Nanowire based electrodes engineering for photocatalysis – BEEP

The wetting and absorption models are carried out by phase field and FDTD method respectively.
The fabrication of the electrodes combines several experimental techniques such as molecular beam epitaxy and photodeposition.
Characterizations by electron microscopy, photoluminescence, photo-current measurements come to qualify the quality of the electrodes.

A. Wetting engineering: the electrolyte wetting on the nanowire array has been investigated using a phase field model. The model has been developed in the framework of the project, leading to a better understanding of the physical mechanism involved during the photoeletrolysis process and predictive model on the wetting behaviour.
B. Photon engineering: model for the light absorption have been obtained using finite time domain difference and clearly demonstrate the superior light absorption of nanowires compared to their equivalent planar counterpart.
C. Growth of GaAs and GaPnanowires: the quality of GaAs nanowires has been improved thanks to a better understanding of the growth mechanism leading to high purity of the crystal structure (zinc blende vs wurtzite phase). To achieve such control of the crystal phase we developed a model for the vapor liquid solid growth combined with in situ (electron diffraction) and ex situ (electron microscopy and photoluminescence) experimental techniques.
D. Doping of III-V nanowires: N doping was studied in details for GaP nanowires. Nanowires were doped with Si using Au as a catalyst. As Si exhibit an amphoteric behaviour and the dopant incorporation can be difficult we have install a Te evaporation cell in the MBE reactor in order to be sure of the N doping. This dopant is expected to be efficient also on III-V nanowires grown by self-assisted method.
E. Oxide shell:the TiO2 shell has been grown using an ex situ Atomic Layer Deposition. The photoanode lifetime was significantly improved (keeping good electrode efficiency) by optimizing the TiO2 shell thickness.
F. Catalyst deposition and photoelectrochemistry: Photodeposition of catalyst have been performed and significant improvement of the photoanode efficiency were obtained.

Increase the efficiency and lifetime of the photoelectrode (cathode & anode).

1. “Wurtzite phase control for self-assisted GaAs nanowires grown by molecular beam epitaxy”, T.Dursap, M. Vettori, C. Botella, P. Regreny, N. Blanchard, M. Gendry, N. Chauvin, M. Bugnet, A. Danescu, J. Penuelas, Submitted (2020) arxiv.org/abs/2007.16047

2. “Approximation of surface diffusion flow: a second order variational Cahn-Hilliard model with degenerate mobilities”, E. Bretin, S. Masnou, A. Sengers, G. Terii, Submitted (2020)
arxiv.org/abs/2007.03793

3. “Crystal phase engineering of self-catalyzed GaAs nanowires using a RHEED diagram” T. Dursap, M. Vettori, A. Danescu, C. Botella, P. Regreny, G. Patriarche, M. Gendry, J. Penuelas, Nanoscale Advances 2, 2127 (2020)
arxiv.org/abs/1912.06502

According to the Paris Agreement adopted in 2015 by 195 countries, limiting greenhouse gas emissions is a worldwide priority in order to significantly reduce the global warming. To make this objective a reality, a tremendous effort for developing alternative energies to replace fossil fuels is pursued. Solar fuel cells are one of the most promising approaches of alternative energy production. The production of these solar fuels mimics photosynthesis, and consists in harvesting the solar energy with a photosensitive unit, such as a semiconductor, and using a catalyst to store this energy into chemical bonds either through water splitting for renewable hydrogen production, or through CO2 reduction for hydrocarbon production.
Within this context, III-V nanowires (NWs) based photoelectrodes are particularly attractive for many reasons: 1) due to a high surface/volume ratio NWs offer more catalytic sites to induce photoelectrochemical reactions compared to their planar counterparts, and thus appear to be promising since the solar fuel is produced at the photoelectrode surfaces. Moreover, compared to thin film devices, much less material could be needed which is of major interest to reduce the manufacturing cost. 2) III-V NWs can be easily n- or p-doped and/or stacked in order to engineer the band structure for efficient charge separation and collection. The band bending that naturally occurs at the NW surface is advantageous for PEC water splitting wherein oxidation and reduction reactions take place at different electrodes. 3) Heterostructured III-V NWs can be grown on low-cost Si substrates, allowing the fabrication of monolithic PECs. 4) Light absorption can be significantly improved in NW arrays, particularly from concepts of photonic crystals. 5) The electrolyte wetting on the NW array could be controlled to a large extent, depending on the NW density and morphology.
The main objective of the BEEP project is to fabricate and investigate the full applicability of semiconducting NWs as photoelectrode for water splitting, from growth, characterization and device fabrication to operation.
The project is organized in three objectives: 1) Photonic & wetting engineering. 2) Studying the Ageing mechanism & engineering the photoelectrode interface. 3) Fabrication of a photoanode / photocathode tandem device. These objectives will be realized by a consortium (INL, SPEC, LCBM, MATEIS, ICJ) having complementary expertise in mathematics, physics and chemistry and recognized experience. 4 tasks have been identified: Management (task 0), Design (task 1), Fabrication (task 2) and Characterization (task 3).

The impact of BEEP will be twofold: firstly, we will show the full potentiality of semiconducting nanowire-based electrodes for water splitting with the objective to obtain the highest efficiency without photoelectrode degradation. Secondly by demonstrating a PEC working without any external energy we will prove the viability of our approach using low cost materials, both as semiconductors (Si substrate) and catalysts.