Solid State Nucl. Magn. Reson. 23 (2003) 88–106
The effects of different heat treatment and
atmospheres on the NMR signal and structure of
TiO2–ZrO2–SiO2 sol–gel materials
P.N. Gunawidjaja,a M.A. Holland,b G. Mountjoy,b
D.M. Pickup,b R.J. Newport,b and M.E. Smitha,*
b
a
Department of Physics, University of Warwick, Coventry CV4 7AL, UK
School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NR, UK
Received May 1, 2002; revised August 29, 2002
Abstract
The effects of different heat treatment schemes (i.e. successively or directly heated to
particular temperatures) and atmospheres (air or nitrogen) on the solid-state NMR spectra
obtained from (TiO2)0.15(ZrO2)0.05(SiO2)0.80 sol–gel materials are investigated. A combination
of 1H, 13C, 17O and 29Si NMR is used. 29Si MAS NMR indicates that the extent of
condensation of the silica-based network strongly depends on the maximum temperature the
sample has experienced, but the condensation is largely independent of the details of the heat
treatment scheme and atmosphere used. For sol–gel produced silicate-based materials the
results show that the equilibrium structure at each temperature is reached rapidly compared to
the time (2 h) spent at that temperature. The 17O NMR results confirm that a nitrogen
atmosphere does significantly reduce loss of 17O from the structure but care must be taken
since there could be differential loss of 17O from the regions having different local structural
characteristics.
r 2003 Elsevier Science (USA). All rights reserved.
Keywords: Silicate sol–gel; Heat treatment;
17
O; Multinuclear solid-state NMR; Amorphous structure
1. Introduction
Sol–gel formation of materials is widely used and extensively studied by a variety
of techniques [1]. There are many systems, such as silica containing with a few mol%
*Corresponding author. Fax: +44-24-7669-2016.
E-mail address: [email protected] (M.E. Smith).
0926-2040/03/$ - see front matter r 2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S0926-2040(02)00019-X
P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
89
TiO2 or ZrO2, which can be usefully produced by sol–gel methods. These materials
can have very interesting physical properties which for TiO2/ZrO2-doped SiO2
include ultra-low thermal expansion (ULE) [1], chemical resistance for coatings [2],
catalysis [3] and high-refractive indices. In the optical industry [4], these materials are
produced as anti-reflective thin film coatings with tailored refractive indices. The
properties of these materials are strongly dependent on their chemical composition,
homogeneity and texture. Sol–gel processing, based on hydrolysis of metal alkoxide
precursors, and subsequent condensation, offers a technique that can combine
atomic level mixing, without the need for very high processing temperatures, with a
high degree of porosity. Although much work exists on the sol–gel process there is
comparatively little work on the details of the structure and how it evolves with
temperature. To improve understanding of such complex materials requires a
combination of various advanced structural probe techniques. Techniques that have
been applied to TiO2/ZrO2-doped SiO2 materials now include diffraction [5], X-ray
absorption (both XANES and EXAFS) [6–12], SANS [13], SAXS [14], FT-IR
[8,10,15], XPS/XAES [16], and DRIFTS and FT-Raman [17]. Our research
programme has involved a range of these techniques applied to TiO2–SiO2 and
ZrO2–SiO2 xerogels to probe the structural details of these amorphous materials,
and to investigate the different roles that titanium and zirconium play in such
structures.
NMR is a good atomic scale probe of the structure of these materials. 29Si NMR is
sensitive to the connectivity of the SiO4 units [18] so it has been widely used in both
the TiO2–SiO2 and ZrO2–SiO2 systems [15,19–25]. 29Si NMR is readily extended to
characterising similar local structural units in related hybrid siloxane–oxide gel
materials [26,27]. Other nuclei (1H, 13C) are readily accessible in such gel-derived
oxide materials and have been reported from TiO2 [28], ZrO2 [29], MgO [30], ZrO2/
TiO2–SiO2 [17] and siloxane–ZrO2 [25].
However in these materials, the most direct NMR probe of the bonding
arrangement throughout the structure is 17O. The major handicap is the low natural
abundance (0.037%) of 17O so that isotopic enrichment is usually required. The sol–
gel process lends itself naturally to such enrichment by using 17O-labelled water in
the hydrolysis process. There are well-defined signals from the different fragments
present with Si–O–Si showing a second-order quadrupolar lineshape [18,31] around
0 ppm [32]. The other metal environments OTi3, OTi4 [28,33,34], OZr3 and OZr4
[25,29,35] have well-separated shifts. Detection of oxygen bridges between the
silicate network and the metal in the system is also very important. For Ti–O–Si
linkages there has been a lot of 17O NMR work to calibrate their NMR interactions
by looking at crystalline compounds with known oxygen bridges (e.g. [20,36,37]).
Then in gel-derived materials atomic mixing can be identified through the presence
of Ti–O–Si [15,20,38] or Zr–O–Si [21,22,25]. For example in SiO2 with 8 mol% TiO2
added, even after heating up to 750oC, the only 17O signals detected were Si–O–Si
and Si–O–Ti, whereas a B41 mol% TiO2-containing sample showed clear signs of
phase separation after heating through the appearance of Ti–O–Ti signals [20].
In previous studies the heat treatment and atmospheres used for the different
experimental approaches have not necessarily been the same. For most experiments
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P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
the initial gel is directly heated to the target temperature (here termed direct heating)
under an air atmosphere, which can be regarded as typical processing conditions.
However given the expense of the 17O isotope smaller samples are usually
successively heated to the various temperatures (here termed stepped heating) under
a nitrogen atmosphere, with NMR experiments carried out between each heat
treatment. A nitrogen atmosphere is used so as to reduce the amount of oxygen
exchange with the atmosphere. In this paper, the effects of various heat treatment
schemes (e.g. directly or stepped) and different atmospheres on the structure and
intensity of the 17O NMR signal from a (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogel is
studied in detail. 1H, 13C, 17O and 29Si NMR is reported to obtain a broad NMR
perspective of the structure and its change with heat treatment.
2. Experimental details
2.1. Sample preparation and thermal analysis
The sample was prepared by reacting tetraethyl orthosilicate, TEOS (Aldrich,
98%), with zirconium (IV) n-propoxide Zr(OPrn)4 (70 wt% solution in propan-1-ol,
Aldrich) and titanium (IV) butoxide, Ti(OBui)4 (Alfa, 99%). HCl (Fisons) was used
as a catalyst to promote the hydrolysis and condensation reactions, and ethanol was
used as a mutual solvent. An organic modifier, acetylacetone (acac), was used to
control the reactivity of the Ti and Zr alkoxides by the formation of larger, slower
reacting molecules by ligand exchange. A modified method of Yoldas [39] was used
to promote homogeneity within the mixed oxide samples and is based upon
procedures previously described [40,41]. This involves prehydrolysis of the TEOS to
maximise the number of Si–OH groups before mixing with the modified Zr(OPrn)4
and Ti(OBui)4 precursors, which are nevertheless still more reactive than TEOS; the
aim being to encourage Ti–O–Si and Zr–O–Si, bonding as opposed to Ti–O–Ti, Ti–
O–Zr or Zr–O–Zr bonding which could lead to phase separation. The chosen
prehydrolysis conditions were TEOS:EtOH:H2O in the presence of HCl (pH=1),
stirring for 30 min. The appropriate quantity of Zr(OPrn)4 and Ti(OBui)4 was slowly
added to the prehydrolysed TEOS solution while stirring, with the zirconium source
added 1 min before the titanium. The stoichiometry of the modified alkoxide
precursors was adjusted to achieve a total metal content of 20 with 15 mol% of TiO2
and 5 mol% of ZrO2. Note that all reagents were loaded in a dry box under a
nitrogen atmosphere and transferred using syringes to avoid absorption of moisture
from the atmosphere. The sample was prepared using 20 mol% 17O-enriched water
(D-Chem). The resulting sol was then left to gel; this typically took a few days. The
sample was then aged for 1 week before being air dried for several days, finely
ground and then pumped under vacuum for 24 h to remove any excess solvent, this is
termed the initial gel.
The sample was divided into seven portions, one of which was unheated, the
others were heated directly to 2501C, 5001C or 7501C either under an air or N2
atmosphere. Heat treatment was performed at a heating rate of 21C/min with the
P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
91
final temperature of a particular sample maintained for 2 h. For comparison both
samples that had been heated to 2501C, were further heated to 5001C and then to
7501C to give the stepped heating procedure. Hence a total of 11 different samples
were examined.
Thermogravimetric analysis was carried out using a Stanton Redcroft TG 750/770
apparatus. Analysis was performed on the (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogels, in
flowing air or N2 with a heating rate of 21C/min up to B10001C.
2.2. Magic angle spinning NMR
1
H NMR spectra of 17O-enriched xerogels were acquired on a Chemagnetics CMX
360 spectrometer operating at 360.13 MHz, using a Bruker 4-mm double-bearing
probe. The 1H NMR spectra were collected using magic angle spinning (MAS) at
typically B12 kHz with a 5 ms (B901 tip angle) pulse and a 20 s recycle delay to
ensure complete relaxation. Chemical shift values were referenced externally to TMS
(d=0 ppm). Each spectrum was the result of summing together B64 FIDs. To
determine the proton content each sample was weighed and a reference spectrum was
measured for adamantane (C10H16), which was also weighed. The probe was
carefully retuned to try and reproduce as closely as possible the tune and match
conditions between each experiment. The integrated signal was then normalised to
take into account the amount of sample so that the absolute level of protons can be
deduced.
13
C MAS NMR spectra of (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogels were acquired on
a Bruker MSL-300 spectrometer operating at 75.47 MHz, using a Bruker 7-mm
double-bearing probe. The 13C spectra were collected using CP MAS at B5 kHz
with a 7 ms pulse, a 10 s recycle delay and 1 ms contact time. Spectra were acquired
under 1H decoupling with an rf field strength of 36 kHz. Chemical shifts were
referenced externally to adamantane (d=38.56 ppm for the most intense peak
relative to TMS) and each spectrum was the result of adding B2000 FIDs.
29
Si MAS NMR spectra of samples were acquired on a Bruker MSL-300
spectrometer operating at 59.62 MHz using a Bruker 7-mm double-bearing probe.
The 29Si spectra were collected using MAS of B5 kHz with a 2 ms (B301 tip angle)
pulse and a 30 s recycle delay. The pulse delay was sufficient to prevent saturation.
Chemical shifts were referenced externally to TMS (d=0 ppm) and each spectrum
was the result of summing B2000 scans.
17
O NMR spectra of 17O-enriched xerogels were acquired on Chemagnetics CMX
240 and 360 spectrometers operating at 32.27 and 48.18 MHz, respectively, using a
Bruker 4-mm double-bearing probe. The 17O MAS NMR spectra were recorded
using a spin-echo y–t–2y pulse sequence with y=901. The t delay value was
determined in MAS experiments by the spinning frequency of the rotor
(R0B12 kHz) and was therefore 80–85 ms (equivalent to 1/R0). The relaxation time
of 17O increased strongly as the sample was heated. In the initial gel T1 for 17O was
B0.1 s so that a recycle delay of 0.5 s was employed to give relaxed spectra.
However, as the sample was heated and protons were lost T1 increased such that in
samples that had been heated to 7501C T1 was 3.5–4.5 s. In all, 4000–180,000 scans
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P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
were added to obtain the final spectrum. Chemical shifts were referenced externally
to distilled water (d=0 ppm, pH=7.170.2). The samples were carefully weighed and
the system retuned as reproducibly as possible so that the absolute level of the
oxygen signal, and how this changed with temperature, could be determined.
3. Results and data analysis
3.1. Thermal analysis
The loss of the organic component along with hydroxyl groups from the structure
can be followed by thermal analysis (TGA). Fig. 1 shows the TGA curves for the
initial gels heated under air (a) and nitrogen (b) from room temperature up to
10001C. Up to B2001C the two curves are essentially identical. Then the air-heated
sample shows a greater mass loss, with a small feature at 4001C, which is probably
associated with combustion of the remaining organic fragments. There is then a
more gradual mass loss and it is slightly greater in the air-heated sample. By 7501C
the mass of both samples has stabilised with the air-heated sample showing a mass
loss of 26.7% compared to 24.4% for the sample heated under nitrogen.
3.2.1. 29Si MAS NMR
Fig. 2 shows some typical 29Si MAS NMR spectra from the (TiO2)0.15(ZrO2)0.05
(SiO2)0.80 xerogel. In the initial xerogel (Fig. 2(a)) there are two resolved peaks of
comparable amplitude separated by B8 ppm. There is also a lower amplitude
shoulder at positive shift. These can be identified with the differently connected SiO4
species which are termed Qn units (Qn stands for a SiO4 unit with n bridging
oxygens). To quantify the Qn distribution the 29Si MAS NMR spectra were
deconvolved by Gaussian fitting using WinFit software [42]; three signals could be
distinguished at typically 91, 100 and 108 ppm which correspond to Q2, Q3 and
Q4, respectively. As the samples are heated all schemes show the same general
behaviour with increased heat treatment resulting in a higher Q4-content, with a
typical spectrum shown in Fig. 2(b) for a sample step-heated to 7501C under an N2
atmosphere. By 7501C the Q4 species is dominant (B80%) with the remainder of the
silicon as Q3 and no Q2. The results of the Gaussian fitting of the 29Si MAS NMR
spectra are summarised in Table 1. These fits show the general trend of increasing
connectivity with increasing heat treatment. Some other clear trends can be seen in
this data. The linewidth of the Q4 species is typically larger than that for either Q2 or
Q3. The width of the Q2 peak remains approximately constant, whereas the
linewidths associated the Q3 and Q4 resonances tend to increase with heat treatment
(an apparent exception to this is the Q3 in the sample step heated to 5001C). The
isotropic shifts of all species tend to negative shift with heat treatment, with this
change really marked in the Q3 and Q4 species on heating from 5001C to 7501C.
P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
93
Fig. 1. TGA traces of (TiO2)0.15(ZrO2)0.05(SiO2)0.80 heated under (a) air and (b) N2.
3.2.2. 13C CP MAS NMR
The 13C CP MAS NMR spectra recorded at 7.05 T are shown in Fig. 3, which
compares the influence of nitrogen and air with the direct heating scheme. Although
the sample has been reacted and dried there is a strong 13C signal from the unheated
xerogel, which must come from residual organic fragments from the alkoxide
precursor and acac. The spectra are dominated by peaks at between B5 and B45
ppm. These resonances can be assigned to alkyl parts of the alkoxide chain and acac.
The resonances observed between B45 and B115 ppm can be assigned to C–O
bonds and the peaks observed between B180 and B200 ppm are from C ¼ O bonds.
After heating to 2501C the samples heated under different atmospheres show very
different 13C CP MAS NMR spectra. The sample heated under an air atmosphere
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Fig. 2. 29Si MAS NMR spectra and their deconvolution by Gaussian fitting for the (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogels in (a) the unheated form and (b) after successive heat treatment to 7501C under N2.
shows a significantly greater loss of intensity compared to the sample heated under
nitrogen with the air-heated sample having only 4075% of the integrated intensity
of the corresponding nitrogen-heated sample. Hence the loss of organic fragments is
encouraged under air at this particular temperature. After heating to 5001C there is a
very large loss of 13C NMR signal with very little CP signal detectable, and no
significant difference between spectra from samples heated under the different
atmospheres and heat treatment schemes. These results suggest that, by 5001C in all
samples, all the organic fragments have been removed. With CP detection there is the
possibility that some deprotonated carbon species could remain and yet not be
P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
95
Table 1
29
Si MAS NMR data for (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogels given different heat treatment under air
and nitrogen atmospheres giving the spectral deconvolution into different Qn speciesa
Heat treatment (1C) Atmosphere Q2
and method
Unheated
Direct 250
Direct 500
Step 500
Direct 750
Step 750
Q3
Q4
fwhm d (ppm) I (%) fwhm d (ppm) I (%) fwhm d (ppm) I (%)
(Hz)
(Hz)
(Hz)
—
Air
N2
Air
N2
Air
N2
Air
N2
Air
N2
680
680
680
690
690
680
680
—
—
—
—
90.7
90.5
89.4
91.0
87.3
92.1
94.0
—
—
—
—
13.2
10.7
10.6
8.5
5.1
7.8
6.7
—
—
—
—
480
540
590
715
670
610
600
760
740
740
740
100.2
98.8
98.4
97.6
96.8
101.8
101.6
102.7
100.3
105.1
106.3
a
fwhm, d and I represent the linewidth full-width half-maximum,
intensity, respectively. Errors: fwhm 725 Hz, d71.5 ppm, i72.5%.
39.9
25.2
24.9
17.6
20.8
20.5
17.9
21.4
18.8
18.3
18.7
550
700
730
700
740
700
740
800
740
740
740
108.4
106.6
106.8
108.7
106.0
109.9
109.4
111.2
110.3
111.8
112.7
46.9
64.1
64.5
74.0
74.1
71.2
75.4
78.6
81.2
81.7
81.3
29
Si chemical shift and relative
Fig. 3. 13C CP MAS NMR spectra of the (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogels (a) unheated and after
direct heat treatment to 2501C under (b) N2 and (c) air ( 3), and to 5001C under (d) N2 ( 30) and (e) air
( 30). Asterisks denote spinning sidebands.
detected. Such unprotonated carbons typically have very long T1 relaxation times
[18] and are consequently difficult to detect. It is likely that there is a small amount of
a carbon species (e.g. coke) present since the samples heated to higher temperatures,
especially under nitrogen, tended to be grey.
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3.2.3. 1H MAS NMR
The integrated intensities of the 1H NMR spectra were calculated directly using
the spectrometer software. As samples were run on different days normalisation was
carried out by weighing each sample and comparing the result with the integrated
intensity of a reference adamantane spectrum. The 1H MAS NMR results obtained
at 8.45 T for step-heating under nitrogen and air are shown in Fig. 4. In the dried,
unheated xerogels, the spectra are dominated by a resonance which peaks close to
6 ppm and can be assigned to protons in C–H bonds, along with a second, lower
intensity peak at higher shift value. After heating to 2501C there is an interesting
difference between the samples step-heated under nitrogen (Fig. 4(b)) and air
(Fig. 4(c)). Under the nitrogen atmosphere the form of the spectrum is very similar
to that at room temperature but with the total signal intensity reduced by B3273%.
However under air there is a 4973% loss of total signal and this difference in loss of
intensity is largely due to the greatly reduced intensity of the 6 ppm peak in the airheated sample. The remaining resonance observed at 10 ppm can be assigned to O–H
bonds. Further heating causes even greater signal loss (Table 2) with only the –OH
signal eventually remaining. Indeed, a small signal of between 1% and 6% of the
original signal intensity from this species exists even after heating to 7501C. For both
atmospheres, stepped heat treatment (i.e. greater total heating time) leads to the
removal of more OH groups than direct heating. For both regimes heating under air
leads to more OH loss than heating under nitrogen.
Fig. 4. 1H MAS NMR spectra of the (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogels after various heat treatment:
(a) unheated, direct 2501C (b) N2 and (c) air, step 5001C (d) N2 ( 8) and (e) air ( 8), and step 7501C (f)
N2 ( 64) and (g) air ( 64).
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97
Table 2
Amount of hydrogen in (TiO2)0.15(ZrO2)0.05(SiO2)0.80 samples as determined by 1H NMR
Heat treatment (1C)
Total amount (mol/g)
Air
Unheated
250
Direct 500
Step 500
Direct 750
Step 750
N2
3
1.7470.10 10
8.6070.40 104
2.1170.32 104
1.1570.17 104
5.3771.07 105
1.7970.54 105
1.1870.06 103
1.7870.27 104
1.9770.30 104
1.0870.16 104
4.4070.88 105
Fig. 5. 17O MAS NMR spectra at 8.45 T of the (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogels after various heat
treatment: (a) unheated, direct 2501C (b) N2 and (c) air, direct 5001C (d) N2 and (e) air, and direct 7501C
(f) N2 and (g) air. Asterisks and the dotted lines indicate the positions of spinning sidebands (note that
these sidebands come from different sites and different transitions).
3.2.4. 17O MAS NMR
The 17O MAS NMR spectra recorded at 8.45 T are presented in Fig. 5. In the
dried, unheated xerogels, the spectra are dominated by a resonance which peaks at
just below 0 ppm and shows a typical second-order quadrupolar lineshape. This
resonance is due to Si–O–Si bridges with a contribution from Si–OH groups [32].
This signal has spinning sidebands at B260 and B–240 ppm. Both samples also have
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resonances between B100 and B300 ppm which can be assigned to Ti/Zr–O–Si by
comparison to solution state NMR studies [27]. It is also possible that there is some
very weak intensity in the region above 400 ppm (Fig. 5) although the signal-to-noise
ratio does not allow this to be identified unequivocally. Any signal in this region
would come from Ti/Zr–O–Ti/Zr fragments. Results after heating to 2501C show
that samples heated under an air atmosphere tend to lose significant intensity from
the Ti–O–Si/Zr–O–Si region of the spectrum compared to samples heated under
nitrogen. The intensity of the Si–O–Si/Si–O–H region is approximately constant.
After heating to 5001C the sample heated under air surprisingly appears to regain
some intensity in the Ti–O–Si/Zr–O–Si region, and again under nitrogen there is
more signal than from the air-heated samples for both directly and step-heated
samples. There is further signal loss after heating to 7501C. Samples heated under a
nitrogen atmosphere still show a measurable signal intensity, with the sample that
has been directly heated (i.e. less integrated heat treatment) showing a larger signal
than from the sample that was step-heated. An additional signal is present (Fig. 5)
with a shift greater than 400 ppm, due to the formation of Ti/Zr–O–Ti/Zr bonding.
Under air there is a very much weaker signal, and no signal could be observed in the
region above 400 ppm. For the sample step-heated under air, no measurable signalto-noise could be accumulated in any reasonable time.
17
O MAS NMR spectra were also recorded at a lower field of 5.6 T (Fig. 6). The
same information is present as at the higher field but there are now different
apparent relative intensities as the various NMR interactions scale differently with
Fig. 6. 17O MAS NMR spectra at 5.6 T of the (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogels after various heat
treatment: (a) unheated, direct 2501C (b) N2 and (c) air, direct 5001C (d) N2 and (e) air, and direct 7501C
(f) N2 and (g) air. Asterisks and the dotted lines indicate the positions of spinning sidebands (note that
these sidebands come from different sites and different transitions).
P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
99
field. The width of the second-order quadrupolar broadened Si–O–Si resonance
decreases from 226 ppm at 5.6 T to 103 ppm at 8.45 T, showing almost exactly the
expected n2
o dependence [31]. The M–O–Si signals have approximately the same
linewidth (in ppm) as at the higher field indicating that chemical shift dispersion is
the dominant linebroadening mechanism for these peaks.
The normalised integrated signal intensity at 5001C and 7501C is compared to the
initial unheated gel in Fig. 7(a). There is a large loss of oxygen under both
Fig. 7. (a) Comparison of the normalised intensity of 17O MAS NMR spectra from some of the xerogels
and (b) some 17O spectra plotted on a normalised intensity scale.
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atmospheres with a maximum of 2773% remaining even after heating to 5001C.
The samples heated at 7501C under nitrogen show comparable signal intensities to
those observed after heating to 5001C, but the air sample shows a measurable
decrease in intensity. The effect that this loss of signal has on the actual spectra
observed is shown in Fig. 7(b) where normalised spectra are displayed. Heating
typically reduces the 17O NMR signal but under air the loss is much more significant.
The sample heated directly to 7501C under nitrogen shows the least loss of signal,
with only 3173% of the signal from the initial gel remaining. Under nitrogen stepheating (i.e. the sample has had more integrated heat treatment) there is a 2573%
greater loss of signal compared to direct heating. However, the justification for
heating under nitrogen to preserve the 17O signal is evident. This is illustrated by the
sample directly heated 7501C under the two atmospheres. The air-heated sample has
only a third of the signal intensity of the sample heated under nitrogen. Given a
nucleus such as oxygen where signal-to-noise ratio is a primary concern in an NMR
experiment such a difference is important for the spectroscopy.
4. Discussion
The 29Si MAS NMR results from this TiO2–ZrO2–SiO2 xerogel exhibit a general
trend towards more polymerised Qn species as the heat treatment is increased.
These results are as expected from previous observations of gels composed
predominantly of SiO2. As a sample is heated to progressively higher temperatures,
the residual organic fragments and hydroxyl groups are driven out of the
structure, producing condensation of the silicate network. The resulting increase in
the cross-linking of the network leads to an increase in the proportions of higher n
Qn species, until at 7501C Q4 is dominant. There has been some confusion in the
literature of these and related gels as to the exact identity of the local structural units
the different Qn species. In the initial gel even after allowing significant time for
reaction and then subsequent room temperature drying the Q2,3 mainly result from
residual organic fragments and hydroxyls, such that the Q3 species are probably
dominated by Q3(OH). Although the proton content, and hence hydroxyl content, is
low after heating to 7501C (a few % of the initial signal) there will still be some
Q3(OH).
The different metals added to the system play different roles with respect to their
network forming/modifying behaviour. The addition of each M4+ ion would form
four non-bridging oxygens (nbo) if it were acting just as a network modifier. Hence,
at the concentration of metal ions added to the sample under discussion here, and
assuming that they all act as modifiers, then the average n for the Qn species present
would be low (the detail would depend on how closely the metal followed a binary or
random model of forming nbo). Since Q4 is so dominant in the observed spectra, this
means that a lot of the added metal must act as a network former. Also, for titanium
entering the network it has been previously observed that the different Q4(mTi), i.e. a
Q4 species with m next-nearest-neighbour titaniums in the network, have a negligible
effect on the shift. This observation has been confirmed by calculations of the
P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
101
isotropic chemical shift which show that the effect of 1 next-nearest-neighbour
titanium atom on the isotropic 29Si shift is only B1 ppm [43]. Titanium and
zirconium show contrasting behaviour in their effect on the network. Zirconium that
remains in the silica host network acts as a network modifier whereas titanium may
be part of the network. Hence the presence of zirconium tends to produce more of
the lower n Qn species (cf. Refs. [15,20–22]). The content of lower n Qn species
observed for the sample studied here is intermediate to that reported for silica when
either 20% of only zirconia or titania is added.
One of the main purposes of this study is to investigate the effect of different
heating regimens and annealing atmospheres on the atomic scale structure of the gel.
The most noticeable feature of Table 1 is that for a given temperature there is a high
degree of consistency of the Qn speciation between the different heating schemes and
atmospheres. These observations suggest that the equilibrium Qn distribution is
attained rapidly at a particular temperature, such that the same structure is present
under both direct and step-heating. Also the atmosphere appears to have no
profound effect on the Q speciation so that the results from heating under nitrogen
and air can be directly compared.
1
H and 13C NMR are good probes of the residual groups from the initial gel. A
combination of thermal analysis and 13C NMR shows that removal of most of the
organic fragments occurs between 2001C and 3001C. For the first heat treatment
2501C was an opportune temperature to choose as it straddles the point when the
bulk of the organics are lost under nitrogen and air. The air-heated sample loses
most of its organic content before 2501C while for nitrogen it is just above this
temperature. This can also be seen from the TGA trace where the mass loss under
nitrogen occurs 20–301C above that from the sample heated under air. The results
also show that the air atmosphere allows more complete removal of hydroxyls than
nitrogen. It has been a common finding of our work that some residual hydroxyls
persist in these sol–gel samples even at the highest temperatures.
17
O MAS NMR results from mixed oxide sol–gel samples provide very clear and
important information on the degree of atomic mixing of the oxides. However the
results herein show that this information should be interpreted with some caution.
The spectrum from the unheated sample shows distinct resonances from Si–O–Si
(B0 ppm), and intensity in the region B150–300 ppm must come from Ti/Zr–O–Si
bonds. The Si–O–Si signal can be simulated with parameters typical of 17O from a
silica gel with the quadrupole coupling constant wQB5.0 MHz and disoB47 ppm
[32,44]. The 150–300 ppm feature has at least two peaks that possibly come from Zr–
O–Si at 165 ppm and Ti–O–Si at 265 ppm. Deconvoluting this region is complicated
by the strong overlap of the resonances and the presence of a spinning sideband from
the Si–O–Si resonance. However in the initial gel around 7072% of the intensity is
in the Si–O–Si/Si–OH region.
Heating the samples shows a change of both the total intensity, with a decrease in
the signal in all samples, and a change of the apparent distribution. In the sample
heated under air at 2501C there is a significant reduction of the intensity from Ti/Zr–
O–Si bonds, while the intensity of Si–O–Si/Si–O–H bonds is largely preserved. It
should be noted that for the air-heated sample the apparent ‘‘reappearance’’ of the
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Ti/Zr–O–Si signal at higher temperature is largely illusory, as it is the loss of
intensity from the Si–O–Si at this temperature that makes the Ti/Zr–O–Si signal
appear larger again. It is clear from these data that there is a differential
loss of oxygen from the different bonds in samples heated under air. The signal
from the Ti/Zr–O–Si bonds is lost more rapidly at first (i.e. up to 2501C),
and although there is then continued loss of oxygen from all oxygen linkages
in the sample at higher temperatures, there is increasing loss from the Si–O–Si bonds.
After heating the samples under air or N2 to 5001C, either directly or stepwise the
numbers and types of the various oxygen sites are found to be similar. After heating
to 7501C there are some significant changes in intensity, and differences between
heating under air and nitrogen. For the sample heated under nitrogen there is a
prominent additional resonance at B460 ppm. This resonance must result from
some Ti/Zr–O–Ti/Zr connections forming. It is known from binary TiO2/ZrO2–SiO2
samples that when phase separation occurs peaks corresponding to M–O–M bonds
are observed. In ZrTiO4, which includes the whole range of (Ti/Zr)–O–(Ti/Zr)
bonds, a broad range of intensity from 380 to 620 ppm is observed [45]. The
appearance of intensity B460 ppm suggests that at 7501C some phase separation
occurs. However, it should be noted that significant intensity is still present from
Ti/Zr–O–Si bonds so that there is a large fraction of the added metal that remains
part of the silicate phase and is not present as a separate phase. In the air-heated
sample there is some intensity in this region but it is very weak. The observed
intensity is centred at 460 ppm, nearer to the signal observed in binary TiO2/ZrO2–
SiO2 samples when ZrO2 rather than TiO2 is added, suggests that zirconium is
dominating the phase separation.
There is apparently a large difference in the 17O MAS NMR spectra from the
samples heated under nitrogen and air. For example, at 7501C the sample heated
under air has a prominent Si–O–Si signal and perhaps some very weak intensity in
other regions. Taken at face value this 17O NMR spectrum would suggest that only
Si–O–Si bonding is present, which is not the case as zirconium and titanium are
present, and X-ray absorption spectroscopy shows that phase separation is occurring
[46]. Hence there must be some Ti/Zr–O–Si and Ti/Zr–O–Ti/Zr linkages present.
However, so much 17O has been lost at this point that the signal of this sample is very
weak. Although this poor signal-to-noise ratio makes it difficult to be certain there is
no evidence (Fig. 5(g)) of any intensity from oxygen linkages that involve titanium or
zirconium. Hence as the metals are still present in the sample, and alternative
evidence of phase separation exists [46], it appears that the signal from Ti/Zr–O–Si
and Ti/Zr–O–Ti/Zr linkages must be preferentially lost. Hence care must be taken in
interpreting such 17O NMR spectra. The absence of a 17O signal cannot be taken to
indicate with absolute certainty the absence of a particular species. If the loss is
preferential from different fragments then it makes quantitative analysis of the
oxygen distribution in such samples difficult. Furthermore, care has to be taken in
the choice of the atmosphere to best preserve the oxygen distribution. However, the
observation of an 17O NMR signal is able to confirm, for example, direct bonding of
a metal to silicon and/or phase separation, and in all samples where other techniques
indicate some phase separation a corresponding 17O signal has always been observed
P.N. Gunawidjaja et al. / Solid State Nucl. Magn. Reson. 23 (2003) 88–106
103
to date when there has been sufficient overall signal-to-noise ratio from the sample
[9,10,20,22].
Multinuclear NMR results show that for a (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogel
the heat treatment atmosphere has very little influence on the Qn-distribution for
silicon, and it appears that the equilibrium silicate structure is reached quickly at any
given temperature (certainly compared to the 2 h period employed in this study). The
majority of the organic fragments are lost at a temperature between 2001C and
3001C, and the hydroxyls are progressively lost but with some remaining even after
heating to 7501C. Oxygen NMR can show in detail when phase separation occurs,
although a nitrogen atmosphere should be used to preserve more oxygen in the
sample in order to give the best sensitivity for detecting different oxide fragments. As
it appears that oxygen is lost differentially, especially under air, the signal observed
from those fragments attached to the metal must represent the minimum actually
present in the sample (compared to the Si–O–Si signal intensity).
5. Conclusion
A comparison has been made of the effect of various heat treatment schemes (e.g.
step- or directly heated) and different atmospheres (air or nitrogen) on the structure
of a (TiO2)0.15(ZrO2)0.05(SiO2)0.80 xerogel using NMR and thermal analysis. The
extent of condensation of the silicate network, based on 29Si MAS NMR was found
to be similar irrespective of different heat treatment or atmosphere used. This
suggests that an equilibrium silicate network structure is reached on timescales
shorter than 2 h. Thermal analysis and 13C NMR shows that there is a significant
mass loss associated with the organic fragments between 2001C and 2501C for the
sample heated under air. The same loss occurs 20–301C higher for the sample heated
under nitrogen. 17O MAS NMR shows that in the initial gel Si–O–Si and Ti/Zr–O–Si
are present, with the possibility of some Ti/Zr–O–Ti/Zr. There is little sign of any
significant phase separation even after heating up to 5001C. However, after heating
at 7501C the sample heated under nitrogen shows an additional resonance with a
shift >400 ppm, which indicates some phase separation. All the indications from
NMR and other measurements are that the underlying equilibrium silicate structure
of gels heated under air and nitrogen are the same after the removal of the organic
fragments (i.e. X5001C). Hence a nitrogen atmosphere should be used, as it
preserves significantly more oxygen within the sample than heating under air, so that
the different oxygen-containing fragments can be best detected.
Acknowledgments
The EPSRC is thanked for its support of the collaboration between Warwick and
UKC for characterisation of sol–gel produced materials through Grants GR/
N64151 and GR/N64247. MES thanks the EPSRC for the support of NMR
instrumentation at the University of Warwick. G. West is thanked for helping with
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the thermal analysis. The referee is thanked for their careful reading of the
manuscript and their useful suggestions.
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