13740
J. Phys. Chem. C 2007, 111, 13740-13746
Structural Characterization of SiO2 and Al2O3 Zener-Pinned Nanocrystalline TiO2 by NMR, XRD and Electron Microscopy Luke A. O’Dell,† Shelley L.P. Savin,‡ Alan V. Chadwick,‡ and Mark E. Smith*,† Department of Physics, UniVersity of Warwick, CoVentry, CV4 7AL, United Kingdom, and School of Physical Sciences, UniVersity of Kent, Canterbury, Kent, CT2 7NH, United Kingdom ReceiVed: May 23, 2007; In Final Form: July 9, 2007
Nanocrystalline TiO2 samples were prepared using sol-gel techniques in a pure form and also Zener pinned with either silica or alumina to reduce the growth of the crystallites during the annealing process and to stabilize the anatase phase at high temperatures. These samples were studied using 17O, 27Al, and 29Si nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and electron microscopy. The silica pinning phase was found to successfully restrict nanocrystal growth as well as stabilize the anatase phase at temperatures up to 800 °C. The alumina phase had less of a pinning effect, and it reacted with the TiO2 to form tialite. 17O NMR relaxation time measurements on enriched samples showed that the presence of the pinning phases also reduced the activation energy for the oxygen ion diffusion mechanism.
1. Introduction TiO2 thin films, which are most commonly manufactured using the sol-gel method,1 are used in applications such as solar cells, lithium batteries, and humidity or oxygen sensors.2,3 TiO2 can exist in three different crystalline phases, each consisting of TiO6 octahedra arranged in different formations, with the sol-gel method tending to produce the anatase phase after heating at 350 °C.3-5 Depending on crystallite size, distribution, and arrangement, this can transform into the rutile phase at temperatures between 400 and 1000 °C,6 but usually between 600 and 800 °C in the nanocrystalline case.3,7 In some cases, the third phase, brookite, is reported to appear as an intermediate between anatase and rutile TiO2 at temperatures below 500 °C. However, brookite rarely occurs in a pure form, and anatase and rutile are the most common polymorphs.6 In many applications of TiO2, one of these phases, in particular, may be advantageous. For example, the anatase phase has been shown to be more efficient than the rutile phase in photocatalysis,8,9 and smaller crystal sizes are also advantageous in this application.10 Anatase nanocrystals have been reported to be most stable at extremely small diameters (below 5-13 nm11,12); however, they have been reported to exist at up to 80 nm.13 Rutile crystals are known to grow much faster than anatase,14 so, generally, rutile nanocrystals exhibit larger diameters.11 To utilize nanocrystalline anatase TiO2 in applications that require high temperatures, a method is required to prevent the anatase-to-rutile phase transition and restrict the growth of the crystals when the material is heated. Zener pinning15 has already been shown to restrict the growth of solgel prepared nanocrystalline metal oxides during the annealing stage.16-19 This process involves the addition of a small amount of a second phase, such as silica or alumina, which exists as discrete particles at the nanocrystal interfaces and “pins” the grain boundaries in place by restricting surface diffusion and * To whom correspondence should be addressed. E-mail: M.E.Smith.1@ warwick.ac.uk, tel: +44 (0)24 7652 2380, fax: +44 (0)24 7669 2016. † University of Warwick. ‡ University of Kent.
reducing the radius of curvature of the nanocrystals. In this paper, nanocrystalline TiO2 samples have been manufactured with either 15% silica or 10% alumina by weight, amounts chosen in light of previous work.20 These pinning phases have been characterized by 29Si and 27Al magic angle spinning (MAS) and nuclear magnetic resonance (NMR) respectively, and the TiO2 nanocrystals have been studied using X-ray diffraction (XRD) and 17O NMR to determine what effect the pinning particles have on the TiO2 phases present at each temperature and on the oxygen ion mobility within the samples. Titania-silica nanocomposites have previously been shown to be more efficient photocatalysts than pure TiO2, and TiO-Si bonding, which has been studied using 17O NMR,21 is already reported to restrict nanocrystal growth.22 TiO2 containing 10% silica has been reported to show anatase nanocrystals with diameters of 16 nm after annealing at 500 °C.22 The presence of highly dispersed alumina has also been shown to stabilize that anatase phase and enhance catalytic properties.23 A TiO2 sample containing 20% alumina remained as anatase up to 1100 °C, with the alumina phase existing as γ-Al2O3 at 800 °C, δ-Al2O3 and Al2TiO5 at 900 °C, and R-Al2O3 at 1100 °C.23 So, Zener pinning nanocrystalline TiO2 with silica or alumina should not only be advantageous in restricting the growth of the crystals, but the presence of the second phase may also improve the material’s photocatalytic properties. 2. Experimental Details 2.1. Sample Preparation. To prepare the unpinned sol-gel TiO2 samples, 92.3 mL titanium iso-propoxide (Aldrich Chemical Co.) was added to a 500 mL beaker. Water was added and the solution was stirred until the solution gelled. The white solid was filtered and dried at 80 °C. To manufacture the silica-pinned TiO2 samples, 83 mL of titanium iso-propoxide and 15 mL of tetraethylorthosilicate (TEOS) were mixed and stirred for 1 h. A 10% v/v solution of ammonia solution in water was prepared and was added dropwise to the alkoxide solution until it gelled. The white solid was then dried at 80 °C.
10.1021/jp0739871 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/23/2007
SiO2 and Al2O3 Zener-Pinned Nanocrystalline TiO2 For the alumina-pinned TiO2 samples, 100 mL of titanium iso-propoxide was added to a 500 mL beaker, and water was added while stirring until the solution gelled. The resultant white solid was filtered and dried at 80 °C and then ground using a pestle and mortar. Approximately 100 mL of butan-2-ol was added to form a slurry, followed by the addition of 15 mL of aluminum tri-sec-butoxide, and the mixture was stirred for 1 h. Water was then added until the solution gelled. The solid was then dried at 80 °C. The unpinned, silica-pinned, and aluminapinned powders were then ground using a pestle and mortar, and portions were annealed for 1 h in air at 400, 600, 800, 1000, and 1200 °C. For the 17O-enriched TiO2 sample, 0.25 g of 70% 17Oenriched water was added to 1.97 g of titanium iso-propoxide, and the solution was stirred until it gelled. For the 17O-enriched silica-pinned TiO2 sample, 0.339 g of TEOS and 1.97 g of titanium iso-propoxide were mixed for 60 min. A 0.255 g portion of 70% 17O-enriched water was added, and the solution was stirred until it gelled. For the 17O-enriched alumina-pinned TiO2 sample, 0.295 g of aluminum tri-sec-butoxide and 1.97 g of titanium isopropoxide were mixed for 60 min. A 0.252 g portion of 70% 17O-enriched water was added, and the solution was stirred until it gelled. The 17O-enriched gels were placed in containers covered in aluminum foil with holes pierced in the top, and they were then dried at 60 °C overnight. 2.2. Nuclear Magnetic Resonance. The 29Si MAS NMR spectra were obtained using a 7.05 T magnet (with a 29Si Larmor frequency of 59.6 MHz) with a 7 mm MAS probe spinning the samples at 4 kHz. These were referenced to the signal from tetramethylsilane, which was set to 0 ppm. The pulse width used was around 1.5 µs, which produced a tip angle of ∼30°. The recycle delay was set to 20 s to produce fully relaxed spectra, with a preacquisition delay of 30 µs, and the average acquisition time for each sample was generally longer than for the other nuclei at 24-48 h because of the relatively low natural abundance of the 29Si isotope (4.7%) and the small amount of silica in the samples (15% by weight). The 27Al MAS NMR experiments were conducted in a 14.1 T magnetic field (with an 27Al Larmor frequency of 156.4 MHz) with a 3.2 mm probe spinning samples at ∼20 kHz. These spectra were referenced against yttrium-aluminum garnet (YAG), with the signal corresponding to the octahedrally coordinated aluminum site, set to 0.7 ppm so that chemical shifts reported are relative to the primary shift reference of 1M [Al(H2O)6]3+ at 0 ppm. A pulse width of 9 µs produced a tip angle of 90° in this liquid sample. For the solid samples, a pulse width of 0.5 µs was used with a preacquisition delay of 7.5 µs and a recycle delay of 1 s, with an overall acquisition time of 5-15 min for each experiment. This recycle delay did not change the relative intensities of the different aluminum environments observed. Certain 27Al MAS NMR spectra were also recorded with similar parameters at 18.8 T (with an 27Al Larmor frequency of 208.5 MHz) using a 2.5 mm probe. Static 17O NMR spectra were recorded at various temperatures using a custom-built probe (described previously17) in a 7.05 T magnetic field, corresponding to a 17O Larmor frequency of 40.7 MHz. The pulse width used was 8 µs, which was determined to be approximately equal to a 90° tip angle. Elevated temperatures were achieved using a 7 mm diameter rf coil and a 22 Ω nickel-chromium resistive heating coil wrapped in a bifilar manner (axis aligned with the magnetic field) with a diameter of ∼25 mm. The sample temperature was varied by adjusting the power supplied to this coil. Temperatures were reproducible
J. Phys. Chem. C, Vol. 111, No. 37, 2007 13741 to an accuracy of approximately (5 °C. The T1 measurements were conducted using the saturation recovery technique, in which a train of (∼64) 90° pulses was used to saturate the magnetization, and subsequently, another 90° pulse was applied after a delay time τ; the free indution decay (FID) signal was measured in the usual way. The saturation pulses were separated by 500 µs (roughly the length of the FID), and values of τ ranged from 0.0001 to 10 s, depending on the temperature of the sample. A decreasing signal-to-noise ratio with increasing temperature meant that a single T1 measurement took up to 24 h to complete. H2O was used as a reference, with the single sharp resonance from this sample set to 0 ppm. All NMR spectra were recorded using the Spinsight program and Varian Chemagnetics CMX, Infinity or Infinity Plus spectrometers. The dmfit200324 and QuadFit25 computer programs were used to simulate the spectra and fit the peaks. 2.3. X-ray Diffraction. XRD experiments were conducted on a Philips PW 1720 diffractometer with Cu KR radiation (1.541 Å), and the resultant data were analyzed using the Traces v3.0 (Diffraction Technology, Pty) software, which also estimated the average metal oxide nanocrystal sizes via the Scherrer relation. 2.4. Electron Microscopy. Electron micrographs were obtained using a Zeiss SUPRA 55VP scanning electron microscope (SEM) with a 10 keV electron beam and a Jeol 2000FX transmission electron microscope (TEM) with a 200 keV beam. Samples were prepared for the TEM by sonic dispersion in acetone and subsequent deposition onto a holey carbon grid. 3. Results and Discussion 3.1. Silica-Pinned TiO2. Figure 1a shows the 29Si MAS NMR spectra obtained from the silica-pinned TiO2 samples annealed at various temperatures. Each spectrum shows a broad peak at around -100 ppm, which is a composite peak consisting of individual overlapping resonances representing SiO4 tetrahedra of various Qn species, where n is the number of oxygen atoms bridging to other tetrahedral units (0 e n e 4).26 The peak positions, relative intensities, and widths found by simulating the various Qn peaks (see Figure 1(b)) are given in Table 1. The grain/crystallite sizes and phases of TiO2 determined by XRD in the unpinned, silica-pinned, and alumina-pinned samples are given in Table 2. As is clear from Figure 1, the broad 29Si MAS NMR peak gradually moves to a more negative chemical shift as the annealing temperature increases. This is, in part, due to the organic groups and hydroxyl groups left over from the sol-gel process being removed from the system during heating. As the data in Table 1 shows, the Qn species tend toward higher n values with increasing annealing temperature, showing that the silica network is becoming more interconnected. The peak widths stay relatively broad across the range of temperatures and Qn species. This is a reflection of the extremely small dimensions of the silica pinning phase, resulting in a large surface area-to-volume ratio and, hence, a relatively large amount of disorder present, even at higher annealing temperatures (pure sol-gel silica not confined between crystals of another phase shows much narrower Q3 and Q4 peaks at these temperatures16). The small size of these silica particles would also mean that a significant amount of surface Si-O-H groups exist. These groups would also account for the lower-order Qn species. The presence of Si-O-Ti bonding is also likely, although a signal from a Q3(OH) group has a similar chemical shift to that from a Q3(Ti) group (∼ -100 ppm),26 so these would overlap in these spectra (i.e., the effect on the 29Si
13742 J. Phys. Chem. C, Vol. 111, No. 37, 2007
O’Dell et al. TABLE 1: The 29Si MAS NMR Spectra Peak Detailsa peak position ppm (( 2 ppm)
peak width Hz (( 30 Hz)
relative intensity % (( 2%)
Qn species
80
-99 -89 -82 -61
940 890 440
29 62 9
3 2 1 0
400
-108 -100 -91 -61
750 1020 750 3
22 59 18
4 3 2 0
600
-109 -100 -91 -61
700 850 730
26 53 22
4 3 2 0
800
-110 -101 -91 -61
740 850 670
36 50 14
4 3 2 0
1000
-110 -100 -61
880 760 280
72 24 4
4 3 0
1200
-109 -100 -61
870 880 150
78 17 5
4 3 0
annealing temperature °C
a In the samples annealed up to 800 °C, the peak at -61 ppm was too small as compared to the background noise to reliably fit.
TABLE 2: The Average TiO2 Crystal Sizes and Their Phasesa as Determined by XRDb average particle size/nm annealing temperature °C
TiO2
TiO2-SiO2
TiO2-Al2O3
80 400 600 800 1000 1200
4 A (2) 10 A (2) 55 A (2) 404 R (20) 449 R (20) 809 R (30)
Amorphous Amorphous 6 A (2) 10 A (2) 26 A 32 R (2) 150 R (10)
5 A (2) 9 A (2) 12 A (2) 27 A 238 R (2,20) 337 R (20) 1011 R (50)
Figure 1. (a) 29Si MAS NMR spectra obtained from the silica-pinned titania samples. Values to the left are the average titania crystallite diameters as obtained by XRD (“A” denotes the Anatase phase, “R” the rutile phase), and the temperatures on the left are the temperatures at which the samples were annealed. (b) A simulation of the 400 °C sample showing the individual Q2, Q3, and Q4 peaks. a
Anatase (A) and Rutile (R), in brackets.
Figure 2. TEM images of the silica-pinned titania samples after annealing at (a) 800 °C, (b) 1000 °C, and (c) 1200 °C.
chemical shift of an SiO4 tetrahedron having a next nearest neighbor titanium that is part of the network is very small).26 The diameters and phases of the TiO2 crystals given on the left in Figure 1 (obtained by XRD) show that the rutile phase begins to appear between 800 and 1000 °C and that all of the anatase TiO2 has fully transformed to rutile by 1200 °C and is accompanied by significant grain growth. Nonetheless, the sample that was annealed at 800 °C showed anatase nanocrystals that are just 10 nm in diameter, as compared with 404 nm rutile
b
Approximate uncertainties are given
in the unpinned sample, a very clear demonstration of the effectiveness of the silica pinning phase. Figure 2 shows various TEM images of the material after annealing at different temperatures. The 800 °C sample (Figure 2a) shows TiO2 nanocrystals with diameters of e10 nm, in good agreement with the 10 nm anatase nanocrystals indicated by the XRD results. The 1000 °C sample (Figure 2b) is less homogeneous in appearance because of the presence of both anatase and rutile nanocrystals. The 1200 °C sample (Figure 2c) consists of only rutile, and as expected, the crystal sizes have dramatically increased to 20-150 nm in diameter. Highlighted in Figure 2c is what appears to be a glassy second phase coating one of the TiO2 crystallites, which is also present in slightly larger quantities at the corners of the grains. This is the silica phase, and it was observed to vary between 5 and 20 nm in thickness. The peak present in Figure 1 at a chemical shift of δ ) -61 ppm represents a Q0 species (i.e., an isolated SiO4 tetrahedron). This peak seems to be present across the entire range of annealing temperatures, but it only becomes narrow enough to reliably distinguish from the background noise at 1000 °C and above. In the sample annealed at 1200 °C, this environment accounts for ∼5% of the silicon sites present. This Q0 environ-
SiO2 and Al2O3 Zener-Pinned Nanocrystalline TiO2
J. Phys. Chem. C, Vol. 111, No. 37, 2007 13743
Figure 3. 27Al MAS NMR spectra obtained from the alumina-pinned titania samples at 14.1 T.
ment might suggest the presence of a crystalline titanium silicate such as TiSiO4; however, no such material exists because titanium atoms cannot substitute for silicon atoms in a crystalline silica structure. Titania and silica may form solid solutions,27 but these tend to be glassy and would not give rise to a narrow peak. Therefore, this feature most likely arises from some crystalline impurity in the samples (such as a small amount of forsterite, known to give a peak at -62 ppm), which may have been mistakenly introduced during the sample preparation or annealing stage, and this peak was not observed in a second sample annealed at 1200 °C. This feature can therefore be ignored for the purposes of this system. 3.2. Alumina-Pinned TiO2. Figure 3 shows a stacked plot of the 27Al MAS NMR spectra obtained from the aluminapinned TiO2 samples, and Table 3 shows some simulation data for a selection of annealing temperatures. The 600 and 1200 °C samples were analyzed at two fields to allow more unambiguous peak fitting, and the two-field simulation for the 600 °C sample is shown in Figure 4. These simulations were done using the QuadFit program25 with a Gaussian distribution in quadrupolar coupling constants (χQ) and fixed values for both the asymmetry parameter (ηQ) and the isotropic chemical shift (δISO) for each distinct aluminum site. The unheated sample consists of a main peak at δ ) 9 ppm, which corresponds to boehmite (AlOOH).26,28 A smaller, wider peak is present to the right of this at ∼0 ppm, and it is visible as a shoulder on the side of the main peak. This peak narrows significantly in the 400 °C spectrum, remains this way until 1000 °C, where it begins to broaden, and has almost disappeared completely in the 1200 °C sample. This peak is likely to be a titanium aluminate such as Al2TiO5 (tialite),29 which contains a single octahedral aluminum site (previously reported to produce a 27Al MAS NMR chemical shift of ∼6 ppm30) and is known to thermally decompose into R-Al2O3 (corundum) and rutile TiO2 at temperatures between 900 and 1280 °C.31 This phase accounts for about 30% of the aluminum content in this sample. Also present in the unheated sample is ∼2% five- and ∼3% fourcoordinated aluminum, present as peaks at roughly 35 and 60 ppm, respectively. These two peaks become much more intense at 400 and 600 °C, indicating dehydration of the boehmite to form an amorphous alumina. At 1000 °C, a mixture of corundum (present as a narrow peak at 14 ppm and presumably decomposed from the tialite) and a transition alumina (present as two broad peaks representing both tetrahedral and octahedral
Figure 4. 27Al MAS NMR spectra and simulations of the aluminapinned titania sample annealed at 600 °C at (a) 14.1 T and (b) 18.8 T. The top line is the experimental data, the bottom line shows the simulated peaks, and the middle line is the sum of the individual peaks.
aluminum environments) are present. The most likely transition alumina phase is θ, which contains little disorder and immediately precedes corundum formation when boehmite is annealed. By 1200 °C, only the corundum and a small amount of tialite remain. The TiO2 crystallite sizes in Table 2 show that, although the alumina pinning phase produced smaller TiO2 crystals than in the unpinned sample from 400 to 1000 °C, overall, it was less successful than the silica phase; this result is consistent with previous studies where the silica had a stronger pinning effect in nanocrystalline SnO216, ZrO217, MgO17, and Ga2O319 systems. Figure 5 shows TEM images of the alumina-pinned samples annealed at 800, 1000, and 1200 °C. The 800 °C sample (Figure 5a) consists of relatively large rutile grains, between 100 and 200 nm in diameter, surrounded by much smaller anatase nanocrystals approximately 20-30 nm in diameter, which is in good agreement with the XRD results in Table 2. Also present are, what appear to be, agglomerations of tiny particles several nm in diameter, identified as the alumina. The 1000 °C sample (Figure 5b) again shows the contrast in size between the large rutile and smaller anatase crystals. In the 1200 °C sample, the tiny alumina particles have agglomerated into large regions of crystalline corundum. Figure 6 is an SEM image showing this corundum existing as discrete particles situated at the interfaces between the larger TiO2 grains. Although these particular TiO2 grains are too large to be strictly called “nanocrystalline”, this figure provides direct evidence of the pinning phase existing as particles between the metal oxide grains.
13744 J. Phys. Chem. C, Vol. 111, No. 37, 2007
O’Dell et al.
Figure 5. TEM images of the alumina-pinned titania after annealing at (a) 800 °C, (b) 1000 °C, and (c) 1200 °C.
Figure 6. An SEM image of the alumina-pinned TiO2 sample annealed at 1200 °C, showing discrete particles of R-Al2O3 in-between the larger titania crystallites.
3.3. The 17O-Enriched Samples. The 17O static NMR spectra obtained from the 70% 17O-enriched TiO2 samples are shown in Figure 7 for samples heated in situ. For the unpinned TiO2 sample (Figure 7a), a dramatic change in the line shape occurred between 335 and 475 °C, indicating the anatase-to-rutile phase transformation. This phase transition was not observed at 610 °C or below in the silica-pinned sample (Figure 7b), but it occurred in the alumina-pinned sample between 480 and 600 °C (Figure 7c). The alumina phase, therefore, has had the effect of raising the transition temperature. The XRD results quoted in Table 2 suggest that the transition occurs between 600 and 800 °C, but it is possible that smaller rutile nanocrystals that formed below 600 °C were not observed by the XRD. Table 2 also suggests that the transition occurred at a much higher temperature in the silica-pinned TiO2 than in the alumina-pinned system, and this is confirmed by these 17O NMR results. It is
known that the anatase nanocrystals are more stable at lower crystallite sizes, and the rutile phase becomes more favorable as they grow;5,11 therefore, these transition temperatures can be explained by the fact that the alumina and silica phases have a weak and strong pinning effect on the TiO2 nanocrystals, respectively. The anatase TiO2 phase shows a 17O static NMR peak centered at ∼350 ppm, whose center of gravity moves to a more positive shift as the annealing temperature increases and hydroxyl groups (which would appear at a shift closer to 0 ppm32) are removed. Indeed, at the lower temperatures the TiO2 may be amorphous because of these hydroxyl groups, rather than anatase. The rutile phase shows a slightly narrower line centered at ∼350 to ∼500 ppm. TiO2 nanoparticles are known to show 17O NMR shifts between 370 and 740 ppm (including both surface and bulk species),33 depending upon the size of the crystals, preparation conditions, and heat treatment. The anatase and rutile 17O MAS NMR shifts are only separated by 35 ppm,34,35 so in a static 17O NMR spectrum the signals from each phase would overlap significantly. This makes the static 17O spectra of samples featuring both phases very difficult to deconvolute into the two separate signals. The difference in chemical shift of the center of gravity of the 600 °C peak for the unpinned and alumina-pinned samples (approximately 450 and 400 ppm, respectively) may be due to differing amounts of remaining anatase or hydroxyl groups still present in the samples or the presence of Ti-O-Al bonding in the alumina-pinned sample (e.g., in the tialite phase). Figure 8 shows the 17O T1 relaxation data obtained for the unpinned, silica-pinned, and alumina-pinned TiO2 samples on the high-temperature side of the T1 minimum (only these values were used to calculate the activation energies for the oxygen mobility mechanism). The observed T1 minima and activation energies are given in Table 4. These EA values are an order of magnitude smaller than those previously reported for a singlecrystal of TiO2 (2.6 eV, determined from tracer diffusion measurements36) or a microcrystalline TiO2 sample (1.78 eV, determined from electrical conductivity measurements37). It is difficult to confirm that this difference is due to the nanocrystalline nature of these samples alone. There may be other contributing factors, such as the differing experimental techniques used to make the measurements (which are sensitive to different motions and length scales) or the way in which the samples were prepared. It is possible that the samples in this current study may show significantly improved ionic conductivity because of their nanocrystalline nature, and conductivity measurements are currently being carried out on them. Whereas the positions of the T1 minima are very similar in all three systems, the activation energies are significantly lower in the
TABLE 3. Some 27Al MAS NMR Peak Simulation Dataa annealing temperature °C
δISO ppm
peak width Hz (( 30 Hz)
80
9 (1) 0 (1) 34 (2) 64 (2)
1150 1930 5340 9980
600
-1 (1) 13 (1) 45 (1) 75 (1)
3 7 13 9
1200
5 (1) 14 (1)
7 0.5
mean χQ MHz (( 1.0 MHz)
∆χQ MHz (( 1.0 MHz)
relative intensity % (( 2%)
Al coordination
63 32 2 3
6 6 5 4
3 5 7 7
29 18 48 7
6 6 5 4
6 0.5
11 89
6 6
a The 80 °C spectrum was fitted using Lorentzian peaks, and the other two spectra were fitted using two-field data with distributions in χQ. Errors for the chemical shift are shown in brackets.
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J. Phys. Chem. C, Vol. 111, No. 37, 2007 13745
Figure 8. The 17O NMR T1 relaxation times obtained for the 17Oenriched TiO2 samples at various temperatures. Lines show linear fits to the data. Error bars are omitted here, but uncertainties in the T1 minima and calculated EA values are given in Table 4.
TABLE 4. The 17O NMR T1 Minima and Activation Energies Obtained from the 17O-enriched TiO2 Samplesa
a
TiO2 system
T1 minimum K
EA meV
Unpinned silica-pinned alumina-pinned
380 (10) 400 (40) 350 (20)
180 (20) 120 (33) 100 (8)
Uncertainties are shown in brackets.
of EA (∼100 meV, determined using the same experimental method) within experimental uncertainty. The details of the nature of the oxygen motion and the effect of Zener pinning on this are still unclear; however, this data suggests that in situ solid-state 17O NMR measurements could provide new insight into a property that is crucial to a number of the applications of this material. 4. Conclusions
Figure 7. 17O static NMR of the 70% 17O-enriched TiO2 samples, (a) unpinned, (b) silica-pinned, and (c) alumina-pinned. Temperatures shown on the right are in situ.
Zener-pinned TiO2 samples than in the unpinned system. The larger EA value in the unpinned system must be due to the more significant growth of these TiO2 nanocrystals in the temperature range 100 to 600 °C. This is dissimilar to a sol-gel prepared ZrO2 system studied previously,17,38 in which the presence of the silica pinning phase had seemingly no effect on the value
Zener pinning with silica restricted the growth of the solgel TiO2 nanocrystals during the annealing process and stabilized the anatase phase at temperatures up to 800 °C. The silica pinning particles underwent a steady increase to higher-order Qn speciation with increasing annealing temperature as organic and hydroxyl groups were removed and the silica network became more connected. In the silica-pinned titania annealed at 1200 °C, the silica appeared in the TEM images as a glassy coating varying between 5 and 20 nm in thickness around some of the surfaces of the nanocrystals, and it aggregated at the corners of the grains. Around 30% of the alumina pinning phase reacted with the TiO2 nanocrystals to form Al2TiO5 (tialite), which mostly decomposed to form corundum and rutile between 1000 and 1200 °C. The rest of the alumina remained amorphous until 1000 °C, where it formed a more crystalline transition alumina phase. By 1200 °C this alumina phase, too, had converted into corundum. The activation energies for the oxygen ion motion in these samples were an order of magnitude smaller than those previously reported for single-crystal36 or microcrystalline37 TiO2 samples. The EA values for the Zener-pinned TiO2 systems were significantly less than that of the unpinned system. Conversely, in a previously reported silica-pinned nanocrystalline ZrO2
13746 J. Phys. Chem. C, Vol. 111, No. 37, 2007 system,17 the EA was found to be equal to that of an unpinned sol-gel ZrO238, within experimental uncertainty. It is therefore unclear whether or not the pinning phase has a significant effect on the oxygen ion mobility mechanism in nanocrystalline metal oxide systems in general, and there is much potential for further work in this area. Acknowledgment. The Engineering and Physical Sciences Research Council (EPSRC) is thanked for funding the work on nanocrystalline materials through grants GR/S61881 and GR/ S61898. M.E.S. thanks both EPSRC and the University of Warwick for partial funding of NMR equipment at Warwick. The authors would like to thank Dr. T. J. Bastow, Commonwealth Scientific and Industrial Research Organization (CSIRO) Division of CSIRO Manufacturing and Materials Technology (CMMT), Melbourne, Australia, for advice on hightemperature NMR and the loan of a probe. Steve York is also thanked for his help with electron microscopy. References and Notes (1) Imao, T.; Noma, N.; Ito, S. J. Sol-Gel Sci. Technol. 2006, 38, 197-202. (2) Falaras, P.; Xagas, A. P. J. Mater. Sci. 2002, 37, 3855-3860. (3) Reddy, K. M.; Reddy, C. V. G.; Manomara, S. V. J. Solid State Chem. 2001, 158, 180-186. (4) Zhang, Y.; Weidenkaff, A.; Reller, A. Mater. Lett. 2002, 54, 375381. (5) Barnard, A. S.; Zapol, P. Phys. ReV. B 2004, 70, 235403. (6) Li, J. G.; Ishigaki, T. Acta Mater. 2004, 52, 5134-5143. (7) Zhang, W. F.; He, Y. L.; Zhang, M. S.; Yin, Z.; Chen, Q. J. Phys. D: Appl. Phys. 2000, 33, 912-916. (8) Mazali, I. O.; Filho, A. G. S.; Viana, B. C.; Filho, J. M.; Alves, O. L. J. Nano. Res. 2006, 8, 141-148. (9) Watson, S.; Beydoun, D.; Scott, J.; Amal, R. J. Nano. Res. 2004, 6, 193-207. (10) Gao, L.; Zhang, Q. Scr. Mater. 2001, 44, 1195-1198. (11) Zhu, K. R.; Zhang, M. S.; Hong, J. M.; Yin, Z. Mater. Sci. Eng. A 2005, 403, 87-93. (12) Hu, Y.; Tsai, H. L.; Huang, C. L. Mater. Sci. Eng. A 2003, 344, 209-214. (13) Grujic´-Brojcˇin, M., Sˇ c´epanovic´, M. J.; Dohcˇevic´-Mitrovic´, Z. D.; Hinic´, I.; Matovic´, B; Stanisˇic´, G.; Popovic´, Z. V. J. Phys. D: Appl. Phys. 2005, 38, 1415-1420.
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