Zirconia) Colloids - Langmuir

The structure of titania/zirconia colloids has been investigated using small-angle neutron scattering (SANS). The colloids were produced by: (i) hydro...
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Langmuir 1998, 14, 3538-3544

Structure of Multicomponent (Titania/Zirconia) Colloids J. R. Bartlett,*,† D. Gazeau,‡ Th. Zemb,‡ and J. L. Woolfrey† Materials Division, ANSTO, Lucas Heights, NSW 2234, Australia, and Service de Chimie Mole´ culaire, CEA, CE Saclay, 91191 Gif sur Yvette Cedex, France Received March 14, 1997. In Final Form: April 8, 1998 The structure of titania/zirconia colloids has been investigated using small-angle neutron scattering (SANS). The colloids were produced by: (i) hydrolysis of a mixture of titanium and zirconium alkoxides and peptization of the resulting hydrolyzate with nitric acid (homogeneous) and (ii) hydrolysis of a titanium alkoxide and peptization of the resulting hydrolyzate with aqueous zirconium(IV) nitrate solution (heterogeneous). The final titania/zirconia and metal oxide/nitrate mole ratios were 16.0 and 10.0, respectively. The sols were concentrated by evaporation, dried under controlled conditions and redispersed in D2O/H2O mixtures. FT-Raman spectra of the sols, and XRD powder patterns from the gels, showed that the material was crystalline anatase and amorphous zirconia. Both TEM and XRD line broadening indicate that the crystallite size of the dried titania gels is ∼8 nm. The results of SANS contrast variation experiments are described. The minimum-contrast points for the homogeneous and heterogeneous colloids, determined by two different methods, gave similar results which differed significantly from the expected value, due to the sorption of nitrate counterions and hydroxyl species on the surface of the colloids. In both systems, the scattering at minimum contrast was consistent with a network of unidimensional zirconia, with a typical diameter of ∼1.5 nm. At full contrast, the homogeneous colloids have fractal dimensions (df of 1.6) similar to those from static light scattering, that is, consistent with an open, extended, chainlike aggregate structure. However, the heterogeneous colloids have higher fractal dimensions (df of 2.2-2.4), due to zirconia crystallites packing the interstices between the titania crystallites, that is, consistent with a loosely packed, spherical structure. The apparent fractal dimension of the heterogeneous colloids in H2O (df of 2.4) is higher than that observed in D2O (df of 2.2), whereas no such effect is observed for the homogeneous colloids. These results infer that, in the homogeneous colloids, the zirconia is segregated within the matrix of the titania crystallites (on the ∼1 nm scale), whereas, in the heterogeneous colloids, the zirconia is segregated on the surface of the titania crystallites (on the ∼10 nm scale).

1. Introduction During the past 20 years, the demand for ceramics with high purity, homogeneity, and well-controlled, tailored properties has led to a renewed interest in sol-gel technology.1 One of the main advantages inherent in the sol-gel process is the mixing of multicomponent systems on a molecular, or at least nanometer, scale. However, the most severe constraint on industrial scale sol-gel processing is the cost of precursor materials, such as alkoxides. In the preparation of multicomponent colloids, it is essential to promote cohydrolysis of the alkoxides, to ensure a homogeneous distribution of the components. However, this is not always possible due to the constraints of chemistry and cost. In the present study, TiO2/ZrO2 mixed oxide colloids were prepared by two different methods: (i) hydrolysis of a mixture of titanium and zirconium alkoxides and peptization of the resulting hydrolyzate with nitric acid (homogeneous) and (ii) hydrolysis of a titanium alkoxide and peptization of the resulting hydrolyzate with aqueous zirconium(IV) nitrate (heterogeneous). Cohydrolysis of mixed alkoxides is often used to prepare multicomponent nanoparticles, which are considered to be homogeneous on a molecular scale, but in the case of systems containing zirconia, the alkoxide precursors are very expensive. Alternatively, a hydrolyzate of one of the * To whom correspondence should be addressed. † ANSTO. ‡ CEA. (1) In Better Ceramics Through Chemistry I-VI; Materials Research Society Proceedings 32, 73, 121, 180, 271; Materials Research Society: Pittsburgh, PA, 1984, 1986, 1988, 1990, 1992, 1994.

materials (in this case, titania) can be peptized with the hydrolyzable inorganic salts of the other component (in this case zirconium(IV) nitrate), for example,

xZr4+ + mH2O h [Zrx(OH)m](4x-m)+ + mH+ Such an approach leads to heterogeneous particles on a molecular scale but provides much cheaper materials. This study investigates the degree of “homogeneity” and the differences in the structure of zirconia in TiO2/ZrO2 colloids prepared by the two different methods. 2. Experimental Section 2.1. Chemicals. Tetraisopropyltitanate (TPT) and tetrabutyl zirconate (TBZ) were obtained from Hu¨ls Troisdorf and were used as received. D2O, with an isotopic purity of 99.8%, was obtained from Euriso-top. 2.2. Sol Preparation. Homogeneous Sol. Appropriate quantities of TPT and TBZ were mixed (16.0 mol of Ti per mol of Zr) and rapidly added to a large excess of water (40 mol of H2O per mol of alkoxide). The resulting TiO2/ZrO2 hydrolyzate was thoroughly washed with water and peptized at 45 °C with dilute nitric acid (0.1 mol of HNO3 per mol of (Ti + Zr)). Heterogeneous Sol. TPT was rapidly added to a large excess of water (40 mol of H2O per mol of alkoxide). The resulting hydrolyzate was thoroughly washed with water and peptized with an acidic zirconia sol at 45 °C, yielding a final Ti/Zr mole ratio of 16.0 and a [NO3]-/(Ti + Zr) mole ratio of 0.1. The sols were dried under ambient conditions, and the resulting gels were redispersed in pure H2O or pure D2O, yielding stock solutions containing 5 vol % oxide. Homogeneous or heterogeneous sols containing 5 vol % oxide and 0-96 vol % D2O-in-H2O were subsequently obtained by mixing appropriate quantities of the stock solutions.

S0743-7463(97)00282-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/05/1998

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Table 1. Scattering Parameters of Species in the Homogeneous and Heterogeneous TiO2/ZrO2 Sols

species H2O D2O TiO2 ZrO2 [NO3](TiO2)0.94(ZrO2)0.06 (TiO2)0.88(ZrO2)0.06(HNO3)0.07 (Ti0.88Zr0.06)O1.82(OH)0.09(HNO3)0.07 homogeneous colloids homogeneous colloids heterogeneous colloids heterogeneous colloids a

scattering length (10-13 cm)

density (g/cm3)

molecular volume (10-23 cm3)

-1.677 19.15 8.172 18.77 26.68 8.797

0.998 1.105 3.84 5.6

2.998 3.009 3.454 3.654 4.900 3.466

3.95

vol % D2O in solvent phase to extinguish scattering

42.1 82.0 86.4 44.6 48.4 49.4 50a 52b 49a 51b

Minimum-contrast point calculated using the invariant method. b Minimum-contrast point calculated using the Guinier method.

2.3. Characterization. Small-angle neutron scattering (SANS) experiments were undertaken using the PAXE spectrometer at the Laboratoire Leon Brillouin. All scattering experiments were undertaken at ambient temperature, using sample-to-detector distances of 1, 3, or 5 m, with de Broglie wavelength either 7 or 12 Å. The data obtained at each detector distance for a given sample were combined into a single spectrum using a least-squares procedure. The effective q range investigated was 6 × 10-3 to 6 × 10-1 Å-1. The samples were held in 1 mm quartz cuvettes during analysis, and scattering intensities were normalized and scaled against the scattering of pure H2O (10/4π cm-1).2 The scattering lengths used were taken from the standard compilation of Koester and Yelon,3 and the average values for the “natural abundance” isotopic mixtures were used in all cases. Densities of the sols were measured at 20.0 ( 0.1 °C using a PAAR DMA 60 densitometer, equipped with a DMA 602 density measuring cell. The molecular volumes of the scattering species were calculated from their respective densities and molecular weights,4 and are included in Table 1. The classical method of contrast variation,5 which involves factorizing the contrast term in the scattered intensity as the product of a contrast and an angular function (i.e. a function of q), was used in this study. This factorization is valid for interacting particles,6 correlated wavelets,7 or disordered materials,8 provided that the scattering length variations inside the particles are negligible compared to the particle/solvent scattering “contrast term”. This latter condition can be experimentally verified from the linearity of plots of I(q)0.5 against contrast. Two cases in which such a condition applies were investigated: (i) near full contrast, where internal heterogeneity may be neglected; and (ii) at the contrast match point, where only internal heterogeneity is observed, yielding scattering intensities 3 orders of magnitude lower than those obtained near full contrast.

3. Results FT-Raman spectra of the sols, and XRD powder patterns from the gels, showed that the materials were crystalline anatase and amorphous zirconia. Variations in log(I(q)‚qn) with q2 for the homogeneous and heterogeneous sols at full contrast are given in Figure 1. The radius-of-gyration (Rg) of the crystallites in the (2) Jacrot, B.; Zaccai, G. Biopolymers 1981, 20, 2413. (3) Koester and Yelon. Standard Compilation of Cross-Sections for Neutron Scattering; ILL: Grenoble, France. (4) CRC Handbook of Chemistry and Physics, 74th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1994. (5) Williams, C. E. In Neutrons, X-Rays and Light Scattering; Lindner, P., Zemb, Th., Eds.; Elsevier: Amsterdam, 1991; p 101. (6) Belloni, L. In Neutrons, X-Rays and Light Scattering; Lindner, P., Zemb, Th., Eds.; Elsevier: Amsterdam, 1991; p 135. (7) Berk, N. F. Phys. Rev. Lett. 1987, 58, 2718. (8) Beaucage, G.; Ulibarri, T. A.; Black, E.; Schaefer, D. W. In Organic Hybrid Materials; Mark, J., Ed.; American Chemical Society Symposium Series; American Chemical Society: Washington, DC, 1994.

Figure 1. Guinier plots of log(I(q)‚qn) versus q2, for “fullcontrast” aggregated particles in (A) homogeneous sols (9, n ) 1.6) and (B) heterogeneous sols (0, n ) 2.2).

homogeneous sol, calculated from the slope of the corresponding line of best fit, was 4 nm. The scattering lengths and molecular volumes of scattering species in the sols are included in Table 1, while variations in scattering invariant and scattering intensity at q ) 0 (calculated by extrapolation from the appropriate Guinier plots) are given in Figure 2 as a function of the D2O content in the solvent phase. The minimum-contrast point was observed at the same solvent composition in both systems (i.e. 50 ( 2% D2O). In addition, the Guinierextrapolation and invariant methods both yield comparable results, within experimental error (Table 1). The intensities of scattering from the homogeneous and heterogeneous sols, at selected D2O/H2O mole ratios wellremoved from the minimum-contrast point, are shown in Figures 3 and 4, respectively, while the corresponding scattering exponents are summarized in Tables 2 and 3,

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Figure 2. Variations in scattering intensity at zero q (calculated via Guinier extrapolations, 4) and invariant q (9) as a function of D2O content in the solvent for (A) homogeneous and (B) heterogeneous sols.

respectively. The homogeneous sols exhibited two distinct scattering regimes, with scattering exponents of ∼ -1.6 and -4. In contrast, the heterogeneous sols exhibited scattering exponents of ∼ -2.4 and -3.5 with solvent compositions below the minimum-contrast point and -2.2 and -4 above the minimum-contrast point. Variations in the scattering intensity with q for the heterogeneous sol at the minimum-contrast point are illustrated in Figures 5 and 6. The scattering intensity is modeled with “infinite” cylinders (7.5 Å radius) and spheres (15 Å radius) in parts A and B, respectively, of Figure 5, “infinite” lamellae of thickness 7.5 Å in Figure 5C, and “infinite” cylinders with radii of 5-40 Å in Figure 6. The corresponding data for the homogeneous sol at minimum-contrast are given in Figure 7. The calculated minimum-contrast solvent compositions for the principal components in the sols (including TiO2, ZrO2, (TiO2)0.96(ZrO2)0.06, [NO3]-, etc.) are included in Table 1. 4. Discussion In the present study, SANS data from three key colloid/ solvent compositions have been investigated: (i) “fullcontrast” scattering in H2O-rich solvent mixtures, where strong scattering from both titania and zirconia species is expected; (ii) “full-contrast” scattering in D2O-rich solvent mixtures, where titania scatters strongly but scattering from zirconia is essentially extinguished; (iii) minimum-contrast scattering, where scattering from a completely homogeneous TiO2/ZrO2/HNO3 colloid would be extinguished (Any scattering observed under these conditions arises from heterogeneities in the structure of the colloids on length scales of 2π/q). In addition, scattering from colloids in a range of D2O/

Figure 3. Variations in scattering intensity (I/cm-1) with scattering wave vector (q/Å-1) for homogeneous TiO2/ZrO2 sols (5 vol % oxide) in D2O/H2O mixtures containing 25, 80, or 96% D2O.

H2O compositions was investigated, to identify the minimum-contrast composition. 4.1. “Full-Contrast” Scattering. The variations in scattering intensities for the homogeneous and heterogeneous sols with q, under conditions well removed from the minimum-contrast point, are illustrated in Figures 3 and 4, respectively. The homogeneous systems exhibited well-defined fractal and Porod regions both above and below the minimum-contrast point, with slopes of -1.6 and approximately -4.0, respectively (Table 2). As expected, the crossover between these regions is observed at a characteristic 2π/q of ∼100 Å, in agreement with the size of the crystallites (∼8 nm) as measured by TEM, XRD line broadening,9 and Guinier extrapolation (see section 4.2). A Porod slope near -4.0 indicates the presence of a sharp interface between the solid and solvent phases. Such a sharp interface exists in the homogeneous system on length scales (∼60 Å) smaller than the dimensions of

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Figure 4. Variations in scattering intensity (I/cm-1) with scattering wave vector (q/Å-1) for heterogeneous TiO2/ZrO2 sols (5 vol % oxide) in D2O/H2O mixtures containing 0, 20, 80, or 96% D2O. Table 2. Variations in Scattering Exponent with q for the Homogeneous Sol, at Selected D2O/H2O Ratios % D2O

1st scattering exponent

25 80 96

-1.6 -1.6 -1.6

2π/q (Å) max min 593 593 593

204 204 204

2nd scattering exponent -3.8 -4.3 -3.8

2π/q (Å) max min 108 85 85

48 45 37

the crystallites (∼80 Å). On larger length scales (>170 Å), the scattering detects the aggregate structure of the colloid, with a measured fractal dimension (df) of 1.6. Comparable values of df have been measured in lightscattering experiments on related TiO2 systems.9 In contrast, the heterogeneous system exhibited significantly different scattering behavior (Figure 4 and Table 3): (i) Below the minimum-contrast point, where the solvent contrast is substantially different from those of both titania and zirconia, the scattering exponent -2.4 was observed over the size range 140-450 Å, and the scattering exponent -3.5 was observed over the range 35-120 Å. No sharp solid/solvent interface was evident over the entire length scale investigated (35-440 Å). The value -2.4 is significantly higher than that observed over the corresponding q range for the homogeneous colloids (see above), indicating that the crystallites are more tightly packed in the heterogeneous system. If there is no contribution at high q due to internal heterogeneities, this would suggest that the voids between the titania crystallites in the heterogeneous colloids are partially filled by zirconia crystallites on the surface of the titania crystallites and (9) Bartlett, J. R.; Woolfrey, J. L. In Better Ceramics Through Chemistry IV; Zelinski, Brinker, C.J., Clark, D. E., Ulrich, D. R., Eds.; Materials Research Society Proceedings 180; Symposium Materials Research Society: Pittsburgh: PA, 1990; p 191.

that the attractive forces leading eventually to sol aggregation are stronger in this latter system. The absence of a sharp solid/solvent interface, at length scales exceeding 35 Å, suggests that the zirconia crystallites are smaller than 35 Å. (ii) Above the minimum-contrast point, where the solvent contrast is well removed from that of titania but comparable to that of zirconia, fractal structures with df ) 2.2 are evident, consistent with a loosely packed, spherical structure. The apparent fractal dimension is lower than that observed on the corresponding length scales at D2O/ H2O ratios below the minimum-contrast point, see above; the scattering from the zirconia crystallites has been systematically extinguished due to the small contrast difference between the zirconia crystallites and the solvent phase, and hence, the structure appears less compact. Similarly, a more well-defined solid/solvent interface is evident on length scales of 30-60 Å (q range 0.1-0.2 Å-1; scattering exponent ∼ -4.0). Assuming that interactions between particles can be neglected in the limited low-q range measured, the homogeneous sample shows behavior similar to diffusionlimited cluster-cluster aggregation,10 while the heterogeneous sample is more compact and closer to either diffusion-limited monomer-cluster or reaction-limited cluster-cluster aggregation. Since the TiO2-to-ZrO2 mole ratios for both sol systems are essentially identical, the above data suggest that the homogeneous sol contains either (i) ZrO2 crystallites with dimensions comparable to that of the TiO2 crystallites (which should exhibit a smooth solid/solution interface on the same characteristic length scales as those for the titania crystallites) (ii) or ZrO2 species contained within (10) Brinker, C. J.; Scherer, G. W. Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990.

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Table 3. Variations in Scattering Exponent with q for the Heterogeneous Sol, at Selected D2O/H2O Ratios 1st 2nd 3rd 2π/q (Å) 2π/q (Å) 2π/q (Å) % scattering scattering scattering D2O exponent max min exponent max min exponent max min 0 20 80 96

-2.39 -2.43 -2.20 -2.22

444 444 444 444

141 141 141 141

-3.6 -3.4 -2.6 -2.9

100 122 131 102

35 40 66 56

-4.1 -3.9

63 54

32 32

Figure 5. Variations in scattering intensity (I/cm-1) with scattering wave vector (q/Å-1) for heterogeneous TiO2/ZrO2 sols at the minimum-contrast point, modeled with (A) “infinite” ZrO2 cylinders of radius 7.5 Å, (B) ZrO2 spheres of radius 15 Å, or (C), ZrO2 lamellae of thickness 7.5 Å.

the matrix of the TiO2 crystallites. These alternatives are discussed in section 4.3. 4.2. Contrast Variation and Determination of Minimum-Contrast Point. In the SANS contrastvariation experiment, the scattering from the colloids under study is investigated at different D2O/H2O volume ratios, to determine the ratio at which a minimum in the scattering intensity is obtained. This minimum is generally determined by plotting the square root of the intensity at q ) 0 (I0, obtained from a Guinier plot, e.g. Figure 1) against the volume fraction of D2O. In the absence of long-range interactions, the intensity of scattering can be approximated by11

I(q) )

I0

e-q d n q

2 2

ln{I(q)‚qn} ) ln{I0} - q2d2 from which the value of I0 can be obtained from a plot of log{I(q)‚qn} against q2. Here, d is a characteristic dimension of the scattering species (e.g. radius-of-gyration of the crystallite or its cross-section) and n is dependent on the geometry of the scattering species. Three cases are classically considered, namely globular particles (n ) 0), needle-like particles (n ) 1), and platelets (n ) 2). In the present study, the scattering diverged significantly for both the homogeneous and heterogeneous sols at low q using these standard models. However, an excellent linear (11) Guinier A.; Fournet, G. Small Angle X-ray Scattering; John Wiley and Sons: London, 1955; p 25.

fit was obtained when n was set equal to the fractal dimension of the scattering species (Figure 1), yielding an apparent minimum contrast at 50 ( 2 vol % D2O for both sols. The validity of this novel approach, which involves dividing the scattering intensity by the dimensionality (following Teixeira12), has been investigated in two ways: (i) The radius-of-gyration, Rg, of the primary crystallites in the homogeneous colloids can be calculated from the slope of a conventional Guinier plot, using the linear relationship between intensity and q2:

ln(I(q)) ) ln(I0) -

Rg2 2 q 3

The apparent value of Rg obtained from Figure 1 was ∼8 nm, in good agreement with the values obtained by XRD line broadening and TEM analysis of dried gels.9 Note that the Guinier method cannot be used to determine Rg for the heterogeneous colloids, since the radius-of-gyration of such core-shell particles cannot be readily defined. (ii) An alternative method for locating the minimumcontrast point involves plotting the square root of the scattering invariant, ∆, as a function of the D2O volume fraction:13 (12) Teixeira, J. In On Growth and Form; Stanley, H. E., Ostrowsky, N., Eds.; Nijhoff: Dordrecht, 1986; p 145. (13) Stuhrmann, H. B.; Miller, A. J. Appl. Crystallogr. 1978, 11, 325.

Multicomponent (Titania/Zirconia) Colloids

Figure 6. Variations in scattering intensity (I/cm-1) with scattering wave vector (q/Å-1) for heterogeneous TiO2/ZrO2 sols at the minimum-contrast point, modeled with “infinite” zirconia cylinders of radius 5, 7.5, 10, 12.5, 15, 20, 30, and 40 Å.

Figure 7. Variations in scattering intensity (I/cm-1) with scattering wave vector (q/Å-1) for homogeneous TiO2/ZrO2 sols at the minimum-contrast point, modeled with “infinite” ZrO2 cylinders of radius 7.5 (s) to 10 Å (- - -).

∆)

∫qq

I(q)q2 dq

max

min

These data are compared to the results obtained from the Guinier extrapolations in Figure 2. Since the invariant does not vanish at minimum contrast (due to particle inhomogeneity), such an approach systematically overestimates the absolute value of the data near the match point. The effect of this systematic error can be seen in Figure 2A. However, the Guinier and invariant methods yield comparable minimum-contrast points for both the homogeneous and heterogeneous colloids. In addition, the minimum-contrast points for the sols are essentially identical, as expected on the basis of their essentially identical bulk compositions (Homogeneous: Guinier, 52%;

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Invariant, 50%. Heterogeneous: Guinier, 51%; Invariant, 49%, Table 1). The measured minimum-contrast point (50 ( 2 vol % D2O) is significantly different from that expected for scattering from pure (TiO2)0.94(ZrO2)0.06 (44.6%, Table 1). Two factors contribute to this apparent discrepancy: (i) Conductivity measurements indicate that ∼70% of the HNO3 initially added during peptization is sorbed on the (TiO2)0.94(ZrO2)0.06 particles. The calculated minimumcontrast point for such a colloid (i.e. (TiO2)0.88(ZrO2)0.06(HNO3)0.07) is 48.4% D2O. (ii) Titration of the homogeneous gels with NaOH revealed that the gels also contained approximately 0.1 mol of acidic hydroxyl species per mol of (TiO2)0.94(ZrO2)0.06. The calculated minimum-contrast point for the hydroxylated colloid (i.e. (Ti0.88Zr0.06O1.82(OH)0.09(HNO3)0.07) is 49.4% D2O (Table 1), in excellent agreement with the measured values for both the homogeneous and heterogeneous sols. It is evident that sorbed species (such as nitrate anions) and surface hydroxyls can significantly influence the value of the minimum-contrast point in metal oxide colloids, which typically exhibit high solid/solution interfacial surface areas (and correspondingly high sorption capacities). To our knowledge, this is the first report of such effects in metal oxide systems. 4.3. Scattering at Minimum Contrast. The intensity of the residual scattering from the heterogeneous and homogeneous sols under minimum-contrast conditions, as a function of q, is illustrated in Figures 5 and 7, respectively. No excess scattering was observed for either system at high q, reflecting the absence of continuous shell core scattering species with shell thicknesses > 9 Å. Since the expected intensity of scattering from the 8 nm TiO2/HNO3 crystallite is negligible under these conditions, the residual scattering from the heterogeneous colloids was modeled using discrete ZrO2 lamellae, spheres, and “infinite” cylinders, with the appropriate volume fraction and contrast for ZrO2. The optimum fits obtained using each model are shown in Figure 5: (i) The lamellar model provided a very poor fit to the experimental data, regardless of the thickness chosen, and can be rejected immediately. (ii) A better fit was obtained using spheres with the diameter 15-20 Å, although significant differences between the model and experimental data are still evident. (iii) “Infinite” cylinders, with the diameter 1520 Å, provided an extremely good fit to the data. In addition, the well-defined linear region in Figure 5 exhibits a slope of -1, indicating that a local cylindrical topology is by far the best approximation. Note that the topological models tested involved monodisperse sizes. Reliable intensity measurements at minimum contrast are only possible for q < 0.2 Å-1, and no independent data regarding size distributions are available for these mixed-oxide particles. These data suggest that the residual scattering observed under minimumcontrast conditions, within the q range 10-1 to 10-2 Å-1, is due to the presence of a network of ZrO2 cylinders, with the diameter 15-20 Å. Previous SAXS and TEM studies of ZrO2 crystallites produced by forced hydrolysis of Zr(IV) salt solutions, and associated formation and polymerization of [Zr4(OH)8(H2O)16]8+ tetramers, revealed the formation of rodlike species with similar dimensions in such systems.14,15 (14) Bleier, A.; Cannon, R. M. In Better Ceramics Through Chemistry II; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Materials Research Society Proceedings 73; Materials Research Society: Pittsburgh, PA, 1986; p 71. (15) Singhal, A.; Toth, L. M.; Beaucage, G.; Lin, J.-S.; Peterson, J. J. Colloid Interface Sci. 1997, 194, 470.

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It would be anticipated that a population of such small crystallites would exhibit a very low-potential-energy barrier to aggregation,16 suggesting that the zirconia species either would be located on the surface of the larger titania crystallites or would aggregate in solution to produce larger zirconia species. The predicted effect of unconstrained aggregation of such zirconia cylinders on the scattering intensity is illustrated in Figure 6. These data demonstrate that the least-squares difference between the experimentally observed scattering intensity and the model data increases significantly with increasing cylinder radius. The results suggest that the zirconia forms a fractal network of high-aspect-ratio species, with an apparent radius of ∼7.5 Å, on the surface of the larger titania crystallites. The presence of a fractal network of zirconia cylinders on the surface of a titania core is also evident from the scattering behavior of the heterogeneous system under full-contrast conditions at high q (see section 4.1): (i) In pure H2O, where (Fsolvent - Fcolloids)2 . 0, the scattering contains contributions from both titania and zirconia. Under such conditions, the scattering species (titania/zirconia) exhibit an apparent fractal dimension of 2.4 ( 0.02. (ii) In D2O/H2O mixtures containing >80 vol % D2O, the scattering from zirconia is essentially extinguished (Table 1). Under these conditions, the scattering species (titania) exhibit an apparent fractal dimension of 2.2 ( 0.02, consistent with a more loosely packed structure (since zirconia species filling the voids between titania crystallites are not detected). Under minimum-contrast conditions, the homogeneous colloids exhibited similar scattering behavior to that of the heterogeneous particles, suggesting that they also contain a fractal network of high-aspect-ratio zirconia cylinders. This suggests that segregation also occurs during hydrolysis of the mixed TPT/TBZ alkoxide, leading to formation of a discrete zirconia network. However, the apparent fractal dimension of the homogeneous colloids did not change and was 1.6 for all solvent compositions well removed from the minimum-contrast point. This indicates that the zirconia particles are not located on the surface of the titania crystallites but are distributed within the interior of the titania crystallite matrix. The results indicate that segregation occurred even during hydrolysis of the mixed TPT/TBZ alkoxides, presumably due to the different rates of hydrolysis of the component alkoxides, but the zirconia crystallites are distributed within the interior of the titania crystallite matrix on the ∼1 nm scale. DLVO theory suggests that the relatively low barrier to aggregation between the TiO2 and ZrO2 particles produced in the homogeneous system leads to rapid coagulation and associated incorporation of the small ZrO2 crystallites into the TiO2 crystallite matrix.7 This segregation effect could be further reduced, or eliminated, by using component alkoxides based on the (16) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638.

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same alcohol (e.g. titanium and zirconium butoxides), which should hydrolyze at similar rates to produce (Ti,Zr)O2 species rather than discrete TiO2 and ZrO2 crystallites. 5. Conclusions The structure of titania/zirconia colloids has been investigated using small-angle neutron scattering. The colloids were produced either by hydrolysis of a mixture of titanium and zirconium alkoxides and peptization of the resulting hydrolyzate with nitric acid (homogeneous) or hydrolysis of a titanium alkoxide and peptization of the resulting hydrolyzate with zirconium(IV) nitrate (heterogeneous). Dry gels were redispersed in D2O/H2O mixtures, yielding a series of sols with a constant oxide volume fraction (0.05) and a varying D2O volume fraction (0-0.95). (1) FT-Raman spectra of the sols, and XRD powder patterns from the gels, showed that the materials were crystalline anatase and amorphous zirconia. Both TEM and XRD line broadening indicate that the crystallite size of the dried titania gels is ∼8 nm. (2) At full contrast, the homogeneous colloids have fractal dimensions (df of 1.6) consistent with those from static light scattering, that is, consistent with an open, extended, chainlike aggregate structure. The heterogeneous colloids have higher fractal dimensions (df of 2.22.4), due to zirconia crystallites packing the interstices between the titania crystallites, that is, consistent with a loosely packed, spherical structure. The denser networks in the heterogeneous colloids appear to arise from stronger interparticle interactions. (3) The apparent fractal dimension of the heterogeneous colloids in H2O (df of 2.4) is higher than that observed in D2O (df of 2.2), whereas no such effect is observed for the homogeneous colloids. (4) SANS contrast variation experiments showed that the points of minimum contrast for the homogeneous and heterogeneous colloids differed significantly from the expected values, due to the sorption of nitrate counterions and hydroxyl species on the surface of the colloids. (5) In both cases, the scattering at minimum contrast was consistent with a fractal network of unidimensional zirconia, with a typical diameter of ∼1.5 nm, that is, consistent with an open, extended, chainlike aggregate structure. (6) The results infer that, in the homogeneous colloids, the zirconia is segregated within the matrix of the titania crystallites (on the ∼1 nm scale), whereas, in the heterogeneous colloids, the zirconia is segregated on the surface of the titania crystallites (on the ∼10 nm scale). (7) Segregation occurred even during hydrolysis of the mixed TPT/TBZ alkoxides, presumably due to the different rates of hydrolysis of the component alkoxides. Acknowledgment. The authors gratefully acknowledge the assistance of Frederic Ne´ and Dr. Jose´ Teixeira. LA970282P