Maria Antoniadou

Solar light-responsive Pt/CdS/TiO2 photocatalysts for hydrogen production and simultaneous degradation of inorganic or organic sacrificial agents in wastewater.

Photocatalytic degradation of waste material in aqueous solutions and simultaneous production of hydrogen was studied with the double purpose of environmental remediation and renewable energy production. Both powdered and immobilized Pt/CdS/TiO(2) photocatalysts were used to oxidize model inorganic (S(2-)/SO(3)(2-)) and organic (ethanol) sacrificial agents/pollutants in water. Powdered Pt/CdS/TiO(2) photocatalysts of variable CdS content (0-100%) were synthesized by precipitation of CdS nanoparticles on TiO(2) (Degussa P25) followed by deposition of Pt (0.5 wt %) and were characterized with BET, XRD, and DRS. Immobilized photocatalysts were deposited either on plain glass slides or on transparent conductive fluorine-doped SnO(2) electrodes. The results show that it is possible to produce hydrogen efficiently (20% quantum efficiency at 470 nm) by using simulated solar light and by photocatalytically consuming either inorganic or organic substances. CdS-rich photocatalysts are more efficient for the photodegradation of inorganics, while TiO(2)-rich materials are more effective for the photodegradation of organic substances.

Study of hybrid solar cells made of multilayer nanocrystalline titania and poly(3-octylthiophene) or poly-(3-(2-methylhex-2-yl)-oxy-carbonyldithiophene)

Hybrid solar cells have been constructed by using nanocrystalline titania and hole-transporting polymers. Titania was deposited on fluorine-doped tin-oxide transparent electrodes in three layers: a blocking layer and two nanostructured layers, giving densely packed or open structures. Open structures produced higher currents due to better polymer penetration and larger oxide-polymer interface. Cells based on the dithiophene-unit-containing polymer gave higher open-circuit voltage. Efficient cells could be made only in the presence of a dye sensitizer and a lithium salt. Cells were neither sealed nor encapsulated and their components were deposited under ambient conditions except for the metal back electrode, which was deposited under vacuum. Cells demonstrated a transient behavior in two stages: initially an increase of both current and voltage followed by an increase in voltage and a drop in current. Both quantities were stabilized at values approximately established within a few days. These values remained stable for several months when the cells were stored in the dark.

Photocatalysis and photoelectrocatalysis using nanocrystalline titania alone or combined with Pt, RuO2 or NiO co-catalysts

Photocatalytic mineralization of ethanol in the presence of oxygen has been studied in aqueous photocatalyst suspensions by employing either pure nanocrystalline titania or TiO2 combined with Pt, RuO2 or NiO co-catalysts. Combined photocatalysts demonstrated a diverse behavior. Highest mineralization rates were obtained with Pt/TiO2 and lowest with RuO2/TiO2 and NiO/TiO2. These results were related with the photocatalysts’ behavior when used as photoanodes for the production of electricity in a photoactivated fuel cell running with ethanol as fuel. The highest current was obtained with pure titania. The current dropped in the case of Pt/TiO2 and became much lower in the case of RuO2/TiO2 and NiO/TiO2 photoanodes. Both current and voltage were lower in the presence of oxygen than in its absence. It is concluded that the presence of electron scavengers, like O2, and/or the use of efficient photocatalysts, like titania-supported Pt, yield less electric power but assist ethanol mineralization process.

Photocatalytic and photoelectrochemical hydrogen production by photodegradation of organic substances

Commercial nanocrystalline titania (titanium dioxide, TiO2) has been used to make TiO2 films, which were employed to photodegrade several organic substances under photocatalytic (PC) or photoelectrochemical (PEC) operation. Hydrogen was produced during both operations while electricity was additionally produced during the PEC operation. Both processes were studied as typical examples of the current trend in the effort to produce useful forms of energy by photodegradation of organic waste materials.

Quantum Dot Sensitized Titania as Visible-light Photocatalyst for Solar Operation of Photofuel Cells

Abstract Nanocrystalline titania photoanodes, sensitized with CdS, ZnS, CdSe, Sb2S3 and PbS quantum dots have been employed in photofuel cells functioning with alkaline electrolyte and ethanol as fuel. Ethanol was photocatalytically oxidized producing electric current. It was found that medium band gap semiconductors, obtained by mixing ZnS and CdS, were the best sensitizers since they combine sensitization capacity in the Visible with sufficient oxidative power for oxidizing ethanol. However, in the presence of small band gap semiconductors, like CdSe, Sb2S3 and PbS, the oxidative power is diminished and the system demonstrates a poor behavior. Nanocrystalline titania sensitized by ZnS-CdS makes photoanodes with remarkable stability in the presence of ethanol, which acts as sacrificial agent.

Photo-Fuel-Cells: An Alternative Route for Solar Energy Conversion

The present work introduces photo-fuel-cells (PFCs) as an alternative means of solar energy conversion with simultaneous degradation of water soluble wastes. A PFC takes the structure of a standard photoelectrochemical cell. The photoanode carries the photocatalyst, which is a nanostructured oxide semiconductor, typically, nanoparticulate titania combined with a quantum dot sensitizer, which provides functionality in the Visible. Only medium bandgap semiconductors like CdS or combined CdS-ZnS may act as sensitizers. Low bandgap semiconductors like CdSe or PbS cannot be employed as sensitizers because of their low oxidation capacity that affects the oxidation capacity of the combined photocatalyst. This is important since the PFC functions by photocatalytic degradation of the fuel and its functionality is preserved, thanks to the sacrifice of the fuel. The principal function of the PFC is to produce electricity; however, it may also be used to produce solar fuels, for example, hydrogen. In that case, the cell functions only under bias. When it is operated to solely produce electricity, then the cathode electrode must be aerated. The present work is a short review of our recent experience in the study of photo-fuel-cells and proposes some measures for the improvement of their performance.