Upconverting photon harvester for photocatalysis – UPHocat

To apply these ideas to photocatalysis, it was necessary to have nanocrystals with up-conversion efficiencies close to those observed in single crystals. We have thus developed a specific synthesis method, using homemade original precursors. These precursors must be totally deprived of water, in order to obtain nanocrystals without OH groups on the surface, which have the particularity to decrease the light emission. To highlight and quantify the photon additions we have combined several light sources (laser, diodes) in the infrared and blue spectral range in order to highlight the UV emission resulting from the different absorption steps, and then the dependence of this UV emission on the light flux density. In classical upconversion systems using a single excitation wavelength, the excitation intensity must be very high to observe a UV emission. By combining several beams of different colors, we have shown that the mechanism, in principle non-linear, becomes linear with respect to each useful wavelength and that the necessary intensities are thus divided by 1000. It is the same for a broad spectrum solar lighting.

We have developed a method for the synthesis of completely anhydrous upconverted nanoparticles and have thus succeeded in obtaining nanoparticles of a few tens of nm in diameter with upconversion yields comparable to those of the bulk and of the order of 100 times higher than those observed and published with nanoparticles obtained by conventional means
We then conducted a study of the frequency conversion processes of these particles. We have shown that the classical upconversion process observed for the generation of UV photons from IR photons can be broken down into several steps. The first step allows to populate a first excited level located at low energy. The second step consists in the simultaneous absorption of 2 infrared photons which allows to directly access the UV emitting level. This second step is particularly efficient because the transition between these two levels is a transition with a large effective absorption cross section. We can therefore consider other UV generation processes in these systems by combining IR photons and visible photons.
This led us to conduct experiments using several sources of excitation. By decomposing the excitation we were able to demonstrate that we were dealing with a sequential process of photon absorption. We then demonstrated that by using a light similar to the sunlight, including all the wavelengths useful to the sequential mechanism, we can generate with these nanoparticles, UV photons in excess.

The UPHOCAT project has allowed us to validate the approach and to establish the proof of concept of combining the upconversion process with a photocatalyst. We were able, first, to synthesize upconversion nanoparticles with a record conversion efficiency for small size nanoparticles (< 20nm) and without surface passivation. Then we have demonstrated how the non-linearity of the upconversion process can be reduced by combining different excitation wavelengths and in particular by using a solar excitation.These results allow us to consider many perspectives. For example it is possible to reconsider the use of very efficient photcatalysts but requiring harder UVs.

3 publications :
(1) G. Ledoux et al. Journal of Physical Chemistry C 122 (2018) 888 10.1021/acs.jpcc.7b10113
(2) B. Purohit, et al ACS Photonics, 6 (2019) 3126 10.1021/acsphotonics.9b01151
(3) B. Purohit, et al. Materials Today Chemistry 17 (2020) 100326 10.1016/j.mtchem.2020.100326
3 invited conferences et 10 other presentations
1 highlight from the CNRS www.inp.cnrs.fr/fr/cnrsinfo/des-nanocristaux-pour-creer-des-uv-partir-du-rayonnement-solaire

The aim of the project is to design multimaterials allowing the conversion of the solar photons which are not used by classical titania-based photocatalysts into useful ones. Our approach is to use luminescent materials to convert visible and IR into UV photons via upconversion (UC).
Some teams, including ours, have tried this approach in the past few years and have indeed demonstrated that combining an upconverting material with titania leads to IR photocatalysis. However since the process is highly non-linear (up to the 5th order) the results were obtained under laser excitation and such a conversion has not been detected under a standard solar irradiation. Our project UPHocat aims at bypassing this non-linear bottleneck by modifying 2 key aspects of the process:
- combining the absorption of different wavelengths to significantly decrease the order of non linearity of the UC process and making it compatible with the excitation density of the sun.
- replace the radiative energy transfer to TiO2 via an emission and absorption of UV photons by a more efficient process of non-radiative energy transfer (ET).
These two aspects have been already demonstrated separately and we want to combine and optimize them. We have recently validated the first point. We have shown that combining an upconverting material which contains an energy transfer mediator in contact with TiO2, the photocatalytic yield under solar excitation is significantly enhance. Regarding the second point, it was demonstrated in 2011 that in a core/shell structure, it is possible to combine excitation of high energy levels (above the band gap of TiO2) by infrared light, with the transport of this excitation over several nanometers.
The titania-based system is mainly a model to demonstrate the proof of concept of photon harvesting and tuning capabilities of a core shell structure to adapt a given excitation for a given photocatalytic system. In the long term, the project will create the conditions to engineer on demand structures adapted for a given photocatalytic problem through 3 instruments:
- An experimental setup dedicated to the measure of transfer rates between different electronic system over a wide range of wavelength and times.
- A simulation tool able to predict the energy migration between materials in a core (multiple) shell structure. The spectroscopic parameters of the model will be defined on the basis of the spectroscopic measurement (previous item).
- The chemical synthetic strategies to develop those systems.
The project combines theoretical predictions, synthesis and performance analysis. It is thus interdisciplinary in nature since it brings together physicists, able to simulate and measure the energy transfer parameters in order to design the structure, with chemists able to synthesize those structures and optimize size shape and interfaces of the complex systems.