Hiroshi Hada

Photochemical formation of colloidal metals

Colloidal silver and gold have been formed by irradiation of AgClO4 and HAuCl4 solutions with 253.7 nm light in the presence of protective agents such as sodium dodecylsulphate, sodium alginate and colloidal silica. Absorption maxima of colloidal silver and gold have been observed at 390–420 and 540–560 nm, respectively. Colloidal metals have been purified by dialysis and their amounts analysed quantitatively. The quantum yields of metal deposition are 0.053–0.15 for colloidal silver and 0.030–0.039 for colloidal gold. The quantum yield is increased several times by the use of protective agents, with the exception of a AgClO4–sodium alginate solution.

Formation of silver metal films by photolysis of silver salts of high molecular weight carboxylic acids

Abstract The photochemical formation of silver metal films from silver salts of high molecular weight carboxylic acids has been studied. On irradiation with a low pressure mercury lamp in wet air at room temperature, the films first became yellow-brown in colour owing to the formation of colloidal silver. After prolonged irradiation, the irradiated surface of the silver salt films of natural high molecular weight carboxylic acids (silver alginate, silver pectate) readily changed into silver mirrors. When silver alginate films were photolysed at 77 K, a broad absorption band below 600 nm and a small peak near 335 nm, probably due to silver clusters, appeared. In this method, silver atoms in the various aggregation states (atoms, clusters, colloids, bulk metal) can be formed by changing photolysis conditions. The preparation of matrix-supported silver films reported by other research groups has been reviewed briefly.

Photoreduction of silver ion on the surface of titanium dioxide single crystals

Mean quantum yields ranging from 0.0053 to 0.16 have been determined for the reduction of Ag+ by irradiation with 365 nm light on a TiO2 single-crystal surface in aqueous solutions containing 1 × 10–5 to 1 × 10–2 mol dm–3 AgNO3. The electrochemical behaviour of the same crystal electrode has been investigated in order to clarify the mechanism of silver reduction. It is proposed that some conduction-band electrons, produced in the TiO2 by irradiation, transfer to the Ag+ adsorbed on the TiO2 surface. A convenient expression for the dependence of the mean quantum yield on the Ag+ concentration is derived from the viewpoint of semiconductor electrochemistry.

Transfer of photoelectrons and photoholes through a AgBr/AgCl interface, and relative locations of the energy bands

The transfer of photoelectrons and photoholes through the interface between AgBr and AgCl thin‐sheet crystals was studied by the transient dc pulse method. Photoelectrons were transferred very easily with near perfect freedom in both directions from AgBr to AgCl and from AgCl to AgBr. The photohole transfer, however, was allowed only from AgCl to AgBr, and the transfer from AgBr to AgCl was effectively blocked at the interface. These results were carefully analyzed in terms of the relative locations of the energy bands of AgBr and AgCl. It was concluded that the bottom of conduction band of AgBr was likely to lie very close to that of AgCl, while the top of valence band of AgBr was separated from that of AgCl by an amount nearly equal to the difference in the band‐gap energies of AgBr and AgCl.

Preparation and luminescence properties of the J aggregate of cyanine dyes in phospholipid Langmuir- Blodgett films

Abstract The J aggregate of two cyanine dyes—dye I (5,6,5′,6′-dibenzo-1,1′-diethyl-2,2′- cyanine chloride) and dye II (5,5′-dichloro-3,3′-diethyl-9-phenyl-thiacarbocyanine chloride)—can be incorporated into acidic-phospholipid-containing Langmuir- Blodgett (LB) films. Dipalmitoyl-phosphatidylcholine (DPPC) and dipalmitoyl- phosphatidic acid (DPPA) were used for preparation of the LB films. An observation that J aggregation was difficult in the absence of negatively charged lipid species (DPPA) in the membrane reconfirms the importance of electrostatic interaction between negative charge at the matrix molecules and positive charge at the chromophore. The luminescence properties of the J aggregate in the more elaborate LB films with dye I and dye II aggregates at the same lipid surface were examined and interpreted in terms of excitation energy transfer from dye I to dye II aggregates.

Uniform chromophore distribution in Langmuir–Blodgett films of poly(vinyl octanal acetal)

Uniform chromophore distribution in Langmuir–Blodgett (LB) monolayers has been attained using a preformed polymer, poly(vinyl octanal acetal), containing the pyrene chromophore as a fluorescence probe.

Measurement of the lifetime of silver atoms on silver bromide grain surfaces in photographic emulsion by the multiflash method

The multiflash exposure method, which is a new useful and powerful tool for studying the nature of the photolytic silver atom on silver halide grain surface, is reported. The lifetime of the photolytic silver atom and the corresponding activation energy are easily and quantitatively determined by this method. The lifetime at 20 °C and the apparent activation energy were 1.1 s and 0.7 eV, respectively, on (100) surface of a chemically unsensitized silver bromide grain in a wet air environment. In a vacuum environment the lifetime was prolonged to 300 s and the activation energy was decreased to 0.40 eV.

Study on the early stage of photochemical vapor deposition of amorphous silicon from disilane on a SiO2 substrate

The nucleation in the very early stage of photochemical vapor deposition of amorphous silicon on a SiO2 substrate from disilanes was studied by using a chemical amplification technique to detect the small Si nuclei. Phenomena characteristics of the nucleation process were observed and the effects of some fundamental deposition parameters on the nucleation revealed interesting features of the nucleation chemistry. In particular, we found two different regions of substrate temperature; in the low‐temperature region up to about 150 °C the initial nucleation rate decreased as a whole with substrate temperature, while above 150 °C the tendency was apparently reversed, suggesting a change of nucleation mechanism. Information about the production, transportation, and identity of active species serving as nucleation precursors was obtained from the effects of gas pressure and addition of nitric oxide as a radical scavenger upon the nucleation efficiency. Based on the known chemistry of silanes and related species...

Oxidation kinetics of large photolytic silver clusters in redox buffer solutions

AbstractOxidation kinetics of relatively large silver clusters, formed by the excess exposure beyond the shoulder region of characteristic curves, in redox buffer solutions was elucidated by investigating the effects of the redox potential and silver ion activity of the buffer solution on the oxidation rates of various size silver clusters. A single-step oxidation of Agn cluster proceeds via the electron transfer front Agn to the buffer solution and the subsequent release of a silver ion from Agn+ to produce Agn-1, and Ag+ . The ionization potential of Agn and the dissociation energy of Agn+ to Agn-1, and Ag+ are the dominant factors inlluettcing the rate of the oxidation. It was concluded that the ionization potential is decreased with cluster size while the dissociation energy has the reverse trend. The trend can be predicted from the classical electromagnetics and thermodynamics and is also consistent with the results of various molecular orbital calculations on metal clusters. The kinetics of the sing...

The Measurement of the Time of Flash-Light Induced Latent Image Formation

AbstractAhigh intensity shorl flash-light exposure produces a number of photoelectrons and positive holes in a grain, and starts the processes of latent image formation. A realtime pursuit of the processes is attempted by measuring the Dember elTect with another light flash delayed for various time intervals from the first high intensity flash exposure. The processes of the flash-light II1duced latent image formation continue up to IOO~IS at a room temperature and is gradually prolonged at lower temperatures. InterpretatIon IS given in terms of the aggregation of photolytic silver atoms and the relaxation or deactivation processes of the positive holes.