Preparation of Stable Silver Nanoparticles Having Wide Red-To-Near-Infrared Extinction

The synthesis of silver nanoparticles (AgNPs) within the interlayer space of transparent layered titania nanosheet (TNS) films is investigated. A considerable number of silver ions (≈70% against the cation exchange capacity of the TNS) are intercalated in the TNS films using methyl-viologen-containing TNSs as a precursor. The silver ion (Ag+)-containing TNS films are treated with aqueous sodium tetrahydroborate (NaBH4), resulting in a gradual color change to bright blue. Various structural analyses clearly show that crystalline AgNPs are generated within the interlayer space of the TNSs. The NaBH4-treated films show intense and characteristic near-infrared (NIR) extinction spectra up to 1800 nm. The stability of the AgNPs within the TNS against oxygen and moisture is also investigated, and 96% and 82% of the AgNPs remain after standing in air for 1 month and 1 year, respectively. The NIR extinctions of the AgNP-containing TNS films are further extended by employing different preparation procedures, for example, using sintered TNS films as starting materials and irradiating the Ag+-containing TNSs with ultraviolet (UV) light. The obtained AgNP-containing TNS films exhibit photochemical activities in the production of hydrogen from ammonia borane under visible-light irradiation and the decomposition of nitrogen monoxide under UV-light irradiation.

Photochemical reduction of CO₂ using TiO₂: effects of organic adsorbates on TiO₂ and deposition of Pd onto TiO₂

Reduction of CO(2) using semiconductors as photocatalysts has recently attracted a great deal of attention again. The effects of organic adsorbates on semiconductors on the photocatalytic products are noteworthy. On untreated TiO(2) (P-25) particles a considerable number of organic molecules such as acetic acid were adsorbed. Although irradiation of an aqueous suspension of this TiO(2) resulted in the formation of a significant amount of CH(4) as a major product, it was strongly suggested that its formation mainly proceeded via the photo-Kolbe reaction of acetic acid. Using TiO(2) treated by calcination and washing procedures for removal of the organic adsorbates, CO was photocatalytically generated as a major product, along with a very small amount of CH(4), from an aqueous suspension under a CO(2) atmosphere. In contrast, by using Pd (>0.5 wt %) deposited on TiO(2) (Pd-TiO(2)) on which organic adsorbates were not detected, CH(4) was the main product, but CO formation was drastically reduced compared with that on the pretreated TiO(2). Experimental data, including isotope labeling, indicated that CO(2) and CO(3)(2-) are the main carbon sources of the CH(4) formation, which proceeds on the Pd site of Pd-TiO(2). Prolonged irradiation caused deactivation of the photocatalysis of Pd-TiO(2) because of the partial oxidation of the deposited Pd to PdO.

Photoinduced electron transfer between the anionic porphyrins and viologens in titania nanosheets and monodisperse mesoporous silica hybrid films

Photoinduced electron transfer between an anionic porphyrin derivative (tetrakis(p-carboxyphenyl)porphyrin; H(2)TCPP(4-)) and an electron accepting methyl viologen (MV(2+)) was investigated in two different nanoscale configurations, i.e., layered titania nanosheet (TNS) photocatalysts and ammonium-functionalized monodisperse mesoporous silica (AMMSS) particles. Cationic MV(2+) intercalated within the TNS interlayers while anionic H(2)TCPP(4-) was accommodated within AMMSS nanocavities to form (MV(2+)-TNS)/(H(2)TCPP(4-)-AMMSS) hybrid films. Upon irradiation with UV light and excitation of the TNS in the (MV(2+)-TNS)/(H(2)TCPP(4-)-AMMSS) hybrid films, the consumption of H(2)TCPP(4-) and the formation of a one-electron reduced MV(2+) (MV(+·)) were simultaneously observed. No consumption of H(2)TCPP(4-) was observed when an electrically insulating poly(styrene) (PS) was also introduced at the interface. These results suggest that photoinduced electron transfer occurred at the interface between the TNS and the AMMSS.

Photocatalytic reduction of CO₂: from molecules to semiconductors

We are facing three serious problems related to fossil resources, i.e., shortage of energy, shortage of carbon resources, and the global worming problem. Development of practical systems for converting CO₂ to useful chemicals using solar light, i.e., photocatalytic CO₂ reduction systems, should be one of the best solutions for these problems. In this article, we review photocatalytic CO₂ reduction systems, which are classified in two categories: (1) homogeneous reaction systems mainly using transition metal complexes, and (2) heterogeneous systems mainly using inorganic semiconductor as a light absorber.