Sol-Gel법에 의해 제조된 titania 나노입자의 결정상과 기공특성

송기창, Sotiris E. Pratsinis*

건양대학교 화학공학과, 스위스 연방공과대학 (ETH)*

Phase Composition and Pore Structure of Sol-Gel Derived Titania Nanoparticles

Ki-Chang Song, Sotiris E. Pratsinis*

Department of Chemical Engineering, Konyang University

Institute of Process Engineering, Swiss Federal Institute of Technology (ETH)*

INTRODUCTION

Titania powders are widely used as a white pigment, an adsorbent, and a major component in many electronic devices (e.g. capacitors and varistors), and have recently become of interest as functional materials such as a photosemiconductor and a photocatalyst.1-5) The most common procedures of preparing the titania powders have been based on the hydrolysis of acidic solutions of Ti(IV) salts,6-7) or by gas-phase oxidation reaction of TiCl4 at high temperatures.3,8) Recently, however, methods of vapor-phase hydrolysis,9) pyrolysis,10) and sol-gel synthesis2,11) of organic precursors (e.g. titanium tetraisopropoxide, TTIP), have been developed to synthesize titania nanoparticles with high purity.

Sol-gel synthesis provides a way to make nanoparticles by a chemical reaction in solution starting with metal alkoxides as a precursor. An advantage of the sol-gel synthesis is that it is possible to obtain large amounts of powder with a high level of chemical purity. In this sol-gel synthesis two simultaneous reactions - hydrolysis and polycondensation - take place when TTIP reacts with water. The polycondendation reaction induces polymerization forming higher molecular weight products (nuclei and subsequently particles). These two reactions are sensitive to the reaction variables, namely, water concentration, type and amount of catalysts, type and amount of solvents, and reaction temperature. Thus, the properties of titania powders depend strongly on these reaction variables.

In this study, we investigate the effect of initial water concentration, type and amount of catalysts, type and amount of alcoholic solvents added during hydrolysis of TTIP, and reaction temperature on the texture and phase composition of product titania powders.

EXPERIMENTAL

Titanium tetraisopropoxide (TTIP; Ti(OC3H7)4, 97%, Aldrich) was used as a precursor. To moderate its high reactivity, the TTIP was first diluted in anhydrous ethyl alcohol (molar ratio of ethyl alcohol/TTIP of 5) prior to reaction with water. In this study, HCl and NH4OH were used as hydrolysis catalysts. The molar ratio of catalyst/TTIP was varied in the range of 0 to 0.5. Different amounts of the hydrolysis catalysts were mixed with a constant amount of water (molar ratio of water/TTIP of 20). The hydrolysis was carried out at room temperature by adding distilled water containing various catalyst concentrations to the mixed solution (ethyl alcohol and TTIP) with a vigorous stirring for 1 h. On adding water into the mixed solution, a slurry containing white titanium hydroxide precipitate was formed. Clear liquid was removed from the slurry by separation in a super-speed centrifuge (Rotina35, Hettich) at 9,000 rpm for 10 min. The filtered powder was dried at 150oC for 24 h, followed by calcination at different temperatures ranging from 300 to 800oC for 1 h at a heating rate of 10oC/min.

The specific surface area of the powders was performed on a BET surface area analyzer (ASAP2010, Micromeritics) using nitrogen as adsorbent at liquid nitrogen temperature. All powders were degassed at 150oC prior to measurement.  The specific surface area was determined by the multipoint BET method using the adsorption data in the relative pressure (P/Po) range of 0.05-0.25. The desorption isotherm was used to determine the pore size distribution using the Barrett, Joyner, and Halenda (BJH) method with cylindrical pore size. An X-ray diffractometer (D5000, Siemens) was used for identification of  crystalline species in the calcined powders. Ni-filtered CuK□ radiation was used as the X-ray source for diffraction angles 2 between 20 to 60o. The weight fractions of rutile present in the tiatnia powders were calculated by:

X = 1/[1 + 0.8IA/IR]

where X is the weight fraction of rutile in the powders, while IA and IR are the X-ray integrated intensities of the strongest peaks corresponding to anatase (2 = 25.3o for the (101) reflection of anatase) and rutile (2 = 27.5o for the the (110) reflection of rutile), respectively.

RESULTS AND DISCUSSION

The XRD patterns of the HN0.0, H0.30, and N0.50 powders are shown in Figs. 1a-c, as a function of calcination temperature.  Figure 1a shows that the HN0.0 powder made without catalyst is amorphous  at 150oC, while anatase appears at 300oC. A phase transformation from anatase to rutile begins at 600oC and is not completed till 800oC. For the H0.30 powder made with HCl/TTIP=0.30, anatase peaks are  already  present at 150oC  in Fig. 1b. The  phase  transformation   from  anatase  to  rutile occurs  at  400oC  and is completed at 700oC. It is apparent that the transformation temperature of anatase to rutile is 200oC lower for the H0.30  than for the HN0.0 powder. On the other hand, the N0.50 powder made with NH4OH/TTIP=0.50 remains amorphous till 300oC and is crystallized to anatase at 400oC in Fig. 1c, which is about 100oC higher than the crystallization temperature of the HN0.0 powder. It is also seen in the N0.50 powder that the phase transformation from anatase to rutile begins at 700oC and is not completed till 800oC. As a result,  the transformation temperature of anatase to rutile is 100oC higher for the NH4OH catalyzed than the non-catalyzed powder. Clearly, the presence of HCl enhances the amorphous to anatase and anatase to rutile phase transformations, while the NH4OH catalyst inhibits them.

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Figure 1. The XRD patterns of the a) HN0.0, b) H0.30, and C) N0.50 powders as a function of calcination temperature.