Photoelectrochemical CO2 conversion with tunable semiconductor nanostructures

• Applicants: Dr. Lutz Geelhaar; Matthew Mayer, Ph.D.
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2020 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 442704684

• Final Report Year: 2025

Final Report Abstract

Chemical conversion reactions can be driven by energy from sunlight using photoelectrochemistry. The active component is a photoelectrode made of a semiconductor material. The electrode’s electronic structure and morphology can influence reactivity and selectivity in ways that are difficult to quantify and deconvolute. To study the effects of these parameters on the electrochemical conversion of carbon dioxide, we developed electrodes based on nanowire ensembles of the material (In,Ga)N, which we use as a model system allowing for systematic variation of electronic structure and morphology. The first goal was to develop a synthesis procedure to produce (In,Ga)N nanowire ensembles with tunable morphology, specifically the length and diameter of the nanowires as well as the spacing between them. We optimized a method based on metal nanoparticle dewetting templates, followed by top-down etching, which is capable of producing nanostructured electrodes with tunable, uniform structure over significant electrode sizes (>1 cm²), providing simplicity and scalability in comparison with more sophisticated approaches such as e-beam lithography. When we applied this method to thick (In,Ga)N layers we discovered that their internal quantum efficiency can be substantially improved by annealing. Furthermore, we showed that light absorption is enhanced in nanowire arrays compared to planar layers. The next goal was to characterize the semiconductor using electrochemistry, including the use of the Mott-Schottky approach to determine carrier concentrations and electronic band structure. This approach takes advantage of the semiconductor-liquid junction, enabling the characterization of nanostructures which is difficult to accomplish using standard solid-solid electric contact methods (e.g. Hall effect). However, we found evidence that aqueous electrolytes do not fully wet the nanowire sidewalls (and the space between nanowires) which led to erroneous results. By studying the effects of different types of non-aqueous electrolytes, we could overcome this limitation and obtain conclusive analysis on a range of electrode morphologies. Finally, we aimed to investigate the activity of (In,Ga)N electrodes for the electrochemical conversion of carbon dioxide to value-added chemicals. By making the material n-type or ptype, we compared the activity of dark electrodes and illuminated photoelectrodes (respectively), observing varied product selectivity depending on the electronic configuration and morphology. Uniquely, we observed the formation of acetate as product on bare GaN electrodes, a valuable product previously only observed from copper-based electrodes in electrochemical CO2 conversion.

Publications

• Controlling the dimensions of top-down GaN nanowire ensembles via self-assembled metal islands, Poster presentation, Nanowire Week 2022, Chamonix, France, 25- 29 April 2022.
  J. Kang, M. Oliva, T. Auzelle, A. Tahraoui, M. Gómez Ruiz, J. Lähnemann, O. Brandt & L. Geelhaar
  
• Phenomena affecting the carbon dioxide reduction activity of semiconductor photoelectrodes, poster presentation, 23rd International Conference on Photochemical Conversion and Storage of Solar Energy (IPS23), Lausanne, Switzerland, 2-5 Aug. 2022.
  M. Mayer, M. Barzgar Vishlaghi, J. Kang, P. Bogdanoff, L. Geelhaar & O. Brandt
  
• Tuning the selectivity of GaN nanowire photocathodes for CO2 reduction by modifying the morphology and co-catalysis, poster presentation, GDCh Electrochemistry 2022, Berlin, Germany, 27-30 Sept. 2022.
  M. Barzgar Vishlaghi, J. Kang, M. Mayer, P. Bogdanoff, L. Geelhaar & O. Brandt
  
• Applying Line-of-Sight quadrupole mass spectrometry to monitor in-situ changes in In incorporation during epitaxy of thick (In,Ga)N films, Poster presentation, International Workshop on Nitride Semiconductors 2022, Berlin, Germany, 9-14 October 2022
  J. Kang, T. Auzelle, M. Oliva, M. Gómez Ruiz, A. Campbell, P. John, D. V. Dinh, J. Lähnemann, O. Brandt & L. Geelhaar
  
• Combining metal dewetting and digital etching for the fabrication of uniform large-area top-down GaN nanowire arrays with independently tunable diameter and spacing, Oral presentation, Nanowire Week 2023, Atlanta, USA, 9-13 October 2023.
  J. Kang, R. M. Jose, M. Oliva, T. Auzelle, O. Brandt & L. Geelhaar
  
• Electrocatalytic conversion of carbon dioxide: understanding dynamic behaviors of catalysts and cells, plenary lecture, SALSA Make & Measure 2023:Interfaces symposium, Berlin, 13 - 15 Sept. 2023.
  M. Mayer
  
• Top-down (In,Ga)N nanowires fabricated from molecular beam epitaxial layers with high structural perfection, Poster presentation, 14th International Conference on Nitride Semiconductors 2023, Fukuoka, Japan, 12-17 November 2023.
  J. Kang, M. Gómez Ruiz, A. Trampert, D. V. Dinh, A. Campbell, M. Oliva, P. John, J. Lähnemann, T. Auzelle, O. Brandt & L. Geelhaar
  
• Tuning the selectivity of GaN nanowire photocathodes for CO2 reduction by modifying the morphology and co-catalysis, poster presentation, NanoGe MATSUS fall meeting 2023, Torremolinos, Spain, 18 July 2023.
  M. Barzgar Vishlaghi, J. Kang, M. Mayer, P. Bogdanoff, L. Geelhaar & O. Brandt
  
• Scalable top-down fabrication of (In,Ga)N nanowires for epitaxial layers, Oral presentation, Compound Semiconductor Week 2024, Lund, Sweden, 3- 6 Jun 2024.
  J. Kang, A. Campbell, M. Gómez Ruiz, D. V. Dinh, P. John, T. Auzelle, O. Brandt, J. Lähnemann & L. Geelhaar
  
• Strongly enhanced internal quantum efficiency from thermal annealing of thick (In,Ga)N layers grown by molecular beam epitaxy, Poster presentation, 12th International Workshop on Nitride Semiconductors 2024, Hawaii, USA, 3-8 November 2024.
  J. Kang, A. Campbell, J. Lähnemann, L. Geelhaar & O. Brandt
  
• Uniform large-area surface patterning achieved by metal dewetting for the top-down fabrication of GaN nanowire ensembles. Nanotechnology, 35(37), 375301.
  Kang, Jingxuan; Jose, Rose-Mary; Oliva, Miriam; Auzelle, Thomas; Ruiz, Mikel Gómez; Tahraoui, Abbes; Lähnemann, Jonas; Brandt, Oliver & Geelhaar, Lutz
  
• Drastic enhancement of the internal quantum efficiency of thick (In,Ga)N layers by thermal annealing, Poster presentation, 22nd Euro MBE Workshop, Auron, France, 9-13 March 2025.
  J. Kang, A. Campbell, J. Lähnemann, L. Geelhaar & O. Brandt
  
• Growth of compositionally uniform InxGa1−xN layers with low relaxation degree on GaN by molecular beam epitaxy. Journal of Physics D: Applied Physics, 58(14), 14LT01.
  Kang, Jingxuan; Ruiz, Mikel Gómez; Dinh, Duc Van; Campbell, Aidan F.; John, Philipp; Auzelle, Thomas; Trampert, Achim; Lähnemann, Jonas; Brandt, Oliver & Geelhaar, Lutz