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J. Phys. Chem. B 2006, 110, 15134-15139
Relative Brookite and Anatase Content in Sol-Gel-Synthesized Titanium Dioxide Nanoparticles Sara L. Isley and R. Lee Penn* UniVersity of Minnesota, Department of Chemistry, 207 Pleasant Street SE, Minneapolis, Minnesota 55455 ReceiVed: March 7, 2006; In Final Form: June 1, 2006
Sol-gel synthesis of titania typically produces a mixture of brookite and anatase. Rietveld refinements were used to systematically track the brookite content and particle size as functions of synthetic variables. Results demonstrate that brookite content and anatase particle size decrease with decreasing Ti/H2O ratios. In syntheses at pH 3, the addition of HCl resulted in increased amorphous content compared to samples synthesized using HNO3. Similar amorphous contents were observed for particles prepared at pH 6-9. Hydrothermal aging for 4 h at 200 °C of sol-gel products containing substantial amorphous titania resulted in higher brookite content than did hydrothermal aging of sol-gel products containing little to no amorphous titania. Finally, dialysis prior to hydrothermal aging appeared to inhibit phase transformation from brookite to anatase in aged materials. Results presented demonstrate that considerable control over the relative anatase and brookite contents can be achieved through control of synthetic variables.
Introduction Nanoparticle size, shape, microstructure, and phase are known to strongly influence nanocrystal properties such as color,1,2 chemical reactivity,3,4 and catalytic behavior.5-7 Previous work addressing the link between sol-gel variables (e.g., temperature, pH, addition rates) and particle properties has demonstrated that small changes in synthetic procedures can yield large changes in the physical and chemical properties of the products.8-15 For example, Sugimoto et al. showed that the morphology of titania particles produced in the presence of triethanolamine was pH dependent.8 At pH 9.6, cubic particles were produced, and, at pH 11.5, ellipsoidal particles were produced.8 Understanding the link between synthetic variables and the physical and chemical properties of the product materials is critical to achieving control over the properties of nanoparticles. Titanium dioxide is commonly used in applications including photocatalysis,6,11,16-18 energy conversion,19-21 sensors,20,22,23 thin-film batteries,21 and pigments.24,25 Sol-gel synthesis, which involves the forced hydrolysis of dissolved titanium (IV) precursors in solution, is often employed to produce titanium dioxide.26 The three most commonly encountered titanium dioxide polymorphs are anatase, rutile, and brookite. In general, anatase is the primary product of such sol-gel syntheses;9,27 however, significant amounts of brookite are frequently observed.11,28,29 Neither the effect of synthetic variables on the brookite content nor how the presence of brookite affects the properties of the materials is currently well understood. Thus, the motivation of this work is to explore the link between synthetic variables and brookite content. Finally, the relative phase percentages are routinely estimated by measuring the heights of characteristic powder X-ray diffraction (XRD) peaks.9,10,17,30-32 The substantial number of overlapping peaks for anatase and brookite makes such an approach semiquantitative, at best, and can result in significant underestimates of the brookite content in sol-gel-synthesized * Corresponding author. E-mail:
[email protected]. Phone: 612626-4680. Fax: 612-626-7541.
titanium dioxide particles.27 To adequately determine the relative amounts of these polymorphs, a more quantitative approach must be applied. Here, we use the Rietveld refinement method,33 which is a whole pattern fitting method that systematically varies constraints in a simulated pattern to minimize differences from that of the experimental pattern. Rietveld refinements have been successfully employed to quantitatively evaluate the phase content of nanoparticle samples, such as TiO2,11,34,35 MgO,34 BaTiO3,36,37 and ZrO2.34 This paper presents the use of Rietveld refinements to quantify anatase and brookite phases. Quantitative results show that the Ti/H2O ratio employed in sol-gel synthesis strongly impacts final brookite content. The effect of the temperature of synthesis, hydrothermal aging time, aqueous solution pH, catalyst identity, and dialysis are also examined. Experimental Methods Titanium dioxide particles were prepared via the sol-gel method38 using two different procedures modified from the literature. The first was adapted from Gnanasekar et al.17 and Sugimoto et al.8 In an acid-washed, 500 mL round-bottom flask, 125 mL of isopropyl alcohol (Fisher, HPLC Grade) and 12.5 mL of titanium isopropoxide (Ti-Iso; Aldrich) were combined. A Teflon-coated stir bar was used to ensure continuous mixing throughout the entire procedure. Aqueous solutions of strong acid or base catalysts were prepared using Milli-Q purified water (Millipore Corporation, 18 MΩ cm resistivity). Three catalysts at five different pHs were used: nitric acid (Mallinckrodt; pH 0, 1.5, and 3), hydrochloric acid (Mallinckrodt; pH 0 and 3), and sodium hydroxide (Mallinckrodt, AR ACS; pH 6 and 9). The appropriate aqueous solution was added dropwise with vigorous and continuous mixing to the contents of the 500 mL round-bottom flask over a period of 20-60 min. The Ti/H2O ratio, temperature of synthesis, aqueous solution pH, and catalyst identity were varied. The sample identifications in Table 1 detail these synthetic variables. In every case, a white sol formed upon addition of the aqueous solution. The reaction flask was then allowed to come to room temperature, if necessary. The sol was
10.1021/jp061417f CCC: $33.50 © 2006 American Chemical Society Published on Web 07/14/2006
Brookite Content in Sol-Gel Titania
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TABLE 1: Effect of Synthetic Variables on Brookite Content and Particle Size sample
bc0a (wt %)
aps0a (nm)
bps0a (nm)
bc4a (wt %)
aps4a (nm)
bps4a (nm)
1:4 (HNO3;pH0)0°C 1:5 (HNO3;pH0)0°C 1:8 (HNO3;pH0)0°C 1:10 (HNO3;pH0)0°C 1:10 (HNO3;pH0)0°CD 1:20 (HNO3;pH0)0°C 1:700 (HNO3;pH1.5)0°CDb 1:5 (HNO3;pH 0)22°C 1:5 (HNO3; pH 0)22°C D 1:10 (HNO3; pH 0)22°C D 1:10 (HNO3; pH 3)0°C 1:10 (HNO3; pH 3)0°C D 1:10 (HCl; pH 0)0°C 1:10 (HCl; pH 0)0°C D 1:10 (HCl; pH 3)0°C 1:10 (HCl; pH 3)0°C D 1:10 (NaOH; pH 6)0°C 1:10 (NaOH; pH 6)0°C D 1:10 (NaOH; pH 9)0°C 1:10 (NaOH; pH 9)0°C D
32.8 ( 4.7 29.8 ( 11.5 23.6 ( 1.5 11.8 ( 6.9
7.8 ( 2.6 7.0 ( 0.2 6.5 ( 3.0 5.2 ( 0.3
5.2 ( 1.4 4.7 ( 1.7 6.5 ( 0.9 5.0 ( 1.4
17.9 ( 7.9 13.4 ( 7.0 34.4 ( 17.4
5.0 ( 0.2 4.2 ( 0.1 6.4 ( 0.9
6.4 ( 0.5 6.7 ( 1.8 3.3 ( 1.2
20.8 ( 3.5 35.7 ( 14.6
5.6 ( 0.1 6.6 ( 0.9
5.0 ( 0.7 4.6 ( 2.6
15.0 ( 5.8
5.6 ( 0.3
6.3 ( 2.3
28.4 ( 0.6 23.1 ( 4.0 18.3 ( 4.2 21.6 ( 4.3 20.9 ( 5.6 20.2 ( 6.0 18.1 ( 7.5 4.7 ( 1.1 19.3 ( 1.7 14.3 ( 1.6 18.6 ( 5.3 29.5 ( 0.6
11.9 ( 0.1 10.9 ( 0.3 8.9 ( 2.4 8.5 ( 2.3 10.5 ( 2.5 7.8 ( 2.5 8.4 ( 0.2 10.4 ( 0.1 11.7 ( 0.3 7.6 ( 0.0 11.4 ( 0.5 11.5 ( 0.0
10.7 ( 0.3 9.4 ( 0.7 7.8 ( 2.3 4.2 ( 2.2 8.0 ( 1.1 6.3 ( 0.5 8.4 ( 1.8 10.5 ( 4.0 9.0 ( 0.5 6.7 ( 1.3 7.6 ( 3.6 8.7 ( 0.1
12.3 ( 7.1
7.8 ( 0.2
9.5 ( 0.9
41.9 ( 3.4
9.5 ( 0.3
9.8 ( 0.2
44.1 ( 2.8
9.7 ( 0.3
9.5 ( 0.1
38.9 ( 4.8
10.2 ( 0.6
9.2 ( 1.0
|- - - - - - - - amorphous + anatase - - - - - - - -| |- - - - - - - - amorphous + anatase - - - - - - - -| |- - - - - - - - amorphous + anatase - - - - - - - -|
a
bc denotes the brookite content (( standard deviation), aps the anatase particle size (( standard deviation), and bps the brookite particle size (( standard deviation) as determined from XRD data. The subscript 0 or 4 denotes the aging time, in hours, at 200 °C. b Sample synthesized via the Gribb and Banfield method.39
heated to boiling (∼83 °C) and allowed to reflux for 12-24 h. A cold-water condenser was employed to prevent concentration of the sol due to evaporation. The final product was a white gel in all cases. The second procedure used a method adapted from Gribb and Banfield.39 To an acid-washed 2 L round-bottom flask, 1 L of 32 mM nitric acid (Mallinckrodt) prepared using Milli-Q water was added. The pH was adjusted to 1.5 by adding 1 M nitric acid while monitoring with a pH meter (VWR SB20). The flask was placed in an ice bath (∼2 °C) to chill. While mixing with a Teflon-coated stir bar, a mixture of 225 mL ethanol (AAPER) and 25 mL Ti-Iso was added dropwise over a period of 3 h. Upon addition of the ethanol/Ti-Iso mixture, a milky white suspension formed. After the reaction flask warmed to room temperature, the suspension was heated to boiling (∼78 °C) and allowed to reflux for 8 h. A cold-water condenser was employed to prevent concentration of the product. The cold-water condenser was then removed, and heating continued for 24 h until the volume was reduced by one-half. This synthetic method yielded a milky-white suspension of titanium dioxide particles. Following sol-gel synthesis, a subset of samples were dialyzed (Spectra/Por MWCO ) 2000 dialysis bags) against Milli-Q water, which was changed 15 times over the course of 3-7 days, to remove the byproducts of synthesis. Suspensions were placed in plastic bottles for storage. Hydrothermal aging was performed by placing 3 mL of suspension and 5 mL of Milli-Q water into the Teflon liner of a Parr Instrument (model 4744) autoclave bomb. Suspensions were aged in an oven at 200 °C for 2-8 h, after which time the autoclave bombs were removed from the furnace and allowed to cool to room temperature. Samples are identified by synthetic variables according to AgingTime this notation: Ti:H2O(Catalyst;pHX)SynthesisTemp Dialysis. Dialyzed particles are noted with a “D”. Finally, unaged samples are denoted by “0 h” in the “Aging Time” superscript. For XRD analysis, several drops of suspension were placed onto a zero-background, quartz slide and allowed to dry. A minimum of three diffraction patterns were collected for each sample using a PANalytical X’Pert Pro diffractometer equipped with a high-speed X’Celerator detector and a Co KR radiation
source (45 kV, 40 mA). Each scan was run in the continuous scanning mode with an effective step size of 0.016°, an effective dwell time of 765 s, a 0.5° divergent slit, and a 1° anti-scattering slit. Diffraction patterns were compared to International Centre for Diffraction Data (ICDD) powder diffraction files (PDF) #01073-1764 for anatase and #00-029-1360 for brookite. In addition, quantitative phase compositions of the samples were determined by Rietveld refinements33 using X’Pert High Score Plus (version 2.0.1) software. Refinements were performed using the known crystal structures for rutile, anatase, and brookite as starting points.40 The parameters refined were zero shift (°2θ), background, scale factor, preferred orientation, W profile parameter, unit cell parameters, and peak shapes, while the thermal parameters, fractional atomic coordinates, U and V profile parameters, and the extinction, porosity, and roughness parameters were fixed. Goodness-of-fit (GOF) and R weighted profile (Rwp) values were monitored to ensure accurate fits between the observed and calculated data and ranged from 2.5 to 25 for GOFs and from 1.5 to 3.6 for Rwp’s. Phase percentages are reported with error bars, which represent the standard deviation calculated from the results obtained from the minimum of three diffraction patterns collected for each sample. No rutile was detected in any of the samples. Patterns collected over a 2θ range of 24-110° were compared to those collected over a 2θ range shortened to 24-62°. The patterns collected over a shorter 2θ range showed no significant differences in the results for phase content and particle size. Therefore, for efficiency, the majority of XRD patterns were collected using the 24-62° 2θ range. Figure 1 shows a typical XRD pattern and the result from the Rietveld refinement, the difference between these profiles, and the reference PDFs for anatase and brookite. Finally, average particle sizes were calculated according to the Scherrer equation41 using the full width at half-maximum of the anatase (101), brookite (120), brookite (111), and brookite (121) peaks after correcting for instrumental broadening. Samples were prepared for transmission electron microscopy (TEM) by diluting suspensions using Milli-Q water, aquasonicating (VWR Aquasonic model 150HT) for 45 s, and then placing one drop onto a 3 mm holey carbon-coated copper grid (SPI supplies). Samples were examined using a Phillips CM 30 TEM, an FEI T12 TEM, or an FEI F30 HRTEM. All TEM
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Isley and Penn
Figure 1. (a) Observed (×××) and calculated (---) XRD patterns, resulting from the Rietveld analysis of data from the 1:20 2h (HNO3;pH0)0°C sample. Refinement GOF was 2.5. (b) Lines correspond to PDF of anatase (#01-073-1764, black) and brookite (#00029-1360, gray). (c) Difference profile between the observed experimental pattern and the calculated Rietveld pattern.
images were collected using a Gatan CCD camera and Digital Micrograph (version 3.3.1 or 3.8.2) software. Particle sizes were determined from TEM images by measuring along two directions (length and width) from the calibrated TEM images. Results and Discussion The ratio of brookite to anatase in titanium dioxide synthesized using the sol-gel method depends strongly on three synthetic variables: the Ti/H2O ratio, the pH, and the catalyst used. Table 1 summarizes the quantitative results obtained from the Rietveld refinements performed on XRD patterns of each sample. In general, brookite content increases (from 13 to 33 wt %) with increasing Ti/H2O ratio (from 1:700 to 1:4), with increasing pH (from 0 to 9), and with the use of HCl as compared to HNO3. In addition no systematic change in the brookite content was observed for changing the temperature during the sol-gel synthesis. Figure 2 shows XRD patterns of samples prepared using Ti/ H2O ratios ranging from 1:4 to 1:700 while holding other variables constant. In general, the material synthesized using more water contains a smaller fraction of brookite. Yanagisawa and Ovenstone concluded that increasing water content leads to increased sample crystallinity.12 The work here demonstrates that H2O is likewise a major factor in determining the phase of the particles. Li et al. noted that, in the sol-gel process, the water content determines the initial species formed during hydrolysis and therefore strongly impacts the resultant phase produced.11 In this case, increased water content results in an increased anatase-to-brookite ratio. Scherrer line-broadening analysis of the anatase (101) peak shows that the anatase particle size decreases with increasing H2O content: the average anatase particle size drops from 7.8 ( 2.6 nm at the lowest water concentration to 4.2 ( 0.1 nm at the highest. This result is consistent with the discussion of Bischoff and Anderson.42 In their work, they comment that faster nucleation rates can result from excess water content.42 This faster rate of nucleation leads to the formation of smaller crystallites.30,42 While the trend observed for decreasing anatase particle size was clear, no similar trend (within error) was noted for brookite particle size. This suggests that the nucleation rate of brookite, in contrast to the nucleation rate of anatase, may be independent of water concentration. Hydrothermal aging is a commonly performed post-sol-gel synthesis step. It typically results in a more crystalline product and larger crystallites.12,20,43-45 Figure 3a shows the XRD patterns for 1:5(HNO3;pH0)0°C aged at 200 °C from 0 to 8 h.
Figure 2. XRD patterns of titania particles synthesized with various Ti/H2O ratios. Samples hydrothermally aged for 0 h (thin solid line) or 4 h (thick solid line) at 200 °C. Anatase (#01-073-1764, black) and brookite (#00-029-1360, gray) PDFs have been included for comparison.
Results here reveal that aging the titanium dioxide nanoparticles for 8 h at 200 °C shows no significant change in the brookite content (Figure 3b). This is in agreement with the results of Penn and Banfield, who found that hydrothermal aging of titanium dioxide particles at 250 °C resulted in little change in the brookite content when aging times were short (0-8 h).44 While Penn and Banfield’s absolute brookite content values may be artificially low as a result of not being quantified using Rietveld refinements, the observed trend of brookite content slowly decreasing over tens of hours of aging at 250 °C is most certainly sound. Not surprisingly, hydrothermal aging results in larger particles. Again, using Scherrer line-broadening analysis of the anatase and brookite peaks, on average, both anatase and brookite particle sizes increase by approximately 65%. Representative data is presented in Figure 3b, showing that aging the particles for 8 h increases the average particles size by 90%. Therefore, hydrothermal aging at 200 °C for short times (0-8 h) does not result in substantial changes in brookite content, but can be used to tailor the size of the nanoparticles while maintaining the brookite content. TEM images (Figure 4) are useful for evaluating the particle size and size distribution as well as morphology. Unfortunately, distinguishing between anatase and brookite from TEM images or microdiffraction is challenging, however, because the materials have similar appearances in most orientations. Orienting an adequate number of nanoparticles in the electron microscope so as to individually identify them as anatase or brookite by high-resolution or microdiffraction is time-consuming and costprohibitive. Therefore, particle sizes determined from TEM data represent the average size, irrespective of the phase of the particle (Figure 5). Consequently, XRD results are most useful for phase composition and average particle size determinations for each phase. However, TEM images are still useful for comparison to the XRD results to obtain information regarding particle morphology and to evaluate whether the particles may
Brookite Content in Sol-Gel Titania
Figure 3. (a) XRD patterns of titanium dioxide particles synthesized with a Ti/H2O ratio of 1:5 and hydrothermally aged at 200 °C in autoclave bombs for 0-8 h. Anatase (#01-073-1764, black) and brookite (#00-029-1360, gray) PDFs have been included for comparison. (b) Observation of brookite content and particle size evolution over time. Brookite contents (9), anatase particle sizes ([), and brookite particle sizes (2) were derived from Rietveld refinements performed on corresponding XRD patterns.
have grown by oriented aggregation, are intergrown,44 or are extensively twinned.46 The majority of the particles in the 4h 1:5(HNO3;pH0)0°C sample, shown in Figure 4a, are primary particles with an average width of 8.4 ( 3.3 nm and an average length of 11.7 ( 5.1 nm (Figure 5). This agrees well with the values calculated via XRD line-broadening analysis, which were 11.9 ( 0.1 nm for anatase and 10.7 ( 0.3 nm for brookite. In addition to particle size, TEM images give an indication of morphology. For example, Figure 4b is a higher magnification image of a single crystal of anatase with a unique morphology that is consistent with oriented aggregation. In general, low pHs (0 and 3) during sol-gel synthesis result in samples with more crystalline titania. Figure 6 shows a comparison of the XRD patterns of the pre-aged samples synthesized using pH values ranging from 0 to 9 and catalysis by HNO3, HCl, and NaOH. XRD patterns of samples prepared at low pH (0 and 3) using HNO3 (Figure 6a,b) are indicative of well-crystallized anatase and brookite with little to no amorphous material apparent. Interestingly, the XRD pattern for the sample prepared at pH 0 and using HCl (Figure 6c) is similar, while the pattern for the sample prepared at pH 3 and using HCl (Figure 6d) is indicative of a high amorphous content and very little crystalline anatase. This result suggests that chloride inhibits nucleation and growth of crystalline titanium dioxide. The XRD patterns for the pH 6 and 9 samples (Figure 6e,f, respectively) are also indicative of high amorphous material content. Quantitating the brookite and anatase contents for samples containing substantial amorphous material proved unsuccessful. However, qualitative comparisons yield a fairly
J. Phys. Chem. B, Vol. 110, No. 31, 2006 15137
Figure 4. (a) TEM image of titanium dioxide nanoparticles from 4h sample 1:5(HNO3;pH0)0°C . (b) High-resolution image of a single crystal of anatase formed by oriented aggregation.
Figure 5. Histogram of particle lengths (black) and widths (white) of 4h the 1:5(HNO3;pH0)0°C sample as determined from TEM data. Particle sizes reported are average sizes irrespective of the phase of the particle. The sample had a mean particle length of 11.7 ( 5.1 nm (median length of 11.4 nm) and a mean particle width of 8.4 ( 3.3 nm (median width 8.5 nm). 1054 total particles were counted and the average aspect ratio was 1.42.
clear trend regarding the impact of pH and catalyst choice on crystallinity. The observed pH trend is consistent with the results of Bischoff and Anderson42 and those of Lee and Zuo.30 Both groups found that, as the pH is decreased by way of acid addition, the solubility of titania increases, causing the preferential dissolution of the amorphous titania, which results in the crystalline material growing at the expense of the amorphous material.30,42 The results in Table 1 show that brookite content in hydrothermally aged materials increases with increasing pH. For those samples exhibiting substantial amorphous content prior
15138 J. Phys. Chem. B, Vol. 110, No. 31, 2006
Figure 6. XRD patterns of titanium dioxide nanoparticles synthesized 0h with different catalysts and pH values. (a) 1:10(HNO3;pH0)0°C (b) 0h 0h 0h 1:10(HNO3;pH3)0°C (c) 1:10(HCl;pH0)0°C (d) 1:10(HCl;pH3)0°C (e) 0h 0h 1:10(NaOH;pH6)0°C (f) 1:10(NaOH;pH9)0°C . Anatase (#01-073-1764, black) and brookite (#00-029-1360, gray) PDFs have been included for comparison.
to hydrothermal aging, surprisingly high brookite content (approximately 40%) is observed after aging. This comparison suggests that it is the high amorphous content of the sol-gel products that leads to the high brookite content observed after hydrothermal aging. It is important to note that all suspensions in this series were dialyzed prior to aging to remove the side products of sol-gel synthesis. This allows a more direct comparison regarding the impact of synthetic variables without the complication of substantial differences in suspension chemistry during the aging step. These results are consistent with previous work showing that the use of HCl as a catalyst promotes brookite growth.10,47 Interestingly, despite the use of a dialysis step to remove counterions and byproducts of synthesis, the average anatase particle size for hydrothermally aged materials depends strongly on the catalyst used during the sol-gel synthesis. When HNO3 is used as the catalyst, there is no observable trend with respect to anatase particle size and increasing pH. In contrast, when HCl or NaOH is used as the catalyst, there is an observable trend of increasing particle size with increasing pH. As shown 4h D, has an in Table 1, the low pH sample, 1:10(HCl;pH0)0°C anatase particle size of 7.8 ( 0.2 nm that increases to 10.2 ( 4h 0.6 nm for the high pH sample, 1:10(NaOH;pH9)0°C D. Pottier et al. observed a similar trend when using HCl and NaOH as catalysts: the anatase particle size increased as the pH increased.48 In comparing brookite particle sizes after hydrothermal aging, no apparent dependence on either aqueous solution pH or the catalyst used was observed. Three sol-gel preparations (1:10(HNO3;pH0)0°C, 1:5(HNO3;pH0)22°C, and 1:10(HNO3;pH3)0°C) were examined to determine the effect of dialysis on the products of aging. Since dialysis alone is not expected to affect the brookite content, after dialysis, the particles were hydrothermally aged for 4 h at 200 °C and the brookite and anatase contents were quantified. In the case of 1:10(HNO3;pH0)0°C, hydrothermal aging resulted in a net phase transformation from anatase to brookite in both the dialyzed and undialyzed samples. However, in the cases of the other two preparations, dialyzing prior to hydrothermal aging appeared to inhibit phase transformation from brookite to anatase during the hydrothermal aging step. In both 1:5(HNO3;pH0)22°C and 1:10(HNO3;pH3)0°C, samples
Isley and Penn that were dialyzed showed substantially less conversion from brookite to anatase (i.e., higher brookite contents) compared to those samples that were not dialyzed. In the samples discussed here, particles in the suspensions that were not dialyzed are expected to be more strongly agglomerated than those in the dialyzed suspensions because dialysis removes the side products (such as NO3-, etc.) of sol-gel synthesis and, thus, substantially reduces the ionic strength. This drop in ionic strength is expected to result in stronger electrostatic repulsions because the electric double-layer thickness increases as the ionic strength decreases.49 These results are consistent with those of Lee and Zuo, who showed that strongly agglomerated samples favored phase transformation.30 While dialysis prior to aging appears to have an effect on the brookite content of the aged products, it has no effect on final particle sizes over the short aging times discussed here. Although preliminary results point toward a possible trend, the relationship between brookite content, particle size, and dialysis must be explored further. Conclusions The influence of titania sol-gel synthesis variables on the brookite content and particle size was systematically studied. Rietveld refinements were used to determine peak breadths and to quantify the relative brookite content of synthetic products before and after hydrothermal aging. The brookite content and particle sizes are most strongly influenced by the Ti/H2O ratio, the pH of the aqueous catalyst solution, and the choice of catalyst used during sol-gel synthesis. In general, low pH during synthesis produces more crystalline products, and high pH produces less crystalline products. The most crystalline products were produced using a pH 0 HNO3 solution during sol-gel synthesis. In addition to high pH values, the use of HCl at pH 3 also resulted in a less crystalline sample compared to the little to no amorphous content produced at pH 3 with HNO3. Results showed that, as the crystallinity of pre-aged material decreased, the brookite content of hydrothermally aged samples increased. Employing dialysis to remove the byproducts of sol-gel synthesis prior to hydrothermal aging appears to inhibit phase transformation from brookite to anatase. Substantial control over the relative brookite content can be achieved by modifying sol-gel synthesis and hydrothermal aging conditions. Experiments further examining the effects of the temperature of synthesis and dialysis on the relative brookite content are currently under way. Acknowledgment. We thank the University of Minnesota and the National Science Foundation (Grants Career-036385 and MRI EAR-0320641) for funding. Danny Vereen, supported by the National Science Foundation (MRSEC Summer program), is thanked for his contribution to this work. References and Notes (1) Scheinost, A. C.; Schulze, D. G.; Schwertmann, U. Clays Clay Miner. 1999, 47, 156. (2) He, Y. Q.; Liu, S. P.; Kong, L.; Liu, Z. F. Spectrochim. Acta, Part A 2005, 61, 2861. (3) Anschutz, A. J.; Penn, R. L. Geochem. Trans. 2005, 6, 60. (4) Meusel, I.; Hoffmann, J.; Hartmann, J.; Libuda, J.; Freund, H.-J. J. Phys. Chem. B 2001, 105, 3567. (5) Beyers, E.; Cool, P.; Vansant, E. F. J. Phys. Chem. B 2005, 109, 10081. (6) Gao, L.; Zhang, Q. Scr. Mater. 2001, 44, 1195. (7) Uekawa, N.; Kajiwara, J.; Kakegawa, K.; Sasaki, Y. J. Colloid Interface Sci. 2002, 250, 285. (8) Sugimoto, T.; Zhou, X.; Muramatsu, A. J. Colloid Interface Sci. 2003, 259, 53.
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