Size-selective photocatalytic reactions by titanium(iv) oxide coated with a hollow silica shell in aqueous solutions

Abstract

A novel core-shell composite photocatalyst, commercially available titanium(IV) oxide (TiO 2 ) particles directly incorporated into a hollow silica shell, was fabricated by successive coating of TiO 2 with a carbon layer and a silica layer followed by heat treatment to remove the carbon layer. The composite induced efficient photocatalytic reactions when relatively small substrates were used, such as methanol dehydration and decomposition of acetic acid, without any reduction in the intrinsic activity of original TiO 2 but did not exhibit efficient photocatalytic activity for decomposition of large substrates, methylene blue and polyvinylalcohol. The unique size-selective properties of the composites are due to their structural characteristics, i.e., the presence of a pore system and a void space in the silica shell and between the shell and medial TiO 2 particles, respectively. The loading of alkylsilyl groups on the surface of the composite led to highly photostable floatability: the floated sample also induced efficient photocatalytic reaction for decomposition of acetic acid while retaining floatation at the gas-water interface.

Preparation, characterization and photocatalytic performance of titania particles encapsulated in hollow silica shells as an efficient photocatalyst for redox-combined stereoselective synthesis of l-pipecolinic acid from l-lysine

“…9 (d)) clearly indicated the deposition of fine Pt particles onto TiO 2 without any collapse of the SiO 2 shells during the platinization process. Similar results were obtained in our previous study and can be attributed to the presence of pores in the SiO 2 shell and void spaces between the shell and core TiO 2 particles [22]. These structures led to efficient mass transfers through an SiO 2 shell to supply Pt particles to the naked active surface of the TiO 2 core.…”

“…The utilization of certain oxide particles during the hydrothermal reaction of glucose gave rise to a uniform layer of PS on the particles instead of formation of PS spheres [21][22][23][24]. Heat treatment at 873 K under vacuum resulted in conversion of the PS layer to an amorphous carbon layer [22]. Also an important point is the use of APS-treated TiO 2 ; use of bare TiO 2 failed to produce surface-covered particles and resulted in the formation of PS spheres.…”

“…Stirring the reaction mixture by rotating the autoclave was indispensable for the uniform coating of particles, presumably because the PS formation proceeds spontaneously and PS particles could be obtained in the absence of TiO 2 particles. The utilization of certain oxide particles during the hydrothermal reaction of glucose gave rise to a uniform layer of PS on the particles instead of formation of PS spheres [21][22][23][24]. Heat treatment at 873 K under vacuum resulted in conversion of the PS layer to an amorphous carbon layer [22].…”

“…However, upon inspection with SEM, it was observed that a hollow core-shell structure, as was seen for SiO 2 /void/TiO 2 prepared by the standard procedure, was not formed. It has already been reported [22] the TiO 2 surface must be treated with APS before the carbon-layer coating for uniform coverage of carbon on the surface of TiO 2 .…”

“…On the other hand, we have recently reported [21,22] the fabrication of a novel core-shell composite photocatalyst that consists of commercially available TiO 2 particles incorporated in a hollow silica shell (SiO 2 /void/TiO 2 ). The composite hollow core-shell particles possessed size-selective properties in the photodecomposition of organic compounds; SiO 2 /void/TiO 2 showed photocatalytic activity for decomposition of small substrates such as acetic acid while retaining the activity of original bare TiO 2 , whereas negligible activity for polymers such as poly(vinyl alcohol) was observed; i.e., SiO 2 /void/TiO 2 exhibited molecular size selectivity [23,24].…”