Photocatalytic Air and Water Purification
The air and water pollution is a serious problem for our society. Photocatalysis is considered to be a rather efficient process for removal or degradation of the environmentally hazardous and toxic substances being present both indoor and outdoor in the world. Therefore, the main focus of the current research activities in our group in the field of photochemistry is photocatalysis. Photocatalysis belongs to the so-called Advanced Oxidation Processes (AOPs) being used for the mineralization of toxic angd difficult biodegradable organic substances containing in water and air by the hydroxide radicals. Hereby, the pollutant molecules can be completely oxidized in the presence of atmospheric oxygen, i.e., the so-called combustion reaction takes place under formation of carbon dioxide, water and possibly other minerals and nontoxic products. During the photocatalytic reactions both water and atmospheric oxygen are often transformed to the more reactive species, i.e., the previously mentioned hydroxid radicals under illumination of the employed photocatalyst such as titanium dioxide (TiO2). For instance, titanium dioxide is a semiconductor with a band gap of 3.2 eV absorbing light in the wavelength range between 200 and 400 nm. Therefore besides the artificial light source also UV-A light of the sun can be utilized for driving of the photocatalytic reactions when TiO2 is used as a photocatalyst. This specific form of photocatalysis is called “solar catalytic water and air purification”. In order to evaluate the photocatalytic activity of different catalysts various model air pollutants such as formaldehyde, acetaldehyde or nitrogen oxides are employed in our laboratories.
Photocatalytic Water Splitting
The photoinduced splitting of water offers a possibility to utilize the sun light for the direct production of molecular hydrogen (H2) as a source of energy. Hydrogen, with an energy content of 33.3 kWh/kg, is one of the most storable energy sources offering an alternative for the fossil energy sources because of its renewable production from water. In this way hydrogen has a potential not only to change our current energy supply fundamentally but also to solve the long-term issues related to it.
In order to split water into molecular hydrogen and molecular oxygen, the energetic position of the bottom of the conduction band of a semiconductor has to be located more negatively than the reduction potential of protons (H+) to molecular hydrogen (0 V vs. NHE at pH 0), while the top of the valance band has to be located more positively than the oxidation potential of water to oxygen (+1,23 V vs. NHE bei pH 0).
In our laboratories, the qualitative and quantitative analysis of the product formation, i.e., molecular hydrogen and molecular oxygen is mainly performed by means of the mass spectrometer in aqueous semiconductor suspensions. Additionally, for a deeper understanding of the mechanism behind the water splitting reaction experiments with deuterated water (D2O) are carried out.
In our group, water splitting reactions can additionally be investigated by means of the photoelectrochemical studies using electrodes. Various materials employed as photoanodes and photocathodes can be characterized in terms of the position of their energy bands (Impedance spectroscopy), in terms of the required potentials for the hydrogen and oxygen evolution as well as for the determination of quantum yields of such reactions.
Kinetic of the fast Photocatalytic Processes
Besides applied research, the study of the physical-chemical fundamentals of photocatalysis is one of the central tasks in our research group. Such investigations are carried out by means of the laser flash photolysis spectroscopy. Thereby, the absorption of the formed active species can be observed after the excitation of a photocatalyst by a nanosecond pulsed laser. The lifetime of these active species varies dependent on the photocatalyst properties and its photocatalytic activity. Therefore, the knowledge gained from such investigations is essential for the development of the new semiconductor materials. Moreover, time-resolved measurements in the presence of various reduction and oxidation agents allow elucidating the mechanisms of the photocatalytic processes. Additionally, the available EPR equipment (electron paramagnetic resonance spectroscopy) in our working group allows both the qualitative and quantitative analysis of the reaction intermediates and end products for the deeper understanding of the mechanisms of the studied photocatalytic reactions.
Dye Sensitized Solar Cells
Dye sensitized solar cells are a new type of organic-inorganic hybrid solar cells, which are intensively studied since 25 years. Contrary to the conventional silicon-based photovoltaics, these cells do not use black silicon as light absorber but an organic dye, which is adsorbed on the surface of a nanostructured titanium dioxide layer. This modern technology does not only provide relatively low production costs, but is also very versatile in architectonical applications due to the availability of different colors.
Our current research in this field deals with the new synthesis routes for the preparation of nanoscaled titanium dioxide materials. Furthermore, our research focus here is on the construction and photoelectrochemical testing of the dye sensitized solar cells in close cooperation with partners from the industry.