Systematic Size Control of Uniform Ti02 Nanoparticles by the Gel-Sol Method and Application of the Uniform TiOz Nanoparticles as a Catalyst for Water Photolysis

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T. Sugimoto, X. Zhou, H. Chen, and A. Muramatsu
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Particle Formation
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Uniform anatase-type TiOl nanoparticles were prepared by Gel-Sol method, in which a condensed aqueous solution of Ti-triethanolamine (TEOA) complex is first aged at 100 “C for 24 h for the hydrolysis of the Ti-TEOA complex to Ti(OH)4 gel network, and then aged at 140 “C for 72 h for the nucleation and growth of the final product by gradual dissolution of the Ti(OH)4 gel. The Ti-TEOA complex was previously prepared by mixing titanium isopropoxide (TIPO) directly with TEOA in a dry box at a molar ratio 1:2. Typically, uniform TiOz particles of ca. 21 nm in mean diameter were obtained by aging an aqueous solution of 0.25 mol dm” TIP0 stabilized with 0.50 mol dmT3 TEOA (initial pH = 9.5) at 100 “C for 24 h, followed by aging at 140 “C for 72 h. Effect of pH on the particle size was remarkable, since the mean diameter of cuboidal particles increased from ca. 5 to 30 nm with increasing pH from 1 to 11.5. However, no TiO2 particles were obtained over pH 12 even after the 2nd aging for 72 h. If TIP0 is not stabilized by TEOA in advance, Ti(OH)4 flock is formed instantly on mixing TIP0 with water at room temperature, and it is completely converted to ill-defined polydispersed TiOl particle of ca. 9 nm in mean diameter after the 1 st aging at 100 “C for 24 h. From the reduction of the reaction rate with increasing pH, irrespective of the presence or absence of TEOA, the increasing particle size with pH is basically elucidated in terms of the reduction of the nucleation rate by the lowered concentration of precursor complexes to TiOZ particles with increasing pH. In addition, this pH effect on the final particle size of TiOz was pronounced by the presence of TEOA, since TEOA liberated after the 1st aging significantly lowered the solubility of the produced Ti(OH)4 gel by adsorption, and since this etTect of TEOA was 111 enhanced with increasing pH. The particle size was also varied systematically by adding different amounts of seeds. Dramatic increase in the formation of TiOz with the increasing amount of seeds provided us with information that the dissolution of the Ti(OH)4 gel is not the rate-determining step of the particle growth of TiOz, and that the particles are grown by deposition of monomeric solute and not by aggregative deposition of particulate matters such as hypothetical primary particles of TiO2. It was also found that the particle shape was changed from cuboidal form to rod-like one when the pH was increased to 11 S. This result was explained in terms of the adsorption of TEOA to the crystal planes parallel to the c-axis of the anatase crystals.

As an application of thus-prepared well defined TiOz nanoparticles, their catalytic performance as a photocatalyst for water photolysis was studied. For this purpose, a special reactor was designed for precise measurement of the quantum efficiency of the photo-excited electrons and holes. Using this reactor with Pt/TiOz catalysts prepared by the selective deposition of Pt onto TiOz particles precisely controlled in size and shape, we performed preliminary experiments for the effects of the particle size and shape of TiOz, Pt loading, pH, atmospheric pressure, and concentration of electrolyte on the quantum efficiency. As a consequence, we found significant effects of these factors. However, we also found that the most imminent issue to be resolved is the surface modification of the Ti02 particles to introduce separate electron and hole trapping centers for the prevention of the drastic recombination of photoelectrons and holes.