2, University of Oxford, Oxford, , United Kingdom
Well-defined, organically-modified ZnO nanoparticles were prepared via an efficient hydrolysis route, without the need for surfactant co-ligands, washing or size-selection steps. The products have a narrow size distribution and are soluble in organic solvents. The synthesis involves reacting a mixture of alkylzinc carboxylate complex and excess diethylzinc with water to yield carboxylate-capped ZnO nanoparticles. Varying the ratio of the different organometallic species enables control of either size or degree of surface modification.
The bottom-up synthesis of ligand-stabilized functional nanoparticles from molecular precursors is widely applied but is difficult to study mechanistically. In this organometallic system, which avoids excess ligand, 31P NMR spectroscopy can be used to follow the trajectory of phosphinate ligands during the synthesis. Initially, we established the structures of a range of ligated zinc oxo clusters, containing 4, 6 and 11 zinc atoms, showing that the clusters interconvert rapidly and self-assemble in solution based on thermodynamic equilibria rather than nucleation kinetics. Subsequently, we identified these clusters in situ during the synthesis of phosphinate-capped zinc oxide nanoparticles. Unexpectedly, the ligand is sequestered to a stable Zn11 cluster during the majority of the synthesis and only becomes coordinated to the nanoparticle surface, in the final step. In addition to a versatile and accessible route to (optionally doped) zinc clusters, the findings provide an understanding of the role of well-defined molecular precursors during the synthesis of small (2–4 nm) nanoparticles.
These stabilised ZnO nanoparticles can be combined with copper nanoparticles to form colloidal catalysts. In a slurry phase reactor, these systems have proven to be highly active in methanol synthesis compared to the analogue heterogeneous Cu/ZnO/Al2O3. The increased catalytic activity appears to correlate with a higher exposed surface area and with the selection of the capping ligand, which plays a role in supporting re-structuring and stability.