Passivation, phase, and morphology control of CdS nanocrystals using fluorinated aromatic amines

Nanocrystals are widely explored for a range of medical, imaging, sensing, and energy conversion applications. CdS nanocrystals have been reported as excellent photocatalysts, with thin film CdS also highly important in photovoltaic devices. To maximise catalytic activity of nanocrystals, control over phase, facet, and morphology are vital. Here, CdS nanocrystals were synthesised by the solvothermal decomposition of a Cd xanthate single source precursor. To attempt to control CdS nanocrystal surfaces and morphology, the solvent used in the nanocrystal synthesis was altered from pure trioctylphosphine oxide (TOPO) to a mixed TOPO:fluorinated aromatic amine (either 3-fluorobenzyl amine (3-FlBzAm) or 3-fluoroaniline (3-FlAn)), which also provides a sensitive NMR-active probe moiety (mono-fluorinated capping ligands on the CdS nanocrystal surface). Powder X-ray diffraction found that the CdS nanocrystals synthesised from TOPO:3-FlAn solvent mixtures were predominantly cubic whilst the TOPO:3-FlBzAm synthesised nanocrystals were predominantly hexagonal. Raman spectroscopy identified hexagonal CdS in all samples, indicating a likely mixture of phases in at least some of the synthesised systems. Solid-state NMR of ¹¹³Cd, ¹⁹F, ¹³C, and ¹H was employed to investigate the local Cd environments, surface ligands, and ligand interactions. This showed there was a mixture of CdS phases present in all samples and that surfaces were capped with TOPO:fluorinated aromatic amine mixtures, but also that there was a stronger binding affinity of 3-FlBzAm compared with 3-FlAn on the CdS surface. This work highlights that fluorinated aromatic amines can be used to passivate NC surfaces and also control NC properties through their influence during NC growth.