Organometallic routes to colloidal nanomaterials for photoelectrochemical applications
Keywords: layered metal hydroxides
copper oxide
colloids
photoelectrochemistry
zinc hydroxide
organometallics
nanomaterials
nanoparticles

This thesis describes the synthesis of inorganic semiconductor and layered zinc hydroxide colloidal nanomaterials that could, in future, be used in printing of photoelectrochemical devices. The colloidal nanomaterials are synthesised using organometallic precursors which are reacted with stoichiometric/sub-stoichiometric equivalents of ligand to provide well-defined, monodisperse, phase-controlled copper (Cu), cuprous oxide (Cu2O) and cuprous sulfide (Cu2S) nanoparticles or monolayer exfoliated layered zinc hydroxide (LZH) nanosheets.
Chapter 1 provides a general introduction to the field of colloidal nanomaterials, with a comparative overview of different synthetic approaches, recent developments and applications. This is followed by a concise literature review of organometallic synthetic routes to make cuprous oxide (Cu2O), copper sulfide (Cu2-xS) and zinc oxide (ZnO) nanoparticles. It also sets out the thesis aims and objectives. Chapter 2 describes the spectroscopic, microscopy and X-ray diffraction techniques used to characterise the inorganic nanomaterials described within this thesis.
Chapter 3 describes the preparation of copper and cuprous oxide nanoparticles starting from mesitylcopper(I). The chapter focusses on changing the capping ligand so as to deliver nanoparticles that are soluble in polar solvents. It also describes the most reproducible and scalable synthesis of the cuprous oxide nanoparticles. The ligand consists of an alkyl ether carboxylate namely, 2-[2-(2-methoxyethoxy)ethoxy]acetic acid, which furnish the resulting nanoparticles with high solubility in solvents such as alcohols and water. These ligands also deliver particles with consistent morphology, small size (~3 nm) and narrow dispersity. The carboxylate ligands can be readily removed after the particles are deposited onto substrates using low temperature (<200 °C) annealing conditions or displaced by reaction with solutions containing trimethyloxonium tetrafluoroborate.
Chapter 4 describes the synthesis of cuprous sulfide nanoparticles starting from organocopper precursors and by reaction with a range of sulfidising agents. Indirect routes include subjecting copper or cuprous oxide nanoparticles to anion exchange reactions or to reaction with different sulfidising reagents (i.e. S(NH4)2, S8). The preferred direct route was established by reacting mesitylcopper with bis(trimethyl silyl)sulfide or hydrogen sulfide, in the presence of substoichiometric amounts of dithiocarboxylic acid or dithiophosphoric acid as ligands, to deliver exclusively chalcocite phase (Cu2S) nanoparticles with small sizes (3-4 nm) and solubility in polar media. The chapter also presents information regarding the colloidal stability in relation to thio- or oxo-containing ligands.
Chapter 5 describes a bottom-up route to make layered zinc hydroxides nanosheets via the hydrolysis of organozinc reagents in the presence of a stoichiometric quantity of carboxylic acid ligand. The route provide access to both soluble monolayer nanosheets and functional nanosheets. A series of carboxylate ligands, with alkyl ether chains, are used to make layered zinc hydroxide nanosheets showing spontaneous exfoliation and high solubilities in alcohols or water (180 mg mL-1). Altering the carboxylate ligand allows the introduction of functional groups in the layered structures.
Chapter 6 describes preliminary investigation of the suitability of cuprous oxide or sulfide nanoparticles as colloidal ‘inks’ for thin-film fabrication. Different deposition techniques yield films which are assessed for homogeneity, transparency and thickness. Thin-films of cuprous oxide are tested as photocathodes. Methods to improve the film deposition and device architectures are also discussed.
Chapter 7 provides an overall conclusion and outlook on the thesis.
Chapter 8 describes the experimental protocols employed both in nanomaterial synthesis and outlines characterisation data.