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X-ray photoelectron spectroscopy study of vanadium germanate glass
Journal of Non-Crystalline North-Holland. Amsterdam
Solids
93 (1987)
45-52
45
X-RAY PHOTOELECTRON SPECTROSCOPY OF VANADIUM GERMANATE GLASS E.E. KHAWAJA,
Z. HUSSAIN,
Departntenr of Physics. Saudi Arabiu
King
Fahd
MS. JAZZAR
Universi!,~
Received 12 November 1986 Revised manuscript received 9 April
01 Petroleum
STUDY
and O.B. DABBOUSI & Minerals,
Dhohran
31261,
1987
Reduction of vanadium ions in 50 mol’% vanadium germanate glass under the influence of annealing the sample in air was studied by Auger and X-ray photoelectron spectroscopy. It was concluded that V5+ reduces mostly to V3+ and to a small extent to V4’. The same conclusion was drawn from optical absorption data.
1. Introduction For vanadium germanate glasses the electrical conductivity measured by Chung and Mackenzie [l] was much smaller than that measured by Rao [2]. Khan et al. [3] measured electrical conductivity of annealed and unannealed vanadium germanate glasses. These measurements clearly elaborated the discrepancies in the two results. The conductivity of annealed samples at room temperature was found to be about five orders of magnitude greater than that of unannealed samples containing a similar amount of vanadium oxide. The increase in conductivity on annealing was attributed to the change in microstructure of the glasses. This was also predicted by Chung and Mackenzie [l]. However, Khawaja et al. [4] reported a detailed study of 50 mol% vanadium germanate glasses using electron paramagnetic resonance, optical absorption, differential scanning calorimetry and electron diffraction techniques. They concluded that the increase in the electrical conductivity of the annealed samples could be attributed to the increase of reduced valence states of vanadium ions (V3+ and V4+) which accompany the microstructure formation and are not necessary to the structural change alone. In the present work, effect of annealing on the binding states of vanadium in 50 mol% vanadium germanate glass has been studied using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The object was to verify our earlier observations [4] that vanadium was reduced upon annealing the glass. Pure vanadium pentoxide (not mixed with other materials to form a glass) has long been studied extensively because of its interesting physical properties 0022-3093/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
46
E. E. Khawaja
er al. / Specrroscopy
01 uarladium-gen,lNnare
glass
[5-91. v,o,, which is known as a good catalyst, in fact, owes its catalytic properties to the presence of vanadium atoms in different oxidation states as a consequence of nonstoichiometry (oxygen deficiency) or even a phase mixture with lower oxide V,O,, [8]. Th e reduction of vanadium pentoxide to lower oxides is known to occur as a result of various treatments such as heating, participation in catalytic oxidation reaction [lo-131 and electron or ion bombardment [9]. However, little research has been devoted to glasses containing vanadium pentoxide. Amorphous transition metal oxides are known to exhibit semiconductivity properties arising from a hopping process of impaired electrons between transition metal ions in different valence states [1,14,15]. Several transition metal oxides when heated with glass-forming substances like P]O,, TeO,, GeO,, etc. form glasses on quenching the melt. The loss of oxygen from the melt produces, lower valency transition metal ions. Electrical conduction in glasses containing vanadium is suggested [1,14,16] to occur by electron hopping from an ion of low (V4’) to an ion of high valence state (V5+).
2. Experimental Glass was prepared by heating a 50 mol% mixture of analytical grade vanadium pentoxide and germanium dioxide in an alumina crucible. Prior to melting the mixture, the crucible was placed in a furnace maintained at 400 o C for 1 h in order to minimize material volatilization. The crucible was then transferred to a melting furnace maintained at a temperature of 1100 o C and left for 3 h with frequent stirring. The homogenized melts were then cast onto a steel plate mould (pre-heated to 300’ C). The samples to be annealed were transferred to an annealing furnace maintained at 300” C for 2 h and were than allowed to cool slowly. The glass discs were ground into a fine powder which was embedded in a substrate of metallic indium for XPS and AES measurement. Photoemission measurements were performed in a VG Scientific ESCALAB MKII system. The very high efficiency spectrometer is equipped with three detectors and adjustable inlet and outlet slits. The samples were loaded through a fast entry air lock into the preparation chamber and finally into the analysis vessel. A rack-and-pinion sample transfer mechanism allows exchange of samples between preparation and analysis vessel. These chambers are isolated from each other through a gate valve. The base pressures in the spectrometer and the preparation vessels were 6 x lo-” mbar and 2 x lo-‘O mbar, respectively. XPS study was performed using Mg Ka radiation at a photon energy of 1253.6 eV. The analyzer was operated in the constant resolution mode with a pass energy of 20 eV and an exit slit of 10 x 3 mm2. AES measurements were performed in the undifferential pulse counting mode by using Mg K~z X-rays as excitation source. The pass energy of the analyzer operated again in the constant resolution mode was 20 eV.
E.E. Khawaja
er al. / Specrroscopy
of vauadium-germanare
glass
47
In order to avoid any shift due to charging and to bring energies to the same reference point, all energies are stated with reference to C Is at a binding energy of 284.6 eV. This is a fortuitous peak which arises due to hydrocarbon contaminants in the vacuum and it is generally accepted to be independent of the chemical state of the sample under investigation. The powder samples were sparingly embedded in indium substrates to avoid unnecessary charging. As a result, the photoemission spectra have small peaks arising due to substrate indium, which could also be employed as an energy reference. Changing the reference to In3d 5,2 (BE = 443.6 eV) provided similar results.
3. Results
XPS spectra of unannealed (UN) and annealed (AN) specimensof the glass were taken under identical experimental conditions. The photoelectron peaks near V2p from UN and AN samples are compared in fig. 1. The positions of various peaks on the binding energy (BE) scale are listed in table 1. All energies are referenced to C 1s at. a binding energy of 284.6 eV. The decrease in binding energy of V2p on going from UN to AN samples is due to the reduction of vanadium ions. Kasperkiewiecz et al. [7] have studied the photoelectron spectra of pure VZOs and VO, and found that the V2p peak shifts to lower binding energy (- 0.3 eV) on going from VZO, to VOz. Wachs
2.20
-
I
1
,
1
I
x104 1.98 1.76
.
1.54
,.
520
525
Binding
Fig.
1. Photoelectron
spectra
excited
by Mg
energy
(eV)
Ka radiation samples.
for (a) unannealed
and (b) annealed
48 Table 1 Core-level samples
binding
E. E. Kltawaja
er al. / Specrroscop~
energies,
sepaiations
energy
Level
Unannealed (UN) BE (KE) (eV)
V2P,,2 “2Pl,Z 01s “U-3M2.3hs) V(L2M,,M,,)
517.3 524.5 530.7 786.3 (467.3) 826.6 (427.0)
* All peaks are referenced parentheses.
oJ oanadium-germanare
and Auger
lines for annealed
*
IO C Is at 284.6 eV. The kinetic
glass
Annealed BE (KE)
and unannealed
glass
(AN) (eV)
516.7 523.9 530.4 785.8 (467.8) 825.8 (427.8) energy
of Auger
peaks
is also given
in
and Chen [13] have found a shift of - 1.7 eV, for the same peak, to the lower energy on going from VzO, to Vz03. We looked for another method to confirm the reduction of vanadium ions upon annealing, and turned to Auger transitions excited by photons in the kinetic energy range of 400-540 eV. It has been pointed out by Tompkins et al. [9] that in pure and stoichiometric Vz05, the 3d-derived electronic levels are completely empty and lie above a rather large energy gap (2.5 eV). In the lower oxides such as VO,, V,O, and V,O,, these 3d states are partially occupied and become part of the valence band states. It is therefore expected that the contribution of the L,MljM,, and L,M,,M,, Auger transitions, that involve these valence 3d states, to the main L,M,, V Auger line, will be absent in V,O, and will appear upon the phase transition from V,O, to VO,, VzO, and/or V,O,,. Therefore Auger transitions that involve these 3d-derived states are fingerprints that can be used for phase identification. The spectra in fig. 2 compare Auger transitions from vanadium in the UN and AN samples. The spectra were taken using Mg KCX radiation under identical experimental conditions. It is observed that the Auger peaks shift to lower binding energy on going from UN to AN samples (table 1). In addition, there are significant differences in the fine structure of vanadium L,MzjV Auger transition occurring at a binding energy of about 786 eV, corresponding to a kinetic energy of about 468 eV [fig. 21. For UN samples the spectrum [fig. 2(a)] near L,M,Y transitions is dominated by one single peak at a kinetic energy of 466.9 eV [17,18]. The effect of annealing is seen to be the occurrence of an additional component (marked by an arrow in fig. 2) at 471.9 eV, together with the shift of the main peak to 467.4 eV. Tompkins et al. [9], in their study of reduction of pure V,O, to the lower oxide V,O,, under the influence of electron bombardment, observed the similar shift in energy and development of the new component. They interpreted this new structural feature as the contribution of the 3d-derived orbitals to L,M,Y Auger transitions through processes of L,M,,M,, type. Such transitions are absent in V,O, where the M,, states are empty, but become
E. E. Klmvuja 1.60
-
1.44
,.
1.28
‘.
I
et al. / Specrroscopy
I
of vanudium-germmare
,
glass
I
49 I
x 105
520
500
480 Kinetic
Fig. 2. Auger
electron
spectra
in the XPS mode
460 energy
440
420
400
(eV)
for (a) unannealed
and (b) annealed
samples
significant in the lower oxides, where MdS states are partially occupied and are therefore part of the valence band [9]. Following Tompkins et al. [9] we could say that vanadium ions in the glass were reduced upon annealing. The same conclusion was drawn above from the study of photoelectron spectra.
4. Discussion Khawaja et al. [4], in their recent study of 50 mol% vanadium germanate glass, concluded that the increase in electrical conductivity of the glass upon annealing could be attributed to an increase in V4+ and v-7+ content which accompanied the microstructure formation. The present results support their view, as is discussed below. The observed decrease in the binding energy of V2p on going from UN to AN samples is about 0.6 eV [table 11. Kasperkiewicz et al. [7] found a decrease of about 0.3 eV in going from pure V,O, to pure VOZ. Wachs and Chen [13] observed a shift to lower energy of about 1.7 eV on changing from pure VzO, to VzO,. In some of the earlier studies [1,16], carried out on vanadium in various glasses, it was assumed that V4+ was the only reduced state that existed in the glasses. However, in the present case even if all the Vs+ were reduced to V4+ upon annealing the sample, the observed shift of 0.6 eV is too large as compared with 0.3 eV, observed by Kasperkiewicz et al. [7] on going from pure
E. E. Khawaja
50
Ed al. / Spectroscopy
of oatladium-gernlanare
glass
VzO, to VOz. In the light of the above mentioned shifts it is plausible to say that Vsc in the UN sample were reduced to a lower oxidation state, probably V3+ after annealing. This conclusion is in agreement with a separate study on the same sample made by Khawaja et al. [4] using electron paramagnetic resonance (EPR). The C values, defined as the ratio of V4+ concentration to the total concentration of vanadium ions, were found to be 0.0085 and 0.015 for UN and AN samples, respectively. It may be mentioned that energy level separations of paramagnetic ions of the order of 10 cm-’ (8.25 cm-’ for V3+ in AlzO, [19]) in general cannot be readily observed with conventional paramagnetic resonance techniques. Since the V4’ content in the glass is small, we conclude that the reduction of V5+ is mostly to V3+. Further support to this comes from the optical absorption spectra, as discussed below. Figure 3 shows optical absorption spectra of the same composition as that of the glass studied in the present work as measured by Khawaja et al. [4] from thin blown film. These measurements were for the same sample, made before and after annealing. In another study [20] it has been reported that vanadium in various glasses can exist in three oxidation states, namely V’+, V4+ and V3+. The absorption band at 350 nm has been assigned [20] to V’+, the bands at 425 nm and 645 nm to V”, and the band at 1100 nm to V4+. The spectrum given in ref. [20] (fig. 2.8) indicates that absorption in the region from 400 to 900 nm has a major contribution from V3+ ions with possibly a small contribution from the tail of the band at 1100 nm, associated with V4+. The bands shown in this figure are well resolved because the vanadium oxide content in these glasses is very small (0.5 wtW). In our glass sample, since the vanadium oxide content is about a hundred times more, it is therefore
2.5-
(b)
550
650
750
850
Wavelength Fig. 3. Optical
absorption
spectra
of thin blown
film (a) before
and (b) after annealing.
E. E. Khowaja
er al. / Spectroscopy
of oanadiuln-germanare
glass
51
expected that the bands would not be resolved. It follows from the above that the increase in absorption in the sample upon annealing is indicative of increased V3+ content in the AN sample, The electrical conductivity of the annealed sample at room temperature was found to be about five orders of magnitude greater than that of the unannealed sample [3]. Formation of microstructure and the reduction of vanadium ions upon annealing the sample could both be responsible for the increase in the conductivity. It is well know that crystallization can increase the conductivity [15]. Furthermore, it has been well established that the conductivity of vanadium glasses (or in general glasses containing transition metal ions) depends upon the existence of vanadium in different valency states [1.14-161. It is accepted that these materials are electronic conductors in which the transport mechanism involves the exchange of electrons between lower and higher valency states (i.e.. V4+ + V5+, the other possible channel could be V3+ + V4’). The conductivities [21] of both single crystal and molten V,O, indicate that the compound is an oxygen-deficient semiconductor in which oxygen deficiency is compensated by reduction of some vanadium ions to lower oxidation states [22]. The observed increase in the reduced valence states together with microstructure formation could thus account for the increase in conductivity upon annealing the sample. It may not be easy to separate the contribution of each of the two effects. However, the optical absorption does suggest that the more dominant of these may be reduction, as discussed below. Two general regions of absorption (fig. 3) are of interest in the glasses. First, the high absorption region on the lower wavelength side which corresponds to interband transitions of electrons. Sanchez et al. [23] attributed the strong absorption to be arising from allowed charge transfer transitions from the 02d valence band to the empty V3p conduction band. The second region is the broad absorption tail that extends to longer wavelengths (say, from 550 nm to 900 nm). The effect of reduction of vanadium ions in this region would be an increase in the absorption (absorption bands corresponding to V3+ lie in this region). On the other hand, microstructure formation should reduce the long wavelength tail because there exist sharp edges in the density of states in the valence and the conduction bands in crystalline semiconductors, whereas in amorphous semiconductors some states are extended in the forbidden gap. Furthermore, Anderson and Compton, in their study of V,O,-P20, glasses [24], reported that the tail for samples of comparable thickness is much stronger than that observed in crystalline VZO,. The net effect, as is clear in fig. 3, is a large increase in the absorption tail. This may suggest that the more dominant of the two effects is the reduction of vanadium ions. 5. Conclusion The Auger and X-ray photoelectron spectroscopy study of 50 mol% vanadium germanate glass confirms the reduction of vanadium ions in the
52
E. E. Khawaja
er al. / Specrroscopy
of vanadium-germanare
glass
glass produced upon annealing the sample. It was concluded that the reduction of V 5+ was mostly to V 3c. Optical absorption results also supported this conclusion. Thus, the increase in the electrical conductivity of the annealed sample could be attributed to the increase of reduced valence states of vanadium ions which accompany the microstructure formation, and not solely to the structural change as was described by others. This work was supported by the KFUPM Research Committee through project #PHY/ARPES/71. The assistance of Mr. Sobingsobing for carrying out part of the experimental work is greatly appreciated.
References [l] C.H. Chung and J.D. Mackenzie. J. Non-Cryst. Solids 42 (1980) 357. [2] B.V.J. Rao. J. Amer. Geram. Sot. 48 (1965) 311. [3] M.N. Khan. E.E. Khawaja. D. Save. A.A. Kutub and C.A. Hogarth. Int. J. Electronics 56 (1984) 395. [4] E.E. Khawaja, F. Tegally, J.S. Hwang, A.S.W. Li and A.A. Kutub, J. Mat. Sci. 20 (1985) 3074. [5] J.B. Goodenough, Prog. Sol. St. Chem. 5 (1971) 45. (61 R. Dziembaj and J. Piwowarczyk, J. Sol. St. Chem. 21 (1977) 387. (71 J. Kasperkiewicz, J.A. Kovacich and D. Lichtman, J. Electron Spectr. Rel. Phen. 32 (1983) 123. [8] I.M. Curelaru, E. Suoninen and E. Minni, J. Chem. Phys. 78 (1983) 2262. [9] H.G. Tompkins, I.M. Curelaru, K.S. Din and E. Suoninen, Appl. Surface Sci. 21 (1985) 280. [lo] H. Flood and O.J. Kleppa. J. Am. Chem. Sot. 69 (1947) 998. [ll] V. Satava. Coil. Czech. Chem. Commun. 24 (1959) 3297. 1121 L. Fiermans et al., J. Microsc. Spectr. Electron 4 (1979) 543. [13] I.E. Wachs and S.S. Chen. Appl. Surface Sci. 20 (1984) 181. [14] I.G. Austin and N.F. Mott, Advan. Phys. 18 (1969) 41. [15] L. Murawski, C.H. Chung and J.D. Mackenzie, J. Non-Cryst. Solids 32 (1979) 91. [16] V.K. Dhawan. A. Mansingh and M. Sayer, J. Non-Cryst. Solids 51 (1982) 87. [17] Tompkins et al. [9] have assigned the peak near about kinetic energy of 470 eV to L,M,,M,, transitions which is in contradiction with the generally accepted assignment for this peak to L,MzsV transition [18]. Peaks corresponding to L,M2sM,, transition occur at much lower kinetic energy (at about 430 eV [18]). However, the conclusion of ref. [9] that the additional peak near 473 eV could be used as a fingerprint for phase identification is still valid. [18] D. Briggs and M.P. Seah, eds., Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy (Wiley, New York, 1983). [19] R.R. Joyce and P.L. Richards, Phys. Rev. 179 (1969) 375. [20] C.R. Bamford, Colour Generation and Control in Glass (Elsevier, Amsterdam, 1977) p. 52. [21] T. Allersima, R. Hakim, T.N. Kennedy and J.D. Mackenzie, J. Chem. Phys. 46 (1967) 154. [22] R. Dziembaj, J. Sol. St. Chem. 26 (1978) 159. [23] C. Sanchez, F. Babonneau, R. Morineac. J. Livage and J. Bullet, Phil. Mag. B47 (1983) 279. [24] G.D. Anderson and W.D. Compton, J. Chem. Phys. 52 (1970) 6166.
Solids
93 (1987)
45-52
45
X-RAY PHOTOELECTRON SPECTROSCOPY OF VANADIUM GERMANATE GLASS E.E. KHAWAJA,
Z. HUSSAIN,
Departntenr of Physics. Saudi Arabiu
King
Fahd
MS. JAZZAR
Universi!,~
Received 12 November 1986 Revised manuscript received 9 April
01 Petroleum
STUDY
and O.B. DABBOUSI & Minerals,
Dhohran
31261,
1987
Reduction of vanadium ions in 50 mol’% vanadium germanate glass under the influence of annealing the sample in air was studied by Auger and X-ray photoelectron spectroscopy. It was concluded that V5+ reduces mostly to V3+ and to a small extent to V4’. The same conclusion was drawn from optical absorption data.
1. Introduction For vanadium germanate glasses the electrical conductivity measured by Chung and Mackenzie [l] was much smaller than that measured by Rao [2]. Khan et al. [3] measured electrical conductivity of annealed and unannealed vanadium germanate glasses. These measurements clearly elaborated the discrepancies in the two results. The conductivity of annealed samples at room temperature was found to be about five orders of magnitude greater than that of unannealed samples containing a similar amount of vanadium oxide. The increase in conductivity on annealing was attributed to the change in microstructure of the glasses. This was also predicted by Chung and Mackenzie [l]. However, Khawaja et al. [4] reported a detailed study of 50 mol% vanadium germanate glasses using electron paramagnetic resonance, optical absorption, differential scanning calorimetry and electron diffraction techniques. They concluded that the increase in the electrical conductivity of the annealed samples could be attributed to the increase of reduced valence states of vanadium ions (V3+ and V4+) which accompany the microstructure formation and are not necessary to the structural change alone. In the present work, effect of annealing on the binding states of vanadium in 50 mol% vanadium germanate glass has been studied using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The object was to verify our earlier observations [4] that vanadium was reduced upon annealing the glass. Pure vanadium pentoxide (not mixed with other materials to form a glass) has long been studied extensively because of its interesting physical properties 0022-3093/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
46
E. E. Khawaja
er al. / Specrroscopy
01 uarladium-gen,lNnare
glass
[5-91. v,o,, which is known as a good catalyst, in fact, owes its catalytic properties to the presence of vanadium atoms in different oxidation states as a consequence of nonstoichiometry (oxygen deficiency) or even a phase mixture with lower oxide V,O,, [8]. Th e reduction of vanadium pentoxide to lower oxides is known to occur as a result of various treatments such as heating, participation in catalytic oxidation reaction [lo-131 and electron or ion bombardment [9]. However, little research has been devoted to glasses containing vanadium pentoxide. Amorphous transition metal oxides are known to exhibit semiconductivity properties arising from a hopping process of impaired electrons between transition metal ions in different valence states [1,14,15]. Several transition metal oxides when heated with glass-forming substances like P]O,, TeO,, GeO,, etc. form glasses on quenching the melt. The loss of oxygen from the melt produces, lower valency transition metal ions. Electrical conduction in glasses containing vanadium is suggested [1,14,16] to occur by electron hopping from an ion of low (V4’) to an ion of high valence state (V5+).
2. Experimental Glass was prepared by heating a 50 mol% mixture of analytical grade vanadium pentoxide and germanium dioxide in an alumina crucible. Prior to melting the mixture, the crucible was placed in a furnace maintained at 400 o C for 1 h in order to minimize material volatilization. The crucible was then transferred to a melting furnace maintained at a temperature of 1100 o C and left for 3 h with frequent stirring. The homogenized melts were then cast onto a steel plate mould (pre-heated to 300’ C). The samples to be annealed were transferred to an annealing furnace maintained at 300” C for 2 h and were than allowed to cool slowly. The glass discs were ground into a fine powder which was embedded in a substrate of metallic indium for XPS and AES measurement. Photoemission measurements were performed in a VG Scientific ESCALAB MKII system. The very high efficiency spectrometer is equipped with three detectors and adjustable inlet and outlet slits. The samples were loaded through a fast entry air lock into the preparation chamber and finally into the analysis vessel. A rack-and-pinion sample transfer mechanism allows exchange of samples between preparation and analysis vessel. These chambers are isolated from each other through a gate valve. The base pressures in the spectrometer and the preparation vessels were 6 x lo-” mbar and 2 x lo-‘O mbar, respectively. XPS study was performed using Mg Ka radiation at a photon energy of 1253.6 eV. The analyzer was operated in the constant resolution mode with a pass energy of 20 eV and an exit slit of 10 x 3 mm2. AES measurements were performed in the undifferential pulse counting mode by using Mg K~z X-rays as excitation source. The pass energy of the analyzer operated again in the constant resolution mode was 20 eV.
E.E. Khawaja
er al. / Specrroscopy
of vauadium-germanare
glass
47
In order to avoid any shift due to charging and to bring energies to the same reference point, all energies are stated with reference to C Is at a binding energy of 284.6 eV. This is a fortuitous peak which arises due to hydrocarbon contaminants in the vacuum and it is generally accepted to be independent of the chemical state of the sample under investigation. The powder samples were sparingly embedded in indium substrates to avoid unnecessary charging. As a result, the photoemission spectra have small peaks arising due to substrate indium, which could also be employed as an energy reference. Changing the reference to In3d 5,2 (BE = 443.6 eV) provided similar results.
3. Results
XPS spectra of unannealed (UN) and annealed (AN) specimensof the glass were taken under identical experimental conditions. The photoelectron peaks near V2p from UN and AN samples are compared in fig. 1. The positions of various peaks on the binding energy (BE) scale are listed in table 1. All energies are referenced to C 1s at. a binding energy of 284.6 eV. The decrease in binding energy of V2p on going from UN to AN samples is due to the reduction of vanadium ions. Kasperkiewiecz et al. [7] have studied the photoelectron spectra of pure VZOs and VO, and found that the V2p peak shifts to lower binding energy (- 0.3 eV) on going from VZO, to VOz. Wachs
2.20
-
I
1
,
1
I
x104 1.98 1.76
.
1.54
,.
520
525
Binding
Fig.
1. Photoelectron
spectra
excited
by Mg
energy
(eV)
Ka radiation samples.
for (a) unannealed
and (b) annealed
48 Table 1 Core-level samples
binding
E. E. Kltawaja
er al. / Specrroscop~
energies,
sepaiations
energy
Level
Unannealed (UN) BE (KE) (eV)
V2P,,2 “2Pl,Z 01s “U-3M2.3hs) V(L2M,,M,,)
517.3 524.5 530.7 786.3 (467.3) 826.6 (427.0)
* All peaks are referenced parentheses.
oJ oanadium-germanare
and Auger
lines for annealed
*
IO C Is at 284.6 eV. The kinetic
glass
Annealed BE (KE)
and unannealed
glass
(AN) (eV)
516.7 523.9 530.4 785.8 (467.8) 825.8 (427.8) energy
of Auger
peaks
is also given
in
and Chen [13] have found a shift of - 1.7 eV, for the same peak, to the lower energy on going from VzO, to Vz03. We looked for another method to confirm the reduction of vanadium ions upon annealing, and turned to Auger transitions excited by photons in the kinetic energy range of 400-540 eV. It has been pointed out by Tompkins et al. [9] that in pure and stoichiometric Vz05, the 3d-derived electronic levels are completely empty and lie above a rather large energy gap (2.5 eV). In the lower oxides such as VO,, V,O, and V,O,, these 3d states are partially occupied and become part of the valence band states. It is therefore expected that the contribution of the L,MljM,, and L,M,,M,, Auger transitions, that involve these valence 3d states, to the main L,M,, V Auger line, will be absent in V,O, and will appear upon the phase transition from V,O, to VO,, VzO, and/or V,O,,. Therefore Auger transitions that involve these 3d-derived states are fingerprints that can be used for phase identification. The spectra in fig. 2 compare Auger transitions from vanadium in the UN and AN samples. The spectra were taken using Mg KCX radiation under identical experimental conditions. It is observed that the Auger peaks shift to lower binding energy on going from UN to AN samples (table 1). In addition, there are significant differences in the fine structure of vanadium L,MzjV Auger transition occurring at a binding energy of about 786 eV, corresponding to a kinetic energy of about 468 eV [fig. 21. For UN samples the spectrum [fig. 2(a)] near L,M,Y transitions is dominated by one single peak at a kinetic energy of 466.9 eV [17,18]. The effect of annealing is seen to be the occurrence of an additional component (marked by an arrow in fig. 2) at 471.9 eV, together with the shift of the main peak to 467.4 eV. Tompkins et al. [9], in their study of reduction of pure V,O, to the lower oxide V,O,, under the influence of electron bombardment, observed the similar shift in energy and development of the new component. They interpreted this new structural feature as the contribution of the 3d-derived orbitals to L,M,Y Auger transitions through processes of L,M,,M,, type. Such transitions are absent in V,O, where the M,, states are empty, but become
E. E. Klmvuja 1.60
-
1.44
,.
1.28
‘.
I
et al. / Specrroscopy
I
of vanudium-germmare
,
glass
I
49 I
x 105
520
500
480 Kinetic
Fig. 2. Auger
electron
spectra
in the XPS mode
460 energy
440
420
400
(eV)
for (a) unannealed
and (b) annealed
samples
significant in the lower oxides, where MdS states are partially occupied and are therefore part of the valence band [9]. Following Tompkins et al. [9] we could say that vanadium ions in the glass were reduced upon annealing. The same conclusion was drawn above from the study of photoelectron spectra.
4. Discussion Khawaja et al. [4], in their recent study of 50 mol% vanadium germanate glass, concluded that the increase in electrical conductivity of the glass upon annealing could be attributed to an increase in V4+ and v-7+ content which accompanied the microstructure formation. The present results support their view, as is discussed below. The observed decrease in the binding energy of V2p on going from UN to AN samples is about 0.6 eV [table 11. Kasperkiewicz et al. [7] found a decrease of about 0.3 eV in going from pure V,O, to pure VOZ. Wachs and Chen [13] observed a shift to lower energy of about 1.7 eV on changing from pure VzO, to VzO,. In some of the earlier studies [1,16], carried out on vanadium in various glasses, it was assumed that V4+ was the only reduced state that existed in the glasses. However, in the present case even if all the Vs+ were reduced to V4+ upon annealing the sample, the observed shift of 0.6 eV is too large as compared with 0.3 eV, observed by Kasperkiewicz et al. [7] on going from pure
E. E. Khawaja
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Ed al. / Spectroscopy
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glass
VzO, to VOz. In the light of the above mentioned shifts it is plausible to say that Vsc in the UN sample were reduced to a lower oxidation state, probably V3+ after annealing. This conclusion is in agreement with a separate study on the same sample made by Khawaja et al. [4] using electron paramagnetic resonance (EPR). The C values, defined as the ratio of V4+ concentration to the total concentration of vanadium ions, were found to be 0.0085 and 0.015 for UN and AN samples, respectively. It may be mentioned that energy level separations of paramagnetic ions of the order of 10 cm-’ (8.25 cm-’ for V3+ in AlzO, [19]) in general cannot be readily observed with conventional paramagnetic resonance techniques. Since the V4’ content in the glass is small, we conclude that the reduction of V5+ is mostly to V3+. Further support to this comes from the optical absorption spectra, as discussed below. Figure 3 shows optical absorption spectra of the same composition as that of the glass studied in the present work as measured by Khawaja et al. [4] from thin blown film. These measurements were for the same sample, made before and after annealing. In another study [20] it has been reported that vanadium in various glasses can exist in three oxidation states, namely V’+, V4+ and V3+. The absorption band at 350 nm has been assigned [20] to V’+, the bands at 425 nm and 645 nm to V”, and the band at 1100 nm to V4+. The spectrum given in ref. [20] (fig. 2.8) indicates that absorption in the region from 400 to 900 nm has a major contribution from V3+ ions with possibly a small contribution from the tail of the band at 1100 nm, associated with V4+. The bands shown in this figure are well resolved because the vanadium oxide content in these glasses is very small (0.5 wtW). In our glass sample, since the vanadium oxide content is about a hundred times more, it is therefore
2.5-
(b)
550
650
750
850
Wavelength Fig. 3. Optical
absorption
spectra
of thin blown
film (a) before
and (b) after annealing.
E. E. Khowaja
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glass
51
expected that the bands would not be resolved. It follows from the above that the increase in absorption in the sample upon annealing is indicative of increased V3+ content in the AN sample, The electrical conductivity of the annealed sample at room temperature was found to be about five orders of magnitude greater than that of the unannealed sample [3]. Formation of microstructure and the reduction of vanadium ions upon annealing the sample could both be responsible for the increase in the conductivity. It is well know that crystallization can increase the conductivity [15]. Furthermore, it has been well established that the conductivity of vanadium glasses (or in general glasses containing transition metal ions) depends upon the existence of vanadium in different valency states [1.14-161. It is accepted that these materials are electronic conductors in which the transport mechanism involves the exchange of electrons between lower and higher valency states (i.e.. V4+ + V5+, the other possible channel could be V3+ + V4’). The conductivities [21] of both single crystal and molten V,O, indicate that the compound is an oxygen-deficient semiconductor in which oxygen deficiency is compensated by reduction of some vanadium ions to lower oxidation states [22]. The observed increase in the reduced valence states together with microstructure formation could thus account for the increase in conductivity upon annealing the sample. It may not be easy to separate the contribution of each of the two effects. However, the optical absorption does suggest that the more dominant of these may be reduction, as discussed below. Two general regions of absorption (fig. 3) are of interest in the glasses. First, the high absorption region on the lower wavelength side which corresponds to interband transitions of electrons. Sanchez et al. [23] attributed the strong absorption to be arising from allowed charge transfer transitions from the 02d valence band to the empty V3p conduction band. The second region is the broad absorption tail that extends to longer wavelengths (say, from 550 nm to 900 nm). The effect of reduction of vanadium ions in this region would be an increase in the absorption (absorption bands corresponding to V3+ lie in this region). On the other hand, microstructure formation should reduce the long wavelength tail because there exist sharp edges in the density of states in the valence and the conduction bands in crystalline semiconductors, whereas in amorphous semiconductors some states are extended in the forbidden gap. Furthermore, Anderson and Compton, in their study of V,O,-P20, glasses [24], reported that the tail for samples of comparable thickness is much stronger than that observed in crystalline VZO,. The net effect, as is clear in fig. 3, is a large increase in the absorption tail. This may suggest that the more dominant of the two effects is the reduction of vanadium ions. 5. Conclusion The Auger and X-ray photoelectron spectroscopy study of 50 mol% vanadium germanate glass confirms the reduction of vanadium ions in the
52
E. E. Khawaja
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glass produced upon annealing the sample. It was concluded that the reduction of V 5+ was mostly to V 3c. Optical absorption results also supported this conclusion. Thus, the increase in the electrical conductivity of the annealed sample could be attributed to the increase of reduced valence states of vanadium ions which accompany the microstructure formation, and not solely to the structural change as was described by others. This work was supported by the KFUPM Research Committee through project #PHY/ARPES/71. The assistance of Mr. Sobingsobing for carrying out part of the experimental work is greatly appreciated.
References [l] C.H. Chung and J.D. Mackenzie. J. Non-Cryst. Solids 42 (1980) 357. [2] B.V.J. Rao. J. Amer. Geram. Sot. 48 (1965) 311. [3] M.N. Khan. E.E. Khawaja. D. Save. A.A. Kutub and C.A. Hogarth. Int. J. Electronics 56 (1984) 395. [4] E.E. Khawaja, F. Tegally, J.S. Hwang, A.S.W. Li and A.A. Kutub, J. Mat. Sci. 20 (1985) 3074. [5] J.B. Goodenough, Prog. Sol. St. Chem. 5 (1971) 45. (61 R. Dziembaj and J. Piwowarczyk, J. Sol. St. Chem. 21 (1977) 387. (71 J. Kasperkiewicz, J.A. Kovacich and D. Lichtman, J. Electron Spectr. Rel. Phen. 32 (1983) 123. [8] I.M. Curelaru, E. Suoninen and E. Minni, J. Chem. Phys. 78 (1983) 2262. [9] H.G. Tompkins, I.M. Curelaru, K.S. Din and E. Suoninen, Appl. Surface Sci. 21 (1985) 280. [lo] H. Flood and O.J. Kleppa. J. Am. Chem. Sot. 69 (1947) 998. [ll] V. Satava. Coil. Czech. Chem. Commun. 24 (1959) 3297. 1121 L. Fiermans et al., J. Microsc. Spectr. Electron 4 (1979) 543. [13] I.E. Wachs and S.S. Chen. Appl. Surface Sci. 20 (1984) 181. [14] I.G. Austin and N.F. Mott, Advan. Phys. 18 (1969) 41. [15] L. Murawski, C.H. Chung and J.D. Mackenzie, J. Non-Cryst. Solids 32 (1979) 91. [16] V.K. Dhawan. A. Mansingh and M. Sayer, J. Non-Cryst. Solids 51 (1982) 87. [17] Tompkins et al. [9] have assigned the peak near about kinetic energy of 470 eV to L,M,,M,, transitions which is in contradiction with the generally accepted assignment for this peak to L,MzsV transition [18]. Peaks corresponding to L,M2sM,, transition occur at much lower kinetic energy (at about 430 eV [18]). However, the conclusion of ref. [9] that the additional peak near 473 eV could be used as a fingerprint for phase identification is still valid. [18] D. Briggs and M.P. Seah, eds., Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy (Wiley, New York, 1983). [19] R.R. Joyce and P.L. Richards, Phys. Rev. 179 (1969) 375. [20] C.R. Bamford, Colour Generation and Control in Glass (Elsevier, Amsterdam, 1977) p. 52. [21] T. Allersima, R. Hakim, T.N. Kennedy and J.D. Mackenzie, J. Chem. Phys. 46 (1967) 154. [22] R. Dziembaj, J. Sol. St. Chem. 26 (1978) 159. [23] C. Sanchez, F. Babonneau, R. Morineac. J. Livage and J. Bullet, Phil. Mag. B47 (1983) 279. [24] G.D. Anderson and W.D. Compton, J. Chem. Phys. 52 (1970) 6166.