Facet-Selective Deposition of Metal Nanoparticles on Decahedral-Shaped Anatase Titania Photocatalyst Particles

Abstract

It has been reported that faceted titanium(IV) oxide (titania) particles, e.g., decahedral-shaped anatase titania particles (DAPs) exposing two {001} and eight {101} facets, show high-level photocatalytic activity, presumably because photogenerated electrons and positive holes migrate to {101} and {001} facets, respectively, to result in charge separation.1,2 This speculation seemed to be based on scanning electron microscopic and/or transmission electron microscopic observations in which most metal and metal-oxide particles deposited on DAPs, through photocatalytic reduction and oxidation of precursors, were seen on {101} and {001} facets, respectively.1,2 However, these studies may include some problems, e.g., (1) there are no statements on the number of metal/metal-oxide particles deposited on the two kind of facets, only showing a single or few microscopic images. (2) there is a possibility that deposited metal particles were desorbed and adsorbed again on the {101} or {001} facets. In this study, metal particles were deposited on DAPs using two methods, observed by scanning electron spectroscopy (SEM) and facet-dependent deposition was quantitatively and precisely measured. DAPs were prepared by a gas-phase reaction of titanium(IV) chloride (TiCl4) with oxygen.3 Gold or platinum particles were deposited on the DAPs through (1) photocatalytic reduction of hexachloroplatinic acid (H2PtCl6) or tetrachloroauric acid (HAuCl4) or (2) mixing a gold or platinum colloid with DAPs suspensions in the dark or under mercury-lamp irradiation. The metal deposited DAPs was washed with water, then the suspension was dried by freeze drying under vacuum for 24 h. The images of the metal particles deposited on the DAPs were observed by SEM, and then the numbers of the metal particles on two kinds of facets were counted for more than 100 DAP particles. Figure 1 shows an SEM image of the platinum colloid-deposited DAPs prepared under mercury-lamp irradiation. A ratio of deposition density (s) was calculated as a measure of facet-selective metal deposition using the following equation: s = p {001} / (p {101}×R ave), (1) where p {001} and p {101} are the number of the metal particles on each facets, and R ave is the average area ratio of {001} and {101} facets (Table 1). If s is smaller than 1, it means that metal particles tend to be deposited preferentially on the {101} facet, and vice versa. In the case of the photocatalytic metal deposition from H2PtCl6 or HAuCl4, s was smaller than 1, as was reported previously. On the other hand, in the case of mixing of a gold or platinum colloid both under irradiation and in the dark, s was also smaller than 1, even though prepared without photocatalytic reaction, i.e., mixing of a metal colloid in the dark exhibited {101}-facet selectivity. These results suggest the difference in ability of metal-particle adsorption between {101} and {001} facets. Therefore, it cannot be excluded the mechanism in which {101}-preferred photocatalytic deposition of metal particles proceeds through facet-selective adsorption of once-deposited metal particles on DAPs. Figure 1