J. Phys. Chem. C 2008, 112, 4469-4474
4469
Titanium Dioxide Nanoparticles: Effect of Sol-Gel pH on Phase Composition, Particle Size, and Particle Growth Mechanism Sara L. Isley and R. Lee Penn* UniVersity of Minnesota, Department of Chemistry, 207 Pleasant Street SE, Minneapolis, Minnesota 55455 ReceiVed: NoVember 13, 2007; In Final Form: January 15, 2008
This work investigates the effect of pH during sol-gel synthesis on the brookite content and average anatase and brookite particle sizes. In general, anatase is the primary product of such sol-gel syntheses; however, significant amounts of brookite are frequently observed. Rietveld refinements of powder X-ray diffraction (XRD) patterns enabled tracking of phase composition and the Scherrer equation was used to determine average anatase and brookite sizes from the full widths at half-maximum of XRD peaks. Furthermore, two nanoparticle growth models are employed to fit nanoparticle growth data in order to elucidate growth mechanisms operating during hydrothermal aging. In general, an increase in pH during sol-gel synthesis results in an increase in brookite content and an increase in average anatase particle size. Results from hydrothermal aging of dialyzed sol-gel products show that the average particle size and dominant particle growth mechanism depend strongly on the pH employed during synthesis (pH -0.5 to pH 3), despite dialysis prior to aging, which increased the pH of all suspensions from -0.5-3 to 5.4-5.7. Interestingly, the dominant anatase particle growth mechanism during hydrothermal aging also depends on the sol-gel pH, with growth dominated by coarsening for particles synthesized at pH -0.5 and by simultaneous oriented aggregation and coarsening for particles synthesized at pH 3. This difference is most likely due to the differences in average particle sizes and phase composition of the products of each sol-gel synthesis. Substantial control over the brookite content, particle size, and particle growth mechanism can be achieved by varying the pH of the sol-gel synthesis.
Introduction Titanium dioxide is used in many applications including thinfilm batteries,1 photocatalysis,2 and pigments.3 It is well-known that the titanium dioxide polymorphs anatase and brookite have dramatically different chemical and physical properties. For example, recent results have shown that photocatalytic activity decreases with increasing brookite to anatase ratio.4 In addition, Ruiz et al. found that anatase is the desirable phase for CO gas sensing, with rutile being less sensitive.5 Finally, Ranade et al. showed that the surface enthalpy of anatase is 0.4 ( 0.1 J/m2, whereas it is 1.1 ( 0.1 J/m2 for brookite.6 Previous research has shown that sol-gel conditions have an impact on both the titania product mixture and the average particle sizes. For example, it was found that decreasing the Ti/H2O molar ratio (from 1:4 to 1:700) resulted in both decreasing brookite content and anatase particle size in solgel synthesized titanium dioxide nananoparticles.7 In addition, hydrothermal aging is a common postsynthesis step, and variables such as pH,5 dialysis,7 and temperature8 have been shown to influence particle growth and phase transformation. Thus, one motivation of this work was to investigate the link between synthetic variables and the resultant phase composition and average particle sizes as a function of sol-gel pH both before and after hydrothermal aging. Particle growth during hydrothermal aging typically follows two major mechanisms, coarsening and aggregation. Coarsening (also known as Ostwald ripening) is growth of larger particles at the expense of smaller particles, whereas aggregation is growth by the combination of primary particles into larger * Corresponding author. E-mail:
[email protected]. Phone: 612626-4680. Fax: 612-626-7541.
secondary particles. Oriented aggregation is a special case of aggregation by which secondary crystals composed of oriented primary crystals are produced.9 The relative contribution to growth by coarsening and aggregation offer the potential for improved control over phase content and particle size. Therefore, a second motivation for this work was to examine the relationship between the sol-gel synthesis conditions and the mechanism by which particle growth proceeds during hydrothermal aging. This paper presents results tracking particle sizes and crystalline phases as a function of sol-gel pH. In addition, aging studies were performed using two samples that were dialyzed so as to remove the byproducts of synthesis and nitric acid: one synthesized at pH -0.5 and one synthesized at pH 3. Particle growth data was fitted to both the classical coarsening and the simultaneous oriented aggregation/coarsening growth models. Results demonstrate that the dominant growth mechanism during hydrothermal aging of dialyzed suspensions depends strongly on the pH employed during sol-gel synthesis. Experimental Methods Titania nanoparticles were prepared by the sol-gel method10 as described by Isley and Penn.7 The pH of the acid solution was varied between -0.5 and 3, and a Ti/H2O ratio of 1:8.5 was used. After synthesis, suspensions were dialyzed to remove byproducts of synthesis and nitric acid. The resulting suspensions were then hydrothermally aged so as to determine the influence of synthetic conditions on phase composition and particle growth. Sol-Gel Synthesis. In an ice bath (∼0 °C), a mixture of 125 mL of isopropyl alcohol (Fisher, HPLC Grade) and 12.5
10.1021/jp710844d CCC: $40.75 © 2008 American Chemical Society Published on Web 02/29/2008
4470 J. Phys. Chem. C, Vol. 112, No. 12, 2008 mL of titanium isopropoxide (Ti-Iso; Aldrich) was added to a 500 mL round-bottom flask with stirring and was chilled for 30 min. A Ti/H2O molar ratio of 1:8.5 was used for all preparations. A nitric acid (Mallinckrodt; ACS grade) solution was prepared using Milli-Q purified water (Millipore Corporation, 18 MΩ cm resistivity) at pH -0.5, 0, 1.5, or 3, as indicated. The acid solution was added dropwise to the isopropyl alcohol/ Ti-Iso solution with continuous stirring by a Teflon-coated stir bar. Then, the reaction vessel was allowed to come to room temperature. The resulting sol was heated to boiling and allowed to reflux for 24 h, and a cold-water condenser was employed to prevent concentration. During the refluxing step, a milkywhite suspension formed. Samples were then dialyzed (Spectra/ Por MWCO ) 2000 dialysis bags) against Milli-Q water to remove byproducts of synthesis and nitric acid. The water was changed at least fifteen times over the course of 5-10 days. The pH of the suspensions after dialysis ranged from pH 5.4 to 5.7. Suspensions were placed in plastic bottles for storage and hereafter are referred to as As-synthesized and are labeled SG pH (e.g., SG -0.5). Hydrothermal Aging. Hydrothermal aging was performed by placing 3 parts suspension and 5 parts Milli-Q water by volume into the Teflon liner of a Parr Instrument autoclave bomb. Suspensions were aged in an oven at 200 °C for 2-168 h, after which time the autoclave bombs were removed from the furnace and allowed to cool to room temperature. All samples had an average pH of 5.7 after hydrothermal aging. Hydrothermally aged samples are identified as SG pH AgedAging Time (h) (e.g., SG -0.5 Aged48). X-ray Diffraction (XRD). For X-ray analysis several drops of suspension were placed onto a zero-background quartz slide and allowed to air-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) over a 2θ range of 24-62°. Patterns were collected in the continuous scanning mode with a step size of 0.016°, a dwell time of 765 s, a 0.5° divergent slit, and a 1° anti-scattering slit. Experimental patterns were compared to International Centre for Diffraction Data (ICDD) powder diffraction files (PDF) #01-073-1764 for anatase, #00-029-1360 for brookite, and #01-072-1148 for rutile. Quantitative phase compositions were determined by the Rietveld method,11 which is a whole pattern fitting method that systematically varies constraints in a simulated pattern to minimize differences from that of the experimental pattern. The refinements were performed using X’Pert High Score Plus (version 2.0.1) software and the known crystal structure data for anatase, brookite, and rutile as starting points.12 The parameters refined were zero shift (°2θ), background, scale factor, preferred orientation, extinction coefficient, unit cell parameters, peak shapes, and W, U, and V profile parameters. 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 1 to 7 and 1 to 4, respectively. Errors reported for the phase compositions represent the standard deviation calculated from the results of at least three refinements performed for each sample. No rutile was detected in any of the samples. The Scherrer equation13 was used to calculate average particle sizes from the full widths at half-maximum of the anatase (101), brookite (120), brookite (111), and brookite (121) peaks after correcting for instrumental broadening. Transmission Electron Microscopy (TEM). Samples were prepared for transmission electron microscopy (TEM) by
Isley and Penn diluting a small amount of suspension in Milli-Q water, aquasonicating (VWR Aquasonic model 150HT) and placing one droplet onto a 3 mm holey carbon-coated copper grid (SPI supplies). Samples were allowed to air-dry and were subsequently imaged with an FEI Tecnai F30 FEGTEM equipped with a Gatan charge-coupled device (CCD) camera. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) measurements were performed on an SSX-100 system (Surface Science Instruments) equipped with a monochromated Al KR X-ray source, a hemispherical sector analyzer (HSA), and a resistive anode detector. Aged48 suspensions were dried in air before analysis, resulting in fine white powders. The four TiO2 samples were mounted on a single sample stage using double-sided carbon tape. The X-ray power was 200 W, and the spot size was approximately 1 × 1 mm2. Survey spectra were obtained at 150 eV pass energy using 1 eV/step, and high-resolution spectra were collected at 50 eV pass energy using 0.1 eV/step. For the samples prepared at pH -0.5, 0, and 1.5, a low-energy electron flux (12 eV) was used for charge neutralization. The pH 3 sample was sufficiently conductive and no neutralization was used. The base pressure of the XPS system was 1.0 × 10-9 Torr. During the data collection, the pressure was 1.3-1.4 × 10-8 Torr because of sample degassing although the samples had been previously evacuated for 2 h in the introduction chamber before transfer into to the analysis chamber. The curve fittings were conducted using the ESCA 2005 software provided with the XPS system. Charge compensations were performed by setting the adventitious C 1s peaks for C-H/C-C to 285.0 eV, and a GaussianLorentzian model with a ratio of 80:10 was applied. Zeta Potential. Samples were prepared for zeta potential measurements by diluting the Aged48 suspensions 100-fold with Milli-Q water. All diluted suspensions had a pH of 6 and were analyzed at 25 °C. Zeta potentials were measured with a ZetaPlus meter (Brookhaven Instruments Corp.) using 2 cycles of 10 runs each. Final experimental zeta potentials were calculated as an average of the 20 runs and are reported to one standard deviation. Results and Discussion X-ray diffraction patterns of titanium dioxide particles synthesized at pH -0.5, 0, 1.5, and 3 are presented in Figure 1. The increase in intensity of the characteristic brookite peak at 36° 2θ indicates an increase in brookite content with increased sol-gel pH. The substantial number of overlapping XRD peaks for anatase and brookite can result in significant underestimates of the brookite content based on peak height intensities. Therefore, to quantify the relative amounts of these polymorphs, Rietveld refinements were used. Rietveld refinements systematically vary constraints to minimize differences between the experimental powder X-ray diffraction pattern and the simulation. Quantitative phase compositions determined from Rietveld refinements are presented in Figure 2, and the error bars represent the standard deviation calculated from the results of at least three refinements of each sample. Overall, the results show that the brookite content increased with increased solgel pH and was 12.1 ( 2.0% for SG -0.5 and 43.7 ( 4.6% for SG 3. In general, hydrothermal aging for 48 h at 200 °C resulted in a drop in brookite content despite the fact that the suspensions were dialyzed before hydrothermal aging (pH ranged from 5.4 to 5.7). In addition, only modest increases in the pH during hydrothermal aging were observed, with post aging pHs ranging from 5.5 to 6.0. When comparing the As-synthesized and Aged48
Influence of pH on Nano-TiO2
Figure 1. XRD patterns of titania particles synthesized under various pH conditions. Samples labeled Aged48 were hydrothermally aged for 48 h at 200 °C. Anatase (#01-073-1764, black) and brookite (#00-0291360, gray) PDFs are shown as stick patterns.
Figure 2. Brookite content of As-synthesized (open squares) and Aged48 (closed squares) samples determined by Rietveld refinement versus pH. Aged48 samples were hydrothermally aged for 48 h at 200 °C. Error bars represent the standard deviation calculated from the results obtained from a minimum of three refinements performed for each sample.
materials, hydrothermal aging of the dialyzed samples results in little to no decrease in brookite content for SG 3 Aged48 and SG 1.5 Aged48 and large drops in brookite for SG 0 Aged48 and SG -0.5 Aged48 (Figure 2). Isley and Penn showed that, as the pH employed during sol-gel synthesis increased from 0 to 9, the amorphous content in sol-gel synthesized titanium dioxide increased, which led to an increase in brookite content in hydrothermally aged samples.7 Extrapolation of the trend to the pH range studied here, pH -0.5 to pH 3, predicts low brookite content at pH -0.5 and high brookite content at pH 3, as observed. In addition to phase composition, the second major difference observed is that the average anatase and brookite particle sizes increased with increased sol-gel pH (Figure 3). The SG -0.5 sample had the smallest average anatase particle size at 5.0 ( 0.1 nm, which was followed by an increase to 6.2 ( 0.2 nm in the SG 0 sample. The SG 1.5 and SG 3 samples are statistically indistinguishable, both with a maximum average anatase particle size of 7.5 ( 0.2 nm. This trend of increased anatase size with increased synthetic pH is consistent with previous results.14 For the brookite particle sizes, the SG -0.5 and SG 0 samples are similar (within error) with sizes of 3.1 ( 0.6 nm and 3.2 ( 0.1 nm, respectively. When comparing average brookite particle sizes for SG 1.5 and SG 3, it is SG 1.5 that has the largest average brookite particle size (7.5 ( 0.4 nm) and is SG 3 that has a slightly smaller average brookite particle size (6.0 ( 0.4
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Figure 3. (a) Anatase particle sizes of As-synthesized (open diamonds) and Aged48 (closed diamonds) and (b) brookite particle sizes of Assynthesized (open triangles) and Aged48 (closed triangles) samples versus pH. Aged48 samples were hydrothermally aged for 48 h at 200 °C. Note that the percent brookite in the SG -0.5 Aged48 sample was 0-1%, which resulted in insufficient peak intensity for size determination. Error bars represent the standard deviation calculated from the results obtained from a minimum of three refinements performed for each sample.
Figure 4. Brookite content of SG -0.5 Aged0-168 (closed squares) and SG 3 Aged0-168 (open squares) samples determined by Rietveld refinement versus aging time at 200 °C. Error bars represent the standard deviation calculated from the results obtained from a minimum of three refinements performed for each sample.
nm). Hydrothermal aging for 48 h at 200 °C caused both the anatase and the brookite particle sizes to increase (Figure 3), with the largest increase in anatase particle size observed for SG -0.5 Aged48. An aging study was performed using SG -0.5 and SG 3 to investigate why the SG -0.5 Aged48 sample showed such a dramatic increase in particle size as compared with the other samples. The particles were hydrothermally aged for 0-168 h at 200 °C, and both phase content and particle sizes were monitored. Figure 4 presents the phase composition results for the aging study. Both SG -0.5 Aged0-168 and SG 3 Aged0-168 samples show a decrease in brookite percentage over time. The SG 3 Aged0-168 shows a much slower rate (0.07 wt % brookite/ h) of brookite to anatase phase transformation as compared with SG -0.5 Aged0-168 (0.2 wt % brookite/h). In SG 3 Aged0-168, the brookite content drops from 43.7 ( 4.6% to 32.1 ( 1.9% over 168 h, while in SG -0.5 Aged0-168, the brookite content drops from 12.1 ( 2.0% to undetectable levels just after 84 h. Figure 5 presents a graph of anatase particle size versus hydrothermal aging time. SG -0.5 Aged0-168 shows a continual and steep increase in anatase particle size from 5.0 ( 0.1 nm at 0 h to 27.6 ( 0.2 nm at 168 h, whereas the SG 3 Aged0-168 shows an initial steep increase followed by a plateau at approximately 14 nm. The data was fit using two particle growth models: the classic coarsening model and the simultaneous
4472 J. Phys. Chem. C, Vol. 112, No. 12, 2008
Isley and Penn
Figure 5. Anatase particle sizes of SG -0.5 Aged0-168 (closed diamonds) and SG 3 Aged0-168 (open diamonds) samples versus aging time at 200 °C. Error bars represent the standard deviation calculated from the results obtained from a minimum of three refinements performed for each sample. Data from each sample was fit following either the classic coarsening or simultaneous coarsening/oriented aggregation growth model and the best fits are presented. The SG -0.5 Aged0-168 sample fit equally well to the reaction limited (n ) 2) classic coarsening (solid black line) and simultaneous oriented aggregation/ coarsening (dashed gray line) growth models (n ) 2, reaction limited). The best fit for SG 3 Aged0-168 was the simultaneous oriented aggregation/coarsening growth model (n ) 3, diffusion limited), which is shown as a solid gray line.
oriented aggregation/coarsening model. The classical coarsening model15-17
Dt ) Do + kct1/n
(1)
where Dt is the particle diameter at time t, Do is the particle diameter at time zero, kc is the coarsening growth rate constant, and n is the growth limited parameter. The growth limited parameter is an integer that describes the mechanism of growth (2 for reaction limited growth, 3 for volume diffusion-limited growth, 4 for grain boundary diffusion-limited growth, and 5 for dislocation-pipe-diffusion-limited growth).15-17 The simultaneous oriented aggregation/coarsening growth model18 is described by eq 2,
Do(x2k1t + 1) 3
Dt )
(k1t + 1)
+ k2t
1/n
(2)
where again Dt is the particle diameter at time t, Do is the particle diameter at time zero, and n is the growth limited parameter. In the simultaneous oriented aggregation/coarsening growth model, k1 is the growth rate constant for oriented aggregation, and k2 is the growth rate constant for simultaneous oriented aggregation and coarsening. This model predicts growth that is initially dominated by oriented aggregation resulting in a rapid increase in particle size. As growth proceeds, the coarsening mechanism begins to dominate, and a plateau in particle growth is observed. The particle growth models were applied using both n ) 2 (reaction limited) and n ) 3 (diffusion limited), and fits were evaluated by minimizing the average absolute value of the difference in particle sizes between the experimental data and the growth model. The best fits are presented in Figure 5 and the corresponding rate constants are tabulated (Table 1). All other fits and corresponding rate constants are contained in Supporting Information. SG 3 Aged0-168 anatase particle sizes fit best to diffusion limited (n ) 3) simultaneous oriented aggregation/coarsening growth, whereas SG -0.5 Aged0-168 fit equally well to reaction limited (n ) 2) classic coarsening and simultaneous oriented aggregation/coarsening particle growth. The simultaneous oriented aggregation/coarsening model fits SG -0.5 Aged0-168 with a very small value of k1, suggesting that the contribution to overall growth by oriented aggregation
Figure 6. (a) Representative TEM image of titanium dioxide nanoparticles from SG 3 Aged4. (b) High-resolution image from SG 3 Aged2 of a single-crystal particle with a morphology that is consistent with oriented aggregation.
TABLE 1: Calculated Titanium Dioxide Nanoparticle Growth Rate Constants from the Best Fit Growth Models sample SG -0.5 Aged0-168 SG 3 Aged0-168
kca (nm min-1/n)
k1a (min-1)
k2a (nm min-1/n)
2.39 × 10-1
8.96 × 10-5 1.08 × 10-1
2.33 × 10-1 2.50 × 10-1
a kc is calculated from classic coarsening model, and k1 and k2 are calculated from the simultaneous oriented aggregation/coarsening model (n ) 2 for SG -0.5 Aged0-168 and n ) 3 SG 3 Aged0-168).
is small. Therefore, it is hypothesized that coarsening is the dominant growth mechanism for SG -0.5 Aged0-168, while both oriented aggregation and coarsening are important growth mechanisms for SG 3 Aged0-168. Samples from SG -0.5 Aged0-168 and SG 3 Aged0-168 were characterized by HRTEM. A representative image of SG 3 Aged4 is shown in Figure 6a, and a high-resolution image of a single anatase crystal, with features consistent with growth by oriented aggregation (i.e., dimples and defects, like edge dislocations9), is shown in Figure 6b. A representative image of SG -0.5 Aged48 is shown in Figure 7, and particles range from approximately 5 to 60 nm in size. Many particles are strongly faceted, and no clear evidence of substantial growth by oriented aggregation was observed. These results support the conclusion that during hydrothermal aging SG -0.5 Aged0-168 growth is dominated by coarsening, whereas SG 3 Aged0-168 growth is dominated by simultaneous oriented aggregation and coarsening. The dramatic difference in growth curves of SG -0.5 Aged0-168 and SG 3 Aged0-168 and the substantial difference in the rate of brookite to anatase conversion at first seemed surprising because the hydrothermal aging step was performed using dialyzed suspensions. The pH of the suspensions ranged from 5.4 to 5.7 after dialysis and from 5.5 to 6.0 after
Influence of pH on Nano-TiO2
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( (
) )
2Vmγ cS rSRT ) 2Vmγ cL 1+ rLRT 1+
Figure 7. Representative TEM image of titanium dioxide nanoparticles from the SG -0.5 Aged48 sample. Many of the particles show strong faceting.
(5)
By using the surface energies calculated by Naicker et al.,20 the anatase particles in SG -0.5 are predicted to be 1.5 times more soluble than the anatase particles in SG 3, and the brookite particles in SG -0.5 are predicted to be 1.8 times more soluble than the brookite particles in SG 3. Thus, solubility differences alone predict a larger kc for SG -0.5 Aged0-168 than for SG 3 Aged0-168. Furthermore, the higher solubility of the SG -0.5 brookite particles likely explain the faster rate of brookite to anatase conversion with hydrothermal aging. Other potential differences caused by the variation in solgel pH could include surface hydroxylation. SG -0.5 Aged48, SG 0 Aged48, SG 1.5 Aged48, and SG 3 Aged48 were characterized by XPS, and the Ti 2p and O 1s spectra are shown in Figure 8 for SG -0.5 Aged48 and SG 3 Aged48. Survey spectra for SG -0.5 Aged48 and SG 3 Aged48 are shown in Supporting
hydrothermal aging. Within this pH range at 200 °C, holding variables like phase composition and particle size constant, no significant difference in TiO2 solubility due to pH is predicted. However, nanoparticle growth in SG -0.5 Aged0-168 is clearly dominated by coarsening, whereas in SG 3 Aged0-168, growth by oriented aggregation is important to overall growth. When comparing SG -0.5 and SG 3, there is a large difference in anatase particle size (4 nm vs 7 nm), brookite particle size (3 nm vs 6 nm), and brookite content (12% vs 42%). The form of the coarsening rate constant, kc, is shown as eq 3,
8cVm2γ kc ) 54πηaNa
(3)
where c is the solubility, Vm is the molar volume, γ is the surface energy of the solid, η is the solvent viscosity, a is the solvated ion radius, and Na is Avogadro’s number.15,16 Both the phase composition and the particle size significantly affect the rate constant for coarsening. For example, Vm varies with phase composition,19 γ varies with phase composition and particle size,20 and c varies with both phase composition and particle size. In addition, solubility is dependent on both phase composition and particle size, and, as solubility increases, eq 3 predicts that kc will also increase. By assuming the particles are small and spherical, the solubility, cd, of a particle with a given diameter d is given by the equation
(
cd ) c ∞ 1 +
)
2Vmγ rRT
(4)
where c∞ is the solubility at a flat surface, γ is the surface energy of the solid, Vm is the molar volume, r is the radius of the particle, R is the gas constant, and T is the temperature.15,16 From eq 4, it is clear that, as the particle size decreases, the solubility will increase. So for a particle of size S and a particle of size L, where S < L, a solubility ratio can be written (eq 5).
Figure 8. (a) Ti 2p XPS and (b) O 1s XPS experimental data (black squares) of samples SG -0.5 Aged48 and SG 3 Aged48 and peak fitting (gray lines).
4474 J. Phys. Chem. C, Vol. 112, No. 12, 2008 Information. The Ti 2p1/2 and Ti 2p3/2 peak positions (Figure 8a) and the spin-orbital splitting agree well with those found in the literature for titanium dioxide.21-24 The O 1s region of the spectra is shown in Figure 8b, and a main peak at 530 eV and a shoulder located on the higher binding energy side of the main peak can be discerned. The binding energy of the main peak agrees well with the literature values for bulk O2- in titanium dioxide.21 The shoulder, which is assigned to the surface OH species, is located at 531.4 eV for SG -0.5 Aged48 and at 531.1 eV for SG 3 Aged48. In addition, the relative intensity of the shoulder with respect to the main O2- peak at 530 eV is greatest for SG 3 Aged48, followed by SG 1.5 Aged48, then SG 0 Aged48, and finally SG -0.5 Aged48, indicating a higher degree of hydroxylation of exposed O2- anions in SG 3 Aged48 as compared with SG -0.5 Aged48. These XPS results are puzzling, as the hydrothermal aging pH matched for both samples, and equilibrium is expected between the solution and the surface. This leads us to two hypotheses, one being that the surface hydroxylation is size dependent. This might explain why the SG 3 Aged48 sample of 13.5 nm is more hydrated than the SG -0.5 Aged48 of 17.4 nm although this size difference is rather modest. A second and more plausible hypothesis is that brookite is more hydroxylated than is anatase, which would thus explain why SG 3 Aged48 with 41% brookite has a stronger surface OH peak than SG -0.5 Aged48 with only 1.2% brookite. Finally, zeta potential measurements were performed to gain further insight into the fundamental differences between SG -0.5 Aged48 and SG 3 Aged48. The zeta potential for both samples was positive but was greater for SG -0.5 Aged48 (22 ( 3 mV) than for that of SG 3 Aged48 (11 ( 6 mV). While the error of the SG 3 Aged48 is larger than that of SG -0.5 Aged48, it is within the range of other zeta potential measurement reported in previous research.25 The larger zeta potential of SG -0.5 Aged48 indicates greater surface charge, which would result in stronger particle-particle electrostatic repulsion. This is consistent with the observation of little to no particle growth by oriented aggregation. SG 3 Aged48 had a much lower zeta potential indicating a lower surface charge density, which is consistent with the observation that the contribution to overall growth by oriented aggregation is substantial. Conclusions Acid solution pH strongly influences phase composition and particle size of sol-gel synthesized titanium dioxide nanoparticles. In general, anatase and brookite particle sizes and brookite content increase with increasing sol-gel pH in the range of -0.5 to 3. The products of hydrothermal aging depend most strongly upon the initial phase composition and average particle sizes after sol-gel synthesis, which are most sensitive to the pH employed during sol-gel synthesis. In general, hydrothermal aging of dialyzed suspensions resulted in a decrease in brookite content and an increase in particle sizes. In the case of titanium dioxide prepared using a sol-gel pH of -0.5, particle growth during hydrothermal aging was dominated by coarsening, and the resulting product was 100% anatase. In the case of titanium dioxide synthesized using a sol-gel pH of 3, particle growth during hydrothermal aging proceeded by both oriented aggregation and coarsening. The primary differences between these two sol-gel synthesized materials were smaller particle size, lower brookite content, and higher surface charge for the material prepared at pH -0.5. The smaller particle size is expected to lead to greater titanium dioxide solubility, which is consistent with the increase in the rate of growth by coarsening, and the greater surface charge is expected to lead to greater electrostatic repulsions between particles, which is consistent with the decrease in the rate of growth by oriented aggregation. Finally,
Isley and Penn the conversion rate of brookite to anatase was greatest for the sample prepared at pH -0.5. This relatively rapid loss of brookite is also consistent with increased solubility because the brookite particles were substantially smaller than the anatase nanoparticles and because brookite is somewhat more soluble than anatase. This work demonstrates that substantial control over the brookite content, particle size, and particle growth mechanism can be achieved simply by changing the pH of the sol-gel synthesis. Acknowledgment. We thank the University of Minnesota and the National Science Foundation (Grant Career-036385) for funding. Parts of this work were carried out in the Institute of Technology Characterization Facility, University of Minnesota, which receives partial support from NSF through the NNIN program. Erik Anderson, supported by the National Science Foundation (MRSEC Summer program), is thanked for his contribution to this work. In addition, Kyle Bantz is thanked for running the zeta potential measurements, and Bing Luo is thanked for collecting and analyzing the XPS data. Supporting Information Available: Particle growth data from each sample was fit following either the classic coarsening or simultaneous coarsening/oriented aggregation growth model, and the best fits are presented in body of the paper. All fits are shown in Supporting Information Figures 1 and 2. In addition, all rate constants calculated from the growth models are presented in Supporting Information Table 1. Last, XPS spectra for the Ti 2p and O 1s peaks from samples SG -0.5 Aged48 and SG 3 Aged48 are shown in the body of the paper. The survey spectra for those samples are presented in Supporting Information Figure 3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Graetzel, M. Analusis 1996, 24, M17. (2) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (3) Braun, J. H. J. Coating Technol. 1997, 69, 59. (4) Isley, S. L.; Anderson, E. R.; Penn, R. L. ECS Trans. 2006, 3, 37. (5) Ruiz, A. M.; Sakai, G.; Cornet, A.; Shimanoe, K.; Morante, J. R.; Yamazoe, N. Sens. Actuator B 2004, 103, 312. (6) Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Banfield, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.; Doran, G. S.; Whitfield, H. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6476. (7) Isley, S. L.; Penn, R. L. J. Phys. Chem. B 2006, 110, 15134. (8) Liu, H.-Z.; Hu, W.-B.; Gu, M.-Y.; Wu, R.-J. Wuji Cailiao Xuebao 2002, 17, 429. (9) Penn, R. L. J. Phys. Chem. B 2004, 108, 12707. (10) Brinker, J. C. Sol-gel Science: The Physics and Chemistry of Solgel Processing; Harcourt Brace Jonvanovich: Boston, 1990. (11) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. (12) Wyckoff, R. W. G. Crystal Structures. Vol. 1. 2nd ed, 1963. (13) Scherrer, P. Go¨ttinger Nachrichten 1918, 2, 98. (14) Pottier, A.; Cassaignon, S.; Chaneac, C.; Villain, F.; Tronc, E.; Jolivet, J.-P. J. Mater. Chem. 2003, 13, 877. (15) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35. (16) Wagner, C. Z. Elektrochem. 1961, 65, 581. (17) Banfield, J. F.; Zhang, H. ReV. Mineral Geochem. 2001, 44, 1. (18) Huang, F.; Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2003, 107, 10470. (19) Matthews, A. Am. Miner. 1976, 61, 419. (20) Naicker, P. K.; Cummings, P. T.; Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2005, 109, 15243. (21) Venezia, A. M.; Liotta, F. L.; Pantaleo, G.; Beck, A.; Horvath, A.; Geszti, O.; Kocsonya, A.; Guczi, L. Appl. Catal. 2006, 310, 114. (22) Liu, C.; Fu, Q.; Wang, J. B.; Zhao, W. K.; Fang, Y. L.; Mihara, T.; Kiuchi, M. J. Korean Phys. Soc. 2005, 46, S104. (23) Zhao, Z.; Tay, B. K.; Yu, G. Appl. Opt. 2004, 43, 1281. (24) Li, G.; Li, L.; Boerio-goates, J.; Woodfield, B. F. J. Mater. Res. 2003, 18, 2664. (25) Yezeka, L.; Rowella, R. L.; Larwab, M.; Chibowskib, E. Colloids Surf. A 1998, 141, 67.
