from MoO3-Bi2O3 and MoO3Bi2O3-CuO systems have been studied by comparative analysis of crystalline and amorphous phases using X-ray photoelectron spectroscopy. According to the XPS data the amorphous networks of both glass systems consist mainly of MoO4 tetrahedra as well as of small amounts of MoO6 octahedra. Based on the O1s spectra analysis it was suggested that the MoO6 octahedra are connected by bridging Mo\O\Mo bonds, while the MoO4 tetrahedra participate in the formation of mixed type Bi\O\Mo bonds. The presence of mixed Bi\O\Mo bonds with a relatively significant covalent character is crucial for the glass formation. Mainly Mo6+ and Bi3+ ions were detected in MoO3-Bi2O3 glasses and additionally Cu+ and Cu2+ ions were observed in MoO3-Bi2O3-CuO amorphous samples. UV-visible diffuse reflectance analysis confirmed the XPS data, revealing the presence of Mo6+, Bi3+ and Cu2+ ions in the glasses. Formation of MoO4 tetrahedral units (absorption band at 260 nm) in the amorphous network was also found by optical absorption spectra. The role of MoO3 as glass former was confirmed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The properties of glass systems depend on their composition and to a considerable extent on their structure. In the search of new functional glass materials, the systematic investigations of composition-structureproperties relationships are both of fundamental and of technological importance. Among the other experimental methods, X-ray photoelectron spectroscopy (XPS) has been extensively applied to the interpretation of the structure and electronic state of glasses. A key concept in XPS is the so called chemical shift, which represents the change in the binding energy of certain electron core-level. The chemical shift could result from a difference in the actual oxidation state as well as a difference in the molecular environment [1,2]. The binding energy increases typically with an increase in the oxidation state of a metal atom [2]. However there is no straightforward correlation between binding energy and structural coordination. For example the Mo3d5/2 binding energy of octahedrally coordinated Mo6+ ions has been measured to be with 1 eV higher compared to that of the Mo ions in tetrahedral oxygen environment, existing in the network of MoO3-CuO glasses [3]. A change of Ti 2p3/2 binding energy in the opposite direction has been observed in the transition of Ti4+ ions from octahedral to tetrahedral coordination, respectively from 458.3 eV to 460.5 eV [4]. This complexity in the interpretation of XPS results can be significantly avoided by using standard crystalline compounds, whose structural and electronic properties of the building units have been previously clarified. Therefore in the ⁎ Corresponding author. E-mail address: [email protected] (M. Milanova).
present paper we study the local structure and the connectivity in the amorphous network of binary MoO3-Bi2O3 glasses by comparative analysis of appropriate crystalline and amorphous phases using X-ray photoelectron spectroscopy. The Bi4f and Cu2p photoelectron spectra of ternary MoO3-Bi2O3-CuO glasses with one and the same MoO3 content (60 mol%) are also discussed here in details in order to obtain more information about the valence state and structural role of these oxides in the network of ternary glasses. 2. Experimental procedure The glass formation in the MoO3-Bi2O3 and in the MoO3-Bi2O3-CuO systems was achieved at high cooling rates (104-105 K/s) by using a roller-quenching technique. The reference crystalline phases Bi2Mo3O12, and Bi2Mo2O9 were obtained by crystallization of glasses having the corresponding compositions (75MoO3·25Bi2O3 and 66MoO3·33Bi2O3). The detailed preparation procedures applied were already described in Refs. [5–7]. The X-ray photoelectron (XPS) measurements were carried out on an ESCALAB Mk II (VG Scientific) electron spectrometer with Al Kα excitation source and a total instrumental resolution of 1.16 eV (as it was measured with the FWHM of Ag3d5/2 photoelectron line). The energy scale was corrected with respect to the C1s - peak maximum at 285.0 eV for electrostatic charging. The processing of the measured spectra included a subtraction of X-ray satellites and Shirley-type background. The peak positions and areas were evaluated by a symmetrical Gaussian-Lorentzian curve fitting. The relative concentrations of the different chemical species were determined by normalization of the peak areas in regard to their
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Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
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M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
photoionization cross-sections calculated by Scofield [8]. The absorption spectra of the MoO3-Bi2O3 and MoO3-Bi2O3-CuO samples were measured at room temperature using Evolution 300 UV–Vis Spectrophotometer in the 190–1100 nm wavelength range.
Table 1 Concentrations (in at.%) of Bi and Mo atoms in different chemical states. Additionally the total oxygen amount (at.%) is shown in the last column. The corresponding Bi4f7/2 and Mo3d5/2 binding-energies (in eV) are shown above the concentration values. Samples
3. Results
Crystal Bi2Mo2O9
3.1. MoO3-Bi2O3 glasses
Glass MoO3: Bi2O3 = 66:33
MoO3:Bi2O3 mol %
Mo3d
66:33
crystal
glass
66:33
glass
70:30
Glass MoO3: Bi2O3 = 75:25 Crystal Bi2Mo3O12
Bi (2)
Mo (1)
Mo (2)
O
159.6 19.5 159.5 18.5 159.7 17.0 159.7 14.5 159.7 14.5
157.7 0.5 157.8 1.0 158.0 1.0 157.8 0.5 157.8 0.5
232.5 18.0 232.4 17.0 232.5 17.0 232.7 19.0 232.8 20.0
-
62.0
233.3 0.5 233.2 1.0 233.3 1.5 -
63.0 64.0 64.5 65
established the existence of two types of oxygen atoms in the structure of Bi2Mo2O9 crystal marked as O(1) and O(3) in Table 2 with a concentration ratio of О(1):О(3) = 1:8. The O(1) involves oxygen surrounded by three Bi atoms (marked as Bi3O bonding configuration) and the O(3) involves oxygen participating in the mixed Bi\O\Mo bonds [9]. In addition to their fitting contributions a third peak denoted as O(6) at 532.1 eV was obtained and it was attributed to adsorbed OH groups on the surface (Fig. 3). Similarly, considering the crystallographic studies of Bi2Mo3O12 crystal [10,11] we suppose the existence of three kinds of oxygen atoms (denoted as O(2), O(3) and O(4)) in a concentration ratio of 2:7:3 participating in different chemical bonds namely short Mo\O, Bi\O\Mo and Mo2O2, respectively. The latter notification (Mo2O2) refers to edge bonds between two Mo polyhedra [10,11]. Note that both Bi2Mo2O9 and Bi2Mo3O12 crystals have a common type of oxygen atom O(3), involved in the Bi\O\Mo bonds. The O1s signal deconvolutions for both crystals gave similar binding energy of 530.4530.5 eV for this kind of oxygen, which is a convincing validation of the above described fitting approach. The O1s spectra of glasses were fitted not only on the basis of the O1s results of the crystalline samples of the same compositions but also having in mind the previous studies [3,6]. Table 2 summarizes the concentrations (at. %) and the О1s binding energies of oxygen atoms incorporated in different chemical bonds of the studied crystalline and glassy samples. The measured (actual) compositions of all samples were calculated using the total areas of the MoO3:Bi2O3 mol %
Bi4f
66:33
crystal
66:33
glass
70:30
glass
75:25
glass
75:25
crystal
glass
75:25
crystal
75:25 225
Glass MoO3: Bi2O3 = 70:30
Intensity (arb. units)
Intensity (arb. units)
The main approach, applied in the present study, consists of comparative analysis of X-ray photoelectron spectra of Bi2Mo3O12 and Bi2Mo2O9 crystalline standards and those of amorphous phases of the same MoO3-Bi2O3 compositions. The spectra deconvolutions were made on the basis of the crystallographic data analysis of Bi2Mo3O12 and Bi2Mo2O9 crystals, available in the literature, concerning the number of the different atoms in their unit cells, the type of bonding and the Me\O (Me_Mo; Bi) bond distances in the unit cell [9–11]. In this way the Mo3d spectra of both crystalline phases were well fitted using only one pair of 3d5/2 and 3d3/2 peaks (Fig. 1). The measured Mo3d5/2 binding energies of Bi2Mo2O9 at 232.5 eV and of Bi2Mo3O12 crystal at 232.8 eV, respectively are in good agreement with those found in the literature [12]. However the Mo3d spectra of the glasses with the identical compositions (Fig. 1) can be fitted with two pairs of 3d5/2 and 3d3/2 peaks, characteristic of Mo atoms in two different coordination states (see Discussion section). The binding energies of the Mo3d5/2 peak contributions and their corresponding concentrations, obtained by measuring the areas of Mo3d5/2 + 3/2 peaks, are shown in Table 1. The Bi4f spectral regions of all MoO3-Bi2O3 samples can be well fitted with two pairs of 4f7/2 and 4f5/2 peaks, evidencing the presence of Bi atoms in two different chemical states (Fig. 2). Their concentrations (in at. %), as well as the corresponding binding energies of Bi4f7/2 are listed in Table 1. Fig. 3 displays the O1s photoelectron spectra of all the studied crystalline and amorphous samples. These spectra were fitted taking into account the crystallographic data for both Bi2Mo3O12 and Bi2Mo2O9 crystals determining the number of different oxygen atoms involved in various bonds in the structures through analysis of the Me\O (Me_Mo; Bi) bond distances in the crystal unit cells [9–11]. Thus we
eV at.% eV at.% eV at.% eV at.% eV at.%
Bi (1)
230
235
240
Binding Energy (eV) Fig. 1. Mo3d-spectral regions of two crystal samples and glasses with different nominal MoO3:Bi2O3 ratios.
155
160
165
170
Binding Energy (eV) Fig. 2. Bi4f-spectral regions of two crystal samples and glasses with different nominal MoO3:Bi2O3 ratios.
Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
MoO3:Bi2O3 mol %
Intensity (arb. units)
Table 3 Comparison between nominal and measured MoO3: Bi2O3 compositions in mol%.
O1s crystal
66:33
glass
66:33
glass
70:30
Samples
Nominal composition MoO3: Bi2O3 mol%
Measured composition MoO3: Bi2O3 mol%
Crystal Bi2Mo2O9 Glass MoO3: Bi2O3 = 66:33 Glass MoO3: Bi2O3 = 70:30 Glass MoO3: Bi2O3 = 75:25 Crystal Bi2Mo3O12
66:33 66:33 70:30 75:25 75:25
64.3:35.7 64.2:35.8 66.7:33.3 73.2:26.8 72.7:27.3
incorporated in different chemical bonds in the investigated glasses. Their concentrations (in at.%) and the corresponding O1s binding energies (eV) are given in Table 5. In contrast to the binary MoO3-Bi2O3 glasses (see Section 3.1) now, in the presence of CuO, the nominal and measured MoO3:Bi2O3 ratios are not in such good agreement (Table 6).
glass
75:25
3
3.3. UV-visible diffuse reflectance analysis
crystal
Mo3d and Bi4f peaks normalized with respect to their photoionization cross-sections [8] and the results (in mol.%) shown in Table 3 are in good agreement with the nominal compositions.
Fig. 6 shows the UV–vis absorption spectra of crystalline Bi2Mo3O12 and glass with the same composition (75MoO3 · 25Bi2O3) and as well as that of ternary 60MoO3 · 30CuO · 10Bi2O3 glass. All spectra exhibit an absorption band at 260 nm. In the spectrum of the crystalline phase (Fig. 6(a)) not well resolved peak located above 300 nm is observed, that is shifted to longer wavelength (370 nm) in the spectrum of the glass sample with the same composition (Fig. 6(b)). Stronger absorption in the 450–700 nm range is observed in the spectrum of the 75MoO3 · 25Bi2O3 glass sample in comparison with the spectrum of its crystalline analog. The UV spectrum of CuO-containing glass (Fig. 6(c)) displays highly intensive and very broad absorption band in the visible region.
3.2. Comparison with MoO3-CuO-Bi2O3 glasses
4. Discussion
The Bi4f and Cu2p photoelectron spectra of ternary MoO3-Bi2O3CuO glasses with one and the same MoO3 content (60 mol%) are shown in Figs. 4 and 5, respectively, while the Mo3d and O1s regions have already been reported elsewhere [6]. Similar to the MoO3-Bi2O3 glasses (Fig. 2) a significantly dominating Bi 4f7/2 peak at about 159.2159.3 eV is observed with all ternary glasses. The Cu2p3/2 spectra show a two-peak structure (Fig. 5), indicating the presence of Cu+ and Cu2 + ions in these glasses [13]. The Cu2p3/2 binding energy of Cu+ ions is found to be ~ 932.1 eV while Cu2+ ions are characterized with a peak at about 934.2 eV as well as with a broad intensive satellite structure centered at ~942 eV (Fig. 5). The surface content of Cu+ ions is approx. 10-15% of the total Cu amount (Table 4). In the same table the corresponding Mo3d5/2, Bi4f7/2 and Cu2p3/2 binding energies of the different chemical states of Mo, Bi and Cu are represented. As it has been shown earlier [6] the O1s spectrum of MoO3-Bi2O3CuO glasses can be fitted with four peaks attributed to oxygen atoms,
4.1. MoO3-Bi2O3 glasses
75:25 525
530
535
Binding Energy (eV) Fig. 3. O1s-spectral regions of two crystal samples and glasses with different nominal MoO3:Bi2O3 ratios. The ratios of О(2):О(3):О(4) = 2:7:3 and О(1):О(3) = 1:8 are supposed for the Bi2Mo3O12 and Bi2Mo2O9 crystals, respectively.
As it was mentioned in the previous section the interpretation of the XPS results is based on the comparative analysis of the photoelectron spectra of Bi2Mo3O12 and Bi2Mo2O9 crystalline samples and that of amorphous phases with equivalent MoO3-Bi2O3 compositions. According to the crystallographic data the Bi2Mo2O9 crystal contains only Mo atoms in tetrahedral coordination against oxygen [9]. Their Mo3d5/2 binding energy was measured to be centered at 232.5 eV in the photoelectron spectrum (Fig. 1). Similar binding energy has been obtained earlier for the tetra-coordinated Mo atoms in MoO3-CuO glasses and in CuMoO4 crystal and has been attributed to the Mo6+ ions in tetrahedral oxygen environment (MoO4) [3]. Based on these earlier observations we can also propose the existence mainly of hexavalent molybdenum atoms in the glasses discussed in the present paper. However as it was shown in the literature, for example by Cozar et al. [14],
Table 2 Concentrations (in at.%) of oxygen atoms incorporated in different chemical bonds of studied crystal and glass samples. The corresponding O1s binding-energies (in eV) are shown above the concentration values. The ratios of О(2):О(3):О(4) = 2:7:3 and О(1): О(3) = 1:8 are supposed for the Bi2Mo3O12 and Bi2Mo2O9 crystals, respectively. Samples
Crystal Bi2Mo2O9 Glass MoO3: Bi2O3 = 66:33 Glass MoO3: Bi2O3 = 70:30 Glass MoO3: Bi2O3 = 75:25 Crystal Bi2Mo3O12
eV at.% eV at.% eV at.% eV at.% eV at.%
O(1)
O(2)
Bi3O
Mo\O
529.1 5.9 529.1 5.0 529.1 3.5 -
-
-
529.7 9.8 529.7 11.2 529.6 15.7 529.6 9.6
O(3)
O(4)
O(5)
O(6)
Bi\O\Mo
Mo2O2
Mo\O\Mo
ads.OH
530.4 47.1 530.4 37.7 530.5 35.7 530.5 30.3 530.5 33.8
-
-
-
531.2 2.4 531.2 4.6 531.2 9.4 -
532.0 9.1 532.1 8.0 532.1 9.1 532.1 9.2 532.1 7.1
530.9 14.5
Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
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M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
MoO3:CuO:Bi2O3 mol %
Intensity (arb. units)
Bi 4f
60:10:30
60:20:20
60:30:10 150
155
160
165
170
175
Binding Energy (eV) Fig. 4. Bi4f-spectral regions of glasses with different nominal MoO3:CuO:Bi2O3 ratios.
the low local symmetry of the Mo polyhedra in some glasses can be an indication for the presence of Mo5 +. Therefore we cannot exclude completely the existence of some amount of Mo atoms in valence state lower than Mo6+. The Mo3d5/2 binding energy of Bi2Mo3O12 crystal is measured to be 232.8 eV, which is higher than that found for Bi2Mo2O9, most probably because of the presence of oxygen ion in the vicinity of each MoO4 tetrahedron at a distance of ca. 2.2 Å. The presence of this fifth oxygen ion is often considered as evidence that molybdenum in the structure of Bi2Mo3O12 is in 5-fold coordination geometry (MoO5) [10,11]. However the Mo3d5/2 binding energy of Bi2Mo3O12 crystal still is in a range characteristic of tetrahedrally coordinated Mo6 + ions [3]. The deconvolution of the Mo3d core-level peak of MoO3-Bi2O3 glasses into two contributions indicates that the Mo atoms in glasses exist in two different coordination states (denoted as Mo(1) and Mo(2) in Table 1) in contrast to the reference crystalline
Cu 2p3/2
MoO3:CuO:Bi2O3 mol %
Intensity (arb. units)
60:10:30
60:20:20
60:30:10
925
930
935
940
945
950
Binding Energy (eV) Fig. 5. Cu 2p3/2 -spectral regions of glasses with different nominal MoO3:CuO:Bi2O3 ratios.
Table 4 Binding energies (in eV) of Mo3d5/2, Bi4f7/2 and Cu2p3/2 core-levels of Mo, Bi and Cu atoms in different chemical states. The Cu1+: Cu2+ ratio (in %) is shown in the last column. Samples
Mo(1) Mo(2) Bi(1)
60MoO3 · 30CuO · 10Bi2O3 60MoO3 · 20CuO · 20Bi2O3 60MoO3 · 10CuO · 30Bi2O3
232.0 232.1 232.3
233.1 233.3 233.4
Bi(2)
Cu1+
Cu2+
Cu1+: Cu2+
157.5 159.2 932.1 934.3 9:91 157.5 159.3 932.1 934.2 8:92 157.4 159.3 932.0 934.1 14:86
compounds. Similar to the crystalline phase results we suppose that the dominant Mo3d5/2 peak at 232.4-232.8 eV is characteristic of Mo6 + ions in tetrahedral oxygen environment (MoO4), whereas the higher binding energy contribution at 233.2-233.3 eV can be assigned to the Mo6+ ions in octahedral coordination (MoO6). Evidence for the latter supposition is that the Мo3d5/2 binding energy at 233. 6 eV was found for the pure MoO3 oxide, whose crystalline structure consists of MoO6 octahedra only [3]. The rising intensity of Mo3d5/2 peak characteristic for MoO6 polyhedra with increasing MoO3 concentrations (Fig. 1, Table 1), indicates the increase of the number of MoO6 units in the glass structure. This assumption is in agreement with the previous infrared spectral results of MoO3-Bi2O3 glasses reported by Iordanova et al. [5]. However, still a significantly dominant feature is that of tetrahedrally coordinated Mo6+ ions. The binding-energy values of the predominant Bi4f peak contribution (Fig. 2) reported in Table 1 and denoted as Bi(1) are in good agreement with those for other bismuth-oxide compounds and glasses indicating the presence of Bi3 + ions [12,15–18]. The low bindingenergy peak denoted as Bi(2) in Table 1 could be related to the existence of a small amount of bismuth, reduced to metallic state (Bio) most probably due to the thermal reduction of bismuth ions during meeting processes [19,20]. Several types of oxygen atoms incorporated in different chemical bonds of the studied crystals and glass samples are supposed, taking into account the analysis of crystallographic data for Bi2Mo2O9 and Bi2Mo3O12 (Section 3.1). Considering these results, represented in Table 2, no significant shift of O1s-peak positions in the spectra of glasses and their crystalline analogs is visible, which gives us the reason to claim that the type of bonding in the amorphous network is similar to that in the crystalline structure. However, changes in the concentrations of different O atoms are observable, indicating that some structural transformation is occurring during the transition from crystalline into amorphous state. From Table 2 it becomes obvious that O(4) atoms are absent in the glass network. The O(4) is attributed to the oxygen in edge shared MoO5 units existing in the structure of Bi2Mo3O12 [10,11]. Also the presence of O(5), related to oxygen from corner shared MoO6 [3], could be observed in the glass samples, but it is not found in the structure of crystalline analogs. The concentration of O(1) oxygen atoms connected with the Bi\O\Bi bonds is smaller in the glasses in comparison with the respective crystalline compound, while the amount of O(2), ascribed to the short Mo\O bonds, increases in the glasses simultaneously with the increase of MoO3 initial amounts. In both crystalline phases and glass samples, the highest amounts of oxygen atoms are involved in mixed type Bi\O\Mo bonds. These concentrations decrease slightly after the transition from crystalline into glassy state. Based on the abovementioned observations the following conclusions can be drawn: (i) the transition to an amorphous structure leads to the breaking of edge bonds (Mo2O2) between molybdenum polyhedra; (ii) the MoO3-Bi2O3 glass network consists mainly of MoO4 tetrahedra as well as of a small amount of MoO6 octahedra; (iii) the MoO6 are connected by bridging Mo\O\Mo bonds, while some MoO4 tetrahedra participate in the formation of mixed type Bi\O\Mo bonds, which are predominant in the MoO3-Bi2O3 glass network; and (iv) the presence of mixed Mo\O\Bi bonding is crucial for the glass formation. It should be pointed out that these heteroatomic Mo\O\Bi linkages cannot be regarded as the typical bridging bond (as for an example is Mo\O\Mo
Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
5
Table 5 Concentrations (in at. %) of oxygen atoms incorporated in different chemical bonds of studied glass samples. The corresponding O1s binding energies (eV) are shown above the concentration values. Samples
O(1)
O(2)
O(3)
O(4)
Me (Me′)\O\Me (Me′)
Me (Me′)\O\Mo
Mo\O\Mo
ads.OH
531.5 22.0 531.6 15.5 531.4 2.0
532.1 19.0 532.1 13.0 532.2 15.0
+
2+
Me_Cu ; Cu 60MoO3 · 30CuO · 10Bi2O3
eV at.% eV at.% eV at.%
60MoO3 · 20CuO · 20Bi2O3 60MoO3 · 10CuO · 30Bi2O3
+
; Me′_Bi
2+
Me_Cu ; Cu
529.0 8.0 529.2 10.4 529.1 11.0
; Me′_Bi
530.5 51.0 530.5 61.0 530.4 72.0
bonding) and there are no typical non-bridging bonds too. Following the classification of Dimitrov and Komatsu [21] these bonds can be considered as semi-covalent bonds, assuming the existence of strong interionic interaction. A good agreement between the nominal and measured compositions is found with all studied MoO3-Bi2O3 glasses (Table 3). Considering that XPS is a surface sensitive method giving information about the upper Mo\O\Bi layers within a depth of few nm, we suppose that Mo:Bi ratios on the surface and in the bulk of the glass samples do not differ significantly. This is in good agreement with earlier XPS study of bismuth molybdates, which found equal Bi:Mo ratios on the surface and in the bulk [22].
Considering the calculated concentrations, based on XPS data, it should be pointed out that the addition of CuO could change the MoO3:Bi2O3 ratio, although not so strongly expressed (Tables 3 and 6). This interesting fact can be explained having in mind that XPS is a surface-sensitive method and the main contribution to the signal is coming from the uppermost surface layer, whereas in the depth of the sample the signal is decaying exponentially depending on the mean free path of the emitted electrons. Therefore we can conclude that the results for the nominal and actual ratios (see Tables 3 and 6) represent a comparison between bulk and surface compositions, respectively. In this aspect the tables outline one apparent difference between both binary MoO3-Bi2O3 and ternary MoO3-Bi2O3-CuO glasses, namely a slightly different surface MoO3:Bi2O3 ratio, compared to the bulk ratio for CuOcontaining glass.
4.2. Comparison with MoO3-CuO-Bi2O3 glasses 4.3. UV-visible diffuse reflectance analysis The XPS results were also supported by UV-visible diffuse reflectance analysis performed on crystalline Bi2Mo3O12 and glass having the same composition (75MoO3 · 25Bi2O3). The absorption band at 260 nm, observed in both spectra, can be assigned to the isolated MoO4 tetrahedra [26,27] as they were also detected by XPS. The poorly resolved absorption peak above 300 nm, detected in the spectrum of crystalline phase, is difficult to be assigned precisely as charge transfer UV–vis bands of both Mo6+ and Bi3+ ions are also located in this spectral range [26–29]. The observed shift of the latter band to longer wavelengths in the spectrum of glass sample and the increasing absorption in the 450–700 nm range in comparison with the spectrum of its crystalline analog are due to the structural changes as a result of transformation into amorphous state and increasing structural disorder [30]. The presence of the absorption band at 260 nm also in the UV-visible 24 22 20 18
Kubelka-Munk units
According to our previous study on MoO3-Bi2O3-CuO glasses [6] there are several types of O atoms participating in different bonds. In this paper, we give the numerical results of the O1s fitting procedure (Table 5), which ensures more precise information about the bonding configuration and structure of the glasses. As it is seen from Table 5 the addition of CuO to the MoO3-Bi2O3 glass does not lead to any noticeable differences in the O1s peak positions of O(1), O(2) and O(3) oxygen atoms in comparison with those found for binary glasses. This indicates the presence of the same type of bonding in both groups of glasses (similar middle range order). However the contents of various oxygen atoms in the binary and ternary glasses are different. The concentrations of O(3) oxygen atoms grow up with the increase of CuO at the expense of Bi2O3 content. The observation evidences that the introduction of CuO to binary MoO3-Bi2O3 glasses with the same MoO3 content (60 mol%) does not change the way of glass network formation, but inhibits MoO6 → MoO4 transformation. Clear preference for the formation of heteroatomic bonds is also observable (Table 5) indicating the crucial role of this type of bonding in the MoO3-CuO-Bi2O3 glass network. It is mainly the Bi3+ ions (denoted as Bi(2) in Table 4) that are detected in the ternary glasses. A small amount of Bio (indicated as Bi(1)) is observed most probably as a consequence of the thermal reduction during the melting of the glasses [19,20]. The observed asymmetry and satellite peaks in the Cu2p spectra of all studied glass samples (Fig. 5) are indications of the presence of both Cu+ and Cu2+ ions. This result is in good agreement with previous XPS measurements of other glasses containing CuO, where monovalent and divalent copper ions were also supposed to exist [3,23–25].
260
16 14 12
340 370
10
c
8 6 4
Table 6 Comparison between nominal and measured MoO3: Bi2O3 ratios. Samples
60MoO3 · 30CuO · 10Bi2O3 60MoO3 · 20CuO · 20Bi2O3 60MoO3 · 10CuO · 30Bi2O3
b
2
Nominal ratio
Measured ratio
MoO3: Bi2O3
MoO3: Bi2O3
6 3 2
4.8 2.5 1.5
a
0 200
400
600
800
1000
Wavelength, nm Fig. 6. Diffuse-reflectance UV–vis spectra of: a) crystalline Bi2Mo3O12; b) 75MoO3 · 25Bi2O3 glass; and c) 60MoO3 · 30CuO · 10Bi2O3 glass.
Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032
6
M. Milanova et al. / Journal of Non-Crystalline Solids xxx (2014) xxx–xxx
absorption spectrum of ternary glass (Fig. 6(c)) is most probably due to the MoO4 groups, which are the predominant structural units building up the amorphous network of these glasses [5]. The broad absorption band in the visible region, noticed in the spectrum of 60MoO3 · 10Bi2O3 · 30CuO glass, is most probably due to the presence of copper ions because a broad visible band characteristic for Cu2+ ions is detected in the optical spectra of copper doped bismuth-borate glasses [31]. 5. Conclusions Using X-Ray photoelectron spectroscopy it has been found out that the local structure and the connectivity in the amorphous network of binary MoO3-Bi2O3 glasses are similar to those of crystalline Bi2Mo2O9 and Bi2Mo3O12 because no chemical shifts of the Mo3d and Bi4f binding energies have been found in the glass and crystalline phases. The MoO4 tetrahedral units have been identified as the main building blocks of the amorphous network of MoO3-Bi2O3 glasses. This fact has been confirmed by UV-visible diffuse reflectance measurements, where an absorption band at 260 nm attributed to MoO4 tetrahedra has been observed. Additionally the existence of a small amount of MoO6 octahedra has also been indicated by XPS. The O1s spectra analysis suggests that the MoO6 octahedra are connected by bridging Mo\O\Mo bonds, while MoO4 tetrahedra participate in the formation of mixed Bi\O\Mo bonds. The presence of mixed Bi\O\Mo bonds with a relatively significant covalent character is crucial for the glass formation. The addition of CuO to the MoO3-Bi2O3 glass does not lead to the noticeable differences in the Mo3d, Bi4f and O1s peak positions, compared with those found for binary glasses, which indicates similar short and middle range order. Besides Mo6+ and Bi3+ both Cu+ and Cu2+ ions are also observed in the case of MoO3-Bi2O3-CuO amorphous samples. In all studied glasses the role of MoO3 as glass former has been confirmed. Acknowledgment This research work has been funded by the Bulgarian Ministry of Education, Project No. BG051PO001-3.3-05/0001 “Science and Business”, within the Operational Program “Development of Human Resources”.
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Please cite this article as: M. Milanova, et al., X-ray photoelectron spectroscopic studies of glasses in the MoO3-Bi2O3 and MoO3-Bi2O3-CuO systems, J. Non-Cryst. Solids (2014), http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.032