1962
J. Phys. Chem. C 2007, 111, 1962-1968
Phase Stability and Transformation in Titania Nanoparticles in Aqueous Solutions Dominated by Surface Energy Michael P. Finnegan,†,‡ Hengzhong Zhang,†,* and Jillian F. Banfield† Department of Earth and Planetary Science, 307 McCone Hall, UniVersity of CaliforniasBerkeley, Berkeley, California 94720, and Materials Science Program, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706 ReceiVed: June 19, 2006; In Final Form: December 11, 2006
The surface free energy of small particles in an aqueous solution consists of the electrostatic energy of charged surfaces and the interfacial energy. For nanoparticles in an aqueous solution, the two terms can be modified by solution chemistry and be manipulated to control phase stability and transformation kinetics. Here we show that the phase stability of titania (TiO2) nanoparticles strongly depends on the solution pH. At small sizes, rutile is stabilized relative to anatase in very acidic solutions, whereas in very basic solutions anatase is stabilized relative to rutile and brookite. Rutile is the stable phase at large particle sizes regardless of pH. These results indicate that the activity of potential determining ions (protons or hydroxyl groups) is a factor that can determine the phase stability of nanoparticulate titania in aqueous solutions at pH values far from the point of zero charge of titania. The phase transformation proceeds via a dissolution-precipitation mechanism under hydrothermal conditions.
Introduction Nanometer-scale oxides exhibit many novel properties compared to their bulk counterparts. These novel properties are explored to improve photovoltaic devices1,2,3 capacitor dielectrics,4 optical adsorption,5 and photocatalysts6 and to mitigate pollution both in solution and gaseous environments via enhanced and/or selected adsorption.7 Different oxides (e.g., Al2O3, ZrO2, and TiO2) have unique properties conducive to use in specific applications. Controlling phase, morphology, and particle size of an oxide can further optimize its suitability for a desired application.2,8,9 Therefore, a comprehensive understanding of thermodynamic phase stability and the kinetic factors that impede or promote thermodynamic equilibrium is necessary for control of structures and properties of nanometer-scale oxides. The Gibbs free energy (G) of a nanoparticle system comprises the bulk free energy (Gb) and the surface free energy (Gs): G ) Gb + Gs. Since the fraction of atoms exposed on the surface is high in nanoparticles, Gs can be an important determinant of the nanoparticle phase stability. Inequality in the specific surface energies of two polymorphs may cause a reversal in the relative magnitudes of their Gibbs free energies at some nanoparticle sizes and hence can change their relative phase stabilities.10 Phase stability reversal in nanocrystalline titania as a function of particle size was suggested based on experimental evidence by Gribb11 (1997) and proven by thermodynamic analysis by Zhang and Banfield12 (1998) and also calorimetric determination.13 Structures and properties of nano-TiO2 have been studied extensively; many thermodynamic and kinetic relationships have been documented.11,14 Experiments show that the phase transformation sequences in air among the three polymorphs of * Corresponding author. Phone: 510-643-9120. E-mail: eps.berkeley.edu. † University of CaliforniasBerkeley. ‡ University of WisconsinsMadison.
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TiO2sanatase, brookite, and rutilesis size dependent.15 This is consistent with thermodynamic calculations that predict that in air anatase is most stable below 11 nm, brookite between 11 and 35 nm, and rutile above 35 nm. The surface environment is generally not considered as a variable in nanoparticle phase stability analyses. However, it has been demonstrated that the surface environment can exert a fundamental control on nanoparticle structure even at room temperature.16,17 In an aqueous solution, together with the surface charge of the nanoparticles, the interfacial energy changes with solution chemistry. The surface charge and/or the interfacial tension may play a majority role in determination of the free energy and hence determine the phase stability in aqueous solutions. The surface free energy of nanoparticles in a solution is the sum of the surface electrostatic energy (σψΑ) and the interfacial energy (γA)
Gs ) γA + σψA
(1)
where σ is the surface charge per unit area (C/m2), ψ the surface potential (V), A the total surface area (m2), and γ the interfacial tension (J/m2) of nanoparticles. At pH values far from the point of zero charge (PZC), the surface charge is high, while the interfacial tension is low due to the adsorption of potential determining ions (PDI).18 Thus Gs is mainly determined by the surface charge term, σψΑ. At pH values close to the PZC, the surface charge is negligible; the predominant contribution to Gs comes from the interfacial tension term, γA. Consequently, the phase stability of small particles in a solution may be determined predominantly by the surface charge at pH values far away from the pHPZC and by the interfacial tension at pH values close to the pHPZC. For two nanoparticulate phases in a solution, as their surface charges are likely to differ at different pH values, their relative surface energies can cross and therefore their relative phase stabilities can reverse at a certain pH value. A few published results are consistent with the above general thermodynamic considerations. It was reported that amorphous
10.1021/jp063822c CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
Phase Stability in Titania Nanoparticles titania transforms to rutile in acidic hydrothermal conditions.19 Aruna et al. noted that hydrolyzed Ti-alkoxide groups precipitate as rutile at low pH values and as anatase at high pH values.20 It has also been proposed that titania nanotubes, possibly formed when single sheets delaminate from anatase crystals and roll up,21 adopt more anatase-like structure at high pH and rutilelike sheet structure at low pH.22 Very recently, the question of how phase stability and crystal morphology depend upon surface protonation has been considered via simulations that utilize calculated free energies and surface tensions for different anatase and rutile surfaces.23 These authors predict that the critical size for the anatase to rutile transition increases as the degree of surface protonation increases and suggest that surface chemistry induced phase transitions may be possible. However, no systematic experimental study of the phase stability and transformation kinetics of nanocrystalline titania over a wide pH range and at various temperatures has been conducted. In this work, we synthesized nanometerscale titania and carried out a series of hydrothermal treatments over the pH range from 1 to 12 and at temperatures of 105, 200, and 250 °C. The results support the prediction of a dependence of phase stability on solution pH. Experimental Section Nanocrystalline titania was synthesized using a sol-gel method. A volume of 225 mL of ethanol solution containing titanium isopropoxide (Ti[OCH(CH3)2]4) (∼10% in volume) was dripped into a 2.25 L HCl aqueous solution (pH ) 1.10, precooled to 4-6 °C) under magnetic stirring. Titania precipitated as the result of the hydrolysis of titanium isopropoxide. The precipitates were separated from the solution by filtration and dried at 80 °C. The obtained titania powder was purified using the following procedure. The titania powder was reintroduced into 100 mL of deionized (DI) water, forming a colloid suspension with a pH of 2.1. The colloid suspension was poured into a dialysis tube made of a Spectra/Por membrane (molecular weight cut off of 3500 Daa). The tube was placed into a DI water bath under slow magnetic stirring. After the water in the bath was changed four times, the colloid turned into gel, indicating that the pH of the colloid had risen to near the pHPZC of titania. The measured pH of the colloid in the dialysis tube and that of the water in the bath were all 5.2. This value is in fairly good agreement with the pHPZC of titania determined by numerous researchers.24-31 The dialyzed titania was dried at 30 °C for 2 days, yielding ∼3 g of as-synthesized titania powders for hydrothermal experiments. The powder was examined by powder X-ray diffraction (XRD). XRD patterns were collected using a Brukker Baker diffractometer (Cobalt target, 45 kV, 35 mA) over a 2θ range of 24-68° with a step size of 0.01° and a dwell time of 1 s per step. Instrumental broadening was determined using the peak widths of a bulk rutile sample. After fitting the Pearson VII functions to chosen XRD peaks, the full width at the halfmaximum (fwhm) and the integrated intensity of each peak were obtained. The particle size of titania was calculated from the fwhm data (after correction for instrumental broadening) using the Scherrer equation (Scherrer constant 0.9). Following the literature,32 the integrated areas/fwhm of rutile (110) peak, brookite (121) peak, and anatase (101) peak were used to calculate the phase contents/average particle sizes of the samples. Hydrothermal experiments were carried out in Teflon cups enclosed in general purpose acid digestion bombs (Parr Instrument Co.). About 30 mg of the as-synthesized titania sample
J. Phys. Chem. C, Vol. 111, No. 5, 2007 1963
Figure 1. TEM image of anatase nanoparticles synthesized by hydrolysis of titanium isopropoxide at 5 °C and pH ) 1. Sizes of several distinct particles are noted.
was put into a Teflon cup containing 9 mL of DI water. The pH of the suspension in the cup was adjusted to an array of target values (i.e., 1-12) with either HCl or NaOH. After the suspension was sonicated for 15 min, the assembled bomb was put into an electric furnace held at 105, 200, or 250 °C for a required time. Then the bomb was removed from the furnace and cooled in air. The pH of the reacted suspension was determined again. Titania powders were separated from the suspension by centrifugation for phase content and particle size determination using XRD. Phase reversal experiments were carried out subsequent to hydrothermal treatments. Samples hydrothermally treated at a low pH value were then re-treated hydrothermally at a high pH value. If there was a conversion of titania to a stable phase during the initial treatment, this subsequent treatment aims to see if the conversion can be reversed. Microstructures of selected samples were examined using a JEOL ARM-800kV electron microscope operated at 800 kV. The pH-dependent surface charge of nanocrystalline anatase was measured by potentiometric titrations.33 The titrations were performed employing continuous in-situ pH monitoring with anatase samples having average particle sizes of ∼3-4 nm. Results The as-synthesized titania sample contains approximately 85% anatase and 15% brookite by XRD analysis. The average particle size of the anatase determined from XRD peak broadening is 3.5 nm. This is in good agreement with the average size determined by TEM (Figure 1). TEM imaging also shows that anatase particles are nearly spherical. The size and morphology of the brookite particles are difficult to determine accurately because of the low brookite content. The average diameter of brookite was estimated to be 3-5 nm based on XRD peak broadening. Results of the hydrothermal treatment of samples are shown in Figures 2-4. At 105 °C, no significant transformation of anatase was observed at pH 1-3 for periods of up to 500 h (Figure 2a); at pH values greater than 6, a fraction of the nanocrystalline anatase (up to ∼ 40%) transformed to brookite (Figure 2b). At 200 °C and pH 1, anatase transformed completely to rutile after about 500 h (Figure 3a). However, as the pH value increased close to the pHZPC of titania (pH ∼ 5), the transformation rate decreased dramatically, approaching zero. At pH values above the pHZPC (Figure 3b), formation of rutile was not
1964 J. Phys. Chem. C, Vol. 111, No. 5, 2007
Figure 2. Anatase content as a function of hydrothermal treatment time at 105 °C and different pH values. In panel a, diamond denotes pH ) 1.0; triangle, 1.8-1.9; square, 2.0-2.2; and circle, 2.6-3.1. No rutile was detected, and the content of anatase is almost invariant. In panel b, diamond denotes pH ) 11.6-11.7; triangle, 10.0; and square, 6.0. No rutile was detected. The loss of anatase content is due to its transformation to brookite.
Figure 3. Rutile content (a) and anatase content (b) as a function of hydrothermal treatment time at 200 °C and different pH values. In panel a, diamond denotes pH ) 1.0; triangle, 1.9-2.1; square, 2.3-2.6; and circle, 4.4-5.6. In panel b, diamond denotes pH ) 12.0-12.6; triangle, 10.7-10.8; square, 8.1-8.3; and circle, 4.3-4.6. No rutile was detected in panel b. The loss or gain of anatase content is due to its transformation to or from brookite.
detected. In vicinity of the pHZPC (i.e., pH ) 4.3-4.6), anatase particles remained essentially untransformed. At pH ) 8.18.3, a fraction of the anatase transformed to brookite; at pH ) ∼10.7, anatase partially transformed to brookite and then the brookite transformed back to anatase again. At pH > ∼12, the
Finnegan et al.
Figure 4. Rutile content (a) and anatase content (b) as a function of hydrothermal treatment time at 250 °C and different pH values. In panel a, diamond denotes pH ) 0.8-1.2; triangle, 1.8-2.1; square, 2.53.2; dark circle, 4.8-5.4; and gray circle, 6.4-6.9. In panel b, diamond denotes pH ) 10.8-11.7 and triangle 9.5-9.8. No rutile was detected in panel b.
brookite nanoparticles in the as-synthesized sample transformed rapidly and completely to anatase. At 250 °C, the conversion from anatase to rutile is faster than at 200 °C at pH values close to or below the pHPZC (Figure 4a). For instance, at 250 °C and pH ) 1.8-2.1, the assynthesized nano-titania transformed to 100% rutile after 53 h (Figure 4a). In contrast, at 200 °C and pH ) 1.9-2.1, it transformed to only 24% rutile after 500 h (Figure 3a). As the pH increased, the transformation percentage from the assynthesized material to rutile reduced significantly at both 200 °C and 250 °C (Figures 3a and 4a). However, the conversion to rutile is still higher at a higher temperature. At 250 °C and pH ∼ 11, the as-synthesized material converted quickly and fully to pure anatase (Figure 4b), resembling the behavior at 200 °C and pH ∼ 12 (Figure 3b). Samples coarsened at 250 for 961 h in the pH range 10-12 showed no evidence of transformation to rutile (Figure 4b). Nanocrystalline titania coarsened in all hydrothermal experiments. Particularly, the rutile phase formed by phase transformation from anatase is always coarsely crystalline with average diameters >30 nm. Above experimental results suggest that the anatase structure may be stabilized at small sizes at high pH values, though rutile is more stable as a bulk phase at ambient conditions. To further test this hypothesis, we conducted experiments to examine whether small rutile/brookite particles formed at a low pH transform to anatase by heat-treatment at a high pH. Phase pure nano-anatase synthesized at pH 5 and purified by dialysis at pH 1 was used as the starting material. This starting material was hydrothermally treated at 200 °C and pH ) 0.15 for 1, 2, 4.5, 21, and 241 h to generate mixtures of nano-anatase, rutile, and/or brookite. Rutile was formed from nano-anatase after such hydrothermal treatments (Table 1). Formation of brookite was also observed for the samples treated for 2 and 4.5 h (Table 1). Subsequently, the five samples generated from above acidic hydrothermal processing (i.e., pretreatment) were transferred into
Phase Stability in Titania Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 5, 2007 1965
TABLE 1: Phase Contents and Particle Lengths of Anatase and Rutile Before and After Coarsening in a Basic Solution (pH ) 11.5) at 200 °C for 744 h anatase pretreatment time (h)c 1 2 4.5 21 241
content (wt %) before after 100 48.6d1 51.8d2 33.3 40.0
90.5 78.9 55.0d3 0 0
rutile length (nm)a before after 5.8 6.3 8.6 7.2 6.7
16.2 13.3 10.2 -
content (wt %) before after
length (nm)b before after
0.0 15.5d1 27.0d2 66.0 60.0
11.0 24.0 >30 >30
9.5 21.1 26.6d3 100 100
11.8 13.9 21.4 >30 >30
a Length of anatase along the 〈101〉 direction. b Length of rutile along the 〈110〉 direction. c Each sample was pretreated at 200 °C in an acidic solution (pH ) 0.15) before coarsening in the basic solution. d Sample also contained brookite (1, 35.9%; 2, 21.2%; 3, 18.4%).
five basic (pH ) 11.5) solutions and hydrothermally treated at 200 °C for 744 h. The phase contents and particle sizes of the treated samples are listed in Table 1. For the sample pretreated for 1 h, ∼ 10% rutile was formed after the basic treatment. The particle size of anatase is ∼16 nm and that of rutile is ∼12 nm. For the sample pretreated for 2 h, the formation of rutile (∼21-16% ) 5%) decreased, signaling a transformation trend toward anatase. Indeed, the percentage of anatase increased (∼79-49% ) 30%) due to transformation from brookite. The sizes of anatase and rutile are all ∼14 nm. For the sample pretreated for 4.5 h, the formation of rutile almost ceased (∼2727% ) 0%) and anatase content increased ∼3% due to transformation from brookite (∼55-52% ) 3%). The particle size of rutile is ∼21 nm and that of anatase is ∼10 nm. For samples pretreated longer than 21 h, all anatase transformed to rutile with large particle sizes (>30 nm). These results show that the relative phase stability of various titania phases are size dependent at a given pH. In addition, results show that at relatively small sizes (anatase 0. As the solution pH changes, the interfacial tension and hence the (γsA) term also changes (Figure 6b). At pH values different from pHPZC, the concentration of adsorbed PDI increases and the interfacial tension between nanoparticle surfaces and the solution decreases according to the Gibbs adsorption equation.18,39 At the pHZPC, the interfacial tension is maximum. The interfacial tensions for both anatase and rutile near the pHPZC have been reported to be G(nano-anatase), anatase nanoparticles are more stable than rutile nanoparticles. This thermodynamic consideration explained well our experimental observations (above). 2. Transformation Mechanism. At lower temperatures (
