Nanoparticles based on organic semiconductors with long-lasting reactive charges

Nanoparticles based on organic semiconductors with long-lasting reactive charges

Credit: Kosco et al.

Due to their advantageous properties, organic semiconductors could be very promising photocatalysts for producing solar fuels. In fact, these materials can be synthesized to absorb visible light, while simultaneously maintaining desirable energy levels to drive various processes. While photocatalysts based on organic semiconductors have achieved promising results, understanding of the physics behind their operation is still relatively limited.

Researchers from King Abdullah University of Science and Technology (KAUST), Imperial College London and the University of Oxford have attempted to develop organic semiconductor-based photocatalysts capable of efficiently harvesting light. solar energy and can thus be used to produce hydrogen in a more sustainable way. Their most recent article, published in natural energyshows that heterojunction organic semiconductor nanoparticles can generate remarkably durable reactive charges, so they could effectively drive the sacrificial evolution of hydrogen.

“We chose to use organic semiconductors to fabricate our photocatalysts because their band gaps can be synthesized to absorb strongly in the visible spectrum,” said Jan Kosco, one of the researchers who conducted the study. TechXplore. “All other things being equal, the more light there is photocatalyst absorbs, the more efficiently it can convert solar energy into hydrogen.”

The most stable photocatalysts made from inorganic semiconductors, such as TiO2 and SrTiO3 absorb almost exclusively UV wavelengths and have little or no activity under visible light. This can be problematic, as less than 5% of solar energy is transported via UV wavelengths. This basically limits the efficiency of these inorganic semiconductor based photocatalysts to less than 5%.

Kosco and his colleagues set out to explore the potential of organic semiconductors to drive hydrogen evolution and the photophysics underlying their operation. Their study is based on their previous work on bulk heterojunction organic semiconductor nanoparticle photocatalysts.

“It is important to develop photocatalysts active in a wide range of UV-visible-infrared wavelengths to maximize sunlight absorption,” Kosco explained. “We were first surprised when we saw that the PM6:PCBM nanoparticles showed a higher H2 rate of evolution than PM6:Y6 NPs.”

When they began conducting their experiments, Kosco and his colleagues expected to find that PM6:Y6 nanoparticles were more active than PM6:PCBM nanoparticles because Y6 is known to absorb much more of the solar spectrum than PCBM. However, when they measured the external quantum efficiencies (EQEs) of PM6:Y6 and PM6:PCBM nanoparticles, they found that the latter are able to convert a greater fraction of the solar energy they absorb the charges which produce hydrogen.

“In other words, we found that PM6:PCBM nanoparticles have higher EQEs,” Kosco said. “This allows them to produce more hydrogen than PM6:Y6 nanoparticles, even though they absorb less light.”

After measuring the nanoparticle EQS, Kosco and his colleagues probed them using a series of ultrafast and operando spectroscopic methods. Their hope was to uncover the mechanisms underlying the higher EQEs they observed in PM6:PCBM nanoparticles.

“We used these techniques to track the photophysical processes responsible for converting photons into catalytically active charges on timescales ranging from picoseconds to seconds,” Kosco said. “These methods revealed that PM6:PCBM nanoparticles are more efficient at converting absorbed photons into long-lived catalytically active fillers, and we believe this is the main reason for their high H.2 production efficiency.”

Kosco and his colleagues also imaged the nanoparticles using cryogenic transmission electron microscopy (Cryo-TEM). It is an advanced electron microscopy technique that allows researchers to quickly freeze a sample and capture images of it under cryogenic conditions, thereby preserving its native structure. Using Cryo-TEM, the team was able to generate images of the nanoparticles that clearly captured their internal morphology with nanoscale resolution, while suspended in vitrified water.

“We expected charges to form in our nanoparticles, due to the type II heterojunction present inside,” Kosco explained. “However, we did not expect the charges to ‘live’ so long inside the nanoparticles. The photogenerated charges typically recombine on the microsecond scale, but we have observed charges in our nanoparticles even a few seconds after photoexcitation.”

The lifetime of the photogenerated charges that the researchers observed in their experiments is extremely long compared to that typically exhibited by organic semiconductors. This remarkably long lifespan could be the main factor behind their high performance, as it extends the time it takes for loads to participate in Redox reactions on the surface of nanoparticles which are known to be relatively slow moving.

“We hope that this new class of highly active organic semiconductor photocatalysts will accelerate the development of efficient visible-light active hydrogen evolution photocatalysts for global water splitting Z schemes,” Kosco said. “In a Z-scheme of water splitting, a hydrogen-evolving photocatalyst is coupled to an oxygen-evolving photocatalyst, and together the two photocatalysts drive the overall splitting of water into H2 and O2. This is analogous to how photosystem 1 and photosystem 2 convert sunlight into chemical energy during photosynthesis in green plants.”

In the future, the promising photocatalysts identified by Kosco and his colleagues could be used to create new, more efficient solar fuel technologies. While researchers have so far primarily assessed their potential to drive the hydrogen evolution reaction, to be applied in real-world settings, this reaction would need to be coupled with oxygen evolution processes, to split water in H2 and O2.

“We are now continuing to develop photocatalysts for H2 evolution, O2 evolution and CO2 reduction to synthetic fuels,” added Kosco.


More efficient photocatalysts could unlock the potential of solar energy


More information:
Jan Kosco et al, Generation of long-lived charges in organic semiconductor heterojunction nanoparticles for efficient photocatalytic hydrogen evolution, natural energy (2022). DOI: 10.1038/s41560-022-00990-2

Jan Kosco et al, Photocatalytic enhanced hydrogen evolution from organic semiconductor heterojunction nanoparticles, Natural materials (2020). DOI: 10.1038/s41563-019-0591-1

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Quote: Semiconductor-Based Organic Nanoparticles with Long-Lasting Reactive Loads (April 11, 2022) Retrieved April 11, 2022 from https://techxplore.com/news/2022-04-semiconductor-based-nanoparticles-long-lasting- reactive.html

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