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X-ray photoelectron spectroscopy (XPS) and magnetic susceptibility studies of copper–vanadium phosphate glasses
Journal of Non-Crystalline Solids 262 (2000) 66±79
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X-ray photoelectron spectroscopy (XPS) and magnetic susceptibility studies of copper±vanadium phosphate glasses G.D. Khattak a, M.A. Salim a, L.E. Wenger b,*, A.H. Gilani a b
a Department of Physics, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Department of Physics and Astronomy, Wayne State University, 666 W. Hancock Ave., Detroit, MI 48202, USA
Received 19 April 1999; received in revised form 8 November 1999
Abstract Vanadium phosphate glasses containing CuO with the chemical composition V2 O5 0:6ÿx P2 O5 0:4 CuOx , where x 0.0, 0.1, 0.2 and 0.3, have been prepared and investigated by X-ray photoelectron spectroscopy (XPS) and magnetization measurements. The core-level binding energies of O 1s, P 2p, P 2s, Cu 2p and V 2p have been measured with the P 2p and P 2s peaks shifting by about 2 eV towards lower binding energies in the CuO±vanadium phosphate glasses from their values in P2 O5 , while the V 2p peaks shift by 0.3 eV towards lower binding energies from V2 O5 and the Cu 2p peaks shift by 0.8 eV towards higher binding energies from CuO. These shifts can be accounted for by changes in the next-nearest neighbor environment around the P, Cu and V atoms and the reduction in the valence state of the Cu and V ions. The O 1s spectrum is ®tted to two peaks and the variation in the ratio of the peak areas is discussed in terms of the local structure as well. In addition, the Cu 2p3=2 and Cu 2p1=2 peak show doublet structures in the XPS spectra which are associated with the presence of both Cu and Cu2 in these glasses; although the XPS results indicate that more than 90% of the Cu ions exist in the Cu2 state. Likewise a multivalent state for the V ions is indicated by an asymmetry and broadening in the V 2p spectra as the CuO concentration in the glasses increases. This tendency is also observed in the magnetic susceptibility results as the ratios V5 =Vtotal determined from XPS are in qualitative agreement with those determined by using the susceptibility measurements combined with inductively coupled plasma (ICP) spectroscopy results. Furthermore the susceptibility data appear to follow a Curie±Weiss behavior v C= T ÿ h for temperatures above 40 K with negative Curie temperatures indicating that the predominate magnetic interactions between the Cu2 ±Cu2 and Cu2 ±V4 exchange pairs are antiferromagnetic in nature. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 61.43.-j; 75.20.CK
1. Introduction Oxide glasses containing transition metal (TM) ions have potential use in numerous technological
* Corresponding author. Tel.: +1-313 577 2720; fax: +1-313 577 3932. E-mail address: [email protected] (L.E. Wenger).
applications [1±5] owing to their semiconducting and magnetic properties. These properties are determined by the presence of multivalent states associated with the TM ions in the vitreous matrix. It is known that glasses containing a single TM oxide exhibit an electronic conductivity which arises from electron hopping between the TM ions in dierent valence states. When the glass contains two dierent TM oxides [6±10], the situation is
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 7 0 0 - 0
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
more complex because electron hopping between ions of two dierent metals can also occur [7,11± 13]. For example, studies of internal friction and XANES spectra of V2 O5 ±P2 O5 ±Fe2 O3 glasses [13] suggest the existence of V4 ! Fe3 transitions. Other considerations include reduction±oxidation (or ÔredoxÕ) interactions between the two transition metals, changes in the activation energy due to additional distortions of lattice sites, and any phase separation. In many of these previous studies [6±8,14], electron paramagnetic resonance (EPR) has been found to be a useful technique for examining the states of dissimilar TM ions in glasses as well as the interactions between unlike magnetic ions in disordered solids. In the present work we have extended the available information concerning interactions between unlike magnetic ions in vitreous matrices by using magnetic susceptibility measurements combined with inductively coupled plasma (ICP) spectroscopy and X-ray photoelectron spectroscopy (XPS) to determine the ratio of dierent valence states in vanadium phosphate glasses containing CuO and to identify the interactions between the V4 (3d1 ) and Cu2 (3d9 ) ions. It may be expected that electron exchange between V4 and Cu2 ions will lead to a decrease of V4 ion concentration and to the formation of V4 ±Cu2 and Cu2 ±Cu2 exchange pairs. Any deviation from a Curie±Weiss behavior v C= T ÿ h should indicate the presence of an interaction between the magnetic ions with the sign of the paramagnetic Curie temperature, h, being indicative of the type of interaction, i.e., ferromagnetic or antiferromagnetic. 2. Experimental procedure 2.1. Glass preparation The glasses were prepared by melting dry mixtures of reagent grade V2 O5 , P2 O5 and CuO in alumina crucibles with the batch composition V2 O5 0:6ÿx P2 O5 0:4 CuOx where x 0.00, 0.10, 0.20 and 0.30. Since the oxidation and reduction reactions in a glass melt are known to depend on the size of the melt, the sample geometry, whether
67
the melt is static or stirred, thermal history and quenching rate, all glass samples were prepared under similar conditions to minimize these factors [15]. Approximately 30 g of chemicals were thoroughly mixed to obtain homogenized batches. The crucible containing the batch mixture was placed in a furnace, heated at 300°C for an hour and then transferred to an electrically heated melting furnace maintained at temperatures ranging from 1100°C to 1200°C depending on the composition of the batch. The melt was left for about 3.5 h under atmospheric conditions in the furnace during which the melt was occasionally stirred with an alumina rod. The homogenized melt was then cast onto a stainless steel plate mold to form glass buttons and glass rods of approximately 5 mm diameter for XPS measurements. The batch and actual compositions of the glasses studied are listed in Table 1 with the actual compositions determined by ICP spectroscopy. 2.2. Magnetization measurements The temperature-dependent dc magnetic susceptibility was measured using a SQUID magnetometer in a magnetic ®eld of 10.0 Oe over a temperature range 5±300 K. The susceptibility of the sample holder is negligible below 100 K for all samples and less than a 5% correction at the highest temperature for samples with a total susceptibility less than 1 10ÿ8 emu/Oe. The overall accuracy of the susceptibility measurements is estimated to be approximately 3% due to the uncertainty of the magnetometer calibration. 2.3. X-ray photoelectron spectroscopy (XPS) High-resolution photoelectron spectra were collected on a spectrometer equipped with dual aluminum±magnesium anodes using Al Ka radiation hm 1486:6 eV as described elsewhere [16]. The electron analyzer was set at a bandpass energy of 20 eV with the energy scale of the spectrometer calibrated using the core level of Cu 2p3=2 (932.4 eV) and the energy separation between Cu 2p3=2 and Cu 2p1=2 (19.8 eV). The charging of nonconducting glass samples was avoided by ¯ooding the sample with a separate source of low-energy
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G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Table 1 Batch and actual composition of various vanadium phosphate glasses containing CuO Batch
Actual (from ICP)
P2 O5
V2 O5
CuO
P2 O5
V2 O5
CuO
0.400 0.400 0.400 0.400
0.600 0.500 0.400 0.300
0.000 0.100 0.200 0.300
0.3976 0.4062 0.4007 0.3868
0.6024 0.4939 0.4056 0.3157
0.0000 0.0998 0.1937 0.2975
electrons. The energy and intensity of these external electrons were adjusted to obtain the best resolution as judged by the narrowing of the fullwidth at half-maximum (FWHM) of the photoelectron peaks. At the optimum settings of the neutralizing gun (electron kinetic energy between 5 and 10 eV and electron emission current at the sample between 1 and 5 nA), the position of the adventitious C 1s line was within 0:5 of 284.6 eV. This peak arises due to hydrocarbon contamination and its binding energy is generally accepted as remaining constant, irrespective of the chemical state of the sample. For the sake of consistency, all energies are reported with respect to the C 1s transition at 284.6 eV. Glass rods of approximately 5 mm diameter were broken by vacuum cleavage inside the preparation chamber of the spectrometer in order to obtain fresh glass surfaces. The typical time required to collect an XPS spectrum for a sample was about 4 h and the base pressure in the analysis chamber during these measurements was less than 5 10ÿ11 mbar. 3. Results A typical wide-scan X-ray photoelectron spectrum for the sample V2 O5 0:4 P2 O5 0:4 CuO0:2 is shown in Fig. 1 with similar spectra being obtained for the other glass compositions. These low-resolution spectra were obtained in about 1 h using non-monochromatic Al Ka. Apart from the photoelectrons and Auger transitions of the glass constituents, the C 1s transition is evident. This feature at 284.6 eV is the peak usually associated with the hydrocarbon contamination, is almost always present on samples introduced from the laboratory environment or from a glove box [17] and is used as an energy reference. The peak po-
sitions and FWHM are listed in Table 2 for core levels Cu 2p, V 2p, P 2p, P 2s and O 1s. In Fig. 2, high-resolution spectra for the O 1s core level for the glasses are compared with the spectrum of P2 O5 powder which consists of two overlapping peaks. It is well-known that the orthorhombic P2 O5 is based on an assembly of discrete molecules consisting of PO4 tetrahedra shared by three neighboring tetrahedra while the other oxygen atom is double-bonded to a phosphorus atom [18]. From the charge density on the non-bridging oxygen atom being about )0.96 and on the bridging oxygen atom )0.65, a shift of the non-bridging atom peak towards a lower binding energy with respect to the bridging-oxygen atom peak is expected [19]. Therefore, the lower binding energy peak O 1s (1) at 531.7 eV is associated with the non-bridging oxygen (P¸O bonds) and the higher energy peak O 1s (2) at 533.5 eV with the bridging oxygen (P±O±P bonds) for the P2 O5 powder. In addition, the ratio of the areas under the P¸O and P±O±P peaks should be 1:1.5 as there should be one non-bridging oxygen atom and 1.5 bridging oxygen atom for every phosphorus atom. A quick visual inspection of the P2 O5 spectrum would suggest that the ratio of the two peaks is in reasonable agreement with this prediction. In comparison to P2 O5 , the O 1s spectra for all the glasses show a sizeable increase in the relative intensity of the lower binding energy peak as compared to the higher binding energy peak, but essentially the binding energies, i.e., the peak positions of the O 1s peaks, remain unchanged. This change in relative intensities of the two peaks indicates an increase in the number of non-bridging bonds in the glasses. Furthermore, each spectrum appears to consist of essentially a single, broad peak whose width (FWHM) varies from 1.70 for
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
69
Fig. 1. A low-resolution XPS spectrum from a fractured surface of a CuO±vanadium phosphate glass with x 0.2 obtained using Al Ka radiation (hm 1486.6 eV).
x 0.00 to 2.49 for x 0.30. (See Table 2 for peak positions and FWHM.) Fig. 3 shows the core level spectra of Cu 2p for the glasses and CuO powder with similar spectra for vanadium shown in Fig. 4. The peak positions for these transition metal oxides are in good agreement with those reported in the literature [16,20]. However, the Cu 2p peaks for glasses shift by 0.8 eV towards higher energies from its value for CuO while the V 2p peaks for the glasses shift by 0.3 eV towards lower energies from its value for V2 O5 . Furthermore, the Cu 2p spectra for the glasses exhibit spin-orbit components, Cu 2p3=2 and Cu 2p1=2 , at binding energies of 933.5 eV and 953.7 eV, respectively, with satellites at about 10 eV higher binding energies. Moreover, the Cu 2p1=2 peaks show a doublet structure marked A and B in Fig. 3 with a separation between the A and B of about 3 eV. These doublets can be
associated with monovalent and divalent Cu ions as these are the only two oxidation states in which Cu exists in various glasses [21]. It is well-known that copper compounds containing Cu2 have strong satellites as one can see in the CuO spectra of Fig. 3 (labeled as Sat), while compounds with just Cu have no satellites [22±25]. A decrease in the intensity of the satellite peak is observed upon going from CuO to the CuO±vanadium phosphate glasses, which indicates that some Cu2 ions in the CuO are reduced to Cu in the glass. Moreover this tendency in the satellite structure of Fig. 3 is also re¯ected in the trend of peak B while the intensity of peak A grows with increasing copper concentration in the glasses. Thus, it may be concluded that the peaks A and B are associated with Cu and Cu2 , respectively. Assuming the Cu 2p spectrum is composed of two overlapping peaks, each component peak was
± 953.64 953.98 953.75 952.96 ± ± (2.70) (4.83) (4.86) (3.13)
Cu 2p1=2 (FWHM) ± 19.80 20.12 19.77 19.84 ± ±
DCu 2p 0:2 eV 517.16 516.95 516.88 516.90 ± ± 517.38 (1.35)
(2.15) (2.15) (2.20) (2.20)
V 2p3=2 (FWHM) 7.40 7.45 7.45 7.35 ± ± 7.50
524.56 524.40 524.33 524.25 ± ± 524.88 (2.05)
(2.25) (2.40) (2.55) (2.60)
DV 2p 0:2 eV
V 2p1=2 (FWHM) 133.61 133.45 133.38 133.38 ± 135.37 ±
P 2p
(2.00)
(1.80) (2.00) (2.10) (2.10)
190.99 190.79 190.86 191.00 ± 192.87 ±
P 2s
(2.25)
(2.13) (2.25) (2.26) (2.20)
530.91 (1.70) 530.75 (2.10) 530.98(1.95) 531.45 (2.49) ± 533.53 (3.40) ±
O 1s (FWHM)
DCu 2p and DV 2p are the peak energy separations Cu 2p3=2 ±Cu 2p1=2 and V 2p3=2 ±V 2p1=2 , respectively. The uncertainity in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
a
± 933.84 933.86 933.98 933.12 ± ±
0.0 0.1 0.2 0.3 CuO P2 O5 V2 O5
(2.68) (4.32) (4.46) (2.92)
Cu 2p3=2 (FWHM)
x
Table 2 Peak positions (in eV) relative to C 1s (284.6 eV) and their corresponding FWHM given in the parentheses for the core levels Cu 2p, V 2p, P 2p, P 2s and O 1sa
70 G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Fig. 2. High-resolution O 1s spectra for the CuO±vanadium phosphate glasses and P2 O5 powder.
®tted to a sum of weighted Lorentzian±Gaussian peaks with a linear sloping background by means of a least squares ®tting program [26]. A best ®t to the experimental data was found by varying the peak position, width and intensity of each of the two component peaks. An example of the resulting ®t is shown in Fig. 5 for x 0.3. Using these peak areas, the ratio of Cu2 ions was calculated as follows: Cu / (area under peak A) AA ; Cu2 / (area under peak B + area under satellite peak) AB ; Thus, C Cu2 = Cu Cu2 AB = AA AB , with the calculated results displayed in Table 3. The validity of the above relation was checked by taking XPS spectra for the Cu2 O and CuO powders and then a mixture of the two. For a mixture which contained an equal concentration
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Fig. 3. High-resolution Cu 2p spectra for the CuO±vanadium phosphate glasses and CuO powder.
of Cu and Cu2 , a value of C of 0.55 was obtained instead of 0.50. The commercial Cu2 O (reagent grade) used in the present work was supplied by BDH Laboratory and the manufacturer claims that the powder contains a minimum of 94% Cu2 O with less than 5% CuO. Also, the presence of a weak satellite associated with the core level Cu 2p in the XPS spectrum of the Cu2 O, con®rms the presence of CuO. It may be noted that in highpurity Cu2 O, no satellite peaks have been observed [22±25]. Therefore, the higher value of C is partly due to CuO in the Cu2 O powder and partly due to the uncertainty in the XPS measurements and analysis technique. Thus, the above procedure should yield values of C with an uncertainty of less than 10%. Since vanadium can also exist in two oxidation states V5 and V4 [21], a double peak structure
71
Fig. 4. High-resolution V 2p spectra for the CuO±vanadium phosphate glasses and V2 O5 powder.
Fig. 5. High resolution Cu 2p3=2 spectra (points) for a CuO±vanadium phosphate glass (x 0.3) and the resulting peaks from a least-square curve ®tting (solid lines).
might be expected in the V 2p spectra as well. Although distinct peaks for these states are not observed in Fig. 4, the presence of an asymmetry
72
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Table 3 Peak positions (in eV) from the curve ®tting of the Cu 2p3=2 peak and the ratio C Cu2 = Cu Cu2 AB = AA AB a x
Cu 2p3=2 (A) FWHM Peak Area
Cu 2p3=2 (B) FWHM Peak Area
Cu 2p3=2 (Sat) FWHM Peak Area
Cu2 =Cutotal 0:03
0.1
931.54 1.38 134.672 931.76 1.56 869.169 931.98 1.98 1489.19
934.72 3.96 2307.96 934.78 3.71 7265.58 934.91 3.52 9322.42
943.82 4.85 1230.40 942.85 5.11 3099.43 942.36 5.21 4823.85
0.9633
0.2 0.3
0.9226 0.9048
a The FWHM and the area are given by the two lower sets of numbers, respectively. The uncertainity in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
and small broadening in the V 2p spectra suggest the existence of some non-equivalent vanadium atoms. Hence, each V 2p3=2 spectrum was similarly ®tted to two Lorentzian±Gaussian peaks with the lower binding energy and the higher binding energy peaks corresponding to V4 and V5 , respectively [26]. The resulting ratios of V5 =Vtotal are given in Table 4. A similar ®tting procedure was carried out for the V 3p and V 2s spectra and the results are presented in Tables 5 and 6, respectively. There is qualitative agreement among the various determinations that a decrease in the V5 =Vtotal ratio occurs with increasing CuO content. The variation in the binding energies of the P 2p and P 2s electrons have also been examined. Figs. 6 and 7 compare high-resolution XPS spectra of P 2p and P 2s core levels for the glasses to those for P2 O5 powder. Each spectrum appears to consist of a single peak whose width remains essentially unchanged; however, both the P 2p and P 2s peaks for the glasses shift by about 2 eV to lower energies as presented in Table 2. The magnetic susceptibility results are shown in Figs. 8 and 9 as plots of the inverse mass magnetic susceptibility, vÿ1 , for the glasses as a function of temperature, T. The susceptibility data appear to follow a Curie±Weiss behavior v C= T ÿ h over most of the temperature range and the deviation from this behavior below 40 K is probably due to interactions between the magnetic ions. From the slope and intercept along the temperature axis, Curie constants, Cexpt , (see Table 7) and
paramagnetic Curie temperatures h of 1.6, )4.8, )12.3 and )24.5 K are determined for samples with x 0.0, 0.1, 0.2 and 0.3, respectively. The presence of two types of TM ions makes it dicult to determine the concentration of these magnetic species from the susceptibility data accurately. However, we have attempted to ®nd the ratio of V5 =Vtotal and the molar fraction of each constituent by using the susceptibility measurements combined with ICP spectroscopy data as explained in Appendix A. These results are also shown in Table 7. 4. Discussion As mentioned in the results section, a shift of 2 eV in the binding energies of the O 1s peaks is observed in the glasses as compared to that of P2 O5 while these O 1s peaks remain unaected by varying CuO content. Furthermore, the O 1s spectra for all the glasses show a sizeable increase in the relative intensity of lower binding energy peak as compared to the higher binding energy peak from that for P2 O5 . This change in relative intensities of the two peaks indicates an increase in the number of non-bridging bonds in the glasses. In order to quantify this change in relative intensities, each O 1s spectrum was also ®tted to two Lorentzian±Gaussian peaks with a linear slopping background by means of a least square ®t [26]. The ratio of the integrated areas of these peaks A 2 =A 1 , as summarized in Table 8, represents the
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
73
Table 4 Peak positions (in eV) from the curve ®tting of the V 2p3=2 peak and the ratio V5 =Vtotal a DV 2p3=2 (0:2 eV)
V5 /Vtotal ( 0.03)
517.35 1.68 4192.14
1.11
0.7546
516.04 1.44 1768.99
517.16 1.80 4468.29
1.12
0.7164
516.88 2.20
516.60 2.05 8447.48
517.40 2.93 14510.30
0.80
0.6320
516.90 2.20
516.83 1.95 4951.48
517.30 3.62 7008.55
0.47
0.5860
x
V 2p3=2 FWHM
V 2p3=2 (1) FWHM Peak Area
0.0
517.16 2.15
516.24 1.19 1363.21
0.1
516.95 2.15
0.2
0.3
V 2p3=2 (2) FWHM Peak Area
a
The FWHM and the area are given by the two lower sets of numbers, respectively. The uncertainty in the peak positions is 0:1 eV and in FWHM is 0:2 eV. Table 5 Peak positions (in eV) from the curve ®tting of the V 3p3=2 peak and the ratio V5 =Vtotal a x
V 3p FWHM
0.0
42.41 4.00
0.1
42.05 3.90
40.60 1.72 115.289
0.2
41.42 4.45
40.63 2.51 272.678
0.3
4 1.37 4.25
40.65 2.97 163.811
DV 3p (0:2 eV)
V5 =Vtotal 0:03
42.55 3.80 943.992
1.98
0.9298
42.44 3.49 797.474
1.84
0.8737
1.70
0.8141
1.57
0.7572
V 3p (1) FWHM Peak Area
V 3p (2) FWHM Peak Area
40.57 1.72 71.3081
42.33 4.82 1194.21 42.22 4.79 510.973
a The FWHM and the area are given by the two lower sets of numbers, respectively. The uncertainty in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
relative concentrations of the bridging oxygen atoms (P±O±P) to other oxygen bonds (P¸O, P±O±V, P±O±Cu, Cu±O±Cu and Cu±O±V etc.). This analysis indicates that about 77% of the oxygen atoms involve bridging P±O±P bonds for these CuO±vanadium phosphate glasses independent of the copper concentration as the ratio A 2 =A 1 remains constant within the uncertainity ?tul> of these determinations. Since the FWHM
of the O 1s peak for the glasses depends on the glass composition and ranges from 1.70 eV for x 0.0 to 2.49 eV for x 0.3, the electronic density of the valance shell on the non-bridging atoms must be dierent from that on the bridging atoms. The 2 eV shifts to lower binding energies of P 2p and P 2s electrons for the glasses probably arise from a dierence in the molecular environment
74
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Table 6 Peak positions (in eV) from the curve ®tting of the V 2s peak and the ratio V5 =Vtotal a x
V 2s (1) FWHM Peak Area
V 2s (2) FWHM Peak Area
DV 2s (0:2 eV)
V5 =Vtotal 0:03
0.0
629.95 3.63 405.989
631.66 5.91 1753.65
1.71
0.8120
0.1
630.49 3.87 680.535
631.64 7.49 1761.51
1.15
0.7213
631.62 5.72 4608.96
1.50
0.6594
2.11
0.5374
0.2
0.3
630.12 4.12 2380.49 629.49 4.95 661.169
631.60 5.82 768.137
a The FWHM and the area are given by the two lower sets of numbers, respectively. The uncertainty in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
Fig. 6. High-resolution P 2p spectra for the CuO±vanadium phosphate glasses and P2 O5 powder.
Fig. 7. High-resolution P 2s spectra for the CuO±vanadium phosphate glasses and P2 O5 powder.
132.91 134.45 a
2.672 2.828
Note: Upper set of numbers based on batch analysis of composition. Lower set of numbers based on ICP analysis of composition.
0.410 0.403 0.295 0.281 0.151 0.162 0.49 0.48 1.336 1.257
1:923 10ÿ3
0.144 0.153
142.89 142.24 0.411 0.379 0.196 0.217 0.156 0.143 0.60 0.64 1:506 10ÿ3
0.237 0.261
152.75 151.87 0.411 0.448 0.098 0.092 0.168 0.170 0.322 0.290
163.18 0.410 ± 0.148 0.442
MWs (g/mol) P2 O5
0.3
surrounding the P atom between the P2 O5 and CuO±vanadium phosphate glass structures as one ®nds the next-nearest neighbors in P2 O5 are all P atoms which are replaced by V or Cu atoms in the glass structures. Since the Pauling electronegativities of V and Cu are lower than that of P ( 2.1), V and Cu have less of an anity for electrons than P. Thus the electron density at the P atom increases with increased V/Cu content which leads to a decrease in P 2p and P 2s binding energies in these highly concentrated vanadium phosphate glasses.
0.3
Fig. 9. The inverse of the mass magnetic susceptibility a function of temperature for CuO±vanadium phosphate glasses at the lowest temperatures. The solid lines represent Curie± Weiss ®ts to the high temperature data.
0.828 0.920
100
3.311 3.420
80
0.2
60
T (K)
0.4
40
0.66 0.63
20
1:182 10ÿ3
0
0.387 0.365
x = 0.30
0
3.869 3.647
x = 0.20
0.1
x = 0.10
0.5
x = 0.00
0.75
50000
7:354 10ÿ4
χ-1 (g Oe / emu)
100000
±
( V2 O5 ) 0.60 - x . (CuO)x . ( P2 O5 )0.40
4.356
150000
±
Fig. 8. The inverse of the mass magnetic susceptibility as a function of temperature for CuO±vanadium phosphate glasses in a magnetic ®eld of 10.0 Oe.
0.6
300
CuO
200
T (K)
V2 O4
100
Molar fraction ICP and susceptibility
0
V2 O5
x = 0.30
0
CuO
x = 0.20
V2 O5
x = 0.10
V5 =Vtotal
x = 0.00
100000
Cexpt (emu K/g Oe)
200000
NCu (1021 Cu ions/g)
χ-1 (g Oe / emu)
300000
NV (1021 V ions/g)
( V2 O5 ) 0.60 - x . (CuO)x . ( P2 O5 )0.40
75
Molar fraction batch
400000
Table 7 Determinations of V5 =Vtotal and molar fraction of V2 O5 , V2 O4 , CuO and P2 O5 from susceptibility results combined with ICP data for CuO±vanadium phosphate glassesa
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G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Table 8 Comparison of the binding energies from the O 1s peaks (in eV) and the corresponding ratio A 2 /A 1 of the area of the O 1s (2) peak to the area of the O 1s (1) peaka
a
x
O 1s (p) FWHM Peak Area
0.0
530.82 1.84 11453.50
0.1
O 2s (p) FWHM Peak Area
DO 1s (0:2 eV)
A 2 =A 1 0:3 eV)
531.86 2.93 8771.88
1.04
0.7659
530.55 2.06 14626.00
531.68 2.86 11411.40
1.13
0.7802
0.2
530.84 2.14 40167.80
531.78 2.95 30959.6
0.94
0.7708
0.3
530.94 2.10 24618.90
531.90 3.11 18537.90
0.96
0.7530
P2 O5
531.88 2.06 38469.3
533.59 2.22 58479.9
1.71
1.5202
The uncertainity in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
The shifts in binding energies for the 2p3=2 and 2p1=2 core levels of V (0.3 eV decrease) and Cu (0.8 eV increase) in the glasses result not only from a change in the molecular environment from V2 O5 and CuO but also from a reduction in the formal oxidation state of the ions [27]. Utilizing an explanation similar to the one given in the preceding paragraph, both V and Cu atoms are surrounded by an increasing number of P atoms and a decreasing number of V or Cu atoms at the nextnearest neighbor positions as the P2 O5 content is increased. Thus the electron density of the V and Cu atoms would decrease while their 2p binding energies should increase. However, binding energies typically decrease if the oxidation state of the transition metal ion is reduced to a lower valence state, e.g., V5 ! V4 and Cu2 ! Cu . Thus the change in molecular environment must be the more dominate mechanism responsible for the increases in the binding energies of the Cu 2p levels while the reduction of the oxidation state of V ions is more responsible for the shifts in the V 2p levels. Another feature of the Cu 2p spectra is that the energy separation between the 2p3=2 and 2p1=2
peaks remains essentially unchanged in going from CuO to the CuO±vanadium phosphate glass system. It has been previously shown that the change in the 2p3=2 ±2p1=2 separation for TM ions in oxide glasses can occur for dierent spin states of the ions [28]. The lack of any signi®cant change in the 2p3=2 ±2p1=2 separations in the present investigation for both the CuO±vanadium phosphate glasses and CuO powder suggests that the Cu ions are predominantly in one oxidation state, probably Cu2 , with only small amounts of the other valence state (Cu ) being present. As seen from Table 3, more than 90% of Cu exists as Cu2 in the glasses in agreement with our previous investigations [20]. The V spectra also revealed some broadening and asymmetry which suggests the existence of more than a single valence state for the V ion. As displayed in Tables 4±6, the ratio of V5 =Vtotal determined from the curve ®tting of various V spectra decreases with increasing copper concentration in the vanadium phosphate glasses. This tendency is also observed from the analysis of the magnetic susceptibility data in combination with
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
the ICP results as described in Appendix A and presented in Table 7. The decrease in V5 with CuO content may arise from similar eects as previously found from EPR investigations on the V2 O5 0:55ÿx TeO2 0:45 CuOx glass system [10]. This study revealed that (i) the addition of a small amount (0.5 mol%) of CuO had the eect of oxidizing V4 to V5 as revealed by the absence V4 resonance, (ii) large concentrations of CuO produced exchange eects of the antiferromagnetic type and a g-shift in the Cu2 resonance and (iii) progressive delocalization of V4 unpaired electrons led to a hopping-type conduction. Likewise, EPR data on CuO 2V2 O5 x 2B2 O3 K2 O3 1ÿx glasses with 0 6 x 6 0:40 [4] gave evidence for the presence of magnetic interactions between the TM ions. Our magnetic susceptibility data also support the conclusion of the aforementioned EPR studies, i.e., the existence of exchange interactions. Since the sign of the paramagnetic Curie temperature is indicative of the type of magnetic interaction, the negative sign implies that the ion±ion interaction is predominately antiferromagnetic in the CuO±vanadium phosphate glasses while the interaction is weakly ferromagnetic in nature for the vanadium phosphate glass (x 0). Also since the Curie temperature is a rough measure of the strength of the interaction between the magnetic ions in the sample, a higher value implies a stronger interaction and/or more ions participating in the interaction. From the experimental values of the Curie temperature increasing with increasing copper content in the glass, it can be concluded that copper is behaving paramagnetically in the host glass network and that the ion±ion exchange interaction is antiferromagnetic and gets stronger with increasing copper content. 5. Conclusions The XPS spectra for O 1s, P 2p, P 2s, Cu 2p and V 2p core levels of the copper±vanadium phosphate glass network have been studied. The binding energies of both P 2p and P 2s for these glasses are found to decrease by 2 eV in comparison to those for pure P2 O5 powder as a result of changes
77
in the next-nearest neighbor environment between the powder and glass structures giving rise to an increase in the electron density at the P atom site. While the binding energies of the V 2p decrease by 0.3 eV from its value in V2 O5 , the Cu 2p level increases by 0.8 eV from its value in CuO. These energy level shifts are explained in terms of the shift resulting from the change in the next-nearest neighbor environment dominating over the shift arising from a reduction in the formal oxidation state of the ions for the Cu sites and vice versa for the V sites. The appearance of multivalent states for both Cu and V ions is con®rmed by other features in the XPS spectra, the observation of a doublet structure in the Cu 2p3=2 and Cu 2p1=2 peak spectra and an asymmetry and broadening in the V 2p spectra as the CuO concentration in the glasses increases. This tendency is also observed in the magnetic susceptibility results as the V5 =Vtotal ratios determined from XPS are in qualitative agreement with those found from susceptibility measurements combined with ICP results. Furthermore the Curie±Weiss behavior, v C= T ÿ h, above 40 K with negative Curie temperatures indicates that the predominate magnetic interactions between the Cu2 ±Cu2 and Cu2 ±V4 exchange pairs are antiferromagnetic in nature. Thus the properties of the vanadium phosphate glasses when doped with CuO are signi®cantly aected by a reduction in the oxidation state of the V ions as well as by the local molecular environment. Acknowledgements The support of the KFUPM Physics Department and Research Committee (Grant PH/ PHYSPROP/43) and the WSU Institute for Manufacturing Research is greatly acknowledged. The assistance of Mr M.A. Khan for experimental work is appreciated. Appendix A To determine the V4 =Vtotal ratio from magnetic susceptibility and ICP data, we will de®ne
78
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
V4 =Vtotal V5 =Vtotal NV NCu Cexpt
ratio of number of V4 ions to total number of V ions ratio of number of V5 ions to total number of V ions 1 ÿ V4 =Vtotal number of vanadium ions/gram of sample from ICP number of copper ions/gram of sample from ICP Curie constant (emu: K/Oe: gram) from v-vs-T results.
Assuming Cu only exists in Cu2 state peff 1:9 lB and V is in either the V5 peff 0 or V4 peff 1:8 lB states, the Curie constant is given by 4 2 peff Cu2 V Cexpt NCu NV 3kB Vtotal
4
2
peff V : 3kB
A:1
Thus the V4 =Vtotal ratio can be determined from Eq. (1) by using the experimental determined values for Cexpt , NV and NCu . Using the resulting value of the V4 =Vtotal ratio, the molar fraction of V2 O5 , V2 O4 , P2 O5 and CuO as well as the molecular weight MWs of the glass sample can be determined. Recall that one mole of these CuO±vanadium phosphate glasses will have Avogadro number, NA , of molecules 1 V4 NA NV MWV2 O4 2 Vtotal 1 V4 NV 1 ÿ MWV2 O5 Vtotal 2 1 NCu MWCuO NP MWP2 O5 ; A:2 2 where NP is the number of P ions per gram and is determined from Eq. (A.2). Then 4 1 V NV MWs NA 2 Vtotal 4 1 V 1 NCu NP ; NV 1 ÿ Vtotal 2 2 A:3
and the molar fraction of each constituent will be 1 V4 MWS NV2 O4 NV NA 2 Vtotal NV2 O5
1 2
NCuO NCu
1 ÿ V4 MWS NV Vtotal NA MWS NA
1 MWS NP2 O5 NP : NA 2
References [1] J.D. Mackenzie, in: Modern Aspects in the Vitreous State, vol. 3, Butterworth, London, 1964. [2] N.F. Mott, J. Non-Cryst. Solids 1 (1964) 1. [3] D. Adler, in: Amorphous Semiconductors, CRC, Cleveland, OH, 1971. [4] I.G. Austine, E.J. Garbett, in: P.G. LeComber, J. Mart (Eds.), Electronic and Structural Properties of Amorphous Semiconductors, Academic Press, London, 1973. [5] L. Murawski, C.H. Chung, J.D. Mackenzie, J. Non-Cryst. Solids 32 (1979) 91. [6] L.D. Bogomolova, J. Non-Cryst. Solids 30 (1979) 379. [7] L.D. Bogomolova, M.P. Glassova, J. Non-Cryst. Solids 37 (1980) 423. [8] L.D. Bogomolova, M.P. Glassova, O.E. Dubatovko, S.I. Reiman, S.N. Spasibkina, J. Non-Cryst. Solids 58 (1983) 71. [9] I. Ardelean, O. Cozar, Gh. Ilonca, J. Non-Cryst. Solids 68 (1984) 33. [10] C.S. Sunandana, K.S. Rao, Phys. Stat. Solid A 90 (1985) 681. [11] A.K. Bandyopadhyay, J.O. Isard, J. Phys. D 10 (1977) L99. [12] W. Chomka, D. Samatowicz, O. Gzowski, L. Murawski, J. Non-Cryst. Solids 45 (1981) 145. [13] O. Gzowski, L. Murawski, W. Lizak, H. Binczycka, J. Sawicki, J. Phys. D 14 (1981) L77. [14] B.-S. Bae, M.C. Weinberg, J. Am. Ceram. Soc. 74 (1991) 3039. [15] E. Culear, J. Non-Cryst. Solids 223 (1998) 47. [16] E.E. Khawaja, Z. Hussain, M.S. Jazzar, O.B. Dabbousi, J. Non-Cryst. Solids 93 (1987) 45. [17] G.E. Mullenberg (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin±Elmer, Norwalk, CT, 1970. [18] M. Jansen, B. L uer, Z. Kristallogr. 177 (1986) 149. [19] B.M.J. Smets, D.M. Krol, Phys. Chem. Glasses 25 (1984) 113.
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79 [20] G.D. Khattak, E.E. Kawaja, L.E. Wenger, D.J. Thompson, M.A. Salim, A.B. Hallak, M.A. Daous, J. Non-Cryst. Solids 194 (1996) 1. [21] C.R. Bamford, in: Color Generation and Control in Glass, Elsevier, Amsterdam, 1977. [22] G.A. Vernon, G. Stucky, T.A. Carlson, Inorg. Chem. 15 (1976) 278. [23] K.S. Kim, J. Electron, Spectrosc. Rel. Phenomen. 3 (1974) 217.
79
[24] S. Larsson, M. Bragga, Chem. Phys. Lett. 28 (1977) 596. [25] G. Van der Laan, C. Westra, C. Haas, G.A. Smatzky, Phys. Rev. B 23 (1981) 4369. [26] A. Proctor, P.M.A. Sherwood, Anal. Chem. 52 (1980) 2315. [27] D. Briggs, J.C. Riviere, in: D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester, 1988, p. 119. [28] D. Briggs, V.A. Gibson, Chem. Phys. Lett. 25 (1974) 493.
www.elsevier.com/locate/jnoncrysol
X-ray photoelectron spectroscopy (XPS) and magnetic susceptibility studies of copper±vanadium phosphate glasses G.D. Khattak a, M.A. Salim a, L.E. Wenger b,*, A.H. Gilani a b
a Department of Physics, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Department of Physics and Astronomy, Wayne State University, 666 W. Hancock Ave., Detroit, MI 48202, USA
Received 19 April 1999; received in revised form 8 November 1999
Abstract Vanadium phosphate glasses containing CuO with the chemical composition V2 O5 0:6ÿx P2 O5 0:4 CuOx , where x 0.0, 0.1, 0.2 and 0.3, have been prepared and investigated by X-ray photoelectron spectroscopy (XPS) and magnetization measurements. The core-level binding energies of O 1s, P 2p, P 2s, Cu 2p and V 2p have been measured with the P 2p and P 2s peaks shifting by about 2 eV towards lower binding energies in the CuO±vanadium phosphate glasses from their values in P2 O5 , while the V 2p peaks shift by 0.3 eV towards lower binding energies from V2 O5 and the Cu 2p peaks shift by 0.8 eV towards higher binding energies from CuO. These shifts can be accounted for by changes in the next-nearest neighbor environment around the P, Cu and V atoms and the reduction in the valence state of the Cu and V ions. The O 1s spectrum is ®tted to two peaks and the variation in the ratio of the peak areas is discussed in terms of the local structure as well. In addition, the Cu 2p3=2 and Cu 2p1=2 peak show doublet structures in the XPS spectra which are associated with the presence of both Cu and Cu2 in these glasses; although the XPS results indicate that more than 90% of the Cu ions exist in the Cu2 state. Likewise a multivalent state for the V ions is indicated by an asymmetry and broadening in the V 2p spectra as the CuO concentration in the glasses increases. This tendency is also observed in the magnetic susceptibility results as the ratios V5 =Vtotal determined from XPS are in qualitative agreement with those determined by using the susceptibility measurements combined with inductively coupled plasma (ICP) spectroscopy results. Furthermore the susceptibility data appear to follow a Curie±Weiss behavior v C= T ÿ h for temperatures above 40 K with negative Curie temperatures indicating that the predominate magnetic interactions between the Cu2 ±Cu2 and Cu2 ±V4 exchange pairs are antiferromagnetic in nature. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 61.43.-j; 75.20.CK
1. Introduction Oxide glasses containing transition metal (TM) ions have potential use in numerous technological
* Corresponding author. Tel.: +1-313 577 2720; fax: +1-313 577 3932. E-mail address: [email protected] (L.E. Wenger).
applications [1±5] owing to their semiconducting and magnetic properties. These properties are determined by the presence of multivalent states associated with the TM ions in the vitreous matrix. It is known that glasses containing a single TM oxide exhibit an electronic conductivity which arises from electron hopping between the TM ions in dierent valence states. When the glass contains two dierent TM oxides [6±10], the situation is
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 7 0 0 - 0
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
more complex because electron hopping between ions of two dierent metals can also occur [7,11± 13]. For example, studies of internal friction and XANES spectra of V2 O5 ±P2 O5 ±Fe2 O3 glasses [13] suggest the existence of V4 ! Fe3 transitions. Other considerations include reduction±oxidation (or ÔredoxÕ) interactions between the two transition metals, changes in the activation energy due to additional distortions of lattice sites, and any phase separation. In many of these previous studies [6±8,14], electron paramagnetic resonance (EPR) has been found to be a useful technique for examining the states of dissimilar TM ions in glasses as well as the interactions between unlike magnetic ions in disordered solids. In the present work we have extended the available information concerning interactions between unlike magnetic ions in vitreous matrices by using magnetic susceptibility measurements combined with inductively coupled plasma (ICP) spectroscopy and X-ray photoelectron spectroscopy (XPS) to determine the ratio of dierent valence states in vanadium phosphate glasses containing CuO and to identify the interactions between the V4 (3d1 ) and Cu2 (3d9 ) ions. It may be expected that electron exchange between V4 and Cu2 ions will lead to a decrease of V4 ion concentration and to the formation of V4 ±Cu2 and Cu2 ±Cu2 exchange pairs. Any deviation from a Curie±Weiss behavior v C= T ÿ h should indicate the presence of an interaction between the magnetic ions with the sign of the paramagnetic Curie temperature, h, being indicative of the type of interaction, i.e., ferromagnetic or antiferromagnetic. 2. Experimental procedure 2.1. Glass preparation The glasses were prepared by melting dry mixtures of reagent grade V2 O5 , P2 O5 and CuO in alumina crucibles with the batch composition V2 O5 0:6ÿx P2 O5 0:4 CuOx where x 0.00, 0.10, 0.20 and 0.30. Since the oxidation and reduction reactions in a glass melt are known to depend on the size of the melt, the sample geometry, whether
67
the melt is static or stirred, thermal history and quenching rate, all glass samples were prepared under similar conditions to minimize these factors [15]. Approximately 30 g of chemicals were thoroughly mixed to obtain homogenized batches. The crucible containing the batch mixture was placed in a furnace, heated at 300°C for an hour and then transferred to an electrically heated melting furnace maintained at temperatures ranging from 1100°C to 1200°C depending on the composition of the batch. The melt was left for about 3.5 h under atmospheric conditions in the furnace during which the melt was occasionally stirred with an alumina rod. The homogenized melt was then cast onto a stainless steel plate mold to form glass buttons and glass rods of approximately 5 mm diameter for XPS measurements. The batch and actual compositions of the glasses studied are listed in Table 1 with the actual compositions determined by ICP spectroscopy. 2.2. Magnetization measurements The temperature-dependent dc magnetic susceptibility was measured using a SQUID magnetometer in a magnetic ®eld of 10.0 Oe over a temperature range 5±300 K. The susceptibility of the sample holder is negligible below 100 K for all samples and less than a 5% correction at the highest temperature for samples with a total susceptibility less than 1 10ÿ8 emu/Oe. The overall accuracy of the susceptibility measurements is estimated to be approximately 3% due to the uncertainty of the magnetometer calibration. 2.3. X-ray photoelectron spectroscopy (XPS) High-resolution photoelectron spectra were collected on a spectrometer equipped with dual aluminum±magnesium anodes using Al Ka radiation hm 1486:6 eV as described elsewhere [16]. The electron analyzer was set at a bandpass energy of 20 eV with the energy scale of the spectrometer calibrated using the core level of Cu 2p3=2 (932.4 eV) and the energy separation between Cu 2p3=2 and Cu 2p1=2 (19.8 eV). The charging of nonconducting glass samples was avoided by ¯ooding the sample with a separate source of low-energy
68
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Table 1 Batch and actual composition of various vanadium phosphate glasses containing CuO Batch
Actual (from ICP)
P2 O5
V2 O5
CuO
P2 O5
V2 O5
CuO
0.400 0.400 0.400 0.400
0.600 0.500 0.400 0.300
0.000 0.100 0.200 0.300
0.3976 0.4062 0.4007 0.3868
0.6024 0.4939 0.4056 0.3157
0.0000 0.0998 0.1937 0.2975
electrons. The energy and intensity of these external electrons were adjusted to obtain the best resolution as judged by the narrowing of the fullwidth at half-maximum (FWHM) of the photoelectron peaks. At the optimum settings of the neutralizing gun (electron kinetic energy between 5 and 10 eV and electron emission current at the sample between 1 and 5 nA), the position of the adventitious C 1s line was within 0:5 of 284.6 eV. This peak arises due to hydrocarbon contamination and its binding energy is generally accepted as remaining constant, irrespective of the chemical state of the sample. For the sake of consistency, all energies are reported with respect to the C 1s transition at 284.6 eV. Glass rods of approximately 5 mm diameter were broken by vacuum cleavage inside the preparation chamber of the spectrometer in order to obtain fresh glass surfaces. The typical time required to collect an XPS spectrum for a sample was about 4 h and the base pressure in the analysis chamber during these measurements was less than 5 10ÿ11 mbar. 3. Results A typical wide-scan X-ray photoelectron spectrum for the sample V2 O5 0:4 P2 O5 0:4 CuO0:2 is shown in Fig. 1 with similar spectra being obtained for the other glass compositions. These low-resolution spectra were obtained in about 1 h using non-monochromatic Al Ka. Apart from the photoelectrons and Auger transitions of the glass constituents, the C 1s transition is evident. This feature at 284.6 eV is the peak usually associated with the hydrocarbon contamination, is almost always present on samples introduced from the laboratory environment or from a glove box [17] and is used as an energy reference. The peak po-
sitions and FWHM are listed in Table 2 for core levels Cu 2p, V 2p, P 2p, P 2s and O 1s. In Fig. 2, high-resolution spectra for the O 1s core level for the glasses are compared with the spectrum of P2 O5 powder which consists of two overlapping peaks. It is well-known that the orthorhombic P2 O5 is based on an assembly of discrete molecules consisting of PO4 tetrahedra shared by three neighboring tetrahedra while the other oxygen atom is double-bonded to a phosphorus atom [18]. From the charge density on the non-bridging oxygen atom being about )0.96 and on the bridging oxygen atom )0.65, a shift of the non-bridging atom peak towards a lower binding energy with respect to the bridging-oxygen atom peak is expected [19]. Therefore, the lower binding energy peak O 1s (1) at 531.7 eV is associated with the non-bridging oxygen (P¸O bonds) and the higher energy peak O 1s (2) at 533.5 eV with the bridging oxygen (P±O±P bonds) for the P2 O5 powder. In addition, the ratio of the areas under the P¸O and P±O±P peaks should be 1:1.5 as there should be one non-bridging oxygen atom and 1.5 bridging oxygen atom for every phosphorus atom. A quick visual inspection of the P2 O5 spectrum would suggest that the ratio of the two peaks is in reasonable agreement with this prediction. In comparison to P2 O5 , the O 1s spectra for all the glasses show a sizeable increase in the relative intensity of the lower binding energy peak as compared to the higher binding energy peak, but essentially the binding energies, i.e., the peak positions of the O 1s peaks, remain unchanged. This change in relative intensities of the two peaks indicates an increase in the number of non-bridging bonds in the glasses. Furthermore, each spectrum appears to consist of essentially a single, broad peak whose width (FWHM) varies from 1.70 for
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
69
Fig. 1. A low-resolution XPS spectrum from a fractured surface of a CuO±vanadium phosphate glass with x 0.2 obtained using Al Ka radiation (hm 1486.6 eV).
x 0.00 to 2.49 for x 0.30. (See Table 2 for peak positions and FWHM.) Fig. 3 shows the core level spectra of Cu 2p for the glasses and CuO powder with similar spectra for vanadium shown in Fig. 4. The peak positions for these transition metal oxides are in good agreement with those reported in the literature [16,20]. However, the Cu 2p peaks for glasses shift by 0.8 eV towards higher energies from its value for CuO while the V 2p peaks for the glasses shift by 0.3 eV towards lower energies from its value for V2 O5 . Furthermore, the Cu 2p spectra for the glasses exhibit spin-orbit components, Cu 2p3=2 and Cu 2p1=2 , at binding energies of 933.5 eV and 953.7 eV, respectively, with satellites at about 10 eV higher binding energies. Moreover, the Cu 2p1=2 peaks show a doublet structure marked A and B in Fig. 3 with a separation between the A and B of about 3 eV. These doublets can be
associated with monovalent and divalent Cu ions as these are the only two oxidation states in which Cu exists in various glasses [21]. It is well-known that copper compounds containing Cu2 have strong satellites as one can see in the CuO spectra of Fig. 3 (labeled as Sat), while compounds with just Cu have no satellites [22±25]. A decrease in the intensity of the satellite peak is observed upon going from CuO to the CuO±vanadium phosphate glasses, which indicates that some Cu2 ions in the CuO are reduced to Cu in the glass. Moreover this tendency in the satellite structure of Fig. 3 is also re¯ected in the trend of peak B while the intensity of peak A grows with increasing copper concentration in the glasses. Thus, it may be concluded that the peaks A and B are associated with Cu and Cu2 , respectively. Assuming the Cu 2p spectrum is composed of two overlapping peaks, each component peak was
± 953.64 953.98 953.75 952.96 ± ± (2.70) (4.83) (4.86) (3.13)
Cu 2p1=2 (FWHM) ± 19.80 20.12 19.77 19.84 ± ±
DCu 2p 0:2 eV 517.16 516.95 516.88 516.90 ± ± 517.38 (1.35)
(2.15) (2.15) (2.20) (2.20)
V 2p3=2 (FWHM) 7.40 7.45 7.45 7.35 ± ± 7.50
524.56 524.40 524.33 524.25 ± ± 524.88 (2.05)
(2.25) (2.40) (2.55) (2.60)
DV 2p 0:2 eV
V 2p1=2 (FWHM) 133.61 133.45 133.38 133.38 ± 135.37 ±
P 2p
(2.00)
(1.80) (2.00) (2.10) (2.10)
190.99 190.79 190.86 191.00 ± 192.87 ±
P 2s
(2.25)
(2.13) (2.25) (2.26) (2.20)
530.91 (1.70) 530.75 (2.10) 530.98(1.95) 531.45 (2.49) ± 533.53 (3.40) ±
O 1s (FWHM)
DCu 2p and DV 2p are the peak energy separations Cu 2p3=2 ±Cu 2p1=2 and V 2p3=2 ±V 2p1=2 , respectively. The uncertainity in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
a
± 933.84 933.86 933.98 933.12 ± ±
0.0 0.1 0.2 0.3 CuO P2 O5 V2 O5
(2.68) (4.32) (4.46) (2.92)
Cu 2p3=2 (FWHM)
x
Table 2 Peak positions (in eV) relative to C 1s (284.6 eV) and their corresponding FWHM given in the parentheses for the core levels Cu 2p, V 2p, P 2p, P 2s and O 1sa
70 G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Fig. 2. High-resolution O 1s spectra for the CuO±vanadium phosphate glasses and P2 O5 powder.
®tted to a sum of weighted Lorentzian±Gaussian peaks with a linear sloping background by means of a least squares ®tting program [26]. A best ®t to the experimental data was found by varying the peak position, width and intensity of each of the two component peaks. An example of the resulting ®t is shown in Fig. 5 for x 0.3. Using these peak areas, the ratio of Cu2 ions was calculated as follows: Cu / (area under peak A) AA ; Cu2 / (area under peak B + area under satellite peak) AB ; Thus, C Cu2 = Cu Cu2 AB = AA AB , with the calculated results displayed in Table 3. The validity of the above relation was checked by taking XPS spectra for the Cu2 O and CuO powders and then a mixture of the two. For a mixture which contained an equal concentration
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Fig. 3. High-resolution Cu 2p spectra for the CuO±vanadium phosphate glasses and CuO powder.
of Cu and Cu2 , a value of C of 0.55 was obtained instead of 0.50. The commercial Cu2 O (reagent grade) used in the present work was supplied by BDH Laboratory and the manufacturer claims that the powder contains a minimum of 94% Cu2 O with less than 5% CuO. Also, the presence of a weak satellite associated with the core level Cu 2p in the XPS spectrum of the Cu2 O, con®rms the presence of CuO. It may be noted that in highpurity Cu2 O, no satellite peaks have been observed [22±25]. Therefore, the higher value of C is partly due to CuO in the Cu2 O powder and partly due to the uncertainty in the XPS measurements and analysis technique. Thus, the above procedure should yield values of C with an uncertainty of less than 10%. Since vanadium can also exist in two oxidation states V5 and V4 [21], a double peak structure
71
Fig. 4. High-resolution V 2p spectra for the CuO±vanadium phosphate glasses and V2 O5 powder.
Fig. 5. High resolution Cu 2p3=2 spectra (points) for a CuO±vanadium phosphate glass (x 0.3) and the resulting peaks from a least-square curve ®tting (solid lines).
might be expected in the V 2p spectra as well. Although distinct peaks for these states are not observed in Fig. 4, the presence of an asymmetry
72
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Table 3 Peak positions (in eV) from the curve ®tting of the Cu 2p3=2 peak and the ratio C Cu2 = Cu Cu2 AB = AA AB a x
Cu 2p3=2 (A) FWHM Peak Area
Cu 2p3=2 (B) FWHM Peak Area
Cu 2p3=2 (Sat) FWHM Peak Area
Cu2 =Cutotal 0:03
0.1
931.54 1.38 134.672 931.76 1.56 869.169 931.98 1.98 1489.19
934.72 3.96 2307.96 934.78 3.71 7265.58 934.91 3.52 9322.42
943.82 4.85 1230.40 942.85 5.11 3099.43 942.36 5.21 4823.85
0.9633
0.2 0.3
0.9226 0.9048
a The FWHM and the area are given by the two lower sets of numbers, respectively. The uncertainity in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
and small broadening in the V 2p spectra suggest the existence of some non-equivalent vanadium atoms. Hence, each V 2p3=2 spectrum was similarly ®tted to two Lorentzian±Gaussian peaks with the lower binding energy and the higher binding energy peaks corresponding to V4 and V5 , respectively [26]. The resulting ratios of V5 =Vtotal are given in Table 4. A similar ®tting procedure was carried out for the V 3p and V 2s spectra and the results are presented in Tables 5 and 6, respectively. There is qualitative agreement among the various determinations that a decrease in the V5 =Vtotal ratio occurs with increasing CuO content. The variation in the binding energies of the P 2p and P 2s electrons have also been examined. Figs. 6 and 7 compare high-resolution XPS spectra of P 2p and P 2s core levels for the glasses to those for P2 O5 powder. Each spectrum appears to consist of a single peak whose width remains essentially unchanged; however, both the P 2p and P 2s peaks for the glasses shift by about 2 eV to lower energies as presented in Table 2. The magnetic susceptibility results are shown in Figs. 8 and 9 as plots of the inverse mass magnetic susceptibility, vÿ1 , for the glasses as a function of temperature, T. The susceptibility data appear to follow a Curie±Weiss behavior v C= T ÿ h over most of the temperature range and the deviation from this behavior below 40 K is probably due to interactions between the magnetic ions. From the slope and intercept along the temperature axis, Curie constants, Cexpt , (see Table 7) and
paramagnetic Curie temperatures h of 1.6, )4.8, )12.3 and )24.5 K are determined for samples with x 0.0, 0.1, 0.2 and 0.3, respectively. The presence of two types of TM ions makes it dicult to determine the concentration of these magnetic species from the susceptibility data accurately. However, we have attempted to ®nd the ratio of V5 =Vtotal and the molar fraction of each constituent by using the susceptibility measurements combined with ICP spectroscopy data as explained in Appendix A. These results are also shown in Table 7. 4. Discussion As mentioned in the results section, a shift of 2 eV in the binding energies of the O 1s peaks is observed in the glasses as compared to that of P2 O5 while these O 1s peaks remain unaected by varying CuO content. Furthermore, the O 1s spectra for all the glasses show a sizeable increase in the relative intensity of lower binding energy peak as compared to the higher binding energy peak from that for P2 O5 . This change in relative intensities of the two peaks indicates an increase in the number of non-bridging bonds in the glasses. In order to quantify this change in relative intensities, each O 1s spectrum was also ®tted to two Lorentzian±Gaussian peaks with a linear slopping background by means of a least square ®t [26]. The ratio of the integrated areas of these peaks A 2 =A 1 , as summarized in Table 8, represents the
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
73
Table 4 Peak positions (in eV) from the curve ®tting of the V 2p3=2 peak and the ratio V5 =Vtotal a DV 2p3=2 (0:2 eV)
V5 /Vtotal ( 0.03)
517.35 1.68 4192.14
1.11
0.7546
516.04 1.44 1768.99
517.16 1.80 4468.29
1.12
0.7164
516.88 2.20
516.60 2.05 8447.48
517.40 2.93 14510.30
0.80
0.6320
516.90 2.20
516.83 1.95 4951.48
517.30 3.62 7008.55
0.47
0.5860
x
V 2p3=2 FWHM
V 2p3=2 (1) FWHM Peak Area
0.0
517.16 2.15
516.24 1.19 1363.21
0.1
516.95 2.15
0.2
0.3
V 2p3=2 (2) FWHM Peak Area
a
The FWHM and the area are given by the two lower sets of numbers, respectively. The uncertainty in the peak positions is 0:1 eV and in FWHM is 0:2 eV. Table 5 Peak positions (in eV) from the curve ®tting of the V 3p3=2 peak and the ratio V5 =Vtotal a x
V 3p FWHM
0.0
42.41 4.00
0.1
42.05 3.90
40.60 1.72 115.289
0.2
41.42 4.45
40.63 2.51 272.678
0.3
4 1.37 4.25
40.65 2.97 163.811
DV 3p (0:2 eV)
V5 =Vtotal 0:03
42.55 3.80 943.992
1.98
0.9298
42.44 3.49 797.474
1.84
0.8737
1.70
0.8141
1.57
0.7572
V 3p (1) FWHM Peak Area
V 3p (2) FWHM Peak Area
40.57 1.72 71.3081
42.33 4.82 1194.21 42.22 4.79 510.973
a The FWHM and the area are given by the two lower sets of numbers, respectively. The uncertainty in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
relative concentrations of the bridging oxygen atoms (P±O±P) to other oxygen bonds (P¸O, P±O±V, P±O±Cu, Cu±O±Cu and Cu±O±V etc.). This analysis indicates that about 77% of the oxygen atoms involve bridging P±O±P bonds for these CuO±vanadium phosphate glasses independent of the copper concentration as the ratio A 2 =A 1 remains constant within the uncertainity ?tul> of these determinations. Since the FWHM
of the O 1s peak for the glasses depends on the glass composition and ranges from 1.70 eV for x 0.0 to 2.49 eV for x 0.3, the electronic density of the valance shell on the non-bridging atoms must be dierent from that on the bridging atoms. The 2 eV shifts to lower binding energies of P 2p and P 2s electrons for the glasses probably arise from a dierence in the molecular environment
74
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Table 6 Peak positions (in eV) from the curve ®tting of the V 2s peak and the ratio V5 =Vtotal a x
V 2s (1) FWHM Peak Area
V 2s (2) FWHM Peak Area
DV 2s (0:2 eV)
V5 =Vtotal 0:03
0.0
629.95 3.63 405.989
631.66 5.91 1753.65
1.71
0.8120
0.1
630.49 3.87 680.535
631.64 7.49 1761.51
1.15
0.7213
631.62 5.72 4608.96
1.50
0.6594
2.11
0.5374
0.2
0.3
630.12 4.12 2380.49 629.49 4.95 661.169
631.60 5.82 768.137
a The FWHM and the area are given by the two lower sets of numbers, respectively. The uncertainty in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
Fig. 6. High-resolution P 2p spectra for the CuO±vanadium phosphate glasses and P2 O5 powder.
Fig. 7. High-resolution P 2s spectra for the CuO±vanadium phosphate glasses and P2 O5 powder.
132.91 134.45 a
2.672 2.828
Note: Upper set of numbers based on batch analysis of composition. Lower set of numbers based on ICP analysis of composition.
0.410 0.403 0.295 0.281 0.151 0.162 0.49 0.48 1.336 1.257
1:923 10ÿ3
0.144 0.153
142.89 142.24 0.411 0.379 0.196 0.217 0.156 0.143 0.60 0.64 1:506 10ÿ3
0.237 0.261
152.75 151.87 0.411 0.448 0.098 0.092 0.168 0.170 0.322 0.290
163.18 0.410 ± 0.148 0.442
MWs (g/mol) P2 O5
0.3
surrounding the P atom between the P2 O5 and CuO±vanadium phosphate glass structures as one ®nds the next-nearest neighbors in P2 O5 are all P atoms which are replaced by V or Cu atoms in the glass structures. Since the Pauling electronegativities of V and Cu are lower than that of P ( 2.1), V and Cu have less of an anity for electrons than P. Thus the electron density at the P atom increases with increased V/Cu content which leads to a decrease in P 2p and P 2s binding energies in these highly concentrated vanadium phosphate glasses.
0.3
Fig. 9. The inverse of the mass magnetic susceptibility a function of temperature for CuO±vanadium phosphate glasses at the lowest temperatures. The solid lines represent Curie± Weiss ®ts to the high temperature data.
0.828 0.920
100
3.311 3.420
80
0.2
60
T (K)
0.4
40
0.66 0.63
20
1:182 10ÿ3
0
0.387 0.365
x = 0.30
0
3.869 3.647
x = 0.20
0.1
x = 0.10
0.5
x = 0.00
0.75
50000
7:354 10ÿ4
χ-1 (g Oe / emu)
100000
±
( V2 O5 ) 0.60 - x . (CuO)x . ( P2 O5 )0.40
4.356
150000
±
Fig. 8. The inverse of the mass magnetic susceptibility as a function of temperature for CuO±vanadium phosphate glasses in a magnetic ®eld of 10.0 Oe.
0.6
300
CuO
200
T (K)
V2 O4
100
Molar fraction ICP and susceptibility
0
V2 O5
x = 0.30
0
CuO
x = 0.20
V2 O5
x = 0.10
V5 =Vtotal
x = 0.00
100000
Cexpt (emu K/g Oe)
200000
NCu (1021 Cu ions/g)
χ-1 (g Oe / emu)
300000
NV (1021 V ions/g)
( V2 O5 ) 0.60 - x . (CuO)x . ( P2 O5 )0.40
75
Molar fraction batch
400000
Table 7 Determinations of V5 =Vtotal and molar fraction of V2 O5 , V2 O4 , CuO and P2 O5 from susceptibility results combined with ICP data for CuO±vanadium phosphate glassesa
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
76
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
Table 8 Comparison of the binding energies from the O 1s peaks (in eV) and the corresponding ratio A 2 /A 1 of the area of the O 1s (2) peak to the area of the O 1s (1) peaka
a
x
O 1s (p) FWHM Peak Area
0.0
530.82 1.84 11453.50
0.1
O 2s (p) FWHM Peak Area
DO 1s (0:2 eV)
A 2 =A 1 0:3 eV)
531.86 2.93 8771.88
1.04
0.7659
530.55 2.06 14626.00
531.68 2.86 11411.40
1.13
0.7802
0.2
530.84 2.14 40167.80
531.78 2.95 30959.6
0.94
0.7708
0.3
530.94 2.10 24618.90
531.90 3.11 18537.90
0.96
0.7530
P2 O5
531.88 2.06 38469.3
533.59 2.22 58479.9
1.71
1.5202
The uncertainity in the peak positions is 0:1 eV and in FWHM is 0:2 eV.
The shifts in binding energies for the 2p3=2 and 2p1=2 core levels of V (0.3 eV decrease) and Cu (0.8 eV increase) in the glasses result not only from a change in the molecular environment from V2 O5 and CuO but also from a reduction in the formal oxidation state of the ions [27]. Utilizing an explanation similar to the one given in the preceding paragraph, both V and Cu atoms are surrounded by an increasing number of P atoms and a decreasing number of V or Cu atoms at the nextnearest neighbor positions as the P2 O5 content is increased. Thus the electron density of the V and Cu atoms would decrease while their 2p binding energies should increase. However, binding energies typically decrease if the oxidation state of the transition metal ion is reduced to a lower valence state, e.g., V5 ! V4 and Cu2 ! Cu . Thus the change in molecular environment must be the more dominate mechanism responsible for the increases in the binding energies of the Cu 2p levels while the reduction of the oxidation state of V ions is more responsible for the shifts in the V 2p levels. Another feature of the Cu 2p spectra is that the energy separation between the 2p3=2 and 2p1=2
peaks remains essentially unchanged in going from CuO to the CuO±vanadium phosphate glass system. It has been previously shown that the change in the 2p3=2 ±2p1=2 separation for TM ions in oxide glasses can occur for dierent spin states of the ions [28]. The lack of any signi®cant change in the 2p3=2 ±2p1=2 separations in the present investigation for both the CuO±vanadium phosphate glasses and CuO powder suggests that the Cu ions are predominantly in one oxidation state, probably Cu2 , with only small amounts of the other valence state (Cu ) being present. As seen from Table 3, more than 90% of Cu exists as Cu2 in the glasses in agreement with our previous investigations [20]. The V spectra also revealed some broadening and asymmetry which suggests the existence of more than a single valence state for the V ion. As displayed in Tables 4±6, the ratio of V5 =Vtotal determined from the curve ®tting of various V spectra decreases with increasing copper concentration in the vanadium phosphate glasses. This tendency is also observed from the analysis of the magnetic susceptibility data in combination with
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
the ICP results as described in Appendix A and presented in Table 7. The decrease in V5 with CuO content may arise from similar eects as previously found from EPR investigations on the V2 O5 0:55ÿx TeO2 0:45 CuOx glass system [10]. This study revealed that (i) the addition of a small amount (0.5 mol%) of CuO had the eect of oxidizing V4 to V5 as revealed by the absence V4 resonance, (ii) large concentrations of CuO produced exchange eects of the antiferromagnetic type and a g-shift in the Cu2 resonance and (iii) progressive delocalization of V4 unpaired electrons led to a hopping-type conduction. Likewise, EPR data on CuO 2V2 O5 x 2B2 O3 K2 O3 1ÿx glasses with 0 6 x 6 0:40 [4] gave evidence for the presence of magnetic interactions between the TM ions. Our magnetic susceptibility data also support the conclusion of the aforementioned EPR studies, i.e., the existence of exchange interactions. Since the sign of the paramagnetic Curie temperature is indicative of the type of magnetic interaction, the negative sign implies that the ion±ion interaction is predominately antiferromagnetic in the CuO±vanadium phosphate glasses while the interaction is weakly ferromagnetic in nature for the vanadium phosphate glass (x 0). Also since the Curie temperature is a rough measure of the strength of the interaction between the magnetic ions in the sample, a higher value implies a stronger interaction and/or more ions participating in the interaction. From the experimental values of the Curie temperature increasing with increasing copper content in the glass, it can be concluded that copper is behaving paramagnetically in the host glass network and that the ion±ion exchange interaction is antiferromagnetic and gets stronger with increasing copper content. 5. Conclusions The XPS spectra for O 1s, P 2p, P 2s, Cu 2p and V 2p core levels of the copper±vanadium phosphate glass network have been studied. The binding energies of both P 2p and P 2s for these glasses are found to decrease by 2 eV in comparison to those for pure P2 O5 powder as a result of changes
77
in the next-nearest neighbor environment between the powder and glass structures giving rise to an increase in the electron density at the P atom site. While the binding energies of the V 2p decrease by 0.3 eV from its value in V2 O5 , the Cu 2p level increases by 0.8 eV from its value in CuO. These energy level shifts are explained in terms of the shift resulting from the change in the next-nearest neighbor environment dominating over the shift arising from a reduction in the formal oxidation state of the ions for the Cu sites and vice versa for the V sites. The appearance of multivalent states for both Cu and V ions is con®rmed by other features in the XPS spectra, the observation of a doublet structure in the Cu 2p3=2 and Cu 2p1=2 peak spectra and an asymmetry and broadening in the V 2p spectra as the CuO concentration in the glasses increases. This tendency is also observed in the magnetic susceptibility results as the V5 =Vtotal ratios determined from XPS are in qualitative agreement with those found from susceptibility measurements combined with ICP results. Furthermore the Curie±Weiss behavior, v C= T ÿ h, above 40 K with negative Curie temperatures indicates that the predominate magnetic interactions between the Cu2 ±Cu2 and Cu2 ±V4 exchange pairs are antiferromagnetic in nature. Thus the properties of the vanadium phosphate glasses when doped with CuO are signi®cantly aected by a reduction in the oxidation state of the V ions as well as by the local molecular environment. Acknowledgements The support of the KFUPM Physics Department and Research Committee (Grant PH/ PHYSPROP/43) and the WSU Institute for Manufacturing Research is greatly acknowledged. The assistance of Mr M.A. Khan for experimental work is appreciated. Appendix A To determine the V4 =Vtotal ratio from magnetic susceptibility and ICP data, we will de®ne
78
G.D. Khattak et al. / Journal of Non-Crystalline Solids 262 (2000) 66±79
V4 =Vtotal V5 =Vtotal NV NCu Cexpt
ratio of number of V4 ions to total number of V ions ratio of number of V5 ions to total number of V ions 1 ÿ V4 =Vtotal number of vanadium ions/gram of sample from ICP number of copper ions/gram of sample from ICP Curie constant (emu: K/Oe: gram) from v-vs-T results.
Assuming Cu only exists in Cu2 state peff 1:9 lB and V is in either the V5 peff 0 or V4 peff 1:8 lB states, the Curie constant is given by 4 2 peff Cu2 V Cexpt NCu NV 3kB Vtotal
4
2
peff V : 3kB
A:1
Thus the V4 =Vtotal ratio can be determined from Eq. (1) by using the experimental determined values for Cexpt , NV and NCu . Using the resulting value of the V4 =Vtotal ratio, the molar fraction of V2 O5 , V2 O4 , P2 O5 and CuO as well as the molecular weight MWs of the glass sample can be determined. Recall that one mole of these CuO±vanadium phosphate glasses will have Avogadro number, NA , of molecules 1 V4 NA NV MWV2 O4 2 Vtotal 1 V4 NV 1 ÿ MWV2 O5 Vtotal 2 1 NCu MWCuO NP MWP2 O5 ; A:2 2 where NP is the number of P ions per gram and is determined from Eq. (A.2). Then 4 1 V NV MWs NA 2 Vtotal 4 1 V 1 NCu NP ; NV 1 ÿ Vtotal 2 2 A:3
and the molar fraction of each constituent will be 1 V4 MWS NV2 O4 NV NA 2 Vtotal NV2 O5
1 2
NCuO NCu
1 ÿ V4 MWS NV Vtotal NA MWS NA
1 MWS NP2 O5 NP : NA 2
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