Titanium dioxide (TiO₂) nanostructured materials have attracted a great deal of attention because of their numerous applications. However, TiO₂ applications depend strongly on the material’s high homogeneity and definite phase composition, morphology, particle size, high surface area and porosity, which are dependent on the sample history, the method of preparation and heat treatment. We synthesised TiO₂ nanopowder with an anatase structure by the sol-gel method using TiCl₄-ethanol solution as a precursor in an argon gas environment, with and without applying ultrasonic waves. Our results show that the use of ultrasonic waves (after aging) has a significant effect on the homogeneity and size of TiO₂ nanoparticles. A smaller crystallite size was obtained using ultrasonic waves. For this purpose, the average diameter of TiO₂ nanoparticles was decreased by about 3 nm. The synthesised powder was characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

Titanium dioxide (TiO₂) nanostructured materials have attracted a great deal of attention because of their numerous applications in various fields, such as in photocatalysts and self-cleaning,^1,2 dye-sensitised solar cells,^3,4,5,6 gas sensing and sensor devices,^7,8 electroluminescent hybrid devices,⁹ energy-storage technologies,¹⁰ electrodes in lithium batteries¹¹ and water-splitting catalysts for generating hydrogen.^12,13 TiO₂ has recently been tested as a dielectric material for the next generation of ultrathin capacitors¹⁴ and as a photonic crystal for photonic band-gap materials.^15,16 TiO₂ has three main structures: rutile, anatase and brookite.^17,18,19 The anatase phase of TiO₂ is useful in photocatalytic applications because of its higher electron mobility, lower fixed dielectric properties and lower density than the other phases.^17,20

TiO₂ applications depend strongly on the material’s high homogeneity and definite phase composition,²¹ morphology, particle size, high surface area and porosity.^19,20,22,23 All of the above parameters depend on the sample history,²⁴ the method of preparation²⁵ and the heat treatment used.²⁶ Some parameters play a more important role than others. For example, the size or surface area to volume ratio of the nanoparticles is an important factor for their application in catalysis.^20,27,28 Thus, it is very important to develop synthetic methods in which the crystalline phase as well as the size and morphology of the TiO₂ nanocrystals can be controlled. Having fine and homogeneous nanoparticles with controlled diameters is necessary for using this material as a good photocatalyst.^17,20,29

TiO₂ nanoparticles can be synthesised using various methods, such as the sulphate process,³⁰ the chloride process,³⁰ impregnation,³¹ co-precipitation,³² the hydrothermal method,^33,34,35 direct oxidation of TiCl₄,³⁶ the metal organic chemical vapour deposition method³⁷ and the sol-gel method.^38,39 The sol-gel method is one of the most convenient ways to synthesise various metal oxides because of its low cost, ease of fabrication and low processing temperatures.⁴⁰ It is worth mentioning that only a few investigations have so far dealt with a comparison between the crystallisation process of TiO₂ in the presence or absence of ultrasonic irradiation after gelatinisation with the sol-gel method. Some investigations have recently used ultrasonic irradiation in the crystallisation process of TiO₂.^{22,23,41,42,43,44,45,46,47,48,49}

In the present work, TiO₂ nanoparticles (anatase phase) were synthesised using the sol-gel process on their TiCl₄ precursor both with and without ultrasonic irradiation at 40 kHz (low intensity) and for specific gelatinisation times. Here, we compare the size and morphology of the particles in the presence and absence of ultrasonic irradiation.

TiCl₄ (99.5% Merck, Hohenbrunn, Germany) and ethanol solutions (99.8% Merck, Darmstadt, Germany) with a certain ratio in an argon gas environment were used without any further purification. All of the chemicals were analytical grade.

At room temperature under argon gas, 2 ml of TiCl₄ was slowly added drop wise into 20 ml of ethanol. A light yellow solution was obtained after adding all the TiCl₄. The pH value of the solution was between 1.5 and 2.0. The solution was then gelatinised for different periods (24 h, 72 h and 120 h) and each prepared solution was subjected to an aging process for 3 h. We prepared each gel solution in two ways, (1) without using ultrasonic waves and (2) with ultrasonic waves at a frequency of 40 kHz and a power of 60 W for

30 min. Each sol-gel solution was vaporised at 80 °C until a dry gel was obtained. Finally, the dry-gel precursor was calcined at 400 °C for 1 h in air to form TiO₂ powder. To promote the decomposition of organic components in the precursor, the initial heating rate was maintained at 5°C/min.

Phase identification and crystallite size determination of the products were achieved using X-ray diffraction (XRD)^{17,18,19,20,21,22} on a GBC-MMR diffractometer (Melbourne, Australia) at a scan rate of 10°/min and Cu–Kα line (λ = 0.1541056 nm) radiation with a working voltage of 30 kV. The particle morphology was investigated using a Philips XL30 scanning electron microscope (Eindhoven, the Netherlands) and a Philips CM120 transmission electron microscope⁵⁰ operating at 16 kV and 100 kV, respectively.

Using the sol-gel method, the diameter of TiO₂ nanoparticles can be controlled by adjusting several physical and chemical factors, such as gelatinisation time and calcination temperature,^17,18 and other methods can be used for reducing the size of the particles.

In order to synthesise TiO₂ nanoparticles, one can combine TiCl₄ and ethanol under special conditions. At first a large amount of HCl gas and TiCl_x(OH)_4-x is produced in combining TiCl₄ and ethanol. In the mixing process, the solution TiCl_x(OH)_4-x absorbs a small amount of water from the atmosphere and forms Ti-OH bonds with the remaining ethanol in the solution. In the polymerisation and hydrolysis process, these Ti-OH bonds form …-Ti-O-…-Ti-OH strings. In the hydrolysis process and while mixing, these long strings form smaller Ti-O-Ti strings. In closing and with development of Ti-O-Ti strings, three-dimensional polymers are produced to form Ti(OH)₄ matter. TiO₂ nanoparticles are formed when Ti(OH)₄ molecules are under specific (critical) thermal conditions according to the following equation^17,18,29:

Ti(OH)₄ à TiO₂ + 2H₂O [Eqn 1]

The …-Ti-O-…-Ti-OH strings break and produce a large number of smaller Ti-O-Ti strings, which help to form more Ti(OH)₄ molecules. The Ti(OH)₄ molecules emerging at a high rate in solution cause the formation of more TiO₂ nanoparticles in a short time.

Ultrasonic waves have a monotonous effect on all components of the solution made by the sol-gel method and cause the breaking of loose links of large nanometric colloids in solution, thus allowing the production of smaller nanoparticles. The high local pressure and temperature break the links in long polymer strings as well as the weak links binding smaller particles that form large colloidal masses. Very small bubbles are produced in the fluid when it is affected by ultrasonic waves. These bubbles collapse when they grow beyond a critical size and this creates high temperatures of 5000 °K and pressures of 10⁸ Pa in the area of collapse. The ultrasonic waves create three distinct states within the fluid, (1) that inside the bubbles (in their gaseous phase) where extremely high temperatures and pressures are produced as they collapse; (2) spaces between the bubbles within the fluid, where temperature is less than that inside the bubbles but is still high; and (3) the overall volume of fluid, where temperatures are equal to the ambient temperature.¹⁷ As the bubbles collapse, there is a high heating to cooling rate of about 10¹⁰ K/s – 10¹¹ K/s in the boundaries between the bubbles and fluid (active area). It was found that applying ultrasonic waves after the gel solution has been mixed is an ideal technique to reduce the diameter of the TiO₂ nanoparticles formed.

The samples prepared at different gelatinisation times (24 h, 72 h and 120 h) and calcined at 400 °C were compared using XRD. Phase identification using XRD relies mainly on the position of the peaks in a diffraction profile and to some extent on the relative intensities of these peaks.^{17,18,19,20,21,22,40} The half width of the peaks decreases slightly when the size of the crystallites decreases. Figures 1 and 2 show the XRD patterns for powder samples of TiO₂ obtained at a calcination temperature of 400 °C and gelatinisation times of 24 h, 72 h and 120 h, with and without ultrasonic waves, respectively. All the observed peaks in the XRD spectra are related to the anatase phase. In Figure 2, the crystallite diameters relative to those in Figure 1 were estimated for most sharp peaks using Debye-Scherrer’s equation:

S=K.l/(b cosq) [Eqn 2]

where S is the crystallite size, λ = 1.54056 Å (the wavelength of the X-ray radiation), K is a constant taken as 0.94, θ is the diffraction half angle and β is the line width at half maximum height.

Figure 3 and Table 1 show a comparison of the average diameter of synthesised crystallites with and without the use of ultrasonic waves. The crystallite dimensions evaluated from the line profile analysis of the XRD peaks indicated that all prepared samples were nanocrystalline in the anatase phase and that crystallites with a mean diameter of less than 15 nm were prepared in the presence of ultrasonic waves. By applying ultrasonic waves, the average diameter of the TiO₂ nanoparticles was reduced by about 3 nm during each gelatinisation time of 24 h, 72 h and 120 h.

Both Figure 1 and Figure 2 show that, as gelatinisation time increases, the beam plates’ diffraction peak of the anatase phase becomes sharper and the line width at half maximum height decreases. Therefore, by increasing the gelatinisation time, the size of the crystallites became larger. Figure 4 shows scanning electron microscope (SEM) images of TiO₂ powder samples prepared using ultrasonic waves and different gelatinising times. With increased gelatinisation time, the boundaries between the nanometric grains became more specific and the shape of the particles became more spherical.

Figure 5 compares SEM images of TiO₂ powder samples prepared using a gelatinisation time of 24 h, with and without ultrasonic waves. The images of the surface of the powder samples show clearly that using ultrasonic waves resulted in greater homogeneity in the average size of the particles and resulted in smaller particles. Transmission electron microscopy (TEM) also was used to investigate the particle size. Figure 6 shows a TEM image of nanoparticles synthesised using a gelatinisation time of 120 h, a calcination temperature of 400 °C and ultrasonic waves. The nanoparticles produced had a diameter of 14 nm – 15 nm.

The above results show that if gelatinisation times are increased, the average size of TiO₂ nanoparticles increases, irrespective of whether or not ultrasonic waves are applied. In addition, if ultrasonic waves are used, the average diameter of TiO₂ nanoparticles decreases by about 3 nm. Thus, smaller and more controlled nanoparticles can be produced using ultrasonic waves.

Figure 1: X-ray diffraction patterns for TiO2 powder samples synthesised at a calcination temperature of 400 °C and gelatinisation times of (a) 120 h, (b) 72 h and (c) 24 h, in the absence of ultrasonic waves.

Figure 2: X-ray diffraction patterns for TiO2 powder samples synthesised at a calcination temperature of 400 °C and gelatinisation times of (a) 120 h, (b) 72 h and (c) 24 h, in the presence of ultrasonic waves.

Figure 3: Crystallite size at different gelatinisation times after calcination for 1 h at 400 °C with (squares) and without (circles) ultrasonic irradiation.

Figure 4: Scanning electron microscopy images of TiO2 powder samples synthesised at a calcination temperature of 400 °C for 1 h and gelatinisation times of (a) 120 h, (b) 72 h and (c) 24 h, in the presence of ultrasonic waves.

Figure 5: Scanning electron microscopy images of TiO2 powder samples synthesised at a calcination temperature of 400 °C for 1 h and a gelatinisation time of 24 h (a) in the presence of and (b) in the absence of ultrasonic irradiation.

Figure 6: Transmission electron microscopy image of TiO2 powder samples synthesised at a calcination temperature of 400 °C for 1 h and a gelatinisation time of 120 h in the presence of ultrasonic irradiation.

We successfully prepared TiO₂ nanoparticles using TiCl₄ and ethanol as precursors by a sol-gel route, in the presence and absence of ultrasonic waves at room temperature. The use of ultrasonic waves led to smaller particle size, low-dimensional particle shape (spherical) and improved particle morphology. The fact that nanoparticles can be produced using ultrasonic waves, as reported in this paper, strongly supports the proposition that ultrasonic irradiation has great potential to control the formation of inorganic nanoparticles by influencing the organic reaction pathway.

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