Photocatalysis is applied for the activation of a chemical reaction through absorption of light aiming for the production of hydrogen directly from water and sunlight, as well as to the removal of pharmaceutical and other organic pollutants from waste water.

In spite of these efforts over more than 4 decades, the efficiency of photocatalytic and even more of photosynthetic converters remains poor, or their practical use is limited by low stability or high cost. Many oxides only absorb in the UV part of the solar spectrum, others suffer from low carrier mobility. So far all devices are characterized by rather low quantum efficiencies typically in the percent range as they do not provide electronic asymmetry for ‘vectorial’ separation of photo-generated electrons and holes. There is also a strong need to control the architecture and the morphology of the structured materials in order to adopt the transport pathways to the diffusion lengths for charge carriers. Controlled engineering of interfaces and surfaces including the application of co-catalysts is also needed to control surface reactions (multi electron transfer vs. recombination).

The objective of the present project is to overcome the issues described above, by using a ‘knowledge-based design’ approach: In our view, the key points are (i) the suitable overall ‘tuning’ of the semiconducting and catalytic materials and (ii) using hetero-structures of tandem or ‘Janus’ type for separation of charge carriers.

In order to develop efficient and reusable photocatalysts for application in water treatment and H2-production, there is a great need to tailor the chemical, structural and electronic properties of material and enhance its photocatalytic activity. In order to tackle the limitations described above and to improve the efficiency of photocatalytical devices, the following innovations will be implemented in the framework of this PhD projects:

  • design of heterogeneous structures based on (i) tandem particles and (ii) ‘Janus’ structures,  for efficient charge carrier separation (Fig. 1)
  • identification of wide-band light absorbers with reduced electronic bandgap of 2-3 eV (doped CuNbO3, BiVO3, CoVO3, and related compounds) based on ternary metals oxides containing only M-ions with d0 d10 or d(t2g)6 electron configuration
  • identification of semiconductor tandem pairs which bridge the energy difference between the redox potentials of the catalytic reaction
  • adaption of the semiconductor diffusion length to the absorption length and the particle morphology
  • combination of adapted hetero-contacts including also metallic co-catalysts based for example on heterostructures of BiVO3 or CuNbO3

Implementation of such advanced heterostructures will be based on the development of specific processing routes to control their size, shape and heterocontacts.In order to ensure the success of the project, the PhD candidate will:

(1) Make an exhaustive review of the state-of-the-art related to heterostructured metal oxides of low bandgaps (e.g. BiVO3, CuNbO3, CoVO3)  semiconductors and select the most suitable candidates for the devellopment process ;

(2) Develop adequate protocols and start the synthesis of the morphology-controlled heterostructures using wet chemical and vacuum based techniques;

3) The characterisation of the materials covers a wide, interdisciplinary range of structural, chemical, electrical and performance properties:

  • chemical and structural properties: elemental microanalysis, infrared and Raman spectroscopy, electron microscopy, X-ray diffraction (UB, TUD)
  • opto-electronic properties diffuse-reflectance UV-visible spectroscopy, (photo)electric measurements, electronic bulk and surface structures (electronic DOS, band alignment, interface dipole potential, and work function will be addressed by surface science experiments using photoelectron spectroscopy (XPS, UPS) including synchrotron techniques) (TUD)
  • electrochemical and photocatalytic properties: UV-visible spectroscopic measurement of dye photodegradation, photodegradation (UB), ATR-FTIR investigation of adsorption and photodegradation of pollutants,
  • device efficiency will be tested in industry labs: UV photoreactors for the design of solar water treatment and photocatalytic efficiencies in hydrogen production (BASF/Evonik)

The research will involve two Academic research groups with a wide interdisciplinary expertise (TUDA surface science division and UB Molecular Science) fully exploiting the complementary know-how of the partners in synthesis and characterization.

Moreover, the research will be performed in a close connections with the industrial partner BASF, who is already active in the photocatalysis for the treatment of water. Depending on the outcome of this collaboration the above-mentioned routes will be adapted to the practical issues and to the industrial’s expectations.

The control of the architecture of the heterostructured metal oxides materials through templating processes will bring a significant and innovative contribution to the field of chemistry of materials and will open new pathways in many other research fields. This will broaden the scientific background of the candidate.

Project Partners and their roles

Department of Materials Science, Technical University (TU) Darmstadt, Germany: thin film deposition of oxides, manufacturing of heterocontacts, chemical, structural, and electronic characterization

Institute for Molecular Science, University Bordeaux, France: Synthesis of defined nanoparticles and heterostructures by wet chemical techniques, photocatalytic evaluation

Industry Partner: BASF, Germany: Test of performance, assessment of industrial applicability

Host Country (employment): Germany

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