J. Phys. Chem. 1995,99,395-401
395
and 29Si Solid State NMR Study of Atomic Scale Structure in Sol-Gel-Prepared TiO2-Si02 Materials 170
Peter J. Dirken,? Mark E. Smith,*J*$and Harold J. Whitfield* Physics Laboratory, University of Kent, Canterbury, Kent CT2 7NR, U.K., CSIRO Division of Materials Science and Technology, Private Bag 33, Rosebank MDC, Clayton, Victoria 3169, Australia, and Department of Applied Physics, Royal Melboume Institute of Technology, Box 2476W, Melboume, Victoria 3001, Australia Received: August 4, 1994@
Solid state 1 7 0 and 29Simagic angle spinning (MAS) NMR spectroscopy has been applied to the study of the structure of three Ti-Si mixed oxides and their gel precursors. Gels were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) and titanium isopropoxide (Ti(OPr")4). Both one-stage (addition of the Ti(OPr")4 to the TEOS) and two-stage (partial hydrolysis of the TEOS before addition of the Ti(OPrn)4) procedures were used. 29Si MAS NMR spectra hardly show any differences between the one- and two-stage gels. All consist of Q2, Q3, and Q4 Si04 species becoming Q4 after heat treatment at 600 "C. This study shows that 170MAS NMR is a valuable tool in the characterization of amorphous oxides and that it is much more informative than 29Si MAS NMR. 1 7 0 MAS NMR shows that titanium is incorporated only as Ti-0-Si bonds in a two-stage gel, whilst samples that show phase separation also have Ti-0-Ti bonds. Ti-0-Si linkages persist until 600 "C and in conjunction with the 29Si NMR data indicate that titanium enters the silica network. The assignment of the Ti-0-Si resonance is confirmed by studying the mineral fresnoite (Ba2TiSi~Og).
Introduction Titania-silica mixed oxide materials are of significant technological importance. In the optical industry they are used as thin films with tailored refractive indices.' Silica glasses with a few mole percent Ti02 are used as ultralow thermal expansion (ULE) glasses2 Mixed titanium-silicon oxides are also important as catalysts and catalyst support material^.^ The properties of titania-silica binaries are strongly dependent on their chemical composition, homogeneity, and texture, which in turn depend on synthesis conditions. Homogeneity at the atomic level is especially important. Sol-gel synthesis, based on hydrolysis and subsequent condensation of metal alkoxide precursors, is a relatively new method that combines atomic level mixing with a high degree of porosity.2 Moreover, synthesis temperatures can be kept much lower than those of standard high-temperature methods. A problem in sol-gel science is that hydrolysis rates of the various metal alkoxides can be very different.4 In the case of titania-silica binary gels, the rate of hydrolysis of Ti alkoxides is much higher than that of Si alkoxides. As a result, phase separation could occur in the gels forming Ti-rich and Si-rich regions. Clearly, this is not desirable for atomic scale homogeneity. Fortunately, this problem may be circumvented by partially hydrolyzing the Si alkoxide prior to mixing it with the Ti alkoxide (two-stage hydr~lyisis).~ One of the challenges in sol-gel science is to show that atomic mixing occurs during hydrolysis and that it persists during annealing of the gels to form consolidated glasses. Since the products are X-ray amorphous, standard X-ray diffraction methods cannot be used. Therefore, the study of the synthesis of titania-silica materials has been undertaken by many
* Author to whom correspondence should be addressed at the University
of Kent.
University of Kent. CSIRO Division of Materials Science and Technology -_ Royal Melbourne Institute of Technology. @Abstractpublished in Advance ACS Abstractr, December 1, 1994. 0
*
0022-365419512099-0395$09.0010
spectroscopic methods that probe short-range order, the most important being X-ray absorption (XANES, EXAFS),6-g Raman,g-loFT-IR,8,10small-angle neutron scattering (SANS),ll XPS/XAES,12and 29SiNMR spectro~copy.'~ XANESEXAFS studies on Ti-Si mixed oxide glasses indicate that Ti is in fourfold coordination.6-8 This is inferred from a short Ti-0 bond distance (1.82 A) derived from the EXAFS spectra, combined with one strong Ti K pre-edge peak and a negative Debye-Waller factor.8 As the Ti02 content is increased, more Ti goes into sixfold coordination, and eventually at 15 wt % Ti02, nanosize crystalline titania is formed. The introduction of Ti02 into Si02 glasses results in a strong FT-IR peak at 950 cm-' and a Raman band at 1000-1 100 cm-'.*-'O These bands are attributed to Ti-Si mixing at an atomic level. A Ti-Si solid solution at low levels (I 15 mol % T i 0 3 is also supported by XPS/XAES studies and Raman studies on Ti-containing silicate melts like CaMgSizO6 (diopside) and NaAlSi3Og (albite).14 However, some studies show no evidence of Ti in tetrahedral c~ordination.'~-'~ Walther et al.13 studied the influence of synthesis conditions on the structure of Si02 gels and glasses with 20 mol % Ti02 using 1H-29Si cross-polarization (CP) MAS NMR spectroscopy. These authors observed an increase in intensity for Q2 and Q3 species in two-stage gels as compared to one-stage gels (all dried at 120 "C). They argue that creating a Si-0-Ti bond has the same effect on the 29Si resonance as a Si-0-H bond. As a result, incorporation of titanium into the silica network would lead to more Q2 and Q3 intensity. Additionally, line widths are broader in the case of the two-stage gel, which it is argued also indicates Ti-0-Si bridges. There is no reason to expect that the hydrolysis rates, and therefore the Q-distributions, will be identical in one- and two-stage gels. Thus, changes in the 29SiNMR peak intensities cannot be unambiguously attributed to a different Q-distribution resulting from differences in hydrolysis or the presence of Si-0-Ti bonds. Here 1 7 0 MAS NMR is used as a direct probe for Ti-0-Si linkages and a comparison is made with the results obtained 0 1995 American Chemical Society
396 J. Phys. Chem., Vol. 99,No. 1, 1995
Dirken et al.
TABLE 1: 29SiNMR Dataa
Q'
temperature
Q3
Q2
fwhh(Hz) 6(ppm) Z(%)
Q4
fwhh(Hz)
6(ppm)
I(%)
25 100 350 450 600
430
-92.2
10
520 680 820 850
-102 -101.8 -100.3 -100.5
53 44 35 20
25 100 250 350
475 600
-92 -93.5
12 9
525 550 725 800.
-101.2 -101 -101.5 -100
750 700
-92 -94.5
26 21
600 570 950 950
-99.5 -101 -101 -101
("C) A
fwhh(Hz) 6(ppm)
I(%) fwhh(Hz)
6(ppm)
I(%)
630 700 860 950 1175
-111.4 -110.8 -109.3 -110 -109.5
37 56 65 80 100
54 40 49 33
700 825 790 900
-109 -109 -110 -108.5
34 51 51 67
39 26 39 26
800 950 900 950 1000
-106.5 -107 -109.5 -109.5 -110
30 53 6.1 74 100
B
C
25 100 350 450 600
750
-84
5
fwhh, 6, and Z represent the line width, 29Sichemical shift, and relati.ve intensity, respectively. Errors: fwhh f25-50 Hz; 6 f 0.5 ppm; I f
5%. by 29SiMAS NMR and 1H-29Si CP MAS NMR. The model proposed by Walther et al.13 is used as the basis for this study. It is demonstrated that 29SiNMR, being based on next nearest neighbor (nnn) interactions, is much less informative than 1 7 0 NMR, which depends on nearest neighbor (nn) interactions, and that 1 7 0 has a large chemical shift range. Oxygen-17 usually only exhibits a modest quadrupolar intera~tion,'~ which does not interfere with spectral interpretation. Previous I7O NMR work on titanates and silicates has shown that oxygens in Ti0-Ti and Si-0-Si linkages have very different isotropic chemical shifts (6 500-600 ppm and 30-90 ppm, respectively).20 Also, oxygens in OTi3 and OTi4 can readily be distinguished (6 530 and 370 ppm, respectively)?0-22 The only drawback of 1 7 0 NMR is its poor sensitivity due to the low natural abundance of 1 7 0 (0.037%). This can be overcome by hydrolyzing the gels with 170-enrichedwater. The relative stability of the C - 0 bond compared to Ti-0 or Si-0 ensures that 1 7 0 is efficiently incorporated int the Ti-Si mixed oxide.20 Three gels are studied here; the fiist two have 7 mol % TiO2, one prepared with a one-stage hydrolysis procedure and the other with a two-stage procedure. The third gel is a two-stage 41 mol % Ti02 gel. Preliminary 1 7 0 work on the three (unheated) samples showed the existence of Ti-0-Si and Si-0-Si bridges in the two-stage gel with 7 mol % Ti02.22 Although the 29Si NMR spectra of these samples are very similar, the 1 7 0 spectra are distinctly different. Both the 41 mol % Ti02 gel and the one-stage gel give four 170signals, three of which could easily be assigned to oxygens in OTi3 (6 530 ppm), OTb (6 % 370 ppm), and Si-0-Si (6 % 0 ppm) sites. The presence of OTi3 and OT4 sites points to phase separation in these gels into Ti-rich and Si-rich domains. The fourth signal at 260-280 ppm had not previously been observed. It is assigned to oxygens in Ti-0-Si bonds for two reasons. Firstly, its chemical shift is intermediate to the shift of oxygen in Si0-Si and Ti-0-Ti linkages. Secondly, the spectrum of the 7 mol % titania two-stage gel (most likely to be atomically mixed) contains only this signal in addition to that from Si0-Si linkages. In this study, the structure of Ti02-SiO2 gels and its evolution with temperature are examined in detail by 29Siand 1 7 0 NMR. New evidence is presented for the assignment of the 260-280 ppm signal to oxygens in Ti-0-Si linkages by examination of the mineral fresnoite (Ba2TiSizOs). lH-170 CP MAS NMR results demonstrate the presence of OH groups.
-
Experimental Methods The SiO2-TiOz mixed gels were prepared by hydrolysis of titanium n-propoxide (Ti(OPrnj4)and tetraethoxy orthosilicate (TEOS)with 10 atom % enriched H2170. Sample A (7 mol % Ti02) was prepared in two steps. First, the water and part of the ethanol were added to the TEOS. Secondly, the Ti(OPrn)4 in the remainder of the ethanol was added dropwise under continuous stirring. The resulting yellow solution was held in a dry nitrogen atmosphere for 48 h. Ethanol and propanol were removed from the viscous product under vacuum at room temperature, yielding a brittle, pale-yellow glass. Sample B (7 mol % TiO2) was prepared in one step by adding Ti(OPrn)4 rapidly to an acidified TEOS solution. At this stage immediate precipitation of a white powder occurred. The mixture was allowed to stand in a dry nitrogen atmosphere,so that hydrolysis could continue. After 48 h the excess solvent had evaporated, leaving a damp product. Sample C (41 mol % TiO2) was prepared in two steps. Firstly, half of the ethanol was mixed with the TEOS to which then was added water and HCl. After 30 min, the Ti(OPPk in the remainder of the ethanol was slowly added to the TEOS mixture to give a yellow solution which immediately began to gel. After 4 h a stiff gel had formed. The gel was dried under dry nitrogen for 90 h. After drying, all powders were subjected to heat treatments at successively higher temperatures (see Table 1) for 2 h periods under a flowing atmosphere of dry nitrogen. Fresnoite (Ba2TiSi~Os)~~ was prepared by taking BaC03 dissolved in glacial acetic acid, adding f i s t TEOS and then Ti(OPr)4, both in propanol, followed by hydrolysis with 170enriched water. The gel was dried to give a white powder which was ground and then heated at 800 "C for 30 min. X-ray diffraction (Philips PW3710 using Co Ka radiation) confirmed the sample to be fresnoite. The 1 7 0 NMR spectra were acquired on a Bruker MSL 400 spectrometer operating at 54.2 MHz using MAS at typically 5 kHz with a 1.6 ps (45") pulse and 2.5 s recycle delay. The delay was sufficient to prevent saturation. Additionally some 1 7 0 spectra were acquired at 11.7 and 14.1 T (Larmor frequencies 67.784 and 81.34 MHz, respectively) with fast (11-15 kHz) MAS and the other experimental conditions similar to those at 9.4 T. The 29Si NMR spectra were acquired at 9.4 T at a frequency of 79.46 MHz using a 2.5 ps (%30°) pulse and 1520 s recycle delays to prevent saturation. Some CP NMR spectra were also collected using contact times of ~ 4 0 p0s for 1 7 0 and 1.5 ms for 29Si with a 'H field of ca. 1.6 mT. The
J. Phys. Chem., Vol. 99, No. 1, 1995 397
Sol-Gel-Repared TiOz-SiOa Materials
l'l ~
~
40
l
~
60
l
40
~
l
-100
~
l
-120
~
-140
l
~
l
~
-180
'si chemical shift (ppm) Figure 1. 29SiMAS Nh4R spectra of sample A (7 mol % Ti02 twostage preparation) form (a) a gel (25 "C) which was then heated for successive periods of 2 h at (b) 100 "C,(c) 350 "C, (d) 450 "C, and (e) 600 "C.
spectra were referenced externally to H20 (170resonance at 0 ppm) and TMS (29Siresonance at 0 ppm).
Results and Discussion The 29SiNMR spectra of the unheated gels consist of three signals at -92, -102, and -112 ppm with varying intensities ((Figure la) for sample A). Each resonance represents a specific degree of Si-0-Si polymerization. The -92 ppm signal comes from Si sites in a Q2 configuration, and the -102 and -112 ppm signals come from Q3 and Q4 configurations, respectively (Q" stands for an Si04 unit with n bridging oxygen^^^). Progressive condensation of the Qo site in TEOS results in an increase in species with a larger degree of connectivity, i.e. Q1-Q4.2 The 29SiNTvlR spectra were deconvolved by Gaussian fitting, with the results summarized in Table 1. It shows that the 29Si MAS NMR spectra of samples A and B are very similar. Yet, their preparation is distinctly different. Sample B was prepared by rapid mixing of the metal &oxides, while sample A was the result of very slow addition of the Ti(OPP)4 to partially hydrolyzed TEOS. The only difference between the 29Sispectra is the fact that the Q3 and Q4 lines in the spectrum of sample A are somewhat better resolved. Table 1 shows that this is a result of the combination of a smaller line width and more negative chemical shift of the Q4 line of sample A. Walther et al.13 studied one- and two-stage gels (dried at 120 "C) with 29Si MAS NMR (with and without 'H-CP). They concluded from an increase of the Q2 and Q3 signal intensities in both the CP and non-CP spectra, combined with an increase in their line widths for the two-step gel, that it contains Si0-Ti bonds, indicative of atomic mixing. This is supported by FT-IR results, showing an increase in a characteristic 945-
l
960 cm-I vibration band in the two-step However, the line widths of the one-stage gel in this study (sample B) are a little higher (475-700 Hz) than those of the two-stage gel (sample A, 430-630 Hz). Our 29Si NMR results of sample A and B are very similar, however, and therefore do not allow for an unambiguous interpretation in terms of Si-0-Ti bridges. For 41 mol % Ti02 content (sample C), no complete solid solution can occur with Si02, the upper limit being 10-15 wt % (7.7-11.7 mol %).7J2 The Q-species distribution of sample C is indeed different from that of samples A and B. Firstly, it is the only sample which contains Qt sites. Secondly, the intensity of the Q2line is significantly higher than that for the other two samples. The differences are probably caused by the much higher titania content for sample C. The condensation of the TEOS species is slowed down by the reaction of Ti(OPrn)4 with water. The water is used for the Ti condensation and is not available for Si condensation. Of course the condensationreaction itself creates water, but because of the very high Ti(OPrn)4 content (41 mol %) even this water is possibly mostly used in the Ti condensation and the Si condensation is slowed down. Since the Q-species distribution in sample C is different from that in samples A and B, which are themselves similar, and since sample C is phase separated, the differences observed by Walther et al.13may be a result of more subtle differences than Si-0-Ti formation. Itl has pointed ~ l ~bridge l ~ l ~ ~ been l ~ l ~out l from ~ EXAFS and XANES studies in solution that the hydrolysis behavior of a metal alkoxide (for instance TEOS) when added to another metal alkoxide can be very complex,25even showing that no Si-0-Ti bridges are formed in solution but that only the alkoxy groups exchange (transesterification). Therefore it appears difficult to draw unambiguous conclusions on atomic mixing purely from changes in Q-species distributions of 29Si NMR.
It must be emphasized that although some M-0-M bridges (M = Ti or Si) will have formed in the unheated gels, there is still a high content of hydroxyls and organic species. This is confirmed by lH-13C CP NMR (not shown), which shows various organic species to be present, consistent with earlier work on titania and zirconia gels.21s26The 1H-29Si CP MAS NMR spectra (not shown) on sample A are comparable to those of Walther et al.13 They show large signals from Q2 and Q3 species at small contact times, but no Q1 species are observed. Q2 and Q3 species here are strongly bonded to hydroxyls, and therefore their signals are preferentially enhanced at short contact times. At longer contact times, the Q4 signal increases in intensity at the expense of the Q2 and Q3 signals, eventually becoming equal to that of the single-pulse spectrum. The 170MAS NMR spectra of samples A-C are shown in Figures 2-4. All the spectra are dominated by a resonance with its center of gravity at approximately -10 ppm and its spinning sidebands. Fast MAS was only available for the unheated gels, and the spectra have been presented in a preliminary communication.22 However, for most of the 1 7 0 spectra presented here, only relatively slow spinning speeds (5-6 kHz) were available. These speeds are not sufficient to concentrate the intensity into the center band alone, and the spectrum is left with a set of spinning sidebands. However, by comparing the spectra with the high-speed MAS spectra and by judicious choice of spinning speed, the overlap of spinning sideband intensity with the center bands is minimized and interpretation becomes possible. The sidebands were confiied by repeating the experiments at various spinning speeds. However, no attempt was made to quantitatively deconvolve the 1 7 0 NMR spectra due to the extensive overlap of the
398 J. Phys. Chem., Vol. 99, No. 1, 1995
Dirken et al.
1
1
>/ik *
*
* I I*
*
I ' l i r ~ r i r i l ~ l ~ l ' r ~ r ~ r ' r '~~ ~
800
600
400
200
0
-200
400
800
800
I
~ 400
I 200
~
I I* I 0
~ -200
I
~
I
-400
"0 chemical shift (ppm) Figure 2. 170MAS NMR spectra of sample A (7 mol % Ti02 twostage preparation) from (a) a gel (25 "C) which was then heated for successive periods of 2 h at (b) 100 "C, (c) 350 "C, (d) 450 "C, and (e) 600 "C. Spinning sidebands are denoted by *, and center bands, by 4.
Figure 3. "0 MAS NMR spectra of sample B (7 mol % Ti02 onestage procedure) from (a) a gel (25 "C) which was then heated for successive 2 h period at (b) 100 "C, (c) 200 "C, and (d) 350 "C. Spinning sidebands are denoted by *, and center bands, by A.
sidebands, some of which, from the low-intensity peaks, are weak and would make integration uncertain. The -10 ppm signal is attributed to oxygens in Si-0-Si and Si-OH configuration^.^^ The contribution of oxygens in Si-OH bonds is made clear by comparing the single-pulse MAS spectrum to the 'H-170 CP MAS spectrum (Figure 5). 'H-X CP NMR uses the protons with their high gyromagnetic ratio and short relaxation time to enhance the signal-to-noise ratio of the X nucleus (here 170).28 More importantly, however, CP favors those oxygens closest to protons, thereby giving some idea of which oxygens are associated with protons.29 In Figure 5 the CP spectrum is more shielded than the single-pulse spectrum. Additionally its shape has changed into a more Gaussian form. At a contact time of a few hundred microseconds, the signal comes predominantly from oxygens in SiOH bonds. Therefore in the region around 0 ppm, the single pulse spectrum has contributions from oxygens in Si-0-Si and Si-OH bonds. No signal indicating the presence of TiOH is detected. This could be caused by the fact that Ti(OPP)4 condenses much more rapidly than TEOS, so that the Ti-OH bonds are no longer present. The low Ti02 content and unknown CP characteristics of oxygen in a Ti-OH bond make observation difficult. In contrast to the 29Sispectra, the 1 7 0 spectra show distinct differences. Samples B and C have, in addition to the resonance at -10 ppm, signals at 280,370, and 560 ppm, whereas sample A only has one additional signal at 280 ppm. The 560 and 370 ppm signals can be assigned to oxygens in OTi3 and OT4 sites.20,21The 280 ppm signal represents oxygens located in Si-0-Ti bridges.22 The present study gives additional evidence that the 280 ppm signal is indeed from Si-0-Ti bonds,
by studying the mineral fresnoite (Ba~TiSizOg)with I7O MAS NMR (Figure 6). Fresnoite has four inequivalent oxygen sites including a well defined Si-0-Ti linkage and has Ti in an unusual five-coordinati~n.~~ The 170spectrum shows signals at ca. 0 and 190 ppm. The signal at 0 ppm can be assigned to oxygens associated with the Si207 group. This signal may be a powder pattem caused by the second-order quadrupolar interaction with a low asymmetry parameter.30 The splitting and width both give a quadrupole coupling constant e2qQ/h of -3 MHz. This value lies in the range for nonbridging oxygens (nbo) in silicates of 1.5-3.2 M H z . ~ ' , ~ There * is a second, narrower resonance at 190 ppm, which is ascribed to the Si0-Ti linkages present. Given the large shift range of oxygen, although this peak is not exactly at 280 ppm, it is sufficiently close to make the assignment consistent. The shift difference can most probably be attributed to the effect of the Ti coordination on the Ti-0 bond strength. In the gels the most likely coordination number of titanium is 46-8912(vide infra), while in fresnoite it is 5. The 29SiMAS NMR spectrum from fresnoite showed a single resonance at -82.0 ppm, consistent with the Q1 Si207 grouping. The intensity of the 280 ppm signal in the spectrum of sample B is much lower than that of sample A. The content of Si0-Ti bonds must therefore be much lower in sample B, and this agrees with this gel being phase separated. Ti-0-Si bonds in phase-separated gels are most likely formed at the interfaces between Ti02 and Si02 domains. The intensity of the 280 ppm signal in sample C simply reflects the higher Ti02 content but is still in approximately the same relative proportion to the OTi3 and OT4 signals for sample B.
'70chemical shift (ppm)
~
I
~
J. Phys. Chem., Vol. 99, No. I, 1995 399
Sol-Gel-Prepared TiO;?-SiOa Materials
1
n
1
i i
1
1
,
1
,
600
1
,
1
,
1
,
,
,
)
200
400
1
1
,
0
1
1
1
1
1
(
1
400
-200
"0 chemical shift (ppm)
Figure 5. Single-pulse I7O MAS NMR spectrum of sample A (a) together with its 'H-I'O CP MAS NMR spectrum (b). h
'I
I
n
~
~
~ 600
I
~ 400
I 200
~
~
400
I
200
~
I
0
~
I
-200
~
-400
I
~
I
~
I
~
I
~
800
"0 chemical shift (ppm)
I 0
I
Figure 6. I7OMAS NMR spectrum of the mineral fresnoite (T3a2TiSizOS) at a 11.7 T magnetic field.
* I I*
800
~
600
~ -200
~
~
400
"0 chemical shift (ppm) Figure 4. I7O MAS NMR spectra of sample C (41 mol % Ti02 prepared using a two-stage procedure) from (a) a gel (25 "C) immediately after drying and (b) vacuum pumped for a further 40 h, which was then heated for 2 h periods at (c) 100 "C, (d) 200 "C, (e) 350 "C,and (0 450 "C and at 600 "C for (g) 3 h and (h) 7 h. Spinning sidebands are denoted by *, and center bands, by 1.
Summarizing the I7O NMR results from the unheated gels, it is clear that samples B and C are phase separated into Ti02 and Si02 domains, while in sample A Ti and Si are atomically mixed. In sample A, Ti-0-Si bonds are present together with Si-OH and Si-0-Si bonds. In this sample no Ti-0-Ti bonds are observed, The development of the intensities, chemical shifts, and line widths of the 29SiNMR spectra with heat treatment for these three samples is given in Table 1. It must be stressed that all the samples under study are X-ray amorphous. Study of the annealing of the gels is important to see what kind of hightemperature structure the gels have and how it evolves from the low-temperature gel. An important question for instance is whether atomic mixing between Si and Ti is retained and if Ti02 and Si02 domains form. If so, does Ti substitute for Si in the network or do Ti05 and/or Ti06 units form? Also nanoscale crystallization might be observed by NMR long before it is observable with X-ray diffraction. The general trend observed in all three samples is an increase of more polymerized Q" species as the temperature increases. The loss of protons and organic groups with heat treatment leads the silica I to further ~ Icondensation ~ I of ~ I network. ~ I Eventually, ~ I all ~ the Si is in a Q4configuration (except for sample B, for which data are only available to a temperature of 350 "C). This is in contrast to the 29Si NMR results of Walther et al.,I3 who observed 7.7%Q3 and 0.8% Q2in the MAS spectra and 24.0% and 2.5%, respectively, in the CP MAS spectra of a sample heated to 600 "C. The fact that Q2 and Q3 are observed by Walther et al. must mean that in their samples some titanium is acting as a network modifier (Le. TiO6). In all samples, line
I
~
400 J. Phys. Chem., Vol. 99, No. 1, 1995
Dirken et al.
widths increase significantly as the temperature increases, which is consistent with previous re~u1ts.l~ This effect can be attributed to further cross-linking within the network, causing more possible nnn environments for the Si atoms. The only difference between samples A and B is that in sample B the Q4 signal increases much faster in intensity than in sample A with heat treatment. The differences in atomic mixing could be responsible for this effect. In sample C there is also an increase in connectivity with increasing temperature, but the process is much slower. Q2 species persist until temperatures of at least 200 "C. This could be the result of the initial low degree of polymerization as evidenced from the presence of Q' sites and high Q2 content. Again it is difficult to extract any unequivocal information about effects of temperature on the phase-separated domains from 29SiN M R alone. The 1 7 0 MAS NMR spectra of samples A-C as a function of temperature are given in Figures 2-4. The form of the 1 7 0 MAS N M R spectra needs careful explanation. The quadrupolar nature of 170(nuclear spin I = 5 / 2 ) means that if the quadrupolar coupling constant (e2qQ/h)becomes large enough, second-order effects become i m p ~ r t a n t . ~MAS ~ , ~ can ~ only partially remove these second-order effects even when the MAS rate exceeds the residual second-order line width of the (l/z, -l/z) transition of the center band (=(e2qQ/h)2(6 7 ) 2 / 2 8 5 0 v ~ At ) . 9.4 T MAS speeds of 5-6 kHz are only just sufficient to cause narrowing of the Si-0-Si resonance. Two other effects are apparent in the sidebands. The fist-order sidebands do not appear to be symmetrically displaced about the center band, as it is the isotropic chemical shift ( ~ 4 ppm) 0 not the center of gravity that is important. The line shapes of the fiist and second sidebands are completely different; the fist is a single Gaussianlike line, and the second is a distinct powder pattem, which has been observed before for quadmpolar systems.34 The line shape of the center band is not that of a classical second-order powder pattem; Le., the right-hand singularity is too intense, but this can be explained by the presence of the closely attendant first-order sidebands and the overlap of Si-0-Si and Si-OH resonances. The line width of the Si-0-Si resonance is dominated by a single quadrupolar coupling constant even in the amorphous gel. In most gels the line width can be described by quadrupolar (Av,) and chemical shift dispersion effects (Avcs),so that the total line width in hertz ( A v ) may be written as
+
AV
+
= A V ~ B ~ -A~V ~ ~ B ,
(1)
in which Avq and Avcsare in hertz per tesla. For the OTi3 and OTi4 resonances in pure Ti02 gels, chemical shift dispersion dominates the line width, so that the line width is almost exactly proportional to Bo. The same has been found here and is also nearly true for the Ti-0-Si resonance as well, so that quadrupolar effects have little influence. This is important in aiding interpretation, for example allowing the identification of the peak position with the chemical shift. Note this is in contrast to the case of the Si-0-Si resonance, which is dominated by quadrupolar effects. In sample A, Si-0-Ti bonds clearly remain present up to 600 "C. Atomic mixing of Si and Ti is therefore believed to persist during the annealing process. No OTi3 or OT4 sites are formed, c o n f i i g this assumption. This is quite surprising, since at this temperature, 29SiNMR shows that only Q4 species are present; Le., the silica network is fully polymerized. These results indicate that, in atomically mixed silica-titania glass, titanium is in the silica network, most probably in a fourfold Coordination. It is surprising that this appears to have so little influence on the 29Sichemical shift. The presence of titanium in the network is consistent with X-ray absorption spe~troscopy6-~
and X P S / X A E S 1 2 results. The trend in the chemical shift of oxygen in Ti-0-Si bonds with varying Ti coordination number also agrees with a fourfold coordination. In the case of titanite (CaTiSi205) Ti is in sixfold coordination and the 170chemical shift of the oxygen in the Ti-0-Si bond is 160 In fresnoite, Ti is in fivefold coordination and the 170chemical shift of the oxygen in the Ti-0-Si bond is 190 ppm. In our glass, it is 280 ppm, which is in the right direction for fourfold coordination. Although Ti has a preference for sixfold coord i n a t i ~ n , ~because ~ . ~ ~ of its longer Ti-0 bonds, fourfold coordinated Ti occurs in B a ~ T i 0 4 R , ~b~~ T i 0 3 C~AlTi04:~ ,~~ BilZTi020$1 Ni-Ti and some garnets.37 Furthermore, restrictions on coordination number, bond angle, and bond length are much less severe in amorphous materials than in crystalline phases. According to the model of Walther et al., Ti-0-Si bonds would contribute to the Si intensity of Q2 and Q3 units. Ti is believed to have the same effect on the chemical shift of an Si it is bonded to as an OH. The arguments and references cited for this assumption are not clear. There is only a difference of 0.3 in electronegativity between Ti and Si (1.5 and 1.8 on Pauling's scale, respectively). In fact, Ti has the same electronegativity as Al, and A1 is known to shift the Si resonance by only 5 The Si signal has a chemical shift of -109.5 ppm. If Ti has the same effect on the silicon shift as Al, a shoulder at -105 ppm should be observed, which is not the case. A possible explanation is that because Ti is incorporated into the silica network, the bond lengths and bond angles change in such a way that the effect of the difference in the covalency of the bond is compensated and the silicon resonance is not significantly affected. In sample B, intensities of the OTi3, OTi4, and Si-0-Ti signals decrease as a function of temperature. Possible reasons for this effect are the loss of 1 7 0 from the sample and a significant increase in relaxation times. Sample C shows some interesting phenomena related to the annealing process. First of all,there is a decrease in the intensity of the signal due to S-0-Ti bonds, probably due to the growth of the individual domains, causing a decrease in their interfacial surface area. Less interface will then lead to fewer possibilities for Si-0-Ti bond formation. A second feature is the development of the OTi3 and OT4 sites as a function of temperature, especially as compared to the pure Ti02 system.26 In the pure Ti02 system, crystallization of anatase occurs at a temperature of approximately 300 "C, as evidenced by XRD and a very sharp 1 7 0 line at 557 ppm. At lower temperatures, the OTi3/OTh intensity ratio gradually increases up until anatase crystallizes when no OTi4 sites remain. In case of the TiOz-Si02 mixed system, however, the OTi3/OT4 intensity ratio remains approximately constant up until 450 "C. Only at 600 "C does this ratio increase significantly. Further heat treatment at 600 "C removes all OT4. In the case of sample C there is a decrease in the signal-to-noise ratio with increasing temperature. Loss of 1 7 0 from the sample may be responsible, in combination with an increase in the 1 7 0 relaxation time as protons are lost from the sample. Furthermore, the OTi3 signal from anatase-like domains in the gel remains very broad, indicating that they have yet to crystallize. This is quite surprising, since the pure Ti02 gels already crystallized at 300 "C. Apparently, mixing of Si and Ti stabilized the metastable disordered anatase-like domains in the gel. In fact NMR and XRD show no crystallization of any kind in samples A and C up to 600 "C.
Conclusions This study shows that 1 7 0 NMR is extremely useful in elucidating the structural role of titanium in amorphous Ti-Si
Sol-Gel-Prepared Ti02-SiOz Materials mixed oxides. Although 29Si NMR effectively indicates the degree of polymerization of the silica part of the system, it is not very sensitive at distinguishing phase-separated and nonphase-separated gels and glasses. 170NMR directly reveals the phase-separated nature of our 7 mol % Ti02 one-stage and 41 mol % Ti02 two-stage gels and glasses through signals for OTi3 and OTi4 sites. Atomic mixing between titania and silica is directly shown in our 7 mol % Ti02 two-stage gel by the presence of signals for Ti0-Si linkages and the absence of Ti-0-Ti bonds. If this sample is heated to 600 "C, these Ti-0-Si bonds are retained in the glass, indicating that Ti and Si are still atomically mixed. The fact that at this temperature 29Si NMR shows only Q4Si species indicates that Ti replaces Si in the network. Its most probable average coordination number is four. No crystallization has been observed in any sample by X-ray diffraction. Line widths in our 1 7 0 NMR spectra, which are dominated by chemical shift dispersion, indicate that the gels and glasses are still highly disordered even after heat treatment to 600 "C. 170NMR has been shown to be an extremely sensitive probe of oxide gels. Acknowledgment. The authors would like to thank Dr. T. J. Bastow (CSIRO) for his interest and encouragement in this project and the Royal Society and the EPSRC for funding this work. The ESPRC is also thanked for access to the ultrahighfield NMR facility at the University of Edinburgh. The authors are also most grateful to Jonathan Stebbins and Peter Fisk (Stanford University) for communicating their results prior to publication. References and Notes (1) Schultz, P. C.; Smyth, H. T. In Amorphous Materials; Douglas, E. W., Ellis, B. Eds.; Wiley: London, 1972. (2) Brinker, C. J.; Scherer, G.W. Sol-gel Science: the Physics and chemistry of Sol-Gel Processing; Academic Press, Inc.: San Diego, CA, 1990. (3) Itoh, M.; Hattori, H.; Tanabe, K. J. Catal. 1974, 35, 225. (4) Yoldas, B. E. J. Mater. Sci. 1977, 12, 1203. (5) Basil, J. D.; Lin, C.-C. In Ultrastructure Processing of Advanced Ceramics; MacKenzie, J. D., Ulrich, D. R., Eds.;Wiley: New York, 1988. (6) Sandstrom, D. R.; Lytle, F. W.; Wei, P. S. P.; Greegor, R. B.; Wong, J.; Schultz, P. J . Non-Cryst. Solids 1980, 41, 201. (7) Greegor, R. B.; Lytle, F. W.; Sandstrom, D. R.; Wong, J.; Schultz, P. J . Non-Cryst. Solids 1983, 88, 27. (8) Liu, Z.; Davis, R. J. J . Phys. Chem. 1994, 98, 1253. (9) Best, M. F.; Condrate, R. A. J . Mater. Sci. Lett. 1985, 4, 994. (10) Schraml-Marth, M.; Walther, K. L.; Wokaun, A.; Handy, B. E.; Baiker, A. J . Non-Cryst. Solids 1992, 143, 93.
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