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EPR studies on the dynamics of the structure of copper (II) doped silica gels
Journal of Non-Crystalline Solids 341 (2004) 170–177 www.elsevier.com/locate/jnoncrysol
EPR studies on the dynamics of the structure of copper (II) doped silica gels Sudip Mukherjee, A.K. Pal
*
Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received 14 July 2003 Available online 4 July 2004
Abstract Detailed X-band EPR studies on 1.0 mol% copper oxide doped silica gels, oven dried as well as calcined at various temperatures up to 1000 °C have been carried out in the temperature range 10.4–300 K. Spin Hamiltonian and line-width parameters are determined by computer simulation of the observed spectra. Important information derived on the dynamics of the porous structure of Cu-doped silica gel in the light of the EPR parameters is as follows: (i) Three kinds of Cu2þ species (including one Cu-pair) are present in the 100 °C oven dried silica gel. (ii) Two kinds of slow rotating isolated Cu2þ species (one Cu2þ pair) occur in the 400 °C calcined sample at room temperature. The Cu-pair species agglomerate at low temperatures ( 6 200 K) to form CuO clusters which remain undetected apparently because of their large line-width. (iii) Only one kind of Cu-species is detected in the 700 °C calcined silica glass at all measuring temperatures. The Cu2þ -pair/CuO cluster spectrum is absent. (iv) The isotropic EPR spectrum due to rapidly rotating Cu2þ -species occurring in very large pores of silica glass calcined at 800 °C has been obtained at room temperature. (v) Pore collapse starts at 900 °C, and results in the observance of a very broad EPR line (5000 Oe) for 900 and 1000 °C calcined samples. This shows that the pore collapse and the formation of superparamagnetic nano-sized CuO particles in the silica glass matrix take place simultaneously. Ó 2004 Elsevier B.V. All rights reserved. PACS: 76.30.Fc; 75.50.Kj; 75.10.Dg
1. Introduction The sol–gel method delivers various advantages over the traditional melt processing techniques such as greater homogeneity, high purity, lower processing temperatures and better control over the glass properties. Of the many glasses, silica glass obtained via sol–gel route has been most extensively studied by various physical and chemical methods and important results on the microstructural behavior of silica glass have been obtained [1–14]. Brinker et al. [5] made a detailed study on sol–gel transition in silica gel by using DTA, TGA, dilatometry, SEM, TEM and SAXS. Their principal observations are as follows: The liquid in the porous gel is a mixure of water, residual alkoxy groups and C2 H5 OH. The prominent endothermic DTA features in *
Corresponding author. Tel.: +91-33 2473 4971; fax: +91-33 2473 2805. E-mail address: [email protected] (A.K. Pal). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.04.016
the 100–150 °C temperature region indicate expulsion of physically adsorbed water. The carbonisation of residual alkoxy groups takes place in the temperature range 200– 300 °C. The exothermic combustion of carbon occurs between 275 and 400 °C while the temperature corresponding to the onset of rapid shrinkage varies in the range 600–900 °C depending on acid/base catalyzed series of silica gel. The Raman spectra of monolithic dry SiO2 gels, measured as a function of temperature, show that both surface and bulk Si–OH groups are present [4]. The total OH content decreases with increasing temperature. The combined results of Raman, density and specific surface area measurements show that during densification two partly related processes occur, viz. pore collapse and the condensation of surface Si–OH groups to Si–O–Si bonds. Raman scattering studies [6] revealed the presence of water in small quantity, probably trapped in silica pores of small size (<2 nm) up to the temperature as high as 700 °C. Roussett et al. [10] observed that at temperatures higher than 400 °C, the
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Si–O–H and Si–O–CH2 –CH3 groups which covered the pore surfaces reacted to form bridges. This has also been confirmed by 29 Si NMR [9]. Poulose et al. [14] observed from their spectroscopic studies that as calcination temperature was increased to 1000 °C, the powdered Cu-doped silica glass showed absorption spectra similar to that of Cuþ or Cu2 O. EPR studies particularly using Cu2þ ion as probe have been very useful in the understanding of magnetic properties of Cu incorporated in SiO2 gel-glass as well as porous gel-glass structure. Darab and MacCrone [7] incorporated copper (II) complexes into a silica sol–gel system and studied various structural elements local to Cu(II) ions in silica gel by Cu(II) EPR during drying and calcination of the gel. SQUID magnetometer studies [8] demonstrated the strange occurrence of ferromagnetism in porous silica gels containing small amounts of copper (of the order of few wt%) calcined and baked up in the temperature range 400–800 °C. The ferromagnetic susceptibility showed a maximum just after heating at 650 °C and disappeared after heating at 750 °C. The same group of scientists also carried out EPR experiments [8] and EPR and Si NMR experiments [9] and observed that a large decrease of the total EPR intensity was introduced by heating and came to the conclusion that the Cu2þ ions become attached to SiO2 chains at higher temperature and were not observable by EPR. EPR studies carried out so far [7–9] have been limited only to the liquid nitrogen temperature range. However, for better appraisal of the dynamics of gel-glass structure of silica gel in the presence of copper(II) impurity ions EPR studies on Cu(II) doped monolithic silica gels over a wider range of low temperature are very much desired. With this object in view, present X-band EPR investigations at a large number of temperatures in the temperature interval 10.4–300 K on 1.0 mol% copper(II) doped silica gel-glass both oven dried and calcined at several temperatures up to 1000 °C have been undertaken. A rigorous computer simulation method has been adopted for the analysis of the observed EPR spectra to obtain spin Hamiltonian parameters including linewidth. The parameters so derived will be helpful in understanding the changes in the gel-glass structure occurred as a function of calcination temperature.
2. Experimental Copper doped silica gel is prepared from silicon tetraethylorthosilicate (TEOS) and dopant copper nitrate trihydrate (1.0 mol% Cu(II)) essentially following the method of Sakka and Kamiya [3]. The molar ratio of water and TEOS is kept at 20 while that of catalyst HCl and TEOS is 0.01. Ethanol is used as solvent. The solution, poured in several small pyrex beakers and covered with polythene sheet, is kept in the atmosphere
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Table 1 EPR spectra recorded for calcined Cu(II):SiO2 gel-glass at various low temperature Samples calcined at temperatures (°C)
EPR spectra recorded at temperatures (K)
400 700 800 900 1000
290, 290, 300, 300, 300,
249, 200, 150, 98, 48, 10.4 200, 98, 47, 10.4 77 77 77
to form stiff gel. Transparent gel with slight bluish tint is formed in 7/8 days. Monolithic gels derived from the solution are allowed to dry further at room temperature for 4–5 weeks. Dried gels are then kept inside an oven maintained at 100 °C for 2 days. Oven dried gels are then transferred to a temperature programmed furnace. Heat treatment of these samples is then performed in air at several pre-selected temperatures up to 1000 °C, the heating procedure being similar to that followed by Krol and Lierop [4]. The silica glass has been found to remain monolithic up to the highest calcination temperature (1000 °C) employed in our experiment. EPR spectra of the oven dried gels and calcined glass are recorded at several temperatures from room temperature down to 10.4 K with on X-band EPR spectrometer with 100 kHz magnetic field modulation in a helium cryocooler system. The specimens for studies were taken in the form of small cakes cut from dried gel and calcined monolithic glass and attached to the tip of a sapphire rod, fixed to the vacuum shroud of the cryocooler. The position of the vacuum shroud was adjusted within the rectangular X-band cavity (working in TE102 mode) such that the tip of the rod containing the glass specimen was at maximum sensitive position within the cavity for observing Cu(II) signal. Before recording EPR spectra at a given temperature, the temperature is maintained constant at least for 20 min. For oven dried (100 °C) gel, EPR spectra are recorded at the following temperatures (K): 290, 234, 230, 226, 220, 215, 210, 205, 200, 195, 190, 179, 160, 140, 119, 100, 10.6. Temperatures at which calcination of Cu(II) doped SiO2 gels take place and at which EPR spectra are recorded are shown in the Table 1.
3. Theory Computation method followed is outlined below: Under the axial ligand field symmetry, the spin Hamiltonian for Cu2þ sites (S ¼ 1/2, I ¼ 3/2) may be written as Hs ¼ gk bHz Sz þ g? bðHx Sx þ Hy Sy Þ þ Ak Sz Iz þ A? ðSx Ix þ Sy Iy Þ:
ð1Þ
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The symbols in Eq. (1) have their usual meaning. Including magnetic hyperfine interaction to second order, the resonance field is approximately given by [15] 1
the next giving rise to a narrow distribution in gk . DH ðmI Þ is expressed in terms of the fluctuation of gk (i.e. dgk ) [17]:
2
H ¼ H0 KmI =gb ð4H0 Þ ðgbÞ A2? ðA2k þ K 2 Þ 1
DH ðmI Þ ¼ dgk
2
K 2 ½IðI þ 1Þ m2I ð2H0 Þ ðgbÞ ðA2k A2? Þ K 2 gk2 g?2 g4 Z 2 ð1 Z 2 Þm2I ;
ð2Þ
hm , g2 ¼ g?2 þ ðgk2 g?2 ÞZ 2 , Z ¼ cos h, Ak and where H0 ¼ gb A? are hyperfine structure constants in energy units. The line-shape of Cu2þ doped in a polycrystalline host (powder pattern) corresponds to the total absorptions from the statistical ensemble of identical sites, each randomly orientated with respect to the applied magnetic field and described by [16] Z 1 1 2 2 IðHa Þ ¼ g? ½8ð2I þ 1Þ S 0 ðHa H Þ½ðgk =g? Þ þ 1dZ: 0
ð3Þ 0
The first derivative S ðHa H Þ expresses a convolution function, which reflects the line-width and shape for a single crystal. Usually, the line-shape function such as Lorentzian is applied, 2 2
S 0 ðxÞ ¼ 2p1 a3 x½1 þ ðx=aÞ ;
a ¼ 31=2
DH : 2
ð4Þ
DH is the separation between points of maximum and minimum slope. In the case of EPR spectrum of Cu2þ ions in a crystalline environment with a precisely fixed set of coordinating ligands, the line-width of each parallel hyperfine component is the same. However, in a glassy environment the line-width of the parallel hyperfine component has been observed to increase with increasing values of mI . The line-width ðDH ðmI ÞÞ dependence on mI arises from fluctuations in both the ligand fields and bond co-valencies from one copper(II) complex to
½hm þ mI ðgk P Ak Þ ; gk2 b
ð5Þ
where P is a constant (0.036 cm1 ).
4. Results and discussion 4.1. Silica gel oven dried (100 °C) (Fig. 1; Table 2) On inspection of Table 2 (Fig. 1) where the computer fitted spin Hamiltonian parameters have been tabulated, following facts emerge: (i) At room temperature (290 K) the EPR spectrum is devoid of any hyperfine (hf) structure and is nearly isotropic. (ii) At 234 K, two sets of anisotropic Cu(II) spectra (C1 ; C2 ) could be identified. Anisotropic g-spectra of C1 and C2 Cu2þ sites show monotonous changes down to 140 K. (iii) Hf structures of C1 and C2 spectra make their appearance at 200 and 140 K respectively. (iv) Spin Hamiltonian parameters (gk , Ak ) associated with C1 and C2 sites are significantly different. Characteristic unequal hf line-widths (progressively increasing from low to high magnetic field) for both C1 and C2 sites indicate that the Cu2þ complexes are in glassy environment. This feature of hf line-width has been satisfactorily explained by taking into account of variation in ligand field i.e. introducing non-zero value of dgk i.e. gk spread. (v) Computer deconvolution also reveals the presence of a third spectra (C3 ) at 220 K. Principal g-values (gk and g? ) of C3 Cu(II) species are remarkably different from those of C1 and C2 species. There is one possibility that C3 species are, in fact, Cu(II)-pairs and so derived principal g-values pertain to those of Cu(II)-pairs. Further, if it is assumed that the
Table 2 Values of Spin-Hamiltonian parameters (gk , g? , Ak , A? ), dgk and line-widths (DHk , DH? ) of Cu2þ doped dried (100 °C) SiO2 gel (1.0 mol%) Sites
290 K
234 K
220 K
200 K
140 K
10.6 K
C1
gk ¼ 2:195 g? ¼ 2:185 DHk ¼ 170 DH? ¼ 95
gk ¼ 2:350 g? ¼ 2:080 DHk ¼ 100 DH? ¼ 50
gk ¼ 2:350 g? ¼ 2:075 DHk ¼ 100 DH? ¼ 50
gk ¼ 2:400 g? ¼ 2:100 Ak ¼ 140 A? ¼ 10 dgk ¼ 0:03
gk ¼ 2:410 g? ¼ 2:080 Ak ¼ 140 A? ¼ 5 dgk ¼ 0:03
gk ¼ 2:410 g? ¼ 2:080 Ak ¼ 145 A? ¼ 5 dgk ¼ 0:03
C2
–
gk ¼ 2:228 g? ¼ 2:154 DHk ¼ 100 DH? ¼ 85
gk ¼ 2:340 g? ¼ 2:150 DHk ¼ 170 DH? ¼ 120
gk ¼ 2:350 g? ¼ 2:100 DHk ¼ 170 DH? ¼ 120
gk ¼ 2:350 g? ¼ 2:08 Ak ¼ 160 A? ¼ 3 dgk ¼ 0:03
gk ¼ 2:355 g? ¼ 2:08 Ak ¼ 160 A? ¼ 5 dgk ¼ 0:03
C3
–
–
gk ¼ 2:230 g? ¼ 2:140 DHk ¼ 100 DH? ¼ 75
gk ¼ 2:230 g? ¼ 2:140 DHk ¼ 100 DH? ¼ 90
gk ¼ 2:250 g? ¼ 2:130 DHk ¼ 120 DH? ¼ 90
gk ¼ 2:260 g? ¼ 2:120 DHk ¼ 130 DH? ¼ 95
Ak , A? , DHk and DH? are expressed in Oe.
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4.2. Silica glass calcined at 400 °C (Fig. 2(a); Table 3) 290 234 230 226 220 215 210 205 200 195 190 179 160 140 119 100 10.6
2000
2500
3000
3500
4000
At 290 K (RT), the C01 spectrum shows g-anisotropy but hf structure is absent (Fig. 2(a); Table 3). This means that Cu(II) species responsible for C01 spectrum are in some kind of rotational motion, the rate of which is such as to average out the hf structure but not g-anisotropy. But hf lines appear at 200 K, indicating that the motion has been slowed down to the extent that the hf structure appears. The line-widths of the four hf lines in the Ak direction vary in the range 2.2–4.7 mT indicating thereby that Cu(II) species represented by C01 spectrum are in glassy environment. Another spectrum (C03 ), is also observed but its gk and g? values are significantly different from those of C01 spectrum. On the other hand, these g-values are close to those of C3 species in oven dried silica gel. So the C03 spectrum may be taken as to represent Cu(II) pair species occurring in large silica Cu(II) ions most probably, have octapores (120 A). hedral conformation with O ions (not H2 O) as ligands. It is likely that with lowering of temperature pair species progressively agglomerate to form CuO clusters and that CuO clusters are small in number as well as have quite large line-width as to escape detection at low temperatures 6 200 K. 4.3. Silica glass calcined at 700 °C (Fig. 2(a); Table 3)
Magnetic Field (Oe) Fig. 1. EPR spectra of 1.0 mol% copper doped oven dried silica gel.
individual Cu(II) units of the copper pairs are under the spell of ligand fields similar to these C1 and C2 species and that the ionic gk -values of the Cu(II) pairs are in the range of those of C1 and C2 species i.e. 2.41–2.35, then it can be shown that the angles between the tetragonal axes of individual units of a Cu(II) pair will lie between 94° (gk ¼ 2:41) and 79° (gk ¼ 2:35). The non-observation of hyperfine structure of C3 species down to 10.6 K provides further support for the presence of Cu(II)-pair species in the silica dried gel (Table 2). To understand the significance of the above observations, it is very much relevant to consider the important findings of Burlamacchi and Martini [18] on the EPR spectral dynamics of aqueous Cu(II) ions in silica gels of known monodispersed pore sizes. C1 and C2 species could then be assigned to the Cu(II) species of the surface of the confined in a region within 20–3 A silica pores. In this region the surface forces are not strong enough as to completely arrest the motion of Cu(II) species and a glassy region is formed giving rise to glassy EPR spectra at low temperatures. In the above C3 spectrum has been assigned to Cu(II) pairs. This results from agglomeration of Cu(II) species in the surface-force-free regions of large silica pores (120 A) occurring at low temperature ( 6 220 K).
Only one kind of glassy spectrum (C001 ) with varying hf line-width has been observed in the temperature range 290–10.4 K. The spectrum obviously arise from nearly isolated Cu(II) species present in the glassy region of silica pores of medium diameter. The intensity of the spectrum is much reduced compared to the spectra observed for equal mass of the silica glass calcined at 400 °C. This confirms the fact that most of the Cu(II) species become attached to SiO2 network formed on the surface of interconnected silica pores of large diameters [8,9] and give rise to ferromagnetism but EPR-inactive. 4.4. Silica glass calcined at 800 °C (Fig. 2(b); Table 3) Density and nitrogen adsorption isotherm measurements of pure SiO2 glass [4] showed that in transition from 700 to 800 °C calcined glass mean pore radius undergoes enormous enhancement i.e. from 10 to 23 A. This is suggestive of the fact that a large number of small pores collapse to give rise to larger pores and some copper(II) ions are incorporated in them. The Cu(II) ions situated in the central region of the larger pores have free movement of rotation type at RT and if the frequencies of their motion are greater than the EPR frequency, the anisotropy of the spectrum is averaged out and isotropic g-spectrum (C0 ) is expected as has been the case (Table 3). Along with the isotropic spec00 trum, anisotropic spectrum (C000 1 ), similar to that (C1 )
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(i):290K (i):249K
(iii):77K
(i):200K
(i):10K
(ii):290K
(iii):300K
(ii):10K
2600
(a)
2800
3000
3200
3400
3600
2400
2600
2800
(b)
Magnetic field (Oe)
3000
3200
3400
Magnetic field (Oe)
(iv):300K
(iv):77K
(v):77K 0
(c)
2000
4000
6000
8000
10000
Magnetic field (Oe)
Fig. 2. (a): EPR spectra of 1.0 mol% copper doped silica gel calcined at (i) 400 °C, (ii) 700 °C. (b): EPR spectra of 1.0 mol% copper doped silica gel calcined at (iii) 800 °C. (c): EPR spectra of 1.0 mol% copper doped silica gel calcined at (iv) 900 °C, (v) 1000 °C.
observed for 700 °C calcined sample, is also present at RT. The C0 isotropic spectrum is absent at 77 K. One
possibility is that the motion of Cu(II) ions responsible for C0 spectrum at RT becomes frozen at 77 K and the
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175
Table 3 Values of spin-Hamiltonian parameters (gk , g? , Ak , A? ), dgk and line-widths (DHk , DH? ) of Cu2þ doped SiO2 gel (1.0 mol%) at different calcined temperature Calcining temperature (°C)
Sites
Expt. temperature 290 K
249 K
200 K
10.4 K
400
C01
gk ¼ 2:39 g? ¼ 2:08 DHk ¼ 100 DH? ¼ 50
gk ¼ 2:40 g? ¼ 2:08 DHk ¼ 100 DH? ¼ 50
C03
gk ¼ 2:23 g? ¼ 2:18 DHk ¼ 140 DH? ¼ 100
gk ¼ 2:24 dg? ¼ 2:15 DHk ¼ 120 DH? ¼ 100
gk ¼ 2:405 g? ¼ 2:08 Ak ¼ 140 A? ¼ 10 dgk ¼ 0:03 –
gk ¼ 2:41 g? ¼ 2:08 Ak ¼ 145 A? ¼ 5 dgk ¼ 0:03 –
290 K
10.4K
gk ¼ 2:39 g? ¼ 2:08 Ak ¼ 145 A? ¼ 5 dgk ¼ 0:03
gk ¼ 2:41 g? ¼ 2:074 Ak ¼ 150 A? ¼ 5 dgk ¼ 0:03
700
C001
300 K
77 K
800
C000 1
gk ¼ 2:39 g? ¼ 2:08 Ak ¼ 140 A? ¼ 1 dgk ¼ 0:03
C0
gk ¼ 2:17 g? ¼ 2:17 DHk ¼ 200 DH? ¼ 200
gk ¼ 2:405 g? ¼ 2:065 Ak ¼ 150 A? ¼ 1 dgk ¼ 0:03 –
300 K
77 K
900
C4
gk ¼ 2:42 g? ¼ 2:15 DH ¼ 850
gk ¼ 2:42 g? ¼ 1:7 DH ¼ 5000
300 K
77 K
1000
C5
Not detectable
gk ¼ 2:425 g? ¼ 1:7 DHk ¼ 5700 DH? ¼ 5600
Ak , A? , DHk and DH? are expressed in Oe.
spectrum (C0 ) turns anisotropic and it coalesces with the C000 1 spectrum giving rise to only one spectrum. 4.5. Silica glass calcined at 900 °C (Fig. 2(c); Table 3) Anisotropic EPR spectrum with hf structure is absent. Instead, at RT a broad anisotropic signal (DH ¼ 850 Oe) with gk ¼ 2:42 and g? ¼ 2:15 is observed. At 77 K the spectrum becomes enormously broad (DHk ¼ DH? ¼ 5000 Oe) having abnormally low g? value <2.0 (gk ¼ 2:42;g? ¼ 1:70). The above observations are explained as follows: (i) Raman study in conjuction with density and specific area measurements [4] revealed that the condensation of surface Si–OH groups to Si–O–Si bonds and pore collapse took place almost simultaneously at 900 °C. It is thus evident that when the calcination temperature is less than 900 °C, some Si–OH groups are present which are responsible for the observed anisotropic Cu(II) spectrum with hf structure. So, the absence of hf structured EPR spectrum
for the 900 °C calcined silica glass is consistent with the complete absence of Si–OH groups. (ii) After collapse of most of the pores at 900 °C calcination temperature, it is natural to expect that agglomeration of CuO molecular units occur at several locations in SiO2 glass network, resulting in the formation of nano-sized particles of CuO. Kindo et al. [19] found that bulk CuO, which becomes antiferromagnetic at 230 K (TN ), exhibits unusually broad paramagnetic resonance with the line-width of about 16 KOe and g-value of about 2.0 at room temperature at the 45 GHz region under pulsed magnetic field. One-dimensional spin correlation has also been established. Below TN , zero field antiferromagnetic resonance in the infrared was also observed. In the present case, on the other hand, the line-width is comparatively quite small at RT (850 Oe as against 16 000 Oe). However, the increase of line-width on lowering the temperature from RT to 77 K is quite enormous (850–5000 Oe) but at the same time no AF transitional effect as in bulk CuO has been detected. Only a small g-shift has
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been observed. It follows, therefore, that EPR spectrum of copper doped silica glass calcined at 900 °C is quite distinct from that of bulk CuO compared but, at the same time, is of unusual nature. So, it is quite reasonable to infer that CuO particles grown in silica glass on calcination at 900 °C are nano-sized and become superparamagnetic at low temperatures. 4.6. Silica glass calcined at 1000 °C (Fig. 2(c); Table 3) In contrast to what has been observed for 900 °C calcined silica glass, 1000 °C calcined sample does not show any EPR signal at RT. The EPR signal is likely be very weak because of the fact that majority conversion of Cu2þ ions to Cuþ ions takes place at 1000 °C i.e. generation of Cu2 O. This is confirmed from spectroscopic studies [14] as well as from our powder XRD measurements. It is known that densification of silica glass is nearly complete at 1000 °C calcination temperature [4]. So, it is reasonable to infer that CuO clusters are somewhat bigger nano-particles at higher calcination temperature. Bigger CuO nano-particles are also expected to have very much broader line-width at RT as to escape detection. However, at 77 K, a very broad signal, larger than those for 900 °C calcined sample, (DHk ¼ 5700 Oe and DH? ¼ 5600 Oe) has been observed because of superparamagnetism.
5. Concluding remarks The present work by Cu(II) EPR on the dynamics of local structure in 1.0 mol% copper doped porous silica gel-glass in the temperature range 10.4–300 K has revealed the following: (i) Three kinds of Cu(II) species (including one due to copper pairs) having different rates of rotational motions are present in the pores of oven dried (100 °C) silica gel. (ii) In silica glass calcined at 400 °C two types of Cu(II) species including Cu-pairs having much slower rotational motion have been detected at room temperature. Cu-pairs agglomerate at low temperature ( 6 200 K) to form CuO clusters. (iii) EPR-wise only one kind of isolated Cu(II) species has been detected in silica glass calcined at 700 °C. Since majority number of Cu(II) ions are shown to be engaged in forming ferromagnetic planer layers in the 500–750 °C temperature range [8,9], very few Cu-pairs are available for EPRdetection. (iv) Average pore radius is more than doubled (10–23 in the transition of the calcination temperature A) from 700 to 800 °C [4]. Consistent with this finding,
isotropic EPR spectrum originating from free moving Cu(II) species present in very large pores of silica glass calcined at 800 °C has been observed at RT. (v) In case of silica glass calcined at 900 and 1000 °C, a very broad line having width 5000 Oe has been detected at 77 K which most likely has its origin in superparamagnetic CuO nano-particles present in the silica glass. For confirmation, experiments on heavily doped silica glass (Cu(II) ions having concentration of the order of 10 mol% or so) are desirable which are in progress. (vi) It is relevant in this connection to mention the Cu2þ EPR findings of Darab and MacCrone [7] and Duval et al. [9] in the context of the present work. Their methods of sol–gel preparation are quite different from what followed by us [3]. Also our method of thermal processing of gel samples [4] is distinct from those practised by them. Spin Hamiltonian parameters determined for Cu2þ doped silica dried gel by computer deconvolution of the EPR spectra as well as those calcined at various temperatures are distinctly different from those determined by Darab and MacCrone [7] and Duval et al. [9]. Since their works were confined to calcination temperatures up to 600 and 750 °C respectively, they have missed the superparamagnetic resonance of CuO nano-particles obtained at calcination temperatures 900 and 1000 °C observed by us. The presence of Cu(II)-pair spectra in Cu(II):silica gelglass at low temperatures has also escaped detection of previous workers.
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EPR studies on the dynamics of the structure of copper (II) doped silica gels Sudip Mukherjee, A.K. Pal
*
Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received 14 July 2003 Available online 4 July 2004
Abstract Detailed X-band EPR studies on 1.0 mol% copper oxide doped silica gels, oven dried as well as calcined at various temperatures up to 1000 °C have been carried out in the temperature range 10.4–300 K. Spin Hamiltonian and line-width parameters are determined by computer simulation of the observed spectra. Important information derived on the dynamics of the porous structure of Cu-doped silica gel in the light of the EPR parameters is as follows: (i) Three kinds of Cu2þ species (including one Cu-pair) are present in the 100 °C oven dried silica gel. (ii) Two kinds of slow rotating isolated Cu2þ species (one Cu2þ pair) occur in the 400 °C calcined sample at room temperature. The Cu-pair species agglomerate at low temperatures ( 6 200 K) to form CuO clusters which remain undetected apparently because of their large line-width. (iii) Only one kind of Cu-species is detected in the 700 °C calcined silica glass at all measuring temperatures. The Cu2þ -pair/CuO cluster spectrum is absent. (iv) The isotropic EPR spectrum due to rapidly rotating Cu2þ -species occurring in very large pores of silica glass calcined at 800 °C has been obtained at room temperature. (v) Pore collapse starts at 900 °C, and results in the observance of a very broad EPR line (5000 Oe) for 900 and 1000 °C calcined samples. This shows that the pore collapse and the formation of superparamagnetic nano-sized CuO particles in the silica glass matrix take place simultaneously. Ó 2004 Elsevier B.V. All rights reserved. PACS: 76.30.Fc; 75.50.Kj; 75.10.Dg
1. Introduction The sol–gel method delivers various advantages over the traditional melt processing techniques such as greater homogeneity, high purity, lower processing temperatures and better control over the glass properties. Of the many glasses, silica glass obtained via sol–gel route has been most extensively studied by various physical and chemical methods and important results on the microstructural behavior of silica glass have been obtained [1–14]. Brinker et al. [5] made a detailed study on sol–gel transition in silica gel by using DTA, TGA, dilatometry, SEM, TEM and SAXS. Their principal observations are as follows: The liquid in the porous gel is a mixure of water, residual alkoxy groups and C2 H5 OH. The prominent endothermic DTA features in *
Corresponding author. Tel.: +91-33 2473 4971; fax: +91-33 2473 2805. E-mail address: [email protected] (A.K. Pal). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.04.016
the 100–150 °C temperature region indicate expulsion of physically adsorbed water. The carbonisation of residual alkoxy groups takes place in the temperature range 200– 300 °C. The exothermic combustion of carbon occurs between 275 and 400 °C while the temperature corresponding to the onset of rapid shrinkage varies in the range 600–900 °C depending on acid/base catalyzed series of silica gel. The Raman spectra of monolithic dry SiO2 gels, measured as a function of temperature, show that both surface and bulk Si–OH groups are present [4]. The total OH content decreases with increasing temperature. The combined results of Raman, density and specific surface area measurements show that during densification two partly related processes occur, viz. pore collapse and the condensation of surface Si–OH groups to Si–O–Si bonds. Raman scattering studies [6] revealed the presence of water in small quantity, probably trapped in silica pores of small size (<2 nm) up to the temperature as high as 700 °C. Roussett et al. [10] observed that at temperatures higher than 400 °C, the
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Si–O–H and Si–O–CH2 –CH3 groups which covered the pore surfaces reacted to form bridges. This has also been confirmed by 29 Si NMR [9]. Poulose et al. [14] observed from their spectroscopic studies that as calcination temperature was increased to 1000 °C, the powdered Cu-doped silica glass showed absorption spectra similar to that of Cuþ or Cu2 O. EPR studies particularly using Cu2þ ion as probe have been very useful in the understanding of magnetic properties of Cu incorporated in SiO2 gel-glass as well as porous gel-glass structure. Darab and MacCrone [7] incorporated copper (II) complexes into a silica sol–gel system and studied various structural elements local to Cu(II) ions in silica gel by Cu(II) EPR during drying and calcination of the gel. SQUID magnetometer studies [8] demonstrated the strange occurrence of ferromagnetism in porous silica gels containing small amounts of copper (of the order of few wt%) calcined and baked up in the temperature range 400–800 °C. The ferromagnetic susceptibility showed a maximum just after heating at 650 °C and disappeared after heating at 750 °C. The same group of scientists also carried out EPR experiments [8] and EPR and Si NMR experiments [9] and observed that a large decrease of the total EPR intensity was introduced by heating and came to the conclusion that the Cu2þ ions become attached to SiO2 chains at higher temperature and were not observable by EPR. EPR studies carried out so far [7–9] have been limited only to the liquid nitrogen temperature range. However, for better appraisal of the dynamics of gel-glass structure of silica gel in the presence of copper(II) impurity ions EPR studies on Cu(II) doped monolithic silica gels over a wider range of low temperature are very much desired. With this object in view, present X-band EPR investigations at a large number of temperatures in the temperature interval 10.4–300 K on 1.0 mol% copper(II) doped silica gel-glass both oven dried and calcined at several temperatures up to 1000 °C have been undertaken. A rigorous computer simulation method has been adopted for the analysis of the observed EPR spectra to obtain spin Hamiltonian parameters including linewidth. The parameters so derived will be helpful in understanding the changes in the gel-glass structure occurred as a function of calcination temperature.
2. Experimental Copper doped silica gel is prepared from silicon tetraethylorthosilicate (TEOS) and dopant copper nitrate trihydrate (1.0 mol% Cu(II)) essentially following the method of Sakka and Kamiya [3]. The molar ratio of water and TEOS is kept at 20 while that of catalyst HCl and TEOS is 0.01. Ethanol is used as solvent. The solution, poured in several small pyrex beakers and covered with polythene sheet, is kept in the atmosphere
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Table 1 EPR spectra recorded for calcined Cu(II):SiO2 gel-glass at various low temperature Samples calcined at temperatures (°C)
EPR spectra recorded at temperatures (K)
400 700 800 900 1000
290, 290, 300, 300, 300,
249, 200, 150, 98, 48, 10.4 200, 98, 47, 10.4 77 77 77
to form stiff gel. Transparent gel with slight bluish tint is formed in 7/8 days. Monolithic gels derived from the solution are allowed to dry further at room temperature for 4–5 weeks. Dried gels are then kept inside an oven maintained at 100 °C for 2 days. Oven dried gels are then transferred to a temperature programmed furnace. Heat treatment of these samples is then performed in air at several pre-selected temperatures up to 1000 °C, the heating procedure being similar to that followed by Krol and Lierop [4]. The silica glass has been found to remain monolithic up to the highest calcination temperature (1000 °C) employed in our experiment. EPR spectra of the oven dried gels and calcined glass are recorded at several temperatures from room temperature down to 10.4 K with on X-band EPR spectrometer with 100 kHz magnetic field modulation in a helium cryocooler system. The specimens for studies were taken in the form of small cakes cut from dried gel and calcined monolithic glass and attached to the tip of a sapphire rod, fixed to the vacuum shroud of the cryocooler. The position of the vacuum shroud was adjusted within the rectangular X-band cavity (working in TE102 mode) such that the tip of the rod containing the glass specimen was at maximum sensitive position within the cavity for observing Cu(II) signal. Before recording EPR spectra at a given temperature, the temperature is maintained constant at least for 20 min. For oven dried (100 °C) gel, EPR spectra are recorded at the following temperatures (K): 290, 234, 230, 226, 220, 215, 210, 205, 200, 195, 190, 179, 160, 140, 119, 100, 10.6. Temperatures at which calcination of Cu(II) doped SiO2 gels take place and at which EPR spectra are recorded are shown in the Table 1.
3. Theory Computation method followed is outlined below: Under the axial ligand field symmetry, the spin Hamiltonian for Cu2þ sites (S ¼ 1/2, I ¼ 3/2) may be written as Hs ¼ gk bHz Sz þ g? bðHx Sx þ Hy Sy Þ þ Ak Sz Iz þ A? ðSx Ix þ Sy Iy Þ:
ð1Þ
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The symbols in Eq. (1) have their usual meaning. Including magnetic hyperfine interaction to second order, the resonance field is approximately given by [15] 1
the next giving rise to a narrow distribution in gk . DH ðmI Þ is expressed in terms of the fluctuation of gk (i.e. dgk ) [17]:
2
H ¼ H0 KmI =gb ð4H0 Þ ðgbÞ A2? ðA2k þ K 2 Þ 1
DH ðmI Þ ¼ dgk
2
K 2 ½IðI þ 1Þ m2I ð2H0 Þ ðgbÞ ðA2k A2? Þ K 2 gk2 g?2 g4 Z 2 ð1 Z 2 Þm2I ;
ð2Þ
hm , g2 ¼ g?2 þ ðgk2 g?2 ÞZ 2 , Z ¼ cos h, Ak and where H0 ¼ gb A? are hyperfine structure constants in energy units. The line-shape of Cu2þ doped in a polycrystalline host (powder pattern) corresponds to the total absorptions from the statistical ensemble of identical sites, each randomly orientated with respect to the applied magnetic field and described by [16] Z 1 1 2 2 IðHa Þ ¼ g? ½8ð2I þ 1Þ S 0 ðHa H Þ½ðgk =g? Þ þ 1dZ: 0
ð3Þ 0
The first derivative S ðHa H Þ expresses a convolution function, which reflects the line-width and shape for a single crystal. Usually, the line-shape function such as Lorentzian is applied, 2 2
S 0 ðxÞ ¼ 2p1 a3 x½1 þ ðx=aÞ ;
a ¼ 31=2
DH : 2
ð4Þ
DH is the separation between points of maximum and minimum slope. In the case of EPR spectrum of Cu2þ ions in a crystalline environment with a precisely fixed set of coordinating ligands, the line-width of each parallel hyperfine component is the same. However, in a glassy environment the line-width of the parallel hyperfine component has been observed to increase with increasing values of mI . The line-width ðDH ðmI ÞÞ dependence on mI arises from fluctuations in both the ligand fields and bond co-valencies from one copper(II) complex to
½hm þ mI ðgk P Ak Þ ; gk2 b
ð5Þ
where P is a constant (0.036 cm1 ).
4. Results and discussion 4.1. Silica gel oven dried (100 °C) (Fig. 1; Table 2) On inspection of Table 2 (Fig. 1) where the computer fitted spin Hamiltonian parameters have been tabulated, following facts emerge: (i) At room temperature (290 K) the EPR spectrum is devoid of any hyperfine (hf) structure and is nearly isotropic. (ii) At 234 K, two sets of anisotropic Cu(II) spectra (C1 ; C2 ) could be identified. Anisotropic g-spectra of C1 and C2 Cu2þ sites show monotonous changes down to 140 K. (iii) Hf structures of C1 and C2 spectra make their appearance at 200 and 140 K respectively. (iv) Spin Hamiltonian parameters (gk , Ak ) associated with C1 and C2 sites are significantly different. Characteristic unequal hf line-widths (progressively increasing from low to high magnetic field) for both C1 and C2 sites indicate that the Cu2þ complexes are in glassy environment. This feature of hf line-width has been satisfactorily explained by taking into account of variation in ligand field i.e. introducing non-zero value of dgk i.e. gk spread. (v) Computer deconvolution also reveals the presence of a third spectra (C3 ) at 220 K. Principal g-values (gk and g? ) of C3 Cu(II) species are remarkably different from those of C1 and C2 species. There is one possibility that C3 species are, in fact, Cu(II)-pairs and so derived principal g-values pertain to those of Cu(II)-pairs. Further, if it is assumed that the
Table 2 Values of Spin-Hamiltonian parameters (gk , g? , Ak , A? ), dgk and line-widths (DHk , DH? ) of Cu2þ doped dried (100 °C) SiO2 gel (1.0 mol%) Sites
290 K
234 K
220 K
200 K
140 K
10.6 K
C1
gk ¼ 2:195 g? ¼ 2:185 DHk ¼ 170 DH? ¼ 95
gk ¼ 2:350 g? ¼ 2:080 DHk ¼ 100 DH? ¼ 50
gk ¼ 2:350 g? ¼ 2:075 DHk ¼ 100 DH? ¼ 50
gk ¼ 2:400 g? ¼ 2:100 Ak ¼ 140 A? ¼ 10 dgk ¼ 0:03
gk ¼ 2:410 g? ¼ 2:080 Ak ¼ 140 A? ¼ 5 dgk ¼ 0:03
gk ¼ 2:410 g? ¼ 2:080 Ak ¼ 145 A? ¼ 5 dgk ¼ 0:03
C2
–
gk ¼ 2:228 g? ¼ 2:154 DHk ¼ 100 DH? ¼ 85
gk ¼ 2:340 g? ¼ 2:150 DHk ¼ 170 DH? ¼ 120
gk ¼ 2:350 g? ¼ 2:100 DHk ¼ 170 DH? ¼ 120
gk ¼ 2:350 g? ¼ 2:08 Ak ¼ 160 A? ¼ 3 dgk ¼ 0:03
gk ¼ 2:355 g? ¼ 2:08 Ak ¼ 160 A? ¼ 5 dgk ¼ 0:03
C3
–
–
gk ¼ 2:230 g? ¼ 2:140 DHk ¼ 100 DH? ¼ 75
gk ¼ 2:230 g? ¼ 2:140 DHk ¼ 100 DH? ¼ 90
gk ¼ 2:250 g? ¼ 2:130 DHk ¼ 120 DH? ¼ 90
gk ¼ 2:260 g? ¼ 2:120 DHk ¼ 130 DH? ¼ 95
Ak , A? , DHk and DH? are expressed in Oe.
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4.2. Silica glass calcined at 400 °C (Fig. 2(a); Table 3) 290 234 230 226 220 215 210 205 200 195 190 179 160 140 119 100 10.6
2000
2500
3000
3500
4000
At 290 K (RT), the C01 spectrum shows g-anisotropy but hf structure is absent (Fig. 2(a); Table 3). This means that Cu(II) species responsible for C01 spectrum are in some kind of rotational motion, the rate of which is such as to average out the hf structure but not g-anisotropy. But hf lines appear at 200 K, indicating that the motion has been slowed down to the extent that the hf structure appears. The line-widths of the four hf lines in the Ak direction vary in the range 2.2–4.7 mT indicating thereby that Cu(II) species represented by C01 spectrum are in glassy environment. Another spectrum (C03 ), is also observed but its gk and g? values are significantly different from those of C01 spectrum. On the other hand, these g-values are close to those of C3 species in oven dried silica gel. So the C03 spectrum may be taken as to represent Cu(II) pair species occurring in large silica Cu(II) ions most probably, have octapores (120 A). hedral conformation with O ions (not H2 O) as ligands. It is likely that with lowering of temperature pair species progressively agglomerate to form CuO clusters and that CuO clusters are small in number as well as have quite large line-width as to escape detection at low temperatures 6 200 K. 4.3. Silica glass calcined at 700 °C (Fig. 2(a); Table 3)
Magnetic Field (Oe) Fig. 1. EPR spectra of 1.0 mol% copper doped oven dried silica gel.
individual Cu(II) units of the copper pairs are under the spell of ligand fields similar to these C1 and C2 species and that the ionic gk -values of the Cu(II) pairs are in the range of those of C1 and C2 species i.e. 2.41–2.35, then it can be shown that the angles between the tetragonal axes of individual units of a Cu(II) pair will lie between 94° (gk ¼ 2:41) and 79° (gk ¼ 2:35). The non-observation of hyperfine structure of C3 species down to 10.6 K provides further support for the presence of Cu(II)-pair species in the silica dried gel (Table 2). To understand the significance of the above observations, it is very much relevant to consider the important findings of Burlamacchi and Martini [18] on the EPR spectral dynamics of aqueous Cu(II) ions in silica gels of known monodispersed pore sizes. C1 and C2 species could then be assigned to the Cu(II) species of the surface of the confined in a region within 20–3 A silica pores. In this region the surface forces are not strong enough as to completely arrest the motion of Cu(II) species and a glassy region is formed giving rise to glassy EPR spectra at low temperatures. In the above C3 spectrum has been assigned to Cu(II) pairs. This results from agglomeration of Cu(II) species in the surface-force-free regions of large silica pores (120 A) occurring at low temperature ( 6 220 K).
Only one kind of glassy spectrum (C001 ) with varying hf line-width has been observed in the temperature range 290–10.4 K. The spectrum obviously arise from nearly isolated Cu(II) species present in the glassy region of silica pores of medium diameter. The intensity of the spectrum is much reduced compared to the spectra observed for equal mass of the silica glass calcined at 400 °C. This confirms the fact that most of the Cu(II) species become attached to SiO2 network formed on the surface of interconnected silica pores of large diameters [8,9] and give rise to ferromagnetism but EPR-inactive. 4.4. Silica glass calcined at 800 °C (Fig. 2(b); Table 3) Density and nitrogen adsorption isotherm measurements of pure SiO2 glass [4] showed that in transition from 700 to 800 °C calcined glass mean pore radius undergoes enormous enhancement i.e. from 10 to 23 A. This is suggestive of the fact that a large number of small pores collapse to give rise to larger pores and some copper(II) ions are incorporated in them. The Cu(II) ions situated in the central region of the larger pores have free movement of rotation type at RT and if the frequencies of their motion are greater than the EPR frequency, the anisotropy of the spectrum is averaged out and isotropic g-spectrum (C0 ) is expected as has been the case (Table 3). Along with the isotropic spec00 trum, anisotropic spectrum (C000 1 ), similar to that (C1 )
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(i):290K (i):249K
(iii):77K
(i):200K
(i):10K
(ii):290K
(iii):300K
(ii):10K
2600
(a)
2800
3000
3200
3400
3600
2400
2600
2800
(b)
Magnetic field (Oe)
3000
3200
3400
Magnetic field (Oe)
(iv):300K
(iv):77K
(v):77K 0
(c)
2000
4000
6000
8000
10000
Magnetic field (Oe)
Fig. 2. (a): EPR spectra of 1.0 mol% copper doped silica gel calcined at (i) 400 °C, (ii) 700 °C. (b): EPR spectra of 1.0 mol% copper doped silica gel calcined at (iii) 800 °C. (c): EPR spectra of 1.0 mol% copper doped silica gel calcined at (iv) 900 °C, (v) 1000 °C.
observed for 700 °C calcined sample, is also present at RT. The C0 isotropic spectrum is absent at 77 K. One
possibility is that the motion of Cu(II) ions responsible for C0 spectrum at RT becomes frozen at 77 K and the
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175
Table 3 Values of spin-Hamiltonian parameters (gk , g? , Ak , A? ), dgk and line-widths (DHk , DH? ) of Cu2þ doped SiO2 gel (1.0 mol%) at different calcined temperature Calcining temperature (°C)
Sites
Expt. temperature 290 K
249 K
200 K
10.4 K
400
C01
gk ¼ 2:39 g? ¼ 2:08 DHk ¼ 100 DH? ¼ 50
gk ¼ 2:40 g? ¼ 2:08 DHk ¼ 100 DH? ¼ 50
C03
gk ¼ 2:23 g? ¼ 2:18 DHk ¼ 140 DH? ¼ 100
gk ¼ 2:24 dg? ¼ 2:15 DHk ¼ 120 DH? ¼ 100
gk ¼ 2:405 g? ¼ 2:08 Ak ¼ 140 A? ¼ 10 dgk ¼ 0:03 –
gk ¼ 2:41 g? ¼ 2:08 Ak ¼ 145 A? ¼ 5 dgk ¼ 0:03 –
290 K
10.4K
gk ¼ 2:39 g? ¼ 2:08 Ak ¼ 145 A? ¼ 5 dgk ¼ 0:03
gk ¼ 2:41 g? ¼ 2:074 Ak ¼ 150 A? ¼ 5 dgk ¼ 0:03
700
C001
300 K
77 K
800
C000 1
gk ¼ 2:39 g? ¼ 2:08 Ak ¼ 140 A? ¼ 1 dgk ¼ 0:03
C0
gk ¼ 2:17 g? ¼ 2:17 DHk ¼ 200 DH? ¼ 200
gk ¼ 2:405 g? ¼ 2:065 Ak ¼ 150 A? ¼ 1 dgk ¼ 0:03 –
300 K
77 K
900
C4
gk ¼ 2:42 g? ¼ 2:15 DH ¼ 850
gk ¼ 2:42 g? ¼ 1:7 DH ¼ 5000
300 K
77 K
1000
C5
Not detectable
gk ¼ 2:425 g? ¼ 1:7 DHk ¼ 5700 DH? ¼ 5600
Ak , A? , DHk and DH? are expressed in Oe.
spectrum (C0 ) turns anisotropic and it coalesces with the C000 1 spectrum giving rise to only one spectrum. 4.5. Silica glass calcined at 900 °C (Fig. 2(c); Table 3) Anisotropic EPR spectrum with hf structure is absent. Instead, at RT a broad anisotropic signal (DH ¼ 850 Oe) with gk ¼ 2:42 and g? ¼ 2:15 is observed. At 77 K the spectrum becomes enormously broad (DHk ¼ DH? ¼ 5000 Oe) having abnormally low g? value <2.0 (gk ¼ 2:42;g? ¼ 1:70). The above observations are explained as follows: (i) Raman study in conjuction with density and specific area measurements [4] revealed that the condensation of surface Si–OH groups to Si–O–Si bonds and pore collapse took place almost simultaneously at 900 °C. It is thus evident that when the calcination temperature is less than 900 °C, some Si–OH groups are present which are responsible for the observed anisotropic Cu(II) spectrum with hf structure. So, the absence of hf structured EPR spectrum
for the 900 °C calcined silica glass is consistent with the complete absence of Si–OH groups. (ii) After collapse of most of the pores at 900 °C calcination temperature, it is natural to expect that agglomeration of CuO molecular units occur at several locations in SiO2 glass network, resulting in the formation of nano-sized particles of CuO. Kindo et al. [19] found that bulk CuO, which becomes antiferromagnetic at 230 K (TN ), exhibits unusually broad paramagnetic resonance with the line-width of about 16 KOe and g-value of about 2.0 at room temperature at the 45 GHz region under pulsed magnetic field. One-dimensional spin correlation has also been established. Below TN , zero field antiferromagnetic resonance in the infrared was also observed. In the present case, on the other hand, the line-width is comparatively quite small at RT (850 Oe as against 16 000 Oe). However, the increase of line-width on lowering the temperature from RT to 77 K is quite enormous (850–5000 Oe) but at the same time no AF transitional effect as in bulk CuO has been detected. Only a small g-shift has
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been observed. It follows, therefore, that EPR spectrum of copper doped silica glass calcined at 900 °C is quite distinct from that of bulk CuO compared but, at the same time, is of unusual nature. So, it is quite reasonable to infer that CuO particles grown in silica glass on calcination at 900 °C are nano-sized and become superparamagnetic at low temperatures. 4.6. Silica glass calcined at 1000 °C (Fig. 2(c); Table 3) In contrast to what has been observed for 900 °C calcined silica glass, 1000 °C calcined sample does not show any EPR signal at RT. The EPR signal is likely be very weak because of the fact that majority conversion of Cu2þ ions to Cuþ ions takes place at 1000 °C i.e. generation of Cu2 O. This is confirmed from spectroscopic studies [14] as well as from our powder XRD measurements. It is known that densification of silica glass is nearly complete at 1000 °C calcination temperature [4]. So, it is reasonable to infer that CuO clusters are somewhat bigger nano-particles at higher calcination temperature. Bigger CuO nano-particles are also expected to have very much broader line-width at RT as to escape detection. However, at 77 K, a very broad signal, larger than those for 900 °C calcined sample, (DHk ¼ 5700 Oe and DH? ¼ 5600 Oe) has been observed because of superparamagnetism.
5. Concluding remarks The present work by Cu(II) EPR on the dynamics of local structure in 1.0 mol% copper doped porous silica gel-glass in the temperature range 10.4–300 K has revealed the following: (i) Three kinds of Cu(II) species (including one due to copper pairs) having different rates of rotational motions are present in the pores of oven dried (100 °C) silica gel. (ii) In silica glass calcined at 400 °C two types of Cu(II) species including Cu-pairs having much slower rotational motion have been detected at room temperature. Cu-pairs agglomerate at low temperature ( 6 200 K) to form CuO clusters. (iii) EPR-wise only one kind of isolated Cu(II) species has been detected in silica glass calcined at 700 °C. Since majority number of Cu(II) ions are shown to be engaged in forming ferromagnetic planer layers in the 500–750 °C temperature range [8,9], very few Cu-pairs are available for EPRdetection. (iv) Average pore radius is more than doubled (10–23 in the transition of the calcination temperature A) from 700 to 800 °C [4]. Consistent with this finding,
isotropic EPR spectrum originating from free moving Cu(II) species present in very large pores of silica glass calcined at 800 °C has been observed at RT. (v) In case of silica glass calcined at 900 and 1000 °C, a very broad line having width 5000 Oe has been detected at 77 K which most likely has its origin in superparamagnetic CuO nano-particles present in the silica glass. For confirmation, experiments on heavily doped silica glass (Cu(II) ions having concentration of the order of 10 mol% or so) are desirable which are in progress. (vi) It is relevant in this connection to mention the Cu2þ EPR findings of Darab and MacCrone [7] and Duval et al. [9] in the context of the present work. Their methods of sol–gel preparation are quite different from what followed by us [3]. Also our method of thermal processing of gel samples [4] is distinct from those practised by them. Spin Hamiltonian parameters determined for Cu2þ doped silica dried gel by computer deconvolution of the EPR spectra as well as those calcined at various temperatures are distinctly different from those determined by Darab and MacCrone [7] and Duval et al. [9]. Since their works were confined to calcination temperatures up to 600 and 750 °C respectively, they have missed the superparamagnetic resonance of CuO nano-particles obtained at calcination temperatures 900 and 1000 °C observed by us. The presence of Cu(II)-pair spectra in Cu(II):silica gelglass at low temperatures has also escaped detection of previous workers.
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