The sustainable production of hydrogen from non-fossil sources is urgently needed for a future hydrogen economy. If we could produce hydrogen sustainably, save and convert efficiently in fuel cells, a technological dream finally come true. Until then, however, still need a lot of highly complex materials science problems are solved. Engineers and chemists working with this target in the Materials Research Department at the RUB together.
An intriguing possibility is to produce hydrogen, the catalytic photoelectrochemical splitting of water (H2O) into hydrogen (H2) and oxygen (O2) that was described as about 40 years ago.
A functional unit for water splitting consists of two electrodes, an external contact and an electrically conductive liquid (electrolyte), which produces the electrical contact between the electrodes inside the cell . In the solar water splitting takes place at the anode and the oxygen evolution at the cathode, hydrogen evolution. The photoactive anode consists of a semiconducting material: it has a so-called valence band, which is filled with electrons, and a so-called conduction band, which is empty. In energy supply by light negatively charged electrons from the valence band are raised into the conduction band energy. Leaving a positively charged hole. The electron-hole pairs can be separated by the developing photovoltage of each other, since the valence and conduction band bending near the surface and thus creates an electric field, move in which the negatively charged electrons and positively charged holes away from each other. The electrons are conducted through an external contact (eg, a transparent conductive oxide) from the anode to the cathode. There, two electrons are transferred into the electrolyte to reduce by two H + ions to form H2. At the anode does a reverse process. This can take up the holes in the valence band of the semiconductor electrode, electrons from the electrolyte. This is H2O to produce H + ions oxidized to O2. The electrical circuit is closed by the diffusion of H + ions from the anode to the cathode.
So far it has not been able to translate this effect into a marketable product. To allow for technological use, first, new materials are developed, the most difficult and diverse requirements. The requested materials must not only exhibit semiconductor properties to take advantage of the photoelectric effect, the incident light can. To be effective as much light into chemical energy change, must also absorb the requested material as large a proportion of the solar spectrum. These requirements are similar to those in part of materials for photovoltaic applications. Both the excited electrons in the conduction band and the holes in the valence band must diffuse to the surface of the semiconductor where the electrons for reduction reactions and oxidation reactions for the holes are available. Consequently, the material must not only absorb the photons optimally. The energy levels of the valence and conduction band must also be adapted to the desired response - the so-called band gap between the two should be neither too large nor too small. To materials with smaller band gap are often unstable and prone to corrosion. For raising the electrons to a large band gap of the sunlight is not strong enough.
As there are electrochemical processes are always at a phase boundary between an electron conductor and an ionic conductor have the desired materials for solar water splitting in an electrolyte and also during prolonged exposure to sunlight over their service life, ie over the years, his stable. To ensure this stability is a big problem. Everyone knows the effect that fade objects that are exposed for a prolonged period of sun and aging. Of course, the materials sought should also be as non-toxic, readily available and inexpensive. All these requirements are also part opposite: For example, materials that would satisfy some required properties, well, just not stable enough.
In the Materials Research Department, we are pursuing the development of new materials for solar water splitting, the following concept: In the group of Prof. Ludwig (Micro and Nano Materials) thin-film materials libraries are made (see article on page 18). These materials include a variety of well-defined libraries, different materials, eg Metal-oxynitride-systems, which are produced in a single experiment. The examined materials are deposited on a platinum thin-film electrode. The material thus produced libraries are then examined together with the groups Schuhmann and Muhler on their photocatalytic properties.
In the group of Prof. Muhler (Chemical Engineering) potential materials for water splitting are examined for their catalytic properties. Photocatalysts are used to bridge the energy barrier that is at the beginning of reaction, the desired reaction and thus to enable energy-efficient. The semiconductor material, e.g. Its titanium dioxide (TiO2), as described above must be able to absorb light, giving rise to electron-hole pairs. It is important that the formed electrons and holes can diffuse to the surface without recombining previously and thus annihilate each other. The electrons are absorbed by the co-catalyst nanoparticles (mostly metals such as rhodium, platinum, gold) at the surface and reduce the co-catalyst water to hydrogen (Fig. 4). Simultaneously oxidize the holes at the semiconductor surface water to oxygen. Accordingly, formed during the irradiation, a mixture of hydrogen and oxygen in a ratio of 2:1.
If a catalyst is suitable for water splitting is examined in a three-phase reactor: The catalytic converter is in the form of non-soluble (dispersed) solid particles, water is the liquid phase and the reactor is passed through an inert (inert) carrier gas that the gases hydrogen and oxygen transported to the analysis. The used multi-channel detector allows the continuous determination of the concentrations of the gases formed, so that the activity of the catalyst can be determined. The challenge is to find a highly active and stable system for water splitting using visible light.
The research group of Prof. Schuhmann (electroanalysis and sensors), meanwhile, is devoted to the measurement of the photocurrent at the different materials as a function of applied voltage. The higher the photocurrent, the greater is the potential of the material, to produce hydrogen. Initially, this was the thin-film materials libraries sawed and examined the resulting samples individually. In these experiments, there are often surprises, so as to solve Materials sometimes simply when they are measured in an unsuitable electrolyte. The electrolyte is necessary, since pure water has a high electrical resistance would be closed and the circuit would not be so closed. The real aim of the project is to find a material that generates hydrogen without an external electric voltage.
At the beginning of the project we developed a simple system to measure the potential dependent photocurrent for individual samples and tested it on a reference system. We chose tungsten oxide (WO3) as reference material. Of WO3 has already announced that it has right semiconducting properties and that it is still stable in aqueous solutions, the most well. However, even with WO3 still some unclear aspects which require closer examination. Thus, the influence of the WO 3 layer thickness on the photocurrent was not yet clear. At relatively large layer thicknesses, the incident light due to the increased absorption volume is utilized better. At the same time, however, also take the sheet resistance and the probability of recombination of electron-hole pairs. Both causes a decrease in the photocurrent.
It is a natural assumption that these effects combine in such a way that a maximum photocurrent results from an optimum layer thickness. To find this optimum layer thickness were determined by reactive magnetron sputtering WO 3 samples (Fig. 5) made with a layer thickness gradient. The color of each area depends not only on the material but, due to interference effects from the film thickness. Since the production parameters huge influence on the microstructure of thin films have, the layer thickness gradient were at various process gas pressures of argon-oxygen (Ar/O2)-gas mixture from 3 to 40 x 10-3 mbar separated.
At a constant voltage of 1 V show the photocurrent measurements (Fig. 6) a pronounced dependence on the thickness and the process gas pressure. Two cases can be distinguished: curves with a maximum at low film thicknesses (~ 200 nm) and curves with a maximum at large layer thicknesses (~ 500 to 600 nm). The maxima in the photocurrent at low layer thicknesses occur in the samples at low pressures (3 and 13 x 10-3 mbar) is deposited. This also confirms the conjecture that there exists an optimum layer thickness, which is for WO 3 at about 150 to 200 nm. However, the optimum layer thickness seems to turn a function of chamber pressure in the production of the material exhibit. In the samples that were sputtered at higher pressures, the photocurrent increases steadily and reaches a saturation at about 500 nm.
We could explain this behavior with the help of transmission electron microscopy (TEM). The TEM sample cross-section image of a can at 40 x 10-3 mbar layer deposited WO3-filamentary pores seen running perpendicular to the silicon wafer. These pores are formed by the increased Sputtergasdruck and effectively increase the chemically active surface which is in contact with the electrolyte. The magnification of the wetted interface between sample and electrolyte in electrochemical processes is a widely used technique to increase the chemical reaction rate.
To enable efficient characterization of materials libraries, we have automated in the group of Prof. Schuhmann the photoelectrochemical measurements. We have a so-called grid cell drops (Fig. 1 and 2) developed with an integrated light guide, which is associated with a xenon lamp. At the trial the voltage can be applied while allowing the counter electrode and reference electrode in the cell drops, the current measurement. A Teflon tip selects an area of the sample of approximately 0.5 mm in diameter, so that a large number of different material compositions can be studied on a produced by the group of Prof. Ludwig thin-film materials library (see Figure 7). Between the measuring points can be exchanged automatically with a syringe pump connected to the electrolyte. The test setup allows you to automate a complete library of materials to test their photocurrent properties. The composition range can be covered here would be reached with the previously presented at the wafer strip electrodes only under extreme time and cost of materials.
Now is the expanding research on new material systems, which will stand the quaternary system W-Fe-Ti-O in the focus of the work. So far, only the binary metal oxides of titanium dioxide (TiO2), iron (III) were oxide (Fe2O3) and tungsten oxide (WO3) were examined. TiO 2 has the best catalytic properties, whereas the band gap of the Fe2O3 comprehensive utilization of the sun allows the incoming energy. By a combination of these three compounds, we expect the discovery of a new material that both absorbs sufficient light in the visible spectrum and is stable against corrosion and has a band gap that allows the water splitting without external electrical voltage.
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