Improving photocatalytic splitting of water using biomimetic quantum dot assemblies

Increasing energy demands and worsening air and water quality call for the development of cleaner and more efficient technologies than those currently available. In a process known as photocatalysis, solar energy can be used to drive chemical reactions that can be used to split water into oxygen and hydrogen for clean energy production, or to break down harmful chemicals in liquids and gases for air and water purification. As such, efficient photocatalysis has the potential to tackle the above mentioned issues. However, the efficiency of artificial photocatalytic systems and other relevant devices remains low. This is in contrast to naturally occurring photosynthetic systems (PS), in which a very high proportion of absorbed solar energy is utilised in life-sustaining chemical reactions. The aim of the project is thus to mimic the structure and function of the PS, to develop an efficient photocatalytic platform - particularly for the water splitting reaction - built from nanophotonic elements.

The efficiency of the splitting reaction will be improved using two approaches. The first is to improve the efficiency of sunlight collection and its subsequent transfer to a key location - the equivalent of the reaction centre of the PS - where the chemical reaction occurs. The second task is to improve the yield of the reaction itself.

The light harvesting equivalent of the PS will be built from semiconductor quantum dots (QDs) of gradually increasing sizes where similar structures in 2D devices have been shown to be very efficient at light energy absorption and its transfer across the QD layers. Similar to the PS processes, this directional transfer of the energy occurs by cascaded energy transfer and can be exploited to deliver the harvested energy to the reaction centre. These particles will be arranged into concentric patterns of decreasing diameters that will resemble the structure and function of naturally occurring PS.

The reaction centre will be a metallic nanoparticle, with the metal chosen for its ability to act as a photocatalyst in a chemical reaction. The confinement of the dimension of the of the metal to the nanoscale will, in addition, allow its plasmonic response to be exploited for enhanced collection of solar energy (due to confinement effects), enhanced transfer of energy along the QD layers (by plasmon-enhanced energy transfer) and improved photocatalytic efficiency (by hot carrier generation in the metal). Rhodium, previously used in several photocatalytic devices for water splitting, also possesses plasmonics resonances which can be extended to the visible range. As such it is well suited for this application and will be the first metal to be employed. It should be noted that several methods for the fabrication for the light-harvesting QD-based antenna have already been tested. Although not yet optimised, these tests show promising results and provide a strong starting point for the proposed project.

This project will use combinatory top-down and bottom-up fabrication processes to construct a device with a unique bio-inspired design composed from semiconducting QDs and a metallic nanoparticle as well. Designed structures can find direct application in photocatalysis, and other light-harvesting applications can also be explored. Therefore, the work to be conducted fits well within the EPSRC remit, primarily in the priority research areas of Catalysis, Photonics and Plasmonics.