THE KINETICS OF TiO2 NANOPARTICLE FABRICATION BY SOL-GEL METHOD

ABSTRACT

This paper studies a technology for obtaining TiO₂ submicron sheets of homogeneous rhombic shape by sol-gel method. The work also presents the mechanism of reactions for producing TiO₂ sheets and the kinetics, in particular, of the particle formation rate as a function of time. The characteristics of the obtained TiO₂ submicron sheets have been proved using scanning electron microscopy.

АННОТАЦИЯ

В данной работе приведена технология получения субмикронных листов TiO₂ однородной ромбической формы методом золь-геля. Показано механизм реакций получения листов TiO₂ и кинетику, в частности, скорости формирования частиц от времени. Характеристики полученных субмикронных листов TiO₂ доказано с помощью сканирующей электронной микроскопии.

Keywords: Nanoparticle; Titanium dioxide; Particle formation; Sol-gel process; kinetics, time.

Ключевые слова: Наночастица; Диоксид титана; Образование частиц; Золь-гель процесс; кинетика, время.

Introduction

Titanium dioxide (TiO₂) has emerged as one of the most extensively studied nanomaterials in recent years due to its myriad advantageous properties [1-2], including chemical stability [3], biocompatibility [4], and unique optical and electrical attributes [5]. TiO₂ exists in three polymorphic phases: anatase [6], rutile [7], and brookite [8], each with distinct tetragonal and rhombic crystal structures. While rutile is the most thermodynamically stable phase [9], anatase and brookite are metastable [10] and transition to the rutile phase at temperatures around 600°C [11]. The respective band gaps for anatase, brookite, and rutile are 3.60, 3.30, and 3.39 eV [12]. TiO₂ nanoparticles do not absorb light in the visible spectrum [13] but instead refract or transmit it.

TiO₂ finds applications in a wide range of industries, including paint, cosmetics, and food production [14]. Rutile-structured TiO₂ is often preferred in these applications due to its enhanced stability and superior light-scattering properties. Brookite TiO₂ is commonly employed in electrochemical electrodes, capacitors, and solar panels [15], whereas anatase TiO₂ is widely used as a photocatalyst for water and air purification [16], as well as in medical applications for disease treatment.

As a semiconductor material, TiO₂ possesses exceptional photocatalytic [17] capabilities, making it suitable for a diverse array of applications such as white pigments, cosmetics, catalysis supports, and photocatalysts [18-20]. Research over the past two decades has shown that the physicochemical characteristics of nano-TiO₂ are strongly dependent on factors like size, shape, and crystalline phase [21]. These parameters can be effectively modulated by controlling crystallization and phase transformation processes during synthesis. Therefore, considerable efforts have been dedicated to identifying synthesis methods capable of producing TiO₂nano powders with desired properties. The choice of synthesis method is critical, as it significantly influences the crystalline phase and other characteristics of the resulting material.

In recent years, various technological approaches based on sol-gel technology have been developed for TiO₂ synthesis. Sol-gel technology offers a promising route for generating large quantities of nanoparticles and materials [22-25]. It leverages inorganic titanium precursors such as titanium chloride and titanium alkoxides, with the alkoxide route being favored for its ability to produce [26] purer products without generating hazardous by-products.

Although sol-gel techniques have been employed to synthesize amorphous TiO₂ with spherical morphologies using titanium tetra ethoxide hydrolyzed in mixed solvent systems like acetonitrile and octanol, effective control over nanoparticle morphology during synthesis remains inadequately explored. One critical aspect influencing the physicochemical properties of these nanoparticles is their morphology. The aim of this study is to synthesize submicron TiO₂ particles using sol-gel technology under simplified conditions and to investigate the kinetics and mechanisms governing their synthesis.

Experimental section

TiO₂ nanoparticles are synthesized using simple zol-gel technique at room temperature. Firstly, 1 ml titanium tetrabutoxide (TBT) was added into mixture of 50ml asetonitrile and 150 ml ethanol after stirring for 5 min, 0.4μl deionized (DI) water (18.2 MΩ•cm) was added into the above solution by stirring consciously. The sol-gel synthesis was conducted at 30°C for 3 h. Finally, the obtained particles were washed few times with ethanol and water.

The morphologies and microstructures were characterized by scanning electron microscopy (SEM) JEOL JSM 6700 with an acceleration voltage of 5 kV and transmission electron microscopy (TEM) a JEOL JEM 2200 FS transmission electron microscope operated at an accelerating voltage of 200 kV.

Results and discussion

For the synthesis of TiO₂ nanosheets, we initially prepared a mixture of ethanol and acetonitrile in a 3:1 ratio. Subsequently, titanium butoxide (TBT) was added to the flask, followed by the addition of water a few minutes later. After the nucleation and aggregation processes were complete (as outlined in Figure 2), TEM and SEM images (Figure 1) revealed that the resulting TiO₂ nanosheets possessed a regular rhombic shape. Figure 1 shows the results of SEM and TEM of TiO₂ submicron particles synthesized at room temperature using the sol-gel technique. The obtained nanoparticles have a homogeneous rhombic shape with two obtuse angles and two acute angles, and their average size is around 300-350 nm.

Figure 1. The TEM and SEM electron microscopy results of the obtained TiO₂ submicron sheets

The sol-gel synthesis involves two main steps in the formation of nanoparticles. The first step is nucleation, and the second is aggregation, during which particles collide and bind together. The possible mechanisms can be illustrated Figure 2. In sol-gel synthesis titanium alkoxides with the composition Ti-(OC₄H₉)₄ readily undergo hydrolysis upon contact with water.

Figure 2. Schematic illustration of the synthetic procedure for TiO₂ submicron sheets

The mechanism of TiO₂ nanoparticles generation in the sol-gel approach is as follows:

Titanium alkoxide containing Ti-(OC₄H₉)₄ readily interact with water to generate a (OH)-group compound:

Titanium dioxide is generated as a result of the abundance of water:

Intermolecular condensation of partly hydrolyzed products occurs when the amount of water available for the hydrolysis and condensation processes is insufficient:

To study the formation of regular rhombic titanium dioxide submicron sheets, we aimed to isolate primary particles. The experiment was conducted at low temperatures (0-5°C) to slow down the reaction rate, thereby facilitating the observation of primary nanoparticles. The formation and growth of nanoparticles is shown in Figure 3.

Figure 3. The Scan electron micrographs resultants of the TiO₂ particles at the different times: 5min, 10min, 15min, 20min, 25min

SEM images presented in Figure 3 illustrate the morphology evolution of TiO₂ prepared at reaction times of 5, 10, 15, 20, and 25 minutes, respectively. Initially (at 5 minutes), TiO₂ nanoparticles appeared to be spherical in shape. Upon increasing the reaction time to 10 minutes and beyond, the primary particles started to aggregate, leading to a TiO₂ layer on the surface. As the reaction time continued to increase, the size of the TiO₂ particles gradually increased as well. The results reveal that the rate of hydrolysis and the amount of water are important factors in creating uniform rhombic morphologies because with a smaller amount of water, the hydrolysis process progresses in steps, allowing for orderly aggregation.

Conclusion

Uniform rhombic TiO₂ submicron sheets with sizes from 300 to 350 nm and thickness 30nm were obtained by the sol-gel method. A typical TiO₂ submicron sheet was generated efficiently at room temperature without the use of a template. In addition, the formation and growth of nanoparticles over time was studied. According to the findings of the studies, the size of primary particles grows with increasing response time, which is consistent with the prediction of the particle growth model.

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