Abstract

KINETICS AND CATALAYSIS; REACTION ENGINEERING; AND MATERIALS SCIENCE

Temperature and reaction time effects on the structural properties of titanium dioxide nanopowders obtained via the hydrothermal method

G. C. CollazzoI^* * To whom correspondence should be addressed ; S. L. Jahn^I; N. L. V. Carreño^II; E. L. Foletto^I

^IDepartment of Chemical Engineering, Federal University of Santa Maria, CEP: 97105-900, Santa Maria - RS, Brazil. E-mail: gabicollazzo@yahoo.com.br

^IIInstitute of Chemistry and Geoscience, Federal University of Pelotas, CEP: 96010-900, Pelotas - RS, Brazil

ABSTRACT

An investigation was carried out on the synthesis of titanium dioxide nanopowders via the hydrothermal method, examining the influence of temperature (150ºC and 200ºC) and reaction time (6 to 36 h). The resulting powders were characterized by X-ray diffraction, infrared absorption spectrophotometry, transmission electron micrographs and surface area. The X-ray diffraction revealed that there was formation of a pure crystalline phase, consisting only of anatase for all conditions of synthesis. Both temperature and reaction time proved to have a slight influence on the crystallite size (from 9 to 17 nm) and a significant influence on the surface area (from 86 to 168 m².g^-1).

Keywords: Titanium dioxide; Hydrothermal synthesis; Nanocrystals.

INTRODUCTION

Crystalline titanium dioxide (TiO₂) occurs in three different structures: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic) (Castañeda et al., 2002; Su et al., 2006a; Wang et al., 2007). The anatase and rutile forms have been extensively studied and, as semiconductors, are used for a variety of applications (Hidalgo et al., 2007). The anatase form appears to be the most active form of these two phases (Kabra et al., 2004; Linsebigler et al., 1995). Anatase is a metastable phase that can be converted to rutile when heated at high temperatures (> 600ºC) (Castañeda et al., 2002). In addition to providing an excellent thermal stability, high photosensitivity (Ho and Yu, 2006; Venkatachalam et al., 2007) and high photoreactivity, as a chemically inert, nontoxic and relatively low-cost material, TiO₂ has been used in many industrial applications. Some applications include: white pigment, gas sensor, corrosion inhibitor and optical coating (Sankapal et al., 2005), solar cells (Regan and Grätzel, 1991), high constant dielectrics and high resistivity dielectrics (Sankapal et al., 2005). TiO₂ has also been used to decompose carbon dioxide and generate hydrogen gas (Fox and Dulay, 1993), decompose pollutants (Hidalgo et al., 2007), and as a photocatalyst (Bouzaida et al., 2004; Guettai and Amar, 2005; Ho and Yu, 2006; Sahel et al., 2007; Sleiman et al., 2007).

Different methods have been employed for the synthesis of titanium dioxide, as follows: chemical precipitation (Pedraza and Vasquez, 1999; Scolan and Sanchez,1998), the sol-gel method (Gartner et al., 2004; Su et al., 2006b), hydrothermal (Wu et al., 2002) and solvothermal processes (Kim et al., 2003; Yin et al., 2003), combustion method (Nagaveni et al., 2003), a microemulsion-mediated process (Hong et al., 2003), electrochemical synthesis (Karuppuchamy et al., 2006), fungus-mediated synthesis (Jha et al., 2009) and chemical vapour deposition (CVD) (Jones and Chalker 2003). Venkatachalam et al. (2007) reported the preparation of titanium dioxide nanoparticles via the sol-gel method with different reaction parameters, such as different hydrolyzing agents, molar ratio, aging time and calcination temperature. They obtained nanocrystals of titanium dioxide with high surface area and different ratios of anatase /rutile.

Wang et al. (2007) reported the synthesis of anatase TiO₂ nanocrystals obtained by the sol-gel method using titanium tetraisopropoxide in ethylene glycol. Titanium dioxide nanoparticles were prepared by amine-assisted sol-gel precipitation of an aqueous Ti⁴⁺ solution and further hydrothermal treatment (Hidalgo et al., 2007). The method led to high surface area values and well crystallized anatase with small crystallite size. Kim et al. (2003) investigated a solvothermal synthetic method to obtain TiO₂ nanoparticles in toluene solutions with titanium isopropoxide (TIP) as precursor. The product exhibited a uniform anatase structure with mid-sized particles on the nano-scale level.

Hydrothermal synthesis has become one of the most promising and important methods employed in nanomaterial production and nanotechnology (Pavasupree et al., 2008). The process includes a simple route and produces high crystallinity oxides under moderate conditions, i.e., low temperatures and short reaction times. Due to its characteristics, this is one the most used methods to produce ceramic materials, allowing the control of the particle size, morphology and phase composition (Su et al., 2006; Wang, 2007).

The aim of this study was to investigate the effects of both temperature and reaction time on the TiO₂ physical properties synthesized by the hydrothermal process. Short (6 and 12 h) and long (24 and 36 h) reaction times were investigated at low synthesis temperatures (150 and 200ºC).

MATERIALS AND METHODS

The reagents used for the synthesis of titanium dioxide nanopowders were titanium (IV) isopropoxide (C₁₂H₂₈O₄Ti) (Aldrich, 97% purity) and glacial acetic acid (CH₃COOH) (Aldrich, 99.7% purity). Through hydrothermal treatment, the synthesis of titanium dioxide was performed at different reaction times (6, 12, 24 and 36h) and at different temperatures (150 and 200ºC). The molar composition of the reaction mixture was 1TiO₂:10CH₃COOH:150H₂ O. Glacial acetic acid (36 mL) was slowly added to 20 mL of titanium (IV) isopropoxide in a water bath at 0ºC, under constant stirring. Afterwards, 170 mL of deionized water were gently added to the mixture, also under constant stirring. After vigorous agitation for one hour, the solution underwent ultrasonic treatment for 30 min. Once again, vigorous agitation was applied for 5 h. Afterwards, the solution was poured into Teflon jars and placed in stainless steel autoclaves (Bayati et al., 2008). The autoclaves were placed in an oven previously heated to 70°C in order to carry out the ageing process for 12 h. Following the ageing process, the hydrothermal treatment was carried out at 150°C and 200°C for 6, 12, 24 and 36 h. Subsequently, the autoclaves were removed from the oven and cooled in running water. Eventually, the precipitate was washed with distilled water, centrifuged and dried in an oven at 100°C for 12 h. The solid sample obtained was finely ground using a mortar and pestle and stored in plastic containers.

The resultant powder was characterized by X-ray diffraction, infrared spectroscopy and surface area measurements. X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance diffractometer. The X-ray source was Cu-Kα radiation, powered at 40 kV and 40 mA. Data were collected from 20 to 80º (2θ) with a step size of 0.05º and a count time of 35s. The average nanocrystal size was determined using the Sherrer equation (Ikezawa et al., 2001): D = K.λ/ (β.cosθ), where D is the average crystallite size, K is the Sherrer constant (0.90), λ is the wavelength of the X-ray radiation (0.15425 nm for Cu-Kα ), β is the peak width at half height and θ corresponds to the peak position (in the current study, 2θ = 25.35).

By means of infrared spectroscopy, infrared spectra of all samples pressed into KBr pellets were recorded by a Bruker tensor 27 FTIR spectrometer. IR spectra were measured in the range 4000-400 cm^-1.

The transmission electron micrographs (TEM) were obtained using a model JEM-2100 microscope. Test samples were prepared by ultrasonic dispersion of the powder in alcohol. A drop of the resulting suspension was spread onto a carbon-coated grid and allowed to dry at room temperature before testing.

The BET surface areas were obtained from nitrogen adsorption isotherms at 77 K, conducted on a Quantachrome Autosorb Automated Gas Sorption system, at a relative pressure (P/Po) from 0 to 0.99.

RESULTS AND DISCUSSION

Figures 1(a) and 1(b) show a series of XRD patterns of the prepared TiO₂ samples following the hydrothermal treatment at 150ºC and 200ºC at various time periods. All diffraction peaks show the complete formation of the crystalline anatase phase and were indexed according to the JCPDS card No. 89-4921. The nanocrystalline anatase structure was confirmed by (101), (004), (200), (105), (211), (204) and (116) diffraction peaks. No peaks related to either the rutile or brookite phases were observed in the XDR patterns. The hydrothermal synthesis route presents the advantage of obtaining phase-pure TiO₂ nanoparticles at both lower temperatures and reaction times. From the diffractogram, the widths of the bases of the peaks and their intensity characterize the size of the nanocrystals, because small crystals may promote a more intense spreading due to internal reflections that occur in the system. Considering the diffractograms shown in Fig. 1 (a) and (b), it can be observed that the samples obtained at 150°C present slightly broader peaks, which are also reduced in intensity compared to samples synthesized at 200°C, revealing that the nanocrystals obtained at lower temperatures have a smaller size. This fact is evidenced by the calculation of the average crystallite size using the Scherrer equation, as shown in Table 1. Comparing the samples synthesized at both temperatures, larger crystallite sizes were observed in all samples treated at 200ºC at all reaction times. The crystallite sizes increase considerably as the temperature increases due to the nanocrystal coalescence that takes place. In addition, in Table 1, it is observed that the average crystallite size increases with reaction time. An increased contact time favours greater coalescence, allowing time to control the nanocrystal growth. Comparing all samples synthesized under all synthesis conditions, the sample synthesized at the shortest time and lowest temperature (6 h and 150ºC) presented the smallest crystallite size of 8.9 nm, while the one synthesized under the highest synthesis conditions (36 h and 200ºC) presented the biggest crystallite size of 17.2 nm.

Fig. 2 shows the isothermal adsorption/desorption of nitrogen related to the titanium dioxide samples synthesized at temperatures of 150 ºC and 200°C and at 6 and 36 h reaction time. All isothermal adsorption and desorption curves show a type IV isotherm and hysteresis loops typical of mesoporous materials (Rouquerol et al., 1999). The isotherms of the other samples (not shown here) showed similar behaviour. With respect to the samples obtained by the hydrothermal method, the surface area data, the average pore diameters and the total pore volume are summarized in Table 1. Analysing the results for the surface area, at a synthesis temperature of 150ºC, the surface area ranged from 169 (6 h) to 158 m².g^-1 (36 h), which corresponds to a difference of only 6%. However, nanocrystals synthesized at 200ºC had a surface area that ranged from 129 (6 h) to 86 m².g^-1 (36 h), which corresponds to a difference of 33%. The results show that the reaction time has little influence on the surface area at lower temperatures; however, it has a significant influence at higher temperatures. Comparing the areas from the samples obtained at a reaction time of 6 h, a variation from 169 m².g^-1 (150ºC) to 129 m².g^-1 (200ºC) is observed, which corresponds to a difference of 23%. Concerning the samples synthesized at the longest reaction time, 36 h, the surface areas range from 158 (150ºC) to 86 m².g^-1 (200ºC), which corresponds to a difference of 45%. Thus, temperature has been shown to influence significantly the surface area over long treatment periods. The highest surface area values were observed for mild treatment conditions (temperature and time) because, under those conditions, smaller crystallite sizes are found. In general, hydrothermal treatment leads to high surface area values, as has been widely reported in the literature for hydrothermal synthesis (Kolen'ko et al., 2004; Kominami, et al., 1997).

The samples synthesized at 150ºC presented an average pore size ranging from 6.3 nm (6h) to 8.3 nm (36h), and the samples synthesized at 200ºC presented an average pore size ranging from 9.6 nm (6h) to 13.6 nm (36h). In this study, the values found for pore diameter ranged from 2 to 50 nm; hence, the material is regarded as being mesoporous (Gregg and Sing, 1982). Pore diameter increases with time and temperature conditions due to the increase in the crystal size of TiO₂. The characterization of mesoporosity is probably due to a variety of accumulated pore voids between the particles. Mesoporous TiO₂ materials have been receiving considerable attention due to their remarkable mesostructure and their synthesis has been extensively studied under different conditions. Huang et al. (2005) prepared mesoporous TiO₂ materials by the sol-gel process at room temperature using tetrabutyl titanate as the precursor in the absence of any templates. Antonelli and Ying (1995) synthesized mesoporous TiO₂ materials by a modified sol-gel method using phosphorus surfactants as templates. Pavasupree et al. (2008) synthesized mesoporous anatase TiO₂ nanopowder by the hydrothermal method at 130ºC for 12h. All researchers mentioned in the current study observed that type IV isotherms exhibit hysteresis loops that are typical of mesoporous material. For all samples, pore volumes were approximately 0.30 cm³.g^-1.

Fig. 3 shows the micrographs of TiO₂ samples treated hydrothermally at 150 and 200°C for 6 and 36 h. TEM images indicate that the TiO₂ nanoparticles are reasonably uniform in size for a given synthesis condition. The images demonstrate the formation of crystalline TiO₂ aggregates with a size of the nanoparticles of about 10-20 nm. The size of the nanoparticles obtained by TEM micrographs is consistent with the crystallite size estimated from XRD analysis.

In Fig. 4, bands in the region 400 a 800 cm^-1, which are assigned to vibrations of the Ti-O bonds of the crystalline anatase phase, can be observed in both samples at all reaction times. The band near 500 cm^-1 is assigned to a vibration of pure titanium dioxide for the crystalline anatase phase (Ivanova and Harizanova, 2001; Ayers and Hunt, 1998). Vibrations ranging from 770 to 800 cm^-1 are typical of Ti-O bonds for TiO_n compounds with n < 6 (Ivanova and Harizanova, 2001; Karuppuchamy and Jeong, 2006). In addition, changes in the intensity of the bands ranging from 400 to 800 cm^-1 were observed. The increase of both reaction time and temperature resulted in more intense bands. This fact may be due to the increase of crystallite size.

CONCLUSIONS

This study investigated the synthesis of titanium dioxide anatase phase, with high surface area and nanosized crystallites, obtained by a hydrothermal process. It was shown that the reaction time exerts little influence on the surface area at lower temperatures, but has a significant influence at higher temperatures. Temperature was shown to exert a significant influence on the surface area under elevated thermal conditions. A small range of crystallite size values may result in a significant variation of surface area. Both temperature and reaction time were shown to have little influence on the crystallite size. A major advantage of this route is that the synthesis of titanium dioxide anatase phase can be performed under moderate temperatures and reaction times.

ACKNOWLEDGEMENTS

The authors would like to thank the Brazilian agency CAPES for financial support.

(Submitted: May 15, 2010 ; Revised: February 21, 2011 ; Accepted: March 9, 2011)

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