This work studied the generation of solar fuels (CO2 to CO reduction and H2 evolution) on photocatalysts comprised of nanoparticulate ZnSe Quantum Dots (QDs) in aqueous (ascorbate) solution.

A continuous-flow setup for photocatalysis was successfully developed which enables in-line gas chromatography of multiple samples in parallel with high sensitivity and operates in an automated fashion. This setup formed a robust experimental protocol for photocatalytic light experiments throughout this dissertation.

First, ligand-free ZnSe-BF4 QDs were examined as light absorbers in combination with a range of molecular catalysts (co-catalysts) based on earth-abundant metal complexes. After an initial co-catalyst screening, three co-catalysts were studied in-depth comprising a phosphonated Ni(cyclam), a Co(quarterpyridine) and a Co(tetraphenylporphyrin) functionalised with three sulfonate groups and one amine group. The latter hybrid photocatalyst exhibited the highest photocatalytic activity (18.6 µmol CO, 27.8 µmol H2) reaching an unprecedented TON (CO) of 619 after 1000 min of irradiation with a CO (vs. H2) selectivity of > 40%. This hybrid photocatalyst showed a distinct induction period which was assigned to slow initial Co(III) to Co(II) reduction and can be accelerated by priming the catalyst in ascorbic acid solution. The insights demonstrate that the photocatalytic activity is not limited to one type of molecular catalyst and that ZnSe QDs are a particularly versatile light-absorber platform to drive a range of molecular co-catalysts based on different catalyst classes and anchoring strategies.

Subsequently, the QDs were used for CO2 reduction in the absence of a molecular co-catalyst by employing an organic surface modification strategy: The chemical environment of the QD surface was modified through design of a capping ligand, which incorporates an imidazolium motif and binds to the QD surface via a thiol group. The ligand capping suppressed H2 evolution and promoted photocatalytic CO2 reduction. The ligand-QD interactions were characterised quantitatively using 1H-NMR spectroscopy and isothermal titration calorimetry which quantified the number of strongly interacting ligands with the QD surface (12 to 17 ligands). Transient absorption spectroscopy and DFT calculations were used to propose a mechanism for the QD-surface promoted reaction in which the imidazolium ligand plays a key role in stabilising a surface-adsorbed CO2- intermediate. Thus, for the first time QDs have been rendered active towards CO2 reduction by means of an organic surface-modification strategy and the results establish capping ligands as a powerful tool to modify the secondary coordination sphere and therefore the product selectivity of colloidal photocatalysts. Furthermore, dithiols, a class of capping ligands not examined in the context of CO2 reduction previously, were found to enhance CO2 reduction on ZnSe QDs as well. A length dependence of the dithiols was found in which shorter dithiols (ca. 4 Å) promote CO2 reduction on the QD surface, whereas longer dithiols (6-8 Å) enhance CO2 reduction in the presence of an additional molecular co-catalyst. The QD-dithiol interactions were studied with 1H-NMR spectroscopy which revealed a solvation sphere dominated by hydrophobic interactions. Additional control experiments and DFT simulations point towards an influence through non-covalent interactions in the secondary coordination sphere to explain the enhanced CO2 reduction.

Finally, the ZnSe QDs operated in a photoelectrochemical setup by deposition on a p-type semiconductor CuCrO2, forming a QD-sensitised photocathode. The CuCrO2-ZnSe photocathode exhibited photocurrents of up to 15 µA cm-2 and was active in controlled potential photoelectrolysis for the reduction of aqueous protons to evolve H2 (38 nmol H2 after 4 h irradiation).

Overall, this work examined ZnSe QDs in aqueous solution as a versatile and efficient platform free of precious metals for photocatalytic CO2 reduction alongside H2 evolution. This work also unveiled organic ligand capping to modify the chemical environment on colloidal photocatalysts, thus enabling control over the product selectivity (H2 vs. CO).