Absorption and photoconductivity spectra of amorphous multilayer structures

The experimental results regarding optical absorption and steady-state photoconductivity of amorphous single-layer structures (Al–As0.40S0.30Se0.30–Al, Al–Ge0.09As0.09Se0.82–Al, and Al–Ge0.30As0.04S0.66–Al) and of an amorphous heterostructure (Al–As0.40S0.30Se0.30/Ge0.09As0.09Se0.82/Ge0.30As0.04S0.66–Al) at different values of the voltage, with positive or negative polarity, applied to the illuminated top Al electrode are presented and discussed. The complex structure of the photocurrent spectra is attributed to the different values of the optical bandgap of the involved amorphous layers (E g ≈ 2.0 eV for As0.40S0.30Se0.30 and Ge0.09As0.09Se0.82 and E g ≈ 3.0 eV for Ge0.30As0.04S0.66). The obtained experimental results are discussed taking into account the light absorption depending on the nature and the thickness of each amorphous layer, on the wavelength, and on contact phenomena at the interfaces between different layers and between the amorphous layers and the metal electrodes with different work functions.

SYNTHESIS, PHOTO- AND ELECTROCATALYTIC PROPERTIES OF NANOSTRUCTURED Ce–TiO2 FILMS

Electrocatalytic films based on nanodispersed titanium dioxide modified by Ce(III) were synthesized by sol-gel method and characterized by X-ray diffraction and ultraviolet-visible photocurrent spectra. The average size of nanoparticles was no more thаn 11 nm. The XRD results indicated that TiO2 and Ce–TiO2 electrodes with Ce concentrations up to 5 % calcined at 500°C consisted of anatase as the unique phase. The photocurrent spectra of the Ce–TiO2 electrodes (0 ≤ Ce concentration ≤ 2% mol.) showed a stronger current in the UV range and a shift in the flat-band potential (Еfb) towards more negative values than that of TiO2 electrodes. Electrocatalytic properties of TiO2 and Ce–TiO2 electrodes were investigated in the process of oxygen electroreduction. It has been found by I–E curves measurements that the potential of oxygen reduction changes with the film composition. Modifying of TiO2 films by Ce(III) improves catalytic activity of Ce–TiO2 electrodes ( Ce concentration up to 2% mol.) in the reaction of oxygen electroreduction, that appears in decreasing of oxygen reduction potential EO2 and increasing of dynamic range of O2 electroreduction potentials. The high electrocatalytic activity of Ce–TiO2 electrodes in the oxygen reduction process may be due to the formation of catalytically active centers which activity may decrease in the presence of an amorphous phase. The correlation between photo- and electrocatalytic properties and structural changes occurring in Ce–TiO2 films on increasing the cerium content is observed. The electrodes investigated were distinguished by a high sensitivity to dissolved oxygen ((4-5)∙10-6 g·l-1) and high reproducibility of characteristics in long-time cycling. These electrodes promise much as reusable electrode materials in electrochemical sensors for the determination of oxygen in liquids.