Efficient charge separation and transport are important factors in achieving high efficiency in TiO2 photocatalysis and dye-sensitized TiO2 applications. Mesoporous TiO2 (meso-TiO2) consisting of compactly packed nanoparticles can be a promising candidate for such purposes. In the present study, meso-TiO2 exhibiting an enhanced photocatalytic activity for H2 evolution was synthesized by a simple and facile, template-free nonhydrothermal method. The photocatalytic activity for H2 evolution of meso-TiO2 was the highest compared to nonporous colloidal-TiO2 and commercial Degussa P25 (P25) and Hombikat UV-100 (HBK) samples. The highest activity of meso-TiO2 was further supported by photocurrent generation and photoluminescence. Under visible light irradiation, the dye-sensitized TiO2 system also exhibited a higher activity for H2 production with meso-TiO2 compared with colloidal-TiO2. The activity of meso-TiO2 exhibited a unique dependence on Pt cocatalyst loading. Under both UV and visible light irradiation, the highest activity for H2 production was obtained around 0.1 wt % Pt and further increase reduced the activity, whereas other nonporous TiO2 samples exhibited a typical saturation behavior with increasing Pt load. The enhanced photocatalytic activity of meso-TiO2 is ascribed to the compact and dense packing of TiO2 nanoparticles forming a uniform agglomerate, which enables efficient charge separation through interparticle charge transfer. Finally, the present simple synthesis method we developed is advantageous over other methods because it eliminates the use of templates and a hydrothermal process, which is highly favored for the scale-up production of the meso-TiO2 photocatalyst.

The photocatalytic production of H 2 in aqueous TiO 2 colloid (with methanol as an electron donor) was greatly accelerated by the in situ agglomeration of the colloid although such an agglomeration should reduce the photocatalytic activity in most other cases because of the reduction of the surface area. The in situ agglomeration occurred after an induction period of 3 h and was ascribed to the pH increase which was resulted from the photocatalytic reduction of nitrate (incorporated from the synthetic step of TiO 2 sol) to ammonia. The agglomeration occurred at pH close to the isoelectric point of colloidal TiO 2 which was 6.9 as measured by the -potential. It is proposed that the charge separation is facilitated by electron hopping from particle to particle when TiO 2 nanoparticles are connected with each other within the agglomerates. This behavior was further supported by the photocurrent collection measurement (mediated by the methyl viologen MV 2+ /MV + redox couple in the colloidal solution), which also showed a rapid increase in the photocurrent after the agglomeration of TiO 2 nanoparticles. When the colloid of TiO 2 was initially coagulated at around pH 6, the production of hydrogen increased linearly with time without showing an induction period and the collected photocurrent showed an immediate increase upon irradiation. To understand the role of the agglomerated state, the colloidal TiO 2 (well-dispersed) and the suspension of commercial TiO 2 (agglomerated) systems were compared and discussed for their photocatalytic behaviors. The present study demonstrates that the degree of agglomeration of TiO 2 nanoparticles is a critical parameter in determining the efficiency of the charge separation and the photocatalytic hydrogen production.

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White-light emission is generated by combining blue and blue green to yellow emissions of Ce 3+ and Eu 2+ , respectively, in a single host lattice of Sr 2 SiO 4 . The excitation is in the near-UV region ͑350-370 nm͒. The role of concentration of Eu 2+ on the photoluminescence emission intensity in Sr 2-x Eu x SiO 4 ͓x = 0.0025, 0.005, 0.0075, and 0.01͔ is studied, and it is found that the critical concentration is 0.0025 mol. Energy migration over Eu 2+ sites occurs, resulting in concentration quenching. Ce 3+ exhibits a high absorption in the near-UV region. Energy transfer from Ce 3+ to Eu 2+ occurs in Sr 2 SiO 4 :Eu 2+ , Ce 3+ . Optimization of concentration of Eu 2+ to produce white light in Sr 1.98-x Eu x Ce 0.01 Li 0.01 SiO 4 shows that the optimum concentrations of Ce 3+ and Eu 2+ are 0.01 and 0.0025 mols, respectively. Partial energy transfer from Ce 3+ to Eu 2+ is responsible for the white-light generation. The results reveal that Sr 2 SiO 4 :Eu 2+ , Ce 3+ is an efficient "single host lattice phosphor" for solid-state lighting technology using UV light-emitting diode ͑LED͒ to generate white light.

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