© Copyright 2004 by the American Chemical Society
VOLUME 108, NUMBER 39, SEPTEMBER 30, 2004
LETTERS Synthesis of Rutile (r-TiO2) Nanocrystals with Controlled Size and Shape by Low-Temperature Hydrolysis: Effects of Solvent Composition Wei Wang,*,† Baohua Gu,† Liyuan Liang,†,§ William A. Hamilton,‡ and David J. Wesolowski# EnVironmental Sciences, Condensed Matter Sciences, and Chemical Sciences DiVisions, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: July 1, 2004; In Final Form: August 10, 2004
A new methodology was developed to synthesize uniform rutile (R-TiO2) nanocrystals by the thermohydrolysis of titanium(IV) chloride in hydrochloric acid-alcohol aqueous solutions at 40-90 °C. Depending on the acidity, the solvent used, and the aging temperature, rod-shaped rutile nanocrystals in sizes ranging from ∼100 nm to 800 nm were obtained. Nanocrystal size and shape are shown to be strongly and systematically related to the type and the concentration of alcohol used in the alcohol-water solvent, as well as the presence or absence of both cationic and anionic surfactants. No other titania phases, such as anatase or brookite, were detected using X-ray diffraction, transmission electron microscopy, and Raman spectroscopy.
Introduction Titania (TiO2) is widely used in various technological applications such as catalysis, sensors, and white pigment for paints or cosmetics.1-7 Other uses for titania are as dyesensitized TiO2 nanoparticle energy converters for photovoltaic cells,8,9 catalysts to photochemically degrade organic pollutants,10,11 and electrodes in lithium batteries.12 More recently, TiO2 has been tested as a dielectric material for the next generation of ultrathin capacitors13 and as a photonic crystal for photonic band-gap material.14,15 The applications for TiO2 are strongly dependent on the crystalline structure, morphology, and size of the particles.10,16-18 TiO2 has three crystalline polymorphs: anatase, rutile, and brookite. Each phase exhibits different physical properties, such as refractive index, chemical reactivity, and photochemical reactivity. Thus, it is important * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † Environmental Science Division. § Now with School of Engineering, Cardiff University, P.O. Box 925, Cardiff CF24 0YF, U.K. ‡ Condesed Matter Division. # Chemical Sciences Division.
to develop synthetic methods in which the crystalline form as well as the size and shape of the TiO2 nanocrystals can be controlled. Many studies have been conducted on the synthesis and morphology control of TiO2 nanoparticles. TiO2 nanoparticles are obtained via the hydrolysis of titanium salts, such as titanium(IV) chloride (TiCl4),19,20 titanium(IV) sulfate (Ti(SO4)2),21,22 and titanium alkyloxide (Ti(OR)4) in solution.23,24 Usually, the anatase and rutile polymorphs are produced,25-29 but the synthesis of monomineralic, well-shaped rutile nanocrystals via hydrolysis in solution is much more difficult than synthesis of anatase nanocrystals. This may be a kinetic effect but may also represent an equilibrium thermodynamic effect of nanoscale systems. Recent findings by Navrotsky’s group30 indicate that, although bulk anatase is metastable, relative to rutile, the much lower surface energy of anatase results in a reversal in thermodynamic stabilities in the limit of very small nanoparticles. Rutile can be obtained via the high-temperature calcination of anatase nanoparticles; however, calcination unavoidably leads to agglomeration and growth of the nanocrystallites.21-33 Recent
10.1021/jp0470952 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/01/2004
14790 J. Phys. Chem. B, Vol. 108, No. 39, 2004
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Figure 1. Transmission electron microscopy (TEM) images of TiO2 nanocrystals prepared by thermal hydrolysis of 0.1 mol/L TiCl4 in acidic solution (a) with 3:1 ethanol:water volume ratio and (b) in the absence of alcohol. Inset shows a high-resolution TEM image, giving a distance of 0.234 nm between atomic planes. TEM images of these synthesized nanocrystals were taken with a Hitachi model HF-2000 electron microscope operated at 200 kV.
Figure 2. Variation of the TiO2 precipitation temperature with the alcohol:water volume ratios in the thermohydrolysis of 0.1 mol/L TiCl4 in acidic solution.
reports34-38 suggest that hydrothermal methods under acidic conditions can be used to produce rutile nanocrystallites. Hydrolysis of TiCl4 in aqueous solution can form rutile nanocrystals at relatively low temperatures.34-38 However, this method produces only polydispersed spherical or irregularly shaped particles. Aruna and Zaban39 reported the hydrothermal synthesis of rutile nanoparticles from titanium isopropoxide (Ti(OC3H7)4) in the presence of nitric acid in an autoclave at 250 °C. Yanagida et al.40 reported rutile synthesis via hydrothermal processes starting from amorphous TiO2, followed by autoclaving at 150 °C or 250 °C in the presence of citric and nitric acids. Obviously, the development of a one-step method for preparation of pure rutile nanocrystals at low temperatures with well-defined shape and size would be significant. Results and Discussion In this report, we present a method to synthesize rutile nanoparticles with a rodlike shape by thermal hydrolysis of TiCl4 in hydrochloric acid (HCl)-alcohol aqueous solution at a relatively low temperature. In this process, no seed nanocrys-
Figure 3. Variation of the resulting TiO2 nanocrystal size with different alcohol:water volume ratios. Nanocrystals were prepared by thermal hydrolysis of 0.1 mol/L TiCl4 in acidic solution, and the nominal average particle sizes were evaluated by DLS measurement.
tals37 or mineralizers29 need to be added. The effects of solvent composition, temperature, and surfactant additives on the formation and morphology of particles are examined. TiCl4 was used as a starting material for the synthesis of TiO2 nanoparticles. TiCl4 could not be directly dissolved in water because of its rapid exothermic reaction, which produces orthotitanic acid, releases large amounts of heat, and thus prevents the homogeneous precipitation of TiO2 particles. A mixture of HCl and water (volume ratio of 1:19, 36% HCl: H2O) was therefore used, and the dissolution was performed at ice-cooled temperatures with vigorous stirring. The initial TiCl4 concentration was 1.0 mol/L in the HCl solution. This solution (10 mL) was then mixed with 90 mL of an alcohol-water mixture with varying alcohol:water ratios, giving the final TiCl4 concentration of 0.1 mol/L. Heating was applied to the reaction mixture at a rate of 2 °C/min. The transparent solution became turbid and white at a certain temperature, indicating the formation of TiO2 particles. This temperature is defined as the precipitation temperature of the solution. After the reaction
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J. Phys. Chem. B, Vol. 108, No. 39, 2004 14791
Figure 4. XRD pattern of TiO2 nanocrystals prepared by thermal hydrolysis of 0.1 mol/L TiCl4 in acidic solution with a 3:2 ethanol:water volume ratio at 75 °C.
Figure 5. Raman spectrum of TiO2 nanocrystals prepared by thermohydrolysis of 0.1 mol/L TiCl4 in an acidic solution with a 3:1 ethanol: water volume ratio.
mixture was aged at a constant temperature for 30 min, the particles were separated by centrifuge and then washed in alcohol and water. The reaction temperatures varied from ∼40 °C to 90 °C, depending on the type of alcohol and the alcohol: water ratio. This synthesis method produced TiO2 nanocrystals with welldefined rodlike shapes (length:width ratio of ∼5:1). For example, thermal hydrolysis of TiCl4 in a 3:1 (by volume) ethanol-water solution produced ∼150 nm × 30 nm particles
(Figure 1a), whereas, in the absence of alcohol, the thermal hydrolysis of TiCl4 leads to irregular shapes with high polydispersity of the resulting particles (see Figure 1b). Alcohols significantly affect the formation of nanocrystals. At a fixed TiCl4 concentration, the colloidal precipitation temperature increased as the alcohol:water ratio increased in the presence of methanol and ethanol but decreased in the presence of 2-propanol (Figure 2). The alcohol:water ratio also controls the size of the resulting nanocrystals. With an increase in water content, the average size of the nanocrystals decreased, as measured by dynamic light scattering (DLS), using a Brookhaven 90Plus/BI-MAS instrument (Figure 3). At a fixed alcohol:water ratio, the average particle size decreased with the type of alcohol, in the following order: 2-propanol > ethanol > methanol. However, the morphology of these nanoparticles remained unchanged. The powder X-ray diffraction (XRD) pattern (determined using a Philips X’PERT system) of the as-synthesized TiO2 nanocrystals is shown in Figure 4. All the diffraction peaks clearly are attributable to rutile; no other phases of TiO2, such as anatase or brookite, could be detected via XRD. The structures of the nanocrystals were further identified by Raman spectroscopy using a Renishaw Raman imaging microscope that was equipped with a 785-nm NIR laser. The clearly visible Raman bands at 607, 442, 323, and 244 cm-1 are characteristic vibrations of rutile (Figure 5).41-44 No additional Raman bands were observed that could be assigned to anatase or brookite. It is known that the presence of inorganic and organic anionic species in the starting solution affects nucleation, crystal growth, and the morphology of the particles.45-48 Therefore, additional
Figure 6. TEM images of TiO2 nanocrystals prepared by thermohydrolysis of 0.1 mol/L TiCl4 in an acidic 2-propanol:water (3:1) mixture in the presence of (a) cetyltrimethylammonium bromide (CTAB, 10-4 mol/L) and (b) sodium dodecyl sulfate (SDS, 10-4 mol/L).
14792 J. Phys. Chem. B, Vol. 108, No. 39, 2004 batches of TiO2 nanocrystals were prepared in the presence of surfactants. In the synthesis, surfactant was first mixed with the alcohol-water mixture, and then acidic TiCl4 solution was added. The surfactant concentration was 10-4 M in the reaction mixture. It was found that the anionic surfactant (sodium dodecyl sulfate, SDS) indeed affected particle morphology. Interestingly, the cationic surfactant (cetyltrimethylammonium bromide, CTAB) also affected particle morphology in a similar way. Because TiO2 colloidal particles carry a positive surface charge at pH
