Elucidating the photochemistry of inorganic nanostructures

Nanostructures are systems with one or more dimensions (particle-size, rod diameter, film thickness, pore-size) ranging from 1E-10 metres, the scale of atomic bonds, to 1E-7, the size of a typical biological virus. Most of the time such nanostructures are in their low energy ground state, but when they absorb light some electrons from the ground state can be excited to form a so-called excited state, which lies higher in energy. Excited states, however, are not stable and typically in 1E-15 to 1E-6 seconds the excited electrons will fall back to the ground state, filling the holes that they left upon excitation.The relaxation of an excited state can follow different paths: Firstly, the nanostructure can reemit light in a process called photoluminescence (PL). Secondly, the nanostructure can undergo a chemical reaction which results in a permanent rearrangement of its atoms. Thirdly, the excited electrons and/or holes can be transferred to a molecule adsorbed on the nanostructure and fourthly the nanostructure can heat up. These different relaxation paths have major practical implications. PL in inorganic nanostructures is successfully exploited in applications such as lasers and energy efficient solid state lighting. The transfer of excited electrons or holes to an adsorbed molecule is a critical step in both heterogeneous photocatalysis and in dye-sensitised solar cells. Finally, the structural changes induced by light impose a limit to the service life of solar cells and other devices that are routinely exposed to direct intense sunlight. Now, in spite of the enormous practical importance of the applications discussed above, fundamental knowledge of the different excited state relaxation paths is limited. For example the final structure of the excited state is often unknown. This knowledge gap arises because the inherent disorder of nanostructures and the short lifetimes of excited states make it difficult to characterise the relevant processes in experiment. As a result progress in photoactive materials development for these applications has been mostly through trial and error. The aim of my fellowship is to close the knowledge gap by using theoretical methods to generate microscopic insight into the photophysics and photochemistry of inorganic nanostructures. This will allow me to answer important practical questions such as on which part of the nanostructures the excited electrons are likely to get trapped, which material properties determine what relaxation path is dominant and how these could be successfully tuned experimentally, thus replacing serendipity by insight.In practice this means I will employ a theoretical method named time-dependent density functional theory (TD-DFT) to probe the geometry and chemical nature of the relaxed excited state in different nanostructures. This method, when properly validated, gives accurate results whilst at the same time being computationally cheap enough to efficiently study the systems of interest. Furthermore, where possible I will compare the obtained results, for example predicted PL spectra, to those obtained by my experimental collaborators. In a first step, I will study stoichiometric nanostructures for a range of sizes, shapes and compositions. Building on this work, I will then switch my attention to the fate of excited states in nanostructures that miss some atoms, are doped with foreign atoms or have molecules adsorbed on their surface. Study of these latter systems is especially important for understanding photocatalysis, an application that has to date not been studied with theoretical methods such as TD-DFT. Finally, as a latter part of the proposed work, I will apply the developed theoretical approach to realistic photocatalytic systems studied by my collaborators in the laboratory. Pooling our experimental and theoretical results will allow us to find, for example, new and improved water splitting catalysts for renewable hydrogen production.

Elucidating the photochemistry of inorganic nanostructures and thus gaining a better understanding of the microscopic processes that underlie the use of photoactive materials in a large number of practical and commercial applications will benefit the UK through impact on UK industry, health, energy security and the environment. The photochemistry of inorganic nanostructures is successfully exploited in a range of commercial products and processes including solar cells, solid-state lighting and photocatalysis. Several UK based companies have a strong market position in this field of industrial endeavour, e.g. Pilkington with their Activ (TM) self-cleaning glass. Photoactive materials also find application in several areas that impact on healthcare, namely in light-activated antimicrobial coatings, in biomarkers and in nanoclusters used to kill cancer cells by local heating. For example, nosocomial infections, infections due to treatment in hospital, result in 1% of all UK deaths every year according to the UK Health Protection Agency. Light-activated antimicrobial coatings based on titanium dioxide are studied experimentally (for instance by my collaborator Prof. Parkin) as a means of reducing these infections by eradicating microbial colonisation on surfaces, such as that of catheters, without resorting to antibiotics. The photoactivity of inorganic nanostructures impact the future energy security of the UK as they are employed in a range of renewable energy sources, for example photovoltaics (solar-cells) and the photocatalytic water splitting for renewable hydrogen production. The excited state properties of inorganic nanostructures are also harnessed in solid-state lighting to generate light more energy efficiently than now old-fashioned incandescent light bulbs. Finally, the photochemistry of inorganic nanostructures is used in the purification of drinking water, sewage and air. Here photocatalysis is employed to remove either organic (e.g. pesticides) or biological (e.g. bacteria) contaminants that are difficult to eliminate by other means. For all the above applications my proposed research will provide fundamental insight into the microscopic processes that occur after irradiation with light, important information for the optimisation or engineering of devices and processes that is currently lacking because it is difficult to obtain experimentally. More importantly perhaps, I will develop for the first time a computational screening method, analogous to what has now become routine for ordinary heterogeneous catalysts, which will subsequently allow other researchers to use their computer to screen for the optimal nanostructure for their system before studying the best candidates in the lab. Results of my work that are of interest to UK industry will be disseminated through EPSRC sponsored bodies such as the UK Semiconductor Photochemistry Network and the Molecular Modelling and Materials Science Engineering Doctorate centre at UCL. For the work on antimicrobial coatings the large concentration of researchers at UCL (e.g. in the Chemistry department and Eastman Dental School) and their industrial contacts will be beneficial to the efficient communication of new exciting ideas originating from my research. In the field of energy the UCL Energy Institute will fulfil a similar function. An additional contribution to the UK knowledge economy will come from the training of the two PhD students in this developing and practically important field of research. Beyond their in-group training the students will visit my collaborator Prof. Sousa in Barcelona and will attend a time-dependent density functional theory summer school in Benasque. Furthermore they will gain practical experience in the laboratories of my collaborators at UCL and in Paris. I will also communicate my research area through talks at public events and the UCL schools programme.