Univ. Paris-Saclay

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Graphite electrodes patterning decorated with hematite nanorods for solar water splitting liquid microcell fabrication

Contact: STANESCU Dana, , dana.stanescu@cea.fr, +33 1 69 08 75 48
Liquid three-electrodes micron-sized electrochemical cells are required to realize operando microscopy studies of photoelectrodes during the solar water splitting reactions. For that purpose, we propose an internship on the development of patterned graphite electrodes decorated with hematite nanorods deposited by aqueous chemical growth.
Possibility of continuation in PhD: Oui
Deadline for application:11/04/2020

Full description:
Renewable energy sources, only 20% of the present mankind’s global energy consumption, will constitute a reliable answer to the energy demand if they reduce carbon dioxide emissions into the atmosphere. Hydrogen appears to be an efficient and sustainable energy carrier since its specific energy is around 120 MJ/Kg, higher than in hydrocarbons (46 MJ/Kg) or lithium air batteries (8 MJ/Kg). Nowadays, more than 95% of total hydrogen production (ca. 50 millions of tonnes per year) depends on the fossil fuels industry, mainly steam methane reforming. Therefore, the carbon impact is huge. Hydrogen can also be produced by water electrolysis. A significant energy input is however necessary to produce the voltage bias necessary to initiate redox reaction (1.23 V). A novel idea, inspired by photosynthesis, is the water photoelectrolysis (or solar water splitting) where sunlight is used to reduce the voltage bias necessary to split hydrogen from water molecule [1].
In most cases, a photoelectrochemical cell consists of an n-type semiconductor photoanode (the working electrode) associated with a conventional metal cathode (the counter electrode) and a reference electrode, the macroscopic parameter that confirms hydrogen production being the generated photocurrent [2]. For this internship, we propose to develop patterned micron-sized working electrodes on SiN, as nanostructured graphite films decorated with hematite nanorods. The optimization of micron-sized working electrodes fabrication process belongs to long-time study, the fabrication of micron-sized photoelectrochemical cells defined between two sealed SiN membranes, for X-rays microscopy. These cells will allow to realize operando microscopy studies (i.e. in the presence of an electrochemical reaction and light) using the STXM (Scanning Transmission X-rays Microscope) at HERMES beamline at the SOLEIL Synchrotron [3]. The graphite electrodes will be obtained by pyrolysed photoresist films (PPF) and evaporation, comparing their physico-chemical properties in order to define the best approach. Parameters like pyrolysis temperature and time, photoresist thickness or spin-coating parameters will be varied in order to optimize graphite adherence to the SiN layer and its electrical conductivity. Hematite nanorods will be deposited by using a simple and versatile method, the Aqueous Chemical Growth (ACG) [4] the parameters for an optimum growth of the hematite nanorods are already known from precedent studies.
The intern will have several missions: a) the elaboration of a protocol insuring reproducible elaboration of patterned graphite electrodes decorated with hematite nanorods on SiN layers and SiN membranes; b) the characterization of the efficiency of these electrodes during the photoelectrolysis by photocurrent measurements; c) the measurements of the photoanodes deposited on SiN membranes with the STXM and the characterization of the chemical/structural/electronic properties of hematite nanorods decorating the graphite electrodes. Combined laboratory (lithography, chemical growth, photocurrent measurements, SEM - Scanning Electron Microscopy) and synchrotron soft X-rays microscopies (STXM - Scanning Transmission X-ray Microscopy and XPEEM - X-ray PhotoEmission Electron Microscopy) techniques will be used to realize this study.

resp. SPEC: Dana Stanescu (dana.stanescu@cea.fr)
resp. SOLEIL: Stefan Stanescu (stefan.stanescu@synchrotron-soleil.fr)

[1] A. Fujishima and K. Honda, Nature, 1972, 238, 37, 10.1038/238037a0
[2] M. Rioult, H. Magnan, D. Stanescu, A. Barbier, J.Phys.Chem.C, 2014, 118 (6), pp. 3007–3014, 10.1021/jp500290j
[3] R. Belkhou, S. Stanescu, S. Swaraj, A. Besson, M. Ledoux, M. Hajlaoui, D. Dalle, J. Synchrotron Radiat., 2015, 22 (4): 968-979, 10.1107/S1600577515007778
[4] L. Vayssieres, Int. J. Nanotechnol. 2004, 1, 10.1504/IJNT.2004.003728; L. Vayssieres, Appl. Phys. A, 2007, 89, 1–8, 10.1007/s00339-007-4039-0
Technics/methods used during the internship:
Lithography, chemical growth, photocurrent measurements, SEM, STXM, XPEEM

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