Light trapping in hematite-coated transparent particles for solar fuel generation

Hematite (alpha-Fe2O3), due to its abundance and low-cost, is an attractive compound for photoelectrochemical splitting of water to produce hydrogen. However, one major obstacle preventing hematite from achieving the target efficiencies comprises its significantly smaller minority carrier transport distance relative to its optical absorption depth in the visible part of the optical spectrum. Here, we combine host-guest and Mie resonance concepts to achieve significant optical absorption in extremely thin layers of hematite. We propose and theoretically evaluate transparent particles coated with an extremely thin hematite shell as building blocks for hematite photoanodes. By full-field optical simulations we found out that maximal optical absorption is achieved when the particle supports two to three Mie resonance modes above the hematite optical absorption edge. Optical absorption efficiencies integrated over the air mass 1.5 global (AM1.5G) irradiance spectrum, AM1.5, reach more than 2 mA cm(-2) within a 10 nm thick hematite shell of a particle with optimal dimensions and AM1.5 comes close to 5 mA cm(-2) in a 25 nm thick hematite shell. Furthermore, we evaluate the performance when the particles are part of an array or stacked atop each other. The concept introduced here could be useful for improving optical absorption in semiconductors with extremely short carrier transport distances.

Photoelectrochemical properties of sensitized thin films

Mohamad Hojeij

In recent years, the fabrication of novel ultrathin film systems featuring multi-functionalities has been extensively studied due to their potential applications in photonics, sensors and solar cells. In a general sense, the primary goal of the present thesis was to develop dye-sensitized ultrathin film-modified electrodes based on the layer-by-layer methodology for future light energy conversion applications. In this work, we show that stable and uniform ultrathin multilayer films were successfully assembled by the electrostatic adsorption of alternating monolayer of poly-L-lysine (pLys) and poly-L-glutamic acid onto modified gold electrodes. The multilayer films become electroactive when ferri/ferrocyanide ions are loaded into the film and behave as photoactive when sensitized by Cadmium selenide (CdSe) quantum dots. Photocurrent responses originating from the CdSe sensitized ultrathin multilayer film are observed and were dependent on the applied potential, the thickness of the film and the presence of quenchers in the organic phase. From the obtained photocurrent and the kinetic analysis, two different regimes were observed for the potential distribution in the film: (i) the strong potential dependence regime observed in case of thin films where the system reacts as a modified electrode and (ii) the weak potential dependence regime observed in case of thick films where the potential drop occurs mainly in the first 5 layers and the system behave as supported Two Immiscible Electrolyte Solutions (ITIES). Thereafter, three-dimensional assembly of nanoparticles and pLys was found to be an efficient way to increase the photocurrent efficiency of the system. Large photocathodic and photoanodic currents were recorded in the case of CdSe and CdSe/CdS sensitized films, respectively. The kinetic analysis of photoinduced processes and coupled reactions revealed that the multilayer films behave as one electronic unit, i.e. homogeneous nanoporous semiconducting photoelectrodes. When sensitizing the multilayer film by porphyrin molecules instead of CdSe quantum dots, mainly cathodic photocurrent is observed and is dependent on the applied potential and the thickness of the film. Depending its thickness, the films vary from that of a modified electrode to that of a supported liquid film. Under illumination, the maximum photocurrent responses were reached only after five bilayers and the photocurrent efficiency was weaker than in case of CdSe nanoparticles. Furthermore, another way to increase the photocurrent efficiency is by increasing the specific surface area of the substrate. Reticulated Vitreous Carbon (RVC) foam with properties of high porosity was used as the substrate and the photocurrent efficiency is increased by an order of magnitude in comparison with a planar electrode. The photostability of CdSe nanoparticles is studied in different solvent and under different environment. In presence of light and oxygen, the photo-oxidation of CdSe particles occurs. When the particles are synthesized in water and present lattice defects, a luminescence photo-enhancement is observed due to the photo-oxidation. On the other hand, the photo-oxidation of organic soluble CdSe particles with high quantum yield and with no structure defects is translated by a photobleaching. Consequently, in both cases, the size of particles is decreased and a large absorption and emission blue shift is observed. The size decreases and the blue-shift can be precisely controlled by controlling the irradiation time by a pulsed nano-laser. The presence of ZnS shell around the particles can decrease the photo-oxidation processes. Moreover, adding a sacrificial holes acceptor (n-butylamine) is shown to be an efficient way to prevent the photo-oxidation of the particles even in presence of oxygen. The photophysical properties of cadmium telluride (CdTe) and gold particles are studied at liquid|liquid interfaces. First, time-resolved fluorescence of CdTe nanoparticles adsorbed at the liquid/liquid interface were probed by Time Correlated Single Photon Counting (TCSPC), which was compared with CdTe nanoparticles originally synthesized in aqueous solution and those transferred from the aqueous to organic solvents by using tetraoctylammonium bromide (TOAB) as a phase transfer reagent. Finally, the formation of metal-like liquid films using gold nanoparticles at liquid|liquid interfaces was successfully obtained. The optical properties of the meltal-like liquid film are studied by Surface Plasmon Resonance (SPR) technique at liquid|liquid interface. General experiments and simulations were carried out in order to determine the best parameters to optimize the SPR technique at liquid|liquid interface.

EPFL2009

Optical Excitation of a Nanoparticle Cu/p-NiO Photocathode Improves Reaction Selectivity for CO2 Reduction in Aqueous Electrolytes

Xueqian Li, Giulia Tagliabue

We report the light-induced modification of catalytic selectivity for photoelectrochemical CO2 reduction in aqueous media using copper (Cu) nanoparticles dispersed onto p-type nickel oxide (p-NiO) photocathodes. Optical excitation of Cu nanoparticles generates hot electrons available for driving CO2 reduction on the Cu surface, while charge separation is accomplished by hot-hole injection from the Cu nanoparticles into the underlying p-NiO support. Photoelectrochemical studies demonstrate that optical excitation of plasmonic Cu/p-NiO photocathodes imparts increased selectivity for CO2 reduction over hydrogen evolution in aqueous electrolytes. Specifically, we observed that plasmon-driven CO2 reduction increased the production of carbon monoxide and formate, while simultaneously reducing the evolution of hydrogen. Our results demonstrate an optical route toward steering the selectivity of artificial photosynthetic systems with plasmon-driven photocathodes for photoelectrochemical CO2 reduction in aqueous media.

2020

Photoelectrochemical properties of sensitized thin films

Mohamad Hojeij

EPFL2009