Sunlight hitting a tiny plastic particle can split water into hydrogen fuel, but the process is too inefficient to be useful—and researchers now plan to spy on the electrons’ spins inside those particles to find out why. The problem is that organic semiconductor nanoparticles, though cheap and tunable, still convert too little sunlight into hydrogen to compete with fossil fuels. Current methods cannot see exactly where energy is lost as charges move through the material. This project uses electron spin resonance (ESR) spectroscopy to track the unpaired electron spins that appear on donor and acceptor molecules after light absorption, revealing the molecular-level dynamics that determine whether a charge reaches the particle surface to make hydrogen or gets wasted. If successful, the work will identify the specific loss pathways and structural requirements for efficient photocatalysis. That knowledge could guide the rational design of better organic semiconductors, accelerating the development of cost-competitive solar hydrogen production. This is fundamental science—no immediate consumer product—but similar spin-based spectroscopy has previously unlocked breakthroughs in organic electronics. A deeper understanding of spin dynamics here could ultimately help turn sunlight into storable fuel, addressing the intermittency that limits solar power’s role in the energy grid.
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Our ability to evolve into an environmentally sustainable society crucially relies on technologies for clean and efficient energy conversion. With the sun as the world's largest renewable energy source, solar energy technology has the potential to significantly contribute to meeting the world's increasing energy demand. In fact, photovoltaic technologies aimed at converting solar energy to electricity already constitute an important fraction of the renewable energy market. Overcoming the major challenge of inherent intermittency of solar energy however also requires the development of technologies for the conversion of solar energy into chemical fuels that can be stored and distributed as needed. Photocatalytic water splitting for renewable hydrogen production offers a promising avenue for sustainably meeting the fuel-based energy demand with solar energy and a wide range of different photocatalytic materials are being developed and studied for this purpose. However, they are currently still far from achieving the conversion efficiencies required to enable real-world impact. In recent years, organic semiconductors originally developed as donors and acceptors for organic photovoltaics have been demonstrated to enable remarkably efficient hydrogen production when used in the form of heterojunction nanoparticles. The vast scope for synthetic tuneability, and experience in material optimisation from organic photovoltaics, makes these organic semiconductor heterojunction nanoparticles extremely promising for solar fuel applications. In order to fully realise their potential, an in-depth understanding of the photophysical and photochemical processes determining efficient photocatalytic conversion is now essential. This project aims to study the photocatalytic mechanism in organic semiconductor heterojunction nanoparticles exploiting an angle that has so far been neglected: the electron spin. The charges generated on donor and acceptor molecules by photoinduced charge separation and required to travel to the nanoparticle surface to participate in the hydrogen generation reaction have associated unpaired electron spins, therefore using these spins as selective probes with Electron Spin Resonance (ESR) spectroscopy provides an opportunity to get unique mechanistic insights with unprecedented detail at the molecular level. The proposed research aims to investigate the nature and dynamics of different spin states generated upon photoexcitation in organic semiconductor heterojunction nanoparticles using a range of advanced ESR techniques, and to identify their role in the photocatalytic mechanism. We will explore a series of different donor and acceptor molecules and nanomorphologies, to correlate the information on electronic structure and molecular environment of catalytically relevant states gained through ESR spectroscopy with solar-to-hydrogen conversion efficiencies. Exploiting the unique insights enabled by directly probing electron spins, we will identify limiting loss pathways and requirements for efficient photocatalytic conversion. The resulting in-depth understanding of the fundamental photophysical and photochemical processes will then directly inform the design of improved organic semiconductors and nanoparticles for photocatalytic hydrogen generation. This spectroscopy-driven synergistic approach to rational and targeted material design has the potential to enable further progress in the development of viable and cost-competitive technologies for solar-driven hydrogen production.
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